Reconciling the post-perovskite phase with seismological observations of lowermost mantle structure

Abstract

The discovery of post-perovskite (pPv), the high pressure polymorph of (Mg x ,Fe 1-x )SiO 3 perovskite (Pv), may have profound implications for the thermal, chemical and dynamical structure of the deep Earth, if it can be demonstrated that pPv currently exists in the lower mantle. The requisite basis for such a demonstration is seismological observation of elastic velocity and density structures diagnostic of or at least compatible with the expected properties of pPv occurrence in a realistic lower mantle chemical and thermal environment. A critical assessment of seismological observations versus predictions for a lower mantle model with pPv is undertaken, with the limitations and robust attributes of the seismological data being summarized. The existence of a seismic velocity discontinuity several hundred kilometers above the core-mantle boundary is a primary line of evidence for the presence of a Pv-pPv phase change. However, some attributes of the discontinuity, such as localized P wave reflections from a large P velocity increase and the sharpness of observed P and S velocity increases, reveal inconsistencies with expected properties of a Pv-pPv phase transition. Lateral variations in temperature can produce complex phase boundary structure that explains variable S wave observations, but such elastic heterogeneity intrinsically complicates testing the pPv hypothesis. The combination of rapidly expanding seismological data sets and new high resolution data analysis procedures reveal multiple seismic discontinuities near the base of the mantle; in some cases these may be consistent with forward and reverse Pv-pPv transformations, bounding a pPv layer or lens in D″ above the CMB. Other phase changes or compositional contrasts could also be involved. At present, existence of pPv in the deep Earth is quite plausible, but not yet conclusively demonstrated. Future seismological and mineral physics research directions for further testing the hypothesis that pPv is present in the lower mantle are suggested.

Seismic Detection of Post-perovskite Inside the Earth

“…At the time of the modelling of Sidorin et al (1999a, b), there were limited observations of the D″ discontinuity in the central Pacific; subsequently, studies have found that the discontinuity in this region is too shallow to be compatible with Sidorin et al's models, and this may be indicative of chemical effects on the phase boundary (e.g. Lay and Garnero 2007;Ohta et al 2008). Furthermore, Hutko et al (2006) have mapped 100-km amplitude undulations in the depth of D″ reflections over lateral distances of 200-300 km, which would require temperature variations of 500-1000 K that are geodynamically difficult to explain.…”

“…References for D″ observations given in Sect. 13.3.1 in D″ structure can be ascribed to different stacking techniques, and subjective data selection between different studies (Wysession et al 1998;Lay and Garnero 2007).…”

Some recent advances in understanding the mineralogy of Earth's deep mantle

“…Although our understanding of this phase transition and its implications is still at an early stage, the CaIrO 3 -type phase of (Mg,Fe)SiO 3 appears to provide some of the right characteristics to match seismic observations of D 00 (Wookey et al 2005;Lay & Garnero 2007;Shim 2008). While there remain considerable uncertainties in laboratory pressure determinations at these extreme conditions, the transformation occurs at pressures near those found at the top of the D 00 layer for expected mantle compositions (Murakami et al 2004;Oganov & Ono 2004;Wookey et al 2005).…”

“…A seismic discontinuity marks the top of D 00 and the region itself is characterized by strong lateral heterogeneity and other anomalous properties (Garnero 2000;Lay & Garnero 2007). In contrast to the bulk of the lower mantle, the core-mantle boundary region exhibits pronounced seismic anisotropy (Kendall 2000).…”

Between a rock and a hot place: the core–mantle boundary

“…This mineral phase not only provides a compelling explanation for many of the seismic properties we observe (Hernlund et al 2005;Wookey et al 2005;Mao et al 2006;Lay & Garnero 2007;Wookey & Kendall 2007), but also provides us with some unique and exciting opportunities. Among these is the possibility of its usage as a 'geothermometer'.…”