Pre-Historic Fragment Of Zircon Provides Evidence Of Cool Early Earth [VIDEO]

By Shweta Iyer on February 25, 2014 10:42 AM EST

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With the help of a tiny fragment of zircon, an international team of researchers is trying to understand how our planet became habitable. (Photo: Photo courtesy of <a href=&)

The Earth formed approximately 4.54 billion years ago and much of it was molten due to frequent volcanic eruptions and collisions with other planetary bodies. There was no oxygen or atmosphere till the Earth started to cool down. Scientists for long have been studying metasedimentary and volcanic rocks to estimate the age of the Earth. Now, with the help of a tiny fragment of zircon, an international team of researchers is trying to understand how our planet became habitable.

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According to a press release Sunday, the researchers' study of zircon has helped portray how the Earth's crust first formed at least 4.4 billion years ago, just 160 million years after the formation of our solar system. "This confirms our view of how the Earth cooled and became habitable," said lead researcher John Valley. "This may also help us understand how other habitable planets would form."

Valley and his team conducted experiments on a microscopic piece of zircon extracted from Western Australia's Jack Hills region. The study confirms that the zircon had crystallized 4.4 billion years ago and that makes the crystal used by Valley, as the oldest known material of any kind formed on Earth. Their study corroborates with earlier studies, which used lead isotopes to date the Australian zircons and identify them as the oldest bits of the Earth's crust. Valley's study strengthens the theory of "cool early Earth", where temperature had cooled sufficiently for liquid water to exist on the surface.

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. Contrary to prediction, the lead atoms in the zircon were not randomly distributed but were clumped together, like "raisins in a pudding," noted Valley.

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. This allows us to read a new page of the thermal history recorded by these tiny zircon time capsules", said Valley.

The formation, isotope ratio, and size of the clumps, which are less than 50 atoms in diameter, indicate that existing geochronology methods provide reliable and accurate estimates of the sample's age. Valley and his group also measured oxygen isotope ratios, which give evidence of early homogenization and later cooling of the Earth.

Valley explained that the cooling may have started when the Earth collided with a Mars-sized object to create the Moon about 4.5 billion years ago. "The Earth was assembled from a lot of heterogeneous material from the solar system 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."

Listen to John Valley discuss the findings in the video below:

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