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

Abstract

Recent development in laser-ablation Lu-Hf dating has 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 crystallisation ages in unfoliated granitoids at temperatures of at least ∼660 °C. Above ∼730 °C, the Lu-Hf system records isotopic resetting by volume diffusion. Hence, in our observations for apatite of ‘typical’ volumes in granitoids (∼0.01-0.03 mm 2 ), the closure temperature of the Lu-Hf system is between ∼660 and ∼730 °C, consistent with theoretical calculations. In foliated granites, the Lu-Hf system records the timing of recrystallisation, 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. Supplementary material at https://doi.org/10.6084/m9.figshare.c.6365962

Evaluating the multiple sulfur isotope signature of Eoarchean rocks from the Isua Supracrustal Belt (Southwest‐Greenland) by MC‐ICP‐MS: Volcanic nutrient sources for early life

“…A Rb–Sr down‐hole fractionation (DHF) correction was applied in LADR using the DHF behaviour of NIST‐610. Secondary reference materials Bund‐1b and Taratap were used for accuracy verification with reference ages of 286.2 ± 2.2 and 497.1 ± 0.6 Ma, respectively (Black, 2007; Glorie, Gilbert, et al., 2024; Glorie, Hand, et al., 2024). All Rb–Sr dates were calculated as inverse isochrons in IsoplotR (Vermeesch, 2018) using the Rb–Sr decay constant of 1.3972 ± 0.0045 × 10 −11 a −1 (Villa et al., 2015).…”

Detrital Garnet Geochronology by In Situ U‐Pb and Lu‐Hf Analysis: A Case Study From the European Alps

“…The method involves the addition of NH 3 (supplied as a 1:9 NH 3 :He mix for safety reasons) into the reaction cell of the mass spectrometer (at a rate of 3 mL min −1 ) to promote efficient formation of the Hf((NH)(NH 2 )(NH 3 ) 3 ) + reaction product as a direct proxy for 176 Hf. Equivalent reaction products for isobars 176 Lu and 176 Yb are negligible, allowing 176 Hf and 176 Lu to be effectively separated and measured free from isobaric interferences (Glorie, Hand, et al., 2023; Simpson et al., 2021, 2022). Following Simpson et al.…”

“…Resulting Lu‐Hf dates were calculated as 2‐point (inverse) isochron ages in IsoplotR (Vermeesch, 2018), where the second point comprised an initial 177 Hf/ 176 Hf anchor of 3.55 ± 0.05, which spans the entire range of initial 177 Hf/ 176 Hf ratios of the terrestrial reservoir (e.g., Glorie, Hand, et al., 2023; Spencer et al., 2020). The obtained inverse isochron age for secondary reference material BP‐1 garnet was 1,752 ± 21 Ma (2σ uncertainty including propagated uncertainty from Hogsbo), which is in good agreement with previously published Lu‐Hf dates (1,745 ± 14 Ma and 1,744 ± 13 Ma; Glorie, Mulder, et al., 2023; Simpson et al., 2023, respectively).…”

Testing in-situ apatite Lu–Hf dating in polymetamorphic mafic rocks: a case study from Palaeoproterozoic southern Australia

“…U-Pb system in apatite has been primarily used to constrain the timing of low-to moderate-temperature metamorphic and metasomatic processes (e.g., Kirkland et al 2017Kirkland et al , 2018Hall et al 2018;Henrichs et al 2018;Glorie et al 2019), with estimates for the U-Pb closure temperature from 375 to 550 °C (Schoene and Bowring 2007;Chew et al 2011;Cochrane et al 2014). However, recent advances utilising the Lu-Hf isotopic system have demonstrated the potential of apatite to date magmatic and higher temperature metamorphic processes (e.g., Barfod et al 2005;Simpson et al 2021;Gillespie et al 2022;Glorie et al 2022Glorie et al , 2023.…”

“…It is not uncommon for the Lu-Hf system in apatite to record older ages compared to the U-Pb system (e.g., Gillespie et al 2022;Glorie et al 2023;Kharkongor et al 2023), with estimates for the Lu-Hf closure temperature between 660 and 730 °C (Cherniak 2000;Barfod et al 2005;Chew and Spikings 2015;Glorie et al 2023). Lu-Hf dates recovered from predominantly undeformed granitoids are in general agreement with primary crystallisation ages obtained from magmatic and high-temperature metamorphic geochronometers such as zircon (e.g., Simpson et al 2021;Gillespie et al 2022;Glorie et al 2023). However, apatite is more susceptible to recrystallisation during deformation, and it has been shown that in-situ Lu-Hf dates from apatite in deformed rocks tend to reflect recrystallisation instead of primary magmatic crystallisation (Glorie et al 2023).…”