CO2 Induced a Small Water Solubility in Orthopyroxene and Its Implications for Water Storage in the Upper Mantle

To investigate the effect of CO2 on water solubility in orthopyroxene coexisting with H2O‐CO2 as a buffering fluid, high‐pressure experiments were conducted at pressures of 1.5 and 3 GPa and a temperature of 1100 °C. The experiments were performed in a Walker‐type multianvil assembly using natural orthopyroxene with various CO2 to CO2‐H2O molar ratios as a starting material. The water contents were measured by polarized Fourier transform infrared spectrometry. At 1.5 GPa and 1100 °C, the H2O solubility decreased with increasing CO2 content in the fluid. The water solubility c(H2O) could be quantitatively determined based on water fugacity f(H2O) as c(H2O) = 25.21 × f(H2O)1.24. The addition of 57% CO2 dramatically reduced the water solubility in orthopyroxene from 184 to 90 ppm. In contrast, at 3 GPa and 1100 °C, the water solubility did not change with the CO2 content in the starting material because CO2 is unstable in bulk peridotite due to the reaction between CO2 and olivine at pressures exceeding 2.2 GPa. This study confirms that the additional component in the aqueous fluid can change the water activity and fugacity, thereby directly lowering the water storage capacity in mantle minerals. As a result, previous estimates of the maximum water storage capacity in the shallow mantle may be overestimated by a factor of 3. Key Points Additional component in aqueous fluids strongly reduces the water capacity of orthopyroxene Water solubility of orthopyroxene was quantitatively determined in terms of water fugacity at 1.5 GPa The previously reported maximum water storage capacity in the shallow upper mantle may be overestimated by a factor of 3