Trace Element Analyses of Fluid-Bearing Fibrous Diamonds from Jwaneng (Botswana) by Neutron Activation Analysis

Abstract

Diamonds provide valuable information about the fluid regime of the mantle. Trace element concentrations, which are attributed to impurities in diamonds, have been the subject of earlier studies by neutron activation analysis (Fesq et al., 1973; Bibby, 1979). Following these investigations, Navon et al. (1988) were able to recognize that most of the impurities, which were attributed earlier to the diamond itself, are actually located in microinclusions. These micro-inclusions contain mantle fluids that vary in their composition linearly betweena carbonatitic and a hydrous endmember (Schrauder and Navon, 1994). Neutron activation analysis (NAA) is a powerful tool for the determination of the bulk trace elemental content of a diamond, because the carbon matrix itself is 'transparent' to neutrons. Thus, clean sample preparation is necessary. Recent improvements in detector technology make it possible to lower detection limits and to determine trace element abundances even in 'clean' diamonds devoid of visible inclusions (Hart et aL, 1991). Bulk analyses of diamonds have previously been carried out with a variety of techniques, but a precise description of the analyzed material was often lacking. In this study, we analyzed thirteen diamonds of cubic shape and fibrous internal texture from Jwaneng, and one clear octahedron as a blank sample. The diamonds were characterized optically and by infrared (IR) spectroscopy and studied by electron microprobe (EMP) and proton microprobe before NAA (Schrauder and Navon, 1994; Navon et al., in prep.).