Geochemistry G3 Volume 9 Geophysics Geosystemspeople.uncw.edu/lamaskint/GLY 445-545 FALL 2013... · 207Pb intensities that are

Enhanced precision, accuracy, efficiency, and spatialresolution of U-Pb ages by laser ablation–multicollector–inductively coupled plasma–mass spectrometry

George E. Gehrels, Victor A. Valencia, and Joaquin RuizDepartment of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721, USA([email protected])

[1] U-Th-Pb geochronology by laser ablation–multicollector–inductively coupled plasma–massspectrometry initiated during the mid to late 1990s as a reconnaissance tool, capable of generating agesof only moderate precision from relatively large volumes of zircon. New developments in instrumentationand experimental methodology, as described herein and by other researchers, now make it possible it tocorrect for common Pb accurately (using measured 204Pb), to acquire geochronologic information rapidly(30–40 unknowns/h), to generate U-Pb ages with an accuracy of better than 1% for most zircon standards,and to conduct analyses on much smaller (e.g., 10 mm by 6 mm) volumes of material. These capabilities aredriving important advances in many aspects of Earth science research.

Components: 6453 words, 13 figures.

Keywords: geochronology; LA-ICPMS.

Index Terms: 1115 Geochronology: Radioisotope geochronology; 1194 Geochronology: Instruments and techniques; 1040

Geochemistry: Radiogenic isotope geochemistry.

Received 31 August 2007; Revised 29 November 2007; Accepted 22 December 2007; Published 20 March 2008.

Gehrels, G. E., V. A. Valencia, and J. Ruiz (2008), Enhanced precision, accuracy, efficiency, and spatial resolution of U-Pb

ages by laser ablation–multicollector–inductively coupled plasma–mass spectrometry, Geochem. Geophys. Geosyst., 9,

Q03017, doi:10.1029/2007GC001805.

1. Introduction

[2] U-Th-Pb geochronology is becoming an increas-ingly important tool in many aspects of Earthscience research because technical developmentshave provided opportunities for improved precisionand accuracy, enhanced spatial resolution, and moreefficient data acquisition. Some of the most excitingadvances in geochronology are being driven bylaser ablation-inductively coupled plasma-massspectrometers [Gunther et al., 1997; Gunther andHeinrich, 1999; Horn et al., 2000; Jackson et al.,2001; Horstwood et al., 2003; Kosler and Sylvester,

2003;Woodhead et al., 2004; Simonetti et al., 2005,2006;Chang et al., 2006;Gehrels et al., 2006;Hornand von Blanckenburg, 2007].

[3] The Arizona LaserChron Center (ALC) con-ducts U-Th-Pb geochronology with a multicollectorinductively coupled plasma-mass spectrometer(GVI Isoprobe) that is coupled to a 193 nm Excimerlaser ablation system (New Wave Instruments andLambda Physik). These instruments have beenparticularly successful because they (1) can deter-mine U-Th-Pb ages very efficiently, (2) generateages with a precision and accuracy that is appropri-

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ate for most geochronologic problems in Earthscience, (3) provide geochronologic informationwith a horizontal resolution of �10 mm and a depthresolution of �1 mm, (4) provide opportunities fordevelopment of new geochronological techniquesand applications, (5) are highly amenable to multi-user operation, and (6) provide an excellent toolfor training researchers in the theory and practice ofU-Th-Pb geochronology. This article describes theanalytical methods that are used for U-Th-Pb geo-chronologic research at the ALC, with emphasis ontechnical developments that provide enhanced pre-cision, accuracy, efficiency, and spatial resolution ofU-Th-Pb ages and on the types of scientific advan-ces that result from these developments.

2. Sample Preparation and LaserAblation

[4] Zircon is commonly used for U-Th-Pb geo-chronology because it is present in many crustalrocks, contains moderate concentrations of U andTh (typically tens to thousands of ppm) butvery little Pb (ppb-ppt) when it crystallizes and isresistant to alteration and disturbance of the U-Th-Pb isotopic system [Harley and Kelly, 2007]. Formost applications in our lab, zircon crystals areextracted from a rock sample by standard mineralseparation techniques and mounted in a 1-inch-diameter epoxy plug, the surface of which is sandeddown to expose the interior portions of most grains.Along with the unknowns are fragments of a stan-dard zircon crystal that has been dated by isotopedilution–thermal ionization (ID-TIMS). The stan-dard crystals are used to constrain Pb/(U-Th) frac-tionation and U and Th concentrations.

