Intercomparison of Boron Isotope and Concentration Measurements. Part I: Selection, Preparation and Homogeneity Tests of the Intercomparison Materials

GEOSTANDARDSNEWSLETTERThe Journal of Geostandards and Geoanalysis

Intercomparison of Boron Isotope and ConcentrationMeasurements. Part I: Selection, Preparation andHomogeneity Tests of the Intercomparison Materials

Vol. 27 — N° 1 p . 2 1 - 3 9

In 1999 the Istituto di Geoscienze e Georisorse(IGG), with the support of the International AtomicEnergy Agency (IAEA), undertook the collection, preparation and distribution of eight geologicalmaterials intended for a blind interlaboratory comparison of measurements of boron isotopiccomposition and concentration. The materials came from Italian sources and consist of three natural waters (Mediterranean seawater and twogroundwaters) and five rocks and minerals (tourmaline, basalt, obsidian, limestone and clay).The solid materials were crushed, milled and mixed,in preparation for distribution. Extensive assays performed at the IGG on these materials demonstrated that their boron isotopic and chemical compositions are homogeneous.

Additional homogeneity tests were carried out onsolid material fragments at the GeoForschungsZentrumPotsdam, with the specific objective of investigatingthe suitability of some of them for the calibration insitu of micro-analytical techniques. Two materials,B4 (tourmaline) and B6 (obsidian), proved to be isotopically homogeneous and may become excellent references for in situ microanalyses of boron isotopes.

The materials described here were used as the basisof a major laboratory intercomparison study andare now available for further distribution from eitherthe IAEA (solid materials) or the IGG (waters).

Keywords: boron isotopes, boron concentration, reference materials, in situ boron microanalysis, homogeneity.

L’Istituto di Geoscienze e Georisorse (IGG), avec le soutien de l’Agence Internationale de l’EnergieAtomique (AIEA), a entrepris en 1999 la récolte, la préparation et la distribution de huit matériauxgéologiques dans le but de conduire une comparaisonen aveugle des mesures de composition isotopiqueet de concentration du bore entre différents laboratoires. Les matériaux, qui dérivent tous de sites italiens, comprennent trois eaux naturelles (une eau de la Mer Méditerranée et deux eaux souterraines) et cinq roches et minéraux(tourmaline, basalte, obsidienne, calcaire et argile).Pour la distribution, les matériaux solides ont étéréduits en fragments, broyés et mélangés et leurhomogénéité de composition isotopique et chimiquedu bore a été vérifiée par des essais exhaustifs.

D’autres essais d’homogénéité ont été effectués auGeoForschungsZentrum Potsdam sur des fragmentsdes matériaux solides avec le but particulier d’étudierla possibilité d’utilisation de ces matériaux pour lacalibration in situ des techniques microanalytiques.Deux matériaux, le B4 (tourmaline) et le B6 (obsidienne), se sont révélés isotopiquement homogènes et donc peuvent devenir d’excellentsmatériaux de référence pour les microanalyses isotopiques in situ du bore.

Les matériaux, qui sont décrits dans cette note, ontété utilisés pour une étude d’intercomparaison desmesures isotopiques et chimiques du bore entre différents laboratoire. Ils sont maintenant à dispositionà l’AIEA (matériaux solides) et à l’IGG (eaux) pourleur distribution ultérieure.

Mots-clés : isotopes du bore, concentration du bore,matériaux de référence, microanalyses in situ du bore, homogénéité.

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0303

Sonia Tonarini (1)*, Maddalena Pennisi (1), Alessandra Adorni-Braccesi (1), Andrea Dini (1), Giorgio Ferrara (1), Roberto Gonfiantini (1)*, Michael Wiedenbeck (2) and Manfred Gröning (3)

(1) Istituto di Geoscienze e Georisorse, Area di Ricerca del CNR, Via G. Moruzzi 1, I-56124 Pisa, Italy(2) GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany(3) International Atomic Energy Agency, Division of Physical and Chemical Sciences, Wagramerstrasse 5, P.O. Box 100, A-1400 Vienna, Austria* Corresponding authors: e-mail [email protected] and [email protected]

Received 30 Sep 02 — Accepted 12 Feb 03

Boron has two stable isotopes, 11B and 10B, whichoccur in nature with a relative abundance of ≈ 4:1. Theelaboration of the geochemical framework for boronisotopes began with the observation that isotopic frac-tionation occurs during adsorption (Schwarcz et al.1969 and the subsequent theoretical and experimen-tal work by H. Kakihana and co-workers). Variations inthe boron isotope ratio in nature are mostly producedby the equilibrium

H3BO3 (trigonal) + OH- = H4BO4- (tetrahedral)

From spectroscopic data on molecular vibrations,Kakihana and Kotaka (1977) and Kakihana et al.(1977) computed that the 11B/10B ratio is higher in thetrigonal form by about 20‰ at room temperature, andthat this isotopic fractionation decreases by 2‰ from 0to 40 °C. Subsequently an empirical fractionationfactor of 30‰ at 2 °C was observed between seawater, enriched in 11B, and the alteration products ofmarine basalts where tetrahedral borate is preferen-tially fixed, though it is not known whether equilibriumconditions were achieved (Spivack and Edmond 1987).

Due to the wide variations of 11B/10B ratio in nature- about 100‰ (Coplen et al. 2002) - and the goodaccuracy by which this ratio can be determined, boronisotopes have found major applications in rock andwater geochemical studies (see the review articles byBassett (1990), Leeman and Sisson (1996), and Palmerand Swihart (1996)) and in environmental investiga-tions (for example, Vengosh et al. 1994).

Over the past two decades, significant technicalimprovements were introduced in the methods ofboron isotope ratio determination in natural materials,which contributed considerably to the understandingof boron isotope fractionations occurring in geoche-mical processes. There are essentially three massspec t rometr ic analy t ical methods used in boronisotope geochemistry (see reviews by Aggarwal andPalmer 1995, Palmer and Swihart 1996):

- Thermal ionisation mass spectrometry (TIMS) inwhich either positive ions (PTIMS) such as Cs2BO2

+, ornegative ions (NTIMS) such as BO2

-, are generated bya filament ion source;

- Inductively coupled plasma-mass spectrometry(ICP-MS) in which monoatomic ions are generated bya plasma torch ion source. This method has beenimproved significantly in recent years with the introduction

of an ion multicollection system (MC-ICP-MS), allowingthe simultaneous detection of both boron isotopes;

- Secondary ion mass spectrometry (SIMS) or ionmicroprobe, which utilizes a focused ion beam for thein situ production of ions from the polished surface ofa solid sample.

Until now, interlaboratory calibration and compa-rison of boron isotopic results were based solely onthe reference material NIST SRM 951 (boric acid) dis-tributed by the National Institute for Standards andTechnology of the US Department of Commerce. NISTSRM 951 is used for normalizing results and establi-shing correc t ion factors for isotope fract ionation,which may occur during chemical t reatment andmass spec t romet r ic measurement . The potent ia lsources of isotopic fractionation occurring duringextraction and purification of boron from differentnatural compounds were discussed by Aggarwal andPalmer (1995).

Through a consensus agreement within the geo-chemical community, the isotopic ratio RNIST SRM 951 ofNIST SRM 951 also defines the “zero” of the boronisotope δ-scale:

(1)

where R is 11B/10B. RNIST SRM 951 has a certified value of4.04362 ± 0.00137 (2s) (Catanzaro et al. 1970). Itmust be stated, however, that subsequently, Spivackand Edmond (1986) found RNIST SRM 951 = 4.04558 ±0.00033, i.e. (0.48 ± 0.35)‰ more than Catanzaro’svalue. Moreover, Leeman et al. (1991) reported valuesof 4.04990 ± 0.00012 and 4.05339 ± 0.00040 fromtwo distinct solutions of NIST SRM 951, i.e. (1.55 ±0.34) and (2.42 ± 0.35)‰, respectively, different fromthe NIST certified value; the reason for these discre-pancies remains unknown.

Despite the fact that it consists of the simplestchemical form of boron, NIST SRM 951 alone is insuffi-cient to guarantee a reliable interlaboratory calibra-tion of boron isotopic results free from systematic errors.Another reference material with a 11B/10B ratio sometens of per mil different from that of NIST SRM 951would be desirable in order to fix the so-called stret-ching factor of the δ-scale, as has already been donefor oxygen and hydrogen isotopes (Gonfiantini 1978)and recently for sulfur isotopes (Ding et al. 2001). A

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δ11

B = Rsample

RNIST SRM 951

- 1 x 1000

second boric acid reference sample, distributed by theInstitute of Reference Materials and Measurements ofthe European Union Joint Research Centre, with thecode IRMM-011, has a 11B/10B ratio equal to 4.0443± 0.0052 (2s) (De Bièvre 1993): as this is practicallyidentical to the ratio of NIST SRM 951, it will not helpto solve the problem.

Clearly, materials covering a broad spectrum ofgeochemical matrices and a wide range of boronisotopic compositions and concentrations are neededto assess whether results obtained by different labo-ratories and with different analytical techniques areconsistent , and to help to identify any sources ofd i screpancies and devise correc t ion procedures .Repeated analyses of such materials would provideeach laboratory with the opportunity to check thereproducibil i ty of boron isotopic composit ion andconcentration measurements under different matrixconditions.

Until now, only three additional materials havebeen available for this purpose: (i) the JB-2 referencematerial (island arc tholeiitic basalt distributed by theGeological Survey of Japan), which is used for boronisotope normalisation in si l icates on the basis ofmeasurements performed by Nakamura et al. (1992),Tonarini et al. (1997) and Kasemann et al. (2001), andfor assessing measurement uncertainty (Benton etal. 1999, Deyhle 2001); (ii) the NIST synthetic silicateglasses NIST SRM 610 and 612, the isotopic composi-tion of which has been determined by Kasemann et al.(2001); and (iii) ocean water, the dissolved boronof which is isotopically uniform on a global scale(Spivack and Edmond 1987) and can be used as acomparison sample for boron isotopic measurementsin natural water samples.

Despite the wide range of boron isotopic variationsin nature and the large number of isotopic dataalready available in the scientific literature, no inter-comparison of boron isotope measurements in geolo-gical materials has ever been performed prior to theone described here. The objectives of the intercompa-rison exercise were to obtain information on thedegree of agreement between boron isotope deter-minations on various types of natural materials car-ried out in different laboratories and with differentanalytical techniques, and to identify any systematicdeviation between laboratories. These objectives dicta-ted the choice of the intercomparison materials, thepreparation of which was a three-step process:

(i) Selection and collection of suitable, chemicallystable geological materials, which should have an ele-mental and isotopic composition that is as homogeneousas possible, and be available in adequate quantities. Theboron isotopic composition and concentration of the materialsselected should encompass the range of natural variations.

(ii) Preparation of the intercomparison materials informs that are appropriate for distribution to laboratories,and that ensure a fully homogeneous isotopic and che-mical composition, suitable for the most precise currentanalytical procedures. The amounts prepared should besufficient to cover the needs of the scientific communityfor many years to come (future distribution to be assuredthrough the IAEA’s reference materials programme).

(iii) Execution of assays to prove the isotopic andchemical homogeneity of the intercomparison materials.

The intercomparison exercise was launched bydisseminating the information through the Isogeochem,TIMS and Plasmachem e-mail lists in late 1999, andjournals such as The Geochemical News (AmericanGeochemical Society) and Water and EnvironmentNews (IAEA) in early 2000. Information was also sentdirectly to all laboratories known to be working in boronisotope geochemistry. Twenty seven laboratories fromtwelve countries agreed to take part in the exercise, butonly fifteen laboratories submitted their measurementresults to the IAEA by the final deadline of July 31, 2001.

We describe in this paper the selection and prepa-ration of the intercomparison materials and the testsperformed at the IGG and the GeoForschungsZentrumPotsdam to check their homogeneity. We also presentthe technical procedures used in the tests and the mea-surements performed, and discuss some of the analyticalproblems that we have encountered. The detailed resultsof the intercomparison exercise are reported anddiscussed in a companion paper (Gonfiantini et al.2003). The exercise was organized by the former Istitutodi Geocronologia e Geochimica Isotopica, Pisa - nowmerged into the new Istituto di Geoscienze e Georisorse( IGG) - on behal f , and wi th the suppor t , o f theInternational Atomic Energy Agency (IAEA), Vienna.

Selection, preparation and homogeneity tests of the intercomparison materials

Eight geological materials (three waters and f iverocks and minerals) were selected for the intercomparison


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exercise. The materials, all from Italian sources whoseapproximate locations are shown in Figure 1, consist ofthree natural waters (B1 to B3) and five rocks orminerals (B4 to B8). Information on the origin and cha-racteristics of the intercomparison materials is givenbelow. The amounts prepared are reported in Table 1,together with the approximate isotopic compositionand concentration of boron obtained from preliminaryassays. We believe that these materials, whose numberhad to be kept within reasonable limits, meet reasona-bly well the needs of the boron isotope geochemicalcommunity.

Waters: origin and characteristics

Manifestly, all fluid materials are isotopically andchemical ly homogeneous . Thus , the three watersamples were only filtered through a 0.45 µm mem-brane filter and acidified to pH = 1.5 with 6.6 mol l-1,B-free HCl. They were stored at room temperature, indarkness, in large polyethylene bottles. Their chemicalcomposition is reported in Table 2.

Material B1: Mediterranean Sea water: A sampleof Mediterranean sur face water was collec ted inSeptember 1999 north of Elba Island, Ligurian Sea(approximate Lat. 43°0’N, Long. 10°15’E). The chlorideconcentration was 20.4 g l-1, i .e . higher than theocean mean concent rat ion (19.5 g l -1) becauseevaporat ion exceeds f resh water in f low in to theMediterranean Sea. The sample is believed to berepresentative of Western Mediterranean water.

Some boron isotopic values on Mediterranean sea-water have been reported in the literature. Vengosh etal . (1992) quote a δ11B of 37.7‰ for the EasternMediterranean Sea. Lecuyer et al. (2000) report amean value of 40.26 ± 0.29‰ for eleven samples,presumably from the region between the Gulf of Lionand Corsica. Tonarini et al. (unpublished data) found amean value of 39.6 ± 0.3‰ for two samples from thesouthern Tyrrhenian Sea. These values indicate that theboron isotopic composition of Mediterranean seawateris close to that of the ocean, which is practicallyconstant worldwide at ≈ 39.5‰ (Spivack and Edmond1987). Thus, B1 represents the upper limit of the δ11Brange of most natural waters . More posit ive δ11Bvalues - up to almost +60‰ - have been reported forbrines from Australian lakes (Vengosh et al. 1991a),

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B8

B2,B3

B1

B4B7

B5

B6

Figure 1. Map of sampling site localities

for the intercomparison materials.

Table 1.Materials selected for the intercomparison of measurements on boron isotope composition and concentration

Code Type and origin Approx. B Approximate Amountconcentration (*) δ11B (‰) prepared

B1 Surface sea water, Ligurian Sea (Western Mediterranean) 5 39 45 l

B2 Groundwater, alluvial aquifer, Cecina R. lower basin (Tuscany) < 1 12 45 l

B3 Groundwater, alluvial aquifer, Cecina R. upper basin (Tuscany) 2 - 22 45 l

B4 Tourmaline, Elba Island (Tuscan Archipelago) 28700 - 8.5 0.8 kg

B5 Basalt, Etna Volcano (Sicily), erupted July 22, 1998 8 - 4 22 kg

B6 Obsidian, Lipari Island (Aeolian Archipelago, Sicily) 210 - 1 10 kg

B7 Miocene marine limestone, Maiella (Abruzzo) 2 5 11 kg

B8 Pliocene clay, Montelupo (Tuscany) 100 - 5 9 kg

(*) B concentration is expressed in mg l-1 for materials B1-B3 and in mg kg-1 for materials B4-B8.

but are presumably rare. The boron concentration ofB1 is about 5 mg l-1, which is typical of ocean water.

Material B2: groundwater: Groundwater collectedin September 1999 from a shallow well in the floodplain of the lower course of the River Cecina, whichflows E-W about 40 km south of Pisa. The sample isbelieved to come mainly from recent river bank fil-tration. The δ11B of ≈ 12‰ and boron concentration of0.2 mg l-1 fall well within the range typical of ground-water.

Material B3: groundwater: Groundwater collectedin September 1999 from a well dug about 300 mfrom Cecina River on the Il Querceto Farm, which islocated 20 km downstream of the Larderello geother-mal area in the upper basin of the river. The ground-water has a relatively high B concentration (about 2mg l-1) with an uncommonly low δ11B of about -22‰.This value is close to the lower limit of the δ11B rangeof natural waters. A more negative value, -27‰, wasrecently observed in groundwater from a well in thesame area (Pennisi et al. unpublished data).

Samples B2 and B3 were selected in the course ofan isotopic investigation, which is still in progress, onsources and fate of boron dissolved in groundwater

from the alluvial aquifer recharged by the River Cecina(preliminary report: Pennisi et al. 1999).

