Joël Radford-Knoery

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.

Hydrothermal exploration near the Azores Triple Junction: tectonic control of venting at slow-spreading ridges?

Abstract Simultaneous acoustic imaging of the seafloor and detection of particle-rich plumes in the overlying water column have been used to identify and determine the tectonic setting of high-temperature ‘black smoker’ hydrothermal activity along 200 km of the Mid-Atlantic Ridge between 36° and 38°N. Using this approach, we have identified hydrothermal signals at 7 different locations. These results indicate a higher incidence of hydrothermal activity along this section of ridge axis than has been reported elsewhere along slow spreading ridges. Our data show that the majority of hydrothermal sites here are located near to non-transform offsets rather than at the centres of individual ridge segments. We suggest that this intersection of fabrics, associated with ridge discontinuities and the spreading process, is instrumental in focussing hydrothermal flow at these localities. Future strategies of exploration for hydrothermal activity on slow-spreading ridges may need to be revised accordingly.

Non-transform offsets along the Mid-Atlantic Ridge south of the Azores (38°N–34°N): ultramafic exposures and hosting of hydrothermal vents

Ten contiguous non-transform offsets (NTOs) along the Mid-Atlantic Ridge (MAR) south of the Azores (between 38°N and 35°40′N) have been studied in detail using swath bathymetric, acoustic backscatter and deep-tow high-resolution sidescan sonar (TOBI) data. In contrast with discontinuities studied elsewhere at slow-spreading ridges, these left-lateral NTOs are consistently broader and larger, with complex structural fabrics accommodating the offset. They are characterized by a range of elevated and faulted massifs detached from their segment flanks, with an irregular acoustic backscatter pattern. Some of these massifs have been explored and sampled recently during dive cruises revealing that they are composed of upper mantle peridotites and lower crustal rocks, and sometimes associated with high-temperature hydrothermal venting. Water column surveys adjacent to these massifs show high CH4 and low TDM (total dissolvable manganese) concentrations, possibly resulting from the process of serpentinization of ultramafic rocks. The correlation between the shallow dome-like shaped massifs and the high concentrations of CH4 (associated with low levels of Mn) is of particular interest to predict the outcrop of ultramafic rocks within the NTOs where no geological data are available. The exposure of the ultramafic massifs within the NTOs is favored by low magmatic supply and low-angle detachment faulting occurring at segment ends. The pervasive fracturing and faulting at these discontinuities favor circulation of hydrothermal fluids and occurrence of high-temperature vent sites.

Detailed Study of Three Contiguous Segments of the Mid-Atlantic Ridge, South of the Azores (37° N to 38°30′ N), Using Acoustic Imaging Coupled with Submersible Observations

Using a new tool of seafloor characterisation (sonar images from FARA-SIGMA cruise; Needham et al., 1992), coupled with submersible observations (DIVA1 cruise) we compare, at different scales of observation, three contiguous segments of the Mid-Atlantic Ridge, South of the Azores Triple Junction, between 37° N and 38°30′ N.The two northernmost segments (‘38°20′ N’ and Menez-Gwen) show unusual morphological features for the MAR; the rift valley is absent and the present-day magmatism is focused on shallow axial volcanoes. On the third segment (Lucky Strike), the morphology is the one usually found on the MAR. On the Menez-Gwen and ‘38°20′ N’ segments, volcanic constructional activity can obliterate, during periods of high magmatic supply, the morphology inherited from tectonic activity. The dive results constrain the recent evolution of each segment and show that a temporal variability in volcanic dynamics exists. On the three segments, outcrops of eruptive lavas alternate with large areas of explosive volcanic ejecta. This cycle in volcanic activity is influenced by changes in water depth, both spatially (i.e. between segments) and temporally (i.e. for the same segment through time).Each segment has known a specific history in its accretionary processes with a succession of tectonic and volcanic predominance and changes in its volcanic phases between volcanic ejecta and effusive dynamics.The hydrothermal activity is focused at the central part of each segment and is controlled by the presence of fresh lava and major tectonic features.

Mantle 3He in hydrothermal vents and plume of the Lucky Strike site (MAR 37°17′N) and associated geothermal heat flux

Abstract The helium isotopes composition of hydrothermal fluids at the Lucky Strike vent field has been determined by mass spectrometry. [4He] end-member concentration is in the 1.2 × 10−5–2.6 × 10−5 cm3 STP/g range and the 3He/4He ratio, R, is 8.13 ± 0.05 × Ra. The helium-3 budget over the active vent field was estimated from the 3He mapping of the plume advected along the rift valley, using a 1-D advection box-model constrained with current velocity measurements. This leads to a 3He flux of 2.3 × 10−8 mol/s. The scaling of the heat flux from the Lucky Strike site to that of 3He, using the 3He/heat ratio measured in the hydrothermal fluids (5.2 × 10−18–7.7 × 10−18 mol/J) allows an initial evaluation of the total geothermal heat output from the site, in the 3800 ± 1200 MW range. This flux, of the same order as that of the Endeavour segment (Juan de Fuca Ridge), indicates the present high level of the hydrothermal activity in this section of the Mid-Atlantic Ridge.

