Degassing mechanisms of noble gases from carbonado diamonds

Diamonds are unique samples for inferring ancient mantle conditions, because of their enormous stability, antiquity and also of mantle origin. However, as temperatures in the mantle where diamonds have existed are very high (greater than 1000 C) and residence time of some diamonds could be more than a few billion years, it is imperative to examine whether or not diamonds have retained their pristine characteristics, especially those of noble gases, under such extreme conditions. As discussed in a review article by Ozima, there are rather large variations in the diffusivity of helium in diamonds obtained in the early determinations. The data are also limited to temperatures higher than 1200 C. In the present study, we conducted more refined diffusion experiments for He using carbonado diamonds, which have large amounts of radiogenic (4)He approximately 10(exp -2) cu cm STP/g. On the basis of the experimentally estimated diffusion coefficients, we will discuss retentivity of He in diamonds in the mantle condition.