Abstract. Compositionally dependent apatite fission track (AFT) annealing is a common
but underappreciated cause for AFT age dispersion in sedimentary samples. We
present an interpretation and modelling strategy for samples with variable
apatite composition that exploits multikinetic AFT annealing to obtain
thermal histories that can provide more detail and better resolution
compared to conventional methods. We illustrate our method using a Permian
and a Devonian sample from northern Yukon, Canada, both with complicated
geological histories and long residence times in the AFT partial annealing
zone. Effective Cl values (eCl; converted from rmr0 values) derived
from detailed apatite elemental data are used to define AFT statistical
kinetic populations with significantly different total annealing
temperatures (∼110–185 ∘C) and ages that agree
closely with the results of age mixture modelling. These AFT populations are
well resolved using eCl values but exhibit significant overlap with respect
to the conventional parameters of Cl content or Dpar. Elemental analyses
and measured Dpar for Phanerozoic samples from Yukon and the Northwest
Territories confirm that Dpar has low precision and that Cl content
alone cannot account for the compositional and associated kinetic
variability observed in natural samples. An inverse multikinetic AFT model,
AFTINV, is used to obtain thermal-history information by simultaneously
modelling multiple kinetic populations as distinct thermochronometers with
different temperature sensitivities. A nondirected Monte Carlo scheme
generates a set of statistically acceptable solutions at the 0.05
significance level and then these solutions are updated to the 0.5 level
using a controlled random search (CRS) learning algorithm. The smoother,
closer-fitting CRS solutions allow for a more consistent assessment of the
eCl values and thermal-history styles that are needed to satisfy the AFT
data. The high-quality Devonian sample (39 single-grain ages and 202 track
lengths) has two kinetic populations that require three cycles of heating
and cooling (each subsequent event of lower intensity) to obtain
close-fitting solutions. The younger and more westerly Permian sample with
three kinetic populations only records the latter two heating events. These
results are compatible with known stratigraphic and thermal maturity
constraints, and the QTQt software produces similar results. Model results
for these and other samples suggest that elemental-derived eCl values are
accurate within the range 0–0.25 apfu (atoms per formula unit, with rmr0 values of 0.73–0.84),
which encompasses most of the data from annealing experiments. Outside of
this range, eCl values for more exotic compositions may require adjustment
relative to better-constrained apatite compositions when trying to fit
multiple kinetic populations. Our results for natural and synthetic samples
suggest that an element-based multikinetic approach has great potential to
dramatically increase the temperature range and resolution of thermal histories
relative to conventional AFT thermochronology.
