Hydrothermal oxidation of Os

A series of hydrothermal diamond anvil cell experiments was conducted to investigate the mobility of Os and Ir in saline and oxidizing hydrothermal fluids as function of fluid pH at temperatures ranging from 500 degrees C to 1000 degrees C and pressures resembling upper mantle/lower crust environments (29-1343 MPa). The composition of reactant fluids was monitored in real time and at in situ conditions by Raman vibrational spectroscopy. Results revealed the formation of Os=O volatile aqueous species under oxidizing redox conditions (similar to+5 Delta FMQ) and at temperatures exceeding 850 degrees C. These species were detected in an immiscible phase separated from the homogeneous fluid at low pressure conditions (<150 MPa). Based on previous experimental and theoretical studies, the Os-bearing species observed in situ are attributed to the presence of OsO4(g). Volatile transport of Os in high temperature/pressure magmatic fluids, therefore, may include oxide aqueous species along with Os-Cl complexation. Possible evidence of Ir oxidation was provided by the formation of H-2(aq) in Ir-H2O experiments at 700-800 degrees C and pressures of 543-793 MPa. Experimental data support the role of slab-derived fluids in yielding elevated Os/Ir ratios in metasomatized xenoliths relative to primitive upper mantle composition through the enhanced mobility of Os in the form of Cl- and oxygen-bearing volatile aqueous complexes. (C) 2019 Elsevier Ltd. All rights reserved.