HE, AR, O, SR AND ND ISOTOPE CONSTRAINTS ON THE ORIGIN AND EVOLUTION OF MOUNT ETNA MAGMATISM

The 0.5 Ma history of the origin and evolution of Mount Etna, Sicily has been investigated by analysing the isotopic composition of He, Ar, O, Sr and Nd in 21 selected lava samples. The near constancy of the isotopic compositions of oxygen (delta O-18 = 5.4 +/- 0.3parts per thousand) and of helium trapped in olivine phenocrysts (6.7 +/- 0.4 R(a)) is interpreted as evidence of a single mantle source, despite drastic petrological changes during the volcano's history. He analyses performed by crushing cogenetic pyroxene and olivine phenocrysts show a tendency to lower He-3/He-4 ratios in pyroxenes. This is best explained by crystallization of pyroxenes at a depth shallower than that of olivines and/or by exchange of helium trapped in pyroxenes with atmospheric or radiogenic He before eruption. Sr-87/Sr-86 ratios of recent lavas tend to increase with time and to correlate with Rb/Th ratios, and, for historical lavas, these variations are tentatively attributed to shallow selective contamination from underlying sediments. Based on the similarity of the He-3/He-4 ratios at Etna to those of European mantle xenoliths [1], we propose that the 'baseline' geochemical signature of isotopic tracers at Etna reflects the composition of the subcontinental mantle. Comparison to other southern Italian active volcanoes (Etna, Vulcano Ischia, Campi Flegrei, Vesuvius) shows gradual dilution of the predominantly mantle Etnean end member by more radiogenic Sr and He and material with higher delta O-18 and C/He-3, which is reasonably explained by the progressively important influence of subducted continental crust.