Robust laser ablation Lu–Hf dating of apatite: an empirical evaluation

Abstract

Abstract Recent developments in laser-ablation Lu–Hf dating have opened a new opportunity to rapidly obtain apatite ages that are potentially more robust to isotopic resetting compared to traditional U–Pb dating. However, the robustness of the apatite Lu–Hf system has not been systematically examined. To address this knowledge gap, we conducted four case studies to determine the resistivity of the apatite Lu–Hf system compared to the zircon and apatite U–Pb system. In all cases, the apatite U–Pb system records a secondary (metamorphic or metasomatic) overprint. The apatite Lu–Hf system, however, preserves primary crystallization ages in unfoliated granitoids at temperatures of at least c. 660°C. Above c. 730°C, the Lu–Hf system records isotopic resetting by volume diffusion. Hence, in our observations for apatites of ‘typical’ grain sizes in granitoids ( c. 0.01–0.03 mm 2 ), the closure temperature of the Lu–Hf system is between c. 660 and c. 730°C, consistent with theoretical calculations. In foliated granites, the Lu–Hf system records the timing of recrystallization, while the apatite U–Pb system tends to record younger cooling ages. We also present apatite Lu–Hf dates for lower crustal xenoliths erupted with young alkali basalts, demonstrating that the Lu–Hf system can retain a memory of primary ages when exposed to magmatic temperatures for a relatively short duration. Hence, the apatite Lu–Hf system is a new insightful addition to traditional zircon (or monazite) U–Pb dating, particularly when zircons/monazites are absent or difficult to interpret due to inheritance or when U and Pb isotopes display open system behaviour. The laser-ablation-based Lu–Hf method allows campaign-style studies to be conducted at a similar rate to U–Pb studies, opening new opportunities for magmatic and metamorphic studies.