Melting Curve of Potassium Carbonate K2CO3 at High Pressures

Melting of carbonated rocks in the mantle influences the Earth’s deep carbon cycle and the long-term evolution of the atmosphere. Previous studies of the high-pressure melting curve of K2CO3 have yielded inconsistent results, with discrepancies of nearly 200 °C at 3 GPa and more than 400 °C at 12 GPa. Here, we report constraints on the melting curve of K2CO3 at pressures up to 20 GPa from in situ ionic conduction experiments and Pt sphere experiments. To help resolve the large discrepancies, we tested the ionic conduction method against the well-established differential thermal analysis (DTA) method and conventional Pt sphere method at the ambient pressure of 1 bar. Furthermore, ionic conduction experiments were conducted on sodium chloride (NaCl) to reduce uncertainties in pressure calibration of the multi-anvil press. We also modified the configuration of the in situ ionic conduction experiments to minimize the influence of thermal gradient on melting point determination. Finally, we inspected the effect of water by varying the initial sample state and container wall thickness in the Pt sphere experiments and applied X-ray radiography as a reliable and efficient method to examine the products. Compared with the results from the ionic conduction experiments, the melting point of K2CO3 from the Pt sphere experiments was found to be 200~400 °C lower, likely due to a small amount of water trapped by hygroscopic K2CO3 in closed platinum (Pt) capsules. We find that anhydrous K2CO3 remains more refractory than Na2CO3 at elevated pressures.