Earth’s oldest rocks retell the story of the planet’s earliest history

A 4-billion-year-old granitic rock from Northwest Territories, Canada. (Image by Li Xianhua’s group, Chinese Academy of Sciences).

An international team of researchers obtained new geochemical evidence from earth’s oldest-known rocks—found in remote lake regions of northern Canada—which presents a different picture of the planet’s earliest history.

In a paper published in the journal Science Advances, the scientists say that the oldest samples they collected contradict previous research that argued that subduction and recycling were operating from as early as ~4.3 Ga ago. Since earth itself is only 4.5 Ga old, such a claim infers plate tectonic activity from almost day one.

But the samples collected by the team led by Li Xianhua show no sign of material recycling at 4.0 Ga.

“The earliest evidence we find for surface recycling into magmas isn’t until 3.8 Ga,” the researcher said.

Plate tectonics, which cycles critical biogeochemical elements and maintains the planetary thermostat, is in part responsible for life on the blue planet. Subduction—the destructive force of plate tectonics that pushes one plate beneath another—is the most telltale sign of plate tectonics’ great recycling program.

According to Xianhua, silicon and oxygen isotopes in granitic rocks are tracers of surface material recycling in magma. On ancient earth, seawater was saturated with Si and rich in heavy Si due to the lack of lifeforms to consume it. Thus, if any heavy Si materials from the seafloor were recycled back into magma chambers by subduction, then heavy Si isotopes would be detected in granitic rock samples.

“One of the difficulties in applying this technique to ancient rocks is identifying the primary Si isotope composition,” Zhang Qing, lead author of the paper, said. “This is because these rocks have been reworked by heat and pressure repeatedly throughout earth’s long history.”

Zircon, the most abundant datable mineral in granitic rocks, is also conveniently resistant to weathering and later alteration. 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 absence of a heavy Si signature in the 4.0 Ga rocks means the oldest samples didn’t require subduction.

“Nonetheless, given that the oldest rocks are from a single locality, no subduction needed for one small area doesn’t mean no plate subduction on the planet at 4.0 Ga,” co-author Allen Nutman said.

After careful filtering, though, the data revealed a distinct shift at 3.8 Ga in both Si and O isotopes. For this reason, the study concludes that a possible change in earth’s geodynamics, such as the onset of plate subduction, occurred at 3.8 Ga.

“It was already amazing that these oldest rocks are preserved,” co-author Ross Mitchell said. “And now we learn that they also tell a tectonic coming-of-age story as well.”