Composition Of Lower Mantle Research Articles

The Composition of Earth's Lower Mantle

Determining the composition of Earth's lower mantle, which constitutes almost half of its total volume, has been a central goal in the Earth sciences for more than a century given the constraints it places on Earth's origin and evolution. However, whether the major element chemistry of the lower mantle, in the form of, e.g., Mg/Si ratio, is similar to or different from the upper mantle remains debated. Here we use a multidisciplinary approach to address the question of the composition of Earth's lower mantle and, in turn, that of bulk silicate Earth (crust and mantle) by considering the evidence provided by geochemistry, geophysics, mineral physics, and geodynamics. Geochemical and geodynamical evidence largely agrees, indicating a lower-mantle molar Mg/Si of ≥1.12 (≥1.15 for bulk silicate Earth), consistent with the rock record and accumulating evidence for whole-mantle stirring. However, mineral physics–informed profiles of seismic properties, based on a lower mantle made of bridgmanite and ferropericlase, point to Mg/Si ∼ 0.9–1.0 when compared with radial seismic reference models. This highlights the importance of considering the presence of additional minerals (e.g., calcium-perovskite and stishovite) and possibly suggests a lower mantle varying compositionally with depth. In closing, we discuss how we can improve our understanding of lower-mantle and bulk silicate Earth composition, including its impact on the light element budget of the core. ▪The chemical composition of Earth's lower mantle is indispensable for understanding its origin and evolution.▪Earth's lower-mantle composition is reviewed from an integrated mineral physics, geophysical, geochemical, and geodynamical perspective.▪A lower-mantle molar Mg/Si of ≥1.12 is favored but not unique.▪New experiments investigating compositional effects of bridgmanite and ferropericlase elasticity are needed to further our insight.

Discriminating lower mantle composition

Constraining Earth's bulk composition is fundamental to understanding our planet's formation and evolution. While the lower mantle accounts for a majority of the bulk silicate Earth, it is also the least accessible. As experimental and theoretical mineral physics constraints on mineral elasticity at lower mantle temperatures and pressures have improved, comparisons between predicted seismic velocity and density profiles for hypothesized bulk compositions and 1D seismic models have become commonplace. However, the degree to which a given composition is a better or worse fit than another composition is not always reported, nor are the influences of the assumed temperature profile and other uncertainties discussed. Here we compare seismic velocities and densities for perovskitite, pyrolite, and harzburgite bulk compositions calculated using advanced ab initio techniques to explore the extent to which the associated uncertainties affect our ability to distinguish between candidate compositions. We find that predicted differences between model compositions are often smaller than the influence of temperature uncertainties and therefore these comparisons lack discriminatory power. The inability to distinguish between compositions is largely due to the high sensitivity of seismic properties to temperature accompanied by uncertainties in the mantle geotherm, coupled with diminished sensitivity of seismic velocity to composition toward the base of the mantle. An important exception is the spin transition in (Mg,Fe)O-ferropericlase, which is predicted to give a distinct variation in compressional wave velocity that should distinguish between relatively ferro-magnesian and silica-rich compositions. However, the absence of an apparent spin transition signature in global 1D seismic profiles is a significant unresolved issue in geophysics, and it has important geochemical implications. The approach we present here for establishing discriminatory power for such comparisons can be applied to any estimate of seismic velocities and associated uncertainties, and offers a straightforward tool to evaluate the robustness of model comparisons.

Thermal and magnetic evolution of a crystallizing basal magma ocean in Earth's mantle

We present the thermochemical evolution of a downward crystallizing basal magma ocean (BMO) overlying the liquid outer core and probe its capability to dissipate enough power to generate and sustain an early dynamo. A total of 61 out of 112 scenarios for a BMO with imposed, present-day heat flux, QBMO, values of 15, 18, and 21 TW and radiogenic heat, Qr, values of 4, 8, and 12 TW fully crystallized during the age of the Earth. Most of these models are energetically capable of inducing magnetic activity for the first 1.5 Gyrs, at least, with durations extending to 2.5 Gyrs; with final core-mantle boundary (CMB) temperatures of 4400 ± 500 K -well within current best estimates for inferred temperatures. None of the models with QBMO = 12 TW achieved a fully crystallized state, which may reflect a lower bound on the present-day heat flux across the CMB. BMO-powered dynamos exhibit strong dependence on the partition coefficient of iron into the liquid layer and its associated melting-point depression for a lower mantle composition at near-CMB conditions -parameters which are poorly constrained to date. Nonetheless, we show that a crystallizing BMO is a plausible mechanism to sustain an early magnetic field.

Electronic spin transition in FeO2 : Evidence for Fe(II) with peroxide O22−

The discovery of FeO$_{2}$ containing more oxygen than hematite (Fe$_{2}$O$_{3}$) that was previously believed to be the most oxygen rich iron compounds, has important implications on the study of the deep lower mantle compositions. Compared to other iron compounds, there are limited reports on FeO$_{2}$ making studies of its physical properties of great interest in fundamental condensed matter physics and geoscience. Even the oxidation state of Fe in FeO$_{2}$ is the subject of debate in theoretical works and there have not been reports from experimental electronic and magnetic properties measurements. Here, we report the pressure-induced spin state transition from synchrotron experiments and our computational results explain the underlying mechanism. Using density functional theory and dynamical mean field theory, we calculated spin states of Fe with volume and Hubbard interaction $U$ change, which clearly demonstrate that Fe in FeO$_{2}$ consists of Fe(II) and peroxide O$_{2}^{2-}$. Our study suggests that localized nature of both Fe 3$d$ orbitals and O$_{2}$ molecular orbitals should be correctly treated for unveiling the structural and electronic properties of FeO$_{2}$.

Confirming a pyrolitic lower mantle using self‐consistent pressure scales and new constraints on CaSiO3 perovskite

AbstractIn this study, we have examined the lower mantle composition and mineralogy by modeling the density (ρ), bulk sound velocity (VΦ), and dlnρ/dlnVΦ profiles of candidate lower mantle minerals using literature and new experimental equation of state (EoS) results. For CaSiO3 perovskite, complimentary synchrotron X‐ray diffraction measurements in a laser‐heated diamond anvil cell were conducted up to 156 GPa between 1200 K and 2600 K to provide more reliable constraints on the thermal EoS parameters. These new experimental results as well as literature P‐V‐T data sets are systematically analyzed using an internally self‐consistent pressure scale. We have modeled ρ, VΦ, and dlnρ/dlnVΦ profiles of the lower mantle with representative pyrolitic and chondritic compositional models in which the effect of Fe spin transition in ferropericlase is also taken into account. Our modeling results show that a pyrolitic lower mantle with an aggregate mineralogy of 75 vol % bridgmanite, 17 vol % ferropericlase, and 8 vol % CaSiO3 perovskite produces ρ and VΦ profiles in better agreement with preliminary reference Earth model than a lower mantle with a chondritic composition. The modeled ρ, VΦ, and dlnρ/dlnVΦ are mainly affected by the relative ratio of bridgmanite and ferropericlase but are not sensitive to the variation of the CaSiO3 perovskite content. In addition, the spin crossover of Fe in ferropericlase can greatly raise the value of dlnρ/dlnVΦ in the middle lower mantle, which is useful in detecting the presence of ferropericlase in the region. Based on these new mineral physical constraints and radial seismic structure, our study suggests the lower mantle is pyrolitic, which is chemically indistinguishable from the upper mantle.

Velocity structure and composition of the lower mantle with spin crossover in ferropericlase

AbstractThe composition of the Earth's lower mantle (LM) is critical in understanding the Earth's interior and dynamics. Previous reports on the composition of the LM are controversial. The composition of LM, constrained here using high‐temperature and high‐pressure data of the velocities and density of minerals from first‐principles calculations, spans a large range: from pyrolite with ~ 15% ferropericlase (Fp) in weight (wt %) to perovskitic‐rich composition with ~ 10 wt % Fp. Any composition well constrained by preliminary reference Earth model (PREM) has a sufficient amount of Fp to exhibit the positive temperature dependence of VΦ (= ) at the middle LM, leading to the insensitivity of VP to temperature. The depth at which VP is insensitive to temperature variation deepens significantly with increasing temperature because the spin transition in Fp has the positive Clapeyron slope (~17 MPa/K). The Large Low Shear Velocity Provinces below Africa is estimated to be ~ 750 K higher than the ambient mantle.

