Abstract:
The diffusion of defects (e.g., vacancies and interstitials) and elements used for dating (e.g., He and Pb) in a mineral structure is a thermal process: This is the primary assumption used to determine the age and thermal history of minerals. For
instance, thermal history reconstruction, through the number and length distribution of tracks produced by spontaneous fission of 238U, is obtained by assuming a thermal event to be the only energy source for shortening of fission tracks. Here, we
report a new, non-thermal energy source that induces additional shortening of fission tracks by the irradiation of alpha-recoils
from the alpha-decay of 238U and 232Th. We simulate alpha-decay induced track-shortening by combining ion accelerator
irradiations with transmission electron microscopy. This allows for the first observation of track-shrinkage during in situ
ion irradiation. We show that rather than alpha-particles, alpha-recoils induce a significant shortening of fission tracks by
nuclear-collisions. The shortening of track-length can be quantified as a function of alpha-decay event dose. However, apatite
is less sensitive than zircon to this non-thermal process. The findings exemplify the interactions among different types of selfirradiation from alpha-particles, alpha-recoils and fission-fragment nuclei in single mineral grains and have important implications for the use of zircon and apatite for radiometric dating and thermochronology.
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