Currently, Earth stands alone as the only identified planet sustaining life, largely thanks to the dynamics of plate tectonics. This process plays a crucial role in recycling vital biogeochemical elements and keeping the planet’s temperature stable. Subduction, the destructive force of plate tectonics that pushes one plate under another, is the most obvious sign of plate tectonics’ great recycling program.
However, how deep in Earth’s past can we find traces of plate tectonics? Have tectonic plates always worked, as they do today, by processes such as subduction and surface material recycling?
Previous studies using numerical geodynamic modeling have argued that subduction and recycling work from about ~4.3 Ga (GA stands for “giga-years”, a unit of time equal to one billion years). Since the Earth itself is only 4.5 Ga years old, such a claim justifies plate tectonics almost from day one.
However, new geochemical evidence from Earth’s oldest known rocks found in remote lake regions of northern Canada paint an entirely different picture of Earth’s earliest history.
The study presenting this evidence Science AdvisorAprece On June 30, Prof. from the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS). It was carried out by researchers led by Li Xianhua in collaboration with colleagues from China as well as Australia and Canada.
Co-author of the study, Prof. “Our earliest samples show no signs of superficial material recycling at 4.0 Ga,” LI said. “And the earliest evidence we’ve found for surface recycling to magmas is not up to 3.8 Ga.”
Silicon (Si) and oxygen (O) isotopes in granite rocks are traces of surface material recycling in magma. In the Old World, 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 is recycled into magma chambers by subduction, heavy Si isotopes will be detected in granite rock samples.
“One of the challenges of applying this technique to ancient rocks is determining the primary Si isotope composition. 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.
The most abundant dateable mineral in granite rocks, zircon is suitably resistant to weathering and subsequent changes. Applying 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 researchers’] The study proposed systematic screening criteria to evaluate the data. “I must thank them for their careful consideration of the zircon Si and O isotope data,” said an anonymous reviewer of the paper.
The absence of a heavy signature of Si in the 4.0 Ga rocks means that the earliest samples did not require submersion.
“Still, the absence of subduction for a small area does not mean that there is no plate subduction on the planet at 4.0 Ga, given that the oldest rocks are from a single region,” said co-author Allen Nutman of the University of Wollongong, Australia.
Still, after careful filtering, the data revealed a significant shift at 3.8 Ga in both Si and O isotopes. Therefore, based on the available data, the study concludes that a possible change in Earth’s geodynamics, such as the onset of plate subduction, occurred at 3.8 Ga.
“The fact that these oldest rocks were preserved was already surprising, and we are now learning that they also tell a tectonic aging story,” said co-author Ross Mitchell of IGGCAS.
Reference: By Qing Zhang, Lei Zhao, Dawn Zhou, Allen P. Nutman, Ross N. Mitchell, Yu Liu, Qiu-Li Li, Hui “No evidence of on-shell recycling in Si-O isotopes of Earth’s oldest rocks 4 Ga ago ” -Min Yu, Billy Fan, Christopher J. Spencer and Xian-Hua Li, June 30, 2023, Science Advances.
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