HR: 0800h
AN: V31B-0482    [Abstracts]
TI: Magma storage conditions and leucite stability in the 79 AD Vesuvius "white pumice" phonolite
AU: * Larsen, J F
EM: faust@gi.alaska.edu
AF: Geophysical Institute, University of Alaska, Fairbanks, Fairbanks, AK 99775, United States
AU: Shea, T
EM: tshea@hawaii.edu
AF: Department of Geology and Geophysics, School of Ocean and Earth Science and Technology (SOEST) University of Hawaii at Manoa, Honolulu, HI 96822, United States
AU: Gurioli, L
EM: gurioli@hawaii.edu
AF: Department of Geology and Geophysics, School of Ocean and Earth Science and Technology (SOEST) University of Hawaii at Manoa, Honolulu, HI 96822, United States
AB: This study focuses on constraining the pressure and temperature stability curve of leucites and other phases that crystallized in the phonolitic magmas from the 79 AD Vesuvius Pompei eruption, using pumice from the EU1 and EU2 pumice fall layers as starting material. The experiments were conducted under H₂O saturated conditions at PH₂O between 25 and 200 MPa, temperatures between 800 and 1000 ^oC, and fO₂ equaling Ni-NiO plus 0.5 log unit, using Rene-style, Waspaloy cold-seal and TZM alloy pressure vessels. The powdered pumice samples were loaded into 4 or 5 mm diameter Ag or Au capsules with 10 wt. % de-ionized water, equilibrated under isobaric and isothermal conditions for 100 to 200 hours, and quenched either by submerging the vessel in cold water or by flipping the TZM vessel to allow the capsule to rapidly quench against the water-cooled pressure seal. Both EU1 and EU2 compositions crystallized plagioclase, brown mica, pyroxene (and possibly amphibole), garnet, sanidine, and leucite in approximate order from highest equilibration PH₂O and T to the lowest. The stability fields of leucite in both compositions are similar, crystallizing at approximately 125 MPa at 800 ^oC and 75 MPa at 880 ^oC. Anorthitic plagioclase and possibly pyroxene appear on the liquidus at 150 MPa and 950 ^oC. Plagioclase is replaced by sanidine at lower temperatures, as seen by an increase in sanidine abundance below ~880 ^oC and evidence for sanidine intergrown with anorthite crystals in some experiments. Previous studies have estimated the location of the 79 AD Vesuvius magma chamber to be between 3 and 6 km depth, based on the metamorphic stratigraphy and melt inclusion volatile contents . Our preliminary results are in good agreement, predicting storage pressures between 100 to 150 MPa (roughly 3 to 6 km) assuming a lithostatic pressure gradient. Refinement of the phase diagrams and additional experiments will allow for a more detailed comparison between the EU1 and EU2 magmas, as well as the tephriphonolite magma that produced the grey pumice (EU3).
DE: 1012 Reactions and phase equilibria (3612, 8412)
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3630 Experimental mineralogy and petrology
DE: 8428 Explosive volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting