Lead isotope determinations by inductively-coupled plasma mass spectrometry (ICP-MS): potential of sector field instruments

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NGU- BULL436,200 0 -PAGE203

Lead isotope determinations by inductively-coupled plasma mass spectrometry (ICP-MS): potential of sector field instruments

BELlN DAFLEM,ANDREASGRIMSTVEDT&NIGELCOOK

Flem,B.,Grimstvedt,A.& Cook,N.J.2000:Lead isot opedeterminat ionsby inductively-coup led plasmamass spectro metry(ICP-MS):potentialofsecto r fieldinstr uments.Norges geologiske underse kelseBulletin436,203-207.

Lead isotope measurementshave been carried out using ahigh -resolut ion induc tively-coup led plasmamass spectrometer(Finnigan MAT ELEMENT, configuredwithCD-1opt io n). The metho dcanbeapplied to all typesof geologicalsamples brought intosolution form.Three commonlead standa rdsareused fordocument ation ofthe method.Detection limit s for204pb,206pb,207Pband208Pbliebetween0.124and2.86pg mr¹.Optimisation of scanningcondit ionsatacount rate ofapproximately200,000cpsfor206Pband207pb,ledtoarelativestandard deviatio nofO.OS%forthemean207Pb/2o6Pbisotoperatio,basedon15consecut ivemeasurements.Themeth odcan beroutinelyapplied togaingeochronologicaland petrol ogicaldata forgeneticst udies ofmineraldeposits,including depositsofindust rialminerals.Because ofspace charge effectsinthe skimmerconeregion and mass-dependent sensitivityof themass spectro mete r,themeasuredint ensityrati o ofsamples must generallybe calib rated using externalorintern al standardsof known isotope rati osto correctformassbiasesand massfractiona t ion.

BelindaFlem,Andreas Grimst vedt&Nigel Cook,Geol ogi cal SurveyofNorw ay,N-7497Tron d heim,Norway.

Introduction

Geoch ronol ogi cal,isotopi c and petr ological dataare important elementsinstudies of mineraldeposits.Thisistruefor all typ esof mineraldeposit s,bot h metalliferousoresand depos- itsofindustri alminerals.Alternatively,geochron ologicaland isotopedata are usefultoolsfor prospecting,asthey can help esta b lish geolog icalcorrelati on s onthe basisofageand/or isot opi c signat ure,andaid ident ificatio nof form ati on s offe r- ing econom icpote nt ial. Thispaper describesdevelopment work carriedout at theGeolog icalSurvey of Norway touse inductively-cou pled plasma massspectrometry(lCP-MS)as a met hod for lead isoto pe measurements.The possibilit yto usethetechniqu ein investigationsof mineraldepositsona routine basis rep resents an important ad dit ional tool for unravelling the geolog ical framewor k hosting econom ic depositsofindust rial mineralssuch as limeston e, dolo mit e, talc andot her mineral products.

Leadisotopes canbemeasured inmin eralswhose U/Pb and Th/Pb ratiosaresufficiently lowsuch that theisotopic com posit ion will not change appreciably wit h tim e.Exam- ples include thecommonestleadmineral,galena(PbS).Pb- Pb isoch ron sfor such minerals, and forwho le rockswithsim- ilarlylowrati os,can giveanage offormation.Such meth od s havebeen inwideusageforseveral decadesand havebeen app liedto meta lliferousores(e.g.,Bjerlykkeetal. 1993, Bill- strorn&Vivallo 1994);toabroadrangeofigneousrocks,fro m granite to basalt;and also to varioustypesofsedimentary rocks.Reliabl e agescanbeobtainedfor rockswit ha simple crystallisation histor y.How ever,becauseof the sig nifica nt differenceingeochemicalbehaviour and mobility of U and Pb,maj orprob lemscanarise ininterpret ationofsit uat ionsin

which thePbisoto pesystemat icshavebeenreset.The appli- cat ionof Pb isotopesforgeoch ronologyhasdeclinedinpop - ularit y in recentyears.However, stu dies of Pbisot ope sys- tematicsareanincreasingl yusedtool in petrog eneticstudies inwhich thegoalisto establish the lead source.Lead isotop es,andot her rad iogenicisotoperatios,areused to iden- tify source region sin the crustas wellasmixing andcontam- inatio n processes. Modern review s of Pb/Pb isot op e method ology(Faure1986,Roll inson1993)showhowPb/Pb geoch ronolo gy is applied to giveagesof metamorphism(Le.

timeofresetting),and morerarelyto give ages of crustal for- mati on.

Det ermin ati on of lead isotope ratios in geolog ical sam- plesmaybe carriedout using a variety ofanalyt ical methods, e.g.,TIMS(t hermalionisationmass spect romet ry).In thepast decade orso,inductively-coupledplasmamass spect ro met ry (lCP-MS) has esta b lished it self as an excellent method for determi nati on of Pbisoto pe abundances.Measurem ent of Pbisotoperat ioswithICP-M Shasbeenshownto be possib le onall typesofenvi ronm entalsamples.Thesecanrangefrom atmospheri c aerosols(e.g., Surges & Barrie1987).gasoline (e.g., Lord 1994),fresh and salinewaters(e.g.,Halicz et al.

1994) and digested samples(e.g. Widenbecketal. 1995).If the ICP-MS isusedin combin ation wit h alaser prob e,the det erm in ationof Pbisotope ratio s canalso becarried outon solidsam ples,perm itti nginsitudeterminationof Pbisot op e systemati cs in individ ual mineral grains (e.g., Feng et al.

1993).

Application ofICP-MS methodstoPb isotopedet erm ina- tionsis adv ant ageousin st udiesof min eral resources and metallogenyinthatrapid,reliable analysisofsam plesisper- mit te d atrelative ly low cost.Furth erm ore, if thetechn ique

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NGU-BULL436,2000-PAGE204 BELlND AFLEM,AN D REAS GRIMS7VEDT&NIGELCO O K

Measuredmassrange

Ta ble1.Operatingcond itionsoftheICP-M5.

Counting time oneach isotope perscan 0.05s Totalcounting time oneach isotope 30s

Detection mode Counting

204Pb 203.905-204.041

206Pb 205.906-206.043

207Pb 206.907-207.045

208Pb 207.907-208.046

[1]

capacitivecoupli ng from theload coil intotheICP.TheICPis thensustained by'pure'inductive coupling;secondarydis- charge between the ICPand mass spectrometer is elimi- nated.TheCD-1 option isusedforthe widestrange of sample matr ices and instrument al operating cond it ions, inclu din g coldplasma ICP-MS.Themostim po rt antoperat ing parame- tersare givenin Table 1.TheFin nigan MATELEMENTisasec- tor field,high-resolutionICPMS,but all measurementswere carriedoutata massresolut ionof300(massof isotopeM/ massdifference ofisotopes, LlM).The high resolutio n(mass resolut ion8000-9000)wasonly usedfor checkingof isob aric interference. The instru ment operat ing parameters were tuned on a 1 ng ml! solut ion of NBS-98l to give a peak heightof approximately700,000 cps for208Pb.

Threecommon leadisotope standa rds havebeen used (NBS-98l,NBS·982and NBS-983).Aspecifi camou ntof each standardwas digested in an auto claveat200 kPafor30 min- uteswit h 7 NHN0₃.Aft er cooling toroomtem perat ure,the solut io nwasdilute d to theconcentrationsgiven inTab le2.

Nitricacidwas added toa concentration of0.24Mfor solution conservation.Asamp le blank wasprepared in thesame wayto checkand correctforcontaminationduringdigestion and dilut ion.Theblank solution wasalsoused to estima tethe limitsofdetection(LOO).

The limitsof detection (LOO) for all Pbisoto peswere est ablishedfrom thesensit ivit ydat agiven inTab le2and 1S successive measurem entson the blank sample. LOOdeter- mination isherebased on 3timesthe stand ard deviation mult ip lied by2°.5,dividedbythe sensitivity,S;equation[1].

