Crystallisation, cooling or contamination: interpreting dispersion in metamorphic 40Ar/39Ar ages

Warren, Clare; Mcdonald, Christopher; Mottram, Catherine and Kelley, Simon (2014). Crystallisation, cooling or contamination: interpreting dispersion in metamorphic 40Ar/39Ar ages. In: Thermo2014: The 14th International Conference on Thermochronology, 8-12 Sep 2014, Chamonix-Mont Blanc, France.



Mica 40Ar/39Ar data, most generally determined by multi- or single-grain step-heating experiments, are commonly used to constrain the timing of cooling in high-temperature metamorphic terranes in conjunction with the Dodson (1973) closure temperature formulation1. The pitfalls of this approach are well documented: blind associations between a mineral 40Ar/39Ar ‘date’ and a single temperature have previously led to many spurious tectonic interpretations. Experiments investigating the differences in 40Ar/39Ar ages, and age spreads, between step-heating, single grain fusion and intra-grain laser ablation methods from samples collected from different metamorphic terranes show that step heating may mask inter- and intra-grain age variability. This variability provides critical spatial information about where and how Ar is hosted, mobilised and removed during a metamorphic cycle.

Age dispersions far in excess of the range expected from variations in grain size in a fully open system have been observed in single grain fusion data from different metamorphic settings. This spread may be caused by differences in timing of crystallisation, variable amounts of contamination or variably inefficient diffusion. A robust interpretation requires a combination of detailed petrology, PT-path modelling, geochemical data and insights from diffusion modelling. The assumptions inherent in the Dodson closure temperature formulation - thermally activated volume diffusion, no initial Ar in the grain and an open grain boundary network that can efficiently remove Ar from the local rock volume1 - can be tested using a combination of in-situ laser ablation spot profiling and single grain fusion methods. These highlight the spatial distribution of Ar both within and between different K-bearing and Kfree minerals. Our data suggest that there is no simple lithologic or metamorphic grade pattern in the way that Ar is produced, stored, (re)cycled and removed during a metamorphic cycle. Each sample need careful investigation in order to most robustly link 40Ar/39Ar age to metamorphic stage.

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