Application of laser ablation inductively coupled plasma (dynamic reaction cell) mass spectrometry...

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Chemical Geology 21

The application of laser ablation-inductively coupled plasma-mass

spectrometry to in situ UndashPb zircon geochronology

Simon E Jacksona Norman J Pearsona William L Griffinab Elena A Belousovaa

aGEMOC National Key Centre Department of Earth and Planetary Sciences Macquarie University North Ryde NSW 2109 AustraliabCSIRO Exploration and Mining North Ryde NSW 2113 Australia

Received 21 May 2003 accepted 7 June 2004

Abstract

This paper reports new developments in in situ UndashPb zircon geochronology using 266 and 213 nm laser ablation-inductively

coupled plasma-mass spectrometry (LA-ICP-MS)

Standard spot ablation (spot diameters 40ndash80 Am) was employed with no sampling strategies employed specifically to

minimise elemental fractionation Instead He ablation gas and carefully replicated ablation conditions were employed to

maintain constant ablation-related elemental fractionation of Pb and U between analyses Combining these strategies with

calibration on a new zircon standard (GJ-1) allows elemental fractionation and instrumental mass bias to be corrected efficiently

and accurate 206Pb238U and 207Pb235U ratios to be measured with short-term precision (2 rsd) of 19 and 30

respectively

Long-term precision (2 rsd) of the technique (266 nm ablation) based on 355 analyses of the 91500 zircon (1065 Ma)

standard over more than a year was 38 40 and 14 for the 206Pb238U 207Pb235U and 207Pb206Pb ratios respectively

Long-term precision (2 rsd) for the 206Pb238U 207Pb235U and 207Pb206Pb ratios of the Mud Tank zircon (732 Ma) was 39

41 and 17 respectively (359 analyses) Selective integration of time-resolved signals was used to minimise the effect of Pb

loss and common Pb enrichments on the measured ages The precision and accuracy of our data compare very favourably with

those obtained using more involved procedures to correct or minimise ablation- and ICP-MS-induced biases

213 nm laser ablation produced comparable precision to 266 nm ablation using generally smaller spot sizes (40ndash50 vs

60ndash80 Am) and offered significant advantages in terms of ablation duration and stability particularly for small zircons

(b60 Am) For the 91500 zircon but not the Mud Tank zircon 213 nm ablation also produced significantly older and more

accurate PbU ages This suggests that shorter wavelength ablation may have reduced a matrix-dependent elemental

fractionation difference between sample and standard

The accuracy and precision of the technique for young zircons are demonstrated by analysis of three zircon populations

ranging in age from 417 to 7 Ma In each case the zircons have yielded concordant ages or common Pb discordia which give

concordia intercept ages that are in agreement with independently determined ages for the same samples Application of Terandash

0009-2541$ - s

doi101016jch

Correspon

E-mail addr

1 (2004) 47ndash69

emgeo200406017

ding author

ess sjacksonelsmqeduau (SE Jackson)

SE Jackson et al Chemical Geology 211 (2004) 47ndash6948

Wasserburg diagrams [Earth Planet Sci Lett 14 (1972) 281] was found to be the most useful approach to handling common Pb

contributions that were not removed by selective integration of signals

Keywords Laser ablation-ICP-MS UndashPb geochronology Zircon Calibration

1 Introduction

The first demonstrations of the potential of LA-

ICP-MS to perform in situ 207Pb206Pb determinations

on zircon with sufficient precision to be a useful tool

for dating Proterozoic and older zircons took place in

the early 1990s (Fryer et al 1993 Feng et al 1993)

However it is only since the development of UV laser

ablation and high-sensitivity ICP-MS instrumentation

in the mid-1990s that the technique has been applied

widely to in situ zircon dating using the PbU decay

schemes (eg Hirata and Nesbitt 1995 Jackson et

al 1996 Fernandez-Suarez et al 1998 Horn et al

2000 Ketchum et al 2001 Li et al 2001 Kosler et

al 2002 Tiepolo et al 2003 Tiepolo 2003) The

measurement of 207Pb235U and 206Pb238U ratios

allows assessment of concordance and extraction of

a true age from zircon populations that have suffered

variable Pb loss from grains or parts thereof

The increasing use of LA-ICP-MS UndashPb dating

derives from the fact that it is the cheapest most

widely available and fastest technique for in situ UndashPb

dating While it is not as well suited as SIMS for

applications requiring high-resolution sampling (eg

dating complex zircons with small cores andor

overgrowths) it is extremely well suited to projects

needing large numbers of analyses (eg detrital

zircon studies) Despite increasing usage however

the LA-ICP-MS technique is not universally accepted

as a robust technique for zircon dating The most

frequently cited drawbacks of UndashPb dating using LA-

ICP-MS are as follows (1) fractionation of Pb relative

to U during the ablationtransport and ionisation

processes (eg Hirata and Nesbitt 1995 Fryer et

al 1995) and (2) difficulty in performing a useful

common Pb correction based on 204Pb due to the

overwhelming isobaric interference from Hg

It is clear that elemental fractionation during

ablation is related to differential volatilisation and

condensation processes but the there has been

considerable debate about the possible mechanisms

involved (see Jackson 2001 for a review) Possible

mechanisms that give rise to ablation time-dependent

elemental fractionation include the following (1)

dynamic differential volatilisationcondensation pro-

cesses within or close to an ablation pit related to

progressive defocusing of the laser as it penetrates

into the sample (Hirata and Nesbitt 1995) or to the

evolving aspect ratio of the pit (Eggins et al 1998

Mank and Mason 1999) (2) partitioning of elements

preferentially into a particulate (refractory elements)

or vapour phase (volatile elements) which are differ-

entially transported (Outridge et al 1997) and (3)

differential volatilisation of elements during incom-

plete vapourisation of particulates due to insufficient

residence time in the ICP (Guillong and Gunther

A wide variety of procedures has been invoked in

previous LA-ICP-MS UndashPb studies to minimise

andor correct for ablation-related elemental fractiona-

tion in addition to the inherent mass bias of the ICP-

MS instrument bActive focussingQ (raising the sample

stage during ablation to maintain constant laser focus

on the ablation surface) was used together with

standardisation on a NIST glass reference material

by Hirata and Nesbitt (1995) This method has not

been widely adopted because of the design limitations

of LA hardware and because it requires time-consum-

ing optimisation of the rate of sample height adjust-

ment for different ablation conditions

A more practical approach to preventing formation

of deep craters and maintaining constant focus

conditions is to use raster ablation in which the

sample is constantly moved laterally during ablation

(eg Li et al 2001 Horstwood et al 2001 Kosler et

al 2002) The main drawback of this method is that it

compromises the spatial resolution attainable (or

requires use of a very small beam which reduces

the ablation rate and thus the signalnoise ratio) Use

of an external standard or an alternate correction

SE Jackson et al Chemical Geology 211 (2004) 47ndash69 49

procedure is still required to correct instrumental mass

Several studies have made use of a bjet cellQ whichintroduces the carrier gas as a high-velocity jet

directly onto the ablation site (Jackson et al 1996

Horn et al 2000 Jackson 2001) to reduce fractio-

nation together with calibration against a zircon

standard (eg Fernandez-Suarez et al 1998

Ketchum et al 2001) While the jet cell can produce

a significant reduction in fractionation (ca 50)

