Some Thoughts on the Kinetics of Peridotite Carbonation

We discuss formation of ophicalcite, which Evans and Trommsdorff informally called ophi$#!t. I often interpret "fools rush in where angels fear to tread" as a call to action. Now, we're hoping that this well-trained audience will help us find the correct path. Given concern about atmospheric CO₂ and global climate, one can dream of creating ophicalcite for carbon storage. Adding 1 wt% CO₂ to the peridotite of the Samail ophiolite in Oman would consume ¼ of atmospheric CO₂.  Converting all olivine to magnesite + quartz would consume ~10¹⁷ kg of CO₂. Worldwide, there are several ophiolites this size plus seafloor peridotite exposures. Rate, not capacity, is the key.

           High pH, Ca-OH, low carbon waters form via alteration of peridotite, and precipitate travertine where they combine with atmospheric CO₂. Clarke & Fontes 1990 dated a travertine terrace in Oman, and underlying veins in peridotite, using ¹⁴C; terrace and veins were<45,000 years old. We thought carbonate veins in peridotite far from springs were Cretaceous, formed by hydrothermal alteration at an ocean ridge or during ophiolite obduction. Instead, ¹⁴C data reveal that these are also <45,000 years old. Peridotite carbonation in Oman consumes ~10⁷ kg of CO₂ per year. The increase in atmospheric CO₂ due to anthropogenic input is ~10¹³ kg per year. To provide a "significant" carbon sink, the rate in Oman must increase by a factor of 10⁴ to 10⁶.

           Drilling and hydrofracture can expose sub-surface peridotite to fluid at elevated pressure and temperature. Supplying pure CO₂ also enhances rates. Thermal models incorporating advection, diffusion and exothermic heating reveal a self-heating regime where rapid reaction sustains high temperature. Together, these could yield a factor of >10⁶ increase in carbonation rates. We are also investigating less efficient, but less expensive carbonation via reaction between shallow sea water and submarine peridotite.