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Accuracy?

Accuracy?. Accuracy?. Possibilities: 1) Geochemical 2) Analytical. - beam damage What are we doing to these things?. - background Unexpected interferences? Details of background shape?. - other factors. How would the probe see discordancy resulting from Pb loss?.

kendall-savage
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Accuracy?

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  1. Accuracy?

  2. Accuracy?

  3. Possibilities: 1) Geochemical 2) Analytical - beam damage What are we doing to these things? - background Unexpected interferences? Details of background shape? - other factors

  4. How would the probe see discordancy resulting from Pb loss?

  5. Let’s assess some of the details of X-ray counting and analysis...

  6. Voltage Issues: spatial resolution absorption corrections Excitation potentials and interferences

  7. X-ray counting follows Poisson statistics, at high count rates:

  8. High Current Beam Effects Charging Slight deflection off normal incidence • Reduction of electron potential at specimen Distortion of excitation volume and (Z) shapes

  9. High Current Beam Effects Specimen damage? Bright spots after analysis • Potential thermal effects • T = 4.8Eoi / kd • Eo =accelerating potential • i = beam current • k = thermal conductivity • d = beam diameter • at 200nA, 1 um beam diameter, 15 kV • k ~ 6-10 E –3 W cm-1 K-1 T = 1500 – 3000 K uncoated

  10. 200 nA, 40 min. Electron Dose Damage effects? [LGG246-5m1 C-coat]

  11. Absorbed Current (200nA 30 min. C-coat)

  12. Changes in major element composition of monazite due to electron exposure (200 nA, 40 min.)

  13. Changes in major element composition of monazite due to electron exposure (200 nA, 40 min.)

  14. Changes in major element composition of monazite due to electron exposure (200 nA, 40 min.)

  15. Absorbed Current (200nA 30 min. Au-coat)

  16. 200 nA, 40 min. Electron Dose Damage effects? [LGG246-5m1 Au-coat]

  17. Changes in major element composition of monazite due to electron exposure (200 nA, 40 min.)

  18. Effect of beam exposure on Th / U

  19. Backgrounds and Interferences Some we know well from the tables: Th Mb on U Ma Y Lg on Pb Ma Choose analytical lines and background points based on tables and WDS scans

  20. A look at potential overlaps and background interferences - LGG 246-5 m1 Scan pt.

  21. A look at potential overlaps and background interferences

  22. A look at potential overlaps and background interferences

  23. A look at potential overlaps and background interferences

  24. Choosing background points

  25. Choosing background points

  26. Choosing background points Background interferences very difficult - must go to differential mode!

  27. Choosing background points

  28. REE Choosing background points Ce La1,2 (2) La La1 (2) La Lb1 (2)

  29. Choosing background points Th Mg U Mb Monazite from pegmatite associated with ~ 1400 Ma reactivation event in NM No interference correction for Th on U Mb = 1342 +/- 5 Ma Interference corrected = 1358 +/- 5 Ma

  30. Back to LGG 246-5 (Grand Canyon) Ce La1,2 (2) La La1 (2) La Lb1 (2) Isotopic Age ~ 1685 Ma Integral mode: 1581 +/- 3 Charging??

  31. Au Coat

  32. Au coat Ce La1,2 (2) La La1 (2) La Lb1 (2)

  33. Au coat Isotopic Age ~ 1685 Ma Integral (C) Differential (Au) 1586 +/- 3 Ma 1642 +/- 5 Ma ppm bkg (c/s/nA) ppm bkg (c/s/nA) Th 74848 +/- 2324 0.443 70862 +/- 193 0.379 U 2844 +/- 52 1.100 3259 +/- 51 1.0109 Pb 6252 +/- 176 0.285 6146 +/- 196 0.232 With U corrected for Th overlap: U  2976 +/- 39 ppm Age  1658 +/- 5 Ma Th Mg U Mb

  34. Background curvature…. We know the Brehmsstrahlung will have a natural curvature - expressed by modified Kramers’ Law: NEE = kEZ[(Eo -E)/E]E NE = # photons from E to E+ E Eo = acc. potential Z = ave. atomic number kE = Kramers’ constant The actual background intensity as a function of wavelength in WDS is, however, highly dependent on spectrometer efficiency...

  35. Use WDS scans to obtain background Regress selected background regions using either Polynomial or exponential models

  36. LGG 246-5m1 With U corrected for Th overlap: U  2587 +/- 141 ppm Age  1692 +/- 2 Ma (high Th core) Average for entire grain: 1686 +/- 3 Ma Isotopic Age ~ 1685 Ma Integral (C) Scanned - Differential (Au) 1586 +/- 3 Ma 1672 +/- 2 Ma ppm bkg (c/s/nA) ppm bkg (c/s/nA) Th 74848 +/- 2324 0.443 74678 +/- 238 0.365 U 2844 +/- 52 1.100 2885 +/- 142 1.006 Pb 6252 +/- 176 0.285 6606 +/- 229 0.206

  37. LGG 245 m3 - what if we go back to one of the ones that initially looked good? Old system (2 pt. backgrounds): Age = 1683 +/- 7 Ma New system (scanned backgrounds - overlap correction): Age  1688 +/- 4 Ma

  38. Summary for Wards Monazite Scanned backgrounds and Th corrected: 1399 +/- 4 Ma Th Mg U Mb Monazite from pegmatite associated with ~ 1400 Ma reactivation event in NM No interference correction for Th on U Mb = 1342 +/- 5 Ma Interference corrected = 1358 +/- 5 Ma

  39. Black Hills - PR-1 SHRIMP II 1761 +/- 11 Ma 1716 +/- 12 Ma (overgrowths) EMPA - three grains 1774 +/- 5 Ma 1760 +/- 16 Ma 1787 +/- 9 Ma 1691 +/- 6 Ma (overgrowths)

  40. Overlaps…continued Th Mg K Ka1 REE L-energies = 4.7-7.9 KeV Excitation potential K (K-shell) = 3.6 KeV

  41. Procedure • Full thin-section map to find monazite (C-coated) • Select grains for detailed mapping based on texture, etc. • Map individual monazite grains for Y, Th, Pb, and U • Identify compositional domains and obtain major element compositions • Apply Au-coat • Run background scans in each domain (200nA, 15 kV) • Pick background regions in each scan and regress - usually exponential • Enter background intensities into trace program with appropriate major element compositions • Analyze trace elements (Y, Th, Pb, U - 200nA, 15kV, 600-900 sec.) A number of analyses should be obtained from each domain • Calculate ages for points in a domain and apply standard error of the mean to estimate the precision

  42. Other considerations: • Effect of major element compositional variation • Choice of Matrix corrections and physical constants • Assessment of Pb diffusion - closure temperature • What role do fluids play in monazite reactions?

  43. Sources of Error • Counting statistics • C = counts • R = count rate • T = count time • For the count rate:

  44. Calibration…. If the count times are equal for peak and background… And the combined relative error for unknown and standard becomes… Calibrate on high concentration standards.

  45. Other sources of error: • Compositional domain boundary effects • Grain edge difficulties (including fluorescence and polishing artifacts!) • Distortion of excitation volume due to induced potentials • Accuracy of background fluorescence corrections • Detector dead time constants • Discrepancies between high and low current measurement • Counter tube charge build-up • Internal counter pressure changes

  46. Standard error of the mean Quantifying error: • Propagation of count statistics through age equation • Monte - Carlo error estimation

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