The thicknesses and Clapeyron slopes of mantle phase boundaries strongly influence the seismic detectability of the boundaries and convection in the mantle. The unusually large positive Clapeyron slope found for the boundary between perovskite (Pv) and post-perovskite (pPv)1,2,3 (the ‘pPv boundary’) would destabilize high-temperature anomalies in the lowermost mantle4, in disagreement with the seismic observations5. Here we report the thickness of the pPv boundary in (Mg0.91Fe2+0.09)SiO3 and (Mg0.9Fe3+0.1)(Al0.1Si0.9)O3 as determined in a laser-heated diamond-anvil cell under in situ high-pressure (up to 145 GPa), high-temperature (up to 3,000 K) conditions. The measured Clapeyron slope is consistent with the D′′ discontinuity6. In both systems, however, the pPv boundary thickness increases to 400–600 ± 100 km, which is substantially greater than the thickness of the D′′ discontinuity (<30 km)7. Although the Fe2+ buffering effect of ferropericlase8,9,10 could decrease the pPv boundary thickness, the boundary may remain thick in a pyrolitic composition because of the effects of Al and the rapid temperature increase in the D′′ layer. The pPv boundary would be particularly thick in regions with an elevated Al content and/or a low Mg/Si ratio, reducing the effects of the large positive Clapeyron slope on the buoyancy of thermal anomalies and stabilizing compositional heterogeneities in the lowermost mantle. If the pPv transition is the source of the D′′ discontinuity, regions with sharp discontinuities may require distinct compositions, such as a higher Mg/Si ratio or a lower Al content.
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This work is supported by the US National Science Foundation (NSF) grant EAR0738655 (S.-H.S.) and a US Department of Energy (DOE) National Nuclear Security Administration Stewardship Science Graduate Fellowship (K.C.). A. Kubo and B. Grocholski assisted in X-ray measurements. Discussion with T. L. Grove and R. D. van der Hilst improved the paper. This work was performed in the GeoSoilEnviroCARS sector of the Advanced Light Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the NSF and the DOE. Use of the APS is supported by the DOE.
Author Contributions K.C. and S.-H.S. prepared and made the measurements on (Mg0.9Fe0.1)(Al0.1Si0.9)O3 and (Mg0.91Fe0.09)SiO3, respectively. V.P. assisted in the synchrotron measurements. K.C. and S.-H.S. conducted the data analysis and calculations. S.-H.S. and K.C. wrote the paper. All authors discussed the results and commented on the manuscript.
This file contains Supplementary Notes and Data, Supplementary References, Supplementary Figures 1-5 with Legends and Supplementary Table 1.
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Phase transition and thermal equations of state of (Fe,Al)-bridgmanite and post-perovskite: Implication for the chemical heterogeneity at the lowermost mantle
Earth and Planetary Science Letters (2018)