What is 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).
Instead of absolute concentrations, the revised method essentially hinges on correctly determining the Th/U ratio.
First, it dramatically increases sample throughput. Measuring the U and Th content of zircon by isotope dilution requires dissolution in hydrofluoric acid at high temperature and pressure using a Parr bomb for up to 48 hours.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.Because any analytical uncertainty in the age standard propagates into the unknown age, the choice of standard is very important.