Slow Spreading Centers Research Articles

Crustal accretion in a slow-spreading center of Meso-Tethyan Ocean: Constraints from cumulates in the Dongco ophiolite (Central Tibet)

Crustal accretion in a slow-spreading center of Meso-Tethyan Ocean: Constraints from cumulates in the Dongco ophiolite (Central Tibet)

The spreading rate dependence of the distribution of axial magma lenses along mid-ocean ridges

Seismically imaged axial melt lenses (AMLs) are seen almost everywhere along the axis of fast-spreading ridges but at only a few localized segment centers on slow-spreading ridges. Standard models assuming that AMLs form when melt percolating upward pools where freezing produces an impermeable cap do not explain this fundamental observation. To tackle this long-standing problem, we combine a crustal density model and a thermal model with a recent mechanical model for sill formation. The mechanical model predicts that AMLs form below the axial lithosphere but only if the average density of the axial brittle lithosphere is not greater than the magma density. For standard thermal models, crustal density structures inferred from seismic velocity data and normal crustal thicknesses, AMLs are found to be stable along all of a ridge segment for spreading rates greater than about 50 mm/y. To explain slow-spreading observations, we assume that a share of the melt produced by the mantle upwelling all along a segment is focused to the segment center. Some of this melt partially crystallizes, releasing latent heat, before the evolved magma flows along the axis to build the crust away from the segment center. This "extra" heat, beyond what is supplied by the magma that builds the crust near the segment center, results in the lithosphere thin enough for stable melt lenses at the segment center. Our results are consistent with observations and offer a quantitative explanation of the marked difference in the distribution of AMLs along fast- versus slow-spreading centers.

Geophysical evidence for a serpentine mud volcano in the relict slow-spreading center of the South China Sea

The seamounts formed in spreading centers are of basaltic with prominent gravity peaks and high density. But Longmen Seamount, 1.5 km high and ∼ 20 km wide, located in the relict slow-spreading center of the South China Sea, shows a weak gravity peak with a low-amplitude gravity variation (∼19 mGal). To reveal the geological origin of the unusual seamount, we conducted pre-stack depth-migrated image and 3D forward gravity modeling for Longmen Seamount based on the constraint from reflection and refraction seismic data. Our results show that bulk density and seismic velocity of Longmen Seamount is as low as ∼2.1 g/cm3, and 1.6–3.2 km/s, respectively, equivalent to that of poorly consolidated sediments. Furthermore, typical structural elements of the mud volcano system, such as a reflection blank zone, up-bent and downward bending reflections, are observed beneath LM Seamount. For the crust beneath LM Seamount is thin (1.6 km) and highly faulted revealed by numerous clear discontinuous reflections, it is easy for seawater to infiltrate through the thin crust and leads to a high degree of serpentinization of the upper mantle rocks. Combined with serpentine mud volcanoes featuring low bulk density and low seismic velocity, Longmen Seamount is most probably a serpentine mud volcano in a slow-spreading mid-ocean center. For the discovered serpentine mud volcanoes are all restricted to the convergent plate margins, this study will lead to a new understanding of the mechanism of serpentine mud volcanism.

Magmatism at oceanic core complexes on the ultraslow Southwest Indian Ridge: Insights from near-seafloor magnetics

Abstract Oceanic core complexes (OCCs) and detachment faults play a key role in crustal accretion at slow and ultraslow spreading centers. We investigated the effect of different magma supply at three OCCs of the Southwest Indian Ridge using high-resolution deep-sea bathymetric and magnetic data. The average equivalent thickness of extrusive basalt deduced from the magnetic anomalies, a proxy for magma supply, decreases from west to east, from the Yuhuang (49.25°E) to Longqi (49.65°E) to Junhui (51.75°E) OCCs. Conversely, serpentinite outcrops become more abundant, the domal OCC morphology flattens as the footwall rotation (measured by the magnetization vector inclination) increases, and hydrothermal evidence becomes sparse. Combined with results from the amagmatic easternmost Southwest Indian Ridge, our study shows that the magma supply controls the character and evolution of the OCCs and detachment faults on the Southwest Indian Ridge.

Sulfide enrichment along igneous layer boundaries in the lower oceanic crust: IODP Hole U1473A, Atlantis Bank, Southwest Indian Ridge

Reactive porous or focused melt flows are common in crystal mushes of mid-ocean ridge magma reservoirs. Although they exert significant control on mid-ocean ridge magmatic differentiation, their role in metal transport between the mantle and the ocean floor remains poorly constrained. Here we aim to improve such knowledge for oceanic crust formed at slow-spreading centers (approximately half of present-day oceanic crust), by focusing on specific igneous features where sulfides are concentrated. International Ocean Discovery Program (IODP) Expedition 360 drilled Hole U1473A 789 m into the lower crust of the Atlantis Bank oceanic core complex, located at the Southwest Indian Ridge. Coarse-grained (5–30 mm) olivine gabbro prevailed throughout the hole, ranging locally from fine- (<1 mm), to very coarse-grained (>30 mm). We studied three distinct intervals of igneous grain size layering at 109.5–110.8, 158.0–158.3, and 593.0–594.4 meters below seafloor to understand the distribution of sulfides. We found that the layer boundaries between the fine- and coarse-grained gabbro were enriched in sulfides and chalcophile elements. On average, sulfide grains throughout the layering were composed of pyrrhotite (81 vol.%; Fe1-xS), chalcopyrite (16 vol.%; CuFeS2), and pentlandite (3 vol.%; [Ni,Fe,Co]9S8), which reflect paragenesis of magmatic origin. The sulfides were most commonly associated with Fe-Ti oxides (titanomagnetites and ilmenites), amphiboles, and apatites located at the interstitial positions between clinopyroxene, plagioclase, and olivine. Pentlandite exsolution textures in pyrrhotite indicate that the sulfides formed from high-temperature sulfide liquid separated from mafic magma that exsolved upon cooling. The relatively homogenous phase proportion within sulfides along with their chemical and isotopic compositions throughout the studied intervals further support the magmatic origin of sulfide enrichment at the layer boundaries. The studied magmatic layers were likely formed as a result of intrusion of more primitive magma (fine-grained gabbro) into the former crystal mush (coarse-grained gabbro). Sulfides from the coarse-grained gabbros are Ir-Platinum Group Element-rich (PGE; i.e., Ir, Os, Ru) but those from the fine-grained gabbros are Pd-PGE-rich (i.e., Pd, Pt, Rh). Notably, the sulfides from the layer boundaries are also enriched in Pd-PGEs, and therefore elevated sulfide contents at the boundaries were likely related to the new intruding melt. Because S concentration at sulfide saturation level is dependent on the Fe content of the melt, sulfide crystallization may have been caused by FeO loss, both via crystallization of late-precipitating oxides at the boundaries, and by exchange of Fe and Mg between melt and Fe-bearing silicates (olivine and clinopyroxene). The increased precipitation of sulfide grains at the layer boundaries might be widespread in the lower oceanic crust, as also observed in the Semail ophiolite and along the Mid-Atlantic Ridge. Therefore, this process might affect the metal budget of the global lower oceanic crust. We estimate that up to ∼20% of the Cu, ∼8% of the S, and ∼84% of the Pb of the oceanic crust inventory is accumulated at the layer boundaries only from the interaction between crystal mush and new magma.

Protracted hydrothermal alteration recorded at the microscale in the Chenaillet ophicarbonates (Western Alps): Insights from in situ δ18O thermometry in serpentine, carbonate and magnetite

