Carbonate-rich melt formation and connectivity in Earth’s upper mantle

The formation and transport of low-degree, carbonate-rich melt influences the physical and chemical characteristics of the Earth’s upper mantle. However, the amount of carbonate-rich melt at depth is debated, with estimates from seismic studies higher than those from electromagnetic observations. This discrepancy limits our understanding of melt distribution and transport in the mantle. We investigated melting carbonated peridotite and carbonated basalt using electrical measurements at 7–15 GPa and up to 1585°C. For the two compositions considered, the temperature dependence of conductivity can be reproduced with an Arrhenius equation below the solidus, and conductivity decreases by a factor of up to 10 at the solidus temperature. This conductivity drop is explained by the simultaneous decrease in solid matrix conductivity due to the removal of incompatible charge-carrier ions (e.g., H, Na, K) to carbonate-rich melt and the anisotropic distribution of the highly mobile melt within the sample. Based on our observations, we re-evaluate melt-content estimates in the upper mantle and suggest that, at the spatial scale probed by electromagnetic measurements, carbonate-rich melt channelization within a depleted mantle is unlikely to significantly increase mantle conductivity, thereby reconciling electrical and seismological studies.