OH Solubility Of Olivine In The Peridotite-COH System Under Oxidizing Conditions

H2O and CO2 are major constituents of fluids in the mantle at?30-150 km depth,which may strongly influence water solubility in nominally anhydrous olivine;however,available hydrogenation experiments of olivine have been nearly exclusively conducted in coexistence with H2O with no CO2.In this study,we investigated the impacts of CO2 on water dissolution in olivine by H-annealing natural olivine under peridotite-and fluid-saturated conditions.Experiments were carried out at 1.5-5 GPa and 1100-1300?,with Ni-NiO as oxygen fugacity buffer,with either H2O or H2O-CO2 as buffering fluid.There is no change in composition of olivine during the experiments.In all the runs,the infrared spectra of the hydrated olivine are dominated by prominent OH bands from?3650 to 3000 cm-1,at both low frequency(<3450 cm-1)and high frequency(>3450 cm-1).H2O solubility is?120-370 ppm for the olivine in coexisting with H2O,and?65-180 ppm for the olivine in coexisting with H2O-CO2.Due to the water partitioning between minerals and coexisting fluid,the H2O solubility of olivine is reduced by a factor of?2 when CO2 is present in the buffering fluid,and the measured H2O solubility shows independence on fluid composition(the molar ratio of CO2 to CO2 + H2O is?0.2?0.5)at given pressure,temperature and oxygen fugacity.Olivine equilibrated in the shallow mantle is probably characterized by OH groups in the wavenumber?3650-3000 cm-1,and the intensity of OH bands at low frequency may be higher than or comparable to those at higher frequencies.The previous estimates may have been overestimated the water storage capacity in the shallow mantle by a factor of at least?4 if the observed effect of CO2 on water solubility is correct.Our research results have significant influence on the understanding of partial melting,electrical conductivity anomalies and metasomatism in the shallow mantle.