Effect of Al on the sharpness of the MgSiO_3 perovskite to post-perovskite phase transition

By means of static ab-initio computations we investigate the influence of Al on the recently discovered perovskite to post-perovskite phase transition in MgSiO_3. We examine three substitution mechanisms for Al in the two structures: MgSi → AlAl; SiSiO → AlAl□; and Si → AlH. The substitutions introducing oxygen vacancies (highly unfavorable, energetically) and water (favorable) both lower the 0 Kelvin transition pressure, whereas charge coupled substitution increases it relative to 105 GPa for pure MgSiO_3. From the transition pressures for 0, 6.25, and 100 mol% charge coupled Al_2O_3 incorporation and simple solution theories, we estimate the phase diagram of Al-bearing MgSiO_3 at low Al concentrations. Assuming the Clapeyron slope is independent of Al concentration, we find the perovskite-to-post-perovskite transition region to span 127–140 GPa, at 6.25 mol% Al_2O_3. When the upper pressure limit is bounded by the core-mantle boundary, the phase coexistence region has width 150 km

Water solubility in octahedrally-coordinated silicates

The incorporation of water in nominally anhydrous silicates is associated with mechanical weakening, lowered melting points, and can serve as a significant volatile reservoir in the planet. We have performed first-principles calculations addressing the substitution mechanism of H_2O in octahedrally coordinated silicates. The solubility of H_2O in these silicates results from combining static calculations with a statistical-mechanical model for the entropy of solution. We address the H_2O solubility in three silicates: stishovite (SiO_2), Mg- and Ca- pervoskite, with H solution mechanisms constrained by charge balance in the structure. In the CMASH system, the only stable solution mechanisms are Al+H = Si or 2H=Ca or Mg

On the sharpness of the perovskite/post-perovskite transition in the Earth's mantle

The phase transition of pure MgSiO_3 perovskite (Pbnm) to the post-perovskite (Cmcm) structure has been recently reported to occur at pressures and temperatures corresponding to the Earth’s lowermost mantle [Murakami et al. 2004, Tsuchiya et al. 2004, Oganov and Ono 2004]. We use ab initio calculations to assess whether this transition survives, in the Earth, for more realistic mantle compositions containing Al, Fe^(2+), and Fe^(3+). We estimate phase coexistence pressures as functions of minor element concentration, and from this we obtain the effects of Al and Fe on the depth and sharpness of the transition. For a pyrolitic mantle composition, with all the Al partitioned into MgSiO_3, we find that Al preferentially partitions into perovskite, and increases the transition pressure by approximately 5 GPa. The transition takes place over a depth range of width 225 km. Fe competes with Al by lowering the transition pressure [Mao et al. 2004], so that post-perovskite is likely to exist in the lower mantle. However, the transition is still smooth, and not likely to explain the sharp discontinuities observed seismically at the base of the mantle. Our results suggest the geodynamical implications of the post-perovskite phase transition require reevaluation