Magnesite as a Deep Carbon Reservoir

Researchers aimed to determine the oxygen fugacity at which diamonds oxidize to magnesite in the presence of high-pressure mantle minerals, such as wadsleyite, ferropericlase and perovskite.

magnesiteMantle rocks show evidence of minerals made of carbon with different oxidation states  -- minerals such as diamond, carbonates and carbides - with the stability of each depending on the redox conditions occurring in Earth's interior. Although many experimental studies performed in recent decades have provided important information on the pressure and temperature conditions critical for the origin of CO2-rich magmas (kimberlitic and carbonatitic), the role played by the oxygen fugacity in experimental petrology has not been considered.

Experiments performed by Vincenzo Stagno (GL-CIW) during his research at the Bayerisches Geoinstitut (Bayreuth) and in collaboration with the Geodynamic Research Centre (Ehime University) aimed to determine the oxygen fugacity at which diamonds oxidize to magnesite (MgCO3) in the presence of high-pressure mantle minerals, such as wadsleyite, ferropericlase and perovskite. Performed in multi-anvil apparatus, the experiments used sintered diamond anvils to reproduce Earth's interior pressures corresponding to depths of ~1250 km where temperatures are above 1600oC.

The results of this study [1], combined with current knowledge of the mantle redox profile, indicate that oxidation - and therefore mobilization - of carbon likely occurs at relatively shallow mantle depths <200 km. This finding infers that carbon is immobilized as diamond and can reside in Earth's interior for millions to billions of years.

It is known that oxygen fugacity decreases with depth until metallic iron forms in peridotitic assemblages at about 300 km. However, the carbonate inclusions (solid and melt) found in natural diamonds from the transition zone and lower mantle represent unique evidence that deep portions of the mantle have undergone oxidation phenomena - likely due to fluid subduction into Earth's interior.

The authors' experimental results, applicable to inclusions of solid carbonate in diamonds from 660 km and deeper, prove for the first time that oxygen fugacity has increased by at least 2 log units with respect to previous predictions, causing oxidation of minerals such as perovskite with a significant ferric iron content of ~70%. These conditions would make magnesite a possible candidate for a deep carbon reservoir in Earth and provide an estimate of ferric iron in mantle minerals involved in oxidation-reduction phenomena.

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