ИСТИНА

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.