Greenstone belts are dominated by mafic volcanic rocks with geochemical characteristics that indicate a range of possible geodynamic influences. Many analogies with modern tectonic settings have been suggested. Increasing exploration of the modern oceans and comprehensive sampling of volcanic rocks from the sea floor are now providing unique opportunities to characterize different melt sources and petrogenesis that can be more closely compared to greenstone belts. In this study, we have compiled high-quality geochemical analyses of more than 2,850 unique samples of submarine mafic volcanic rocks (<60 wt % SiO2) from a wide range of settings, including mid-ocean ridges, ridge-hotspot intersections, intraoceanic arc and back-arc spreading centers, and ocean islands. The compiled data show significant geochemical variability spanning the full range of compositions of basalts found in greenstone belts. This diversity is interpreted to be due to variable crustal thickness, dry melting versus wet melting conditions, mantle mixing, and contamination. In particular, different melting conditions have been linked to mantle heterogeneity, complex mantle flow regimes, and short-lived tectonic domains, such as those associated with diffuse spreading, overlapping spreading centers, and triple junctions. These are well documented in the microplate mosaics of the Western Pacific.

Systematic differences in mafic volcanic rock compositions in modern oceanic settings are revealed by a combination of principal components analysis and unsupervised hierarchical clustering of the compiled data. Mafic volcanic rocks from most arc-back arc systems have strongly depleted mantle signatures and well-known subduction-related chemistry such as large ion lithophile element (LILE) enrichment in combination with strong negative Nb-Ta anomalies and low heavy rare earth elements (HREEs). This contrasts with mafic volcanic rocks in Archean greenstone belts, which show no, or at least weaker, subduction-related chemistry, a less depleted mantle, less wet melting, and variable crustal contamination. The differences are interpreted to be the result of the lower mantle temperatures, thinner crust, and subduction-related processes of present-day settings. However, mafic rocks that are geochemically identical to those in Archean greenstone belts occur in many modern back-arc basins, including the Lau basin, East Scotia ridge, Bransfield Strait, and Manus basin, which are characterized by fertile mantle sources, high heat flow, and complex spreading regimes typical of small-scale microplate mosaics. These types of settings are recognized as favorable for volcanogenic massive sulfide (VMS) deposits in modern and ancient greenstone belts, and therefore the particular geochemical signatures of the mafic volcanic rocks are potentially important for area selection in base metal exploration.

For over 40 years, geochemical signatures of volcanic rocks have been used to infer possible geodynamic settings of ancient greenstone belts and their contained mineral deposits (e.g., Lesher et al., 1986; Paradis et al., 1988; Kerrich and Wyman, 1997; Hart et al., 2004; Piercey, 2011). Comparisons with the modern oceans, however, have been limited by a lack of samples. This is changing with increased ocean exploration and the availability of high-precision analytical data on submarine volcanic rocks from diverse locations. In a companion paper, we compiled data on more than 2,200 samples of felsic volcanic rocks from 70 different locations in the oceans and compared them to Archean rhyolites and dacites that host some of the world’s most important volcanogenic massive sulfide (VMS) deposits. We found significant geochemical diversity in the modern samples, spanning the full range of compositions of felsic volcanic rocks found in greenstone belts (Fassbender et al., 2023). The present study takes a broader look at the geochemistry of mafic volcanic rocks (<60 wt % SiO2), which are found in nearly every niche of the modern oceans and are the dominant lithology of greenstone belts. We compiled high-quality geochemical analyses of more than 2,800 unique samples of submarine mafic volcanic rocks from mid-ocean ridges (MORs), ridge-hotspot intersections, intraoceanic arc and back-arc spreading centers, and ocean islands (Fig. 1) and compare these data to well-studied assemblages of the Abitibi greenstone belt of Canada, the largest and best-preserved Neoarchean greenstone belt in the world. Precise analogs of ancient greenstone belts in modern settings seem unlikely due to the hotter mantle in the Archean, thicker oceanic crust, a potential lack of continental basement, and subduction regimes that were in their infancy (e.g., Bédard et al., 2013; Wyman, 2013). However, many of the fundamental petrogenetic indicators are very similar between modern and ancient systems. Major and trace element geochemistry have been widely used to infer conditions, such as high heat flow associated with rifting, shallow depths of melting, and increased fractionation, that are favorable for the formation of VMS deposits (e.g., Galley et al., 1995; Gibson et al., 2007; Piercey, 2011). However, the most common lithotectonic discrimination diagrams for mafic volcanic rocks have been developed by inspection of relatively small data sets. Here, we use a much larger compilation to identify the geochemical characteristics of mafic volcanic rocks in a full range of modern settings. We also used agglomerative hierarchical clustering and principal component analysis (PCA) to identify geochemical differences and similarities between the data sets. The classes identified in the data reflect highly variable melt sources and processes at different stages of basin evolution and reveal unexpected complexity in rapidly evolving microplate domains. We suggest these findings have implications for understanding similar magmatic processes in older terranes such as the Abitibi greenstone belt.

Fig. 1.

Global map of mafic volcanic rock samples from the oceans compiled for this study. Six different tectonic settings are represented, including intraoceanic back-arc basins (orange: Manus basin, Lau basin, Mariana Trough, East Scotia Ridge, New Hebrides arc-back arc), intraoceanic island arcs (yellow: Mariana arc, Izu-Bonin, Fiji, Tonga Kermadec arc, Lesser Antilles volcanic arc), mid-ocean ridges (olive: Juan de Fuca Ridge, East Pacific Rise (EPR), Galapagos spreading center, Pacific Antarctic Rise, Mid-Atlantic Ridge, South Indian Ridge), ridge-hotspot intersections (purple: Ascension Island, Iceland), ocean islands (green: Hawaii, Marquesas Archipelago) and intracontinental arc-back arc basins (blue: Antarctic Peninsula, Okinawa Trough). Details of the samples are provided in Appendix Table A1.

Fig. 1.

Global map of mafic volcanic rock samples from the oceans compiled for this study. Six different tectonic settings are represented, including intraoceanic back-arc basins (orange: Manus basin, Lau basin, Mariana Trough, East Scotia Ridge, New Hebrides arc-back arc), intraoceanic island arcs (yellow: Mariana arc, Izu-Bonin, Fiji, Tonga Kermadec arc, Lesser Antilles volcanic arc), mid-ocean ridges (olive: Juan de Fuca Ridge, East Pacific Rise (EPR), Galapagos spreading center, Pacific Antarctic Rise, Mid-Atlantic Ridge, South Indian Ridge), ridge-hotspot intersections (purple: Ascension Island, Iceland), ocean islands (green: Hawaii, Marquesas Archipelago) and intracontinental arc-back arc basins (blue: Antarctic Peninsula, Okinawa Trough). Details of the samples are provided in Appendix Table A1.

Mid-ocean ridges account for 75% of global mafic magmatism, with the balance in back-arc basins, volcanic arcs, and ocean islands (Perfit and Davidson, 2000). Mafic melts in these different settings are produced by partial melting of the mantle due to (1) increased temperatures, as in hotspot-related regimes, (2) decreasing pressure, as in mid-ocean ridge spreading centers, and (3) addition of volatiles in subduction zones. These major differences in melting process and different sources involved result in very different trace element geochemical signatures of the basalts (e.g., Arevalo and McDonough, 2010; Hofmann, 2014; Ueki et al., 2018). In the following, we review the systematics of mafic melt production in modern oceanic settings (Fig. 1) and summarize the key geochemical characteristics (Tables 1 and 2). As shown in our previous paper (Fassbender et al., 2023), the petrogenesis of the mafic rocks has an important impact on felsic rock compositions, as fractionation of mafic melts or melting of the associated basaltic crust are the main sources of felsic magma.

Table 1.

Summary Characteristics of Mafic Volcanic Rock Compositions in Modern Oceanic Settings

Geodynamic settingLithotectonic assemblagesExample locationsGeochemical signaturesPetrogenesisReferences
Mid-ocean ridge (MOR)Extensional zones, overlapping spreading centers, propagating rift tips, ridge-transform intersectionsEast Pacific Rise, Juan de Fuca Ridge, Galapagos spreading center, Pacific Antarctic Rise48.4–51.5 wt % SiO2; 0.87–3.27 wt % TiO2; 0.01–0.41 Th/Yb; 1.76–11.35 Ba/Nb; 0.24–5.58 Nb/Yb; 0.57–2.54 La/SmDecompression melting by adiabatic rise of mantle materialFreund et al., 2013,Gale et al., 2013 
Intraoceanic arc and back-arcBack-arc spreading centersLau basin (Valu Fa rift and spreading center, Central Lau spreading center, Northwest Lau spreading center, Northeast Lau spreading center) Manus basin (Manus spreading center) East Scotia Ridge (East Scotia back-arc spreading center) Mariana arc (Mariana Trough back-arc spreading center)48.5–58.4 wt % SiO2; 0.41–2.40 wt % TiO2; 0.02–2.33 Th/Yb; 3.88–191.0 Ba/Nb; 0.20–22.9 Nb/Yb; 0.47–5.0 La/SmCombination of wet and dry melting varying with distance from the dehydrating slabFretzdorff et al., 2002,Pearce et al., 2005,Fretzdorff et al., 2006,Langmuir et al., 2006,Beier et al., 2010,Escrig et al., 2012,Jenner et al., 2012,Lytle et al., 2012,Haase et al., 2022 
 Arc riftsIzu-Bonin (Aogashima rift, Myjoin rift, Torishima rift, Sumisu rift) Manus basin (Pual rift, Susu rift)47.5–58.9 wt % SiO2; 0.34–1.60 wt % TiO2; 0.32–0.57 Th/Yb; 15.3–362.4 Ba/Nb; 0.16–0.96 Nb/Yb; 0.90–2.84 La/SmWet melting induced by fluids released from dehydrating subducting slabBeier et al., 2010,Beier et al., 2015,Haraguchi et al., 2017 
 Arc and back-arc volcanoesLau basin (Niuatahi Volcano, Niuatoputapu Volcano) Kermadec arc (Raoul, Healy, Monowai, Rumble) Lesser Antilles arc (Montserrat)48.1–58.9 wt % SiO2; 0.34–1.42 wt % TiO2; 0.11–1.29 Th/Yb; 36.7–302.4 Ba/Nb; 0.11–1.89 Nb/Yb; 0.85–3.71 La/SmWet and dry meltingHaase et al., 2002 Zellmer et al., 2003 Todd et al., 2011 Park et al., 2015,Beier et al., 2017 
Ocean island volcanism and hot-spot associationHotspot relatedHawaii (Waianae Volcano) Marquesas Archipelago (Hane, Hanatetena, Hikitau)42.7–51.2 wt % SiO2; 2.48–4.46 wt % TiO2; 0.30–6.57 Th/Yb; 4.52–11.67 Ba/Nb; 4.38–47.74 Nb/Yb; 2.03–6.88 La/SmDeep crustal melting and low-degree partial meltingvan der Zander et al., 2010,Chauvel et al., 2012 
 Hotspot and MOR relatedAscension fracture zone (Ascension Island) Iceland43.7–54.78 wt % SiO2; 1.57–3.87 wt % TiO2; 0.66–1.87 Th/Yb; 5.16–8.45 Ba/Nb; 10.23–23.11 Nb/Yb; 3.20–4.72 La/SmLow-degree partial melting of enriched mantle materialJicha et al., 2013,Zellmer et al., 2008 
Intracontinental back-arcEarly-stage back-arc riftRyukyu arc-back arc system (Okinawa Trough)45.9–58.6 wt % SiO2;0.58–2.75 wt % TiO2; 0.08–5.06 Th/Yb; 4.30–190.1 Ba/Nb; 0.42–2.31 Nb/Yb; 1.17–6.40 La/SmCombination of wet and dry melting varying with distance from the dehydrating slab, with variable assimilation of crustal materialShinjo et al., 2000
 Extending marginal basinAntarctic Peninsula (Bransfield Strait)  Keller et al., 2002,Fretzdorff et al., 2004 
Geodynamic settingLithotectonic assemblagesExample locationsGeochemical signaturesPetrogenesisReferences
Mid-ocean ridge (MOR)Extensional zones, overlapping spreading centers, propagating rift tips, ridge-transform intersectionsEast Pacific Rise, Juan de Fuca Ridge, Galapagos spreading center, Pacific Antarctic Rise48.4–51.5 wt % SiO2; 0.87–3.27 wt % TiO2; 0.01–0.41 Th/Yb; 1.76–11.35 Ba/Nb; 0.24–5.58 Nb/Yb; 0.57–2.54 La/SmDecompression melting by adiabatic rise of mantle materialFreund et al., 2013,Gale et al., 2013 
Intraoceanic arc and back-arcBack-arc spreading centersLau basin (Valu Fa rift and spreading center, Central Lau spreading center, Northwest Lau spreading center, Northeast Lau spreading center) Manus basin (Manus spreading center) East Scotia Ridge (East Scotia back-arc spreading center) Mariana arc (Mariana Trough back-arc spreading center)48.5–58.4 wt % SiO2; 0.41–2.40 wt % TiO2; 0.02–2.33 Th/Yb; 3.88–191.0 Ba/Nb; 0.20–22.9 Nb/Yb; 0.47–5.0 La/SmCombination of wet and dry melting varying with distance from the dehydrating slabFretzdorff et al., 2002,Pearce et al., 2005,Fretzdorff et al., 2006,Langmuir et al., 2006,Beier et al., 2010,Escrig et al., 2012,Jenner et al., 2012,Lytle et al., 2012,Haase et al., 2022 
 Arc riftsIzu-Bonin (Aogashima rift, Myjoin rift, Torishima rift, Sumisu rift) Manus basin (Pual rift, Susu rift)47.5–58.9 wt % SiO2; 0.34–1.60 wt % TiO2; 0.32–0.57 Th/Yb; 15.3–362.4 Ba/Nb; 0.16–0.96 Nb/Yb; 0.90–2.84 La/SmWet melting induced by fluids released from dehydrating subducting slabBeier et al., 2010,Beier et al., 2015,Haraguchi et al., 2017 
 Arc and back-arc volcanoesLau basin (Niuatahi Volcano, Niuatoputapu Volcano) Kermadec arc (Raoul, Healy, Monowai, Rumble) Lesser Antilles arc (Montserrat)48.1–58.9 wt % SiO2; 0.34–1.42 wt % TiO2; 0.11–1.29 Th/Yb; 36.7–302.4 Ba/Nb; 0.11–1.89 Nb/Yb; 0.85–3.71 La/SmWet and dry meltingHaase et al., 2002 Zellmer et al., 2003 Todd et al., 2011 Park et al., 2015,Beier et al., 2017 
Ocean island volcanism and hot-spot associationHotspot relatedHawaii (Waianae Volcano) Marquesas Archipelago (Hane, Hanatetena, Hikitau)42.7–51.2 wt % SiO2; 2.48–4.46 wt % TiO2; 0.30–6.57 Th/Yb; 4.52–11.67 Ba/Nb; 4.38–47.74 Nb/Yb; 2.03–6.88 La/SmDeep crustal melting and low-degree partial meltingvan der Zander et al., 2010,Chauvel et al., 2012 
 Hotspot and MOR relatedAscension fracture zone (Ascension Island) Iceland43.7–54.78 wt % SiO2; 1.57–3.87 wt % TiO2; 0.66–1.87 Th/Yb; 5.16–8.45 Ba/Nb; 10.23–23.11 Nb/Yb; 3.20–4.72 La/SmLow-degree partial melting of enriched mantle materialJicha et al., 2013,Zellmer et al., 2008 
Intracontinental back-arcEarly-stage back-arc riftRyukyu arc-back arc system (Okinawa Trough)45.9–58.6 wt % SiO2;0.58–2.75 wt % TiO2; 0.08–5.06 Th/Yb; 4.30–190.1 Ba/Nb; 0.42–2.31 Nb/Yb; 1.17–6.40 La/SmCombination of wet and dry melting varying with distance from the dehydrating slab, with variable assimilation of crustal materialShinjo et al., 2000
 Extending marginal basinAntarctic Peninsula (Bransfield Strait)  Keller et al., 2002,Fretzdorff et al., 2004 
Table 2.

Interpreted Significance of Trace Element Indicators of Mafic Volcanic Rocks from Different Modern Oceanic Sttings (see text for details and references)

Elements/ratiosUnderlying behaviorResultInterpretation
LILEs (K, Rb, Cs, Sr, Ba)Incompatible behavior; high fluid mobilityHigh LILEs in enriched sources (e.g., OIBs), in fractionated melts, and in low-degree partial melts; significant LILE enrichment in arc-related magmasLILE enrichment controlled by dehydration of subducted slab; low LILEs in MOR settings reflecting lack of fluid-mobilized elements and depleted mantle
HFSEs (Zr, Nb, Hf, REE)Incompatible behavior; low fluid mobilityHigh HFSEs in enriched sources and in fractionated melts; high HREEs in low-degree partial melts; significant HFSE depletion in arc-related magmasHFSEs controlled by residual enrichment compared to fluid-mobile elements and by enriched mantle sources
Nb-Ta anomalyIncompatible behavior; low fluid mobilityHigh Nb-Ta in enriched sources in fractionated melts and in low-degree partial melts; significant Nb-Ta depletion in arc-related magmasNb-Ta depletion due to selective enrichment of nearby elements in the multielement diagram by fluids from a dehydrating slab
Th/YbTh mobilized in melts only at high subduction temperatures compared to Yb; crustal assimilation of Th-enriched, subduction-related crustHigh Th/Yb in continental arc crust, proximal to subduction zones, and in areas of crustal assimilation settings; low Th/Yb in oceanic arcs and as a result of dry meltingHigh volatile input and high Th/Yb controlled by dehydration and melting of a subducted slab at high temperature; contamination by subduction related crust
Ba/NbBa mobilized in melts and fluids over wide range of subduction temperatures compared to NbHigh Ba/Nb proximal to subduction zones; low Ba/Nb distal to subduction zonesHigh volatile input controlled by dehydration of a subducted slab
Th/NbTh mobilized in melts only at high subduction temperaturesHigh Th/Nb proximal to subduction zones; low Th/Nb distal to subduction zonesHigh volatile input controlled by dehydration of a subducted slab at high temperature
La/SmSlight difference in incompatible behaviorHigh La/Sm in enriched sources (e.g., OIBs); low La/Sm in depleted sources (e.g., MORs)LREE enrichment originating from less depleted mantle (e.g., OIBs); variable enrichment and mostly flat LREE patterns due to mantle heterogeneity and degrees of depletion
Nb/YbSlight difference in incompatible behaviorHigh Nb/Yb in enriched sources (e.g., OIBs); low Nb/Yb in depleted sources (e.g., MORs)Enriched mantle sources and low degrees of melting
Elements/ratiosUnderlying behaviorResultInterpretation
LILEs (K, Rb, Cs, Sr, Ba)Incompatible behavior; high fluid mobilityHigh LILEs in enriched sources (e.g., OIBs), in fractionated melts, and in low-degree partial melts; significant LILE enrichment in arc-related magmasLILE enrichment controlled by dehydration of subducted slab; low LILEs in MOR settings reflecting lack of fluid-mobilized elements and depleted mantle
HFSEs (Zr, Nb, Hf, REE)Incompatible behavior; low fluid mobilityHigh HFSEs in enriched sources and in fractionated melts; high HREEs in low-degree partial melts; significant HFSE depletion in arc-related magmasHFSEs controlled by residual enrichment compared to fluid-mobile elements and by enriched mantle sources
Nb-Ta anomalyIncompatible behavior; low fluid mobilityHigh Nb-Ta in enriched sources in fractionated melts and in low-degree partial melts; significant Nb-Ta depletion in arc-related magmasNb-Ta depletion due to selective enrichment of nearby elements in the multielement diagram by fluids from a dehydrating slab
Th/YbTh mobilized in melts only at high subduction temperatures compared to Yb; crustal assimilation of Th-enriched, subduction-related crustHigh Th/Yb in continental arc crust, proximal to subduction zones, and in areas of crustal assimilation settings; low Th/Yb in oceanic arcs and as a result of dry meltingHigh volatile input and high Th/Yb controlled by dehydration and melting of a subducted slab at high temperature; contamination by subduction related crust
Ba/NbBa mobilized in melts and fluids over wide range of subduction temperatures compared to NbHigh Ba/Nb proximal to subduction zones; low Ba/Nb distal to subduction zonesHigh volatile input controlled by dehydration of a subducted slab
Th/NbTh mobilized in melts only at high subduction temperaturesHigh Th/Nb proximal to subduction zones; low Th/Nb distal to subduction zonesHigh volatile input controlled by dehydration of a subducted slab at high temperature
La/SmSlight difference in incompatible behaviorHigh La/Sm in enriched sources (e.g., OIBs); low La/Sm in depleted sources (e.g., MORs)LREE enrichment originating from less depleted mantle (e.g., OIBs); variable enrichment and mostly flat LREE patterns due to mantle heterogeneity and degrees of depletion
Nb/YbSlight difference in incompatible behaviorHigh Nb/Yb in enriched sources (e.g., OIBs); low Nb/Yb in depleted sources (e.g., MORs)Enriched mantle sources and low degrees of melting

