Geochemistry of hypabyssal kimberlites from Lac de Gras, Canada: Comparisons to a global database and applications to the parent magma problem

Lithos 112S (2009) 236–248

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Geochemistry of hypabyssal kimberlites from Lac de Gras, Canada: Comparisons to a global database and applications to the parent magma problem B.A. Kjarsgaard a,⁎, D.G. Pearson b, S. Tappe c, G.M. Nowell b, D.P. Dowall b a b c

Geological Survey of Canada, 601 Booth Street, Ottawa, ON, Canada K1A 0E8 Earth Sciences, Durham University, South Road, Durham, DH1 3LE, United Kingdom Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada T6G 2E3

a r t i c l e

i n f o

Article history: Received 24 October 2008 Accepted 3 June 2009 Available online 21 June 2009 Keywords: Ultramafic magma Volatiles Crustal contamination Mantle assimilation

a b s t r a c t We present 104 whole-rock geochemical analyses of hypabyssal kimberlite from the Lac de Gras field. Screens using Yb versus Al2O3 and ln Si/Al versus ln Mg/Yb effectively discriminate crustally contaminated samples. The remaining “non-contaminated” kimberlites samples have variable (5 to 50%) entrainment of cratonic peridotite. It is problematic to effectively screen for small amounts (b 5%) of digested crust in samples with higher (N 20%) contents of peridotite contamination. We utilize the Lac de Gras data suite to calculate, by two different methods, parent magma compositions and identify two (and potentially three) geochemically distinct parent magma types. The Lac de Gras parent magma compositions are compared to those calculated from other localities in Canada, Greenland, South Africa and Russia. Together, these calculated parent magmas define a range, albeit limited, of viable, yet distinct, kimberlite parent magma compositions. Geochemically, kimberlite parent magmas have high volatile contents (H2O and CO2), high MgO, and low SiO2, Al2O3 and alkalis, with K N Na and Na + K/Al b 1. It is difficult to reconcile differences between various calculated kimberlite parent magma compositions from different cratonic areas as merely due to the effects of craton specific lithospheric mantle contamination, indicating the intra- and intercratonic variation of parent magma compositions reflect differing source region characteristics and/or partial melting regimes. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.

1. Introduction Kimberlites are typically considered to be silica-undersaturated, alkaline, potassic, CO2-rich rocks. Whole rock major- and traceelement geochemical studies have been performed on suites of kimberlite from Lesotho (Gurney and Ebrahim, 1973), and a variety of other southern African occurrences (e.g., Fesq et al., 1975; Clement, 1982) in the 1970's and 80's. In these studies, many of the samples analysed were massive volcaniclastic kimberlite and kimberlite breccias that contained high proportions of entrained crustal material. These data confirmed the broadly high-Mg and Si-poor geochemical nature of kimberlites determined in earlier studies (e.g., Wagner, 1914). Smith et al. (1985) recognized the need for geochemical studies on samples less influenced by crustal input and provided a high quality dataset for a suite of southern African hypabyssal kimberlites. This study importantly identified, separated and distinguished Gp Ia, Gp Ib and Gp II kimberlites in the Kaapvaal craton. Spriggs (1988) made the first detailed geochemical studies of circum-cratonic kimberlites, from Namibia, focusing on hypabyssal varieties. Taylor et al. (1994) examined whole-rock geochemical data from kimber-

⁎ Corresponding author. E-mail address: [email protected] (B.A. Kjarsgaard).

lites from West Africa, making detailed comparison with Kaapvaal kimberlites. More recently, le Roex et al. (2003), Harris et al. (2004) and Becker and le Roex (2006) have published analyses of kimberlites from southern Africa, and Price et al. (2000) and Kopylova et al. (2008) from the Jericho kimberlite in Nunavut, Canada. A significant body of work has also been published on Na-, Cl-, and CO2-rich kimberlites from Udachnaya-East, Russia (M. Kamenestsky et al., 2004; V. Kamenetsky et al., 2007a, b). These recent studies all focused on analysing ‘fresh’, apparently littlecontaminated hypabyssal kimberlite samples and utilized the ICP-MS technique for trace-element determination. The data obtained from these studies were gathered with the purpose of understanding kimberlite primary magma compositions, and constraints on the source region. From the above summary of previous research in kimberlite geochemistry, it is clear that the great majority of effort has been expended on kimberlites from southern Africa, despite the significant number of localities recognized from other parts of the world. In this study, we utilize 176 whole-rock geochemical analyses from North America, which includes data for 104 new samples from the Lac de Gras kimberlite field, central Slave craton, Northwest Territories, Canada. The new Lac de Gras data, with 72 additional analyses from other North American localities, 157 analyses from southern African kimberlites and 55 analyses from other kimberlite localities worldwide, defines a data set of 388 analyses. This new data set of analyses of hypabyssal

0024-4937/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2009.06.001

B.A. Kjarsgaard et al. / Lithos 112S (2009) 236–248 Table 1 Sample locality sites in the Lac de Gras field, including number of samples analysed, and those considered, fresh, contaminated, or rejected. Kimberlite

# analyses

‘Fresh’

‘Contaminated’

Rejected

A154 Aaron Anaconda Anne Arnie DD39 DO27 Don Finlay Grizzly Koala West Leslie Misery Misery East Pigeon Porpoise Rat Rattler Roger T21 T36 T237 T19 T35 TR107 SUM

3 3 9 3 5 6 1 3 3 16 5 8 5 7 2 5 2 4 3 3 5 2 2 1 1 107

3 3 9 3 5 6 0 0 3 16 0 8 5 7 0 0 2 4 0 0 0 0 0 1 1 76

0 0 0 0 0 0 1 3 0 0 5 0 0 0 2 5 0 0 3 3 4 2 0 0 0 28

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 2 0 0 3

Gp I kimberlites presented herein enables a much more objective examination of kimberlite bulk compositions using a dataset not dominated by samples from the Kaapvaal craton. We have specifically excluded samples that exhibit obvious crustal contamination, utilizing a variety of major- and trace-element screens and we consider only archetypal kimberlites (usually referred to as Gp I kimberlites) in the database. Hypabyssal kimberlites with high modal proportions of calcite or dolomite have also been excluded on the basis that these represent fractionated kimberlites, due to magmatic differentiation via crystal — liquid, flowage seggregation and/or filter pressing processes (e.g., Mitchell, 1986; Kjarsgaard 2007; Mitchell, 2008). We utilize this worldwide data set to examine the major element content of kimberlites and conclude that these rocks, as sampled at the

