Experimental constraints on pre-eruption conditions of plinian events at vesuvius.

Crystallisation experiments are being performed on white phonolitic pumice samples of the Mercato (8010 BP) Avellino (3360 BP), Pompei (79 AD), and Pollena (472 AD) eruptions in order to constrain the pre-eruption conditions of major Plinian eruptions at Vesuvius. Experiments were done in an IHPV with an H2-membrane, at 200 MPa, betwen 900 and 800°C, fO2 around NNO+1, and aH2O varying between 1 and 0.3. Run durations were 6-10 days. The quench was isobaric. Run products were characterized by SEM and EMPA techniques. Phases crystallised are: sanidine, plagioclase, leucite, nepheline, analcime, scapolite, clinopyroxene, amphibole, biotite and garnet. Under water-saturation, the liquidus phase is clinopyroxene for all compositions but the temperature of its crystallisation decreases from ca 940°C for the Pollena, to around 840°C for all other compositions. Sanidine is the first tectosilicate to appear in the crystallisation sequence, followed by plagioclase, ± leucite, analcime and finally nepheline, except at temperatures higher than 850°C in both the Pollena and Pompei compositions, where sanidine appears next to leucite. Leucite is not present in both the Mercato and Avellino compositions, which have lower bulk K2O/Na2O ratios than the Pompei and Pollena bulk rocks. Similarly, garnet is present in the Pompei and Pollena compositions, being in probable reaction relationship with amphibole, while it is absent in Mercato and Avellino, at least under the conditions so far investigated. Amphibole stability is restricted to temperatures below 830°C for Pompei, 825°C for Mercato, 810°C for Avellino, and possibly below 800°C for Pollena. In all cases, amphibole stability is weakly dependent on variations in aH2O. Assuming pre-eruptive storage at 200 MPa, amphibole presence in phenocryst-poor phonolitic pumices of Vesuvius indicates temperatures lower than 830°C and water-rich conditions, possibly close to saturation. Experiments are underway at 100 MPa to refine these preliminary estimates.