Abbreviations: HFSEs = high field strength elements, LILEs = large ion lithophile elements, LREEs = light rare earth elements, MOR = mid-ocean ridge, OIB = ocean island basalt, REEs = rare earth elements

Mid-ocean ridge basalts

Mid-ocean ridges are dominated by tholeiitic basalt (mid-ocean ridge basalt [MORB]) produced by decompression partial melting of the mantle (McKenzie and O’Nions, 1991). High degrees of partial melting result in low incompatible element concentrations, including light rare earth elements (LREEs) and large ion lithophile elements (LILEs), relative to the more compatible heavy rare earth elements (HREEs) and high field strength elements (HFSEs) (Hofmann, 2014). Complexity is introduced by varying magmatic heat input and hydrothermal cooling, which change with spreading rate (Michael and Cornell, 1998; Haase et al., 2005). End member examples include the fast-spreading ridges such as the East Pacific Rise at 20-21°S and the Pacific-Antarctic Rise at 37°S, which have stable axial melt lenses developed in a warm lithosphere and thickened oceanic crust, and slow-spreading ridges such as the Mid-Atlantic Ridge at 45°N, where magma chambers are dispersed in a colder lithosphere and magmatism is episodic. These differences result in distinct geochemical signatures, especially at slow-spreading ridges (White and Klein, 2014 and references therein) where greater tectonic extension can lead to strong hydrothermal cooling and fractionation. Mafic melts at ridge-hotspot intersections are produced by lower degrees of partial melting in thicker crustal settings, resulting in intermediate incompatible element concentrations (Jicha et al., 2013). Isotopic systems such as 87Sr/86Sr and 143Nd/144Nd show that mantle heterogeneity is also important.

Ocean island basalts

Hotspot volcanism, such as at Hawaii and the Marquesas Archipelago, is characterized by deeply sourced alkaline melts produced by low degrees of partial melting of the mantle at high pressure and high temperature (Hofmann, 2014; Haase et al., 2019). This results in the enrichment of incompatible elements (LREEs, LILEs) relative to compatible elements (HREEs, HFSEs), producing the typical signatures of ocean island basalt (OIB). Additional complexity is introduced by mantle reservoirs with higher 87Sr/86Sr and lower 143Nd/144Nd than at MORs (Jackson et al., 2010; Price et al., 2014).

Intraoceanic arc and back-arc basalts

Arc volcanism in intraoceanic settings, such as the Tofua and Izu Bonin arcs, is mainly calc-alkaline. Melting in the sub-arc mantle is triggered by addition of volatiles from a dehydrating slab, which lowers the solidus temperature of the mantle (e.g., Hawkesworth et al., 1993; Langmuir et al., 2006). The volatiles are sourced from sediments, pore fluids, and hydrous minerals (from earlier alteration of the oceanic crust) and are released into the mantle wedge via pervasive or channeled flow during dehydration of the slab. The melts have high water contents, strong LILE enrichment, and high oxidation states. Certain trace elements, such as Ba, are mobile in the slab-derived fluids and become enriched in fluid-fluxed melts, while Nb and Yb are immobile, resulting in high Ba/Nb and Ba/Yb ratios (e.g., Pearce et al., 2005).

Back-arc rifts and spreading centers, such as in the Lau basin and Manus basin, produce both calc-alkaline and tholeiitic melts, depending on the volatile input from the slab (Langmuir et al., 2006). This mainly varies with proximity of the spreading center to the adjacent arc and trench, but also with the age of the subducting lithosphere, the angle of subduction, the rate of slab rollback, the trajectory of the overriding plate, and different mantle flow regimes (Sdrolias and Müller, 2006; Castillo, 2012; Schmidt and Poli, 2014; Schellart, 2020). These, in turn, vary with the progress of basin opening, which begins with initial rifting and eventually leads to back-arc spreading (Taylor and Martinez, 2003; Pearce et al., 2005; Langmuir et al., 2006). With basin opening, melting regimes that may initially overlap become separated into zones of hydrous melting proximal to the arc and dry (MORB-like) melting of depleted mantle in the deeper back-arc basin. At the Valu Fa Ridge in the Lau basin and the Eastern Rifts of the Manus basin, calc-alkaline melts are produced where the rifts are propagating into arc crust. At more distal spreading centers, such as the Northwest Lau spreading center and the Manus spreading center in the Manus basin, the melts are tholeiitic.

Boninites

Boninitic melts have been found in a wide range of modern oceanic settings, including back-arc spreading centers and mantle plumes (Cooper et al., 2010; Resing et al., 2011; Golowin et al., 2017; Pearce and Reagan, 2019; Pearce and Arculus, 2020). They are produced by melting of previously depleted mantle material. In subduction zones, early rollback of the subducting slab triggers melting that leaves behind a depleted mantle (Arculus et al., 2019). Melting of this more refractory material is promoted by eventual addition of volatiles and increased heat. The resulting melts are widely interpreted to be indicative of subduction initiation (Arculus et al., 2019; Pearce and Arculus, 2020). However, similar melts are known from back-arc spreading centers and plume-related regimes (Pearce and Reagan, 2019). In the Fonualei rift and spreading center of the northern Lau basin, boninites are variably interpreted to be products of melting of subarc mantle that was depleted by earlier episodes of back-arc spreading or by melting of refractory material related to the Samoan plume (Falloon et al., 2007; Cooper et al., 2010; Resing et al., 2011; Escrig et al., 2012). In mantle-plume settings, the melting is thought to be initiated by a pulse of hot material from the plume (Pearce and Arculus, 2020). The origins of these melts are of particular interest, as they have been widely recognized in greenstone belts (Kerrich et al., 1998).

Geochemical analyses of mafic lavas (<60 wt % SiO2) from 433 locations in the oceans were compiled from the literature (Fig. 1). From an original database of more than 3,800 analyses, a subset of 2,857 unique samples was selected for this study. The rejected samples included duplicate samples, repeat analyses of the same sample, samples analyzed by inappropriate methods, and samples with location errors or incomplete descriptions. Only samples with complete major and trace element data were retained; every sample in the final data set was analyzed for at least Ti, Al, Ba, Th, Zr, Nb, La, Y, and Yb. No samples were included where the concentration of any element was equal to or below its detection limit or exceeded the upper limits of detection or where the limits of detection were not specified. To check for altered samples, we compared the data to fresh rock compositions of basalts and basaltic andesites, including immobile element ratios and REE profiles. In particular, we excluded samples with unusually high or low mobile element concentrations, such as Si, Na, K, and Eu. The final database is provided in Appendix Table A1.

The MORBs in the database have low SiO2 (48.7–52.1 wt %), low K2O (0.01–0.47 wt %), and intermediate TiO2 (0.55–2.57 wt %) and are depleted in LREEs. Mafic volcanic rocks from ridge-hotspot intersections have variable SiO2 concentrations (43.7–54.8 wt %) but high K2O (0.2–2.3 wt %), high TiO2 (1.6–3.9 wt %), and LREE enrichment. Ocean island basalts also have high K2O (0.2–2.0 wt %), high TiO2 (2.4–4.5 wt %), and LREE enrichment but low SiO2 concentrations (42.4–51.2 wt %). Samples from intraoceanic back-arc and island arc assemblages have high SiO2 concentrations (44.9–58.9 wt %), intermediate K2O (0.01–1.1 wt %), and low TiO2 (0.1–2.4 wt %) and are only moderately LREE enriched. Samples from intracontinental arc-back arc assemblages have high SiO2 concentrations (45.9–58.6 wt %), intermediate K2O (0.2–1.9 wt %), intermediate TiO2 (0.6–2.8 wt %), and moderate LREE enrichment. Figure 2 (see App. Fig. A1 for details) shows the range of mafic volcanic rock compositions in the database on a total alkali-silica plot (after Le Maitre, 1989) and an immobile element plot (after Winchester and Floyd, 1977). The original Winchester and Floyd (1977) diagram is based on a limited number of samples, and so the full range of modern oceanic basalts considered here is not well classified by the depicted field boundaries. Pearce (1996) used a more comprehensive data set; however, some of the modern oceanic basalts are still not well classified (App. Fig. A2). Data for mafic volcanic rocks from the Abitibi greenstone belt are shown for comparison and discussed further below.

Fig. 2.

A) Total alkali-silica plot (after Le Maitre, 1989) showing the distribution of modern oceanic mafic volcanic rocks (see App. Fig. A1 for details). Samples from ridge-hotspot intersections and ocean islands have a dominantly alkaline affinity. Mid-ocean ridge basalts are subalkaline with low SiO2 concentrations. Samples from intraoceanic back-arc assemblages plot in a strong linear array from low SiO2 to high SiO2 with increasing Na2O + K2O concentrations. Samples from intraoceanic island arc assemblages have a wider range of SiO2 concentrations with a narrower range of Na2O + K2O. Samples from intracontinental arc-back arc assemblages have the highest SiO2 concentrations and a wide range of Na2O + K2O. The range of compositions of mafic volcanic rocks in the Abitibi greenstone belt is shown for comparison (black symbols beneath the other data points: see text for discussion and App. Fig. A1 for details). B) Discrimination plot of Winchester and Floyd (1977) showing the distribution of modern oceanic mafic volcanic rocks (see App. Fig. A1 for details). Mid-ocean ridge mafic volcanic rocks from the East Pacific Rise, Pacific-Antarctic Rise, and Juan de Fuca Ridge have low Nb/Y and high Zr/TiO2 and plot in the basalt/andesite field. Samples from different back-arc assemblages, such as the Lau basin and Manus basin, and island-arc assemblages, such as Izu Bonin, plot mostly in the basalt field with a range of Zr/TiO2 and Nb/Y ratios extending into the andesite/basalt field. Mafic volcanic rocks from intracontinental arc-back arc assemblages, such as the Okinawa Trough and Western Antarctic Peninsula, have a very wide range of Zr/TiO2 and Nb/Y spanning the entire field of basalt and andesite (beneath the mid-ocean ridge samples: see App. Fig. A1 for details). Note that the boundaries in Winchester and Floyd (1977) are based on a limited number of oceanic arc samples and therefore are not fully representative of the samples in this study. As a result, many of the new samples are misclassified.

Fig. 2.

A) Total alkali-silica plot (after Le Maitre, 1989) showing the distribution of modern oceanic mafic volcanic rocks (see App. Fig. A1 for details). Samples from ridge-hotspot intersections and ocean islands have a dominantly alkaline affinity. Mid-ocean ridge basalts are subalkaline with low SiO2 concentrations. Samples from intraoceanic back-arc assemblages plot in a strong linear array from low SiO2 to high SiO2 with increasing Na2O + K2O concentrations. Samples from intraoceanic island arc assemblages have a wider range of SiO2 concentrations with a narrower range of Na2O + K2O. Samples from intracontinental arc-back arc assemblages have the highest SiO2 concentrations and a wide range of Na2O + K2O. The range of compositions of mafic volcanic rocks in the Abitibi greenstone belt is shown for comparison (black symbols beneath the other data points: see text for discussion and App. Fig. A1 for details). B) Discrimination plot of Winchester and Floyd (1977) showing the distribution of modern oceanic mafic volcanic rocks (see App. Fig. A1 for details). Mid-ocean ridge mafic volcanic rocks from the East Pacific Rise, Pacific-Antarctic Rise, and Juan de Fuca Ridge have low Nb/Y and high Zr/TiO2 and plot in the basalt/andesite field. Samples from different back-arc assemblages, such as the Lau basin and Manus basin, and island-arc assemblages, such as Izu Bonin, plot mostly in the basalt field with a range of Zr/TiO2 and Nb/Y ratios extending into the andesite/basalt field. Mafic volcanic rocks from intracontinental arc-back arc assemblages, such as the Okinawa Trough and Western Antarctic Peninsula, have a very wide range of Zr/TiO2 and Nb/Y spanning the entire field of basalt and andesite (beneath the mid-ocean ridge samples: see App. Fig. A1 for details). Note that the boundaries in Winchester and Floyd (1977) are based on a limited number of oceanic arc samples and therefore are not fully representative of the samples in this study. As a result, many of the new samples are misclassified.

Figure 3 shows the range of magmatic affinities of modern oceanic rock (after Ross and Bédard, 2009). The mafic volcanic rocks from Juan de Fuca Ridge and the East Pacific Rise, Galapagos spreading center, Pacific Antarctic Rise, Mid-Atlantic Ridge, and South Indian Ridge plot mainly in the tholeiitic field. Samples from ridge-hotspot intersections and ocean islands, such as Ascension Island, Iceland, Hawaii, and volcanoes of the Marquesas Archipelago, are mostly transitional to alkaline basalts, basanites, and basaltic trachytes. Samples from intraoceanic back-arc assemblages, such as in the Manus basin, Lau basin, Mariana Trough, East Scotia Ridge, and New Hebrides, are similar to MORBs (i.e., dominantly tholeiitic basalts). Samples from island-arc assemblages, such as the Mariana, Izu-Bonin, Fiji, Tonga-Kermadec, and Lesser Antilles volcanic arcs, span the range from tholeiitic to calc-alkaline basalts and andesites. Mafic volcanic rocks from intracontinental arc-back arc settings, such as the Okinawa Trough and Western Antarctic Peninsula, are mostly calc-alkaline andesitic basalts. The corresponding tectonic affiliations are shown in the discrimination plot of Pearce (2014).

Examples of geochemical and tectonic affinity diagrams for mafic volcanic rocks showing the classifications of modern oceanic basalts. A) Geochemical affinity diagram of Ross and Bédard (2009) showing trace element data for mafic volcanic rocks from the Mid-Atlantic Ridge, Iceland, Ascension Island, Lesser Antilles volcanic arc, East Scotia Ridge, and Antarctic Peninsula. The samples range from dominantly tholeiitic to calc-alkaline; the tholeiitic array includes samples from the Mid-Atlantic Ridge and East Scotia Ridge, and the calc-alkaline array includes samples from Ascension Island and Antarctic Peninsula. The transitional array includes samples from Iceland, East Scotia Ridge, the Antarctic Peninsula, and the Lesser Antilles arc. B) Lithotectonic discrimination plot of Pearce (2014) showing trace element data for the same mafic volcanic rocks in (A). The normal mid-ocean ridge basalt (N-MORB) and ocean island basalt (OIB) array includes mainly samples from the Mid-Atlantic Ridge and East Scotia Ridge. Samples from Ascension Island plot in the OIB array. The oceanic arc and continental arc fields include samples from the Lesser Antilles and Antarctic Peninsula. However, there is overlap with the enhanced (E)-MORB to ocean island basalt (OIB) array. C) Trace element data for mafic volcanic rocks from Juan de Fuca Ridge, the East Pacific Rise, the Galapagos spreading center, the Pacific Antarctic Rise, the Marquesas Archipelago, and Hawaii on the geochemical affinity diagram of Ross and Bédard (2009). Samples from Juan de Fuca, the East Pacific Rise, the Galapagos spreading center, and the Pacific Antarctic Rise are almost all tholeiitic. Samples from the Marquesas Archipelago and Hawaii plot mostly in the calc-alkaline array. D) Trace element data for mafic volcanic rocks in (C) on the lithotectonic discrimination plot of Pearce (2014). Samples from Juan de Fuca Ridge, the East Pacific Rise, the Galapagos spreading center, and the Pacific Antarctic Rise plot in the N-MORB to E-MORB array. Samples from the Marquesas Archipelago and Hawaii plot in the OIB array. E) Trace element data for mafic volcanic rocks from the Manus basin, Mariana arc-back arc, New Hebrides arc-back arc, Izu-Bonin, and Okinawa Trough on the geochemical affinity diagram of Ross and Bédard (2009). Samples from the Manus basin and the Mariana arc-back arc are tholeiitic to calc-alkaline. Samples from the New Hebrides arc-back arc and the Izu-Bonin are tholeiitic to transitional. Samples from the Okinawa Trough plot mostly in the transitional array. F) Trace element data for mafic volcanic rocks in (E) on the lithotectonic discrimination plot of Pearce (2014). Samples from the Manus basin, the New Hebrides arc-back arc, and Izu-Bonin plot in the N-MORB array. Samples from the Mariana arc-back arc plot in the E-MORB and oceanic arc arrays. Samples from the Manus basin, the New Hebrides arc-back arc, and the Izu Bonin mainly plot in the oceanic arc field. A few samples from the Manus Basin, the Mariana arc-back arc, and the New Hebrides arc-back arc plot in the continental arc field. Samples from the Okinawa Trough plot only in the continental arc field. G) Trace element data for mafic volcanic rocks from Fiji, Tonga-Kermadec, and the South Indian Ridge on the geochemical affinity diagram of Ross and Bédard (2009). Samples from Fiji and Tonga-Kermadec plot in the tholeiitic to calc-alkaline array. Samples from South Indian Ridge are mostly tholeiitic. H) Trace element data for mafic volcanic rocks from (G) on the lithotectonic discrimination plot of Pearce et al. (2014). Samples from Fiji and Tonga-Kermadec plot in the N-MORB to OIB array and oceanic and continental arc fields. Samples from the South Indian Ridge plot in the N-MORB to E-MORB array.

