Project

Back to overview

Influence of Magnetic Fabric on Strong Magnetic Anomalies

Applicant Biedermann Andrea Regina
Number 155517
Funding scheme Early Postdoc.Mobility
Research institution Department of Structural Engineering Norwegian University of Science & Technology
Institution of higher education Institution abroad - IACH
Main discipline Geophysics
Start/End 01.10.2014 - 31.03.2016
Show all

All Disciplines (2)

Discipline
Geophysics
Mineralogy

Keywords (4)

hemo-ilmenite; magnetic anisotropy; total field anomaly; magnetic fabric

Lay Summary (German)

Lead
An verschiedenen Orten auf der Erde, und auch auf Mars, ist das an der Oberfläche gemessene Magnetfeld stärker oder schwächer als erwartet. Diese Abweichungen (magnetische Anomalien), entstehen, wenn Gesteine in der Erdkruste stark magnetisiert sind. Ist die Magnetisierung entgegengesetzt zum heutigen Magnetfeld, entsteht eine negative Anomalie und die Stärke des Magnetfelds kann auf 50% des erwarteten Werts reduziert werden. Solche stark negativen Anomalien stehen in Zusammenhang mit Verwachsungen von bestimmten Mineralien, Hämatit und Ilmenit, die eine ausserordentlich starke und stabile Magnetisierung aufweisen. Die Mineralien sind plättchenförming und besitzen deshalb in verformten Gesteinen eine bevorzugte Orientierung. Das Hauptziel des Projekts ist es herauszufinden, wie sich die Orientierung der Plättchen auf die Stärke der Anomalie auswirkt. Die Ergebnisse können dabei helfen, magnetische Anomalien auf Mars besser zu verstehen.
Lay summary

Das Ziel dieses Projekts ist es, stark negative magnetische Anomalien besser zu verstehen. Insbesondere wird erforscht, wie sich die Orientierung der plättchenförmigen magnetisierten Mineralien Hämatit und Ilmenit auf die gemessene Intensität des Magnetfelds an der Erdoberfläche auswirkt. Dazu wird eine stark negative Anomalie in Südnorwegen und Gesteinsproben in deren Umgebung anhand von gesteinsmagnetischen und mikroskopischen Methoden untersucht. Das Hauptaugenmerk liegt darauf, wie die Hämatit-Ilmenit-Verwachsungen im Gestein orientiert sind, und wie sich diese Orientierung auf die Stärke der Anomalie auswirkt. Die Erkenntnisse, die in Norwegen gewonnen werden, können bei der Interpretation anderer negativer Anomalien auf der Erde und auf Mars angewendet werden.

Direct link to Lay Summary Last update: 18.07.2014

Responsible applicant and co-applicants

Publications

Publication
Effects of magnetic anisotropy on total magnetic field anomalies
Biedermann A.R., McEnroe S.A. (2017), Effects of magnetic anisotropy on total magnetic field anomalies, in Journal of Geophysical Research - Solid Earth, JB014647.
Effect of magnetic anisotropy on the natural remanent magnetization in the MCU IVe' layer of the Bjerkreim Sokndal Layered Intrusion, Rogaland, Southern Norway
Biedermann Andrea R., Jackson Mike, Bilardello Dario, McEnroe Suzanne A. (2017), Effect of magnetic anisotropy on the natural remanent magnetization in the MCU IVe' layer of the Bjerkreim Sokndal Layered Intrusion, Rogaland, Southern Norway, in Journal of Geophysical Research - Solid Earth, early view.
Anisotropy of magnetic susceptibility and remanence in layered intrusions – Reinfjord Ultramafic Complex and Bjerkreim Sokndal Layered Intrusion
Biedermann Andrea Regina (2016), Anisotropy of magnetic susceptibility and remanence in layered intrusions – Reinfjord Ultramafic Complex and Bjerkreim Sokndal Layered Intrusion.
Magnetic fabrics in the Bjerkreim Sokndal Layered Intrusion, Rogaland, southern Norway: Mineral sources and geological significance
Biedermann A.R., Heidelbach F., Jackson M., Bilardello D., McEnroe S.A. (2016), Magnetic fabrics in the Bjerkreim Sokndal Layered Intrusion, Rogaland, southern Norway: Mineral sources and geological significance, in Tectonophysics, 688, 101-118.

Collaboration

Group / person Country
Types of collaboration
Bayerisches Geoinstitut, University Bayreuth (Florian Heidelbach) Germany (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
- Research Infrastructure
Norwegian Geological Survey (Morgan Ganerød) Norway (Europe)
- Research Infrastructure
Institute for Rock Magnetism, University of Minnesota (Mike Jackson & Dario Bilardello) United States of America (North America)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
- Research Infrastructure
Department of Earth Sciences, Uppsala University (Bjarne Almqvist) Sweden (Europe)
- Research Infrastructure

