EVIDENCE FOR ERUPTION OF A SUPERHYDROUS MAGMA FROM SHIVELUCH VOLCANO, KAMCHATKA

Shiveluch volcano is one of the most active volcanoes of Kamchatka, and has produced almost twice as many large (VEI 4 or higher) eruptions than any other volcano in the world during the Holocene. Shiveluch erupts predominantly andesites, however tephrochronological studies have identified at least two mafic eruptive events in recent Shiveluch history. These mafic tephras (erupted at 3600 yr BP and 7600 yr BP) are extremely magnesian but moderately enriched in silica at 52-54% wt % SiO₂. We present an extended petrologic study of the basaltic andesite tephra erupted at 3600 yr BP. We build on previous studies of Volynets et al. (1997) and Ponomareva et al. (2007), present new data on phenocryst chemistry as well as mineral inclusion chemistry, and XANES data for Fe³⁺/Fe_tot in amphibole.

Phenocryst phases are dominantly amphibole and olivine, with minor amounts of clinopyroxene and phlogopite. Olivine ranges from Fo₈₉ to Fo₉₂, uncorrected for any amount of ferric iron in the magma. Amphibole Mg numbers range from 62-80, but cluster around 74-78. Some olivine phenocrysts have inclusions of primary amphibole and phlogopite. Phlogopite phenocrysts that are present in the tephra are almost completely chloritized, but some primary phlogopite is preserved. No plagioclase phenocrysts are found in this tephra, although rare xenocrysts are found which range in anorthite content from 22-70%, and are clearly not in equilibrium with the host magma. Plagioclase is present along with magnetite and clinopyroxene in the groundmass.

Based on olivine and amphibole chemistry and co-existence, we argue that this magma is superhydrous (> 9 wt% H2O) at depth and relatively cold (T<1150ºC), and must be erupted rapidly to the surface. Lack of breakdown rims around amphibole crystals, lack of plagioclase in the phenocryst assemblage, and the magnesian nature of the olivine coexisting with amphibole and phlogopite suggests that the magma must have contained at least 10-12wt% dissolved H₂O at depth.