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Earth's surface, generally show a number of similarities, such as elevated H2O and CO2 contents, are MgO-rich, Al-, Na- and K-poor, and silicaundersaturated. In contrast, previous workers have suggested that kimberlites are CO2-rich, alkaline potassic undersaturated rocks. However, we also observe that there are important regional differences in the geochemistry of hypabyssal kimberlites (e.g. Lac de Gras, South Africa, Greenland). We then examine the relationship between kimberlite whole-rock geochemistry and the problem of kimberlite parent magma composition. While it has been suggested by Mitchell (2008) that hypabyssal kimberlite compositions are not representative of primary magma composition due to the effects of mantle and crustal contamination and assimilation, the first step to solve the primary magma problem is to resolve the composition(s) of near surface parent magma composition, which is the focus of this contribution. 2. Analytical methods The Lac de Gras kimberlite suite was prepared for whole-rock geochemistry by selecting samples with minimal surficial alteration that contained the least visible crustal fragments. In most cases, additional post-coarse crushing hand picking was performed to remove any obvious additional crustal fragments. This careful selection and preparation resulted in only three samples being rejected as obviously crustally contaminated (Table 1). The samples were analysed at the Geological Survey of Canada. Major elements were determined by wavelength dispersive X-ray fluorescence spectrometry on lithium metaborate/tetraborate fused disks. H2O (total), and CO2 (total) were determined using combustion followed by infrared spectrometry. CO2 (carbonate and carbon) was determined by acid evolution followed by infrared spectrometry. Fluorine and chlorine were determined using a pyrohydrolysis method followed by ion chromatography. Trace-element determination for Yb is based on total dissolution of the sample using nitric, perchloric and hydrofluoric acids followed by a lithium metaborate fusion of any residual material. Analysis was done using ICP-MS. A subset of samples were analysed for major elements by X-ray fluorescence spectrometry at Acme Analytical Labs, Vancouver. Replicate trace-element analyses for the majority of the samples were undertaken at Durham University using ICP-MS following methods described in Ottley et al. (2003). Limits of detection for the above analytical methods are detailed in Appendix A and the data set is tabulated in Appendix B.

Fig. 1. Map of the central Slave Province showing the locations of the kimberlites (diamonds) in the Lac de Gras area (Kjarsgaard et al., 2002). Kimberlite localities studied are shown as filled circles: (1) Porpoise; (2) Anaconda; (3) Rattler; (4) Rat; (5) DD39; (6) Pigeon; (7) Koala West; (8) Leslie; (9) Grizzly; (10) Roger; (11) Arnie; (12) Aaron; (13) Misery East; (14) Misery; (15) A154S; (16) TR107; (17) Don; (18) Finlay; (19) Anne; (20) DO27; (21) T35; (22) T36; (23) T19; (24) T237 and (25) T21.

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3. Whole rock geochemistry of Lac de Gras kimberlites 3.1. Sample suite and background information An original starting suite of one hundred and seven samples of hypabyssal kimberlite from twenty-five bodies in the Lac de Gras field, Northwest Territories, Canada were examined in this study (Fig. 1, Table 1). The kimberlite samples are from the BHP-Billiton Ekati Mine claim block, De Beers Canada Exploration Hardy Lake claim block and the Kennecott Canada (RTZ) Diavik Mine claim block. The samples are from a variety of different types of hypabyssal kimberlite including discrete dykes, dykes cutting volcaniclastic pipes, small magmatic bodies adjacent to or underneath volcaniclastic pipes, larger magmatic bodies that dominate a pipe and kimberlite — country rock breccia dykes. The kimberlite samples studied are variably fresh, with a subset of samples exhibiting partial to near-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 15–45 modal% forsterite macrocrysts), although the Don, Porpoise, Roger and Rat kimberlite samples are poor in forsterite macrocrysts (b10 modal%). A subset of samples from the Aaron, Pigeon, Misery East, Leslie and Grizzly localities also contain variable, but minor amounts (b3 modal%) of macrocrystic phlogopite. All of the samples contain variable proportions of phenocrysts/microphenocrysts of forsterite, spinel and calcite ± phlogopite, set in a finer-grained groundmass of phlogopite-kinoshitalite mica, spinel, monticellite, perovskite, apatite, calcite and serpentine. In three samples (Rat, Rattler and Anaconda), dolomite and talc are also observed in the groundmass. Additional information on the petrography, mineral chemistry and stable isotopic systematics for a subset of the studied Lac de Gras hypabyssal kimberlite suite has been previously reported in detail by Armstrong et al. (2004) and Wilson et al. (2007). The majority of Lac de Gras kimberlites studied by Wilson et al. (2007) had bulk calcite and micro-sample calcite that preserved their primary (1100 to 750 °C), magmatic (δ18O from 6 to 9‰) isotopic signatures. A subset of samples with higher δ18O values from 9 to 14‰ for whole-rock calcite and micro-sample calcite derived from calcite + serpentine segregations, correlates with more abundant calcite + serpentine segregations and concomitant higher degrees of alteration of forsterite

macrocrysts and phenocrysts. Isotopic modelling indicated that the elevated δ18O values reflect a second generation of calcite that formed from deuteric fluids at temperatures between 500 to 100 °C. Importantly, Wilson et al. (2007) concluded that combined isotopic measurements, petrographic observations and isotopic modelling were consistent with carbonate in the Lac de Gras kimberlites forming from a combination of magmatic and deuteric processes in a closed system, without the need to introduce external (i.e., meteoric) fluids to the kimberlite magma, or during post-emplacement cooling. A significant number of the studied Lac de Gras hypabyssal kimberlite samples studied are considered petrographically as “fresh” on the basis of non-altered, 50 µm olivine and 20 µm monticellite grains in the groundmass, coupled with b2% alteration rinds on olivine macrocrysts. In this regard, combined petrographic and stable isotopic studies indicate a number of the Lac de Gras kimberlites certainly represent minimally altered hypabyssal kimberlites. 3.2. Quantifying crustal contamination A significant issue with the whole-rock chemical analysis of hypabyssal kimberlite is the degree of crustal contamination. Preliminary screening of the sample suite resulted in the rejection of 3 samples (Table 1) with e.g., high Al2O3 (N4.5 wt.%). A widespread approach in kimberlite geochemistry studies is to apply the Clement Contamination Index (C.I.) as a means of discriminating for crustal contamination in samples that lack obvious petrographic evidence for contamination. Application of this index to the 104 samples remaining after our initial screening produced C.I. values between 1.38 to 0.81. Typically, C.I. values b1 are considered uncontaminated (Clement, 1982; Mitchell, 1986), an approach taken recently by Nowicki et al. (2008), although Clement (1982) reported on apparently uncontaminated kimberlites with C.I. values up to 1.5. For our Lac de Gras sample suite, approximately half the samples would be considered contaminated using a C.I. cut-off value of 1. Recently, le Roex et al. (2003) have suggested that examination of a combination of SiO2, Al2O3, MgO, Pb and HREE can provide more robust screens for crustal contamination. In this study we utilize bivariate plots of Yb versus Al2O3 and ln Mg/Yb versus ln Si/Al as crustal contamination screens (Fig. 2A, B). The natural log element ratio pairs are used to minimize the effects of closure, a significant problem when

Fig. 2. Bivariate plot of Al2O3 versus Yb (A) and ln Si/Al versus ln Mg/Yb (B) for hypabyssal kimberlite samples from the Lac de Gras area. Note the bimodal distribution of “noncontaminated” and crustally contaminated samples. In (B) the Lac de Gras parent magma composition is from Dowall (2004). Crustal and/or mantle contamination are shown as labeled vectors: crust = (5G, 10G and 20G); cratonic peridotite = (10P, 20P, 30P, 40P, 50P, 60P and 70P); crust and peridotite (30P5G, 30P10G). Data for cratonic peridotite composition from Pearson and Nowell (2002), Pearson et al. (2005) and Pearson and Wittig (2008); data for Lac de Gras Slave crust granodiorite composition from Nowicki et al. (2008).

