Spin transition in magnesiowüstite and ultralow thermal conduction in ultralow velocity zones

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Hsieh, Wen-Pin ; Deschamps, Frédéric ; Tsao, Yi-Chi ; Pease, Allison ; Dorfman, Susannah M. ; Bausch, Hannah J. ; Wang, Fei:
Spin transition in magnesiowüstite and ultralow thermal conduction in ultralow velocity zones.
In: Nature Communications. Bd. 16 (2025) . - 10431.
ISSN 2041-1723
DOI: https://doi.org/10.1038/s41467-025-65430-7

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Abstract

Above Earth’s core-mantle boundary (CMB), seismic studies revealed numerous enigmatic, small-sized patches of ultralow velocity zones (ULVZs) with anomalously lower velocities and higher density than ambient mantle. These regions may be enriched in Fe-rich oxides, and their thermal conductivity Ʌ would critically influence regional heat-flux and thermochemical evolution around CMB, but remains poorly constrained. Here we experimentally show that Ʌ of (Mg0.75,Fe0.25)O, ɅFp25, and (Mg0.25,Fe0.75)O, ɅFp75, both decrease across an iron spin-transition at 573 K, while such reduction is ~30–40% smaller than at room temperature, suggesting their Ʌ are less-affected across the spin-transition under deep-mantle’s high temperatures. The temperature dependences of low-spin ɅFp25 and ɅFp75 (T –0.39 and T –0.23, respectively) are weaker than the conventional T –0.5 for high-spin state. If made of Fe-rich oxides (e.g., Fp75), ULVZs should have an ultralow thermal conductivity ( ~ 3.4 W m−1 K−1). Such strong thermal insulation enhances local temperature, vitalizing regional mantle dynamics and thermochemical evolution, and growth of thermal plumes. The significant Ʌ discontinuity across CMB would induce heterogeneous amplitude and pattern of CMB heat-flux, potentially impacting geodynamo and geomagnetic stability.