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Book Chapter

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

By
D. F. Mark
D. F. Mark
1
Isotope Geosciences Unit, Scottish Universities Environmental Research Centre (SUERC), Rankine Avenue, East Kilbride G75 0QF, UK
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P. Lindgren
P. Lindgren
2
School of Geographical and Earth Sciences, University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow G12 8QQ, UK
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A. E. Fallick
A. E. Fallick
1
Isotope Geosciences Unit, Scottish Universities Environmental Research Centre (SUERC), Rankine Avenue, East Kilbride G75 0QF, UK
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Published:
January 01, 2014

Abstract

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.

Supplementary material:

Raw 40Ar/39Ar data are available at http://www.geolsoc.org.uk/SUP18633.

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Contents

Geological Society, London, Special Publications

Advances in 40Ar/39Ar Dating: From Archaeology to Planetary Sciences

F. Jourdan
F. Jourdan
Curtin University, Australia
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D. F. Mark
D. F. Mark
Scottish Universities Environmental Research Centre, UK
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C. Verati
C. Verati
University of Nice, France
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Geological Society of London
Volume
378
ISBN electronic:
9781862396623
Publication date:
January 01, 2014

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