As we all remember from school, some 71 per cent of the surface of the globe is covered by water. But what if all that H2O was just the tip of the unfrozen iceberg? Imagine if, deep within our planet, just as much water again was sloshing around in a giant subterranean ocean.
It’s enough to send Jules Verne devotees dashing for their swimsuits, but this theory is very seriously being suggested by scientists, excited by the results of tests done after a most unlikely discovery that took place in 2014.
Numerous individual moments of astonishing serendipity had to come together for a tiny and ostensibly worthless 3mm shard of brown diamond to travel from the centre of the Earth and end up on mantle geochemist Graham Pearson’s desk at the University of Alberta. That this particular fragment was uncovered at all, amid the immensity of Juína in western Brazil’s Mato Grosso, was incredible, but what was found inside it could change the way we look at our planet.
When Pearson and his team analysed the diamond using high-tech light-scattering spectroscopy, they found embedded in it a fleck of crystal called ringwoodite, which had never before been seen in a terrestrial rock. And an even bigger surprise was to come, with the discovery that the ringwoodite contained water. Considering where this rock had come from, Pearson knew he’d just found a potential answer to a very long-standing enigma: what exactly does lie under the skin of the world we live on?
Ringwoodite is the high-pressure phase of a shapeshifting forsterite, which reverts to its olivine state once the extreme forces that created it have dissipated. To chance across it, you’d have to be in the right place at the right time—immediately after some extraordinary event that generated the requisite conditions for it to morph—and then you’d have to act fast. It’s the geochemical equivalent of finding a snowflake in the desert after a once-in-a-millennia storm.
Amazingly, it has happened before, in Australia in 1879, when fragments of a meteorite were collected soon after they fell and were found to contain ringwoodite formed by impact pressure. But this discovery was different. This one didn’t come from outer space, it came from the bowels of the planet, spat out by an ultra-deep diatreme volcano. And it was encased in a protective diamond, which meant it was suspended in time and wouldn’t change form and spill the message it was carrying.
Most diamonds are formed between 150 to 200 kilometres below the planet’s surface, but Pearson’s prize pebble was from an ultra-deep region, the so-called ‘transition zone’, a little-understood space between the upper and lower mantles within the Earth’s core. To reach this zone you’d need to dig a hole between 410 and 660 kilometres deep, which explains why no one really knows what’s down there. Well, until now that is.
Pearson was being cautiously conservative, but if calculations based on the H2O content of a 40-micrometre grain of ringwoodite found within a 0.09-gram diamond are correct, the answer to that question about the make-up of our planet has been revealed by his crystal haul, which indicates that transition zone in the guts of our planet could be a whopping great bladder full of water.