Intermediate Spreading Ridges Research Articles

Beyond spreading rate: Controls on the thermal regime of mid-ocean ridges.

The thermal state of mid-ocean ridges exerts a crucial modulation on seafloor spreading processes that shape ~2/3 of our planet's surface. Standard thermal models treat the ridge axis as a steady-state boundary layer between the hydrosphere and asthenosphere, whose thermal structure primarily reflects the local spreading rate. This framework explains the deepening of axial melt lenses (AMLs)-a proxy for the basaltic solidus isotherm-from ~1 to ~3 km from fast- to intermediate-spreading ridges but fails to account for shallow crustal AMLs documented at slow-ultraslow spreading ridges. Here, we show that these can be explained by a numerical model that decouples the potentially transient ridge magma supply from spreading rate, captures the essential physics of hydrothermal convection, and considers multiple modes of melt emplacement. Our simulations show that melt flux is a better thermal predictor than spreading rate. While multiple combinations of melt/dike emplacement modes, permeability structure, and temporal fluctuations of melt supply can explain shallow crustal AMLs at slow-ultraslow ridges, they all require elevated melt fluxes compared to most ridge sections of comparable spreading rates. This highlights the importance of along-axis melt focusing at slow-ultraslow ridges and sheds light on the natural variability of their thermal regimes.

Influence of late Pleistocene sea-level variations on midocean ridge spacing in faulting simulations and a global analysis of bathymetry

It is established that changes in sea level influence melt production at midocean ridges, but whether changes in melt production influence the pattern of bathymetry flanking midocean ridges has been debated on both theoretical and empirical grounds. To explore the dynamics that may give rise to a sea-level influence on bathymetry, we simulate abyssal hills using a faulting model with periodic variations in melt supply. For 100-ky melt-supply cycles, model results show that faults initiate during periods of amagmatic spreading at half-rates >2.3 cm/y and for 41-ky melt-supply cycles at half-rates >3.8 cm/y. Analysis of bathymetry across 17 midocean ridge regions shows characteristic wavelengths that closely align with the predictions from the faulting model. At intermediate-spreading ridges (half-rates >2.3 cm/y and [Formula: see text]3.8 cm/y) abyssal hill spacing increases with spreading rate at 0.99 km/(cm/y) or 99 ky (n [Formula: see text] 12; 95% CI, 87 to 110 ky), and at fast-spreading ridges (half-rates >3.8 cm/y) spacing increases at 38 ky (n [Formula: see text] 5; 95% CI, 29 to 47 ky). Including previously published analyses of abyssal-hill spacing gives a more precise alignment with the primary periods of Pleistocene sea-level variability. Furthermore, analysis of bathymetry from fast-spreading ridges shows a highly statistically significant spectral peak (P < 0.01) at the 1/(41-ky) period of Earth's variations in axial tilt. Faulting models and observations both support a linkage between glacially induced sea-level change and the fabric of the sea floor over the late Pleistocene.

Effective global mixing of the highly siderophile elements into Earth’s mantle inferred from oceanic abyssal peridotites

