The origin of Triassic/Jurassic kimberlite magmatism, Canada: Two mantle sources revealed from the...

Article

Volume 12, Number 9

10 September 2011

Q09005, doi:10.1029/2011GC003659

ISSN: 1525‐2027

The origin of Triassic/Jurassic kimberlite magmatism,Canada: Two mantle sources revealed from the Sr‐Ndisotopic composition of groundmass perovskite

S. E. ZurevinskiDepartment of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3,Canada

Now at Department of Geology, Lakehead University, Thunder Bay, Ontario P7B 5E1, Canada([email protected])

L. M. Heaman and R. A. CreaserDepartment of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3,Canada

[1] The crystallization ages and the Sr and Nd isotopic compositions of groundmass perovskite from awell‐established, SE trending, Triassic‐Jurassic corridor of kimberlite magmatism in central and easternNorth America were determined to investigate the origin of this magmatism. The results obtained fromkimberlite fields located along this corridor are interpreted to indicate that at least two distinct mantlesources contributed to this magmatism. The most primitive Rankin Inlet and Timiskaming kimberlites havea relatively unradiogenic strontium isotopic signature (0.7032–0.7036), interpreted to be derived from recycledand metasomatized oceanic lithosphere in the deep mantle. In contrast, the Attawapiskat and Kirkland Lakekimberlites have CHUR‐like (Chondritic Uniform Reservoir) signatures (0.7040–0.7042) interpreted tohave an origin in the asthenosphere. The progressive decrease in the age of magmatism from the TriassicRankin Inlet kimberlites to the Miocene Great Meteor seamount, combined with the similarity in the isotopiccomposition of these diverse magmas along the proposed >3000 km long hot spot track, provides strongevidence in support of a common mantle plume origin for both the continental and oceanic components.

Components: 12,200 words, 4 figures, 2 tables.

Keywords: Sr‐Nd isotope; geochronology; hot spots; kimberlite; perovskite.

Index Terms: 1038 Geochemistry: Mantle processes (3621); 1040 Geochemistry: Radiogenic isotope geochemistry; 1115Geochronology: Radioisotope geochronology.

Received 14 April 2011; Revised 19 July 2011; Accepted 24 July 2011; Published 10 September 2011.

Zurevinski, S. E., L. M. Heaman, and R. A. Creaser (2011), The origin of Triassic/Jurassic kimberlite magmatism, Canada:Two mantle sources revealed from the Sr‐Nd isotopic composition of groundmass perovskite, Geochem. Geophys. Geosyst., 12,Q09005, doi:10.1029/2011GC003659.

1. Introduction

[2] Kimberlites are ultramafic rocks derived fromsmall‐volume mantle‐derived magmas enriched in

alkalis and volatiles [Mitchell, 1986, 1995]. Thereare numerous theories on: (1) the source compo-nents of kimberlite magma; and (2) the role that thesubcontinental lithospheric mantle (SCLM) plays

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http://dx.doi.org/10.1029/2011GC003659

in the formation and/or contamination of kimberlitemagmas during transport to the earth’s surface.Kimberlites are complex rocks that contain a mix-ture of primary minerals that crystallize directlyfrom the magma, xenocrystic material derived fromcontamination by mantle and crustal rocks duringtransport to the surface, and alteration mineralassemblages that form from interaction with fluidsduring and after emplacement. Deciphering the ori-gin and primary isotopic composition of such com-plex rocks is very challenging. Many studies havepointed out the challenges of obtaining primary Srisotopic compositions of whole rock kimberlitesbecause of contamination and alteration, and con-cluded that whole rock kimberlite isotope geo-chemistry needs to be assessed with caution [e.g.,Heaman, 1989; Paton et al., 2007a; Malarkey et al.,2010]. It has been shown that primary unalteredminerals that have crystallized directly from kim-berlite magma prior to entrainment of crustal mate-rial, such as perovskite, are the only materials thatfaithfully record the primary isotopic signature ofthe kimberlite magma, especially the Sr isotopicsignature [e.g., Heaman, 1989; Paton et al., 2007a;Yang et al., 2009; Malarkey et al., 2010]. The pri-mary isotopic signature of a kimberlite magma isof great importance in unraveling the nature ofthe mantle source of this magmatism (i.e., from thelithosphere or asthenosphere) and to evaluate therole of mantle and/or crustal contamination duringemplacement.

[3] Perovskite (CaTiO3) is a common matrix min-eral and has a well‐established crystallization his-tory, occurring in many kimberlites. It containsabundant U, Th, Sr and REE and as a result ofthese geochemical traits perovskite is useful forobtaining U‐Pb dates, interpreted to closely con-strain the emplacement age of kimberlitic rocks,and more recently, Sr and Nd isotopic compositions[Heaman, 1989; Paton et al., 2007a; Yang et al.,2009; Wu et al., 2010; Malarkey et al., 2010].Perovskite is generally resistant to alteration, pre-serving primary isotopic compositions of the kim-berlite magma from which it crystallized. In thisstudy we investigate the U‐Pb age, as well as Srand Nd isotopic composition of groundmass perov-skite from 16 kimberlites sampled along the well‐established, SE trending Triassic‐Jurassic corridorof kimberlite magmatism in central and easternNorth America. This corridor of kimberlitesrecords ∼100 m.y. of magmatic activity preservedin several clusters and fields, younging from NWto SE and includes: (1) the 225–170 Ma RankinInlet field, Nunavut; (2) the ∼180–150 Ma Atta-

wapiskat field, James Bay Lowlands, Ontario; and(3) the 165–125 Ma Kirkland Lake and Timis-kaming kimberlite fields, central Ontario/Quebec(Figure 1). The known length of this corridor isapproximately 2000 km and has been interpreted byHeaman and Kjarsgaard [2000] to reflect the con-tinental expression of magmatism linked to either asingle or multiple mantle plume hot spot track(s), apattern geographically coincident with independentestimates for the timing and location of the conti-nental extension of both the Great Meteor and Verdehot spot tracks [Crough et al., 1980; Crough, 1981;Morgan, 1983]. Along this corridor, the kimberlitemagmatism was emplaced into Archean crustalblocks: the Churchill, Superior and Southern geo-logical provinces, all with distinctive crustal andlithospheric mantle histories. The Superior craton isdescribed as an Archean block which acted as anucleus onto which younger Precambrian provinceswere amalgamated in the Proterozoic [Corriganet al., 2009]. The Churchill craton is made up ofthe Rae and Hearne structural provinces, Archeancratonic rocks which were reworked during thePaleoproterozoic [Corrigan et al., 2009]. This corri-dor of kimberlite magmatism provides an idealsetting in which to test the relative contributions oflithospheric versus asthenospheric mantle sourcesin their genesis. For example, the isotopic compo-sitions of lithosphere‐derived kimberlite magmasshould be high and variable (e.g., Group II kimberlitesin southern Africa [Smith, 1983; Woodhead et al.,2009]), whereas an origin in the asthenospherewould be expected to be much more homogeneousand independent of the craton transected. In thisstudy, the U‐Pb age as well as Sr and Nd isotopiccompositions of perovskite isolated from a numberof kimberlites along this Triassic to Jurassic corri-dor have been determined to evaluate the origin ofthe magmatism.

1.1. Geologic Overview of theTriassic‐Jurassic Kimberlitesin Central and Eastern Canada

[4] The Rankin Inlet kimberlite field (Nunavut,Figure 1) mainly contains evolved sparsely macro-crystic, oxide‐rich calcite hypabyssal kimberlite andmacrocrystic oxide‐rich monticellite phlogopitehypabyssal kimberlite [Zurevinski et al., 2008].The Rankin Inlet kimberlites intrude rocks of themetamorphosed Archean Rankin Inlet Group aswell as Archean metaplutonic rocks. Twenty sevenprecise U‐Pb perovskite and Rb‐Sr phlogopiteemplacement ages for these Churchill Province

GeochemistryGeophysicsGeosystems G3G3 ZUREVINSKI ET AL.: TRIASSIC/JURASSIC KIMS 10.1029/2011GC003659

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kimberlites (representing 85% of the known kimber-lites on the Shear Minerals Rankin Inlet property)indicate that magmatism spans ∼55 million years(225–170 Ma), with the majority of the kimber-lites (n = 19) emplaced between 204 and 181 Ma[Zurevinski et al., 2008].