[5] It is also possible to analyze grains in situ in athin section (e.g., in cases where the petrographiccontext is critical) or loose grains attached to tapeon a glass slide (e.g., when specific grains will beremoved and reanalyzed for higher precision byID-TIMS or for fission track or (U-Th)/He thermo-chronologic analyses). In the case of thin sections,standards are inserted into holes drilled adjacent tothe unknowns, and there is little additional varia-tion in Pb/(U-Th) fractionation. For loose grains,there is considerably (�2X) greater uncertainty inPb/(U-Th), presumably due to nonlaminar flow ofcarrier gas across the sample surface and thecomplexity of interactions between the laser beamand an irregular crystal surface.

[6] Cathodoluminescence (CL) images are ac-quired for most samples because they enable

placement of laser pits in specific portions ofcrystals, and because variations in CL texture aidin interpreting the origin (e.g., igneous, metamor-phic, or hydrothermal) of the zircons [Hanchar andMiller, 1993; Nasdala et al., 2003; Corfu et al.,2003]. Such images need to be used with caution indetrital zircon analyses, however, because selec-tion/rejection of grains according to CL character-istics can yield a biased age spectra.

[7] Laser ablation (LA) takes place with a beamdiameter of either 35 or 25 mm for most applica-tions, or with a beam diameter of 15 or 10 mm iffiner spatial resolution is needed. With a 35 or25 mm beam, the laser is set at a repetition rate of8 Hz and fluence of �4 J/cm2, which ablates at arate of �1 mm/s and yields an average pit depth of�12 mm. This generates a signal of �100,000 cpsper ppm for U in zircon. For smaller beam sizes, theablation rate is reduced to �0.5 mm/s by reducingthe laser fluence and repetition rate, and average pitdepth is�6 mm. In both cases the ablated material isremoved from the ablation chamber in He carrier gas(following Eggins et al. [1998] and Gunther andHeinrich [1999]), mixed with Ar, and passedthrough the plasma of the inductively coupledplasma–mass spectrometry (ICP-MS).

3. Isotopic Analysis

[8] Isotopic analysis is performed with a multi-collector inductively coupled plasma-mass spec-trometer (GVI Isoprobe) equipped with an Soption interface (Figure 1). The instrument isequipped with a collision cell operated with anargon flow rate of 0.24 mL/min to create a uniformenergy distribution, and the accelerating voltage is�6 kV. Collectors include nine Faraday detectorsand four low-side channeltron multipliers, all ofwhich are moveable, as well as an axial Dalyphotomultiplier.

3.1. Collector Configurations

[9] Two different collector configurations are usedto accommodate the wide range of signal intensi-ties that result from variations in U concentration,age, and rate of ablation (Figure 1). For samplesanalyzed with a 35 or 25 mm beam, Pb isotopemeasurement is challenging because crystals thatare old and/or of high U concentration commonlygenerate 206Pb intensities that are >1,000,000 cps,which is too high for continuous measurement witha channeltron. Conversely, crystals that are youngand/or of low U concentration commonly generate

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207Pb intensities that are <50,000 cps, which is toolow for reliable measurement with a Faraday col-lector equipped with a 1 � 1011 ohm resistor. Wehave accordingly developed a configuration inwhich 238U, 232Th, 208Pb, and 206Pb are measuredin Faraday collectors equipped with 1 � 1011 ohmresistors, 207Pb is measured with a Faraday collec-tor equipped with a 1 � 1012 ohm resistor, and204Pb is measured with a channeltron multiplieradjusted to have a gain of 1.0 relative to theFaraday collectors (‘‘large zircon’’ configurationof Figure 1).

