High-spatial resolution U-Pb dating of phosphate minerals in Martian meteorite Allan Hills 84001

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  • KOIKE MIZUHO
    Atmosphere and Ocean Research Institute, The University of Tokyo
  • OTA YOSHIHIRO
    Atmosphere and Ocean Research Institute, The University of Tokyo
  • SANO YUJI
    Atmosphere and Ocean Research Institute, The University of Tokyo
  • TAKAHATA NAOTO
    Atmosphere and Ocean Research Institute, The University of Tokyo
  • SUGIURA NAOJI
    Department of Earth and Planetary Science, School of Science, The University of Tokyo

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Abstract

Phosphate minerals, which are ubiquitous in terrestrial and extra-terrestrial rocks, are important carriers of trace elements, including U and Th, which provide chronological information because of their radioactive decay. Abundance and grain sizes of the phosphates are limited in extra-terrestrial materials. Therefore, high resolution and minimally or non-destructive analytical methods must be used for age determination. For this study, we conducted U-Pb dating using a NanoSIMS for three phosphate grains from an old Martian meteorite: ALH 84001. The 238U-206Pb and 207Pb-206Pb isochron ages were found to be 3850 ± 170 Ma and 4002 ± 52 Ma, respectively, suggesting a concordant signature at approx. 4.0 Ga. A total Pb/U isochron age of 3990 ± 160 Ma is consistent with a previous SHRIMP U-Pb age of 4018 ± 81 Ma (Terada et al., 2003). Moreover, heterogeneous distributions of U are observed in these grains, which might have been preserved since igneous crystallization of the phosphates because the diffusion of U in the mineral is considerably slow.

Journal

  • GEOCHEMICAL JOURNAL

    GEOCHEMICAL JOURNAL 48 (5), 423-431, 2014-09-20

    GEOCHEMICAL SOCIETY OF JAPAN

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