BurnMan: A lower mantle mineral physics toolkit

We present BurnMan, an open-source mineral physics toolbox to determine elastic properties for specified compositions in the lower mantle by solving an Equation of State (EoS). The toolbox, written in Python, can be used to evaluate seismic velocities of new mineral physics data or geodynamic models, and as the forward model in inversions for mantle composition. The user can define the composition from a list of minerals provided for the lower mantle or easily include their own. BurnMan provides choices in methodology, both for the EoS and for the multiphase averaging scheme. The results can be visually or quantitatively compared to observed seismic models. Example user scripts show how to go through these steps. This paper includes several examples realized with BurnMan: First, we benchmark the computations to check for correctness. Second, we exemplify two pitfalls in EoS modeling: using a different EoS than the one used to derive the mineral physical parameters or using an incorrect averaging scheme. Both pitfalls have led to incorrect conclusions on lower mantle composition and temperature in the literature. We further illustrate that fitting elastic velocities separately or jointly leads to different Mg/Si ratios for the lower mantle. However, we find that, within mineral physical uncertainties, a pyrolitic composition can match PREM very well. Finally, we find that uncertainties on specific input parameters result in a considerable amount of variation in both magnitude and gradient of the seismic velocities.

Thermal conductivity of MgO, MgSiO3 perovskite and post-perovskite in the Earth's deep mantle

We report lattice thermal conductivities of MgO and MgSiO3 in the perovskite and post-perovskite structures at conditions of the Earth's lower mantle, obtained from equilibrium molecular dynamics simulations. Using an advanced ionic interaction potential, the full conductivity tensor was calculated by means of the Green–Kubo method, and the conductivity of MgSiO3 post-perovskite was found to be significantly anisotropic. The thermal conductivities of all three phases were parameterized as a function of density and temperature. Assuming a Fe-free lower-mantle composition with mole fractions xMgSiO3=0.66 and xMgO=0.34, the conductivity of the two-phase aggregate was calculated along a model geotherm. It was found to vary considerably with depth, rising from 9.5W/(mK) at the top of the lower mantle to 20.5W/(mK) at the top of the thermal boundary layer above the core–mantle boundary. Extrapolation of experimental data suggests that at deep-mantle conditions, the presence of a realistic amount of iron impurities lowers the thermal conductivity of the aggregate by about 50% (Manthilake et al., 2011a). From this result and our thermal conductivity model, we estimate the heat flux across the core–mantle boundary to be 10.8TW for a Fe-bearing MgO/MgSiO3 perovskite aggregate and 10.6TW for a Fe-bearing MgO/MgSiO3 post-perovskite aggregate.

The thermo-chemical and physical structure beneath the North American continent from Bayesian inversion of surface-wave phase velocities

[1] We jointly invert local fundamental-mode and higher-order surface-wave phase-velocities for radial models of the thermo-chemical and anisotropic physical structure of the Earth's mantle to ∼1000 km depth beneath the North American continent. Inversion for thermo-chemical state relies on a self-consistent thermodynamic method whereby phase equilibria and physical properties (P-, S-wave velocity and density) are computed as functions of composition (in the Na2O-CaO-FeO-MgO-Al2O3-SiO2 model system), pressure and temperature. We employ a sampling-based strategy to solve the non-linear inverse problem relying on a Markov Chain Monte Carlo method to sample the posterior distribution in the model space. A range of models fitting the observations within uncertainties are obtained from which any statistics can be estimated. To further refine sampled models we compute geoid anomalies for a collection of these and compare with observations, exemplifying a posteriori filtering through the use of additional data. Our thermo-chemical maps reveal the tectonically stable older eastern parts of North America to be chemically depleted (high Mg#) and colder (>200°C) relative to the active younger regions (western margin and oceans). In the transition zone the thermo-chemical structure decouples from that of the upper mantle, with a relatively hot thermal anomaly appearing beneath the cratonic area that likely extends into the lower mantle. In the lower mantle no consistent large-scale thermo-chemical heterogeneities are observed, although our results do suggest distinct upper and lower mantle compositions. Concerning anisotropy structure, we find evidence for a number of distinct anisotropic layers pervading the mantle, including transition zone and upper-most lower mantle.

