Hydrogen Partitioning Between Olivine and Orthopyroxene: Implications for the Lithosphere-Asthenosphere Structure

Hydrogen solubility was determined in olivine and orthopyroxene under water-saturated conditions at P = 3-5 GPa and T = 1373-1573 K. For olivine, polycrystalline samples were prepared from San Carlos olivine, and for orthopyroxene synthetic samples were prepared from oxide mixture containing 1.5-5 wt% of Al2O3. Olivine and orthopyroxene were placed next to each other and annealed under various pressure and temperature conditions for 3-5 hr. Hydrogen content was measured across each sample by FTIR spectroscopy. Under the water-saturated conditions, the hydrogen solubility in olivine increases with pressure and temperature similar to previous results. Hydrogen solubility in Al2O3-bearing orthopyroxene also increases with temperature and pressure for a fixed Al2O3 content. Based on these observations we calculated the partition coefficients of hydrogen between orthopyroxene and olivine assuming the fugacity dependence of hydrogen solubility in olivine and Al2O3-bearing orthopyroxene reported by previous studies. We find that the partition coefficient depends weakly on temperature but strongly on pressure and water fugacity. Our results are extended to an open system where Al2O3 content in orthopyroxene changes with pressure and temperature. At relatively low pressures and low water fugacity (in the lithosphere (shallower than similar to 50 km)), the partition coefficient is high and a majority of hydrogen is present in orthopyroxene. Consequently, the influence of water on the bulk physical properties is small. In contrast, at higher pressures and higher water fugacity (in the asthenosphere), the partition coefficient is smaller and a substantial amount of hydrogen is present in olivine. Consequently, hydrogen has a strong effect on the bulk properties of the asthenosphere reducing viscosity and increasing electrical conductivity.