Examples of geochemical and tectonic affinity diagrams for mafic volcanic rocks showing the classifications of modern oceanic basalts. A) Geochemical affinity diagram of Ross and Bédard (2009) showing trace element data for mafic volcanic rocks from the Mid-Atlantic Ridge, Iceland, Ascension Island, Lesser Antilles volcanic arc, East Scotia Ridge, and Antarctic Peninsula. The samples range from dominantly tholeiitic to calc-alkaline; the tholeiitic array includes samples from the Mid-Atlantic Ridge and East Scotia Ridge, and the calc-alkaline array includes samples from Ascension Island and Antarctic Peninsula. The transitional array includes samples from Iceland, East Scotia Ridge, the Antarctic Peninsula, and the Lesser Antilles arc. B) Lithotectonic discrimination plot of Pearce (2014) showing trace element data for the same mafic volcanic rocks in (A). The normal mid-ocean ridge basalt (N-MORB) and ocean island basalt (OIB) array includes mainly samples from the Mid-Atlantic Ridge and East Scotia Ridge. Samples from Ascension Island plot in the OIB array. The oceanic arc and continental arc fields include samples from the Lesser Antilles and Antarctic Peninsula. However, there is overlap with the enhanced (E)-MORB to ocean island basalt (OIB) array. C) Trace element data for mafic volcanic rocks from Juan de Fuca Ridge, the East Pacific Rise, the Galapagos spreading center, the Pacific Antarctic Rise, the Marquesas Archipelago, and Hawaii on the geochemical affinity diagram of Ross and Bédard (2009). Samples from Juan de Fuca, the East Pacific Rise, the Galapagos spreading center, and the Pacific Antarctic Rise are almost all tholeiitic. Samples from the Marquesas Archipelago and Hawaii plot mostly in the calc-alkaline array. D) Trace element data for mafic volcanic rocks in (C) on the lithotectonic discrimination plot of Pearce (2014). Samples from Juan de Fuca Ridge, the East Pacific Rise, the Galapagos spreading center, and the Pacific Antarctic Rise plot in the N-MORB to E-MORB array. Samples from the Marquesas Archipelago and Hawaii plot in the OIB array. E) Trace element data for mafic volcanic rocks from the Manus basin, Mariana arc-back arc, New Hebrides arc-back arc, Izu-Bonin, and Okinawa Trough on the geochemical affinity diagram of Ross and Bédard (2009). Samples from the Manus basin and the Mariana arc-back arc are tholeiitic to calc-alkaline. Samples from the New Hebrides arc-back arc and the Izu-Bonin are tholeiitic to transitional. Samples from the Okinawa Trough plot mostly in the transitional array. F) Trace element data for mafic volcanic rocks in (E) on the lithotectonic discrimination plot of Pearce (2014). Samples from the Manus basin, the New Hebrides arc-back arc, and Izu-Bonin plot in the N-MORB array. Samples from the Mariana arc-back arc plot in the E-MORB and oceanic arc arrays. Samples from the Manus basin, the New Hebrides arc-back arc, and the Izu Bonin mainly plot in the oceanic arc field. A few samples from the Manus Basin, the Mariana arc-back arc, and the New Hebrides arc-back arc plot in the continental arc field. Samples from the Okinawa Trough plot only in the continental arc field. G) Trace element data for mafic volcanic rocks from Fiji, Tonga-Kermadec, and the South Indian Ridge on the geochemical affinity diagram of Ross and Bédard (2009). Samples from Fiji and Tonga-Kermadec plot in the tholeiitic to calc-alkaline array. Samples from South Indian Ridge are mostly tholeiitic. H) Trace element data for mafic volcanic rocks from (G) on the lithotectonic discrimination plot of Pearce et al. (2014). Samples from Fiji and Tonga-Kermadec plot in the N-MORB to OIB array and oceanic and continental arc fields. Samples from the South Indian Ridge plot in the N-MORB to E-MORB array.

Extended trace element plots of the mafic volcanic rocks in different settings are shown in Figure 4. They are grouped according to typical MOR-like signatures (Fig. 4A, B), intraoceanic arc-back arc rift and spreading center signatures (Fig. 4C, D), and arc-back arc signatures (Fig. 4E, F). The MOR mafic volcanic rocks, represented by the East Pacific Rise and Mid-Atlantic Ridge, have mostly flat REE patterns and no negative Nb-Ta anomaly and plot in the N-MORB array of Pearce (2014). Samples from ridge-hotspot intersections and ocean islands have more steeply dipping REE patterns, no negative Nb-Ta anomaly, and plot in the E-MORB array. Mafic volcanic rocks from intraoceanic back-arc assemblages, represented by the East Scotia Ridge, have mostly flat REE patterns with strong negative Nb-Ta anomalies and low HREE concentrations and plot in the oceanic arc array. Samples from the terminations of back-arc spreading centers, represented by the Northeast Lau spreading center, the Southern East Scotia Ridge, and the northern volcanic tectonic zone of the Mariana Trough, all have weak negative Nb-Ta anomalies and dipping REE patterns and plot between the E-MORB and continental arc arrays. Samples from back-arc rifts that are propagating into arc crust, represented by the Fonualei rift and spreading center, have even stronger negative Nb-Ta anomalies and low HREE concentrations. Mafic volcanic rocks from intracontinental arc-back arc settings, represented by the Okinawa Trough, have dipping REE patterns and intermediate negative Nb-Ta anomalies and overlap with the continental arc array.

Fig. 4.

A) Primitive mantle-normalized trace element plot comparing a subset of mafic volcanic rocks from mid-ocean ridges, ridge-hotspot intersections, and ocean islands (individual samples are transparent; averages are nontransparent). Samples from the East Pacific Rise have no negative Nb-Ta anomaly and a rising rare earth element (REE) pattern, whereas samples from Iceland, Hawaii, and Ascension Island have elevated REEs and dipping REE patterns. B) Lithotectonic discrimination plot of Pearce (2014) of the samples in A) showing a strong linear array from normal mid-ocean ridge basalt (N-MORB) to ocean island basalt (OIB). Samples from the East Pacific Rise plot mostly in the N-MORB array, whereas samples from Iceland, Hawaii and Ascension Island plot mostly in the array from enhanced (E)-MORB to OIB. C) Primitive mantle-normalized trace element plot comparing a subset of mafic volcanic rocks from the Mid-Atlantic Ridge and representative back-arc rifts and back-arc spreading centers. Samples from the Mid-Atlantic Ridge have a positive Nb-Ta anomaly and flat REE pattern, whereas samples from the back-arc rifts and spreading center (Ngatoroirangi rift, Fonualei rift and spreading center, and East Scotia Ridge) have negative Nb-Ta anomalies and flat REE patterns. D) Lithotectonic discrimination plot of the samples in (C) showing differences in the samples from arcs with strong fluid-fluxed components (Fonualei spreading center) and those with weaker fluid-fluxed components (East Scotia Ridge, Ngatoroirangi rift). Samples from the Ngatoroirangi rift, Fonualei rift and spreading center, and East Scotia Ridge plot mostly within the oceanic arc field. Samples from the Mid-Atlantic Ridge plot in a strong linear array within the N-MORB field. E) Primitive mantle-normalized trace element plot comparing a subset of mafic volcanic rocks from an intracontinental back arc rift (Okinawa Trough), the Vate and Futuna Troughs, and propagating rifts in nascent back-arc crust (Northeastern Lau spreading center, Southern East Scotia Ridge, northern volcanic tectonic zone – Mariana Trough). The samples from the Northeast Lau spreading center are a subset of the Tonga-Kermadec samples plotted in Figure 3. The samples from the Southern East Scotia Ridge are a subset from the southern termination of the East Scotia Ridge arc. The samples from the northern volcanic tectonic zone are a subset of back-arc basin samples at the northern termination of the Mariana Trough back arc. The samples from the Vate and Futuna Trough are a subset of the New Hebrides samples. All samples have weak negative Nb-Ta anomalies and shallowly dipping REE patterns. F) Lithotectonic discrimination plot of the samples in (E) showing composition intermediate between the continental arc and the MORB fields.

Fig. 4.

A) Primitive mantle-normalized trace element plot comparing a subset of mafic volcanic rocks from mid-ocean ridges, ridge-hotspot intersections, and ocean islands (individual samples are transparent; averages are nontransparent). Samples from the East Pacific Rise have no negative Nb-Ta anomaly and a rising rare earth element (REE) pattern, whereas samples from Iceland, Hawaii, and Ascension Island have elevated REEs and dipping REE patterns. B) Lithotectonic discrimination plot of Pearce (2014) of the samples in A) showing a strong linear array from normal mid-ocean ridge basalt (N-MORB) to ocean island basalt (OIB). Samples from the East Pacific Rise plot mostly in the N-MORB array, whereas samples from Iceland, Hawaii and Ascension Island plot mostly in the array from enhanced (E)-MORB to OIB. C) Primitive mantle-normalized trace element plot comparing a subset of mafic volcanic rocks from the Mid-Atlantic Ridge and representative back-arc rifts and back-arc spreading centers. Samples from the Mid-Atlantic Ridge have a positive Nb-Ta anomaly and flat REE pattern, whereas samples from the back-arc rifts and spreading center (Ngatoroirangi rift, Fonualei rift and spreading center, and East Scotia Ridge) have negative Nb-Ta anomalies and flat REE patterns. D) Lithotectonic discrimination plot of the samples in (C) showing differences in the samples from arcs with strong fluid-fluxed components (Fonualei spreading center) and those with weaker fluid-fluxed components (East Scotia Ridge, Ngatoroirangi rift). Samples from the Ngatoroirangi rift, Fonualei rift and spreading center, and East Scotia Ridge plot mostly within the oceanic arc field. Samples from the Mid-Atlantic Ridge plot in a strong linear array within the N-MORB field. E) Primitive mantle-normalized trace element plot comparing a subset of mafic volcanic rocks from an intracontinental back arc rift (Okinawa Trough), the Vate and Futuna Troughs, and propagating rifts in nascent back-arc crust (Northeastern Lau spreading center, Southern East Scotia Ridge, northern volcanic tectonic zone – Mariana Trough). The samples from the Northeast Lau spreading center are a subset of the Tonga-Kermadec samples plotted in Figure 3. The samples from the Southern East Scotia Ridge are a subset from the southern termination of the East Scotia Ridge arc. The samples from the northern volcanic tectonic zone are a subset of back-arc basin samples at the northern termination of the Mariana Trough back arc. The samples from the Vate and Futuna Trough are a subset of the New Hebrides samples. All samples have weak negative Nb-Ta anomalies and shallowly dipping REE patterns. F) Lithotectonic discrimination plot of the samples in (E) showing composition intermediate between the continental arc and the MORB fields.

Lithotectonic discrimination diagrams, such as those of Pearce (2014) and Ross and Bédard (2009), rely on a limited number of trace elements and trace element ratios that can be interpreted in nonunique ways. For example, the Zr/Y ratio is controlled by very slight differences in compatibility (Pearce and Norry, 1979), and the Nb/Yb ratio is strongly dependent on highly variable source depletion (Pearce, 2008). Th/Yb ratios reflect not only volatile input but also assimilation of subduction-related crust (Pearce, 2014). Increased Th may also be related to higher temperatures that are required to mobilize this element. These difficulties can be overcome by considering multiple trace element concentrations simultaneously, as we did in our study of oceanic rhyolites and dacites (Fassbender et al., 2023). We applied multivariate statistical techniques (principal component analysis [PCA] and agglomerative hierarchical clustering) to identify and verify the geochemical differences between sample suites using multielement data rather than individual element ratios. In this study we chose nine elements: Ti, Al, Ba, Th, Zr, Nb, La, Y, and Yb. All were recalculated to elemental concentrations in ppm, and centered-log ratio (CLR) transformations were applied to overcome closure effects (Aitchison, 1982; see App. Table A2). The PCA and clustering were performed with the R statistical software package (R Core Team, 2020).

Principal component analysis

We used PCA to illustrate the minimum number of “components” that account for the largest variance in the data set (Bartholomew, 2010). Each principal component (PC) is affected by all the variables; the first and second components, PC1 and PC2, account for most of the variance in the data set (66 and 22%, respectively; see App. Table A3). The contribution of the different elements is described in terms of loadings of the element on each component. In Figure 5, the red labels represent the loadings of the elements for the entire data set (see App. Table A4). The plotted points are individual sample scores in PC1-PC2 space, given in Appendix Table A5. Yb and Y have strong positive loadings on PC1, whereas Th and La have strong negative loadings, similar to the felsic volcanic rocks (Fassbender et al., 2023). Zr and Nb have strong positive loadings on PC2, whereas Ba and Al have strong negative loadings. The PCA shows very consistent behavior among the mobile elements (e.g., Ba and La) and less mobile elements (Th and Al).

Fig. 5.

Principal component plots of the major and trace element geochemistry of modern oceanic mafic volcanic rocks. The principal component analysis (PCA) was conducted using nine elements (Ti, Al, Y, Zr, Nb, Ba, La, Yb, Th) for all samples in the data set. PC1 and PC2 account for 66 and 28%, respectively, of the total variance of the data set. The red element labels correspond to loadings for the entire data set (see App. Table A4); the symbols are individual sample scores given in Appendix Table A5. For illustration purposes, the different clusters of samples identified in this study are outlined and labeled (C1-C7: App. Table A6). See Figure 6 for the explanation of the polygons. The characteristics of each cluster and their relationship to specific lithotectonic assemblages are summarized in Table 3 and discussed in the text. A) PCA plot of samples from the Mid-Atlantic Ridge, Iceland, Ascension Island, Lesser Antilles volcanic arc, East Scotia Ridge, and Antarctic Peninsula. Samples from the Mid-Atlantic Ridge have a wide range of loadings on PC1 and positive loadings on PC2 spanning C1 to C4. Samples from the ridge-hotspot intersection at Iceland have intermediate loadings on PC1 and PC2 and fall mainly in C3. Samples from the ridge-hotspot intersection at Ascension Island have large negative loadings on PC1 and strong positive loadings on PC2 and fall in C4. Samples from the intraoceanic island arc of the Lesser Antilles have intermediate loadings on PC1 and negative loadings on PC2 and fall mainly in C7. Samples from the intraoceanic back-arc of the East Scotia Ridge have intermediate loadings on PC1 and PC2, spanning C2 and C3. Samples from the intracontinental arc-back arc of the Antarctic Peninsula have intermediate to negative loadings on PC1 and intermediate to positive and negative loadings on PC2 and include samples in C2, C4, and C7. B) Mid-ocean ridge mafic volcanic rocks from the Pacific Antarctic Rise, Juan de Fuca Ridge, and the Galapagos spreading center have intermediate loadings on PC1 and positive loadings on PC2, spanning C1 to C3. Samples from the East Pacific Rise have a range of mostly positive loadings on PC1 and PC2 and fall in C1 and C2. Samples from the Marquesas Archipelago have strong negative loadings on PC1 and positive loadings on PC2 and fall in C4. Samples from Hawaii have large negative loadings on PC1 and strong positive loadings on PC2 and fall mainly in C3. C) Samples from the Manus basin have a wide range of loadings on PC1 and PC2 and include samples in C1, C6, and C7. Samples from the Mariana Trough and New Hebrides have intermediate loadings on PC1 and intermediate to negative loadings on PC2 and include samples in C2, C3, and C7. Samples from the Izu-Bonin arc have a narrow range of intermediate loadings on PC1 and PC2 and fall between C2 and C7. Three samples from the Okinawa Trough have negative loadings on PC1 and intermediate loadings on PC2 and fall between C3 and C7. D) Samples from intraoceanic island arc assemblages from Fiji and Tonga-Kermadec have a wide range of loadings on PC1 and PC2 and do not belong to a particular cluster. Mid-ocean ridge mafic volcanic rocks from the South Indian Ridge have positive loadings on PC1 and PC2, similar to the other mid-ocean ridge assemblages.

Fig. 5.

Principal component plots of the major and trace element geochemistry of modern oceanic mafic volcanic rocks. The principal component analysis (PCA) was conducted using nine elements (Ti, Al, Y, Zr, Nb, Ba, La, Yb, Th) for all samples in the data set. PC1 and PC2 account for 66 and 28%, respectively, of the total variance of the data set. The red element labels correspond to loadings for the entire data set (see App. Table A4); the symbols are individual sample scores given in Appendix Table A5. For illustration purposes, the different clusters of samples identified in this study are outlined and labeled (C1-C7: App. Table A6). See Figure 6 for the explanation of the polygons. The characteristics of each cluster and their relationship to specific lithotectonic assemblages are summarized in Table 3 and discussed in the text. A) PCA plot of samples from the Mid-Atlantic Ridge, Iceland, Ascension Island, Lesser Antilles volcanic arc, East Scotia Ridge, and Antarctic Peninsula. Samples from the Mid-Atlantic Ridge have a wide range of loadings on PC1 and positive loadings on PC2 spanning C1 to C4. Samples from the ridge-hotspot intersection at Iceland have intermediate loadings on PC1 and PC2 and fall mainly in C3. Samples from the ridge-hotspot intersection at Ascension Island have large negative loadings on PC1 and strong positive loadings on PC2 and fall in C4. Samples from the intraoceanic island arc of the Lesser Antilles have intermediate loadings on PC1 and negative loadings on PC2 and fall mainly in C7. Samples from the intraoceanic back-arc of the East Scotia Ridge have intermediate loadings on PC1 and PC2, spanning C2 and C3. Samples from the intracontinental arc-back arc of the Antarctic Peninsula have intermediate to negative loadings on PC1 and intermediate to positive and negative loadings on PC2 and include samples in C2, C4, and C7. B) Mid-ocean ridge mafic volcanic rocks from the Pacific Antarctic Rise, Juan de Fuca Ridge, and the Galapagos spreading center have intermediate loadings on PC1 and positive loadings on PC2, spanning C1 to C3. Samples from the East Pacific Rise have a range of mostly positive loadings on PC1 and PC2 and fall in C1 and C2. Samples from the Marquesas Archipelago have strong negative loadings on PC1 and positive loadings on PC2 and fall in C4. Samples from Hawaii have large negative loadings on PC1 and strong positive loadings on PC2 and fall mainly in C3. C) Samples from the Manus basin have a wide range of loadings on PC1 and PC2 and include samples in C1, C6, and C7. Samples from the Mariana Trough and New Hebrides have intermediate loadings on PC1 and intermediate to negative loadings on PC2 and include samples in C2, C3, and C7. Samples from the Izu-Bonin arc have a narrow range of intermediate loadings on PC1 and PC2 and fall between C2 and C7. Three samples from the Okinawa Trough have negative loadings on PC1 and intermediate loadings on PC2 and fall between C3 and C7. D) Samples from intraoceanic island arc assemblages from Fiji and Tonga-Kermadec have a wide range of loadings on PC1 and PC2 and do not belong to a particular cluster. Mid-ocean ridge mafic volcanic rocks from the South Indian Ridge have positive loadings on PC1 and PC2, similar to the other mid-ocean ridge assemblages.

