Experimental constraints on pre-eruption conditions of Vesuvius phonolites.

We are currently establishing the phase relationships of Vesuvius phonolites (white pumices of Mercato, Avellino, Pompei and Pollena eruptions) and tephrites with the aim of defining the P-T-fH2O-fO2 pre-eruption conditions of erupted magmas. Here we will present the results so far obtained on the phonolites. We used dry starting glasses prepared from fusing twice powdered pumice at 1 bar, to which various amounts of H2O and CO2 were added to achieve fluid saturation, loaded to Au capsules. Experiments were performed at 1-3 kbar, 750-900°C, H2Omelt 1-10 wt% and NNO-NNO+1, using an IHPV fitted with a H2-membrane, with Ar-H2 mixtures as pressure media. Run durations were 7-30 days, and were ended by isobaric quenches. Phase relationships were established either isobarically (2 kb, 750-900°C) or isothermally (800°C, 1-3 kb). Phases crystallised are: sanidine, plagioclase, leucite, nepheline, analcime, scapolite, sodalite, biotite, amphibole, garnet, clinopyroxene, magnetite. Under water-rich conditions, clinopyroxene is the liquidus phase in all compositions. In Pompei and Pollena phonolites, however, it reacts out to garnet at temperatures below 800-820°C. Amphibole is not stable at temperatures above 825°C at 2 kb in Avellino, Mercato and Pompei pumices, and it is always absent in Pollena at any P-T so far explored. Experiments realised below 2 kb and 800°C do not crystallise amphibole in any compositions, except in Pompei at low H2Omelt. Similarly, at 3 kb and 800°C, amphibole is absent in the Pompei. Leucite crystallises only in Pompei and Pollena, and is present at and below 2 kb. At 2 kb, garnet crystallisation is restricted to below 825°C in Pompei and below 800°C in both Mercato and Avellino pumices, whereas it is stable up to 900°C in Pollena. The presence of amphibole in Mercato, Avellino and Pompei pumices constrains pre-eruption temperatures of the magmas to be below 825°C. For Pompei, amphibole and leucite occurrences suggest a pressure of magma storage of 2.1±0.2 kb. At this pressure, the phase assemblage of Pompei white pumice is reproduced at 815±10°C, H2Omelt of 6-6.5 wt% (XH2Ofluid of 0.8±0.05), at an fO2 of NNO+0.5±0.5, in agreement with melt inclusion constraints (Cioni, 2000). For both Mercato and Avellino, experiments are not yet conclusive with respect to pressure. If, however, the magma reservoir that fed these two eruptions was at a similar pressure depth than that of the Pompei event then phase equilibria imply pre-eruption temperatures < 780°C for Mercato and < 750°C for Avellino with, in both cases, H20melt higher than 6 wt% (XH2Ofluid> 0.8) which contrast with melt inclusion data for Avellino (3.1±0.7 wt%, Signorelli et al., 1999). Additional experiments will test these estimates. For Pollena, the presence of amphibole in the pumice shows that there too additional experiments are needed. Ignoring this problem, phase equilibria suggest temperatures higher than 800°C, H2Omelt < 6 wt% (XH2Ofluid<0.8) and pressure below 3 kb. Cioni, R, 2000, Volatile content and degassing processes in the AD 79 magma chamber at Vesuvius (Italy), Contrib. Miner. Petrol., 140, 40-54. Signorelli, S., Vaggelli, G. and Romano, C., 1999, Pre-eruptive volatile (H2O, F, Cl and S) contents of phonolitic magmas feeding the 3550-year old Avellino eruption from Vesuvius, southern Italy. J. Volc. Geotherm. Res., 93, 237-256.