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Geoscience Canada

Modern Analytical Facilities

U-Pb Geochronology Using 193 nm Excimer LA-ICP-MSOptimized for In Situ Accessory Mineral Dating in ThinSectionsChris R.M. McFarlane et Yan Luo

Volume 39, numéro 3, 2012

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Éditeur(s)The Geological Association of Canada

ISSN0315-0941 (imprimé)1911-4850 (numérique)

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Citer cet articleMcFarlane, C. R. & Luo, Y. (2012). U-Pb Geochronology Using 193 nm ExcimerLA-ICP-MS Optimized for In Situ Accessory Mineral Dating in Thin Sections.Geoscience Canada, 39(3), 158–172.

Résumé de l'articleLe présent article constitue un abrégé sur l’application de la spectroscopie demasse à plasma inductif à ablation par laser (ICP-MS) à la géochronologie UPbdes minéraux accessoires en plaques minces polies normales, en utilisant uneversion moderne d’un laser excimère ArF d’ablation avec la spectrométrie demasse quadripolaire à plasma à couplage inductif (QICPMS). Au cours des cinqdernières années environ, les manufacturiers de Q-ICPMS (Agilent, ThermoScientific, PerkinElmer, Bruker Daltonics, par exemple) ont présenté denouveaux instruments ou interfaces de meilleure sensibilité et de moindrebruit de fond par rapport aux générations d’instrument précédentes. Lessystèmes laser excimère ArF construits par Resonetics (RESOlutionMC M-50 etS-50), newWave Research (NWR-193MC), Photon Machines (ExciteMC et AnalyteG2MC), et Coherent (GeoLasMC) ont eux-aussi amélioré leurs capacités dans unepériode relativement courte. Il y a également eu une avancée significative dansle développement de logiciels et de contrôle du laser, ce qui, combiné à destraitements de données hors ligne sophistiqués a augmenté l'efficacité globalede la technique.Cet article commence par un rappel de la bibliographie de base et de la théoriede l'interaction laser-cible pour les lasers nanosecondes et femtosecondes.Nous traitons ensuite des améliorations de sensibilité de l’ICPMS, del’importance de la cellule d’ablation par laser, des dispositifs de lis-sage, et descontrôles synchronisés matériels et logiciels. Nous donnons également desexemples de la façon dont ces progrès récents ont considérablement augmentél'efficacité (par exemple le coût par analyse), la précision et l'exactitude de lagéochronologie U-Pb in-situ sur minéraux accessoires par ICP-MS à ablationpar laser excimère 193 nm. Les jeux de données de la démonstration quiproviennent de l’instrumentation installée à l’University of New Brunswick,donnent des exemples des capacités générales de l’ICP-MS à ablation par laserexcimère pour l’analyse géochronologique U-Pb in situ.

MODERN ANALYTICAL FACILITIES

U-Pb Geochronology Using193 nm Excimer LA-ICP-MSOptimized for In SituAccessory Mineral Datingin Thin Sections

Chris R.M. McFarlane* and YanLuoDepartment of Earth Sciences, University ofNew Brunswick, Fredericton, NB, CanadaE3B 5A3crmm@unb.ca and yanluo@unb.ca

*Corresponding Author

SUMMARYThis article is designed to provide asynopsis of the application of LA-ICP-MS to U-Pb geochronology ofaccessory minerals, in standard pol-ished thin sections, using modern 193nm ArF excimer laser ablation (LA)and quadrupole inductively coupledplasma mass spectrometry (Q-ICP-MS)instrumentation. During about the lastfive years, Q-ICP-MS manufacturers(e.g. Agilent, Thermo Scientific, PerkinElmer, Bruker Daltonics) have intro-duced new instruments or interfaceswith higher sensitivity and lower back-grounds, compared to the previousgeneration of instruments. ArFexcimer laser systems built by Resonet-

ics (RESOlution™ M-50 and S-50),NewWave Research (NWR-193™),Photon Machines (Excite™ and Ana-lyte G2™), and Coherent (GeoLas™),have also expanded their capabilities ina relatively short period of time.There has also been a significant leapin software development and laser con-trol, which, when matched with sophis-ticated offline data processing hasincreased the overall efficiency of thetechnique.

This article begins with abackground section designed to pro-vide the basic bibliography and theoryof laser-target interaction for nanosec-ond and femtosecond lasers. We thendescribe enhancements in ICP-MS sen-sitivity, the importance of the laser-ablation cell, smoothing devices, andsynchronized hardware and softwarecontrols. We also provide examples ofhow these recent advances have dra-matically increased the efficiency (e.g.cost per analysis), precision and accura-cy of in situ U-Pb geochronology ofaccessory minerals using 193 nmexcimer LA-ICP-MS. Demonstrationdatasets are based on the instrumenta-tion installed at the University of NewBrunswick, and provide examples ofthe general capabilities of excimer LA-ICP-MS for detailed in situ U-Pbgeochronology.

SOMMAIRELe présent article constitue un abrégésur l’application de la spectroscopie demasse à plasma inductif à ablation parlaser (ICP-MS) à la géochronologie U-Pb des minéraux accessoires enplaques minces polies normales, enutilisant une version moderne d’unlaser excimère ArF d’ablation avec laspectrométrie de masse quadripolaire àplasma à couplage inductif (Q-ICPMS). Au cours des cinq dernières

années environ, les manufacturiers deQ-ICPMS (Agilent, Thermo Scientific,PerkinElmer, Bruker Daltonics, parexemple) ont présenté de nouveauxinstruments ou interfaces de meilleuresensibilité et de moindre bruit de fondpar rapport aux générations d’instru-ment précédentes. Les systèmes laserexcimère ArF construits par Resonetics(RESOlutionMC M-50 et S-50),newWave Research (NWR-193MC), Pho-ton Machines (ExciteMC etAnalyteG2MC), et Coherent (GeoLasMC)ont eux-aussi amélioré leurs capacitésdans une période relativement courte.Il y a également eu une avancée signi-ficative dans le développement de logi-ciels et de contrôle du laser, ce qui,combiné à des traitements de donnéeshors ligne sophistiqués a augmenté l'ef-ficacité globale de la technique.

