Some samples of peridotites that we have collected over many dredges are not really peridotites, but what are called serpentinites. Serpentinites are peridotites that have been hydrated by interaction with seawater and / or fluid circulating in the oceanic crust. This hydration process called serpentinization, is to transform the magmatic ferromagnesian minerals of the peridotite, olivine and pyroxene, a new mineral, serpentine. Serpentine is a magnesium silicate hydrate (Mg 3 If 2 O 5 (OH) 4 ), iron content in olivine initial will be used to form an iron oxide, magnetite (Fe 3 O 4 ). The two newly formed minerals will fit together into a mesh texture characteristic of serpentinite (green area with fine black veins of magnetite in the picture), thereby changing the primary texture of the peridotite.
The serpentinization reaction is exothermic and the heat released by serpentinization total of 1 kg of peridotite is 250 joules, which can raise the temperature one liter of water about 50 degrees in conditions of ordinary temperature and pressure. In addition, serpentinization is a process that, in addition lead to a transformation of the rock mineralogy initial changes the physical properties (density, velocity of propagation of seismic waves, magnetic properties, rheology ,...). In fact, during serpentinization, the volume of the rock increases by 30%, thus decreasing the density of 3300 kg / m 3 for a fresh peridotite at 2600 kg / m 3 for serpentinite.
same time, the speed propagation of seismic waves decreases during serpentinization: about 8 km / s in fresh peridotites, about 5.5 km / s in the serpentinites. The formation of magnetite during serpentinization is accompanied by a sharp increase in magnetic susceptibility of the rock, ie its ability to magnetized. Finally, the serpentine is less resistant to deformation than olivine or pyroxene. Indeed, 15-20% serpentine in peridotite and sufficient to reduce the resistance of rock deformation, and may in particular facilitate the movement of faults.
A final consequence of serpentinization, linked to the formation of magnetite, is the production of hydrogen (H 2 ). This hydrogen, often associated with methane (CH 4 ), is found in high concentrations in fluids from hydrothermal systems located on massive serpentinized peridotite along slow spreading ridges. The hydrogen and methane can then be used as an energy source by microorganisms colonizing the hydrothermal system.
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