Role of Defects and Radiation Damage on He Diffusion in Magnetite: Implication for (U-Th)/He Thermochronology

Abstract

The discovery of He retentivity in magnetite has opened up the use of the magnetite (U-Th)/He method as a thermochronometer to date the exhumation of mafic and ultramafic rocks, and also as a chronometer to date magnetite crystallization during serpentinization. However, published He diffusion data reveal more complex behavior than expected. To resolve this issue and generalize the understanding of He retention in magnetite, we conducted a multiscale theoretical study. We investigated the impact of natural point-defects (i.e., vacancies unrelated to radiation damage) and defects associated with radiation damage (i.e., vacancies and recoil damage that form amorphous zones) on He diffusion in magnetite. The theoretical results show that He diffusion is purely isotropic, and that defect-free magnetite is more He diffusive than indicated by experimental data on natural specimen. Interestingly, the obtained theoretical trapping energy of vacancies and recoil damage are very similar to those obtained from experimental diffusion data. These results suggest that He diffusion in magnetite is strongly controlled by the presence of vacancies and radiation damage, even at very low damage dose. We propose that, when using magnetite (U-Th)/He thermochronometry, the impact of vacancies and radiation damage on He retention behavior should be integrated.

Hematite Frictional Behavior and He Loss From Comminution During Deformation Experiments at Slow Slip Rates

“…Thus, during fault slip, He loss may arise from the formation of extensive new grain surfaces that intersect and expose He where it was previously trapped within interstitial space in the crystal lattice (Figure 7). Density Functional Theory and Kinetic Monte Carlo simulations of He diffusion in goethite and magnetite suggest He is trapped in crystallographic defects and alpha recoil damage (Bassal, Heller, et al., 2022; Bassal, Roques, et al., 2022). If He in hematite is trapped preferentially in recoil damage, we speculate that He loss from gouge could reflect the preferential formation and or propagation of microcracks along damage sites; these microcracks would expose He thereby facilitating He loss during slip and comminution.…”

(U-Th)/He Geochronology Constraints on Lateritic Duricrust Formation on the Guiana Shield

“…The He diffusion leads to underestimated (U-Th)/He ages due to the polycrystalline nature of supergene Fe-oxides (Figure 5H) (Shuster et al, 2005). Bassal et al (2022) proposed that He retention is linked to the damage dose and the Al-substitution of the Fe-oxide. In addition, porosity and crystallinity of the different generations can impact He retention and lead to younger ages (Farley, 2018).…”

“…30% of Al-substitution in the nodular and vacuolar Fe-crusts), that should decrease goethite crystallinity and consequently may increase He retention, due to smaller crystallite size (Fitzpatrick and Schwertmann, 1982;Schulze, 1984;Schwertmann and Carlson, 1994;Vasconcelos et al, 2013). Therefore, we systematically applied the correction for He diffusion to our samples following the consideration of Bassal et al (2022) (Supplementary Table S4). The different grains of each subsample usually display homogenous U and Th concentrations within the same range of values and with similar Th/U ratios (Figure 7), attesting to similar weathering history.…”

“…As only one aliquot is characterized with an age younger than 0.8 Ma, the age is reported with the <0.8 Ma value. In addition, a correction of the He loss associated with samples of polycrystallinity and Al content has been applied with 5% ± 5% for the Al-rich samples and 15% ± 15% for the Al-poor samples following the proposition of Bassal et al (2022).…”