Earth Mantle: Clear indication of water reservoir

Seismic measurements confirm the existence of large amounts of water in the mantle rock

In such a diamond press, the ringwoodite was heated under high pressure. © Steve Jacobsen / Northwestern University
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Underground Reservoir: In the mantle, there is indeed a huge reservoir of water. This is confirmed by laboratory tests and, for the first time, seismic measurements. In some places under North America, US researchers have detected a telltale smelting of mantle rock - an indication of water in the rock. The earthly water cycle reaches deep into the mantle, according to the researchers in the journal Science.

A few months ago, researchers found the first indication that there could be large amounts of water in the Earth's mantle. Proof of this was a tiny grain of the mineral ringwoodite, which had once been carried from the depths by a volcano to the surface. The ringwoodite contained 1.5% by weight of water bound as a hydroxyl molecule. Since this mineral occurs in the transition zone from the upper to the lower mantle in 410 to 660 kilometers in depth in greater quantities, this could mean that the mantle is there much more water-rich than assumed. Three times as much water as in all the oceans together could be stored there.

Water leakage melts mineral

"But whether or not this ringwoodite sample was actually representative of the Earth's interior was not known, " explains Steve Jacobsen of Northwestern University in Evanston, one of the first authors. The missing evidence of the presence of so much water in the earth's mantle is provided by Jacobsen, seismologist Brandon Schmandt of the University of New Mexico in Albuquerque and her colleagues.

The starting point was observations of hydrated ringwoodite produced in the laboratory. With the help of a diamond press, Jacobsen and his team set high pressure and temperatures of 1, 600 degrees Celsius - the conditions prevailing in the transition zone of the jacket. The ringwood is transformed into other mineral forms. But something else happened: there were zones on the tiny mineral crumb where the mineral had melted. For this so-called dehydration melting, the water bound in the ringwood wood is responsible, as the researchers explain. If the water is missing, this form of melting does not occur.

Ringwood produced in the laboratory, typical of which is the bluish color of the mineral Steve Jacobsen / Northwestern Un

Melt pockets in mantle rock

Exactly this dehydration melting the scientists showed shortly thereafter in the earth's mantle. This was achieved with the help of USArray, a dense network of more than 2, 000 seismometers distributed across the US. When earthquake waves propagate through the Earth's interior, they are characteristically altered by different types of rock, as well as solid and molten rock. The seismometer grid absorbs these altered waves and thus allows a kind of X-ray view into the earth's mantle. display

And this actually revealed a melting of the rock 660 kilometers deep, and thus at the lower edge of the transition zone in the mantle. "We have found evidence of extensive smelting of North American mantle rock at the exact depth at which ringwoodite is dehydrated, " says Jacobsen. The zones of molten rock were found mainly where rock is pressed down along the tectonic plate boundaries.

Water cycle reaches into the earth's interior

According to the researchers, these results suggest that there is indeed an extensive water reservoir in the transition zone of the mantle. "Scientists have been searching for this lack of deep water for decades, " says Jacobsen. Because for a long time one suspects that the water cycle of our planet also includes the earth's interior.

This reservoir in the depths is fed by submerged rock from the surface and in turn gives off water when the rock comes back to the surface in the course of millions of years. "Now we finally see evidence for such a water cycle that encompasses the whole earth, " states Jacobsen. (Science, 2014; doi: 10.1126 / science.1253358

(Science, 13.06.2014 - NPO)