Method Development

We have developed a significantly simplified method for in-situ U-Th-He dating removing the need to know any absolute concentrations or ablation pit volumes. This is done by calculating the normalised U, Th, and He concentrations of a conventionally dated calibration standard from its measured Th/U ratio and known U-Th-He age, and scaling these concentrations to the raw U, Th, and He signals of the sample. The Th/U ratio of the standard can be determined from its measured 208Pb/206Pb ratio, removing the need to use NIST glass as a reference material.

Because the U-Th-He age equation is scale invariant, it does not matter if a mineral's U, Th, and He contents are expressed as atomic abundances or concentrations. They can even be renormalised to unity and plotted on a ternary diagram (Vermeesch, 2010). To calculate a helium age, it is not necessary to know the absolute concentrations of U, Th, and He. 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.