Detailed morphological studies including phase-volume estimation were performed on a natural fluid inclusion in an eclogite-facies rock collected from the Sanbagawa metamorphic belt in Japan. The studied fluid inclusion, which is composed of H2O–NaCl liquid and a CH4 bubble, was picked up by using a focused ion beam (FIB) system and investigated by synchrotron-radiation-based X-ray microtomography (XCT), which provides a submicrometer spatial resolution. By using the FIB-XCT technique, we can perform detailed three-dimensional morphological studies on tiny objects (<10 μm) in combination with several other analytical techniques, such as Raman spectroscopy. The XCT image of the fluid inclusion (~7 μm in size) clearly shows a faceted shape, which corresponds fairly well to the interfacial relationship of α-quartz. This euhedral morphology also satisfies the crystal orientation of the host quartz, and hence is considered a negative crystal. The XCT determination of liquid and vapor-phase volumes in the fluid inclusion is in excellent accordance with the value estimated by the combination of microthermometry and the equation of state of the H2O–NaCl–CH4 system. Furthermore, this agreement indicates that XCT observation is applicable for volumetric analysis of more complex multi-phase systems, regardless of their compositions and phase numbers.

Combining the volume fraction of the fluids with the characterization and densimetry of each liquid/gas phase, we can accurately estimate the bulk composition and molar volume of the fluid inclusion. The isochore calculation of the estimated bulk properties shows lower-P conditions than the peak P-T conditions of the rock, although the inclusion was trapped during the pressure-increasing stage. This is possibly caused by density re-equilibration during decompression and cooling of the host metamorphic rock. The estimated isochore and the decompression P-T path from literature intersect at 0.2–0.3 GPa and 300°C, which is consistent with the fluid density closure temperature proposed in the literature for quartz-hosted fluid inclusions.

The present study demonstrates powerful potential of the FIB-XCT technique for performing morphological observations on tiny fluid inclusions a few micrometers across. With increasing availability of high-resolution XCT systems, the FIB-XCT technique combined with existing fundamental methods provides a new investigative tool for fluid inclusion studies, and may be especially valuable for complex inclusions that are difficult to analyze by other methods.

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