Evaluation and restoration of the 1970 volcanic gas analyses from Mount Etna, Sicily

Abstract

The 1970 Mount Etna volcanic gas analyses (Huntingdon, 1973) are among the most reduced volcanic gas samples ever reported. They contain 20–40% H 2 , 2–3.5% CO, and 2–5% H 2 S. Calculated oxygen fugacities for most of the analyses are well below quartz-fayalite-magnetite, several are more reduced than magnetite-wustite and all are many orders of magnitude less than those measured by Sato and Moore (1973) in the gas-streams of the collection sites at the time the samples were taken. The analyses show no similarity to calculated equilibrium compositions at any temperature. Deviations between analytical and equilibrium compositions indicate the gases have undergone extensive reduction involving mainly loss of oxygen. There also is limited evidence of sulfur loss. The reduced analyses are not the products of unusually reduced lavas, but originated from reactions of the erupted gases with the metal sampling device used in the collection procedure. The oxygen deficiencies of the analyses have been restored using the atomic hydrogen, carbon and sulfur data of Huntingdon and the oxygen fugacity data of Sato and Moore. The restored analyses are much more representative of the erupted gases which were remarkably rich in CO 2 (15–35%) and SO 2 (15–35%), and they show relatively steady compositions at each collection site over periods of observation ranging from hours to days.