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granodiorite), or mantle and crust together (Fig. 2A, B). With respect to the Lac de Gras kimberlite data set, the addition of crust produces vectors away from the “non-contaminated” samples towards the crustally contaminated samples, while the addition of 10–60% cratonic peridotite produces a vector that covers the range of “non-contaminated” Lac de Gras samples (Fig. 2A, B). The combined addition of 20 to 30% peridotite and 5% crust generates kimberlite signatures that are highly problematic to differentiate from “non-contaminated” samples from the Lac de Gras kimberlite suite using any of the screens above. We thus suggest that it may be extremely difficult to detect by geochemical means the effects of 5% (and possibly 10%) added crust in hypabyssal kimberlites containing high peridotite input, as could be the case for many of the studied “non-contaminated” Lac de Gras hypabyssal samples. 3.4. Major element systematics — a focus on Lac de Gras kimberlites

Fig. 3. Bivariate plot of Clement's Contamination Index versus ln Si/Al for hypabyssal kimberlite samples from the Lac de Gras area. Note the bimodal distribution of “noncontaminated” and crustally contaminated samples in terms of ln Si/Al, but the wide variation in C.I. for ‘non-contaminated’ samples, including many samples that have C.I. N1, a typical screen for crustal contamination.

examining major element data. Our data set is bimodal for both these elemental screens, with samples falling into groups that seem to clearly represent either crustally contaminated kimberlite samples or “non-contaminated” kimberlite samples. We use “non-contaminated” kimberlite in a liberal sense, because although we believe the “noncontaminated” kimberlites are not significantly contaminated by crustal material, we are cogent that combined lithospheric mantle and crustal contamination can mask crustal contamination and hence it is impossible to rule out using any current geochemical screen (see Section 3.3). The natural log of Si/Al is a useful screen that is very sensitive to the addition of high Al crustal rocks and neatly divides the Lac de Gras kimberlite samples into two distinct groups that are a function of their contamination (Fig. 2B). Crustally contaminated rocks that are identified using this measure are also classified as being significantly contaminated on the basis of Yb abundances. If, however, we examine the range of C.I. values for kimberlites within the two groups defined from ln Si/Al considerations we see that they largely overlap in C.I. values (Fig. 3). Furthermore, a significant number of “non-contaminated” samples, identified on the basis of the ln Si/Al parameter, actually have CI N 1, i.e., they would be flagged as contaminated if we used only this index according to the suggestions of Mitchell (1986) and Nowicki et al. (2008). One reason for this limitation of the C.I. is that crustal contamination and peridotite incorporation have mutually opposing effects (see below) that severely limit the sensitivity of this index. On this basis we suggest that the sole use of Clement's C.I. as a screen for identifying crustally contaminated kimberlites be abandoned, or is at least utilized in conjunction with other geochemical screens. 3.3. Combined crustal and mantle contamination An important additional problem regarding contamination of kimberlite is the combined effects of contamination by crustal material with entrainment and/or digestion of cratonic peridotite. In the simplest terms, addition of cratonic peridotite to kimberlite lowers C.I., while the addition of crust has the opposite effect. As an example we utilize the postulated Lac de Gras kimberlite parent magma composition of Dowall (2004) as a starting point to illustrate the effects of adding cratonic peridotite, or crust (average Lac de Gras

Representative bivariate plots of data from Lac de Gras samples are shown in Fig. 4 and compared to other recently acquired high quality analytical data from southern African kimberlites (le Roex et al., 2003; Harris et al., 2004; Becker and le Roex, 2006). For the Lac de Gras samples, there is a broad positive correlation between MgO and SiO2 (Fig. 4A) with crustally contaminated samples typically exhibiting lower MgO with variable, but typically lower SiO2 concentrations at a given MgO content as compared to the “non-contaminated” samples. A plot of Al2O3 versus MgO re-iterates the fundamental high Al (contaminated) versus low Al (“non-contaminated”) nature of the Lac de Gras suite (see also Fig. 2A). For the “non-contaminated” samples, there appears to be two sub-populations, separated at ~ 34.5 wt.% MgO. The low-MgO group exhibits decreasing MgO with decreasing SiO2 and Al2O3; the high MgO group exhibits increasing MgO with decreasing Al2O3 and increasing (albeit quite scattered) SiO2 (Fig. 4A, B). For the high MgO group the variations in MgO, SiO2 and Al2O3 can be ascribed to greater levels of peridotite assimilation, consistent with much higher abundances of olivine macrocrysts (25–50 modal % olivine) in samples from the Aaron, Arnie, Grizzly, Leslie and Finlay kimberlites. Accounting for the systematic variation of MgO, SiO2 and Al2O3 in the low-MgO group could theoretically be accounted for by olivine and phlogopite fractionation, but phlogopite macrocrysts and or/phenocrysts are not common in these samples (Armstrong et al., 2004; Wilson et al., 2007). However, approximately half of the samples in the low-MgO group are mineralogically distinct in being dolomite-bearing, as opposed to the exclusively calcite-bearing high MgO group. The majority of “non-contaminated” Lac de Gras kimberlite samples are low in TiO2 (0.3 to 0.7 wt.%) with variable K2O (0.1 to 1.9 wt.%) contents. Ilmenite megacrysts are absent to very rare in the low-Ti samples, and groundmass perovskite is scarce. The subset of “non-contaminated” samples that are dolomite-bearing have elevated TiO2 (0.9 to 1.7 wt.%) at variable K2O (0.1 to 1.3 wt.%) contents. In general, the crustally contaminated samples have elevated TiO2 and/or K2O levels (Fig. 4C). With respect to southern African hypabyssal kimberlites, Lac de Gras samples range to higher MgO (~40 wt.% versus 35 wt.%) and SiO2 (~38 wt.% versus 36 wt.%) concentrations (Fig. 4A), and at similar levels of alumina have a wider range of MgO (Fig. 4B). Lac de Gras kimberlites also exhibit significant differences to southern African kimberlites with respect to TiO2 – K2O relationships (Fig. 4C). In general all the Lac de Gras samples are low-Ti (maximum 1.7 wt.% TiO2) with varying K2O contents as compared to southern African samples that have titanium concentrations from ~ 1 to 3.7 wt.% TiO2 (Fig. 4C). A subset of the Lac de Gras low-Ti samples have high enough potassium to fall in the Gp II kimberlite field (as defined by Smith et al., 1985). There is a strong negative correlation between CaO and SiO2, similar to that exhibited by southern African kimberlites (le Roex et al., 2003). The Lac de Gras samples have slightly higher silica contents at a given calcium concentration and also range to lower CaO

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Fig. 4. Bivariate plot of MgO versus SiO2 (A), MgO versus Al2O3 (B), K2O versus TiO2 (C) and, SiO2 versus CaO (D) for hypabyssal kimberlite samples from the Lac de Gras area. Data have been subdivided on the plots into “non-contaminated” and crustally contaminated samples. For comparison, compositions of South Africa kimberlites (data from le Roex et al., 2003; Harris et al., 2004; Becker and le Roex, 2006) are indicated by oval dashed lines in (A), (B) and (D) and within the short dashed line in (C). In (C) the long dashed line (after Smith et al., 1985) separates low-K, high-Ti Gp I kimberlites from high-K, low-Ti Gp II kimberlites. See text for details.

