HR: 1340h
AN: V43A-1552    [Abstracts]
TI: FTIR Study of Microdiamonds From Garnet-Biotite Gneisses of the Kokchetav Massiv, Kazakhstan
AU: * Kikuchi, M
EM: m-kikuchi@suou.waseda.jp
AF: Department of Earth Sciences, Waseda University, 1-6-1 Nishiwaseda Shinjuku-ku, Tokyo, 169-8050 Japan
AU: Kagi, H
EM: kagi@eqchem.s.u-tokyo.ac.jp
AF: Laboratory for Earthquake Chemistry, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan
AU: Ogasawara, Y
EM: yoshi777@waseda.jp
AF: Department of Earth Sciences, Waseda University, 1-6-1 Nishiwaseda Shinjuku-ku, Tokyo, 169-8050 Japan
AU: Dobrzhinetskaya, L
EM: larissa@ucr.edu
AF: Department of Earth Sciences, University of California, Riverside, 900 University Avenue Riverside, CA, 92521 United States
AB: Microdiamonds from ultrahigh pressure metamorphic (UHPM) rocks of the Kokchetav massif Kazakhstan have been studied extensively by Fourier Transformed Infrared (FTIR) spectroscopy (De Corte et al., 2000, Cartigny et al., 2001) and transmission electron microscopy (Dobrzhinetskaya et al., 2001-2005; Hwang et al. 2005). The origin of these diamonds was proposed to be a COH supercritical fluid, or a melt of carbonate or silicate. The physical properties of fluids and melts are similar above a supercritical point. Nonetheless their diverse compositions are an important part of the geological history of metasediments subducted into mantle depths. Previous FTIR studies were carried out on large diamonds (100-300 microns) from garnet clinopyroxenites. They revealed that diamonds contain nitrogen, molecular water, C-H, and radicals of OH- and CO3. Small diamonds (1-50 microns) have not been studied yet because of the detection limits of FTIR spectroscopy and technical problems associated with well-polished samples. However, small diamonds are important since they may serve as a role of the primary nuclei around which a continuous growth of large diamonds occurs. Two-stage growth of Kokchetav diamonds was recognized recently (Ishida et al., 2003). Here we report our preliminary results of FTIR studies of ~30 and 60 micron diamonds from the Kokchetav garnet-biotite gneisses. More than 300 grains of yellow and translucent diamonds were separated from 200 grams of powdered gneiss by thermochemical digesting (Dobrzhinetskaya et al., 2005). Three or five grains of such diamonds were picked up by a steel needle. They were placed together on a diamond plate (window) made from a diamond crystal free of nitrogen and other impurities. FTIR spectra were determined after an accumulation of more than 1000 scans with a Thermo Nicolet Continu_Em microscope using an Avatar 370 FTIR spectrometer. All spectra show two nitrogen bands at 1132 cm-1 and 1282 cm-1, suggesting that the diamonds belong to Type IaA-Ib like other diamonds from UHPM terranes. Some spectra show no band at 1132 cm-1, which indicates the absence of low-temperature nitrogen aggregation known as defect C or type Ib. The 60-micron size diamond exhibits a similar ratio of two bands to that of previously published data (De Corte et al., 2000). Each spectrum contains a narrow H band and a broad OH band. Bands attributed to H2O and CO32- also were evident in some spectra. In addition, silicate bands most likely derived from silicates that survived thermochemical digesting were present. The impurities and inclusions of H, H2O and CO32- in small microdiamonds suggest that COH fluid played a significant role at the early stages of diamond growth.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3615 Intra-plate processes (1033, 8415)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3654 Ultra-high pressure metamorphism
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005