Other
Scientific paper
Apr 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.1747w&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 7, pp.1747-1757
Other
4
Scientific paper
Stepped heating experiments in the range of 1200-1700°C were performed on terrestrial diamonds, some containing substantial cosmic ray-produced 3 He, in order to study the diffusion coefficients and release characteristics of 3 He and 4 He. Cosmogenic 3 He is shown to be much more retentive than 4 He in the same sample, with as little as 0.3% of the total 3 He, and up to 27.4% of the 4 He released by 1700°C from diamonds in which cosmogenic 3 He predominates (bulk 3 He / 4 He = 176 R A ). A tentative determination, from the data, of the site activation energy for cosmogenic 3 He gives ~ 150 kJ/mol. The diffusion equation, with D 0 = 6.1 × 10 -11 cm 2 / s , yields an extrapolated diffusivity at 1200°C of 1.9 × 10 -16 cm 2 /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 3 He, apparent D ( 3 He ) D ( 4 He ), with measured and inferred diffusivities at 1200°C of 10 -13 to 10 -10 cm 2 /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 3 He, 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.
Lal Devendra
Rison William
Wacker John F.
Wiens Roger C.
No associations
LandOfFree
Helium isotope diffusion in natural diamonds does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Helium isotope diffusion in natural diamonds, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Helium isotope diffusion in natural diamonds will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1256611