Thermodynamics of mantle mineralsI. Physical properties (original) (raw)
Related papers
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.
A mineralogical model for density and elasticity of the Earth's mantle
Geochemistry, Geophysics, Geosystems, 2007
1] We present a thermodynamic model of high-pressure mineralogy that allows the evaluation of phase stability and physical properties for the Earth's mantle. The thermodynamic model is built from previous assessments and experiments in the five-component CFMAS system (CaO-FeO-MgO-Al 2 O 3 -SiO 2 ), including mineral phases that occur close to typical chemical models of the mantle and reasonable mantle temperatures. In this system we have performed a system Gibbs free energy minimization, including pure end-member phases and a nonideal formulation for solid solutions. Solid solutions were subdivided into discrete pseudocompounds and treated as stoichiometric phases during computation of chemical equilibrium by the simplex method. We have complemented the thermodynamic model with a model of shear wave properties to obtain a full description of aggregate elastic properties (density, bulk, and shear moduli) that provide a useful basis for the consideration of seismic and geodynamic models of the Earth's mantle. The thermodynamic model described here is made available for research and training purposes through a Web interface (http://www.earthmodel.org). We examine its validity in light of experiments from mineral physics and briefly discuss inferences for mantle structure.
The physical and chemical composition of the lower mantle
Philosophical Transactions of The Royal Society A: Mathematical, Physical and Engineering Sciences, 2005
This article reviews some of the recent advances made within the field of mineral physics. In order to link the observed seismic and density structures of the lower mantle with a particular mineral composition, knowledge of the thermodynamic properties of the candidate materials is required. Determining which compositional model best matches the observed data is difficult because of the wide variety of possible mineral structures and compositions. State-of-the-art experimental and analytical techniques have pushed forward our knowledge of mineral physics, yet certain properties, such as the elastic properties of lower mantle minerals at high pressures and temperatures, are difficult to determine experimentally and remain elusive. Fortunately, computational techniques are now sufficiently advanced to enable the prediction of these properties in a self-consistent manner, but more results are required.
Temperature and pressure derivatives of elastic constants with application to the mantle
International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, 1989
The temperature and pressure derivatives of the elastic moduli M of solids can be cast into the form of dimensionless logarithmic anharmonic {DLA} parameters, a In M/8 In p = {M} at constant temperature, pressure, or entropy (T, P, S), where p 1s the density. These parameters show little variation from material to mat_erial and_ an'. expected t_o s?ow _little variation with temperature at high temperature. Most of the available denvatlve data for 10mc sohds has been renormalized and analyzed for dependency on 10n type, crystal structure, and other parameters. The {DLA} parameters exhibit little variation and little cNrelation with crystal structure for most close-packed halides and oxides. There are small systematic vanat10ns with 10mc radms, Griineisen's y, and the bulk modulus-rigidity ratio (K/G). Temperature and pressure derwatlves are correlated because of the importance of the volume-dependent, or extrinsic, terms. The mtnns1c terms { K}v and { G}v are also highly correlated, even for open-packed structures, where a In M/aaT = {M}v. These correlations make it possible to estimate the derivatives of highpressure phases. The spine! forms of olivine are predicted to have "normal" derivatives, and therefore the magnitude of the modulus or velocity jump associated with the olivine-spine! transition near 400 km should be similar to that measured in th~ laborat?~Y• The actual size of the 400-km discontinuity is much less, md1_catmg the presence of subst~ntial _quantlhes of mmerals other than olivine in the upper mantle or trans1t10n reg10n. Rece_nt calculat10ns m apparent support of a homogeneous olivine-rich (>60%) mantle are based on choices for the denvatJves of /3and y-Mg 2 Si0 4 , which are unlike other ionic crystals. There is no evidence that these phases should be anomalous in their physical properties. The temperature and pressure derivatives of ionic crystals depend on the nature of the ions and their coordination.