Agglomerative hierarchical clustering

To identify geochemically distinct groups of samples in the data set, we used unsupervised agglomerative hierarchical clustering of the CLR-transformed data (App. Table A6). This type of cluster analysis can identify subtle differences in geochemical signatures between sample types, particularly for volcanic rocks that show small but systematic variations between lithotectonic settings. In Fassbender et al. (2023), we performed cluster analysis on the PCA results for the felsic volcanic rocks rather than the raw geochemical data. Here, we performed clustering on the CLR-transformed trace elements of the mafic volcanics, which yielded much better separation of individual clusters. The analysis was performed in R using the hierarchical clustering function, “hclust.” Agglomerative hierarchical clustering uses a set of queries to classify samples in a way that maximizes the differences between the groups. In this type of analysis, no preassigned groups or clusters are identified (i.e., the analysis is unsupervised). The analysis starts by assuming that every individual sample forms its own cluster, and then the statistical differences (or “distances”) from all other samples are calculated. The two samples with the closest distance to each other form the next level of clusters. The distances are calculated using Ward’s criterion, which determines the center of each cluster and then the sum of the squared distances of individuals from the center. Each sample was assigned to one cluster (App. Table A6). The smallest number of clusters representing all of the samples was determined using an iterative approach discussed below. The workflow is presented in Appendix Figure A3 and can be replicated with the code and instructions provided in the Appendix. Seven different clusters (C1-C7) were selected to train our classifier and are outlined in Figures 5 and 6.

Fig. 6.

A) Summary of hierarchical clustering classification overlain on PC1-PC2. Different clusters of samples from the hierarchical cluster analysis are outlined by the labeled polygons (C1-C7). The assigned target classes are listed in Appendix Table A6. The lines dividing the clusters were drawn outward from the center of the plot, first between samples in C1 and C2, and then sequentially to C7. The outer boundary encloses the samples with the highest and lowest scores in PC1 and PC2. B) Numbers of samples in each target class of the hierarchical clustering colored according to the different geodynamic settings in (A). See the text for discussion of the inferred melting conditions and sources.

Fig. 6.

A) Summary of hierarchical clustering classification overlain on PC1-PC2. Different clusters of samples from the hierarchical cluster analysis are outlined by the labeled polygons (C1-C7). The assigned target classes are listed in Appendix Table A6. The lines dividing the clusters were drawn outward from the center of the plot, first between samples in C1 and C2, and then sequentially to C7. The outer boundary encloses the samples with the highest and lowest scores in PC1 and PC2. B) Numbers of samples in each target class of the hierarchical clustering colored according to the different geodynamic settings in (A). See the text for discussion of the inferred melting conditions and sources.

Training a classifier

The results of the unsupervised clustering can be used to train a classifier. We did this by performing a supervised classification on the newly defined clusters identified above. We then conducted a blind test to determine if the classification scheme can accurately predict geodynamic influences in the mafic volcanic rock geochemistry. Supervised classification is a machine-learning task used for large data sets that sorts samples into known classes based on a “training” data set. The training is an iterative process by which a machine-learning classifier, such as random forest (RF), learns what elements achieve the best classification outcome and then builds a model that can be used as a classification scheme. Random forest has proven to be particularly useful for supervised classification based on multivariate data sets (Breiman, 2001; Fernandez-Delgado et al., 2014). The steps involve initial processing of the data to create the training set, training of the classifier, and then evaluation of its prediction success. The input is a selection of parameters (i.e., the analyzed elements) and the predetermined clusters (i.e., target classes). The starting database is randomly split into the training data set, used to build and validate the RF classifier, and the blind test data used to evaluate the prediction rate.

RF functions using multiple decision trees with true or false outcomes (e.g., “are SiO2 concentrations <60 wt % ?”; if yes, the sample is a mafic rock; if not, the sample is a felsic rock). The decision points are referred to as “nodes,” and the cutoffs are determined by the classifier’s ability to separate the training data. For example, the correct cutoff for SiO2 is identified by testing several SiO2 concentrations until the best separation of the clusters is achieved. “Pure” separation is achieved if the cut-off value of 60 wt % SiO2 correctly identifies all basalts. At every node, the “impurity” of the separation can be measured in terms of the probability of a parameter falsely classifying a sample. This is commonly determined using the “Gini index”:

Ginit=c=1jgc1gc

gc=ncn

where “t” is the node in the decision tree (e.g., Ti), “c” is the target class, “j” refers to the possible classes, “gc” is the relative frequency of classification, “nc” is the number of samples belonging to target class “c,” and “n” is the total number of samples evaluated at this node. Nodes, or parameters (e.g., Ti), with a low Gini index have a high probability of correctly classifying a sample and therefore greater importance in making the decision.

Machine-learning classifiers may work very well with the training data but perform poorly in blind tests. Random forest overcomes this problem by using a large number of uncor-related decision trees operating as a “committee” that casts votes for each decision. Many decision trees are created by randomly selecting elements from a training data set as the nodes. The least number of decision trees (usually 500 or more) is selected for the final random forest model. Typically, after this many decision trees, the performance of the classifier does not improve, and the smallest number is chosen to lower computational cost and minimize any potential artifacts in the data analysis. Each decision tree is then “trained” with randomly chosen samples equal to the total number of samples in the training data base, a method commonly known as “bootstrapping.” During this process samples can be picked multiple times, and a pool of samples that were not picked and therefore not involved in training (so-called “out of bag” samples) is left behind. This is typically about one third of all samples. These “out of bag” samples are used to validate the classification scheme (see below). To classify an unknown sample, each of the randomized decision trees casts a single “vote” for a single target class or cluster from the training set. The votes cast by all of the decision trees for each cluster are given in percentages (summing to 100%), and RF uses the majority vote to finally classify a sample (Breiman, 2001). A disadvantage of the RF classifier is that it will force a decision regarding the classification of a sample even if the sample does not match any of the target classes. Forced decisions commonly return a randomly distributed set of votes.

We selected 80% of our database at random (see App. Table A7) to train and validate our RF classifier; the remaining 20% was used for the blind test (see App. Table A8), which is a standard approach (Gholamy et al., 2018). We used the target classes identified by the unsupervised hierarchical clustering to train the classifier. Different numbers of samples assigned to the target classes in the training data set can create a class imbalance and lead to poor performance of the classifier. To overcome this, we equalized the number of samples in each target class in the training set using the synthetic minority oversampling technique (SMOTE: Chawla et al., 2002). SMOTE randomly “removes” samples from classes with too many samples, and “creates” new samples for classes that have too few by slightly altering the geochemical composition of individual samples until each class contains the same number of samples. In our study, this resulted in a modified training set with 2,288 samples (App. Table A9).

To validate the model, we used the subset of samples remaining from the bootstrapping procedure (i.e., “out of bag” samples). Of the “out of bag” samples from all target classes, 97.2% were correctly classified (App. Table A10). The probability of misclassification by a particular node, averaged over the entire RF classification scheme, is referred to as the Mean Decrease Gini. This parameter identified Nb, Th, Y, Yb, and Al as the most important variables in determining whether a sample is classified correctly (Fig. 7). Nodes corresponding to La and Zr have low Mean Decrease Gini values and therefore less influence in the correct classification of a sample. We then conducted a blind test on the 20% of the original samples in the database that were not used in the training set (see App. Table A8; none were created by SMOTE) to determine how well the classifier performs on new data. The test samples were correctly classified in almost every case, identifying 96% or 547 of 569 samples (App. Table A11). Because RF does not use any element independently, this significantly reduces the effect of potentially anomalous behavior of any individual element for individual samples. To exclude any potential control of Ba mobility on RF classification, we created a second classifier excluding Ba yielding almost identical results (App. Table A12).

Fig. 7.

Plot of the “Mean Decrease Gini” value for individual elements for the classification of a sample into a target class or “cluster” (see text for discussion). The Mean Decrease Gini value corresponds to the probability of a correct classification of a sample and identifies Nb, Ba, Yb, Y, Th, and Ti as the most important variables in determining the accuracy of the classification. Al, Zr, and La have low Mean Decrease Gini values and therefore less influence in the correct classification of a sample.

Fig. 7.

Plot of the “Mean Decrease Gini” value for individual elements for the classification of a sample into a target class or “cluster” (see text for discussion). The Mean Decrease Gini value corresponds to the probability of a correct classification of a sample and identifies Nb, Ba, Yb, Y, Th, and Ti as the most important variables in determining the accuracy of the classification. Al, Zr, and La have low Mean Decrease Gini values and therefore less influence in the correct classification of a sample.

We identified seven target classes of mafic volcanic rocks from the modern oceans, corresponding to distinct geodynamic settings and different melting conditions. Based on these target classes, we constructed an RF classifier that accurately identifies rocks from those settings. Here, we discuss the tectonic and petrogenetic significance of the target classes and then assess the application of the RF classifier to ancient volcanic rocks.

In the PCA, we interpreted the loadings on PC1 to represent different mantle sources, from depleted MORs to enriched ocean island settings, whereas the loadings on PC2 mainly reflect different melting regimes, from dry melting at MORs and distal back-arc basin spreading centers to wet melting in near-arc environments. Mafic volcanic rocks from MORs and mature back-arc spreading centers have positive loadings on PC1 and PC2 (App. Table A5) and belong to target class C1 (Table 3; App. Table A6). These rocks have low Nb and La, typical of MOR magmas and an incompatible element-depleted mantle source. Mafic rocks from less-depleted MORs and back-arc spreading centers have intermediate positive loadings on PC1 and positive loadings on PC2 and belong to target class C2. Mafic rocks from enriched MORs and back-arc spreading centers have slightly negative loadings on PC1 and positive loadings on PC2 and belong to target class C3, with low to intermediate Nb and La. Mafic rocks from ocean island-like settings have negative loadings on PC1 and positive loadings on PC2 and belong to target class C4, with mantle sources that are highly enriched in incompatible elements (i.e., high Nb and La). Mafic rocks from immature back-arc rifts have small positive loadings on PC1 and negative loadings on PC2 and belong to target class C5, influenced by at least some fluid-fluxed melt components and having intermediate Al and Ba. Mafic rocks from arc volcanoes and arc-related rifts have intermediate loadings on PC1 and negative loadings on PC2 and belong to target class C6, reflecting mostly fluid-fluxed melts. These rocks have high Al and Ba, typical of island arcs. Finally, mafic volcanic rocks from nascent back-arc rifts, especially in continental margin settings, have small loadings on PC1 and negative loadings on PC2 and belong to target class C7, including both fluid-fluxed melt components and enriched mantle or crustal sources. These rocks have high Ba, Nb, Th, and La, at least partly due to crustal contamination.

Table 3.

Summary Attributes of the Target Classes of Oceanic Mafic Volcanic Rocks According to Tectonic Setting

ClusterLithotectonic assemblagesType examplesTrace element characteristicsPCA loadings
C1Depleted mid-ocean ridges and mature back-arc spreading centersEast Pacific Rise; Central Lau spreading centerVery low La/Sm, very low La/Yb, very low Ba/Nb, very low Th/Yb, very low Nb/Yb; no negative Nb-Ta anomalyVery high Yb and Y loadings on PC1 and low Th and La loadings on PC2
C2Undepleted mid-ocean ridges and mature back-arc spreading centersMid-Atlantic Ridge; Central Scotia back-arc spreading centerLow La/Sm, low La/Yb, very low Ba/Nb, low Th/Yb, low Nb/Yb; no negative Nb-Ta anomalyHigh Yb and Y loadings on PC1 and low Th and La loadings on PC2
C3Enriched mid-ocean ridges and mature back-arc spreading centersGalapagos spreading center; Northwestern Lau spreading center, Rochambeau riftIntermediate La/Sm, intermediate La/Yb, very low Ba/Nb, intermediate Th/Yb, intermediate Nb/Yb; no negative Nb-Ta anomalyIntermediate Yb, Y, Th, and La loadings on PC1 and PC2
C4Ocean island related settingsAscension Island, HawaiiHigh La/Sm, high La/Yb, low Ba/Nb, high Th/Yb, high Nb/Yb; no negative Nb-Ta anomalyHigh Nb and Zr loadings on PC2 and low Al and Yb loadings on PC1
C5Back-arc and arc-related riftsMyojin rift (Izu-Bonin), Ngatoroirangi rift (Havre Trough)Intermediate La/Sm, intermediate La/Yb, intermediate Ba/Nb, intermediate Th/Yb, low Nb/Yb; moderate negative Nb-Ta anomalyVery high Al loadings on PC1 and low La and Nb loadings on PC2
C6Arc volcanoes and arc-related riftsFonualei rift and spreading center (Lau basin), Tofua volcano (Tofua arc)Intermediate La/Sm, high Ba/Nb, high Th/Yb, low Nb/Yb, strong negative Nb-Ta anomalyHigh Al and Ba loadings on PC1 and low Zr and Nb loadings on PC2
C7Nascent back-arc rift and continental margin crustBransield StraitHigh La/Sm, intermediate Ba/Nb, high Th/Yb, intermediate Nb/Yb, moderate negative Nb-Ta anomalyHigh Ba, Th and La loadings on PC1 and low Yb and Ti loadings on PC2
ClusterLithotectonic assemblagesType examplesTrace element characteristicsPCA loadings
C1Depleted mid-ocean ridges and mature back-arc spreading centersEast Pacific Rise; Central Lau spreading centerVery low La/Sm, very low La/Yb, very low Ba/Nb, very low Th/Yb, very low Nb/Yb; no negative Nb-Ta anomalyVery high Yb and Y loadings on PC1 and low Th and La loadings on PC2
C2Undepleted mid-ocean ridges and mature back-arc spreading centersMid-Atlantic Ridge; Central Scotia back-arc spreading centerLow La/Sm, low La/Yb, very low Ba/Nb, low Th/Yb, low Nb/Yb; no negative Nb-Ta anomalyHigh Yb and Y loadings on PC1 and low Th and La loadings on PC2
C3Enriched mid-ocean ridges and mature back-arc spreading centersGalapagos spreading center; Northwestern Lau spreading center, Rochambeau riftIntermediate La/Sm, intermediate La/Yb, very low Ba/Nb, intermediate Th/Yb, intermediate Nb/Yb; no negative Nb-Ta anomalyIntermediate Yb, Y, Th, and La loadings on PC1 and PC2
C4Ocean island related settingsAscension Island, HawaiiHigh La/Sm, high La/Yb, low Ba/Nb, high Th/Yb, high Nb/Yb; no negative Nb-Ta anomalyHigh Nb and Zr loadings on PC2 and low Al and Yb loadings on PC1
C5Back-arc and arc-related riftsMyojin rift (Izu-Bonin), Ngatoroirangi rift (Havre Trough)Intermediate La/Sm, intermediate La/Yb, intermediate Ba/Nb, intermediate Th/Yb, low Nb/Yb; moderate negative Nb-Ta anomalyVery high Al loadings on PC1 and low La and Nb loadings on PC2
C6Arc volcanoes and arc-related riftsFonualei rift and spreading center (Lau basin), Tofua volcano (Tofua arc)Intermediate La/Sm, high Ba/Nb, high Th/Yb, low Nb/Yb, strong negative Nb-Ta anomalyHigh Al and Ba loadings on PC1 and low Zr and Nb loadings on PC2
C7Nascent back-arc rift and continental margin crustBransield StraitHigh La/Sm, intermediate Ba/Nb, high Th/Yb, intermediate Nb/Yb, moderate negative Nb-Ta anomalyHigh Ba, Th and La loadings on PC1 and low Yb and Ti loadings on PC2

Abbreviations: PCA = principal component analysis, PC = principal component

The cluster analysis illustrates the significant geodynamic control on the trace element signatures (Figs. 5, 6, and 8): C1, C2, and C3 are MOR-like assemblages; C4 corresponds to ocean island-like assemblages; and C5-C7 correspond to arc and back-arc assemblages (Table 3). The type localities for clusters C1-C3 are Juan de Fuca Ridge, the East Pacific Rise, the Galapagos spreading center, the Pacific Antarctic Rise, the Mid-Atlantic Ridge, and the South Indian Ridge as well as mature back-arc spreading centers such as the Central Lau spreading center (Lau basin) and the Manus spreading center (Manus basin). Samples in C1 (e.g., East Pacific Rise and Central Lau spreading center) have very low La/Sm, La/Yb, Th/Yb, Nb/Yb, and Ba/Nb, and no negative Nb-Ta anomaly. Samples in C2 (e.g., Mid-Atlantic Ridge and Central Scotia back-arc spreading center) have low La/Sm, La/Yb, Th/Yb, and Nb/Yb and very low Ba/Nb, with no negative Nb-Ta anomaly. Samples in C3 (e.g., Galapagos spreading center and Northwest Lau spreading center) have intermediate La/Sm, La/Yb, Th/Yb, and Nb/Yb, very low Ba/Nb, and no negative Nb-Ta anomaly. The type localities for cluster C4 are the Samoa hotspot, Hawaii and the Marquesas Archipelago (ocean island settings), Ascension Island (a ridge-hotspot intersection), and other settings with ocean island-like signatures, such as the Mid-Atlantic Ridge north of Iceland. Samples in C4 have high La/Sm, La/Yb, Th/Yb and Nb/Yb, low Ba/Nb, and no negative Nb-Ta anomaly. Samples in C5 include mafic volcanic rocks from the Myojin rift (Izu-Bonin) and Ngatoroirangi rift (Havre Trough), which have intermediate La/Sm, La/Yb, and Th/Yb, low Nb/Yb, intermediate Ba/Nb, and moderate negative Nb-Ta anomalies. Samples from the Fonualei rift and spreading center (Lau basin) and the Tofua volcanic in C6 have intermediate La/Sm, high Th/Yb, low Nb/Yb, high Ba/Nb, and strong negative Nb-Ta anomalies. Finally, samples from the nascent back-arc rift of the Northeast Lau spreading center (Lau basin) and continental margin arc settings, such as the Bransfield Strait, fall in C7 and have high La/Sm and Th/Yb, intermediate Nb/Yb and Ba/Nb, and moderate negative Nb-Ta anomalies. The results of training the RF classifier using these different clusters is discussed below.

Fig. 8.

Selected trace element ratios for samples assigned to each target cluster (C1-C7). The black circle represents the mean and the notch the median of the data. The lower boundary of the box is the 25th percentile and the upper boundary of the box is the 75th percentile. A) Ba/Nb ratios showing the distinct differences between clusters C1-C4 (samples with low Ba/Nb) and C5-C7 (samples with high Ba/Nb). The high Ba/Nb values of C5-C7 are interpreted to indicate volatile input from dehydration of the subducted slab. B) Ba/Yb ratios showing the same differences between C1-C4 and C5-C7. C) Th/Yb ratios showing increasing values from C1 to C4 and C5 to C7. High Th/Yb ratios are interpreted to indicate volatile input at high temperatures and assimilation of crustal material. D) Nb/Yb ratios showing increasing values from C1 to C4 and C5 to C7. High Nb/Yb is interpreted to indicate enriched mantle sources and low degrees of melting. E) La/Sm ratios showing increasing values from C1 to C4 and C5 to C7, similar to Nb/Yb. F) La/Yb ratios showing increasing values from C1 to C4 and C5 to C7. See Table 2 for details.

Fig. 8.

