Typical chronometers

LA-ICPMS and SIMS challenges from ~

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

LA-ICPMS schematics

Pit depths (0.25 μm to 10 μm)

Laser Ablation systems mass analyzer detector He in (carrier gas) Ar in (mix gas) laser Sample Cell ion source

SIMS instruments

Pink = ion beam source red, green, blue = material you are analyzing

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

The problem:

LA-ICPMS data Sample name204206/204 Arendal Burke Elk Mountain Jefferson County Madagascar Smithsonian CePO

Experimental set-up

Something within monazite apparently affects the measurement of Pb

Dissolved monazite mass scans

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 P 2 O 5 species

LASS: Laser ablation split stream (John Cottle will happily discuss this tomorrow)

SIMS

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

SHRIMP-RG

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

U-Pb ≠ Th-Pb in many instances