Spreading Southwest Indian Ridge Research Articles

Microseismicity and lithosphere thickness at a nearly-amagmatic oceanic detachment fault system

Oceanic detachment faults play a central role in accommodating the plate divergence at slow-ultraslow spreading mid-ocean ridges. Successive flip-flop detachment faults in a nearly-amagmatic region of the ultraslow spreading Southwest Indian Ridge (SWIR) at 64°30’E accommodate ~100% of plate divergence, with mostly ultramafic smooth seafloor. Here we present microseismicity data, recorded by ocean bottom seismometers, showing that the axial brittle lithosphere is on the order of 15 km thick under the nearly-amagmatic smooth seafloor, which is no thicker than under nearby volcanic seafloor or at more magmatic SWIR detachment systems. Our data reveal that microearthquakes with normal focal mechanisms are colocated with seismically-imaged damage zones of the active detachment fault and of antithetic hanging-wall faults. The level of the hanging-wall seismicity is significantly higher than that documented at more magmatic detachments of slow-ultraslow ridges, which may be a unique feature of nearly-amagmatic flip-flop detachment systems.

Shallow recycling of lower continental crust: The Mahoney Seamount at the Southwest Indian Ridge

The Mahoney Seamount is a recently discovered volcanic edifice located 4 km north of the ultra-slow spreading Southwest Indian Ridge (SWIR). The SWIR is one of the slowest spreading ridges worldwide with a full spreading rate of ∼14 mm/year and low magmatic productivity. We report that highly vesicular basalts from the Mahoney Seamount have unradiogenic Nd-Hf together with radiogenic Sr isotopic compositions. Their distinct low 206Pb/204Pb isotope signature combined with high 207Pb/204Pb and 208Pb/204Pb is best explained by melting of a mantle that has been strongly influenced by stranded lower continental crust. The geographic distribution of the isotopic variability favors the idea of shallow recycling of lower continental crust isolated for a longer period contributing to melts forming Mahoney Seamount through off-axis fault systems. The isotopic composition of Mahoney Seamount lavas shares many characteristics with EM-1 sources and the DUPAL signature. Previous isotopic studies of the SWIR basalts proposed recycling of ancient subcontinental lithospheric mantle (SCLM) or pelagic sediments with oceanic crust to be responsible for this enriched isotopic signature. Lu/Hf and Sm/Nd ratios of pelagic sediments would result in decoupled 143Nd/144Nd and 176Hf/177Hf ratios. This decoupling is also observed in Ejeda-Bekily dikes from Madagascar, but those are believed to sample the SCLM dispersed in the Indian Ocean. However, Mahoney Seamount shows no decoupling in those isotopic systems and the restricted occurrence of the extreme lower continental crustal signature at Mahoney Seamount implies that the enriched isotopic signature has a different origin.

Plate-driven micro-hotspots and the evolution of the Dragon Flag melting anomaly, Southwest Indian Ridge

Micro hotspots represent excess volcanism at the ultraslow spreading Southwest Indian Ridge (SWIR) unrelated to mantle hotspots, but to focused melt flow in the mantle, wide volcano spacing, and/or increased mantle fertility. Individual micro hotspot can reflect variations in regional mantle fertility, simultaneously affecting 100's of km of ridge, or extreme melt focusing to isolated segments in regions with unusually thick lithosphere. The Dragon Flag melting anomaly, erupting isotopically moderately depleted low-K tholeiite, is the best example of the former: an enormous ridge-centered volcano at 50.5°E. It is far from Crozet, and lies at the apex of a V-shaped trace consisting of ∼1 km anomalously elevated seafloor that extends ∼400 km to the northwest and southeast, indicating a sudden onset of excess volcanism at ∼11-8 Ma. This trend is opposite to that predicted by the hotspot framework, with inconsistent geochemistry. Narrowgate at 14.7°E is one of many micro hotspots that represent isolated large volcanic centers bounded by long amagmatic ridge sections. It also lies at the apex of an eastward V-shaped bathymetric trend, but erupts large volumes of alkali basalt.Both micro hotspot varieties are an order of magnitude smaller than Wilson's classic hotspots, and are not fixed in the hotspot referenced frame. They can grow and remain stationary for extended periods, or migrate freely with respect to each other; driven by the evolving plate tectonic stress field. Dragon Flag, represents a plate reorganization during a prolong period of enhanced melt supply. Narrowgate, on the other hand represents excess volcanism due to extreme deep melt focusing of low degree melt from an unusually wide region in the mantle to an isolated volcanic segment where thick lithosphere caps melting at great depth. To constrain the origins of the micro hotspots six examples are evaluated in terms of their major and isotopic composition, prior plate history, depth and extent of melting and tectonic context. This includes new major, trace element, and heavy isotope data for the Dragon Flag Supersegment, and unpublished data for the Joseph Mayes and Narrowgate segments.

