The microscopic minerals extracted from an old outcrop in Jack Hills, Western Australia, have been the subject of intense geological study, as they appear to carry traces of the Earth’s magnetic field dating back 4.2 billion years. . It’s almost a billion years earlier than when the magnetic field was supposed to have originated, and almost when the planet itself was formed.
But as intriguing as this original story may be, a team led by MIT has now found evidence to the contrary. In an article published in Scientists progress, the team examined the same type of crystals, called zircons, extracted from the same outcrop, and concluded that the zircons they had collected were not reliable as old magnetic field recorders.
In other words, the jury is still out on whether the Earth’s magnetic field existed more than 3.5 billion years ago.
“There is no solid evidence of a magnetic field before 3.5 billion years ago, and even if there were a field, it will be very difficult to find evidence in the zircons of Jack Hills, ”says Caue Borlina, a graduate student at MIT. Department of Earth, Atmospheric and Planetary Sciences (EAPS). “This is an important result in the sense that we know what not to look for anymore. “
Borlina is the first author of the article, which also includes Professor Benjamin Weiss from EAPS, Principal Investigator Eduardo Lima and Researcher Jahandar Ramezan from MIT, as well as others from the University of Cambridge, the Harvard University, University of California at Los Angeles, University of Alabama and Princeton University.
A field, stirred
It is believed that the Earth’s magnetic field plays an important role in making the planet habitable. Not only does a magnetic field define the direction of our compass needles, it also acts as a shield in a way, deflecting the solar wind which could otherwise eat away at the atmosphere.
Scientists know that today the Earth’s magnetic field is powered by the solidification of the planet’s liquid iron core. The cooling and crystallization of the nucleus stirs the surrounding liquid iron, creating powerful electric currents that generate a magnetic field extending far into space. This magnetic field is known as geodynamo.
Multiple data sources have shown that the Earth’s magnetic field existed at least 3.5 billion years ago. However, the planet’s core would have started to solidify only 1 billion years ago, which means that the magnetic field must have been driven by another mechanism before 1 billion years ago. Pinning it just when the magnetic field formed could help scientists understand what started it.
Borlina says the origin of the Earth’s magnetic field could also shed light on the first conditions under which the Earth’s first life forms settled.
“During the first billion years of Earth, between 4.4 billion and 3.5 billion years, this is when life emerged,” said Borlina. “Whether you have a magnetic field at the time has different implications for the environment in which life emerged on Earth. This is the motivation of our work. “
“I can’t trust zircon”
Scientists have traditionally used minerals in ancient rocks to determine the orientation and intensity of the Earth’s magnetic field over time. As rocks form and cool, the electrons in the individual grains can move in the direction of the surrounding magnetic field. Once the rock has cooled above a certain temperature, known as the Curie temperature, the orientations of the electrons are virtually etched in stone. Scientists can determine their age and use standard magnetometers to measure their orientation, to estimate the strength and orientation of the Earth’s magnetic field at a given time.
Since 2001, Weiss and his group have been studying the magnetization of Jack Hills rocks and zircon grains, with the difficult goal of determining whether they contain ancient records of the Earth’s magnetic field.
“Jack Hills zircons are among the weakest magnetic objects studied in the history of paleomagnetism,” says Weiss. “In addition, these zircons contain the oldest known terrestrial materials, which means that many geological events could have reset their magnetic records. “
In 2015, a separate research group that had also started studying Jack Hills zircons argued that they had found evidence of magnetic material in 4.2 billion year-old zircons – the first evidence that the Earth’s magnetic field could have existed before 3.5 billion years ago.
But Borlina notes that the team has not confirmed whether the magnetic material they detected actually formed during or after the formation of the zircon crystal 4.2 billion years ago – a goal that he and his team took it for their new paper.
Borlina, Weiss and their colleagues had collected rocks from the same Jack Hills outcrop, and from these samples extracted 3,754 grains of zircon, each about 150 microns long – about the width of a human hair. Using standard dating techniques, they determined the age of each zircon grain, which ranged from 1 billion to 4.2 billion years.
About 250 crystals were over 3.5 billion years old. The team isolated and imaged these samples, looking for signs of cracks or secondary materials, such as minerals that may have been deposited on or inside the crystal after its complete formation, and looked for evidence that ‘They have been significantly heated in the last billion years since their formation. Of these 250, they identified only three zircons which were relatively free of such impurities and therefore could contain suitable magnetic records.
The team then performed detailed experiments on these three zircons to determine what types of magnetic materials they might contain. They ultimately determined that a magnetic mineral called magnetite was present in two of the three zircons. Using a high-resolution quantum diamond magnetometer, the team examined the cross sections of each of the two zircons to map the location of the magnetite in each crystal.
They discovered magnetite found along cracks or damaged areas inside the zircons. These cracks, says Borlina, are pathways that allow water and other elements to enter the interior of the rock. These cracks could have let in secondary magnetite which deposited in the crystal much later than when the zircon was originally formed. In any case, Borlina says the evidence is clear: these zircons cannot be used as reliable recorders for the Earth’s magnetic field.
“This is proof that we cannot trust these zircon measurements for recording the Earth’s magnetic field,” said Borlina. “We have shown that before 3.5 billion years ago, we still do not know when the earth’s magnetic field started. “
Despite these new results, Weiss points out that previous magnetic analyzes of these zircons are still very valuable.
“The team that brought back the original magnetic study on zircon deserves a lot of credit for trying to solve this extremely difficult problem,” said Weiss. “Thanks to all the work of the two groups, we now understand much better how to study the magnetism of ancient geological materials. We can now begin to apply this knowledge to other mineral grains and to the grains of other planetary bodies. “
New Research Provides Evidence Of A Strong Early Magnetic Field Around The Earth
“Reassess the evidence for a Hadean-Eoarchean dynamo” Scientists progress (2020). avances.sciencemag.org/content/6/15/eaav9634
Massachusetts Institute of Technology
The existence of a magnetic field beyond 3.5 billion years is still under debate (2020, April 8)
retrieved April 8, 2020
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