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 remanence-bearing minerals. The subtraction of normalized AMR from AMS, in theory and under limited practical circumstances, may isolate the paramagnetic + diamagnetic anisotropy contribution and thus the orientation distribution of the matrix minerals (Borradaile et al., 1999. Geol, Sec. Lend., Sp. Publ. 151, 139-145), Limitations include the great sensitivity of the subtraction process to the precision of the definition of the respective (AMS, AMR) tensors, and a requirement that single-domain and superparamagnetic grains are absent. The latter is particularly important for superparamagnetic minerals because iron oxides may be part of the orientation distribution of the main group of remanence-bearing minerals, although they would be excluded from the AMR fabric. Low ratios of saturation isothermal remanence to induced susceptibility characterize those rare rocks in which superparamagnetic behavior is a significant contribution. (C) 2000 Elsevier Science B.V, All rights reserved.