HR: 08:45h
AN: V51E-04    [Abstracts]
TI: Microdiamonds Formation During
AU: * Ogasawara, Y
EM: yoshi777@waseda.jp
AF: Department of Earth Sciences, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyko, 169-8050 Japan
AB: The previous studies on the Kokchetav UHPM rocks by the authors group demonstrated the following subjects on the formation of microdiamonds. Microdiamond is highly abundant (max. ca. 2700carat/ton) in dolomite marble that has diopside (with K-bearing silicate lamella)-dolomite-garnet assemblage and was stable at XCO2=0.1. The microdiamonds in dolomite marble are classified into 3- types; S, R and T. The dominant type S (ca. 80%) indicates that it grew at two stages, the core and rim stages. R-type grew mainly at the core stage, and T-type grew at the rim stage (Ishida et al., 2003; Yoshioka & Ogasawara, 2005). One of the possible source of carbon for the 2nd stage growth is a fluid during UHPM. Some domains of dolomite marble contacting with dolomitic marble lack diamond, and indicate are lower XCO2 than diamond-bearing marble. No diamond occurs in dolomitic marble that is a product of strong H2O-rich fluid effect during UHPM (Ogasawara & Aoki, 2005); Ti-clinohumite-aragonite corresponds to extremely low-XCO2 (=<0.01), and the TiO2 carrier could be a H2O-rich fluid. Hydroxyl in lamellar-free diopside was confirmed as > 850ppm (Kikuchi & Ogasawara, in press). Low XCO2 condition corresponds to relatively oxidized conditions (Ogasawara et al., 2000). Extremely low-XCO2 conditions are unsuitable for diamond formation. A small amount of microdiamond (61 grains) occurs in diopside (with lamellar) in some layers of titanite-bearing calcite marble. Other domain contains titanite with coesite exsolution and the precursor silica-excess composition of titanite gave the minimum pressure as 6 GPa (Ogasawara et al., 2002). The presence of titanite (including relic aragonite + rutile) indicates very low-XCO2 (ca. 0.02). All grains of microdiamond are similar to R-type in morphology. No evidence for the 2nd stage diamond growth was observed. No diamond occurs in garnet- clinopyroxene rock like _gskarns_h. This rock has UHP evidence; coesite exsolution in titanite and K- bearing-silicate lamella in clinopyroxene, and is a product of the metasomatism (indicating very low XCO2) under UHP. The lack of diamond is consistent with other diamond-free carbonate rocks that were stable under extremely low- XCO2. The 2nd abundant diamond-bearing rock is pelitic gneiss. The characteristic features of microdiamonds show the strong contrast with those in dolomite marble. The dominant morphology is rounded to cuboidal form with rugged surface (> 80%); this corresponds to R- type. S-type is rare in pelitic gneiss. Morphology of microdiamond is controlled by growth and/or dissolution. Rounded grain with smooth surface may show the resorption after diamond growth. The absence of S-type diamond is a great difference from the microdiamond in dolomite marble, and indicates that fluid played different roles in both two diamond-bearing rocks; 1) carbon dissolved into aqueous fluid in pelitic gneiss, and 2) carbon precipitated from fluid to form microdiamond at the 2nd stage in dolomite marble. The microdiamond formed at the 2nd stage might be closely related to UHPM fluid. A model for fluid evolution from CO2-rich to H2O-rich during prograde stage can explain the metamorphic history of the Kokchetav carbonate rocks. Such UHPM fluid evolution may be caused by the dehydration in gneisses/eclogites surrounding the carbonate. Summarizing these, ``Intraslab UHP metasomatism" could be proposed. References: Ishida et al. (2003) J. Metamorphic Geol., 21. Kikuchi & Ogasawara (in press) GSA Special Papers. Ogasawara et al. (2000) Island Arc, 9, 400-416. Ogasawara et al. (2002) Am. Min., 87, 454-461. Ogasawara & Aoki (2005) Int. Geol. Rev., 47 (in press). Yoshioka & Ogasawara (2005) Int. Geol. Rev., 47, 703-715.
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