In situ La Ba geochronology by LA-ICP-MS/MS: A new method to rapidly date LREE-rich minerals
Date
2025
Authors
Glorie, S.
Gilbert, S.E.
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Journal article
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Chemical Geology, 2025; 695:123045-1-123045-14
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Stijn Glorie, Sarah E. Gilbert
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Abstract
The La–Ba decay system has been demonstrated for dating LREE-rich minerals such as monazite, bastnaesite and allanite. However, due the laborious isotope dilution process and the limitations of a long half-life and low parent isotope abundance, the method is not used routinely. We present an analytical approach for in situ La–Ba geochronology using LA-ICP-MS/MS technology. Using a mixture of N₂O and H₂ in the reaction cell, Ba mostly reacts to BaOH, while La and Ce preferentially make oxide reaction products, enabling separate detection. Isobaric interferences of ¹³⁸⁺¹⁷LaO(H) and ¹³⁸⁺¹⁷CeO(H) on ¹³⁸⁺¹⁷BaOH are minimised by supplying a maximum of 18 ml.min¯¹ H₂ gas via 2 mass-flow controllers (while reducing N₂O) in the reaction cell. NIST61X glasses are not suitable to calibrate La/Ba ratios for minerals with wt% LREE and ppm Ba concentrations. We developed a La (1000 ppm)-Ba(100 ppm) glass to correct La/Ba ratios for pulse/analog offsets. Using our most optimised analytical workflow, we report accurate La–Ba dates (within 1 % of reference values) for bastnaesite and allanite reference materials. The La–Ba dates for a range of monazites are offset by an average of 7.5 % from reference ages. Correcting for matrix-induced fractionation to the TS reference monazite, and considering fully propagated uncertainties, the monazite La–Ba dates are in good agreement with reference values. The main advantage of the in situ La–Ba method over the Lu–Hf method is that 1–3 % uncertainties can be achieved with laser beams as small as 20–30 μm, opening up new opportunities to accurately date LREE minerals at high spatial resolution.
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© 2025 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).