Melting of H₂O and CO₂-bearing eclogite at 4–6 GPa and 900–1200 °C: Implications for the generation of diamond-forming fluids

Eclogites play a significant role in geodynamic processes, transferring large amounts of basaltic material and volatiles (chiefly CO₂ and H₂O species) into the earth's mantle via subduction. Previous studies of eclogite melting focused on two end member systems: either carbonated or hydrous eclogites. Here we focus on the hydrous carbonated eclogitic system in order to define the position of its solidus and determine the near solidus fluid and melt compositions at 4–6 GPa and 900–1200 °C. Experiments were performed on a rocking multi-anvil press. The total dissolved solids in the equilibrated fluids were analyzed following the cryogenic technique using a LA-ICP-MS. H₂O and CO₂ content were determined by mass balance calculations. Solid phases were chemically characterized using an EPMA. Garnet and clinopyroxene are present in all experiments, assembling the eclogitic rock. A carbonate phase was detected at all temperatures at 4 GPa and at temperatures below 1200 °C at 5 and 6 GPa. Coesite was observed at all pressures below 1200 °C. The solidus was crossed between 1000 and 1100 °C at 4 and 5 GPa. At 6 GPa we observed a relatively smooth decrease in the H₂O and CO₂ content of the fluid phase with rising temperature, suggesting the presence of a supercritical fluid. The second critical endpoint is thus defined in this system at ∼5.5 GPa and 1050 °C. The composition of fluids and melts reported in this study indicates that the hydrous carbonated eclogite system is a plausible source-rock for high density fluids (HDFs) found in microinclusions in diamonds, specifically for the intermediate compositions along the array spanned between low-Mg carbonatitic HDFs and hydrous-silicic ones. Our results suggest that the whole array reflects melting in a heterogeneous mantle. Melting of water-rich eclogite produces silicic HDFs, carbonate-rich zones will produce carbonatitc HDFs, while source-rocks with varying H₂O/CO₂ ratios produce intermediate compositions.