Cet article commence par unrappel de la bibliographie de base et dela théorie de l'interaction laser-ciblepour les lasers nanosecondes et fem-tosecondes. Nous traitons ensuite desaméliorations de sensibilité de l’ICP-MS, de l’importance de la cellule d’ab-lation par laser, des dispositifs de lis-sage, et des contrôles synchronisésmatériels et logiciels. Nous donnonségalement des exemples de la façondont ces progrès récents ont consid-érablement augmenté l'efficacité (parexemple le coût par analyse), la préci-sion et l'exactitude de la géochronolo-gie U-Pb in-situ sur minéraux acces-soires par ICP-MS à ablation par laserexcimère 193 nm. Les jeux de donnéesde la démonstration qui proviennent del’instrumentation installée à l’Universityof New Brunswick, donnent desexemples des capacités générales del’ICP-MS à ablation par laser excimèrepour l’analyse géochronologique U-Pbin situ.

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INTRODUCTIONThere are numerous landmark refer-ences that chronicle LA-ICP-MS devel-opment and applications to U-Pbgeochronology. Most of these are ref-erenced in two Mineralogical Associa-tion of Canada Short Course Series(volumes 29 and 40; (Sylvester 2001;Sylvester 2008) . These provide histori-cal perspectives and a thorough treat-ment of LA-ICP-MS analytical tech-niques and applications. More recently,a wide variety of studies have appearedin such journals as the Journal of Ana-lytical Atomic Spectrometry (JAAS),Analytical Chemistry, Applied Spec-troscopy, and Chemical Geology. Arecent Open Access (OA) review byKoch and Gunther (2011) also pro-vides a useful ‘state-of-the-art’ review,including some remarks on the designand performance of sampling cells andthe relative merits of femtosecondlaser systems:

http://www.ingentaconnect.com/content/sas/sas/2011/00000065/

00000005/art00006.Other review papers include those ofCocherie and Robert (2008) whoreview laser ablation zircon U-Pb dat-ing compared to ion-microprobe tech-niques, and Becker (2002), who pro-vides a good review of the intrinsicaccuracy, precision, and mass discrimi-nation behaviour of plasma-sourcemass spectrometers. The University ofArizona Laserchron facility also pro-vides a wealth of information about U-Pb dating techniques applicable to awide range of accessory minerals:

https://sites.google.com/a/laserchron.org/laserchron/home.

Studies focusing on zircondating by LA-ICP-MS are plentiful, butrecent studies of allanite (Pal et al.2011; Darling et al. 2012; Gregory etal. 2012), apatite (Chew et al. 2011),xenotime (Liu et al. 2011), and rutile(Zack et al. 2011) have greatly extend-ed the range of dateable accessoryminerals and associated host rocktypes.

There has been a recent prolif-eration of 193 nm ArF excimer LA-ICP-MS systems in Earth Sciencesdepartments across Canada. Systemshave recently been installed (or are dueto be installed) at: University of NewBrunswick (source of data for thispaper), Université de Québec à

Chicoutimi, University of Ottawa, theGeological Survey of Canada inOttawa, Laurentian University, Univer-sity of Alberta, and the University ofBritish Columbia. Some of these sys-tems are tailored to suit a particularresearch focus so potential users needto assess whether the instrumentationis capable of carrying out their desiredanalyses.

Potential users should also beaware of the cost involved with LA-ICP-MS projects. The most significantconsumable costs are associated withconsumption of high-purity gases: 1)the ICP-MS torch and Ar carrier gastogether typically consume Ar at a rateof 18 to 20 L/min; 2) He used as acarrier gas is consumed at between 0.5to 1.0 L/min; 3) the ArF premix usedto create 193 nm laser pulses needs tobe replenished approximately everytwo weeks depending on laser usage;and 4) low-grade N2 used for laserbeam line purge is consumed at 5L/min when the laser is in operation.Taken together, yearly gas consump-tion for excimer LA-ICP-MS facilitiesis about $25–30K. Other consumablesinclude valuable reference materials,Hg traps, polishing supplies, ICP-MSconsumables (e.g. torch, sampler, skim-mer and detector), cleaning supplies,tubing, vacuum pump consumables,etc. Additional costs are associatedwith salary recovery for technician timeand yearly maintenance and repair billsfor both the laser and ICP-MS. Thus,the actual cost of running a LA-ICP-MS facility can be quite high (up to ~$100K/year) and so lab fee structuresreflect this large consumable burden.Nonetheless, the productivity of LA-ICP-MS still makes this technology themost cost effective dating techniquethat exists today

Fundamentals of Laser-ablationThere are currently two types of laserablation systems being used for U-Pbgeochronology: 1) nanosecond (ns)lasers (common); and 2) femtosecond(fs) lasers (currently rare).

The first type includes 266nm, 213 nm, and 193 nm lasers. Theexact mechanism used to generate ns-laser pulses of a specific wavelengthand energy is discussed elsewhere (e.g.Basting and Marowsky 2005). It isworth noting, however, that 193 nm

excimer (i.e. ‘excited-dimer’) lasers aregenerated using an ArF gas (the ‘pre-mix’), which, when pumped above itsground state results in photon emis-sion at a 193 nm fundamental frequen-cy. This means that in contrast tosolid-state Nd:YAG laser sources, nocomplex optical quintupling methodsare necessary and this greatly simplifiesthe optical design and maintenance of193 nm ArF excimer laser systems.Excimer laser pulses have higher ener-gy density compared to 266 nm and213 nm UV lasers and by varying laseroperating conditions and using opticalattenuators, 193 nm excimer lasers candeliver a wide range of fluences (<1 to40 J/cm2) to the target. This widedynamic range is necessary becauseablation thresholds depend strongly onthe composition of the target (i.e.absorption, electrical conductivity).For example, controlled ablation oftranslucent apatite typically requiresfluence >10 J/cm2 whereas zircon,monazite, and titanite can be ablatedover a range of fluences (2 to 8 J/cm2)that can be fine tuned to achieve anablation rate (e.g. 0.5 μm/s) suitablefor in situ analyses in polished thin sec-tions.

