Serpentinization

serpentinite from Styria Province, Austria Serpentinization is a hydration and metamorphic transformation of ferromagnesian minerals, such as olivine and pyroxene, in mafic and ultramafic rock to produce serpentinite. Minerals formed by serpentinization include the serpentine group minerals (antigorite, lizardite, chrysotile), brucite, talc, Ni-Fe alloys, and magnetite. The mineral alteration is particularly important at the sea floor at tectonic plate boundaries.

serpentinite from Styria Province, Austria]] Serpentinization is a hydration and metamorphic transformation of ferromagnesian minerals, such as olivine and pyroxene, in mafic and ultramafic rock to produce serpentinite. Minerals formed by serpentinization include the serpentine group minerals (antigorite, lizardite, chrysotile), brucite, talc, Ni-Fe alloys, and magnetite. The mineral alteration is particularly important at the sea floor at tectonic plate boundaries.

Breakdown of diopside and formation of rodingites

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. These fluids are highly reactive and may transport calcium and other elements into surrounding mafic rocks. Fluid reaction with these rocks may create metasomatic reaction zones enriched in calcium and depleted in silica, called rodingites.

Formation of magnetite and hydrogen

In most crustal rock, the chemical activity of oxygen is prevented from dropping to very low values by the fayalite-magnetite-quartz (FMQ) buffer. The very low chemical activity of silica during serpentinization eliminates this buffer, creating highly reducing conditions that allow water to oxidize ferrous ( ) ions in fayalite. This reaction modifies minerals and liberates hydrogen gas: Studies of serpentinites suggest that in nature iron minerals are first converted to ferroan brucite, that is, brucite containing , which then undergoes the Schikorr reaction in the anaerobic conditions of serpentinization: Maximum reducing conditions, and the maximum rate of production of hydrogen, occur when the temperature of serpentinization is between and when fluids are carbonate undersaturated. If the original ultramafic rock (the protolith) is peridotite, which is rich in olivine, considerable magnetite and hydrogen are produced. When the protolith is pyroxenite, which contains more pyroxene than olivine, iron-rich talc is produced with no magnetite and only modest hydrogen production. Infiltration of silica-bearing fluids during serpentinization can suppress both the formation of brucite and the subsequent production of hydrogen. Chromite present in the protolith will be altered to chromium-rich magnetite at lower serpentinization temperatures. At higher temperatures, it will be altered to iron-rich chromite (ferrit-chromite). During serpentinization, the rock is enriched in chlorine, boron, fluorine, and sulfur. Sulfur will be reduced to hydrogen sulfide and sulfide minerals, though significant quantities are incorporated into serpentine minerals, and some may later be reoxidized to sulfate minerals such as anhydrite. The sulfides produced include nickel-rich sulfides, such as mackinawite.

Methane and other hydrocarbons

Laboratory experiments have confirmed that at a temperature of and pressure of 500 bars, olivine serpentinizes with release of hydrogen gas. Abiotic methane formation related to serpentinization has been shown in much lower temperatures and pressure systems, associated with ultramafic rocks in ophiolites, and in continental systems such as the greenstone belts of the Canadian Shield. In addition, methane and complex hydrocarbons are formed through reduction of carbon dioxide. The process may be catalyzed by magnetite formed during serpentinization. One reaction pathway is:

Metamorphism at higher pressure and temperature

Lizardite and chrysotile are stable at low temperatures and pressures, while antigorite is stable at higher temperatures and pressure. Its presence in a serpentinite indicates either that serpentinization took place at unusually high pressure and temperature or that the rock experienced higher grade metamorphism after serpentinization was complete. Infiltration of -bearing fluids into serpentinite causes distinctive talc-carbonate alteration. Brucite rapidly converts to magnesite and serpentine minerals (other than antigorite) are converted to talc. The presence of pseudomorphs of the original serpentinite minerals shows that this alteration takes place after serpentinization. Serpentinite may contain chlorite (a phyllosilicate mineral), tremolite (Ca 2 (Mg 5.0-4.5 Fe 2+ 0.0-0.5 )Si 8 O 22 (OH) 2 ), and metamorphic olivine and diopside (calcium-rich pyroxene). This indicates that the serpentinite has been subject to more intense metamorphism, reaching the upper greenschist or amphibolite metamorphic facies. Above about , antigorite begins to break down. Thus serpentinite does not exist at higher metamorphic facies.

Extraterrestrial production of methane by serpentinization

The presence of traces of methane in the atmosphere of Mars has been hypothesized to be a possible evidence for life on Mars if methane was produced by bacterial activity. Serpentinization has been proposed as an alternative non-biological source for the observed methane traces. 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. 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. A model suggests that the ocean on Enceladus has an alkaline pH of 11–12. The high pH is interpreted to be a key consequence of serpentinization of chondritic rock, that leads to the generation of , a geochemical source of energy that can support both abiotic and biological synthesis of organic molecules.

Environment of formation

of the Gros Morne National Park, Newfoundland (Labrador, Canada). Ophiolites characteristically have a serpentinite component.]] Serpentinization occurs at mid-ocean ridges, in the forearc mantle of subduction zones, in ophiolite packages, and in ultramafic intrusions.

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