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

Lead Research Organisation: Imperial College London
Department Name: Earth Science and Engineering

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 grin 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

10 25 50
 
Description Rafal Dunin-Borkowski in DE 
Organisation Julich Research Centre
Country Germany 
Sector Public 
PI Contribution man power and intellectual ideas
Collaborator Contribution expertise and access to the best microscope facilities in Europe
Impact see publications
Start Year 2010