AmericanChemicalSociety.com

The ultimate goal of this project is to better understand diffusion of helium in apatite, because the U-Th/He dating of apatite is proving of great value in determining the low-temperature thermal history of rocks and sedimentary basins. In this project year, we continued to investigate the solubility of helium in apatite, which is turning out to be more interesting and more significant than we had thought. Our original intention was simply to saturate annealed apatites with helium, irradiate them, and then investigate the diffusion kinetics of helium in this material as a function of fission-track damage alone, having eliminated alpha-recoil damage by annealing. However, it has proven difficult to get consistent results from the saturation experiments, and we observe complex behavior of the samples beyond simple Henry's Law solubility. This year in collaboration with Bruce Watson and Jay Thomas at RPI, we subjected samples to far higher helium partial pressures of 30 to 100 bars. Although the scatter is very large, it appears that by 30 bars the samples were saturated. Thus, we have only managed to bracket the likely Henry's Law pressure range (10 mbar, far too low; 30 bars, far too high). Also, we see unusually large scatter in measuring treated samples, far outside normal analytical uncertainty, suggesting that the solution process is not simple and other material properties are coming into play. We have requested a no-cost one year extension to this project to (a) conduct additional runs to assess Henry's Law behavior, (b) understand the origins of the large analytical scatter, and (c) finally use treated samples that we understand to do the proposed radiation-damage study.