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Sample records for active mid-ocean ridge

  1. Hydrothermal activity at the Arctic mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Pedersen, Rolf B.; Thorseth, Ingunn H.; Nygård, Tor Eivind; Lilley, Marvin D.; Kelley, Deborah S.

    Over the last 10 years, hydrothermal activity has been shown to be abundant at the ultraslow spreading Arctic Mid-Ocean Ridges (AMOR). Approximately 20 active and extinct vent sites have been located either at the seafloor, as seawater anomalies, or by dredge sampling hydrothermal deposits. Decreasing spreading rates and decreasing influence of the Icelandic hot spot toward the north along the AMOR result in a north-south change from a shallow and magmatically robust to a deep and magmatically starved ridge system. This contrast gives rise to large variability in the ridge geology and in the nature of the associated hydrothermal systems. The known vent sites at the southern part of the ridge system are either low-temperature or white smoker fields. At the deep, northern parts of the ridge system, a large black smoker field has been located, and seawater anomalies and sulfide deposits suggest that black smoker-type venting is common. Several of these fields may be peridotite-hosted. The hydrothermal activity at parts of the AMOR exceeds by a factor of 2 to 3 what would be expected by extrapolating from observations on faster spreading ridges. Higher fracture/fault area relative to the magma volume extracted seems a likely explanation for this. Many of the vent fields at the AMOR are associated with axial volcanic ridges. Strong focusing of magma toward these ridges, deep rifting of the ridges, and subsequent formation of long-lived detachment faults that are rooted below the ridges may be the major geodynamic mechanisms causing the unexpectedly high hydrothermal activity.

  2. The Mid-Ocean Ridge.

    ERIC Educational Resources Information Center

    Macdonald, Kenneth C.; Fox, Paul J.

    1990-01-01

    Described are concepts involved with the formation and actions of the Mid-Ocean Ridge. Sea-floor spreading, the magma supply model, discontinuities, off-axis structures, overlaps and deviation, and aquatic life are discussed. (CW)

  3. The Mid-Ocean Ridge

    SciTech Connect

    Macdonald, K.C. ); Fox, P.J. )

    1990-06-01

    The Mid-Ocean Ridge girdles the earth like the seam of a baseball. For more than 75,000 kilometers, this submerged range of razorback mountains--many higher than the greatest peaks on land--marks the restless boundary between continental plates. An analysis of this huge structure reveals a fascinating picture of how it is created by magma welling up as the plates pull apart. The paper discusses sea-floor spreading, the magma supply model, types of discontinuities, off-axis structures, small overlaps and DEVALs (slight DEViations in Axial Linearity), and aquatic life.

  4. Rapid cooling rates at an active mid-ocean ridge from zircon thermochronology

    USGS Publications Warehouse

    Schmitt, Axel K.; Perfit, Michael R.; Rubin, Kenneth H.; Stockli, Daniel F.; Smith, Matthew C.; Cotsonika, Laurie A.; Zellmer, Georg F.; Ridley, W. Ian

    2011-01-01

    Oceanic spreading ridges are Earth's most productive crust generating environment, but mechanisms and rates of crustal accretion and heat loss are debated. Existing observations on cooling rates are ambiguous regarding the prevalence of conductive vs. convective cooling of lower oceanic crust. Here, we report the discovery and dating of zircon in mid-ocean ridge dacite lavas that constrain magmatic differentiation and cooling rates at an active spreading center. Dacitic lavas erupted on the southern Cleft segment of the Juan de Fuca ridge, an intermediate-rate spreading center, near the intersection with the Blanco transform fault. Their U–Th zircon crystallization ages (29.3-4.6+4.8 ka; 1δ standard error s.e.) overlap with the (U–Th)/He zircon eruption age (32.7 ± 1.6 ka) within uncertainty. Based on similar 238U-230Th disequilibria between southern Cleft dacite glass separates and young mid-ocean ridge basalt (MORB) erupted nearby, differentiation must have occurred rapidly, within ~ 10–20 ka at most. Ti-in-zircon thermometry indicates crystallization at 850–900 °C and pressures > 70–150 MPa are calculated from H2O solubility models. These time-temperature constraints translate into a magma cooling rate of ~ 2 × 10-2 °C/a. This rate is at least one order-of-magnitude faster than those calculated for zircon-bearing plutonic rocks from slow spreading ridges. Such short intervals for differentiation and cooling can only be resolved through uranium-series (238U–230Th) decay in young lavas, and are best explained by dissipating heat convectively at high crustal permeability.

  5. Rapid cooling rates at an active mid-ocean ridge from zircon thermochronology

    NASA Astrophysics Data System (ADS)

    Schmitt, Axel K.; Perfit, Michael R.; Rubin, Kenneth H.; Stockli, Daniel F.; Smith, Matthew C.; Cotsonika, Laurie A.; Zellmer, Georg F.; Ridley, W. Ian; Lovera, Oscar M.

    2011-02-01

    Oceanic spreading ridges are Earth's most productive crust generating environment, but mechanisms and rates of crustal accretion and heat loss are debated. Existing observations on cooling rates are ambiguous regarding the prevalence of conductive vs. convective cooling of lower oceanic crust. Here, we report the discovery and dating of zircon in mid-ocean ridge dacite lavas that constrain magmatic differentiation and cooling rates at an active spreading center. Dacitic lavas erupted on the southern Cleft segment of the Juan de Fuca ridge, an intermediate-rate spreading center, near the intersection with the Blanco transform fault. Their U-Th zircon crystallization ages (29.3 - 4.6 + 4.8 ka; 1σ standard error s.e.) overlap with the (U-Th)/He zircon eruption age (32.7 ± 1.6 ka) within uncertainty. Based on similar 238U- 230Th disequilibria between southern Cleft dacite glass separates and young mid-ocean ridge basalt (MORB) erupted nearby, differentiation must have occurred rapidly, within ~ 10-20 ka at most. Ti-in-zircon thermometry indicates crystallization at 850-900 °C and pressures > 70-150 MPa are calculated from H 2O solubility models. These time-temperature constraints translate into a magma cooling rate of ~ 2 × 10 - 2 °C/a. This rate is at least one order-of-magnitude faster than those calculated for zircon-bearing plutonic rocks from slow spreading ridges. Such short intervals for differentiation and cooling can only be resolved through uranium-series ( 238U- 230Th) decay in young lavas, and are best explained by dissipating heat convectively at high crustal permeability.

  6. Magnetic Measurements of Bacterial Activity in Mid Oceanic Ridge Basalt Samples.

    NASA Astrophysics Data System (ADS)

    Carlut, J.; Horen, H.; Brunet, F.; Janots, D.; Pozzi, J.

    2004-12-01

    Magnetic properties of the young oceanic crust gives insights on the 3D structure at ridges axis and on variations of the Earth's magnetic field. Accurate interpretations depend on our understanding of the magnetic stability of titanomagnetite crystals carrying the magnetic signal. The mean remanent intensity of young ocean basalts shows significant variations with age and it has been suggested that rapid chemical alteration of titanomagnetite could be responsible. Ubiquitous presence of bacteria in deep marine environment and their ability to interact with iron oxides suggest a possible important impact of these life forms on the stability of titanomagnetites. In vivo experiments have been conducted on more than 15 fresh lava macro samples with a batch of selected sulfate reducing bacteria of the desulfovibrio genus, relevant to the deep biosphere. Remanent magnetization of samples was measured at regular time interval, in addition to more complete magnetic and chemical characterization at beginning and end of experiments. After a few months results show a significant contribution of bacterial activity on magnetic properties of samples, in particular a well defined drop in remanence. Our results suggest a strong impact of bacteria on the alteration rate of iron oxides carrying the magnetic signal within the oceanic crust.

  7. Dynamical instability produces transform faults at mid-ocean ridges.

    PubMed

    Gerya, Taras

    2010-08-27

    Transform faults at mid-ocean ridges--one of the most striking, yet enigmatic features of terrestrial plate tectonics--are considered to be the inherited product of preexisting fault structures. Ridge offsets along these faults therefore should remain constant with time. Here, numerical models suggest that transform faults are actively developing and result from dynamical instability of constructive plate boundaries, irrespective of previous structure. Boundary instability from asymmetric plate growth can spontaneously start in alternate directions along successive ridge sections; the resultant curved ridges become transform faults within a few million years. Fracture-related rheological weakening stabilizes ridge-parallel detachment faults. Offsets along the transform faults change continuously with time by asymmetric plate growth and discontinuously by ridge jumps. PMID:20798313

  8. Dynamical instability produces transform faults at mid-ocean ridges.

    PubMed

    Gerya, Taras

    2010-08-27

    Transform faults at mid-ocean ridges--one of the most striking, yet enigmatic features of terrestrial plate tectonics--are considered to be the inherited product of preexisting fault structures. Ridge offsets along these faults therefore should remain constant with time. Here, numerical models suggest that transform faults are actively developing and result from dynamical instability of constructive plate boundaries, irrespective of previous structure. Boundary instability from asymmetric plate growth can spontaneously start in alternate directions along successive ridge sections; the resultant curved ridges become transform faults within a few million years. Fracture-related rheological weakening stabilizes ridge-parallel detachment faults. Offsets along the transform faults change continuously with time by asymmetric plate growth and discontinuously by ridge jumps.

  9. Active spreading processes at ultraslow mid-ocean ridges: The 1999-2001 seismo-volcanic episode at 85°E Gakkel ridge, Arctic Ocean

    NASA Astrophysics Data System (ADS)

    Schlindwein, Vera; Riedel, Carsten; Korger, Edith; Läderach, Christine

    2010-05-01

    The rate of magma and crustal production at mid-ocean ridges is thought to decrease with decreasing spreading rate. At ultraslow spreading rates below 10-20 mm/y full rate, heat loss by conduction greatly reduces melt production with less melt produced at increasingly greater depths. Gakkel Ridge, the actively spreading mid-ocean ridge in the Arctic Ocean, opens at rates of 14 mm/y in the west decreasing to less than 6 mm/y at its eastern termination and demonstrates that magma production is not only a function of spreading rate. Whereas amagmatic spreading takes place at rates of about 12-10 mm/y, focussed melt production occurs at even lower spreading rates in long-lived discrete volcanic centres. One such centre is the 85°E volcanic complex at eastern Gakkel ridge where in 1999 a teleseismically recorded earthquake swarm consisting of more than 250 earthquakes over 9 months signalled the onset of an active spreading episode. The earthquake swarm is believed to be associated with volcanic activity although no concurrent lava effusion was found. We analysed the teleseismic earthquake swarm together with visual observation and microseismic data recorded at this site in 2001 and 2007 and noted the following characteristics which may be indicative for volcanic spreading events at the still poorly explored ultraslow spreading ridges: - unusual duration: The 1999 earthquake swarm lasted over 9 months rather than a few weeks as observed on faster spreading ridges. In addition, in 2001 seismoacoustic sounds which we interpret as gas discharge in Strombolian eruptions and a giant event plume maintained over more than one year indicate waxing and waning volcanic activity since 1999. - unusual strength: The earthquake swarm was detected at teleseismic distances of more than 1000 km and included 11 events with a magnitude >5. No other confirmed mid-ocean ridge eruption released a comparable seismic moment. Rather than focussing in a narrow area or showing pronounced

  10. The timescales of magma evolution at mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Brandl, Philipp A.; Regelous, Marcel; Beier, Christoph; O'Neill, Hugh St. C.; Nebel, Oliver; Haase, Karsten M.

    2016-01-01

    Oceanic crust is continuously created at mid-ocean ridges by decompression melting of the upper mantle as it upwells due to plate separation. Decades of research on active spreading ridges have led to a growing understanding of the complex magmatic, tectonic and hydrothermal processes linked to the formation of new oceanic igneous crust. However, less is known about the timescales of magmatic processes at mid-ocean ridges, including melting in and melt extraction from the mantle, fractional crystallisation, crustal assimilation and/or magma mixing. In this paper, we review the timescales of magmatic processes by integrating radiometric dating, chemical and petrological observations of mid-ocean ridge basalts (MORBs) and geophysical models. These different lines of evidence suggest that melt extraction and migration, and crystallisation and mixing processes occur over timescales of 1 to 10,000 a. High-resolution geochemical stratigraphic profiles of the oceanic crust using drill-core samples further show that at fast-spreading ridges, adjacent flow units may differ in age by only a few 100 a. We use existing chemical data and new major- and trace-element analyses of fresh MORB glasses from drill-cores in ancient Atlantic and Pacific crust, together with model stratigraphic ages to investigate how lava chemistry changes over 10 to 100 ka periods, the timescale of crustal accretion at spreading ridges which is recorded in the basalt stratigraphy in drilled sections through the oceanic crust. We show that drilled MORBs have compositions that are similar to those of young MORB glasses dredged from active spreading ridges (lavas that will eventually be preserved in the lowermost part of the extrusive section covered by younger flows), showing that the dredged samples are indeed representative of the bulk oceanic crust. Model stratigraphic ages calculated for individual flows in boreholes, together with the geochemical stratigraphy of the drilled sections, show that at

  11. Migration of mid-ocean-ridge volcanic segments

    USGS Publications Warehouse

    Schouten, Hans; Dick, H.J.B.; Klitgord, Kim D.

    1987-01-01

    The propagation of small-offset volcanic spreading-centre segments along mid-ocean ridge crests may reflect absolute motion of the plate boundary relative to the underlying mesospheric frame. Such a relationship could be caused by a purely vertical flow of the mantle under spreading centres and would have value in constraining past plate motions from non-transform trends generated during along-ridge propagation and in linking the major-element variability of oceanic crust and upper mantle to the bulk composition and temperatures of mantle ascending under mid-ocean ridges. ?? 1987 Nature Publishing Group.

  12. Mantle anisotropy beneath the Earth's mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Nowacki, Andy; Kendall, J.-Michael; Wookey, James

    2012-02-01

    Observations of seismic anisotropy at oceanic spreading centres offer insights into mid-ocean ridge processes and the formation of new plates. Here, remote observations of seismic anisotropy beneath mid-ocean ridges are made using measurements of source-side shear wave splitting. Over 100 high-quality measurements are made using earthquakes that occur near mid-ocean ridges and transform faults, but are observed at teleseismic distances. In general, for off-axis ridge events, the polarisation of fast shear waves, ϕ″, is approximately parallel to the spreading direction. Nearer the ridge (<~50 km), ϕ″ becomes more scattered and is often ridge-parallel. Delay times, δt, tend to increase from < 1 s near the ridge axis to ˜ 3 s further away. Slow-spreading regions (Gakkel and Southwest Indian Ridges) show smaller amounts of splitting than faster spreading centres. At transform zones, the pattern is more complex. Coverage beneath the East Pacific Rise is especially good, and we observe a systematic increase in delay times in S wave splitting measurements compared to previous SKS splitting observations made at ocean-bottom seismometers. One compatible explanation is the presence of horizontally-aligned, connected layers of melt at depth; this is also compatible with other observations of the 'LAB' discontinuity and surface-wave derived measurements of radial anisotropy.

  13. The structure of mid-ocean ridges

    NASA Technical Reports Server (NTRS)

    Solomon, Sean C.; Toomey, Douglas R.

    1992-01-01

    Recent research results on the structure of midocean ridges are reviewed. The new view of ridge-axis crustal structure obtained from high-resolution seismology is reviewed, emphasizing the variation of that structure with spreading rate and along-axis at a given spreading rate. Recent results on upper mantle structure beneath ridges are examined, including variations with seafloor age, indications from anisotropy for directions of mantle flow, and long-wavelength along-axis variations in structure and their implications for lateral heterogeneity in mantle temperature and composition.

  14. The Galapagos Spreading Center. Galapagos Rifts Expedition--Grades 9-12. Mid-Ocean Ridges.

    ERIC Educational Resources Information Center

    National Oceanic and Atmospheric Administration (DOC), Rockville, MD.

    This activity introduces students to the basic concept of seafloor spreading, the processes involved in creating new seafloor at a mid- ocean ridge, the Galapagos Spreading Center system, and the different types of plate motion associated with ridge segments and transform faults. The activity provides learning objectives, a list of needed…

  15. Model of Methane Hydrate Formation in Mid-ocean Ridges

    NASA Astrophysics Data System (ADS)

    Dmitrievsky, A. N.; Balanyuk, I. E.; Sorokhtin, O. G.; Matveenkov, V. V.; Dongaryan, L. Sh.

    2003-04-01

    MODEL OF METHANE HYDRATE FORMATION IN MID-OCEAN RIDGES A.N. Dmitrievsky, I.E. Balanyuk, O.G.Sorokhtin, V.V. Matveenkov, and L.Sh. Dongaryan P.P.Shirshov Institute of Oceanology Russian Academy of Sciences Moscow, Russia, balanyuk@sio.rssi.ru One among the most perspective direction in studying the ocean floor is the research of hydrothermal fields within the most active zones — rift valleys, where the processes of spreading of the ocean floor, uplift of the deep matter to the surface of the ocean floor, and creation of the new oceanic crust occur. Volcanic activity in these zones is accompanied with the formation of the hydrothermal system executing separation, transfer, and precipitation of a series of chemical elements. It is known that ore deposits with high concentration of iron, manganese, copper, nickel, cobalt are formed as a result of hydrothermal activity. It is much less known that hydrothermal activity in these zones has important but not so evident result — the formation of hydrocarbons in the form of methane hydrates. We propose the hypothesis of formation of methane hydrate deposits over the shallow slopes of the mid-oceanic ridges as an outcome of the action of two factors: the thermal convection of water in fractured-porous rocks of the crust and the reaction of serpentinization of the crust. The intensive exhalation of hydrocarbons takes place in the process of serpentinization. The conditions of water convection in the porous media are favorable for the formation and accumulation of methane hydrates in the near-surface layers of the oceanic crust. The carbonic-acid gas dissolved in the seawater is involved into the process of methane hydrate formation. It was established that the most favorable conditions for this mechanism are over the slopes of the Mid-Atlantic Ridge. All types of water areas where gas hydrates occur can be conditionally subdivided into following geodynamic zones: the abyssal basins of the inner and marginal seas, the

  16. Hydrothermal activity in Tertiary Icelandic crust: Implication for cooling processes along slow-spreading mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Pałgan, D.; Devey, C. W.; Yeo, I. A.

    2015-12-01

    Known hydrothermal activity along the Mid-Atlantic Ridge is mostly high-temperature venting, controlled by volcano-tectonic processes confined to ridge axes and neotectonic zones ~15km wide on each side of the axis (e.g. TAG or Snake Pit). However, extensive exploration and discoveries of new hydrothermal fields in off-axis regions (e.g. Lost City, MAR) show that hydrothermalism may, in some areas, be dominated by off-axis venting. Little is known about nature of such systems, including whether low-temperature "diffuse" venting dominates rather than high-temperature black-smokers. This is particularly interesting since such systems may transport up to 90% of the hydrothermal heat to the oceans. In this study we use Icelandic hot springs as onshore analogues for off-shore hydrothermal activity along the MAR to better understand volcano-tectonic controls on their occurrence, along with processes supporting fluid circulation. Iceland is a unique laboratory to study how new oceanic crust cools and suggests that old crust may not be as inactive as previously thought. Our results show that Tertiary (>3.3 Myr) crust of Iceland (Westfjords) has widespread low-temperature hydrothermal activity. Lack of tectonism (indicated by lack of seismicity), along with field research suggest that faults in Westfjords are no longer active and that once sealed, can no longer support hydrothermal circulation, i.e. none of the hot springs in the area occur along faults. Instead, dyke margins provide open and permeable fluid migration pathways. Furthermore, we suggest that the Reykjanes Ridge (south of Iceland) may be similar to Westfjords with hydrothermalism dominated by off-axis venting. Using bathymetric data we infer dyke positions and suggest potential sites for future exploration located away from neotectonic zone. We also emphasise the importance of biological observations in seeking for low-temperature hydrothermal activity, since chemical or optical methods are not sufficient.

  17. Who cares about Mid-Ocean Ridge Earthquakes? And Why?

    NASA Astrophysics Data System (ADS)

    Tolstoy, M.

    2004-12-01

    Every day the surface of our planet is being slowly ripped apart by the forces of plate tectonics. Much of this activity occurs underwater and goes unnoticed except for by a few marine seismologists who avidly follow the creaks and groans of the ocean floor in an attempt to understand the spreading and formation of oceanic crust. Are marine seismologists really the only ones that care? As it turns out, deep beneath the ocean surface, earthquakes play a fundamental role in a myriad of activity centered on mid-ocean ridges where new crust forms and breaks on a regular basis. This activity takes the form of exotic geological structures hosting roasting hot fluids and bizarre chemosynthetic life forms. One of the fundamental drivers for this other world on the seafloor is earthquakes. Earthquakes provide cracks that allow seawater to penetrate the rocks, heat up, and resurface as hydrothermal vent fluids, thus providing chemicals to feed a thriving biological community. Earthquakes can cause pressure changes along cracks that can fundamentally alter fluid flow rates and paths. Thus earthquakes can both cut off existing communities from their nutrient source and provide new oases on the seafloor around which life can thrive. This poster will present some of the fundamental physical principals of how earthquakes can impact fluid flow, and hence life on the seafloor. Using these other-wordly landscapes and alien-like life forms to woe the unsuspecting passerby, we will sneak geophysics into the picture and tell the story of why earthquakes are so fundamental to life on the seafloor, and perhaps life elsewhere in the universe.

  18. Arctic Lena Trough -- NOT a Mid-Ocean Ridge

    NASA Astrophysics Data System (ADS)

    Snow, J. E.; Hellebrand, E.; Handt, A. V.; Nauret, F.; Gao, Y.; Feig, S.; Jovanovic, Z.

    2005-12-01

    The North American-Eurasian plate boundary traverses the Atlantic and Arctic oceans. Over most of that length, it is a Mid-Ocean Ridge that spreads between about 23 mm/yr (MAR) and 10 mm/yr (Gakkel Ridge) full rate. The northern MAR and the Gakkel ridge are connected by a deep linear feature called Lena Trough. Until about 10 million years ago, Lena Trough was not an oceanic domain at all, but a continental shear zone through a narrow isthmus of continental crust that connected the American and Eurasian plates. Its opening was, significantly, the most recent and final event in the separation of the North American from the Eurasian continent, and opened the gateway for deep water circulation between the Arctic and North Atlantic oceans. Models for the tectonic configuration of Lena Trough have until now differed only in the number and length of fracture zones and spreading segments thought to be present. Lena Trough is a deep fault-bounded basin with depths of 3800-4200m, and irregular, steep valley sides that are oblique to the spreading direction. Basement horst structures outcrop as sigmoidal ridges with steeply dipping sides project out of the valley floor, but these are not traceable to any parallel structures on either side. Ridge-orthogonal topography is simply absent (ie no segments trending parallel nor fracture zones perpendicular to Gakkel Ridge). Most faults trend approximately SSE-NNW, an obliquity with respect to Gakkel Ridge (SW-NE) of about 55 degrees. The basement ridges are composed nearly entirely of mantle peridotite, as are the valley walls. Only at the northern and southern extremities of Lena Trough do basalts appear at all. The Northern basalts show strong chemical affinities to those of Gakkel Ridge, and can be considered a part of the Gakkel volcanic system. The rare southernmost basalts, however, are quite unique. They are alkali basalts with K2O up to 2 weight percent, highly incompaitble element enriched and occupy a corner of isotope

  19. Mid-ocean ridges: discontinuities, segments and giant cracks.

    PubMed

    Macdonald, K C; Scheirer, D S; Carbotte, S M

    1991-08-30

    Geological observations reveal that mid-ocean ridges are segmented by numerous rigid and nonrigid discontinuities. A hierarchy of segmentation, ranging from large, long-lived segments to others that are small, migratory, and transient, determines the pattern and timing of creation of new ocean floor. To the extent that spreading segments behave like giant cracks in a plate, the crack propagation force at segment tips increases with segment length, which may explain why long segments tend to lengthen and prevail over shorter neighboring segments. Partial melting caused by decompression of the upper mantle due to plate separation and changes in the direction of spreading result in the spawning of new short segments so that a balance of long and short segments is maintained.

  20. The petrogenesis of primary mid-ocean ridge basalts

    NASA Technical Reports Server (NTRS)

    Elthon, Don

    1990-01-01

    The nature of primary mid-ocean ridge basalts (MORB) is reviewed from the primary-magma composition point of view. The concept of primary MORB magma used in the study stipulates that melting of the mantle produces a discrete identifiable magma that separates from the mantle and ascends toward the surface. Constraints from abyssal peridotites are considered along with constraints from high-pressure phase equilibria studies with emphasis on partial melting of mantle peridotites, basalt-peridotite sandwich techniques, high-pressure experiments on MORB-type compositions, and constraints on the pressure of origin from mineral compositions. Compositional variations in primitive MORB glasses are discussed, and possible models for the origin of these glasses are presented.

  1. Crustal processes of the mid-ocean ridge.

    PubMed

    1981-07-01

    Independent geological and geophysical investigations of the Mid-Ocean Ridge system have begun to focus on the nature of the magma chamber system underlying its central axis. Thermal models predict the existence of a steady-state chamber beneath a thin crustal lid ranging in thickness from 2 to 13 kilometers. The only aspect of the system that these models fail to account for is the extremely slow spreading rates. Seismological studies reveal the existence of a low-velocity zone beneath segments of the East Pacific Rise, which is thought to correspond to a chamber system having a half-width of approximately 5 to 10 kilometers. These estimates compare favorably with those derived separately through petrological investigations of deep-sea drilling results, various sampling programs, and field and laboratory studies of ophiolites. The chamber is thought to be wing-shaped and to remain continuously open; it is thought to be fed from the center while simultaneously solidifying at the sides as spreading carries the two halves apart. Progressive fractionation occurs by crystal settling coupled with repeated replenishment and magma mixing in an open steady-state system. Near-bottom studies reveal that the zone of extrusion above the chamber is narrow, but its eruptive history is cyclic in nature, in conflict with the predictions of a steady-state model. On-bottom gravity data at 21 degrees N on the East Pacific Rise reveal a negative gravity anomaly that may be related to the uppermost part of the chamber. The anomaly is only 2 kilometers wide and 1 kilometer below the sea floor. This feature may be associated with a short-term upper magma reservoir. The cyclic volcanic activity is directly related to the active phase of hydrothermal circulation responsible for the observed negative thermal anomaly. The volume of water associated with this circulation is equal to the entire ocean volume passing through the accretion zone approximately every 8 million years. This is about 0

  2. Crustal processes of the Mid-Ocean Ridge

    USGS Publications Warehouse

    Ballard, Richard D.; Craig, H.; Edmond, J.; Einaudi, M.; Holcomb, R.; Holland, H.D.; Hopson, C.A.; Luyendyk, B.P.; Macdonald, K.; Morton, J.; Orcutt, J.; Sleep, N.

    1981-01-01

    Independent geological and geophysical investigations of the Mid-Ocean Ridge system have begun to focus on the nature of the magma chamber system underlying its central axis. Thermal models predict the existence of a steady-state chamber beneath a thin crustal lid ranging in thickness from 2 to 13 kilometers. The only aspect of the system that these models fail to account for is the extremely slow spreading rates. Seismological studies reveal the existence of a low-velocity zone beneath segments of the East Pacific Rise, which is thought to correspond to a chamber system having a half-width of approximately 5 to 10 kilometers. These estimates compare favorably with those derived separately through petrological investigations of deep-sea drilling results, various sampling programs, and field and laboratory studies of ophiolites. The chamber is thought to be wing-shaped and to remain continuously open; it is thought to be fed from the center while simultaneously solidifying at the sides as spreading carries the two halves apart. Progressive fractionation occurs by crystal settling coupled with repeated replenishment and magma mixing in an open steady-state system. Near-bottom studies reveal that the zone of extrusion above the chamber is narrow, but its eruptive history is cyclic in nature, in conflict with the predictions of a steady-state model. On-bottom gravity data at 21 ??N on the East Pacific Rise reveal a negative gravity anomaly that may be related to the uppermost part of the chamber. The anomaly is only 2 kilometers wide and 1 kilometer below the sea floor. This feature may be associated with a short-term upper magma reservoir. The cyclic volcanic activity is directly related to the active phase of hydrothermal circulation responsible for the observed negative thermal anomaly. The volume of water associated with this circulation is equal to the entire ocean volume passing through the accretion zone approximately every 8 million years. This is about 0

  3. Hydrothermal Exploration of Mid-Ocean Ridges: Where Might the Largest Sulfide Deposits Occur?

    NASA Astrophysics Data System (ADS)

    German, C. R.; Petersen, S.; Hannington, M. D.

    2015-12-01

    We review the distribution of modern-day seafloor hydrothermal activity along the global mid-ocean ridge crest (MOR) and the mineral deposits being formed at those sites. To date, one form of hydrothermal activity - "black smoker" venting - has been prospected for along >30% of the global mid ocean ridge crest and some important trends have emerged. Submarine venting can occur along all mid-ocean ridges, of all spreading rates, in all ocean basins. While the abundance of currently active venting (from water column signals), scales linearly with seafloor spreading rate (a proxy for magmatic heat-flux) there is an "excess" of high temperature venting along slow and ultra-slow spreading ridges when compared to early predictions. Consistent with this, no more than half of the sites responsible for "black smoker" plume signals along the slow spreading Mid Atlantic Ridge are associated with magmatic systems with the other half hosted under tectonic control. The latter appear both to be longer-lived than, and to give rise to much larger sulfide deposits than, their magmatic counterparts - presumably as a result of sustained fluid flow. Where these tectonic-hosted systems also involve water-rock interaction with ultramafic sources, seafloor massive sulfide deposits exhibit highly concentrated Cu and Au in surface samples (>10wt.% average Cu content and >3ppm average Au). Intriguingly, first detailed examinations of hydrothermally active sites along ultraslow-spreading ridges seems to indicate that they may depart beyond the slow-spreading Mid-Atlantic Ridge pattern. Hydrothermal plume distributions may follow the same (~50:50) distribution of "black smoker" plume signals between magmatic and tectonics settings, but the first three "black smoker" sites tracked to source have all revealed large polymetallic sulfide deposits - in both magmatic as well as tectonic settings. Could ultra-slow ridges represent the richest mineral resource potential along the global MOR?

  4. Periodic Magmatic Events on Slow-Spreading Mid-Ocean Ridges: Evidence from the North Kolbeinsey Ridge, Atlantic.

    NASA Astrophysics Data System (ADS)

    Devey, C. W.; Yeo, I. A.

    2015-12-01

    The majority of the Earth's solid surface is produced by volcanic eruptions at mid-ocean ridges. Slow-spreading mid-ocean ridge eruptions are thought to be characterized by cyclic or periodic volcanism separated by periods of tectonic extension. Here we present high-resolution acoustic sidescan data from the North Kolbeinsey Ridge, a shallow slow-spreading ridge where high glacial and steady post-glacial sedimentation rates allow relative flow ages to be determined using backscatter amplitude as a proxy for sediment thickness and hence age. We identify a suite of young lava flows within the axial valley, suggesting that a significant length of the segment was magmatically active at the same time. This suite of flows represents the largest magmatic event in the last 7 kyr but still do not constitute enough volume to maintain the thickness of seismic layer 2A, suggesting that larger volume, periodic magmatic events are required to maintain crustal thickness.

  5. Extinct mid-ocean ridges and insights on the influence of hotspots at divergent plate boundaries

    NASA Astrophysics Data System (ADS)

    MacLeod, Sarah; Dietmar Müller, R.; Williams, Simon; Matthews, Kara

    2016-04-01

    We review all global examples of confirmed or suspected extinct mid-ocean ridges that are preserved in present-day ocean basins. Data on their spreading rate prior to extinction, time of cessation, length of activity, bathymetric and gravity signature are analysed. This analysis identifies some differences between subgroups of extinct ridges, including microplate spreading ridges, back-arc basin ridges and large-scale mid-ocean ridges. Crustal structure of extinct ridges is evaluated using gravity inversion to seek to resolve a long-standing debate on whether the final stages of spreading leads to development of thinned or thickened crust. Most of the ridges we assess have thinner crust at their axes than their flanks, yet a small number are found to have a single segment that is overprinted by an anomalous feature such as a seamount or volcanic ridge. A more complex cessation mechanism is necessary in these cases. The location of spreading centres at their time of cessation relative to hotspots was also evaluated using a global plate reconstruction. This review provides strong evidence for the long-term interaction of spreading centres with hotspots and plate boundaries have been frequently modified within the radius of a hotspot zone of influence.

  6. Long-term interaction between mid-ocean ridges and mantle plumes

    NASA Astrophysics Data System (ADS)

    Whittaker, J. M.; Afonso, J. C.; Masterton, S.; Müller, R. D.; Wessel, P.; Williams, S. E.; Seton, M.

    2015-06-01

    Plate tectonic motions are commonly considered to be driven by slab pull at subduction zones and ridge push at mid-ocean ridges, with motion punctuated by plumes of hot material rising from the lower mantle. Within this model, the geometry and location of mid-ocean ridges are considered to be independent of lower-mantle dynamics, such as deeply sourced plumes that produce voluminous lava eruptions--termed large igneous provinces. Here we use a global plate model to reconstruct the locations of large igneous provinces relative to plumes and mid-ocean ridges at the time they formed. We find that large igneous provinces repeatedly formed at specific locations where mid-ocean ridges and plumes interact. We calculate how much mantle material was converted to oceanic lithosphere at the mid-ocean ridges and find that slowly migrating ridge systems that have been stabilized by upwelling plumes have extracted large volumes of material from the same part of the upper mantle over periods up to 180 million years. The geochemical signatures of mid-ocean ridge basalts and seismic tomographic data show that upper-mantle temperatures are elevated at significant distances from ridge-plume interactions, indicating a far-field, indirect influence of plume-ridge interactions on the upper-mantle structure. We conclude that strong feedbacks exist between the dynamics of slowly migrating ridges and deeply sourced plumes.

  7. Hafnium isotope results from mid-ocean ridges and Kerguelen.

    USGS Publications Warehouse

    Patchett, P.J.

    1983-01-01

    176Hf/177Hf ratios are presented for oceanic volcanic rocks representing both extremes of the range of mantle Hf-Nd-Sr isotopic variation. Hf from critical mid-ocean ridge basalts shows that 176Hf/177Hf does indeed have a greater variability than 143Nd/144Nd and 87Sr/86Sr in the depleted mantle. This extra variation is essentially of a random nature, and can perhaps be understood in terms of known Rb/Sr-Sm/Nd-Lu/Hf fractionation relationships. At the other extreme of mantle isotopic compositions, 176Hf/177Hf ratios for igneous rocks from the Indian Ocean island of Kerguelen show a closely similar variation to published 143Nd/144Nd ratios for the same samples. Comparison of Hf-Nd-Sr isotopic relatonships for Tristan da Cunha, Kerguelen and Samoa reveals divergences in the mantle array for ocean-island magma sources, and perhaps suggests that these irregularities are largely the result of an extra component of 87Sr/86Sr variation.-G.R.

  8. Melt retention and segregation beneath mid-ocean ridges.

    PubMed

    Faul, U H

    2001-04-19

    Geochemical models of melting at mid-ocean ridges-particularly those based on trace elements and uranium-decay-series isotopes-predict that melt segregates from the matrix at very low porosities, of order 0.1%. Some of these models also require that the melt ascends rapidly. But these predictions appear to conflict with seismic data obtained by the mantle electromagnetic and tomography (MELT) experiment. These data reveal, beneath the East Pacific Rise (at 17 degrees S), a region of low velocities several hundred kilometres wide, which is best explained by the presence of 1-2% melt, distributed on a grain scale in disk-shaped geometries. Here I show that these apparently contradictory constraints can be reconciled by taking into account the geometry and resulting permeability of the intergranular network of melt, together with the changing character of the melt as it ascends. A deep, volatile-rich melt with low viscosity and density is mobile at 0.1% porosity, but basaltic melt only becomes mobile at a porosity above 1%. While the volumetric contribution of the volatile-rich melt to the erupted basalts is small, the isotopic disequilibria (except for radium) generated by porous flow of this melt are preserved if melt transport is rapid at the onset of high-productivity melting. Also, because of incomplete extraction, some melt is retained in a broad zone, consistent with the MELT observations.

  9. The detectability of melt channels beneath slow- and fast-spreading mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Weatherley, S.; Nowacki, A.; Katz, R. F.; Wookey, J.

    2012-12-01

    At mid-ocean ridges, the oceanic crust is created by the intrusion and extrusion of melt, drawn upwards from below the ridge axis in high-porosity channels which form by reactive flow. These channels have drastically different seismic properties to the surrounding mantle material, which has the potential to explain widespread observations of seismic anisotropy in surface and body waves travelling through mid-ocean ridges and the oceanic lithosphere. In order to investigate these processes, we use a 2D numerical model of coupled magma/mantle dynamics. The model is based on conservation statements for mass, momentum, energy and composition in a two-phase system with two components in local thermodynamic equilibrium. One component is more fusible than the other, and a network of magma-rich channels are nucleated when heterogeneities rich in fusible material are introduced into the system. Whilst most channels focus melt towards the axis, pockets of high porosity (analogous to melt) are also rifted away from the ridge axis. We model ridges with full spreading rates between 40 and 160 mm/a, reflecting most ridge systems currently active. Shear wave splitting measurements at mid-ocean ridges reveal a pattern of increasing splitting with distance from the ridge axis, and greater splitting beneath fast-spreading ridges. Surface wave studies show that horizontally-polarised waves (SH) travel faster than those polarised vertically (SV) in the region of 50-100 km beneath the ocean floor. Neither of these observations can be wholly explained by the alignment of mineral phases due to mantle flow beneath ridges. We convert the thermodynamic ridge model to elasticity using effective medium theories, taking porosity as a proxy for melt, and using melt flow lines to predict the orientation of the elasticity. Seismic forward modelling then predicts the amount and orientation of shear wave splitting and surface wave anisotropy, which can be compared to observations. Early results

  10. A correlation between mid-ocean-ridge basalt chemistry and distance to continents.

    PubMed

    Humler, Eric; Besse, Jean

    2002-10-10

    To fully understand the structure and dynamics of the Earth's convecting mantle, the origins of temperature variations within the mantle need to be resolved. Different hypotheses have been proposed to account for these temperature variations: for example, heat coming from the decay of radioactive elements or heat flowing out of the Earth's core. In addition, theoretical studies suggest that the thermal properties of continental masses can affect mantle convection, but quantitative data that could allow us to test these models are scarce. To address this latter problem, we have examined the chemistry of mid-ocean-ridge basalt--which reflects the temperature of the source mantle--as a function of the distance of the ridge from the closest continental margin. No correlation is observed for oceanic ridges close to subduction zones or hotspots; subduction zones probably inhibit thermal transfer between the mantle beneath continents and ocean, whereas hotspots influence the major-element chemistry of ridge basalts, which makes their interpretation with respect to mantle temperature more difficult. However, we do observe a significant correlation for mid-oceanic basalts from the Atlantic and Indian oceans. From this, we conclude that the location of continental masses relative to active ridges influences the large-scale thermal structure of the mantle and we estimate that the mantle cools by 0.05 to 0.1 degrees C per kilometre from the continental margins.

  11. Evidence of active ground deformation on the mid-ocean ridge: Axial seamount, Juan de Fuca Ridge, April-June 1988

    SciTech Connect

    Fox, C.G. )

    1990-08-10

    Since September 1987 a precision bottom pressure recorder (BPR) has been deployed within the summit caldera of Axial seamount. The instrument is capable of measuring pressure of 1 mbar resolution and recording these measurements at 64 samples per hour for up to 15 months. Any significant change in the pressure record should indicate a change of depth associated with vertical ground movement, commonly indicative of active inflation or deflation of underlying magma bodies. Results from the first 9 months of the BPR deployment revealed a significant change in pressure, which is interpreted to represent a 15-cm subsidence of the caldera floor during two 2- to 3-week periods in April-June 1988. Also during these periods, an anomalous decline in temperature at the site was recorded that is correlated with an apparent increase in current velocity at the Axial Seamount Hydrothermal Emissions Study (ASHES) vent field, suggesting vigorous advection of cold water into the caldera. Concurrent oceanographic data from Geosat and from current meter arrays do not indicate any large-scale oceanographic phenomena capable of generating these simultaneous events. One mechanism to explain simultaneous ground subsidence and temperature decline at the caldera center and increased bottom current at the caldera margin is the generation of a buoyant parcel of heated water in response to the intrusion or the eruption of magma associated with volcanic deflation. Similar volcanic events also may have generated large midwater plumes that have been described previously along the southern Juan de Fuca Ridge.

  12. Calcium isotopic compositions of mid-ocean ridge basalts

    NASA Astrophysics Data System (ADS)

    Zhu, H.; Zhang, Z.; Sun, W.; Wang, G. Q.

    2015-12-01

    Previous studies have demonstrated that Earth's mantle has heterogeneous calcium isotopic compositions. But the reason why mantle has its heterogeneity remains uncertain. In general, δ44/40Ca values of mantle xenolith samples have a variation of >0.45‰. While ultramafic rocks, especially dunites, have higher δ44/40Ca values than volcanic rocks, and there is a positive correlation between δ44/40Ca and Ca/Mg. These phenomena imply that the heterogeneity of Ca isotopic compositions of mantle xenolith samples might result from different degrees of melt extraction, as indicated by large Ca isotopic fractionation between co-existing clinopyroxene and orthopyroxene. However, because ancient marine carbonate has its own unique calcium isotopic characteristics, recycling of even a small amount of ancient marine carbonates into the mantle could also cause the heterogeneity of Ca isotopes in Earth's mantle. This could be the reason why oceanic island basalts (OIB) have lighter Ca isotopic compositions than the mantle xenolith. Thus, the lighter Ca isotopic compositions in the mantle source cannot only be ascribed to magmatic processes. Therefore, it is more important to know calcium isotopic characteristics during partial melting and oceanic crust contamination.Mid-ocean ridge basalts (MORB) are formed from the partial melts of the upper mantle and are rarely affected by crustal contamination. Different types of MORB, including D-MORB, N-MORB and E-MORB, have experienced different degrees of partial melting and contamination of enriched end-members. Here we report calcium isotopic characteristic of different types of MORB, we believe it will be very helpful to understand the behaviors of Ca isotopes during partial melting and it is possible to provide further information to discover the reason why calcium isotopic compositions is heterogeneous in Earth's mantle. This work was supported by Natural Science Foundation of China (No. 41373007, No. 41490632 and No. 91328204

  13. Controls on melting at spreading ridges from correlated abyssal peridotite - mid-ocean ridge basalt compositions

    NASA Astrophysics Data System (ADS)

    Regelous, Marcel; Weinzierl, Christoph G.; Haase, Karsten M.

    2016-09-01

    Variations in the volume and major element composition of basalt erupted along the global mid-ocean ridge system have been attributed to differences in mantle potential temperature, mantle composition, or plate spreading rate and lithosphere thickness. Abyssal peridotites, the residues of mantle melting beneath mid-ocean ridges, provide additional information on the melting process, which could be used to test these hypotheses. We compiled a global database of abyssal peridotite compositions averaged over the same ridge segments defined by Gale et al. (2013). In addition, we calculated the distance of each ridge segment to the nearest hotspots. We show that Cr# in spinel in abyssal peridotites is negatively correlated with Na90 in basalts from the same ridge segments on a global scale. Ridge segments that erupt basalts apparently produced by larger degrees of mantle melting are thus underlain by peridotites from which large amounts of melt have been extracted. We find that near-ridge hotspots have a more widespread influence on mid-ocean ridge basalt (MORB) composition and ridge depth than previously thought. However, when these hotspot-influenced ridge segments are excluded, the remaining segments show clear relationships between MORB composition, peridotite composition, and ridge depth with spreading rate. Very slow-spreading ridges (<20 mm/yr) are deeper, erupt basalts with higher Na90, Al90, K90/Ti90, and lower Fe90, Ca90/Al90, and expose peridotites with lower Cr# than intermediate and fast-spreading ridges. We show that away from hotspots, the spreading-rate dependence of the maximum degree of mantle melting inferred from Cr# in peridotites (FM) and the bulk degree of melting inferred from Na90 in basalts (FB) from the same ridge segments is unlikely to be due to variations in mantle composition. Nor can the effects of dynamic mantle upwelling or incomplete melt extraction at low spreading rates satisfactorily explain the observed compositions of abyssal

  14. The influence of ridge migration on the magmatic segmentation of mid-ocean ridges.

    PubMed

    Carbotte, S M; Small, C; Donnelly, K

    2004-06-17

    The Earth's mid-ocean ridges display systematic changes in depth and shape, which subdivide the ridges into discrete spreading segments bounded by transform faults and smaller non-transform offsets of the axis. These morphological changes have been attributed to spatial variations in the supply of magma from the mantle, although the origin of the variations is poorly understood. Here we show that magmatic segmentation of ridges with fast and intermediate spreading rates is directly related to the migration velocity of the spreading axis over the mantle. For over 9,500 km of mid-ocean ridge examined, leading ridge segments in the 'hotspot' reference frame coincide with the shallow magmatically robust segments across 86 per cent of all transform faults and 73 per cent of all second-order discontinuities. We attribute this relationship to asymmetric mantle upwelling and melt production due to ridge migration, with focusing of melt towards ridge segments across discontinuities. The model is consistent with variations in crustal structure across discontinuities of the East Pacific Rise, and may explain variations in depth of melting and the distribution of enriched lavas.

  15. Mineralogy of the mid-ocean-ridge basalt source from neodymium isotopic composition of abyssal peridotites.

    PubMed

    Salters, Vincent J M; Dick, Henry J B

    2002-07-01

    Inferring the melting process at mid-ocean ridges, and the physical conditions under which melting takes place, usually relies on the assumption of compositional similarity between all mid-ocean-ridge basalt sources. Models of mantle melting therefore tend to be restricted to those that consider the presence of only one lithology in the mantle, peridotite. Evidence from xenoliths and peridotite massifs show that after peridotite, pyroxenite and eclogite are the most abundant rock types in the mantle. But at mid-ocean ridges, where most of the melting takes place, and in ophiolites, pyroxenite is rarely found. Here we present neodymium isotopic compositions of abyssal peridotites to investigate whether peridotite can indeed be the sole source for mid-ocean-ridge basalts. By comparing the isotopic compositions of basalts and peridotites at two segments of the southwest Indian ridge, we show that a component other than peridotite is required to explain the low end of the (143)Nd/(144)Nd variations of the basalts. This component is likely to have a lower melting temperature than peridotite, such as pyroxenite or eclogite, which could explain why it is not observed at mid-ocean ridges. PMID:12097907

  16. Variations in mid-ocean ridge CO2 emissions driven by glacial cycles

    NASA Astrophysics Data System (ADS)

    Burley, Jonathan M. A.; Katz, Richard F.

    2015-09-01

    The geological record documents links between glacial cycles and volcanic productivity, both subaerially and, tentatively, at mid-ocean ridges. Sea-level-driven pressure changes could also affect chemical properties of mid-ocean ridge volcanism. We consider how changing sea-level could alter the CO2 emissions rate from mid-ocean ridges on both the segment and global scale. We develop a simplified transport model for a highly incompatible trace element moving through a homogeneous mantle; variations in the concentration and the emission rate of the element are the result of changes in the depth of first silicate melting. The model predicts an average global mid-ocean ridge CO2 emissions rate of 53 Mt/yr or 91 Mt/yr for an average source mantle CO2 concentration of 125 or 215 ppm by weight, in line with other estimates. We show that falling sea level would cause an increase in ridge CO2 emissions about 100 kyrs after the causative sea level change. The lag and amplitude of the response are sensitive to mantle permeability and plate spreading rate. For a reconstructed sea-level time series of the past million years, we predict variations of up to 12% in global mid-ocean ridge CO2 emissions.

  17. Mineralogy of the mid-ocean-ridge basalt source from neodymium isotopic composition of abyssal peridotites.

    PubMed

    Salters, Vincent J M; Dick, Henry J B

    2002-07-01

    Inferring the melting process at mid-ocean ridges, and the physical conditions under which melting takes place, usually relies on the assumption of compositional similarity between all mid-ocean-ridge basalt sources. Models of mantle melting therefore tend to be restricted to those that consider the presence of only one lithology in the mantle, peridotite. Evidence from xenoliths and peridotite massifs show that after peridotite, pyroxenite and eclogite are the most abundant rock types in the mantle. But at mid-ocean ridges, where most of the melting takes place, and in ophiolites, pyroxenite is rarely found. Here we present neodymium isotopic compositions of abyssal peridotites to investigate whether peridotite can indeed be the sole source for mid-ocean-ridge basalts. By comparing the isotopic compositions of basalts and peridotites at two segments of the southwest Indian ridge, we show that a component other than peridotite is required to explain the low end of the (143)Nd/(144)Nd variations of the basalts. This component is likely to have a lower melting temperature than peridotite, such as pyroxenite or eclogite, which could explain why it is not observed at mid-ocean ridges.

  18. Time-Critical Studies: Rapid response to Transient Dynamic Mid-Ocean Ridge Events

    NASA Astrophysics Data System (ADS)

    Cowen, J. P.; Baker, E. T.; Dziak, R. P.; Lilley, M. M.

    2003-12-01

    The Time-Critical Studies (TCS) Theme of Ridge 2000 focuses on observations of the immediate geochemical and geobiological consequences of magmatic and tectonic events along the global mid-ocean ridge system. To date funding has centered on the Juan de Fuca and Gorda Ridges which are within the range of the U.S. Navy's Northeast Pacific Sound Surveillance System (SOSUS). NOAA's T-Phase Monitoring Program has accessed SOSUS in real-time since 1993, providing the TCS community with detection of seismicity associated with eruptive or tectonic activity along these two ridges. This remote detection of earthquake swarms along the N.E. Pacific mid-ocean ridge coupled to NSF funding for pre-event staging equipment and supplies has allowed directed and increasingly well-organized field responses to the event site. Major rapid and follow-up response cruises have been successfully mounted to 1993 CoAxial, 1996 and 2001 Gorda Ridge, the 1998 Axial Volcano, and 2001 Middle Valley magmatic episodes. The logistical approach required to study these events has been greatly facilitated by the RIDGE/Ridge 2000 programs and collaboration between university, NOAA and Canadian investigators. Not only have our studies of these events significantly impacted our ideas on the nature of crustal accretion, but they also have led to the discovery and preliminary documentation of a previously unrecognized biomass reservoir that lives below the seafloor and is swept out during these cataclysmic events, and to increased appreciation of the formation and thermal, chemical and biogeochemical implications of the 'Event Plumes' commonly associated with sea floor magmatic events. Rapid shore-to-event site response is an important aspect of TCS. Proposals to enhance the event detection and response effort are welcome at any Ridge 2000 target date. The Ridge 2000 program recognizes that even the most rapid ship response will miss the earliest subsurface and water column expressions of magmatic events

  19. Geophysical evidence for reduced melt production on the Arctic ultraslow Gakkel mid-ocean ridge.

    PubMed

    Jokat, W; Ritzmann, O; Schmidt-Aursch, M C; Drachev, S; Gauger, S; Snow, J

    2003-06-26

    Most models of melt generation beneath mid-ocean ridges predict significant reduction of melt production at ultraslow spreading rates (full spreading rates &<20 mm x yr(-1)) and consequently they predict thinned oceanic crust. The 1,800-km-long Arctic Gakkel mid-ocean ridge is an ideal location to test such models, as it is by far the slowest portion of the global mid-ocean-ridge spreading system, with a full spreading rate ranging from 6 to 13 mm x yr(-1) (refs 4, 5). Furthermore, in contrast to some other ridge systems, the spreading direction on the Gakkel ridge is not oblique and the rift valley is not offset by major transform faults. Here we present seismic evidence for the presence of exceptionally thin crust along the Gakkel ridge rift valley with crustal thicknesses varying between 1.9 and 3.3 km (compared to the more usual value of 7 km found on medium- to fast-spreading mid-ocean ridges). Almost 8,300 km of closely spaced aeromagnetic profiles across the rift valley show the presence of discrete volcanic centres along the ridge, which we interpret as evidence for strongly focused, three-dimensional magma supply. The traces of these eruptive centres can be followed to crustal ages of approximately 25 Myr off-axis, implying that these magma production and transport systems have been stable over this timescale. PMID:12827194

  20. Geophysical evidence for reduced melt production on the Arctic ultraslow Gakkel mid-ocean ridge.

    PubMed

    Jokat, W; Ritzmann, O; Schmidt-Aursch, M C; Drachev, S; Gauger, S; Snow, J

    2003-06-26

    Most models of melt generation beneath mid-ocean ridges predict significant reduction of melt production at ultraslow spreading rates (full spreading rates &<20 mm x yr(-1)) and consequently they predict thinned oceanic crust. The 1,800-km-long Arctic Gakkel mid-ocean ridge is an ideal location to test such models, as it is by far the slowest portion of the global mid-ocean-ridge spreading system, with a full spreading rate ranging from 6 to 13 mm x yr(-1) (refs 4, 5). Furthermore, in contrast to some other ridge systems, the spreading direction on the Gakkel ridge is not oblique and the rift valley is not offset by major transform faults. Here we present seismic evidence for the presence of exceptionally thin crust along the Gakkel ridge rift valley with crustal thicknesses varying between 1.9 and 3.3 km (compared to the more usual value of 7 km found on medium- to fast-spreading mid-ocean ridges). Almost 8,300 km of closely spaced aeromagnetic profiles across the rift valley show the presence of discrete volcanic centres along the ridge, which we interpret as evidence for strongly focused, three-dimensional magma supply. The traces of these eruptive centres can be followed to crustal ages of approximately 25 Myr off-axis, implying that these magma production and transport systems have been stable over this timescale.

  1. Mid-ocean ridge jumps associated with hotspot magmatism

    NASA Astrophysics Data System (ADS)

    Mittelstaedt, Eric; Ito, Garrett; Behn, Mark D.

    2008-02-01

    Hotspot-ridge interaction produces a wide range of phenomena including excess crustal thickness, geochemical anomalies, off-axis volcanic ridges and ridge relocations or jumps. Ridges are recorded to have jumped toward many hotspots including, Iceland, Discovery, Galápagos, Kerguelen and Tristan de Cuhna. The causes of ridge jumps likely involve a number of interacting processes related to hotspots. One such process is reheating of the lithosphere as magma penetrates it to feed near-axis volcanism. We study this effect by using the hybrid, finite-element code, FLAC, to simulate two-dimensional (2-D, cross-section) viscous mantle flow, elasto-plastic deformation of the lithosphere and heat transport in a ridge setting near an off-axis hotspot. Heating due to magma transport through the lithosphere is implemented within a hotspot region of fixed width. To determine the conditions necessary to initiate a ridge jump, we vary four parameters: hotspot magmatic heating rate, spreading rate, seafloor age at the location of the hotspot and ridge migration rate. Our results indicate that the hotspot magmatic heating rate required to initiate a ridge jump increases non-linearly with increasing spreading rate and seafloor age. Models predict that magmatic heating, itself, is most likely to cause jumps at slow spreading rates such as at the Mid-Atlantic Ridge on Iceland. In contrast, despite the higher magma flux at the Galápagos hotspot, magmatic heating alone is probably insufficient to induce a ridge jump at the present-day due to the intermediate ridge spreading rate of the Galápagos Spreading Center. The time required to achieve a ridge jump, for fixed or migrating ridges, is found to be on the order of 10 5-10 6 years. Simulations that incorporate ridge migration predict that after a ridge jump occurs the hotspot and ridge migrate together for time periods that increase with magma flux. Model results also suggest a mechanism for ridge reorganizations not related to

  2. Hafnium isotope results from mid-ocean ridges and Kerguelen

    USGS Publications Warehouse

    Jonathan, Patchett P.

    1983-01-01

    176Hf/177Hf ratios are presented for oceanic volcanics representing both extremes of the range of mantle HfNdSr isotopic variation. Hf from critical mid-ocean ridgebasalts shows that 176Hf/177Hf does indeed have a greater variability than 143Nd/144Nd and 87Sr/86Sr in the depleted mantle. This extra variation is essentially of a random nature, and can perhaps be understood in terms of known Rb/SrSm/NdLu/Hf fractionation relationships. At the other extreme of mantle isotopic composition, 176Hf/177Hf ratios for igneous rocks from the Indian Ocean island of Kerguelen show a closely similar variation to published 143Nd/144Nd ratios for the same samples. Comparison of HfNdSr c relationships for Tristan da Cunha, Kerguelen and Samoa reveals divergences in the mantle array for ocean island magma sources, and perhaps suggests that these irregularities are largely the result of an extra component of 87Sr/86Sr variation. ?? 1983.

  3. Mid-ocean ridge dynamics - Observations and theory

    SciTech Connect

    Phipps morgan, J. )

    1991-01-01

    Recent observational and theoretical investigations of midocean-ridge extension and its relation to melting, magmatic, deformation, and hydrothermal processes are discussed in a critical review of U.S. research from the period 1987-1990. Topics examined include segmentation, along-axis crustal variations and gravity, axial crust and lithosphere structure and seismics, ophiolite studies, and the interaction of ridge and continental rift studies. Consideration is given to theoretical models of axial topography; mantle flow, melting, and melt migration; mantle rheology and flow beneath a midoceanic ridge; upwelling structure and segmentation; the role of the lithosphere in shaping ridge segmentation; thermal stress and the origin of fracture zones; and hydrothermal cooling. A comprehensive bibliography is provided.

  4. Modes of faulting at mid-ocean ridges.

    PubMed

    Buck, W Roger; Lavier, Luc L; Poliakov, Alexei N B

    2005-04-01

    Abyssal-hill-bounding faults that pervade the oceanic crust are the most common tectonic feature on the surface of the Earth. The recognition that these faults form at plate spreading centres came with the plate tectonic revolution. Recent observations reveal a large range of fault sizes and orientations; numerical models of plate separation, dyke intrusion and faulting require at least two distinct mechanisms of fault formation at ridges to explain these observations. Plate unbending with distance from the top of an axial high reproduces the observed dip directions and offsets of faults formed at fast-spreading centres. Conversely, plate stretching, with differing amounts of constant-rate magmatic dyke intrusion, can explain the great variety of fault offset seen at slow-spreading ridges. Very-large-offset normal faults only form when about half the plate separation at a ridge is accommodated by dyke intrusion.

  5. Helium isotope geochemistry of mid-ocean ridge basalts from the South Atlantic

    NASA Astrophysics Data System (ADS)

    Graham, David W.; Jenkins, William J.; Schilling, Jean-Guy; Thompson, Geoffrey; Kurz, Mark D.; Humphris, Susan E.

    1992-05-01

    We report new helium isotope results for 49 basalt glass samples from the Mid-Atlantic Ridge between 1°N and 47°S. 3He/ 4He in South Atlantic mid-ocean ridge basalts (MORB) varies between 6.5 and 9.0 R A (R A is the atmospheric ratio of 1.39 × 10 -6), encompassing the range of previously reported values for MORB erupted away from high 3He/ 4He hotspots such as Iceland. He, Sr and Pb isotopes show systematic relationships along the ridge axis. The ridge axis is segmented with respect to geochemical variations, and local spike-like anomalies in 3He/ 4He , Pb and Sr isotopes, and trace element ratios such as (La/Sm) N are prevalent at the latitudes of the islands of St. Helena, Tristan da Cunha and Gough to the east of the ridge. The isotope systematics are consistent with injection beneath the ridge of mantle "blobs" enriched in radiogenic He, Pb and Sr, derived from off-axis hotspot sources. The variability in 3He/ 4He along the ridge can be used to refine the hotspot source-migrating-ridge sink model. MORB from the 2-7°S segment are systematically the least radiogenic samples found along the mid-ocean ridge system to date. Here the depleted mantle source is characterized by 87Sr/ 86Sr of ˜ 0.7022, Pb isotopes close to the geochron and with 206Pb/ 204Pb of ˜ 17.7, and 3He/ 4He of 8.6-8.9 R A. The "background contamination" of the subridge mantle, by radiogenic helium derived from off-ridge hotspots, displays a maximum between ˜ 20 and 24°S. The He sbnd Pb and He sbnd Sr isotope relations along the ridge indicate that the 3He/ 4He ratios are lower for the hotspot sources of St. Helena, Tristan da Cunha and Gough than for the MORB source, consistent with direct measurements of 3He/ 4He ratios in the island lavas. Details of the He sbnd Sr sbnd Pb isotope systematics between 12 and 22°S are consistent with early, widespread dispersion of the St. Helena plume into the asthenosphere, probably during flattening of the plume head beneath the thick lithosphere

  6. Redox heterogeneity in mid-ocean ridge basalts as a function of mantle source.

    PubMed

    Cottrell, Elizabeth; Kelley, Katherine A

    2013-06-14

    The oxidation state of Earth's upper mantle both influences and records mantle evolution, but systematic fine-scale variations in upper mantle oxidation state have not previously been recognized in mantle-derived lavas from mid-ocean ridges. Through a global survey of mid-ocean ridge basalt glasses, we show that mantle oxidation state varies systematically as a function of mantle source composition. Negative correlations between Fe(3+)/ΣFe ratios and indices of mantle enrichment--such as (87)Sr/(86)Sr, (208)Pb/(204)Pb, Ba/La, and Nb/Zr ratios--reveal that enriched mantle is more reduced than depleted mantle. Because carbon may act to simultaneously reduce iron and generate melts that share geochemical traits with our reduced samples, we propose that carbon creates magmas at ridges that are reduced and enriched.

  7. Mantle segmentation along the Oman ophiolite fossil mid-ocean ridge.

    PubMed

    Le Mée, Laurent; Girardeau, Jacques; Monnier, Christophe

    2004-11-11

    It has been difficult to relate the segmentation of mid-ocean ridges to processes occurring in the Earth's underlying mantle, as the mantle is rarely sampled directly and chemical variations observed in lavas at the surface are heavily influenced by details of their production as melt extracted from the mantle. Our understanding of such mantle processes has therefore relied on the analysis of pieces of fossil oceanic lithosphere now exposed at the Earth's surface, known as ophiolites. Here we present the phase chemistry and whole-rock major- and trace-element contents of 174 samples of the mantle collected along over 400 km of the Oman Sultanate ophiolite. We show that, when analysed along the fossil ridge, variations of elemental ratios sensitive to the melting process define a three-dimensional geometry of mantle upwellings, which can be related to the segmentation observed in modern mid-ocean ridge environments.

  8. Pb isotopes in sulfides from mid-ocean ridge hydrothermal sites

    SciTech Connect

    LeHuray, A.P.; Church, S.E.; Koski, R.A.; Bouse, R.M.

    1988-04-01

    The authors report Pb isotope ratios of sulfides deposited at seven recently active mid-ocean ridge (MOR) hydrothermal vents. Sulfides from three sediment-starved sites on the Juan de Fuca Ridge contain Pb with isotope ratios identical to their local basaltic sources. Lead in two deposits from the sediment-covered Escanaba Trough, Gorda Ridge, is derived from the sediments and does not appear to contain any basaltic component. There is a range of isotope ratios in a Guaymas Basin deposit, consistent with a mixture of sediment and MOR basalt Pb. Lead in a Galapagos deposit differs slightly from known Galapagos basalt Pb isotope values. The faithful record of Pb isotope signatures of local sources in MOR sulfides indicates that isotope ratios from ancient analogues ca be used as accurate reflections of ancient oceanic crustal values in ophiolite-hosted deposits and continental crustal averages in sediment-hosted deposits. The preservation of primary ophiolitic or continental crustal Pb isotope signatures in ancient MOR sulfides provides a powerful tool for investigation of crustal evolution and for fingerprinting ancient terranes.

  9. Segmentation of mid-ocean ridges attributed to oblique mantle divergence

    NASA Astrophysics Data System (ADS)

    Vanderbeek, Brandon P.; Toomey, Douglas R.; Hooft, Emilie E. E.; Wilcock, William S. D.

    2016-08-01

    The origin of mid-ocean ridge segmentation--the systematic along-axis variation in tectonic and magmatic processes--remains controversial. It is commonly assumed that mantle flow is a passive response to plate divergence and that between transform faults magma supply controls segmentation. Using seismic tomography, we constrain the geometry of mantle flow and the distribution of mantle melt beneath the intermediate-spreading Endeavour segment of the Juan de Fuca Ridge. Our results, in combination with prior studies, establish a systematic skew between the mantle-divergence and plate-spreading directions. In all three cases studied, mantle divergence is advanced with respect to recent changes in the plate-spreading direction and the extent to which the flow field is advanced increases with decreasing spreading rate. Furthermore, seismic images show that large-offset, non-transform discontinuities are regions of enhanced mantle melt retention. We propose that oblique mantle flow beneath mid-ocean ridges is a driving force for the reorientation of spreading segments and the formation of ridge-axis discontinuities. The resulting tectonic discontinuities decrease the efficiency of upward melt transport, thus defining segment-scale variations in magmatic processes. We predict that across spreading rates mid-ocean ridge segmentation is controlled by evolving patterns in asthenospheric flow and the dynamics of lithospheric rifting.

  10. Seismotectonics of mid-ocean ridge propagation in Hess Deep.

    PubMed

    Floyd, Jacqueline S; Tolstoy, Maya; Mutter, John C; Scholz, Christopher H

    2002-11-29

    Hydroacoustic data from the eastern equatorial Pacific reveal low-magnitude seismicity concentrated at the propagating tip of the Galapagos Rise in Hess Deep. The patterns of seismicity and faulting are similar to those observed in the process zone of laboratory-scale propagating tensile cracks. Because the fracture energy required for propagation scales with crack length and process zone size, it follows that ridges can propagate stably in the brittle crust without exceptional resisting forces as proposed by previous models based on linear elastic fracture mechanics.

  11. Discovery of a black smoker vent field and vent fauna at the Arctic Mid-Ocean Ridge

    PubMed Central

    Pedersen, Rolf B.; Rapp, Hans Tore; Thorseth, Ingunn H.; Lilley, Marvin D.; Barriga, Fernando J. A. S.; Baumberger, Tamara; Flesland, Kristin; Fonseca, Rita; Früh-Green, Gretchen L.; Jorgensen, Steffen L.

    2010-01-01

    The Arctic Mid-Ocean Ridge (AMOR) represents one of the most slow-spreading ridge systems on Earth. Previous attempts to locate hydrothermal vent fields and unravel the nature of venting, as well as the provenance of vent fauna at this northern and insular termination of the global ridge system, have been unsuccessful. Here, we report the first discovery of a black smoker vent field at the AMOR. The field is located on the crest of an axial volcanic ridge (AVR) and is associated with an unusually large hydrothermal deposit, which documents that extensive venting and long-lived hydrothermal systems exist at ultraslow-spreading ridges, despite their strongly reduced volcanic activity. The vent field hosts a distinct vent fauna that differs from the fauna to the south along the Mid-Atlantic Ridge. The novel vent fauna seems to have developed by local specialization and by migration of fauna from cold seeps and the Pacific. PMID:21119639

  12. Discovery of a black smoker vent field and vent fauna at the Arctic Mid-Ocean Ridge.

    PubMed

    Pedersen, Rolf B; Rapp, Hans Tore; Thorseth, Ingunn H; Lilley, Marvin D; Barriga, Fernando J A S; Baumberger, Tamara; Flesland, Kristin; Fonseca, Rita; Früh-Green, Gretchen L; Jorgensen, Steffen L

    2010-11-23

    The Arctic Mid-Ocean Ridge (AMOR) represents one of the most slow-spreading ridge systems on Earth. Previous attempts to locate hydrothermal vent fields and unravel the nature of venting, as well as the provenance of vent fauna at this northern and insular termination of the global ridge system, have been unsuccessful. Here, we report the first discovery of a black smoker vent field at the AMOR. The field is located on the crest of an axial volcanic ridge (AVR) and is associated with an unusually large hydrothermal deposit, which documents that extensive venting and long-lived hydrothermal systems exist at ultraslow-spreading ridges, despite their strongly reduced volcanic activity. The vent field hosts a distinct vent fauna that differs from the fauna to the south along the Mid-Atlantic Ridge. The novel vent fauna seems to have developed by local specialization and by migration of fauna from cold seeps and the Pacific.

  13. Controls on extrusion at mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Buck, W. Roger; Carbotte, Suzanne M.; Mutter, Carolyn

    1997-10-01

    A magma lens can erupt to form extrusives only if it is under greater pressure than the static pressure in a column of magma reaching from the lens to the surface. The excess pressure results partly from overburden pressure caused by the presence of high- and low-density rocks (dikes and extrusives, respectively) above the lens. The thicker the pile of low-density extrusives, the lower the average overburden density. Thus, extrusion above a lens should be self-regulating, in that thickening the extrusive layer reduces the driving pressure for subsequent eruptions. Flexural stresses may affect extrusion by altering the pressure on a magma chamber. For ridges lacking an axial valley, we predict that deeper magma lenses should correlate with thicker extrusive layers, consistent with recent observations.

  14. Sea level forcing of mid-ocean ridge magmatism on Milankovitch timescales

    NASA Astrophysics Data System (ADS)

    Lund, D.; Asimow, P.

    2008-12-01

    It is well-documented that Iceland experienced a pulse of elevated volcanism immediately following the last deglaciation (Maclennan et al., 2002). Modeling results suggest ice sheet retreat depressurized the mantle thus enhancing melt production and the supply of magma to the surface (Jull and McKenzie, 1996). Here we take a similar approach, but instead model the effect of glacial-interglacial changes in sea level on mantle melting at mid-ocean ridges. Loading rates reaching ±2 cm/year of water are comparable to the tectonic unloading rate of ~2 cm/year of mantle rock that drives magmatic activity at a slow-spreading ridge. Although the magnitude of sea level forcing is smaller than subglacial forcing, the sea level effect is globally distributed and could have significant consequences for ocean crust architecture and geothermal heat delivery to the deep ocean. We use a model of melt production based on analytical corner flow velocities coupled to the pMELTS model (Ghiorso et al. 2002; Asimow et al. 2004) of melting of the Workman and Hart (2006) depleted upper mantle source composition. For simplicity we assume that the hydrostatic pressure signal from sea-level variation is felt instantaneously by the entire melting regime, and that melts migrate from source to ridge axis at a constant rate. We neglect crustal magmatic and hydrothermal processes that might damp or delay the signal. We examined mid-ocean ridge systems with half-spreading rates from 30 mm/yr to 100 mm/yr and melt migration rates from 2.5 to 50 m/yr. For the case of 30 mm/yr half-spreading rate and 10 m/yr melt migration, we find that the rate of melt delivery to the crust varies ±30% relative to steady state conditions when the model is driven by a record of sea-level variability for the last 140 kyr. Notably, we simulate that melt delivery increased by ~30% beginning at 75 kyr BP, coincident with a rapid decrease in sea level of approximately 60 m. We also estimate a ~30% increase in melt

  15. Recycled dehydrated lithosphere observed in plume-influenced mid-ocean-ridge basalt.

    PubMed

    Dixon, Jacqueline Eaby; Leist, Loretta; Langmuir, Charles; Schilling, Jean-Guy

    2002-11-28

    A substantial uncertainty in the Earth's global geochemical water cycle is the amount of water that enters the deep mantle through the subduction and recycling of hydrated oceanic lithosphere. Here we address the question of recycling of water into the deep mantle by characterizing the volatile contents of different mantle components as sampled by ocean island basalts and mid-ocean-ridge basalts. Although all mantle plume (ocean island) basalts seem to contain more water than mid-ocean-ridge basalts, we demonstrate that basalts associated with mantle plume components containing subducted lithosphere--'enriched-mantle' or 'EM-type' basalts--contain less water than those associated with a common mantle source. We interpret this depletion as indicating that water is extracted from the lithosphere during the subduction process, with greater than 92 per cent efficiency.

  16. Composition and depth of origin of primary mid-ocean ridge basalts

    NASA Astrophysics Data System (ADS)

    Presnall, D. C.; Hoover, J. D.

    1984-09-01

    Some workers have held that mid-ocean ridge basalts are fractionated from high pressure (15 30 kbar) picritic primary magmas whereas others have favored primary magmas generated at about 10 kbar with compositions close to those of mid-ocean ridge basalts. Of critical significance are presumed differences in composition between experimentally determined primary magmas and the least fractionated mid-ocean ridge basalts. To evaluate the significance of these differences, all based on electron microprobe analyses, we consider three sources of uncertainty: (1) analytical uncertainties for a single microprobe laboratory, (2) systematic interlaboratory analytical differences, and (3) real variations in the possible compositions of primary magmas that can be produced from a peridotite source at a given pressure. The first source of error is surprisingly large and can account for a substantial part of the total variation of normative quartz (hypersthene calculated as equivalent olivine and quartz) in FAMOUS basalts. The second is not as serious but remains undetermined for many laboratories. The third is potentially the largest but is not yet fully documented. The least fractionated FA-MOUS basalts have high mg numbers (70 73) compatible with derivation from the mantle by direct partial melting with little or no subsequent fractional crystallization. Because of the wide range of normative quartz content in these basalts, it appears necessary to consider them as representatives of multiple parental magmas. When all the sources of uncertainty are taken into account, we conclude that the experimental data by various investigators are all fairly consistent and favor derivation of the least fractionated mid-ocean ridge basalts by at most only a small amount of fractional crystallization from primary magmas having a wide range of normative quartz content and generated over a range of pressures from about 7 11 kbar.

  17. Circumventing shallow air contamination in Mid Ocean Ridge Basalts

    NASA Astrophysics Data System (ADS)

    Mukhopadhyay, Sujoy; Parai, Rita; Tucker, Jonathan; Middleton, Jennifer; Langmuir, Charles

    2016-04-01

    Noble gases in mantle-derived basalts provide a rich portrait of mantle degassing and surface-interior volatile exchange. However, the ubiquity of shallow-level air contamination frequently obscures the mantle noble gas signal. In a majority of samples, shallow air contamination dominates the noble gas budget. As a result, reconstructing the variability in heavy noble gas mantle source compositions and inferring the history of deep recycling of atmospheric noble gases is difficult. For example, in the gas-rich popping rock 2ΠD43, 129Xe/130Xe ratios reach 7.7±0.23 in individual step-crushes, but the bulk composition of the sample is close to air (129Xe/130Xe of 6.7). Here, we present results from experiments designed to elucidate the source of shallow air contamination in MORBs. Step-crushes were carried out to measure He, Ne, Ar and Xe isotopic compositions on two aliquots of a depleted popping glass that was dredged from between the Kane and Atlantis Fracture Zones of the Mid-Atlantic Ridge in May 2012. One aliquot was sealed in ultrapure N2 after dredge retrieval, while the other aliquot was left exposed to air for 3.5 years. The bulk 20Ne/22Ne and 129Xe/130Xe ratios measured in the aliquot bottled in ultrapure N2 are 12.3 and 7.6, respectively, and are nearly identical to the estimated mantle source values. On the other hand, step crushes in the aliquot left exposed to air for several years show Ne isotopic compositions that are shifted towards air, with a bulk 20Ne/22Ne of 11.5; the bulk 129Xe/130Xe, however, was close to 7.6. These results indicate that lighter noble gases exchange more efficiently between the bubbles trapped in basalt glass and air, suggesting a diffusive or kinetic mechanism for the incorporation of the shallow air contamination. Importantly, in Ne-Ar or Ar-Xe space, step-crushes from the bottled aliquot display a trend that can be easily fit with a simple two-component hyperbolic mixing between mantle and atmosphere noble gases. Step

  18. Permeability of asthenospheric mantle and melt extraction rates at mid-ocean ridges.

    PubMed

    Connolly, James A D; Schmidt, Max W; Solferino, Giulio; Bagdassarov, Nikolai

    2009-11-12

    Magmatic production on Earth is dominated by asthenospheric melts of basaltic composition that have mostly erupted at mid-ocean ridges. The timescale for segregation and transport of these melts, which are ultimately responsible for formation of the Earth's crust, is critically dependent on the permeability of the partly molten asthenospheric mantle, yet this permeability is known mainly from semi-empirical and analogue models. Here we use a high-pressure, high-temperature centrifuge, at accelerations of 400g-700g, to measure the rate of basalt melt flow in olivine aggregates with porosities of 5-12 per cent. The resulting permeabilities are consistent with a microscopic model in which melt is completely connected, and are one to two orders of magnitude larger than predicted by current parameterizations. Extrapolation of the measurements to conditions characteristic of asthenosphere below mid-ocean ridges yields proportionally higher transport speeds. Application of these results in a model of porous-media channelling instabilities yields melt transport times of approximately 1-2.5 kyr across the entire asthenosphere, which is sufficient to preserve the observed (230)Th excess of mid-ocean-ridge basalts and the mantle signatures of even shorter-lived isotopes such as (226)Ra (refs 5,11-14).

  19. What processes at mid-ocean ridges tell us about volcanogenic massive sulfide deposits

    NASA Astrophysics Data System (ADS)

    Cathles, Lawrence M.

    2011-07-01

    Episodic seafloor spreading, ridge topography, and fault movement at ridges find (more extreme) analogs in the arc and back-arc setting where the volcanogenic massive sulfide (VMS) deposits that we mine today were formed. The factors affecting sulfide accumulation efficiency and the extent to which sulfides are concentrated spatially are the same in both settings, however. The processes occurring at mid-ocean ridges therefore provide a useful insight into those producing VMS deposits in arcs and back-arcs. The critical observation investigated here is that all the heat introduced by seafloor spreading at mid-ocean ridges is carried out of the crust within a few hundred meters of the ridge axis by ˜350°C hydrothermal fluids. The high-temperature ridge hydrothermal systems are tied to the presence of magma at the ridge axis and greatly reduce the size and control the shape of axial magma intrusions. The amount of heat introduced to each square kilometer of ocean crust during its formation can be calculated, and its removal by high-temperature convection allows calculation of the total base metal endowment of the ocean basins. Using reasonable metal deposition efficiencies, we conclude that the ocean floor is a giant VMS district with metal resources >600 times the total known VMS reserves on land and a copper resource which would last >6,000 years at current production rates.

  20. High Tech High School Interns Develop a Mid-Ocean Ridge Database for Research and Education

    NASA Astrophysics Data System (ADS)

    Staudigel, D.; Delaney, R.; Staudigel, H.; Koppers, A. A.; Miller, S. P.

    2004-12-01

    Mid-ocean ridges (MOR) represent one of the most important geographical and geological features on planet Earth. MORs are the locations where plates spread apart, they are the locations of the majority of the Earths' volcanoes that harbor some of the most extreme life forms. These concepts attract much research, but mid-ocean ridges are still effectively underrepresented in the Earth science class rooms. As two High Tech High School students, we began an internship at Scripps to develop a database for mid-ocean ridges as a resource for science and education. This Ridge Catalog will be accessible via http://earthref.org/databases/RC/ and applies a similar structure, design and data archival principle as the Seamount Catalog under EarthRef.org. Major research goals of this project include the development of (1) an archival structure for multibeam and sidescan data, standard bathymetric maps (including ODP-DSDP drill site and dredge locations) or any other arbitrary digital objects relating to MORs, and (2) to compile a global data set for some of the most defining characteristics of every ridge segment including ridge segment length, depth and azimuth and half spreading rates. One of the challenges included the need of making MOR data useful to the scientist as well as the teacher in the class room. Since the basic structure follows the design of the Seamount Catalog closely, we could move our attention to the basic data population of the database. We have pulled together multibeam data for the MOR segments from various public archives (SIOExplorer, SIO-GDC, NGDC, Lamont), and pre-processed it for public use. In particular, we have created individual bathymetric maps for each ridge segment, while merging the multibeam data with global satellite bathymetry data from Smith & Sandwell (1997). The global scale of this database will give it the ability to be used for any number of applications, from cruise planning to data

  1. Unraveling the unusual morphology of the Cretaceous Dirck Hartog extinct mid-ocean ridge

    NASA Astrophysics Data System (ADS)

    Watson, S. J.; Whittaker, J. M.; Halpin, J.; Williams, S.; Milan, L. A.; Daczko, N. R.; Wyman, D. A.

    2015-12-01

    The Perth Abyssal Plain (PAP), offshore southwest Australia formed during Mesozoic East Gondwana breakup and Kerguelen plume activity. This study combines petrographic and geochemical data from the first samples ever to be dredged from the flanks of the Dirck Hartog Ridge (DHR), a prominent linear bathymetric feature in the central PAP, with new bathymetric profiles across the PAP to better constrain the formation of the early Indian Ocean floor. The DHR exhibits high relief and distinctive asymmetry that is unusual compared to most active or extinct spreading centres and likely results from compression and deformation of the recently extinct DHR during changes in relative motion of the Indian plate (110 - 100 Ma). Exhumation of gabbros in the southern DHR and an increase in seafloor roughness towards the centre of the PAP, likely result from a half spreading rate decrease from 35 mm/yr (based on magnetic reversals) to 24 mm/yr at ~114 Ma. The results support a slowdown of spreading prior to full cessation at ~102 Ma. The composition of basaltic samples varies along the DHR: from sub-alkaline dolerites with incompatible element concentrations most similar to depleted-to-normal mid-ocean ridge basalts in the south, to alkali basalts similar to ocean island basalts in the north. Therefore, magma sources and degrees of partial melting varied in space and time, a result supporting the interpretation that the DHR is an extinct spreading ridge rather than a pseudofault. The enriched alkali basalt signatures may be attributed to melting of a heterogeneous mantle or to the influence of the Kerguelen plume over distances greater than 1000 km. The results demonstrate the significance of regional tectonic plate motions on the formation and deformation of young ocean crust, and provide insight into the unique DHR morphology.

  2. Imaging the deep seismic structure beneath a mid-ocean ridge: the MELT experiment

    PubMed

    1998-05-22

    The Mantle Electromagnetic and Tomography (MELT) Experiment was designed to distinguish between competing models of magma generation beneath mid-ocean ridges. Seismological observations demonstrate that basaltic melt is present beneath the East Pacific Rise spreading center in a broad region several hundred kilometers across and extending to depths greater than 100 kilometers, not just in a narrow region of high melt concentration beneath the spreading center, as predicted by some models. The structure of the ridge system is strongly asymmetric: mantle densities and seismic velocities are lower and seismic anisotropy is stronger to the west of the rise axis.

  3. Melting Systematics in Mid-ocean Ridge Basalts and Implications for Global CO2 Fluxes

    NASA Astrophysics Data System (ADS)

    Behn, M. D.; Grove, T. L.; Wanless, V. D.; Brown, S. M.

    2015-12-01

    We present a new model for anhydrous melting in the spinel and plagioclase stability fields that provides enhanced predictive capabilities for the major element compositional variability found in mid-ocean ridge basalts (MORBs). The melting model is coupled to geodynamic simulations of mantle flow and mid-ocean ridge temperature structure to investigate global variations in MORB chemistry and crustal thickness as a function of mantle potential temperature, spreading rate, mantle composition, and the pattern(s) of melt migration. To constrain global variations in mantle melting parameters we incorporate evidence from both MORB major element compositions and seismically determined crustal thicknesses. Specifically, we show that to explain the global data set of crustal thickness, Na8, Fe8, Si8, Ca8/Al8, and K8/Ti8 (oxides normalized to 8 wt% MgO) requires a relatively narrow zone over which melts are pooled to the ridge axis. In all cases, our preferred model involves melt transport to the ridge axis over relatively short horizontal length scales (~25 km), implying that although melting occurs over a wide region, up to 20-40% of the total melt volume is not extracted, and will eventually refreeze and refertilize the lithosphere. We further incorporate constraints from melt inclusion datasets to constrain the global mid-ocean ridge CO2 degassing flux. Our estimates indicate that ~3.4 x1014 g/yr of CO2 are released by MORB melting, however, more than half of this CO2 may remain trapped in the lithospheric mantle.

  4. Does sea level influence mid-ocean ridge magmatism on Milankovitch timescales?

    NASA Astrophysics Data System (ADS)

    Lund, David C.; Asimow, Paul D.

    2011-12-01

    Magma production at mid-ocean ridges is driven by seafloor spreading and decompression melting of the upper mantle. In the special case of Iceland, mantle melting may have been amplified by ice sheet retreat during the last deglaciation, yielding anomalously high rates of subaerial volcanism. For the remainder of the global mid-ocean ridge system, the ocean may play an analogous role, with lowering of sea level during glacial maxima producing greater magma flux to ridge crests. Here we show that the mantle decompression rate associated with changes in sea level is a substantial fraction of that from plate spreading. Modeled peaks in magma flux occur after sea level drops rapidly, including the Marine Isotope Stage (MIS) 5/4 and 3/2 transitions. The minimum in simulated flux occurs during the mid-Holocene, due to the rapid sea level rise at the MIS 2/1 boundary. The model results are highly sensitive to melt migration rate; rates of ˜1 m/yr produce small signals, while those >5 m/yr yield substantial anomalies. In the latter case, sea level-driven magma flux varies by 15-100% relative to the long-term average, with the largest effect occurring at slow-spreading ridges. We suggest that sedimentary time series of hydrothermal particle flux, oceanic Os isotopic ratio, and oceanic radiocarbon may serve as proxies for magma-flux variations at mid-ocean ridges. Although well-dated records are rare, preliminary data from the Pacific and Atlantic suggest hydrothermal metal flux was elevated during MIS 2 and 4, broadly consistent with our modeling results.

  5. Microbial diversity of Loki's Castle black smokers at the Arctic Mid-Ocean Ridge.

    PubMed

    Jaeschke, A; Jørgensen, S L; Bernasconi, S M; Pedersen, R B; Thorseth, I H; Früh-Green, G L

    2012-11-01

    Hydrothermal vent systems harbor rich microbial communities ranging from aerobic mesophiles to anaerobic hyperthermophiles. Among these, members of the archaeal domain are prevalent in microbial communities in the most extreme environments, partly because of their temperature-resistant and robust membrane lipids. In this study, we use geochemical and molecular microbiological methods to investigate the microbial diversity in black smoker chimneys from the newly discovered Loki's Castle hydrothermal vent field on the Arctic Mid-Ocean Ridge (AMOR) with vent fluid temperatures of 310-320 °C and pH of 5.5. Archaeal glycerol dialkyl glycerol tetraether lipids (GDGTs) and H-shaped GDGTs with 0-4 cyclopentane moieties were dominant in all sulfide samples and are most likely derived from both (hyper)thermophilic Euryarchaeota and Crenarchaeota. Crenarchaeol has been detected in low abundances in samples derived from the chimney exterior indicating the presence of Thaumarchaeota at lower ambient temperatures. Aquificales and members of the Epsilonproteobacteria were the dominant bacterial groups detected. Our observations based on the analysis of 16S rRNA genes and biomarker lipid analysis provide insight into microbial communities thriving within the porous sulfide structures of active and inactive deep-sea hydrothermal vents. Microbial cycling of sulfur, hydrogen, and methane by archaea in the chimney interior and bacteria in the chimney exterior may be the prevailing biogeochemical processes in this system.

  6. Geophysical and geochemical evidence for deep temperature variations beneath mid-ocean ridges.

    PubMed

    Dalton, Colleen A; Langmuir, Charles H; Gale, Allison

    2014-04-01

    The temperature and composition of Earth's mantle control fundamental planetary properties, including the vigor of mantle convection and the depths of the ocean basins. Seismic wave velocities, ocean ridge depths, and the composition of mid-ocean ridge basalts can all be used to determine variations in mantle temperature and composition, yet are typically considered in isolation. We show that correlations among these three data sets are consistent with 250°C variation extending to depths >400 kilometers and are inconsistent with variations in mantle composition at constant temperature. Anomalously hot ridge segments are located near hot spots, confirming a deep mantle-plume origin for hot spot volcanism. Chemical heterogeneity may contribute to scatter about the global trend. The coherent temperature signal provides a thermal calibration scale for interpreting seismic velocities located distant from ridges.

  7. Sensitivity of seafloor bathymetry to climate-driven fluctuations in mid-ocean ridge magma supply

    NASA Astrophysics Data System (ADS)

    Olive, Jean-Arthur; Behn, Mark; Ito, Garrett; Escartin, Javier; Buck, Roger; Howell, Samuel

    2016-04-01

    Abyssal hills are the most common topographic feature on the surface of the solid Earth, yet the detailed mechanisms through which they are formed remain a matter of debate. Classical seafloor observations suggest hills acquire their shape at mid-ocean ridges through a combination of normal faulting and volcanic accretion. However, recent studies have proposed that the fabric of the seafloor reflects rapid fluctuations in ridge magma supply caused by oscillations in sea level modulating the partial melting process beneath the ridge [Crowley et al., 2015, Science]. In order to move this debate forward, we propose a modeling framework relating the magma supply of a mid-ocean ridge to the morphology of the seafloor it produces, i.e., the spacing and amplitude of abyssal hills. We specifically assess whether fluctuations in melt supply of a given periodicity can be recorded in seafloor bathymetry through (1) static compensation of crustal thickness oscillations, (2) volcanic extrusion, and (3) fault growth modulated by dike injection. We find that topography-building processes are generally insensitive to fluctuations in melt supply on time scales shorter than ~50-100 kyr. Further, we show that the characteristic wavelengths found in seafloor bathymetry across all spreading rates are best explained by simple tectono-magmatic interaction models, and require no periodic (climatic) forcing. Finally, we explore different spreading regimes where a smaller amplitude sea-level signal super-imposed on the dominant faulting signal could be most easily resolved.

  8. Interactions among mid-ocean ridges, plumes and Large Igneous Provinces

    NASA Astrophysics Data System (ADS)

    Whittaker, J. M.; Afonso, J. C.; Masterton, S. M.; Müller, D.; Wessel, P.; Williams, S.; Seton, M.

    2015-12-01

    Plate tectonic motions are commonly considered to be driven by slab pull at subduction zones and ridge push at mid-ocean ridges (MORs), with motion punctuated by plumes of hot material rising from the lower mantle. Within this model, the geometry and location of MORs are considered to be independent from deeply sourced mantle plumes commonly implicated in the formation of Large Igneous Provinces (LIPs). Here we reconstruct the absolute locations of LIPs and MORs relative to plume locations, and find that LIPs predominantly form episodically at specific locations of MOR-plume interaction. Analysis of MOR and continental diverging plate boundary locations since 180 Myr reveals that long-standing MOR-plume interactions are enabled by slowly migrating ridge systems. We calculate how much mantle material was converted to oceanic lithosphere at the MORs and calculate that slowly migrating MORs have extracted large volumes of material from the same part of the upper mantle over periods up to 180 million years. The geochemical signatures of mid-ocean ridge basalts and seismic tomographic data show that upper-mantle temperatures are elevated at significant distances from ridge-plume interactions, indicating a far-field, indirect influence of plume-ridge interactions on the upper-mantle structure. In summary, long-standing interaction between divergent plate boundaries and mantle plumes, leading to the formation of LIPs, occurs in a much more systematic way than previously appreciated. The interaction of the surface spreading system with deeply sourced mantle plumes is an overlooked yet important aspect of the mantle circulation system, with likely implications for understanding a range of Earth system processes, such as evolution of the plate-mantle system, supercontinent assembly and dispersal, and patterns of ridge morphology and geochemistry.

  9. Three-dimensional passive mantle flow beneath mid-ocean ridges: an analytical approach

    NASA Astrophysics Data System (ADS)

    Ligi, Marco; Cuffaro, Marco; Chierici, Francesco; Calafato, Antonino

    2008-11-01

    We discuss theoretical and computational method on plate-driven mantle flow beneath mid-ocean ridges. We consider a steady-state flow induced by motion of overlying rigid plates in an incompressible viscous mantle beneath a generic ridge-transform-ridge plate boundary. No assumption of orthogonal and symmetric spreading at ridge axis is made. Analytical solutions for viscosity flow in a half-space and in a layered viscosity mantle beneath an infinitesimal thickness lithosphere and beneath plates that thicken with increasing age, are presented. Numerical calculations were carried out using a standard fast Fourier transform algorithm. The difficulty of using standard Fourier methods to predict accurately the mantle flow field in the proximity of the plate boundaries is overcome by applying the Gegenbauer reconstruction post-processing technique to the Fourier pseudo-spectral solutions. Finally, we present some examples of flow computations. We consider, for both models, two different ridge-transform-ridge geometries consisting of 100 and 1000 km offsets of two ridge segments spreading at 15 mm/yr half rate. We found a significant difference in the flow structure between the two flow models close to ridge axis and ridge-transform intersections. The proposed model and methods are useful for fast mantle flow calculations to investigate melting processes beneath spreading centres, and to predict the relationship between mantle temperature, crustal thickness and geochemistry of the oceanic crust.

  10. Sediment distribution on the mid-ocean ridges with respect to spreading of the sea floor.

    PubMed

    Ewing, J; Ewing, M

    1967-06-23

    An abrupt change in sediment thickness between the crests and flanks of the mid-ocean ridges can be interpreted as a major discontinuity in the rates either of spreading of the sea floor or of accumulation of sediment. The preferable interpretation of the data is that the process of spreadig of the sea floor is intermittent and that the present cycle of spreading commenced around 10 million years ago. following a long period Of quiescence during which most of the observed sediments were deposited.

  11. Quantifying melt production and degassing rate at mid-ocean ridges from global mantle convection models with plate motion history

    NASA Astrophysics Data System (ADS)

    Li, Mingming; Black, Benjamin; Zhong, Shijie; Manga, Michael; Rudolph, Maxwell L.; Olson, Peter

    2016-07-01

    The Earth's surface volcanism exerts first-order controls on the composition of the atmosphere and the climate. On Earth, the majority of surface volcanism occurs at mid-ocean ridges. In this study, based on the dependence of melt fraction on temperature, pressure, and composition, we compute melt production and degassing rate at mid-ocean ridges from three-dimensional global mantle convection models with plate motion history as the surface velocity boundary condition. By incorporating melting in global mantle convection models, we connect deep mantle convection to surface volcanism, with deep and shallow mantle processes internally consistent. We compare two methods to compute melt production: a tracer method and an Eulerian method. Our results show that melt production at mid-ocean ridges is mainly controlled by surface plate motion history, and that changes in plate tectonic motion, including plate reorganizations, may lead to significant deviation of melt production from the expected scaling with seafloor production rate. We also find a good correlation between melt production and degassing rate beneath mid-ocean ridges. The calculated global melt production and CO2 degassing rate at mid-ocean ridges varies by as much as a factor of 3 over the past 200 Myr. We show that mid-ocean ridge melt production and degassing rate would be much larger in the Cretaceous, and reached maximum values at ˜150-120 Ma. Our results raise the possibility that warmer climate in the Cretaceous could be due in part to high magmatic productivity and correspondingly high outgassing rates at mid-ocean ridges during that time.

  12. Lava Cones and Shields on Intermediate-Rate Mid-Ocean Ridges

    NASA Astrophysics Data System (ADS)

    Clague, D. A.; Paduan, J. B.; Caress, D. W.

    2014-12-01

    Most eruptions of basalt along mid-ocean ridges produce either sheet flows or pillow mounds and ridges. Rare eruptions on the Juan de Fuca and Gorda Ridges and on the Alarcon Rise (northern East Pacific Rise) produce volcanic cones or shields from point sources. Bathymetric maps at 1-m resolution from an autonomous underwater vehicle enabled classification of these ~circular features. The most common are 290-510m across and <50m tall cones with craters or tumulus-like inflated flows on their summits. There are 8 of these on the upper south rift, caldera floor, and southwest caldera rim on Axial Seamount; one on North Cleft segment near the 1986 pillow mounds; and one in the axial graben on northern Endeavour Segment. Hundreds of smaller pillow mounds lack craters or tumulus-like inflated flows at their summits. Three 660-1300m across circular cones have either a crater or an inflated tumulus-like structure at their flat to slightly domed summits. One in the axial graben on the northern Endeavour Segment is dissected by extensional faulting. Cage Seamount on the Coaxial Segment south of the 1993 pillow ridge is the most voluminous at 1100m across and >200m tall. Two flat-topped cones are located near the center of Alarcon Rise. A low-relief shield volcano on the northern Alarcon Rise is ~1700m across and only ~45m tall, and is cut by numerous faults and fissures. Two other shields, 860m and 1700m across and 50-70m tall, occur south of the 1996 North Gorda pillow mounds. These shields are decorated with small pillow mounds. Five 100-250m across and 15-30m deep pits collapsed on the northern shield. These constructional cones and shields form during eruptions where the initial fissure consolidated to a point, indicative of long duration activity. They are constructed during uncommon eruptions with flux larger than produces pillow mounds and smaller than produces sheet flows. They are a submarine equivalent of subaerial shield-building eruptions.

  13. Serpentinization-assisted deformation processes and characterization of hydrothermal fluxes at mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Genc, Gence

    Seafloor hydrothermal systems play a significantly important role in Earth’s energy and geochemical budgets and support the existence and development of complex biological ecosystems by providing nutrient and energy to microbial and macrafaunal ecosystems through geochemical fluxes. Heat output and fluid flow are key parameters which characterize hydrothermal systems at oceanic spreading centers by constraining models of hydrothermal circulation. Although integrated measurements of heat flux in plumes are critically important as well, quantification of heat flux at discrete sources (vent orifices versus patches of seafloor shimmering diffuse flow) from direct measurements is particularly essential for examining the partitioning of heat flow into focused and diffuse components of venting and determining geochemical fluxes from these two modes of flow. Hydrothermal heat output also constrains the permeability of young oceanic crust and thickness of the conductive boundary layer that separates magmatic heat source from overlying hydrothermal circulation. This dissertation will be fundamentally focused on three main inter-connected topics: (1) the design and development of direct high- or low-temperature heat flow measuring devices for hydrothermal systems, (2) the collection of new heat output results on four cruises between 2008 and 2010 at several distinct hydrothermal sites along mid-ocean ridges (MORs) to estimate total heat output from individual vent structures such as Dante, Hulk or the whole vent field (e.g., Main Endeavour Vent Field (MEF)), the partitioning between focused and diffuse hydrothermal venting in MEF, and determination of initial estimates of geochemical flux from diffuse hydrothermal fluids which may be influenced by the activity in subsurface biosphere and finally (3) the deformation and uplift associated with serpentinization at MORs and subduction zones. Despite extensive efforts spent for the last couple of decades on heat flow measurement

  14. Age determination of mid-ocean ridge basalts by radiocarbon dating of lithified carbonate crusts

    SciTech Connect

    Kuptsov, V.M.; Bogdanov, Yu.A.; Palkina, A.M.; Lisitsyn, A.P.

    1986-01-01

    The processes that take place in the mid-ocean ridges are the key to their understanding of the evolution of the earth's crust and mantle. Mid-ocean ridge volcanism supplies vast masses of mantle material, forming new oceanic crust. In recent years, comprehensive study has been made of such processes. The problems of geochronology have an important place in these investigations, since only a study of the events in their time sequence will enable them to make a valid estimate of the intensity of these global processes. In 1980, crusts were obtained by the Pikar combined expedition in the Red Sea rift in the 18/sup 0/ study area on the lower tectonic terrace, in the axial zone, and in three deep water basins. Manned deep water submersible, dredges, trawls, bottom samplers, and impact tubes brought up basalts covered with lithified crusts, and also separate lithified crusts, collected from the basalt basement during sampling. The authors have dated the crusts by the radiocarbon method using the benzene technique. Results of the analysis give ages ranging from 2980 to 20,700 years. Results are discussed. The use of lithified carbonate crusts for determining the age of the basalts is effective within the range of the radiocarbon dating method (up to 40,000-45,000 years). This time interval is inaccessible for determinations by other methods of nuclear geochronology, which makes the method especially valuable. 1 reference, 2 figures, 1 table.

  15. Rapid hydrothermal cooling above the axial melt lens at fast-spreading mid-ocean ridge

    PubMed Central

    Zhang, Chao; Koepke, Juergen; Kirchner, Clemens; Götze, Niko; Behrens, Harald

    2014-01-01

    Axial melt lenses sandwiched between the lower oceanic crust and the sheeted dike sequences at fast-spreading mid-ocean ridges are assumed to be the major magma source of oceanic crust accretion. According to the widely discussed “gabbro glacier” model, the formation of the lower oceanic crust requires efficient cooling of the axial melt lens, leading to partial crystallization and crystal-melt mush subsiding down to lower crust. These processes are believed to be controlled by periodical magma replenishment and hydrothermal circulation above the melt lens. Here we quantify the cooling rate above melt lens using chemical zoning of plagioclase from hornfelsic recrystallized sheeted dikes drilled from the East Pacific at the Integrated Ocean Drilling Program Hole 1256D. We estimate the cooling rate using a forward modelling approach based on CaAl-NaSi interdiffusion in plagioclase. The results show that cooling from the peak thermal overprint at 1000–1050°C to 600°C are yielded within about 10–30 years as a result of hydrothermal circulation above melt lens during magma starvation. The estimated rapid hydrothermal cooling explains how the effective heat extraction from melt lens is achieved at fast-spreading mid-ocean ridges. PMID:25209311

  16. Rapid hydrothermal cooling above the axial melt lens at fast-spreading mid-ocean ridge.

    PubMed

    Zhang, Chao; Koepke, Juergen; Kirchner, Clemens; Götze, Niko; Behrens, Harald

    2014-09-11

    Axial melt lenses sandwiched between the lower oceanic crust and the sheeted dike sequences at fast-spreading mid-ocean ridges are assumed to be the major magma source of oceanic crust accretion. According to the widely discussed "gabbro glacier" model, the formation of the lower oceanic crust requires efficient cooling of the axial melt lens, leading to partial crystallization and crystal-melt mush subsiding down to lower crust. These processes are believed to be controlled by periodical magma replenishment and hydrothermal circulation above the melt lens. Here we quantify the cooling rate above melt lens using chemical zoning of plagioclase from hornfelsic recrystallized sheeted dikes drilled from the East Pacific at the Integrated Ocean Drilling Program Hole 1256D. We estimate the cooling rate using a forward modelling approach based on CaAl-NaSi interdiffusion in plagioclase. The results show that cooling from the peak thermal overprint at 1000-1050°C to 600°C are yielded within about 10-30 years as a result of hydrothermal circulation above melt lens during magma starvation. The estimated rapid hydrothermal cooling explains how the effective heat extraction from melt lens is achieved at fast-spreading mid-ocean ridges.

  17. Rapid hydrothermal cooling above the axial melt lens at fast-spreading mid-ocean ridge.

    PubMed

    Zhang, Chao; Koepke, Juergen; Kirchner, Clemens; Götze, Niko; Behrens, Harald

    2014-01-01

    Axial melt lenses sandwiched between the lower oceanic crust and the sheeted dike sequences at fast-spreading mid-ocean ridges are assumed to be the major magma source of oceanic crust accretion. According to the widely discussed "gabbro glacier" model, the formation of the lower oceanic crust requires efficient cooling of the axial melt lens, leading to partial crystallization and crystal-melt mush subsiding down to lower crust. These processes are believed to be controlled by periodical magma replenishment and hydrothermal circulation above the melt lens. Here we quantify the cooling rate above melt lens using chemical zoning of plagioclase from hornfelsic recrystallized sheeted dikes drilled from the East Pacific at the Integrated Ocean Drilling Program Hole 1256D. We estimate the cooling rate using a forward modelling approach based on CaAl-NaSi interdiffusion in plagioclase. The results show that cooling from the peak thermal overprint at 1000-1050°C to 600°C are yielded within about 10-30 years as a result of hydrothermal circulation above melt lens during magma starvation. The estimated rapid hydrothermal cooling explains how the effective heat extraction from melt lens is achieved at fast-spreading mid-ocean ridges. PMID:25209311

  18. Water-rich basalts at mid-ocean-ridge cold spots.

    PubMed

    Ligi, Marco; Bonatti, Enrico; Cipriani, Anna; Ottolini, Luisa

    2005-03-01

    Although water is only present in trace amounts in the suboceanic upper mantle, it is thought to play a significant role in affecting mantle viscosity, melting and the generation of crust at mid-ocean ridges. The concentration of water in oceanic basalts has been observed to stay below 0.2 wt%, except for water-rich basalts sampled near hotspots and generated by 'wet' mantle plumes. Here, however, we report unusually high water content in basaltic glasses from a cold region of the mid-ocean-ridge system in the equatorial Atlantic Ocean. These basalts are sodium-rich, having been generated by low degrees of melting of the mantle, and contain unusually high ratios of light versus heavy rare-earth elements, implying the presence of garnet in the melting region. We infer that water-rich basalts from such regions of thermal minima derive from low degrees of 'wet' melting greater than 60 km deep in the mantle, with minor dilution by melts produced by shallower 'dry' melting--a view supported by numerical modelling. We therefore conclude that oceanic basalts are water-rich not only near hotspots, but also at 'cold spots'.

  19. Phase equilibria constraints on the chemistry of hot spring fluids at mid-ocean ridges

    SciTech Connect

    Seyfried, W.E. Jr.; Ding, K.; Berndt, M.E. )

    1991-12-01

    Recent advances in experimental and theoretical geochemistry have made it possible to assess both homogeneous and heterogeneous equilibria involving a wide range of aqueous species at temperatures and pressures appropriate to model hydrothermal alteration processes at mid-ocean ridges. The authors have combined selected aspects of the chemistry of hot spring fluids with constraints imposed by a geologically reasonable assemblage of minerals in the system Na{sub 2}O-K{sub 2}O-CaO-MgO-FeO-Fe{sub 2}O{sub 3}-Al{sub 2}O{sub 3}-SiO{sub 2}-H{sub 2}O-HCl-H{sub 2}S to assess the effect of temperature on the composition of the aqueous phase and the activities of mineral components in plagioclase and epidote solid solutions. Assuming fO{sub 2(g)} and fS{sub 2(g)} controlled by pyrite-pyrrhotite-magnetite equilibria, a constant dissolved Ca concentration, and a dissolved Cl concentration equivalent to that of seawater, increasing temperature from 250 to 400C at 500 bars results in systematic changes in the composition of mineral phases, which in turn constrain pH and the distribution of aqueous species. The model predicts that dissolved concentrations of Fe, SiO{sub 2}, K, H{sub 2}S, and H{sub 2} increase, while Na and pH{sub (25C)} decrease with increasing temperature. That many hot springs vent fluids are characterized by variable degrees of conductive heat loss renders measured temperatures unreliable as indicators of the maximum temperature of subseafloor hydrothermal alteration processes. The implications of this are significant for hot spring fluids which reveal large Cl variations relative to seawater, since likely mechanisms to account for such variability typically require temperatures in excess of those inferred for subseafloor reaction zones by simply correcting measured temperatures for the effects of adiabatic cooling.

