IN-SITU LU-HF GARNET GEOCHRONOLOGY BY LA-ICP-MS/MS (Invited Presentation)

Lu–Hf geochronology is a powerful tool to date a variety of geological processes, by targeting high-Lu low-Hf minerals such as apatite, xenotime and lawsonite, and importantly for the metamorphic petrologist, garnet. Highly precise isotope dilution geochronology has been instrumental in developing our understanding of the timing and durations of metamorphism, and has proven itself useful for dating a wide variety of lithologies and geological processes. This traditional application of the dating method requires chemical separation of the isobaric parent (¹⁷⁶Lu) and daughter (¹⁷⁶Hf) isotopes prior to analysis. The recent development of in-situ (laser-ablation based) Lu–Hf geochronology by LA–ICP–MS/MS and ammonia (NH₃) reaction gas allows the resolution of ¹⁷⁶Lu, ¹⁷⁶Hf and ¹⁷⁶Yb interference during analysis, with the use of a reaction/collision cell. Hf reacts with the NH₃ to form high-order reaction products which can be measured independently of Lu and Yb. This method offers a number of advantages including rapid analysis with high spatial resolution, as well as targeted control on textural relationships of the analysed mineral, the simultaneous collection of trace and major element data, and the ability to include or exclude mineral inclusions from data signals. For garnet, in-situ Lu–Hf geochronology is an important tool to directly date metamorphism and couple the timing of garnet growth with estimated P–T conditions, distinguish polymetamorphism in single grains or samples, and to undertake rapid campaign-style geochronology across large metamorphic terranes. Some applications of these types of strategies will be presented, including data from different lithologies and metamorphic facies, with a focus on strengths and limitations of the method.