The partitioning of chalcophile elements between sediment melts and fluids at 3 GPa, 950-1050 °C with implications for slab fluids in subduction zones

There is growing evidence for the importance of slab sediment components in transferring trace element signatures to the arc mantle. The chalcophile elements are of particular interest due to their differing compatibility in sulfide and sulfate phases, the stability of which are dependent on subducted sediment redox. Many studies have shown that chalcophile elements are compatible in saline fluids at shallow levels of arcs (<10 km depth) but there is comparatively limited work on chalcophile element fluid partitioning at deeper conditions relevant to subduction. We performed experiments at 3 GPa and 950-1050 °C on fluid-saturated pelite melt compositions doped with S, V, Sc, Ce, Mo, As, Sb and Pb at reduced and oxidized conditions where pyrrhotite (Po) and anhydrite (Anh) are stable, respectively. We determined the trace element composition of quenched melts in our fluid-saturated experiments by LA-ICP-MS and derive the coexisting fluid composition by mass balance. The concentration of lithophile elements V and Sc in melts and fluids are unaffected by redox, however Ce concentration is higher in melts and lower in fluids when Po rather than Anh is stable. Of the chalcophile elements, S is in higher concentration in the melts and fluids when Anh is stable compared to Po. Molybdenum, As, Sb and Pb, are enriched in melt and depleted in fluid in the presence of Anh. The partition coefficients determined for these elements are the first reported at 3 GPa in reduced fO2 conditions where Po is present (at 1000 °C, Dfluid/melt Ce = 4 ± 0.3, Dfluid/melt Mo = 319 ± 43, Dfluid/melt As = 669 ± 134, Dfluid/melt Sb = 1437 ± 208, Dfluid/melt Pb = 73 ± 4; all errors are 1σ) and in oxidized conditions where Anh is present (at 1000 °C, Dfluid/melt Ce = 150 ± 48, Dfluid/melt Mo = 3.7 ± 1.9, Dfluid/melt As = 2.5 ± 1.7, Dfluid/melt Sb = 12 ± 5.1, Dfluid/melt Pb = 3.8 ± 1.8). The contrasting behavior of Mo and Ce in fluids versus melts is consistent with the notion that elevated Mo in lavas from the Lesser Antilles arc is associated with reduced fluids extracting Mo from subducted black shales. The Mo/Ce ratio is a promising tool in other arcs to infer the role that fluid may or may not play for chalcophile enrichment in the sub-arc mantle. Unlike melts, fluids in equilibrium with reduced, Po-bearing sediments can endow the arc mantle in chalcophile elements without concomitant oxidation.