TRACE ELEMENTS IN GEM-QUALITY DIAMONDS – ORIGIN AND EVOLUTION OF DIAMOND-FORMING FLUIDS

Diamond crystallizes from a fluid/melt phase - relics of which may become trapped and, thus preserved in the form of inclusions. To understand the origin of diamonds it is essential to constrain the nature of these fluids. As fibrous diamonds trap a high but variable density of fluid inclusions they have been extensively studied. Constraining the nature of the diamond-forming fluid or melt for high purity gem diamonds, however, is hampered by the sparsity of high quality trace element data, mainly due to the extremely low concentrations of impurities they contain. The recent discovery of fluids in gem diamonds showing similar major element chemistry to fluid-rich diamonds suggests that many diamonds may share a common spectrum of parental fluids.

Here we present trace element data from ultra-pure gem diamonds originating from the Victor Mine, Ontario, Canada, and “near gem” diamonds from diamondiferous mantle xenoliths from the Finsch and Newlands kimberlites, SA, analyzed using the “off-line” laser ablation pre-concentration approach. These data can be compared to trace element data for gem diamonds and to fibrous diamonds, to further investigate the similarities between diamond-forming fluids for fibrous and gem diamonds.

The analysed region of each diamond was optically free of solid inclusions or other impurities at the 10 micron scale. Elemental abundances in the gem diamonds are very low (0.1 to 0.0001 x chondrite), which is significantly lower than values reported for fibrous stones but consistent with “closed-cell” ablation data previously reported for other gem diamonds from mines known to produce high quality stones. Distinct differences are evident in the trace element patterns of Victor diamonds containing silicate inclusions compared with sulphide-bearing diamonds that have less fractionated patterns, with more widely varying Ni, Fe and Co concentrations. These observed differences, however, are subtle - therefore, it seems likely that the fluids in the Victor diamonds have broadly the same origin and the silicate and sulphide-bearing diamonds formed by gradations of the same processes.

We observe clear similarities in the trace element patterns of fibrous and gem diamonds, indicating that invisible fluid inclusions are being analyzed and supporting a common origin for the fluids.