Evidence for crystals from the lower mantle: baddeleyite megacrysts of the Mbuji Mayi kimberlite

We report results of a transmission electron microscopy study on previously dated centimeter-sized baddeleyite xenocrysts (ZrO2) from the Mbuji Mayi kimberlite, DR Congo [Sch?rer et al., Chem. Geol. 143 (1997) 1¯16]. Crystallographic analysis of their microstructures reveals that these megacrysts transformed to the present monoclinic state from a cubic parent phase, for which we infer a lower mantle origin because of its high temperature stability field. Our results substantiate ideas of solid material transfer across the 670 km mantle discontinuity and as such are in agreement with previous phase-petrologic investigations of tiny ferro-periclase inclusions in diamonds [Scott Smith et al., in: Kimberlites I: Kimberlites and Related Rocks, pp. 121¯142; Harte and Harris, Miner. Mag. 58A (1994) 384¯385; Kesson and Fitzgerald, Earth Planet. Sci. Lett. 111 (1991) 229¯240; Joswig et al., Earth Planet. Sci. Lett. 173 (1999) 1¯6] for which a lower mantle origin has also been proposed. In contrast to these inclusions, the baddeleyites further record chronological information and hence, potentially provide new insights into the history of mantle dynamics.