Nature and Evolution of Primitive Vesuvius Magmas: an Experimental Study

Two mafic eruptive products from Vesuvius, a tephrite and a trachybasalt, have been crystallized in the laboratory to constrain the nature of primitive Vesuvius magmas and their crustal evolution. Experiments were performed at high temperatures (from 1000 to a parts per thousand yen1200A degrees C) and both at 0 center dot 1 MPa and at high pressures (from 50 to 200 MPa) under H2O-bearing fluid-absent and H2O- and CO2-bearing fluid-present conditions. Experiments started from glass except for a few that started from glass plus San Carlos olivine crystals to force olivine saturation. Melt H2O concentrations reached a maximum of 6 center dot 0 wt % and experimental fO(2) ranged from NNO - 0 center dot 1 to NNO + 3 center dot 4 (where NNO is nickel-nickel oxide buffer). Clinopyroxene (Mg# up to 93) is the liquidus phase for the two investigated samples; it is followed by leucite for H2O in melt < 3 wt %, and by phlogopite (Mg# up to 81) for H2O in melt > 3 wt %. Olivine (Fo(85)) crystallized spontaneously in only one experimental charge. Plagioclase was not found. Upon progressive crystallization of clinopyroxene, glass K2O and Al2O3 contents strongly increase whereas MgO, CaO and CaO/Al2O3 decrease; the residual melts follow the evolution of Vesuvius whole-rocks from trachybasalt to tephrite, phonotephrite and to tephriphonolite. Concentrations of H2O and CO2 in near-liquidus 200 MPa glasses and primitive melt inclusions from the literature overlap. The earliest evolutionary stage, corresponding to the crystallization of Fo-rich olivine, was reconstructed by the olivine-added experiments. They show that the primitive Vesuvius melts are trachybasalts (K2O similar to 4 center dot 5-5 center dot 5 wt %, MgO = 8-9 wt %, Mg# = 75-80, CaO/Al2O3 = 0 center dot 9-0 center dot 95) that crystallize Fo-rich olivine (90-91) as the liquidus phase between 1150 and 1200A degrees C and from 300 to < 200 MPa. Primitive Vesuvius melts are volatile-rich (1 center dot 5-4 center dot 5 wt % H2O and 600-4500 ppm CO2 in primitive melt inclusions) and oxidized (from NNO + 0 center dot 4 to NNO + 1 center dot 2). Assimilation of carbonate wall-rocks by ascending primitive magmas can account for the disappearance of olivine from crystallization sequences and explains the lack of rocks representative of olivine-crystallizing magmas. A correlation between carbonate assimilation and the type of feeding system is proposed: carbonate assimilation is promoted for primitive magma batches of small volumes. In contrast, for longer-lived, large-volume, less frequently recharged, hence more evolved, cooler reservoirs, magma-carbonate interaction is limited. Primitive magmas from Vesuvius and other Campanian volcanoes have similar redox states. However, the Cr# of Vesuvius spinels is distinctive and therefore the peridotitic component in the mantle source of Vesuvius differs from that of the other Campanian magmas.