Thursday, October 22, 2009

Earth's oldest diamonds found in Australia

The diamonds are about 1 billion years older than the oldest known diamonds found in terrestrial rock [File photo]. (Reuters: Juda Ngwenya)
Map: Perth 6000
The oldest known diamonds, almost as old as the Earth itself, have been found in Australia and could hold the key to unlocking how the planet's crust evolved.
The 4 billion-year-old diamonds were found trapped inside zircon crystals from the Jack Hills region, about 900 kilometres north of Perth.
They are thought to be about 1 billion years older than any found in terrestrial rock.
The diamonds "introduce a new dimension to the debate on the origin of these zircons and the evolution of the early Earth", the researchers write in the journal Nature.
Australian and German scientists who found the diamonds say the discovery supports the view that Earth in the Hadean era about 4.4 billion years ago was not a hellish zone but was cooler, like Earth today.
But Dr Ian Williams of the Australian National University disagrees.
In a commentary in the same issue of the journal, he writes that the find will instead "necessitate a careful reassessment" of the 'cool Earth' theory.
Co-author Dr Alexander Nemchin, from the Department of Applied Geology at Curtin University of Technology, says the researchers are still "stunned" by the find, and is not surprised by Dr Williams's criticism of their position.
"It is still a very controversial area generally," the self-proclaimed 'cool Earth' sceptic said.
"There are several groups that have agreed to disagree and it has been like that for the past 10 years.
"At this stage we are just saying we have found these diamonds."
The researchers used Raman laser technology to analyse 1,000 randomly chosen zircon grains, ranging in age from 4.3 to 3.1 billion-years-old.
They found diamond inclusions in 45 of the grains.
Dr Nemchin says the diamonds are about 1 billion years older than the oldest known diamonds found in terrestrial rock.
Hot or cool
The commonly held theory that the Hadean period was a time of extreme volcanic activity and frequent meteoric impacts was first challenged in 2001 when researchers measured oxygen levels in 4.3 billion-year-old zircon crystals from Jack Hills.
They found high levels of an oxygen isotope that could only have come from clay.
Because clay forms in water at the Earth's surface the researchers concluded there were oceans present at the time.
Ancient zircon crystals are the oldest identified fragments of the Earth's crust and date back to the first 500 million years of the planet's history.
Because of their durability under extreme geological conditions, they preserve unique information about the earliest evolution of the Earth.
Finding 4 billion-year-old diamonds within the zircon grains "places fairly narrow constraints on the conditions that existed very early in the history of the planet", Dr Nemchin said.
He says diamonds are formed when pressure increases faster than temperature.
These conditions occur on the planet during meteoric impacts or when the carbon, from which diamond is formed, is buried deep under thick continental plates.
"Based on what little we know about our diamonds, we believe the impact origin is unlikely, which leaves the cool crust," Dr Nemchin said.
"If that is correct we [have] another piece of evidence that the early Earth may be somewhat similar to the more modern Earth."
But he says a problem for this theory is that the zircon crystals show no signs of being subjected to high pressure.
In his journal commentary, Dr Williams highlights this point and says the researchers hypotheses are not "entirely convincing".
"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 said.
"This would undermine other inferences based on the assumption that the zircons preserve their original chemical and isotopic compositions, including the deduction that the early Earth was cool."
He suggests the researchers should test the diamonds' carbon isotopic composition and whether nitrogen is present as single or paired atoms.
This would indicate the time spent in the mantle and whether it was under relatively high or low temperatures.
Dr Nemchin says analysis of the carbon isotopes would be the next obvious step and could provide clues about the possible existence of life forms 4.2 billion years ago.
By ABC Science Online reporter Dani Cooper

Posted Thu Aug 23, 2007 5:39pm AEST
Tags: science-and-technology, geology, research, australia, wa, perth-6000

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