The hydrous, high-pressure mineral lawsonite is important in volatile and element cycling between the crust and mantle in subduction zones and may also influence the rheology and deformation behavior of the subducted crust and associated sediments. However, despite its potential geochemical and geodynamic significance, little is known about the trace element affinity and the types and origins of zoning patterns in lawsonite. To evaluate the significance of trace element variations and zoning in lawsonite, we conducted a geochemical and microstructural study of lawsonite in a suite of different rock types from the Sivrihisar Massif, Turkey, one of the few places in the world where pristine lawsonite has survived in eclogite during exhumation from depths of ∼75–80 km. Lawsonite in metamafic, metasedimentary (impure quartzite and quartz-rich schists), and metasomatic chlorite-rich rocks contains Fe, Ti, and/or Cr as major constituents (substituting for Al) and commonly displays zoning in these elements. Intragrain variations (up to two orders of magnitude) in rare earth elements and other trace elements are also common and in some cases correlate with transition-metal zoning. For some elements (e.g., Ti), uptake was crystallographically controlled, whereas for others, compositional variations may reflect changes in the local metamorphic environment, such as the growth or breakdown of other mineral phases that compete for trace elements (garnet, titanite, epidote-group minerals, apatite) or shifts in the bulk-rock composition during subduction. Deformation may have assisted the mobilization of some elements during and after crystal growth, including relatively immobile elements such as Ti. Intersample variations in lawsonite composition likely reflect variations inherited from the protolith. Lawsonite from Sivrihisar metamafic rocks has high Sr/Pb, whereas lawsonite from quartz-rich metasediments yielded lower Sr/Pb, with a few exceptions that may indicate interactions between oceanic crust and sediments during metamorphism. This study shows that lawsonite composition, zoning, and microstructure can be used to track processes during subduction metamorphism and deformation and can potentially be used to document fluid-rock interaction within and between different lithologic layers.

Water is transported into the deep parts of subduction systems via hydrous phases such as lawsonite, phengite, amphibole, epidote-group minerals, talc, chlorite, and serpentine. Because these hydrous phases form as a result of fluid-rock interaction, their compositions and microstructures may provide a record of fluid compositions and sources as well as fluid transport pathways in the subducted slab. Of these phases, lawsonite [CaAl2Si2O7(OH)2·H2O] is of particular importance to fluid processes and element cycling in subduction zones because it is abundant over a wide range of depths (and may be the main hydrous phase at pressures [P] >2.5 GPa) (e.g., Pawley, 1994; Schmidt and Poli, 1994), has a high water content (11.5 wt%), and is a significant reservoir for trace elements in high-pressure assemblages, particularly rare earth elements (REEs), Sr, Pb, Th, and U (Tribuzio et al., 1996; Ueno, 1999; Spandler et al., 2003; Martin et al., 2014; Vitale Brovarone et al., 2014). It can also be used to date subduction metamorphism (Mulcahy et al., 2009, 2014; Vitale Brovarone and Herwartz, 2013), to document the deformation behavior of subducted oceanic crust and associated sedimentary rocks (Teyssier et al., 2010; Kim et al., 2013, 2015; Cao et al., 2014; Cao and Jung, 2016; Whitney et al., 2014), and to interpret seismic properties of subducted slabs (e.g., Abers and Sarker, 1996; Hacker, 1996; Connolly and Kerrick, 2002; Hacker et al., 2003; Fujimoto et al., 2010; Chantel et al., 2012; Mookherjee and Bezacier, 2012; Reynard and Bass, 2014). Lawsonite dehydration has also been proposed as a driving force for some intermediate-depth earthquakes in subduction zones (e.g., Kita et al., 2006; Abers et al., 2013). It is therefore important to understand the chemical and physical properties and behavior of lawsonite.

In comparison to other hydrous phases in subduction systems, relatively little is known about compositional variations and zoning of lawsonite because it has been proposed to exhibit little compositional variation (e.g., Pawley, 1994) and because it is rarely preserved in subduction-related rocks exhumed to the Earth’s surface (Zack et al., 2004; Whitney and Davis, 2006). Furthermore, many lawsonite-bearing localities preserve lawsonite only in texturally and/or spatially restricted sites, such as inclusions in garnet (Zhang and Meng, 2006; Tsujimori et al., 2006) or in a xenolith (Usui et al., 2006) (Fig. 1). In some cases, the (former) presence of lawsonite can only be inferred from rectangular or prismatic-shaped aggregates of epidote + paragonite + quartz ± albite ± talc (at lower pressures; e.g., Ballèvre et al., 2003) or epidote + kyanite + quartz/coesite ± garnet ± omphacite (at higher pressures), from phase equilibria modeling, or from mass-balance calculations (e.g., Guo et al., 2013) (Fig. 1). The scarcity of well-preserved lawsonite, particularly in eclogite, has prevented a comprehensive understanding of the compositions, substitution mechanisms, trace element affinity, and types and origins of zoning patterns in lawsonite.

One of the few places in the world with fresh, unaltered lawsonite in eclogite- and blueschist-facies rocks is the Sivrihisar Massif of the Tavşanlı Zone, Turkey (Davis and Whitney, 2006, 2008) (Fig. 1), which contains a coherent sequence of metamafic and metasedimentary rocks that were metamorphosed and deformed at or near the top of a subducting slab (Teyssier et al., 2010; Whitney et al., 2014). Lawsonite is abundant in the Sivrihisar Massif and occurs in blueschist- and eclogite-facies metamafic, metasedimentary (calc-schist, quartzite), and metasomatic rocks, as well as in lawsonite-rich veins and layers developed at mafic pod margins (Figs. 2, 3). Petrographic and textural evidence suggests that lawsonite was stable along the prograde metamorphic path (inclusions in garnet), at the peak (matrix grains), and along the retrograde path. The preservation of lawsonite in a variety of rock types, coupled with its stability over a significant portion of the subduction and exhumation cycle in the Sivrihisar Massif, provides an opportunity to document in a systematic way the composition and zoning of lawsonite and to interpret these features in the context of subduction metamorphism and deformation.

In this study, we integrate results of major and trace element compositional analyses of lawsonite by electron microprobe (EMP) and laser ablation–inductively coupled mass spectrometry (LA-ICPMS) with microstructural analysis of lawsonite by electron backscatter diffraction (EBSD) to characterize different types of zoning and to document compositional variations in lawsonite from different rock types. This data set is used to evaluate the controlling factors in lawsonite composition and zoning at eclogite- and blueschist-facies conditions and to investigate the utility of lawsonite as a monitor of metamorphic, fluid, and deformation processes over a range of depths during subduction and exhumation.

The Tavşanlı Zone of western Turkey is a Late Cretaceous paleosubduction zone (Okay and Kelley, 1994; Okay, 1998; Sherlock et al., 1999; Seaton et al., 2009, 2014; Mulcahy et al., 2014; Fornash et al., 2016) formed during the closure of the Neo-Tethys Ocean (Okay, 1980a, 1984, 1986) and is exposed as a 50-km-wide and 350-km-long, east-west–trending, high-pressure–low-temperature (HP-LT) metamorphic belt. It consists of a coherent continental blueschist-facies sequence overlain by an accretionary complex and large ophiolite slabs (Okay, 1998; Okay and Whitney, 2010). Undeformed Eocene granitoids (ca. 48–53 Ma; Harris et al., 1994; Okay, 1998; Sherlock et al., 1999) locally intruded the HP-LT sequence and the overlying ophiolite.

