Is There a Carbonated Mid‐Lithosphere Discontinuity in Cratons?

The mid‐lithosphere discontinuity (MLD), identified by a sharp velocity drop at ∼70–100 km depths within the cratonic lithosphere is key to comprehending the chemical composition and thermal structure of the cratonic lithosphere. The MLD is widely accepted to be caused by composition anomalies, such as hydrous minerals, which show low velocities and high electrical conductivities. However, noticeable high‐electrical conductivity anomalies have not been detected in the most cratonic lithosphere. Dolomite has an electrical conductivity similar to olivine and can be originated by carbonatitic melts trapped at ∼80–140 km depths. Here we investigated the elasticity of dolomite under mantle conditions using ab initio calculations and found dolomite exhibits significantly lower velocities than the primary minerals in the lithospheric mantle. Therefore, the dolomite enrichment might provide a good explanation for the observed velocity drop of the MLD in cratonic regions where no high‐conductivity anomaly has been detected, such as the northern Slave craton. Plain Language Summary: Seismological observations have detected a significant mid‐lithosphere discontinuity (MLD) in the cratonic lithosphere, marked by a velocity reduction of ∼2–7% at ∼70–100 km depths. The MLD might reveal the cratonic lithosphere structure and dynamics and its origin is crucial for us to comprehend the formation and evolution histories of cratons. During the formation and evolution of cratons, volatiles‐rich melts can be introduced by the processes of plate subduction or mantle upwelling. Therefore, hydrous minerals and carbonates frozen from volatiles‐rich melts are the pivotal candidates with low velocity. Hydrous minerals usually exhibit high electrical conductivities and their enrichment could cause obvious electrical conductivity anomalies. However, obvious high‐conductivity anomalies have not been detected in many cratonic lithospheres. In contrast, carbonates show electrical conductivity similar to that of olivine, and carbonatitic melts could be trapped at ∼80–140 km depths due to the "decarbonization front" and "redox freezing front," reacting with pyroxene and olivine in the surrounding mantle to produce dolomite as the cratonic lithosphere cools down. Dolomite exhibits significantly lower velocities than the main silicates within the cratonic lithosphere and its enrichment could account for the observed velocity drop at the MLD depths in the cratonic regions where no high‐electrical conductivity anomalies have been found. Key Points: The elasticities of dolomite are determined at P‐T conditions corresponding to the cratonic lithosphere using ab initio calculationsThe velocities of dolomite are significantly lower than those of the main minerals in the lithospheric mantleThe enrichment of dolomite can explain the velocity drop of the mid‐lithosphere discontinuity in cratonic regions without high electrical conductivity anomalies [ABSTRACT FROM AUTHOR]