Testing the reproducibility of in situ Lu[sbnd]Hf dating using Lu-rich garnet from the Tørdal pegmatites, southern Norway

Abstract

In situ LuHf geochronology offers the potential for direct dating of garnet within petrographic context. However, the method requires matrix-matched standards to calibrate measured Lu/Hf ratios. In order to assess the accuracy of this calibration, as well as the reproducibility of the resulting LuHf dates, two Lu-rich (up to 1 wt% Lu) garnet samples from the Tørdal pegmatite field in southern Norway were analysed over six analytical sessions. Multi-session isochron LuHf dates of 930.3 ± 1.4 Ma (MSWD = 1.0, 164 analyses) and 930.3 ± 1.7 Ma (MSWD = 1.5; 125 analyses) were obtained, conform with previously published age constraints. The relative standard deviation of 0.1% between the analytical sessions indicates that the dates are highly reproducible. For each individual analytical session, age uncertainties of 0.3–0.6% were achieved by measuring high Lu count rates in analog detection mode. We further describe a calibration strategy that deals with LuHf datasets where Lu is measured in both pulse and analog detector mode. Given the high analytical precision and reproducibility, we suggest that the Tørdal garnets are suitable reference materials for future LA-ICP-MS/MS LuHf studies.