Melt inclusion geochemistry and computer modeling of trachyte petrogenesis at Ponza, Italy

Abstract

The Punta della Guardia trachytic unit on the island of Ponza, Italy is thought to represent the latest stage in the evolution of a parent basaltic melt in a shallow magma chamber undergoing fractional crystallization. To test this hypothesis, melt inclusions (MI) have been analyzed to constrain the crystallization history of the Ponza trachyte. The trachyte is a weakly porphyritic lava that contains phenocrysts of plagioclase, K-feldspar, clinopyroxene and biotite. Melt inclusions were observed and measured in plagioclase, clinopyroxene and K-feldspar. The crystallized devitrified melt inclusions were heated to dissolve (melt) the crystals, quenched to a homogeneous glass, and then analyzed for major elements by electron microprobe. Two different heating protocols, one involving rapid heating in a one-atmosphere furnace, and the other involving slower heating in a microscope heating stage, were used to homogenize the inclusions in order to investigate possible effects of heating rates on inclusion composition. No detectable difference was found. Melt inclusion compositions suggest that the magma evolved from an early phase dominated by clinopyroxene±plagioclase crystallization to a late stage in which melt composition was controlled by precipitation of two feldspars. The potential basaltic parent magma does not crop out on Ponza, although more primitive and coeval basalts and trachybasalts are found on nearby Ventotene Island, which is considered to be co-magmatic with Ponza. Using the program MELTS, we have compared results obtained from melt inclusions in the Ponza trachyte with melt compositions predicted for fractional crystallization of a magma having the composition of the Ventotene trachybasalt. Crystallization trends predicted by MELTS are consistent with data from melt and host phases, although differences in measured and predicted compositions are significant in some cases. In spite of these differences, the model results and measured compositions are consistent with the hypothesis of a genetic link between the magmatism of Ponza and Ventotene.