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In hypabyssal and crater-facies kimberlites of the Lac de Gras kimberlite field, perovskite occurs as reaction-induced rims on earlier-crystallized Ti-bearing minerals (magnesian ilmenite and priderite), inclusions in atoll spinels and discrete crystals in a serpentine-calcite mesostasis. The mineral is associated with spinels, apatite, monticellite, phlogopite, baryte, Fe-Ni sulphides, ilmenite, diopside and zircon. Uncommon accessory phases found in an assemblage with perovskite include titanite, monazite-(Ce), witherite, strontium-apatite, khibinskite, djerfisherite, wollastonite, pectolite, suolunite, hydroxy-apophyllite and bultfonteinite. Three types of perovskite can be distinguished on the basis of composition: (I) REE-Nb-Al-poor perovskite with relatively high Sr and K contents (up to 2.2 and 0.6 wt.% oxides, 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 rims 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) found as rims on perovskite I and II. The overwhelming majority of perovskite 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 precipitation of MUM spinels. Perovskite II crystallized after magnesiochromite, pleonaste and MUM (magnesian ulvöspinel-magnetite) spinels, under increasing fO2. 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 (lueshite), Na0.5LREE0.5TiO3 (loparite), and CaFe0.5Nb0.5O3 (latrappite).
The discovery of the Lac de Gras kimberlite field was the culmination of nearly thirty years of reconnaissance and research involving geochemical survey, indicator-mineral sampling and, subsequently, drilling, magnetic and electromagnetic techniques (Fipke et al., 1995; Pell 1997a). The Lac de Gras field is part of the larger Slave province (Northwest Territories, Canada) that includes at least six other kimberlite fields and should be classified as a Type III kimberlite province (Mitchell, 1986). The Lac de Gras field is located in the central Slave Craton (Fig. 1⇓) and comprises kimberlites ranging from Late Cretaceous to Early Tertiary in age (Davis and Kjarsgaard, 1997). Most of the kimberlite intrusions are situated in a NW-trending zone that is roughly parallel to the Bathurst fault (Pell, 1997a). The Slave Craton consists predominantly of Late Archaean granitic rocks intruding supracrustal metasedimentary and metavolcanic rocks (2.7–2.6 Ga), and an ancient sialic basement (~4.0 Ga). In addition, five distinct swarms of Proterozoic diabase dykes ranging from 2.2 to 1.3 Ga have been identified in the vicinity of Lac de Gras (LeCheminant et al., 1996). The existing isotopic and geological evidence suggests that the ancient sialic rocks underlie only the western part of the Craton, whereas its eastern part represents a comparatively younger, juvenile crust (Pell, 1997a). The kimberlitic rocks have been observed in both structural domains, where they intrude the older granitic, metasedimentary rocks and diabase dykes (Pell, 1997a). Relatively few kimberlite bodies are exposed at the surface; many intrusions are overlain by a blanket of Quaternary glacial sediments. The first diamondiferous kimberlite intrusion was discovered in 1991 at Point Lake. Subsequently, nearly 200 intrusions have been discovered in the area. Detailed accounts of the history of diamond exploration at Lac de Gras can be found in Fipke et al. (1995) and Pell (1997a).
Very little information on the petrology and mineralogy of the Lac de Gras kimberlites is available in the literature. Most of the kimberlite bodies are relatively small (<120,000 sq. m) vents infilled with resedimented volcaniclastic and pyroclastic material. The crater-facies rocks commonly exhibit stratification and contain fragments of wood, siltstone and mudstone (Pell, 1997a). Hypabyssal-facies kimberlites are less common at Lac de Gras and typically form dykes and sills. In some intrusions, the presence of diatreme-facies lithologies composed of macrocrystal kimberlite and heterolithic breccia has been postulated (Pell, 1997b). However, in a detailed petrographic study of the Lac de Gras kimberlites, Masun (1999) has shown that diatreme-facies rocks are characteristically absent from the area, and suggested that the Lac de Gras kimberlites probably formed by phreato-magmatic excavation and subsequent infilling of the vents. Some data on the whole-rock chemistry and compositional variation of rock-forming minerals from the Lac de Gras kimberlites have been provided by Pell (1997b) and Masun (1999). According to Masun (1999), the enrichment of spinels from hypabyssal-facies rocks in Al (so called pleonaste trend) indicates extensive contamination of a kimberlitic magma with wall-rock material. These data are consistent with generally high contamination indices of the Lac de Gras rocks (Pell, 1997b).
