Slow-spreading Southwest Indian Ridge Research Articles

Fe-Si Oxides Tracing the Ongoing Low-T° Hydrothermal Alteration of Exhumed Serpentinites at the Ultraslow-spreading Southwest Indian Ridge

On the smooth seafloor of exhumed mantle-rocks of the slow-spreading Southwest Indian Ridge, the secondary oxide and carbonate mineralizations record specific chemical signatures of the alteration of serpentinites by low-temperature hydrothermal seawater-dominated fluids. The uncommon characteristics of Fe-oxides forming nodule cortex or thick encrustments are their high Si, low Mn contents, trace elements (e.g. Cr) and REE behavior. Oxides composition evolves through time. Associated to oxides, carbonates (aragonite, calcite, dolomite) characterize the matrix of young indurated serpentinite breccias found at ridge axis. Such mineralogical and geochemical sequences trace, as proxies, the ongoing mantle-exhumation and serpentinization process linked to the detachment faults, and the late alteration reactions that impact on crust–ocean transfers and oceanic budget.

Geochemistry of abyssal peridotites from the super slow-spreading Southwest Indian Ridge near 65°E: Implications for magma source and seawater alteration

The geochemical characteristics of abyssal peridotite samples from one dredge station (27 degrees 49.74'S, 65 degrees 02.14'E, water depth 4473 m) on the super slow-spreading Southwest Indian Ridge (SWIR) near 65 degrees E were investigated. Abyssal peridotites recovered from this site were comprised mainly of lizardite, chlorite, carbonate and magnetite with minor amounts of talc, pyroxene phenocrysts and sparse olivines. Serpentinites exhibit talc veins and major serpentine derived from serpentinization with relict olivine granuloblasts. Olivine grains in serpentinites display exsolution lamellae, indicating the occurrence of talc reduction or decompression during seawater-rock interaction. Pyroxene shows clear cleavage in two directions, with clinopyroxene or orthopyroxene exsolution lamellae. By contrast, bulk rock trace element patterns of serpentinites reveal depletion in most incompatible elements, similarly to the depleted mid-ocean ridge basalt mantle composition, indicating that the SWIR peridotites originated from a depleted mantle source magma and have experienced partial melting. Meanwhile, Rb, Ba, U, Pb, Sr, Li anomalies and the Ce/Pb ratio suggest that these serpentinites have been strongly altered by seawater.

Protracted timescales of lower crustal growth at the fast-spreading East Pacific Rise

The formation of oceanic crust at mid-ocean ridges is a fundamental component of plate tectonics. A large fraction of the new crust is created when magmas in the lower crust cool to form gabbroic rocks. The duration of magmatism during formation of the new gabbroic crust is expected to vary with plate-spreading rate and has been constrained by dating gabbroic rocks at the slow-spreading Southwest Indian Ridge and Mid-Atlantic Ridge1, 2, 3, 4. Here we present high-precision U–Pb dating of zircon minerals from gabbroic rocks formed at the fast-spreading East Pacific Rise and exposed at Hess Deep. We find that the zircons formed between 1.420 and 1.271 (±0.006–0.081) million years ago. Within individual samples, the zircon minerals exhibit a range in formation dates of up to 0.124 Myr, consistent with either protracted crystallization from a magma or assimilation into the magma of older zircons from adjacent rocks. The variability of zircon dates is comparable to that measured at the slow-spreading Mid-Atlantic Ridge3. We conclude that the timescales of magmatic processes in the lower crust may be similar at slow- and fast-spreading ridges, implying that the duration of crust formation at mid-ocean ridges is not only controlled by spreading rate.

