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Publication - Dr Tim Tomkinson

    The paradox between low shock-stage and evidence for compaction in CM carbonaceous chondrites explained by multiple low-intensity impacts

    Citation

    Lindgren, P, Hanna, RD, Dobson, KJ, Tomkinson, T & Lee, MR, 2015, ‘The paradox between low shock-stage and evidence for compaction in CM carbonaceous chondrites explained by multiple low-intensity impacts’. Geochimica et Cosmochimica Acta, vol 148., pp. 159-178

    Abstract

    Petrographic analysis of eight CM carbonaceous chondrites (EET 96029,
    LAP 031166, LON 94101, MET 01072, Murchison, Murray, SCO 06043, QUE
    93005) by electron imaging and diffraction, and X-ray computed
    tomography, reveals that six of them have a petrofabric defined by shock
    flattened chondrules. With the exception of Murchison, those CMs that
    have a strong petrofabric also contain open or mineralized fractures,
    indicating that tensional stresses accompanying the impacts were
    sufficient to locally exceed the yield strength of the meteorite matrix.
    The CMs studied span a wide range of petrologic subtypes, and in common
    with Rubin (2012) we find that the strength of their petrofabrics
    increases with their degree of aqueous alteration. This correspondence
    suggests that impacts were responsible for enhancing alteration,
    probably because the fracture networks they formed tapped fluid
    reservoirs elsewhere in the parent body. Two meteorites that do not fit
    this pattern are MET 01072 and Murchison; both have a strong petrofabric
    but are relatively unaltered. In the case of MET 01072, impact
    deformation is likely to have postdated parent body aqueous activity.
    The same may also be true for Murchison, but as this meteorite also
    lacks fractures and veins, its chondrules were most likely flattened by
    multiple low intensity impacts. Multiphase deformation of Murchison is
    also revealed by the microstructures of calcite grains, and
    chondrule-defined petrofabrics as revealed by X-ray computed tomography.
    The contradiction between the commonplace evidence for
    impact-deformation of CMs and their low shock stages (most belong to S1)
    can be explained by most if not all having been exposed to multiple low
    intensity (i.e.,

    Full details in the University publications repository