The earliest terrestrial crystals bear indications that continents were present and sinking into the mantle during the Hadean eon, over 4 billion years ago. If this holds true, it necessitates a reassessment of the early Earth’s appearance and dynamics, simultaneously extending the timeframe during which life could have arisen. The findings of this investigation are detailed in the journal Nature.
While the age of the oldest known terrestrial rocks is debated, settling around 4.16 billion years, even older minerals exist. These are microscopic crystals known as zircons, which originated within highly ancient, eroded rock formations; the durable zircons are the only surviving remnants. It remains uncertain how widespread these zircons were during the initial 500 million years of Earth’s existence; the sole location where they have survived 4.4 billion years of geological activity is the Jack Hills region of Western Australia.
Such tiny crystals, having detached from their primary bedrock, cannot resolve every major question about the world of that era. Nevertheless, analysis of their chemical makeup suggests that geological processes did not follow a singular global pattern.
Zircons incorporate certain elements while excluding others, and the captured elements provide clues about the magma in which they crystallized. “They are tiny time capsules, packed with an immense amount of data,” noted Professor John Valley of the University of Wisconsin-Madison.
Valley is part of a research team that contrasted the chemical signatures of zircons from the Jack Hills with those from the South African greenstone belt, which formed near the close of the Hadean, ending 4.03 billion years ago. Their compositions differ substantially enough to demand an explanation, particularly regarding the niobium-to-uranium and scandium-to-ytterbium ratios.
“In the Jack Hills sample area, we found that the majority of our zircons do not look like they originate from the mantle,” Valley explained. “They appear consistent with continental crust. It suggests they formed above a subduction zone.” Modern subduction zones involve one tectonic plate overriding another, drawing it down into the mantle.
Today, we would anticipate crystals formed oceans apart to exhibit distinct characteristics, but this wasn’t the case for the early Earth, conjectured to have been so hot from constant asteroid impacts that it was covered by a magma ocean. Models of its cooling typically predict the formation of a rigid outer layer, which geologists term a “stagnant lid.” Only later, according to these models, did this lid break apart into tectonic plates that moved relative to each other and beneath one another, building the continental crust we inhabit.
One possible explanation considered is an error in the analysis conducted on one set of crystals, but Valley is skeptical of this. “I believe the South African data is correct, and our data is also correct,” he stated. “This implies that the Earth during the Hadean was not covered by a uniform, static crust.”
Other celestial bodies in the inner Solar System, such as Venus and the Moon, maintain stagnant lids. Even Io, characterized by intense volcanic activity, possesses one, which generally supports the notion that Earth once did too. However, some geologists are dubious of this idea or suspect the stagnant phase concluded much sooner than generally assumed, around 3.8 billion years ago. Evidence supporting their viewpoint has been accumulating in recent years.
This does not imply that modern plate tectonics was fully operational when the 4.4-billion-year-old Jack Hills zircons formed. Instead, Valley and his co-authors propose a scenario where mantle plumes, similar to those generating oceanic islands, partially melted the base of the crust, initiating a process that pulled surface material downwards. This surface material was rich in water delivered by comets, and it altered the chemistry of the mantle it mixed with. “Water causes melting, and that leads to the production of granites,” Valley commented.
Despite being denser than biological matter, granites are less dense than the rock composing oceanic crust. This difference results in granite-rich continents floating atop the ocean floor.
“This is evidence for the earliest continents and mountain ranges,” Valley concluded. “It’s not plate tectonics, but surface material is being drawn down into the mantle. We might have had an environment similar to a stagnant lid existing alongside a subduction-like environment, just in different locations.”
Even the most stagnant models of a stagnant lithospheric slab allow for portions of the plate to be drawn into the mantle. However, the authors emphasize that “Subduction is fundamentally different from stagnant-lid processes such as delamination.” Delamination only involves the lower portion of the plate, which is dry, whereas subduction incorporates water-bearing surface rocks.
Some theories regarding the origin of life propose it began near deep-sea volcanic vents; others favor an emergence within lakes on volcanic islands. Nevertheless, if continents existed at that time, even relatively small ones, they would have offered far more diverse environments than small and inherently unstable oceanic islands allow.
“We estimate that for about 800 million years in Earth’s history, the surface was habitable, but we lack fossil evidence, and we don’t know precisely when life first emerged on Earth,” Valley remarked.
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