Present-day Earth mantle structure set up by crustal pollution of the basal magma ocean.

The crystallization of a global magma ocean during early terrestrial planet evolution and the subsequent segregation of a longer-lived "basal magma ocean" (BMO) atop the core set up the evolution of the mantle-atmosphere system. Although seismic evidence for a BMO exists on present-day Mars and the Moon, the Earth's BMO is (near-)completely solidified. Seismically observed "large low-velocity provinces" (LLVPs) are thought to have resulted from the canonical "fractional" style of BMO crystallization. However, we show using thermodynamic modeling that BMO fractional crystallization yields lowermost-mantle densities much higher than those of LLVPs. In turn, pollution of the BMO by progressive addition of recycled basaltic crust and related "reactive crystallization" can reconcile LLVP volumes, densities, and compositions. This model also makes testable predictions of the compositions of "ultralow-velocity zones," enigmatic deep Earth seismic domains, and possible BMO remnants. The critical role of crustal pollution elucidates the survival of a BMO on Mars, but implies an Earth-like fate for any Venusian BMO.