Discovery of the "mother" rock from which abiotic methane is produced, produced without the action of organic substances. To say so, a research conducted by INGV in collaboration with an international team. The findings were published in Nature's Scientific Reports journal
Identified in chromitites, igneous rocks rich in metals, the "mother" rock (source-rock) of abiotic methane, a type of natural gas different from the one commonly used as an energy source and which could also be present on Mars. This was discovered by a team of researchers from the National Institute of Geophysics and Volcanology (INGV), the University of Patras, the University Brunei Darussalam and the Carnegie Institution for Science in Washington.
The results of the research Widespread abiotic methane in chromitites, were published in Nature's Scientific Reports journal Link.
“Abiotic methane”, says Monia Procesi, INGV researcher, “can originate from non-sedimentary rocks formed at great depths within the earth's crust. Unlike common methane biotic, produced by bacteria or by the degradation of organic matter, this gas is produced only by chemical reactions between hydrogen and carbon dioxide, at temperatures that can be lower than 150°C. The process takes place within geological formations, called ophiolites, which represent sections of oceanic crust brought to the outcrop on the continents as a result of tectonic processes".
In the last 30 years, scientific literature has documented the presence of abiotic methane in gaseous manifestations on the earth's surface, in water sources and underground aquifers, without however understanding from which rock this gas comes.
“To identify the exact source of the gas,” continues Procesi, “we conducted multiple analyzes on all the rocks that make up the ophiolite sequence in Greece, known as serpentinites, peridotites, chromites, gabbros, rodingites and basalts. The rocks”, explains the INGV expert, “were crushed and the gas released was analyzed with various instruments. Only chromitites have been seen to host considerable quantities of methane and other hydrocarbons such as ethane and propane, all of abiotic origin. This appears to be due to the presence of hydrogen and carbon dioxide which react with some metals, such as chromium and ruthenium".
Once identified in the chromitite the source-rock which generates abiotic methane, the research group, with further high-resolution Raman spectroscopy investigations, was able to understand that these rocks are very porous and fractured and the gas is hosted, and probably produced, precisely in these fractures, where it is most easy passage of hydrogen and carbon dioxide.
“The implications of the research are many”, explains the researcher, “and range from the balance of atmospheric methane, to the origin of life and the study of methane on rocky planets. Chromitites, in fact, may represent a new geological source of methane for the atmosphere: recently, thanks to other studies by INGV, geological emissions have been considered among the natural sources of greenhouse gases, but chromitites were not yet contemplate. Furthermore, the abiotic conversion of carbon dioxide into methane is considered a fundamental step in the origin of life, as methane is a source of energy for microbial forms, and therefore chromitites may be the place where the first prebiotic processes occurred on Earth . The study also provides a possible explanation for the presence of methane on Mars, which has rocks similar to terrestrial chromitites” concludes the expert.
Extended
G. Ethiopian, E. Ifandi, M. Nazzari, M. Procesi, B. Tsikouras, G. Ventura, A. Steele, R. Tardini, P. Szatmari
Recurring discoveries of abiotic methane in gas seeps and springs in ophiolites and peridotite massifs worldwide raised the question of where, in which rocks, methane was generated. Answers will impact the theories on life origin related to serpentinization of ultramafic rocks, and the origin of methane on rocky planets. Here we document, through molecular and isotopic analyzes of gas liberated by rock crushing, that among the several mafic and ultramafic rocks composing classic ophiolites in Greece, ie, serpentinite, peridotite, chromitite, gabbro, rodingite and basalt, only chromitites, characterized by high concentrations of chromium and ruthenium, host considerable amounts of 13C-enriched methane, hydrogen and heavier hydrocarbons with inverse isotopic trend, which is typical of abiotic gas origin. Raman analyzes are consistent with methane being occluded in widespread microfractures and porous serpentine- or chlorite-filled veins. Chromium and ruthenium may be key metal catalysts for methane production via Sabatier reaction. Chromitites may represent source rocks of abiotic methane on Earth and, potentially, on Mars.
Figure 1 - CH4 (methane) detected by Raman within the veins of the chromitites of the Vourinos ophiolite sequence.
Figure 2 - Methane inside the ophiolite rocks of Vourinos (a) and Othrys (b). Methane is found only within chromite-rich layers in the ultramafic body. Peridotites and other rocks overlying or underlying chromitites, i.e. gabbros and basalts, do not contain methane.