Petrological notes on the 1983 lavas at Mount Etna, Sicily, with reference to their REE and Sr?Nd isotope composition

Abstract

The 1983 hawaiite of Mount Etna was sampled and analyzed for groundmass and mineral compositions, rare-earth-element concentrations and Sr-Nd isotope ratios. Microprobe data for coexisting mineral phases and glass show crystallization temperatures of around 1100° C from a rather differentiated hawaiite magma at rather highfO2 (10−8 at 1100° C), well above the QFM buffer. The hawaiites are characterized by a marked enrichment in the light REE similar to other alkaline magmas: the (Ce)N/(Yb)N is greater than 10, a feature these hawaiites have in common with alkaline magmas erupted earlier on Mount Etna. Since the Ce/Yb ratio cannot be affected by fractionation of clinopyroxene and plagioclase, it is taken as an accurate reflection of the LREE-enriched nature of the hawaiites. From this point of view, the Etnean hawaiites are identical to within-plate alkaline magmas erupted on the Hawaiian islands. This similarity extends to the Nd-Sr isotope features. Two hawaiites have87Sr/86Sr=0.70346 and 0.70352 and143Nd/144Nd=0.51286 and 0.51284. These data indicate a source similar to oceanic-island basalts, a source depleted in Rb/Sr and Nd/Sm for some period of time. The Sr isotope data are identical to that previously reported for Mount Etna. Extraction of hawaiites from depleted source regions requires either recent enrichment events, mixing of asthenospheric and lithospheric melts, or variable degrees of melting. At present, the data do not allow a clear decision. The peculiar tectonic setting of Mount Etna, between the relatively undeformed African foreland and the active Aeolian volcanic islands, may suggest contributions to the source region from subduction and within-plate processes. Etnean lavas have a geochemical imprint of subduction-related enrichment processes, and they also share petrological and chemical features identical to oceanic-island basalts whose source region has been affected by within-palte enrichment processes.