Hydrothermal signatures in sediments from eastern Southwest Indian Ridge 63°E to 68°E

Hydrothermal vent incidence is proportional to the spreading rate of the mid-oceanic ridges (MORs) (Baker et al., 1996). However, the ultra-slow spreading Southwest Indian Ridge (SWIR) is unique and has a relatively higher incidence of hydrothermal vent fields compared with other ultra-slow spreading ridges (German et al., 1998; Baker and German, 2004). In the present study, sediment samples from the less studied easternmost section of the ultra-slow spreading SWIR are investigated to decipher the signatures of hydrothermal activity. The sediments are mainly composed of calcium carbonate (55–85%), Fe-Mn-(oxyhydr)oxide (13–27%), and residual aluminosilicate phases (2–17%). The metalliferous nature, low Mn concentration (393–1772 ppm), negative Ce, and positive Y anomalies in the shale-normalized rare earth element (REE) pattern of the bulk sediments provide evidence for the presence of hydrothermal inputs in most of the sediments. The fractionation among lighter REEs over heavier REEs in bulk sediments indicates the high-temperature hydrothermal plume fall-out to the sediments. The leached fraction also shows similar negative Ce and positive Y anomalies, and indicates that the Fe-Mn-(oxyhydr)oxide precipitates in these sediments may be dominantly of hydrothermal origin. In addition to hydrothermal Fe-Mn-(oxyhydr)oxide phases, the residual fraction of the sediments also consists of minor quantities of hydrothermally sourced sulfate phases. Otherwise, the residual fraction is significantly dominated by mid-oceanic ridge basalt (MORB) derived components in addition to serpentinized peridotite and terrigenous material.Among the studied sediments, sample E-3-16 from Fuji Dome (influenced by hydrothermal activity) has high zinc concentrations, which indicates the possibility of a new vent field in the vicinity. The present study also introduces a new geochemical tool (Zn/Fe vs Co/Zn proxy) for assessing the contribution of hydrothermal components in the sediments. The Zn/Fe vs Co/Zn proxy will be more useful in MOR settings hosting sulfides dominated by sphalerite.The influence of hydrothermal activity in most of the studied sediments from the eastern SWIR indicates that the ultra-slow spreading eastern SWIR has the potential to possess a larger number of hydrothermal vent fields.

Osmium isotope compositions and highly siderophile element abundances in abyssal peridotites from the Southwest Indian Ridge: Implications for evolution of the oceanic upper mantle

Abyssal peridotites are extensively exposed on the seafloor of amagmatic segments at the ultraslow spreading Southwest Indian Ridge (SWIR). This study presents abundances of highly siderophile elements (HSEs; Os, Ir, Ru, Pt, Pd and Re) and Re–Os isotope compositions as well as major and trace element concentrations of nine spinel-facies peridotites from the Dragon Bone amagmatic segment (~53°E) and the easternmost part (~63.5°E and 69.5°E) of the SWIR. These peridotites have experienced intense serpentinization and seafloor weathering in addition to variable degrees of melt depletion (bulk rock Al2O3 = 0.74–2.73 wt%). Low-temperature alterations, however, have a negligible effect on Os isotope compositions and concentrations of platinum-group elements. The 63.5°E peridotites displayed sub-chondritic Os/Ir and chondritic Pt/Ir ratios and slightly variable Re/Ir and Pd/Ir ratios. The observed positive covariance between 187Re/188Os ratios and γOs, Re and Pd abundances can be explained most likely by interaction of mantle residues with radiogenic basaltic melts, which is consistent with the petrographic observations indicative of melt infiltration. The 53°E peridotites, on the other hand, displayed larger variations in Os isotopes and relative HSE abundances, indicating a more complex evolutionary history. High Re abundances and 187Os/188Os ratios in some 53°E samples attest to Re and radiogenic Os addition, probably through reaction with S-undersaturated hydrous melts. Specifically, sample 21V-S9-D5‐2 exhibited a high 187Os/188Os ratio (0.148) and enrichment of Cu and Pt abundances, implying interaction with an oxidizing agent. Although the Re–Os isotopes of SWIR peridotites were superimposed by variable metasomatic processes, they preserve the ancient Os isotope signature (~1Ga) and record the melting episode at ~0.7 Ga during the closure of the Mozambique Ocean. We infer that ancient depleted and hence buoyant mantle exists beneath the SWIR, which probably supports the weakly magmatic rifted Marion Rise.

