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

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 SiO 2 /C = 3/2. The sample was ablated by the laser and simultaneously combusted by laser ablation with excess O 2 to produce CO 2 . Replicate analyses on the two types of standards under O 2 -atmospheric condition (8-20 torr) are reproducible to ±0.1‰ (1σ) for δ 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 CO 2 . In this case, the amounts of CO 2 produced were far smaller (<1%) than those of CO 2 produced with excess O 2 and the δ 13 C 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 δ 13 C analyses of organic carbon can be ignored because it only result in a little positive shift (<0.2‰) in δ 13 C values. Application of the laser microprobe technique on δ 13 C analyses of organic carbon to five late Archean black shale samples (Jeerinah Formation, Hamersley Basin, Western Australia) gives δ 13 C values that are reproducible to ±0.1-0.3‰, and the mean δ 13 C values range from -37.2 to -39.1‰ which are very close to the δ 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‰.cryogenically and subjected to a mass spectrometer for isotope analysis. Another advanced technique for analyzing δ 13 C of organic carbon is GC-IRMS (gas chromatography-isotope ratio mass spectrometry) that is conducted using a mass spectrometer equipped with a gas chromatograph (GC) via a CuO/Pt combustion furnace operated at 850°C or 900°C (Revill et al., 1994). These techniques require large amounts (>10 mg) of powdered rock samples and are not suitable for in situ microanalysis.

Atomic spectrometry update. Environmental analysis

“…Laser-based stable isotope methods offer several advantages for these types of minerals, which may only be available in situ in small quantities. 438,439 McConville and colleagues 440 studied the S isotope content of diagenetic pyrite using LA and an ion microprobe, as well as bulk analysis. Laser sampling at the scale of 100-500 mm seemed to be adequate to characterise the S isotope variations in this material.…”