(2 wt.%) contents at higher SiO2 (38 wt.%) levels (Fig. 4D). These latter samples are all extremely olivine macrocryst rich (i.e., the high MgO group). In summary, although there are a number of broad similarities between the major element geochemistry of southern African and Lac de Gras kimberlites, there are also some significant differences. 3.5. Measured alumina, alkalis, and peralkalinity for Lac de Gras kimberlites Histograms of Al2O3, K2O and Na2O (all in wt.%) for Lac de Gras kimberlites are shown in Fig. 5. The distribution of alumina in the Lac de Gras suite is strongly bimodal, with low Al2O3 contents in “noncontaminated” samples and higher Al2O3 contents in crustally contaminated samples (Fig. 5A; see also Figs. 2A, 4B). Potassium contents are very variable, but “non-contaminated” samples typically have lower concentration levels (majority of the data b0.75 wt.% K2O) as compared to the crustally contaminated samples, which cover the concentration range, and include the highest K2O levels (Fig. 5B; Appendix B). The Lac de Gras samples have low sodium contents (maximum of 0.32 wt.% Na2O; Appendix B); most “non-

contaminated” samples have b0.11 wt.% Na2O which is exceptionally low, but the crustally contaminated samples are not obviously enriched in sodium (Fig 5C; Appendix B). For silica-undersaturated rocks, alkalinity is defined on the basis of molar (Na + K)/Al (Shand, 1922; Sørensen, 1974). For all Lac de Gras kimberlite samples the peralkalinity index is b1, with “non-contaminated” and crustally contaminated samples having overlapping ranges (0.08 to 0.86 and 0.05 to 0.93, respectively; Fig. 5D) and the “non-contaminated” samples having a mode at molar (Na + K)/Al of ~ 0.35. Clearly, with (Na + K)/Al b1, the Lac de Gras kimberlites cannot be considered alkaline rocks, by definition. 3.6. Measured H2O and CO2 concentrations for Lac de Gras kimberlites Measured CO2 contents for Lac de Gras “non-contaminated” and crustally contaminated kimberlite samples are quite variable (1.1– 15.1 wt.% and 3.6–13.8 wt.% CO2, respectively; Appendix B), with dolomite-bearing kimberlites dominating the CO2-rich end of the spectrum (7.8–15.1 wt.% CO2). Measured H2O contents are less variable. The “non-contaminated” and crustally contaminated

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Fig. 5. Histograms of wt.% Al2O3 (A), wt.% K2O (B), wt.% Na2O (C) and peralkalinity index [molar (Na + K)/Al] (D) for hypabyssal kimberlite samples from the Lac de Gras area. Data have been subdivided on the plots into “non-contaminated” and crustally contaminated samples.

kimberlite samples have similar water contents, containing 6.1–9.3 and 3.7–10.7 wt.% H2O, respectively (Appendix B). Examination of the Lac de Gras H2O and CO2 data on a wt.% [H2O/(CO2 + H2O)] basis demarcates the H2O-rich nature of the calcite-bearing kimberlites, in contrast to the CO2-rich nature of the subordinate dolomite-bearing kimberlites (Fig. 6A; see also Armstrong et al., 2004; Wilson et al., 2007). On a molar [H2O/(CO2 + H2O)] basis, the water-rich nature of the Lac de Gras kimberlites is further substantiated (Fig. 6B) and we note the majority of Lac de Gras samples (including some of the dolomite-bearing varieties), have molar [H2O/(CO2 + H2O)] N 0.5. These rocks are thus best described as volatile-rich (H2O and CO2), emphasizing their water-rich nature. 3.7. Calculation of Lac de Gras kimberlite parent magma composition(s) Two different techniques have been applied to determine parent magma compositions for the Lac de Gras kimberlites. We utilize parent magma composition here as the composition of the kimberlite magma in the upper crust, in contrast to the primary magma composition that represents the composition of the kimberlite

magma generated at the source region. Representative thin sections from the HL-10 and HL-12 kimberlites in the Hardy Lake cluster and the Leslie kimberlite from the Lac de Gras cluster were scanned (Appendix C) and image analysis software utilized to determine the area % of olivine (and peridotite garnet). In addition, 97 to 111 of the largest olivine grains (and any garnet) in each thin section were analysed by electron microprobe. Mean olivine composition ranged from Fo91.2 to Fo91.8 and lie within the previously reported range for olivine macrocrysts from Lac de Gras kimberlites (Armstrong et al., 2004) and olivine from peridotite xenoliths from Lac de Gras (Pearson et al., 1999; Menzies et al., 2004). The composition of the parent magma was determined by a subtraction technique [whole-rock composition minus (area % of olivine X the mean determined olivine composition)]. Results of the calculations are listed in Table 2. This subtraction technique is similar to that utilized by Nielsen and Jensen (2005) in their study of the Majuagaa kimberlite dyke in Greenland, but we utilized image analysis to determine area % olivine. Analysis of two thin sections from HL10 (HL10-1A; HL10-1B), separated in space by ~10 cm along the length of the split drill core utilized for the wholerock analysis are quite similar, with 24.0% and 25.2% olivine and mean

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Fig. 6. Histograms of H2O/(H2O + CO2) (A), and molar H2O/(H2O + CO2) (B) for hypabyssal kimberlite samples from the Lac de Gras area. Data have been subdivided on the plots into “non-contaminated” and crustally contaminated samples.

olivine compositions of Fo91.23 and Fo91.29, respectively, suggesting the determined area % and mean olivine forsterite composition is representative. However, we note that no orthopyroxene or clinopyroxene was observed in the examined thin sections, and the amount of peridotite garnet is highly variable, ranging from 0 to 25% (as a percentage of the olivine area determined by image analysis). If the olivine macrocrysts represent dissaggregated peridotite, then the olivine subtraction method does not truly construe subtraction of mantle because garnet, orthopyroxene and clinopyroxene are not represented. There is certainly a statistical support problem related to the wide variation in garnet observed. Furthermore, a range of garnet peridotite compositions (i.e., dunite, harzburgite, lherzolite; e.g., Pearson et al., 1999; Menzies et al., 2004;) have likely been incorporated into Lac de Gras kimberlite magmas en route from the source region. As an alternate approach, we have subtracted an average cratonic mantle peridotite composition (data from Pearson and Nowell, 2002; Pearson et al., 2005; Wittig et al., 2008) and we assume that measured olivine represents approximately 80% of any entrained peridotite (Lac de Gras peridotites range from 55–95 modal % olivine; Pearson et al., 1999). Results from these calculations are listed in Table 2. With peridotite subtraction, concentrations of SiO2, Al2O3, and MgO are lower and TiO2, FeOt, CaO, K2O, P2O5, H2O and CO2

contents are higher, as compared to olivine subtraction alone. For the minor elements and volatiles, this is mainly due to subtraction of 25% more ultramafic material in the peridotite calculation versus the olivine calculation (Table 2). An alternate method to determine the primary magma composition was suggested by Dowall (2004), who contended that the Lac de Gras parent magma composition could be determined by defining a composition intermediate between the least mantle contaminated and the least crustally contaminated samples. Using this tactic, concentration levels for major elements were determined via examination of mixing curves between kimberlite and Lac de Gras crust and mantle compositions (Dowall, 2004), and bivariate plots (e.g., Figs. 2A, 4C), bivariate ratio plots (e.g., Fig. 2B) and histograms (e.g., Fig. 5A) for “non-contaminated” kimberlites as compared to crustally contaminated kimberlites. Parent magma composition determinations were made separately for low- and high-Ti Lac de Gras kimberlites (Table 3). These techniques are similar to those employed by le Roex et al. (2003), Harris et al. (2004) and Becker and le Roex (2006) to determine parent magma compositions for South African kimberlites. For the Lac de Gras low-Ti parent magma composition, a comparison to the range of individual element concentrations as determined by the olivine and peridotite subtraction