  20. Sensitivity of seafloor bathymetry to climate-driven fluctuations in mid-ocean ridge magma supply.

    PubMed

    Olive, J-A; Behn, M D; Ito, G; Buck, W R; Escartín, J; Howell, S

    2015-10-16

    Recent studies have proposed that the bathymetric fabric of the seafloor formed at mid-ocean ridges records rapid (23,000 to 100,000 years) fluctuations in ridge magma supply caused by sealevel changes that modulate melt production in the underlying mantle. Using quantitative models of faulting and magma emplacement, we demonstrate that, in fact, seafloor-shaping processes act as a low-pass filter on variations in magma supply, strongly damping fluctuations shorter than about 100,000 years. We show that the systematic decrease in dominant seafloor wavelengths with increasing spreading rate is best explained by a model of fault growth and abandonment under a steady magma input. This provides a robust framework for deciphering the footprint of mantle melting in the fabric of abyssal hills, the most common topographic feature on Earth. PMID:26472905

  1. Oceanic phosphorus imbalance: Magnitude of the mid-ocean ridge flank hydrothermal sink

    NASA Astrophysics Data System (ADS)

    Wheat, C. Geoffrey; McManus, James; Mottl, Michael J.; Giambalvo, Emily

    2003-09-01

    We present a new estimate for the crustal phosphorous sink that results from reactions among seawater, basalt, and sediment blanketing low temperature mid-ocean ridge flank hydrothermal systems. New estimates for global hydrothermal power output, sediment thickness, and the dissolved phosphate concentrations in basement formation fluids indicate that fluid flow through ridge flanks removes 2.8 × 1010 mol P yr-1. This value is larger (130%) than the riverine dissolved flux of inorganic phosphate and is as much as 35% of the sedimentary P sink. The concordant seawater flux (2.1 × 1016 kg yr-1) is 65% of the riverine fluid flux and circulates a fluid volume equivalent to the entire ocean in about 70,000 yr. Additional sampling of seafloor springs is required to further constrain the range of calculated phosphate fluxes; nevertheless the modern phosphorus budget is clearly unbalanced with total sinks outpacing sources.

  2. Sensitivity of seafloor bathymetry to climate-driven fluctuations in mid-ocean ridge magma supply.

    PubMed

    Olive, J-A; Behn, M D; Ito, G; Buck, W R; Escartín, J; Howell, S

    2015-10-16

    Recent studies have proposed that the bathymetric fabric of the seafloor formed at mid-ocean ridges records rapid (23,000 to 100,000 years) fluctuations in ridge magma supply caused by sealevel changes that modulate melt production in the underlying mantle. Using quantitative models of faulting and magma emplacement, we demonstrate that, in fact, seafloor-shaping processes act as a low-pass filter on variations in magma supply, strongly damping fluctuations shorter than about 100,000 years. We show that the systematic decrease in dominant seafloor wavelengths with increasing spreading rate is best explained by a model of fault growth and abandonment under a steady magma input. This provides a robust framework for deciphering the footprint of mantle melting in the fabric of abyssal hills, the most common topographic feature on Earth.

  3. Grain Size as a Control for Melt Focusing Beneath Mid-Ocean Ridges

    NASA Astrophysics Data System (ADS)

    Turner, A.; Katz, R. F.; Behn, M. D.

    2015-12-01

    Grain size is a fundamental control on both the rheology and permeability of the mantle. These properties, in turn, affect the transport of melt beneath mid-ocean ridges. Previous models of grain size beneath ridges have considered only the single-phase problem of dynamic recrystallisation and the resultant pattern of grain-size variation [1,2]. These models have not coupled the spatially variable grain-size field to two-phase (partially molten) mechanics to investigate the implications of spatially variable grain size on melt transport. Here, we present new results from numerical models that investigate the consequences of this coupling. In our two-dimensional, two-phase model the grain-size is coupled to both the permeability and rheology. The rheology is strain-rate and grain-size dependent. For simplicity, however, the grain-size field is not computed dynamically — rather, it is imposed from a single-phase, steady-state model [1] that is based on the "wattmeter" theory [3]. Our calculations predicts that a spatially variable grain size field can promote focusing of melt towards the ridge axis. This focusing is distinct from the commonly discussed, sub-lithospheric decompaction channel [4]. Furthermore, our model predicts that the shape of the partially molten region is sensitive to rheological parameters associated with grain size. The comparison of this shape with observations [5] may help to constrain the rheology of the upper mantle beneath mid-ocean ridges. References: [1] Turner et al., Geochem. Geophys. Geosyst., 16, 925-946, 2015. [2] Behn et al., EPSL, 282, 178-189, 2009. [3] Austin and Evans, Geology, 35:343-346, 2007. [4] Sparks and Parmentier, EPSL, 105, 368-377, 1991. [5] Key et al., Nature, 495, 499-502, 2013.

  4. Variations in mid-ocean ridge magmatism and carbon emissions driven by glacial cycles

    NASA Astrophysics Data System (ADS)

    Katz, R. F.; Burley, J. M.; Huybers, P. J.; Langmuir, C. H.; Crowley, J. W.; Park, S. H.; Carbotte, S. M.; Ferguson, D.; Proistosescu, C.; Boulahanis, B.

    2015-12-01

    Glacial cycles transfer ˜5×10^19 kg of water between the oceans and ice sheets, causing pressure changes in the upper mantle with consequences for the melting of Earth's interior. Forced with Plio-Pleistocene sea-level variations, theoretical models of mid-ocean ridge magma/mantle dynamics predict temporal variations up to 10% in melt supply to the base of the crust. Moreover, a transport model for a perfectly incompatible element suggests that CO2 emissions from mid-ocean ridges could vary by a similar proportion, though with a longer time-lag.Bathymetry from the Australian-Antarctic ridge shows statistically significant spectral energy near the Milankovitch periods of 23, 41, and 100 thousand years, which is consistent with model predictions. These results suggest that abyssal hills record the magmatic response to changes in sea level. The mechanism by which variations in the rate of melt supply are expressed in the bathymetry is not understood.The same pressure variations that modulate the melting rate could also modulate the depth of the onset of silicate melting. As ice sheets grow and sea level drops, this onset deepens, causing melting at the base of the silicate melting regime. Excess highly incompatible elements like CO2 enter the melt and begin their journey to the ridge axis. Tens of thousands of years later, this additional CO2 flux is emitted into the climate system. Because of its delay with respect to sea-level change, the predicted variation in CO2 emissions could represent a restoring force on climate (and sea-level) excursions. This mechanism has a response time determined by the time scale of melt transport; it potentially introduces a resonant frequency into the climate system.

  5. Bathymetric Constraints on Dynamic Topography and Mantle Flow from Asymmetric Subsidence Across the Mid-Ocean Ridges

    NASA Astrophysics Data System (ADS)

    Watkins, C. E.; Conrad, C. P.

    2015-12-01

    Although stresses from convective flow in the mantle should deflect Earth's surface vertically, the amplitude of such deflections remains unclear despite their importance for historic shoreline and sea-level reconstruction. Indeed, obtaining topographic constraints on this dynamically-supported topography is challenging, especially on continents where isostatic topography is enlarged by the long and complicated history of tectonic deformation. In contrast, the well-understood half-space cooling of young seafloor offers a relatively weak filter for the signal of dynamic topography. In this study we attempt to constrain both the sources and scales of dynamic topography by comparing deviations in unperturbed seafloor bathymetry with predictions of dynamic topography from a numerical mantle flow model. Here we define unperturbed seafloor as regions free from the direct influence of volcanic hotspots, oceanic plateaus, tectonic plate boundaries, or sub-lithospheric thermal alteration. We identify dynamic deflections of the seafloor by focusing on regions where normal seafloor resides on both sides of a mid-ocean ridge, specifically across the East Pacific Rise and the Mid-Atlantic Ridge south of the Icelandic hotspot. Across both mid-ocean ridges the magnitude of the subsidence is greater in the direction of South America than away from it, which is consistent with long-wavelength dynamic subsidence of the seafloor above subduction-induced mantle downwelling. We compared models of dynamic topography driven by both negative and positive density heterogeneity (which induce upwelling and downwelling, respectively) in both the upper and lower mantle to the asymmetrical seafloor subsidence across these mid-ocean ridges. We used spherical harmonics up to degree eight for each model prediction of dynamic topography, and found that peak-to-peak amplitudes of 700-1200m sourced mainly from density contrasts in the lower mantle best explain the observed ridge asymmetries. Dynamic

  6. Dacite petrogenesis on mid-ocean ridges: Evidence for oceanic crustal melting and assimilation

    USGS Publications Warehouse

    Wanless, V.D.; Perfit, M.R.; Ridley, W.I.; Klein, E.

    2010-01-01

    Whereas the majority of eruptions at oceanic spreading centers produce lavas with relatively homogeneous mid-ocean ridge basalt (MORB) compositions, the formation of tholeiitic andesites and dacites at mid-ocean ridges (MORs) is a petrological enigma. Eruptions of MOR high-silica lavas are typically associated with ridge discontinuities and have produced regionally significant volumes of lava. Andesites and dacites have been observed and sampled at several locations along the global MOR system; these include propagating ridge tips at ridge-transform intersections on the Juan de Fuca Ridge and eastern Gal??pagos spreading center, and at the 9??N overlapping spreading center on the East Pacific Rise. Despite the formation of these lavas at various ridges, MOR dacites show remarkably similar major element trends and incompatible trace element enrichments, suggesting that similar processes are controlling their chemistry. Although most geochemical variability in MOR basalts is consistent with low-pressure fractional crystallization of various mantle-derived parental melts, our geochemical data for MOR dacitic glasses suggest that contamination from a seawater-altered component is important in their petrogenesis. MOR dacites are characterized by elevated U, Th, Zr, and Hf, low Nb and Ta concentrations relative to rare earth elements (REE), and Al2O3, K2O, and Cl concentrations that are higher than expected from low-pressure fractional crystallization alone. Petrological modeling of MOR dacites suggests that partial melting and assimilation are both integral to their petrogenesis. Extensive fractional crystallization of a MORB parent combined with partial melting and assimilation of amphibole-bearing altered crust produces a magma with a geochemical signature similar to a MOR dacite. This supports the hypothesis that crustal assimilation is an important process in the formation of highly evolved MOR lavas and may be significant in the generation of evolved MORB in

  7. The Cobb-Eickelberg seamount chain: Hotspot volcanism with mid-ocean ridge basalt affinity

    SciTech Connect

    Desonie, D.L.; Duncan, R.A. )

    1990-08-10

    Cobb hotspot, currently located beneath Axial seamount on the Juan de Fuca ridge, has the temporal but not the isotopic characteristics usually attributed to a mantle plume. The earlier volcanic products of the hotspot, form eight volcanoes in the Cobb-Eickelberg seamount (CES) chain, show a westward age progression away from the hotspot and a westward increase in the age difference between the seamounts and the crust on which they formed. These results are consistent with movement of the Pacific plate over a fixed Cobb hotspot and eventual encroachment by the westwardly migrating Juan de Fuca ridge. CES lavas are slightly enriched in alkalies and incompatible elements relative to those of the Juan de Fuca ridge but they have Sr, Nd, and Pb isotopic compositions virtually identical to those found along the ridge. Therefore, Cobb hotspot is a stationary, upper mantle melting anomaly whose volcanic products show strong mid-ocean ridge basalt (MORB) affinity. These observations can be explained by low degrees of partial melting of entrained heterogeneous upper mantle MORB source material within a thermally driven lower mantle diapir or by an intrinsic MORB-like composition of the deeper mantle source region from which northeast Pacific plumes rise.

  8. Noble metals in mid-ocean ridge volcanism: A significant fractionation of gold with respect to platinum group metals

    NASA Technical Reports Server (NTRS)

    Crocket, James H.

    1988-01-01

    Hydrothermal precipitates, black smoker particulate, and massive sulphide dredge samples from the Explorer Ridge on the Juan de Fuca Plate and the TAG hydrothermal area on the Mid-Atlantic Ridge were analyzed for selected noble metals including Au, Ir and Pd by radiochemical neutron activation analysis. The preliminary results indicate that gold contents may reach the ppm range although values in the neighborhood of 100 to 200 ppb are more typical. The platinum group elements (PGE) represented by Ir and Pd are typically less than 0.02 ppb and less than 2 ppb respectively. These abundances represent a significant enrichment of gold relative to the PGE in comparison with average noble metal abundances in mid-ocean ridge basalts (MORB). A partial explanation of this distinctive fractionation can be found in the concepts of sulfur-saturation of basic magma in mid-ocean ridge (MOR) settings, and the origin of MOR hydrothermal fluids. Experimental and petrological data suggest that MORBs are sulfur-saturated at the time of magma generation and that an immiscible sulfide component remains in the mantle residue. Hence, MORBs are noble metal-poor, particularly with respect to PGE. Consequently, black smoker fluids can be expected to reflect the low Ir and Pd contents of the rock column. The average Au content of MORB is 1.3 ppb, and so the rock column is not significantly enriched in Au. The generation of fluids which precipitate solids with 200 ppb Au is apparently dependent on highly efficient fluid chemistry to mobilize Au from the rock column, high Au solubility in seawater hydrothermal fluids and efficient precipitation mechanisms to coprecipitate Au on Fe, Zn and Cu sulfides. Significant differences in these parameters appear to be the ultimate cause of the strong Au-PGE fractionation in the MOR setting. It does not appear from the current data base that MOR hydrothermal fluids are significant contributors to the Ir enrichment seen in Cretaceous-Tertiary boundary

  9. The structure and dynamics of mid-ocean ridge hydrothermal systems.

    PubMed

    Coumou, D; Driesner, T; Heinrich, C A

    2008-09-26

    Sub-seafloor hydrothermal convection at mid-ocean ridges transfers 25% of the Earth's heat flux and can form massive sulfide ore deposits. Their three-dimensional (3D) structure and transient dynamics are uncertain. Using 3D numerical simulations, we demonstrated that convection cells self-organize into pipelike upflow zones surrounded by narrow zones of focused and relatively warm downflow. This configuration ensures optimal heat transfer and efficient metal leaching for ore-deposit formation. Simulated fluid-residence times are as short as 3 years. The concentric flow geometry results from nonlinearities in fluid properties, and this may influence the behavior of other fluid-flow systems in Earth's crust. PMID:18818356

  10. Seismic structure of the lithosphere beneath the ocean islands near the mid-oceanic ridges

    NASA Astrophysics Data System (ADS)

    Haldar, C.; Kumar, P.; Kumar, M. Ravi

    2013-10-01

    Deciphering the seismic character of the young lithosphere near the mid-oceanic ridges (MOR) is a challenging endeavor. In this study, we determine the seismic structure of the oceanic plate near the MORs, using the P-to-s conversions isolated from good quality data recorded at 5 broadband seismological stations situated on the ocean Islands in their vicinity. Estimates of the crustal and lithospheric thickness values from waveform modeling of the P receiver function stacks reveal that the crustal thickness varies between 6 and 8 km with the corresponding depths to the lithosphere asthenosphere boundary (LAB) varying between 43 and 68 km. However, the depth to the LAB at Macquire Island is intriguing in view of the observation of a thick (~ 87 km) lithosphere beneath a relatively young crust. At three other stations i.e., Ascension Island, Sao Jorge and Easter Island, we find evidence for an additional deeper low velocity layer probably related to the presence of a hotspot.

  11. The structure and dynamics of mid-ocean ridge hydrothermal systems.

    PubMed

    Coumou, D; Driesner, T; Heinrich, C A

    2008-09-26

    Sub-seafloor hydrothermal convection at mid-ocean ridges transfers 25% of the Earth's heat flux and can form massive sulfide ore deposits. Their three-dimensional (3D) structure and transient dynamics are uncertain. Using 3D numerical simulations, we demonstrated that convection cells self-organize into pipelike upflow zones surrounded by narrow zones of focused and relatively warm downflow. This configuration ensures optimal heat transfer and efficient metal leaching for ore-deposit formation. Simulated fluid-residence times are as short as 3 years. The concentric flow geometry results from nonlinearities in fluid properties, and this may influence the behavior of other fluid-flow systems in Earth's crust.

  12. Estimation of S, F, Br, Cl, and P Fluxes at Mid Ocean Ridges

    NASA Astrophysics Data System (ADS)

    Kagoshima, T.; Sano, Y.; Takahata, N.; Jung, J.; Amakawa, H.; Kumagai, H.

    2011-12-01

    It is known that superficial volatile elements have been accumulated by degassing from the solid Earth. Studies that use noble gases as tracers have been conducted to investigate the degassing history of the Earth, which suggests that the significant degassing occurred in the early Earth. C and N fluxes were well documented in literatures. In order to estimate the other fluxes of volatile elements from mantle, we measured S, F, Br, Cl, and P concentrations trapped in vesicles in mid-ocean ridge basalts (MORBs) and back-arc basin basalts (BABBs). Highly reactive elements such as sulfur and fluorine in the sample were extracted into the alkali solution at the crushing: crush leaching. We selected fresh glassy aliquots from MORBs and BABBs and put them in a crusher with an iron ball and 1-2 cm^{3} of 4M aqueous sodium hydroxide. Then, we crushed the samples with the solution kept frozen at the liquid nitrogen temperature to avoid the adhesion of the solution to the inside of the crusher. After crushing, we froze the solution again, and introduced volatiles in the gas phase into the purification vacuum line. At first, we removed volatile elements except He and Ne by a U-tube and activated charcoals at the temperature of liquid nitrogen and hot Ti-getters. We measured ^{4}He/^{20}Ne ratio of the gas by a quadrupole mass spectrometer and removed Ne by a cryogenic charcoal trap at 40K. Then, the He fraction was introduced into VG5400 mass spectrometer and the ^{3}He/^{4}He ratio was measured. Immediately after the sample analysis, we measured HESJ (He standard of Japan; 20.4 R_{A}) for calibration. For sulfur measurement, 1-2 cm^{3} of 30% hydrogen peroxide were added into the filtered sodium hydroxide solutions. Thus all sulfur compounds were quantified as sulfate ions and F, Br, Cl, and P were measured by ion chromatography (Dionex-320). We collected fine powders of basalt glass by filtering through a stainless steel sieve and measured their weight to estimate the

  13. Vapour undersaturation in primitive mid-ocean-ridge basalt and the volatile content of Earth's upper mantle.

    PubMed

    Saal, Alberto E; Hauri, Erik H; Langmuir, Charles H; Perfit, Michael R

    2002-10-01

    The analysis of volatiles in magmatic systems can be used to constrain the volatile content of the Earth's mantle and the influence that magmatic degassing has on the chemistry of the oceans and the atmosphere. But most volatile elements have very low solubilities in magmas at atmospheric pressure, and therefore virtually all erupted lavas are degassed and do not retain their primary volatile signatures. Here we report the undersaturated pre-eruptive volatile content for a suite of mid-ocean-ridge basalts from the Siqueiros intra-transform spreading centre. The undersaturation leads to correlations between volatiles and refractory trace elements that provide new constraints on volatile abundances and their behaviour in the upper mantle. Our data generate improved limits on the abundances of carbon dioxide, water, fluorine, sulphur and chlorine in the source of normal mid-ocean-ridge basalt. The incompatible behaviour of carbon dioxide, together with the CO(2)/Nb and CO(2)/Cl ratios, permit estimates of primitive carbon dioxide and chlorine to be made for degassed and chlorine-contaminated mid-ocean-ridge basalt magmas, and hence constrain degassing and contamination histories of mid-ocean ridges.

  14. Mid-Ocean Ridge Mantle Processes Constrained by the FAIM Seismic Refraction Experiment

    NASA Astrophysics Data System (ADS)

    Collins, J. A.; Lizarralde, D.; Gaherty, J. B.; Hirth, G.; Kim, S.

    2003-12-01

    The seismic structure (e.g. velocity, velocity gradients, anisotropy) of the shallow upper mantle of oceanic lithosphere constrains the degree of melt extraction at mid-ocean ridges and the shear deformation of the lithosphere. The variation of fine-scale structure with depth is difficult to resolve with teleseismic data but, with some notable exceptions, seismic refraction techniques - which have excellent depth resolution - have not been used to elucidate oceanic upper mantle structure. Here we report further results from the FAIM (Far-offset Active-source Imaging of the Mantle) seismic refraction experiment conducted along an 800-km-long flow-line transect and a 150-km-long ridge-parallel transect in the western north Atlantic. Sixteen short-period, vertical-component ocean-bottom seismometers (OBS) were deployed along the spreading-parallel transect in 3-km-separated pairs spaced 80-120 km apart, and 3 OBS were deployed orthogonal to the primary transect to a maximum distance of 350 km. Shot spacing was 1 km. Coherent P-wave arrivals were observed to 350-km distance, and the amplitude of this phase and its increasing and fast apparent-velocity suggest that energy is propagating to ˜24 km depth below the Moho. Two-dimensional travel-time analyses predict velocities of ˜8.25-8.55 km/s over the uppermost 24 km of the mantle on the flow-line-parallel transect and velocities of ˜8.05-8.25 km/s over the uppermost 10 km of the mantle on the ridge-parallel transect. At face value, these values indicate P-wave anisotropy is at least ˜3.0% throughout the upper 10 km of the mantle. However, velocities in only two orthogonal directions cannot constrain the precise direction and magnitude of anisotropy. To improve this estimate we analyzed the travel times of P-waves with good signal to noise from shots at ranges of 75-250 km, with an azimuth range spanning ˜100\\deg. When plotted as a function of propagation azimuth, these travel times display a strong variation, with

  15. The Depth of Detachment Faulting at Mid-Ocean Ridges : Evidence From Zircon Geo- and Thermochronometry

    NASA Astrophysics Data System (ADS)

    Grimes, C. B.; John, B. E.; Cheadle, M. J.; Reiners, P. W.; Wooden, J. L.

    2008-12-01

    Pb/U and (U-Th)/He zircon ages determined from evolved samples of gabbroic crust exposed in the footwalls of large-offset, low-angle normal faults near the Atlantis and Fifteen-Twenty Transforms on the Mid-Atlantic Ridge (MAR; ODP Holes 1275D and 1270D, IODP Hole U1309D), provide new constraints on the depth of detachment faulting at mid-ocean ridges. Ti-in-zircon crystallization temperatures, taken with the closure temperature of the (U-Th)/He system in zircon bracket the acquisition temperature of magnetic remanence; collectively these three chronometers define a cooling history for footwall gabbro sections over the temperature range of ~900°-220° C. Time-averaged cooling rates over 900°- 220° C from all holes investigated range from 1025(+645, -330)° C/m.y. to 2110(+1600, -720)° C/m.y. Assuming the gabbroic footwall was denuded along a single, continuous fault system, the time interval defined by the difference in Pb/U and (U-Th)/He ages for zircon from rocks beneath the fault can be used to estimate the distance between the 900° and 200° C isotherms along the fault system, and therefore the length-scale of the fault system while it was active, if the fault slip rate is known. As these large-offset faults serve as the plate boundary, the fault slip rate is equivalent to the plate-spreading rate during formation of the footwall. During formation of the Atlantis Massif core complex (30° N, MAR), accretion was asymmetric, with spreading partitioned on the North American plate at a rate approaching the full spreading rate of 24 mm/yr. This rate, along with a cooling time interval of 0.42±0.09 Ma implies that a single, continuous fault system would have had a length of 10±2.3 km between the 900° and 200° C isotherms while active. Lengths of fault systems determined at ODP Holes 1275D and 1270D are 9.5±3.2 km and 5.0±2.9 km, respectively, assuming a fault slip rate equivalent to 65% of the full plate spreading rate (consistent with asymmetric accretion

  16. The Arctic Mid-Ocean Ridge Expedition -AMORE 2001- Seafloor Spreading at the Top of the World

    NASA Astrophysics Data System (ADS)

    Michael, P. J.; Thiede, J.; Dick, H. J.; Goldstein, S. L.; Graham, D.; Jokat, W.; Langmuir, C. H.; Muhe, R.; Snow, J. E.

    2001-12-01

    From the end of July until early October, 2001 a remarkable expedition was undertaken to map and sample the submarine Gakkel Ridge and its surrounding basins. Preliminary results of this expedition are presented at this meeting. Gakkel Ridge extends 1800 km from north of Greenland to Laptev Sea, all of it beneath Arctic sea ice. It is the most remote and slowest spreading ridge (western end =1.6 cm/yr; eastern end=0.6 cm/yr) portion of the global mid-ocean ridge system. The Arctic Mid-Ocean Ridge Expedition (AMORE) was an international effort involving Germany's research icebreaker RV POLARSTERN and the new U.S. research icebreaker USCGC HEALY on its maiden scientific voyage. This historic and highly successful expedition fulfilled goals set forth by InterRidge in charting and sampling Gakkel Ridge. Over 130 sites along 1000 km of Gakkel Ridge were sampled for igneous rocks in permanently ice-covered waters. Distinctive geochemical trends and anomalies in basalts analyzed onboard show that the extent of mantle melting is low and varies along axis although not systematically with spreading rate. They also show systematic variations in source composition. Peridotites are less refractory and less altered than most other oceanic mantle peridotites. Surprisingly, the ships' bottom mapping sonar systems generated superb maps of the seafloor even while the ships were breaking ice. Although they cover a narrower region than SCICEX maps (Cochran et al., in prep.) they have better resolution and navigation and they cover the western part of the ridge. Geological features can be interpreted more confidently than with SCICEX data. There is an abrupt change in ridge depth, character and morphology at about 3 degr East. From 8 degr W to 3 degr E there are elongate constructional ridges which are nearly continuous and not offset from each other. From 3 degr E to 70 degr E the ridge has deep axial clefts that are separated by shallower, evenly-spaced volcanic saddles, sometimes

  17. Incorporation of seawater into mid-ocean ridge lava flows during emplacement

    USGS Publications Warehouse

    Soule, S.A.; Fornari, D.J.; Perfit, M.R.; Ridley, W.I.; Reed, M.H.; Cann, J.R.

    2006-01-01

    Evidence for the interaction between seawater and lava during emplacement on the deep seafloor can be observed in solidified flows at a variety of scales including rapid quenching of their outer crusts and the formation of lava pillars through the body of the flow. Recently, an additional interaction, incorporation of heated seawater (vapor) into the body of a flow, has been proposed. Large voids and vesicles beneath the surface crusts of mid-ocean ridge crest lobate and sheet lava flows and lava drips found within those cavities have been cited as evidence for this interaction. The voids resulting from this interaction contribute to the high porosity of the shallow ocean crust and play an important role in crustal permeability and hydrothermal circulation at mid-ocean ridges, and thus it is important to understand their origin. We analyze lava samples from the fast-spreading East Pacific Rise and intermediate-spreading Galapagos Spreading Center to characterize this process, identify the source of the vapor, and investigate the implications this would have on submarine lava flow dynamics. We find that lava samples that have interacted with a vapor have a zone of increased vesicularity on the underside of the lava crust and a coating of precipitate minerals (i.e., crystal fringe) that are distinct in form and composition from those crystallized from the melt. We use thermochemical modeling to simulate the reaction between the lava and a vapor and find that only with seawater can we reproduce the phase assemblage we observe within the crystal fringes present in the samples. Model results suggest that large-scale contamination of the lava by mass exchange with the vapor is unlikely, but we observe local enrichment of the lava in Cl resulting from the incorporation of a brine phase separated from the seawater. We suggest that high eruption rates are necessary for seawater incorporation to occur, but the mechanism by which seawater enters the flow has yet to be

  18. Does presence of a mid-ocean ridge enhance biomass and biodiversity?

    PubMed

    Priede, Imants G; Bergstad, Odd Aksel; Miller, Peter I; Vecchione, Michael; Gebruk, Andrey; Falkenhaug, Tone; Billett, David S M; Craig, Jessica; Dale, Andrew C; Shields, Mark A; Tilstone, Gavin H; Sutton, Tracey T; Gooday, Andrew J; Inall, Mark E; Jones, Daniel O B; Martinez-Vicente, Victor; Menezes, Gui M; Niedzielski, Tomasz; Sigurðsson, Þorsteinn; Rothe, Nina; Rogacheva, Antonina; Alt, Claudia H S; Brand, Timothy; Abell, Richard; Brierley, Andrew S; Cousins, Nicola J; Crockard, Deborah; Hoelzel, A Rus; Høines, Åge; Letessier, Tom B; Read, Jane F; Shimmield, Tracy; Cox, Martin J; Galbraith, John K; Gordon, John D M; Horton, Tammy; Neat, Francis; Lorance, Pascal

    2013-01-01

    In contrast to generally sparse biological communities in open-ocean settings, seamounts and ridges are perceived as areas of elevated productivity and biodiversity capable of supporting commercial fisheries. We investigated the origin of this apparent biological enhancement over a segment of the North Mid-Atlantic Ridge (MAR) using sonar, corers, trawls, traps, and a remotely operated vehicle to survey habitat, biomass, and biodiversity. Satellite remote sensing provided information on flow patterns, thermal fronts, and primary production, while sediment traps measured export flux during 2007-2010. The MAR, 3,704,404 km(2) in area, accounts for 44.7% lower bathyal habitat (800-3500 m depth) in the North Atlantic and is dominated by fine soft sediment substrate (95% of area) on a series of flat terraces with intervening slopes either side of the ridge axis contributing to habitat heterogeneity. The MAR fauna comprises mainly species known from continental margins with no evidence of greater biodiversity. Primary production and export flux over the MAR were not enhanced compared with a nearby reference station over the Porcupine Abyssal Plain. Biomasses of benthic macrofauna and megafauna were similar to global averages at the same depths totalling an estimated 258.9 kt C over the entire lower bathyal north MAR. A hypothetical flat plain at 3500 m depth in place of the MAR would contain 85.6 kt C, implying an increase of 173.3 kt C attributable to the presence of the Ridge. This is approximately equal to 167 kt C of estimated pelagic biomass displaced by the volume of the MAR. There is no enhancement of biological productivity over the MAR; oceanic bathypelagic species are replaced by benthic fauna otherwise unable to survive in the mid ocean. We propose that globally sea floor elevation has no effect on deep sea biomass; pelagic plus benthic biomass is constant within a given surface productivity regime. PMID:23658696

  19. Does presence of a mid-ocean ridge enhance biomass and biodiversity?

    PubMed

    Priede, Imants G; Bergstad, Odd Aksel; Miller, Peter I; Vecchione, Michael; Gebruk, Andrey; Falkenhaug, Tone; Billett, David S M; Craig, Jessica; Dale, Andrew C; Shields, Mark A; Tilstone, Gavin H; Sutton, Tracey T; Gooday, Andrew J; Inall, Mark E; Jones, Daniel O B; Martinez-Vicente, Victor; Menezes, Gui M; Niedzielski, Tomasz; Sigurðsson, Þorsteinn; Rothe, Nina; Rogacheva, Antonina; Alt, Claudia H S; Brand, Timothy; Abell, Richard; Brierley, Andrew S; Cousins, Nicola J; Crockard, Deborah; Hoelzel, A Rus; Høines, Åge; Letessier, Tom B; Read, Jane F; Shimmield, Tracy; Cox, Martin J; Galbraith, John K; Gordon, John D M; Horton, Tammy; Neat, Francis; Lorance, Pascal

    2013-01-01

    In contrast to generally sparse biological communities in open-ocean settings, seamounts and ridges are perceived as areas of elevated productivity and biodiversity capable of supporting commercial fisheries. We investigated the origin of this apparent biological enhancement over a segment of the North Mid-Atlantic Ridge (MAR) using sonar, corers, trawls, traps, and a remotely operated vehicle to survey habitat, biomass, and biodiversity. Satellite remote sensing provided information on flow patterns, thermal fronts, and primary production, while sediment traps measured export flux during 2007-2010. The MAR, 3,704,404 km(2) in area, accounts for 44.7% lower bathyal habitat (800-3500 m depth) in the North Atlantic and is dominated by fine soft sediment substrate (95% of area) on a series of flat terraces with intervening slopes either side of the ridge axis contributing to habitat heterogeneity. The MAR fauna comprises mainly species known from continental margins with no evidence of greater biodiversity. Primary production and export flux over the MAR were not enhanced compared with a nearby reference station over the Porcupine Abyssal Plain. Biomasses of benthic macrofauna and megafauna were similar to global averages at the same depths totalling an estimated 258.9 kt C over the entire lower bathyal north MAR. A hypothetical flat plain at 3500 m depth in place of the MAR would contain 85.6 kt C, implying an increase of 173.3 kt C attributable to the presence of the Ridge. This is approximately equal to 167 kt C of estimated pelagic biomass displaced by the volume of the MAR. There is no enhancement of biological productivity over the MAR; oceanic bathypelagic species are replaced by benthic fauna otherwise unable to survive in the mid ocean. We propose that globally sea floor elevation has no effect on deep sea biomass; pelagic plus benthic biomass is constant within a given surface productivity regime.

  20. Does Presence of a Mid-Ocean Ridge Enhance Biomass and Biodiversity?

    PubMed Central

    Priede, Imants G.; Bergstad, Odd Aksel; Miller, Peter I.; Vecchione, Michael; Gebruk, Andrey; Falkenhaug, Tone; Billett, David S. M.; Craig, Jessica; Dale, Andrew C.; Shields, Mark A.; Tilstone, Gavin H.; Sutton, Tracey T.; Gooday, Andrew J.; Inall, Mark E.; Jones, Daniel O. B.; Martinez-Vicente, Victor; Menezes, Gui M.; Niedzielski, Tomasz; Sigurðsson, Þorsteinn; Rothe, Nina; Rogacheva, Antonina; Alt, Claudia H. S.; Brand, Timothy; Abell, Richard; Brierley, Andrew S.; Cousins, Nicola J.; Crockard, Deborah; Hoelzel, A. Rus; Høines, Åge; Letessier, Tom B.; Read, Jane F.; Shimmield, Tracy; Cox, Martin J.; Galbraith, John K.; Gordon, John D. M.; Horton, Tammy; Neat, Francis; Lorance, Pascal

    2013-01-01

    In contrast to generally sparse biological communities in open-ocean settings, seamounts and ridges are perceived as areas of elevated productivity and biodiversity capable of supporting commercial fisheries. We investigated the origin of this apparent biological enhancement over a segment of the North Mid-Atlantic Ridge (MAR) using sonar, corers, trawls, traps, and a remotely operated vehicle to survey habitat, biomass, and biodiversity. Satellite remote sensing provided information on flow patterns, thermal fronts, and primary production, while sediment traps measured export flux during 2007–2010. The MAR, 3,704,404 km2 in area, accounts for 44.7% lower bathyal habitat (800–3500 m depth) in the North Atlantic and is dominated by fine soft sediment substrate (95% of area) on a series of flat terraces with intervening slopes either side of the ridge axis contributing to habitat heterogeneity. The MAR fauna comprises mainly species known from continental margins with no evidence of greater biodiversity. Primary production and export flux over the MAR were not enhanced compared with a nearby reference station over the Porcupine Abyssal Plain. Biomasses of benthic macrofauna and megafauna were similar to global averages at the same depths totalling an estimated 258.9 kt C over the entire lower bathyal north MAR. A hypothetical flat plain at 3500 m depth in place of the MAR would contain 85.6 kt C, implying an increase of 173.3 kt C attributable to the presence of the Ridge. This is approximately equal to 167 kt C of estimated pelagic biomass displaced by the volume of the MAR. There is no enhancement of biological productivity over the MAR; oceanic bathypelagic species are replaced by benthic fauna otherwise unable to survive in the mid ocean. We propose that globally sea floor elevation has no effect on deep sea biomass; pelagic plus benthic biomass is constant within a given surface productivity regime. PMID:23658696

  1. Grain-size dynamics beneath mid-ocean ridges: Implications for permeability and melt extraction

    PubMed Central

    Turner, Andrew J; Katz, Richard F; Behn, Mark D

    2015-01-01

    Grain size is an important control on mantle viscosity and permeability, but is difficult or impossible to measure in situ. We construct a two-dimensional, single phase model for the steady state mean grain size beneath a mid-ocean ridge. The mantle rheology is modeled as a composite of diffusion creep, dislocation creep, dislocation accommodated grain boundary sliding, and a plastic stress limiter. The mean grain size is calculated by the paleowattmeter relationship of Austin and Evans (2007). We investigate the sensitivity of our model to global variations in grain growth exponent, potential temperature, spreading-rate, and mantle hydration. We interpret the mean grain-size field in terms of its permeability to melt transport. The permeability structure due to mean grain size may be approximated as a high permeability region beneath a low permeability region. The transition between high and low permeability regions occurs across a boundary that is steeply inclined toward the ridge axis. We hypothesize that such a permeability structure generated from the variability of the mean grain size may focus melt toward the ridge axis, analogous to Sparks and Parmentier (1991)-type focusing. This focusing may, in turn, constrain the region where significant melt fractions are observed by seismic or magnetotelluric surveys. This interpretation of melt focusing via the grain-size permeability structure is consistent with MT observation of the asthenosphere beneath the East Pacific Rise. Key Points: The grain-size field beneath MORs can vary over orders of magnitude The grain-size field affects the rheology and permeability of the asthenosphere The grain-size field may focus melt toward the ridge axis PMID:26693211

  2. Vertical Seafloor Geodesy at Two Mid-ocean Ridge Sites: Recent Results and Lessons Learned (Invited)

    NASA Astrophysics Data System (ADS)

    Nooner, S. L.; Chadwick, B.; Webb, S. C.

    2013-12-01

    Precise measurements of ambient seawater pressure can be used as a proxy for seafloor depth and can be used to track vertical movements of the seafloor with time. We have employed two measurement techniques simultaneously to track both episodic and long-term deformation signals at active volcanic sites on mid-ocean ridges. The first technique is through the use of Bottom Pressure Recorders (BPRs), which are instruments that sit on the seafloor recording pressure continuously for 1-3 years until they are recovered for data download and battery replacement. BPRs are essential for measuring episodic events but suffer from slow instrument drift that is indistinguishable from long-term deformation. To track the long-term deformation signals and quantify drift in the BPRs, we developed a technique using ROV deployed Mobile Pressure Recorders (MPRs). In 2000 we began making MPR measurements on top of seafloor benchmarks at Axial Seamount on the Juan de Fuca Ridge after it's 1998 eruption. The combined BPR and MPR measurements have allowed us to observe and quantify an entire eruption cycle at the intermediate spreading Axial Seamount. From 2008-2011 we established another geodetic network at the fast spreading East Pacific Rise (EPR) at the site of a 2005/2006 eruption near 9°50' N. Here we show the results to date from both Axial Seamount and the EPR, and discuss lessons learned during the last 14 years. Measurements at Axial Seamount were all made using ROVs, while measurements at the EPR were made using the manned submersible Alvin in 2008 and 2009 and the Jason ROV in 2011. Our observations at Axial Seamount have enabled us to characterize its eruption cycle into 4 distinct phases: 1.) pre-eruption short-term rapid inflation, 2.) co-eruption deflation, 3.) rapid post-eruption reinflation, and 4.) subsequent long-term steady inflation. The transition between the phases 3 and 4 was not captured after the 1998 eruption and is an important impetus for continued

  3. Earthquake-induced changes in a hydrothermal system on the Juan de Fuca mid-ocean ridge

    PubMed

    Johnson; Hutnak; Dziak; Fox; Urcuyo; Cowen; Nabelek; Fisher

    2000-09-14

    Hydrothermal vents on mid-ocean ridges of the northeast Pacific Ocean are known to respond to seismic disturbances, with observed changes in vent temperature. But these disturbances resulted from submarine volcanic activity; until now, there have been no observations of the response of a vent system to non-magmatic, tectonic events. Here we report measurements of hydrothermal vent temperature from several vents on the Juan de Fuca ridge in June 1999, before, during and after an earthquake swarm of apparent tectonic origin. Vent fluid temperatures began to rise 4-11 days after the first earthquake. Following this initial increase, the vent temperatures oscillated for about a month before settling down to higher values. We also observed a tenfold increase in fluid output from the hydrothermal system over a period of at least 80 days, extending along the entire ridge segment. Such a large, segment-wide thermal response to relatively modest tectonic activity is surprising, and raises questions about the sources of excess heat and fluid, and the possible effect on vent biological communities.

  4. Melting and Reactive Flow of Carbonated Peridotite Beneath Mid-Ocean Ridges

    NASA Astrophysics Data System (ADS)

    Keller, T.; Katz, R. F.

    2015-12-01

    The mantle carbon reservoir is four orders of magnitude more massive than that of the atmosphere and ocean combined. The behaviour of carbon in the mantle, especially its transport and extraction, is thus of crucial importance to understanding the coupling between the deep interior and the surface environment of Earth. Laboratory experiments indicate that even small concentrations of carbon dioxide (and other volatiles like H2O) in the upper mantle significantly affect silicate melting [HK96,DH06] by stabilising carbon-rich melt at high pressure. The presence of carbon in the mantle substantially extends the region where partial melt is stable and has important consequences for the dynamics of magma transport and chemical differentiation [H10,DH10]. We have developed theory and numerical implementation to simulate thermo-chemically coupled magma/mantle dynamics in terms of a two-phase (rock+melt), three component (dunite+MORB+carbonated MORB) physical model. The fluid dynamics is based on McKenzie's equations [McK84]. The thermo-chemical formulation of the system is represented by a novel, disequilibrium, multi-component melting model based on thermodynamic theory [RBS11]. This physical model is implemented as a parallel, two-dimensional, finite-volume code that leverages tools from the PETSc toolkit. First results show that carbon and other volatiles cause a qualitative difference to the style of melt transport, potentially enhancing its extraction efficiency - measured in the carbon mass flux arriving at the mid-ocean ridge axis - by at least an order of magnitude. The process that controls magma transport in our models is a volatile flux-induced reactive infiltration instability, causing carbonated melt to rise from depth in localized channels. These results add to our understanding of melt formation and transport at mid-ocean ridges (the most important magmatic system in the mantle) and may have important implications for subduction zones. REFERENCESHK96 Hirth

  5. Using submarine lava pillars to record mid-ocean ridge eruption dynamics

    USGS Publications Warehouse

    Gregg, Tracy K.P.; Fornari, Daniel J.; Perfit, Michael R.; Ridley, W. Ian; Kurz, Mark D.

    2000-01-01

    Submarine lava pillars are hollow, glass-lined, basaltic cylinders that occur at the axis of the mid-ocean ridge, and within the summit calderas of some seamounts. Typically, pillars are ~1-20 m tall and 0.25-2.0 m in diameter, with subhorizontal to horizontal glassy selvages on their exterior walls. Lava pillars form gradually during a single eruption, and are composed of lava emplaced at the eruption onset as well as the last lava remaining after the lava pond has drained. On the deep sea floor, the surface of a basaltic lava flow quenches to glass within 1 s, thereby preserving information about eruption dynamics, as well as chemical and physical properties of lava within a single eruption. Investigation of different lava pillars collected from a single eruption allows us to distinguish surficial lava-pond or lava-lake geochemical processes from those operating in the magma chamber. Morphologic, major-element, petrographic and helium analyses were performed on portions of three lava pillars formed during the April 1991 eruption near 9°50'N at the axis of the East Pacific Rise. Modeling results indicate that the collected portions of pillars formed in ~2-5 h, suggesting a total eruption duration of ~8-20 h. These values are consistent with observed homogeneity in the glass helium concentrations and helium diffusion rates. Major-element compositions of most pillar glasses are homogeneous and identical to the 1991 flow, but slight chemical variations measured in the outermost portions of some pillars may reflect post-eruptive processes rather than those occurring in subaxial magma bodies. Because lava pillars are common at mid-ocean ridges (MORs), the concepts and techniques we present here may have important application to the study of MOR eruptions, thereby providing a basis for quantitative comparisons of volcanic eruptions in geographically and tectonically diverse settings. More research is needed to thoroughly test the hypotheses presented here. (C) 2000

  6. A Global Comparison of Mid-Ocean Ridge Processes from Seismic Anisotropy

    NASA Astrophysics Data System (ADS)

    Eakin, C. M.; Rychert, C.; Harmon, N.

    2015-12-01

    As oceanic plates form it is thought that they will inherit an internal anisotropic fabric due to basal shear, developing a fast seismic axis parallel with the seafloor spreading direction. Studying this process in-situ however has proven difficult given the lack of seismic instrumentation over the oceans. In this study we instead utilise the worldwide distribution of seismicity along mid-ocean ridges (MOR), as well as the source-side shear wave splitting technique, to characterise seismic anisotropy and therefore upper mantle dynamics beneath MOR earthquakes. To achieve this we collate over 80 "null" seismic stations distributed globally that appear lie above effectively isotropic upper mantles. These then allow us to then isolate the anisotropic signature beneath the earthquake or source, instead of beneath the receiver, by using direct teleseismic S phases. Globally on average we find that around 40% of measurements from ridge earthquakes produce splitting of the shear wave, with the other 60% recording a null result (i.e. lack of splitting), likely reflecting complex processes happening beneath MORs. Of the shear wave splitting seen, the magnitude is moderate (delay times average 1.2s), suggesting considerable seismic anisotropy beneath some MOR locales. The distribution of nulls versus splits is not homogeneous however, with localised pockets of coherent nulls or coherent splits with similar fast directions. The seismic properties appear to change abruptly over relatively short distances, such as across adjacent transform faults, or on different limbs of a triple junction, suggesting local ridge processes control the anisotropy rather than the larger scale mantle flow.

  7. Energy landscapes shape microbial communities in hydrothermal systems on the Arctic Mid-Ocean Ridge.

    PubMed

    Dahle, Håkon; Økland, Ingeborg; Thorseth, Ingunn H; Pederesen, Rolf B; Steen, Ida H

    2015-07-01

    Methods developed in geochemical modelling combined with recent advances in molecular microbial ecology provide new opportunities to explore how microbial communities are shaped by their chemical surroundings. Here, we present a framework for analyses of how chemical energy availability shape chemotrophic microbial communities in hydrothermal systems through an investigation of two geochemically different basalt-hosted hydrothermal systems on the Arctic Mid-Ocean Ridge: the Soria Moria Vent field (SMVF) and the Loki's Castle Vent Field (LCVF). Chemical energy landscapes were evaluated through modelling of the Gibbs energy from selected redox reactions under different mixing ratios between seawater and hydrothermal fluids. Our models indicate that the sediment-influenced LCVF has a much higher potential for both anaerobic and aerobic methane oxidation, as well as aerobic ammonium and hydrogen oxidation, than the SMVF. The modelled energy landscapes were used to develop microbial community composition models, which were compared with community compositions in environmental samples inside or on the exterior of hydrothermal chimneys, as assessed by pyrosequencing of partial 16S rRNA genes. We show that modelled microbial communities based solely on thermodynamic considerations can have a high predictive power and provide a framework for analyses of the link between energy availability and microbial community composition. PMID:25575309

  8. Energy landscapes shape microbial communities in hydrothermal systems on the Arctic Mid-Ocean Ridge

    PubMed Central

    Dahle, Håkon; Økland, Ingeborg; Thorseth, Ingunn H; Pederesen, Rolf B; Steen, Ida H

    2015-01-01

    Methods developed in geochemical modelling combined with recent advances in molecular microbial ecology provide new opportunities to explore how microbial communities are shaped by their chemical surroundings. Here, we present a framework for analyses of how chemical energy availability shape chemotrophic microbial communities in hydrothermal systems through an investigation of two geochemically different basalt-hosted hydrothermal systems on the Arctic Mid-Ocean Ridge: the Soria Moria Vent field (SMVF) and the Loki's Castle Vent Field (LCVF). Chemical energy landscapes were evaluated through modelling of the Gibbs energy from selected redox reactions under different mixing ratios between seawater and hydrothermal fluids. Our models indicate that the sediment-influenced LCVF has a much higher potential for both anaerobic and aerobic methane oxidation, as well as aerobic ammonium and hydrogen oxidation, than the SMVF. The modelled energy landscapes were used to develop microbial community composition models, which were compared with community compositions in environmental samples inside or on the exterior of hydrothermal chimneys, as assessed by pyrosequencing of partial 16S rRNA genes. We show that modelled microbial communities based solely on thermodynamic considerations can have a high predictive power and provide a framework for analyses of the link between energy availability and microbial community composition. PMID:25575309

  9. Energy landscapes shape microbial communities in hydrothermal systems on the Arctic Mid-Ocean Ridge.

    PubMed

    Dahle, Håkon; Økland, Ingeborg; Thorseth, Ingunn H; Pederesen, Rolf B; Steen, Ida H

    2015-07-01

    Methods developed in geochemical modelling combined with recent advances in molecular microbial ecology provide new opportunities to explore how microbial communities are shaped by their chemical surroundings. Here, we present a framework for analyses of how chemical energy availability shape chemotrophic microbial communities in hydrothermal systems through an investigation of two geochemically different basalt-hosted hydrothermal systems on the Arctic Mid-Ocean Ridge: the Soria Moria Vent field (SMVF) and the Loki's Castle Vent Field (LCVF). Chemical energy landscapes were evaluated through modelling of the Gibbs energy from selected redox reactions under different mixing ratios between seawater and hydrothermal fluids. Our models indicate that the sediment-influenced LCVF has a much higher potential for both anaerobic and aerobic methane oxidation, as well as aerobic ammonium and hydrogen oxidation, than the SMVF. The modelled energy landscapes were used to develop microbial community composition models, which were compared with community compositions in environmental samples inside or on the exterior of hydrothermal chimneys, as assessed by pyrosequencing of partial 16S rRNA genes. We show that modelled microbial communities based solely on thermodynamic considerations can have a high predictive power and provide a framework for analyses of the link between energy availability and microbial community composition.

  10. Elastic thickness control of lateral dyke intrusion at mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Grandin, Raphaël; Socquet, Anne; Doubre, Cécile; Jacques, Eric; King, Geoffrey C. P.

    2012-02-01

    Magmatic accretion at slow-spreading mid-ocean ridges exhibits specific features. Although magma supply is focused at the centre of second-order segments, melts are episodically distributed along the rift toward segment ends by lateral dyke intrusions. It has been previously suggested that an along-axis downward topographic slope away from the magma source is sufficient to explain lateral dyke propagation. However, this cannot account for the poor correlation between dyke opening and surface elevation in the 2005-2010 series of 14 dyke intrusions of Afar (Ethiopia). Using mechanical arguments, constrained by both geodetic and seismological observations, we propose that the large dykes that initiate near the mid-segment magma source are attracted toward segment ends as a result of a thickening of the elastic-brittle lithosphere in the along-rift direction. This attraction arises from the difference of elastic resistance between the segment centre where the lithosphere is thermally weakened by long-term focusing of melts, and comparatively "colder", hence stronger segment ends. The axial topographic gradient in magmatic rifts may be more likely explained as an incidental consequence of these variations of along-axis elastic-brittle thickness, rather than the primary cause of lateral dyke injections.

  11. Interaction of sea water and lava during submarine eruptions at mid-ocean ridges

    USGS Publications Warehouse

    Perfit, M.R.; Cann, J.R.; Fornari, D.J.; Engels, J.; Smith, D.K.; Ridley, W.I.; Edwards, M.H.

    2003-01-01

    Lava erupts into cold sea water on the ocean floor at mid-ocean ridges (at depths of 2,500 m and greater), and the resulting flows make up the upper part of the global oceanic crust. Interactions between heated sea water and molten basaltic lava could exert significant control on the dynamics of lava flows and on their chemistry. But it has been thought that heating sea water at pressures of several hundred bars cannot produce significant amounts of vapour and that a thick crust of chilled glass on the exterior of lava flows minimizes the interaction of lava with sea water. Here we present evidence to the contrary, and show that bubbles of vaporized sea water often rise through the base of lava flows and collect beneath the chilled upper crust. These bubbles of steam at magmatic temperatures may interact both chemically and physically with flowing lava, which could influence our understanding of deep-sea volcanic processes and oceanic crustal construction more generally. We infer that vapour formation plays an important role in creating the collapse features that characterize much of the upper oceanic crust and may accordingly contribute to the measured low seismic velocities in this layer.

  12. Interaction of sea water and lava during submarine eruptions at mid-ocean ridges.

    PubMed

    Perfit, Michael R; Cann, Johnson R; Fornari, Daniel J; Engels, Jennifer; Smith, Deborah K; Ridley, W Ian; Edwards, Margo H

    2003-11-01

    Lava erupts into cold sea water on the ocean floor at mid-ocean ridges (at depths of 2,500 m and greater), and the resulting flows make up the upper part of the global oceanic crust. Interactions between heated sea water and molten basaltic lava could exert significant control on the dynamics of lava flows and on their chemistry. But it has been thought that heating sea water at pressures of several hundred bars cannot produce significant amounts of vapour and that a thick crust of chilled glass on the exterior of lava flows minimizes the interaction of lava with sea water. Here we present evidence to the contrary, and show that bubbles of vaporized sea water often rise through the base of lava flows and collect beneath the chilled upper crust. These bubbles of steam at magmatic temperatures may interact both chemically and physically with flowing lava, which could influence our understanding of deep-sea volcanic processes and oceanic crustal construction more generally. We infer that vapour formation plays an important role in creating the collapse features that characterize much of the upper oceanic crust and may accordingly contribute to the measured low seismic velocities in this layer.

  13. Geochemistry of Andaman Ophiolite: Evidence for a Mid-Oceanic Ridge origin

    NASA Astrophysics Data System (ADS)

    Bhattacharya, S.; Ray, J. S.

