HELIUM AND CARBON ISOTOPES IN INDIAN DIAMONDS

We report the first helium and carbon isotopic measurements for Indian diamonds. The helium concentrations and isotope ratios in ten diamonds from documented sites in Andhra Pradesh are generally in the ranges of values for diamonds from other locations. Five samples are alluvial, from various sites in the Krishna River gravels; the rest were mined from three kimberlite pipes. Despite the limited statistics our results lead to fairly plausible conclusions: 1. (1) most of the 3He in the alluvial diamonds is of cosmogenic origin, and 2. (2) the alluvial diamonds may also have a significant 4He component due to implantation of α particles during storage in a secondary matrix (conglomerates and/or alluvium). Carbon isotope ratios in these diamonds are within the "normal" mantle range (δ13C = -4 to-7‰ plus one sample at -10‰) except for a single sample, the only spectral Type II diamond, which has δ13C = -26.5‰ This sample, a mined kimberlite specimen, has the lowest 4He content so far recorded in diamonds (0.018 μcc/g). © 1990.