Boron and Oxygen Isotope Composition of Certified Reference Materials NIST SRM 610/612 and Reference...

GEOSTANDARDSNEWSLETTERThe Journal of Geostandards and Geoanalysis

Boron and Oxygen Isotope Composition of Certified Reference Materials NIST SRM 610/612 and Reference Materials JB-2 and JR-2

Vol. 25 — N° 2-3 p . 4 0 5 - 4 1 6

We present data on the concentration, the isotopecomposition and the homogeneity of boron in NIST silicate glass reference materials SRM 610 and SRM612, and in powders and glasses of geological reference materials JB-2 (basalt) and JR-2 (rhyolite).Our data are intended to serve as references for bothmicroanalytical and wet-chemical techniques. The δ11Bcompositions determined by N-TIMS and P-TIMS agreewithin 0.5‰ and compare with SIMS data within2.5‰. SIMS profiles demonstrate boron isotope homogeneity to better than δ11B = 2‰ for both NISTglasses, however a slight boron depletion was detectedtowards the outermost 200 µm of the rim of eachsample wafer. The boron isotope compositions of SRM610 and SRM 612 were indistinguishable. Glassesproduced in this study by fusing JB-2 and JR-2 powderalso showed good boron isotope homogeneity, bothwithin and between different glass fragments. Theirmajor element abundance as well as boron isotope compositions and concentrations were identical tothose of the starting composition. Hence, referencematerials (glasses) for the in situ measurement of boronisotopes can be produced from already well-studiedvolcanic samples without significant isotope fractionation.Oxygen isotope ratios, both within and between wafers,of NIST reference glasses SRM 610 and SRM 612 areuniform. In contrast to boron, significant differences inoxygen isotope compositions were found between thetwo glasses, which may be due to the different amountsof trace element oxides added at ten-fold differentconcentration levels to the silicate matrix.

Keywords: reference material, in situ analysis, boron isotopes, oxygen isotopes, NIST SRM 610, NIST SRM 612, JB-2, JR-2.

Nous présentons des données de concentration, decomposition isotopique et d’homogénéité du bore dansles matériaux de référence suivants - verres silicatésNIST SRM 610 et SRM 612, poudres et verres debasaltes JB-2 et rhyolite JR-2. Nos données pourraientservir de référence pour des microanalyses et pour destechniques par voie humide. Les compositions isotopiquesde δ11B déterminées par N-TIMS et P-TIMS sont enaccord entre elles (0.5‰) et avec les données SIMS(2.5‰). Les profils SIMS mettent en évidence unehomogénéité du bore de δ11B = 2‰ pour tous les verresNIST, même si un léger appauvrissement en bore a étédétecté dans les 200 µm de la bordure externe dechaque pastille. Il n’y a pas de différence de compositionisotopique du bore entre SRM 610 et SRM 612. Lesverres produits dans cette étude par fusion de poudresJB-2 et JR-2 montrent également une bonne homogénéitéisotopique du bore à la fois dans et entre les fragmentsde verre. La teneur en éléments majeurs ainsi que lacomposition isotopique du bore est équivalente à celledes échantillons de départ. Ainsi, des matériaux deréférence (verres) pour la mesure in situ des isotopes dubore peuvent être produits sans fractionnement isotopique significatif à partir d’échantillons volcaniquesbien étudiés. Les rapports isotopiques d’oxygène, danset entre les pastilles des verres NIST SRM 610 et SRM612, sont uniformes. A la différence du bore, des différences significatives de la composition isotopiqued’oxygène ont été trouvées entre les deux verres, etpeuvent être dues aux différentes teneurs en oxydesd’éléments en traces ajoutés à des concentrations différentes (x 10) de celle de la matrice silicatée.

Mots-clés : matériau de référence, analyse in situ, isotopes du bore, isotopes de l’oxygène, NIST SRM 610, NIST SRM 612, JB-2, JR-2.

4 0 5

1201

Simone Kasemann (1)§, Anette Meixner (1), Alexander Rocholl (1), Torsten Vennemann (2), Martin Rosner (1), Axel K. Schmitt (1) and Michael Wiedenbeck (1)*

(1) GeoForschungsZentrum Potsdam, Telegrafenberg 1, D-14473 Potsdam(2) Institute for Geochemistry, University of Tübingen, Wilhelmstrasse 56, D-72076 Tübingen§ Present address: European Commission Joint Research Centre (IRMM), 2440 Geel, Belgium* Corresponding author. e-mail: [email protected]

Received 22 Nov 00 — Accepted 30 Apr 01

The rapid growth in the application of microana-lytical in situ techniques such as secondary ion massspectrometry (SIMS) and laser ablation inductively cou-pled plasma-mass spectrometry (LA-ICP-MS) has led toan increased demand for suitable reference materials.The basic requirements for microanalytical referencematerials include (a) a well-characterized chemicalcomposition, (b) documented homogeneity and (c) suffi-cient quantity to assure their continuing availability tomicroprobe laboratories. While much effort has beeninvested in recent years towards establishing referencematerials for the in situ analysis of trace elements (e.g.NBS 1970, Jochum et al. 2000), little attention has beendevoted towards characterizing such samples for isotopesystems of geochemical interest. Here we present newdata on the abundance, isotope composition andhomogeneity for boron and oxygen in NIST certifiedreference materials SRM 610 and SRM 612 determinedby spectroscopic and mass spectrometric methods.

Additionally, we have melted sub-gramme sizedcharges of rock powders of geochemical referencematerials JB-2 (basalt) and JR-2 (rhyolite) from theGeological Survey of Japan. The two resulting glasssamples, JB-2G and JR-2G, were studied with respectto their boron concentration and isotope compo-sition. These glasses are valuable for microanalyticalcalibration purposes of basaltic and rhyolitic samples,but also to investigate possible matrix effects duringSIMS analysis.

With respect to wet-chemical methods of boronisotope determination (TIMS), Tonarini et al. (1997)pointed out the need for establishing internationalre fe rence mate r ia l s , i n o rde r to eva lua te bo thanalytical uncertainty and interlaboratory bias. Ourboron data on the artificial and natural referencematerials contribute towards this goal. Here we presentadditional boron isotope data that complement theprevious data set on JB-2 (Nakamura et al. 1992,Tonarini et al. 1997, Kasemann et al. 2000) and alsoprovide the first data on JR-2.

Samples

NIST certified reference materials SRM 610 andSRM 612 are synthesised Si-Na-Ca-Al-oxide glassesthat were produced in kilogramme quantities for theU.S. National Institute of Standards and Technology(NIST; formerly National Bureau of Standards, NBS).The glasses were produced from a single “pure” sup-port matrix and doped with sixty one trace elements

at nominal concentrations of 500 (SRM 610) and 50µg g-1 (SRM 612), respect ively (NBS 1970, Kane1998). These glasses are widely distributed and aresti l l commercially available. They have been wellcharacterized with respect to their trace element com-position (Pearce et al. 1997, Rocholl et al. 1997, 2000)and homogeneity (Hinton et al. 1995, Hinton 1999,Rocholl et al. 2000). While data on the composition ofa few radiogenic isotopes exist for these glasses(Woodhead and Hergt 2000), until now no data onstable isotopes have been reported.

Basalt JB-2 and rhyolite JR-2 are natural geolo-gical reference materials supplied by the GeologicalSurvey of Japan as powdered samples. Both powdersare well characterized with respect to their major andtrace element composition (e.g. Govindaraju 1994,Imai et al. 1995, GSJ 2000). In addition, the boronisotope composition of JB-2 has been determined bythree different laboratories (Nakamura et al. 1992,Tonarini et al. 1997, Kasemann et al. 2000), showingno significant bias between the different laboratories.

We produced JB-2G and JR-2G glass chargesfrom rock powders in a resistance strip heater appa-ra tus a t the Ins t i tu t fü r P laneto logie , Univers i tä tMünster, following a procedure described by Brown(1977). Briefly, about 40 mg of rock powder wasmelted in an argon atmosphere at 0.2 MPa on a tan-talum strip filament. In a first step, the sample washeated at ~ 1000 °C for 15 s (to drive off gasses andto avoid boil over) and then, in a second step, com-pletely melted at > 1800 °C. Typically, run times of 40s resulted in complete fusion and homogenisation ofthe melt. Quenching was performed by rapidly reducingthe applied current with simultaneous argon flushing.In some cases, this fusion procedure resulted in loss ofiron from the melt to the tantalum filament within anarrow zone of about 250 µm along the contact.Note, however, that electron and ion probe micro-analysis showed that the inner parts of the glassf ragments were homogeneous with respect to bothmajor element and boron isotopic composition (seesection “micro-beam homogeneity tests”).

Analytical methods

Sample dissolution

Boron was chemically separated from its silicatematrix by alkaline fusion according to a method modi-fied after Owens et al. (1982). The advantage of this

4 0 6


method includes a relatively simple chemical dissolu-tion procedure with a low degree of loss or fractiona-tion of boron, even at low concentration levels. Thesample powder (≤ 0.5 g) was mixed with K2CO3

(ratio 1:4) in a platinum crucible, covered with a lidand fused in a muff le furnace at 950 °C for 30minutes. After cooling, the fusion cake was dissolvedin suprapure H2O at room temperature and storedovernight at 50 °C in the oven. After cooling to roomtemperature, the sample solution was transferred intopolypropylene cups, covered and heated to 105 °Cfor three hours. Possible precipitates were removedfrom the cooled solution by filtering and re-washingwith suprapure H2O. The filtrate was neutralized byultra-clean HClO4 (70% m/m), mixed with mannitolsolution (1 µg B : 20 µg mannitol) and left overnightto allow for degassing. The KClO4 precipitate wasremoved by filtering and the filtrate was diluted to100 ml in ultrapure water. Twenty ml of the samplesolut ion was then loaded on a cat ion-exchangecolumn (30 ml Bio-Rad columns filled with 6 ml AG50W-X8). Boron was eluted with ultrapure H2O (10ml) and collected in a polyethylene flask. Quantitativeboron retention was monitored by replicate analysesof reference materials passing through the entire che-mical procedure and was estimated to be ca. 95%.

Total boron blanks for this entire procedure weretypically less than 200 ng. Following the reasoning ofTonarini et al. (1997), the minimum amount of sampleboron (δ11B ≈ ± 0‰) necessary to maintain a negli-gible blank contribution (δ11B ≈ ± 10‰) was calcula-ted to be about 10 µg. Hence, for a 0.5 g sample,blank contributions appear insignif icant at boronconcentrations ≥ 20 µg g-1. This condition was fulfilledby all samples analysed in this study.

Inductively coupled plasma-atomic emission spectroscopy (ICP-AES)

Subsequent to the chemical separation proceduredescribed above, boron concentrations were deter-mined by ICP-AES at the GeoForschungsZentrumPotsdam, using a VARIAN Liberty 200 instrument .Calibration was carried out by means of a HCl stan-dard solution (0.05 mol l-1) doped with 1 µg g-1 eachof B and Fe. All boron determinations were performedat emission line 249.773 nm. Possible Fe interferenceswere monitored at emission line 238.204 nm.

Replicate analyses of single sample solutions showedan external precision in measurements of better than 1%.

Overall, the reproducibility of the total procedure, inclu-ding sample digestion and chemical separation, is betterthan 5% (1s) for samples containing > 30 µg g-1 B andabout 5-10% for samples containing < 30 µg g-1 B.Based on the comparison with various GSJ referencematerials, our overall accuracy is estimated to better than10% and, in the case of JB-2, better than 1%.

Positive thermal ionisation mass spectrometry (P-TIMS)

For P-TIMS analysis, the sample dissolution wasbasically identical to the procedure described above. Itdiffered in that the boron extraction started with thealkaline filtrate being processed by a three step (anion-cation-anion) ion-exchange separation as described inTonarini et al. (1997). To avoid boron loss during theseparation process, the sample was mixed with manni-tol solution (1 µg B : 40 µg mannitol). The formation ofCs2BO2

+ complexes (the boron species measured byTIMS) was forced by adding Cs2CO3 solution to theboron sample (1 mol B : 2 mol Cs) and evaporatingthe mixture to dryness. Isotope analyses were carriedout on a static multi-collector Finnigan MAT 262 massspectrometer using a double collector geometry, follo-wing the method reported in Nakano and Nakamura(1998). Together with graphite (40 µg) the sample, dis-solved in HCl (6 mol l-1) and containing > 250 ng totalboron, was loaded on a previously degassed singletantalum filament. The sample was first loaded on thefilament before complete dryness of the graphite, thendried at 1.0 A and then heated at 1.8 A for 10 s. Theboron was detected as Cs2BO2

+ complexes at masses308 and 309. Running temperatures were < 900 °C;ion beam intensities on mass 309 varied between 1and 3 pA. Under these conditions, the experimentallydetermined mean fractionation correction factor wasca. 0.9973. The raw data were corrected to the valuesof a NIST SRM 951 standard solution, which was runconcurrently. In all cases, the correction was small andvaried between 0.001 and 0.1‰. Typically, isotoperatios were recorded for at least half an hour in analyti-cal blocks of twenty scans each. Recorded data wereautomatically corrected for isobaric interference fromCs2

10B16O17O+ on mass 309.

The instrumental precision was evaluated by analy-sis of the boric acid reference material NIST SRM 951(formerly NBS SRM 951). Table 1 shows that P-TIMSmeasurements of the 11B/10B ratio, carried out over aperiod of six months, replicate within 0.02% (onerelative standard deviation, RSD). (For comparison, the


4 0 7

precision of single analyses is about one order ofmagnitude better and varies between 1s (mean) =0.002 and 0.009%). The measurements did not revealany correlation between the isotopic composition mea-sured and the amount of boron loaded (250, 500,1000 ng). The average 11B/10B ratio was 4.05451 ±0.00071 (1s) and differs significantly from the certifiedvalue (4.04362 ± 0.00069; Catanzaro et al. 1970).Similar differences from the certified value have alsobeen reported by Nakamura et al. (1992) and Nakanoand Nakamura (1998).

Negative thermal ionisationmass spectrometry (N-TIMS)

For N-TIMS analysis, chemical extraction of boronwas identical to the fusion/ion-exchange proceduredescribed above. Isotope analysis was performed on astatic multicollector VG SECTOR 54-30 mass spectro-meter, following the method described by Hemmingand Hanson (1994). Briefly, 3 - 25 ng of chemicallyextracted boron, together with 0.5 µl of boron-freeocean water serving as the emitter substance, was loa-ded on a degassed Re single filament and evapora-ted at 0.7 A to complete dryness and then heated at1.2 A for 30 s. Analysis was carried out at filamenttemperatures of about 950 to 1000 °C. At these tem-peratures the experimentally determined fractionationcoefficient was about 1.0070. Boron isotopes wereregistered as BO2

- complexes on masses 42 and 43.Ion beam intensities on mass 43 were typically ~ 10pA; ion counting was started at intensities of about 2pA. A complete measurement of a single sample typi-cally comprised 80 blocks of 12 cycles each, takingabout four hours of data acquisition.

All analyses, including those of the chemicallyunprocessed standard solution of NIST SRM 951, weresubject to both isotopic fractionation and, starting atthe beginning of each measurement, a significantCNO- interference on mass 42, which has also beenreported by Hemming and Hanson (1994) and Klötzli

(1992). To suppress the fractionation to an insignificantlevel, Hemming and Hanson (1994) chose sea wateras the matrix solution. These authors also reported thatthe CNO- interference may be monitored by CN- atmass 26, but suggested that any run displaying suchan interference should be discarded. We were unableto monitor precisely and correct for the CNO- interfe-rence by means of CN-, because the rate of decreasediffered distinctly between both molecules, with the lat-ter decaying faster. To correct for this interference andfractionation, we have, therefore, chosen an alternativeapproach by applying an extrapolation technique. Thismethod is depicted in Figure 1 and involves a linearregression correction to time zero in order to obtain theinitial 11B/10B ratio at the start of each analytical run.This approach a assumes Rayleigh-type, steady statefractionation and takes advantage of the fact that theCNO- interference decreases exponentially within oneto two hours to a negligible level. Accordingly, thelinear regression involves only values obtained fromCNO- free scans. In detail, the average 43/42 ratiosof each block were plotted against the cumulativeelectrical charge. The interference was then eliminated

4 0 8


Table 1.Boron isotope composition of reference material NIST SRM 951 (formerly NBS SRM 951)

Sample Method 11B/10B 1s 1 RSD (%) Range N

NIST 951 P-TIMS 4.0545 0.0007 0.02 4.0535-4.0562 24

NIST 951 N-TIMS 4.0157 0.0026 0.06 4.0115-4.0210 39

Data were obtained by P-TIMS and N-TIMS over 6 and 23 months, respectively.s standard deviation. RSD relative standard deviation. N number of analyses.

4.04

4.02

4.00

3.98

3.96

0 100 200 300 400 500 600

�

�

�

CNO interference absent-

�

CNO interference present-

Regression line

Figure 1. Linear regression correction to derive the initial11B/10B ratio at the beginning of an analytical run: the

fractionation curve is extrapolated to time zero (electrical

charge = 0 As). Sample NIST SRM 951.

Cumulative electrical charge (nAs)

11B/

10B

from the boron signal by extrapolating the linear (i.e.interference-free) part of the curve to the y-axis. Thedata obtained in such a way were finally normalizedto the concurrently analysed NIST SRM 951 standard.

The in-run precision of the method was determinedby replicate analysis of a standard solution NIST SRM951 (Table 1). N-TIMS analyses, carried out over aperiod of nearly two years, yield an average 11B/10Bratio of 4.0157 ± 0.0026 (1s) and replicate within0.06% (1 RSD), indicating a slightly higher analyticaluncertainty as compared to P-TIMS. Note that theapplied extrapolation technique does not give anerror for a single analysis. Instead, the in-run precisionfor rock samples analysed is estimated on the basisof replicate analyses of NIST SRM 951 to be about0.06% (1 RSD).

The possible effects of sample dissolution, chemicalextraction of boron and mass spectrometric analysis onthe isotope composition have been tested by compa-ring results for chemically processed and unprocessedNIST SRM 951 solutions. It was found that the isotopecompositions were identical within analytical uncertainty.Hence, possible isotope fractionation originating fromthe chemical treatment and the associated blankcontribution appears to be of low significance. This isalso indicated by the close agreement of our P-TIMSand N-TIMS results (see discussion).

Microanalytical methods

All four glasses were analysed to assess homoge-neity of both boron concentration and isotope compo-sition, using the Cameca ims 6f ion microprobe at theGeoForschungsZentrum Potsdam. The polished, clea-ned and gold-coated samples were sputtered with a12.5 kV, 40 nA 16O- primary beam focused to a 20-25µm spot size. Positive secondary ions (10B+, 11B+, 28Si+)were accelerated to 10 keV and no energy filteringwas applied. Operating at a mass resolution of ~

1600 was sufficient to eliminate interferences from9Be1H+ and 10B1H+. The image field was 150 µm. Thebest analytical precision was obtained using an energywindow of 50 eV and the 150 µm and 1800 µmcontrast and field apertures, respectively. Prior to eachanalysis, the samples were pre-sputtered for 5 min toremove surface contamination. The B-isotope ratioswere measured in 80 cycles, each cycle consisting of10 and 20 s count times on 11B+ and 10B+, respectively.The total analysis time was ca. 55 min and the in-runprecision (1s) of a single analysis was typically ≤

1‰ at high B concentrations (> 100 µg g-1) and ≤2‰ at low concentrations (~ 30 µg g-1). Measuredboron concentration and isotope ratios were normali-zed to reference material NIST SRM 610 by applyingthe respective values obtained by TIMS in the courseof this study.

Major element compositions and homogeneity ofthe prepared JB-2G and JR-2G glasses were determi-ned using a wavelength dispersive 4-spectrometerCAMECA SX-100 microprobe. Elements were measu-red for 20 s each applying an acceleration voltage of15 kV, a probe current of 20 nA and a beam diameterof 10 µm. Analyses were performed as profiles acrossthe glass fragments, corresponding to the ion micro-probe profiles. Calibration was achieved using naturaland synthetic minerals and oxides (for details seeJochum et al. 2000). The overall analytical uncertaintywas estimated to be < 2% relative, for elements atoxide concentrations > 3% m/m.

Oxygen isotope analyses

The oxygen isotope composition (16O, 17O, 18O) ofthe NIST SRM 610 and SRM 612 glasses were mea-sured at the University of Tübingen, using a methodsimilar to that described by Rumble and Hoering (1994),where a CO2- laser was used to heat indiv idualsample chips (here ca. 0.5-2 mg) supported on a Pt-sample holder in a F2 atmosphere. The extracted O2

was co l lec ted on a molecu la r s ie ve (13X) andanalysed as O2 on an isotope ratio mass spectrometer(Finnigan MAT 252). We analysed three dif ferentwafers of SRM 610 and SRM 612, each providedby the GeoForschungsZentrum Potsdam and theUniversities of Heidelberg and Karlsruhe. Separateanalyses were performed on four individual chips fromeach wafer. No analyses were conducted on eitherJB-2G or JR-2G.

Results

Boron: concentration, isotope composition and analytical uncertainty

Boron concentration data obtained by ICP-AES arelisted in Table 2. The long-term external reproducibility(including sample digestion and chemical separation)was ca. 5% (1 RSD) for samples with boron concentrations> 30 µg g-1. For example, twenty boron concentrationanalyses of JB-2 carried out over the course of threeyears replicate to within 5.4% (1 RSD) and JR-2 data


4 0 9

replicate to within 4.4%. Our results are in goodagreement wi th publ i shed re ference values . Forexample, the concentration obtained for JB-2 (30.4 µgg-1) is identical to that proposed by the GeologicalSurvey of Japan (30.2 µg g-1; GSJ 2000), and theconcentrations of both NIST glasses are identical within2% to the “preferred averages” of Pearce et al. (1997).Within 10%, the boron concentration obtained for JR-2(159 ± 7 µg g-1) overlaps with the respective referencevalue (145 µg g-1; GSJ 2000). Given a possibleuncertainty of a few percent for the reference valueand taking into account possible heterogeneities forthis rhyolitic rock, both data sets agree well. With respectto JR-2G, only a single concentration determinationhas been carr ied out . Al though the dif ference inconcentration relative to JR-2 is nearly 10%, the valuesoverlap within errors.

Table 3 presents the average boron isotope com-position of the reference materials investigated. Theisotope data are listed as δ11B values, i.e. normalizedto the concurrently analysed reference material NISTSRM 951. The table also provides an estimate of theanalytical uncertainty, which is reflected by both the

precision and the range of the data. Typically, the“external” precision indicating the scatter betweendata from independent chemical dissolution/separa-tion aliquots is better than 0.8‰ (1s), the exceptionbeing the two N-TIMS data of JR-2 (1.3‰). Note thatthe precision obtained for the concurrently analysedNIST SRM 951 is more than one order of magnitudebetter and has, therefore, only a small additional effecton the overall uncertainty of the data. The externalprecision is controlled by a combination of various che-mical, mass spectrometrical and instrumental effectsand comparable for all samples analysed, regardlessof their boron content. The “internal”, or in-run precisionof a single analysis is better than or similar to theexternal precision (not shown). For example, the in-runprecision of P-TIMS data ranges between 0.04 and0.25‰ (1s mean) for SRM 610/612 and between0.05 and 0.62‰ for JB-2 and JR-2/JR-2G. Based onthe range of both the external and internal precision,we estimate the overall analytical uncertainty for ourmean values at 1s = 0.8‰.

P-T IMS (δ11B = -0 .78‰) and N-T IMS (δ11B =-1.31‰) data of NIST SRM 610 are identical within

4 1 0


Table 2.Boron concentration in reference materials analysed

Sample B (µg g-1) 1s 1 RSD (%) Range N

JB-2 30.4 1.6 5.4 27-33 20

JR-2 158 7 4.4 152-169 5

JR-2G 174 - - - 1

NIST SRM 610 357 - - - 1

NIST SRM 612 35 - - - 1

Data were obtained by ICP-AES over a period of three years.Abbreviations as in Table 1.

Table 3.Boron isotope composition of reference materials analysed as determined by TIMS

Sample Method δ11B (‰) 1s Range N

NIST SRM 610 P-TIMS -0.78 0.70 -2.02 - +0.04 8

NIST SRM 610 N-TIMS -1.31 0.68 -2.12 - -0.37 6

NIST SRM 612 P-TIMS -1.07 0.85 -2.00 - +0.14 7

JB-2 P-TIMS +7.14 0.77 +5.97 - +8.24 9

JB-2 N-TIMS +7.12 0.34 +6.66 - +7.47 4

JR-2 P-TIMS +2.91 0.48 +2.28 - +3.52 7

JR-2 N-TIMS +2.51 1.30 +1.49 - +3.42 2

JR-2G P-TIMS +2.57 0.53 +1.92 - +3.49 12

Abbreviations as in Table 1.

