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INTER-LABORATORY NOTE

Precise lead isotope ratios in Australian galena

samples by high resolution inductively coupled

plasma mass spectrometry

Ashley T. Townsend*a, Zongshou Yub, Peter McGoldrickb and James A. Huttona

aCentral Science L aboratory, University of T asmania, GPO Box 252-74, Hobart, T asmania, 7001, Australia

bCentre for Ore Deposits and Exploration Studies, University of T asmania, GPO Box 252-79, Hobart, T asmania, 7001, Australia

High resolution inductively coupled plasma mass spectrometry   After careful consideration of all possible causes of instrumental

bias, the precision obtained from the analysis of NIST Standard(HR-ICP-MS) was used to measure Pb isotope ratios in

standard solutions and Pb mineral digests. The RSD values   Reference Material (SRM ) 981 Natural Lead ranged from 0.04to 0.12%, depending on the ratio considered. The accuracyobtained for   208Pb/204Pb,   207Pb/204Pb,   206Pb/204Pb,   208Pb/206Pb

and   207Pb/206Pb were 0.13, 0.11, 0.11, 0.046 and 0.048%,   and precision reported may be the best obtained from a

quadrupole instrument. However, the methodology and detailrespectively (values as 1s). These values were obtained from30 analyses of three diff erent standard sample types (multi-   required to obtain such results could not be called routine.

High resolution ICP-MS (HR-ICP-MS) is a relatively newelement standard, NIST SRM 981 and a Broken Hill galena

digest). Based on 39 analyses of 11 galena samples from   technique employing a magnetic sector mass analyser. To dateit has been applied in a variety of areas such as environmentaldiff erent locations around Australia, the diff erence between

HR-ICP-MS and conventional TIMS values for   208Pb/204Pb,   and biological analysis and the monitoring of radionuclides(see the review by Becker and Dietze9). Only a limited number207Pb/204Pb,   206Pb/204Pb,   208Pb/206Pb and   207Pb/206Pb ratios

was generally better than±0.2%. This paper outlines a very   of studies have been reported on measuring isotope ratios.10–14

With regard to Pb isotope ratios, Vanhaecke  et al.10 obtainedsimple and rapid analytical method for the measurement of Pb

isotope ratios, and is one of the first studies to use   a precision of 0.04% (n=10) for the   206Pb/ 207Pb ratio in astandard lead solution. Little has been reported about the PbHR-ICP-MS to measure Pb isotope ratios in galena and

galena-bearing ores.   isotope ratio analysis of geological matrices.14  It should also

be noted, however, that double focusing multiple collectorKeywords:   L ead isotope ratios; high resolution inductively

ICP-MS instruments have been used recently to measurecoupled plasma mass spectrometry; galena; ores

highly accurate isotope ratios (including Pb), and may seeincreased use in future work.15,16

In this paper, a fast and reliable method for the preciseThe study of the isotopic composition of Pb in rocks and

measurement of Pb isotope ratios in geological samples byminerals began over 80 years ago and was initially used as a

HR-ICP-MS is reported. The technique was applied to thegeochronological tool.1   Subsequent work showed that for

measurement of the Pb isotope composition of galena andmany types of geological materials, Pb isotope geochronology

galena-bearing ores from Broken Hill and Western Tasmania,was complicated by the presence of ‘common’ Pb ( i.e., non-

previously measured by TIMS.radiogenic Pb).2  Hence today, Pb isotope geochronology usesminerals that formed with very little Pb. 3  However, a second

EXPERIMENTALimportant practical application of Pb isotopes is in studyingand exploring for mineral deposits.4  The Pb isotope composi-

Reagentstion of rocks and Pb-bearing minerals can be used to trace

High purity HCl was prepared by doubly quartz distillingthe sources of metal in ores and to help discriminate barrenanalytical-reagent grade acid (Merck, Darmstadt, Germany),from mineralised ore systems.

