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.