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 postperovskite (pPv)1-3 (the pPv boundary) would destabilize hightemperature anomalies in the lowermost mantle4, in disagreement with the seismic observations5. Here we report the thickness of the pPv boundary in (Mg0.91Fe 2+0.09)SiO3 and (Mg0.9Fe 3+0.1)(Al0.1Si0.9)O3as 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 D99 discontinuity6. In both systems, however, the pPv boundary thickness increases to 400-600+6100 km, which is substantially greater than the thickness of the D discontinuity (≥30 km)7. Although the Fe2+ buffering effect of ferropericlase8-10 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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