Two mafic 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 > 1200°C) and both at 0.1 MPa and at high pressures (from 50 to 200 MPa) under H2O-bearing fluid-absent and H2O- and CO2-bearing fluid-present. Experiments started from glass except a few which started from glass plus San Carlos crystals to force olivine saturation. Melt H2O concentrations reached a maximum of 6.0 wt% and experimental fO2 ranged from NNO-0.1 to NNO+3.4. 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 (Fo85) crystallized spontaneously in only one experimental charge. Plagioclase was not found. Upon progressive crystallization of clinopyroxene, glass K2O and Al2O3 strongly increase while MgO, CaO and CaO/Al2O3 decrease, and residual melts follow the evolution of Vesuvius whole-rocks, from trachybasalt, tephrite, phonotephrite to tephriphonolite. Concentrations of H2O and CO2 in near-liquidus 200 MPa glasses and primitive glass inclusions from the literature overlap. The earliest evolutionary stage, corresponding to the crystallization of Fo-rich olivine, was reconstructed by the olivineadded experiments. They show that primitive Vesuvius melts are trachybasalts (K2O ~ 4.5-5.5 wt%, MgO = 8-9 wt%, Mg# = 75-80, CaO/Al2O3 = 0.9-0.95) which crystallize Fo-rich olivine (90-91) as the liquidus phase between 1150 and 1200°C and from 300 to < 200 MPa. Primitive Vesuvius melts are volatile-rich (1.5-4.5 wt% H2O; 600-4500 ppm CO2 in primitive glass inclusions) and oxidized (from NNO+0.4 to NNO+1.2). Assimilation of carbonate wallrocks 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, colder 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 the other Campanian magmas.

Nature and Evolution of Primitive Vesuvius Magmas: an Experimental Study / M. Pichavant;B. Scaillet;A. Pommier;G. Iacono-Marziano;R. Cioni. - In: JOURNAL OF PETROLOGY. - ISSN 0022-3530. - STAMPA. - 55:(2014), pp. 2281-2310. [10.1093/petrology/egu057]

Nature and Evolution of Primitive Vesuvius Magmas: an Experimental Study

CIONI, RAFFAELLO
2014

Abstract

Two mafic 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 > 1200°C) and both at 0.1 MPa and at high pressures (from 50 to 200 MPa) under H2O-bearing fluid-absent and H2O- and CO2-bearing fluid-present. Experiments started from glass except a few which started from glass plus San Carlos crystals to force olivine saturation. Melt H2O concentrations reached a maximum of 6.0 wt% and experimental fO2 ranged from NNO-0.1 to NNO+3.4. 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 (Fo85) crystallized spontaneously in only one experimental charge. Plagioclase was not found. Upon progressive crystallization of clinopyroxene, glass K2O and Al2O3 strongly increase while MgO, CaO and CaO/Al2O3 decrease, and residual melts follow the evolution of Vesuvius whole-rocks, from trachybasalt, tephrite, phonotephrite to tephriphonolite. Concentrations of H2O and CO2 in near-liquidus 200 MPa glasses and primitive glass inclusions from the literature overlap. The earliest evolutionary stage, corresponding to the crystallization of Fo-rich olivine, was reconstructed by the olivineadded experiments. They show that primitive Vesuvius melts are trachybasalts (K2O ~ 4.5-5.5 wt%, MgO = 8-9 wt%, Mg# = 75-80, CaO/Al2O3 = 0.9-0.95) which crystallize Fo-rich olivine (90-91) as the liquidus phase between 1150 and 1200°C and from 300 to < 200 MPa. Primitive Vesuvius melts are volatile-rich (1.5-4.5 wt% H2O; 600-4500 ppm CO2 in primitive glass inclusions) and oxidized (from NNO+0.4 to NNO+1.2). Assimilation of carbonate wallrocks 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, colder 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 the other Campanian magmas.
2014
55
2281
2310
M. Pichavant;B. Scaillet;A. Pommier;G. Iacono-Marziano;R. Cioni
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Utilizza questo identificatore per citare o creare un link a questa risorsa: https://hdl.handle.net/2158/915130
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