Stephen Stackhouse

Efficacy of the post-perovskite phase as an explanation for lowermost-mantle seismic properties

Constraining the chemical, rheological and electromagnetic properties of the lowermost mantle (D″) is important to understand the formation and dynamics of the Earths mantle and core. To explain the origin of the variety of characteristics of this layer observed with seismology, a number of theories have been proposed, including core–mantle interaction, the presence of remnants of subducted material and that D″ is the site of a mineral phase transformation. This final possibility has been rejuvenated by recent evidence for a phase change in MgSiO3 perovskite (thought to be the most prevalent phase in the lower mantle) at near core–mantle boundary temperature and pressure conditions. Here we explore the efficacy of this ‘post-perovskite’ phase to explain the seismic properties of the lowermost mantle through coupled ab initio and seismic modelling of perovskite and post-perovskite polymorphs of MgSiO3, performed at lowermost-mantle temperatures and pressures. We show that a post-perovskite model can explain the topography and location of the D″ discontinuity, apparent differences in compressional- and shear-wave models and the observation of a deeper, weaker discontinuity. Furthermore, our calculations show that the regional variations in lower-mantle shear-wave anisotropy are consistent with the proposed phase change in MgSiO3 perovskite.

On the application of computer simulation techniques to anionic and cationic clays: A materials chemistry perspective

The use of computational methods for the study of clay minerals has become an essential adjunct to experimental techniques for the analysis of these poorly ordered materials. Although information may be obtained through conventional methods of analysis regarding macroscopic properties of clay minerals, information about the spatial arrangement of molecules within the interlayers is hard to obtain without the aid of computer simulation. The interpretation of experimental data from techniques such as solid-state nuclear magnetic resonance or neutron diffraction studies is considerably assisted by the application of computer simulations. Using a series of case studies, we review the techniques, applications and insight gained from the use of molecular simulation applied to the study of clay systems (particularly for materials applications). The amount of information that can be gleaned from such simulations continues to grow, and is leading to ever larger-scale and hence more realistic classical and quantum mechanical studies which promise to reveal new and unexpected phenomena.

Elastic anisotropy of FeSiO3 end‐members of the perovskite and post‐perovskite phases

The athermal elastic constants of the perovskite and post-perovskite polymorphs of pure end-member FeSiO3 were calculated from ab initio calculations. We predict that incorporating ten mole percent FeSiO3 together with four mole percent Al2O3 into MgSiO3 reduces the perovskite to post-perovskite phase transition pressure by 5 GPa. Small changes in the seismic properties of the post-perovskite phase due to the incorporation of iron and alumina are compatible with observations for the lower mantle. MgSiO3 post-perovskite enriched in fifty percent or more iron may be responsible for ultra-low velocity zones at the base of the mantle.

First-principles calculation of the elastic moduli of sheet silicates and their application to shale anisotropy

Abstract The full elastic tensors of the sheet silicates muscovite, illite-smectite, kaolinite, dickite, and nacrite have been derived with first-principles calculations based on density functional theory. For muscovite, there is excellent agreement between calculated properties and experimental results. The influence of cation disorder was investigated and found to be minimal. On the other hand, stacking disorder is found to be of some relevance for kaolin minerals. The corresponding single-crystal seismic wave velocities were also derived for each phase. These revealed that kaolin minerals exhibit a distinct type of seismic anisotropy, which we relate to hydrogen bonding. The elastic properties of a shale aggregate was predicted by averaging the calculated properties of the contributing mineral phases over their orientation distributions. Calculated elastic properties display higher stiffness and lower p-wave anisotropy. The difference is likely due to the presence of oriented flattened pores in natural samples that are not taken into account in the averaging.

Shear-induced material transfer across the core-mantle boundary aided by the post-perovskite phase transition

We present a novel mechanical model for the extraction of outer core material upwards across the CMB into the mantle side region of D” and subsequent interaction with the post-perovskite (ppv) phase transition. A strong requirement of the model is that the D” region behaves as a poro-viscoelastic granular material with dilatant properties. Using new ab-initio estimates of the ppv shear modulus, we show how shear-enhanced dilation promoted by downwelling mantle sets up an instability that drives local fluid flow. If loading rates locally exceed c. 10−12 s−1, calculated core metal upwelling rates are >10−4 m/s, far in excess of previous estimates based on static percolation or capillary flow. Associated mass flux rates are sufficient to deliver 0.5% outer core mass to D” in < 106 yr, provided the minimum required loading rate is maintained. Core metal transported upwards into D” may cause local rapid changes in electrical and thermal conductivity and rheology that if preserved, may account for some of the observed small wavelength heterogeneties (e.g. PKP scattering) there.

High-pressure, temperature elasticity of Fe- and Al-bearing MgSiO3: implications for the Earth’s lower mantle

Fe and Al are two of the most important rock-forming elements other than Mg, Si, and O. Their presence in the lower mantles most abundant minerals, MgSiO3 bridgmanite, MgSiO3 post-perovskite and MgO periclase, alters their elastic properties. However, knowledge on the thermoelasticity of Fe- and Al-bearing MgSiO3 bridgmanite, and post-perovskite is scarce. In this study, we perform ab initio molecular dynamics to calculate the elastic and seismic properties of pure, Fe3+- and Fe2+-, and Al3+-bearing MgSiO3 perovskite and post-perovskite, over a wide range of pressures, temperatures, and Fe/Al compositions. Our results show that a mineral assemblage resembling pyrolite fits a 1D seismological model well, down to, at least, a few hundred kilometers above the core�mantle boundary, i.e. the top of the D � region. In D � , a similar composition is still an excellent fit to the average velocities and fairly approximate to the density. We also implement polycrystal plasticity with a geodynamic model to predict resulting seismic anisotropy, and find post-perovskite with predominant (001) slip across all compositions agrees best with seismic observations in the D � .

Equations of state and stability of MgSiO3 perovskite and post-perovskite phases from quantum Monte Carlo simulations

We have performed quantum Monte Carlo (QMC) simulations and density functional theory calculations to study the equations of state of MgSiO3 perovskite (Pv, bridgmanite) and post-perovskite (PPv) up to the pressure and temperature conditions of the base of Earths lower mantle. The ground-state energies were derived using QMC simulations and the temperature-dependent Helmholtz free energies were calculated within the quasiharmonic approximation and density functional perturbation theory. The equations of state for both phases of MgSiO3 agree well with experiments, and better than those from generalized gradient approximation calculations. The Pv-PPv phase boundary calculated from our QMC equations of state is also consistent with experiments, and better than previous local density approximation calculations. We discuss the implications for double crossing of the Pv-PPv boundary in the Earth.

Mineral physics: The spin deep within

The electronic configuration of iron impurities in lower-mantle minerals influences their physical properties, but it is not well constrained. New studies suggest that ferrous iron in silicate phases exists mainly in an intermediate spin state.

The spin deep within

The electronic configuration of iron impurities in lower-mantle minerals influences their physical properties, but it is not well constrained. New studies suggest that ferrous iron in silicate phases exists mainly in an intermediate spin state.

The rational design, synthesis and demonstration of the recognition and binding of a diaza-dioxa-12-crown-4 diphosphonate macrocycle to all crystal growth faces of barium sulfate

Computer-aided molecular design and virtual screening of a series of amino phosphonic acid derivatives were used to probe the probable interaction of these compounds as potential crystal growth inhibitors of barium sulfate, as judged by their ability to bind efficiently to all of the possible growing faces. As a result, a diphosphonic acid derivative of a 1,7-dioxa-4,10-diaza-12-crown-4 system 5 was proposed as a potential inhibitor of barium sulfate crystallisation. A subsequent synthesis of this macrocycle was developed, together with other larger-ring oxa-aza crown derivatives. Macrocycle 5 proved to be a highly efficient inhibitor of barium sulfate crystal growth at a level of 0.096 mM, as evidenced by the changes brought about in crystal morphology. Work was therefore undertaken to probe the mechanism of action of 5 using adsorption isotherms, mixed flow reactor and atomic force microscopy (AFM) measurements. It was possible to show that 5 inhibits effectively in solution by covering the growing surfaces, as observed on the 001 surface, effectively inhibiting two-dimensional nucleation as well as monolayer-step growth.