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

Abstract

Recent advances in laser ablation reaction-cell mass spectrometry have opened the possibility for rapid and efficient Lu-Hf age dating of a variety of minerals. Of particular interest is the important metamorphic mineral, garnet, which is traditionally dated using solution-based Lu-Hf or Sm-Nd methods that are time consuming and often lack micron scale spatial resolution. In this study, we analyse garnets from samples from the ‘Moine Supergroup’ Scotland with the novel laser ablation inductively coupled quadrupole tandem mass spectrometry (LA-ICP-Q-MS/MS) Lu-Hf dating method, aiming to reveal terrane scale geochronological information in polymetamorphic rocks. We demonstrate that laser ablation Lu-Hf dating of garnet can accurately reveal the timing of different metamorphic events within single multi-growth garnets with precision typically as low as ∼1.5% (2σ, including systematic uncertainties). Garnets analysed from our samples produced dates including 957 ± 33 Ma, corresponding to the Renlandian Orogeny, 808 ± 14 Ma, corresponding to the Knoydartian Orogeny, and 471 ± 8 Ma, corresponding to the Caledonian Orogeny; all events which have previously been recorded from this rock suite. Furthermore, laser ablation Lu-Hf dating of garnet can provide insight into micron-scale processes such as Lu diffusion and garnet recrystallization. We recommend a series of sequential protocols, from sample selection through to laser ablation analysis, which should be used to maximize the likelihood of producing well constrained garnet Lu-Hf dates.