DIAMOND INTRAGRANULAR δ13C VARIABILITY MEASURED BY TOF-SIMS

This study evaluates the viability of microanalysis by Time of Flight Secondary Ion Mass Spectrometery (ToF-SIMS) to determine in situ δ¹³C variability in diamonds. Diamonds for this study include five diamonds from the North West Territories, Canada and one diamond from Arkansas, USA. The North West Territory diamonds are from the Diavik and Ekati mines and the Arkansas diamond is from the Prairie Creek Lamproite. The Canadian diamonds have shown δ¹³C intragranular variability of ≤ 1‰, determined by other SIMS methods. While diamonds from Prairie Creek have no published high spatial resolution δ¹³C data. We use previous Canadian diamond analyses as a control to evaluate the ToF-SIMS for high spatial resolution diamond analysis. Magnet Cove Carbonatite samples of known carbon isotopic composition were used as an internal standard for ToF-SIMS calibration. We present an assessment of the viability of ToF-SIMS as a new methodology for measuring δ¹³C variability in diamonds. Also, determine if ToF-SIMS has similar analytical precision and spatial resolution as other SIMS methods for δ¹³C analysis of diamonds. With a broader goal of establishing cheaper and faster methodologies to characterize mantle flux spatially and temporally.

Diamonds were cut and polished along the longest dimension that intersects the center and mounted in indium with the carbonatite standard. The internal structure of diamonds established with a secondary electron microscope and Fourier transform infrared spectroscopy. The ToF-SIMS for this study uses a Bi⁺ primary ion beam. Analyses were run with the standard practice of two blocks of five cycles with four seconds per cycle. Focused beam for ToF-SIMS operated with a current of 30 nA and ion extraction at 30 keV. Beam spot sizes range from 50 to 100 µm with δ¹³C value precision ≤ 0.5‰. This study focuses on beam spot traverses from core to rim for each diamond and grid analyses for a subset population based on internal structure. Traverses include 10 to 20 spots at spot size intervals per diamond. Grid analysis maps arranged as an array with spot size intervals. Traverses characterize carbon equilibrium within the mantle. ToF-SIMS trace element maps and δ¹³C value maps derive partition coefficients of trace element impurities and diffusivity coefficients for carbon.