Ar. Chakhmouradian et Rh. Mitchell, Occurrence, alteration patterns and compositional variation of perovskite in kimberlites, CAN MINERAL, 38, 2000, pp. 975-994
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.