3.95-billion-year-old rocks in China reveal clues to Earth’s earliest tectonics

Scientists have discovered 3.95-billion-year-old rocks in northern China that reveal tectonic activity about 4 million years ago. Earth formed about 4.5 billion years ago. However, rocks older than 3.9 billion years are exceptionally rare. The study noted that the only rocks of this age have been found in Canada’s Acasta Gneiss Complex. Researchers led by Hangqiang Xie of the Chinese Academy of Geological Sciences identified 3.95-billion-year-old trondhjemitic gneiss in eastern Hebei, within the North China Craton. The rocks contained chemical evidence suggesting ancient oceanic crust was buried deep beneath Earth’s surface before partially melting. The team provided new evidence about the formation of Earth’s earliest crust and when subduction may have begun. Subduction refers to process when one section of the Earth’s crust sinks beneath another and is associated with modern plate tectonics. The discovery adds to evidence that Earth’s crust was undergoing complex geological changes during the planet’s earliest history. Ancient rocks discovered in China The team collected three samples from the Shimen Complex for detailed analysis. Zircon contains uranium, which gradually decays into lead, which allowed scientists to determine the rocks’ age, the study reported. The team also analyzed oxygen and hafnium isotopes within the crystals and used computer models to investigate how they were formed. The researchers found that the zircon crystals contained elevated oxygen isotope values, averaging +6.77 ± 0.23 percent, alongside positive hafnium isotope values, according to the study. These chemical signatures suggest the rocks originated from basaltic material that underwent low-temperature alteration near Earth’s surface. The hafnium measurements also indicate that the source material originated from a mantle depleted during the Hadean, Earth’s earliest geological eon. Researchers modeled the temperature and pressure conditions that produced the rocks. Their calculations showed that the original magma formed through partial melting of basaltic crust at pressures exceeding 1.6 gigapascals and temperatures between 1,364°F and 1,508°F. According to the model, however, approximately 8 percent to 16 percent of the source material melted. The material may have been carried more than 30 miles beneath Earth’s surface before melting. That subduction, according to researchers, could explain how material originally exposed to surface conditions reached such depths. “We propose that this rapid burial was most likely achieved through a transient subduction event,” the authors said. New clues about Earth’s earliest crust The latest research into the geological complex provided evidence about the early evolution of Earth’s mantle and continental crust. According to study, hafnium isotope analysis suggested that material had begun separating from Earth’s mantle to form new crust more than 4.3 billion years ago. The researchers contrasted these findings with 4-billion-year-old rocks from Canada’s Acasta Gneiss Complex, which previous research suggested formed under lower-pressure conditions. The Chinese rocks instead show chemical signatures consistent with formation at much higher pressures. The researchers proposed that a temporary subduction event may have occurred approximately 3.95 billion years ago, providing new evidence about tectonic processes during Earth’s earliest history. The findings were recently published in Nature. Get the latest in engineering, tech, space & science - delivered daily to your inbox.Originally from LA, Maria Mocerino has been published in Business Insider, The Irish Examiner, The Rogue Mag, Chacruna Institute for Psychedelic Plant Medicines, and now Interesting Engineering.