OCCURRENCE, ALTERATION PATTERNS AND COMPOSITIONAL VARIATION OF PEROVSKITE IN KIMBERLITES

Abstract

The present work summarizes a detailed investigation of perovskite from a representative collection of kimberlites, including samples from over forty localities worldwide. The most common modes of occurrence of perovskite in archetypal kimberlites are discrete crystals set in a serpentine–calcite mesostasis, and reaction-induced rims on earlier-crystallized oxide minerals (typically ferroan geikielite or magnesian ilmenite). Perovskite precipitates later than macrocrystal spinel (aluminous magnesian chromite), and nearly simultaneously with “reaction” Fe-rich spinel ( sensu stricto ), and groundmass spinels belonging to the magnesian ulvospinel – magnetite series. In most cases, perovskite crystallization ceases prior to the resorption of groundmass spinels and formation of the atoll rim. During the final evolutionary stages, perovskite commonly becomes unstable and reacts with a CO 2 - rich fluid. Alteration of perovskite in kimberlites involves resorption, cation leaching and replacement by late-stage minerals, typically TiO 2 , ilmenite, titanite and calcite. Replacement reactions are believed to take place at temperatures below 350°C, P a (Mg 2+ ) values. Perovskite from kimberlites approaches the ideal formula CaTiO 3 , and normally contains less than 7 mol.% of other end-members, primarily lueshite (NaNbO 3 ), loparite (Na 0.5 Ce 0.5 TiO 3 ), and CeFeO 3 . Evolutionary trends exhibited by perovskite from most localities are relatively insignificant and typically involve a decrease in REE and Th contents toward the rim (normal pattern of zonation). A reversed pattern is much less common, and probably results from re-equilibration of perovskite with a kimberlitic magma modified by assimilation or contamination processes. Oscillatory zonation on a fine scale is comparatively uncommon, and involves subtle variations in LREE , Th, Nb and Fe. Relatively high levels of LREE , Th and Nb observed in perovskite from some occurrences (Lac de Gras and Kirkland Lake in Canada, Obnazhennaya in Yakutia) probably result from inherent enrichment of the host kimberlites in “incompatible” elements. In some cases (Benfontein in South Africa), differentiation processes may have contributed to the accumulation of “incompatible” elements in perovskite.