Garnet Lu-Hf speed dating: A novel method to rapidly resolve polymetamorphic histories

• DOI: 10.1016/j.gr.2023.04.011
• Publication Date: 2023-05-02T20:53:07Z
• Authors: Alexander Simpson; Stijn Glorie; Martin Hand; Carl Spandler; Sarah Gilbert
• Keywords: Metamorphism; LA-ICP-MS/MS; Garnet; Lu-Hf; Moine Supergroup; 01 natural sciences; 0105 earth and related environmental sciences

GARNET LU-HF SPEED DATING: A NOVEL METHOD TO RAPIDLY RESOLVE POLYMETAMORPHIC HISTORIESAlexander Simpsona,, Stijn Gloriea,, Martin Handa,, Carl Spandlera, Sarah Gilberta,aDepartment of Earth Sciences, School of Physical Sciences, The University of Adelaide, Adelaide SA-5005, AustraliaAbstractGarnet is a remarkably useful mineral in the study of crustal and lithospheric-scale tectonic systems. A vast body of knowledge exists on its thermodynamic properties, meaning garnet is widely used for determining pressure-temperature conditions.  Traditional methods to date garnet (e.g. Lu-Hf and Sm-Nd) are time consuming and commonly volumetrically imprecise.  Consequently, garnet is generally now dated indirectly using co-existing minerals such as zircon and monazite that can be efficiently analysed using U-Pb age-dating, with their chemical compositions linked to garnet via inferred equilibrium elemental relationships.  The disadvantage of this approach is the age of garnet is inferred, rather than directly determined, and it can be difficult to prove garnet grew contemporaneously with the dated U-Pb minerals. In situ Lu-Hf laser ablation dating of garnet is a comparatively recent development that embodies the efficiency of LAICPMS U-Pb dating. Here, we explore the use of laser ablation garnet Lu-Hf dating to rapidly reveal the metamorphic history of polymetamorphic terranes, producing accurate dates (in excellent agreement with published ages) with precisions as low as 1.5%. For instance, garnets analysed from the Moine supergroup, NW Scotland produce ages that coincide with all major orogenic evets (e.g. ~950 Ma Renlandian Orogeny, 810 - 740 Ma Knoydartian orogeny, and 470 – 420 Ma Caledonian Orogeny). In certain cases, two of these orogenic events are preserved in a single garnet grain, and the high spatial resolution of the laser method allows to accurately resolve such multi-growth histories. In addition, we demonstrate that the laser ablation Lu-Hf technique has the potential to evaluate micron-scale isotopic disturbances (such as diffusion) in garnet.