HR: 0800h
AN: T21A-0503    [Abstracts]
TI: Occurrence of microdiamond in UHP calcite marble from the Kokchetav Massif
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: Adachi, T
EM: t-adatchi@fuji.waseda.jp
AF: Department of Earth Sciences, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo, 169-8050 Japan
AU: Aoki, K
EM: kazumasa@asagi.waseda.jp
AF: Department of Earth Sciences, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo, 169-8050 Japan
AB: Small amount of microdiamond was discovered in UHP calcite marble from Kumdy-kol, the Kokchetav Massif northern Kazakhstan. UHP calcite marble is characterized by 1) pure calcite (after aragonite) is only one carbonate phase, 2) presence of titanite, K-feldspar and diopside with phengite and K-feldspar lamella, 3) coesite exsolution from supersilicic titanite. This marble contains UHP evidence as coesite of exsolution origin in titanite and lamella texture in diopside. Minimum P condition was constrained as 6 GPa by precursor composition of titanite. So far, we have described this calcite marble as diamond-free UHP carbonate rock (Ogasawara et al., 2002); however, our recent observation found microdiamond in some domains (B-type) of this rock. Microdiamond occurs only in diopside and its amount is very low compared with diamond-bearing dolomite marble. We found 61 microdiamond grains in two thin sections (c.f. in dolomite marble, highest concentration domain contains over 4000 grains in one thin section). Microdiamond grains are heterogeneously distributed in some layers. The domain (A-type) containing relatively large amount of titanite does not contain diamond. The microdiamond in calcite marble shows 3 to 20 micrometers in diameter, rounded or cubic form, pale yellowish color and translucent character. Laser Raman spectroscopy indicated that FWHM of a Raman band at about 1332 cm-1 ranges from 2.99 to 3.64 cm-1 (average: 3.28 cm-1) and this average value is the smallest among core of S-type, rim of S-type, R-type and diamond in gneiss. Based on the morphology and other features except for FWHM of Raman band, the microdiamond in calcite marble is similar to R-type in dolomite marble (Ishida et al., 2003). No S-type diamond (_gStar_h-shaped form with core and rim) occurs in this calcite marble, and this indicates that the second stage growth of microdiamond probably from multicomponent aqueous fluid had not occurred in calcite marble. Low amount of microdiamond occurs only in diopside, no diamond occurs in titanite (stable at extremely low-XCO2 conditions), and lack of the second stage growth of microdiamond in calcite marble; all these features may be related with extremely low-XCO2 condition under UHP metamorphism. One of the possible explanations for this diamond occurrence is that microdiamonds in calcite marble are relic crystal formed before the second stage growth (the rim stage of S-type microdiamond in dolomite marble). Therefore, it is very important to confirm whether these microdiamond in B-type calcite marble had been growing or dissolving under such extremely low-XCO2 conditions. References Ishida et al. (2003): Journal of Metamorphic Geology, Vol. 21, p. 515-522. Ogasawara et al. (2000): The Island Arc, Vol. 9, p. 400-416. Ogasawara et al. (2002): American Mineralogist, Vol. 87, p. 454-461.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3620 Crystal chemistry
DE: 3660 Metamorphic petrology
DE: 1212 Earth's interior--composition and state (8105)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting