Radon loss from zircon: Effects of microstructure on radon emanation and diffusion.

摘要

Radon emanation from rocks and minerals is ubiquitous, but the mechanisms of radon loss are not well understood. Quantification of radon emanation rates from zircon has potential bearing on the reliability of U-Pb ages of zircon bearing rocks. The 238U decay chain includes 222Rn, a noble gas, which has a half-life of 3.82 days and can escape from the crystal structure of zircon if sufficient pathways exist, or by recoil if the parent 238U was very near the outer edge of the crystal. Loss of 222Rn ultimately leads to a deficiency of 206Pb, resulting in discordance between 238U-206Pb, 235U-207Pb, and 232Th-208Pb ages. In order to evaluate the factors affecting radon loss from zircon, we performed two experiments: one focused on the effect of microstructure on room temperature 222Rn emanation, and the other to investigate 222Rn loss by high temperature diffusion. Large (~100 g) single crystal zircon samples from each of three localities were selected for this study: Mud Tank, Malawi, and Bancroft. The zircons were pulverized and five grain sizes (500 mum, 250-500 mum, 125-250 mum, 63 - 125 mum, and < 63 mum) were separated from each. Room temperature radon emanation rates were measured for an aliquot of each grain size. To investigate the effects of microstructure, in particular fission track density, separate aliquots were heated to temperatures of 200°C, 300°C, 400°C, 600°C, and 800°C for six hours after which they were cooled to room temperature and radon emanation rates were measured. Fission track densities were measured after the same annealing steps in the Mud Tank zircon, allowing quantification of 222Rn emanation rate as a function of fission track density. In general, radon emanation rates decrease with decreasing fission track density, but increase when all fission tracks are annealed, suggesting the possibility of using 222Rn to assess defect density within crystals. To investigate diffusive loss of 222Rn, we heated separate aliquots of each grain size of the Bancroft zircon to 975°C for different durations. 222Rn loss after heating was assessed by measuring the activity of the daughter products of 222Rn (214Bi and 214Pb), in addition to 226Ra, 228Ra, 234Th, and 210Pb, using gamma spectroscopy before and after heating. Results indicate slow diffusion of 222Rn, and suggest there may be structural changes in the zircon lattice at long heating durations. Results of both experiments have implications for U/Th-Pb geochronology (i.e., discordant ages), and noble gas escape systematics in zircon (i.e., volume diffusion or fast pathway escape).