Whatever the ns-laser source,the energy contained in each pulse istransferred into the target lattice overns-timescales. When the energy of thelaser pulses exceeds a target’s ablationthreshold, the solid’s binding energy isovercome causing it to thermallydecompose into a mixture of atoms,ions, and fragments. The particle sizedistribution generated by ns-LA-ICP-MS peaks in the range of 60 to 100nm, and 193 nm ArF excimer laserstypically generate fewer μm-scale frag-ments, compared to 213 nm and 266nm wavelengths (Guillong et al. 2003).The short timescales for interaction ofthe incoming pulse (e.g. between 4 nsand 20 ns for excimer lasers) allowsmost of the energy to be absorbedwithin the target area. Nonetheless,the target area is subject to zone heatingthat can cause preferential evaporationof volatile elements, such as Pb. Thisablated material rapidly expands into atransient plasma plume above the tar-get area. This plume interacts withsubsequent laser pulses in a processknown as plasma shielding which furtheralters the size and composition of the

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GEOSCIENCE CANADA Volume 39 2012 161

aerosol. Outer-shell electron transi-tions in the plasma plume also releasephotons with wavelengths characteris-tic of the elements present in theplume. This is the basis of laser-induced breakdown spectroscopy(LIBS).

The net result of these ther-mally activated laser-target interactionprocesses is ‘non-stoichiometric’ abla-tion and the occurrence of elementalfractionation. For U-Pb geochronolo-gy applications, this element fractiona-tion is manifested as increasing Pb/Uas craters deepen. Eggins et al. (1998)have elegantly demonstrated anddescribed the processes occurring atthe ablation site that give rise to thislaser-induced elemental fractionation(LIEF). This study revealed the pres-ence of a volatile-element-enrichedcondensate blanket surrounding abla-tion craters and related its formation tocondensation of volatile elements (e.g.Pb) out of the expanding plasmaplume. Figure 1 shows a typical con-densate blanket surrounding craters inzircon analysed by the UNB laser abla-tion system. This observation suggeststhat heavier elements with greater iner-tia are actually preferentially entrainedin the carrier gas so that some otherprocess must account for the observedincrease in Pb/U as a function of timeduring ablation of craters. Carefulinspection of craters by Eggins et al.(1998) revealed, however, that heavierrefractory elements (e.g. U) appear tobe preferentially deposited (e.g. byvapour deposition) on the inner wallsof the craters as the plasma plumeexpands from progressively deeper intothe material. Their results also suggestthat whereas the ablation process isquantitative for shallow craters (i.e.most of the laser interaction volume isconverted to aerosol), deep craters (i.e.depth/diameter >>1) are subject toplasma re-sampling and re-depositionthat prevents a large fraction of theablated volume from becomingaerosolized and entrained in the carriergas.

These are some of the charac-teristics of ns-laser ablations systemsthat cause Pb/U fractionation resultingfrom laser–target interactions indrilling mode. The following moviesavailable on YouTube demonstratehow three laser operating conditions

(19 μm crater @ 10 Hz, 26 μm crater@ 5 Hz, and 45 μm crater @ 4 Hz)can influence the time-dependentincrease in 206Pb/238U during ablation ofzircon 91500 standards:

http://youtu.be/OLYwy7VQKgUhttp://youtu.be/HOh8MMFMZOw.

In contrast, femtosecond laserpulses transfer energy to a material in afundamentally different way. Ratherthan being a thermal decompositionprocess, femtosecond laser energy isbelieved to be absorbed primarily byelectrons (e.g. photon–electron interac-tions). This energy absorption andtransfer takes place over picosecondtimescales such that bonds are brokenwithout heating the lattice (i.e. a dise-quilibrium process). Binding energiesare thus overcome without significantheating of the lattice: the material‘explodes’ into an aerosol composed ofa mixture of atoms, ions, and smallmolecular fragments. As a result, inter-element fractionation from laser-targetinteractions is predicted to be negligi-ble. This has obvious advantages forU-Pb geochronology. The significantly

higher cost (currently) of turn-keyfemtosecond LA-ICP-MS systems(Photon Machines FS198.G2,NewWave Research NWR-femto,Allied Spectra J100) has limited theirdeployment to a relatively small cohortof institutions. In North America,there are only a handful of femtosec-ond LA-ICP-MS systems operated byEarth Sciences departments (Universityof Windsor, University of Wisconsin,UC Santa Barbara, and UC Berkeley)with other experimental systemshoused and operated by physics andmaterials science departments.

There are, however, manyother sources of inter-element frac-tionation within a LA-ICP-MS systemindependent of the laser instrumenta-tion. These include: 1) deposition ofmaterial on the inner surfaces of thecell (e.g. upper window or walls); 2)fractionation in the sample tubing (e.g.gravitational settling and electrostaticprocesses); and 3) mass-fractionationin the ICP-MS plasma and the ion-optical system. Some of theseprocesses are not time-dependent and

Figure 1. Reflected light photomicrograph (Zeiss AxioImager, 50 x objective) ofablation blankets surrounding 33 μm diameter craters in zircon. The largest parti-cles visible are ~500 nm in size. The majority of the blanket is composed of muchfiner particles (<100 nm) that are just barely resolvable optically. This blanket ispresumably formed from volatile-enriched nanoscale-particles that have condensedout of the expanding plasma plume. The particle size fraction carried away by thein-cell carrier gas is expected to be <100 nm.

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so can be compensated for by matrix-matched external standardization.

