Stable isotope composition of magmatic and deuteric carbonate phases in hypabyssal kimberlite, Lac de Gras field, Northwest Territories, Canada

Chemical Geology 242 (2007) 435 – 454 www.elsevier.com/locate/chemgeo

Stable isotope composition of magmatic and deuteric carbonate phases in hypabyssal kimberlite, Lac de Gras field, Northwest Territories, Canada Mark R. Wilson a,⁎, Bruce A. Kjarsgaard b , Bruce Taylor b a

Department of Earth Sciences, Memorial University, St. John's, Newfoundland, Canada A1B 3X5 b 601 Booth St., Geological Survey of Canada, Ottawa, Ontario, Canada Received 18 May 2006; received in revised form 30 April 2007; accepted 3 May 2007 Editor: R.L. Rudnick

Abstract The oxygen and carbon isotope compositions of carbonates from thirteen, Late Cretaceous to Paleogene hypabyssal intrusions from the Lac de Gras kimberlite field indicate that the carbonates are well-preserved and have a complex paragenetic formation history. Kimberlites at ten localities studied are calcite-bearing based on the presence of phenocrysts, microphenocrysts and/or millimeter-scale, calcite-only segregations of Sr-rich calcite. The calcite-bearing kimberlites also contain large (cm-scale), Sr-poor calcite + serpentine segregations that are texturally distinct from the smaller, Sr-rich calcite-only segregations. Kimberlites at three other localities studied are dolomite-bearing based on the presence of calcite + dolomite segregations, some of which preserve complex, oscillatory and banded textures of calcite, dolomite, and magnesian calcite. δ18O values for whole-rock calcite in calcite-bearing kimberlite and for micro-samples of calcite from individual segregations in calcite- and dolomite-bearing kimberlite generally vary from 6 to 9‰, consistent with their formation at magmatic temperatures (e.g., N 750 °C) from a kimberlite melt. In contrast, higher δ18O values of 9 to 14‰ characterize micro-samples of calcite from calcite + serpentine segregations in calcite-bearing kimberlite, as well as whole-rock dolomite and micro-samples of dolomite from segregations in dolomite-bearing kimberlite. These data are consistent with formation from deuteric fluids (late-stage, magmatic) at sub-solidus temperatures (e.g., 500 to 100 °C). These higher δ18O values for whole-rock calcite also correlate with increased intensity of forsterite alteration, increased abundance of calcite + serpentine segregations, textural overprint of primary groundmass minerals by anhedral calcite and serpentine, and development of atoll-spinels, all interpreted as a result of deuteric fluids. In contrast, whole-rock calcite in dolomite-bearing kimberlite has notably lower δ18O values of 1.5 to 5.5‰ interpreted to have formed from, or been overprinted by, locally-derived, high CO2/H2O deuteric fluids at 500 to 100 °C. δ13C values of the various types of calcite and dolomite (−8 to − 3 ‰) have no systematic variation with δ18O values and, therefore, there is no isotopic record of volatile degassing from these hypabyssal bodies. © 2007 Elsevier B.V. All rights reserved. Keywords: Kimberlite; Stable isotopes; Calcite; Dolomite; Primary carbonate; Lac de Gras; Slave Province

⁎ Corresponding author. E-mail address: [email protected] (M.R. Wilson). 0009-2541/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2007.05.002

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1. Introduction Armstrong et al. (2004) documented the major (Ca, Mg, Fe) and trace element (Sr, Ba, Mn) contents of different textural varieties of carbonate phases in hypabyssal kimberlite from eight bodies from the Lac de Gras field. They recognized that these hypabyssal kimberlites contained a variety of carbonates, including calcite, dolomite, magnesian calcite and magnesite. Differences are such that calcite-bearing and dolomitebearing varieties of kimberlite can be recognized. Moreover, Armstrong et al. (2004) presented backscatter electron (BSE) images that documented unusual and complex oscillatory zoning and banded textures in carbonate segregations within dolomite-bearing kimberlites. Further, Armstrong et al. (2004) found wellpreserved idiomorphic crystals of Sr and Ba-rich calcite in the Lac de Gras kimberlites. They interpreted this calcite to be a magmatic phase. Previous stable isotope studies of kimberlites and many carbonatite bodies have shown that, in general, carbonates do not preserve high-temperature, magmatic oxygen isotope compositions. In the case of kimberlites, the vast majority of carbonates analyzed previously have δ18O values N 9‰, consistent with either formation at lower temperatures, isotopic disturbance, or formation during weathering. The degree of preservation of textural detail and trace element characteristics of carbonate in

the Lac de Gras kimberlites makes them particularly ideal candidates for a stable isotope investigation. This stable isotope study of carbonate in kimberlite used the same drill core samples and polished slabs described by Armstrong et al. (2004) from the BHP-Billiton Ekati Mine claim block (Leslie, Porpoise, Anaconda, Grizzly, Rat, Koala West, Misery East, Rattler) with additional samples from the Roger, Pigeon, Aaron kimberlites, and the Ann and Don kimberlites from the DeBeers Canada Exploration Hardy Lake claim block. All kimberlite localities are within the central Slave Craton, approximately 360 km northeast of Yellowknife, N.W.T. (Fig. 1). Tabular carbonate phenocrysts and microphenocrysts within kimberlite represent primary crystallization products (e.g., see discussion in Mitchell, 1986). Calcite + serpentine and calcite segregations in kimberlites, and their possible genetic implications have also been previously reported by Clement (1975), Clarke and Mitchell (1975), Mitchell (1975), Clement and Skinner (1979), Clement (1982), and Mitchell (1986; 1994). The number of stable isotope studies of carbonate in kimberlite are few (e.g., Deines and Gold, 1973; Sheppard and Dawson, 1975; Kobelski et al., 1979; Kirkley et al., 1989). Much of the data reported in these studies fall outside the range of mantle carbonatite isotopic values which makes the interpretation of latestage magmatic processes, preservation and low-temperature isotopic overprinting problematic.

Fig. 1. Generalized geologic map of the Central Slave Province (Kjarsgaard et al., 2002) showing the locations of the kimberlite in the Lac de Gras area. Kimberlite localities studied are shown with circles: (1) Anaconda, (2) Porpoise, (3) Rattler, (4) Rat, (5) Pigeon, (6) Leslie, (7) Koala West, (8) Grizzly, (9) Roger, (10) Aaron, (11) Misery East, (12) Ann and (13) Don. Geology Key: dark grey, metagreywacke; medium grey, two-mica and biotite granite; light grey, granodiorite–tonalite–diorite; diamond symbols, kimberlites not part of this study.

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This study interprets new stable isotope analyses of well-preserved, Sr-rich (in excess of 2.6 wt.% SrO; Armstrong et al., 2004) calcite phenocrysts, groundmass carbonate, and both serpentine-absent and serpentine-rich carbonate segregations. We report 95 whole-rock isotopic analyses of carbonate and 19 analyses of micro-samples of calcite and dolomite segregations, sampled in situ, guided by back-scattered electron (BSE) images. The whole-rock and micro-sample isotopic results indicate that much of the carbonate in these kimberlites has preserved its magmatic isotopic composition. 2. Analytical techniques Previous petrographic, electron microprobe and scanning electron microscope (SEM) analysis by Armstrong et al. (2004) identified the carbonate mineralogy of samples from eight of the kimberlites used in this study. New petrographic and SEM analyses of carbonates from five additional kimberlites have been added. X-Ray diffraction (XRD) analyses of whole-rock powders have verified the calcite or calcite + dolomite nature of the samples (Armstrong et al., 2004). Sample preparation for stable isotope work and mass spectroscopy were performed at the Geological Survey of Canada (GSC). Oxygen and carbon isotope compositions are reported in the familiar δ notation relative to the SMOW and PDB reference standards, respectively. Whole-rock powders of calcite-bearing kimberlites were reacted at 25 °C with 100% phosphoric acid for 18 to 21 h, releasing CO2 for isotopic analysis (e.g., McCrea, 1950). The fine grained nature of calcite and dolomite in the groundmass of dolomite-bearing kimberlites required a selective extraction method based on the differential reaction rates of calcite and dolomite, an approach used in the past (e.g., Northrup and Clayton, 1966; Schwarz, 1966) and, more recently, by Al-Aasm et al. (1990) and Santos and Clayton (1995). In this study, CO2 from sequential extractions, or splits, collected after acidification of carbonate mixtures in whole-rock powders for 1, 3, 18 and 64 h were used to resolve the isotopic compositions of calcite, magnesian calcite and dolomite from one another. This sequential extraction technique was also employed on ten of the calcite-bearing kimberlites to test for small amounts of dolomite not detected by XRD. In addition to the whole-rock powders, micro-samples (typically 0.05 to 0.1 mg) of carbonate segregations and calcite + serpentine segregations were separated from 100 μm thick polished slabs of kimberlite using a binocular microscope and a scalpel blade held in a simple, custommade jig. Selection of the carbonate material for the microsamples was guided by BSE images of the slabs and

