NUMERICAL MODELLING OF MAGNETIC SUSCEPTIBILITY IN FERROMAGNETIC ROCKS: THE ROLE OF MAGNETIC INTERACTIONS

The Anisotropy of Magnetic Susceptibility (AMS) in rocks due to non-uniform spatial distribution of magnetite grains has been investigated by Hargraves et al. (1991) and Stephenson (1994). Theoretical studies that consider similar grains and infinite geometry have shown that the AMS results more from magnetic interactions than from grain shape anisotropy, provided that neighbouring grains are close enough from each others, hence that magnetic interaction between them takes place (Stephenson, 1994; Canon Tapia, 1996). Based on the former theoretical basis, we present a numerical model in which each grain has its own magnetic anisotropy, volume, orientation and location in space. We will compare our results with the infinite model of interacting spheres (Stephenson, 1994), and with the experiments of Grégoire et al. (1995) and with new detailed experiments. The natural case study of Grégoire et al. (1998) will be discussed in the framework of the Total ASM tensor recently introduced by Canon Tapia (2001).

Canon Tapia, E., 1996. Single-grain versus distribution anisotropy: a simple three-dimensional model. Phys. Earth Planet. Inter. 94, 149-158.

Canon Tapia E., 1996. 2001. Factors affecting the relative importance of shape and distribution anisotropy in rocks : theory and experiments. Tectonophysics 340, 117-131.

Grégoire, V. et al., 1995. Shape anisotropy versus magnetic interactions of magnetite grains : experiments and application to AMS in granitic rocks. Geophys. Res. Lett., 22, 2765-2768.

Grégoire, V. et al., 1998. Magnetite grain fabric and distribution anisotropy vs. rock magnetic fabric: a 3D case study. J. Struct. Geol. 20, 937-944.

Hargraves, R.B. et al, 1991. Distribution anisotropy: the cause of AMS in igneous roks? Geophys. Res. Lett. 18, 2193-2196.

Stephenson, A., 1994. Distribution anisotropy: 2 simple models for magnetic lineation and foliation. Phys. Earth. Planet. Inter. 82, 49-53.