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Publication - Dr Tim Tomkinson

    Significance of the cosmogenic argon correction in deciphering the 40Ar/39Ar ages of the Nakhlite (Martian) meteorites


    Cohen, BE, Cassata, W, Mark, DF, Tomkinson, T, Lee, MR & Smith, CL, 2015, ‘Significance of the cosmogenic argon correction in deciphering the 40Ar/39Ar ages of the Nakhlite (Martian) meteorites’.


    All meteorites contain variable amounts of cosmogenic 38Ar and 36Ar
    produced during extraterrestrial exposure, and in order to calculate
    reliable 40Ar/39Ar ages this cosmogenic Ar must be removed from the
    total Ar budget. The amount of cosmogenic Ar has usually been calculated
    from the step-wise 38Ar/36Ar, minimum 36Ar/37Ar, or average
    38Arcosmogenic/37Ar from the irradiated meteorite fragment. However, if
    Cl is present in the meteorite, then these values will be disturbed by
    Ar produced during laboratory neutron irradiation of Cl. Chlorine is
    likely to be a particular issue for the Nakhlite group of Martian
    meteorites, which can contain over 1000 ppm Cl [1]. An alternative
    method for the cosmogenic Ar correction uses the meteorite's exposure
    age as calculated from an un-irradiated fragment and step-wise
    production rates based on the measured Ca/K [2]. This calculation is
    independent of the Cl concentration. We applied this correction method
    to seven Nakhlites, analyzed in duplicate or triplicate. Selected
    samples were analyzed at both Lawrence Livermore National Laboratory and
    SUERC to ensure inter-laboratory reproducibility. We find that the
    cosmogenic argon correction of [2] has a significant influence on the
    ages calculated for individual steps, particularly for those at lower
    temperatures (i.e., differences of several tens of million years for
    some steps). The lower-temperature steps are more influenced by the
    alternate cosmogenic correction method of [2], as these analyses yielded
    higher concentrations of Cl-derived 38Ar. As a result, the Nakhlite data
    corrected using [2] yields step-heating spectra that are flat or nearly
    so across >70% of the release spectra (in contrast to
    downward-stepping spectra often reported for Nakhlite samples), allowing
    for the calculation of precise emplacement ages for these meteorites.
    [1] Cartwright J. A. et al. (2013) GCA, 105, 255-293. [2] Cassata W. S.,
    and Borg L. E. (2015) 46th LPSC, Abstract #2742.

    Full details in the University publications repository