HR: 16:15h
AN: V54B-02    [Abstracts]
TI: Diopside and Coesite Lamellae in Cr-spinel of Podiform Chromitite in Luobusa Ophiolite, Tibet: Evidence for Ultra-High Pressure Origin of the Chromite
AU: * Yamamoto, S
EM: syamamot@geo.titech.ac.jp
AF: Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, 152-8551 Japan
AU: Komiya, T
EM: tkomiya@geo.titech.ac.jp
AF: Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, 152-8551 Japan
AU: Takafuji, N
EM: takafuji@geo.titech.ac.jp
AF: Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, 152-8551 Japan
AU: Maruyama, S
EM: smaruyam@geo.titech.ac.jp
AF: Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, 152-8551 Japan
AB: The Luobusa ophiolite lies in the Indus-Tarlung Zangbo suture zone, which separates Indian continent from Eurasia. In this study, we discovered needle-shaped clinopyroxene and coesite within Cr-spinel of podiform chromitite in the Luobusa ophiolite, Southern Tibet. The Cr-spinels contain abundant cpx-needles, which are almost 1μ m wide and a few ten μ m long under the microscopic observation. Crystal orientation of the cpx-neeldes seems random at a glance, but some cpx needles are parallel each other. These textures of the cpx needles within Cr-spinel are very similar to that of pyroxene lamellae within garnet in ultramafic mantle xenoliths (Haggerty and Sautter 1990). Hence, the similarity suggests that these cpx needles were formed as exsolution lamellae from the host Cr-spinel. We investigated the chemical composition, crystal structure of the needle in Cr-spinel and the topotaxy between the needles and the host Cr-spine using SEM-EDS, Laser-Raman and Analtical Transmission Electron Microscopy (ATEM) at Tokyo Institute of Technology. EDS spectra of ATEM showed that most needles have diopside composition. A rod-shaped SiO ₂ phase was also observed and identified as coesite by electron diffraction pattern. Examination by selected area electron diffraction and nano-beam electron diffracrion showed that the topotaxy between diopside and Cr-spinel and between coesite and Cr-spinel are [100]sp // [103]di and [011]sp // [010]di, [100]sp // [012]coe and [110]sp // [100]coe respecivly. These observations indicate that diopside and coesite were formed as exsolution lamellaes from Cr-spinel. Although it is well-known that ilmenite occurs as exsolution lamellae in Cr-spinel from layered intrusion (e.g. Rollinson et al., 2002), diopside and coesite lamellae in Cr-spinel have not been known yet. Previous experimental and mineralogical works showed that CaO and SiO₂ were not incorporated in Cr-spinel. How were the diopside and coesite lamellae generated? Recently, high-pressure polymorphs of Cr-spinel, CaFe₂O₄ (CF) and CaTi₂O₄ (CT) structures, were discovered in the shock veins of the Suizhou meteorite (Chen et al. 2003a). And then, they performed laser-heated diamond anvil cell experiments to confirm that Cr-spinel transforms to CF at 12.5 GPa, and to CT structures above 20GPa. They mentioned the possibility that the Cr-spinel polymorphs, CF and CT phases, incorporate Ca, Si, Ti, and Fe as CaFe2O4 and CaTi₂O₄ solid solutions (Chen et al. 2003b). It suggests that diopside and SiO₂ component can be solved in high-pressure polymorphs of Cr-spinel. Actually coesite lamallae is the direct evidence of the high-pressure condition for the formation of these lamellaes. On the basis of these arguments, we conclude that the Cr-spinel with the diopside and coesite lamellae were formed as the CaO- and SiO₂-bearing polymorphs under the high-pressure condition, over 12.5 GPa.
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3621 Mantle processes (1038)
DE: 3654 Ultra-high pressure metamorphism
DE: 3694 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005