Electrical Conductivity of (Mg, Fe)CO₃ at the Spin Crossover and Its Implication for Mid‐Mantle Geomagnetic Heterogeneities

Abstract (Mg, Fe)CO 3 is an important deep carbon carrier and plays a vital role in our understanding of lower‐mantle carbon reservoirs. The electrical conductivity (EC) of FeCO 3 was measured at 126−2000 K up to 83 GPa in diamond‐anvil cells using a standard four‐probe van der Pauw method. Moreover, the EC of FeCO 3 increases by ∼6 orders of magnitude from 300 to 1500 K at 10−20 GPa, indicating a strong effect of high temperature. The EC of Fe 0.65 Mg 0.35 CO 3 was measured up to 60 GPa at 300 K, the EC values of (Mg, Fe)CO 3 are proportional to iron content and increase by 2–3 orders of magnitude at 300 K across the spin crossover. The EC values of (Mg, Fe)CO 3 and FeCO 3 + Fe 3 O 4 ± C mixtures surpass that of bridgmanite, ferropericlase and davemaoite by ∼1–4 orders of magnitude at depths of 800–2,000 km. This result sheds insights into the genesis of local geomagnetic heterogeneities in the mid‐lower mantle.