6533b859fe1ef96bd12b7621

RESEARCH PRODUCT

Plume chemistry provides insights into mechanisms of sulfur and halogen degassing in basaltic volcanoes

subject

description

Abstract This paper deals with sulfur, chlorine and fluorine abundances in the eruptive volcanic plume of the huge October 2002–January 2003 eruption of Mount Etna, aiming at relating the relevant compositional variations observed throughout with changes in eruption dynamics and degassing mechanisms. The recurrent sampling of plume acidic volatiles by filter-pack methodology revealed that, during the study period, S/Cl and Cl/F ratios ranged from 0.1–6.8 and 0.9–5.6, respectively. Plume S/Cl ratios increased by a factor of ∼10 as volcanic activity drifted from paroxysmal lava fountaining (mid- and late November) to passive degassing and minor effusion (early January), and then decreased to the low values (S/Cl=0.1) typical of the final stages of the eruption. Parallel variations in chlorine to fluorine ratios were also observed. A theoretical model is proposed for quantitative interpretation of these changes in plume composition. The model calculates the composition of a volatile phase exsolving from an ascending Etna magma, based on knowledge of solubilities and abundances in the undegassed melt of sulfur and halogens [T.M. Gerlach, EOS 72 (1991), 249, 254–255]. According to this model, degassing of Etnean basaltic melt at high pressures and depths (>100 MPa, 3 km) is likely to release a CO2+H2O-rich vapor phase with S/Cl molar ratios ∼1. Extensive sulfur and chlorine degassing from the melt would take place at shallower depth (P