Diving into the Earth's interior allows scientists to unlock the secrets of diamond formation

Understanding the global carbon cycle provides scientists with vital clues to the livability of the planet.

This is why the Earth has a stable mild climate and a low carbon dioxide atmosphere compared to that of Venus, for example, which is in a drifting greenhouse state with high surface temperatures and a high temperature. atmosphere thick in carbon dioxide.

A major difference between Earth and Venus is the existence of active plate tectonics on Earth, which makes our environment unique within our solar system.

But the atmosphere, the oceans and the earth's crust are only part of the story. The mantle, which accounts for 75% of the Earth's volume, potentially contains more carbon than all other reservoirs combined.

Carbon – one of the essential building blocks of organic life – is absorbed into the Earth's interior by subduction, where it significantly lowers the melting point of the solid mantle, forming carbonated melts (rocks carbon-rich melts) in the shallow mantle, thus feeding the surface volcanoes. Carbonated minerals can also be transported much deeper into the Earth, reaching the lower mantle, but the rest is uncertain.

Answering this question is fraught with difficulty – the conditions deep in the Earth are extreme and samples taken in the mantle are rare. The solution is to recreate these conditions in the laboratory with the help of a sophisticated technology.

That's what a team of experimental geoscientists from the University of Bristol did. Their results, published in open access in Letters of Earth and Planetary Science, discover new clues about what happens to carbonate minerals when they are transported in the mantle by subduction of the oceanic crust (where one of the tectonic plates of the Earth slides one under the other).

Their findings uncovered a carbonate subduction barrier beyond 1000 km deep, where it reacts with silica in the oceanic crust to form diamonds that are stored in the deep Earth at geologic time scales. .

Dr. James Drewitt of the Faculty of Earth Sciences explains, "Do carbonate minerals remain stable in the lower mantle of the Earth and, if not, what changes in pressure / temperature do we need to trigger reactions between the minerals and what they look like are the questions we wanted to find answers to – and the only way to get those answers was to replicate the conditions of the Earth's interior. "

Dr. Drewitt and his team have subjected synthetic carbonate rocks to very high pressures and temperatures, comparable to deep terrestrial conditions of up to 90 GPa (about 900,000 atmospheres) and 2,000 degrees C with the help of of a laser-etched diamond anvil cell. They found that the carbonate remained stable to a depth of 1,000 to 1,300 km, almost halfway to the core.

Under these conditions, the carbonate then reacts with the surrounding silica to form a mineral called bridgmanite, which forms most of the Earth's mantle. The carbon released by this reaction is in the form of solid carbon dioxide. As the surrounding warm mantle warms the subducted slab, this solid carbon dioxide decomposes to form super-deep diamonds.

Dr. Drewitt adds, "The super-deep diamonds may eventually come to the surface in ascending mantle plumes, and this process could represent one of the super-deep diamond sources we find on the surface and that is the only evidence direct composition of the deep earth.

"It's exciting, because the deepest men ever to drill are about 12 km, less than half the depth of the Earth's crust. It is a bit ridiculous compared to the enormous scale of the Earth's mantle, which reaches almost 3,000 km deep.

The team used a diamond anvil cell to generate pressures equivalent to those found at these depths, loading the specimens under a microscope into a pressure chamber drilled in a metal seal, which is then compressed between diamond-shaped anvils cut into brilliant of precious quality. The crystal structure of these samples was then analyzed by X-ray diffraction in the British Oxfordshire synchrotron facility.

Dr. Drewitt is now considering applying these high-pressure, high-temperature experiments and advanced computer simulation techniques to other minerals and materials, adding, "In addition to carbon, there are potentially several oceans of deeply transported water. in the coat; once released, this will cause the melting of the upper and lower mantle of the Earth.

"However, we can not adequately test or understand current models of the dynamic behavior of this water-rich melting rock because we know neither their composition nor their physical properties." Experiments under extreme conditions and advanced computer simulations we are currently working on will help to solve these problems. "

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More information:
James W. E. Drewitt et al. The fate of the carbonate in the oceanic crust is subducted in the lower mantle of the Earth, Letters of Earth and Planetary Science (2019). DOI: 10.1016 / j.epsl.2019.01.041