46796-AC2
Impacts of Fission-Track Damage on Helium Diffusion Kinetics in Apatite and Zircon
U-Th/He dating of apatite is proving to be an extremely useful and widely applied tool for quantifying the thermal histories of rocks and geologic features like sedimentary basins, something of considerable significance to hydrocarbon exploration. Critical to the use of this tool is an understanding of the diffusion systematics of helium in the mineral apatite. Our goal is to examine the role that fission-track damage plays in establishing internal traps for 4-He; such traps would alter the manner in which He escapes from grains, particularly under conditions of slow cooling or reheating. In this first year of the project, we have been developing the materials and methods needed to unambiguously establish that fission-track damage is an issue (as opposed to the more common but less energetic alpha-track damage). This work has included development of a precise vacuum step-heating furnace that uses a diode laser to apply square-pulse heating to samples, and work to develop techniques for introducing helium into apatites that contain only fission-track damage, and no alpha damage. This is not easy to do because all natural samples will contain both alpha and track damage, and any heating to anneal tracks will expunge all helium from the sample. Ion implantation of helium will introduce particle damage of a sort that would complicate our results. Thus, we attempted to introduce helium into standard samples via in-diffusion, placing annealed samples into our vacuum extraction system and then pressurizing it with pure 4-He. Our results for apatite parallel empirical inferences suggesting very low noble-gas solubility in minerals, as we were unable to introduce any helium into our samples above blank levels. For the primary purposes of this project, we will have to try an alternative experimental protocol involving slow-neutron irradiation of natural samples to create varying densities of induced fission tracks. Because we can detect low levels of helium, we will use geologically young samples in which the alpha-track dose is low, and then look for the influence of fission-track density on diffusion kinetics. However, our foray into trying to introduce helium into apatite did in itself yield a publishable result, as we were able to place a strong constraint on helium solubility in this mineral. This is significant because all U-Th/He age determinations to date have proceeded on the assumption that the high mobility of helium permits one to discount the presence of any initial helium in a sample when performing an age calculation. Although this assumption seems to work for most samples, it remains a weak link in the method as practitioners expand use of the method to a broader array of geological environments. Our results provide experimental data that verify that in normal crustal environments, apatite solubility is sufficiently low that excess 4-He should not impact age determinations, even in young rocks.
