University of Vienna: Following the traces of carbon in the depths of the earth

Whether as melts or solid minerals, carbonates are probably the most important carriers of carbon in the interior of our Earth. However, how carbonates react under high pressure and at different temperatures – and thus also the question of what form they take in the depths of the Earth’s mantle – is still largely unexplored.

In a joint project funded by the Austrian Science Fund (FWF), researchers from the Department of Mineralogy and Crystallography are now working with Siberian colleagues to investigate how artificially produced potassium-containing carbonates react to different pressure and temperatures. The project is also funding a PhD position at the Vienna International School for Earth and Space Sciences.

“The occurrence of carbon in the interior of the planet, its distribution between the metallic core and the Earth’s mantle, as well as the exchange of carbon stored in the depths of our Earth with the atmosphere and biosphere are currently important research questions in relation to the global carbon cycle,” explains Ronald Miletich, Professor of Mineralogy and Crystallography at the Faculty of Earth Sciences, Geography and Astronomy at the University of Vienna.

Large reservoir of carbon in the Earth’s mantle
In the so-called oxidic mantle of our planet, CO2 and carbonates are, presumably, the most essential components. “They take the form of CO2-containing fluids, melts, as well as solid carbonate mineral phases and form a large reservoir in the Earth’s mantle”, says the crystallographer. We have evidence that they are present there: “Due to smallest inclusions in natural diamonds, some of which are believed to have formed as far as the lower mantle just before the core-mantle boundary at a depth of 2,750 km, we know that these carbonates exist, and also that these dense high-pressure carbonates always have significant sodium and potassium contents”, explains Miletich.

These minerals formed under high pressure in diamonds are a rare natural evidence for the presence of CO2 in the depths of the Earth’s mantle. However, it is still unclear whether the carbonates are already enclosed as crystalline components in the depths during the growth of the diamonds, or whether the growing diamond trapped carbonate melt droplets that crystallised later in the course of the cooling. “Yet, this question is crucial to be able to determine the conditions of formation beyond doubt,” emphasises Miletich, deputy head of the Department of Mineralogy and Crystallography.

Artificial carbonates
In a new FWF Joint Project, together with researchers from the Sobolev Institute of the Siberian branch of the Russian Academy of Sciences in Novosibirsk, he is therefore investigating the stability of the crystalline phases of artificial carbonates and whether they transform into other forms that are not yet known. “We cannot reach these depths of the Earth, but with the help of our research partner in Siberia, we are able to produce these carbonates artificially and then study them in the so-called diamond anvil cell under comparable conditions”, says Miletich.

The collaboration with the Russian Academy of Sciences was initiated through this joint project. The crystallographer appreciates this, “Of course, we have known our colleagues in Novosibirsk for a long time, but rather as ‘scientific competitors’ in this field – now, however, we are pooling our interests and pulling together.”

Pooling our interests

The synthesis of the carbonates takes place in Siberia using the technique of the multi-anvil press. Afterwards, the artificially produced carbonates are examined in the diamond anvil cell using established high-pressure techniques and heating techniques that can be combined. “For this, we use X-ray diffraction methods and vibration spectroscopy, among other things”, explains Miletich. The experimental investigations in the diamond anvil cells are carried out in the laboratories both in Vienna and in Novosibirsk, supplemented by measurements with synchrotron radiation at suitable large European research institutions.

The existence of potassium-containing carbonates has been proven by studies on diamond inclusions. Potassium itself has the effect of lowering the melting point of carbonate phases, so that potassium-rich carbonates melt at lower temperatures, or that their existence as crystalline solids would suggest higher pressures. The stability of these crystalline phases, possible transformations into further forms not yet known will be investigated in this study. In addition, the so-called thermomechanical properties will be determined, that are needed to model the formation conditions.

PhD position at VISESS
The project will also fund a doctoral position at the Vienna International School for Earth and Space Sciences. “After all, this new doctoral school at our Faculty offers particularly good opportunities for cooperation across disciplinary boundaries – either with lithosphere research, Earth sciences or even planetary geology”, emphasises the crystallographer.

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