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TitleRadiation-enhanced fission track annealing revisited and consequences for apatite thermochronometry
 
AuthorMcDannell, K TORCID logo; Issler, D RORCID logo; O'Sullivan, P B
SourceGeochimica et Cosmochimica Acta vol. 252, 2019 p. 213-239, https://doi.org/10.1016/j.gca.2019.03.006 Open Access logo Open Access
Image
Year2019
Alt SeriesNatural Resources Canada, Contribution Series 20180216
PublisherElsevier BV
Documentserial
Lang.English
Mediapaper; on-line; digital
File formatpdf; html
Subjectsgeochronology; geochemistry; fission-track dates; thermal analyses; bedrock geology; lithology; igneous rocks; intrusive rocks; granitic rocks; metamorphic rocks; chlorine geochemistry; rare earths geochemistry; thermal history; radiometric dating; modelling; Methodology
Illustrationsgeoscientific sketch maps; tables; plots; frequency distribution diagrams; diagrams
ProgramGEM2: Geo-mapping for Energy and Minerals TransGEM
Released2019 03 13
AbstractApatite 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 craton region (Northwest Territories) show a broad range of single grain effective uranium concentrations (eU) (<1 to ~300 ppm) and some of the oldest reported AFT ages in North America. Although most of our samples are characterized by near-endmember fluorapatite composition with implied low track retentivity (<0.1 apfu Cl, rmr0 ~0.85-0.82), single-grain AFT ages are statistically overdispersed and ages decrease with increasing eU content. This eU-age relationship is resonant of the Hendriks and Redfield (2005) argument for alpha-radiation enhanced fission track annealing (REA) and is analogous to the negative age-eU correlations observed in published zircon and titanite (U-Th)/He data from slowly-cooled cratonic rocks. In all cases, the samples fail the canonical chi-square test (<5%), generally considered to indicate that the ages are unlikely to be drawn from a single Poissonian distribution with a discrete mean value and may represent multiple populations. The high intra-sample age variability for low-Cl bedrock apatites with protracted histories (>200-500 m.y.) at <100 ºC 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, chemical composition (e.g. low Cl and REE enrichment), and thermal history is implied when eU and rmr0 are positively correlated. 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 due to the modal nature of central age determination. REA is also supported by and compatible with materials science and nuclear waste studies of radiation damage in different apatite groups, therefore it is crucial that bedrock samples exhibiting high age scatter are evaluated in terms of intra-sample compositional heterogeneity. Samples with relatively low Cl concentrations are especially prone to greater REA effects on AFT grain 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 history modeling and interpretation.
Summary(Plain Language Summary, not published)
Apatite fission track (AFT) analysis of granitoid and metamorphic bedrock samples from the Canadian Shield show a broad range of single grain effective uranium concentrations (eU) from <1 to ~300 ppm and are among some of the oldest reported AFT ages in North America. Typically, our samples characterized by low effective Cl (<0.1 apfu) and variable eU between single grains establish a correlation between high eU and younger AFT ages, while low eU grains are older. This eU-age relationship is resonant of the Hendriks and Redfield (Earth and Planetary Science Letters, 236, 443-458, 2005) argument for a-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 rocks. 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.
GEOSCAN ID311290

 
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