TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 52, 2018

Micro-excavation and direct chemical analysis of individual fluid inclusion by cryo-FIB-SEM-EDS: application to the UHP talc-garnet-chloritoid schist from the Makbal Metamorphic Complex, Kyrgyz Tian-Shan

Kenta Yoshida1*, Rustam Orozbaev2, Takao Hirajima3, Akira Miyake3, Akira Tsuchiyama3, Apas Bakirov2, Akira Takasu4 and Kadyrbek Sakiev2

1Research and Development Center for Ocean Drilling Science, Japan Agency for Marine-earth Science and Technology, Yokosuka 237-0061, Japan
2Institute of Geology, Kyrgyz National Academy of Sciences, 30 Erkindik Avenue, 720040 Bishkek, Kyrgyzstan
3Department of Geology and Mineralogy, Kyoto University, Kyoto 606-8502, Japan
4Department of Geoscience, Shimane University, Matsue 693-8501, Japan.

(Received May 10, 2017; Accepted August 1, 2017)

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.
Key words: fluid inclusion, focused ion beam, Raman spectroscopy, ultrahigh-pressure metamorphism, Kyrgyz Tian-Shan

*Corresponding author E-mail: yoshida_ken@jamstec.go.jp

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