Rhenium solubility and speciation in aqueous fluids at high temperature and pressure

Abstract

In order to characterize rhenium transport via infiltration of fluids in the Earth's interior, the solubility and solution mechanisms of ReO2 in aqueous fluids were determined to 900 °C and about 1710 MPa by using an externally–heated hydrothermal diamond anvil cell. In order to shed light on how Re solubility and solution mechanisms in aqueous fluids can be affected by interaction of Re with other solutes, compositions ranged from the comparatively simple ReO2–H2O system to compositionally more complex Na2O–ReO2–SiO2–H2O fluids. Fluids in the ReO2–SiO2–H2O, SiO2–H2O, Na2O–SiO2–H2O, and Na2O–ReO2–H2O systems also were examined. The presence of Na2O enhances the ReO2 solubility so that in Na2O–ReO2–H2O fluids, for example, Re solubility is increased by a factor of 10–15 compared with the Re solubility in Na2O-free ReO2–H2O fluids. The SiO2 component in ReO2–SiO2–H2O causes reduction of ReO2 solubility compared with ReO2–H2O fluids. The ReO2 solubility in the Na-bearing Na2O–ReO2–SiO2–H2O fluids is greater than that in fluids in both the ReO2–H2O and ReO2–SiO2–H2O systems. Rhenium is dissolved in aqueous fluid as ReO4-complexes with Re in fourfold coordination with oxygen. Some, or all, of the oxygen in these complexes is replaced by OH-groups depending on whether Na2O also is present. It is proposed that during dehydration of hydrated subduction zone mineral assemblages in the upper mantle, the alkali/alkaline earth ratio of the source of the released aqueous fluid affects the extent to which Re (and other HFSE) can be transported into an overlying peridotite mantle wedge. The infiltration of such fluids will, in turn, affect the Re content (and Re/Os ratio) of magma formed by partial melting of this peridotite wedge.