Macrofaunal burrowing enhances deep-sea carbonate lithification on the Southwest Indian Ridge

Abstract. Deep-sea carbonates represent an important type of sedimentary rock due to their effect on the composition of the upper oceanic crust and their contribution to deep-sea geochemical cycles. However, the role of deep-sea macrofauna in carbonate lithification remains poorly understood. A large lithified carbonate area, characterized by thriving benthic faunas and a tremendous amount of burrows, was discovered in 2008, blanketing the seafloor of the ultraslowly spreading Southwest Indian Ridge (SWIR). Benthic inhabitants – including echinoids, polychaetes, gastropods and crustaceans – are abundant in this carbonate lithified area. The burrowing features within these carbonate rocks, as well as the factors that may influence deep-sea carbonate lithification, were examined. We suggest that burrowing in these carbonate rocks enhances deep-sea carbonate lithification. We propose that active bioturbation may trigger the dissolution of the original calcite and thus accelerate deep-sea carbonate lithification on mid-ocean ridges. Macrofaunal burrowing provides a novel driving force for deep-sea carbonate lithification at the seafloor, illuminating the geological and biological importance of bioturbation in global deep-sea carbonate rocks.

Geochemical impacts of hydrothermal activity on surface deposits at the Southwest Indian Ridge

Submarine hydrothermal circulation has attracted much scientific interest since seafloor hydrothermal activity was first observed in the 1970s; an area of particular interest is the impact of exported inorganic and organic materials from hydrothermal vent systems into the open ocean. In 2007, the first active hydrothermal vent field, with vent fluid temperatures up to 379 °C, was discovered at the ultraslow spreading Southwest Indian Ridge (SWIR), where active vents are much less abundant than fast spreading ridges, and the effect of hydrothermal extrusion on surface sediments is not fully understood. To explore how geochemical proxy signatures respond to hydrothermal activity, we investigated the distributions of elements, minerals and lipids in surficial normal marine sediments, metalliferous sediments and low-temperature hydrothermal deposits collected from the SWIR. The results showed different effects of hydrothermal activity on the surface deposits. The normal marine sediments were predominantly calcium carbonate characterized by > 42% CaO and > 90% calcite, with a significant autochthonous marine contribution to organic matter (OM) and a predominance of lower molecular weight alkanols and fatty acids; they were uninfluenced by hydrothermal activity but received some terrigenous input represented by abundant high molecular weight n-alkanes with an odd-over-even predominance. The near-field metalliferous sediments and hydrothermal deposits were very different. Some near-field metalliferous sediments were influenced by low-temperature hydrothermal activity, and their distributions of elements and minerals were similar to those of hydrothermal deposits, which were characterized by abundant Fe/Si and opal/nontronite. Other near-field metalliferous sediments were evidently influenced by mixing of high-temperature hydrothermal sulfides typically containing abundant Cu/Zn. With respect to the organic matter assemblages, near-field deposits contained little evidence for thermal maturation of organic matter and all were characterized by a strong microbial signature, including hopanoids, isoprenoidal and non-isoprenoidal dialkyl glycerol ether lipids, and low molecular weight n-alkanes with an even carbon number predominance. The far-field metalliferous sediments, despite the influence of non-buoyant plumes and slightly higher concentrations of hydrothermal-derived metals (e.g., Fe, Cu and Zn), had the same distribution of organic lipids and major mineral composition (> 90% calcite) as did normal marine sediments. Thus, the influence of non-buoyant plume inputs appears to have been minimal possibly due to the dilution of in situ microorganisms by normal marine organisms in sediment and seawater. Furthermore, these characteristics indicate inorganic indices based on abundant metal elements derived from the hydrothermal systems (such as Fe/Cu/Zn content, ∑REE/Fe, the ternary diagram of Fe, Cu×100 and Ca) are more sensitive, serving as better proxies than organic matter assemblages to differentiate the effects of diverse hydrothermal activity on surface deposits.

Chalcophile elemental compositions of MORBs from the ultraslow-spreading Southwest Indian Ridge and controls of lithospheric structure on S-saturated differentiation

