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

[Ron Baalke]

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."