Detection of the spin crossover in ferropericlase in the Earth’s lower mantle; an interdisciplinary approach.

Abstract:

The high- to low-spin crossover in Fe²⁺ cations in ferropericlase (Fp: (Mg,Fe)O) affects material properties including density, viscosity, elasticity, thermal conductivity, and element partitioning. It is therefore important to identify the crossover in respect to the nature of LLSVPs, upwellings, and slabs, as well as the long-term composition, structure, and evolution of the planet. The crossover, occurring at conditions corresponding to the mid-lower mantle, has been shown, experimentally and theoretically, to have a positive dp/dT-slope and to widen with increasing T. Therefore, the lateral T variations determine the depth (pressure) of the onset and depth range of the crossover, which decreases the bulk modulus and has an insignificant effect on the shear modulus. Despite these fundamental predictions, it has eluded seismic observation, including 1D radial profiles. Because Fp may constitute around 23 mol% in depleted peridotite (bulk of the convecting mantle, including subducted slabs), this offers a distinct target for crossover detection. The onset of the mixed spin region is expected to occur at shallower depths in cold domains, and at deeper, and across a wider depth range, in warm mantle domains. We separate mantle T domains to their equivalent anomalous seismic wavespeed expressions (i.e. fast and slow), and quantitatively and qualitatively compare multiple global seismic velocity tomography models based on a ‘vote map’. We show a decoupling of shear (S-wave) and compressional (P-wave) wavespeed anomalies in the lower mantle for both fast wavespeed anomalies (>+1sigma) at depths greater than ~1400 km and for slow anomalies (<-1sigma) greater than ~ 1800 km, as predicted earlier. This is the first clear observational evidence, consistent with the occurrence of a Fp spin transition. The absence of an equivalent signal in the ambient (±0.5sigma) and average seismic domain suggests that the lower mantle is dominated by rheologically strong bridgmanite-rich Fp-poor domains. These ambient domains are confined to the regions between the slow upwelling columns above the two antipodal LLSVPs and the high-velocity longitudinal belt of sinking mantle under the Arctic, east Asia, Australia, Antarctica and the Americas.