Spin transition-induced anomalies in the lower mantle: Implications for mid-mantle partial layering

Recent advances in high pressure mineral physics reveal that the thermoelastic properties of the lower mantle may significantly change as a consequence of the spin transition in ferric iron in the major lower mantle minerals, ferropericlase (Fp) and perovskite (Pv). The spin transition in iron introduces anomalies in the thermoelastic properties, like the bulk modulus, which influences the mixing and the style of mantle convection and the thermal history of the planet. Employing high resolution axi-symmetric spherical models, we explore the influence of the spin transition-induced anomalies in density and viscosity on mantle dynamics. Model results reveal that both a viscosity increase in the lower regions of the mantle and a negative spin transition-induced density anomaly can slow sinking slabs they approach mid-mantle depths; however, more profoundly, the density anomaly may halt slabs at mid-mantle depths for tens of millions of years, and up to similar to 200 Myr before they finally penetrate to the lower depths. This phenomenon is consistent with the nature of sinking slabs inferred from high resolution tomographic images which show slab stagnations at mid-mantle depths (from 1200 to around 1600 km).