and high purity HF was prepared from analytical-reagentAt present, thermal ionisation mass spectrometry (TIMS) isgrade acid (Merck) that had been further purified  via  a Teflonthe technique of choice for all types of geological Pb isotopedistillation system. High purity HNO

3 (Mallinckrodt, St. Louis,measurements. However, the relatively high cost of TIMS

MO, USA) was used as received for sample digestion andinstrumentation and the (necessary) extensive chemical pre-solution acidification. Ultra-pure de-ionised water (≥18 MV)treatment of the sample prior to analysis have imposedwas prepared   via   a glass distillation unit followed by furtherserious limitations on the routine use of TIMS techniques inpurification with a Modulad water purification systemgeochemical exploration applications.(Continental Water System, Melbourne, Australia).Quadrupole based ICP-MS instruments have been used in

many studies to measure Pb isotope ratios.5–8  However, owingto the design and manner in which quadrupole mass analysers

Standards and sample preparationoperate, the technique has been found to be limited in both

precision and accuracy. The precision of isotope ratios meas- Instrument tuning and general Pb solutions were prepared bydilution from a Perkin-Elmer (Norwalk, CT, USA) 10 mg g−1ured on quadrupole ICP-MS instruments is typically 0.1%

RSD, whereas for ratios involving a low abundance isotope multi-element solution containing Pb (Cat. No. N930–0233).

A stock standard solution of NIST SRM 981 Natural Leadsuch as  2 04Pb, a precision of 0.2–1% RSD is usually obtained.8Recently, Begley and Sharp8  undertook an elegant study of was prepared by dissolving 0.05 g of Pb in 5 ml of dilute

HNO3

  (1+4) with heating in a Savillex screw-topped TeflonPb isotope ratios using a quadrupole ICP-MS instrument.

Journal of Analytical Atomic Spectrometry, August 1998, Vol. 13 (809–813)   809

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beaker.17  After cooling to room temperature, 1 ml of concen-Table 2   Method parameters for Pb isotope ratio measurements

trated HNO3

 was added before dilution to 100 ml total volume(in polycarbonate). The stock standard solution was diluted Isotopes considered   201Hg or   202Hg,   204Pb,   206Pb,   207Pbto working concentrations (generally 10–50 ng g−1 ) before   and   208Pb

Scan type Magnet fixed at   201Hg. Electric scansanalysis. This reference solution was used as the externalover other mass rangescalibration standard in this study.

Mass scanning window 3%Galena sample solutions were prepared as follows. SamplesMagnet settle time 0.1 s

were dried at 105 °C overnight and 0.1 g portions were addedDwell time per measurement 0.001 s

to Savillex screw-topped Teflon beakers. A 5 ml volume of  Scan duration per sweep 5×0.025 s

HF-HNO3 –HCl (2+1+3) acid mixture was added to each   Number of sweeps 2000Measurement time 250 ssample18 with overnight heating on a hot-plate at 130 °C. The

per replicateresulting solution was twice evaporated to incipient dryness,Detector mode Countingwith the addition of 1 ml of HNO

3 after each drying stage. All

Integration type Averagesolutions were stored in sealed polycarbonate containers.

Sample uptake and 2 minGalena solutions were diluted to a Pb concentration between equilibration time40 and 50 ng g−1   for HR-ICP-MS analysis.   Rinse time bet ween 3 –5 min (with 5% HNO

3)

each sampleTotal time per sample   ~15 min

Instrumentation   Measured dead time In the range 20–50 ns, depending ondetector settings

Pb isotope ratios were measured on an Element HR-ICP-MSinstrument (Finnigan MAT, Bremen, Germany). This instru-

ment, utilising a magnetic sector mass analyser of reversedRES ULTS AND DIS CUS SIONNier–Johnson geometry, has pre-defined resolution settings

(m/Dm  at 10% valley definition) of 300 (low), 3000 (medium) Mass discrimination correctionand 7500 (high). A resolution of 300 was used in this work,

Isotope ratio values obtained using ICP-MS may deviate fromproviding flat-topped peaks and maximum instrument sensi-the ‘true’ value as a function of the diff erence in mass betweentivity (typically greater than 106 counts s−1 for 10 ng g−1 115Inthe two isotopes measured. This eff ect is defined as mass biasor 7×105   counts s−1   for 10 ng g−1 2 08Pb). Instrument andand its value may be either positive or negative.23 The bias ormethod settings are outlined in Tables 1 and 2. A standarddiscrimination can only be determined experimentally and willMeinhard nebuliser and Scott double pass water cooled sprayvary with the configuration of the individual ICP-MS system. 22chamber were employed. Isotopes of interest were analysedIn this study, instrument mass discrimination was correctedusing electric scanning with the magnet held at fixed mass.by the use of a NIST SRM 981 external standard. UnknownThe secondary electron multiplier detector (with discretesamples to be measured were typically analysed betweendynodes) was operated in the counting mode. Instrumentalternate standard reference solutions. The use of this correc-tuning and optimisation were performed daily using a 10 ngtion method allows any measured diff erences in a Pb standardg−1   multi-element solution containing Pb. Further detailssample to correct the ratios found in unknown Pb solutionsconcerning this instrument have been reported elsewhere.19–21

(i.e., Pb standards correcting for Pb unknowns). Also, similarAll isotope ratio values were adjusted for instrument dead isotope abundances are typically measured for both standardstime.22   Each Pb isotope ratio was considered in dead time

and unknowns. However, on the negative side, there is a timedeterminations. The dead time was measured regularlydelay between each standard and sample in which changes inthroughout the study using three Pb solutions of diff erentplasma conditions can occur. The other option of using a Tlconcentrations, namely 10, 25 and 50 ng g−1   Pb. Dead timeinternal standard for mass discrimination correction was notvalues in the range 20–50 ns were typically found, dependinginvestigated in this study. This method involves measuringon the detector settings. Analyses were also blank corrected.another isotope (Tl) which may behave in a diff erent fashionto Pb whilst also increasing the analysis time for each scan(time which could be spent accumulating Pb data). The internalcorrection approach has an advantage, however, of continu-

Table 1   Typical instrumental configuration and settingsously monitoring the mass discrimination for each individualsample. Both methods have been used in previous Pb isotopeInstrument Finnigan MAT Element

Resolution (m/Dm) 300 ( low)   studies6–8,11 and were outlined in detail by Begley and Sharp.8Rf power 1250 W

Nebuliser Meinhard concentricGeneral analytical and methodology considerationsCoolant argon flow rate   ~12.5 l min−1*

Auxiliary argon flow rate   ~1.1 l min−1*Method parameters were systematically varied and tested inNebuliser argon flow rate   ~1.1 l min−1*order to obtain the most accurate and precise Pb isotopeSpray chamber Scott (double pass) type cooled

to 3.5–5 °C   ratios. The first variable considered was the number of scansSampler cone Nickel, 1.1 mm aperture id   for each sample. The ideal situation would be to acquire asSkimmer cone Nickel, 0.8 mm aperture id many scans as possible in the shortest time interval, henceInstrument tuning Performed using a 10 ng g−1

approaching the behaviour of a multiple collector system. Themulti-element solution

number of sweeps across the Pb isotopes was varied betweenIon transmission Typically 106  counts s−1  per 10 ng g−1500 ( total analysis time 62.5 s) and 3000 (375 s), with 2000115In or 7×105  counts s−1  for

10 ng g−1 208Pb   scans (250 s) proving optimum considering Pb signal stabilities.Scan type Fixed magnet with electric scan over All scanning protocols were at a rate of 8 sweeps s−1, keeping

small mass range the dwell time for each mass segment constant. VanhaeckeIon sampling depth Adjusted daily*

et al.10   used 1200 scans in 2 min to measure   206Pb/ 207PbIon lens settings Adjusted daily†

isotope ratios in a standard Pb solution.* Adjusted in order to obtain maximum signal intensity. † AdjustedVanhaecke   et al.11  also found that when considering both

in order to obtain maximum signal intensity and optimum resolution.Pb or Cu isotope ratios at resolution 3000 using HR-ICP-MS,

810   Journal of Analytical Atomic Spectrometry, August 1998, Vol. 13

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concentration range 10–50 ng g−1. Work is continuing to

increase the concentration range for which accurate Pb ratioscan be obtained.

