Abstract. Age dispersion is a common feature of apatite fission track (AFT) and apatite (U-Th)/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. Here we investigate, using synthetic data, how multikinetic AFT annealing behaviour (defined using the rmr0 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 generates synthetic AFT and AHe data for three different apatite populations with significantly different annealing kinetics. The synthetic data are used as input for inverse modelling (Bayesian QTQt model) that attempts 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 (using varying rmr0 values from the AFT data for the He radiation damage model) and constraints are included. In contrast, a more conventional monokinetic interpretation that ignores multikinetic AFT behaviour yields incorrect thermal solutions that fail to adequately reproduce all the data. The AFT data are reproduced well but the AHe data are not. Under these conditions, incorporation of constraints can be very 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 age scatter. A future companion paper will explore multikinetic AFT methodology and application to detrital apatite samples from Yukon, Canada.
Brain and Human Body Modeling 2020