Iron-spin transition controls structure and stability of LLSVPs in the lower mantle

Abstract:

Seismic tomography models have revealed that there exist two large low shear velocity provinces (LLSVPs) beneath Africa and the Pacific Ocean in the Earth’s lower mantle. Waveform modeling results suggest both LLSVPs have steep sharp side boundaries，which imply that they probably are compositionally heterogeneous from the ambient mantle. When applying the surface plate motion history in the last a few hundred million years (Ma) as the driving mechanism, numerical modeling has successfully reproduced the geographical distribution of the two LLSVPs in thermochemical mantle convection. However, two prominent seismic features of the LLSVPs, the steep side boundaries and the high elevation, can hardly be obtained in previous geodynamic models. Here, we include in our mantle convection model the effects of iron-spin transition of ferropericlase which substantially change physical property of the mantle. Our results show that iron-spin transition plays a dominant role in controlling the structure and stability of LLSVPs. Large chemical blocks with steep side boundaries and high elevations up to ~1200 km above core-mantle boundary (CMB) emerge in our models with the volume content of ferropericlase ~20%. Such blocks cause a shear wave velocity decrease of ~3.5% which is consistent with the seismic observations. 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 large-scale mantle motion.