Badly Behaved Detrital (U‐Th)/He Ages: Problems With He Diffusion Models or Geological Models?

Abstract

Dispersion in apatite (U‐Th)/He ages makes extracting thermochronological information challenging. This is particularly problematic for constraining the thermal history of sedimentary basins, as the total amount of heating during burial can be small and at the limits of the sensitivity of the apatite (U‐Th)/He system in apatite, but undetectable with other thermochronometric systems. Here we explore the importance of the differences in predepositional thermal histories for individual crystals and the effect of radiation damage‐related diffusion during both the predepositional and postdepositional thermal history on the dispersion of apatite (U‐Th)/He ages. Different predepositional thermal histories can lead to a large spread of detrital ages due to radiation damage and its effect on He diffusivity. The resulting ages may appear scattered, and relationships between age and [eU], a conventional proxy for accumulated radiation damage, may be nonexistent or nonsensical. Furthermore, other variables that are expected to control He diffusivity could vary as a function of source region, resulting in false conclusions about controls on He diffusivity and the applicability of the apatite (U‐Th)/He system in general. We suggest that in some situations in which detrital apatite (U‐Th)/He age does not correlate with [eU], it is not because of problems with the thermochronometric system, but problems with the assumed geological model.

Innovations in (U–Th)/He, Fission Track, and Trapped Charge Thermochronometry with Applications to Earthquakes, Weathering, Surface‐Mantle Connections, and the Growth and Decay of Mountains

“…Thermal history information should only be derived from bedrock samples that exhibit date-eU and/or date-grain size patterns, including uniform dates regardless of eU or grain size values (Flowers & Kelley, 2011). Samples yielding overdispersed (U-Th)/He dates uncorrelated with eU or grain size should either not be modeled or more information is required to understand the date scatter (Flowers & Kelley, 2011;Fox et al, 2019).…”

Sediment provenance and routing evolution in the Late Cretaceous–Eocene Ager Basin, south‐central Pyrenees, Spain

“…Hence, it should be considered whether partial/total ZHe resetting of radiation damaged zircons might have occurred, below nominal closure temperatures, in slowly coooled bedrock samples prior to tectonic unroofing or after depositional burial in the basin (Fosdick et al, ; Reiners, ). Radiation damaged zircons in the source region are susceptible to resetting at lower temperatures and may have been delivered to the basin before exhumation of the nominal fossil partial retention zone (PRZ) (Fox, Dai, & Carter, ; Guenthner et al, ). Post depositional resetting is unlikely, expect in heavily radiation damaged zircons, as estimates of basin fill thickness of <3.5 km and apatite fission‐track thermal modeling results from the Ager Basin suggest maximum burial temperatures of <140°C (Fillon et al, ; Gómez‐Gras et al, ).…”

Resolving the effects of 2-D versus 3-D grain measurements on apatite (U–Th) ∕ He age data and reproducibility

“…6). The U, Th, and Sm concentrations, often combined into a single term, "effective uranium" (eU), have been used a proxy for radiation damage within a crystal, and age versus eU correlations are commonly used for interpretation of age scatter and thermal history inverse modeling (e.g., Guenthner et al, 2013;Fox et al, 2019). Therefore, accurate knowledge of volume has cascading effects from mass to eU concentration and age interpretation (Fig.…”

“…It has been shown that due to uncertainties in grain geometry, stopping distances, and parent nuclide zonation and variability, this correction can contribute > 50 % of the total analytical uncertainty (Farley and Stockli, 2002). Similarly, low error, highly dispersed apatite (U-Th) / He ages are problematic for robust interpretation and time-temperature modeling (e.g., Fox et al, 2019). The observation that the scatter of measured ages in even well-understood samples exceeds expectation based on analytical errors, combined with the knowledge that the above simplifications will not always hold, has led to the practice of Published by Copernicus Publications on behalf of the European Geosciences Union.…”

“…In practice, for the determination of a correct He age, the grain dimensions and shape must be measured to compute an F T correction factor prior to He and U, Th, and Sm analysis, assuming either parent nuclide homogeneity or prescribing an assumed or measured 1-D or 2-D parent nuclide zonation (Farley et al, 1996;Farley, 2002). While not directly related to the computation of He ages, these same grain dimensions are also used to calculate grain size parameters for the purpose of calculating isotopic and/or elemental concentrations and for age interpretation and diffusion or thermal history modeling (Shuster et al, 2006;Flowers et al, 2007Gautheron et al, 2009;Flowers and Kelley, 2011). For example, the grain mass, which is used to calculate the grain U, Th, Sm, and He concentrations, is often derived from the grain volume and an assumed density.…”