Abstract
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 Kr2+ (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.
Highlights
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]
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[1,2,3,4]
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[5,6,7]: The thermally enhanced Pb diffusion during a reheating event leads to the redistribution of the radiogenic 207Pb and 206Pb within the nano-clusters produced by alpha-decay
Summary
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]. We demonstrate how alpha-particles can cause the recovery of radiation damage in apatite using in situ TEM consecutive ion-irradiation experiments (see Method section).
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