Ancient zircons date the beginning of plate tectonics 3.6 billion years ago – an event critical to making the earth hospitable to life

Zircons examined by the research team, photographed with cathodoluminescence, a technique with which the team was able to visualize the inside of the crystals with a special scanning electron microscope. Dark circles on the zircons are the cavities left by the laser for analyzing the age and chemistry of the zircons. Scientists, led by Michael Ackerson, a research geologist at the Smithsonian National Museum of Natural History, are providing new evidence that modern plate tectonics, a characteristic of the Earth and its unique ability to support life, was about 3.6 billion years ago originated. The study, published May 14 in the journal Geochemical Perspective Letters, uses zircons, the oldest minerals ever found on Earth, to look back at the planet’s ancient past. The team tested more than 3,500 zircons, each with just a few human hairs, by beaming 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 that were tested, about 200 were eligible for study due to the billions of years of devastation these minerals had suffered since their formation. Photo credit: Michael Ackerson, Smithsonian

The oldest minerals on earth date back to 3.6 billion years before plate tectonics began

Ancient zircons from the Jack Hills of Western Australia set the date of an event that was crucial in making the planet hospitable for life.

Scientists, led by Michael Ackerson, a research geologist at the Smithsonian National Museum of Natural History, are providing new evidence that modern plate tectonics, a characteristic of the Earth and its unique ability to support life, was about 3.6 billion years ago originated.

Earth is the only planet known to harbor complex life, and this ability is due in part to another characteristic that makes the planet unique: plate tectonics. No other planetary body known to science has the dynamic crust of the earth, which is divided into continental plates that move, break, and collide with each other over eons. Plate tectonics provides a link between the chemical reactor of the Earth’s interior and its surface, which has evolved the habitable planet people enjoy today, from oxygen in the atmosphere to concentrations of climate-regulating carbon dioxide. But when and how plate tectonics began has remained a mystery and is buried under billions of years of geological time.

The study, published May 14, 2021 in the journal Geochemical Perspectives Letters, uses zircons, the oldest minerals ever found on Earth, to look back at the planet’s ancient past.

Jack Hills from Western Australia

The Jack Hills in Western Australia, where the zircons studied were examined, were taken from 15 grapefruit-sized rocks collected by the research team. Scientists, led by Michael Ackerson, a research geologist at the Smithsonian National Museum of Natural History, are providing new evidence that modern plate tectonics, a characteristic of the Earth and its unique ability to support life, was about 3.6 billion years ago originated. The study, published May 14 in the journal Geochemical Perspective Letters, uses zircons, the oldest minerals ever found on Earth, to look back at the planet’s ancient past. Photo credit: Dustin Trail, University of Rochester

The oldest zircons in the study, which came from the Jack Hills of Western Australia, were about 4.3 billion years old – meaning these near-indestructible minerals were formed when the earth itself was in its infancy and only about 200 million Years old. Together with other ancient zircons that originated in the Jack Hills and spans the earliest history on earth up to 3 billion years ago, these minerals provide what researchers can best come up with in a continuous chemical record of the nascent world.

“We’re reconstructing how the earth changed from a molten ball of stone and metal to what we have today,” said Ackerson. “None of the other planets have continents or liquid oceans or life. In a way, we are trying to answer the question of why the earth is unique and we can to some extent answer that with these zircons. “

To look billions of years into Earth’s past, Ackerson and the research team collected 15 grapefruit-sized stones from the Jack Hills and reduced them to their smallest components – minerals – by grinding them into sand with a machine called a chipmunk. Fortunately, zircons are very dense, which makes it relatively easy to separate them from the rest of the sand using a technique similar to panning for gold.

Polished slice of a rock collected from Jack Hills of Western Australia

A thin, polished slice of rock that originated in the Jack Hills of Western Australia. Using a special microscope equipped with a polarizing lens, the research team was able to examine the intricate internal structure of the quartz that makes up the rock, including unique features that allowed them to spot ancient zircons (magenta mineral in the center of the red-outlined) to identify inset on the right photo). Scientists, led by Michael Ackerson, a research geologist at the Smithsonian National Museum of Natural History, are providing new evidence that modern plate tectonics, a characteristic of the Earth and its unique ability to support life, was about 3.6 billion years ago originated. The study, published May 14 in the journal Geochemical Perspective Letters, uses zircons, the oldest minerals ever found on Earth, to look back at the planet’s ancient past. To look billions of years into Earth’s past, Ackerson and the research team collected 15 grapefruit-sized stones from the Jack Hills and reduced them to their smallest components – minerals – by grinding them into sand with a machine called a chipmunk. Fortunately, zircons are very dense, which makes it relatively easy to separate them from the rest of the sand using a technique similar to panning for gold. Photo credit: Michael Ackerson, Smithsonian

The team tested more than 3,500 zircons, each with just a few human hairs, by beaming 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 that were tested, about 200 were eligible for study due to the billions of years of devastation these minerals had suffered since their formation.

“Unraveling the secrets of these minerals is no easy task,” said Ackerson. “We analyzed thousands of these crystals to get a handful of useful data points, but each sample has the potential to tell us something completely new and to reshape our understanding of the origins of our planet.”

The age of a zircon can be determined with great precision because each contains uranium. The famous radioactive nature of uranium and the well-quantified rate of decay allow scientists to reverse engineer how long the mineral has existed.

The aluminum content of each zircon was also of interest to the research team. Tests on modern zirconia show that high aluminum zirconia can only be made in a limited number of ways, allowing researchers to use the presence of aluminum to infer what geologically could have been going on at the time the zircon was formed .

After analyzing the results of hundreds of beneficial zircons among the thousands tested, Ackerson and his co-authors found a significant increase in aluminum concentrations about 3.6 billion years ago.

“This shift in composition likely marks the beginning of plate tectonics in the modern style and could potentially signal the emergence of life on Earth,” Ackerson said. “But we need to do a lot more research to determine the links between this geological shift and the origins of life.”

The line of inference that connects high aluminum zircons to the onset of dynamic crust with plate tectonics is as follows: One of the few ways that high aluminum zirconia can form is to melt rocks deeper below the surface of the earth.

“It’s really difficult to get aluminum into zircon because of its chemical bonds,” said Ackerson. “They must have pretty extreme geological conditions.”

Ackerson argues that this sign that rocks were melting deeper below the surface of the earth caused the planet’s crust to thicker and cool, and that this thickening of the earth’s crust was a sign of the transition to modern plate tectonics in the Was in progress.

Previous research on the 4 billion year old Acasta Gneiss in northern Canada also suggests that the earth’s crust is thickening and the rocks are melting deeper in the planet.

“The Acasta Gneiss results give us more confidence in our interpretation of the Jack Hills Zircons,” said Ackerson. “Today these places are separated by thousands of kilometers, but they tell us a pretty consistent story that something globally significant happened about 3.6 billion years ago.”

This work is part of the museum’s new initiative called Our Unique Planet, a public-private partnership that supports research into some of the most enduring and meaningful questions about what makes the earth so special. Other research will examine the source of Earth’s liquid oceans and how minerals might have helped spark life.

Ackerson said he hoped to pursue these results by searching the ancient Jack Hills zircons for traces of life and examining other very ancient rock formations to see if they too were showing signs of thickening of the earth’s crust about 3.6 billion years ago exhibit.

Reference: “Formation of Peraluminous Crustal Magmas and Effects on Early Earth” by MR Ackerson, D. Trail, and J. Buettner, May 14, 2021, Geochemical Perspectives Letters.
DOI: 10.7185 / Geochemlet.2114

Funding and support for this research was provided by the Smithsonian and the National Aeronautics and Space Administration (NASA).

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