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The massive sulphide deposit on the Mid-Atlantic ridge is dependent on the presence of anhydrite and grows as an in situ breccia pile. This layer is formed during successive periods of hydrothermal activity that creates new hydrothermal precipitates and strengthens the previous deposit. During periods of hydrothermal inactivity, the brecciation of the deposits is initiated by the breakup of the sulphide chimneys, rupturing of anhydrite, and disordering by faulting. The high temperature within the mound is due to the hydrothermal circulation of the sea water through permeable ocean crust.

Serpentinization involves the hydrolysis and transformation of primary ferromagnesian minerals such as olivine ((Mg,Fe)2SiO4) and pyroxenes ((Mg,Fe)SiO3) to produce H2-rich fluids and a variety of secondary minerals over a wide range of environmental conditions. The continual and elevated production of H2 is capable of reducing carbon, thus initiating an inorganic pathway to produce organic compounds. The production of H2 and H2-dependent CH4 in serpentinization systems has received significant interdisciplinary interest, especially with regard to the abiotic synthesis of organic compounds and the origins and maintenance of life in Earth's lithosphere and elsewhere in the Universe. Here, serpentinization with an emphasis on the formation of H2 and CH4 are reviewed within the context of the mineralogy, temperature/pressure, and fluid/gas chemistry present in planetary environments. Whether deep in Earth's interior or in Kuiper Belt Objects in space, serpentinization is a feasible process to invoke as a means of producing astrobiologically indispensable H2 capable of reducing carbon to organic compounds. Key Words: Serpentinization-Fischer-Tropsch-type synthesis-Hydrogen formation-Methane formation-Ultramafic rocks. Astrobiology 15, 587-600. [ABSTRACT FROM AUTHOR]