Determination of noble metals in sulphide inclusions from diamonds using inductively coupled plasma-mass spectrometry

Microscopic inclusions of Fe or Ni-rich sulphides within crack-free natural diamonds provide a unique means to study the distribution and chemistry of the noble metals (Re, Au and the platinum-group elements: Ru, Rh, Pd, Os, Ir and Pt) in the Earth's mantle. In a feasibility study, we have analysed for the noble metals (with the exception of Os) and base metals (Fe, Ni and Cu) in two single, large Fe-rich sulphide inclusins (22 and 30 μg) and a composite sample of 23 small inclusions (53 μg) extracted from diamonds from the Orapa mine in Botswana. The analytical procedure utilises acid dissolution of the sulphide sample followed by inductively coupled plasma-mass spectrometry. Since the whole sample is analysed, this eliminates problems of inter-sample variation which can plague microprobe analyses for the noble metals. The results of this study show that it is possible to detect and quantify noble metal concentrations of < 10 mg kg^-1 in both the individual inclusions and the composite sample. The base metals can also be detected and quantified, but the analytical precision is much poorer than that attainable using energy dispersive X-ray spectrometry. The three samples appear to show a consistent pattern of enrichment of the light platinum-group elements (Ru, Rh and Pd) and Au, over Re and the heavy platinum-group elements (Ir and Pt), which may be geochemically significant in terms of noble metal fractionation between sulphide and alloy phases in the mantle. However, uncertainties in the Fe, Ni and Cu concentrations (and hence the stoichiometry of the sulphide) prevents a more detailed assessment of this question at present.