Evolving fluid composition during prograde metamorphism in subduction zones: A new approach using carbonate-bearing assemblages in the pelitic system |
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| Authors: | Atsushi Goto Keitaro Kunugiza Soichi Omori |
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| Institution: | aPetrology Course, School of Science, University of Hyogo (Branch of Himeji Institute of Technology), 2167, Shosha Himeji, Hyogo 671-2201, Japan;bDepartment of Environment and Humanity, Faculty of Human Development, University of Toyama, 3190, Gofuku Toyama 930-8555, Japan;cDepartment of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan |
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| Abstract: | The changing XCO2 in fluids during the progressive metamorphism in Sanbagawa belt of the Cretaceous subduction zone, Japan, was estimated by a newly proposed method. The subduction zone meta-sediments are characterized commonly by four-phase assemblages in the CaO–NaAlO2–KAlO2–Al2O3 system with excess quartz and a CO2–H2O binary fluid phase. Using the common assemblage of calcite–albite–muscovite–clinozoisite, XCO2 of the fluid was estimated to be from about 0.0001–0.0005 (the lowest grade chlorite zone), through 0.004–0.01 (garnet zone), 0.01–0.05 (albite–biotite zone) to 0.06–0.2 (oligoclase–biotite zone).The paragenetic relationship of meta-sediments from the subduction zones was compared in a wide P–T range to cover the stability fields of aragonite and jadeite. As a result, an excellent P–T–XCO2 relationship was delineated to serve as a quantitative monitor for the evolving fluid composition during the progressive metamorphism in subduction zones. |
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| Keywords: | Calcite/aragonite Metamorphic fluid composition Meta-sediments Phase relations Subduction zone metamorphism XCO2 |
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