On early Earth, the surface and deep interior are typically seen as separate realms. It is widely believed that subduction—the process by which ocean floors sink into the planet—began relatively late.
Debris from volcanic rocks, some of the oldest known to exist, indicates that the connection between these layers formed much earlier. Material from the surface penetrated deep into the Earth long before textbooks suggest such processes began.
These rocks are komatiites—lavas so intensely hot that virtually nothing similar has formed since. They erupted at temperatures exceeding 1600°C, surpassing anything modern volcanoes can produce.
Almost all komatiites date back to the Archean eon—Earth’s early era, from 4 to 2.5 billion years ago—when the planet’s internal temperatures were far higher. Forming beneath the crust, they carry chemical signatures from regions rarely accessed by ordinary rocks.
A research team led by Zheng-Yu Long from the Paris Institute of Earth Physics (IPGP) analyzed 50 komatiite samples collected from ten locations worldwide. The findings were published in the journal Nature Communications.
The oldest eruptions originated from a 3.6-billion-year-old formation in South Africa. The most recent eruptions occurred just 89 million years ago.
The key lies in potassium isotopes. Potassium exists in two slightly different weight forms, and the balance between them shifts depending on what the element has passed through, especially in contact with water.
When Long’s team performed their analysis, three sites stood out as distinctly different. Their komatiites contained significantly more heavy potassium than typical mantle rocks—even more than seawater, which is naturally rich in heavy isotopes.
This combination is unusual. The deep mantle from which these lavas rose should fall roughly in the middle of the potassium isotope range, not at its extreme. This discrepancy demands an explanation.
Long’s team first considered the simplest possibilities. Weathering and chemical breakdown were obvious suspects. But these processes remove heavy potassium, lowering rather than raising the readings. Contamination from continental crust had a similar effect, reducing values wherever it seeped into the environment.
Even a catastrophe from Earth’s earliest days—the collision that formed the Moon—could not have shifted potassium enough to account for the observed data.
One scenario was harder to rule out. The heavy potassium could have been a relic from the planet’s earliest moments, locked in a mantle layer that was never mixed. But these rocks also carry other traces from that era, such as tungsten and neodymium. Potassium lacks either, so the team turned to a more recent mechanism.
The data fit subduction—the sinking of ocean floor into the mantle, dragging surface material downward. As the descending plate heats up, it releases hot fluids that preferentially carry away heavy potassium.
Such fluid infiltration into the mantle saturates it with heavy potassium, matching the signature seen in these komatiites. Even faint traces can disrupt the balance while barely affecting the rest of the rock.
The three most notable sites were already well known. These ancient lava fields in South Africa and Canada, according to earlier studies, stood out for their unusual moisture content due to water trapped within their crystals.
An independent study conducted several years ago argued that some of these lavas formed from crust soaked in seawater and pulled into the deep mantle. The presence of potassium provides a second, separate line of evidence supporting this hypothesis.
Transmitting this signal from the surface to its source takes time. The recycled material likely sank to the depths and remained there for many millions of years before rising back to the surface within immense plumes of molten rock.
According to modeling results, in the most water-rich regions, melting occurs hundreds of kilometers deep, near the mantle transition zone. This layer lies between 400 and 660 kilometers deep, far within Earth’s interior.
Whether the potassium resided at such great depths or was picked up as the lava rose, the team cannot yet say. The same clue recently revealed surface water lurking beneath modern Asia in a study focused on younger rocks.
When all these pieces come together, one conclusion stands out. At least 3.6 billion years ago, water and the materials it carried began detaching from Earth’s surface and being drawn into the planet’s interior.
This does not prove the existence of modern plate tectonics. However, it does show that the young planet was already efficiently pulling surface water into its depths.
This timing aligns with other evidence, including an analysis suggesting that continents were drifting at modern-like speeds as early as 3.2 billion years ago.
Earth remains habitable partly due to the recycling of water and gases between the surface and mantle, regulating the melting and flow of rocks. Detecting this process at such an early stage reshapes the timeline.
The mechanism that makes a habitable world possible may have been functioning within the first billion years of Earth’s existence.
The findings provide researchers with a long-lasting tool. Heavy potassium in ancient lava persists for eons, so the same test can now be used to examine other old rocks, helping determine when the surface first reached Earth’s depths.
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