[5] The Attawapiskat kimberlites, located in theJames Bay lowlands of Ontario (Figure 1), intrudea thick Phanerozoic carbonate sequence [Fowleret al., 2001]. The kimberlite cluster at Attawapiskatincludes 19 kimberlites, and 16 have been classifiedas diamondiferous, including the 15 ha economicVictor kimberlite [Webb et al., 2004]. Three arche-typal “Group I” crater facies kimberlite samplesfrom the Attawapiskat kimberlite field (MacFayden,Bravo and Charlie) have been previously dated(U‐Pb perovskite) and are Jurassic, with ages in the180–176 Ma range [Heaman and Kjarsgaard,2000]. Rb‐Sr phlogopite and U‐Pb perovskite dat-ing of five Attawapiskat kimberlites indicate agesthat range between 180 to 155 Ma [Kong et al.,1999].

[6] Kimberlites from the Kirkland Lake and Timis-kaming fields are predominantly tuffisitic and hyp-

abyssal kimberlite. Kimberlites from Kirkland Lakeare hosted within the Abitibi Greenstone belt of theSuperior Province [Sage, 1996]. Kimberlites fromTimiskaming intrude Paleoproterozoic rocks (e.g.,Huronian Supergroup sediments) of the Southernprovince [McClenaghan et al., 1999]. The DiamondLake kimberlite pipe (Kirkland Lake) consistsmainly of kimberlite breccia and a 2 m wide hyp-abyssal kimberlite (HK) dyke, intruding a northtrending Proterozoic diabase dyke [Sage, 1996]. TheBuffonta kimberlite dyke (Kirkland Lake) crosscutsArchean mafic volcanic rocks along a shear‐hostedquartz vein. The Peddie kimberlite is a phlogopitemacrocrystic monticellite hypabyssal kimberlite thatintrudes Precambrian diabase sills and Paleozoiccarbonate rocks [Sage, 1996]. Kimberlites fromKirkland Lake have emplacement ages ranging from165 to 152 Ma (n = 13), and Timiskaming rangingfrom 154 to 125Ma (n = 9) [Heaman, 1989;Heamanand Kjarsgaard, 2000; Heaman et al., 2004].

[7] There is general agreement that the final con-trols of kimberlite emplacement for each of the kim-berlite fields are related to older crustal structures[Heaman and Kjarsgaard, 2000, and referencestherein]. For example, the Rankin Inlet kimberlites

Figure 1. Map of Eastern North America showing the SE trending Triassic‐Jurassic corridor of kimberlitic andbasaltic magmatism.


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are associated with the Pyke fault; the Attawapiskatkimberlites are associated with the Winisk Riverstructural zone; the Kirkland Lake kimberlites werecontrolled by the Porcupine‐Destor and Cadillacfault systems; and the Timiskaming kimberlites werecontrolled by the Lake Timiskaming fault system[Sage, 1996; Heaman and Kjarsgaard, 2000].

1.2. Previous Isotopic Studies of KimberliticPerovskite

[8] Perovskite is a common accessory mineral thatoccurs in the groundmass of many kimberlites. It isstoichiometrically close to the end‐member CaTiO3

[Chakhmouradian and Mitchell, 2000]. It is typi-cally present in low abundance (<10 vol. %) andoccurs most commonly in hypabyssal kimberlite,either randomly in groundmass or as necklacetextures surrounding earlier crystallizing olivines[Mitchell, 1986]. In diatreme and crater zone rocksperovskite occurs in the matrix of pelletal andjuvenile lapilli, respectively [Chakhmouradian andMitchell, 2000]. The average size of groundmassperovskite is ∼20–50 mm. There is general agree-ment that perovskite is resilient to alteration andcan be an early to mid crystallizing phase in a kim-berlite magma [e.g., Veksler and Teptelev, 1990;Chakhmouradian and Mitchell, 2000; Malarkeyet al., 2010]. Perovskite is an ideal mineral to inves-tigate in kimberlites because it crystallizes directlyfrom the kimberlite magma, can be used to deter-mine the kimberlite emplacement age, and alsoprovide information about the origin of the magmafrom Sr and Nd isotopic compositions on the samemineral fraction.

[9] Heaman [1989] investigated a suite of NorthAmerican and South African kimberlitic perovskite,determining the U‐Pb emplacement ages as well astheir initial Sr‐Nd‐Pb isotopic compositions, andconcluded that perovskite is the mineral of choicein kimberlites for Sr isotopic tracer studies basedon the following observations: (1) it is a majorcarrier of Sr (500–9000 ppm); (2) it has very lowRb/Sr ratios (<0.001) because of low Rb contents(0.05–1.63 ppm), therefore the initial Sr composi-tions are insensitive to age corrections; and (3) itrecords a much more reliable primary initial Sr sig-nature than whole rock kimberlite (i.e., initial Srratio is largely insensitive to crustal contamination).For these reasons the initial Sr isotopic composi-tions of perovskite were interpreted to be a reliablerecord of the primary host kimberlite magma signa-ture (87Sr/86Srinitial = 0.70341–0.70485). Heaman[1989] also reported Sm and Nd concentration

data, as well as Nd isotopic data for perovskiteand showed that the initial Nd isotopic composi-tions of perovskite were comparable to whole rockdata for kimberlites (147Sm/144Nd = 0.06–0.09,143Nd/144Ndinitial = 0.51248–0.51286). Based on asmall number of samples, Heaman [1989] inter-preted the isotopic variations of Sr from differentkimberlites to reflect either local isotopic hetero-geneities in the mantle source region or the docu-mentation of progressive modification of isotopicallyuniform kimberlite magma through the SCLM andcontinental crust. Heaman [1989] also showed thatthe initial Sr and Nd isotopic compositions displayedlittle variation even with a large range in emplace-ment ages.

[10] Smith [1983] and Alibert and Albarède [1988]showed many examples of the usefulness of wholerock Sr and Nd isotopic studies on kimberlites.These classic Sr‐isotope studies have been revisitedand comparisons of Sm‐Nd and Rb‐Sr data ofwhole rock kimberlites and kimberlitic perovskite[Paton et al., 2007a; Woodhead et al., 2009; Yanget al., 2009;Malarkey et al., 2010] have shown thatperovskite is a useful mineral for obtaining primarySr isotopic signatures. Woodhead et al. [2009]concluded that in the case of Group I South Afri-can kimberlites, the Sr isotope data variations donot correlate with age and reflect substantial dif-ferences in the nature of the magma source regions.

2. Methods

[11] Kimberlite samples (drill core) ranging from0.1 to 1 kg were crushed to ∼2 cm and then pul-verized to a fine powder using a tungsten carbideshatterbox. Perovskite was separated using standardheavy liquid and magnetic separation techniques.Liberated perovskite grains varied in size from 20to 100 mm, handpicked with a binocular microscopeat 100× magnification.

[12] All grains with visible inclusions or imperfec-tions were excluded. The fractions were washed in4N HNO3, then given a 45 s bath in an ultrasoniccleaner, and then rinsed with millipore water. Cali-brated 150Sm‐149Nd and 235U‐205Pb tracer solutionswere added prior to dissolution. Perovskite frac-tions were dissolved in Teflon bombs in 48% HFand 7N HNO3 (50:50), and were placed in an ovenat ∼230°C for ∼100 h. The analytical proceduresfor purifying uranium and lead by anion exchangechromatography and determining their isotopic com-positions using a VG354 thermal ionization massspectrometer are outlined byHeaman and Kjarsgaard


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Tab

le1.

U‐PbPerov

skite

ResultsforSelectedKim

berlitesa

Weigh

t(mg)

U(ppm

)Th

(ppm

)Pb

(ppm

)Th/U

TCPb

(pg)

206Pb/

204Pb

238U/204Pb

206Pb/

238U

206Pb/

238U

Age

(Ma)

Chu

rchillProvince,

Nun

avut

05FWR00

5‐A‐2

5240

163

221

1.6

174

286.78

±1.01

7547

.83±29

.06

0.03

56±0.00

0122

5.3±0.6

04KD23

0‐A‐(S)

3925

517

6024

6.9

9125

6.30

±2.02

6959

.37±67

.10

0.03

42±0.00

0221

6.7±2.4

04KD23

0‐A‐1(L)

3637

524

1940

6.5

134

247.60

±2.94

6527

.15±10

3.2

0.03

51±0.00

0322

2.4±4.0

CD

009‐2‐(S)

3791

4405

1548

.412

764

.69±0.46

1670

.66±14

.85

0.02

77±0.00

0317

6.1±3.2

CD

009(L)

9011

722

7614

19.5

322

81.77±0.34

2044

.15±9.70

0.03

10±0.00

0119

6.5±1.4

CD

024–

226

.0m

3313

222

2240

16.8

500

34.92±0.08

545.25

±3.69

0.03

03±0.00

0419

2.4±4.8

Attawapiskat,Ontario

Bravo

7525

837

2547

14.4

772

63.12±0.27

1588

.08±16

.51

0.02

81±0.00

0317

8.9±3.7

Charlie

9519

139

8042

20.8

693

62.39±0.16

1660

.46±16

.41

0.02

65±0.00

0316

8.4±3.4

KirklandLake,

Ontario

Buffonta

9933

592

718

.222

540

.82±0.16

915.87

±10

.72

0.02

45±0.00

0315

5.7±3.9

Buzz

178

4130

05

7.4

396

47.14±0.17

1153

.92±12

.15

0.02

49±0.00

0315

8.4±3.7

Diamon

dLake(small)

