HR: 12:10h
AN: V42B-08    [Abstracts]
TI: Trachyte Obsidian Blocks in the Lag-Breccia of Ignimbrite Campana (Campi Flegrei, Italy). Additional Experimental Data related to the magma ascent conditions.
AU: * Trigila, R C
EM: raffaello.trigila@uniroma1.it
AF: Raffaello C. Trigila, Dipartimento di Scienze della Terra, Università di Roma, La Sapienza, P.le A. Moro 5., Roma, RM 00185, Italy
AU: Dolfi, D
EM:
AF: Daniela Dolfi, Dipartimento di Scienze Geologiche, Università di Roma Tre, L.go S.Leonardo Murialdo 1., Roma, RM 00146, Italy
AB: The Obsidian blocks occurring frequently in the Lag-breccia of the Ignimbrite Campana super-eruption (IC) have been found to mime strictly the magma composition and the phenocrysts assemblage of the main eruptive ash- flow unit, possibly representing a quenched fraction of the magmatic system. To date, in spite of the several investigations on the erupted rocks, conclusive data on the fissural vents location, intracrustal reservoir(s) depth and dissolved volatiles in the melt, are controversial. The obsidian blocks show, typically, millimetric euhedral sanidine phenocrysts (up to 10-12 vol.%) regularly associated with salitic pyroxene, corroded bytownite, euhedral andesine, ulvo-spinel, apatite and biotite (totalling 4-6 vol.%). All these phases, but the bytownite, appear to be in thermodynamic equilibrium with the melt as shown by the equilibrium experiments performed at subliquidus T and confining P(H2O) of 50, 100, and 200 MPa. At 100 and 200 MPa, in particular, the entry order of mineral phases is corresponding to the obsidian one, with near liquidus co-crystallization of pyroxene, plagioclase and ulvospinel. Crystallisation is scarce until the appearance of sanidine(40-60°C below the liquidus), which brings quickly to the complete solidification of the magmatic system. Other experiments at the same P's and T's just above liquidus were performed to determine the H2O solubility in the melt both under equilibrium conditions (respectively 2.6, 4.7, and 6.9 wt.%) and under slow decompression gradients. In these last experiments performed by decreasing the confining P from 100 to 50 MPa with gradients up to 0.02 MPa/s and delay times before quenching from 0 to 12 h, no vesiculation was observed at the SEM scale, despite the amount of dissolved H2O (3.4wt.%) resulted significantly higher than the equilibrium value at the final experimental P. The effect of slow decompression rates at shallow depths keeps the volatiles into the melt enhancing the magma ascent already triggered at higher depths by the increase of the dissolved H2O due to the extensive crystal-liquid fractionation from a parent magma basaltic-trachyandesitic in composition (Fowler et al., J. Petrol., 2007).
DE: 8412 Reactions and phase equilibria (1012, 3612)
DE: 8428 Explosive volcanism
DE: 8445 Experimental volcanism
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly