PETROGENESIS OF VESUVIUS HISTORICAL LAVAS CONSTRAINED BY PEARCE ELEMENT RATIOS ANALYSIS AND EXPERIMENTAL PHASE-EQUILIBRIA

Petrogenesis of Vesuvius historical lavas erupted iii the interval 163 1-1944 was investigated by petrological methods and experimental phase equilibria. Lavas are strongly porphyritic with principal phenocrysts of clinopyroxene, leucite and calcic plagioclase. Olivine, biotite, F e-Ti oxides and apatite are present occasionally. Their bulk compositi on defines a chemical group spanning the leucite-tephrite and phonolit e-tephrite regions of the TAS (total alkalies vs. silica) diagram. Thr ough PERs (Pearce element ratios) analysis it has been possible to det ermine the lava differentiation relationships (controlled by cpx joine d by minor lc, plag and Ti-mt) and to estimate the extent of different iation processes (about 25% of initial mass). Chemistry of cpx indicat es a multistage crystallization process in the upper crust. The crysta llization environmental parameters deduced from the experimental cpx a nd liquids are consistent with the existence of a shallow magma chambe r within the carbonate rocks of the Triassic basement where the nearly dry Vesuvius magma becomes CO2 saturated.The generally small volume o f the products from single eruptions, and Vesuvius lavas geochemical a nd petrographic features suggest magma reservoir is zoned with respect to temperature and phenocrysts abundance. The dynamics of magma withd rawal controls mixing of the melt fractions proceeding from different areas of the magma chamber and, therefore, it produces the within-flow chemical differences. Chemical variations among the lavas of differen t eruptive cycles are instead believed to be controlled essentially by the speed of magma ascent through the upper crust allowing differenti al removals of cpx phenocrysts crystallized from the magma on its way to the surface.