301-8 Bridging the Gap: Evidence for a Single Gap Model in Apatite Fission Tracks

Session: Geochronology (Posters)

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Abstract:

Fission track thermochronology is based on the analysis of damage trails (tracks) left behind by the spontaneous fission of uranium, which are revealed for analysis by etching. Researchers can model the thermal history of rocks by measuring the distribution of track lengths because they anneal (shorten) within certain temperature windows. Fission tracks generally anneal from the tips inward. However, at advanced stages of annealing shortening accelerates, which has been attributed to the formation of unetchable gaps or pinching of the track in the middle at one or more places. Recent TEM work, inspired by the Ewing research group, has proposed that a tiny gap forms at the site of the track-forming event, spanning the ~50-nm-scale distance required for the fission fragments to ionize fully. This suggests that each track may have one built-in gap. Initially, the gap may be so narrow that the etchant traverses it nearly instantaneously, but with increasing annealing may grow and require progressively longer to etch through, leading to noticeable differences in the degree of etching on either side of the gap.

This study investigates the hypothesis that each fission track contains a single gap at the fission site. Two slabs of Durango apatite cut parallel to the C-axis were fully annealed, and new tracks were induced in a nuclear reactor. These tracks were then thermally annealed to a point where gaps have been observed to occur. The slabs were mounted and etched, and confined tracks were picked according to a standard set of criteria: (1) tracks have to display pinching where one side of the track is wider than the other, and (2) they need to be sufficiently etched for accurate measurement. We have measured 56 confined fission tracks in one sample and over 100 tracks in the second sample. After images are taken of these tracks, track diameters are measured at various locations along each track by drawing circles using Adobe Illustrator to infer etching rates. We then etch the tracks for 10 more seconds, and retake images and measurements to see how the track lengths and diameters have changed.

Verification of the single-gap hypothesis will provide new insights into track structure, leading to better models for how tracks anneal and more accurate interpretations of the etching process.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6926

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