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Jean Luc Charlou, Ifremer Centre de Brest (France)
Jean Pierre Donval, Ifremer Centre de Brest (France)
Cecile Konn, Stockholm University (Sweden)
Yves Fouquet, Ifremer Centre de Brest (France)
Philippe Jean-Baptiste, CEA-CNRS 1572 (France)
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During the last past fifteen years, many oceanographic cruises have been conducted along the mid-Atlantic Ridge (MAR) between 12°N and the Azores Triple Junction for exploring the ridge axial valley at different scales. These works achieved under the auspices of the FARA and the EC MAST II and MAST III programmes permitted the discovery of numerous large CH4 anomalies on a lot of ridge segments, the study of seawater-rock exchanges, the discovery of many diffuse and very active hot vent hydrothermal fields, demonstrating an intense methane degassing along the MAR. This methane degassing is related to the nature of seawater-rock interaction, including interactions in hydrothermal systems hosted primarily in basaltic rocks and interactions of seawater with ultramafic rocks. Many ultramafic-hosted hydrothermal vent fields are now known and already sampled along the MAR, including Rainbow, 36°14'N; Lost City, 30°N; Logachev I and II, 14°45'N; Ashaze I and II, 13°N; 8°S site. From tracer anomalies in the seawater column, we have evidence of other new potentially active or diffuse areas, many of them linked to ultramafic outcrops. Between 12°N and the Azores Triple Junction, CH4 anomalies over axial ultramafic sites are common and point to the association of high temperature hydrothermal activity, mantle degassing through focussed or diffuse flow (either hot or cold) indicative of ongoing serpentinization process generating the large fluxes of energy and gas observed at the ridge axis. Phase separation and seawater-peridotite interaction are the two processes controlling the geochemistry of fluids which exhibit variable salinity and mineral composition, but all show a strong enrichment in H2 and hydrocarbons. Particularly, H2 represents 40 to 70% of the total gas volume extracted from fluids. Isotopic data show clearly that CH4 is abiogenic with d-13 C varying from -6 to -18 per mil (PDB). In addition, the progressive isotopic trends for the series of C1 to C4 alkanes indicate the hydrocarbon formation occurs by a catalytic way. This is clearly the result of hydration of olivine with conversion of Fe(II) in olivine to Fe(III) in magnetite during serpentinization, process which leads to production of H2 and conversion of dissolved CO2 to reduced-C species including methane, ethane, propane and straight chain hydrocarbons in ultramafic rocks through Fischer-Tropsch type (FTT) reactions. These results show that hydrogen and methane issued from the slow-spreading MAR have to be considered in the global budget of the Earth. Methane anomalies in the seawater column, fluid geochemistry, high hydrogen and abiogenic methane fluxes issued from serpentinization along the MAR will be discussed.
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