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\begin{document}

\title{Radiation-enhanced fission track annealing revisited and consequences
for apatite thermochronometry}



\author[1,1,1]{Kalin McDannell}%
\author[1,1,1]{Dale Issler}%
\author[2]{Paul O'Sullivan}%
\affil[1]{Geological Survey of Canada Calgary}%
\affil[2]{Geosep Services}%


\vspace{-1em}



  
  \date{May 24, 2024}


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\selectlanguage{english}
\begin{abstract}
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)
(\textless{}1 to \textasciitilde{}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 (\textless{}0.1 apfu Cl, rmr0
\textasciitilde{}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) Earth
Planet. Sci. Lett. 236 (443-458) argument for \selectlanguage{greek}α-\selectlanguage{english}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 \selectlanguage{greek}χ\selectlanguage{english}2 test (\textless{}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 (\textgreater{}200-500 m.y.) at
\textless{}100\selectlanguage{ngerman}°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, 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. 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 history modeling and
interpretation.%
\end{abstract}\selectlanguage{ngerman}%



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