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Tuesday, February 13, 2018

Scientists: water appeared in the mantle of the Earth 2.7 billion years ago


Researchers from the Institute of Geochemistry and analytical chemistry named. V. I. Vernadsky Russian Academy of Sciences together with colleagues from France and Germany confirmed the existence of deposits of water in the Earth’s mantle and found that the water did not get there later 2.7 billion years ago. Scientists suggest that water from the mantle have contributed to the emergence of the oceans. About it the correspondent of TASS said Alexander Sobolev, lead author of the study published in the journal Nature.
Water in minerals
In 2014, canadian researchers published an article in which reported that they were the first to establish the presence of water in the rare mineral ringwoodite. It is formed from olivine, the main mineral of the mantle, under the pressure of hundreds of thousands of atmospheres. In the crystal of ringwoodite, is enclosed in a diamond that was 1.5 percent water in the mantle, where temperatures exceed 1.5 thousand degrees Celsius, the water molecules basically “dissolved” in the molten rock, or are in insignificant concentrations in the solid minerals.

Ringwoodite is an extremely rare find, as he and his fellow wadsleyite “live” on the large depth of 410-660 kilometers in the so-called transit zone between the upper and lower mantle of the Earth. According to the experiments, these minerals can contain hundreds of times more water than olivine. If researchers found the ringwoodite crystal is a typical representative of the transition zone, it will, according to calculations of scientists, should contain several oceans of water.

“The question arises — when it there appeared? The data, published in 2014, received on the inclusion in the diamond, made from deep mantle about 100 million years ago. The age of this diamond and ringwoodite included is unknown. Maybe several hundred million, maybe several billion years. Our research shows that this water existed 2.7 billion years ago,” said Sobolev.
How to “see” into the earth’s mantle
In order to assess the age of deposits of water in the mantle, Sobolev and his colleagues investigated igneous rock komatiite. At the disposal of researchers are samples of magma from the transition zone, which came to the surface in the composition of mantle plumes — hot streams that rise in the mantle from the Earth’s core to its surface at a speed of several meters per year.

“We’re looking for remnants of olivine in these rocks and look inside them to find the inclusions of the melt, which were seized 2.7 billion years ago. These droplets melt in the size thinner than a human hair giving us the right information, as preserved in the pristine olivine. In them we find water in an amount of 0.6% in 20-50 times what might be expected”, — said the scientist.
Where did the oceans
Researchers believe that the transit area in antiquity could serve as the main source of water for the oceans on the Earth’s surface.

Modern mantle plumes colder than those that were billions of years ago, komatiites, formed over 2.5 billion years ago, represent a high degree of mantle melting. More hot plumes passing through the transition area, took water from it and “dragged” along on the surface, while the colder plumes in our time, pass through a transit zone, almost without exchanging with her agent.

“Ancient plumes were at 200 degrees hotter than modern: 1 800 to 1, 600 degrees Celsius. The 200 degrees is enough plumes were partially molten in the transition zone. In this case, the melt absorbs water and brings it to the surface. And modern make water plumes from the lower mantle, where it is not very much,” — said Sobolev.
How and when water got into the mantle?
Until now it was believed that water enters the deep mantle of the Earth by the process of plate tectonics. Going down, the slab carries away what was on its surface, including “pulls” the water in the composition of the hydrous minerals formed at the ocean floor. Part of this water eventually enters the transition zone of earth’s mantle, where it is retained in deeper relatives of the olivine — wadsleyite and ringwoodite.

“However, the movement of lithospheric plates on Earth, apparently, was not started immediately. At least 3 billion years ago, and maybe even later. And we showed that the excess of water was already in the transition zone 2.7 billion years ago. Something doesn’t add up: either plate tectonics began before any water got in there from the beginning, the Earth was formed. This is one of the major issues that emerged as a result of our research,” explained the scientist.

In order to examine this question, Sobolev and his colleagues are currently conducting a similar study on komatiite age of 3.5 billion years.

“I think the water was probably brought by asteroids or comets in the early stages of formation of the Earth, about four billion years ago when the planet had already formed as a ball. Question, when did it happen and how much water, is not solved yet, but I think that we will raise in the near future. This helps us support the Russian science Foundation and an exceptional ability to analyze microscopic quantities of substances,” concluded the researcher.

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