[meteorite-list] Confirmed: Earth's Oldest Rock in Australia

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 24 Feb 2014 14:06:25 -0800 (PST)
Message-ID: <201402242206.s1OM6PgR021472_at_zagami.jpl.nasa.gov>

http://www.abc.net.au/science/articles/2014/02/24/3950076.htm

Confirmed: Earth's oldest rock in Australia
Stuart Gary
Australian Broadcasting Company (ABC)
24 February 2014

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.

Second dating

Zircon's crystal structure has specific sites where only atoms of a given
size and charge will fit.

These locations concentrate uranium atoms and exclude lead, so the only
lead found at these sites is generated by the radioactive decay of uranium.

As uranium transforms into lead, it emits alpha particles which cause
the lead atoms generated from the uranium to recoil and move into other
parts of the crystal, where they accumulate.

"If that happens, the places where the lead has been removed, will appear
to be younger than they are, while places where the lead has migrated
appear older," says Valley.

This problem of lead mobility has led some to question the reliabilty
of the method for dating zircons, however Valley and colleagues have found
it does not affect the isotopic ratios.

Using atom-probe tomography the authors identified the distances that
lead atoms move are so small so as not to affect the analysis.

"We capture both the lead-depleted and lead-enriched domains, so the ratio
we measure is averaged out," says Valley.

"We're getting the true ratio of the parent uranium to daughter lead,
and therefore we're getting the true age."

Complete picture

The age confirmation closes the gap between the Moon-generating impact,
and the formation of Earth's crust, according to Professor Samuel Bowring
of the Massachusetts Institute of Technology.

Bowring, who wrote an accompanying opinion piece on the research, believes
the findings indicate Earth's water didn't need to come from asteroids,
during a period known as the late heavy bombardment 3.9 billion years
ago.

Instead, it suggests water was present in the liquid magma ocean that
formed the zircon crystals.

"We'll never know how much water there really was, but the simplest interpretation
of those zircons coming from granitic rocks, is that we had a hydrous
planet right from the very beginning," says Bowring.

"The water was probably accreted with the rest of the parts of the Earth
as the planet formed."
Received on Mon 24 Feb 2014 05:06:25 PM PST