2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 9
Presentation Time: 10:15 AM

Some Thoughts on the Kinetics of Peridotite Carbonation


KELEMEN, Peter B. and MATTER, Jürg, Earth & Environmental Sciences, Columbia University, Lamont Doherty Earth Observatory, Palisades, NY 10964, peterk@ldeo.columbia.edu

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.