LOD = 30- -J 2 5

Result s and d iscussion

Mass discrim inationeffectshavebeenobserved in measure- mentsof isotopicratios usingICP-MS.Severalprocessescon- tributetothisphenom enon.Thesearemainlyspace charge effects (Columbi c rep ulsions)in the skim mer cone reg io n, which result in preferentialtransmission ofthe heavierions (Platzner 1997),and mass-de pend entsensit ivit y ofthe mass spectro meter. The effectof mass bias should be monit ored, and ifnecessarycorr ected,byusingastandardofknow niso- topiccomposit ionclosetothatof the unknown.The standard should be analysedfrequentlyduring an analyticalrun to monitorchanges in massbias with time.Mass biasis also Escan

300 900Watt

600 Plasmaconditions

Datacollection PlasmaPower

Mass resolution(m/c.M) Scan type

No ofscans

canbeusedin conjunctio nwit halaserprobe, analyses could beperform edinsituon sam plesprepared formineralogical or petrograph ical st udy,thusfurtherreducing preparat ion and analysistime.The method offers,therefore,sig nif icant promise forthefuture.

In this paper, the ap plication of inductiv ely-cou pl ed plasma mass spectr ometry(ICP-MS)to direct determination ofPbisot operat iosin aqueous or digested samplesisinves- tigat ed.Three common lead standa rdswit h different concentration s of Pb isotopes have been stud ied in order to showthe suitabilityof thetechniqu eforrout inedetermina- tion of Pbisotopes.

Instrumental details

Astandard, doubl efocusing,sectorfield,ICP-MS(Finnigan MAT,ELEMENT) instrument isused(Feldmann et al. 1994, Giessmann&Greb 1994).The configuration includedthe

co-

l option from Finnigan,consist ing ofa torchinsert module which replacesthe standard Elem ent ICPtorch holder. A grounded Pt electrode (GuardElectrodeTM) is inserte d between the quartz ICPtorchand the RFload coilto prevent

Table2.Detect io nlimits(LOO)obtainedfrom 15successive mea sureme nts onthe blank sample andsensitivities obtained from calibration togeth er with theconcen trations of thedige stedNB5 standa rds.

Sensitivity St.dev. of Detectio n NB5-981 NBS-982 NB5-983

inte nsities limit

Blank (LOD)

[cps(ngml-')"'] [cps] [pg ml'] [pgrnl''] [pgmr' ] [pgmr ']

Pb204 905094.7 26.4 0.124 15.70 17.3 0.503

Pb206 903388.7 305 1.43 266 635 1356

Pb207 926895.3 264 1.21 243 297 96.5

Pb208 933902.1 631 2.86 577 63S 18.5

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stro ngly depend ent upon theday-to-day tuning of the Y- defl ecti on lenses;themassbias will refl ect how well the Y- defl ecti onlenses were tuned on the act ual day.Massbiasfor theisotop erati os 204PbP 06Pb, 207Pb/206Pband208PbP0 6Pb are given in Tabl e 3forthethree sta ndard materials.Theval- ues arequitehigh,possibly suggest ing that insuff icient time was spent on instrumental tuning and that this must be borne in mind whenunknown sam ples areanalysed.Atypi- cal run procedure would include multipledet ermination of reagent blank s and reference standards alongside the unknown samp les.

To correct systematic deviati on s,corr ection factors are det erminedusing exte rna lor intern al sta ndards,whichallow the conversion of intensityrat iosintoisotoperat ios.External calibrat ion uses certified standards with known isotope rat ios,astheone usedin thispaper.Themeasuredintensity ratiosofthe sam p lestobe exam inedarecorrecte daccording tothe followin gformula:

BELlNDAFLEM,ANDREASGRIMSTVEDT&NIGELCOOK

Table3.Measured isotoperati os andrelative standard deviationof the mean of15 consecutivemeasurements.

