Phase equilibria constraints on pre-eruptive magma storage conditions for the 1956 eruption of Bezymianny Volcano, Kamchatka, Russia

Phase equilibria experiments were performed on andesites from the catastrophic 1956 eruption of Bezymianny Volcano, Kamchatka, Russia, to determine pre-eruptive magma storage conditions. Fifteen experiments were conducted under water-saturated conditions, with oxygen fugacity equal to the Ni–NiO oxygen buffer, at temperatures between 775 and 1100 °C and pressures between 50 and 200 MPa. Simultaneous amphibole and plagioclase crystallization is reproduced at ≤ 850 °C and ≥ 200 MPa. The simultaneous crystallization temperature range of the plagioclase–clinopyroxene–orthopyroxene–Fe–Ti oxide assemblage increases with decreasing pressure, from 840 to 940 °C at 150 MPa to 940–1020 °C at 50 MPa. Melt inclusion compositions in plagioclase phenocrysts and matrix glass match experimental melt compositions reproduced at 50–100 MPa and ≤ 50 MPa, respectively. Presence of the silica phase in groundmass and mature amphibole breakdown rims suggests that magma has been stored at ca. 3 km depth prior to the final ascent for at least 40 days. Syn-eruptive ascent led to decompression-driven crystallization, which caused a temperature increase from 850–900 °C to 950–1000 °C.