Organic compound occurrence in fluids from ultramafic-hosted hydrothermal systems of the Mid-Atlantic ridge: a consequence of H2 production?

A characteristic phenomenon that occurs at slow spreading ridge, e.g. the Mid-Atlantic Ridge, is the formation of large faults, which at some place results in lifting up of the mantle rocks up to the ocean floor. Ultramafic-hosted hydrothermal systems are precisely located on these mantle outcrops, where circulation of seawater along the faults alters the periodites that mainly compose the mantle. The process known as serpentinisation results the production of a large amount of H2, which is a great source of energy for further chemical reactions such as methane and heavier hydrocarbons production. High concentration of CH4 detected in the water column above ultramafic-hosted hydrothermal fields has indeed been suggested to be the abiogenic product of a Fisher-Tropsh synthesis (3H2 + CO  CH4 + H2O), which is supported by the carbon isotopic data. This has lead to the idea of abiogenic formation of larger organic compounds such as hydrocarbons or key molecules for the origin of life issue. Both thermodynamics and laboratory work support this idea, but field data have been lacking up to date. Our study will present results on the organic content of hydrothermal fluids from 6 ultramafic-hosted hydrothermal fields of the MAR. Fluids were collected at Rainbow, Lost City, Logachev I & II and Ashadze I & II during the EXOMAR and the SERPENTINE cruises conducted by IFREMER, France. Innovative and efficient techniques have been developed, used and improved to concentrate, isolate and extract compounds from the fluids by SPE (Solid Phase Extraction)-GC-MS. Organics were listed and all compounds present in deep seawater and blank experiments were discarded as considered contaminants. The remaining appeared to be mainly hydrocarbons and oxygen-bearing compounds that were clearly -identified by comparison of recorded mass spectra with library data.This work is carried out partly within the MoMARnet (Monitoring deep sea floor hydrothermal environments on the Mid-Atlantic Ridge: A Marie Curie Research Training network) framework.