Sulfide globules in mid-ocean ridge basalts (MORBs) from the ultra-slow spreading Southwest Indian Ridge (SWIR) are present in olivine crystals and in the matrix glass, indicating S-saturated fractionation during magmatic evolution. Most of the sulfides were in equilibrium with the magmas based on the Ni contents of the sulfides. The major element trends of MORBs from Zone C (48–51°E) of the SWIR define two distinct liquid lines of descent (LLD), including one which is composed of MORBs depleted in HREE. The MORBs have variable PGE concentrations with Pd contents ranging from 0.056 to 1.572ppb. They are PGE-depleted relative to Ni and Cu with high Cu/Pd ratios (55,000–1,785,000). Quantitative modeling using Pd vs. Ni correlations yields relatively low amounts of segregated sulfides for both groups assuming complete equilibration between the sulfides and the melt. However, the correlation of calculated sulfur contents with MgO suggests much higher amounts of segregated sulfides, indicating that the segregated sulfides are incompletely equilibrated with the magma.There are no obvious correlations between PGE concentrations and spreading rates for MORBs globally. MORBs from the SWIR show slowly decreasing PGE contents during magmatic evolution but exhibit higher sulfide segregation rates compared to MORBs from faster-spreading ridges, largely due to the poor equilibration between the segregated sulfides and the magma. Such incomplete equilibration could result from thicker lithosphere and an absence of steady magma chambers and conduits beneath ultra-slow spreading ridges.

Distribution of large-scale detachment faults on mid-ocean ridges in relation to spreading rates

Large-scale detachment faults on mid-ocean ridges (MORs) provide a window into the deeper earth. They have megamullion on their corrugated surfaces, with exposed lower crustal and upper mantle rocks, relatively high residual Bouguer gravity anomaly and P-wave velocity, and are commonly associated with oceanic core complex. According to 30 detachment faults identified on MORs, we found that their distances to the axis mostly range from 5 to 50 km, half-spreading rates range from 6.8 to 17 mm/a, and activity time ranges from recent to 3 Ma. Most of the detachment faults are developed on the slow spreading Mid-Atlantic Ridge (MAR) and ultra-slow spreading Southwest Indian Ridge (SWIR), with the dominant half-spreading rates of 7-13 mm/a, especially 10-13 mm/a. Furthermore, they mostly occur at the inside corner of one segment end and result in an asymmetric seafloor spreading. The detachment faults on MORs are mainly controlled by the tectonism and influenced by the magmatism. Long-lived detachment faults tend to be formed where the ridge magma supply is at a moderate level, although the tectonism is a first-order controlling factor. At the slow spreading ridges, detachment faults tend to occur where local magma supply is relatively low, whilst at the ultra-slow spreading ridges, they normally occur where local magma supply is relatively high. These faults are accompanied by hydrothermal activities, with their relationships being useful in the study of hydrothermal polymetallic sulfides and their origin.

Heterogeneous upper mantle Ne, Ar and Xe isotopic compositions and a possible Dupal noble gas signature recorded in basalts from the Southwest Indian Ridge

Variations in heavy noble gas (Ne, Ar, Xe) isotopic compositions provide unique insights into the nature of heterogeneities in the mantle. However, few precise constraints on mantle source heavy noble gas isotopic compositions are available due to ubiquitous shallow-level atmospheric contamination. As a result, the extent of heterogeneity in mid-ocean ridge basalt (MORB) mantle source Ne, Ar and Xe isotopic compositions is unknown. Basalts from the ultra-slow spreading Southwest Indian Ridge (SWIR) between 7°E and 25°E exhibit remarkable variability in He isotopic composition: SWIR 4He/3He spans half the total range observed in all mantle-derived basalts. Therefore, basalts from the SWIR provide a unique window into upper mantle heterogeneity and present an ideal opportunity to characterize variations in upper mantle heavy noble gas isotopic composition. Here we present new high-precision Ne, Ar and Xe isotopic compositions as well as He, CO2, Ne, Ar and Xe abundances measured in basalt glasses from the SWIR. After correcting the measured values for shallow-level atmospheric contamination, significant and systematic variations in mantle source Ne, Ar and Xe compositions are observed. We note that large variations in source 40Ar/36Ar and 129Xe/130Xe are observed in basalts removed from the influence of known hotspots, indicating a heterogeneous mid-ocean ridge basalt source. Thus, SWIR heavy noble gas data reveal a greater degree of source heterogeneity than is evident in the 4He/3He systematics alone. The observed heavy noble gas isotopic heterogeneities imply that the average MORB source 40Ar/36Ar and 129Xe/130Xe ratios are not yet well-determined.Variation in MORB source 40Ar/36Ar and 129Xe/130Xe at a given 4He/3He and 21Ne/22Ne may reflect heterogeneous recycling of atmospheric Ar and Xe. In particular, we find low mantle source 40Ar/36Ar and 129Xe/130Xe ratios in the eastern region of the study area, which may reflect the noble gas signature of the Dupal mantle domain. Our observations require that the sampled mantle domain either is very ancient (>4.45Ga) or has been metasomatized by subduction zone fluids carrying recycled atmospheric Ar and Xe. However, our Xe isotopic measurements indicate that differences between MORB and ocean island basalt (OIB) source noble gas compositions cannot be explained by recycling of atmospheric noble gases alone. Instead, a relatively undegassed mantle reservoir is required to account for OIB noble gases. The SWIR data demonstrate that the reservoir supplying primordial noble gases to mantle plumes differentiated from the MORB source early in Earth history, and the two reservoirs have not been homogenized over 4.45Ga of mantle convection.

Characteristics of tectonomagmatic earthquake swarms at the Southwest Indian Ridge between 16°E and 25°E

The ultraslow spreading Southwest Indian Ridge (SWIR) is a prominent end member of the global mid-ocean ridge system. It spreads with a full-rate of 14 mm/y to 16 mm/y and shows several segments of various obliquities. The western SWIR consists of the Oblique and Orthogonal Supersegments lying at an epicentral distance of ~21° to the VNA2 seismic array operated by the German Neumayer station in East Antarctica. The array monitors backazimuth, apparent velocity and signal-to-noise ratio of arriving waves and provides a dataset of seismicity from the western SWIR over several years. Compared to the global seismological network, its detection threshold for earthquakes occurring at the western SWIR is more than 0.5 mb lower enabling a more comprehensive study of mid-ocean ridge processes than the teleseismic earthquake catalogues. We identified a total number of 743 earthquakes occurring at the western part of the SWIR and calculated the body-wave magnitudes (mb) from P-wave amplitude picks on the VNA2 broadband sensor obtaining a magnitude range from mb 3.18 to mb 5.34. In the years of 2001, 2004, 2005 and 2008, significantly increased event rates indicated four earthquake swarms with up to 164 events lasting for several days. All swarms had strong events registered in the International Seismological Centre (ISC) catalogue. The relocalization of these events confirmed that all swarms occurred in the same region on the Orthogonal Supersegment. We analysed event and moment release rate histories, b-values and aftershock decay rates (Modified Omori Law) finding that the swarms of 2001, 2004 and 2005 have similarities in the temporal distribution of seismic moment and event numbers. The swarm of 2008 is smaller with high magnitude events at the swarm’s onset which represent shear failure on normal faults. The application of the Modified Omori Law and the b-value show that the earthquakes of the swarms do not follow the classical mainshock-aftershock pattern of purely tectonic earthquake sequences. At the Orthogonal Supersegment a continuous positive magnetic anomaly along the rift axis, a negative Mantle Bouguer Anomaly, basalts at the seafloor and potentially volcanic edifices indicate robust magmatic crustal accretion. The high-resolution bathymetry shows ubiquitous rift-parallel ridges with steep flanks toward the rift axis indicating high angle normal faults. The high magnitude earthquakes detected teleseismically during the swarms are generated there. We interpret that the swarms are caused by magmatic accretion episodes at a suggested volcanic centre of the Orthogonal Supersegment and that possible magma injection activates the steeply dipping fault planes.

Mineralogical and geochemical features of sulfide chimneys from the 49°39′E hydrothermal field on the Southwest Indian Ridge and their geological inferences

During January–May in 2007, the Chinese research cruise DY115-19 discovered an active hydrothermal field at 49°39′E/37°47′S on the ultraslow spreading Southwest Indian Ridge (SWIR). This was also the first active hydrothermal field found along an ultraslow-spreading ridge. We analyzed mineralogical, textural and geochemical compositions of the sulfide chimneys obtained from the 49°39′E field. Chimney samples show a concentric mineral zone around the fluid channel. The mineral assemblages of the interiors consist mainly of chalcopyrite, with pyrite and sphalerite as minor constitunets. In the intermediate portion, pyrite becomes the dominant mineral, with chalcopyrite and sphalerite as minor constitunets. For the outer wall, the majority of minerals are pyrite and sphalerite, with few chalcopyrite. Towards the outer margin of the chimney wall, the mineral grains become small and irregular in shape gradually, while minerals within interstices are abundant. These features are similar to those chimney edifices found on the East Pacific Rise and Mid-Atlantic Ridge. The average contents of Cu, Fe and Zn in our chimney samples were 2.83 wt%, 45.6 wt% and 3.28 wt%, respectively. The average Au and Ag contents were up to 2.0 ppm and 70.2 ppm respectively, higher than the massive sulfides from most hydrothermal fields along mid-ocean ridge. The rare earth elements geochemistry of the sulfide chimneys show a pattern distinctive from the sulfides recovered from typical hydrothermal fields along sedimentstarved mid-ocean ridge, with the enrichment of light rare earth elements but the weak, mostly negative, Eu anomaly. This is attributed to the distinct mineralization environment or fluid compositions in this area. sulfide chimneys, 49°39′E hydrothermal field, Southwest Indian Ridge, mid-ocean ridge, DY115-19 Chinese cruise

Spreading rate, spreading obliquity, and melt supply at the ultraslow spreading Southwest Indian Ridge

We use bathymetry, gravimetry, and basalt composition to examine the relationship between spreading rate, spreading obliquity, and the melt supply at the ultraslow spreading Southwest Indian Ridge (SWIR). We find that at regional scales (more than 200 km), melt supply reflects variations in mantle melting that are primarily controlled by large‐scale heterogeneities in mantle temperature and/or composition. Focusing on adjacent SWIR regions with contrasted obliquity, we find that the effect of obliquity on melt production is significant (about 1.5 km less melt produced for a decrease of 7 mm/a to 4 mm/a in effective spreading rates, ESR) but not enough to produce near‐amagmatic spreading in the most oblique regions of the ridge, unless associated with an anomalously cold and/or depleted mantle source. Our observations lead us to support models in which mantle upwelling beneath slow and ultraslow ridges is somewhat focused and accelerated, thereby reducing the effect of spreading rate and obliquity on upper mantle cooling and melt supply. To explain why very oblique SWIR regions nonetheless have large outcrops of mantle‐derived ultramafic rocks and, in many cases, no evidence for axial volcanism (Cannat et al., 2006; Dick et al., 2003), we develop a model which combines melt migration along axis to more volcanically robust areas, melt trapping in the lithospheric mantle, and melt transport in dikes that may only form where enough melt has gathered to build sufficient overpressure. These dikes would open perpendicularly to the direction of the least compressive stress and favor the formation of orthogonal ridge sections. The resulting segmentation pattern, with prominent orthogonal volcanic centers and long intervening avolcanic or nearly avolcanic ridge sections, is not specific to oblique ridge regions. It is also observed along the SWIR and the arctic Gakkel Ridge in orthogonal regions underlain by cold and/or depleted mantle.

Asymmetric generation of oceanic crust at the ultra‐slow spreading Southwest Indian Ridge, 64°E

We describe topographic, gravity, magnetic, and sonar data from a Southwest Indian Ridge spreading segment near 64°E, 28°S. We interpret these to reveal crustal structure, spreading history, and volcanic and tectonic processes over the last 12 Myr. We confirm that the crust is some 2 km thicker north of the ridge axis, though it varies along and across axis on scales of ∼10 km and 4 Myr. The plate separation rate remained approximately constant at 13 ± 1 km Myr−1, but half‐spreading rates were up to 40% asymmetric, varying between faster‐to‐the‐north and faster‐to‐the‐south on a 4 Myr timescale. Topography shows a dominant E–W lineation normal to the N–S spreading direction. This is superficially similar to faulted abyssal hill terrain of the Mid‐Atlantic Ridge (MAR), but inferred fault scarps are 3–4 times more widely spaced and have greater offsets. Conjugate pairs of massifs on either plate are interpreted as volcanic constructions similar to the large volcano currently filling the median valley at the segment center. They have temporal spacings of ∼4 Myr and are thought to reflect episodic melt focusing along an otherwise melt‐poor ridge. Additionally, there are places, mainly on the southern plate, where lineated topography is replaced by a much blockier topography and embryonic ocean core complexes similar to those recently reported on the MAR near 13°N. There is generally more extrusive volcanism on the northern plate and more tectonism on the southern one. Extrusive volcanism has propagated westward from the segment center since 2 Ma. The FUJI Dome core complex and adjacent seafloor to its east and west appear to be part of a single coherent block, capped by extrusive rock near the segment center, exposing gabbro via a detachment fault over the Dome and probably exposing deeper crust or upper mantle farther west near the segment end. Magnetic anomalies are continuous along this block. We suggest that at its eastern boundary the detachment is simply welded onto magmatically emplaced crust to the east in a similar way to young crust being welded to the old plate at ridge‐transform intersections.

Focused magmatism versus amagmatic spreading along the ultra‐slow spreading Southwest Indian Ridge: Evidence from TOBI side scan sonar imagery

