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jeffrey-mcgee
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Fundamentals
• Use photons or ions to drill a hole into a sample• All material from that hole is ionized and dumped
into the mass spectrometer• Mass spectrometers employ a magnet (and energy
filters and focusing mechanisms) to direct ions based on mass/charge ratio into detector
• Measure signal intensity • Compare to reference material• Make corrections• Calculate age
Laser Ablation systems
massanalyzer detectorHe in
(carrier gas)
Ar in (mix gas)la
ser
Sample Cell
ion source
Advantages
• Rapid acquisition– ~20 sec drilling typical– ~40 analyses per hour
• Maintain spatial resolution – Typical pit size used to be 20 to 50 μm – Now can be much smaller (7 μm)
• Measure isotopic ratios to calculate date– Evaluate concordance (or disruption) of the system
• Can be measured in situ
Disadvantages
• Must calibrate to reference material• Many age domains smaller than 20 (or 7) μm• Matrix is also dumped into the mass
spectrometer, not just the targeted elements• Interferences can be significant – Most can be corrected for
LA-ICPMS dataSample name
204 206/204
Arendal 0.00114 97.1
Burke 0.00128 97.6
Elk Mountain 0.00097 241.6
Jefferson County
0.00108 143.3
Madagascar 0.00112 88.8
Smithsonian 0.00081 8.3
CePO4 0.00104 1.7
What’s happening?
• Cannot accurately measure Pb isotopes in monazite with Isoprobe
• Some component(s) of monazite produce(s) aberrant measurements– Cannot uniquely determine component(s)– Likely a P2O5 species
Cameca ims1270 Th-Pb ages measured over 5 sessions
Inconsistently accurate data that are precise! (You won’t know when you have accurate data)
No offset position worked for all samples measured. Best measurements inaccurate by 3–4%, or have ± 8% precision
Tried measuring at energy offsets to minimize matrix effects
Things to consider…
• Matrix match with reference material (RM)• Age match with RM• Compositional domains vs. age domains– Are they the same?– Sometimes yes, sometimes no– Growth of compositional domains can be too fast
to record age difference• Using U-Pb of RM to correct Th-Pb– John Cottle and I are measuring ID Th-Pb for a
suite of monazite and allanite RMs