Temperature of Vesuvius magmas

Melt inclusions in Ca-pyroxenes from pyroclasts of several Vesuvius eruptio ns show that pyroxenes (Wo(46-48)-En(45-50) to Wo(50-52)-En(20-25)) formed in shallow reservoirs from K-basalt and/or K-tephrite to K-phonolite magmas over a wide temperature range (800-1200 degrees C). The data set reflects the general modalities of magma evolution at Vesuvius, where the highly evo lved compositions are generally reached under closed-conduit conditions, fo rcing magmatic crystallization under increasing activity of dissolved volat iles. These data allow the establishment of two empirical geothermometric l aws that link temperature either to the equilibrium constant of melt-pyroxe ne pairs or to a compositional parameter of the melt (e.g., CaO). They are used to re construct the thermal and compositional evolution of Vesuvius ma gmas. This evolution is often affected by syneruptive mixing processes in t he magma storage zone that alter preeruptive conditions. The method can hav e general applicability for the definition of empirical thermometric laws.