Comparison between LA-ICP-MS and EPMA analysis of trace elements in diamonds

Abstract

Eight elements were measured in twenty-eight microinclusion-bearing diamonds using both Electron Probe Micro-Analyzer (EPMA) and the cellulose-calibrated Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) methods. Orthopyroxene microinclusions (< 1 µm) found in one of the diamonds have very similar Si atoms per formula unit and Mg/(Mg + Fe) ratios to those of larger orthopyroxene inclusions in diamonds, indicating that the EPMA analysis of the major elements in individual mineral and fluid microinclusions is accurate to better than 15%. For the fluid-bearing diamonds, very good correlations were found between the element/Fe ratios determined by EPMA and LA-ICP-MS for Mg, Ca, Na and K and most diamonds fall on or close to the 1:1 line, validating the accuracy of both techniques. Al/Fe, Ti/Fe and Ba/Fe ratios show good to moderate correlations. LA-ICP-MS analyses of four coated diamonds show that concentrations in the microinclusion-bearing coat are higher than those in the clear core by two orders of magnitude or more. Since most interferences from C–N–O–H molecular ions can be corrected by reference to analyses of pure synthetic diamond, and fewer interferences are expected for the heavier trace elements, the assembled information suggests that LA-ICP-MS technique combined with the cellulose calibration method provides accurate trace-element analyses of diamonds and allows compositional characterization of fluids trapped in them.