[10] The improvement in our ability to measure206Pb/207Pb ages is shown in Figure 2a, which com-pares the precision of 206Pb/207Pb and 207Pb/235Uages resulting from measurement of 207Pb with 1 �1012 ohm versus 1 � 1011 ohm resistors. Analysesare of standard zircons ranging in age from 91 to1065 Ma, using the same operating conditions forboth sets of measurements (Data Set S1 in theauxiliary material1). The 207Pb intensity (in countsper second) is shown for each sample. It is clear thatthe 1 � 1012 ohm resistor yields much betterprecision for count rates below �50,000 cps.[11] For applications where beam size and pitdepth are reduced to improve spatial resolution,channeltrons are used for all Pb isotopes andFaraday collectors are used for 232Th and 238U(‘‘small zircon’’ configuration of Figure 1). By

reducing the laser beam diameter to 15 or 10 mmand the excavation rate to �0.5 mm/s, the intensi-ties of the Pb peaks rarely approach 500,000 cps.As described below, this configuration yields agesthat are similar in precision and accuracy to agesmeasured with the ‘‘large zircon’’ configuration,even though much less zircon (as low as 1–2 ng) isexcavated. This configuration is used only whenthe improved spatial resolution is necessary, how-ever, in an effort to prolong the lifetime of thechanneltrons (�200 � 1011 total counts).

[12] Channeltron linearity over the range of 10,000to 600,000 cps has been evaluated by measurementof 206Pb(channeltron)/

238U(faraday) (in solution) as afunction of 206Pb(channeltron) intensity (Figure 2b),and by comparison of the known age of zirconswith the 206Pb/238U ages of zircons that have beenanalyzed with 206Pb in a channeltron and 238U in aFaraday detector (Figure 2c, data in Data Set S2).These plots show that measured ratios are accurateto within �1.5% over this intensity range, and thatthere is a correlation between intensity and offset.Experiments with channeltron corrections (e.g.,dead time) to account for this nonlinear responseare in progress.

3.2. Wet Versus Dry Plasma

[13] As described by Gunther and Heinrich [1999]and O’Connor et al. [2006], signal intensity issignificantly enhanced (due to more efficient ener-gy transfer to the ablated ions), and the plasma isless affected by the arrival of ablated material

Figure 1. Schematic diagram of the GV Isoprobe used for isotope ratio measurements at the ALC. Also shown arethe two collector configurations used for zircon analyses.

1Auxiliary materials are available at ftp://ftp.agu.org/apend/gc/2007gc001805.

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Figure 2. (a) Comparison of analyses conducted with measurement of 207Pb with Faraday collectors using 1 � 1011

ohm (blue ellipses) versus 1 � 1012 ohm (red ellipses) resistors. All other aspects of acquisition were similar for thetwo data sets. Signal intensity (in counts per second) of 207Pb during analysis with 1 � 1012 ohm resistors isindicated. Data are reported in Data Set S1. (b) Comparison of signal intensity versus 206Pb/238U in a solution, using achanneltron for measurement of 206Pb and a Faraday collector for measurement of 238U. Zero percentvalue corresponds to the signal intensity (�254,000 cps) generated by the calibration standard during a typicalanalysis. (c) Comparison of known age of zircon standards (zero line) with measured 206Pb/238U ages using achanneltron for 206Pb and a Faraday detector for 238U (data reported in Data Set S2). Each symbol represents the ageshift (expressed in %) of the weighted mean of 10 analyses of a sample. The 206Pb/238U ages are calibrated relative toa Sri Lanka zircon, as described in the text, which yielded an average 206Pb intensity of 254,000 cps.


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(because water dominates the plasma loading), ifwater is aspirated during laser ablation analysis.The impact for our system is a 2X increase insensitivity, and elimination of the tendency for204Pb signal intensities to drop when ablated mate-rial is injected into the plasma. Optimal gas flowrates for this configuration are 0.36 L/min for Hecarrier gas, 0.20 L/min for Ar make-up carrier gas(mixed with He 60 cm upstream from the torch),1.0 L/min Ar for intermediate gas, and 14 L/min Arfor coolant gas. Aspiration takes place with amicroconcentric nebulizer with an uptake rate of50 mL/min and an Ar flow rate of 0.34 L/min.

3.3. Data Acquisition

[14] Data acquisition involves (1) a single 12-sintegration on peaks with no laser firing to measureon-peak background intensities, (2) 12 s of laserablation during which intensities are integratedonce per second, and (3) �30 s with no laser firingto allow all sample material to purge through thesystem and to prepare for the next analysis. Thisyields a throughput of 30–40 unknown analysesper hour.

