HR: 15:00h
AN: V13C-07    [Abstracts]
TI: A study of oxygen isotopes and hydroxyl content in minerals of UHP metamorphic rocks from CCSD core samples (734 to 933 m)
AU: * Chen, B
EM: yfzheng@ustc.edu.cn
AF: Univ. Sci. & Technol. China, School of Earth and Space Sciences,, Hefei, 230026 China
AU: Zheng, Y
EM: yfzheng@ustc.edu.cn
AF: Univ. Sci. & Technol. China, School of Earth and Space Sciences,, Hefei, 230026 China
AU: Wu, Y
EM: ybwu@ustc.edu.cn
AF: Univ. Sci. & Technol. China, School of Earth and Space Sciences,, Hefei, 230026 China
AB: Two continuous core segments between eclogite and gneiss from the main hole of CCSD project, at the depths of 734.21 to 737.16m (02CCSD-I) and 929.67 to 932.86m (02CCSD-II), respectively, are systematically analyzed for oxygen isotopes by the laser fluorination technique and for hydroxyl (OH) content in nominally anhydrous minerals by the Fourier transform infrared spectroscopy (FTIR). Oxygen isotope analysis shows that garnet from eclogites in the two core segments has d18O values of +1.0 to +3.8­‰ and -4.1 to -3.3­‰, respectively, and that garnet from gneisses in the two core segments has d18O values as low as -4.0­‰. These suggest that protoliths of both eclogite and gneiss experienced meteoric-hydrothermal alteration before continental subduction. An oxygen isotope front is observed in the contact zone between eclogite and gneiss in the core segment 02CCSD-I. This is interpreted as a result of fluid-rock interaction between eclogite near the contact zone and fluid mainly from gneiss nearby, which is controlled by fluid transport distance. The FTIR analysis shows concentrations of hydroxyl in 50 to 776 ppm for garnet and in 321 to 1636 ppm for omphacite. For quartz, we have only found the characteristic band of hydroxyl at 3596cm-1 but its hydroxyl content is very low. A profile-comparison of oxygen isotope ratio and hydroxyl content shows a consistent change with depth between d18O value and OH content in garnet, omphacite and other minerals. This suggests that retrograde alteration and associated local fluid flow may be the main cause for the observations that the ultrahigh- and high-pressure minerals deviate from oxygen isotope equilibrium fractionations and the degree of disequilibrium is controlled by local water/rock ratios. Therefore, the significant amounts of hydroxyl in garnet and omphacite, together with other nominally anhydrous minerals, may constitute a major source of retrograde fluid by releasing water during decompression exhumation.
DE: 3660 Metamorphic petrology
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting