Metamorphic control of magnetic susceptibility and magnetic fabrics: a 3-D projection
Norihiro Nakamura, Graham J. Borradaile, 2004. "Metamorphic control of magnetic susceptibility and magnetic fabrics: a 3-D projection", Magnetic Fabric: Methods and Applications, F. Martín-Hernández, C. M. Lüneburg, C. Aubourg, M. Jackson
Download citation file:
Magnetic fabric changes due to progressive metamorphism are poorly understood. Bulk magnetic susceptibility (κ) is known to increase with metamorphic grade but anisotropy changes have been neglected. To combine information on anisotropy with bulk susceptibility, we introduce a projection with three axes: κ, ellipsoidal eccentricity (Pj, the so-called ‘anisotropy degree’, despite the fact that this is quantified) and ellipsoid symmetry (Tj) as independent variables. The projection reveals that metamorphic facies can be discriminated successfully in the 3-D projection, with distinct, significant regression surfaces for crustal metamorphic rocks metamorphosed successively in greenschist, amphibolite, and granulite facies. This emphasizes that bulk magnetic susceptibility (κ) and its anisotropies (‘magnetic fabric’) evolve in response to metamorphic process, not just strain. Moreover, post-tectonic granitic plutons, upper mantle harzburgites and serpentinized mantle rocks also have characteristic regression surfaces relating κ, Pj and Tj in the new projection.