Origin of compositional trends in clinopyroxene of oceanic gabbros and gabbroic rocks: A case study using data from ODP Hole 735B

The Ocean Drilling Program Hole 735B near the Southwest Indian Ridge represents the most complete in situ section of oceanic lower crust. The drill core samples are gabbros with several sub-divisions defined by varying amounts of minor phases. These samples have been subject to many studies. Among several yet to be understood phenomena is an apparently unexpected observation in the compositions of the constituent clinopyroxene (cpx). TiO 2 and Na 2O in cpx do not show expected inverse correlations with Mg Cpx #. Instead, both TiO 2 and Na 2O increase with decreasing Mg Cpx #, reaching a maximum at Mg Cpx # = ~ 0.76–0.73. With continued Mg Cpx # decrease, TiO 2 decreases sharply whereas Na 2O declines gently and flattens out. This observation has been previously interpreted as resulting from boundary layer crystallization within a steady-state magma chamber. In this study, we show that the Na 2O–Mg Cpx #and TiO 2–Mg Cpx #co-variations in cpx can be explained as a straightforward consequence of basaltic magma evolution and related cpx crystal stoichiometry control without invoking complex magma chamber processes. Our interpretation is superior to the boundary layer crystallization model (1) because the latter requires a steady-state magma chamber that is inconsistent with the observation of melt emplacement as thin sills, making convective and steady-state magma chambers unlikely at the slow-spreading Southwest Indian Ridge; and (2) because the TiO 2 and Na 2O maxima in cpx correspond to the onset and continued crystallization of Fe–Ti oxides or the basalt–andesite transition stage of tholeiitic basaltic melt evolution.

Petrology of local concentration of chromian spinel in dunite from the slow-spreading Southwest Indian Ridge

This is the first detailed report on local concentration of chromian spinel in a dunite from an ultraslow spreading ridge, the Southwest Indian Ridge (SWIR). The sample was collected from an outcrop with detailed observations using submersible SHINKAI 6500 of the Japanese Marine Science Technology Center. The dunite occurs as a tabular-shaped layer in a lherzolite host outcrop. Spinel is found as a string of small micropods 2–3 centimeters in size. These spinel micropods make a layer in the middle part of a spinel-poor dunite ( 0.4) compared with other peridotite samples in the studied area (lherzolites to harzburgite with low-Cr# spinel, typically Cr# ≤ 0.3). The occurrence and chemical compositions of clinopyroxene in the enstatite-poor harzburgite suggest that some clinopyroxenes crystallized from infiltrated interstitial melts. The host peridotites are interpreted as a residue of relatively low degrees of partial melting consistent with a location along the SWIR far from a mantle hot spot. This was then followed by crystallization of clinopyroxene from interstitial melt in the dunite. Irrespective of their small size, the lithological relationships between the spinel micropods and the host peridotites are the same as those for podiform chromitite in ophiolites and orogenic peridotites. The spinel Cr# in the micropods (0.3) is compatible with the lower range of those in basalts from SWIR far from hot spot. The spinel micropods were mainly formed by interaction between relatively depleted peridotite and a locally significant volume of basaltic melt traversing the upper mantle. This study coupled with the previous works on chromitites suggest that podiform chromites occur in every geodynamic setting, though economic concentrations of chromite (Cr-rich spinel) are unlikely to occur in the mantle at ultraslow spreading ridges.

Contrasting Cooling Rates in the Lower Oceanic Crust at Fast- and Slow-spreading Ridges Revealed by Geospeedometry

Two approaches to determining the high-temperature (� 10008 Ct o � 6008C) cooling rate of the lower oceanic crust and upper mantle are presented and critically evaluated.The first is based on the downtemperature diffusive exchange of Ca between olivine and clinopyroxene. The second, less well-constrained, approach is based on the down-temperature diffusive exchange of Mg and Fe between olivine and spinel. Cooling rates based on olivine^spinel geospeedometry are approximately an order of magnitude faster than those from Ca-in-olivine geospeedometry. In contrast, cooling rates derived from thermochronology and remanent magnetism are approximately an order of magnitude slower than those derived by Ca-in-olivine geospeedometry; this is probably because they record cooling at lower temperatures. Using the Ca-in-olivine geospeedometer, the cooling rate of samples from the lower oceanic crust and upper oceanic mantle formed in the Oman ophiolite and in the three main ocean basins has been determined. Samples from the lower oceanic crust formed at fast-spreading ridges show a large decrease in cooling rate between the top and base of the gabbroic section, with most of the variation occurring within the upper kilometre. This is consistent with vertical heat loss (within the crustal frame of reference) dominating the thermal evolution at fast-spreading ridges. Samples from Ocean Drilling Program Hole 735B, which formed at the slowspreading Southwest Indian Ridge, show no variation in cooling rate over 1500 m depth range and cooled substantially faster than rocks from the deeper portion of the gabbros in the Oman ophiolite, where the change in cooling rate with depth is limited. These observations are consistent with heat loss from small plutons emplaced in cool lithosphere at the slow-spreading ridge. Alternatively, they could be explained by cooling through the Ca-in-olivine closure interval during uplift towards the surface.