Selected trace element ratios for samples assigned to each target cluster (C1-C7). The black circle represents the mean and the notch the median of the data. The lower boundary of the box is the 25th percentile and the upper boundary of the box is the 75th percentile. A) Ba/Nb ratios showing the distinct differences between clusters C1-C4 (samples with low Ba/Nb) and C5-C7 (samples with high Ba/Nb). The high Ba/Nb values of C5-C7 are interpreted to indicate volatile input from dehydration of the subducted slab. B) Ba/Yb ratios showing the same differences between C1-C4 and C5-C7. C) Th/Yb ratios showing increasing values from C1 to C4 and C5 to C7. High Th/Yb ratios are interpreted to indicate volatile input at high temperatures and assimilation of crustal material. D) Nb/Yb ratios showing increasing values from C1 to C4 and C5 to C7. High Nb/Yb is interpreted to indicate enriched mantle sources and low degrees of melting. E) La/Sm ratios showing increasing values from C1 to C4 and C5 to C7, similar to Nb/Yb. F) La/Yb ratios showing increasing values from C1 to C4 and C5 to C7. See Table 2 for details.

Highly variable melt sources and conditions, ranging from the thickness of the crust to the composition of the sub-arc mantle wedge and mantle mixing (e.g., corner flow and incursion through slab tears and windows: Taylor and Martinez, 2003; Haase et al., 2009; Cooper et al., 2010; Price et al., 2017), all contribute to the overall complexity observed in modern oceanic basalts. It can be difficult to unravel all of the possible influences from just a few trace-element ratios. By using nine elements simultaneously, even samples with a weak influence from subduction fluids (target class C5) can be distinguished from samples with a stronger influence (target class C6). We can also distinguish truly arc-related assemblages (e.g., target classes C5 and C6) from mafic volcanic suites that are only affected by crustal contamination, such as the Icelandic basalts in target class C3 (Caracciolo et al., 2022; Fig. 4A). Such distinctions often require difficult isotopic studies to unravel.

Although no prior knowledge of the geodynamic settings of the samples influenced the clustering, the classification outcome presented here corresponds closely to established geochemical characteristics associated with different melt sources. The most important are the absence of negative Nb-Ta anomalies in target classes C1-C4, indicating no subduction-derived fluids; increasing La/Sm and Nb/Yb ratios from target class C1 to C4, indicating the transition from depleted mantle sources to enriched sources; negative Nb-Ta anomalies and LILE enrichments with low Th in target classes C5 and C6, related to subduction zone processes; and weak negative Nb-Ta anomalies and LILE enrichments but high Th in target class C7, with enriched-mantle components and crustal contamination. The increase in La/Sm and Nb/Yb ratios from C1 to C3 reflects undepleted mantle in C2 and more enriched mantle in C3, but they all lack high Ba/Nb and LILE (Pearce, 2008; Hofmann, 2014; White and Klein, 2014). Although the high La/Sm and Nb/Yb ratios are related mainly to enriched mantle sources (e.g., Haase et al., 2019), they may also be produced by different degrees of partial melting and variations in the depth of melting (Engel et al., 1965; Wanless and Shaw, 2012).

Mafic volcanic rocks in MOR-like settings and mature back-arc spreading centers, captured in target classes C1-C3, are products of decompression melting due to adiabatic rise of mantle material into MOR spreading centers (McKenzie and O’Nions, 1991; Hofmann, 2014). Although they all have depleted mantle sources with very low La/Sm, Nb/Yb, and no negative Nb-Ta anomaly, the geochemical signatures are influenced by a range of melting conditions. Increased depth of melting can leave garnet in the melt residue, which results in depletion of Yb (Hofmann, 2014). The transition from C1-C3 to C4 reflects deep, low-degree partial melting of anhydrous depleted mantle, characteristic of hotspots and ocean island settings, which results in high La/Sm, high Nb/Yb, and no negative Nb-Ta anomaly (Chauvel et al., 2012; Jicha et al., 2013; Haase et al., 2020). Melting that is influenced to different degrees by fluids from a dehydrating slab is captured in target classes C5, C6, and C7. In this case, the mafic volcanic rocks are products of mixing between hydrous melts derived from devolatilization of the downgoing slab and a water-poor MOR-like melt with negative Nb-Ta anomalies and LILE enrichment compared to MOR basalts (Taylor and Martinez, 2003; Langmuir et al., 2006). Less fluid from the dehydrating slab at back-arc spreading centers distant from the volcanic arc results in weak negative Nb-Ta anomalies, intermediate Ba/Nb, and low Th/Yb (Fretzdorff et al., 2006; Bezos et al., 2009), which is captured in target class C5. The large volatile fluxes above a dehydrating slab that result in strong negative Nb-Ta anomalies, high Ba/Nb, and intermediate Th/Yb, characteristic of arc volcanoes and arc-related rifts (Keller et al., 2008; Escrig et al., 2012), are captured in target class C6. Arc rifting and initial back-arc spreading may be related to processes entirely within the upper plate (e.g., Caratori Tontini et al., 2019) or to complex mantle flow regimes related to the subducting slab (Heuret and Lallemand, 2005; Schellart, 2008). Finally, in nascent back-arc rifts and continental margin rifts, crustal assimilation produces the high Th/Yb and La/Sm (Fretzdorff et al., 2004; Haase et al., 2020) captured in target class C7.

A case study of the Lau basin

The Lau basin is the type example of an intraoceanic back-arc basin and a key location to explore the geochemical variations in mafic volcanic rocks in response to subduction, microplate interactions, and mantle heterogeneity (Taylor et al., 1996; Keller et al., 2008; Yan et al., 2012; Sleeper and Martinez, 2016). It shows remarkable variations in arc- and non-arc petrogenesis that may be encountered in a single back-arc basin at scales of tens to just a few hundred kilometers. The V-shaped basin is ~450 km wide and ~1,000 km in length (Fig. 9). It is flanked by the active Tofua arc in the east, the Lau Ridge remnant arc in the west, and the Vitiaz lineament or fracture zone in the north (Hawkins, 1995). The opening of the basin, which began approximately 6 m.y. ago and has been propagating southward (Hawkins, 1995; Taylor et al., 1996), is accommodated by the Eastern Lau spreading center in the south, Valu Fa Ridge and Central Lau spreading center in the middle, and the Fonualei rift and spreading center, Mangatolu Triple Junction, Northeast Lau spreading center, and Northwest Lau spreading center in the north. The southern part is tectonically simple, but the north is a complex mosaic of microplates that are variably influenced by the subduction of the Pacific Plate and large-scale transcurrent faulting (Stewart et al., 2022). In addition to the structural complexity, a number of researchers have suggested that the mantle below the back-arc region is characterized by complex flow regimes including contributions from the nearby Samoan mantle plume (Pearce et al., 2007; Escrig et al., 2009, 2012; Price et al., 2014; Yan et al., 2020; Haase et al., 2022). Isotopic studies of Hf, Nd, Sr, and Pb indicate significant mantle mixing and crustal assimilation in the northeastern Lau basin (Regelous et al., 2008; Tian et al., 2011; Price et al., 2014; Nebel and Arculus, 2015).

Fig. 9.

A) Regional bathymetric map of the Lau back-arc basin (Ryan et al., 2009) showing spreading ridges, spreading rates, and rates of retreat of the active arc (in mm/yr; modified from Sleeper and Martinez, 2016). The basin includes at least eight major zones of active extension: ELSC = Eastern Lau spreading center; VFR = Valu Fa Ridge; CLSC = Central Lau spreading center; FRSC = Fonualei rift and spreading center; MTJ = Mangatolu Triple Junction; NELSC = Northeast Lau spreading center; NWLSC = Northwest Lau spreading center; RR = Rochambeau rifts. B) Primitive mantle-normalized trace element plots of Lau Basin mafic volcanic rocks considered in this study. Samples from the RR, NWLSC, and MTJ have no negative Nb-Ta anomaly, flat rare earth element (REE) patterns and high heavy (H)REE concentrations indicating dry melting. Samples from the NELSC have a very weak negative Nb-Ta anomaly, steeply dipping REE patterns and low HREE concentrations indicating limited wet melting. Samples from the FRSC have a strong negative Nb-Ta anomaly, flat REE patterns, and low HREE concentrations (i.e., typical for wet melting). Samples from the CLSC have no negative Nb-Ta anomaly, increasing REE pattern and high HREE concentrations typical for dry melting. Samples from the ELSC have an intermediate negative Nb-Ta anomaly, flat REE pattern and high HREE concentrations expected for limited wet melting. Samples from the VFR have a strong negative Nb-Ta anomaly, flat REE pattern and high HREE concentrations typical for wet melting.

Fig. 9.

A) Regional bathymetric map of the Lau back-arc basin (Ryan et al., 2009) showing spreading ridges, spreading rates, and rates of retreat of the active arc (in mm/yr; modified from Sleeper and Martinez, 2016). The basin includes at least eight major zones of active extension: ELSC = Eastern Lau spreading center; VFR = Valu Fa Ridge; CLSC = Central Lau spreading center; FRSC = Fonualei rift and spreading center; MTJ = Mangatolu Triple Junction; NELSC = Northeast Lau spreading center; NWLSC = Northwest Lau spreading center; RR = Rochambeau rifts. B) Primitive mantle-normalized trace element plots of Lau Basin mafic volcanic rocks considered in this study. Samples from the RR, NWLSC, and MTJ have no negative Nb-Ta anomaly, flat rare earth element (REE) patterns and high heavy (H)REE concentrations indicating dry melting. Samples from the NELSC have a very weak negative Nb-Ta anomaly, steeply dipping REE patterns and low HREE concentrations indicating limited wet melting. Samples from the FRSC have a strong negative Nb-Ta anomaly, flat REE patterns, and low HREE concentrations (i.e., typical for wet melting). Samples from the CLSC have no negative Nb-Ta anomaly, increasing REE pattern and high HREE concentrations typical for dry melting. Samples from the ELSC have an intermediate negative Nb-Ta anomaly, flat REE pattern and high HREE concentrations expected for limited wet melting. Samples from the VFR have a strong negative Nb-Ta anomaly, flat REE pattern and high HREE concentrations typical for wet melting.

Mafic volcanic rocks from spreading centers closest to the Tofua arc (VFR and FRSC) have trace element signatures that reflect the subduction, including strong negative Nb-Ta anomalies, low HREEs, high LILEs, and high Ba/Nb ratios (Figs. 9 and 10). Rocks from the Valu Fa Ridge show the greatest influence of volatiles from the dehydrating slab (Haase et al., 2009), indicated by the high Ba/Nb and Ba/Yb (Fig. 10). With increasing distance from the slab, the amount of fluid that enters the melting regime decreases, resulting in decreasing Ba/Nb, Ba/Yb, Th/Yb, and Th/Yb (Fig. 10). Similarly, mafic rocks from the Fonualei rift and spreading center show decreasing influence of the arc (e.g., decreasing Ba/Nb) from south to north (Fig. 10), while Th/Yb increases due to enriched mantle sources and potential assimilation of preexisting arc crust. In contrast, the dominant source for melts along the Central Lau spreading center and Northwest Lau spreading center is the Indian Ocean mantle, with only a minor subduction component (Tian et al., 2008). The Mangatolu Triple Junction, Fonualei rift and spreading center, Eastern Lau spreading center, and Northeast Lau spreading center show a range of contributions from the slab, the modified mantle wedge, as well as Indian mid-ocean ridge basalt (MORB) (Yan et al., 2012; Zhang et al., 2018). Sampling has revealed complex melt sources in a very small area, with mixing of depleted mantle, enriched mantle, and boninitic sources (Haase et al., 2022). The enriched mantle component, indicated by the high Nb/Yb in Figure 10, has been attributed to Samoan plume material crossing into the northern Lau basin (Price et al., 2014), although this is debated (e.g., Falloon et al., 2008; Lupton et al., 2015; Nebel et al., 2018; Haase et al., 2022). Another possibility is that the enriched melts are related to the subduction of the Louisville Seamount Chain; another is assimilation of crustal material noted above (Beier et al., 2017; Price et al., 2017; Schönhofen, 2021). A contribution from the Samoan plume is also considered to be the source of the high Nb/Yb at the Northwest Lau spreading center and Rochambeau rifts (Zhang et al., 2018). High He isotope ratios, which are a diagnostic feature of the Samoan plume, are well known in mafic melts in the Northwest Lau spreading center and Rochambeau rifts (Lupton et al., 2009, 2015). However, volcanic rocks from the Fonualei rift and spreading center and Northeast Lau spreading center lack the He isotope anomaly. Boninitic melts from the Fonualei rift and spreading center and Northeast Lau spreading center, indicated by elevated SiO2, low TiO2, and low REE concentrations, reflect enhanced melting caused by a high volatile flux from the subduction zone (Falloon et al., 2007; Regelous et al., 2008; Resing et al., 2011, Escrig et al., 2012).

Fig. 10.

Spatial distribution of selected trace element ratios in mafic volcanic rocks of the northern Lau basin illustrating enrichment and depletion patterns in Ba, Nb, Yb, and Th. Samples from the Valu Fa Ridge (VFR), Eastern Lau spreading center (ELSC), and Central Lau spreading center (CLSC) show decreasing Ba/Yb (A), Ba/Nb (B), and Th/Yb (C) with increasing distance from the arc indicating less input of fluids from the dehydrating subducting slab. Nb/Yb ratios (D) show significant variations between the northern and southern Lau basin. Samples from the Fonualei rift and spreading center (FRSC) to Northeast Lau spreading center (NELSC) show decreasing Ba/Nb with distance from the arc but increasing Ba/Yb, Th/Yb, and Nb/Yb. The increasing Ba/Yb, Th/Yb, and Nb/Yb indicate an enriched mantle source. Samples from Northwest Lau spreading center (NWLSC) and Rochambeau rifts (RR) show low Ba/Nb but high Ba/Yb, Th/Yb, and Nb/Yb values indicate no input from arc-related fluids and a notably enriched mantle source. PR = Peggy Ridge.

Fig. 10.

Spatial distribution of selected trace element ratios in mafic volcanic rocks of the northern Lau basin illustrating enrichment and depletion patterns in Ba, Nb, Yb, and Th. Samples from the Valu Fa Ridge (VFR), Eastern Lau spreading center (ELSC), and Central Lau spreading center (CLSC) show decreasing Ba/Yb (A), Ba/Nb (B), and Th/Yb (C) with increasing distance from the arc indicating less input of fluids from the dehydrating subducting slab. Nb/Yb ratios (D) show significant variations between the northern and southern Lau basin. Samples from the Fonualei rift and spreading center (FRSC) to Northeast Lau spreading center (NELSC) show decreasing Ba/Nb with distance from the arc but increasing Ba/Yb, Th/Yb, and Nb/Yb. The increasing Ba/Yb, Th/Yb, and Nb/Yb indicate an enriched mantle source. Samples from Northwest Lau spreading center (NWLSC) and Rochambeau rifts (RR) show low Ba/Nb but high Ba/Yb, Th/Yb, and Nb/Yb values indicate no input from arc-related fluids and a notably enriched mantle source. PR = Peggy Ridge.

Mafic volcanic rocks with these highly variable geochemical signatures have been dredged in almost every back-arc basin investigated in this study, including the western Manus spreading center, the Mariana back-arc spreading center, the East Scotia Ridge, and the Bransfield Strait (Fig. 4). The locations of the anomalous samples are almost always at the terminations of the spreading centers, which are typically associated with complex mantle flow (Pearce and Stern, 2006). These locations are variably affected by localized upwelling of mantle, mantle flow around the edges of the slab, and mantle mixing (Fig. 11; Fretzdorff et al., 2002, 2004; Sinton et al., 2003; Pearce et al., 2005; Pearce and Stern, 2006; Beier et al., 2010; Haase et al., 2020). Strong toroidal flow has been proposed in the northern Lau basin (Schellart and Moresi, 2013) and elsewhere (northern East Scotia Ridge; Fretzdorff et al., 2002). This has been the subject of analog and numerical modeling (Chen et al., 2016; Magni, 2019) that suggest flow through a slab tear at the northern termination of the Tonga Ridge is introducing fertile mantle into the back-arc region.

Fig. 11.

Schematic illustration of general mantle flow at microplate edges and terminating back-arc spreading centers as in the northeast Lau basin. Different mantle flow regimes may be dominated by (1) hydrated mantle, due to dehydration of the subducting slab; (2) asthenospheric dry mantle, due to mantle upwelling; and (3) fertile mantle introduced into the back-arc region via flow around the subducting slab. These three mantle components are all present near the slab tear at the northeast corner of the Tonga Plate. Models suggest the fertile asthenospheric mantle is brought into the back arc by toroidal flow around the slab (e.g., Magni, 2019) and is not related to direct influx from the Samoan hotspot (Haase et al., 2022).

Fig. 11.

Schematic illustration of general mantle flow at microplate edges and terminating back-arc spreading centers as in the northeast Lau basin. Different mantle flow regimes may be dominated by (1) hydrated mantle, due to dehydration of the subducting slab; (2) asthenospheric dry mantle, due to mantle upwelling; and (3) fertile mantle introduced into the back-arc region via flow around the subducting slab. These three mantle components are all present near the slab tear at the northeast corner of the Tonga Plate. Models suggest the fertile asthenospheric mantle is brought into the back arc by toroidal flow around the slab (e.g., Magni, 2019) and is not related to direct influx from the Samoan hotspot (Haase et al., 2022).

The different mantle domains that are suggested by the hierarchical clustering model are most reasonably interpreted in terms of the complex microplate architecture of the basin (Fig. 12; Baxter et al., 2020; Schmid et al., 2020; Stewart et al., 2022). Enriched mantle material at the Northwest Lau spreading center and Rochambeau rifts is reflected in the predominance of samples in target classes C2 and C3. Mafic volcanic rocks of the Fonualei rift and spreading center are mainly in target class C6, with a shift to target class C7 among the samples from the Northeast Lau spreading center, where there is less fluid-fluxed melting, enriched mantle sources, and possible assimilation of crustal material. Low-degree partial melting of fertile mantle is also observed at several locations, producing OIB-like mafic volcanic rocks of target class C4.

Fig. 12.

Map showing the locations of basalt belonging to different target classes in the northern Lau basin. The different microplates of the basin are shown (from Baxter et al., 2020; and Stewart et al., 2022). Samples from the extinct Peggy Ridge (PR) belong to target class C1 (i.e., depleted mid-ocean ridge [MORB]-like samples). Samples from the Northwest Lau spreading center (NWLSC) and the Rochambeau rifts (RR) belong to target classes C2 and C3 (i.e., MORB and E-MORB-like samples). Samples from the Fonualei rift and spreading center (FRSC) belong to target class C6 (i.e., arc-related rifts). Samples from the Mangatolu Triple Junction (MTJ) belong to target classes C2, C3 (i.e., MORB and E-MORB-like samples) and C7 (i.e., nascent back-arc rift). Samples from the Northeastern Lau spreading center (NELSC) belong to target classes C4 (i.e., ocean island affinity) and C7 (i.e., nascent back-arc rift). See text for discussion.

Fig. 12.