Late accretion occurred through addition of massive impactors to Earth, leading to potential heterogeneity in the distribution of highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re) within the mantle. Abyssal peridotites sample the present-day convecting mantle, which make them useful for examining the distribution of the HSE within the mantle. Here we report new HSE abundance data and 187Os/188Os ratios, in conjunction with mineral chemistry and bulk rock major- and trace-element compositions for abyssal peridotites from the fast-spreading Pacific Antarctic Ridge (PAR) and East Pacific Rise (Hess Deep), and for slow to intermediate spreading ridges from the Southwest Indian Ridge, Central Indian Ridge and Mid-Atlantic Ridge. These analyses expand the global abyssal peridotite Os isotope and HSE database, enabling evaluation of potential variations with spreading rate, from ultraslow (<20 mm/yr, full spreading rate) to fast (135–150 mm/yr). Accounting for likely effects from seawater modification and serpentinization, the Pacific data reveals heterogeneous and sometimes significant melt depletion for PAR (3–23% melt depletion; 187Os/188Os from 0.1189 to 0.1336, average = 0.1267 ± 0.0065; 2SD) and Hess Deep abyssal peridotites (15–20% melt depletion; 0.1247 ± 0.0027). Abyssal peridotites from fast to intermediate spreading ridges reveal no systematic differences in the distribution and behavior of the HSE or Os isotopes, or in degrees of melt depletion, compared with slow to ultraslow spreading ridges. These observations arise despite significant differences in melt generation processes at mid-ocean ridges, suggesting that the effects of ancient melt depletion are more profound on HSE compositions in abyssal peridotites than modern melting beneath ridges. Using global abyssal peridotites with Al2O3 content > 2 wt.%, the average composition of the primitive mantle is 0.3 ppb Re, 4.9 ppb Pd, 7.1 ppb Pt, 7.2 ppb Ru, 3.8 ppb Ir and Os, showing no Pd/Ir, but a positive Ru/Ir anomaly, relative to chondrites. There is ∼50% variation of the HSE abundances in the oceanic mantle, with much of this variation being observed at small length scales (<1 km) and due entirely to both modern and more ancient partial melting effects. Consequently, any significant HSE heterogeneities formed during late accretion or early Earth differentiation processes are no longer recognizable in the mantle sampled within ocean basins, implying generally efficient mixing of Earth’s mantle for these elements. By contrast, relatively ancient heterogeneity in Os and other radiogenic isotopes has been effectively preserved in the convecting mantle over the last ∼2 Ga, through recycling processes and through preservation and isolation of melt-depleted refractory residues.

Seafloor hydrothermal activity along mid-ocean ridge with strong melt supply: study from segment 27, southwest Indian ridge

Continuous tow investigations have shown that the present vent field inventory along fast to intermediate spreading ridges may be underestimated by at least 3–6 times, while the limited towed line investigations of venting sites along slow to ultra-slow spreading ridges make it impossible to determine their distribution. The Chinese Dayang cruise has conducted detailed towed line surveys of hydrothermal activity on segment 27 of the ultra-slow spreading southwest Indian ridge in 2015. The results have identified as many as 9 hydrothermal fields along 85-km-long segment, including one confirmed hydrothermal field, three inferred hydrothermal fields and five suspected fields. Hydrothermal activities are not only limited along-axis but also found approximately 10 km away from the axis. These vent fields are likely powered by a seismically identified axial magma chamber, including melt migration along normal faults to flank areas. The calculated hydrothermal activity frequency on segment 27 is approximately 3.6–8 times higher than that calculated from the Interridge database, suggesting that careful system exploration can reveal more hydrothermal activities even on ultra-slow spreading ridges effected by hotspot.

SKS splitting in the Western Indian Ocean from land and seafloor seismometers: Plume, plate and ridge signatures

We present SKS splitting measurements in the Western Indian Ocean, recorded on 20 land and 57 seafloor seismometers deployed by the RHUM-RUM experiment (Réunion Hotspot and Upper Mantle – Réunions Unterer Mantel). We discuss our splitting observations within their geodynamic settings and compare them to SKS splitting parameters predicted from an azimuthally anisotropic Rayleigh wave tomography model that includes the RHUM-RUM data. We find that anisotropic directions poorly correlate with the present-day motion of the Somali plate, which at <2.6 cm/yr may be too slow to cause strongly sheared fabric in the asthenosphere. Fast split directions (Φ) between La Réunion and the Central Indian Ridge (CIR) trend E–W and provide strong, first seismological evidence for near-horizontal flow in the asthenosphere that connects the Réunion mantle upwelling with the CIR, supporting a long-standing hypothesis on plume–ridge interaction. In the vicinity of the Réunion hotspot, we observe a seismic anisotropy pattern indicative of a parabolic asthenospheric flow controlled by the Réunion mantle upwelling and its consecutive asthenospheric spreading. We furthermore observe ridge-normal Φ along the CIR and ridge-parallel Φ along the Southwest Indian Ridge (SWIR), both mainly attributed to asthenospheric mantle flows. In the Mozambique Channel between East-Africa and Madagascar, we attribute E–W trending Φ to frozen lithospheric structures, recording the paleo-orientation of the spreading ridges that enabled Madagascar's separation away from Africa. Based on the synopsis of this and previous SKS splitting studies at mid-ocean ridges, we propose that ridge-normal Φ may develop at fast and intermediate spreading ridges (e.g., CIR and East Pacific Rise) and ridge-parallel Φ could be characteristic to slow spreading ridges (e.g., SWIR, Mid-Atlantic Ridge and the paleo-ridges in the Mozambique Channel).

