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 a s pyrolite)(1-3). But little is known about lower-mantle mineralogy an d phase chemistry(4,5), especially at depth. Here we report diamond-an vil cell experiments at pressures of 70 and 135 GPa (equivalent to dep ths within the Earth of about 1,500 and 2,900 km, respectively) which show that pyrolite would consist solely of magnesian-silicate perovski te (MgPv), calcium-silicate perovskite (CaPv) and magnesiowustite(Mw). Contrary to recent speculation(6,7), no additional phases or dispropo rtionations were encountered and MgPv was found to be present at both pressures. Moreover, we estimate that, at ultrahigh pressures where th ermal expansivities are low, buoyancy forces inherent in subducted sla bs because of their lithology will be of similar magnitude to those re quired for thermally driven upwelling. So slabs would need to be about 850 degrees C cooler than their surroundings if they are to sink to t he base of the mantle. Furthermore, initiation of plume-like upwelling s from the core-mantle boundary, long attributed to superheating, may be-triggered by lithologically induced buoyancy well before thermal eq uilibration 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-te rm repository for ancient slabs.