serpentinization

Serpentinization is a geological hydrothermal alteration process in which ultramafic rocks, especially peridotite and dunite, react with water to form serpentine group minerals. The principal rock-forming minerals, olivine and pyroxene, are hydrated and altered to serpentine minerals such as antigorite, lizardite, and chrysotile. The reaction typically also yields secondary minerals such as magnetite, brucite, and various phyllosilicates, depending on composition and conditions.

This process occurs at low to moderate temperatures and pressures, commonly in suboceanic and subduction zone environments where seawater or hydrothermal fluids infiltrate mantle rocks. Serpentinization is exothermic and often produces highly reducing, alkaline fluids, with pH values frequently above 9. Molecular hydrogen is generated, and in the presence of carbon sources, this can drive the production of methane and support chemosynthetic microbial communities.

Serpentinization has important geological implications: it alters rock properties, influencing mechanical strength, permeability, and seismic velocities; it sustains hydrothermal systems and can drive localized buoyant fluid circulation. The hydrogen-rich fluids are of interest for models of early Earth chemistry and the subsurface biosphere.

Occurrences: it is widely documented on the seafloor at mid-ocean ridges in ultramafic sections of oceanic crust, in ophiolites, and in some subduction zones where mantle rocks are exposed. The process is key to understanding alkaline vent systems and the deep carbon cycle.