Magnetic anisotropy of chloritoid and its significance in magnetic fabric studies of low-grade metamorphic rocks

The magnetocrystalline anisotropy of monoclinic chloritoid, a relatively common mineral in aluminium-rich, metapelitic rocks, has been determined for the first time by measuring the anisotropy of magnetic susceptibility (AMS), using two independent high-field approaches, i.e. (a) directional magnetic hysteresis measurements and (b) torque magnetometry, on a collection of single crystals collected from different tectonometamorphic settings worldwide [Haerinck et al., 2013a]. Magnetic remanence experiments show that all specimens contain ferromagnetic (s.l.) impurities, being mainly magnetite. The determined (paramagnetic) high-field-AMS (HF-AMS) ellipsoids have a highly oblate shape with the minimum susceptibility direction subparallel to the crystallographic c-axis of chloritoid. In the basal plane of chloritoid, though, the HF-AMS can be considered isotropic. The corrected degree of anisotropy (PJHF) is found to be 1.47, which is significantly higher than the anisotropy of most paramagnetic silicates and even well above the frequently used upper limit (i.e. 1.35) for the paramagnetic contribution to AMS of siliciclastic rocks. As there is no apparent relationship between PJHF and the high-field bulk susceptibility, it seems that the remarkably high magnetic anisotropy of chloritoid is not simply the result of more Fe (& Mn) cations and hence, a stronger ferrimagnetic interaction within the basal plane of the chloritoid lattice. Instead, an analysis of the paramagnetic Curie temperature, parallel (θpar.) and perpendicular (θperp.) to the basal plane of the chloritoid crystals, indicates that this pronounced magnetocrystalline anisotropy is related to strong antiferromagnetic exchange interactions in the direction of the crystallographic c-axis (θperp. < 0) and rather weak ferromagnetic exchange interactions within the basal plane (θpar. > 0). As a consequence, chloritoid-bearing metapelites with a pronounced mineral alignment can have a high degree of AMS without the need of invoking a significant contribution of strongly anisotropic, ferromagnetic (s.l.) minerals. This is tested by a magnetic fabric study of a particular stratigraphic horizon of Armorican metasiltstones, that covers both an area with chloritoid and white mica-bearing metasiltstones, associated with an epizonal metamorphic grade, and an area with chlorite and white mica-bearing metasiltstones, associated with an anchizonal metamorphic grade [Haerinck et al., 2013b]. It was found that the epizonal chloritoid-bearing metasiltstones show (dominantly paramagnetic) PJ values up to 1.45, whereas the anchizonal, chlorite and white mica-bearing metasiltstones show PJ values only up to 1.27. These observations clearly show that the presence of chloritoid in low-grade metamorphic rocks has a profound impact on the rock’s magnetic fabric (AMS) which can be attributed to the very high intrinsic magnetic anisotropy of chloritoid. Therefore, our work calls for a revised approach of magnetic fabric interpretations in chloritoid-bearing rocks. Haerinck et al. 2013a, JGR-B, 118, 13-13, doi: 10.1002/jgrb.50276. Haerinck et al. 2013b, JGS of London, 170 (2), 263-280, doi: 10.1144/jgs2012-062.status: publishe