Tectonics at the axis of the very slow spreading Southwest Indian Ridge: Insights from TOBI side‐scan sonar imagery

We present the analysis of the deformation in the axial valley of two contrasted regions of the very slow spreading Southwest Indian Ridge based on side‐scan sonar images. Our objective is to investigate how the obliquity is accommodated along the system. We show that the robust magmatic segments have axial valleys and major faults subperpendicular to spreading. The other sections show fault populations with various degrees of obliquity, often arranged in left‐stepping echelons, accommodating part of the strike‐slip deformation. Side‐scan sonar reveals the presence of a corrugated surface near 59°E interpreted to be an incipient detachment fault. We show that the large width of the SWIR oblique sections, and the difference in tectonic style between the robust volcanic segments and the magma‐starved sections, is accounted for by large variations in the thickness of the brittle lithosphere. We suggest that the emplacement of mantle rocks in the surveyed amagmatic ridge sections can occur by alternating conjugate faults. Serpentinization of outcropping peridotites might also play a significant role in the development of faults in thin crust regions and the distribution of deformation in space and time.

Magmatic srilankite (Ti2ZrO6) in gabbroic vein cutting oceanic peridotites: An unusual product of peridotite-melt interactions beneath slow-spreading ridges

We report srilankite in a gabbroic vein cutting a serpentinized peridotite collected from the Atlantis II Fracture Zone, the slow-spreading Southwest Indian Ridge, using submersible SHINKAI 6500 of the Japanese Marine Science Technology Center. Srilankite occurs in small patches, <30 μm across, always coexisting with ilmenite and rutile. Zircon, apatite, and phlogopite also occur as accessory minerals in the vein. The Zr/Ti ratio of the srilankite is close to the stoichiometric value of one-half (Ti 2.00 Zr 0.98 Hf 0.01 Fe 0.01 O 6 ). Based on petrography, the srilankite appears to have co-crystallized with ilmenite and rutile from melts rather than through metamorphic recrystallization. Mineral assemblages and mineral compositions in the vein indicate that melts that produced the vein have high concentrations of compatible elements (MgO and Cr 2 O 3 ) as well as incompatible elements (high-field strength elements, K 2 O, and H 2 O). On the other hand, TiO 2 -enrichment of minerals in the peridotite host on the periphery of the gabbroic vein may have resulted from interaction with the melts. Geochemical interactions between peridotite and melt in the upper mantle may effectively concentrate incompatible elements in a modified melt, which may precipitate srilankite directly. Physical conditions under slow-spreading ridges, characterized by a highly attenuated magma supply and high rock/melt ratio, favor peridotite-melt interactions

TOBI sidescan sonar imagery of the very slow-spreading Southwest Indian Ridge: evidence for along-axis magma distribution

New deep tow sidescan sonar data from the Southwest Indian Ridge reveal complex volcanic/tectonic interrelationships in the axial zone of this ultra-slow spreading ridge. While some constructional volcanic features resemble examples documented at the slow-spreading Mid-Atlantic Ridge, such as axial volcanic ridges, hummocky and smooth lava flows, their distribution and dimensions differ markedly. The largest axial volcanic ridges occur at segment centres, but fresh-looking volcanic constructions also occur at segment ends and in the deep basins marking the non-transform discontinuities. The orientations of the dominant fault population and main volcanic ridges are controlled by tectonic processes such as orthogonal extension in the sections of the ridge perpendicular to the spreading direction and transtensional extension in the obliquely spreading sections of the ridge. Minor faults and small volcanic ridges striking parallel to the axis in the oblique part of the ridge are not controlled by these extensional regimes. This observation suggests that the ridge axis acts as a zone of weakness and that magmatic processes, with associated fractures opening in response to magma pressure, may control local emplacements of axial volcanic ridges at obliquely spreading ridges. This non-systematic pattern of ridge characteristics suggests an along-axis variation between focused and distributed magmatic supply, a model which is supported by our interpretation of low-amplitude mantle Bouguer anomalies calculated for the area. We propose that a change of the axial segmentation pattern, from two segments to the present-day three segments, may have introduced additional instability into the crustal accretion process.

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.

The geochemical consequences of late-stage low-grade alteration of lower ocean crust at the SW Indian Ridge: results from ODP Hole 735B (Leg 176)

Chemical exchange between oceanic lithosphere and seawater is important in setting the chemical composition of the oceans. In the past, budgets for chemical flux in the flanks of mid-ocean ridges have only considered exchange between basalt and seawater. Recent studies have shown that lower crustal and upper mantle lithologies make up a significant fraction of sea floor produced at the global mid-ocean ridge system. Moreover, the rugged topography of slow spread crust exposing lower crust and upper mantle facilitates prolonged fluid circulation, whereas volcanic ridge flanks are more rapidly isolated from the ocean by a sediment seal. Hence, elemental fluxes during lower crust-seawater reactions must be assessed to determine their role in global geochemical budgets.ODP Hole 735B penetrates more than 1500 m into lower ocean crust that was generated at the very slow spreading Southwest Indian Ridge and later formed the 5-km-high Atlantis Bank on the inside corner high of the Atlantis II Fracture Zone. The gabbroic rocks recovered from Hole 735B preserve a complex record of plastic and brittle deformation and hydrothermal alteration. High-temperature alteration is rare below 600 m below seafloor (mbsf), but the lowermost section of the hole (500–1500 mbsf) has been affected by a complex and multistage low-temperature (<250°C) alteration history probably related to the tectonic uplift of the basement. This low-T alteration is localized and typically confined to fractured regions where intense alteration of the host rocks can be observed adjacent to veins/veinlets filled with smectite, smectite-chlorite mixed layer minerals, or chlorite ± calcite ± zeolite ± sulfide ± Fe-oxyhydroxide.We have determined the bulk chemistry and O and Sr isotope compositions of fresh/altered rock pairs to estimate the chemical fluxes associated with low-temperature interaction between the uplifted and fractured gabbroic crust and circulating seawater. The locally abundant low-temperature alteration in crust at Site 735 has significantly changed the overall chemical composition of the basement. The direction of these changes is similar to that defined for volcanic ridge flanks, with low-temperature alteration of gabbroic crust acting as a sink for the alkalis, H2O, C, U, P, 18O, and 87Sr. The magnitudes of element fluxes are similar to volcanic ridge flanks for some components (C, P, Na) but are one or two orders of magnitude lower for others. The flux calculations suggest that low-temperature fluid circulation in gabbro massifs can result in S uptake (3% of riverine sulfate input) in contrast to the S losses deduced for volcanic ridge flanks.

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.

Segmentation and melt supply at the Southwest Indian Ridge

The segmentation of mid-ocean ridge axes and its variation with spreading rate provide constraints on models of mantle upwelling and the supply of basaltic melt to the crust. We present seismic refraction results from three segments of the very slow spreading Southwest Indian Ridge. From the absence of oceanic layer 3 beneath segment boundaries, and from significant variations in maximum crustal thickness between segments, we infer that melt generation and/or delivery is focused at segment midpoints and varies between segments. Anomalously thin crust—2.0–2.5 km beneath the nontransform segment boundaries and 3.5–6.0 km beneath the segment mid-points—is caused by restricted melt generation resulting from conductive heat loss from the upwelling mantle under the spreading center.

