Latent Fission Tracks

A major issue in thermochronology and U-Th-Pb dating is the effect of radiation damage, created by α-recoils from α-decay events, on the diffusion of radiogenic elements (e.g., He and Pb) in host mineral. Up until now, thermal events have been considered as the only source of energy for the recovery of radiation-damage. However, irradiation, such as from the α-particle of the α-decay event, can itself induce damage recovery. Quantification of radiation-induced recovery caused by α-particles during α-decay events has not been possible, as the recovery process at the atomic-scale has been difficult to observe. Here we present details of the dynamics of the amorphous-to-crystalline transition process during α-particle irradiations using in situ transmission electron microscopy (TEM) and consecutive ion-irradiations: 1 MeV Kr 2+ (simulating α-recoil damage), followed by 400 keV He + (simulating α-particle annealing). Upon the He + irradiation, partial recrystallization of the original, fully-amorphous Durango apatite was clearly evident and quantified based on the gradual appearance of new crystalline domains in TEM images and new diffraction maxima in selected area electron diffraction patterns. Thus, α-particle induced annealing occurs and must be considered in models of α-decay event damage and its effect on the diffusion of radiogenic elements in geochronology and thermochronology. Diffusion kinetics for noble gas thermochronometry is commonly assumed to be solely a function of temperature , and this is the basis for extrapolating the physical mechanisms observed in laboratory experiments to the temperature and time regimes of natural systems 1. However, recent new models have demonstrated that alpha-recoil damage, i.e., isolated defects induced by α-recoils from α-decay events, significantly reduces the diffusion of noble gases (e.g., He) in apatite 1–4. This effect can be reversed by thermal annealing of the radiation damage 2,3. Similar approaches have been used in the determination of the age of the oldest zircon ~4.4 Ga via a new U-Th-Pb method 5–7 : The thermally enhanced Pb diffusion during a reheating event leads to the redistribution of the radiogenic 207 Pb and 206 Pb within the nano-clusters produced by alpha-decay. In addition, the recovery of fission tracks, another type of radiation damage caused by spontaneous fission of 238 U, is generally considered as diffusion-controlled process 8–10 , an ultimate thermal effect. However, under certain radiation conditions , thermally induced diffusion is less significant than radiation-enhanced diffusion 11,12 as more vacancies and interstitials are produced by interactions of energetic ions than those that are thermally activated 13,14. In apa-tite, alpha-decay from U or Th produces a pair of an alpha-particle (energy: ~4.5 MeV He ion; ion range: ~14 µm), and an alpha-recoil (energy: 60–90 keV; ion range: 20–30 nm) that are ejected in opposite directions (Fig. 1a,b). This study addresses the recovery of alpha-recoil damage in apatite by the irradiation of alpha-particles, another source of energy that can drive the recovery process. Despite the importance of radiation effects in reconstructing the thermal histories and age of rocks 1,5,10,15 , mechanisms of radiation damage and damage recovery in minerals are still poorly understood at the atomic-scale. Radiation damage in minerals is dominated by the accumulation of Frenkel defect pairs based on nuclear stopping power, (dE/dx) n , between heavy α-recoils (usually heavier than Pb) and surrounding atoms (Fig. 1a). This is in contrast to the irradiation of alpha-particles, where electronic stopping power, (dE/dx) e , between the

Fission track (FT) thermochornology is essentially based on empirical fits to annealing data of FTs revealed by chemical etching, because, until now, unetched, latent FTs could not be examined analytically at the atomic-scale. The major challenge to such an analysis has been the random orientation of FTs and their extremely small diameters. Here we use high-energy ions (2.2 GeV Au or 80 MeV Xe) to simulate FT formation along specific crystallographic orientations. By combining results from transmission electron microscopy (TEM) of single tracks and small-angle X‑ray scattering (SAXS) for millions of tracks, a precise picture of track morphology as a function of orientation is obtained. High-resolution analysis reveals that orientation affects the shape of tracks in apatite and zircon through the preferential creation of damage along directions with highest atomic density. However, track radius does not depend on orientation, contradicting previous reports. Independent of track orientation, track radii, as measured at each point along the entire length of 80 MeV Xe ion tracks in apatite, can be understood using the thermal spike model of Szenes. Thus, the well-known track annealing anisotropy of apatite is not due to track radius anisotropy. The combination of ion-irradiations with TEM and SAXS analysis provides a unique opportunity to understand and model track formation and annealing under various geologic conditions.