High-temperature geochemistry combined with igneous petrology is an essential tool to infer the conditions of magma generation and evolution in the Earth's interior. During the past thirty years, a large number of geochemical models of the Earth, essentially inferred from the isotopic composition of basaltic rocks, have been proposed. These geochemical models have paid little attention to basic physics concepts, broadband seismology, or geological evidence, with the effect of producing results that are constrained more by assumptions than by data or first principles. This may not be evident to seismologists and geodynamicists. A common view in igneous petrology, seismology, and mantle modeling is that isotope geochemistry (e.g., the Rb-Sr, Sm-Nd, U-Th-Pb, U-Th-He, Re-Os, Lu-Hf, and other less commonly used systems) has the power to identify physical regions in the mantle, their depths, their rheological behavior, and the thermal conditions of magma generation. We demonstrate the fallacy of this approach and the model-dependent conclusions that emerge from unconstrained or poorly constrained geochemical models that do not consider physics, seismology (other than teleseismic travel-time tomography and particularly compelling colored mantle cross sections), and geology. Our view may be compared with computer printers. These can reproduce the entire range of colors using a limited number of basic colors (black, magenta, yellow, and cyan). Similarly, the isotopic composition of oceanic basalts and nearly all their primitive continental counterparts can be expressed in terms of a few mantle end members. The four most important (actually “most extreme”, because some are extraordinarily rare) mantle end members identified by isotope geochemists are DMM or DUM (depleted MORB [mid-ocean-ridge basalt] mantle or depleted upper mantle), HIMU (high mu, where mu = μ = 238 U/ 204 Pb), EMI, and EMII (enriched mantle type I and type II). Other mantle end members, or components, have been proposed in the geochemical literature (e.g., PHeM, FOZO, LVC, PreMa, EMIII, CMR, LOMU, and C), but these can be considered to be less extreme components or mixtures in the geochemical mantle zoo. Assuming the existence of these extreme “colors” in the mantle isotopic printer, the only matter for debate is their location in the Earth's interior. At least three of them need long-term insulation from convection-driven homogenization or mixing processes. In other words, where these extreme isotopic end members are located needs to be defined. In our view, no geochemical, geological, geophysical, or physical arguments require the derivation of any magma from deep mantle sources. Arguments to the contrary are assumption based. The HIMU, EMI, and EMII end members can be entirely located in the shallow non-convecting volume of the mantle, while the fourth, which is by far the more abundant volumetrically (DMM or DUM), can reside in the transition zone. This view is inverted compared with current canonical geochemical views of the Earth's mantle, where the shallowest portions are assumed to be DMM like (ambient mantle) and the EMI-EMII-HIMU end members are assumed to be isolated, located in the deep mantle, and associated with thermal anomalies. We argue that the ancient, depleted signatures of DMM imply long-term isolation from recycling and crustal contamination while the enriched components are not free of contamination by shallow materials and can therefore be shallow.

A comprehensive petrological study carried out on Ethiopian mantle xenoliths entrained in Neogene–Quaternary alkaline lavas overlying the continental flood basalt area (Dedessa River–Wollega region, Injibara-Gojam region) and from the southern Main Ethiopian Rift (Mega-Sidamo region) provides an ideal means to investigate mantle evolution from plume to rift settings. Mantle xenoliths from the plateau area (Injibara, Dedessa River) range in composition from spinel lherzolite to harzburgite and olivine websterite, showing pressure-temperature ( P-T ) equilibrium conditions in the range 1.3–0.9 GPa and 950–1050 °C. These xenoliths show flat chondrite (ch)–normalized bulk-rock rare earth element (REE) patterns, with only few light (L) REE–enriched samples (La N /Yb N up to 7) in the most refractory lithotypes. Clinopyroxene (cpx) REE patterns are mostly LREE depleted (La N /Yb N down to 0.2) or enriched (La N /Yb N up to 4.4). Sr-Nd isotopes of clinopyroxene mainly show compositions approaching the depleted mantle (DM) end member ( 87 Sr/ 86 Sr < 0.7030; 143 Nd/ 144 Nd > 0.5132), or less depleted values ( 87 Sr/ 86 Sr = 0.7033–0.7034; 143 Nd/ 144 Nd = 0.5129–0.5128) displaced toward the enriched mantle components that characterize the Afar plume signature and the related Ethiopian Oligocene continental flood basalts. The 3 He/ 4 He (R a ) values of olivines range from 