Rocks and minerals: preparation and homogeneity tests carried out at IGG, Pisa

Rock and mineral samples were crushed, milledand mixed using the quartering technique used in rockdating. The granularity of the milled materials was asfollows: materials B6, B7 and B8, all deriving fromrelatively friable substances, yielded grains smallerthan 5 µm; material B4, derived from hard tourmalinecrystals, consisted, for about 50% by weight, of grainssmaller than 5 µm, and, for the other half, by grainsup to 40 µm and rare grains of larger dimensions (upto 100 µm); material B5, which was mineralogicallyheterogeneous, consisted of grains that were generallysmaller than 5 µm with few grains up to 40 µm (pro-bably Ti-magnetite grains). The trace element concen-tration of the five solid materials is given in Table 3.

Sol id materials , however, present the issue ofsample homogeneity. In order to increase the chanceof achieving an homogeneous boron isotopic compo-sition at the sub-gram level, the choice of rocks andminerals was guided, whenever possible, by previousstudies and measurements carried out at the IGG,


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Table 2.Chemical composition of water samples prepared for B isotope intercomparison

Major components B1 B2 B3 Trace elements B1 B2 B3(mg l-1) (µg l-1)

Na 11472 31.4 129.9 Ag < 1.25 < 0.05 < 0.05K 475 1.4 8.4 Al - 6.9 2.25Mg 1380 29 51 Ba - 35.8 61.5Ca 444 80.4 129.5 Be < 15 < 0.6 < 0.6Cl- 20400 1323 2146 Cd < 2.5 < 0.1 < 0.1SO4

2- 3042 168.3 251.9 Co < 0.5 0.59 0.63SiO2 - 7.83 19.1 Cr - 1.01 1.35

Cu < 11 1.94 2.08Conductivity (µS cm-1) 67800 640 1530 Fe 1 2.29 3.71

Li 196 8.6 26.1Mn < 10 0.33 0.37Mo 12.6 0.64 0.69Ni < 5.5 3.23 6.51Pb - 2.39 0.69Rb 130.5 0.73 1.32Sr 6800 890 1481Te < 7 < 0.28 < 0.28Tl < 1.5 < 0.6 < 0.6U 3.72 0.45 1.07

Zn < 47 30.5 28.5

Chloride and conductivity were measured by Dr. L. Pierotti at the IGG.All other measurements were performed by Dr. F. Podda by ICP-AES and ICP-MS at the Dipartimento di Scienze della Terra, University of Cagliari, Italy.

indicating that the boron isotopic ratio did not varysignificantly in the source materials.

Assays carried out at the IGG to assess the isotopichomogeneity across individual fragments of the star-ting materials, as well as between both the initial andthe milled materials, are reported in Table 4. Thesedata document that the intercomparison materials areisotopically homogeneous, not only as a consequenceof milling and mixing, but also because the startingsubstances were themselves quite homogeneous.

The boron isotopic composition was determinedby PTIMS (positive thermal ionisation mass spectro-metry), using a VG Isomass 54E mass spectrometer.The measurements were carried out on boron extractedfrom the samples with the ion-exchange procedures

described by Tonarini et al. (1997). The overall accu-racy was evaluated by replicate analyses of the NISTSRM 951 boric acid exposed to the full chemicalprocedure (Table 5). The isotopic fractionation duringthe mass spectrometric analysis was corrected byapplying the factor (RNIST SRM 951 + 0.00079)/RNIST SRM

951(measured), where RNIST SRM 951 = 4.04362 (Catanzaroet al. 1970), RNIST SRM 951(measured) is the mean valueobtained from the NIST SRM 951 measurements and0.00079 is a correction term for the 17O interference.The values obtained directly on aliquots of NIST SRM951 that did not undergo the chemical procedureare also reported in Table 5. As the mean valuesobtained for processed and unprocessed aliquots arevery similar, it can be concluded that no significantisotopic fractionation took place during the chemicalpreparation.

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Table 3.Trace element concentration (in mg kg-1) in solid materials prepared for B isotope intercomparison

B4 B5 B6 B7 B8Tourmaline Basalt Obsidian Limestone Clay

Be 7.1 1.90 8.3 0.05 2.73Sc 74 29 2 2 15V 39 281 0.5 5 116Cr 15 26 1 3 127Co 4 39 0.5 15 21Ni 1 35 1 35 83Rb 2.49 47 274 0.46 119.8Sr 0.53 1201 16.4 224 245Y 16.7 28.3 44 7.2 24.3Zr 6.5 194 183 1.8 87Nb 21.1 45 34 0.09 14Mo 0.1 3.1 6.7 0.03 0.31Cs 1.54 1.03 18.3 0.05 7.4Ba 0.19 632 17.5 6.7 355La 12.5 56 65 4.7 32Ce 38 107 124 1.44 64Pr 4.8 12.6 13.4 0.9 7.7Nd 15.4 48 44.5 3.9 28.9Sm 4.0 9.2 8.8 0.75 5.6Eu < d.l. 2.73 0.15 0.18 1.12Gd 2.4 7.6 6.6 0.84 4.5Tb 0.53 1.1 1.19 0.13 0.73Dy 3.0 5.6 7.0 0.83 4.2Ho 0.53 1.03 1.45 0.18 0.83Er 1.58 2.58 4.3 0.49 2.24Tm 0.34 0.38 0.74 0.07 0.36Yb 2.53 2.07 4.4 0.38 2.08Lu 0.38 0.31 0.69 0.07 0.31Hf 1.66 4.3 6.5 0.03 2.42Ta 14.9 2.26 2.6 0.01 1.09Tl 0.02 0.10 0.93 0.03 0.72Pb 4.4 6.9 33 0.4 22.7Th 0.99 7.6 53 0.1 9.9U 3.9 2.23 15.4 1.65 2.25

The trace element determinations were performed with ICP-MS by M. D’Orazio at the Dipartimento di Scienze della Terra, University of Pisa, Italy.

Aliquots of GSJ JB-2 were also analysed using afusion and ion-exchange procedure (Table 5, Figure 2).The mean δ11B obtained was 7.33 ± 0.37 (2s), which isnot statistically different from the values of Nakamura etal. (1992) and Kasemann et al. (2001). The IGG in-runanalytical precision for individual analyses of GSJ JB-2can vary from 0.15 to 0.3‰ (1s), depending on the

amount and nature of impurities extracted from thesample together with boron. For comparison, the repea-tability - i.e. the closeness of agreement between theresults of successive measurements of the same measu-rand carried out under the same conditions of measure-ment (ISO 1993, Kane 2001) - of 11B/10B determinationsin samples undergoing alkaline fusion was 0.2-0.3‰ (1s).


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Table 4.Results of assays performed at the IGG to check homogeneity of concentration and isotopic composition of boron in milled materials and large fragments of the starting materials before milling

B concentration a δ11B b

(mg kg-1) (‰)

Milled material Fragments Milled material Fragments

B4 – Tourmaline 28683 27824 -8.40 ± 0.45 -8.99 ± 0.1228498 28181 -8.57 ± 0.27 -8.95 ± 0.2228892 - -8.77 ± 0.30 -9.36 ± 0.23

- - -8.82 ± 0.40 -8.63 ± 0.42- - -8.54 ± 0.22 -8.41 ± 0.15- - - -8.75 ± 0.18

Mean value 28691 28002 -8.62 -8.85Std. deviation 197 252 0.17 0.33

B5 – Basalt 8.33 8.65 -4.22 ± 0.49 -4.22 ± 0.498.21 8.42 -4.35 ± 0.35 -3.97 ± 0.648.5 8.26 -3.53 ± 0.40 -3.48 ± 0.54

8.38 8.58 -3.93 ± 0.45 -3.68 ± 0.36- - -4.15 ± 0.62 -

Mean value 8.42 8.48 -3.95 -3.84Std. deviation 0.15 0.17 0.32 0.32

B6 – Obsidian 202.5 196.9 -1.23 ± 0.54 -2.44 ± 0.82199.8 208 -1.38 ± 0.45 -1.30 ± 0.44210.5 217.2 -1.80 ± 0.47 -1.30 ± 0.30

- - -1.83 ± 0.43 -Mean value 205.8 207.4 -1.61 -1.68Std. deviation 7.5 10.2 0.44 0.66

B7 – Limestone 2 1.96 5.95 ± 1.01 6.23 ± 0.651.99 1.99 6.00 ± 0.86 -2.01 - 5.38 ± 0.47 -1.97 - 5.19 ± 0.88 -- - 5.28 ± 1.15 -

Mean value 1.99 1.98 5.67 -Std. deviation 0.02 0.02 0.44 -

B8 – Clay 98 99.2 -4.74 ± 0.40 -4.32 ± 0.4298.8 83 -4.67 ± 0.25 -4.69 ± 0.40

101.2 78 -4.89 ± 0.37 -4.80 ± 0.32101.4 77 -5.14 ± 0.30 -

- 83 - -- 82 - -

Mean value 99.7 80.6 c -4.75 -4.6Std. deviation 1.5 2.9 c 0.25 0.25

a The concentrations were measured by isotope dilution and have a mean internal uncertainty of 0.1%. Valuesin italics of B8 fragments were determined by PGNAA (prompt gamma neutron activation analysis): these values are interconsistent but show a systematic deviation with respect to the ID measurements.

b δ11B values were determined by PTIMS and are affected by an internal uncertainty ranging from 0.1 to 1.2‰ (2s), depending on boron concentration, mineralogical matrix and mass spectrometer working conditions. The internal uncertainty mean value is 0.4‰ (2s).

c PGNAA values only (in italics).

