> Writing in the journal Nature, scientists said they had found an elusive mineral pointing to the existence of a vast reservoir deep in Earth’s mantle, 400 to 600 kilometres beneath our feet.
Not too surprising. Hydrogen and oxygen are two of the three most common elements in space, and in the solar system, being far more common than silicon and iron that make up most of the Earth’\s mantle and core. So having trapped water in the mantle is expected. It would be rather peculiar if all of it out-gassed before or during the impact that formed the Moon, and little would have out-gassed from the deep mantle since then.
> It may hold as much water as all the planet’s oceans combined, they believe.
That small? I would have expected more.
> The evidence comes from a water-loving mineral called ringwoodite that came from the so-called transition zone sandwiched between the upper and lower layers of Earth’s mantle, they said. Analysis shows a whopping 1.5 per cent of the rock comprises molecules of water.
That’s new.
> until now, ringwoodite has only ever been found in meteorites. Geologists had simply been unable to delve deep enough to find any sample on Earth.
Not surprising.
> In 2008, amateur gem-hunters digging in shallow river gravel in the Juina area of Mato Grasso, Brazil, came across a tiny, grubby stone called a brown diamond. Measuring just three millimetres across and commercially worthless, the stone was acquired by the scientists when they were on a quest for other minerals. But the accidental acquisition turned out to be a bonanza. In its interior, they found a microscopic trace of ringwoodite – the very first terrestrial evidence of the ultra-rare rock.
That is startling. It proves, quite apart from the ultra-rare rock, that mantle convection has in the past penetrated to a great depth. Good on the scientists for studying the mineralogy of supposedly worthless diamonds. That really is the best way to get rock samples from as deep as possible in the mantle, because natural diamonds always form at great depth and are hard enough to survive a trip to the surface. Think of them as a natural geological sample-container.