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

Journal Article · Fri Sep 01 00:00:00 EDT 1989 · Geochimica et Cosmochimica Acta; (USA)

DOI:https://doi.org/10.1016/0016-7037(89)90358-X· OSTI ID:7164359

McConville, P; Reynolds, J H ^[1]

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1. Univ. of California, Berkeley (USA)

Pursuant to the discovery elsewhere of cosmogenic {sup 10}Be in alluvial diamond fragments from Zaire, noble gas measurements were made on two identical splits of a finely powdered, harshly acid-washed sample derived from selected (for clarity) fragments of a single alluvial diamond from Sierra Leone (sample LJA {yields} L4 and L5). 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. {sup 3}He was exceptionally abundant, {sup 4}He exceptionally depleted, possibly from the acid wash, and the ratio {sup 3}He/{sup 4}He almost unprecedentedly high at an R/R{sub a} value of 246 {plus minus} 16. The results support the hypothesis that excess {sup 3}He 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{sub a} value of 1,000. This 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 belongs to a distinct population of {sup 40*}Ar-rich diamonds consisting mostly of cubic diamonds for Zaire.

• OSTI ID: 7164359
• Journal Information: Geochimica et Cosmochimica Acta; (USA), Vol. 53:9; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
DIAMONDS
INCLUSIONS
ISOTOPE RATIO
ORIGIN
QUANTITATIVE CHEMICAL ANALYSIS
ALLUVIAL DEPOSITS
CHEMICAL COMPOSITION
COSMOCHEMISTRY
GEOCHEMISTRY
GEOLOGIC HISTORY
HELIUM 3
HELIUM 4
ZAIRE REPUBLIC
AFRICA
CARBON
CHEMICAL ANALYSIS
CHEMISTRY
DEVELOPING COUNTRIES
ELEMENTAL MINERALS
ELEMENTS
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
GEOLOGIC DEPOSITS
HELIUM ISOTOPES
ISOTOPES
LIGHT NUCLEI
MINERALS
NONMETALS
NUCLEI
STABLE ISOTOPES
580000* - Geosciences
640106 - Astrophysics & Cosmology- Cosmology