Laser microprobe technique for stable carbon isotope analyses of organic carbon in sedimentary rocks

A technique for analyzing stable carbon isotope composition of organic carbon using a Nd-YAG laser microprobe system has been developed. Analyses were performed on graphite rod and silica-graphite discs made from mixtures of silica glass and graphite powders with a weight ratio as SiO2/C = 3/2. The sample was ablated by the laser and simultaneously combusted by laser ablation with excess O-2 to produce CO2. Replicate analyses on the two types of standards under O-2-atmospheric condition (8-20 torr) are reproducible to +/-0.1 parts per thousand(1 sigma) for delta(13)C, which is in agreement with accepted precision by the conventional method. In order to examine the matrix effect by other silicate minerals in natural samples during laser ablation, the silica-graphite disc samples were also combusted by laser ablation without excess O-2 to produce CO2. In this case, the amounts of CO2 produced were far smaller (<1%) than those of CO2 produced with excess O-2 and the delta(13)C values range from -18.9 to -7.5 parts per thousand. Considering the mass balance, we conclude that the matrix effects of silica or other silicates on the delta(13)C analyses of organic carbon can be ignored because it only result in a little positive shift (<0.2 parts per thousand) in delta(13)C values. Application of the laser microprobe technique on delta(13)C analyses of organic carbon to five late Archean black shale samples (Jeerinah Formation, Hamersley Basin, Western Australia) gives delta(13)C values that are reproducible to +/-0.1-0.3 parts per thousand, and the mean delta(13)C values range from -37.2 to -39.1 parts per thousand which are very close to the delta(13)C values of the kerogens extracted from these shales. The analytical results demonstrate that the laser microprobe technique developed in this study is effective for the in situ isotope analyses of organic carbon in sedimentary rocks with a good precision of +/-0.1 parts per thousand.