Geochemistry of volcanic gas at the 2012–13 New Tolbachik eruption, Kamchatka

Abstract

We report measurements of the chemical and isotopic composition of gas emitted from the lava flow at the 2012–13 New Tolbachik eruption. Gas and condensate samples were taken from two vents over a lava tube in May 2013. The 1030°C gas sample was collected in evacuated Giggenbach bottle from a periodically pumping-to-venting outlet above active lava flow ~300m from Naboko cone. Concentrations of major components in the 1030°С gas sample are (mol%): 95.5 H2O, 0.47 CO2, 2.01 SO2, 1.18 HCl, 0.34 HF that are within a range of gas compositions for subduction zone volcanoes. Isotopic analysis of He gives a corrected to atmosphere R/Ra ratio=7.24 (He/Ne ratio=1.41) that is close to MORB values. The 1030°C condensate contained 9.7ppm Cu, 2.5ppm Zn, 1.5ppm Tl, 20ppb Re and 3ppb Au, and can be considered as a representative sample for the metal composition of exsolved magmatic gases at the 2012–13 Tolbachik eruption. Isotopic data on the 1030°C condensate (δ18O=6.4‰, δD=−32‰) indicate a magmatic source. Another condensate sample taken at 690°C was found to be drastically different from the magmatic 1030°C condensate. We suggest that the disproportional enrichment in trace elements of this 690°C condensate as compared to the 1030°C condensate could result from evaporation at forced pumping during sampling and possible dissolution of earlier precipitated sublimates in the gas conduit. Unusual isotopic composition of the 690°C condensate (δ18О=18.9‰, δD=−68.5‰) can be explained by the isotopic exchange between volcanic vapor and atmospheric O2 (δ18О=23.5‰).