Field dependence of magnetic anisotropy in pyrrhotite: effects of texture and grain shape

Abstract

To obtain detailed rock fabric analyses of drill cores from the pilot hole of the German Continental Deep Drilling Program (KTB), magnetic anisotropy properties have been investigated. Although anisotropy of magnetic susceptibility (AMS) measurements have already been made on a large number of KTB samples, this study presents data of isothermal remanent magnetization (IRM) anisotropies induced in pyrrhotite bearing rocks at various field strengths. Thereby the anisotropy properties of the ferrimagnetic minerals have been assessed independently of the paramagnetic contributions which often dominate the AMS characteristics. The acquisition of IRM at inducing fields between 1 and 500 mT has been measured parallel to the three principal axes of the AMS ellipsoid on three pyrrhotite bearing gneisses and it has been found tha the degree of IRM anisotropy decreases with increasing field strength. For two samples prolate anisotropies turn into oblate anisotropies. To elucidate the respective roles of texture (preferred orientation of crystallographic axes) and grain shape anisometry in the field-dependent effects, measurements have been made on a spherical and a cylindrical specimen cut from a massive (polycrystalline) pyrrhotite ore for which the texture had been determined by X-ray goniometry. The results suggest that IRM anisotropies at low fields are controlled by texture but become entirely dominated by grain shape as the field strength increases. For comparison, the anisotropies of anhysteretic remanent magnetization (ARM) are independent of the applied d.c. field, with smaller degrees of anisotropy than for low-field IRMs and orientations of the ellipsoid that reflect the texture only for the spherical ore specimen. Texture determinations on pyrrhotite-bearing rocks by low-field IRM anisotropy measurements are more rapid than X-ray goniometry studies and can be applied on rocks with low pyrrhotite concentrations where X-ray analysis fails.