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 compositio n by ICP-MS. Based on these data we present new evidence on the high v olatility of both alkali and some heavy metals. In contrast, both lant hanides and actinides are not produced as significant gaseous compound s at magmatic temperatures and can be used as tracers of spattered mat erials. All metals of each chemical family maintained systematic trend s during the study period, at both summit craters and effusive vents l ocated 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 enrichmen ts are recorded in aerosols sampled in summit crater plumes according to the primary degassing type occurring at these vents. While a primar y degassing type still occurs at both Bocca Nuova and Voragine during the post-eruptive period, Sud-Est Crater exhibits a transition from pr imary to secondary type according to the variations of the volcanic ac tivity level. We confirm that the alkali metals fractionate systematic ally within their family as they cross the melt-vapour interface, the heavier members being always more enriched than the lighter ones in ae rosols. We also show that this fractionation is not related to the vol canic activity and the degassing type but seems to be a significant fe ature of the alkali family itself. Estimation of new emanation coeffic ients 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, 24 0 t/yr and 1600 t/yr, respectively. Mount Etna is a very important sou rce 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. (C) 1998 Elsevier Science B.V. All rights reserved.