Melt inclusion investigation of the volatile behaviour in historic alkali basaltic magmas of Etna

Crystallization paths of basaltic (1763 eruption) and hawaiitic (1865 and 1329 eruptions) scoria from Etna were deduced from mineralogy and melt inclusion chemistry. The volatile behaviour was investigated through the study of melt inclusions trapped in the phenocrysts and those of the whole rocks and the matrix glasses. The results from the 1763 eruption point to the early crystallization of olivine Fo 81.7 from a water-rich alkaline basalt, with high Cl (1750–2000 ppm) and S (2100–2400 ppm) concentrations. The hawaiitic melt inclusions trapped in olivine Fo 74, salite and plagioclase are characterized by a decrease in Cl/K₂O and S/K₂O ratios. In each investigated system there is good correlation between K₂O and P₂O₅. In the whole rocks, Cl ranges from 980 to 1680 ppm, from basaltic to hawaiitic lavas, whereas S (110–136 ppm) remains low. Cl and S behaviour in the 1763 magma suggests an early degassing stage of Cl and S, with CO₂ and a water-rich gaseous phase for a pressure close to 100 MPa, consistent with a permanent outgassing at the summit craters of Etna. During the eruption, the sulphur remaining in the hawaiitic liquid is lost, and the degassing of chlorine is limited. Such a degassing model can be extended to the 1865 and 1329a.d. eruptions.