Helium isotopes at the Rainbow hydrothermal site (Mid-Atlantic Ridge, 36°14'N)

The 3He/4He ratio and helium concentration have been measured in the vent fluids and the dispersing plume of the Rainbow hydrothermal site, on the Mid-Atlantic Ridge (MAR). The 3He/4He ratio (7.51 Ra) and 3He end-member concentration (25 pmol/kg) are in the range of observed values elsewhere on mid-ocean ridges, pointing to the relative homogeneity of the upper mantle with respect to helium isotope geochemistry. 3He is linearly correlated with methane and manganese throughout the plume, with CH4/3He and Mn/3He ratios identical to those measured in the hot fluids. The bulk residence time of the plume in the rift valley estimated from the plume 3He budget is V20 days. The 3He flux transported by the plume, calculated from current-meter data, is 12.3: 3 nmol/s, requiring a flux of 490 kg/s of high-temperature fluid. The scaling of the heat flux emitted by the Rainbow site to that of 3He, using the 3He/heat ratio measured in the hydrothermal fluids (9.3 :2U10318 mol/J), indicates a heat output of 1320 MW. With a regional spreading rate of V25 mm/year, we calculate that the annual 3He flux for this section of the MAR is of the order of 0.5:0.2 mmol per kilometre of ridge per millimetre of newly formed crust. This figure compares well with the flux calculated for the neighbouring Lucky Strike segment. Although the uncertainties remain large, both fluxes are V40-50% above the world average (0.33 mmol/km/mm), thus supporting earlier suggestions that the intensity of the hydrothermal activity south of the Azores is higher than expected from the regional spreading rate.

FAMOUS and AMAR segments on the Mid-Atlantic Ridge: ubiquitous hydrothermal Mn, CH4, δ3He signals along the rift valley walls and rift offsets

Dynamic hydrocast experiments enabled Mn (TDM), CH4 concentrations and δ3He ratio to be recorded through vertical cross-sections of hydrothermal plumes along the FAMOUS segment and the southern part of the AMAR segment on the Mid-Atlantic Ridge between 36°N and 37°N. Mn, CH4 and δ3He figures all along both segments are well above the seawater background in the open ocean: they are interpreted to be the result of time-integrated hydrothermal discharges dispersed and mixed in a closed basin delineated by the rift valley and the segment ends. Hydrothermal activity along the FAMOUS and AMAR segments appears to be similar. A comparison of the residence times of the three tracers from the dispersed, time-integrated signals is proposed. Although the background values in these closed basin are high, some proximal and (or) large hydrothermal inputs, overprinted on the general time-integrated plume, can be detected (i.e. the Rainbow site south of AMAR). Based on the depth and the location of plumes, hydrothermal activity is not, by far, limited to the neo-volcanic inner floor of the valley and should involve the walls and complex offsets of the rift valley. Considering the Mn and CH4 concentrations in these plumes, two types of ocean–mantle interaction may be represented: hot, focused discharges on ultramafic exposures (Rainbow site) and low-temperature diffuse serpentinisation.

Distribution of dissolved sulfide, methane, and manganese near the seafloor at the Lucky Strike (37°17′N) and Menez Gwen (37°50′N) hydrothermal vent sites on the mid-Atlantic Ridge

During the 1994 DIVA-1 cruise, the distribution of sulfide, methane and total dissolvable manganese (TDM) in the water column was examined near the seafloor at two locations on the mid-Atlantic ridge (MAR) affected by hydrothermal venting. Samples for this study were obtained using a miniature rosette mounted on the Nautile submersible and hydrographic data was collected using a CTD probe. Water samples were analyzed on board ship (sulfide and methane) or at the shore laboratory (TDM). The results presented here show that nanomolar level sulfide measurements can be used to describe hydrothermal plumes. In the first study area, called the Lucky Strike hydrothermal field (MAR at 37°17′N), the distribution of hydrothermal tracers (<5 m above the seafloor) was strongly influenced by the location of known hydrothermal vents. A study of water samples collected above the Tour Eiffel smoker shows that for the buoyant plume, sulfide is rapidly removed, while TDM and methane behave similarly (conservatively) upon mixing with ambient seawater. At the Menez Gwen hydrothermal field, which was the second site and which was discovered during the DIVA-1 cruise, a distribution map (3 km×4 km) of sulfide, TDM, and methane concentrations was completed. The highest tracer concentrations were found in close proximity to the vent field and ca. 1 km to the west of the venting site. These and other data are evidence for possibly additional venting on the western wall of the axial graben of the Menez Gwen segment.

Extensive magmatic and hydrothermal activity documented in Manus Basin

In 1995, a joint French-Japanese cruise (ManusFlux) explored the Manus Spreading Center and some sites of the South Eastern Rift in the Manus Basin off the coast of Papua New Guinea (Figure 1) with the Japanese submersible [Auzende et al., 1996; Gamo et al., 1997]. In the framework of this joint project, called New STARMER, the French and Japanese teams completed the Manaute cruise with the R/V LAtalante and the Ifremer submersible Nautile in April and May 2000. The Manaute cruise explored and sampled volcanic and hydrothermal sites in the eastern part of Manus Basin and on the spreading axis of the Western Ridge (WR). The eastern part of the Manus Basin (Figure 2) was interpreted as a stretched-back arc basin crust showing alternatively elongated ridges and isolated volcanoes where accretion and hydrothermal activity are distributed [Martinez and Taylor, 1996; Binns et al., 1993].