Methane and serpentinization anomalies
Around fifteen years ago, geochemists started to take an active interest in the interactions of seawater with mantle rock along slow spreading mid-ocean ridges and in particular along the Mid-Atlantic Ridge. The exploratory work conducted in these zones has shown that the countless number of methane anomalies observed along the axis of the ridge are closely related to the presence of outcrops of mantle rock on the summits and walls of the rift and near the ocean floor in the axial valley. This alliance is a consequence of the hydration process of mantle rocks by reacting with seawater. Tectonic movement in effect provokes fracturing of the crust allowing seawater to penetrate to the mantle. The reaction of seawater with peridotite rocks of the mantle is a process called serpentinization and it causes an increase in the volume of rocks, physically pushing them to the outcrop accompanied by chemical reactions of significant consequence.
An example of gas composition in the fluid of Rainbow (36°14’N-MAR). In both cases, serpentinization produces large quantities of hydrogen.
From mantle minerals to oil
In the presence of water, olivine and/or pyroxene, the main mantle minerals, are oxidized during the serpentinization process. This exothermal reaction may locally induce 100 °C of excess heat. Water is reduced to the state of molecular hydrogen and oxidized iron (FeII) to iron (FeIII) in the form of magnetite. Hydrogen is produced in great quantities during serpentinization and then reacts with CO2 (released from minerals and present in seawater) to firstly generate methane and then heavier hydrocarbons (straight chain hydrocarbons, aromatic hydrocarbons, fatty acids…). This phenomenon occurs through a reaction catalysed by elements present and enriched in mantle rock such as nickel, cobalt and chrome.
This is a well-known process as it is currently used in industry to produce synthetic petroleum. This type of catalytic reaction at high temperature and pressure demonstrates that it is possible to synthesize organic molecules of abiotic origin from minerals present in the Earth’s mantle.
From observing to modeling: further scientific studies
To understand this natural process and estimate the quantities of hydrogen, methane and hydrocarbons formed, three domains of study co-exist. Firstly, oceanographers supply the field data, measures and studies of the phenomena at sea and the analysis of natural samplings.
They pursue further experiments in the lab: the scientists make peridotites interact with seawater in the presence of different catalyzers at high temperature and pressure. This helps to understand the reactive mechanisms.
Lastly, thermodynamic modeling taking into account all the chemical reactions possible in this environment, allows to predict the chemical, mineral and organic species synthesized in this hydrothermal context and to orient our in situ research.
Towards new energy resources
So the Earth has a natural source of hydrogen and gas other than that derived from the burial and maturation of organic matter in sedimentary basins…In fact, trace amounts of oil of inorganic origin have already been detected in mantle environments of mid-ocean ridges.
How much hydrogen and gas is generated abiogenically from minerals? Ongoing exploration of slow-spreading ridges and thorough knowledge of the deep ocean will be requisite to establishing preliminary estimates. The fate of these gigantic hydrogen and methane emissions resulting from serpentinization on slow spreading mid-ocean ridges and their transformation in the ocean environment are all the more subject of study as oil sources diminish and identifying new energy sources is becoming increasingly complicated.
There is also the outstanding question of the most favorable reactive spheres for their synthesis in the natural environment. Moreover, the coming years of exploration will enable scientists to verify whether frozen gas hydrates also exist on the sedimented flanks of slow spreading ocean ridges similar to those already discovered on sedimented continental margins. This hypothesis is supported by pressure and temperature conditions, the presence of hydrothermal convection resulting in water circulation through the fractured crust associated with intense production of methane through serpentinization.