Rock weathering and plate tectonics is vital for life. They both regulate the planet’s surface temperature and provide bio-essential nutrients. But how and when these critical processes got started on Earth is still mystery. And is is possible that they may date back to Earth's infancy—the Hadean Eon over 4 billion years ago?
The central problem is that Earth doesn’t preserve any rocks of this age and so geologists must rely exclusively on the oldest Hadean materials that are preserved: the Jack Hills zircon from the remote outback of Western Australia. Not surprisingly, the Jack Hills zircon have been both lauded and questioned for containing (or not) potential evidence of Earth’s earliest signs of sediments and plate tectonics.
Close-up of a Jack Hill zircon. The grain is ~200 micrometers across.
For years geologists have tried to determine if the Jack Hills zircon crystallized from a magma with sediments in its source or not. Granitic magma containing sediments is known as sediment-derived, or “S-type”, granite. But so far, using traditional geochemical proxies, the results have been mixed.
Magma melt incorporating sediment (so-called “S-type” granite) from the Himalaya (left) and the discovery site of the Jack Hills zircon in remote Western Australia (right).
A traditional discrimination diagram using a small handful of trace element has been used to argue for only rare, or almost no, S-type granite on early Earth. At low phosphorous (P) contents, however, the different types of zircon sources unfortunately overlap, which presents a particular problem because low-P zircon account for >95% of the most ancient detrital zircon. Clearly more zircon trace element information is needed.
In a paper published in the journal PNAS, a research team led by Professor Ross Mitchell from IGGCAS, have employed a precise machine learning method to recognize Hadean S-type zircon to address this issue. This new approach distinguishes S-type zircon from non-S-type zircon, especially with the ability to identify low-P S-type zircon in the Hadean. While previous work argues for rare S-type granite on early Earth, this is the first study to suggest abundant existence of Hadean S-type zircon. With a much high accuracy of 96%, they demonstrate not only the existence, but the abundant existence of Jack Hills S-type zircon, and dating as far back as 4.24 billion years ago.
"That means that the weathering of crust, the deposition of sedimentary rocks, and their burial and incorporation into magma sources may have begun since over 4 billion years ago,” said Mitchell.
The machine-learning-identified changes in Hadean S-type granite coincides with other geochemical characteristics, such as Hf isotope cycling and elevated δ18O values, consistently indicating a connection with continental collision and mountain building.
S-type detrital zircon on early Earth and the evolution of magma sources.
Variation of the proportion of S-type detrital zircon throughout Earth history. Time is in billions of years.
Further classifying global detrital zircon throughout all geologic time, they found that such high peaks of S-type zircon proportions occur repeatedly since 4 billion years ago. They surprisingly found that these peaks are consistent with supercontinents occurring,
"It looks like a cycle, with a strikingly similar ~600 million year period as the supercontinent cycle. As the continents of the world collide, the amount of S-type zircon increases, and then drops down during breakup”, explained first-author Jilian Jiang.
This matching pattern through time hints that the process of plates moving and colliding, driven by subduction, has been happening almost since Earth's earliest days. That means subduction-driven plate tectonics has operated since the Hadean.
"S-type granites kill two birds with one stone, by being a signature of both surface weathering (to generate the sediments) and plate subduction (to get the sediments into magma chambers),” Mitchell explained. “With both these processes in place by essentially day one, the Hadean Earth would have been habitable, permitting the origin of life not only at hydrothermal vents, but also Darwin’s warm pools.”
Bruce Watson, a geochemist from Rensselaer Polytechnic Institute who conducted some of the seminal work on the Jack Hills zircon and was not involved in the study, said, “The authors introduce a new strategy with their machine-learning approach, which should set the stage (and a standard) for future work on early-Earth zircons.”
This study demonstrates the potential for machine learning in exploring and uncovering knowledge that had previously been elusive with traditional methods. Further studies using AI-based knowledge mining should provide new insights into Earth’s ancient history in the future.