    2015-12-01

    "Ophiolite conundrum" deals with the unresolved puzzle of the origin of ophiolite, whether they have primarily formed in mid-oceanic ridge (MOR) or supra subduction zone (SSZ) settings. This attracts considerable debate because most of the ophiolites are located along the present day suture zones. The Andaman Ophiolite is a Cretaceous complex that occurs on the Andaman accretionary prism of the Indian-Eurasian convergent plate boundary. Here, we present whole rock trace element and isotope (Sr-Nd) data from the crustal section of the ophiolite comprising of pillow basalt, basalt, dolerite and gabbro from Rutland Island, south and middle Andaman Islands. Trace element patterns for a majority of our samples show N-MORB affinity barring enrichments in Rb and Ba, which could be attributed to secondary alteration. Comparison of trace element patterns and isotopic compositions of these rocks (eNd (t = 95 Ma) = 7.5 to 9.4; 87Sr/86Sri = 0.703 to 0.704) with that of the 127- 64 Ma Indian Ocean MORB suggest strong similarities thus, implying that a large section of Andaman Ophiolite represents the subducting Indian Oceanic Plate. If so, then this ophiolite complex possibly represents an obducted Indian oceanic lithosphere that formed at the Indian Ocean MOR.

  14. Helium, heat, and the generation of hydrothermal event plumes at mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Lupton, John E.; Baker, Edward T.; Massoth, Gary J.

    1999-09-01

    Hydrothermal event plumes are unique water-column features observed over mid-ocean ridges, presumably generated by the sudden release of large volumes of hot, buoyant fluid. Although the specifics of event plume generation are unknown, event plumes have been attributed to the rapid emptying of a hydrothermal reservoir or to rapid heat extraction from a recently emplaced dike or seafloor lava flows. The chemical and thermal signatures of event plumes as compared to the underlying steady-state plumes offer important clues to the generation of event plumes. Event plumes have low 3He/heat ratios of ˜0.4 × 10-17 mol J-1, similar to vent fluids from mature hydrothermal systems. In contrast, the steady-state plumes found beneath the event plumes have elevated and variable 3He/heat ratios of 2 to 5 × 10-17 mol J-1. Fluids collected directly over fresh lava flows have even higher 3He/heat ratios of 2 to 8 × 10-17 mol J-1, up to 30 times the event plume values. These disparate 3He/heat ratios place strong constraints on models of event plume generation, especially models which rely on heat extraction from seafloor eruptions. Published by Elsevier Science B.V.

  15. A novel microbial habitat in the mid-ocean ridge subseafloor

    PubMed Central

    Summit, Melanie; Baross, John A.

    2001-01-01

    The subseafloor at the mid-ocean ridge is predicted to be an excellent microbial habitat, because there is abundant space, fluid flow, and geochemical energy in the porous, hydrothermally influenced oceanic crust. These characteristics also make it a good analog for potential subsurface extraterrestrial habitats. Subseafloor environments created by the mixing of hot hydrothermal fluids and seawater are predicted to be particularly energy-rich, and hyperthermophilic microorganisms that broadly reflect such predictions are ejected from these systems in low-temperature (≈15°C), basalt-hosted diffuse effluents. Seven hyperthermophilic heterotrophs isolated from low-temperature diffuse fluids exiting the basaltic crust in and near two hydrothermal vent fields on the Endeavour Segment, Juan de Fuca Ridge, were compared phylogenetically and physiologically to six similarly enriched hyperthermophiles from samples associated with seafloor metal sulfide structures. The 13 organisms fell into four distinct groups: one group of two organisms corresponding to the genus Pyrococcus and three groups corresponding to the genus Thermococcus. Of these three groups, one was composed solely of sulfide-derived organisms, and the other two related groups were composed of subseafloor organisms. There was no evidence of restricted exchange of organisms between sulfide and subseafloor habitats, and therefore this phylogenetic distinction indicates a selective force operating between the two habitats. Hypotheses regarding the habitat differences were generated through comparison of the physiology of the two groups of hyperthermophiles; some potential differences between these habitats include fluid flow stability, metal ion concentrations, and sources of complex organic matter. PMID:11226209

  16. A Climactic Feedback? Variations in Mid-Ocean Ridge CO2 Emissions Driven by Glacial Cycles

    NASA Astrophysics Data System (ADS)

    Burley, J. M.; Katz, R. F.; Huybers, P. J.

    2015-12-01

    Changes in sea level associated with glacial cycles affect the pressure beneath a mid-ocean ridge (MOR) [1,2,3]. Pressure controls the depth of first melting, and therefore the rate of change of pressure controls the rate of change of the depth of first melting. The changing depth of first melting alters the effective rate at which mantle, and thus CO2, enters the melting region. Melt then transports CO2 to the ridge axis, where it enters the climate system. We calculate that the lag between sea level change and consequent variation in MOR CO2 emissions is 40-120 kyrs[4], similar to the timescale of glacial cycles. Could these variations in MOR CO2 emissions feed back on climate and lead to ice-age pacing at a small multiple of the obliquity period? [5]To test this hypothesis we begin with a climate model comprised of a global energy balance and a 1D ice sheet. The ice sheet flows under its own weight, accumulates due to precipitation, and melts in response to the local energy balance[6]. This model broadly replicates Early Pleistocene 40 kyr glacial cycles. We extend the model to include a variable greenhouse effect, according to atmospheric CO2, and variable MOR CO2 emissions driven by sea level. The lag between sea level change and MOR CO2 emissions is controlled by mantle permeability. If this model does not demonstrate MOR CO2 emissions altering glacial cycles, it would suggest this hypothesised feedback mechanism can be rejected. References[1] Huybers & Langmuir 2009; 10.1016/j.epsl.2009.07.014[2] Lund & Asimow 2011; 10.1029/2011GC003693[3] Crowley et al 2015; 10.1126/science.1261508[4] Burley & Katz 2015; 10.1016/j.epsl.2015.06.031[5] Huybers (in prep.)[6] Huybers & Tziperman 2008; 10.1029/2007PA001463

  17. Variations in Mid-Ocean Ridge CO2 Emissions Driven By Glacial Cycles

    NASA Astrophysics Data System (ADS)

    Burley, J. M.; Katz, R. F.; Huybers, P. J.

    2014-12-01

    Glacial cycles impact continental volcanism through pressure changes associated with growth and retreat of ice sheets [e.g. Iceland - Jull, 1996]. Similarly, changes in sea level accompanying glacial cycles modulate mid-ocean ridge (MOR) volcanism by pressure changes and their influence on melt production [Crowley 2014; Lund 2011; Huybers 2009]. CO2 transport through the upper mantle is sensitive to mantle melting because CO2 partitions completely into the melt phase when present. Melt then transports CO2 to the ridge axis, where it enters the climate system. We present models of CO2 transport that investigate how sea level modulates the rate of CO2 emission from MORs. The total carbon reservoir in the mantle is circa 10^7 GtC [Dasgupta 2010], orders of magnitude more than the oceans (40,000 GtC) and atmosphere (600 GtC). Changes in the rate of CO2 emission from the solid Earth therefore have the potential to significantly affect the surface carbon system. We have developed an analytical model of CO2 transport from the depth of first silicate melting (~60km) to the ridge axis, enabling a calculation of CO2 emission rate for a generic section of MOR. The model assumes homogeneous mantle and energy-conserving melt production from a simplified 2-component mantle; CO2 is taken as a perfectly incompatible trace element. Pressure variations modulate the depth of initial silicate melting and hence the flux of CO2 into the melting regime. The model can also be applied to any species that is strongly partitioned into the melt (eg. Uranium, Thorium, Niobium, Barium, Rubidium). Results suggest that changing sea level over the past Myr could have altered the CO2 emissions from MOR by ~8%. The magnitude of variation in emissions is sensitive to the mantle permeability, the ridge spreading rate, and the rate of change of sea level. The travel time of melt through the mantle causes a delay between sea-level change and the CO2 response of the MOR. This delay is sensitive to plate

  18. Flow rate perturbations in a black smoker hydrothermal vent in response to a mid-ocean ridge earthquake swarm

    NASA Astrophysics Data System (ADS)

    Crone, Timothy J.; Wilcock, William S. D.; McDuff, Russell E.

    2010-03-01

    Although there is indirect evidence for strong connections between tectonic processes and mid-ocean ridge hydrothermal flow, there are no direct observations of these links, primarily because measuring flow in these systems is difficult. Here we use an optical analysis technique to obtain a 44 day record of flow rate changes in a black smoker vent in the Main Endeavour field of the Juan de Fuca Ridge. We show that variations in the flow rate coincide with an earthquake swarm observed using an ocean bottom seismometer array. These observations indicate that connections between tectonics and flow are indeed strong, that hydraulic connections within this hydrothermal system are long ranging, and that enhanced tidal pumping of fluids may be initiated by earthquake activity. Because the effects of the swarm cross over an intervening vent field, we infer that the upflow zones feeding this field are narrow. Using the time lag between the swarm onset and the flow rate changes we estimate that the bulk permeability of the crust on the Endeavour segment ranges from 3.0 × 10-13 m2 to 6.0 × 10-12 m2.

  19. Thermal response of mid-ocean ridge hydrothermal systems to perturbations

    NASA Astrophysics Data System (ADS)

    Singh, Shreya; Lowell, Robert P.

    2015-11-01

    Mid-ocean ridges are subject to episodic disturbances in the form of magmatic intrusions and earthquakes. Following these events, the temperature of associated hydrothermal vent fluids is observed to increase within a few days. In this paper, we aim to understand the rapid thermal response of hydrothermal systems to such disturbances. We construct a classic single-pass numerical model and use the examples of the 1995 and 1999 non-eruptive events at East Pacific Rise (EPR) 9°50‧N and Main Endeavour Field (MEF), respectively. We model both the thermal effects of dikes and permeability changes that might be attributed to diking and/or earthquake swarms. We find that the rapid response of vent temperatures results from steep thermal gradients close to the surface. When the perturbations are accompanied by an increase in permeability, the response on the surface is further enhanced. For EPR9°50‧N, the observed ~7 °C rise can be obtained for a ~50% increase in permeability in the diking zone. The mass flow rate increases as a result of change in permeability deeper in the system, and, therefore, the amount of hot fluid in the diffused flow also increases. Using a thermal energy balance, we show that the ~10 °C increase in diffuse flow temperatures recorded for MEF after the 1999 event may result from a 3-4 times increase in permeability. The rapid thermal response of the system resulting from a change in permeability also occurs for cases in which there is no additional heat input, indicating that hydrothermal systems may respond similarly to purely seismic and non-eruptive magmatic events.

  20. The Thermal Response of Mid-Ocean Ridge Hydrothermal Systems to Perturbations

    NASA Astrophysics Data System (ADS)

    Singh, S.; Lowell, R. P.

    2014-12-01

    Mid-ocean ridges are subject to episodic disturbances in the form of magmatic intrusions and earthquakes. Following these events, the temperature of associated hydrothermal vent fluids is observed to increase within a few days. In this paper, we aim to understand the rapid thermal response of hydrothermal systems to such disturbances. We construct a classic single-pass numerical model and use the examples of the 1995 and 1999 non-eruptive events at East Pacific Rise 9⁰50' N and Main Endeavour Field, respectively. We model both the thermal effects of dikes and permeability changes that might be attributed to diking and/or earthquake swarms. We find that the rapid response of vent temperatures results from steep thermal gradients close to the surface. When the perturbations are accompanied by an increase in permeability, the response on the surface is enhanced further. For East Pacific Rise 9⁰50' N, the observed ~7°C rise can be obtained for a ~ 50% increase in permeability in the diking zone. The mass flow rate increases as a result of change in permeability deeper in the system, and, therefore, the amount of hot fluid in the diffused flow also increases. Using a thermal energy balance, we show that the ~ 10 ⁰C increase in diffuse flow temperatures recorded for MEF after the 1999 event may result from a 3-4 times increase in permeability. The rapid thermal response of the system resulting from a change in permeability also occurs for cases in which there is no additional heat input, indicating that hydrothermal systems may respond similarly to purely seismic and non-eruptive magmatic events.

  1. Incorporation of seawater into mid-ocean ridge lava flows during emplacement

    NASA Astrophysics Data System (ADS)

    Soule, S. A.; Fornari, D. J.; Perfit, M.; Cann, J. R.; Montési, L.; Ridley, W. I.

    2005-12-01

    Mid-ocean ridge (MOR) lava flows erupted at ~2000-3500 m below sea level contain large, cm- to m-sized cavities close to the upper surfaces of the flows. Within these cavities, lava drips (similar to lava stalactites found in terrestrial lava tubes) hang from the base of the upper crust. In order for these features to form, the cavities must exist as vapor-filled open space under hydrostatic pressures up to 35 MPa, and must maintain near-magmatic temperatures to attain the observed morphology of the drips. We have examined numerous lobate and sheet lava crusts from the fast-spreading East Pacific Rise in order to constrain the nature of the vapor that filled the cavities beneath these crusts, the extent of the interaction between the vapor and the liquid lava, and the impact of the vapor phase on flow emplacement. On the inner surfaces of large vesicles and cavities within the flow are mineral phases that appear to have grown at the interface between the liquid lava and vapor. These mineral phases have compositions distinct from those found in the flow interior (e.g., pure albite and forsterite), and show mineral growth habits indicative of undercooling of the lava. Preliminary thermodynamic modeling of the gas-liquid-solid interaction suggests that vapor derived from heated seawater is capable of producing the observed mineral compositions. In addition, we have found that quenched glass adjacent to analyzed vesicles can be enriched in Cl by an order of magnitude over ambient glass within the same sample suggesting that local mass exchange between the seawater and the lava has occurred. Here we present results of these analyses and accompanying thermodynamic modeling that lead us to conclude that the vapor filling the cavities is derived directly from seawater. In addition, we present numerical models investigating mechanisms for seawater incorporation and its impact on flow emplacement.

  2. Serpentinization Rates at Slow-Spreading Mid-Ocean Ridges: From Sample Scale to Plate-Boundary

    NASA Astrophysics Data System (ADS)

    Cannat, M.; Rouméjon, S.

    2015-12-01

    About 25% of the crust formed at slow spreading mid-ocean ridges includes a component of tectonically exhumed and partially serpentinized mantle-derived peridotites. Exhumation occurs along large offset normal faults (detachment faults). In this presentation we outline a conceptual and testable model for serpentinization at slow-spreading ridges, based on petrological observations of samples of variably serpentinized peridotites from the Mid Atlantic and Southwest Indian ridges, on tectonic and geophysical data and on current interpretations of plate-boundary processes at slow spreading ridges. Serpentinization at mid-ocean ridges is a heterogeneous and multistage process occurring along fractures, in the footwall of axial detachments. The initial and most pervasive stage of serpentinization results in the typical serpentine mesh texture. We propose that it occurs when hydrothermal fluids reach extensively microfractured fresh peridotite. Subsequent stages of serpentinization appear to involve higher fluid-rock ratio along larger fractures and veins that are spaced by at least a few decimeters. Experimental data indicate that each stage of serpentinization identified in a given sample may have occurred at very high rates relative to the rates of tectonic exhumation. However, serpentinization (initial and later stages) may be a protracted process at the km-scale, associated with complex fracturation of the detachment's footwall due to combined tectonic and reaction-induced stresses. We outline possible consequences of this conceptual model in terms of crustal structure, hydrogen production and of the relations between serpentinization and black smoker-type hydrothermal circulation at slow-spreading ridges.

  3. Insights on the Nature of the Geophysical Crust at a Melt-starved Slow-spreading Mid-oceanic Ridge

    NASA Astrophysics Data System (ADS)

    Momoh, E. I.; Cannat, M.; Leroy, S. D.; Watremez, L.; Singh, S. C.

    2015-12-01

    Of the 7200 km long very slow-spreading (14 mm/yr) Southwest Indian ridge (SWIR), which forms the plate boundary between Africa and the Antarctic, the easternmost portion represents an endmember of the global mid-ocean ridge system in terms of low melt supply. It is characterized by 30 to 70 km-wide corridors of nearly amagmatic spreading, separating domains of focused volcanic activity. Our study concerns one of these amagmatic corridors. Volcanic features that typically characterize other ridge systems are scarce and the seafloor is mostly made of exhumed mantle-derived sepentinized periodotites. Long-offset detachment faults are proposed to be responsible for exhumation, and provide the context for serpentinization. In this poster presentation, we use seismic constraints to discuss the kilometer-scale characteristics of the geophysical crust formed at the melt-poor divgergent plate boundaries. To supply these constraints, we use seismic data acquired during the recent Sismosmooth cruise (2014; E64o20' to E65o). We show seismic reflection results from three 2D coincident across-axis lines, and two 2D axis-parallel lines processed in a self-similar manner and merged to improve the information content. From these, we show evidence for the expression of the active detachment fault responsible for mantle exhumation. This fault can be followed to ~ 7.7 km at depth. To give an insight to the nature of the subsurface, we examine tomography results from wide-angle seismic refraction data, which suggests a thin geophysical crust. This is similar to models of geophysical crust proposed from gravity studies in the study area.

  4. Extreme incompatibility of helium during mantle melting: Evidence from undegassed mid-ocean ridge basalts

    NASA Astrophysics Data System (ADS)

    Graham, David W.; Michael, Peter J.; Shea, Thomas

    2016-11-01

    We report total helium concentrations (vesicles + glass) for a suite of thirteen ultradepleted mid-ocean ridge basalts (UD-MORBs) that were previously studied for volatile contents (CO2, H2O) plus major and trace elements. The selected basalts are undersaturated in CO2 + H2O at their depths of eruption and represent rare cases of undegassed MORBs. Sample localities from the Atlantic (2), Indian (1) and Pacific (7) Oceans collectively show excellent linear correlations (r2 = 0.75- 0.92) between the concentrations of helium and the highly incompatible elements C, K, Rb, Ba, Nb, Th and U. Three basalts from Gakkel Ridge in the Arctic were also studied but show anomalous behavior marked by excess lithophile trace element abundances. In the Atlantic-Pacific-Indian suite, incompatible element concentrations vary by factors of 3-4.3, while helium concentration varies by a factor of 13. The strong correlations between the concentrations of helium and incompatible elements are explained by helium behavior as the most incompatible element during mantle melting. Partial melting of an ultradepleted mantle source, formed as a residue of earlier melt extraction, accounts for the observed concentrations. The earlier melting event involved removal of a small degree melt (∼1%) at low but non-zero porosity (0.01-0.5%), leading to a small amount of melt retention that strongly leveraged the incompatible element budget of the ultradepleted mantle source. Equilibrium melting models that produce the range of trace element and helium concentrations from this source require a bulk solid/melt distribution coefficient for helium that is lower than that for other incompatible elements by about a factor of ten. Alternatively, the bulk solid/melt distribution coefficient for helium could be similar to or even larger than that for other incompatible elements, but the much larger diffusivity of helium in peridotite leads to its more effective incompatibility and efficient extraction from a

  5. Fluid composition of the sediment-influenced Loki's Castle vent field at the ultra-slow spreading Arctic Mid-Ocean Ridge

    NASA Astrophysics Data System (ADS)

    Baumberger, Tamara; Früh-Green, Gretchen L.; Thorseth, Ingunn H.; Lilley, Marvin D.; Hamelin, Cédric; Bernasconi, Stefano M.; Okland, Ingeborg E.; Pedersen, Rolf B.

    2016-08-01

    The hydrothermal vent field Loki's Castle is located in the Mohns-Knipovich bend (73°N) of the ultraslow spreading Arctic Mid-Ocean Ridge (AMOR) close to the Bear Island sediment fan. The hydrothermal field is venting up to 320° C hot black smoker fluids near the summit of an axial volcanic ridge. Even though the active chimneys have grown on a basaltic ridge, geochemical fluid data show a strong sedimentary influence into the hydrothermal circulation at Loki's Castle. Compelling evidence for a sediment input is given by high alkalinity, high concentrations of NH4+, H2, CH4, C2+ hydrocarbons as well as low Mn and Fe contents. The low δ13C values of CO2 and CH4 and the thermogenic isotopic pattern of the C2+ hydrocarbons in the high-temperature vent fluids clearly point to thermal degradation of sedimentary organic matter and illustrate diminution of the natural carbon sequestration in sediments by hydrothermal circulation. Thus, carbon-release to the hydrosphere in Arctic regions is especially relevant in areas where the active Arctic Mid-Ocean Ridge system is in contact with the organic matter rich detrital sediment fans.

  6. Evidence from gabbro of the Troodos ophiolite for lateral magma transport along a slow-spreading mid-ocean ridge.

    PubMed

    Abelson, M; Baer, G; Agnon, A

    2001-01-01

    The lateral flow of magma and ductile deformation of the lower crust along oceanic spreading axes has been thought to play a significant role in suppressing both mid-ocean ridge segmentation and variations in crustal thickness. Direct investigation of such flow patterns is hampered by the kilometres of water that cover the oceanic crust, but such studies can be made on ophiolites (fragments of oceanic crust accreted to a continent). In the Oman ophiolite, small-scale radial patterns of flow have been mapped along what is thought to be the relict of a fast-spreading mid-ocean ridge. Here we present evidence for broad-scale along-axis flow that has been frozen into the gabbro of the Troodos ophiolite in Cyprus (thought to be representative of a slow-spreading ridge axis). The gabbro suite of Troodos spans nearly 20 km of a segment of a fossil spreading axis, near a ridge-transform intersection. We mapped the pattern of magma flow by analysing the rocks' magnetic fabric at 20 sites widely distributed in the gabbro suite, and by examining the petrographic fabric at 9 sites. We infer an along-axis magma flow for much of the gabbro suite, which indicates that redistribution of melt occurred towards the segment edge in a large depth range of the oceanic crust. Our results support the magma plumbing structure that has been inferred indirectly from a seismic tomography experiment on the slow-spreading Mid-Atlantic Ridge.

  7. Evidence from gabbro of the Troodos ophiolite for lateral magma transport along a slow-spreading mid-ocean ridge.

    PubMed

    Abelson, M; Baer, G; Agnon, A

    2001-01-01

    The lateral flow of magma and ductile deformation of the lower crust along oceanic spreading axes has been thought to play a significant role in suppressing both mid-ocean ridge segmentation and variations in crustal thickness. Direct investigation of such flow patterns is hampered by the kilometres of water that cover the oceanic crust, but such studies can be made on ophiolites (fragments of oceanic crust accreted to a continent). In the Oman ophiolite, small-scale radial patterns of flow have been mapped along what is thought to be the relict of a fast-spreading mid-ocean ridge. Here we present evidence for broad-scale along-axis flow that has been frozen into the gabbro of the Troodos ophiolite in Cyprus (thought to be representative of a slow-spreading ridge axis). The gabbro suite of Troodos spans nearly 20 km of a segment of a fossil spreading axis, near a ridge-transform intersection. We mapped the pattern of magma flow by analysing the rocks' magnetic fabric at 20 sites widely distributed in the gabbro suite, and by examining the petrographic fabric at 9 sites. We infer an along-axis magma flow for much of the gabbro suite, which indicates that redistribution of melt occurred towards the segment edge in a large depth range of the oceanic crust. Our results support the magma plumbing structure that has been inferred indirectly from a seismic tomography experiment on the slow-spreading Mid-Atlantic Ridge. PMID:11343114

  8. Lava Flow Ages and Geologic Mapping on Mid-ocean Ridges

    NASA Astrophysics Data System (ADS)

    Clague, D. A.; Paduan, J. B.; Dreyer, B. M.; Caress, D. W.

    2010-12-01

    Geologic mapping of mid-ocean ridges has been hindered by a lack of high-resolution bathymetry and age data. Autonomous underwater vehicles (AUV) with multibeam sonars now produce maps with 1-m resolution. MBARI has collected data since 2006 along the Juan de Fuca and Gorda Ridges, including the 1998 eruptions in summit caldera and upper south rift zone on Axial Seamount, the 1993 and 1982-1991 eruptions on the CoAxial segment, the 1986 pillow mounds and “young sheet flow” on the north Cleft segment, the 1996 eruption on the North Gorda segment, and part of the Endeavour Ridge. The 1-m data allows identification of flow internal structure, boundaries, and emplacement sequences using superposition and abundance of fissures. Geologic maps of young volcanoes on land are constructed using the same principles, constrained by observations of flow contacts and 14C age dates on charcoal from beneath flow margins. In the deep sea, we collect sediment on top of the flows that contains planktic and benthic foraminifera that can be dated using AMS 14C dating. We sampled sediment on flows from the Axial, CoAxial, and North Cleft areas using 30-cm long pushcores deployed from remotely operated vehicles (ROVs). The coring is done with collection of flow samples for chemistry and video observations to confirm contact locations and flow superposition. Cores are inserted until they hit the underlying lava and can be recovered between pillow lobes when the sediment is >~10 cm thick. We recover the basal 1 cm of sediment, sieve to recover foraminifera, and hand-pick for 14C dating. The North Gorda neovolcanic zone at ~3150 m lacks carbonate sediment and therefore ages. Ages of planktic foraminifera are marine calibrated in years before present (aBP). Benthic foraminifera are calibrated against planktic foraminifera from 5 samples. 14C ages obtained from basal sediment from over 40 sites represent minimum ages as there is probably a small amount of unrecovered basal sediment. Ages

  9. Investigation of turbulent flows and near-bottom hydrothermal plumes at mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Zhang, X.; Lin, J.; Jiang, H.

    2015-12-01

    We investigate the characteristics of turbulent flows within near-bottom hydrothermal plumes at mid-ocean ridges through quantitative analysis of video images from manned submersibles using the Particle Image Velocimetry (PIV) method. High-quality video images of near-bottom hydrothermal vents were selected from the Data Library and Archives of the Woods Hole Oceanographic Institution (WHOI), consisting of multiple examples of vent fields in the Atlantic, Pacific, and Indian Oceans. Selected video segments of near-bottom hydrothermal plumes were decomposed into a series of still-image frames at a typical time interval of 1/30 second between consecutive frames. The PIV method was then used to track the motion of individual turbulent parcels, which were identified based on their relatively high concentration of optically visible particles than the surrounding water column. Finally, the velocity fields of the individual turbulent parcels, as well as the integrated fluxes of the composing hydrothermal plume, were calculated. Preliminary investigation of hydrothermal plumes at the TAG area of the Mid-Atlantic Ridge revealed significant spatial and temporal variations in the fluid dynamics of turbulent fluid parcels and near-bottom hydrothermal plumes: (1) Each rising hydrothermal plume is composed of a string of turbulent fluid parcels of variable sizes with a typical dimension of several cm. The calculated instantaneous velocities of individual turbulent parcels could reach tens of cm/s. (2) Turbulent fluid parcels within the hydrothermal plume were observed to grow rapidly through coalescing with adjacent parcels and interacting with ambient water column. (3) The cross-sectional dimensions of the near-bottom hydrothermal plumes were observed to increase several times within upwelling distance of tens of cm, indicating rapid entrainment of ambient fluids into the rising plumes. The overall vertical fluxes of the rising plumes are calculated to have changed

  10. On the limits and limitations of the ophiolite - mid-ocean ridge analogy: Oman vs the East Pacific Rise

    NASA Astrophysics Data System (ADS)

    MacLeod, Christopher J.; Lissenberg, C. Johan

    2015-04-01

    Throughout the near half century since Ian Gass proposed that ophiolite complexes formed by a process directly analogous to seafloor spreading, the study of ophiolites has been central to the development of our conceptual understanding of the mechanisms of formation of oceanic lithosphere at mid-ocean ridges. This role has been affected little by the recognition - on the basis of their geochemistry - that most ophiolites must have formed by spreading above subduction zones rather than at 'proper' open-ocean mid-ocean ridges. Why? - because we will never be able to gain access to the internal structure of modern ocean lithosphere to the extent we can by walking over the largest and best-preserved ophiolites (e.g. Cyprus, Oman, Newfoundland etc.). Ophiolites will always provide vital insights into the mechanisms of formation of lithosphere formed at submarine volcanic spreading centres. To what extent, however, can we be confident that what we learn from ophiolite studies is directly applicable to modern open-ocean mid-ocean ridges? Exactly how far can we press the analogy? To a first order it is reasonable to assume that the physical processes of crustal formation at an open-ocean mid-ocean ridge and at a supra-subduction zone spreading centre should be closely comparable: the presence of an organised sheeted dyke complex, representing 100% extension accompanied by magmatism, is convincing evidence for seafloor spreading. But does this mean the processes of crustal formation are identical in these different geodynamic environments? In this presentation we compare the 'crown jewel' of ophiolites, Oman, with the East Pacific Rise to explore the veracity of the widely-held belief that Oman represents a direct analogue for lithosphere formed at a fast-spreading (open-ocean) mid-ocean ridge. Whereas the mantle source of the axial volcanic suite in Oman is very similar to that of mid-ocean ridge basalt, we have recently shown (MacLeod et al. 2013, Geology v.41, p.459

  11. The Complex History of Alarcon Rise Mid-Ocean Ridge Rhyolite Revealed through Mineral Chemistry

    NASA Astrophysics Data System (ADS)

    Dreyer, B. M.; Portner, R. A.; Clague, D. A.; Daczko, N. R.; Castillo, P.; Bindeman, I. N.

    2014-12-01

    enrichments in zircon 176Hf/177Hf and whole rock 207,206Pb/204Pb may indicate an enriched MORB mantle component. In conclusion, mid-ocean rhyolite at Alarcon formed from a variety of petrogenetic processes including magma-mixing, assimilation, and crystallization following partial melting of slightly heterogeneous mantle source(s).

  12. Halogen systematics (Cl, Br, I) in Mid-Ocean Ridge Basalts: A Macquarie Island case study

    NASA Astrophysics Data System (ADS)

    Kendrick, Mark A.; Kamenetsky, Vadim S.; Phillips, David; Honda, Masahiko

    2012-03-01

    The abundance of halogens (Cl, Br and I) in the Earth's mantle has the potential to provide information about mantle metasomatism and volatile recycling in subduction zones. Basalt melts sample different parts of the Earth's mantle, but few data are available for Br or I in basalt melts, and the relative partitioning behaviour of these elements has not been investigated rigorously. To address these issues, we determined the abundances of Cl, Br and I in enriched Mid-Ocean Ridge Basalt (E-MORB) glasses from Macquarie Island in the southwest Pacific. The Macquarie Island glasses are fairly typical of Pacific MORB with MgO of ˜5.5. to 9 wt%, 87Sr/86Sr of 0.70257-0.70276, 143Nd/144Nd of 0.51300-0.51306 and 3He/4He of ˜8 Ra. These glasses provide a unique opportunity to investigate halogen partitioning behaviour, because their variable MgO contents and trace element signatures (e.g. La/Sm ˜1.4-7.9) result from different degrees of partial melting and fractional crystallisation. The combined measurement of Br/Cl, I/Cl and K/Cl, together with correlations between Cl and other trace elements, demonstrate that the halogens in the Macquarie Island glasses had a mantle source and were not influenced by seawater contamination. Log-log correlation diagrams indicate that Cl, Br and K were not statistically fractionated during partial melting, crystallisation or degassing of CO2 from these melts. The behaviour of I is less well constrained and minor fractionation of I/Cl cannot be precluded during multi-stage melting and enrichment processes. The data indicate the mantle source of the Macquarie Island glasses was characterised by K/Cl of 13 ± 4, Br/Cl of (3.7 ± 0.5) × 10-3 and I/Cl of (130 ± 100) × 10-6 (2σ uncertainties; weight ratios). The K/Cl ratio of the Macquarie Island glasses is equivalent to the median of all published MORB data. This suggests the Br/Cl and I/Cl values may also be representative of average MORB mantle values.

  13. Low temperature hydrothermal oil and associated biological precursors in serpentinites from Mid-Ocean Ridge

    NASA Astrophysics Data System (ADS)

    Pasini, Valerio; Brunelli, Daniele; Dumas, Paul; Sandt, Christophe; Frederick, Joni; Benzerara, Karim; Bernard, Sylvain; Ménez, Bénédicte

    2013-09-01

    The origin of light hydrocarbons discovered at serpentinite-hosted mid-ocean hydrothermal fields is generally attributed to the abiogenic reduction of carbon (di)oxide by molecular hydrogen released during the progressive hydration of mantle-derived peridotites. These serpentinization by-products represent a valuable source of carbon and energy and are known to support deep microbial ecosystems unrelated to photosynthesis. In addition, the pool of subsurface organic compounds could also include materials derived from the thermal degradation of biological material. We re-investigate the recently described relics of deep microbial ecosystems hosted in serpentinites of the Mid-Atlantic Ridge (4-6°N) in order to study the ageing and (hydro)thermal degradation of the preserved biomass. An integrated set of high resolution micro-imaging techniques (Scanning Electron Microscopy, High Resolution Transmission Electron Microscopy, Raman and Fourier Transform Infra-Red microspectroscopy, Confocal Laser Scanning Microscopy, and Scanning Transmission X-ray Microscopy at the carbon K-edge) has been applied to map the distribution of the different organic components at the micrometer scale and to characterize their speciation and structure. We show that biologically-derived material, containing aliphatic groups, along with carbonyl and amide functional groups, has experienced hydrothermal degradation and slight aromatization. In addition, aliphatic compounds up to C6-C10 with associated carboxylic functional groups wet the host bastite and the late serpentine veins crosscutting the rock. These compounds represent a light soluble organic fraction expelled after biomass degradation through oxidation and thermal cracking. The detected complex organic matter distribution recalls a typical petroleum system, where fossil organic matter of biological origin maturates, expelling the soluble fraction which then migrates from the source to the reservoir. Ecosystem-hosting serpentinites

  14. Biogenic iron oxyhydroxide formation at mid-ocean ridge hydrothermal vents: Juan de Fuca Ridge

    NASA Astrophysics Data System (ADS)

    Toner, Brandy M.; Santelli, Cara M.; Marcus, Matthew A.; Wirth, Richard; Chan, Clara S.; McCollom, Thomas; Bach, Wolfgang; Edwards, Katrina J.

    2009-01-01

    Here we examine Fe speciation within Fe-encrusted biofilms formed during 2-month seafloor incubations of sulfide mineral assemblages at the Main Endeavor Segment of the Juan de Fuca Ridge. The biofilms were distributed heterogeneously across the surface of the incubated sulfide and composed primarily of particles with a twisted stalk morphology resembling those produced by some aerobic Fe-oxidizing microorganisms. Our objectives were to determine the form of biofilm-associated Fe, and identify the sulfide minerals associated with microbial growth. We used micro-focused synchrotron-radiation X-ray fluorescence mapping (μXRF), X-ray absorption spectroscopy (μΕXAFS), and X-ray diffraction (μXRD) in conjunction with focused ion beam (FIB) sectioning, and high resolution transmission electron microscopy (HRTEM). The chemical and mineralogical composition of an Fe-encrusted biofilm was queried at different spatial scales, and the spatial relationship between primary sulfide and secondary oxyhydroxide minerals was resolved. The Fe-encrusted biofilms formed preferentially at pyrrhotite-rich (Fe 1-xS, 0 ⩽ x ⩽ 0.2) regions of the incubated chimney sulfide. At the nanometer spatial scale, particles within the biofilm exhibiting lattice fringing and diffraction patterns consistent with 2-line ferrihydrite were identified infrequently. At the micron spatial scale, Fe μEXAFS spectroscopy and μXRD measurements indicate that the dominant form of biofilm Fe is a short-range ordered Fe oxyhydroxide characterized by pervasive edge-sharing Fe-O 6 octahedral linkages. Double corner-sharing Fe-O 6 linkages, which are common to Fe oxyhydroxide mineral structures of 2-line ferrihydrite, 6-line ferrihydrite, and goethite, were not detected in the biogenic iron oxyhydroxide (BIO). The suspended development of the BIO mineral structure is consistent with Fe(III) hydrolysis and polymerization in the presence of high concentrations of Fe-complexing ligands. We hypothesize that

  15. Biogenic iron oxyhydroxide formation at mid-ocean ridge hydrothermal vents: Juan de Fuca Ridge

    SciTech Connect

    Toner, Brandy M.; Santelli, Cara M.; Marcus, Matthew A.; Wirth, Richard; Chan, Clara S.; McCollom, Thomas; Bach, Wolfgang; Edwards, Katrina J.

    2008-05-22

    Here we examine Fe speciation within Fe-encrusted biofilms formed during 2-month seafloor incubations of sulfide mineral assemblages at the Main Endeavor Segment of the Juan de Fuca Ridge. The biofilms were distributed heterogeneously across the surface of the incubated sulfide and composed primarily of particles with a twisted stalk morphology resembling those produced by some aerobic Fe-oxidizing microorganisms. Our objectives were to determine the form of biofilm-associated Fe, and identify the sulfide minerals associated with microbial growth. We used micro-focused synchrotron-radiation X-ray fluorescence mapping (mu XRF), X-ray absorption spectroscopy (mu EXAFS), and X-ray diffraction (mu XRD) in conjunction with focused ion beam (FIB) sectioning, and highresolution transmission electron microscopy (HRTEM). The chemical and mineralogical composition of an Fe-encrusted biofilm was queried at different spatial scales, and the spatial relationship between primary sulfide and secondary oxyhydroxide minerals was resolved. The Fe-encrusted biofilms formed preferentially at pyrrhotite-rich (Fe1-xS, 0<_ x<_ 0.2) regions of the incubated chimney sulfide. At the nanometer spatial scale, particles within the biofilm exhibiting lattice fringing and diffraction patterns consistent with 2-line ferrihydrite were identified infrequently. At the micron spatial scale, Fe mu EXAFS spectroscopy and mu XRD measurements indicate that the dominant form of biofilm Fe is a short-range ordered Fe oxyhydroxide characterized by pervasive edge-sharing Fe-O6 octahedral linkages. Double corner-sharing Fe-O6 linkages, which are common to Fe oxyhydroxide mineral structures of 2-line ferrihydrite, 6-line ferrihydrite, and goethite, were not detected in the biogenic iron oxyhydroxide (BIO). The suspended development of the BIO mineral structure is consistent with Fe(III) hydrolysis and polymerization in the presence of high concentrations of Fe-complexing ligands. We hypothesize that

  16. Evidence from three-dimensional seismic reflectivity images for enhanced melt supply beneath mid-ocean-ridge discontinuities

    PubMed

    Kent; Singh; Harding; Sinha; Orcutt; Barton; White; Bazin; Hobbs; Tong; Pye

    2000-08-10

    Quantifying the melt distribution and crustal structure across ridge-axis discontinuities is essential for understanding the relationship between magmatic, tectonic and petrologic segmentation of mid-ocean-ridge spreading centres. The geometry and continuity of magma bodies beneath features such as overlapping spreading centres can strongly influence the composition of erupted lavas and may give insight into the underlying pattern of mantle flow. Here we present three-dimensional images of seismic reflectivity beneath a mid-ocean ridge to investigate the nature of melt distribution across a ridge-axis discontinuity. Reflectivity slices through the 9 degrees 03' N overlapping spreading centre on East Pacific Rise suggest that it has a robust magma supply, with melt bodies underlying both limbs and ponding of melt beneath large areas of the overlap basin. The geometry of melt distribution beneath this offset is inconsistent with large-scale, crustal redistribution of melt away from centres of upwelling. The complex distribution of melt seems instead to be caused by a combination of vertical melt transport from the underlying mantle and subsequent focusing of melt beneath a magma freezing boundary in the mid-crust.

  17. Spreading-rate dependence of melt extraction at mid-ocean ridges from mantle seismic refraction data.

    PubMed

    Lizarralde, Daniel; Gaherty, James B; Collins, John A; Hirth, Greg; Kim, Sangmyung D

    2004-12-01

    A variety of observations indicate that mid-ocean ridges produce less crust at spreading rates below 20 mm yr(-1) (refs 1-3), reflecting changes in fundamental ridge processes with decreasing spreading rate. The nature of these changes, however, remains uncertain, with end-member explanations being decreasing shallow melting or incomplete melt extraction, each due to the influence of a thicker thermal lid. Here we present results of a seismic refraction experiment designed to study mid-ocean ridge processes by imaging residual mantle structure. Our results reveal an abrupt lateral change in bulk mantle seismic properties associated with a change from slow to ultraslow palaeo-spreading rate. Changes in mantle velocity gradient, basement topography and crustal thickness all correlate with this spreading-rate change. These observations can be explained by variations in melt extraction at the ridge, with a gabbroic phase preferentially retained in the mantle at slower spreading rates. The estimated volume of retained melt balances the approximately 1.5-km difference in crustal thickness, suggesting that changes in spreading rate affect melt-extraction processes rather than total melting. PMID:15592410

  18. Spreading-rate dependence of melt extraction at mid-ocean ridges from mantle seismic refraction data.

    PubMed

    Lizarralde, Daniel; Gaherty, James B; Collins, John A; Hirth, Greg; Kim, Sangmyung D

    2004-12-01

    A variety of observations indicate that mid-ocean ridges produce less crust at spreading rates below 20 mm yr(-1) (refs 1-3), reflecting changes in fundamental ridge processes with decreasing spreading rate. The nature of these changes, however, remains uncertain, with end-member explanations being decreasing shallow melting or incomplete melt extraction, each due to the influence of a thicker thermal lid. Here we present results of a seismic refraction experiment designed to study mid-ocean ridge processes by imaging residual mantle structure. Our results reveal an abrupt lateral change in bulk mantle seismic properties associated with a change from slow to ultraslow palaeo-spreading rate. Changes in mantle velocity gradient, basement topography and crustal thickness all correlate with this spreading-rate change. These observations can be explained by variations in melt extraction at the ridge, with a gabbroic phase preferentially retained in the mantle at slower spreading rates. The estimated volume of retained melt balances the approximately 1.5-km difference in crustal thickness, suggesting that changes in spreading rate affect melt-extraction processes rather than total melting.

  19. Halogen and trace element geochemistry in Mid-Ocean Ridge basalts from the Australian-Antarctic Ridge (AAR)

    NASA Astrophysics Data System (ADS)

    Yang, Y. S.; Seo, J. H.; Park, S. H.; Kim, T.

    2015-12-01

    Australian-Antarctic Ridge (AAR) is an extension of easternmost SE Indian Mid-Ocean Ridge (MOR).We collected volcanic glasses from the "in-axis" of the KR1 and KR2 MOR, and the overlapping zones of the KR1 MOR and the nearby seamounts ("KR1 mixing"). We determined trace and halogen elements in the glasses. Halogen concentrations and its ratios in the glasses are important to understand the mantle metasomatism and volatile recycling. 52 of the collected glasses are "primitive" (higher than 6 wt% MgO), while 3 of them have rather "evolved" composition (MgO wt% of 1.72, 2.95 and 4.15). K2O concentrations and Th/Sc ratios in the glasses show a negative correlation with its MgO concentration. Incompatible element ratios such as La/Sm are rather immobile during a magma differentiation so the ratios are important to understand mantle composition (Hofmann et al. 2003). La/Sm ratios in the glasses are 0.95 ~ 3.28 suggesting that the AAR basalts can be classified into T-MORB and E-MORB (Schilling et al., 1983). La/Sm ratios are well-correlated with incompatible elements such as U, Ba, Nb, and negatively correlated with compatible elements such as Sc, Eu2+, Mg. The AAR glasses contain detectable halogen elements. The "KR1 mixing" glasses in halogen elements are more abundant than "in-axis" the glasses. Cl is the least variable element compared to the other halogens such as Br and I in the AAR. The "KR1 mixing" glasses have the largest variations of Br/Cl ratios compared to the "in-axis" glasses. The Cl/Br and Th/Sc ratios in the "in-axis" glasses and in the "KR1 mixing" glasses show positive and negative correlations, respectively. The Br-rich glasses in the "KR1 mixing" zone might be explained by a recycled Br-rich oceanic slab of paleo-subduction or by a hydrothermal alteration in the AAR. I composition in the glasses does not show a correlation other trace elements. The K/Cl and K/Ti ratios in the AAR glasses are similar to the basalts from the Galapagos Spreading Center

  20. Loki's Castle: A sediment-influenced hydrothermal vent field at the ultra-slow spreading Arctic Mid-Ocean Ridge

    NASA Astrophysics Data System (ADS)

    Baumberger, T.; Frueh-Green, G. L.; Pedersen, R.; Thorseth, I. H.; Lilley, M. D.; Moeller, K.

    2010-12-01

    The chemical composition as well as the stable and radiogenic isotope signatures of hydrothermal fluids from the Loki’s Castle vent field, located at the Mohns-Knipovich bend in the Norwegian-Greenland Sea (73°N), are substantially different from sediment-starved mid-ocean ridge hydrothermal systems. Geochemical studies of the hydrothermal vent fluids and the adjacent rift valley sediments provide insights into the influence of sediments on the hydrothermal fluid composition and provide constraints on acting redox conditions. Additionally, they reflect the degree of fluid-rock-sediment interaction at this arctic hydrothermal vent field. Here we present an overview of the geochemical characteristics of the hydrothermal and sedimentary components at Loki’s Castle, obtained during expeditions in 2008, 2009 and 2010, with emphasis on the stable and radiogenic isotope signatures. We compare these data with other sediment-influenced and sediment-starved mid-ocean ridge hydrothermal systems. The hydrothermal vent fluids are characterized by a pH of ˜ 5.5 and by elevated concentrations of methane, hydrogen and ammonia, which reflect a sedimentary contribution. δ13CDIC (dissolved inorganic carbon) are depleted relative to mantle carbon values, consistent with an organic carbon input. The δ18OH2O values of the vents fluids are enriched compared to background bottom seawater, whereas the δD values are not. 87Sr/86Sr ratios are more radiogenic than those characteristic of un-sedimented mid-ocean ridge vent fluids. S-isotope data reflect mixing of a MORB source with sulphide derived from reduced seawater sulphate. To document the background sediment input of the ridge system, short gravity cores and up to 18 m long piston cores were recovered from various localities in the rift valley. The pore-fluid isotope chemistries of the sediments show vertical gradients that primarily reflect diagenesis and degradation of organic matter. The vertical gradient is locally enhanced

  1. A Novel Approach to Characterize Compositional Variability through Time at Mid-Ocean Ridges

    NASA Astrophysics Data System (ADS)

    Dreyer, B. M.; Clague, D. A.; Gill, J. B.

    2013-12-01

    The temporal, spatial, and compositional variability of eruptive products illuminates the scales of magmatic sources and processes. Prior methods to evaluate this variability rely on extensive sampling of lava flows. We describe a novel approach to assess the variability of submarine volcanics using material recovered in pushcores at a mid-ocean ridge. Pushcores collected from the west flank of Endeavour in 2011 with ROV Doc Ricketts were extruded in 1- or 2-cm intervals and volcaniclastic glass shards and foraminifera were extracted for electron microprobe and radiocarbon analyses, respectively. 14C ages and glass compositions from two cores provide chemical stratigraphies we compare to compositions of proximal lavas to assess local variability and volcaniclast dispersal. A 20-cm core from the north slope of Summit Seamount ~100m above the axial valley floor has a basal age of 4.5ka and 10-12 cm is ~1.5ka. Shards in the bottom 6cm are chemically distinct from the lava flow that it overlies. These basal shards are bimodal in MgO and the more differentiated mode is similar to flows on the valley floor on the opposite side of the faulted remnants of Summit Seamount. Most shards in the top 14 cm are similar to the underlying lava flow and have uniform MgO of 7.3 ×0.1. This composition is dominant in shards from 6-12 cm, which may represent a single eruption. Nearby flows encompassing three additional distinct lava types are also present as rare random shards; one additional lava type ~1km distant is absent. A 24-cm core from NW of Main Endeavour Field overlies a compositionally distinct low pillow mound on the west flank; no shards with this composition are found. The core is dated at 10.5ka (basal), ~4.8ka (15-17 cm), and ~2.0ka (9-11 cm). From 10.5 to 4.8 ka, most shards are compositionally similar to, but slightly more evolved than, extensive sheet lavas that flowed around the pillow mounds. A subordinate amount of shards that are compositionally similar, but more

  2. Correlation Length Scales of Isotopic Variations Along Mid-Ocean Ridges and Upper Mantle Dynamics

    NASA Astrophysics Data System (ADS)

    Graham, D. W.; Spera, F. J.

    2003-12-01

    How isotopic variations in basalts erupted at the Earth's surface are linked to convective mixing in the underlying mantle is a central problem in geodynamics. The objective of this study is to quantify the length scales of upper mantle heterogeneity through spatial statistical analysis of MORB. We define a characteristic length scale, the scale of segregation L, computed from the spatial self-correlations for 3He/4He, 87Sr/86Sr, 143Nd/144Nd and 206,207,208Pb/204Pb in "zero age" lavas from mid-ocean ridges. Our working hypothesis is that small scale convection in the upper mantle controls dispersion of geochemical tracers. Differences in L between ocean basins may then be quantitatively related to unsteadiness in this convection, due to thickening of the lithosphere, plume impingement, or lateral temperature/compositional differences between continental and oceanic lithosphere induced by batholith formation. The correlation coefficient R, and the separation distance r, are calculated for every i,j pair of points. Ri,j is given by the product of the deviations in isotope composition from the population mean, normalized to the population variance, and R(r) is computed as an ensemble average. The total number of point pairs (N) for n sample locations is given by N=n(n-1)/2. For the global MORB data set (n=1265 and 735 for Sr and He, respectively), N exceeds 105 (799480 and 269745, respectively). A value of R(r) close to 1 indicates that an isotope ratio above (or below) the population average is likely to be associated with an above (or below) average value at a distance r away. A value of R(r) close to zero implies a random relationship, and a value close to -1 implies an anti-correlation. R(r) approaches unity at small r by definition, as points close together are from the same "clump" of mantle. The value of r at which R first goes to zero is denoted as r*. On a diagram of R(r) vs. r (the correlogram), the integral of R(r) from r=0 to r=r* is the scale of

  3. Investigating Earthquake Stress Drops on Mid-Ocean Ridge Transform Faults (Invited)

    NASA Astrophysics Data System (ADS)

    Boettcher, M. S.; Moyer, P. A.; McGuire, J. J.; Collins, J. A.

    2013-12-01

    A key question concerning the development of mid-ocean ridge transform faults (RTFs) is why have full fault ruptures not been observed in the historic record? Similarly, why do the rupture areas of the largest earthquakes on RTFs not scale directly with area above the 600°C isotherm? Recent studies have shown that Blanco, Discovery, Gofar, Heezen, Tharp, and Hollister RTFs all have multiple rupture patches on a single fault segment that repeatedly fail in characteristic largest (Mc) earthquakes. We develop a scaling relation for the stress drop of repeating Mc earthquakes assuming full-coupling on Mc rupture patches, such that slip (Dc) in Mc earthquakes is given by the product of the repeat time (tR) and plate tectonic slip (V), and assuming that slip scales with the square root of rupture area (Ac), Dc = ΔσAc1/2μ-1, where μ is the shear modulus. Using the definition of seismic moment, Mc = μAcDc, we directly solve for stress drop given observed repeat times: Δσ = μVtR3/2Mc-1/2. For stress drops in the range of 1-2 MPa, slip in repeating Mc earthquakes on each of the RTFs noted above is approximately equal to the accumulated plate tectonic motion. We analyze the source parameters of 3.0 < Mw < 5.0 earthquakes recorded in 2008 during a yearlong ocean bottom seismic (OBS) experiment on Gofar transform fault to determine the stress drop of earthquakes in both repeating Mc patches and the rupture barriers between the rupture patches. The OBS deployment captured the end of a seismic cycle, including a foreshock sequence that was both extensive (~20,000 earthquakes within the week prior to the mainshock) and localized (within a ~10 km region), as well as the Mw 6.0 mainshock and its aftershock sequence [McGuire et. al, 2012]. The foreshocks occurred in a rupture barrier on the western segment of Gofar and the aftershocks occurred in the rupture patch. Using waveforms recorded with a sample rate of 50 Hz on OBS accelerometers, we investigate the corner

  4. Comparison of Oceanic and Continental Ultramafic Hosted Hydrothermal Sulfide Deposits under Slow-spreading Mid-ocean Ridge Setting

    NASA Astrophysics Data System (ADS)

    Li, Honglin; Li, Jianghai; Zhang, Huatian

    2013-04-01

    Recently, slow-spreading mid-ocean ridges have attracted lots of researchers, especially in the MAR (Mid-Atlantic Ridge) and the Indian Ocean Ridge. People have found many hydrothermal vents or hydrothermal sulfide deposits around MAR, such as TAG, Rainbow and Lost City. The slow-spreading ridges are characterized by variations in magmatic, tectonic, and alteration processes along ridge segments. Because of the difficulties of the seafloor exploration, we need an example on the continent for compare with the hydrothermal sulfide deposits on mid-ocean ridge. So we found De'erni Cu (Co) ore deposit on the north of Tibet. De'erni Cu (Co) ore deposit is a typical VHMS developing in the north of Tibet, China. The ore body is hosted by the ultramafic rocks of the A'nyemagen ophiolite suite, which is the symbol of the residual crust of Paleo-Tethys Ocean. Through the detailed geological analysis to De'erni Cu (Co) ore deposit, lots of reminded geological records of submarine hydrothermal system, including: 1) thin-layer exhalative rock covering on the ore body; 2) the colloform structure, raspberry-like structure and breccia structure reserved in the porous-type ores; 3) the main mineral composition; 4) the calcite and felsic cement in the synchronization with the pyrite clast; 4) the ore zonality similar to the TAG hydrothermal sulfide deposit. According to the TiO2 content in the MORB basalts, the approximate half-spreading rate is 1.1-2.5cm/a, of the Paleo-Tethys Ocean represented by the De'erni ophiolite. Comparing to the mineralization processes of present mid-ocean hydrothermal sulfide deposits, we insist that De'erni Cu (Co) deposit has experienced three stages: submarine exhalation stage, cooling deposition and subduction emplacement. And the OCC (Oceanic Core Complex) may be the host setting of the submarine hydrothermal exhalation stage. Compared to other similar sulfide deposits on the continent in the world, De'erni Cu (Co) sulfide deposit has a younger age

  5. Geodynamic modeling of the capture and release of a plume conduit by a migrating mid-ocean ridge

    NASA Astrophysics Data System (ADS)

    Hall, P. S.

