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Last Updated: Thursday, 23 August 2007, 17:33 GMT 18:33 UK
Diamond record of ancient Earth
Zircon with diamond inclusion (Martina Menneken)
The ancient zircons crystals include even smaller diamonds
Tiny diamonds found in Australia suggest the early Earth was not a hellish world for as long as previously supposed, the journal Nature reports.

The miniature gems, from Jack Hills in the west of the country, are encased in zircon crystals that have been dated up to 4.25 billion years ago.

Scientists say their analysis of the diamonds suggests the planet had cooled sufficiently by then to form a crust.

This shell may even have been moving and exchanging material with the deep.

In other words, the diamonds could be the earliest evidence for plate tectonics, the theory used today to explain how the continents drift across the surface of the globe and rocks are recycled into the interior.

"We have compared these diamonds with known diamonds and so far it seems like the most similar diamonds are ultra-high pressure diamonds which form in a subduction process," explained Martina Menneken from Westfalische-Wilhelms University in Muenster, Germany.

"Today these form in plate tectonics surroundings. The implication is: did we have plate tectonics at this early stage of the Earth? I still think this is controversial; we cannot prove it and we need to do further research," she told the BBC's Science In Action Programme.

Rock survivors

Scientists describe the first phase in Earth history as the Hadean, when the planet would have begun its existence nearly 4.6 billion years ago as a sphere of molten rock circling the proto-Sun.

The traditional view was that it took several hundred million years for the Earth to cool enough for a crust to form and for oceans to condense out of a thick atmosphere.

That view has been challenged in the past 20 years with the discovery of zircons in Western Australia. These minute crystals - made of zirconium, silicon and oxygen, among other elements - are the tough remnants of ancient rocks that have long since disappeared. Today, these crystals are incorporated into more recent rocks, such as those at Jack Hills.

Artist's concept of the early Earth (MARK GARLICK/SPL)
A seething, molten sphere: How quickly did things cool down?
Scientists have used radioactive dating to put some of the zircons' formation as far back as 4.4 billion years ago; and it has become clear from these crystals' geochemistry that the Earth would also have had to have been cooler and wetter much earlier because they show evidence of growing out of low-temperature magma that had been in contact with water.

The new analysis by Menneken and colleagues looks at the diamond inclusions in the zircons.

It is not clear how the diamonds got into the zircons. Normally, diamonds will form under intense pressure, from deep burial or perhaps from meteoroid impacts.

The team says that from its examination, deep burial looks the more obvious contender - and that supports the notion of a crust that was re-cycled deep within the Earth.

Geoscience 'presents'

This suggestion is problematic, however, for some commentators, who point out that the zircon crystals themselves show no signs of being exposed to very high pressures.

Dr Ian Williams, of the Australian National University, says this conundrum could in fact lead some to take diamond analysis as support for a hotter early Earth.

Jack Hills (BBC)
"If the evidence of the diamonds is correct and the Hadean zircons did crystallise from magmas at high pressure, then those magmas could not have been crustal melts," he tells Nature.

This would undermine other inferences about the zircons, he said, in particular that they formed at lower temperatures and at shallower depths within the Earth.

What is certain, say researchers, is that the tiny zircons - they are about 0.3mm across - continue to give fascinating insights into the Earth's beginnings.

"Any information about the very early Earth is fantastic; it's like a Christmas present for geoscientists," said Dr Martin Van Kranendonk, a senior geologist with the Geological Survey of Western Australia.

"[The Nature study] provides a new constraint for geoscientists to consider how the Earth formed into the planet it is today. It's another piece in the early Earth puzzle."


SEE ALSO
Sea floor records ancient Earth
23 Mar 07 |  Science/Nature
Ancient rocks 'built by microbes'
07 Jun 06 |  Science/Nature
Earth's oldest object on display
09 Apr 05 |  Americas
World's 'oldest' volcanic rocks
05 Dec 02 |  Science/Nature
Ancient crystal questions Earth's history
11 Jan 01 |  Science/Nature
World's deepest rocks recovered
19 May 00 |  Science/Nature
The history of rock
03 Nov 99 |  Science/Nature

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