Original paper

Magmatic crystallisation of Cr-Al diopside and Al-Fe3+ diopside from the ancient alkaline basalts (Mt. Etna, Sicily)

Nazzareni, Sabrina; Bus, Tiziana; Cristofolini, Renato

Abstract

The petrography of basalts from Ancient Alkaline Centres of Mt. Etna (Sicily) shows several disequilibrium textures, particularly in pyroxene and plagioclase. As regards the former, this disequilibrium is shown by complex zoning: Cr-Al diopside (Di) composition for the core and Al-Fe3+ Di for the rim, with different thicknesses. More rarely, homogeneous Al-Fe3+ Di phenocrysts are found. MELT calculations indicate that the only pyroxene to be in equilibrium with the bulk rock composition is Al-Fe3+ Di, in agreement with petrographic observations. A crystal-chemical study performed on zoned and unzoned phenocrystic pyroxene suggests similar conditions of crystallisation pressure, and a magmatic origin for both compositions. The clinopyroxene geobarometer of Nimis (1999) was applied and maximum values around 0.5 (┬▒ 0.2) GPa were obtained. A similar crystallisation pressure is suggested both by kink-band deformation of olivine, and olivine-hosted CO2 inclusions in one of the studied basalts. Excluding variations of crystallisation pressure as responsible for the disequilibrium between the two pyroxenes, this may be ascribed to time-related changes in the geochemical character of the melt. Clinopyroxene crystal chemistry, host-rock petrology, and fluid-inclusion data are also consistent with seismic data suggesting a complex reservoir system feeding Etnean activity at depths around 15-20 km.

Keywords

alkali basaltsphase relationsclinopyroxene crystal chemistrygeobarometryetnaitaly