Partitioning of trace elements between anhydrite and sediment melts: Implications for subducting sediment redox and Ce/Mo at arcs

Subducted sediments impart geochemical and redox fingerprint to arc magmas and affect long-term geochemical evolution of the deep mantle. Yet the redox state of subducting sediments remains unconstrained. Large ion lithophile (LILE) to chalcophile element (ChE) ratios of arc magmas may serve as a redox proxy of downgoing sediments because of the difference in compatibility of these groups of elements between sulfide (e.g., pyrrhotite: Po) and sulfate (e.g., anhydrite: Anh) whose presence depends on the redox state of the subducting sediment [e.g. 1]. However, evaluating LILE/ChE magmatic ratios require complete understanding of element partitioning between residual phases in the sediment and sediment partial melts or fluids. Although previous studies have explored trace element partitioning between sulfide and silicate melts [2], similar data are not available for anhydrite-melt systems at the conditions of slab melting.

We performed piston-cylinder experiments in Au capsules at 2 GPa and 900-1000 °C to investigate partitioning of 26 lithophile and chalcophile elements between Anh and a hydrous silicic melt. Phases were analyzed using EPMA and LA-ICPMS. Sr, Y, Ba, and the REEs are compatible in anhydrite while the other lithophile and chalcophile elements behave oppositely. Ce and Mo have D^anh/melt values of 40 and 0.3, respectively and hence by using the Ce/Mo ratio at the arcs we can fingerprint processes involved in modifying the sub-arc mantle by reduced or oxidized slab components. The bulk D for Ce and Mo were calculated using the mineralogical modes from partial melting experiments carried out on Globally Subducting Sediments (GLOSS). The bulk D_Ce/bulk D_Mo values of 3.88 and 0.41 for Anh-saturated and Po-saturated sediment systems, respectively, indicates preferential partitioning of Ce in Po-saturated sediment melts, whereas, Mo gets partitioned into Anh-saturated melts. Arcs featuring sediment signature like Greater Antilles show that the Ce/Mo ratio can be described by the mixing of DMM melt and Po-saturated sediment slab melt, whereas for lavas from Marianas, the majority of the Ce/Mo values needs the mixing of DMM melt and Anh-saturated sediment slab melt which corroborates with previous findings [1].

[1] Skora et al., 2017, GCA [2] Stimac & Hickmott, 1994, Chem Geol