Primitive magmatism of Mt. Etna: insights from mineralogy and melt inclusions

Abstract

Data on the mineralogy and petrology of primitive ancient and prehistoric 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), transitional (Paterno) and alkaline (Mt. Maletto, Mt. Spagnolo and Timpa di Acireale) basalts has been undertaken, using the electron microprobe and heating/freezing stages. The maximum Fo content in olivine increases 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 primary. Clinopyroxene Mg# values (maximum and range) correspond closely to those of olivine, suggesting their early co-crystallization. Orthopyroxene (Mg# 86 mol.%) has been found as inclusions in the most magnesian olivine in tholeiitic lavas only. Crystallization temperatures determined from melt inclusion studies revealed no contrast between samples of different affinity, and range from 1240° to 1100°C. The pressure of crystallization is believed to be higher than 2 kbar (up to 6 kbar), as indicated by the density of primary CO 2 inclusions in olivine, and indirectly, by the early clinopyroxene crystallization. Melt inclusions in olivine from tholeiitic lavas form a continuous trend from Q-normative, low potassium and low phosphorus (0.2 wt%) to Si-undersaturated, Ne-normative compositions, enriched in K 2O and P 2O 5 (∼ 1.6 wt%). The range in melt inclusion compositions cannot be accounted for by crystal fractionation and was probably generated during partial melting. The melts reconstructed from melt inclusions in olivine phenocrysts from a Mt. Maletto alkaline lava define a compositional trend consistent with early clinopyroxene + olivine fractionation. The melt equilibrated with the most primitive olivine (Fo 90.5), clinopyroxene (Mg# 92) and Cr-spinel (Cr# 80) is considered to be a near-primary melt (Mg# 71–74 mol.%; CaO Al 2O 3 ∼ 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 confidently ruled out. Comparison of characteristic features of the magmatism of Mt. Etna and adjacent areas (Mt. Iblei and the Aeolian arc) testifies to progressive depletion of a mantle source by continuous magma extraction, and its heterogeneous chemical and modal modification.