[15] Ion intensities achieved during laser ablationof a typical zircon (564Ma,�518 ppmU, 206/204 =�16,000) are shown on Figure 3. Important valuesand patterns are as follows:

[16] 1. The 204 intensity has a large spike duringthe first �0.2 s due to the presence of common Pbon the surface of the sample mount.

[17] 2. Background 204 intensity is �310 cps.Most of this 204 is Hg, as indicated by a back-ground 202/204 ratio that is indistinguishable fromnatural Hg and by a low 206/204 ratio. Reducingthis background 204Hg is one of our constantchallenges. Useful strategies include using researchgrade (99.999% purity) He carrier gas, replacementof the Au hexapole rods with Al rods, using Al(rather than Ni) cones, avoiding analysis of mountsthat have been coated with gold, and insertion of anHg trap (made from gold-coated quartz beads,available from Brooks-Rand Corporation, http://www.brooksrand.com) into the He carrier gas line.

[18] 3. Peak 204 intensity is �620 cps, which istypical for an average zircon crystal. Given that202Hg does not increase in intensity during abla-tion, this 204 must be Pb.

[19] 4. The 207Pb intensity has a slower responsethan the other signals.

[20] 5. U, Th, and Pb decrease in intensity duringmost of the analysis but at different rates. Thesetrends result from increasing degrees of interactionbetween the ablated material and the sample sur-face within the pit as pit depth increases [Guntherand Hattendorf, 2001; Kosler and Sylvester, 2003].

[21] 6. All intensities return to approximately back-ground values within several seconds after the laserceases firing.

4. Data Processing

[22] All aspects of data reduction are conductedoff-line with an Excel spreadsheet (‘‘agecalc’’)equipped with VBA macros. This system is fullyautomated to import data from Isoprobe files,perform all necessary corrections, and calculateages, uncertainties, and error correlations. Follow-ing extraction from a set of Isoprobe files, only threecorrections are applied prior to age calculation.

4.1. Depth-Related Fractionation

[23] Because the first few seconds of acquisitionhave rapid changes in intensity, delayed response

Figure 3. Ion intensities generated by laser ablation ofa 564 Ma zircon with 518 ppm U and a 206Pb/204Pb of�16,000. The laser was fired for 12 s (starting at 0 s).Data from the first 3 s are ignored due to the rapidlychanging signal intensities, the large spike in 204Pb, andthe delayed response of the 207Pb collector (due to thelonger time constant of the 1 � 1012 ohm resistor). Agesare calculated from data for seconds 4–12.


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in the 207Pb signal, and a large spike in 204Pb, thefirst 3 s of data are not used in calculating ages.The remaining 9 s of data are extracted fromIsoprobe files as nine 1 s integrations, and isotoperatios are calculated from these integrated intensi-ties. Because little time-dependent fractionation isapparent in 206Pb/207Pb, 206Pb/204Pb, and208Pb/204Pb, the values returned for these ratiosare simple averages and standard deviations.Depth-dependent changes in 206Pb/238U and208Pb/232Th are accounted for by least squaresprojection back to the initial ratio (fourth secondof acquisition), and the uncertainty of this value iscalculated as the standard deviation of this initialintercept.

4.2. Common Pb Correction

[24] The analytical procedures outlined above havebeen developed in order to generate reliable 204Pbmeasurements because accurate common Pb cor-rection is essential for robust (U-Th)/Pb geochro-nology [Mattinson, 1987]. For example, if a206Pb/238U age is calculated without a commonPb correction, the age will be off by 0.2% if thetrue 206Pb/204Pb is 10,000, 0.4% for a 206Pb/204Pbof 5000, and as much as 1.2% for a 206Pb/204Pb of1000. The accuracy of our measurements is shownon Figure 4, which plots the measured 206Pb/204Pbfrom our laboratory against the 206Pb/204Pb deter-mined by ID-TIMS on zircons (and SRM 610 glass)

from the same samples. The data for these analysesare presented in Data Set S3.

[25] Because the composition of common Pb in azircon crystal is commonly unknown, e.g., fordetrital minerals, the common Pb composition isinterpreted from Stacey and Kramers [1975] andconservative uncertainties of 1.0 for 206Pb/204Pb, 0.3for 207Pb/204Pb, and 2.0 for 208Pb/204Pb (2-sigma)

Figure 4. Measured 206Pb/204Pb from analysis by LA-ICPMS at the ALC (during five different sessions, utilizingall three different collector configurations) and from analysis by ID-TIMS. Data and explanations are provided inData Set S3. The general correspondence of values indicates that all three of our collector configurations yield robust206Pb/204Pb measurements.

