Mantle rock and carbon dioxide sequestration

The peridotite mantle sequence of ophiolites often shows signs of having been altered by processes that form calcite and magnesite (CaCO3 and MgCO3) veins. It is a mundane feature and few geologists have paid it any heed, other than to note the veining. Such theories as there are generally suggest that the veining took place at the time of obduction of the ophiolitic masses onto continental margins, which was generally accompanied by some metamorphism. Nonetheless, the veins must have taken up carbon dioxide from some reservoir, either hydrothermal fluids derived from seawater or groundwater, but ultimately from the atmosphere: there are no primary carbonates in ophiolites. Dating the veins was deemed impossible, but someone had a go at veins in the Oman ophiolite using the 14C method (Keleman, P.B. & Matter, J. 2008. In situ carbonation of peridotite for CO2 storage. Proceedings of The National Academy of Sciences of the USA, v. 105, p. 17295-17300), discovering a great surprise; the veins are very much younger than the Eocene age of ophiolite emplacement. Their ages span 1.6 to 43 ka, about the same as the period over which a surface tufa deposit formed. Calcite and magnesite form by the breakdown of olivine and clinopyroxene in the presence of slightly acid water in which CO2 is dissolved, their young ages suggesting the veins formed during weathering by rainwater, the tufa deposits probably forming through related processes. Keleman and Matter estimated the volume of veins in peridotites exposed in new road cuttings at about 1%. The 15 m thick weathering horizon in the exposed Oman peridotite therefore corresponds to about 1012 kg of CO2, which accumulated at an average rate of around 4 x107 kg of CO2 per year. If this could be increased by 100 thousand times, the Oman peridotite could sequester about 10% of anthropogenic emissions. Is that possible?

Higher temperatures could speed up the carbonation reactions. The reactions are exothermic and sustaining a temperature around 185ºC is feasible by stimulating the reactions through shallow drilling and pumping carbon dioxide and water into shattered rock. Interestingly, the reactions might be capable of limited geothermal power generation. The potential absorption by such a plant in the Oman ophiolite could be up to 1 billion tonnes of CO2, and there are many other ophiolites rich in olivine. But that is not the end of the story: other olivine breakdown reactions involving water generate hydrogen, as discovered by Australian hydrogeologist Gordon Stanger. While conducting his PhD field work in Oman as part of the Open University Oman Ophiolite Project, Stanger discovered natural springs from which hydrogen gas was bubbling (Stanger, G. 1986. The hydrogeology of the Oman mountains. Unpublished PhD thesis, The Open University, Milton Keynes, UK).


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