Yves Fouquet

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

The Flores diving cruise was part of the MAST III-AMORES (1995–1998) program funded by the European Union. One of the major achievements of the Flores cruise was the discovery of the Rainbow hydrothermal field hosted in ultramafic rocks south of the Amar segment on the Mid-Atlantic ridge (MAR). The Rainbow hydrothermal fluids exhibit temperatures of 365 °C, pH of 2.8, high chlorinity (750 mmol/kg), and low silica (6.9 mmol/kg). The uniformity in endmember major, minor, trace element concentrations and gas contents suggests that all Rainbow fluids originate from the same deep source. Although H2S content is relatively low (1.20 mmol/kg), all vent fluids show extraordinary high H2 (16 mmol/kg), CH4 (2.5 mmol/kg) and CO (5 μmol/kg) endmember concentrations compared to fluids collected from other vent sites along the MAR. Hydrogen represents more than 40% of the total gas volume extracted from the fluids. At Rainbow, H2 production is likely associated with alteration of olivine and orthopyroxene minerals during serpentinization. Given that exposures of ultramafic rock may be common, particularly along slow-spreading ridges, the production of H2 may have important implications for microbial activity at and beneath the seafloor.

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

Fluids were collected from the Rainbow vent field (36°14′N) on the Mid-Atlantic Ridge (MAR) during the 1997 diving FLORES cruise. This vent field, in ultramafic rocks at a depth of 2300 m, is composed of ∼10 black smokers emitting acidic (pH∼2.8) fluids at 365 °C. The low pH of the hot-temperature Rainbow fluids likely results from seawater–ultramafic rock interaction that releases H+ ions into reducing hydrothermal fluids. Fluid chemistry is strongly influenced by phase separation generating Cl-rich brines (ClEM=750 mM) strongly enriched with Mn, Fe, Co, Ni, Cu, Zn, Ag, Cd, Cs, Pb, Y, and rare earth elements (REE). REE and transition metal abundance (particularly Fe and Mn) in the Rainbow fluids is dramatically higher than in other MAR fluids. The abundance of trace element and REE enrichment is due to the greater solubility of these elements that is strongly favored by Cl-complexation at low-pH and high-temperature conditions. Chondrite-normalized REE patterns show strong LREE enrichment with evidence of the typical Eu anomaly. This REE partitioning suggests that either (1) ultramafic rocks represent only a part of rocks leached during hydrothermal alteration and/or (2) that the unique Rainbow fluid temperature, pH, and redox state issued from the ultramafic character of leached substratum can produce unique REE partitioning.

Yttrium and rare earth elements in fluids from various deep-sea hydrothermal systems

Rare earth element (REE) and yttrium (Y) concentrations were measured in fluids collected from deep-sea hydrothermal systems including the Mid-Atlantic Ridge (MAR), i.e., Menez Gwen, Lucky Strike, TAG, and Snakepit; the East Pacific Rise (EPR), i.e., 13°N and 17–19°S; and the Lau (Vai Lili) and Manus (Vienna Woods, PacManus, Desmos) Back-Arc Basins (BAB) in the South-West Pacific. In most fluids, Y is trivalent and behaves like Ho. Chondrite normalized Y-REE (Y-REEN) concentrations of fluids from MAR, EPR, and two BAB sites, i.e., Vai Lili and Vienna Woods, showed common patterns with LREE enrichment and positive Eu anomalies. REE analysis of plagioclase collected at Lucky Strike strengthens the idea that fluid REE contents, are controlled by plagioclase phenocrysts. Other processes, however, such as REE complexation by ligands (Cl−, F− SO42−), secondary phase precipitation, and phase separation modify REE distributions in deep-sea hydrothermal fluids. REE speciation calculations suggest that aqueous REE are mainly complexed by Cl− ions in hot acidic fluids from deep-sea hydrothermal systems. REE concentrations in the fluid phases are, therefore, influenced by temperature, pH, and duration of rock-fluid interaction. Unusual Y-REEN patterns found in the PacManus fluids are characterized by depleted LREE and a positive Eu anomaly. The Demos fluid sample shows a flat Y-REEN pattern, which increases regularly from LREE to HREE with no Eu anomaly. These Manus Basin fluids also have an unusual major element chemistry with relatively high Mg, SO4, H2S, and F contents, which may be due to the incorporation of magmatic fluids into heated seawater during hydrothermal circulation. REE distribution in PacManus fluids may stem from a subseafloor barite precipitation and the REE in Demos fluids are likely influenced by the presence of sulfate ions.

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

During the DIVA 1 cruise in May 1994, a series of 19 dives was conducted using the French submersible Nautile at the topographic highs of three volcanic segments centered at 37°17′N, 37°50′N and 38°20′N, respectively, south of the Azores Triple Junction (ATJ) on the Mid-Atlantic Ridge (MAR). Hydrothermal fluids were collected at the Lucky Strike (LS) hydrothermal field, discovered at 37°17′N in 1993, and at a new hydrothermal site called Menez Gwen (MGw) discovered at 37°50′N during this cruise. Both systems are relatively shallow compared to other MAR systems with seafloor depths of 1700 and 850 m, respectively, characteristics which make them unique among the already known sites on the MAR. The characteristics of the LS fluids are the same as in 1993 and include temperatures ranging from 170°C to 324°C, variable chlorinities lower than seawater, low hydrogen sulfide (<3.0 mmol/kg), high Ba concentrations (up to 80 μmol/kg), low metal concentrations and high gas contents, and distinct chemical end-members indicative of significant geographic control of the venting system. In contrast, the very clear MGw fluids at 37°50′N show a rather uniform exit temperature (285°C) and chemical compositions with chlorinities (360–380 mmol/kg) lower than at LS and corresponding lower concentrations of cations, H2S (1.8 mmol/kg), metals and silica (8–11.5 mmol/kg), due to the lower temperature (T) and pressure (P) of the system. In the two systems, fluid chemistry is strongly affected by phase separation. At the low-pressure conditions of these sites, phase relations in the NaCl–H2O system dictate production of an extremely low salinity. The higher salinities observed in fluids thus indicate that mixing of extremely low salinity vapor with single-phase hydrothermal seawater is the dominant process controlling Cl concentrations. Depletions and enrichments of elements in solutions are also explained by the involvement of fluid-rock reactions. The enrichment of Ba, K, Cs and Rb in the fluids is linked to the enriched character of these elements in basalts. Relatively high pH and Ca, low Li, Sr and metals are related to the highly altered basaltic substrate in the reaction zone. Low Sr/Ca and high Ca/Na ratios are explained by albitization within the crust. The increase of degassing when approaching the Azores may be related to the carbon-enriched basaltic crust near the Azores hot spot. Isotopic ratios show that CO2, CH4 and helium gases have a magmatic signature. Compared to other deeper sites on the MAR, all fluids collected at LS and MGw are gas-enriched, except for H2S and helium, whose both concentration and isotopic ratio compare well with other hydrothermal fluids. The high CH4 concentration associated with unsaturated hydrocarbons and the high CH4/3He ratios also suggest a contribution of CH4 generated by serpentinization of ultramafic rocks by Fischer–Tropsch catalysis of CO2 reduction.

Intense CH4 plumes generated by serpentinization of ultramafic rocks at the intersection of the 15°20′N fracture zone and the Mid-Atlantic Ridge

