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

A technique for analyzing stable carbon isotope composition of organic carb on 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 ablat ion 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 wit h accepted precision by the conventional method. In order to examine the ma trix effect by other silicate minerals in natural samples during laser abla tion, the silica-graphite disc samples were also combusted by laser ablatio n without excess O-2 to produce CO2. In this case, the amounts of CO2 produ ced were far smaller (<1%) than those of CO2 produced with excess O-2 and t he delta(13)C values range from -18.9 to -7.5 parts per thousand. Consideri ng 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 beca use 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 org anic carbon to five late Archean black shale samples (Jeerinah Formation, H amersley Basin, Western Australia) gives delta(13)C values that are reprodu cible to +/-0.1-0.3 parts per thousand, and the mean delta(13)C values rang e 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 deve loped in this study is effective for the in situ isotope analyses of organi c carbon in sedimentary rocks with a good precision of +/-0.1 parts per tho usand.