Figure 5. ID-TIMS data for the Sri Lanka zirconcrystal that is used to correct for elemental and isotopicfractionation at the ALC. All uncertainties are at 2-sigma.Analytical techniques are described by Gehrels [2000],and the data are reported in Table S1.


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are assigned [Mattinson, 1987]. These uncertaintiesare propagated through all age calculations.

4.3. Fractionation Correction

[26] Because fractionation of U, Th, and Pb occursduring laser ablation, as summarized by Guntherand Hattendorf [2001] and Kosler and Sylvester[2003], measured isotopic ratios for unknowns arecorrected by comparison with matrix-matchedstandards that are analyzed once between everythree to five unknowns. The primary standard usedfor zircon analyses is a Sri Lanka zircon crystal thatyields an ID-TIMS age of 563.5 ± 3.2Ma (2-sigma).The ID-TIMS analyses are shown in Figure 5 andreported in Table S1.

[27] Figure 6 is a plot showing 206Pb/238U and206Pb/207Pb fractionation factors for a session in-volving analysis of 200 unknowns in which stand-ards were analyzed once between every fourunknowns. Each blue diamond is a standard (plot-ted as the known value divided by the measuredvalue), the thick red line is the sliding windowaverage of the closest 8 standards, the thin red linesshow the standard error for this set of standards, andthe vertical gray lines show the magnitude of a ±2%error about the average. Each unknown is accord-ingly adjusted for the closest 8 unknowns using asliding window average. The total fractionation,transient variation in fractionation during a session,and scatter shown on these plots is typical for azircon analysis with a beam diameter of 35mmusingmixed 1 � 1011 and 1 � 1012 ohm resistors.

4.4. U and Th Concentrations

[28] U and Th concentrations are determined as ameans of understanding discordance patterns (e.g.,

high U zircons are more susceptible to Pb loss) andbecause U/Th is a useful indicator of whethermetamorphic fluids were present during zirconcrystallization [Williams, 2001; Rubatto, 2002;Rubatto et al., 2001; Hoskin and Schaltegger,2003; Harley et al., 2007]. The concentration ofU and Th in unknowns is determined by compar-ison with the Sri Lanka zircon standard, which hasan average U concentration of 518 ppm and Thconcentration of 68 ppm (Table S1). U and Thconcentration is determined by calculating theaverage intensity/concentration of 238U and 232Thfor the standard analyses in a session, and thenadjusting unknowns by this factor according totheir measured 238U and 232Th intensities. U andTh concentrations are also calculated by compari-son with chips of SRM 610 trace element glass,which are included on most mounts. In this case,the measured intensity of U and Th in the glass iscompared with the known concentrations of 461and 457 ppm (respectively), and this factor is thenapplied to the unknowns. In most cases, the twomethods yield similar U and Th concentrations.The accuracy of our determinations of U concen-tration and U/Th is better than 20% based onanalyses of zircon standards that have been ana-lyzed in our laboratory and by ID-TIMS (Figure 7and Data Set S4).

5. Calculation of Ages and Uncertainties

[29] Ages are calculated from the isotope ratiosfollowing correction for collector gains, on-peakbackgrounds, depth-related fractionation, commonPb, and elemental/isotopic fractionation. Uncer-tainties are propagated as either measurementerrors or systematic errors.

Figure 6. Fractionation factors for 206Pb/238U and 206Pb/207Pb using the Sri Lanka zircon standard described above.See text for explanation.


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[30] Measurement (or internal or random) errorsarise from measurements that pertain to only asingle analysis: these include 206Pb/238U and206Pb/204Pb for 206Pb/238U ages, 206Pb/207Pb and206Pb/204Pb for 206Pb/207Pb ages, and all three for

207Pb/235U ages. The 206Pb/238U -207Pb/235U errorcorrelation is calculated following Ludwig [1980,2003].

