HR: 0800h
AN: V31C-0608    [Abstracts]
TI: xMELTS: A thermodynamic model for the estimation of magmatic phase relations over the pressure range 0-30 GPa and at temperatures up to 2500 C
AU: * Ghiorso, M S
EM: ghiorso@ofm-research.org
AF: OFM-Research, 7336 24th Ave NE, Seattle, WA 98115, United States
AU: Hirschmann, M M
EM: Marc.M.Hirschmann-1@umn.edu
AF: University of Minnesota, Geology and Geophysics, Minneapolis, MN 55455, United States
AU: Grove, T L
EM: tlgrove@MIT.EDU
AF: Massachusetts Institute of Technology, Earth, Atmospheric and Planetary Sciences, Cambridge, MA 02139, United States
AB: A thermodynamic model for multicomponent silicate liquids in the system SiO₂-TiO₂-Al₂O₃- Fe₂O₃-Cr₂O₃-FeO-MgO-CaO-Na₂O-K₂O-P₂O₅-H₂O is calibrated over the pressure range 0-27.5 GPa and temperature range 700-2500°C from previously published experimental data (LEPR, http://lepr.ofm-research.org) on liquid-solid phase equilibria. The liquid model is combined with thermodynamic models for relevant igneous solid solutions - including a new model for majoritic garnets - to facilitate the calculation of phase relations by minimization of thermodynamic potentials. Extension to high pressures is achieved by adopting the liquid EOS of Ghiorso (2004, AJS 304, 637-838) and the Universal EOS for solid phases. The Gibbs free energy of the liquid is described as a regular associated solution, which permits a more physical description of the configurational entropy. The model includes both the speciation of water as molecular and hydroxyl units and multiple species of oxidized iron. The latter makes possible an internally consistent description of melt redox equilibria over the oxygen fugacity range from the iron- wüstite buffer to air. The model is calibrated from approximately 20000 statements of phase equilibria, including 6100 between endmember components of clinopyroxene and liquid, 3550 between olivine and liquid, 2800 orthopyroxene-liquid, 2750 feldspar-liquid, 2200 spinel-liquid, 950 garnet-liquid, 350 liquid-rhombohedral oxide (largely ilmenite_ss), 650 statements of water saturation, and a host of miscellaneous phases coexisting with liquid, importantly quartz, leucite, nepheline, (Mg,Fe²⁺)-perovskite, and (Mg,Fe²⁺)- wüstite. Calibrant liquids span naturally occurring compositions, but do not include data on simple systems (e.g., CMAS). xMELTS supersedes MELTS (Ghiorso and Sack, 1995, CMP 119, 197-212) and pMELTS (Ghiorso et al., 2002, G³ 10.1029/2001GC000217) for the calculation of magmatic phase equilibria from crustal conditions down to the base of the transition zone of the Earth's mantle.
DE: 3611 Thermodynamics (0766, 1011, 8411)
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3618 Magma chamber processes (1036)
DE: 3621 Mantle processes (1038)
DE: 3630 Experimental mineralogy and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting