The Origins of Seismic Heterogeneities in the Lower Mantle: Insights from Mineral Physics

Abstract:

Seismic studies have revealed numerous heterogeneities with different length scales in the lower mantle, among which the Large Low Shear Velocity Provinces (LLSVPs) beneath Africa and the Pacific in the lowermost mantle are the largest and most enigmatic. They hold key to understanding the chemical composition and thermal structure of the deep Earth, and may help us reconstruct the evolution history of the planet. Systematic observations suggest that the chemical compositions of LLVSPs are distinct from the surrounding mantle, but the origin of the heterogeneities has long been debated, with hypotheses ranging from subducted oceanic crust and to primordial materials such as metallic melts [1].

Chemical heterogeneities could have been produced by redox reactions in the magma ocean of early Earth. Ferrous iron (Fe²⁺) in silicate melts has been found to disproportionate to ferric iron (Fe³⁺) plus metallic iron (Fe⁰) at high pressures [2]. Segregation of precipitated Fe⁰ from the magma ocean into the core would enrich Fe³⁺ in the mantle. Bridgmanite, the dominant lower-mantle mineral, is the major host of Fe³⁺. It exhibits various geophysical properties that depend on chemical compositions. Here we combine high P-T experiments and ab initio calculations to investigate the stability of Fe³⁺-bearing bridgmanite. We observe the formation of an Fe³⁺-rich and obtain its high P-T elastic properties using ab initio calculations. We find that oxidized domains with a certain fraction of such Fe³⁺-rich bridgmanite would have higher density and lower shear wave velocity than a pyrolitic lower mantle, and they can explain the seismic features of the LLSVPs. Geodynamic simulations demonstrate that dense Fe³⁺-rich domains could form large-scale thermochemical piles in the deep mantle throughout Earth’s history, without being mixed into the background mantle. The LLSVPs may represent the oxidizing domains in the lower mantle, and they would have profound effects on volatile cycles and the deep reservoirs of redox-sensitive elements.

[1] McNamara, A. K. A review of large low shear velocity provinces and ultra-low velocity zones.

[2] Armstrong, K., Frost, D. J., McCammon, C. A., Rubie, D. C. & Boffa Ballaran, T. Deep magma ocean formation set the oxidation state of Earth’s mantle. Science 365, 903–906 (2019).