HR: 11:35h
AN: MR32A-06    [Abstracts]
TI: Large Grüneisen Gamma of Dense Silicate Liquids: More Experiments and a First Self- consistent Model
AU: * Asimow, P D
EM: asimow@caltech.edu
AF: California Institute of Technology, Division of Geological and Planetary Sciences 170-25, Pasadena, CA 91125, United States
AU: Mosenfelder, J L
EM: jed@gps.caltech.edu
AF: California Institute of Technology, Division of Geological and Planetary Sciences 170-25, Pasadena, CA 91125, United States
AU: Ahrens, T J
EM: tja@gps.caltech.edu
AF: California Institute of Technology, Division of Geological and Planetary Sciences 170-25, Pasadena, CA 91125, United States
AU: Sun, D
EM: sdy@gps.caltech.edu
AF: California Institute of Technology, Division of Geological and Planetary Sciences 170-25, Pasadena, CA 91125, United States
AB: The Grüneisen parameter, γ, of solid materials normally decreases upon compression, approximately as γρ^q = constant where q=1. However, multiple lines of evidence now indicate the opposite behavior in silicate liquids, in which γ increases upon compression (i.e., q<0). This was observed in shock-melted (Mg,Fe)₂SiO₄ liquid by Brown et al. [1] via comparison of the Hugoniot and release velocity. We observed the same behavior in Mg₂SiO₄ liquid (q ≤ -1.5) from comparison of the Hugoniots of forsterite and wadsleyite [2]. First-principles molecular dynamics simulations of MgSiO₃ liquid [3] confirm that γ increases with density and show that γ in the liquid phase mimics solids with similar Si coordination state. Hence a continuous increase in γ of silicate liquids to lowermost mantle pressures, well beyond the range where transition to six-coordination of Si is complete, suggests that even higher-coordinated species are forming in the melt and by extension there may be 8- coordinated silicate minerals with stability fields beginning not very far above the Earth's core-mantle boundary pressure [4]. We present new experimental evidence for this behavior in another liquid composition. The Hugoniot of 1400°C anorthite-diopside eutectic liquid was measured at low pressure by Rigden et al. [5] and extended to 110 GPa by our recent work. We collected a Hugoniot point on a solid aggregate of the same composition initially at room temperature, shocked into the melt regime at 133 GPa. The difference in internal energy between this point and the hot liquid Hugoniot allows determination of the γ of this aluminosilicate liquid at 50% compression; the result fits q = -1.85±0.2, entirely consistent with the behavior of enstatite, forsterite, and Fe- bearing olivine liquids. We suggested on the basis of an approximate calculation that the large γ of dense silicate liquids yields a liquid isentrope steeper than the liquidus of a lower mantle magma ocean [2]. Here we show a preliminary self-consistent thermodynamic model of the MgO-SiO₂ binary that matches the phase diagrams of MgO, Mg₂SiO₄, MgSiO₃, and SiO₂ in the lower mantle, that incorporates negative q in the γ model of the liquid, and that allows calculation of pressure-entropy diagrams showing how model isentropes behave during cooling. We find that for peridotite or chondritic compositions, perovskite crystallization begins at an entropy maximum near 60 GPa. The consequences for geochemical evolution depend on whether these crystals remain turbulently suspended or fractionate [6]; in the case of suspension our model shows that the mush transition affects the entire lower mantle over a rather narrow range in potential temperature. Below this point the solidus does not have a maximum and normal decompression melting behavior is observed. 1. Brown et al., in High-Pressure Research in Mineral Physics, M.H. Manghnani and Y. Syono, Editors. 1987, AGU: Washington, DC. p. 373-384. 2. Mosenfelder et al., J. Geophys. Res., 2007. 112: p. B06208. 3. Stixrude & Karki, Science, 2005. 310(5746): p. 297-299. 4. Akins & Ahrens, Geophys. Res. Lett., 2002. 29(10): 1394-1397. 5. Rigden et al. J. Geophys. Res. 1988. 93(B1): p. 367-382. 6. Solomatov & Stevenson. J. Geophys. Res., 1993. 98(E3): p. 5375-5390.
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
DE: 3939 Physical thermodynamics
DE: 3944 Shock wave experiments
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting