CO2

From the articl Popular Mechianics they said.

"In a study published November 11 in the Proceedings of the National Academy of Sciences, Kelemen and Columbia geochemist Jurg Matter suggest the natural process of removing CO2 from the air could be accelerated 100,000 fold, enough to make a significant dent in global warming. They calculate that Oman's peridotite alone could sequester 4 billion tons of CO2 per year, one-eighth of the 30 billion tons of CO2 humans emit annually. The researchers suggest that CO2 captured from power plants and other sources could be pumped down boreholes into peridotite. Using fracturing technology borrowed from the petroleum industry to shatter the rock and expose more of its surface area, CO2 would seep into the peridotite hundreds of feet below the ground. Heat would be added initially to accelerate chemical reactions. But as new carbonate rock begins forming, the process could start feeding on itself, with new carbonate rock continually fracturing the host rock further, and the heat from the reaction supplementing the deep-Earth's heat".