    2011-12-01

    plates over the relatively stationary, long-lived conduits of mantle plumes. However, paleomagnetic data from the Hawaii-Emperor Seamount Chain suggests that the Hawaiian hotspot moved rapidly (~40 mm/yr) between 81 - 47 Ma [Tarduno et al., 2003]. Recently, Tarduno et al. [2009] suggested that this period of rapid motion might be the surface expression of a plume conduit returning to a largely vertical orientation after having been captured and tilted as the result of being "run over" by migrating mid-ocean ridge. I report on a series of analog geodynamic experiments designed to characterize the evolution of a plume conduit as a mid-ocean ridge migrates over. Experiments were conducted in a clear acrylic tank (100 cm x 70 cm x 50 cm) filled with commercial grade high-fructose corn syrup. Plate-driven flow is modeled by dragging two sheets of Mylar film (driven by independent DC motors) in opposite directions over the surface of the fluid. Ridge migration is achieved by moving the point at which the mylar sheets diverge using a separate motor drive. Buoyant plume flow is generated using a small electrical heater placed at the bottom of the tank. Plate velocities and ridge migration rate are controlled and plume temperature monitored using LabView software. Experiments are recorded using digital video which is then analyzed using digital image analysis software to track the position and shape of the plume conduit throughout the course of the experiment. The intersection of the plume conduit with the surface of the fluid is taken as an analog for the locus of hotspot volcanism and tracked as a function of time to obtain a hotspot migration rate. Results show that the plume conduit experiences significant tilting immediately following the passage of the migrating ridge.

  6. Correlating microbial community profiles with geochemical data in highly stratified sediments from the Arctic Mid-Ocean Ridge.

    PubMed

    Jorgensen, Steffen Leth; Hannisdal, Bjarte; Lanzén, Anders; Baumberger, Tamara; Flesland, Kristin; Fonseca, Rita; Ovreås, Lise; Steen, Ida H; Thorseth, Ingunn H; Pedersen, Rolf B; Schleper, Christa

    2012-10-16

    Microbial communities and their associated metabolic activity in marine sediments have a profound impact on global biogeochemical cycles. Their composition and structure are attributed to geochemical and physical factors, but finding direct correlations has remained a challenge. Here we show a significant statistical relationship between variation in geochemical composition and prokaryotic community structure within deep-sea sediments. We obtained comprehensive geochemical data from two gravity cores near the hydrothermal vent field Loki's Castle at the Arctic Mid-Ocean Ridge, in the Norwegian-Greenland Sea. Geochemical properties in the rift valley sediments exhibited strong centimeter-scale stratigraphic variability. Microbial populations were profiled by pyrosequencing from 15 sediment horizons (59,364 16S rRNA gene tags), quantitatively assessed by qPCR, and phylogenetically analyzed. Although the same taxa were generally present in all samples, their relative abundances varied substantially among horizons and fluctuated between Bacteria- and Archaea-dominated communities. By independently summarizing covariance structures of the relative abundance data and geochemical data, using principal components analysis, we found a significant correlation between changes in geochemical composition and changes in community structure. Differences in organic carbon and mineralogy shaped the relative abundance of microbial taxa. We used correlations to build hypotheses about energy metabolisms, particularly of the Deep Sea Archaeal Group, specific Deltaproteobacteria, and sediment lineages of potentially anaerobic Marine Group I Archaea. We demonstrate that total prokaryotic community structure can be directly correlated to geochemistry within these sediments, thus enhancing our understanding of biogeochemical cycling and our ability to predict metabolisms of uncultured microbes in deep-sea sediments.

  7. Mantle properties and the MOR process: a new and versatile model for mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Osmaston, Miles

    2014-05-01

    Introduction. First I summarize the reasons why a radical departure from the current MOR model is now essential. I then outline the new model and its apparent versatility, not only in providing the observed contrasting spreading-rate-dependent characteristics but also some of the other common features of the MOR system which warrant clearer explanation. Ophiolites have been thought to provide on-land guidance but turn out to be a non-mid-ocean variant, outside the scope of this presentation. Seismic anisotropy and mantle mobility. Ever since the 1969 discovery [1] of seismic anisotropy in the uppermost oceanic mantle, this has been attributed to the shearing of olivine in a convectively driven MOR-divergent flow beneath the flanks. This would imply a high degree of rheological mobility of this mantle, but new constraints on its rheological properties and dynamical behaviour have come from two directions and need to be taken into account in forming a model. 1. Contrary to the seismologists' rule-book, the oceanic seismological Low Velocity Zone (LVZ) is no longer to be thought of as mobile, because the presence of interstitial melt strips out the water-weakening of the mineral structure [2, 3]. So we require a substitute for the divergent-flow model for MORs which, we find, also has other, apparently unrecognized, dynamical inconsistencies. One of these [4] is that there are in the record many rapid changes of spreading rate and direction, and ridge jumps. This cannot happen with a process driven by slow-to-change body forces, such as thermal convection. 2. My work on the global dynamic pattern for the past 150Ma (I will show examples) has shown [4 - 7] that the tectospheres of cratons must extend to very close to the bottom of the upper mantle (660km). The metasomatism of kimberlite xenoliths from >180km depth suggests that the reason for this downwards extent of 'keels' is the same as [3]. Phase changes. Another geodynamically important property apparently

  8. Advanced Seismic Studies of the Endeavour Ridge: Understanding the Interplay among Magmatic, Hydrothermal, and Tectonic Processes at Mid-Ocean Ridges

    NASA Astrophysics Data System (ADS)

    Arnoux, G. M.; VanderBeek, B. P.; Morgan, J. V.; Hooft, E. E. E.; Toomey, D. R.; Wilcock, W. S. D.; Warner, M.

    2014-12-01

    At mid-ocean ridges magmatic, hydrothermal, and tectonic processes are linked. Understanding their interactions requires mapping magmatic systems and tectonic structures, as well as their relationship to hydrothermal circulation. Three-dimensional seismic images of the crust can be used to infer the size, shape, and location of magma reservoirs, in addition to the structure of the thermal boundary layer that connects magmatic and hydrothermal processes. Travel time tomography has often been used to study these processes, however, the spatial resolution of travel time tomography is limited. Three-dimensional full waveform inversion (FWI) is a state-of-the art seismic method developed for use in the oil industry to obtain high-resolution models of the velocity structure. The primary advantage of FWI is that it has the potential to resolve subsurface structures on the order of half the seismic wavelength—a significant improvement on conventional travel time tomography. Here, we apply anisotropic FWI to data collected on the Endeavour segment of the Juan de Fuca Ridge. Starting models for anisotropic P-wave velocity were obtained by travel time tomography [Weekly et al., 2014]. During FWI, the isotropic velocity model is updated and anisotropy is held constant. We have recovered low-velocity zones approximately 2-3 km beneath the ridge axis that likely correspond to a segmented magma-rich body and are in concert with those previously resolved using multi-channel seismic reflection methods. The segmented crustal magma body underlies all five known high-temperature hydrothermal vent fields along the Endeavour segment. A high-velocity zone, shallower than the observed low-velocity zones, underlies the southernmost hydrothermal vent field. This may be indicative of waning hydrothermal activity in which minerals are crystallizing beneath the vent field. Our FWI study of the Endeavour Ridge will provide the most detailed three-dimensional images of the crustal structure to

  9. Mid-Ocean Ridge Magma Supply and Glacial Cycles: Long Time Series Studies of Crustal Thickness and Seafloor Topography

    NASA Astrophysics Data System (ADS)

    Boulahanis, B.; Carbotte, S. M.; Huybers, P. J.; Langmuir, C. H.; Han, S.; Aghaei, O.; Canales, J. P.; Nedimovic, M. R.; Menke, W. H.

    2015-12-01

    Glacial loading has been shown to modulate volcanic melt generation in subaerial systems, and recent studies suggest that eustatic sea level fluctuations induced by glacial cycles may influence mantle-melting regimes at mid-ocean ridges. Models predict temporal variation in crustal thickness, and seafloor topography, linked to sea level change. Recent studies of bathymetry as a proxy for crustal thickness show significant spectral energy at periodicities linked to Milankovitch cycles of 23, 41, and 100ka (Crowley et al., 2015; Tolstoy, M., 2015). In this study we investigate climate driven periodicity in mid-ocean ridge magma supply utilizing basement topography and crustal thickness data. We use multichannel seismic reflection (MCS) data from two prior studies of the flanks of the Juan de Fuca (JdF) ridge, and 3D MCS data from the Northern East Pacific Rise (EPR) 9°37-57'N. The JdF datasets extend to crustal ages up to 8.78 Ma, and EPR data to ~180 ka. By performing spectral analysis on these data along with dO18 climate records from Lisiecki and Raymo (2005) for the last 5.32ma and Zachos et al. (2001) for earlier times we investigate intervals of similar periodicities in order to identify potential links between climate and magma supply to mid-ocean ridges. Further analysis is undertaken to determine whether depth to basement and crustal thickness are correlated within and across datasets, and whether significant spectral peaks occur in basement and crustal thickness data outside of known climate cycles. Initial results show significant spectral energy in basement depth at the 100ky cycle in the 0-1Ma time series, when eccentricity is understood to have the most impact. Long-term temporal variability is apparent in JdF data, with low relief abyssal hills (~70m on average) present 1-3.2Ma and 6-8.78Ma, but higher relief bathymetry (~200m) from 3.2-6Ma. These subsets align well with previously identified climatic subgroups (Zachos et al., 2001), correlating both

  10. Crustal accretion along the global mid-ocean ridge system based on basaltic glass and olivine-hosted melt inclusion compositions

    NASA Astrophysics Data System (ADS)

    Wanless, V. D.; Behn, M. D.

    2015-12-01

    The depth and distribution of crystallization at mid-ocean ridges controls the overall architecture of the oceanic crust, influences hydrothermal circulation, and determines geothermal gradients in the crust and uppermost mantle. Despite this, there is no overall consensus on how crystallization is distributed within the crust/upper mantle or how this varies with spreading rate. Here, we examine crustal accretion at mid-ocean ridges by combining crystallization pressures calculated from major element barometers on mid-ocean ridge basalt (MORB) glasses with vapor-saturation pressures from melt inclusions to produce a detailed map of crystallization depths and distributions along the global ridge system. We calculate pressures of crystallization from >11,500 MORB glasses from the global ridge system using two established major element barometers (1,2). Additionally, we use vapor-saturation pressures from >400 olivine-hosted melt inclusions from five ridges with variable spreading rates to constrain pressures and distributions of crystallization along the global ridge system. We show that (i) crystallization depths from MORB glasses increase and become less focused with decreasing spreading rate, (ii) maximum glass pressures are greater than the maximum melt inclusion pressure, which indicates that the melt inclusions do not record the deepest crystallization at mid-ocean ridges, and (iii) crystallization occurs in the lower crust/upper mantle at all ridges, indicating accretion is distributed throughout the crust at all spreading rates, including those with a steady-state magma lens. Finally, we suggest that the remarkably similar maximum vapor-saturation pressures (~ 3000 bars) in melt inclusion from all spreading rates reflects the CO2 content of the depleted upper mantle feeding the global mid-ocean ridge system. (1) Michael, P. & W. Cornell (1998), Journal of Geophysical Research, 103(B8), 18325-18356; (2) Herzberg, C. (2004), Journal of Petrology, 45(12), 2389.

  11. Mantle properties and the MOR process: a new and versatile model for mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Osmaston, Miles

    2014-05-01

    Introduction. First I summarize the reasons why a radical departure from the current MOR model is now essential. I then outline the new model and its apparent versatility, not only in providing the observed contrasting spreading-rate-dependent characteristics but also some of the other common features of the MOR system which warrant clearer explanation. Ophiolites have been thought to provide on-land guidance but turn out to be a non-mid-ocean variant, outside the scope of this presentation. Seismic anisotropy and mantle mobility. Ever since the 1969 discovery [1] of seismic anisotropy in the uppermost oceanic mantle, this has been attributed to the shearing of olivine in a convectively driven MOR-divergent flow beneath the flanks. This would imply a high degree of rheological mobility of this mantle, but new constraints on its rheological properties and dynamical behaviour have come from two directions and need to be taken into account in forming a model. 1. Contrary to the seismologists' rule-book, the oceanic seismological Low Velocity Zone (LVZ) is no longer to be thought of as mobile, because the presence of interstitial melt strips out the water-weakening of the mineral structure [2, 3]. So we require a substitute for the divergent-flow model for MORs which, we find, also has other, apparently unrecognized, dynamical inconsistencies. One of these [4] is that there are in the record many rapid changes of spreading rate and direction, and ridge jumps. This cannot happen with a process driven by slow-to-change body forces, such as thermal convection. 2. My work on the global dynamic pattern for the past 150Ma (I will show examples) has shown [4 - 7] that the tectospheres of cratons must extend to very close to the bottom of the upper mantle (660km). The metasomatism of kimberlite xenoliths from >180km depth suggests that the reason for this downwards extent of 'keels' is the same as [3]. Phase changes. Another geodynamically important property apparently

  12. [sup 226]Ra-[sup 230]Th disequilibrium in axial and off-axis mid-ocean ridge basalts

    SciTech Connect

    Volpe, A.M.; Goldstein, S.J. Los Alamos National Lab., NM )

    1993-03-01

    The authors describe [sup 226]Ra-[sup 230]Th disequilibrium in mid-ocean ridge basalt (MORB) glasses from the Juan de Fuca, Gorda, and East Pacific ridges. These first mass spectrometric measurements of [sup 226]Ra in MORB glasses at sub-picogram abundance levels confirm the large excesses over [sup 230]Th determined by radon-emanation techniques and alpha spectrometry. All off-axis MORB glasses have [sup 226]Ra-[sup 230]Th and [sup 234]U-[sup 238]U in secular equilibrium. This suggests that magmatic processes, not secondary post-eruption alteration, generate [sup 238]U-series disequilibrium in these MORB. Least evolved, N-MORB from axial valleys have ([sup 226]Ra/[sup 230]Th) between 2.2-2.3. Differentiated and enriched E-type MORB have consistently low ([sup 226]Ra/[sup 230]Th) ratios compared with N-MORB from the same ridge sections. Ra-Th fractionation may be less pronounced, or magma residence-transit periods may be long for differentiated MORB. Also, E-MORB may be generated by different melt extraction volumes and rates. Estimated [sup 226]Ra-[sup 230]Th ages for N-MORB agree with location on and off ridge segments, and with Th-U model ages. These preliminary results show that [sup 226]Ra-[sup 230]Th disequilibrium could be used to quantify volcanic episodicity at ocean ridges. 39 refs., 6 figs., 4 tabs.

  13. The quantum event of oceanic crustal accretion: impacts of diking at mid-ocean ridges.

    PubMed

    Delaney, J R; Kelley, D S; Lilley, M D; Butterfield, D A; Baross, J A; Wilcock, W S; Embley, R W; Summit, M

    1998-07-10

    Seafloor diking-eruptive events represent the irreducible, quantum events of upper oceanic crustal accretion. They record events by which a large portion of the oceanic crust has formed through geological history. Since 1993, the U.S. Navy's real-time Sound Surveillance System has allowed location of ongoing acoustic signatures of dike emplacement and basalt eruptions at ridge crests in the northeast Pacific. These diking-eruptive events trigger a sequence of related, rapidly evolving physical, chemical, and biological processes. Magmatic volatiles released during these events may provide nutrients for communities of subsea-floor microorganisms, some of which thrive in high-temperature anaerobic environments. Many of the organisms identified from these systems are Archaea. If microorganisms can thrive in the water-saturated pores and cracks within deep, volcanically active portions of our planet, other hydrothermally active planets may harbor similar life forms.

  14. The Quantum Event of Oceanic Crustal Accretion: Impacts of Diking at Mid-Ocean Ridges

    PubMed

    Delaney; Kelley; Lilley; Butterfield; Baross; Wilcock; Embley; Summit

    1998-07-10

    Seafloor diking-eruptive events represent the irreducible, quantum events of upper oceanic crustal accretion. They record events by which a large portion of the oceanic crust has formed through geological history. Since 1993, the U.S. Navy's real-time Sound Surveillance System has allowed location of ongoing acoustic signatures of dike emplacement and basalt eruptions at ridge crests in the northeast Pacific. These diking-eruptive events trigger a sequence of related, rapidly evolving physical, chemical, and biological processes. Magmatic volatiles released during these events may provide nutrients for communities of subseafloor microorganisms, some of which thrive in high-temperature anaerobic environments. Many of the organisms identified from these systems are Archaea. If microorganisms can thrive in the water-saturated pores and cracks within deep, volcanically active portions of our planet, other hydrothermally active planets may harbor similar life forms.

  15. Seismic structure and crustal accretion along an intermediate-rate mid-ocean ridge segment

    NASA Astrophysics Data System (ADS)

    Weekly, Robert Todd

    Epicenters and magnitudes for 36,523 earthquakes recorded along the Endeavour segment between August 2003 and October 2006 are automatically determined using a local ocean-bottom seismometer (OBS) network. The catalog is dominated by two swarm sequences in January and February 2005 in the vicinity of the Endeavour overlapping spreading center, which included earthquakes in West Valley, the northern portion of the Endeavour segment, southwest Endeavour Valley and the Endeavour vent fields. These swarms are attributed to volcanism including a dike intrusion on the northern Endeavour in February 2005 and smaller diking events on the propagating tip of the West Valley segment in both swarms. The dike on the northern Endeavour propagated to the south, which is inconsistent with magma sourced from the axial magma chamber beneath the elevated central portion of the segment. Following the swarms, seismic activity on the Endeavour segment decreased on average to ˜15% of pre-swarm values and almost ceased at the segment ends. I infer that a six-year non-eruptive event that started with a swarm in 1999 and finished with the 2005 swarms ruptured the entire segment and relieved plate-spreading stresses. The inferred coupling between the 1999 and 2005 events, the observation of extensive precursory activity prior to the 2005 swarms, and the interaction between seismically active regions during the swarms is consistent with static triggering with delays influenced by viscoelastic relaxation, hydraulic diffusion and magma withdrawal and replenishment. The isotropic and anisotropic P-wave velocity structure of the upper oceanic crust on the Endeavour Segment of the Juan de Fuca Ridge is studied using refracted travel time data collected by an active-source, three-dimensional tomography experiment. The isotropic velocity structure is characterized by low crustal velocities in the overlapping spreading centers (OSCs) at the ends of the segment. These low velocities are indicative of

  16. Osmium isotopic constraints on the nature of the DUPAL anomaly from Indian mid-ocean-ridge basalts.

    PubMed

    Escrig, S; Capmas, F; Dupré, B; Allègre, C J

    2004-09-01

    The isotopic compositions of mid-ocean-ridge basalts (MORB) from the Indian Ocean have led to the identification of a large-scale isotopic anomaly relative to Pacific and Atlantic ocean MORB. Constraining the origin of this so-called DUPAL anomaly may lead to a better understanding of the genesis of upper-mantle heterogeneity. Previous isotopic studies have proposed recycling of ancient subcontinental lithospheric mantle or sediments with oceanic crust to be responsible for the DUPAL signature. Here we report Os, Pb, Sr and Nd isotopic compositions of Indian MORB from the Central Indian ridge, the Rodriguez triple junction and the South West Indian ridge. All measured samples have higher (187)Os/(188)Os ratios than the depleted upper-mantle value and Pb, Sr and Nd isotopic compositions that imply the involvement of at least two distinct enriched components in the Indian upper-mantle. Using isotopic and geodynamical arguments, we reject both subcontinental lithospheric mantle and recycled sediments with oceanic crust as the cause of the DUPAL anomaly. Instead, we argue that delamination of lower continental crust may explain the DUPAL isotopic signature of Indian MORB.

  17. The crustal structure of central East Greenland-II: From the Precambrian shield to the recent mid-oceanic ridges

    NASA Astrophysics Data System (ADS)

    Schmidt-Aursch, Mechita C.; Jokat, Wilfried

    2005-02-01

    We present a 3-D crustal model of the East Greenland Fjord Region between 69°N and 74°N. The model covers the Precambrian shield and the Caledonian orogenic belt, the adjoining Devonian and Mesozoic basins, the continent-ocean transition and the Cenozoic oceanic areas as far as the Kolbeinsey and the Mohns mid-oceanic ridges. Existing seismic models of the crustal structure are extrapolated into adjacent areas using 3-D gravity modelling. For this purpose, we compile a new regional-scale Bouguer anomaly map. The Precambrian shield, west of the Caledonian orogen (approximately west of 32°W), shows a mean thickness of 35 km with only small-scale undulations. This thickness is at the lower limit of the global range in shield thickness. The Caledonian orogen exhibits a pronounced mountain root with overall crustal thicknesses up to 51 km. Beside the Urals, the East Greenland Caledonides are one of the two Palaeozoic mountain belts where a crustal root has preserved to the present day. Continuation of the crustal model to the east, beyond the continent-ocean transition, yielded thicknesses of the crystalline oceanic crust from 9 km near the Kolbeinsey Ridge to 3 km west of the Mohns Ridge. Differences in the thermal structures of the old continental and the young oceanic lithosphere are responsible for the low-density mantle beneath the oceanic crust, which is also demonstrated by 3-D gravity modelling.

  18. Modes and implications of mantle and lower-crust denudation at slow-spreading mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Schroeder, Timothy John

    Slow-spreading mid-ocean ridges (<5 cm/yr) have intermittent magma supply, and accommodate spreading by a combination of magmatism and tectonic extension (Smith and Cann, 1993, Cannat, 1993). Extension at mid-ocean ridges is most commonly manifested by slip on high angle (˜60°) normal faults that dip into, and define the rift valley walls (Smith and Cann, 1993). Less commonly, extension occurs by long periods of slip along low-angle normal faults that penetrate to structurally deep levels of oceanic lithosphere and denude gabbro and/or pendotite to the seafloor in domal massifs termed "oceanic core complexes" (Dick et al., 1981; Dick et al., 1991; Tucholke et al., 1998; Mutter and Karson, 1992; Cann et al., 1997; MacLeod et al., 2002). This dissertation addresses processes and implications of tectonic extension at two oceanic core complexes. Atlantis Massif (30°N, Mid-Atlantic Ridge) is formed dominantly of serpentinized peridotite with lesser gabbro, and Atlantis Bank (57°E, Southwest Indian Ridge) is dominated by gabbro. Localization of brittle strain at Atlantis Massif occurred by reaction-softening processes associated with metasomatic alteration of peridotite and serpentmite to amphibole-, chlorite- and talc-bearing assemblages. Ductile strain at Atlantis Massif and Atlantis Bank is localized into intervals of highly-fractionated, oxide-rich gabbro. Two-oxide geothermometry of gabbro indicates that it was not penetratively deformed below ˜500°C. Denuded peridotite at Atlantis Massif is host to hydrothermal circulation driven in part by exothermic serpentinization reactions. Serpentinization decreases the seismic velocity of peridotite and leads to acquisition of a magnetic signature. Venting of highly-alkaline, methane- and hydrogen-rich serpentinization-derived fluids leads to lithification of seafloor carbonate ooze by precipitation of carbonate cement in a zone of mixing with "normal" seawater. This process may be the primary depositional mechanism of

  19. Interaction of a mantle plume and a segmented mid-ocean ridge: Results from numerical modeling

    NASA Astrophysics Data System (ADS)

    Georgen, Jennifer E.

    2014-04-01

    Previous investigations have proposed that changes in lithospheric thickness across a transform fault, due to the juxtaposition of seafloor of different ages, can impede lateral dispersion of an on-ridge mantle plume. The application of this “transform damming” mechanism has been considered for several plume-ridge systems, including the Reunion hotspot and the Central Indian Ridge, the Amsterdam-St. Paul hotspot and the Southeast Indian Ridge, the Cobb hotspot and the Juan de Fuca Ridge, the Iceland hotspot and the Kolbeinsey Ridge, the Afar plume and the ridges of the Gulf of Aden, and the Marion/Crozet hotspot and the Southwest Indian Ridge. This study explores the geodynamics of the transform damming mechanism using a three-dimensional finite element numerical model. The model solves the coupled steady-state equations for conservation of mass, momentum, and energy, including thermal buoyancy and viscosity that is dependent on pressure and temperature. The plume is introduced as a circular thermal anomaly on the bottom boundary of the numerical domain. The center of the plume conduit is located directly beneath a spreading segment, at a distance of 200 km (measured in the along-axis direction) from a transform offset with length 100 km. Half-spreading rate is 0.5 cm/yr. In a series of numerical experiments, the buoyancy flux of the modeled plume is progressively increased to investigate the effects on the temperature and velocity structure of the upper mantle in the vicinity of the transform. Unlike earlier studies, which suggest that a transform always acts to decrease the along-axis extent of plume signature, these models imply that the effect of a transform on plume dispersion may be complex. Under certain ranges of plume flux modeled in this study, the region of the upper mantle undergoing along-axis flow directed away from the plume could be enhanced by the three-dimensional velocity and temperature structure associated with ridge-transform-ridge

  20. Geochemical heterogeneity within mid-ocean ridge lava flows: insights into eruption, emplacement and global variations in magma generation

    NASA Astrophysics Data System (ADS)

    Rubin, K. H.; Smith, M. C.; Bergmanis, E. C.; Perfit, M. R.; Sinton, J. M.; Batiza, R.

    2001-06-01

    Compositional heterogeneity in mid-ocean ridge (MOR) lava flows is a powerful yet presently under-utilized volcanological and petrological tracer. Here, it is demonstrated that variations in pre- and syn-eruptive magmatic conditions throughout the global ridge system can be constrained with intra-flow compositional heterogeneity among 10 discrete MOR flows. Geographical distribution of chemical heterogeneity within flows is also used along with mapped physical features to help decipher the range of conditions that apply to seafloor eruptions (i.e. inferred vent locations and whether there were single or multiple eruptive episodes). Although low-pressure equilibrium fractional crystallization can account for much of the observed intra-flow compositional heterogeneity, some cases require multiple parent magmas and/or more complex crystallization conditions. Globally, the extent of within-flow compositional heterogeneity is well correlated (positively) with estimated erupted volume for flows from the northern East Pacific Rise (EPR), and the Mid Atlantic, Juan de Fuca and Gorda Ridges; however, some lavas from the superfast spreading southern EPR fall below this trend. Compositional heterogeneity is also inversely correlated with spreading rate. The more homogeneous compositions of lavas from faster spreading ridges likely reflect the relative thermal stability and longevity of sub-ridge crustal magma bodies, and possibly higher eruption frequencies. By contrast, greater compositional heterogeneity in lavas at slower spreading rates probably results from low thermal stability of the crust (due to diminished magma supply and greater hydrothermal cooling). Finally, the within-flow compositional variations observed here imply that caution must be exercised when interpreting MOR basalt data on samples where individual flows have not been mapped because chemical variations between lava samples may not necessarily record the history of spatially and temporally distinct

  1. Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere

    NASA Astrophysics Data System (ADS)

    Schlindwein, Vera; Schmid, Florian

    2016-07-01

    Along ultraslow-spreading ridges, where oceanic tectonic plates drift very slowly apart, conductive cooling is thought to limit mantle melting and melt production has been inferred to be highly discontinuous. Along such spreading centres, long ridge sections without any igneous crust alternate with magmatic sections that host massive volcanoes capable of strong earthquakes. Hence melt supply, lithospheric composition and tectonic structure seem to vary considerably along the axis of the slowest-spreading ridges. However, owing to the lack of seismic data, the lithospheric structure of ultraslow ridges is poorly constrained. Here we describe the structure and accretion modes of two end-member types of oceanic lithosphere using a detailed seismicity survey along 390 kilometres of ultraslow-spreading ridge axis. We observe that amagmatic sections lack shallow seismicity in the upper 15 kilometres of the lithosphere, but unusually contain earthquakes down to depths of 35 kilometres. This observation implies a cold, thick lithosphere, with an upper aseismic zone that probably reflects substantial serpentinization. We find that regions of magmatic lithosphere thin dramatically under volcanic centres, and infer that the resulting topography of the lithosphere-asthenosphere boundary could allow along-axis melt flow, explaining the uneven crustal production at ultraslow-spreading ridges. The seismicity data indicate that alteration in ocean lithosphere may reach far deeper than previously thought, with important implications towards seafloor deformation and fluid circulation.

  2. Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere.

    PubMed

    Schlindwein, Vera; Schmid, Florian

    2016-07-14

    Along ultraslow-spreading ridges, where oceanic tectonic plates drift very slowly apart, conductive cooling is thought to limit mantle melting and melt production has been inferred to be highly discontinuous. Along such spreading centres, long ridge sections without any igneous crust alternate with magmatic sections that host massive volcanoes capable of strong earthquakes. Hence melt supply, lithospheric composition and tectonic structure seem to vary considerably along the axis of the slowest-spreading ridges. However, owing to the lack of seismic data, the lithospheric structure of ultraslow ridges is poorly constrained. Here we describe the structure and accretion modes of two end-member types of oceanic lithosphere using a detailed seismicity survey along 390 kilometres of ultraslow-spreading ridge axis. We observe that amagmatic sections lack shallow seismicity in the upper 15 kilometres of the lithosphere, but unusually contain earthquakes down to depths of 35 kilometres. This observation implies a cold, thick lithosphere, with an upper aseismic zone that probably reflects substantial serpentinization. We find that regions of magmatic lithosphere thin dramatically under volcanic centres, and infer that the resulting topography of the lithosphere-asthenosphere boundary could allow along-axis melt flow, explaining the uneven crustal production at ultraslow-spreading ridges. The seismicity data indicate that alteration in ocean lithosphere may reach far deeper than previously thought, with important implications towards seafloor deformation and fluid circulation. PMID:27362231

  3. Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere

    NASA Astrophysics Data System (ADS)

    Schlindwein, Vera; Schmid, Florian

    2016-07-01

    Along ultraslow-spreading ridges, where oceanic tectonic plates drift very slowly apart, conductive cooling is thought to limit mantle melting and melt production has been inferred to be highly discontinuous. Along such spreading centres, long ridge sections without any igneous crust alternate with magmatic sections that host massive volcanoes capable of strong earthquakes. Hence melt supply, lithospheric composition and tectonic structure seem to vary considerably along the axis of the slowest-spreading ridges. However, owing to the lack of seismic data, the lithospheric structure of ultraslow ridges is poorly constrained. Here we describe the structure and accretion modes of two end-member types of oceanic lithosphere using a detailed seismicity survey along 390 kilometres of ultraslow-spreading ridge axis. We observe that amagmatic sections lack shallow seismicity in the upper 15 kilometres of the lithosphere, but unusually contain earthquakes down to depths of 35 kilometres. This observation implies a cold, thick lithosphere, with an upper aseismic zone that probably reflects substantial serpentinization. We find that regions of magmatic lithosphere thin dramatically under volcanic centres, and infer that the resulting topography of the lithosphere–asthenosphere boundary could allow along-axis melt flow, explaining the uneven crustal production at ultraslow-spreading ridges. The seismicity data indicate that alteration in ocean lithosphere may reach far deeper than previously thought, with important implications towards seafloor deformation and fluid circulation.

  4. Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere.

    PubMed

    Schlindwein, Vera; Schmid, Florian

    2016-06-29

    Along ultraslow-spreading ridges, where oceanic tectonic plates drift very slowly apart, conductive cooling is thought to limit mantle melting and melt production has been inferred to be highly discontinuous. Along such spreading centres, long ridge sections without any igneous crust alternate with magmatic sections that host massive volcanoes capable of strong earthquakes. Hence melt supply, lithospheric composition and tectonic structure seem to vary considerably along the axis of the slowest-spreading ridges. However, owing to the lack of seismic data, the lithospheric structure of ultraslow ridges is poorly constrained. Here we describe the structure and accretion modes of two end-member types of oceanic lithosphere using a detailed seismicity survey along 390 kilometres of ultraslow-spreading ridge axis. We observe that amagmatic sections lack shallow seismicity in the upper 15 kilometres of the lithosphere, but unusually contain earthquakes down to depths of 35 kilometres. This observation implies a cold, thick lithosphere, with an upper aseismic zone that probably reflects substantial serpentinization. We find that regions of magmatic lithosphere thin dramatically under volcanic centres, and infer that the resulting topography of the lithosphere-asthenosphere boundary could allow along-axis melt flow, explaining the uneven crustal production at ultraslow-spreading ridges. The seismicity data indicate that alteration in ocean lithosphere may reach far deeper than previously thought, with important implications towards seafloor deformation and fluid circulation.

  5. Experimental and theoretical constraints on the origin of mid-ocean ridge geothermal fluids

    SciTech Connect

    Berndt, M.E.

    1987-01-01

    Hydrothermal experiments were performed using basalt, diabase, and two synthetic plagioclase bearing assemblages and Na-Ca-K-Cl fluids of seawater chlorinity at conditions from 350 to 425/sup 0/C and 250 to 400 bars. Dissolved Ca, Na, SiO/sub 2/, and pH appear to be controlled by equilibrium with plagioclase and epidote. Fluids reacting with diabase at low fluid/rock ratios (0.5-1) remain undersaturated with respect to quartz due to formation of olivine hydration products, whereas fluids reacting with basalt become supersaturated with respect to quartz due to breakdown of fractionated glass and formation of amphibole. High SiO/sub 2/ activities during basalt alteration, leads to high Ca and base metal concentrations and low pH compared to diabase alteration at the same conditions. Dissolved Li, K, Rb, and Ba concentrations reach higher levels during basalt alteration than during diabase alteration. Since these elements avoid incorporation into crystalline phases during solidification of magmas they are concentrated in the glass which is easily altered by fluids and explains their increased mobility during basalt alteration. Na-Ca-pH-SiO/sub 2/ relationships in vent fluids can be used to constrain reaction zone conditions assuming the fluids are equilibrated with plagioclase and epidote. The temperatures predicted by such models are higher than measured vent fluid temperatures. Dissolved Sr/Ca ratios for ridge crest fluids are similar to those produced during diabase alteration and higher than those produced during basalt alteration. This observation supports deep-seated reaction of the hydrothermal fluids with diabase dikes and/or gabbro for vent fluid origin. Only 4% of the Sr initially present in basalt is mobilized during hydrothermal alteration even after 800 hours of reaction.

  6. Enhanced volcanic CO2 degassing at oceanic hotspots and mid-ocean ridges in response to falling sea level

    NASA Astrophysics Data System (ADS)

    Hasenclever, Jörg; Knorr, Gregor; Rüpke, Lars; Köhler, Peter; Morgan, Jason; Garofalo, Kristin; Barker, Stephen; Lohmann, Gerrit; Hall, Ian

    2016-04-01

    Evidence from paleo-climate proxy data as well as results from geodynamical and biogeochemical modelling point to complex interactions between sea level variations, pressure-release melting of oceanic mantle, associated volcanic degassing, and atmospheric CO2 concentrations. Ice core data shows that the orbital component in global temperature records gradually declined between ˜85,000-70,000 yr BP, while atmospheric CO2 - instead of continuing its long-term correlation with Antarctic temperatures - remained relatively stable for several thousand years. Based on 2-D and 3-D geodynamical models we show that the massive (60-100 m) sea level drop during this period of Earth history led to a significant increase in magma and possibly CO2 fluxes along mid-ocean ridges (MOR) and especially oceanic hotspot volcanoes. We assess the MOR magma and CO2 fluxes using 2-D thermo-mechanical models that solve for wet melting of the mantle and the partitioning of highly incompatible carbon dioxide into the melt. These models have been run at various MOR opening rates, and we integrate these results with the global distribution of spreading rates to compute baseline fluxes as well as enhanced fluxes during the sea level fall. Furthermore we conducted more than 120 3-D simulations of rising and melting mantle plumes to construct a four-dimensional parameter space that covers a wide range of plume buoyancy fluxes, plume excess temperatures, lithosphere thicknesses and plate speeds. Using published data on 43 oceanic hotspots and locating them in the parameter space we derive a global hotspot-melting model that predicts magma and CO2 fluxes before and during the sea level drop. We find that, during a 80 m sea level drop over 10 kyr, global degassing at MOR and oceanic hotspots increases by 26 % and 36 %, respectively. Biogeochemical carbon cycle modelling further shows that the combined predicted increase in volcanic emissions along the global mid-ocean ridge system and at oceanic

  7. Some Approaches to Modeling Diffuse Flow at Mid-Ocean Ridges

    NASA Astrophysics Data System (ADS)

    Farough, A.; Lowell, R. P.; Craft, K.; Germanovich, L. N.

    2011-12-01

    To obtain a sound understanding of subsurface temperatures and the extent of the subsurface biosphere in young oceanic crust, one must understand the mechanisms of diffuse flow at oceanic spreading centers. Mathematical modeling of diffuse flow at oceanic spreading centers has received relatively little attention compared to high-temperature black smoker discharge, in part because the temperature and fluid flow data required to constrain the models are scarce. We review a number of different approaches to modelling diffuse flow: (1) The simplest method considers 1-D steady-state uniform upflow from below subject to a heat transfer boundary condition at the surface, which represents the effects of mixing of hydrothermal fluid with seawater. These models, in which the heat transfer coefficient and the velocity of the ascending fluid are constrained by observed diffuse flow vent temperature and heat flux, typically result in a steep temperature gradient near the seafloor and subsurface biological activity may be limited to the upper few cm of the crust. (2) A related method uses data on the partitioning of heat flux between focused and diffuse flow and chemical data from the focused and diffuse flow components in a two-limb single pass modeling approach to determine the fraction of high-temperature fluid that is incorporated in the diffuse flow. Using data available from EPR 950', the Main Endeavour Field, and ASHES vent field at Axial Volcano on the Juan de Fuca Ridge in conjunction with Mg as a passive tracer, we find that the mixing ratio of high temperature in diffuse flow is <10%. The high-temperature contribution to the diffuse heat flux remains large, however, and high-temperature vent fluid ultimately contributes ~ 90% of the total heat output from the vent field. In these models mixing between high-temperature fluid and seawater may occur over a considerable depth, and the subsurface biosphere may be ~ 100 m deep beneath diffuse flow sites. (3) Finally, in

  8. Along-strike magma mixing beneath mid-ocean ridges - Effects on isotopic ratios

    NASA Technical Reports Server (NTRS)

    Kenyon, P. M.; Turcotte, D. L.

    1987-01-01

    The effects of mixing processes on the isotopic variability of midocean ridge basalts are studied. The processes considered are porous flow dispersion and convective mixing in magma chambers. Porous flow dispersion is capable of mixing magmas over distances of only a few tens of meters. Convective mixing, on the other hand, is found to produce continuous magma chambers, where mixing is limited by convective processes, and for discontinuous chambers, where mixing is limited by chamber size. Preliminary comparison of the calculations with observations along the midocean ridges shows that the calculations are consistent with the existence of a correlation between bathymetry and isotopic ratio at long, but not at short, wavelengths. They are also capable of explaining a decrease in isotopic variability with increasing spreading rate.

  9. Electrical properties of the mantle upwelling zone beneath a mid-ocean ridge: An application of vertical gradient sounding

    NASA Astrophysics Data System (ADS)

    Jegen, Marion Dorothea

    On mid-ocean ridges, as adjacent plates move apart, the mantle material rises to fill the void created. During its ascent the solidus of the material is crossed and melting occurs. The melt itself is eventually emplaced at the ridge axis producing new oceanic crust. The understanding of the flow of the solid and molten material is hampered by the lack of knowledge of vital model parameters such as the connectivity of the partial melt. Connectivity is related to the permeability in the upwelling region. It therefore controls the migration pattern of the buoyant melt, the flow of the solid phase material, and the mantle upwelling mechanism. Changes in the geometry of the distribution of melt in the solid material have a large impact on the electrical conductivity. I have measured the conductivity of the upwelling region to constrain possible partial melt geometries. I present results of vertical gradient sounding (VGS) experiments on the Endeavour and Explorer ridge, which are part of the Juan de Fuca and its northern extension, the Explorer ridge, respectively. The VGS method is a natural source EM method based entirely on measurements of the magnetic fields. Electrical responses of the 1D layered normal seafloor combined with a 2D region representing the mantle upwelling zone and proposed upwelling mechanisms are derived. A comparison of the synthetic response of a range of models with data measured on the Endeavour segment shows that the conductivity in the upwelling region is very high (in the order of 1 to 5 ohm m depending on the shape of the upwelling region). The results of this experiment suggest that the pore space containing the conductive melt is well connected. The melt must be able to move freely through the upwelling region. The experiments support so called melt migration models. The data measured on the Explorer segment yielded a different conductivity model. The data do not require the presence of a pronounced 2D conductivity anomaly at depth and

  10. Loki's Castle: Discovery and geology of a black smoker vent field at the Arctic Mid-Ocean Ridge

    NASA Astrophysics Data System (ADS)

    Pedersen, R.; Thorseth, I. H.; Lilley, M. D.; Barriga, F. J.; Früh-Green, G.; Nakamura, K.

    2010-12-01

    Previous attempts to locate hydrothermal vent fields and unravel the nature of venting at the ultraslow spreading and magma starved parts of the Arctic Mid Ocean Ridge (AMOR) have been unsuccessful. A black smoker vent field was eventually discovered at the Mohns-Knipovich bend at 73.5°N in 2008, and the field was revisited in 2009 and 2010. The Loki’s Castle vent field is located on the crest of an axial volcanic ridge that is bordered by a tectonic terrain dominated by core complexes to the NW, and a ridge flank that is buried by sediments from the Bear Island Fan to the SE. Fluid compositions are anomalous to other basalt-hosted fields and indicate interactions with sediments at depths. The vent field is associated with an unusually large hydrothermal deposit, which documents that extensive venting occurs at ultraslow spreading ridges despite the strongly reduced magmatic heat budget. ROV surveys have shown that venting occurs in two areas separated by around 100 m. Micro-bathymetry acquired by a Hugin AUV documents that two 20-30 tall mounds that coalesce at the base have developed around the vent sites. The micro-bathymetry also shows that the venting is located above two normal faults that define the NW margin of a rift that runs along the crest of the volcano. The black smoker fluids reach 317 °C, with an end-member SiO2 content of 16 mmol/kg. End-member chlorinity is around 85% of seawater suggesting that the fluids have phase-separated at depth. The fluid compositions indicate that the rock-water reactions occur around 2 km below the seafloor. The crustal thickness is estimated to be 4 +/- 0.5 km in the area. Whereas the depth of the reaction zone is comparable with faster spreading ridges, the fraction of crust cooled convectively by hydrothermal circulation is two times that of vent fields at ridges with normal crustal thickness.

  11. Mantle rock exposures at oceanic core complexes along mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Ciazela, Jakub; Koepke, Juergen; Dick, Henry J. B.; Muszynski, Andrzej

    2015-12-01

    The mantle is the most voluminous part of the Earth. However, mantle petrologists usually have to rely on indirect geophysical methods or on material found ex situ. In this review paper, we point out the in-situ existence of oceanic core complexes (OCCs), which provide large exposures of mantle and lower crustal rocks on the seafloor on detachment fault footwalls at slow-spreading ridges. OCCs are a common structure in oceanic crust architecture of slow-spreading ridges. At least 172 OCCs have been identified so far and we can expect to discover hundreds of new OCCs as more detailed mapping takes place. Thirty-two of the thirty-nine OCCs that have been sampled to date contain peridotites. Moreover, peridotites dominate in the plutonic footwall of 77% of OCCs. Massive OCC peridotites come from the very top of the melting column beneath ocean ridges. They are typically spinel harzburgites and show 11.3-18.3% partial melting, generally representing a maximum degree of melting along a segment. Another key feature is the lower frequency of plagioclase-bearing peridotites in the mantle rocks and the lower abundance of plagioclase in the plagioclase-bearing peridotites in comparison to transform peridotites. The presence of plagioclase is usually linked to impregnation with late-stage melt. Based on the above, OCC peridotites away from segment ends and transforms can be treated as a new class of abyssal peridotites that differ from transform peridotites by a higher degree of partial melting and lower interaction with subsequent transient melt.

  12. Poroelastic response of mid-ocean ridge hydrothermal systems to ocean tidal loading: Implications for shallow permeability structure

    NASA Astrophysics Data System (ADS)

    Barreyre, Thibaut; Sohn, Robert A.

    2016-02-01

    We use the time delay between tidal loading and exit-fluid temperature response for hydrothermal vents to model the poroelastic behavior and shallow upflow zone (SUZ) effective permeability structure of three mid-ocean ridge (MOR) sites with different spreading rates. Hydrothermal vents at Lucky Strike field exhibit relatively small phase lags corresponding to high SUZ effective permeabilities of ≥ ~10-10 m2, with variations that we interpret as resulting from differences in the extrusive layer thickness. By contrast, vents at East Pacific Rise site exhibit relatively large phase lags corresponding to low SUZ effective permeabilities of ≤ ~10-13 m2. Vents at Main Endeavour field exhibit both high and low phase lags, suggestive of a transitional behavior. Our results demonstrate that tidal forcing perturbs hydrothermal flow across the global MOR system, even in places where the tidal amplitude is very low, and that the flow response can be used to constrain variations in SUZ permeability structure beneath individual vent fields.

  13. Seismic structure of the lithosphere and upper mantle beneath the ocean islands near mid-oceanic ridges

    NASA Astrophysics Data System (ADS)

    Haldar, C.; Kumar, P.; Kumar, M. Ravi

    2014-05-01

    Deciphering the seismic character of the young lithosphere near mid-oceanic ridges (MORs) is a challenging endeavor. In this study, we determine the seismic structure of the oceanic plate near the MORs using the P-to-S conversions isolated from quality data recorded at five broadband seismological stations situated on ocean islands in their vicinity. Estimates of the crustal and lithospheric thickness values from waveform inversion of the P-receiver function stacks at individual stations reveal that the Moho depth varies between ~ 10 ± 1 km and ~ 20 ± 1 km with the depths of the lithosphere-asthenosphere boundary (LAB) varying between ~ 40 ± 4 and ~ 65 ± 7 km. We found evidence for an additional low-velocity layer below the expected LAB depths at stations on Ascension, São Jorge and Easter islands. The layer probably relates to the presence of a hot spot corresponding to a magma chamber. Further, thinning of the upper mantle transition zone suggests a hotter mantle transition zone due to the possible presence of plumes in the mantle beneath the stations.

  14. Cu, Fe, and Zn Isotope Variations Within a High-Temperature Mid-Ocean Ridge Sulfide Structure

    NASA Astrophysics Data System (ADS)

    Ewing, S. M.; Nelson, B. K.; Kelley, D. S.; Nielsen, D. C.

    2006-12-01

    Hydrothermal processes at mid-ocean ridges play an important role in controlling the transition metal budget of seawater and the crust through which it circulates. Preliminary work has shown stable metal isotope variations accompany these processes. We report Cu, Zn, and Fe isotope analyses of transects through a high temperature sulfide structure ("Fin") collected during the 1998 Edifice Rex Sulfide Recovery Project. We analyzed two horizontal transects through the sulfide edifice, from inner conduit to outer surface. Transects A and F are 9 and 6 cm in length, respectively. Each displays radially zoned mineralogy progressing from a chalcopyrite (ccp) zone through zones of zinc sulfide, pyrite-anhydrite (pyr-anh) matrix, zinc sulfide-anhydrite (zns-anh) matrix, to an outer well-cemented silica (Si) zone. Additional ccp and pyr-anh zones appear in transect A resulting from a smaller breakout conduit. In transect A, Cu displays the most isotopic variation, with little variation in Fe and Zn isotopes. From the inner ccp zone outward, the Cu isotope profile shows a 0.4‰ (±0.05‰ 2σ) increase in the first pyr-anh zone over the coarse-grained ccp zone. The δ65Cu drops by 0.6‰ in the secondary ccp zone and recovers to values of the innermost wall in the following zone where it is constant until the outermost portion of the Si rich zone, which shows a 1.3‰ increase over inner zone values. The Zn isotope profile has a total variation of 0.27‰ (±0.05‰ 2σ), with a 0.2‰ increase in the first pyr- anh zone followed by a .27‰ decrease in the adjacent zone, and recovering to its heaviest values in the second pyr-anh zone. The Zn profile lacks any significant increase of the δ^{64}Zn in the outermost zones. The Fe isotope profile shows very little variation across the chimney wall, but does have a sharp 0.7‰ (±0.1‰ 2σ) increase in the δ56Fe in the well-cemented Si rich zone. In transect F, the Cu isotope profile again shows the most variation, but

  15. Link between Mid-Ocean Ridge kinematics and uplift of passive continental margins

    NASA Astrophysics Data System (ADS)

    Døssing, A.; Japsen, P.; Nielsen, T.; Thybo, H.; Dahl-Jensen, T.

    2012-04-01

    Tectonic models predict post-rift subsidence of rift margins after initial flexural rebound and transgression of a sedimentary wedge over the subsiding mar¬gin as the lithosphere cools with time. However, studies of North Atlantic rifted margins show that thermal subsidence following breakup at the Paleocene-Eocene transition was interrupted by significant uplift movements. These vertical movements represent a long-standing enigma and they have been linked to sea-level fluctuations, climate deterioration and tectonics but as yet they remain unexplained. Here we combine regional Multi-Channel Seismic reflection data across the NE Greenland Shelf, the Greenland Fracture Zone (GFZ) and continental East Greenland Ridge (EGR) in the northern NE Atlantic and stratigraphic data from a drill core. We show that a mid-Miocene change from down-faulting to uplift along the GFZ-EGR correlates with significant uplift of the NE Greenland margin. This tectonic change is associated with a regional unconformity that marks the first occurrences of mass-¬wasted deposits in the deep sea off the NE Greenland Shelf and the development of prograd¬ing mega-sequences and angular truncation of hemipelagic sediments below the unconformity, respectively, on the outer and inner NE Greenland Shelf. We attribute the tectonic changes at the GFZ to the development of a modern, continuous spreading system along the Mohns-Knipovich Ridge segments that led to an opening of the Fram Strait corridor, to large-scale changes in ocean circulation and climate and possibly to medium-scale (20-30 m) sea-level fluctuations. While these consequences of the tectonic changes may have affected the amplitude of uplift in NE Greenland, they cannot explain the uplift at the GFZ-EGR in deep sea. We therefore find that plate-tectonic changes produced the driving force for the mid-Miocene uplift in NE Greenland.

  16. The effects of magmatic processes and crustal recycling on the molybdenum stable isotopic composition of Mid-Ocean Ridge Basalts

    NASA Astrophysics Data System (ADS)

    Bezard, Rachel; Fischer-Gödde, Mario; Hamelin, Cédric; Brennecka, Gregory A.; Kleine, Thorsten

    2016-11-01

    Molybdenum (Mo) stable isotopes hold great potential to investigate the processes involved in planetary formation and differentiation. However their use is currently hampered by the lack of understanding of the dominant controls driving mass-dependent fractionations at high temperature. Here we investigate the role of magmatic processes and mantle source heterogeneities on the Mo isotope composition of Mid-Ocean Ridges Basalts (MORBs) using samples from two contrasting ridge segments: (1) the extremely fast spreading Pacific-Antarctic (66-41°S) section devoid of plume influence and; (2) the slow spreading Mohns-Knipovich segment (77-71°N) intercepted by the Jan Mayen Plume (71°N). We show that significant variations in Mo stable isotope composition exist in MORBs with δ98/95Mo ranging from - 0.24 ‰ to + 0.15 ‰ (relative to NIST SRM3134). The absence of correlation between δ98/95Mo and indices of magma differentiation or partial melting suggests a negligible impact of these processes on the isotopic variations observed. On the other hand, the δ98/95Mo variations seem to be associated with changes in radiogenic isotope signatures and rare earth element ratios (e.g., (La/Sm)N), suggesting mantle source heterogeneities as a dominant factor for the δ98/95Mo variations amongst MORBs. The heaviest Mo isotope compositions correspond to the most enriched signatures, suggesting that recycled crustal components are isotopically heavy compared to the uncontaminated depleted mantle. The uncontaminated depleted mantle shows slightly sub-chondritic δ98/95Mo, which cannot be produced by core formation and, therefore, more likely result from extensive anterior partial melting of the mantle. Consequently, the primitive δ98/95Mo composition of the depleted mantle appears overprinted by the effects of both partial melting and crustal recycling.

  17. Conductive heat loss in recent eruptions at mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Johnson, Paul; Hutnak, Michael

    A new technique for measuring conductive heat flow from unsedimented volcanic rocks on the sea floor has been tested on two new eruption sites in the NE Pacific. This technique consists of isolating the surficial rocks from sea water using water-saturated urethane foam as an insulating thermal blanket. The thermal gradient transferred from the outcrop to the thermal blanket is a quantitative measurement of the conductive heat flow that takes place in unsedimented volcanic areas. We deployed two thermal blankets at 13 sites on the 1993 and 1996 Juan de Fuca/Gorda Ridge flows and found (1) a factor of 10 decrease in heat flow over a period of 12 months on the 1993 CoAxial flow, (2) a value of 6950 mW/m² on the 8 month old Gorda flow, and (3) measurements of heat flow versus age-since-eruption indicate that newly extruded volcanic units are quite permeable to fluid circulation and cool rapidly by convection in only a few years. These new heat flux data confirm that the extrusive volcanic layer is not the primary heat source for long-lived, high temperature hydrothermal systems, which must instead rely on a more isolated thermal reservoir within the lower crustal rocks.

