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Nature and Evolution of Primitive Vesuvius Magmas: an Experimental Study

Abstract : Two mafic eruptive products from Vesuvius, a tephrite and a trachy-basalt, 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 !12008C) and both at 0Á1 MPa and at high pressures (from 50 to 200 MPa) under H 2 O-bearing fluid-absent and H 2 O-and CO 2 -bearing fluid-present conditions. Experiments started from glass except for a few that started from glass plus San Carlos olivine crys-tals to force olivine saturation. Melt H 2 O concentrations reached a maximum of 6Á0 wt % and experimental fO 2 ranged from NNO ^ 0Á1 to NNO þ 3Á4 (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 H 2 O in melt 53 wt %, and by phlogopite (Mg# up to 81) for H 2 O in melt 43 wt %. Olivine (Fo 85) crystallized spontaneously in only one ex-perimental charge. Plagioclase was not found. Upon progressive crys-tallization of clinopyroxene, glass K 2 O and Al 2 O 3 contents strongly increase whereas MgO, CaO and CaO/Al 2 O 3 decrease; the residual melts follow the evolution of Vesuvius whole-rocks from trachybasalt to tephrite, phonotephrite and to tephriphonolite. Concentrations of H 2 O and CO 2 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 re-constructed by the olivine-added experiments. They show that the primitive Vesuvius melts are trachybasalts (K 2 O $ 4Á5^5Á5 wt %, MgO ¼ 8^9 wt %, Mg# ¼ 75^80, CaO/Al 2 O 3 ¼ 0Á9^0Á95) that crystallize Fo-rich olivine (90^91) as the liquidus phase between 1150 and 12008C and from 300 to 5200 MPa. Primitive Vesuvius melts are volatile-rich (1Á5^4Á5 wt % H 2 O and 600^ 4500 ppm CO 2 in primitive melt inclusions) and oxidized (from NNO þ 0Á4 to NNO þ1Á2). 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 pro-posed: 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.
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Michel Pichavant, Bruno Scaillet, Anne Pommier, Giada Iacono-Marziano, Raffaello Cioni. Nature and Evolution of Primitive Vesuvius Magmas: an Experimental Study. Journal of Petrology, Oxford University Press (OUP), 2014, 55 (11), pp.2281-2310. ⟨10.1093/petrology/egu057⟩. ⟨insu-01093796⟩

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