The study of serpentinites and ophicarbonates from ophiolitic terrains provides a three-dimensional perspective on the hydration and carbonation processes affecting modern oceanic lithosphere. The Chenaillet ophiolite (western Alps) is interpreted as a fragment of an oceanic core complex that resembles a modern slow spreading center, and it was weakly affected by Alpine metamorphism. Ophicarbonates from the Chenaillet ophiolite were targeted in this study for in situ analysis by Secondary Ion Mass Spectrometry (SIMS) of oxygen and carbon isotopes in serpentine, calcite, dolomite and magnetite. The high spatial resolution of SIMS allowed us to target different serpentine, carbonate and magnetite generations intergrown at scales ≤ 50 μm, and reveal systematic zoning in δ18O with a range of 5.8‰ in serpentine (from 3.0 to 8.8‰, V-SMOW), 21.2‰ in carbonate (9.4 to 30.6‰), and 5.6‰ in magnetite (–5.0 to –10.6‰). Coupled analysis of oxygen isotopes in seven different touching-pairs of co-crystallized serpentine + carbonate and serpentine + magnetite provides independent constraints on both the temperatures and δ18O(water) values during serpentinization and carbonation responsible for the formation of the Chenaillet ophicarbonates. The new stable isotope data and thermometric estimates can be directly linked to textural and petrographic observations. These new results identify at least four different stages of hydrothermal alteration in the Chenaillet ophicarbonates: (1) peridotite hydration during seafloor exhumation at temperatures down to 200–130 °C and water δ18O values varying from 5 to 2‰, as documented by serpentine + magnetite in mesh textures; (2) carbonation during exhumation near the seafloor at temperatures as low as 10 °C assuming water δ18O values of –1‰, as documented by the highest oxygen isotope ratios in texturally older calcite; (3) serpentinization and carbonation at temperatures up to 240 °C and water δ18O values of 2-3‰, as documented by serpentine + magnetite in veins crosscutting mesh textures (T = 192 ± 66 °C, δ18O(water) = 2 ± 1‰, 2 standard deviation), serpentine + magnetite (T = 182 ± 32 °C, δ18O(water) = 2 ± 1‰) and serpentine + dolomite (T = 243 ± 79 °C, δ18O(water) = 3 ± 2‰) in recrystallized hourglass domains within serpentinite clasts, serpentine + dolomite (T = 229 ± 50 °C, δ18O(water) = 3 ± 1‰) and serpentine + calcite (T = 208 ± 40 °C, δ18O(water) = 2 ± 1‰) within the fine-grained calcite matrix surrounding serpentinite clasts; (4) late stage carbonation at temperatures down to 70–40 °C assuming water δ18O values of 3 to –1‰, as documented by the highest oxygen isotope ratios in a large calcite vein crosscutting both serpentinite clasts and fine-grained carbonate matrix. We suggest that the textural and isotopic observations are consistent with a protracted serpentinization and carbonation of the lithospheric mantle that started during progressive exhumation to the seafloor and continued due to interaction with hot and isotopically shifted seawater, which circulated at depth in the oceanic crust and was then discharged near the seafloor, similar to modern mid-ocean ridge venting systems.

Ocean crust accretion along a high-temperature detachment fault in the Oman ophiolite: A structural and petrological study of the Bahla massif

The Bahla massif exposes the lower crustal section of the Oman ophiolite located close to the thrust front of the Semail nappe. It is affected by intense faulting previously attributed to tectonic events that dismembered a classical ophiolitic sequence during or after the obduction. Here we show that most of this complexity is primary, inherited from syn-accretion tectonics. The crustal section is exposed in a 15 by 8 km tectonic enclave surrounded by mantle peridotite. Its northern boundary corresponds to a major, steeply dipping normal fault striking WNW-ESE, at low angle to the paleo-ridge axis. Movement along this fault was accommodated by intense plastic deformation of the crustal cumulates and adjacent mantle peridotites at temperature conditions ≥900 °C. The thickness of the deformed zone reaches several hundred meters. The flattening of the cumulate layering away from the fault is correlated to a decrease in the deformation intensity. Undeformed olivine-gabbro dykes cross-cut this “tectonic Moho” indicating that the tilting occurred before the end of the igneous activity. To the southwest, the crustal enclave is bounded by a NW-SE trending transtentional shear zone that was active in the amphibolite to greenschist facies and was intensely injected by syn- to post-kinematic gabbronorite and tonalite/trondhjemite dykes and plugs. The age of one felsic sample (95.214 ± 0.032 Ma, high-precision UPb zircon dating) is within error of the age of intrusive felsic intrusions into the mantle and lowermost axial crust from the length of the Oman ophiolite, which slightly post-dates the mean crystallization age of the Semail crust (V1 magmatism; 96.1–95.6 Ma). Other contacts are low temperature features including cataclastic faults, serpentine‑carbonate breccias and flat-lying décollements.Parent melts of the Bahla crustal cumulates were more siliceous and hydrous, i.e. more andesitic, than typical mid-ocean ridge basalt (MORB) as deduced from the frequent occurrence of early crystallizing orthopyroxene (opx) and late crystallizing amphibole. Some facies such as cumulate harzburgite and opx-troctolite have not been documented elsewhere in the Oman ophiolite and may be specific to the tectonic context in which the frontal massifs accreted. The chemical composition of the lower crustal cumulates can be accounted for by the hybridization in various proportions between MORB and a primitive andesite from a depleted source whose origin can be looked for in melts from a nascent subduction zone or from high temperature hydrothermal processes.The structure of the Bahla lower crustal section is reminiscent of the plutonic growth faults documented along present-day slow-spreading centres in both mid-ocean ridge and back arc settings. The distinctive characteristics of the Moho and lower crustal section in the Bahla massif are tentatively related to their position at the leading edge of the ophiolite, i.e. closer to the Arabian continental margin at the time of accretion than the massifs from the internal part of the ophiolite that have a more continuous and less deformed lower crust. It indicates that the style of crustal accretion may have changed during the opening of the oceanic basin from which the Oman ophiolite issued.

Role of compaction in melt extraction and accumulation at a slow spreading center: Microstructures of olivine gabbros from the Atlantis Bank (IODP Hole U1473A, SWIR)

The exposure of gabbroic sequences at Oceanic Core Complexes (OCC) along ultraslow- to slow-spreading ridges permits the study of the processes forming the lower oceanic crust. On top of the Atlantis Bank OCC along the ultraslow-spreading Southwest Indian Ridge, IODP Expedition 360 drilled Hole U1473A, mainly composed of primitive olivine gabbros interspersed with more evolved Ti-Fe oxide-bearing gabbros and minor felsic veins. These rocks record a complex history of protracted magmatism during continuous uplift and deformation of the gabbroic sequence. Extensive crystal-plastic deformation is dominantly recorded in the shallower sections of the drillhole, whereas the deeper sections better preserve primary magmatic features. We focus on microstructures, including intra-crystalline deformation of rock-forming minerals, and plagioclase crystallographic preferred orientations of olivine gabbros lacking evidence for exhumation-related crystal plastic deformation, to gain insights on the relationship between compaction, melt migration and melt accumulation during the early magmatic history of this section of lower oceanic crust. Olivine gabbros are characterized by ubiquitous grain-size variations, from coarse- to fine-grained intervals. Minerals in coarse-grained intervals show intra-crystalline deformation, while fine-grained crystals lack internal strain. Bent coarse-grained plagioclase associated with weak magmatic foliation and lack of lineation suggest that the coarse-grained intervals were deformed under weak compaction. On the other hand, crystallographic preferred orientations of undeformed fine-grained plagioclase show weak lineations, likely indicative of non-coaxial strain. We thereby infer that the coarse-grained intervals underwent ongoing weak compaction from the stage of olivine + plagioclase ± clinopyroxene crystal mush to the melt-poor stage, and that this process likely aided melt extraction and accumulation in discrete melt-rich zones where crystals orientated in the direction of magmatic flow. Crystallization of melts in the melt-rich zones ultimately formed the fine-grained intervals at different depths in Hole U1473A. This indicates that processes of compaction can lead to local chemical and grain-size heterogeneities in a lower crustal section, while had a minor role in the melt movement at larger scales (e.g., the whole crystal mush) within the oceanic crust.

13 million years of seafloor spreading throughout the Red Sea Basin

The crustal and tectonic structure of the Red Sea and especially the maximum northward extent of the (ultra)slow Red Sea spreading centre has been debated—mainly due to a lack of detailed data. Here, we use a compilation of earthquake and vertical gravity gradient data together with high-resolution bathymetry to show that ocean spreading is occurring throughout the entire basin and is similar in style to that at other (ultra)slow spreading mid-ocean ridges globally, with only one first-order offset along the axis. Off-axis traces of axial volcanic highs, typical features of (ultra)slow-spreading ridges, are clearly visible in gravity data although buried under thick salt and sediments. This allows us to define a minimum off-axis extent of oceanic crust of <55 km off the coast along the complete basin. Hence, the Red Sea is a mature ocean basin in which spreading began along its entire length 13 Ma ago.

Recovery of hydrothermal vent communities in response to an induced disturbance at the Lucky Strike vent field (Mid-Atlantic Ridge)

So far, the natural recovery of vent communities at large scales has only been evaluated at fast spreading centers, by monitoring faunal recolonisation after volcanic eruptions. However, at slow spreading ridges, opportunities to observe natural disturbances are rare, the overall hydrothermal system being more stable. In this study, we implemented a novel experimental approach by inducing a small-scale disturbance to assess the recovery potential of vent communities along the slow-spreading northern Mid-Atlantic Ridge (nMAR). We followed the recovery patterns of thirteen Bathymodiolus azoricus mussel assemblages colonising an active vent edifice at the Lucky Strike vent field, in relation to environmental conditions and assessed the role of biotic interactions in recolonisation dynamics. Within 2 years after the disturbance, almost all taxonomic richness had recovered, with the exception of a few low occurrence species. However, we observed only a partial recovery of faunal densities and a major change in faunal composition characterised by an increase in abundance of gastropod species, which are hypothesised to be the pioneer colonists of these habitats. Although not significant, our results suggest a potential role of mobile predators in early-colonisation stages. A model of post-disturbance succession for nMAR vent communities from habitat opening to climax assemblages is proposed, also highlighting numerous knowledge gaps. This type of experimental approach, combined with dispersal and connectivity analyses, will contribute to fully assess the resilience of active vent communities after a major disturbance, especially along slow spreading centers targeted for seafloor massive sulphide extraction.

