Ultraslow-spreading Mid-ocean Ridges Research Articles

Teleseismic Indication of Magmatic and Tectonic Activities at Slow- and Ultraslow-Spreading Ridges

Magmatic and tectonic processes in the formation of oceanic lithosphere at slow–ultraslow-spreading mid-ocean ridges (MORs) are more complicated relative to faster-spreading ridges, as their melt flux is overall low, with highly spatial and temporal variations. Here, we use the teleseismic catalog of magnitudes over 4 between 1995 and 2020 from the International Seismological Center to investigate the characteristics of magmatic and tectonic activities at the ultraslow-spreading Southwest Indian Ridge and Arctic Gakkel Ridge and the slow-spreading North Mid-Atlantic Ridge and Carlsberg Ridge (total length of 14,300 km). Using the single-link cluster analysis technique, we identify 78 seismic swarms (≥8 events), 877 sequences (2–7 events), and 3543 single events. Seismic swarms often occur near the volcanic center of second-order segments, presumably relating to relatively robust magmatism. By comparing the patterns of seismicity between ultraslow- and slow-spreading ridges, and between melt-rich and melt-poor regions of the Southwest Indian Ridge with distinct seafloor morphologies, we demonstrate that a lower spreading rate and a lower melt supply correspond to a higher seismicity rate and a higher potential of large volcano-induced seismic swarms, probably due to a thicker and colder lithosphere with a higher degree of along-axis melt focusing there.

Abundant off-axis hydrothermal activity in the 29–30 ridge segment of the Southwest Indian Ridge: evidence from ferromanganese crusts

In the ultra-slow spreading mid-ocean ridge, seafloor hydrothermal ventings mostly occur in the off-axis region. The plume of hydrothermal venting provides Fe, Mn and other metal materials for the growth of ferromanganese crust in the surrounding seamounts, showing unique geochemical characteristics that are different from that of hydrogenetic crust. Based on five samples of ferromanganese crusts, major, trace and rare earth element analysis was carried out to identify their material sources. Combined with the investigation data of water column and seafloor camera photos by deep towed hydrothermal detection system, the potential of hydrothermal activity in the 29–30 ridge segment of the Southwest Indian ridge was evaluated. The results showed that the ferromanganese crust in the study area had significantly higher Fe/Mn value (average 1.9), relatively higher Si and Al contents, and significantly higher Ca/P value (average 9.3),without significant phosphorylation. ferromanganese crust in the study area have significantly lower Co and Ni contents (about 1600 mg/kg on average), and relatively lower Sr, Ba, Pb, Cu, Zr and Mo contents (between 100-1000 mg/kg on average). The contents of W, Th and Te are also relatively low (average content between 10–50 mg/kg); The total rare earth element content of the crust in the study area is relatively low (about 928 mg/kg on average), and the light rare earth is relatively enriched. The standardized rare earth curve of the shale shows a left-leaning pattern as a whole, showing the enrichment of heavy rare earth relative to the shale. The Co content and rare earth element content of the ferromanganese crusts in this area are significantly lower than those of hydrogenetic crusts. The discrimination diagrams of ternary and bivariate material sources reveal that they have mixed hydrothermal and hydrogenetic origins. The three crust samples of S1, S2, and S5 are located within 2 km of the known hydrothermal fields, indicating a correlation between ferromanganese crust and the location of hydrothermal activity. There is no known hydrothermal field near the S3 and S4 stations. Altered rocks and water column turbidity anomalies were found near S3 station, and large areas of altered rocks and suspected hydrothermal biological remains were also found near S4 station, indicating that hydrothermal activity may exist in both areas. In addition to the two new hydrothermal fields identified in this article, the spatial frequency of hydrothermal activity in the study area reaches 15 sites/100 km, which is significantly higher than other well investigated oceanic ridges. Out of the eight hydrothermal fields in the study area, seven are located in the off axis region, mainly because the hydrothermal activity in this area is controlled by high angle and large offset normal faults and one-way detachment faults. This also indicates that the off-axis region of the Southwest Indian Ridge has high potential for hydrothermal activity.

