Joel Ita

Petrology, elasticity, and composition of the mantle transition zone

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-Al2O3-SiO2 system and estimating the elasticity of the relevant minerals. Mie-Gruneisen 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 Al 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.

The influence of thermodynamic formulation on simulations of subduction zone geometry and history

We introduce varying approximations to the thermodynamic properties of a convecting system similar to Earths mantle. A realistic geometry, rheology and faults are included to insure that the variations we see are robust in a dynamic, self-buffering environment. We find two levels of response in the system. First, some approximations lead to very different patterns of flow in the simulations. Second, other sets of approximations lead to similar patterns of flow, but the time history of the flow varies significantly with the formulation used. Last, we note that only minor changes to an existing code would be needed to integrate a complex thermodynamic model.

Effects of Pressure on Diffusion and Vacancy Formation in MgO from Nonempirical Free-Energy Integrations

The free energies of vacancy pair formation and migration in MgO were computed via molecular dynamics using free-energy integrations and a nonempirical ionic model with no adjustable parameters. The intrinsic diffusion constant for MgO was obtained at pressures from 0 to 140GPa and temperatures from 1000 to 5000K. 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. {copyright} {ital 1997} {ital The American Physical Society}

Effect of slab rheology on mass transport across a phase transition boundary

In this study we explore the impact of a strongly temperature-dependent viscosity on convective flow in the presense of an endothermic phase transition. Temperature-dependent viscosity strengthens the upper thermal boundary layer and its associated downwellings (slabs). Three temperature-dependent rheologies ranging from weakly to strongly temperature-dependent, as well as a constant viscosity are considered in a two-dimensional Cartesian model. All other properties of the flow and phase transition are held constant. The slab is located in the center of the computational grid to avoid the direct influence of a sidewall on the downgoing flow. We find that when holding the volume averaged Rayleigh number constant, the strength of the downwelling does not noticeably affect its ability to penetrate the phase transition as observed by the isotherms. Stronger downwellings, however, do maintain a more coherent, narrow, slablike shape and significantly increase the vertical mass flux compared to weaker downwellings which tend to build up in a large pool of cold material above the phase transition. Compared to the strong slabs, there is widespread deformation of the cold downwelling material above the phase transition. This effect is most noticeable at large negative values of the Clapeyron slope, when the constant viscosity models are strongly layered. In contrast, we find that whether the slab descends along the side of the box or in the middle of the box has a significant influence on the ability of the slab to penetrate the phase boundary as observed by the isotherms, but the variation in the mass flux diagnostic is minor. We conclude that neither slab penetration nor mass flux diagnostics is a sufficient indicator of long-term mixing of material between the upper and lower mantle.

Diffusion in MgO at high pressure: Implications for lower mantle rheology

Theoretical calculations of diffusion in periclase were performed to model the rheological properties of the lower mantle. Gibbs free energy values derived from molecular dynamics simulations using a non-empirical interatomic potential to accurately predict diffusion in MgO at zero pressure. Diffusion coefficients were computed for pressures and temperatures up to 140 GPa and 5000 K. We find that Mg vacancies required for diffusive transport are likely extrinsic (due to impurities), and the resulting O vacancies would then be too few to support bulk O transport. Estimates of viscosity from these results are consistent with those inferred for the lower mantle from geophysical observations if grain boundary diffusion is responsible for O transport.

Pressure-volume-temperature equation of state of MgSiO3 perovskite from molecular dynamics and constraints on lower mantle composition

