Thallium isotopes as a potential tracer for the origin of cratonic eclogites

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Geochimica et Cosmochimica Acta 73 (2009) 7387–7398

Thallium isotopes as a potential tracer for the originof cratonic eclogites

Sune G. Nielsen a,b,*, Helen M. Williams a,b, William L. Griffin a,Suzanne Y. O’Reilly a, Norman Pearson a, Fanus Viljoen c

a GEMOC, Department of Earth and Planetary Sciences, Macquarie University, 2109 NSW, Australiab Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK

c Department of Geology, University of Johannesburg, Auckland Park 2006, Johannesburg, South Africa

Received 6 April 2009; accepted in revised form 1 September 2009; available online 6 September 2009

Abstract

Cratonic eclogites are inferred to originate either from subducted ocean crust or mantle melts accreted onto the roots ofcontinents. These models have different implications for the growth of continents, but it is currently difficult to determine theorigin of individual eclogite suites.

Upper ocean crust altered at low temperatures and marine sediments both display high thallium (Tl) concentrations andstrongly fractionated Tl isotope signatures relative to the ambient upper mantle. In this study we carry out the first exami-nation of the suitability of Tl isotopes as a tracer for an ocean-crust origin of cratonic eclogites. We have analysed the Tlisotope composition of clinopyroxene and garnet in six eclogites from the Kaalvallei and Bellsbank kimberlite pipes in SouthAfrica. Minerals were pre-cleaned with an HCl leaching technique and the leachates display variably light Tl isotope ratios.These most likely reflect low-temperature hydrothermal alteration occurring after eruption of the kimberlite that carried theeclogites to the surface.

The leached mineral pairs all display identical Tl isotope ratios, strongly suggesting that the source of the analysed Tl isidentical for each mineral pair. It is, however, not possible to exclude the possibility that the analysed Tl originates from kim-berlitic material that was not removed by the cleaning procedure.

Only one of the six samples exhibits a Tl isotope composition different from ambient mantle. Assuming that the Tl isotopesignatures indeed represent the eclogite minerals and not any form of contamination, the Tl isotope composition in this sam-ple is consistent with containing a minor component (<3%) of ocean crust altered at low temperatures.

Thallium isotopes may become one of the most sensitive indicators for the presence of low-T altered ocean crust because ofthe stark contrast in Tl concentration and isotopic composition between the mantle and altered ocean crust. In fact, no otherchemical or isotopic tracer could have provided an indication that any of the samples studied here had a subduction origin.However, much work is still required before it becomes clear if Tl isotope measurements are a viable means to establish theorigin of cratonic eclogites.� 2009 Elsevier Ltd. All rights reserved.

0016-7037/$ - see front matter � 2009 Elsevier Ltd. All rights reserved.

doi:10.1016/j.gca.2009.09.001

* Corresponding author. Address: Department of Earth Sciences,University of Oxford, Parks Road, Oxford OX1 3PR, UK. Tel.:+44 1865272026.

E-mail address: [email protected] (S.G. Nielsen).

1. INTRODUCTION

From experimental evidence it has long been known thatbasaltic rocks that form the upper part of subducting oce-anic crust transform at high pressures (depths >100 km)into eclogite (Ringwood and Green, 1966). Due to theirconnection with subduction zones, and potentially with

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7388 S.G. Nielsen et al. / Geochimica et Cosmochimica Acta 73 (2009) 7387–7398

hot-spot volcanism (Hofmann and White, 1982), eclogitesare important recorders of tectonic processes in the Earth.There are two main settings in which eclogites are foundat the surface of the Earth. One is in orogenic zones whereone tectonic plate is thrust above another and therebyexposes otherwise deep-seated material; these orogeniceclogites are often clearly related to subduction processes.Secondly, eclogites occur as xenoliths included in rapidlyascending host magmas, such as kimberlites, that mainlyerupt within continents.

Kimberlitic (or cratonic) eclogites are interestingbecause they represent samples of the lithospheric mantlekeels that underlie the continents to depths up to 250 kmand contribute to their stability. The occurrence of eclogitexenoliths in the subcontinental lithospheric mantle raisesquestions about the ultimate fate of subducted slabs, andtheir role in the origin of continental roots. Two generalmodels can be invoked to explain the occurrence of cratoniceclogites. One suggests that they originate from magmasthat crystallize at high pressures underneath the pre-existingcontinental roots where the eclogite mineralogy (clinopy-roxene + garnet) is stable (O’Hara, 1969; Caporuscio andSmyth, 1990; Griffin and O’Reilly, 2007). The other infersthat the continental roots grew by “stacking” of subductedslabs beneath pre-existing continental crust, and interpretsthe eclogites as fragments of these ancient slabs (Macgregorand Manton, 1986; Jacob et al., 1994; Barth et al., 2001).The stacking model assumes that flat-slab subduction hasdominated at least early Earth history, a scenario signifi-cantly different to modern subduction, which sometimescan be seen to penetrate into the lower mantle (Zhao,2004). The melt accretion model, on the other hand, mayimply a more continuous growth of continental roots overEarth’s history. However, using current petrological andgeochemical methods it can be difficult to determine if sam-ples have one or the other petrogenetic origin. The presenceof accessory phases such as coesite (Schulze et al., 2000)suggests a protolith that was silica saturated, which is notpossible when melting peridotite at pressures higher than0.8 GPa (Green and Falloon, 1998) and hence may be anindicator of an ocean-crust origin. This is also the casefor the occurrence of Sr and Eu anomalies, which may besigns of plagioclase accumulation and hence the formerpresence of this low-pressure mineral (Jacob et al., 2003,2005). However, coesite as well as Sr and Eu anomaliesare relatively rare in cratonic eclogite xenoliths, and couldhave metasomatic origins (Griffin and O’Reilly 2007).Perhaps the most compelling support for an ocean-crustorigin for cratonic eclogites comes from mass-independentsulphur isotope analyses of sulphide inclusions in eclogiticdiamonds (Farquhar et al., 2002), which require the sulphurto be cycled through the Archean atmosphere. However, todate such evidence has only been found in four diamondsfrom the Orapa kimberlite pipe and it is therefore difficultto use this information to derive the larger scale origin ofcontinental roots.

Perhaps the most convincing indication of an ocean-crust origin for cratonic eclogites in general is offered bythe wide range in oxygen-isotope ratios found in manyxenolith suites (Garlick et al., 1971; Jacob et al., 1994,

2004; Beard et al., 1996; Viljoen et al., 1996; Schulzeet al., 2003), which contrasts with the homogeneousoxygen-isotope composition (d18O = 5.5 ± 0.4) of mostmantle rocks (Mattey et al., 1994). Oxygen isotopes areknown to be significantly fractionated during hydrothermalalteration of the ocean crust, with shallow low-temperature(low-T) alteration producing d18O-values higher than nor-mal mantle and deeper high-T alteration resulting in valueslower than the mantle (Alt et al., 1986, 1996a; Staudigelet al., 1995; Hart et al., 1999). The variability in O-isotopecomposition is, therefore, generally regarded as a signatureinherited from alteration of oceanic basalts by circulatingseawater (Jacob, 2004 and references therein). However,by analogy to carbon isotopes measured in kimberliticdiamonds (Maruoka et al., 2004), fluid–solid reactionscould in principle also explain the variable O-isotope signa-tures in these eclogites (Griffin and O’Reilly, 2007). It istherefore necessary to develop new tools that can comple-ment O-isotope analyses in order to evaluate how muchof the O-isotope variation ultimately might originate fromocean-crust alteration processes.

The stable isotope geochemistry of thallium (Tl) hasconsiderable potential as an isotopic tracer of ocean crustrecycling processes. Thallium is a heavy, volatile, andhighly incompatible trace metal with two isotopes (203Tl,205Tl). In a series of recent publications it has been shownthat Tl is isotopically fractionated in ferro-manganese(Fe–Mn) marine sediments (Rehkamper and Halliday,1999; Rehkamper et al., 2002, 2004) and MORBs alteredat low-temperature by seawater (Nielsen et al., 2006c), withe205Tl values of about +10 and �15, respectively(e205Tl = 10,000 � (205Tl/203Tlsample � 205Tl/203TlSRM 997)/(205Tl/203TlSRM 997)). In contrast, samples from most otherenvironments, including unaltered ocean crust (Nielsenet al., 2006b, c), high-temperature altered ocean crust (Niel-sen et al., 2006c), hydrothermal fluids (Nielsen et al.,2006c), the continental crust (Nielsen et al., 2005), rivers,and mineral aerosols (Nielsen et al., 2005), display essen-tially invariant Tl isotope compositions (e205Tl � �2 ± 1).Another compelling feature of Tl geochemistry is that bothFe–Mn sediments and low-T altered basalts are highly en-riched in Tl (McGoldrick et al., 1979; Jochum and Verma,1996; Hein et al., 2000; Rehkamper et al., 2002, 2004; Niel-sen et al., 2006c) compared to the primitive mantle(McDonough and Sun, 1995). Therefore, if either of thesetwo components is contained within eclogitic material, evenin volumetrically minor quantities, Tl isotope compositionsdistinct from normal mantle should be observed. This is agreat advantage compared to oxygen because oxygen con-centrations are similar in most geologic materials, andtherefore a large component (>10%) of altered ocean crustwould be required to produce a measurable change in theisotope composition.

In this contribution we report the first Tl isotope mea-surements in cratonic eclogites, in order to explore if Tl iso-tope ratios are a useful tool in tracing ocean crust in theselithologies. The Tl isotope analyses are complemented bymore conventional oxygen isotope measurements and laserablation trace element concentration determinations of themain mineral phases.

Thallium isotopes in cratonic eclogites 7389

2. MATERIALS AND METHODS

2.1. Samples

Six samples were investigated in this study. Five of themare from the Kaalvallei kimberlite pipe, South Africa, andwere provided by De Beers. These are from the same suiteof samples as those described by Viljoen et al. (2005). Thesixth sample is from the Bellsbank kimberlite pipe northof Kimberley, South Africa. All samples are bi-mineralicand consist of coarse-grained garnet and omphacitic pyrox-ene. The samples were selected based on the degree of alter-ation and foreign material visible in hand specimens. It wasimportant to keep these at an absolute minimum due to thevery large quantities of pure mineral separates that wererequired to perform the Tl isotope analyses. Detaileddescriptions of hand samples and thin sections are givenby Williams et al. (2009).