0.5‰ and, within error, are indistinguishable from SRM612 (δ11B = -1.07‰). Hence, the δ11B values of bothNIST glasses are somewhat lower by about 1‰ ascompared to those of standard solution NIST SRM 951.As to JB-2, isotope compositions obtained by P-TIMS(δ11B = 7.14‰) and N-TIMS (δ11B = 7.12‰) are indistin-guishable. These values are also in good agreementwith data published by Nakamura et al. (1992) (δ11B =7.08 ± 0.08‰) and Tonarini et al. (1997) (δ11B = 7.23± 0.25‰). The isotope compositions of JR-2G (δ11B =2.57‰) and powder sample (δ11B=2.91‰) obtainedby P-TIMS are identical within 0.4‰. The N-TIMS (δ11B= 2.51‰) value also compares well, although it is assi-gned with a large error. No published isotope data onJR-2 are available for comparison.

Micro-beam homogeneity tests

We have checked the chemical and isotopic homo-geneity of all glassy samples by means of electronprobe microanalysis (EPMA) and/or secondary ionmass spectrometry (SIMS). SIMS profiles across NISTSRM 610 and SRM 612 glass wafers (Figure 2) indica-ted slight boron depletions of a few percent towardsthe outermost 200 µm of both glass wafers. However,in contrast to boron concentration measurements, noisotopic heterogeneity was detectable in either case.Depletion of boron (and F) in the outermost 10 µm of

another SRM 610 wafer has also been reported byHinton (1999). In accordance with our findings, Hintondid not observe any heterogeneity in boron distributionaside from the rim. This suggests both isotopic homo-geneity and uniform abundance of boron for indivi-dual SRM 610/612 wafers at the sub-mm to cm scale(with the outermost 200 µm being the exception).

Major element compositions for the synthesisedJB-2G and JR-2G glasses are shown in Table 4. Thistable includes values for the starting composition of therespective sample powders (GSJ 2000) and the com-position of the prepared glasses along with variationsbetween rim and core areas of glass fragments. In thecase of the rhyolitic sample JR-2G, profiles across differentfragments indicate uniform major element compositionand no difference is observed between the composi-tions of glass and starting composition. The exceptionis a systematic depletion of iron of about 15-20% inthe glass. This is a commonly observed phenomenonoccurring during rock fusion and, in our case, we attri-


4 1 1

Figure 2. Boron isotope and concentration profiles across NIST

SRM 610 and SRM 612 wafers. Note the slight boron depletion

of a few percent towards the outermost rim of each wafer

(a) which contrasts with the homogeneous isotope composition

throughout each sample (b). All isotopic values were normalized

to the “recommended value” for SRM 610 (Table 6).

Distance from rim (µm)

B (µ

g g

-1) S

RM61

2

δ11B

(‰)

B (µ

g g

-1) S

RM 6

10

��

330

350

370

-4.0

-2.0

0.0

2.0

4.0

0 200 4000 8000 12000

390

16000

SRM 610 SRM 612

35

37

39

�

(b)

(a)

SRM 610 SRM 612

33

Table 4.Major element composition of powders and glasses of geological reference samples

JB-2 JB-2G

(powder)* Core Rim §

SiO2 53.25 53.0 ± 0.5 45.4TiO2 1.19 1.26 ± 0.06 0.85Al2O3 14.64 14.8 ± 0.2 12.9Fe2O3tot 14.25 10.5 ± 0.6 0.81MnO 0.22 0.26 ± 0.03 0.11MgO 4.62 4.92 ± 0.23 2.74CaO 9.82 9.91 ± 0.24 6.61Na2O 2.04 2.01 ± 0.05 2.22K2O 0.42 0.41 ± 0.03 0.51Total 100.45 97.1 ± 0.6 72.1

JR-2 JR-2G

(powder)* Average

SiO2 75.69 77.2 ± 0.6TiO2 0.07 0.05 ± 0.01Al2O3 12.72 13.0 ± 0.2Fe2O3tot 0.77 0.64 ± 0.04MnO 0.11 0.13 ± 0.03MgO 0.04 0.04 ± 0.01CaO 0.50 0.54 ± 0.03Na2O 3.99 3.81 ± 0.05K2O 4.45 4.61 ± 0.09Total 98.34 100.0 ± 0.6

* Starting composition (GSJ 2000).§ Ta droplets-bearing.Concentrations in % m/m. Uncertainties refer to 1s of > 30 singleanalyses.

bute this to a loss of iron to the tantalum strip heaterf i lament . The abundance of the remaining majorelements that could be measured with good precisiondiffer from the starting powder by between 0.2 and2.5%. In agreement with the homogeneous majorelement composition, no variations in B concentrationor isotope composition were found, neither within norbetween different glass fragments. In a few cases,insignificant amounts of tiny and relatively silica-poor(SiO2 = 67% m/m) melt droplets were observed tohave segregated from the bulk melt during the fusionprocedure. It was found that, within error, these dropletshad the same boron concentration and isotope com-position as the bulk glass.

In contrast to JR-2G, significant differences in thechemical composition of core and rim exist in manyJB-2G fragments (Table 4), where the outermost 250µm are highly heterogeneous and extremely depletedin iron. In a few cases, an apparent depletion affectsall major elements due to the “diluting” effect ofTa -metal droplets formed during fusion. With theexception of Fe, the overall composition of the interiorof different JB-2G fragments are, within error, identicalto the starting composition. With respect to boron

concentration and isotope composition, the central (Ta-droplets free) parts of the glasses are also homoge-neous, while the outermost (Ta-droplets bearing) ~ 250µm show slight apparent increases in B concentrationby > 1 µg-1 and a decrease in δ11B values by > 3‰.

Oxygen isotope composition and homogeneity

Three independent wafers of both SRM 610 andSRM 612 were analysed four times each (Table 5).Oxygen isotope analyses replicate within our typicalexternal analytical precision of 0.2‰ (1s). Hence, bothwithin-wafer and between-wafer measurements showthat the SRM glasses have homogeneous oxygen iso-tope compositions on the mg scale. Note that the δ18Ovalues of SRM 610 and SRM 612 glasses differ by0.6‰, and their δ17O values by 0.2‰ (Table 5, Figure3). In accordance with most other terrestrial samples,δ17O values correspond to 0.516 x δ18O in all SRMsamples analysed.

Discussion

N-TIMS vs. P-TIMS comparisonand accuracy of TIMS data

Several processes related to sample dissolutionand chemical separation of boron are capable ofchanging the or ig inal i so tope composi t ion o f asample. These include enhanced blanks or incompleteboron recovery combined with isotope fractionationduring chromatographic separation. In addition, thecomposition of the chemical matrix loaded on the

4 1 2


Table 5.Oxygen isotope composition of NIST SRM 610 and SRM 612

Sample* N § δ18O # δ17O #(‰, VSMOW) (‰, VSMOW)

Reference materialsDoer Qtz 6 12.12 ± 0.25 6.24 ± 0.19NIST-28 4 9.65 ± 0.19 4.99 ± 0.21

NIST SRM 610GFZ-1 4 10.84 ± 0.06 5.51 ± 0.13GFZ-2 4 11.01 ± 0.18 5.60 ± 0.03HD 4 10.87 ± 0.14 5.55 ± 0.15

Simple mean 10.91 ± 0.09 5.55 ± 0.05

NIST SRM 612GFZ-1 4 10.22 ± 0.09 5.28 ± 0.06GFZ-2 4 10.43 ± 0.27 5.40 ± 0.12KA 4 10.26 ± 0.26 5.26 ± 0.22

Simple mean 10.33 ± 0.15 5.34 ± 0.08

* Provided by the GeoForschungsZentrum Potsdam (GFZ-1, GFZ-2) andthe Universities of Heidelberg (HD) and Karlsruhe (KA).

§ Number of analyses.# Uncertainties refer to 1 standard deviation.SRM 610 and SRM 612 data are normalised to quartz reference materialsNIST SRM 28 and Doer Qtz (Fiebig et al. 1999). Accepted δ18O values forthese are 9.7‰ and 12.1‰ relative to VSMOW, respectively. Acceptedδ17O value for NIST SRM 28 is 5.0‰. Precision and accuracy are 1s ≈ 0.2‰.

Figure 3. Oxygen isotope composition of different samples

of NIST SRM 610 and SRM 612. The data indicate significant

isotopic differences between both glasses. Error bars

represent 1s repeatability.

δ18O

(‰) V

SMO

V

10.0

10.5

11.0

GFZ-1 GFZ-2 HD GFZ -1 GFZ -2 KA

SRM 610

SRM 612

filament may control both the formation of interferingisobaric molecules and the evaporation/ionisation/fractionation behaviour of boron during mass spectro-metric analysis. Although the number of boron isotopestudies has rapidly increased in recent years andalthough quite dif ferent chemical extraction tech-niques and/or mass spectrometric conditions havebeen employed in these studies, there exists little inter-laboratory and, to our knowledge, no methodicalcomparison of isotope data. Such comparisons are,however, crucial if “reference values” are to be achie-ved. The data obtained in this s tudy al low bothmethodical (N-TIMS vs. P-TIMS) and interlaboratory(JB-2) comparison.

It has been noted that P-TIMS and N-TIMS dataof NIST SRM 610 are indistinguishable within 0.5‰(Table 3). Similarly, P-TIMS and N-TIMS data for JB-2and JR-2 are identical to within 0.02 and 0.4‰,respectively. In all cases, these differences appear tobe sy s temat i c i n tha t N-T IMS ana l y s i s y ie ldeds l ight ly lower δ11B values . We speculate that , i fthese dif ferences are real and in fact systematic,they may be attributed to an under-corrected contri-but ion of the CNO- molecule to mass 42. Note,however, that the isotopic dif ferences are alwayswell within the assumed overall analytical uncertaintyestimated at 1s ≈ 0.8‰, i.e. they are obscured bythe analytical error and cannot, therefore, be distin-guished with confidence.

To evaluate the accuracy of our data, we havecompared our results with published values. Thefirst δ11B data on JB-2, published by Nakamura etal. (1992) gave 7.09 ± 0.08‰, using a HF diges-t ion and P-TIMS. Boron extraction via fusion/ion-exchange wa s ca r r i ed ou t b y Tona r i n i e t a l .( 19 9 7 ) a n d Ka s e m a n n e t a l . ( 2 0 0 0 ) . T h e s eauthors report δ11B values of 7.23 ± 0.24‰ (P-TIMS) and 6.9 ± 0.4‰ (N-TIMS), respectively. Inth is s tudy, δ11B values for the JB-2 powder are7.14 ± 0 .37‰ (P-T IMS) and 7.12 ± 0 .27‰ (N-TIMS) , which is wi thin the publ ished range andindicates good precis ion and accuracy for dataob ta ined by d i f f e ren t ana l y t i ca l me thods andlabora to r ies . Fu r thermore , the accordance be t -ween P-TIMS and N-TIMS data and their excellentagreement with published values support the viewthat possible analytical effects on the boron isoto-pe composition (which would be expected to dif-fer between di f ferent methods and laborator ies)are only of minor significance.

TIMS vs. SIMS comparison

Figure 4 and Table 6 allow a comparison of SIMSand TIMS data on the boron isotope composition ofthe reference materials analysed. For SIMS analysis,SRM 610 served as our reference material, applyingthe isotope composition determined by TIMS. Note thatno TIMS values are avai lable for JB-2G. For thissample we have, therefore, applied the correspondingreference value of the powder, assuming that glassfusion had no effect on the boron isotope composition(see below). With the exception of sample JR-2, SIMSand TIMS data agree to within 2‰, and this agree-ment is independent of boron concentration. We can,however, not exclude the possibility that the slightlyhigher isotope values for JB-2G and JR-2G obtainedby SIMS might be the result of a matrix effect, resultingfrom the use of the synthetic SRM 610 glass as referencematerial. This glass is characterized by an unusualmajor element composition, which is not matchedby any natural rock. Note that such a matrix effect onthe boron isotope composition is reported for SIMSanalyses of tourmalines of variable major elementcompos i t ion (Chauss idon and A lbarède 1992) .However, in the present study the differences betweenSIMS and TIMS data are within the range of analyticaluncertainty. Therefore, any possible matrix effectsappear to be small, to a first approximation, as com-pared to the analytical uncertainties. This implies thatour B-isotope determinations on the reference glassesare sufficiently accurate to be useful for geochemicalstudies.


