Highly magnetised and differentiated basalts at the 18–19° S propagating spreading centre in the North Fiji Basin

Geochemical and magnetic properties of basalts dredged along a propagating spreading centre (central part of the North Fiji Basin) have been analysed. Two phenomena appear to be important in controlling variations of natural remanent magnetisation (NRM): 1) an extensive magma differentiation to Fe-Ti basalts increases with the intensity of NRM; and both increase towards the propagator tip, 2) low temperature oxidation (maghemitisation) seems to have already occurred along the ridge axis for the zero age basalts. This paper shows that despite the apparent lack of correlation between NRM and the maghemitisation process (masked by the effect of extensive magma differentiation), the latter involves a change of the domain state of magnetic carriers, from pseudo single-domain to single-domain. We separated the NRM into 4 partial NRMs (pNRM) depending on the coercivity of grains. This analysis showed that the contribution to the remanence of grains with high coercivity increases with maghemitisation, whereas the contribution of grains with weak coercivity decreases.Despite the relatively high degree of maghemitisation, the variations of natural remanent magnetisation intensity principally reflect the variations of magmatic processes. This joint work on magnetic and magmatic properties of basalts collected on the NS-propagating spreading centre from the North Fiji Basin reinforces the magnetic telechemistry hypothesis of Vogt (1979).     

Rocks from the Gibbs fracture zone and the Minia seamount near 53°N in the Atlantic Ocean

Rocks dredged and drilled from both the rift mountains of the Mid-Atlantic Ridge (Minia seamount) and from the northern wall and the median ridge of the adjacent Gibbs fracture zone near 53°N include tholeiites, serpentinized and mylonitized peridoties, and gabbroic rock.The tholeiites include: (1) pyroxene-tholeiites, commonly without phenocrysts and containing less than 15% Al₂O₃; (2) plagioclase-tholeiites with small (1.1–2.2 mm length) plagioclase phenocrysts and an Al₂O₃ content varying between 14–17%; and (3) high-alumina plagioclase tholeiites with large (> 2.5 mm length) plagioclase phenocrysts and more than 17% Al₂O₃. The apparently transitional differences among the three groups support the possibility that differentiation by crystal fractionation of the high-alumina plagioclase-tholeiites gave rise to the plagioclase-tholeiites with less Al₂O₃ and smaller phenocrysts and to the pyroxene-tholeiites. A small portion of the basalts ampled show effects of low-grade metamorphism. The peridotites may represent evidence of intrusive emplacement of peridotitic material beneath the tholeiitic rocks.     

Spreading rate dependence of the axial topography and gravity of the Mid‐Atlantic ridge between 22°N and 38°N

A method is developed to quantify the relationship between the ridge axial topography and gravity and the spreading rate along the Mid‐Atlantic Ridge between 22 and 38°N. This relationship reflects the variations of slope of the best‐fit line of topography and gravity spectra with the spreading rate of the ridge segments. The slope of the best‐fit line of topography spectrum becomes smaller as the spreading rate increases, indicating that with increasing spreading rate more energy of the ridge axial topography shifts into high‐frequency bands. The spreading rate dependence of the ridge axial topography may be explained by an anomalous thermal structure beneath the ridge. No significant correlation was found between the slope of the best‐fit line of gravity spectrum and the spreading rate in this region. The lack of spreading rate dependence of the ridge axial gravity may be attributable to the isostatic compensation of the spreading center.     

Study on the porosity of a metalliferous sediment core from the East Pacific Rise 13°N

研究了采自东太平洋海隆13°N（EPR13°N）的一个热液成因含金属箱式沉积物岩心E272的孔隙率变化特征。样品孔隙率在70．O％～85．2％之间,顶部层位比底部层位高14％左右。孔隙率随深度增加而指数式单调递减,平均递减梯度约为-0.31％／cm。该岩心上部层位呈红棕色,下部层位呈黄绿色,表明该岩心经历了显著的早期化学成岩作用。前人关于该岩心的元素地球化学,粒度和年代学数据,共同证明该岩心孔隙率随深度变化特征受稳态压缩过程控制。其孔隙率随深度的变化关系符合稳态压缩模型的经验拟合公式。E272顶部沉积物．水界面处溶解物质与海水的交换以离子扩散机制为主,而岩芯内部溶解物质的迁移则可能主要受自下而上的孔隙水流动机制控制,这与岩芯中活动元素含量上部高,下部低的分布模式一致。该研究对将来进一步讨论E272岩心在早期成岩作用中的化学过程具有指导意义。     

