Rock of ages
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| Guest writer: |
 Stephen Brook was appointed science writer of The Australian newspaper in March 1999 and has been based in the paper's Canberra Bureau since January. Prior to moving to Canberra he worked out of the paper's Sydney Bureau. He completed a Bachelor of Arts degree in history and politics in 1994, joining News Ltd the next year. After completing a cadetship with The Australian in 1996, he was appointed to the paper's Perth Bureau in March 1997. During the next two years he reported on the trial of Alan Bond and the release of disgraced former Premier Brian Burke, as well as touring the State with the West Australian Symphony Orchestra and jackarooing on a sheep station. |
| Research team: |
| Professor Ken Collerson, Dr Balz Kamber and Sarath Hapugoda |
| Funding: |
| Unfunded |
| Email/Web link: |
| k.collerson@mailbox.uq.edu.au www.earthsciences.uq.edu.au |
| Faculty of Engineering, Physical Sciences & Architecture |
About 34 million years ago, something stirred deep beneath the surface of the Earth, causing some of the most violent volcanic eruptions ever seen on the planet.
The violent disruptions were caused by melting in depths of the lower mantle, 670kms to 2900kms below the surface of the Earth.
The rising gas-rich magma generated tremendous pressures in the transition zone between the lower and upper mantle. Something had to give.
The eruptions forced deep mantle rocks from depths of between 400kms and 670kms towards the Earth's surface at supersonic speeds.
The magma and rock fragments exploded out of the Earth's crust, possibly reaching as high as the stratosphere before falling back to the surface.
Despite the tremendous forces involved, such was the distance to the surface that the journey probably took less than a week.
Now the dark, glossy, reddish black rocks, rich in garnet and some as large as 30cms in diameter, sit in the office of the Department of Earth Sciences Professor Ken Collerson, atop a filing cabinet to the right of his often-open door.
They are the deepest rocks ever seen — about twice as deep as anything previously discovered.
A paper about them, written by Professor Collerson, research fellow Dr Balz Kamber and PhD student Sarath Hapugoda was published in the May 19 edition of the prestigious international journal Science, the peer-reviewed journal of the American Association for the Advancement of Science.
Their co-researcher was Dr Quentin Williams, a specialist in mineral physics, from the Earth Sciences Department at the University of California in Santa Cruz.
The discovery won the researchers world-wide attention, and rivals Professor Collerson's discovery of the world's oldest mantle rocks, the 3.9 billion-year-old rocks from northern Labrador in Canada, which was published in the British journal Nature in 1991.
"I have had a few fairly lucky discoveries in my career," Professor Collerson said, as he detailed the story of how basic research at the University over the past two years led to the realisation of the great significance of the rocks and also speculation about the lucrative new mining industry their discovery could create.
In 1998, several prospectors sent samples of garnet from the island of Malaita, east of Papua New Guinea, to Professor Collerson in the hope they might be valuable.
Malaita, part of the Solomon Islands, has been treated warily by geologists since locals were reputed to have killed a British tax collector in the 1920s.
The death resulted in the Island being shelled by an Australian warship at the request of the British.
And recent political unrest in the Solomon Islands is making access difficult again.
Professor Collerson performed numerous chemical analyses on minerals in the rocks, found they were interesting but not valuable and promptly forgot about them.
Six months later, more samples arrived, sent by a small, New Guinea-based company.
"That's when I started to realise there were some very strange minerals in these rocks," Professor Collerson said.
The minerals were identical in composition to minerals that had been created artificially under incredible pressures, similar to those found very deep inside the Earth.
"Some of these minerals haven't even been named," he said.
Analyses in the University's world-class Centre for Microscopy and Microanalysis and at the University of California in Santa Cruz, where Professor Collerson taught before returning to Australia in the early 1990s, revealed the garnet contained patches of majorite, a silica-rich form of mineral garnet, a rock which only forms under extreme pressures.
This majorite garnet contained a lot more silica and less aluminium and chromium than typically occurs in garnet.
Professor Collerson and colleagues were able to use this information to calculate the pressures the rocks must have experienced.
They found the pressure was 22 or 23 gigapascals, equal to 250,000 times the pressure of the atmosphere at the Earth's surface.
These same pressures occur in the transition zone, which is only 10.4 percent of the distance from the surface to the centre of the core.
As pressure increases beneath the surface, so does density, as mineral atomic lattices become tightly packed.
Sudden changes of rock density occur in the transition zone, which separates the upper and lower mantle.
At such depths and temperatures, solid rock takes on a plastic, slowly creeping form. In places, the mantle rock apparently cracks, allowing gas-rich kimberlite magma from the lower mantle to erupt and form volcanic pipes, providing the route for the transition-zone garnet to reach the surface.
As well as majorite, the second group of garnet samples sent to Professor Collerson contained micro-diamonds, measuring up to 100 microns in diameter.
Until now, diamonds have never been found in the oceanic crust, which has an average thickness of about 8kms and lies beneath the world's islands and its oceans.
Diamonds have always been found in land that is part of the thicker and older continental crust, which has an average thickness of 35kms but can get up to 100kms.
In places such as South Africa, the diamonds were carried to the surface from depths greater than 150kms beneath the surface by pipes of kimberlite, a type of magma commonly associated with bringing diamonds to the surface and similar to the deep volcanic pipes on Malaita.
Professor Collerson said the micro-diamonds were "causing a rethink about where we find diamonds. I think what we have got is a paradigm shift for diamond exploration. There's no guarantee there are going to be big diamonds there, but it's a good chance, more than a chance."
But what caused the formation and eruption of kimberlite melts from the lower mantle?
The eruptions were possibly connected with transfer of energy at the very centre of the Earth, 6370kms from the surface.
"As kimberlites only erupt periodically throughout Earth history, maybe it's something to do with major changes in the Earth's dynamo that's triggered these deep eruption events," Professor Collerson said.
The most profound event at these depths would be the rotation of the inner core in relation to the outer core, generating the Earth's magnetic field and causing major shifts in the Earth's orientation.
The research by Professor Collerson on the transition zone rocks will reveal new information about the chemical stratification of the planet, previously only able to be studied via waves from earthquakes, laboratory experiments or by studying minute traces of minerals embedded in diamonds.
Professor Collerson likened the discovery to scientists repairing the damaged lens of the Hubble space telescope.
"In addition to getting the chemistry of these rocks, we will also get their physical properties and they can be linked in equations used by seismologists to establish the temperature and density of the interior of the Earth.
The images of the lower mantle were rather blurred like the images of the Hubble space telescope before the lenses were fixed. Now Earth scientists will be able to get significantly better resolution of images of the deep interior of planet Earth," he said.
