Jakarta –
An international team of researchers has obtained new geochemical evidence from the oldest rocks found in a remote lake area in northern Canada. These rocks present a different picture of Earth’s early history.
In a paper published in the journal Science Advances, scientists say the oldest samples collected contradict previous research that suggested subduction and recycling occurred about 4.3 Gigaannum (Ga) ago. Ga is a unit of time equivalent to 1 billion years.
Since the Earth itself is only 4.5 billion years old, the statement infers plate tectonic activity almost from day one.
However, samples collected by a team led by Li Xianhua showed no signs of recycling of the material at 4.0 Ga.
“The first evidence we found of surface recycling in magma occurred just 3.8 billion years ago,” the researchers said.
Tectonic plates, which recycle important biogeochemical elements and maintain the planet’s thermostat, are partly responsible for life on the Blue Planet.
Subduction, or the destructive force of tectonic plates that push one plate under another plate, is the clearest sign of the great recycling activity of tectonic plates.
According to Xianhua, silicon and oxygen isotopes in granitic rocks are tracers of the recycling of surface material into magma. On early Earth, seawater was saturated with silicon (Si) and rich in heavy Si due to the lack of living creatures consuming it.
So, if heavy Si material from the seafloor is recycled into the magma chamber through subduction, then heavy Si isotopes will be detected in the granite rock samples.
“One of the difficulties in applying this technique to ancient rocks is identifying the primary isotopic composition of Si,” said Zhang Qing, lead author of the paper cited by Mining.com.
“This is because these rocks have been repeatedly reworked by heat and pressure over Earth’s long history,” he said.
Zircon, the most abundant mineral in granitic rocks, is also resistant to weathering and subsequent changes. The application of very high precision analytical techniques to zircon can provide the most reliable constraints on whether the detected Si isotopic composition represents a primary signature.
The absence of heavy traces of Si in the 4.0 Ga rocks means that the older samples did not require subduction.
“However, given that the oldest rocks come from a single location, the absence of subduction required for a small area does not mean that there is no plate subduction on the planet to a depth of 4.0 Ga,” said the co-author Allen Nutman.
However, after careful filtering, the data showed a real shift in the 3.8 Ga isotopes of Si and O (oxygen). Therefore, the study concluded that possible changes in Earth’s geodynamics, such as the onset of plate subduction, occurred at 3.8 Ga.
“It’s amazing that these older rocks are preserved. And now we know that they also tell the story of the tectonic future,” said co-author Ross Mitchell.
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(rn/rn)
2024-01-17 01:06:00
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