The quest for primary magnetisation in Earth's oldest materials

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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.