The oldest rocks force to change the history of the Earth

2023-07-05 12:33:02

MADRID, 5 Jul. (EUROPA PRESS) –

New geochemical evidence obtained from the oldest known rocks on Earth, found in remote lake regions of northern Canada, change the oldest history of the planet.

Previous studies using numerical geodynamic models have argued that subduction and recycling of surface material had been operating for about 4.3 billion years. Since the Earth itself is 4.5 billion years old, such a claim argues for plate tectonics from almost the origin of our planet.

But a new study substantially delays the onset of plate tectonics. “Our oldest samples show no signs of recycling of surface material from 4 billion years ago,” he said. it’s a statement co-author and Professor LI Xianhua from the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS). “And the earliest evidence we found for surface recycling into magmas It’s not until 3.8 billion years ago.”

Silicon (Si) and oxygen (O) isotopes in granitic rocks are tracers of the recycling of surface material into magma. On ancient Earth, seawater was saturated with Si and rich in heavy Si due to the lack of life forms to consume it. Therefore, if any heavy Si material from the seafloor were to be recycled back to the magma chambers by subduction, then heavy Si isotopes would be detected in the granitic rock samples.

“One of the difficulties in applying this technique to ancient rocks is identifying the primary isotopic composition of Si. This is because these rocks have been repeatedly reworked by heat and pressure throughout Earth’s long history.” said ZHANG Qing of IGGCAS, lead author of the study, which is published in Science.

Zircon, the most abundant datable mineral in granitic rocks, is also conveniently resistant to weathering and subsequent alteration. The application of ultra-high precision analytical techniques to zircon can provide the most reliable constraints on whether the detected Si isotope composition represents the primary signature.

“The study [de los investigadores] proposed systematic detection criteria to evaluate the data. I must congratulate them on their careful evaluation of their Si and O zircon isotope data,” said an anonymous reviewer of the paper.

The absence of a strong Si signature in 4 billion-year-old rocks means that the oldest samples did not require subduction.

“However, since the oldest rocks come from a single locality, subduction is not needed for a small area does not mean that there was no plate subduction on the planet 4 billion years ago,” said co-author Allen Nutman of the University of Wollongong in Australia.

However, after careful filtering, the data revealed a distinct change 3.8 billion years ago in the Si and O isotopes. For this reason, based on current data, the study concludes that a possible change in the geodynamics of the Earth, as the start of plate subduction, It happened 3.8 billion years ago.

“It was already amazing that these older rocks were preserved,” said co-author Ross Mitchell of IGGCAS, “and now we know they also tell a mature tectonic story.”