Carbon isotopic analyses of microdiamonds

Analytical methods Each microdiamond was sectioned, polished and analysed by cathodoluminescence and FTIR spectrometric methods prior to C isotopic study. In preparation for C isotopic analysis, the stones were pressed into a substrate of soft indium metal filling recessed mounts made of stainless steel or aluminium, and gold-coated. In each mount, a plate of a synthetic diamond standard 'syn-A' was placed alongside the unknowns. 13C/12C analyses were done using the Edinburgh CAMECA ims-4f ion probe, following the method developed for large diamonds by Harte and Otter (1992). A 0.5nA primary Cs+ beam was employed, sputtering C~ ions from a lOpm triangular-shaped area on the cut surface. Typical count-rates for 12Cand 13C" of 106 and 104 counts per second, respectively, were obtained from unknowns and standards alike. A single analysis constituted a one second count on f2C" followed by a six second count on 13C", repeated 50 times. A deadtime correction of 13.5 ns was applied to the raw counts, and a mean l3C/12C ratio was calculated for each data set. This ratio was then normalized for instrumental isotopic mass fractionation, based on the data obtained for the syn-A standard and its known composition (Fig.l), then converted to 513C notation, which represents permil deviation from the 13C/I2C ratio of the international PDB standard. Estimated uncertainties on individual analyses range from ±1.2 to 1.8 %o.