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 uniformGives predominantly singly charged ionsSamples are introduced at atmospheric pressure and can be readily interchangedMost elements are ionised >99%Fractionation (Mass bias) in an ICP is stable and easily corrected

Sensitivity

Ion:Atom % Ionisation cps/ppmSr 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

Sensitivity

10 8

10 9

10 10

10 11

250 150 100 50 0

Li

Mg

Fe

Co Sr

Rh Cs

Nd

Tb

Pb

Os

Hf

Th

U

In cp

s/pp

m

3-4 eV 5-6 eV 6-7 eV

7-8 eV

8-9 eV

200

Ionization Potential

Mass

Transmission efficiency of IsoProbe with CETAC MCN6000

0.001

0.01

0.1

1

0 50 100 150 200 250

MASS

ion

/ato

m e

ffic

ien

cy (

%)

Sr

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.

50m 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

0

0.5

1

1.5

2

2.5

0 20 40 60

Seconds

206

Pb

Inte

nsi

ty

Isotopic variation down hole

207Pb/206Pb v Time

0.17

0.18

0.19

0.2

0.21

0.22

0 20 40 60

seconds

207P

b/20

6P

b

208Pb/206Pb down hole208Pb/206Pb v Time

0

0.02

0.04

0.06

0.08

0.1

0.12

0 20 40 60

Seconds

208 P

b/20

6 Pb

Down hole isotope variation zircon KC81135

second 206Pb volts 208Pb/206Pb 207Pb/206Pb 206Pb/204Pb0-10 0.85 0.06174 0.20076 671.610-20s 0.48 0.01663 0.18596 1582.620-30s 0.49 0.01754 0.18259 2537.830-40s 0.65 0.00472 0.18309 3418.240-50s 1.23 0.00324 0.18223 5454.850-60s 1.74 0.00436 0.18279 6537.8

0.077

0.0775

0.078

0.0785

0.079

0.0795

0 5 10 15 20

Analysis No.

207

Pb

/206

Pb

ra

tio

Analysis 1-15 on 24th FebAnalysis 16-19 on 23rd Feb

TIMS Data

Uqz-1 Zircon, 48ppm Pb

Ion:Atom Laser Transport EfficiencyIon:Atom Laser Transport EfficiencyLead Isotope Measurements - Laser Ablation - Micromass IsoProbe

Zircon ZirconSAMPLE NAME NIST 610 NIST 612 BCR2G (3.9 Ga) (1.0 Ga)

pit depth 2.00E-04 2.00E-04 2.00E-04 2.00E-06 2.00E-06radius 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+00ablated 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+06grams 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-05mass 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-19Amps (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+11Volts 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 1.0% 0.9% 0.7% 0.7% 0.77%

ZIRCON 91500ZIRCON 91500

Age 1062+/-0.4Myr (TIMS)

12ppm Pb

207Pb/206Pb 0.07488+/-0.00001 TIMS

Laser Ablation ICP-MS vs TIMSLaser Ablation ICP-MS vs TIMSPrecision vs Sample Size Zircon 91500

8.58E-08

9.38E-07 0.0008

0.074

0.0745

0.0750

0.0755

0.0760

0.0765

0.00000001 0.0000001 0.000001 0.00001 0.0001 0.001 0.01

Log Sample size (grams)

20

7P

b/2

06P

b

TIMS samples are circa. 1000 x larger

and 50 x more precise than 50m ablations

TIMS

50m100m

Sample size in grams

1.2

1.15

1.1

1.05

0.95

0.9

0.85

1

0.0680000

0.0700000

0.0720000

0.0740000

0.0760000

0.0780000

0.0800000

207 P

b/20

6 Pb

50mAverage = 0.072148% St. Dev. = 3.72

100mAverage = 0.072291

200mAverage = 0.073638

300mAverage = 0.074703

St. Dev. = 0.87

Ag

e (Ga)

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

TIMS Age = 1062 +/-0.4Ma

0.07488

91500 with Laser Induced Fractionation91500 with Laser Induced Fractionation

25 & 50m pits in 91500 zircon

25m Spots in 91500 <1pg Pb25m Spots in 91500 <1pg Pb

0.0650000

0.0700000

0.0750000

0.0800000

0.0850000

0.0900000

0 5 10 15

207P

b/20

6P

b

Average = 0.75936St. Dev. = 4.4% (n=15)

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

Age = 1070 +/-30Ma

TIMSTIMS

Analysis no.Analysis no.

0.0650000

0.0700000

0.0750000

0.0800000

0.0850000

0.0900000

0 2 4 6 8 10 12 14 16 18

207P

b/20

6P

b

Average = 0.07495St. Dev. =.00082 (1.1%; n=15)

Weighted Mean = 0.07494 +/- 45 (0.61%)TIMS = 0.07488 +/- 1

TIMS

Age = 1067 +/-12MaTIMS Age = 1062.4 +/-0.4Ma

Analysis no.Analysis no.

50m Spots in 9150050m Spots in 91500

Zircon prior to ablationZircon prior to ablation

100m100m

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

Profile 2

0.150000

0.200000

0.250000

0.300000

0.350000

0.400000

0.450000

0 20 40 60 80 100 120 140 160 180 200

Depth (mm)

20

7P

b/2

06

Pb

0.150000

0.200000

0.250000

0.300000

0.350000

0.400000

0.450000

0 20 40 60 80 100 120 140 160 180 200

Depth (mm)

207P

b/2

06P

b

SummarySummary

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

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

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 2Summary 2

We have demonstrated that accurate and precise Pb isotopic ratios can be determined from less than 1pg total Pb in 25m 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 10m resolution.

We have demonstrated that accurate and precise Pb isotopic ratios can be determined from less than 1pg total Pb in 25m 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 10m resolution.