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Diamonds and carbonates in the Earth’s interior: the deep carbon cycle

By Vincenzo Stagno (25.03.2011)

Industrialisation has led to the release of large amounts of CO2 into the atmosphere over the last 100 years. The cycling of this CO2 through the atmosphere, oceans and biosphere is a major concern to humanity.  However, the amount of carbon stored in the mantle, the silicate portion of the Earth’s interior, is probably much greater than in these surface reservoirs. The processes of subduction and volcanic eruption lead to the cycling of carbon between the surface and the mantle, which consequently forms an important reservoir in the Earth’s carbon cycle. The subduction into the mantle of oceanic crust, which contains carbon as a result of sedimentation and interaction with seawater, is, for example, an important natural form of carbon sequestration.

The concentration and residence time of carbon in the mantle will depend on the form in which carbon is stable at the pressures, temperatures and redox conditions of the interior. Redox conditions will control whether carbon exists in reduced elemental form as graphite or diamond or as oxidised carbonate or CO2-bearing species. In the form of diamond or graphite carbon is immobile and can reside in the interior of the Earth for millions to billions of years. However oxidation to CO2-bearing species or carbonate can promote the formation of magmas, which can erupt to the surface, effectively mobilising and removing carbon from the mantle. This oxidation process therefore has important implications for the deep carbon cycle.

Fig.1. Left: A diamond from Brazil observed using an optical microscope. Right: Bright Field image showing many mineral inclusions with different contrast and relief (1 is solid carbonate).[Bildunterschrift / Subline]: Fig.1. Left: A diamond from Brazil observed using an optical microscope. Square indicates the selected area for investigation by Transmission Electron Microscopy. Right: Bright Field image showing many mineral inclusions with different contrast and relief (1 is solid carbonate). The arrow shows nano-inclusions oriented parallel to the investigated micro-inclusions.

To determine the conditions at which elemental carbon becomes oxidised to form carbonate minerals and carbonate-bearing melts in the Earth I have performed experiments in a multianvil apparatus at the Bayerisches Geoinstitut (University of Bayreuth). The multianvil press allows us to reproduce the pressures and temperatures in the Earth’s interior to depths of 700 km, where temperatures are above 1600°C. My results describe the stability field of graphite and diamond with respect to the redox state of the mantle. By comparing this stability field with the proposed redox state of the interior the stable form of carbon as a function of depth and environment can be determined. The results indicate that oxidation, and therefore mobilisation, of carbon must occur at relatively shallow depths in the mantle <200 km, inferring that carbon is immobilised as diamond throughout most of the interior.

These experimental observations were also supported by the investigation of mineral inclusions within natural diamonds from Brazil (Fig. 1). We identified solid carbonates (of micrometer size) within diamond grains and using a model determined from my experimental studies we could precisely determine the redox conditions at which the diamonds formed. The results can be used to understand the diamond formation process in the Earth.

Fig.2. Left: Vincenzo Stagno within a group of colleagues of the Geodynamic Research Centre (Ehime University, Japan). Right: Mineral assemblage under lower mantle conditions.[Bildunterschrift / Subline]: Fig.2. Left: Vincenzo Stagno within a group of colleagues of the Geodynamic Research Centre (Ehime University, Japan) carrying out experiments with a multianvil apparatus at the SPring-8 synchrotron facility in Japan simulating the pressure/temperature conditions within the Earth’s lower mantle. Right: Mineral assemblage under lower mantle conditions with: magnesite (mst), Fe-periclase (Fp), perovskite (Pv), diamond (C) and small metal grains, as a run product of a high pressure experiment observed by Scanning Electron Microscopy.

During my graduate studies I had the unique opportunity to perform experiments during a three months stay at the Geodynamics Research Center (GRC) at Ehime University in Japan, in collaboration with a highly experienced staff of researchers (Fig. 2). Using a modified multianvil press that employs sintered diamond anvils I was able to perform experiments at pressures compatible with the Earth’s mid lower mantle (~1200 km) by which we obtained information about carbonate stability in the Earth’s lower mantle. My results indicate that diamond is likely to be the main stable form of carbon at conditions deep into the lower mantle.

My doctoral research was supported by the EU Marie Curie project “Atomic to Global” for two years and in the final year by a grant from the DFG. During my studies I appreciated the high scientific level and stimulating environment provided by the Geoinstitut in Bayreuth and in particular it was a great advantage to be able to collaborate and discuss with colleagues within the Elite Network of Bavaria. For the future I would appreciate to be able to promote further collaboration with the Japanese research institute.

Vincenzo Stagno
Vincenzo Stagno
* 1979, Palermo, Italy

  • 2011
  • PhD in Experimental Geosciences at Bavarian Geoinstitute, University of Bayreuth.
  • 2006
  • Master degree (' Laurea') in Geology at Universita' degli Studi, Palermo
  • 1998
  • High school degree ('Maturita') at Liceo Scientifico ‘A.Einstein’, Palermo