Fast and slow spreading ridges: Structure and hydrothermal activity, ultramafic topographic highs, and CH₄ output

Abstract

Different parts of the world ridge system have quite different morphologies, which reflect different constructional processes. It appears that hydrothermal circulation at all spreading centers is an important exchange process between the ocean and the newly formed oceanic crust. This hydrothermal circulation may vary according to morphology and crustal composition and may also affect ridge constructional processes. The TAG (26°N) and Snake Pit (23°N) hydrothermal sites on the Mid‐Atlantic Ridge (MAR) display Mn/CH 4 ratios in overlying seawater similar to those of nonsedimented East Pacific Rise sites, i.e., about 0.2 mol/L. In contrast, large methane anomalies with very low Mn/CH 4 ratios of 0.005 mol/L are associated with ultramafic topographic highs near 15°N, close to the axis and on ultramafic walls of the rift valley. The association of ultramafic bodies and CH 4 anomalies in seawater indicates active serpentinization processes. CH 4 is produced during serpentinization according to the Fisher Tropsch reaction (Charlou et al., 1991). Many ultramafic rock assemblages have been sampled on the MAR away from fracture zones, in particular by the Deep Sea Drilling Project. Changes of mechanical properties and of density of uplifted deep material in the accreting plate boundary zone caused by serpentinization may play an important role in the construction of slow spreading ridges.