Temperature of Vesuvius magmas

Melt inclusions in Ca-pyroxenes from pyroclasts of several Vesuvius eruptions show that pyroxenes (Wo -En 46–48 -En ) formed in shallow reservoirs from K-basalt 20–25 to Wo and/or K-tephrite to K-phonolite magmas over a wide temperature range (800–1200 °C). The 45–50 50–52 data set reflects the general modalities of magma evolution at Vesuvius, where the highly evolved compositions are generally reached under closed-conduit conditions, forcing magmatic crystallization under increasing activity of dissolved volatiles. These data allow the establish- ment of two empirical geothermometric laws that link temperature either to the equilibrium constant of melt-pyroxene pairs or to a compositional parameter of the melt (e.g., CaO). They are used to reconstruct the thermal and compositional evolution of Vesuvius magmas. This evo- lution is often affected by syneruptive mixing processes in the magma storage zone that alter preeruptive conditions. The method can have general applicability for the definition of empir- ical thermometric laws.