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Table 5.Boron isotopic analysis of NIST SRM 951 and GSJ JB-2 reference materials

11B/10B Internal uncert. 11B/10B δ11Bmeasured (1s) corrected (‰)

Reference material samples without chemistryNIST SRM 951 4.0507 0.0004 4.0445 0.21

4.0500 0.0007 4.0437 0.034.0508 0.0004 4.0445 0.234.0496 0.0003 4.0433 -0.074.0499 0.0005 4.0436 0.004.0503 0.0010 4.0440 0.104.0512 0.0004 4.0449 0.324.0491 0.0010 4.0428 -0.194.0515 0.0008 4.0452 0.404.0512 0.0006 4.0449 0.324.0497 0.0005 4.0435 -0.044.0502 0.0010 4.0439 0.084.0506 0.0006 4.0443 0.184.0478 0.0008 4.0415 -0.514.0499 0.0006 4.0436 0.00

Mean value 4.05017 0.00061 4.04390 0.07Standard deviation 0.00094 - 0.00094 0.23

Reference material samples through full rock chemistryNIST SRM 951 4.0495 0.0009 4.0432 -0.10

4.0508 0.0009 4.0445 0.234.0511 0.0010 4.0448 0.304.0504 0.0005 4.0441 0.134.0497 0.0005 4.0434 -0.054.0502 0.0007 4.0439 0.084.0506 0.0009 4.0443 0.184.0506 0.0009 4.0443 0.184.0488 0.0009 4.0425 -0.274.0487 0.0009 4.0424 -0.294.0508 0.0006 4.0445 0.234.0495 0.0005 4.0432 -0.104.0510 0.0006 4.0447 0.284.0511 0.0014 4.0448 0.304.0485 0.0009 4.0422 -0.344.0494 0.0011 4.0431 -0.124.0487 0.0010 4.0424 -0.294.0490 0.0008 4.0427 -0.224.0504 0.0008 4.0441 0.134.0489 0.0007 4.0426 -0.24

Mean value 4.04990 0.00081 4.04360 0.00Standard deviation 0.00091 - 0.00091 0.23

GSJ JB-2 analysisSep 1999 4.0802 0.0011 4.0739 7.49Sep 1999 4.0801 0.0010 4.0738 7.46Oct 1999 4.0796 0.0005 4.0733 7.34Nov 1999 4.0809 0.0010 4.0746 7.66Nov 1999 4.0797 0.0008 4.0734 7.36Nov 1999 4.0792 0.0007 4.0729 7.24Jan 2000 4.0788 0.0011 4.0725 7.14Jan 2000 4.0790 0.0013 4.0727 7.19Jan 2000 4.0801 0.0008 4.0738 7.46Feb 2000 4.0796 0.0004 4.0733 7.34Feb 2000 4.0790 0.0013 4.0727 7.19Mar 2000 4.0780 0.0006 4.0717 6.94May 2000 4.0794 0.0008 4.0731 7.29Jun 2000 4.0802 0.0011 4.0739 7.49Mean value 4.07960 0.00087 4.0733 7.33Standard deviation 0.00073 - 0.00073 0.18

Reference material samples through full water chemistryNIST SRM 951 4.0485 0.0015 4.0422 -0.34

4.0489 0.002 4.0426 -0.244.0499 0.0018 4.0436 0.004.0494 0.002 4.0431 -0.12

Mean value 4.04920 0.00110 4.04290 -0.18Standard deviation 0.00061 - 0.00061 0.15

Rocks and minerals: homogeneity tests carried out at GFZ Potsdam

Further homogeneity tests at the nanogram orsmaller sampling size were conducted on four of thesolid materials at the GeoForschungsZentrum Potsdam(GFZ) using SIMS (secondary ion mass spectrometry)and EPMA (electron probe microanalysis), the results ofwhich are presented in Tables 6, 7 and 8. Further infor-mation on the material structure and homogeneity wasobtained by BSE (back-scattering electrons) and CL(cathode luminescence) imaging of polished fragmentswith a SEM (scanning electron microscope; Figure 3).The analytical details are given below.

SIMS measurements: The use of SIMS to determinethe isotopic composition and concentration of boron ingeological materials has been steadily increasing inrecent years. However, SIMS is severely hampered bythe need for well characterised reference materialsthat match chemically and structurally the samples tobe analysed. The primary goal of this investigationwas, therefore, to determine which of the solid materialsselected for this intercomparison are suitable forcalibrating in situ boron geochemical measurements.

Of the five rocks and minerals used to prepareintercalibration materials, two were a priori intrinsicallyunsuitable as reference samples for in situ measure-ments: B5 (Mt. Etna basalt), which is highly porphyriticand thus heterogeneous, and B8 (clay), because of itsinherent fine-grained and friable nature. The otherthree samples were investigated using both SIMS andEPMA at the GFZ Potsdam.

SIMS was used to assess the homogeneity of samplefragments at a 20 µm sampling scale. A Cameca ims

6f probe was used to measure the secondary ionintensity ratio between 11B+ and a major element (Si orCa) and also, where possible, between 11B+ and 10B+

(Tables 6 and 7). Specific analytical conditions were


2 9

6.0

6.5

7.0

7.5

8.0

8.5

9.0

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

GSJ JB-2 basalt

Average 7.33 ± 0.37 (2s)

Figure 2. Corrected boron isotopic ratios

measured in the GSJ reference material

rock JB-2 over the period during which

boron isotopic homogeneity tests were

being conducted on intercomparison

materials. Uncertainty bars represent 2s.

Analysis #

δ11B

(‰)

Table 6.Results from SIMS homogeneity tests carried outat GFZ on B4 (tourmaline) and B6 (obsidian)

B4 - Tourmaline

Fragment 11B+/10B+ 1s (%) 11B+/28Si+ 1s (%)

1 3.804 0.25 0.242 0.231 3.809 0.26 0.242 0.221 3.811 0.28 0.242 0.231 3.819 0.26 0.242 0.222 3.820 0.31 0.239 0.242 3.810 0.26 0.239 0.192 3.812 0.23 0.244 0.243 3.816 0.29 0.241 0.243 3.813 0.23 0.240 0.18

mean 3.813 0.26 0.241 0.22s.d. (%) 0.13 - 0.64 -

B6 - Obsidian

Fragment 11B+/10B+ 1s (%) 11B+/30Si+ 1s (%)

1 3.977 0.27 0.017 0.21 3.979 0.23 0.017 0.191 3.971 0.28 0.017 0.171 4.004 0.26 0.016 0.211 3.993 0.29 0.016 0.221 3.995 0.32 0.016 0.221 4.003 0.33 0.016 0.212 3.999 0.35 0.016 0.232 3.972 0.32 0.016 0.242 3.979 0.40 0.016 0.272 3.978 0.40 0.016 0.252 3.992 0.39 0.016 0.292 3.989 0.33 0.016 0.222 3.992 0.40 0.016 0.25

mean 3.987 0.33 0.016 0.23s.d. (%) 0.28 - 1.92 -

Measured 11B+/10B+ and 11B+/Si+ ratios are not corrected for machine- and matrix-induced fractionations.

3 0


Table 7.Results of homogeneity tests performed at the GFZ on material B7 (limestone) using SIMS (above) and EPMA (below)

Fragment 11B+/42Ca+ 1s (%)

1 9.21E-06 8.31 1.94E-05 5.41 1.01E-04 2.21 2.60E-05 7.32 4.13E-05 7.22 1.10E-05 5.03 1.36E-05 4.33 5.76E-05 3.6

mean 3.49E-05 5.4s.d. (%) 90.1 -

Na2CO3 MgCO3 CaCO3 SrCO3 BaCO3 FeCO3 MnCO3

Fragment 1max 0.19 0.88 99.31 0.48 0.05 0.07 0.06min 0.00 0.38 96.82 0.00 0.00 0.00 0.00Mean 0.04 0.65 97.75 0.18 0.01 0.02 0.02Std. deviation 0.05 0.15 0.52 0.10 0.01 0.03 0.02



Cumulativemax 0.19 1.10 99.91 0.48 0.11 0.12 0.06min 0.00 0.11 94.27 0.00 0.00 0.00 0.00Mean 0.03 0.64 98.04 0.15 0.03 0.02 0.02Std. deviation 0.03 0.20 1.10 0.09 0.04 0.03 0.02

Values in % m/m.

