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The effect of sulphur on the phase equilibria of vesuvius phonolites : preliminary results.

Abstract : Crystallisation experiments have been performed on white phonolitic pumice samples of the Mercato (8010 BP), Avellino (3360 BP), Pompei (79 AD), and Pollena (472 AD) eruptions of Vesuvius to determine how the addition of sulphur to the system changes phase equilibria and compositions with respect to the sulphurfree system (experiments being undertaken by B. Scaillet and co-workers); the goal being to better constrain the pre-eruption conditions in the magma chamber preceding major phonolitic eruptions at Vesuvius, as well as to constrain the atmospheric sulphur yields of these eruptions. The experiments have been performed in an IHPV with a hydrogen membrane at 200 MPa, a maximum temperature of 930 °C, ƒO2 between NNO-4 and NNO+2, with 1 wt% S added to half of the capsules. Run durations were 6-10 days with an isobaric quench. The experimental products were identified by SEM and EMPA techniques. The addition of sulphur to the Avellino composition causes the appearance of sulphur-bearing scapolite and (Ca, Na2, K2)SO4 at 930 °C; the sulphur-free sample of the Avellino pumice is crystalfree for the same conditions. Preliminary results on the Pompei pumice at the same conditions demonstrate an increased stability of biotite when sulphur is introduced to the system. For the Pollena pumice the addition of sulphur favours the substitution of Mg for Fe2+ in garnet/pyroxene which have crystallised at 930 °C, NNO+2. The sulphur contents of the co-existing silicate melts for the four phonolites are typically between 1000-2000 ppm S at 930°C at NNO-NNO+2.
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Submitted on : Monday, July 10, 2006 - 9:02:01 AM
Last modification on : Friday, April 5, 2019 - 8:08:24 PM

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D.H.S. Moncrieff, Bruno Scaillet, Michel Pichavant. The effect of sulphur on the phase equilibria of vesuvius phonolites : preliminary results.. Geophysical Research Abstract, 2001, Nice, France. ⟨hal-00084662⟩

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