Garnet xenocryst from petit-spot lavas as an indicator for off-axis mantle refertilization at intermediate spreading ridges

Studies of lithospheric mantle from (ultra)slow spreading ridges have shown that melt extraction at mid-ocean ridges may be incomplete, producing metasomatism/refertilization of the shallow lithospheric mantle. However, it remains unclear whether similar processes operate off axis and could affect the cooling lithosphere. Here, we report the discovery of a garnet xenocryst in a petit-spot lava sampled on the top of the downgoing Pacific plate in front of Japan. The trace-element composition of this garnet xenocryst, in particular the low chromium, excludes a peridotitic origin, while the flat mid– to heavy rare earth element pattern does not support direct crystallization from melt percolating through the oceanic lithosphere. Garnet formation is therefore interpreted as formed by a subsolidus reaction of a plagioclase-bearing cumulate during the progressive off-axis cooling of the lithosphere. Combining lithosphere cooling models and the specific physical conditions required for subsolidus formation of garnet in tholeiitic systems (0.7–1.2 GPa) indicates that melt percolation to produce plagioclase-bearing cumulate occurs >150 km off axis. These conditions support that some low-degree melts produced off axis are not collecting to form mid-oceanic ridge basalt (MORB), but percolate and crystallize during the cooling and thickening of the lithospheric mantle. The demonstration of mantle refertilization affecting Pacific lithosphere off axis is critical because such a process could explain the presence of metasomatic domains with distinct physical and chemical properties in the depleted oceanic lithosphere.

Hydrothermal processes in the Edmond deposits, slow- to intermediate-spreading Central Indian Ridge

Abstract The Edmond hydrothermal field, located on the Central Indian Ridge (CIR), has a distinct mineralization history owing to its unique magmatic, tectonic, and alteration processes. Here, we report the detailed mineralogical and geochemical characteristics of hydrothermal metal sulfides recovered from this area. Based on the mineralogical investigations, the Edmond hydrothermal deposits comprise of high-temperature Fe-rich massive sulfides, medium-temperature Zn-rich sulfide chimney and low-temperature Ca-rich sulfate mineral assemblages. According to these compositions, three distinctive mineralization stages have been identified: (1) low-temperature consisting largely of anhydrite and pyrite/marcasite; (2) medium-high temperature distinguished by the mineral assemblage of pyrite, sphalerite and chalcopyrite; and (3) low-temperature stage characterized by the mineral assemblage of colloidal pyrite/marcasite, barite, quartz, anglesite. Several lines of evidence suggest that the sulfides were influenced by pervasive low-temperature diffuse flows in this area. The hydrothermal deposits are relatively enriched in Fe (5.99–18.93 wt%), Zn (2.10–10.00 wt%) and Ca (0.02–19.15 wt%), but display low Cu (0.28–0.81 wt%). The mineralogical varieties and low metal content of sulfides in the Edmond hydrothermal field both indicate that extensive water circulation is prevalent below the Edmond hydrothermal field. With regard to trace elements, the contents of Pb, Ba, Sr, As, Au, Ag, and Cd are significantly higher than those in other sediment-starved mid-ocean ridges, which is indicative of contribution from felsic rock sources. Furthermore, the multiphase hydrothermal activity and the pervasive water circulation underneath are speculated to play important roles in element remobilization and enrichment. Our findings deepen our understanding about the complex mineralization process in slow- to intermediate-spreading ridges globally.

Electrical properties of seafloor massive sulfides

Seafloor massive sulfide (SMS) deposits are increasingly seen as important marine metal resources for the future. A growing number of industrialized nations are involved in the surveying and sampling of such deposits by drilling. Drill ships are expensive and their availability can be limited; seabed drill rigs are a cost-effective alternative and more suitable for obtaining cores for resource evaluation. In order to achieve the objectives of resource evaluations, details are required of the geological, mineralogical, and physical properties of the polymetallic deposits and their host rocks. Electrical properties of the deposits and their ore minerals are distinct from their unmineralized host rocks. Therefore, the use of electrical methods to detect SMS while drilling and recovering drill cores could decrease the costs and accelerate offshore operations by limiting the amount of drilling in unmineralized material. This paper presents new data regarding the electrical properties of SMS cores that can be used in that assessment. Frequency-dependent complex electrical resistivity in the frequency range between 0.002 and 100 Hz was examined in order to potentially discriminate between different types of fresh rocks, alteration and mineralization. Forty mini-cores of SMS and unmineralized host rocks were tested in the laboratory, originating from different tectonic settings such as the intermediate-spreading ridges of the Galapagos and Axial Seamount, and the Pacmanus back-arc basin. The results indicate that there is a clear potential to distinguish between mineralized and non-mineralized samples, with some evidence that even different types of mineralization can be discriminated. This could be achieved using resistivity magnitude alone with appropriate rig-mounted electrical sensors. Exploiting the frequency-dependent behavior of resistivity might amplify the differences and further improve the rock characterization.

