Mineral Compositions as Governed by Spin-spin Interactions: Evidence for a Compositionally Layered, Inhomogeneous Mantle

The hypothesis that the Earth’s mantle is composed of chemically distinct layers has long provided a solution to reconcile chemical differences between mid-ocean ridge basalts and ocean-island basalts. In particular, a perovskitic lower mantle can account for the “missing silicon problem”, whereby the upper mantle Mg/Si ratios are systematically higher than chondritic values. Knowledge of the bulk chemical and mineralogical profile of the mantle is crucial in constraining models of Earth’s convection and dynamics in time, which controls the evolution of igneous processes over Earth’s history. The main argument against compositional layering in Earth’s mantle has been observations from seismic waves that subducted slabs reach the lower mantle, which imply the existence of a component of whole mantle convection. However, recent geodynamical modeling has shown that subducted slabs can reach the lower mantle in models including chemical layering. Here, we provide new theoretical evidence for a chemically layered mantle, with a perovskitic lower mantle. Our results are obtained from a novel approach for predicting mineral compositions, which is based on the observation that exchange interactions play an important role in determining optimal compositions in Hubbard insulators as well as weak-metals. Our model also provides a solution for the existence of the enigmatic large-low-shear-velocity provinces. We provide evidence for the stability of anomalously Fe-rich brigdmanite, which implies the existence of chemical gradients in the lower mantle. This in turn offers a mechanism for the origin of partial melting and mantle plumes in the lower mantle