First terrestrial discovery of an extremely rare mineral called ringwoodite confirms theory about huge water ‘reservoirs’ 410 to 660 km beneath the surface of our planet, says a team of researchers led by Prof Graham Pearson from the University of Alberta, Canada.
Ringwoodite is a form of the mineral peridot, believed to exist in large quantities under high pressures in the transition zone.
Ringwoodite has been found in meteorites but, until now, no terrestrial sample has ever been unearthed because scientists haven’t been able to conduct fieldwork at extreme depths.
Analysis of the mineral shows it contains a significant amount of water – 1.5 per cent of its weight.
The mineral was found in 2008 in the Juina area of Mato Grosso, Brazil, where artisan miners unearthed the host diamond from shallow river gravels.
The diamond had been brought to the Earth’s surface by a volcanic rock known as kimberlite – the most deeply derived of all volcanic rocks.
“This sample really provides extremely strong confirmation that there are local wet spots deep in the Earth in this area. That particular zone in the Earth, the transition zone, might have as much water as all the world’s oceans put together,” Prof Pearson said.
The discovery was almost accidental in that scientists had been looking for another mineral when they paid about USD 20 for a 3-mm-wide, dirty-looking brown diamond.
The ringwoodite itself is invisible to the naked eye, buried beneath the surface, so it was fortunate that it was found by Prof Pearson’s team member, John McNeill, in 2009.
“It’s so small, this inclusion, it’s extremely difficult to find, never mind work on, so it was a bit of a piece of luck, this discovery, as are many scientific discoveries,” said Prof Pearson, who is the first author of a paper appearing in the journal Nature.
The sample underwent years of analysis using spectroscopy and X-ray diffraction before it was officially confirmed as ringwoodite.
The discovery confirms about 50 years of theoretical and experimental work by geophysicists, seismologists and other scientists trying to understand the makeup of the Earth’s interior.
Scientists have been deeply divided about the composition of the transition zone and whether it is full of water or desert-dry. Knowing water exists beneath the crust has implications for the study of volcanism and plate tectonics, affecting how rock melts, cools and shifts below the crust.
“One of the reasons the Earth is such a dynamic planet is because of the presence of some water in its interior. Water changes everything about the way a planet works,” Prof Pearson concluded.
D.G. Pearson et al. 2014. Hydrous mantle transition zone indicated by ringwoodite included within diamond. Nature 507, 221–224; doi: 10.1038/nature13080