4 1 3

12

6 8

10

8

2

0

-2

0 2 4-2-4

-4

-6

“Assigned”�compositionSRM 6101:1

-6

4

6

10

SRM 612

JR-2G

JB-2G

Figure 4. Comparison of SIMS and TIMS data. Thick and thin

error bars represent in-run and between-run 1s precision,

respectively. SIMS values are normalised to NIST SRM 610

δ11B = -1.05‰; Table 6).

δ11B (‰) TIMS

δ11B

(‰) S

IMS

Isotopic difference between NIST SRM 610 and SRM 612

The similar preparation process of NIST SRM 610and SRM 612 glasses, including the identical supportmatrix and trace element pool (Kane 1998), wouldpredict an identical B-isotope composition for bothSRMs. Our boron data confirm this expectation, in thatthe boron isotope compositions of both NIST glassesare indistinguishable.

In contrast to boron, systematic isotopic differencesbetween the SRMs 610 and 612 have been observedfor oxygen. This is confirmed by replicate analyses offour random fragments from each of three independentwafers of both samples. Table 5 and Figure 3 indicatethat the δ18O values of SRM 610 and SRM 612 glassesdiffer by 0.6‰, and their δ17O values by 0.2‰. Wesuggest that this may ref lec t the ten-t imes higherdoping of trace element oxides in SRM 610, with theadded trace element oxides having had higher δ18Oand δ17O values as compared to the support matrix.

Effects of glass fusion on boron isotopic composition and homogeneity

Potentially, partial loss of an element from a matrixmay result in isotopic fractionation. Partial loss of boronduring glass production has been observed both inthe SRM 610 and SRM 612 and is reflected by both itslow abundance relative to refractory trace elements(Pearce et al. 1997) and its depletion in the outermostrim of the glass wafers (Hinton 1999). Ion probe profilesacross the glass wafers demonstrate isotopic homoge-neity for both SRM 610 and SRM 612, suggesting thatglass manufacturing was not accompanied by signifi-cant isotopic fractionation (see above).

ICP-AES and TIMS analyses of JR-2 and JR-2G showthat glass fusion resulted neither in a loss of boron(Table 2) nor in any significant isotope fractionation(Table 3). This is also suggested by ion probe profileswhich demonstrate a good overall boron isotope homo-geneity throughout, regardless of the presence or absen-ce of the silica-poor melt droplets which, in a few cases,formed during fusion. Overall isotope homogeneity hasalso been found in the internal parts of each JB-2Gfragment. In contrast, the Ta droplet-bearing parts of theoutermost contact zone show 2% higher boron contentsand some 4‰ lower δ11B values. At this point it is notclear if these isotopic differences are real or due tosome matrix effect occurring during ion probe analysis.In any case, the ion probe data suggest that the rims ofglass fragments should be avoided for analysis.

In summary, we conclude that the applied method offusing tens of milligrammes of silicate powder has no signi-ficant influence on the boron isotope composition and iscapable of producing a homogeneous glass with respectto both boron concentration and isotopic composition.

Recommended values and concluding remarks

A summary of our results is compiled in Table 6.This table includes boron and oxygen isotope data forthe NIST certified reference materials SRM 610 andSRM 612. These glasses are commonly used as micro-analyt ical calibration and reference materials forvarious in situ methods, including ion and laser beamtechniques. Comparison of concentration and isotopedata obtained in this study with the published valuesof geochemical reference materials JB-2 and JR-2powders demonstrates their accuracy. Also listed arethe mean boron isotope compositions obtained by

4 1 4


Table 6.Data summary and recommended values

Sample B (µg g-1) δ11B (‰) δ18O (‰) δ17O (‰)Literature ICP-AES SIMS N-TIMS P-TIMS Rec. Value $ SIMS

NIST SRM 610 356 ± 7 * 357 ± 17 # -1.31 ± 0.68 -0.78 ± 0.70 -1.05 # 10.91 ± 0.09 5.55 ± 0.05NIST SRM 612 34.7 ± 3.2 * 35 ± 2 37 ± 1 - -1.07 ± 0.85 -1.07 -0.7 ± 0.8 10.33 ± 0.15 5.34 ± 0.08JB-2 30.2 ** 30 ± 2 - 7.12 ± 0.34 7.14 ± 0.77 7.13 - - -JB-2G - - 37 ± 2 - - - 7.9 ± 1.5 - -JR-2 145 ** 158 ± 7 - 2.51 ± 1.30 2.91 ± 0.48 2.71 - - -JR-2G - 174 ± 10 § 136 ± 10 - 2.57 ± 0.53 2.57 4.9 ± 1.3 - -

Uncertainties refer to 1 standard deviation.# No data, because TIMS and ICP-AES values have been used as references for SIMS analysis.* Pearce et al. (1997). ** GSJ (2000). § Single analysis with estimated uncertainty. $ Recommended or preferredvalue. Based on the scatter between independent dissolutions/chemical separations, the overall analytical uncertainty is estimated to 1s = 0.8‰.

P-TIMS and N-TIMS, respectively, which suggest that apossible bias between both techniques is of minorsignificance. The average of P- and N-TIMS data arepresented as “recommended values”. The assumedoverall analytical uncertainty of the “recommendedvalues” is estimated at 1s (δ11B) ≈ 0.8‰. This errorreflects the mean repeatability and is based on thescatter between independent chemical dissolutionand/or separation aliquots. Table 6 also presents theboron isotope and concentration compositions deter-mined by SIMS, which demonstrate that this techniqueis a useful tool for the in situ study of boron isotopicvariations in geological materials. This conclusion is ofespecial interest in the light of extremely diverse majorelement compositions (possibly resulting in enhancedmatrix effects) of the materials studied.

Acknowledgements

We thank A. Deutsch for access to the resistor hea-ter at the Institut für Planetologie, Universität Münster.The paper benefited greatly from critical comments bytwo anonymous reviewers.

References

Brown R.W. (1977)A sample fusion technique for whole rock analysis withthe electron microprobe. Geochimica et CosmochimicaActa, 41, 435-438.

Catanzaro E.J., Champion C.E., Garner E.L.,Marinenko G., Sappenfield K.M. and Shields W.R. (1970)Boric acid: Isotopic and assay standard reference materials. US National Bureau of Standards, SpecialPublication, 260-17, 70pp.

