Organics in hydrothermal fluids from ultramafic-hosted vents of the Mid-Atlantic Ridge

Abstract: The process of serpentinization at slow spreading ridges is due to the circulation of seawater through outcropping mantle rocks. The peridotites of the mantle are altered via fluid-rock interaction generating, in the presence of Fe bearing minerals, high CH4- and H2-concentrations in the water column. CH4 has been suggested to be the abiogenic product of a Fisher-Tropsch Type (FTT) synthesis (4H2 + CO2 ? CH4 + 2H2O), which is supported by the carbon isotopic data. This has lead to the idea of abiotic formation, likely via FTT, of higher molecular weight organic compounds such as hydrocarbons and other key molecules that could have played a role in the origin of life. Both thermodynamics and laboratory work support this idea yet field data have been lacking. This study focuses on determining the organic content of fluids from ultramafic-hosted hydrothermal systems from the Mid-Atlantic Ridge (MAR) and on understanding the processes involved in the production of these organics. Fluids were collected from the Rainbow and Lost City hydrothermal field in the MAR region during the EXOMAR cruise conducted by IFREMER, France. The two sites and fluids from them have different chemical, geological and physical features (e.g. pH, temperature, depth, petrology, mineral geochemistry). A Solid Phase Extraction - Gas Chromatography - Mass Spectrometry (SPE-GC-MS) technique was developed and used to concentrate, separate and extract compounds from the fluids. Extracts from deep seawater controls and blanks were compared to the samples. After organic compounds present in deep seawater and blank experiments were discarded as likely contaminants, those remaining appeared to be mainly hydrocarbons and oxygen-bearing compounds. They were clearly identified by comparison of recorded mass spectra with library data. In order to establish whether the compounds were biogenic or abiogenic in origin, compound specific carbon isotope ratio measurements were made. The ?13C values were in the range of -40 to -30‰ and did not show a pattern. It is not possible at this point to draw conclusions from the carbon isotopic data about the origin of the compounds present in the fluids. There are other lines of evidence that suggest a likely biogenic origin for certain groups of compounds whereas other groups appear to have a mix of both origins.This first step allowed an overview of the fluids composition. Subsequent effort has been to optimize the sample conditioning methods in order to separate and concentrate targeted compounds. The main focus was to separate fatty acids from the other compounds in order to get higher ?13C signals. The new protocols have been used in 2007 during the SERPENTINE and MOMARDREAMnaut cruises conducted by IFREMER. I am currently analyzing the GC-MS and GC-IRMS data. This work is carried out partly within the MoMARnet (Monitoring deep sea floor hydrothermal environments on the Mid-Atlantic Ridge: Marie Curie Research Training network’) framework.