Depletion, Density, and Deposits through the mantle transition zone (MTZ)

We present phase diagrams of variously enriched and depleted mantle rocks down to 800 kilometers depth and explore density as the parameter governing convection and compositional stratification. Some results are surprising and not all are included in present concepts and models: (1) Primitive and enriched mantle compositions are buoyant in the uppermost lower mantle compared to depleted mantle, especially, when they are warmer, but also at identical temperatures. Hence, if the upper mantle is depleted compared to the lower, a petrological lower-upper-mantle boundary (LUMB) can be expected several tens of kilometers below the seismic one. (2) Depleted compositions show the slope-break of the 660 phase transitions at higher temperatures. Hence, the uppermost lower mantle would be an excellent trap for very hot depleted mantle, which could be relevant for komatiite generation. (3) Primitive and enriched compositions experience negative thermal expansion at high temperatures in the upper MTZ, i.e. they display a density minimum at slightly elevated temperatures. The dynamic consequences for plume rise are enormous and explored in a complementary contribution (Vesterholt and Nagel). The key phase for effects above is garnet, which (1) is stable in the uppermost lower mantle, (2) relatively dens in the upper, but buoyant in the lower mantle, and (3) may become more abundant with temperature. Depending on bulk rock composition, garnet is stable in the uppermost 70-150 kilometers of the lower mantle causing a reversal of the expected density-order in that depth interval. Our present work includes studying seismic footprints of stratification scenarios.