Stepped heating experiments in the range of 1200 to 1700C were performed on terrestrial diamonds, some containing substantial cosmic ray-produced 3He, in order to study the diffusion coefficients and release characteristics of 3He and 4He. Cosmogenic 3He was shown to be much more retentive than 4He in the same sample, with as little as 0.3% of the total 3He, and up to 27.4% of the 4He released by 1700C from diamonds in which cosmogenic 3He predominates (bulk 3He 4He = 176RA). A tentative determination, from the data, of the site activation energy for cosmogenic 3He gave ~150kJ/mol. The diffusion equation, with D0 = 6.1 x 10-11cm2/s, yielded an extrapolated diffusivity at 1200C of 1.9 x 10-16cm2/s. Other helium components were difficult to distinguish from each other by their apparent diffusion coefficients. In diamonds expected to have relatively low proportions of cosmogenic 3He, apparent D(3He) ≈ D(4He), with measured and inferred diffusivities at 1200C of 10-13 to 10-10cm2/s. The higher apparent diffusivities of these components indicated a difference in distribution and siting relative to the homogeneous cosmogenic component. Diffusion coefficients from repeated isothermal extractions were successively lower, possibly indicating that near-surface or dislocation sites were depleted at lower temperatures. It was predicted that surface-correlated helium, such as that implanted from external radioactive decays, should be easily distinguishable by studies comparing grain sizes. Helium sited in isolated inclusions may be expected to have lower effective D than cosmogenic 3He, since removal from the inclusions was the rate-limiting step in low solubility systems (Trull and Kurz, 1993). However, this was not observed in the present study. Bulk retention of trapped mantle helium over the age of most diamonds (> 1Ga) would require effective diffusion coefficients at least several orders of magnitude lower than that inferred for the cosmogenic helium component at 1200C, typical of mantle temperatures.

Helium Isotope Diffusion in Natural Diamonds. Wiens, R.C., Lal, D., Rison, W., Wacker, J.F.: Geochimica et Cosmochimica Acta, 1994, 58[7], 1747-57