All ratio measurements were dead time corrected. Even withthis adjustment, it was found that the accuracy of eachmeasurement could be further improved when the Pb concen-

tration in the NIST SRM 981 external calibration standardand the unknown sample were closely matched. As such theconcentration of Pb in each sample was often adjusted prior

to ratio determinations.Along with the four Pb isotopes of interest,   202Hg and   201Hg

were also measured as   204Pb (1.4% abundant) has an isobaricinterference from   204Hg (6.85% abundant) in resolution 300.For geological samples that were previously ground in tungsten

Fig. 1   Measured   208Pb/ 204Pb ratios   versus   Pb concentration in   crushing mills,   201Hg was used to correct   204Pb, as   202Hgstandard solutions. Ratios were measured over two consecutive days.   suff ers interference by   186W16O.7 For other samples   202Hg wasNote that the values shown have not been corrected for any mass used. It was expected that during sample digestion any volatilediscrimination.

Hg would be driven off   during the multiple drying stages,while the Hg5Pb ratio in rocks and sulfides is generally low(1510–100).24  No samples analysed in this study were foundthe precision was improved when the mass scanning windowto have appreciable levels of Hg or W. However, it waswas reduced from 20 to 10% (the percentage window value isimportant to include these isotopes in method developmentbased on the expected mass width of a peak in the respectivework for future samples when more appreciable Hg or Wresolution). It was suggested that when smaller mass windows

concentrations may be encountered.were used, only the centre of each peak was scanned, minimis-ing the influence of any mass calibration instabilities. Althoughmore crucial at resolution 3000 away from the flat-toppedpeaks of resolution 300, a similar systematic study was under-

Precisiontaken in this work, considering Pb ratios at low resolution.Mass windows between 1 and 10% were considered, and it Lead isotope ratio precision data for 10 consecutive measure-

ments of three diff erent standard solutions are given in Table 3.was found that a mass scanning window of 3% gave isotopesignals with minimum variation over the total number of scans. Precisions obtained for ratios involving the low abundance

204Pb isotope were 0.10–0.13% for   208Pb/ 204Pb, 0.10–0.11%The working Pb concentration range for which accurate Pb

isotope ratios could be obtained was also investigated. At Pb for   207Pb/ 204Pb and 0.10–0.11% for  2 06Pb/ 204Pb (values as 1s).Improved precisions were found for those ratios involving theconcentrations below 10 ng g−1, ratios with   204Pb as the basis

were found to be limited by counting statistics, resulting in more abundant isotopes, namely 0.041–0.046% for 20 8Pb/ 206Pband 0.044–0.048% for   207Pb/ 206Pb. No diff erence in measure-less accurate results (a concentration of 10 ng g−1 corresponded

to approximately 15 000 counts s−1   of   204Pb). This is shown ment precision was apparent between standard Pb solutions

and more ‘complex’ galena digest samples. Theoreticalmore clearly in Fig. 1 for the  208

Pb/ 204

Pb ratio measured inPb standard solutions of various concentrations analysed over precisions based on Poisson counting statistics were alsocalculated and are shown in Table 3 (based on   208Pb signalstwo consecutive days. Similar results were found for the other

Pb ratios considered. This has not been noted as a problem of the order of 1×106 –3×106  counts s−1 ). In agreement withother workers,13  external precisions approximately 2–3 timesin other HR-ICP-MS studies as the ratios investigated have

typically involved   206Pb,   207Pb and   208Pb isotopes only.10,14   greater than that predicted from counting statistics alone werefound, suggesting noise contributions from other sources suchAt the other extreme, Pb concentrations above 50 ng g−1 were

not considered as the signal from   208Pb required measurement as the sample introduction system and plasma flicker. Theprecisions obtained for ratios referenced to   206Pb are similarin the analogue detector mode to avoid detector saturation

