Earth’s oldest crystals reveal age of plate tectonics
Earth’s constantly moving tectonic plates have created the unique habitable planet we recognize today.
A new study, led by geologist Michael Ackerson from the Smithsonian’s National Museum of Natural History in Washington has predicted that our planet’s tectonic plates — large slabs of Earth’s crust lying over a liquid mantle — began moving roughly 3.6 billion years ago, when Earth was just under one billion years old.
Peering this far back into the planet’s ancient past requires studying material that can stand the test of time, in this case billions of years. For this, the scientists turned to zircons — the oldest and one of the toughest minerals on Earth.
These tiny crystal time capsules are virtually indestructible and can be more than four billion years old. They’re one of our only windows into the primitive stages of our planet’s formation.
“We are reconstructing how the Earth changed from a molten ball of rock and metal to what we have today,” Ackerson said in a statement. “None of the other planets have continents or liquid oceans or life. In a way, we are trying to answer the question of why Earth is unique, and we can answer that to an extent with these zircons.”
The Jack Hills of Western Australia where the zircon samples were collected. (Image credit: Dustin Trail/University of Rochester)
So, how do you find minerals that are billions of years old? The research team turned their sights to the Jack Hills of Western Australia, where some of the oldest rocks in the world are found. There they collected 15 grapefruit-size chunks of ancient rock.
The researchers ground down the rocks and sifted out the zircons from the rest of the material, using a technique similar to gold panning. The zircons were then zapped with a laser and analyzed using a process called mass spectrometry, which reveals the chemical makeup and age of the minerals.
Some of the zircons were 4.3 billion years old and would have formed when Earth was in its infancy, around 200 million years old. These tiny minerals — only a couple of human hairs wide — reveal a lot about the environmental conditions at the time of their formation.
Thin slice of rock collected from the Jack Hills of Western Australia shows the internal structure of quartz that makes up the rock, including ancient zircons (magenta mineral in the center of the red-outlined inset image). (Image credit: Michael Ackerson/Smithsonian)
In this study, the scientists had collected enough ancient zircons that formed between 4.3 and three billion years ago to produce a continuous chemical record of the infant Earth.
The age of the zircons was calculated from the uranium content within the mineral. Uranium is radioactive and makes for a great “geological clock” due to its well-quantified rate of decay. By analyzing the uranium content currently present and working backwards, scientists can determine when the zircons formed.
While uranium sheds light on the age of the zircons, the aluminum content revealed details about the processes involved in their creation — and consequently, plate tectonics. The scientists discovered a rise in aluminium concentration in zircons around 3.6 billion years ago.
“This compositional shift likely marks the onset of modern-style plate tectonics,” Ackerson said in the statement.
The reasoning behind this association is that one of the ways high-aluminium zircons can form is by rocks melting deep below Earth’s surface. The melting of rocks at greater depths is evidence that the crust was thickening and beginning to cool, a geological indicator that the transition to modern plate tectonics was thoroughly underway.
This work was published May 14 in the journal Geochemical Perspective Letters.
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