An in situ technique for (U-Th-Sm)/He and U-Pb double dating

Abstract

We report on a new laser-based technique for rapid, quantitative and automated in situ double dating (U-Pb and (U-Th-Sm)/He) of minerals, for applications in geochronology, thermochronology and geochemistry. In situ laser microanalysis offers several advantages over conventional bulk crystal methods in terms of spatial resolution, productivity, and safety. This new approach/methodology utilizes an interoperable and integrated suite of analytical instruments including a 193 nm ArF excimer laser system, quadrupole ICP-MS, quadrupole helium mass spectrometry system and swappable flow-through and ultra-high vacuum analytical chambers. We describe the analytical protocols for zircon analysis including grain mounting in Teflon, parameters for parent and daughter isotopic measurement, and standard development, and provide a freeware application for determining (U-Th-Sm)/He 'pairwise' ages from analytical data. The in situ double dating method described is applied to the Ellendale lamproite pipe and country rocks, Western Australia and successfully replicates conventional U-Pb and (U-Th-Sm)/He age variations determined previously by conventional techniques.