[S-EM16] 地磁気・古地磁気・岩石磁気・環境磁気

[SEM16-P10] Strong Anisotropy in Magnetic Susceptibility in Serpentinite Mylonites from Happo Ultramafic Complex

*渡辺 了¹、川崎 一雄¹、道林 克禎² (1.富山大学学術研究部都市デザイン学系、2.名古屋大学大学院環境学研究科地球惑星科学系)

キーワード：帯磁率異方性、蛇紋岩、マグネタイト

We study magnetic susceptibility of three antigorite-bearing serpentinite mylonite samples, which are collected from the Happo ultramafic complex, central Japan. Mylonite samples show well-developed foliation and lineation. Rock samples are mostly composed of antigorite, olivine and magnetite. The antigorite content is 37, 61 and 80 vol.% in samples HPS-M, HKB-D and HKB-B. respectively. The magnetite content increases from 1.1 to 7.8 vol.% with the antigorite content. Antigorite grains are strongly oriented, whereas olivine and magnetite grains are weakly oriented. The c-axes of antigorite are aligned perpendicular to the foliation plane and the b-axes subparallel to the lineation. The mean bulk susceptibility is quite high (~0.1 SI) and increases with increasing magnetite content. The direction of the maximum susceptibility is parallel to the lineation and the minimum susceptibility perpendicular to the foliation. The corrected anisotropy degree is quite high (2.222~3.782) and shape parameter is 0.197~0.464 (oblate). Since magnetite is weak in crystalline anisotropy, shape anisotropy must dominate the observed anisotropy. X-ray micro-CT images show magnetite grains form oblate clusters. To quantitatively understand the cause of strong anisotropy, we analyze the shape of clusters by fitting ellipsoids and evaluate the effect of their shape on the anisotropy in magnetic susceptibility. Major axes are aligned parallel to the lineation, while minor axes are aligned perpendicular to the foliation. Ratios of axes (b/a and c/a) determine the degree and shape factor of anisotropy. The oblate shape of magnetite clusters reasonably reproduces the observed anisotropy in magnetic susceptibility.