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Thermochronology, Lecture 5

Thermochronology, Lecture 5. Mass spectrometry , Part 1 TIMS Part 2 - ICP etc. Homework 4 today. Two prior questions. Accurate measurements??? Is the 235 U/ 238 U constant through time? At one time? Why?. Standards. Analyzed every day - provide a test of accuracy

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Thermochronology, Lecture 5

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  1. Thermochronology, Lecture 5 Mass spectrometry , Part 1 TIMS Part 2 - ICP etc

  2. Homework 4 today

  3. Two prior questions • Accurate measurements??? • Is the 235U/238U constant through time? At one time? Why?

  4. Standards • Analyzed every day - provide a test of accuracy • The ultimate accuracy of the standard numbers are arguable - thus making absolute ages (among other things) prone to systematic errors. • Systematic errors do not hamper the validity of data in a relative way.

  5. U-Pb • Decay route                 t1/2, Byr                      Decay const., yr-1 • 238U  -  206Pb                          4.47                           1.55125 H 10-10 • 235U  -  207Pb                          0.704                         9.8485  H 10-10 • 232Th -  208Pb                        14.01                          0.49475 H 10-10

  6. Equations Do not need parent/daughter ratios

  7. 235U/238U does change through time • You will calculate how. (homework 4 Tuesday); • Is there any place on Earth that will have a different 235U/238U????

  8. Nuclear reactors • Self sustaining but non-exlosive chain reaction, usually started by fission of 235U when struck by a neutron; • Use U artificially enriched in 235 by 3% compared to natural ratios

  9. Natural reactors?

  10. Oklo- A natural reactor • Strange as it may sound, there is a natural reactor; • Parts of the U deposit called Oklo in Gabon, Africa; • The key observation made in the early 70’s is that parts of this reservoir have significantly higher than present day 238/235 ratios !!!!! • The 235 depletion hypothesized to have been due to fission reactions. • Other isotopic anomalies around Oklo provide further evidence for the fossil reactor interpretation.

  11. Interpretations • Parts of the deposit self -started nuclear reactions similar to the ones in man-made facilities! • How long did it burn for? Based on reservoir volumes etc, they could have been active for as much as 1 my each! • When did this happen? How do we get that info?

  12. Age of nuclear reactions @ Oklo • U-Pb geochron on non-modified U-rich materials yield 1.7-8 Ga = age of deposit formation • Age of seds poorly know but ~ 1.8 Ga; • Self ignition could not have taken place later than some 1.5 Ga, due to constraints in global 238/235. • Moderated by presence of water • Probably soon after deposit formation. • How can one best determine the timing?

  13. Other similar reactors might exist in old large U deposits (Wittwatersrand, etc) but have not yet been identified

  14. MASS SPEC TECHNIQUES A very brief intro.. TIMS, SIMS, SHRIMP, ICP-MS MCICPMS, …. LA-MCICPMS….?? What are these?

  15. Mass spectrometry • A brief guide through how do we get isotopic analyses • What is being analyzed? • How is it analyzed? • How do we ensure that we get the correct answers? • What are the differences and similarities between all these tools that you may have heard of: TIMS, SIMS, ICP, MC-ICP etc?

  16. The clean labs

  17. Today: TIMS • Stands for thermal ionization mass spectrometry and is by far the most used tool in heavy isotope measurements • A solid sample is being uploaded onto a filament (Re, Ta) and heated up to over 1000 to 2000 0C at which T the molecules ionize in the mass spec; • The sample is being accelerated into a magnet equiped flight tube (which deflects various isotopes) and analyzed in “collectors” or cups at the end of the flight.

  18. Chemical separation

  19. Loading Single or triple filaments, various tricks to prevent quick sample burnout, oxidation, etc

  20. What is a mass spec

  21. Ions are counted in several collectors

  22. Analysis protocol • Collect ratios; • Optimize for the radiogenic isotopic ratios • Collect isotopic ratios that can signal interferences • Collect stable isotopic ratios that can be used for fractionation correction - what is that?

  23. Isotopic fractionation • Because the potential energy well of the bond involving the lighter isotope is always shallower than for the heavier, the bond with the lighter isotope is more readily broken. Hence it is preferentially released from the hot filament, causing isotopic fractionation.

  24. Fractionation Correction • Luckily there are other isotopes of most elements of interest - stable isotopes that formed only during nucleosynthesis. Two such isotopes should always have the same ratio on Earth; • They, too, will fractionate in the mass spec; the deviation of the measured to true value of this ratio is applied to the ratio of radiogenic isotopes; • This “trick” has improved the accuracies and precisions of isotopic ratios by hundreds of times.

  25. Example - Sr • Four isotopes, 84, 86. 87(from 87Rb) and 88; • We measure 87/86 and do a normalization based on the 86/88 ratio • Nominal 86Sr/88Sr=0.1194

  26. Corrections • Linear • Power law • Exponential Want to learn more bout this? - Wasserburg et al, 1981, pdf available in “library” online

  27. Next.. • New alternatives to TIMS measurements: ICP, Ion microprobe, etc.

  28. What is an ICP-MS? • A mass spec that uses a carrier gas to introduce a sample into the instrument (usually Ar): • Has high mass resolution, much better than tims • Ionization takes place in plasma, resulting in much higher (approaching 100%) ionization efficiencies; • Consequently, the sample needed is much smaller than TIMS • Also the mass res is used to easily avoid interferences even on samples on which chem sep has not been performed.

  29. Measure • An isotope is counted (as ions); • Choose an isotope free of interferences • Measure counts, compare to counts in a well-calibrated standard • Ultimately use the measurement to calculate trace element concentrations.

  30. Isotopic ratios? • Can do but with limited precision • Signal very unstable at second- tens of second time scales; • Serious fractionation effects

  31. Other advantages • Good to excellent mass res • Great ionization efficiency • Can attach laser and input sample through laser ablation, thus eliminating the need to dissolve samples.

  32. A hybrid ICP-TIMS • Attach multi collectors at the end of the tube (typically 10 or more) • Attach a magnet for mass deflections like a standard TIMS instrument has; • Can also use laser ablation for certain applications

  33. Mc-ICP schematics

  34. Overall results • Great tool for most geochronological applications • Faster, cheaper, similar quality data for Nd, Hf, Sr, common Pb isotopes • Provides the ability to do in-situ U-Pb age measurements on Zircons and Th-Pb on monazites

  35. New developments • Other isotopic systems are being explored with this type of instrument, e.g those systems that were hampered by poor ionization in the TIMS; • New laser ablation exploratios; can we do Sr isotopes in-situ? etc

  36. Limitations • Still suffer from large fractionations in the mass spec, not yet understood in terms of their physics • Some applications are prone to memory effects in the mass spec

  37. TIMS_ICP comparison

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