Theoretical aspects of particle movement in flowing magma: implications for the anisotropy of magnetic susceptibility of dykes
Edgardo Cañón-Tapia, Jazmín Chávez-Álvarez, 2004. "Theoretical aspects of particle movement in flowing magma: implications for the anisotropy of magnetic susceptibility of dykes", Magnetic Fabric: Methods and Applications, F. Martín-Hernández, C. M. Lüneburg, C. Aubourg, M. Jackson
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Most studies of the anisotropy of magnetic susceptibility (AMS) of dykes have assumed that the axes of maximum susceptibility (kmax) should define an opposed imbrication pointing along the direction of magma flow, and that this orientation should be preserved along the dyke. This assumption is partially based on a model predicting the orientation of ellipsoidal particles floating in a moving liquid although the model actually predicts a cyclic movement of the particles that has been overlooked in msot AMS studies without further justification. The consequences of considering the full rotation of the ellipsoidal particles, as actually predicted by the theory, in the expected AMS of dykes are examined in this work. The complete version of the motion of ellipsoidal particles is then incorporated in a model of magma movement that takes into consideration the distribution of shear deformation within the dyke as predicted from the velocity gradient of the moving magma. Results of this model show that both particle elongation and the amount of shear that is sampled will affect the quality of the AMS results. By paying attention to the systematic variations of the AMS predicted by the theory, however, it is possible to devise sampling schemes that can be used to add more confidence to the interpretation of the AMS results. Although based on an idealized scenario of magma movement within a dyke, the model developed here explains satisfactorily the sometimes observed variation of AMS along flow direction in one dyke, and provides a simple explanation for many of the ‘abnormal’ magnetic fabrics that have been reported in dyke swarms around the world.
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Fabric is a ubiquitous and significant feature of geological materials. The processes involved in the formation and deformation of rocks and sediments leave their mark on the orientations of the constituent mineral grains. Petrofabrics thus provide essential keys to understanding the history of geological materials. Magnetic anisotropy is directly related to petrofabric, and has become one of the most rapid, sensitive and widely used tools for its characterization. The relationship between magnetic fabric and petrofabric is complex and depends on various factors including the composition, concentration and grain size of mineral grains. Ongoing research in geological applications is paralleled by studies of the fundamental mineral magnetic phenomena involved.
The papers in this book represent the current state of investigations in magnetic anisotropy studies as a discipline that integrates geological interpretations, mineral fabric development, technical advances and rock-magnetic properties.