Superplumes – Post-Perovskite Phase Transition and the Nature of the Dʺ Layer

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Superplumes – Post-Perovskite Phase Transition and the Nature of the Dʺ Layer

It has been shown by recent experimental and theoretical studies that MgSiO₃ perovskite, which is a dominant mineral in the lower mantle, that at temperatures and pressures expected to exist in the Dʺ layer region it transforms to a post-perovskite phase. For a long time the Dʺ layer has been the most enigmatic region in the interior of the Earth, as it is here that there a large number of seismic anomalies that are not consistent with the known properties of MgSiO₃ perovskite. Understanding of the nature of the lowermost mantle has been significantly improved by the discovery of a phase transformation from perovskite to post-perovskite. Additionally the hot thermal boundary layer at the base of the mantle is destabilised by the post-perovskite transition and this induces the formation of mantle plumes, as it has a large positive Clapeyron slope. Chemical compositions of such plumes are naturally distinct from the bulk of the mantle, if the plumes are sourced from the core-mantle boundary (CMB) region. Hirose suggests the Dʺ layer most likely has significant chemical heterogeneities at various scales that are caused by slabs that have been subducted, partial melting, and reaction with the outer core.

It is suggested that many mantle plumes originate in the core-mantle boundary region (e.g., Zhao, 2004). The Dʺ layer, which occupies the bottom 100 km of the mantle, is one of the least understood regions in the interior of the Earth. It has been shown by seismic observations that there are a number of unexplained features, which have not been reconciled with the known properties of MgSiO₃ perovskite that have long been believed to be stable to the bottom of the mantle. In the Dʺ region phase transition of a particular mantle mineral was not known until recently.

The search for phase transitions at high P-T condition corresponding to the CMB region have been enabled by developments in X-ray diffraction (XRD) measurements at synchrotron facilities combined with laser-heated diamond-anvil cell (LHDAC) techniques. A drastic change in the XRD pattern above 125 Gpa and 2,500 K, corresponding to a depth of 2,700 km in the mantle (several 100 km above the CMB), led to the discovery of the phase transformation from MgSiO₃ perovskite to post-perovskite (Murakami et al., 2004). The post-perovskite transition was confirmed later by theory, and its elastic properties were also calculated (Tsuchiya et al., 2004a, b; Iitaka et al., 2004; Oganov & Ono, 2004).

There are significant implications for the nature and dynamics of the lower mantle that follow from the discovery of this phase transition (Hirose et al., 2006b). Hirose points out that there are a number of anomalies in the Dʺ layer that cannot be due to thermal anomaly alone. These anomalies were often interpreted as chemical heterogeneities as a phase transition was unknown (Karato & Karki, 2001). Many of these observations may, however, be reconciled with the post-perovskite phase (Wookey et al., 2005). This phase transition is a significantly large exothermic reaction that destabilises the bottom thermal boundary layer, though the increase of density at the phase change is small (Nakagawa & Tackley, 2004; Matyska & Yuen, 2005).

In order to understand the nature of mantle plumes an understanding of thermal and chemical anomalies in Dʺ is very important. This is a review by Hirose of the experimental and theoretical studies that have been carried out on the post-perovskite phase transition and its implications for the Dʺ layer. It is naturally expected that the Dʺ region includes large chemical heterogeneities as it is a thermal and compositional boundary. In this paper Hirose discusses the chemical heterogeneities that are possible, as well as their origins that may be reflected in the distinct chemical compositions of plumes that originate from the CMB region.

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