Instability of perovskite in a CO2-rich environment: Examples from carbonatite and kimberlite

Intricate multiphase pseudomorphs after perovskite (Nb-, LREE-poor) from ca lcite carbonatite (Sebljavr complex, Kola Peninsula, Russia) and serpentine calcite kimberlite (Iron Mountain, Wyoming) are described. In the kimberli te, the major products of perovskite replacement are (in order of crystalli zation): kassite, anatase and titanite plus calcite, ilmenite, LREE-Ti oxid e [? lucasite-(Ce)]. In the carbonatite, perovskite is initially replaced b y anatase plus calcite and, subsequently, ilmenite and ancylite-(Ce). In bo th instances, the development of calcite and Ti-bearing phases after perovs kite involved initial progressive leaching of Ca2+ from the structure follo wed by crystallization of ilmenite and LREE minerals in the final stages, a fter the precipitation of groundmass calcite. The formation of kassite and titanite in the pseudomorphs in kimberlite was controlled by a lower Ca lea ch-rate and higher activity of SiO2 in this system, compared with the carbo natite. The similarity between the two types of pseudomorphs results from t he instability of Nb-LREE-poor perovskite in a CO2-rich fluid at low temper atures. Perovskite is considered an unsuitable host for radioactive Sr-90 a nd REE isotopes owing to the low resistance of CaTiO3 to leaching and incom patibility of Sr and the rare earths with the products of perovskite replac ement.