Map showing the locations of basalt belonging to different target classes in the northern Lau basin. The different microplates of the basin are shown (from Baxter et al., 2020; and Stewart et al., 2022). Samples from the extinct Peggy Ridge (PR) belong to target class C1 (i.e., depleted mid-ocean ridge [MORB]-like samples). Samples from the Northwest Lau spreading center (NWLSC) and the Rochambeau rifts (RR) belong to target classes C2 and C3 (i.e., MORB and E-MORB-like samples). Samples from the Fonualei rift and spreading center (FRSC) belong to target class C6 (i.e., arc-related rifts). Samples from the Mangatolu Triple Junction (MTJ) belong to target classes C2, C3 (i.e., MORB and E-MORB-like samples) and C7 (i.e., nascent back-arc rift). Samples from the Northeastern Lau spreading center (NELSC) belong to target classes C4 (i.e., ocean island affinity) and C7 (i.e., nascent back-arc rift). See text for discussion.

Comparison with Archean mafic volcanic rocks

The volcanic and sedimentary rocks of the AGB are widely interpreted to be part of an intraoceanic arc-back arc system with similarities to modern oceanic microplate mosaics (Jackson et al., 1994; Percival et al., 2012; Wyman, 2013). However, other authors have suggested significant differences between Archean and post-Archsean magmatism and tectonics, with some Archean terranes possibly having no modern analog (e.g., Bédard et al., 2013). Submarine volcanism in the Abitibi greenstone belt produced seven temporally distinct assemblages between 2795 and 2695 Ma, traditionally divided into the Northern and Southern Volcanic Zones (Fig. 13A). We used the RF classifier developed in this paper to test whether Archean mafic volcanic rocks of the Abitibi greenstone belt can be classified along the same geochemical lines as modern samples and thereby possibly interpreted in terms of processes like those observed in the modern oceans. The comparison complements our earlier study of modern felsic volcanic rocks, which showed significant geochemical diversity similar to Archean rhyolites from the Abitibi greenstone belt (Fassbender et al., 2023).

Fig. 13.

A) Geologic assemblage map of the Abitibi greenstone belt (AGB) showing the locations of mafic volcanic rock samples compiled for this study (modified from Thurston et al., 2008; Monecke et al., 2017; Dube and Mercier-Langevin, 2020). B) Pie chart of the random forest classification results for the entire AGB data set, colored according to the different target cdlasses. The majority of the samples are classified as target class C2, C3, and C7. C) Number of samples in each target class identified among the different assemblages of the AGB. The distribution of the different target classes corresponds generally to the north and south rift zones of the Abitibi previously identified by Mole et al. (2022). The Blake River Assemblage (Noranda Volcanic Complex) and Deloro Assemblage have the highest proportion of samples belonging to target class C7, consistent with a combination of arc-like magmas influenced by wet melting and probable crustal contamination. The Kidd Munro Assemblage has a high proportion of samples belonging to target classes C2 and C3, similar to ridge-hotspot intersections. The Tisdale Assemblage has a high proportion of samples belonging to target class C6 and C7, consistent with a combination of arc-like magmas influenced by wet melting and probable crustal contamination. Stoughton-Roquemaure has the highest proportion of samples in target class C2 and C3 similar to ridge-hotspot intersections and mid-ocean ridge (MOR)-like settings. The Deloro Assemblage has the highest proportion of samples belonging to target class C7 consistent with a combination of arc-like magmas influenced by wet melting and probable crustal contamination. Sample details are provided in Appendix Table A14.

Fig. 13.

A) Geologic assemblage map of the Abitibi greenstone belt (AGB) showing the locations of mafic volcanic rock samples compiled for this study (modified from Thurston et al., 2008; Monecke et al., 2017; Dube and Mercier-Langevin, 2020). B) Pie chart of the random forest classification results for the entire AGB data set, colored according to the different target cdlasses. The majority of the samples are classified as target class C2, C3, and C7. C) Number of samples in each target class identified among the different assemblages of the AGB. The distribution of the different target classes corresponds generally to the north and south rift zones of the Abitibi previously identified by Mole et al. (2022). The Blake River Assemblage (Noranda Volcanic Complex) and Deloro Assemblage have the highest proportion of samples belonging to target class C7, consistent with a combination of arc-like magmas influenced by wet melting and probable crustal contamination. The Kidd Munro Assemblage has a high proportion of samples belonging to target classes C2 and C3, similar to ridge-hotspot intersections. The Tisdale Assemblage has a high proportion of samples belonging to target class C6 and C7, consistent with a combination of arc-like magmas influenced by wet melting and probable crustal contamination. Stoughton-Roquemaure has the highest proportion of samples in target class C2 and C3 similar to ridge-hotspot intersections and mid-ocean ridge (MOR)-like settings. The Deloro Assemblage has the highest proportion of samples belonging to target class C7 consistent with a combination of arc-like magmas influenced by wet melting and probable crustal contamination. Sample details are provided in Appendix Table A14.

We used data from Mole et al. (2021) and several data sets of the Ontario Geological Survey (MRD 378, 292, 362, 377, 393, 355, 085), the Geological Survey of Canada (Open File 6623), and the SIGEOM (Système d’information géominière of Québec). The samples include tholeiitic to calc-alkaline volcanic rocks from the Selbaie area, Geant-Dormant, and Joutel in the Northern Volcanic Zone (NVZ), dominantly tholeiitic rocks from the northeast Matagami area, and tholeiitic to calc-alkaline volcanic rocks from from the Hunter Mine Group, Val-d’Or, Noranda, Bousquet, the western Blake River Group, and the Timmins-Porcupine area, including Kidd Creek and Kamiskotia, in the Southern Volcanic Zone (SVZ) (Fig. 13A). These locations host some of the world’s most important volcanogenic massive sulfide deposits (e.g., Franklin et al., 2005). Following the same QA/QC applied to the modern volcanic rocks, 5,746 samples were selected for the final database (see App. and App. Table A13). Effects of alteration are minimized, although the larger range of K2O and Na2O in the Abitibi greenstone belt samples (e.g., Fig. 2) suggests that some altered samples are present. In our analysis of the RF classifier, we showed that Ba mobility had no effect on classification performance for modern sea-floor basalts (App. Table A12). Barium mobility in the older Abitibi greenstone belt samples may lead to lower prediction rates for individual classes. To exclude samples that may have received lower prediction rates due to alteration, only classification results with a prediction rate >50% were included in the analysis. In the Abitibi greenstone belt data set, of the 5,746 samples in the database, 1,065 were not classified (i.e., no class received more than 50% of the votes for the sample).

The mafic volcanic rocks from the Abitibi greenstone belt are distinguished from modern ocean floor basalts by more variable K2O and Na2O, lower TiO2, and flat to steeply dipping REE patterns. The larger range of Zr/TiO2 could be related to higher temperatures, more contamination in the Abitibi greenstone belt, or a sampling bias. Extraction of melts from the Archean mantle required melting at very high temperatures under mostly dry melting conditions (Prior et al., 1999; Piercey, 2011; Thurston, 2015). However, examples of negative Nb-Ta anomalies and LILE enrichment, which are the dominant features of melting hydrated crust in modern submarine arcs, are found in every assemblage of the Abitibi greenstone belt (Ayer et al., 2002). Samples of mafic volcanic rocks from the Val-d’Or formation, for example, have strong negative Nb-Ta anomalies, low REE concentrations and LILE enrichment (Fig. 14), interpreted to indicate an arc-like regime (Scott et al., 2002). Samples from Noranda have overall weaker negative Nb-Ta anomalies with slightly dipping REE patterns that have been interpreted as originating in a back-arc regime (e.g., Yang and Scott, 2003). Samples from north east of Matagami lack negative Nb-Ta anomalies and LILE enrichment, and they have significantly higher REE concentrations that have been attributed to melt generation at an oceanic spreading center (Hart et al., 2004). The weak negative Nb-Ta anomalies and higher Th concentrations in Abitibi mafic volcanic rocks are commonly cited as important differences compared to basalts from modern oceanic subduction zones (Bédard et al., 2013). Arc-like geochemical signatures could have been created in a number of different ways, including assimilation of hydrated basalt (e.g., amphibole-containing altered crustal components) and/or ilmenite fractionation (e.g., Haase et al., 2005; Pearce, 2008; Mole et al., 2021). The example of Icelandic basalts perhaps best illustrates this. They have high Th/Yb and plot close to the continental arc field in the lithotectonic discrimination diagram of Pearce (2014) (Fig. 4B), although they lack the Ba enrichment that is typical for arc-like rocks. Mafic volcanic rocks nearly identical to those from the Eastern Blake River Group occur at the terminations of modern back-arc spreading centers (e.g., in the Northeast Lau spreading center, the southern East Scotia Ridge, the western Bransfield Strait, and the southern and northern Mariana Trough; Figs. 4 and 14). Similar rocks are clearly identified in the Abitibi greenstone belt by the RF classifier developed from modern oceanic basalts.

Fig. 14.

A) Primitive mantle-normalized trace element plot of representative mafic volcanic rocks from northeast Matagami, Noranda, and Val-d’Or (individual samples are transparent; averages are nontransparent). Samples from northeast Matagami have no negative Nb-Ta anomaly and flat rare earth element (REE) patterns, similar to the Northwest Lau spreading center and Rochambeau Rifts. Samples from Noranda have weak negative Nb-Ta anomalies and dipping REE patterns, suggesting limited wet melting. Samples from Val-d’Or have strong negative Nb-Ta anomalies and flat REE patterns characteristic of wet melting. B) Plot of mafic volcanic rocks from northeast Matagami, Noranda, and Val-d’Or on the tectonic discrimination diagram of Pearce (2014). Samples from northeast Matagami plot mostly in the E-MORB array, whereas samples from Noranda plot within or slightly below the continental arc field. Samples from Val-d’Or plot between the oceanic arc field and N-MORB array. Fields for modern oceanic mafic volcanic rocks from the Fonualei rift and spreading center and Northeast Lau spreading center of the Lau basin, as well as the Pacific-Antarctic Rise (see Fig. 3), are shown for comparison.

Fig. 14.

A) Primitive mantle-normalized trace element plot of representative mafic volcanic rocks from northeast Matagami, Noranda, and Val-d’Or (individual samples are transparent; averages are nontransparent). Samples from northeast Matagami have no negative Nb-Ta anomaly and flat rare earth element (REE) patterns, similar to the Northwest Lau spreading center and Rochambeau Rifts. Samples from Noranda have weak negative Nb-Ta anomalies and dipping REE patterns, suggesting limited wet melting. Samples from Val-d’Or have strong negative Nb-Ta anomalies and flat REE patterns characteristic of wet melting. B) Plot of mafic volcanic rocks from northeast Matagami, Noranda, and Val-d’Or on the tectonic discrimination diagram of Pearce (2014). Samples from northeast Matagami plot mostly in the E-MORB array, whereas samples from Noranda plot within or slightly below the continental arc field. Samples from Val-d’Or plot between the oceanic arc field and N-MORB array. Fields for modern oceanic mafic volcanic rocks from the Fonualei rift and spreading center and Northeast Lau spreading center of the Lau basin, as well as the Pacific-Antarctic Rise (see Fig. 3), are shown for comparison.

In the Abitibi greenstone belt data set, Nb, Ba, Yb, Y, and Th had the greatest influence on the classification of the rocks by the RF classifier. Of the 5,746 samples in the database, 1,065 were not classified (i.e., no class received more than 50% of the votes). This indicates either fundamental differences from modern oceanic mafic volcanic rocks or geochemical variation due to alteration and metamorphism. The remaining 4,681 samples were assigned to a target class comparable to that of modern oceanic basalts. Rocks belonging to C2, C3, C5, C6, and C7 were confidently identified in the Abitibi greenstone belt data set (Fig. 13B; App. Table A14). They can be broadly divided into two groups, which in the modern classification would correspond to dry melting (C2 and C3) and wet melting conditions (C5, C6, and C7). The latter could be interpreted as dehydration of hydrated basalts in a downgoing slab or, in other models, hydrated basaltic crust that sank into the mantle during delamination. None of the samples were fully classified (100% of the votes) into C1 (i.e., depleted MORS) or C4 (i.e., ocean island settings). Samples from Kidd Creek were classified as belonging to target class C2 and C3 (Figure 13C), indicating similarities to ridge-hotspot intersections, such as Iceland, as previously suggested by Prior et al. (1999). The samples from the Blake River Group (and Noranda specifically) show greater geochemical variability. Samples from the western part of the Blake River Group are mostly classified in C2 (i.e., undepleted MOR volcanic suites), whereas samples from the Eastern Blake River Group and Noranda are mostly classified in C7. The latter are most similar to mafic volcanic rocks from nascent back-arc rifts as in the Northeast Lau spreading center and the eastern Manus basin, as previously recognized by Yang and Scott (2003). A high proportion of mafic volcanic rocks in the Bousquet and Val d’Or districts were classified in target classes C5, C6, and C7 (i.e., intraoceanic back-arc and arc-related rifts). This is in agreement with previous studies characterizing the Val-d’Or formation as arc-like crust (Scott et al., 2002).

We compared the major and trace element geochemistry of modern submarine mafic volcanic rocks from different geodynamic settings using PCA, agglomerative hierarchical clustering, and RF. Automated hierarchical clustering reveals significant geochemical diversity related to mantle heterogeneity, highly variable melting regimes, and contributions of fluids from the subducting slab. Back-arc basalt and MOR-like mafic volcanic rocks, such as at the Northwest Lau spreading center and the Pacific Antarctic Rise, as well as ridge-hotspot intersections such as in Iceland, all share enriched mantle signatures typical of spreading centers far from any subduction. However, significant variation in the trace element geochemistry is found in complex microplate mosaics at very small scales (e.g., distances of less than 100 km along the Fonualei rift and spreading center and the Northeast Lau spreading center). The greatest diversity occurs at the terminations or propagating tips of back-arc spreading centers, such as the Northeast Lau spreading center, the northern and southern East Scotia Ridge, and the northern and southern Mariana Trough, owing to complex mantle flow regimes.

Comparisons with mafic volcanic rocks of the Abitibi greenstone belt indicate magmas generated above the hot Archean mantle were most similar geochemically to those of modern back-arc spreading centers with weak arc signatures and enriched mantle sources, similar to the Northwest Lau spreading center of the Lau basin and, in the open ocean, similar to the Pacific Antarctic Rise and Iceland. The mafic volcanic rocks from these settings bear a strong similarity to volcanic rocks from northeast of Matagami and from Kidd Creek. Significant geochemical diversity in mafic rock compositions in the Abitibi greenstone belt, including in the Eastern Blake River Group at Noranda and at Val d’Or, occur at about the same scale as in the northern Lau basin. We interpret these variations to reflect mantle heterogeneity and mantle flow regimes influenced by microplate formation. The ability to recognize possible ancient analogs in greenstone belts raises the possibility of improved area selection for mineral exploration, focusing on the enhanced magmatic productivity of back-arc spreading centers with enriched mantle sources, at the terminal or propagating tips of arc-related rifts, and at ridge-hotspot intersections where mantle upwelling likely provided the heat for melting.

We thank two anonymous referees for the helpful comments that improved the manuscript. This research was funded by the Canada First Research Excellence Fund (CFREF, Metal Earth at Laurentian University) and the Marine Mineral Resources Group at the GEOMAR Helmholtz Centre for Ocean Research Kiel. The Natural Sciences and Engineering Research Council of Canada (NSERC), the Helmholtz Association, and the German Ministry of Science and Education (BMBF, grant 03G0267) are acknowledged for the support of this work through research grants to the authors. The project was also supported by the NSERC Collaborative Research and Training Experience program (iMAGE-CREATE) on Marine Geodynamics and Georesources. This is contribution MERC-ME-2023-27.

1.
Aitchison
,
J.
,
1982
,
The statistical analysis of compositional data: Journal of the Royal Statistical Society: Series B (Methodological)
, v.
44
, p.
139
160
.
2.
Arculus
,
R.J.
,
Gurnis
,
O.
,
Ishizuka
,
O.
,
Reagan
,
M.K.
,
Pearce
,
J.A.
, and
Sutherland
,
R.
,
2019
,
How to create new subduction zones: A global perspective
:
Oceanography
 , v.
32
, p.
160
174
.
3.
Arevalo
,
R.
, and
McDonough
,
W.F.
,
2010
,
Chemical variations and regional diversity observed in MORB
:
Chemical Geology
 , v.
271
, p.
70
85
.
4.
Ayer
,
J.
,
Amelin
,
Y.
,
Corfu
,
F.
,
Kamo
,
S.
,
Ketchum
,
J.
,
Kwok
,
K.
, and
Trowell
,
N.
,
2002
,
Evolution of the southern Abitibi greenstone belt based on U-Pb geochronology: Autochthonous volcanic construction followed by plutonism, regional deformation and sedimentation
:
Precambrian Research
 , v.
115
, p.
63
95
.
5.
Bartholomew
,
D.J.
,
2010
,
Principal components analysis
, in Peterson et al, ed.,
International encyclopedia of education
 , 3rd ed:
Elsevier Science
, p.
374
377
.
6.
Baxter
,
A.T.
,
Hannington
,
M.D.
,
Stewart
,
M.S.
,
Emberley
,
J.M.
,
Breker
,
K.
,
Krätschell
,
A.
,
Petersen
,
S.
,
Brandl
,
P.A.
,
Klischies
,
M.
,
Mensing
,
R.
, and
Anderson
,
M.O.
,
2020
,
Shallow seismicity and the classification of structures in the Lau back-arc basin
:
Geochemistry, Geophysics, Geosystems
 , v.
21
, article e2020GC008924.
7.
Bédard
,
J.H.
,
Harris
,
L.B.
, and
Thurston
,
P.C.
,
2013
,
The hunting of the snArc
:
Precambrian Research
 , v.
229
, p.
20
48
.
8.
Beier
,
C.
,
Turner
,
S.P.
,
Sinton
,
J.M.
, and
Gill
,
J.B.
,
2010
,
Influence of subducted components on back-arc melting dynamics in the Manus Basin
:
Geochemistry, Geophysics, Geosystems
 , v.
11
, article Q0AC03.
9.
Beier
,
C.
,
Bach
,
W.
,
Turner
,
S.
,
Niedermeier
,
D.
,
Woodhead
,
J.
,
Erzinger
,
J.
, and
Krumm
,
S.
,
2015
,
Origin of silicic magmas at spreading centres—an example from the South East rift, Manus basin
:
Journal of Petrology
 , v.
56
, p.
255
272
.
10.
Beier
,
C.
,
Turner
,
S.P.
,
Haase
,
K.M.
,
Pearce
,
A.J.
,
Münker
,
C.
, and
Regelous
,
M.
,
2017
,
Trace element and isotope geochemistry of the northern and central Tongan Islands with an emphasis on the genesis of high Nb/Ta signatures at the northern volcanoes of Tafahi and Niuatoputapu
:
Journal of Petrology
 , v.
58
, p.
1073
1106
.
11.
Bézos
,
A.
,
Escrig
,
S.
,
Langmuir
,
C.H.
,
Michael
,
P.J.
, and
Asimow
,
P.D.
,
2009
,
Origins of chemical diversity of back-arc basin basalts: A segment-scale study of the Eastern Lau spreading center
:
Journal of Geophysical Research
 , v.
114
, article B06212.
12.
Breiman
,
L.
,
2001
,
Random forests
:
Machine Learning
 , v.
45
, p.
5
32
.
13.
Caracciolo
,
A.
,
Halldórsson
,
S.A.
,
Bali
,
E.
,
Marshall
,
E.W.
,
Jeon
,
H.
,
Whitehouse
,
M.J.
,
Barnes
,
J.D.
,
Guofinnsson
,
G.H.
,
Kahl
,
M.
, and
Hartley
,
M.E.
,
2022
,
Oxygen isotope evidence for progressively assimilating trans-crustal magma plumbing systems in Iceland
:
Geology
 , v.
50
, p.
796
800
.
14.
Caratori Tontini
,
F.
,
Bassett
,
D.
,
de Ronde
,
C.E.J.
,
Timm
,
C.
, and
Wysoczanski
,
R.
,
2019
,
Early evolution of a young back-arc basin in the Havre Trough
:
Nature Geoscience
 , v.
12
, p.
856
862
.
15.
Castillo
,
P.R.
,
2012
,
Adakite petrogenesis
:
Lithos
 , v.
134–135
, p.
304
316
.
16.
Chauvel
,
C.
,
Maury
,
R.C.
,
Blais
,
S.
,
Lewin
,
E.
,
Guillou
,
H.
,
Guille
,
G.
,
Rossi
,
P.
, and
Gutscher
,
M.-A.
,
2012
,
The size of plume heterogeneities constrained by Marquesas isotopic stripes
:
Geochemistry, Geophysics, Geosystems
 , v.
13
, article Q07005.
17.
Chawla
,
N.V.
,
Bowyer
,
K.W.
,
Hall
,
L.O.
, and
Kegelmeyer
,
W.P.
,
2002
,
SMOTE: Synthetic minority over-sampling technique
:
Journal of Artificial Intelligence Research
 , v.
16
, p.
321
357
.
18.
Chen
,
Z.
,
Schellart
,
W.P.
,
Strak
,
V.
, and
Duarte
,
J.C.
,
2016
,
Does subduction-induced mantle flow drive backarc extension?
:
Earth and Planetary Science Letters
 , v.
441
, p.
200
210
.
19.
Cooper
,
L.B.
,
Plank
,
T.
,
Arculus
,
R.J.
,
Hauri
,
E.H.
,
Hall
,
P.S.
, and
Parman
,
S.W.
,
2010
,
High-Ca boninites from the active Tonga Arc
:
Journal of Geophysical Research
 , v.
115
, p.
1
23
.
20.
Dubé
,
B.
, and
Mercier-Langevin
,
P.
,
2020
,
Gold Deposits of the Archean Abitibi Greenstone Belt, Canada: Society of Economic Geologists
,
Special Publication
 
