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Abstract Recycling of oxidized sulfur from subducting slabs to the mantle wedge provides simultaneous explanations for the elevated oxygen fugacity (fO2) in subduction zones, their high hydrothermal and magmatic sulfur outputs, and the enriched sulfur isotopic signatures (i.e., δ34S > 0‰) of these outputs. However, a quantitative understanding of the abundance and speciation of sulfur in slab fluids consistent with high pressure experiments is lacking. Here we analyze published experimental data for anhydrite solubility in H2O‐NaCl solutions to calibrate a high‐pressure aqueous speciation model of sulfur within the framework of the deep earth water model. We characterize aqueous complexes, required to account for the high experimental anhydrite solubilities. We then use this framework to predict the speciation and solubility of sulfur in chemically complex fluids in equilibrium with model subducting mafic and ultramafic lithologies, from 2 to 3 GPa and 400 to 800°C at logfO2from FMQ‐2 to FMQ+4. We show that sulfate complexes of calcium and sodium markedly enhance the stability of sulfate in moderately oxidized fluids in equilibrium with pyrite atfO2conditions of FMQ+1 to +2, causing large sulfur isotope fractionations up to 10‰ in the fluid relative to the slab. Such fluids could impart oxidized, sulfur‐rich and high δ34S signatures to the mantle wedge that are ultimately transferred to arc magmas, without the need to invoke34S‐rich subducted lithologies.
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Variable Sulfur Release Predicted for Hot and Cold Subducting Slabs
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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- Award ID(s):
- 1725301
- PAR ID:
- 10111065
- Date Published:
- Journal Name:
- Goldschmidt Abstracts
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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