Evolution of enhanced magmatism at the ultraslow spreading Southwest Indian Ridge between 46°E and 53.5°E

The Indomed-Gallieni supersegment (46-52°E) of the Southwest Indian Ridge (SWIR) has been of interest as a melt-rich endmember of ultraslow spreading mid-ocean ridges, attributed to enhanced magmatism since 11 Ma. This provides an excellent opportunity to study the evolution and the impact of enhanced magmatism at mid-ocean ridges. Here we combine shipboard bathymetry and gravity data over the SWIR 46-53.5°E. We illustrate that the enhanced magmatism initiated in the western part of the Indomed-Gallieni supersegment with decreasing crustal productions and no propagation. In the eastern part, the enhanced magmatism propagated to the east at decreasing propagating velocities, producing an increasingly thick crust. The present propagation tip is at 51.9°E, ∼30 km to the west of the Gallieni transform fault (TF). The most magmatically-robust area of the propagation has been located near 50.5°E since ∼6 Ma. We hypothesize that a mantle melting anomaly with heterogenous upwelling rates and/or geometry was responsible for the change of the enhanced magmatism. The enhanced magmatism plays significant roles in ridge segmentation and seafloor spreading modes. The degree of along-axis melt focusing was lowered, causing the segmentation to be typical of slow spreading ridges. As magmatism dramatically decreases over 160 km along the ridge axis from the most robust (producing a 9.5-km-thick crust at 50.5°E) to nearly-amagmatic near the Gallieni TF, this ridge section exhibits the full spectrum of ultraslow spreading modes, changing from magmatically-controlled (intense volcanism and almost no faulting) to tectonically-dominated (smooth ridges and extensive peridotites outcrops) modes.

Ultramafic-Hosted Ni-Cu-Co-(As) Mineralization from an Ancient Oceanic Transform Fault Zone in the Troodos Ophiolite, Cyprus: An Analogue for Ultramafic Sea-Floor Massive Sulfide Mineralization?

Abstract Accumulations of sulfide minerals that are enriched in Ni-Cu-Co-(As) occur as sea-floor massive sulfide (SMS) deposits associated with ultramafic rock types on the sea floor and in ophiolite terranes as Outokumpu-type mineralization. In this study we focus on similar mineralization at Lakxia tou Mavrou in the Limassol Forest Complex of Cyprus, which represents the on-land exposure of an oceanic transform fault zone preserved within the Troodos ophiolite. Mineralization here consists of massive lenses of pyrrhotite associated with veins of isocubanite, chalcopyrite, Co pentlandite, and chrome spinel hosted in serpentinized mantle peridotite. We reexamine the field context of mineral occurrences and use in situ mineral chemistry, element mapping, and sulfur isotope ratios (δ34S) to constrain metal sources and provide an updated paragenetic model for Lakxia tou Mavrou. Highly variable S/Se ratios (304–108,571), a depletion in platinum group elements relative to mantle values, and an average δ34S value of –3.7 ± 2.4‰ (1σ, n = 17) in sulfide minerals support a hybrid hydrothermal and magmatic origin for the mineralization. Metals at Lakxia tou Mavrou were sourced from both the serpentinization of peridotites and from crosscutting intrusions, with later intrusions into the already serpentinized mantle lithosphere host providing a heat source to drive prolonged hydrothermal circulation. The reexamination of the field context of mineralization shows that the Ni-Cu-Co-(As) mineralization at Lakxia tou Mavrou originally formed because of the fault-guided intrusion of hot primitive magma bodies into serpentinized shallow mantle lithosphere in the active domain of an ocean-floor transform fault zone. The mineralization was subsequently partially disrupted by structures related to emplacement of the Troodos ophiolite. We show that the relationship between serpentinization, magmatism, and hydrothermal circulation at Lakxia tou Mavrou can be used to understand the formation of ultramafic-hosted SMS deposits in transform fault and other ultramaficdominated slow- and ultraslow-spreading mid-ocean ridge settings.

Weathering-driven porosity generation in altered oceanic peridotites

Ultramafic rocks exposed at slow and ultra-slow spreading mid-ocean ridges represent a significant and extremely reactive portion of the oceanic lithosphere. Thus, mechanistic understanding of the processes by which seawater infiltrates into and reacts with these rocks is essential for constraining their contribution to the chemistry of the oceans and the coupled carbonate-silicate cycle. Recent observations indicate that nanoscale processes contribute to seawater-driven alteration of ultramafic rocks, but conventional petrographic and tomographic observations of the associated physical features are challenging to link to these nanoscale features. Moreover, multiple generations and varying conditions of fluid infiltration often obscure the relative roles of higher-temperature serpentinization, where reactions are mostly isochemical, and lower-temperature weathering reactions, where observations suggest the release of massive amounts of magnesium. Here we bridge these scales and investigate the specific role of weathering processes in dissolution-driven porosity generation by integrating focused ion beam scanning electron microscopy nanotomography and micro-computed X-ray tomography imaging of the pore structures preserved in drill cores of serpentinized oceanic peridotites. Relict olivine crystals in all imaged samples contain abundant etch pits, and those in the higher-resolution FIB-SEM imagery show the presence of channel-like dissolution structures. The pore channels preferentially affect olivine along grain boundaries and show anisotropic distribution likely controlled by crystallographic features. The pores formed via olivine dissolution are interpreted to result from dissolution of serpentinized peridotite at conditions where serpentine and carbonate precipitation are kinetically inhibited, i.e., at weathering conditions. Importantly, the calculated connectivity of the imaged pore structures increases as the scale of investigation increases, suggesting that weathering-driven olivine dissolution facilitates further seawater infiltration and olivine dissolution, a positive feedback that can sustain continued magnesium extraction until the rocks are ultimately cut off from seawater circulation via sedimentation. Thus, while much attention has been directed towards constraining geochemical fluxes from the higher-temperature alteration of ultramafic rocks, our results support literature studies suggesting that mineral dissolution, and hence elemental fluxes, are significant at the lower temperatures of seafloor weathering. Our data thus provide mechanistic evidence of the physical process contributing to the observed elemental loss from weathered oceanic peridotites.

Shear wave velocity structure and crustal lithology beneath the ultraslow spreading Southwest Indian Ridge at 50° E

SUMMARY Shear wave velocities provide an important constraint on crustal lithology. Limited crustal shear wave data are available from the ultraslow spreading mid-ocean ridges. We combine observations of both compressional (P) and shear (S) waves in ocean–bottom seismometer data from the Southwest Indian Ridge to determine crustal P-wave velocity (Vp), S-wave velocity (Vs), Vp/Vs and Poisson's ratio variations along the ridge at 49°17′E–50°49′E. Similar layered crustal structures were revealed beneath both the magmatically robust segment centres (Vp/Vs of 1.76–1.94, Poisson's ratio of 0.26–0.32) and the non-transform discontinuity (NTD) between them (Vp/Vs of 1.76–2.03 and Poisson's ratio of 0.26–0.34). Because laboratory measurements show an overlap in Poisson's ratio between mafic igneous and ultramafic rocks, particularly at Vp values typical of oceanic Layer 3, it can be difficult to distinguish crustal composition using this parameter only. However, our observed Vp gradients of 0.1 ± 0.1 s−1 suggest that in this area, oceanic Layer 3 consists primarily of mafic igneous rocks both at segment centres and at the NTD. Oceanic crustal Layers 2A and 2B above are likely also to consist of mafic igneous rocks, with some evidence for increased fracturing at the NTD.

New Opportunities and Untapped Scientific Potential in the Abyssal Ocean

The abyssal ocean covers more than half of the Earth’s surface, yet remains understudied and underappreciated. In this Perspectives article, we mark the occasion of the Deep Submergence Vehicle Alvin’s increased depth range (from 4500 to 6500 m) to highlight the scientific potential of the abyssal seafloor. From a geologic perspective, ultra-slow spreading mid-ocean ridges, Petit Spot volcanism, transform faults, and subduction zones put the full life cycle of oceanic crust on display in the abyss, revealing constructive and destructive forces over wide ranges in time and space. Geochemically, the abyssal pressure regime influences the solubility of constituents such as silica and carbonate, and extremely high-temperature fluid-rock reactions in the shallow subsurface lead to distinctive and potentially unique geochemical profiles. Microbial residents range from low-abundance, low-energy communities on the abyssal plains to fast growing thermophiles at hydrothermal vents. Given its spatial extent and position as an intermediate zone between coastal and deep hadal settings, the abyss represents a lynchpin in global-scale processes such as nutrient and energy flux, population structure, and biogeographic diversity. Taken together, the abyssal ocean contributes critical ecosystem services while facing acute and diffuse anthropogenic threats from deep-sea mining, pollution, and climate change.