The composition of the lower mantle can be investigated by examining densities and seismic velocities of compositional models as functions of depth. In order to do this it is necessary to know the volumes and thermoelastic properties of the compositional constituents under lower mantle conditions. We determined the thermal equation of state (EOS) of MgSiO3 perovskite using the nonempirical variational induced breathing (VIB) interatomic potential with molecular dynamics simulations at pressures and temperatures of the lower mantle. We fit our pressure-volume-temperature results to a thermal EOS of the form P(V, T) = P0(V, T0) + ΔPth(T), where T0 = 300 K and P0 is the isothermal Universal EOS. The thermal pressure ΔPth can be represented by a linear relationship ΔPth = a + bT. We find V0 = 165.40 A3, KT0 = 273 GPa, KT0′ = 3.86, a = −1.99 GPa, and b = 0.00664 GPa K−1 for pressures of 0–140 GPa and temperatures of 300–3000 K. By fixing V0 to the experimentally determined value of 162.49 A3 and calculating density and bulk sound velocity profiles along a lower mantle geotherm we find that the lower mantle cannot consist solely of (Mg,Fe)SiO3 perovskite with XMg ranging from 0.9–1.0. Using pyrolitic compositions of 67 vol % perovskite (XMg = 0.93–0.96) and 33 vol % magnesiowustite (XMg = 0.82–0.86), however, we obtained density and velocity profiles that are in excellent agreement with seismological models for a reasonable geotherm.

Using eigenfunctions of the two‐point correlation function to study convection with multiple phase transformations

We describe and illustrate a new approach for extracting and understanding the pattern of flow in complex, time-dependent convection. In this approach we calculate the eigenfunctions of the two-point correlation function and show that the two-point correlation can be efficiently characterized by the dominant few eigenmodes. We apply this methodology to extract structural information from convection models with two phase transformations.

Subduction and volatile recycling in earth’s mantle

The subduction of water and other volatiles into the mantle from oceanic sediments and altered oceanic crust is the major source of volatile recycling in the mantle. Until now, the geotherms that have been used to estimate the amount of volatiles that are recycled at subduction zones have been produced using the hypothesis that the slab is rigid and undergoes no internal deformation after subduction. We consider the effects of the strength of the slab using two‐dimensional calculations of a slab‐like thermal downwelling with an endothermic phase change. Because the rheology and composition of subducting slabs are uncertain, we consider a range of Clapeyron slopes which bound current laboratory estimates of the spinel to perovskite plus magnesiowustite phase transition and simple temperature‐dependent rheologies based on an Arrhenius law diffusion mechanism. Phase transitions can have two pronounced effects on subducting slab deformation and the resulting geotherms. First, an endothermic phase transformati...

Regionalized temperature variations in the upper 400 km of the Earth's mantle

Abstract Tectonically regionalized variations in the temperature of the upper 400 km of the Earths mantle are estimated from analysis of global seismic travel-time data cataloged by the International Seismological Centre (ISC). Seismic parameter profiles are determined from estimates of P and S velocities obtained by tau inversion. Summary phase diagrams for the olivine and pyroxene-garnet subsystems are constructed in conjunction with a thermodynamic potential formulation that allows self-consistent determination of density, bulk modulus and adiabats throughout the pressure and temperature regimes of the mantle. Perturbations in estimated seismic parameters are expressed in terms of variations in temperature using the model temperature derivatives of the bulk modulus and density at a given temperature and pressure. Confidence bounds on the velocity estimates are used to place corresponding bounds on the constructed seismic parameters. A simple differential relationship is solved iteratively to obtain a temperature variation for a given variation in seismic parameter. This approach allows the estimation of a range of seismically determined temperature variations by employing a given compositional model. Results indicate that whereas the P and S velocity variations in the upper mantle are consistent with the tectonic regionalization, variations in V p V s ratios are irregular. This leads to unstable estimates of the seismic parameters and thus estimates of mean temperature anomalies, typically within 600°C of the weighted mean, that are inconsistent with the regionalized seismic data. A comparison of two compositional models is used to show the trade-off with estimated temperature variations. A refined regionalization and analysis of a larger ISC data set are suggested to stabilize the S velocity inversion, reduce statistical uncertainties on the seismic parameters, and thus improve constraints on estimated temperature variations.

The Effects of Subduction Zones on Teleseismic SH Waves: A Numerical Study

One of the most important tasks of seismology is to image the current state of Earth’s convective system. It is still an open and much debated question whether Earth’s mantle convects as a whole or whether there is layered convection bounded by the seismic discontinuity observed at 670km depth (e.g. [13],[12],[22],[9],[21],[1],[6],[16]). Recently, further evidence [16] to claims made earlier [4] was found indicating that at least in some subducting regions slabs — manifested by high-velocity anomalies — penetrate deep into the lower mantle, favoring the concept of whole mantle convection.