Blueschist-facies rocks from the western part of the belt record pressure-temperature (P-T) conditions of up to 2.4 GPa and 430 °C (Okay and Kelley, 1994; Okay, 2002) to 470–550 °C (Plunder et al., 2013), whereas blueschists from a more southeastern portion of the belt record lower P (0.9–1.1 GPa) and T (375–450 °C) (Droop et al., 2005). Preserved lawsonite eclogite occurs only in the Sivrihisar Massif, located where the Tavşanlı Zone changes from an east-west to a NE-SW trend, and records maximum P-T conditions of 2.4–2.5 GPa and 550 °C, corresponding to depths of ∼75–80 km (Davis and Whitney, 2006, 2008; Whitney and Davis, 2006). The Sivrihisar Massif thus represents one of the deepest-formed known occurrences of fresh lawsonite eclogite exhumed from an oceanic subduction zone (Whitney et al., 2014).

In the Sivrihisar Massif, lawsonite-bearing rocks are well exposed near the villages of Halilbağı and İkipınar, where they occur as meter- to kilometer-scale metasedimentary (calc-schist, quartzite) and metamafic layers (blueschist). Metasedimentary rocks contain HP minerals such as lawsonite, sodic pyroxene, sodic amphibole, and phengitic white mica, and quartzite also contains piemontite (Mn-rich epidote) and spessartine-rich garnets. Hundreds of mafic pods, including lawsonite eclogite pods, occur throughout the field area, where they are hosted within blueschist, calc-silicate, and quartzite (Davis and Whitney, 2006). Some pods have chlorite + epidote–rich margins that contain coarse-grained patches of lawsonite and crosscutting, monomineralic lawsonite veins.

This sequence of HP-LT rocks is in fault contact with a low-grade metamorphosed ultramafic and mafic complex to the north. A mylonite zone that extends ∼150 m south from the fault developed under HP-LT conditions (Teyssier et al., 2010; Whitney et al., 2014). Meter-scale antigorite serpentinite lenses occur within several hundred meters of this contact and are bordered by lawsonite + chlorite ± garnet assemblages (Zack, 2013; Whitney et al., 2014).

Ten samples of metamafic rocks (four eclogite, five blueschist, one chlorite + epidote–rich pod margin) and three samples of quartz-rich metasedimentary rocks collected near the villages of Halilbağı and İkipınar were selected for detailed major and trace element composition and/or microstructural analysis (Table 1; Fig. 2). Sample locations and a summary of the analytical methods used are provided in Table S1 [footnote 1] for all samples for which compositional (± microstructural) data are presented. We note that the names “blueschist” and “eclogite” are used to describe metamafic samples consisting of glaucophane-rich and garnet + omphacite–rich assemblages, respectively, and do not necessarily reflect the metamorphic facies in which the rock equilibrated; i.e., some garnet + glaucophane–rich assemblages (blueschist) and garnet + omphacite–rich assemblages (eclogite) may be cofacial, with differences in mineral assemblage reflecting differences in bulk composition. In addition to metamafic and metasedimentary rocks, lawsonite-rich veins and layers (Davis and Whitney 2008) and lawsonite + chlorite–rich rocks associated with serpentinite lenses were also collected to investigate the composition of lawsonite associated with metasomatic features (Fig. 3).

Metamafic Rocks

Lawsonite composes ∼10%–50% of the mode in Tavşanlı metamafic rocks (Okay, 1980a; Davis and Whitney, 2006, 2008), although modal abundances vary over millimeter to centimeter scales in some samples with compositional banding. Fresh lawsonite eclogite typically consists of omphacite + garnet + lawsonite + phengite, minor amounts of epidote-group minerals, and accessory rutile (rimmed by titanite), apatite, florencite (an aluminous REE-rich phosphate), and/or zircon (Figs. 2A–2C). Minor, texturally late glaucophane is common. Lawsonite occurs as subhedral to euhedral rhombs (Fig. 2A) or as polycrystalline aggregates (Fig. 2B). Retrogressed eclogite largely consists of lawsonite and chlorite (Fig. 2D) with minor epidote (with allanitic cores) and accessory rutile (rimmed by titanite) and zircon. Relict omphacite, zoned amphibole, and highly fractured garnet partly replaced by chlorite are also present (Fig. 2E). Lawsonite occurs as inclusions in garnet and as a matrix phase (Figs. 2D, 2E). Lawsonite blueschist consists of glaucophane + lawsonite + phengite ± garnet ± omphacite ± quartz ± chlorite, with minor amounts of epidote-group minerals (commonly with allanitic cores or rims) and calcite (Figs. 2F–2J). The dominant accessory phases include titanite, apatite, Fe-oxides, and/or zircon. Chlorite + epidote–rich assemblages along pod margins, interpreted as retrogressed eclogite (Davis and Whitney, 2006, 2008), consist of coarse-grained lawsonite in a fine-grained matrix of epidote, chlorite, and clinopyroxene with accessory Fe-Ti oxides, apatite, and titanite (Figs. 2K, 2L). A centimeter-scale, cross-cutting, monomineralic lawsonite vein (sample SV12-24B) was also sampled from the margin of this pod (Fig. 2K).

Metasedimentary Rocks

Lawsonite-bearing quartz-rich metasedimentary rocks consist of quartz + phengite + lawsonite (3%–5% modal abundance) ± garnet ± clinopyroxene ± sodic amphibole ± calcite, with minor amounts of epidote-group minerals, including piemontite, and accessory Fe-Ti oxides (Fig. 2M). In calcite-bearing samples, titanite is also present as an accessory phase and is spatially associated with calcite-rich domains.

Lawsonite + Chlorite–Rich Rocks

Lawsonite + chlorite–rich rocks occur throughout the Tavşanlı Zone (Zack, 2013; Plunder et al., 2013; Whitney et al., 2014) and are typically associated with serpentinite lenses (Fig. 3A). They are largely composed of lawsonite in a chlorite-rich matrix with accessory titanite, although some samples also contain epidote-group minerals, clinopyroxene, and garnet. Lawsonite is typically coarse-grained (>0.5 mm; Figs. 3B–3E) and slightly pink in hand sample, reflecting the presence of Cr (Fig. 3B). Three lawsonite + chlorite–rich rocks were sampled: two from the Halilbağı region of the Sivrihisar Massif (samples SV12-21D, SV13-17A) and one from farther west in the Tavşanlı Zone (sample TZ10-2.2c; for location, see stop 2.2 in Okay and Whitney [2010]) (Table 1).

Lawsonite-Rich Veins and Layers

Two lawsonite-rich veins and layers were sampled from the margins of metamafic pods: lawsonite + garnet + phengite layer sample SV03-103C is from the margin of the lawsonite eclogite pod studied by Davis and Whitney (2008) and from which fresh lawsonite eclogite samples SV12-13E (core) and SV03-103A and SV03-305 (margin) were collected; and monomineralic lawsonite vein sample SV12-24B was collected from the epidote + chlorite–rich pod margin from which sample SV12-24 was obtained (Fig. 2K). Trace element analyses of lawsonite in sample SV03-103C were previously presented by Martin et al. (2014).

To evaluate the major and trace element affinities of lawsonite in different bulk compositions and assemblages, the composition of lawsonite in different rock types and in different textural sites within each sample was determined using EMP analysis (major elements) and LA-ICPMS (trace elements). X-ray element maps were also obtained to examine zoning patterns and trends in relation to mineral assemblages and textures, as this could provide information about reaction history and mechanisms.

