[meteorite-list] Age of Oldest Rocks Off By Millions of Years

[Ron Baalke]

http://www.newscientist.com/article/dn21644-age-of-oldest-rocks-off-by-millions-of-years.html  

Age of oldest rocks off by millions of years
by Lisa Grossman
New Scientist
29 March 2012

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."

Chronometer shortage

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.

Youthful meteorites

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.

Mass extinction

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.

Journal references: Science, DOI: 10.1126/science.1215507 and
10.1126/science.1215510