Post-perovskite

Post-perovskite is a high-pressure phase of MgSiO₃, is composed of the prime oxide constituents of the Earth's rocky mantle (MgO and SiO₂), and its pressure and temperature for stability imply that it is likely to occur in portions of the lowermost few hundred km of Earth's mantle. MgSiO₃ has the same crystal structure as the synthetic solid compound CaIrO₃, and is often referred to as the "CaIrO₃-type phase of MgSiO₃" in the literature. The crystal system of post-perovskite is orthorhombic, its space group is Cmcm, and its structure is a stacked SiO₃-octahedral sheet along the b axis. The name "post-perovskite" derives from the name of the stable phase of MgSiO₃ throughout most of Earth's mantle, termed perovskite. The prefix "post-" refers to the fact that it occurs after perovskite as pressure increases (and historically, the progression of high pressure mineral physics). At upper mantle pressures, nearest Earth's surface, MgSiO₃ persists as enstatite, a mineral which can be found in natural rocks.

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History

The CaIrO₃-type phase of MgSiO₃ phase was discovered in 2004 using the laser-heated diamond anvil cell (LHDAC) technique by a group at the Tokyo Institute of Technology and, independently, by researchers from the Swiss Federal Institute of Technology (ETH Zurich) and Japan Agency for Marine-Earth Science and Technology who used a combination of quantum-mechanical simulations and LHDAC experiments. The results from each group were published within less than a month's period; the TIT group's paper appeared in the journal Science^[1]. The ETH/JAM-EST collaborative paper appeared in the journal Nature^[2]. In 2006, Nature reported that the earlier discovery of a CaIrO₃-type phase of Fe₂O₃ by S. Ono has become an issue of contention.

Nomenclature

Most geophysicists use the terminology "post-perovskite." The word has become commonly heard in the discussions of scientists in Earth Science departments around the world, and those not specialized in deep Earth research have the impression that it is either the biggest geoscience fad they've witnessed in decades, or that it may actually be as important and revolutionary as their deep Earth colleagues claim.

However, in earlier versions of their manuscript, Oganov and Ono attempted to name the new MgSiO3 mineral phase "Birchite" (in deference and respect to renowned Harvard geophysicist Francis Birch). However, the International Mineralogical Association (IMA), established to control mineral names and to whose will all scientific journals give consent on this issue, has very strict rules for the naming of a mineral. One of the seemingly impossible tasks of naming a MgSiO₃-post-perovskite mineral is that, according to the rules of the IMA, firm evidence in actual natural samples of its occurrence in nature is required. These rules are a historical throwback to the old days, when all known mineral phases were stable (or at least sufficiently meta-stable to be discovered) at Earth's surface. These rules have, however, been relaxed somewhat over the years, such that micro-structural evidence of a higher pressure phase's occurrence in a hand sample is sufficient. Yet this appears unlikely for MgSiO₃-post-perovskite, which only becomes stable at pressures greater than one million atmospheres: conditions which are not likely to have been preserved in any hand sample.

Importance in Earth's Mantle

Post-perovskite phase is stable above 125 GPa at 2500 K, and exhibits a positive Clapeyron slope such that the transformation pressure increases with temperature. Because these conditions correspond to a depth of about 2700 km and the highly variable D" seismic discontinuity occurs at similar depths, post-perovskite is considered to be a primary suspect for inducing seismic discontinuities in this region. Post-perovskite also holds great promise for mapping experimentally determined information regarding the temperatures and pressures of its transformation into direct information regarding temperature variations in the D" layer once the seismic discontinuities attributed to this transformation have been sufficiently mapped out. Such information can be used, for example, to: 1) better constrain the amount of heat leaving Earth's core, 2) determine whether or not subducted slabs of oceanic lithosphere reach the base of the mantle, 3) help delineate the degree of chemical heterogeneity in the lower mantle, and 4) find out whether or not the lowermost mantle is unstable to convective instabilities that result in upwelling hot thermal plumes of rock which rise up and possibly trace out volcanic hot spot tracks at Earth's surface. For these reasons the finding of the MgSiO₃-post-perovskite phase transition is considered by many geophysicists to be the most important discovery in deep Earth science in several decades, and was only made possible by the concerted efforts of mineral physics scientists around the world as they sought to increase the range and quality of LHDAC experiments.

Physical Properties

The sheet structure of post-perovskite makes the compressibility of the b axis higher than that of the a or c axis. This anisotropy may yield the morphology of a platy crystal habit parallel to the (010) plane; the seismic anisotropy observed in the D" region might be explained by this characteristic. More recently, theoretical calculations ascribed the D" layer anisotropy to texturing of post-perovskite crystals along the {110} planes. Stacking faults along these planes generate a polytypic series of structures, the end members of which are perovskite and post-perovskite. These MgSiO3 polytypes may be chemically or thermally stabilised and are quite possible as mineral phases of the lower mantle. The theoretically predicted mechanism of plastic deformation has been later confirmed by experiments, and there is a growing experimental evidence for the predicted polytypes.

Chemical Properties

Another potentially important effect that needs to be better characterized for the post-perovskite phase transition is the influence of other chemical components that are known to be present to some degree in Earth's lowermost mantle. For example, introducing FeO allows for a limited solid solution in the binary system MgSiO₃-FeSiO₃ in both perovskite and post-perovskite phases. The phase transition pressure (characterized by a two-phase loop in this system), is thought to decrease as the FeO content increases, though the magnitude of this effect is debated. Other components, such as Al₂O₃ or the more oxidized Fe₂O₃ also affect the phase transition pressure, and might have strong mutual interactions with one another. The influence of variable chemistry present in the Earth's lowermost mantle upon the post-perovskite phase transition raises the issue of both thermal and chemical modulation of its possible appearance (along with any associated discontinuities) in the D" layer.

Summary

Experimental and theoretical work on the perovskite/post-perovskite phase transition continues, while many important features of this phase transition remain ill-constrained. For example, the Clapeyron slope (characterized by the Clausius-Clapeyron_relation) describing the increase in the pressure of the phase transition with increasing temperature is known to be relatively high in comparison to other solid-solid phase transitions in Earth, however, the experimentally determined value varies from about 5 MPa/K to as high as 13 MPa/K. The difference between these estimates arises primarily because different materials are used as pressure standards in LHDAC experiments. A well-characterized equation of state for the pressure standard, when combined with high energy synchrotron generated X-ray diffraction patterns of the pressure standard (which is mixed in with the experimental sample material), yields information on the pressure-temperature conditions of the experiment. However, as these extreme pressures and temperatures have not been sufficiently explored in experiments, the equations of state for many popular pressure standards are not yet well-characterized and often yield different results. Another source of uncertainty in LHDAC experiments is the measurement of temperature from a sample's thermal radiation, which is required to obtain the pressure from the equation of state of the pressure standard. In laser-heated experiments at such high pressures (over 1 million atmospheres), the samples are necessarily small and numerous approximations (e.g., grey body) are required to obtain estimates of the temperature.

References

1. ^ Murakami, M., K. Hirose, N. Sata, Y. Ohishi, and K. Kawamura, Phase transition of MgSiO₃ perovskite in the deep lower mantle, Science, 304: 855-858, 2004
2. ^ Oganov, A.R. and S. Ono, Theoretical and experimental evidence for a post-perovskite phase of MgSiO₃ in Earth's D" layer, Nature, 430: 445-448, 2004