Strong Anisotropy in Magnetic Susceptibility and Distribution of Magnetite Grains in Serpentinite Mylonites
Abstract
Magnetic susceptibility and petrofabrics were studied on three antigorite-bearing serpentinite mylonite samples, which were collected from the Happo ultramafic complex, central Japan. Mylonite samples show well-developed foliation and lineation. The foliation is made by the compositional layering, while the lineation by a mineral elongation direction. 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 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 degree of anisotropy P (=K1/K3) is quite high (2.2~3.8) and the shape parameter T (Jelinek, 1981) is 0~0.5 (oblate). Both the shape preferred orientation of individual magnetite grains and their distribution anisotropy contribute to the anisotropy in magnetic susceptibility (AMS). Magnetite grains are small (~1 μm) and equidimensional in HPS-M, while large (~100 μm) and elongated parallel to the foliation in HKB-D and HKB-B. The planar distribution of magnetite grains is clearly imaged by X-ray microCT. AMS is mainly due to the distribution anisotropy of magnetite grains in HPS-M, since magnetite grains are aligned along elongated olivine grains. In HKB-D and HKB-B, the shape preferred orientation and anisotropic distribution contribute to the strong AMS.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMGP23B0804W
- Keywords:
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- 1518 Magnetic fabrics and anisotropy;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1519 Magnetic mineralogy and petrology;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1540 Rock and mineral magnetism;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1594 Instruments and techniques;
- GEOMAGNETISM AND PALEOMAGNETISM