Correction to “Spiess Ridge: An axial high on the slow spreading Southwest Indian Ridge” by Neil C. Mitchell and Roy A. Livermore

Correction to “Spiess Ridge: An axial high on the slow spreading Southwest Indian Ridge” by Neil C. Mitchell and Roy A. Livermore

Spiess Ridge: An axial high on the slow spreading Southwest Indian Ridge

We report recent mapping of Spiess Ridge with the Hawaii‐MR1 sidescan sonar. Spiess Ridge is an unusual elongate 90 by 50 km volcanic feature on the slow spreading Southwest Indian Ridge near the Bouvet hotspot. The northwest half of the ridge has a narrow ∼7‐km neovolcanic zone in MR1 sonar images and a simple magnetic anomaly sequence including possible anomalies C1n and C2n. In contrast, the southeast half shows extrusive volcanism over &gt;40 km with distributed eruption sites, a broadened central magnetic anomaly with no anomaly C2n, and a volcanic ridge radiating from the summit of Spiess Ridge, oblique to the spreading orthogonal trend. The images show no evidence for large‐offset normal faults or an axial rift valley typical of slow spreading ridges. Overall, Spiess Ridge has an appearance very unlike that of either fast or slow spreading ridges and more transitional between a spreading ridge and a seamount. We compare the morphology of Spiess Ridge to other large volcanic structures in oceanic rifts. Spiess Ridge and Bouvet Island represent localized zones of excess melting along the Southwest Indian Ridge and we discuss their origin in the light of current ideas on ridge‐hotspot interaction.

Variations of backscatter strength along the super slow-spreading Southwest Indian Ridge between 57°E and 70°E

Simrad EM12 backscatter strength data of the Southwest Indian Ridge (SWIR), between 57°E and 70°E, are used to reveal the along-axis segmentation of this super slow-spreading ridge. The backscatter properties of different geologic domains, like bathymetric highs and oblique basins within the rift valley, are characterized using 66 small test sites. We show that backscatter strength is higher on bathymetric swells, corresponding to segment centres, and lower in deep oblique basins corresponding to axial non-transform discontinuities and fracture zones. This contrast between segment centres and discontinuities is produced by both a thicker sediment cover and less frequent volcanic eruptions at segment ends. Using the model of Mitchell (1993), sediments have been estimated to be 2 to 5 m thicker in these areas than at segment centres. The distribution of the seamounts within the rift valley is controlling the long-wavelength variations of the mean backscatter strength calculated along the axis. Lower densities of seamounts and thicker sediments are producing lower and heterogeneous reflectivity levels in the deepest part of the axial valley floor between 61°45′E and 63°45′E. We propose that cooler mantle temperatures inducing construction of fewer volcanoes occur beneath this part of the ridge. The mean backscatter strength along the SWIR axis decreases dramatically toward the Rodrigues Triple Junction suggesting that volcanic production is reduced between 68°20′E and 69°20′E and that the transition from amagmatic tectonic deformation at the triple junction to new seafloor spreading occurs between 69°20′E and 70°E.

Seamount volcanism at the super slow-spreading Southwest Indian Ridge between 57° and 70°

Using high-resolution bathymetric data of the Southwest Indian Ridge axial valley floor, 523 seamounts were identified between the Atlantis II fracture zone (57°) and the Rodrigues triple junction (70°). We find that the seamount population has a much larger spatial density (∼ 103 per 10 3 km 2 ) and shorter characteristic height (∼ 48 m) to the west of 62° than to the east (∼ 8 per 10 3 km 2 and ∼ 105 m, respectively). These densities of seamounts are much lower than those found on the Mid-Atlantic Ridge, suggesting that magma production is attenuated under the Southwest Indian Ridge compared with faster spreading oceanic ridges. We propose that cooler mantle temperatures, which induce lower degrees of partial melting and less magmatic supply to the ridge axis, result in construction of taller and fewer seamounts between 62° and 70°.