Table 8.EPMA results obtained at GFZ on B4 (tourmaline), B5 (porphyritic basalt) and B6 (obsidian)

B4 Fragment 1 (n = 10) B4 Fragment 2 (n = 10)

max min mean s.d. max min mean s.d.

B2O3 10.79 10.12 10.4 0.19 11.55 10.59 10.90 0.28

Na2O 1.95 1.76 1.84 0.05 2.36 2.12 2.23 0.09

SiO2 34.85 34.19 34.56 0.21 35.22 34.61 34.91 0.19

MgO 2.04 1.76 1.93 0.08 2.40 0.37 1.35 0.66

Al2O3 34.88 34.35 34.72 0.15 40.07 32.83 35.27 2.49

K2O 0.05 0.03 0.04 0.01 0.05 0.02 0.03 0.01

CaO 0.40 0.34 0.36 0.02 0.86 0.27 0.55 0.21

MnO 0.27 0.15 0.18 0.04 1.12 0.53 0.81 0.22

Cr2O3 0.01 0.00 0.00 0.00 0.02 0.00 0.00 0.01

FeO 13.79 13.31 13.48 0.15 14.18 6.46 11.74 2.66

Total - - 97.51 0.35 - - 97.79 0.86


3 1

Table 8 (continued).EPMA results obtained at GFZ on B4 (tourmaline), B5 (porphyritic basalt) and B6 (obsidian)

B4 Fragment 3 (n = 10) B5 Porphyritic Basalt (n = 17)

max min mean s.d. max min mean s.d.

B2O3 11.08 10.58 10.79 0.19 SiO2 55.61 47.32 52.60 2.35Na2O 1.92 1.72 1.83 0.07 Al2O3 28.90 9.04 19.74 5.17SiO2 34.71 34.14 34.4 0.16 TiO2 2.88 0.08 1.18 0.84MgO 1.83 1.02 1.51 0.27 MgO 9.10 0.09 2.88 2.33Al2O3 35.07 34.64 34.85 0.12 CaO 16.90 4.56 9.33 2.99K2O 0.05 0.03 0.04 0.01 Na2O 6.05 2.25 4.89 0.93CaO 0.35 0.3 0.33 0.01 K2O 5.86 0.49 2.76 1.65MnO 0.23 0.14 0.18 0.03 Cl 0.27 0.00 0.10 0.09Cr2O3 0.02 0.00 0.01 0.01FeO 15.22 13.7 14.25 0.4

Total - - 98.19 0.26 Total - - Heterog. Heterog.

B6 Obsidian (n = 19)

max min mean s.d.

SiO2 75.94 74.23 75.25 0.43Al2O3 13.19 12.99 13.10 0.07TiO2 0.11 0.04 0.08 0.02MgO 0.06 0.03 0.04 0.01CaO 0.79 0.70 0.74 0.02Na2O 3.52 3.33 3.44 0.05K2O 5.51 5.21 5.37 0.09Cl 0.36 0.29 0.33 0.02

Total - - 98.28 0.46

Values in % m/m.

Figure 3. Back-scattered electron microscope (BSE) and cathodoluminescence (CL) images

of fragments of solid materials B4 to B7. Width of field of view for all images is 1 mm.

B4: Tourmaline BSE

B6: Obsidian BSE B6: Obsidian CL B7: Limestone BSE

B4: Tourmaline CL B5: Basalt BSE

selected for each sample in order to comply with thematrix composition and the bulk boron content. Allanalyses were conducted at the moderate mass reso-lution of M/∆M ≥ 1850 which is sufficient to eliminateall significant isobaric interferences. Analyses were alsocarried out with a 12.5 kV 16O- primary ion beam anda nominal 10 kV secondary ion extraction potential.The ion counting mode was used for data acquisitionand the analyses were preceded by a 3 minute pre-burn. Spot spacing for SIMS analyses typically rangedfrom 500 to 1000 µm. Other basic SIMS analyticalparameters for each of the three samples were:

- B4 tourmaline: 4 nA primary beam intensity, pro-ducing a ~ 15 µm diameter crater; 80 V energy offset,50 cycles of the sequence 9.5 bkg, 10B+ (16 s), 11B+

(8 s), 28Si+ (2 s), equivalent to a total analysis time of27 minutes.

- B6 obsidian: 12 nA primary beam intensity, pro-ducing a ~ 20 µm diameter crater; 0 V energy offset,50 cycles of the sequence 9.5 bkg, 10B+ (16 s), 11B+

(8 s), 30Si+ (2 s), equivalent to a total analysis time of27 minutes.

- B7 limestone: 20 nA primary beam intensity, pro-ducing a ~ 20 µm diameter crater; 0 V energy offset,20 cycles of the sequence 10.75 bkg, 11B+ (30 s),42Ca+ (2 s), equivalent to a total analysis time of14 minutes. Note that, due to its low boron content,the 10B+ counting rate for B7 was only 10 ions s-1.Therefore, only the test of boron concentration homo-geneity was possible but not a precise measurement ofthe isotopic ratio.

In conclusion, the SIMS measurements revealed anacceptable degree of homogeneity for the tourmalineand obsidian samples (Table 6). In contrast, the mea-surements indicated that the boron concentration ishighly variable in the limestone sample (Table 7), inagreement with the heterogeneity observed with theelectron microscope (Figure 3f).

Electron probe microanalysis: Wavelength disper-sive EPMA was used to further characterise samples B4to B7 and to test the homogeneity of major elementd is t r ibut ion at the µm-level spat ial resolut ion . ACameca SX50 electron probe, equipped with the PC2(2d = 9.56 nm) crystal for the quantitative analysis oflight elements, was used for the tourmaline sample B4.The boron determination was performed with a 40nA/10 kV probe, while other elements were measured

with a 20 nA/15 kV probe focused to a 15 µm diame-ter. The data show a zoned distribution of the majorelements in the tourmaline crystals, which is particularlyevident for Mg, Al and Fe (Table 8).

A Cameca SX100 electron probe was used for theMt. Etna basalt (B5) and the obsidian (B6) samples(Table 8). The analyses were performed with a 20nA/15 kV probe focused to a 15 µm diameter beam.For B5, the goal was to document the overall chemicalheterogeneity of this crystal-rich sample. In contrast, B6was found to be chemically very homogeneous, whichis consistent with the BSE and CL images obtainedfrom chips of this sample (Figures 3d and 3e).

Finally, a Cameca SX100 electron probe was usedto analyse the composi t ion of the B7 l imes tonesample, with a 20 nA/15 kV probe focused to a 20µm diameter beam. The data (Table 7) show a limiteddegree of variation, with calcite/aragonite being thedominant phase. Our EPMA data failed to reflect thehigh degree of heterogeneity of the boron contentshown by SIMS.

Rocks and minerals: origin and characteristics of materials

Material B4: Elba tourmaline: Material B4 wasderived from a composite sample of euhedral crystalsof schorl tourmaline from the Rosina pegmatite dykehosted in monzogranite, near San Piero in Campo,Elba Island. The age of the formation is 6.9 Ma (Dini etal. 2002). The well-formed, prismatic crystals, 2-3 cm insize, were hand-picked from the coarse grained portionof the pegmatitic body. The composite sample wasnearly pure, but, after a first crushing, some quartz andfeldspar impurities were removed by magnetic separa-tion. The material was then milled in an agate mill.

Material B4 was collected from the same site as theschorl N1/b studied by Tonarini et al. (1998), whichhad a fully uniform boron isotopic composition, even inthe co-existing evolved crystals of elbaite. Incidentally,this fact demonstrates that the processes producingchemical differentiation during tourmaline crystallisa-tion are not inherently accompanied by boron isotopefractionation. The BSE images do not show any ele-mental zoning or alteration, whereas the CL images doreveal a detectable banding (Figures 3a and 3b).

The PTIMS measurements confirmed that the milledmaterial and spot samples from different fragments of

3 2


schorl crystals from the same pegmatite dyke yieldconsistent δ11B values with standard deviations compa-rable to the observed internal uncertainty of individualmeasurements (Table 4, Figure 4a). The isotopic homo-gene i t y o f B4 was fu r the r con f i rmed by S IMSmeasurements carried out at the GFZ, for which thein-run precision and the external reproducibility weresimilar (Table 6). This tourmaline is therefore a goodsource for intercomparison/intercalibration material forboron isotope measurements, in spite of its non-homo-geneous chemical composition. Its mean δ11B-value,around -9‰, places B4 right in the middle of the δ11Brange of pegmatitic tourmalines (Jiang and Palmer 1998).

