Scientists led by Michael Ackerson, a research geologist at the Smithsonian National Museum of Natural History, provide new evidence that modern plate tectonics, a defining feature of the Earth and its unique ability to support life, arose there about 3.6 billion years ago.
Earth is the only planet known to harbor complex life, and this ability is based in part on another characteristic that makes the planet unique: plate tectonics. No other planetary body known to science has the Earth’s dynamic crust, which is divided into continental plates that move, fracture, and collide over eons. Plate tectonics provide a connection between the chemical reactor of Earth’s interior and its surface that shaped the habitable planet people enjoy today, from oxygen in the atmosphere to regulating carbon dioxide concentrations. the climate. But when and how plate tectonics began has remained a mystery, buried under billions of years of geological time.
The study, published May 14 in the journal Geochemical Perspective Letters, uses zircons, the oldest minerals ever found on Earth, to look back on the planet’s ancient past.
The oldest zircon in the study, which came from the Jack Hills of Western Australia, was around 4.3 billion years old – meaning these nearly indestructible minerals formed when the Earth itself was in it. in its infancy, only about 200 million years old. Along with other ancient zircons collected from the Jack Hills spanning Earth’s earliest history up to 3 billion years ago, these minerals provide what researchers have closest to a continuous chemical record of the nascent world. .
“We are reconstructing how the Earth went from a molten ball of rock and metal to what we have today,” Ackerson said. “None of the other planets have liquid continents or oceans or life. In a way, we are trying to answer the question of why Earth is unique, and we can answer it to some extent with these zircons. “
To look at billions of years in Earth’s past, Ackerson and the research team collected 15 grapefruit-sized rocks in the Jack Hills and chopped them into their smallest constituent parts – minerals – by grinding them into sand with a machine called chipmunk. Fortunately, zircons are very dense, which makes them relatively easy to separate from the rest of the sand using a technique similar to gold panning.
The team tested more than 3,500 zircons, each only protruding a few human hairs, by blasting them with a laser and then measuring their chemical composition with a mass spectrometer. These tests revealed the age and underlying chemistry of each zircon. Of the thousands tested, about 200 were fit for study due to the ravages of billions of years these minerals have endured since their inception.
“Unlocking the secrets contained in these minerals is no easy task,” Ackerson said. “We’ve analyzed thousands of these crystals to find a handful of useful data points, but each sample has the potential to tell us something completely new and reshape the way we understand the origins of our planet.”
The age of a zircon can be determined with great precision because each one contains uranium. The famous radioactive nature of uranium and its well-quantified decay rate allow scientists to reverse-engineer the lifespan of the mineral.
The aluminum content of each zircon was also of interest to the research team. Modern zircon testing shows that high aluminum content zircons can only be produced in a limited number of ways, allowing researchers to use the presence of aluminum to infer what might have happened, geologically speaking, at the time of the formation of zircon.
After analyzing the results of the hundreds of useful zircons out of the thousands tested, Ackerson and his coauthors deciphered a marked increase in aluminum concentrations about 3.6 billion years ago.
“This compositional change likely marks the start of modern-style plate tectonics and could potentially signal the emergence of life on Earth,” Ackerson said. “But we will need to do a lot more research to determine how this geological change relates to the origins of life.”
The inference line that connects high aluminum zircons to the appearance of a dynamic crust with plate tectonics goes like this: one of the few ways for high aluminum zircons to form is to melt rocks deeper below the Earth’s surface.
“It’s really hard to get aluminum into zircons because of their chemical bonds,” Ackerson said. “You must have some pretty extreme geological conditions.”
Ackerson explains that this sign that rocks were melting deeper below the Earth’s surface meant that the planet’s crust was getting thicker and starting to cool, and that this thickening of the Earth’s crust was a sign that the transition to tectonics modern plates were in progress.
Previous research on the 4 billion year old Acasta gneiss in northern Canada also suggests that the earth’s crust was thickening and causing rocks to melt deeper in the planet.
“The Acasta Gneiss results give us more confidence in our interpretation of Jack Hills zircons,” Ackerson said. “Today these places are thousands of miles apart, but they tell us a pretty consistent story – that about 3.6 billion years ago, something globally significant was happening. .
This work is part of the museum’s new initiative called Our Unique Planet, a public-private partnership, which supports research on some of the most enduring and important questions about what makes Earth special. Further research will focus on the source of Earth’s liquid oceans and how minerals may have helped start life.
Ackerson said he hoped to follow these findings by looking for traces of life in ancient zircons at Jack Hills and examining other extremely ancient rock formations to see if they also show signs of thickening of the earth’s crust. about 3.6 billion years ago.
Funding and support for this research was provided by the Smithsonian and the National Aeronautics and Space Administration (NASA).