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Bart W.H. Hendriks, Geological Survey of Norway (Norway)
Ray Donelick, Apatite to Zircon Inc. (United States)
Paul O'Sullivan, Apatite to Zircon Inc. (United States)
Tim Redfield, Geological Survey of Norway (Norway)
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Apatite fission-track (AFT) analyses using a LA-ICP-MS based approach on samples from Finland and Canada reveal an inverse correlation between single-grain AFT ages and the present-day concentration of alpha-emitter actinides (U,Th,Sm) in part of these samples. Because of the single-grain approach, this inverse correlation cannot be explained simple by intra-grain variations in temperature history as a function of radio-active heat production.
We see three possible explanations for this observation: 1) the observed inverse correlation is related to problems observing high track densities, which would explain why old samples with high alpha-emitter concentrations are rarely encountered, 2) grains with higher concentrations of alpha-emitters have experienced a high degree of radiation enhanced annealing , leading to younger AFT ages, or 3) because Rare Earth Elements (REE's, specifically La and Ce) correlate positively with the affected single-grain AFT ages, the observed (U,Th,Sm) dependency might be due to a REE control upon fission tracks in apatite that so-far has gone undetected for these elements in the laboratory setting. The first possibility has been eliminated in a first re-examination of grain mounts where the inverse correlation of alpha-emitters with single-grain AFT age was observed. Very few uncountable grains were encountered, which implies that the presence of uncountable grains (due to high track densities) is not the primary explanation. An explanation based on radiation enhanced annealing is compatible with results on radiation effects in fluorapatite studied in the context of nuclear waste sequestration. Experiments attempting to quantify the radiation enhanced annealing on fission track in apatite are currently underway. A systematic control of rare earth elements on fission track annealing behavior has not been observed in laboratory studies, but may exist when low temperatures ( ∼60°C) are maintained over very long periods of geological time.
Neither of the two explanations that seem most likely (radiation enhanced annealing and a systematic control by rare earth elements) are incorporated in existing annealing models. Using such models to model thermal histories on samples with pre-Mesozoic AFT ages will likely engender incorrect geological conclusions.
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