Halogens in Mount Etna volcanic gas plume: insights into degassing processes

The passive and eruptive plume gas emissions from Mount Etna volcano, in Southern Italy, represent a persistent source of volcanic halogenidric acids (HCl, HF, HBr and HI) to the Earth's atmosphere. Etna's halogen source strength has been repeatedly characterized over the past few years [Francis et al., 1998; Caltabiano et al., 2004; Aiuppa et al., 2005], and the pre-eruptive Cl and F contents in Etna's basalts have been well constrained by measurements on both silicate melt inclusions and matrix glasses [Metrich et al., 2004; Spilliaert et al ., 2006a]. However, the mechanisms driving halogen degassing upon magma ascent and decompression are not entirely understood, and the significance of volatile ratios SO2/HCl and SO2/HF in the volcanic gas plume still a matter of debate [Aiuppa et al., 2002, 2004; Spilliaert et al., 2006b]. Here, we review a set of halogen measurements carried out in the Mount Etna volcanic gas plume during 2003-2007, and demonstrate that a large compositional range (e.g., SO2/HCl ratios from 0.4 to 12; CO2/HCl ratios from 0.1 to 52) characterize the sustained quiescent emissions from the volcano. By contrasting the volcanic gas data against model equilibrium compositions calculated by the Moretti et al. [2003] saturation model, we also attempt at a quantitative interpretation of the degassing process at Etna. The saturation models calculates the composition of a gas phase (in the H2O-CO2- SO2-HCl-HF system) at equilibrium with Etna's magmas at given set of P-T-X conditions, and takes into account halogen saturation by making use of the most recent experimental determinations of Cl and F partitioning between coexisting fluid and basaltic melt [Alletti et al., 2006, 2007]. Based on model calculations, we propose that the observed SO2/HCl and SO2/HF plume ratios at Etna derive from low pressure (P less than 10 MPa) open-system degassing of magmas feeding the upper conduit system of the volcano.