HR: 1340h
AN: V13C-0566    [Abstracts]
TI: The Role of Liquid Composition on the Argon Solubility of Silicate Melts
AU: * Marrocchi, Y
EM: ym@physics.wustl.edu
AF: CRPG-CNRS, 15, rue Notre Dame des Pauvres BP20, Vandoeuvre-LŠs-Nancy, 54501 France
AU: * Marrocchi, Y
EM: ym@physics.wustl.edu
AF: Washington University, Laboratory for Space Sciences 1 Brookings Drive, Saint Louis, MO 63130 United States
AU: Toplis, M J
EM: toplis@pontos.cst.cnes.fr
AF: CRPG-CNRS, 15, rue Notre Dame des Pauvres BP20, Vandoeuvre-LŠs-Nancy, 54501 France
AU: Toplis, M J
EM: toplis@pontos.cst.cnes.fr
AF: DTM-OMP, 14, Avenue Belin, Toulouse, 31400 France
AB: Interpreting the abundance and ratios of noble gases in magmatic rocks requires an understanding of how these elements behave as a function of pressure, temperature and composition. In particular, knowledge of gas solubilities in the melt phase is critical. Previous studies have concluded that noble gas solubility in silicate melts is a function of ionic porosity, a measure of the 'free space' in the liquid. However, data for simplified synthetic melts such as CMAS do not overlap the trend of solubility as a function of ionic porosity observed for natural liquids. To understand the discrepancies between data for natural and synthetic melts we have determined the argon solubility of 38 liquids in the system NCMAS. The compositions were chosen to cover a wide range in molar silica content, molar Al/Si, average polymerization state, and identity of the cation which charge-balances Al. Beads of liquid, approximately 2 mm in diameter, were held for 24 hours at 1873 K and 1 bar under a flow of pure Ar. After quenching, glass chips typically 1 to 7 mg were analysed using a conventional noble gas mass spectrometer with a Nier type source (VG 5400 at the CRPG, Nancy, France). The gas was released from the chips by melting with a CO2 laser for periods of 1 to 5 minutes. The results confirm that liquid composition significantly affects argon solubility. When our data are considered in isolation, the covaration of argon solubility and ionic porosity is excellent. However, when literature data are added, no clear correlation is apparent. Based upon this observation and consideration of the temperature dependence of argon solubility, it is concluded that ionic porosity is not a universally applicable parameter for predicting noble gas solubility. Two new models for argon solubility are proposed, both employing the notion of partial molar argon solubilities. The model which works best uses tetrahedral rather than oxide units as the melt components, and successfully reproduces reported solubilities for Al-free and Al-bearing synthetic systems as well as data for natural liquids.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1036 Magma chamber processes (3618)
DE: 1037 Magma genesis and partial melting (3619)
DE: 3630 Experimental mineralogy and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005