20 4Pbl 207Pbl 208Pbl

206Pb ²⁰⁶Pb ²⁰⁶Pb

NBS-981 Meanmeasured 0.057925 0.92135 2.1987 value

Certificatevalue 0.059042 0.91464 2.1681 Bias0/0 -1.890/0 +0.733 +1.41 Experimental%RSD 0.08 0.05 0.07 NBS-982 Meanmeasured 0.026576 0.46783 1.00973

value

Certificatevalue 0.027219 0.46707 1.00016

Bias0/0 -2.36 +0.163 +0.957

Experimental%RSD 0.06 0.04 0.05 NBS-983 Meanmeasured 0.000298 0.070906 0.013095

value

Certificatevalue 0.000371 0.071201 0.013619

Bias0/0 -19.7 -0.414 -3.85

Experimental%RSD 0.68 0.05 0.19

NGU-BULL 436, 2000-PAGE205

nati on ettec tot theisot op etobe determin ed.I hishastobe check ed in each individu al case.Externalcalibration isthere- forenorm ally usedin preferenceto inte rnalsta ndar disatio n (Finniga n 1998).

It mayhappenthat avalueappears in a rat io determination which departsconside rab ly from ot hervalues in that measurement series.If thenumber ofva lues is sma ll,suchan out lyi ngvalue canaffec t thestand arddeviati on tosuchan extent that itis reasonab leto conclude thatthemeanrati o recalcul at edwit hout the out lyingvaluewouldbetter express the truemean.Severalstat isticalcrite riafor rejec t io nofout- lying values exist, e.g., the Chauve net rej ect ion crit eria (Plat zner 1997).lnthepresentst udy,howe ver,no rejec t ionof measurem ent shasbeenmade,sincetheaimistodevelop a meth odfor Pbisoto pe measurem ent stobeused onarou- tinebasis.

Vanh aecke etal.(1996) reporte d206Pb/207Pbratios wit h relat ive sta ndard deviati on (RSD)of0.1% in a 20-40 J..lg

1 -

¹

solutio nof Pborwhen the signalinte nsityexceeds200,000 cps.The sta ndard NBS-981has a Pbconcent rat io nof 1.1.J..lg 1-¹and bytuningtheinstrum entto anopt ima lsig nalinten- sity,nearly 200,000cpswere obta ined forthenuclides206Pb and207Pb. Valu es ofprecision obta ined in thepresentwork are com parable wit h those of Vanhaecke et al. (1996), alt hough theisoto picconcent ratio nsare 20-40timeslow er.

Percenta geRSD valuesare in the sameorderof magn it ude;

0.2% RSDfor sing le measurements and 0.05% RSDfor the mean (Tab le3).Thereport ed precisionisthusfully accepta- blefor routine app licat io n.

Thedet ecti onlimitsfortheisotopes areest imated to be bet w een0.124and2.86pg

rnl'

(Table 2). Thelowestdetec- tion limit (0.124 pg rnl")is obtained for 204Pb, the isotope which usually hasthe lowest abundance. In the NBS-983 solution,204Pbispresent ata concent rat ionof0.503 pg

rn l'.

i.e.4timeshigherthanthedetect ionlimit. Theexperimental RSDfortheratio204PbP06Pb in NBS-983solution is,at0.68%, quite high. However, since the inst rumental intensity for 204Pb is approaching thelowerlim it of detecti on, RSDmay belimitedbycountingstatistics.

The st andard deviati on on the count rate, based on counting stat ist ics givenbyJen kinsand DeVries(1970) is:

H

²

^R

l3J

(J = - <=:}

Var (R )

= (5 = -

R

T

R

T

.z . Ibx

standard l2J

lax

standard

lax

sample

Ibx

sample

ax

bx

Theuncert aintyin the isoto pe ratio,Z, canbe exp ressed as a com binat io n of the individual cont ribut ions.Expressing whereRisthecounting rat e andTisthecountingtime.The netintensityfor204Pb(R₂₀₄₎isthepeak inte nsity(1204)minus the backgroundintensit y(lBk)'Theisot op eratio isthengiven by:

whereZisthe cert ified isotoperati o and I isthemeasured inte nsity.Elem ents wit h known isotoperati os,whosemass neighbour sthemassof the elem ent to bedeterm in ed,can beusedfor inte rna lstandardisat ion(e.g.203TI/205TIcan be usedtodet erm in etheisotoperat io206PbP07Pb).Theknown isot ope rati oisthenusedto correct formassbiasin therat io to bedet ermined.