[31] Systematic (or external) errors include fourcontributions, as follows: (1) uncertainties in decayconstants for 238U and 235U, which are 0.16% and0.21%, respectively [Jaffey et al., 1971] (includinga factor of 1.5X to account for systematic errors inthe original Jaffey et al. measurements [Mattinson,1987]), (2) uncertainty in the age of the standardused for fractionation correction, (3) averageuncertainty of the fractionation correction (slidingwindow standard error shown on plots of Figure 6),and (4) average uncertainty that arises from thecomposition of common Pb (described above). Formost analyses, these systematic errors are �1%(2-sigma) for both 206Pb/238U and 206Pb/207Pb ages.

[32] Ages are reported on the basis of 206Pb/238Ufor ages that are less than �1.2 Ga and on the basisof 206Pb/207Pb for ages that are older than �1.2 Ga.This is due primarily to the fact that 206Pb/238Uages are more precise for younger systems whereas206Pb/207Pb ages or more precise for older systems(Figure 8). A second important factor is that206Pb/207Pb ages are less sensitive to Pb loss,which is more common in older systems. Ourstrategy for determining which age to use, forexample in a detrital study, is to determine a cutoffnear �1.2 Ga that does not artificially divide acluster of analyses.

[33] For analyses of grains that are interpreted to becogenetic (e.g., from an igneous rock), the weightedmean of a set of 206Pb/238U or 206Pb/207Pb ages iscalculated using Ludwig [2003]. For most samples

Figure 7. Plots comparing the U concentration and U/Th of zircon standards determined in our laboratory andby ID-TIMS. Gray shaded region shows an error of 20%from perfect correspondence. LA-ICPMS and availableID-TIMS data are presented in Data Set S4.

Figure 8. Plot of 5200 206Pb/238U and 206Pb/207Pb ages selected at random from samples analyzed during spring2007. Uncertainties are shown at 1-sigma in both Ma and percent and include only measurement (internal) errors.Solid blue line is a least squares regression of the 206Pb/238U ages. Solid red line is a power law fit of the 206Pb/207Pbages.


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the MSWD of the weighted mean is �1.0 and theuncertainty ranges from �0.5% to 2% (2-sigma)depending primarily on age and U concentration.Systematic errors are not included in the uncertaintyassigned to each analysis because uncertaintiesarising from decay constants, age of the standard,common Pb composition, and elemental/isotopicfractionation do not decrease as the number ofanalyses increases. Rather, systematic errors arepropagated separately and added quadratically tothe uncertainty of the weighted mean. Addition ofthe systematic errors yields a final age uncertaintyof 1–2% (2-sigma) for most analyses.

6. Reproducibility, Precision, andAccuracy

[34] The precision of our U-Pb age determinationsis shown on Figure 8. Figure 8 indicates that most206Pb/238U ages and >1 Ga 206Pb/207Pb ages havean uncertainty of 0.5–2% (at 1-sigma level),whereas <1.0 Ga 206Pb/207Pb ages have consider-ably greater uncertainty. The greater uncertainty foryoung 206Pb/207Pb ages is due to the relativeinsensitivity of 206Pb/207Pb for young systems, aswell as the difficulty of measuring small 207Pbsignals.

[35] Secondary zircon standards are commonlyanalyzed in an effort to ensure accuracy and toevaluate reproducibility. As an example of the useof secondary standards, Figure 9 shows 792 anal-yses of standard zircon ‘‘49127’’ (136.6 Ma) thatwere conducted by M. Grove and D. Kimbrough(written communication, 2007) during four differ-

ent sessions in fall 2006 and spring 2007. Thisanalysis shows that the measurement techniquesdescribed above are reproducible within and be-tween sessions.

[36] The accuracy of our methods is determined byanalyses of zircons that are well characterized byID-TIMS (Figure 10 and Data Set S5). Thesestandards have been analyzed during five separatesessions, with 10 analyses of each sample duringeach session, and no analyses discarded. Three setsof analyses were conducted utilizing Faraday col-lectors for 206Pb, 207Pb, and 208Pb, one data setwith 50� 20 mm pits and two sets with 35� 12 mmpits. Two sets of analyses were conducted with allPb isotopes measured with channeltrons and�15 � 6 mm pits. Plotted are averages and stan-dard deviations (at 2-sigma, including random and

Figure 9. Plot of 792 measurements of standard zircon49127 conducted during four separate sessions in fall2006 and spring 2007 (analyzed by D. Kimbrough andM. Grove, written communication, 2007). Theseanalyses are used as a secondary standard to assessreproducibility and precision.