125

5570

710

12.8

444

43.05±0.11

984.92

±9.99

0.02

50±0.00

0315

9.1±3.8

Diamon

dLake(large)

162

1943

32.2

319

34.27±0.10

624.39

±6.72

0.02

54±0.00

0416

1.4±4.9

Morissette

Creek

137

135

2104

2715

.612

2745

.54±0.08

949.24

±9.14

0.02

54±0.00

0316

1.6±3.9

Tim

iskaming,

Ontario

Glin

kers

4813

322

3321

16.8

235

48.02±0.29

1442

.61±18

.11

0.02

05±0.00

0213

0.9±3.1

Peddie

6611

811

7916

10.0

357

52.29±0.16

1381

.08±14

.35

0.02

45±0.00

0315

6.1±3.3

OPAP

3537

790

921

.411

035

.82±0.47

762.06

±15

.62

0.02

27±0.00

0414

4.9±5.4

a Perov

skite

238U/204Pband

206U/204Pbratio

swerecorrectedforfractio

natio

n,blankandspike.Thconcentrationestim

ated

from

amou

ntof

208Pband

206Pb/

238Uage.Age

uncertaintiesrepo

rted

at2sigm

a.TCPb,

totalcommon

Pb.


5 of 19

[2000]. In this study the 206Pb/238U date is takenas the best estimate for the timing of perovskitecrystallization as it is least sensitive to the com-mon lead correction. Atomic ratios are corrected formass spectrometer fractionation (Pb‐0.088% / amu;U‐0.155% / amu), blank (5 pg Pb; 1 pg U), andspike contribution. In addition, the 206Pb/238U ratioswere corrected for the presence of initial commonPb using the two stage average crustal Pb model ofStacey and Kramers [1975]. The errors reported inTable 1 and the uncertainties associated with agedeterminations are quoted at two sigma (assumingl238U − 1.55125 × 10−10 yr−1; l235U − 9.8485 ×10−10 yr−1 [Jaffey et al., 1971]).

[13] The Sr and Nd isotopic compositions of perov-skite were determined on the same fractions pre-pared for U‐Pb geochronology. The column washescontaining Sr and REE from the uranium and leadpurification were loaded onto a separate standardcation exchange column with an initial separationof the rare earth elements as a group then furtherpurification and isolation of Sr, following the pro-cedures of Holmden et al. [1996]. The purified Srwas loaded onto a single Re filament employing atantalum gel loading method [Creaser et al., 2004]and the Sr isotopic compositions were determinedusing a Sector54 thermal ionization mass spec-trometer in static multicollector mode. Accuracy ofthe mass spectrometer measurements was moni-tored using the NIST SRM 987 Sr isotopic standard(0.71022 ± 0.00002 average value obtained, n = 12).To confirm the procedure for measuring multipleisotope systems on a single perovskite fraction, twofractions of the Ice River in‐house perovskite stan-dard were analyzed and the 87Sr/86Sr results obtainedare 0.70288 ± 0.00002 and 0.70276 ± 0.00002, inagreement with the initial strontium isotopic com-positions obtained for two ijolite samples and oneperovskite melteigite sample from the intrusion(0.70276–0.70282 [Locock, 1994]). Published laserablation ICP‐MS data for fragments of Ice Riverperovskite standard yielded a similar average87Sr/86Sr value of 0.70293 ± 0.00002 (n = 32 [Yanget al., 2009]).

[14] Nd and Sm were separated using Di (2‐ethylhexyl phosphate) chromatography (HDEHP)[Creaser et al., 1997]. The purified Sm and Nd wasanalyzed via solution‐mode using a Nu PlasmaMC‐ICP‐MS in the Radiogenic Isotope Facility atthe University of Alberta. Accuracy of the Nd iso-topic composition was monitored using the La JollaNd isotopic standard (0.511850 ± 17). Repeat analy-ses of a 200 ppb solution of the in‐house alpha Nd

standard during this study yielded 143Nd/144Nd =0.512270 ± 20 (n = 60).

3. Results

3.1. U‐Pb Perovskite Geochronology

[15] Although U‐Pb perovskite ages have beenreported previously for the 16 kimberlite samplesinvestigated in this study [Heaman and Kjarsgaard,2000; Zurevinski et al., 2008], most samples havebeen reanalyzed in order to confirm the age ofperovskite crystallization for fractions investigatedhere, and ensure the correct formation age is used tocalculate initial Nd isotopic compositions. Multi-grain fractions (up to 400 single grains) were ana-lyzed due to the small grain size of the perovskite(20–100 mm) and results are presented in Table 1.

[16] Perovskite from the Rankin Inlet kimberlitesoffers a unique perspective to this Sr and Nd iso-tope study, due to the presence of multiple agepopulations of perovskite occurring in a singlekimberlite intrusion. Previous studies of perovskiteage dating of different phases within a singlekimberlite have suggested a minimum 4 m.y., mostlikely 6–7 m.y. difference (Orion Kimberlite, Sas-katchewan, Canada), therefore this may not beunusual for kimberlites [Harvey et al., 2009]. Asdescribed by Zurevinski et al. [2008], some RankinInlet kimberlites contain two distinct populations ofperovskite (on the basis of size, color and habit).U‐Pb results indicated that in some cases, the dif-ferent populations yielded similar emplacement agesand a weighted average of the 206Pb/238U dates isinterpreted as the best estimate for the emplace-ment age of that particular intrusion, while in othercases, a range of perovskite dates were obtained.In these cases, the older perovskite was inter-preted as containing an inherited perovskite com-ponent derived from xenolithic kimberlite (kimberlite“autoliths” are present in these kimberlites andthey may be unrelated to the host kimberlite). Thisinterpretation is feasible based on the fact that theemplacement ages for Rankin Inlet kimberlitesspan nearly 55 m.y. from 225 to 170 Ma, thereforeyounger kimberlite magmas could intrude througholder kimberlites. Samples from Rankin Inlet kim-berlites were selected to encompass the spectrum ofknown kimberlite emplacement ages in the field andto investigate details of individual kimberlites thatrecord multiple perovskite dates. One example ofthe latter is kimberlite CD 009. In this kimberlitetwo perovskite fractions were selected, separated on


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the basis of size (Small and Large) and color, andthey record distinct U‐Pb dates of 176.1 ± 3.2 Maand 196.5 ± 1.4 Ma, respectively [Zurevinskiet al., 2008]. Kimberlite sample 05FWR005‐A‐2was selected because it is the oldest known kim-berlite on the property (225.3 ± 0.6 Ma) andkimberlite CD 024 is one of the younger kimberlitesyielding a single perovskite population with an age of192.4 ± 4.8 Ma [Zurevinski et al., 2008].

[17] A single perovskite fraction was analyzed fromeach of the two Attawapiskat kimberlites, Bravoand Charlie. Both kimberlites contain abundanteuhedral brownish black perovskite with moderateU concentrations (258 and 191 ppm, respectively)and moderate Th/U (14.4 and 20.8, respectively).The 206Pb/238U model age obtained for the Bravoperovskite fraction in this study is 178.9 ± 3.7 Ma,in excellent agreement with the two previouslyreported perovskite U‐Pb ages for the Bravo kim-berlite (179.4 ± 2.2, 175.7 ± 1.8 Ma [Heaman andKjarsgaard, 2000]). The weighted mean of thethree reported perovskite 206Pb/238U dates is 177.4 ±1.3 Ma (MSWD = 3.8) and we recommend this asthe current best estimate for the emplacement agefor the Bravo kimberlite. The perovskite 206Pb/238Umodel age obtained in this study for a second frac-tion selected from the Charlie kimberlite is 168.4 ±3.4 Ma. This age is younger than the previouslyreported perovskite U‐Pb age of 179.9 ± 1.6 Ma[Heaman and Kjarsgaard, 2000]. The cause forthis age difference is unknown, and the possibilitythat there could be multiple age populations ofperovskite in this sample of Charlie kimberliteneeds to be explored further. The age of 168.4 Maobtained here for the Charlie perovskite is used tocalculate initial Nd isotopic composition (howeverthe use of the older age of 179.9 ± 1.6 Ma wouldhave a negligible effect on the calculated initial Ndcomposition).

