Zircon gives clues to the past.
Two of the solar system's best natural timekeepers have been caught misbehaving, suggesting that the accepted ages for the oldest known rock samples are off by a million years or more.
According to two new studies, a radioactive version of the element samarium decays much more quickly than previously thought, and different versions of uranium don't always appear in the same relative quantities in earthly rocks.
Both elements are used by geologists to date rocks and chart the history of events on our planet and in the solar system.
"If you have a critical event in Earth's history, something like an extinction event or a climate change shift or a meteorite impact, you need to know the absolute age with the most confidence," says Joe Hiess
of the British Geological Survey, who led one of the studies. "In Earth sciences there's a need to be able to define what happened first and what happened second."
Geochemists age rocks by measuring the ratio of radioactive isotopes - versions of the same element with different atomic masses - in them. Because the elements decay from one isotope, or element, to another at a constant rate, knowing the ratio in a particular rock gives its age.
Different elements and isotopes decay at vastly different rates. Scientists pick one that suits the timescale of interest. One of the favourites for tracing events in the early solar system, such as when the Earth's crust differentiated from its mantle or when the lava oceans on the moon solidified, is samarium-146, a hard shiny metal found in many minerals in the Earth's crust.
"In this time window, there are not many other chronometers," says Michael Paul
of the Hebrew University in Jerusalem.
Scientists have measured samarium-146's half-life - the time taken for exactly half of a sample of atoms to decay radioactively - four times over the past 60 years, and got different answers each time. The two most recent measurements seemed to converge on a half-life of 103 million years, plus or minus 5 million years. But Paul and colleagues suspected that that number wasn't quite right. So they used a technique called accelerator mass spectrometry, which Paul says is less likely to be skewed by experimental errors.
They found that the half-life is just 68 million years, 30 per cent shorter than thought. That means that every rock dated by samarium-146 decay - which include some of the oldest on Earth and the moon, and even some Martian meteorites - formed 20 million to 80 million years earlier than thought.
In a solar system that's 4.5 billion years old, tens of millions of years "is a lot", Paul says. "It means everything was forming more quickly."
There was a second, separate hiccup. Earth's oldest rocks are also aged using isotopes of uranium, which decay into isotopes of lead. Until a few years ago, geochemists assumed that the ratio of uranium-238 to uranium-235 was constant - 137.88 - in all rocks, and therefore the ratio of lead isotopes was the only measurement needed to date the rocks. But high-precision measurements of early materials found in meteorites or rocks formed in oceans showed differences.
Hiess and colleagues made the most wide-ranging study of uranium isotope ratios yet, using 45 samples of zircon from all over the world (pictured, above right). Zircon was one of the first minerals to solidify on Earth, it resists weathering and melting, and it holds on to uranium well, so it's a good candidate for dating old rocks.
The team found that, while most of their samples had similar uranium ratios, some were wildly different.
"It's no longer safe to assume that it doesn't vary. It clearly does," says Gregory Brennecka
of Arizona State University, who was not involved in either study. "Nobody thought that was the case five years ago."
The team produced a new, average figure for the uranium ratios. It shifts the ages of Earth's oldest rocks slightly, by just under a million years, Hiess says. The oldest rocks will have the biggest corrections: sediments that are 4.4 billion years old are now younger by 700,000 years. "To put it into a human perspective, if the Earth was only 18 years old, we have taken 1 day off the life of its oldest materials," Hiess says.
Now that scientists know they need to measure the ratio of uranium isotopes in all of their samples - as well as the ratio of lead isotopes - they'll be able to date rocks more accurately.
That's important for putting events in order. If a mass extinction occurred just before a meteorite struck, say, that paints a different picture than if the meteorite hit first.
"These are two big steps in improving the way we do geochronology, both in the solar system and terrestrial rocks," Brennecka says.
DOI: 10.1126/science.1215507 and 10.1126/science.1215510