Zoned hibonites from Punalur, South India

Abstract

Zoned hibonites from a kalsilite, leucite, spinel, corundum, perovskite gneiss from the southern Indian granulite terrain near Punalur, Kerala, have rims that are the most Ti-rich yet recorded (0.83-0.87 Ti atoms per 19 O) and are essentially free of REE elements (ZREE < 0.01 atoms per 19 O) while the cores are the most REE-rich compositions yet recorded (YREE = 0.55-0.65 atoms per 19 O). Within the limits of analytical uncertainty, the compositions of the hibonite can be related to the theoretical end-member CaA112019 by the substitutions REE R 2+ ~ CaA1 and R2+R 4+ ~ AI2 with the REE-rich cores containing in excess of 50% of the REE R2+A111019 end member. Minor substitution of Na for Ca occurs in the rims, while non-stoichiometry in both the cores and rims is indicated by partial 12-fold site occupancy. Ion-microprobe analysis of the REE-rich hibonites reveals strong enrichment in LREE with La/Lu c. 250 000.

Mg–Al Sapphirine- and Ca–Al Hibonite-bearing Granulite Xenoliths from the Chyulu Hills Volcanic Field, Kenya

“…The studied hibonite is very poor in La, Ce, Th and Y. These low abundances are rather uncommon for terrestrial hibonites, as the latter, except for one locality (Punalur, India), are enriched in LREE and Th (Maaskant et al, 1980;Santosh et al, 1991;Rakotondrazafy et al, 1996). Low concentrations of La, Ce, Th and Y are typical, however, for hibonite from meteorites (e.g.…”