This is a false-color microscope image of a 4-billion-year-old zircon. Image credit: John Valley.
Jack Hills Zircon: Scientists Discover Oldest-Known Fragment of Earth
A small fragment of a mineral called zircon extracted from a remote rock outcrop in Australia confirms that the Earth’s crust first formed at least 4.4 billion years ago (Hadean eon).
Zircons are the oldest known materials on our planet. They offer a window in time back as far as 4.4 billion years ago, when the planet was a mere 160 million years old.
Because zircons are exceptionally resistant to chemical changes, they have become the gold standard for determining the age of ancient rocks.
The new study, published in the journal Nature Geoscience, shows that zircon crystals from Western Australia’s Jack Hills region crystallized 4.4 billion years ago, strengthening the theory of a ‘cool early Earth,’ where temperatures were low enough for liquid water, oceans and a hydrosphere not long after the planet’s crust congealed from a sea of molten rock.
“The study reinforces our conclusion that Earth had a hydrosphere before 4.3 billion years ago,” and possibly life not long after,” said lead author Prof John Valley from the University of Wisconsin-Madison.
The microscopic zircon crystal used by Prof Valley in this study is now confirmed to be the oldest known material of any kind formed on Earth.
A timeline of the history of our planet places the formation of the Jack Hills zircon and a “cool early Earth” at 4.4 billion years. Image credit: Andree Valley.
The study was conducted using a new technique called atom-probe tomography that, in conjunction with secondary ion mass spectrometry, permitted the scientists to accurately establish the age and thermal history of the zircon by determining the mass of individual atoms of lead in the sample. Instead of being randomly distributed in the sample, as predicted, lead atoms in the zircon were clumped together, like ‘raisins in a pudding.’
The clusters of lead atoms formed 1 billion years after crystallization of the zircon, by which time the radioactive decay of uranium had formed the lead atoms that then diffused into clusters during reheating.
“The zircon formed 4.4 billion years ago, and at 3.4 billion years, all the lead that existed at that time was concentrated in these hotspots,” Prof Valley said.
“This allows us to read a new page of the thermal history recorded by these tiny zircon time capsules.”
“The Earth was assembled from a lot of heterogeneous material from the Solar System.”
“The early Earth experienced intense bombardment by meteors, including a collision with a Mars-sized object about 4.5 billion years ago that formed our Moon, and melted and homogenized the Earth. Our samples formed after the magma oceans cooled and prove that these events were very early.”