Geophysical Journal International, 2009
The joint interpretation of seismic and geodynamic data requires mineral physical parameters linking seismic velocity to density perturbations in the Earth's mantle. The most common approach is to link velocity and density through relative scaling or conversion factors: R ρ/s = dlnρ/dlnV S . However, the range of possible R ρ/s values remains large even when only considering thermal effects. We directly test the validity of several proposed depthdependent conversion profiles developed from mineral physics studies for thermally-varying properties of mantle materials. The tests are conducted by simultaneously inverting shear wave traveltime data and a diverse suite of convection-related constraints interpreted with viscous-flow response functions of the mantle. These geodynamic constraints are represented by surface spherical harmonic basis functions (up to harmonic degree 16) and they consist of the global free-air gravity field, tectonic plate divergences, dynamic surface topography and the excess ellipticity of the core-mantle boundary (CMB). The tests yield an optimum 1-D thermal R ρ/s profile that is generally compatible with all considered data, with the exception of the dynamic surface topography that is most sensitive to the shallow upper mantle. This result is not surprising given that cratonic roots are known to be compositionally-distinct from the surrounding ambient mantle. Moreover, it is expected that the temperature-dependence of the thermal R ρ/s in the upper mantle is significant due to the temperature-dependence of seismic attenuation or Q. Therefore, a simple 1-D density-velocity relationship is insufficient. To address this problem, we obtained independent estimates of the first-order correction factors to the selected R ρ/s profile within the cratonic roots and in the ambient (thermal) upper mantle. These correction factors, defined as ∂ R ρ/s /∂lnV S , are highly negative within the cratons signifying considerable compositional buoyancy. This result confirms that the negative buoyancy associated with the low temperatures in the cratons is significantly counteracted by the composition-induced positive buoyancy resulting in near-neutral buoyancy of the cratonic roots. Within the ambient upper mantle, the correction factors are found to be positive which is consistent with the expected behaviour of the R ρ/s relationship in thermally-varying upper-mantle material. We obtain a significantly greater reconciliation of the global gravity anomalies and dynamic surface topography when applying these laterally-varying corrections compared to a simple 1-D R ρ/s relationship. Inversion for a fully 3-D R ρ/s relationship reveals secondary effects including additional compositional variation within the cratonic roots and the deep-mantle superplume structures. We estimate the relative magnitude of the thermal and compositional (non-thermal) contributions to mantle density anomalies and conclude that thermal effects dominate shear wave and density heterogeneity throughout the non-cratonic mantle. We also demonstrate the potential pitfalls of scaling a purely seismically-derived model to obtain density rather than performing a true joint inversion to obtain velocity and density simultaneously.
Inferring the thermochemical structure of the upper mantle from seismic data
Geophysical Journal International, 2009
We test a mineral physics model of the upper mantle against seismic observations. The model is based on current knowledge of material properties at high temperatures and pressures. In particular, elastic properties are computed with a recent self-consistent thermodynamic model, based on a six oxides (NCFMAS) system. We focus on average structure between 250 and 800 km. We invert normal modes eigenfrequencies and traveltimes to obtain best-fitting average thermal structures for various compositional profiles. The thermochemical structures are then used to predict long-period waveforms, SS precursors waveforms and radial profiles of attenuation. These examples show the potential of our procedure to refine the interpretation combining different data sets.
Mineral physics in thermo-chemical mantle models
The mantle structures observed by seismic tomography can only be linked with convection models by assuming some relationships between temperature, density and velocity. These relationships are complex and non linear even if the whole mantle has a uniform composition. For example, the density variations are not only related to the depth dependent thermal expansivity and incompressibility, but also to the distribution of the mineralogical phases that are themselves evolving with temperature and pressure. The geochemical observations indicate that the mantle cannot be homogeneous but is composed with various reservoirs of different compositions, although their sizes, origins and topologies are still questionable. In this paper, we present a stoichiometric iterative method to compute the equilibrium mineralogy of mantle assemblages by Gibbs energy minimization. The numerical code can handle arbitrary elemental composition in the system MgO, FeO, CaO, Al O¡ and SiO and reaches the thermodynamic equilibrium by choosing the abundances of 31 minerals belonging to 14 possible phases. The code can deal with complex chemical activities for minerals belonging to solid state solutions. We illustrate our approach by computing the phase diagrams of various compositions with geodynamic interest (pyrolite, harzburgite and oceanic basalt). Our simulations are in reasonable agreement with high pressure and high temperature experiments. We predict that subducted oceanic crust remains significantly denser than normal mantle even near the core mantle boundary. We then provide synthetic tomographic models of slabs. We show that properties computed at thermodynamic equilibrium are significantly different from those computed at fixed mineralogy. Although the accuracy of our results is limited by the uncertainties on the thermodynamic parameters and equations of states of each individual mineral, future geodynamic models will need to include these mineralogical aspects to interpret the tomographic results as well as to explain the geochemical observations.