maintaining precise alignment of the carrier gas jet

onto the ablation site is difficult and represents a

serious limitation to this cell design

Horn et al (2000) used an experimentally derived

mathematical procedure that relates fractionation to

spot geometry (at constant energy density) to correct

for ablation-related elemental fractionation together

with use of a Tl235U spike to correct for instrumental

mass bias This method requires elemental fractiona-

tion to be extremely reproducible from day to day It is

better suited to the flat energy profiles of excimer laser

systems (eg Horn et al 2000) than NdYAG

systems (Tiepolo et al 2003) for which the energy

distribution within the laser beam can vary signifi-

cantly depending upon laser tuning and operating

conditions

In this study we used a high-sensitivity quadrupole

ICP-MS coupled to a custom-built UV laser ablation

microprobe based on a frequency quadrupled NdYAG

laser (k=266 nm) Also evaluated was a commercial

frequency-quintupled (k=213 nm) laser ablation sys-

tem We describe and evaluate a simple technique that

employs spot ablations with no specific strategies to

minimise ablation-related elemental fractionation

Fractionation and instrumental mass bias are corrected

by direct calibration against a new zircon standard

analysed under carefully matched conditions using He

as the ablation gas to increase the reproducibility of the

PbU fractionation Time-resolved data acquisition is

employed to evaluate zircon homogeneity and to allow

selective integration of signals to minimise common

Pb contributions and Pb loss and thus to maximise

concordance TerandashWasserburg diagrams (Tera and

Wasserburg 1972) are also employed to assess and

correct for residual common Pb contributions These

procedures are evaluated using data for five zircons

ranging in age from 1065 to 7 Ma Two of these

zircons have been analysed more than 400 times over

more than a year by two instrument operators

allowing an exhaustive evaluation of the long-term

precision and accuracy of the technique

2 Analytical techniques

21 Sample preparation

All zircons were mounted in epoxy in 25-cm-

diameter circular grain mounts and polished until the

zircons were just revealed Images of the zircons were

obtained using the back-scatter electron (BSE) detector

on a Cameca SX50 electron microprobe The BSE

detector is a light-sensitive diode so the image

obtained of the internal structure of the zircon is a

combination of the variation in mean atomic number

(composition) as well as the cathodoluminescence

(CL) BSECL images were taken of all analysed

zircons to study the internal structure of the grains prior

to LA-ICP-MS analysis Grain mounts containing the

samples and standards were cleaned in 1 N nitric acid

immediately prior to analysis to remove surface Pb

contamination This removed the necessity for pre-

ablating the samples to remove surface contamination

22 Instrumentation

The laser ablation microprobe uses a focussed laser

beam to ablate a small amount of a sample contained

in a closed cell The ablated material is transported

from the cell in a carrier gas (Ar or He) to an ICP-MS

for isotopic quantification For the spot diameters

(mostly 50ndash80 Am) and ablation times (ca 60 s) used

in this study ablated masses of zircon were approx-

imately 500ndash1500 ng

Two laser ablation (LA) systems were used in this

study Most analyses were performed using a custom-

built UV LA sampler that has been described by

Norman et al (1996) The system incorporates a

frequency-quadrupled NdYAG laser (k=266 nm) and

beam delivery optics that attenuate the beam to the

required fluence and steer the beam down the photo-

tube of a petrographic microscope The microscope

incorporates specialised optics that focus the laser and

provide a high-quality image of the sample which

greatly facilitates locating sample sites in petrographic

mounts The sample mount and standardisation materi-

als were ablated in a sample cell approximately 8 cm in

diameter with a Teflon insert that holds up to four 25-

mm sample mounts A 05-mm restriction on the gas

inlet nozzle results in a jet-like flow across the samples

which results in amuchmore stable ablation signal than

a comparable cell without a restricted inlet nozzle The

ablated sample was transferred to the ICP-MS through

3 mm id PVC tubing that was cleaned by soaking in

02 N nitric acid prior to use and replaced on a monthly

Ablation was performed in a He carrier gas (except

where noted otherwise) as first described by Eggins et

al (1998) The He carrier exiting the sample cell was

combined with Ar in a 30 cm3 mixing chamber prior to

entering the ICP The gas plumbing configuration

includes a two-way valve on the Ar line so that after

each ablation the user may switch the Ar to combine

with the He before entering the sample cell The ArndashHe

mixture flushes ablated particles from the cell much

faster than He alone resulting in a more rapid return of

signals to background levels and increased sample

throughput Operating conditions of the LA systems

are reported in Table 1

Constant LA operating conditions were maintained

throughout each analytical brunQ (18ndash22 analyses)

because elemental fractionation is extremely sensitive

to laser pulse energy and focus Moderate- to high-

power frequency-quadrupled NdYAG lasers require a

significant period (minutes) to attain stable output after

the initiation of lasing due largely to the thermal-

equilibration time of the temperature-sensitive 4th

harmonic crystal Constant ablation conditions were

achieved by leaving the laser firing continuously

throughout a run and initiating and terminating ablation

by unblocking and blocking the beam path

All analyses were performed with the laser focused

above the sample (typically ca 150ndash250 Am) This was

found to be a more robust method than bactivefocusingQ (Hirata and Nesbitt 1995) for reducing the

relative change in focus of the laser beam as it

penetrates into the sample since it does not require

optimisation of the vertical translation rate of

the stage for different laser operating conditions

(Jackson 2001) While greater defocusing reduces

ablation time-dependent fractionation of Pb and U it

increases the ablation spot size The degree of

defocusing was therefore set to give the maximum

crater diameter possible for a set of zircons The

266 nm laser system was used mostly for large zircons

where large spot sizes (60ndash80 Am in diameter) were

appropriate

Also evaluated in this study was a commercial

LUV213 laser ablation system (k=213 nm) (New

Wave ResearchMerchantek) The LUV213 was used

primarily for analysis of small zircons (b60 Am)

which can be difficult to ablate controllably at 266 nm

due to the high transmittance of the laser beam

through the grain into the underlying epoxy mounting

medium Due to the greater absorption of the 213 nm

laser beam the incidence of catastrophic ablation was

significantly reduced Most of the 213 nm laser data

presented here are for smaller spots (40ndash50 Am) than

produced with the 266 nm laser

The LUV213 was used with an in-house built

sample cell similar to that used on the 266 nm laser

except for a smaller cell diameter (ca 6 cm)

necessitated by space limitations Much lower beam

energies enter the harmonic generators in this system

compared to the 266 nm laser sampler due to the

lower output energy of the laser and the optical

configuration of the system Thus the 213 nm laser

sampler does not require a significant stabilisation

period after initiating ablation and no pre-ablation

warm up was used

The ICP-MS used was an Agilent 4500 a high-

sensitivity quadrupole ICP-MS that provides a sensi-

tivity of in excess of 200 million counts per second

(cps)ppm for mono-isotopic heavy elements (atomic

mass N85) in standard solutionmode Current detection

limits for these elements using laser ablation sampling

are typically b10 ppb for a 60-s analysis at 40ndash50 Amsampling resolution ICP-MS operating conditions

were generally optimised using continuous ablation

of our in-house GJ-1 zircon standard to provide

maximum sensitivity for the high masses (PbndashU) while

maintaining low oxide formation (ThO+Th+b15)

Few analyses give signals in excess of 2000000 cps

and thus all readings were made in pulse counting

mode employing a dead-time correction applied

automatically by the ICP-MS operating system

Instrumental background was established by the

standard procedure of measurement of a bgas blankQie analysis of the carrier gas with no laser ablation

(eg Hirata andNesbitt 1995 Fernandez-Suarez et al

1998 Horn et al 2000 Kosler et al 2002 Tiepolo et

al 2003 Tiepolo 2003) Adoption of this procedure

Table 1

LA-ICP-MS operating conditions and data acquisition parameters

ICP-MS

Model Agilent 4500

Forward power 1350 W

Gas flows

Plasma (Ar) 16 lmin

Auxiliary (Ar) 1 lmin

Carrier (He) 09ndash12 lmin

Make-up (Ar) 09ndash12 lmin

Shield torch Used for most

analyses

Expansion

chamber

pressure

350ndash360 Paa

LA Custom system LUV 213

Wavelength 266 nm 213 nm

Repetition rate 5 and 10 Hz 5 and 10 Hz

Pre-ablation laser

warm up

Laser fired

continuously

Pulse duration

(FWHM)

9 ns 5 ns

Apertured beam

diameteriris

setting

4 mm 15

expander setting

Focusing objective 10 fl = 20 mm 5 fl = 40 mm

Degree of

defocusing

150ndash250 Am(above sample)

Not known

Spot size 60ndash80 Am 40ndash50 AmIncident

pulse energy

ca 035 ca 01 mJ

Energy density

on sample

ca 12 J cm2 ca 8 J cm2

Data acquisition parameters

Data acquisition

protocol

Time-resolved analysis

Scanning mode Peak hopping 1 point

per peak

Detector mode Pulse counting

dead-time

correction applied

Isotopes

determined

206Pb 207Pb 208Pb232Th 238U

Dwell time

per isotope

15 30 10 10 15 ms

respectively

Quadrupole

settling time

ca 2 ms

Timescan ca 89 ms

Data acquisition (s) 180 s (60 s gas blank

up to 120 s ablation)

Table 1 (continued)

Samples and standrds

Mounts 25 mm diameter polished

grain mounts

Standard Gem zircon bGJ-1Q 609 Maa Pirani vacuum gauge may not be calibrated accurately for