Density profile of pyrolite under the lower mantle conditions

The pyrolite model is one of the possible compositions of the Earth's lower mantle. The lower mantle's composition is generally modelled by comparing seismic observations with mineral physics data of possible lower mantle end‐member phases. Here, we report the compression behavior of a natural KLB‐1 peridotite (a representative composition of the pyrolite model) in a quasi‐hydrostatic environment at simultaneous high pressure (P) and temperature (T), covering the entire range of lower mantle P‐T conditions up to 112 GPa. This is the first experimentally determined density profile of pyrolite under the lower mantle conditions. The results allow us to directly compare the measured density of peridotite mantle along the geotherm with the Preliminary Reference Earth Model (PREM) derived from seismic observations, without extrapolation. The comparison shows significant mismatch between the two, which calls for a re‐evaluation of the PREM density model or a non‐pyrolite lower mantle composition.

Elasticity of MgO to 130 GPa: Implications for lower mantle mineralogy

The aggregate shear wave velocities of MgO (periclase) have been determined throughout Earth's lower mantle pressure regime approaching 130 GPa using Brillouin spectroscopy in conjunction with synchrotron X-ray diffraction technique in a diamond anvil cell apparatus. We found that the extrapolations of the high-pressure shear wave velocities and shear moduli to ambient pressure are highly consistent with earlier studies. However, the measurements over a wide pressure range revealed that the pressure derivative of the shear modulus (d G/d P = G 0′) of MgO is 1.92(2), which is distinctly lower than that of previous lower-pressure experiments. Compared with the previous results on (Mg,Fe)O ferropericlase, there is no clear correlation between iron content and G 0′. We calculate that the shear wave velocity profile of lower mantle along the adiabatic geotherm applied by the lower G 0′ value of periclase can remarkably well reproduce the global seismological 1-D velocity profile model with uniform composition model. The best-fitting result indicates the possibility of a lower mantle mineralogy with ~ 92 vol.% silicate perovskite phase, implying that the bulk composition of lower mantle is likely not to be pyrolitic but more chondritic. The present acoustic measurements performed over the large pressure range have thus led us to a better understanding of compositional model of the Earth's lower mantle.

On the bulk composition of the lower mantle: predictions and limitations from generalized inversion of radial seismic profiles

SUMMARY We examine the problem of obtaining the thermal structure and bulk chemical composition of the lower mantle from its seismologically determined velocity and density profiles, and the most recent results on the elastic properties of the relevant phases (including, of particular importance, shear moduli). A novel aspect of this paper is the application of an iterative technique solving generalized non-linear inverse problem, which allows us to simultaneously consider a complex chemical system (the MgO‐FeO‐SiO2‐Al2O3‐CaO system, which includes all major components in the lower mantle), and to rigorously evaluate the full covariance and resolution matrices. The effects of experimental uncertainties in the shear moduli are carefully accounted for. We show that although the a posteriori uncertainties in the results for lower-mantle compositions are relatively large, the averaged lower-mantle Mg/Si ratio should be lower than 1.3 in order to satisfactorily fit the 1-D seismic profiles. Two distinct families of best-fitting models are determined. The first is based upon a value for the pressure derivative of the perovskite shear modulus that is representative of various existing experimental measurements (µ � = 1.8). Under this assumption, it is not possible to match the lower mantle seismic properties with an adiabatic geotherm and uniform chemical composition. Instead, this family of solutions is characterized by a geotherm with large temperature gradients (dT/dz increases from 0.5 to 0.9 K km −1 between 800 and 2700 km and the temperature reaches 3400 K at the depth of 2700 km), and a depth dependent bulk composition with an Mg/Si ratio decreasing from 1.18 ± 0.14 to 1.03 ± 0.16 between 800 and 2700 km. The second family of solutions is obtained when we attempt to fit the lower mantle with a simpler compositional and thermal structure. This can only be done when the pressure derivative of the shear modulus for perovskite is close to the most recent values obtained by Brillouin spectroscopy, that is, with a µ � close to 1.6 instead of 1.8. The resulting temperature gradient is 0.25 K km −1 in the upper part of the lower mantle and 0.5 K km −1 below 1700 km depth; the geotherm reaches 2800 K at a depth of 2700 km. Corresponding Mg/Si ratio remains rather constant and close to 1.16 throughout the lower mantle. We show that the temperature gradient is strongly correlated with the pressure derivative µ � of the shear modulus of perovskite: lower values of µ � imply lower thermal gradients. We also discuss the importance of the Bullen parameter as an additional constraint. In order to refine conclusions on the lower-mantle structure, additional independent observables, such as accurate observations on electrical conductivity and 1-D Q profiles, are necessary.