A detailed magnetic study of the Reykjanes Ridge between 63°00′N and 63°40′N

Immediately southwest of Iceland, the Reykjanes Ridge consists of a series ofen échelon, elongate ridges superposed on an elevated, smooth plateau. We have interpreted a detailed magnetic study of the portion of the Reykjanes Ridge between 63°00N and 63°40N on the Icelandic insular shelf. Because the seafloor is very shallow in our survey area (100–500 m), the surface magnetic survey is equivalent to a high-sensitivity, nearbottom experiment using a deep-towed magnetometer. We have performed two-dimensional inversions of the magnetic data along profiles perpendicular to the volcanic ridges. The inversions, which yield the magnetization distribution responsible for the observed magnetic field, allow us to locate the zones of most recent volcanism and to measure spreading rates accurately. We estimate the average half spreading rate over the last 0.72 m.y. to have been 10 mm/yr within the survey area. The two-dimensional inversions allow us also to measure polarity transition widths, which provide an indirect measure of the width of the zone of crustal accretion. We find a mean transition width on the order of 4.5±1.6 km. The observed range of transition widths (2 to 8.4 km) and their mean value are characteristic of slow-spreading centers, where the locus of crustal accretion may be prone to lateral shifts depending on the availability of magmatic sources. These results suggest that, despite the unique volcanotectonic setting of the Reykjanes Ridge, the scale at which crustal accretion occurs along it may be similar to that at which it occurs along other slow-spreading centers. The polarity transition width measurements suggest a zone of crustal accretion 4–9 km wide. This value is consistent with the observed width of volcanic systems of the Reykjanes Peninsula. The magnetization amplitudes inferred from our inversions are in general agreement with NRM intensity values of dredge samples measured by De Boer (1975) and ourselves. Our thermomagnetic measurements do not support the hypothesis that the low amplitude of magnetic anomalies near Iceland is the result of a high oxidation state of the basalts. We suggest that the observed reduction in magnetic anomaly amplitude toward Iceland may be the result of an increase in the size of pillows and other igneous units.     

Contrasted sulphide chemistries in the environment of 13°N EPR vent fauna

To improve our understanding of the environmental constraints exerted on vent fauna, we investigated sulphide chemistry in the habitats of Riftia pachyptila and Alvinella pompejana, at the Genesis and Elsa EPR 13°N sites. Temperature, pH and sulphide measurement series were acquired in situ, around the organisms, from the submersible Nautile. Hot fluid samples were also collected to evaluate end-member composition at these sites. Under the assumption of conservative mixing, pH, total sulphide concentration and sulphide speciation gradients in relation to temperature were modelled. From the comparison of measured and calculated concentrations, deviation from conservative behaviour was highlighted for total sulphide versus temperature. While the observed sulphide depletion around tubeworms suggests significant subsurface removal or biological consumption, the apparent sulphide enrichment in the alvinellid environment may reveal either conductive cooling of diffusing fluids or a secondary sulphide source. The calculated sulphide speciation appears to be contrasted at the two sites studied. Because of the low iron content in Genesis fluid, iron sulphide would not constitute a dominant sulphide species and the toxic H₂S form would be predominant in the mixing zone. By contrast, iron is expected to play a dominant role in sulphide speciation at the Elsa site where the end-member is iron rich. With respect to sulphide, the conditions encountered in the different habitats considered in this study are strongly contrasted. A low fluid flux was observed in the R. pachyptila habitat, contrasting with previous ideas, and suggests that sulphide availability could be a major limiting factor. Particularly, the bioavailable HS⁻ form is expected to vary weakly along the mixing gradient. In contrast, sulphide in the A. pompejana environment is shown to be particularly high, about one order of magnitude higher than observed for other Eastern Pacific alvinellids. At Genesis, because of the acidic pH and low iron conditions encountered, exposure to high levels of toxic sulphide is expected. A. pompejana thus appears to be particularly tolerant to such toxic conditions, but, as previously suggested, less severe conditions may also be found when iron is rich enough in the medium to dominate sulphide chemistry.     

