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An Introduction to U-Pb Geochronology (from a SHRIMPer’s point of view) Ian S. Williams

EURISPET 2008. An Introduction to U-Pb Geochronology (from a SHRIMPer’s point of view) Ian S. Williams Research School of Earth Sciences. Decay constant. Time. Parent atoms. Daughter atoms. Geochronology: The concept. Radioactive parent. P. D/P = e l t - 1. l. Stable daughter.

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An Introduction to U-Pb Geochronology (from a SHRIMPer’s point of view) Ian S. Williams

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  1. EURISPET 2008 An Introduction to U-Pb Geochronology (from a SHRIMPer’s point of view) Ian S. Williams Research School of Earth Sciences

  2. Decay constant Time Parent atoms Daughter atoms Geochronology: The concept Radioactive parent P D/P = elt - 1 l Stable daughter D

  3. ln(D/P + 1) t = l Geochronology: The concept Radioactive parent P l Stable daughter D

  4. ln(2) T1/2 = l Geochronology: The concept Radioactive parent After one half life D = P P l Stable daughter D

  5. Geochronology in practice Several natural radioisotopes have half lives suitable for geochronology Half life 40K 40Ar (and 40Ca) 1.25 Ga 87Rb 87Sr 48.8 Ga 238U 206Pb 4.47 Ga 235U 207Pb 704 Ma 147Sm 143Nd 106 Ga

  6. Geochronology in practice Some initial assumptions • No daughter isotope was present in the system to start with, or if some was present, the amount can be measured. • The system has remained closed. No parent or daughter isotopes have been added from external sources, no parent or daughter isotopes have been lost.

  7. Geochronology in practice K-Ar and Rb-Sr mineral ages • High abundance host minerals (e.g micas) • High radioactive parent content • Low initial daughter content • Reasonably simple sample preparation • Reasonably simple analysis • Good analytical precision (~0.5%)

  8. Closure temperatures Minerals only retain radiogenic daughter products below their closure temperatures

  9. Closure temperatures Approximate closure temperatures of commonly dated minerals Hornblende K-Ar500˚C Muscovite Rb-Sr 500˚C Muscovite K-Ar 400˚C Biotite K-Ar350˚C Biotite Rb-Sr 350˚C

  10. Rb-Sr whole-rock analysis Concept Although individual minerals might ‘leak’, the whole rock remains closed

  11. Rb-Sr whole-rock analysis The Isochron Isochrons give the age, initial isotopic composition and a test that the system has remained closed Slope = et - 1 Mineral 3 Mineral 2 Whole rock 3 Whole rock 2 87Sr/86Sr Whole rock 1 Mineral 1 Initial 87Sr/86Sr t = 0 87Rb/86Sr

  12. p p n n 39K 39K 39K 39K 39K 39K 39K 39Ar 39Ar The 40Ar-39Ar technique 39K is converted to 39Ar by fast neutron irradiation. A neutron is captured and a proton is lost. 39Ar becomes a proxy for K.

  13. Plateau age The 40Ar-39Ar technique 40Ar 39Ar Age Ar is released from progressively more retentive domains by step heating 100 0 Percent gas release

  14. The U-Pb technique Half life U-Pb is a paired decay scheme: Two isotopes of U decay to two isotopes of Pb at different rates 238U206Pb4.47 Ga 235U207Pb704 Ma

  15. The U-Pb technique The decays take place via many intermediate radioactive daughter products

  16. or mixing The U-Pb technique The Wetherill concordia 206Pb/238U 4000 To common Pb 3500 Loss of radiogenic daughter can be detected as discordance Concordant 3000 2500 Isotopic disturbance 2000 Recent Pb loss 207Pb/235U

  17. The U-Pb technique The Tera-Wasserburg concordia 207Pb/206Pb To common Pb Loss of radiogenic daughter can be detected as discordance 4000 Concordant Recent Pb loss 3500 3000 Isotopic disturbance 2500 2000 1500 238U/206Pb

  18. The U-Pb technique Mineral U-Pb geochronology, pros and cons • High radioactive parent content • Low initial daughter content • High closure temperatures (e.g. >900˚C) • Isotope Dilution gives exceptional analytical precision (~0.02%) • Loss of radiogenic daughter is detectable • Low abundance host minerals (e.g. zircon, monazite) • Difficult chemistry • Difficult mass spectrometry

  19. The U-Pb technique ID-TIMS Duluth Anorthosite zircon 206Pb/238U Isotope Dilution Thermal Ionisation Mass Spectrometry is extremely precise 207Pb/235U Paces & Miller, 1993

  20. The U-Pb technique ID-TIMS Duluth Anorthosite zircon 206Pb/238U Isotope Dilution Thermal Ionisation Mass Spectrometry is extremely precise 207Pb/235U Paces & Miller, 1993

