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    Grain size matters: Implications for element and isotopic mobility in titanite

    Access Status
    Fulltext not available
    Authors
    Kirkland, C.
    Spaggiari, C.
    Johnson, T.
    Smithies, R.
    Danišík, Martin
    Evans, Noreen
    Wingate, M.
    Clark, C.
    Spencer, C.
    Mikucki, E.
    McDonald, B.
    Date
    2016
    Type
    Journal Article
    
    Metadata
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    Citation
    Kirkland, C. and Spaggiari, C. and Johnson, T. and Smithies, R. and Danišík, M. and Evans, N. and Wingate, M. et al. 2016. Grain size matters: Implications for element and isotopic mobility in titanite. Precambrian Research. 278: pp. 283-302.
    Source Title
    Precambrian Research
    DOI
    10.1016/j.precamres.2016.03.002
    ISSN
    0301-9268
    School
    Department of Applied Geology
    URI
    http://hdl.handle.net/20.500.11937/16591
    Collection
    • Curtin Research Publications
    Abstract

    The U–Pb isotopic signature of titanite collected across an exhumed refractory lower crustal block within the Albany–Fraser Orogen, Australia, records thermal overprints not apparent in a suite of other U–Pb chronometers. This helps to reconcile a dichotomy within the geochronological record of two adjacent zones within the orogen. The zircon U–Pb record for the older Biranup Zone preserves widespread overprinting at 1225–1140 Ma (Stage II), whereas the younger Fraser Zone records an older 1330–1260 Ma (Stage I) tectonothermal event. Titanite in the Fraser Zone also predominantly records a U–Pb age of 1299 ± 14 Ma, reflecting the interval of closure to radiogenic Pb mobility. Nonetheless, small titanite grains reveal subsequent overprinting with a mean reset age of 1205 ± 16 Ma. By contrast, titanite from metasedimentary rocks within the adjacent Biranup Zone principally record U–Pb ages of 1200–1150 Ma, interpreted as dating cooling after prolonged Stage II metamorphism. Interestingly, titanite also preserves domains with old apparent ages.These domains have a statistically significant association with lower U content and also indicate reduced Sm/Yb ratios and are interpreted to have lost U but acquired HREE (e.g. Yb) more rapidly than MREE (e.g. Sm). The old apparent ages are interpreted as artefacts of a Stage II U redistribution process, leading to unsupported radiogenic Pb. In addition, titanite grain size has a strong effect on the preservation or resetting of metamorphic U–Pb ages. Thermochronological modelling based on apparent age versus grain size relationships indicates that complete resetting of small titanite grains requires overprinting temperatures of 695–725 °C during Stage II in the Fraser Zone. This result is similar to estimates from the Biranup Zone based on phase equilibrium modelling that indicates pressures and temperatures of 6.5–8.5 kbar and 675–725 °C. An in situ U–Pb analysis strategy for titanite that targets a range of grain sizes has the potential to reveal differential resetting and place important controls on thermal history.

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