High Hydrogen and abiotic hydrocarbons from new ultramafic hydrothermal sites between 12°N and 15°N on the Mid Atlantic Ridge- Results of the Serpentine cruise (March 2007)

New hydrothermal fields were recently explored and sampled between 12° and 17°N (MAR) on the Mid-Atlantic Ridge as a part of the French-Russian cooperative SERPENTINE diving cruise. In addition to the Logachev I site (14°45N) previously studied and revisited during this cruise, new smoking areas, called Ashaze I and 2 (12°58N) and Logachev 2 (14°43N) were discovered in ultramafic environments, as previoulsly found at Rainbow (36°14N), Lost City (30°N), and Logachev 1 (14°45N). Very strong anomalies in temperature, nephelometry, CH4 (from 1 to 120 µl/l), helium, were found in the seawater column above the Ashaze and Logachev high-temperature fields. The fluid endmembers at these sites exhibit different temperature (310 to 370°C), and different chemical characteristics: pH (3.5 to 4.0), chloride (150 to 620 mM), signifying that phase separation is occurring and controlling the fluid chemistry. All fluids are issued from ultramafics and controlled by seawater-peridotite interaction They show low silica (5 to 10 mM), low H2S (<0.5 mM) and are extraordinary enriched in hydrogen gas. Gas bubbles are observed coming out from Ashaze 1 vents and pulses of clear fluid were observed venting from Logachev 2. All fluids issued from ultramafics contain very high concentrations in H2 (70 per cent of total gas), CO2, CH4. Preliminary calculations show that one vent at Ashaze 1 field produces 1 million of cubic meters of natural H2 per year. CH4 is clearly abiogenic with d13C varying from -6 to -14 per mil (PDB). In addition, the progressive isotopic trends for the series of C1 to C4 alkanes indicate that hydrocarbon formation occurs by way of polymerization of CH4 precursors. The serpentinization process is observed here up to 4080m at Ashaze 1, the deepest venting area so far know in ocean, and generates high hydrogen and abiogenic hydrocarbons during the hydration of olivine and pyroxen minerals through catalytic reactions (Fischer-Tropsch type reactions) as previously observed at all ultramafic sites on the slow-spreading mid Atlantic Ridge between 12°N and the Azores Triple Junction. More information is available at http://www.ifremer.fr/serpentine.