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.
Formation and petrology
Serpentinization is a form of low-temperature (0 to ~600 °C) metamorphism of ferromagnesian minerals in mafic and ultramafic rocks, such as dunite, harzburgite, or lherzolite. These are rocks low in silica and composed mostly of olivine (), pyroxene (), and chromite (approximately ). Serpentinization is driven largely by hydration and oxidation of olivine and pyroxene to serpentine group minerals (antigorite, lizardite, and chrysotile), brucite (), talc (), and magnetite (). Under the unusual chemical conditions accompanying serpentinization, water is the oxidizing agent, and is itself reduced to hydrogen, . This leads to further reactions that produce rare iron group native element minerals, such as awaruite () and native iron; methane and other hydrocarbon compounds; and hydrogen sulfide.
During serpentinization, large amounts of water are absorbed into the rock, increasing the volume, reducing the density and destroying the original structure. The density changes from with a concurrent volume increase on the order of 30-40%. The reaction is highly exothermic, releasing up to per mole of water reacting with the rock, and rock temperatures can be raised by about , providing an energy source for formation of non-volcanic hydrothermal vents. The hydrogen, methane, and hydrogen sulfide produced during serpentinization are released at these vents and provide energy sources for deep sea chemotroph microorganisms.
Formation of serpentine minerals
Olivine is a solid solution of forsterite, the magnesium endmember of , and fayalite, the iron endmember, with forsterite typically making up about 90% of the olivine in ultramafic rocks. Serpentine can form from olivine via several reactions:
Reaction 1a tightly binds silica, lowering its chemical activity to the lowest values seen in common rocks of the Earth's crust. Serpentinization then continues through the hydration of olivine to yield serpentine and brucite (Reaction 1b). The mixture of brucite and serpentine formed by Reaction 1b has the lowest silica activity in the serpentinite, so that the brucite phase is very important in understanding serpentinization. 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.
A similar suite of reactions involves pyroxene-group minerals:
Reaction 2a quickly comes to a halt as silica becomes unavailable, and Reaction 2b takes over. When olivine is abundant, silica activity drops low enough that talc begins to react with olivine:
This reaction requires higher temperatures than those at which brucite forms.
The final mineralogy depends both on rock and fluid compositions, temperature, and pressure. Antigorite forms in reactions at temperatures that can exceed during metamorphism, and it is the serpentine group mineral stable at the highest temperatures. Lizardite and chrysotile can form at low temperatures very near the Earth's surface.
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, allowing serpentinization to produce highly reducing conditions. Under these conditions, water is capable of oxidizing ferrous () ions in fayalite. The process is of interest because it generates hydrogen gas:
However, studies of serpentinites suggest that iron minerals are first converted to ferroan brucite, that is, brucite containing , which then undergoes the Schikorr reaction in the anaerobic conditions of serpentinization: