(Corrects kilometre to mile conversion in paragraph 3)
WASHINGTON, March 13 (Reuters) - A dirty-looking, commercially worthless brown diamond found in Brazil is yielding priceless information about the composition of a layer deep inside our planet.
The diamond contained a mineral called ringwoodite that entrapped a significant amount of water, an international team of scientists reported this week.
The discovery of the water-rich mineral indicates that mammoth amounts of water are trapped in a zone 410 to 660 kilometers (255 to 410 miles) beneath the surface between Earth's upper and lower mantle, the researchers said.
Ringwoodite is a form of the mineral peridot, thought to exist in large quantities under high pressures deep underground. The mineral was found to contain water amounting to 1.5 percent of its weight, the scientists said.
"This sample really provides extremely strong confirmation that there are local wet spots deep in the Earth in this area," said University of Alberta diamond scientist Graham Pearson, who led the research published in the journal Nature.
"That particular zone in the Earth, the transition zone, might have as much water as all the world's oceans put together," Pearson said in a statement.
This water is not in the form of underground liquid oceans, but rather is locked inside minerals, the scientists said.
The diamond, formed deep underground, was discovered in 2008 in the Juina area of Mato Grosso, Brazil, where miners retrieved it from shallow river gravels. It had been transported to the planet's surface by a volcanic rock known as kimberlite, the scientists said.
Ringwoodite has been seen in meteorites but this was the first terrestrial sample ever found, they said, because it is simply too hard to do scientific fieldwork at extreme depths.
There has been a debate among some scientists about the composition of Earth's transition zone and whether it is full of water or not. Determining that water exists there has implications for the study of volcanism and plate tectonics, according to the researchers.
"One of the reasons the Earth is such a dynamic planet is the presence of some water in its interior," Pearson said. "Water changes everything about the way a planet works." (Reporting by Will Dunham; Editing by Marguerita Choy)