Technical Note: Reducing age dispersion in in-situ (U-Th)/He dating: How to optimize ablation pit geometries with OptiPit

Abstract. Radiometric dating requires the measurement of related parent and daughter isotopes to produce meaningful ages. The alpha ejection during the decay of uranium and thorium spatially separates the parent and daughter nuclides analysed in (U-Th)/He dating by a few tens of microns within the crystals. In-situ (U-Th)/He dating applies two superimposed laser ablation spots to analyse the liberated parent and daughter nuclides sequentially, assuming that the analysed He is derived from the analysed U and Th. However, computer simulation considering alpha ejection visualizes the helium-source volume around the helium-pit and reveals that in-situ (U-Th)/He dating undermines that assumption. Only a fraction of the parent nuclei that produced the measured volume of helium are probed during analysis. Furthermore, while those parent nuclei contribute equally to the analysed radionuclide budget, their contribution to the helium budget is a complex function depending on the relative location of the alpha emitter to the laser ablation spots, as well as the geometry of the latter. This geometry controls the overdispersion of in-situ (U-Th)/He ages produced by radionuclide zonation, which can be reduced to a minimum by optimizing the geometries. This contribution serves as a guideline on how to minimize age overdispersion produced by the interplay of radionuclide zonation and the geometry of the laser ablation spots and demonstrates how to assess the reliability of in-situ (U-Th)/He ages by optimizing the laser pit geometry.