Figure 10. Comparison of LA-ICPMS ages with ID-TIMS ages for well-characterized zircons that range inage from 28 to 1434 Ma (data in Data Set S5). Eachsquare is the weighted mean of a set of 10 LA-ICPMSmeasurements, and error bars show the standarddeviation (expressed at 2-sigma) of the weighted mean.No analyses were rejected from any of the sessions. Allages shown are 206Pb/238U ages. Analyses wereconducted during five different sessions betweenNovember 2005 and March 2007. The average ageoffset for all analyses is 0.15% and all means are within2% of the ID-TIMS ages.


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systematic errors) of 206Pb/238U ages for 10 anal-yses from each sample. The average precision of allanalyses is 2.1% (1-sigma).

[37] This analysis shows that a set of 10 analysesyields an average age that is within 2% of theknown (ID-TIMS) age for all standards, that mostsamples are within 1%, and that the average agesfor R33 and Temora (perhaps the best behaved ofthe various standards) are accurate to within�0.3%. The average offset of all analyses is0.15%, which indicates that there is no significantbias in our analytical methods.

[38] Of interest are the systematic shifts of the LA-ICPMS ages of some samples relative to the ID-TIMS ages (Figure 10). Compositional analysesindicate that these shifts may be related to traceelement concentrations (especially Nd), as sug-gested by Black et al. [2004], although physicalaspects of the zircons (e.g., density of inclusions orfractures) may also be important. More detailedanalyses of the chemical and physical natureof zircons, and perhaps treatment by chemicalabrasion and/or high-temperature annealing, mayyield improvements in the precision and accuracyof U-Pb ages by LA-ICPMS.

7. Spatial Resolution

[39] The spatial resolution of laser ablation (beamsize down to 8 mm with our system) enables U-Pbages to be determined with a horizontal resolutionof �10 mm. The vertical resolution is on the orderof 4–6 mm for an entire analysis (when analysesare conducted with a reduced ablation rate and Pb

isotopes are measured with channeltrons), althougheach 1-s integration within an analysis yields ageinformation with a spatial resolution of 0.5 to 1 mm.This is still considerably larger than analysis bySIMS, where a vertical resolution of less than0.1 mm is readily achievable [e.g., Breeding etal., 2004].

8. Applications

8.1. Detrital Zircon Provenance Studies

[40] Most of the geochronologic analyses con-ducted at the ALC are on detrital zircon grains,as this application takes maximum advantage ofthe high efficiency of laser ablation-ICPMS tech-niques. As described by Gehrels et al. [2006], 100unknowns are analyzed per sample in an effort torecognize all of the major age components present,in approximately their original proportions. Agesare portrayed on a relative age probability plot, andimportant age peaks are recognized as containing atleast three overlapping analyses. Programs forplotting, analyzing, and statistically comparingage spectra are available at the ALC web site(http://www.geo.arizona.edu/alc).

[41] U-Th-Pb geochronologic analyses conductedat the ALC are contributing to the rapid advancesin detrital zircon provenance research given that�40,000 analyses of detrital zircon grains areconducted each year, with samples gathered frommany different regions of the world. An example ofa detrital zircon data set that has important tectonicimplications is shown in Figure 11. These datademonstrate that Greater Himalayan and Tethyan

Figure 11. Relative age probability plot of detrital zircon grains from Lesser Himalayan strata, Greater Himalayanstrata, and Tethyan strata in the Nepal Himalaya [from Gehrels et al., 2003]. The Greater Himalayan strata arestructurally juxtaposed over rocks of the Lesser Himalaya along the Main Central Thrust. Differences in detritalzircon age spectra of Lesser Himalayan strata and Greater Himalayan/Tethyan strata suggest that the Main CentralThrust is a fundamental crustal boundary, separating Lesser Himalayan strata that accumulated on the Indian cratonfrom a Greater Himalayan/Tethyan terrane that originated in the paleo-Tethys ocean basin.


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strata in the Himalayan mountain system bear littleresemblance to strata of the Lesser Himalaya,which requires large-scale tectonic transport ofthe Greater Himalaya over the Lesser Himalayaalong the Main Central Thrust [Gehrels et al.,2003].

