Niobian ilmenite, hydroxylapatite and sulfatian monazite: Alternative hosts for incompatible elements in calcite kimberlite from Internatsional'naya,Yakutia

Hypabyssal calcite kimberlite from the Internatsional' naya intrusion in Ya kutia, Russia, consists of serpentinized olivine and laths of primary Sr-en riched calcite immersed in a mesostasis of serpentine and secondary Sr-poor calcite. Accessory groundmass minerals include spinel, pyrite, pyrrhotite, nickeliferous sulfides, baddeleyite, ilmenite, hydroxylapatite and monazit e-(Ce). An unusual feature of this mineral assemblage is the absence of per ovskite, which normally serves as a major depository for the rare-earth ele ments and Nh in kimberlites. At Internatsional'naya, these incompatible ele ments are concentrated in alternative mineral hosts, primarily ilmenite-gro up minerals and phosphates. Ilmenite-group minerals are represented by macr ocrystal chromian ferroan geikielite, mantles of manganoan ilmenite on tita niferous spinels, and manganoan niobian ilmenite; the latter mineral occurs as platy crystals and radiating clusters (less than or equal to 100 mu m) in the groundmass. The most evolved compositions of ilmenite are depleted i n Mg, Cr and Fe3+, and contain up to 9.5 wt.% MnO, 12.5 wt.% Nb2O5 and 0.8 wt.% Ta2O5. Accommodation of Nb and Ta in ilmenite is accompanied by the cr eation of vacancies in the site normally occupied by divalent cations: (Fe, Mn)(2+) + 2Ti(4+) <-> square + 2(Nb,Ta)(5+). Hydroxylapatite is confined to the interstices between calcite laths, and serves as a host for light rare -earth elements (less than or equal to 2.1 wt.% LREE2O3) and Sr (less than or equal to 0.8 wt.% SrO). Monazite is unusually enriched in S (6.8-8.4 wt. % SO3), Sr and Ca (less than or equal to 4.9 and 7.1 wt.% oxides, respecti vely), and depleted in Th. Incorporation of these elements in the mineral s tructure involves the complex substitution Ce3+ + P5+ <-> Ca2+(Sr2+) + S6+. The occurrence of these minerals in the kimberlite from Internatsional'nay a reflects enrichment of a parental magma in incompatible elements, and pro bably assimilation of sulfate-bearing brine from the sedimentary country-ro cks.