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compositional data from microprobe analyses. The microsamples were reacted in small-volume reaction vessels (∼8 cm3) with 4 drops of 100% phosphoric acid. The sequential extraction procedure was used to ensure that pure calcite or dolomite CO2 fractions were obtained for isotopic analysis. Repeat analyses of a GSC laboratory calcite standard (CC-1) revealed that the micro-sample procedure had no effect on the carbon isotopic composition of the CO2 produced, but there was a small, relatively consistent increase of approximately 0.5‰ in the oxygen isotope value (Table 1) compared to analyses of averagedsized samples prepared in regular-volume vessels using the routine technique. The source of this systematic difference may be the oxygen isotope fractionation associated with the solubility of small CO2 samples in phosphoric acid. Regardless, the isotopic difference between gas extracted using the micro-sample or routine technique was reproducible and not large enough to affect interpretation of the results. A phosphoric acid fractionation factor of 1.01025 was used for calcite (Friedman and O'Neill, 1977), and 1.01178 for dolomite (Rosenbaum and Sheppard, 1986). 3. Geologic setting and carbonate petrography The samples examined in this study are all magmatic (hypabyssal) kimberlite though they occur as: (1) discrete dykes (e.g., Anaconda, Koala West); (2) dykes cutting volcaniclastic pipes (e.g., Rat, Rattler); (3) small magmatic bodies adjacent to or underneath Table 1 Oxygen and carbon isotopic compositions of a Geological Survey of Canada calcite reference material (CC-I)a prepared in a regular-volume and small-volume reaction vessel δ13C

δ18O

CO2

Regular-volume vessel runs Average (n = 12) −4.84 +/− 0.07

7.17 +/− 0.27

50

Small-volume vessel runs Run # 1 −4.81 Run # 2 −4.72 Run # 3 −4.78 Run # 4 −4.75 Run # 5 −4.57 Run # 6 −5.08 Run # 7 −4.71 Run # 8 −4.73 Run # 9 −4.68 Run # 10 −4.71 Run # 11 −4.51 Average (n = 11) −4.69 +/− 0.27

7.42 7.35 7.61 7.68 7.81 7.04 7.38 7.50 7.86 7.84 7.90 7.68 +/− 0.51

13.0 6.0 4.0 2.1 2.0 1.7 1.5 1.2 1.2 1.2 0.6

a

Note: the GSC laboratory reports a nominal composition for CC-I of δ13C = − 4.90 and δ18O = +7.10. The average values (with 2 sigma errors) measured for analyses in each type of vessel and the number of μmol of CO2 produced are given.

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Table 2 Summary of mineralogy and petrographic features of kimberlite samples from Lac de Gras Locality

Sample #

Drill core #, depth

Macrocrysts Forsterite

Ann Don Aaron

Grizzly

Koala W

Leslie

Misery E

Pigeon Porpoise

Roger

Anaconda

Rat

Rattler

21 22 17 11 18 1 13 24 27 19 6 9 30 10 14 3 25 26 8 29 20 7 28 15 31 12 39 38 5 16 35 2 32 40 33 4 36 23 37

HL10-2B HL11-3B DDH97-57 at 62.6 DDH97-57 at 63.3 DDH97-57 at 74.8 DDH92-02 at 306.75 DDH92-02 at 312.34 DDH92-02 at 320.55 KWDC-01 at 52.25 KWDC-01 at 81.5 KWDC-01 at 100.5 LDC-09 at 15.0 LDC-09 at 20.5 LDC-09 at 28.9 DDH93-13 at 14.9 DDH93-13 at 17 DDH93-13 at 25.75 DDH93-13 at 36 DDH97-54 at 311.82 DDH97-54 at 312.12 DDH94-11 at 81.1 DDH94-11 at 87.7 DDH94-11 at 96.25 DDH94-11 at 106.8 DDH94-17 at 131.9 DDH94-17 at 134.1 DDH94-17 at 139.7 DDH96-17 at 162.65 DDH96-17 at 170.4 DDH96-17 at 170.9 DDH96-17 at 174.7 DDH94-08 at 257.75 DDH94-08 at 260.15 DDH94-08 at 262.35 DDH94-08 at 269.40B DDH96-13 at 233.25 DDH96-13 at 234.5 DDH96-13 at 237.3 DDH96-13 at 238.6

X (20–40; Cc) X (20–40; S) X (1; Cc) X (2; Cc) X (10; S) X (3; S) X (3; S) X (2; S) X (10–20; S) X (5; S) X (5; S) X (5; Cc, S) X (5; S) X (2–10; S) X (1–5; S) X (10–20; S) X (40–60; S, Cc) X (3–10; S) X (10–30; S) X (20–60; Cc, S) X (80–100; Cc, S) X (50–100; S, Cc) X (10–30; Cc) X (20–60; Cc) X (10–50; S) X (10–30; S) X (10–60; Cc) X (1; Cc) X (1; Cc) X (1; Cc) X (5; Cc) X (5; S, Cc) X (80–100; Cc, S) X (5; S, Cc) X (5; Cc) X (1; S, Cc) X (1; S) X (1; S, Cc) X (1; S, Cc)

Phenocrysts, microphenocrysts, groundmass Phlogopite

X X X X

X

X X X

Forsterite

X (100; S) X (50–100; S, Cc) X (2; Cc, S) X (5; Cc, S) X (10–50; S) X (5–95; S, Cc) X (5–15; S, Cc) X (5; S, Cc) X (10–100; S, Cc) X (1–3; S) X (5–10; S) X (3 –10; Cc) X (3 –5; Cc) X (3–5; Cc) X (10–100; S, Cc) X (50–100; Cc; S) X (80–100; Cc; S) X (10–100; S, Cc) X (70–100; S) X (50–100; S,Cc) X (100; Cc) X (100; S) X (50–100; Cc, S) X (50–100; Cc, S) X (60–100; Cc, S) X (60–100; Cc, S) X (60–100; Cc) X (1; Cc) X (1; Cc) X (1; Cc) X (5–10; Cc, S) X (5–10; Cc, S) X (100; Cc) X (5–10; Cc, S) X (5; Cc) X (3; Cc, S) X (5; S) X (5; Cc, S) X (3; Cc, S)

Phlogopite

x

x x x x

X X X x x x X X

x x x x x x

Spinel (a = atoll texture) X–a X–a X X X X X X x–a X X X x–a X X X X–a X X X–a X–a X–a x–a X–a X–a X–a X–a X–a X–a X X–a x–a X X X X–a X X–a X

xPhenocrysts, = present, minor. X = common. groundmass X = abundant.For olivine, number(s) in brackets is the average percent of the crystal which is altered to serpentine microphenocrysts, Types of segregations or calcite e.g. X (1; S, Cc) means 1% alteration to serpentine plus calcite is typical; X (70–100; S) means 70–100% alteration to serpentine is typical. PhlogopiteMonticellite Perovskite Apatite Serpentine Talc Carbonate Carbonate Calcite Calcite + Calcite + Calcite + microphenocrysts (groundmass) only serpentine dolomite dolomite + kinoshitalite (f = fresh) (p = poikilitic) serpentine

volcaniclastic pipes (e.g., Misery East, Pigeon); (4) large magmatic bodies that dominate a pipe (e.g., Aaron, Anne, Don, Grizzly, Leslie, Roger), and; (4) kimberlitecountry rock breccia dyke (e.g., Porpoise). The kimberlite samples studied are variably fresh, with a subset of samples exhibiting partial to complete replacement of the primary mineral assemblage by serpentine and/or carbonates. Most samples are typical

of hypabyssal kimberlite i.e. they are forsterite macrocrystic (typically 30–60 modal % forsterite macrocrysts), although the Don, Porpoise, Roger and Rat kimberlite samples are poor in forsterite macrocrysts (i.e.b 10 modal %). The Aaron, Pigeon, Misery East, Leslie and Grizzly kimberlites also contain variable, but minor amounts of macrocrystic phlogopite. All the samples contain variable proportions of phenocryst/microphenocrysts of forsterite,

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Table 2 (continued) Phenocrysts, microphenocrysts, groundmass

Types of segregations

PhlogopiteMonticellite Perovskite Apatite Serpentine Talc Carbonate Carbonate Calcite Calcite + Calcite + Calcite + kinoshitalite (f = fresh) microphenocrysts (groundmass) only serpentine dolomite dolomite + (p = poikilitic) serpentine X–p X–p

x x x x x x X x x–p x X–p X–p X–p X–p X x x x–p X X X–p x–p x x x x x x x x x x X–p X–p x

x x X–f X–f X x–f x–f X–f x x x–f X–f X–f X–f x–f x–f x x–f x–f x–f x x–f x X–f X–f X–f x–f x–f x x–f X x x X x x–f x x

x X x x x x x x x x x x x x x x x x x x X x x x x x x x x x

x

X X X X X X X X X X x x x x x x x x x x x X x x x x x x x x x x x x x x x x x

X X x x X x X x x x x

x X X x X X X X X x x X x x x x x x x X x x x x X

x x x x x x x x x x x x x x x x X X x x x x x x x x x x X x x x

x x x X x x

spinel, phlogopite and calcite, set in a finer grained groundmass which may contain phlogopite–kinoshitalite mica, spinel, monticellite, perovskite, apatite, carbonates, serpentine and talc. Refer to Table 2 for a summary of petrographic observations. Four types of carbonate identified include: (1) carbonate phenocrysts and microphenocrysts; (2) carbonate segregations, a term used to describe small (0.2 to

x x x X x X X X X X x x x x X x x X X x x X X X x x x X X X X X X X X X X X X

x x x x X X X x x X X X X X x X x X X x X x x

X X x X x x X x x x x x x x x X X X X x x x X X x x X

x

x

X X X

x X X

X x X X X X X

x X

3 mm), irregular-shaped features comprised almost entirely of carbonate (i.e. serpentine absent) of which there are two main compositional varieties, a high Sr–Ba calcite-only segregation and a calcite + dolomite segregation; (3) carbonate + serpentine segregations, a term used to describe cm scale, irregular-shaped features consisting of variable proportions of low-Sr calcite and serpentine and, rare, calcite + dolomite + serpentine segregations,

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and; (4) groundmass calcite (Fig. 2). Additional petrographic descriptions of carbonate can be found in Armstrong et al. (2004). Phenocrysts occur in some samples as tabular-tosubhedral Sr–Ba calcite grains, approximately 0.5 mm in size (e.g., Grizzly, Leslie, Misery East bodies; Fig. 2A).

