Improving TIMS monazite geochronology (?)

1. Improving TIMS monazite geochronology (?)

2. Why TIMS? • High-precision • typically better than 0.2% • Benchmark ages of reference materials • LA-ICPMS • SIMS • EPMA • No matrix correction / background calibration • No need to assume concordance

3. Challenges with TIMS • Minimal spatial resolution • However, studies have successfully: • Broken off tips (Schärer & Allègre, 1982) • Used X-ray maps to guide micro-drilling of compositional domains (Corrie & Kohn, 2007) • Weeks of analysis time • Currently, no widely available Th–Pb spike* • Therefore only U–Pb (and Pb–Pb) ages • *Cottle and Peterman are currently preparing a calibrated U–Th–Pb spike specifically for monazite

4. 6 samples analyzed in study

5. Broad range in age and composition

6. Method development • Iterative process • Typical single step dissolution accomplished at 180°C in 12M HCl for 24 hours • Our variables: • Acid strength (HCl) 12M, 6M and 3.1M • Initial T at 120, 100 and 80°C • Dissolution times of 12 and 6 hours

8. Jefferson County Jefferson County Annealed; Pre-etch Annealed; Post-etch Amelia Amelia Not-annealed; etched Annealed; etched

9. Conditions used for analysis • Parallel digestion of annealed (1000°C, 48 hours in air) with not-annealed grains (or fragments) • Starting T: 80°C • Acid: 3.1M HCl • Duration of step: initially 12 hours • After 5 steps, reduced time to 6 hours and started increased T by 10°C (to ensure sufficient dissolution to measure precisely via TIMS) • Geochemist rule of thumb—increase of 10°C nearly equivalent to doubling the step time

10. Why 1000°C, 48 hours in air? Experiments demonstrate structural recovery if annealed in air. Recrystallization occurs if annealing is fluid-mediated.

11. Structural recovery of monazite accomplished by simple heating; “defects completely disappear”

12. Conditions used for analysis • Parallel digestion of annealed (1000°C, 48 hours in air) with not-annealed grains (or fragments) • Starting T: 80°C • Acid: 3.1M HCl • Duration of step: initially 12 hours • After 5 steps, reduced time to 6 hours and started increased T by 10°C (to ensure sufficient dissolution to measure precisely via TIMS) • Geochemist rule of thumb—increase of 10°C nearly equivalent to doubling the step time

13. Amelia N Amelia A

14. Age data presented • 238U–206Pb age spectra • 235U–207Pb, where appropriate • Plotted following 40Ar/39Ar convention • X-axis = percent of sample • Y-axis = age • Height of box = uncertainty

15. Example age diagram Not shaded = not included Imprecise; ~10% of mnz Shaded = included in calc’ed age

16. Compositional data presented • Data collected from column elutions via ICPMS with internal and external standards • Also follows 40Ar/39Ar convention • X-axis = percent of sample • Y-axis = compositional ratio • Used blank-corrected ratios (normalized to 31P) because each step dissolved different amounts of monazite (concentration therefore not useful)

17. 4 selected ratios Reflect changes in compositional domains (as per EPMA data)

18. Amelia – ages

20. Annealed vs. not-annealed

21. Burke – ages

23. Annealed vs. not-annealed • *note change in scale on axes

24. Jefferson County – ages

26. Annealed vs. not-annealed • *note change in scale on axes

27. Major findings (and how it may apply to your research)

28. Annealing repairs dislocations and lattice damage induced by alpha-recoil and fission. Results in slower dissolution rate

29. Not-annealed samples ( n = 3):heterogeneity in first step, largely homogeneous chemical spectra Amelia Burke Jefferson County

30. Annealed samples (n = 6): complicated compositional spectra Amelia Burke Jefferson County These are the pairs to the not-annealed aliquots

31. 3 additional samples Elk Mountain Madagascar 554 These samples do not have not-annealed aliquots

32. Annealing affects chemistry… and age?

33. Suggestions for your research, should you choose to use TIMS • Don’t anneal • Apparently induces recrystallization • Use multi-step TIMS or use x-ray maps to guide microdrilling • Single step TIMS will yield a precise age that may be quite inaccurate