Electrical Conductivity of (Mg, Fe)CO 3 at the Spin Crossover and Its Implication for Mid‐Mantle Geomagnetic Heterogeneities

(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. Geomagnetic observations reveal electrical heterogeneities in the mid‐lower mantle. However, some of the aforementioned geomagnetic anomalies are higher than the electrical conductivity (EC) values of major lower‐mantle minerals such as ferropericlase, bridgmanite, and davemaoite. This calls for the existence of high conductive mineral(s) in the region where subducting slabs can bring certain amounts of Fe‐bearing and/or carbon‐bearing materials. In this study, we report that EC values of (Mg, Fe)CO 3 and FeCO 3 + Fe 3 O 4 ± C mixtures are 1–4 orders of magnitude greater than that of the major minerals at depths of 800–2,000 km. This suggests (Mg, Fe)CO 3 ‐bearing and its decomposed mixtures subducting patches may induce local geomagnetic heterogeneities at these depths. In particular, these materials may contribute to the geomagnetic anomalies observed at ∼1,300 km depths and assist in identifying potential carbon‐rich regions in the lower mantle. Electrical conductivity (EC) of FeCO 3 was measured at 126−2000 K and 0–83 GPa using the van der Pauw method in diamond‐anvil cells EC of (Mg, Fe)CO 3 is proportional to iron content and increases by 2–3 orders of magnitude at the spin crossover High conductivity of (Mg, Fe)CO 3 and FeCO 3 + Fe 3 O 4 ± C mixtures may contribute to local geomagnetic heterogeneities in the mid‐lower mantle