  18. Novel microbial assemblages inhabiting crustal fluids within mid-ocean ridge flank subsurface basalt.

    PubMed

    Jungbluth, Sean P; Bowers, Robert M; Lin, Huei-Ting; Cowen, James P; Rappé, Michael S

    2016-08-01

    Although little is known regarding microbial life within our planet's rock-hosted deep subseafloor biosphere, boreholes drilled through deep ocean sediment and into the underlying basaltic crust provide invaluable windows of access that have been used previously to document the presence of microorganisms within fluids percolating through the deep ocean crust. In this study, the analysis of 1.7 million small subunit ribosomal RNA genes amplified and sequenced from marine sediment, bottom seawater and basalt-hosted deep subseafloor fluids that span multiple years and locations on the Juan de Fuca Ridge flank was used to quantitatively delineate a subseafloor microbiome comprised of distinct bacteria and archaea. Hot, anoxic crustal fluids tapped by newly installed seafloor sampling observatories at boreholes U1362A and U1362B contained abundant bacterial lineages of phylogenetically unique Nitrospirae, Aminicenantes, Calescamantes and Chloroflexi. Although less abundant, the domain Archaea was dominated by unique, uncultivated lineages of marine benthic group E, the Terrestrial Hot Spring Crenarchaeotic Group, the Bathyarchaeota and relatives of cultivated, sulfate-reducing Archaeoglobi. Consistent with recent geochemical measurements and bioenergetic predictions, the potential importance of methane cycling and sulfate reduction were imprinted within the basalt-hosted deep subseafloor crustal fluid microbial community. This unique window of access to the deep ocean subsurface basement reveals a microbial landscape that exhibits previously undetected spatial heterogeneity. PMID:26872042

  19. Novel microbial assemblages inhabiting crustal fluids within mid-ocean ridge flank subsurface basalt

    PubMed Central

    Jungbluth, Sean P; Bowers, Robert M; Lin, Huei-Ting; Cowen, James P; Rappé, Michael S

    2016-01-01

    Although little is known regarding microbial life within our planet's rock-hosted deep subseafloor biosphere, boreholes drilled through deep ocean sediment and into the underlying basaltic crust provide invaluable windows of access that have been used previously to document the presence of microorganisms within fluids percolating through the deep ocean crust. In this study, the analysis of 1.7 million small subunit ribosomal RNA genes amplified and sequenced from marine sediment, bottom seawater and basalt-hosted deep subseafloor fluids that span multiple years and locations on the Juan de Fuca Ridge flank was used to quantitatively delineate a subseafloor microbiome comprised of distinct bacteria and archaea. Hot, anoxic crustal fluids tapped by newly installed seafloor sampling observatories at boreholes U1362A and U1362B contained abundant bacterial lineages of phylogenetically unique Nitrospirae, Aminicenantes, Calescamantes and Chloroflexi. Although less abundant, the domain Archaea was dominated by unique, uncultivated lineages of marine benthic group E, the Terrestrial Hot Spring Crenarchaeotic Group, the Bathyarchaeota and relatives of cultivated, sulfate-reducing Archaeoglobi. Consistent with recent geochemical measurements and bioenergetic predictions, the potential importance of methane cycling and sulfate reduction were imprinted within the basalt-hosted deep subseafloor crustal fluid microbial community. This unique window of access to the deep ocean subsurface basement reveals a microbial landscape that exhibits previously undetected spatial heterogeneity. PMID:26872042

  20. Novel microbial assemblages inhabiting crustal fluids within mid-ocean ridge flank subsurface basalt.

    PubMed

    Jungbluth, Sean P; Bowers, Robert M; Lin, Huei-Ting; Cowen, James P; Rappé, Michael S

    2016-08-01

    Although little is known regarding microbial life within our planet's rock-hosted deep subseafloor biosphere, boreholes drilled through deep ocean sediment and into the underlying basaltic crust provide invaluable windows of access that have been used previously to document the presence of microorganisms within fluids percolating through the deep ocean crust. In this study, the analysis of 1.7 million small subunit ribosomal RNA genes amplified and sequenced from marine sediment, bottom seawater and basalt-hosted deep subseafloor fluids that span multiple years and locations on the Juan de Fuca Ridge flank was used to quantitatively delineate a subseafloor microbiome comprised of distinct bacteria and archaea. Hot, anoxic crustal fluids tapped by newly installed seafloor sampling observatories at boreholes U1362A and U1362B contained abundant bacterial lineages of phylogenetically unique Nitrospirae, Aminicenantes, Calescamantes and Chloroflexi. Although less abundant, the domain Archaea was dominated by unique, uncultivated lineages of marine benthic group E, the Terrestrial Hot Spring Crenarchaeotic Group, the Bathyarchaeota and relatives of cultivated, sulfate-reducing Archaeoglobi. Consistent with recent geochemical measurements and bioenergetic predictions, the potential importance of methane cycling and sulfate reduction were imprinted within the basalt-hosted deep subseafloor crustal fluid microbial community. This unique window of access to the deep ocean subsurface basement reveals a microbial landscape that exhibits previously undetected spatial heterogeneity.

  1. Low seismic velocities below mid-ocean ridges: Attenuation versus melt retention

    NASA Astrophysics Data System (ADS)

    Goes, Saskia; Armitage, John; Harmon, Nick; Smith, Hannah; Huismans, Ritske

    2012-12-01

    The first comprehensive seismic experiment sampling subridge mantle revealed a pronounced low-velocity zone between 40 and 100 km depth below the East Pacific Rise (EPR) that has been attributed to substantial retained melt fractions of 0.3-2%. Such high melt fractions are at odds with low melt productivity and high melt mobility inferred from petrology and geochemistry. Here, we evaluate whether seismic attenuation can reconcile subridge seismic structure with low melt fractions. We start from a dynamic spreading model which includes melt generation and migration and is converted into seismic structure, accounting for temperature-, pressure-, composition-, phase-, and melt-dependent anharmonicity, and temperature-, pressure-, frequency- and hydration-dependent anelasticity. Our models predict a double low-velocity zone: a shallow—approximately triangular—region due to dry melting, and a low-velocity channel between 60 and 150 km depth dominantly controlled by solid state high-temperature seismic attenuation in a damp mantle, with only a minor contribution of (<0.1%) melt. We test how tomographic inversion influences the imaging of our modeled shear velocity features. The EPR experiment revealed a double low-velocity zone, but most tomographic studies would only resolve the deeper velocity minimum. Experimentally constrained anelasticity formulations produce VSas low as observed and can explain lateral variations in near-ridge asthenospheric VS with ±100 K temperature variations and/or zero to high water content. Furthermore, such QS formulations also reproduce low asthenospheric VS below older oceans and continents from basic lithospheric cooling models. Although these structures are compatible with global QS images, they are more attenuating than permitted by EPR data.

  2. The axial melt lens as a processor of evolved melts at fast-spreading mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Loocke, M. P.; Lissenberg, J. C. J.; MacLeod, C. J.

    2015-12-01

    The axial melt lens is a steady-state, generally magma-rich body located at the dyke-gabbro transition at mid-crustal levels beneath intermediate- and fast-spreading ridges. It is widely believed to be the reservoir from which mid-ocean ridge basalt (MORB) erupts. Using a remotely-operated vehicle, cruise JC21 to the Hess Deep Rift recovered the first comprehensive sample suite of the uppermost plutonics from a fast-spreading ridge. We present the results of a detailed microanalytical investigation of 23 samples (8 dolerites, 14 gabbronorites, and 1 gabbro) recovered by ROV dive 78 from a section traversing the transition from the uppermost gabbros into the sheeted dykes. With the exception of a single olivine-bearing sample (78R-6), dive 78 is dominated by evolved, varitextured (both in hand sample and thin section) oxide gabbronorites. Full thin section quantitative element maps were acquired on serial thin sections from each sample using the analytical scanning electron microscope in the at Cardiff University. The resulting maps were post-processed in MatlabTM to determine the full distribution of plagioclase compositions across entire thin sections (typically 500,000 analyses per sample); an approach we term 'quantitative assessment of compositional distribution' (QACD). By so doing we are able to conduct the first fully rigorous assessment of gabbro compositions, and, by extension, melt compositions present at this level beneath the ridge axis. Critically, we only found 2 grains of high-An plagioclase (An>80) in all of the samples (N = 51). These occur as cores within a sample dominated by lower-An plagioclase. Instead, the vast majority (75%) of plagioclase within the samples have compositions of An65 or lower; compositions too evolved to be in equilibrium with MORB. The most primitive sample, 78R-6, is an olivine-bearing gabbronorite with Fo67 olivine, and plagioclase ranging from An52-77 (median An = 65). These data are difficult to reconcile with models in

  3. Macrofauna of shallow hydrothermal vents on the Arctic Mid-Ocean Ridge at 71N

    NASA Astrophysics Data System (ADS)

    Schander, C.; Rapp, H. T.; Pedersen, R. B.

    2007-12-01

    Deep-sea hydrothermal vents are usually associated with a highly specialized fauna and since their discovery in 1977, more than 400 species of animals have been described. Specialized vent fauna includes various animal phyla, but the most conspicuous and well known are annelids, mollusks and crustaceans. We have investigated the fauna collected around newly discovered hydrothermal vents on the Mohns Ridge north of Jan Mayen. The venting fields are located at 71°N and the venting takes place within two main areas separated by 5 km. The shallowest vent area is at 500-550 m water depth and is located at the base of a normal fault. This vent field stretches approximately 1 km along the strike of the fault, and it is composed of 10-20 major vent sites each with multiple chimney constructions discharging up to 260°C hot fluids. A large area of diffuse, low- temperature venting occurs in the area surrounding the high-temperature field. Here, partly microbial mediated iron-oxide-hydroxide deposits are abundant. The hydrothermal vent sites do not show any high abundance of specialized hydrothermal vent fauna. Single groups (i.e. Porifera and Mollusca) have a few representatives but groups otherwise common in hydrothermal vent areas (e.g. vestimentifera, Alvinellid worms, mussels, clams, galathaeid and brachyuran crabs) are absent. Up until now slightly more than 200 species have been identified from the vent area. The macrofauna found in the vent area is, with few exceptions, an assortment of bathyal species known in the area. One endemic, yet undescribed, species of mollusc has been found so far, an gastropod related to Alvania incognita Warén, 1996 and A. angularis Warén, 1996 (Rissoidae), two species originally described from pieces of sunken wood north and south of Iceland. It is by far the most numerous mollusc species at the vents and was found on smokers, in the bacterial mats, and on the ferric deposits. A single specimen of an undescribed tanaidacean has also

  4. Variations in Seismic Anisotropy and Olivine LPO in Peridotites From Four Mid-Ocean Ridges: the Effect of Melt?

    NASA Astrophysics Data System (ADS)

    Achenbach, K. L.; Cheadle, M. J.; Dick, H. J.; Swapp, S.

    2008-12-01

    We present results of an electron backscatter diffraction (EBSD) study of abyssal peridotites from four mid- ocean ridges. Included in the study are peridotites from: a) the Mid-Atlantic Ridge (MAR) at 15°39' N (full spreading rate ~2.6 cm/year), b) the Atlantis II Fracture Zone (57°E) on the Southwest Indian Ridge (SWIR) (full spreading rate ~1.4 cm/year), c) the Gakkel Ridge at 84°38'N and 4°13'E (full spreading rate ~1.2 cm/year), and d) Hess Deep (2°N) on the East Pacific Rise (EPR) (full spreading rate ~12.3 cm/year). All samples preserve distinct crystal lattice-preferred orientations (LPOs) indicative of high-temperature (>1100°C) deformation in the dislocation creep regime. LPOs were used to predict seismic properties following the method of Mainprice (1990). The samples from the MAR and SWIR preserve olivine [010] axes perpendicular to the foliation, and olivine [100] axis maxima within the plane of foliation, indicative of slip in the (010)[100] or "Type A" slip system commonly observed in high- temperature environments of low differential stress and low water content. Predicted P-wave anisotropy ranges from 5.8 to 8.6%. The Gakkel Ridge sample also preserves an olivine [010] axis maxima perpendicular to the foliation, but has a girdle of olivine [100] axes within the plane of the foliation. Holtzman et al. (2003) observed this latter LPO pattern during deformation experiments of olivine + MORB, and suggested that it is characteristic of olivine deformation in the presence of oriented melt pockets. Predicted P-wave anisotropy is 6.8%. The EPR sample preserves both olivine [010] and [100] axis maxima within the plane of the foliation and olivine [001] axis maxima perpendicular to the foliation. We interpret the EPR olivine slip system as (001)[100], which is in agreement with findings by Boudier et al. (1996) from the same sample suite. This"Type E" slip system is thought to form in high-temperature low diferential stress environments, either with

  5. Quantitative and phylogenetic study of the Deep Sea Archaeal Group in sediments of the Arctic mid-ocean spreading ridge

    PubMed Central

    Jørgensen, Steffen L.; Thorseth, Ingunn H.; Pedersen, Rolf B.; Baumberger, Tamara; Schleper, Christa

    2013-01-01

    In marine sediments archaea often constitute a considerable part of the microbial community, of which the Deep Sea Archaeal Group (DSAG) is one of the most predominant. Despite their high abundance no members from this archaeal group have so far been characterized and thus their metabolism is unknown. Here we show that the relative abundance of DSAG marker genes can be correlated with geochemical parameters, allowing prediction of both the potential electron donors and acceptors of these organisms. We estimated the abundance of 16S rRNA genes from Archaea, Bacteria, and DSAG in 52 sediment horizons from two cores collected at the slow-spreading Arctic Mid-Ocean Ridge, using qPCR. The results indicate that members of the DSAG make up the entire archaeal population in certain horizons and constitute up to ~50% of the total microbial community. The quantitative data were correlated to 30 different geophysical and geochemical parameters obtained from the same sediment horizons. We observed a significant correlation between the relative abundance of DSAG 16S rRNA genes and the content of organic carbon (p < 0.0001). Further, significant co-variation with iron oxide, and dissolved iron and manganese (all p < 0.0000), indicated a direct or indirect link to iron and manganese cycling. Neither of these parameters correlated with the relative abundance of archaeal or bacterial 16S rRNA genes, nor did any other major electron donor or acceptor measured. Phylogenetic analysis of DSAG 16S rRNA gene sequences reveals three monophyletic lineages with no apparent habitat-specific distribution. In this study we support the hypothesis that members of the DSAG are tightly linked to the content of organic carbon and directly or indirectly involved in the cycling of iron and/or manganese compounds. Further, we provide a molecular tool to assess their abundance in environmental samples and enrichment cultures. PMID:24109477

  6. Degassing history of a mid-ocean ridge rhyolite dome on the Alarcon Rise, Gulf of California

    NASA Astrophysics Data System (ADS)

    Portner, R. A.; Dreyer, B. M.; Clague, D. A.; Lowenstern, J. B.; Head, J. W., III; Saal, A. E.

    2014-12-01

    A 2350 meter deep rhyolite lava dome and surrounding intermediate-mafic complex on the Alarcon Rise mid-ocean ridge in the Gulf of California was sampled extensively during a 2012 MBARI expedition. The dome is predominantly composed of sparsely vesicular (<10%) obsidian with local deposits of pumiceous breccia. Pumiceous lapilli comprise highly vesicular (40-60%) fracture networks that separate non-vesicular obsidian "pseudoclasts". Textures and major element geochemistry suggest that both lithologies originated from the same magma that formed the majority of the dome. This is corroborated by comparable major element compositions (~75% SiO2) and near-equilibrium phenocryst assemblages including olivine (Fo10) and plagioclase (An17). Attenuated total reflectance (ATR) and transmission FTIR spectroscopy was used to measure H2O concentrations in olivine and plagioclase melt inclusions as well as host glasses (CO2 was below detection, <30 ppm). Rhyolite host glass contains 1.5-2.0 wt% H2O, similar to nearby andesite and dacite. These concentrations agree with saturation limits for H2O (1.7%) at the depth of Alarcon Rise, but are slightly less than what is predicted by fractional crystallization modeling. Melt inclusions from plagioclase and olivine in rhyolite contain a maximum of 3.5-4.5% H2O suggesting that up to 3.0% H2O exsolved into bubbles during a 3 km ascent. Hydrostatic pressures (23 MPa) at the eruptive vent would have permitted 53% vesiculation in agreement with petrographic observations. Although ~50% vesiculation and exsolved H2O contents of 3.0 wt% are less than the ideal threshold for magmatic fragmentation, the presence of highly vesicular ash particles representing fragmented pumiceous breccia argues otherwise. We posit that decoupled volatiles from a deeper magma body migrated through fracture networks to the surface causing mild explosivity.

  7. Spreading-rate dependent mid-ocean ridge processes expressed in Western Atlantic lithosphere

    NASA Astrophysics Data System (ADS)

    Kim, Sangmyung David

    The Far-Offset Active-Source Imaging of Mantle (FAIM) experiment was conducted along an 800-km-long transect in the Western Atlantic to study the evolution of 108-157 m.y. lithosphere. The main transect (Line 1) crosses a transition from slow (13-14 mm/yr in half rate) to ultra-slow (˜8 mm/yr) paleo spreading rates, and thus represents an ideal setting to study spreading-rate dependent processes as expressed in lithospheric structure. This thesis presents results of four analyses efforts along this transect. We present a crustal model based on seismic refraction and wide-angle traveltime modeling, we extend the crustal model to an upper lithosphere density model using gravity constraints, we constrain Poisson's ratio in oceanic Layer 3 using converted shear-wave phases, and we consider regional lithospheric structure by analysis of geoid/topography ratios. The crustal model indicates that a transition in crustal thickness accompanies the spreading-rate change, with the crust produced at slow rates being 1.0-1.5 km thinner. The gravity modeling shows that a density model can be constructed that simultaneously satisfies observed gravity, seismic constraints on crustal thickness, and our expectation of isostasy if ˜1.3 km of low-density material is distributed into the upper 30-60 km of the mantle. This amount of material (˜1.3 km) roughly equals the difference in thickness between slow and ultra-slow spreading crust, suggesting that the thinner crust formed during very slow spreading arises due to melt retention in the mantle rather than decreased mantle melting. Modeling of mode-converted S-wave phases reveals a uniform Poisson's ratio (˜0.27) in the lower crust. Along with the observation of sharp crust/mantle boundary, this result suggests that crust along the FAIM transect is primarily melt-derived igneous crust. Geoid versus topography relationships along Line 1 and nearby parallel tracks show abrupt changes that may originate from lateral changes in mantle

  8. Interaction of mantle plumes and migrating mid-ocean ridges: Implications for the Gal{acute a}pagos plume-ridge system

    SciTech Connect

    Ito, G.; Lin, J.; Gable, C.W.

    1997-07-01

    We investigate the three-dimensional interaction of mantle plumes and migrating mid-ocean ridges with variable viscosity numerical models. Numerical models predict that along-axis plume width W and maximum distance of plume-ridge interaction x{sub max} scale with (Q/U){sup 1/2}, where Q is plume source volume flux and U is ridge full spreading rate. Both W and x{sub max} increase with buoyancy number {Pi}{sub b} which reflects the strength of gravitational- versus plate-driven spreading. Scaling laws derived for stationary ridges in steady-state with near-ridge plumes are consistent with those obtained from independent studies of {ital Ribe} [1996]. In the case of a migrating ridge, the distance of plume-ridge interaction is reduced when a ridge migrates toward the plume because of the excess drag of the faster moving leading plate and enhanced when a ridge migrates away from the plume because of the reduced drag of the slower moving trailing plate. Given the mildly buoyant and relatively viscous plumes investigated here, the slope of the lithospheric boundary and thermal erosion of the lithosphere have little effect on plume flow. From observed plume widths of the Gal{acute a}pagos plume-migrating ridge system, our scaling laws yield estimates of Gal{acute a}pagos plume volume flux of 5{endash}16{times}10{sup 6}km{sup 3}m.y.{sup {minus}1} and a buoyancy flux of {minus}2{times}10{sup 3}kgs{sup {minus}1}. Model results suggest that the observed increase in bathymetric and mantle-Bouguer gravity anomalies along Cocos Plate isochrons with increasing isochron age is due to higher crustal production when the Gal{acute a}pagos ridge axis was closer to the plume several million years ago. The anomaly amplitudes can be explained by a plume source with a relatively mild temperature anomaly (50{degree}{endash}100{degree}C) and moderate radius (100{endash}200 km). (Abstract Truncated)

  9. Investigations of a novel fauna from hydrothermal vents along the Arctic Mid-Ocean Ridge (AMOR) (Invited)

    NASA Astrophysics Data System (ADS)

    Rapp, H.; Schander, C.; Halanych, K. M.; Levin, L. A.; Sweetman, A.; Tverberg, J.; Hoem, S.; Steen, I.; Thorseth, I. H.; Pedersen, R.

    2010-12-01

    The Arctic deep ocean hosts a variety of habitats ranging from fairly uniform sedimentary abyssal plains to highly variable hard bottoms on mid ocean ridges, including biodiversity hotspots like seamounts and hydrothermal vents. Deep-sea hydrothermal vents are usually associated with a highly specialized fauna, and since their discovery in 1977 more than 400 species of animals have been described. This fauna includes various animal groups of which the most conspicuous and well known are annelids, mollusks and crustaceans. The newly discovered deep sea hydrothermal vents on the Mohns-Knipovich ridge north of Iceland harbour unique biodiversity. The Jan Mayen field consists of two main areas with high-temperature white smoker venting and wide areas with low-temperature seepage, located at 5-700 m, while the deeper Loki Castle vent field at 2400 m depth consists of a large area with high temperature black smokers surrounded by a sedimentary area with more diffuse low-temperature venting and barite chimneys. The Jan Mayen sites show low abundance of specialized hydrothermal vent fauna. Single groups have a few specialized representatives but groups otherwise common in hydrothermal vent areas are absent. Slightly more than 200 macrofaunal species have been identified from this vent area, comprising mainly an assortment of bathyal species known from the surrounding area. Analysis of stable isotope data also indicates that the majority of the species present are feeding on phytodetritus and/or phytoplankton. However, the deeper Loki Castle vent field contains a much more diverse vent endemic fauna with high abundances of specialized polychaetes, gastropods and amphipods. These specializations also include symbioses with a range of chemosynthetic microorganisms. Our data show that the fauna composition is a result of high degree of local specialization with some similarities to the fauna of cold seeps along the Norwegian margin and wood-falls in the abyssal Norwegian Sea

  10. Zinc, copper, and lead in mid-ocean ridge basalts and the source rock control on Zn/Pb in ocean-ridge hydrothermal deposits

    USGS Publications Warehouse

    Doe, B.R.

    1994-01-01

    The contents of Zn, Cu, and Pb in mid-ocean ridge basalts (MORB) and the MORB source-rock control on Zn/Pb in ocean-ridge hydrothermal deposits are examined. The values of Zn, Cu, and Pb for submarine mid-ocean ridge basalts (MORB) are, respectively (in ppm): average MORB-75, 75, and 0.7; West Valley, Juan de Fuca Ridge (JFR)-87, 64, and 0.5; southern JFR-120 and 0.5; and 21??N, East Pacific Rise (EPR)-73, 78, and 0.5. Values of Zn/Pb range from about 100-240 and Cu/ Pb from 100-156. In this study, Zn is found to correlate positively with TiO2 + FeO (mean square of weighted deviates, MSWD, of 1.6 for JFR basalt), and inversely with Mg number (MSWD of 3.5). Therefore, contrary to statements in the literature that Zn should be compatible in MORB, Zn is a mildly incompatible element and must be enriched in the glass phase relative to olivine as Zn does not fit into the other major phenocryst phase, plagioclase. In the source of MORB, Zn likely is most enriched in oxides: spinel, magnetite, and titanomagnetite. Copper generally does not correlate well with other elements in most MORB data examined. When differentiation is dominated by olivine, Cu has a tendency to behave incompatibly (e.g., at Mg numbers > 70), but, overall, Cu shows some tendency towards being a compatible element, particularly along the Mid-Atlantic Ridge, a behavior presumably due to separation of sulfides in which Cu (but not Zn) is markedly enriched. Copper thus may be in dispersed sulfides in the source of MORB. Ocean ridges provide important data on source-rock controls for sulfide deposits because, in sediment-starved ridges, much is known about the possible source rocks and mineralization is presently occurring. In contrast to Zn/Pb ~5 in continental hot Cl-rich brines, Zn/Pb in the hottest sediment-starved ridge black smoker hydrothermal fluids at 21 ??N, EPR is about 110, similar to local MORB (145), but Cu/Pb is closer to 30, possibly due to subsurface deposition of Cu. At the JFR, the best

  11. Biosignatures in chimney structures and sediment from the Loki's Castle low-temperature hydrothermal vent field at the Arctic Mid-Ocean Ridge.

    PubMed

    Jaeschke, Andrea; Eickmann, Benjamin; Lang, Susan Q; Bernasconi, Stefano M; Strauss, Harald; Früh-Green, Gretchen L

    2014-05-01

    We investigated microbial life preserved in a hydrothermally inactive silica–barite chimney in comparison with an active barite chimney and sediment from the Loki's Castle low-temperature venting area at the Arctic Mid-Ocean Ridge (AMOR) using lipid biomarkers. Carbon and sulfur isotopes were used to constrain possible metabolic pathways. Multiple sulfur (dδ34S, Δ33S) isotopes on barite over a cross section of the extinct chimney range between 21.1 and 22.5 % in δ34S, and between 0.020 and 0.034 % in Δ33S, indicating direct precipitation from seawater. Biomarker distributions within two discrete zones of this silica–barite chimney indicate a considerable difference in abundance and diversity of microorganisms from the chimney exterior to the interior. Lipids in the active and inactive chimney barite and sediment were dominated by a range of 13C-depleted unsaturated and branched fatty acids with δ13C values between -39.7 and -26.7 %, indicating the presence of sulfur-oxidizing and sulfate-reducing bacteria. The majority of lipids (99.5 %) in the extinct chimney interior that experienced high temperatures were of archaeal origin. Unusual glycerol monoalkyl glycerol tetraethers (GMGT) with 0–4 rings were the dominant compounds suggesting the presence of mainly (hyper-) thermophilic archaea. Isoprenoid hydrocarbons with δ13C values as low as -46 % also indicated the presence of methanogens and possibly methanotrophs.

  12. Quantitative estimate of heat flow from a mid-ocean ridge axial valley, Raven field, Juan de Fuca Ridge: Observations and inferences

    NASA Astrophysics Data System (ADS)

    Salmi, Marie S.; Johnson, H. Paul; Tivey, Maurice A.; Hutnak, Michael

    2014-09-01

    A systematic heat flow survey using thermal blankets within the Endeavour segment of the Juan de Fuca Ridge axial valley provides quantitative estimates of the magnitude and distribution of conductive heat flow at a mid-ocean ridge, with the goal of testing current models of hydrothermal circulation present within newly formed oceanic crust. Thermal blankets were deployed covering an area of 700 by 450 m in the Raven Hydrothermal vent field area located 400 m north of the Main Endeavour hydrothermal field. A total of 176 successful blanket deployment sites measured heat flow values that ranged from 0 to 31 W m-2. Approximately 53% of the sites recorded values lower than 100 mW m-2, suggesting large areas of seawater recharge and advective extraction of lithospheric heat. High heat flow values were concentrated around relatively small "hot spots." Integration of heat flow values over the Raven survey area gives an estimate of conductive heat output of 0.3 MW, an average of 0.95 W m-2, over the survey area. Fluid circulation cell dimensions and scaling equations allow calculation of a Rayleigh number of approximately 700 in Layer 2A. The close proximity of high and low heat flow areas, coupled with previous estimates of surficial seafloor permeability, argues for the presence of small-scale hydrothermal fluid circulation cells within the high-porosity uppermost crustal layer of the axial seafloor.

  13. Insights into mantle heterogeneities: mid-ocean ridge basalt tapping an ocean island magma source in the North Fiji Basin

    NASA Astrophysics Data System (ADS)

    Brens, R., Jr.; Jenner, F. E.; Bullock, E. S.; Hauri, E. H.; Turner, S.; Rushmer, T. A.

    2015-12-01

    The North Fiji Basin (NFB), and connected Lau Basin, is located in a complex area of volcanism. The NFB is a back-arc basin (BAB) that is a result of an extinct subduction zone, incorporating the complicated geodynamics of two rotating landmasses: Fiji and the Vanuatu island arc. Collectively this makes the spreading centers of the NFB the highest producing spreading centers recorded. Here we present volatile concentrations, major, and trace element data for a previously undiscovered triple junction spreading center in the NFB. We show our enrichment samples contain some of the highest water contents yet reported from (MORB). The samples from the NFB exhibit a combination of MORB-like major chemical signatures along with high water content similar to ocean island basalts (OIB). This peculiarity in geochemistry is unlike other studied MORB or back-arc basin (to our knowledge) that is not attributed to subduction related signatures. Our results employ the use of volatiles (carbon dioxide and water) and their constraints (Nb and Ce) combined with trace element ratios to indicate a potential source for the enrichment in the North Fiji Basin. The North Fiji Basin lavas are tholeiitic with similar major element composition as averaged primitive normal MORB; with the exception of averaged K2O and P2O5, which are still within range for observed normal MORB. For a mid-ocean ridge basalt, the lavas in the NFB exhibit a large range in volatiles: H2O (0.16-0.9 wt%) and CO2 (80-359 ppm). The NFB lavas have volatile levels that exceed the range of MORB and trend toward a more enriched source. In addition, when compared to MORB, the NFB lavas are all enriched in H2O/Ce. La/Sm values in the NFB lavas range from 0.9 to 3.8 while, Gd/Yb values range from 1.2 to 2.5. The NFB lavas overlap the MORB range for both La/Sm (~1.1) and Gd/Yb (~1.3). However, they span a larger range outside of the MORB array. High La/Sm and Gd/Yb ratios (>1) are indications of deeper melting within the

  14. Melting and reactive flow of a volatilized mantle beneath mid-ocean ridges: theory and numerical models

    NASA Astrophysics Data System (ADS)

    Keller, Tobias; Katz, Richard F.

    2015-04-01

    Laboratory experiments indicate that even small concentrations volatiles (H2O or CO2) in the upper mantle significantly affect the silicate melting behavior [HK96,DH06]. The presence of volatiles stabilizes volatile-rich melt at high pressure, thus vastly increasing the volume of the upper mantle expected to be partially molten [H10,DH10]. These small-degree melts have important consequences for chemical differentiation and could affect the dynamics of mantle flow. We have developed theory and numerical implementation to simulate thermo-chemically coupled magma/mantle dynamics in terms of a two-phase (rock+melt), three component (dunite+MORB+volatilized MORB) physical model. The fluid dynamics is based on McKenzie's equations [McK84], while the thermo-chemical formulation of the system is represented by a novel disequilibrium multi-component melting model based on thermo-dynamic theory [RBS11]. This physical model is implemented as a parallel, two-dimensional, finite-volume code that leverages tools from the PETSc toolkit. Application of this simulation code to a mid-ocean ridge system suggests that the methodology captures the leading-order features of both hydrated and carbonated mantle melting, including deep, low-degree, volatile-rich melt formation. Melt segregation leads to continuous dynamic thermo-chemical dis-equilibration, while phenomenological reaction rates are applied to continually move the system towards re-equilibration. The simulations will be used first to characterize volatile extraction from the MOR system assuming a chemically homogeneous mantle. Subsequently, simulations will be extended to investigate the consequences of heterogeneity in lithology [KW12] and volatile content. These studies will advance our understanding of the role of volatiles in the dynamic and chemical evolution of the upper mantle. Moreover, they will help to gauge the significance of the coupling between the deep carbon cycle and the ocean/atmosphere system. REFERENCES

  15. Volatile Content of the Mid-ocean Ridge Mantle Inferred from Off-axis Seamounts and Intra-transform Lavas

    NASA Astrophysics Data System (ADS)

    Shimizu, K.; Saal, A. E.; Hauri, E. H.; Nagle, A.; Forsyth, D. W.; Niu, Y.

    2011-12-01

    H2O/Ce ratios in our samples positively correlate with Th/La, Sm/Yb, and isotope ratios suggesting that the enriched mantle component is also enriched in volatile contents. S/Dy ratios are the exception, with relatively constant values in both enriched, and depleted basalts. Although it has been argued that correlation between Sr, Nd and Pb isotope ratios and fractionation corrected major element in seamount samples indicate different mantle lithologies under the mid-ocean ridges, we will show that such correlation might be an artifact of ignoring the effect of water during the correction for fractional crystallization. [1] Dixon et al. (1995) J. Pet., 36, 1607-1631. [2] Liu et al. (2007) Geochim Cosmochim Ac., 71, 1783-1799.

  16. Partial Melt Systems in Plate-Driven Corner Flow: Evaluating the Formation of Porosity Bands as a Mechanism for Magma Focusing at Mid-Ocean Ridges

    NASA Astrophysics Data System (ADS)

    Gebhardt, D.; Butler, S. L.

    2015-12-01

    The imposition of an external shear on a system of partial melt will result in compaction of the solid matrix and concentration of the interstitial liquid melt leading to the formation of regions of contrasting high and low porosity. In experiments, direct and torsional shear geometries have demonstrated that these regions of varying porosity form in bands orientated at low angles relative to the shear plane. A variety of numerical models have been employed to recreate these experimental results. Simple shear, pure shear and torsional shear geometries have been used in both linear and nonlinear numerical settings to model the formation of the porosity bands. In this contribution the numerical models utilize a shear geometry derived from the velocity field of the plate-driven corner flow of a mid-ocean ridge. Motivation for using the velocity field of a mid-ocean ridge comes from evidence that suggests the existence of lateral melt channeling from either side of the ridge axis. Imposing the shear from a mid-ocean ridge corner flow allows for the evaluation of the resulting porosity bands in terms of suitability for channeling melt laterally toward the ridge axis. This is done using both slow and fast spreading ridge geometries. The degree of similarity between previous numerical and experimental results has been found to be greatly influenced by the imposed viscosity law of the solid matrix phase. In order to keep this in mind, the numerical models in this contribution use three different matrix viscosity laws: strain-rate independent, strain-rate dependent and anisotropic. Of these rheologies, strain-rate independence results in the poorest orientation for channeling melt directly to the ridge axis. The strain-rate dependent and anisotropic viscosities present more favorable direct-channeling orientations for the fastest growing porosity bands, but in both cases the background flow will rotate bands to less ideal orientations over time. However, these less

  17. Results from a 14-month hydroacoustic monitoring of the three mid-oceanic ridges in the Indian Ocean

    NASA Astrophysics Data System (ADS)

    Royer, J.-Y.; Dziak, R. P.; Delatre, M.; Chateau, R.; Brachet, C.; Haxel, J. H.; Matsumoto, H.; Goslin, J.; Brandon, V.; Bohnenstielh, D. R.

    2009-04-01

    From October 2006 to January 2008, an hydroacoustic experiment in the Indian Ocean was carried out by the CNRS/University of Brest and NOAA/Oregon State University to monitor the low-level seismic activity associated with the three contrasting spreading ridges and deforming zones in the Indian Ocean. Three autonomous hydrophones were moored in the SOFAR channel by R/V Marion Dufresne for 14 months in the Madagascar Basin, and northeast and southwest of Amsterdam Island, complementing the two permanent hydroacoustic stations of the Comprehensive nuclear-Test-Ban Treaty Organization (CTBTO) located near Diego Garcia Island and off Cape Leeuwin. The three instruments successfully collected 14 month of continuous acoustic records. Combined with the records from the permanent stations, the array detected 1780 acoustic events consisting mostly of earthquake generated T-waves, but also of iceberg tremors from Wilkes Land, Antarctica. Within the triangle defined by the temporary array, the three ridges exhibit contrasting seismicity patterns. Along the Southeast Indian ridge (SEIR), the 272 acoustic events (vs 24 events in the NEIC catalog) occur predominantly along the transform faults ; only one ridge segment (76˚E) displays a continuous activity for 10 months. Along the Central Indian Ridge (CIR), seismicity is distributed along fracture zones and ridge segments (269 events vs 45 NEIC events), with two clusters of events near the triple junction (24-25S) and south of Marie-Celeste FZ (18.5S). Along the Southwest Indian Ridge (SWIR), the 222 events (vs 31 NEIC events) are distributed along the ridge segments with a larger number of events west of Melville FZ and a cluster at 58E. The immediate vicinity of the Rodrigues triple junction shows periods of quiescence and of intense activity. Some large earthquakes (Mb>5) near the triple junction (SEIR and CIR) seem to be preceded by several acoustic events that may be precursors. Finally, off-ridge seismicity is mostly

  18. Constraints On Fluid Evolution During Mid-Ocean Ridge Hydrothermal Circulation From Anhydrite Sampled by ODP Hole 1256D

    NASA Astrophysics Data System (ADS)

    Smith-Duque, C.; Teagle, D. A.; Alt, J. C.; Cooper, M. J.

    2008-12-01

    Anhydrite is potentially a useful mineral for recording the evolution of seawater-derived fluids during mid- ocean ridge hydrothermal circulation because it exhibits retrograde solubility, and hence may precipitate due to the heating of seawater or the sub-surface mixing of seawater with black smoker-like fluids. Here we provide new insights into the chemical and thermal evolution of seawater during hydrothermal circulation through analyses of anhydrite recovered from ODP Hole 1256D, the first complete penetration of intact upper oceanic crust down to gabbros. Previously, crustal anhydrite has been recovered only from Hole 504B. Measurements of 87Sr/86Sr, major element ratios, Rare Earth Elements and δ18O in anhydrite constrain the changing composition of fluids as they chemically interact with basalt. Anhydrite fills veins and pore-space in the lower lava sequences from ~530 to ~1000 meters sub- basement (msb), but is concentrated in the lava-dike transition (754 to 811 msb) and uppermost sheeted dikes. Although present in greater quantities than in Hole 504B, the amount of anhydrite recovered from the Site 1256 crust is low compared to that predicted by models of hydrothermal circulation (e.g., Sleep, 1991). Two distinct populations of anhydrite are indicated by measurements of 87Sr/86Sr suggesting different fluid evolution paths within Site 1256. One group of anhydrites have 87Sr/86Sr of 0.7070 to 0.7085, close to that of 15 Ma seawater (0.70878), suggesting that some fluids penetrate through the lavas and into the sheeted dikes with only minimal Sr-exchange with the host basalts. A second group, with low 87Sr/86Sr between 0.7048 and 0.7052, indicates precipitation from a fluid that has undergone far greater interaction with basalt. This range is close to that estimated from Sr-isotopic analyses of epidote for the Hole 1256D hydrothermal fluids (87Sr/86Sr ~0.705). Sr/Ca and 87Sr/86Sr indicate a similar relationship to that seen at ODP Hole 504B suggesting that

  19. Petrogenesis of the Conejo volcanic suite, southern California: Evidence for mid-ocean ridge continental margin interactions

    NASA Astrophysics Data System (ADS)

    Hurst, Richard W.

    1982-05-01

    The evolution of the southern California Borderland during mid-Miocene time was dependent on the interactions between the North American continent and the converging East Pacific Rise. Evidence from geological and geochemical investigations of the Conejo volcanic suite, whose petrogenesis is linked to these interactions, suggests an intimate relation between the subducting ridge and the volcanic rocks. The whole-rock chemistry of this suite (K2O < 0.4%; 87Sr/86Sr = 0.70248 0.70372, average = 0.70306; TiO2 = 1.2% 1.8%) and restricted range of pyroxene and plagioclase compositions support a model in which active ridge volcanism, fractionation, and mixing of primitive liquids with later differentiates can account for the observed petrologic evolution. The interaction of the ridge with the subduction zone may result in the cessation of subduction, the preferential subduction of the trenchward ridge flank, or localized volcanism in the vicinity of the ridge subduction zone intersection.

  20. Diversity of microbial communities of Loki's Castle black smoker field at the ultra-slow spreading Arctic Mid-Ocean Ridge

    NASA Astrophysics Data System (ADS)

    Jaeschke, A.; Bernasconi, S. M.; Thorseth, I. H.; Pedersen, R.; Früh-Green, G.

    2010-12-01

    Here we present an organic geochemical study of Loki’s Castle, a black smoker field recently discovered at the Arctic Mid-Ocean Ridge (AMOR) in the Norwegian-Greenland Sea at around 73.2°N. Located at the Mohn-Knipovich Ridge, which is one of the slowest spreading ridge segments on Earth, Loki’s Castle is the most northerly major hydrothermal vent field known to date. The vent field is composed of five actively venting (320°C) black-smoker chimneys that tower on top of a large mound of hydrothermal sulfide deposits. Loki’s Castle is a basalt-hosted hydrothermal system, but high methane and ammonium contents in the vent fluids strongly indicate a sedimentary component below the volcanic ridge. In 2009, another site of low-temperature hydrothermal venting hosting numerous barite chimneys was discovered in the vicinity of the black smokers, which probably results from subsurface mixing of diffuse hydrothermal fluid with seawater. In our study, variations in microbial communities associated with the formation of actively venting, sulfide and sulfate chimneys in this essentially unexplored ultraslow spreading ridge system are assessed based on biomarker lipid and compound-specific carbon isotope analyses. Lipid extracts from an active, high-temperature sulfide chimney yielded abundant archaeal di- and tetraether lipids as well as irregular isoprenoidal hydrocarbons (PMIs) that are associated with archaeal methanogens and methanotrophs. Predominant archaeal biomarker lipids include archaeol, sn-2-hydroxyarchaeol as well as glycerol dialkyl glycerol tetraethers (GDGTs) containing 0-4 cyclopentyl moieties. In addition, GDGTs with an additional covalent bond between the isoprenoid hydrocarbon chains, so-called H-shaped GDGTs, containing 0-4 cyclopentyl rings were also found to be abundant components and are indicative of hyperthermophilic methanogens. Biomarkers characteristic of eukaryotes (sterols) and bacteria (fatty acids and hopanoids) were less prevalent in

  1. Spatial and temporal distribution of the seismicity along two mid-oceanic ridges with contrasted spreading rates in the Indian Ocean

    NASA Astrophysics Data System (ADS)

    Tsang-Hin-Sun, E.; Perrot, J.; Royer, J. Y.

    2015-12-01

    The seismicity of the ultra-slow spreading Southwest (14 mm/y) and intermediate spreading Southeast (60 mm/y) Indian ridges was monitored from February 2012 to March 2013 by the OHASISBIO array of 7 autonomous hydrophones. A total of 1471 events were located with 4 instruments or more, inside the array, with a median location uncertainty < 5 km and a completeness magnitude of mb = 3. Both ridges display similar average rates of seismicity, suggesting that there is no systematic relationship between seismicity and spreading rates. Accretion modes do differ, however, by the along-axis distribution of the seismic events. Along the ultra-slow Southwest Indian Ridge, events are sparse but regularly spaced and scattered up to 50 km off-axis. Along the fast Southeast Indian Ridge, events are irregularly distributed, focusing in narrow regions near the ridge axis at segment ends and along transform faults, whereas ridge-segment centers generally appear as seismic gaps (at the level of completeness of the array). Only two clusters, 6 months apart, are identified in a segment-center at 29°S. From the temporal distribution of the clustered events and comparisons with observations in similar mid-oceanic ridge setting, both clusters seem to have a volcanic origin and to be related to a dike emplacement or a possible eruption on the seafloor. Their onset time and migration rate are comparable to volcanic swarms recorded along the Juan de Fuca Ridge. Overall, the rate of seismicity along the two Indian spreading ridges correlates with the large-scale variations in the bathymetry and shear-wave velocity anomaly in the upper mantle, suggesting that the distribution of the low-magnitude seismicity is mainly controlled by along-axis variations in the lithosphere rheology and temperature.

  2. Plagioclase and epidote buffering of cation ratios in mid-ocean ridge hydrothermal fluids: Experimental results in and near the supercritical region

    SciTech Connect

    Berndt, M.E.; Seyfried, W.E. Jr. ); Janecky, D.R. )

    1989-09-01

    Experiments have been performed with Na-Ca-K-Cl fluids of seawater chlorinity and diabase, basalt, and plagioclase bearing mineral mixtures at 350-425{degree}C and 250-400 bars to help constrain hydrothermal alteration processes at mid-ocean ridges. Dissolved Ca, Na, and pH for all experiments responded systematically to differences in dissolved SiO{sub 2} concentrations and the compositions of plagioclase reactants. Diabase alteration at low fluid/rock mass ratios (0.5 to 1) produces fluids undersaturated with respect to quartz during hydration of primary olivine and orthopyroxene, whereas basalt alteration under similar conditions yields fluids slightly supersaturated with respect to quartz during breakdown of glass to smectite and amphibole. Fluid chemistry in all experiments appears to approach a partial equilibrium state with the albite and anorthite components in plagioclase and approaches a pH consistent with plagioclase alteration to epidote. Trace element data from vent fluids, specifically B and Sr, together with major element chemistry, provides evidence that the reaction zone for black-smoker fluids at mid-ocean ridges is composed of only slightly altered diabase and is characterized by small amounts of epidote, nearly fresh plagioclase and clinopyroxene, and partially to completely hydrated olivine and orthopyroxene. Using equilibrium between plagioclase, the dominant reactant, and epidote, the dominant reaction product in experiments, the authors estimate that temperatures in reaction zones are in excess of 375{degree}C for most vent systems. These temperatures are higher than measured vent temperatures, suggesting that hot spring fluids commonly loose heat during ascent to the sea floor.

  3. Composition of plume-influenced mid-ocean ridge lavas and glasses from the Mid-Atlantic Ridge, East Pacific Rise, Galápagos Spreading Center, and Gulf of Aden

    NASA Astrophysics Data System (ADS)

    Kelley, Katherine A.; Kingsley, Richard; Schilling, Jean-Guy

    2013-01-01

    The global mid-ocean ridge system is peppered with localities where mantle plumes impinge on oceanic spreading centers. Here, we present new, high resolution and high precision data for 40 trace elements in 573 samples of variably plume-influenced mid-ocean ridge basalts from the Mid-Atlantic ridge, the Easter Microplate and Salas y Gomez seamounts, the Galápagos spreading center, and the Gulf of Aden, in addition to previously unpublished major element and isotopic data for these regions. Included in the data set are the unconventional trace elements Mo, Cd, Sn, Sb, W, and Tl, which are not commonly reported by most geochemical studies. We show variations in the ratios Mo/Ce, Cd/Dy, Sn/Sm, Sb/Ce, W/U, and Rb/Tl, which are expected not to fractionate significantly during melting or crystallization, as a function of proximity to plume-related features on these ridges. The Cd/Dy and Sn/Sm ratios show little variation with plume proximity, although higher Cd/Dy may signal increases in the role of garnet in the mantle source beneath some plumes. Globally, the Rb/Tl ratio closely approximates the La/SmN ratio, and thus provides a sensitive tracer of enriched mantle domains. The W/U ratio is not elevated at plume centers, but we find significant enrichments in W/U, and to a lesser extent the Mo/Ce and Sb/Ce ratios, at mid-ocean ridges proximal to plumes. Such enrichments may provide evidence of far-field entrainment of lower mantle material that has interacted with the core by deeply-rooted, upwelling mantle plumes.

  4. Genesis of andesitic boninitic magmas at mid-ocean ridges by melting of hydrated peridotites: Geochemical evidence from DSDP Site 334 gabbronorites

    NASA Astrophysics Data System (ADS)

    Nonnotte, Philippe; Ceuleneer, Georges; Benoit, Mathieu

    2005-08-01

    The gabbronoritic cumulates drilled at DSDP Site 334 (Mid-Atlantic Ridge off the FAMOUS area) are neither crystallization products of the associated basalts, nor from any MORB composition documented along ocean ridges. Their parent melts are richer in SiO 2 than MORB at a given MgO content, as attested by the crystallization sequence starting with an olivine + calcic and sub-calcic pyroxene assemblages. These melts are issued from a source highly depleted in incompatible elements, likely residual peridotite left after MORB extraction. To understand the role of water in the genesis of these lithologies whose occurrence in a mid-ocean ridge setting is rather puzzling, we performed a geochemical study on clinopyroxene separates following an analytical protocol able to remove the effects of water rock interactions post-dating their crystallization. Accordingly, the measured isotopic signatures can be used to trace magma sources. We find that Site 334 clinopyroxenes depart from the global mantle correlation: normal MORB values for the 143Nd / 144Nd ratio (0.51307-0.51315) are associated to highly radiogenic 87Sr / 86Sr (0.7034-0.7067) ratios. This indicates that the parent melts of Site 334 cumulates are issued from a MORB source but that seawater contamination occurred at some stage of their genesis. The extent of contamination, traced by the Sr isotopic signature, is variable within all cumulates but more developed for gabbronorites sensus stricto, suggesting that seawater introduction was a continuous process during all the magmatic evolution of the system, from partial melting to fractional crystallization. Simple masse balance calculations are consistent with a contaminating agent having the characters of a highly hydrated (possibly water saturated) silica-rich melt depleted in almost all incompatible major, minor and trace elements relative to MORB. Mixing in various proportions of contaminated melts similar to the parent melts of Site 334 cumulates with MORB can

  5. Hydrothermal Exploration of the Mid-Cayman Spreading Center: Isolated Evolution on Earth’s Deepest Mid-Ocean Ridge?

    NASA Astrophysics Data System (ADS)

    German, C. R.; Bowen, A.; Coleman, M. L.; Huber, J. A.; Seewald, J.; van Dover, C.; Whitcomb, L. L.; Yoerger, D.; Connelly, D.; Honig, D. L.; Jakuba, M.; Kinsey, J. C.; McDermott, J.; Nakamura, K.; Sands, C.; Smith, J.; Sylva, S.

    2009-12-01

    We report the first systematic exploration for and characterization of hydrothermal vents and vent ecosystems on the short (~110 km), deep (> 5000 m), ultra-slow-spreading (<20 mm yr-1) Mid-Cayman Rise in the Caribbean Sea. This work was carried out aboard the RV Cape Hatteras in October-November 2009 as part of the ChEss Project of the Census of Marine Life, funded through NASA’s ASTEP program and represents the first scientific field program funded to use WHOI’s new hybrid deep submergence vehicle, Nereus, first in AUV mode then in ROV mode. Prior to this work, evidence for hydrothermal venting had been found on every active spreading center investigated, including the comparably ultra-slow spreading ridges in the SW Indian Ocean and in the Arctic (Mohns, Knipovich & Gakkel Ridges). The organisms colonizing vents are renowned for their endemicity, their adaptations to the extreme chemical and physical conditions encountered and for differences in species level from one ocean basin to another. Consequently, the identification of any organisms colonizing vents of the Mid-Cayman Spreading Center offers a critical opportunity to build upon our understanding of the dispersion of vent species and the potential role of the rise of the Isthmus of Panama (dating from ~5 Ma) as a vicariant event leading to the evolutionary divergence of Atlantic and Pacific vent faunas. Further, the MCSC is so deep that any vents present may occur at depths greater than all previously known vent systems, extending the known limits to life on our planet in terms of pressure, temperature, and vent-fluid chemistry. Finally, hydrothermal circulation through ultramafic rocks can generate abiotic synthesis of organic matter: an analog for the prebiotic basis for the origin of life on early Earth and Mars. In future years of this 4-year study, therefore, we will also aim to assess the relative importance of abiotic organic synthesis versus recycling of bio-organic material and/or chemical

  6. Zn isotope composition in hydrothermal systems on the mid-ocean ridge and its implication for oceanic cycling of Zinc

    NASA Astrophysics Data System (ADS)

    Li, Xiaohu; Wang, Jianqiang; Lei, Jijiang; Yu, Xing; Wang, Hao; Chu, Fengyou

    2016-04-01

    Seafloor hydrothermal systems play an important role on the oceanic biogeochemical cycles of Zn and its isotopes. However, for the Zn isotopic systems in hydrothermal systems we know too little of the distribution of Zn isotopes in variable hydrothermal products and its impact on modern oceanic mass balance. We have measured Zn isotopes in hydrothermal products such as oxidation products of chimney sulfides and hydrothermal sediments from the active hydrothermal field on the Mid-Atlantic Ridge in order to better understand the oceanic biogeochemical cycles of Zn isotopes. We present isotopic data for Zn in sulfides and sediments, which yield δ66Zn=+0.11±0.08‰(2SD,n=23)and range from -0.14‰ to +0.38‰.We found that δ66Zn values of our samples were lighter or similar to chimney sulfides from the high-temperature hydrothermal vent, but much lighter than hydrothermal fluids and chimney sulfides from the low-temperature hydrothermal vent. We also compared our results with δ66Zn values of the Fe-Mn crusts, nodules and oceanic carbonate as heavy Zn isotope sink, which implies that Zn isotopes output to hydrothermal sediment and oxidation products of chimney sulfides as a missing light sink can explain the heavy isotopic composition of the oceans.

  7. How the growth and freeboard of continents may relate to geometric and kinematic parameters of mid-ocean spreading ridges

    USGS Publications Warehouse

    Howell, D.G.

    1989-01-01

    If the volume of continents has been growing since 4 Ga then the area of the ocean basins must have been shrinking. Therefore, by inferring a constant continental freeboard, in addition to constant continental crustal thicknesses and seawater volume, it is possible to calculate the necessary combinations of increased ridge lengths and spreading rates required to displace the seawater in the larger oceans of the past in order to maintain the constant freeboard. A reasonable choice from the various possibilities is that at 4 Ga ago, the ridge length and spreading rates were ca. 2.5 times greater than the averages of these parameters during the past 200 Ma. By 2.5 Ga ago the ridge length and spreading rate decreased to about 1.8 times the recent average and by 1 Ga ago these features became reduced to approximately 1.4 times recent averages. ?? 1989.