P– V– T equation of state of MgSiO 3 perovskite and MgO periclase: Implication for lower mantle composition

The P– V– T equation of state (EOS) for magnesium silicate perovskite (MgSiO 3) and periclase (MgO) was obtained by fitting measured P, V, T data. The EOS of MgSiO 3 perovskite was determined by applying the Mie–Grüneisen–Debye approach, based on three alternative bulk modulus values ( K T0 = 240, 250 and 261 GPa), yielding substantially different values for high-order thermoelastic parameters such as the Grüneisen parameter ( γ) and the temperature derivative of bulk modulus ((∂ K T /∂ T) P ). Combined with the EOS of magnesiowüstite, we calculated the density ( ρ), bulk modulus ( K S) and seismic parameter ( ϕ) for simplified pyrolitic lower mantle along a plausible adiabat (temperature at 670 km, T a = 1900 K), and compared these values to those of a seismological model (PREM). The results based on K T0 of 261 GPa, which has been commonly used, showed good agreement with PREM, within 0.5% for ρ, and 2–3% for K S and ϕ, throughout the lower mantle. The EOS obtained using the recent K T0 data (240–250 GPa), substantially overestimated K S and ϕ by 5–10%, requiring a progressive enrichment in (Mg,Fe)O with increasing depth. In addition, a more realistic vibrational density of states (VDOS) model (Kieffer-type model) was applied to calculate the EOS. The effect of VDOS on thermoelastic parameters was not large enough to modify those conclusions.

Lower mantle composition and temperature from mineral physics and thermodynamic modelling

SUMMARY A generalized inverse method is applied to infer the radial lower mantle composition and temperature profile from seismological models of density and bulk sound velocity. The computations are performed for a five-component system, MgO‐FeO‐CaO‐Al2O3‐SiO2 and three phases: (Mg,Fe,Al)(Si,Al)O3 perovskite, (Mg,Fe)O magnesiowustite, and CaSiO3 perovskite. A detailed review of the elasticity data set used to compute the elastic properties of mineral assemblages is given. We consider three different a priori compositional models—pyrolite, chondritic and a model based on cosmic abundances of elements—as a priori knowledge for the inversions in order to investigate the sensitivity of any given best-fit solution to the assumed initial composition. Consistent features in all inversions, independent of the a priori model, are a total iron content of X Fe � 0.10 ± 0.06 and a subadiabatic temperature gradient over most of the lower mantle depth range. A peculiar correlated behaviour of the two most sensitive parameters (iron content and temperature) is found below the 660 km discontinuity: over the depth range from 660 km down to 1300 km. Significantly, we find that the bulk composition inferred from any given inversion is strongly dependent on the choice of a priori model. Equally satisfactory fits to the lower mantle bulk sound velocity and density profiles can be obtained using any of the a priori models. However, the thermal structure associated with these compositional models differs significantly. Pyrolite yields a relatively cool geotherm (T 660 � 1800 K and X Pv � 0.64), while perovskite-rich models such as chondritic or cosmic models yield hot geotherms (T 660 � 2500 K and X Pv � 0.84 for the latter), but all of the geotherms are subadiabatic. The results of inversions are virtually unaffected by the partitioning of iron between perovskite and magnesiowustite. Out of the five oxide components considered in our models, the bulk Al2O3 and CaO contents of the mineral assemblages are least well constrained from our inversions. Our results show that a major shortcoming of lower mantle compositional and thermal models based on inversions of bulk sound velocity and density is the strong dependence of the final solution on the a priori model. That is, a wide variety of best-fit compositional and thermal models can be obtained, all of which provide satisfactory fits to global average seismic models. It is, in fact, this non-uniqueness that dominates the resulting a posteriori uncertainties and prevents a clear discrimination between different compositional models. Independent constraints on the thermal structure or on the shear properties of lower mantle assemblages are needed to infer lower mantle composition with a higher degree of certainty.

Shear waves in the diamond-anvil cell reveal pressure-induced instability in (Mg,Fe)O.

The emerging picture of Earth's deep interior from seismic tomography indicates more complexity than previously thought. The presence of lateral anisotropy and heterogeneity in Earth's mantle highlights the need for fully anisotropic elasticity data from mineral physics. A breakthrough in high-frequency (gigahertz) ultrasound has resulted in transmission of pure-mode elastic shear waves into a high-pressure diamond-anvil cell using a P-to-S elastic-wave conversion. The full elastic tensor (c(ij)) of high-pressure minerals or metals can be measured at extreme conditions without optical constraints. Here we report the effects of pressure and composition on shear-wave velocities in the major lower-mantle oxide, magnesiowüstite-(Mg,Fe)O. Magnesiowüstite containing more than approximately 50% iron exhibits pressure-induced c(44) shear-mode softening, indicating an instability in the rocksalt structure. The oxide closer to expected lower-mantle compositions ( approximately 20% iron) shows increasing shear velocities more similar to MgO, indicating that it also should have a wide pressure-stability field. A complete sign reversal in the c(44) pressure derivative points to a change in the topology of the (Mg,Fe)O phase diagram at approximately 50-60% iron. The relative stability of Mg-rich (Mg,Fe)O and the strong compositional dependence of shear-wave velocities (and partial differential c(44)/ partial differential P) in (Mg,Fe)O implies that seismic heterogeneity in Earth's lower mantle may result from compositional variations rather than phase changes in (Mg,Fe)O.

Constraints on lower mantle composition from molecular dynamics simulations of MgSiO3 perovskite

Abstract We have carried out molecular dynamics simulations of MgSiO 3 perovskite (pv) in order to determine its equation of state and elastic parameters under pressure and temperature conditions of the Earth’s lower mantle. Combining this information with density and elastic constants for (Mg,Fe)O magnesiowustite (mw) and including the effects of iron on the density of perovskite, we investigated possible compositional models for the lower mantle by calculating seismic velocities and densities as functions of depth and comparing them to the globally averaged seismological models ak135-f and PREM. The calculations were done along several possible geothermal gradients, both adiabatic and non-adiabatic, and for a range of Mg–Fe partitioning coeffcients, K Fe–Mg pv–mw =0.10–0.35. Our results indicate that the most probable compositional models for the lower mantle are pyrolitic, with X Mg ≈0.87 and X pv ≈0.54. For increasing temperatures at the top of the lower mantle, the best fitting models increase slightly in both iron and silica content.

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 Å3, 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 Å3 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 % magnesiowüstite (XMg = 0.82–0.86), however, we obtained density and velocity profiles that are in excellent agreement with seismological models for a reasonable geotherm.

Silicate perovskite analogue ScAlO 3: temperature dependence of elastic moduli

The elastic moduli of ScAlO 3 perovskite, a very close structural analogue for MgSiO 3 perovskite, have been measured between 300 and 600 K using high precision ultrasonic interferometry in an internally heated gas-charged pressure vessel. This new capability for high temperature measurement of elastic wave speeds has been demonstrated on polycrystalline alumina. The temperature derivatives of elastic moduli of Al 2O 3 measured in this study agree within 15% with expectations based on published single-crystal data. For ScAlO 3 perovskite, the value of (∂ K S/∂ T) P is −0.033 GPa K −1 and (∂ G/∂ T) P is −0.015 GPa K −1. The relative magnitudes of these derivatives agree with the observation in Duffy and Anderson [Duffy, T.S., Anderson, D.L., 1989. Seismic velocities in mantle minerals and the mineralogy of the upper mantle. J. Geophys. Res. 94, 1895–1912.] that |(∂ K S/∂ T) P| is typically about twice |(∂ G/∂ T) P|. The value of (∂ K S/∂ T) P for ScAlO 3 is intermediate between those inferred less directly from V( P, T) studies of Fe-free and Fe- and Al-bearing MgSiO 3 perovskites [Wang, Y., Weidner, D.J., Liebermann, R.C., Zhao, Y., 1994. P– V– T equation of state of (Mg,Fe)SiO 3 perovskite: constraints on composition of the lower mantle. Phys. Earth Planet. Inter. 83, 13–40; Mao, H.K., Hemley, R.J., Shu, J., Chen, L., Jephcoat, A.P., Wu, Y., Bassett, W.A., 1991. Effect of pressure, temperature and composition on the lattice parameters and density of (Mg,Fe) SiO 3 perovskite to 30 GPa. J. Geophys. Res. 91, 8069–8079; Zhang, Weidner, D., 1999. Thermal equation of state of aluminum-enriched silicate perovskite. Science 284, 782–784]. The value of |(∂ G/∂ T)| P for ScAlO 3 is similar to those of most other mantle silicate phases but lower than the recent determination for MgSiO 3 perovskite [Sinelnikov, Y., Chen, G., Neuville, D.R., Vaughan, M.T., Liebermann, R.C., 1998. Ultrasonic shear wave velocities of MgSiO 3 perovskite at 8 GPa and 800K and lower mantle composition. Science 281, 677–679]. Combining the results from the previous studies and current measurements on ScAlO 3 perovskite, we extracted the parameters ( q and γ 0) needed to fully specify its Mie–Grüneisen–Debye equation-of-state. In this study, we have demonstrated that acoustic measurements of K S( T), unlike V( P, T) data, tightly constrain the value of q. It is concluded that ScAlO 3 has ‘normal’ γ 0 (∼1.3) and high q (∼3.6). The high value of q indicates that ScAlO 3 has very strong intrinsic temperature dependence of the bulk modulus; similar behaviour has been observed in measurements on Fe- and Al-bearing silicate perovskites (Mao et al., 1991; Zhang and Weidner, 1999).

Sound wave velocities and elastic constants for Magnesiowüstite using gigahertz interferometry

Ultrasonic interferometry at near‐gigahertz frequencies is used to investigate longitudinal sound wave velocities parallel to [100] and [111] and the elastic constant c11 for high quality single crystals of magnesiowüstite (Mg,Fe)O. The measurements were made at room pressure and temperature on samples spanning the full compositional range from periclase (MgO) to wüstite (Fe1‐xO). A strong P‐wave anisotropy of ∼10% was measured for periclase, increasing about 1% for Mg0.94Fe0.06O then decreasing linearly to ∼0% for wüstite. The elastic modulus c11 of magnesiowüstite decreases linearly with iron content by −0.72(3) GPa/mol% FeO with the exception of a sharp increase for pure periclase. Thus, the addition of a small amount of iron to periclase has a major effect on its elastic behavior. This observation indicates that the use of a simple interpolation of elastic constant data between periclase and wüstite [Jackson, 1998] may yield erroneous predictions for lower mantle compositions based on seismic data.

Ultrasonic shear wave velocities of MgSiO3 perovskite at 8 GPa and 800 K and lower mantle composition

Ultrasonic interferometric measurements of the shear elastic properties of MgSiO3 perovskite were conducted on three polycrystalline specimens at conditions up to pressures of 8 gigapascals and temperatures of 800 kelvin. The acoustic measurements produced the pressure (P) and temperature (T) derivatives of the shear modulus (G), namely ( partial differentialG/ partial differentialP)T = 1.8 +/- 0.4 and ( partial differentialG/ partial differentialT)P = -2.9 +/- 0.3 x 10(-2) gigapascals per kelvin. Combining these derivatives with the derivatives that were measured for the bulk modulus and thermal expansion of MgSiO3 perovskite provided data that suggest lower mantle compositions between pyrolite and C1 carbonaceous chondrite and a lower mantle potential temperature of 1500 +/- 200 kelvin.