Melting curve of H2O to 90 GPa measured in a laser-heated diamond cell

We report new data on the melting curve of H2O in the range 20-90 GPa and 1000-2400 K obtained in a laser-heated diamond anvil cell. We found a marked discrepancy between our present results and previous work which covered the pressure range up to 38 GPa. The melting curve shows a discontinuous change in slope at about 43 GPa, indicating a first-order phase transition most likely to the ice X phase. We could find no evidence for a strong dissociation of H2O up to 200 K above the melting temperatures.