ArHe mixtures

followed experiments to determine whether an

bablation blankQ (a blank measured while ablating)

might be a more appropriate measure of the true

background An ablation blank would take into account

any contribution to background derived from particles

released from the sample cell or transfer tubing wall by

the shock wave induced by laser ablation However

multiple ablations of high-purity synthetic fused silica

produced Pb Th and U count rates that were higher

than the gas background by an amount (median values

of b5 cps on 206Pb 207Pb and 238U b10 cps on 208Pb

and 232Th) that could reasonably have been derived

from Pb Th and U in the silica and in any case would

not significantly affect the vast majority of UndashPb

analyses (see the discussion of potential effect on208Pb232Th determination in Section 24)

For UndashPb work low gas background signals for Pb

are essential Low Pb gas backgrounds were achieved

by (1) careful attention to cleanliness of the LA system

(acid cleaning sample cell and inserts samples

delivery tubing) (2) dedicating the ICP-MS to laser

ablation analysis since analysis of solutions invariably

has a detrimental and long-term effect on Pb back-

grounds and (3) use of a liquid rather than a

compressed Ar supply Under these conditions typical

gas blank 208Pb signals of ca 30ndash40 cps were achieved

routinely Backgrounds were b10 cps for the other

elements of interest except Hg (ca 300 cps on 202Hg)

23 Data acquisition

Data acquisition parameters are listed in Table 1

Data were acquired on five isotopes using the

instrumentrsquos time-resolved analysis data acquisition

software The time-resolved analysis software reports

signal intensity data (cps) for each mass sweep

performed by the mass spectrometer This data

acquisition protocol allows acquisition of signals as a

function of time (ablation depth) and subsequent

recognition of isotopic heterogeneity within the abla-

tion volume (eg zones of Pb loss or common Pb

related to fractures or areas of radiation damage also

inclusions inherited cores etc) The signals can then

be selectively integrated Useful data could not be

acquired for 204Pb due to the large isobaric interference

from Hg a significant contaminant apparently derived

from the Ar supply or Ar supply piping and fittings Hg

signals could not be reduced sufficiently using filters to

allow useful analyses (see below)

A fast peak hopping protocol (dwell time per

isotope from 10 to 30 ms) was used to ensure

representative measurement of rapidly transient sig-

nals typical of laser ablation sampling This protocol

resulted in a full mass sweep time of ca 89 ms Given

the instrument-set mean quadrupole settling time of

ca 2 ms this is a reasonable compromise between the

conflicting ideals of maximum possible scanning

speed (to approximate simultaneous detection) and

overall counting efficiency (duty cycle) Each 3-min

analysis consisted of ca 60 s of measurement of

instrumental background (ie analysis of carrier gas

no ablation) followed by the ablation event (up to

120 s) giving a total analysis time of 3 min

Mass discrimination of the mass spectrometer and

residual elemental fractionation were corrected by

calibration against a standard zircon GJ-1 Samples

were analysed in brunsQ of ca 18ndash22 analyses

which included 10 unknowns A typical run com-

menced with two analyses of the zircon standard (or

more in the event of disagreement between the first

two analyses) This was followed by analyses of the

two near-concordant zircons 91500 (Wiedenbeck et

al 1995) and Mud Tank (Black and Gulson 1978)

which are analysed in every run in our laboratory as

an independent control on reproducibility and

accuracy These were followed by two more

analyses of the GJ-1 standard then up to 10

unknowns followed by two (or more) analyses of

the GJ-1 standard This protocol results in a

minimum of six analyses of the zircon standard in

each run A minimum of six analyses is required to

identify and reject anomalous analyses of the stand-

ard and to correct drift in isotope ratios during the

run (typically 2 h)

24 GJ-1 standard

Fractionation of Pb and U during laser ablation and

inherent mass bias of all mass spectrometers must be

corrected to produce accurate ages In this study both

effects were corrected by external standardisation

Since the degree of ablation-related fractionation is

significantly matrix-dependent a zircon standard was

used The standard employed in this work was a large

(1 cm) gem quality pink zircon GJ-1 one of a bag of

similar pink (and yellow) zircons acquired from a

Sydney gem dealer The grain shows no zoning under

CL imaging LA-ICP-MS trace element analyses

show that the zircon is relatively low in Th with

mean U and Th contents of 230 and 15 ppm

respectively The chondrite-normalised REE pattern

is characterised by very low La (b01 chondrite) a

strong positive Ce anomaly no Eu anomaly and Lu at

ca 280 times chondrite

TIMS analyses were performed on eight aliquots

(two fragments from each of four grains) at the

University of Oslo by F Corfu The results are

presented in Table 2 and in Fig 1 TIMS analyses

provide a highly precise 207Pb206Pb age of

6085F04 Ma 206Pb204Pb ratios in excess of

146000 and U contents ranging 212ndash422 ppm

making it a potentially highly suitable standard The

disadvantage of this zircon standard is that it is not

concordant and while TIMS 206Pb238U and207Pb235U ratios for fragments of individual grains

vary by b06 there are small variations in these

ratios between grains (ca 1)

For standardising UndashPb analyses several large

grains (up to 1 cm in diameter) were investigated for

isotopic homogeneity by multiple LA-ICP-MS anal-

yses The grain that provided the most reproducible

ratios was subsequently adopted as the calibration

standard For the 207Pb206Pb 207Pb235U and206Pb238U ratios the weighted means of the TIMS

values have been adopted as the working ratios (see

Table 2) 208Pb232Th ratios were not measured

directly by TIMS but model ratios have been

calculated from the 208Pb206Pb ratios Using the

mean of the TIMS model ratios (003011) as the

working value for GJ-1 has resulted consistently in

young LA-ICP-MS208 Pb232 Th ages for all zircons

that we have measured relative to their TIMS UndashPb

ages On the basis of LA-ICP-MS calibration against

other zircons a value of 003074 has been adopted in

our laboratory as the working value for the208Pb232Th ratio in GJ-1 This value has been

employed in this study The difference between this

Table 2

TIMS analyses of the GJ-1 zircon standard

Apparent age

Fraction Weight

(Ag)Pb

ThU Pbcom

206204 207235

207235

2r (abs)

206238

2r (abs)

rho 207206

207206

2r (abs)

208232

(model)

206238

207235

207206

(1) (2) (2) (2) (3) (4) (5) (6) (7) (6) (7) (6) (7) (8)

GJ-1-4 51 2949 195 215 006 9 420650 08125 00022 009801 000025 098 006012 000003 0030262 6027 6038 6080

GJ-1-4 52 1519 193 212 006 14 146133 08126 00018 009796 000020 098 006016 000003 0030249 6025 6039 6094

GJ-1-3 53A 6119 279 313 002 26 452072 08067 00034 009729 000040 099 006013 000003 0030048 5985 6006 6083

GJ-1-3 53B 6119 279 313 002 22 533252 08064 00030 009725 000035 099 006013 000003 0030036 5983 6004 6084

GJ-1-2 56 3144 374 422 002 13 612660 08034 00030 009689 000035 099 006014 000003 0029928 5962 5988 6086

GJ-1-2 59 2876 333 373 002 11 610787 08068 00027 009729 000031 099 006014 000003 0030047 5985 6007 6088

GJ-1-1 61A 4800 202 224 003 39 170396 08119 00033 009792 000039 099 006013 000003 0030237 6022 6035 6083

GJ-1-1 61B 4800 201 224 003 12 543384 08074 00027 009738 000032 099 006013 000003 0030074 5991 6010 6084

Wt Mean 08093 00009 009761 000011 006014 000001 0030110

91500 58A 738 141 77 035 55 11494 18476 00100 017890 000096 099 007491 000005 0053879 10609 10626 10660

91500 58B 738 140 77 034 10 62869 18543 00037 017948 000032 097 007493 000004 0054054 10641 10650 10667

Eight aliquots (two fragments from each of four grains) Zircon 91500 was also analysed for quality control purposes Analyses performed at the University of Oslo by Fernando

(1) One zircon fragment in each fraction A and B denote fractions split after spiking dissolution and HCl re-equilibration but before chemical separation

(2) Weights better than 05 U and Pb concentrations probably F10 (spike concentration uncertainty)

(3) ThU model ratio inferred from 208206 ratio and age of sample

(4) Pbc=initial common Pb

(5) Total common Pb in sample (initial+blank)

(6) Raw data corrected for fractionation and blank

(7) Corrected for fractionation spike blank and initial common Pb (estimated from Stacey and Kramers (1975) model)

(8) Error calculated by propagating the main sources of uncertainty error does not include uncertainty of UndashPb ratio in spike about 01 based on spike calibration results

