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By Jennifer Chu MIT News
A study reveals that 1 to 2% of the world's oldest mantle rocks are made from diamonds.
According to a new study, there could be more than one quadrillion of study from MIT and other universities. But the new results are unlikely to trigger a diamond rush. Scientists believe that valuable minerals are buried more than 100 miles below the surface, far beyond any drilling expedition.
The ultradeep cache can be disseminated in cratonic roots – the oldest and most immobile rock sections the center of most continental tectonic plates. In the form of inverted mountains, the cratons can extend up to 200 miles across the Earth's crust and into its mantle; geologists refer to their deepest sections as "roots".
In the new study, scientists estimate that cratonic roots can contain 1 to 2 percent of diamonds. Considering the total volume of cratonic roots in the Earth, the team estimates that about a quadrillion (1016) tons of diamonds are scattered within these ancient rocks, 90 to 150 miles below. from the surface
"This shows that the diamond is not perhaps this exotic mineral, but on the scale of things [geological] it is relatively common," says Ulrich Faul, Researcher at the Department of Earth Sciences, Atmosphere and Planets at MIT. "We can not reach them, but there are many more diamonds than we had ever imagined before."
Faul's co-authors include scientists from the University of California at Santa Barbara, the Institute of Globe Physics, Paris, the University of California at Berkeley, the University of California, and the University of California. Polytechnic School, Washington Carnegie Institution, Harvard University, University of Science and Technology es de Chine, Bayreuth University, University of Melbourne and University College London
A sonic error
Faul and his colleagues came to their conclusion after Asked about an anomaly in the seismic data. In recent decades, organizations such as the United States Geological Survey have maintained global records of seismic activity – essentially sound waves that cross the Earth and are triggered by earthquakes, tsunamis, explosions and other sources. Seismic receivers around the world capture sound waves from such sources at different speeds and intensities that seismologists can use to determine the origin of an earthquake for example.
Scientists can also use these seismic data to build an image of what the interior of the Earth might look like. Sound waves move at different speeds across the Earth, depending on the temperature, density, and composition of the rocks they pass through. Scientists have used this relationship between seismic velocity and rock composition to estimate the types of rocks that make up the Earth's crust and parts of the upper mantle, also known as the lithosphere.
However, using seismic data to map the interior of the Earth. Scientists have been unable to explain a curious anomaly: Sound waves tend to accelerate considerably by passing through the roots of ancient cratons. The cratons are known to be cooler and less dense than the surrounding mantle, which in turn would produce slightly faster sound waves, but not as fast as measured.
"Measured velocities are faster than we think we can reproduce with reasonable assumptions about what is there," says Faul. "Then we have to say," There is a problem. That's how the project began. "
Diamonds in the deep
The team sought to identify the composition of cratonic roots that could explain the spikes. seismic velocities. To do this, the team's seismologists first used USGS seismic data and other sources to generate a three-dimensional model of the seismic wave velocities traveling through the main cratons of the Earth.
Next, Faul and other past measured sound velocities across many types of minerals in the laboratory, used this knowledge to assemble virtual rocks, made from various combinations of minerals. Then, the team calculated the speed at which the sound waves cross each virtual rock and found only one type of rock producing the same velocities as those measured by the seismologists: 1 to 2 % of diamond, in addition to the typical peridotite of the upper mantle of the Earth) and minor amounts of eclogite (representing the subducted oceanic crust). This scenario represents at least 1,000 times more diamonds than people had expected before.
"Diamond in many ways is special," says Faul. "One of its special properties is, the speed of sound in the diamond is more than twice as fast as in the dominant mineral in the rocks of the upper mantle, olivine."
The researchers found that a composition of 1 to 2% just enough to produce the higher sound velocities measured by seismologists. This small diamond fraction would not change either the overall density of a craton, which is naturally less dense than the surrounding mantle.
"They are like pieces of wood floating on the water," says Faul. "The cratons are a little less dense than their environment, so they are not brought back into the Earth but remain floating on the surface, so they preserve the oldest rocks. that you just needed 1 to 2% diamonds for the cratons to be stable and not flowing. "
In a way, Faul says that cratonic roots made in part of diamond make sense. Diamonds are forged in the environment at high pressure and high temperature of the deep Earth and only bring it closer to the surface by volcanic eruptions that occur every few tens of millions of years. These eruptions excavate geological "pipes" made up of a type of rock called kimberlite (named after the city of Kimberley, South Africa, where the first diamonds of this type of rock were found). Diamond, as well as magma from the depths of the Earth, can spread through kimberlite pipes on the surface of the Earth.
Kimberlite pipes were found on the edges of cratonic roots, as in some areas. from Canada, Siberia, Australia and South Africa. It would be logical, then, that cratonic roots should contain diamond in their makeup.
"It's circumstantial evidence, but we've pieced it all together," says Faul. "We have gone through all the different possibilities, from all angles, and it's the only one that remains a reasonable explanation."
This research was supported, in part, by the National Science Foundation.
"Reprinted with permission from MIT News"
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