Tracking pre-eruptive magmatic H2O evolution from the mantle to the surface at Klyuchevskoy volcano (Kamchatka arc)

The maximum dissolved H₂O content of magmas plays a pivotal role in the generation, evolution, ascent, and eruption of arc magmas. However, the pre-eruptive dissolved H₂O content of magmas is difficult to estimate for many reasons including but not limited to nearly complete degassing of magmas during ascent, eruption, and cooling. A novel olivine-based hygrometer utilizes the effect of magmatic H₂O on Ca-partitioning (Ol/melt) and allows reconstruction of the primary magmatic H₂O contents of partially degassed glassy melt inclusions (MIs). Olivine is one of the first crystallizing phases in primitive magmas and is stable at a wide range of P-T conditions during magma ascent to the surface. While MIs hosted in olivine often preserve the signature of physico-chemical processes of magma generation and evolution, they are leaky with respect to dissolved water. As H₂O solubility drops during ascent, prolonged storage in the shallow crust allows H₂O to diffuse out of the melt inclusion. In contrast to H₂O, the diffusivity of Ca in olivine is very slow, and its partitioning depends on melt H₂O content, making Ca contents in olivine a potentially more reliable recorder of original magmatic H₂O contents. Here, we show how magmatic H₂O contents change during primitive magma evolution by applying the Ca-in-olivine hygrometer to Klyuchevskoy volcano. Klyuchevskoy does not have a seismically or petrologically observable crustal magma chamber and represents the rapid end-member of magma source to surface transport times, making it ideal to test the robustness of the Ca-in-olivine hygrometer. We compared our results to direct FTIR/SIMS measurement of H₂O in olivine-hosted MIs that have presumably experienced minimal H₂O loss during the very rapid ascent. The two methods agree well, calculated pre-eruptive water contents using the Ca-in-olivine hygrometer track the maximum of the directly measured H₂O content preserved in the MIs, and water contents vary systematically with the Forsterite content of the host olivines. These results indicate that the Ca-in-olivine geohygrometer may allow the reconstruction of magmatic H₂O contents for systems where MIs are affected by degassing. Moreover, the hygrometer can be applied to deduce how magmatic water varied during magma ascent and evolution.