A team of researchers led by Ho-Kwang Mao of the Carnegie Institution in the United States appears to have uncovered a mystery that has long troubled geologists. The authors explained what mineral the zones of slowing seismic waves at the core-mantle boundary consist of and reproduced the process of its formation in the laboratory. If the conclusions of the scientists are correct, then such zones turn out to be a giant storage of oxygen, which can influence the history of life on Earth. A scientific article with the results of the study was published in the journal Nature on November 22, 2017.
How do we know what is under our feet? This simple question is a headache for geologists. The journey to the center of the Earth exists only in the pages of science fiction books, but in reality, the record drilling depth barely exceeds 12 kilometers.
The scientists came to the aid of seismic waves, which spread in different ways in rocks of different densities, they allow conducting "ultrasound" into the very depths of our planet. However, the information that can be obtained from such a scan is incomplete. Often the answers received only raise new questions. Actually, like in any other field of science.
At a depth of 2900 km, the core-mantle boundary is located. Geologists have long discovered areas in which seismic waves move unusually slowly. They were called ultra-low speed zones, in English ultra-low speed zones, hence the abbreviation UVZ. But what is it? What mysterious material slows down the vibration of the earth so much?
This issue has long been discussed by scientists, and many hypotheses have already been proposed. However, they all have one drawback: it is necessary to explain how the necessary substance as a whole ended up at such a depth.
Ho-Kwan's team seems to have killed two birds with one stone. The researchers found a suitable mineral and a simple mechanism for its formation right on the spot. True, this success led to the question of the need to revise something in the history of the Earth's biosphere.
The mineral that the authors liked is iron peroxide FeO2Hx (the number of hydrogen atoms can be different). Talking about the work of their predecessors, they say that they are stable in the conditions of the lower mantle.
But where can you get it from? To answer this question, scientists have recreated a small portion of the infernal depths in the laboratory. Diamond grippers compressed the substance, creating a pressure of 860 thousand atmospheres. A laser beam penetrating transparent diamond plates heated the reagents to 1900 degrees Celsius.
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Under these conditions, as the authors found, iron peroxide is formed from the simplest components: iron and water. There is as much iron in the core of the Earth as is necessary - in fact, it is mainly made of this metal. And water? According to the authors, annually 300 million tons of water sinks into the depths of the mantle due to the movement of tectonic plates. According to their calculations, a tenth of the water in the World Ocean would be enough to create the entire observed UVZ. For comparison: according to some estimates, there is 90 times more water in the Earth's interior, including the crust, mantle and core, than on the surface.
It turns out that the necessary mineral is formed at the border of the mantle and the core from substances that are enough there. But is this really responsible for the picture that is observed on seismograms?
Researchers have tested this as well. Using experiments and theoretical calculations, they found that the required characteristics (speed of sound, density, etc.) have a mixture in which 40-50% is iron peroxide, and the rest is in the usual rocks of the lower mantle. The properties of such a mixture are in excellent agreement with the observational data from UVZ. It looks like the secret of the mysterious "slowdown zones" has been revealed.
But this discovery raises other interesting questions. As explained in a press release from the study, iron peroxide deposits represent a giant reservoir of oxygen. From time to time, oxygen can be released and reach the surface. The implications of such events and their possible link to climate change, mass extinctions and other grand events remain to be studied.