Channelling of hydrothermal fluids during the accretion and evolution of the upper oceanic crust: Sr isotope evidence from ODP Hole 1256D

ODP Hole 1256D in the eastern equatorial Pacific is the first penetration of a complete section of fast spread ocean crust down to the dike–gabbro transition, and only the second borehole to sample in situ sheeted dikes after DSDP Hole 504B. Here a high spatial resolution record of whole rock and mineral strontium isotopic compositions from Site 1256 is combined with core observations and downhole wireline geophysical measurements to determine the extent of basalt–hydrothermal fluid reaction and to identify fluid pathways at different levels in the upper ocean crust.The volcanic sequence at Site 1256 is dominated by sheet and massive lava flows but the Sr isotope profile shows only limited exchange with seawater. However, the upper margins of two anomalously thick (>25 m) massive flow sequences are strongly hydrothermally altered with elevated Sr isotope ratios and appear to be conduits of lateral low-temperature off-axis fluid flow. Elsewhere in the lavas, high 87Sr/86Sr are restricted to breccia horizons. Mineralised hyaloclastic breccias in the Lava–Dike Transition are strongly altered to Mg-saponite, silica and pyrite, indicating alteration by mixed seawater and cooled hydrothermal fluids. In the Sheeted Dike Complex 87Sr/86Sr ratios are pervasively shifted towards hydrothermal fluid values (∼0.705). Dike chilled margins display secondary mineral assemblages formed during both axial recharge and discharge and have higher 87Sr/86Sr than dike cores, indicating preferential fluid flow along dike margins. Localised increases in 87Sr/86Sr in the Dike–Gabbro Transition indicates the channelling of fluids along the sub-horizontal intrusive boundaries of the 25 to 50 m-thick gabbroic intrusions, with only minor increases in 87Sr/86Sr within the cores of the gabbro bodies.When compared to the pillow lava-dominated section from Hole 504B, the Sr isotope measurements from Site 1256 suggest that the extent of hydrothermal circulation in the upper ocean crust may be strongly dependent on the eruption style. Sheet and massive flow dominated lava sequences typical of fast spreading ridges may experience relatively restricted circulation, but there may be much more widespread circulation through pillow lava-dominated sections. In addition, the Hole 1256D sheeted dikes display a much greater extent of Sr-isotopic exchange compared to dikes from Hole 504B. Because seawater-derived hydrothermal fluids must transit the dikes during their evolution to black smoker-type fluids, the different Sr-isotope profiles for Holes 504B and 1256D suggest there are significant variations in mid-ocean ridge hydrothermal systems at fast and intermediate spreading ridges, which may impact geochemical cycles of elements mobilised by fluid–rock exchange at different temperatures.

Along‐axis hydrothermal flow at the axis of slow spreading Mid‐Ocean Ridges: Insights from numerical models of the Lucky Strike vent field (MAR)

AbstractThe processes and efficiency of hydrothermal heat extraction along the axis of mid‐ocean ridges are controlled by lithospheric thermal and permeability structures. Hydrothermal circulation models based on the structure of fast and intermediate spreading ridges predict that hydrothermal cell organization and vent site distribution are primarily controlled by the thermodynamics of high‐temperature mid‐ocean ridge hydrothermal fluids. Using recent constraints on shallow structure at the slow spreading Lucky Strike segment along the Mid‐Atlantic Ridge, we present a physical model of hydrothermal cooling that incorporates the specificities of a magma‐rich slow spreading environment. Using three‐dimensional numerical models, we show that, in contrast to the aforementioned models, the subsurface flow at Lucky Strike is primarily controlled by across‐axis permeability variations. Models with across‐axis permeability gradients produce along‐axis oriented hydrothermal cells and an alternating pattern of heat extraction highs and lows that match the distribution of microseismic clusters recorded at the Lucky Strike axial volcano. The flow is also influenced by temperature gradients at the base of the permeable hydrothermal domain. Although our models are based on the structure and seismicity of the Lucky Strike segment, across‐axis permeability gradients are also likely to occur at faster spreading ridges and these results may also have important implications for the cooling of young crust at fast and intermediate spreading centers.