  8. Plume Capture by Divergent Plate Motions: Implications for the Distribution of Hotspots, Geochemistry of Mid-Ocean Ridge Basalts, and Heat Flux from the Core-Mantle Boundary

    NASA Astrophysics Data System (ADS)

    Jellinek, A. M.; Richards, M. A.

    2001-12-01

    The coexistence of mantle plumes with plate-scale flow is problematic in geodynamics. Significant problems include the fixity of hotspots with respect to plate motions, the spatial distribution and duration of hotspots, the geophysical and geochemical signatures of plume-ridge interactions, and the relation between mantle plumes and heat flux across the core-mantle boundary. We present results from laboratory experiments aimed at understanding the effects of an imposed large-scale circulation on thermal convection at high Rayleigh number (up to 109) in a fluid with a strongly temperature-dependent viscosity. In a large tank, a layer of corn syrup is heated from below while being stirred by large-scale flow due to the opposing motions of a pair of conveyor belts immersed in the syrup at the top of the tank. Three regimes are observed, depending on the velocity ratio V of the imposed horizontal flow velocity to the rise velocity of plumes ascending from the hot boundary. When V<<1, large scale circulation has a negligible effect and convective upwelling occurs as randomly-spaced axisymmetric plumes that interact with one another. When V>10, plume instabilities are suppressed entirely and the heat flux from the hot lower boundary is carried by a central sheet-like upwelling. At intermediate V, ascending plumes are advected along the bottom boundary layer, and the heat flux from the boundary is found to scale (according to a simple boundary layer theory) with V and the ratio of the viscosity of cold fluid above the thermal boundary layer to the viscosity of the hottest fluid in contact with the bottom boundary. For large viscosity ratios (10-100), only about 1/5th or less of the total heat flux from the hot boundary layer is carried by plume instabilities, even for modest imposed horizontal flow velocities (V of order 1). When applied to Earth, our results suggest that plate-scale flow focuses ascending mantle plumes toward mid-ocean ridges, and that plumes may be

  9. Comment on “Sensitivity of seafloor bathymetry to climate-driven fluctuations in mid-ocean ridge magma supply”

    NASA Astrophysics Data System (ADS)

    Huybers, Peter; Langmuir, Charles; Katz, Richard F.; Ferguson, David; Proistosescu, Cristian; Carbotte, Suzanne

    2016-06-01

    Olive et al. (Reports, 16 October 2015, p. 310) argue that ~10% fluctuations in melt supply do not produce appreciable changes in ocean ridge bathymetry on time scales less than 100,000 years and thus cannot reflect sea level forcing. Spectral analysis of bathymetry in a region they highlight as being fault controlled, however, shows strong evidence for a signal from sea level variation.

  10. Response to Comment on "Sensitivity of seafloor bathymetry to climate-driven fluctuations in mid-ocean ridge magma supply".

    PubMed

    Olive, J-A; Behn, M D; Ito, G; Buck, W R; Escartín, J; Howell, S

    2016-06-17

    Huybers et al present new bathymetric spectra from an intermediate-spreading ridge as evidence for a primary contribution of sea level cycles to the morphology of the seafloor. Although we acknowledge the possibility that sea level-modulated magmatic constructions may be superimposed on a first-order tectonic fabric, we emphasize the difficulty of deciphering these different contributions in the frequency domain alone.

  11. Response to Comment on "Sensitivity of seafloor bathymetry to climate-driven fluctuations in mid-ocean ridge magma supply".

    PubMed

    Olive, J-A; Behn, M D; Ito, G; Buck, W R; Escartín, J; Howell, S

    2016-06-17

    Huybers et al present new bathymetric spectra from an intermediate-spreading ridge as evidence for a primary contribution of sea level cycles to the morphology of the seafloor. Although we acknowledge the possibility that sea level-modulated magmatic constructions may be superimposed on a first-order tectonic fabric, we emphasize the difficulty of deciphering these different contributions in the frequency domain alone. PMID:27313035

  12. Response to Comment on "Sensitivity of seafloor bathymetry to climate-driven fluctuations in mid-ocean ridge magma supply".

    PubMed

    Olive, J-A; Behn, M D; Ito, G; Buck, W R; Escartín, J; Howell, S

    2016-07-15

    Tolstoy reports the existence of a characteristic 100 thousand year (ky) period in the bathymetry of fast-spreading seafloor but does not argue that sea level change is a first-order control on seafloor morphology worldwide. Upon evaluating the overlap between tectonic and Milankovitch periodicities across spreading rates, we reemphasize that fast-spreading ridges are the best potential recorders of a sea level signature in seafloor bathymetry. PMID:27418498

  13. Comment on "Sensitivity of seafloor bathymetry to climate-driven fluctuations in mid-ocean ridge magma supply".

    PubMed

    Huybers, Peter; Langmuir, Charles; Katz, Richard F; Ferguson, David; Proistosescu, Cristian; Carbotte, Suzanne

    2016-06-17

    Olive et al (Reports, 16 October 2015, p. 310) argue that ~10% fluctuations in melt supply do not produce appreciable changes in ocean ridge bathymetry on time scales less than 100,000 years and thus cannot reflect sea level forcing. Spectral analysis of bathymetry in a region they highlight as being fault controlled, however, shows strong evidence for a signal from sea level variation. PMID:27313034

  14. Response to Comment on "Sensitivity of seafloor bathymetry to climate-driven fluctuations in mid-ocean ridge magma supply".

    PubMed

    Olive, J-A; Behn, M D; Ito, G; Buck, W R; Escartín, J; Howell, S

    2016-07-15

    Tolstoy reports the existence of a characteristic 100 thousand year (ky) period in the bathymetry of fast-spreading seafloor but does not argue that sea level change is a first-order control on seafloor morphology worldwide. Upon evaluating the overlap between tectonic and Milankovitch periodicities across spreading rates, we reemphasize that fast-spreading ridges are the best potential recorders of a sea level signature in seafloor bathymetry.

  15. Lava accretion system around mid-ocean ridges: Volcanic stratigraphy in the Wadi Fizh area, northern Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Kusano, Yuki; Adachi, Yoshiko; Miyashita, Sumio; Umino, Susumu

    2012-05-01

    Detailed lithological study combined with geochemical variations of lavas reveals the across-axis accretionary process at Wadi Fizh in the northern Oman ophiolite. The >900 m thick V1 sequence is divided into the lower V1 (LV1), middle V1 (MV1) and upper V1 (UV1) sequence by 0.4 m and 0.8 m thick umbers at 410 mab (meters above the base of the extrusive rocks) and 670 mab, respectively. The lowest part of the LV1 (LV1a) consists of lobate sheet and pillow lava flows extruded on the relatively flat ridge crest. Elongate pillows at 230 mab are flows draping downslope from the ridge crest and characterize the lithofacies on the ridge flank. Just above a jasper layer at 270 mab, 130 m thick evolved lavas were transported from the crest and emplaced on the ridge flank (LV1b). Off-axial accretionary processes recorded in the MV1 resulted in alternating flows of less evolved, depleted lava and evolved lava, suggesting that the MV1 off-axial lava sequence comprises flows emanated from both on- and off-axis source vents. The less evolved and depleted UV1 flows suggest independent sources distinct from the axial lavas. The Lasail Unit is regarded as a subunit of the V1 because it is comparable to the UV1 in the geological, petrological, and geochemical characteristics. The broad compositional range of the V1 sequence endorses a view that the Wadi Fizh area corresponds to a segment end of the Oman paleospreading system accompanied by off-axis volcanism as in segment boundaries of the present East Pacific Rise.

  16. Patterns of distribution of deepwater demersal fishes of the North Atlantic mid-ocean ridge, continental slopes, islands and seamounts

    NASA Astrophysics Data System (ADS)

    Bergstad, Odd Aksel; Menezes, Gui M. M.; Høines, Åge S.; Gordon, John D. M.; Galbraith, John K.

    2012-03-01

    Basin-scale spatial and depth-related distribution patterns of deepwater demersal fishes were analysed using bottom trawl datasets from the North Atlantic continental margin, slopes of oceanic islands and seamounts, and the mid-Atlantic Ridge. Depth-stratified presence-absence data for 593 species were compiled from fisheries-independent trawl studies with full species lists. The datasets comprised trawls conducted on the upper continental slope (200 m) to abyssal depths, and 750 m wide depth strata were used. Number of species and families declined with depth in all areas. Species number was highest in the western North Atlantic, significantly lower on the mid-Atlantic Ridge and eastern North Atlantic. Observed species numbers are also low in southern areas (Bahamas, NW Africa, southerly seamounts), but the sampling effort in these waters has been much less than in northern sites. Fish assemblages vary by depth, latitude and longitude, and the study corroborates earlier suggestions that assemblages are broadly distributed in relation to regional circulation and watermass features. The mid-Atlantic Ridge assemblages between Iceland and the Azores are most similar to those on eastern North Atlantic slopes and rises, rather dissimilar to all others, including western Atlantic, Greenland, northwest Africa and Azorean seamount and island assemblages. Across the North Atlantic differences between subareas are strongest at slope depths, much less pronounced at the less speciose rise and abyssal depths. Demersal fish biomass estimates suggest that the American slope (New England) has low biomass compared with Newfoundland and European areas, and that the supposedly oligotrophic mid-Atlantic Ridge has a level of biomass similar to or higher than the European margin.

  17. Directional dispersal between mid-ocean ridges: deep-ocean circulation and gene flow in Ridgeia piscesae.

    PubMed

    Young, C R; Fujio, S; Vrijenhoek, R C

    2008-04-01

    This study examined relationships between bathymetrically induced deep-ocean currents and the dispersal of the hydrothermal vent tubeworm Ridgeia piscesae along the northeast Pacific ridge system. A robust diagnostic model of deep-ocean circulation in this region predicted strong southeasterly currents following contours of the Blanco Transform Fault, a 450-km lateral offset that separates the Gorda and Juan de Fuca ridge systems. Such currents should facilitate the southward dispersal of R. piscesae larvae. Immigration rates for populations north and south of the Blanco Transform Fault were estimated from molecular population genetic data. Mitochondrial DNA evidence revealed population subdivision across the Blanco Transform Fault, and a strong directional bias in gene flow that was consistent with predictions of the circulation model. The distribution of mitochondrial diversity between the northern and southern populations of R. piscesae suggests that the Gorda Ridge tubeworms have maintained larger effective population sizes than the northern populations, a pattern that also exists in co-occurring limpets. Together, these data suggest that the northern vent fields may experience a higher frequency of habitat turnover and consequently more rapid losses of genetic diversity.

  18. Microbial Life in the Subseafloor at Mid-Ocean Ridges: A Key to Understanding Ancient Ecosystems on Earth and Elsewhere?

    NASA Astrophysics Data System (ADS)

    Baross, J. A.; Delaney, J. R.

    2001-12-01

    Some planets and moons in our solar system were similar to Earth in their geological properties during the first few hundred million years after accretion. This is the period when life arose and became established on Earth. It follows that understanding the geophysical and geochemical characteristics of early Earth could provide insight into life-supporting environments on other solar bodies that have not evolved "Garden of Eden" conditions. Hydrothermal systems are primordial and their emergence coincided with the accumulation of liquid water on Earth. The interactions of water and rock associated with hydrothermal systems result in predictable suites of dissolved elements and volatiles. While the concentrations of these chemicals vary at different vent locations and were certainly different during the early Archaean, the overall chemical composition of aqueous hydrothermal fluid is likely to be the same because of the basaltic nature of oceanic crust. In present-day hydrothermal systems, those environments not contaminated by electron acceptors produced from pelagic photosynthesis would most closely mimic the earliest conditions on Earth. These conditions include the subseafloor and high temperature, anaerobic environments associated with hydrothermal systems. The microorganisms associated with these environments derive energy from sulfur, iron, hydrogen and organic compounds. New seafloor eruptions and diffuse flow vents provide unprecedented access to deep subseafloor microbial communities. For example, 12 new eruptions have occurred in the past 15 years including five in the Northeast Pacific. Hyperthermophiles were isolated from 5-30oC diffuse vent fluids from new eruption sites at CoAxial within months of the June, 1993 eruption and from the 1998 eruption at Axial Volcano, and from plume fluids within days of the February, 1996 eruption at the N. Gorda Ridge. The presence of such organisms in fluids that are 20 to 50°C below their minimum growth temperature

  19. Ultramafic-hosted Hydrothermal Systems at Mid-Ocean Ridges: Serpentinization, Chloritization and Geochemical Controls on Mass-Transfer Processes

    NASA Astrophysics Data System (ADS)

    Seyfried, W. E.; Pester, N. J.; Ding, K.

    2012-12-01

    Recent studies of seafloor hydrothermal systems associated with the slow spreading Mid-Atlantic Ridge have provided a wealth of information on the complex interplay between tectonic and magmatic processes that ultimately govern the chemical and physical evolution of these systems. The Lost City hydrothermal field (LCHF)(30°N) and the Rainbow hydrothermal system (36°N), for example, provide contrasting styles of heat and mass transfer that result in very different constraints on the composition of hydrothermal fluids. Hydrothermal fluids were sampled and analyzed during a series of ROV (Jason II) supported dives in 2008 to these and related vent sites along the northern MAR. In addition to deployment of conventional vent fluid sampling devices, in-situ chemical sensor systems were also used to better constrain pH and redox reactions. The general characteristics of the Lost City hydrothermal field, which is offset approximately 15km from the MAR owing to tectonic effects imposed by the emplacement of the Atlantis Massif, have been extensively reviewed in recent years. Vent fluids issuing from this peridotite-hosted system reveal temperatures of approximately 90-100°C, high concentrations of dissolved hydrogen and methane, and pH measured (25°C) values that exceed 10. The relatively low vent fluid temperatures notwithstanding, phase equilibria constraints imposed by dissolved Ca and sulfate suggest temperatures of approximately 200°C at depth, below the seafloor. New data for dissolved silica indicate a hydrothermal "root zone" lacking brucite, but where fluid chemistry and pH is buffered by serpentine-diopside-fluid equilibria. Consistent with previously published strontium and boron isotope measurements, data reported here for trace alkali elements (Cs, Rb, Li) indicate high fluid/rock mass ratios. Variably low dissolved Fe concentrations are broadly consistent with constraints imposed by magnetite-fluid equilibria at the high measured dissolved H2

  20. Palagonitization of Basalt Glass in the Flanks of Mid-Ocean Ridges: Implications for the Bioenergetics of Oceanic Intracrustal Ecosystems.

    PubMed

    Türke, Andreas; Nakamura, Kentaro; Bach, Wolfgang

    2015-10-01

    When basalt is exposed to oxygenated aqueous solutions, rims of palagonite form along fractures at the expense of glass. We employed electron microprobe and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of fresh glass and adjacent palagonite crusts to determine the geochemical changes involved in palagonite formation. Samples were retrieved from drill cores taken in the North Pond Area, located on the western flank of the Mid-Atlantic Ridge at 22°45'N and 46°05'W. We also analyzed whole rock powders to determine the overall crust-seawater exchange in a young ridge flank. Radioactive elements are enriched in palagonite relative to fresh glass, reaching concentrations where radiolytic production of molecular hydrogen (H2) may be a significant energy source. Based on these results, we hypothesize that microbial ecosystems in ridge flank habitats undergo a transition in the principal energy carrier, fueling carbon fixation from Fe oxidation in very young crust to H2 consumption in older crust. Unless the H2 is swept away by rapid fluid flow (i.e., in young flanks), it may easily accumulate to levels high enough to support chemolithoautotrophic life. In older flanks, crustal sealing and sediment accumulation have slowed down seawater circulation, and the significance of radiolytically produced H2 for catalytic energy supply is expected to increase greatly. Similar habitats on other planetary surfaces are theoretically possible, as accumulation of radiolytically produced hydrogen merely requires the presence of H2O molecules and a porous medium, from which the hydrogen is not lost. PMID:26426282

  1. Serpentinization of mantle-derived peridotites at mid-ocean ridges: Mesh texture development in the context of tectonic exhumation

    NASA Astrophysics Data System (ADS)

    Rouméjon, Stéphane; Cannat, Mathilde

    2014-06-01

    slow spreading ridges, axial detachment faults exhume mantle-derived peridotites and hydrothermal alteration causes serpentinization in a domain extending more than 1 km next to the fault. At the microscopic scale, serpentinization progresses from a microfracture network toward the center of olivine relicts and forms a mesh texture. We present a petrographic study (SEM, EBSD, and Raman) of the serpentine mesh texture in a set of 278 abyssal serpentinized peridotites from the Mid-Atlantic and Southwest Indian Ridges. We show that serpentinization initiated along two intersecting sets of microfractures that have consistent orientations at the sample scale, and in at least one studied location, at the 100 m scale. We propose that these microfractures formed in fresh peridotites due to combined thermal and tectonic stresses and subsequently served as channels for serpentinizing fluids. Additional reaction-induced cracks developed for serpentinization extents <20%. The resulting microfracture network has a typical spacing of ˜60 µm but most serpentinization occurs next to a subset of these microfractures that define mesh cells 100-400 µm in size. Apparent mesh rim thickness is on average 33 ± 19 µm corresponding to serpentinization extents of 70-80%. Published laboratory experiments suggest that mesh rims formation could be completed in a few years (i.e., quasi instantaneous at the plate tectonic timescale). The depth and extent of the serpentinization domain in the detachment fault's footwall are probably variable in time and space and as a result we expect that the serpentine mesh texture at slow spreading ridges forms at variable rates with a spatially heterogeneous distribution.

  2. Palagonitization of Basalt Glass in the Flanks of Mid-Ocean Ridges: Implications for the Bioenergetics of Oceanic Intracrustal Ecosystems.

    PubMed

    Türke, Andreas; Nakamura, Kentaro; Bach, Wolfgang

    2015-10-01

    When basalt is exposed to oxygenated aqueous solutions, rims of palagonite form along fractures at the expense of glass. We employed electron microprobe and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of fresh glass and adjacent palagonite crusts to determine the geochemical changes involved in palagonite formation. Samples were retrieved from drill cores taken in the North Pond Area, located on the western flank of the Mid-Atlantic Ridge at 22°45'N and 46°05'W. We also analyzed whole rock powders to determine the overall crust-seawater exchange in a young ridge flank. Radioactive elements are enriched in palagonite relative to fresh glass, reaching concentrations where radiolytic production of molecular hydrogen (H2) may be a significant energy source. Based on these results, we hypothesize that microbial ecosystems in ridge flank habitats undergo a transition in the principal energy carrier, fueling carbon fixation from Fe oxidation in very young crust to H2 consumption in older crust. Unless the H2 is swept away by rapid fluid flow (i.e., in young flanks), it may easily accumulate to levels high enough to support chemolithoautotrophic life. In older flanks, crustal sealing and sediment accumulation have slowed down seawater circulation, and the significance of radiolytically produced H2 for catalytic energy supply is expected to increase greatly. Similar habitats on other planetary surfaces are theoretically possible, as accumulation of radiolytically produced hydrogen merely requires the presence of H2O molecules and a porous medium, from which the hydrogen is not lost.

  3. Mechanisms of magma generation beneath hawaii and mid-ocean ridges: uranium/thorium and samarium/neodymium isotopic evidence.

    PubMed

    Sims, K W; Depaolo, D J; Murrell, M T; Baldridge, W S; Goldstein, S J; Clague, D A

    1995-01-27

    Measurements of uranium/thorium and samarium/neodymium isotopes and concentrations in a suite of Hawaiian basalts show that uranium/thorium fractionation varies systematically with samarium/neodymium fractionation and major-element composition; these correlations can be understood in terms of simple batch melting models with a garnet-bearing peridotite magma source and melt fractions of 0.25 to 6.5 percent. Midocean ridge basalts shows a systematic but much different relation between uranium/thorium fractionation and samarium/neodymium fractionation, which, although broadly consistent with melting of a garnet-bearing peridotite source, requires a more complex melting model. PMID:17788786

  4. Mechanisms of magma generation beneath hawaii and mid-ocean ridges: uranium/thorium and samarium/neodymium isotopic evidence.

    PubMed

    Sims, K W; Depaolo, D J; Murrell, M T; Baldridge, W S; Goldstein, S J; Clague, D A

    1995-01-27

    Measurements of uranium/thorium and samarium/neodymium isotopes and concentrations in a suite of Hawaiian basalts show that uranium/thorium fractionation varies systematically with samarium/neodymium fractionation and major-element composition; these correlations can be understood in terms of simple batch melting models with a garnet-bearing peridotite magma source and melt fractions of 0.25 to 6.5 percent. Midocean ridge basalts shows a systematic but much different relation between uranium/thorium fractionation and samarium/neodymium fractionation, which, although broadly consistent with melting of a garnet-bearing peridotite source, requires a more complex melting model.

  5. Plume-proximal mid-ocean ridge origin of Zhongba mafic rocks in the western Yarlung Zangbo Suture Zone, Southern Tibet

    NASA Astrophysics Data System (ADS)

    He, Juan; Li, Yalin; Wang, Chengshan; Dilek, Yildirim; Wei, Yushuai; Chen, Xi; Hou, Yunling; Zhou, Aorigele

    2016-05-01

    The >2000 km-long Yarlung Zangbo Suture Zone (YZSZ) in southern Tibet includes the remnants of the Mesozoic Neotethyan oceanic lithosphere, and is divided by the Zhada-Zhongba microcontinent into northern and southern branches in its western segment. Zircon U-Pb dating of a doleritic rock from the northern branch has revealed a concordant age of 160.5 ± 1.3 Ma. All of the doleritic samples from the northern branch and the pillow basalt and gabbro samples from the southern branch display consistent REE and trace element patterns similar to those of modern OIB-type rocks. The geochemical and Sr-Nd-Pb isotopic signatures of these OIB-type rocks from the western segment are identical with those of OIB-type and alkaline rocks from other ophiolite massifs along the central and eastern segments of the YZSZ, suggesting a common mantle plume source for their melt evolution. The enriched Sr-Nd-Pb isotopic character of the gabbroic dike rocks from the southern branch points to a mantle plume source, contaminated by subducted oceanic crust or pelagic sediments. We infer that the mafic rock associations exposed along the YZSZ represent the remnants of a Neotethyan oceanic lithosphere, which was developed as part of a plume-proximal seafloor-spreading system, reminiscent of the seamount chains along-across the modern mid-ocean ridges in the Pacific Ocean.

  6. Mathematical modeling of diffuse flow in seafloor hydrothermal systems: The potential extent of the subsurface biosphere at mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Lowell, R. P.; Houghton, J. L.; Farough, A.; Craft, K. L.; Larson, B. I.; Meile, C. D.

    2015-09-01

    We describe a variety of one- and two-dimensional mathematical modeling approaches to characterizing diffuse flow circulation at mid-ocean ridge hydrothermal systems. The goal is to estimate the potential extent of the sub-seafloor microbial biosphere based on subsurface contours of the 120 °C isotherm as determined from the various models. The models suggest that the sub-seafloor depth for microbial life may range from less than 1 m in some places to the thickness of crustal layer 2A of ∼ 500 m in others. This depth depends primarily on how diffuse flow is driven. The 120 °C isotherm tends to be much deeper if diffuse flow is induced as boundary layer flow near high-temperature plumes, than if it results from conductive cooling or mixing near the seafloor. Because the heat flow alone may not allow identification of the flow regime in the subsurface, we highlight the use of chemical tracers as an additional constraint that sheds light into the flow and reaction patterns associated with vents. We use thermodynamic modeling, which connects the temperature of the diffuse fluid to its chemical composition. As the temperature-composition relationships differ for mixing versus conductive heating and cooling, the fluid geochemistry can shed light on subsurface transport. Using methane as an example, the geochemical models indicate subsurface microbial methane production and consumption in different regions of the vent field near EPR 9 °50‧ N.

  7. Geology of the Early Archean Mid-Ocean Ridge Hydrothermal System in the North Pole Dome, Pilbara Craton, Western Australia

    NASA Astrophysics Data System (ADS)

    Kitajima, K.; Maruyama, S.

    2007-12-01

    An Archean hydrothermal system in the North Pole Dome, Pilbara Craton is associated with extensive fluid circulation driven by numerous extensional fracture systems and the underlying heat source. The fracture system is now occupied by abundant fine-grained quartz aggregate, hence we call this as silica dikes. Some of the fracture system extends deeper structural levels as listric normal faults down to 1000 m depth in the MORB crust. Barite-bearing fine-grained quartz predominant mineralogy indicates the extensive development of fracturing and quenching in a short time. Accompanying the fluid circulation, the extensive metasomatism proceeded to form the four different chemical courses, (1) silicification, (2) carbonation, (3) potassium-enrichment, and (4) Fe- enrichment. Silicification occurs along the silica dikes, carbonated greenstones are distributed relatively shallower level. Potassium-enriched (mica-rich) greenstones occur at the top of the greenstone sequence, and Fe-enriched (chlorite-rich) greenstones are distributed at lower part of the basaltic greenstones. The down going fluid precipitated carbonate-rich layer at shallow levels, whereas depleted in SiO2. Then, the fluid went down to more deeper level, and was dissolved SiO2 at high temperature (~350°C) and chlorite-rich greenstone was formed by water-rock interaction. The upwelling fluid precipitated dominantly SiO2 and formed silica dikes. Silica dikes cement the fractures formed by extensional faulting at earliest stage of development of oceanic crust. Therefore, the hydrothermal system must have related to normal fault system simultaneously with MORB volcanism. Particularly the greenish breccia with cherty matrix (oregano chert) was formed at positions by upwelling near ridge axis. After the horizontal removal of MORB crust from the ridge-axis with time, the propagating fracture into deeper levels, transports hydrothermal fluids into 500-1000 m depth range where metasomatic element exchange between

  8. Atmospheric contamination of the primary Ne and Ar signal in mid-ocean ridge basalts and its implications for ocean crust formation

    NASA Astrophysics Data System (ADS)

    Stroncik, N. A.; Niedermann, S.

    2016-01-01

    Both, terrestrial and extra-terrestrial applications of noble gases have demonstrated their importance as tracers for source identification, process characterisation and mass and heat flux quantification. However, the interpretation of noble gas isotope data from terrestrial igneous rocks is often complicated by the ubiquitous presence of heavy noble gases (Ne, Ar, Kr, Xe) with an atmospheric origin. Up to now there has been no consensus on how atmospheric noble gases are entrained into igneous rocks. Suggested processes range from contamination during sample preparation to mantle recycling through subduction. Here we present Ne, Ar, Mg, K, and Cl data of fresh glasses from the Mid-Atlantic Ridge north and south of the Ascension Fracture Zone which show that incorporation of atmospheric noble gases into igneous rocks is in general a two-step process: (1) magma contamination by assimilation of altered oceanic crust results in the entrainment of noble gases from air-equilibrated seawater; (2) atmospheric noble gases are adsorbed onto grain surfaces during sample preparation. This implies, considering the ubiquitous presence of the contamination signal, that magma contamination by assimilation of a seawater-sourced component is an integral part of mid-ocean ridge basalt evolution. Combining the results obtained from noble gas and Cl/K data with estimates of crystallisation pressures for the sample suite shows that the magma contamination must have taken place at a depth between 9 and 13 km. Taking thickness estimates for the local oceanic crust into account, this implies that seawater penetration in this area reaches lower crustal levels, indicating that hydrothermal circulation might be an effective cooling mechanism even for the deep parts of the oceanic crust.

  9. Noble gas isotope signals of mid-ocean ridge basalts and their implication for upper mantle structure

    NASA Astrophysics Data System (ADS)

    Stroncik, Nicole A.; Niedermann, Samuel

    2016-04-01

    The geochemical structure of the upper mantle in general and its noble gas isotopic structure in particular have been the subject of countless studies, as both provide important insights into mantle dynamic processes and are essential for the formulation of mantle geodynamic models. This contribution presents a noble gas study of basaltic glasses derived from the Mid-Atlantic-Ridge (MAR) between 4 and 12° S, an area well known for its high degree of lithophile isotope heterogeneity and exhibiting anomalous crustal thickness. The Sr, Nd, Pb and Hf isotopies along this segment of the MAR range from ultra-depleted (more than NMORB) to highly enriched, and different concepts have been proposed to explain the observed isotopic signatures. Here we show that the high degree of heterogeneity is not confined to the isotopes of the lithophile elements, but is also shown by the noble gas isotopes and noble gas interelement ratios, such as e.g. 3He/22NeM or 4He/40Ar*. 3He/4He, 21Ne/22Neextra and 40Ar/36Ar range from 7.3 to 9.3 RA, from 0.05 to 0.08, and from 346 to 37,400, respectively. Nevertheless, the majority of the Ne isotope data are clearly aligned along a single mixing line in the Ne-three-isotope diagram, represented by the equation 20Ne/22Ne=70.5 x 21Ne/22Ne + 7.782, with a slope distinctly different from that of the MORB line, indicating that the ultra-depleted material is characterised by a significantly more nucleogenic 21Ne/22Ne isotopy than the normal depleted mantle. We show, based on covariations between 3He/4He and 21Ne/22Neextra with 206Pb/204Pb and 178Hf/177Hf, that the ultra-depleted material erupted at this MAR segment was most likely produced by an ancient, deep melting event. This implies that isotopic heterogeneities in the upper mantle are not solely caused by the injection of enriched materials from deep-seated mantle plumes or by crustal recycling but may also be due to differences in the depth and degree of melting of upper mantle material within

  10. Coupled major and trace elements as indicators of the extent of melting in mid-ocean-ridge peridotites.

    PubMed

    Hellebrand, E; Snow, J E; Dick, H J; Hofmann, A W

    2001-04-01

    Rocks in the Earth's uppermost sub-oceanic mantle, known as abyssal peridotites, have lost variable but generally large amounts of basaltic melt, which subsequently forms the oceanic crust. This process preferentially removes from the peridotite some major constituents such as aluminium, as well as trace elements that are incompatible in mantle minerals (that is, prefer to enter the basaltic melt), such as the rare-earth elements. A quantitative understanding of this important differentiation process has been hampered by the lack of correlation generally observed between major- and trace-element depletions in such peridotites. Here we show that the heavy rare-earth elements in abyssal clinopyroxenes that are moderately incompatible are highly correlated with the Cr/(Cr + Al) ratios of coexisting spinels. This correlation deteriorates only for the most highly incompatible elements-probably owing to late metasomatic processes. Using electron- and ion-microprobe data from residual abyssal peridotites collected on the central Indian ridge, along with previously published data, we develop a quantitative melting indicator for mantle residues. This procedure should prove useful for relating partial melting in peridotites to geodynamic variables such as spreading rate and mantle temperature. PMID:11287951

  11. Lithium isotope evidence for subduction-enriched mantle in the source of mid-ocean-ridge basalts.

    PubMed

    Elliott, Tim; Thomas, Alex; Jeffcoate, Alistair; Niu, Yaoling

    2006-10-01

    'Recycled' crustal materials, returned from the Earth's surface to the mantle by subduction, have long been invoked to explain compositional heterogeneity in the upper mantle. Yet increasingly, problems have been noted with this model. The debate can be definitively addressed using stable isotope ratios, which should only significantly vary in primitive, mantle-derived materials as a consequence of recycling. Here we present data showing a notable range in lithium isotope ratios in basalts from the East Pacific Rise, which correlate with traditional indices of mantle heterogeneity (for example, 143Nd/144Nd ratios). Such co-variations of stable and radiogenic isotopes in melts from a normal ridge segment provide critical evidence for the importance of recycled material in generating chemical heterogeneity in the upper mantle. Contrary to many models, however, the elevated lithium isotope ratios of the 'enriched' East Pacific Rise lavas imply that subducted ocean crust is not the agent of enrichment. Instead, we suggest that fluid-modified mantle, which is enriched during residency in a subduction zone, is mixed back into the upper mantle to cause compositional variability. PMID:17024091

  12. Lithium isotope evidence for subduction-enriched mantle in the source of mid-ocean-ridge basalts.

    PubMed

    Elliott, Tim; Thomas, Alex; Jeffcoate, Alistair; Niu, Yaoling

    2006-10-01

    'Recycled' crustal materials, returned from the Earth's surface to the mantle by subduction, have long been invoked to explain compositional heterogeneity in the upper mantle. Yet increasingly, problems have been noted with this model. The debate can be definitively addressed using stable isotope ratios, which should only significantly vary in primitive, mantle-derived materials as a consequence of recycling. Here we present data showing a notable range in lithium isotope ratios in basalts from the East Pacific Rise, which correlate with traditional indices of mantle heterogeneity (for example, 143Nd/144Nd ratios). Such co-variations of stable and radiogenic isotopes in melts from a normal ridge segment provide critical evidence for the importance of recycled material in generating chemical heterogeneity in the upper mantle. Contrary to many models, however, the elevated lithium isotope ratios of the 'enriched' East Pacific Rise lavas imply that subducted ocean crust is not the agent of enrichment. Instead, we suggest that fluid-modified mantle, which is enriched during residency in a subduction zone, is mixed back into the upper mantle to cause compositional variability.

  13. Coupled major and trace elements as indicators of the extent of melting in mid-ocean-ridge peridotites.

    PubMed

    Hellebrand, E; Snow, J E; Dick, H J; Hofmann, A W

    2001-04-01

    Rocks in the Earth's uppermost sub-oceanic mantle, known as abyssal peridotites, have lost variable but generally large amounts of basaltic melt, which subsequently forms the oceanic crust. This process preferentially removes from the peridotite some major constituents such as aluminium, as well as trace elements that are incompatible in mantle minerals (that is, prefer to enter the basaltic melt), such as the rare-earth elements. A quantitative understanding of this important differentiation process has been hampered by the lack of correlation generally observed between major- and trace-element depletions in such peridotites. Here we show that the heavy rare-earth elements in abyssal clinopyroxenes that are moderately incompatible are highly correlated with the Cr/(Cr + Al) ratios of coexisting spinels. This correlation deteriorates only for the most highly incompatible elements-probably owing to late metasomatic processes. Using electron- and ion-microprobe data from residual abyssal peridotites collected on the central Indian ridge, along with previously published data, we develop a quantitative melting indicator for mantle residues. This procedure should prove useful for relating partial melting in peridotites to geodynamic variables such as spreading rate and mantle temperature.

  14. Hydrothermal and magmatic couplings at mid-ocean ridges : controls on the locations of high-temperature hydrothermal vent fields

    NASA Astrophysics Data System (ADS)

    Rabinowicz, M.; Fontaine, F. J.; Cannat, M.; Escartin, J.

    2012-12-01

    The heat output and thermal regime of oceanic spreading centers are strongly controlled by boundary layer processes between the hydrothermal system and the underlying crustal magma chamber, which remain to be fully understood. In thermal models, the dynamical interactions between the hydrothermal system and the deeper part of the lithosphere affected by processes such as magma chamber convection, magma crystallization and latent heat release, or simple conduction, is usually not considered and a ad-hoc temperature or heat flux is prescribed at the base of the hydrothermal layer. In this work we develop original two-dimensional numerical models of the interactions between a shallow cellular hydrothermal (porous) system at temperatures <700°C in the upper crust, and a deeper magmatic (viscous) layer at temperatures up to 1200°C representing the lower crust. Our formalism allows for a dynamical interface between the two layers, which is fluctuating according to the dynamics of each layer. We systematically investigate the range of permeability and viscosity that characterized the dynamics of the porous and magmatic systems, respectively. An intriguing and highly debated question that we investigate is about the genesis of focused (i.e., kilometer-wide), hundreds-of-mega-watt (MW) powerfull, high-temperature (300-400°C) hydrothermal fields such as those discovered along the East Pacific Rise at 9°50'N or along the Juan de Fuca ridge/Endeavour segment for example. One hypothesis is that these fields require the formation of "elongated" hydrothermal convection cells that cool the crust on 5-10 kms, but the processes controlling the formation of such large aspect ratio (length/height) are poorly constrain. Our models show that such cells naturally arise from the dynamical coupling between a « low-viscosity », convecting lower-crust and a low-permeability upper hydrothermal layer. They also predict along-axis variations in the depth of the axial magma lens (AMC

  15. Hydrothermal and magmatic couplings at mid-ocean ridges : controls on the locations of high-temperature hydrothermal vent fields

    NASA Astrophysics Data System (ADS)

    Fontaine, Fabrice; Rabinowicz, Michel; Cannat, Mathilde; Escartin, Javier

    2013-04-01

    The heat output and thermal regime of oceanic spreading centers are strongly controlled by boundary layer processes between the hydrothermal system and the underlying crustal magma chamber, which remain to be fully understood. In thermal models, the dynamical interactions between the hydrothermal system and the deeper part of the lithosphere affected by processes such as magma chamber convection, magma crystallization and latent heat release, or simple conduction, is usually not considered and a ad-hoc temperature or heat flux is prescribed at the base of the hydrothermal layer. In this work we develop original two-dimensional numerical models of the interactions between a shallow cellular hydrothermal (porous) system at temperatures <700°C in the upper crust, and a deeper magmatic (viscous) layer at temperatures up to 1200°C representing the lower crust. Our formalism allows for a dynamical interface between the two layers, which is fluctuating according to the dynamics of each layer. We systematically investigate the range of permeability and viscosity that characterized the dynamics of the porous and magmatic systems, respectively. An intriguing and highly debated question that we investigate is about the genesis of focused (i.e., kilometer-wide), hundreds-of-mega-watt (MW) powerfull, high-temperature (300-400°C) hydrothermal fields such as those discovered along the East Pacific Rise at 9°50'N or along the Juan de Fuca ridge/Endeavour segment for example. One hypothesis is that these fields require the formation of "elongated" hydrothermal convection cells that cool the crust on 5-10 kms, but the processes controlling the formation of such large aspect ratio (length/height) are poorly constrain. Our models show that such cells naturally arise from the dynamical coupling between a « low-viscosity », convecting lower-crust and a low-permeability upper hydrothermal layer. They also predict along-axis variations in the depth of the axial magma lens (AMC

  16. Two-component mantle melting-mixing model for the generation of mid-ocean ridge basalts: Implications for the volatile content of the Pacific upper mantle

    NASA Astrophysics Data System (ADS)

    Shimizu, Kei; Saal, Alberto E.; Myers, Corinne E.; Nagle, Ashley N.; Hauri, Erik H.; Forsyth, Donald W.; Kamenetsky, Vadim S.; Niu, Yaoling

    2016-03-01

    We report major, trace, and volatile element (CO2, H2O, F, Cl, S) contents and Sr, Nd, and Pb isotopes of mid-ocean ridge basalt (MORB) glasses from the Northern East Pacific Rise (NEPR) off-axis seamounts, the Quebrada-Discovery-GoFar (QDG) transform fault system, and the Macquarie Island. The incompatible trace element (ITE) contents of the samples range from highly depleted (DMORB, Th/La ⩽ 0.035) to enriched (EMORB, Th/La ⩾ 0.07), and the isotopic composition spans the entire range observed in EPR MORB. Our data suggest that at the time of melt generation, the source that generated the EMORB was essentially peridotitic, and that the composition of NMORB might not represent melting of a single upper mantle source (DMM), but rather mixing of melts from a two-component mantle (depleted and enriched DMM or D-DMM and E-DMM, respectively). After filtering the volatile element data for secondary processes (degassing, sulfide saturation, assimilation of seawater-derived component, and fractional crystallization), we use the volatiles to ITE ratios of our samples and a two-component mantle melting-mixing model to estimate the volatile content of the D-DMM (CO2 = 22 ppm, H2O = 59 ppm, F = 8 ppm, Cl = 0.4 ppm, and S = 100 ppm) and the E-DMM (CO2 = 990 ppm, H2O = 660 ppm, F = 31 ppm, Cl = 22 ppm, and S = 165 ppm). Our two-component mantle melting-mixing model reproduces the kernel density estimates (KDE) of Th/La and 143Nd/144Nd ratios for our samples and for EPR axial MORB compiled from the literature. This model suggests that: (1) 78% of the Pacific upper mantle is highly depleted (D-DMM) while 22% is enriched (E-DMM) in volatile and refractory ITE, (2) the melts produced during variable degrees of melting of the E-DMM controls most of the MORB geochemical variation, and (3) a fraction (∼65% to 80%) of the low degree EMORB melts (produced by ∼1.3% melting) may escape melt aggregation by freezing at the base of the oceanic lithosphere, significantly enriching it in

  17. Implications of the Iceland Deep Drilling Project for improving understanding of hydrothermal processes at slow spreading mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Elders, Wilfred A.; Friðleifsson, Guðmundur Ómar

    The Iceland Deep Drilling Project (IDDP) is investigating producing geothermal energy from magma-hydrothermal systems at supercritical conditions. This requires drilling to depths of 4 to 5 km where temperatures should be in the range of 450-600°C or higher. Modeling studies suggest producing superheated steam from supercritical temperatures >450°C could increase power output tenfold relative to steam produced from a 300°C reservoir. The first IDDP well was drilled at Krafla within a volcanic caldera in the active rift zone of NE Iceland. At the end of June 2009, drilling was terminated at only 2100 m depth when a >900°C rhyolitic magma flowed into the drill hole. The well was completed with a casing cemented a few meters above the magma. Depending on the result of a planned flow test, there is the possibility of creating the world's hottest "Engineered Geothermal System" by injecting water in a nearby well to produce superheated steam from the magma. An advantage of such a strategy would be that the acidic gases likely to be given off by the magma could be neutralized by injecting suitably treated water. Two new wells, ˜4 km deep, are planned to be drilled during 2010-2012 at the Hengill and the Reykjanes geothermal fields in SW Iceland to explore for supercritical zones. The Reykjanes geothermal system produces hydrothermally modified seawater. This presents an ideal situation to study a high-temperature magma-hydrothermal system at depth analogous to those responsible for the black smokers at submarine divergent plate margins.

  18. A non-traditional isotope study of plagiogranite and rhyo-dacite suites: insights into mid ocean ridge magma chamber processes (Invited)

    NASA Astrophysics Data System (ADS)

    Lundstrom, C.; Huggett, N.; Perfit, M. R.; Zambardi, T.

    2013-12-01

    The eruption of silicic volcanic rocks in the mid-ocean ridge environment, although rare, provides an important observation for understanding MOR magma chamber processes. Although axially erupted MORB are quite homogeneous geochemically, this likely reflects mixing and reaction from the magma lens axis at only slightly variable temperature. Differentiation occurs along the colder 'wings' of the magma lens and rhyo-dacite suites show mixing trends with normal MORB. Both plagiogranites in ophiolites and erupted rhyodacites provide evidence of extensive magma evolution at MORs. Yet, whether silicic eruptives in MOR environments are equivalent to plagiogranites is unknown. Here we compare the non-traditional isotope ratios of Fe and Si in a plagiogranite suite from Troodos (Cyprus) with rhyo-dacite suites from the Galapagos Spreading Center, the Juan de Fuca-Blanco ridge-transform intersection [RTI], and the 9°N EPR overlapping spreading center [OSC]) to shed light on the origin of oceanic silicic rocks. Within the eastern Troodos Ophiolite we collected 14 samples from a 700m transect from gabbros through to plagiogranites (which immediately transition to sheeted dikes). Indeed, Troodos geologic maps show plagiogranites ubiquitously occur at the upper gabbro-sheeted dike contact. Silica content ranges from ~45 wt. % in the gabbros to 75 wt. % in the most evolved plagiogranites. Notably, iron ratios increase (become heavier) with increasing, up-section distance from the gabbros (d56Fe ranges from -0.05 to 0.40). This spatial pattern is consistent with the prediction of thermal diffusion occurring within a temperature gradient in a MOR magma lens although other explanations are possible. Because there is a spatial progression in SiO2 from gabbro to plagiogranite, these isotope ratios also increase with SiO2. Notably, within the 3 silicic MOR systems, d56Fe and d30Si exhibit the same relationship becoming isotopically heavier with increasing silica content (d30Si ranges

  19. An empirical method for calculating melt compositions produced beneath mid-ocean ridges: Application for axis and off-axis (seamounts) melting

    NASA Astrophysics Data System (ADS)

    Niu, Yaoling; Batiza, Rodey

    1991-12-01

    We present a new method for calculating the major element compositions of primary melts parental to mid-ocean ridge basalt (MORB). This model is based on the experimental data of Jaques and Green (1980), Falloon et al. (1988), and Falloon and Green (1987, 1988) which are ideal for this purpose. Our method is empirical and employs solid-liquid partition coefficients (Di) from the experiments. We empirically determine Di = ƒ(P,F) and use this to calculate melt compositions produced by decompression-induced melting along an adiabat (column melting). Results indicate that most MORBs can be generated by 10-20% partial melting at initial pressures (P0) of 12-21 kbar. Our primary MORB melts have MgO = 10-12 wt %. We fractionate these at low pressure to an MgO content of 8.0 wt % in order to interpret natural MORB liquids. This model allows us to calculate Po, Pƒ, To, Tƒ, and F for natural MORB melts. We apply the model to interpret MORB compositions and mantle upwelling patterns beneath a fast ridge (East Pacific Rise (EPR)8°N to 14°N), a slow ridge (mid-Atlantic Ridge (MAR) at 26°S), and seamounts near the EPR (Lament seamount chain). We find mantle temperature differences of up to 50°-60°C over distances of 30-50 km both across axis and along axis at the EPR. We propose that these are due to upward mantle flow in a weakly conductive (versus adiabatic) temperature gradient. We suggest that the EPR is fed by a wide (-100 km) zone of upwelling due to plate separation but has a central core of faster buoyant flow. An along-axis thermal dome between the Siqueiros transform and the 11°45' Overlapping Spreading center (OSC) may represent such an upwelling; however, in general there is a poor correlation between mantle temperature, topography, and the segmentation pattern at the EPR. For the Lament seamounts we find regular across-axis changes in Po and F suggesting that the melt zone pinches out off axis. This observation supports the idea that the EPR is fed by a

  20. An empirical method for calculating melt compositions produced beneath mid-ocean ridges: for axis and off-axis (seamounts) melting application

    NASA Astrophysics Data System (ADS)

    Batiza, Rodey

    1991-12-01

    We present a new method for calculating the major element compositions of primary melts parental to mid-ocean ridge basalt (MORB). This model is based on the experimental data of Jaques and Green (1980), Falloon et al. (1988), and Falloon and Green (1987, 1988) which are ideal for this purpose. Our method is empirical and employs solid-liquid partition coefficients (Di) from the experiments. We empirically determine Di=f(P,F) and use this to calculate melt compositions produced by decompression-induced melting along an adiabat (column melting). Results indicate that most MORBs can be generated by 10-20% partial melting at initial pressures (P0) of 12-21 kbar. Our primary MORB melts have MgO=10-12 wt %. We fractionate these at low pressure to an MgO content of 8.0 wt% in order to interpret natural MORB liquids. This model allows us to calculate Po, Pf, To, Tf, and F for natural MORB melts. We apply the model to interpret MORB compositions and mantle upwelling patterns beneath a fast ridge (East Pacific Rise (EPR) 8°N to 14°N), a slow ridge (mid-Atlantic Ridge (MAR) at 26°S), and seamounts near the EPR (Lamont seamount chain). We find mantle temperature differences of up to 50°-60°C over distances of 30-50 km both across axis and along axis at the EPR. We propose that these are due to upward mantle flow in a weakly conductive (versus adiabatic) temperature gradient. We suggest that the EPR is fed by a wide (~100 km) zone of upwelling due to plate separation but has a central core of faster buoyant flow. An along-axis thermal dome between the Siqueiros transform and the 11°45' Overlapping Spreading Center (OSC) may represent such an upwelling; however, in general there is a poor correlation between mantle temperature, topography, and the segmentation pattern at the EPR. For the Lamont seamounts we find regular across-axis changes in Po and F suggesting that the melt zone pinches out off axis. This observation supports the idea that the EPR is fed by a broad

  1. Modeling mid-ocean ridge hydrothermal response to earthquakes, tides, and ocean currents: a case study at the Grotto mound, Endeavour Segment, Juan de Fuca Ridge

    NASA Astrophysics Data System (ADS)

    Xu, G.; Bemis, K. G.

    2014-12-01

    Seafloor hydrothermal systems feature intricate interconnections among oceanic, geological, hydrothermal, and biological processes. The advent of the NEPTUNE observatory operated by Ocean Networks Canada at the Endeavour Segment, Juan de Fuca Ridge enables scientists to study these interconnections through multidisciplinary, continuous, real-time observations. The multidisciplinary observatory instruments deployed at the Grotto Mound, a major study site of the NEPTUNE observatory, makes it a perfect place to study the response of a seafloor hydrothermal system to geological and oceanic processes. In this study, we use the multidisciplinary datasets recorded by the NEPTUNE Observatory instruments as observational tools to demonstrate two different aspects of the response of hydrothermal activity at the Grotto Mound to geological and oceanic processes. First, we investigate a recent increase in venting temperature and heat flux at Grotto observed by the Benthic and Resistivity Sensors (BARS) and the Cabled Observatory Vent Imaging Sonar (COVIS) respectively. This event started in Mar 2014 and is still evolving by the time of writing this abstract. An initial interpretation in light of the seismic data recorded by a neighboring ocean bottom seismometer on the NEPTUNE observatory suggests the temperature and heat flux increase is probably triggered by local seismic activities. Comparison of the observations with the results of a 1-D mathematical model simulation of hydrothermal sub-seafloor circulation elucidates the potential mechanisms underlying hydrothermal response to local earthquakes. Second, we observe significant tidal oscillations in the venting temperature time series recorded by BARS and the acoustic imaging of hydrothermal plumes by COVIS, which is evidence for hydrothermal response to ocean tides and currents. We interpret the tidal oscillations of venting temperature as a result of tidal loading on a poroelastic medium. We then invoke poroelastic

  2. A possible difference in cooling rates recorded in REE in coexisting pyroxenes in peridotites from supra-subduction ophiolites and mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Dygert, N. J.; Liang, Y.; Kelley, K. A.

    2013-12-01

    Recently a REE-in-two-pyroxene thermometer was developed for mafic and ultramafic rocks [1]. This new thermometer is based on temperature sensitive REE partition coefficients between coexisting pyroxenes and calibrated against laboratory partitioning data. Because REE diffusion rates in pyroxene are relatively slow, the thermometer reads a higher temperature than major element based pyroxene thermometers. The difference between major and trace element derived temperatures depends primarily on cooling rate. Here we report new trace element data for peridotites from Trinity and Josephine ophiolites and a modern supra-subduction zone (SSZ) ophiolite analogue (the Mariana trench) determined by laser ablation ICP-MS. We inverted temperatures from the new data and globally distributed ophiolitic peridotite from eight literature studies (Figure 1). Data quality was carefully monitored leaving temperatures from 65 samples. Individual ophiolites usually have temperatures clustered within a range of a few hundred degrees, but the temperature range for the global dataset is greater than 700°C (688-1401°C). Temperatures calculated for the same samples using the two pyroxene thermometer of Brey and Köhler [2] are considerably lower (564-1049°C). REE temperatures are plotted against the major element temperatures [2] in Figure 1. Abyssal peridotites reported in [1] are shown by the peach field. Much of the ophiolite data plots farther from the blue 1:1 line than the abyssal peridotites, suggesting SSZ lithospheric mantle may cool more rapidly at those ophiolites. Fast cooling can be attributed to one or more dynamic differences between mid-ocean ridge (MOR) environments and supra-subduction environments, such as enhanced hydrothermal circulation, thinner oceanic crust, or rapid cooling due to basin closure and obduction. We note that several ophiolites appear to cool more slowly than the abyssal peridotites, however in those samples geochemical evidence suggests secondary

  3. An experimental study of focused magma transport and basalt-peridotite interactions beneath mid-ocean ridges: implications for the generation of primitive MORB compositions

    NASA Astrophysics Data System (ADS)

    Lambart, Sarah; Laporte, Didier; Schiano, Pierre

    2009-04-01

    We performed experiments in a piston-cylinder apparatus to determine the effects of focused magma transport into highly permeable channels beneath mid-ocean ridges on: (1) the chemical composition of the ascending basalt; and (2) the proportions and compositions of solid phases in the surrounding mantle. In our experiments, magma focusing was supposed to occur instantaneously at a pressure of 1.25 GPa. We first determined the equilibrium melt composition of a fertile mantle (FM) at 1.25 GPa-1,310°C; this composition was then synthesised as a gel and added in various proportions to peridotite FM to simulate focusing factors Ω equal to 3 and 6 (Ω = 3 means that the total mass of liquid in the system increased by a factor of 3 due to focusing). Peridotite FM and the two basalt-enriched compositions were equilibrated at 1 GPa-1,290°C; 0.75 GPa-1,270°C; 0.5 GPa-1,250°C, to monitor the evolution of phase proportions and compositions during adiabatic decompression melting. Our main results may be summarised as follows: (1) magma focusing induces major changes of the coefficients of the decompression melting reaction, in particular, a major increase of the rate of opx consumption, which lead to complete exhaustion of orthopyroxene (and clinopyroxene) and the formation of a dunitic residue. A focusing factor of ≈4—that is, a magma/rock ratios equal to ≈0.26—is sufficient to produce a dunite at 0.5 GPa. (2) Liquids in equilibrium with olivine (±spinel) at low pressure (0.5 GPa) have lower SiO2 concentrations, and higher concentrations in MgO, FeO, and incompatible elements (Na2O, K2O, TiO2) than liquids produced by decompression melting of the fertile mantle, and plot in the primitive MORB field in the olivine-silica-diopside-plagioclase tetrahedron. Our study confirms that there is a genetic relationship between focused magma transport, dunite bodies in the upper mantle, and the generation of primitive MORBs.

