Project

Discipline

ultrabasic rocks; EBSD; pyroxene; anisotropy of magnetic susceptibility; feldspar; amphibole

Lead
Lay summary
This project has two main parts. The first part is interested in gaining a deeper understanding of how anisotropy of magnetic susceptibility (AMS) arises in minerals from the amphibole, pyroxene and feldspar groups and olivines. This will involve evaluating the magnetic properties in relationship to chemical composition and the spatial distribution of Fe in the crystal structure, and modeling dipolar interactions between Fe cations. The second part of the project will involve modeling AMS of mantle rocks from the AMS of single crystal of constituent minerals and their distribution. Modeled results will be compared to measurements. This work will involve determining high-field AMS in order to isolate the paramagnetic fabric, as well as the preferred crystallographic orientation with EBSD for minerals that contribute to the susceptibility; lithologies will include deformed peridotites, amphibolites and pryoxenites.

Direct link to Lay Summary

Last update: 21.02.2013

Active participation

Title	Year

Number	Title	Start	Funding scheme

The proposed project is a continuation of a two-year project, which started in October, 2010, that is examining the magnetic anisotropy in single crystals of pyroxene, amphibole and feldspar. A consistent anisotropy that is related to crystallographic structure has been identified in each group. Most amphibole minerals have their minimum susceptibility normal to the crystallographic b-c plane and the maximum axis along [010]. Clinopyroxene has its intermediate axis of susceptibility close to [010], and the maximum and minimum axes in the a-c crystallographic plane. Feldspars, which possess a weak intrinsic anisotropy, have their minimum axes associated with the b-crystallographic axis. The continuation project has two goals: 1) to obtain a better physical understanding of the origin of anisotropy in amphibole, pyroxene and feldspar, based on the distribution of chemical elements within the crystal structure; and 2) to apply the results of the single crystal study to understand factors that contribute to physical anisotropies in ultrabasic rocks. The intrinsic magnetic anisotropy of a crystal is strongly dependent on the arrangement of atoms in the lattice structure. The distribution of iron in defined planes or along specific crystallographic axes, can lead to higher susceptibility in those directions. We propose to do high-resolution chemical mapping, using scanning transmission X-ray microspectroscopy (STXM) (PolLUX beamline) at PSI, to evaluate the distribution of iron cations in single crystals. This method has a spatial resolution of 10 - 15 nm, and can distinguish between ferric and ferrous iron, which is an important aspect due to the fundamental difference in their magnetic anisotropies. This method, however, cannot distinguish coordination between atoms. For this purpose we want to explore the use of X-ray magnetic circular dichroism to test for exchange coupling of spins of the iron atoms. To date, only a few studies have been conducted on paramagnetic minerals; however, the ability to use high fields and low temperature has potential to provide new insights into the origin of magnetic anisotropy in paramagnetic minerals. In the second part of the project we propose to use the intrinsic anisotropy of single crystals to examine how these contribute to the overall magnetic anisotropy in ultrabasic rocks. Rock samples will be selected to cover different compositions that are composed of minerals whose intrinsic anisotropy has been measured in the first part of the project. Lithologies will include peridotite (olivine), gabbros (olivine and pyroxenes), pyroxenite, amphibolites and anorthosite (plagioclase). The magnetic anisotropy of these rocks will be modeled based on the orientation distributions of their constituent phases and single crystal properties; modeled values will be compared to AMS measurements of the bulk rock.