In situ Lu–Hf dating of allanite by LA-ICP-MS/MS: Implications for geochronology

Abstract

Allanite is a common REE-rich accessory mineral found in various igneous and metamorphic rocks, and can record a variety of geological processes. Therefore, allanite geochronology has the potential to answer a range of important geological questions. Allanite U–Th–Pb geochronology is hampered by common open-system behavior in the system. In this study, we demonstrate the feasibility of in situ allanite Lu–Hf dating by LA–ICP–MS/MS. A total of nine allanite samples from different rock types (e.g., granite, pegmatite) were investigated. These allanite samples have ages ranging from ca. 2650 to ca.100 Ma, and Lu contents ranging from several to hundreds of μg g⁻¹ levels and relatively high Lu/Hf ratios (> 30). At a mass shift of +82, the isobaric interferences ¹⁷⁶Lu and ¹⁷⁶Yb have extremely low reaction rates of ∼0.003 % and ∼ 0.0003 %, respectively, indicating the isobaric interference corrections are insignificant for the allanite samples (mean ¹⁷⁵Lu/¹⁷⁷Hf = ∼814; mean ¹⁷²Yb/¹⁷⁷Hf = ∼719). A two-step calibration strategy was proposed for the ¹⁷⁶Lu/¹⁷⁶Hf and ¹⁷⁷Hf/¹⁷⁶Hf ratio corrections using NIST SRM 610 (for instrument drift) and LE2808 (for matrix effect). The nine allanite samples contain common Hf contents (f_176Hf) of ∼5 % to >90 %, and the data are plotted in inverse isochron diagrams. Unanchored inverse isochron ages exhibit a large deviation in accuracy (5–10 %), whilst anchored isochron ages have a better accuracy of <5 %. The uncertainty of anchor initial ¹⁷⁶Hf/¹⁷⁷Hf values was investigated and, in general, was insignificant (< 2.8 %) for samples with f_176Hf < 80 %. Our results demonstrate that in situ allanite Lu–Hf dating by LA-ICP-MS/MS is feasible and can yield precise and accurate ages (< 5 %). Lu–Hf geochronometer captures the high-temperature process and may be more resistant during the late thermal event. Thus, it provides an alternative solution for those samples which are suffered by open-system behavior in U-Th-Pb system.