DEEP EARTH, DEEP TIME: IMPLICATIONS OF RECENT MINERAL PHYSICS DISCOVERIES

Motohiko Murakami's discovery of MgSiO₃ post-perovskite (PPV) provided several critical clues that allowed us to begin unraveling the nature of features observed in the deep mantle. One of the primary themes that emerges from models for the occurrence of PPV inside colder (seismically fast) portions of the lowermost mantle (e.g., slabs ponded just above the core) is a core-mantle heat flow that was higher than expected. These kinds of results arise from relating seismic discontinuities associated with PPV to constraints on the P-T phase diagram. The initial clues about high heat flow from PPV are broadly consistent with even more recent experimental constraints on heat loss by conduction from Earth's core, and the thermal conductivity of the deep mantle. With a high core-mantle heat flow, Earth's solid inner core must be young (~1Ga), and vigorous convective cooling of the deep Earth by mantle convection must have occurred throughout the Precambrian in order to sustain the geodynamo that produced Earth's magnetic field since at least ~3.5 Ga. However, such high ancient heat flows in the deep Earth imply very high temperatures in the early core and deep mantle; so high, in fact, that extensive melting in the early deep mantle is inescapable. Such a scenario for the early deep Earth's secular evolution is only plausible if melts were dense and gravitationally stable. Recent discoveries regarding the density of melts in the deep mantle indicate that melts formed above ~76 GPa (below ~1800 km depth) are indeed dense and gravitationally stable. Furthermore, we can now consider how dense magmas would have crystallized over time through the Archean, and predict that dense cumulates of FeO-rich rock should have formed in the Precambrian and remained stable at the base of the mantle. Such a model can explain the presence of two large low shear velocity provinces beneath the Pacific and Africa, as well as the existence of ultralow-velocity zones at the core-mantle boundary as the mushy residuum of a once vast basal magma ocean. Thus the present seismological structure of the deep mantle can now be linked to ancient processes occurring in the deep Earth. It was the discovery of PPV that provided the initial spark for this revolution.