Technical note: In situ U–Th–He dating by 4He/3He laser microprobe analysis
Abstract. In-situ U–Th–He geochronology is a potentially disruptive technique that combines laser ablation inductively coupled plasma mass spectrometry with laser microprobe noble gas mass spectrometry. Despite its potential to revolutionise (detrital) thermochronology, in-situ U–Th–He dating is not widely used, due to persistent analytical challenges. The main issue is that currently used approaches to in-situ dating require that the U, Th and He measurements are expressed in units of molar concentration, in contrast with conventional methods, which use units of molar abundance. Whereas molar abundances can be reliably determined by isotope dilution, accurate concentration measurements are not so easy to obtain. In the absence of matrix-matched U,Th-concentration standards and accurate He-ablation pit measurements, the accuracy of in-situ U–Th–He dates often leaves much to be desired. This paper proposes a solution to this problem, by using proton-induced 3He as a proxy for ablation pit volume, and by pairing samples with a standard of known U–Th–He age. Thus, the U–Th–He age equation can be solved using relative rather than absolute concentration measurements. Pilot experiments show that the new method produces accurate results. However, it is prone to overdispersion, which is attributed to gradients in the proton fluence. These gradients can be measured and their effect can be removed by fixing the geometry of the sample and the standard during the proton irradiation.
