Probing the magma plumbing of Erebus volcano, Antarctica, by open-path FTIR spectroscopy of gas emissions

Abstract

Gas emissions from Erebus volcano, Antarctica, were measured by open-path Fourier transform infrared spectroscopy to understand degassing of its magmatic system. Two degassing phonolite lava lakes were present in the summit crater during observation in December 2004. We report analyses of H 2O, CO 2, CO, SO 2, HF, HCl and OCS, (in order of molar abundance) in the plumes. Variations in the proportions of these species strongly reflect the dynamics of degassing, and sourcing of gas from different depths in the magmatic network. The highest observed ratios of CO 2 and H 2O are consistent with gas extracted from the melt at a depth of up to ∼ 2 km below the lava lakes. Magma degassing above this depth contributes to a higher H 2O/CO 2 proportion in the airborne plume. The ratio therefore reflects the balance of deeper vs. shallower contributions of volatiles and, possibly, a combination of closed- and open-system degassing. We observe a strong contrast in HF content in emissions from the two lava lakes, which we attribute to differing levels of magma ascent and/or cooling and crystallization of the magma supply. Fluxes of all gas species were determined using independent SO 2 flux determinations and measured gas ratios. In the case of CO 2 and water, ∼ 1 and ∼ 0.4 m 3 s − 1 , respectively, of parental basanite magma are required to sustain the calculated output. The discrepancy between the two figures is readily explained by sequestration of part of the magma supply at depth such that it only partially degasses its complement of water.