To determine whether some lawsonite zoning types are crystallographically controlled and whether deformation affects trace element distribution in lawsonite, the crystallographic orientations of lawsonite crystals displaying core-to-rim, sector, and oscillatory zoning from ten samples representing different rock types (six metamafic rocks, one quartz-rich metasediment, two lawsonite + chlorite–rich rocks, and one lawsonite-rich layer) were measured using EBSD. Grain-scale EBSD maps and orientation contrast images were also obtained for individual lawsonite crystals to evaluate whether there was any relationship between microstructural features (subgrains, grain boundaries, twinning) and zoning patterns. All compositional and microstructural analyses were conducted in situ on polished thin sections cut parallel to the lineation and perpendicular to foliation.

Electron Microprobe (EMP) Analysis (Major Elements)

Major element compositions and element maps were obtained with a JEOL JXA-8900 electron microprobe at the Department of Earth Sciences, University of Minnesota (Minneapolis, Minnesota, USA). Major element analyses were performed with a 15 kV accelerating voltage, 15 nA beam current, and defocused 5–10 μm beam diameter to minimize beam damage to minerals. Natural mineral standards were used in calibrations. Oxygen and OH were calculated by cation stoichiometry and included in the matrix corrections. Lawsonite mineral formulas were calculated on an eight-oxygen basis. X-ray element maps were acquired using a 15 kV accelerating voltage, 100 nA beam current, focused beam, dwell time of 50 ms, and stage-rastering step sizes ranging from 1 to 6 μm depending on lawsonite grain size.

Laser Ablation–Inductively Coupled Mass Spectrometry (LA-ICPMS) (Trace Elements)

Laser ablation–inductively coupled mass spectrometry (LA-ICPMS) trace element analyses of lawsonite were acquired at the Research School of Earth Sciences, Australian National University (Acton ACT). Trace elements were measured on an argon fluoride excimer laser coupled to a quadrupole ICPMS Agilent 7700, using the setup described by Eggins et al. (1998). The laser was tuned to a frequency of 5 Hz and energy of 50 mJ, and spot sizes ranged from 28 μm to 47 μm depending on the size of the grain and compositional zoning features. Counting times were 20 s for the background and 50 s for sample analysis. The internal isotope used to quantify the analyses was 29Si. The U.S. National Institute of Standards and Technology (NIST) 612 glass was used as a primary standard (Jochum and Stoll, 2008; GeoReM 2902), and the Columbia River Basalt BCR-2G glass was used as a secondary standard. The effects of small inclusions of rutile, zircon, and allanite on the analyses were removed manually by examining the time-resolved spectra for each analysis prior to data reduction. The data were reduced with the freeware Iolite (Paton et al., 2011) and its data reduction scheme for trace elements (Woodhead et al., 2007). Accuracy and reproducibility of the secondary standard were generally within 10% of the reference value.

Electron Backscatter Diffraction (EBSD)

The crystallographic orientation and microstructures of lawsonite were analyzed with a JEOL 6500 field-emission gun scanning electron microscope and the Oxford Instruments HKL Channel 5 software in the Characterization Facility of the College of Science and Engineering, University of Minnesota. Scanning electron microscope conditions were 70° tilt, 20 kV accelerating voltage, and ∼15 nA beam current. The phase details and solution models used to index the lawsonite and the sample and acquisition reference frame are provided in Figure S1 in the Supplemental Materials1.

The results of the major and trace element zoning and composition analyses are summarized in Table 2 and presented in Table S2 (footnote 1), organized by rock type. For lawsonite in which Fe, Ti, and/or Cr oxide content is >0.5 wt%, composition data from microprobe analyses are reported; for lawsonite in which Fe, Ti, and/or Cr oxide content is <0.5 wt%, composition data from LA-ICPMS analyses are reported. Rare earth element concentrations and trace element ratios were normalized to the chondritic and primitive mantle compositions of Sun and McDonough (1989).

Lawsonite Major Element Composition

Lawsonite from all studied samples contains FeO* (asterisk indicates that all iron is reported as FeO) and, more rarely, TiO2 and/or Cr2O3 as impurities at the weight percent level, and displays zoning in these elements (Table 2; Fig. 4). The most common zoning types are core-to-rim (Fe, Ti), sector (Fe, Ti), and oscillatory (Fe, Cr) (Figs. 4A–4L). In zoned lawsonite, Fe, Ti, and Cr typically covary with Al. The types of zoning observed within some samples is heterogeneous at millimeter to centimeter scales: some individual lawsonite grains display different types of zoning patterns, and within some grains, different elements display different zoning types. For example, some lawsonite grains in blueschist sample SV13-01 display oscillatory zoning in Fe and Cr, whereas others display Fe and Ti sector zoning and variations in Cr concentrations that mimic the inclusion pattern (Table 2). In the lawsonite-rich layer sample (SV03-103C), a single lawsonite grain displays core-to-rim zoning in Fe, sector zoning in Ti, and oscillatory zoning in Cr (Table 2).

Intersample variations in the overall range of Fe, Ti, and Cr concentrations in lawsonite also occur. In general, lawsonite in the Halilbağı metasomatic lawsonite + chlorite–rich rocks contains the highest TiO2 concentrations (maximum of 1 wt%) measured in this study (Table 2). Similar TiO2 contents were documented in lawsonite from lawsonite metasomatites in Corsica (Vitale Brovarone et al., 2014). No rigorous correlation between FeO* or Cr2O3 content in lawsonite and rock type is observed.

Lawsonite Trace Element Composition

Results from LA-ICPMS analysis of lawsonite from the Tavşanlı Zone shows that lawsonite can incorporate a variety of trace elements, including REEs (light REEs [LREEs], middle REEs [MREEs], and heavy REEs [HREEs]), some transition metals (Sc, V, Mn, Ni), and high-field strength elements (HFSEs; Th, U), as well as Sr, Pb, Y, and P (Table 2; Table S2 [footnote 1]). The concentrations of large-ion lithophile elements (LILEs; Rb, Ba, Cs) and other HFSEs (Zr, Hf, Ta) in lawsonite were at or below detection level in all rock types analyzed (Table S2).

Rare earth element concentrations vary at both the sample and the grain scale. Lawsonite from metabasaltic rocks typically contains REE concentrations of 1×–1000× chondrite (Figs. 5A–5C). In general, lawsonite in blueschist and retrogressed eclogite contains higher concentrations of REE (∼10×–1000× chondrite) than lawsonite in fresh eclogite and shows REE patterns that are enriched in the LREEs relative to the MREEs and/or HREEs ([La/Dy]N > 1 [N indicates a chondrite-normalized concentration]; Fig. 5A). Lawsonite from metasedimentary rocks contains the highest concentration of REEs, and displays chondrite-normalized REE patterns that are enriched in LREEs relative to HREEs ([La/Yb]N > 1; Figs. 5A, 5C) and exhibit a negative Eu anomaly. Lawsonite from the lawsonite-rich layers and veins displays similar REE patterns as lawsonite from metasedimentary samples (enrichment in LREEs over HREEs, negative Eu anomaly), but has lower overall REE concentrations (∼100× chondrite; Fig. 5A). Lawsonite in the metasomatic lawsonite + chlorite–rich rocks displays the lowest overall REE concentrations (∼1×–100× chondrite) and the greatest diversity of REE patterns (flat, MREE-enriched, and/or HREE-enriched; Figs. 5A–5C). At the grain scale, lawsonite REE concentrations typically vary by one to two orders of magnitude. In some cases, these intragrain variations are correlated with major or minor element (Fe, Ti, Cr) zoning or the textural position of the analysis (i.e., core versus rim), whereas in others, there is no obvious correlation between variations in REE concentrations and the spatial position of the analysis.

Transition metals, including V (32–1551 ppm), Sc (1–50 ppm), and Mn (6–365 ppm), are also incorporated into the lawsonite structure at trace levels in some grains. As with Fe, Ti, and Cr, there are few correlations between transition metal content in lawsonite and rock type. In general, lawsonite in the quartz-rich metasediments has the lowest overall V concentrations (Table 2). Similar V concentrations are observed in lawsonite from lawsonite-rich veins and layers and the chlorite + epidote–rich pod margin (Table 2). Manganese concentrations are highest in lawsonite in impure quartzite sample SV10-06 (73–365 ppm) and show a negative correlation with Fe. With few exceptions (samples TZ10-2.2c, SV13-17A, SV13-01), Sc concentrations are generally low in lawsonite (<10 ppm) and show small intrasample variations (standard deviations are generally <3 ppm). Although Ni concentrations are generally at or below detection levels in most samples, in blueschist sample SV13-01, Cr-rich lawsonite tends to be Ni rich (16–53 ppm; Table S2 [footnote 1]).Yttrium concentrations range from 3 to 537 ppm and generally mirror the trends observed in Mn contents, with the highest concentrations in lawsonite from impure quartzite sample SV10-06 (113–537 ppm). Concentrations of Th and U in lawsonite are highly variable at both the sample and grain scale, and range from below detection level to 127 ppm and 13 ppm, respectively. In contrast to most trace elements, Sr and Pb contents are correlated with rock type, such that lawsonite in metasediments has lower Sr/Pb than lawsonite from metamafic rocks (Table 2). Lawsonite from the monomineralic lawsonite vein (sample SV12-24B) and its host chlorite + epidote–rich pod has Sr/Pb similar to those observed in metasedimentary rocks, whereas lawsonite from lawsonite-rich layer sample SV03-103C and metasomatic lawsonite + chlorite–rich rocks has Sr/Pb similar to that in metamafic rocks (Table 2).

Lawsonite from all rock types displays large variations in the concentrations of transition metals (Fe, Ti, Cr) and other trace elements (REEs, Sr, Pb) and commonly displays zoning in these elements. Within a sample, and even within a single lawsonite grain, chondrite-normalized REE concentrations vary by one to two orders of magnitude, consistent with the results of trace and REE analyses conducted on lawsonite from other HP-LT terranes (e.g., Spandler et al., 2003; Martin et al., 2011, 2014; Vitale Brovarone et al., 2014). REE trends also vary markedly, displaying patterns that range from flat to LREE-, MREE-, and/or HREE-enriched (Fig. 5).

To understand the processes giving rise to these compositional variations, we first review Fe, Ti, and Cr substitution in lawsonite. We then examine the types of zoning patterns and their relationship (if any) to microstructural features to consider processes acting at the grain scale. Finally, we consider processes that may have operated at a larger scale to affect the major and trace element systematics of lawsonite, such as rock composition (protolith), metamorphic assemblage, and fluid-rock interaction among different rock types.

Fe, Ti, and Cr Substitution in Lawsonite

Although the major element composition of lawsonite has been reported as near end-member (Pawley, 1994; Okay and Kelley, 1994), the lawsonite crystal structure can accommodate a variety of different elements as both major and trace elements (Martin et al., 2014). The most common impurities at the weight percent level are Fe (maximum reported ∼8 wt%; Maekawa et al., 1992), Ti (maximum reported ∼1 wt%; Vitale Brovarone et al., 2014; this study), and Cr (maximum reported ∼8 wt%; Mevel and Kienast, 1980; Vitale Brovarone et al., 2014). Some lawsonite also contains LREEs at weight percent levels (Ueno, 1999). These elements are likely incorporated into the lawsonite crystallographic structure via substitution on the octahedral Ca2+ site, the octahedral Al3+ site, and/or the tetrahedral Si4+ site. On the basis of similarities in charge and/or ionic radii, previous studies have suggested that Sr2+, Pb2+, and REE3+ reside on the octahedral Ca site (Ueno, 1999; Martin et al. 2011), whereas Ti4+ (Ueno, 1999) and Cr3+ reside on the octahedral Al site (Sherlock and Okay, 1999).

In Sivrihisar lawsonite, the most common impurity is Fe (0.5–3 wt% FeO*; Table 2), which covaries with Al, suggesting that Fe substitutes for Al3+ and is therefore likely ferric iron. Plots of the sum of Fe, Ti, and Cr versus Al atoms per formula unit in lawsonite show a negative correlation, which is consistent with previous results indicating that these elements substitute for Al (Fig. 6).

There are few studies of Fe, Ti, and Cr systematics in lawsonite. In Corsican metasomatic rocks, Ti-rich lawsonite occurs in low-grade and highly altered rocks, whereas Fe- and Cr-rich lawsonite occurs in higher-grade rocks (Vitale Brovarone et al., 2014). A positive correlation between Fe content in lawsonite and metamorphic grade was also documented by Maruyama and Liou (1988) in Franciscan metabasites. In the Sivrihisar rocks, there is no difference in the Fe (or Cr) content of lawsonite in blueschist- versus eclogite-facies rocks (Fig. 7), although this may reflect the fact that some eclogite and blueschist may be cofacial (Davis and Whitney, 2006). In lawsonite from the western part of the Tavşanlı Zone, Plunder et al. (2015) observed a correlation between lawsonite Fe content and oxide assemblage, such that lawsonite in metabasalt containing titanite had lower FeO* contents (0–1.5 wt%) than lawsonite from impure quartzite (inferred metachert) containing hematite (1–2.2 wt% FeO*). Therefore, another possibility is that intersample variations in Fe content reflect variations in the oxidation state of the protolith.

Lawsonite containing high concentrations of Cr is not common; nearly all major element analyses of lawsonite from the Sivrihisar Massif and in the published literature show higher concentrations of Fe and Ti than Cr (Fig. 7). Chromium-rich lawsonite from the Sivrihisar Massif and elsewhere (Sherlock and Okay, 1999; Vitale Brovarone et al., 2014) commonly displays oscillatory zoning, possibly suggesting growth in a fluid-rich environment.

Lawsonite Zoning Patterns

The most common types of major and minor element zoning in lawsonite are oscillatory (Sherlock and Okay, 1999; Vitale Brovarone et al., 2014; this study), core-to-rim (this study), and sector (Ueno, 1999; Tsujimori and Ernst, 2014; Vitale Brovarone et al., 2014; this study). Similar zoning has also been observed for trace elements (Y, Sr, Th, REEs) (Ueno, 1999; Martin et al., 2011, 2014; Tsujimori and Ernst, 2014; Vitale Brovarone et al., 2014; this study). Understanding the types and mechanisms of zoning in lawsonite, particularly in Fe, Ti, Cr, and REEs, is important because these elements have been used to track changes in mineral parageneses (Martin et al., 2011), monitor metamorphic grade (Maruyama and Liou, 1988; Vitale Brovarone et al., 2014), and date subduction zone metamorphism (Mulcahy et al., 2009, 2014; Vitale Brovarone and Herwartz, 2013).

Some individual lawsonite grains display similar zoning patterns in different elements (e.g., core-to-rim zoning in Fe, Ti, and Cr), or display different zoning patterns in different elements (e.g., core-to-rim zoning in Fe, sector zoning in Ti, oscillatory zoning in Cr), demonstrating that the uptake and distribution of different elements may be controlled by different mechanisms. In the following sections, we discuss each zoning type and consider crystallographic and local environmental factors that may have resulted in such zoning.

