Petrology and volatile content of magmas erupted from Tolbachik Volcano, Kamchatka, 2012-13

We report petrography, and bulk rock, mineral and glass analyses of eruptive products of the 2012-13 eruption of Tolbachik volcano, Central Kamchatka Depression, Russia. Magmas are shoshonitic in composition, with phenocrysts of olivine and plagioclase; clinopyroxene phenocrysts are scarce. Samples collected as bombs from the active vent, from liquid lava at the active lava front, and as naturally solidified “toothpaste” lava allow us to quantify changes in porosity and crystallinity that took place during 5.25 km of lava flow and during solidification. Olivine-hosted melt inclusions from rapidly-cooled, mm-size tephra have near-constant H2O contents (1.19 ± 0.1 wt%) over a wide range of CO2 contents (< 900 ppm), consistent with degassing. The groundmass glasses from tephras lie at the shallow end of this degassing trend with 0.3 wt% H2O and 50 ppm CO2. The presence of small saturation, rather than shrinkage, bubbles testifies to volatile saturation at the time of entrapment. Calculated saturation pressures are 0.3 to 1.7 kbar, in agreement with the depths of earthquake swarms during November 2012 (0.6 to 7.5 km below the volcano). Melt inclusions from slowly-cooled and hot-collected lavas have H2O contents that are lower by an order of magnitude than tephras, despite comparable CO2 contents. We ascribe this to diffusive H2O loss through olivine host crystals during cooling. The absence of shrinkage bubbles in the inclusions accounts for the lack of reduction in dissolved CO2 (and S and Cl). Melt inclusions from tephras experienced < 3 wt% post-entrapment crystallisation. Melt inclusion entrapment temperatures are around 1080 °C. Compared to magmas erupted elsewhere in the Kluchevskoy Group, the 2012-13 Tolbachik magmas appear to derive from an unusually H2O-poor and K2O-rich basaltic parent.