Magma changes at Mount Etna: the 2001 and 2002–2003 eruptions

The 2001 and 2002–2003 Etna eruptions were strongly explosive and their lavas (trachybasalts and alkali basalts) exhibit quite unusual mineralogy and geochemistry. In addition to plagioclase, clinopyroxene and olivine, some of these lavas contain amphibole megacrysts which are shown to be formed in a volatile-rich magma at the top of a shallow chamber, and relicts of hypersthene phenocrysts resulting from an earlier, high pressure stage of crystallization (1.1–1.4 GPa, i.e. 30–40 km depth). Variations observed in whole-rock chemistry, trace element contents, 87Sr/ 86Sr ratios and 238U– 230Th– 226Ra– 210Pb disequilibria are consistent with arrival of new basaltic magma from depth, which intruded two other batches of slightly differentiated magmas (trachybasalts) occupying the upper parts of the volcanic conduits. The 2001 basaltic magma presents geochemical signatures slightly different from those of the 1974 lava, which heralded a drastic change in chemical composition and eruptive activity of the past 30 years. 210Pb– 226Ra disequilibria strongly suggest that Ra, K, Rb, Cs and 87Sr enrichments which characterize the new magma(s) took place at least several decades before eruption, and are thus deep-seated phenomena occurring in the mantle source during partial melting. After a beginning of crystallization at depth (hypersthene), this magma then ascended within a few months in upper crustal levels, triggering dike emplacement, degassing and eruption. The presently exceptional Etna's activity seems to result, therefore, from regional mantle processes that control both tectonics and magma ascent. After about three centuries of nearly steady state behaviour of the deep magma reservoir, the present period of activity since 1971 corresponds to an increased influx from the mantle of (a) distinct basaltic magma(s), which escape(s) mixing in the deep reservoir. Its geochemical signatures and higher production rate could be explained by melting of a fluid-enriched, metasomatized portion of the mantle.