Helium isotopic variability within single diamonds from the Orapa kimberlite pipe

The distribution and isotopic composition of helium has been measured in a suite of well-characterized one-carat diamonds from the Orapa kimberlite, Botswana. Crushing of the diamonds in vacuo indicates that most of the helium is contained by the matrix (generally greater than 90 %), rather than by the inclusions. We suggest that this internal isotopic variability is the result of stepwise graphitization: the first heating step initiates graphitization, which nucleates around defects, and the second heating step graphitizes the relatively defect-free regions of the diamond. The 3He/4He ratio measured, using the partial graphitization technique, differs by up to a factor of 100 within a single specimen. The 3He contents of the monocrystalline diamonds are relatively constant (at ≅ 3x10-13 cm3 STP/gram) and indicate that most of the isotopic variability is due to radiogenic 4He. Although this may not be true of all diamonds, nuclear reactions in the crust and mantle (including spallation reactions at the surface) can explain many of the high 3He/4He ratios previously reported for diamonds. (orig./SHOE)