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

摘要

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 degrees C gas sample was collected in evacuated Giggenbach bottle from a periodically pumping to-venting outlet above active lava flow similar to 300 m from Naboko cone. Concentrations of major components in the 1030 degrees C 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/R-a ratio = 724 (He/Ne ratio = 1.41) that is close to MORB values. The 1030 degrees C condensate contained 9.7 ppm Cu, 2.5 ppm Zn, 1.5 ppm Tl, 20 ppb Re and 3 ppb 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 degrees C condensate (delta O-18 = 6.4%a, delta D = -32%) indicate a magmatic source. Another condensate sample taken at 690 degrees C was found to be drastically different from the magmatic 1030 degrees C condensate. We suggest that the disproportional enrichment in trace elements of this 690 degrees C condensate as compared to the 1030 degrees 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 degrees C condensate (delta O-18 = 18.9%., delta D = -68.5%) can be explained by the isotopic exchange between volcanic vapor and atmospheric O-2 (delta O-18 = 23.596.). (C) 2016 Published by Elsevier B.V.