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High Precision Pb Isotope Ratio Measurements of Zircon. Zenon Palacz , Simon Meffan-Main, Patrick Turner, & Fadi Abou-Shakra. Introduction. Lead isotope ratios conventionally determined in Zircons by TIMS/SHRIMP MC-ICPMS compliments conventional methods and expands geochronological horizons.

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High Precision Pb Isotope Ratio Measurements of Zircon

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High Precision Pb Isotope Ratio Measurements of Zircon

Zenon Palacz, Simon Meffan-Main,

Patrick Turner, & Fadi Abou-Shakra


Introduction

Lead isotope ratios conventionally determined in Zircons by TIMS/SHRIMP

MC-ICPMS compliments conventional methods and expands geochronological horizons.


Advantages of an ICP as an Ion Source

  • The degree of elemental ionisation across the periodic table

    is relatively high and uniform

  • Gives predominantly singly charged ions

  • Samples are introduced at atmospheric pressure and can be

    readily interchanged

  • Most elements are ionised >99%

  • Fractionation (Mass bias) in an ICP is stable and easily

    corrected


Sensitivity

Ion:Atom % Ionisation cps/ppm

Sr 1:124 0.08% 2.74 x 109

Nd 1:310 0.32% 6.74 x 109

Hf 1:400 0.25% 4.18 x 109

Os 1:500 0.20% 3.05 x 109

Pb 1:110 0.92% 1.34 x 1010

Th 1:345 0.29% 3.75 x 109

U 1:200 0.50% 6.31 x 109


11

10

In

Pb

10

10

Nd

U

Rh

Hf

Cs

cps/ppm

Tb

Th

Sr

Os

Co

Mg

3-4 eV

9

10

5-6 eV

Ionization

Potential

6-7 eV

Fe

7-8 eV

Li

8-9 eV

8

10

200

0

50

100

150

250

Mass

Sensitivity


1

0.1

ion/atom efficiency (%)

0.01

Sr

0.001

0

50

100

150

200

250

MASS

Transmission efficiency of IsoProbe with CETAC MCN6000


Cetac LSX-200 Laser Ablation

  • Nd-YAG UV 216nm

  • In-Situ Analyses from Polished thin sections or loose grains

  • Speed of Analyses

  • Spatial Flexibility in 3 Dimensions


Zircon Geochronology

High Precision Spatial Resolution isotope ratios essential for zircon age determinations in many situations.

SHRIMP - SIMS spot size down to 10microns.

LA-MC-ICPMS - Offers High Precision Isotope Ratios and simplified sample introduction. STATIC analysis.


50mm pit in zircon during ablation


Zircon KC81-135 (TIMS data)

  • Multigrain fraction 0.8% discordant data

  • 207Pb/206Pb 0.18071 ± 0.00018

  • 208Pb/206Pb 0.0055

  • 206Pb/204Pb 26240

  • Pb 514ppm

  • U 993ppm

  • Age 2659 ± 2Ma

  • (Fernando Corfu per.com. Nov. 1996)

  • Multigrain fraction 0.8% discordant data

  • 207Pb/206Pb 0.18071+/-0.00018

  • 208Pb/206Pb 0.0055

  • 206Pb/204Pb 26240

  • Pb 514ppm

  • U 993ppm

  • Age 2659+/-2Ma

  • (Fernando Corfu per.com. Nov. 1996)


206Pb (volts) down hole.

206Pb Intensity v Time

2.5

2

1.5

206Pb Intensity

1

0.5

0

0

20

40

60

Seconds


207Pb/206Pb v Time

0.22

0.21

0.2

207Pb/206Pb

0.19

0.18

0.17

0

20

40

60

seconds

Isotopic variation down hole


208Pb/206Pb v Time

0.12

0.1

0.08

208Pb/206Pb

0.06

0.04

0.02

0

0

20

40

60

Seconds

208Pb/206Pb down hole


second

206Pb volts

208Pb/206Pb

207Pb/206Pb

206Pb/204Pb

0-10

0.85

0.06174

0.20076

671.6

10-20s

0.48

0.01663

0.18596

1582.6

20-30s

0.49

0.01754

0.18259

2537.8

30-40s

0.65

0.00472

0.18309

3418.2

40-50s

1.23

0.00324

0.18223

5454.8

50-60s

1.74

0.00436

0.18279

6537.8

Down hole isotope variation zircon KC81135


0.0795

Analysis 1-15 on 24th Feb

0.079

Analysis 16-19 on 23rd Feb

0.0785

207Pb/206Pb ratio

0.078

TIMS Data

0.0775

0.077

0

5

10

15

20

Analysis No.

