Serpentinization
Serpentinization is a hydration and metamorphic transformation of ferromagnesian minerals, such as olivine and pyroxene, in mafic and ultramafic rock to produce serpentinite.
[3][4] Serpentinization is a form of low-temperature (0 to ~600 °C) [5] metamorphism of ferromagnesian minerals in mafic and ultramafic rocks, such as dunite, harzburgite, or lherzolite.
[1][6] During serpentinization, large amounts of water are absorbed into the rock, increasing the volume, reducing the density and destroying the original structure.
[14] However, the brucite is often blended in with the serpentine such that it is difficult to identify except with X-ray diffraction, and it is easily altered under surface weathering conditions.
[18] Ultramafic rocks often contain calcium-rich pyroxene (diopside), which breaks down according to the reaction: This raises both the pH, often to very high values, and the calcium content of the fluids involved in serpentinization.
[19] In most crustal rock, the chemical activity of oxygen is prevented from dropping to very low values by the fayalite-magnetite-quartz (FMQ) buffer.
[20] The very low chemical activity of silica during serpentinization eliminates this buffer, creating highly reducing conditions[14] that allow water to oxidize ferrous (Fe2+) ions in fayalite.
This reaction modifies minerals and liberates hydrogen gas:[1][21][22] Studies of serpentinites suggest that in nature iron minerals are first converted to ferroan brucite, that is, brucite containing Fe(OH)2,[23] which then undergoes the Schikorr reaction in the anaerobic conditions of serpentinization:[24][25] Maximum reducing conditions, and the maximum rate of production of hydrogen, occur when the temperature of serpentinization is between 200 and 315 °C (392 and 599 °F)[26] and when fluids are carbonate undersaturated.
[33][34] In 2022 it was reported that microscopic examination of the ALH 84001 meteorite, which came from Mars, shows that indeed the organic matter it contains was formed by serpentinization, not by life processes.
[35][36] Using data from the Cassini probe flybys obtained in 2010–12, scientists were able to confirm that Saturn's moon Enceladus likely has a liquid water ocean beneath its frozen surface.
[37] The high pH is interpreted to be a key consequence of serpentinization of chondritic rock, that leads to the generation of H2, a geochemical source of energy that can support both abiotic and biological synthesis of organic molecules.
[37][38] Serpentinization occurs at mid-ocean ridges, in the forearc mantle of subduction zones, in ophiolite packages, and in ultramafic intrusions.
Xenoliths of harzburgite and (less commonly) dunite are occasionally erupted by the mud volcanoes, giving clues to the nature of the protolith.
[47] The serpentinization can produce an inverted Moho discontinuity, in which seismic velocity abruptly decreases across the crust-mantle boundary, which is the opposite of the usual behavior.