Core-to-Rim Zoning

Lawsonite from nearly every rock type studied displays core-to-rim zoning in Fe, with Fe-poor cores and Fe-rich rims (Table 2; Figs. 4A, 4B, 4F). The absolute variation in Fe content varies between samples and between grains within a sample, although the latter may to some extent reflect sectioning effects. In contrast to core-to-rim zoning in other minerals such as garnet (in which zoning is commonly continuous), the change in Fe content in lawsonite is typically abrupt, showing either a well-defined core and rim region (Figs. 8A, 8B) or an Fe-poor core that transitions to a rim with oscillatory variations (Fig. 8C). Core-to-rim zoning is commonly concentric and centered (i.e., symmetric with respect to grain boundaries), but zoning in some samples is truncated at grain boundaries (Fig. 4A), indicating that lawsonite has been modified by post-crystallization processes such as dissolution.

In some cases, core-to-rim zoning is accompanied by a change in inclusion assemblage. For example, lawsonite from blueschist sample SV13-07 has an Fe-poor, Ti-rich core with abundant glaucophane and titanite inclusions and an inclusion-poor Fe-rich, Ti-poor rim (Figs. 4B, 4C). In this sample, lawsonite inclusions in garnet display different zoning patterns than matrix lawsonite; lawsonite inclusions have Ti-rich and titanite-bearing margins (Fig. 4E), whereas matrix lawsonite displays Ti-rich and titanite-bearing cores (Fig. 4C).

These features suggest that Fe zoning may be related to changes in reaction history. Results of phase equilibria modeling and the presence of epidote inclusions in garnet, omphacite, and lawsonite indicate that prograde P-T paths passed through the epidote stability field or close to the epidote-lawsonite stability boundary (Davis and Whitney, 2006). Therefore, one possibility to explain the observed core-to-rim increase in Fe in lawsonite would be the breakdown of coexisting Fe3+-bearing epidote. Lawsonite that grew concurrently with epidote along the prograde path would be poor in Fe3+ relative to epidote. Upon breakdown of epidote, lawsonite would take up the Fe, resulting in an increase in Fe3+ toward the lawsonite rim. It is also possible that some intragrain variations in Fe3+ reflect changes in oxygen fugacity (fO2) during metamorphism. Okay (1980b) suggested that hydrous phases may be more sensitive to fluctuations in fO2, and that core-to-rim variations in the Fe2+/Fe3+ ratio of sodic amphiboles from the western part of the Tavşanlı Zone were recording changes in fO2. Sodic amphiboles from the Sivrihisar region have similar zoning patterns to those documented in that study (Davis and Whitney, 2006), and in the studied samples, Fe zoning in coexisting lawsonite and sodic amphibole is similar (i.e., both record rimward increases in Fe3+).

Some lawsonite grains from both metamafic and metasedimentary rocks exhibit core-to-rim variation in REEs. In quartzite sample SV10-06 and retrogressed eclogite sample SV08-4A, lawsonite records a core-to-rim decrease in REE concentrations with no change in the overall shape of the REE pattern (Figs. 9A, 9B). In the latter sample, lawsonite inclusions in garnet have REE concentrations similar to those in the core regions of matrix lawsonite (100×–1000× chondrite; Fig. 9B). In these samples, the rimward depletion in REEs may be related to the depletion of these elements from the rock matrix during progressive crystallization of lawsonite. In other samples, however, lawsonite records a core-to-rim variation in the shape of the REE pattern and/or an increase in REE concentrations (Figs. 9C, 9D, 9E). These zoning trends cannot be explained by the continued fractionation of REEs from the matrix during lawsonite growth, and instead likely reflect the growth or breakdown of other phases that compete for REEs in the rock. Besides lawsonite, the other primary REE hosts in blueschist- and eclogite-facies metamafic and metasedimentary rocks are epidote-group minerals (LREEs), titanite (MREEs), garnet (HREEs), and apatite (LREEs to MREEs) (Tribuzio et al. 1996; Hermann, 2002; Zack et al., 2002a; Spandler et al., 2003; John et al., 2008; El Korh et al., 2009; Beinlich et al., 2010; Guo et al., 2012, 2013), of which epidote, titanite, and garnet are the most common in the studied samples.

In lawsonite from garnet blueschist sample SV13-06, the shape of the REE pattern changes from core to rim such that the Fe-rich rim is enriched in MREEs (Fig. 9C), likely as a result of the breakdown of an MREE-rich phase such as titanite or apatite. In several samples, lawsonite records a core-to-rim increase in REEs, particularly LREEs, that correlates with the core-to-rim increase in Fe (Figs. 9D, 9E). This trend is consistent with the interpretation that Fe zoning in some lawsonite may be related to epidote stability, as epidote breakdown releases not only Fe3+ but also LREEs, Sr, Pb, Th, and U, which can be incorporated into newly formed lawsonite (e.g., Hermann, 2002; Spandler et al., 2003; El Korh et al., 2009; Guo et al., 2013). Therefore, the Fe- and LREE-poor lawsonite cores may represent lawsonite that crystallized in equilibrium with epidote, and the Fe- and LREE-rich lawsonite rims may represent lawsonite that crystallized during epidote destabilization or dissolution. Although some lawsonite with rimward increases in Fe also record increases in Sr and/or Pb content, this relationship is not observed in all samples, perhaps reflecting the presence of sector or oscillatory zoning in these elements or variations in these elements at scales smaller than the spot size used for LA-ICPMS analysis (28–48 μm).

In sample SV03-103C, one lawsonite grain records a core-to-rim reversal in the sign of the Eu anomaly, with the core recording a weak positive Eu anomaly and the rim recording a negative anomaly. In some Corsican metasomatic rocks, lawsonite also records a core-to-rim change in the sign of the Eu anomaly, but the trend is opposite to that of Sivrihisar lawsonite: in the Corsican samples, the core records a negative Eu anomaly and the rim has a positive Eu anomaly. Martin et al. (2011) attributed the change in the sign of the Eu anomaly to lawsonite-core growth during prograde blueschist-facies (plagioclase-stable) metamorphism and lawsonite-rim growth during eclogite-facies (plagioclase-absent) metamorphism, suggesting that lawsonite zoning can be used to track changes in P-T conditions. Although there is some textural evidence for retrograde transformation of eclogite to blueschist in the Sivrihisar rocks (Davis and Whitney, 2006, 2008; Whitney et al., 2014), there is no plagioclase present in our sample or adjacent blueschist, so it is unlikely that the core-to-rim change in the sign of the Eu anomaly is related to a transition from plagioclase-absent (eclogite-facies) to plagioclase-stable (blueschist-facies) conditions. In this case, the shift in Eu anomaly could be (1) related to a change in oxidation state, as the switch in Eu anomaly also correlates with core-to-rim zoning in Fe3+; (2) due to a change in bulk-rock Eu as a result of external fluid flux (Vitale Brovarone et al., 2014); or (3) inherited from epidote that formed by prograde reaction of plagioclase.

