In situ LaBa geochronology by LA-ICP-MS/MS: A new method to rapidly date LREE-rich minerals

Abstract

The LaBa 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 LaBa 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 LaBa dates (within 1 % of reference values) for bastnaesite and allanite reference materials. The LaBa 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 LaBa dates are in good agreement with reference values. The main advantage of the in situ LaBa method over the LuHf 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.