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1. CSEs in high pressure rocks from exhumed terranes

   Walters, JB; Cruz-Uribe, AM; Marschall, HR (August 2018, Goldschmidt Abstracts)

   Despite their geochemical and economic importance, very little work has focused on the behavior of subducted chalcophile and siderophile elements (CSE). Here we present the first survey of these elements in metasediments, metabasites, and hybrid mélange rocks from exhumed terranes worldwide. These samples represent greenschist- to eclogite-facies conditions. EPMA X-ray maps display significant Co, Ni, and As zoning in pyrite; however, no zoning was observed in pyrrhotite or chalcopyrite. In situ LA- ICP-MS analyses of sulfides reveal Co, Ni, Cu, Zn, As, Pb, and Cr at concentrations above 10 μg/g, whereas Ga, Ge, Mo, Ag, Cd, In, Sn, Sb, Tl, and Bi are typically below 1 μg/g. Pyrite is enriched in Co, As, Zn, and Cr relative to pyrrhotite and chalcopyrite, whereas pyrrhotite contains abundant Ni. Pyrite is also enriched in Cu relative to pyrrhotite. Amphiboles and phyllosilicates were found to contain up to hunderds of μg/g of Ni, Cr, and Zn, and tens of μg/g of Co and Ga. In eclogites, Co in silicates mainly occurs in garnet, whereas Ni, Ga, and Zn occur in pyroxene. Both phases contain similar concentrations of Cr and Ge. Most silicates were found to have less than 1 μg/g of Cu; Cu in garnet was below detection, and As was below detection in all silicates. Contrasting behavior of Co and Ni is displayed in hybrid mélange samples. Transects of pyrite in chlorite schists show no correlation between these two elements, consistent with the hightened fluid mobility of Co over Ni observed in hydrothermal ore deposits. In one glacophane-omphacite rock, Co and Ni are anti-correlated. This behavior may be explained by alternation between fluid-buffered conditions, in which cobalt is mobile, and rock-buffered conditions, in which reactions with silicates release Ni. Matrix sulfides are absent in most eclogite-facies samples. Sulfide breakdown during subduction likely drives the release of As, Pb, and Cu into fluids that flux the overlying mantle, whereas both silicates and sulfides may contribute Co to these fluids. The elements Cr, Zn, Ga, and Ge likely persist into the eclogite facies, but may also be released during silicate dehydration. 

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2. Accessory phases as recorders of subduction redox: Sulfide–oxide– silica equilibria during high-pressure metamorphism

   Walters, JB; Cruz-Uribe, AM; Marschall, HR (December 2018, 2018 Fall Meeting, AGU)

   Examination of a global suite of eclogite-facies metabasites and metasediments suggests that eclogites tend to exhibit reduced mineral assemblages relative to their protoliths. High-pressure rocks tend to lack sulfides and Fe3+-bearing oxides in the eclogite facies. We suggest that eclogite-facies mineral assemblages are consistent with prograde reactions that balance the oxidation of S2- or S- to S6+ by reducing Fe3+in silicates or oxides: (1)8Fe3+Si O (OH) +S2-=8Fe2+Si O +SO 2-+(H O) abc de42f The oxidation of one mole of S2-or S-is balanced by the reduction of 7 to 8 moles of Fe3+, and typical S concentrations in the oceanic crust are capable of fully reducing the entire Fe3+ budget of metabasites. As most eclogite facies rocks do not preserve peak metamorphic sulfides, petrographic evidence for prograde S oxidation reactions are cryptic; however, textures associated with sulfate reduction in response to influx of external fluids are common (reaction 1 in reverse). These reactions produce Fe3+-rich phases and are observed in both metasedimentary and metabasic rocks across a range of retrograde P-T paths (blueschist to granulite facies). For example, high-P calc- schists exhibit reaction textures that suggest the breakdown of garnet and white mica to produce pyrite + chalcopyrite + epidote + biotite + magnetite. Our thermodynamic models of aS2 and aO2 at subduction zone P-T conditions suggest assemblages of this type are indicative of aO2 0.7 to 4.5 log units above the quartz-fayalite-magnetite buffer. In rehydrated eclogites, pyrite is commonly associated with the breakdown of garnet + omphacite to amphibole + pyrite. Additionally, direct precipitation of sulfide from sulfate is observed in two samples: 1) The retrograde assemblage pyrite + ilmenite + gypsum occurs in one retrogressed metagabbroic eclogite, and 2) Coronas of secondary pyrite + barite + gypsum enclose early retrograde pyrite in a retrogressed garnet blueschist. In many eclogites, S- is reduced to S2- as pyrite is replaced by pyrrhotite, chalcopyrite, and mixed valence Co-Ni sulfides. These reactions are balanced by oxidation of divalent to trivalent Fe-Co-Ni. Reactions of this type are consistent with increasing aS2 during retrograde metamorphism. Thus, ample evidence exists for oxidized S-bearing fluids released from subducting slabs. 

