ИСТИНА

The amount of H2O that cycles through convergent margins controls the path and time that magmas take from the subducting slab-mantle interface to Earth’s surface. Maximum dissolved H2O content in magmas plays a pivotal role in the generation, and evolution of arc melts. A novel olivine-based hygrometer [1] utilizes the effect of magmatic H2O on CaO-partitioning (Ol/melt) and allows reconstruction of the magmatic H2O contents of partially degassed glassy melt inclusions. The low diffusivity of Ca in olivine makes Ca a more reliable recorder of original H2O contents compared to melt inclusions that may have experienced diffusive water loss during magma ascent and degassing. Here, we show how magmatic H2O contents change during primitive magma evolution by applying the Ca-in-olivine hygrometer to Klyuchevskoy volcano and comparing it to H2O in olivine-hosted MIs that have experienced presumably little H2O loss due to rapid ascent [2, 3, 4]. The two methods agree well, where our Ca-in-olivine estimates track the maximum H2O content preserved in the melt inclusions that varies with evolving Fo content of the host olivines. [1] Gavrilenko et al. (2016) JPet. 57, 1811-1832. [2] Churikova et al. (2007) CMP 154, 217-239. [3] Portnyagin et al. (2007) EPSL 255, 53-69. [4] Auer et al. (2009) CMP 157, 209-230.