Geochemical characteristics of back-arc basin lower crust and upper mantle at final spreading stage of Shikoku Basin: an example of Mado Megamullion

This paper explores the evolutional process of back-arc basin (BAB) magma system at final spreading stage of extinct BAB, Shikoku Basin (Philippine Sea) and assesses its tectonic evolution using a newly discovered oceanic core complex, the Mado Megamullion. Bulk and in-situ chemical compositions together with in-situ Pb isotope composition of dolerite, oxide gabbro, gabbro, olivine gabbro, dunite, and peridotite are presented. Compositional ranges and trends of the igneous and peridotitic rocks from the Mado Megamullion are similar to those from the slow- to ultraslow-spreading mid-ocean ridges (MOR). Since the timing of the Mado Megamullion exhumation corresponds to the very end of the Shikoku Basin opening, the magma supply was subdued and highly episodic, leading to extreme magma differentiation to form ferrobasaltic, hydrous magmas. In-situ Pb isotope composition of magmatic brown amphibole in the oxide gabbro is identical to that of depleted source mantle for mid-ocean ridge basalt (MORB). In the context of hydrous BAB magma genesis, the magmatic water was derived solely from the MORB source mantle. The distance from the back-arc spreading center to the arc front increased away through maturing of the Shikoku Basin to cause MORB-like magmatism. After the exhumation of Mado Megamullion along detachment faults, dolerite dikes intruded as a post-spreading magmatism. The final magmatism along with post-spreading Kinan Seamount Chain volcanism were introduced around the extinct back-arc spreading center after the opening of Shikoku Basin by residual mantle upwelling.

Serpentinization-Driven H2 Production From Continental Break-Up to Mid-Ocean Ridge Spreading: Unexpected High Rates at the West Iberia Margin

Molecular hydrogen (H2) released during serpentinization of mantle rocks is one of the main fuels for chemosynthetic life. Processes of H2 production at slow-spreading mid-ocean ridges (MORs) have received much attention in the past. Less well understood is serpentinization at passive continental margins where different rock types are involved (lherzolite instead of harzburgite/dunite at MORs) and the alteration temperatures tend to be lower (<200°C vs. >200°C). To help closing this knowledge gap we investigated drill core samples from the West Iberia margin. Lherzolitic compositions and spinel geochemistry indicate that the exhumed peridotites resemble sub-continental lithospheric mantle. The rocks are strongly serpentinized, mainly consist of serpentine with little magnetite, and are generally brucite-free. Serpentine can be uncommonly Fe-rich, with XMg = Mg/(Mg + Fe) < 0.8, and shows distinct compositional trends toward a cronstedtite endmember. Bulk rock and silicate fraction Fe(III)/∑Fe ratios are 0.6–0.92 and 0.58–0.8, respectively; our data show that 2/3 of the ferric Fe is accounted for by Fe(III)-serpentine. Mass balance and thermodynamic calculations suggest that the sample’s initial serpentinization produced ∼120 to >300 mmol H2 per kg rock. The cold, late-stage weathering of the serpentinites at the seafloor caused additional H2 formation. These results suggest that the H2 generation potential evolves during the transition from continental break-up to ultraslow and, eventually, slow MOR spreading. Metamorphic phase assemblages systematically vary between these settings, which has consequences for H2 yields during serpentinization. At magma-poor rifted margins and ultraslow-spreading MORs, serpentine hosts most Fe(III). Hydrogen yields of 120 to >300 mmol and 50–150 mmol H2 per kg rock, respectively, may be expected at temperatures of <200°C. At slow-spreading MORs, in contrast, serpentinization may produce 200–350 mmol H2, most of which is related to magnetite formation at >200°C. Since, in comparison to slow-spreading MORs, geothermal gradients at magma-poor margins and ultraslow-spreading MORs are lower, larger volumes of low-temperature serpentinite should form in these settings. Serpentinization of lherzolitic rocks at magma-poor margins should produce particularly high amounts of H2 under conditions within the habitable zone. Magma-poor margins may hence be more relevant environments for hydrogenotrophic microbial life than previously thought.

Resource estimation of the sulfide-rich deposits of the Yuhuang-1 hydrothermal field on the ultraslow-spreading Southwest Indian Ridge

Abstract Seafloor massive sulfide (SMS) deposits are important deep-sea mineral resources expected to occur predominantly on slow- and ultraslow-spreading mid-ocean ridges. Resource estimates are already available for some of the largest SMS deposits on slow-spreading ridges but not on ultraslow-spreading ridges. Based on geological mapping and sampling, this study investigates the distribution and content of sulfide-rich deposits in the Yuhuang-1 hydrothermal field (YHF), located on the ultraslow-spreading Southwest Indian Ridge. The sulfide-rich deposits in the YHF are composed of two areas ~500 m apart: the southwest sulfide area (SWS) and the northeast sulfide area (NES). We calculated the volume of sulfide-rich moundsin the YHF and arrived at a total accumulation of ~10.6 × 106 tons, including at least ~7.5 × 105 tons of copper and zinc and ~18 tons of gold. Furthermore, considering the coverage of layered hydrothermal sediment mixed with sulfide-rich breccias, which may have underlying massive sulfide deposits, the maximum total mass was estimated at ~45.1 × 106 tons. This suggests that the YHF is one of the largest SMS deposits worldwide and confirm that ultraslow-spreading ridges have the greatest potential to form large-scale SMS deposits.

