Mantle Bouguer Anomaly Along an Ultra Slow-Spreading Ridge: Implications for Accretionary Processes and Comparison with Results from Central Mid-Atlantic Ridge

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Melt supply variations to a magma-poor ultra-slow spreading ridge (Southwest Indian Ridge 61° to 69°E)

International audienceThe Southwest Indian Ridge (SWIR) to the east of the Melville Fracture zone receives anomalously low volumes of melt on average. However, a small number of ridge segments appear to receive more melt than this regional average. We use off-axis bathymetry, gravity, and magnetic data to show that this melt distribution pattern, quite distinct from what is observed at the Mid-Atlantic Ridge (MAR), has been a characteristic of the easternmost SWIR for at least the past 10 myr. We also show that segments of the easternmost SWIR are substantially shorter lived than most segments of the MAR. Melt distribution in our SWIR study area is therefore both more focused and more variable in time than at the MAR. We tentatively propose a mechanism by which strong and transient melt-focusing events could be initiated by a localized increase in the volume of melt supplied by the melting mantle to the base of the axial lithosphere, causing thermal thinning of this lithosphere and along-axis melt migration. These two processes may combine to effectively focus larger volumes of melt toward the center of future thick crust segments. Rapid melt extraction by dikes that feed large volcanic constructions on the seafloor, followed by tectonic disruption of these volcanic constructions by deep-reaching faults, may then cool the axial lithosphere back to its original thickness and end the melt-focusing events. The easternmost SWIR is also characterized by a common departure from isostatic compensation of seafloor topography and by a pronounced asymmetry of crustal thickness and seafloor relief between the two ridge flanks. At the faster spreading MAR, similar characteristics are found near the ends of ridge segments. We propose that spreading at the ultra-slow SWIR during periods when the melt supply is low (i.e., most of the time for the easternmost SWIR) is dominated by large offset asymmetric normal faulting, with significant flexural uplift of the footwalls. Faults face either north or south, and changes in fault polarity are frequent, both along axis and along flow lines (i.e., with time). Producing large faults and maintaining high uncompensated reliefs require the axial lithosphere to be thick, a predictable characteristic for this ultra-slow ridge, which has an anomalously low regionally averaged melt supply

Tectonic interpretation of the Andrew Bain transform fault: Southwest Indian Ocean

International audience[1] Between 25°E and 35°E, a suite of four transform faults, Du Toit, Andrew Bain, Marion, and Prince Edward, offsets the Southwest Indian Ridge (SWIR) left laterally 1230 km. The Andrew Bain, the largest, has a length of 750 km and a maximum transform domain width of 120 km. We show that, currently, the Nubia/Somalia plate boundary intersects the SWIR east of the Prince Edward, placing the Andrew Bain on the Nubia/Antarctica plate boundary. However, the overall trend of its transform domain lies 10° clockwise of the predicted direction of motion for this boundary. We use four transform-parallel multibeam and magnetic anomaly profiles, together with relocated earthquakes and focal mechanism solutions, to characterize the morphology and tectonics of the Andrew Bain. Starting at the southwestern ridge-transform intersection, the relocated epicenters follow a 450-km-long, 20-km-wide, 6-km-deep western valley. They cross the transform domain within a series of deep overlapping basins bounded by steep inward dipping arcuate scarps. Eight strike-slip and three dip-slip focal mechanism solutions lie within these basins. The earthquakes can be traced to the northeastern ridge-transform intersection via a straight, 100-km-long, 10-km-wide, 4.5-km-deep eastern valley. A striking set of seismically inactive NE-SW trending en echelon ridges and valleys, lying to the south of the overlapping basins, dominates the eastern central section of the transform domain. We interpret the deep overlapping basins as two pull-apart features connected by a strike-slip basin that have created a relay zone similar to those observed on continental transforms. This transform relay zone connects three closely spaced overlapping transform faults in the southwest to a single transform fault in the northeast. The existence of the transform relay zone accounts for the difference between the observed and predicted trend of the Andrew Bain transform domain. We speculate that between 20 and 3.2 Ma, an oblique accretionary zone jumping successively northward created the en echelon ridges and valleys in the eastern central portion of the domain. The style of accretion changed to that of a transform relay zone, during a final northward jump, at 3.2 Ma

Magmato-Tectonic Cyclicity at the Ultra-Slow Spreading Southwest Indian Ridge: Evidence from Variations of Axial Volcanic Ridge Morphology and Abyssal Hills Pattern.

article initialement paru à Geochem. Geophys. Geosyst. en 2003 et traduit en chinoisInternational audienc

Focused volcanism and growth of a slow spreading segment (Mid-Atlantic Ridge, 35°N)

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Detection and phylogenetic identification of labeled prokaryotic cells on mineral surfaces using Scanning X-ray Microscopy