Scientific events

Active participation

Title Type of contribution Title of article or contribution Date Place Persons involved
Seminar at the Institute for Roock Magnetism, University of Minnesota Individual talk Magnetic anisotropy – from single crystals to total field anomalies 01.02.2017 Minneapolis, United States of America Biedermann Andrea Regina;
15th Castle Meeting Talk given at a conference Effect of magnetic anisotropy on remanent magnetization in the Bjerkreim Sokndal Layered Intrusion, Southern Norway 21.08.2016 Dinant, Belgium Biedermann Andrea Regina;
EGU General Assembly 2016 Talk given at a conference Magnetic Fabrics in the Bjerkreim Sokndal Layered Intrusion, Southern Norway 16.04.2016 Wien, Austria Biedermann Andrea Regina;
AGU Fall Meeting 2015 Poster Magnetic Fabrics and Natural Remanent Magnetization in the MCU IVe’ Unit of the Bjerkreim Sokndal Layered Intrusion 14.12.2015 San Francisco, United States of America Biedermann Andrea Regina;
Seminar at the Institute for Rock Magnetism, University of Minnesota Individual talk Magnetic Fabrics in Layered Intrusions: Seiland Igneous Province & Bjerkreim Sokndal Layered Intrusion 08.12.2015 Minneapolis, United States of America Biedermann Andrea Regina;
Seminar at the Norwegian Geological Survey, to introduce them to magnetic fabrics Individual talk Magnetic Anisotropy 24.03.2015 Trondheim, Norway Biedermann Andrea Regina;
Young Trondjemites - Conference for Young Geoscientists, organized by the Norwegian Geological Survey Talk given at a conference Magnetic anisotropy and its applications: Case studies 20.03.2015 Trondheim, Norway Biedermann Andrea Regina;


Self-organised

Title Date Place
PhD Retreat for PhDs and postdocs at the IGB/NTNU as they have no seminars / I gave them a talk on 'Magnetic anisotropy, what does it tell us?' 25.09.2015 Røros, Norway

Abstract

The magnetic field of the Earth as measured at the surface is composed of contributions from different sources: the core field, which is responsible for the largest part of the signal, and smaller contributions from external sources and the Earth’s crust. The crustal field usually causes spatial variations of a few 100s of nT to the total magnetic field. Very strong magnetic anomalies of 1000 to 10 000s of nT are observed in specific areas, such as the Sokndal Region, Rogaland, in Norway, the Peculiar Knob deposit in Australia, the Adirondack Mountains, USA or the Sulu ultra-high-pressure zone in China. Variations of the magnetic field associated with crustal sources can be isolated from the total magnetic field and used for interpretation of structures in the shallow sub-surface. Generally, positive magnetic anomalies are associated with rocks with high magnetite contents yielding induced magnetization, or remanent magnetization parallel, or close to today’s geomagnetic field. If the remanent magnetization is opposite, or at a large angle to the direction of the ambient geomagnetic field, negative anomalies (magnetic lows) are observed. In certain areas, for example the Sokndal Region of Norway, the negative anomalies can be very prominent. This requires a very strong remanent magnetization, stable enough to not demagnetize during geological timescales. It has been proposed that this strong and stable negative magnetization is carried by ilmenite with hematite exsolution lamellae, or hematite with ilmenite lamellae, in a phenomenon described as lamellar magnetism. A similar mechanism was proposed to be responsible for strong remanent magnetization in the Sulu metamorphic belt in China. Lamellar magnetism could also be the cause of the strong magnetic anomalies on Mars. The Bjerkreim-Sokndal intrusion in Rogaland, Norway, is a magmatic suite of rocks on which lamellar magnetism has been studied and proposed as the source for the unusually strong negative magnetic anomalies. These rocks have strong mineral fabrics and contain hemo-ilmenite as well as orthopyroxene with hemo-ilmenite exsolution. The Unit IVe of the Bjerkreim-Sokndal intrusion forms a syncline, with weakest deformation fabrics at the hinge and stronger deformation on the limbs of the fold. The orthopyroxenes have a strong lattice-preferred orientation and have their c-axis parallel to the steep lineation of the norites in the syncline, and their a-axis parallel to foliation. Iron oxides within pyroxene crystals are restricted in their growth by the silicate lattice and grow in certain orientation. The Bjerkreim-Sokndal orthopyroxenes host rods and blades of ilmenite, where the rods are along the c-axis of the orthopyroxene and blades parallel to the b-c plane. The a-axis of ilmenite is parallel to the c-axis of the orthopyroxene, and hematite-exsolutions are common within the ilmenite. It has been shown that the lattice-preferred orientation of hemo-ilmenite with respect to the magnetizing field has a strong influence on the intensity of remanent magnetization. The easy axes of magnetization in hematite lie within the basal plane of the oxide. Thus, the observed NRM intensities strongly depend on the orientation of the hematite basal planes with respect to the magnetizing field. The dependence of the remanent magnetization on lattice-preferred orientation of the hemo-ilmenite is of importance when interpreting magnetic anomalies over strongly deformed rocks. This study will focus on the effects of magnetic anisotropy or mineral fabric on strong magnetic anomalies with a special focus on the analysis of rocks and interpretation of the anomalies in southern Norway. These results can then be applied to interpretation of anomalies in other areas of similar geology, or to areas with different anisotropic rocks, such as ore deposits or fault zones. The results of this study will have important implications for the interpretation of strong magnetic anomalies over deformed hemo-ilmenite- or ilmeno-hemtatie-bearing rocks not only on Earth, but potentially also on Mars.
-