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U-Pb

U-Pb. The Decay. 238 U → 206 Pb λ = 1.551 x 10 -10 a -1 therefore t 1/2 = 4.47 Ga α & β decays 235 U → 207 Pb λ = 9.849 x 10 -10 a -1 therefore t 1/2 = 707 Ma α & β decays 238 U much more abundant than 235 U (137.88x). ln2/ λ. ln2/ λ. What controls the rate?.

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U-Pb

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  1. U-Pb

  2. The Decay • 238U → 206Pb λ= 1.551 x 10-10 a-1 therefore t1/2= 4.47 Ga α & β decays • 235U → 207Pb λ= 9.849 x 10-10 a-1 therefore t1/2= 707 Ma α & β decays • 238U much more abundant than 235U (137.88x) ln2/λ ln2/λ

  3. What controls the rate? Don’t disregard the α particles! What are α particles? Why are they important in geology? U-Th-(He) →~α Tc for zircon ~ 190-170 °C Low-T thermochron www.ead.anl.gov/pub/doc/NaturalDecaySeries.pdf

  4. Forms a very tight crystal structure→once U is in it stays in and Pb* tends to stay in. Excludes common lead during crystalization. 206Pb/204Pb ~1000. Why is that good? Zircon TC U-Pb ~800°C→ What are we dating? Chemically and mechanically resistant. U~100 ppm Why zircon is king ZrSiO4

  5. Why zircon is king • Zr4+ ionic radius 0.80 Å • U4+ ionic radius 0.89 Å • Th4+ ionic radius 0.972 Å • Pb2+ (Pb4+ rare) ionic radius 1.19 Å Th4+, U4+ substitute forZr4+ b/c about the same size w/ HFS and are all highly incompatible. U4+ ZrSiO4 Pb

  6. No more damn decay equations!Ok just a couple. • 207Pb* = 235U (eλ235t-1) 206Pb* = 238U (eλ238t-1) Rearrange… • 207Pb*/235U = (eλ235t-1) 206Pb*/238U = (eλ238t-1) Then start plugging in for t

  7. U-Pb concordia method of dating 206/207 isotope composition

  8. Ages based on concentration of 206/ conc. of 238 isotopic composition of 206/207 Don’t measure 207/235 b/c way more 238 than 235. So: →207/235 = [(206/238)/(206/207)]x137.88 Age is ‘concordant’ if both (238U→206Pb & 235U→207Pb) yield the same age Actual Dating is a little more tricky! 206/207

  9. Common Lead • All minerals incorporate some initial Pb as they form. • The problem…if 204Pb is getting to the crystal then some 206Pb and 207Pb in getting in as well. Why? • Should we just pack up and go home? U4+ * ZrSiO4

  10. Common Lead Correction Strategy involves using a 2-stage model to infer the composition of common Pb when the mineral crystallized (Stacey&Kramers, 1975) 3.7 Ga

  11. Common Lead Correction • Range of uncertainty= 206Pb/204Pb ~ 18.60-18.80 207Pb/204Pb ~ 15.60-15.67 • It is necessary to assign an uncertainty to the assumed common Pb composition. ±1.0 for 206/204 ±0.3 for 207/204 • The effect of these uncertainties depends on the proportion of common Pb relative to radiogenic Pb in the crystal. The more common Pb present, the greater the effect of the uncertainty in the common Pb composition.

  12. Common Pb composition: Effect of error depends on the amount of common Pb relative to radiogenic Pb (measured as 206/204)

  13. 206sample = 206radiogenic + 206 initial 206r = 206s – 206i 206r/204 = 206s/204 – 206i/204 (206/204)r = (206/204)s – (206/204)i (207/204)r = (207/204)s – (207/204)i The correction for common Pb is simple once a composition is assumed

  14. Discordant ages: Lead Loss • Although Zircons form a tight crystal structure Pb can be removed • Pb w/ a 2+ charge (rather than 4+ as U and Th) and a larger ionic radius can be more mobile • Pb can be removed by diffusion or fluids U4+ Pb