2.2. Sample preparation

Kimberlitic eclogites are always contaminated to somedegree by their host kimberlite magmas in cracks and veinspassing through the sample. Therefore it is important toobtain a quantitative separation of eclogite and hostmatrix. Previous studies have shown that the analysis ofpure mineral separates yields the most reliable results(Jacob, 2004). In the samples studied here no other primarymineral phases than clinopyroxene and garnet were present.Samples were crushed in a hammer mortar and opticallypure fractions were then separated by hand-picking underethanol using a binocular microscope. Extra care was takento avoid any alteration products during mineral separation.However, Tl may be so enriched in the kimberlite host mag-ma compared to the eclogite that a volumetrically insignif-icant contamination can become significant for the Tlbudget. Therefore, in a further attempt to prevent contam-ination from the host kimberlite magma we employed aprocedure for cleaning the mineral grains prior to analysis.Previous studies have utilised relatively intense leachingmethods involving strong HCl and dilute HF (Machadoet al., 1986; Jacob et al., 1994) to remove potential contam-inants. However, we opted to be more cautious because anysignificant leaching (and thereby alteration) of the crystallattice could lead to stable isotope fractionation and there-by erroneous results. We therefore ultrasonicated the min-eral separates in �3.5 M HCl for 2 times 1 h at roomtemperature. After the first leaching period the supernatantand particulates in suspension were removed, the mineralsrinsed in 18 MX water, and fresh acid was supplied. Thetwo leachates and their water rinses were then combinedand evaporated to dryness. The leachates were then dis-solved in a 1:5 mixture of concentrated HF and HCl, drieddown, and dissolved in �10 ml of 1 M HCl. Small aliquotsof the leachates were taken out for trace element analysis byICP-MS. The residual cleaned minerals were dissolvedusing a standard HF:HNO3 technique, which we have pre-viously employed for powdered silicates (Nielsen et al.,2007). For all samples, thallium was separated from samplematrices using a two-column chemistry procedure following

methods described elsewhere (Rehkamper and Halliday,1999; Nielsen et al., 2004). Total procedural Tl blanks dur-ing this study were <3 pg, which is insignificant comparedto the indigenous Tl present in the samples.

2.3. Mass spectrometry

Thallium isotope compositions were determined with aNu Instruments multiple collector inductively coupled plas-ma-mass spectrometer (MC-ICPMS) at the GeochemicalAnalysis Unit, ARC National Key Centre GEMOC atMacquarie University using external normalisation toNIST SRM 981 Pb as well as standard-sample bracketing(Rehkamper and Halliday, 1999; Nielsen et al., 2004). Ra-tios are reported relative to the NIST SRM 997 Tlstandard.

Due to the very low Tl concentrations in several of thesamples, total Tl ion beam intensities as low as0.06 � 10�11 A were recorded in this study. In a prior con-tribution the reproducibility of Tl isotope Acompositions attotal Tl ion beam intensities down to 0.2 � 10�9 A wasinvestigated (Nielsen et al., 2006a). It was found that thefollowing errors apply: ±2 e205Tl for ion beams between0.2 and 0.5 � 10�9 A, ±1.5 e205Tl for 0.5–1.0 � 10�11 A,and ±1 e205Tl for total Tl ion beams of >1.0 � 10�11 A. To-tal ion beams below 0.2 � 10�11 A were assigned an arbi-trary error of ±3 e205Tl (Nielsen et al., 2006a), but thismay be an underestimation particularly for analyses withion beam intensities below 0.1 � 10�11 A. To make a betterquantification of the uncertainties at these ion beam inten-sities, we analysed the isotope composition of NIST 997 Tlat �0.1 � 10�11 A bracketed by standards of normal con-centration. In order to avoid degeneration of the countingstatistics the NIST 981 Pb concentration was kept at a nor-mal level for both sets of standards. The results of theseanalyses are listed in Table 1 and show that the approxi-mate uncertainty for measurements performed at such ionbeam intensities is about ±5 e205Tl (2sd), which is similarto the precision reported for d234U measured at similarion beam intensities (Andersen et al., 2004). There may bea small systematic offset in the isotope compositions mea-sured at low ion beam intensities possibly related to peaktailing from the large Pb ion beams in the close vicinityof the Tl ion beams. However, this effect is not resolvablewithin the reproducibility of the measurements and we havetherefore assigned an uncertainty of ±5 e205Tl to the twosample analyses performed at <0.1 � 10�11 A.

Due to the quantitative yields of Tl from the columnchemistry procedure, Tl concentrations could be deter-mined by monitoring the 205Tl signal intensities of the sam-ples during the isotopic measurements. A known quantityof NIST SRM 981 Pb was added to the sample Tl andthe measured 205Tl/208Pb ratios were converted into Tlabundances by assuming that Tl ionises 5% more efficientlythan Pb. As the measured 205Tl/208Pb ratios of mixed stan-dard solutions of Tl and Pb were observed to vary signifi-cantly, depending on the instrumental operatingconditions, it is estimated that the Tl concentrations deter-mined in this way are only accurate to ±25% (2sd)(Rehkamper et al., 2002; Nielsen et al., 2007).

Table 1Thallium isotope compositions of standards run at low ion beamintensities.

