Zircon M257 - a Homogeneous Natural Reference Material for the Ion Microprobe U-Pb Analysis of Zircon

Zircon M257 - a Homogeneous Natural Reference Materialfor the Ion Microprobe U-Pb Analysis of Zircon

Vol. 32 — N° 3 p . 2 4 7 - 2 6 5

We introduce and propose zircon M257 as a futurereference material for the determination of zircon U-Pb ages by means of secondary ion mass spectrometry. This light brownish, flawless, cut gemstone specimen from Sri Lanka weighed 5.14 g(25.7 carats). Zircon M257 has TIMS-determined,mean isotopic ratios (2s uncertainties) of 0.09100 ± 0.00003 for 206Pb/238U and 0.7392 ± 0.0003 for207Pb/235U. Its 206Pb/238U age is 561.3 ± 0.3 Ma(unweighted mean, uncertainty quoted at the 95%confidence level); the U-Pb system is concordantwithin uncertainty of decay constants. Zircon M257contains ~ 840 μg g-1 U (Th/U ~ 0.27). The materialexhibits remarkably low heterogeneity, with a virtual absence of any internal textures even incathodoluminescence images. The uniform, moderatedegree of radiation damage (estimated from theexpansion of unit-cell parameters, broadening of Raman spectral parameters and density) corresponds well, within the “Sri Lankan trends”,with actinide concentrations, U-Pb age, and the calculated alpha fluence of 1.66 x 1018 g-1. This,and a (U+Th)/He age of 419 ± 9 Ma (2s), enables

Nous présentons et proposons à la communauté lezircon M257 qui pourrait devenir un matériau deréférence pour la détermination des âges U-Pb sur zircon par spectrométrie de masse à ions secondaires (SIMS). Ce spécimen scié d'une gemmebrun clair, sans défauts, provenant du Sri Lanka,pèse 5.14 g (25.7 carats). Les moyennes des rapports isotopiques (et les incertitudes associées2s) mesurées par TIMS sur ce zircon M257 sont0.09100 ± 0.00003 en 206Pb/238U et 0.7392 ±0.0003 en 207Pb/235U. Son âge 206Pb/238U est de561.3 ± 0.3 Ma (moyenne non pondérée, incertitude:95% intervalle de confiance); le système U-Pb y est concordant, aux erreurs sur les constantes dedésintégration près. Le zircon M257 contient ~ 840μg g-1 U (Th/U ~ 0.27). Ce matériel montre de trèsfaibles hétérogénéités, avec une quasi absence de textures internes, même à l'examen par cathodoluminescence. Le degré de dommage liéaux radiations (estimé par l'expansion des paramètres de la cellule unitaire, l'élargissementdes paramètres des spectres Raman et la densité)est modéré et uniforme. Il se corrèle bien, (sur

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0908

Lutz Nasdala (1)*, Wolfgang Hofmeister (2), Nicholas Norberg (1)**, James M. Mattinson (3),Fernando Corfu (4), Wolfgang Dörr (5)***, Sandra L. Kamo (6), Allen K. Kennedy (7), Andreas Kronz (8), Peter W. Reiners (9), Dirk Frei (10), Jan Kosler (11), Yusheng Wan (12), Jens Götze (13), Tobias Häger (2), Alfred Kröner (2, 12) and John W. Valley (14)

(1) Institut für Mineralogie und Kristallographie, Universität Wien, Althanstr. 14, A-1090 Wien, Austria(2) Institut für Geowissenschaften, Johannes Gutenberg-Universität, Becherweg 21, D-55099 Mainz, Germany(3) Department of Earth Science, University of California, Santa Barbara, CA 93106, USA(4) Department of Geosciences, University of Oslo, Postbox 1047 Blindern, N-0316 Oslo, Norway(5) Institut für Geowissenschaften und Lithosphärenforschung, Justus-Liebig-Universität, D-35390 Gießen, Germany(6) Jack Satterly Geochronology Laboratory, Department of Geology, University of Toronto, 22 Russell St., Toronto, Ontario,

M5S 3B1, Canada(7) Department of Imaging and Applied Physics, Curtin University of Technology, Kent St., Bentley, 6102, Western Australia(8) Geowissenschaftliches Zentrum, Universität Göttingen, Goldschmidtstrasse 1, D-37077 Göttingen, Germany(9) Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA(10) Department of Geological Mapping, Geological Survey of Denmark and Greenland, Øster Voldgade 10,

DK-1350 Copenhagen K, Denmark(11) Department of Earth Science, University of Bergen, Allegaten 41, N-5007 Bergen, Norway(12) Beijing SHRIMP Center, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, 100037 Beijing, China(13) Institut für Mineralogie, TU Bergakademie Freiberg, Brennhausgasse 14, D-09596 Freiberg, Germany(14) Department of Geology & Geophysics, University of Wisconsin, Madison, WI 53706, USA* Corresponding author. e-mail: [email protected]** Present address: Sektion 4.1, Helmholtz-Zentrum Potsdam, Deutsches Geoforschungszentrum, D-14473 Potsdam, Germany*** Present address: Geozentrum, Johann-Wolfgang-Goethe-Universität, Altenhöferallee 1, D-60438 Frankfurt, Germany

© 2008 The Authors. Journal compilation © 2008 International Association of Geoanalysts

GEOSTANDARDS and

RESEARCHGEOANALYTICAL

SIMS (secondary ion mass spectrometry), and inparticular the SHRIMP (sensitive high mass resolutionion microprobe) technique, has revolutionised thedetermination of zircon U-Pb ages because it was thefirst method offering the possibility to derive spatially-resolved age information from micro-areas of polishedsample surfaces (e.g., Compston et al. 1984, Williams1998, compare also Andersen and Hinthorne 1972).The accuracy of U-Pb ages determined by SIMS analy-sis of zircon, however, depends strongly on the qualityof the reference material used. Quality features comprisein particular (i) the reliability of isotopic data obtainedfor the reference material, (ii) the isotopic homogeneityof the reference material, i .e., the degree to whichthese data represent individual chips selected to beused as reference samples, and (iii) the performanceof the reference material during analysis. The latterincludes sufficient count rates for Pb isotopes, theabsence of notable matrix effects (i.e., similar structuralstate of reference and unknowns) and a negligibleconcentration of non-radiogenic Pb. The search for asuitable SIMS reference material is therefore deman-ding. Any potential candidate should be homoge-neous, contain sufficiently high concentrations of U andradiogenic Pb but should not be affected by significantradiation damage, and have a (close to) concordantU-Pb system (Pidgeon 1997, Kennedy 2000).

Here we present analytical work characterisingzircon M257, mainly in view of its potential use as aSIMS U-Pb reference material. Our analyses focusedon the determinat ion of re l iable “ recommendedmean values” for U-Pb age, isotopic ratios and Uconcentration, and the investigation of internal homo-geneity/heterogeneity of the specimen, in order toobtain rel iable information on whether randomly

selected chips will be representative of the analyticalmeans.

General characterisation

General description and preparation

Zircon M257 was an oval shaped, cut s tone(Figure 1a) with a weight of 5.1412 g (or 25.706carats) ; i t s longest dimension was 19.5 mm. Thespecimen originated from the collection of the Institutfür Edelsteinforschung Idar-Oberstein und Mainz,Germany. It was originally found in a secondary pla-cer deposi t in the Sri Lankan Highland Complex(Kröner et al. 1994). The crystal displayed light brow-nish colour and was flawless and transparent (Figure1a). Neither internal cracks, fractures nor significantinclusions were seen under the binocular microscope.Prior to purchase, the specimen was selected from asuite of fifteen zircon gemstones from several deposits.Select ion was made after thorough inspection intransmitted and cross-polarised light, using immersionliquids, by means of a cathodoluminescence (CL)microscope to check roughly the extent of zoning andheterogeneity, and with a first set of Raman spectro-scopic measurements. The CL inspection was doneusing an optical microscope equipped with a vacuumchamber connected to an elec tron beam source,which allowed visual inspection of the CL patternsand colours emi t ted f rom macroscopic samples .Sample M257 was selected because it appearedhomogeneous and showed a uni form, moderatedegree of radiation damage.

