HR: 0800h
AN: MR11A-0922    [Abstracts]
TI: Free SiO2 from Mantle of Ophiolite in Tibet, China
AU: * Bai, W
EM: yfren@ccsd.org.cn
AU: Yang, J
EM: yangjsui@ccsd.org.cn
AU: Ren, Y
EM: yfren@ccsd.org.cn
AU: Fang, Q
EM: yfren@ccsd.org.cn
AU: Yan, B
EM: yfren@ccsd.org.cn
AU: Rong, H
EM: ronghe@ccsd.org.cn
AB: Free SiO$_{2}$ and their host minerals were discovered from podiform chromitites of the Luobusa ophiolite, Tibet. The ophiolitic body is about 200 km east-southeast of Lhasa in the Indus-Yarlung Zangbo suture zone. It is a fault-bounded slab, 1-2 km thick, with about 70 km$^{2}$ in area. Luobusa ophiolitic body consists of harzburgite, cumulative and m"Żlange phases. Numerous ultra-high pressure minerals including diamond have been recovered in the podiform chromitites. Since upper mantle peridotites are silica-unsaturated, free SiO$_{2}$ and olivine never coexist in peridotite. Therefore, in mantle peridotite and ultramafic rocks, primary free SiO$_{2}$ has not been found yet. But recently, free SiO$_{2}$ (quartz and coesite) was discovered from the podiform chromitites in mantle peridotites of Luobusa ophiolite. The free silica presented as inclusions in OsRuIr alloys and PtFe alloys, or as intergrowth with TiO$_{2}$ and FeO. These occurrences of free silica show their mantle origin and excluding the possibilities of secondary genesis or contamination. Early high pressure and temperature experiments proved that under the conditions of the lower mantle, olivine from the upper mantle is decomposed to mixtures of periclase and stishovite. Therefore, the free SiO$_{2}$ from the podiform chromitites in mantle peridotite in Tibetan ophiolite is most likely derived from the lower mantle. Besides the free SiO$_{2}$, other simple oxides, such as FeO, MgO, CaO, Al$_{2}$O$_{3}$, Cr$_{2}$O$_{3}$, Fe$_{2}$O$_{3}$ and Fe$_{3}$O$_{4}$, are also found in the same sample from Luobusa chromitites. How is the free SiO$_{2}$ in Tibetan ophiolite protected and transported from the lower mantle to the upper mantle? A reasonable explanation is that silica must be contained within residual mineral and the latter was brought up to oceanic mantle depth by an upwelling plume from the core-mantle boundary.
DE: 3625 Descriptive mineralogy
DE: 3630 Experimental mineralogy and petrology
DE: 3672 Planetary mineralogy and petrology (5410)
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting