Giant Impact Origin for the Large Low Shear Velocity Provinces

The Moon has been widely recognized for decades to have formed from a "Giant Impact," when a planetary embryo named Theia collided with the proto-Earth toward the end of its accretion. However, the composition and mass of Theia remain controversial, with Theia's mass estimated as ~0.1 to > 0.4 Earth masses (Cameron and Ward 1976; Canup 2012). Direct samples of Theia would constrain these, and a variety of Giant Impact models predict that large, intact samples of Theia's mantle should persist today in the Earth's mantle. We propose that these sank to the core-mantle boundary and were collected by mantle convection after billions of years to form the largest structures in the mantle, the Large Low-Shear-Velocity Provinces (LLSVPs). This hypothesis is supported by isotopic ratios of He and Ne in ocean island basalts sourced from LLSVPs, as we hypothesize Theia would have ingassed solar nebula noble gases. LLSVPs comprise ~6% the mass of Earth's mantle, which could represent 13-60% of the mass of Theia's mantle, depending on Theia's size. Moreover, our systematic geodynamic modeling shows that the density anomaly needed to form LLSVP-like thermochemical piles is ~2.5%, consistent with recent estimates of Theia's mantle's density assuming chemically reduced conditions (Desch and Robinson 2019). Our results shed light on the origins of LLSVPs and the long-lasting effects of the Giant Impact on the thermal and chemical evolution of the Earth.