Occurrence, alteration patterns and compositional variation of perovskite in kimberlites

The present work summarizes a detailed investigation of perovskite from a r epresentative 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 me sostasis, and reaction-induced rims on earlier-crystallized oxide minerals (typically ferroan geikielite or magnesian ilmenite). Perovskite precipitat es later than macrocrystal spinel (aluminous magnesian chromite), and nearl y simultaneously with "reaction" Fe-rich spinel (sensu stricto), and ground mass spinels belonging to the magnesian ulvospinel - magnetite series. In m ost cases, perovskite crystallization ceases prior to the resorption of gro undmass spinels and formation of the atoll rim. During the final evolutiona ry stages, perovskite commonly becomes unstable and reacts with a CO2-rich fluid. Alteration of perovskite in kimberlites involves resorption, cation leaching and replacement by late-stage minerals, typically TiO2, ilmenite, titanite and calcite. Replacement reactions are believed to take place at t emperatures below 350 degreesC, P < 2 kbar, and over a wide range of a(Mg2) values. Perovskite from kimberlites approaches the ideal formula CaTiO3, and normally contains less than 7 mol.% of other end-members, primarily lue shite (NaNbO3), loparite (Na0.5Ce0.05TiO3), and CeFeO3. Evolutionary trends exhibited by perovskite from most localities are relatively insignificant and typically involve a decrease in REE and Th contents toward the rim (nor mal pattern of zonation). A reversed pattern is much less common, and proba bly results from re-equilibration of perovskite with a kimberlitic magma mo dified by assimilation or contamination processes. Oscillatory zonation on a fine scale is comparatively uncommon, and involves subtle variations in L REE, Th, Nb and Fe. Relatively high levels of LREE, Th and Nb observed in p erovskite from some occurrences (Lac de Gras and Kirkland Lake in Canada, O bnazhennaya in Yakutia) probably result from inherent enrichment of the hos t kimberlites in "incompatible" elements. In some cases (Benfontein in Sout h Africa), differentiation processes may have contributed to the accumulati on of "incompatible" elements in perovskite.