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

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.