“The results demonstrate that methane readily forms by the reaction of marble with iron-rich minerals and water under conditions typical in Earth’s upper mantle,” said Laurence Fried, of Livermore’s Chemistry and Materials Science Directorate. “This suggests that there may be untapped methane reserves well below Earth’s surface. Our calculations show that methane is thermodynamically stable under conditions typical of Earth’s mantle, indicating that such reserves could potentially exist for millions of years.” The study is published in the Sept. 13-17 early, online edition of the PNAS.
The mantle is a dense, hot layer of semi-solid rock approximately 2,900 kilometers thick. The mantle, which contains more iron, magnesium and calcium than the crust, is hotter and denser because temperature and pressure inside Earth increase with depth. Because of the firestorm-like temperatures and crushing pressure in Earth’s mantle, molecules behave very differently than they do on the surface. “When we looked at the samples under these pressures and temperatures, they revealed optical changes indicative of methane formation,” Fried said. “At temperatures above 2,200 degrees Fahrenheit, we found that the carbon in calcite formed carbon dioxide rather than methane. This implies that methane in the interior of Earth might exist at depths between 100 and 200 kilometers. This has broad implications for the hydrocarbon reserves of the planet and could indicate that methane is more prevalent in the mantle than previously thought. Due to the vast size of Earth’s mantle, hydrocarbon reserves in the mantle could be much larger than reserves currently found in Earth’s crust.”