The MgSiO₃system at high pressure: Thermodynamic properties of perovskite, postperovskite, and melt from global inversion of shock and static compression data

Abstract

[1] We present new equation-of-state (EoS) data acquired by shock loading to pressures up to 245 GPa on both low-density samples (MgSiO 3 glass) and high-density, polycrystalline aggregates (MgSiO 3 perovskite + majorite). The latter samples were synthesized using a large-volume press. Modeling indicates that these materials transform to perovskite, postperovskite, and/or melt with increasing pressure on their Hugoniots. We fit our results together with existing P-V-T data from dynamic and static compression experiments to constrain the thermal EoS for the three phases, all of which are of fundamental importance to the dynamics of the lower mantle. The EoS for perovskite and postperovskite are well described with third-order Birch-Murnaghan isentropes, offset with a Mie-Grüneisen-Debye formulation for thermal pressure. The addition of shock data helps to distinguish among discrepant static studies of perovskite, and for postperovskite, constrain a value of K 0 significantly larger than 4. For the melt, we define for the first time a single EoS that fits experimental data from ambient pressure to 230 GPa; the best fit requires a fourth-order isentrope. We also provide a new EoS for Mg 2 SiO 4 liquid, calculated in a similar manner. The Grüneisen parameters of the solid phases decrease with pressure, whereas those of the melts increase, consistent with previous shock wave experiments as well as molecular dynamics simulations. We discuss implications of our modeling for thermal expansion in the lower mantle, stabilization of ultra-low-velocity zones associated with melting at the core-mantle boundary, and crystallization of a terrestrial magma ocean.

Shock compression of preheated silicate liquids: Apparent universality of increasing Grüneisen parameter upon compression

“…Comparison of Hugoniot points with multiple initial states achieving solid shock states in the Mgperovskite or post-perovskite structures gave high-P values of γ for those solids [5]. The analysis yielded decreasing γ upon compression for both solids, showing again that the increase for silicate liquids is a robust feature required by the data.…”

“…Hugoniot data on MgSiO 3 from initial states including glass, enstatite, porous enstatite, and highly porous oxide mix were assigned to liquid shock states by Mosenfelder et al [5]. Data cover a large range in internal energy and tightly constrain the value of γ at high P. The best fitting value of q, for ambient properties from [6], is -1.71 ± 0.31.…”

Reconciling magma‐ocean crystallization models with the present‐day structure of the Earth's mantle

“…This firstorder question has important implications for planetary thermochemical evolution, because freezing from mid-mantle depths outwards implies that a basal magma ocean (BMO) is chemically and thermally isolated in the deep mantle (Labrosse et al, 2007). The depth range of possible BMO formation depends on the poorly-constrained crystallization behavior of silicate liquids at high pressures (i.e., the relationship between the solidus and adiabiat) (Fiquet et al, 2010;Mosenfelder et al, 2009;Stixrude et al, 2009), as well as the density difference between crystals and liquids as a function of depth (Andrault et al, 2017). In any case, the surficial MO will ultimately evolve independently of the (putatively present) BMO.…”

Terrestrial aftermath of the Moon-forming impact

“…On the other hand, it is also conceivable that freezing began within the middle of the lower mantle [35]. In this case, tidal dissipation would have been concentrated initially in the middle of the mantle.…”

“…[35]). Thus, ignoring the heating from the core, we can assume that the mantle froze from the bottom up as it cooled (figure 3).…”