Abundant groundmass microphenocrysts (tabular-shaped, 5–50 μm in size; Fig. 2B) of Sr–Ba calcite characterize the majority of the samples, and have trace element compositions that mimic those of calcite phenocrysts and calcite-only segregations from the same sample (Armstrong et al., 2004). In dolomite-bearing kimberlite,

M.R. Wilson et al. / Chemical Geology 242 (2007) 435–454

groundmass calcite is commonly rimmed by dolomite. Ten of the thirteen sample localities studied were calcite-bearing kimberlite containing as few as 3 or 4, or up to a dozen or more, calcite-only segregations per thin section (Table 2). The calcite-only segregations exhibit a relatively simple Sr–Ba calcite mineralogy (4,900– 11,000 ppm Sr; Armstrong et al. 2004) with no associated serpentine. They commonly have small regions of exsolved Sr or Ba-rich carbonate, but are otherwise quite homogeneous in appearance in BSE images (Fig. 2C). Some of the calcite-only segregations in the Leslie, Misery East and Grizzly bodies also contain subordinate apatite, Ba-phlogopite and monticellite. Three of the thirteen sample localities studied were dolomite-bearing kimberlite characterized by calcite + dolomite segregations, and no calcite-only segregations. In the Rat and Rattler bodies, the segregations have a complex mineralogy and preserve detailed textural banding that is absent in the homogeneous, calcite-only segregations described above (see also, Armstrong et al., 2004). The calcite + dolomite segregations typically have either a discontinuous layer of microcrystalline Sr–Ba calcite, or inwardly projecting euhedral rhombs of Sr–Ba calcite along the walls of the segregations (Fig. 2D). The Sr–Ba calcite layer and euhedral rhombs are overgrown by oscillatory bands of dolomite, multiple generations of parallel bands of magnesian calcite and, more rarely, dolomite–magnesite solid solutions. Multiple generations of microcrystalline dolomite generally occupy the centres of the segregations. The modal mineralogy of calcite + dolomite segregations typically includes approximately 10– 15% Sr–Ba calcite, 15–20% magnesian calcite, and 65–70% dolomite, with no silicate minerals. In the Anaconda kimberlite, the calcite + dolomite segregations contain more Sr–Ba calcite than found in the Rat or Rattler bodies, although multiple 50–100 μm wide bands of dolomite, and even narrower bands of magnesian calcite and magnesite, occur near the walls of the segregations. The textural appearance of the calcite + dolomite segregations is interpreted to be an open space (i.e. void, vug), in-filling feature (Armstrong et al., 2004).

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Calcite + serpentine segregations were in general observed in all of the calcite-bearing kimberlites studied, though they were relatively rare or absent in individual samples from the Leslie, Porpoise and Misery East localities and abundant in others (e.g., Ann, Don and Roger). The segregations are composed mostly of featureless aggregates of microcrystalline (1–10 μm), low-Sr calcite (b 2500 ppm Sr; Armstrong et al., 2004) and serpentine (Fig. 2E; F). The calcite + serpentine segregations are commonly five to ten times larger (i.e. cm scale) than the calcite-only or calcite + dolomite segregations described previously. They are distinguished by the ubiquitous presence of serpentine, either as small (5–10 μm) inclusions, or as mm-scale patches (Fig. 2G). In addition, there is a continuum between calcite + serpentine segregations that contain no other minerals, and those that contain numerous 20–50 μm grains of phlogopite, apatite, spinel, perovskite and monticellite. The complete continuum is observed within a single thin section. The textural appearance of calcite + serpentine segregations is interpreted to result from overprinting of the groundmass, i.e., not replacing forsterite, nor as an infilled void or vug. In three Anaconda samples and one Rat and one Porpoise sample, large cm-scale calcite +dolomite+ serpentine segregations were observed (Table 2). Groundmass carbonate consists of fine anhedral grains of calcite. In the dolomite-bearing kimberlites, there is a higher modal proportion of groundmass calcite than in the calcite-bearing kimberlites (Table 2). In some dolomite-bearing kimberlites (e.g. Anaconda and Rat), the entire groundmass has been texturally overprinted by calcite, leaving a residual assemblage of spinel plus calcite plus apatite. As well, in dolomite-bearing kimberlites, talc has been observed in association with serpentine and dolomite or calcite in the groundmass and in fractures in forsterite (Fig. 2H). 4. Stable isotope results 4.1. Calcite-bearing kimberlite Twenty-seven whole-rock samples of drill core from ten calcite-bearing kimberlite localities were analyzed

Fig. 2. Representative reflected light (RL) and back-scattered electron (BSE) images of carbonate in Lac de Gras kimberlite (locality and sample number are cited; abbreviations include: cc, calcite; dol, dolomite; Mg-cc, magnesian calcite; serp, serpentine; Fo, forsterite; ph, phlogopite; mont, monticellite). (A) BSE image of a blocky, zoned, primary phenocryst of Sr–Ba calcite (cc, Grizzly, #1; modified from Armstrong et al., 2004); (B) BSE image of tabular, Sr–Ba calcite phenocryst and microphenocryst (cc, Misery East, #3); (C) BSE image of Sr–Ba calcite-only segregation (cc, Misery East, #3; modified from Armstrong et al., 2004); (D) BSE image of a calcite + dolomite segregation (dol, Rattler, #3) with a Sr–Ba calcite rhomb (cc) and Sr–Ba calcite layer (cc) along the wall, overgrown by alternating bands of dark dolomite and magnesian calcite, with a dark dolomitefilled interior; (E) RL image of calcite + serpentine segregation (cc, Grizzly, #13); (F) BSE image of calcite + serpentine segregation (cc, Aaron, #17); (G) BSE image of a calcite + serpentine segregation (cc, Koala, #19) with patches of intergrown serpentine and inclusions of phlogopite and monticellite; (H) BSE image of talc in a fracture in forsterite (Rat, #2) with dolomite and calcite.

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(Table 3). The δ13C and δ18O values for calcite vary from − 10.1 to − 3.3 and 4.1 to 13.9‰, respectively, but the majority of the samples have isotopic compositions comparable to those compositions accepted for primary carbonate in mantle carbonatites (δ13C and δ18O of − 6.5 to − 4 and 6 to 9.2, respectively; Fig. 3). Dolomite was not observed in XRD patterns of the whole-rock powders, and the amount of CO2 yielded after 69 h was

typically b 4% of the total CO2 in the sample indicating that dolomite, if present in the samples at all, was very minor. The calcite-dominant nature of the samples was further supported by the fact that the isotopic compositions of splits of CO2 extracted after acidification for 3 and 21 h did not vary significantly (i.e.b 0.5‰). Ten micro-samples of calcite from eight calcitebearing kimberlite localities were analyzed (Table 4).

Table 3 Whole-rock stable isotopic compositions of carbonate from calcite-bearing kimberlite localities from the Lac de Gras kimberlite field Locality

Drill core # at depth(m)

Sample #

Split #

Reaction time

δ13C

δ18O

CO2

Fraction %

Misery East Misery East Misery East

DDH93-3 at 14.9 DDH-93-13 at 17 DDH93-13 at 25.75

Misery East

DDH93−13 at 36

Koala West

KWDC-01 at 52.25

Koala West Koala West Grizzly Grizzly Grizzly

KWDC-01 at 100.5 KWDC-01 at 81.5 DDH-92-02 at 306.75 DDH92-02 at 312.34 DDH92-02 at 320.55

Porpoise Porpoise

DDH-94-11 at 87.7 DDH94-11 at 96.25

Porpoise Porpoise

DDH94-11 at 106.8 DDH-94-11-81.10 DDH-94-11-81.10 (duplicate run)

Pigeon Pigeon

DDH-97-54 at 311.82 DDH97-54 at 312.12

Leslie Leslie

LDC-09 at 15.0 LDC-09 at 20.5

Leslie Aaron Aaron Aaron Roger

LDC-09 at 28.9 DDH-97-57 at 63.3 DDH97-57 at 62.6 DDH97-57 at 74.8 DDH94-17 at 131.9

Roger Roger

DDH-94-17 at 134.1 DDH94-17 at 139.7

Hardy Lk (Ann) Hardy Lk (Don)

Hl-10-2-B HL-11-3b

14 3 25 25 25 26 26 26 27 27 27 6 19 1 13 24 24 7 28 28 28 15 20 20 20 20 8 29 29 9 30 30 30 10 11 17 18 31 31 12 39 39 21 22

1 1 1 2 3 1 2 3 1 2 3 1 1 1 1 1 2 1 1 2 3 1 1 1 2 3 1 1 2 1 1 2 3 1 1 1 1 1 2 1 1 2 1 1