The basic disad vant age of inte rna l sta ndardisatio n is linkedtothe condit io n thatthemassdiscrim inat ion effec t for the inte rna lstandard mustcorrespo nd tothemassdiscrimi-

R'04Ph

1

₂₀₄ -

1

Bk

Z = - - =

- =-'---""--

R

²⁰⁶Pb R206Pb

l4J

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NGU-BULL436,2000 -PAGE206 BELlNDAFLEM,ANDREA SGRIM S7VED T&NIGELCOO K

the uncertainties as variance and by the different ial of the dependent variab leyields:

[5]

Table4.Measured inte nsitiesfor204Pb and206Pb.Tisthe total counting time pr.isot o pe,Riare netinte nsi t ies(as count rat es),,;are peak intensi- ties,an dZis the isotop erat io.

The relative uncertainty (expressed as standard dev ia- tion)can beobtainedfrom equation [5] when each part ial derivat ive and the coun ting erro rgiven in equation [3] are introduced:

Ifnreplicates are run,the standard deviationof the mean is obtained by dividingby the square root ofn.Usingthe data forsam ple NBS-983given in Table 4 and equation[61,the relative standard deviation of the isotope ratio204Pb/2o6Pb can be estima te das:

Acknowledgements

Theau t ho rswish tothan kBjern Sundvo ll,Mine ralogisk-GeologiskMuse- um,Univer sityof Oslo ,forsup ply ing thethree com monleadstandard s.

Colle ag uesin theNGU labora tory,andespec iallyEliHe y br at enand SivH.

Dun das,are th anked fortheir contributionstodevelop mentwor k.Con- st ruct iv ereview s byBernardBing enandJoakim Mansfeld help edtosig- nificantlyimprovethe pape r.

Conclusio ns

The isot ope rat iosof Pbin NBS-981,NBS-982and NBS-983 were measured by sect or field ICP-MS.The levelsof precision obt aine daresuffi cient ly good to allow meani ng ful interpre- tati oninisot o pe stu dies ofmineral deposits. The method is easy, fast,relativelyinexpensiveandcan beperform edin situ . Age determinations based solely on calculated Pbisot o pe models are gene rall y somewhatconjectural if they are not suppo rt ed by independent geochronolog ical info rmati o n.

Howeve r,ifsta ndar d mate rials are availabl e,it should be easy to extendthismethodto include the det erm in at ion ofThand Uisotop es,thuspermi tting U-Pband Th-Pbdatin g methods to be develo pe dusing the same instrumentation.

Significant mass fractionation ofPbiso t op es is observed.

Itis thus importan tto run standardsof known iso to pi c com- positionwhichisclose in isotopiccompositionto that of the unknow ns,regularly thro ug ho ut the sampleseriesto correct fo r massbias, whic h appearsto be strong ly dependent on the day-to -daytuning of the instr umen t.

Optimis at ion of scanni ng conditions led to a relative sta ndar d dev iat io n(basedon 15 consecutivemeasurements) for207PbPo6Pb whichwas 0.05%ofthe mean,a valuecom- parable with the best valuesreported in thelit erat ure . Com- pared to aFinnigan ICP-MSwithout the CD-1electrode,the newCD-1 optio n allows the same precision for the determ i- nation of isotope ratios in solutio ns in which the concentra- tio n of Pb is 20-40timeslow er(Vanhaecke et al.1996).

Using theCD-1 option,detection lim its for Pb iso to p es are ext remelylow.The204Pbisoto p e,whi ch usuallyhas the lo w est abundance,hasthe best lim itof detection(0.124 pg

mr\

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[7]

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2774

0.000298

1[( 1 ² ¹

^Bk⁾

I ]

^[6J

T R

₂₀₄

+ Rio 4 + R

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284 649

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365

(5:

= ~ Va r( Z ) ⁼

Z Z 2

O't O'z I

~

¹ ^[(

-6 )]

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-f;z -Jl5

^J)

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BELlNDAFLEM,ANDREASGRIMSTVEDT&NIGELCOOK

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Lead isotope determinations by inductively-coupled plasma mass spectrometry (ICP-MS): potential of sector field instruments