A tiny grain of zircon extracted from a remote outcrop in Australia is the oldest bit of rock on Earth at 4.4 billion years old, a new study has confirmed.
An international team led by University of Wisconsin-Madison geoscience Professor John Valley reveals data which confirm the Earth’s crust first formed at least 4.4 billion years ago.
This was just 160 million years after the formation of the solar system.
“This confirms our view of how the Earth cooled and became habitable,” says Valley, a geochemist whose studies of zircons, the oldest known terrestrial materials, have helped portray how the Earth’s crust formed during the first geologic eon of the planet.
“This may also help us understand how other habitable planets would form.”
The new study, published in the journal Nature Geoscience, confirms that zircons from Western Australia’s Jack Hills region crystallised 4.4 billion years ago.
This builds on earlier studies that used lead isotopes to date the Australian zircons and identify them as the oldest bits of the Earth’s crust.
The microscopic zircon crystal used by Valley and his group in the current study is now confirmed to be the oldest known material of any kind formed on Earth.
The study, according to Valley, strengthens the theory of a “cool early Earth,” where temperatures were low enough for liquid water and oceans 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, says Valley.
The study was conducted using a new technique called atom-probe tomography which, in conjunction with secondary ion mass spectrometry, permitted the scientists to accurately establish the age of the zircon by determining the mass of individual atoms of lead.
Instead of being randomly distributed in the sample, as predicted, lead atoms in the zircon were clumped together like raisins in a pudding.
“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,” Valley says.
“This allows us to read a new page of the thermal history recorded by these tiny zircon time capsules.”
The formation, isotope ratio and size of the clumps — less than 50 atoms in diameter — become in effect a clock, says Valley.
Valley says that the early Earth experienced intense bombardment by meteors, including a collision with a Mars-sized object about 4.5 billion years ago which formed the moon.
“Our samples formed after the magma oceans cooled and prove that these events were very early.”
This 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: