The EMSO-Azores deep sea observatory: 8 years of operation

The EMSO-Azores deep sea observatory is a component of the EMSO ERIC. It focuses on two main questions: What are the feedbacks between volcanism, deformation, seismicity, and hydrothermalism at a slow spreading mid-ocean ridge and how does the hydrothermal ecosystem couple with these sub-seabed processes? The infrastructure comprises 2 sea monitoring nodes, autonomous instruments and a set of site studies experiments. It has been deployed in 2010 in the Lucky Strike vent field and acquires multidisciplinary data since then.Peer Reviewe

Abyssal hill characterization at the ultraslow spreading Southwest Indian Ridge

International audienceThe morphology of the flanks of the Southwest Indian Ridge holds a record of seafloor formationand abyssal hill generation at an ultraslow spreading rate. Statistical analysis of compiled bathymetry andgravity data from the flanks of the Southwest Indian Ridge from 54°E to 67°E provides estimates of abyssalhill morphologic character and inferred crustal thickness. The extent of the compiled data encompasses aspreading rate change from slow to ultraslow at 24 Ma, a significant inferred variation in sub-axis mantletemperature, and a patchwork of volcanic and non-volcanic seafloor, making the Southwest Indian Ridge anideal and unique location to characterize abyssal hills generated by ultraslow spreading and to examine theeffect of dramatic spreading rate change on seafloor morphology. Root mean square abyssal hill height inultraslow spreading seafloor ranges from 280 m to 320 m and is on average 80 m greater than foundfor slow-spreading seafloor. Ultraslow spreading abyssal hill width ranges from 4 km to 12 km, averaging8 km. Abyssal hill height and width increases west-to-east in both slow and ultraslow spreadingseafloor, corresponding to decreasing inferred mantle temperature. Abyssal hills persist in non-volcanic seafloorand extend continuously from volcanic to non-volcanic terrains. We attribute the increase of abyssalhill height and width to strengthening of the mantle portion of the lithosphere as the result of cooler subaxialmantle temperature and conclude that abyssal hill height is primarily controlled by the strength ofthe mantle component of the lithosphere rather than spreading rate

Rare gas systematics on Lucky Strike basalts (37°N, North Atlantic): Evidence for efficient homogenization in a long-lived magma chamber system?

International audienceWe present rare gas data in fresh glasses from the Lucky Strike segment located on the Mid Atlantic Ridge (∼37.3°N), close to the Azores plateau. We analyzed the helium and neon isotopes in 28 samples by melting as well as He‐Ne‐Ar‐Kr‐Xe isotopes in 9 samples by crushing. Samples were collected during the Graviluck06, MOMAR08, and Bathyluck09 cruises over a ridge length of ∼13 km (mean sample spacing of ∼500 m), and at depths ranging from 1550 m to 2174 m. The helium isotopic ratio varies between 84,410 and 88,235 (R/Ra between 8.19 and 8.56). The samples having the “most” primitive helium isotopic ratio are the enriched samples (e.g. high K2O/TiO2) although the difference to the depleted samples is small. It appears that all of our samples derive from the same and well‐homogenized magma chamber. Neon isotopes clearly show the influence of the Azores hotspot, which is not seen with helium because of lower 3He/22Ne in the plume source compared to the MORB source

Extremely thin crust in the Indian Ocean possibly resulting from Plume–Ridge Interaction

International audienceThe thickness of the crust created at ocean spreading centres depends on the spreading rate and melt production in the mantle. It is ~5–8 km for a crust formed at slow and fast spreading centres and 2–4 km at ultra-slow spreading centres away from hotspots and mantle anomalies. The crust is generally thin at the fracture zones and thick beneath hotspots and large igneous provinces. Here we present results for the crust generated at the fast Wharton spreading centre 55–58 Ma ago using seismic reflection and refraction data. We find that the crust over a 200 km segment of the Wharton Basin is only 3.5–4.5 km thick, the thinnest crust ever observed in a fast spreading environment. A thin crust could be produced by the presence of depleted and/or cold mantle. Numerical simulations and recent laboratory experiments studying the impact of a hot plume under a lithosphere show that a curtain of weak cold downwellings of depleted mantle material is likely to develop around the edges of the hot plume pond. Because of a strongly temperature-dependent viscosity of lithospheric material, the hotter, therefore less viscous, bottom of the lithosphere can be mobilized by an impinging plume. If sampled by a spreading centre, the locally cold and depleted mantle should result in low production of melt. We suggest that the observed thin crust in the Wharton Basin is likely to have been formed by the interaction between the Kerguelen mantle plume and the Wharton spreading centre ~55 Ma ago

