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    A terrestrial perspective on using ex situ shocked zircons to date lunar impacts

    Access Status
    Open access via publisher
    Authors
    Cavosie, Aaron
    Erickson, Timmons
    Timms, Nicholas Eric
    Reddy, Steven
    Talavera, Cristina
    Montalvo, S.
    Pincus, M.
    Gibbon, R.
    Moser, D.
    Date
    2015
    Type
    Journal Article
    
    Metadata
    Show full item record
    Citation
    Cavosie, A. and Erickson, T. and Timms, N.E. and Reddy, S. and Talavera, C. and Montalvo, S. and Pincus, M. et al. 2015. A terrestrial perspective on using ex situ shocked zircons to date lunar impacts. Geology. 43 (11): pp. 999-1002.
    Source Title
    Geology
    DOI
    10.1130/G37059.1
    ISSN
    0091-7613
    School
    Department of Applied Geology
    URI
    http://hdl.handle.net/20.500.11937/44255
    Collection
    • Curtin Research Publications
    Abstract

    Deformed lunar zircons yielding U-Pb ages from 4333 Ma to 1407 Ma have been interpreted as dating discrete impacts on the Moon. However, the cause of age resetting in lunar zircons is equivocal; as ex situ grains in breccias, they lack lithologic context and most do not contain microstructures diagnostic of shock that are found in terrestrial zircons. Detrital shocked zircons provide a terrestrial analog to ex situ lunar grains, for both identifying diagnostic shock evidence and also evaluating the feasibility of dating impacts with ex situ zircons. Electron backscatter diffraction and sensitive high-resolution ion microprobe U-Pb analysis of zircons eroded from the ca. 2020 Ma Vredefort impact structure (South Africa) show that complete impact-age resetting did not occur in microstructural domains characterized by microtwins, planar fractures, and low-angle boundaries, which record ages from 2890 Ma to 2645 Ma. An impact age of 1975 ± 39 Ma was detected in neoblasts within a granular zircon that also contains shock microtwins, which link neoblast formation to the impact. However, we show that granular texture can form during regional metamorphism, and thus is not unique to impact environments. These results demonstrate that dating an impact with ex situ shocked zircon requires identifying diagnostic shock evidence to establish impact provenance, and then targeting specific age-reset microstructures. With the recognition that zircon can deform plastically in both impact and magmatic environments, age-resetting in lunar zircons that lack diagnostic shock deformation may record magmatic processes rather than discrete impacts. Identifying shock microstructures that record complete age resetting for geochronological analysis is thus crucial for constructing accurate zircon-based impact chronologies for the Moon, Earth, or other planetary bodies.

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