HR: 0800h
AN: GP11C-0841    [Abstracts]
TI: Paleodirectional and Paleointensity Variations During the Brunhes-Matuyama Polarity Reversal From the Lava Sequence in Punaruu Valley, Tahiti Island
AU: * Mochizuki, N
EM: nmochi@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551 Japan
AU: Tsunakawa, H
EM: htsuna@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551 Japan
AU: Kurata, M
EM: mkurata@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551 Japan
AU: Yamazaki, T
EM: toshi-yamazaki@aist.go.jp
AF: Institute for Marine Resources and Environment, Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Oda, H
EM: hirokuni-oda@aist.go.jp
AF: Institute for Marine Resources and Environment, Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AB: The polarity reversal is considered to show a large drop in the geocentric axial dipole (GAD) moment. Since GAD is the greatest component of the main field, its large change possibly gives key information about the geodynamo. To clarify behaviors of the geomagnetic field during the reversal, we have studied the Brunhes-Matuyama (B-M) polarity reversal recorded in 21 lavas of the lava succession in the northern side of Punaruu valley, Tahiti Island. Although Chauvin et al. (1990) reported the B-M reversal and older events from the lava sequence in the southern side of the same valley, only a few paleointensities have been obtained for the B-M reversal record. We firstly carried out rock magnetic experiments. Curie temperatures were observed to be 500-600 and/or 100-200 $\deg$C, suggesting that a single phase of titanium-poor or titanium-rich titanomagnetite, or a mixture of them was contained in the samples. Hysteresis parameters of the samples were mostly plotted in PSD or SD regions of the Day plot. The samples were subjected to thermal demagnetization, or alternating field (AF) demagnetization following the low temperature demagnetization (LTD). Secondary components were generally erased at low temperatures ($\leq$ 300-400 $\deg$C) or low AFs ($\leq$ 10 mT). Several lavas showed significant amount of secondary components, so that a high coercivity or a high blocking-temperature component was carefully extracted as a primary one. For paleointensity determination, the double heating technique of the Shaw method combined with LTD (LTD-DHT Shaw method) was applied to 107 samples from those lavas. This is because AF demagnetization was more effective for removal of secondary components than thermal one and also because the reliability of the method was supported by recent studies of historical lavas. Mean paleodirections (N$\geq$3) were obtained for 18 lavas and mean paleointensities (N=2-6) for 13 lavas. The results show two stages of paleodirection: the directionally stable period of the reversed polarity and the directionally unstable period of reversed-intermediate-normal-reversed (R-I-N-R) polarity change. The second reversed polarity was recorded in the uppermost lava, suggesting another reversal to the Brunhes normal polarity after the studied sequence. While the field strength seems to be weak (4.7 $\mu$T) for the directionally unstable period, it varies in oscillation-like manner between 5.9 and 42.9 $\mu$T for the directionally stable period. During the directionally stable period, virtual geomagnetic pole (VGP) latitudes repeatedly changed between 60 and 90$\deg$S. When compared with these VGPs, the oscillation-like change in virtual dipole moments shows a strong correlation. This behavior is suggested to be one of the characteristics of the reversal process.
DE: 1521 Paleointensity
DE: 1535 Reversals (process, timescale, magnetostratigraphy)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting