Researchers discovered 3.95 billion-year-old rocks in northern China’s Shimen Complex, offering new clues about Earth’s early crust. The team used zircon crystals to study geological processes from long ago. "We propose that this rapid burial was most likely achieved through a transient subduction event," the researchers said about the findings.
Earth is approximately 4.5 billion years old, according to estimates. However, rocks dating back to the planet's earliest days are extremely rare. Hence the excitement over the discovery in northern China of some of the oldest known rocks on Earth, dating back 3.95 billion years. And one of the reasons for all the attention is that they're revealing new clues about how Earth's crust formed and evolved.
Researchers found the rocks in the Shimen Complex in Hebei province and collected three samples for detailed analysis. Scientists have been searching for rocks older than 3.8 billion years in China for decades.
Dating ancient rocks
The team, led by Hangqiang Xie of the Chinese Academy of Geological Sciences in Beijing, dated the rocks by analyzing tiny zircon crystals trapped inside them. These contain uranium, which slowly decays into lead at a known rate.
But the researchers wanted to know more than just the rocks' age.
More specifically, they were interested in how the rocks had formed and what they could reveal about the geological processes taking place on Earth nearly 4 billion years ago.
To find out, they analyzed their chemical composition, including different forms of oxygen and hafnium in zircon crystals. The researchers also used computer models to investigate the conditions under which the rocks formed.
The results, published in the journal Nature, suggest that a process similar to modern-day plate tectonics may have been taking place in at least one part of Earth nearly 4 billion years ago. A Research Briefing in the same edition provides further insights into the findings.
Deep beneath the surface
The analysis revealed that the rocks had originally formed from material that had been in contact with water at or near Earth's surface. Meanwhile, computer models suggested this material was then carried more than 50 kilometers (31 miles) below Earth's surface, where intense heat and pressure caused it to melt partially.
The researchers believe this could have happened through a process called subduction. In plate tectonics, this is where one section of the Earth's crust sinks beneath another, as they wrote in their paper. "We propose that this rapid burial was most likely achieved through a transient subduction event."
Analysis of hafnium isotopes in the zircon crystals also suggested that the process of separating material from Earth's mantle to form new crust had begun more than 4.3 billion years ago.
This was hundreds of millions of years before the rocks in China formed, suggesting that Earth's crust was already evolving very early in the planet's history.