HIGH INTERNAL-PRESSURES IN DIAMOND FLUID INCLUSIONS DETERMINED BY INFRARED-ABSORPTION

FLUID inclusions in mantle-derived minerals provide a rare opportunity to study deep-seated mantle fluids. Among mantle minerals, only diamond possesses sufficient strength to encapsulate and transport to the surface fluids that were trapped at depths below 100 km (ref. 1). Previous studies of the bulk composition 2 and internal morphology 3 of micro-inclusions in cubic and coated diamonds suggested that they contain fluids, but their depth of origin could not be determined. Here I report infrared spectroscopic evidence for residual internal pressures of 1.5-2.1 GPa within these micro-inclusions, higher than any reported previously for fluid inclusions. Extrapolation to mantle temperatures indicates fluid pressures of 4-7 GPa, comparable to those estimated on the basis of mineral equilibria between crystalline inclusions in diamond 4. These pressures fall within the diamond stability field in the upper mantle, suggesting that the deep-seated fluids, rich in H2O, CO2, SiO2, and K2O, are probably the mother solutions from which cubic, and possibly eclogitic diamonds grow.