HR: 0830h
AN: S21E-0358    [PDF]
TI: Nanocrystalline inclusions of ilmenite and ilmenite with perovskite structure in olivine from an ilm.-ga.-peridotite, kimberlite pipe Udachnaya, Yakutia.
AU: * Wirth, R
EM: wirth@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg Div.4, Potsdam, D-14473 Germany
AU: Matsyuk, S
EM:
AF: Inst. Geochemistry and Mineralogy, Ukrainian Acad. of Sciences, Palladina 34, Kiev, 02164 Ukraine
AB: Oriented olivine grains (010) from a mantle-derived xenolith from the kimberlite pipe Udachnaya have been studied by transmission electron microscopy (TEM). The host xenolith is an ilmenite-garnet peridotite with the mineral assemblage gt + ol + cpx + accessory ilm. The investigated olivine grains exhibit a typical grain size of about 1--2 mm. Crystallisation of the ilm-ga-peridotites occurred at 35-40 kbar and 1300-1400$\deg$C according to graphite pyrope facies conditions [1, 2]. The investigated olivine contains inhomogeneously distributed nanometer sized inclusions of ilmenite always intergrown with a small lamella of magnetite. The grain size of ilmenite is in the range of 50-70 nm and the magnetite lamella is about 10-20 nm thick. The orientation relationship between olivine-ilmenite and magnetite is given by: [010]ol // [112]mgt // [1-210]ilm (00-1)ol // (11-1)mgt // (000-1)ilm A second type of inclusions has the chemical composition of ilmenite, however, the electron diffraction pattern suggests a different structure, that of perovskite. The diffraction pattern can only be indexed assuming a monoclinic perovskite structure with doubled ao lattice spacing. The doubled ao spacing is present in the diffraction pattern as additional weak reflections with 1.1 nm spacing, equivalent with 2ao. The orientation relationship between ilmenite with perovskite structure and olivine is given by: [010]ol // [0-10]perovskite (10-1)ol // (100)perovskite It is suggested that in a first step Fe-rich ilmenite with monoclinic perovskite structure exsolved from olivine. The high Fe concentration causes the monoclinic distortion of the lattice. During uplift the perovskite structure is transformed into the ilmenite structure exsolving the excess iron as a small platelet of magnetite. [1] Lazzko E.E. (1979) Nedra, Moscow, 192 pp (in Russian.[2] Sobolev, V.S., Dobretsov,N.L.,Sobolev, N.V. (1972) Classification of deep seated xenoliths and the type of the upper mantle. Geol. Geophys. N12, 37-42 in Russian)
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3625 Descriptive mineralogy
DE: 5112 Microstructure
SC: Seismology [S]
MN: 2003 Fall Meeting