Serpentinization and associated hydrogen and methane fluxes at slow spreading ridges
Résumé
We estimate that domains with frequent outcrops of mantle-derived ultramafics represent about 20–25% of the seafloor created at ridges spreading at rates less than 40 mm a−1 and form at asymmetric detachment faults along about 14,660 km of the present-day ridge system. Ridge serpentinization extends to a maximum of 3 to 4 km into the footwall of axial detachment faults and consumes about 0.18 km3 of fresh peridotite per year. We estimate serpentinization-related hydrogen and methane fluxes with two independent methods, one based on the proportion of
serpentine in slow spreading crust, and the other based on estimated regional hydrothermal heat fluxes. Our results yield “initial” (prior to methane production) hydrogen fluxes ranging between 0.33 and 4.1 107 mol a−1 and per kilometer of ridge axis, in domains where mantle exhumation occurs and for the average exhumation rate of 11.4 mm a−1. Geological considerations lead us to prefer an intermediate value around 107
mol a−1 km−1, corresponding to a global hydrogen flux of about 16.7 1010 mol a−1. Based on methane/hydrogen concentration ratios in ultramafichosted vent fluids, this would correspond to a global methane flux of about 0.4 megaton (Mt) a−1. These estimates correspond to averages over the time it takes for the exhumed material to leave the axial domain. In the discussion, we relate fluid circulation, serpentinization, and hydrogen production, to the distribution of brittle fracture and faulting in the detachment fault’s footwall. We distinguish two principal settings, depending on whether melt is being emplaced and cooled in this fractured and permeable footwall domain.
Domaines
Tectonique
Origine : Fichiers produits par l'(les) auteur(s)
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