The macroscopic density was determined as 4.63g cm-3 by weighing the stone in water (with minute

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GEOSTANDARDS and



us to exclude any unusual thermal history or heattreatment, which could potentially have affected theretention of radiogenic Pb. The oxygen isotope ratio of this zircon is 13.9‰ VSMOW suggesting ametamorphic genesis in a marble or calc-silicateskarn.

Keywords: zircon, reference material, M257, U-Pb, geochronology, SIMS, secondary ion mass spectrometry.

l'alignement “Sri Lanka”) avec les concentrations enactinides, l'âge U-Pb et la fluence calculée de 1.66x 1018 g-1. Ceci, couplé à son âge (U+Th)/He de419 ± 9 Ma (2s), nous permet d'exclure l'existenced'événement thermique passé, ou de traitement parla chaleur, qui aurait potentiellement pu affecter le processus de rétention de Pb radiogénique. Le rapport isotopique de l'oxygène de ce zircon est 13.9‰ VSMOW, suggérant une génèse parmétamorphisme, au sein d'un marbre ou d'un skarn calco-silicaté.

Mots-clés : zircon, matériau de référence, M257, U-Pb,géochronologie, SIMS, spectrométrie de masse à ionssecondaires.Received 12 Mar 08 — Accepted 28 Jul 08

amount of detergent to decrease surface tension) andin air. As accumulation of radiation damage in zirconresults in volume expansion and density decrease(from ~ 4.68 g cm-3 to well below 4 g cm-3; Hollandand Gottfried 1955, Murakami et al. 1991), the abovedensity value points to a rather moderate degree ofmetamictisation of zircon M257.

The stone was then sliced (with a 200 μm, dia-mond-trimmed metal wire) and slices were crushed.One ca. 8 x 10 mm sized slice, representing the cross-section of one half of the stone, was then used to pro-duce a large, doubly polished thin section (Figure 1b).The thickness of this section was not reduced below45-47 μm, because (i) fractures began to develop asa result of strain induced by the sawing and/or poli-shing process and (ii) the section should remain thickenough for analysis by laser ablation-inductively cou-pled plasma-mass spectrometry (LA-ICP-MS). The ideabehind the production of such a large section was toobtain a sufficiently large sample for homogeneitytests , even though this consumed a considerableamount of material.

Chemical composition

Major and trace element contents were measuredby means of electron probe microanalysis and LA-ICP-

MS. Multiple measurements along two 8 mm and 6mm long (nearly perpendicularly intersecting) traversesacross the large thin section (Figure 1b) were perfor-med to characterise the chemical composition and tocheck for potential heterogeneity.

Wavelength-dispersive X-ray analysis of majorand t race e lemen t s was pe r fo rmed a t t heGeowissenschaftliches Zentrum, Universität Göttingen,Germany. Analyses were performed by a Jeol 8900 RLelectron microprobe, with an accelerating voltage of20 kV and a beam current of 80 nA. The fully focusedelectron beam yielded an excitation area of approxi-mately 2 μm in diameter. Element-specific countingtimes varied from 300 s (background 2 x 150 s) for U-Mα and Th-Mα to 15 s (background 2 x 5 s) for mainelements (Zr-Kα, Si-Kα). For the calibration procedure,the following synthetic materials were used: ZrSiO4 forZr and Si, HfSiO4 for Hf, yttrium aluminium garnet(YAG) for Y, Yb-glass for Yb, apatite for P, wollastonitefor Ca, Al2O3 for Al, haematite for Fe, ThSiO4 for Th,and UO2 for U. Data processing was done using theCITZAF routine in the JEOL software, which is based onthe Φ(ρZ) method (Armstrong 1991, 1995).

Actinide concentrations (U, ~ 812 μg g-1; Th, ~ 225μg g-1) were well above the detection limits of theelectron probe but the accuracies of the determined

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GEOSTANDARDS and



Figure 1. Images of zircon M257, originating from the Ratnapura district, Sri Lanka. (a) Image of the clear

transparent, brown cut gemstone. (b) Interference colour of a large thin section (thickness 45-47 μm) of

zircon M257, observed in cross-polarised light. The image was produced by combining thirty-six single

photomicrographs. Note the lack of growth zones or other internal textures.

values were somewhat limited because the electronmicroprobe reference materials (synthetic UO2 andThSiO4) differed notably from the unknowns, with acti-nide contents being three orders of magnitude higherin the former. To reduce these uncertainties, U and Thdata for M257 were reduced again and calibratedagainst M127, another homogeneous Sri Lankan zirconof similar chemical composition, which was analysed(sixty-three single analyses) during the same electronmicroprobe session as M257. The U and Th concentra-t ions o f M127 were determined be forehand byconventional solution ICP-MS. Direct determination ofU and Th using this technique, however, was not donefor M257, simply because too much material (~ 300mg) would have been consumed.

Solution ICP-MS analysis of zircon M127 was per-formed at GEUS (Nationale Geologiske Undersøgelserfor Danmark og Grønland), Copenhagen, Denmark.The sample was ground to powder (i.e., particle sizes ofless than 63 μm) using a tungsten carbide ball mill,subsequently dried at 110 °C for 2 hours, and thenignited in an electric furnace at 1000 °C for 1 hour. Ahomogeneous glass disc was produced by fusing theignited powder together with a borate flux. A fragmentof the glass disc was then dissolved in a HF-HNO3 mix-ture, evaporated to dryness and subsequently twice re-dissolved with HNO3 and evaporated to dryness. Thedry residue was then dissolved in HNO3 and diluted,

and the resulting solution was analysed for trace ele-ments using a Perkin Elmer 6100 DRC quadrupole ICP-MS. This method is a modified version of the methoddescribed by Turner et al. (1999), whereby the use of aglass disc ensures the complete digestion of the zircon.Routine analysis of international and in-house qualitycontrol materials demonstrated that the repeatabilityand accuracy were usually better than 5% relative forthe majority of the elements analysed. Based on the Uconcentration of M127, electron probe microanalysisdata of M257 were re-calibrated, and a U concentra-tion of 863 μg g-1 was calculated (Table 2).

Selected trace element contents were determinedby laser ablation ICP-MS at the University of Bergen,Norway. A ThermoFinnigan Element2 single-collectordouble focusing magnetic sector system coupled to aNewWave/Merchantek UP213 laser was used. Datawere acquired with a laser frequency of 5 Hz and aspot size of 53 μm, using He to flush the sample cell.The NIST SRM 610 glass (values of Pearce et al. 1997)was used as primary calibrant, and BCR-2G (unpubli-shed values, Memorial University of Newfoundland)was used as a quality control material. Data for thegas blank were acquired for 45 s, followed by 120 sablation. Silicon was used as internal calibrant to cor-rect for differences in ablation yields between glassreference materials and zircon samples. Final concen-tration calculations from time resolved signals were

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GEOSTANDARDS and



Table 1.Chemical composition of zircon M257

Electron probe microanalysis results LA-ICP-MS results(n = 76) (n = 16)

Oxide Concentration [% m/m]* Element Concentration [μg g-1]*

ZrO2 66.4 ± 0.2 Nb 0.19 ± 0.06HfO2 1.38 ± 0.01 La 0.37 ± 0.08SiO2 32.7 ± 0.1 Ce 5.04 ± 0.34P2O5 n.d. Pr 0.20 ± 0.07FeO n.d. Nd 1.39 ± 0.45Al2O3 n.d. Sm 1.57 ± 0.26CaO n.d. Eu 0.38 ± 0.10Y2O3 0.019 ± 0.005 Gd 3.96 ± 1.04Yb2O3 n.d. Tb 1.18 ± 0.11ThO2 0.026 ± 0.002 Dy 12.6 ± 0.8UO2 0.092 ± 0.003 Ho 4.44 ± 0.26Total 100.6 ± 0.2 Er 19.5 ± 1.5

Tm 4.51 ± 0.26Yb 45.0 ± 2.2Lu 7.20 ± 0.42Hf 10610 ± 465Pb 83.2 ± 3.7Th 229 ± 8U 853 ± 24

n.d. = not detected or average below 0.005% m/m. * Quoted uncertainties are 2s .

performed using the Glitter software package (vanAchterbergh et al. 2001).

