DEGASSING MECHANISMS OF NOBLE GASES FROM CARBONADO DIAMONDS

Abstract

Two carbonado diamonds were studied for helium diffusion by employing incremental stepwise-heating for temperature ranges from 350 to 1800°C. Arrhenius plots of experimental data demonstrate that there exist two degassing mechanisms of helium from diamond; the lower temperature degassing (T < 1600°C) is volume diffusion and the higher temperature degassing (T > 1600°C) is essentially governed by graphitization of diamond. Diffusivity calculated from the observed release rates of the lower temperature range assuming spherical grains and initially homogeneous helium distributions defines Arrhenius relations with an activation energy of 103.5 +/- 4.8 kJ/mol and a pre-exponential factor, D0 = 10-13.2 +/- 0.2 cm2/s in carbonado diamond. Using these values, we can estimate the diffusion coefficient of He to be 3 x 10-17 cm2/s at 1300°C.