Thermochemical remanent magnetisations: How do they affect ancient magnetic field intensities from the Earth and Solar System?

Ancient records of magnetic fields stored in rocks and meteorites hold the key to answering some of the most fundamental questions in Earth and Planetary Sciences including the evolution of the Earth's Core and geodynamo, and the formation of the Solar System. In particular, it is the estimates of ancient field intensities that allows us to solve many of these questions, from constraining theories of Solar evolution, to ideas that link the start of the geodynamo to the beginning of life on Earth.

To recover ancient field intensities, we study igneous rocks that have recorded thermoremanent magnetisations (TRM) during cooling. A TRM is the remanent magnetisation recorded by magnetic minerals as they cool from above the Curie temperature (~600 C) in weak magnetic fields like the Earth's. The Curie temperature is a key parameter that defines the maximum temperature at which a material exhibits magnetisation. During TRM acquisition it is assumed that the magnetic minerals are chemically stable, and do not physically or chemically alter during cooling. Such TRMs can be stable for times greater than the age of the Universe.

The magnetic mineral in igneous rocks, particularly basalts, is usually titanomagnetite Fe2.4Ti0.6O4. Basalts are ubiquitous on Earth, for example, most of the top of the ocean crust (70% of the Earth's surface) is basalt. It has been known for many decades that as Fe2.4Ti0.6O4 cools it unmixes (exsolves) into a magnetic magnetite phase (Fe3O4) and a non-magnetic ulvöspinel phase (Fe2TiO4). The unmixing has been extensively studied since the 1950s and has been shown to occur at temperatures above and below the Curie temperature. The exact temperature at which unmixing stops depends on many factors like the cooling rate, with slower cooling rates more likely to give rise to exsolution structures at low temperatures.

For many years palaeomagnetists who study ancient field intensities have assumed that exsolution processes stop at temperatures above the Curie temperature, and that rocks acquire TRMs; however, there is growing evidence to suggest that the minerals continue to unmix below the Curie temperature, thereby chemically alerting and recording another type of magnetic remanent magnetisation termed a thermochemical remanent magnetisation (TCRM). This is a problem, as methods for ancient magnetic field intensity determination assume that rocks carry a TRM not a TCRM.

The Earth Science community maintains a database of global ancient field intensities. Analysis for this proposal indicates at least ~51% of the 4293 intensity estimates (site-level) in the database collected over the last 60 years, could be compromised by the incorrect assumption that the magnetisation is a TRM when it is in fact a TCRM. This maybe the reason for the large scatter found in the database.

Hitherto little attempt has been made to determine the effect of TCRM on ancient field intensity determination, primarily because of the complexity of the problem. In recent years the PI, CoIs, Visiting Fellow and Project Partners, have developed new nanometric imaging, numerical algorithms (MERRILL) and magnetic measurement protocols to study TRM acquisition, that now make the TCRM problem tractable. We aim to nanometrically image magnetic structures in Ti-rich iron oxides during unmixing at temperature, to allow us to understand how the magnetisation is affected by the unmixing process. We will combine this information with nanometric chemical mapping to build numerical models, using a new multiphase addition to MERRILL. The numerical model will allow us to: (1) make predictions which we will ground-truth against magnetic measurements, (2) determine the stability of TCRM on geological timescales, and (3) to determine the contribution of TCRM to ancient magnetic field intensity determinations. We will use the results to develop new ancient field intensity estimations protocols and provide corrections to legacy data.

Who might benefit from this research?
This is a discovery-led project with an interdisciplinary team of scientists of diverse expertise, attempting to better understand how Earth minerals record variation in the geomagnetic field. The end users are primarily within the academic community, however there will be development of micromagnetic modelling methods, that will be of interest to industry. The past changes in the geomagnetic field intensity during have profound implications for our understanding of deep Earth processes, Earth evolution and beginnings of life. These are stories that will be of great interest to the both school children and the general public.

How might they benefit from this research?
1. Industry
There is a large multi-national manufacturing industry concerned with the design and manufacture of high-stability and high-density magnetic data storage. There has been great interest over the last 30 years on initial design exploration using micromagnetic models. Much of the research being done in micromagnetics in the Earth Science community has focused on non-ideal magnetic materials, which are not the immediate focus of current research in the industrial sector, however unexpected commercial innovations have benefited from the work done in the Earth Sciences. Our proposed research addresses the issue of resilience of magnetic recordings to chemical change in the recording media. The magnetic storage industry see stability of their products as a key advantage over solid-state devices, and so are likely to be interested in the techniques we develop.

2. General public and Schools
This scientific background to this project is the need to provide evidence for some of the key questions surrounding the evolution of the Earth and its magnetic field. This evidence not only defines the start of plate tectonics and the nucleation of the inner core, but also the emergence of the protective envelope of the geomagnetic field that provided an environment conducive to the emergence of life.

Interaction with identified beneficiaries.
1. Industry
There already exists a large academic community in materials science and physics that are carrying out micromagnetic models targeted at advancement of industrial recording technologies. The aim in this case is to maintain an interdisciplinary approach to our work so that the developments in micromagnetic modelling are shared across the different communities. To engage with this group, we propose to run an add-on workshop at the 2022 Magnetism and Magnetic Materials Conference in New Orleans, which is a mixed academic and industry meeting.

2. General public and Schools
We intend to develop and deliver a comprehensive and innovative package of public outreach activities, designed to maximize the impact of our research both on the public understanding of science and on UK school-level science education.

Media outreach:
Muxworthy has written for The Times Eureka Magazine. Such direct contacts will be maintained and other contact to the media via newspapers (local and national).

Outreach on the internet and social media:
If funded we will start a website, that will be dedicated to the project, and fully maintained and updated. In order to communicate important and often complex concepts to the general public, novel approaches will be considered. A new Twitter project account will be set up.

Outreach via Exhibitions, science fairs, public events:
We plan to deliver a set of public talks as part of the public lectures and seminars of the Geological Society etc. In year three we will submit a proposal to the Royal Society Summer Science Exhibition.

Schools Outreach:
Through Excitec and Imperial College's "ReachOut Lab", the PI will help bring awareness about the natural environment and geological processes to schools and pupils in London. Similarly, Co-I Williams participates in Sci-fun in Scotland.