  21. The U-Pb technique ID-TIMS Duluth Anorthosite zircon 206Pb/238U Isotope Dilution Thermal Ionisation Mass Spectrometry is extremely precise 207Pb/206Pb age 1099.01 ± 0.58 Ma 207Pb/235U Paces & Miller, 1993

  22. The U-Pb technique ID-TIMS Duluth Anorthosite zircon 206Pb/238U Isotope Dilution Thermal Ionisation Mass Spectrometry is extremely precise Concordia age 1099.24 ± 0.26 Ma 207Pb/235U Paces & Miller, 1993

  23. The U-Pb technique ID-TIMS Duluth Anorthosite zircon 206Pb/238U The accuracy of isotope dilution analyses is now limited mainly by uncertainty in the decay constants 207Pb/206Pb age 1099.0 ± 5.1 Ma Concordia age 1099.8 ± 0.7 Ma 207Pb/235U Paces & Miller, 1993

  24. The U-Pb technique ID-TIMS Mundil et al., 2003, Triassic platform carbonates, northern Italy Age differences can nevertheless be measured with high precision

  25. The U-Pb technique Micro-analysis Zircon Accurate age measurements on complex crystals requires micro-sampling 50 µm Transmitted light, crossed polarisers

  26. Laser Laser ablation ICP-MS

  27. Laser ablation ICP-MS ArF Excimer (193 nm) laser Lasers allow high precision micro-sampling, ~100 ng per analysis 100 µm

  28. Secondary Ion Mass Spectrometry SIMS Ion microprobes sample on an even smaller scale, ~2 ng per analysis

  29. Secondary Ion Mass Spectrometry How does an ion microprobe work? Chemical and isotopic analyses Chemical analyses

  30. Secondary Ion Mass Spectrometry How does an ion microprobe work? High-energy primary ions bombard the target surface

  31. Secondary Ion Mass Spectrometry How does an ion microprobe work? Secondary ions and neutrals of atoms and molecules are ejected

  32. Secondary Ion Mass Spectrometry How does an ion microprobe work? Secondary ions and neutrals of atoms and molecules are ejected

  33. How does an ion microprobe work? Energy analyser Magnet 1 METRE Primary ion source The sputtered secondary ions are analysed by a large, high-resolution mass spectrometer Ion counter Sample

  34. Primary Ion source Energy analyser Magnet Ion Counters Sample chamber How does an ion microprobe work?

  35. Why is the SHRIMP so big? The secondary ion spectrum is complex, even from relatively simple minerals

  36. Why is the SHRIMP so big? Pb in zircon 206Pb The mass spectrometer needs high mass resolution to resolve the small mass differences between atoms and molecules HfSi HfSi HfO2 207Pb HfO2 208Pb Zr2O Zr2O HfSi

  37. Electrostatic Analyser (energy) Quadrupole Magnet (momentum) Why is the SHRIMP so big? The double-focusing mass spectrometer separates the ions by energy and momentum Sample

  38. SHRIMP U-Pb dating in practice Sample 1 ln Pb+/U+ SHRIMP Pb/U measurements must be calibrated against natural mineral standards Sample 2 Sample 206Pb+/238U+ Standard analyses Standard for sample 2 Standard for sample 1 ln UO+/U+

  39. SHRIMP U-Pb dating in practice Pb/U ages can be measured with better than 1% precision ln Pb/Ustd ln Pb/Uunk ln UO/U

  40. SHRIMP U-Pb dating in practice Paterson Volcanics Individual analyses are less precise than ID-TIMS analyses, but they are equally accurate ID-TIMS 328 ± 2 Ma SHRIMP 329 ± 4 Ma Claoué-Long et al., 1995

  41. SHRIMP U-Pb applications Mineral ages of igneous rocks Zircon, monazite, baddeleyite, titanite, perovskite, allanite

  42. 3995 3984 3532 3600 3591 SHRIMP U-Pb applications Zircon ages of high grade orthogneiss protoliths 100 µm

  43. SHRIMP U-Pb applications The Acasta Gneiss, Canada, the oldest-known rock in the world Bowring & Williams, 1999

  44. SHRIMP U-Pb applications Cooma biotite schist Provenance of sedimentary rocks Zircon, monazite, titanite, rutile

  45. 0.5% 1% 1% 2% 2% Illustrating large data sets The relative probability histogram Allocate each measurement a Gaussian curve of unit area reflecting its uncertainties Relative probability 1%

  46. Illustrating large data sets The relative probability histogram Sum the curves to show the relative probability of the various ages Relative probability

  47. Illustrating large data sets The relative probability histogram The histogram displays the probable age distribution, considering the uncertainties

  48. SHRIMP U-Pb applications Neoproterozoic metasediments in Scotland Tectonic history and correlation of sedimentary rocks MOINE DALRADIAN

  49. SHRIMP U-Pb applications Deposition ages from diagenetic overgrowths Xenotime, monazite

  50. SHRIMP U-Pb applications Jillamatong S-type granodiorite Ages of the components in magmas from inherited zircon cores Cathodoluminescence

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