6.6 to 8.9 R a , overlapping typical depleted mantle values. These characteristics suggest that most xenoliths reflect complex asthenosphere-lithosphere interactions due to refertilization processes by mafic subalkaline melts that infiltrated and reacted with the pristine peridotite parageneses, ultimately leading to the formation of olivine-websterite domains. On the other hand, mantle xenoliths from the southern Main Ethiopian Rift (Mega-Sidamo region) consist of spinel lherzolite to harzburgites showing various degree of deformation and recrystallization, coupled with a wider range of P-T equilibrium conditions, from 1.6 ± 0.4 GPa and 1040 ± 80 °C to 1.0 ± 0.2 GPa and 930 ± 80 °C. Bulk-rock REE patterns show generally flat heavy (H) REEs, ranging from 0.1 chondritic values in harzburgites up to twice chondritic abundances in fertile lherzolites, and are variably enriched in LREE, with La N /Yb N up to 26 in the most refractory lithologies. The constituent clinopyroxenes have flat HREE distributions and La N /Yb N between 0.1 and 76, i.e., in general agreement with the respective bulk-rock chemistry. Clinopyroxenes from lherzolites have 87 Sr/ 86 Sr = 0.7022–0.7031, 143 Nd/ 144 Nd = 0.5130–0.5138, and 206 Pb/ 204 Pb = 18.38–19.34, and clinopyroxenes from harzburgites have 87 Sr/ 86 Sr = 0.7027–0.7033, 143 Nd/ 144 Nd = 0.5128–0.5130, and 206 Pb/ 204 Pb = 18.46–18.52. These range between the DM and high-μ (HIMU) mantle end members. The helium isotopic composition varies between 7.1 and 8.0 R a , comparable to the xenoliths from the plateau area. Regional comparison shows that HIMU-like alkali-silicate melt(s), variably carbonated, were among the most effective metasomatizing agent(s) in mantle sections beneath the southern Main Ethiopian Rift, as well as along the Arabian rifted continental margins and the whole East African Rift system. The different types of metasomatic agents recorded in Ethiopian mantle xenoliths from the continental flood basalt area and the rift systems clearly reflect distinct tectonomagmatic settings, i.e., plume-related subalkaline magmatism and rift-related alkaline volcanism, with the latter extending far beyond the influence of the Afar plume.

We investigated the petrogenetic characteristics of the Paleogene Veneto volcanic province and compared them with other intraplate magmatic occurrences of the Adria–North Africa plate since Late Cretaceous time. Veneto volcanic province magmas were erupted through a transtensional rift system that resulted from intra-plate reactions to the Alpine collisional events. The lavas, mostly basic in composition, encompass a wide range of serial affinities from (mela)-nephelinites to quartz-normative tholeiites. Nephelinites and basanites often carry spinel-peridotite mantle xenoliths that have rheologic and thermobarometric characteristics that indicate an origin from the mechanical boundary layer at depths not exceeding 50–60 km. Incompatible element patterns of the most primitive Veneto magmas, together with their isotopic signature ( 87 Sr/ 86 Sr 0.70315–0.70386; 143 Nd/ 144 Nd 0.51279–0.51298; 206 Pb/ 204 Pb 18.8–19.8), share geochemical characteristics with other magmatic occurrences of Adria–North Africa domains, and they show a clear affinity with intraplate sodic lavas, particularly HIMU (high U/Pb = high µ) and, to a lesser extent, enriched-mantle–ocean-island basalt (EM2-OIB) magmas. An integrated petrogenetic model, generally applicable for Adria–North Africa domains, suggests that most of the magmas were generated within the spinel-peridotite lithospheric mantle, from progressively deeper sources (30–100 km) and with a concomitant decrease in the degrees of partial melting (25%–3%) from quartz-normative tholeiites to nephelinites. The modeled magma sources invariably require enrichments in incompatible elements and metasomatic phases comparable (or equivalent) to those observed in some mantle xenoliths associated with the Veneto volcanic province lavas. Two kinds of mantle sources were identified: lherzolites bearing amphibole ± phlogopite for tholeiites to basanites, and lherzolites bearing amphibole ± phlogopite plus carbonatitic components for nephelinites. The elemental and isotopic characteristics of these mantle sources correspond to variable mixing of HIMU and, to a lesser extent, EM2 metasomatic components with a pristine depleted-mantle (DM) lithosphere. The HIMU metasomatizing agents may possibly be related to the mantle plume that is thought to extend from the eastern Atlantic to Europe and the Mediterranean, including Adria–North Africa domains, since the Late Cretaceous. These components more effectively accumulated in the lower lithospheric portion, i.e., the thermal boundary layer, whereas older metasomatic EM2 components may have been better preserved in the upper, more rigid, mechanical boundary layer.

