Raman imaging of fluid inclusions in garnet from UHPM rocks (Kokchetav massif, Northern Kazakhstan)

Confocal Raman imaging of fluid inclusions in garnet porphyroblasts from diamond-grade metamorphic calc-silicate rocks from the Kumdy-Kol microdiamond deposit (Kokchetav Massif, Northern Kazakhstan) reveals that these fluid inclusions consist of almost pure water with different step-daughter phases (e.g., calcite, mica and rare quartz). These fluid inclusions are characterized by negative crystal shape of the host-garnet and they exclusively occur within the core of garnet porphyroblasts. These observations are consistent with their primary origin, most likely at ultrahigh-pressure (UHP) metamorphic conditions. The euhedral newly formed garnet, different in color and composition, was found to be associated with these fluid inclusions. It is proposed that newly formed garnet and water fluid inclusions appear by reaction between the hydrous fluid and the garnet-host. These fluid inclusions provide an unequivocal record of almost pure H2O fluids, indicating water-saturated conditions within subducted continental crust during prograde stage and/or ultrahigh-P metamorphism.

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► Fluid inclusions in UHPM minerals were studied with WITec alpha300R Confocal Microscope.
► We found liquid H2O, phlogopite and calcite within the fluid inclusions.
► H2O-fluid was a stable phase at peak metamorphic conditions (4–6 GPa and 950–100 °C).