By Deborah Netburn, Los Angeles Times
The oldest known material on Earth is a tiny bit of zircon crystal that has remained intact for an incredible 4.4 billion years, a study confirms.
The ancient remnant of the early Earth may change the way we think about how our planet first formed.
The crystal is the size of a small grain of sand, just barely visible to the human eye. It was discovered on a remote sheep farm in western Australia, which happens to sit on one of the most stable parts of our planet.
“The Earth’s tectonic processes are constantly destroying rocks,” said John Valley, a professor of geoscience at the University of Wisconsin-Madison, who discovered and dated the crystal. “This may be the one place where the oldest material has been preserved.”
The crystal is so much older than anyone expected that Valley and his team have had to date it twice. They published their first paper about this grain of zircon in 2001. At that time they determined it was 4.4 billion years old by measuring how many of the uranium atoms in the rock had decayed into lead.
Geologists have used this technique, known as the uranium lead system, for decades to date rocks on Earth and from space, but because nobody had ever found anything on Earth that was this old, the initial findings were questioned. Maybe, some said, the lead atoms had moved around in the zircon, making it seem like there was more lead and giving the scientists an inaccurate date.
On Sunday, Valley and his colleagues published a paper in Nature Geoscience that proves the zircon is as old as they say. This time around, they used a new method called atom-probe tomography that let them see individual atoms of lead in the sample and see whether they had moved. They found that the lead atoms do indeed move around over time, but on such a small scale that the movement would not interfere with the overall dating process.
“We have a zircon that is 4.4 billion years old,” Valley said.
And now the fun begins, because this small piece of ancient rock has big implications for how and when the Earth’s crust started to form.
Like the rest of the solar system, scientists say, Earth formed about 4.567 billion years ago.
One theory suggests that in the frenzy of those early days 4.5 billion to 4.4 billion years ago, Earth was struck by an object the size of Mars. The impact altered Earth’s tilt and caused a chunk of the planet to vaporize into space; some of that dust later became the moon.
The blow also caused the rest of Earth to be covered in a hot magma ocean. This is called the Hadean period.
Scientists were not sure how long the Hadean period lasted, but this ancient piece of zircon suggests the Earth’s crust had started to cool and form by 4.4 billion years ago.
“What we can say is that 4.4 billion years ago, the continental crust had started for form,” Valley said, “and that the real Hadean period only lasted for a very short time. By 4.4 billion years, it was over.”
Photo: cobalt123 via Flickr