Developing a novel magmatic hygrometer using hydrogen in plagioclase

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Direct analysis of volatile concentrations in nominally anhydrous minerals (NAMs) represents a promising alternative to standard methods, as NAMs can incorporate trace concentrations of hydrogen, primarily in the form of hydroxyl defects, within their crystal structures (e.g., Bell and Rossman, 1992; Johnson and Rossman, 2004; Aubaud et al., 2004). NAMs can serve as a proxy for determining magmatic water concentrations if a partition coefficient between the hydrogen in the mineral of interest and melt is known at the relevant conditions. Previous studies on NAMs have typically utilised olivine and pyroxene (e.g., Wade et al., 2008; Lloyd et al., 2016), as their early crystallization from mafic melts enables insight into the water contents during the early stages of magma differentiation. However, comparably little work has been done on feldspars, despite their ubiquitous presence in volcanic rocks over a wide compositional and temperature range. Hence, this project aims to develop a novel technique of estimating magmatic water contents using plagioclase feldspar, by quantifying hydrogen partitioning between plagioclase and silicate melts and hydrogen diffusion through plagioclase.