ABSTRACT

Direct chemical analysis of a single fluid inclusion was conducted by micro-excavation at cryo-temperatures. A scanning electron microscope (SEM) equipped with a focused ion beam (FIB), an energy dispersive X-ray spectrometer (EDS), and a cold stage, were used to analyze the chemical composition of the sample fluid inclusion, which was trapped in the ultrahigh-pressure (UHP) talc-garnet-chloritoid schist obtained from the Makbal metamorphic complex, Kyrgyz. Conventional techniques such as microthermometry and cryo-temperature and room-temperature Raman spectroscopy enabled the detection of NaCl and CaCl₂ as solute species in the fluid inclusions, as well as high salinity of 20.5 mass% CaCl₂ and 1.7 mass% NaCl with the assumption of a NaCl-CaCl₂-H₂O ternary system. However, additional chemical analysis using the present cryo-FIB-SEM-EDS system further revealed the presence of K as a solute element, which is hardly identified by conventional techniques. Petrographic examination of the fluid inclusions indicated that they had been trapped during the exhumation stage of the UHP talc-garnet-chloritoid schist, possibly originating from the decompression breakdown of lawsonite. The data acquired from the present study challenges previous reports of the existence of simple chemical system of aqueous fluids in high-pressure and UHP metamorphic terrane in eastern Asia. This is imperative because misidentification of solute species introduces errors into salinity estimation, resulting in inaccuracy propagation in quantitative analytical processes such as LA-ICP-MS. A detailed fluid inclusion petrography would thus require accurate quantitative analysis involving FIB-based sample-preparation and SEM-EDS analysis.