Magma Reservoir Formation and Evolution at a Slow-Spreading Center (Atlantis Bank, Southwest Indian Ridge)

Several ODP-IODP expeditions drilled oceanic core complexes (OCCs) interpreted as exhumed portions of lower crust close to the ridge axis, and provide the community with invaluable sampling opportunity for further constraining magmatic processes involved in the formation of the slow-spreading lower oceanic crust. ODP Hole 735B presents the most primitive lithologies sampled at Atlantis Bank OCC (Southwest Indian Ridge) in a ~250 m thick section that was previously interpreted as a single crustal intrusion. We combined detailed structural and petrographic constraints with whole rock and in situ mineral analyses of this section in order to precisely determine the processes of emplacement, crystallization and melt migration within the lower crust. The lower half of the unit is comprised of alternating olivine gabbros and troctolites showing intrusive contacts, magmatic fabrics and crystal-plastic fabrics. Such structures and primitive lithologies are lacking in the upper half, rather uniform, gabbroic sequence. Whole rock compositions highlight the cumulative character of both lower and upper units and a great compositional variability in the lower sequence, whereas the upper sequence is homogeneous and differentiates up-section. In situ analyses of mineral phases document magma emplacement processes and provide evidence for ubiquitous reactive porous flow during differentiation. We show that the whole section, and related geochemical unit, constitutes a single magmatic reservoir, in which the lower unit is formed by stacked primitive sills formed by repeated recharge by primitive melts and melt-present deformation. Recharge led to partial assimilation of the crystallizing primitive cumulates, and hybridization with their interstitial melts. Hybrid melts were progressively collected in the overlying mushy part of the reservoir (upper unit), whereas the sills' residual hybrid melts differentiated by reactive porous flow processes under a predominantly crystallization regime. Similarly, hybrid melts’ evolution in the upper unit was governed by upward reactive porous flow, and progressive differentiation and accumulation of evolved melts at the top of the reservoir. Our results provide the community with the first integrated model for magma reservoir formation in the lower slow-spreading oceanic crust that can potentially be applied to other magmatic lower crust sections.

Crustal structure of the central sector of the NE Brazilian equatorial margin

Abstract The central equatorial Brazilian margin is divided into the Amazon and Barreirinhas divergent segments separated by the Pará-Maranhão transform segment. Analysis of regional 2D seismic lines allowed the definition of the crustal architecture of the margin. In the study area, the Barreirinhas segment has a proximal domain with a 30–35 km-thick continental crust, a 20–40 km-wide necked domain where the crust thins to 10 km, and an outboard domain with hyperextended continental crust. The Pará-Maranhão and Amazon segments consist of exhumation domains and their transition to ocean crust. Their structural styles indicate that this is a magma-poor passive margin with oceanic crust formed in a slow spreading centre. The Pará-Maranhão segment is bounded by two branches of the Saint Paul Fracture Zone that displace crustal domains with structures that document the transition from the distal part of a transform margin to an oceanic fracture zone. Two groups of post-rift volcanic complexes have been identified in the exhumation and oceanic domains, and whose distribution is controlled by the fracture zones. Late Cretaceous–Recent gravitationally-driven slide systems and mass-transport deposits indicate long-lived margin collapse and sediment redistribution fundamentally controlled by the underlying crustal structure of this part of the northeastern Brasilian passive margin.

Volcanic-Tectonic Structure of the Mount Dent Oceanic Core Complex in the Ultraslow Mid-Cayman Spreading Center Determined From Detailed Seafloor Investigation.

The flanks of the ultraslow‐spreading Mid‐Cayman Spreading Center (MCSC) are characterized by domal massifs or oceanic core complexes (OCCs). The most prominent of these, Mount Dent, comprises lower‐crustal and upper‐mantle lithologies and hosts the Von Damm vent field ~12 km west of the axial deep. Here, presented autonomous underwater vehicle‐derived swath sonar (multibeam) mapping and deep‐towed side‐scan sonar imagery lead to our interpretation that: (i) slip along the OCC‐bounding detachment fault is ceasing, (ii) the termination zone, where detachment fault meets the hanging wall, is disintegrating, (iii) the domed surface of the OCC is cut by steep north‐south extensional faulting, and (iv) the breakaway zone is cut by outward facing faults. The Von Damm vent field and dispersed pockmarks on the OCC's south flank further suggest that hydrothermal fluid flow is pervasive within the faulted OCC. On the axial floor of the MCSC, bright acoustic backscatter and multibeam bathymetry reveal: (v) a volcanic detachment hanging wall, (vi) a major fault rifting the southern flank of Mount Dent, and (vii) a young axial volcanic ridge intersecting its northern flank. These observations are described by a conceptual model wherein detachment faulting and OCC exhumation are ceasing during an increase in magmatic intrusion, brittle deformation, and hydrothermal circulation within the OCC. Together, this high‐resolution view of the MCSC provides an instructive example of how OCCs, formed within an overall melt‐starved ultraslow spreading center, can undergo magmatism, hydrothermal activity, and faulting in much the same way as expected in magmatically more robust slow‐spreading centers elsewhere.

Crustal structure of an extinct, late Jurassic-to-earliest Cretaceous spreading center and its adjacent oceanic crust in the eastern Gulf of Mexico

An extinct, late Jurassic-to-earliest Cretaceous ridge-and-fracture zone geometry in the western Gulf of Mexico (GOM), and extinct seafloor ridge segments in the eastern Gulf of Mexico (EGOM), were previously identified using the vertical gradient of satellite-derived free-air gravity data. Circular gravity anomaly lows, and magnetic anomaly highs, over the center of spreading ridge segments are interpreted as large volcanic centers that erupted within a late Jurassic-to-earliest Cretaceous, slow-spreading center. Detailed mapping of oceanic basement using oil industry seismic data indicates that the EGOM oceanic ridge system is characterized by 30-60-km-long spreading ridge segments, that include 15-km-wide, 2-km-high axial volcanoes in their centers, and nodal basins at their ends. Stratigraphic evidence from seismic reflection data tied to a deepwater well indicates that volcanism along the spreading ridge ended around the same time (Berriasian), or slightly after (Valanginian), the cessation of seafloor spreading in the EGOM. Flowlines of late Jurassic-to-earliest Cretaceous seafloor, based on a pole of rotation from the geometry of GOM spreading ridges and fracture zones, show a good match with gravity and magnetic anomalies along the Florida and Yucatan conjugate margins of the EGOM. Mapping of age-dated, stratigraphic downlaps onto the oceanic crust is consistent with an interpreted ridge jump at the beginning of seafloor spreading (Kimmeridgian) to the southwest, and in the same southwestward direction of a previously inferred mantle plume in the central GOM. Our 3-D gravity structural inversion of the Moho requires 6.4 km thick oceanic crust in the northwestern EGOM, and 5.5 km thick oceanic crust in southeastern EGOM. We interpret this along-ridge, thickness variation to reflect faster spreading and thicker oceanic crust farther from the opening pole located in the southeastern GOM.

Oblique seafloor spreading across intermediate and superfast spreading centers

We show that oblique seafloor spreading occurs in several regions where obliquity, α, was not recognized before. These include the slow spreading centers of the Red Sea (α≈20°), intermediate spreading centers of the Cocos–Nazca plate boundary between 91°W and 94°W (α≈9°), and superfast spreading centers of the East Pacific Rise at the Nazca–Pacific plate boundary between 29°S and 32°S (α≈10°) and perhaps between ≈16°S and ≈22°S (α≈4°). Thus, oblique spreading occurs across slow, intermediate, and superfast spreading centers, but not across fast spreading centers. Across intermediate and superfast spreading centers, oblique spreading does not occur in staircase-mode plate geometries, but does occur in oblique-mode plate geometries. Across slow and intermediate spreading centers, obliquity tends to decrease with increasing spreading rate, while across fast and superfast spreading centers it tends to increase with increasing spreading rate. Oblique spreading at intermediate and superfast spreading centers may be related to magma overpressure or to unusual directions of remote tectonic stress or to ongoing plate boundary reorganizations or to some combination of these. Using prior methods, we estimate the ratio of overpressure to remote tectonic stress for several spreading centers. In particular we show along a segment of the Cocos–Pacific plate boundary that magma overpressure is only one-fourth as large as remote tectonic stress, consistent with a prior inference from other observations. The highest obliquity occurs along ridge segments lying 200 km to 1500 km from a mantle plume, but not all ridge segments near plumes spread obliquely. For one set of estimates of plume fluxes, the rate of plume flux delivered to ridges correlates positively and significantly with spreading rate.

