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Published byLina Bertrand Modified over 2 years ago

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Improving TIMS monazite geochronology (?)

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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

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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

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6 samples analyzed in study

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Broad range in age and composition

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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

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

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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 thumbincrease of 10°C nearly equivalent to doubling the step time

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

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Structural recovery of monazite accomplished by simple heating; defects completely disappear

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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 thumbincrease of 10°C nearly equivalent to doubling the step time

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Amelia N Amelia A

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

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Example age diagram Imprecise; ~10% of mnz Shaded = included in calced age Not shaded = not included

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

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

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Amelia – ages

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Annealed vs. not-annealed

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Burke – ages

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

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Jefferson County – ages

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

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

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

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Not-annealed samples ( n = 3): heterogeneity in first step, largely homogeneous chemical spectra AmeliaBurkeJefferson County

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Annealed samples (n = 6): complicated compositional spectra These are the pairs to the not-annealed aliquots AmeliaBurkeJefferson County

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3 additional samples These samples do not have not-annealed aliquots Elk MountainMadagascar554

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Annealing affects chemistry… and age?

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Suggestions for your research, should you choose to use TIMS Dont 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

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