Hydrothermal activity along the slow-spreading Lucky Strike ridge segment (Mid-Atlantic Ridge) : distribution, heatflux, and geological controls

Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 431 (2015): 1730185, doi:10.1016/j.epsl.2015.09.025.We have reviewed available visual information from the seafloor, and recently acquired microbathymetry for several traverses across the Lucky Strike segment, to evaluate the distribution of hydrothermal activity. We have identified a new on-axis site with diffuse flow, Ewan, and anactive vent structure ~1.2 km from the axis, Capelinhos. These sites are minor relative to the Main field, and our total heatflux estimate for all active sites (200-1200 MW) is only slightly higher than previously published estimates. We also identify fossil sites W of the main Lucky Strike field. A circular feature ~200 m in diameter located on the flanks of a rifted off-axis central volcano, is likely a large and inactive hydrothermal edifice, named Grunnus. We find no indicator of focused hydrothermal activity elsewhere along the segment, suggesting that the enhanced melt supply and the associated melt lenses, required to form central volcanoes, also sustain hydrothermal circulation to form and maintain large and long-lived hydrothermal fields. Hydrothermal discharge to the seafloor occurs along fault traces, suggesting focusing of hydrothermal circulation in the shallow crust along permeable fault zones.This work has been partly financed by ANR (France) Mothseim Project NT05-3 42213 toJE, and by EU-RTN-MOMARNET to MC. The French Ministry of Research financed ship, ROV and AUV time (Graviluck’06, MOMAR’08, Bathyluck’09, MOMARSAT cruises in 2010-2015

Quantifying diffuse and discrete venting at the Tour Eiffel vent site, Lucky Strike hydrothermal field

Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 13 (2012): Q04008, doi:10.1029/2011GC003991.The relative heat carried by diffuse versus discrete venting of hydrothermal fluids at mid-ocean ridges is poorly constrained and likely varies among vent sites. Estimates of the proportion of heat carried by diffuse flow range from 0% to 100% of the total axial heat flux. Here, we present an approach that integrates imagery, video, and temperature measurements to accurately estimate this partitioning at a single vent site, Tour Eiffel in the Lucky Strike hydrothermal field along the Mid-Atlantic Ridge. Fluid temperatures, photographic mosaics of the vent site, and video sequences of fluid flow were acquired during the Bathyluck'09 cruise (Fall, 2009) and the Momarsat'10 cruise (Summer, 2010) to the Lucky Strike hydrothermal field by the ROV Victor6000 aboard the French research vessel the “Pourquoi Pas”? (IFREMER, France). We use two optical methods to calculate the velocities of imaged hydrothermal fluids: (1) for diffuse venting, Diffuse Flow Velocimetry tracks the displacement of refractive index anomalies through time, and (2) for discrete jets, Particle Image Velocimetry tracks eddies by cross-correlation of pixel intensities between subsequent images. To circumvent video blurring associated with rapid velocities at vent orifices, exit velocities at discrete vents are calculated from the best fit of the observed velocity field to a model of a steady state turbulent plume where we vary the model vent radius and fluid exit velocity. Our results yield vertical velocities of diffuse effluent between 0.9 cm s−1 and 11.1 cm s−1 for fluid temperatures between 3°C and 33.5°C above that of ambient seawater, and exit velocities of discrete jets between 22 cm s−1 and 119 cm s−1 for fluid temperatures between 200°C and 301°C above ambient seawater. Using the calculated fluid velocities, temperature measurements, and photo mosaics of the actively venting areas, we calculate a heat flux due to diffuse venting from thin fractures of 3.15 ± 2.22 MW, discrete venting of 1.07 ± 0.66 MW, and, by incorporating previous estimates of diffuse heat flux density from Tour Eiffel, diffuse flux from the main sulfide mound of ∼15.6 MW. We estimate that the total integrated heat flux from the Tour Eiffel site is 19.82 ± 2.88 MW and that the ratio of diffuse to discrete heat flux is ∼18. We discuss the implication of these results for the characterization of different vent sites within Lucky Strike and in the context of a compilation of all available measurements of the ratio of diffuse to discrete heat flux.E. Mittelstaedt was supported by the International Research Fellowship Program of the U.S. National Science Foundation (OISE-0757920). Funding for the 2006, 2008, 2009, and 2010 cruises was provided by CNRS/ IFREMER through the MoMAR program (France), by ANR (France), the Mothseim Project NT05–3 42213 to J. Escartín and by grant CTM2010–15216/MAR from the Spanish Ministry of Science to R. Garcia and J. Escartín. T. Barreyre was supported by University Paris Diderot (Paris 7 – France) and Institut de Physique du Globe de Paris (IPGP, France).2012-10-1

