Nanocrystalline (Mg, Fe, Cr)TiO3 perovskite inclusions in olivine from a mantle xenolith, Udachnaya-East kimberlite pipe, Siberia

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).