The anisotropy of magnetic susceptibility (AMS) in low-grade, cleaved pelitic rocks: influence of cleavage/bedding angle and type and relative orientation of magnetic carriers

Abstract

Cambrian and Silurian, low-grade, pelitic rocks of the single-phase deformed Brabant Massif consistently have a maximum magnetic susceptibility axis (K1) parallel to the cleavage/bedding intersection. In contrast, the minimum susceptibility axis (K3) either coincides with the bedding pole, with the cleavage pole or occupies an intermediate position. Anisotropy of anhysteretic remanence (AARM) and X-ray pole figure goniometry allow the distinguishing of the orientation distributions of the ferromagnetic and paramagnetic (white mica and chlorite) carriers, respectively. Mismatches between K3 and the poles to the macroscopic fabric elements (i.e. bedding and cleavage) are attributed to different orientations of the different magnetic (s.l.) carriers. A strong relationship exists between the cleavage/bedding angle and the shape parameter: low, respectively high angles leading to oblate, respectively prolate susceptibility ellipsoids. However, differences are observed between the Cambrian and Silurian samples in terms of the shape parameter and the behaviour of the degree of anisotropy with changing cleavage/bedding angle. This is tentatively attributed to differences in relative orientation and mineralogy of the magnetic (s.l.) carriers. These results demonstrate the influence of the relative orientation of the different carriers on AMS and suggest that, although being a petrofabric tool, AMS cannot be used as a strain gauge in the case of composite magnetic fabrics.

Complexity of the Anisotropy of Magnetic Susceptibility in Single-Phase Deformed, Low-Grade, Cleaved Mudstone

“…Recently, similar trends were observed on the basis of natural samples [5]. In the present paper, we present new data that confirm earlier conclusions [4,5] and further emphasise the complexity of AMS.…”

“…3) [5]. A similar relationship was observed between T and the angle between two synthetic magnetite orientation distributions [4].…”

“…If not, one may attempt to further unravel AMS in terms of the preferred orientation of the different magnetic carriers, using techniques such as phyllosilicate X-ray pole figure goniometry or AARM. However, also the interpretation of these methods in relation to AMS is not always straightforward [5].…”

“…Pole figure measurements were performed using Fe-filtered Co-radiation (40kV x 30 mA). Complete normalised pole figures were obtained by combining incomplete pole figure measurements, performed in transmission mode, on two mutually perpendicular sections of the sample, both perpendicular to the main foliation (bedding or cleavage) [15,5]. Fig.…”

Unraveling magnetic fabrics

“…For this reason, a comparison is only valid if the deformation is homogeneous on the same order of size. Debacker et al (2004) also noted a discrepancy between texture and magnetic fabric in mudstones from the Brabant massif. They defined the texture using recrystallized fine-grained (b) (a) Fig.…”