Instability of perovskite in a CO (sub 2) -rich environment; examples from carbonatite and kimberlite

Intricate multiphase pseudomorphs after perovskite (Nb-, LREE-poor) from calcite carbonatite (Sebljavr complex, Kola Peninsula) and serpentine-calcite kimberlite (Iron Mountain, Wyoming) are described. In the kimberlite, the major products of perovskite replacement are (in order of crystallization): kassite, anatase and titanite plus calcite, ilmenite and LREE-Ti oxide. In the carbonatite, perovskite is initially replaced by anatase plus calcite and, subsequently, ilmenite and ancylite-(Ce). In both cases the development of calcite and Ti-bearing phases after perovskite involved initial progressive leaching of Ca (super 2+) from the structure followed by crystallization of ilmenite and LREE minerals in the final stages, after the precipitation of groundmass calcite. The formation of kassite and titanite in the pseudomorphs in kimberlite was controlled by a lower Ca leach-rate and higher activity of SiO (sub 2) in this system, compared with the carbonatite. The composition of ilmenite from carbonatites and kimberlites is considered; ilmenite compositions from Sebljavr plot along the ilmenite-geikielite join, whereas those from Irom Mountain are significantly enriched in pyrophanite.