Several structural studies of tourmaline have shownthat boron content is close to the stoichiometric compo-sition. Nevertheless, recent studies (Marler and Ertl2002, and references therein) have reported theoccurrence of tourmalines with a significant fraction ofexcess boron in tetrahedral sites, which may haveimpl icat ions for the boron iso topic composi t ion .Although no exhaustive study was carried out on B4, a

SIMS invest igation on tourmalines from the samepegmatitic complex did not detect any tetrahedrallycoordinated boron in the Elba tourmalines (Aurisicchioet al . 1999), where boron concentration is nearlyconstant. Significant variations affect the concentrationof other major elements, as proved by EPMA analysescarried out on three fragments of tourmaline B4 byWilliam P. Leeman at Rice University, USA, and micro-probe analyses by Tonarini et al. (1998) on differentschorl crystals from the same dyke. In particular, theEPMA data on mineral fragments obtained at the GFZshowed that their iron and aluminium contents areinversely correlated, although the overall compositionremains within the schorl range (Table 8). Somewhatsimilar results were also obtained by Thomas Zack(pe r s . comm. ) a t t he Memor ia l Un i ve r s i t y o fNewfoundland, Canada.

Material B5: Etna basalt: This material derivesfrom porphyritic hawaiite (basalt) erupted in July 22,1998, from Mt. Etna’s main crater. δ11B and boronconcentration in different rock fragments as well as inthe milled material range from -3.48 to -4.35‰ andfrom 8.21 to 8.65 µg g-1, respectively (Table 3). Otherδ11B values from Mt. Etna hawaiites range between-3.38 and -4.35‰, a span which is only slightly greaterthan the measurement uncertainty limits (Tonarini et al.2001). Similar spans of δ11B values observed in rockfragments and in milled material suggest that any iso-topic heterogeneity present in this material is small(Figure 4b). Rather unexpectedly, the δ11B-value of B5falls in the range typical for MORB (Mid Ocean RidgeBasalt) and is higher than that of OIB (Ocean IslandBasalt; Chaussidon and Marty 1995).

Back-scattered electron (BSE) images of materialB5 (Figure 3c) document the highly porphyritic natureof this sample. As this sample is highly heterogeneousat the < 100 µm scale no SIMS data were collected onit. The chemical variations revealed in B5 by EPMAappear normal for a porphyritic rock (Table 8). Thus,this material is clearly unsuited for intercomparisonwork involving micro-sampling techniques, but at the100 mg sampling sizes these variations appear to bemore than adequately smoothed out.

Material B6: Lipari obsidian: This well-knownobsidian was collected from the Rocche Rosse flow ofLipari Island (Aeolian Archipelago, southern TyrrhenianSea), dated to 1400 ± 450 years BP (Bigazzi andBonadonna 1973). This obsidian, composed of morethan 98% glass, was selected because of its very


3 3

Figure 4. Comparison of boron isotopic compositions

(as δ11B‰) measured in fragment (open diamond) and

milled materials (filled circle) for (a) B4 tourmaline and

(b) B5 basalt. Uncertainty bars and uncertainties of

averages represent 2s.

Analysis #

δ11B

(‰)

δ11B

(‰)

-10

-9.5

-9

-8.5

-8

-7.5

-7

0 2 4 6 8 10 12

Average = -8.62 ± 0.17

Average = -8.85 ± 0.33

B4 tourmaline

a

-5.5

-5

-4.5

-4

-3.5

-3

-2.5

0 2 4 6 8 10

Average = -3.95 ± 0.32

Average = -3.84 ± 0.33

B5 basalt

b

homogeneous texture and boron content (≈ 200 mgkg-1, i.e. within the optimal range of measurement).

Again, the boron isotopic and concentration valuesobtained on the milled material were similar to thosefrom chips of the starting material (Table 4, Figure 5a),implying that B6 is homogeneous at the sub-mg sam-pling scale. The spread of values of this material,however, is somewhat wider than for other materials,which may be due to its high sil ica content: after

undergoing alkali fusion, the latter could cause difficul-ties during the ion exchange purification procedure(see section below on analytical problems encounte-red at IGG).

Back-scattered electron and cathodo-luminescence(CL) images of individual chips of B6 starting materialdo not reveal any heterogeneity (Figure 3d and 3e).SIMS analyses are also consistent with the materialbeing homogeneous in both boron isotopic composi-tion and concentration at the sampling scale of thistechnique (Table 6). Finally, the EPMA analyses onchips from the B6 starting material confirm a fullyhomogeneous chemical composition (Table 8). Thus,material B6 appears to be eminently suitable as areference material for SIMS measurement.

Material B7: Maiella l imestone: The star t ingmaterial is a bioclastic calcarenite of Miocene age,consisting of nearly pure calcite with rare glauconitespots and a low boron content (about 2 mg kg-1). Itwas collected in a quarry at Lettomanoppello, 30 kmSW of Pescara, in the foothills of the Maiella Massif,Abruzzo. The material shows practically no alterationand empty pore space occupies a considerable volume;calcareous fossils are clearly visible in the sample(Figure 3f). The homogeneity of the boron isotopiccomposition was not extensively tested in the startingmaterial since its low boron concentration would haveentailed considerable analytical work. Data from mul-tiple aliquots of the milled material do, however, indi-cate a high level of homogeneity (Table 4).

The δ11B of material B7 was ≈ 6‰ (Figure 5b), i.e.somewhat below that of modern marine carbonates,ranging from 10 to 30‰ (Vengosh et al . 1991b,Hemming and Hanson 1992, Gaillardet and Allègre1995). The 11B depletion in old carbonate, paralleledby the drop in boron content from the 30-50 mg kg-1

typical of modern carbonates to 2 mg kg-1, suggestsa boron loss during diagenesis, accompanied by anisotopic fractionation (with 11B being preferentiallyremoved).

SIMS analyses of the starting material revealed asignificant scatter of boron concentration values at theng sampling scale (Table 7). SIMS isotopic determina-tions were not possible because of the low boroncontent . EPMA analyses on the s tar t ing mater ialshowed some scatter in the data, with CaCO3 rangingbetween 94.3 and 99.9% and the rest consistingmainly of MgCO3 (Table 7).

3 4


Figure 5. Comparison of boron isotopic compositions

(as δ11B‰) measured in fragment (open diamond) and

milled materials (filled circle) for (a) B6 obsidian, (b) B7

limestone and (c) B8 clay. Uncertainty bars and

uncertainties of averages represent 2s.

Analysis #

δ11B

(‰)

δ11B

(‰)

δ11B

(‰)

-3

-2.5

-2

-1.5

-1

-0.5

0

0 2 4 6 8

B6 obsidianAverage = -1.61 ± 0.44

Average = -1.68 ± 0.66a

4

4.5

5

5.5

6

6.5

7

0 2 4 6 8

B7 limestone

Average = 5.67 ± 0.44 b

-6

-5.5

-5

-4.5

-4

-3.5

-3

0 2 4 6 8

B8 clay

Average = -4.75 ± 0.25

Average = -4.60 ± 0.25c

Material B8: Montelupo clay: This Pliocene claywas co l lec ted f rom a depos i t near Mon te lupoFiorentino, 20 km W of Florence, which has been usedfor centuries for earthenware pottery manufacture. Theclay was heated for three days at 120 °C to remove thebulk of the water, and then milled and homogenised.Major components were SiO2 60% m/m, Al2O3 18%m/m, FeO 6% m/m and K2O 4% m/m.

The δ11B values of three samples of the startingmaterial closely agreed with those obtained on themilled material (Table 4, Figure 5c). The boron concen-tration was measured in five samples of the startingmaterial by PGNAA (prompt gamma neutron activa-tion analysis) by Actlabs, Ontario, Canada. The valuesobtained were consistent within the analytical uncer-tainty, but systematically about 20% lower than thevalues obtained by isotope dilution on the milledmaterial. The reason for this discrepancy is unknown.

Both δ11B (≈ -5‰) and boron content (100 mgkg-1) are well within the range of clays defined byIshikawa and Nakamura (1993).

Determination of the isotopic compositionof the intercomparison materials: problems encountered at IGG

Due to the great variety of rock composit ions,matrix properties, and boron concentrations, no singleanalytical procedure can be applied equally to all theintercomparison materials. Each material may presentspecific analytical problems related to the chemicaltreatment and/or the mass spectrometric measurement.It could therefore prove very difficult, or even impos-sible, to keep measurement uncertainty at the samegood level for all materials. Some details on specificanalytical problems encountered at IGG are given below.

Waters

During the repeated PTIMS measurements of boronextracted from water samples, the occurrence of spu-rious peaks at various m/e values was observed. Itwas impossible to identify the nature and origin ofthese peaks, some of which interferred with those to bemeasured, but they were certainly the cause of thepoor repeatability of some results, for which the stan-dard deviation was 2 to 5 times larger than normal.