[18] Five perovskite fractions were analyzed fromfour Kirkland Lake kimberlites (Buffonta, Buzz,Diamond Lake (2 fractions), and Morissette Creek).Diamond Lake perovskite was separated into twofractions on the basis of size and color; (1) a browneuhedral fraction of smaller (∼30 mm) cubic crys-tals and (2) a fraction consisting of larger (∼60 mm)yellow euhedral cubes. The 206Pb/238U dates forboth perovskite fractions are within error (159.1 ±3.8 and 161.4 ± 4.9 Ma, respectively). Thereforethe weighted average 206Pb/238U date of 160.0 ±2.9 Ma (2s error) is interpreted as the emplace-ment age of the Diamond Lake kimberlite. Thepreviously reported age of 152.6 ± 2.2 Ma [Heamanand Kjarsgaard, 2000] is younger than the ages

determined in this study. A fraction of perovskiteselected from the Buzz kimberlite in this studyyields a 206Pb/238U model date of 158.4 ± 3.7 Ma(2s), which is in good agreement with the previ-ously reported perovskite date of 153.5 ± 1.3 Ma[Heaman and Kjarsgaard, 2000]. The Buffontakimberlite perovskite 206Pb/238U model date obtainedin this study is 155.7 ± 3.9 Ma (2s), which is inagreement with the previously reported age of153.4 ± 2.6 Ma [Heaman and Kjarsgaard, 2000].The weighted mean 206Pb/238U date for these twofractions of 154.1 ± 2.2 Ma is considered to bethe current best estimate for the emplacement ageof the Buffonta kimberlite. The Morissette Creekperovskite 206Pb/238U model age from this study is161.6 ± 3.9 Ma. This age is almost within uncer-tainty of the previously reported age for MorissetteCreek of 155.6 ± 2.0 Ma [Heaman and Kjarsgaard,2000]. The U‐Pb perovskite age results obtainedin this study confirm that these Kirkland Lake kim-berlites were emplaced between 162 and 152 Ma.

[19] Three kimberlites from the Timiskaming fieldwere analyzed (Glinkers, Peddie and OPAP). The206Pb/238U model age for the Glinkers kimberlite is130.9 ± 3.1 Ma, which is in agreement with thepreviously reported ages of 133.9 ± 2.4 Ma and133.9 ± 2.0 Ma [Heaman and Kjarsgaard, 2000].The weighted average 206Pb/238U date for thesethree perovskite fractions is 133.3 ± 1.4 Ma and isconsidered the current best estimate for the emplace-ment age of the Glinkers kimberlite. The 206Pb/238Umodel date for perovskite from the Peddie kim-berlite in this study is 156.1 ± 3.3 Ma, which isin agreement with the previously reported age of153.6 ± 2.4 Ma [Heaman and Kjarsgaard, 2000].The weighted average 206Pb/238U date of 154.5 ±1.9 Ma is considered the current best estimate forthe emplacement age of the Peddie kimberlite. The206Pb/238U model age for the OPAP kimberlite is144.9 ± 5.4 Ma. The U‐Pb perovskite and Rb‐Srphlogopite age results indicate that there are multiplekimberlite emplacement events within this field,with the emplacement ages ranging from 155 Mato 125 Ma [Heaman and Kjarsgaard, 2000;Heamanet al., 2004].

3.2. Perovskite Sr and Nd IsotopicCompositions

[20] The Sr and Nd isotopic compositions weredetermined for 20 perovskite fractions isolated from16 kimberlites (Table 2) in the following fields;Rankin Inlet (n = 4), Attawapiskat (n = 3), KirklandLake (n = 5) and Timiskaming (n = 4). Multiple


7 of 19

perovskite analyses were conducted on four sam-ples to test for age and isotopic variability withinan individual sample (Table 2). Overall, the perov-skite strontium isotopic compositions obtained formultiple kimberlites within an individual fieldshow a relatively narrow range (Figure 2a); RankinInlet (0.70317–0.70359), Attawapiskat (0.70401–0.70419), and Timiskaming (0.70337–0.70364;excluding Peddie). The perovskite compositions

obtained for the Kirkland Lake kimberlites show alarger range (0.70407–0.70509), including a perov-skite analysis previously reported from the UpperCanada Mine kimberlite [Heaman, 1989].

[21] The most notable trend in the perovskite stron-tium isotope data in Figure 2a is the inverse cor-relation between initial 87Sr/86Sr and kimberliteemplacement age for three of the kimberlite fields.

Figure 2. (a) A diagram of 87Sr/86Sri versus Age (Ma) showing the Sr evolution of two reservoirs on earth, ChondriteUniform Reservoir (CHUR) and Depleted Mantle (DM) [Workman and Hart, 2005]. (b) A diagram of 143Nd/144Ndiversus Age (Ma) showing the Nd evolution of CHUR and DM [Workman and Hart, 2005]. In both: Rankin Inlet(Churchill), Attawapiskat, Kirkland Lake and Timiskaming perovskite analyses are shown (this study), along withMonteregian Hills (whole rock 87Sr/86Sri [Foland et al., 1988]); PC, Prairie Creek Lamproite [Heaman, 1989]; NES,New England Seamounts (whole rock [Taras and Hart, 1987]); GMS, Great Meteor Seamount (whole rock[Geldmacher et al., 2006]); VL, Varty Lake (perovskite [Heaman, 1989]); CV, Cape Verde (whole rock [Holm et al.,2006]).


8 of 19

Tab

le2.

SrandNdIsotop

icRatiosforKim

berliticPerov

skite

a

Age

(Ma)

87Sr/86Sr (i)

Sm

(ppm

)Nd

(ppm

)147Sm

/144Nd

143Nd/144Nd (c)

143Nd/

144Nd (i)

"Nd (T=o)

"Nd (T)

Chu

rchillProvince,

Nun

avut

05FWR00

5‐A‐2

225.3b

0.70

328±0.00

002

286

1991

0.08

697

0.51

286±0.00

001

0.51

273±0.00

001

4.3

7.4

04KD23

0‐A‐(S)

216.7b

0.70

356±0.00

001

447

3501

0.07

714

0.51

296±0.00

001

0.51

285±0.00

002

6.3

9.6

04KD23

0‐A‐(L)

222.4b

0.70

359±0.00

001

318

2444

0.07

863

0.51

276±0.00

001

0.51

265±0.00

001

2.5

5.8

CD

009(S)

196.5b

0.70

346±0.00

002

283

2571

0.06

660

0.51

283±0.00

001

0.51

275±0.00

001

3.7

7.0

CD

009(L)

196.5b

0.70

327±0.00

001

356

3156

0.06

816

0.51

302±0.00

005

0.51

294±0.00

005

7.6

10.9

CD

024–

226

.0m

192.4b

0.70

317±0.00

002

406

3510

0.06

994

0.51

241±0.00

001

0.51

232±0.00

001

−4.5

−1.4

Attawapiskat,Ontario

MacFayden

177.3c

0.70

401±0.00

004

4033

3377

00.07

220

0.51

298±0.00

003

0.51

290±0.00

003

6.7

9.5

Bravo

(WA)

177.4

0.70

419±0.00

003

175

1599

0.06

610

0.51

319±0.00

002

0.51

311±0.00

002

10.7

13.7

Bravo‐2

(WA)

177.7

0.70

413±0.00

003

5451

50.06

350

0.51

296±0.00

002

0.51

288±0.00

002

6.3

9.3

Charlie

168.4

0.70

413±0.00

002

538

4428

0.07

343

0.51

280±0.00

001

0.51

272±0.00

002

3.2

5.8

KirklandLake,

Ontario

Buffonta(W

A)

154.1

0.70

509±0.00

002

348

2721

0.07

732

0.51

278±0.00

002

0.51

270±0.00

003

2.8

5.2

Buzz(W

A)

158.4

0.70

417±0.00

003

254

1828

0.08

411

0.51

280±0.00

001

0.51

271±0.00

002

3.2

5.5

Diamon

dLake(small)(W

A)

160.0

0.70

451±0.00

002

269

2062

0.07

898

0.51

279±0.00

002

0.51

268±0.00

001

2.9

5.3

Diamon

dLake(large)(W

A)

160.0

0.70

451±0.00

003

108

709

0.09

182

0.51

277±0.00

001

0.51

270±0.00

002

2.7

4.8

Morissette

Creek

161.6

0.70

421±0.00

002

521

3827

0.08

227

0.51

292±0.00

001

0.51

283±0.00

002

5.5

7.8

Tandem

164.6c

0.70

501±0.00

004

2120

20.06

200

0.51

276±0.00

008

0.51

269±0.00

008

2.3

5.1

Tim

iskaming,

Ontario

Glin

kers

(WA)

133.3

0.70

364±0.00

003

524

4900

0.06

460

0.51

286±0.00

001

0.51

281±0.00

001

4.4

6.7

OPAP

144.9

0.70

338±0.00

002

115

1159

0.06

005

0.51

304±0.00

002

0.51

298±0.00

002

7.9

10.4

MacLean

141.9c

0.70

337±0.00

001

317

2959

0.06

490

0.51

300±0.00

001

0.51

294±0.00

001

7.1

9.5

Peddie(W

A)

154.5

0.70

457±0.00

002

449

3466

0.07

830

0.51

294±0.00

002

0.51

286±0.00

003

6.0

8.3

a WA,weigh

tedaverage

206Pb/

238U

age(see

text

fordetails);l1

47Sm

=6.54

×10

−12y−

1;143Nd/

144Nd C

HUR=0.51

2638

,147Sm/144Nd C

HUR=0.19

67.

bZurevinskiet

al.[200

8].

c Heaman

andKjarsga

ard[200

0].


