Recovering ancient records of Earth’s magnetic field is challenging because the magnetization in rocks is often reset by heating during tectonic burial over their long and complex geological histories. Geoscientists from MIT and elsewhere show that rocks from the Isua Supracrustal Belt in West Greenland have experienced three thermal events throughout their geological history. The first event was the most significant, and heated the rocks up to 550 degrees Celsius around 3.7 billion years ago. The subsequent two events did not heat the rocks in the northernmost part of the area above 380 degrees Celsius. The authors use multiple lines of evidence to test this claim, including paleomagnetic field tests, the metamorphic mineral assemblages across the area, and the temperatures at which radiometric ages of the observed mineral populations are reset. They use these lines of evidence to argue that an ancient, 3.7-billion-year-old record of Earth’s magnetic field may be preserved in the banded iron formations in the northernmost part of the field area.
In the new study, University of Oxford’s Professor Claire Nichols and colleagues examined an ancient sequence of iron-containing rocks from Isua, Greenland.
Iron particles effectively act as tiny magnets that can record both magnetic field strength and direction when the process of crystallization locks them in place.
The researchers found that rocks dating from 3.7 billion years ago captured a magnetic field strength of at least 15 microtesla comparable to the modern magnetic field (30 microtesla).
These results provide the oldest estimate of the strength of Earth’s magnetic field derived from whole rock samples, which provide a more accurate and reliable assessment than previous studies which used individual crystals.
“Extracting reliable records from rocks this old is extremely challenging, and it was really exciting to see primary magnetic signals begin to emerge when we analyzed these samples in the…
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