Zoisite-aqueous fluid trace element partitioning with implications for subduction zone fluid composition

Abstract

Zoisite-fluid trace element partition coefficients have been determined and are used to model fluids generated during the breakdown of hydrous phases in a subducting slab. Partition coefficients were determined for Rb, Ba, Th, U, Nb, Ta, Pb, Sr, and nine rare earth elements (REE) at 750–900 °C and 2.0 GPa. Our results show that Sr and Pb are extremely compatible in zoisite relative to other high-pressure phases (DSr > 100, DPb > 10). The zoisite-fluid partition coefficients are combined with previously determined partition coefficients for eclogite-facies minerals (garnet, cpx, rutile, phengite) to generate reaction-specific partition coefficients as well as bulk eclogite-fluid partition coefficients. Batch equilibrium models are then used to approximate the conditions of fluid formation in the slab. Two kinds of fluids are modeled — 1) fluids resulting from specific zoisite breakdown reactions, and 2) combined fluids from a variety of continuous and discontinuous reactions that have had the opportunity to equilibrate with a residual zoisite-bearing eclogite assemblage. We find that fluids generated during individual reactions have much more extreme trace element enrichments than those that have equilibrated with the eclogitic slab as a whole. In particular, fluids generated during zoisite-breakdown reactions tend to be strongly enriched in Pb and Sr. Fluids of extreme composition from individual dehydration reactions may be preserved as fluid inclusions and veins in exhumed eclogites. Fluids that have equilibrated with zoisite-bearing eclogite have a more moderate trace element enrichment pattern that is more consistent with the fluid component sampled by island arc basalts (IAB) in the zone of melting beneath the volcanic front. We propose that fluids generated in the slab, which may initially have extreme patterns of trace element enrichment and fractionation, and may show extreme variation on a small spatial scale, are ultimately at least partially equilibrated with the bulk eclogite. Subsequent mixing with partial melts of the sediment layer and exchange with the mantle wedge en route to the melting region serve to further homogenize the fluids. Therefore, we expect that “metamorphic” subduction zone fluids recorded by inclusions and veins in eclogite will differ quite markedly in composition from “metasomatic” subduction zone fluids that contribute to the formation of island arc basalts, even though their origin may be the same.