Improving TIMS monazite geochronology (?)

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

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

6 samples analyzed in study

Broad range in age and composition

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

Jefferson County

Jefferson County

Annealed;Pre-etch

Annealed;Post-etch

Amelia

Not-annealed;etched

Amelia

Annealed;etched

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

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

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

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

a b c d e f g h i j k l m n0

10

20

30

40

50

60

70

80

90

100

Dissolution Rates

Jeff N

SmithN

Burke N

JeffA

SmithA

BurkeA

Dissolution steps

Am

ou

nt

dis

solv

ed

(%

)

Amelia N

Amelia A

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

Example age diagramImprecise; ~10% of mnz

Shaded = included

in calc’ed age

Not shaded = not included

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)

4 selected ratios

Reflect changes in compositional domains (as per EPMA data)

Amelia – ages

Annealed vs. not-annealed

Burke – ages

Annealed vs. not-annealed

*note change in scale on axes

Jefferson County – ages

Annealed vs. not-annealed

*note change in scale on axes

Major findings

(and how it may apply to your research)

Annealing repairs dislocations and lattice damage induced by alpha-recoil and fission.

Results in slower dissolution rate

Not-annealed samples ( n = 3):heterogeneity in first step,

largely homogeneous chemical spectraAmelia Burke Jefferson County

Annealed samples (n = 6): complicated compositional spectra

These are the pairs to the not-annealed aliquots

Amelia Burke Jefferson County

3 additional samples

These samples do not have not-annealed aliquots

Elk Mountain Madagascar 554

Annealing affects chemistry… and age?

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