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 ∼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. Plain Language Summary: Water has strong influence on a number of physical and chemical properties of minerals and rocks, and therefore distribution of hydrogen in the deep Earth has been studied extensively. For the upper mantle, some rock samples are available and we can directly determine the hydrogen content in these minerals. However, samples collected from various localities have experienced a broad range of thermodynamic conditions since they were at chemical equilibrium when partial melting (including cooling) occurred and water loss during transportation to the surface. Therefore, a correction needs to be applied to the influence of thermodynamic history if one wants to learn hydrogen distribution in Earth from these rock samples. So, the purpose of this study is to determine the hydrogen partition coefficient between olivine and orthopyroxene at a variety of thermodynamic conditions including pressure, temperature and water fugacity. Key Points: Hydrogen solubility was determined in olivine and orthopyroxene under water‐saturated conditions at P = 3–5 GPa and T = 1373–1573 KWe found that hydrogen content in Al2O3‐bearing orthopyroxene increases with temperature and pressure for a fixed Al2O3 contentThe hydrogen partition coefficient between olivine and orthopyroxene depends weakly on temperature but strongly on pressure and water fugacity [ABSTRACT FROM AUTHOR]