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| Titel |
Parent zonation in thermochronometers - resolving complexity revealed by ID-TIMS U-Pb dates and implications for the application of decay-based thermochronometers |
| VerfasserIn |
Andre Navin Paul, Richard Spikings, David Chew, J. Stephen Daly, Alexey Ulyanov |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250148417
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| Publikation (Nr.) |
 EGU/EGU2017-12672.pdf |
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| Zusammenfassung |
| High temperature (>350℃) U-Pb thermochronometers primarily use accessory
minerals such as apatite, titanite and rutile, and assume that daughter isotopes are
lost by thermally activated volume diffusion while the parent remains immobile.
Studies exploiting such behaviour have been successfully used to reconstruct thermal
histories spanning several hundred million years (e.g. Cochrane et al., 2014). However,
outliers in date (ID-TIMS) vs diffusion length space are frequently observed, and
grains are frequently found to be either too young or too old for expected thermal
history solutions using the diffusion data of Cherniak et al. (2010). These deviations
of single grain apatite U-Pb dates from expected behaviour could be caused by a
combination of i) metamorphic (over-)growth, ii) fluid-aided Pb mobilisation during
alteration/recrystallization, iii) parent isotope zonation, iv) metamictisation, and v) changes in
diffusion length with time (e.g. fracturing). We present a large data set from the
northern Andes of South America, where we compare apatite U-Pb ID-TIMS-(TEA)
data with LA-ICP-MS element maps and in-situ apatite U-Pb LA-(MC)-ICP-MS
dates. These are combined with U-Pb zircon and 40Ar/39Ar (muscovite) data to
attempt to distinguish between thermally activated volume diffusion and secondary
overgrowth/recrystallization. We demonstrate that in young (e.g. Phanerozoic) apatites that
have not recrystallized or experienced metasomatic overgrowths, U-Pb dates are dominantly
controlled by volume diffusion and intra-crystal uranium zonation. This implies that
ID-TIMS analyses of apatites with zoned parent isotope distributions will not usually
recover accurate thermal history solutions, and an in-situ dating method is required.
Recovering the uranium distribution during in-situ analysis provides a means to
account for parent zonation, substantially increasing the accuracy of the modelled
t-T-paths. We present in-situ data from apatites where scatter in date v diffusion length
scale is observed and compare t-T-paths from single grain and in-situ modelling.
Modelling of in-situ data will further show if all apatites from a single hand specimen
record the same thermal history using Cherniak et al. (2010) diffusion data, or if
the Pb-in-apatite diffusion parameters are a function of composition. U zonation
is ubiquitous in the studied rocks (Triassic apatites extracted from peraluminous
leucosomes), implying that these conclusions may also apply to lower temperature
thermochronometers that are based on uranium decay, such as (U-Th)/He dating. |
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