Oxygen fugacity range of subducting crust inferred from fractionation of trace elements during fluid-present slab melting in the presence of anhydrite versus sulfide

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Subducted slabs, which include altered-oceanic crust and overlying sediments, impart geochemical and redox fingerprints to arc magmas and affect long-term geochemical evolution of the deep mantle. Yet, the oxygen fugacity of the subducting slab remains poorly constrained. Light Rare Earth Element (LREE) to chalcophile element (ChE) ratios of arc magmas may serve as redox proxies for downgoing slabs because of the difference in the compatibility of these groups of elements between sulfide (e.g., pyrrhotite: Po) and sulfate (e.g., anhydrite: Anh) whose presence in the subducting crustal lithologies depend on the oxygen fugacity of the subducting slab. However, evaluating LREE/ChE of the arc magma require a complete understanding of element partitioning between residual phases in the subducting slab and the slab derived partial melts or fluids. Although previous studies have explored trace element partitioning between sulfide and silicate melts, similar data are not available for anhydrite-melt systems at the conditions of fluid-present slab melting. Here we performed laboratory experiments at 2 GPa and 900–1000 °C to investigate the partitioning of 26 lithophile and chalcophile elements between anhydrite and a hydrous silicic slab melt. Phases were analyzed using EPMA and LA-ICPMS. Sr, Y, Ba, and the REEs were found to be compatible in anhydrite while the other lithophile and chalcophile elements behave oppositely. Since Ce is compatible in anhydrite and Mo and Cu are not, we can use the Ce/Cu and Ce/Mo ratios of the near-primary arc magmas to try to fingerprint processes involved in modifying the sub-arc mantle by reduced or oxidized slab melt components. The bulk D (D¯) for Ce, Cu, and Mo were calculated using the mineralogical modes from partial melting experiments carried out on subducting sediments and Altered-Oceanic Crust (AOC). The differences in D¯Ce/D¯Mo and D¯Ce/D¯Cu values for anhydrite-saturated and sulfide-saturated subducting lithologies indicate preferential partitioning of Ce in sulfide-saturated sediment melts, whereas Mo and Cu are partitioned into anhydrite-saturated melts. Arc localities featuring slab melt signature like Marianas, L. Antilles, Kurile, Cascades, and Mexico show that the Ce/Mo and Ce/Cu ratios can be attributed to the mixing of depleted mantle-derived melts and sulfide-saturated slab melts for some arcs (e.g., Mexico and Cascades), whereas for lavas from Marianas and Kurile, most of the Ce/Mo and Ce/Cu values needs the mixing of depleted mantle-derived melts and anhydrite-saturated slab melts. Our experimental data and the resulting geochemical modeling, therefore, suggest that oxygen fugacity of subducting crusts is likely variable from one subduction zone to another.