The magmatic H₂O pathway of ascending arc magmas recorded by Ca-in-olivine hygrometry: advantages, complications, and perspectives

The Ca-in-olivine geohygrometer, first calibrated in 2016 (Gavrilenko et al. J Petrol, 57(9):1811–1832, 2016a), has since been widely applied to diverse datasets, providing significant insights into magmatic H₂O contents. Building on extensive experience with this method, this study reviews the application of this petrological tool, summarizing its key features, strengths, and limitations. Using a large dataset of olivine-hosted melt inclusions (MIs) from Klyuchevskoy volcano, we highlight the method's advantages and challenges, propose strategies for optimizing its use, and suggest potential improvements for Ca-in-olivine hygrometry. Applying the Ca-in-olivine geohygrometer to extensive MI datasets for a given arc volcano can reveal the H₂O content variation during this magma evolution, showing magmatic H₂O accumulation at greater depth due to incompatible H₂O behavior, and then a degassing trend at shallow depth when H₂O saturation is reached. While effective for evolved compositions (Fo < ~ 85), the method underestimates magmatic H₂O content in primitive compositions (Fo > ~ 85). Based on the 1-atm and high-pressure experiments with Klyuchevskoy compositions, combined with secondary fluorescence modeling around olivine-hosted MIs, we suggest that refining current Ca partitioning models (olivine/melt) and routinely measuring CaO in host olivine for reported MIs can improve the method's accuracy and broaden its applicability in magmatic studies. These findings aim to enhance the accuracy and applicability of this technique in studying magmatic processes.