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

(Mg, Fe)CO₃ is an important deep carbon carrier and plays a vital role in our understanding of lower-mantle carbon reservoirs. The electrical conductivity (EC) of FeCO₃ 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 FeCO₃ 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_0.65Mg_0.35CO₃ was measured up to 60 GPa at 300 K, the EC values of (Mg, Fe)CO₃ 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)CO₃ and FeCO₃ + Fe₃O₄ ± 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.