Synthetic fluid inclusions XXIII. Effect of temperature and fluid composition on rates of serpentinization of olivine

Serpentinization, i.e. the hydrothermal alteration of ultramafic rocks, is an important and ubiquitous geologic process that occurs at slow- and ultraslow-spreading mid-ocean ridges, magma-poor passive margins, and subduction zones. While serpentinization occurs over a wide temperature range and involves diverse fluid compositions, few experimental studies systematically examined the effects of temperature and fluid composition on the kinetics of serpentinization of olivine, and published rates diverge greatly. We present results of an experimental study using synthetic fluid inclusions in Mg-rich olivine as micro-batch reactors to monitor the effects of temperature (100–350 °C), fluid composition (H2O-MgCl-NaCl, H2O-NaCl, H2O-MgCl), and total salinity on serpentinization rates of olivine under closed system conditions.Petrographic observations and Raman analyses of the experimental run products revealed the alteration of olivine to produce serpentine minerals, brucite, and magnetite. The salinity of the aqueous fluid in the inclusions increased as H2O was removed from the solution and incorporated into the hydrous product phases and served as a proxy for reaction progress. The fastest serpentinization rates were found at 250 °C in the presence of a seawater-like aqueous solution. This result further suggests that serpentinization of olivine is fastest at lower temperatures (∼250 °C) and shallower depths in the oceanic lithosphere than suggested by previous studies (∼300 °C). Moreover, our experiments show that serpentinization rates decrease by several orders of magnitude as salinity and the concentration of dissolved Mg increase. These effects may reconcile some of the differences observed when comparing results of previously published rates obtained from experiments involving fluids of different compositions. We constructed a quantitative model based on the principle of detailed balance to predict variations in serpentinization rates (J±) from low temperatures to 320 °C. It suggests that at 25 °C, serpentinization rates are 4 to 5 orders of magnitude slower than at 250 °C. Moreover, the temperature dependence model has been coupled with the kinetic effects derived from experiments involving different fluid composition to calculate the amount of time required for serpentinization of olivine having different reactive surface areas. The model shows that between 200 and 300 °C serpentinization is fast on geologic timescales when rocks with large reactive surface areas interact with a seawater-like aqueous solution (complete serpentinization in days to decades).

Seafloor expression of oceanic detachment faulting reflects gradients in mid-ocean ridge magma supply

Oceanic detachment faulting is a major mode of seafloor accretion at slow and ultraslow spreading mid-ocean ridges, and is associated with dramatic changes in seafloor morphology. Detachments form expansive dome structures with corrugated surfaces known as oceanic core complexes (OCCs), and often transition to multiple regularly-spaced normal faults that form abyssal hills parallel to the spreading axis. Previous studies have attributed these changes to along-axis gradients in lithospheric strength or magma supply. However, despite the recognition that magma supply can influence fault style and seafloor morphology, the mechanics controlling the transition from oceanic detachment faults to abyssal hill faults and the relationship to along-axis variations in magma supply remain poorly understood. This study investigates this issue using two complementary modeling approaches. The first consists of semi-analytical, two-dimensional (2-D) cross-axis models designed to address the fundamental mechanical controls on the longevity of normal faults. These 2-D model sections are juxtaposed in the along-axis direction to examine the response of the plan-view pattern of faults to along-axis variations in magmatic accretion in the absence of along-axis mechanical coupling. The second approach uses three-dimensional (3-D), time-dependent numerical models that simulate faulting and magma intrusion in a visco-elasto-plastic continuum. The primary variable studied through both approaches is the along-axis gradient in the fraction M of seafloor spreading that is accommodated by magmatism. The 2-D and 3-D results predict different abyssal hill spacing and orientation, however the plan-view geometry of self-emerging detachment faults predicted by the 3-D numerical models are well explained by the juxtaposed 2-D models. This indicates a first-order control by cross-axis effects of changing values of M. These models are also shown to explain the along-axis extent and plan-view curvature of the well-developed 13°20′N and Mt. Dent OCCs (Mid-Atlantic Ridge and Cayman Rise) in terms of quantifiable along-axis gradients in magma emplacement rates.