The analysis of the Towed Ocean Bottom Instrument (TOBI) side scan sonar images along the Southwest Indian Ridge between 63°40′E and 65°40′E reveals strong focusing of magmatic activity and long amagmatic accretionary ridge segments. Fresh‐looking volcanic terrains are observed at distinct locations along the axis separated by highly tectonized and sedimented terrains of an along‐axis extent as much as 82 km. The largest tectonized section corresponds to a dramatically thin crust area with moderate magnetization anomalies. We suggest that seafloor spreading is mainly amagmatic in this tectonized section of the Southwest Indian Ridge with upper mantle rocks exposed at the seafloor. Amagmatic accretionary ridge segments of such dimensions are quite distinct from what is observed at the Mid‐Atlantic Ridge but are also recognized at the Gakkel Ridge and may thus be characteristic of ultra‐slow spreading ridges. The correlation between the distribution of fresh‐looking volcanic terrains, the occurrence of shallow areas crowned by axial volcanic ridges, and high magnetization values suggests a shallow segmentation of the ridge mainly related to variation in the thickness and/or the intrinsic magnetization of the basaltic source layer. By contrast, strong along‐axis variations of the gravity‐derived crustal thickness are shrunken in length relative to this shallow segmentation of the ridge and occur only beneath the elevated segments. Adjacent to these elevated segments, small bathymetric swells with fresh‐looking volcanic constructions do not correspond to thicker crust areas. This suggests a highly focused melt supply beneath the elevated segments which may feed volcanic constructions up to 60 km from the center of these segments by shallow lateral melt migration in the crust, probably through large dikes. Neither the ultra‐slow spreading rate nor the ridge obliquity explains the variation of the magmatic vigor along the ridge. Mantle source heterogeneities together with lower mantle temperatures beneath the easternmost Southwest Indian Ridge could partly control its segmentation.

Magmato‐tectonic cyclicity at the ultra‐slow spreading Southwest Indian Ridge: Evidence from variations of axial volcanic ridge morphology and abyssal hills pattern

On‐axis deep tow side scan sonar data are used together with off‐axis bathymetric data to investigate the temporal variations of the accretion processes at the ultra‐slow spreading Southwest Indian Ridge. Differences in the length and height of the axial volcanic ridges and various degrees of deformation of these volcanic constructions are observed in side scan sonar images of the ridge segments. We interpret these differences as stages in an evolutionary life cycle of axial volcanic ridge development, including periods of volcanic construction and periods of tectonic dismemberment. Using off‐axis bathymetric data, we identify numerous abyssal hills with a homogeneous size for each segment. These abyssal hills all display an asymmetric shape, with a steep faulted scarp facing toward the axis and a gentle dipping volcanic slope facing away. We suggest that these hills are remnants of old split axial volcanic ridges that have been transported onto the flanks and that they result from successive periods of magmatic construction and tectonic dismemberment, i.e., a magmato‐tectonic cycle. We observe that large abyssal hills are in ridge sections of thicker crust, whereas smaller abyssal hills are in ridge sections of thinner crust. This suggests that the magma supply controls the size of abyssal hills. The abyssal hills in ridge sections of thinner crust are regularly spaced, indicating that the magmato‐tectonic cycle is a pseudoperiodic process that lasts ∼0.4 m.y., about 4 to 6 times shorter than in ridge sections of thicker crust. We suggest that the regularity of the abyssal hills pattern is related to the persistence of a nearly constant magma supply beneath long‐lived segments. By contrast, when magma supply strongly decreases and becomes highly discontinuous, regular abyssal hills patterns are no longer observed.

Sr, Nd and Pb isotopic composition of in situ lower crust at the Southwest Indian Ridge: results from ODP Leg 176

