A simple method for in-situ U-Th-He dating

Pressing research questions in tectonic geomorphology and detrital studies call for ever increasing volumes of thermochronological data which the current practice of whole grain degassing and dissolution of U-Th-He-bearing minerals cannot deliver. In-situ U-Th-He dating by laser ablation holds the potential solution to this problem but adoption of this new method has been slow due to the steep hardware requirements and complex analytical procedure. We here present 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/206 Pb ratio, removing the need to use NIST glass as a reference material. Corrections for variable ablation depths between the standard and the unknown can reach the 10% level. We introduce a method to determine this correction by LA-ICP-MS, using the strength of the ablated 29Si signal. The effectiveness of the proposed method has been demonstrated on three samples of gem-quality Sri Lanka zircon, which yield ages that are in agreement with previously published conventional U-Th-He measurements.