Fig. 8-7 Multiple gamma-ray detection system; GEMINI-II Gamma-rays emitted from the sample are efficiently detected by 19 gamma-ray detectors.

Fig. 8-8 Two dimensional matrix(left) and an enlarged part(right) obtained from NAAMG for a geological sample A geological sample has been irradiated with neutrons at the research reactor JRR-3M. The gamma-rays subsequently emitted by the sample were measured with GEMINI-II. In the two dimensional matrix, a small peak from Ir can be detected.

Fig. 8-9 Geological mapping around F/F boundary Almost 1000 ppt Ir concentration can be seen at the F/F boundary while other ages show 100 to 200 ppt.

In 1980, L. Alvarez et al., found iridium (Ir) anomalies in the Cretaceous period and the Tertiary period section (K/T boundary), which was formed ca. 65 million years ago, and proposed a meteorite impact hypothesis for the cause of dinosaur extinction based on this discovery. Similar mass extinctions had happened at other times, so scientists have tried to find the same anomalies in other sections as well. However, Ir content in the earth's crust is less than 100 ppt (one ten-thousandth of a ppm), so it is very difficult to analyze such a small amount of Ir in geological samples without a complicated chemical procedure. In this study, using neutron activation analysis with multiple gamma-ray detection methods (NAAMG) which have been developed at JAERI, a non-destructive, ultra-high sensitivity determination of Ir has been realized. The multiple gamma-ray detection method is performed with an array of 19 gamma-ray detectors, named GEMINI-II (Fig. 8-7). Using this system, very weak gamma-rays emitted from trace amounts of elements can be detected (Fig. 8-8). The Ir concentration has been determined by means of the NAAMG method for rock samples across the Frasnian/Famenian (F/F) boundary strata, which formed 370 million years ago. The results are shown in Fig. 8-9. An anomalously high concentration (ca. 1000 ppt) was detected for 1 sample at the F/F boundary. Based on these results, an Ir anomaly in the F/F boundary strata has been confirmed. However, the concentration of Ir at the F/F boundary is only 10% of that at the K/T boundary which was discovered by Alvarez et al. According to our results, geologists have proposed a new hypothesis that the mass extinction in the late Devonian happened through the out-flux out of a large amount of methane from gas hydrate which had been accumulating in the ocean depths, and was released by a meteorite impact.

Reference Y. Hatsukawa et al., Ir Anomalies in Marine Sediments: Case Study for the Late Devonian Mass Extinction Event, Geochim. Cosmochim. Acta, 67, A138 (2003).

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