New Technique Allows for More Accurate Geochronology Dating of Geological Systems

A team of researchers at the Chinese Academy of Sciences in Beijing has successfully applied a new technique for in situ Lu–Hf geochronology using laser ablation inductively coupled plasma–tandem mass spectrometry (LA-ICP-MS/MS) analysis, according to a paper published in the Journal of Analytical Atomic Spectrometry (1). The technique uses high-purity NH₃ for the reaction, allowing for more accurate dating of Paleozoic-Precambrian xenotime, apatite, and garnet, which can be used to constrain the aging of geological systems.

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In situ LA-ICP-MS/MS involves using a laser beam to vaporize a small amount of the sample, which is then carried into an ICP-MS instrument. The sample is then ionized and separated by mass, allowing for the measurement of the isotopic ratios of the sample. The lutetium–hafnium (Lu–Hf) geochronology technique applied with LA-ICP-MS/MS uses the decay of lutetium to hafnium to determine the age of the sample. By measuring the ratios of lutetium and hafnium isotopes, the age of the sample can be determined with a high degree of accuracy. The technique is particularly useful for samples with complex temporal records or a lack of traditional U-rich accessory minerals, providing a new tool for geochronology research.

The team found that high-purity NH₃ was more effective in the reaction than the commonly used 1:9 NH₃–He mixture, and an 80% improvement in sensitivity was achieved using an N₂ flow rate of 4.0 mL/min. Lutetium, ytterbium, and hafnium reaction products with NH₃ were identified in the mass range from 175–300 amu, and the reaction product of ^(176+82)Hf was measured for the separation of ¹⁷⁶Hf from ¹⁷⁶Lu and ¹⁷⁶Yb. Isobaric interferences ¹⁷⁶Lu and ¹⁷⁶Yb had extremely low reaction rates, which were only required to be corrected for samples with extremely high ¹⁷⁵Lu/¹⁷⁷Hf and ¹⁷²Yb/¹⁷⁷Hf ratios.

The team also observed a matrix-induced bias of ¹⁷⁶Lu/¹⁷⁷Hf ratios between NIST SRM 610 and the samples, which required further correction using matrix-matched reference materials. For xenotime, the accuracy of the common-hafnium corrected single-spot ages was generally better than 1.5%, comparable to those obtained by in situ uranium–lead analysis. The precisions of common-Hf corrected single-spot ages were in a range of 1.5–8.1% and 9.2–36.0% for xenotime and apatite samples, respectively. For garnet, the analytical uncertainties of the isochron ages were in a range of 3.5–10%, which could be further improved using a sensitivity-enhanced instrument and/or enlarged sampling volume.

The team's findings suggest that the new in situ Lu–Hf dating technique may be especially useful for determining the age of samples with complex temporal records or lack of traditional uranium-rich accessory minerals, such as zircon. Furthermore, the team revealed that the xenotime reference materials can be used as calibrators for apatite Lu–Hf dating.

The researchers hope that their new technique will allow for more accurate geochronology dating of geological systems, which could provide important insights into the aging of our planet.

Reference

(1) Wu, S.; Wang, H.; Yang, Y.; Niu, J.; Lan, Z.; Zhang, L.; Huang, C.; Xie, L.; Xu, L.; Yang, J.; Wu, F. In situ Lu–Hf geochronology with LA-ICP-MS/MS analysis. J. Anal. At. Spectrom. 2023, ASAP. DOI: 10.1039/D2JA00407K