HR: 09:15h
AN: U41A-06    [PDF]
TI: Shock-induced Melting of MgSiO$_3$ (perovskite): Implications for Density of Ultramafic Liquids above Core-Mantle Boundary
AU: Akins, J A
EM: jakins@gps.caltech.edu
AF: California Institute of Technology, Div. Geol. Planet. Sci. 1200 E. California Blvd., Pasadena, CA 91125
AU: * Ahrens, T J
EM: tja@caltech.edu
AF: California Institute of Technology, Div. Geol. Planet. Sci. 1200 E. California Blvd., Pasadena, CA 91125
AU: Asimow, P D
EM: asimow@gps.caltech.edu
AF: California Institute of Technology, Div. Geol. Planet. Sci. 1200 E. California Blvd., Pasadena, CA 91125
AB: New shock Hugoniot data for crystal enstatite (En99) at 110 to 170 GPa are concordant with diamond anvil- and Brillouin- based equations of state for perovskite structure. Above 170 GPa transition to a phase 2-3% denser than perovskite occurs. The densities achieved are too high to be explained by transition to any known solid phase, or, via disproportionation, to MgO + SiO$_2 $(CaCl$_2$), or, MgO + SiO$_2$ (melt). We propose on the similar basis of of the shock-induced melting of stishovite, that like SiO$_2$, shock-induced MgSiO$_3$(perovskite) becomes super-heated (at $\sim$5500-5800 K) and subsequently completely melts at Hugoniot pressures above 170 GPa.From the inferred melting entropy of 1.1 R / (mole-atom), the recent high-pressure x-ray thermal expansion data for MgSiO$_3 $(perovskite) indicating a value of $\sim$ 1x10$^{-5 }$/ K above 100 GPa, and the observation that the bulk sound velocity of crystals are approximately unchanged upon melting. The MgSiO$_3 $(perovskite) melting curve peaks at $\sim$ 90 GPa and has a negative slope of -10 to -16 K/GPa at higher pressures. As a result, molten MgSiO$_3$ is negatively buoyant under lower-most mantle conditions and may account for the possible partially molten (silicate) in the ULVZ (indicated by data and analysis of Helmberger, Garnero and Williams).
DE: 1015 Composition of the core
DE: 3924 High-pressure behavior
DE: 7207 Core and mantle
DE: 8115 Core processes (1507)
DE: 8124 Earth's interior--composition and state (old 8105)
SC: U
MN: 2003 Fall Meeting