Electrical Conductivity of Amorphous and Molten CaCO3 at High Pressures and Its Implications for Mantle Conductivity Anomalies

Impedance spectrometry experiments have been conducted on CaCO3 up to 15 GPa and 2,100 K to identify its state under high pressure. The melting temperature of CaCO3 was also determined by the falling of a Re sphere observed via X-ray radiography. The phase transition from aragonite to the amorphous phase does not cause a leap in the Electrical conductivity (EC), while a drastic increase in the EC, by 1.5–2.0 log units, only occurs with the onset of melting. The EC of amorphous CaCO3 is comparable to other hydrous mantle minerals at similar pressure and temperature conditions. The required fraction of amorphous CaCO3 implies that it can be excluded from the potential origins responsible for the observed high EC anomalies in the upper mantle. If the conductivity anomalies are induced by the presence of carbonate, a low-degree melting of carbonate-bearing peridotite is anticipated.