Oscillatory Zoning

Oscillatory zoning in lawsonite was first documented by Sherlock and Okay (1999) in blueschist-facies metamafic rocks from the Tavşanlı Zone. In that study, oscillatory variations in Cr (ranging from 3 to 6 wt% Cr2O3) were attributed to the breakdown of inferred precursor Cr-rich phases such as chromite (which is not observed in the rocks) and concomitant fluid-rock interaction during subduction metamorphism. Oscillatory Cr zoning in lawsonite was further documented in eclogite-facies veins from Corsica (Vitale Brovarone et al., 2014), where high Cr2O3 contents (up to 8 wt%) were interpreted as resulting from interactions with fluids derived from the adjacent serpentinite.

Oscillatory Cr zoning in Sivrihisar lawsonite occurs in alternating lawsonite-rich and glaucophane-rich layers at eclogite pod margins, as well as in metamafic rocks and in some metasomatic lawsonite + chlorite–rich rocks adjacent to serpentinite (Figs. 4H, 10A–10C). However, Cr-rich (>1 wt% Cr2O3) lawsonite has only been observed in blueschist (sample SV13-01) and chlorite + talc–rich pod rinds sampled in the HP shear zone near the fault contact of the unit (Teyssier et al., 2010; Whitney et al., 2014). Chromium-rich lawsonite in SV13-01 contains overall higher concentrations of LREEs, Ni, Sr, Y, and Pb than Cr-poor lawsonite (Table 2; Table S2 [footnote 1]) and also tends to lack the Fe and Ti sector zoning observed in Cr-poor lawsonite, suggesting that it represents a distinct population of lawsonite. In metamorphic minerals, oscillatory zoning is commonly interpreted as resulting from repeated fluctuations in pressure, temperature, fluid supply or composition, and/or rates of thrusting (Jamtveit, 1991; Yardley et al., 1991; Kohn, 2004). Because the Sivrihisar Massif records only a single subduction-exhumation cycle, and compositional oscillations are not observed in coexisting minerals, the most plausible explanation for the presence of Cr oscillatory zoning is the involvement of a Cr-rich fluid. Although Cr is commonly assumed to be immobile, Cr-rich minerals and mineral domains have been documented in HP metasomatic features and environments, including eclogite-facies veins, serpentinite mélanges, and shear zones (Tsujimori and Liou, 2004; Spandler et al., 2011; Angiboust et al., 2014; Vitale Brovarone et al., 2014), indicating that Cr may be mobilized by fluids in subduction zones.

In the eclogite-facies veins from Monviso (Western Alps, northwestern Italy), Cr-rich domains in garnet, clinopyroxene, and rutile are also enriched in Ni, B, Sb, As, and/or LREEs (relative to MREEs) (Spandler et al., 2011). Because serpentinites commonly have high concentrations of these elements (Tenthorey and Hermann, 2004; Deschamps et al., 2011, 2013), this elemental association has been used to infer the presence of fluids derived from ultramafic sources. One possible source for Cr-rich fluids in the blueschist mylonite zone in the Sivrihisar Massif could therefore be the nearby (i.e., within ∼3 m) antigorite serpentinite bodies. It is unknown if the Cr- and Ni-rich lawsonite contains elevated concentrations of B and Sb, as these elements were not measured in this study. However, As concentrations in lawsonite were below detection level for all analyzed lawsonite, indicating either that lawsonite is not a host for this element or that the fluids were derived from other Cr- and Ni-rich rocks, such as metagabbro, that would not be expected to contain appreciable amounts of As.

Oscillatory zoning in other elements, such as Fe, is also observed in Sivrihisar lawsonite (Fig. 10). In the case of oscillatory Fe zoning, variations in Fe occur either across the entire grain (Fig. 10) or only in the Fe-rich rim (Fig. 8C). In some samples that display both Fe and Cr oscillatory zoning, Fe and Cr concentrations are correlated such that high concentrations of Fe3+ coincide with high concentrations of Cr3+ (Figs. 10A, 10C). This observation is consistent with the hypothesis that fluctuations in the fO2 of the fluid might play a role in the generation of oscillatory zoning in lawsonite, with oxidizing fluids facilitating the Cr3+-Al3+ substitution in lawsonite (Sherlock and Okay, 1999).

Sector Zoning

Sector zoning develops when elements are preferentially incorporated onto specific growth surfaces and these differences are preserved in the crystal as a result of rapid growth and/or slow intracrystalline diffusion rates (Watson and Liang, 1995). Calcium, Ti, Sr, and REE sector zoning has previously been described in lawsonite (Ueno, 1999; Tsujimori and Ernst, 2014; Vitale Brovarone et al., 2014). In Sivrihisar lawsonite, Ti sector zoning is most common, although other elements, such as Fe and Sr, can also display sector zoning (Figs. 4G, 4J, 4K, 4L, 11). The shape of the sectors can vary: boundaries between adjacent sectors are straight (Figs. 11B, 11D, 11E) or curved (Figs. 4J, 4K), or transition from a curved boundary in the core to a straight boundary in the rim (Fig. 11A). As the boundary between two adjoining sectors represents the intersection of two growth planes, its geometry will depend on the growth rates, and thus growth mechanisms, of the two planes. Straight boundaries develop when the growth rates of adjoining faces remain constant, whereas curved boundaries develop if the growth rates change at different rates during crystal growth. Although the boundaries between Ti-rich and Ti-poor sectors are either curved or straight, the boundaries between Fe-rich and Fe-poor sectors are always curved (Figs. 4J, 4K), implying that particular growth conditions may be needed for the development of Fe sector zoning.

Ueno (1999) studied the compositional differences between sectors in lawsonite in the Sanbagawa pelitic schists and proposed that the {100} sector shows enrichment in REEs, the {001} sector shows enrichment in Ti, and the {010} sector records a near-ideal lawsonite composition (following the nomenclature adopted by Ueno [1999], in which sectors are denoted as {100}, {010}, and {001} according to the axis along which they grew). In that study, crystallographic directions were inferred from petrographic observation. Although lawsonite commonly forms euhedral rhombs, it can be difficult to identify crystallographic axes from petrographic observation alone, in part because the two-dimensional cross-sectional geometry of lawsonite can vary and/or be modified by post-crystallization processes. We therefore conducted EBSD analyses of lawsonite displaying sector zoning to determine crystallographic orientation. Results show that sector zoning is typically observed in lawsonite crystals cut perpendicular to the crystallographic a- and b-axes (Fig. 11). Titanium-rich sectors tend to grow in the direction of the a-axis (observed if the crystal is intersected perpendicular to the b-axis) or in the direction of the b-axis (observed if the crystal is intersected perpendicular to the a-axis) (Fig. 11), whereas Ti-poor sectors tend to grow in the direction of the c-axis. When both Ti and Fe or Sr sector zoning is present in lawsonite, the Fe- and/or Sr-rich sectors tend to occur in the Ti-poor sectors.

A notable feature of Ti sector zoning in Sivrihisar lawsonite is the irregularity of Ti concentrations within Ti-rich sectors, which generally vary from ∼1000 to 4000 ppm regardless of rock type (Table 2; Figs. 4, 11). Irregular Ti concentrations have been documented in sector-zoned metamorphic tourmaline (van Hinsberg et al., 2006) and garnet (Carlson, 2002), in which variations in Ti were interpreted as reflecting a preexisting heterogeneity overgrown by the mineral. This interpretation is consistent with observations from sector-zoned lawsonite from blueschists that show low Ti concentrations in Fe-rich regions which may represent preexisting minerals that were overgrown by lawsonite (Figs. 4F, 4G). Preservation of these heterogeneities implies slow diffusion rates for Ti in lawsonite. In some Sivrihisar lawsonite, Ti variation within a sector appears to be oscillatory, suggesting that intrasector variations in Ti may be related to variations in Ti availability. The major sources of Ti in the Sivrihisar rocks are rutile, titanite, and Fe-Ti oxides, so the periodic availability of Ti could be related to the destabilization of these phases. A similar mechanism was proposed by Vitale Brovarone et al. (2014) to explain high Ti concentrations (∼1 wt%) in lawsonite from metasomatic rocks in Corsica. In that lawsonite, Ti concentrations were positively correlated with Nb, which also resides in Ti-bearing phases (e.g., Zack et al., 2002b), lending support to the interpretation that rutile was the source of Ti. However, no correlation between Ti and Nb concentrations or any other HFSE element has been observed thus far in Sivrihisar lawsonite. Samples displaying sector zoning in Fe do not appear to display similar intrasectoral compositional heterogeneities (Figs. 4J, 4K).

