Diffusion of helium in radiation-damaged zircon

Abstract

Diffusion of helium was characterized in natural zircon that had been implanted with Kr at a range of energies to simulate the effects of alpha recoil in natural radiation damage. Polished slabs of zircon, oriented either normal or parallel to c, were implanted with Kr+ ions at a series of energies to produce a damaged region ∼ 1μm in thickness, then implanted with 100keV 3He at a dose of 1×1015 3He/cm2, which placed the 3He within the region damaged by the Kr ion implantation. The implanted zircons were annealed isothermally in Pt capsules in 1-atm furnaces at temperatures ranging from 51 to 252°C and times ranging from 15min to 3 weeks. 3He distributions following experiments were measured with Nuclear Reaction Analysis using the reaction 3He(d,p)4He. For diffusion in zircon normal to c, we obtain the following Arrhenius relation:D=1.7×10−8 exp(−71±2kJ mol−1/RT) m2sec−1.This activation energy for diffusion is about half of the value for He diffusion in zircon from the same sample locality but without Kr implantation, measured with an otherwise similar experimental and analytical approach (Cherniak et al., 2009). He diffusivities in the Kr-implanted material are also significantly faster. Further, He diffusivities normal and parallel to c in the Kr-implanted zircon are similar, in contrast to the marked diffusional anisotropy (with differences of two orders of magnitude for diffusion normal and parallel to c) observed for He diffusion in this zircon in samples not implanted with Kr.These data indicate that heavily damaged zircon will have very limited retentivity of helium. Only those zircons larger than a few hundred μm will have mean closure temperatures exceeding typical earth surface temperatures; mean closure temperatures for He in highly damaged zircon will be about ∼200°C lower than those for relatively undamaged zircon. At 200°C, equivalent amounts of He will be lost from severely damaged zircon at a rate about a million times faster than from relatively undamaged zircon. For retentivity of He comparable to that for low-damage zircon at 200°C, the severely damaged zircon would require residence temperatures of ∼ -10°C.