GP43A-1235
Rock-magnetic properties of single zircon crystals sampled from the Tanzawa tonalitic pluton, central Japan
Thursday, 17 December 2015
Poster Hall (Moscone South)
Masahiko Sato, Geological Survey of Japan, Tsukuba, Japan, Shinji Yamamoto, University of Tokyo, Bunkyo-ku, Japan, Yuhji Yamamoto, Kochi Core Center, Kochi, Japan, Masao Ohno, Kyushu University, Fukuoka, Japan, Hideo Tsunakawa, Tokyo Institute of Technology, Tokyo, Japan and Shigenori Maruyama, Earth-Life Science Institute, Tokoyo, Japan
Abstract:
This paper reports on the rock-magnetic properties of single zircon crystals, which are essential for future work establishing the paleointensity method using single zircon crystals. Zircon crystals used in this study were sampled from the Nakagawa River, which crosses the Tanzawa tonalitic pluton in central Japan. Rock-magnetic measurements were conducted on 1037 grains of zircons, but many of these measurements are below the limits of the sensitivity of the magnetometers employed. Isothermal remanent magnetizations (IRMs) of 876 zircon crystal are below the practical resolution of this study; we infer that these crystal contain no or only minute quantities of ferromagnetic minerals. The other zircon crystals contain enough magnetic minerals to be measured in the DC SQUID magnetometer. For 81 zircon crystals, IRM intensity (MIRM) are larger than 4 × 10−12 Am2, while natural remanent magnetization (NRM) intensity (MNRM) are below 4 × 10−12 Am2, indicating that these crystals are inappropriate for the paleomagnetic study. For the samples that had values of MNRM ≥ 4 × 10−12 Am2 and MIRM ≥ 4 × 10−12 Am2, combining the rock-magnetic parameter, we proposed the sample selection criteria for future study of paleointensity experiments using single zircon crystals. In the case that the samples had high Bc values (>10 mT) or high MNRM/MIRM values (>~0.1), main remanence carriers are probably pyrrhotite and these samples are inappropriate for the paleointensity study. In the case that the samples had low Bc values (<10 mT) and low MNRM/MIRM values (<~0.1), main remanence carrier seem to be nearly pure magnetite with PSD grain sizes and these samples are expected to appropriate for the paleointensity study. Total thermoremanent magnetization (TRM) acquisition experiments were also carried out for 12 samples of the zircon crystals satisfying the above criteria. The TRM intensity was comparable with that of NRM, and a rough estimation of the paleointensity using NRM/TRM ratios shows field intensities consistent with the average geomagnetic field intensity at the Tanzawa tonalitic pluton for last 5 Myr.