KEY DEVELOPMENTS: SENSITIVITY,SAMPLE CELLS, AND SOFTWARECONTROL

SensitivityChanges to quadrupole-ICP-MS ionoptical system and interface designsrecently introduced by Agilent (e.g.7700x), Thermo (X-SeriesII), Perkin-Elmer (NexION 300), and Bruker Dal-tonics (Aurora M90) have resulted ininstruments with inherently higher sen-sitivity than previous generations.Additional steps can be taken to fur-ther enhance the sensitivity of LA-ICP-MS by changing the conditions of:1) the ‘dry’ plasma, and 2) the vacuumin the interface region between thecones and the extraction lens. First,adding a small amount of high-purityH2 or N2 to the aerosol carrier gas canbe used to significantly increase thetemperature of the plasma and therebyenhance its ionizing efficiency (Guil-long and Heinrich 2007). Other stud-ies have recently shown that mixturesof high-purity CH4 + Ar and CH3OH(methanol) + Ar may have even largereffects on sensitivity enhancement(Fliegel et al. 2011), but this approachcan also generate unwanted polyatomicinterferences. Using N2 has merit as itcan be obtained at purities of the samelevel as the He carrier gas (e.g.99.999%) and is inert but results in sig-nificantly increased nitride interfer-ences. The amount of N2 or other gasmixture required to maximize sensitivi-ty varies from instrument to instru-ment. Typical flows for N2 arebetween 2 to 5 mL/min added to theHe + Ar carrier gas. Figure 2 showsthe effect on 207Pb intensity of adding2.8 mL/min N2 to an Agilent 7700x.These data were acquired under thesame analytical conditions used for U-Pb geochronology of zircon. It shouldbe noted that sensitivity on NIST610could be significantly increased simplyby increasing the laser fluence (e.g. to 8J/cm2), but this would compromise thegoal of keeping ablation craters shal-low for in situ work in thin sections.Notice that the robustness of the plas-ma (monitored as oxide productionand U/Th ratio ≈ 1) remains higheven under maximum sensitivity condi-

tions. The much greater ionizationefficiency under these conditions doesenhance the production of doubly-charged species (more ions reach theirsecond ionization potential), and maycontribute to the formation of molec-ular ion interferences (e.g. possible176Hf14N14N interference on 204Pb in zir-con).

Another technique that canenhance sensitivity is the use of a larg-er interface rotary pump, or multiplerotary pumps, to increase the vacuumin the region between the sample coneand the first ion extraction lens. Ther-mo Scientific have introduced a similarupgrade for their magnetic-sector ICP-MS line called the ‘Jet Interface’ thatfeatures a larger ‘dry’ vacuum pumpand redesigned cones (see, for exam-ple, Newman 2012). We use a manual-ly controlled second external rotarypump that can be turned on toincrease sensitivity of Pb and U. InFigure 2, the effect of turning the sec-ond rotary pump on is illustrated bythe sharp jump in 207Pb intensity. Theeffect is instantaneous because operat-ing the second pump immediately low-ers the pressure in the interface regionfrom 2.3 ×10-2 kPa to 1.9 ×10-2 kPa onour Agilent 7700x ICP-MS. The com-bination of N2 and second rotarypump allows us to obtain sensitivity asmeasured on the NIST610 silicate glassof ~2700 icps/ppm for 207Pb at a spotdiameter of 33 μm, pulse energy of 4

J/cm2, and repetition rate of 5 Hz.The zircon 91500 standard containsonly about 1 ppm 207Pb such that underthese analytical conditions we canachieve a net intensity of ~2500 icpsfor 207Pb.

For U-Pb geochronology ofyoung accessory minerals, such astitanite, rutile, and allanite, which cancontain a significant common-Pb com-ponent (e.g. 207Pb*/204Pbc <100), it maybe necessary to quantitatively subtractthe common-Pb component from eachof the radiogenic daughter products.This is most accurately accomplishedby direct measurement of the 204Pb sig-nal during ablation. The presence oftrace-level impurities of Hg in the car-rier gases (with 204Hg isobarically inter-fering with 204Pb) requires that Hg beeither removed from the gas usingimpurity traps (e.g. gold-coated sandtraps) or peak-stripped from the netmass 204 peak by monitoring 202Hg andassuming an instrumental 204Hg/202Hgvalue. Even so, some authors (Simon-etti et al. 2006) suspect the existence ofadditional polymolecular interferenceson 204Pb, casting doubt on the reliabilityof this approach. Alternatively, manylabs simply ignore 204Pb and rely oncommon-Pb corrections, such as themethod proposed by Andersen (2002),which is based on the measured 208Pbsignal, U/Th ratio, an estimate of theage of the target, and the model com-mon-Pb composition (e.g. Kramers

Figure 2. Effect on 207Pb signal, oxide production (ThO/Th) and U/Th ratio ofadding N2 to the carrier gas mixture and increasing the interface region vacuum.Moving average shown in black for oxide production and U/Th data.

GEOSCIENCE CANADA Volume 39 2012 163

and Tolstikhin 1997). In the UNB lab,we use dedicated high-capacity (up to20 L/min) Hg traps manufacturered byVici Metronics on all of the gas linesthat enter the laser and ICP-MS.These ensure that 204Hg gas back-grounds remain <150 icps under thehighest sensitivity conditions.

Sample CellsThe sample cell is arguably one of themost important components of anylaser ablation system. The cell holdsthe samples and standards in a con-trolled atmosphere and presents thetargets to the focused laser beam forablation. Ideally, all ambient air mustbe removed from the cell (e.g. byrepeated evacuation) and then continu-ously flushed with an ablation carriergas (typically pure He or He + Ar).Aerosols generated during ablationshould be fine grained, uniform in par-ticle size distribution, and ideally betransported to the ICP-MS as quicklyand efficiently as possible (fast cellwashout and high sensitivity) with min-imal contact with the cell walls or roof(which can cause additional inter-ele-ment fractionation). The conditions ofaerosol generation and transportshould also be identical anywhere inthe cell to maximize its useable area.Whereas numerous custom-designedcells have been described in the litera-ture, which are capable of fast washoutor high sensitivity, commercially avail-able cells come in two main configura-tions: 1) open cylindrical designs, and2) two-volume cells. It is now widelydocumented that large-volume opencells can achieve fast washout and highsensitivity only for parts of the cellthat lie between the inlet and outlet forthe carrier gas. Transport of aerosol isefficient within this central channel,but inefficient and variable away fromit, which limits the useful working areawithin these cells (Fisher et al. 2011;Koch and Gunther 2011). To circum-vent the problem of maintaining con-stant washout and sensitivity through-out the cell, while also providing alarge enough area to accommodategeologic samples (e.g. thin sections,polished slabs) and standards, two-vol-ume low-volume (2VLV) cells havebeen developed. The most commonlyused, the Australian National Universi-ty (ANU) Helex design (Eggins et al.