(9) Model 208Pb232Th ratio calculated from 208206 ratio assuming same degree of discordance as UndashPb ratios

SEJackso

alChem

icalGeology211

(2004)47ndash69

Fig 1 UndashPb concordia plot of TIMS analyses of the GJ-1 zircon standard

value and the TIMS model 208Pb232Th ratio could be

explained by an interference as small as ca 10 cps on

the LA-ICP-MS 208Pb signals for GJ-1 Thus a

possible explanation is a small contribution to the208Pb signal from ablation-induced enhancement of

the blank (median value of 7 cps in our experimentsmdash

see discussion in Section 22) possibly with additional

minor contributions to the 208Pb signal from poly-

atomic ion interference(s) (eg 96Zr216O+ 176Yb16O2

+176LuO2

+ 176HfO2+) Due to the very low Th (ca 15

ppm) and thus 208Pb contents of the GJ-1 zircon the

relative effect of these interferences would be much

larger on this zircon than on most others that we have

analysed This would explain the slightly young

measured 208Pb232Th ages for zircons calibrated

against GJ-1 using the TIMS model 208Pb232Th ratio

Further work is required to establish unequivocally the

true 208Pb232Th ratio of the GJ-1 zircon as some

common Pb corrections that are not based on 204Pb

determination (eg Andersen 2002) rely on accurate

determination of the 208Pb232Th ratio

25 Samples

The samples for which data are presented in this

study are two zircons 91500 (Wiedenbeck et al

1995) and Mud Tank (Black and Gulson 1978)

which are analysed in every analytical run in our

laboratory as quality control standards and three

young zircon samples (417ndash7 Ma) analysed numer-

ous times in a single analytical session The 91500

and Mud Tank zircons have each been analysed more

than 400 times over more than a year by two

instrument operators using both 266 and 213 nm

ablation systems The three zircons analysed repeat-

edly in a single analytical session were the Temora

zircon distributed by Geoscience Australia as a

microbeam UndashPb standard the Walcha Road zircon

(Flood and Shaw 2000) and the Gunung Celeng

(Indonesia) zircon

3 Data processing

31 GLITTER software

The raw ICP-MS data were exported in ASCII

format and processed using GLITTER (Van Achter-

bergh et al 2001) an in-house data reduction

program GLITTER calculates the relevant isotopic

ratios (207Pb206Pb 208Pb206Pb 208Pb232Th206Pb238U and 207Pb235U where 235U=238U13788)

for each mass sweep and displays them as a coloured

pixel map and as time-resolved intensity traces

Ratios were examined carefully for anomalous

portions of signal related to zones of Pb loss and

or common Pb gain inherited cores and occasional

surface Pb contamination not removed by acid

cleaning For both laser systems ratios generally

stabilised within 5ndash10 s of initiating ablation after

which data could be integrated The most concordant

segments of each ablation signal were generally

selected for integration Because of the ablation time

dependence of elemental fractionation GLITTER

automatically uses for each selected ablation time

segment of an unknown the identical integrated

ablation time segments of the standard zircon

analyses (relative to the commencement of ablation)

Net background-corrected count rates for each

isotope were used for calculation GLITTER corrects

the integrated ratios for ablation-related fractionation

and instrumental mass bias by calibration against the

zircon standard using an interpolative correction

(usually linear) for drift in ratios throughout the

run based on the six or more analyses of the

standard It then calculates ratios ages and errors

GLITTER does not apply a common Pb correction

Calculated ratios were exported and concordia ages

and diagrams were generated using Isoplot v 249

(Ludwig 2001)

32 Error propagation

In GLITTER isotope ratios are derived from

background-subtracted signals for the relevant iso-

topes Uncertainties in these ratios combine the

uncertainties of signal and background arising from

counting statistics and are added in quadrature The

same propagation is used for unknowns and standard

analyses The standard ratios are interpolated

between standard measurements to estimate the

standard ratios at the time of the measurement of

the unknowns Uncertainties in the standard ratio

measurements are propagated through this procedure

to estimate the standard ratio uncertainties relevant to

each unknown ratio measurement Relative uncer-

tainties estimated for the standard ratios are com-

bined with the unknown ratio uncertainties in

quadrature A further 1 uncertainty (1r) is

assigned to the measured TIMS values of the isotope

ratios for the standard and propagated through the

error analysis

33 Common Pb correction

In conventional TIMS analysis 204Pb is measured

to correct for common Pb present in the sample or

added during preparation of the sample for analysis

Initial attempts to measure 204Pb during this study

proved fruitless owing to the overwhelming contribu-

tion to the signal from 204Hg (isotopic abun-

dance=687) Typical gas blank signals at mass

204 were ca 300 cps Previous attempts to lower Pb

backgrounds on a VG PQII+bSQ ICP-MS at Memorial

University of Newfoundland using a Hg trap con-

sisting of glass tubes of gold-coated sand on the

carrier gas line resulted in a ca 50 reduction in Hg

signal that was still insufficient to allow a useful 204Pb

correction (Jackson unpublished data) Addition of

extra traps on the carrier gas and other ICP gas lines

resulted in little additional reduction in Hg intensity

suggesting that some of the Hg background is long-

term instrument memory In light of the similar Hg

background signals on the Agilent 4500 ICP-MS no

attempt was made to filter Hg on the ICP-MS used in

this study

In this study two main approaches to common Pb

reduction correction have been employed (1) selec-

tive integration of time-resolved signals and (2) Terandash

Wasserburg diagrams The common Pb correction

procedure described by Andersen (2002) was also

tested This procedure determines the amount of

common Pb by solving the mass-balance equations

for lead isotopes in a zircon and correcting the data

back to a three-dimensional Pb loss discordia line

However this correction is very sensitive to the

measured 208Pb232Th ratio Because of the low 208Pb

and 232Th concentrations and the uncertainty in the

true 208Pb232Th ratio of the GJ-1 standard this

method could not be used effectively in this study

331 Selective integration of time-resolved signals

The ability to selectively integrate LA-ICP-MS

time-resolved signals is frequently overlooked The

ablation surface penetrates into the sample at a rate

that is on the order of 01 Ampulse (05 Ams at 5 Hz

repetition rate) that is in any analysis the sampling

occurs on a scale where it may encounter significant

chemical or isotopic variations related to alteration

inclusions fractures and inherited cores and on a time

scale where transient signals related to these features

are commonly resolvable using a fast data acquisition

protocol Each analysis therefore records a profile of

the elemental and isotopic composition of the sample

with depth In many zircons common Pb and Pb loss

occur in restricted domains (along fractures zircon

rims) which can be recognised easily in time-resolved

signals of ablations that penetrate into such a domain

Using appropriate software signals and their ratios

can be displayed and selectively integrated so that

only the most isotopically concordant portions of

signals are integrated thereby hugely reducing the

incidence of analyses affected by common Pb and Pb

loss Fig 2 shows a striking example of an ablation

which contains useful data from 65 to 95 s at which

point the laser beam penetrated a zone which has

undergone significant radiogenic Pb loss and common

Pb gain with relative magnitudes that result in lower206Pb238U and 208Pb232Th ratios but higher207Pb206Pb and 207Pb235U ratios Selectively inte-

grating the data from ca 65 to 95 s produces an

Fig 2 Time-resolved isotope ratio traces for an ablation of a zircon from

95 s at which point the laser beam penetrated a domain that had under

analysis that is within error of the concordia at the

correct age for the sample

332 TerandashWasserburg diagrams

For young zircons containing a significant amount

of common Pb plotting of data on TerandashWasserburg

diagrams (Tera and Wasserburg 1972) was found to

be the most effective method of evaluating and

correcting for contributions from common Pb This

is discussed more fully below with reference to the

Walcha Road and Gunung Celeng zircons

4 Results

41 Short-term precision (266 and 213 nm)

The short-term (2 h) precision of the method has

been evaluated by multiple analyses of an isotopically

homogeneous grain of our zircon standard GJ-1 The

266 nm laser ablation has been compared with 213 nm

ablation using both Ar and He as ablation gases The

GJ-1 zircon was analysed 10 times using nominally

the Walcha Road pluton Useful data were recorded between 65 and

gone substantial loss of radiogenic Pb and gain of common Pb

Fig 3 Precision expressed as relative standard deviation (1r) of the measured ratios for ablation in Ar and He using 266 and 213 nm laser

ablation of GJ-1 zircon under nominally the same focusing and irradiance conditions (10 analyses of each combination of laser and ablation

gas) 60 s ablations ca 50 Am spot size

the same conditions (spot size=50 Am pulse energy

025 mJ measured at the sample 10 Hz repetition rate

60 s of data integrated) for each laser system External

precisions (1r) for the four combinations of laser and

ablation gas are presented in Fig 3

The most striking feature of the data is the dramatic

improvement in precision for ablations in He com-

pared to ablations in Ar For ablations in He

Fig 4 Signals for ablation of GJ-1 zircon in Ar and He (266 nm laser) A

signals

precisions on both PbU ratios were close to 1

with very slightly improved RSDs for 213 nm

ablations compared to 266 nm Comparison of

ablation signals (266 nm) (Fig 4) show that ablation

in He resulted in generally larger more stable signals

with very significantly less short-term (1 s) noise

reflecting the higher proportion of small particles

(b05 Am) in the ablation volume that is characteristic

blation in He produces generally larger more stable and less noisy

of ablation in He (Horn and Gunther 2003) However

the ca 2-fold greater signal intensities for ablations in

He cannot explain the 3- to 5-fold improvement in

precisions for the UndashPb ratios compared to ablation in

Ar Comparison of PbU fractionation trends (Fig 5)

reveals that while ablation in He did not result in

reduced PbU fractionation relative to ablation in Ar

it did produce significantly more reproducible fractio-

nation trends particularly during the first 60 s of

ablation The reason for the initial drop in PbU ratio

for ablations in Ar may relate to a burst of large

particles in the early stages of ablation Incomplete

vaporisation of these particles in the ICP results in

preferential volatilisation of the more volatile ele-

ments (eg Pb) over more refractory elements (eg

U) (Guillong and Gunther 2002) and thus high PbU

ratios in the early stages of an ablation The larger

proportion of small particles that are transported

during ablation in He might mask this effect

Ablation in He resulted in substantially improved

precisions for 207Pb206Pb measurements compared to

ablation in Ar reflecting higher signals and reduced

short-term noise in signals The 207Pb206Pb ratios

were for all lasergas combinations much more

precise than the PbU ratios suggesting that the

limiting factor on precision of PbU ratios was not

counting statistics but reproducibility of PbU frac-

tionation during ablation together with spatial varia-

Fig 5 Measured 206Pb238U ratios for ablation of GJ-1 zircon in Ar and H

laser) Note the two Ar analyses highlighted which show significantly dif

tions in the PbU ratios within the sample

Interestingly the 266 nm laser produced an approx-

imately twofold improvement in precision of the207Pb206Pb ratio for ablations in Ar and He

compared to the 213 nm laser a statistic that cannot

be explained fully by the 30 higher count rates

Based on the data above all further analyses were

performed using He as the ablation gas To

determine the precision of the method over the

course of a typical analytical run 20 consecutive

analyses of the GJ-1 zircon standard were performed

(266 nm laser) using typical ablation conditions (60 s

ablation 50 Am spot diameter) The mean ratios for

the first and last four analyses of the GJ-1 standard

were used to calibrate the run and correct any drift

in ratios throughout the run The external precisions

(2r) on the 206Pb238U 207Pb235U and 207Pb206Pb

ratios were 19 30 and 24 respectively (Fig

6) These compare with mean internal precisions

(2r) for the 20 analyses of 07 19 and 19

which are close to the predicted precisions based on

counting statistics alone (06 17 and 18

respectively) Again small differences in elemental

fractionation between analyses together with real

variations in the PbU ratios within the sample can

explain the significantly higher external precisions

than internal precisions for the 206Pb238U and207Pb235U ratios

e (five analyses of each) using identical ablation conditions (266 nm

ferent fractionation trends during the first 50 s of ablation

Fig 6 Concordia plot of 20 consecutive analyses of gem zircon standard GJ-1 demonstrating 2r reproducibility of 206Pb238U and 207Pb235U

ratios of better than 2 and 4 respectively

42 Long-term precision and accuracy

The long-term precision and accuracy of the

technique has been established via repeated analyses

of two zircons 91500 (Wiedenbeck et al 1995) and

Mud Tank (Black and Gulson 1978) which are

analysed as unknowns for quality control purposes in

every analytical run conducted in our laboratories

421 91500

The 91500 zircon one of the most widely used

zircon reference materials in existence is derived

from a single large crystal in a syenite from Renfrew

County Ontario It has a 207Pb206Pb age based on 11

TIMS determinations of 10654F03 Ma (Wieden-

beck et al 1995) Reported U concentrations of the

aliquots analysed ranged from 71 to 86 ppm

The data presented here were acquired by two

analysts between May 2001 and October 2002 The

266 nm and 213 nm laser systems were used for 372

and 88 analyses respectively Because of the smaller

spot sizes used for the 213 nm laser analyses the

magnitudes of the signals for these analyses were on

average ca 50ndash60 of the signals for the 266 nm

laser analyses There are no systematic differences in

the data from the two operators and their data have

been pooled Two highly anomalous analyses one

from each laser system (207Pb206Pb ratio N6 and N20

sd from the mean 213 and 266 nm data respec-

tively) were rejected during data acquisition and are

not discussed further

A frequency distribution diagram of the 266 nm

laser 206Pb238U data (Fig 7) reveals a slightly skewed

bell curve with a low age tail and a few outliers on

each side of the curve The low age outliers are

presumed to be related largely to Pb loss The older

outliers do not show high 207Pb206Pb ages which

would accompany a common Pb problem or inherited

component They are reversely discordant due most

probably to redistribution of Pb within the zircon

crystal or to anomalous laser-induced PbU fractiona-

tion A summary of the data for the 91500 zircon is

presented in Table 3 (complete data set may be

accessed from the online data repository (see Appen-

dix A)) with anomalous analyses that lie outside the

bell curvemdashages greater than 1110 Ma (four analyses)

and less than 990 Ma (12 analyses)mdashrejected from

statistical analysis

laser 206Pb238U data shows a generally similar age

Fig 7 Frequency distribution diagram for 371 206Pb238U age determinations of the 91500 zircon using the 266 nm laser ablation system

MeanF2 sd and weighted meanFuncertainty at 95 confidence based on 355 analyses as discussed in the text

distribution to the 266 nm laser data including several

positive outliers (Fig 8) However in contrast to the

266 nm laser data there is no tailing on the low age

side of the bell curve For statistical analysis only the

four positive outliers with ages greater than 1110 Ma

were rejected (Table 3)

The precisions of the 207Pb206Pb 206Pb238U and207Pb235U ages derived using the two laser systems are

remarkably similar and are almost all within a factor of

2 of the measured short-term precisions of the

technique reported above 207Pb206Pb ages for both

systems are in close agreement with the TIMS age

However statistically significant differences exist in

the PbU ages produced by the two laser systems

(differences of 11 and 9 Ma for the weighted mean206Pb238U and 207Pb235U ages respectively) The

Table 3

Summary of LA-ICP-MS and TIMS UndashPb isotopic ages for the 91500 zi

Isotopic ratios Mean age (Ma)

266 nm (n=355) 213 nm (n=83) 266 nm (n=355) 213

Mean 2 rsd Mean 2 rsd Mean 2 sd Me

207Pb206Pb 00749 14 00750 13 1065 28 106207Pb235U 18279 40 18491 44 1055 27 106206Pb238U 01771 38 01789 37 1051 37 106208Pb232Th 00532 72 00538 155 1048 74 105

213 nm laser 206Pb238U weighted mean age is in

perfect agreement with the TIMS 206Pb238U age but

the 266 nm laser age for this system is 12 Ma younger

Several explanations exist for the low age skew

exhibited by the 266 nm laser data not displayed in

the 213 nm laser data and the consequent discernibly

younger age produced Some of the early 266 nm laser

analyses of the 91500 zircon were performed on a

different grain mount than that used for the 213 nm

analyses However there are no discernible differences

in the ages obtained for the two mounts The skew may

therefore be a function of the larger spot size and

greater penetration depth of the 266 nm laser spots

which might have resulted in increased incidence of

intercepting domains (fractures) along which Pb loss

has occurred There is also a possibility that the

rcon TIMS data from Wiedenbeck et al 1995

Weighted mean age (Ma)Ferrors

(at 95 confidence)