Tl ion beam intensity �10�11 A e205Tl

0.12 �3.10.11 �7.00.12 �1.40.11 �4.80.11 �0.60.10 �4.00.11 �1.60.10 �4.30.11 �0.10.10 1.40.10 �3.90.11 �4.60.11 �2.60.11 �5.80.11 �2.50.10 �2.10.12 �3.10.11 �7.00.11 �2.80.10 3.10.10 �5.80.11 �2.40.10 �2.80.11 �3.80.10 �0.20.10 �6.30.10 �1.60.10 �5.70.10 3.40.10 �8.80.11 �1.3

Average �3.02sd 5.2


2.4. Determination of trace element concentrations

Trace element abundances of mineral leachates (Table 2)were determined by solution ICP-MS using a 100 ppb mul-ti-element solution as a standard. In order to monitorinstrument drift the standard was run before and after thesamples. Such a method does not produce highly precisedata and we therefore estimate an uncertainty of about±20% (2 se). In addition we have not attempted to use thesedata for any quantitative modelling. As the absolute massof the leached material is unknown these concentrationsare reported relative to the Mg abundance in ng/mg ofMg. The leachate from sample 382 was not analysed fortrace elements.

3. RESULTS AND DISCUSSION

3.1. Validation of leaching technique

The thallium isotope compositions of leachates and cor-responding minerals are summarised in Table 3 and Fig. 1.The most notable feature of the data is that leachates areconsistently isotopically lighter (i.e. have lower e205Tl) thanthe minerals. In addition, a large proportion of the mineral

samples are indistinguishable from the average mantle com-position (Fig. 1) as defined by mid ocean ridge basalts(Nielsen et al., 2006b). In principle, the systematic discrep-ancy between minerals and leachates can be interpreted intwo ways: a) that Tl isotopes are fractionated during thecleaning process, such that 203Tl is preferentially leachedout of the mineral phase, or b) that the low e205Tl repre-sents the removal of various materials not indigenous tothe main crystals, presumably kimberlitic material or alter-ation products. If interpretation a) is correct, this would se-verely compromise the aim of determining the Tl isotopecomposition of the in-situ mineral, particularly as the leach-ates contain a large fraction of the total Tl recovered fromthe samples (Table 3).

There are several lines of evidence that strongly suggestthat b) is more likely. Firstly, the leachates are highly en-riched in incompatible trace elements such as Ba and Rb(Ba/Mg � 1–10 � 10�3 and Rb/Mg � 0.1–1 � 10�3; Table2), which are commonly associated with kimberlitic mag-mas (Jacob, 2004) and are also highly fluid-mobile. In con-trast these elements are either at or below detection limits inthe LA-ICP-MS analyses of the minerals (Williams et al.,2009). The detection limits for Ba and Rb are approxi-mately 0.2 and 0.05 ppm, respectively, resulting in maxi-mum Ba/Mg and Rb/Mg ratios on the order of 3 � 10�6

and 1 � 10�6. Thus, Ba and Rb abundances are at leastthree orders of magnitude higher in the leachates than inthe minerals themselves. Similarly, all leachates displayenrichment in light rare earth elements (LREE) whereasthe laser-probe analyses show REE patterns characteristicfor clinopyroxene and garnet in equilibrium with each other(Fig. 2a–b). This is confirmed by comparing REE partitioncoefficients for garnet and clinopyroxene (Halliday et al.,1995) with the relative REE abundances in the samples(Fig. 3). This indicates that the main contributor to theleachates was not clinopyroxene and garnet, but more likelyone or more secondary mineral phases. Lastly, all mineralpairs exhibit identical Tl isotope compositions within error,whereas their respective leachates are variable (Fig. 1). Itappears highly unlikely that the leaching procedure ex-tracted variable amounts of fractionated Tl from the crys-tals but left them with identical Tl isotope compositions.We conclude that the leaching procedure did not compro-mise the Tl contained within the main mineral phases, butinstead removed small amounts of alteration productsand/or kimberlitic material that would have otherwise ren-dered the Tl isotope ratios of the minerals erroneous.

3.2. Origin of fractionated Tl isotope signatures in leachates

In order to investigate the origin and isotopic mass bal-ance of the Tl in the leached material we sieved the samplematerial remaining after mineral picking and analysed Tlisotope compositions and concentrations of the size frac-tion <150 lm. The grain size of the main minerals in thesamples was at least 500 lm (Williams et al., 2009), whereasany secondary material, including both the host kimberliteand later alteration products, is situated in small cracks andveins. It is therefore reasonable to assume that the smallersize fraction has a significant component of such material.

Table 2Trace element abundances in ng/mg of Mg in mineral leachates.

