Defining the mechanisms that disturb the Sm-Nd isotopic systematics of the Martian meteorites: Examples from Dar al Gani 476 and Allan Hills 77005

Abstract

Microbeam studies of Martian meteorites Dar al Gani (DaG) 476 and Allan Hills (ALH) 77005 have been conducted to identify potential causes of disequilibrium exhibited in their Sm-Nd isotopic systematics. Olivine and maskelynite mineral fractions on the DaG 476 isochron are displaced relative to their positions as dictated by measured mineral compositions. The olivine mineral fractions from ALH 77005 not only have a relatively low Sm/Nd ratio, but appear to contain an unradiogenic component that shifts the olivine mineral fraction off the isochron defined by the pyroxene and maskelynite mineral fractions. Trace components such as melt inclusions, impact melt, high-Si mesostasis, and altered olivine were analyzed using scanning electron microscopy, quantitative electron microscopy, and secondary ion mass spectrometry to determine their potential for disturbing the isotopic systematics of the mineral fractions, assuming that the mineral fractions were not completely pure. Mixing models indicate that the presence of melt inclusions in the DaG 476 olivine mineral fraction lowered its Sm/Nd ratio. The maskelynite mineral fraction contains a related but more evolved mesostasis component that raised the Sm/Nd ratio of the fraction. The position of two olivine mineral fractions below the ALH 77005 isochron is interpreted to reflect small additions of impact melt with a light rare earth element enriched pattern and a non-indigenous, unradiogenic Nd component. Furthermore, the presence of rare earth elements in olivine and maskelynite from both igneous and non-igneous components such as melt inclusions, mesostasis, and impact melt is observed on a fine (<30 μm) scale. Despite the addition of this material, the Sm-Nd ages are not affected. This study demonstrates that detailed mineral separation procedures as employed by modern geochronology laboratories permit reliable ages to be derived from shocked and altered samples.