Results are presented in Tables 1 and 2. Zircon M257is relatively low in non-formula elements, with hafniumbeing the only element in the % m/m range. The averageU concentration was determined at ~ 840 μg g-1 (Table2). This value is higher than in most other SIMS zirconreference materials, which is considered an advantagebecause of higher count rates and hence better Poissonstatistics. The Th/U ratio averages ~ 0.27 (determinedconsidering the data presented in Table 2, and the TIMSresults in Table 4). This ratio corresponds to typical valuesof Sri Lankan zircon samples (e.g., Murakami et al. 1991,Zhang et al. 2000, Nasdala et al. 2004) but is notablyabove 0.11 (note that in some of the earlier studies of SriLankan zircon, Th was assumed as 1/9 of the U concen-tration; see for instance Holland and Gottfried 1955).

Somewhat “regular” concentrations of trace ele-ments are also indicated by the typical blue CL colour(Figure 2), and CL and photoluminescence (PL) spectraof M257. Cathodoluminescence images and spectrawere taken at TU Bergakademie Freiberg, Germany,using a HC1-LM CL microscope. This system consistedof a conventional optical microscope, hot-cathodeelectron gun, vacuum chamber (< 10-6 bar), and dis-persive spectrometer. The accelerating voltage was 14kV and the beam current density was 10 mA mm-2.Real-colour CL images were taken using a Kappa 961-1138 CF 20 DXC camera attached to the microscope.Colours of images were manually corrected in the soft-ware according to the CL colour observed in the bino-cular microscope (Götze 1998, Götze at al . 1999).Photoluminescence spectra were obtained with Ar+

488 nm laser excitation, using an edge-filter basedRenishaw RM1000 spectrometer with quasi-confocal

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GEOSTANDARDS and



Table 2.Determination of U concentration and Th/U ratio of M257

Laboratory Technique No. of U* [μg g-1] Th/U* U reference materialanalyses (with U concentration)

Univ. Göttingen Electron probe microanalysis 76 812 ± 28 (751-879) 0.277 ± 0.024 Synth. UO2 (88.15% m/m)76 863 ± 29 (798-934) 0.281 ± 0.024 Zircon M127 (923 μg g-1)

Curtin Univ., Perth SHRIMP 36 837 ± 38 (766-914) 0.275 ± 0.002 Zircon CZ3 (551 μg g-1)Beijing center SHRIMP 11 834 ± 15 (809-855) 0.273 ± 0.004 Zircon CZ3 (551 μg g-1)GEUS, Copenhagen LA-ICP-MS 80 824 ± 59 (721-939) 0.302 ± 0.014 Zircon GJ-1 (230 μg g-1)Univ. of Bergen LA-ICP-MS 16 853 ± 24 (813-907) 0.269 ± 0.007 NIST SRM 610 (457.1 μg g-1)

Suggested values**: 840 0.27

* Quoted errors are 2s . Ranges of measured U concentrations are given in brackets.** For the determination of the suggested Th/U value, TIMS results (cf. Table 4 below) were considered, too, whereas the GEUS result was

disregarded.

Figure 2. (a) Cathodoluminescence spectrum and (b) laser-excited (Ar+ 488nm) PL spectrum of zircon

M257. Inset: CL image of a chip of M257 embedded in epoxy, showing the typical blue emission

colour. For band assignment see Gaft et al. (2000).

Emission wavelength (nm)

Inte

nsity

(a.u

.)

arrangement of the optical pathway (lateral resolution~ 2 μm). The CL spectrum (Figure 2a) was dominatedby the blue broad-band CL emission (Remond et al.1992) with weak, superimposed groups of Dy3+ bands(Gaft et al. 2000). In contrast, the PL spectrum (Figure2b) showed several emission bands of trivalent rareearth elements, and a broad-band emission in thenear infrared, which was assigned to Fe3+ (Gaft et al.2000). The obvious difference in CL and PL spectralcharacteristics is not caused by differences in the che-mical composition (note that the two spectra shown inFigures 2a and 2b were obtained from the same zir-con chip) but is due to different responses of lumines-cence centres to different excitations (Gaft et al. 2000).

Structural state and radiation damage

The structural state of natural zircon is stronglyaffected by the metamictisation process, i.e., accumula-tion of radiation damage generated by alpha-decayevents of U and Th, and their unstable daughternuclei. This process causes dramatic changes in physi-cal and chemical properties, such as a strong decrea-se of the ability to retain radiogenic lead and helium(Nasdala et al. 1998, 2004, Reiners 2005). The struc-tural state was quantified by X-ray powder diffraction(Holland and Gottfried 1955) analysis and Ramanspectroscopy (Nasdala et al. 1995).

X-ray diffraction was undertaken using a PHILLIPSX’Pert diffractometer (r = 171.9 mm) with Cu Kα1,2 radia-tion at 40 kV and 40 mA. The system was equippedwith a sample spinner, an automatic divergence slit, asecondary graphite monochromator and a scintillationcounter. Addit ionally, a receiving sl i t of 0.01 mmwhich corresponds to 0.033° 2θ, an anti-scatter slit (4°)and two 0.04 rad soller sl i ts on each side of thesample stage were used. The measurements were per-formed in step scan mode (0.02° 2θ steps, 45 secondsper step) over a range of 5 to 140° 2θ at 295 K and 1bar. Unit-cell parameters were determined by Rietveldrefinement, using the Topas 3 (Bruker AXS) software.The unit-cell dimensions were determined at a0 =6.626(1) Å and c0 = 6.030(1) Å, which results in a unit-cell volume of 264.69(10) Å3. For comparison, a minuteamount of M257 was annealed at 1300 °C for fourdays, to reconstitute the structure, and then analysedagain. The annealed sample yielded a0 = 6.6047(3) Å,c0 = 5 .9801(5 ) Å , and V = 260 .863(4 ) Å3.Consequently, the natural sample had experiencedvolume swelling of ~ 1.47% v/v, which corresponds toa mildly to moderately radiat ion-damaged state

(Holland and Gottfried 1955, Nasdala et al. 2004)well below the first percolation point (Salje et al. 1999).The latter describes the beginning inter-connection ofamorphous damage clusters, which results in the for-mation of a three-dimensional network of crystalline-to-amorphous boundaries (i.e., potential nanometre-scalepathways for the mobilisation of radiogenic elements).

The self-irradiation causing this radiation damagewas quantified from the U and Th concentrations andthe zircon age (cf. below) by calculating the time-integrated alpha fluence (Dα) according to (Murakamiet al. 1991, Nasdala et al. 2001)

(1)

with cU and cTh , actinide concentrations in μg g-1; NA,Avogadro’s Number; M238, M235, and M232, molecularweights o f i so topes ; λ238, λ235 and λ232, decayconstants; t, zircon age in Ma. The calculated fluenceof 1.66 x 1018 alpha-events per gram is rather mode-rate (note that Sri Lankan zircon is transformed into anearly amorphous state after about 1019 alpha-eventsper gram). More importantly, the unit-cell parametersof M257 correspond very well to the calculated alphafluence. Data for zircon M257 plot within the trend ofpreviously studied, untreated zircon samples from SriLanka (Figures 3a-b). The same was found to be truefor the sample density (Figure 3c).

Raman micro-spec t roscopy was per formed atUnivers i tät Mainz , Germany, us ing a Jobin YvonLabRAM-HR system. For experimental detai ls seeNasdala et al. (2004); the correction of band widths forthe apparatus function has been discussed in detail byNasdala et al. (2001). The degree of radiation dama-ge was determined from the full width at half-maximum(FWHM) of the ν3(SiO4) Raman band (internal anti-symmetric stretching of SiO4 tetrahedrons; B1g mode).This band was observed at a Raman shift of 1001.3 ±0.5 cm-1, its corrected FWHM was determined at 11.7± 1.0 cm-1. These values reconfirm the mildly to mode-rate ly radiat ion-damaged s tate o f z i rcon M257(Nasdala et al. 1995, 2004). Furthermore, they correspondvery well to the calculated alpha fluence (Figure 3d).

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GEOSTANDARDS and



7 . cU . 0.0072 . NA

106 . M235

. e λ235 t - 1 +

8 . cU . 0.9928 . NA

106 . M238

. e λ238 t - 1

Dα = 6 . cTh . NA

106 . M232

. e λ232 t - 1 +

To evaluate the retention of radiogenic helium,(U+Th)/He analyses were undertaken at the Universityo f A r i zona , U .S .A . Fo r exper imen ta l de ta i l s seeNasdala et al. (2004) and Reiners (2005). Results aresummarised in Table 3. Three analyses yielded anaverage He age of 419 ± 9 Ma for zircon M257. Thisvalue is consistent with previously published heliumages for mildly to moderately radiation-damagedzircon samples from Sri Lanka (Hurley 1954, Nasdalaet al. 2004).

All of our observations show that zircon M257 can-not have experienced any “unusual” thermal history,which also allows us to exclude any heat-treatment.Any young heating event would have caused (at leastpartial) thermal annealing of the radiation damage,resulting in miscorrelation of the observed parameters(unit-cell dimensions, density, Raman band parameters)and the calculated self-irradiation dose. Also, as radio-genic helium is known to diffuse out of zircon at eleva-ted temperatures (Reiners et al . 2005), the “regular”

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GEOSTANDARDS and



260

265

270

275

0 1 2 3 45.96

6.00

6.08

6.04

6.12

M257

Murakami et al. (1991)

Nasdala et al. (2004)

Holland (1964)

(a)

(b)

0 1 2 3 4 5 6

10

0

20

30

7

4. 8

4. 6

4. 4

4. 2



Zhang et al. (2000)

M257

Murakami et al. (1991)

(c)

(d)

Figure 3. Unit-cell parameters (a, b), density (c), and width of the main Raman band at ~ 1000 cm-1 (d) of zircon

samples from Sri Lanka, plotted against the time-integrated alpha dose. Note that parameters of M257 plot well

within the “Sri Lankan” trends (visualised by dashed arrows).

Alpha fluence (x1018 g-1) Alpha fluence (x1018 g-1)

C o(Å

)

ν 3(S

iO4) F

WH

M (c

m-1

)D

ensi

ty (g

cm

-3)

Uni

t-ce

ll vo

lum

e (Å

3)

Table 3.(U+Th)/He age of zircon M257

Sample name 4He [pmol] U [ng] Th [ng] Age (Ma)

M257zA 6.01 ± 0.07 2.43 ± 0.05 0.639 ± 0.01 417 ± 9M257zB 16.3 ± 0.19 6.48 ± 0.12 1.71 ± 0.03 422 ± 9M257zC 20.1 ± 0.23 8.07 ± 0.15 2.11 ± 0.04 419 ± 9

Average age of three analyses: 419 ± 6 Ma (2s)

All uncertainties are 2s.

helium age of M257 excludes any thermal treat-ment of M257 after sampling.

Internal homogeneity

Internal homogeneity is a crucial quality parameterfor any potential SIMS reference zircon. Homogeneity(especially of U-Pb isotopic ratios and U concentration)is important first in view of the necessity that suggestedmean values for M257 must be representative for anysmall (~ 100 μm) chip that is included in a SIMSmount. Second, homogeneity in the U content is also ofadvantage insofar as lateral variations in U (and,hence, variations of the self-irradiation dose) must resultin heterogeneous metamictisation, which is connectedwith heterogeneous volume expansion. This processleads to the build-up of strain and eventual formationof fractures (Chakoumakos et al. 1987, Lee and Tromp1995), which then act as pathways for any secondaryalteration process. Even if no leaching of radiogenic Pb(affecting the U-Pb isotopic ratios) in micro-areas adja-cent to fractures has occurred, these areas need to beavoided because their sputter behaviour under the oxy-gen ion beam in the SIMS may differ from non-fractu-red parts of the material (especially if fractures are filledwith secondary minerals; compare Sláma et al. 2008).

The degree of chemical and structural homogenei-ty/heterogeneity of zircon M257 was studied using arange of imaging and micro-analytical methods. Theapplied techniques included transmitted light opticalmicroscopy, electron microprobe line-scanning, andback-scattered electrons (BSE) and CL imaging usingthe electron microprobe, hot-cathode CL imaging, andmultiple CL spectroscopy analyses, Raman microprobeline-scanning, and multiple laser-induced photolumi-nescence (PL) spectroscopy analyses. Multiple LA-ICP-MS analyses of isotopic ratios, and laser fluorinationfor analysis of oxygen isotope ratio are discussedseparately below. Homogeneity checks were done onthe 10 mm thin section (Figure 1b) and eight smallerchips between 1.5 mm (inset in Figure 2a) and 0.3 mmin size. These eight chips were derived from differentareas of the initial gemstone, in order to sample theentire specimen. In addition, a number of Raman mea-surements were done on each slice produced from theinitial gemstone, and especially on all unpreparedchips that were subjected to SHRIMP, TIMS, (U+Th)/He,and oxygen isotope analysis.

Inspection of the large thin section in cross-polari-sed, transmitted light (Figure 1b) revealed no signifi-cant internal variations of interference colours, which

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GEOSTANDARDS and



0 1 2 3 4 5 6

0.02

0.00

0.08

16

14

12

10

3 4(Si O ) mode

0.04

0.06

0.10

UO2

ThO 2

7 8

3.6

3.8

3.4

3.2

Sm ( G3+ 4 6

5/2 7/2H transition)

Figure 4. Results of micro-PL, Raman

microprobe and electron probe

microanalysis (from top to bottom) line scans

across 8 mm long traverses (same section as

shown in Figure 1b). Variations of observed

Raman band widths and concentrations of

actinide oxides are below the analytical

uncertainties (visualised by grey bars).

Lateral distance (mm)

Oxi

de

cont

ent (

% m

/m)

FWH

M (c

m-1

)In

t. (x

10

3cp

s)

2 5 5

GEOSTANDARDS and



Table 4.Results of TIMS U-Pb determinations of M257

Laboratory 207Pb/235U 2s (abs.) 206Pb/238U 2s (abs.) ρ (err. corr.) 207Pb*/206Pb* 2s (abs.)

University of California 0.7387 0.0011 0.09093 0.00014 0.95 0.05892 0.00004at Santa Barbara 0.7390 0.0011 0.09097 0.00014 0.95 0.05891 0.00003(CA-TIMS)* 0.7405 0.0011 0.09107 0.00014 0.95 0.05897 0.00003

0.7392 0.0011 0.09099 0.00014 0.95 0.05892 0.000030.7394 0.0011 0.09098 0.00014 0.95 0.05894 0.000030.7397 0.0011 0.09105 0.00014 0.95 0.05892 0.000030.7394 0.0011 0.09101 0.00014 0.95 0.05892 0.000030.7394 0.0011 0.09102 0.00014 0.95 0.05892 0.000030.7391 0.0011 0.09096 0.00014 0.95 0.05893 0.000030.7388 0.0011 0.09093 0.00014 0.95 0.05892 0.000030.7396 0.0011 0.09102 0.00014 0.95 0.05893 0.000030.7396 0.0011 0.09103 0.00014 0.95 0.05893 0.000030.7388 0.0011 0.09093 0.00014 0.95 0.05893 0.000030.7385 0.0011 0.09088 0.00014 0.95 0.05894 0.000030.7393 0.0011 0.09099 0.00014 0.95 0.05893 0.000030.7393 0.0011 0.09101 0.00014 0.95 0.05892 0.000030.7394 0.0011 0.09101 0.00014 0.95 0.05893 0.000030.7392 0.0011 0.09097 0.00014 0.95 0.05893 0.00003

CA-TIMS means 0.7393 - 0.09099 - - - -

Universität Gießen 0.7423 0.0019 0.09145 0.0002 0.84 0.05887 0.00008(ID-TIMS) 0.7395 0.0019 0.09111 0.0002 0.85 0.05887 0.00008

0.7407 0.0018 0.09124 0.0002 0.93 0.05888 0.000050.7384 0.0018 0.09095 0.0002 0.92 0.05888 0.00005

Universität Gießen 0.7376 0.0013 0.09084 0.00012 0.79 0.05889 0.00006(ID-TIMS) 0.7380 0.0012 0.09090 0.00011 0.75 0.05888 0.00006