International audiencehe involvement of intraterrestrial microbes in geochemical cycles is now well recognized. However, owing to the small number of appropriate methods for probing these ecosystems, the exploration of their metabolic diversity, energy sources, and biogeochemical transformations remains limited. Here we demonstrate the ability of scanning X-ray microscopy using synchrotron radiation to localize and characterize the phylogenetic affiliation of individual prokaryotic cells on various mineral surfaces (e.g. carbonates, basaltic glass) when combined with a newly developed protocol based on fluorescence in situ hybridization coupled to ultra-small immunogold. The possibility to associate simultaneously the phylogenetic identification of microorganisms with the chemical and structural characteristics of associated mineral phases (i.e. inorganic substrate and biomineralizations), offers great interest for assessing the geochemical impact of subsurface microbial communities and unraveling microbe-mineral interactions in the deep biosphere

Structural Iron (II) of Basaltic Glass as an Energy Source for Zetaproteobacteria in an Abyssal Plain Environment, Off the Mid Atlantic Ridge

International audienceTo explore the capability of basaltic glass to support the growth of chemosynthetic microorganisms, complementary in situ and in vitro colonization experiments were performed. Microbial colonizers containing synthetic tholeitic basaltic glasses, either enriched in reduced or oxidized iron, were deployed off-axis from the Mid Atlantic Ridge on surface sediments of the abyssal plain (35 • N; 29 • W). In situ microbial colonization was assessed by sequencing of the 16S rRNA gene and basaltic glass alteration was characterized using Scanning Electron Microscopy, micro-X-ray Absorption Near Edge Structure at the Fe-K-edge and Raman microspectroscopy. The colonized surface of the reduced basaltic glass was covered by a rind of alteration made of iron-oxides trapped in a palagonite-like structure with thicknesses up to 150 μm. The relative abundance of the associated microbial community was dominated (39% of all reads) by a single operational taxonomic unit (OTU) that shared 92% identity with the iron-oxidizer Mariprofundus ferrooxydans PV-1. Conversely, the oxidized basaltic glass showed the absence of iron-oxides enriched surface deposits and correspondingly there was a lack of known iron-oxidizing bacteria in the inventoried diversity. In vitro, a similar reduced basaltic glass was incubated in artificial seawater with a pure culture of the iron-oxidizing M. ferrooxydans DIS-1 for 2 weeks, without any additional nutrients or minerals. Confocal Laser Scanning Microscopy revealed that the glass surface was covered by twisted stalks characteristic of this iron-oxidizing Zetaproteobacteria. This result supported findings of the in situ experiments indicating that the Fe(II) present in the basalt was the energy source for the growth of representatives of Zetaproteobacteria in both the abyssal plain and the in vitro experiment. In accordance, the surface alteration rind observed on the reduced basaltic glass incubated in situ could at least partly result from their activity

Ridge segmentation and the magnetic structure of the Southwest Indian Ridge (at 50°30'E, 55°30'E and 66°20'E): Implications for magmatic processes at ultraslow-spreading centers.

which is observed along the traces of the largest amagmatic discontinuities. By contrast, in thicker crust areas, the upper mantle rocks are shielded from the alteration and the serpentinization process may be delayed resulting, as on the Mid-Atlantic Ridge, in slightly more positive magnetization values along the traces of axial discontinuities, regardless of polarity

Biogeochemical insights into microbe–mineral–fluid interactions in hydrothermal chimneys using enrichment culture

International audienceActive hydrothermal chimneys host diverse microbial communities exhibiting various metabolisms including those involved in various biogeochemical cycles. To investigate microbe-mineral-fluid interactions in hydrothermal chimney and the driver of microbial diversity, a cultural approach using a gas-lift bioreactor was chosen. An enrichment culture was performed using crushed active chimney sample as inoculum and diluted hydrothermal fluid from the same vent as culture medium. Daily sampling provided time-series access to active microbial diversity and medium composition. Active archaeal and bacterial communities consisted mainly of sulfur, sulfate and iron reducers and hydrogen oxidizers with the detection of Thermococcus, Archaeoglobus, Geoglobus, Sulfurimonas and Thermotoga sequences. The simultaneous presence of active Geoglobus sp. and Archaeoglobus sp. argues against competition for available carbon sources and electron donors between sulfate and iron reducers at high temperature. This approach allowed the cultivation of microbial populations that were under-represented in the initial environmental sample. The microbial communities are heterogeneously distributed within the gas-lift bioreactor; it is unlikely that bulk mineralogy or fluid chemistry is the drivers of microbial community structure. Instead, we propose that micro-environmental niche characteristics, created by the interaction between the mineral grains and the fluid chemistry, are the main drivers of microbial diversity in natural systems