Abstract As part of the FARA French-US Program designed to study the Mid- Atlantic Ridge (MAR) between 15°N and the Azores, twenty-three dives with the submersible Nautile were conducted during the French-US Faranaut 15N cruise on the eastern and western parts of the 15°20′N Fracture Zone/Ridge axis intersection. South of the eastern ridge-transform fault intersection, nine Nautile dives were made within the rift valley and along the western rift valley wall. CH4 concentrations in the bottom waters reach 53.2 nmol/kg along faulted zones on top and on the east flank of the ultramafic inner corner high (15°05′N, 44°59′W) where serpentinized rocks outcrop. No 3He anomaly is associated with methane, ruling out any primary mantle component. Fourteen dives were also made in the rift valley to the north, close to the western intersection of the 15°20′N Transform. High CH4 anomalies (up to 22 nmol/kg) are also present in the bottom waters of the rift valley northern segment on both the western and eastern valley walls and on the inner high adjacent to the eastern wall where ultramafic rocks outcrop. Seven vertical hydrocasts carried out in the axial valley (4500 m deep) show an intense CH4 anomaly, with a maximum (35.8 nmol/kg) at 3200 m depth. This CH4 concentration is among the highest found along the Mid-Atlantic Ridge Charlou and Donval 1993 , Charlou et al 1997 . CH4 concentrations of 9.9–14.9 nmol/kg are also present on the western wall along the 3200 m isobath. The high CH4 concentrations correspond to only weak 3He anomalies. This CH4-rich plume is also associated with active fault zones that expose peridotite rocks. CH4 output from ultramafic outcrops on the western and eastern intersections of the 15°20′N Fracture Zone with the MAR is believed to reflect ongoing serpentinization. These results associated with many other CH4 anomalies measured in the water column above ultramafic outcrops found between 12°N and the Azores most likely reflect serpentinization processes extending along the whole slow spreading Mid-Atlantic ridge.

Ultramafic and gabbroic exposures at the Mid-Atlantic Ridge: geological mapping in the 15°N region

Abstract The outcrops of mantle-derived ultramafic rocks in the 15°N region are the most extensive yet reported for the Mid-Atlantic Ridge. North of the Fifteen Twenty fracture zone, these outcrops form a belt at least 20 km long along the west wall of the axial valley and also crop out on the east axial valley wall. Ultramafic rocks also crop out extensively south of the Fifteen Twenty fracture zone. Based on dredging and on a morphological analysis of the bathymetric map, we propose that ultramafic outcrops may be common in the crust formed between 14°30′N and 15°50′N during at least the past 2.4 m.y. Moderately dipping fault planes and large expanses of tectonic breccia have been observed during dives on the ultramafic outcrops. Diving observations also show that the ultramafic rocks are capped, in stratigraphic contact, by a thin layer of basalt. This suggests that these rocks were tectonically emplaced at the axial seafloor, or very close to it, then uplifted in the footwall of the faults that bound the axial valley. The occurrence of ultramafic rocks on both walls of the axial valley may be due to frequent changes of faulting polarity in the axial region: instead of one master shear zone, there would be a complex array of cross-cutting conjugate faults and shear zones that could jump inward in the axial domain as spreading proceeds.

Hydrothermal vents near a mantle hot spot: the Lucky Strike vent field at 37'N on the Mid-Atlantic Ridge

The Lucky Strike hydrothermal field occurs in the summit basin of a large seamount that forms the shallow center of a 65 km long ridge segment near 37°N on the Mid-Atlantic Ridge. The depth and chemistry of the ridge segment are influenced by the Azores hot spot, and this hydrothermal field is the first Atlantic site found on crust that is dominated by a hot spot signature. Multiple hydrothermal vents occur over an area of at least 300 m by 700 m. Vent morphologies range from flanges and chimneys with temperatures of 200–212°C, to black smoker chimneys with temperatures up to 333°C. Cooler fluids from northern vents have higher chlorinities and lower gas volumes, while hotter, southern fluids have chlorinities 20% below seawater with higher gas volumes, suggesting phase separation has influenced their compositions. All gas volumes in fluids are higher than those at TAG and Snake Pit hydrothermal fields. Black smokers exhibit their typical mineralogy, except that barite is a major mineral, particularly at lower-temperature sites, which contrasts with previously investigated Atlantic sites. The fluid chemistry, distribution of the relict sulfide deposits on the seamount summit in the areas investigated using DSV Alvin, and contact relationships between active vent sites and surrounding basaltic and sulfide substrate suggest that the hydrothermal system has a long history and may have recently been rejuvenated. Fauna at the Lucky Strike vent sites are dominated by a new species of mussel, and include the first reported sea urchins. The Lucky Strike biological community differs considerably from other vent fauna at the species level and appears to be a new biogeographic province. The Lucky Strike field helps to constrain how variations in the basaltic substrate influence the composition of hydrothermal fluids and solids, because basalt compositions at Lucky Strike are 10–30 times enriched in incompatible elements compared to other Atlantic hydrothermal sites such as TAG, Snake Pit and Broken Spur. The incompatible element