Table 2 Lac de Gras calculated parent magma compositions, utilizing the olivine subtraction method, and peridotite subtraction method, based on area % of olivine. Sample

SiO2 TiO2 Al2O3 FeOt MnO MgO CaO Na2O K2O P2O5 H2O CO2

HL12-3

HL12-3

HL10-1B

HL10-1B

HL10-1A

HL10-1A

Leslie B-1

Minus 37.6%

Minus 47%

Minus 25.2%

Minus 31.5%

Minus 24.0%

Minus 30.0%

Minus 26.9%

Minus 33.6%

Olivine

Peridotite

Olivine

Peridotite

Olivine

Peridotite

Olivine

Peridotite

34.76 0.58 3.42 7.32 0.18 28.79 6.90 0.15 2.19 0.67 9.53 5.51

31.75 0.65 3.14 7.81 0.17 28.07 7.45 0.15 2.54 0.78 11.10 6.41

28.79 0.92 3.41 8.39 0.21 28.18 9.25 0.18 0.88 0.81 11.50 7.49

26.30 0.98 3.27 8.92 0.20 27.94 9.75 0.18 0.95 0.88 12.50 8.14

29.11 0.90 3.36 8.34 0.20 28.49 9.10 0.17 0.86 0.79 11.31 7.36

26.67 0.96 3.22 8.89 0.20 28.34 9.56 0.18 0.93 0.86 12.23 7.96

29.28 0.76 2.15 8.10 0.19 34.38 8.41 0.20 0.35 0.56 8.94 6.68

27.07 0.81 1.87 8.55 0.18 34.33 8.88 0.20 0.38 0.61 9.79 7.31

See text for further details.

Leslie B-1

B.A. Kjarsgaard et al. / Lithos 112S (2009) 236–248 Table 3 Lac de Gras calculated parent magma compositions, determined from analysis of parent magma – contaminant mixing curves, bivariate plots, bivariate ratio plots and histograms. Sample

LDG low Ti

LDG low Ti

LDG low Ti

Table 3 min

Parent magma

Table 3 max

Parent magma

SiO2 TiO2 Al2O3 FeOt MnO MgO CaO Na2O K2O P2O5 H2O CO2

26.30 0.58 1.87 7.32 0.17 27.94 6.90 0.15 0.35 0.56 8.94 5.51

31.79 0.72 3.08 8.28 0.19 30.77 9.23 0.10 1.03 0.97 8.72 5.13

34.76 0.98 3.42 8.92 0.21 34.38 9.75 0.20 2.54 0.88 12.50 8.14

27.46 1.12 2.54 7.29 0.17 27.46 14.24 0.09 0.61 0.71 6.10 12.20

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data ranges reported for kimberlite analyses from Udachnaya-East, Russia (M. Kamenestsky et al., 2004; V. Kamenetsky et al., 2007a, b). 4.1. Measured alumina, alkalis and peralkalinity

LDG hi Ti

See text for further details.

methods (Table 2) is listed in Table 3. The Lac de Gras low-Ti kimberlite parent magma is typically intermediate in composition to the listed high – low range via the subtraction methods, with the exception of Na2O, P2O5, H2O and CO2 (Table 3). 4. Comparison of Lac de Gras kimberlite data with a global kimberlite dataset Data from the Lac de Gras hypabyssal kimberlite samples was combined with data from other North American localities to form the North America kimberlite data set (NA; n = 176). This is compared with kimberlite data from southern African (SA; n = 157) and other localities worldwide (LW; n = 55). The three data sets are listed in Appendix B. Graphical analysis of the data sets was undertaken using relative probability plots which are less susceptible to artifacts from binning choices compared to histograms, especially for large data sets. To the relative probability plots, we have added bars that represent

Kimberlites worldwide have low alumina contents (modes of 2.4, 2.0 and 2.2 wt.% Al2O3 for SA, NA and LW, respectively; Fig. 7A). Potassium contents are very low (modes of 0.9, 0.4 and 0.5 wt.% K2O for SA, NA and LW, respectively; Fig. 7B) and sodium contents are exceptionally low (modes of 0.08, 0.06 and 0.07 wt.% Na2O for SA, NA and LW, respectively; Fig. 7C). In general kimberlites worldwide have very similar Na2O contents, which are exceptionally low, with the exception of samples from Udachnaya-East that have 3.23–4.96 wt.% Na2O (Fig. 7C; M. Kamenestsky et al., 2004; V. Kamenetsky et al., 2007a, b). Southern African kimberlites have slightly elevated K2O (and Al2O3) contents compared to North American and other worldwide kimberlite localities. The Udachnaya-East samples also have elevated K2O contents, however, these lie within the defined range for kimberlites worldwide (Fig. 7B). For kimberlites worldwide, the combined alkali and alumina contents define (Na + K)/Al as being b1 (main mode of ~ 0.25 to 0.75; Fig. 7D) and therefore, by definition, kimberlites cannot be termed alkaline rocks. Contamination of kimberlite by crust (other than some evaporites) will not increase the peralkalinity. Kimberlites that are contaminated by cratonic peridotite will not have their Na–K–Al systematics affected significantly, due to the lower concentration levels in the entrained/digested mantle xenoliths (Pearson et al., 2005; Pearson and Wittig, 2008) than in the kimberlite magma; Na is essentially not affected, Al will be slightly diluted, and K to a slightly greater degree. In contrast, the Narich, high-K kimberlites reported from Udachnaya-East (M. Kamenestsky et al., 2004; V. Kamenetsky et al., 2007a, b) are quite distinct with extremely high molar (Na + K)/Al values of 3.74–5.45 (Fig. 7D). These peralkalinity levels are exceptional; in rare cases primitive alkaline silicate rocks such as olivine melilitite and melilite nephelinites have (Na + K)/Al values of 1.50 to 1.75, but typical peralkalinity values for these rock types is near unity (Peterson and Kjarsgaard,

Fig. 7. Relative probability plots for alumina as wt.% Al2O3 (A), potassium as wt.% K2O (B) sodium as wt.% Na2O (C) and peralkalinity [molar (Na + K)/Al] (D). For comparison, the concentration levels reported for Udachnaya-East kimberlites are shown as solid bars labelled U-E in (A) and (B). In (C), the Udachnaya-East Na2O contents lie at much higher concentration as indicated by the arrow with wt.% levels indicated on the figure; in Fig. 7 (D), the Udachnaya-East peralkalinity index plots off the figure and is indicated by an arrow with recorded range.

