Thickness and Clapeyron Slope of the Postperovskite Phase Boundary in the Earth's Mantle

Krystle Catalli, Massachusetts Institute of Technology

The thicknesses and Clapeyron slopes of phase boundaries between minerals in the Earth’s mantle strongly influence the seismic detectability of the boundaries as well as convection in the mantle. The unusually large positive slope found for the perovskite to postperovskite boundary should destabilize high-temperature anomalies in the lowermost mantle, which is incompatible with seismic observations. We report the thickness of the perovskite-to-postperovskite boundary in (Mg0.91Fe0.09)SiO3 and (Mg0.9Fe0.1)(Al0.1Si0.9)O3 determined in the laser-heated diamond-anvil cell (DAC) at in situ high pressure–temperature conditions up to 3000 K and 145 GPa. The measured Clapeyron slope is consistent with the D'' seismic discontinuity. However, in both systems the postperovskite boundary thickness increases to 400-600±100 km, which is substantially larger than the thickness of the D'' discontinuity (<30 km) measured in seismology. Although the Fe2+ buffering effect of ferropericlase could decrease the postperovskite boundary thickness, the boundary should remain thick in a pyrolitic composition because of the effects of Al and the rapid temperature increase in the D'' layer. The postperovskite boundary will be particularly thick in regions with elevated Al content and/or low Mg/Si ratio, reducing the effects of the large positive Clapeyron slope of the pPv boundary on the buoyancy of thermal anomalies and stabilizing the compositional heterogeneities at 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 low Al content.

Abstract Author(s): Krystle Catalli, Sang-Heon Shim, and Vitali Prakapenka