New age data for Mid‐Atlantic Ridge hydrothermal sites: TAG and Snakepit chronology revisited

The chronologies of TAG and Snakepit hydrothermal fields have been established using 210Pb/Pb, 230Th/234U and 14C dating. At the TAG field, a Mn-oxide record, indicative of low temperature events, began at least 125,000 years and possibly 140,000 years ago with maximum intensities at 15,000, 7000 and 4000 years before present. High temperature events, giving rise to sulfide deposits, began about 100,000 years ago and have been intermittent to the present day. A presently active site has experienced intermittent pulses of activity every 4000 to 6000 years over the past 20,000 years. Decrease in activity is often marked by low temperature aragonite precipitation in chimney conduits at 4000, 7000 and 9000 years ago. After a period of quiescence lasting about 4000 years this site was reactivated about 50 years ago. The Snakepit field is much younger and no sulfides older than 4000 years have been recovered. Relict sulfide deposits are dated between 2000 and 4000 years old indicating this site was active during a quiescent period at TAG. Reactivation of Snakepit. took place about 80 years ago, and is presently concurrent with that of TAG. Comparison with hydrothermal sites on the East Pacific Rise suggests that on slow spreading ridges the major fracture systems focussing the hydrothermal discharge can be reactivated at intervals and new deposits precipitated on top of older ones, while on faster spreading ridges each pulse of activity is separated in space and time resulting in discrete deposits.

Hydrothermal activity in the Lau back-arc basin:Sulfides and water chemistry

The submersible Nautile completed 22 dives during the Nautilau cruise (R/V Nadir, April 17-May 10, 1989) for a detailed investigation of the southern Lau basin near Tonga. The objective of the scientific team from France, Germany, and Tonga was to understand the process of sea-floor ore formation associated with hydrothermal circulation along the Valu Fa back-arc ridge behind the Tonga- Kermadec trench. The four diving areas, between lat 21°25′S and 22°40′S in water ∼2000 m deep, were selected on the basis of results from cruises of the R/V JeanCharcot and R/V Sonne. The Nadir cruise provided proof of hydrothermal activity—in all four areas, over more than 100 km—as indicated by the widespread occurrence of hydrothermal deposits and by heat flow, conductivity, and temperature measurements near the sea bottom. The most spectacular findings were high-temperature white and black smokers and associated fauna and ore deposits. Hydrothermal water chemistry and sulfide composition data presented here indicate that this hydrothermal field is very different from the hydrothermal fields in oceanic ridges. This difference is seen in the water chemistry of the hydrothermal fluid (pH = 2 and high metal content) and the chemical composition of sulfides (enrichment in Ba, As, and Pb).

Recent tectonic, magmatic, and hydrothermal activity on the East Pacific Rise between 17°S and 19°S: Submersible observations

The objective of the Naudur cruise (December 1993) of the submersible Nautile was to study the interaction among magmatic, tectonic, and hydrothermal processes at a very fast spreading mid-ocean ridge axis. Twenty-three dives were completed, both along and across the axis, in four areas located between 17°10′ and 18°45′S on the East Pacific Rise. Rock, sulfides, water, and biological samples have been collected along each of the segments. Two main types of segments have been distinguished, characterized either by the predominance of present-day volcanic activity or by predominant tectonic activity. Linked to both types of activity, 69 hydrothermal sites have been discovered and sampled. They comprise four types, interpreted as successive evolutionary stages. The first are shimmering water sites which occur immediately after the formation of lava lakes and are characterized by large surface area and poorly developed associated fauna. The second, in areas dominated by recent volcanic activity, have waters venting directly from lava fissures and more focused discharge areas through black smoker chimneys. The third stage is represented by more mature hydrothermal vents and deposits, along the faults bounding the eastern side of the axial graben in tectonic-dominated areas. The associated fauna is well developed. The fourth stage corresponds to the reactivation of volcanic activity with lava flows, young black smokers, and diffuse venting associated with the faults bounding the axial graben. Fluids collected range from 200° to 340°C and show a wide variability in chemical and gas composition. Within each of the explored areas, evidence of recent volcanic activity has been observed.