A newly published study into the start date for the Earth's magnetic field has provoked claims of foul play among rival academics.
In 2015, a group of researchers led by the US University of Rochester reckoned they found samples of zircon minerals lodged in rocks from Jack Hills, a region in Western Australia, that contained proof the Earth's magnetic field took shape quickly after the planet formed in the Solar System some 4.5 billion years ago. The results were published in a previous paper in Science.
Now, a separate study led by MIT claims the zircon crystals collected from Jack Hills are unreliable sources of evidence.
"There is no robust evidence of a magnetic field prior to 3.5 billion years ago, and even if there was a field, it will be very difficult to find evidence for it in Jack Hills zircons," said Cauê Borlina, a graduate student at MIT's Department of Earth, Atmospheric, and Planetary Sciences (EAPS). "It's an important result in the sense that we know what not to look for anymore."
This dismissal of earlier research led to a swift rebuttal from the Rochester team.
"This group has been trying to prove that our results are wrong for a long time," John Tarduno, professor of geophysics at the University of Rochester, who led the previous study, told The Register. "In my opinion, they have not reported an objective analysis."
He added: "In our 2015 study, we carefully selected zircons that are not compromised by alteration. In contrast, the samples they have presented are altered. Moreover, the techniques they apply are inferior and enhance alteration, leading to failed experiments."
The MIT team's idea rests on the fact that the electrons within grains of zircon are affected by Earth's magnetic field. When the rocks cool down enough to dip below the Curie temperature – the limit that locks its magnetic properties in place – scientists can measure the strength and orientation of Earth's magnetic field at that particular time. The telltale sign is found by studying the remnant magnetism in magnetite, a natural magnetic material formed from iron.
Borlina and his colleagues collected 3,754 tiny zircon grains measuring around 150 micrometers long. They estimated that they were anywhere up to 4.2 billion years old and these samples were whittled down further to 250 which were older than 3.5 billion years.
They probed these zircon crystals carefully looking for signs that the samples may have been heated in the past by other geologic events or for impurities. Secondary minerals might have been deposited inside them after the rocks had formed. These were discarded leaving just three samples left.
These were then studied using a diamond magnetometer to map out where magnetite had formed inside the zircon. The researchers found the magnetic mineral along cracks within the crystals. These cracks create pathways for the magnetite to settle.
That would mean that the magnetite was only embedded later after the zircon had formed, leading the researchers to believe that even though the rocks may date as far back as 4.2 billion years old the magnetic material inside them is much more recent.
"The Jack Hills zircons are some of the most weakly magnetic objects studied in the history of paleomagnetism," said Benjamin Weiss, co-author of the paper and a professor at MIT's Department of EAPS. "Furthermore, these zircons include the oldest known Earth materials, meaning that there are many geological events that could have reset their magnetic records."
"This is evidence we can't trust these zircon measurements for the record of the Earth's magnetic field," Borlina added. "We've shown that, before 3.5 billion years ago, we still have no idea when Earth's magnetic field started."
The team still believes that the previous 2015 study is valuable, however, as it shows that zircon crystals may not be suitable to answer the question of how old Earth's magnetic field really is.
"We believe that zircons overall are the problem," Borlina told The Register. "Essentially zircons trap a high concentration of uranium when they form, and as the uranium decays, it creates radiation damage, which in turn produces these pathways for secondary minerals to make their way into the zircon. The older the grain, the more radiation damage, the more pathways for secondary material to get in the zircon."
Borlina added: "As of now, we have evidence that [Earth's magnetic field] existed for the last 3.5 billion years. We are not sure what happened before that."
In contrast, Rochester's Tarduno argued that Earth must have had a magnetic field soon after it formed, otherwise our oceans and our atmosphere would have been stripped by the Sun's harmful rays that Earth's magnetic field deflects.
"The very existence of our ocean and atmosphere supports our claims of an ancient magnetic field ... if Earth lacked a magnetic field in the Hadean, the early atmosphere would have been vulnerable to erosion from the intense solar wind streaming from the young Sun," he said.
"Our most recent results indicate the magnetic field not only existed, but that it was strong and able to protect Earth's hydrosphere." ®
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