The magma plumbing system of Bezymianny Volcano: Insights from a 54 year time series of trace element whole-rock geochemistry and amphibole compositions

Abstract Samples from 33 individual eruptions of Bezymianny volcano between 1956 and 2010 provide an opportunity to study in detail the temporal evolution of an arc volcano. Major element and ICP-MS trace element analyses show that the eruptive products shifted progressively from relatively silicic magma in 1956 (~ 60.4% SiO 2 ) to more mafic compositions (e.g. 56.8% SiO 2 in 2010). Amphibole compositions changed concurrently from low-Al 2 O 3 to high-Al 2 O 3 . Whole rock element-element variation diagrams show tight compositional arrays, some with a distinct kink in the late 1970s, which cannot be reproduced by fractionation of a single magma along a liquid line of descent. Amphibole thermobarometry indicates amphibole crystallization in two separate reservoirs, one between 200 and 300 MPa, and another between 500 and 750 MPa. Liquid compositions calculated from the amphibole analyses show that liquids stored in each reservoir become increasingly mafic from 1956 to 2010, suggesting that each reservoir received magma inputs from more mafic sources throughout the eruptive cycle. End member mixing analysis of the dataset allows calculation of three end member compositions that can be combined in varying proportions to reproduce major and trace element whole rock compositions. The end-member mixing proportions vary systematically between 1956 and 2010, with maxima for end-members A, B and C during 1956, 1977, and 2010, respectively. Major element compositions of phenocrysts, combined with published trace element partition coefficients, show that each end member may have evolved from a common parental magma by fractionation of three different mineral assemblages, possibly due to different pressures of crystallization and volatile contents. The petrologic data are consistent with three magma reservoirs at different depths. Magmas from the shallowest reservoir erupted first, with increasing proportions of deeper reservoirs over time. Past studies have demonstrated similarities in eruptive style between Bezymianny and both Mount St. Helens and Soufriere Hills volcanoes. Contrasting time series data show that the magma plumbing systems of Mount St. Helens and Bezymianny have little in common, while Soufriere Hills may be a better analog. Similar eruptive styles at the surface do not necessarily reflect similar systems of magma supply and evolution at depth.