Initial Water Content in Pyroxenes Constrained by Experimental and Numerical Simulations: Reassessment of Clinopyroxene as a Standard for Estimating Water Content in the Earth's Upper Mantle

Abstract The absence of detectable hydrogen diffusion profiles in clinopyroxenes (Cpx) was used to support the reliability of clinopyroxenes as proxies for estimating water content, thereby allowing estimation of water concentration in their parental magma based on water content in clinopyroxenes. Despite many studies on partitioning of water between nominally anhydrous minerals and silicate liquid, significant discrepancies remain due to unclear effects of pressure and water content on water partitioning behavior. This study presents new experimental data on water partitioning between clinopyroxene and orthopyroxene (Opx) () up to 7.0 GPa. found that both pressure and water content show same level of influence on as the Al 2 O 3 content of pyroxenes, which can be quantified as ( r 2 = 0.77). However, this improved still cannot account for the wide variation in water content ratio between clinopyroxene and orthopyroxene in natural samples (, 0.5 to ∼24). Therefore, we conducted numerical modeling of hydrogen diffusion to test the hypothesis that the variation in was caused by diffusive water loss. Using Luobusa peridotite as a case study and considering the factors that affect diffusion coefficients of hydrogen in pyroxenes, we propose that diffusive water loss at relatively high‐temperature conditions can explain the large , while low‐temperature water re‐equilibration processes are necessary to rationalize the rarity of diffusion profiles in clinopyroxenes. This study underscores that the absence of observable diffusion profiles does not necessarily validate clinopyroxenes as reliable proxies for estimating water content, and their initial water content can be assessed using method presented herein.