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.

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High Precision Pb Isotope Ratio Measurements of Zircon

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High precision pb isotope ratio measurements of zircon

High Precision Pb Isotope Ratio Measurements of Zircon

Zenon Palacz, Simon Meffan-Main,

Patrick Turner, & Fadi Abou-Shakra


Introduction

Introduction

Lead isotope ratios conventionally determined in Zircons by TIMS/SHRIMP

MC-ICPMS compliments conventional methods and expands geochronological horizons.


High precision pb isotope ratio measurements of zircon

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


High precision pb isotope ratio measurements of zircon

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


High precision pb isotope ratio measurements of zircon

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


High precision pb isotope ratio measurements of zircon

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

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

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.


High precision pb isotope ratio measurements of zircon

50mm pit in zircon during ablation


High precision pb isotope ratio measurements of zircon

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)


High precision pb isotope ratio measurements of zircon

206Pb (volts) down hole.

206Pb Intensity v Time

2.5

2

1.5

206Pb Intensity

1

0.5

0

0

20

40

60

Seconds


High precision pb isotope ratio measurements of zircon

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


208 pb 206 pb 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


High precision pb isotope ratio measurements of zircon

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


High precision pb isotope ratio measurements of zircon

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


Ion atom laser transport efficiency

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

ZIRCON 91500

  • Age 1062+/-0.4Myr (TIMS)

  • 12ppm Pb

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


Laser ablation icp ms vs 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

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


High precision pb isotope ratio measurements of zircon

25 & 50mm pits in 91500 zircon


25 m m spots in 91500 1pg pb

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.


50 m m spots in 91500

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

Zircon prior to ablation

100mm


High precision pb isotope ratio measurements of zircon

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


Summary

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

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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