Scientists unlock the secrets of Earth’s earliest rocks

In Earth's early days, more than 4 billion years ago, the surface was a dangerous and unpredictable place. Violent volcanoes, crashing meteorites, and constant tectonic activity repeatedly resurfaced the youthful planet. Almost all of the rocks from that time have been annihilated or transformed beyond recognition. One mineral, however, small but almost indestructible, survived the mayhem and now tells us part of Earth's early history.

This mineral, zircon, develops in magma and can exist for billions of years. Some of the oldest dated zircon crystals—over 4.3 billion years old—are located in Western Australia's Jack Hills. These crystals are among the few remaining records from the Hadean eon, a mysterious era soon after the creation of Earth.

Scientists have been examining the isotopes and trace elements in these zircons for decades, but there was much they didn't understand about the very rocks in which they crystallized. What did the crust look like back then? What minerals did it contain? And how did it form?

Today, researchers have taken a giant leap towards solving these questions. Their breakthrough doesn't come from a new rock discovery—but from artificial intelligence.

A team led by Professors Jia Liu and Qunke Xia at Zhejiang University used machine learning to study ancient rocks. They reconstructed the geochemical makeup of crustal rocks that no longer exist.

The group included PhD students Denggang Lu, Zhikang Luan, Jingjun Zhou, and Tianting Lei. They were attempting to figure out what types of rocks existed in the Hadean period, over 4 billion years ago, by examining zircons and the molten rock—or magma—in which they were created.

To achieve this, the researchers assembled the largest dataset yet of zircon samples from across the globe and the rocks they were hosted in. The dataset included over 14,000 zircon samples paired with 823 rock samples. Using this information, they built supervised machine learning models to uncover patterns between the chemical signatures of the zircons and the rocks in which they formed.

This kind of modeling allowed them to recreate the chemical compositions of rocks that existed billions of years ago but were erased by time. The virtual reconstructions gave scientists a kind of time machine—a way to "see" rocks that no longer exist. "We don't know what the actual rocks of the Hadean crust looked like, because we don't have any," PhD student Denggang Lu said. "But zircons give us a window into that unseen world."

The findings challenged some of the traditional knowledge about early Earth. Some had thought that the Hadean crust might have been andesite-like, a silica-rich rock formed at subduction zones. The new research painted a different picture. The crust would have been more felsic—rich in lighter minerals like quartz and feldspar—and made up largely of tonalite, trondhjemite, granodiorite (TTG), and potassic granites.

The scientists concluded that SiO₂ content in the parent magmas varied from 58 to 78 weight percent. These magmas also had potassium-to-sodium (K₂O/Na₂O) ratios of 0.1 to 1.2 and strontium-to-yttrium (Sr/Y) ratios of 1 to 103. Based on these values, the ancient rocks were not formed at deep oceanic subduction zones but likely formed from impact thickened crust. That type of crust is created when two continents collide with one another—like the formation of the Himalayas, but on a much older Earth.

So where would these TTG and granite rocks have come from? From the study, they would have formed by the partial melting of mafic proto-crust. That is, when darker, magnesium- and iron-rich rocks were heated, they partially melted to form lighter, silica-rich rocks. Some of these melts formed TTGs, while others, especially those from more potassium-rich sources, formed granites.

This petrogenetic model explains the origin of rocks from magma and leads to a new view of early continental crust. Instead of deep subduction zones recycling crust back into the mantle, the early Earth may have had plate collision-thickened crust. These collisions most likely produced enough heat and pressure for felsic crust to form near the planet's surface.

The significance of these discoveries is enormous. Because the Earth's oldest preserved rocks date to about 4.03 billion years ago, the interval between Earth's formation and then had been termed the planet's "missing chapter." Because nearly all rock records from that time have been destroyed, scientists had little direct evidence of the character of the surface.

“The Hadean is a key period for understanding the origin of Earth’s continents,” said Professor Liu. “But rocks from this time are incredibly rare. So far, the only known samples are from the Acasta region in Canada and date to about 4.03 billion years ago.”

Zircons have been the exception. The tiny crystals are chemically inert and can resist intense heat and pressure. They are time capsules, locking in chemical information from the molten formations where they formed. Using AI to decode this information has allowed scientists to recreate something that would otherwise be lost forever.

During a time of rapid AI development, using machine learning to discover the relationship between zircons and their rocks is simply exciting," described Professor Liu. "It provides us with a chance to push the known rock record back by nearly 400 million years and examine how the earliest crust may have evolved."

This study is a paradigm shift for geology. It shows that artificial intelligence can be leveraged to complete important gaps in the rock record—especially for times when there are no rocks left. The machine learning algorithms the team used were capable of predicting many major and trace elements in the parental magmas of the Jack Hills zircons. These predictions are very much in line with what scientists would expect from real rock samples run through a laboratory.

By combining AI with traditional geochemical analysis, scientists have a powerful new tool. The technique could be applied to study not only the Hadean but other ancient time periods for which there is no physical evidence. It also opens up greater application of zircon in geology. Once considered a mere method of rock dating, zircon now offers a way of chemically reconstructing rocks that no longer exist.

More broadly, this research supports the idea that early plate tectonics—or at least nascent expressions of it—may have been active during the Hadean eon. That idea recalibrates our models of the onset of continental crust generation and may influence speculation regarding the start of plate tectonics on Earth.

Liu, Xia, and their students' research opens up new possibilities for scientists trying to find out about the early Earth. Their model can be tweaked with still more zircon data, which will lead to increasingly accurate reconstructions of early crust. In time, the method can be used to examine ways Earth's early crust differed from that of other rocky worlds like Mars or Venus.

For now, the group has given science a clearer picture of a period long thought to be impenetrable. With machine learning and the power of ancient zircons, the missing chapter in Earth's early history is no longer quite so missing.

Research findings are available online in the journal National Science Review.

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