An optimized chromatography method and MC-ICP-MS technique for apatite Lu–Hf geochronology

Abstract

Apatite is a common accessory mineral found in almost all igneous, metamorphic, and clastic sedimentary rocks. It contains concentrations of lutetium and hafnium amenable to dating by the Lu–Hf isochron method. However, the application of this method has been hampered by chemical difficulties associated with the extraction of a clean Hf concentrate from apatite. A novel method of efficient chemical separation utilizing Ln and DGA-normal extraction chromatography columns has been developed for precise isotope analysis and is presented in this study. The routine analysis of sub-ng amounts of Hf maintained high yield close to 100%. The purification of Hf with a DGA-normal chromatography column effectively eliminated the interfering elements (Yb, Lu, and W), typically resulting in less than 1 pg of Lu in the analyzed apatite Hf solution. Apatite Lu–Hf isotope compositions of Otter Lake, Bancroft, and Durango apatites were determined using the reported methodology and yielded isochron ages of 1047.6 ± 3.4 Ma, 1092 ± 17 Ma, and 31.1 ± 1.1 Ma, respectively. All apatite Lu–Hf isochrons are characterized by phenomenally high apatite ¹⁷⁶Lu/¹⁷⁷Hf and ¹⁷⁶Hf/¹⁷⁷Hf ratios. Such apatite Lu–Hf isochrons are shown to be not significantly dependent on the initial isotopic composition in determining accurate and precise ages, allowing for anchoring the low end of the isochron with an estimated isotopic composition.