Ethane and heavier hydrocarbons can be synthesized under the pressure-temperature conditions of the upper mantle (the layer of Earth under the crust and on top of the core) without organic matter, Carnegie Institution’s Geophysical Laboratory with colleagues from Russia and Sweden have found.

(A. Kolesnikov and V. Kutcherov)

Experiments point to the possibility of an inorganic source of hydrocarbons at great depth in the Earth’s upper mantle, which underlies the crust at depths of about 12 to 37 miles beneath the continents.

These would be “hydrocarbons that come from simple reactions between water and rock and not just from the decomposition of living organisms,” stated Dr. Russell Hemley of the Carnegie Institution’s Geophysical Laboratory and co-author of a study published in the September 13-17, early, on-line edition of the Proceedings of the National Academy of Sciences.

With a diamond anvil cell, the scientists squeezed materials common at Earth’s surface–iron oxide (FeO), calcite (CaCO3) and water– to pressures ranging from 50,000 to 110,000 times the pressure at sea level. They heated the samples using two techniques–focused laser light and the so-called resistive heating method–to temperatures up to 2,700 degrees F (1500 degrees C). The researchers found that methane was formed by reducing the carbon in calcite over a wide range of temperatures and pressures.

Carnegie Institution news release