Simultaneous in situ Analysis of U-Pb Age and Hf Isotopes of Zircon by Laser AblationSector-Field (MC-) ICP-MSJ. B. Schwieters, C. Bouman, M. Deerberg, J. D.Wills, Thermo Fisher Scientific, Bremen, GermanyA. Gerdes, Institut für Geowissenschaften, Johann Wolfgang Goethe Universität, Frankfurt am Main, Germany

IntroductionThe combination of laser ablation with multicollectorinductively coupled plasma mass spectrometers (LA-MC-ICP-MS) has been shown to produce high quality zircon age data from U-Th-Pb isotopic studies.These results however may not correctly indicate thenumber of zircon forming events, especially those arisingfrom multiple growth and alteration processes. Thislimitation can be overcome by combining results from Pb-U and Lu-Hf analyses made on small, well definedzircon areas.

Static measurements of Pb-U and Lu-Hf can not beachieved in a single MC-ICP-MS analysis due to the largemass range that would need to be scanned. This haspreviously meant sequential analyses of the Pb-U and theLu-Hf systems at the same zircon site1, e.g. an initial Pb-Uanalysis by laser ablation coupled to sector-field ICP-MS(LA-ICP-SFMS) and a subsequent LA-MC-ICP-MSanalysis for Lu-Hf2. The sequential analyses were made as closely as possible in the same area (growth domain) of the zircon, for example a 40 µm Lu-Hf spot was drilledon top of a 30 µm U-Pb spot)2.

In order to overcome the limitations of sequentialsampling of U-Th-Pb and Hf isotopes from different sites in zircons, a fast scanning single collector and a multicollector inductive coupled plasma mass spectro-meter have been coupled in parallel to a New WaveUP213 213 nm UV laser ablation system. A low volumelaser ablation sample chamber with fast response (< 1 s)and rapid washout time was fitted instead of the standardsample chamber in order to enable sequential sampling of heterogeneous grains during time-resolved dataacquisition on both instruments. The ablated material was transported in helium, split after the ablation cell viaa Y-shaped connector and introduced simultaneously intothe two mass spectrometers, the Thermo ScientificELEMENT XR for U-Th-Pb analysis and ThermoScientific NEPTUNE for high precision Hf isotopeanalysis.

For accurate age dating of zircons the control of common lead contaminations is essential. This can bedone by monitoring mass 204Pb which is not part of the U-Pb decay scheme. Since the natural abundance of 204Pbis so small every common lead contribution measured on204Pb is magnified by about a factor of 15 to 18 on 206Pband 207Pb intensities. This means the small 204Pb signal hasto be measured precisely and therefore the highestsensitivity is required to achieve the smallest statisticalerror. In comparison to quadrupole ICP-MS instruments3

the ELEMENT XR sector-field ICP-MS is at least oneorder of magnitude more sensitive and as a consequencethe statistical uncertainty on 204Pb is significantlyimproved.

Instrumentation

U-Th-Pb analysis Hf isotope analysis

Instrument ELEMENT XR NEPTUNE

Scan mode E-Scan Static

Scanned masses 202, 204, 206, 207, 171, 173, 175, 176,

208, 235, 232, 238 177, 178, 179, 180

Mass resolution 300 300

Dead time 18 ns 20 ns

Dwell time/isotope 4 ms 1 s

Settling time/isotope 1 ms –

Number of scans 1080 50

Integration time 1 s (= 15 scans) 1 s

Background 20 s

Ablation time 50 s 50 s

Carrier gas 0.5 L/min He (+1.1 L/min Ar)

Laser UP213 SA (New Wave Research; Nd:YAG, 213 nm)

Spot size 55 to 80 µm

Laser settings 5 Hz

Laser fluence ~ 3 J/cm2

Drill speed 0.6 µm/s

Table 1: Operating conditions and instrument settings.

Key Words

• ELEMENT XR

• NEPTUNE

• Isotope Ratio

• Laser Ablation

• Zircon

ApplicationNote: 30170

Figure 1: Screenshot from the ELEMENT software showing instrumentsettings and sensitivity obtained for La and Th in NIST 612 using the lasersampling parameters as shown.

The primary advantages of ICP-SFMS offers overother ICP-MS instrumentation for U-Th-Pb analyses areshown in Figure 1 and can be summarized as:

• The characteristic flat-top peaks of sector-field massspectrometers in low mass resolution allow the reliableuse of single point per peak analyses.

• The high sensitivity of ICP-SFMS allows for increasedaccuracy on the common lead correction that is madewhen measuring 202Hg as well as 204Pb+Hg. With lesssensitive instrumentation small common leadcontaminations detected at mass 204 would beotherwise magnified on masses 206Pb and 207Pb, leadingto worsened Pb-U dating accuracy.