1998) has been adopted by Resonetics(in collaboration within Laurin TechnicPty) and more recently by PhotonMachines (e.g. Analyte G2 in collabora-tion with ANU Research School ofEarth Sciences). A similar ‘large for-mat cell’ was also developed by NewWave Research, but deployment of thiscell has been limited. One of the prin-ciple benefits of the 2VLV cell is thatablation takes place within a low-vol-ume (<1 cm3) funnel, which can movearound inside the cell (in the ANUHelex and Laurin Technic M-50designs the cell actually moves aroundthe funnel which is fixed). Thisensures that ablation conditions,washout times, and transport efficiencyare identical anywhere in the cell. Thelow-volume of the 2VLV design alsoensures that aerosol washout isextremely fast (several orders of mag-nitude in <1 second). This means thatbackground levels are re-establishedonly a few seconds after the cessationof ablation, so that the next target inthe sequence can be started almostimmediately. This innovation signifi-cantly increases the number of analy-ses that can be conducted per hour. Ina recent detrital zircon study using aResonetics M-50-LR laser ablation sys-

tem equipped with a Laurin TechnicPty 2VLV cell, 97 analyses (76unknown zircons, 18 zircon standards,and 3 NIST glasses), each comprising a15-second ablation followed by a 20-second washout period, were complet-ed in just over 1 hour. The detrital zir-cons occur in a single, heavy-minerallayer in a thin section of tourmalinizedmetaquartzite from the lower Aldridgemember of the Belt-Purcell basin westof Kimberley, BC. The results of thisablation sequence are shown in Figure3.

Signal SmoothingThe use of fast-washout sample cellswith excimer-based lasers can lead toundesired oscillation of the time-resolved ICP-MS signal during ablationat repetition rates less than 4 Hz, whenthe transfer rate of aerosol from thecell (e.g. several orders of magnitude in<1 second) approaches that of thedelay between pulses. Inline smooth-ing devices are, therefore, necessary toeither lengthen or homogenize individ-ual pulses prior to introduction to theICP-MS torch. For example, the ANUHelex and Laurin Technic Pty designsuse a high efficiency smoothing devicecalled the ‘squid’ that first divides the

Figure 3. The rapid washout time of the 2VLV cell allows large datasets, such asthis detrital zircon provenance curve, to be constructed in about 1 hour.

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incoming aerosol gas stream intonumerous different tubes each of dif-ferent path length, and then recom-bines the separate paths into a‘stretched’ output signal. Other manu-facturers provide baffled cylindricalmixing bulbs designed to homogenizetransient pulses before continuing onto the ICP-MS torch. Smoothing ofthis nature is critical for isotope meas-urements by single-collector-ICP-MS(either quadrupole or magnetic sector)in which ions are counted sequentially:significant variations in intensity (e.g.oscillating signal) during the measure-ments sequence (e.g. 238U might becounted 0.1 seconds after 206Pb) willlead to high internal errors that cancompromise the final propagated pre-cision. The importance of signalsmoothing using the ‘squid’ is wellillustrated by Müller et al. (2009). Theyalso demonstrated a phenomenonknown as spectral skew, which ariseswhen the laser repetition rate and ICP-MS sampling time give rise to long-

period (e.g. several second) oscillationsor ‘beating’ of the signal. This spectralskew can be eliminated by changingeither the ICP-MS dwell times or laserrepetition rate.

Software Control

Ablation Modes and Ablation TimingLaser ablation system control softwarehas been engineered with synchronizedstage motion, laser triggering, and laserpulse timing to facilitate a wide rangeof laser ablation techniques. The sim-plest application of this synchroniza-tion involves automated execution oflarge ablation sequences pre-pro-grammed with crater diameter, laserenergy, repetition rate, ablation dura-tion and timing constraints for back-grounds and sample pre-cleaning oper-ations. Figure 4 shows some of thewide variety of standard ablationmodes typically available on modernlaser ablation systems. The ability to

mask the laser beam using narrow rec-tangular slits enables high spatial reso-lution linescans across compositionaldomains. The rotating rectangular slitavailable on the Resonetics systemadditionally enables synchronizationbetween stage motion, laser firing, androtation angle of the slit. Thisapproach is used to ensure that thenarrow direction of the slit is alwaysparallel to the direction of travel,thereby yielding the highest possiblespatial resolution.

Synchronization with ICP-MSThe recent development and dissemi-nation of Iolite™ software (Paton etal. 2010, 2011) has allowed both manu-facturers and end-users to change theirapproach to laser ablation. Iolite™represents a significant departure fromother widely used U-Pb data reductionschemes such as LAMDATE (Košleret al. 2002) and GLITTER™ (Griffinet al. 2008), in which data areprocessed on a spot-by-spot basis.

Figure 4. Ablation modes available on most new laser ablation systems. Depending on textural variations and compositionalzoning within a target grain, different ablation modes can be used to target specific domains.

