Andrea R. Biedermann

Anisotropy of magnetic susceptibility in natural olivine single crystals

Mantle flow dynamics can cause preferential alignment of olivine crystals that results in anisotropy of physical properties. To interpret anisotropy in mantle rocks, it is necessary to understand the anisotropy of olivine single crystals. We determined anisotropy of magnetic susceptibility (AMS) for natural olivine crystals. High-field AMS allows for the isolation of the anisotropy due to olivine alone. The orientations of the principal susceptibility axes are related to the olivine’s crystallographic structure as soon as it contains >3 wt % FeO. The maximum susceptibility is parallel to the c axis both at room temperature (RT) and at 77 K. The orientation of the minimum axis at RT depends on iron content; it is generally parallel to the a axis in crystals with 3–5 wt % FeO, and along b in samples with 6–10 wt % FeO. The AMS ellipsoid is prolate and the standard deviatoric susceptibility, k0, is on the order of 8*10210 m3/kg for the samples with <1wt % FeO, and ranges from 3.1*1029 m3/kg to 5.7*1029 m3/kg for samples with 3–10 wt % FeO. At 77 K, the minimum susceptibility is along b, independent of iron content. The shape of the AMS ellipsoid is prolate for samples with <5 wt % FeO, but can be prolate or oblate for higher iron content. The degree of anisotropy increases at 77 K with p0 7757.160.5. The results from this study will allow AMS fabrics to be used as a proxy for olivine texture in ultramafic rocks with high olivine content.

Magnetic anisotropy in clinopyroxene and orthopyroxene single crystals

Pyroxenes constitute an important component in mafic igneous and metamorphic rocks. They often possess a prismatic habit, and their long axis, the crystallographic c axis, helps define a lineation in a textured rock. Anisotropy of magnetic susceptibility (AMS) serves as a fabric indicator in igneous and metamorphic rocks. If a rock’s AMS is carried by pyroxenes, it can be related to their crystallographic preferred orientation and degree of alignment. This requires knowing the intrinsic AMS of pyroxene single crystals. This study provides a comprehensive low-field and high-field AMS investigation of chemically diverse orthopyroxene and clinopyroxene crystals in relation to crystal structure, chemical composition, oxidation state of Fe, and the possible presence of ferromagnetic inclusions. The paramagnetic anisotropy, extracted from high-field data, shows clear relationships to crystallographic directions and Fe concentration both in clinopyroxene and orthopyroxene. In the diopside-augite series, the intermediate susceptibility is parallel to b, and the maximum is at 45° to the c axis. In aegirine, the intermediate axis remains parallel to b, while the maximum susceptibility is parallel to c. The AMS of spodumene depends on Fe concentration. In enstatite, the maximum susceptibility aligns with c and the minimum with b, and in the case of hypersthene, the maximum susceptibility is normal to the exsolution lamellae. Magnetite inclusions within augite possess a ferromagnetic anisotropy with consistent orientation of the principal susceptibilities, which dominates the low-field anisotropy. These results provide better understanding of magnetic anisotropy in pyroxenes and form a solid basis for interpretation of magnetic fabrics in pyroxene-bearing rocks.

Magnetic anisotropy in natural amphibole crystals

Abstract Anisotropy of magnetic susceptibility (AMS) is often used as a proxy for mineral fabric in deformed rocks. To do so quantitatively, it is necessary to quantify the intrinsic magnetic anisotropy of single crystals of rock-forming minerals. Amphiboles are common in mafic igneous and metamorphic rocks and often define rock texture due to their general prismatic crystal habits. Amphiboles may dominate the magnetic anisotropy in intermediate to felsic igneous rocks and in some metamorphic rock types, because they have a high Fe concentration and they can develop a strong crystallographic preferred orientation. In this study, the AMS is characterized in 28 single crystals and 1 crystal aggregate of compositionally diverse clino- and ortho-amphiboles. High-field methods were used to isolate the paramagnetic component of the anisotropy, which is unaffected by ferromagnetic inclusions that often occur in amphibole crystals. Laue imaging, laser ablation-inductively coupled plasma-mass spectrometry, and Mössbauer spectroscopy were performed to relate the magnetic anisotropy to crystal structure and Fe concentration. The minimum susceptibility is parallel to the crystallographic a*-axis and the maximum susceptibility is generally parallel to the crystallographic b-axis in tremolite, actinolite, and hornblende. Gedrite has its minimum susceptibility along the a-axis, and maximum susceptibility aligned with c. In richterite, however, the intermediate susceptibility is parallel to the b-axis and the minimum and maximum susceptibility directions are distributed in the a-c plane. The degree of anisotropy, kʹ, increases generally with Fe concentration, following a linear trend: kʹ = 1.61 × 10-9 Fe - 1.17 × 10-9 m3/kg. Additionally, it may depend on the Fe2+/Fe3+ ratio. For most samples, the degree of anisotropy increases by a factor of approximately 8 upon cooling from room temperature to 77 K. Ferroactinolite, one pargasite crystal and riebeckite show a larger increase, which is related to the onset of local ferromagnetic (s.l.) interactions below about 100 K. This comprehensive data set increases our understanding of the magnetic structure of amphiboles, and it is central to interpreting magnetic fabrics of rocks whose AMS is controlled by amphibole minerals.

Origin of magnetic fabrics in ultramafic rocks

The magnetic fabric of a rock, defined by the anisotropy of magnetic susceptibility (AMS), is often used as a tectonic indicator. In order to establish a quantitative relationship between AMS and mineral texture, it is important to understand the single crystal intrinsic AMS of each mineral that contributes to the AMS of the rock. The AMS and crystallographic preferred orientation (CPO) of amphiboles, olivine and pyroxenes has been analyzed in a series of amphibolites, peridotites and pyroxenites that do show preferred mineral alignment. The CPO of each mineral phase was determined based on electron backscatter diffraction (EBSD). Whole- rock AMS was computed based on the CPO and single crystal AMS of the respective minerals. A comparison between measured and modelled magnetic anisotropy shows that the directions of the principal susceptibility axes agree well in amphibolite and peridotite. Pyroxenite is a good example for competing AMS fabrics in polyphase rocks.

Effects of magnetic anisotropy on total magnetic field anomalies

Modeling of magnetic anomaly data is a powerful technique to gain information on the shape of subsurface rock bodies. Most models are based on the assumption that the magnetization in the source body is parallel to the direction of the Earths magnetic field. It has long been recognized that remanent magnetization affects the magnetization direction, intensity and shape of the anomaly, and therefore the interpreted structure. The effects of anisotropy, however, have only received little attention so far. This study uses synthetic models and a case study to investigate how anisotropy affects magnetization and anomalies over a thick dipping sheet, and determines expected errors in interpreted magnetic properties and geometry of the source body for various anisotropy degrees and field inclinations. Anisotropy affects both the shape and amplitude of anomalies. For an oblate uniaxial fabric with the minimum susceptibility normal to the sheet and P = 1.5, errors in interpreted dip are up to 12°, depending on the field inclination, dip, and profile orientation, and errors in estimated mean susceptibility are up to -30%/+20 %, if anisotropy is not taken into account during modeling. These effects are larger for higher degrees of anisotropy. A case study over the MCU IVe’ layer in the Bjerkreim Sokndal layered intrusion, Norway, investigates the contributions of (an)isotropic induced and remanent magnetizations to the total field anomalies. There, the influence of anisotropy is mainly related to remanence deflection. The results shown here will help to further improve interpretation of magnetic potential field data.

Influence of static alternating field demagnetization on anisotropy of magnetic susceptibility: Experiments and implications

Anisotropy of magnetic susceptibility (AMS) indicates the preferred orientation of a rocks constituent minerals. However, other factors can influence the AMS, e.g. domain wall pinning or domain alignment in ferromagnetic minerals. Therefore, it is controversial whether samples should be alternating field (AF) demagnetized prior to AMS characterization. This may remove the influence of natural remanent magnetization (NRM) or domain wall pinning on AMS; however, it may also result in field-induced anisotropy. This study investigates the influence of stepwise AF and low-temperature demagnetization on mean susceptibility, principal susceptibility directions, AMS degree and shape for sedimentary, metamorphic and igneous rocks. Alternating fields up to 200 mT were applied along the sample x, y and z axes, rotating the order for each step, to characterize the relationship between AMS principal directions and the last AF orientation. The changes in anisotropy, defined by the mean deviatoric susceptibility of the difference tensors, are between <2% and 270% of the AMS in NRM-state. Variations in AMS parameters range from small changes in shape to complete reorientation of principal susceptibility axes, with the maximum susceptibility becoming parallel to the last AF direction. This is most prevalent in samples with low degrees of anisotropy in the NRM-state. No clear correlations were found between field-induced anisotropy and hysteresis properties. Therefore, we propose that future studies check any samples whose AMS is carried by ferromagnetic minerals and low anisotropy degrees for AF-induced artifacts. These results highlight the need for understanding the AMS sources and carriers prior to any structural interpretation.

Petrofabric development during experimental partial melting and recrystallization of a mica‐schist analogue

Petrofabric development during experimental partial melting and recrystallization of a mica-schist analogue