(under normal operating conditions and detector settings, to that found by Vanhaecke et al.10 using a similar HR-ICP-MS

instrument to measure the   206Pb/ 207Pb ratio of a standard Pb50 ng g−1   of   208Pb corresponded to 3×106 –3.5×106   countss−1). Rather than limit the accuracy of the ratio determinations solution. The external precisions given in Table 3 are also

similar to the best obtained using a quadrupole ICP-MS byit was decided to make all measurements in the same detectormode (counting). Hence ratios were only determined in the Pb Begley and Sharp.8

Table 3   Pb isotope ratio precision and accuracy data for a standard multi-element solution, NIST SRM 981 and a Broken Hill galena digest.Samples were analysed at a Pb concentration of 45 ng g−1

208Pb/ 204Pb   207Pb/ 204Pb   206Pb/ 204Pb   208Pb/ 206Pb   207Pb/ 206Pb

Multi-element standard solution— Precision for 10 measurements, RSD (%) 0.11 0.11 0.10 0.041 0.047Theoretical RSD (%)* 0.054 0.055 0.055 0.015 0.016

NIST SRM 981 Natural L ead— Precision for 10 measurements, RSD (%) 0.10 0.11 0.11 0.046 0.048Theoretical RSD (%)* 0.049 0.050 0.050 0.014 0.015

Broken Hill galena— Measured ratio 35.520 15.353 15.985 2.222 0.961Reference value† 35.599 15.370 15.994 2.226 0.961ICP-MS bias from TIMS value (%)   −0.22   −0.11   −0.06   −0.18 0Precision for 10 measurements, RSD (%) 0.13 0.10 0.11 0.043 0.044Theoretical RSD (%)* 0.052 0.053 0.053 0.015 0.016

* From Poisson counting statistics. † From TIMS analysis.

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    2

    1    8 .    0

    6    3

    1    8 .    0

    7    5

   −    0 .    0

    7

    2 .    0

    9    2    9

    2 .    0

    9    7    4

   −    0 .    2

    1

    0 .    8

    6    2    7

    0 .    8

    6    2    0

     +    0 .    0

    8

    E    B    7

    2    0    8    7

    E    l    l    i   o    t    B   a   y

    2

    3    7 .    9

    0    0

    3    7 .    8

    7    6

     +    0 .    0

    6

    1    5 .    5

    8    7

    1    5 .    5

    7

     +    0 .    1

    1

    1    8 .    0

    9    5

    1    8 .    0

    9    8

   −    0 .    0

    2

    2 .    0

    9    4    6

    2 .    0

    9    2    8

     +    0 .    0

    9

    0 .    8

    6    1    3

    0 .    8

    6    0    3

     +    0 .    1

    2

    E    B    7

    2    0    9    6

    E    l    l    i   o    t    B   a   y

    2

    3    8 .    0

    8    3

    3    8 .    0

    4    1

     +    0 .    1

    1

    1    5 .    6

    2    8

    1    5 .    6

    0    5

     +    0 .    1

    5

    1    8 .    1

    9    8

    1    8 .    1

    9    1

     +    0 .    0

    4

    2 .    0

    9    5    3

    2 .    0

    9    1    2

     +    0 .    2

    0

    0 .    8

    5    8    8

    0 .    8

    5    7    8

     +    0 .    1

    2

    B    H    2

    B   r   o    k   e   n    H    i    l    l

    1    5

    3    5 .    5

    3    0

    3    5 .    5

    9    9

   −    0 .    1

    9

    1    5 .    3

    8    4

    1    5 .    3

    7

     +    0 .    0

    9

    1    6 .    0

    0    4

    1    5 .    9

    9    4

     +    0 .    0

    6

    2 .    2

    2    0    8

    2 .    2

    2    5    8

   −    0 .    2

    3

    0 .    9

    6    1    3

    0 .    9

    6    1    0

     +    0 .    0

    3

    *

      n   =    N   u   m    b   e   r   o    f   a   n   a    l   y   s   e   s   p   e   r    f   o   r   m   e    d   o   n   e   a   c    h

   s   a   m   p    l   e    (   a   v   e   r   a   g   e   v   a    l   u   e   s   s    h   o   w   n    ) .

