Springs and aquifers that originate within ophiolites, mountains comprised of old seafloor, contain highly alkaline water that often is bubbling with methane. Scientists have discovered these springs in 17 countries around the world. Existing evidence suggests that the gas forms from low-temperature chemical reactions within the rock but no one had pinpointed its exact source.
In a new paper in Scientific Reports , Deep Energy Community member Giuseppe Etiope (Istituto Nazionale di Geofisica e Vulcanologia, Italy), Reservoirs and Fluxes Community member Andrew Steele (Carnegie Institution of Washington, USA), and DCO member Peter Szatmari, (Petrobras Research and Development Center, Brazil), have identified chromitite, a chromium-rich rock that contains trace amounts of the rare element ruthenium. Chromitite, which often forms layers within ophiolites, is a source of methane and other hydrocarbons. The researchers crushed up different rocks that make up ophiolites to discover that the chromitite layer is the only one to hold significant amounts of methane. The finding that chromitites are a source of subsurface methane may have important implications for the carbon cycle and for establishing the conditions that led to the origin of life.
In a previous study, Etiope worked with DCO member Edward Young (University of California Los Angeles, USA) to analyze methane from ophiolites, using Young’s Panorama mass spectrometer . The instrument can measure tiny differences between the masses of atoms that make up a sample, which can be used to extrapolate the source of a methane gas sample. These analyses suggest that the gas in ophiolites formed from chemical reactions below 150 degrees Celsius, which point to the Sabatier reaction as the likely cause. In the Sabatier reaction, metals within rock catalyze the reaction between carbon dioxide and hydrogen, making methane. The hydrogen comes from serpentinization reactions occurring inside ophiolites, when the mantle rock peridotite interacts with groundwater.
To find the source rock where the Sabatier reaction occurs, the researchers sampled rocks that make up the layers of four ophiolites in Greece. They selected these locations because they already had measured abiotic methane from the associated springs and had access to the different rock types from outcrops and mines. Using a special mill, the researchers crushed more than 60 samples of rock, including serpentinite, peridotite, chromitite, gabbro, rodingite, and basalt, and extracted the released gas. Two labs, one at Istituto Nazionale di Geofisica e Vulcanologia, and a second at GEO-data in Germany, independently analyzed the same rocks to find the same results.
“The results are very clear,” said Etiope. “We discovered that only chromitite contains methane as well as other hydrocarbons, like ethane and propane. Once gas forms, it may migrate to rocks that act as reservoirs, and then seep to the surface through springs.”
Invariably, these hyper-alkaline springs occur near chromium mines or in areas with rocks rich in chromium and ruthenium, which act as catalysts in the Sabatier reaction. Ruthenium is especially efficient and can catalyze the reaction to make methane and other small hydrocarbons at temperatures below 150 degrees Celsius. “We analyzed ruthenium in these rocks and we see a clear correlation between the amount of methane and the amount of ruthenium present,” said Etiope.
Next, the researchers hope to repeat the experiments in similar rocks from other locations. One possible location is the Bushveld Complex in South Africa, the site of the largest ruthenium mine on Earth. Unexpected explosions have occurred there, suggesting that chromitite in the complex may yield considerable amounts of methane.
The researchers also are interested in investigating further how chromitite stores the gas. The rocks appear to be surprisingly porous and hold methane in veins and holes in the rock. Preliminary estimates suggest that as much as half of the rock's volume is pore space, but this question needs further study.
These findings expose a previously unaccounted for piece of the global carbon budget and may have implications for the location of the origins of life on Earth. The methane and hydrogen produced by these reactions are excellent sources of energy for microbes. Additionally, the Sabatier reaction converts inorganic carbon into organic compounds, which is an important pre-cellular step in the rise of cells.
Scientists have detected chromitites containing ruthenium in Martian meteorites, suggesting that they may yield methane on other planets. “If chromitites produce methane on Earth, they can do the same on Mars,” said Etiope.