Uqz-1 Zircon, 48ppm Pb


Lead Isotope Measurements - Laser Ablation - Micromass IsoProbe

Zircon

Zircon

(3.9 Ga)

(1.0 Ga)

SAMPLE NAME

NIST 610

NIST 612

BCR2G

pit depth

2.00E-04

2.00E-04

2.00E-04

2.00E-06

2.00E-06

radius

2.00E-05

2.00E-05

2.00E-05

1.20E-05

2.00E-05

pi

3.14E+00

3.14E+00

3.14E+00

3.14E+00

3.14E+00

ablated volume

2.51E-13

2.51E-13

2.51E-13

9.05E-16

2.51E-15

density

2.80E+06

2.80E+06

2.80E+06

3.50E+06

3.50E+06

grams of ablated material

7.04E-07

7.04E-07

7.04E-07

3.17E-09

8.80E-09

concentration of element

4.27E-04

3.90E-05

1.30E-05

1.50E-04

4.80E-05

mass of element

2.08E+02

2.08E+02

2.08E+02

2.08E+02

2.08E+02

Avogardo #

6.03E+23

6.03E+23

6.03E+23

6.03E+23

6.03E+23

# of ions

8.71E+11

7.95E+10

2.65E+10

1.38E+09

1.22E+09

ablation time (seconds)

6.00E+01

6.00E+01

6.00E+01

1.00E+00

1.00E+00

# of ions/sec

1.45E+10

1.33E+09

4.42E+08

1.38E+09

1.22E+09

coulomb charge

1.60E-19

1.60E-19

1.60E-19

1.60E-19

1.60E-19

Amps (charge/sec)

2.32E-09

2.12E-10

7.07E-11

2.20E-10

1.96E-10

resistor

1.00E+11

1.00E+11

1.00E+11

1.00E+11

1.00E+11

Volts

2.32E+02

2.12E+01

7.07E+00

2.20E+01

1.96E+01

total beam (volts)

2.40E+00

2.00E-01

5.00E-02

1.50E-01

1.50E-01

Total ion transport efficiency

0.77%

1.0%

0.9%

0.7%

0.7%

Ion:Atom Laser Transport Efficiency


ZIRCON 91500

  • Age 1062+/-0.4Myr (TIMS)

  • 12ppm Pb

  • 207Pb/206Pb 0.07488+/-0.00001 TIMS


Precision vs Sample Size Zircon 91500

0.0765

TIMS samples are circa. 1000 x larger

0.0760

and 50 x more precise than 50mm ablations

0.0755

Sample size in grams

50mm

100mm

8.58E-08

207Pb/206Pb

0.0750

9.38E-07

0.0008

TIMS

0.0745

0.074

0.00000001

0.0000001

0.000001

0.00001

0.0001

0.001

0.01

Log Sample size (grams)

Laser Ablation ICP-MS vs TIMS


91500 with Laser Induced Fractionation

0.0800000

1.2

IsoProbe 207Pb/206Pb Age (St. Dev. of 43) = 1008 +/- 46Ma

TIMS Age = 1062 +/-0.4Ma

0.0780000

1.15

300mm

Average = 0.074703

1.1

100mm

0.0760000

St. Dev. = 0.87

Average = 0.072291

0.07488

1.05

Age (Ga)

207Pb/206Pb

0.0740000

200mm

1

Average = 0.073638

0.0720000

0.95

0.0700000

50mm

0.9

Average = 0.072148

% St. Dev. = 3.72

0.0680000

0.85


25 & 50mm pits in 91500 zircon


25mm Spots in 91500 <1pg Pb

Average = 0.75936

St. Dev. = 4.4% (n=15)

Weighted Mean = 0.0751 +/-10 (1.4%)

0.0900000

Age = 1070 +/-30Ma

0.0850000

0.0800000

TIMS

207Pb/206Pb

0.0750000

0.0700000

0.0650000

0

5

10

15

Analysis no.


50mm Spots in 91500

0.0900000

Age = 1067 +/-12Ma

Average = 0.07495

TIMS Age = 1062.4 +/-0.4Ma

St. Dev. =.00082 (1.1%; n=15)

0.0850000

Weighted Mean = 0.07494 +/- 45 (0.61%)

TIMS = 0.07488 +/- 1

0.0800000

207Pb/206Pb

0.0750000

TIMS

0.0700000

0.0650000

0

2

4

6

8

10

12

14

16

18

Analysis no.


Zircon prior to ablation

100mm


3-D mapping of isotopic ratios using 50mm pits in zircon


Summary

LA-MC-ICPMS - Offers high precision Isotope Ratios and simplified sample introduction.

MC-ICPMS offers alternatives to conventional geochronological methods and expands geochronological horizons.


Summary 2

We have demonstrated that accurate and precise Pb isotopic ratios can be determined from less than 1pg total Pb in 25mm ablation pits within a 15ppm zircon with Faraday ion collection.

With multiple ion counting LA-ICPMS provides a tool to obtain reliable and accurate geochronology data from zircons at 10mm resolution.


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