23
, p.
669
708
.
21.
Engel
,
A.E.J.
,
Engel
,
C.G.
, and
Havens
,
R.G.
,
1965
,
Chemical characteristics of oceanic basalts and the upper mantle
:
Geological Society of America Bulletin
 , v.
76
, p.
719
734
.
22.
Escrig
,
S.
,
Bézos
,
A.
,
Goldstein
,
S.L.
,
Langmuir
,
C.H.
, and
Michael
,
P.J.
,
2009
,
Mantle source variations beneath the Eastern Lau spreading center and the nature of subduction components in the Lau basin-Tonga arc system
:
Geochemistry, Geophysics, Geosystems
 , v.
10
, article Q04014.
23.
Escrig
,
S.
,
Bézos
,
A.
,
Langmuir
,
C.H.
,
Michael
,
P.J.
, and
Arculus
,
R.
,
2012
,
Characterizing the effect of mantle source, subduction input and melting in the Fonualei spreading center, Lau basin: Constraints on the origin of the boninitic signature of the back-arc lavas
:
Geochemistry, Geophysics, Geosystems
 , v.
13
, article Q10008.
24.
Falloon
,
T.J.
,
Danyushevsky
,
L.V.
,
Crawford
,
T.J.
,
Maas
,
R.
,
Woodhead
,
J.D.
,
Eggins
,
S.M.
,
Bloomer
,
S.H.
,
Wright
,
D.J.
,
Zlobin
,
S.K.
, and
Stacey
,
A.R.
,
2007
,
Multiple mantle plume components involved in the petrogenesis of subduction-related lavas from the northern termination of the Tonga arc and northern Lau basin: Evidence from the geochemistry of arc and backarc submarine volcanics
:
Geochemistry, Geophysics, Geosystems
 , v.
8
, article Q09003.
25.
Falloon
,
T.J.
,
Danyushevsky
,
L.V.
,
Crawford
,
A.J.
,
Meffre
,
S.
,
Woodhead
,
J.D.
, and
Bloomer
,
S.H.
,
2008
,
Boninites and adakites from the northern termination of the Tonga trench: Implications for adakite petrogenesis
:
Journal of Petrology
 , v.
49
, p.
697
715
.
26.
Fassbender
,
M.L.
,
Hannington
,
M.
,
Stewart
,
M.
,
Brandl
,
P.A.
,
Baxter
,
A.T.
, and
Diekrup
,
D.
,
2023
,
Geochemical signatures of felsic volcanic rocks in modern oceanic settings and implications for archean greenstone belts
:
Economic Geology
 , v.
118
, p.
319
345
.
27.
Fernandez-Delgado
,
M.
,
Cernadas
,
E.
,
Barro
,
S.
, and
Amorim
,
D.
,
2014
,
Do we need hundreds of classifiers to solve real world classification problems?
:
Journal of Machine Learning Research
 , v.
15
, p.
3133
3181
.
28.
Franklin
,
J.M.
,
Gibson
,
H.L.
,
Jonasson
,
I.R.
, and
Galley
,
A.G.
,
2005
,
Volcanogenic massive sulfide deposits
:
Economic Geology
 , v.
100
, p.
523
560
.
29.
Fretzdorff
,
S.
,
Livermore
,
R.A.
,
Devey
,
C.W.
,
Leat
,
P.T.
, and
Stoffers
,
P.
,
2002
,
Petrogenesis of the back-arc East Scotia Ridge, South Atlantic Ocean
:
Journal of Petrology
 , v.
43
, p.
1435
1467
.
30.
Fretzdorff
,
S.
,
Worthington
,
T.J.
,
Haase
,
K.M.
,
Hekinian
,
R.
,
Franz
,
L.
,
Keller
,
R.A.
, and
Stoffers
,
P.
,
2004
,
Magmatism in the Bransfield basin: Rifting of the south Shetland arc?
:
Journal of Geophysical Research
 , v.
109
, article B12208.
31.
Fretzdorff
,
S.
,
Schwarz-Schampera
,
U.
,
Gibson
,
H.L.
,
Garbe-Schönberg
,
C.-D.
,
Hauff
,
F.
, and
Stoffers
,
P.
,
2006
,
Hydrothermal activity and magma genesis along a propagating back-arc basin: Valu Fa ridge (southern Lau basin)
:
Journal of Geophysical Research
 , v.
111
, article B08205.
32.
Freund
,
S.
,
Beier
,
C.
,
Krumm
,
S.
, and
Haase
,
K.M.
,
2013
,
Oxygen isotope evidence for the formation of andesitic-dacitic magmas from the fastspreading Pacific-Antarctic Rise by assimilation-fractional crystallisation
:
Chemical Geology
 , v.
347
, p.
271
283
.
33.
Gale
A.
,
Dalton
,
C.A.
,
Langmuir
,
C.H.
,
Su
,
Y.
, and
Schilling
,
J.-G.
,
2013
,
The mean composition of ocean ridge basalts
:
Geochemistry, Geophysics, Geosystems
 , v.
14
, p.
489
518
.
34.
Galley
,
A.G.
,
Watkinson
,
D.H.
,
Jonasson
,
I.R.
, and
Riverin
,
G.
,
1995
,
The subsea-floor formation of volcanic-hosted massive sulfide: Evidence from the Ansil Deposit, Rouyn-Noranda, Canada
:
Economic Geology
 , v.
90
, p.
2006
2017
.
35.
Gholamy
,
A.
,
Kreinovich
,
V.
, and
Kosheleva
,
O.
,
2018
,
Why 70/30 or 80/20 relation between training and testing sets: A pedagogical explanation
:
University of Texas at El Paso, Departmental Technical Reports (CS)
 ,
1209
(https://scholarworks.utep.edu/cs_techrep/1209).
36.
Gibson
,
H.L.
,
Allen
,
R.L.
,
Riverin
,
G.
, and
Lane
,
T.E.
,
2007
,
The VMS model: Advances and application to exploration targeting
:
Proceedings of Exploration
 , v.
7
, p.
713
730
.
37.
Golowin
,
R.
,
Portnyagin
,
M.
,
Hoernle
,
K.
,
Hauff
,
F.
,
Gurenko
,
A.
,
Garbe-Schönberg
,
D.
,
Werner
,
R.
, and
Turner
,
S.
,
2017
,
Boninite-like intraplate magmas from Manihiki Plateau require ultra-depleted and enriched source components
:
Nature Communications
 , v.
8
, article 14322.
38.
Haase
,
K.M.
,
Worthington
,
T.J.
,
Stoffers
,
P.
,
Garbe-Schönberg
,
D.
, and
Wright
,
I.
,
2002
,
Mantle dynamics, element recycling, and magma genesis beneath the Kermadec Arc-Havre Trough
:
Geochemistry, Geophysics, Geosystems
 , v.
3
, article 107.
39.
Haase
,
K.M.
,
Stroncik
,
N.
,
Hekinian
,
R.
, and
Stoffers
,
P.
,
2005
,
Nb-depleted andesites from the Pacific-Antarctic Rise as analogs for early continental crust
:
Geological Society of America
 , v.
33
, p.
921
924
.
40.
Haase
,
K.M.
,
Fretzdorff
,
S.
,
Mühe
,
R.
,
Garbe-Schönberg
,
D.
, and
Stoffers
,
P.
,
2009
,
A geochemical study of off-axis seamount lavas at the Valu Fa Ridge: Constraints on magma genesis and slab contributions in the southern Tonga subduction zone
:
Lithos
 , v.
112
, p.
137
148
.
41.
Haase
,
K.M.
,
Beier
,
C.
, and
Kemner
,
F.
,
2019
,
A comparison of the magmatic evolution of Pacific intraplate volcanoes: Constraints on melting in mantle plumes
:
Frontiers in Earth Science
 , v.
6
, article 242.
42.
Haase
,
K.M.
,
Gress
,
M.U.
,
Lima
,
S.M.
,
Regelous
,
M.
,
Beier
,
C.
,
Romer
,
R.L.
, and
Bellon
,
H.
,
2020
,
Evolution of magmatism in the New Hebrides island arc and in initial back-arc rifting, SW Pacific
:
Geochemistry, Geophysics, Geosystems
 , v.
21
, article e2020GC008946.
43.
Haase
,
K.M.
,
Schönhofen
,
M.V.
,
Storch
,
B.
,
Beier
,
C.
,
Regelous
,
M.
,
Rubin
,
K.
, and
Brandl
,
P.A.
,
2022
,
Effects of the hydrous domain in the mantle wedge on magma formation and mixing at the Northeast Lau spreading center, SW Pacific
:
Geochemistry, Geophysics, Geosystems
 , v.
23
, article e2021GC010066.
44.
Haraguchi
,
S.
,
Kimura
,
J.-I.
,
Senda
,
R.
,
Fujinaga
,
K.
,
Nakamura
,
K.
,
Takaya
,
Y.
, and
Ishii
,
T.
,
2017
,
Origin of felsic volcanism in the Izu arc intra-arc rift
:
Contributions to Mineralogy and Petrology
 , v.
172
, p.
1
21
.
45.
Hart
,
T.R.
,
Gibson
,
H.L.
, and
Lesher
,
C.M.
,
2004
,
Trace element geochemistry and petrogenesis of felsic volcanic rocks associated with volcanogenic massive Cu-Zn-Pb sulfide deposits
:
Economic Geology
 , v.
99
, p.
1003
1013
.
46.
Hawkesworth
,
C.
,
Gallagher
,
K.
,
Hergt
,
J.M.
, and
McDermott
,
F.
,
1993
,
Mantle and slab contributions in arc magmas
:
Annual Review of Earth and Planetary Sciences
 , v.
21
, p.
175
204
.
47.
Hawkins
,
J.W.
,
1995
,
The geology of the Lau basin
, in
Taylor
,
B.
, ed.,
Backarc basins: Tectonics and magmatism
 :
Springer
, p.
63
138
.
48.
Heuret
,
A.
, and
Lallemand
,
S.
,
2005
,
Plate motions, slab dynamics and back-arc deformation
:
Physics of the Earth and Planetary Interiors
 , v.
149
, p.
31
51
.
49.
Hofmann
,
A.W.
,
2014
,
Sampling mantle heterogeneity through oceanic basalts: Isotopes and trace elements
, in
Holland
,
H.D.
, and
Turekian
,
K.K.
, eds.,
Treatise on geochemistry
 , 2nd ed.:
Elsevier
, p.
67
101
.
50.
Jackson
,
M.G.
,
Hart
,
S.R.
,
Konter
,
J.G.
,
Koppers
,
A.A.P.
,
Staudigel
,
H.
,
Kurz
,
M.D.
,
Blusztajn
,
J.
, and
Sinton
,
J.M.
,
2010
,
Samoan hot spot track on a “hot spot highway”: Implications for mantle plumes and a deep Samoan mantle source
:
Geochemistry, Geophysics, Geosystems
 , v.
11
, article Q12009.
51.
Jackson
,
S.L.
,
Fyon
,
A.
, and
Corfu
,
F.
,
1994
,
Review of Archean supracrustal assemblages of the southern Abitibi greenstone belt in Ontario, Canada: Products of microplate interaction within a large-scale plate-tectonic setting
:
Precambrian Research
 , v.
65
, p.
183
205
.
52.
Jenner
,
F.E.
,
Arculus
,
R.J.
,
Mavrogenes
,
J.A.
,
Dyriw
,
N.J.
,
Nebel
,
O.
, and
Hauri
,
E.H.
,
2012
,
Chalcophile element systematics in volcanic glasses from the northwestern Lau basin
:
Geochemistry, Geophysics, Geosystems
 , v.
13
, article Q06014.
53.
Jicha
,
B.R.
,
Singer
,
B.S.
, and
Valentine
,
M.J.
,
2013
,
40Ar/39Ar geochronology of subaerial Ascension Island and a re-evaluation of the temporal progression of basaltic to rhyolitic volcanism
:
Journal of Petrology
 , v.
54
, p.
2581
2596
.
54.
Keller
,
R.A.
,
Fisk
,
M.R.
,
Smellie
,
J.L.
,
Strelin
,
J.A.
, and
Lawver
,
L.A.
,
2002
, c:
Journal of Geophysical Reasearch
, v.
107
, B8, p.
17
.
55.
Keller
,
N.S.
,
Arculus
,
R.J.
,
Hermann
,
J.
, and
Richards
,
S.
,
2008
,
Submarine back-arc lava with arc signature: Fonualei spreading center, northeast Lau basin, Tonga
:
Journal of Geophysical Research: Solid Earth
 , v.
113
, article B08S07.
56.
Kerrich
,
R.
, and
Wyman
,
D.A.
,
1997
,
Review of developments in trace-element fingerprinting of geodynamic settings and their implications for mineral exploration
:
Australian Journal of Earth Sciences
  v.
44
, p.
465
487
.
57.
Kerrich
,
R.
,
Wyamn
,
D.
,
Fan
,
J.
, and
Bleeker
,
W.
,
1998
,
Boninite series: Low Ti-tholeiite associations from the 2.7 Ga Abitibi greenstone belt
:
Earth and Planetary Science Letters
 , v.
164
, p.
303
316
.
58.
Langmuir
,
C.H.
,
Bézos
,
A.
,
Escrig
,
S.
, and
Parman
,
S.W.
,
2006
,
Chemical systematics and hydrous melting of the mantle in back-arc basins
:
American Geophysical Union, Geophysical Monograph
 
