Zircon fragment said to be the oldest piece of Earth
The tiny fragment of zircon discovered in a rock on a Jack Hills sheep ranch in Australia might not look like anything special to the untrained eye. Scientists, however, believe this pretty, blue crystal is the “oldest piece of Earth”. They estimate the age of the find to be 4.4billion years old.
According to their report in yesterday’s journal of Nature Geoscience, John Valley, a University of Wisconsin geoscience professor and a team of international scientists say the fragment is a significant key to determining how the planet evolved to support life. Valley utilized lead isotopes in order to date the zircon and prove it to be both the oldest known substance formed on the planet as well as an actual piece of the Earth’s crust.
Valley reported that their research supports the theory of “a cool early Earth” where the temperatures were cool enough for the existence of water, oceans and an entire hydrosphere shortly after the crust of the planet congealed out of a body of molten rock. He states: “The study reinforces our conclusion that Earth had a hydrosphere before 4.3 billion years ago.” He also told the press: “we have no evidence that life existed then. We have no evidence that it didn’t. But there is no reason why life could not have existed on Earth 4.3 billion years ago.”
The research team employed a new scientific technique known as atom-probe tomography along with standard ion mass spectrometry thus allowing them to accurately determine both the thermal history and age of the zircon by establishing the mass of the individual atoms of lead within the fragment. Valley explained: “Instead of being randomly distributed in the sample as predicted, lead atoms in the zircon were clumped together, like raisins in a pudding,”
The lead atom clusters shaped one billion years following the zircon’s crystallization, by which period the radioactive breakdown of uranium had birthed the atoms of lead that then disseminated into clusters throughout reheating. Valley added: “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.”
The research team was thus able to easily document the actual formation, the size of the clumps (under 50 atoms in diameter) and the isotope ratio. They measured the oxygen isotope ratios which further support the “cooling” theory.
Valley elaborated: “The Earth was assembled from a lot of heterogeneous material from the solar system.”
He explained that when the planet was in its “infancy” it was bombarded by meteors and was even struck by an object the size of Mars approximately 4.5 billion years ago. He concluded that this collision “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.”
(Image courtesy of LiveScience)