The 1956 eruption of Bezymianny volcano (Kamchatka). Part I - Petrological constraints on magma storage and eruptive dynamics

Bezymianny volcano is an andesitic island arc stratovolcano that started to erupt in 1955 after ~ 1000 years of dormancy. The March 30, 1956, the climactic phase of the eruption was preceded by a four-month-long emplacement of a shallow cryptodome, which triggered a flank collapse violently decompressing the magma into a laterally-directed blast followed by an explosive phase emplacing extensive pumice concentrated pyroclastic density currents (pumice C-PDC). Aiming at constraining the plumbing system below Bezymianny volcano prior to the 1956 eruption, we performed a multiphase textural and petrological study using dense to vesiculated clasts of the blast and pumice samples from the post-blast C-PDC deposits. We inferred the pressure and temperature conditions of magma storage using sample vesicularity, amphibole destabilization rims, volatile contents in melt inclusions, microlite textures, and phase compositions (phenocrysts, microlites, and glasses). We propose a three-level magma storage characterized by a deep reservoir (≥ 200–350 MPa, ≥ 850°C, 4–8 wt% H ₂ O and CO ₂ up to 1500 ppm, where amphibole is stable), a shallow reservoir (50–100 MPa, 850–900°C, 1.5-4.0 wt% H ₂ O and CO ₂  < 250 ppm, where amphibole is unstable and quartz crystallizes) in which the pre-cryptodome magma resided and from which the post-blast pumiceous magma originated, and a subsurface cryptodome (< 25 MPa, ~ 900°C, cristobalite crystallized) from which the blast was initiated. This plumbing system provides the framework for constraining the timescales of the 1956 eruptive dynamics (companion paper).