Numerical investigations of apatite ⁴He/³He thermochronometry

Abstract

Apatite 4He/3He thermochronometry has the potential to constrain cooling histories for individual samples provided that several presently untested assumptions are valid. Here we simulate the sensitivity of 4He/3He spectra to assumptions regarding geometric model, crystallographic anisotropy, broken grain terminations, parent nuclide zonation, and the accuracy of results obtained from analyses of aggregates of multiple crystals. We find that 4He/3He spectra obtained from a cylinder with isotropic diffusion are almost indistinguishable from those obtained from an equivalent sphere with an equivalent initial 4He distribution. Under similar conditions anisotropic diffusion from the cylinder can greatly bias 4He/3He spectra, but only if diffusion is >10 times faster in the axial than the radial direction. Existing data argue against anisotropy of this magnitude. We find that analysis of apatites with broken terminations will also bias 4He/3He spectra, but not greatly so. In contrast, we find that zonation of a factor of 3 in parent nuclide concentration produces 4He/3He spectra that deviate substantially from the homogeneous model. When parent nuclides are highly concentrated near the grain rim and/or cooling is fast, the resulting 4He/3He spectra will be readily identified as aberrant. However, more subtle zonation, higher concentrations in the grain interior, or samples that have cooled slowly regardless of zonation style can yield 4He/3He spectra that look acceptable but will lead to inaccurate thermochronometric interpretation if parent homogeneity is assumed. Finally, we find that analysis of an aggregate of crystals with identical 4He distributions can yield 4He/3He spectra (and diffusion Arrhenius arrays) that are very different from those that would be obtained on the individual crystals if even small variations in He diffusion exist among the grains. Overall, our observations suggest that modeling tools that assume spherical geometry and isotropic diffusion are appropriate for interpreting apatite 4He/3He spectra. However, it is essential to analyze only individual crystals and to assess the degree of parent nuclide zonation in those crystals.

Thermochronology in Orogenic Systems

“…Several applications have demonstrated the utility of the apatite 4 He/ 3 He technique for tectonic and landscape evolution studies (Colgan et al, 2008;Schildgen et al, 2010;. Recently, Farley et al (2010) explored a variety of issues that could undermine the reliability of thermal histories deduced from apatite 4 He/ 3 He experiments under certain circumstances. They found that the 4 He/ 3 He studies of multigrain aliquots and grains that are highly zoned in U and Th may yield uninterpretable results.…”

Incorporating 3‐D parent nuclide zonation for apatite 4He/3He thermochronometry: An example from the Appalachian Mountains

“…Numerical experiments have explored the sensitivity of stepwise degassing 4 He/ 3 He data to anisotropic diffusion, broken crystals, and spatially variable 4 He production; the results highlight a need to account for these complexities in certain cases [ Farley et al ., ]. In addition, the influence of radiation damage accumulation and annealing causes spatial variability in He diffusion kinetics, and when simplified using a spherical approximation, such variability can be treated numerically [ Farley et al ., ; Flowers and Farley , ; Fox and Shuster , ].…”

U and Th zonation in apatite observed by laser ablation ICPMS, and implications for the (U–Th)/He system

“…As a practical matter we adopt a spherical geometry to allow us to use widely available computational tools, especially the HeFTy helium production-diffusion code (Ketcham, 2005). The validity of the spherical approximation has been demonstrated by numerous previous workers (Meesters and Dunai, 2002;Farley et al, 2010;Gautheron and Tassan-Got, 2010;Watson et al, 2010).…”

“…This modification can affect the 4 He ages since it is this gradient that governs diffusive loss. Similarly the 4 He/ 3 He technique, which documents the 4 He concentration gradient, is sensitive to the parent nuclide distribution (Shuster and Farley, 2004;Farley et al, 2010). Finally, because He diffusion rates are sensitive to radiation damage density in apatite , intracrystalline variations in U and Th distribution can cause intracrystalline variations in He diffusivity.…”