FTIR remote sensing of fractional magma degassing at Mount Etna, Sicily

Abstract

Abstract The chemical composition of volcanic gas emissions from each of the four summit craters of Mount Etna was measured remotely in May 2001, using a Fourier transform infrared (FTIR) spectrometer operated on the upper flanks of the volcano. The results reveal constant HCl/HF ratio but distinct SO 2 /HCl and SO 2 /HF ratios in the emitted gases, which, in the light of melt inclusion data for Etna basalts, can be interpreted in terms of escape of gases from partially, and variably, degassed magma at different depths beneath the summit. Gases released from the three main summit craters (Bocca Nuova, Voragine, and Northeast) had an identical composition, controlled by bulk degassing of a single magma body that had previously lost c. 25% of its original sulphur. The similar gas composition at all three main craters suggests that these are connected to a central conduit system that branches at relatively shallow depth. Measurements of the bulk volcanic plume on the same day, c. 7 km downwind, show that degassing from these craters dominated the total gas output of the volcano, and that no significant chemical evolution occurred within the plume over a time-scale of c. 12 min. Weaker gas emissions from the Southeast crater were comparatively depleted in SO 2 (SO 2 /HCl and SO 2 /HF ratios a factor of two lower), implying that this crater is fed either by a separate conduit or by a branch of the central conduit whose geometry favours solubility-controlled volatile fractionation. Still lower SO 2 /HCl and SO 2 /HF ratios measured for residual degassing of a lava flow erupted from the Southeast crater verify the lower solubility and earlier escape of sulphur compared to halogens at Etna. Fractional magma degassing is also implied by strong chemical contrasts between the bulk volcanic plume and fissure gas emissions measured during the July-August 2001 flank eruption. These results highlight the ability of FTIR spectrometry to detect fine spatial and temporal variations in magma degassing processes, and thereby constrain models of shallow plumbing systems.