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Geologists suggest early continents formed through mantle plumes, not plate collisions

Geologists from the University of Hong Kong (HKU) have made a breakthrough in understanding how Earth's early continents formed during the Archean time, more than 2.5 billion years ago. Their findings, recently in Science Advances, suggest that early continental crust likely formed through deep Earth processes called mantle plumes, rather than the plate tectonics that shape continents today.

Unlike other planets in our solar system, Earth is a unique planet with continental crust—vast landmasses with granitoid compositions that support life. However, the origin of these continents has remained a mystery. Scientists have long debated whether early continental crust formed through plate tectonics, i.e., the subduction and collision of giant slabs of Earth's crust, or through other processes that do not involve plate movement.

This study, led by Drs Dingyi Zhao and Xiangsong Wang in Mok Sau-King Professor Guochun Zhao's Early Earth Research Group at the HKU Department of Earth and Planetary Sciences, together with international collaborators, has uncovered strong evidence that a distinct geodynamic mechanism shaped Earth's formative years.

Rather than the plate tectonic processes we see today, the research points to a regime dominated by mantle plumes—towering columns of hot, molten rock ascending from deep within Earth. It also identifies a phenomenon known as sagduction, wherein surface rocks gradually descend under their weight into the planet's hotter, deeper layers. These findings shed new light on the dynamic processes that governed the early evolution of Earth's lithosphere.

Studying ancient rocks to understand the deep past

The team analyzed ancient granitoid rocks called TTGs (tonalite–trondhjemite–granodiorite), which make up a large part of the oldest continental crust. These rocks, found in northern China, date back around 2.5 billion years. Using advanced techniques, the researchers studied tiny minerals within the rocks, known as zircons, which preserve chemical signatures from the time the rocks were formed.

By measuring the water content and oxygen isotope composition of these zircons, the team found that the rocks were formed in dry, high-temperature environments, unlike those typically found in zones where tectonic plates collide and one sinks below the other (subduction zones). The oxygen signatures also indicate a mixture of molten oceanic rocks and sediments, consistent with rocks formed above mantle plumes rather than subduction zones.

The researchers proposed a two-stage model to explain their findings. About 2.7 billion years ago, a mantle plume caused thick piles of basalt (Fe- and Mg-rich volcanic rock) to form on the seafloor. Then, about 2.5 billion years ago, another mantle plume brought heat that caused the lower parts of these basalts to melt partially. This process produced the lighter TTG rocks that eventually formed continental crust.

Implications for Earth and planetary science

"Our results provide strong evidence that Archean continental crust did not have to be formed through subduction," explained Dr. Dingyi Zhao, postdoctoral fellow of the Department of Earth and Planetary Sciences and the first author of the paper. "Instead, a two-stage process involving mantle plume upwelling and gravitational sagduction of greenstones better explains the geochemical and geological features observed in the Eastern Block."

The study distinguishes between two coeval Archean TTG suites—one plume-related and the other arc-related— by comparing their zircon water contents and oxygen isotopes. Professor Guochun Zhao emphasized, "The TTGs from the Eastern Block contain markedly less water than those formed in a supra-subduction zone in the Trans-North China Orogen, reinforcing the interpretation of a non-subduction origin."

"This work is a great contribution to the study of early Earth geodynamics," co-author Professor Fang-Zhen Teng from the University of Washington added. "Our uses of zircon water and oxygen isotopes have provided a powerful new window into the formation and evolution of early continental crust."

This study not only provides new insights into understanding the formation of Archean continental crust, but also highlights the application of water-based proxies in distinguishing between tectonic environments. It contributes to a growing body of evidence that mantle plumes played a major role in the formation of the early continental crust.