GEOSCIENCE CANADA Volume 39 2012 165

-short pulse excimer

-flat craters

m and up

SEM overlays

-flat craters

Neptune

Element

Analyte 193

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#140, 5700 1 Street S.W., Calgary, AB T2H 3A9 . isomass@isomass.com Tel: (403) 255-6631 . Fax: (403) 255-6958 . Toll Free: 1-800-363-7823

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Iolite, which runs as a plugin in Wave-metrics Igor Pro 6.2x™, allows time-series data (e.g. cps vs. time for eachanalyte) for primary standards, qualitycontrol standards, and unknowns to becollected and reduced as a single out-put file that could represent severalhours of continuous data collection.Such long acquisitions are not unusualfor solution-based ICP-MS, where inte-grated autosamplers and pre-definedsequences are used to analyze largebatches of liquid samples. Laser abla-tion system manufacturers have nowadopted a similar approach, in whichthe user builds up a sequence ofnumerous, pre-defined ablations, whichcan be run automatically or semi-auto-matically (i.e. with user doing the finalfine-positioning) while the ICP-MScollects data continuously over thesame time interval. A time-lapsemovie that shows a 20 minute ablationsequence in which Temora zircon wasanalyzed using zircon 91500 as a stan-dard is available at:

http://youtu.be/EcR-JwYy-boThe movie shows various

modes of illumination, the growth ofablation blankets (visible in reflectedlight), and the plasma plume (with illu-mination off) that is generated withineach laser pulse. At the end of thesequence, the laser software generates a.csv log file that records the laser ‘on’and ‘off ’ timestamps, which Iolite™uses to synchronize with the ICP-MSoutput file. The laser log file alsorecords all other variables as metadata,such as target labels, raster scan speed,repetition rate, laser energy, and cellpressure.

By synchronizing the lasercontrol and ablation sequencing withthe intensity data measured by theICP-MS, Iolite™ can also be used to‘auto-integrate’ the icps versus timedata related to each type of ablation(e.g. standards, unknowns). For targetgrains that contain complex zoning,evidence for heterogeneous Pb-loss, orthat require a common-Pb correction,VizualAge™ (Petrus and Kamber2012) is used to manually select theintervals within the signal that yieldeither the most concordant data or thelowest internal errors on ratios such as207Pb/206Pb. The automatic integrationfeature also allows the definition of acontinuous baseline for the entire ana-

lytical sequence. One of the numerousadvantages of this approach is the abil-ity to accurately correct for instrumentdrift (as recorded by time-dependentchanges in the standards) and to usedifferent splines to accurately modeland subtract the gas background. Theability to accurately and rapidly reducelaser ablation data using two synchro-nized files (one from the laser and onefrom the ICP-MS) ensures that theresults of each ablation sequence canbe processed and inspected before thenext batch of data is collected. Thisapproach is not quite ‘on-the-fly’, butthis could easily be achieved throughincreased collaboration between laserand ICP-MS instrument manufacturers.

Elemental Fractionation Correc-tions

Drilling mode analysisAccurate and precise U-Pbgeochronology using 193 nm ArFexcimer LA-ICP-MS in ‘drilling’ modeis dependent on external standardiza-tion to correct for both LIEF at theablation site, within the transport tub-ing connecting the laser cell and theICP-MS, fractionation within the ICP-MS torch, and for mass-discriminationwithin the ICP-MS ion optical system.Testing the effectiveness of LIEF cor-rections is relatively straightforward. Ifa standard and unknown are sufficient-ly matrix-matched such they displaysimilar LIEF, then application of thecorrection will yield constant 206Pb/238U,207Pb/235U and 208Pb/232Th as a functionof time, assuming the unknown has aconstant age within the volume of thecrater. The power of Iolite™ softwaredescribed above is its ability to fit dif-ferent functions (e.g. linear, exponen-tial, power law) to the measured LIEFfor Pb/U and Pb/Th. The goodness-of-fit of different models can be easilyassessed and, if necessary, customizedfunctions can be implemented to bestdescribe the fractionation relationship(Paton et al. 2010).

Once the standard andunknowns are corrected for LIEF, thedifference in the measured versus trueisotopic ratios for the standard arisingfrom mass-discrimination induced bythe ICP-MS are used to further correctthe unknowns. These corrections canbe significant (e.g. a few percent) for

Pb/U and Pb/Th ratios because of thehigher transmission efficiency throughthe ICP-MS ion optic system of U andTh relative to Pb. In contrast, themeasured 207Pb/206Pb may differ by<1% compared to the true ratiobecause laser-induced fractionation ofisotopes of the same element andmass-discrimination between mass 206and 207 during transmission throughthe ICP-MS are minimal. Figure 5shows how this process works for amonazite analyzed with a 45 μm craterin a sequence that was standardizedagainst the Trebilcock monazite. Thefinal corrected ratio overlaps with theknown value as measured independent-ly by SHRIMP.

Well characterized, matrix-matched standards exist for zircon (e.g.91500, QGNG, Temora, FC-1) andbaddeleyite (FC-1), but there are nowell-characterized and widely distrib-uted matrix-matched standards formost of the other accessory mineralsthat are routinely dated (i.e. monazite,titanite, allanite, apatite, perovskite, andrutile). In most cases, matrix-matchedstandards are developed in-house andtheir efficacy demonstrated by thereproduction of concordant ages onwell-established quality control materi-al. For example, the 416 Ma Temorazircon standard is commonly used totest the accuracy and precision of zir-con U-Pb dating methods. A typicalresult for Temora analyzed on the Uni-versity of New Brunswick LA-ICP-MSsystem is shown in Figure 6.

If the matrices do not exhibitcomparable ablation characteristics (e.g.analyzing metamict zircon using a crys-talline zircon standard) then normaland reverse discordance is likely tooccur, as a result of mismatched Pb/Ufractionation correction. In thesecases, minimizing laser-induced frac-tionation by rastering would yield themost concordant results and many LA-ICP-MS labs adopt this approach.

In many cases, there are nomatrix-matched standards that are suit-able for LA-ICP-MS geochronology.This often arises for accessory mineralsthat contain significant amounts ofcommon-Pb. In such cases, it is verydifficult to find an isotopically homo-geneous and concordant standard thatcan be used to make the correctionsdescribed above. A rare exception in

GEOSCIENCE CANADA Volume 39 2012 167

use at UNB and elsewhere is the ~520Ma Khan Mine titanite, which wasformed from a very high U/Pb sourcerock. No such standards currentlyexist for other common-Pb bearingminerals such as apatite and allanite.

Raster mode analysisBy using raster-mode ablation, therebyminimizing LIEF, it is usually possibleto use matrix-mismatched standards toreliably correct for instrumental mass-discrimination. For example, Chew etal. (2011) have recently developedapatite dating methods using rasteranalysis that rely on zircon 91500 as anexternal standard. Zircon is used inpreference to other potential standardsbecause it shares similar absorbance toapatite and so both minerals ablateunder similar fluence conditions.

Image OverlaysLaser ablation manufacturers now pro-vide software interfaces that enableimage files from other instruments tobe overlain (e.g. as semi-transparentlayers) on top of the basic video feedfrom the laser optical system. Internalzoning information in target materialsrecorded using polarizing microscopy,backscattered electron (BSE) imaging,cathodoluminescence imaging (CL), orcompositional maps (from either thelaser itself or from electron microbeaminstruments), can be overlain in thelaser software to help guide the place-ment of ablations in the sequence.This ability is particularly useful whenanalyzing complexly zoned accessoryminerals that may contain several gen-erations of inherited components.

The UNB laser ablation sys-tem is highly integrated with bothSEM and optical imaging instruments.The principle imaging instrument is aZeiss AxioImager, which is equippedwith a motorized X-Y stage. This toolallows us to create seamless mosaics(using the AxioVision software‘Mosaix’ routine) of entire thin sec-tions or polished slabs, augmentedwhen necessary by high magnificationtransmitted and reflected light imagesof the most important targets.Because the objectives are calibrated,the AxioImager is also used to measurethe size of targets and to determinethe diameter of the crater or raster pat-tern that can be used for each grain.

Figure 6. Results of a quality-control run on Temora zircon using a 24 μm diame-ter crater. The calculated Concordia age overlaps within error the true age, con-firming the accuracy of the analytical method.

Figure 5. Raw, LIEF-corrected, and mass-discrimination-corrected time-series for206Pb/238U ratio for a monazite unknown analyzed with a 45 μm crater in a sequencestandardized using Trebilcock monazite. The green bar (the height of which inIolite is proportional to the error) is the final integration period used to calculatethe error for the final 206Pb/238U ratio.

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This ability to integrate different kindsof macro- and micro-imaging ensuresthat internal zoning features in acces-sory minerals can be identified andaccurately targeted for LA-ICP-MS.Figure 7 shows two levels of imageoverlays that have been ‘georeferenced’to the underlying laser system stagecoordinates to help accurately positioncraters on a small monazite grain.

Offline Definition of AblationSequences Some laser ablation system manufac-turers (e.g. Resonetics) are also nowproducing ‘offline’ versions of theirablation software that allow users toconstruct ablation sequences prior tothe actual analytical session. In thisapproach, images of the target area arefirst imported into the offline software,and the relative locations of ablationpoints, lines, paths, or polygonsdefined. The offline definition ofthese points can include such parame-ters as target labels, repetition rate,ablation time, etc. The image and thepredefined ablation sequence are then

saved as a single file. The com-bined image and its predefinedablation sequence are then import-ed into the online laser software atthe start of an analytical session.Figure 8 shows a seamless mosaicof a zircon-rich heavy-mineral layerin a polished thin section. The rela-tive X-Y position of pre-definedablation targets and the sequence ofanalysis (connected by lines) areshown. The image and storedpoints are then ‘georeferenced’ tothe laser system X-Y stage using theonline software. The only remain-ing step is to define a series ofstandard ablations and verify theICP-MS tuning. This approach sig-nificantly increases the number ofsamples that can be analyzed duringan analytical session.

APPROACH AND APPLICATIONSTO IN SITU U-PbGEOCHRONOLOGYIncreasing LA-ICP-MS sensitivity asdescribed above facilitates datingmost accessory minerals with 10s to

Figure 7. Example of multi-layer overlays at progressively higher levels ofdetail. This process is used to ensure the laser crater is positioned so as to sam-ple uniform elemental or isotopic domains.

Figure 8. Offline ablation sequence definedfor a heavy-mineral layer in a quartzite.

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100s of ppm Pb and U concentrationsusing crater diameters <45 μm. Acces-sory minerals with higher U concentra-tions (e.g. 100s to 1000s of ppm U inmonazite, allanite) can be successfullyanalyzed using craters <20 μm indiameter. In all cases, laser fluence andrepetition rate can be optimized toensure that craters are also half asdeep as the diameter of the crater (i.e.depth/diameter ≤0.5). Figure 9 showsa crossed polarized photomicrographof craters produced in epidote–allaniteusing a 33 μm diameter crater and 30second ablation time at 4 Hz repetitionrate. The decrease in interferencecolour between the top (2nd order pink)

and bottom (2nd order blue-green) ofthe crater indicates depths of ~10 μm.This level of horizontal and verticalspatial resolution is critical for mini-mizing mixing between different agedomains. Ablating shallow craters alsohelps reduce the magnitude of LIEFand, together with efficient samplesmoothing, prevents the rapid loss ofanalyte intensity.

These ablation conditions aretypically matched with an ICP-MStime-resolved analyse (TRA) methodthat takes advantage of the very fastpeak-hopping capabilities of thequadrupole mass filter. For U-Pbgeochronology, the dwell times (mea-

sured in ms) are typically: 207Pb > 206Pb> 208Pb ≥ 232Th ≥ 238U. This orderreflects: 1) the diminishing abundanceof Pb isotopes; 2) the need to ensurethat the 207Pb/206Pb ratio has the high-est precision, and; 3) the higherabsolute concentration and ion trans-mission through the ICP-MS of U andTh. Dwell times are typically 10-20 msfor 208Pb, 232Th, and 238U and somewhatlonger dwell times of 30-80 ms for206Pb, 207Pb and 204Pb (if the latter iseven analyzed). The goal of thissetup is to ensure that a single sweepof the quadrupole is almost instanta-neous, that a large number of measure-ments per second are obtained, andthat a sufficient number of ions arecounted for each mass (see, for exam-ple, Halicz et al. 1996). Adding upindividual dwell times plus quadrupolesettling time (<1 ms) typically yieldstotal integration times ≤0.25 secondsso that a 35 second ablation will yield~140 independent measurements ofthe 207Pb/206Pb, 206Pb/238U and208Pb/232Th ratios (207Pb/235U is calculat-ed offline using the natural abundanceof 235U/238U = 1/137.88).

