Cosmogenic helium and volatile-rich fluid in Sierra Leone alluvial diamonds

Abstract

Pursuant to the discovery elsewhere of cosmogenic 10Be in alluvial diamond fragments from Zaire, noble gas measurements were made on two identical splits of a finely powdered, harshly acidwashed sample derived from selected (for clarity) fragments of a single alluvial diamond from Sierra Leone (sample LJA → L#4 and L#5). Essentially identical results were obtained for both splits. Isotopic ratios for Ar, Kr, and Xe were atmospheric and their elemental abundances were high relative to published data, owing to shock implantation in the crushing as verified in a supplementary experiment. No neon was detected above blank level. 3He was exceptionally abundant, 4He exceptionally depleted, possibly from the acid wash, and the ratio 3He 4He almost unprecedentedly high at an R/R a value of 246 ± 16. The results support the hypothesis that excess 3He in diamonds is cosmogenic, although a cosmic-ray exposure of 5, 35, or (impossibly) 152 Ma for cyclic gardening of the sample to a maximum depth of 0, 4.6 m, or 20 m, respectively, is required. Troublesome for the cosmogenic hypothesis is a sample from very deep in the Finsch mine, South Africa, found by Zadnik. et al. (1987) to have an R/R a value of 1000. Our paper includes histograms of noble gas data published prior to mid-1988 for diamonds of known provenance. The Sierra Leone diamond studied in the supplementary experiment (sample LJE2 → L#6 and L#7) belongs to a distinct population of 40∗Ar-rich diamonds consisting mostly of cubic diamonds from Zaire. The latter stones, the “supplementary” Sierra Leone diamond, and a milky Brazilian diamond measured by Honda et al. (1987) all display upper mantle signatures, in common with MORB glasses and CO 2 gas wells, for 3He 4He , 40∗Ar 4He , excess 129,134,136Xe/ 132Xe, and to a lesser extent in concordance, 136∗Xe 4He .