Uptake of carbon dioxide by rocks!

In 2008, Kelemen and Matter concluded that the rate of natural carbonation of peridotite, a type of rock commonly found in the Earth’s mantle, can be enhanced to produce a significant CO₂ sink. They found that >1 billion tons of CO₂ per year could be consumed by the rock reacting with the CO₂ in the atmosphere to form solid carbonate minerals.

Mantle peridotite is usually found 6km below the seafloor or 40km below the land surface (Kelemen and Matter, 2008). However, sometimes the rock can be exposed when tectonic plates collide and push the rock to the surface. The exposed mantle is called ophiolite. It is composed mainly of the minerals olivine and pyroxene which react with water and CO₂ to make hydrous silicates, iron oxide, and carbonates (Kelemen and Matter, 2008).

Kelemen and Matter’s research was on the Samail ophiolite in Oman. They observed that the peridotite was crisscrossed by carbonate veins. These veins were previously believed to have been the same age as the rock. Kelemen and Matter (2008) found that the veins had a 14C age of approximately 26,000 years. They used the ages along with the volume of the carbonate veins to calculate the rate of CO₂ uptake. Therefore they estimated that the Samail ophiolite naturally absorbs 10,000 to 100,000 tons of CO₂ per year.

This image shows the white carbonate veins weathering out in peridotite north of the village Batin, Oman with a pencil for scale (Kelemen and Matter, 2008)

If this rate can be increased by 100,000 times then there is a potential for around 4 billion tons of CO₂ per year to be absorbed through carbonation of the Samail ophiolite (Kelemen and Matter, 2008). In their paper, Kelemen and Matter (2008) explain the rate of carbonation can be increased 100,000 times by increasing the depth of the weathering horizon from 15m to 3km in the peridotite by drilling and hydraulic fracture. Another method for increasing the rate of carbonation is to increase the temperature of the peridotite and inject CO₂ rich fluids. This would jump start a chain reaction which would naturally produce heat after it was started and therefore needs minimal energy input.

I think this manipulation of a natural process could work very well in absorbing the CO₂ from the atmosphere. The study by Kelemen and Matter (2008) shows that a substantial amount of CO₂ could be absorbed from just one area. If other large ophiolites, such as the ones in Papua New Guinea and New Caledonia, were manipulated in the same way then a lot of atmospheric CO₂ could be stored. This method could also be used in connection with the CCS method mentioned in a previous post because the liquid CO₂ could be injected into the peridotite and used to jump start the chain reaction. As always caution is required because of the unknown effects of enhancing carbonation. For example, the formation of solids beneath the ground could cause earthquakes but these may not be felt on the surface. Also the ophiolites may have a limit to the extent of carbonation and therefore cannot substitute a reduction in CO₂ emissions.

Kelemen, P.B, and J. Matter (2008) ‘In situ carbonationof peridotite for CO₂ storage’ PNAS, 105, 45, pp.17295-17300

www.pnas.org/cgi/doi/10.1073/pnas.0805794105

doi: 10.1073/pnas.0805794105