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

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 CO2. Inclusions consist of glass + vapor (CO2) hosted by clinopyroxene with kaersutite, magnetite, spinel and apatite daughter minerals (type I), and liquid (CO2) + vapor (CO2) + 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 (Th) (n=128) of type II CO2 + glass inclusions display a bimodal fluid density distribution, with high-density (0.61–0.75 g/cm3) CO2 fluids in the nodules, and low-density (0.23–0.61 g/cm3) CO2 fluids in the phenocrysts. A two-step degassing process is proposed with high-density immiscible CO2 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 CO2 is recorded around 2.8 kbar, most likely prior to eruption.