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Book Chapter

Applications: Planetary sciences

Published:
January 01, 2014

Abstract

The 40Ar/39Ar ages of a group of Martian meteorites called shergottites are systematically older by about 25% or more than ages obtained using Sm–Nd and other radiometric dating methods. The older 40Ar/39Ar ages indicate the presence of 40Ar not derived in situ from the radiogenic decay of 40K. The ‘excess’ argon can be associated with several different components, including the Martian atmosphere and mantle. We discuss the sources of Ar in shergottites, and the methods used to separate and identify them.

Abstract

Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of NW Africa (NWA) 4797. Shock heating of NWA 4797 completely melted and vesiculated precursor igneous plagioclase, which cooled to an assemblage of plagioclase crystals with interstitial glasses of variable composition (Ca/K ratios). Using a focused ultraviolet laser beam, is has been possible to distinguish between argon isotopic signatures from groundmass minerals (igneous olivine + pyroxene), plagioclase and a shock vein. This study focuses on the potential for this meteorite to shed light on shock ages of shergottites.

Apparent 40Ar/39Ar ages of groundmass minerals show that there are large amounts of excess argon in this phase, yielding a wide range of calculated ages from 690 ± 30 Ma to several apparent ages older than 4.5 Ga. A traverse of laser-probe extractions across the 1 mm-diameter shock vein in NWA 4797 yielded apparent 40Ar/39Ar ages younger than the groundmass. A signature of the Martian atmosphere, identified by 40Ar/36Ar ratios of 1600–1900, was not found in the NWA 4797 shock vein. This is distinct from other shergottites where the products of shock melting contain a nearly pure sample of Martian atmosphere. We attribute this to a distinct formation mechanism, and hence gas-trapping mechanism, of the NWA 4797 shock vein.

We undertook 44 analyses of plagioclase areas identified by SEM analysis. Ages ranged from 45 ± 27 to 3771 ± 109 Ma and yield an average age of 375 ± 77 Ma, considerably younger than ages obtained in this study from either the groundmass or the shock vein. A plot of age v. 37Ar/39Ar for plagioclase showed a continuum of ages from the oldest to youngest ages measured. Older ages are correlated with higher Ca/K ratios of plagioclase, indicating contamination from groundmass minerals rich in excess argon. The youngest ages correlate to plagioclase extractions with the lowest Ca/K ratios, interpreted to have crystallized from a nearly pure plagioclase melt with contributions from a K-rich mesostasis. We see no evidence for multiple shock events in NWA 4797. Rather, we favour the interpretation that the cosmic-ray exposure (CRE) age of 3.0±0.5 Ma, obtained on NWA 4797 in this study using cosmogenic 38Ar, approximates the timing of shock melting in this meteorite.

Supplementary material:

Laser probe argon isotopic data for NWA 4797 obtained in this study are available at http://www.geolsoc.org.uk/SUP18602.

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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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