In situ Lu–Hf geochronology with LA-ICP-MS/MS analysis

Abstract

Lu–Hf geochronology is useful for constraining the evolution of geological systems. In this study, the in situ LA-ICP-MS/MS Lu–Hf dating technique was successfully applied to Paleozoic-Precambrian xenotime, apatite and garnet. For an iCap TQ ICP-MS/MS instrument (Thermo Fisher, USA), 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⁻¹. Lutetium, Yb and Hf reaction products with NH₃ were identified in the mass range from 175–300 amu. The reaction product of ^(176+82)Hf (expressed for ¹⁷⁶Hf(¹⁴N¹H)(¹⁴N¹H₂)(¹⁴N¹H₃)₃, mass shift by +82) is measured for the separation of ¹⁷⁶Hf from ¹⁷⁶Lu and ¹⁷⁶Yb. Isobaric interferences ¹⁷⁶Lu and ¹⁷⁶Yb have extremely low reaction rates of ∼0.0034% and ∼0.00036% (at mass shift +82), which are only required to be corrected for the samples (e.g., xenotime) with extremely high ¹⁷⁵Lu/¹⁷⁷Hf and ¹⁷²Yb/¹⁷⁷Hf ratios. A matrix-induced bias of ¹⁷⁶Lu/¹⁷⁷Hf ratios was observed between NIST SRM 610 and the samples, which required further correction using matrix-matched reference materials. For xenotime, the accuracy of the common-Hf corrected single-spot ages is generally better than 1.5%, comparable to those obtained by in situ U–Pb 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. For garnet, the analytical uncertainties of the isochron ages are in a range of 3.5–10%, which could be further improved using a sensitivity-enhanced instrument and/or enlarged sampling volume. Our study revealed that the xenotime reference materials can be used as calibrators for apatite Lu–Hf dating. The novel in situ Lu–Hf dating technique may be especially useful for determining the age of the samples with complex temporal records or lack of traditional U-rich accessory minerals (e.g., zircon).