Methane and methane carbon isotope ratios in the Northeast Atlantic including the Mid-Atlantic Ridge (50°N)

Previous work has shown that methane anomalies frequently occur within the rift valley of the Mid-Atlantic Ridge (MAR). The plumes appear confined within the high, steep walls of the valley, and it is not known whether methane may escape to the open ocean outside. In order to investigate this question, the concentration and ¹³C/¹²C ratio of methane together with CCl₃F concentration were measured in the northeastern Atlantic including the rift valley near 50°N. This segment contained methane plumes centered several 100 m above the valley floor with δ¹³C values mostly between –15‰ and –10‰. A limited number of helium isotope measurements showed that δ³He increased to 17% at the bottom of the valley, which suggests the helium and methane sources may be spatially separated. In the eastern Atlantic away from the ridge (48°N, 20°W), the methane concentration decreased monotonically from the surface to the bottom, but the methane δ¹³C exhibited a mid-water maximum of about –25‰. The bottom water methane contained a significantly lower δ¹³C of about –36‰. Thus, it appears that isotopically heavy methane escapes from the MAR into North Atlantic Deep Water (NADW) that contacts the ridge crest while circulating to the east. The formation of NADW supplies isotopically light methane that dilutes the input of heavy carbon from the ridge. We employed a time-dependent box model to calculate the extent of isotope dilution and thereby the flux of MAR methane into the NADW circulation. The degree of methane oxidation, which affects the ¹³C/¹²C of methane through kinetic isotope fractionation, was estimated by comparing methane and CFC-11 model results with observations. The model calculations indicate a MAR methane source of about 0.06×10⁻⁹ mol L⁻¹ yr⁻¹ to waters at the depth of the ridge crest. Assuming this extends to a 500 m thick layer over half of the entire Atlantic, the amount of methane escaping from the MAR to the open ocean is estimated to be about 1×10⁹ mol yr⁻¹. The total production of methane within the rift valley is likely much greater than the flux from the valley to the outside because of local oxidation. This implies that serpentinization of ultramafic rocks supports much of methane production in the rift valley because the amount expected from basalt degassing in association with mantle helium (<0.6×10⁹ mol CH₄ yr⁻¹) is less than even the net amount escaping from the valley. The model results also indicate the methane specific oxidation rate is about 0.05 yr⁻¹ in open waters of the northern Atlantic.     

Seasonal variability of δ15N in sinking particles in the Benguela upwelling region

Temporal changes in δ¹⁵N values of sinking particles collected with sediment traps in the Benguela upwelling regime off southwest Africa mirrored variations in the input of inorganic nitrogen to the surface water. Reductions in δ¹⁵N (to as low as 2.5‰) corresponded to low sea surface temperatures during austral spring and late austral autumn/early winter, indicating increased nitrate availability due to the presence of recently upwelled water. High particulate fluxes accompanied the low δ¹⁵N values and sea surface temperatures, reflecting increased productivity, fueled by the upwelled nutrients. High δ¹⁵N values (up to 13.1‰) coincided with high sea surface temperatures and low particle fluxes. In this area, the seaward extension of upwelling filaments, which usually occurs twice yearly, brings nutrient-rich water to the euphotic zone and leads to elevated productivity and relatively lower δ¹⁵N values of the particulate nitrogen. Satellite images of ocean chlorophyll show that productivity variations coincide with δ¹⁵N changes. The observed isotopic pattern does not appear to have been caused by variations in the species composition of the phytoplankton assemblage. Calculations based on δ¹⁵N of the sinking particulate nitrogen show that the surface nitrate pool was more depleted during late austral summer/early fall and mid-winter and that supply exceeded demand during the intense spring bloom and in late austral fall. The main uncertainty associated with these estimates is the effect of diagenesis on δ¹⁵N and possible variability in preservation of the isotope signal between periods of high and low particle flux.     

