HR: 17:00h
AN: V44A-04    [Abstracts]
TI: P, T conditions of the CCSD (Chinese Continental Scientific Drilling) eclogites; importance of ferric estimation based on Mössbauer and micro-XANES analysis
AU: * Okamoto, K
EM: kokamoto@mail.saitama-u.ac.jp
AF: Dept. Geoscience, Saitama University, Shimo-Okubo 255, Sakura-ku, Saitama, 338-8570, Japan
AU: Jahn, B
EM: jahn@eath.sinica.edu.tw
AF: Institute of Earth Science, Academia Sinica, P.O. Box 120, Academic Road, Sec. 2, Taipei, 115, Taiwan
AU: Yui, T
EM: tfyui@eath.sinica.edu.tw
AF: Institute of Earth Science, Academia Sinica, P.O. Box 120, Academic Road, Sec. 2, Taipei, 115, Taiwan
AU: Akasaka, M
EM: akasaka@riko.shimane-u.ac.jp
AF: Department of Geoscience, Shimane University, Nishikawatsu 1060, Matsue, 690-8504, Japan
AU: Xu, Z
EM: xuzhiqin@ccsd.org.cn
AF: Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing, 100037, China
AB: Recent UHP experiments in C-O-H fluid-bearing MORB system have revealed that phase change and fluid composition depend on oxygen fugacity (e.g. Molina and Poli, 2000). If oxygen fugacities represented by the equilibrium NNO (Ni-NiO) or FMQ (fayalie-magnetite-quartz) are assumed to be the average condition of UHP metamorphism, then the phase assemblages of UHP rocks are expected to have graphite/diamond only, graphite/diamond +carbonates, or carbonates only depending on the bulk compositions (Poli and Fumagalli, 2004). CCSD (Chinese Continental Scientific Drilling) samples are probably the best for identification of C- species in UHP rocks. We investigated nine eclogites from various depths (170 to 2000 m). Under the microscope, the eclogites contain garnet, clinopyroxene. quartz and rutile with or without phengite, graphite, apatite, zircon, pyrite, talc, and K-feldspar. Graphite is always recognized with pyrite, suggesting oxygen fugacity was low (NNO) at UHP stage. The presence of graphite suggests that the eclogites released H2O-rich (CO2- poor) fluids and melt at UHP stage. The studied eclogites contain CaEs component in clinopyroxene. Therefore, Fe3+ content can not be calculated based on EPMA analysis. We estimated Fe3+/Fe2+ based on Mossbauer and micro-XANES analysis. With ignoring the Fe3+, P-T conditions based on the assemblage of phengite-garnet-cpx- (coesite) can be estimated as P=3-5 GPa, and T = 850–950 oC. However, the Fe3+ estimation from the clinopyroxenes based on Mössbauer and micro-XANES analysis corrects the P, T condition as 3-4 GPa, and 650 -780 oC. The geothermobarometry based on the kyanite-garnet-cpx- phengite-coesite assemblage (Ravna and Terry, 2004) is reliable because temperature estimation is independent from Fe3+ content in clinopyroxene. The estimation from one eclogite gives P=3.4 GPa, and T=750oC, quite consistent with the above estimation. The thermometry based on Zr concentration from rutile also gives estimated as 600 to 730 oC. Ref. M & Poli (2002) EPSL, 176, 295; P & F (2004) EMU notes in miner. vol. 5. 307; R & T (2004) JMG, 22, 579; .Z & K (1996) Petrology and Structural Geology 7, Kluwer Academic Press, 49.
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3652 Pressure-temperature-time paths
DE: 3654 Ultra-high pressure metamorphism
DE: 3900 MINERAL PHYSICS
DE: 8100 TECTONOPHYSICS
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting