New signs of a shielding magnetic field found in Earth's oldest rock crystals

 sciencemag.org  01/20/2020 20:00:00 

Earth's magnetic field produces an aurora when disturbed by the solar wind.

tiziano valeno/iStock

By Colin Barras

Earths earliest lands, hot and hellish, were sheltered from above. Researchers have found more evidence that our planet had a strong magnetic field 4.2 billion years ago, three-quarters of a billion years earlier than previously thought and just 350 million years after the Earth formed. The field would have shielded Earth, protecting its atmosphere from being stripped away by high-energy particles from the sunand perhaps helping life gain a foothold. 

With few surviving rocks to study, geologists struggle to reconstruct the time known as the Hadean, which ran from 4.55 billion years ago to 4 billion years ago. But fragmentaryand controversialclues can be found in younger, 3-billion-year-old rocks from the Jack Hills of Western Australia. These rocks contain tiny crystals of a hardy mineral called zircon, which are chips off an even older block: 4.2-billion-year-old Hadean rocks that formed from cooling magma.

The crystals also preserve evidence of an ancient magnetic field, according to an international team led by John Tarduno, a geophysicist at the University of Rochester, New York. Not all researchers are convinced by the result, because it would push back the accepted birth date of Earths magnetic field by 750 million years. Theres been a large research group trying to prove our results wrong, says Tarduno. Thats part of science.

Tarduno and his colleagues first reported the fields in an analysis published in 2015 in Science. Trapped inside about two dozen zircons were even tinier grains of an iron-containing mineral, magnetite, that effectively turned each crystal into a miniscule bar magnet. The researchers found that the magnetic fields held by the grains were all aligned, which occurs only if magnetite is exposed to a magnetic field as it cools. 

A zircon crystal fits comfortably inside the "o" on a U.S. dime's lettering.

Julie Mansy; Rory Cottrell/University of Rochester; John Tarduno/University of Rochester

Tardunos team says the magnetization was imprinted 4.2 billion years ago, when the original zircon-containing rock first cooled. However, if the magnetite grains, at any point in their history, became hot enoughabove about 600Cthey would have lost the magnetic alignment and gained a new one as they cooled down again. And just such a heating event may have occurred about 2.6 billion years ago, says Benjamin Weiss, a geologist at the Massachusetts Institute of Technology (MIT) and leader of a group that has challenged Tardunos claims. These zircons have mind-bogglingly long and largely unknown histories, he says.

In the Proceedings of the National Academy of Sciences today, Tardunos team marshals a new line of evidence to show that the magnetization happened in the Hadean instead of much later. Zircon crystals contain zones rich in lithium ions that, when heated, can bleed over time into adjacent areas through a process called chemical diffusion. Tarduno and his colleagues measured the lithium concentrations across the boundaries of these zones in three of their zircon crystals. In two, they found limited evidence of diffusion. Tarduno says its a sign they were never heated up above 600C in their 4.2-billion-year historyand that their magnetic signature is original. I think thats a remarkable finding.

Whats more, the single zircon crystal that did show signs of heating is an outlier in another way: It preserves an unusually weak magnetic signal. In 2015 Tarduno and his colleagues concluded this might be evidence of a strangely variable magnetic field 4.2 billion years ago. But now, he says, it seems that this particular zircon gained its magnetism later, and its weak signal can be ignored in discussions of Earths earliest magnetic field. Doing so, says Tarduno, suggests Hadean Earth actually had a magnetic field, and a churning dynamo in its core, as strong as the one operating today.

Mark Harrison, a geologist at the University of California, Los Angeles, isnt convinced by the new analysis, because it takes both high temperatures and long periods of time for the lithium ions to diffuse across a boundary. If a heating incident brought the zircons above 600C but persisted for only 10,000 yearsa blink of an eye for a geologistit would be enough time to reset the zircons magnetism but not to diffuse lithium. I dont think this is even remotely smoking gun evidence, he says.

MIT geologist Claire Nichols hasnt been involved in any of the Jack Hills studies but she did recently argue that Earths magnetic field was in place at least 3.7 billion years ago after analyzing rocks in Greenland. Its great to see different research groups pushing each other to use more and more advanced techniques, she says of the new study. It gives us the best chance of understanding the earliest record of the geodynamo.
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