Other Zoning Types

Some lawsonite grains display irregular compositional variation, including in elements such as Ti that in other cases display geometrically organized zoning (core-to-rim, oscillatory, sector). Results from EBSD and EMP analyses show that, in some cases, there is a correlation between zoning and the presence of subgrain (defined here as a misorientation <10°) and grain (defined here as a misorientation >10°) boundaries. In layered eclogite and blueschist sample SV08-76, for example, the shape of a Ti-rich sector is distorted in the region where a subgrain is present (Figs. 12A, 12B). The boundary between the Fe-poor core and Fe-rich rim is also irregular adjacent to the subgrain (Fig. 12C), perhaps indicating deformation-assisted mobilization of Fe and Ti in lawsonite. Further study is needed to understand the influence of deformation on compositional zoning and element mobility in lawsonite.

In eclogite pod margin sample SV03-305 (Davis and Whitney, 2008), collected from the same pod as eclogite samples SV12-13E and SV03-103A, Ti (but not Fe or Cr) is concentrated along lawsonite grain boundaries (Figs. 12D, 12E, 12F). This may indicate that Ti was mobilized in fluids and concentrated along grain boundaries that served as pathways for Ti-bearing fluids. The fact that Fe and Cr are not similarly concentrated suggests that the Ti was released by the breakdown of a Ti-rich and Fe- and Cr-poor mineral phase. Alternatively, the composition and/or nature of the metamorphic fluids could have preferentially mobilized Ti relative to Fe and Cr, as experimental results and empirical observations from exhumed high-pressure rocks have shown that Ti, commonly assumed to be fluid immobile, can be soluble in the presence Cl- or F-rich brines or dissolved albite components (Gao et al., 2007; Antignano and Manning, 2008; Rapp et al., 2010).

In most cases, however, lawsonite grains with little to no misorientation display distinct and irregular zoning features that require other explanations. Lawsonite from quartz-rich rock sample SV10-03B displays Fe-rich and Al-poor regions that are typically located in or near the geometric center of the grain but that in some cases are located near the rim; some lawsonite crystals contain more than one Fe-rich region, and these regions may be irregularly shaped, tabular, or rounded (Figs. 4N, 4O). One hypothesis to account for these observations is that lawsonite formed as a pseudomorph after a precursor Fe-rich phase (such as an epidote-group mineral) and that subsequent lower-Fe lawsonite grew around this initial crystal. Similar zoning features were observed in blueschists (Figs. 4A, 4F) and a lawsonite grain from a lawsonite + phengite + garnet + omphacite layer from the western part of the Tavşanlı Zone (Fig. S2 [footnote 1]). In some lawsonite from blueschist sample SV13-01, Cr variations in the interior of the grain appear to mimic the inclusion pattern (Fig. 4I), indicating that the Cr variations may be inherited from the distribution of Cr in the fabric that the lawsonite overgrew. Yang and Rivers (2001) and Martin (2009) proposed a similar mechanism to explain the heterogeneous distribution of Cr in metamorphic garnets from pelitic schists.

Controls on Lawsonite Composition and Compositional Variation

The few published studies of lawsonite geochemistry to date document that lawsonite is a major reservoir for trace elements in all rock types in which it occurs (eclogite, blueschist, quartz-rich metasediment, metasomatic lawsonitite) (e.g., Spandler et al., 2003; Martin et al., 2011, 2014; Vitale Brovarone et al., 2014). In this paper, we extend the trace element data set for lawsonite in eclogite- and blueschist-facies metamafic and metasedimentary (impure quartzite) rocks and lawsonite-rich veins, layers, and metasomatic (lawsonite + chlorite–rich) rocks.

Rare Earth Element Composition

Trace element analysis of lawsonite from metamafic rocks in the Sivrihisar Massif shows that chondrite-normalized REE concentrations are typically ∼10×–1000× chondrite (Figs. 5, 9B–9D). These concentrations are similar to those documented in lawsonite in metamafic rocks from other oceanic and continental subduction complexes, such as the Franciscan Complex (California, western USA; Mulcahy et al., 2009), New Caledonia (Spandler et al., 2003), the Ligurian Alps (northwestern Italy; Tribuzio et al., 1996), and the North Qilian belt (China; Xiao et al., 2013). Although lawsonite from metamafic rocks from these areas all have similar concentrations of REEs, the shape of the chondrite-normalized REE pattern varies. For example, in Franciscan metabasalt, lawsonite displays high MREE to HREE concentrations relative to LREE, whereas lawsonite from Sivrihisar metamafic rocks displays more LREE-enriched patterns (Figs. 5A, 5C). This difference may reflect variations in reaction history, as the Franciscan blueschist formed on the retrograde path after garnet breakdown (Mulcahy et al., 2009) and therefore lawsonite likely incorporated HREEs liberated by garnet. In contrast, Sivrihisar lawsonite likely grew either concurrently with garnet or after significant garnet growth, resulting in a relative depletion in HREEs.

Compared to lawsonite from metamafic rocks, lawsonite from Sivrihisar metasedimentary rocks typically yields higher LREE contents (100×–10,000× chondrite; Table 2; Figs. 5A, 5C), similar to results obtained for lawsonite in metasedimentary rocks in other HP-LT terranes (Ueno, 1999; Spandler et al., 2003; Martin et al., 2014). One explanation for this enrichment is that there are fewer phases competing with lawsonite for available trace elements (Martin et al., 2014). Another factor may be that the modal abundance of lawsonite in metasedimentary rocks is lower than in metamafic rocks (∼3%–5% in Sivrihisar quartz-rich metasedimentary rocks versus ≥20% in metamafic rocks), so available trace elements are distributed over fewer grains.

Lawsonite + chlorite–rich rocks interpreted to represent metasomatic reaction between serpentinite and metamafic or metapelitic rocks occur in the Tavşanlı Zone (Plunder et al., 2013), including the Sivrihisar Massif (Zack, 2013, Whitney et al., 2014), and in Corsica (Martin et al., 2011; Vitale Brovarone et al., 2014). Possible pseudomorphic equivalents have also been described in the Southern Ural Mountains, Russia (Beane and Liou, 2005).

Lawsonite in Corsican lawsonite + chlorite–rich rocks contains high concentrations of REEs (up to 1000×–10,000× chondrite), Th, and U (Martin et al., 2011; Vitale Brovarone et al., 2014). Lawsonite in Tavşanlı lawsonite + chlorite–rich rocks typically contains much lower trace element concentrations, particularly for LREEs (∼10×–100× chondrite; Table 2; Fig. 5). One explanation for this difference is that Corsican lawsonite + chlorite–rich rocks formed as a result of interactions between serpentinite and adjacent REE-enriched metasedimentary rocks (Martin et al., 2011; Vitale Brovarone et al., 2014), whereas field relations in the Sivrihisar Massif suggest that the lawsonite + chlorite–rich rocks formed as a result of metasomatic interactions between serpentinite and metabasalt, the latter of which has lawsonite REE concentrations similar to those of lawsonite from the lawsonite + chlorite–rich rocks (Fig. 5).

