Phyllosilicate preferred orientation as a control of magnetic fabric: evidence from neutron texture goniometry and low and high-field magnetic anisotropy (SE Rhenohercynian Zone of Bohemian Massif)

Abstract

The low-and high-field magnetic anisotropy (AMS, HFA) of the Rhenohercynian mudstones and greywackes is compared to the theoretical anisotropy calculated from neutron texture goniometry measurements. The magnetic anisotropy is predominantly carried by the paramagnetic phyllosilicates in the form of chlorite/mica stacks and the ferromagnetic contribution is insignificant. The respective principal directions of the theoretical anisotropy and the AMS and HFA are sub-parallel; magnetic foliation reflects the orientation of the maximal concentration of phyllosilicate basal planes, magnetic lineation is subparallel to the intersection axis of those planes. For the purpose of quantitative comparison, the infrequently used standard deviatoric susceptibility as a measure of the HFA degree is employed. A very good linear correlation of the degree of theoretical anisotropy and the measured AMS and HFA is found. The prolate and oblate shapes of the respective fabric ellipsoids are reasonably well correlated. Neutron texture goniometry justifies the use of the conventional magnetic anisotropy technique for the assessment of the mineral fabric of studied rocks. When compared with other works relating the magnetic anisotropy to the mineral preferred orientation (examined by e.g. U-stage or X-ray texture goniometry) neutron texture goniometry seems to be a preferable and very precise method fabric analysis.

Mathematical simulations of anisotropy of magnetic susceptibility on composite fabrics

“…In another approach, Hrouda and Schulmann [1990] proposed an inverse method to generate a theoretical orientation tensor of the minerals from the intrinsic AMS of constituent minerals. The method was later extended to correlate AMS with deformation in phyllosilicate-bearing rocks [Hrouda, 1993;Chadima et al, 2004].…”

Paramagnetic metamorphic mineral assemblages controlling AMS in low-grade deformed metasediments and the implications with respect to the use of AMS as a strain marker

“…A paramagnetic fabric, on the other hand, results from intrinsic crystal lattice anisotropy of the paramagnetic minerals and their degree of preferred orientation, and so is more likely to yield reliable fabric information than the ferromagnetic fabric (see Hrouda et al 1997). Such an approach reveals a good agreement between the (paramagnetic) phyllosilicate fabric, determined by X-ray or neutron diffraction texture analysis, and AMS, particularly in low-grade pelitic rocks (see Lüneburg et al 1999;Chadima et al 2004;Hansen et al 2004;Cifelli et al 2005Cifelli et al , 2009). However, the paramagnetic fabric can also result from various mineral phases that do not necessarily reflect the same geological event (Debacker et al 2004(Debacker et al , 2009).…”

Preferred mineral orientation of a chloritoid-bearing slate in relation to its magnetic fabric

“…Note that such a link between mineral preferred orientations and magnetic properties is well established for slates in which the magnetic properties are dominated by the paramagnetic phyllosilicate minerals (e.g. Richter et al, 1993;Siegesmund et al, 1995;Chadima et al, 2004;Hansen et al, 2004;Martín-Hernandez et al, 2005;Cifelli et al, 2009;Oliva-Urcia et al, 2010).…”