We compare the predictions of compositional models of the mantle transition zone to observed seismic properties by constructing phase diagrams in the MgO-FeO-CaO-AI203-SiO 2 system and estimating the elasticity of the relevant minerals. Mie-Grfineisen and Birch-Murnaghan finite strain theory are combined with ideal solution theory to extrapolate experimental measurements of thermal and elastic properties to high pressures and temperatures. The resulting thermodynamic potentials are combined with the estimated phase diagrams to predict the density, seismic parameter, and mantle adiabats for a given compositional model. We find that the properties of pyrolite agree well with the observed density and bulk sound velocity of the upper mantle and transition zone. Piclogite significantly underestimates the magnitude of the 400-km velocity discontinuity and overestimates the velocity gradient in the transition zone. Substantially enriching piclogite in AI provides an acceptable fit to the observations. Invoking a chemical boundary layer between the uppermost mantle and transition zone leads to poor agreement with observed seismic properties for the compositions considered. Within the transition zone, the dissolution of garnet to Ca-perovskite near 18 GPa may explain the proposed 520-km seismic discontinuity. Below 700 km depth, all compositions disagree with observed bulk sound velocities, implying that the lower mantle is chemically distinct from the upper mantle.Paper number 92JB00068. 0148-0227/92/92JB-00068 $05.00 mantle convection, but nearly all data currently available permit several interpretations. The inferred penetration of subducting slabs into, and in some cases through, the transition zone is easily accommodated by whole mantle convection models but cannot rule out substantial chemical stratification imposed by semipermeable (leaky) transition zone boundary layers [Isacks and Molnar, 1971; Jarrard, 1986; Creager and Jordan, 1984, 1986; Silver and Chan, 1986]. Some slabs are apparently prevented from reaching the lower mantle and are significantly deformed by its upper boundary, suggesting a substantial change in material properties at 660 km depth and a stratified mantle [Giardini and Woodhouse, 1984; Zhou and Clayton, 1990]. A tenfold to 30-fold increase in viscosity at 660 km depth, which does not require multiple-layer convection or compositional layering, may also explain slab deformation and is independently inferred from geoid observations [Vassiliou et al., 1984; Hager and Richards, 1989]. At the same time, the absence of slab deformation at 400 km is not inconsistent with compositional change at that boundary provided the attendant changes in density and viscosity are not large enough to significantly impede the slab's descent. Many geochemical observations are most easily explained by the survival of two or more physically distinct reservoirs, such as multiple convective layers [DePaolo, 1983; All•gre and Turcotte, 1985; Zindler and Hart, 1986; Silver et al., 1988]. However, inefficient mixing in t...

The free energies of vacancy pair formation and migration in MgO were computed via molecular dynamics using free-energy integrations and a non-empirical ionic model with no adjustable parameters. The intrinsic diffusion constant for MgO was obtained at pressures from 0 to 140 GPa and temperatures from 1000 to 5000 K. Excellent agreement was found with the zero pressure diffusion data within experimental error. The homologous temperature model which relates diffusion to the melting curve describes well our high pressure results within our theoretical framework. 61.72.Bb,62.50.+p,66.30.Dn,91.60.Ed,91.60.Gf

Recent convection calculations have demonstrated that an endothermic phase transition can greatly decrease the vertical flow through the transition in a convecting system, in some cases leading to a layered flow. Using reasonable estimates of both the Rayleigh number and Clapeyron slope of the spinel to perovskite plus magnesiowüstite phase change, these results suggest that the 670‐km phase change has a strong effect on mantle convection. This so‐called “dynamic layering” phenomenon is further investigated with a compressible finite element code using a two‐dimensional, Cartesian geometry. We find a weak sensitivity of the pattern of flow to the form of the equations, considering Boussinesq, extended Boussinesq, and anelastic compressible forms of the governing equations, assuming that the thermodynamic properties (thermal expansivity, heat capacity, and latent heat) remain constant. The pattern of flow, however, depends strongly on the initial conditions, boundary conditions and equation of state. We compare the simple equation‐of‐state formulations used in previous work with a self‐consistent equation of state based on Debye and Birch‐Murnaghan finite strain theory under a Mie‐Grüneisen formulation. A thermal expansion coefficient that decreases monotonically with depth and is unaffected by changes in phase or temperature greatly enhances dynamic layering. This trend is reversed when the temperature, pressure, and phase dependence of thermodynamic properties such as thermal expansivity, entropy, and heat capacity is introduced. At moderate Rayleigh numbers, the pattern of the flow is strongly influenced by the pattern of the initial condition (i.e., the location of upwellings and downwellings); however, it is not sensitive to the thickness of the initial thermal boundary layers. The sensitivity of the flow to the pattern of the initial condition can potentially bias mass fluxes, especially for moderate Rayleigh number calculations.

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