NANOCRYSTALLINE (MG, FE, CR)TIO3 PEROVSKITE INCLUSIONS IN OLIVINE FROM A MANTLE XENOLITH, UDACHNAYA-EAST KIMBERLITE PIPE, SIBERIA

Abstract

Olivine grains from an ilmenite–garnet–peridotite nodule from the kimberlite pipe Udachnaya–Wostotschnaya, Siberia have been investigated by TEM methods (electron diffraction, HREM, AEM, EELS). The nodule is a high-temperature garnet peridotite, which equilibrated at about 1200–1300 °C and 200 km depth. TEM investigation of several olivine grains revealed nanometre-sized inclusions of (Mg,Fe,Cr)TiO3 perovskite together with ilmenite. Both types of inclusions exhibit a typical grain size of about 50 nm. The crystal structure of the perovskite inclusions is monoclinic with a0=1.095 nm, b0=0.5169 nm, c0=0.743 nm and the monoclinic angle β=95°. The orientation relationship between perovskite and olivine is (101)ol//(300)per and [010]ol//[01¯0]per. This observation combined with the experimentally determined ilmenite/perovskite phase boundary [A. Metha, K. Leinenweber, A. Navrotsky, M. Akaogi, Calorimetric study of high-pressure polymorphism in FeTiO3: stability of the perovskite phase, Phys. Chem. Minerals, 21 (1994) 207–212] [8] allows the use of perovskite in olivine as a geobarometer. The presence of perovskite in olivine suggests that the peridotite originated at pressures of 8–10 GPa, significantly higher than the pressure of last silicate equilibration (4.5–6.5 GPa).