Ancient Rocks Shed Light on Early Plate Tectonics – Earth’s Most Ancient Rocks Provide Insights into Early Geological Activities
Earth’s eldest rock: A 4-billion-year-old granitic formation found in the Northwest Territories of Canada. Image Credit: Research Group of Professor Li Xianhua
At present, Earth remains the solitary known planet that supports life, largely attributed to the dynamics of plate tectonics. This process plays a crucial role in recycling essential biogeochemical elements and maintaining the planet’s temperature stability. Subduction, the forceful action of plate tectonics that pushes one plate beneath another, stands as the primary indicator of this extensive recycling system.
Yet, how far back in Earth’s history can we trace signs of plate tectonics? Were tectonic plates always functioning in the manner they do today, involving processes like subduction and the recycling of surface materials?
Earlier investigations utilizing numerical geodynamic simulations have contended that subduction and recycling were in operation as early as around 4.3 billion years ago (referred to as Ga, equivalent to a billion years). Considering Earth’s age of 4.5 billion years, this proposition suggests that plate tectonics were active almost from the very beginning.
However, fresh geochemical evidence derived from Earth’s earliest-known rocks, situated in remote lake regions in northern Canada, presents a notably different narrative of the planet’s early history.
This research, detailed in the publication “Science Advances” on June 30, was carried out by a team of researchers led by Professor Li Xianhua from the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS), in collaboration with colleagues from Australia, Canada, and China.
“Our oldest samples, dating back to 4.0 billion years, exhibit no indications of surface material recycling,” remarked Professor Li, one of the co-corresponding authors of the study. “And the earliest evidence of surface material recycling into molten rock is only apparent at 3.8 billion years.”
Between approximately 4.0 and 2.5 billion years ago, the seawater contained abundant silicon (Si), resulting in the seafloor becoming enriched in heavy Si. The study notes a transition between the period prior to 4.0 billion years, when heavy Si wasn’t integrated into granitic magma, and the period around 3.8 billion years, when this integration occurred. Image Credit: Research Group of Professor Li Xianhua
Isotopes of silicon (Si) and oxygen (O) within granitic rocks serve as indicators of surface material recycling within magma. In ancient times, the oceans contained a high concentration of Si and heavy Si due to the absence of lifeforms that would consume them. Consequently, if heavy Si materials from the seafloor were reprocessed into magma chambers through subduction, traces of heavy Si isotopes would be discernible in samples of granitic rock.
“One of the challenges in applying this methodology to ancient rocks is determining the initial Si isotope composition. This is complicated by the fact that these rocks have undergone repeated transformations due to heat and pressure over Earth’s extensive history,” explained Zhang Qing, the lead author of the study from IGGCAS.
Zircon, a prolific mineral in granitic rocks that can be accurately dated, is also resistant to weathering and subsequent changes. Precise analytical techniques applied to zircon offer reliable insights into whether the identified Si isotope composition represents the primary signature.
“The study introduces systematic criteria for evaluating the data. Their thorough assessment of zircon Si and O isotope data is commendable,” commended an anonymous peer reviewer of the paper.
The absence of a significant heavy Si signal in rocks dated to 4.0 billion years suggests that subduction wasn’t a requisite process for those ancient samples.
“However, it’s important to note that the oldest rocks come from a single location. The lack of subduction in this small area doesn’t necessarily imply an absence of plate subduction across the planet at 4.0 billion years,” explained co-author Allen Nutman from the University of Wollongong in Australia.
After meticulous filtering, the data disclosed a notable shift at 3.8 billion years in both Si and O isotopes. Thus, based on the current data, the study proposes that a potential shift in Earth’s geodynamic processes, such as the initiation of plate subduction, occurred around 3.8 billion years ago.
“It was already remarkable that these ancient rocks were preserved,” added Ross Mitchell, a co-author from IGGCAS, “and now we uncover that they also narrate a story of tectonic maturation.”
Reference: “No evidence of supracrustal recycling in Si-O isotopes of Earth’s oldest rocks 4 Ga ago” by Qing Zhang, Lei Zhao, Dawn Zhou, Allen P. Nutman, Ross N. Mitchell, Yu Liu, Qiu-Li Li, Hui-Min Yu, Billy Fan, Christopher J. Spencer and Xian-Hua Li, 30 June 2023, Science Advances.
DOI: 10.1126/sciadv.adf0693
Table of Contents
Frequently Asked Questions (FAQs) about Geological Evolution
What is the significance of Earth’s oldest rocks in understanding early tectonics?
The oldest-known rocks found in remote regions of Canada offer insights into Earth’s early geological activities, particularly the onset of plate tectonics. These rocks provide geochemical evidence that suggests subduction and surface material recycling didn’t start until around 3.8 billion years ago, challenging previous models that proposed an earlier start.
How do isotopes of silicon and oxygen contribute to understanding early tectonics?
Isotopes of silicon (Si) and oxygen (O) in granitic rocks act as tracers of surface material recycling within magma. The presence of heavy Si isotopes indicates recycling from the seafloor. By analyzing these isotopes in ancient rocks, researchers can determine when such recycling processes began and whether plate tectonics were active.
What is subduction, and how does it relate to plate tectonics?
Subduction is a geological process in which one tectonic plate is forced beneath another, often leading to the recycling of surface materials into Earth’s mantle. It’s a fundamental aspect of plate tectonics, driving the movement of plates and influencing geological activities like earthquakes, volcanic eruptions, and mountain formation.
How were these findings obtained, and what methods were used?
Researchers conducted geochemical analyses on ancient granitic rocks, particularly focusing on isotopes of silicon and oxygen. By carefully evaluating these isotopes in zircon minerals, researchers were able to identify patterns that indicate the presence or absence of surface material recycling and subduction processes in Earth’s early history.
Why is the age of Earth’s oldest rocks significant?
Earth’s oldest rocks provide a window into its early geological evolution. By studying these rocks and the processes recorded within them, scientists can gain valuable insights into the conditions and mechanisms that shaped Earth’s surface and internal dynamics billions of years ago.
More about Geological Evolution
- Science Advances: No evidence of supracrustal recycling in Si-O isotopes of Earth’s oldest rocks 4 Ga ago
- Chinese Academy of Sciences: Institute of Geology and Geophysics
- University of Wollongong: Allen Nutman