    †      D   =    D    i      ff   e   r   e   n   c   e    b   e    t   w   e   e   n    I    C    P  -    M    S   a   n    d    T    I    M    S   v   a    l   u   e   s .

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measured in Pb mineral concentrates using HR-ICP-MS.

Based on the analysis of a series of standard reference solutions,this comparatively fast analytical method provided Pb ratioswith precisions of 0.13% RSD or better for ratios to   204Pb.For ratios to   206Pb, the precisions obtained were of the orderof 0.045%. TIMS and HR-ICP-MS were found to be in

agreement to an accuracy of ±0.2%, irrespective of the ratioconsidered. This level of accuracy is adequate for many geoch-emical applications. The relative ease and speed with which

these values were obtained suggest that HR-ICP-MS may seeincreased use for geochemical Pb isotope ratio measurements.

The authors acknowledge the facilities and support providedFig. 2   Comparison between average   206Pb/ 204Pb ratio values by the Central Science Laboratory and Centre for Ore Depositsobtained  via  HR-ICP-MS and TIMS techniques (for purposes of scale

and Exploration Studies ( CODES), University of Tasmania.the Broken Hill galena sample has been omitted).

Ross Large, Paul Kitto, Bruce Gemmel and David Steele are

thanked for their support, samples and related information.Comparison with TIMS values and accuracy of the analytical

method

REFERENCESIn order to study the accuracy of the present method, the Pbisotope ratios of 11 galena and galena-rich ore samples were   1 Holmes, A., T he Age of the Earth, Harper, New York, 1913, p. 194.

2 Faure, G.,   Principles of Isotope Geology, Wiley, New York,measured by TIMS at CSIRO North Ryde and by HR-ICP-MS1977, p. 464.at the University of Tasmania. Samples were typically analysed

3 Schaltegger, U., Schneider, J. L., Maurin, J. C., and Corfu, F.,

2–3 times by HR-ICP-MS, with the Pb concentration of the Earth Planet. Sci. L ett., 1996,  144 , 403.NIST SRM 981 external calibration standard and the unknown 4 Gulson, B. L.,   L ead Isotopes in Mineral Exploration, Elsevier,Pb sample being closely matched. Comparative data are given   Amsterdam, 1986, p. 245.in Table 4. There is very good agreement between the two   5 Smith, R. G., Brooker, E. J., Douglas, D. J., Quan, E. S. K., and

Rosenblatt, G.,  J. Geochem. Explor., 1984,  21 , 385.techniques and this is highlighted in Fig. 2, showing6 Date, A. R., and Cheung, Y. Y., Analyst, 1987,  112 , 1531.HR-ICP-MS values for the   206Pb/ 204Pb ratio plotted against7 Longerich, H. P., Wilton, D. H. C., and Fryer, B. J.,  J. Geochem.

TIMS data (206Pb/ 204PbHR-ICP-MS

=0.98×206Pb/ 204PbTIMS+

Explor., 1992,  43 , 111.0.45,   r2=0.983 for   n=10). The ratio precision from 8 Begley, I. S., and Sharp, B. L., J. Anal. At. Spectrom., 1997, 12, 395.HR-ICP-MS was about 0.05% and that from TIMS was about   9 Becker, J. S., and Dietze, H.-J., J. Anal. At. Spectrom., 1997, 12, 881.0.02–0.05%.15 A similar figure was presented in work by Date   10 Vanhaecke, F., Moens, L., and Dams, R.,   Anal. Chem., 1996,

