Whewellite-Bearing Fluid Inclusions in Zircons From a Stromatic Migmatite in the Chinese Sulu Ultrahigh-Pressure Metamorphic Belt

Abstract

Abstract Abundant primary fluid and melt inclusions occur in zircons from a stromatic migmatite sample in the Chinese Sulu ultrahigh-pressure (UHP) metamorphic belt, reflecting an immiscible melt-fluid coexistence during the partial melting process. Raman analysis shows that the fluid inclusions include whewellite, highly disordered carbonaceous matter (CM), diamond, calcite, dawsonite, quartz, cristobalite, H2O-bearing silica glass, muscovite, chalcopyrite, molybdenite, pyrite/pyrrhotite, magnetite, hematite, rutile, CO2 and H2O. The inclusions were entrapped during the exhumation of the rock from UHP to granulite-facies metamorphic conditions. The presence of whewellite suggests that the primary fluids during the entrapment of the inclusions contained some kind of dicarboxylic compound. The highly disordered CM still includes aliphatic hydrocarbon species revealed by the C-H stretching bands at ~2886 and ~2941 cm−1, and a subsidiary band at 1250 cm−1 on the low-frequency side of the D band at 1350 cm−1, indicating that it was derived from incomplete carbonization of an organic compound. The identification of whewellite as well as highly disordered CM in the fluid inclusions provides evidence that abiotic synthesis of organic compounds can happen at elevated pressures and temperatures where both melt and fluids are stable, and in which magnetite and sulphides may play a catalyzing role.