18 18 3 21 69 3 21 69 3 21 69 18 18 18 18 3 69 18 3 21 69 18 18 3 21 69 18 3 69 18 3 21 69 18 18 18 18 3 21 18 3 69 18 18

− 5.92 − 6.26 − 7.74 − 7.34 − 8.87 − 5.40 − 5.52 − 5.30 − 5.11 − 5.45 − 5.59 − 4.99 − 5.30 − 5.53 − 5.76 − 5.17 − 5.37 − 6.79 − 6.42 − 6.66 − 6.55 − 5.27 − 5.51 − 5.52 − 5.53 − 5.43 − 7.76 − 4.45 − 6.60 − 4.22 − 5.37 − 5.45 − 5.20 − 5.88 − 4.64 − 5.41 − 5.54 − 3.29 − 3.18 − 3.48 − 3.45 − 3.56 − 10.06 − 4.92

6.14 5.81 11.01 10.85 12.07 6.53 7.33 11.00 7.97 8.35 11.56 7.52 7.24 8.61 10.31 8.62 9.02 7.40 7.54 7.34 8.09 9.74 4.06 4.07 4.58 7.16 5.94 13.91 11.58 8.24 6.81 6.92 8.55 7.96 9.59 6.60 8.90 11.85 12.27 12.51 12.50 13.52 11.37 9.62

70 92 134 31 3 110 27 5 130 43 8 118 126 89 101 90 8 196 148 43 8 134 150 116 35 5 83 86 3 187 86 27 6 207 82 79 79 125 24 89 134 3 110 89

100.0 100.0 79.8 18.5 1.8 77.5 19.0 3.5 71.8 23.8 4.4 100.0 100.0 100.0 100.0 91.8 8.2 100.0 74.4 21.6 4.0 100.0 100.0 74.4 22.4 3.2 100.0 96.6 3.4 100.0 72.3 22.7 5.0 100.0 100.0 100.0 100.0 83.9 16.1 100.0 97.8 2.2 100.0 100.0

The time (in hours) that the samples were reacted with phosphoric acid and the amounts (μmol) of CO2 gas produced are shown. Duplicate analyses are indicated. Select samples have multiple splits of gas extracted at different reaction times and the fraction % of CO2 produced for each split is reported.

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analyses of calcite from calcite + serpentine segregations with δ13C and δ18O values from − 6.0 to − 4.8 and 9.9 to 13.9‰, respectively (Fig. 4). 4.2. Dolomite-bearing kimberlite

Fig. 3. Whole-rock δ13C and δ18O values for calcite from calcitebearing kimberlite. The field of primary carbonatite carbonate (CBT) is shown for reference (e.g., Clarke et al., 1991). Sample numbers shown correspond to those in Table 3. The values plotted are for the calcite fraction (split #1) from Table 3.

Five analyses of calcite from calcite-only segregations have δ13C and δ18O values from − 7.2 to − 3.8 and 6.8 to 8.2‰, respectively, significantly different from five

Twelve whole-rock samples of drill core from three dolomite-bearing kimberlite localities were analyzed (Table 5). SEM/microprobe and XRD analysis of wholerock powders indicated that the samples contain mixtures of calcite, dolomite and magnesian calcite. Sequential CO2 extraction (see Al-Aasm et al., 1990) indicated some samples contained either very little calcite (e.g., #32 and #40, Table 5) or very little dolomite (e.g., #23 and #36, Table 5). For the eight samples containing calcite + dolomite mixtures, acidification for 1 h yielded δ13C and δ18O values for the calcite fraction that varied from − 7.9 to − 3.4 and 1.2 to 5.4‰, respectively, similar to results following acidification for 3 h (Fig. 5). The yields of CO2 for the 1-and 3-hour fractions indicate that calcite typically comprises 30– 50% of the total carbonate in most of the samples of dolomite-bearing kimberlite (Table 5). Acidification for 64 or 70 h yielded δ13C and δ18O values for the dolomite fraction that varied from − 6.6 to −3.5 and 8.7 to 12.3‰, respectively. This dolomite fraction typically comprised

Table 4 Stable isotopic compositions of micro-samples of carbonate sampled in situ from segregations in calcite-bearing and dolomite-bearing kimberlites from the Lac de Gras kimberlite field Micro-samples from dolomite-bearing kimberlite Locality Anaconda Anaconda Rat Rattler Rattler Rattler Rattler Rattler

Drill core # at depth(m) DDH96-17 at 170.9 DDH96-17 at 170.4 DDH-94-08 at 257.75 DDH-96-13 at 233.25 DDH-96-13 at 233.25 DDH-96-13 at 233.25 DDH-96-13 at 233.25 DDH96-13 at 237.3

Sample # 16 5 2 4 4 4 4 23

Micro-samples from calcite-bearing kimberlite Koala West KWDC-01 at 81.5 19 Koala West KWDC-01 at 100.5 6 Misery East DDH-93-13 at 17 3 Misery East DDH-93-13 at 17 3 Grizzly DDH92-02 at 312.34 13 Aaron DDH97-57 at 62.6 17 Aaron DDH97-57 at 74.8 18 Hardy Lk (Don) HL-11-3b 22 Pigeon DDH-97-54 at 311.82 8 Porpoise DDH-94-11 at 87.7 7

Reaction time 62 62 62 64 63 3.5 3 18

δ13C − 3.36 − 3.11 − 3.72 − 7.59 − 5.66 − 4.50 − 5.76 − 7.49

δ18O 12.96 11.76 13.91 11.13 11.96 8.83 7.93 9.15

CO2 0.3 0.3 0.2 0.8 0.6 0.6 0.3 2.0

Description of micro-sample Dolomite in calcite + dolomite segregation Dolomite in calcite + dolomite segregation Dolomite in calcite + dolomite segregation Dolomite in calcite + dolomite segregation #1 Dolomite in calcite + dolomite segregation #2 Calcite in calcite + dolomite segregation #2 Calcite in calcite + dolomite segregation #3 Calcite in small calcite-only segregation

18 18 18 18 18 18 18 18 18 18

− 5.59 − 3.75 − 6.02 − 5.87 − 4.76 − 5.93 − 4.84 − 5.00 − 6.03 − 7.22

12.69 8.17 9.93 7.67 13.89 11.88 12.15 6.84 7.31 7.66

0.9 0.4 1.5 1.1 1.2 5.5 1.8 0.3 0.3 5.5

Calcite in large calcite + serpentine segregation Calcite in small calcite-only segregation Calcite in large calcite + serpentine segregation Calcite in small calcite-only segregation Calcite in large calcite + serpentine segregation Calcite in large calcite + serpentine segregation Calcite in large calcite-serpentine segregation Calcite in small calcite-only segregation Calcite in small calcite-only segregation Calcite in small calcite-only segregation

The samples analyzed were typically b0.1 mg in size, prepared in small-volume reaction vessels. The time (in hours) that the samples were reacted with phosphoric acid and the amounts (μmol) of CO2 gas produced are given. Micro-samples include calcite and dolomite from calcite + dolomite segregations; and calcite from calcite-only and calcite + serpentine segregations.

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Fig. 4. δ13C and δ18O values for micro-samples of calcite from calciteonly and calcite + serpentine segregations in calcite-bearing kimberlites. The isotopic whole-rock composition of calcite (Table 3) is joined with a tie-line to the micro-sample separated from the same sample. Samples are numbered as in Table 4.

10–50% of the total carbonate in the whole-rock sample (Table 5). Though the CO2 extracted after 18 or 21 h of acidification was derived mostly from a magnesian calcite, the measured isotopic values (Table 5; Fig. 5) are approximations owing to variable contamination of the fraction by CO2 from both calcite and dolomite end members in the whole-rock powders. Eight micro-samples of calcite and dolomite from three dolomite-bearing kimberlite localities were analyzed (Table 4). Five analyses of dolomite from calcite + dolomite segregations have δ13C and δ18O values from −7.6 to −3.1 and 11.1 to 13.9‰, respectively, similar to those for whole-rock dolomite. Three analyses of calcite from a calcite-only segregation, and from two calcite + dolomite segregations, have δ13C and δ18O values from −7.5 to −4.5 and 7.9 to 9.2‰, respectively (Fig. 6). 5. Discussion 5.1. Stable isotopic variation in Lac de Gras carbonate Many of the analyses of whole-rock calcite in calcitebearing kimberlite and calcite from calcite-only segregations, and from calcite + dolomite segregations have δ18O values from 6 to 9‰, and δ13C values from − 7 to − 4‰, comparable to those of primary carbonate in mantle carbonatites (i.e., δ13C and δ18O of − 6.5 to − 4 and 6 to 9.25, respectively; CBT field, Fig. 7). The

clustering of analyses within the CBT field (Fig. 7) distinguishes the distribution of calcite analyses from Lac de Gras from the majority of analyses of other carbonates published for kimberlites worldwide. The isotopic compositions of the calcites from Lac de Gras indicate that much of the calcite formed from a kimberlite melt as a magmatic phase at sub-liquidus (e.g., 1100–750 °C) temperatures and, importantly, preserved its magmatic isotopic composition. This finding is consistent with the exceptional degree of preservation of macrocrystic and phenocrystic forsterite in many of the thin sections examined, and the conclusions reported by Armstrong et al. (2004) for calcite in several Lac de Gras kimberlites based, in part, on textures and the elevated Sr and Ba contents of calcite. Some whole-rock δ18O values of calcite from calcitebearing kimberlite are between 9 and 14‰, which overlap the range of δ18O values for calcite in calcite + serpentine segregations and for dolomite from dolomitebearing kimberlite. Equilibrium fractionation of oxygen isotopes between kimberlite melt, an exsolved fluid and carbonate at magmatic temperatures cannot account for these high δ 18 O values (i.e. N 9‰). Instead, the carbonates with high δ18O values could have formed at lower, subsolidus temperatures from either magmatic volatiles no longer in equilibrium with the melt (closed system model), or externally-derived, non-magmatic fluids (open system model). Alternatively, the isotopic compositions reflect a magmatic process such as differentiation, assimilation or degassing (see discussions by Sheppard and Dawson, 1975; Deines, 1989; Kirkley et al. 1989; Santos and Clayton, 1995; Pandit et al. 2002). It is also possible, but less likely, that the carbonates with high δ18O values represent a late, variable isotopic alteration by one or more secondary processes such as weathering (e.g., Deines, 1989; Pandit et al. 2002). Similarly, high temperature, magmatic fractionation of oxygen isotopes between kimberlite melt and carbonate cannot account for the low δ18O values (2 to 5.5‰; Table 5) of whole-rock calcite in dolomite-bearing kimberlites (Fig. 5). Therefore, carbonates from Lac de Gras with δ18O values outside the range of 6 to 9‰ likely reflect formation or alteration by one or more processes involving fluids of variable composition and temperature. We distinguish between magmatic and hydrothermal carbonate as did Santos and Clayton (1995). Magmatic carbonates are those which formed at magmatic temperatures in isotopic equilibrium with kimberlite melt, whereas fluid-related (or, hydrothermal) carbonates are those which formed from (magmatic) deuteric fluids that persisted in the porous

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Table 5 Whole-rock stable isotopic compositions of carbonate from dolomite-bearing kimberlite localities from the Lac de Gras kimberlite field Locality

Drill core # at depth(m)

Sample #

Split #

Reaction time

δ13C

δ18O

CO2

Fraction %

Rat

DDH-94-08 at 257.75 DDH-94-08 at 257.75 (duplicate run)

Rat

DDH94-08 at 260.15

Rat

DDH94-08 at 269.40B

Rat

DDH94-08 at 262.35

Rattler

DDH-96-13 at 233.25 DDH-96-13 at 233.25 (duplicate run)

Rattler

DDH96-13 at 234.5

Rattler

DDH96-13 at 237.3 DDH96-13 at 237.3 (duplicate run)

Rattler

DDH96-13 at 238.6

Anaconda

DDH96-17 at 162.65

Anaconda

DDH96-17 at 170.4 DDH96-17 at 170.4 (duplicate run)

Anaconda

DDH96-17 at 170.9 DDH96-17 at 170.9 (duplicate run)

Anaconda

DDH96-17 at 174.7

2 2 2 2 32 32 32 32 33 33 33 33 40 40 40 40 4 4 4 4 36 36 36 36 23 23 23 23 37 37 37 37 38 38 38 38 5 5 5 5 16 16 16 16 16 16 16 35 35 35 35

1 1 2 3 1 2 3 4 1 2 3 4 1 2 3 4 1 1 2 3 1 2 3 4 1 1 2 3 1 2 3 4 1 2 3 4 1 1 2 3 1 2 3 1 2 3 4 1 2 3 4

1 3 18 64 1 3 18 70 1 3 21 70 1 3 21 70 1 3 18 64 1 3 21 70 1 3 18 64 1 3 21 70 1 3 21 70 1 3 18 64 3 18 64 1 3 21 70 1 3 21 70

− 4.94 − 4.90 − 4.14 − 3.51 − 5.12 − 4.52 − 2.83 − 1.82 − 4.57 − 4.60 − 4.30 − 3.79 − 1.22 − 2.03 − 2.11 − 1.93 − 7.89 − 7.81 − 7.34 − 6.57 − 6.15 − 6.20 − 6.14 − 5.97 − 7.11 − 7.12 − 6.78 – − 7.19 − 7.28 − 7.09 − 6.41 − 3.35 − 3.85 − 4.57 − 5.49 − 4.39 − 4.33 − 3.88 − 3.97 − 4.32 − 4.55 − 5.04 − 4.30 − 4.44 − 4.57 − 5.03 − 3.53 − 3.61 − 3.62 − 3.98

5.35 6.08 6.91 9.59 11.80 10.90 10.82 9.18 5.29 5.82 7.20 8.65 12.17 10.09 9.86 10.31 3.33 4.60 8.58 11.71 8.14 8.08 7.97 9.61 10.16 10.30 10.52 – 2.25 2.43 5.24 8.97 4.53 5.31 8.53 10.37 3.30 4.08 9.92 12.27 2.20 9.36 12.31 1.21 3.07 7.92 10.32 4.59 4.43 6.58 9.73

73 116 68 29 26 38 160 189 132 57 152 56 2.5 20 95 112 119 114 44 16 148 63 51 7.5 120 96 14 b1 168 66 81 23 139 61 125 73 122 132 60 40 106 46 29 114 44 97 57 182 105 133 29

– 54.5 31.9 13.6 6.3 9.2 38.7 45.8 33.2 14.4 38.3 14.1 1.1 8.7 41.4 48.8 – 65.5 25.3 9.2 54.9 23.4 18.9 2.8 – 87.3 12.7 – 49.7 19.5 24.0 6.8 34.9 15.3 31.4 18.3 – 56.9 25.9 17.2 58.6 25.4 16.0 36.5 14.1 31.1 18.3 40.5 23.4 29.6 6.5

The time (in hours) that the samples were reacted with phosphoric acid and the amounts (μmol) of CO2 gas produced are shown. Duplicate analyses are indicated. Samples have multiple splits of gas extracted at different reaction times and the fraction % of CO2 produced for each split is reported. SEM results and XRD patterns for these samples show the carbonates are mixtures of calcite, magnesian calcite, and dolomite. Short reaction times yield CO2 predominantly from calcite whereas long reaction times reflect the dolomite fraction.

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matrix of the crystalline kimberlite at subsolidus temperatures, or from water-rock interaction with unspecified fluids (e.g., non-magmatic, meteoric) at subsolidus temperatures. 5.2. Method and parameters for oxygen isotope modeling of carbonate Deines (1989) reviewed the oxygen isotope fractionation between calcite and fluid and showed the pronounced effect of water/rock interaction involving fluids of variable CO2/H2O ratios on the δ18O value of carbonatites at subsolidus temperatures. Santos and Clayton (1995) extended this work and used variable CO2/H2O and fluid/rock ratios in carbonatites to model a wide range of theoretical δ18O values for calcite formed at 500 to 100 °C from fluids outgassed from a magma chamber. These studies highlighted the potential importance of CO2-rich fluids in forming carbonates of widely varying δ18O and δ13C values, and, especially carbonates with low δ18O values. In the absence of mineral equilibria (cf. Taylor and Bucher-Nurminen, 1986), such isotopic modeling in magmatic rocks depends critically on assumptions of CO 2/H2O ratios when independent evidence to quantify this parameter is difficult to constrain and is commonly absent.

Fig. 5. Whole-rock δ13C and δ18O values for sequential extractions of calcite, magnesian calcite, and dolomite (Table 5). Tie-lines join the sequential aliquots of CO2 gas from individual samples. Fields encompassing Lac de Gras whole-rock data for dolomite and for primary carbonatite carbonate (CBT) are shown for reference. Sample numbers shown correspond to those in Table 5.

Fig. 6. δ13C and δ18O values for micro-samples of calcite and dolomite from calcite + dolomite segregations in dolomite-bearing kimberlite. The whole-rock calcite and dolomite compositions (Tables 3 and 5) are joined with a tie-line to the micro-sample separated from the same sample. Fields encompassing Lac de Gras whole-rock data for dolomite and for primary carbonatite carbonate (CBT) are shown for reference. Sample numbers shown correspond to those in Table 4.

In this study of carbonates from Lac de Gras, the equations of Santos and Clayton (1995) were utilized to model isotopic exchange during fluid-rock interaction,

Fig. 7. δ13C and δ18O values for kimberlite carbonate from literature (n = 142), and for whole-rock calcite and calcite from calcite-only and calcite + dolomite segregations from Lac de Gras (this study). The field of primary carbonate from carbonatites (CBT) is shown for reference.

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but in a slightly different fashion, using different isotope fractionation factors and assumptions. The goal of the modeling was to test whether magmatic CO2–H2O fluids, in equilibrium with kimberlite melt, could account for the observed range of δ18O values for carbonate at Lac de Gras without the need to introduce external (i.e. non-magmatic) fluids to the kimberlites. The oxygen isotope models attempt to constrain the nature of the fluids based on aspects of the geologic setting of the Lac de Gras kimberlites, petrographic observations and whole-rock concentrations of CO2 and H2O. The two principal equations used for isotopic modeling of CO2–H2O fluids interacting with a rock are (Santos and Clayton, 1995): Drock−fluid ¼ ð2NCO2 Drock−CO2 þ NH2O Drock−H2O Þ= ð2NCO2 þ NH2O Þ

ð1Þ


 d18 Ofrock ¼ ðF=RÞ d18 Oifluid þ Drock−fluid þ d18 Oirock = ½1 þ F=R ð2Þ The first equation was used to calculate the oxygen isotope fractionation factor between kimberlite and a mixed CO2–H2O fluid (△rock–fluid), assuming a molar CO2/H2O ratio (i.e., NCO2 and NH2O) in the fluid. In this case, a whole-rock δ 18 O value of 5.7‰ was assumed for the kimberlite, and this was combined with fractionation factors for peridotite-CO2 and peridotiteH2O (Zhao and Zheng, 2003) to calculate the initial oxygen isotopic composition of an equilibrium magmatic CO2–H2O fluid (δ 18 O i fluid). Kimberlite-fluid fractionation factors have not been published, hence those for peridotite-fluid were used because they are the closest analogue (i.e. forsterite dominated) available in the literature. Eq. (2) was modified from Santos and Clayton (1995) by substituting carbonate terms for the rock terms. This modified equation (below) was used to calculate the final δ18O value of the carbonate (δ18Ofcarbonate) in the kimberlite, either as calcite (δ18Ofcalcite) or as dolomite (δ18Ofdolomite), for a range of temperatures and fluid/rock (F/R) ratios:
d18 Ofcarbonate ¼ ½ðF=RÞ d18 Oifluid þ Dcarbonate−fluid þd18 Oicarbonate =½1 þ F=R

447

△carbonate–fluid term was calculated using fractionation factors for dolomite–CO2 and dolomite–H2O (Zheng, 1999). Eq. (3) assumes that the magmatic CO2–H2O fluid either precipitates carbonate directly, or continues to exchange oxygen isotopes with existing carbonate over a range of temperatures as the kimberlite cools, possibly even as low as 80 °C, below which the rate of isotope exchange is too slow (Zheng and Hoefs, 1993). The initial δ18O value of the calcite (δ18Oicarbonate) was assumed to be 7‰ based on δ18O values of microsamples of calcite-only segregations, presumed to be the least likely to be affected by later isotopic exchange with fluids. The modeled δ18O value of magmatic CO2–H2O fluid (δ18Oifluid) increased from 8.2 to 9.6‰ over the temperature range of 1100 to 750 °C (at CO2/H2O of 0.25; see Fig. 8), assuming complete isotopic exchange between kimberlite melt and fluid. Below solidus temperatures (i.e., b 750 °C), the models assume that the δ18Oi fluid value of the fluid remained constant. This assumption is justified because the kimberlites studied are small-volume magmatic bodies that likely cooled quickly due to their shallow emplacement depths (b500 m; Stasiuk et al., 2003; Nowicki et al., 2004), and the low rate of isotopic re-equilibration between crystallized silicates and deuteric fluid. Above 750 °C, the isotope models presume a fluid/ rock (F/R) molar oxygen ratio of 1.0, equivalent to a whole-rock volatile content of 15 wt.% in the kimberlite (see data in Armstrong et al., 2004). At sub-solidus temperatures, isotopic exchange with the fluid is

ð3Þ

In the case of calcite, the △carbonate–fluid term in Eq. (3) was calculated from Eq. (1) upon substitution of oxygen isotope fractionation factors (△) for calcite–CO2 and calcite–H2O (Zheng, 1999). Similarly, for dolomite, the

Fig. 8. Whole-rock, molar CO2/H2O volatile ratios (unpublished data, B. Kjarsgaard) and whole-rock δ18O values for calcite in calcite-bearing (Table 3) and dolomite-bearing (Table 5) kimberlite from Lac de Gras.

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presumed to be dominated by only the carbonate fraction of the kimberlite, so the isotopic models use a F/R ratio of 7 that is approximately equivalent to a whole-rock volatile content of 15 wt.% and a modal carbonate content of 15% (calcite-bearing kimberlite) to 30% (dolomite-bearing kimberlite). At a temperature of 200 °C, for example, changing the F/R from a value of 7 to 20 changes the calculated δ18Ofcarbonate value for calcite from 12.1 to 12.5‰, and for dolomite from 9.7 to 10.0‰, indicating the results of the isotopic models are not strongly affected by modest changes in the F/R term. Use of the fractionation factor (△rock–fluid) term in (1) makes the final δ18O value of the calcite or dolomite fraction (δ18Ofcarbonate) in Eq. (3) strongly dependent on temperature, and on the CO2/H2O ratio of the magmatic fluid. Although there is no independent constraint on the CO2/H2O ratio of the magmatic fluid, the measured, whole-rock volatile concentrations in different varieties of kimberlite may provide some insight. Armstrong et al. (2004) noted that calcite-bearing kimberlites from Lac de Gras typically have twice the H2O and only half the CO2 of dolomite-bearing kimberlites. For the purposes of modeling, we assume that the measured, whole-rock CO2/ H2O ratios of the kimberlites (Fig. 8) reflect those of the magmatic fluids which, in turn, constrain calculations using Eq. (1) and Eq. (3), respectively. A wide range of molar CO2/H2O ratios of 0.25, 0.5, 1.5 and 10 were used to model isotopic variations in calcite-bearing and dolomite-bearing kimberlites. At magmatic temperatures (above 750 °C), for example, changing the molar CO2/ H2O ratios between 0.25 and 10 has limited influence on the calculated δ18Ofcarbonate value for calcite; the values range between 7 and 8‰ (i.e. consistent with the CBT box). However, at subsolidus temperatures, at 200 °C for example, doubling CO2/H2O from a value of 0.25 to 0.50 changes the calculated δ18Ofcarbonate value for calcite from 14.1 to 11.1‰, and for dolomite from 12.9 to 10.0‰, indicating the results of the isotopic models are sensitive to changes in the CO2/H2O term at lower temperatures. 5.3. Results of oxygen isotope modeling of Lac de Gras carbonates The δ18O values measured for carbonate from Lac de Gras are compared in Fig. 9 with calculated δ18O values representing isotopic equilibrium with magmatic and/or deuteric CO2–H2O fluids at formation temperatures of 100 to 1100 °C. The model δ18O values span a wide range, from approximately 2 to 17‰, and are strongly dependent on the assumed CO2/H2O ratio. Given the assumptions stated above for the models, the results shown in Fig. 9 suggest that magmatic carbonate in

kimberlite should have δ18O values between about 6.9 and 8.5‰ at temperatures of 1100–750 °C. Indeed, the majority of whole-rock calcite, and micro-sample calcite from calcite-only and calcite + dolomite segregations in the Lac de Gras kimberlite suite fall within this magmatic range, having preserved their primary, magmatic isotopic signatures (cf. Fig. 7). The δ18O values for a subset of whole-rock calcite in calcite-bearing kimberlite (Table 3), and for all of the micro-samples of calcite from calcite + serpentine segregations (Table 4), exceed the modeled upper limit for magmatic calcite (i.e. 8.5‰; Fig. 9). Similarly, analyses of whole-rock dolomite and micro-samples of dolomite from calcite + dolomite segregations also exceed the calculated magmatic value (i.e. 8.2‰; Fig. 9). These carbonates could have formed from, or re-equilibrated with, deuteric CO2–H2O fluids with a molar CO2/H2O ratio below about 0.5, at 100 to 500 °C (Fig. 9). Lower assumed CO2/ H2O ratios yield higher formation temperatures for these carbonates, but a CO2/H2O ratio of 0.25 gives formation temperatures for calcite + serpentine segregations in Lac de Gras that are consistent with those (i.e.b 500 °C) for serpentine stability in kimberlites (Mitchell, 1986; Chernovsky et al., 1988). At Lac de Gras, the whole-rock calcite fraction of dolomite-bearing kimberlites has δ18 O values from 2 to 5.5‰, below the lowest δ18 O value published previously for carbonate in kimberlite (e.g., 6.9‰, Wesselton Pipe; Sheppard and Dawson, 1975), and outside the CBT field for carbonatite. Even if the modeling parameters are changed (e.g., kimberlite δ18 O value of 5.3 to 5.7‰, calcite of 6 to 7‰, and the F/R ratio reduced by two-thirds), the lowest δ 18 O calculated value of magmatic carbonate remains too high (i.e. 6.4‰) to account for any of the whole-rock calcite in dolomite-bearing kimberlite. One possible explanation of the low δ18 O values, invoked in other studies, is that an influx of low- 18 O meteoric waters exchanged oxygen with the calcite at elevated, but still sub-solidus, temperatures (e.g. Sheppard and Dawson, 1975; Taylor, 1977; Deines, 1989; Pandit et al. 2002). This does not explain, however, why only dolomite-bearing kimberlites, and not calcite-bearing kimberlites, would have been affected. Another explanation could involve equilibrium with deuteric fluids having elevated CO2/ H2O ratios (e.g., Santos and Clayton, 1995). Whole-rock CO2 and H2O concentrations clearly indicate that dolomite-bearing kimberlites have higher CO2/H2O ratios than calcite-bearing kimberlite (Fig. 8). The measured CO2–H2O contents for dolomite-bearing kimberlite suggest an upper limit of about 1.5 for the molar CO2/H2O ratio (Fig. 8), but using this ratio for

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Fig. 9. Compilation of δ18O values for magmatic and deuteric types of kimberlite carbonate from Lac de Gras. The results of four isotope models (see text) showing the predicted δ18O values of carbonate at temperatures from 1100 to 100 °C with variable fluid CO2/H2O ratios (0.25 to 10) are shown for reference. The range of δ18O values for primary carbonate from carbonatite (CBT) is provided. Abbreviations: cc = calcite; dol = dolomite; segs = segregations; kimb = kimberlite; serp = serpentine.

modeling cannot reproduce any of the measured low δ18O values for calcite in dolomite-bearing kimberlite (Fig. 9). However, if a CO2/H2O ratio of 10 is applied over a temperature range of 500 to 100 °C, the measured range of low δ18O values is exactly reproduced (Fig. 9). The modeling suggests that in dolomite-bearing kimberlites, the magmatic fluid originally had a CO2/H2O ratio of ∼ 0.25 which evolved to a much higher CO2/ H2O ratio (∼ 10) that, in turn, can account for the low δ18O values of calcite in the groundmass (Fig. 9). This CO2-rich deuteric fluid did not affect the isotopic compositions of groundmass dolomite, or of carbonates in the calcite + dolomite segregations. This likely reflects the higher stability of dolomite with regard to isotopic re-equilibration with fluids, and for calcite in segregations, isotopic preservation is likely a result of their large, mm-scale size compared to groundmass calcite. 5.4. Carbon isotope variation of Lac de Gras carbonate In magmatic systems, crystal fractionation, degassing of volatiles and the effects of deuteric fluids have been

shown to produce variations of δ13C in carbonate from carbonatites and kimberlites (e.g., Deines, 1989; Kirkley et al., 1989; Santos and Clayton, 1995; Castorina et al., 1997). At Lac de Gras, the present study analyzed carbonates from thirteen individual localities with up to four analyses from a single locality and one or two analyses from each of the other localities. In this suite of Lac de Gras samples there are no consistent co-variations of oxygen and carbon isotope compositions apparent among samples, as have been observed in studies of other localities (e.g. Deines, 1989), or as predicted by isotopic modeling (Santos and Clayton, 1995). Fig. 10 summarizes the δ13C values for calcite and dolomite from Lac de Gras kimberlites. The average δ13C value for carbonate from the thirteen Lac de Gras kimberlite localities is −5.3 +/− 1.5 (1σ, n = 54). This average and, indeed, the general variation of the Lac de Gras data, are comparable to carbonates from kimberlites in southern Africa and North America, for which the average δ13C value is −5.4 +/− 1.8 (1σ, n = 142; Deines and Gold, 1973; Sheppard and Dawson, 1975; Kobelski et al., 1979; Kirkley et al., 1989). The δ13C values for Lac

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Fig. 10. Compilation of δ13C values for magmatic and deuteric types of kimberlite carbonate from Lac de Gras. The range of δ13C values for primary carbonate from carbonatites (CBT) is shown for reference, as well as the mean value and two-sigma standard deviation for kimberlitic carbonate published in the literature (n = 142; data are from Deines and Gold, 1973; Sheppard and Dawson, 1975; Kobelski et al., 1979; Kirkley et al., 1989). Abbreviations are the same as in Fig. 9.

de Gras carbonate are typical of magmatic or mantlederived carbonate with no discernable carbon isotope evidence for crustal or marine carbon. 5.5. Integration of isotopic data and petrography/ mineralogy on carbonate formation 5.5.1. Calcite-bearing kimberlites In the least-altered samples of calcite-bearing kimberlites (i.e.b10% alteration of forsterite macrocrysts), whole-rock δ13C and δ18O values for high Sr–Ba calcite plot within the range of compositions for carbonatite carbonate (Fig. 3). Micro-samples of calcite from calcite-only segregations (Fig. 4) also preserve magmatic δ13C and δ18O values, consistent with their magmatic, high Sr–Ba compositions (Armstrong et al., 2004). In contrast, highly altered samples of calcite-bearing kimberlite (e.g., Roger #12, 31 and 39; Hardy Lake #21 [Anne]; Pigeon #29; Misery East #25; Table 2) are characterized by 50–100% replacement of forsterite phenocrysts by calcite plus serpentine and with forsterite macrocrysts being variably (10–60%), but highly replaced by serpentine plus calcite (Table 2). Furthermore, highly altered samples also contain a greater number of cm-scale calcite + serpentine segregations, and the groundmass has local veinlets of calcite and a ubiquitous, but variable textural overprint of calcite and serpentine. The whole-rock calcite of these

altered samples has elevated δ18O values (e.g., 11 to 14‰) consistent with formation at subsolidus temperatures from deuteric fluids. Moderately altered samples of calcite-bearing kimberlite (e.g., Grizzly #13; Hardy Lake #22 [Don]; Porpoise #15) are characterized by 15–100% replacement of forsterite phenocrysts by calcite and serpentine and less than 50% replacement of forsterite macrocrysts by serpentine or calcite (Table 2). These samples contain fewer calcite + serpentine segregations and the groundmass has less serpentine compared to the more strongly altered samples. These samples also have whole-rock δ18O values of calcite intermediate (e.g., 9 to 11‰) between those of the fresh and highly altered samples. However, micro-samples of calcite from calcite-only segregations in these altered samples have preserved their high temperature, magmatic isotopic signatures (i.e. δ18O values from 6.8 to 8.2‰), whereas microsamples of calcite from calcite + serpentine segregations (i.e. δ18O values from 9.9 to 13.9‰) record a low temperature, deuteric history. For samples with calcite + serpentine segregations, there is a complete continuum within a single thin section between segregations that contain only calcite and serpentine, and those that also include numerous 10–50 μm inclusions of phlogopite, apatite, spinel, perovskite and monticellite. We interpret the continuum as representing incomplete or variable degrees of alteration and textural overprinting of the kimberlite

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groundmass mineralogy by deuteric fluids. With increasing alteration, there is a distinctive corresponding increase in the development and formation of atolltextured spinel (accompanied by serpentine) in the groundmass (Table 2) interpreted to result from interaction with deuteric fluids. The rims surrounding atoll-spinel are close to end member magnetite in composition (Armstrong et al., 2004) indicating a relatively oxidizing deuteric fluid. In one case, barite was noted in a calcite + serpentine segregation (Aaron #17) further suggesting an oxidizing deuteric fluid. In summary, higher δ18O values of whole-rock calcite correlate with increased intensity of forsterite alteration, increased abundance of calcite + serpentine segregations, textural overprint of primary groundmass minerals by anhedral calcite and serpentine, and development of atoll-spinels, all interpreted as a result of deuteric fluids. 5.5.2. Dolomite-bearing kimberlites Sr–Ba calcite rhombs are well-preserved in calcite + dolomite segregations in dolomite-bearing kimberlites (e.g., Fig. 2D; see also Armstrong et al., 2004). Microsamples of these rhombs have magmatic δ13C and δ18O values (e.g., δ18O values from 7.9 to 8.8‰; δ13C from − 5.8 to − 4.5; Fig. 6) indistinguishable from those in calcite-bearing kimberlites suggesting a common early history of magmatic formation. δ18O values of whole-rock dolomite and microsamples of dolomite in dolomite-bearing kimberlite overlap those for calcite in calcite + serpentine segregations in calcite-bearing kimberlite. In addition, the presence of rare calcite + dolomite + serpentine segregations (Table 2) is texturally analogous to calcite + serpentine segregations, further supporting a deuteric origin for dolomite in dolomite-bearing kimberlite. Additional textural evidence for a subsolidus, deuteric origin is provided by the degree of textural preservation of delicate, acicular carbonate needles, and preservation of finely laminated magnesian calcite bands in the segregations (Fig. 2D) which indicates the kimberlite was mechanically rigid (i.e. subsolidus) at the time of dolomite precipitation (see also Armstrong et al., 2004). The textural evidence is consistent with the modeled isotopic formation temperatures for dolomite (∼400 to 100 °C; Fig. 9). The forsterite macrocrysts and phenocrysts in dolomite-bearing kimberlites are almost universally fresh (Table 2). The very low modal abundance of serpentine in dolomite-bearing kimberlite is interpreted to result from the formation of dolomite as the stable magnesium phase instead of serpentine. This possibly reflects the slightly higher CO2/H2O ratio of the

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deuteric fluid associated with dolomite-bearing kimberlite, consistent with that inferred by the whole-rock volatile data and the isotope models (Figs. 8 and 9). The processes responsible for producing these different deuteric fluids are difficult to constrain uniquely. However, previous studies have shown that calciteand dolomite-bearing kimberlites have different whole-rock major and trace element geochemical signatures (Dowall et al., 2003; Dowall, 2004) and spinel chemistry (Armstrong et al., 2004) that collectively suggest these kimberlites originated from different parental magmas (Nowicki et al., 2004). It is possible that the difference in CO2/H2O ratio of the deuteric fluids was influenced by parental magma compositions. Dolomite-bearing kimberlites contain a higher modal abundance of anhedral, groundmass calcite than calcite-bearing kimberlites (Table 2). In some dolomite-bearing kimberlites (e.g., Anaconda and Rat), the primary (i.e. magmatic), groundmass silicate phases have been texturally overprinted leaving a residual assemblage of oxide minerals, apatite and calcite (+/− serpentine, talc). These texturally overprinted kimberlites notably contain calcite with low δ 18 O values (i.e. b 6‰). The co-occurrence of dolomite and low δ18O calcite is ubiquitous in the Anaconda and Rat kimberlites, but neither of these phases is observed in calcite-bearing kimberlite. The origin of the low δ18O groundmass calcite remains problematic. If one assumes that the isotopic modeling undertaken is valid, then the low δ18O calcite formed from a deuteric fluid with a CO2/H2O ratio of ∼10, at temperatures between 500–100 °C (Fig. 9). Although there is no direct evidence to quantify the inferred high CO2/H2O ratio of the model, certainly, the presence of talc as an alteration product in dolomitebearing kimberlite is consistent with much higher CO2/H2O ratio deuteric fluids. Talc likely formed as a product of the reaction involving serpentine and CO2rich fluid (e.g., Trommsdorff and Connolly, 1990). In contrast, the absence of talc in calcite-bearing kimberlite indicates a comparably lower CO2/H2O deuteric fluid. 5.5.3. Spatial distribution and relative timing of deuteric fluids Primary, magmatic calcite is found in the majority of samples studied; however, based on petrographic observations and isotopic results, all of the kimberlites were affected to varying degrees by deuteric fluids. It is noteworthy that individual samples from the freshest calcite-bearing kimberlites are strongly altered (by deuteric