Preserved organic matter in a fossil Ocean Continent Transition in the Alps: the example of Totalp, SE Switzerland

Evidence from ultraslow spreading mid-ocean ridges and both fossil and present-day Ocean–Continent Transitions (OCT) demonstrates that mantle serpentinization resulting from the interaction of mantle rock and water during tectonic exhumation is widespread. Observations at white smokers in modern ocean settings suggest that methane produced by serpentinization can support methanotrophic bio-systems, which use methane as the only source of carbon. An important question is whether such bio-systems are more generally pervasive in their association with serpentinized mantle in the subsurface. In this study, we examined whether there is evidence for such a methanotrophic system in exhumed serpentinized mantle at a magma-poor rifted continental margin, by probing for characteristic biological markers in these and associated sedimentary rocks in the Totalp unit of SE Switzerland. This unit represents a remnant of the former OCT of the southern Alpine Tethyan margin and was chosen because of its mild Alpine tectonic and low-grade metamorphic overprint during Alpine orogeny, hence giving potential for the preservation of indigenous organic matter (OM). Totalp samples are characterized by low organic carbon contents of 11–647 ppm. The majority of the samples contain hydrocarbons in the form of n-alkanes in the range C17–C36. Some sediments contain isoprenoids, for example pristane and phytane and a suite of steranes that are consistent with a marine origin for the OM preserved in the rocks. Traces of marine planktonic and bacterial OM are preserved in the serpentinized mantle and overlying sediments of this ancient Tethyan OCT, but there is no evidence that the OM has been generated from methanotrophic bio-systems.

Geochemistry of hydrothermal vent fluids and its implications for subsurface processes at the active Longqi hydrothermal field, Southwest Indian Ridge

The Longqi hydrothermal field at 49.6°E on the Southwest Indian Ridge was the first active hydrothermal field found at a bare-rock ultra-slow spreading mid-ocean ridge. Here we report the chemistry of the hydrothermal fluids, for the first time, that were collected from the S zone and the M zone of the Longqi field by gas-tight isobaric samplers by the HOV “Jiaolong” diving cruise in January 2015. According to H2, CH4 and other chemical data of the vent fluid, we suggest that the basement rock at the Longqi field is dominantly mafic. This is consistent with the observation that the host rock of the active Longqi Hydrothermal field is dominated by extensively distributed basaltic rock. It was very interesting to detect simultaneously discharging brine and vapor caused by phase separation at vents DFF6, DFF20, and DFF5 respectively, in a distance of about 400m. Based on the end-member fluid chemistry and distance between the vents, we propose that there is a single fluid source at the Longqi field. The fluid branches while rising to the seafloor, and two of the branches reach S zone and M zone and phase separate at similar conditions of about 28–30.2MPa and 400.6–408.3°C before they discharge from the vents. The end-member fluid compositions of these vents are comparable with or within the range of variation of known global seafloor hydrothermal fluid chemical data from fast, intermediate and slow spreading ridges, which confirms that the spreading rate is not the key factor that directly controls hydrothermal fluid chemistry. The composition of basement rock, water-rock interaction and phase separation are the major factors that control the composition of the vent fluids in the Longqi field.

Observations of fauna attending wood and bone deployments from two seamounts on the Southwest Indian Ridge

The Southwest Indian Ridge is an ultraslow-spreading mid-ocean ridge with numerous poorly-explored seamounts. The benthic fauna of seamounts are thought to be highly heterogeneous, within even small geographic areas. Here we report observations from a two-year opportunistic experiment, which was comprised of two deployments of mango wood and whale bones. One was deployed at 732m on Coral Seamount (~32°S) and the other at 750m on Atlantis Bank (~41°S), two areas with little background faunal knowledge and a significant distance from the continental shelf. The packages mimic natural organic falls, large parcels of food on the deep-sea floor that are important in fulfilling the nutritional needs and providing shelter and substratum for many deep-sea animals. A large number of species colonised the deployments: 69 species at Coral Seamount and 42 species at Atlantis Bank. The two colonising assemblages were different, however, with only 11 species in common. This is suggestive of both differing environmental conditions and potentially, barriers to dispersal between these seamounts. Apart from Xylophaga and Idas bivalves, few organic-fall specialists were present. Several putative new species have been observed, and three new species have been described from the experiments thus far. It is not clear, however, whether this is indicative of high degrees of endemism or simply a result of under-sampling at the regional level.