Kaalvallei A 375 402 423 Bellsbank

CPX GRT CPX GRT CPX GRT CPX GRT GRT

Be 16 11 19 16 13 20 11 15 88Cr 6580 9280 5260 16500 27700 44200 18200 32900 7000Mn 5310 17000 10000 17500 13200 20900 10600 18100 14500Co 506 935 563 675 287 660 347 203 581Ni 7280 24400 12100 22400 7310 9570 4240 2340 5840Cu 6270 10500 4500 3910 738 958 240 139 654Zn 60 51 36 55 65 89 35 59 33Ga 11300 14900 15700 22500 10100 6050 6340 6440 14600As 278 470 260 357 1080 634 234 207 320Se 370 276 79 738 299 202 196 254 231Rb 77 1330 260 560 83 268 261 483 883Sr 5900 3330 15300 5750 11800 2710 6080 7760 5570Y 22 186 51 243 476 180 82 205 179Zr 319 487 103 484 1240 453 600 352 938Nb 2.8 45 4.6 2.9 11 18 2.3 3.2 30Cd 1.1 0.8 0.5 0.8 2.0 1.9 0.7 0.9 0.8Cs 24 63 21 60 6.1 12 51 66 270Ba 3990 7230 10300 1060 3970 1870 1170 1820 2030La 60 472 89 379 2370 1040 756 352 527Ce 122 676 206 547 1430 531 948 547 819Pr 15 49 19 39 265 120 59 39 60Nd 98 248 122 198 1670 757 273 200 274Sm 19 44 28 42 267 113 35 29 29Eu 4.9 13 23 12 51 21 6.5 12 6.2Gd 6.3 20 11 25 91 39 13 14 13Tb 1.4 5.3 2.4 6.6 17 6.9 2.9 3.5 3.6Dy 6.3 30 12.5 41 73 31 14 22 23Ho 1.1 6.4 2.4 8.8 13 6.4 3.1 5.4 5.8Er 2.7 17 6.0 24 29 16.2 8.3 16 18Yb 2.9 23 6.2 33 20 18.5 9.1 19 27Lu 0.3 2.7 0.7 3.8 2.1 2.1 1.2 2.5 3.3Hf 12 7.8 2.8 8.1 18 7.8 7.8 6.4 16.3Ta 3.4 24 0.6 2.1 42 6.8 1.0 0.9 2.5W 787 753 262 838 950 2980 564 1500 947Pb 1.9 1.6 19 6.5 1.9 3.7 2.9 3.2 2.3Th 8.4 42 18 33 322 31 92 36 87U 11 16 13 13 52 7.9 30 12 27


Thallium isotope and concentration data for the<150 lm size fraction are shown in Table 3. Thallium con-centrations are 1–2 orders of magnitude higher than in theclinopyroxene or garnet, which is consistent with Tl mainlyoriginating from secondary materials. The absolute concen-trations recorded should be seen as minima because the sec-ondary material has been diluted with Tl-poor eclogiteminerals. For three of the samples the Tl isotope composi-tions are significantly different to the leachates (Table 3),which suggests that the isotopic variation in the leachatesdoes not simply represent the bulk composition of the sec-ondary material. The low e205Tl-values in the leachatescould be explained by incorporation of isotopically lightTl into more acid-soluble minerals formed by low-T hydro-thermal alteration of the eclogite after the emplacement ofkimberlite. Such hydrothermal activity is known to be aubiquitous feature of kimberlite pipes (Sparks et al.,2006). In this case, the light Tl isotope signatures couldbe produced through a process analogous to that observedin low-T hydrothermal alteration of oceanic crust, where Tl

with compositions as light as e205Tl = �15 is strongly par-titioned from the circulating hydrothermal fluids into clayminerals (Nielsen et al., 2006c). We therefore suggest thatthe highly variable Tl isotope compositions in the leachatesrepresent mixtures of isotopically fractionated alterationproducts and less fractionated kimberlitic material.

3.3. Contamination of residual leached minerals by kimberlite

and/or alteration products

There is convincing evidence that the technique em-ployed for leaching of the mineral separates did not affectthe isotope composition of the Tl situated in the crystalstructure of the minerals. However, it is more difficult to as-sess whether the leaching method successfully removed all

Tl not indigenous to the minerals themselves. All the resid-ual mineral pairs exhibit Tl isotope compositions within er-ror of each other, whereas the leachates are highly variable.It would be fortuitous if the cleaning procedure removedexactly enough isotopically fractionated material from each

Table 3Thallium isotope compositions and concentrations in eclogite minerals.

Group Kaalvallei A 375 382 402 423 BellsbankI I II II II I

CPX GRT CPX GRT CPX GRT CPX GRT CPX GRT CPX GRT

d18Oa 5.4 4.5 5.9 5.1 4.9 4.7 5.3 5.0 5.8 3.8 5.5 5.1Modal mineral abundanceb 0.4 0.6 0.65 0.35 0.8 0.2 0.8 0.2 0.35 0.65 0.55 0.45

Sample weight (g) 0.283 1.124 0.601 1.218 1.203 na 0.819 0.809 0.580 0.323 na 1.898Tl leached (ng) 0.13 0.8 1.0 2.2 1.0 na 1.0 0.2 0.6 0.6 na 2.1Tl in residual mineral (ng) 0.05 1.0 1.1 2.0 2.4 na 0.3 0.4 0.6 0.4 na 3.4Mineral Tl concentration (ppb) 0.2 0.9 1.8 1.6 2.0 na 0.3 0.5 1.0 1.1 na 1.8% Tl in Leachate 72 44 48 52 29 na 77 33 50 60 na 38Leachate e205Tl �11.4 �6.9 �15.9 �8.3 �3.5 na �4.2 �9.0 �4.3 �5.7 na �2.8Error 5 1.5 1.5 1 1.5 na 1.5 3 2 2 na 1Mineral e205Tl 2.3 �3.5 �5.3 �4.5 �2.4 na �1.9 �1.7 �2.0 �1.6 na �2.9Error 5 1 1 1 1 na 2 1.5 1.5 2 na 1Calculated bulk e205Tl �2.8 �5.1 �2.4c �1.8 �1.7 �2.9c

Bulk sample e205Tl <150 lm size fraction �4.4 �4.7 �2.3 �1.7 �3.4 �4.0Error 1 1 1 1 1 1Tl conc. (ppb) <150 lm size fraction 132 464 12 80 14 294

na, Not analysed due to insufficient material; CPX, clinopyroxene; GRT, garnet.a d18O data from Williams et al. (2009).b Modal mineral abundances from Williams et al. (2009).c Bulk assumed to be identical to mineral analysed.

