HR: 0800h
AN: GP21A-11    [Abstracts]
TI: Magnetic signature of the Yamato 000593/749/802 paired nakhlites
AU: * Funaki, M
EM: funaki@nipr.ac.jp
AF: National Institute of Polar Research, 9-10 Kaga 1 Itabashi , Tokyo, 173-8515, Japan
AU: Hoffmann, V
EM: viktor.hoffmann@uni-tuebingen.de
AF:  Institute for Geosciences, Univ. Tubingen, Sigwartstrasse 10, , Tubingen, 72076, Germany
AU: Torii, M
EM: torii@big.ous.ac.jp
AF: Okayama University of Science, 1-1 Ridaimae, Okayama, 700-0005, Japan
AB: Yamato 000593 consists of coarse-grained clinopyroxene and augite associated with olivine and minor amounts of Ti-rich magnetite and was classified as a typical nakhlite (Imae et al., 2002). The 87Rb/86Sr age was obtained to 1269±240 Ma by (Nakamura et al., 2002). Yamato 000749 and 000802 are most probably paired with Yamato 0000593. Here we give an overview of the magnetic signature and the NRM carriers of these Martian meteorites. The NRM intensities of 3 samples of Y000593 were between 4.02E-5 and 6.07E-5 Am2/kg. One of the samples having the direction of I=40 and D=215 was cut into 3 oriented subsamples. The directions of the subsamples were scattered to I=-53 and D=113 with α95=57 which is inconsistent with their parents. A subsample was demagnetized by AF field up to 100 mT in steps of 5 mT. The AF demagnetization of the NRM intensity showed a decreasing curve with zigzag variation. The direction has relatively smooth change to 30 mT, while it changed drastically between 30 and 100 mT. Thermal demagnetization of the NRM under vacuumed condition revealed a quick demagnetization of the intensity below 80C, a clearly defined NRM blocking temperature between 280 and 330C and a small directional shift to one direction was observed. The unblocking temperature may be consistent with a monoclinic pyrrhotite Curie point. Although the small intensity variation with a convex curve appeared between 330 and 630C, it may be insignificant due to the large variation of directions. The temperature dependence of HC (6.8 mT) revealed that the coercivity reached almost zero at 550C in the heating curve. A polished surface was prepared for the observations by reflected right microscope. Larger euhedral magnetite grains up to 300μm in diameter with ilmenite exsolution lamellae and elongated-shaped magnetite grains of about one micron appeared in olivine. Small amounts of sulfide grains less than 50μm in diameter were included. When magnetotactic bacteria were applied to the surface compulsively magnetized by IRM, the bacteria made strong clusters on the magnetite grains and small ones on the sulfide grains. Imae et al. (2002) described magnetite grains with ilmenite exsolution lamellae and small amount of pyrrhotite grains. Mikouchi et al. (2003) introduced symplectic inclusions composed of magnetite (less than a few μm) and augite in olivine grains. In the thermomagnetic analysis, pure magnetite and/or low Ti-magnetite are dominating magnetic minerals. The moderate coercivity of HC=6.8 mT seems to be carried by exsolution lamellae in larger magnetite grains. Iron sulfide is confirmed to be monoclinic ferrimagnetic pyrrhotite due to the forming of clusters of magnetotactic bacteria. The NRM was thermally demagnetized between 280 and 330C which consists with a blocking temperature of monoclinic pyrrhotite. Probably MD grains low-Ti magnetite might acquire IRM and/or VRM (demagnetized before 30 mT) in the terrestrial magnetic field.  Fritz et al. (2005) estimated the peak pressure above 40 GPa in nakhlites including Yamato nakhlites. This pressure is one order larger than a high-pressure transformation of monoclinic pyrrhotite (Rochette et al., 2003), providing an explanation for the missing magnetic anomalies on all known impact craters in the Mars crust.
DE: 1540 Rock and mineral magnetism
DE: 5440 Magnetic fields and magnetism
DE: 6225 Mars
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly