Developing a novel magmatic hygrometer using hydrogen in plagioclase

直接分析名义上无水矿物（NAMs）的挥发性浓度代表了标准方法的一种有希望的替代方法，因为NAMs可以在其晶体结构中加入微量的氢浓度，主要以羟基缺陷的形式存在（例如，Bell和Rossman, 1992； Johnson和Rossman, 2004； Aubaud等人，2004）。如果在相关条件下已知感兴趣矿物中氢和熔体之间的分配系数，NAMs可以作为确定岩浆水浓度的代理。先前对NAMs的研究通常使用橄榄石和辉石（例如，Wade等人，2008；Lloyd等人，2016），因为它们从基性熔体中早期结晶，可以了解岩浆分化早期阶段的含水量。然而，相对而言，对长石的研究很少，尽管长石普遍存在于火山岩中，成分和温度范围都很广。因此，本项目旨在开发一种利用斜长石估算岩浆含水量的新技术，通过量化斜长石与硅酸盐熔体之间的氢分配和氢在斜长石中的扩散。

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.