Variability of alkali and heavy metal fluxes released by Mt. Etna volcano, Sicily, between 1991 and 1995

We regularly collected volcanic gases and aerosols at Mt. Etna volcano over a long period (1991–1995) and measured their chemical composition by ICP-MS. Based on these data we present new evidence on the high volatility of both alkali and some heavy metals. In contrast, both lanthanides and actinides are not produced as significant gaseous compounds at magmatic temperatures and can be used as tracers of spattered materials. All metals of each chemical family maintained systematic trends during the study period, at both summit craters and effusive vents located in the Valle del Bove during the 1991–1993 eruption. During the eruption we observed high metal enrichments at effusive vents; these enrichments seem to be characteristic of the secondary degassing of a magma previously degassed under the summit area. Lower metal enrichments are recorded in aerosols sampled in summit crater plumes according to the primary degassing type occurring at these vents. While a primary degassing type still occurs at both Bocca Nuova and Voragine during the post-eruptive period, Sud–Est Crater exhibits a transition from primary to secondary type according to the variations of the volcanic activity level. We confirm that the alkali metals fractionate systematically within their family as they cross the melt–vapour interface, the heavier members being always more enriched than the lighter ones in aerosols. We also show that this fractionation is not related to the volcanic activity and the degassing type but seems to be a significant feature of the alkali family itself. Estimation of new emanation coefficients for rubidium, caesium, tin and thallium (0.07%; 0.15%; 0.50% and 21% respectively) yields annual outputs around 1000 t/yr, 60 t/yr, 240 t/yr and 1600 t/yr, respectively. Mount Etna is a very important source of metals injected into the atmosphere, contributing to the global volcanic budget about 16% for heavy metals and 19% for alkali metals in eruptive periods and only 2% and 4% respectively in quiet periods.