TSK 11 Göttingen 2006 Otto et al. Correlation of magnetic fab- ric and crystallographic pre- ferred orientations of natu- rally deformed carbonate — mica rocks from the Alpi Apuane in Italy and the Damara Orogen in Namibia Poster Michael Otto1 Ann M. Hirt2 Bernd Leiss1 Volkmar Schmidt2 Jens M. Walter3 Scope of the correlation The anisotropy of magnetic susceptibil- ity (AMS) is a time-efficient method to describe crystallographic preferred ori- entations of rocks and has been applied in a wide field of sedimentary, meta- morphic and magmatic geology. The method, however, suffers from limi- tations which mainly result from the interference of diamagnetic, paramag- netic and ferromagnetic fabrics (de Wall 2005) — the term ferromagnetism is used in a wider sense here, including e.g. ferrimagnetism. The AMS is an integral parameter which describes a crystallo- graphic preferred orientation as an el- lipsoid. The quantitative correlation of the AMS with the crystallographic pre- ferred orientations should help to allow a closer view at the applicability and the limitations of the AMS analysis (see also Schmidt et al. 2006 a, b). 1 Geoscience Centre Göttingen, University of Göttingen, 37077 Göttingen, Germany 2 In- stitute of Geophysics, ETH Zurich, 8093 Zurich, Switzerland 3 Forschungszentrum Jülich, 52425 Jülich, Germany Recent advances in AMS analy- sis through new methods for phase separation The separation of ferromagnetic, para- magnetic and diamagnetic partial fab- rics has been a subject of research in re- cent times and has led to several new methods. Martin-Hernandez & Hirt (2001) presented a method for the sep- aration of diamagnetic/paramagnetic and anti-ferromagnetic from the fer- romagnetic phase fabrics using high- field torque measurements and differ- ent field strengths between 0.1 and 1.7T. While ferromagnetic magnetiza- tion saturates at high fields, paramag- netic/diamagnetic magnetization is pro- portional to the field strength. So the separation can be calculated from mea- surements at several fields. Schmidt et al. (2005, 2006 a,b) de- veloped a method for the separation of paramagnetic from diamagnetic fab- ric. It is achieved by the comparison of room-temperature and low-temperature (77K) measurements using the high- field torque method. This method was applied and compared to neutron tex- tures on synthetically-deformed calcite- mica samples. Naturally-deformed rocks To test the application of the new meth- ods on naturally deformed rocks, mica bearing calcite marbles and mylonites from the Alpi Apuane in Italy and dolomite mylonites from the Damara Orogen in Namibia were selected. Se- lection criteria were varying mica con- tents and varying intensities/types of their crystallographic-preferred orienta- tions (CPOs). Quantitative texture analyses were carried out by means of the ‘powder and texture diffractometer SV7’ at the research reactor Jülich 2 of 1 Otto et al. TSK 11 Göttingen 2006 the Research Center Jülich (FRJ-2) in Germany. Due to a low absorption co- efficient of neutrons in condensed mat- ter, neutron diffraction allows a volume- related quantitative texture analysis of the sample cylinders which were also used for AMS measurements. Only this strategy allows a direct correlation of CPOs with the AMS. AMS measure- ments were performed at different field intensities (0.8 to 1.7T) as well as at different temperatures (room tempera- ture and 77K) in order to separate the different magnetic phases. Mica shows a relatively strong paramag- netic anisotropy, whereas calcite is dia- magnetic. First results The 30 samples analysed cover a spec- trum from fine-grained (0.05mm) rocks to coarser grain sizes (max. 0.2mm). In general, grains show isometric to elon- gated shapes; the grain boundaries vary from relatively straight to irregular and lobate. In the different samples, the mica content covers a range from 0 to ca. 50%. Texture analyses reveal a large variety of texture types. The carbonate phases show single c-axis maxima, covering a range from weak to very strong intensity maxima. Other samples show distinct to weak c-axis double maxima. Further- more, some samples show partially de- veloped girdle distributions with mod- erate to weak intensity. In one case, dolomite displays a completely devel- oped girdle distribution. The mica phases show c-axis preferred orienta- tions covering a range of very weak to very strong single maxima. The measured AMS tensors are found to represent the crystallographic preferred orientations of calcite and mica detected by neutron diffraction goniometry. Dif- fering directions of the carbonate and mica phase preferred orientations are generally reflected by the directions of the dia- and paramagnetic AMS. In gen- eral, the eccentricity of the AMS ellip- soid reflects the intensity and type of the texture. The AMS of the ferromagnetic phases was found to be often significantly different from the para-/diamagnetic AMS. Magnetite has been identified by acquisition of isothermal remanent mag- netization (IRM) and thermal demag- netization of a cross-component IRM. The AMS of magnetite is defined by the grain shape rather than crystallo- graphic orientation. Additional low field AMS measurements give results repre- senting directions which are intermedi- ate between paramagnetic/diamagnetic and ferromagnetic AMS. Conclusions The results of this study are based on a large variety of fabric types of carbonate-mica marbles and mylonites, i.e. varying mica content, grain sizes, grain shapes, types and intensities of the crystallographic preferred orienta- tion. The presented first correlations of the AMS and CPO for the single mineral phases in general demonstrate a good matching. Regarding the comparison of texture types and the AMS, limitations are possible. While single c-axis max- ima and girdle-like c-axis distributions can be also distinguished by the AMS, it is obvious that distinguishing between these types and the double c-axis type is not possible at the present stage. References De Wall, H (2005) Die Anisotropie der mag- netischen Suszeptibilität — eine Methode 2 TSK 11 Göttingen 2006 Otto et al. zur Gefügeanalyse. Z. d. dt. Geol. Ges. 155, 287–298 Martin-Hernandez F & Hirt AM (2001) Sep- aration of ferrimagnetic and paramagnetic anisotropies using a high-field torsion mag- netometer. Tectonophysics 337, 209–221 Hrouda F (1982) Magnetic-anisotropy of rocks and its application in geology and geo- physics. Geophysical Surveys 5, 37–82 Lowrie W (1989) Magnetic Analysis of Rock Fabric. In: James, DE The Encyclopedia of Solid Earth Geophysics, 698–706 Schmidt, V, Hirt, AM, Burlini, L & Leiss, B (2005) Crystallographic-Preferred Orien- tations and Anisotropic Magnetic Suscepti- bilities in Experimentally Produced Calcite Samples: Deformation Mechanics, Rheology and Tectonics 2005, Zürich, pp 190 Schmidt V, Hirt AM & Rosselli P (2006a) Sep- aration of magnetic subfabrics by high-field, low-temperature torsion measurements. this volume Schmidt V, Hirt AM, Burlini L, Leiss B & Wal- ter JM (2006b) Measurement of calcite crys- tallographic preferred orientations by mag- netic anisotropy and comparison to diffrac- tion methods. this volume 3