Quantitative relationship between opAMS and ipAMS in some pyrrhotite-bearing rocks

The anisotropy of magnetic susceptibility (AMS) of pyrrhotite-bearing rocks (typically represented by ultramafic rocks) is often composite, carried not only by pyrrhotite, but also by magnetite and mafic silicates. In magnetic fabric studies, it is therefore desirable to separate the AMS component due to pyrrhotite from that due to the rest of the rock. This can be made, in addition to other techniques, by the anisotropy of the out-of-phase component of the AMS (opAMS). For comparative studies, it is vital to know quantitative relationship between opAMS and ipAMS, which is the aim of this paper. This knowledge is useful in interpreting the AMS of rocks in which pyrrhotite is important but not dominant magnetism carrier.

The out-of-phase susceptibility (opMS) of the pyrrhotite-bearing rocks investigated increases significantly with the field intensity within the field range between 10 A/m and 700 A/m. The increase is faster in very low fields (<100 A/m) than in stronger fields. The principal directions of the opAMS are virtually field independent in the entire low-field range used, being also very well parallel to the ipAMS directions. The degree of opAMS is also virtually field independent, but much higher than the degree of ipAMS. The shape parameter in opAMS is also field independent and resembles that in ipAMS.

The Rayleigh Law range, in which magnetization is linearly related to the field, is relatively narrow, less than 40 A/m. Theoretical quadratic relationship was suggested by Markert and Lehman (1996, GJI) between the tensor of initial ipMS and the tensor of Rayleigh coefficient characterizing the opAMS. The tensors are related by a constant c, which in general may or may not be direction independent. The direction independence would give rise to very simple relationship between the respective anisotropy degrees. Our investigations show that the constant c is in case of pyrrhotite direction independent. The tensor of the Rayleigh coefficient can be calculated from the opAMS measurement in one field, while the same tensor determined from field variation of ipAMS requires measurement in multiple fields (in two in minimum).