The composition of gabbroic rocks from the drill core of Hole 735B (ODP Leg 176) at the 11 Ma Atlantis II bank close to the slow spreading Southwest Indian Ridge (SWIR) has been analysed for major and trace elements and Sr, Nd and Pb isotopic composition. The samples are thought to represent much of the mineralogical and geochemical variation in a vertical 1-km section (500–1500 m below the sea floor, mbsf) of the lower ocean crust. Primitive troctolitic gabbros, olivine gabbros and gabbros that have Mg #=84–70, Ca #>61 and low Na # (Na/(Na+Al)) (8–17) are intruded by patches or veins of more evolved FeTi-oxide rich gabbroic and dioritic rocks with Mg # to 20, Ca # to 32, Na #=14–23, TiO 2<7 wt.% and FeO total<18 wt.%. All rocks are adcumulates, and incompatible element concentrations are low, e.g. Pb=0.1–0.7 ppm and U≤0.005 ppm in the primitive rocks and up to 2 ppm Pb and 0.2 ppm U in the evolved. The range of isotopic compositions of the unleached rocks is: 87Sr/ 86Sr=0.70280–0.70299, average 0.70287±0.00005 (1 S.D., N=30 samples) (except one felsic vein with 87Sr/ 86Sr=0.7045), 143Nd/ 144Nd=0.51304–0.51314, average 0.51310±0.00002 (1 S.D., N=28), 206Pb/ 204Pb=17.43–18.55, 207Pb/ 204Pb=15.40–15.61 and 208Pb/ 204Pb=37.19–38.28. The range of Sr and the almost constant Nd isotopic composition resemble that found in the upper 500 m of Hole 735B, while Pb ranges to more radiogenic compositions. In general, there is a decrease in isotopic variation of Sr and Pb as well as μ ( 238U/ 204Pb), U and Pb with depth, with a trend towards relatively unradiogenic compositions. This correlates with a decrease in alteration and frequency of evolved rock-types in the core. Leached samples generally have less radiogenic Pb with values trending towards 206Pb/ 204Pb=17.35, 207Pb/ 204Pb=15.35 and 208Pb/ 204Pb=37.0, while their 87Sr/ 86Sr ratios deviate less systematically from unleached rocks and reach both higher, 0.70307, and lower values, 0.70276. Separated clinopyroxene has elevated 87Sr/ 86Sr up to 0.7035, while plagioclase generally has close to whole rock Sr. Leaching reduced 87Sr/ 86Sr in clinopyroxene and in two (out of nine) cases leached separates and whole rock display isotopic equilibrium. Relatively minor hydrothermal seawater alteration is thought to have increased 87Sr/ 86Sr in the rocks, while a secondary high temperature percolation of a mantle-derived agent is thought to be the cause for the trend towards radiogenic Pb. This material had intermediate 87Sr/ 86Sr and may have originated from non-MORB off axis mantle. The main primary igneous isotopic variation of the gabbros is suggested to have been derived from the MORB-mantle and is defined mainly by leached samples from both ODP Leg 176 and Leg 118 and can be explained by two-component mixing of an end-member with composition like Central Indian Ridge basalts and an end-member with composition unlike any MORB. The latter is characterised by very unradiogenic Pb, in particular 207Pb/ 204Pb, and may have an origin with affinity to old depleted mantle (DM). The isotopic composition of the magmas parental to the FeTi-oxide rich rocks cannot be distinguished from the magmas parental to the primitive gabbros and an intimate relationship is indicated. The small-scale inhomogeneity indicated for the SWIR MORB-mantle at the Atlantis II Fracture Zone was probably inherited by the lower crustal rocks due to small-scale melting and monogenetic magma chambers at this slow spreading ridge.

Evidence from gravity anomalies for interactions of the Marion and Bouvet hotspots with the Southwest Indian Ridge: effects of transform offsets

The ultra-slow spreading Southwest Indian Ridge (SWIR) presents a unique environment to study the interactions between hotspots and ridges with highly segmented geometry. Using recently available satellite free-air gravity and shipboard bathymetry data, we obtain mantle Bouguer (MBA) and residual mantle Bouguer anomalies (RMBA) by removing from free-air gravity the attractions of seafloor topography, sediment thickness variations, a reference crust, and theoretically predicted effects of lithospheric cooling. The Bouvet hotspot, previously observed to be associated with anomalous bathymetry and geochemistry near the Bouvet triple junction, has an MBA axial gravity low of ∼100 mGal, implying pronounced localized crustal thickening. Off-axis, the RMBA lows along previously calculated Bouvet hotspot tracks are variable in amplitude, suggesting the possibilities that Bouvet flux varies in time or that hotspot magmatism is enhanced by proximity to a spreading center. Along-axis geophysical anomalies suggest that the Marion hotspot has a significant effect on accretionary processes in the central portion of the SWIR. Importantly, the Marion axial anomaly appears to be compartmentalized between the Andrew Bain and Discovery II fracture zones, implying that transform offsets play a significant role in governing the distribution of plume material in a highly segmented, ultra-slow spreading system.

Formation of the axial relief at the very slow spreading Southwest Indian Ridge (49° to 69°E)

The comparison of segment lengths, relief, and gravity signature along the very slow spreading Southwest Indian Ridge (SWIR) between 49°E and 69°E suggests that the marked change in segmentation style that occurs across the Melville transform (60°45′E) reflects a change in the modes of formation of the axial topography. We propose that the axial relief east of Melville is largely due to volcanic constructions that load the axial lithosphere from above. By contrast, the axial relief in segments west of the Melville fracture zone appears to be primarily due, as proposed for segments of the faster spreading Mid‐Atlantic Ridge, to along‐axis changes in the depth of the axial valley, and to partial compensation of negative loads (thicker lower crust and/or lighter upper mantle) acting within the plate, or at the bottom of the plate. In terms of geology, this means that the contribution of the uppermost, effusive, part of the crust to along‐axis crustal thickness variations may be greater east of Melville than in other regions of the study area. Regional axial depths suggest that the ridge east of Melville is also characterized by a low melt supply and is underlain by cold mantle. A simple model of mantle melting and regional isostatic compensation suggests that differences in mantle temperature and in melt thickness between this deep eastern ridge region, and the shallower region west of the Gallieni transform (52°20′E), are of the order of 80°C and 4 km, respectively.