Towards a reconstruction of the magmatic feeding system of the 1944 eruption of Mt Vesuvius

摘要

Geochemistry and mineralogy of both juvenile clasts and xenoliths ejected during the 1944 eruption of Mt Vesuvius provide major contraints on the magmatic feeding system. Melt inclusions in phenocrysts of juvenile scoriae highlight that the magmas feeding the eruption underwent differentiation at different pressures. A K-tephritic volatile-rich melt evolved to reach K-phonotephritic composition at pressures higher than 300 MPa before being fed into a very shallow reservoir (P < 100 MPa) in which it mixed with the low-volatile resident K-phonotephritic magma. The newly arrived magma forced the transition from the effusive to the lava fountain phase of the 1944 eruption. The outer portion of the shallow reservoir is formed by a crystallizing margin (glass-bearing fergusites). From this, the transition to the carbonate country rocks occurs throughout a front of infiltration of magmatic melts in porous decarbonating host rocks. Magmatic melts are contaminated by the addition of Ca and Mg deriving from decarbonation reactions and/or melting of host rocks. These modified melts metasomatize the carbonates inducing skarn reactions and forming an endoskarn shell. Isotope (Sr, O) composition of juvenile products and xenoliths marks the amount of magma-carbonate interaction. Isotopic data point out that the crystallizing margin of the chamber may present a very limited contamination by carbonates. This suggests that the main volume of magma hosted in the magma chamber did not suffer any mass exchange with the wall rocks.