The present work is part of a comprehensive study of perovskite group minerals from various kimberlite occurrences (Chakhmouradian and Mitchell, 2000). During preliminary examination of Lac de Gras samples, we noticed that some perovskite in these rocks was unusually enriched in a number of elements atypical of kimberlitic perovskite (e.g. Th and Zr). Most crystals also contain ‘abnormal’ levels of Nb, REE and some other elements that are relatively minor components of perovskite in other kimberlites. The present study is a first detailed description of this unusual perovskite based on petrographic and electron microscopic examination of nearly 20 samples of volcaniclastic and hypabyssal kimberlites. Unfortunately, the very small size of the perovskite crystals, i.e. typically <10 μm across, precluded their examination by X-ray or spectroscopic methods.
At Lac de Gras, perovskite is a ubiquitous accessory mineral in both crater-and hypabyssal-facies rocks. The earliest generation of this mineral occurs as mantles on priderite (Fig. 2a⇓), and small fragments (2–8 μm) enclosed in some atoll spinels. In contrast to the majority of perovskite from Lac de Gras, this generation is very primitive in composition, being virtually devoid of REE and Nb, and enriched in Sr (see below). This perovskite contains relict fragments of primary priderite and barian priderite. It is noteworthy that priderite has not been described in archetypal (or Group 1) kimberlites previously. Hollandite group minerals observed in some highly evolved kimberlites are characteristically devoid of K and belong to the series BaFe3+2 Ti6O16–BaFe2+Ti7O16 (henry-meyerite) (Mitchell, 1995; Mitchell et al., 2000). Priderite observed in the present work is unusual in composition, as it contains high levels of Ca, Na and Mg (Table 1⇓, analyses 1–2). It is also noteworthy that the A sites in this mineral are almost completely occupied by large cations (K, Na, Ca and Ba), probably indicating a high-pressure crystallization environment. We believe that priderite in the Lac de Gras kimberlites represents xenocrysts derived from the upper-mantle rocks, rather than remnants of the macrocryst suite. The Ba-rich variety that precipitated after the early priderite contains ~12–13 wt.% BaO; however, its composition could not be determined accurately because of the small size of the grains. This phase probably represents a member of the solid solution between priderite [ideally K2(Fe,Mg)2Ti6O16] and BaFe3+2 Ti6O16 or BaFe2+Ti7O16. The early primitive perovskite mantling or replacing priderite was subsequently overgrown by a REE-Nb-enriched variety compositionally similar to rims on euhedral groundmass crystals (Fig. 2a⇓)
Mantles of perovskite also occur on fragments of magnesian ilmenite belonging to the macro-cryst suite (Table 1⇑, analysis 3). Commonly, the perovskite rim is separated from the ilmenite core by a thin cavernous zone of magnesian titani-ferous magnetite (Fig. 2b⇑). This magnetite is similar in composition to magnesian ulvöspinel-magnetite (MUM) spinels occurring elsewhere in the groundmass (Table 1⇑, analysis 4). The perovskite rim does not exceed 10 μm in thickness and is identical in composition to REE-Nb-Fe-rich groundmass perovskite (see below).
The most common mode of occurrence of perovskite at Lac de Gras is minute euhedral crystals typically <10 μm across. Larger crystals (up to 45 μm) are extremely rare and were observed in only a few samples of volcaniclastic kimberlites. Most perovskite has a pseudocubic, elongate habit; pseudocubo-octahedral crystals are much less common. In addition to euhedral grains, volcaniclastic kimberlites contain angular fragments of perovskite smaller than 20 μm. Crystals of perovskite are randomly distributed throughout a serpentine-calcite mesostasis. The mineral is associated with spinels, apatite, monticellite, phlogopite, baryte, Fe-Ni sulphides and, less commonly, ilmenite, diopside and zircon. In some samples, we also found minerals that are atypical of or very rare in kimberlites, including titanite, monazite-(Ce), witherite, strontium-apatite, priderite, khibinskite (K2ZrSi2O7), djerfisherite [K6(Fe,Cu,Ni)25S26Cl], wollastonite, pectolite, suolunite [Ca2Si2O5(OH)2 · H2O], hydroxyapophyllite [KCa4Si8O20(OH) · H2O], and bultfonteinite [Ca2SiO2(OH)4]. The Ca silicates, rhombohedral calcite and strontium-apatite crystallized during the terminal evolutionary stages, and occur exclusively in vugs and fractures. It is noteworthy that in the Lac de Gras samples, perovskite is commonly enclosed in a relatively Ti-poor rim of some atoll spinels or trapped in a ‘lagoon’ separating the rim from a zoned core (Fig. 2c⇑). The Lac de Gras perovskite characteristically lacks reaction-induced rims. Such rims are quite common in hypabyssal-and diatreme-facies kimberlites, and typically consist of manganoan ilmenite or TiO2 polymorphs. A detailed account of replacement textures exhibited by perovskite from different kimberlite occurrences is given in Chakhmouradian and Mitchell (2000).