0.7383 0.0011 0.09101 0.00012 0.85 0.05884 0.000050.7385 0.0010 0.09103 0.00011 0.83 0.05884 0.00004

University of Oslo 0.7425 0.0029 0.09134 0.00033 0.97 0.05895 0.00005(ID-TIMS) 0.7395 0.0029 0.09107 0.00033 0.97 0.05889 0.00005

0.7426 0.0029 0.09139 0.00032 0.97 0.05893 0.000050.7397 0.0029 0.09103 0.00033 0.97 0.05893 0.00005

University of Toronto 0.7383 0.0013 0.09088 0.00012 0.92 0.05892 0.00005(ID-TIMS) 0.7363 0.0016 0.09063 0.00016 0.89 0.05891 0.00006

0.7369 0.0017 0.09071 0.00018 0.93 0.05892 0.000050.7367 0.0014 0.09073 0.00013 0.86 0.05889 0.00006

ID-TIMS means 0.7391 - 0.09102 - - - -

206Pb/204Pb 2s (%) 208Pb*/206Pb* Total 204Pb Total Mass of U Th/U Pb[pg] common Pb fragment [μg g-1] [μg g-1]

[pg] [μg]

10460 1.9 0.0844 0.094 7.0 14 883 0.266 79.624450 1.9 0.0844 0.069 5.1 24 896 0.266 80.851813 2.0 0.0845 0.054 4.0 41 865 0.266 78.090090 1.8 0.0843 0.047 3.5 60 892 0.266 80.4112360 2.0 0.0844 0.034 2.5 58 835 0.266 75.2304878 2.1 0.0844 0.024 1.7 108 841 0.266 75.8198807 1.9 0.0844 0.025 1.9 71 891 0.266 80.3249377 2.0 0.0843 0.025 1.8 91 864 0.266 77.9378788 1.9 0.0843 0.020 1.5 116 820 0.266 73.9294118 2.0 0.0843 0.024 1.8 108 834 0.266 75.1180180 1.9 0.0844 0.036 2.7 100 824 0.266 74.268493 2.0 0.0843 0.077 5.7 80 833 0.266 75.1203666 2.0 0.0844 0.023 1.7 71 835 0.266 75.2364964 1.8 0.0844 0.020 1.5 111 838 0.266 75.5332226 2.0 0.0843 0.015 1.1 79 817 0.266 73.6294985 2.0 0.0843 0.022 1.6 99 832 0.265 75.0469484 1.8 0.0843 0.016 1.2 115 796 0.265 71.7123305 1.9 0.0843 0.048 3.5 91 817 0.265 73.6

Mean concordia age of eighteen steps: 561.4 ± 0.5 Ma (95% conf.)

are most sensitive to the structural state (Chakoumakoset al. 1987, Nasdala et al. 1998, Sláma et al. 2008).Note, however, that the transmitted light image shownin Figure 1b is affected by three minor artefacts. First,there is some slight “fading” of observed interferencecolours, which is due to slight variations in the sectionthickness (~ 47 μm near the centre and down to ~ 45μm in the upper left corner) across the ~ 10 mm sec-tion. This non-uniform thickness resulted from mechani-cal hand-polishing on cloth. Second, some weak (nearvertical) striation is due to the imperfect polish of thesection. Third, the sample was too large to be photo-graphed with one single shot even at the lowestmicroscope magnification; it was therefore producedby combining thirty-six single photomicrographs. Slightbrightness variations in single micrographs caused acheckerboard-like pattern. In spite of these artefacts,optical microscopy of the large thin-section indicatedthat zircon M257 is remarkably homogeneous; virtual-ly no lateral variations in optical properties werefound.

The same observations were made using all otherimaging techniques applied to this thin-section andother polished chips of zircon M257. All BSE and CLimages were virtually uniform in greyscale intensity(electron microprobe images) and colour (hot-cathodeCL; inset in Figure 2a). Contrast enhancement merelyresulted in the enhancement of artefacts (such ashigher BSE and CL signal at edges due to chargingeffects), neither internal growth nor other zoning wereobserved. Note that a portion of our CL images wereobtained at the Geowissenschaf t l i ches Zent rumGöttingen, by the same operator and with the samesystem under the same experimental conditions thatwere used to clearly reveal the zoning of the 91500zircon reference material, which is characterised byexceptionally small variations of trace elements ands t ruc tu ra l s ta te ( c f . Wiedenbeck e t a l . 2004) .Considering the high sensitivity of CL to minor sampleheterogeneity (e.g., Corfu et al. 2003), the absenceof any zoning in all of our CL images supports thehomogeneity of zircon M257.

2 5 6

GEOSTANDARDS and



Table 4 (continued).Results of TIMS U-Pb determinations of M257

206Pb/204Pb 2s (%) 208Pb*/206Pb* Total 204Pb Total Mass of U Th/U Pb[pg] common Pb fragment [μg g-1] [μg g-1]

[pg] [μg]

7487 0.58 0.0836 3.1 187.7 293 865 0.263 78.47488 0.57 0.0835 3.1 187.9 294 868 0.263 78.223918 0.61 0.0837 0.9 63.8 296 860 0.264 77.123916 0.59 0.0837 1.0 60.6 297 863 0.264 77.3

Mean concordia age of four analyses: 562.6 ± 0.6 Ma (95% conf.)

7632 0.47 0.0842 3.5 221.9 299 1001 0.265 90.17461 0.46 0.0840 3.6 227.8 301 996 0.265 89.69249 0.42 0.0843 2.9 182.7 299 996 0.265 89.79246 0.41 0.0842 2.8 183.9 302 992 0.265 89.4


385595 4.9 0.0836 0.082 6.1 (532)** 768 0.263 68.9227294 5.6 0.0834 0.139 10.2 (532)** 767 0.263 68.7309155 5.9 0.0835 0.103 7.6 (532)** 768 0.263 68.9346484 5.3 0.0835 0.092 6.7 (532)** 770 0.263 68.8


78726 1.2 0.0843 0.007 0.6 12 667 0.265 6092116 0.9 0.0842 0.017 1.2 34 578 0.265 5280269 1.0 0.0836 0.022 1.7 36 643 0.263 57128482 1.2 0.0837 0.011 0.8 28 640 0.263 57

Mean concordia age of four analyses: 560.4 ± 0.6 Ma (95% conf.)Mean concordia age of all sixteen analyses: 561.1 ± 0.7 Ma (95% conf.)

* Univ. of California: One 1437 mg chip was analysed; reported “mass of fragment” values are the amounts of zircon dissolved per step.Reported U and Pb [μg g-1] values were calculated from combined ID and ICP-AES results, for mass of zircon dissolved. Th/U mass ratios werecalculated from 208Pb*/206Pb* ratios. ** Univ. of Oslo: One 532 mg chip was dissolved (split after dissolution).

We have also applied line-scanning with tech-niques providing a lateral resolution in the micrometrerange. Representative results are shown in Figure 4.The observed lateral variations in electron microprobeand spectroscopic data do not exceed analyticaluncertainties. An apparent indication of heterogeneitywas again due to an artefact , comparable to the“fading” of interference colours in Figure 1b: The upperpart of Figure 4 shows the intensity of the 604 nm PLband (assigned to a Sm3+ emission; compare Figure2b) across an 80 mm traverse, obtained by automatedline-scanning. Due the thickness variation of the largesection (see above), measurements close to the rims ofthe section may have been taken slightly out of focus,which has resulted in some minor intensity loss. For ele-ments detectable by the electron microprobe we havecalculated a homogeneity index. The ratio of the stan-dard deviation of 76 single measurements divided bythe root mean square (RMS) of the uncertainties givenby counting statistics was close to unity for all elementsand did not exceed 1.3. Hence the large thin sectioncan be regarded as homogeneous within the repeata-bility of the used microprobe parameters.

There is, however, a possible indication of heteroge-neity. The U concentrations determined by individualTIMS analyses (Table 4) range from 578 to 1001 μg g-1.On the one hand, this might indicate notable sampleheterogeneity. On the other hand, such variation ofactinide concentrations corresponds to a variation ofself-irradiation doses between 1.18 and 2.04 x 1018

alpha-events per gram, which would be expected toresult in significantly different degrees of radiationdamage (as all parts of crystal M257 have experiencedthe same thermal, and hence the same annealing histo-ry). For instance, the above range of alpha fluencesshould correlate with ν3(SiO4) Raman band FWHMs inthe range 9–16 cm-1 (compare Figure 3d). This, howe-ver, was not observed: All zircon chips that were sent toTIMS laboratories yielded a ν3(SiO4) Raman bandFWHM of 11.7 ± 1.0 cm-1. We need to state that thereremains some disagreement among co-authors of thispaper on how to interpret the obvious contradiction bet-ween apparently heterogeneous elemental compositionand apparently homogeneous structural state of M257.

U-Pb age determination

Isotope dilution age determination

The uranium-lead age of zircon M257 was deter-mined by TIMS analysis. Analyses were conducted in

the geochronology laboratories of the University ofCalifornia at Santa Barbara, U.S.A. (eighteen plateausteps), the Universität Gießen, Germany (eight ana-lyses), the University of Oslo, Norway (four analyses),and the University of Toronto, Canada (four analyses).

Chemica l ab ras ion (CA) T IMS ana l y ses theUniversity of California were performed according tothe experimental procedure described by Mattinson(2005). The results are presented in Table 4 andFigure 5a. For conversion of isotopic ratios (determinedby means of al l four U-Pb analy t ical techniquesapplied in this study) into ages and preparation of theconcord ia p lo t s ( F igures 5a-d) , the 238U decayconstant of Jaffey et al. (1971; 1.55125 x 10-10) andthe revised 235U decay constant of Schoene et al.(2006; 9.8569 x 10-10) were used. Plotting and agecalculations were done using the Excel-based Isoplotprogram (Ludwig 2003). From eighteen CA-TIMS pla-teau s teps , a mean 206Pb/238U isotopic rat io of0.09099 ± 0.00003 (2s) was calculated, which corres-ponds to an age of 561.37 ± 0.19 Ma (uncertaintyquoted at the 95% conf idence level ) . The mean207Pb/235U age was calculated at 561.50 ± 0.15(95% confidence), which indicates that the U-Pb sys-tem of zircon M257 is concordant within uncertaintiesof the decay constants (compare Jaffey et al. 1971,Schoene et al . 2006) . The mean concordia age(Ludwig 1998) was determined at 561.4 ± 0.5 Ma(95% confidence), with a mean square weighteddeviate (MSWD) of concordance calculated at 0.043(Figure 5a). This MSWD value is very small, becauseuncertainties of decay constants were considered,which are quite large compared to the repeatability.Note that the above isotopic ratios are reported asmeasured (i .e., without Th fractionation correction),which is considered most appropriate for a U-Pb refe-rence material. Note, however, that determination of a“true geological” age for young zircon samples shouldinclude a correction for 230Th deficit during zircon crys-ta l l i sa t ion . Account ing for the cor rec t ion (whichassumes exclusion of 75 ± 25% of the equilibriumamount of 230Th) results in very minor changes in theca lcu la ted age va lues (e .g . , +0 .08 Ma fo r the206Pb/238U age).

For TIMS analyses in Toronto, zircon fragments werepre-treated to remove radiation-damaged zones thatmay have lost radiogenic Pb (Mundil et al. 2004,Mattinson 2005). Grains were placed in a muffle fur-nace at ~ 950 °C for 60 hours to partially anneal theradiation damage, followed by leaching in 50% HF in

2 5 7

GEOSTANDARDS and



Teflon® dissolution vessels at 200 °C for 17 hours.Each zircon fragment was cleaned with HNO3, H2Oand acetone, then weighed on a microbalance, andtransferred to a miniaturised Teflon bomb (Krogh1973). The weight results are estimated to be accuratewithin about ± 3-5% (note that these uncertainties onlyaffect U and Pb concentrations, not the isotopic ratiosand age). A 205Pb-235U spike was added to the Teflondissolution capsules during sample loading. Zirconwas dissolved using ~ 0.10 ml of concentrated HF acidand ~ 0.02 ml of 7 mol l-1 HNO3 at 200 °C for fivedays, and re-dissolved in ~ 0.15 ml of 3 mol l-1 HCl to

promote equilibration with the spike. Uranium andlead were isolated from the zircon solutions using 50μl anion exchange columns, deposited onto out-gas-sed rhenium filaments with silica gel (Gerstenbergerand Haase 1997), and analysed with a VG354 massspectrometer using a multi-Pb-dynamic program. Allcommon Pb was assigned to procedural Pb blank.Uranium was measured in static mode or by using theaxial Faraday or axial Daly collector in pulse countingmode. Dead time of the measuring system for Pb was22.8 ns and 20.8 ns for U. The mass discriminationcorrec t ion for the Daly detec tor was constant at

2 5 8

GEOSTANDARDS and



0.734 0.736 0.738 0.740 0.742 0.744 0.746

0.0904

0.0908

0.0912

0.0916

560

562

564

mean Concordia age: 561. 4 ± 0.5 Ma

(95% conf., n = 18)

MSWD (of concordance) = 0.043

probability (of concordance) = 0.84

(a)

0.734 0.736 0.738 0.740 0.742 0.744 0.746

0.0904

0.0908

0.0912

0.0916

560

562

564

mean Concordia age: 561. 1 ± 0 .7 Ma

(95% conf., n = 16)

MSWD (of concordance ) = 0.077

probability (of concordance) = 0.78

(b)

0.64 0.68 0.72 0.76 0.80 0.840.080

540

520

600

mean Concordia age: 561. 9 ± 1 .7 Ma

(95% conf., n = 80)

0.084

0.088

0.092

0.096

0.100

(c)

0.64 0.68 0.72 0.76 0.80 0.840.080

540

520

600

mean Concordia age: 566. 5 ± 1.6 Ma

(95% conf., n = 47)

0.084

0.088

0.092

0.096

0.100

(d)

Figure 5. Results of U-Pb analyses. Ellipses represent 2s uncertainties. Quoted uncertainties of mean ages include

uncertainties of decay constants. (a) Concordia pot of CA-TIMS data obtained at the University of California (eighteen

plateau steps). (b) Concordia plot of TIMS results obtained in Gießen, Oslo, and Toronto (four groups of four individual

analyses each). (c) Concordia plot of LA-ICP-MS data (eighty single analyses). (d) Concordia plot of SHRIMP measurements,

all calibrated vs. CZ3 with an assumed age of 564 Ma (four sessions with 10, 11, 15 and 11 analyses).

207Pb/235U

207Pb/235U 207Pb/235U

207Pb/235U

20

6Pb

/23

8U

20

6Pb

/23

8U

20

6Pb

/23

8U

20

6Pb

/23

8U

0.07%/amu. Amplifier gains and Daly characteristicswere monitored using the NIST SRM 982 Pb referencematerial. Thermal mass discrimination corrections were0.10%/amu.

The procedures in the other two TIMS laboratories(i.e., Gießen and Oslo) were broadly similar. One diffe-rence for the Oslo analyses is that just one fragmentwas used and that it was not treated by chemical ormechanical abrasion before dissolution. Details of theprocedure at the University of Oslo are described byCorfu (2004). At Universität Gießen the analyses weredone on pieces derived from two single fragments;details of the experimental procedure can be found inDoerr et al. (1998, 2002). The results of the TIMS ana-lyses in Toronto, Olso and Gießen, are listed in Table 4and shown in Figure 5b. For these sixteen ID-TIMSanalyses, a mean concordia age of 561.1 ± 0.7 Ma(95% confidence; MSWD = 0.077) was calculated.This age is within uncertainties identical to the aboveCA-TIMS result. Slight differences among laboratoriesare close to analytical uncertainties; they may reflectslight experimental differences (such as the spike used,dead time correction, etc.), which hopefully will be cali-brated in the framework of the EARTHTIME initiative (cf.http://www.earth-time.org/). It is clear, however, thatour characterisation plan would have been improvedby including a traceability material for each laboratoryto run in parallel.

Consider ing al l th i r ty- four T IMS analyses , theunweighted mean 206Pb/238U ratio for M257 is cal-culated at 0.09101 ± 0.00003 (2s), which correspondsto a 206Pb/238U age of 561.28 ± 0.26 Ma (MSWD =3.0), and the mean 207Pb/235U isotopic ratio is calcu-lated at 0.7392 ± 0.0003 (2s), which corresponds to207Pb/235U age of 561.35 ± 0.21 Ma (MSWD = 2.5;all ages quoted at the 95% confidence level). We sug-gest these isotopic ratios and ages to be used asrecommended values when M257 is used as referen-ce material in SIMS zircon dating.

LA-ICP-MS age determination

Uranium-lead dating of M257 was also underta-ken using the LA-ICP-MS technique. The purpose wasto systematically check for potential lateral variationsin isotopic ratios across the sample. Analyses wereca r r ied ou t a t GEUS , Copenhagen , u s ing aThermoFinnigan Element2 single-collector double focu-s ing magne t i c sec to r ICP-MS coup led to aNewWave/Merchantek UP213 laser ablation system. A

total of eighty single spot analyses (beam diameter 30μm) were made across the 10 mm thin section (Figure1b). During 30 s of ablation the laser drilled a pitapproximately 30 μm deep. Sample and referencesamples were held in a teardrop-shaped low-volumeablation cell specially developed for U-Pb-dating(Horstwood et al. 2003). The instrumental mass biason measured isotopic ratios was corrected by matrix-matched external calibration using the GJ-1 zircon(Jackson et al. 2004) that was analysed under exactlythe same conditions as the sample. For further detailsof the experimental procedure see Frei et al. (2006)and Frei and Gerdes (2008).

Results of the LA-ICP-MS analyses are summarisedin Figure 5c. Data for the eighty single analyses arenot presented in detail here; the data (Table S1) areavailable online (see link given at the end of thepaper). The calculated mean concordia age of 561.9± 1.7 Ma (95% confidence) compares well with theTIMS results. Results imply that zircon M257 does notshow internal variations in the U-Pb isotopic systemexceeding the comparably large uncertainties in indivi-dual spot analyses (i.e., about ± 20 Ma; Figure 5c andTable S1).

SHRIMP analysis

In order to check the performance of M257 for ionmicroprobe analysis, we undertook an intercomparisonof SHRIMP measurements of M257 and four other,well-characterised zircon references, namely, CZ3(Pidgeon et al . 1994), 91500 (Wiedenbeck et al .1995), Temora-2 (Black et al. 2004) and Plesovice(Slama et al. 2008). Measurements were undertakenin two sessions using the two SHRIMP II instruments ofthe John de Laeter Centre for Mass Spectrometry atCurtin University, Perth, Western Australia.

Instrumental conditions were described by Kennedyand de Laeter (1994) and de Laeter and Kennedy(1998), and analytical procedures for zircon datingare detailed in Claoué-Long et al. (1995) and Williams(1998). Typically, a 25-30 μm diameter elliptical spotwas used, with a mass-filtered (O2)- primary beam of~ 2-3 nA. Data for each spot were collected in sets ofsix or seven scans through the mass range of 196Zr2O+,204Pb+, background (204.1), 206Pb+, 207Pb+, 208Pb+,238U+, 248ThO+, and 254UO+. It has been proposedthat very low count rates on 204Pb suggest that non-radiogenic lead is surface-related (Kinny 1986), and inthis case Broken Hill lead composition should be used

2 5 9

GEOSTANDARDS and



for 204Pb correction. The correction for Pb/U fractiona-tion was done according to (Claoué-Long et al. 1995)

206Pb+/238U+ = a(238U16O+/238U+)b (2)

using the parameter values of Black et al. (2003). TheExcel-based program Squid (Ludwig 2002) was usedfor data processing. Results are shown in Table 5 andFigure 5d. Problems in the analysis of M257 (as forinstance irregular sputtering behaviour, high 204Pbcount rates, obvious U-Pb discordance) were notobserved. Thus, there appear to be no performanceproblems that would prevent the use of M257 as aSIMS zircon reference material.

During analysis we made a somewhat surprisingobservation. Using CZ3 as the primary reference mate-rial and assuming a concordant age of 564 Ma(Pidgeon et al. 1994), the calculated ages for M257as well as for the other three zircon references appearslightly too old. On the other hand, using M257 as theprimary reference material, all 206Pb/238U ages calcu-lated for CZ3 (Table 5) scatter around 558-559 Maand are thus about 5-6 Ma younger than the recom-mended age. Similar observations were made in twoearlier series of SHRIMP analyses, done on a differentsample mount in two different SHRIMP laboratoriesand using CZ3 as a reference material. These ana-lyses also yielded mean 206Pb/238U ages for M257,apparently too old, of 566.5 ± 3.5 Ma (95% confiden-ce uncertainty, ten analyses; Curtin University) and566.4 ± 3.7 Ma (eleven analyses; Beijing SHRIMPcenter, China) for zircon M257, however again basedon an age of 564 Ma for CZ3. In contrast, all SHRIMPages determined for M257 as well as for Plesovice,Temora-2, and 91500 appear to be correct withinanalytical uncertainties when calibrated against a refe-rence other than CZ3 (Table 5). These observations

suggest that the 206Pb/238U age of CZ3 might besomewhat younger than 564 Ma. These doubts seemto be reinforced by the initial TIMS results for CZ3obtained in the early 1990s at the Max Planck Institutfür Chemie, Mainz, Germany, and Curtin University,Perth (R.T. Pidgeon, personal communication). ElevenTIMS analyses of CZ3 yielded 207Pb/206Pb ages in therange 562.2-567.5 Ma, averaging at 564 Ma (thepublished and recommended value). In contrast, thecorresponding 206Pb/238U ages determined in theseeleven analyses exhibited a slightly larger scatter inthe range 553.0-564.0 Ma, with a mean at 561.5 Ma.This difference suggests that the U-Pb isotopic systemof CZ3 might be slightly discordant.

Oxygen isotope analysis

Four chips of M257 were analysed for oxygen iso-tope ratio at the University of Wisconsin, Madison,U.S.A. (Table 6). The samples were single pieces wei-ghing 0.94 to 1.24 mg. Samples were heated by IRlaser (λ = 10.6 μm) in the presence of BrF5. The evol-ved O2 gas was cryogenically purified, converted toCO2, and analysed by dual inlet gas source massspectrometer as described by Valley et al. (1995).Values of δ18O were normalised to the recommendedvalue of 5.80 for the garnet reference sample UWG-2(Valley et al. 1995), which was analysed four timesbefore, and four times after, analyses of M257 with anaverage correction of 0.37 ± 0.04‰. Four analyses ofthe reference zircon, KIM-5, also bracketed the M257analyses, and the same correction scheme yieldedδ18O = 4.96 ± 0.19‰, in excellent agreement with theaccepted value of 5.09 ± 0.06‰ (Valley 2003). Thesedata yield a value of δ18O = 13.93 ± 0.11‰ VSMOW(Vienna Standard Mean Ocean Water; 1s) for M257and suggest that the crystal may be homogeneous inoxygen isotope ratio at the mm-scale.

2 6 0

GEOSTANDARDS and



Table 5.Results of SHRIMP U-Pb analyses of M257 and other zircon reference materials

Sample Published No. of analyses Determined age* when calibrated versusage (Ma)

Plesovice Temora-2 M257 CZ3 91500

Plesovice 336.5 26 - 337.8 ± 1.8 337.6 ± 1.5 339.8 ± 1.0 335.5 ± 2.1

Temora-2 417 28 414.5 ± 2.4 - 414.9 ± 2.4 419.0 ± 2.4 416.0 ± 3.5

M257 561.3** 25 560.9 ± 3.1 562.7 ± 3.0 - 567.6 ± 3.1 563.5 ± 5.6

CZ3 564 28 557.8 ± 3.0 559.2 ± 3.5 558.3 ± 3.2 - 558.2 ± 4.8

91500 1064 16 1061.7 ± 7.5 1064.5 ± 7.5 1063.8 ± 8.6 1074.8 ± 7.6 -

* Calculated 206Pb/238U ages are weighted mean values. Uncertainties are quoted at the 95% confidence level; they include the uncertaintyof the reference analyses. ** 206Pb/238U TIMS age determined in the present study.