166
, p.
87
146
.
59.
Le Maitre
,
R.W.
,
1989
,
A classification of igneous rocks and glossary of terms: Recommendations of the International Union of Geological Sciences, Subcommission on the Systematics of Igneous Rocks
:
Oxford
,
Blackwell Scientific Publications
,
193
p.
60.
Lesher
,
C.M.
,
Goodwin
,
A.M.
,
Campbell
,
I.H.
, and
Gorton
,
M.P.
,
1986
,
Trace-element geochemistry of ore-associated and barren, felsic metavolcanic rocks in the Superior Province, Canada
:
Canadian Journal of Earth Sciences
 , v.
23
, p.
222
237
.
61.
Lupton
,
J.E.
,
Arculus
,
R.J.
,
Greene
,
R.R.
,
Evans
,
L.J.
, and
Goddard
,
C.I.
,
2009
,
Helium isotope variations in seafloor basalts from the Northwest Lau backarc basin: Mapping the influence of the Samoan hotspot
:
Geophysical Research Letters
 , v.
36
, article L17313.
62.
Lupton
,
J.
,
Rubin
,
K.H.
,
Arculus
,
R.
,
Lilley
,
M.
,
Butterfield
,
D.
,
Resing
,
J.
,
Baker
,
E.
, and
Embley
,
R.
,
2015
,
Helium isotope, C/3He, and Ba-Nb-Ti signatures in the northern Lau basin: Distinguishing arc, back-arc, and hotspot affinities
:
Geochemistry, Geophysics, Geosystems
 , v.
16
, p.
1133
1155
.
63.
Lytle
,
M.L.
,
Kelley
,
K.A.
,
Hauri
,
E.H.
,
Gill
,
J.B.
,
Papia
,
D.
, and
Arculus
,
R.J.
,
2012
,
Tracing mantle sources and Samoan influence in the northwestern Lau back-arc basin
:
Geochemistry, Geophysics, Geosystems
 , v.
13
, article Q10019.
64.
Magni
,
V.
,
2019
,
The effects of back-arc spreading on arc magmatism
:
Earth and Planetary Science Letters
 , v.
519
, p.
141
151
.
65.
McKenzie
,
D.
, and
O’Nions
,
R.K.
,
1991
,
Partial melt distribution from inversion of rare earth element concentrations
:
Journal of Petrology
 , v.
32
, p.
1021
1091
.
66.
Michael
,
P.J.
, and
Cornell
,
W.C.
,
1998
,
Influence of spreading rate and magma supply on crystallization and assimilation beneath mid-ocean ridges: Evidence from chlorine and major element chemistry of mid-ocean ridge basalts
:
Journal of Geophysical Research
 , v.
103
, p.
18,325
18,356
.
67.
Mole
,
D.R.
,
Thurston
,
P.C.
,
Marsh
,
J.H.
,
Stern
,
R.A.
,
Ayer
,
J.A.
,
Martin
,
L.A.J.
, and
Lu
,
Y.J.
,
2021
,
The formation of Neoarchean continental crust in the south-east Superior Craton by two distinct geodynamic processes
:
Precambrian Research
 , v.
356
, article 106104.
68.
Mole
,
D.R.
,
Frieman
,
B.M.
,
Thurston
,
P.C.
,
Marsh
,
J.H.
,
Jorgensen
,
T.R.C.
,
Stern
,
R.A.
,
Martin
,
L.A.J.
,
Lu
,
Y.J.
, and
Gibson
,
H.L.
,
2022
,
Crustal architecture of the south-east Superior craton and controls on mineral systems
:
Ore Geology Reviews
 , v.
148
, article 105017.
69.
Monecke
,
T.
,
Mercier-Langevin
,
P.
,
Dubé
,
B.
, and
Frieman
,
B.M.
,
2017
,
Geology of the Abitibi greenstone belt, Canada: Reviews in
Economic Geology
 , v.
19
, p.
7
49
.
70.
Nebel
,
O.
, and
Arculus
,
R.J.
,
2015
,
Selective ingress of a Samoan plume component into the northern Lau backarc basin
:
Nature Communications
  v.
6
, article 6554.
71.
Nebel
,
O.
,
Sossi
,
P.A.
,
Foden
,
J.
,
Bénard
,
A.
,
Brandl
,
P.A.
, and
Stammeier
,
J.A.
,
2018
,
Iron isotope variability in ocean floor lavas and mantle sources in the Lau back-arc basin
:
Geochimica et Cosmochimica Acta
 , v.
241
, p.
150
163
.
72.
Paradis
,
S.
,
Ludden
,
J.
, and
Gelinas
,
L.
,
1988
,
Evidence of contrasting compositional spectra in comagmatic intrusive and extrusive rocks of the late Archean Blake River Group, Abitibi, Quebec
:
Canadian Journal of Earth Sciences
 , v.
25
, p.
134
144
.
73.
Park
,
J.-W.
,
Campbell
,
I.H.
,
Kim
,
J.
, and
Moon
,
J.-W.
,
2015
,
The role of late sulfide saturation in the formation of a Cu- and Au-rich magma: Insights from the platinum group element geochemistry of Niuatahi-Motutahi lavas, Tonga rear arc
:
Journal of Petrology
 , v.
56
, p.
59
81
.
74.
Pearce
,
J.A.
,
1996
,
A user’s guide to basalt discrimination diagrams: Geological Association of Canada, Short Course
Notes
 , v.
12
, p.
79
113
.
75.
Pearce
,
J.A.
,
2008
,
Geochemical fingerprinting of oceanic basalts with applications to ophiolite classification and the search for Archean oceanic crust
:
Lithos
 , v.
100
, p.
14
48
.
76.
Pearce
,
J.A.
,
2014
,
Immobile element fingerprinting of ophiolites
:
Elements
  v.
10
, p.
101
108
.
77.
Pearce
,
J.A.
, and
Arculus
,
R.J.
,
2020
,
Boninites
, in
Alderton
,
D.
, and
Scott
,
A.E.
, eds.,
Encyclopedia of geology
 , 2nd ed.:
American Press
, p.
113
129
.
78.
Pearce
,
J.A.
, and
Norry
,
M.J.
,
1979
,
Petrogenetic implications of Ti, Zr, Y, and Nb variations in volcanic rocks
:
Contributions to Mineralogy and Petrology
 , v.
69
, p.
33
47
.
79.
Pearce
,
J.A.
, and
Reagan
,
M.K.
,
2019
,
Identification, classification, and interpretation of boninites from Anthropocene to Eoarchean using Si-Mg-Ti systematics
:
Geosphere
  v.
15
, p.
1008
1037
.
80.
Pearce
,
J.A.
, and
Stern
,
R.J.
,
2006
,
Origin of back-arc basin magmas: Trace element and isotope perspectives: American Geophysical Union, Geophysical Monograph
Series
 , v.
166
, p.
63
86
.
81.
Pearce
,
J.A.
,
Stern
,
R.J.
,
Bloomer
,
S.H.
, and
Fryer
,
P.
,
2005
,
Geochemical mapping of the Mariana arc-basin system: Implications for the nature and distribution of subduction components
:
Geochemistry, Geophysics, Geosystems
 , v.
6
, article Q07006.
82.
Pearce
,
J.A.
,
Kempton
,
P.D.
, and
Gill
,
J.B.
,
2007
,
Hf-Nd evidence for the origin and distribution of mantle domains in the SW Pacific
:
Earth and Planetary Science Letters
 , v.
260
, p.
98
114
.
83.
Percival
,
J.A.
,
Skulski
,
T.
,
Sanborn-Barrie
,
M.
,
Stott
,
G.M.
,
Leclair
,
A.D.
,
Corkery
,
M.T.
, and
Boily
,
M.
,
2012
,
Geology and tectonic evolution of the Superior Province, Canada
:
Geological Association of Canada
, Special Paper 49, p.
321
378
.
84.
Perfit
,
M.R.
, and
Davidson
,
J.P.
,
2000
,
Plate tectonics and volcanism
, in
Sigurdsson
,
H.
,
Houghton
,
B.F.
,
McNutt
,
S.R.
,
Rymer
,
H.
,
Stix
,
J.
, and
Ballard
,
R.D.
, eds.,
Encyclopedia of volcanoes
 :
San Diego
,
Academic Press
, p.
89
113
.
85.
Piercey
,
S.J.
,
2011
,
The setting, style, and role of magmatism in the formation of volcanogenic massive sulfide deposits
:
Mineralium Deposita
 , v.
46
, p.
449
471
.
86.
Price
,
A.A.
,
Jackson
,
M.G.
,
Blichert-Toft
,
J.
,
Hall
,
P.S.
,
Sinton
,
J.M.
,
Kurz
,
M.D.
, and
Blusztajn
,
J.
,
2014
,
Evidence for a broadly distributed Samoan-plume signature in the northern Lau and North Fiji basins
:
Geochemistry, Geophysics, Geosystems
 , v.
15
, p.
986
1008
.
87.
Price
,
A.A.
,
Jackson
,
M.G.
,
Blichert-Toft
,
J.
,
Kurz
,
M.D.
,
Gill
,
J.
,
Blusztajn
,
J.
,
Jenner
,
F.
,
Brens
,
R.
, and
Arculus
,
R.
,
2017
,
Geodynamic implications for zonal and meridional isotopic patterns across the northern Lau and North Fiji basins
:
Geochemistry, Geophysics, Geosystems
 , v.
18
, p.
1013
1042
.
88.
Prior
,
G.J.
,
Gibson
,
H.L.
,
Watkinson
,
D.H.
,
Cook
,
R.E.
, and
Hannington
,
M.D.
,
1999
,
Rare earth and high field strength element geochemistry of the Kidd Creek rhyolites, Abitibi greenstone belt, Canada: Evidence for Archean felsic volcanism and massive sulfide ore formation in an Iceland-style rift environment
:
Economic Geology, Monograph
 
10
, p.
457
484
.
89.
R Core Team
,
2020
,
R: A language and environment for statistical computing: R Foundation for Statistical Computing
,
Vienna, Austria
, www.R-project.org.
90.
Regelous
,
M.
,
Turner
,
S.
,
Falloon
,
T.J.
,
Taylor
,
P.
,
Gamble
,
J.
, and
Green
,
T.
,
2008
,
Mantle dynamics and mantle melting beneath Niuafo’ou Island and the northern Lau back-arc basin
:
Contributions to Mineralogy and Petrology
 , v.
156
, p.
103
118
.
91.
Resing
,
J.A.
,
Rubin
,
K.H.
,
Embley
,
R.W.
,
Lupton
,
J.E.
,
Baker
,
E.T.
,
Dziak
,
R.P.
,
Baumberger
,
T.
,
Lilley
,
M.D.
,
Huber
,
J.A.
,
Shank
,
T.M.
,
Butterfield
,
D.A.
,
Clague
,
D.A.
,
Keller
,
N.S.
,
Merle
,
S.G.
,
Buck
,
N.J.
,
Michael
,
P.J.
,
Soule
,
A.
,
Caress
,
D.W.
,
Walker
,
S.L.
,
Davis
,
R.
,
Cowen
,
J.P.
,
Reysenback
,
A.-L.
, and
Thomas
,
H.
,
2011
,
Active submarine eruption of boninite in the northeastern Lau Basin
:
Nature Geoscience
 , v.
4
, p.
799
806
.
92.
Ross
,
P.S.
, and
Bédard
,
J.H.
,
2009
,
Magmatic affinity of modern and ancient subalkaline volcanic rocks determined from trace-element discriminant diagrams
:
Canadian Journal of Earth Sciences
 , v.
46
, p.
823
839
.
93.
Ryan
,
W.B.F.
,
Carbotte
,
S.M.
,
Coplan
,
J.O.
,
O’Hara
,
S.
,
Melkonian
,
A.
,
Arko
,
R.
,
Weissel
,
R.A.
,
Ferrini
,
V.
,
Goodwillie
,
A.
,
Nitsche
,
F.
,
Bonczkowski
,
J.
, and
Zemsky
,
R.
,
2009
,
Global multi-resolution topography synthesis
:
Geochemistry Geophysics Geosystems
 , v.
10
, article Q03014.
94.
Schellart
,
W.P.
,
2008
,
Subduction zone trench migration: Slab driven or overriding-plate-driven?
:
Physics of the Earth and Planetary Interiors
 , v.
170
, p.
73
88
.
95.
Schellart
,
W.P.
,
2020
,
Control of subduction tone age and size on flat slab subduction
:
Frontiers in Earth Science
  v.
8
, p.
18
.
96.
Schelart
,
W.P.
, and
Moresi
,
L.
,
2013
,
A new driving mechanism for backarc extension and backarc shortening through slab sinking induced toroidal and poloidal mantle flow: Results from dynamic subduction models with an overriding plate
:
Journal of Geophysical Research
 , v.
118
, p.
3221
3248
.
97.
Schmid
,
F.
,
Kopp
,
H.
,
Schnabel
,
M.
,
Dannowski
,
A.
,
Heyde
,
I.
,
Riedel
,
M.
,
Hannington
,
M.D.
,
Engels
,
M.
,
Beniest
,
A.
,
Klaucke
,
I.
,
Augustin
,
N.
,
Brandl
,
P.A.
, and
Devey
,
C.
,
2020
,
Crustal structure and evolution of the Niuafo’ou Microplate in the northeastern Lau Basin, Southwestern Pacific
:
Journal of Geophysical Research: Solid Earth
 , v.
125
, article e2019JB019184.
98.
Schmidt
,
M.W.
, and
Poli
,
S.
,
2014
,
Devolatilization during subduction
, in
Holland
,
H.D.
, and
Turekian
,
K.K.
, eds.,
Treatise on geochemistry
 , 2nd ed.:
Elsevier
, p.
669
701
.
99.
Schönhofen
,
M.V.
,
2021
,
Melting and source variability in the Tonga-Kermadec Lau arc-backarc system
: Doctoral thesis,
University of Erlangen and Nuremburge
,
Germany
,
154
p.
100.
Scott
,
C.R.
,
Mueller
,
W.U.
, and
Pilote
,
P.
,
2002
,
Physical volcanology, stratigraphy, and lithogeochemistry of an Archean volcanic arc: evolution from plume-related volcanism to arc rifting of SE Abitibi greenstone belt, Val d’Or, Canada
:
Precambrian Research
 , v.
115
, p.
223
260
.
101.
Sdrolias
,
M.
, and
Müller
,
R.D.
,
2006
,
Controls on back-arc basin formation
:
Geochemistry, Geophysics, Geosystems
 , v.
7
, article Q04016.
102.
Shinjo
,
R.
, and
Kato
,
Y.
,
2000
,
Geochemical constraints on the origin of bimodal magmatism at the Okinawa Trough, an incipient back-arc basin
:
Lithos
 , v.
54
, p.
117
137
.
103.
Sinton
,
J.M.
,
Ford
,
L.L.
,
Chappell
,
B.
, and
McCulloch
,
M.T.
,
2003
,
Magma genesis and mantle heterogeneity in the Manus back-arc basin, Papua New Guinea
:
Journal of Petrology
 , v.
44
, p.
159
195
.
104.
Sleeper
,
J.D.
, and
Martinez
,
F.
,
2016
,
Geology and kinematics of the Niuafo’ou microplate in the northern Lau Basin
:
Journal of Geophysical Research
 , v.
121
, p.
4852
4875
.
105.
Stewart
,
M.S.
,
Hannington
,
M.D.
,
Emberley
,
J.
,
Baxter
,
A.T.
,
Krätschel
,
A.
,
Petersen
,
S.
,
Brandl
,
P.A.
,
Anderson
,
M.O.
,
Mercier-Langevin
,
P.
,
Mensing
,
R.
,
Breker
,
K.
, and
Fassbender
,
M.L.
,
2022
,
A new geological map of the Lau basin reveals crustal growth processes in arc-backarc systems
:
Geosphere
 , v.
18
, p.
910
943
.
106.
Taylor
,
B.
, and
Martinez
,
F.
,
2003
,
Back-arc basin basalt systematics
:
Earth and Planetary Science Letters
 , v.
210
, p.
481
497
.
107.
Taylor
,
B.
,
Zellmer
,
K.
,
Martinez
,
F.
, and
Goodliffe
,
A.
,
1996
,
Sea-floor spreading in the Lau back-arc basin
:
Earth and Planetary Science Letters
 , v.
144
, p.
35
40
.
108.
Thurston
,
P.C.
,
2015
,
Greenstone belts and granite-greenstone terranes: Constraints on the nature of the Archean world
:
Geoscience Canada
 , v.
42
, p.
437
484
.
109.
Thurston
,
P.C.
,
Ayer
,
J.A.
,
Goutier
,
J.
, and
Hamilton
,
M.A.
,
2008
,
Depositional gaps in Abitibi greenstone belt stratigraphy: A key to exploration for syngenetic mineralization
:
Economic Geology
 , v.
103
, p.
1097
1134
.
110.
Tian
,
L.
,
Castillo
,
P.R.
,
Hawkins
,
J.W.
,
Hilton
,
D.R.
,
Hanan
,
B.B.
, and
Pietruszka
,
A.J.
,
2008
,
Major and trace element and Sr-Nd isotope signatures of lavas from the Central Lau basin: Implications for the nature and influence of subduction components in the back-arc mantle
:
Journal of Volcanology and Geothermal Research
 , v.
178
, p.
657
670
.
111.
Tian
,
L.
,
Castillo
,
P.R.
,
Hilton
,
D.R.
,
Hawkins
,
J.W.
,
Hanan
,
B.B.
, and
Pietruszka
,
A.J.
,
2011
,
Major and trace element and Sr-Nd isotope signatures of the northern Lau Basin lavas: Implications for the composition and dynamics of the back-arc basin mantle
:
Journal of Geophysical Research: Solid Earth
 , v.
116
, article B11201.
112.
Todd
,
E.
,
Gill
,
J.B.
,
Wysoczanski
,
R.J.
,
Hergt
,
J.
,
Wright
,
I.C.
,
Leybourne
,
M.I.
, and
Mortimer
,
N.
,
2011
,
Hf isotopic evidence for small-scale heterogeneity in the mode of mantle wedge enrichment: Southern Havre Trough and South Fiji Basin back arcs
:
Geochemistry, Geophysics, Geosystems
 , v.
12
, article Q09011.
113.
Ueki
,
K.
,
Hino
,
Hideitsu
, H., and
Kuwatani
,
T.
,
2018
,
Geochemical discrimination and characteristics of magmatic tectonic settings: A machine-learning-based approach
:
Geochemistry, Geophysics, Geosystems
 , v.
19
, p.
1327
1347
.
114.
Wanless
,
V.D.
, and
Shaw
,
A.M.
,
2012
,
Lower crustal crystallization and melt evolution at mid-ocean ridges
:
Nature Geoscience
 , v.
5
, p.
651
655
.
115.
White
,
W.M.
, and
Klein
,
E.M.
,
2014
,
Composition of the oceanic crust
, in
Holland
,
H.D.
, and
Turekian
,
K.K.
, eds.,
Treatise on geochemistry
 , 2nd ed.:
Elsevier
, p.
457
496
.
116.
Winchester
,
J.A.
, and
Floyd
,
P.A.
,
1977
,
Geochemical discrimination of different magma series and their differentiation products using immobile elements
:
Chemical Geology
 , v.
20
, p.
325
343
.
117.
Wyman
,
D.A.
,
2013
,
A critical assessment of Neoarchean “plume only” geodynamics: Evidence from the Superior Province
:
Precambrian Research
 , v.
229
, p.
3
19
.
118.
Yan
,
Q.
,
Castillo
,
P.R.
, and
Shi
,
X.
,
2012
,
Geochemistry of basaltic lavas from the southern Lau basin: Input of compositionally variable subduction components
:
International Geology Review
 , v.
54
, p.
1456
1474
.
119.
Yan
,
Q.
,
Straub
,
S.
,
Castillo
,
P.
,
Zhang
,
H.
,
Tian
,
L.
, and
Xuefa
,
Shi.
,
2020
,
Hafnium isotope constraints on the nature of the mantle beneath the Southern Lau basin (SW Pacific)
:
Scientific Reports
 , v.
10
, article 17476.
120.
Yang
,
K.
, and
Scott
,
S.D.
,
2003
,
Geochemical relationships of felsic magmas to ore metals in massive sulfide deposits of the Bathurst mining camp, Iberian pyrite belt, Hokuroku district, and the Abitibi belt
:
Society of Economic Geologists, Economic Geology Monograph
 
11
, p.
457
478
.
121.
van der Zander
,
I.
,
Sinton
,
J.M.
, and
Mahoney
,
J.J.
,
2010
,
Late shield-stage silicic magmatism at Wai’anae volcano: Evidence for hydrous crustal melting in Hawaiian volcanoes
:
Journal of Petrology
 , v.
51
, p.
671
701
.
122.
Zellmer
,
G.F.
,
Hawkesworth
,
C.J.
,
Sparks
,
R.S.J.
,
Thomas
,
L.E.
,
Harford
,
C.L.
,
Brewer
,
T.S.
, and
Loughlin
,
S.C.
,
2003
,
Geochemical evolution of the Soufriere Hills volcano, Monsterrat, Lesser Antilles Volcanic arc
:
Journal of Petrology
 , v.
44
, p.
1249
1374
.
123.
Zellmer
,
G.
,
Rubin
,
K.
,
Gronvold
,
K.
, and
Jurado-Chichay
,
Z.
,
2008
,
On the recent bimodal magmatic processes and their rates in the Torfajökull–Veidivötn area, Iceland
:
Earth and Planetary Science Letters
 , v.
269
, p.
388
398
.
124.
Zhang
,
H.
,
Yan
,
Q.
,
Li
,
C.
,
Zhu
,
Z.
,
Zhao
,
R.
, and
Shi
,
X.
,
2018
,
Geochemistry of diverse lava types from the Lau sasin (South West Pacific): Implications for complex back-arc mantle dynamics
:
Geological Journal
 , v.
56
, p.
3643
3659
.

Marc Fassbender is a freelance consulting geologist holding a Ph.D. from the University of Ottawa, Canada. He specializes in host rocks of actively forming VMS deposits on the modern sea floor and their ancient analogues, using petrology, geochemistry, and machine learning. Marc holds a B.Sc. and an M.Sc. in geology from the University of Erlangen-Nuremberg, Germany, where his research focused on HREE enrichment in olivine and petrogenetic evolution of the Vergenoeg F-Fe-REE deposit in South Africa. He has experience in the fields of economic geology, geochemistry, and integrating geochemical and other data sets using machine learning to solve geoscience problems.

Gold Open Access: This paper is published under the terms of the CC-BY-NC license.

Supplementary data