9 of 19

From NW to SE (i.e., oldest to youngest kimber-lites) the average perovskite initial 87Sr/86Sr com-position increases from 0.70339 ± 0.00002 (RankinInlet) to 0.70412 ± 0.00004 (Attawapiskat) to0.70451 ± 0.00003 (Kirkland Lake) (weightedaverages, see Table 2). Interestingly, the range andaverage composition for perovskite from the Atta-wapiskat kimberlites (0.70412 ± 0.00004) and thethree least radiogenic samples from the KirklandLake kimberlites (Buzz, Upper Canada Mine;0.70415 ± 0.00003) are identical. Three of the fourkimberlites investigated from the Timiskaming field(Glinkers, OPAP, MacLean) have low and similarinitial strontium isotopic compositions (0.70346 ±0.00003), identical to the Rankin Inlet perov-skite results. One of the Timiskaming kimber-lites (Peddie) has an emplacement age (154.5 Ma)and strontium isotopic composition (0.70457) sim-ilar to the range observed for the Kirkland Lakekimberlites, plottingwithin theKirkland Lake field inFigure 2a. Most of the perovskite analyses havestrontium isotopic compositions that plot betweenthe CHUR and Depleted Mantle reference evolu-tion lines (dashed lines) in Figure 2a.

[22] The 147Sm/144Nd ratios obtained for most ofthe perovskite fractions analyzed in this study fallwithin the previously reported range for kim-berlite perovskite (0.0623–0.0993 [Heaman, 1989;Wu et al., 2010]): Rankin Inlet (0.0666–0.0870);Attawapiskat (0.0635–0.0734); and Kirkland Lake/Timiskaming (0.0600–0.0918). The perovskite143Nd/144Nd initial isotopic compositions are plottedon a time evolution diagram in Figure 2b and manyof the analyses plot between the reference evolu-tion lines for CHUR and Depleted Mantle (dashedlines). Unlike the strontium isotopic results discussedabove, the neodymium isotopic compositions showconsiderable variation within an individual field andwithin an individual sample. For example, the neo-dymium isotopic results for three of the sampleswhere multiple perovskite fractions were analyzed(CD 009, 04KD230, Bravo) do not agree withinanalytical uncertainty (connected with tie lines inFigure 2b) and indicate that neodymium may bea sensitive monitor of lithosphere contaminationduring kimberlite magma evolution. An interestingfinding is that the initial epsilon Nd values ("NdT)for kimberlitic perovskite vary considerably between−1.3 and +13.7 (Table 2); Rankin Inlet ("NdT = −1.3to +10.7); Attawapiskat ("NdT = +6.0 to +13.7);Kirkland Lake ("NdT = +4.8 to +7.8) and Timis-kaming ("NdT = +6.7 to +10.4). Considering all thedata together the majority of perovskite fractions

have positive "NdT values between +2.3 and +9.6(Table 2).

4. Discussion

4.1. Perovskite as a Monitor of KimberliteMagma Evolution

[23] Perovskite is a relatively common accessorymineral in kimberlites, ultramafic lamprophyres andcarbonatites and its utility as both a chronometer[Heaman, 1989; Smith et al., 1989] and a petroge-netic tool capable of tracing magma sources in themantle through its Sr and Nd isotopic composition[Heaman, 1989] have been known for some time.There has been renewed interest in this mineralsince it has been shown to be amenable to in situU‐Pb dating [e.g.,Kinny et al., 1997;Cox andWilton,2006; Batumike et al., 2008; Simonetti et al., 2008;Yang et al., 2009; Wu et al., 2010], Sr isotope anal-yses [Paton et al., 2007b; Kamenetsky et al., 2009;Woodhead et al., 2009; Yang et al., 2009] and Ndisotope analyses [Wu et al., 2010]. In this study, thegroundmass perovskite strontium and neodymiumisotopic compositions were determined for 16 kim-berlites from four fields in central and easternCanada (Rankin Inlet, Attawapiskat, Kirkland Lakeand Timiskaming) to investigate intrafield isotopicvariations and to ascertain the primary isotopic com-positions of the kimberlite magmas as a proxy forthe nature of their mantle source regions. In gen-eral, perovskite from North American kimberliteshave relatively unradiogenic 87Sr/86Sr isotopic com-positions (0.7032–0.7051) and overlap the composi-tions reported for Group I Cretaceous South Africanwhole rock kimberlites (0.7033–0.7055 [Smith,1983]) and perovskite (0.7035–0.7050 [Heaman,1989; Woodhead et al., 2009]), 478 Ma Mengyinkimberlitic perovskite, China (0.7037–0.7042 [Yanget al., 2009]), 363 Ma Udachynaya kimberlite,Russia (0.70305 [Maas et al., 2008]) and 1.1 GaIndian Narayanpet kimberlites (0.7031–0.7033[Paton et al., 2007a]).

[24] In most cases, the Sr isotopic composition ofperovskite records less radiogenic strontium isoto-pic compositions compared to the host whole rockkimberlite. For example, Maas et al. [2005, 2008]came to the same conclusion as Heaman [1989]that perovskite records a more straightforward androbust Sr isotopic record of kimberlite magma sour-ces. Investigating a kimberlite autolith from theDevonian Udachnaya‐East kimberlite pipe, Yakutia,these authors showed that the weighted average87Sr/87Srinitial of 0.70305 ± 0.00003 for three


10 of 19

perovskite analyses [Maas et al., 2008] is lessradiogenic than the fresh kimberlite autolith whole‐rock host (0.7036–0.7049 [Maas et al., 2005]).

[25] Perhaps the most striking discrepancy betweenperovskite and whole rock kimberlite strontiumisotopic compositions is the study by Paton et al.[2007a] on ∼1.1 Ga kimberlites from the DharwarCraton, India. In this study the perovskite strontiumisotopic compositions obtained for several kim-berlites from the Narayanpet and Wajrakarur fields,indicate a distinct yet internally homogeneous com-position for each field (average 87Sr/86Sr of 0.70321and 0.70246, respectively), significantly lower thanthe majority of host kimberlite whole rock com-positions (0.701–0.709). They also concluded thatperovskite provides the most accurate record ofprimary kimberlite isotopic compositions and issuperior to weathered kimberlite whole rock, high-lighting the immunity of perovskite to late stagecrustal contamination and alteration.

[26] One of the objectives of this study was totest for isotopic variability within kimberlite fieldsand individual kimberlite intrusions. A summary ofperovskite Sr isotopic compositions investigatedfrom several kimberlite fields and clusters, includ-ing several North American examples [Heaman,1989; Malarkey et al., 2010, this study] and a num-

ber of other kimberlite localities worldwide [Patonet al., 2007a; Kamenetsky et al., 2009; Woodheadet al., 2009], are presented in Figure 3. From thesedata two patterns emerge; the first pattern reflectskimberlite fields that have multiple kimberliteswith relatively uniform perovskite strontium isoto-pic compositions, such as the Rankin Inlet (0.7032–0.7036; n = 4), Attawapaskat (0.7040–0.7042;n = 3), and Timiskaming (0.7034–0.7036; n = 3)fields investigated in this study and the Narayanpet(0.7031–0.7033; n = 7) and Wajrakaru (0.7023–0.7025; n = 5) fields in India [Paton et al., 2007a].In addition, to test for within kimberlite isotopicvariability in this study, four duplicate analyses wereconducted on perovskite fractions selected to repre-sent a range in grain size; some of these fractionshave quite distinct U‐Pb dates (e.g., CD009). In allfour duplicate tests (04KD230A, CD009, Bravoand Diamond Lake), the 87Sr/86Sr ratios are indis-tinguishable within analytical uncertainty. In theseexamples there is no evidence at this scale of sam-pling for strontium isotopic variability within asingle kimberlite intrusion. In summary, the rela-tively narrow range in perovskite initial 87Sr/86Srisotope compositions for individual kimberlites andmultiple kimberlites from individual fields is inter-preted to indicate that kimberlitic perovskite canbe a good proxy for the isotopic nature of the

Figure 3. A diagram of the Sr isotopic composition of perovskite isolated from worldwide kimberlites. Data fromRankin Inlet, Attawapiskat, Kirkland Lake and Timiskaming (this study); NA (North American) Other [Heaman,1989]; Mengyin [Yang et al., 2009]; Udachnaya [Maas et al., 2008]; Wajrakarur and Narayanpet [Paton et al.,2007a]; and Group I, II and Transitional kimberlites [Woodhead et al., 2009].


