Helium isotope diffusion in natural diamonds

Abstract

Abstract Stepped heating experiments in the range of 1200–1700°C 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 is 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 1700°C from diamonds in which cosmogenic 3He predominates (bulk 3 He 4 He = 176 R A ) . A tentative determination, from the data, of the site activation energy for cosmogenic 3He gives ~ 150 kJ/mol. The diffusion equation, with D0 = 6.1 × 10−11cm2/s, yields an extrapolated diffusivity at 1200°C of 1.9 × 10−16cm2/s. Other helium components are difficult to distinguish from each other by their apparent diffusion coefficients. In diamonds expected to have relatively low proportions of cosmogenic 3He, apparent D( 3 He) ≈ D( 4 He) , with measured and inferred diffusivities at 1200°C of 10−13 to 10−10 cm2/s. The higher apparent diffusivities of these components indicate a difference in distribution and siting relative to the homogeneous cosmogenic component. Diffusion coefficients from repeat isothermal extractions are successively lower, possibly indicating that near-surface or dislocation sites are depleted at lower temperatures. We predict that surface-correlated helium, such as that implanted from external radioactive decays, should be easily distinguishable by studies comparing grain size separates. Helium sited in isolated inclusions may be expected to have lower effective D than cosmogenic 3He, since removal from the inclusions is the ratelimiting step in low solubility systems (Trull and KURZ, 1993). However, this was not observed in our study. Bulk retention of trapped mantle helium over the age of most diamonds (> 1 Ga) would require effective diffusion coefficients at least several orders of magnitude lower than that inferred for the cosmogenic helium component at 1200°C, typical of mantle temperatures.