Thermal and compositional evolution of the shallow magma chambers of Vesuvius: Evidence from pyroxene phenocrysts and melt inclusions

A large number of clinopyroxene (cpx) phenocrysts and hosted melt inclusions (MI) in pyroclasts from several Vesuvius eruptions were analyzed. Assuming that their temperature of homogenization (T-hom) reflects that of crystallization of the host mineral and that after homogenization and quenching, composition of the MI represents the melt from which the host crystallized, each cpx-MI pair records equilibrium conditions at T-hom. These data were used to discuss the magma evolution within shallow magma chamber. They formed by means of the periodic arrival of mafic batches, recorded by the MI in diopside (Fs(4-7)) crystallized at 1150-1200 degrees C. Under open conduit conditions the chambers are small, and each magma pulse induces thermal and compositional variations recorded by oscillatory zoned pyroxenes (Fs(4-15)). Their MI and T-hom summarize the evolution of the chamber resulting from cycles of magma injection, crystal settling, and magma extraction. The deposits of the explosive eruptions which reopen the conduit reflect variable mixing during magma withdrawal from zoned reservoirs, whose layering was deciphered through MI and T-hom in salite (Fs(15-30)). We suggest that the Vesuvius magma chambers evolve from prolate toward subequant, changing their layering with increasing volume and age: (1) initial stage, high aspect ratio chamber, homogeneous mafic melt (T degrees C similar to 1100) crystal enriched downward;(2) young stage, medium aspect ratio, continuous gradation from mildly evolved (T degrees C similar to 1050) to felsic melt (T degrees C similar to 850-900); and (3) mature stage, low aspect ratio, twofold chamber with stepwise gradient separating lower, convective, mildly evolved portion (T degrees C similar to 1050) from I upper, stratified, felsic portion (T degrees C similar to 800-950).