First demonstration of in situ Lu-Hf dating using LA-ICP-MS/MS applied to monazite

Monazite is a LREE-rich accessory mineral in various igneous and metamorphic rocks; therefore, monazite geochronology has the potential to answer a range of important geological questions. We demonstrate the feasibility of in situ Lu-Hf dating by LA-ICP-MS/MS applied to monazite for the first time. Eight monazite samples were investigated, with ages of 2.65-0.27 Ga and Lu contents of 19-180 mu g g(-1). Although the isobaric interferences of Lu-176 and Yb-176 occur at extremely low reaction rates (reacted/unreacted fractions) for higher mass reaction products (+82 a.m.u), isobaric interference corrections are still needed for the monazite samples due to their high Lu/Hf and Yb/Hf ratios (mean Lu-175/Hf-177 = similar to 24 000; mean Yb-172/Hf-177 = similar to 52 000) when using an iCap TQ instrument. The RW-1, ELK, 44 069, and Maine 1 monazites yield relatively low common Hf contents, whereas the Vermillion, Namaqualand, 117 531, and Maine 2 monazites have high and variable common Hf contents. The results show that in situ monazite Lu-Hf dating by LA-ICP-MS/MS is feasible and can yield precise (0.40-2.96%, 2 relative standard error) and accurate (-0.40 to +1.99%, 2 relative deviation) ages that are comparable to those from the U-Th-Pb system. There are no systematic Lu-Hf age biases with respect of the U-Th-Pb ages and Lu and Hf contents. We report a data reduction scheme (DRS) and Visual Lu-Hf age plugin for Iolite software to process in situ Lu-Hf age data. Lu-Hf geochronology could provide independent age constraints for the monazite, which has previously been successfully dated using the U-Th-Pb system.