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

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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.