Clinopyroxene microtextures reveal incompletely extracted melts in abyssal peridotites.

ABSTRACT Textural evidence is interpreted to suggest that in regions where upwelling rates of the mantle are slow to very slow, a small amount (ϳ2%) of melt was present when plagioclasefree abyssal peridotites entered the conductive regime at the base of the oceanic lithosphere. Upon crystallization, this melt appears to have been undersaturated in orthopyroxene, but precipitated clinopyroxene, Al-rich and Ti-poor spinel, and sulfides. Furthermore, the primary clinopyroxene grains have rare earth element patterns typical of residues of fractional melting, suggesting that the interstitial liquids were incremental partial melts rather than having mid-oceanic-ridge basalt compositions

Monitoring ecological dynamics on complex hydrothermal structures: A novel photogrammetry approach reveals fine‐scale variability of vent assemblages

We set out to characterize the fine-scale processes acting on interannual dynamics of deep-sea vent fauna by using a novel approach involving a 5-yr time series of 3D photogrammetry models acquired at the Eiffel Tower sulfide edifice (Lucky Strike vent field, Mid-Atlantic Ridge). Consistently, with the overall stability of the vent edifice, total mussel cover did not undergo drastic changes, suggesting that they have been at a climax stage for at least 25 yr based on previous data. Successional patterns showed consistency over time, illustrating the dynamic equilibrium of the ecological system. In contrast, microbial mats significantly declined, possibly due to magmatic events. The remaining environmental variability consisted of decimeter-scale displacement of vent outflows, resulting from their opening or closure or from the progressive accretion of sulfide material. As a result, vent mussels showed submeter variability in the immediate vicinity of vent exits, possibly by repositioning in response to that fine-scale regime of change. As former studies were not able to quantify processes at submeter scales in complex settings, this pioneering work demonstrates the potential of 3D photogrammetry models for conducting long-term monitoring in the deep sea. We observed that the ability of mussels to displace may enable them to cope with changing local conditions in a stable system. However, the long-term stability of mussel assemblages questions their capacity to withstand large-scale disturbances and may imply a low resilience of these “climax” communities. This suggests that they may be particularly vulnerable to the negative effects of mining activities in hydrothermal ecosystems

The EGIM, EMSO generic instrument module, step towards standardization

Peer ReviewedPostprint (published version

Hydrothermally-induced melt lens cooling and segmentation along the axis of fast- and intermediate-spreading centers

Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 38 (2011): L14307, doi:10.1029/2011GL047798.The heat output and thermal regime of fast and intermediate spreading centers are strongly controlled by boundary layer processes between the hydrothermal system and the underlying crustal magma chamber (AMC), which remain to be fully understood. Here, we model the interactions between a shallow two-dimensional cellular hydrothermal system at temperatures <700°C, and a deeper AMC at temperatures up to 1200°C. We show that hydrothermal cooling can freeze the AMC in years to decades, unless melt injections occur on commensurate timescales. Moreover, the differential cooling between upflow and downflow zones can segment the AMC into mush and melt regions that alternate on sub-kilometric length scales. These predictions are consistent with along-axis variations in AMC roof depth observed in ophiolites and oceanic settings. In this respect, fine-scale geophysical investigations of the structure of AMCs may help constrain hydrothermal recharge locations associated with active hydrothermal sites