HR: 0800h
AN: T21A-0504    [Abstracts]
TI: UHP Ilmenite Exsolution from Iron-bearing Rutile in Eclogite from the Altyn Tagh, NW China
AU: * Liu, L
EM: liuliang@nwu.edu.cn
AF: Dept. of Geology, Northwest University, Xi'an, 710069 China
AU: * Liu, L
EM: liuliang@nwu.edu.cn
AF: Inst. of Geophys. & Planet. Phys., Univ. of California, Riverside, 92521 United States
AU: Zhang, J
EM: junfeng.zhang@email.ucr.edu
AF: Inst. of Geophys. & Planet. Phys., Univ. of California, Riverside, 92521 United States
AU: Green, H W
EM: hgreen@mail.ucr.edu
AF: Inst. of Geophys. & Planet. Phys., Univ. of California, Riverside, 92521 United States
AU: Jin, Z
EM: zmjin@cug.edu.cn
AF: Dept. of Earth Sciences, China University of Geosciences, Wuhan, 430074 China
AU: Sun, Y
EM: sunyong@nwu.edu.cn
AF: Dept. of Earth Sciences, China University of Geosciences, Wuhan, 430074 China
AB: Rutile transforms to TiO$_{2}$ (II) with the $\alpha$-PbO$_{2}$ structure with increasing pressure. The latter has been found recently in UHP metamorphic rocks as well as in shocked gneisses, suggesting a peak metamorphic pressure in excess of 7-8 GPa. TiO$_{2}$ (II) has been determined experimentally to have a much higher solubility of Fe$_{2}$O$_{3}$ than does rutile. Unlike rutile, that commonly incorporates thousands of ppm hydrogen in its structure, TiO$_{2}$ (II) has essentially no solubility of hydrogen. We report here preliminary study of ilmenite exsolution from rutile in eclogite from the Altyn Tagh, China. The Altyn Tagh has been recognized recently as a new UHP metamorphic terrane of China. Rutile is the most abundant accessory mineral in eclogite. Exsolved ilmenite platelets, 0.5-5$\mu$m thick and 10-100$\mu$m long, and parallel to (010) and (100) of the host rutile, are abundant. The area proportions of the exsolved ilmenite vary from 7.6 to 22.2% with an average of 14%. The reconstructed precursor rutile has a Ti content of 0.93 and a Fe content of 0.07 per formula unit, corresponding to an average Fe$_{2}$O$_{3}$ content of 5.3 wt %. This Fe$_{2}$O$_{3}$ content is significantly higher than those reported from other natural eclogitic rutiles and from experimental rutiles at high pressure. In addition, infrared spectra of these rutiles show that they have a single OH absorption band around 3290 cm$^{-1}$. The average hydroxyl contents in rutile calculated by using three different published coefficients for rutile are 0.028 wt% (Johnson et al., 1973), 0.128 wt% (Hammer and Beran, 1991) and 0.011 wt% (Maldener et al., 2001) respectively. The high calculated Fe$_{2}$O$_{3}$content and low present hydrogen content suggest the earlier presence of TiO$_{2}$ (II) at a pressure greater than 6-7 GPa that, upon transformation to rutile during exhumation, exsolved the observed large amounts of ilmenite. The very large amount of ilmenite observed here suggests that the Fe$_{2}$O$_{3}$ content within the rutile precursor could potentially be used as a barometer for very high pressure metamorphism.
DE: 8102 Continental contractional orogenic belts
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3630 Experimental mineralogy and petrology
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting