Complex evolution processes in the upper feeding system of Mt. Etna (Italy) as revealed by the geochemistry of recent lavas

This comprehensive review, based on a database constituted of original major element compositions for mineral phases together with already published geochemical and isotopic data for lavas and tephra, discusses some aspects of the 2001 eruption at Mt. Etna aimed at clarifying pre- and sin-eruptive differentiation processes. Lavas and tephra with rather distinct petrographic and geochemical features were being emitted for about 20 days of intense activity from two different fracture systems along the southern slopes of the volcano, namely: i) a NNW-SSE oriented one, active at the South East crater (3100 m a.s.l.) and Piano del Lago (2950-2650 m a.s.l.) areas, hereafter called SE-PL; ii) a N-S oriented one, with vents at Mts. Calcarazzzzi (2100 m a.s.l.) and Laghetto (2550 m a.s.l.) areas. Lavas from the SE-PL fractures are characterized by a Porphyritic Index (P.I.) ~30-40 with abundant oscillatory-zoned plagioclase, whereas those from the N-S fracture (both at the C Calcarazz i and Laghetto areas) are oligophyric (P.I. ~10-20) and characterized by i) the peculiar occurrence of Mg-hastingsite megacrysts and ii) large amounts of quartzarenite xenoliths. C Core-rim profiles on plagioclase and clinopyroxene phenocrysts from the N-S fracture exhibit no significant compositional changes in the early-emitted products at the Calcarazzi area (2100 m a.s.l.), whereas An% and Mg# markedly increase at the phenocryst rims of plagioclase and clinopyroxene respectively in the late-emitted lavas by the Laghetto vent (2550 m a.s.l.). The latter products exhibit a more primitive character together with a distinct trace element signature if compared to the lavas emitted from the 2100 m vent. Sr, Nd, Pb and O isotopic compositions reveal also a marked change towards higher Sr isotope ratios and lower Nd-Pb-O ones in the late-emitted volcanics of the Laghetto vent. All these features, exclusive of this vent, support a model of an uprise of more primitive, volatile-rich and isotopically distinct magma into the deeper portion of a closed, 6 km deep, reservoir shortly before the eruption, which led to mixing of the new magma with the residing one. Moreover, major element compositions of residual glasses in tephra grains coming from the Laghetto vent, indicate that, before magma input and mixing, a volatile influx occurred in the system, which caused an enrichment in Ti, Fe, P, K and volatiles (Cl, H2O, etc.) in the northern part of the closed magma body. Volcanological observations suggest that this complex interaction strongly controlled the eruptive behaviour at the Laghetto vent, where the mixed, volatile-enriched products were emitted.