The present status and not-too-distant future of laser ablation (U-Th)/He geochronology

The development of effective protocols for laser-ablation (U-Th)/He geochronology enables a new way of exploring many important problems in the earth sciences. Here we review the preliminary results of several research projects actively being pursued in our laboratory. The high spatial resolution of laser ablation (U-Th)/He geochronology allows for unique and exciting analytical opportunities not possible with conventional (U-Th)/He techniques. In addition, blank measurements are kept consistently low because the sample chamber is not heated during ablation, allowing for very small (near blank level) amounts of gas to be measured reproducibly. As a consquence, ablated volumes as small as 8 x 8 x 0.125 μ m (8μ mm³) yield sufficient helium for a successful analysis of a sample of Jurassic age. This permits the measurement of core ages of very small or heterogeneous accessory mineral grains, in many cases effectively eliminating the need for alpha-ejection corrections that can contribute significantly to the uncertainty of a conventional (U-Th)/He age. The laser microprobe permits the analysis of monazite, xenotime, and other accessory mineral inclusions too small to otherwise date easily. In addition, it permits the dating of individual crystals in petrographic context, a boon to the study of samples with complex thermal histories. In raster mode, with a nominal minimum depth of 0.125 μ m per pass, high-resolution depth profiles can be generated with the laser microprobe. This enables the quantification of experimental diffusion profiles, natural ejection profiles, and the effect of intra-grain parent element heterogeneities on cooling profiles.