Geochemists have determined that a tiny grain of zircon from Western Australia is nearly 4.4 billion years old, pushing back the date when Earth's crust solidified.
As best as geochemists can tell, solid matter started condensing in our solar system 4.567 billion years ago and rapidly assembled into planets. Just how rapidly Earth came together in that chaotic early tumult isn't clear, because no rocks on Earth are as old as the planet itself.
There's been something of a spirited competition among geochemists to identify our planet's "oldest rock." Old rock formations in the undisturbed continental interiors offer the most promising hunting grounds, and much attention has been focused on the Jack Hills region of Western Australia. Ancient sandstone deposits there contains tiny bits of the durable mineral zircon that must have been part of Earth's first crust, and that's where field work in the 1980s turned up zircon grains dating to 3.8 billion years.
Since then there've been various reports of even older specimens. Some zircons have been reported that are older than 4.35 billion years. Meanwhile, two years ago a research team found a rock known as the Nuvvuagittuq greenstone near Canada's Hudson Bay that could have formed up to 4.3 billion years ago.
An article published online this week in Nature Geoscience has created quite a stir among geochemists. There a team led by John Valley (University of Wisconsin, Madison) reports that a tiny grain of zircon plucked from Jack Hills sediments has an age of 4.374 billion years. This result suggests that terra firma existed within about 160 million years of the solar system's formation.
Arguably just as important is the method used to date the zircon. The researchers used a well-known isotopic "clock" involving the decay of uranium-238 to lead-206 (half-life: 4.47 billion years) and uranium-235 to lead-207 (704 million years). But this dating technique only works if none of the lead moved around or escaped since the rock solidified. And, let's face it, 4.4 billion years is a very long time to stay put.
Valley and his team used a technique called atom-probe tomography to map the grain's contents at incredibly tiny scales. This tedious, labor-intensive effort shows that the lead atoms haven't moved much, and consequently they believe their 4.374-billion-year result has an uncertainty of just 6 million years.
The ultimate winning "oldest rock" claim involves more than just bragging rights. A difference of just 100 or 200 million years, notes MIT's Samuel Bowring in an accompanying perspective, is "enormous in terms of modelling the geochemical evolution of Earth and the formation and recycling of the first continental crust."
The new finding supports the idea that the terrestrial crust started to solidify within 100 million years after a huge object, Mars-size or larger, slammed into Earth and ultimately formed the Moon. Whatever nascent crust Earth might have had was destroyed by that collision. But knowing that the final crust (re)formed so soon after the blast should provide new insight into how soon our planet became truly habitable.