All mineral compositions were determined using an Hitachi 570 scanning electron microscope equipped with a LINK ISIS analytical system incorporating a Super ATW Light Element Detector. Raw EDS spectra were acquired for 180 s (live time) with an accelerating voltage of 20 kV and a beam current of 0.86 nA. The spectra were processed with the LINK ISIS-SEMQUANT software, with full ZAF corrections applied. The following standards were employed for the determination of mineral compositions: benitoite (Ba), corundum (Al), loparite-(Ce) (Na, La, Ce, Pr, Nd, Nb), Mn-bearing olivine (Mn), perovskite (Ca, Ti, Fe), orthoclase (K), periclase (Mg), wollastonite (Si), synthetic SrTiO3 (Sr), and metallic Ta, Th and Zr. A multi-element standard for the light REE (loparite) was used, as experience has shown that this gives more accurate data than single-REE standards when using EDS spectrum-stripping techniques. However, peak profiles used for the analytical X-ray lines were obtained on single REE-fluoride standards.
Perovskite from Lac de Gras contains a unique assemblage of minor elements, some of which have unusually high concentrations. In most samples, the following elements are present in detectable amounts: Ca, Ti, LREE, Fe, Nb, Al, Th, Sr, Si and Ta. A number of samples also contain significant Zr and K. With a few exceptions (see below), we analysed only perovskite crystals larger than 8 μm across to avoid excitation of the surrounding silicate matrix by the electron beam. Therefore, we believe that the Si contents observed in the majority of samples (up to 1.0 wt.% SiO2) are real, and not spurious. Si4+ has a much smaller radius in octahedral coordination than Ti4+, and, hence, can be present in perovskite only in minor concentrations. Experimental data on miscibility in the system CaTiO3–CaSiO3 suggest a maximum of 1.9 mol.% CaSiO3 (~0.9 wt.% SiO2) in perovskite crystallized below P = 5.3 GPa (Kubo et al., 1997). However, Mitchell and Chakhmouradian (1999a) have shown that even relatively low-pressure perovskite occurring in the groundmass of olivine lamproites may contain as much as 1.9 wt.% SiO2.
In the Lac de Gras rocks, three types of perovskite can be distinguished on the basis of composition:
REE-, Nb-and Al-poor perovskite with relatively high Sr and K contents (up to 2.2 and 0.6 wt.% oxides, respectively). This perovskite is found as mantles on xenocrystic priderite, and inclusions in atoll spinels (see above). In contrast to the other two types, perovskite I is subhedral and has a noticeably lower average atomic number (AZ).
The overwhelming majority of crystals are poor in Na and Sr (typically <0.6 wt.% oxides, respectively), and rich in Al (0.6–1.4 wt.% Al2O3), Fe (2.6–8.3 wt.% Fe2O3), Nb (1.7–9.1 w t.% Nb2O5) and LREE (5.8–17.0 wt.% ∑LREE2O3). In some intrusions, this perovskite also contains high levels of Th (up to 5.5 wt.% ThO2) and Zr (up to 3.7 wt.% ZrO2). Crystals with the lowest Al, Fe, Nb and LREE contents compositionally approach perovskite typically found in the groundmass of archetypal kimberlites (Chakhmouradian and Mitchell, 2000). Perovskite II occurs as discrete crystals and reaction-induced rims on macrocrystic ilmenite. Textural evidence indicates that crystals of type II precipitated after perovskite I was fragmented and enclosed by spinel.
Perovskite significantly enriched in Na, Sr, Nb and LREE (2.1–3.3, 1.7–3.4, 9.5–13.0 and 11.0–22.6 wt.% oxides, respectively). Apart from elevated Na, Sr and Nb contents, this perovskite differs from the most common type II in being systematically depleted in Al and Th (≤0.6 and 0.4 wt.% oxides, respectively). Compositional type III is found exclusively as rims on perovskite of types I and II. Such rims are relatively common, and appear to be confined to hypabyssal kimberlites. In most cases, the rims are <3 μm thick, and their composition cannot be determined accurately. We found only a few grains with rims >6 μm thick, which explains the paucity of analytical data for perovskite III. It is noteworthy that excitation of the silicate matrix by the electron beam probably contributed to relatively high levels of Si (~1.1 wt.% SiO2) found in some analyses of this perovskite.
We did not observe any exceptions from the relationships described above between the three compositional types of perovskite. Therefore, we propose that perovskite from the Lac de Gras kimberlites shows the overall evolutionary trend from the most primitive type I towards type III showing the highest Na, Nb and LREE contents. Representative compositions of perovskite are given in order in Tables 2⇓–4⇓⇓ from primitive to more evolved compositions.
To establish the complete range of compositional variation exhibited by perovskite from Lac de Gras, the data were recalculated to end-member components. Mitchell (1996) demonstrated that the composition of perovskite from most kimberlite occurrences can be described adequately in terms of three end-members, i.e. CaTiO3, Na0.5LREE0.5TiO3 (loparite) and NaNbO3 (lueshite). The majority of Lac de Gras perovskites contain insufficient Na to ascribe high concentrations of LREE exclusively to the loparite end-member. Only the relatively Na-rich perovskite III appears to have significant loparite contents (Fig. 3a⇓). Most compositions of types II and III demonstrate a reasonable positive correlation between Na and Nb, suggesting that Nb in the Lac de Gras perovskite can be assigned, at least partially, to the lueshite component (Fig. 3b⇓). However, a significant fraction of compositions plot below the CaTiO3-NaNbO3 join indicating that some Nb remains unassigned. It is obvious that, in addition to loparite and lueshite, other perovskite-type end-members have to be included in the recalculation scheme to account for the excess LREE and Nb present in most of the samples. These end-members must also explain the high Fe and Al contents (0.06–0.21 a.p.f.u. Fe+Al) observed in perovskite II.
The available spectroscopic studies (Muir et al., 1984; Mitchell et al., 1998) demonstrate that naturally-occurring perovskite (sensu stricto) incorporates only ferric iron substituting for Ti4+ in the octahedral B site. According to the experimental data of Kimura and Muan (1971), there is no solubility between CaTiO3 and FeTiO3 at atmospheric pressure. At high pressures, Fe2+ substitutes for Ca forming highly ordered perovskite-type structures (Leinenweber et al., 1995). As the Lac de Gras perovskite is a relatively low-pressure phase, and significant variations in iron content (0.05–0.16 a.p.f.u.) rule out the existence of ordering between Ca and Fe in this mineral, we assign all iron present in our samples to octahedrally-coordinated Fe3+. To maintain charge balance, the incorporation of Fe3+ and Al in perovskite requires a coupled substitution in either the A or B site:
As illustrated by Fig. 3c⇑, substitution 1 is more suitable for accommodating the high levels of LREE in the Lac de Gras perovskite. To account for this substitution, several new perovskite end-members such as LaFeO3, LaAlO3, CeFeO3, etc. should be introduced into the recalculation routine. For simplicity, we use the sum of these end-members designated as CeFeO3 (Ce is the dominant rare-earth element, and the amount of Fe in our samples invariably exceeds the Al content). It is noteworthy that synthetic compounds with the general formula AFeO3 (A = Y, La, Ce, ... Yb) are orthorhombic perovskites isostructural with CaTiO 3 (Eibschütz, 1965; Robbins et al., 1969; Marezio et al., 1970). The orthoaluminate end-members AAlO3 (A = La–Nd) also have perovskite-type structures, but may be orthorhombic or rhombohedral in symmetry (e.g. Geller and Bala, 1956).
Substitution mechanism 2 delineates a solid solution between perovskite and the two hypothetical end-members CaFe0.5 Nb0.5 O3 and CaAl0.5 Nb0.5 O3. For the end-member CaFe0.5Nb0.5O3, Mitchell (1996) has suggested the name ‘latrappite’, after the mineral containing the maximum known levels of this component (14–24 mol.%). It has been established experimentally that CaFe0.5Nb0.5O3 and CaTiO3 are isostructural and form a continuous solid solution series (Chakhmouradian and Mitchell, 1998a). The end-member CaAl0.5Nb0.5O3 can be further disregarded, as all Al present in our perovskite samples is assigned to the orthoaluminate end-members and included into the CeFeO3 component (see above). Figure 3d⇑ shows that most of the Lac de Gras perovskite has relatively low latrappite contents (<5 mol.%). Only a few compositions simultaneously enriched in trivalent cations (>0.12 a.p.f.u. Fe+Al) and Nb (>0.09 a.p.f.u.) contain ~15 mol.% latrappite (Table 4⇑, analysis 6; Fig. 4⇓). It is noteworthy that high latrappite contents (>5 mol.%) appear to be restricted to perovskite from hypabyssal kimberlites (cf. Tables 2⇑ and 3⇑ with Table 4⇑, analyses 1–8).
The primitive perovskite of type I contains up to 2.9 mol.% SrTiO3, and negligible amounts of other perovskite-type end-members (Table 2⇑, analyses 1–3). An interesting compositional feature of this perovskite is elevated Na contents (0.04–0.07 a.p.f.u.) that cannot be ascribed entirely to the minor lueshite or loparite components present. In this work, we assign the ‘excess’ Na to the hypothetical end-member NaTiO2.5 that has been observed previously in high-pressure synthetic ‘perovskites’ (Mitchell and Chakhmouradian, 1999b).
As noted above, the end-member Na0.5LREE0.5TiO3 (loparite) is present in appreciable amounts (>5 mol.%) only in perovskite III (Fig. 5⇓). This variety also contains significant NaNbO3, CeFeO3, SrTiO3 (Table 4⇑, Fig. 5⇓) and, in some cases, CaZrO3 contents (up to 5.0 mol.%) Enrichment of perovskite in Zr is atypical of kimberlites, as zircon and baddeleyite (ZrO2) are the principal hosts for this element. Relatively high Zr contents have been observed in perovskite from carbonatite complexes of eastern Siberia (Pozharitskaya and Samoylov, 1972; Chernysheva and Davydova, 1974) and Polino, Italy (Lupini et al., 1992). The highest Zr contents (5.6–6.1 wt.% ZrO2) coupled with elevated levels of Nb and Ta (up to 25.2 and 3.1 wt.% oxides, respectively) were reported in samples from the Ozernyi complex in the Aldan Alkaline Province (Pozharitskaya and Samoylov, 1972). In perovskite from Polino, high Zr contents (1.4–4.4 wt.% ZrO2: authors’ unpubl. data) are accompanied by enrichment in Al, Si, Fe and Th. The two aforementioned localities also differ with respect to the mineral assemblage in which the Zr-rich perovskite is found. In the Aldan carbonatites, this mineral occurs in plutonic diopside–calcite varieties in association with baddeleyite and zirconolite; titaniferous garnet is characteristically absent from this paragenesis. At Polino, small crystals of Zr-rich perovskite are found in the mesostasis of forsterite–monticellite–calcite extrusive carbonatites, typically in association with zirconian schorlomite and magnetite (Lupini et al., 1992). The Polino rocks lack baddeleyite and other Zr oxides. Concomitant enrichment of perovskite and garnet in Zr has been also noted in carbonatites of the Maimecha-Kotuy Province, eastern Siberia (Chernysheva and Davydova, 1974). The Lac de Gras rocks represent yet another type of occurrence of Zr-rich perovskite, i.e. poorly evolved hypabyssal kimberlites devoid of either titanian garnet or Zr-oxide phases. The Lac de Gras perovskite contains up to 3.7 wt.% ZrO2 coupled with high LREE, Nb and Fe contents (Table 4⇑, analyses 4–6, 11–12). Compositions of Zr-rich perovskite from kimberlites and other occurrences are compared in Fig. 6⇓. The Lac de Gras perovskite demonstrates a distinct negative correlation between ZrO2 and TiO2, in accord with the existence of solid solution towards CaZrO3. Correlation between ZrO2 and other minor components in Zr-rich perovskite from different localities is obscured by the complexity of cation substitutions in the A and B sites.
Discussion and conclusions
The paragenetic and compositional diversity of perovskite in the Lac de Gras kimberlites suggest that this mineral crystallized in a wide range of pressure-temperature conditions, and under differing activities of minor elements (Al, Fe, Sr, Nb, LREE, Th and Zr). The earliest and compositionally most primitive generation of perovskite is characteristically enriched in Sr and K. This contrasts with most kimberlitic perovskite that is typically very poor in Sr (< 0.6 wt.% SrO) owing to the preferential partitioning of this element into carbonate minerals and apatite (Mitchell and Chakhmouradian, 1999a; Chakhmouradian and Mitchell, 2000). This conclusion is in agreement with the experimental study of SrTiO3 stability in haplocarbonatite melts (Mitchell, 1997). Therefore, the enrichment of perovskite I in Sr clearly indicates that this generation crystallized prior to primary calcite and apatite. The composition of priderite associated with the early perovskite, and the absence of jeppeite (K2Ti6O13) in this mineral assemblage suggest a high-pressure crystallization environment. The high levels of K and appreciable NaTiO2.5 contents in the early perovskite are consistent with its formation at elevated pressures (Mitchell and Chakhmouradian, 1999b, unpubl. data).
