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Radiation-enhanced fission track annealing revisited and consequences for apatite thermochronometry
  • Kalin McDannell,
  • Dale Issler,
  • Paul O'Sullivan
Kalin McDannell
Geological Survey of Canada Calgary, Geological Survey of Canada Calgary, Geological Survey of Canada Calgary

Corresponding Author:kalin.mcdannell@canada.ca

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Dale Issler
Geological Survey of Canada Calgary, Geological Survey of Canada Calgary, Geological Survey of Canada Calgary
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Paul O'Sullivan
Geosep Services
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Apatite fission track (AFT) analyses for granitoid and metamorphic bedrock samples from the Western Superior Province (Ontario), the Churchill-Rae Province (Melville Peninsula and Southampton Island, Nunavut), and the Slave Province (Northwest Territories) show a broad range of single grain effective uranium concentrations (eU) (less than 1 to ~300 ppm) and some of the oldest reported AFT ages in North America. Although most of our samples have a typical fluorapatite composition (effective Cl less than 0.1 apfu) with implied low track retentivity, single grain AFT ages are overdispersed and decrease with increasing eU content. This eU-age relationship is resonant of the Hendriks and Redfield (Earth and Planetary Science Letters, 236, 443-458, 2005) argument for α-radiation enhanced fission track annealing (REA) and is analogous to the negative age-eU correlation observed in published zircon and titanite (U-Th)/He data from slowly-cooled cratonic rocks. The high intra-sample age variability for low-Cl bedrock apatites with protracted histories (greater than 200-500 m.y.) at less than 100°C since the Precambrian suggests strong REA control on AFT ages. Conversely, some low Cl AFT samples with a narrower eU range show less age dispersion and a weak apparent age-eU correlation. A complex trade-off between radiation damage and chemical composition (e.g. low Cl and REE enrichment) is implied when eU and rmr0 (and equivalent effective Cl) are correlated. In all cases, the samples fail the canonical χ2 test to evaluate if grains are from a single age population (χ2 less than 5%) and have characteristic “open jaw” radial plots, generally considered to indicate multiple age populations. Previous assessments of the influence of REA on AFT age were based on evaluating central age and mean track length, which potentially mask high single-grain age scatter and REA effects. Therefore, it is crucial that bedrock samples exhibiting high age scatter are evaluated in terms of intra-sample compositional heterogeneity. AFT samples with relatively low Cl concentrations are especially prone to greater REA control of cooling ages and this underscores the need for routine acquisition of compositional data for AFT datasets. Our broad range in single-grain AFT ages (with no other clear, strong compositional controls) supports the notion that radiation damage affects both the AFT and (U-Th)/He thermochronometers in slowly-cooled settings and must be accounted for during thermal modeling and interpretation.
May 2019Published in Geochimica et Cosmochimica Acta volume 252 on pages 213-239. 10.1016/j.gca.2019.03.006