Chaussidon M. and Albarède F. (1992)Secular boron isotope variations in the continental crust:An ion microprobe study. Earth and Planetary ScienceLetters, 108, 229-241.

Erickson A.A. and Williams J.P. (1970)A series of trace element standards in a glass matrix forthe National Bureau of Standards. Corning Glass WorksReport, L-1169, 24pp.

Fiebig J., Wiechert U., Rumble D. III and Hoefs J. (1999)High-precision in situ oxygen isotope analysis of quartzusing an ArF laser. Geochimica et Cosmochimica Acta,63(5), 687-702.

Govindaraju K. (1994)1994 compilation of working values and sample description for 383 geostandards. GeostandardsNewsletter, 18 (Special Issue), 158pp.

GSJ (Geological Survey of Japan) (2000)Geochemical Standards DataBase.http://www.aist.go.jp/RIODB/geostand/

Hemming N.G. and Hanson G.N. (1992)Boron isotopic composition and concentration in moderncarbonates. Geochimica et Cosmochimica Acta, 56,537-543.

Hemming N.G. and Hanson G.N. (1994)A procedure for the isotopic analysis of boron by negativethermal ionization mass spectrometry. Chemical Geology,114, 147-156.

Hinton R.W., Harte B. and Witt-Eickschen G. (1995)Ion probe measurements of National Institute ofStandards and Technology Standard Reference MaterialSRM 610 Glass, trace elements. The Analyst, 120,1315-1319.

Hinton R.W. (1999)NIST SRM 610, 611 and SRM 612, 613 multi-elementglasses: Constraints from element abundance ratios measured by microprobe techniques. GeostandardsNewsletter: The Journal of Geostandards andGeoanalysis, 23, 197-207.

Imai N., Terashima S., Itoh S. and Ando A. (1995)1994 compilation of analytical data for minor and traceelements in seventeen GSJ geochemical referencesamples, “Igneous rock series”. Geostandards Newsletter,19, 135-213.

Jochum K.P., Dingwell D.B., Rocholl A., Stoll B.,Hofmann A.W. and thirty one others (2000)The preparation and preliminary characterisation of eightgeological MPI-DING reference glasses for in situmicroanalysis. Geostandards Newsletter: The Journal ofGeostandards and Geoanalysis, 24, 87-133.

Kane J.S. (1998)A history of the development and certification of NISTglass SRMs 610-617. Geostandards Newsletter: TheJournal of Geostandards and Geoanalysis, 22, 15-31.

Kasemann S., Erzinger J. and Franz G. (2000)Boron recycling in the continental crust of the CentralAndes from the Palaeozoic to Mesozoic, NW Argentina.Contributions to Mineralogy and Petrology, 140, 328-343.

Klötzli U.S. (1992)Negative thermal ionisation mass spectrometry: A newapproach to boron isotope geochemistry. ChemicalGeology (Isotope Geoscience Section), 101, 111-122.

Nakamura E., Ishikawa T., Birck J.L. and Allègre C. (1992)Precise boron isotopic analysis of natural samples usinga boron-mannitol complex. Chemical Geology, 94,193-204.

NBS 1970Trace elements in glasses SRMs: SRM 610 through 617,inclusive. National Bureau of Standards (Certificate), 1-4.


4 1 5

references

Nakano T. and Nakamura E. (1998)Static multicollection of Cs2BO2

+ ions for precise boronisotope analysis with positive thermal ionization massspectrometry. International Journal of Mass Spectrometry,176, 13-21.

Owens J.W., Gladney E.S. and Knab D. (1982) Determination of boron in geological materials by inductively-coupled plasma emission spectrometry.Analytica Chimica Acta, 135, 169-172.

Pearce N.J.G., Perkins W.T., Westgate J.A., Gorton M.P., Jackson S.E., Neal C.R. and Chenery S.P. (1997)A compilation of new and published major and traceelement data for NIST SRM 610 and NIST SRM 612 glassreference materials. Geostandards Newsletter: TheJournal of Geostandards and Geoanalysis, 21, 115-144.

Rocholl A.B.E., Simon K., Jochum K.P., Bruhn F.,Gehann R., Kramar U., Luecke W., Molzahn M.,Pernicka E., Seufert M., Spettel B. and Stummeier J. (1997)Chemical characterization of NIST silicate glass certifiedreference material SRM 610 by ICP-MS, TIMS, LIMS,SSMS, INAA, AAS and PIXE. Geostandards Newsletter:The Journal of Geostandards and Geoanalysis, 21, 101-114.

Rocholl A., Dulski P. and Raczek I. (2000)New ID-TIMS, ICP-MS and SIMS data on the trace element composition and homogeneity of NIST certifiedreference material SRM 610-611. GeostandardsNewsletter: The Journal of Geostandards andGeoanalysis, 24, 261-274.

Rumble D. and Hoering T.C. (1994)Analysis of oxygen and sulfur isotope ratios in oxide andsulfide minerals by spot heating with a carbon dioxidelaser in a fluorine atmosphere. Accounts of ChemicalResearch, 27, 237-241.

Tonarini S., Pennisi M. and Leeman W.P. (1997)Precise boron isotopic analysis of complex silicate (rock)samples using alkali carbonate fusion and ion-exchangeseparation. Chemical Geology, 142, 129-137.

Vengosh A., Chivas A.R. and McCulloch M.T. (1989)Direct determination of boron and chlorine isotopic compositions in geological materials by negative thermal-ionization mass spectrometry. Chemical Geology(Isotope Geoscience Section), 79, 333-343.

Woodhead J.D. and Hergt J.M. (2000)Isotopic analysis of NIST reference materials. 4thInternational Conference on the Analysis of Geologicaland Environmental Materials, Geoanalysis 2000, 32(abstract).

Xiao Y.K., Beary E.S. and Fasset J.D. (1988)An improved method for the high-precision isotopic measurement of boron by thermal ionization mass spectrometry. International Journal of Mass Spectrometryand Ion Processes, 85, 203-213.

4 1 6


Boron and Oxygen Isotope Composition of Certified Reference Materials NIST SRM 610/612 and Reference...

Documents

Transcript of Boron and Oxygen Isotope Composition of Certified Reference Materials NIST SRM 610/612 and Reference...