Problems in interpreting AMS parameters in diamagnetic rocks

Abstract

Abstract The AMS parameters widely used to characterize the magnetic fabric of rocks ( L, F, P, T ) are not directly applicable to diamagnetic rocks. Namely, there are two principal methods of their definition: (1) from the signed principal susceptibilities or (2) from the absolute values of the principal susceptibilities. The first method results in L, F and P values less than one in contrast to materials whose susceptibility arises from paramagnetism and/or ferromagnetism sensu lato , where they are greater than one by definition. The ratio parameters decrease with increasing intensity of alignment of the magnetic minerals; the calculated shape parameters are, however, correct. The second method gives L, F and P values greater than one; they increase with increasing preferred orientation, however the ellipsoid shape parameters are inverse. The development of AMS parameters with increasing preferred orientation is modelled mathematically for quartzite/evaporite (para- and/or ferromagnetic AMS superimposed on the isotropic diamagnetic matrix) and for marble/limestone (preferred orientation of calcite in a rock). A rational set of AMS parameters is then recommended.