Mineralogy and geochemistry of hydrothermal precipitates from Kairei hydrothermal field, Central Indian Ridge

The Kairei hydrothermal field was the first confirmed active submarine hydrothermal system on the Central Indian Ridge. It has been suggested to be related to mafic as well as ultramafic host rocks based on vent fluid composition and the presence of ultramafic rocks in its vicinity. In this study, detailed geochemical and mineralogical analyses have been carried out on the hydrothermal precipitates from the Kairei vent field in order to investigate the possible presence of indications for an ultramafic substrate at this vent site. The studied samples included fragments of sulfide chimneys, massive sulfides and talc-bearing and silicified breccias. Three mineralization stages were identified: (1) a high-temperature stage consisting largely of chalcopyrite, isocubanite, and pyrite; (2) a medium to low temperature stage characterized by the mineral assemblages of sphalerite and pyrite; and (3) a weathering stage characterized by secondary Cu-sulfides (bornite, digenite, covellite and idaite), Fe-oxihydroxides, Opal-A, and Cu-chloride (paratacamite and atacamite). The sulfide geochemistry is characterized by high concentrations of Cu and Zn (Cu + Zn up to 29.3 wt.%, n = 17) and Au (mean 5.28 ppm, n = 17), which is comparable to results from seafloor massive sulfides collected from ultramafic-hosted sites in the Atlantic Ocean, but differs from those of typical mafic-hosted deposits. The high concentrations of Cu and Au at the Kairei hydrothermal field could be an indication for the involvement of ultramafic rocks in the subseafloor. Ultramafic-hosted, Au-rich sulfide deposits may not be restricted to the Atlantic Ocean and may be common along all slow- and intermediate-spreading ridges.

Waning magmatic activity along the Southern Explorer Ridge revealed through fault restoration of rift topography

We combine high‐resolution bathymetry acquired using the Autonomous Underwater Vehicle ABE with digital seafloor imagery collected using the remotely operated vehicle ROPOS across the axial valley of the Southern Explorer Ridge (SER) to infer the recent volcanic and tectonic processes. The SER is an intermediate spreading ridge located in the northeast Pacific. It hosts the Magic Mountain hydrothermal vent. We reconstruct the unfaulted seafloor terrain at SER based on calculations of the vertical displacement field and fault parameters. The vertical changes between the initial and the restored topographies reflect the integrated effects of volcanism and tectonism on relief‐forming processes over the last 11,000–14,000 years. The restored topography indicates that the axial morphology evolved from a smooth constructional dome &gt;500 m in diameter, to a fault‐bounded graben, ~500 m wide and 30–70 m deep. This evolution has been accompanied by changes in eruptive rate, with deposition of voluminous lobate and sheet flows when the SER had a domed morphology, and limited‐extent low‐effusion rate pillow eruptions during graben development. Most of the faults shaping the present axial valley postdate the construction of the dome. Our study supports a model of cyclic volcanism at the SER with periods of effusive eruptions flooding the axial rift, centered on the broad plateau at the summit of the ridge, followed by a decrease in eruptive activity and a subsequent dominance of tectonic processes, with minor low‐effusion rate eruptions confined to the axial graben. The asymmetric shape of the axial graben supports an increasing role of extensional processes, with a component of simple shear in the spreading processes.

Gravity crustal models and heat flow measurements for the Eurasia Basin, Arctic Ocean

The Gakkel Ridge in the Arctic Ocean with its adjacent Nansen and Amundsen Basins is a key region for the study of mantle melting and crustal generation at ultraslow spreading rates. We use free-air gravity anomalies in combination with seismic reflection and wide-angle data to compute 2-D crustal models for the Nansen and Amundsen Basins in the Arctic Ocean. Despite the permanent pack-ice cover two geophysical transects cross both entire basins. This means that the complete basin geometry of the world’s slowest spreading system can be analysed in detail for the first time. Applying standard densities for the sediments and oceanic crystalline crust, the gravity models reveal an unexpected heterogeneous mantle with densities of 3.30 × 103, 3.20 × 103 and 3.10 × 103 kg/m3 near the Gakkel Ridge. We interpret that the upper mantle heterogeneity mainly results from serpentinisation and thermal effects. The thickness of the oceanic crust is highly variable throughout both transects. Crustal thickness of less than 1 km dominates in the oldest parts of both basins, increasing to a maximum value of 6 km near the Gakkel Ridge. Along-axis heat flow is highly variable and heat flow amplitudes resemble those observed at fast or intermediate spreading ridges. Unexpectedly, high heat flow along the Amundsen transect exceeds predicted values from global cooling curves by more than 100%.

Geology, Geochemistry and Chromite Mineralization Potential of the Amnay Ophiolitic Complex, Mindoro, Philippines

AbstractThe Amnay Ophiolitic Complex in Mindoro, the Philippines, is considered an emplaced Cenozoic South China Sea oceanic lithosphere as a result of the collision between the Palawan microcontinental block and the Philippine mobile belt. Middle Oligocene sedimentary rocks intercalated with dominantly MORB‐like pillow lavas and volcanic flows suggest the generation of this ophiolite complex in an intermediate spreading ridge within a back‐arc basin setting. The volcanic rock suite geochemistry also manifests a slab component suggesting that it is a supra‐subduction zone ophiolite. Petrography of the gabbros shows a plagioclase‐clinopyroxene crystallization order consistent with a back‐arc basin setting. Spinel and pyroxene geochemistry shows that the lherzolites and aluminous‐spinel harzburgites are products of low degrees of partial melting. The chromitites hosted by the harzburgites could have not been associated with the MORB‐like volcanic suites, gabbros, lherzolites and aluminous‐spinel harzburgites. The chromitites are products of mantle sources that have undergone higher degrees of partial melting that would have involved the presence of water. The study of this ophiolitic complex gives us a glimpse of the characteristics of the South China Sea.

Magmatic and tectonic extension at mid‐ocean ridges: 1. Controls on fault characteristics

We use 2‐D numerical models to explore the thermal and mechanical effects of magma intrusion on fault initiation and growth at slow and intermediate spreading ridges. Magma intrusion is simulated by widening a vertical column of model elements located within the lithosphere at a rate equal to a fraction, M, of the total spreading rate (i.e., M = 1 for fully magmatic spreading). Heat is added in proportion to the rate of intrusion to simulate the thermal effects of magma crystallization and the injection of hot magma into the crust. We examine a range of intrusion rates and axial thermal structures by varying M, spreading rate, and the efficiency of crustal cooling by conduction and hydrothermal circulation. Fault development proceeds in a sequential manner, with deformation focused on a single active normal fault whose location alternates between the two sides of the ridge axis. Fault spacing and heave are primarily sensitive to M and secondarily sensitive to axial lithosphere thickness and the rate that the lithosphere thickens with distance from the axis. Contrary to what is often cited in the literature, but consistent with prior results of mechanical modeling, we find that thicker axial lithosphere tends to reduce fault spacing and heave. In addition, fault spacing and heave are predicted to increase with decreasing rates of off‐axis lithospheric thickening. The combination of low M, particularly when M approaches 0.5, as well as a reduced rate of off‐axis lithospheric thickening produces long‐lived, large‐offset faults, similar to oceanic core complexes. Such long‐lived faults produce a highly asymmetric axial thermal structure, with thinner lithosphere on the side with the active fault. This across‐axis variation in thermal structure may tend to stabilize the active fault for longer periods of time and could concentrate hydrothermal circulation in the footwall of oceanic core complexes.

Utilization of high resolution satellite gravity over the Carlsberg Ridge

The Carlsberg Ridge lies between the equator and the Owen fracture zone. It is the most prominent mid-ocean ridge segment of the western Indian Ocean, which contains a number of earthquake epicenters. Satellite altimetry can be used to infer subsurface geological structures analogous to gravity anomaly maps generated through ship-borne survey. In this study, free-air gravity and its 3D image have been generated over the Carlsberg Ridge using a very high resolution data base, as obtained from Geosat GM, ERS-1, Seasat and TOPEX/POSEIDON altimeter data. As observed in this study, the Carlsberg Ridge shows a slow spreading characteristic with a deep and wide graben (average width *15 km). The transform fault spacing confirms variable slow to intermediate char- acteristics with first and second order discontinuities. The isostatically compensated region of the Carlsberg Ridge could be demarcated with near zero contour values in the free-air gravity anomaly images over and along the Carlsberg Ridge axes and over most of the fracture zone patterns. Few profiles have been generated across the Carlsberg Ridge and the characteristics of slow/interme- diate spreading ridge of various orders of discontinuity could be identified. It has also been observed in zero contour image as well as in the characteristics of valley patterns along the ridge from NW to SE that different spreading rates, from slow to intermediate, are occurring in different parts of the Carlsberg ridge. It maintains the morphology of a slow spreading ridge in the NW, where the wide and deep axial valley (*1.5-3 km) also implies the pattern of a slow spreading ridge. However, a change in the morphology/depth of the axial valley from NW to SE indicates the nature of the Carlsberg Ridge as a slow to intermediate spreading ridge.

Seismic structure of the Endeavour Segment, Juan de Fuca Ridge: Correlations with seismicity and hydrothermal activity

Multichannel seismic reflection data collected in July 2002 at the Endeavour Segment, Juan de Fuca Ridge, show a midcrustal reflector underlying all of the known high‐temperature hydrothermal vent fields in this area. On the basis of the character and geometry of this reflection, its similarity to events at other spreading centers, and its polarity, we identify this as a reflection from one or more crustal magma bodies rather than from a hydrothermal cracking front interface. The Endeavour magma chamber reflector is found under the central, topographically shallow section of the segment at two‐way traveltime (TWTT) values of 0.9–1.4 s (∼2.1–3.3 km) below the seafloor. It extends approximately 24 km along axis and is shallowest beneath the center of the segment and deepens toward the segment ends. On cross‐axis lines the axial magma chamber (AMC) reflector is only 0.4–1.2 km wide and appears to dip 8–36° to the east. While a magma chamber underlies all known Endeavour high‐temperature hydrothermal vent fields, AMC depth is not a dominant factor in determining vent fluid properties. The stacked and migrated seismic lines also show a strong layer 2a event at TWTT values of 0.30 ± 0.09 s (380 ± 120 m) below the seafloor on the along‐axis line and 0.38 ± 0.09 s (500 ± 110 m) on the cross‐axis lines. A weak Moho reflection is observed in a few locations at TWTT values of 1.9–2.4 s below the seafloor. By projecting hypocenters of well‐located microseismicity in this region onto the seismic sections, we find that most axial earthquakes are concentrated just above the magma chamber and distributed diffusely within this zone, indicating thermal‐related cracking. The presence of a partially molten crustal magma chamber argues against prior hypotheses that hydrothermal heat extraction at this intermediate spreading ridge is primarily driven by propagation of a cracking front down into a frozen magma chamber and indicates that magmatic heat plays a significant role in the hydrothermal system. Morphological and hydrothermal differences between the intermediate spreading Endeavour and fast spreading ridges are attributable to the greater depth of the Endeavour AMC and the corresponding possibility of axial faulting.

Origin of high-Al N-MORB by fractional crystallization in the upper mantle beneath the Galápagos Spreading Center

The Galápagos Spreading Center (GSC) includes lavas with chemical compositions ranging from N-MORB to transitional and more enriched MORB. N-MORB dominate the region west of 95.5°W, far from the influence of the Galápagos hotspot. However, some N-MORB glass samples from the GSC have very high Al contents (> 16.0 wt.% Al 2O 3 at > 8.5 wt.% MgO), similar to high-Al N-MORB from other slow and intermediate spreading ridges and close to fracture zones elsewhere. GSC high-Al N-MORB are dominated mineralogically by uniform plagioclase compositions (An 80–82) with only 1–2% olivine (Fo 85–87), and have glass compositions with higher Al 2O 3 and lower SiO 2 than is predicted by normal MORB fractionation trends. Forward modeling using the MELTS and pMELTS algorithms constrained by crustal thickness measurements indicates that high-Al, low-Si MORB can be produced by high-pressure crystallization in the upper mantle using the same source as normal (low-Al) GSC N-MORB. Although high-Al glasses can be obtained by very low extents of partial melting of this mantle source, such melting models result in significant misfits in other major element oxides, especially SiO 2. Models involving significant evolution with up to 20% olivine and clinopyroxene crystallization at pressures of 0.3–0.4 GPa can account for the complete major and selected trace element compositions of these unusual MORB samples. We suggest that high-pressure fractionation is enhanced by conductive cooling of the upper mantle in this area of the GSC, consistent with other recent models correlating mantle crystallization with slow spreading mid-ocean ridges and fracture zones.

Seismic evidence for variations in axial magma chamber properties along the southern Juan de Fuca Ridge

Multichannel seismic data collected along the Cleft segment on the southern Juan de Fuca Ridge show that this intermediate-spreading center is underlain by a mid-crustal reflector interpreted as the top of an axial magma chamber (AMC). The AMC reflection is present along most of the segment, and deepens gently from 2.0 km near the southern end of the segment beneath the RIDGE Cleft Observatory Site, to 2.3 km at the northern end beneath the site of the mid-1980s submarine eruption. We analyzed the one-dimensional seismic structure of the AMC at two locations with contrasting lava chemistry beneath two different hydrothermal vent fields. At the northern site, waveform modeling in the time intercept–slowness ( τ– p) domain indicates that the AMC is ∼ 100 m thick and it is characterized by a decrease in P-wave velocity from 6 km/s to 3.7 km/s. In contrast, the P-wave velocity within the shallower, ∼ 100-m-thick AMC at the southern site is higher (5.0 km/s). The decrease in seismic velocity within the AMC indicates that it is partially molten and that it is not a cracking front as previously suggested for other intermediate-spreading segments. The data show a coherent seismic phase interpreted as the P- to S-wave conversion at the AMC ( P AMC S). Stacking of this event shows that the P AMC S is only present along the northern part of the segment. Our results thus suggest along-axis variations in the crystallinity of the AMC. The AMC along Cleft varies from a high crystal content (< 30% melt) sill at the southern end of Cleft, to a largely melt (60–75%) sill at the source of the 1980s eruption at the northern end. The variations in magma chamber properties inferred from our seismic data correlate with changes in lava chemistry and with the location of hydrothermal plumes, and they all suggest that focused, high-temperature hydrothermal venting along intermediate-spreading ridges is closely linked to the physical state of the underlying magma chamber.

Upper crustal structure and axial topography at intermediate spreading ridges: Seismic constraints from the southern Juan de Fuca Ridge

We use multichannel seismic reflection data to image the upper crustal structure of 0–620 ka crust along the southern Juan de Fuca Ridge. The study area comprises two segments spreading at intermediate rate with an axial high morphology with narrow (Cleft) and wide (Vance) axial summit grabens (ASG). Along most of the axis of both segments we image the top of an axial magma chamber (AMC). The AMC along Cleft deepens from south to north, from 2.0 km beneath the RIDGE Cleft Observatory and hydrothermal vents near the southern end of the segment to 2.3 km at the northern end near the site of the 1980s eruptive event. Along the Vance segment, the AMC also deepens from south to north, from 2.4 to 2.7 km. Seismic layer 2A, interpreted as the basaltic extrusive layer, is 250–300 m thick at the ridge axis along the Cleft segment and 300–350 m thick along the axis of the Vance segment. However, off‐axis layer 2A is similar in both segments (500–600 m), indicating ∼90% and ∼60% off‐axis thickening at the Cleft and Vance segments, respectively. Half of the thickening occurs sharply at the walls of the ASG, with the remaining thickening occurring within 3–4 km of the ASG. Along the full length of both segments, layer 2A is thinner within the ASG, compared to the ridge flanks. Previous studies argued that the ASG is a cyclic feature formed by alternating periods of magmatism and tectonic extension. Our observations agree with the evolving nature of the ASG. However, we suggest that its evolution is related to large changes in axial morphology produced by small fluctuations in magma supply. Thus the ASG, rather than being formed by excess volcanism, is a rifted flexural axial high. The changes in axial morphology affect the distribution of lava flows along the ridge flanks, as indicated by the pattern of layer 2A thickness. The fluctuations in magma supply may occur at all spreading rates, but its effects on crustal structure and axial morphology are most pronounced along intermediate spreading rate ridges.