TIMS age (Ma)

nm (n=83) 266 nm (n=355) 213 nm (n=83) Mean 2r

an 2 sd Mean error Mean error

8 26 1065 25 1068 57 10654 03

3 29 1055 14 1064 33

1 36 1050 19 1061 40 10624 04

8 159 1045 35 1049 16

different mean ages derived from the two laser systems

result from a systematic difference in PbU elemental

fractionation between the 91500 and GJ-1 standard

zircon as a result of a small difference at 266 nm in

laser beam absorption which was significantly reduced

at the more absorbing shorter wavelength (213 nm)

Fig 9 Frequency distribution diagram for 364 206Pb238U age determinatio

MeanF2 sd and weighted meanFuncertainty at 95 confidence based o

422 Mud Tank zircon

The Mud Tank zircon derives from one of only a

few carbonatites known in Australia The Mud Tank

carbonatite is located in the Strangways Range

Northern Territory The zircon dated is a large (ca

1 cm) megacryst A UndashPb concordia intercept age of

ns of the Mud Tank zircon using the 266 nm laser ablation system

n 359 analyses as discussed in the text

732F5 Ma was reported by Black and Gulson (1978)

based on five TIMS analyses (an additional aberrant

analysis was rejected) four of which lie very close to

concordia Errors on individual data points were not

quoted Reported U concentrations of the aliquots

analysed ranged from 6 to 36 ppm However 16 LA-

ICP-MS trace element analyses of our sample ranged

from 11 to 131 ppm (Belousova 2000) and the mean

LA-ICP-MS U signal for the crystal analysed in this

study was 20 higher than the signal for the 91500

zircon (reported U concentrations=71ndash86 ppm Wie-

denbeck et al 1995)

The data presented here were acquired by the same

two analysts and over the same period as the 91500

analyses The 266 nm and 213 nm laser systems were

used for 365 and 73 analyses respectively As for the

91500 analyses signal intensities for the 213 nm laser

analyses were on average ca 60ndash70 of the signals

for the 266 nm laser analyses Again there are no

systematic differences in the data from the two

operators and their data have been pooled One highly

anomalous analysis performed on the 266 nm laser

(207Pb206Pb ratio N7 sd from the mean) was rejected

during data acquisition and is not considered further

A frequency distribution diagram of 266 nm laser206Pb238U data (Fig 9) reveals a symmetrical bell

curve with a few outliers on the lower and upper side of

the curve reflecting significant Pb loss and reverse

discordance respectively For statistical analysis

(Table 4) extreme outliers with ages greater than 780

Ma (four analyses) and less than 680 Ma (one analysis)

were rejected (complete data set may be accessed from

the online data repository (see Appendix A))

A frequency distribution diagram of 213 nm laser206Pb238U data (Fig 10) is a perfectly symmetrical

bell curve with no outliers and all data have been

accepted for statistical analysis The lack of outliers

Table 4

Summary of LA-ICP-MS isotopic ages for the Mud Tank zircon (TIMS a

266 nm (n=359) 213 nm (n=73) 266 nm (n=359) 213

Mean 2 rsd Mean 2 rsd Mean 2 sd Mea

207Pb206Pb 00639 17 00638 19 740 36 735207Pb235Pb 10607 41 10569 53 734 22 732206Pb238U 01204 39 01202 49 733 27 731208Pb232Th 00370 70 00372 97 734 50 738

in the 213 nm laser data may reflect reduced

incidence of ablating through fractures in the zircon

due to the smaller spot size and reduced laser

penetration depth

The data for the two laser systems are compared in

Table 4 Because of the large uncertainty on the TIMS

age reported by Black and Gulson (1978) the Mud

Tank cannot be considered a precisely calibrated

zircon Nevertheless all LA-ICP-MS ages are within

error of the reported TIMS age As for the 91500

zircon data precisions for the two laser systems are

very similar Noticeable in this data set however is

the very close agreement of the 206Pb238U and207Pb235U ages for the two laser systems

43 Application of the technique to young zircons

The precision and accuracy of the technique for

dating younger samples (down to 8 Ma) are discussed

with reference to the Temora Walcha Road and

Gunung Celeng zircons

431 Temora zircon

The Temora zircon derives from a high-level

gabbro-diorite stock in the Lachlan Fold Belt near

Temora NSW Because of its availability and isotopic

integrity this zircon has been developed by Geo-

science Australia as a standard for SHRIMP dating of

Phanerozoic rocks U concentrations range from 60 to

600 ppm and TIMS analyses of 21 single grain

analyses were all concordant and gave an age of

4168F024 Ma (95 confidence limits based on

measurement errors alone) (Black et al 2003)

The grains analysed in this study ranged from ca

50 to 100 Am in diameter and showed little internal

structure under CLBSE Results for 15 analyses

performed using 266 nm laser ablation are presented

ge=732F5 Ma Black and Gulson 1978)

Weighted mean age (Ma)Ferrors (at 95 confidence)

nm (n=73) 266 nm (n=359) 213 nm (n=73)

n 2 sd Mean Error Mean Error

40 7396 28 7356 68

28 7339 11 7312 32

34 7324 14 7306 39

70 7323 26 7324 83

Fig 10 Frequency distribution diagram for 73 206Pb238U age determinations of the Mud Tank zircon using the 213 nm laser ablation system

MeanF2 sd and weighted meanFuncertainty at 95 confidence based on all analyses as discussed in the text

in Table 5 (complete data set may be accessed from

the online data repository (see Appendix A)) All data

lie on or very close to concordia The weighted mean206Pb238U and 207Pb235U ages (4150 and 4154 Ma)

are in good agreement with the TIMS age and the 2rerrors (32 and 34 Ma) on the weighted mean

represent a relative error of ca 08 The weighted

mean 206Pb238U age of the 11 most coincident points

Table 5

LA-ICP-MS ages (266 nm laser) for the Temora zircon (TIMS age 4168

Analysis no 207Pb206Pb F2 sd 206Pb238U F2

TEM-1 421 59 416 10

TEM-2 507 87 410 10

TEM-3 384 91 422 10

TEM-4 408 72 411 9

TEM-5 423 67 413 9

TEM-6 416 61 418 9

TEM-7 434 107 411 10

TEM-8 421 93 423 11

TEM-9 353 94 417 10

TEM-10 377 91 418 10

TEM-11 391 80 418 10

TEM-12 506 106 423 11

TEM-13 396 96 420 11

TEM-14 474 67 405 9

TEM-15 402 79 406 10

Weighted mean 4150 3

when plotted on a TerandashWasserburg diagram is

4167F29 Ma and probably represents the best

estimate of the age of this sample (Belousova and

Griffin 2001) Because of its isotopic homogeneity

the Temora zircon provides a good example of the

precision and accuracy attainable by LA-ICP-MS for

a sample in a single analytical run (15 analyses over

ca 2 h)

plusmn03 Ma Black et al 2003)

sd 207Pb235U F2 sd 208Pb232Th F2 sd

416 10 412 10

425 14 432 13

416 14 420 13

410 11 408 10

415 11 405 11

418 10 415 10

414 16 426 16

423 15 414 14

407 14 415 12

412 14 412 12

414 13 403 13

436 17 440 15

416 15 410 14

415 11 408 10

405 12 401 11

2 4154 32

432 Walcha Road zircon

The late Permian Walcha Road adamellite is a large

I-type zoned pluton in the New England batholith of

northern New South Wales It grades from a mafic

hornblende-biotite adamellite along themargin through

a K-feldspar-phyric adamellite into a fine grained

leuco-adamellite in the core of the pluton (Flood and

Shaw 2000) Zircon U concentrations range from

several hundred ppm in the periphery of the intrusion to

several thousand ppm in the core (S Shaw personal

communication) All zircons are small (most b50 Am)

and are variably metamict This sample therefore

represents a significant challenge for age dating

Fifty-one grains were dated including some from

each of the three phases of the pluton using the 266 nm

laser system with laser focusing conditions adjusted to

give ca 30ndash40 Am spots The 02123 zircon standard

(Ketchum et al 2001) was used for calibration as the

GJ-1 standard had not been developed at the time of

analysis Data from 11 grains were rejected during data

reduction from further consideration because of very

short ablations (b10 s) due to catastrophic failure of

the grain during ablation or extremely disturbed ratios

(N70 discordant) reflecting the high degree of

metamictisation of the grains

Fig 11 TerandashWasserburg plot for 40 zircon analyses from the Walcha roa

dark grey

ATerandashWasserburg plot of the remaining 40 grains

(Fig 11 complete data set may be accessed from the

online data repository (see Appendix A)) reveals a

cluster of points on concordia and an array of points

above concordia defining an apparent common Pb

discordia line Two points to the left of the line are

interpreted to reflect inheritance Points to the right of

the line are interpreted to have lost radiogenic Pb and

gained common Pb (see example shown in Fig 2) In

this case a graphical approach to interpreting the data

was favoured Rejection of all data with a very large

amount of common Pb (207Pb206PbN008) and points

not overlapping at 2r confidence the analyses defining

a common Pb array leaves 26 points that yield a

common Pb-anchored regression intercept age of 249F3

Ma (Fig 12) This is in perfect agreement with a RbSr

isochron age (based on 22 analyses of rock K-feldspar

plagioclase and biotite) of 248F2 Ma (MSWD=084)

