Diamond reveals the oceans that lie beneath
Diamond Oceans The first direct evidence of the vast quantities of water deep in the Earth's mantle has been uncovered by scientists studying a tiny diamond.
The water was found in ringwoodite included in the diamond taken from Juina, Brazil, the researchers reveal in today's Nature journal.
The find also marks the first discovery of terrestrial ringwoodite, thought to be one of the most abundant minerals in the mantle's transition zone, between 410 and 670 kilometres beneath the surface.
"Until now, no one's ever been able to put their hands on a real piece of ringwoodite that's come from inside the planet," says the study's lead author, Dr Graham Pearson of the University of Alberta in Edmonton.
Previously the only samples of ringwoodite available were either manufactured artificially or came to Earth in meteorites.
Ringwoodite is transformed from olivine as "it changes crystalline structure to become denser" under the high temperatures and pressures of the transition zone, says Pearson.
Olivine holds very little water, however once its structure is changed to ringwoodite it acts like a sponge soaking up water in the form of hydroxyl molecules.
Laboratory experiments using artificially made ringwoodite indicate it can contain up to 2.5 per cent of its weight as water.
The ringwoodite discovered by Pearson and colleagues contains about one per cent of its weight as water.
"That might not sound like much, but when you look at how much ringwoodite there is in Earth's mantle, then you're looking at vast amounts of water," says Pearson.
"This sample indicates there could be as much water in the mantle as there is in all the world's oceans."
The authors used X-ray diffraction, Raman and infrared spectroscopic analysis to uncover the mineral inclusion.
According to Pearson, the fact the ringwoodite inclusion was trapped in diamond was key to the discovery.
Diamond is an extremely strong structure allowing little expansion, thereby preventing the high-pressure inclusion from reverting to olivine.
"It's staggering to think that when you hold this little sample of diamond in your hand, that the ringwoodite inclusion is trapped and held in the diamond at a residual pressure of 20,000 atmospheres," says Pearson.
Scientists are yet to determine how the water reached the transition zone as the mantle above and below this region resists holding water.
"One of the things we would like to do is determine the hydrogen to deuterium ratios in the water, which would give us clues as to where the water comes from," says Pearson.
"Was the water transported down by subducting oceanic slabs that interacted with surface water, or is it primordial water, trapped from when the planet formed 4.5 billion years ago."
Determining the ratios could also tell scientists where in the solar system the Earth was made, as these ratios change with distance from the Sun.
"But these would be semi-destructive measurements, and as it's the only ringwoodite sample we have from the deep Earth, we want to preserve it as much as we can," says Pearson.