Serpentinization of oceanic peridotites: Implications for geochemical cycles and biological activity

Abstract

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 H 2 O-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.