  15. Lead loss Which way will they move? Smaller grains lose higher % of Pb than larger grains (high surface/ volume) Move down 206/207 line. Why? b/c natural chemical process cannot fractionate Pb isotopes

  16. A billion years later… Age of crystallization Age of Pb loss (hydrothermal activity)

  17. Inheritance: young crystals growing around older crystals Granite body intrudes through 2.4 Ga country rock and inherits cores from the older rock while growing new younger rims. How will the grains move? Move up 206/207 “mixing” line Grains with inherited cores tend to be larger

  18. Inherited crystals a billion years later Age of inherited component Age of crystalization

  19. CL

  20. Inheritance and Pb-loss Granite body intrudes through 2.4 Ga country rock and inherits cores from the older rock while growing new younger rims.

  21. Inheritance and Pb-loss Hydrothermal activity→ Pb loss a billion years later. Both intercept ages mean what? Absolutely nothing! Zircons are pulled off the “mixing” line along new 206/207 lines

  22. ID-TIMS: Isotope Dilution-Thermal Ionization Mass Spectrometry Procedure: • Air abrasion of grain, if desired. Why? • Dissolve mineral using ultraclean containers and acids (HF-HNO3) • Chemical separation of U-Th-Pb, from everthing else using ion exchange resins (clean lab) • Load separately on filaments

  23. ID-TIMS • Composition measurements → only need measured ratios, fraction correction, and common Pb correction. • Concentration measurements → isotope ratios, fractionation correction, common Pb correction, and spike information • Adding a spike → naturally occurring, but low abundance isotope (208Pb or 235U at hight purity) or manufactured 205Pb

  24. Fractionation Correction • Fraction occurs on the filament → lighter isotopes ionize more easily then heavy isotopes. • For U, Th, Pb → analyze standards and assume that samples run the same as standards. • Pb → NBS-982 where 208=206 • U → NBS U500 where 235=238

  25. Error Analysis • Errors: systematic and analytical How well do we know the decay constants? 235U = ±0.28% (2σ) 238U = ±0.22% (2σ), but may be off by 0.1% • Isotope ratio measurement for ID-TIMS 206Pb/207Pb can be measured to ± 0.1%, 206Pb/238U can be determined to within ± 0.3% (random errors only)

  26. Error Analysis • Isotope fractionation for ID-TIMS adds 0.1% to each measurement, so 0.1% for 206/207 and 0.2% for 206/238 • Common lead correction • Bottom line for ID-TIMS: best possible precision for 206/207 ages is ~5 Ma whereas 206/238 ages cannot be determined to better than 0.5%.

  27. Cross-over occurs at 1 Ga Very young samples and very old samples have the highest precision in % (and Ma) Plot of errors of 206/238 and 206/207 ages for ID-TIMS (1σ)

  28. Applications other than zircon • Sphene (Titanite) CaTiSiO4: U~20ppm and 206/204 of 100. Uncertainties are ~2x the uncertainties for zircon. Tc ~ 500 °C. Used for ‘cooling’ ages of igneous and metamorphics. • Apatite Ca5(PO4)3: U~20ppm and 206/204 of 100. Uncertainties are ~2x the uncertainties for zircon. Tc ~ 400 °C, applied to retrograde thermochronology of igneous and metamorphic rocks.

  29. Applications other than zircon • Xenotime YPO4: ~U~Th~ 100 ppm and 206/204 of 100. Uncertainties can be similar to zircon. Tc~ 400-500 °C. Recently dated as diagenetic xenotime in sedimentary rocks. DZ sometimes have ~10μm xenotime overgrowths • Monazite CeLaThPO4: Th ~1000-10,000 ppm (U=100ppm). Uncertainties can be better than zircon. Tc~700-750 °C. Metamorphic minerals, that records prograde metamorphism. Just to name a few…

  30. The End Thursday LA-MC-ICP-MS with GG

  31. Fig 1: Stacey&Kramers, 1975

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