Zircons, and their microscopic mineral inclusions, from an ancient outcrop of Jack Hills, Western Australia, have been at the centre of an intense geological debate: When did the Earth first create a magnetic field? Previous studies have suggested that these minerals record traces of Earth’s magnetic field dated as far back as 4.2 billion years ago (Ga). However, an international team led by MIT, and including Professor Richard Harrison (Dept of Earth Sciences, University of Cambridge), has now found evidence to the contrary.
In a paper published in Science Advances, the team examined the same minerals, excavated from the same outcrop, and have concluded that the zircons they collected are unreliable as recorders of ancient magnetic fields. In other words, the jury is still out on whether the Earth’s magnetic field existed earlier than 3.5 Ga.
Earth’s magnetic field plays an important role in making our planet habitable; shielding it from solar winds that would otherwise strip off our atmosphere. Scientists know that today the Earth’s magnetic field is powered by the solidification of the planet’s liquid iron core, which drives convection currents in the surrounding liquid iron (i.e., electric currents), generating a magnetic field that stretches far out into space: the geodynamo.
Multiple lines of evidence have shown that the Earth’s magnetic field existed at least 3.5 Ga. However, the planet’s core is thought to have started solidifying only 1 Ga, so Earth’s magnetic field must have been driven by some other mechanism prior to this time. The origin of Earth’s magnetic field could illuminate the early conditions in which Earth’s first life forms took hold.
Scientists have traditionally used minerals in ancient rocks to determine the orientation and intensity of Earth’s magnetic field back through time. As ferromagnetic minerals (e.g., magnetite) form and cool below a certain temperature (Curie temperature), they lock in a natural remanent magnetisation (NRM) based on Earth’s magnetic field at the time of formation. This NRM can be measured using magnetometers in the lab to estimate the strength and orientation of the Earth’s magnetic field at a given point in time.
In 2015, another research group studying the Jack Hills zircons argued that they had found evidence of magnetic material in 4.2-billion-year-old zircons—the first evidence that Earth’s magnetic field may have existed prior to 3.5 Ga. However, the team did not confirm whether the magnetic material had actually formed at the same time as the host zircon crystal, or rather formed later in the zircon’s history.
To interrogate the Jack Hills zircons further and establish the origin of their NRM, Harrison and colleagues collected rocks from the same Jack Hills outcrop and extracted 3,754 zircon grains. Using U–Pb dating they determined the age of each zircon grain, and selected those older than 3.5 billion years: 250 grains.
The team analysed each remaining grain using microscopic imaging techniques to eliminate grains that had likely been heated, or crystallised secondary inclusions, after initial formation of the zircon. Just three zircons remained that were relatively free of such impurities, and therefore could contain suitable magnetic records.
Two of the three grains (dated 3.97 Ga) contained magnetite, which preserved NRM, but had formed along cracks or damaged zones within the zircons. Therefore, the magnetite formed as a result of natural fluid alteration, at an unknown time during the last 3.97 Ga, giving the host zircon a secondary NRM; rather than the desired primary NRM signal.
Richard Harrison concludes:
“These tiny zircons are the oldest crystals on Earth, and provide our best chance of catching Earth’s earliest magnetic field in the act. However, our work reveals that these ancient crystals have an 'Achilles Heel'—damage caused by the radioactive decay of uranium builds up over millions of years, eventually allowing iron to enter the crystal and form new magnetic particles. Although zircons can make great magnetic recorders, they are, unfortunately, recording a much younger field than we hoped they would! There is cause for optimism, however. We have now established the nanoscale tools that will enable us to hunt down primary magnetite. Although the Hadean [> 4.0 Ga] field remains a mystery, younger zircons may still hold the key to unlocking the rest of Earth’s magnetic history."
This article is based on a press release by MIT.
Additional links:
Borlina, C. S. et al., 2020. Reevaluating the evidence for a Hadean-Eoarchean dynamo. Science Advances, 6(15), eaav9634. DOI: 10.1126/sciadv.aav9634