Mineralogy and dynamics of a pyrolite lower mantle

There is a growing consensus that-the Earth's lower mantle possesses a bulk composition broadly similar to that of the upper mantle (known as pyrolite)(1-3). But little is known about lower-mantle mineralogy and phase chemistry(4,5), especially at depth. Here we report diamond-anvil cell experiments at pressures of 70 and 135 GPa (equivalent to depths within the Earth of about 1,500 and 2,900 km, respectively) which show that pyrolite would consist solely of magnesian-silicate perovskite (MgPv), calcium-silicate perovskite (CaPv) and magnesiowustite(Mw). Contrary to recent speculation(6,7), no additional phases or disproportionations were encountered and MgPv was found to be present at both pressures. Moreover, we estimate that, at ultrahigh pressures where thermal expansivities are low, buoyancy forces inherent in subducted slabs because of their lithology will be of similar magnitude to those required for thermally driven upwelling. So slabs would need to be about 850 degrees C cooler than their surroundings if they are to sink to the base of the mantle. Furthermore, initiation of plume-like upwellings from the core-mantle boundary, long attributed to superheating, may be-triggered by lithologically induced buoyancy well before thermal equilibration is attained. We estimate that ascent would commence within similar to 0.5 Gyr of the slab reaching the core-mantle boundary, in which case the lowermost mantle should not be interpreted as a long-term repository for ancient slabs.