Analytical procedures, accuracy, precision and mineral standards

Analytical procedures, accuracy, precision and mineral standards 

Once grains have been selected for analysis and measured for a ejection correction, a (U-Th)/He age determination involves He extraction and measurement, and U and Th measurement on either the same or a separate aliquot. Analytical techniques, especially for the small amounts of He obtained from most samples, have evolved rapidly in the last five years, and so are described in some detail here. 

U and Th are analyzed by isotope dilution inductively-coupled plasma mass spectrometry performed on the same aliquot analyzed for He (Appendix). 

Accuracy and precision. Based on reproducibility of pure standard gases and aqueous standard solutions, the overall analytical precision of He ages determined by this procedure should be about 2% (2o, excluding errors in a-ejection correction) when ages are well above blank levels. Most of this uncertainty arises from the He measurement. In actual practice, we obtain He ages that reproduce to about 6% (2o), demonstrating some natural variability within grain populations. 

Mineral inclusions. In our experience at Caltech the single biggest difficulty in He dating of apatite is the presence of small U-Th-rich inclusions within the dated grains. This difficulty was first noted by Lippolt et al. (1994) and was further described by House et al. (1997). The most common inclusions are zircon and monazite, but we have occasionally encountered xenotime and allanite. We have also observed inclusions of quartz, feldspar, pyrite and graphite( ), but these are unlikely to carry sufficient U and Th to be a problem. 

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