Figure 2: Low volume laser sample chamber used in all analyses.

The advantages of the low volume laser chamber4

(Figure 2) for discrete laser sampling can be seen in Figure3. Uptake delays of 1 s were achievable with the lowvolume cell, compared to 6 s with the standard samplecell. In the low volume cell, signal intensities after the endof the ablation dropped by 2 orders of magnitude in lessthan 4 s and 3 orders in about 6 s. In comparison, thesignal from the standard cell needs more than 3 timeslonger (13 s and 25 s respectively) to drop to the samelevels. Spiking is still observed in the standard cell up to50 s after the end of ablation, while the low volume cellshows no spiking.

Figure 3: Improvement in sample uptake and washout when using the lowvolume laser sample chamber.

Results

Figure 4: Simultaneous U-Pb and Hf isotope analyses in zircon.

U-Pb age 176Hf/177Hf

91500: 1065 Ma5 0.282306 ± 0.0000088

GJ-1: 607 Ma2 0.282103 ± 0.0000081 (unpublished)

Plešovice: 337.1 Ma6 0.282484 ± 0.0000086

Temora 2: 417 Ma7 0.282686 ± 0.0000068

AS3 (~FC1): 1099 Ma9 0.282184 ± 0.0000167

MudTank: 732 Ma10 0.282507 ± 0.0000068

Table 2: Reference U-Pb age and Hf isotope data for the zircons analyzed inFigure 4.

1030

1050

1070

1090

1110

1130

Mean = 1095.6 ± 4.1 [0.37%] 95% conf.MSWD =1.2, probability = 0.20

As3 (~Fc1)

580

590

600

610

620

630GJ-1

1010

1030

1050

1070

1090

1110data-point error ellipses are 2σ error bars are 2σ error bars are 2σ

error bars are 2σ

error bars are 2σ

error bars are 2σ

error bars are 2σ

error bars are 2σ

error bars are 2σ

error bars are 2σ

error bars are 2σ

error bars are 2σ

error bars are 2σ

data-point error ellipses are 2σ

data-point error ellipses are 2σ

1050

1090

0.1701.72 1.76 1.80 1.84 1.88 1.92 1.96 2.00

0.174

0.178

0.182

0.186

0.190

1070

1100

0.28220

0.28224

0.28228

0.28232

0.28236

0.2824091500average =0.282299 ± 0.000026

spot size 65 and 80 µm, respectively

176 Hf

/177 Hf

176 Hf

/177 Hf

176 Hf

/177 Hf

176 Hf

/177 Hf

176 Hf

/177 Hf

176 Hf

/177 Hf

0.28190

0.28194

0.28198

0.28202

0.28206

0.28210

0

GJ-1average = 0.282015 ± 0.000029

spot size 65 and 55 µm, respectively

206 Pb

/238 U

age

(Ma)

206 Pb

/238 U

age

(Ma)

206 Pb

/238 U

age

(Ma)

206 Pb

/238 U

age

(Ma)

206 Pb

/238 U

age

(Ma)

206 Pb

/238 U

age

(Ma)

430

420

410

4000.0635

0.0645

0.0655

0.0665

0.0675

0.0685

0.0695

0.475 0.485 0.495 0.505 0.515 0.525

Temora

GJ-1

0.28238

0.28242

0.28246

0.28250

0.28254

0.28258

average =0.282486 ± 0.000028Plešovice

350

340

330

320

0.050

0.052

0.054

0.056

0.365 0.375 0.385 0.395 0.405 0.415

Plešovice

Concordia age = 337.7 ± 2.0MaMSWDC+E = 1.34

9150091500

322

326

330

334

338

342

346

350

Mean = 338.6 ± 1.6 [0.48%] 95% conf.MSWD = 1.06, probability = 0.39

Plešovice

0.28255

0.28260

0.28265

0.28270

0.28275

0.28280

0.28285

average = 0.282689 ± 0.000049

Temora

400

404

408

412

416

420

424

428

Mean = 414.9 ± 2.0 [0.48%] 95% conf.MSWD = 0.83, probability = 0.62

Temora

0.28206

0.28210

0.28214

0.28218

0.28222

0.28226

0.28230 As3 (~Fc1)average = 0.282185 ± 0.000043

MudTankaverage =0.282522 ± 0.000019

0.28244

0.28248

0.28252

0.28256

0.28260

690

710

730

750

770MudTank

760

720

0.112

0.116

0.120

0.124

0.128

0.92 0.96 1.00 1.04 1.08 1.12 1.16

740

700

MudTank

Concordia age = 729.0 ± 2.6MaMSWDC+E = 1.07

1110

1070

0.170

0.174

0.178

0.182

0.186

0.190

0.194

1.76 1.80 1.84 1.88 1.92 1.96 2.00 2.04 2.08

1090

Concordia age = 1094.9 ±3.3MaMSWDC+E = 1.06

As3 (~Fc1)

Mean = 1065.6 ±4.3 [0.40%] 95% conf.MSWD = 1.5, probability = 0.087

Concordia age = 1066.2 ± 3.3MaMSWDC+E = 0.56

630

610

590

0.093

0.095

0.097

0.099

0.101

0.103

0.76 0.78 0.80 0.82 0.84 0.86 0.88 Mean = 607.0 ±2.4 [0.39%] 95% conf.MSWD =0.63, probability = 0.89

Concordia age = 607.6 ± 2.4MaMSWDC+E = 0.56

spot size 80, 65 and 55 µm, respectively

spot size 65 and 55 µm, respectively

spot size 65 and 55 µm, respectively

GJ-1

206 Pb

/238 U

206 Pb

/238 U

206 Pb

/238 U

206 Pb

/238 U

206 Pb

/238 U

206 Pb

/238 U

207Pb/235U

207Pb/235U

data-point error ellipses are 2σ

207Pb/235U

data-point error ellipses are 2σ207Pb/235U

data-point error ellipses are 2σ

207Pb/235U

207Pb/235U

Concordia age = 414.5 ± 2.0MaMSWDC+E = 0.89

MSWD = 0.91, probability = 0.69Mean = 728.8. ± 2.4 [0.33%] 95% conf.

spot size 65 and 55 µm, respectively

ConclusionsCoupling the Thermo Scientific ELEMENT XR and theNEPTUNE to the laser ablation system was easy toachieve and demonstrated to be a powerful instrumentalsolution for the simultaneous analysis of U-Th-Pb and Hfisotope ratios in small zircons. The low volume laser cellused in this study demonstrated fast sample uptake andfast washout, resulting in low backgrounds withoutspiking signals. U-Pb ages and Hf isotope ratios of allzircons obtained in this study are in very good agreementwith the published data.

References1 Gerdes, A., Zeh, A., 2006. Combined U–Pb and Hf isotope

LA-(MC)-ICP-MS analyses of detrital zircons: comparison with SHRIMP and new constraints for the provenance and age of an Armorican metasediment in Central Germany. Earth and Planetary Sciences Letters 249, 47–61.

2 Gerdes, A., Zeh, A., 2008. Zircon formation versus zircon alteration – New insights from combined U–Pb and Lu–Hf in-situ LA-ICP-MS analyses, and consequences for the interpretation of Archean..., Chemical Geology, doi:10.1016/j.chemgeo.2008.03.005

3 Hong-Lin Yuan, Shan Gao, Meng-Ning Dai, Chun-Lei Zong, Detlef Günther, Gisela Helene Fontaine, Xia-Ming Liu, ChunRong Diwu; Simultaneous determindations of U-Pb age, Hf isotopes and trace element compositions of zircon by excimer laser-ablation quadrupole and multiple-collector ICP-MS; Chem Geol. 247, 110-118.

4 The laser ablation cell is of similar design to that described in: Davide Bleiner, Detlef Günther; Theoretical description and experimental observation of aerosol transport processes in laser ablation ICPMS, J. Anal. At. Spectrom., 2001, 16, 449-456.

5 Wiedenbeck, M. et al, 1995. Three natural zircon standards for U–Th–Pb, Lu–Hf, trace element and REE analyses. Geostandards Newsletter 19, 1-23.

6 Slama, J. et al, 2008. Plešovice zircon – A new natural reference material for U–Pb and Hf isotopic microanalysis. Chemical Geology 249, 1-35

7 Black, L.P. et al, 2004. Improved Pb-206/U-238 microprobe geochronology by the monitoring of a traceelement related matrix effect; SHRIMP, ID-TIMS, ELA-ICP-MS and oxygen isotope documentation for a series of zircon standards. Chemical Geology 205, 115–140.

8 Woodhead, J.D., Hergt, J.M., 2005. A preliminary appraisal of seven natural zircon reference materials for in situ Hf isotope determination. Geostandards and Geoanalytical Research 29 (2), 183–195.

9 Paces, J.B., Miller, J.D., 1993. Precise U–Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochronological insights into physical, petrogenetic, paleomagnetic and tectonomagmatic processes associated with the 1.1 Ga midcontinent rift system. Journal of Geophysical Research 98, 13997–14013

10 Black, L.P., Gulson, B.L., 1978. The age of the Mud Tank carbonatite, Strangways Range, Northern Territory. BMR Journal of Australian. Geology and Geophysics, 3, 227–232.

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