Michael Williams Michael Jercinovic

1. Exploring new frontiers in X-Ray microanalysis:  The Ultrachron trace element microprobe and high resolution in-situ geochronology Michael Williams Michael Jercinovic Geosciences

2. ThMα Geochronology – traditionally using isotopic/mass-spectrometric techniques • IDTIMS • Ion Probe Electron Microprobe (EPMA) • High spatial resolution access to ultra-thin rims, micro-domains, and inclusions • In-situ: relate composition (and age) to micro/macro-structure and mineral paragenesis • Non-destructive • Integrated spatial / compositional / age relationships Monazite: LREE-phosphate with Th and U (→ radiogenic Pb) Common accessory phase in many rocks Fabric former Dissolution/re-precipitation reactions result in polygenetic nature, and ties into overall reaction history Dating events{

3. Radiogenic Pb accumulates as a function of Th and U decay constants and time…

5. Schematic model for evolution of felsic granulites in retrograde shear zones. B. Schematic illustration of corresponding monazite growth.

6. Ultrachron performance – analyses of standards Detection limit: 3 ppm Pb, 4 ppm U achieved in 14 pt analysis (15min each)

7. CeB6 New HV power supply Decouple operation of condensers to optimize brightness down column Current regulation up to 1 microamp The Ultrachron Project Electron optics • Optimize analytical resolution (Smaller phase analysis) for a range of kV and current • High, stable current for trace element analysis • Minimize excitation volume in high Z material Detection BSE and X-Ray optics • Improve precision (Optimize counting - PbMα) • Integrate spectrometers • Improve accuracy – background estimation Techniques • Minimize beam damage • Background • Analytical protocols Improve dynamic range of BSE amplifier BSE shielding for high current applications New high intensity crystals (VLPET) + VL detectors Counters optimized (gas mixture, pressure, HV) Completely dry vacuum system Anticontamination

8. VLPET development

10. Improvement in Precision - GSC 8153 monazite SX50

11. GSC 8153 monazite SX-Ultrachron

12. Th M4-N3 Pb M4-O2 Pb Mβ Pb Mα Th Mζ1 Th Mζ2 La Lβ (2) Pr Lα (2) Ce Lα (2) La Lα (2)

16. New VLPET comparison Measured intensity PbMα on pyromorphite sp3 sp4 sp3 sp4

18. GSC 8153 old VLPET (SP3)

19. GSC 8153 new VLPET 10% improvement in pk/bkg

20. Interference corrections Interferences on PbMα old VLPETnew VLPET Predicted* Measured reduced relative CF CF CF intensity (%) ThMζ0.33 0.41 0.36 12 YLγ0.70 0.88 0.827 LaLα0.14 0.18 0.02 89 *Based on increased count rate

21. Measurement issues: Interferences

22. Natural Monazite

23. Interference effects The case of mutual interference of first order lines

24. Th interferences on U-M region

25. Measurement issues: Background Curvature Effect on net intensity of PbMα

26. Pb Mα

27. How to measure background? Pb Mα

28. Natural Monazite

29. GdPO4 Pb region (PET) PbMα

31. PbMα

32. GSC 8153 scan vs GdPO4

33. PbMα

37. Pb concentration vs. net intensity Black Hills, Leg 8

38. Note: The IDTIMS 207Pb/206Pb age for the Elk Mt. monazite is 1394+/-1 Ma (J. Baldwin and S. Bowring, unpublished data). The IDTIMS age for RT87-17 is 394+/-1Ma (Tucker, et al., 1990).

39. Measurement issues: Counting linearity Calibrate at low current, analyze at high current

43. Intensity current

44. Intensity Adjust dead-time (corrected ≠ imposed) current

45. Intensity Adjust linearity (current cut-off specified) current

46. Measurement issues Fluorescence interference REE-L lines will fluoresce K Kα K-feldspar or mica hosted monazite

47. Fluorescence range

48. Interference effects The case of mutual interference of first order lines

49. K fluorescence effect on U concentration

50. K fluorescence effect on apparent age