A novel in situ methodology for U–Pb, (U–Th)/He, and fission track triple dating†

Abstract

In this study, we present a novel laser-based technique for in situ U–Pb, (U–Th)/He, and fission track (FT) triple dating for apatite and other U-rich accessory minerals. This approach allows for obtaining three ages with different closure temperatures on a single crystal, significantly enhancing spatial resolution and analytical productivity. Our new workflow employs an Autoscan System for FT analysis, a ResoChron system for He determination, and an LA-ICP-MS system for parent isotope and U–Pb age measurements. We describe the sample preparation, instrument parameters, and data reduction for apatite in detail. Using a sample–standard bracketing approach to determine the pairwise factor κ, we significantly improved reproducibility, particularly by replacing volume with depth in helium pit measurement. Additionally, we found that chemical etching in FT dating can reduce (U–Th)/He age by removing the surrounding He, necessitating a second polishing for samples with high track density. This in situ triple dating method has been successfully applied to five large apatite crystals, improving analytical efficiency and resolving grain-to-grain age discrepancy.