Modification of fumarolic gases by the ice-covered edifice of Erebus volcano, Antarctica

Abstract

The chemistry of gases measured in ice caves and from warm geothermal ground at Erebus volcano, Antarctica, shows that gas emissions are dominated by air, with varying amounts of added volcanic CO2. This suggests widespread circulation of air through the volcanic edifice, as well as spatially or temporally varying contributions from magmatic degassing. The resulting gases are further modified by two processes. The first is CO2 dissolution in water, resulting in fractionation from magmatic delta C-13-CO2 values, which are estimated to be around -4%., to heavier values, up to 1%.. Assuming all magmatic CO2 is dissolved in neutral water as HCOi', this requires hydrothermal temperatures of over 120 degrees C. However, other phases such as calcite may be present, implying even higher temperatures, while lower water pH values could result in similar isotope ratios at much lower temperatures, such as 60 degrees C at pH of 53. A large proportion of magmatic CO2 must be lost to this hydrothermal system or to mixing with air. The hydrothermal influence is localized to certain areas on the volcano, which may be associated with high velocity zones identified in previous studies by seismic tomography. Two sites with stronger magmatic signatures, by contrast, are above low velocity zones representing possible shallow magma storage. The second modification is the removal of oxygen from both deeply-sourced and air-derived gases. This is likely due to prevailing conditions in the subsurface, as it is independent of the original source of the gases and of hydrothermal modifications; and thus may affect sites with magmatic, air-like, or hydrothermal signatures. (C) 2019 Elsevier B.V. All rights reserved.