11 of 19

kimberlite mantle source region and these examplesdo not record progressive lithosphere contamination.

[27] In contrast to the above examples of relativelyhomogeneous perovskite strontium isotopic com-positions within a single kimberlite and for multi-ple kimberlites within a field, a second pattern isemerging where there is significant perovskite stron-tium isotopic heterogeneity for multiple kimberliteswithin a single field and even within an individualintrusion. In two North American examples, ElliottCounty and C14, the initial strontium isotopic com-position of perovskite shows significant isotopicvariation within a single kimberlite sample. In thecase of the Elliott County kimberlite (Kentucky),the initial strontium isotopic results obtained fortwo separate whole rock kimberlite‐phlogopite pairanalyses are in good agreement (0.70447 ± 4 and0.70453 ± 5, respectively [Alibert and Albarède,1988]) but are intermediate in composition to twoperovskite analyses from a separate sample of thiskimberlite (0.70403 ± 8 and 0.70485 ± 3 [Heaman,1989]). In detail, perovskite from this sample ofElliott County kimberlite has quite variable mor-phology, U‐Pb age and strontium isotopic com-position [Heaman, 1989; Heaman et al., 2004],documenting a more complicated kimberlite magmaevolution. The least radiogenic perovskite analysiswas interpreted as the best proxy for the primarykimberlite composition [Heaman, 1989] and is lowerthan the whole rock‐phlogopite initial strontium iso-tope compositions but similar to clinopyroxene andoxide separated from the same sample (0.70411 ± 3and 0.70417 ± 3, respectively [Alibert and Albarède,1988]).

[28] The greatest within field isotopic variation inNorth America is observed for the Kirkland Lakefield kimberlites with strontium isotopic composi-tions that vary between 0.7041 (Buzz, MorissetteCreek, Upper Canada Mine) to 0.7051 (Buffonta).This level of variability was also recorded withinindividual perovskite crystals isolated from the C14kimberlite from the Kirkland Lake field (0.7047–0.7056 [Malarkey et al., 2010]). These authorsinterpreted this variation to indicate perovskite canmonitor the changing kimberlite magma composi-tion during lithosphere contamination. In such cases,perovskite may not record primary mantle sourcecompositions [Malarkey et al., 2010] and at bestthe least radiogenic composition may approximatethe primary magma composition. It is likely thatkimberlite fields that record large interfield isotopicvariability have a more complex emplacement his-tory, reflecting either a short length‐scale isotopicheterogeneity in the mantle source region or vari-

able contamination with continental lithosphericmantle or continental crust.

[29] One indication that the subcontinental litho-spheric mantle (SCLM) beneath eastern NorthAmerica has a radiogenic strontium isotopic com-position is the high perovskite strontium isotopiccomposition (0.70691) obtained for the Prairie Creeklampröite in Arkansas (labeled PC in Figures 2aand 2b), a magma interpreted to have originatedfrom the SCLM [Heaman, 1989]. Therefore, oneinterpretation for the isotopic variation in theKirkland Lake kimberlites is that they are derivedlargely from a mantle source with a strontiumisotopic composition of ∼0.7041 (lowest recordedstrontium isotopic composition obtained for threekimberlites) or lower and that the more radio-genic kimberlitic perovskite records variable con-tinental lithosphere contamination as suggestedby Malarkey et al. [2010]. Other examples of inter-field Sr isotopic variability include the CretaceousGroup I kimberlites from Southern Africa (0.7032–0.7048 [Smith, 1983; Woodhead et al., 2009]) andto a lesser extent the Mengyin kimberlites, China(0.7037–0.7042 [Yang et al., 2009]).

[30] Another field in this study with variable inter-field strontium isotopic compositions is the Timis-kaming field (0.7034–0.7046). This variabilitylargely reflects the distinctive Peddie kimberlite, ithas an emplacement age (154.5 Ma) and perovskitestrontium isotope composition (0.70457) that isunlike the other three younger kimberlites investi-gated from this field (133–145 Ma and 0.70337–0.70364, respectively). Unlike the Kirkland Lakefield where there is convincing evidence for kim-berlite contamination by SCLM, we interpret thevariation in age and isotopic compositions observedin the Timiskaming field to indicate kimberlite gen-eration from multiple mantle sources over a 20 m.y.time span. The Kirkland Lake and Timiskamingkimberlite fields are only ∼50 km apart, but con-sidered together two distinct mantle sources canbe identified in this region; (1) the younger (145–133 Ma) kimberlites have a less radiogenic strontiumisotopic composition (0.70346 ± 0.00015; n = 3)and (2) the older kimberlites (165–154 Ma) have amore radiogenic composition (0.70415 ± 0.00007;n = 3).

4.2. Isotopic Nature of Central and EasternNorth American Kimberlite Mantle SourceRegions

[31] One of the most controversial aspects of kim-berlites is the location of their mantle source region


12 of 19

and several hypotheses have been proposed; thesubcontinental lithosphere (e.g., Group II kimberlites[Smith, 1983]), asthenosphere (Group I kimberlites[Smith, 1983]), transition zone [Paton et al., 2007a]and core/mantle boundary. Of these proposed reser-voirs, the SCLM is the best studied because frag-ments entrained in mantle‐derived magmas areavailable for direct isotopic study. A common isoto-pic feature of the SCLM is its heterogeneous andrelatively radiogenic strontium isotopic composi-tion, indicating a complicated history (e.g., meltremoval, subducted oceanic crust (slab) subcretion,magma underplating, metasomatism) that includesthe long‐term enrichment of incompatible elements,such as Rb. For example, there is a large variationin the 87Sr/86Sr isotopic composition of SCLMperidotite xenoliths from all investigated cratonicregions; Kaapvaal (0.7045–0.7118 [Hawkesworthet al., 1990]), Wyoming (0.7066–0.7575 [Carlsonand Irving, 1994]) and Churchill (0.7047–0.7085[Schmidberger et al., 2001]). Kimberlites derivedfrom the SCLM will therefore have a variableand radiogenic strontium isotope composition; afeasible mantle source for the radiogenic Group IIkimberlites in Southern Africa [Smith, 1983]. Animportant finding in this study is that the identicalstrontium isotopic composition of multiple kim-berlite fields intruding distinct cratonic regionsseparated by large distances (>1500 km), such asthe Timiskaming (0.7034–0.7036) and Rankin Inlet(0.7032–0.7036) fields, is not compatible withderivation from the SCLM. It is possible that thetrend to more radiogenic strontium isotopic com-positions for some kimberlites in the Kirkland Lakefield and within a single kimberlite [Malarkeyet al., 2010] could reflect the interaction betweenkimberlite magmas derived at great depth and theSCLM. For these reasons we interpret the majority ofstrontium isotopic signatures obtained for Triassic‐Jurassic kimberlite magmatism in central and easternNorth America to be derived from mantle regionsbelow the SCLM.

[32] The Sr and Nd isotopic results for perovskiteanalyses from Triassic‐Jurassic central to easternCanadian kimberlites and related rocks are shownin Figures 2a and 2b, including results from the VartyLake ultramafic lamprophyre [Heaman, 1989]. TheNd isotopic compositions for all perovskite analy-ses in this study are shown in Figure 2b and mostlyplot between the reference CHUR and DM lines.A few analyses plot above the DM line and oneperovskite sample from the Rankin Inlet kimber-lites (CD 024) plots below the CHUR referenceline. The distinct clustering of perovskite Sr iso-

tope compositions recorded in most of the kim-berlite fields investigated here is not observed inthe Nd isotope data; in fact the Nd isotopic com-position for multiple perovskite fractions from thesame kimberlite can yield quite different results(represented by tie lines in Figure 2b). In general,the perovskite "NdT values are more variable andtypically more positive in the Rankin Inlet andAttawapiskat kimberlites compared to the KirklandLake kimberlites (Table 2) but otherwise the Ndisotope data do not help to resolve the detailednature of the kimberlite source regions and in thefollowing section we focus on the strontium iso-topic results.

[33] The strontium isotopic composition of perov-skite from North American kimberlites and relatedrocks are presented in Figure 2a (diamonds).Excluding three kimberlites from the KirklandLake cluster (Buffonta, Diamond Lake, Tandem)and one kimberlite from the Timiskaming cluster(Peddie), all of which are interpreted to reflect somedegree of lithosphere contamination, the remain-der of the kimberlites plot between the CHUR andDM reference lines (Figure 2a). The most straight-forward interpretation of these data is that theleast radiogenic perovskite analyses (0.7032–0.7036)reflect the isotopic composition of the dominantmantle source region for these kimberlites and thatthe more radiogenic perovskite analyses indicatevariable contamination with SCLM and/or conti-nental crust. However, this interpretation is rejectedbased on the tight clustering of perovskite stron-tium isotopic compositions for kimberlites emplacedover vast distances and long periods of time.

[34] In detail, the Triassic‐Jurassic Central and East-ern North American kimberlites define two distinctkimberlite strontium isotopic signatures (shadedbands in Figure 2a); interpreted to be derived fromtwo mantle reservoirs. Both reservoirs plot betweenthe dashed CHUR and DM reference lines inFigure 2a. For this construction both mantle reser-voirs are assumed to have CHUR‐like 87Rb/86Srcompositions (0.0827). The more radiogenic reser-voir is closest in composition to CHUR (Figure 2a)and includes the Attawapiskat (0.7040–0.7042;n = 3) and the least radiogenic Kirkland Lake(0.7041–0.7042; n = 3) kimberlites. It is interest-ing to note that the Cretaceous Elliott County,Kentucky (0.70403 ± 0.00008; Type D [Heaman,1989]) and Ham, Nunavut (0.70403 ± 0.00004[Heaman, 1989]) kimberlites have identical stron-tium isotopic compositions within analytical uncer-tainty. Therefore, this mantle reservoir has persistedbeneath North America for at least 100 m.y.


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[35] The strontium isotopic composition of perov-skite isolated from kimberlites worldwide is com-piled in Figure 3 and there is an indication thatthe CHUR‐like mantle source identified for theNorth American kimberlites also occurs beneathother cratons. For example, CHUR‐like mantle com-positions are reported for some Cretaceous (e.g.,Dutoitspan, Frank Smith, Jagersfontein, Kamfersdam,Koffiefontein, Kaalvallei, Wesselton) and Cambrian(Oaks, Venetia) Group I kimberlites from SouthernAfrica [Woodhead et al., 2009], the 478 Ma Men-gyin kimberlite, China [Yang et al., 2009], the 1.1 GaNarayanpet kimberlites, India [Paton et al., 2007a]and possibly the 1.2 Ga Premier kimberlite inSouthern Africa [Woodhead et al., 2009]. If cor-rect, this CHUR‐like mantle reservoir must bewidespread and has existed for more than 1 Ga. Weinterpret this mantle source to be in the astheno-sphere and kimberlites with this signature formedbelow the base of the SCLM (∼300 km) and abovethe 670 km discontinuity.

[36] The second mantle source composition is lessradiogenic and includes kimberlites in the RankinInlet (0.7032–0.7036; n = 4) and the youngerkimberlites in the Timiskaming (0.7034–0.7036;n = 3) fields. Perovskite from the 176 Ma VartyLake ultramafic lamprophyre (0.70341 ± 0.00007[Heaman, 1989]) also has an identical composi-tion and could be derived from the same mantlereservoir. This reservoir has also persisted beneathNorth America for nearly 100 m.y. Similar to theCHUR‐like reservoir, this less radiogenic reservoiris also widespread in the mantle. Other kimberlitesworldwide that may have been derived from thissource (Figure 3) are the least radiogenic Creta-ceous Group I kimberlites in Botswana, Lesothoand Southern Africa (e.g., Lethlakhane, Liquoberg,Orapa, K17, Kao, Letseng andMonastery [Woodheadet al., 2009]), 363 Ma Udachnaya, Russia [Maaset al., 2008], and the 1.1 Ga Wajrakarur kimber-lites, India [Paton et al., 2007a]. The origin of thismantle source is unclear but it has many isotopicsimilarities to FOZO [Stracke et al., 2005], includingrelatively unradiogenic strontium (0.7028–0.7035),radiogenic neodymium (0.5129–0.5130), and mod-erately radiogenic 206Pb/204Pb (19.5–20.5). Mostauthors agree that the FOZO reservoir is wide-spread in the mantle (a required mantle componentto explain the isotopic variation in most MORBand OIB lavas) and most likely represents recycledoceanic lithosphere in the deep mantle [Strackeet al., 2005]. It is unclear whether mantle withFOZO isotopic characteristics occurs as isolateddomains scattered throughout the upper and lower

mantle as proposed by Stracke et al. [2005] orwhether this signature could be a feature of themantle at the 670 km discontinuity. In either scenario,it is becoming clear that one type of kimberlitemagmaworldwide involves melting deep, ancient, metaso-matized recycled oceanic lithosphere.

4.3. Origin of Triassic‐Jurassic EasternNorth American Kimberlite Magmatism

[37] Three main models have been invoked toexplain the origin of Eastern North American (ENA)kimberlite magmatism including rifting [Phipps,1988], subduction [Sharp, 1974; McCandless,1999], and mantle melting related to the passage ofone or more mantle plumes [Crough, 1981;Morgan,1983; Sleep, 1990; Heaman and Kjarsgaard, 2000].Rifting associated with the opening of the NorthAtlantic Ocean at about 200 Ma has been proposedbut does not readily explain all the Triassic to Jurassickimberlite magmatism in North America. There areJurassic kimberlites and related rocks occurringon both margins of the Labrador Sea in Labrador[Tappe et al., 2006] and Greenland [Larsen and Rex,1992; Larsen et al., 2009; Secher et al., 2009],therefore some Jurassic kimberlite magmatism inNorth America is related in time to the opening of theNorth Atlantic Ocean. However, the spatial geom-etry of the Triassic/Jurassic corridor of kimberlitemagmatism is not easily explained by this riftingmodel.

[38] There have been two subduction hypothesesproposed for ENA kimberlite magmatism. West‐dipping subduction‐related magmatism linked to thedevelopment of the Appalachian Mountains (i.e.,Paleozoic subduction) was proposed by Sharp [1974]but does not explain the much younger ENA Meso-zoic kimberlite magmatism as subduction ceasedmore than 200 m.y. prior to the Jurassic kimberlitemagmatism. A more recent model involves the sub-duction of the Farallon plate in the Pacific Ocean inan E‐SE direction from 175 to 125 Ma [McCandless,1999]. This “deep‐seated” subduction, where the slabextends ∼2700 km beneath North America [Grandet al., 1997], involves the process of progressiverelease of entrapped fluids from the downgoing slab,promoting small degree partial melts of the over-lying mantle wedge to produce kimberlitic magma.The kimberlites are the result of partial melting,and as subduction continues fluids are released upthe slab, therefore kimberlites would have a young-ing direction toward the trench [McCandless, 1999].This subduction model would generate belts of mag-matism that are roughly parallel to the margin where


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the subduction occurs. Heaman and Kjarsgaard[2000] maintain that the timing and location ofthe Rankin and Attawapiskat kimberlites does notfit this model because these kimberlites occur morethan 2000 km from the site of Farallon subduc-tion at the time when the Farallon plate was justbeginning to impact the tectonics of western NorthAmerica at 200 Ma. Furthermore, the youngingdirection of the Triassic‐Jurassic kimberlite mag-matism is opposite to that predicted for a subduc-tion origin linked to the east‐dipping Farallonplate. In addition, the fluids released from a sub-ducting slab are insufficient in high field strengthelements to represent the precursor metasomaticagent required to generate the geochemical signa-ture of a kimberlitic melt. Although we postulateone type of kimberlite magma can be derivedfrom ancient recycled oceanic lithosphere in thedeep mantle (discussed above), we do not favor anorigin for kimberlites from active subducting oce-anic crust.

[39] The remaining hypothesis to test is that theTriassic to Jurassic corridor of kimberlite magma-tism that currently extends from the 225–190 MaRankin Inlet field in Nunavut to the 154–125 MaTimiskaming field in Ontario could be the conti-nental expression of one or more mantle plume hotspot tracks [Heaman and Kjarsgaard, 2000]. Thesuggestion that the generation of kimberlite mag-mas could be related to hot spot tracks is not new,Crough et al. [1980] pointed out that a numberof known kimberlite occurrences in eastern NorthAmerica, Brazil and Africa are situated along ornear the proposed continental extension of AtlanticOcean mantle plume hot spot tracks. These authorssuggested that kimberlites in Ontario and UpperNew York State could be linked to the Great Meteorhot spot track. The other main continental expres-sion of magmatism along the Great Meteor hot spottrack in North America (Figure 1) includes thedominantly Cretaceous alkaline rocks that com-prise the 124–123 Ma Monteregian Hills in Quebec[Foland et al., 1986] and the 128–114 Ma WhiteMountain magma series in Maine, Vermont andNew Hampshire [e.g., McHone and Butler, 1984;Sleep, 1990]. The oceanic magmatic expressionof the Great Meteor hot spot track from oldest toyoungest includes the 103–82 Ma New EnglandSeamounts [Duncan, 1984], 75–70 Ma Corner Sea-mounts [Duncan, 1984], and the 17–16 Ma GreatMeteor Seamount [Geldmacher et al., 2006]. Inaddition to the perovskite analyses from this studydisplayed in the strontium isotope evolution dia-gram (Figure 2a), all proposed expressions of Great

Meteor hot spot track magmatism are shown. Alsoshown for comparison are the isotopic compositionsfor basalts from the Cape Verde islands [Holm et al.,2006] to assess whether it is possible to distinguishbetween mantle source regions that were melted toproduce the Great Meteor versus the Verde hot spotmagmas. The most striking feature of these data arethat the kimberlite perovskite data summarized abovefor the Rankin Inlet and Timiskaming kimberlitefields and the Varty Lake ultramafic lamprophyreoverlap the least radiogenic samples from the Mon-teregian Hills and White Mountains magma series(0.70315–0.70341; n = 5 [Foland et al., 1988]) andthe range observed for basalts from the New EnglandSeamounts (0.70311–0.70348; n = 6 [Taras andHart, 1987]), the Great Meteor Seamount (0.70310–0.70320; n = 3 [Geldmacher et al., 2006]) and theCape Verde Islands (0.70305–0.70333; n = 23[Holm et al., 2006]). This implies that this mantledomain was relatively homogeneous, long‐lived (i.e.,>200 m.y.), and quite extensive, existing beneathboth the oceanic and continental lithospheres. Otherconclusions that can be drawn from this diagram are:(1) the strontium isotopic data previously reportedfor the New England Seamounts, the Great MeteorSeamount and the Cape Verde Islands are indistin-guishable, therefore they could all be derived fromone mantle source composition and (2) perovskitefrom the Timiskaming, least radiogenic KirklandLake, Ham and Elliott County kimberlites have iden-tical CHUR‐like strontium isotopic compositionsbut they are distinct from the Great Meteor mag-matism and this implies that these kimberlites areformed from a different mantle source; either fromthe same plume but melting occurred at differentdepths or they are unrelated to the Great Meteormantle plume.

[40] Figure 4 is a northwest to southeast cross sec-tion of the Churchill and Superior cratons, whichcompiles information gleaned from the SCLMbased on sampling by a number of kimberlite fieldsthat crudely fall along this transect (Somerset Islandsoutheast to the Kirkland Lake and Timiskamingarea). The position of the cratonic root (SCLMlower boundary) along this corridor has been inter-preted from different sources. Schmidberger et al.[2001] recognized two lithospheric domains on thebasis of the 87Sr/86Sr composition of Archean garnetperidotite and garnet pyroxenite xenoliths from thehost Nikos kimberlite at Somerset Island, a shallow(0.704) and deep (0.706–0.708) zone. Irvine et al.[2003] summarize geothermobarometry estimateswhich indicate pressures representing lithosphericdepths that clearly extend into the diamond stability


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field. Since there are currently no mantle xenolith/xenocryst studies from the Rankin Inlet kimber-lites, the base of the SCLM beneath the Churchillkimberlite field has been extrapolated using theinformation derived from the studies of Irvine et al.[2003] and Schmidberger et al. [2001] at SomersetIsland, the margin of the Churchill craton. TheSCLM lower boundary beneath the Trans‐Hudsonorogen, was estimated to be 180 km by Griffinet al. [2004] using garnet chemistry on xenolithsfrom 7 kimberlites in the Fort à la Corne kimberlitefield. Deep seismic tomography studies show theentire area as a high velocity root (∼250 km thick)that is continuous from the Hearne Domain acrossthe Trans‐Hudson Orogen and into the Superiorcraton [van der Lee, 2001].

[41] As discussed above, the two mantle sourcesfor Triassic to Jurassic kimberlite magmatism inthis study and Group I kimberlites worldwide arehypothesized to be located in the asthenosphere

(CHUR‐like reservoir) and from recycled oceaniclithosphere located in the deep mantle and pos-sibly concentrated at the 670 km discontinuity.These mantle source regions are depicted in Figure 4with the youngest Timiskaming and Rankin Inletkimberlites being derived from the greatest mantledepths and having strontium isotopic composi-tions comparable to Great Meteor hot spot magma-tism (e.g., Great Meteor Seamount, New EnglandSeamounts, least radiogenic Monteregian Hillsintrusions). It is not possible to distinguish GreatMeteor and Cape Verde magmatism based onstrontium or neodymium isotopic compositions soit remains unresolved whether there was one or twomantle plumes involved in generating the Triassic‐Jurassic corridor of kimberlite magmatism. The twomantle sources identified in this study could reflectthe impact of two plumes that initiated mantle melt-ing at different depths but similar observationscould be generated by a single plume that had a

Figure 4. A proposed model of the upper mantle from Somerset Island, Nunavut through to Timiskaming, Ontario.SCLM, subcontinental lithospheric mantle. 87Sr/86Sr ratios from Schmidberger et al. [2001].


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protracted melting history as it traversed the uppermantle. We interpret the fact that the two mantlesources identified in this study are the main globalreservoirs for generating Group I kimberlite magmasto be more consistent with the latter hypothesis.

5. Conclusions

[42] Perovskite is a robust mineral for obtainingthe age of kimberlite emplacement and the primarySr and Nd isotopic signature of the host kim-berlite magma. In turn, these initial 87Sr/86Sr and143Nd/144Nd compositions of primary kimberlitemagma can be used as a proxy to constrain thenature of its mantle source region, as well as pro-vide valuable information on the isotopic archi-tecture of the SCLM and asthenosphere and extentof lithosphere contamination. The most primitiveperovskite Sr and Nd isotopic results obtained inthis study from each kimberlite field along the∼2000 km long corridor of near continuous Triassic/Jurassic kimberlite magmatism in central and east-ern North America are interpreted to indicate thatat least two distinct mantle sources are implicatedin the origin of this kimberlite magmatism. Themost primitive Rankin Inlet and Timiskamingkimberlites have relatively unradiogenic strontiumisotopic signature (87Sr/86Sr = 0.7032–0.7036),whereas the Attawapiskat and Kirkland Lake kim-berlites have a more CHUR‐like signature (0.7040–0.7042). In the Kirkland Lake kimberlite field thereis a large interfield strontium isotopic variation(0.70407 and 0.70509) that is attributed to variablelithosphere contamination.

[43] The most primitive strontium isotopic composi-tions observed within the Rankin Inlet and Timis-kaming kimberlite fields overlap the least radiogenicisotopic compositions previously reported for theMonteregian Hills and White Mountain intrusions.In addition, these most primitive signatures overlapwith the strontium isotopic compositions previouslyreported for basaltic rocks considered to be partof the oceanic expression of the Great Meteor hotspot track, including the New England and GreatMeteor seamounts. The progressive decrease in theage of magmatism from the Triassic Rankin Inletkimberlites to the Miocene Great Meteor sea-mount combined with the similarity in the isotopiccomposition of these diverse magmas along the pro-posed >3000 km long hot spot track, provides strongevidence in support of a common mantle plumeorigin for both the continental and oceanic com-ponents. The isotopic character of this mantlesource is most like FOZO and is interpreted to

reflect some kimberlite generation from recycled,metasomatized oceanic lithosphere located deep inthe mantle. The CHUR‐like strontium isotopic com-positions obtained for a number of North Americankimberlites including the most primitive KirklandLake and Attawapiskat bodies investigated in thisstudy and the Cretaceous Ham and Elliott Countykimberlites are interpreted to indicate the existenceof a second isotopically distinct mantle sourceregion beneath North America that was involved inkimberlite magma generation. This source is inter-preted to be in the asthenosphere. These two mantledomains identified for the North American kimber-lites are proposed as the dominant source for Group Ikimberlite magmas worldwide and have been inexistence for at least 1.2 b.y.

Acknowledgments

[44] We greatly appreciate the generosity of Shear MineralsLtd., for the donation of samples. This project has been sup-ported by the NSERC Discovery grants to L.M.H. and R.A.C.We are grateful to Antonio Simonetti and Gayle Hatchardwho provided helpful suggestions during the isotopic analyses.Reviews by Chad Paton and Bruce Kjarsgaard, as well asrecommendations of the editor Joel Baker, have helped usimprove the paper.

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