Predicting the reliability with which the geomagnetic field can be recorded in igneous rocks

Lead Research Organisation: University of Edinburgh
Department Name: Sch of Geosciences

Abstract

Palaeomagnetic recordings in ancient rocks and meteorites hold the key to answering some of the most fundamental questions in Earth Sciences. Theories regarding the evolution of the geodynamo, the thermal evolution of the Earth's core, plate tectonics and palaeogeography, and the formation of the solar system, are all constrained by observations of the ancient fields trapped in rocks that are hundreds or even thousands of millions of years old. However, not all palaeomagnetic observations are reliable, because the magnetic signal carried by most rocks and meteorites is dominated by a poorly understood thermoremanent magnetisation (TRM) in grains with non-uniform magnetic structures.
Most palaeomagnetic interpretations are based on the assumption that such TRMs are carried by magnetically uniform, single domain (SD) particles, whose behaviour is well described by Néel's SD TRM theories. However, slightly larger grains with non-uniform magnetic structures are ubiquitous in nature. These are termed pseudo-SD (PSD) as they display some characteristics to SD grains (such as a large magnetic remanence), but can have a significantly different recording fidelity. Presently there is no physical model for PSD TRM acquisition therefore we have no means of assessing the stability and reliability of many palaeomagnetic signals.
This proposal will address the urgent need to quantify the fundamental behaviour of PSD TRM. In particular we aim to address two key issues that can affect palaeomagnetic fidelity: (a) PSD stability as a function of time and temperature, and (b) their TRM dependence on cooling rates. This will be achieved by developing a three-dimensional numerical model that incorporates the effects of thermal-fluctuations. It will then be possible to model PSD TRM acquisition and assess the accuracy with which PSD domain states can record a geomagnetic field.
A key aspect of the numerical modelling is validation of the predicted domain structures, as a function of grain size and temperature, against direct nano-metric-scale experimental observations. This will be achieved using a remarkable set of highly characterised artificial samples (produced by an electron lithography process in a previous NERC-funded study) and using the advanced transmission electron microscope (TEM) technique of off-axis electron holography, which is able to image the magnetisation on a nano-metric scale. Experiments will also be conducted on bulk samples, including a suite of already collected lavas.
Once validated, the numerical model will be used to explore the fidelity of TRM recordings and palaeointensity (ancient geomagnetic field intensity) determinations in a range of grain geometries applicable to natural samples containing PSD domain states.
The research will result in a comprehensive understanding of TRM acquisition for PSD grains of magnetite, which are thought to the dominant carrier of palaeomagnetic recordings, and identify how accurately PSD grains can record the ancient field. The predictive micromagnetic model we develop will be able to directly address a number of key issues, for example:
(1) Palaeointensity estimates from PSD magnetites are used to constrain models of the Earth's core dynamics and the Solar System's formation. We will be able to determine whether these palaeointensities are likely to under or over estimate the true value of the ancient field.
(2) Archaen palaeointensity estimates are often determined from PSD magnetite crystals, embedded with in single-silicate crystals extracted from gabrros. The model will allow us to quantify the effect of long-term cooling-rates on TRM intensity, something which cannot be done experimentally.
With increased accuracy of palaeomagnetic observations, a much clearer picture will emerge of the past behaviour of the geomagnetic field, and hence a far better hope of unravelling the true nature of the early universe and the evolution and behaviour of the Earths deep interior.

Planned Impact

Main Beneficiaries
The primary beneficiaries will be academics working in the field of palaeomagnetism and geomagnetism, but will also impact on other academic sectors and industry. Currently geomagnetic field models rely primarily on data derived from direct observations, such as historical ships' logs of field declination. This means there is only a few hundred years of data, from which to predict the behaviour of the field. Further progress relies on the availability of accurate archaeomagnetic and palaeomagnetic observations, much of which come from samples dominated by non-deal recorders (inhomogeneously magnetised) of the geomagnetic field.

Other potential beneficiaries include:

(i) Academic and industrial Geologists who use palaeomagnetic directional analysis to determine the geological history and structure or a region, or who use palaeomagnetic recordings for drill core orientation.

(ii) Materials scientists will benefit from continued development of our numerical modelling, with applications to modelling fidelity of man made recording media.


Engagement Activities

We indent to take the results of our research directly to the end-users by attending specific geological workshops and through direct discussions with industrial consultants at Fugro Robertson Ltd. For academic end users, in addition to peer-reviewed publications and conference presentations, we intend to invite participation in the project through a discussion forum set up on the website and publicise our results via a newsletter 'IRM Quarterly' freely distributed by the Institute for Rock Magnetism at the University of Minnesota.

Wider user interest and engagement activities
Improvements in our understanding and applications of rock magnetic recording mechanisms, will lead to a better understanding of many geological phenomena, for example, the origin of the geomagnetic field. Over the last few years there has been much discussion in the scientific community regarding the stability of the geomagnetic field, and the decline in the field intensity as a precursor to field reversal. This is turn has generated a good deal of public interest on what generates the geomagnetic field, and what might happen if it underwent a field reversal.

To communicate with this group we will:

1) In year two we will submit a proposal to the Royal Society Summer Science Exhibition.

2) Day-to-day aspects of the project will feature on Edinburgh University's GeoWorld Spotlihjt webpage http://www.geos.ed.ac.uk/spotlight/ and through Imperial College's Department of Earth Science and Engineering's "Hot Topics" website articles page: http://www3.imperial.ac.uk/earthscienceandengineering/aboutese/hottopic.

3) Major findings will be disseminated through the Edinburgh University press office.

Milestones/measures of success
The modification of the rockmag.net website will take place on announcement of the success of this grant application. Shortly after the project start date we will publicise its existence using the gpmag-l list server and the IRM Quarterly newsletter. A major measure of success will be the number and frequency of visitors to the website, and much of the traffic can be quantified using 'Google Analytics'.

Further measures of visitors to the general participation in the websites discussion forum, and collection of a wider range of experimental samples from the scientific community.

Publications

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Emmerton S (2013) Correlating biodegradation to magnetization in oil bearing sedimentary rocks in Geochimica et Cosmochimica Acta

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Khakhalova E (2018) Magnetic Vortex States in Small Octahedral Particles of Intermediate Titanomagnetite in Geochemistry, Geophysics, Geosystems

 
Description The Earth's magnetic intensity and direction is recorded by rocks when they are formed. These recorders provide the only direct record of major geological and geophysical events that have occurred duding Earth history. It has been assumed that the magnetic particles that record these events have been small uniformly magnetised mineral grains. What we have shown in this research is that this is unlikely to be true. In fact the magnetic particles are larger and non-uniformly magnetised. More than that they have a far greater magnetic strength and stability. We have also found that there is a narrow grain size range of poor recording fidelity which is likely the course of unreliable paleomagnetic recordings.
Exploitation Route The outcome will be a far better reliability of paleo magnetic recordings, which should allow us to discriminate between competing theories of early Earth evolution. We should have a far better idea of when the geomagnetic field started, which has implications for changes in atmospheric chemistry over 4 billion years ago and the start of life n Earth. We should also be able to determine the thermal and physical evolution of the core and nucleation of the solid inner core of the Earth. We will be better able to answer questions relating to the generation of geomagnetic fields both on Earth and other planetary bodies.
Sectors Education,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology,Other

 
Description The science is being communicated through collaborations in the scientific community as well as peer reviewed publications.
First Year Of Impact 2013
Impact Types Societal

 
Title MERRILL - an opensource micromagnetic modelling package 
Description This in an open-source micromagnetic finite-element modelling pack aimed at the earth science community, and run king a simple text scripting interface. It allows the novice modeller to determine the magnetic characteristics of different minerals and predict their magnetic recording fidelity 
Type Of Material Computer model/algorithm 
Year Produced 2018 
Provided To Others? Yes  
Impact The software package has a growing number of contributors world-wide and an even faster growing number of users. The simple scripting language we employ makes it very easy to use. It has allowed graduate students and experienced academics to enter the numerical modelling discipline without the need for detailed knowledge. It provides a means for them to gain much greater insight to the magnetic properties of their experimental samples. 
URL http://www.rockmag.org
 
Description Joint Project with Scripps Institution of Oceanography 
Organisation University of California, San Diego (UCSD)
Department Scripps Institution of Oceanography
Country United States 
Sector Academic/University 
PI Contribution The grant is a joint NSF-GEO/NERC project. The grant itself is the result of a visit I made to Scripps two years ago. The project. has been high successful with a number of joint articles already published. We are now developing a an open-access database of our model solutions, which already has 4 million entries, and is likely to double in size over the next year. This database will form the foundation for a 'virtual paleoamgnetic laboratory', whereby the model solutions can be data-mined to simulate the common experimental observations. The in turn will provide a quantitative estimate of the reliability of paleo magnetic observations - and hence allows us to provide a better basis upon which to build theories of the early Earth evolution. The Edinburgh team have been responsible for the development of the the micromagnetic modelling programme.
Collaborator Contribution The team in Scripps have been primarily responsible for building the data base of solutions and development of data-mining techniques.
Impact Nagy, L., Williams, W., Tauxe, L., & Muxworthy, A. R. (2019). From Nano to Micro: Evolution of Magnetic Domain Structures in Multidomain Magnetite. Geochemistry Geophysics Geosystems, 20(6), 2907-2918. http://doi.org/10.1029/2019GC008319 Nagy, L., Williams, W., Tauxe, L., Muxworthy, A. R., & Ferreira, I. (2019). Thermomagnetic recording fidelity of nanometer-sized iron and implications for planetary magnetism. Proceedings of the National Academy of Sciences of the United States of America, 116(6), 1984-1991. http://doi.org/10.1073/pnas.1810797116 Shah, J., Williams, W., Almeida, T. P., Nagy, L., Muxworthy, A. R., Kovács, A., et al. (2018). The oldest magnetic record in our solar system identified using nanometric imaging and numerical modeling. Nature Communications, 9(1), 1173. http://doi.org/10.1038/s41467-018-03613-1
Start Year 2017
 
Description Summer School workshop 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact This was a one day workshop on our open source micro magnetic modelling package. The workshop was given by invitation at the Institute of Rock Magnetism Summer School which occurs every two years at the University of Minnesota. It is sponsored by the National Science Foundation and provides training in rock and mineral magnetism to postgraduate students worldwide. It takes about 50 students in each summer school.
Year(s) Of Engagement Activity 2018