Evidence of methane venting and geochemistry of brines on mud volcanoes of the eastern Mediterranean Sea

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Geochemistry of high H2 and CH4 vent fluids issuing from ultramafic rocks at the Rainbow hydrothermal field (36°14′N, MAR)

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Helium isotopic composition of hydrothermal fluids from the Manus back-arc Basin, Papua New Guinea

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Deepwater mantle 3 He plumes over the northern Mid-Atlantic Ridge (36°N-40°N) and the Azores Platform

International audienceAs part of a multidisciplinary project aimed at studying mid-ocean ridge processes near the Azores, fifty water column profiles were analyzed for 3 He/ 4 He ratios in dissolved helium (a well-known hydrothermal tracer) from 36°N to 40°N along the Mid-Atlantic Ridge (MAR) and over the Azores Plateau. As expected, large d 3 He anomalies could be observed over the Rainbow, Lucky Strike, and Menez Gwen hydrothermal sites. The main finding of the present study is the discovery of a large hydrothermal 3 He plume north of the Açor Fracture Zone (north AFZ site), with a CH 4 / 3 He ratio indicative of a basaltichosted hydrothermal system. Clear 3 He and CH 4 anomalies, likely corresponding to unknown venting sites too, were also detected in the Amar Minor segment and south of the Kurchatov Fracture Zone. Evidence for substantial mantle helium degassing was also observed in the deep nodal basins along the Terceira Rift. On the basis of 3 He plumes over the total length of the surveyed segments, the distribution of hydrothermal sites corresponds to a site frequency of 1.3 ± 0.2 site/100 km, in good agreement with the global vent field statistics of Baker and German (2004). For the Rainbow, Lucky Strike, and Menez Gwen sites, the application of a plume model based on the conservation of mass, heat, and momentum shows that the heat output computed by the model is only an estimation of the heat released by the focused part of the flow imputable to one single vent. Applied to the north AFZ venting site for which the height of the plume is not known precisely, the model does not allow us to discriminate between a Menez Gwen/Rainbow type of venting or a more focused vent complex such as the one observed at the TAG site (26°N)

Hydrocarbons and oxidized organic compounds in hydrothermal fluids from Rainbow and Lost City ultramafic-hosted vents

The first building blocks of life could be produced in ultramafic-hosted hydrothermal systems considering the large amounts of hydrogen and methane generated by serpentinisation and Fischer–Tropsch-Type synthesis, respectively, in those systems. The purpose of this study was to detect and characterise organic molecules in hydrothermal fluids from ultramafic-hosted hydrothermal systems in the Mid-Atlantic Ridge (MAR) region. During the EXOMAR cruise 2005, fluids from the Rainbow (36°14′N) and the Lost City (30°N) hydrothermal fields were collected and treated by Stir Bar Sorptive Extraction (SBSE) and Solid Phase Extraction (SPE). The extracts were analysed by Thermal Desorption–Gas Chromatography–Mass Spectrometry (TD–GC–MS) and GC–MS, respectively. Compared to nearby deep seawater, hydrothermal fluids were clearly enriched in organic compounds, with a more diverse spectrum of molecules. We observed a very similar range of organic compounds in fluids from both sites, with a dominance of aliphatic hydrocarbons (C9–C14), aromatic compounds (C6–C16) and carboxylic acids (C8–C18). The occurrence of these compounds is supported by other field studies on serpentinites and sulfide deposits. Literature on thermodynamic data and experimental work has suggested the possible abiogenic origin of hydrocarbons and organic acids. In addition, it has been shown elsewhere that catalytic reactions producing hydrocarbons likely occur at both Lost City and Rainbow hydrothermal fields as suggested by the evolution of δ13C with increasing C number for methane, ethane, propane and butane. In order to investigate the origin of the organic molecules in the fluids, compound-specific carbon isotope ratio measurements were performed on n-alkanes and carboxylic acids, for which the δ13C values were in the range of − 46 to − 20‰ (vs. V-PDB). These preliminary data did not allow conclusive support or rejection of an abiogenic origin of the compounds. Indeed, predicting δ13C signatures in hydrothermal systems is likely to be complicated, due to differences in source δ13C signatures (i.e., of the C building blocks), and a variety of, mostly unknown, fractionation steps which may occur along the synthesis pathways. In addition, even though a fraction of the compounds detected in the fluids is likely abiotically produced, a dominance of biogenic sources and/or processes might hide their characteristic signature.

The rainbow vent fluids (36°14′N, MAR): the influence of ultramafic rocks and phase separation on trace metal content in Mid-Atlantic Ridge hydrothermal fluids

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Hydrothermal activity in the Lau Basin: First results from the NAUTILAU Cruise

The Lau Basin, a back arc spreading center, is one of the most active hydrothermal areas in the ocean. A scientific team from France, Germany, and Tonga investigated the southern Lau Basin near Tonga in 1989 to study the processes of seafloor ore-mineral formation associated with hydrothermal circulation along the volcanic Valu Fa ridge (Ride de Valu Fa in Figure 1), which lies in back of the Tonga-Kermadec trench. Between April 17 and May 10 scientists on the R/V Nadir used the submersible Nautile to make 22 dives in the southern Lau Basin. The cruise was called NAUTILAU, for Nautile in Lau Basin. In addition to the standard equipment of the submersible (video and photo cameras, and temperature probe), a CTD (conductivity-temperature-depth) instrument was integrated with a “mini rosette” water sampling device used for the first time on the Nautile to obtain correlations between the geological observations and the physical and chemical anomalies measured in the seawater

Compared geochemical signatures and the evolution of Menez Gwen (37°50′N) and Lucky Strike (37°17′N) hydrothermal fluids, south of the Azores Triple Junction on the Mid-Atlantic Ridge

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