HR: 14:10h
AN: V53E-03    [Abstracts]
TI: Volatile transfer and recycling at UHP metamorphism; constraint from CCSD (Chinese Continental Scientific Drilling) eclogites
AU: * Okamoto, K
EM: kazu@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, P.O. Box 1-55 Nankang, Taipei, 115 Taiwan
AU: Iizuka, Y
EM: yiizuka@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, P.O. Box 1-55 Nankang, Taipei, 115 Taiwan
AU: Jahn, B
EM: jahn@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, P.O. Box 1-55 Nankang, Taipei, 115 Taiwan
AU: Tzeng-Fu, Y
EM: tfyui@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, P.O. Box 1-55 Nankang, Taipei, 115 Taiwan
AU: Xu, Z
EM: xzq@ccsd.org.cn
AF: Key Laboratory for Continental Dynamics, MLR, Institute of Geology, Chinese Academy of Geological Sciences, Baiwanzhuang Rd. #26, Beijing, 100037 China
AB: Study of dehydration and decarbonation processes of subducting oceanic crust is important to understand the island arc volcanism and recycling of water and carbon to deep mantle. 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, 2002; Crottini and Poli, 2004). If oxygen fugacities represented by the equilibrium NNO (Ni-NiO) or FMQ (fayalie-magnesite-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, EMU notes in miner. vol. 5). C-species are well described in Chinese UHP eclogites (e.g. Zhang and Kai, 1996). However, carbonates can be easily leached from outcrop. Therefore in the worst case, only graphite could be recognized from surface exposures although drilled core samples represent carbonates with graphite (e.g. Sanbagawa schist in Japan, Goto et al., 2000, Ann. Meet. Japan. Petrol. Miner. Mining Geol. Assoc.). From this point of view, 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). Two types of eclogite can be distinguished; dry- and phengite-eclogite. The phengite eclogite is associated with orthogneiss. Under the microscope, the dry eclogites contain relative coarse-grained (> 500 microns across) garnet, clinopyroxene and rutile with or without graphite, quartz, apatite, zircon, and pyrite. The phengite eclogites exhibit garnet, clinopyroxene, rutile, quartz, and phengite with or without graphite, pyrite, talc, apatite, zircon, and K-feldspar. Graphite is always recognized with pyrite, suggesting oxygen fugacity was low (NNO) at UHP stage. Estimated P, T conditions based on the assemblage garnet-clinopyroxene-phengite (Water and Martin, 1993; Ravna and Terry, 2004) gives P = 4 GPa, and T = 850 oC, suggesting that peak P reached graphite-diamond boundary. Temperature conditions of the eclogites exceed wet solidus in MORB composition. The solidus at 3-5 GPa is defined as follows; phengite + clinopyroxene + coesite (quartz) + fluid = melt (Schmidt et al., 2004). This reaction is evidenced by the _gmelt texturE_h: K-feldspar pool containing blocky quartz, clinopyroxene and graphite. The presence of graphite suggests that the eclogites released H2O-rich (CO2-poor) fluids and melt at UHP stage (Crottini and Poli, 2004).
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3630 Experimental mineralogy and petrology
DE: 3660 Metamorphic petrology
DE: 3694 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005