Recovering ancient records of the Earth's magnetic field is difficult because the magnetization of rocks is often reset by heating during burial due to tectonic movements over a long and complex geological history. Geoscientists from MIT and elsewhere have shown that rocks in West Greenland's Isua supercrustal zone have experienced three thermal events throughout their geological history. The first event was the most important, heating rocks to 550 degrees Celsius about 3.7 billion years ago. His two subsequent phenomena did not heat the region's northernmost rocks above 380 degrees Celsius. The authors use multiple lines of evidence to test this claim, including paleomagnetic field tests, metamorphic mineral assemblages across the region, and temperatures at which the radiometric ages of observed mineral assemblages are reset. They use this body of evidence to argue that an ancient record of Earth's magnetic field from 3.7 billion years ago may be preserved in the striated iron layer at the northernmost edge of the magnetic field. .
In a new study, Professor Claire Nicholls from the University of Oxford and colleagues examined a range of ancient iron-bearing rocks from Isua, Greenland.
Once locked in place during the crystallization process, iron particles effectively act as tiny magnets that can record both the strength and direction of a magnetic field.
Researchers found that 3.7 billion-year-old rocks exhibited magnetic field strengths of at least 15 microteslas, comparable to modern magnetic fields (30 microteslas).
These results provide the oldest estimates of the strength of Earth's magnetic field derived from whole rock samples, providing a more accurate and reliable estimate than previous studies using individual crystals.
“It's very difficult to extract reliable records from rocks this old, so it was really exciting to see the primary magnetic signals start to emerge when we analyzed these samples in the lab,” Professor Nichols said. said.
“This is a very important step forward in our efforts to understand the role of ancient magnetic fields in the creation of life on Earth.”
Although the strength of the magnetic field appears to remain relatively constant, the solar wind is known to have been significantly stronger in the past.
This suggests that surface protection from the solar wind may have strengthened over time, thereby allowing life to leave the protection of the oceans and migrate to the continents.
The Earth's magnetic field is created by the mixing of molten iron within a fluid outer core, driven by buoyancy as the inner core solidifies, forming a dynamo.
During the early stages of Earth's formation, a solid inner core had not yet formed, leaving unanswered questions about how the initial magnetic field was maintained.
These new results suggest that the mechanisms driving Earth's early dynamo were as efficient as the solidification processes that generate Earth's magnetic field today.
Understanding how the strength of Earth's magnetic field has changed over time is also key to determining when Earth's interior solid core began to form.
This helps us understand how fast heat is escaping from the Earth's deep interior, which is key to understanding processes such as plate tectonics.
A key challenge in reconstructing Earth's magnetic field back in time is that any event that heats rocks can change the preserved signal.
Rocks in the Earth's crust often have long and complex geological histories that erase information about previous magnetic fields.
However, the Isua supercrustal zone has a unique geology, sitting on a thick continental crust and protected from extensive tectonic movements and deformation.
This allowed scientists to build clear evidence for the existence of magnetic fields 3.7 billion years ago.
The results may also provide new insights into the role of magnetic fields in shaping the development of Earth's atmosphere as we know it, particularly regarding the release of gases into the atmosphere.
“In the future, we hope to expand our knowledge of Earth's magnetic field before oxygen increased in the Earth's atmosphere about 2.5 billion years ago by examining other ancient rock sequences in Canada, Australia, and South Africa. “We believe that this is the case,” the authors said.
“A better understanding of the strength and variability of ancient Earth's magnetic field will help determine whether the planet's magnetic field was important for harboring life on the planet's surface and its role in the evolution of the atmosphere. Masu.”
of study Published in Geophysical Research Journal.
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Claire IO Nichols other. 2024. Possible Archean record of geomagnetism preserved in the Isua supercrustal zone of southwestern Greenland. Geophysical Research Journal 129 (4): e2023JB027706; doi: 10.1029/2023JB027706
Source: www.sci.news
