Serpentinization

Synonym

Serpentinite, Serpentinitization

Required Geological Setting

Accretionary complexes, Serpentine mélange

Occurrence

Rock forming minerals in serpentinite formed by hydration alteration of peridotite. It is considered that peridotite is hydrated by reacting with meteoric or marine water under ground or at near ground surface as serpnetinite is observed near subduction zones and mid-oceanic ridges where peridotite is contacting with meteoric and/or marine water. Serpentinite is also observed along tectonic lines as a matrix of mélange containing high-pressure metamorphic rocks. Protolish of serpentinite altered from harzburgite may be distinguished by bastite pseudomorphs after enstatite phenocrysts. Ni, Cr, and Fe are contained in forsterite in peridotite, whereas they are not in its alteration product, serpentine. Thus Ni, Cr, and Fe-bearing sulfide minerals are formed in serpentinite in many cases. Formation reactions of serpentinite from major constituents of peridotite such as forsterite and orthopyroxene (enstatite) are considered as follows:

2Mg₂SiO₄ (Forsterite) + 3H₂O (Water) = Mg₃Si₂O₅(OH)₄ (Serpentine) + Mg(OH)₂ (Brucite)

Mg₂SiO₄ (Forsterite) + MgSiO₃ (Enstatite) + 2H₂O (Water) = Mg₃Si₂O₅(OH)₄ (Serpentine)

Magnetite is formed from the fayalite component in olivine

3(Mg_0.9,Fe_0.1)₂SiO (Forsterite/Fayalite) + 4.1H₂O (Water) = 1.5Mg₃Si₂O₅(OH)₄ (Serpentine) + 0.9Mg(OH)₂ (Brucite) + 0.2Fe₃O₄ (Magnetite) + 0.4H⁺

Reductive and hyperalkaline fluid is produced by this reaction. This fluid serpentinizes clinopyroxene (diopside) in peridotite.

4Mg₂SiO₄ (Forsterite) + CaMgSi₂O₆ (Diopside) + 5H₂O (Water) + 2H⁺ = 3Mg₃Si₂O₅(OH)₄ (Serpentine) + Ca²⁺

Ca in peridotite is not a major component of minerals of serpentinite formed, and Ca is released in the fluid. As the result, released fluid by serpentinization is reductive, hyperalkaline, and rich in Ca. Rodingite and reductively altered serpentinite is fomed by this reductive, hyperalkaline, Ca-rich fluid. Carbonate minerals such as calcite associated with serpentinite is formed by mixing of this Ca-rich fluid with seawater.

Mineral Assemblages

[ Degree of serpentinization ]

[ Low ] Antigorite, Chrysotile, Lizardite, Pyrrhotite, Pentlandite, Magnetite, (Chalcopyrite)

[ Moderate ] Antigorite, Chrysotile, Lizardite, Pentlandite, Heazlewoodite, Godlevskite, Magnetite, Chromite, (Chalcopyrite)

[ High ] Antigorite, Chrysotile, Lizardite, Pentlandite, Violarite, Polydymite, Millerite, Chromite, (Chalcopyrite, Magnetite)