A natural quartz sample was irradiated using a 200MeV proton beam in order to produce spallogenic 21Ne, 3He and 4He. Their temperature-dependent diffusivities were then determined simultaneously by means of high-precision stepped heating and noble-gas mass spectrometry. The outward mobility of the proton-induced species reflected diffusion through the quartz lattice. At 70 to 400C, the He diffusion coefficients exceeded those of Ne by 5 to 7 orders of magnitude. The diffusion parameters were 153.7kJ/mol and ln(Do/a2) = 15.9ln[/s], and 84.5kJ/mol and ln(Do/a2) = 11.1ln[/s], for proton-induced 21Ne and 3He, respectively. The Ne diffusion parameters also indicated that diffusive loss had to be considered for small (<1mm) quartz grains that had experienced high temperatures. A roughly 70kJ/mol higher activation energy for Ne diffusion, as compared with He diffusion, was suggested to reflect an energy barrier related to its 13% greater diameter. The diffusion parameters for proton-induced 4He were indistinguishable from those for 3He; thus providing no evidence for the commonly expected inverse square-root of mass diffusion-relationship between isotopes. The preliminary indications were that increased exposure to radiation could enhance Ne and He retentivity in quartz at low temperatures.

Diffusion Kinetics of Proton-Induced 21Ne, 3He and 4He in Quartz. D.L.Shuster, K.A.Farley: Geochimica et Cosmochimica Acta, 2005, 69[9], 2349-59