Fission Tracks

Science. Radioactive decay of uranium isotopes in the mineral grain releases alpha particles that pass through the lattice of the crystal, leaving a trace of their path, these paths are “fission tracks” Annealing point, distance from annealing point The number of Uranium fission tracks corresponds to the amount of uranium in the rock Used in Zircon and Apatite, accessory minerals in metamorphic and igneous rocks. AFTA, ZFTA. Termochronologicical technique.

Techniques How are fission tracks measured? In the case of apatite, if the grains are heated above their closure temperature, all existing fission tracks will be annealed and the fission track age will be reset. Uranium-238 decays at a known rate and is the only isotope with a decay rate that is relevant to the production of fission tracks. Chemically etch the fission tracks and use of an optical microscopy to count the fission tracks (generally 1-15 micrometers). F s = ( λ f / λ α ) 238 U(e λ t -1)

Applications Used to figure out what time in the past a mineral was at a certain temperature. Calculate erosion rates, faulting rates If a rock cools quickly as in the case of a volcanic rock or a shallow igneous intrusion, the fission-track ages will date this initial cooling. Has been to applied to diverse areas of earth sciences, such as volcanology, stratigraphy, basin analysis and extraterrestrial bodies. “Fissure Track Dating of Meteorite impacts” The age of the impact craters was estimated by Storzer and Wagner (1977) at approximately 6400 yr, using fission tracks in impact melt droplets.

Sources of Errors? Laboratory Analysis: Relying on looking and counting of the fission tracks under microscopy. Due to annealing points, tracks can be erased. The value of the fission decay constant or U-238 is currently in contention. “not too many, not too few” fission tracks