Thin Crust On the Flanks of the Slow-Spreading Southwest Indian Ridge

SUMMARY The Southwest Indian Ridge marks an end-member of mid-ocean ridge spreading rates, with a current full rate of only 13-15 mm a-'. Most thermal models of mid-ocean ridges suggest a decrease in crustal thickness at such slow spreading rates, because conductive heat loss from the upwelling asthenospheric mantle decreases the volume of melt generated by decompression. Seismic measurements of the thickness of crust formed at very slow-spreading ridges are sparse. We have reanalysed data from a twoship, split spread seismic refraction experiment conducted in 1962 on the southern flank of the Southwest Indian Ridge (Francis & Raitt 1967). We used synthetic seismograms to model amplitude variations, which were carefully recorded by the original investigators. 1- and 2-D modelling suggests that the seismic velocity increases smoothly with depth within the igneous crust, with an unusually high velocity of -6.0 km s--' near the top of the crust, increasing to -7.0 km s-' just above the Moho, and a crustal thickness of 5 km. Converted shear waves yield a Poisson's ratio of 0.30+0.01 in the crust, which is intermediate between values for gabbros and serpentinized upper mantle. The crustal thickness is consistent with a passive mantle upwelling model of melt generation at mid-ocean ridges. The unusual velocity structure may indicate the presence of gabbroic rocks near the seabed, unroofed by extension.

Methane‐rich fluids in the oceanic crust

Analyses of fluid inclusions in plutonic rocks recovered from the slow‐spreading Southwest Indian Ridge (SWIR) record CH4 concentrations of 15–40 times those of hydrothermal vent fluids and of basalt‐hosted volcanic gases and provide the first direct sampling of CO2‐CH4‐H2O‐H2‐C‐bearing fluids in the oceanic crust. Compositional, thermal, and spatial analyses of these fluids are used to model the evolution of volatiles during the crystallization and cooling of mid‐ocean ridge magma chambers and to assess the potential importance of carbon‐bearing fluids in geochemical processes in the lower crustal component of hydrothermal systems. Results from these analyses show that the earliest fluids to be exsolved from the melts are dominated by CO2‐rich vapors, which with progressive fractionation evolved to more H2O‐rich compositions. These later fluids were most likely exsolved under immiscible conditions and involved the development of CO2‐H2O‐rich vapors and CO2‐H2O‐NaCl brines that were trapped during mineral growth, as well as during later high‐temperature fracturing events. CO2 + CH4 ± H2O‐rich fluids in olivine and plagioclase minerals that contain up to 30–50 mol % CO2 and 33 mol % CH4 may reflect respeciation of magmatic CO2 during cooling and attendant graphite precipitation. Phase equilibria suggest that these fluids reequilibrated at ∼500°–600°C and at fO2's ∼ −3 log units below, to close to QFM conditions. Alternatively, the inclusions may record respeciation of magmatic fluids attendant with the inward diffusion of H2 into the inclusions and reduction of entrapped CO2 during degassing and cooling of the gabbros throughout the subsolidus regime. Subsequent seawater reaction, at minimum temperatures of ∼400°C, with mafic‐rich layers within the gabbroic rocks or with ultramafic material that underlies the plutonic sequence resulted in the formation of CH4‐H2O fluids that contain up to 40 mol % CH4, molecular H2, and graphite(?) daughter minerals. These data provide strong evidence that the CH4‐H2O‐rich fluids produced during serpentinization reactions were trapped under equilibrium conditions in the presence of graphite at very near to QFM conditions. The ubiquitous occurrence of CH4‐rich fluids in oceanic crustal layer 3 rocks from the SWIR indicates that these fluids may be a significant and previously unrecognized source for these volatile species in some hydrothermal systems venting on the seafloor and that the deep‐seated fluids most likely play an important role in the transfer of carbon from the lithosphere to the hydrosphere.

Restricted melting under the very slow-spreading Southwest Indian ridge

Restricted melting under the very slow-spreading Southwest Indian ridge