H 2O and CO 2 in parental magmas of Kliuchevskoi volcano inferred from study of melt and fluid inclusions in olivine

Abstract

This paper reports new FTIR data on the H 2O and CO 2 concentrations in glasses of 26 naturally quenched and experimentally partially homogenized melt inclusions in olivine (Fo 85–91) phenocrysts from rocks of the Kliuchevskoi volcano. Measured H 2O concentrations in the inclusions range from 0.02 to 4 wt.%. The wide variations in the H 2O content of the inclusions, which do not correlate with the host olivine composition and contents of major elements in the melts, are explained by the H 2O escape from inclusions via diffusion through the host olivine during the magma eruption and the following cooling. The largest H 2O loss is characteristic of inclusions from lava samples which cooled slowly after eruption. The minimal H 2O loss is observed for inclusions from rapidly quenched pyroclastic rocks. Parental magmas of the Kliuchevskoi volcano are estimated to contain 3.5 wt.% H 2O. The new data imply a 40 °C lower mantle temperatures than that estimated earlier for the Kliuchevskoi primary melts. The concentrations of CO 2 in glasses range from <0.01 to 0.13 wt.% and do not correlate with the type of studied inclusions and their composition. The calculated pressures of melt equilibria with H 2O–CO 2 fluid inside the inclusions are lower than 270 MPa. They are significantly lower than a pressure of 500 MPa calculated from the density (~0.8 g/cm 3) of cogenetic fluid inclusions in high-Fo olivine. The significant pressure drop inside the melt inclusions after their trapping in olivine might be due to the H 2O loss and redistribution of CO 2 from melt to daughter fluid phase. Compared with melt inclusions, cogenetic fluid inclusions provide independent information about the crystallization pressures of olivine and initial CO 2 content in the Kliuchevskoi magma, which were estimated to be at least 500 MPa and 0.35 wt.%, respectively. The maximum CO 2 concentrations in the primary Kliuchevskoi melts are estimated at 0.8–0.9 wt.%. The decompression crystallization of the Kliuchevskoi magmas starts at depths of 30–40 km and proceeds with a continuous decrease in CO 2 content and an increase (up to 6–7 wt.%) and then a decrease (at <300 MPa) in H 2O content in melts, which explains the origin of the whole spectrum of rocks and melt inclusions of the Kliuchevskoi volcano.