PRIMITIVE MAGMATISM OF MT ETNA - INSIGHTS FROM MINERALOGY AND MELT INCLUSIONS

Data on the mineralogy and petrology of primitive ancient and prehisto ric basaltic magmas of Mt Etna (Sicily) are scarce. A systematic study of the mineralogy and magmatic inclusions in olivine phenocrysts from the most Mg-rich tholeiitic (Aci Castello, Aci Trezza and Adrano), tr ansitional (Paterno) and alkaline (Mt. Maletto, Mt. Spagnolo and Timpa di Acireale) basalts has been undertaken, using the electron micropro be and heating/freezing stages. The maximum Fo content in olivine incr eases from 86 mol.% in tholeiites, to 88 mol.% in transitional basalt and up to 89-90.5 mol.% in alkaline basalts. The relationship between Fo of the most primitive olivine from alkaline samples and the Cr# of coexisting spinel indicates that their parental melts were near primar y. Clinopyroxene Mg# values (maximum and range) correspond closely to those of olivine, suggesting their early co-crystallization. Orthopyro xene (Mg# 86 mol.%) has been found as inclusions in the most magnesian olivine in tholeiitic lavas only. Crystallization temperatures determ ined from melt inclusion studies revealed no contrast between samples of different affinity, and range from 1240 degrees to 1100 degrees C. The pressure of crystallization is believed to be higher than 2 kbar ( up to 6 kbar), as indicated by the density of primary CO2 inclusions i n olivine, and indirectly, by the early clinopyroxene crystallization. Melt inclusions in olivine from tholeiitic lavas form a continuous tr end from Q-normative, low potassium and low phosphorus (0.2 wt%) to Si -undersaturated, Ne-normative compositions, enriched in K2O and P2O5 ( similar to 1.6 wt%). The range in melt inclusion compositions cannot b e accounted for by crystal fractionation and was probably generated du ring partial melting. The melts reconstructed from melt inclusions in olivine phenocrysts from a Mt. Maletto alkaline lava define a composit ional trend consistent with early clinopyroxene + olivine fractionatio n. The melt equilibrated with the most primitive olivine (Fo(90.5)), c linopyroxene (Mg# 92) and Cr-spinel (Cr# 80) is considered to be a nea r-primary melt (Mg# 71-74 mol.%; CaO/Al2O3 similar to 1.3), formed by the melting of a clinopyroxene-rich source. The likelihood of a single magma parental to both the tholeiitic and alkaline suites is confiden tly ruled out. Comparison of characteristic features of the magmatism of Mt. Etna and adjacent areas (Mt. Iblei and the Aeolian are) testifi es to progressive depletion of a mantle source by continuous magma ext raction, and its heterogeneous chemical and modal modification.