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TitleModelling competing effects of cooling rate, grain size and radiation damage in low temperature thermochronometers
AuthorKellett, D AORCID logo; Whipp, D; Coutand, I
SourceGeochronology vol. 4, 2022 p. 143-152, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20210364
PublisherEuropean Geosciences Union
Mediapaper; on-line; digital
File formatpdf
Subjectsgeochronology; fission tracks; fission-track dates; zircon; apatite
Illustrationsplots; models
ProgramGEM-GeoNorth: Geo-mapping for Energy and Minerals GEM Program Coordination
Released2022 03 22
AbstractLow-temperature multi-thermochronometry, in which the (U-Th) = He and fission track methods are applied to minerals such as zircon and apatite, is a valuable approach for documenting rock cooling histories and relating them to geological processes. Here we explore the behaviors of two of the most commonly applied low-temperature thermochronometers, (U-Th) = He in zircon (ZHe) and apatite (AHe), and directly compare them against the apatite fission track (AFT) thermochronometer for different forwardmodeled cooling scenarios. We consider the impacts that common variations in effective spherical radius (ESR) and effective uranium concentration (eU) may have on cooling ages and closure temperatures under a range of different cooling rates. This exercise highlights different scenarios under which typical age relationships between these thermochronometers (ZHe > AFT > AHe) are expected to collapse or invert (either partially or fully). We anticipate that these predictions and the associated software we provide will be a useful tool for teaching, planning low-temperature multi-thermochronometry studies, and for continued exploration of the relative behaviors of these thermochronometers in temperature-time space through forward models.
Summary(Plain Language Summary, not published)
Low temperature multi-thermochronometry, in which thermochronometric methods such as (U-Th)/He dating of zircon and apatite, and apatite fission-track dating are combined, is the state-of-the-art for reconstructing rock thermal histories for the upper continental crust. However, our ability to reconstruct thermal histories and make interpretations about the geological significance of measured ages requires modeling of He diffusion and fission track annealing behaviour. Here we use forward models to explore the competing effects that natural ranges in grain size and radioactive content are expected to have on resulting cooling ages and closure temperatures for the (U-Th)/He decay systems in apatite and zircon. We illustrate the relative sensitivity of He diffusion in apatite and zircon to grain size and composition, and their contrasting behaviours with respect to He diffusivity under a wide range of linear cooling histories. Finally, we compare these behaviours with the apatite fission track system, for which ages are not known to be affected by grain size or radioactive decay damage.

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