(S Shaw unpublished data) Although ideally a high-

precision TIMS UndashPb date is required for this sample to

confirm the absolute accuracy of the LA-ICP-MS age

the close agreement of LA-ICP-MS and RbSr ages

suggests that with appropriate protocols LA-ICP-MS

can provide accurate ages even for zircons that have

gained significant amounts of common Pb

d pluton Analyses rejected from the regression in Fig 12 shown in

Fig 12 TerandashWasserburg plot with regression of 26 zircon analyses from the Walcha road pluton

433 Gunung Celeng zircon

The application of LA-ICP-MS to dating very

young zircons is demonstrated using a zircon from a

high-level intrusion emplaced into MiocenendashPliocene

volcanics in the Gunung Celeng area of Java

Fig 13 TerandashWasserburg plot of data

Indonesia The zircons are mostly elongated euhedral

grains with lengthwidth ratios varying from 2 to 4

and widths ranging from 50 to 80 Am Zircons from

these samples show obvious magmatic oscillatory

zoning on the BSECL images Twenty-five grains

from the Gunung Celeng zircon

from the Gunung Celeng area with U concentrations

ranging from ca 50 to 550 ppm (mean ca 335 ppm)

were analysed using the 266 nm laser ablation system

and a spot size of ca 60 Am Four grains ablated

catastrophically and no data were collected The

remaining 21 grains gave usable data (complete data

set may be accessed from the online data repository

(see Appendix A))

A TerandashWasserburg plot of these 21 data points

(Fig 13) shows evidence of common Pb contamina-

tion possibly due to ablation of epoxy mounting the

grains A common Pb-anchored regression of all the

data gave an intercept age of 69F02 Ma

(MSWD=29) Rejecting analyses with 207Pb206PbN

008 which were clearly compromised by a significant

amount of common Pb the remaining eight grains gave

a slightly older weighted mean 206Pb238U age of

71F01 (MSWD=022) which probably represents the

best estimate of the crystallisation age of this sample

Apatite from the same samples gave a UndashHe age of

50F026 Ma (B McInnis pers comm) The UndashHe

age which dates the time that the apatite cooled below

its He closure temperature (75 8C) represents a

minimum age for the Gunung Celeng zircons The

slightly older LA-ICP-MS UndashPb dates are therefore

consistent with this age although a TIMS UndashPb date is

required for this sample to confirm their absolute

accuracy

5 Discussion and conclusions

A new zircon standard GJ-1 has been developed

Multiple LA-ICP-MS analyses of single grains have

produced the best precision of any zircon that we have

studied including the Temora zircon standard This

together with its large grain size (ca 1 cm)

combination of relatively high U content (230 ppm)

and moderate age (ca 609 Ma) extremely high206Pb204Pb (in excess of 146000) make it a

potentially highly suitable standard for in situ zircon

analysis The major drawback of this standard is that it

is not concordant and ID-TIMS analyses reveal small

variations (ca 1) in 206Pb238U and 207Pb235U

ratios between grains While this variation is less than

the analytical precision of the LA-ICP-MS technique

it does ideally require that each individual grain of the

GJ-1 standard be calibrated individually

A number of protocols have been described in the

literature for calibrating UndashPb analyses by LA-ICP-

MS These procedures are required to correct for

elemental fractionation associated with laser ablation

and the sample transport and ionisation processes

together with the inherent mass bias of the ICP-MS

The procedures involve a combination of (1) external

standardisation which corrects both sources of

fractionation but requires a very good matrix-

matched standard a stable laser and ICP-MS and

very careful matching of ablation conditions for

sample and standard (Tiepolo et al 2003 Tiepolo

2003 this study) (2) rastered ablation (eg Li et al

2001 Horstwood et al 2001 Kosler et al 2002)

which significantly reduces the ablation time depend-

ence of elemental fractionation but results in degraded

spatial resolution (or requires use of a very small

beam which reduces the ablation rate and thus the

signalnoise ratio) Moreover while it has been

demonstrated that rastering effectively reduces time-

dependent change in element ratios during ablation it

may not necessarily provide the correct elemental

ratio because of elemental fractionation during

incomplete breakdown of large particles in the ICP

(Guillong and Gunther 2002) This method has been

used with (1) or (4) (below) to correct for instrumental

mass bias (3) an empirical correction for ablation-

related elemental fractionation (Horn et al 2000)

which requires a reproducibility of laser conditions

that is not easily attainable using NdYAG-based

systems (Tiepolo et al 2003) This method has been

used in conjunction with (4) to correct for instrumen-

tal mass bias (Horn et al 2000) (4) use of an on-line

spike (Tl233U or 235U) introduced into the carrier gas

stream to correct for instrumental mass bias (Horn et

al 2000 Kosler et al 2002) Use of a solution-based

spike can result in high Pb backgrounds (Kosler et al

2002) which compromises data for young andor low-

U zircons This method must be used with 1 2 or 3 to

correct for ablation-related bias

The results of this study demonstrate that using a

reliable zircon standard to correct the sources of

isotopic bias together with a stable and sensitive

quadrupole ICP-MS and appropriate time-resolved

data reduction software accurate U-Pb ages can be

produced with a precision (2 rsd) on single spot

analyses ranging from 2 to 4 By pooling 15 or

more analyses 2r precisions below 1 can readily be

achieved These figures of merit compare very

favourably with those produced using rastering or

experimentally derived corrections of PbU fractiona-

tion (eg Kosler et al 2002 Horn et al 2000)

together with on-line TlU spiking This performance

also compares well with that produced using a similar

calibration protocol to the one described on a double

focusing sector field mass spectrometer (single

collector) (Tiepolo et al 2003 Tiepolo 2003)

The main disadvantage of the calibration approach

described here is that identical ablation conditions

(spot size laser fluence integration time) must be

maintained for all analyses in a run This is most

problematic for zircon populations with widely differ-

ent grain sizes and for very small zircons where

short low energy ablations required for the sample

must also be used for the standard

Common Pb contributions can be addressed

adequately using a variety of strategies including

selective integration of signals and graphical evalua-

tion It should also be noted that at a typical analysis

rate of ca 40ndash50 unknowns per day rejection of a few

analyses because of high common Pb is rarely a

serious problem

The 213 nm laser ablation produces slightly better

precision than 266 nm ablation and offers much

better ablation characteristics particularly for small

zircons For zircon 91500 213 nm produced206Pb238U and 207Pb235U ages that were signifi-

