Petrological evidence of rapid evolution of the magma plumbing system of Bezymianny volcano in Kamchatka before the December 20th, 2017 eruption

We used petrological data on samples of the December 20th, 2017 eruption of the Bezymianny volcano (Kamchatka, Russia) to investigate the evolution of the magmatic system during the repose period and the following reactivation of the volcano. The eruptive products are diverse in bulk rock and matrix glass compositions, but are similar in phenocryst proportion, composition, and zoning. Three rock types are distinguished by the presence of either silica-phase (cristobalite or tridymite) or their absence. Rocks without silica phase (Silica-phase Free Host Rocks) are characterized by ~55–56 wt% bulk SiO2 content and have predominantly glassy groundmass with rare microlites in dacitic matrix glass (65.5–70.5 wt% SiO2 and 2.5–3.5 wt% K2O). Cristobalite-bearing rocks are characterized by ~56.6–57.5 wt% bulk silica content and highly crystalline groundmass with many small microlites and rhyolitic matrix glass (76.5–78 wt% SiO2 and 3.8–4.2 wt% K2O) between them. Tridymite-bearing rocks are characterized by ~58.8 wt% bulk silica content and glassy groundmass with rare microlites in rhyolitic matrix glass (77–77.5 wt% SiO2 and ~3.6 wt% K2O).Rhyolite-MELTS simulation and mineral thermobarometry indicate that silica-phase-bearing rock last equilibrated under low-pressure conditions (~15 Mpa) corresponding to the conduit depths, whereas rocks without silica phase crystallized at higher pressure corresponding to shallow magma storage depths (>40–60 Mpa). Bulk rock mass-balance calculations indicate that silica-phase-bearing magmas were produced by 15% of crystal fractionation from the magma without silica phase. Based on this petrological data, we reconstruct the evolution of the magmatic system as follows. During the volcano repose period, undisturbed magma fractionated and stratified due to buoyant ascent of fluid and melt to the chamber roof. Rejuvenation of magmatic system caused pushing the differentiated magma from the top magmatic reservoir to the conduit, where silica phases crystallized. We argue that cristobalite-bearing rocks are likely the remnants of conduit plug, whereas tridymite-bearing rocks represent magma trapped in the conduit under the plug. The renewal of explosive activity in December 2017 was triggered by magma influx from deeper levels of the plumbing system shortly before the explosion, which is supported by the presence of amphibole-bearing mafic enclaves of mid-crustal origin (520–850 Mpa).