A major interfering peak occurred at m/e = 307,produced by ions that may have contained the isotope

10B. This would imply the occurrence at m/e = 308 ofan associated peak, four times higher, due to 11B,which superimposes on the peak to be measured for(133Cs2

10B16O2)+ ions. Ion pairs that produce peaks atm/e = 307 and 308 could be the two isotopic formsof (Cs2BP)+ and (Cs2BCF)+. Another pair of interferingisobaric ions could be (Cs2

10BS)+ and (Cs211BS)+, with

the same m/e ratios, 308 and 309, as the peaks tobe measured. These sulfur-bearing ions, however,would have the same boron isotope ratio as the(Cs2BO2)+ ion pair (isotopic fractionations apart) and,therefore, would have only a minor influence on theisotopic ratio of (Cs2BO2)+ ions. However, ions contai-ning 12C and 32S would be accompanied by ionscontaining the less abundant isotopes 13C and 33Swhich would also have an effect on the data.

None of the above conjectures could be definitelyproven, so no attempt was made to correct the massspectrometric data. Fortunately, during the measure-ment, the spurious ions tend to reduce their contribu-tion and to disappear from the mass spectrometricpeak scan more rapidly than the ions produced bycesium tetraborate. As a consequence, the ion ratio309+/308+ tends to reach a steady value, which isbelieved to reflect the true 11B/10B ratio of the sample.

We tentatively excluded the occurrence of isobaricions formed by organic substances for two reasons.First, organic ions, mainly composed of C, H and O,would have a mass about 0.3 a.u. higher than ionsformed by cesium tetraborate, and induce a degradedpeak shape that was never observed (although, admit-tedly, the peak definition at m/e > 300 is such that theeffect might be hardly detectable). Secondly, in anattempt to eliminate or reduce the possible contribu-tion of organic ions, boron was extracted from the eva-porated water residue by fusion with K2CO3 in a 5:1weight proportion. The K2CO3 melting temperature is891 °C, at which organic matter must be destroyed.Notwithstanding this treatment, the measurements per-formed on the fusion product showed a peak at m/e =307, suggesting a non-organic origin.

The spurious ions might be the product of tracecomponents/contaminants presen t in the watersamples. In an empirical attempt at improving the ana-lytical results, the purification procedure finally adop-ted to separate boron from other anions and cationsincluded an additional final step through the anion-exchange column (AG 1-X8, 200-400 mesh), after theusual steps through anion-exchange (Amberlite IRA


3 5

743) and cation-exchange (Dowex 50W-X8, 200-400mesh) columns. The additional step improved thedegree of sample purification and helped to reducespurious peaks in mass spectrometry.

Rocks and minerals

The problems described in this section equallyapply to the measurements of both boron isotopiccomposition and concentration, the latter being deter-mined by isotope dilution (ID), which also involves the massspectrometric measurement of boron isotope ratios.

Boron extraction: Relatively low boron concentra-t ions determined on mater ial B4 may be due toincomplete alkaline fusion of tourmaline, the stabilityfield of which may extend above the K2CO3 meltingtemperature (Benard et al. 1985). This problem isbelieved to affect only the boron concentration measu-rements, because no isotopic fractionation is expectedto take place during the alkaline fusion.

Incomplete alkaline fusion may also affect thebasalt B5. However, the recovery tests for basic rocksindicate that all boron is extracted by the after-fusiondissolution (Tonarini et al. 1997). This is probably dueto the very low boron partition coefficient between theminerals commonly present in this type of rock and theglass phase, where practically all boron is accommo-dated (Brenan et al. 1998).

Boron purification: Boron separation from thematrix was particularly challenging for material B6(obsidian), which virtually consists of glass with highsilica content (75% m/m). With this type of material,the solution obtained after the alkali fusion may besupersaturated in silica, so that supernatant colloidalsilica containing some boron may form, which cannotbe separated by centrifugation. During purificationthrough boron-specific resin (Amberlite IRA 743), thecolloid is retained on the column, or might even haveformed and precipitated there, thus reducing thethrough-flow and retaining some boron. The formationof colloidal silica can be avoided by diluting the solu-tion in order to avert silica oversaturation.

Mass spectrometry: In the mass spectrometricmeasurement of material B7 (limestone), the ion beamwas sufficiently high but it increased and subsequentlydecreased faster than normal. In a good mass spectro-metric measurement, the ion beam should, after havingreached the maximum intensity, decrease at a rate of

0.5 to 3% per minute during which data acquisitiontakes place. During measurement of B7, however, theion beam decrease rate was always greater than 5%per minute so that the in-run precision dropped by afactor of 2 to 4. This behaviour is believed to be dueto the lack of any other component besides carbonatein the solution deposited on the ion source filament(e.g., no silica present), which might suppress andstabilize the boron evaporation and ionisation rate.

Conclusions

Eight geological materials, all from Italian sources,were selected for intercomparison of boron isotopeand concentration measurements. The materials consistedof three natural waters (Mediterranean seawater andtwo groundwaters with large differences in δ11B and Bcontent) and five rocks and minerals characterized bya great variety of compositions, matrix properties, andboron concentrations.

Prior to distribution, the water samples (isotopicallyand chemically homogeneous by nature) were filteredand acidified. The mineral and rock samples werecrushed, milled and mixed using the quartering tech-nique. Boron isotopic ratio and concentration measure-ments on individual fragments of the starting materials,as well as on the milled materials, proved that theintercomparison samples were isotopically homoge-neous, not only as a consequence of mill ing andmixing, but also because the starting substances werethemselves quite homogeneous.

Further homogeneity tests on four of the solid mate-rials at the nanogram or smaller sampling size usingSIMS and EPMA indicated that both the tourmaline B4and the obsidian B6 are potentially good referencematerials for future in situ microanalyses of boron isotopes.

The materials prepared for this intercomparison ofboron isotope and concentration results are now avai-lable from the IAEA for distribution (with the exceptionof the three waters, which are kept at, and will bedistributed by, the IGG). Any laboratories, interested incontributing to our joint effort to improve the measure-ment intercalibration, are warmly urged to request andmeasure these materials and report their results to theIAEA (attention of Dr. M. Gröning). Comments and sug-gestions addressed to the authors of this report andIAEA are most welcome. We regard this intercompari-son as an initial benchmark, from which the boron iso-tope geochemical community can plan future actions

3 6


for strengthening the comparability of data obtained indifferent laboratories and with different techniques.

Acknowledgements

The financial support of the International AtomicEnergy Agency (IAEA), Vienna, to IGGI, through theTechnical Contract No. 10725/R0/RBF, is gratefullyacknowledged. We acknowledge with thanks thecollaboration of several colleagues and institutions,namely:

The Centro Interdisciplinare di Biologia Marina,Livorno, for collecting the seawater sample B1.

Etta Patacca (University of Pisa), Paolo Scandone(Un i ve r s i t y o f P i sa ) , and Dav id S . Wes te rman(Northwich University, Vermont, USA), for help in procu-ring the intercomparison materials B4 and B7.

Mass imo Pompi l io ( I s t i tu to In te rnaz ionale d iVulcanologia, Catania), for procuring the material B5.

Giuliana De Grandis (Ist i tuto di Geoscienze eGeorisorse, Pisa), for preparing the rock and mineralmaterials for distribution.

Francesca Podda (University of Cagliari) and LisaPierotti (Istituto di Geoscienze e Georisorse, Pisa),for carrying out the chemical analyses of the watersamples.

Massimo D’Orazio (University of Pisa), for carryingout the chemical analyses of the rock and mineralmaterials.

Giancarlo Pardini and Umberto Giannotti (Istitutodi Geoscienze e Georisorse, Pisa), for assistance inmass spectrometry.

Ilona Shäpan (GeoForschungsZentrum Potsdam),for assistance in SIMS analyses.

Oona Appelt and Dieter Rhede (GeoForschungs-Zentrum Potsdam), for performing EPMA analyses.

Ursula Glenz (GeoForschungsZentrum Potsdam), forproviding the electron microscope images.

The final editing of the English was by Mary H.Dickson (IGG).

References

Aggarwal J.K. and Palmer M.R. (1995)Boron isotope analysis: A review. The Analyst, 120,1301-1307.

Aurisicchio C., Ottolini L. and Pezzotta F. (1999)Electron- and ion-microprobe analyses, and genetic inferences of tourmalines of the foitite-schorl solid solution,Elba Island (Italy). European Journal of Mineralogy, 11,217-225.

Bassett R.L. (1990)A critical evaluation of the available measurements forthe stable isotopes of boron. Applied Geochemistry, 5,541-554.

Benard F., Moutou P. and Pichavant M. (1985)Phase relations of tourmaline leucogranites and the significance of tourmaline in silicic magmas. Journal ofGeology, 93, 271-291.

