The need for reliable proxies on pre-eruptive magmatic H2O concentration is driven by its fundamental impact on volcanic petrology and hazards. We establish a hygrometric proxy for dacitic magmas by experimentally determining H2O partitioning between plagioclase and dacitic melt, so far unreported. Plagioclase is an ideal phase due to its widespread occurrence in arc magmas, extended thermal stability, and lower H diffusivity compared to other silicate minerals[1].

We conducted SIMS H2O measurements in plagioclase and coexisting dacitic glass from vapor-saturated experimental runs[2,3]. The starting material of the experiments has a Mount St. Helens dacitic composition (65.2 wt.% SiO2). The chosen experiments were performed in piston-cylinder devices[2,3] at 900-1000 ºC, 400-1310 MPa. Oxygen fugacity was buffered close to the Re-ReO2 fO2 buffer by employing a double-capsule configuration as detailed in the original studies. The experimental temperature was held at supraliquidus condition for 2 h before being adjusted to the intended run conditions. Experiments were run for ca. 120h. This resulted in large (>50 µm), homogeneous plagioclase crystals in equilibrium with the coexisting dacite melt[2,3]. 

First results (66.5-71.7wt.% SiO2 in glass; An43-60) show 3.7-5.6 wt.% H2O in glass and 94-137 µg/g H2O in plagioclase, resulting in DH plag-glass between 2-3*10^(-3). These results show no discernable correlation of DH with experimental temperature, pressure, or FeO in plagioclase. However, we report a negative An#-DH correlation. This is the first report of such correlation as previous studies focused on Na-free systems or used plagioclase seeds of fixed composition.

This method complements existing proxies, such as melt inclusion analysis, H quantification in more rapidly diffusing phases, or plagioclase-melt hygrometry, by partially overcoming some of their challenges.

[1]Johnson & Rossman (2013), Am Mineral, 98, 1779-1787

[2]Blatter, Sisson & Hankins (2017), Contrib Mineral Petrol, 172, 27

[3]Blatter, Sisson & Hankins (2023), Contrib Mineral Petrol, 178, 33