Typical chronometers. LA-ICPMS and SIMS challenges from ~2007-2009.

Typical chronometers

LA-ICPMS and SIMS challenges from ~2007-2009

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

massanalyzer detectorHe in

(carrier gas)

Ar in (mix gas)la

ser

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 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

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 P2O5 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

U-Pb ≠ Th-Pb in many instances