Apatite helium dating
In its simplest form, assuming identical laser ablation rates in the standard and the unknown, the method works as follows: In practice, the standard and the unknown are combined on a pairwise basis.
This is done by calculating the normalised U, Th, and He concentrations of the standard from its known age and measured Th/U ratio (Figure 1.a), and then using this normalised composition as a benchmark against which to compare the U, Th, and He signals of the unknown (Figure 1.b,c).
It suffices that two elemental ratios are known, such as U/He and Th/He, or U/Th and U/He.
This insight forms the basis of the simplified method, which does not require knowledge of any absolute abundances or concentrations, but instead uses the raw mass spectrometer measurements.
To correct for instrument drift and plasma-induced fluctuations in the ICP-MS, the U and Th signals should be measured relative to stoichiometric Si.
Second, the process of in-situ measurements of U and Th content of grains yields U-Th-Pb ages as a by-product.From 1987 until 2000, helium was measured by bulk degassing of multi-grain aliquots in a resistance furnace, and measuring the U-Th content on either the same or a different aliquot by XRF or TIMS (Zeitler et al., 1987; Lippolt et al., 1994; Wolf et al., 1996).In 2000, a method was developed to degas individual mineral grains by means of laser-heating in Pt or Nb ‘micro-furnaces’, followed by acid dissolution and U-Th analysis by isotope dilution in an ICP-MS (House et al., 2000).In order to qualify as a good U-Th-He age standard, a sample must fulfill the following requirements.