GEOPHYSICAL MONOGRAPH SERIES, VOL. 144, PP. 119-136, 2004
Serpentinization of oceanic peridotites: Implications for geochemical cycles and biological activity
Ultramafic rocks are a major component of the oceanic lithosphere and are commonly exposed near and along slow- and ultra-spreading
ridges and in other tectonically active environments. The serpentinization of mantle material is a fundamental process that
has significant geophysical, geochemical and biological importance for the global marine system and for subduction zone environments.
Mineral assemblages and textures are typically complex and reflect multiple phases of alteration, deformation and veining
during emplacement, hydrothermal alteration, and weathering. In this paper, we review mineralogical and geochemical consequences
of serpentinization processes in oceanic upper mantle sequences in different tectonic environments and discuss the relationship
between serpentinization and fluid chemistry. We present phase equilibria that provide models for interpreting mineral-fluid
relationships in oceanic serpentinites and allow the simultaneous evaluation of the conditions for redox, hydration and carbonation
processes. These models predict that serpentinization reactions are sensitive to Si content of ultramafic rocks and that serpentine
phases have an upper stability limit of ∼450°C, where H2O-rich fluids will be dominant. More pervasive serpentinization commences with olivine breakdown reactions below ∼425°C and
leads to progressively more reduced fluids with decreasing temperature. Our calculations indicate that carbonates may have
extensive stability fields in CH4-rich fluids in Si-deficient systems and that they may be significant in generating reducing conditions. If methane formation
driven by serpentinization is common, its contribution to the carbon cycle in submarine biogeochemical systems may be substantial.
Serpentinization may thus be an important process in sustaining diverse microbial communities in subsurface and near-vent
environments and has consequences for the existence of a deep biosphere.
Citation: Früh-Green, G. L.,