HR: 16:15h
AN: MR54A-02    [Abstracts]
TI: Constraints on Thermodynamics of the Lower Mantle from New Shock-wave Experiments in the MgSiO3 and Mg2SiO4 systems
AU: * Mosenfelder, J L
EM: jed@gps.caltech.edu
AF: Department of Geological and Planetary Sciences, California Institute of Technology, M/C 170-25, Pasadena, CA 91125, United States
AU: Asimow, P D
EM: asimow@gps.caltech.edu
AF: Department of Geological and Planetary Sciences, California Institute of Technology, M/C 170-25, Pasadena, CA 91125, United States
AU: Frost, D J
EM: Dan.Frost@Uni-Bayreuth.DE
AF: Bayerisches Geoinstitut, Universitaet Bayreuth, Bayreuth, D-95440, Germany
AU: Rubie, D C
EM: Dave.Rubie@Uni-Bayreuth.DE
AF: Bayerisches Geoinstitut, Universitaet Bayreuth, Bayreuth, D-95440, Germany
AU: Ahrens, T J
EM: tja@gps.caltech.edu
AF: Department of Geological and Planetary Sciences, California Institute of Technology, M/C 170-25, Pasadena, CA 91125, United States
AB: We present the results of recently published [1,2,3] and new shock-wave equation-of-state measurements in the Mg-Si-O system. Dynamic experiments have been performed in our laboratory at pressures up to 245 GPa using both low-pressure starting materials (Mg₂SiO₄ forsterite, MgSiO₃ glass, and MgSiO₃ enstatite) and high pressure polycrystalline aggregates (Mg₂SiO₄ wadsleyite and MgSiO₃ perovskite), synthesized and recovered from high pressure using innovative techniques in the multi-anvil apparatus. All of these phases were characterized by low porosity and high purity with the exception of the perovskite, which contained 15-25% majorite. Differences in initial density and internal energy between these materials lead to distinct Hugoniots, each characterized by multiple phase regimes. By comparing the difference in energy at constant volume between two or more Hugoniots within a given regime, the Grueneisen parameter (gamma) of the phase can be determined. Furthermore, with the exception of perovskite, melting is inferred along all the Hugoniots at high pressures, which provides thermodynamic data for melts at P-T conditions difficult to achieve with other methods. In concert with molecular dynamics simulations [4], the results of our experiments have important implications for processes occurring in the lower mantle. At high pressures corresponding to those of the core-mantle boundary (CMB), significant density increases (which cannot be reconciled with the formation of post-perovskite) occur upon melting. This implies that melts with compositions over the entire Mg/Si range likely for the mantle would be negatively or neutrally buoyant at conditions close to the CMB, supporting the popular hypothesis calling for stagnant, partial melt to explain ultra-low velocity zones at the CMB. Furthermore, whereas gamma for solids normally decreases with increasing pressure, our results indicate the reverse behavior for melts. This unusual thermodynamic behavior substantially changes estimates of the adiabatic temperature gradient in a terrestrial magma ocean, which requires revision of existing models of crystallization behavior and attendant dynamics during cooling in this early period of Earth history. [1] Akins et al., 2004, GRL, 34, doi:10.1029/2004GL020237 [2] Luo et al., 2004, JGR, 109, B05205, doi:10.1029/2003JB002860 [3] Mosenfelder et al., 2007, JGR, 112, B06208, doi:10.1029/2006JB004364 [4] Stixrude and Karki, 2005, Science, 310, 297-299
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