Superplumes – Past-Perovskite Investigated by First Principles

In 2003-2004 the discovery of a high-pressure phase transition in iron free MgSiO₃ perovskite, the most abundant Earth-forming mineral phase, was reported. In this paper Tsuchiya, Tsuchiya & Wentzcovitch summarise their theoretical and computational studies on this phase transition and on the physical properties of this newly found post-perovskite phase. The theoretical approach is based on density functional theory and on the plane-wave pseudopotential method. They focused on the structural, elastic, vibrational, and thermodynamic properties of the post-perovskite phase. For this transition the predicted Clapeyron slope and several properties of the high-pressure phase strongly suggest that this new phase is an important candidate for a primary constituent in Dʺ.

Introduction

The structure and dynamics of the Earth are dominated by phase transitions of Earth-forming materials (e.g., Helffrich & Wood, 2001). Phase transitions of the constituent minerals can be the cause of major changes in seismic velocity travelling through the mantle. Therefore one of the central problems in the Earth sciences for a long time has been the exploration and investigation of high-pressure phase change in the predominant MgO-FeO-SiO₂ system. The high-pressure Pbnm-perovskite polymorph of MgSiO₃ (pv) (Liu, 1974) is believed to be the most abundant mineral in the lower mantle (Knittle & Jeanloz, 1987). The possibility of a phase transition in this polymorph has been controversial for several years. There have been reports of its dissociation into SiO₂ and MgO at 70-80 GPa and 3,000 K (Mead et al., 1995; Saxena et al., 1996), or possibly a phase change above 83 GPa and 1,700 K (Shim et al., 2001), or no phase transition at all (Fiquet et al., 2000) up to 94 GPa and 2,500 K, have appeared in the literature. None of the pressure-temperature conditions in these experiments achieved the thermodynamic state expected in the Dʺ region, though the pressure-temperature (P-T) conditions in these experiments were quite high. A post-perovskite (ppv) transition in MgSiO₃ has recently been found by in situ X-ray diffraction in a diamond anvil cell at ~2,500 K and ~125 KPa (Murakami et al., 2004). This P-T condition is similar to what is expected in the Dʺ layer near the core-mantle boundary (CMB). First principle variable cell shape calculation and its thermodynamic properties, as obtained by means of quasiharmonic free energy calculations, were used to independently identify the ppv structure (Tsuchiya et al., 2004b; Tsuchiya et al., 2005a). The predicted Clapeyron slope of the ppv transition was ~7.5 ± 0.5 MPa/K, a number that according to the authors is surprisingly close to that claimed to be necessary for a solid-solid transition to account for the Dʺ discontinuity (Sidorin et al., 1999). These results, which have also been supported by another study (Organov and Ono, 2004), suggest that the ppv phase might be the most abundant mineral in the Dʺ region. Mantle convection could be enhanced by the ppv phase transition with this large positive Clapeyron slope (Nakagawa & Tackley, 2004) and superplume generation could also be substantially affected (Matyska & Yuen, 2005). It is therefore essential to understand the ppv transition and the properties of the ppv phase for gaining an understanding of the deep lower mantle, particularly of the Dʺ region (Lay & Helmberger, 1982; Lay et al., 1998; Wysession et al., 1998; Wysession et al., 1999). It is also important to understand seismic velocities and elastic anisotropy at relevant pressures to understand the characteristic velocity discontinuities and anisotropy that are observed in the Dʺ. Tsuchiya et al., summarise their first principles computational results that were obtained, with more results still to come, in particular, the structural, thermodynamic and elastic properties of ppv MgSiO₃.

Conclusions

First principles computations have led to the identification of a new polymorph of MgSiO₃ with the CalrO₃ structure that is more stable than the Pbnm-pv phase. A Clapeyron slope of ~7.5 MPa/K at ~2,750 K and ~125 GPa is given by quasiharmonic high-temperature calculations of the thermodynamic phase boundary. Tsuchiya et al., say these P-T conditions are close to those that were anticipated for the Dʺ region and this Clapeyron slope is close to that anticipated if the Dʺ topography were related to a solid-solid transformation. At relevant pressures thermodynamic properties of ppv are very similar to those of pv, with some of them being indistinguishable. There are particular observables in the deep mantle that have never been explained well, the anticorrelated anomaly in V_S and V_Ф and the positive S wave splitting. This type of anticorrelation can be produced by the ppv transition. The very large S wave splitting can be produced by a transversely isotropic medium of ppv with [001] oriented vertically. It is strongly suggested by these results that the ppv transition might be associated with the Dʺ discontinuity and that this polymorph might be the most abundant phase in Dʺ.

Sources & Further reading

1. Taku Tsuchiya, Jun Tsuchiya, Renata M. Wentzcovitch in Yuen, D.A., Maruyama, S, Karato, Shun-ichiro & Windley, B., (Eds), 2007, Superplumes: Beyond Plate Tectonics, Springer.