| Summary: | <p>Age dispersion is a common feature of apatite fission track (AFT) and
apatite (U–Th) <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="1b4178c77ca0d4bfee6c9ddd864f3a43"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-3-321-2021-ie00001.svg" width="8pt" height="14pt" src="gchron-3-321-2021-ie00001.png"/></svg:svg></span></span> He (AHe) thermochronological data, and it can be attributed
to multiple factors. One underappreciated and underreported cause for
dispersion is variability in apatite composition and its influence on
thermal annealing of fission tracks. Using synthetic data we investigate how
multikinetic AFT annealing behaviour, defined using the <span class="inline-formula"><i>r</i><sub>mr0</sub></span> parameter,
can be exploited to recover more accurate, higher-resolution thermal
histories than are possible using conventional interpretation and modelling
approaches. Our forward model simulation spans a 2 Gyr time interval with
two separate heating and cooling cycles and was used to generate synthetic
AFT and AHe data for three different apatite populations with significantly
different annealing kinetics. The synthetic data were then used as input for
inverse modelling in the Bayesian QTQt software to recover thermal-history
information under various scenarios. Results show that essential features of
the dual peak thermal history are captured using the multikinetic AFT data
alone, with or without imposed constraints. Best results are achieved when
the multikinetic AFT data are combined with the AHe data and geologic
constraint boxes are included. In contrast, a more conventional monokinetic
interpretation that ignores multikinetic AFT behaviour reproduces all the
input data but yields incorrect thermal-history solutions. Under these
conditions, incorporation of constraints can be misleading and fail to
improve model results. In general, a close fit between observed and modelled
parameters is no guarantee of a robust thermal-history solution if data are
incorrectly interpreted. For the case of overdispersed AFT data, it is
strongly recommended that elemental data be acquired to investigate if
multikinetic annealing is the cause of the AFT apparent age scatter.
Elemental analyses can also be similarly useful for broadly assessing AHe
data. A future companion paper (Issler et al., 2021) will explore multikinetic AFT methodology and
application to detrital apatite samples from Yukon, Canada.</p>
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