Three compositional varieties of perovskite from kimberlites of the Lac deGras field (Northwest Territories, Canada)

In hypabyssal and crater-facies kimberlites of the Lac de Gras kimberlite f ield, perovskite occurs as reaction-induced rims on earlier-crystallized Ti -bearing minerals (magnesian ilmenite and priderite), inclusions in atoll s pinels and discrete crystals in a serpentine-calcite mesostasis. The minera l is associated with spinels, apatite, monticellite, phlogopite, baryte, Fe -Ni sulphides, ilmenite, diopside and zircon. Uncommon accessory phases fou nd in an assemblage with perovskite include titanite, monazite(Ce), witheri te, strontium-apatite, khibinskite, djerfisherite, wollastonite, pectolite, suolunite, hydroxyapophyllite and bultfonteinite. Three types of perovskit e can be distinguished on the basis of composition: (1) REE-Nb-Al-poor pero vskite with relatively high Sr and K contents (up to 2.2 and 0.6 wt.% oxide s, respectively) occurring as mantles on priderite and inclusions in atoll spinels; (II) perovskite with elevated Al, Fe, Nb and LREE (up to 1.4, 8.3, 9.1 and 17.0 wt.% oxides, respectively) found as discrete crystals and rim s on macrocrystic ilmenite; (III) perovskite significantly enriched in Na, Sr, Nb and LREE (up to 3.3, 3.4, 13.0 and 22.6 wt.% oxides, respectively) f ound as rims on perovskite I and II. The overwhelming majority of perovskit e is represented by discrete crystals of type II. In some occurrences, this type of perovskite also has high Th contents (up to 5.5 wt.% ThO2) and Zr contents (up to 3.7 wt.% ZrO2). Textural evidence indicates that perovskite shows an overall evolutionary trend from the most primitive type I towards type III showing the highest Na, Nb and LREE contents. Perovskite of type I probably crystallized under relatively high pressures prior to the precip itation of MUM spinels. Perovskite II crystallized after magnesiochromite, pleonaste and MUM (magnesian ulvospinel-magnetite) spinels, under increasin g So, The most compositionally evolved type III formed during near-solidus re-equilibration of the earlier-crystallized perovskite. The compositional variation of the Lac de Gras perovskite can be adequately characterized in terms of five major end-members: CaTiO3 (perovskite), CeFeO3, NaNbO3 (luesh ite), Na0.5LREE0.5TiO3 (loparite), and CaFe0.5Nb0.5O3 (latrappite).