Iron-spin Transition Controls the Structure of LLSVPs beneath Africa and the Pacific Ocean

Abstract:

Seismic tomography models have revealed that there exist two large low shear velocity provinces (LLSVPs) in the Earth’s lower mantle beneath Africa and the Pacific Ocean. Waveform modeling results further identify that LLSVPs have sharp side boundaries, suggesting that they are compositionally heterogeneous from the ambient mantle. The structures of LLSVPs are significantly influenced by the whole mantle convection. When imposing the plate motion history in the last 120 Ma at the surface as a driving mechanism, the geographic positions of the two LLSVPs can be well explained using geodynamics modeling. However, two prominent features of LLSVPs, steep side boundaries and high elevations up to the mid-mantle at ~1500 km depth, can hardly be explained by these models. Here, we include the effects of iron spin transition of ferropericlase in our mantle convection model which leads to substantial physical property change for the mantle materials. We found that if the coefficient of thermal expansion decreases by a factor of 2 or 3 from the surface to the core-mantle boundary and chemical density anomaly is between 2.0-2.6%, huge chemical blocks emerge in our models with steep side boundaries and high elevations up to ~1500 km depth. Such blocks cause a decrease of shear wave velocity of ~4% which is comparable to the seismic observations. Our results show that iron spin transition plays a dominant role in shaping the morphology of LLSVPs. Our results also show that these LLSVPs are transient structures in the lower mantle which can typically last for a few hundred million years, before destroyed by mantle motion.