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 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.
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.