ABSTRACT

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 O₂ to produce CO₂. Replicate analyses on the two types of standards under O₂-atmospheric condition (8-20 torr) are reproducible to ±0.1 ‰ (1σ) for δ¹³C, 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 O₂ to produce CO₂. In this case, the amounts of CO₂ produced were far smaller (<1%) than those of CO₂ produced with excess O₂ and the δ¹³C values range from -18.9 to -7.5‰. Considering the mass balance, we conclude that the matrix effects of silica or other silicates on the δ¹³C analyses of organic carbon can be ignored because it only result in a little positive shift (<0.2‰) in δ¹³C values. Application of the laser microprobe technique on δ¹³C analyses of organic carbon to five late Archean black shale samples (Jeerinah Formation, Hamersley Basin, Western Australia) gives δ¹³C values that are reproducible to ±0.1-0.3‰, and the mean δ¹³C values range from -37.2 to -39.1‰ which are very close to the δ¹³C 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‰.