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1995; Dawson, 1998; Keller et al., 2006; Earthchem database, www. earthchemportal.org). 4.2. Measured H2O and CO2 contents While kimberlites are usually considered to be CO2-rich, or volatile-rich (CO2 and H2O), whole-rock H2O and CO2 analyses from hypabyssal kimberlites worldwide indicate that their high measured water contents should receive more attention. Measured H2O contents (mode ~ 7.2 wt.%; Fig. 8A) are greater than CO2 contents (mode ~ 4.8 wt.%; Fig. 8B), a point first observed by Smith et al. (1985) for South African kimberlites. This suggests that, as found for the majority of the Lac de Gras sample suite, many kimberlites worldwide are better termed volatile-rich (H2O and CO2) rocks, emphasizing their water-rich nature. Examination of the H2O and CO2 data on a molar basis [H2O/(CO2 + H2O)] demarcates the H2O-rich nature of many kimberlite rocks (mode of 0.78; Fig. 8C). However, there are certainly

Fig. 8. Relative probability plots for measured wt% H2O (A), wt % CO2 (B), and molar H2O/(CO 2 + H2O) (C) for the global (NA + SA + LW) kimberlite data set. The concentration levels of wt% H2O, wt % CO 2 and molar H2O/(CO2 + H2O) for Udachnaya-East kimberlites are shown as solid bars labelled U-E in (A), (B) and (C). In (C), the dashed line separates CO2-rich and H2O-rich compositions.

kimberlites worldwide that should be termed CO2-rich rocks, to emphasize their volatile-rich nature with high CO2/H2O ratios, e.g., Rat and Anaconda kimberlites in Lac de Gras field (Wilson et al., 2007; this study) and the De Beers, Benfontein, Wesselton and Ondermaajte kimberlites in South Africa. The Udachnaya-East samples reported by M. Kamenetsky et al. (2004) and V. Kamenetsky et al. (2007a,b) have measured CO2 concentrations that are within, but at the high end of the known range (Fig. 8B), however, the reported H2O concentration levels are extremely low (Fig. 8A), such that molar [H2O/(CO2 + H2O)] values are b0.12 i.e., at the extreme low end for kimberlites worldwide (Fig. 8C). 5. Discussion 5.1. Kimberlite parental magma compositions A comparison of calculated kimberlite parental magma compositions for Kimberley, South Africa (le Roex et al., 2003), Uintjiesberg, South Africa (Harris et al., 2004), Majuagaa, Greenland (Nielsen and Jensen, 2005; Nielsen and Sand, 2008), South African Gp I (Becker and le Roex, 2006), Jericho, Canada (Kopylova et al., 2008) and from this study are listed in Table 4; note these parent magma compositions were calculated via a variety of different techniques. While there are a number of similarities for these localities (e.g., low SiO2 and Al2O3, high MgO) there are also typically minor, but variable differences for certain elements (TiO2, Na2O, K2O, P2O5, H2O and CO2) that suggest regional kimberlite parent magma compositional characteristics are important. The two Majuagaa parent magma compositions listed in Table 4 differ in terms of SiO2 and significantly with respect to TiO2 contents. This is related to different proportions of olivine and ilmenite subtracted from the whole-rock composition, and suggests the basis for % mineral(s) subtracted needs to be well constrained. The Udachnaya-East, Russia parent magma (M. Kamenestsky et al., 2004; V. Kamenetsky et al., 2007a, b), although similar in most respects to other calculated parent magma compositions listed in Table 4, is quite distinctive with very high Na2O, CO2 and Cl, and very low H2O. Thus V. Kamenetsky et al. (2007a,b) have suggested that other kimberlites worldwide have lost significant Na2O and Cl; this is discussed further in Sections 5.2 and 5.3. It is tempting to suggest that the different kimberlite parent magma compositions listed in Table 4 are related to contamination of kimberlite primary magma of ‘universal composition’ by lithospheric mantle of differing (region specific) composition. For example, Greenland mantle lithosphere is quite olivine-rich and orthopyroxene-poor, Kaapvaal mantle lithosphere is orthopyroxene-rich (and also contains significant proportions of phlogopite peridotite), while the Slave and Siberian mantle lithospheres have intermediate olivine-orthpyroxene contents as compared to Greenland and the Kaapvaal (Pearson and Wittig, 2008). Furthermore, the incorporation of previously formed megacryst suite minerals (ilmenite, phlogopite, Ti-Cr-garnet, Cr-diopside) into the magma could be of significance; we note that kimberlites in the Kaapvaal often contain megacryst suite minerals while the opposite is true in Lac de Gras. However, the notion that a ‘universal’ kimberlite primary magma composition exists, utilizing the lines of reasoning noted above are difficult to support, based on observations from Lac de Gras. Within the Lac de Gras kimberlite field, we identify two parent magma types, low-Ti and high-Ti (Tables 2, 3, 4) that are compositionally distinct. Furthermore, there are also significant differences in calculated parent magma compositions (e.g., Al2O3 and MgO contents; Table 2) for low-Ti kimberlites from the Hardy Lake cluster (HL-10, HL-12) and low-Ti kimberlites from the Lac de Gras cluster (Leslie) within the Lac de Gras field. Interestingly, within the Lac de Gras field, calculated parent magma compositions appear to have age dependence (geochronology data from Heaman et al., 2004), with low-Ti Hardy Lake kimberlites emplaced at ~ 72 Ma and high-Ti Lac de Gras kimberlites emplaced at

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245

Table 4 Calculated parent magma compositions for Lac de Gras and other localities worldwide. Country

Canada

Canada

Canada

South Africa

South Africa

South Africa

Russia

Greenland

Area

LDG low Ti

LDG hi Ti

Jericho

Group I

Kimberley

Uintjiesberg

Udachnaya-E

Majuagaa (1)

Greenland Majuagaa (2)

SiO2 TiO2 Al2O3 FeOt MnO MgO CaO Na2O K2O P2O5 H2O CO2 mol H2O (H2O + CO2) (Na + K)/Al Cl (ppm) F (ppm) F:Cl

31.79 0.72 3.08 8.28 0.19 30.77 9.23 0.10 1.03 0.97 8.72 5.13 0.81 0.42 b 100–240 474–2532 4–10.5

27.46 1.12 2.54 7.29 0.17 27.46 14.24 0.09 0.61 0.71 6.10 12.20 0.55 0.32

27.73 1.50 1.87 7.06 0.17 27.23 13.36 0.10 1.48 0.68 8.41 10.02 0.67 0.95

26.15 2.58 2.76 9.65 0.19 25.2 13.26 0.16 0.83 2.04 7.33 8.19 0.69 0.42

26.5 2.2 2.2 8.8 nd 26.5 12 nd 1.5 nd 12.3 7 0.81

27 3.23 2.32 9.16 0.19 26.09 14.99 0.06 1.87 0.3 4.92 8.63 0.58 0.92

26.71 1.25 1.75 8.09 nd 31.33 12.19 3.23 1.33 0.49 0.38 9.42 0.09 3.86 2.38 nd

20.48 0.19 2.33 9.62 0.2 25.67 17.96 0.16 0.33 0.83 nd nd

17.47 4.99 2.27 10.61 0.24 23.98 17.27 0.13 0.32 0.81 nd nd

0.27

0.25

Sources of data: Lac de Gras (this study); Jericho, Canada (average of 4 determinations), Kopylova et al. (2008); South Africa Gp I, Becker and le Roex (2006); South Africa Kimberley, le Roex et al. (2003); South Africa Uintjieberg (Harris et al., 2004); Udachnaya-East, Russia, V. Kamenetsky et al. (2007a); Majuagaa, Greenland (1), Nielsen and Jensen (2005); Majuagaa, Greenland (2), Nielsen and Sand (2008).