The formation of gold‐rich seafloor sulfide deposits: Evidence from the Beebe hydrothermal vent field, Cayman Trough

AbstractThe Beebe vent field (BVF) in the Cayman Trough has built an auriferous massive sulfide deposit on the ultraslow spreading mid‐Cayman spreading center. The genesis of auriferous sulfide deposits at mid‐ocean ridges is not fully understood, although there is a growing recognition that slow and ultraslow spreading centers are conducive to gold mineralization. Analysis of hydrothermal precipitates from the BVF indicates that the highest gold contents are present within “beehive diffusers,” which have developed a highly porous pyrrhotite framework. The beehive structure allows vent fluids to effuse slowly while allowing ingress of seawater to cool the fluid. The prevalence of pyrrhotite in the beehive samples, lack of sulfates, association between pyrrhotite and gold grains, and results of thermodynamic modeling suggest gold precipitation occurred under highly reduced conditions even during mixing with seawater. In contrast, high‐temperature chimneys, with a single orifice, maintain high temperatures to the primary vent orifice and much of the gold is lost to seawater. Despite this, both chimney types are relatively gold‐enriched, which points to a further underlying cause for high gold at the BVF such as interaction of hydrothermal fluids with ultramafic lithologies in the basement. The final gold composition of the deposit is partially controlled by loss of gold during mass‐wasting of the material, with gold depletion most prevalent in blocks formed at beehive‐type chimneys. The BVF demonstrates that the overall gold content of a massive sulfide deposit is the sum of basement, precipitation, and surface processes.

Dynamics of a seafloor-spreading episode at the East Pacific Rise.

Seafloor spreading is largely unobserved because 98 per cent of the global mid-ocean-ridge system is below the ocean surface. Our understanding of the dynamic processes that control seafloor spreading is thus inferred largely from geophysical observations of spreading events on land at Afar in East Africa and Iceland. However, these are slow-spreading centres influenced by mantle plumes. The roles of magma pressure and tectonic stress in the development of seafloor spreading are still unclear. Here we use seismic observations to show that the most recent eruption at the fast-spreading East Pacific Rise just North of the Equator initiated at a melt-rich segment about 5 kilometres long. The change in static stress then promoted almost-concurrent rupturing along at least 35 kilometres of the ridge axis, where tectonic stress had built up to a critical level, triggering magma movement. The location of impulsive seismic events indicative of lava reaching the seafloor suggests that lava subsequently erupted from multiple isolated magma lenses (reservoir chambers) with variable magma ascent rates, mostly within 48 hours. Therefore, even at magmatically robust fast-spreading ridges, a substantial portion of the spreading may be due to tectonic stress building up to a critical level rather than magma overpressure in the underlying magma lenses.

Mitochondrial DNA Analyses Indicate High Diversity, Expansive Population Growth and High Genetic Connectivity of Vent Copepods (Dirivultidae) across Different Oceans

Communities in spatially fragmented deep-sea hydrothermal vents rich in polymetallic sulfides could soon face major disturbance events due to deep-sea mineral mining, such that unraveling patterns of gene flow between hydrothermal vent populations will be an important step in the development of conservation policies. Indeed, the time required by deep-sea populations to recover following habitat perturbations depends both on the direction of gene flow and the number of migrants available for re-colonization after disturbance. In this study we compare nine dirivultid copepod species across various geological settings. We analyze partial nucleotide sequences of the mtCOI gene and use divergence estimates (FST) and haplotype networks to infer intraspecific population connectivity between vent sites. Furthermore, we evaluate contrasting scenarios of demographic population expansion/decline versus constant population size (using, for example, Tajima’s D). Our results indicate high diversity, population expansion and high connectivity of all copepod populations in all oceans. For example, haplotype diversity values range from 0.89 to 1 and FST values range from 0.001 to 0.11 for Stygiopontius species from the Central Indian Ridge, Mid Atlantic Ridge, East Pacific Rise, and Eastern Lau Spreading Center. We suggest that great abundance and high site occupancy by these species favor high genetic diversity. Two scenarios both showed similarly high connectivity: fast spreading centers with little distance between vent fields and slow spreading centers with greater distance between fields. This unexpected result may be due to some distinct frequency of natural disturbance events, or to aspects of individual life histories that affect realized rates of dispersal. However, our statistical performance analyses showed that at least 100 genomic regions should be sequenced to ensure accurate estimates of migration rate. Our demography parameters demonstrate that dirivultid populations are generally large and continuously undergoing population growth. Benthic and pelagic species abundance data support these findings.

