Influence of temperature, composition, silica activity and oxygen fugacity on the H₂O storage capacity of olivine at 8 GPa

Abstract

Olivine crystals were grown in the presence of a hydrous silicate fluid during multi-anvil experiments at 8 GPa and 1,000–1,600°C. Experiments were conducted both in a simple system (FeO–MgO–SiO₂–H₂O) and in a more complex system containing additional elements (CaO–Na₂O–Al₂O₃–Cr₂O₃–TiO₂–FeO–MgO–SiO₂–H₂O). Silica activity was buffered by the presence of either pyroxene (high a _SiO2) or ferropericlase (low a _SiO2), and $f_{{\text{O}}_{\text{2}} }$ was buffered by the presence of Ni + NiO or Fe + FeO, or constrained by the presence of Fe₂O₃. Raman spectroscopy was used to identify pyroxene polymorphs in the run products. Clinoenstatite was present in the 1,000°C experiment, and enstatite in experiments at 1,400–1,520°C. The H₂O content of olivine was measured using secondary ion mass spectroscopy, and infrared spectroscopy was used to investigate the nature of hydrous defects. The H₂O storage capacity of olivine decreases with increasing temperature at 8 GPa. In contrast to previous experimental results at ≤2 GPa, no significant effect of varying oxygen fugacity is evident, but H₂O storage capacity is enhanced under conditions of low silica activity. No significant growth of low wavenumber (<3,400 cm⁻¹) peaks, generally associated with high $f_{{\text{O}}_{\text{2}} }$ at low pressure, was observed in the FTIR spectra of olivine from the high $f_{{\text{O}}_{\text{2}} }$ experiments. Our experiments show that previous high pressure H₂O storage capacity measurements for olivine synthesized under more oxidizing conditions than the Earth’s mantle are not likely to be compromised by the $f_{{\text{O}}_{\text{2}} }$ of the experiments. However, the considerable effect of temperature on H₂O storage capacity in olivine must be taken into account to avoid overestimation of the bulk upper mantle H₂O storage capacity.