Wednesday, 18 June 2014 | MYT 4:35 PM

Oceans of water underground, but not a drop to drink

Looking for water? You'll have to dig deep. Deeper. Deeper. Yes, deeper.

Massive amounts of water appear to exist deep beneath the planet’s surface, trapped in a rocky layer of the mantle at depths between 410km to 660km. But don’t expect to quench your thirst down there. 

“It’s no longer liquid water that we’re talking about at these great depths. The weight of hundreds of kilometres of rock and very high temperatures above 1,000 degrees Celsius break down water into its components. And it’s not accessible. It’s not a resource in any way,” Northwestern University geophysicist Steve Jacobsen said in a telephone interview.

Instead of flowing like rivers, frozen like ice, or vapourised like steam, the water in Earth's mantle is locked inside the molecular structure of minerals called ringwoodite and wadsleyite  that possesses the remarkable ability to absorb water like a sponge. 

Squeezing water from a rock: Ringwoodite, a crystal-like polymorph of olivine, has the rare property of being able to store 'dehydrated' elements of water inside its molecular structure. Examples of ringwoodite are extremely rare on the Earth's surface, where they are found mostly in meteorites. But it's apparently abundant deep within the mantle transition zone, between 410km and 660km underground. Shown above is a blue-coloured ringwoodite, measuring less than 150micrometres across, synthetically manufactured in Germany. — Wikipedia

The other surprising thing is how much water actually is down there. “It may equal or perhaps be larger than the amount of water in the oceans,” Jacobsen said. “It alters our thoughts about the composition of the Earth.”

How do it get there?

According to Jacobsen, water is taken down into the mantle with minerals during the process known as plate tectonics — the slow, inexorable movement of the colossal rock slabs that make up the Earth’s surface. When the minerals containing this water reach certain depths, they break down in a process called dehydration and release the water to form magmas. Such “dehydration melting” is common in the shallow mantle and forms the source for magmas in many volcanoes.

In the study, published in the journal Science on June 13, the researchers present evidence that this is also occurring much deeper in the mantle in a region called the “transition zone” between Earth’s upper and lower mantle. The study combined lab experiments involving synthetic ringwoodite being exposed to conditions simulating the heat and pressure of the “transition zone” and observations of events in this zone based on seismic data from a network of more than 2,000 seismometres across the US.

The 'oceans' of ringwoodite and wadsleyite that researchers think may contain more than all the water on the Earth's surface exist in the transition zone between the upper mantle and the lower mantle layers of the planet, shown in the cutaway diagram above. — Reuters

A team led by Jacobsen and University of New Mexico seismologist Brandon Schmandt identified deep pockets of magma, a likely signature of the presence of water at those depths. “Melting of rock at this depth is remarkable because most melting in the mantle occurs much shallower, in the upper 80km,” Schmandt said in a statement. “If there is a substantial amount of H2O in the transition zone, then some melting should take place in areas where there is flow into the lower mantle, and that is consistent with what we found.”

The research built on another study in March showing that a commercially worthless diamond found in Brazil contained ringwoodite that entrapped water amounting to more than 1% of its weight. Ringwoodite has been found in meteorites, but this was the first terrestrial sample because it normally is so deeply buried. – Reuters

Tags / Keywords: Lifestyle , Features , Science , Geology , water , underground , reservoir , ringwoodite , mantle , reservoir


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