Recycling of ancient subduction-modified mantle domains in the Purang ophiolite (southwestern Tibet)

Ophiolites in the Indus–Yarlung Zangbo (IYZ) suture (southern Tibet) have been interpreted as remnants of the Neo-Tethyan lithosphere. However, the discovery of diamonds and super-reducing, ultra-high pressure (SuR-UHP) mineral assemblages (e.g., coesite after stishovite, olivine after wadsleyite, native metals, alloys, and moissanite) in some of these massifs and associated chromitites requires a re-evaluation of their origin and evolution. A new petrological and geochemical study of the Purang ophiolite in the western IYZ suture sheds new lights on these issues.The depleted harzburgites of the Purang massif have low modal contents of clinopyroxene (<2%), and high Cr# [100*Cr3+/(Cr3++Al3+)] in spinel (>40~70) and pyroxenes (>16 in orthopyroxene, and >20 in clinopyroxene), suggesting high degrees of melt extraction (>20%). These features are not consistent with formation in a (ultra-) slow-spreading mid-ocean ridge. These peridotites have high modal contents of orthopyroxene; this, and the extremely high Cr# of spinels in these peridotites, suggests modification in a subduction zone. The clinopyroxene-rich harzburgites and lherzolites contain rare spinel–pyroxene symplectites after garnet. Their clinopyroxenes have low MREE-to-HREE ratios ((Sm/Yb)N<0.1) at relatively high HREE concentrations, and are Na-rich but Nd-poor. The relatively enrichment of Na but depletion of Nd in clinopyroxene cannot be explained by refertilization with MORB melts but are consistent with an origin from Na-rich subcontinental lithospheric mantle (SCLM). All lines of evidence suggest that these peridotites underwent initial melting in the stability field of garnet–facies peridotites, followed by additional melting in the spinel–facies mantle.Whole-rock Os isotopic compositions of the Purang peridotites give ancient TRD model ages (up to 1.3Ga), indicating that the formation of these ancient depletion residues predated the opening of Neo-Tethyan Ocean. These observations, together with recent studies on other IYZ peridotites, suggest that the Purang peridotites are genetically unrelated to the associated mafic crust. Instead, they represent ancient SCLM domains, initially formed beneath a continental margin, and then modified by subduction, before they were incorporated into the Neo-Tethyan ocean basin. This model is consistent with the deep-mantle-recycling model for the presence of SuR-UHP phases in the IYZ ophiolites. The infiltration of MORB melts through these ancient depleted peridotites during their final exhumation in a (ultra-) slow-spreading center may have refertilized them to produce the clinopyroxene-rich peridotites.

Carlsberg Ridge and Mid-Atlantic Ridge: Comparison of slow spreading centre analogues

Eighty per cent of all mid-ocean spreading centres are slow. Using a mixture of global bathymetry data and ship-board multibeam echosounder data, we explore the morphology of global mid-ocean ridges and compare two slow spreading analogues: the Carlsberg Ridge in the north-west Indian Ocean between 57°E and 60°E, and the Kane to Atlantis super-segment of the Mid-Atlantic Ridge between 21°N and 31°N. At a global scale, mid-ocean spreading centres show an inverse correlation between segment length and spreading rate with segmentation frequency. Within this context, both the Mid-Atlantic Ridge super-segment and Carlsberg Ridge are similar: spreading at 22 and 26mm/yr full rates respectively, being devoid of major transform faults, and being segmented by dextral, non-transform, second-order discontinuities. For these and other slow spreading ridges, we show that segmentation frequency varies inversely with flank height and ridge axis depth. Segments on both the Mid-Atlantic Ridge super-segment and Carlsberg Ridge range in aspect ratio (ridge flank height/axis width), depth and symmetry. Segments with high aspect ratios and deeper axial floors often have asymmetric rift flanks and are associated with indicators of lower degrees of melt flux. Segments with low aspect ratios have shallower axial floors, symmetric rift flanks, and evidence of robust melt supply. The relationship between segmentation, spreading rate, ridge depth and morphology, at both a global and local scale, is evidence that rates of melting of the underlying mantle and melt delivery to the crust play a significant role in determining the structure and morphology of slow spreading mid-ocean ridges.