3He/4He values in natural diamonds

Abstract

Values of the 3He/4He ratio in natural diamond crystals appear to be higher than the so‐called primordial ratios observed in meteorites, according to a recent report by Minoru Ozima and Shigeo Zashu of the Geophysical Institute at the University of Tokyo (Science, March 4, 1983). The values obtained from helium fractions of 13 diamonds ranged from less than 10−7 to 3.2±0.25×10−4, indicating a rather large enrichment of primitive helium. The measured ratios are close to those for solar‐type helium.The significance of those determinations could be two‐fold, raising questions about the interpretation of helium isotope ratios from other surface samples and from other terrestrial sources. A possibility is that the diamonds may have originated in a deep, chemically isolated, and decoupled part of the earth's mantle. The problem is that this decoupling would have to be total, implying essentially no communication of the deep mantle with the rest of the earth. Ozima and Zashu prefer to think that the diamonds themselves preserved the helium ratio of a very primitive environment by being unusually depleted in uranium and thorium. This latter explanation, if correct, would mean that in at least one case, i.e., these diamonds, one cannot determine the geological evolution of 3He/4He reliably.