Serpentinization is a process where water reacts with minerals rich in iron and magnesium, such as olivine and pyroxene, in types of rock called mafic and ultramafic. This reaction changes the minerals into a new type of rock called serpentinite. Minerals formed during this process include antigorite, lizardite, chrysotile (all part of the serpentine group), brucite, talc, Ni-Fe alloys, and magnetite. This mineral change is especially significant in areas where tectonic plates meet on the ocean floor.

Formation and petrology

Serpentinization is a type of low-temperature (0 to about 600 °C) change in rocks that contain minerals rich in iron and magnesium, such as dunite, harzburgite, or lherzolite. These rocks have little silica and are mostly made of olivine ((Mg, Fe)₂SiO₄), pyroxene (XY(Si,Al)₂O₆), and chromite (FeCr₂O₄). During serpentinization, water reacts with olivine and pyroxene to form new minerals, including serpentine (antigorite, lizardite, chrysotile), brucite (Mg(OH)₂), talc (Mg₃Si₄O₁₀(OH)₂), and magnetite (Fe₃O₄). Water acts as an oxidizing agent, and in the process, it is reduced to hydrogen gas (H₂). This reaction also creates rare iron minerals, such as awaruite (Ni₃Fe), native iron, methane, other hydrocarbons, and hydrogen sulfide.

Serpentinization causes rocks to absorb large amounts of water, increasing their volume and reducing their density. The density of the rock decreases from 3.3 to 2.5 g/cm³ (0.119 to 0.090 lb/cu in), and the volume increases by about 30-40%. The reaction releases heat, up to 40 kilojoules (9.6 kcal) per mole of water, raising rock temperatures by about 260 °C (500 °F). This heat can power non-volcanic hydrothermal vents. Hydrogen, methane, and hydrogen sulfide released at these vents provide energy for deep-sea microorganisms.

Olivine is a mixture of forsterite (magnesium-rich) and fayalite (iron-rich), with forsterite making up about 90% of olivine in ultramafic rocks. Serpentine forms from olivine through reactions that tightly bind silica, lowering its chemical activity. Another reaction produces serpentine and brucite. Brucite is important in understanding serpentinization but is often mixed with serpentine, making it hard to identify without special tools like X-ray diffraction.

Similar reactions occur with pyroxene minerals. When olivine is abundant, silica activity drops, allowing talc to react with olivine. This reaction requires higher temperatures than brucite formation.

The final minerals in the rock depend on the rock’s and fluid’s composition, temperature, and pressure. Antigorite forms at high temperatures (over 600 °C) and is the serpentine mineral stable at the highest temperatures. Lizardite and chrysotile form at lower temperatures near Earth’s surface.

Ultramafic rocks often contain calcium-rich pyroxene (diopside), which breaks down during serpentinization. This increases the pH of fluids and their calcium content, making them reactive. These fluids can alter surrounding mafic rocks, creating calcium-rich, silica-poor zones called rodingites.

In most rocks, the chemical activity of oxygen is controlled by the FMQ buffer. During serpentinization, low silica activity removes this buffer, creating highly reducing conditions. Water then oxidizes iron in fayalite, producing hydrogen gas.

Studies show that iron minerals in serpentinites first form ferroan brucite (brucite with Fe(OH)₂), which then reacts in anaerobic conditions through the Schikorr reaction. Maximum hydrogen production occurs between 200 and 315 °C (392 and 599 °F) when fluids lack carbonate. If the original rock is peridotite (rich in olivine), magnetite and hydrogen are produced. If the rock is pyroxenite (rich in pyroxene), iron-rich talc forms with little hydrogen. Silica-rich fluids can reduce brucite and hydrogen production.

Chromite in the original rock changes to chromium-rich magnetite at lower temperatures or to iron-rich chromite at higher temperatures. Serpentinization increases chlorine, boron, fluorine, and sulfur in the rock. Sulfur becomes hydrogen sulfide and sulfide minerals, though some sulfur is stored in serpentine. Later, sulfides may oxidize to sulfate minerals like anhydrite. Nickel-rich sulfides, such as mackinawite, are also produced.

