HR: 15:25h
AN: B23D-08    [Abstracts]
TI: Organics in hydrothermal fluids from ultramafics on the Mid-Atlantic Ridge (MAR) - Abiogenic and/or biogenic origin?
AU: * Charlou, J
EM: charlou@ifremer.fr
AF: Dept.G‚osciences Marines, Ifremer, Plouzane, 29280 France
AU: Donval, J
EM:
AF: Dept.G‚osciences Marines, Ifremer, Plouzane, 29280 France
AU: Fouquet, Y
EM:
AF: Dept.G‚osciences Marines, Ifremer, Plouzane, 29280 France
AU: Jean-Baptiste, P
EM:
AF: LSCE, CEA-CNRS 1572, Saclay, 91191 France
AU: Dehairs, F
EM:
AF: Dept. of Chemistry, Vrije Universiteit, Brussels, B1050 Belgium
AU: Holm, N
EM:
AF: Dept.of Geology, University of Stockholm, Stockholm, 10691 Sweden
AU: Godfroy, A
EM:
AF: Dept.G‚osciences Marines, Ifremer, Plouzane, 29280 France
AB: Between 12°N and the Azores Triple Junction along the MAR, CH4 anomalies over axial ultramafic sites are common and point to the association of high or low temperature hydrothermal activity and mantle degassing indicative of ongoing serpentinization process. The general occurrence of isotopically-heavy methane shows the possible abiogenic synthesis of hydrocarbons in hydrothermal systems. The abiogenic formation of CH4 and more complex organic compounds is related to the process of serpentinization of mantellic rocks. Three sites (Logachev, 14°45'N; Rainbow, 36°14'N; Lost City Field, 30°N) are known on the MAR. New fresh fluids were recently sampled at Rainbow and Lost City by the French ROV-Victor during EXOMAR cruise (July 24 to August 28, 2005). The Rainbow and Lost City fluids issued from contrasted ultramafic environments are both enriched in H2, CH4 and hydrocarbons. Hydrogen gas represents more than 40 per cent total gas volume extracted from fluids. SPME (Solid Phase Micro-Extraction) and SBSE (Stir-Bar Sorptive Extraction) extraction techniques were used on board for organic recovery and the analysis was performed on shore by direct GC/MS or by Thermo-Desorption/GC/MS. The hydration of olivine and pyroxen minerals with conversion of Fe(II) to Fe(III) in magnetite during serpentinization leads to production of H2 and conversion of dissolved CO2 to reduced-C species including methane, ethane, propane. In addition heavier straight chain hydrocarbons as alcohols, aldehydes, ketones, aromatics, and cyclic compounds are identified at Rainbow. These compounds may be generated in ultramafic rocks through catalytic reactions (Fischer-Tropsch type reactions), but a biogenic contribution cannot be excluded. Abiogenic organic compounds may be produced from crystalline basement, from volcanic structures, from riftogenic zones and probably from sedimented margins.
DE: 0404 Anoxic and hypoxic environments (4802, 4834)
DE: 0450 Hydrothermal systems (1034, 3017, 3616, 4832, 8135, 8424)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
SC: Biogeosciences [B]
MN: Fall Meeting 2005