Fig. 1. Thallium isotope compositions of eclogite mineral leaches and the residual minerals after leaching. Grey band denotes the averagemantle thallium isotope composition (Nielsen et al., 2006b). Error bars on individual data points are based on the respective ion beamintensities (see text for discussion). CPX, clinopyroxene; GRT, garnet.


mineral to leave them with the same isotope composition.Hence, we infer that alteration products are likely to havebeen removed essentially quantitatively from the minerals.

Still, it cannot be excluded that the residual minerals rep-resent contamination by kimberlite host magma. This couldmost reasonably occur if the leaching procedure selectivelyleft some kimberlite material behind, for example if kimber-lite and eclogite reacted to produce silicate material (Jacobet al., 2005) that is not entirely removable from the eclogite

minerals with the cleaning technique employed here. Unfor-tunately samples of the kimberlite host magma are not avail-able and therefore we cannot evaluate this possibility.

3.4. Isotopic equilibration of eclogite minerals with kimberlite

and/or alteration products

Even if the sample preparation method was successful inremoving non-eclogitic material, it is still critical to evaluate

Fig. 2. Chondrite normalised REE patterns for (a) mineral leaches determined by solution ICP-MS and normalised to Mg (see text fordiscussion) and (b) in-situ mineral analyses by LA-ICP-MS. Chondrite normalising values from McDonough and Sun (1995). LA-ICP-MSdata from Williams et al. (2009).


whether Tl isotopes in the samples are in equilibrium and ifso, with what they equilibrated. All of the analysed mineralpairs display identical Tl isotope compositions (Fig. 1), andit is therefore useful to consider if high-temperature igneousminerals in equilibrium are expected to show any Tl isotopefractionation. From Fig. 3 it is apparent that the rare earthelements appear to be equilibrated (though perhaps not insample Bellsbank), a situation which also applies to Fe iso-topes (Williams et al., 2009). The diffusivity of Tl in igneousminerals is unknown, but is probably best approximated bythe alkali metal Cs due to its high mass and the very similar

ionic radius and charge (Shannon, 1976). Caesium displaysa diffusion rate of about 1 � 10�9 cm2/s in clinopyroxene ata temperature of �1000 �C (Roselieb and Jambon, 1997),which is in the range of temperatures recorded for the sam-ples studied here (Williams et al., 2009). This is at least 6 or-ders of magnitude faster than Fe (Azough and Freer, 2000).Although Tl diffusion may be slower than the alkali metalsdue to its higher mass, it appears unlikely that Tl diffusesslower than Fe, which is in isotopic equilibrium (Williamset al., 2009). Therefore, Tl isotopes are most probably inequilibrium in the studied samples. Equilibrium stable iso-

Fig. 3. Relative REE abundances in clinopyroxene and garnet in eclogite minerals. Also plotted are relative partition coefficients (Hallidayet al., 1995). The similarity between relative partition coefficients and mineral abundances strongly indicate that the REE in garnet andclinopyroxene are in equilibrium.


tope fractionation between mantle minerals has been re-ported for elements as heavy as iron (Zhu et al., 2002; Wil-liams et al., 2004, 2005). Although Tl is much heavier thanFe, equilibrium stable isotope fractionation even at hightemperatures is theoretically possible for Tl (Schauble,2007). However, the calculations presented by Schauble(2007) show that any significant Tl isotope fractionation re-quires the co-existence of both naturally occurring thalliumvalence states, Tl1+ and Tl3+. This situation is improbablein igneous processes because Tl3+ is thermodynamically sta-ble only under very oxidising conditions and even seawateris completely dominated by Tl+ (Nielsen et al., 2009). Thal-lium isotope fractionation in igneous systems is thereforevery unlikely. Consequently, the results for co-existingclinopyroxene and garnet presented here (Table 3, Fig. 1)are consistent with what would be expected for the Tl iso-tope compositions of these minerals.

The high diffusion rate of Cs (and thereby perhaps also Tl)in clinopyroxene warrants a discussion of potential Tl iso-tope equilibration between eclogite minerals and the hostkimberlite magma or post-eruption hydrothermal fluids.Assuming that the Tl diffusivity in clinopyroxene is therate-limiting factor for isotopic re-setting in the eclogite, wecan attempt to assess the feasibility of such a process by com-paring diffusion rates with kimberlite eruption rates. Againapplying the Cs diffusion rate at 1000 �C of 1 � 10�9 cm2/sresults in a Tl isotope re-equlibration time for the samplesof 2.5 � 108 s (for a distance of 0.5 cm). Kimberlite eruptionand ascent rates (up to 20 m/s) are very high (Sparks et al.,2006); an ascent rate of 10 m/s would enable a kimberlitemagma to travel through the entire lithosphere in less than3 h (�1 � 105 s), rendering Tl isotope equilibration betweenkimberlite magma and eclogite unlikely.

Post-emplacement hydrothermal circulation probablypersists for much longer periods, albeit at significantly low-

er temperatures (<500 �C; (Sparks et al., 2006). Diffusiveisotope exchange and fractionation has been observed forLi at such temperatures (Teng et al., 2006), implying thatisotope re-setting is possible in post-emplacement igneoussettings. However, the diffusion rate of Li is clearly manyorders of magnitude faster than Tl due to its very low massand different ionic radius and it is therefore not realistic toconsider this element as an analogue for Tl. Instead, we canconsider the conclusion we reached in Section 3.2 that post-emplacement alteration is associated with considerable Tlisotope fractionation. This would presumably perturb theeclogite minerals if they had been isotopically reset by dif-fusion during low-T hydrothermal circulation. We there-fore conclude that the eclogite minerals are unlikely tohave been isotopically reset by diffusive interaction withkimberlite magma or by post-emplacement hydrothermalcirculation.

4. POTENTIAL IMPLICATIONS OF THE ECLOGITE

TL ISOTOPE COMPOSITIONS

Incompatible trace element concentrations in cratoniceclogites are notoriously difficult to measure in bulk sam-ples, because of their often low abundances and high poten-tial for contamination with kimberlite host magmas (Jacobet al., 1994). In the previous sections various possible inter-pretations of the Tl isotope ratios measured in the six eclog-ite xenoliths were discussed. Though it appears likely thatthe mineral cleaning procedure removed all external con-tamination it cannot be excluded that some kimberlitematerial was left, which could have compromised the min-eral analyses.

In the following section we will briefly discuss the impli-cations of the Tl isotope data assuming that the cleaningprocedure removed all non-eclogite material quantitatively

Table 4Depletion of Cu and Zn in bulk eclogites.

Kaalvallei A 375 382 402 423 Bellsbank

Bulk Cu (ppm) 2.2 4.2 0.75b 0.46b 0.32b 0.76Bulk Zn (ppm) 44.6 34.6 10.0 13.8 13.3 36.1Cu*/Cua 0.029 0.063 0.015 0.015 0.0032 0.0051Zn*/Zna 0.32 0.36 0.11 0.25 0.10 0.13

Bulk Cu and Zn calculated using modal mineral abundances from Williams et al. (2009).a Cu and Zn normalised to primitive mantle (McDonough and Sun, 1995) as well as to the normalised abundances of Sc and Y.b Bulk concentrations estimated by assuming Cu concentrations in garnet similar to the detection limit, which is 0.15 ppm. These bulk

concentrations are therefore maximum estimates.


from the samples and that the analyses represent the true Tlisotope composition of the eclogite minerals.

All samples are devoid of accessory phases and show nomicrostructures suggestive of former oceanic crust (Wil-liams et al., 2009). The absence of Eu anomalies (Fig. 2b)also eliminates this parameter as an indicator for oceancrust. There is, however, a uniform strong depletion ofCu and to a lesser extent Zn compared to the primitivemantle normalised abundances of Sc and Y (Table 4).One of the most notable features of high-T altered oceancrust is a depletion of Cu and Zn caused by the high solu-bility of metals and sulphides in high-T hydrothermal fluids(Alt et al., 1996a, b), thus implying such an origin for thestudied samples. However, low Cu and Zn is also compat-ible with removal of mantle sulphides either through frac-

Fig. 4. Bulk eclogite Tl vs. oxygen-isotope compositions. Bulk isotopabundances (Williams et al., 2009). Shown are also mixing lines between tplate: Fe–Mn crusts, low-T altered ocean crust and high-T altered ocTl = 0.0012 ppm, e205Tl = �2 (Nielsen et al., 2006b), d18O = 5.5 (Mattey2000; Rehkamper et al., 2002), d18O = 0 (it is assumed that Fe–Mn crustaltered ocean crust: Tl = 0.2 ppm, e205Tl = �15 (Nielsen et al., 2006c)Tl = 0.0005 ppm, e205Tl = �2 (Nielsen et al., 2006c), d18O = 4.0 (Alt et al.low-T altered ocean crust that has lost 90% of the original Tl during dehysimilar oxygen concentrations.

tional crystallisation or by partial melt extraction, ascenario specifically proposed for these samples (Williamset al., 2009).

If eclogites are remnants of subducted oceanic crust onemight expect a correlation between oxygen and thalliumisotope compositions towards isotope compositions heavierand lighter than normal mantle, respectively (normal man-tle has d18O � 5.5 and e205Tl � �2), because low-T alteredoceanic crust displays d18O � 10 (Staudigel et al., 1995) ande205Tl � �15 (Nielsen et al., 2006c). Fig. 4 shows the bulkTl and O-isotope compositions calculated from modal min-eral abundances (Williams et al., 2009). Mixing lines be-tween normal mantle and three separate reservoirscontained in a section of ocean crust overlain by marinesediments are also shown. Only two samples, samples 423

e compositions are calculated based on estimated modal mineralhe mantle and three separate bulk reservoirs present in a subductingean crust. The endmember compositions are as follows. Mantle:et al., 1994). Fe–Mn crusts: Tl = 100 ppm, e205Tl = 10 (Hein et al.,

s have d18O identical to the seawater they precipitate from). Low-T, d18O = 10 (Staudigel et al., 1995). High-T altered ocean crust:, 1996b). Additionally, we plot a mixing line between the mantle anddration subduction processes. It is assumed that all reservoirs have


and 375, plot outside the normal mantle region. Sample 423falls on a mixing line towards high-T altered ocean crust(Fig. 4). In addition, sample 423 is also the most depletedin Cu and Zn (Table 4) which might also be consistent withderivation from this portion of ocean crust. However, it hasbeen argued, based on a correlation between O and Fe iso-topes, that the low d18O-values observed in these sampleswere produced by open-system processes occurring in thelithospheric mantle, most likely several melt extractionevents (Williams et al., 2009). Hence, the O-isotope compo-sition together with the low Zn and Cu concentrations arealso compatible with exhaustion of sulphides during re-peated melting events in the lithospheric mantle.

