Ancient zircon crystals discovered in Western Australia have been positively dated to 4.374 billion years, confirming their place as the oldest rock ever found on Earth, according to a new study. The research reported in the journal Nature Geoscience, means Earth began forming a crust far sooner than previously thought, following the giant impact event which created the Earth-Moon system 4.5 billion years ago.
“That age is 300 million years older than the oldest previously dated age [of other rocks], and only 100 million years after the magma ocean,” says the study’s lead author Professor John Valley of the University of Wisconsin. “This is when Earth started making protocontinental crust, which is chemically differentiated from the mantle. The chemical evidence from the zircons is a good fit for what we call intermediate composition … halfway between granite and basalt.”
Valley and colleagues have previously used uranium-lead radioactive dating to determine the age of a zircon crystal sample (named 01JH36-69), which was found 15 years ago in metamorphosed sandstone at Jack Hills, 800 kilometres north of Perth. Uranium radioactively decays into lead at a known rate, allowing age to be determined based on the ratio of uranium to lead in the sample. However, there have been concerns over the accuracy of using this method to date zircon crystals, which means there has been uncertainty about the exact age of the Jack Hills rock. Now, Valley and colleagues have used a new technique to confirm the validity of their original findings. Via Earth’s oldest rock