Geochemical Journal, Vol. 46 (No. 3), pp. 249-253, 2012NOTE
Akizumi Ishida,1 Ko Hashizume2 and Takeshi Kakegawa1
1Department of Earth Science, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
2Department of Earth and Space Sciences, Graduate School of Science, Osaka University, Toyonaka 560-6643, Japan
(Received November 4, 2011; Accepted February 11, 2012)
Nitrogen isotopic analyses were conducted on two kerogenous samples from the Gunflint Formation (ca. 1.9 Ga) using the stepwise combustion technique to evaluate a potential analytical problem for the carbonaceous samples and to assess if this method is appropriate for the analysis of ancient rocks. Two discriminated δ15N plateaux are identified for each sample with mean values of +5.0 and +7.3 for sample 0708, and +6.1 and +5.2 for sample 0704, respectively. The direction of the δ15N shift is opposite in each sample. This characteristic excludes the possibility of analytical artifact as the source of isotope fractionation and metamorphism isotope fractionation. The two δ15N plateaux observed for each of the samples are characterized by different activation energies for the co-released carbon. These results suggest that similarly aged sedimentary rocks may contain at least two types of organic matter that record different source information for δ15N.
Key words: nitrogen isotopic composition, Paleoproterozoic, stepwise combustion method, kerogen, Gunflint Formation