Sample 375 is the only specimen to display a Tl isotopecomposition different to normal mantle. The isotope anom-aly would be compatible with the presence of smallamounts (between 0.1 and 0.3%) of low-T altered oceaniccrust in the eclogite protolith (Fig. 4). Such minor quanti-ties of low-T altered ocean crust would not be detectablewith oxygen isotopes due to the lack of concentration con-trast between the different oxygen reservoirs.

The calculated amount of the low-T altered crustal com-ponent in sample 375 is based on an estimate of the averagecomposition of the volcanic zone (i.e. the upper ca 600 me-ters of ocean crust; Nielsen et al., 2006c). However, the vol-canic zone is extremely heterogeneous in both Tlconcentrations and Tl isotope compositions, with shallowerdepths displaying higher Tl concentrations as well as morefractionated Tl isotope ratios (Nielsen et al., 2006c). Thisestimate must therefore be viewed as semi-quantitative.The amount of low-T altered basalt component is alsohighly dependent on the slab dehydration processes occur-ring during ocean crust subduction. Thallium has beenshown to be a slightly fluid-mobile element in arc settings(Noll et al., 1996) and it is therefore realistic that some ofthe Tl in the slab will be lost during subduction. It is diffi-cult to estimate how much Tl could be lost during subduc-tion, but Noll et al. (1996) concluded that Tl wassignificantly less fluid-mobile than elements such as B andPb. It has been suggested that Pb-loss during subductioncould be between 80–90% (Bach et al., 2003) and it there-fore appears reasonable to set an upper limit of Tl-loss fromthe slab at 90%. If we assume that Tl isotopes are not frac-tionated during fluid loss from the slab then we estimatethat sample 375 contains 1-3% low-T altered ocean crust(Fig. 4). The assumption of insignificant Tl isotope frac-tionation during slab dehydration appears reasonable ashigh-T hydrothermal alteration of ocean crust does notproduce any resolvable isotopic variation (Nielsen et al.,2006c).

5. CONCLUSIONS AND FUTURE WORK

We present the first Tl isotope composition measure-ments of mineral separates in bi-mineralic (clinopyroxeneand garnet) cratonic eclogites. Before dissolution the miner-als were pre-cleaned by ultrasonification in HCl. All theseleachates display isotope compositions variably lighter thanthe residual minerals and we conclude that this reflects theremoval from the mineral surfaces of isotopically fraction-

ated Tl originating from low-temperature post-eruptionhydrothermal alteration. Based on analyses of bulk eclogitematerial and finer size fractions we also conclude that, asexpected, the eclogites contain a component of kimberlitemagma. All residual mineral pairs display identical Tl iso-tope ratios indicating that the source of Tl in the mineralseparates is identical. It is, however, not possible to excludethat the analysed Tl originates from kimberlitic materialthat was not removed by the cleaning procedure.

Only one of the samples investigated here exhibits a Tlisotope composition significantly different to normal man-tle. If it is assumed that the Tl isotope signatures indeedrepresent the eclogite minerals and not any form of contam-ination, the Tl isotope signature in this sample is consistentwith the presence of a minor component (less than 3%) ofocean crust altered at low temperatures. The remainder ofthe samples bear no chemical or physical evidence of for-merly being ocean crust. This, however, does not precludea subduction origin for the studied eclogites, because themajor portion of ocean crust is made up of mafic lithologiesthat have Tl isotope compositions indistinguishable frommantle melts.

The major obstacle encountered in this study was theexceedingly low Tl concentrations in the eclogite minerals.This meant that very large sample sizes were required to ob-tain reasonably precise thallium isotope analyses. This wasthe overriding reason for our inability to determine if allcontamination had been removed from the mineral sepa-rates. The samples examined were selected based on thevery low degree of alteration, which was expected to aidthe attempts to remove secondary material from the eclog-ites. Even though the exceptional preservation did allow forlarge quantities of very fresh material to be separated thelow Tl concentrations still prevented unambiguous results.

For future studies an alternative strategy could be to se-lect eclogite samples that are known to be remnants ofocean crust. Such samples could for example be orogeniceclogites, which have previously been studied for Li iso-topes with the same objective (Zack et al., 2003; Marschallet al., 2007). Firstly, these samples will be more likely tohave higher Tl abundances, thus making Tl isotope analy-ses less challenging. Secondly, it will be important to char-acterise the isotopic composition of subducted materialbefore Tl isotopes can confidently be used as a tracer of oce-anic crust in cratonic eclogites.

Thallium isotopes may become one of the most sensitiveindicators for the presence of low-T altered ocean crust ineclogite material because of the stark concentration and iso-topic contrast between the mantle and altered ocean crust.However, significantly more work is necessary in order toconclude if this isotope tool can provide new informationabout the origin of eclogites.

ACKNOWLEDGMENTS

We would like to thank Dorrit Jacob and Thomas Zack forconstructive reviews of this manuscript and Rich Walker for hishelpful guidance and editorial handling. This research was fundedby a grant to SGN from the Danish Research Agency and usedinstrumentation at GEMOC funded by ARC LIEF and DEST Sys-temic Infrastructure Grants, Macquarie University and industry.


This is publication 604 of the ARC National Key Centre for Geo-chemical Evolution and Metallogeny of Continents (www.es.mq.e-du.au/GEMOC).

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Thallium isotopes as a potential tracer for the origin of cratonic eclogites