68, 567.and Cheung6  using an early model quadrupole ICP-MS. The11 Vanhaecke, F., Moens, L., Dams, R., Papadakis, I., and Taylor, P.,diff erence in Pb ratio values between the two analytical tech-

Anal. Chem., 1997,  69 , 268.niques was generally less than ±0.2%, and was in the range

12 Cornec, F. L., and Correge, T., J. Anal. At. Spectrom., 1997, 12, 969.0.001–0.1 for   208Pb/ 204Pb, 0.004–0.03 for   207Pb/ 204Pb, 13 Sturup, S., Hansen, M., and Mølgaard, C.,  J. Anal. At. Spectrom.,0.003–0.04 for   206Pb/ 204Pb, 0.001–0.008 for   208Pb/ 206Pb and

  1997,  12 , 919.0.0001–0.004 for   207Pb/ 206Pb.   14 Shuttleworth, S., Morris, J., Crombie, K., Ridings, R., andArvidson, R., in   Proceedings of the 3rd International Conferenceon the Analysis of Geological and Environmental Materials,

Geological implications   Colorado,  USA,  June 1997, p. 100.15 Walder, A. J., Platzner, I., and Freedman, P. A.,   J. Anal. At.

Lead isotopes are powerful tools in exploration for mineral Spectrom., 1993,  8 , 19.deposits and the investigation of problems of ore genesis.4   16 Walder, A. J., and Furuta, N., Anal. Sci., 1993,  9 , 675.

17 Potts, P. J. ,   A Handbook of Silicate Rock Analysis, Blackie,However, unlike multi-element geochemical analyses, they areGlasgow, 1987, p. 622.not commonly used when exploring for new deposits owing to

18 Chao, T. T, and Sanzolone, R. F., J. Geochem. Explor., 1992, 44, 65.the expense of TIMS analysis. From an earlier study,4   the19 Feldmann, I., Tittes, W., Jakubowski, N., Stuewer, D., and

Cambrian volcanogenic massive sulfide (VMS) and post-Giessmann, U.,  J. Anal. At. Spectrom., 1994,  9, 1007.

Cambrian (Post C) mineralisation in Western Tasmania can 20 Moens, L., Verrept, P., Dams, R., Greb, U., Jung, G., and Laser,be clearly separated when the Pb isotope ratio precision is less   B.,  J. Anal. At. Spectrom., 1994,  9, 1075.

21 Moens, L., Vanhaecke, F., Riondato, J., and Dams, R.,   J. Anal.than   ±0.1 for   208Pb/ 204Pb and   ±0.05 for   207Pb/ 204Pb andAt. Spectrom., 1995,  10 , 569.206Pb/ 204Pb (in terms of absolute ratio units). Our work has

22 Price Russ, G., in   Applications of Inductively Coupled Plasmashown that Pb isotope ratios can be measured relativelyMass Spectrometry, ed. Date, A. R., and Gray, A. L., Blackie,

quickly by HR-ICP-MS to a precision approaching thatGlasgow, 1989, pp. 90–110.

obtained by TIMS on the same mixed acid digestion solutions 23 Jarvis, K. E., Gray, A. L., and Houk, R. S.,   Handbook of normally used in commercial geochemical analyses. This results   Inductively Coupled Plasma Mass Spectrometry, Blackie, Glasgow,

1992, p. 380.in a reduction in cost when compared with TIMS measure-24 Sewell, D. M., and Wheatley, C. J. V., J. Geochem. Explor., 1994,ments, and opens up the possibility of the routine use of Pb

50, 351.isotopes in geochemical exploration surveys for new mineral

deposits.Paper 8/ 01397G

Received February 18, 1998CONCLUSION   Accepted May 7, 1998

This work has clearly shown that under routine operating

conditions, precise and accurate Pb isotope ratios can be

Journal of Analytical Atomic Spectrometry, August 1998, Vol. 13   813