The relationship between shear and compressional velocities at high pressures: Reconciliation of seismic tomography and mineral physics (original) (raw)
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Physics of the Earth and Planetary Interiors, 1993
Compositional profiles of Earth's mantle require accurate extrapolation of elasticity to simultaneous high pressure P and temperature T, using the first P and T derivatives of elasticity measured near ambient conditions. We present thermodynamic relations that elucidate o 2KT/oPaT for minerals for which data on the volume dependence of (~T/4P)ãre available (KT is the isothermal bulk modulus and S indicates constant entropy). We find that~2KT/aPaT is positive for almost all solids and has values of (0.39 ±0.10, 0.33 ±0.09) X 10 K 'for MgO and olivine, respectively. We suggest that neglect of 2KT/~P~T could cause the olivine content of the upper mantle to be overestimated by 3% at most, if second-order pressure effects are not considered. This result is based on a comparison of the longitudinal velocities of the a and~phases of olivine at 400 km P. T conditions. We show that the isothermal~2KT/)P~T decreases with increasing P. Ab initio model calculations on MgO are consistent with the results from thermodynamics. We show for MgO the theoretical values of (aKT/aP)T and (aG/8P)T, where G is the isotropic shear modulus, along a geotherm.
Lateral Variations in Compressional/Shear Velocities at the Base of the Mantle
Science, 1999
Observations of core-diffracted P ( P diff ) and SH ( SH diff ) waves recorded by the Missouri-to-Massachusetts (MOMA) seismic array show that the ratio of compressional ( P ) seismic velocities to horizontal shear ( SH ) velocities at the base of the mantle changes abruptly from beneath the mid-Pacific ( V P / V S = 1.88, also the value predicted by reference Earth models) to beneath Alaska ( V P / V S = 1.83). This change signifies a sudden lateral variation in material properties that may have a mineralogical or textural origin. A textural change could be a result of shear stresses induced during the arrival at the core of ancient lithosphere from the northern Pacific paleotrench.
Pure and Applied Geophysics, 1998
A brief outline is given on experimental studies carried out in the Minnesota Mineral and Rock Physics Laboratory of microstructural evolution and rheology of mantle mineral aggregates or their analogues, using a simple shear deformation geometry. A simple shear deformation geometry allows us to unambiguously identify controlling factors of microstructural evolution and to obtain large strains at high pressures and temperatures, and thus provides a unique opportunity to investigate the ''structural geology of the mantle.'' We have developed a simple shear deformation technique for use at high pressures and temperatures (pressure up to 16 GPa and temperature up to 2000 K) in both gas-medium and solid-medium apparati. This technique has been applied to the following mineral systems: (i) olivine aggregates, (ii) olivine basaltic melt, (iii) CaTiO 3 perovskite aggregates. The results have provided important data with which to understand the dynamics of the earth's mantle, including the geometry of mantle convection, mechanisms of melt distribution and migration beneath mid-ocean ridges, and the mechanisms of shear localization. Limitations of laboratory studies and future directions are also discussed.
Thermodynamics of mantle mineralsI. Physical properties
Geophysical Journal …, 2005
We present a theory for the computation of phase equilibria and physical properties of multicomponent assemblages relevant to the mantle of the Earth. The theory differs from previous treatments in being thermodynamically self-consistent: the theory is based on the concept of fundamental thermodynamic relations appropriately generalized to anisotropic strain and in encompassing elasticity in addition to the usual isotropic thermodynamic properties. In this first paper, we present the development of the theory, discuss its scope, and focus on its application to physical properties of mantle phases at elevated pressure and temperature including the equation of state, thermochemical properties and the elastic wave velocities. We find that the Eulerian finite strain formulation captures the variation of the elastic moduli with compression. The variation of the vibrational frequencies with compression is also cast as a Taylor series expansion in the Eulerian finite strain, the appropriate volume derivative of which leads to an expression for the Grüneisen parameter that agrees well with results from first principles theory. For isotropic materials, the theory contains nine material-specific parameters: the values at ambient conditions of the Helmholtz free energy, volume, bulk and shear moduli, their pressure derivatives, an effective Debye temperature, its first and second logarithmic volume derivatives (γ 0 , q 0 ), and the shear strain derivative of γ . We present and discuss in some detail the results of a global inversion of a wide variety of experimental data and first principles theoretical results, supplemented by systematic relations, for the values of these parameters for 31 mantle species. Among our findings is that the value of q is likely to be significantly greater than unity for most mantle species. We apply the theory to the computation of the shear wave velocity, and temperature and compositional (Fe content) derivatives at relevant mantle pressure temperature conditions. Among the patterns that emerge is that garnet is anomalous in being remarkably insensitive to iron content or temperature as compared with other mantle phases.
Petrology, elasticity, and composition of the mantle transition zone
Journal of Geophysical Research, 1992
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.
High-temperature elastic constant data on minerals relevant to geophysics
Reviews of Geophysics, 1992
The high-temperature measurements of elastic constants and related temperature derivatives of nine minerals of interest to geophysical and geochemical theories of the Earth's interior are reviewed and discussed. A number of correlations between these parameters, which have application to geophysical problems, are also presented. Of especial interest is a, the volume coefficient of thermal expansion, and a section is devoted to this physical property. Here we show how a can be estimated at very high temperatures and how it varies with density. An estimate of a for Mgperovskite at deep-mantle conditions is made. The formula for the Griineisen ratio -y as a function of V and T is presented, including plots of the numerical values of 'y over a wide T and V range. An example calculation of 'y for MgO is made. The high-T-high-P values of 'y calculated here agree well with results from the ab initio method of calculation for MgO. The use of the thermoelastic parameters is reviewed, showing application to the understanding of thermal pressure, thermal expansivity, enthalpy, and entropy. We review an extrapolation formula to determine K s , the adiabatic bulk modulus, at very high T. We show that the thermal pressure is quite linear with T up to high temperatures (-1800 K), and, as a consequence, the anharmonic contribution to the Helmholtz free energy is sufficiently small, so that it can and should be ignored in thermodynamic calculations for mantle conditions.
Physics of the Earth and Planetary Interiors, 1993
This paper focusses on the physical properties of rocks and property changes occurring during geodynamic processes. It addresses a number of outstanding petrophysical problems in the framework of crustal and mantle deformation. The major themes to be discussed will be the seismic signature of crustal and mantle shear zones, the nature of crustal and mantle anisotropy and the contribution of anisotropic rocks to seismic reflectivity. In particular, seismic shear waves traversing anisotropic strata can reveal important information on the spatial orientation of deformed structures, because the measured seismic anisotropy has a structural origin. Emphasis will be placed on the relations between shear wave splitting and fabric anisotropy, fabric anisotropy and strain, and strain and geologic/tectonic processes. The second part of this paper discusses the effect that metamorphic phase transitions, in crustal and mantle rocks, may have on rock physical properties (velocity, density). Specifically, the possible role that the gabbro/eclogite transition could have in crustal delamination and recycling of material into the mantle will be considered.