Garnet Reference Materials for In Situ Lu-Hf Geochronology

Abstract

In situ garnet Lu-Hf geochronology has the potential to revolutionise the chronology of petrological and tectonic processes, yet there is a paucity of well-characterised reference materials to account for laser-induced matrix-dependant elemental fractionation. Here, we characterise two reference garnets GWA-1 (Lu ~ 7.0 μg g⁻¹) and GWA-2 (Lu ~ 8.5 μg g⁻¹) for in situ garnet Lu-Hf geochronology. Isochron ages from isotope dilution Lu-Hf analyses yield crystallisation ages of 1267.0 ± 3.0 Ma with initial ¹⁷⁶Hf/¹⁷⁷Hf_i of 0.281415 ± 0.000012 (GWA-1), and 934.7 ± 1.4 Ma with ¹⁷⁶Hf/¹⁷⁷Hf_i of 0.281386 ± 0.000013 (GWA-2). In situ Lu-Hf analyses yield inverse isochron ages up to 10% older than the known crystallisation age due to matrix effects between garnet and reference glass (NIST SRM 610) under different instrument tuning conditions. This apparent age offset is reproducible for both materials within the same session and can be readily corrected to obtain accurate ages. Our results demonstrate that GWA-1 and GWA-2 are robust reference materials that can be used to correct for matrix-analytical effects and also to assess the accuracy of in situ Lu-Hf garnet analyses across a range of commonly encountered garnet compositions.