Microbial Sulfur Cycle in Two Hydrothermal Chimneys on the Southwest Indian Ridge

ABSTRACTSulfur is an important element in sustaining microbial communities present in hydrothermal vents. Sulfur oxidation has been extensively studied due to its importance in chemosynthetic pathways in hydrothermal fields; however, less is known about sulfate reduction. Here, the metagenomes of hydrothermal chimneys located on the ultraslow-spreading Southwest Indian Ridge (SWIR) were pyrosequenced to elucidate the associated microbial sulfur cycle. A taxonomic summary of known genes revealed a few dominant bacteria that participated in the microbial sulfur cycle, particularly sulfate-reducing Deltaproteobacteria. The metagenomes studied contained highly abundant genes related to sulfur oxidation and reduction. Several carbon metabolic pathways, in particular the Calvin-Benson-Bassham pathway and the reductive tricarboxylic acid cycles for CO2 fixation, were identified in sulfur-oxidizing autotrophic bacteria. In contrast, highly abundant genes related to the oxidation of short-chain alkanes were grouped with sulfate-reducing bacteria, suggesting an important role for short-chain alkanes in the sulfur cycle. Furthermore, sulfur-oxidizing bacteria were associated with enrichment for genes involved in the denitrification pathway, while sulfate-reducing bacteria displayed enrichment for genes responsible for hydrogen utilization. In conclusion, this study provides insights regarding major microbial metabolic activities that are driven by the sulfur cycle in low-temperature hydrothermal chimneys present on an ultraslow midocean ridge.

Deformation associated to exhumation of serpentinized mantle rocks in a fossil Ocean Continent Transition: The Totalp unit in SE Switzerland

Although the exhumation of ultramafic rocks in slow and ultraslow spreading Mid-Ocean Ridges and Ocean Continent Transitions (OCTs) has been extensively investigated, the deformation processes related to mantle exhumation are poorly constrained. In this paper we present a new geological map and a section across the exhumed serpentinized peridotites of the Totalp unit near Davos (SE Switzerland), and we propose that the Totalp unit is formed by two Alpine thrust sheets. Geological mapping indicates local exposure of a paleo-seafloor that is formed by an exhumed detachment surface and serpentinized peridotites. The top of the exhumed mantle rocks is made of ophicalcites that resulted from the carbonation of serpentine under static conditions at the seafloor. The ophicalcites preserve depositional contacts with Upper Jurassic to Lower Cretaceous pelagic sediments (Bernoulli and Weissert, 1985). These sequences did not exceed prehnite–pumpellyite metamorphic facies conditions, and locally escaped Alpine deformation.Thin mylonitic shear zones as well as foliated amphibole-bearing ultramafic rocks have been mapped. The age of these rocks and the link with the final exhumation history are yet unknown but since amphibole-bearing ultramafic rocks can be found as clasts in cataclasites related to the detachment fault, they pre-date detachment faulting. Our petrostructural study of the exhumed serpentinized rocks also reveals a deformation gradient from cataclasis to gouge formation within 150m in the footwall of the proposed paleo-detachment fault. This deformation postdates serpentinization. It involves a component of plastic deformation of serpentine in the most highly strained intervals that has suffered pronounced grain-size reduction and a polyphase cataclastic overprint.

Continental and oceanic core complexes

Core-complex formation driven by lithospheric extension is a first-order process of heat and mass transfer in the Earth. Core-complex structures have been recognized in the continents, at slow- and ultraslow-spreading mid-ocean ridges, and at continental rifted margins; in each of these settings, extension has driven the exhumation of deep crust and/or upper mantle. The style of extension and the magnitude of core-complex exhumation are determined fundamentally by rheology: (1) Coupling between brittle and ductile layers regulates fault patterns in the brittle layer; and (2) viscosity of the flowing layer is controlled dominantly by the synextension geotherm and the presence or absence of melt. The pressure-temperature-time-fluid-deformation history of core complexes, investigated via field- and modeling-based studies, reveals the magnitude, rate, and mechanisms of advection of heat and material from deep to shallow levels, as well as the consequences for the chemical and physical evolution of the lithosphere, including the role of core-complex development in crustal differentiation, global element cycles, and ore formation. In this review, we provide a survey of ∼40 yr of core-complex literature, discuss processes and questions relevant to the formation and evolution of core complexes in continental and oceanic settings, highlight the significance of core complexes for lithosphere dynamics, and propose a few possible directions for future research.

