Solubility of hydroxyl groups in pyroxenes: Effect of oxygen fugacity at 0.2–3 GPa and 800–1200 °C

Abstract

Clinopyroxene and orthopyroxene are important constituents of the upper mantle, and determine largely the water budget. Despite considerable efforts on water storage in pyroxenes, however, less attention has been paid to the effect of oxygen fugacity, a key thermodynamic factor in Earth sciences. We have systematically assessed the effect of oxygen fugacity on OH dissolution in pyroxenes, by conducting high-pressure and temperature experiments over a wide range of conditions. The runs were carried out at 0.2–3 GPa, 800–1200 °C and peridotite- and H2O-saturated conditions, with gem-quality natural diopside and enstatite single crystals and a fresh peridotite xenolith as the starting materials. The oxygen fugacities were buffered by Fe-FeO, Ni-NiO, Re-ReO2 and Fe3O4-Fe2O3 pairs, by using a modified double-capsule technique. The water contents of the recovered pyroxenes were determined by polarized analyses with a Fourier-transform infrared spectroscopy.The H-annealed pyroxenes show typical OH-related bands. The samples show no zoned OH distribution at sub-grain scales, and the OH contents are independent of run durations, suggesting the attainments of equilibrium OH incorporation. For each starting mineral, the spectral shapes and OH frequency positions are similar throughout the run conditions. The measured H2O contents are ∼14–394 and 34–227 ppm in the H-annealed clinopyroxene and orthopyroxene, respectively. For each mineral under otherwise identical conditions, the OH content increases with increasing pressure or temperature but decreases with increasing oxygen fugacity. In clinopyroxene, the ratio of either the peak heights or integrated absorbances between the bands at ∼3645 and 3435 cm−1 is less sensitive to a change of pressure, particularly above 1 GPa, but is more sensitive to a change of temperature, if the redox state is maintained as in the upper mantle. The OH groups in the spectra of clinopyroxene can be used to estimate the redox state in the source regions where the sample was finally equilibrated, if the equilibrium temperature is determined by other approach(s). On average, the OH content is enhanced by ∼40% and 60% in Fe-poor clinopyroxene and orthopyroxene, respectively, at Fe-FeO buffered reducing than at Ni-NiO buffered oxidizing conditions. This enhancement appears greater if the pyroxenes are more Fe-enriched, likely due to the enhanced reduction of Fe3+ to Fe2+ and the coupled incorporation of H in the structure toward more reducing conditions. The quantitative dependence of OH solubility on oxygen fugacity differs between pyroxenes and olivine (and other nominally anhydrous minerals), and the partitioning of OH between pyroxenes and coexisting olivine and the inventory of OH in the upper mantle are influenced by oxygen fugacity. The OH storage capacity of the shallow mantle is probably greater in Mars than in the Earth, because of the relatively more reduced state and more Fe-enriched pyroxenes in the former. Finally, the enhanced solubility of OH in pyroxenes under very reducing conditions could be important for insights into the early differentiation of Earth’s shallow mantle, in that hydrous melting may have been induced by redox-involved dehydration of pyroxenes when the shallow mantle evolved into its modern oxidizing state.