Experimental study on silicate-melt inclusions in clinopyroxene phenocrysts from Roccamonfina lavas (Italy)

Abstract

Primary silicate-melt inclusions (MI) in phenocrysts from leucite-tephrite, latite, K-basalt, and trachybasalt Roccamonfina lavas have been studied in order to investigate magma crystallization and evolution, and to characterize the composition of parental magmas. The homogenization temperatures (Th) of the clinopyroxene-hosted silicate-melt inclusions range from 1185 ± 5 °C to 1275 ± 5 °C. Both Th and calculated temperatures from re-heated silicate-melt inclusions (RMI) compositions show that leucite-tephrite clinopyroxenes have a lower cristallization temperatures than latite and K-basalt. Electron microprobe analyses (EMPA) of clinopyroxene, feldspar, biotite, olivine phenocrysts and RMI hosted in clinopyroxenes have been performed. Clinopyroxene compositions generally fall in the diopside-salite field. Plagioclase varies from An89 to An43 in latitic lavas and from An90 to An85 in trachybasalt lavas, the biotite is F-rich, and the olivine composition ranges from Fo88–80 to Fo75–60 in phenocryst cores and rims respectively. The reheated clinopyroxene-hosted silicate-melt inclusion analyses show that they define a less evolved magma composition compared with their corresponding bulk rock composition and that they are comparable with Vulsini silicate-melt inclusions in olivine (Kamenetsky et al., 1995), Vesuvius silicate-melt inclusions in clinopyroxene (Lima et al., 1999) and with the least differentiated Roccamonfina lavas (Giannetti and Ellam, 1994). Silicate-melt inclusion major, minor-element and volatiles variations display three different compositional trends, identified as potassic (KS), high potassic (HKS) and intermediate-K, indicating derivation from different parental melts. Experimental study of RMI indicates that the trapped melts in clinopyroxene from different petrographic types of lavas have different volatile contents. Leucite-tephrite has higher S, F and H2O compared with latite and K-basalt. Compositions of the RMI and their host clinopyroxene indicate a complex magmatic history that is not controlled by clinopyroxene fractionation only, but partially also by crystallization of an Al-rich phase and the redox magmas conditions.