Constraining the rate and extent of mantle serpentinization from seismic and petrological data: implications for chemosynthesis and tectonic processes

Abstract

We used seismic velocity as a proxy for serpentinization of the mantle, which occurred beneath thinned but laterally continuous continental crust during continental break up, prior to opening of the Atlantic Ocean. The serpentinized sub-continental mantle is now exhumed, beneath the Iberia Abyssal Plain and was accessed by scientific drilling on Ocean Drilling Program legs 149 and 173. Chromatographic modelling of kinetically limited transport of the serpentinization front yields a front displacement of 2197 ± 89 m, a time-integrated fluid flux of 1098 ± 45 m3 m2 and a Damköhler number of 6.0 ± 0.2. Whether either surface reaction or chemical transport limit the rate of reaction, we calculate timescales for serpentinization of approximately 105–106 years. This yields time-average fluid flux rates for H2O, entering and reacting with the mantle, of 60–600 mol m2 a1 and for CH4, produced as a by-product of oxidation of Fe++ to magnetite and exiting the mantle, of 0.55–5.5 mol m2 a1. This equates to a CH4-flux of 0.18–1.8 Tg a1 for coeval serpentinization of the mantle that was exhumed west of Iberia. This represents 0.03–0.3% of the present-day annual CH4-flux from all sources and a higher fraction of pre-anthropogenic (lower) CH4 levels. CH4 released by serpentinization at or beneath the seafloor could provide substrate for biological chemosynthesis and/or promote gas-hydrate formation. Finally, noting its volumetric extent and rapidity (<106 years), we interpret serpentinization to be a reckonable component of tectonic processes, contributing both diapiric and expansional forces and helping to 'lubricate' extensional processes. Given its anisotropic permeability, actively deforming serpentinite might impede melt migration which may be of interest, given the apparent lack of melt in some rifted margins.

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