Laboratory experiments show that olivine serpentinizes at 300 °C (572 °F) and 500 bars of pressure, releasing hydrogen gas. Methane and complex hydrocarbons form from carbon dioxide reduction. Magnetite formed during serpentinization may catalyze these reactions.

Lizardite and chrysotile form at low temperatures and pressures, while antigorite forms at higher temperatures and pressures. Antigorite’s presence in a serpentinite suggests high-pressure or high-temperature conditions during serpentinization or later metamorphism.

Infiltration of CO₂-rich fluids into serpentinite creates talc-carbonate alteration, converting brucite to magnesite and other serpentine minerals to talc. Pseudomorphs of original minerals show this alteration occurred after serpentinization.

Serpentinite may contain chlorite, tremolite (Ca₂(Mg₅₋₄.₅Fe₀₋₀.₅)Si₈O₂₂(OH)₂), and metamorphic olivine or diopside. These minerals indicate the rock underwent intense metamorphism, reaching the upper greenschist or amphibolite facies.

Above 450 °C (842 °F), antigorite breaks down, so serpentinite does not exist at higher metamorphic grades.

Traces of methane in Mars’ atmosphere may suggest life if produced by bacteria. Serpentinization is proposed as a non-biological source. In 2022, analysis of the ALH 84001 meteorite showed its organic matter formed via serpentinization, not life.

Data from the Cassini probe suggests Saturn’s moon Enceladus has a liquid water ocean with an alkaline pH (11–12). This high pH is linked to serpentinization of chondritic rock, producing hydrogen gas (H₂), a potential energy source for life.

Environment of formation

Serpentinization happens in several places, including mid-ocean ridges, the forearc mantle of subduction zones, ophiolite packages, and ultramafic intrusions.

This process is very suitable at slow to ultraslow spreading mid-ocean ridges. At these locations, the Earth's crust stretches rapidly compared to the amount of magma produced, bringing ultramafic mantle rock close to the surface. Fractures in the rock allow seawater to enter, which helps serpentinization occur.

At slow spreading mid-ocean ridges, serpentinization can shift the seismic Moho discontinuity to the area where serpentinization happens, instead of at the base of the crust as usual. The Lanzo Massif in the Italian Alps has a clear serpentinization front that might be a remnant of the seismic Moho.

Serpentinization is important in subduction zones because it strongly influences the water cycle and geodynamics there. In these areas, mantle rock cools to temperatures where serpentinite is stable, and large amounts of fluid are released from the subducting slab into the ultramafic mantle rock. Evidence of serpentinization in the Mariana Islands is shown by active serpentinite mud volcanoes. These volcanoes sometimes erupt xenoliths of harzburgite and, less often, dunite, which provide information about the original rock type.

Serpentinization reduces the density of the original rock, which can cause uplift or bring serpentinites to the surface. This process is seen in the Presidio of San Francisco, where serpentinite is exposed after subduction stopped.

Serpentinized ultramafic rock is common in ophiolites. Ophiolites are pieces of oceanic lithosphere that have been pushed onto continents through a process called obduction. They usually have layers of serpentinized harzburgite (sometimes called alpine peridotite in older texts), hydrothermally altered diabases and pillow basalts, and deep water sediments containing radiolarian ribbon chert.

Seismic wave studies can identify large areas of serpentinite in the crust and upper mantle because serpentinization greatly affects shear wave velocity. More serpentinization leads to slower shear wave speeds and higher Poisson's ratios. Studies confirm that serpentinization is widespread in forearc mantle. This process can create an inverted Moho discontinuity, where seismic velocity drops suddenly at the crust-mantle boundary, the opposite of normal behavior. Serpentinite is very flexible, forming an aseismic zone in the forearc where serpentinites move at stable plate speeds. The presence of serpentinite may limit how deep megathrust earthquakes can occur, as it prevents rupture into the forearc mantle.