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

Abstract

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 μg g⁻¹. Although the isobaric interferences of ¹⁷⁶Lu and ¹⁷⁶Yb 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 ¹⁷⁵Lu/¹⁷⁷Hf = ∼24 000; mean ¹⁷²Yb/¹⁷⁷Hf = ∼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.