24 March 2011

Journey to the mantle of the Earth by 2020

By
Cosmos Online
A half-century after the first attempt to drill through the ocean crust into the Earth’s mantle, a new campaign armed with improved technology is underway that could reach the mantle by the end of the decade, researchers say.
Chikyu

A view of the drilling derrick on board the Chikyu vessel. Credit: JAMSTEC/IODP

SYDNEY: A half-century after the first attempt to drill through the ocean crust into the Earth’s mantle, a new campaign armed with improved technology is underway that could reach the mantle by the end of the decade, researchers say.

By extracting samples of the mantle, which is nearly 3,000 km thick and contains roughly 68% of the planet’s mass, researchers hope to unearth valuable information about its composition that could yield clues about the evolution of the planet. It could also contribute to our understanding of how the ocean crust is formed, the nature of the crust-mantle boundary and the limits of microbial life under the Earth’s surface.

“If successful this would be the first in situ sampling of the largest part of our planet,” said Damon Teagle, a geochemistry professor at the University of Southampton in England and co-author of the report in Nature.

Ocean floor is optimal

In the past, researchers have been able to study fragments of the mantle thrust to the Earth’s surface by tectonic mountain building, sea-floor spreading, or encased in lava spewed from volcanic eruptions. As a result certain features of the mantle’s dense, rocky composition are already known.

According to the report, however, the processes that force these fragments upward to the surface chemically alter their composition and displace key trace elements that might be useful in reconstructing the Earth’s evolution.

To fill in the gaps, researchers must extract samples of the mantle below the oceanic crust. The ocean floor is optimal for drilling because its crust is a mere six km thick compared to continental crust, which can range from 30 to 60 km in thickness.

The Japanese factor

Teagle, who will lead a run-up expedition this spring to bore further into the oceanic crust than ever before, said the forward push to get this project rolling is coming largely from Japanese funding agencies and the availability of a massive Japanese deep-sea drilling vessel called Chikyu.

The ship has ‘riser drilling’ equipment necessary for the eventual exploration of the mantle. There is an outer pipe around the drill string through which rock cuttings can be transported back to the ship. This allows for better well control and stability, and will help prevent rock cuttings from blocking the drill bit, said Teagle.

But many challenges still exist. “We will need to drill a 6.5 km hole into…the ocean floor in roughly 4,000 m of water … [and] a ship that can be dynamically positioned to stay precisely above a drill hole for many months at a time,” said Teagle. In addition, new tools must be developed to withstand extreme pressure and heat – which can reach upwards of 300 degrees Celsius.

But worth the cost?

Neville Exon, a marine geophysicist at the Australian National University, said the project would be a massive technological undertaking that could carry a price tag upwards of a billion dollars.

Still, he said the prospect of reaching the mantle was an exciting one. “This was the original reason ocean drilling began,” he said.

“This could give us a tremendous insight into how the Earth works because the circulation of the mantle is what drives plate tectonics.”

A fifty-year effort still ongoing

The first attempt to drill the Earth’s mantle was in 1961 with the U.S.-led Project Mohole – an ambitious ocean-drilling endeavour that collapsed in 1966 when costs began spiralling out of control.

Nevertheless, this project – which coincided with the growing scientific acceptance of plate tectonics – proved the viability of ocean basement drilling and spawned an array of international drilling efforts over the following decades and into the present-day.

Over the next three years, geophysical surveys will be conducted at three Pacific Ocean locations to determine the best site to begin exploratory drilling.

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