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

Abstract

Abstract In order to characterize rhenium transport via infiltration of fluids in the Earth's interior, the solubility and solution mechanisms of ReO 2 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 ReO 2 –H 2 O system to compositionally more complex Na 2 O–ReO 2 –SiO 2 –H 2 O fluids. Fluids in the ReO 2 –SiO 2 –H 2 O, SiO 2 –H 2 O, Na 2 O–SiO 2 –H 2 O, and Na 2 O–ReO 2 –H 2 O systems also were examined. The presence of Na 2 O enhances the ReO 2 solubility so that in Na 2 O–ReO 2 –H 2 O fluids, for example, Re solubility is increased by a factor of 10–15 compared with the Re solubility in Na 2 O-free ReO 2 –H 2 O fluids. The SiO 2 component in ReO 2 –SiO 2 –H 2 O causes reduction of ReO 2 solubility compared with ReO 2 –H 2 O fluids. The ReO 2 solubility in the Na-bearing Na 2 O–ReO 2 –SiO 2 –H 2 O fluids is greater than that in fluids in both the ReO 2 –H 2 O and ReO 2 –SiO 2 –H 2 O systems. Rhenium is dissolved in aqueous fluid as ReO 4 -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 Na 2 O 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.