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Typical chronometers

Typical chronometers. LA-ICPMS and SIMS challenges from ~2007-2009. Fundamentals. Use photons or ions to drill a hole into a sample All material from that hole is ionized and dumped into the mass spectrometer

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Typical chronometers

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  1. Typical chronometers

  2. LA-ICPMS and SIMS challenges from ~2007-2009

  3. Fundamentals • Use photons or ions to drill a hole into a sample • All material from that hole is ionized and dumped into the mass spectrometer • Mass spectrometers employ a magnet (and energy filters and focusing mechanisms) to direct ions based on mass/charge ratio into detector • Measure signal intensity • Compare to reference material • Make corrections • Calculate age

  4. LA-ICPMS schematics

  5. Pit depths (0.25 μm to 10 μm)

  6. Laser Ablation systems laser Ar in (mix gas) mass analyzer detector He in (carrier gas) ion source Sample Cell

  7. SIMS instruments

  8. Pink = ion beam sourcered, green, blue = material you are analyzing

  9. Advantages • Rapid acquisition • ~20 sec drilling typical • ~40 analyses per hour • Maintain spatial resolution • Typical pit size used to be 20 to 50 μm • Now can be much smaller (7 μm) • Measure isotopic ratios to calculate date • Evaluate concordance (or disruption) of the system • Can be measured in situ

  10. Disadvantages • Must calibrate to reference material • Many age domains smaller than 20 (or 7) μm • Matrix is also dumped into the mass spectrometer, not just the targeted elements • Interferences can be significant • Most can be corrected for

  11. The problem:

  12. LA-ICPMS data

  13. Experimental set-up

  14. Something within monazite apparently affects the measurement of Pb

  15. Dissolved monazite mass scans

  16. Cannot accurately measure Pb isotopes in monazite with Isoprobe Some component(s) of monazite produce(s) aberrant measurements Cannot uniquely determine component(s) Likely a P2O5 species What’s happening?

  17. LASS: Laser ablation split stream (John Cottle will happily discuss this tomorrow)

  18. SIMS

  19. Cameca ims1270 Th-Pb ages measured over 5 sessions Inconsistently accurate data that are precise! (You won’t know when you have accurate data)

  20. Tried measuring at energy offsets to minimize matrix effects No offset position worked for all samples measured. Best measurements inaccurate by 3–4%, or have ± 8% precision

  21. SHRIMP-RG

  22. Things to consider… • Matrix match with reference material (RM) • Age match with RM • Compositional domains vs. age domains • Are they the same? • Sometimes yes, sometimes no • Growth of compositional domains can be too fast to record age difference • Using U-Pb of RM to correct Th-Pb • John Cottle and I are measuring ID Th-Pb for a suite of monazite and allaniteRMs

  23. U-Pb ≠ Th-Pb in many instances

  24. U-Pb ≠ Th-Pb in many instances

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