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fluids) to serpentine plus calcite-rich assemblages compared to other samples from the same kimberlite body (e.g., Koala West, Grizzly, Aaron, see Table 2). This implies localized deuteric fluids existed within individual kimberlite bodies. The idea of localized deuteric fluids is further supported based on the spatial distribution of dolomite and low δ18O groundmass calcite in both calcite- and dolomite-bearing kimberlites. For example, in the calcitebearing Porpoise kimberlite, consisting of four samples over 25 m of drill core, one sample (#20) has calcite + dolomite and calcite + dolomite + serpentine segregations, and whole-rock calcite with a low δ18O value of 4.1‰. The other three Porpoise samples have no evidence of dolomite or low δ18O calcite. Another example is the calcite-bearing Misery East kimberlite, consisting of four samples over 21 m of drill core, of which one sample (#3) has both calcite phenocrysts and calcite-only segregations texturally rimmed by magnesian calcite (see Armstrong et al., 2004). The whole-rock calcite for this sample also has a low δ18O value of 5.8‰ (Table 3). The other three Misery East samples have no evidence of dolomite or low δ18O calcite. Conversely, in the Rattler dolomite-bearing kimberlite, consisting of four samples, there are two samples (#4 and 37) that contain abundant dolomite and low δ18O calcite, whereas, within the same 4-m interval of drill core, two other samples (#23 and 36) lack dolomite and have calcite with higher, not lower, δ18O values (i.e. 8.1 and 10.1‰; Table 5). Clearly, a majority of the kimberlite bodies have been affected by local or channeled flow of subsolidus, deuteric fluids. In dolomite-bearing kimberlite, dolomite with high δ18O values co-exists with low δ18O calcite. This spatial distribution suggests a genetic relationship; however, one fluid cannot have precipitated both the low δ18O calcite and high δ18O dolomite. The isotope models suggest that the low δ18O calcite formed from deuteric fluids with much higher CO2/H2O ratio (∼10; Fig. 9). The origin of these CO2-rich fluids can be explained by either of two possibilities: 1) an influx of CO2-rich fluid from magmatic (i.e. suprasolidus) degassing of kimberlite at depth; 2) an influx of CO2-rich fluid derived from subsolidus alteration (i.e. serpentinization and dehydration of the fluid) of serpentine-rich kimberlite at depth. Alternatively, an influx of meteoric water at elevated temperatures could be the source of the low δ18O calcite; however, this model would require a heat source to drive this late event. The occurrence of the Rat and Anaconda kimberlites as isolated dykes, emplaced at shallow depths, is inconsistent with an external heat source to drive the meteoric fluid circulation. Furthermore, it remains unclear why an influx of meteoric water and precipitation of low δ18O calcite would be restricted only to dolomitized portions of

dolomite-bearing kimberlites. The preferred interpretation for the source of the low δ18O calcite is an influx of CO2rich fluid from degassing of kimberlite at depth. 5.5.4. Serpentine in calcite-serpentine segregations Previous workers (e.g., Clement and Skinner, 1979; Clement, 1982; Mitchell, 1986, 1994) consider that serpentine in calcite + serpentine segregations and in the groundmass represents a primary mineral that has crystallized from kimberlite magma. In this study, calcite micro-samples from calcite + serpentine segregations have high δ18O values from 9.9 to 13.9‰ and are interpreted to have precipitated from deuteric fluids at low temperatures (500–100 °C). These formation temperatures are consistent with calculated temperatures for serpentine stability (Chernovsky et al., 1988), and are below the solidus temperature of kimberlite magma, i.e. serpentine is not a primary magmatic crystallizing phase in kimberlite. 6. Conclusion The hypabyssal kimberlite intrusions from Lac de Gras contain exceptionally well-preserved carbonate assemblages. Both calcite-and dolomite-bearing kimberlites shared a common early history of magmatic (e.g., 1100– 750 °C), Sr–Ba calcite formation with δ18O values for whole-rock and micro-sample calcite that range from 6 to 9‰. The δ13C values of magmatic calcite vary from −3.5 to −8, similar to those for carbonates in primary igneous carbonatite. Higher δ18O values from 9 to 14‰ for whole-rock calcite and micro-sample calcite from calcite + serpentine segregations in calcite-bearing kimberlite correlate with more abundant calcite + serpentine segregations and higher degrees of alteration of forsterite macrocrysts and phenocrysts. Isotopic modeling suggests the higher δ18O values reflect a second generation of calcite that formed from deuteric fluids at temperatures between 500 to 100 °C. Dolomite-bearing kimberlite localities are rare compared to calcite-bearing kimberlite localities. Isotope models for whole-rock and microsample dolomite from calcite + dolomite segregations with δ18O values from 9 to 14‰ indicate formation from deuteric fluids at temperatures between 400 to 100 °C. The δ18O values, the model temperatures and the rare presence of calcite + dolomite + serpentine segregations all indicate that dolomite is not magmatic, rather it formed from deuteric fluids. High δ18O dolomite and low δ18O whole-rock calcite (from 1.5 to 5.5‰) co-exist in dolomite-bearing kimberlites. Isotopic modeling suggests this low δ18O calcite formed from deuteric fluids with elevated CO2/H2O

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ratios (i.e. ∼10) at 500 to 100 °C. Further support for high CO2/H2O fluids is provided by the existence of talc, found only in dolomite-bearing kimberlite. Lacking evidence to constrain the origin of these high CO2/H2O fluids, the preferred interpretation envisions a flux of CO2-rich fluid from degassing of kimberlite at depth. Combined isotopic measurements, petrographic observations and isotopic modeling are consistent with carbonate at Lac de Gras forming from a combination of magmatic and deuteric processes without the need to introduce external (i.e. non-magmatic) fluids to the kimberlites. Acknowledgements The authors thank BHP-Billiton Diamonds and De Beers Canada Explorations Inc. for access to samples and permission to publish. Technical support at the GSC was generously provided by Hassan Mirnejad, Pat Hunt and Katherine Venance. The manuscript benefited from discussions with John Armstrong and the comments of two reviewers for Chemical Geology. References Al-Aasm, I.S., Taylor, B.E., South, B., 1990. Stable isotopic analysis of multiple carbonate samples using selective acid extraction. Chemical Geology 80, 119–125. Armstrong, J.P., Wilson, M.R., Barnett, R.L., Nowicki, T., Kjarsgaard, B.A., 2004. Mineralogy of primary carbonate-bearing hypabyssal kimberlite, Lac de Gras, Slave Province, Northwest Territories, Canada. Lithos 76, 415–433. Castorina, F., Censi, P., Comin-Chiaramonti, P., Piccirillo, E.M., Alcover Neto, A., Gomes, C.B., Ribeiro de Almeida, T.I., Speziale, S., Toledo, M.C.M., 1997. Carbonatites from eastern Paraguay and genetic relationships with potassic magmatism: C, O, Sr and Nd isotopes. Mineralogy and Petrology 61, 237–260. Chernovsky, J.V., Berman, R.G., Bryndzia, L.T., 1988. Stability, phase relations, and thermodynamic properties of chlorite and serpentine group minerals. In: Bailey, S.W. (Ed.), Hydrous Phyllosilicates. Reviews in Mineralogy, vol. 19. Mineralogical Society of America, pp. 295–346. Clarke, D.B., Mitchell, R.H., 1975. Mineralogy and petrology of the kimberlite from Somerset Island, N.W.T., Canada. Physical Chemistry of the Earth 9, 236–251. Clarke, L.B., Le Bas, M.J., Spiro, B., 1991. Rare earth, trace element and stable isotope fractionation of carbonatites at Kruidfontein, Transvaal, S. Africa. In: Meyer, H., Leonardos, O.H. (Eds.), Kimberlite, Related Rocks and Mantle Xenoliths. Proceedings of the 5th International Conference, Brazil, vol. 1, pp. 100–120. Clement, C.R., 1975. The emplacement of some diatreme facies kimberlites. Physical Chemistry of the Earth 9, 51–59. C.R. Clement (1982). A comparative geological study of some major kimberlite pipes in the Northern Cape and Orange Free State. Unpublished Ph.D. thesis, University of Cape Town, 728 pp. Clement, C.R., Skinner, E.M.W., 1979. A textural classification of kimberlitic rocks, Kimberlite Symposium II, Extended Abstracts, unpaginated, Cambridge.

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