Thermodynamic constraints on mineral carbonation of serpentinized peridotite

Carbonation of serpentinite appears to be a promising process to mitigate rising concentrations of atmospheric CO 2. However, given that chemical affinities of mineral carbonation reactions in serpentinized peridotite change drastically with temperature, protolith and the activities of dissolved constituents, this process is incompletely understood. In this report numerical models are used to investigate the heterogeneous phase equilibria and the speciation of intergranular fluids during mineral carbonation of serpentinite/dunite at 100, 200, 300 and 400 °C. The results suggest that temperature-dependent activity gradients of both aqueous carbon dioxide (aCO 2,aq) and aqueous silica (aSiO 2,aq) determine the carbonation potential of serpentinite. At low to moderate temperatures there is ample thermodynamic driving force for the carbonation of serpentinite, as the gradients in aSiO 2,aq and aCO 2,aq set between talc–quartz–magnesite and serpentine–brucite equilibria are steepest. With increasing temperature these activity gradients become smaller because the stability fields of brucite, serpentine and talc expand toward higher aCO 2,aq and aSiO 2,aq while the stability field of magnesite contracts. In addition, the enhanced solubility of quartz toward higher temperatures shifts the talc–quartz–magnesite equilibrium to very high aCO 2,aq. Field observations of carbonated serpentinite reveal a rather simple sequence of arrested carbonation reactions, which is typically fossilized as spatially distinct metasomatic zones. A zone of serpentinized peridotite is followed by a zone of serpentine + carbonate, then talc + carbonate and finally quartz + carbonate. Our modeling results not only reproduce these observations, they also suggest that complete carbonation is favored at low to moderate temperatures. Since complete carbonation of serpentinite is achieved at relatively small fluid to rock ratios at temperatures ≤ 200 °C, prolonged fluid influx at low temperatures can result in decarbonation and silicification of the altered rock. In contrast, at temperatures ≥ 200 °C decarbonation and silicification reactions are unlikely, even at very high fluid to rock ratios. Since serpentinized peridotite is highly reactive in the presence of CO 2,aq, its widespread exposure at slow- and ultraslow-spreading mid-ocean ridges may represent a significant sink for CO 2 in the global carbon cycle. Our modeling results can be used to evaluate fluid–rock interactions in active or fossil mineral carbonation systems. If the mineralogy is known and the temperature of formation can be constrained independently, the aSiO 2,aq, aCO 2aq, oxygen fugacity (fO 2,g) and pH can be estimated. In turn, if the alteration temperature, aSiO 2,aq and aCO 2,aq are known, governing phase equilibria can be approximated. The latter is a promising tool to monitor the reaction progress during CO 2-sequestration into serpentinized peridotite.

Seismic Arrays on Drifting Ice Floes: Experiences from Four Deployments in the Arctic Ocean

In the last three decades our knowledge about the seismic structure and the seismicity of the world's oceans has increased tremendously through the common availability of ocean bottom seismometers for active and passive seismic studies ( e.g. , Orcutt et al. 1976; Toomey et al. 1985; Tolstoy et al. 2008). The safe recovery of the instruments is essential to access the acquired data. Comparable studies of the polar oceans do not exist, because the perennial sea-ice cover in polar regions prevents a safe recovery of ocean bottom instruments. Hence, the seismic structure and the earthquake activity of large parts of the Arctic Ocean, including the Gakkel Ridge, one of the most prominent members of ultraslow-spreading mid-ocean ridges (< 20 mm/y full rate; Dick et al. 2003), are largely unknown. Mounting conventional land seismometers on drifting sea ice is the only way to acquire seismic data in the Arctic Ocean. The difficult access to the survey area, which requires icebreaking vessels and helicopters, and the cold temperatures and a constantly moving seismometer platform, represent highly unsuitable survey conditions. However, Kristoffersen et al. (1982) made first attempts at recording regional earthquakes in the Arctic by attaching a sonobuoy array to an ice floe. Jokat et al. (2003) demonstrated that the acquisition of refraction seismic data is possible with seismic stations mounted on drifting ice floes. Okal and MacAyeal (2006) report on a deployment of seismometers on icebergs in Antarctica recording a variety of ice-induced (cryogenic) signals (MacAyeal et al. 2009). We deployed seismic arrays on drifting sea ice during four expeditions in 2001, 2007, 2008, and 2009, recording local and regional earthquakes along Gakkel Ridge. In addition, the records contained many sorts of cryogenic signals (“icequakes”) stemming possibly from collisions between ice floes, crack formation, or disintegration of ice floes …