Zircons Are Forever (and the Secrets They Hold).
The formation of continents and mountains as well as the pivotal role plate tectonics played in the transformation of the planet's surface from one of molten lava and rock to one that was hospitable to life has long been understood by scientists. Plate tectonics is the movement of distinct, rigid plates that make up the Earth's crust.
When it started has been less clear.
Scientists from Harvard University have examined zircons, which are tiny grains of sand-sized, extremely uncommon, old, and highly unbreakable crystals, in search of chemical hints concerning the beginning of plate tectonics. According to a study recently published in the journal AGU Advances, there was a significant change in the geochemistry of these zircons 3.8 billion years ago that caused them to resemble zircons created in today's hot settings where plate tectonics occurs significantly more.
The world didn't appear to be as active until 3.8 billion years ago, according to Nadja Drabon, a Harvard assistant professor of Earth and Planetary Sciences and the paper's primary author. "In what are known as subduction zones, a lot of crust is now continually being destroyed and replaced. Numerous [prior] zircons demonstrated that the early crust back then, once it developed, existed for a very long time—in this case, nearly 600 million years. We never produced new granitic crust, despite some internal reworking. Everything then abruptly shifts 3.8 billion years ago.
Consider zircons as little time capsules that still contain chemical traces from the first 500 million years of Earth's history. Some were created in the planet's lava more than 4 billion years ago, when the Earth was still in its infancy geologically speaking. They are the planet's oldest materials we are aware of. By zapping them with lasers, like the researchers did for their examination, their secrets can be discovered.
The researchers discovered that as the planet cooled 3.8 billion years ago, a large amount of new crust was suddenly formed, and zircon geochemical signatures started to resemble those produced in subduction zones, the locations where two colliding tectonic plates meet and one slides under the other and into the mantle where it is recycled (code word for burned to a crisp).
According to the researchers, it is unclear if subduction zones existed 3.8 billion years ago, but it is known that the formation of the new crust was probably caused by some sort of plate tectonics.
The work advances the body of knowledge on the relatively recent (within the 4.5 billion years) tectonic activity on Earth.
It provides suggestions as to how the planet became livable and the circumstances under which the first life forms emerged.
The planet's continents and seas are held within the approximately 15 movable crustal blocks that make up the Earth's outer shell today. Because the process exposed fresh rocks to the atmosphere and sparked chemical reactions that maintained Earth's surface temperature over billions of years, it was essential to the emergence of life and the formation of the planet.
It's difficult to find evidence of when the transformation started because it's so rare. No rock is older than around 4 billion years, and just 5% of all rocks on Earth are older than 2.5 billion years.
The zircons have a role in this.
3,936 new zircons were collected by the scientific team from a 2017 excursion in South Africa, which also included geologists from Stanford and Louisiana State University. 33 of them have a minimum age of 4 billion years. Because of their size, zircons from that era are hard to obtain, so it was quite the haul.
After crushing collected rocks into sand and sorting the resultant discoveries, researchers largely rely on luck. The age of the zircons from South Africa varied from 4.1 billion to 3.3 billion years. The researchers examined the hafnium isotope, oxygen isotope, and trace element compositions of the zircon crystals they had discovered. Each one provided them with a unique puzzle piece.
For instance, the oxygen isotopes provided information about whether there were seas, the hafnium isotopes about the origin and evolution of the Earth's crust, and the trace elements about the crust's composition. According to the data, crust formation began to accelerate about 4 billion years ago.
In order to determine whether they might find evidence of a comparable shift, the researchers also examined data from previous studies on ancient zircons that have been discovered all around the world. When it comes to information on hafnium isotopes, they did.
All of them, according to Drabon, "indicate this transition between 3.8 and 3.6 billion years ago."
The other two geochemical traits, according to Drabon, lacked significant information. He intends to concentrate on them next, including determining when oceans first began to emerge.
In order to determine whether they might find evidence of a comparable shift, the researchers also examined data from previous studies on ancient zircons that have been discovered all around the world. When it comes to information on hafnium isotopes, they did.
All of them, according to Drabon, "indicate this transition between 3.8 and 3.6 billion years ago."
The other two geochemical characteristics, according to Drabon, have little information and he intends to concentrate on them next, including determining when oceans first formed.
Drabon chuckled, "I don't even know where to begin." She said, "There's so much to do."