Smectite formation in metalliferous sediments near the East Pacific Rise at 13°N

A 43 cm long E271 sediment core collected near the East Pacific Rise(EPR) at 13°N were studied to investigate the origin of smectite for understanding better the geochemical behavior of hydrothermal material after deposition.E271 sediments are typical metalliferous sediments. After removal of organic matter, carbonate, biogenic opal,and Fe-Mn oxide by a series of chemical procedures, clay minerals(2 μm) were investigated by X-ray diffraction,chemical analysis and Si isotope analysis. Due to the influence of seafloor hydrothermal activity and close to continent, the sources of clay minerals are complex. Illite, chlorite and kaolinite are suggested to be transported from either North or Central America by rivers or winds, but smectite is authigenic. It is enriched in iron, and its contents are highest in clay minerals. Data show that smectite is most likely formed by the reaction of hydrothermal Fe-oxyhydroxide with silica and seawater in metalliferous sediments. The Si that participates in this reaction may be derived from siliceous microfossils(diatoms or radiolarians), hydrothermal fluids, or detrital mineral phases. And their δ30 Si values are higher than those of authigenic smectites, which implies that a Si isotope fractionation occurs during the formation because of the selective absorption of light Si isotopes onto Feoxyhydroxides. Sm/Fe mass ratios(a proxy for overall REE/Fe ratio) in E271 clay minerals are lower than those in metalliferous sediments, as well as distal hydrothermal plume particles and terrigenous clay minerals. This result suggests that some REE are lost during the smectite formation, perhaps because their large ionic radii of REE scavenged by Fe-oxyhydroxides preclude substitution in either tetrahedral or octahedral lattice sites of this mineral structure, which decreases the value of metalliferous sediments as a potential resource for REE.     

New Observations on the Distribution of Past and Present Hydrothermal Activity in the TAG Area of the Mid-Atlantic Ridge (26°08′ N)

Seafloor acoustic and photographic imagery combined with high- resolution bathymetry are used to investigate the geologic and tectonic relations between active and relict zones of hydrothermal venting in the TAG (Trans-Atlantic Geotraverse) hydrothermal field at 26°08N on the Mid-Atlantic Ridge (MAR). The TAG field consists of a large, currently active, high-temperature mound, two relict zones (the Alvin and Mir zones), and an active low-temperature zone. The active mound and the Alvin relict zone lie along a series of closely-spaced, axis-parallel (NNE-trending) faults in an area of active extension east of the neovolcanic zone. The Alvin zone extends for 2.5 km along these faults from the valley floor onto the eastern wall, and consists of at least five mounds identified using DSL-120 sidescan sonar and bathymetric data. The existence of sulfide structures on most of these mounds is verified with near-bottom electronic still camera (ESC) images from the Argo-II deep-towed vehicle, and is confirmed in at least one case with collected samples. Two of these mounds were previously unidentified. The existence of these mounds extends the length of the Alvin zone by ~0.5 km to the south. Much of the Alvin relict zone appears to be buried by debris from a large mass wasting event on the eastern wall of the median valley. The Mir zone, located on normal fault blocks of the eastern valley wall, cannot be clearly identified in the sidescan data and no structural connections from it to the active mound or Alvin zone can be discerned. The active mound is located at the intersection of an older oblique fault set with the younger axis- parallel faults which extend into the Alvin relict zone, and no fresh volcanics are observed in the vicinity of the mound. The fact that both the active mound and the Alvin relict zone lie along the same set of active, axis-parallel faults suggests that the faults may be a major control on the location of hydrothermal activity by providing pathways for fluid flow from a heat source at the ridge axis.     

Transport of Antarctic Bottom Water through passages in the East Azores Ridge (37° N) in the East Atlantic

The structure of northerly overflow of Antarctic Bottom Water (AABW) through passages in the East Azores Ridge (37° N) in the East Atlantic from the Madeira Basin to the Iberian Basin is studied on the basis of hydrographic measurements carried out by the Institute of Oceanology, Russian Academy of Sciences (RAS) in October 2011, historical World Ocean Data Base 2009, and recent data on the bottom topography. The overflow of the coldest layers of this water occurs through two passages with close depths at 16° W (Discovery Gap) and at 19°30′ W (nameless Western Gap). It is shown that it is likely that the role of the latter passage in water transport was underestimated in earlier publications because the water (2.01°C) found in the region north of the Western Gap was cooler than in the region north of the Discovery Gap (2.03°C). In 2011, we found a decrease of 0.01°C in the AABW temperature near the bottom compared to previous measurements in 1982 (from 2.011°C to 2.002°C). Analysis of the historical database shows that this decrease is most likely caused by the cooling trend in the abyssal waters in the East Atlantic basins.     

