CHEMICAL IMAGING OF DIAMONDS BY TIME OF FLIGHT SECONDARY IONIZATION MASS SPECTROMETRY

Diamonds are the only unaltered mineral from the mantle. This chemical resistance of diamond allows an in-situ study of the diamond source region within the mantle. Many studies define the mantle terrane sourced in each diamondiferous deposit. However, the spatial resolution and analytical precision for such a study require scientists to use a handful of labs around the world. Here we evaluate the use of Time-of-Flight Secondary Ionization Mass Spectrometer (ToF-SIMS) as an alternative method of diamond isotope and trace element analysis and evaluate the feasibility of developing a new methodology in the stable isotope analysis of diamond. We used five diamonds from the Northwest Territories of Canada (Diavik mine) and one from Murfreesboro, Arkansas. Diamonds from the Diavik mine have published high spatial resolution data showing intracrystalline δ¹³C variability. Five diamonds from Diavik were used as a control for evaluating the accuracy of ToF-SIMS analysis. Diamonds from Arkansas have no known high spatial resolution carbon isotope data and was used as an unknown. Experiments with different multi-element carbon-based standards, sputter times, instrument conditions and ion sources were tested. Natural carbonatite sample from Malvern, AR determined was determined to be the most cost effective suitable external standard, and ¹³C/¹²C ratio was used as the internal standard. The precision and accuracy of the ToF-SIMS method, and the homogeneity of the carbonatite multi-element standard, were tested by multiple analyses. Concentrations of ¹³C, ¹²C, ¹⁶O, ¹⁸O, and CH were determined for five of the six diamonds and the carbonatite standard. ¹³C/¹²C ratios and δ¹³C values were determined by GC-IRMS for the carbonatite standard and the comparison of these methods shows the use of carbonatite as acceptable. Evaluation of the ToF-SIMS data includes the possibility to convert spectral intensity to chemical information, quality of spatial resolution, and application of chemical imaging to characterizing intracrystalline variation. We conclude that ToF-SIMS while a valuable chemical imaging technique due to its ability to analyze a region for the entire periodic table with 2 µm spatial resolution, lacks the precision to quantitatively determine carbon isotope values without additional analysis.