Melt-mineral-fluid interactions in ultramafic nodules from alkaline lavas of Mount Etna (Sicily, Italy): Melt and fluid inclusion evidence

Abstract

Clinopyroxene phenocrysts and ultramafic nodules in the “Ancient Alkaline Lavas” of Mount Etna contain melt and fluid inclusions which represent immiscible trapping of basaltic melt and CO 2. Inclusions consist of glass + vapor (CO 2) hosted by clinopyroxene with kaersutite, magnetite, spinel and apatite daughter minerals (type I), and liquid (CO 2) + vapor (CO 2) + glass secondary inclusions (type II) with highly variable fluid/glass ratios. Electron-microprobe investigations on type I melt inclusions indicate a glass of trachitic composition, with extremely high chlorine contents (0.5–1 wt.%), which we believe evolved from an initial melt composition of an alkali-basalt. Homogenization temperatures ( T h) ( n=128) of type II CO 2 + glass inclusions display a bimodal fluid density distribution, with high-density (0.61–0.75 g/cm 3) CO 2 fluids in the nodules, and low-density (0.23–0.61 g/cm 3) CO 2 fluids in the phenocrysts. A two-step degassing process is proposed with high-density immiscible CO 2 fluids, formed at pressures of about 4.3 kbar. The density distribution suggests interaction events between nodules, ascending lavas, and fluid phases, with a peak around 3.5 kbar. A second generation of lower-density CO 2 is recorded around 2.8 kbar, most likely prior to eruption.