New constraints on the width of the zone of active faulting on the East Pacific Rise 8° 30′ N – 10° 00′ N from Sea Beam Bathymetry and SeaMARC II Side-scan Sonar

Sea Beam bathymetry and SeaMARC II side-scan sonar data are used to constrain the width of the zone of active faulting (plate boundary zone) to be 90 km (0.8 Ma) wide along the East Pacific Rise 8° 30N – 10° 00N. Fault scarps, identified on the basis of contoured, shaded relief and slope intensity maps of bathymetry, are measured. These scarp measurements, used in conjunction with data from a separate near-axis study, show that both inward- and outward-facing fault scarps increase in height away from the ridge axis, reaching average heights of 100 m at 0.8±0.2 Ma, 45±10 km from the ridge axis. Beyond this distance, there is no significant increase in scarp height. Earlier studies had suggested that the width of the zone of active faulting for outward-dipping faults might be significantly narrower than for inward-dipping faults. A lower crustal decoupling zone between brittle crust and strong upper mantle is predicted to exist out to 20–200 km from the ridge based on previously published lithospheric models. Such a decoupling zone may explain why outward-dipping faults continue to be active as far off-axis as inward-dipping faults. If the width of the zone of active faulting is controlled by the width of a lower crustal decoupling zone, our observations predict an 90 km wide decoupling zone in the lower oceanic crust at this location.     

Hydrothermal Mineral Assemblages at 71° N of the Mid-Atlantic Ridge (First Results)

Oceanology - The article discusses the preliminary results of plume and bottom sediment studies of the Trollveggen hydrothermal vent field based on data from cruise 68 of the R/V Akademik Mstislav...     

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.     

Bottom fauna associated with mussel beds and alvinellid communities in the hydrothermal field at 9° N of the East Pacific Rise

Samples from mytilid mollusk and alvinellid polychaet communities and associated fauna were collected during cruise 49 of R/V Akademik Mstislav Keldysh at 9° N of the East Pacific Rise. The mytilids Bathymodiolus thermophylus occupied zones of diffuse releases of hydrothermal solutions with temperature anomalies a few degrees in amplitude. The alvinellid assemblages were located in the hottest parts of hydrothermal vents (～ 40°C) over sulfide mounds. The taxonomic compositions of these kinds of assemblages are different. The taxonomic richness of mytilid communities is almost twice higher. A comparison of the samples using Jaccard’s index showed a greater similarity among the alvinellid samples than that between the mytilid ones. With respect to the species number, polychaets formed the most diverse group, while gastropods were the second most diverse. The dominant species had greater proportions in alvinellid communities than in mytilid ones. This study showed great differences between the mytilid and alvinellid associations in their species composition and community structure.     

Pathway and variability of deep circulation around 40°N in the northwest Pacific Ocean

To clarify the global deep-water circulation in the northwest Pacific, we conducted current observations with seven moorings at 40°N east of Japan from May 2007 to October 2008, together with hydrographic observations. By analyzing the data, while taking into consideration that the deep circulation has a northward component in this region and carries low-silica, high-dissolved-oxygen water, we clarified that the deep circulation flows within the region between 144°30′ and 146°10′E at 40°N on and east of the eastern slope of the Japan Trench with marked variability; the deep circulation flows partly on the eastern slope of the trench and mainly to the east during P1 (10 May–24 November 2007), is confined to the eastern slope of the trench during P2 (25 November 2007–20 May 2008), and flows on and to the immediate east of the eastern slope of the trench during P3 (21 May–15 October 2008). Previous studies have identified two branches of the deep circulation at lower latitudes in the western North Pacific; one flows off the western trenches and the other detours near the Shatsky Rise. It was thus concluded that the eastern branch flows westward at 38°N and then northward to the east of the trench, finally joining the western branch around 40°N during P1 and P3, whereas the eastern branch passes westward south of 38°N, joins the western branch around 38°N, and flows northward on the eastern slope of the trench during P2.     

Rate of palagonitization and manganese coating on basaltic rocks from the Rift Valley in the Atlantic Ocean near 36°50′N

Various types of rocks consisting of plagioclase basalts, olivine basalts, pyroxene basalts, picritic basalts and volcanoclastics were found in the Rift Valley between 36°52′N and 36°47′N in the Atlantic Ocean. Palagonite and manganese are the main coating materials encountered on the chilled margins of the pillow-lava fragments. Palagonite is often associated with montmorillonite and chlorite. The thickness of palagonite coating the chilled margins increases in the fragments found from the innerfloor axis towards the walls of the Rift Valley. There is a linear correlation between the thickness of palagonite and that of the manganese encrustation. Considering a rate of manganese and palagonite accumulation of about $\text{3μ}\text{10}{}^3$ year it is speculated that the age of a hill located near the axis of the inner floor near 36°50′N is between 3,000 and 12,000 years. The relative age of the inner floor near the intersection of the Rift Valley walls is less than 120,000 years. The specimens having a thicker manganese and palagonite crust located at 4–6 km away from the inner floor axis are 130,000–482,000 years old.