Interrogating the Effects of Radiation Damage Annealing on Helium Diffusion Kinetics in Apatite

Abstract:

Apatite (U-Th)/He thermochronology is commonly used to study landscape evolution and potential links between climate, erosion and tectonics. The technique relies on a quantitative understanding of (i) helium diffusion kinetics in apatite, (ii) an evolving ⁴He concentration, (iii) accumulating damage to the crystal lattice caused by radioactive decay^[1], and (iv) the thermal annealing of such damage^[2],[3], which are each functions of both time and temperature. Uncertainty in existing models of helium diffusion kinetics has resulted in conflicting conclusions, especially in settings involving burial heating through geologic time. The effects of alpha recoil damage annealing are currently assumed to follow the kinetics of fission track annealing (e.g., reference [3]), although this assumption is difficult to fully validate. Here, we present results of modeling exercises and a suite of experiments designed to interrogate the effects of damage annealing on He diffusivity in apatite that are independent of empirical calibrations of fission track annealing. We use the existing experimental results for Durango apatite^[2] to develop and calibrate a new function that predicts the effects of annealing temperature and duration on measured diffusivity. We also present a suite of experiments conducted on apatite from Sierra Nevada, CA granite to establish whether apatites with different chemical compositions have the same behavior as Durango apatite. Crystals were heated under vacuum to temperatures between 250 and 500°C for 1, 10, or 100 hours. The samples were then irradiated with ~220 MeV protons to produce spallogenic ³He, the diffusant then used in step-heating diffusion experiments. We compare the results of these experiments and model calibrations to existing models.

Citations: ^[1]Shuster, D., Flowers R., and Farley K., (2006), EPSL 249(3-4), 148-161; ^[2]Shuster, D. and Farley, K., (2009), GCA 73 (1), 6183-6196; ^[3]Flowers, R., Ketcham, R., Shuster, D. and Farley, K., (2009), GCA 73, 2347-2365.