February 24, 2014
Australian Rock Determined To Be Oldest Material Formed On Earth
[ Watch the Video: The Oldest Rock On Earth ]
Lee Rannals for redOrbit.com - Your Universe Online
Researchers, publishing a paper in the journal Nature Geoscience, say a rock in Australia is helping to paint a picture of how our planet became habitable 4.4 billion years ago.
John Valley, geoscience professor at the University of Wisconsin-Madison, said that the discovery shows evidence that it was at a much earlier time when the planet was just a fiery ball covered in magma.
"This confirms our view of how the Earth cooled and became habitable," Valley said in a statement. "This may also help us understand how other habitable planets would form."
The study confirms that zircon crystals from Australia’s Jack Hills region crystalized 4.4 billion years ago. This finding builds on previous studies that used lead isotopes to date the zircons and identify them as the oldest parts of the Earth’s crust. The fragment used in the current study is confirmed to be the oldest known material of any kind formed on Earth.
Valley says his team’s findings add more weight to 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 magma.
"The study reinforces our conclusion that Earth had a hydrosphere before 4.3 billion years ago," and possibly life not long after, says Valley.
Researchers used a new technique known as atom-probe tomography for the study, along with secondary ion mass spectrometry. These techniques allowed the team to accurately establish the age and thermal history of the zircon by determining the mass of individual atoms of lead in the sample.
The team predicted that the atoms would be randomly distributed in the sample, but instead they saw them clumped together like “raisins in 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."
He said the formation, isotope ratio and size of the clumps become a clock and verify that existing geochronology methods provide reliable and accurate estimates of the sample’s age.
"The Earth was assembled from a lot of heterogeneous material from the solar system," Valley explains. “Our samples formed after the magma oceans cooled and prove that these events were very early."