The quest for primary magnetisation in Earth's oldest materials

The Earth's magnetic field is generated by the constant churning of liquid iron in its outer core. This "geodynamo" is crucial to modern life: without it our atmosphere would be gradually stripped away by the solar wind and we would be exposed to potentially lethal doses of high-energy cosmic rays (a prospect that awaits astronauts journeying to Mars, which lost its magnetic field along with much of its atmosphere 4 billion years ago).

The geodynamo is likely to have played an equally important role in creating the conditions necessary for the emergence of life on Earth around 4 billion years ago. However, we know little, if anything, about the behaviour of the geodynamo during this critical period. The earliest evidence for the geodynamo comes from rocks that are 3.5 billion years old, but since the Earth formed over 4.5 billion years ago, there is currently a gap of more than one billion years in our knowledge of Earth's magnetic history. This proposal forms part of an international quest to extract pristine magnetic signals from Earth's oldest materials, with the ultimate aim of plugging this gap in the paleomagnetic record.

The lack of magnetic data for the early Earth is easy to explain: rocks of this age are extremely rare, and those that exist have suffered varying degrees of heating and/or chemical alteration during their long geological history. The magnetic signals carried by tiny magnetic mineral grains in ancient rocks become corrupted over time. Too much heating and the primary magnetic signals are destroyed forever. Growth of new magnetic minerals during low-temperature chemical alteration can obscure or replace the primary magnetic signals. Only the most thermally stable magnetic grains, which have been fully protected from chemical alteration, have the potential to cling on to their primary magnetisation.

In the search for ideal magnetic recorders, the "single-crystal" paleomagnetism method has emerged as an exciting prospect. Instead of analysing bulk rocks (which are likely to be dominated by secondary magnetic minerals), measurements are made on single crystals of nominally non-magnetic minerals (e.g. quartz) containing sub-micrometre inclusions of primary magnetic minerals. These magnetic inclusions are not only protected from chemical alteration by their host silicate, but are small enough to contain stable magnetic structures that survive heating to all but the most extreme metamorphic temperatures. Intense attention has recently focussed on single crystals of detrital zircon from the 3 billion year old Jack Hills metaconglomerate. A small fraction of these crystals have been dated to the Hadean era (i.e. more than 4 billion years old). It is claimed that these zircons were magnetised by an active geodynamo 4.2 billion years ago; pushing back the start of the geodynamo by 700 million years. A counter claim asserts, however, that the Jack Hills samples have been pervasively remagnetised, and no longer contain any vestige of primary magnetisation. Resolving this controversy requires in-depth analysis of the magnetic inclusions.

This project will place the single-crystal paleomagnetism method on a sound physical basis. A range of tomographic methods, correlated across multiple length scales, will enable the internal architecture of single crystals to be reconstructed in unprecedented detail, providing evidence to establish the primary/secondary nature of their magnetic inclusions. Fe isotopes will be investigated as a potential method to distinguish high-temperature primary inclusions from the low-temperature products of chemical alteration. The 3D distribution of primary magnetic inclusions will be used to develop computer simulations that recreate the magnetic recording process. These methods will be applied to the Jack Hills zircons and combined with high-resolution magnetic imaging to enable to properties of the Hadean magnetic field to be determined unambiguously.

Who might benefit from this research?

The non-academic beneficiaries of this research are as follows:

1) Earth scientists involved in research and development within industry/geoengineering
2) Instrument manufacturers
3) The general public
4) Schools

How might they benefit from this research?

There is a burgeoning community of Earth scientists in academia, the oil industry, the mining industry, the nuclear industry and geoengineering who are using 3D imaging (tomography) as part of their research and development programmes. Although X-ray tomography is by far the most common tool, there is increasing interest in other forms of tomography (e.g. FIB-nt, STEM tomography, atom probe tomography) that provide much higher resolution information about the chemistry, crystallography and microstructure of natural samples. This project brings together a multi-disciplinary team of world-leading experts in all these techniques, including Earth scientists, materials scientists and chemical engineers. We are perfectly placed, therefore, to capitalise on two key opportunities for impact resulting from our research:

1) bringing the capabilities of multi-scale correlative tomography to the wider attention of Earth scientists both in academia and industry.

2) providing the opportunity for instrument manufacturers to increase their engagement with an emerging community of Earth scientists.

Engagement between instrument manufacturers and the wider Earth science community is mutually beneficial. Earth scientists will be able to discuss their requirements with the instrument manufacturers, make suggestions for adapting their instruments to unique challenges of natural samples, and learn from the experts about the optimal ways to solve their R+D problems. Instrument manufacturers are keen to engage with a new community that represents a significant growth opportunity for their companies.

Our team includes Co-Is with a long history of engagement with tomography instrument manufacturers and a Researcher Co-I who worked as an applications engineer for FEI for ten years. We will leverage the professional network of the team to create mechanisms of engagement between Earth scientists in industry and academia and the instrument manufacturers. We propose to:

1) Organise a 3-day workshop in Cambridge on multi-scale correlative tomography for Earth sciences, with invited speakers from academia, industry and instrument manufacturers.

2) Develop our working relationship with Zeiss X-ray microscopy; communicate to them the hardware/software needs of Earth scientists; work with them to help implement new software tools; provide access to the results of our high-profile research for promotional purposes.

3) Establish a web-site, discussion group, Twitter and Facebook account for Earth scientists across academia and industry interested in tomography.

General public

The scientific context for this project (the role of Earth's magnetic field in establishing the conditions necessary for the emergence of life, and how these conditions compared to those on Mars) are sufficiently high profile to be of interest to the general public. We have existing connections with science journalists working for BBC Science, and will work with them to ensure national and international media coverage for the results of this research (in addition to our long-standing commitment to public outreach via the NanoPaleoMagnetism blog and Twitter account).

Schools

As a fellow of St Catharine's college, the PI has a commitment to outreach and engagement with local primary and secondary schools, and regularly participates in Science Week presentations on a range of topics relating to Earth sciences, materials science and physics. The scientific context of this project is particularly conducive to outreach and engagement, and the PI commits to his continued involvement in this area.