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3. Variable Sulfur Release Predicted for Hot and Cold Subducting Slabs

   Walters, J; Cruz-Uribe, AM; Marschall, HR (August 2019, Goldschmidt Abstracts)

   Here we present the first thermodynamic models that predict the full range of possible S-liberating reactions during the subduction of mafic oceanic crust. Models for MORB and AOC were created in Perple_X utilizing the combined thermodynamic databases of [1] and [2]. Transitions from pyrrhotite to pyrite and pyrite to anhydrite are observed with increasing P-T. The pressure of the pyrite-anhydrite transition depends on initial Fe3+/6Fe: 3.2 GPa for MORB (Fe3+/6Fe = 0.16) and 2.3 GPa for AOC (Fe3+/6Fe = 0.28) at 650 °C. Reactions were monitored along slab-top geotherms for Honshu and Cascadia (D80, [3]). Above the pyrite-anhydrite transition HSO4-, SO42-, and HSO3- are the dominant fluid species (<0.9 mol/kg), whereas HS- is dominant in the sulfide fields (<0.1 mol/kg). Along the Honshu path, oxidized Sspecies increase from 0.05 to 0.4 mol/kg over 550 to 625 °C (82-84 km depth), concurrent with an increase of 70 % in the total fluid volume due to reactions such as lawsonite-out. Sulfur oxidation is balanced by the reduction of Fe3+, with a 48 % decrease in Fe3+/6Fe. In contrast, oxidized S-species increase from 0.0 to 0.4 mol/kg over 600 to 675 °C (68-77 km depth) along the Cascadia path, concurrent with a 15 % increase in total fluid volume. Nearly 85 % of the total fluid released along the Cascadia path occurs where HS- dominates and S concentrations in fluid are low. Our data suggest that slab-derived S-bearing fluids are a viable mechanism for oxidation of arc magmas. Coeval sulfur and water loss along cold P-T paths are expected to result in high fluxes of oxidized sulfur to volcanic arcs, whereas significant dehydration prior to sulfur oxidation will result in low sulfur fluxes along hot P-T paths. This discrepancy is expected to be accentuated by the less oxidized nature of younger oceanic crust at hot subduction zones. [1] Holland & Powell (2011) JMG [2] Servjensky et al. (2014) GCA [3] Syracuse et al. (2010) PEPI 

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4. Equilibrium Silicon Isotope Fractionation in Eclogites and Granites Constrained by Single Crystal X‐Ray Diffraction and the Force Constants Approach

   https://doi.org/10.1029/2023GL107954  

   Zhang, Dongzhou; Xu, Jingui; Dera, Przemyslaw K; Chen, Bin; Chen, Ming (April 2025, Geophysical Research Letters)

   Abstract Using our recently developed X‐ray diffraction basedforce constantsapproach, we have determined the equilibrium Si isotope fractionation between omphacite/garnet, quartz/kyanite, and quartz/zircon at temperatures relevant to the petrogenesis. We find that Na strongly affects the Si isotope fractionation between omphacite and garnet. Our results have suggested that the omphacite and garnet in eclogite collected in the Dabie Mountain, as well as the kyanite and its host quartz veins, are isotopically in equilibrium, which further suggests that the Dabie Mountain eclogites and its host veins underwent the same high pressure‐temperature condition during their formation. The Si isotope fractionation determined by our methods, together with published mass spectroscopy measurements, DFT‐CIPW calculations and sigmoid fitting on various felsic granites, have suggested that the Si isotope fraction between zircon and whole rock “saturates” at ∼0.45‰ at 1000 K when the SiO₂content in the granite is above ∼70 wt%. 

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5. Mantle oxidation by sulfur drives the formation of giant gold deposits in subduction zones

   He, D-Y; Qui, K-F; Simon, AC; Deng, J; Pokrovski, GS; Yu, H-C; Connolly, JAD; Li, S-S; Turner, S; Wang, Q-F; et al (December 2024, PNAS Proceedings of the National Academy of Sciences of the United States of America.)

   Oxidation of the sub-arc mantle driven by slab-derived fluids has been hypothesized to contribute to the formation of gold deposits in magmatic arc environments that host the majority of metal resources on Earth. However, the mechanism by which the infiltration of slab-derived fluids into the mantle wedge changes its oxidation state and affects Au enrichment remains poorly understood. Here, we present the results of a numerical model that demonstrates that slab-derived fluids introduce large amounts of sulfate (S6+) into the overlying mantle wedge that increase its oxygen fugacity by up to 3 to 4 log units relative to the pristine mantle. Our model predicts that as much as 1 wt.% of the total dissolved sulfur in slab-derived fluids reacting with mantle rocks is present as the trisulfur radical ion, S3–. This sulfur ligand stabilizes the aqueous Au(HS)S3– complex, which can transport Au concentrations of several grams per cubic meter of fluid. Such concentrations are more than three orders of magnitude higher than the average abundance of Au in the mantle. Our data thus demonstrate that an aqueous fluid phase can extract 10 to 100 times more Au than in a fluid-absent rock-melt system during mantle partial melting at redox conditions close to the sulfide-sulfate boundary. We conclude that oxidation by slab-derived fluids is the primary cause of Au mobility and enrichment in the mantle wedge and that aqueous fluid-assisted mantle melting is a prerequisite for formation of Au-rich magmatic hydrothermal and orogenic gold systems in subduction zone settings. 

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