~ 60 Ma. In contrast low-Ti Lac de Gras kimberlites were emplaced at ~ 53 Ma. 5.2. Alkali and volatile contents of calculated parental magma compositions A problem with calculated kimberlite parent magma compositions is that the basis for all calculations is measured concentrations from rocks that have been subjected to a wide variety of processes. We recognize that the potentially most significant discrepancies are volatile and alkali concentrations, due to solute-bearing (e.g., Na, K) fluid/gas loss from the magma during crystallization and cooling. However, kimberlites contain alkali- and volatile-bearing minerals (phlogopite, apatite, carbonate phases) that provide important insights to these problems. Measured potassium concentrations of kimberlites (Fig. 7B) are quite similar to calculated parent magma compositions (Table 4) and while K2O concentrations are variable, they are low. The mineral– chemical manifestation of the generally low potassium concentrations in kimberlites is the substitution of Ba for K in phenocrystal and groundmass phlogopite, with typical kimberlite groundmass mica zoned from cores of barian phlogopite through phlogopitekinoshitalitess to kinoshitalite-rich rims (Mitchell, 1995) e.g., as observed in Lac de Gras (Armstrong et al., 2004) and at Somerset Island (Appendix D). These observations suggest residual kimberlite melts that crystallize the groundmass are extremely K-depleted, along with the co-existing fluid phase. Furthermore, the absence of leucite in kimberlites (Woolley et al., 1996) is consistent with a silica-undersaturated magma that is K-poor and H2O-rich, because phlogopite is the stable crystallizing K-rich phase based on the experiments of Luth (1967). The variation in K2O content of kimberlites worldwide can thus likely be ascribed to a combination of variable source region characteristics, coupled with contamination by phlogopite from the megacryst suite and/or phlogopite peridotite xenoliths superimposed on dilution effects from peridotite. Measured sodium concentrations of kimberlites (Fig. 7B) are low and quite similar to calculated parent magma compositions (Table 4), with the exception of the Udachnaya-East kimberlites (M. Kamenestsky et al., 2004; V. Kamenetsky et al. 2007a, b). The absence of nepheline in kimberlite (Woolley et al., 1996) is consistent with the low magmatic Na2O and (Na + K)/Al b 1. However, the suggested high Cl levels in the Udachnaya-East parent magma could stabilize NaCl, at the expense of nepheline. The mineral–chemical manifestation of high Na2O and Cl contents (and low H2O) in kimberlite magma would be the crystal-

lization of chlorine-rich apatite and phlogopite, but these minerals are not reported in the literature as being observed in kimberlites, nor can we find any analyses of apatite or phlogopite with significant chlorine contents. Importantly, microprobe analyses of groundmass phlogopite from Lac de Gras and Somerset Island (with chlorine above detection limits; 0.01%Cl) have 0.43–2.75% F and 0.01–0.10% Cl, with F:Cl ratios of 14–275 (Fig. 9; Armstrong et al., 2004; Appendix D), consistent with measured F and Cl concentration of kimberlites and their high F:Cl ratios (Appendix B; Table 4). Other possible primary magmatic phases crystallizing from a Na- and Cl-rich magma would be sodalite and pectolite (e.g., as seen in damtjernites from Aillik Bay; Tappe et al., 2006), however, sodalite is not reported in kimberlite and pectolite has only been observed in crustally contaminated kimberlite (Scott Smith et al., 1983). We interpret the data as indicative that kimberlite parent magmas are Na- and Cl-poor, with the Na- and Cl-rich Udachnaya-East samples being highly unusual. Calculated kimberlite parent magma compositions from Canada and South Africa have high, but variable H2O and CO2 concentrations

Fig. 9. Bivariate plot of F versus Cl for groundmass phlogopite from Lac de Gras kimberlite (Armstrong et al., 2004; unpublished) and the Jos dyke, Somerset Island (Appendix D).

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(Table 4), with molar H2O/(CO2 + H2O) ranging from 0.55 to 0.58 for parent magmas with high CO2 contents (Lac de Gras high-Ti, Uintjiesberg), through 0.67–0.69 (Jericho, South Africa Gp I) to 0.81 for parent magmas with high H2O contents (Lac de Gras low-Ti, Kimberley). In contrast, the Udachnaya-East parent magma is extremely CO2-rich and H2O-poor with a quite distinctive and low molar H2O/(CO2 + H2O) of 0.09 (M. Kamenestsky et al., 2004; V. Kamenetsky et al., 2007a, b). Two key points regarding mineral stability in kimberlite with respect to volatile speciation of the parent magma and fluid are suggested. A significant observation for kimberlites is the paucity of orthopyroxene xenocrysts observed in concentrate studies, despite clear evidence for the incorporation of substantial amounts of orthopyroxene-rich mantle peridotite (e.g., Schulze, 1995). In order to efficiently resorb orthopyroxene, H2O-rich magmas are required, based on the experimental studies of Kushiro (1970) and Eggler (1973). Calculated molar H2O/(CO2 + H2O) for parent magmas (Table 4) are N0.55, consistent with orthopyroxene dissolution. Furthermore, the ubiquitous presence of low temperature (b500 °C) serpentine (and not talc) in hypabyssal kimberlites requires a high molar H2O/(CO2 + H2O) in the fluid phase (e.g., Chernovsky et al., 1988). Stable isotopic data on groundmass carbonate (δ18O 12.87 to 13.68‰; δ13C −3.22 to − 3.55‰) reported by V. Kamenetsky et al. (2007a) are identical to those reported by Wilson et al. (2007) for low temperature deuteric calcite in calcite + serpentine segregations from Lac de Gras kimberlites. Such compositions can be modelled to have formed at b500 °C from fluids with high H2O/(H2O + CO2) i.e., not from CO2-rich melts or fluids. Calcite from Lac de Gras kimberlites with low δ18O (1.21 to 5.35‰) was modelled to have precipitated at b500 °C from fluids with high CO2 (Wilson et al., 2007). Importantly, the Udachnaya-East groundmass carbonates fall outside the primary carbonatite carbonate box (Clarke et al., 1991), in contrast to magmatic groundmass calcite from the least altered Lac de Gras samples (δ18O 6.84 to 8.17‰; δ13C −3.75 to 7.22‰; Wilson et al., 2007). Based on the above discussion, the stable isotope data presented by V. Kamenetsky et al. (2007a) for the Udachnaya-East samples are not consistent with having preserved their primary magmatic volatile content or speciation. Hence measured H2O + CO2 contents from Udachnaya-East samples should not be taken as a confident representation of kimberlite parent magma volatile composition.

at low CO2 partial pressures. However, diopside is not a groundmass phase in kimberlites (due to expansion of the olivine stability field with high H2O partial pressures; Edgar et al., 1988), and alnöite contains magmatic calcite. These observations suggest the absence of melilite in kimberlite may be related to low magmatic sodium contents, and not caused by the volatile content and speciation of the magma. If this is true, then an upper limit for sodium in kimberlite (based on data from melilite-bearing rocks) is 0.5–1.00 wt.% Na2O, in line with the observation that our minimally contaminated kimberlite compositions contain less than this. Calculated parent melt compositions (Table 4) contain 0.09–0.16 wt.% Na2O; this suggests that 0.34–0.91 wt.% Na2O may have been lost via fluids in equilibrium with the magma. Utilizing a maximum loss of 0.91 wt.% Na2O from the calculated parent magmas in Table 4, re-constituted (higher-Na) Lac de Gras, Greenland and South Africa Gp I parent magmas would still have (Na + K)/Al b1; Jericho (Canada) and Uintjiesberg (S. Africa) become peralkaline with (Na + K)/Al of 1.74 and 1.58, repectively, but are still significantly less peralkaline than the Udachnaya-East parent magma [(Na + K)/ Al = 3.86].

