Geological Society of London
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A high-precision 40Ar/39Ar age for hydrated impact glass from the Dellen impact, Sweden

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posted on 2016-06-21, 11:22 authored by D. F. Mark, P. Lindgren, A. E. Fallick

The dating of terrestrial impact craters and impact glasses that exhibit high degrees of mineralogical complexity can be problematic. However, if the maximum potential of the terrestrial impact crater record is to be realized, accurate and precise ages for crater-forming events are critical. Here we report a high-precision 40Ar/39Ar age for the Dellen impact structure, Sweden. Previous radio-isotopic constraints show a wide variation in age as a result of poor sample characterization and analytical approach. A detailed petrographical and mineralogical study provides a solid foundation for interpretation of step-heating 40Ar/39Ar data, culminating in a statistically robust age of 140.82±0.51 Ma (2σ; full external precision) for the Dellen impact event, for which data disfavour an inherited argon component. Primary hydration of the impact melt during cooling–quenching and entrapment of molecular water promoted rapid loss of inherited 40Ar from the impact melt of rhyolitic composition. Duplicate analyses of the water content and ∂D of the glass give similar values for the former (1.9±0.1 μmol mg−1) but unexpectedly low values for the latter (−159±8‰), with scatter beyond the expected analytical reproducibility due to isotopic heterogeneity. This study highlights that the 40Ar/39Ar technique is unrivalled in its ability to precisely and accurately date the products of hypervelocity collisional events.