8.2. Igneous History

[42] Igneous samples are first imaged with CL todetermine whether there is evidence for inheritedcores and/or younger overgrowths. A beam size isthen selected that allows for analysis of homoge-neous domains. Analyses are conducted until thereare at least 20 measurements for each domain andweighted mean plots are prepared for each. Thepetrogenesis and age significance of each domainis then determined from the CL images and fromexamination of plots of age versus U concentration(for evidence of Pb loss) and age versus U/Th (forevidence of metamorphic fluids during zircongrowth).

[43] U-Th-Pb geochronologic research at the ALCis also contributing to understanding the historyand tectonic significance of magmatism in orogenicbelts around the world through analysis of�10,000igneous zircon grains per year. An example of anigneous data set is shown in Figure 12, whichpresents U-Th-Pb zircon analyses from 63 differentgranitic bodies in the Coast Mountains batholith ofBritish Columbia [Gehrels et al., 2007]. The ageshelp define the main phases of magmatism in this

segment of the batholith, and U-Th values indicatethat two of these phases were associated with large-scale generation of metamorphic fluids.

Figure 12. Plot of U/Pb age and U/Th from zircons extracted from granitic bodies of the Coast Mountains batholithin coastal British Columbia (from G. E. Gehrels, unpublished data, 2007). This plot shows the power of assembling alarge database to reconstruct the magmatic history of a region and the utility of using U/Th from zircons to recognizeperiods of metamorphism.

Figure 13. Age and U/Th map of a zoned zircon grainfrom the Coast Mountains batholith in coastal BritishColumbia (G. E. Gehrels, unpublished data, 2007). Eachof the 84 analyses was conducted with a beam diameterof 10 mm and a pit depth of �4 mm. Such maps, togetherwith CL images, provide a powerful tool for under-standing the petrogenesis of zircons that have experi-enced multiple phases of growth.


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8.3. Age Mapping

[44] The spatial resolution and sample-throughputefficiency of LA-ICPMS make it possible to in-vestigate complex zircon crystals by generating ageand U/Th maps. Figure 13 shows a zircon crystal inwhich two distinct phases of zircon growth areclearly visible in a CL image (from G. E. Gehrels,unpublished data, 2007). Eighty-four analyseswere conducted on this crystal with a laser beamdiameter of 10 mm and a pit depth of �4 mm. Theages clearly demonstrate that there are two phasesof zircon growth, one at 92.6 ± 1.3 Ma and ayounger phase at 58.3 ± 1.1 Ma. The high U/Thvalues (average 20.1) in outer portions demonstratethat zircon growth at �58.3 Ma was accompaniedby metamorphism.

9. Conclusions

[45] Analysis of zircons by LA-MC-ICPMS at theALC yields individual ages with a precision andreproducibility of 1–2% and sets of ages that inmost cases are accurate to better than 1%. Giventhe high efficiency of the described methodology,with a throughput of 30–40 analyses per hour,combined with the fine spatial resolution of laserablation, U-Pb geochronology by LA-MC-ICPMSis poised to have a major impact on the generationand application of U-Pb geochronology in theEarth sciences.

[46] It is also apparent from the recent develop-ment of new instrumentation and new measure-ment strategies, as described herein and by Horn etal. [2000], Jackson et al. [2001], Horstwood et al.[2003], Kosler and Sylvester [2003], Woodhead etal. [2004], Simonetti et al. [2005, 2006], Chang etal. [2006], Gehrels et al. [2006], and Horn and vonBlanckenburg [2007] that there are many opportu-nities to improve the precision, accuracy, efficiency,and spatial resolution of laser ablation-ICPMSgeochronology.

Acknowledgments

[47] The ALC is supported with funds from the National

Science Foundation for acquisition of our LA-ICPMS (EAR-

9976676) and for facility support (EAR-0443387). Postdoc-

toral researcher Scott Johnston and Ph.D. student Alex Pullen

provide invaluable assistance in laboratory operation. Our

instruments are very capably maintained by Mark Baker,

David Steinke, and Ben McElhaney, who are supported by

the University of Arizona. Zenon Palacz and Darren Hutchison

(GV Instruments) were essential in maintaining and develop-

ing new techniques with our Isoprobe.

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