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| Titel |
Comparison of lattice preferred orientation and magnetic fabric of a chloritoid-bearing slate |
| VerfasserIn |
Tom Haerinck, Hans-Rudolf Wenk  , Timothy N. Debacker, Manuel Sintubin |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250089245
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| Publikation (Nr.) |
 EGU/EGU2014-3441.pdf |
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| Zusammenfassung |
| A regional analysis of the anisotropy of the magnetic susceptibility (AMS) on
chloritoid-bearing slates of the Paleozoic Plougastel Formation in the low-grade
metamorphic conditions (epizonal) of the Monts d’Arrée slate belt in Central Armorica
(Brittany, France) reveals very high values for the degree of anisotropy (PJ), up to
1.43 (Haerinck et al. 2013a). In contrast, stratigraphically equivalent slates free of
chloritoid, in the very low-grade metamorphic conditions (anchizonal) of the Crozon
fold-and-thrust belt, show a lower degree of anisotropy, with PJ values up to 1.27.
Classically, very strong magnetic fabrics (i.e. those with PJ above 1.35) are attributed to a
contribution of ferromagnetic (s.l.) minerals. Nonetheless, high-field torque magnetometry
indicates that the magnetic fabric of the chloritoid-bearing slates is dominantly
paramagnetic. The ferromagnetic (sensu lato) contribution to the AMS is less than
10%.
Based on these observations, it would seem that chloritoid has an intrinsic magnetic
anisotropy that is significantly higher than that of most paramagnetic silicates and the
frequently used upper limit for the paramagnetic contribution to the AMS. Using two
independent approaches, i.e. (a) directional magnetic hysteresis measurements, and (b) torque
magnetometry, on a collection of single chloritoid crystals, collected from different
tectonometamorphic settings worldwide, the magnetocrystalline anisotropy of monoclinic
chloritoid has been determined (Haerinck et al. 2013b). The determined paramagnetic
high-field AMS ellipsoids have a highly oblate shape with the minimum susceptibility
direction subparallel to the crystallographic c-axis of chloritoid and the degree of anisotropy
of chloritoid is found to be 1.47 ± 0.06.
The obtained very high magnetocrystalline degree of anisotropy suggests that
chloritoid-bearing slates with a pronounced mineral alignment can have a high degree of
anisotropy (PJ) without the need of invoking a significant contribution of strongly anisotropic
ferromagnetic (s.l.)minerals. To validate this assumption a texture analysis has been
performed on a representative sample of the chloritoid-bearing slates (PJ = 1.40), using hard
X-ray synchrotron diffraction (e.g. Wenk et al. 2010). For estimation of the mineralogical
composition and the preferred orientation a Rietveld refinement of the synchrotron X-ray
diffraction images has been performed. The Rietveld refinement confirms that the slate
contains a significant fraction of chloritoid (21 vol%). The resulting orientation distribution
of both muscovite and chloritoid display an approximate axial symmetric (001) pole figure
pattern with respect to the minimum magnetic susceptibility axis K3, that has an
extremely strong preferred orientation (~36 m.r.d. for muscovite and ~19 m.r.d. for
chloritoid).
It is therefore fair to conclude that the strong preferred orientation of the chloritoid
basal planes parallel to the magnetic fabric, in combination with the pronounced
magnetocrystalline anisotropy of chloritoid, explains the very high values for the degree of
magnetic anisotropy (PJ) observed in the chloritoid-bearing slates.
References
Haerinck et al. 2013a. Journal of the Geological Society, London 170, 263-280,
doi:10.1144/jgs2012-062.
Haerinck et al. 2013b. Journal of Geophysical Research: Solid Earth 118(8), 3886-3898,
doi:10.1002/jgrb.50276.
Wenk et al. 2010. Journal of Structural Geology 32(4), 478-489, doi:10.1016/j.jsg.2010.02.003. |
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