Using this approach allowsArF excimer LA-ICP-MS to achieveshort-term precision, expressed as rela-tive standard deviation (%RSD) for theraw 207Pb/206Pb, 206Pb/238U, and207Pb/235U in the range of 0.5 to 3.5%as shown in Table 1.

Precision on the final correct-ed ratios is typically better than shownin Table 1 because the data are correct-ed for instrument drift, and processingwith Iolite™ and VizualAge™ helpsrefine the standard time integrations toachieve the most coherent population.The precision for the 207Pb/206Pb in thistable was also optimized by using thelongest dwell times for 207Pb (75 ms)

Figure 9. Adjusting ablation conditions (e.g. fluence and repetition rate) can beused to minimize crater depth, as shown here for 33 μm craters in epidote–allanite.The shift in interference colour from 2nd order pink to 2nd order blue-green is con-sistent with crater depths of 10 μm. The diameter of the crater was measuredusing our Zeiss AxioImager A1M polarizing microscope.

Table 1. Precision, expressed as %RSD of raw ratios, for replicate analyses (n) of isotopically homogeneous standards at rou-tinely used laser crater diameters and repetition rates (data obtained using Resonetics M-50-LR laser ablation system and Agilent7700x ICP-MS).

Standard Crater Ø Rep. rate n Ablation Raw Raw Raw(μm) (Hz) time 206Pb/238U 207Pb/235U 207Pb/206Pb

(sec) (%RSD) (%RSD) (%RSD)

Khan Titanite (520 Ma) 45 4 13 30 0.55 0.76 0.5591500 Zircon (1065 Ma) 24 4.5 18 35 0.87 1.04 0.84Trebilcock Monazite (272 Ma) 14 4.5 16 30 1.36 1.76 1.04FC-1 Zircon (1099 Ma) 10 4.5 14 30 3.08 3.34 2.24

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and 206Pb (30 ms) thereby ensuring thaterror ellipses on a conventional Con-cordia diagram have positive error cor-relations and can be plotted using Iso-plot (Ludwig 2003). Assuming thatICP-MS sensitivity is maximized asdescribed above and that 1% RSD onthe raw 207Pb/206Pb is a reasonable min-imum precision, the data in Table 1also demonstrates that there are limitsto crater diameters useable on differentaccessory minerals which is a functionof absolute U concentration and agefor the standard. For example, zirconcan be analyzed with minimum craterdiameters between 24 μm (using91500) and 20 μm (using higher-U FC-1 as a standard) whereas similar preci-sion can be achieved on monaziteusing 15 to 20 μm diameter cratersowing to its significantly higher U andPb* concentration.

Typical workflow for in situ U-Pb accessory mineral geochronologyusing the methods described above isshown in Table 2.

CONCLUSIONSBy combining enhanced ICP-MS sensi-tivity, fast and efficient sample cells,and sophisticated software controls,modern 193 nm ArF excimer LA-ICP-

MS systems are able to:1) Obtain precise and accurate U-Pb

isotope data at crater sizes < 45μm in diameter and <20 μm deepfor most accessory minerals, facili-tating in situ analyses in standard30 μm thick polished thin sections.

2) Use low repetition rates to main-tain shallow craters, thereby mini-mizing LIEF, and reducing theeffects of matrix mismatchesbetween standards and unknowns.This not only leads to increasedaccuracy and precision, but hasalso enabled development of U-Pbdating protocols for a wider rangeof accessory minerals.

3) Identify ablation targets and defineablation sequences offline, therebyincreasing the overall efficiencyand cost-effectiveness of the tech-nique.

4) Seamlessly integrate internal zon-ing information captured usingcombinations of polarizingmicroscopy, SEM-BSE, SEM-CL,EPMA X-ray elemental maps.

5) Rapidly reduce and inspect resultsusing Iolite™ and VizualAge™software to confirm the precisionand accuracy of LA-ICP-MS ana-lytical conditions and assess the

need for further refinement ofages.

ACKNOWLEDGEMENTSThe authors are grateful to S. Jacksonand J. Gagnon for their thoughtfulreview of the original manuscript. Wewould also like to thank M. Hamel(Resonetics LLC) for his technical sup-port over the duration of this project.The UNB laser ablation lab (with co-PIs D. Lentz and C. Shaw) was builtwith the support of the CFI LeadersOpportunity Fund, NBIF ResearchInnovation Fund, Xstrata Zinc, M.Watson, Elmtree Resources, NBDepartment of Natural Resources andwith in-kind support from ResoneticsLLC and Agilent Technologies Canada.Y. Luo was additionally supported by aNBIF Research Assistantships Initia-tive grant. A portion of this researchwas also supported by an NSERC Dis-covery Grant to C.R.M.M.

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Table 2. Typical workflow used for in situ LA-ICP-MS U-Pb geochronology of accessory minerals in standard 30 μm polishedthin sections.

Creation (if necessary) of polished thin section scan or reflected-light mosaic and optical identification of accessory mineraltargets. Estimation of largest crater diameter that can be used to analyze specific compositional domains.

↓

Backscattered electron (BSE) and/or cathodoluminescence (CL) imaging, electron microprobe X-ray mapping to identify textur-al and/or compositional domains.

↓

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↓Check craters optically and correlate measured U-Pb ages with textural or compositional domains.

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Received June 2012Accepted as revised June 2012

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