Limitations of tensor subtraction in isolating diamagnetic fabrics by magnetic anisotropy

The anisotropy of low field susceptibility (AMS) represents the orientation distribution of all minerals in a rock, whereas the anisotropy of magnetic remanence (AMR, preferably anhysteretic) isolates that of the accessory re manence-bearing minerals. The subtraction of normalized AMR from AMS, in th eory and under limited practical circumstances, may isolate the paramagneti c + diamagnetic anisotropy contribution and thus the orientation distributi on of the matrix minerals (Borradaile et al., 1999. Geol, Sec. Lend., Sp. P ubl. 151, 139-145), Limitations include the great sensitivity of the subtra ction process to the precision of the definition of the respective (AMS, AM R) tensors, and a requirement that single-domain and superparamagnetic grai ns are absent. The latter is particularly important for superparamagnetic m inerals because iron oxides may be part of the orientation distribution of the main group of remanence-bearing minerals, although they would be exclud ed from the AMR fabric. Low ratios of saturation isothermal remanence to in duced susceptibility characterize those rare rocks in which superparamagnet ic behavior is a significant contribution. (C) 2000 Elsevier Science B.V, A ll rights reserved.