HR: 0830h
AN: GP21B-0039    [PDF]
TI: Absoulte Paleointensities From 21-84 ka Ontake Volcanic Rocks, Japan - Evidence For Excursion At 80 ka
AU: * Yamamoto, Y
EM: yuhji-yamamoto@aist.go.jp
AF: Geological Survey of Japan, AIST, Tsukuba Central 7, Tsukuba, 305-8567 Japan
AU: Tanaka, H
EM: hidefumi@cc.kochi-u.ac.jp
AF: Faculty of Education, Kochi University, 2-5-1 Akebono-cho, Kochi, 780-8520 Japan
AU: Yamazaki, T
EM: toshi-yamazaki@aist.go.jp
AF: Geological Survey of Japan, AIST, Tsukuba Central 7, Tsukuba, 305-8567 Japan
AB: Tanaka and Kobayashi (2003) reported a paleosecular variation in directions from 21-84 ka volcanic rocks of the Ontake Volcano, Japan. They found two low latitude VGPs from 48 ka (41.9N, 196.2E) and 80 ka (15.9N, 183.3E) lavas, indicating existence of excursions in Japan during the latest Pleistocene. Following this study, we have tried absolute paleointensity measurements on the Ontake volcanic rocks including samples with these two low latitude VGPs. From various rock magnetic experiments, main remanence carrier was judged to be titanomagnetite with minor Ti content. Hysteresis properties suggested that most of the paleomagnetic cores had PSD characteristics, though ratios of Hcr/Hc were beyond the value of PSD-MD threshold (Hcr/Hc=4) for some cores. These evidences indicated that the present samples had ordinary rock magnetic properties for absolute paleointensity measurements. Therefore, we have performed two different methods of the measurements; Coe's version of the Thellier method (Coe, 1967), and double heating technique of the Shaw method combined with low temperature demagnetization (LTD-DHT Shaw method; Tsunakawa et al., 1997; Yamamoto et al., 2003). Until now, the former method has been applied to 71 specimens while the latter to 126 specimens. Each method gave 30 and 54 successful results. Especially, the latter method seemed to be effective for the MD-pronounced specimens. These results ranged from about 10 to 60 ZAm2 in VADM except one site at 84 ka (110 ZAm2). Striking point in these results is that samples from the 80 ka lava with low latitude VGP yielded consistent low paleointensities for both methods; 4.9+/-0.4 uT for the Thellier method (N=4); 5.9+/-0.2 uT for the LTD-DHT Shaw method (N=3). These samples were collected from two outcrops which are 20 m apart across a gully. Since corresponding VDM and VADM is about 12% of the present value, typical for the geomagnetic excursions, the existence of 80 ka excursion is confirmed not only from the paleodirectional evidence but also from the absolute paleointensity data.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1513 Geomagnetic excursions
DE: 1521 Paleointensity
DE: 1560 Time variations--secular and long term
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting