Abstract
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.
Highlights
Anisotropy of magnetic susceptibility (AMS) is often used as a proxy for mineral texture in geologic applications, e.g. to gain information on deformation processes in metamorphic rocks or emplacement and flow processes in igneous rocks [1,2,3]
This study is a further demonstration of modelling magnetic fabrics based on the modal composition and crystallographic preferred orientation (CPO) of relevant phases in a rock [cf. 14]
A good agreement has been found between measured and modelled anisotropy of magnetic susceptibility (AMS) in particular for amphibolite and peridotite, where the AMS is dominated by a single mineral
Summary
Anisotropy of magnetic susceptibility (AMS) is often used as a proxy for mineral texture in geologic applications, e.g. to gain information on deformation processes in metamorphic rocks or emplacement and flow processes in igneous rocks [1,2,3]. AMS in a rock can yield information on the preferred mineral alignment, given that (1) the minerals that carry the AMS can be identified, and (2) their intrinsic AMS is known. Rock texture is usually carried by the abundant paramagnetic minerals, e.g. silicates, while even small amounts of ferromagnetic grains, e.g. iron oxides, often dominate the magnetic susceptibility. It is important to isolate the paramagnetic component of the AMS [4]. Ultramafic minerals, such as amphiboles, pyroxenes and olivine, often control the paramagnetic AMS and texture in mafic and ultramafic rocks [5,6,7,8]
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