The Aeolian Island arc, emplaced on continental lithosphere, is composed of seven islands and several seamounts, which have evidence of magmatic activity from 1.3 Ma (Sisifo seamounts) to present time (Vulcano, Stromboli). The rock compositions belong to different magmatic series and show a large silica range (48–76 wt%). Calc-alkaline and high-K calc-alkaline volcanics are present in all the islands, except for Vulcano. Shoshonitic rocks are only lacking at Alicudi, Filicudi, and Salina. Potassic magmas have been erupted at Vulcano and Stromboli. The different parental magmas originated in a heterogeneous mid-ocean-ridge basalt (MORB)–like mantle wedge, variously metasomatized by subduction-related components (oceanic crust + sediments, released as either fluids or sediment melts). Trace-element and Sr-Nd isotopic ratios show clear geographical west-east variations among calc-alkaline rocks. The composition of the mantle source of Stromboli is strongly influenced by the addition of a sedimentary component recycled into the mantle wedge; it shows evidence of a higher amount (∼2%) than in all the other islands (<0.5%). Furthermore, the islands from the central sector of the arc are characterized by a higher proportion of slab-derived fluids, which promotes a higher degree of melting. In this frame, the high Pb isotopic ratios (HIMU-like [high µ–like]) of the rocks of the central and western branch of the arc are explained with the high 206 Pb/ 204 Pb carried from a fluid component derived from the dehydration of the ancient subducting Ionian oceanic crust. On the contrary, the low Pb isotope signature of Stromboli magmas is dictated by the sediment input, as for Sr and Nd isotopes. Parental shoshonitic magmas of Vulcano are generated by low melting degrees of a MORB-like mantle wedge, metasomatized by crustal contaminant with high fluids/sediment values, whereas Vulcano potassic magmas are interpreted as deriving from the shoshonitic magmas by refilling, tapping, fractionation, assimilation (RTFA) processes. At Stromboli, potassic to calc-alkaline magmas are generated by increasing melting degrees of a heterogeneous veined mantle. The involvement of K-micas in the genesis of potassic magmas (during partial melting of mantle wedge and/or subducted sediments) is also suggested. U-Th disequilibria confirm the higher fluid versus melt proportion in the central than in the western islands. At Stromboli, the 238 U excesses measured in calc-alkaline volcanics suggest a consistent addition of slab-derived fluids in the source, also promoting higher degrees of melting. The shift to the consistent 230 Th excesses in shoshonitic and potassic rocks requires dynamic melting processes capable of producing in-growth of 230 Th. Quantitative modeling suggests lower melting rates for shoshonitic and potassic rocks, which are consistent with the lower melting degree proposed for these magmas.

Ocean island basalt (OIB) and OIB-like basalt are widespread in oceanic and continental settings and, contrary to popular belief, most occur in situations where mantle plumes cannot provide a plausible explanation. They are readily distinguished from normal mid-ocean ridge basalt (N-MORB) through ΔNb, a parameter that expresses the deviation from a reference line (ΔNb = 0) separating parallel Icelandic and N-MORB arrays on a logarithmic plot of Nb/Y versus Zr/Y. Icelandic basalts provide a useful reference set because (1) they are by definition both enriched mid-ocean ridge basalt (E-MORB) and OIB, and (2) they represent a larger range of mantle melt fractions than do intraplate OIBs. Virtually all N-MORB has ΔNb < 0, whereas all Icelandic basalts have ΔNb > 0. E-MORB with ΔNb > 0 is abundant on other sections of ridge, notably in the south Atlantic and south Indian oceans. E-MORB and N-MORB from this region form strongly bimodal populations in ΔNb, separated at ΔNb = 0, suggesting that mixing between their respective mantle sources is very limited. Most OIBs and basalts from many small seamounts, especially those formed on old lithosphere, also have ΔNb > 0. HIMU OIB (OIB with high 206 Pb/ 204 Pb values and therefore a high-µ [U/Pb] source) has higher ΔNb on average than does EM (enriched mantle) OIB, consistent with the presence of recycled continental crust (which has ΔNb < 0) in the EM source. Although EM OIBs tend to have the lowest values, most still have ΔNb > 0, suggesting that a relatively Nb-rich component (probably subducted ocean crust) is present in all OIB sources. The OIB source components seem to be present on all scales, from small streaks or blobs of enriched material (with positive ΔNb) carried in the upper-mantle convective flow and responsible for small ocean islands, some seamounts, and most E-MORB, to large mantle upwellings (plumes), inferred to be present beneath Hawaii, Iceland, Réunion, and Galápagos. It is not possible to identify a point on this continuum at which mantle plumes (if they exist) become involved, and it follows that OIB cannot be a diagnostic feature of plumes. The geochemical similarity of allegedly plume-related OIB and manifestly nonplume OIB is the first part of the OIB paradox. Continental intraplate transitional and alkali basalt in both rift and nonrift (e.g., Cameroon line) settings usually has positive ΔNb and is geochemically indistinguishable from OIB. Continental volcanic rift systems erupt OIB-like basalt, irrespective of whether they are apparently plume-driven (e.g., East Africa, Basin and Range), passive (e.g., Scottish Midland Valley) or somewhere between (e.g., North Sea basin). Magma erupted in passive rifts must have its source in the upper mantle, and yet it is always OIB-like. N-MORB–like magma is only erupted when rifting progresses to continental break-up and the onset of seafloor spreading. Continental OIB-like magma is frequently erupted almost continuously in the same place on a moving lithospheric plate for tens of millions of years, suggesting that its source is coupled in some way to the plate, and yet the Cameroon line (where continental and oceanic basalts are geochemically indistinguishable) suggests that the source is sub-lithospheric. The causes and sources of continental OIB-like magma remain enigmatic and form the second part of the OIB paradox.