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 the serpentinization front yields a front displacement (z) of 2197 ± 89 m. We ignored diffusive broadening of this front in the direction of fluid flow and used the shape of the front to constrain a Damköhler number (ND). This was scaled such that ND = κt, where κ is a rate constant for serpentinization in s-1 and t is time in s. We thus obtained ND = 6.0 ± 0.2. We then estimated values of κ for (1) surface reaction as rate-limiting and (2) chemical transport as rate-limiting. Combining these values with our estimate of the Damköhler number yielded comparable timescales of 10,000 - 1,000,000 years for serpentinization. Combining this timescale with the front displacement yielded a time-averaged volumetric rate for serpentinization of 0.002-0.2 m3-serpentinite.m-2.yr-1.
We then referred to the experimental results of Janecky and Seyfried (1986), which predicted that serpentinization by reaction with seawater probably occurs by a coupled set of reactions, with olivine, orthopyroxene and clinopyroxene dissolving independently and at different rates. Of this set of reactions, olivine dissolution is predicted to occur by the reaction:
2(Mg,Fe)2SiO2 + 2H+ + H2O = (Mg,Fe)3Si2O5(OH)4 + (Mg,Fe)2+. Because lizardite, which is the most abundant of the serpentine minerals typically contains no more than 5 wt. % FeO (Wicks & O’ Hanley 1988), excess Fe2+ will preferentially (with respect to Mg2+) enter the solution. Oxidation of Fe2+ by H2O and CO2 can then produce magnetite and CH4 by the reaction:
12Fe2+ + 14H2O + CO2 = 4Fe3O4 + 24H+ + CH4.
Fe3O4 (magnetite) can be used as a proxy for CH4 produced by this reaction and exiting the mantle. The average mode of magnetite in 16 samples of >95% serpentinized peridotite recovered by scientific drilling within the region of exhumed mantle is 4.5 vol. %. This is equivalent to 1000 mol-Fe3O4.m-3. Combining this value with the time-averaged volumetric rate for serpentinization, we obtain a time-averaged annual production rate for magnetite of 2-200 mol.m-2.yr-1. Finally, based on the stoichiometry of reaction (2), we thus obtain an annual flux rate for CH4 production of 0.5-50 mol.m-2.yr-1.