We present the results of the first systematic study of melt compositions at Pantelleria, based on both melt inclusions and matrix glasses in pantellerites from 10 eruptions during the last eruptive cycle (<45 kyr). We present major and trace element compositions, as well as data on the volatiles sulphur (S), fluorine (F), chlorine (Cl), water (H2O), carbon dioxide (CO2) and lithium (Li) Rare earth element (REE) compositions were inverted using the program INVMEL to establish the melt fraction vs depth relationship in the Pantellerian mantle source region. Inversion indicates that melts are generated by ∼1·7% melting of a light rare earth element (LREE)-enriched mantle source. The source lies principally within the spinel–garnet transition zone, which, on the basis of trace element ratios, shows some affinity to the source of North African magmatism. Major and trace element data indicate a gap in melt compositions at intermediate compositions, consistent with previously published whole-rock data. This gap rules out the possibility of explaining chemical variability in the Pantelleria lavas merely by changes in the crystal content of the magmas. Principal component analysis of major element glass compositions shows that the liquid line of descent for mafic melt compositions is controlled by clinopyroxene, plagioclase, magnetite and olivine crystallization. Alkali feldspar, clinopyroxene, ilmenite and olivine or aenigmatite crystallization controls the liquid line of descent for the silicic melt compositions, with aenigmatite broadly replacing olivine in the most evolved magmas. Trace element modelling indicates that 96% fractional crystallization is required to generate pantellerites from alkali basalts at Pantelleria (through trachytes, generated after 76% fractional crystallization). We have measured pantellerite volatile concentrations in melt inclusions and in matrix glasses from a variety of eruptions. Melt inclusions, on average, contain 350 ppm S, 3500 ppm F and 9000 ppm Cl. We have measured up to 4·9 wt % H2O and 150 ppm CO2 in melt inclusions. Li–H2O systematics and Cl abundances in melt inclusions are consistent with partitioning of Li and Cl into a subcritical hydrosaline fluid at low pressures. The volatiles H2O and CO2 are used to estimate melt equilibration pressures, which reach a maximum of 1·5 kbar. Temperatures of 800°C are calculated for the most evolved pantellerites, using published feldspar–melt geothermometers, and up to 870°C for the least evolved samples. Low melt viscosities are calculated for the range of pantellerite compositions observed and may account for rapid differentiation by crystal settling. Stable density stratification of the magma chamber is reflected in the eruption of generally progressively more fractionated compositions after the Green Tuff eruption during the last eruptive cycle. Some anomalies in this trend may be explained by variation in the relative rates of eruption vs fractionation. The density stratification is expected to be enhanced and further stabilized by the efficient migration of a fluid phase to the roof of the magma chamber. The sulphur data are used in combination with published experimental partitioning data for peralkaline rhyolites to estimate the sulphur yield to the atmosphere for a large pantelleritic eruption similar to the Green Tuff. This is expected to be markedly higher than for a similar-sized metaluminous rhyolitic or dacitic eruption, mainly owing to the higher bulk sulphur content, lower fluid–melt partition coefficients, and rapid differentiation and vapour phase segregation in the magma chamber.
- fractional crystallization
- peralkaline rhyolite