The Earth’s crust may have moved about 3.8 billion years ago.
A change in the chemistry of ancient zircon crystals from different ages past. may be the first sign of subductionthe sliding of one tectonic plate under another, researchers report in April AGU advances. This change may mark the first steps of processes that would evolve towards modernity. plate tectonics on earth (Serial number: 01/13/21). Before then, the early Earth may have had a relatively immobile “protocrust,” like the one seen on Venus today, the team says.
“Earth is actually quite different from other planets in that it has plate tectonics,” says study co-author Nadja Drabon, a geochemist at Harvard University. “We’re trying to figure out, when did it become so special?”
Geologists have long debated what Appearance of the earth’s crust during the Hadean Eonmore than 4 billion years ago (Serial number: 03/16/17). One reason the debate has remained unresolved is that opportunities to investigate that distant past are slim, since few rocks have survived eons of tectonic activity.
But zircons, brilliant and extremely resistant gemstones, are famous for their ability last billions of years (Serial number: 01/31/17). Geologists know this because they can reliably estimate the ages of zircon by measuring the amount of uranium and lead in the crystal. Growing zircons readily capture uranium but tend to exclude lead, so any lead found in the zircon is assumed to have decayed from uranium. Geologists know that decay rate, which is based on the half-life of uranium, so they can estimate the age of the zircons.
Additionally, zircons provide clues about the environments in which they formed, as their growing networks of zirconium, silica, and oxygen capture small amounts of other elements from their environment.
Most of the world’s Hadean zircons have been found at only a dozen sites, with the majority of them being from one site in Australia. But since 2018, Drabon and his colleagues have reported on a new source: the Barberton Makhonjwa Mountains in South Africa. Dozens of zircons collected in that region crystallized between 4.15 and 4 billion years ago, while thousands of others are at least 3.3 billion years old. The findings have provided researchers with a new window roughly 800 million years into the past, straddling the end of the Hadean and the beginning of the Archean Eon.
For the new study, the researchers looked through that window, checking zirconium for trace elements — uranium, scandium, ytterbium, niobium and cerium — and also for different forms of oxygen and hafnium.
Before 3.8 billion years ago, most zircons contained amounts of different forms of hafnium over time that supported the existence of a stable and ancient protocrust. Additionally, these older zircons contained trace elements at levels similar to those found in zircons from the Hawaiian volcanic hotspot. There, new rock is created as a plume of molten material rises from Earth’s mantle and crystallizes, a process that could have occurred in the absence of moving plates.
But in zircons that formed after 3.8 billion years ago, the researchers found no further evidence of a protocrust, and many of the zircons contained trace element signatures that resemble those found at subduction zones today.
That’s a fundamental change, says Jennifer Kasbohm, a geochronologist at Yale University who was not involved in the study. “We really think that subduction is essential for plate tectonics.”
Previous analyzes of zircons from elsewhere, such as Australia and India, Similar compositions have been reportedthe researchers note, suggesting that an early form of plate tectonics may have become widespread about 3.8 billion years ago (Serial number: 9/4/21).
In the future, Drabon plans to investigate whether other minerals were trapped within the zircons during their formation. If so, those hitchhiking fragments could open even more windows into Earth’s early tectonic history.