Genesis of M257

The primary origin of zircon M257 (which, like themajority of Sri Lankan gem zircon specimens, origina-ted from a placer deposit) is not restricted by zoning orradiogenic isotopes. On the one hand, M257 doesnot show growth zoning, which is typical of zircon thatgrew during high-grade metamorphism (Connelly2000), whereas primary magmatic zircon growth nor-mally results in the formation of sharp primary zoning,also referred to as oscillatory zoning (Hoskin 2000).On the other hand, we observed that the properties ofM257 (U-Pb isotopic composition, degree of radiationdamage) fall well into the general trends defined byother, zoned Sri Lankan zircon samples, whose zoningi s gene ra l l y as s igned to igneous g rowth (e .g . ,Chakoumakos e t a l . 1987, K röne r e t a l . 1987,Murakami et al. 1991). Homogeneity on a centimetre-scale may indicate that zircon M257 either had for-med under exceptionally uniform conditions or wasderived from one single, thick growth zone of a verylarge, zoned crystal.

The oxygen isotope composition of M257 providesnew information as to its genesis. The δ18O value of13.9‰ is higher than that of igneous zircons from over1200 rocks of all ages and differing composit ion(Valley et al. 2005). However, this value is similar tothat recently reported for CZ3, which is also from aplacer in Sri Lanka, by Cavosie et al . (2008). As

discussed by Cavosie et al., such high values stronglyargue for a metamorphic origin in a high δ18O proto-lith such as marble or calc-silicate skarn. It is likely thatboth M257 and CZ3 formed in this manner and thatthese conditions contributed to their unusual homoge-neity and their value as reference materials.

Conclusions

Our study has established that zircon M257 consti-tutes a suitable reference material for the ion micropro-be analysis of unknown zircon samples. Advantageousfeatures of M257 include homogeneity and concor-dance of the U-Pb isotopic system, a very low level ofnon-radiogenic Pb, as well as relatively high uraniumand lead concentrations, resulting in high count ratesand good counting statistics.

Zircon M257 shows a remarkably low degree ofinternal heterogeneity. Imaging techniques (BSE, CL)did not reveal any internal textural features. Variationsin chemical composition as determined by multiplemicroprobe (electron probe, LA-ICP-MS) analyses didnot exceed exper imental uncer taint ies . The onlyconcern about potential heterogeneity arises fromvariations in actinide concentrations as determined byTIMS. Also, micro-analyses of the structural state didnot reveal any significant heterogeneity within, andvariations among, zircon chips. There were no detectableinternal variations of optical properties, luminescence

2 6 1

GEOSTANDARDS and



Table 6.Oxygen isotope analyses by laser fluorination

Analysis No. Sample/reference material name Material analysed Mass δ18O[mg] [‰ VSMOW]

1 UWG-2 Garnet reference 0.66 5.712 UWG-2 Garnet reference 0.51 5.783 UWG-2 Garnet reference 0.50 5.834 UWG-2 Garnet reference 0.69 5.795 KIM-5 Zircon reference 1.35 5.196 KIM-5 Zircon reference 1.13 4.997* M257 Zircon 1.21 13.778* M257 Zircon 0.95 14.009* M257 Zircon 1.24 13.9410* M257 Zircon 0.94 14.00

M257 average: 13.93 ± 0.11 (1s)

11 KIM-5 Zircon reference 1.11 4.9412 KIM-5 Zircon reference 0.96 4.9513 UWG-2 Garnet reference 0.67 5.8414 UWG-2 Garnet reference 0.76 5.8515 UWG-2 Garnet reference 0.72 5.8116 UWG-2 Garnet reference 0.59 5.80

* M257 data are corrected to the eight UWG-2 reference analyses.

emissions, and micro-Raman parameters. It may beconcluded that M257 is remarkably homogeneousand may therefore serve as future reference materialfor ion microprobe zircon analysis. Mean values deter-mined in this study are valid for any random chip tobe used as a reference sample.

Alteration textures or features pointing to low-tem-perature alteration have not been found. The degreeof accumulation of radiation damage and retention ofradiogenic Pb and He corresponds to other Sri Lankanzircon samples, which suggests that M257 has notexperienced any unusual thermal history. Any seconda-ry ( thermal and/or chemical) overprint , includingsample treatment to enhance the colour, can thereforebe excluded.

Its homogeneity may also make M257 suitable asa reference material for the micrometre-scale determi-nation of trace actinides, for instance in connectionwith laser microprobe (U+Th)/He dating (Boyce et al.2006). However, such applications must remain limitedas the material from M257 will predominantly be usedin ion microprobe laboratories. We have decided notto provide any material to LA-ICP-MS laboratories butrather to reserve it for SIMS analysis. The reason for thisdecision is that even though M257 was a comparablylarge gemstone with a weight of 25.7 ct [larger thanCZ3, which weighed 4.5 ct (Pidgeon 1997) and haslasted for more than ten years], the consumption needsto be kept low to ensure that material will be availablefor at least one decade.

Zircon M257 will in future be used, and distribu-ted , by SHR IMP labora to r ie s i n Pe r th ( con tac t :[email protected]), Beijing (contact:[email protected]), and St. Petersburg, Russia(Cen t re o f I so top ic Research o f the A l l -Russ ianGeologica l Research Ins t i tu te , VSEGEI ; con tac t :[email protected]). A total of about 3.4 g ofmaterial has been passed to these three laboratories(with the largest fraction, about 2 g, being depositedin Beijing). Another large, single chip 0.4 g in weight iskept in the gemstone collect ion of the Inst i tut fürEde l s te in fo r schung Ida r-Ober s te in und Mainz ,Germany. The remaining amount of about 1.1 g willbe kept, and made available on request, by the leadauthor of this paper. Consequently, about 250 mghave been consumed for the analytical characterisa-tion documented in this study whereas there is still anamount of about 4.9 g of unused material. In addition,epoxy mounts used for SHRIMP, electron microprobe,

and spectroscopic analyses, will also be available forfurther analysis. The large thin section that was usedfor homogeneity checks (Figure 1b) was made avai-lable as natural U reference for (U+Th)/He dating, cur-rently done at the Department of Earth and SpaceSciences, University of California, Los Angeles, U.S.A.(contact: [email protected]).

Future SIMS analysis will further evaluate the varia-bility of δ18O in M257 and if, as seems likely, it proveshomogeneous in oxygen isotope ratio, M257 mightalso be a useful reference material in ion microprobestudies that correlate age and δ18O (i.e., Valley et al.2005, Cavosie et al. 2005).

Acknowledgements

Zircon M257 was made available by the Institutfür Edelsteinforschung Idar-Oberstein und Mainz,Germany. Sample prepara t ion was done by A .Wagner. Mike Spicuzza assisted in oxygen isotopeanalysis at the University of Wisconsin. Thanks are dueto Robert T. Pidgeon for providing original TIMS analy-sis data for CZ3, to C.L. Lengauer for help with therefinement of X-ray data, and to J.M. Hanchar for pro-viding synthetic zircon, hafnon, thorite and uraniumoxide crystals as electron microprobe calibrants. Weare grateful to D.W. Davis and two anonymous revie-wers for helpful comments on the manuscript . Thispaper i s pub l i shed w i th the pe rm i s s ion o f t heGeological Survey o f Denmark and Greenland .Financial support was provided by the ExcellenceCluster “Geocycles” funded by the state of Rheinland-Pfalz, Germany. This paper is Geocycles publicationnumber 466. Partial funding of this research was provi-ded by the European Commission through contract no.MEXC-CT-2005-024878 to L.N., by the FWF AustrianScience Fund through grant P20028-N10 to L.N., andby the U .S . Na t iona l Sc ience Founda t ion , EAR0549672 to J.M.M.

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Supporting information

The following supporting information is availableonline:

Table S1. Laser ablat ion U-Pb data for zirconM257 (Geological Survey of Denmark and Greenland,Copenhagen)

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http://www.blackwell-synergy.com/doi/abs/10.1111/j.1751-908X.914.x(this link will take you to the article abstract).

Please note: Blackwell Publishing are not respon-sible for the content or functionality of any supportinginformation supplied by the authors. Any queries (otherthan missing material) should be directed to the cor-responding author of this article.

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Documents

Zircon M257 - a Homogeneous Natural Reference Material for the Ion Microprobe U-Pb Analysis of Zircon