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Magnetic anisotropy of the common rock-forming minerals: amphibole, pyroxene and feldspar

English title Magnetic anisotropy of the common rock-forming minerals: amphibole, pyroxene and feldspar
Applicant Hirt Ann M.
Number 129806
Funding scheme Project funding (Div. I-III)
Research institution Institut für Geophysik ETH Zürich
Institution of higher education ETH Zurich - ETHZ
Main discipline Geophysics
Start/End 01.10.2010 - 30.09.2012
Approved amount 111'641.00
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All Disciplines (3)

Discipline
Geophysics
Mineralogy
Geology

Keywords (7)

anisotropy of magnetic susceptibility; amphibole; pyroxene; diamagnetic susceptibility; paramagnetic susceptibility; feldspar; magnetic properties

Lay Summary (English)

Lead
Lay summary
Anisotropy of magnetic susceptibility (AMS) is often used as a proxy for deformation. For this, a better understanding of the physical origin of the AMS and the AMS of single mineral phases are needed. AMS depends on the composition, mainly the location and concentration of iron atoms, and the crystallographic structure. In this project, AMS of single crystals from the amphibole, pyroxene and feldspar groups will be examined. Measurements on a torsion magnetometer in high fields and at different temperatures allow for separation of diamagnetic, paramagnetic and ferromagnetic contributions to the AMS. Amphiboles, pyroxenes and feldspars can make up a large part of a rocks composition and often contain iron oxides which are good recorders of the geomagnetic field. These inclusions are considered to be reliable recorders of the paleointensity of the Earth's magnetic field. preferential alignment of ferromagnetic moments may bias the recorded magnetic field. Therefore this project will also investigate the magnetic fabric of the iron oxide inclusions to evaluate whether this affects their ability to record the intensity and direction of the geomagnetic field accurately.
Direct link to Lay Summary Last update: 21.02.2013

Responsible applicant and co-applicants

Employees

Publications

Publication
A method for improving the measurement of low-field magnetic susceptibility anisotropy in weak samples
Biedermann Andrea R. Lowrie William Hirt Ann M. (2013), A method for improving the measurement of low-field magnetic susceptibility anisotropy in weak samples, in Journal of Applied Geophysics, 88, 122-130.
Low-temperature magnetic anisotropy in micas and chlorite
Biedermann A.R., Bender-Koch C., Lorenz W.E.A., Hirt A.M., Low-temperature magnetic anisotropy in micas and chlorite, in Tectonophysics.

Scientific events

Active participation

Title Type of contribution Title of article or contribution Date Place Persons involved
Swiss Geoscience Meeting Talk given at a conference Seismic and magnetic anisotropy of the Finero Peridotite (Ivrea Zone) 17.11.2012 Bern, Switzerland Biedermann Andrea Regina; Hirt Ann M.;
Swiss Geoscience Meeting Poster Magnetic fabrics and anisotropy in rock-forming minerals 17.11.2012 Bern, Switzerland Hirt Ann M.; Biedermann Andrea Regina;
European Geophysical Union General Assembly 2012 Poster Relationship among petrofabric, magnetic anisotropy and seismic anisotropy in dunite 24.04.2012 Wien, Austria Biedermann Andrea Regina; Hirt Ann M.;
European Geophysical Union General Assembly 2012 Poster The Swiss Atlas of Physical Properties of Rocks (SAPHYR) 24.04.2012 Wien, Austria Biedermann Andrea Regina;
IUGG General Assembly Talk given at a conference Magnetic properties of the Møre-Trøndelag Fault Complex 28.06.2011 Melbourne, Australia, Australia Hirt Ann M.; Biedermann Andrea Regina;


Associated projects

Number Title Start Funding scheme
143438 Magnetic anisotropy of the common rock-forming minerals (amphibole, pyroxene and feldspar) and ultrabasic rocks 01.10.2012 Project funding (Div. I-III)
100224 Magnetic and mineral fabrics in carbonate rocks 01.07.2003 Project funding (Div. I-III)

Abstract

The proposed project is a continuation of a series of projects that we have conducted over the past eight years to better understand the physical origin of magnetic anisotropy in rocks. Early studies of the anisotropy of magnetic susceptibility concentrated on using magnetic fabrics as a proxy for deformation and finite strain. It became evident from these investigations that a more comprehensive understanding was needed of the factors that control the magnetic anisotropy in rocks. First and foremost is the need of a reliable data set for the magnetic anisotropy of single mineral phases. In our past work we have developed a measurement procedure using a high-field torsion magnetometer at 293 K and 77 K to separate the contributions of paramagnetic, ferrimagnetic, antiferrimagnetic and/or diamagnetic contributions to the anisotropy of magnetic susceptibility, when the individual magnetic components make a significant contribution to the total anisotropy. This allowed us to isolate the paramagnetic anisotropy in the phyllosilicate minerals: biotite, muscovite, and chlorite, and Fe-bearing carbonate minerals, all of which often contain ferromagnetic (s.l.) inclusions. We have also studied diamagnetic anisotropy in calcite. Composition and crystallographic structure are the main factors that control the magnetic anisotropy in single crystals. Iron is the most important element that affects the strength of the magnetic susceptibility, and its location in the crystallographic structure will determine the magnetic anisotropy. In this project we plan to examine the magnetic anisotropy of single crystals from the amphibole, pyroxene and feldspar groups. These minerals are of interest for two reasons: firstly, they include some of the major rock forming minerals that can make up a significant part of a rock’s composition. Secondly these phases often contain inclusion of iron oxides. It has been recognized since the 1960’s that paramagnetic minerals that contain ferromagnetic inclusions can be good recorders of the Earth’s magnetic field. Some of the earliest studies, which examined the magnetic susceptibility of minerals, concentrated on diamagnetic phases. However, there is only a single reliable study for orthoclase, potassium feldspar. There have been numerous later studies of minerals from the pyroxene and amphibole groups, but published results on their magnetic susceptibilities are highly variable, which could be due to chemical composition or to the inadequate separation of contributions from ferromagnetic inclusions. This study aims to isolate the anisotropy due to the pure diamagnetic or paramagnetic susceptibility of these mineral phases and to relate this to their chemical composition. Our interest in the magnetic anisotropy due to the ferromagnetic magnetic inclusions arises from the fact that the inclusions are often systematically aligned within the silicate host crystals. Any preferential alignment of the silicate crystals themselves within a rock could therefore lead to a bias in the direction and intensity of the recorded field direction. There have only been a few studies made that isolate the ferromagnetic fabric due to the magnetic inclusions. Because the measurement technique will allow the separation of this ferromagnetic component, we will be able to establish a picture about variability in the degree of alignment of the inclusions and any relationship in terms of the chemical composition of the minerals. These results would be important for groups using single crystals to study the paleointensity of the Earth’s field.
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