It was recalled that recent work had revealed a correlation, between radiogenic 4He

concentrations and He diffusivity in natural apatites, which suggested that helium

migration was retarded by radiation-induced damage to the crystal structure. This

would imply that the He diffusion kinetics of an apatite were a changing function

of time and of the effective uranium concentration in a cooling sample.

Experiments were performed here in order to investigate and quantify the

phenomenon by determining the He diffusivities in apatites after systematically

adding or removing radiation damage. Radiation damage was produced in synthetic

and natural apatites by exposure to between 1 and 100h of reactor neutrons. The

samples were then bombarded with a 220MeV proton beam and the resultant

spalled 3He was used as a diffusant in step-heating diffusion experiments. In each

sample, irradiation was found to increase the activation energy, Ea, and frequency

factor, Do/a2, for diffusion and led to a higher He closure temperature, Tc, than that

of the starting material. Thus, 100h in the reactor caused the He closure

temperature to increase by as much as 36C. For a given neutron fluence, the

increase in closure temperature scaled negatively with the initial closure

temperature. This was consistent with the neutron damage being added to any

initial damage. The irradiation produced correlated increases in Ea and ln(Do/a2)

which followed the same pattern as those found in natural apatites. This strongly

suggested that neutron-induced damage imitated the damage that was produced by

U and Th decay in natural apatites. To investigate the possible effect of annealing

out radiation damage, Durango apatite was heated (vacuum, up to 550C, 1 to

350h). The samples were then step-heated using the remaining natural 4He as the

diffusant. A systematic change in Tc was observed at above 290C, with the values

becoming lower with increasing temperature and time. Thus, a reduction in Tc from

71 to 52C occurred in 1h at 375C or 10h at 330C. The variations in Tc were closely

related to the fission-track length reduction predicted using the initial holding time

and temperature. As in the case of the neutron-irradiated apatites, these samples fell on the same Ea−ln(Do/a2) line as did natural samples. It was concluded that

damage-annealing was simply undoing the damage-accumulation effect on He

diffusivity.

The Influence of Artificial Radiation Damage and Thermal Annealing on Helium

Diffusion Kinetics in Apatite. D.L.Shuster, K.A.Farley: Geochimica et

Cosmochimica Acta, 2009, 73[1], 183-96