Within the Tavşanlı Zone, however, there are large variations in the trace element content of lawsonite from lawsonite + chlorite–rich rocks. For example, lawsonite from the sample (TZ10-2.2c) from the western part of the belt contains the highest V concentrations (>1000 ppm) of any sample analyzed in this study, whereas a sample (SV12-21D) from the Sivrihisar region contains some of the lowest V concentrations measured (<300 ppm) (Table 2). Rare earth element patterns also vary markedly, with lawsonite from the western Tavşanlı sample yielding flat patterns, in contrast to Sivrihisar lawsonite that displays patterns that range from flat to MREE- and/or HREE-enriched (Figs. 5B, 5C). In Sivrihisar sample SV12-21D, the shift from HREE-enriched to flat REE patterns correlates with Fe and Ti zoning: Fe- and Ti-poor regions are enriched in HREEs, and Fe- and Ti-rich regions have flat REE patterns. These variations may stem from heterogeneities in the source rocks and/or from variation in the extent of fluid-rock interaction.

Sr and Pb

A possible monitor of lawsonite compositional variation as a function of rock type are the Sr and Pb concentrations of lawsonite. A global review of lawsonite composition shows that lawsonite may contain the majority of Sr and Pb in HP-LT mafic rocks: up to ∼60% of Pb and up to ∼100% of Sr (Martin et al., 2014). Concentrations of Sr and Pb in metabasalt are typically distinct from those in metasedimentary rocks (Martin et al., 2014): lawsonite in blueschist and eclogite tends to yield higher Sr/Pb ratios than lawsonite in metasedimentary rocks. In Sivrihisar rocks, Sr/Pb generally defines two linear arrays, one corresponding to metamafic rocks and the other to quartz-rich metasedimentary rocks (Fig. 13A). Bulk-rock Sr/Pb appears to closely match lawsonite Sr/Pb and plot along the array defined by lawsonite in each sample (Fig. 13A).

In zoned lawsonite from Corsican metasomatic rocks, lawsonite cores yielded Sr/Pb ratios similar to those of metasedimentary rocks (their inferred protolith) and rims yielded Sr/Pb similar to those of metamafic rocks (Martin et al., 2014). The lawsonite cores were interpreted as forming prior to metasomatic interaction with mafic and ultramafic rocks, and the rims as forming during the metasomatic event (Martin et al., 2011). Therefore, intragrain shifts in lawsonite Sr/Pb may be an indicator of changes in the bulk-rock composition resulting from pervasive fluid-rock interactions or mechanical mixing between different rock types during subduction and exhumation (Martin et al., 2014).

In the Sivrihisar samples, no lawsonite recorded significant intragrain shifts in Sr/Pb, indicating either that these rocks did not experience extensive fluid-rock interactions with externally derived fluids or that such interactions occurred early in the subduction history, prior to lawsonite growth. Although no intragrain variations in Sr/Pb were observed, lawsonite from one sample—the chlorite + epidote–rich pod margin (sample SV12-24)—records lawsonite Sr/Pb that is apparently inconsistent with its rock type. Lawsonite from this sample plots along the Sr/Pb array for metasedimentary rocks (Fig. 13A) and also contains REE concentrations similar to those of metasedimentary rocks (∼100×–1000× chondrite; Table 2). Bulk-rock analysis of the monomineralic lawsonite vein from this chlorite + epidote–rich pod (sample SV12-24B) also has Sr/Pb similar to that of metasedimentary rocks (Table 2; Fig. 13A), suggesting that this pod may have experienced interactions with fluids derived from adjacent metasedimentary rocks prior to or during lawsonite growth.

A comparison of Sr and Pb concentrations in lawsonite from the Tavşanlı Zone (including Sivrihisar samples) with published data for lawsonite from other HP-LT and ultrahigh-pressure terranes (Fig. 13B) shows that although Sr and Pb contents are correlated for lawsonite in most samples, the association of Sr/Pb and rock type observed in Tavşanlı samples is not generally applicable. Lawsonite from mafic rocks in other subduction complexes typically defines a flat or much shallower Sr/Pb slope, similar to lawsonite from blueschist sample SV12-59. The only lawsonite that follows the “metamafic” trend that characterizes the majority of Sivrihisar lawsonite is from Corsican metasomatic rocks that had metapelitic protoliths (Vitale Brovarone et al., 2014). As stated earlier, one possible explanation for this observation is that the analyzed Sivrihisar rocks (with the exception of SV12-59) experienced interactions with metasedimentary rocks prior to lawsonite growth, resulting in Sr/Pb ratios between those of metasedimentary rocks and those of (unaltered) metamafic rocks. Additional information about the history of fluid-rock interaction in the Tavşanlı Zone and studies of lawsonite from other HP-LT terranes are needed to understand the Sr/Pb systematics of lawsonite and its utility as an indicator of metasomatic events, especially in terranes with abundant carbonate-rich rocks or sulfides that could serve as additional reservoirs for Sr and/or Pb.

This study of the composition and zoning of lawsonite from different rock types and bulk compositions (metamafic and metasedimentary rocks, lawsonite + chlorite–rich rocks, lawsonite-rich layers and veins) from the Sivrihisar Massif, Turkey, shows that lawsonite can display a diversity of zoning patterns in both major and trace elements, including core-to-rim zoning in Fe, which is described for the first time here. Within a single grain, different elements can display contrasting zoning types, suggesting that the incorporation and distribution of different elements in lawsonite is controlled by different mechanisms. For some elements such as Ti, uptake may be crystallographically controlled (and in some cases modified by deformation), whereas for other elements, intragrain compositional variations may track changes in the local metamorphic environment, such as the presence or absence of other trace element–rich phases (garnet, titanite, epidote, apatite) or changes in the bulk-rock composition. Intersample variations in lawsonite composition (Fe, Sr/Pb) likely reflect variations in the nature of the protolith (i.e., composition and/or oxidation state) or extent of interaction with other rock types. Given the large intra- and intergrain variability possible in lawsonite trace element patterns and concentrations, texturally controlled compositional analyses of lawsonite should be undertaken when using lawsonite compositions to calculate trace element budgets and fluid compositions in subduction zones.

K.F. Fornash acknowledges support from a U.S. National Science Foundation Graduate Research Fellowship (NSF GRFP) and Graduate Research Opportunities Worldwide grant, and from the University of Minnesota for a Doctoral Dissertation Fellowship and Thesis Research Grant. D.L. Whitney acknowledges funding from the College of Science and Engineering at the University of Minnesota. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from the NSF through the Materials Research Science and Engineering Center program. The authors thank S.C. Kruckenberg for EBSD analysis of lawsonite to help with inter-lab verification of lawsonite indexing, S. Penniston-Dorland and C. Spandler for their thorough and helpful reviews, and G. Bebout for patient editorial handling.

1Supplemental Materials. Locations of all studied samples (Table S1), representative major and trace element analyses of lawsonite (Table S2), the solution models and acquisition reference frame for EBSD analyses (Figure S1), and a backscattered electron image of a lawsonite grain (Figure S2). Please visit https://doi.org/10.1130/GES01455.S1 or access the full-text article on www.gsapubs.org to view the Supplemental Materials.
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