Non-Henrian behavior of hydrogen between plagioclase and silicate melt

Abstract

Hydrogen reported in lunar plagioclase (one of nominally anhydrous minerals) was used to quantify the water concentration of the late stage of Lunar Magma Ocean (LMO) based on an oft-cited fixed value of partitioning coefficient between plagioclase and silicate melt. However, the partitioning coefficient of hydrogen between plagioclase and silicate melt has been poorly constrained, especially at the lunar conditions. We conducted a series of water-bearing experiments to determine plagioclase-melt partition coefficients of hydrogen under the late stage of LMO conditions. The water concentrations of plagioclase and coexisting melt were analyzed using Fourier Transform Infrared Spectroscopy. Our new results show that the partitioning behavior of hydrogen between plagioclase and melt does not obey a classical Henry’s law at the water concentration in melt lowering than ~0.7 wt.%, and that the hydrogen partition coefficients do systematically increase with decreasing the water concentrations of the coexisting silicate melt, consistent with the re-evaluating all of the previous data of hydrogen partitioning coefficients between plagioclase and silicate melt. This indicates that the water concentration of silicate melt plays a dominant role in controlling hydrogen partitioning between plagioclase and coexisting silicate melt. This finding suggests that hydrogen partitioning between nominally anhydrous minerals and silicate melt could be far more complicated than previously thoughts, and indicates that it should be in caution when using plagioclase as a watermeter.