The bulk of perovskite in the Lac de Gras rocks crystallized during the precipitation of the groundmass, after aluminous magnesiochromite and pleonaste (ferroan spinel) rimming magnesiochromite in some kimberlites. The occurrence of perovskite II as mantles on the MUM spinel, and euhedral inclusions in the rim of atoll crystals (Fig. 2b,c⇑) indicates that this generation of perovskite precipitated nearly simultaneously with the latest generations of spinel. Hence, the crystallization of perovskite II occurred in the temperature range 600–650°C at oxygen fugacities generally below 10−19 bar (Mitchell, 1986). In contrast to the groundmass perovskite typically found in kimberlites (Mitchell, 1986; Chakhmouradian and Mitchell, 2000), perovskite II contains moderate-to-high Fe3+ contents (0.05–0.16 a.p.f.u.). This unusual compositional feature may reflect some increase in fO2 during precipitation of the perovskite. Silica activities during the crystallization of groundmass did not exceed 10−2.0, as indicated by the occurrence of perovskite, not titanite, and monticellite, not diopside, in the majority of Lac de Gras kimberlites (Mitchell, 1986). The presence of late-stage clinopyroxene and titanite in some samples suggests that a ) could increase SiO2 locally to 10−1.5 owing to contamination of the kimberlite with wallrock material. We believe that contamination of the kimberlitic magma was also responsible for enrichment of the groundmass perovskite in Al (up to 0.05 a.p.f.u.). This conjecture is supported by the occurrence of extremely Al-rich spinels (17–57 wt.% Al2O3) as mantles on magnesiochromite, i.e. pleonaste evolutionary trend (Masun, 1999).
It is not clear whether the contamination processes enhanced the concentrations of incompatible elements in kimberlitic magmas, or if these elements were already present in ‘abnormal’ concentrations. Undoubtedly, perovskite was the most suitable repository for light REE, Nb, Th and Zr in all intrusions, as alternative hosts for these elements such as monazite-(Ce), ground-mass ilmenite and khibinskite are present in the Lac de Gras rocks in negligible amounts. Apatite group minerals may serve as a major host for LREE in some alkaline rocks (e.g. Rønsbo, 1989). However, undersaturated rocks containing both apatite and perovskite typically show preferential partitioning of the light lanthanides into perovskite (Dawson et al., 1994; Lloyd et al., 1996). The high levels of Zr in perovskite, and the absence of baddeleyite, clazirtite and zirconolite in our samples suggest that crystallization of Zr oxides was inhibited by high aCa2+ in the system.
The youngest generation of perovskite is unusual in containing high Na, Nb and LREE contents. The mode of occurrence of perovskite III indicates that it is a product of near-solidus re-equilibration of the earlier-crystallized perovskite of types I and II. The evolutionary trend of increasing Na, Nb and LREE contents has been described in a diversity of alkaline-ultramafic and carbonatitic rocks (Platt, 1994; Chakhmouradian and Mitchell, 1997, 1998b). The late-stage enrichment of perovskite in Na0.5LREE0.5TiO3 and NaNbO3 probably results from interaction of early, compositionally primitive generations of this mineral with a fluid (melt?) enriched in incompatible elements. As Na contents are very low in kimberlites (Mitchell, 1986), the evolutionary trend observed in the Lac de Gras perovskite did not lead to crystallization of Nb-rich loparite or lueshite. As perovskite III is found exclusively in hypabyssal kimberlites, the deuteric fluid enriched in Na, Nb and LREE probably evolved from the kimberlitic magma, rather than being derived from an external source. In addition, the external origin of such fluid is not substantiated by the presence of younger alkaline rocks in the Lac de Gras area.
This work is supported by the Natural Sciences and Engineering Research Council of Canada and Lakehead University (Ontario). The samples investigated in the present study were kindly provided by Kennecott Canada Exploration Inc. We are grateful to Alan MacKenzie for assistance with analytical work, Ann Hammond for masterful sample preparation, and Sam Spivak for drawing the location map. J.B. Dawson and A.P. Jones are thanked for their constructive comments on the early version of this paper.
- Manuscript received 21 January 2000.
- Modified version received 14 September 2000.