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[SEM15-P06] Analysis of the relationship between particle orientation and anisotropy of nagnetic susceptibility in artificial sediments
Keywords:AMS, particle orientation, artificial sediments
Orientation of constituent particles in sediments is one of indicators of depositional environments. Anisotropy of Magnetic Susceptibility (AMS) has been used to analyze the particle orientation. The usage of AMS is based on the general feature that the long axis of magnetic particle is correspond to the maximum axis of the AMS ellipsoid, and orientation of magnetic particles denoted by AMS is regarded to represent dominant orientation of particles in sediments. However, there is no study that evaluate how accurately AMS shows particle orientation in detail. In order to elucidate the relationship between particle orientation and AMS directly we performed AMS measurements and X-ray computed tomography (CT) image analyses on artificial sediments made by controlling depositional condition.
We used fine to coarse-grained sands collected on the beach of Iwase-hama, Toyama city. The main constituent particles were quartz (31%), plagioclase (32%), and rock fragments (33%). Rock magnetic analyses revealed that the main magnetic mineral in the sand samples was magnetite in pseudo-single magnetic domain size, and that about 99% of initial magnetic susceptibility of the sand sample was carried by magnetite.
The sedimentation experiments were conducted under still water and flowing water (flow rate 25, 30, 40, 50 L/min, gradient 1/100) using a experimental channel. Nine samples were collected per experiment using 7cc plastic cubes. AMS was measured with a KLY-3S susceptibility meter. CT images were analyzed by ImageJ to obtained shape, size and directional distribution of particles as particle ellipsoids.
Our current results show the followings:
[1] When deposited under flowing water, the long axes of the particle concentrated parallel to the flow direction, and the particles tiled upstream were somewhat predominant. It is implied that the degree of concentration in orientation may increase as the flow rate increases.
[2] When deposited under flowing water, the minimum axes of the AMS ellipsoid tended to tilt upstream from the vertical direction, while the maximum axis tended to concentrate in an orientation parallel to the flow direction. As the flow rate increases, the minimum axis first tilts from the vertical direction, and then the orientation of the maximum axis concentrates parallel to the flow direction.
[3] The dominant orientation of the long axes of the particle ellipsoids is not always consistent with the maximum axis of the AMS ellipsoid for each cube sample. However, for each depositional experiment, it is implied that an averaged orientation of the long axes of the particle ellipsoids in nine samples tend to coincide with that of the maximum axes of the AMS ellipsoids.
We used fine to coarse-grained sands collected on the beach of Iwase-hama, Toyama city. The main constituent particles were quartz (31%), plagioclase (32%), and rock fragments (33%). Rock magnetic analyses revealed that the main magnetic mineral in the sand samples was magnetite in pseudo-single magnetic domain size, and that about 99% of initial magnetic susceptibility of the sand sample was carried by magnetite.
The sedimentation experiments were conducted under still water and flowing water (flow rate 25, 30, 40, 50 L/min, gradient 1/100) using a experimental channel. Nine samples were collected per experiment using 7cc plastic cubes. AMS was measured with a KLY-3S susceptibility meter. CT images were analyzed by ImageJ to obtained shape, size and directional distribution of particles as particle ellipsoids.
Our current results show the followings:
[1] When deposited under flowing water, the long axes of the particle concentrated parallel to the flow direction, and the particles tiled upstream were somewhat predominant. It is implied that the degree of concentration in orientation may increase as the flow rate increases.
[2] When deposited under flowing water, the minimum axes of the AMS ellipsoid tended to tilt upstream from the vertical direction, while the maximum axis tended to concentrate in an orientation parallel to the flow direction. As the flow rate increases, the minimum axis first tilts from the vertical direction, and then the orientation of the maximum axis concentrates parallel to the flow direction.
[3] The dominant orientation of the long axes of the particle ellipsoids is not always consistent with the maximum axis of the AMS ellipsoid for each cube sample. However, for each depositional experiment, it is implied that an averaged orientation of the long axes of the particle ellipsoids in nine samples tend to coincide with that of the maximum axes of the AMS ellipsoids.