5.3. Potential for alkali loss from magmas to the fluid phase

Aillikites belong to the ultramafic lamprophyre group that are frequently mistaken for kimberlite (cf., Tappe et al., 2005, this issue). Measured whole-rock data for aillikite have lower H2O and higher CO2 contents, with a mode for molar [H2O/(CO2 + H2O)] of ~0.5, significantly lower than main kimberlite mode (compare with Fig. 8C; aillikite data from Tappe et al., 2004, 2006, 2008 and unpublished). In contrast to kimberlites, major element data from aillikite sills and dykes (Tappe, op. cited) have slightly higher Al2O3 (2.7 wt.%; compare with Fig. 7A), and increased levels of K2O (1.5 wt.%; compare with Fig. 7B) and Na2O (0.2 wt.%; compare with Fig. 7C). The aillikite peralkalinity index (Na + K)/Al has a main mode of ~0.5 to 1.2, which is higher than that of kimberlites (0.25 to 0.75; compare with Fig. 7D) with a subset of aillikites being true alkaline rocks with (Na + K)/Al N 1. Notably, these rocks do not contain melilite. These whole-rock geochemical attributes are consistent with aillikites having either different mineral phase assemblages, or similar mineral assemblages with different mineral chemistry, to those from kimberlites (Mitchell et al., 1999; Reguir et al., this issue; Nielsen et al., this issue; Tappe et al., 2005, this issue).

Recently, Kamenetsky et al. (this issue) reported primary and pseudosecondary vapour-rich and multiphase melt inclusions in olivine rims from Majuagaa, Jericho, Gahcho Kué, Aaron and Leslie kimberlites. A variety of alkali chloride and carbonate daughter phases are observed in the inclusions; inclusion homogenization temperatures range from ~650 °C (Majuagaa) to 800 °C (Gahcho Kué). Based on studies at Lac de Gras (Wilson et al., 2007), we believe these inclusions variably represent the lowest temperature residual melts from magmatic crystallization (i.e., 800 °C) and/or subsolidus fluids (i.e., 650 °C). In Section 5.2, we suggested that mineral assemblage and mineral chemistry data for phlogopite are consistent with late stage magmatic kimberlite being poor in K2O, which suggests limited loss of potassium to the co-existing fluid. However, sodium loss is much more difficult to constrain in the absence of any primary Nabearing mineral phases in the kimberlite groundmass. In this regard, the absence of melilite in kimberlite is of interest. Melilite in primitive mantle derived rocks is Mg-rich (akermanite), but contains a minimum of ~15% of the soda-melilite end member. Examination of whole-rock analyses of olivine melilitite and melilite nephelinite (Earthchem database, www.earthchem portal.org) and alnöite with N10 wt.% MgO have N1 wt.% Na2O (and typically N2 wt.% Na2O), with rare examples having 0.5–0.75 wt.% Na2O. In the reaction akermanite + melt= calcite + diopside+ melt (e.g., Yoder, 1979), melilite is stabilized

5.4. Wallrock metasomatism by kimberlite fluids Studies of wallrock adjacent to hypabyssal kimberlite occurrences have noted induration (low-T contact metamorphism) of the country rocks (Wagner, 1914; Dawson and Hawthorne, 1970). More pronounced metasomatic effects of country rocks have also been reported. Masun et al. (2004) describe K-metasomatism (microcline overgrowths on host rock granitoids). Smith et al. (2004) describe metasomatites with compositions intermediate between the kimberlite and granite host rocks, with the metasomatic fluid being CO2-rich, hydrous and having low Na-concentrations. Ferguson et al. (1973) describe granite wall rock alteration at the De Beers pipe and concluded that the metasomatising fluids were CO2- and H2O-rich and depleted in sodium. Together, these studies suggest that kimberlite-derived metasomatic agents are mixed CO2/H2O fluids with low Na2O-contents. These fluid compositions are inconsistent with the suggestion of V. Kamenetsky et al. (2007a,b, this issue) that kimberlite magmas and co-existing fluids are Na2O-, CO2and Cl-rich and H2O-poor. 5.5. Comparison of kimberlite and aillikite data

5.6. Descriptive geochemical terminology for kimberlites It is clear that kimberlites contain an intimate mixture of mantle and crustal components and this will change the original composition of the magma. However, these processes are ubiquitous and in terms of rock classification from geochemistry, current practice is to use the

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compositions of the rocks available (Le Maitre, 2002), rather than to make classifications based on estimations of prior compositions that are subject to large uncertainties. Graphical analysis of the worldwide kimberlite data set using relative probability plots reveals geochemical systematics that are inconsistent with many of the purported hallmarks of kimberlite magma i.e., alkaline, potassic and CO2-rich. For silica-undersaturated rocks, alkalinity is defined on the basis of molar (Na + K)/Al. Since kimberlites are not alkaline (as defined by (Na + K)/Al b 1), this precludes further refining these rocks as being of a ‘sodic’ or ‘potassic’ lineage. Although many kimberlites have molar K/Na N 1 (often with values of 3–30) this is largely a manifestation of their extremely low levels of Na2O. This means that kimberlites may appear to be nominally ultrapotassic, but bulk rocks rarely have N3 wt.% K2O (Figs. 4C, 5B, 7B) and average much lower (typically 0.5–1.5 wt.% K2O). Thus, kimberlites cannot be considered as ultrapotassic rocks, as defined by Foley et al. (1987). Kimberlites have similar potassium contents to those in MORB and lower levels than more common alkaline rocks occurring in continental and oceanic settings, i.e., basanites, OIBs. When compared with other magmatic rocks (other than carbonatites), archetypal kimberlites are certainly quite CO2-rich, but we strongly suggest that these rocks are best described as volatilerich (H2O + CO2), because H2O is typically dominant, often being more abundant on a wt.% basis, and certainly on a molar basis (Fig. 8C). We recognize, however, that there are also numerous kimberlites better described as volatile-rich (CO2 + H2O) in which CO2 is the dominant volatile species. 6. Conclusions Careful screening of hypabyssal kimberlite from the Lac de Gras area, NWT, Canada, coupled with detailed petrographic, mineral chemistry and stable isotope studies allows us to define a suite of fresh to little altered, non-contaminated to minimally contaminated samples. The resultant data set was utilized to calculate, by two different methods, the major element and volatile content of two (and potentially three) distinct kimberlite parent magma compositions within the Lac de Gras field. Significant progress has been made the past decade by a variety of researchers in better understanding kimberlite parent magma compositions. Based on results presented in this study, and comparison with previous work, there appears to be a range, albeit limited, of viable kimberlite parent magma compositions. Geochemically, kimberlite parent magmas have high volatile contents (with variable H2O and CO2 contents), high MgO, and low SiO2, Al2O3 and alkalis, with K N Na. It is difficult to reconcile differences between various calculated kimberlite parent magma compositions from different cratonic areas as merely due to the effects of craton specific lithospheric mantle contamination. Although it could be suggested that calculation of kimberlite primary magma compositions are an intractable proposition, this in itself should provide motivation to do so. Resolution of this issue is needed if we are to adequately understand the genesis of these enigmatic rocks. Acknowledgements NERC, the British Geological Survey and Geological Survey of Canada are acknowledged for funding Dave Dowall's Ph.D. thesis. BHPBilliton, De Beers Canada Exploration and Kennecott Canada are thanked for access to drill core from kimberlites in the Lac de Gras field. De Beers Canada Exploration provided partial funding support in 1997/98 for the image analysis study. Troels Nielsen, Copenhagen, provided a comprehensive and very insightful review of the original manuscript that was exceptionally beneficial to focusing the authors thoughts. Additional comments by an anonymous reviewer resulted in a more detailed consideration of the Udachnaya-East kimberlites. Steve Foley is thanked for consummate editorial handling.

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