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Navigating the modelling puzzle: Using forward and inverse models to make clear decisions when exploring and interpreting cooling ages in both HeFTy and QTQt.
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  • Alyssa Langford Abbey,
  • Kendra Murray,
  • Andrea Stevens Goddard,
  • Mark Wildman
Alyssa Langford Abbey
California State University Long Beach

Corresponding Author:alabbey@berkeley.edu

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Kendra Murray
Idaho State University
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Andrea Stevens Goddard
Indiana University Bloomington
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Mark Wildman
University of Glasgow
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Numerical thermal history modelling has become a core approach used for interpretation of low-temperature thermochronometry data. Modelling programs can find rock time-temperature (t-T) paths that fit the input data while incorporating independent geologic information about a sample’s history and leveraging the factors that impact the kinetics of each thermochronometric system (e.g., grain size, radiation damage, and composition). HeFTy (Ketcham, 2005) and QTQt (Gallagher, 2012) are two of the commonly used tools for both forward and inverse t-T modeling. The modelling process involves making key decisions about (i) data input, (ii) initial set-up of model space and parameters, (iii) kinetic model(s) (i.e. annealing, diffusion, radiation damage), and (iv) additional t-T constraints. In addition, users need to have an understanding of the statistical methods underlying the modelling approach to be able to interpret the model outputs and the relationship between the observed and predicted data. However, these modeling tools currently lack clear and accessible entry-points for all users—experienced and new thermochronologists alike—and thus for many geoscientists, there is a substantial barrier to the modeling, interpretation, and publication of thermochronologic datasets. Here we present a suite of simple forward and inverse models that we recommend everyone perform before embarking on t-T modeling in HeFTy and/or QTQt for the first time. At the core of the exercises are the six different t-T paths used by Wolf et al. (1998) to illustrate the partial-retention behavior of the apatite He system; however, this approach can easily be applied to other systems as well. This exercise not only illustrates the fundamental behavior of thermochronologic systems but also guides users through the main functionality of the modelling programs. Despite the apparent simplicity of this exercise, users will experience most of the challenges and opportunities common to thermal history modeling, including: how to enter data; error handling; how to use geologic constraints in t-T space; the non-unique nature of cooling ages; the power of grain size and eU variability; the limitations on a model’s ability to resolve the ‘right’ rock thermal history; and how to evaluate the sensitivity of model results to all these factors. These exercises were introduced in the Thermo2020/1 Sunday workshops for both QTQt and HeFTy and are more fully fleshed out in two publications currently in preparation.