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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. 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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  1. High Precision Pb Isotope Ratio Measurements of Zircon Zenon Palacz, Simon Meffan-Main, Patrick Turner, & Fadi Abou-Shakra

  2. Introduction Lead isotope ratios conventionally determined in Zircons by TIMS/SHRIMP MC-ICPMS compliments conventional methods and expands geochronological horizons.

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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.

  9. 50mm pit in zircon during ablation

  10. 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)

  11. 206Pb (volts) down hole. 206Pb Intensity v Time 2.5 2 1.5 206Pb Intensity 1 0.5 0 0 20 40 60 Seconds

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. ZIRCON 91500 • Age 1062+/-0.4Myr (TIMS) • 12ppm Pb • 207Pb/206Pb 0.07488+/-0.00001 TIMS

  18. 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

  19. 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

  20. 25 & 50mm pits in 91500 zircon

  21. 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.

  22. 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.

  23. Zircon prior to ablation 100mm

  24. 3-D mapping of isotopic ratios using 50mm pits in zircon

  25. 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.

  26. 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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