Benton L.D., Tomascak P.B. and Helz R.T. (1999)A study of boron isotopes in Kilauea Iki lava lake, Hawaii.Abstract. EOS Transactions, American GeophysicalUnion, 80, F1197.

Bigazzi G. and Bonadonna F. (1973)Fission track dating of the obsidian of Lipari Island (Italy).Nature, 242, 322-323.

Brenan J.M., Neroda E., Lundstrom C.C., Shaw H.F.,Ryerson F.J. and Phinney D.L. (1998)Behaviour of boron, beryllium and lithium during meltingand crystallization: Constraints from mineral-melt partitioning experiments. Geochimica et CosmochimicaActa, 62, 2129-2141.

Catanzaro E.J., Champion C.E., Garner E.L.,Marienko O., 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 Marty B. (1995)Primitive boron isotope composition of mantle. Science,269, 383-386.

Coplen T.B., Hopple J.A., Böhlke J.K., Peiser H.S.,Rieder S.E., Krouse H.R., Rosman K.J.R., Ding T.,Vocke R.D., Jr., Révész K.M., Lamberty A., Taylor P.,and De Bièvre P. (2002)Compilation of minimum and maximum isotope ratios of selected elements in naturally occurring terrestrialmaterials and reagents. Water-Resources InvestigationsReport 01-4222, U.S. Department of Interior-U.U.Geological Survey, Reston, Virginia, 98pp.

De Bièvre P. (1993)Certificate isotopic reference materials IRMM-011. Institutefor Reference Materials and Measurements, EuropeanUnion, Joint Research Centre, Geel, Belgium.


3 7

references

Deyhle A. (2001)Improvements of boron isotope analysis by positive thermal ionization mass spectrometry using static multicollection of Cs2BO2

+ ions. International Journal ofMass Spectrometry, 206, 79-89.

Ding T., Valkiers S., Kipphardt H., De Bièvre P.,Taylor P.D.P., Gonfiantini R. and Krouse R. (2001)Calibrated sulfur abundance ratios of three IAEA sulphurisotope reference materials and V-CDT with a reassessment of the atomic weight of sulfur. Geochimicaet Cosmochimica Acta, 65, 2433-2437.

Dini A., Innocenti F., Rocchi S., Tonarini S. andWesterman D.S. (2002)The magmatic evolution of the late Miocene laccolith-pluton-dyke granitic complex of Elba Island, Italy.Geological Magazine, 139, 257-279.

Gaillardet J. and Allègre C.J. (1995)Boron isotopic composition of corals: Seawater or diagenesis record? Earth and Planetary Science Letters,136, 665-676.

Gonfiantini R. (1978)Standards for stable isotope measurements in naturalcompounds. Nature, 271, 534-536.

Gonfiantini R., Tonarini S., Gröning M., Adorni-Braccesi A., Al Ammar A.S., Astner M.,Bächler S., Barnes R.M., Bassett R.L., Cocherie A.,Deyhle A., Dini A., Ferrara G., Gaillardet J., Grimm J., Guerrot C., Krähenbühl U., Layne G.,Lemarchand D., Meixner A., Northington D.J.,Pennisi M., Reiznerová E., Rodushkin I., Sugiura N.,Surberg R., Tonn S., Wiedenbeck M., Wunderli S.,Xiao Y. and Zack T. (2003)Intercomparison of boron isotope and concentrationmeasurements. Part II: Evaluation of results. GeostandardsNewsletter: The Journal of Geostandards andGeoanalysis, 27, 41-57.

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

Ishikawa T. and Nakamura E. (1993)Boron isotope systematics of marine sediments. Earth andPlanetary Science Letters, 117, 567-580.

ISO (1993)International vocabulary of basic and general terms inmetrology. 2nd Edition, ISO (International Organizationfor Standardization), Geneva.

Jiang S.Y. and Palmer M.R. (1998)Boron isotope systematics of tourmaline from granitesand pegmatites. European Journal of Mineralogy, 10,1253-1265.

Kakihana H. and Kotaka M. (1977)Equilibrium constants for boron isotope-exchange reactions. Bulletin of the Research Laboratory forNuclear Reactors, 2, 1-12.

Kakihana H., Kotaka M., Satoh S., Nomura M. andOkamoto M. (1977)Fundamental studies on the ion-exchange separation ofboron isotopes. Bulletin of the Chemical Society ofJapan, 50, 158-163.

Kane J.S. (2001)The use of reference materials: A tutorial. GeostandardsNewsletter: The Journal of Geostandards andGeoanalysis, 25, 7-22.

Kasemann S., Meixner A., Rocholl A., Vennemann T., Rosner M., Schmitt A.K. andWiedenbeck M. (2001)Boron and oxygen isotopic composition of certified reference materials NIST SRM 610/612 and referencematerials JB-2 and JR-2. Geostandards Newsletter: TheJournal of Geostandards and Geoanalysis, 25,405-416.

Lecuyer C., Reynard B., Albarède F. and Télouk P. (2000)Analysis of geochemical materials by ICP-MS Plasma 54:Methodology and paleo-pH of seawater (Abstract). EOSTransactions, American Geophysical Union, 81 (48), FallMeeting.

Leeman W.P., Vocke R.D. Jr., Beary E.S. andPaulsen P.J. (1991)Precise boron isotopic analysis of aqueous samples: Ionexchange extraction and mass spectrometry. Geochimicaet Cosmochimica Acta, 55, 3901-3907.

Leeman W.P. and Sisson V.B. (1996)Geochemistry of boron and its implications for crustaland mantle processes. In: Grew E.S. and Anowitz L.M.(eds), Boron mineralogy, petrology and geochemistry.Reviews in Mineralogy, 33, Mineralogical Society ofAmerica, Washington D.C., 645-707.

Marler B. and Ertl A. (2002)Nuclear magnetic resonance and infrared spectroscopicstudy of excess-boron olenite from Koralpe, Styria, Austria.American Mineralogist, 87, 364-367.

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

Palmer M.R. and Swihart G.H. (1996)Boron isotope geochemistry: An overview. In: Grew E.S.and Anowitz L.M. (eds), Boron mineralogy, petrologyand geochemistry. Reviews in Mineralogy, 33,Mineralogical Society of America, Washington D.C.,709-744.

Pennisi M., Tonarini S., Gonfiantini R., Grassi S.,Rossi S. and Squarci P. (1999)Boron and strontium isotopes in studying the high boroncontent of the alluvial aquifer of the Cecina River valley,Central Italy. Proceedings of the International Symposiumon Isotope Techniques in Water Resources Developmentand Management, IAEA, Vienna. Book of extendedsynopses, 231-232.

Schwarcz H.P., Agyei E.K. and McMullen C.C. (1969)Boron isotope fractionation during clay adsorption fromsea water. Earth and Planetary Science Letters, 6, 1-5.

3 8


references

Spivack A.J. and Edmond J.M. (1986)Determination of boron isotope ratios by thermal ionization mass spectrometry of the dicesium metaboratecation. Analytical Chemistry, 58, 31-35.

Spivack A.J. and Edmond J.M. (1987)Boron isotope exchange between seawater and theoceanic crust. Geochimica et Cosmochimica Acta, 51,1033-1043.

Tonarini S., Armienti P., D’Orazio M. and Innocenti F. (2001)Subduction-like fluids in the genesis of Mt. Etna magmas:Evidence from boron isotopes and fluid mobile elements.Earth and Planetary Science Letters, 192, 471-483.

Tonarini S., Dini A., Pezzotta F. and Leeman W.P. (1998)Boron isotopic composition of zoned (schorl-elbaite) tourmalines, Mt. Capanne Li-Cs pegmatites, Elba (Italy).European Journal of Mineralogy, 10, 941-951.

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

Vengosh A., Chivas A.R., McCulloch M.T., Starinsky A. and Kolodny Y. (1991a)Boron isotope geochemistry of Australian salt lakes.Geochimica et Cosmochimica Acta, 55, 2591-2606.

Vengosh A., Kolodny Y., Starinsky A., Chivas A.R.and McCulloch M.T. (1991b)Coprecipitation and isotopic fractionation of boron inmodern biogenic carbonates. Geochimica etCosmochimica Acta, 55, 2901-2910.

Vengosh A., Heumann K.G., Juraske S. and Kasher R. (1994)Boron isotopes application for tracing sources of contamination in groundwater. Environmental Scienceand Technology, 28, 1968-1974.

Vengosh A., Starinsky A., Kolodny Y., Chivas A.R.and Raab M. (1992)Boron isotope variations during fractional evaporation ofsea water: New constraints on the marine vs. non-marinedebate. Geology, 20, 799-802.


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Documents

Intercomparison of Boron Isotope and Concentration Measurements. Part I: Selection, Preparation and Homogeneity Tests of the Intercomparison Materials