cantly different (older by ca 10 Ma) than those

produced by 266 nm laser ablation and which

agreed perfectly with TIMS ages The 266 and 213

nm laser data for the Mud Tank zircon analysed in

the same runs are indistinguishable indicating that

the difference is not related to use of different grains

of the GJ-1 zircon standard for the 266 and 213 nm

laser analyses This strongly suggests that the

difference in the 266 and 213 nm ages is related

either to the different ablation volumes produced by

the two laser systems andor to small differences in

ablation behaviour of the 91500 and GJ-1 zircons at

266 nm and that reproducibility of fractionation

between sample and standard is better using the

more strongly absorbed shorter wavelength 213 nm

radiation However in either case there is no

evidence of a similar effect(s) with the Mud Tank

and Temora zircons indicating that the effect is not

universal

For most zircons analysed the external precisions

of the 206Pb238U and 207Pb235U ratios are despite

significantly different count rates very similar and

significantly poorer than the 207Pb206Pb ratios for

the same analyses (see for example Fig 3) This

may in part be due to heterogeneity of the zircons

through redistribution of Pb andor U within the

crystals However a significant contribution to this is

likely to be small differences in PbU fractionation

behaviour from analysis to analysis related to drift

in laser operating conditions slight differences in

laser focus etc If inability to reproduce PbU

fractionation is the major limiting factor on preci-

sion further gains in the technique must come from

improvements in the sampling system rather than in

the detection system In either case the fact that

attainable precisions on the PbU ratios are several

times poorer than predicted on the basis of counting

statistics suggests that use of multi-collector (MC)-

ICP-MS instruments may not yield significantly

better precision than single collector instruments

although simultaneous collection might allow meas-

urement of 204Pb with sufficient precision to provide

a useful common Pb correction

Acknowledgements

We are most grateful to Jerry Yakoumelos of G

and J Gems Sydney for the donation of the GJ-1

standard zircon and to Fernando Corfu for perform-

ing TIMS analyses of the GJ-1 zircon We thank

Ayesha Saeed for providing her LA-ICP-MS data

sets for the 91500 and Mud Tank zircons Brent

McInnes kindly provided the Gunung Celeng zircon

and supporting data and Lance Black provided the

Temora zircon Stirling Shaw kindly allowed us to

use his data set for the Walcha Road zircon and

provided supporting RbndashSr data Chris Ryan and

Esme van Achterberg developed the GLITTER data

reduction program that was used in this study Tom

Andersen kindly provided a copy of his common Pb

correction program CommPbCorr Ashwini Sharma

and Suzy Elhlou are thanked for their assistance with

the LA-ICP-MS analyses The authors would like to

thank Roland Mundil and Takafumi Hirata for their

careful reviews that substantially improved the

manuscript This is publication no 345 from the

ARC National key Centre for Geochemical Evolu-

tion and Metallogeny of Continents (wwwesmq

eduauGEMOC) [PD]

Appendix A Supplementary data

Supplementary data associated with this article can

be found in the online version at doi101016

jchemgeo200406017

References

Andersen T 2002 Correction of common Pb in UndashPb analyses

that do not report 204Pb Chem Geol 192 59ndash79

Belousova EA 2000 Trace elements in zircon and apatite

application to petrogenesis and mineral exploration Unpub-

lished PhD thesis Macquarie University

Belousova EA Griffin WL 2001 LA-ICP-MS age of the

Temora zircon Unpublished data reported to Geoscience

Australia

Black LP Gulson BL 1978 The age of the Mud Tank

carbonatite Strangways Range Northern Territory BMR J

Aust Geol Geophys 3 227ndash232

Black LP Kamo SL Allen CM Aleinikoff JN Davis DW

Korsch RJ Foudoulis C 2003 TEMORA 1 a new zircon

standard for Phanerozoic UndashPb geochronology Chem Geol

200 155ndash170

Eggins SM Kinsley LPJ Shelley JMG 1998 Deposition and

element fractionation processes occurring during atmospheric

pressure sampling for analysis by ICP-MS Appl Surf Sci 129

278ndash286

Feng R Machado N Ludden J 1993 Lead geochronology

zircon by laser probe-inductively coupled plasma mass spec-

trometry (LP-ICPMS) Geochim Cosmachim Acta 57 3479ndash

Fernandez-Suarez J Gutierrez-Alonso G Jenner GA Jackson

SE 1998 Geochronology and geochemistry of the Pola de

Allande granitoids (northern Spain) their bearing on the

CadomianAvalonian evolution of NW Iberia Can J Earth

Sci 35 1439ndash1453

Flood RH Shaw SE 2000 The Walcha Road adamellite a large

zoned pluton in the New England batholith Australia Geol

Soc Australian Abstr 59 152

Fryer BJ Jackson SE Longerich HP 1993 The application of

laser ablation microprobe-inductively coupled plasma-mass

spectrometry (LAM-ICP-MS) to in-situ (U)ndashPb geochronology

Chem Geol 109 1ndash8

Fryer BJ Jackson SE Longerich HP 1995 The design

operation and role of the laser-ablation microprobe coupled with

an inductively coupled plasma-mass spectrometer (LAM-ICP-

MS) in the earth sciences Can Min 33 303ndash312

Guillong M Gqnther D 2002 Effect of particle size distributionon ICP-induced elemental fractionation in laser ablation

inductively coupled plasma mass spectrometry J Anal At

Spectrom 17 831ndash837

Hirata T Nesbitt RW 1995 UndashPb isotope geochronology of

zircon evaluation of the laser probe-inductively coupled

plasma-mass spectrometry technique Geochim Cosmochim

Acta 59 2491ndash2500

Horn I Gqnther D 2003 The influence of ablation carrier gasses

Ar He and Ne on the particle size distribution and transport

efficiencies of laser ablation-induced aerosols implications for

LA-ICP-MS Appl Surf Sci 207 144ndash157

Horn I Rudnick RL McDonough WF 2000 Precise elemental

and isotope ratio determination by simultaneous solution

nebulization and laser ablation-ICP-MS application to UndashPb

geochronology Chem Geol 164 281ndash301

Horstwood MSA Foster GL Parrish RR Noble SR 2001

Common-Pb and inter-element corrected UndashPb geochronology

by LA-MC-ICP-MS VM Goldschmidt Conf Abstr 3698

Jackson SE 2001 The application of NdYAG lasers in LA-ICP-

MS In Sylvester PJ (Ed) Laser Ablation-ICP-Mass Spec-

trometry in the Earth Sciences Principles and Applications

Mineralog Assoc Canada (MAC) Short Course Series vol 29

Mineralogical Association of Canada pp 29ndash45

Jackson SE Horn I Longerich HP Dunning GR 1996 The

application of laser ablation microprobe (LAM)-ICP-MS to in

situ UndashPb zircon geochronology VM Goldschmidt Confer-

ence J Conf Abstr 1 283

Ketchum JWF Jackson SE Culshaw NG Barr SM 2001

Depositional and tectonic setting of the Paleoproterozoic Lower

Aillik Group Makkovik Province Canada evolution of a

passive marginmdashforedeep sequence based on petrochemistry

and UndashPb (TIMS and LAM-ICP-MS) geochronology Precam-

brian Res 105 331ndash356

Kosler J Fonneland H Sylvester P Tubrett M Pedersen R-

B 2002 UndashPb dating of detrital zircons for sediment

provenance studiesmdasha comparison of laser ablation ICP-MS

and SIMS techniques Chem Geol 182 605ndash618

Li X-H Liang X Sun M Guan H Malpas JG 2001 Precise206Pb238U age determination on zircons by laser ablation

microprobe-inductively coupled plasma-mass spectrometry

using continuous linear ablation Chem Geol 175 209ndash219

Ludwig KR 2001 Isoplot v 22mdasha geochronological toolkit for

Microsoft Excel Berkeley Geochronology Center Special

Publication No 1a 53 pp

Mank AJG Mason PRD 1999 A critical assessment of laser

ablation ICP-MS as an analytical tool for depth analysis in silica-

based glass samples J Anal At Spectrom 14 1143ndash1153

Norman MD Pearson NJ Sharma A Griffin WL 1996

Quantitative analysis of trace elements in geological materials

by laser ablation ICPMS instrumental operating conditions

and calibration values of NIST glass Geostand Newsl 20

247ndash261

Outridge PM Doherty W Gregoire DC 1997 Ablative and

transport fractionation of trace elements during laser sampling of

glass and copper Spectrochim Acta 52B 2093ndash2102

Stacey JS Kramers JD 1975 Approximation of terrestrial lead

isotope evolution by a two-stage model Earth Planet Sci Lett

26 207ndash221

Tera F Wasserburg GJ 1972 UndashThndashPb systematics in three

Apollo 14 basalts and the problem of initial Pb in lunar rocks

Earth Planet Sci Lett 14 281ndash304

Tiepolo M 2003 In situ Pb geochronology of zircon with laser

ablation-inductively coupled plasma-sector field mass spectrom-

etry Chem Geol 199 159ndash177

Tiepolo M Bottazzi P Palenzona M Vannucci R 2003 A

laser probe coupled with ICP-double focusing sector-field mass

spectrometer for in situ analysis of geological samples and UndashPb

dating of zircon Can Min 41 259ndash272

Van Achterbergh E Ryan CG Jackson SE Griffin WL 2001

Data reduction software for LA-ICP-MS appendix In Syl-

vester PJ (Ed) Laser Ablation-ICP-Mass Spectrometry in the

Earth Sciences Principles and Applications Mineralog Assoc

Canada (MAC) Short Course Series Ottawa Ontario Canada

vol 29 pp 239ndash243

Wiedenbeck M Alle P Corfu F Griffin WL Meier M

Oberli F von Quadt A Roddick JC Spiegel W 1995

Three natural zircon standards for UndashThndashPb LundashHf trace

element and REE analyses Geostand Newsl 19 1ndash23