  4. A Gradient in Cooling Rate Beneath the Moho at the Oman Ophiolite: Fresh Insights into Cooling Processes at Mid-Ocean Ridges from REE-Based Thermometry

    NASA Astrophysics Data System (ADS)

    Dygert, N. J.; Kelemen, P. B.; Liang, Y.

    2015-12-01

    The Wadi Tayin massif in the southern Oman ophiolite has a more than 10 km thick mantle section and is believed to have formed in a mid-ocean ridge like environment with an intermediate to fast spreading rate. Previously, [1] used major element geothermometers to investigate spatial variations in temperatures recorded in mantle peridotites and observed that samples near the paleo-Moho have higher closure temperatures than samples at the base of the mantle section. Motivated by these observations, we measured major and trace elements in orthopyroxene and clinopyroxene in peridotites from depths of ~1-8km beneath the Moho to determine closure temperatures of REE in the samples using the REE-in-two-pyroxene thermometer [2]. Clinopyroxene are depleted in LREE and have REE concentrations that vary depending on distance from the Moho. Samples nearer the Moho have lower REE concentrations than those deeper in the section (e.g., chondrite normalized Yb ranges from ~1.5 at the Moho to 4 at 8km depth), consistent with near fractional melting along a mantle adiabat. Orthopyroxene are highly depleted in LREE but measurements of middle to heavy REE have good reproducibility. We find that REE-in-two-pyroxene temperatures decrease with increasing distance from the Moho, ranging from 1325±10°C near the Moho to 1063±24°C near the base of the mantle section. Using methods from [3], we calculate cooling rates of >1000°C/Myr near the Moho, dropping to rates of <10°C/Myr at the bottom of the section. The faster cooling rate is inconsistent with conductive cooling models. Fast cooling of the mantle lithosphere could be facilitated by infiltration of seawater to or beneath the petrologic Moho. This can explain why abyssal peridotites from ultra-slow spreading centers (which lack a crustal section) have cooling rates comparable to those of Oman peridotites [3]. [1] Hanghøj et al. (2010), JPet 51(1-2), 201-227. [2] Liang et al. (2013), GCA 102, 246-260. [3] Dygert & Liang (2015

  5. A Dual-Porosity, In Situ Crystallisation Model For Fast-Spreading Mid-Ocean Ridge Magma Chambers Based Upon Direct Observation From Hess Deep

    NASA Astrophysics Data System (ADS)

    MacLeod, C. J.; Lissenberg, C. J.

    2014-12-01

    We propose a revised magma chamber model for fast-spreading mid-ocean ridges based upon a synthesis of new data from a complete section of lower crust from the East Pacific Rise, reconstructed from samples collected from the Hess Deep rift valley during cruise JC21. Our investigation includes detailed sampling across critical transitions in the upper part of the plutonic section, including the inferred axial melt lens (AML) within the dyke-gabbro transition. We find that an overall petrological progression, from troctolite and primitive gabbro at the base up into evolved (oxide) gabbro and gabbronorite at the top of the lower crustal section, is mirrored by a progressive upward chemical fractionation as recorded in bulk rock and mineral compositions. Crystallographic preferred orientations measured using EBSD show that the downward increase in deformation of mush required in crystal subsidence models is not observed. Together these observations are consistent only with a model in which crystallisation of upward migrating evolving melts occurs in situ in the lower crust. Over-enrichment in incompatible trace element concentrations and ratios above that possible by fractional crystallisation is ubiquitous. This implies redistribution of incompatible trace elements in the lower crust by low porosity, near-pervasive reactive porous flow of interstitial melt moving continuously upward through the mush pile. Mass balance calculations reveal a significant proportion of this trace element enriched melt is trapped at mid-crustal levels. Mineral compositions in the upper third to half of the plutonic section are too evolved to represent the crystal residues of MORB. Erupted MORB therefore must be fed from melts sourced in the deeper part of the crystal mush pile, and which must ascend rapidly without significant modification in the upper plutonics or AML. From physical models of mush processes we posit that primitive melts are transported through transient, high porosity

  6. Lava-seawater vapor interaction at the mid-ocean ridge crest: an important volcanic process to explain lava transport and flow morphology on the deep sea floor

    NASA Astrophysics Data System (ADS)

    Ridley, W. I.; Perfit, M.; Fornari, D.; Cann, J.; Smith, D.

    2003-12-01

    Eruption of lava from seafloor vents at the mid-ocean ridge (MOR) crest remains a poorly understood phenomena, despite the fact that it is the dominant volcanic process on earth. During the last decade only a handful of MOR eruptions have been documented using either NOAA-PMEL hydrophone detected events or serendipity, and observations of seafloor manifestations of those effusive events did not capture the actual interaction between erupted lava and near-freezing ambient seawater. Because of the great physical and technological obstacles to actually observing volcanic eruption processes in the deep sea, we must rely on the physical and chemical evidence left behind in the cooled seafloor lava flows to deduce the likely processes that occurred. Based on observations and sampling of numerous lava flows from slow to fast-spreading MORs we find a plethora of delicate macroscopic features preserved on the crusts of lava flows and in lava pillars that suggest intense and extensive interactions between hot magma and seawater during seafloor eruptions resulting in a briny vapor phase. Undersides of many lobate and sheet lava crusts have glassy drips (lava stalactites) and flanges (relict bubble walls) that could only have formed in cavities initially filled with a hot vapor at magmatic temperatures as lava was transported across the seafloor. Detailed petrologic observations of the surfaces of drips and flanges, including the presence of molten salt, exotic Cl- and S-bearing secondary silicates, secondary sulfates and almost pure forsterite, suggest that the vapor phase was flashed seawater. This vapor phase is a key to understanding delicate drip structures formed on lava crusts and the mechanisms by which lava is distributed far from eruptive fissures on the deep sea floor. We suggest that vaporized seawater is incorporated at the flow front as lava moves over the seafloor. The vapor rises as streams of bubbles through the lava behind the flow front and then collects

  7. Arctic Ocean: hydrothermal activity on Gakkel Ridge.

    PubMed

    Jean-Baptiste, Philippe; Fourré, Elise

    2004-03-01

    In the hydrothermal circulation at mid-ocean ridges, sea water penetrates the fractured crust, becomes heated by its proximity to the hot magma, and returns to the sea floor as hot fluids enriched in various chemical elements. In contradiction to earlier results that predict diminishing hydrothermal activity with decreasing spreading rate, a survey of the ultra-slowly spreading Gakkel Ridge (Arctic Ocean) by Edmonds et al. and Michael et al. suggests that, instead of being rare, the hydrothermal activity is abundant--exceeding by at least a factor of two to three what would be expected by extrapolation from observation on faster spreading ridges. Here we use helium-3 (3He), a hydrothermal tracer, to show that this abundance of venting sites does not translate, as would be expected, into an anomalous hydrothermal 3He output from the ridge. Because of the wide implications of the submarine hydrothermal processes for mantle heat and mass fluxes to the ocean, these conflicting results call for clarification of the link between hydrothermal activity and crustal production at mid-ocean ridges.

  8. Petrogenesis of Cretaceous adakite-like intrusions of the Gangdese Plutonic Belt, southern Tibet: Implications for mid-ocean ridge subduction and crustal growth

    NASA Astrophysics Data System (ADS)

    Zheng, Yuan-chuan; Hou, Zeng-qian; Gong, Ying-li; Liang, Wei; Sun, Qing-Zhong; Zhang, Song; Fu, Qiang; Huang, Ke-Xian; Li, Qiu-Yun; Li, Wei

    2014-03-01

    least 70% of the magma source region was juvenile materials. Combined with the presence of HT (high temperature) charnockitic magmatism, HT granulite facies metamorphism, and large volumes of Late Cretaceous batholiths, the oceanic-slab-derived Nuri adakitic rocks indicate a substantial high heat flux in the Gangdese batholith belt during the Late Cretaceous, which may have been related to subduction of a Neo-Tethyan mid-ocean ridge system. According to this model, hot asthenosphere would rise up through the corresponding slab window, and come into direct contact with both the oceanic slab and the base of the overlying plate. This would cause melting of both the oceanic slab and the overlying plate by the addition of heat that was ultimately linked with peak magmatism and the significant growth and chemical differentiation of juvenile crust in southern Tibet during the Late Cretaceous (105-76 Ma). In addition, the petrogenesis of the Langxian adakite-like two-mica granite indicates that the southern Tibetan crust was still of normal thickness prior to the emplacement of these intrusions at ca. 76 Ma. This probably means that large parts of southern Tibet were not very highly elevated prior to the Indian-Asian collision.

  9. δ37Cl of Mid-Ocean Ridge Vent Fluids Determined by a new SIMS Method for Stable Chlorine Isotope Ratio Measurements

    NASA Astrophysics Data System (ADS)

    Bach, W.; Layne, G. L.; von Damm, K. L.

    2002-12-01

    A method has been developed for the direct determination of δ37Cl in natural fluid samples or rock leachates (pyrohydrolysis products) using Secondary Ion Mass Spectrometry (SIMS). Samples as small as 1 μl (<2 μg Cl) are simply dried by evaporation and the residual salts are then admixed with 1 mg of graphite powder and pressed into a small (1mm diameter) pellet amenable to SIMS analysis. Analyses are performed with a large format high-transmission, high-resolution ion microprobe -- the IMS 1270. Mass resolving powers of greater than 5000 are used to exclude isobaric interferences on 35Cl and 37Cl, producing an accurate and reproducible measurement of δ37Cl. Nine analyses of NIST Cl isotope standard 975a yield an external reproducibility of 0.5 ‰ (2σ ). Repeat analyses of samples are reproducible within 1 ‰ . First δ37Cl data for mid-ocean ridge hydrothermal vent fluids from three sites at EPR 9°N and the Logatchev site (MAR 15°N) have been collected. End member δ37Cl compositions for the EPR fluids are between +6.5 and +7.1 ‰ , whereas that of the Logatchev fluid is +4.6 ‰ . Together with pore waters from accretionary prisms, which are depleted in 37Cl (-2.0 to -7.7 ‰ ; Ransom et al., Geology, 23, 715-718, 1995), seawater-derived fluids in the marine environment span range in δ37Cl of 15 ‰ . This variability is remarkably large when compared to >100 analyses of continental waters (formation and oil-field waters, fresh waters, brines, etc.) that cluster around 0 ‰ with a maximum variation of only 5 ‰ . Two observations suggest that the 37Cl enriched nature of the vent fluids is not related to phase separation. (1) Laboratory experiments indicate that the Δ37Cl(vapor-brine) associated with super-critical phase separation of seawater between 420 and 450°C is small (-0.6 to 0.2 ‰ ; Magenheim, PhD Thesis, UCSD, 1995). (2) Conjugate vapor-brine pairs of boiling sampled in 1991 and 1994 at F vent (Von Damm et al. EPSL, 149, 101-111, 1997

  10. Mid-ocean ridge basalt generation along the slow-spreading, South Mid-Atlantic Ridge (5-11°S): Inferences from 238U-230Th-226Ra disequilibria

    NASA Astrophysics Data System (ADS)

    Turner, Simon; Kokfelt, Thomas; Hauff, Folkmar; Haase, Karsten; Lundstrom, Craig; Hoernle, Kaj; Yeo, Isobel; Devey, Colin

    2015-11-01

    U-series disequilibria have provided important constraints on the physical processes of partial melting that produce basaltic magma beneath mid-ocean ridges. Here we present the first 238U-230Th-226Ra isotope data for a suite of 83 basalts sampled between 5°S and 11°S along the South Mid-Atlantic Ridge. This section of the ridge can be divided into 5 segments (A0-A4) and the depths to the ridge axis span much of the global range, varying from 1429 to 4514 m. Previous work has also demonstrated that strong trace element and radiogenic isotope heterogeneity existed in the source regions of these basalts. Accordingly, this area provides an ideal location in which to investigate the effects of both inferred melt column length and recycled materials. 226Ra-230Th disequilibria indicate that the majority of the basalts are less than a few millennia old such that their 230Th values do not require any age correction. The U-Th isotope data span a significant range from secular equilibrium up to 32% 230Th excess, also similar to the global range, and vary from segment to segment. However, the (230Th/238U) ratios are not negatively correlated with axial depth and the samples with the largest 230Th excesses come from the deepest ridge segment (A1). Two sub-parallel and positively sloped arrays (for segments A0-2 and A3 and A4) between (230Th/238U) and Th/U ratios can be modelled in various ways as mixing between melts from peridotite and recycled mafic lithologies. Despite abundant evidence for source heterogeneity, there is no simple correlation between (230Th/238U) and radiogenic isotope ratios suggesting that at least some of the trace element and radiogenic isotope variability may have been imparted to the source regions >350 kyr prior to partial melting to produce the basalts. In our preferred model, the two (230Th/238U) versus Th/U arrays can be explained by mixing of melts from one or more recycled mafic lithologies with melts derived from chemically heterogeneous

  11. DSL-120A High-Resolution, Near-Bottom Side Scan Sonar Imaging of Mid-Ocean Ridge Crests: East Pacific Rise, Galapagos Rift and Mid-Atlantic Ridge

    NASA Astrophysics Data System (ADS)

    Fornari, D.

    2003-04-01

    The DSL-120A 120kHz side-looking sonar system is one of the survey vehicles in the US National Deep Submergence Facility (NDSF) operated by the Woods Hole Oceanographic Institution for the academic community. Survey speeds for the DSL-120A sonar range from 1.0-1.5 knots depending on terrain roughness. The sonar fish is towed approx. 100 m above the seafloor yielding a backscatter imagery swath of 1.2 km and phase-bathymetry swath of approximately 500-600 m. Data resolution is 1 m for side scan imagery and 4 m for phase-bathymetry. DSL-120A data can resolve bathymetric features with linear dimensions more than 25x smaller than those detectable by hull-mounted multibeam sonars at ridge crest seafloor depths. Data resulting from the DSL-120A sonar system have provided the requisite base maps to understand the volcanic and tectonic evolution of seafloor in many geographic and tectonic settings including several segments of the mid-ocean ridge. These maps have provided site-specific information for planning and carrying out detailed photographic imaging, sampling, and other in situ investigations, and ocean crustal drilling. Backscatter images reveal variations in volcanic emplacement style at different spreading rate and tectonic settings along the MOR crest. DSL-120 sonar data have been collected at the Mid-Atlantic Ridge - 37N, the East Pacific Rise (between 9-10N, an overlapping spreading center at 3 20'N, and at 1 45'N) and within the tectonically dominated Galapagos Rift at 97.5W. Despite the 'fast' spreading nature of crust at the EPR between 1-10N, the sonar images show that volcanic emplacement changes dramatically along-strike depending on the local tectonic setting, and on more regional variations that are likely related to magmatic supply along and across adjacent ridge segments boundaries of various scales. Representative side scan images from these areas will be presented to illustrate different modes of volcanic eruption and emplacement styles

  12. Plume capture by divergent plate motions: implications for the distribution of hotspots, geochemistry of mid-ocean ridge basalts, and estimates of the heat flux at the core-mantle boundary

    NASA Astrophysics Data System (ADS)

    Jellinek, A. Mark; Gonnermann, Helge M.; Richards, Mark A.

    2003-01-01

    The coexistence of stationary mantle plumes with plate-scale flow is problematic in geodynamics. We present results from laboratory experiments aimed at understanding the effects of an imposed large-scale circulation on thermal convection at high Rayleigh number (10 6≤Ra≤10 9) in a fluid with a temperature-dependent viscosity. In a large tank, a layer of corn syrup is heated from below while being stirred by large-scale flow due to the opposing motions of a pair of conveyor belts immersed in the syrup at the top of the tank. Three regimes are observed, depending on the ratio V of the imposed horizontal flow velocity to the rise velocity of plumes ascending from the hot boundary, and on the ratio λ of the viscosity of the interior fluid to the viscosity of the hottest fluid in contact with the bottom boundary. When V≪1 and λ≥1, large-scale circulation has a negligible effect on convection and the heat flux is due to the formation and rise of randomly spaced plumes. When V>10 and λ>100, plume formation is suppressed entirely, and the heat flux is carried by a sheet-like upwelling located in the center of the tank. At intermediate V, and depending on λ, established plume conduits are advected along the bottom boundary and ascending plumes are focused towards the central upwelling. Heat transfer across the layer occurs through a combination of ascending plumes and large-scale flow. Scaling analyses show that the bottom boundary layer thickness and, in turn, the basal heat flux q depend on the Peclet number, Pe, and λ. When λ>10, q∝Pe 1/2 and when λ→1, q∝(Pe λ) 1/3, consistent with classical scalings. When applied to the Earth, our results suggest that plate-driven mantle flow focuses ascending plumes towards upwellings in the central Pacific and Africa as well as into mid-ocean ridges. Furthermore, plumes may be captured by strong upwelling flow beneath fast-spreading ridges. This behavior may explain why hotspots are more abundant near slow

  13. Microbial Diversity in Samples of High Temperature Vent Chimneys From the 71 °N Hydrothermal Fields at the Arctic Mid-Ocean Ridge

    NASA Astrophysics Data System (ADS)

    Steinsbu, B. O.; Daae, F.; Ovreaas, L.; Thorseth, I. H.; Pedersen, R. B.

    2007-12-01

    To get a first insight into the diversity of microorganisms present in the recently discovered active hydrothermal fields along the Mohns Ridge in the Norwegian-Greenland Sea, 16S rDNA clone libraries were constructed with DNA extracted from the walls of active smoker pipes from different locations. Enrichments targeting different physiological groups of microorganisms were prepared both under aerobic, micro-aerobic, and strictly anaerobic conditions. Different combinations of substrates and electron acceptors, pH, and temperatures were used. The enrichment cultures were monitored by use of PCR in combination with denaturing gradient gel electrophoresis (PCR-DGGE) and partial 16S rDNA sequencing. Species dominating in the enrichments were isolated, and their 16S rRNA genes were analyzed. The clones obtained from DNA amplified with primers specific for Archaea represented members of the orders Archaeoglobales, Thermococcales, Desulfurococcales, and Thermoproteales, as well as some unidentified groups. Three major fractions of the clones showed highest similarity to hyperthermophiles belonging to the families Pyrodictiaceae and Desulfurococcaceae, and an unidentified group which was given the name "Arctic Ridge Hydrothermal Vent Archaea" (ARHVA). The major fraction of the clones obtained by use of PCR primers specific for Bacteria affiliated with various genera of Aquificales. Clones representing Proteobacteria, Deferribacteres, Bacteroidetes, Deinococcus- Thermus, Chloroflexi and Firmicutes were also detected. Many clones were relatively distantly related to sequences in the GenBank database. Different types of both thermophiles and hyperthermophiles were enriched and isolated. The isolates were phylogenetically affiliated to Thermotogales, Thermales, Nautilales, Aquificales, Archaeoglobales, Thermococcales, and Desulfurococcales. The cultivation experiments documented the presence of microorganisms mediating various metabolic processes including fermentation

  14. Experimental study on weathering of seafloor volcanic glass by bacteria (Pseudomonas fluorescens) - Implications for the contribution of bacteria to the wate-rock reaction at the Mid-Oceanic Ridge setting

    NASA Astrophysics Data System (ADS)

    Chen, Shun; Wu, Zijun; Peng, Xiaotong

    2014-08-01

    biogeochemical cycling process in the Mid-Oceanic Ridge setting.

  15. Sub-km HIMU-type Enriched Mantle at a Mid-ocean Ridge Far From a Plume: Endeavour, JdFR

    NASA Astrophysics Data System (ADS)

    Gill, J. B.; Michael, P. J.; Dreyer, B. M.; Clague, D. A.; Ramos, F. C.

    2015-12-01

    The Endeavour segment of the Juan de Fuca Ridge is characterized by abundant enriched (E) MORB since the currently inflated axial ridge formed <105 years ago, and by the full range of depleted (D) to E-MORB during the last 2300 years in the km-wide axial graben. Two different styles of enrichment of moderately incompatible elements are present. The first characterized basalts across the ~5 km-wide ridge from >10,000 to ~4000 years ago, whereas the second characterizes more recent basalts erupted in the axial graben. We attribute the first to a higher proportion of pyroxenite to enriched peridotite in the mantle source during ridge inflation. The more recent style reflects the reduced role of pyroxenite after the axial graben formed. The enriched component for both styles is a HIMU-type because it has low 87Sr/86Sr and 176Hf/177Hf relative to 143Nd/144Nd, lower 3He/4He (~8.1 RA) than in the more depleted basalts, shallow slopes on Pb isotope diagrams, and high Nb/LREE ratios. It is regionally widespread and shared with the West Valley and Explorer segments to the north. At least 14 different samplings of mantle components occurred within <1 km of ridge length and width during a time when <1 km of upwelling occurred, indicating that the scale of mantle heterogeneity is <1 km in this setting that is far from a plume.

  16. Design of an integrated flow sensor device with application for monitoring in-situ pH and redox of hydrothermal diffuse flow fluids at mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Ding, K.; Seyfried, W. E.; Tan, C.; Jin, B.; Chen, Y.

    2009-05-01

    In-situ measurement of fluid chemistry with chemical sensors at deep sea conditions is always affected by drifting phenomena, which can be especially serious for long-term monitoring studies. Although fundamental changes in sensor design can be used to lessen the effect of this, in-situ calibration is still an unusually effective means to ensure measurement accuracy. With recent development of more reliable valves and pumps applicable for deep sea operation, together with results of laboratory studies showing the viability of solid state chemical sensors, it now possible to achieve in-situ calibration for sensor measurements under challenging chemical and physical conditions. Thus, a new flow device has been designed to facilitate chemical sensor measurements (e.g., pH and redox) in diffuse flow hydrothermal systems at mid-ocean ridges. The integrated calibration and measurement system was developed with a notion of promoting in-situ data acquisition at deep-sea vents for extended periods of time. Accordingly, it enables integration of multiple thermal and chemical sensors, while allowing automated in-situ calibration during deployment. Laboratory tests have successfully demonstrated the effectiveness of this in-situ calibration. For instance in the case of pH measurement with Ir/IrOx pH sensor in NaCl-bearing fluid at 220 bars, the device not only insured reliable measurement in seawater-type fluids, but also revealed effectiveness during measurement of fluids having dissolved CO2 up to 0.52 mol/kg, which is relevant to conditions likely in volcanically active back- arc settings, fluids issuing from cold seeps, as well as monitoring studies in connection with different carbon sequestration scenarios. The sea-going device consists of a sensor cell with limited internal volume of ~ 1 ml for more effectively enhancing interaction between incoming sample fluid and integrated temperature/chemical sensors. The instrument also contains a computer-controlled process

  17. Diversity of Microorganisms Associated With low Temperature Iron Deposits at the 71°N Hydrothermal Vent Field Along the Arctic Mid-Ocean Ridge

    NASA Astrophysics Data System (ADS)

    Ovreas, L.; Johannessen, T.; Jorgensen, S.; Thorseth, I. H.; Pedersen, R. B.

    2007-12-01

    Rust coloured mounds and chimney-like deposits of the newly discovered71°N hydrothermal vent fields at the south-western part of the Mohns Ridge have been investigated. Iron is the fourth most abundant element in the Earth's crust and thus represents one of the most abundant redox active metals widely available for microbial energy generation. Microbial Fe-oxidation is a widespread process in the deep-sea environments, but only recently have studies begun to elucidate these processes and describe the phylogenetic and physiological diversity of the microbial communities that mediate them. Therefore studying the process by which iron is oxidised and how this influence these cold deep-sea communities is of significant importance. We have studied the microbial communities present in these low-temperature rust coloured deposits in order to elucidate the phylogenetic and physiological diversity of the microbial populations inhabiting these deep-sea environments. Polyphasic characterisations by using geochemical and biological analyses have been performed. The deposited material has a highly porous microtexture of branching, twisted filaments resembling stalks of the iron- oxidising Gallionella sp, but numerous other unidentified filamentous structures were also found to be present. Phylogenetic analysis of clone libraries has so far demonstrated that the bacterial community is dominated by members of the Proteobacteria, Planctomycetes and Chloroflexi. The archaeal community consists of both Crenarchaeota and Euryarchaeota. The Crenarchaeota sequences affiliates with other reported uncultivated Deep-Sea archaeal sequences. To further investigate the ecological impact of these iron mounds and their interaction with microorganisms cultivation experiments have been applied. We are specifically focusing on enrichment of iron oxidizing bacteria. Preliminary results indicates that iron oxidizers related to the newly described Mariprofundus ferrooxidans as well as iron reducers

  18. Response to Comment on “Sensitivity of seafloor bathymetry to climate-driven fluctuations in mid-ocean ridge magma supply”

    NASA Astrophysics Data System (ADS)

    Olive, J.-A.; Behn, M. D.; Ito, G.; Buck, W. R.; Escartín, J.; Howell, S.

    2016-07-01

    Tolstoy reports the existence of a characteristic 100 thousand year (ky) period in the bathymetry of fast-spreading seafloor but does not argue that sea level change is a first-order control on seafloor morphology worldwide. Upon evaluating the overlap between tectonic and Milankovitch periodicities across spreading rates, we reemphasize that fast-spreading ridges are the best potential recorders of a sea level signature in seafloor bathymetry.

  19. Response to Comment on “Sensitivity of seafloor bathymetry to climate-driven fluctuations in mid-ocean ridge magma supply”

    NASA Astrophysics Data System (ADS)

    Olive, J.-A.; Behn, M. D.; Ito, G.; Buck, W. R.; Escartín, J.; Howell, S.

    2016-06-01

    Huybers et al. present new bathymetric spectra from an intermediate-spreading ridge as evidence for a primary contribution of sea level cycles to the morphology of the seafloor. Although we acknowledge the possibility that sea level-modulated magmatic constructions may be superimposed on a first-order tectonic fabric, we emphasize the difficulty of deciphering these different contributions in the frequency domain alone.

  20. Complete genome sequence of the heavy metal resistant bacterium Altererythrobacter atlanticus 26DY36(T), isolated from deep-sea sediment of the North Atlantic Mid-ocean ridge.

    PubMed

    Wu, Yue-Hong; Cheng, Hong; Zhou, Peng; Huo, Ying-Yi; Wang, Chun-Sheng; Xu, Xue-Wei

    2015-12-01

    Altererythrobacter atlanticus 26DY36(T) (CGMCC 1.12411(T)=JCM 18865(T)) was isolated from the North Atlantic Mid-Ocean Ridge. The strain is resistant to heavy metals, such as Mn(2+) (200 mM), Co(2+) (2.0mM), Cu(2+) (1mM), Zn(2+) (1mM), Hg(2+) (0.1mM) and Cd(2+) (0.5mM). Here we describe the genome sequence and annotation, as well as the features of the organism. A. atlanticus 26DY36(T) harbors a chromosome (3,386,291 bp) and a circular plasmid (88,815 bp). The genome contains 3322 protein-coding genes (2483 with predicted functions), 47 tRNA genes and 6 rRNA genes. A. atlanticus 26DY36(T) encodes dozens of genes related to heavy metal resistance and has potential applications in the bioremediation of heavy metal-contaminated environments.

  1. Colonization by pioneer populations of ɛ-Proteobacteria and community succession at mid-ocean ridge hydrothermal vents as determined by T-RFLP analysis

    NASA Astrophysics Data System (ADS)

    Moyer, C. L.; Engebretson, J. J.

    2002-12-01

    Terminal-restriction fragment length polymorphism (T-RFLP) patterns were used to track populations of bacteria occurring within multiple bacterial growth chambers (BGCs) deployed at eight diffuse-flow (Tmax=78°C) hydrothermal vent orifices located within the caldera of Axial Volcano, Juan de Fuca Ridge. For comparison, two distal diffuse vents located at the Magic Mountain area on the Explorer Ridge were also examined. Over a five-year sampling period in conjunction with the NeMO (New Millennium Observatory) program, 52 BGCs were recovered after either a short-term (days) or long-term (annual) deployment. Upon recovery, genomic DNA was extracted and amplified using bacterial-specific PCR primers to generate 5' fluorescently-labeled amplicons of small subunit rRNA genes (i.e., SSU rDNAs). These PCR amplicons were digested with multiple tetrameric restriction endonucleases and the respective community diversity and succession patterns were characterized. The average number of populations (a measure of species richness) within the community that developed in short-term deployed BGCs was significantly lower than those detected in long-term deployed BGCs. All short-term BGC communities were dominated by primary colonizers or pioneer populations indicative of ɛ-Proteobacteria, of which, specific phylogenetic groups were recognized at vent sites throughout the five-year sampling period. The long-term BGCs showed evidence of successional events by an increased occurrence of numerous other populations accompanying the pioneer populations of ɛ-Proteobacteria. The discovery that all primary colonizing populations were most similar to known lineages of ɛ-Proteobacteria detected from hydrothermal vents located worldwide provides further evidence that a few cosmopolitan populations are capable of acting as the primary microbial successors of newly-formed hydrothermal vent systems.

  2. Diversity of Planctomycetes in iron-hydroxide deposits from the Arctic Mid Ocean Ridge (AMOR) and description of Bythopirellula goksoyri gen. nov., sp. nov., a novel Planctomycete from deep sea iron-hydroxide deposits.

    PubMed

    Storesund, Julia E; Øvreås, Lise

    2013-10-01

    Planctomycetes form a deep branching and distinct phylum of the domain Bacteria, and represent a fascinating group due to their unusual features such as intracellular compartmentalization and lack of peptidoglycan in their cell walls. The phylum Planctomycetes was described already in 1924, but still the diversity of this phylum represents an enigma and unexploited resource. In this study the diversity of the phylum Planctomycetes in low temperature iron-hydroxide deposits at the Mohns Ridge, a part of the Arctic Mid Ocean Ridge (AMOR), was characterised by descriptive analysis of 16S rRNA gene sequences in combination with isolation of planctomycetes strains. The 16S rRNA gene sequences were affiliated with three order within the phylum Planctomycetes namely the (i)Planctomycetales, (ii) "Candidatus Brocadiales" and (iii) Phycisphaerae in addition to sequences affiliating to hitherto unknown Planctomycetes. The majority of the sequences were affiliated with the CCM11a group (Phycisphaerae), and with the Pir4 group (Planctomycetaceae). Two strains from the order Planctomycetales were isolated. One strain (Plm2) showed high similarity to the previously isolated Planctomyces maris (99 % 16S rRNA sequence identity). The other strain (Pr1d) belonged to the Pir4 group, and showed highest identity with Rhodopirellula baltica (86 %), Blastopirellula marina (86 %) and Pirellula staleyi (85 %). Based on its physiological and biochemical properties, strain Pr1d(T) is considered to represent a new genus of the order Planctomycetales. We propose to classify the novel planctomycete in a new genus and species, Bythoypirellula goksoyri gen. nov., sp. nov., the type strain being Pr1d(T).

  3. Diversity of Planctomycetes in iron-hydroxide deposits from the Arctic Mid Ocean Ridge (AMOR) and description of Bythopirellula goksoyri gen. nov., sp. nov., a novel Planctomycete from deep sea iron-hydroxide deposits.

    PubMed

    Storesund, Julia E; Øvreås, Lise

    2013-10-01

    Planctomycetes form a deep branching and distinct phylum of the domain Bacteria, and represent a fascinating group due to their unusual features such as intracellular compartmentalization and lack of peptidoglycan in their cell walls. The phylum Planctomycetes was described already in 1924, but still the diversity of this phylum represents an enigma and unexploited resource. In this study the diversity of the phylum Planctomycetes in low temperature iron-hydroxide deposits at the Mohns Ridge, a part of the Arctic Mid Ocean Ridge (AMOR), was characterised by descriptive analysis of 16S rRNA gene sequences in combination with isolation of planctomycetes strains. The 16S rRNA gene sequences were affiliated with three order within the phylum Planctomycetes namely the (i)Planctomycetales, (ii) "Candidatus Brocadiales" and (iii) Phycisphaerae in addition to sequences affiliating to hitherto unknown Planctomycetes. The majority of the sequences were affiliated with the CCM11a group (Phycisphaerae), and with the Pir4 group (Planctomycetaceae). Two strains from the order Planctomycetales were isolated. One strain (Plm2) showed high similarity to the previously isolated Planctomyces maris (99 % 16S rRNA sequence identity). The other strain (Pr1d) belonged to the Pir4 group, and showed highest identity with Rhodopirellula baltica (86 %), Blastopirellula marina (86 %) and Pirellula staleyi (85 %). Based on its physiological and biochemical properties, strain Pr1d(T) is considered to represent a new genus of the order Planctomycetales. We propose to classify the novel planctomycete in a new genus and species, Bythoypirellula goksoyri gen. nov., sp. nov., the type strain being Pr1d(T). PMID:24018702

  4. In-Situ pH Measurements in Mid-Ocean Ridge Hydrothermal Vent Fluids: Constraints on Subseafloor Alteration Processes at Crustal Depths

    NASA Astrophysics Data System (ADS)

    Schaen, A. T.; Ding, K.; Seyfried, W. E.

    2013-12-01

    Developments in electrochemistry and material science have facilitated the construction of ceramic (YSZ) based chemical sensor systems that can be used to measure and monitor pH and redox in aqueous fluids at elevated temperatures and pressures. In recent years, these sensor systems have been deployed to acquire real-time and time series in-situ data for high-temperature hydrothermal vent fluids at the Main Endeavour Field (Juan de Fuca Ridge), 9oN (East Pacific Rise), and at the ultramafic-hosted Rainbow field (36oN, Mid-Atlantic Ridge). Here we review in-situ pH data measured at these sites and apply these data to estimate the pH of fluids ascending to the seafloor from hydrothermal alteration zones deeper in the crust. In general, in-situ pH measured at virtually all vent sites is well in excess of that measured shipboard owing to the effects of temperature on the distribution of aqueous species and the solubility of metal sulfides, especially Cu and Zn, originally dissolved in the vent fluids. In situ pH measurements determined at MEF (Sully vent) and EPR 9oN (P-vent) in 2005 and 2008 were 4.4 ×0.02 and 5.05×0.05, respectively. The temperature and pressure (seafloor) of the vent fluids at each of the respective sites were 356oC and 220 bar, and 380oC and 250 bar. Plotting these data with respect to fluid density reveals that the in-situ pH of each vent fluid is approximately 1.5 pH units below neutrality. The density-pH (in-situ) correlation, however, is important because it provides a means from which the vent fluids were derived. Using dissolved silica and chloride from fluid samples at the MEF (Sully) suggest T/P conditions of approximately 435oC, 380 bar, based on quartz-fluid and NaCl-H2O systems. At the fluid density calculated for these conditions, pH (in-situ) is predicted to be ~6.2. Attempts are presently underway to assess the effect of the calculated pH on metal sulfide and silicate (e.g., plagioclase, chlorite) solubility in comparison with

  5. Depth-dependent permeability of hydrothermal discharge zones: measurements through tidal modulation and implications for mid-ocean ridge heat budgets

    NASA Astrophysics Data System (ADS)

    Barreyre, T.; Crone, T. J.; Olive, J. A. L.

    2015-12-01

    The efficiency of hydrothermal circulation as a heat and mass exchanger is strongly modulated by the permeability structure of newly formed oceanic crust, which is known to vary spatially and temporally. Here we use the modulation of discharge temperatures by oscillatory tidal loading to estimate permeability vs. depth profiles beneath hydrothermal discharge zones at three basalt-hosted hydrothermal fields. Phase lags between fluctuations in discharge temperature and seafloor tidal loading have previously been inverted for the permeability of the underlying discharge zone using the poroelastic modulation model of Jupp and Schulz [2004]. This has yielded a wide range of permeability estimates ranging from 10-13­-10-9 m2. Here we extend the model to a stratified medium comprising a top layer (2A) of permeability k2A overlying a bottom layer (2B) of permeability k2B. We apply it to three basalt-hosted hydrothermal fields on a slow- (Lucky Strike), intermediate- (Main Endeavour Field), and fast-spreading (East Pacific Rise, EPR-9ºN) ridge, where stratification is well known from seismic studies. We estimate a layer 2A permeability of ~10-10 m2 at Lucky Strike, in sharp contrast with EPR-9ºN, where k2A ~ 10-13 m2. At the Main Endeavour field, two different sites located above different distinct discharge zones (as indicated by magnetic studies) yield a high and a low permeability. The permeability of layer 2B is not as well constrained as that of layer 2A, with possible values ranging from 10-14-10-12 m2. We note, however, that the variability in measured phase lags across hydrothermal fields is compatible with a uniform layer 2B permeability of ~10-13 m2. Using theoretical scalings for high-Rayleigh porous convection, we demonstrate that the permeability of layer 2B sets the "effective permeability" of the entire convective system, and therefore the efficiency of heat extraction through young oceanic crust. A uniform layer 2B permeability would thus reconcile the

  6. Petrology and Sr, Nd, and Pb isotope geochemistry of mid-ocean ridge basalt glasses from the 11°45'N to 15°00'N segment of the East Pacific Rise

    NASA Astrophysics Data System (ADS)

    Castillo, P. R.; Klein, E.; Bender, J.; Langmuir, C.; Shirey, S.; Batiza, R.; White, W.

    2000-11-01

    Basaltic glasses from the geophysically well-studied section of the East Pacific Rise (EPR) between 11°45'N to 15°00'N range from normal mid-ocean ridge basalts (MORB) to transitional MORB and their major element variations correlate with isotopic and trace element indices of enrichment. To first order, basalts enriched in Na8.0, incompatible elements, 87Sr/86Sr, and 206Pb/204Pb but low in Fe8.0 and 143Nd/144Nd are more prevalent along the shallow portions of the ridge axis. In detail, the samples can be divided into two chemical and geographical Groups: the southern bathymetric dome, extending from the 11°45'N overlapping spreading center to ~14°10'N, and the northern Group, extending from ~14°10'N to the Orozco transform. The boundary between these two Groups is apparent in a change in isotopic composition. Results indicate that there are three mantle source components that produce the compositional variability observed among samples from the 11°45'N to 15°00'N segment of the EPR: a depleted mantle component, a seamount-type enriched mantle component, and an Indian MORB-like mantle component. South of ~14°10'N, the geochemical variability is dominated by binary mixing between a depleted mantle component and an enriched component similar to near-ridge seamounts. North of ~14°10'N, the low 206Pb/204Pb, high 207Pb/204Pb Indian MORB-like component exerts a major influence on the geochemical variability of the axial lavas. Regional averages of major element composition (e.g., Na8.0 and Fe8.0) show relatively limited variability consistent with the restricted range in depth for this region and plot within the Pacific field of the previously defined global trends. Major element variations among individual samples, however, parallel the global array, and their correlation with indices of mantle enrichment supports the idea that the ``Pacific-type local trend'' results from small-scale heterogeneities in the mantle beneath the EPR. Our results also indicate that

  7. Volcanic eruption of the mid-ocean ridge along the East Pacific Rise crest at 9°45-52'N: Direct submersible observations of seafloor phenomena associated with an eruption event in April, 1991

    NASA Astrophysics Data System (ADS)

    Haymon, R. M.; Fornari, D. J.; Von Damm, K. L.; Lilley, M. D.; Perfit, M. R.; Edmond, J. M.; Shanks, W. C.; Lutz, R. A.; Grebmeier, J. M.; Carbotte, S.; Wright, D.; McLaughlin, E.; Smith, M.; Beedle, N.; Olson, E.

    1993-08-01

    In April, 1991, we witnessed from the submersible Alvin a suite of previously undocumented seafloor phenomena accompanying an in-progress eruption of the mid-ocean ridge on the East Pacific Rise crest at 9°45'N-52'N. The volume of the eruption could not be precisely determined, although comparison of pre- and post-eruption SeaBeam bathymetry indicate that any changes in ridge crest morphology resulting from the eruption were < 10 m high. Effects of the eruption included: (1) increased abundance and redistribution of hydrothermal vents, disappearance of numerous vent communities, and changes in characteristics of vent fauna and mineral deposits within the eruption area since December, 1989; (2) murkiness of bottom waters up to tens of meters above the seafloor due to high densities of suspended mineral and biogenic particulates; (3) destruction of a vent community by lava flows, mass wasting, and possible hydrovolcanic explosion at a site known as 'Tubeworm Barbecue' in the axial summit caldera (ASC) at 9°50.6'N; (4) near-critical temperatures of hydrothermal vent fluids, ranging up to 403°C; (5) temporal variations over a 2 week interval in both temperatures and chemical/isotopic compositions of hydrothermal fluids; (6) unusual compositions of end-member vent fluids, with pH values ranging to a record low of 2.5, salinities ranging as low as 0.3 wt% NaCl (one-twelfth that of seawater), and dissolved gases reaching high concentrations (> 65 mmol/l for both CO 2 and H 2S); (7) venting at temperatures above 380°C of visually detectable white vapor that transformed to plumes of gray smoke a few centimeters above vent orifices; (8) disorganized venting of both high-temperature fluids (black and gray smoke) and large volumes of cooler, diffuse hydrothermal fluids directly from the basaltic seafloor, rather than from hydrothermal mineral constructions; (9) rapid and extensive growth of flocculent white bacterial mats (species unknown) on and under the seafloor in

  8. Anhydrite Solubility and Ca Isotope Fractionation in the Vapor-Liquid Field of the NaCl-H2O System: Implications for Hydrothermal Vent Fluids at Mid-ocean Ridges

    NASA Astrophysics Data System (ADS)

    Scheuermann, P.; Syverson, D. D.; Higgins, J. A.; Seyfried, W. E., Jr.

    2015-12-01

    mid-ocean ridges.

  9. Hydrothermal activity on near-arc sections of back-arc ridges: Results from the Mariana Trough and Lau Basin

    NASA Astrophysics Data System (ADS)

    Baker, Edward T.; Massoth, Gary J.; Nakamura, Ko-Ichi; Embley, Robert W.; de Ronde, Cornel E. J.; Arculus, Richard J.

    2005-09-01

    The spatial density of hydrothermal venting is strongly correlated with spreading rate on mid-ocean ridges (with the interesting exception of hot spot-affected ridges), evidently because spreading rate is a reliable proxy for the magma budget. This correlation remains untested on spreading ridges in back-arc basins, where the magma budget may be complicated by subduction-induced variations of the melt supply. To address this uncertainty, we conducted hydrothermal plume surveys along slow-spreading (40-60 mm/yr) and arc-proximal (10-60 km distant) sections of the southern Mariana Trough and the Valu Fa Ridge (Lau Basin). On both sections we found multiple plumes overlying ˜15-20% of the total length of each section, a coverage comparable to mid-ocean ridges spreading at similar rates. These conditions contrast with earlier reported results from the two nearest-arc segments of a faster spreading (60-70 mm/yr) back-arc ridge, the East Scotia Ridge, which approaches no closer than 100 km to its arc. There, hydrothermal venting is relatively scarce (˜5% plume coverage) and the ridge characteristics are distinctly slow-spreading: small central volcanic highs bookended by deep median valleys, and axial melt lenses restricted to the volcanic highs. Two factors may contribute to an unexpectedly low hydrothermal budget on these East Scotia Ridge segments: they may lie too far from the adjacent arc to benefit from near-arc sources of melt supply, and subduction-aided migration of mantle from the Bouvet hot spot may reduce hydrothermal circulation by local crustal warming and thickening, analogous to the Reykjanes Ridge. Thus the pattern among these three ridge sections appears to mirror the larger global pattern defined by mid-ocean ridges: a well-defined trend of spreading rate versus hydrothermal activity on most ridge sections, plus a subset of ridge sections where unusual melt delivery conditions diminish the expected hydrothermal activity.

  10. Carbon and its isotopes in mid-oceanic basaltic glasses

    NASA Technical Reports Server (NTRS)

    Des Marais, D. J.; Moore, J. G.

    1984-01-01

    Sample surface carbon, mantle carbon dioxide in vesicles, and mantle carbon dissolved in glasses, are the three carbon components evident in the 11 mid-oceanic basalts presently analyzed. The total carbon content may be controlled by the depth of the shallowest ridge magma chamber, and carbon isotopic fractionation accompanies magma degassing. Using He-3 and carbon data for submarine hydrothermal fluids, the present day midoceanic ridge carbon flux is approximately estimated to be 1.0 x 10 to the 13th g C/yr, requiring 8 Gyr to accumulate the earth's present crustal carbon inventory.

  11. Carbon and its isotopes in mid-oceanic basaltic glasses

    USGS Publications Warehouse

    Des Marais, D.J.; Moore, J.G.

    1984-01-01

    Three carbon components are evident in eleven analyzed mid-oceanic basalts: carbon on sample surfaces (resembling adsorbed gases, organic matter, or other non-magmatic carbon species acquired by the glasses subsequent to their eruption), mantle carbon dioxide in vesicles, and mantle carbon dissolved in the glasses. The combustion technique employed recovered only reduced sulfur, all of which appears to be indigenous to the glasses. The dissolved carbon concentration (measured in vesicle-free glass) increases with the eruption depth of the spreading ridge, and is consistent with earlier data which show that magma carbon solubility increases with pressure. The total glass carbon content (dissolved plus vesicular carbon) may be controlled by the depth of the shallowest ridge magma chamber. Carbon isotopic fractionation accompanies magma degassing; vesicle CO2 is about 3.8??? enriched in 13C, relative to dissolved carbon. Despite this fractionation, ??13CPDB values for all spreading ridge glasses lie within the range -5.6 and -7.5, and the ??13CPDB of mantle carbon likely lies between -5 and -7. The carbon abundances and ??13CPDB values of Kilauea East Rift glasses apparently are influenced by the differentiation and movement of magma within that Hawaiian volcano. Using 3He and carbon data for submarine hydrothermal fluids, the present-day mid-oceanic ridge mantle carbon flux is estimated very roughly to be about 1.0 ?? 1013 g C/yr. Such a flux requires 8 Gyr to accumulate the earth's present crustal carbon inventory. ?? 1984.

  12. Geochemistry of mid ocean ridge basalts (MORB) from the northern Central Indian Ridge between 7°46 and 13°20 S: Implication of mantle heterogeneity influenced by Reunion hotspot plume?

    NASA Astrophysics Data System (ADS)

    Lee, J.; Lee, I.; Lee, S.; Kim, J.

    2010-12-01

    Between the Rodrigues Triple Junction (RTJ) and Carlsberg Ridge, The Central Indian Ridge (CIR) is a slow-intermediate spreading-rate (~ 43mm/year) plate boundary formed during separation of the Mascarene Plateau from the Chagos-Lacadives Ridge. Although several samplings of basaltic rocks were carried out in previous explorations on Indian Ridge, no systematic studies on the petrogenesis of CIR, especially for north of 18°S have been reported yet. We present the major and trace element composition of lavas dredged along the spreading axis of the northern CIR between 7°46 and 13°20 S. The mineralogy of the CIR MORB mainly consists of tiny needle- and/or lath-like plagioclase microlites (~45%), sub- to anhedral olivine (~15%), small anhedral clino-pyroxene (~10%), and intersertal/intergranular Fe-Ti oxide glass matrix (~30%). In the whole rock and glass chemistry, the samples show the very similar pattern of compositional variation in SiO2, Al2O3, FeO, and CaO concentration against MgO (6.59 ~ 8.68 wt %), although relatively less compatible elements (TiO2, Na2O, K2O, and P2O5) show linear trend. Plottings of Na8.0, Fe8.0, and CaO/Al2O3 vs. depth are examined to check the varying extents of partial melting and the global correlations. The results broadly agree with the global trend of MORB and fall within the dry MORB domain. Concentration of light rare earth elements (LREE) vary widely in the analyzed rock samples ((La/Sm)N ≈ 0.68 ~ 1.41 and (Nb/Yb)N ≈ 0.42 ~ 1.93). In spider diagram normalized by primitive mantle, the volcanic lavas show systematic increase of incompatible element concentration from south (segment 1) to north (segment 5), which could be attributed to the influence of enriched source in south of the study area. However, basalts from segment 2 are significantly enriched in incompatible elements. Plots of the ratios of several incompatible elements versus La/Sm ratio show a linear trend which could be attributed to the mixing of depleted and

  13. He, Ne and Ar isotope signatures of mid-ocean ridge basalts and their implications for upper mantle structure: A case study from the Mid-Atlantic Ridge at 4-12°S

    NASA Astrophysics Data System (ADS)

    Stroncik, Nicole A.; Niedermann, Samuel

    2016-06-01

    The geochemical structure of the upper mantle in general and its noble gas isotopic structure in particular have been the subject of countless studies, as both provide important insights into mantle dynamic processes and are essential for the formulation of mantle geodynamic models. This contribution presents a noble gas study of basaltic glasses derived from the Mid-Atlantic Ridge (MAR) between 4° and 12°S, an area well known for its high degree of lithophile isotope heterogeneity and its anomalous crustal thickness. The Sr, Nd, Pb and Hf isotopic systematics along this segment of the MAR range from strongly depleted (i.e. more depleted than N-MORB) to highly enriched. Different concepts have been proposed to explain the observed isotopic signatures. Here we show that the high degree of heterogeneity is not confined to the isotopes of the lithophile elements, but is also shown by the noble gas isotopes and noble gas interelement ratios, such as 3He/22NeM or 4He/40Ar∗. Helium isotopic ratios (3He/4He), 21Ne/22Neextra and 40Ar/36Ar range from 7.34 ± 0.06 to 9.38 ± 0.08 RA, from 0.039 ± 0.020 to 0.075 ± 0.007, and from 346 ± 5 to 37,400 ± 1300, respectively. However, the majority of the Ne isotope data are clearly aligned along a single mixing line in the Ne three-isotope diagram, represented by the equation 20Ne/22Ne = 70.5 × 21Ne/22Ne + 7.782, with a slope distinctly less than that of the MORB line. This indicates that the strongly depleted material is characterised by a significantly more nucleogenic Ne isotopic composition than the normal depleted mantle. We show, based on covariations between 3He/4He and 21Ne/22Neextra with 206Pb/204Pb and 178Hf/177Hf that the strongly depleted material erupted at this MAR segment was most likely produced by an ancient melting event. This implies that isotopic heterogeneities in the upper mantle are not solely caused by the injection of enriched materials from deep-seated mantle plumes or by crustal recycling but may

  14. Hydrothermal Activity on ultraslow Spreading Ridge: new hydrothermal fields found on the Southwest Indian ridge

    NASA Astrophysics Data System (ADS)

    Tao, C.; Li, H.; Deng, X.; Lei, J.; Wang, Y.; Zhang, K.; Zhou, J.; Liu, W.

    2014-12-01

    Ultraslow spreading ridge makes up about 25% of global mid-ocean ridge length. Previous studies believed that hydrothermal activity is not widespread on the ultraslow spreading ridge owing to lower magma supply. Southwest Indian ridge (SWIR) with the spreading rate between 1.2cm/a to 1.4cm/a, represents the ultraslow spreading ridge. In 2007, Chinese Cruise (CC) 19th discovered the Dragon Flag deposit (DFD) on the SWIR, which is the first active hydrothermal field found on the ultraslow spreading ridge. In recent years, over 10 hydrothermal fields have been found on the SWIR between Indomed and Gallieni transform faults by the Chinese team. Tao et al. (2012) implied that the segment sections with excess heat from enhanced magmatism and suitable crustal permeability along slow and ultraslow ridges might be the most promising areas for searching for hydrothermal activities. In 2014, CC 30thdiscovered five hydrothermal fields and several hydrothermal anomalies on the SWIR. Dragon Horn Area (DHA). The DHA is located on the southern of segment 27 SWIR, with an area of about 400 km2. The geophysical studies indicated that the DHA belongs to the oceanic core complex (OCC), which is widespread on the slow spreading ridges (Zhao et al., 2013). The rocks, such as gabbro, serpentinized peridotite, and consolidated carbonate were collected in the DHA, which provide the direct evidence with the existence of the OCC. However, all rock samples gathered by three TV-grab stations are basalts on the top of the OCC. A hydrothermal anomaly area, centered at 49.66°E,37.80° S with a range of several kms, is detected in the DHA. It is probably comprised of several hydrothermal fields and controlled by a NW fault. New discovery of hydrothermal fields. From January to April 2014, five hydrothermal fields were discovered on the SWIR between 48°E to 50°E during the leg 2&3 of the CC 30th, which are the Su Causeway field (48.6°E, 38.1°S), Bai Causeway field (48.8°E, 37.9 °S), Dragon

  15. Volcanic eruption of the mid-ocean ridge along the East Pacific Rise crest at 9°45-52'N: direct submersible observations of seafloor phenomena associated with an eruption event in April, 1991

    USGS Publications Warehouse

    Haymon, R.M.; Fornari, D.J.; Von Damm, K. L.; Lilley, M.D.; Perfit, M.R.; Edmond, J.M.; Shanks, Wayne C.; Lutz, R.A.; Grebmeier, J.M.; Carbotte, S.; Wright, D.; McLaughlin, E.; Smith, M.; Beedle, N.; Olson, E.

    1993-01-01

    We suggest that, in April, 1991, intrusion of dikes in the eruption area to < 200 m beneath the ASC floor resulted in phase separation of fluids near the tops of the dikes and a large flux of vapor-rich hydrothermal fluids through the overlying rubbly, cavernous lavas. Low salinities and gas-rich compositions of hydrothermal fluids sampled in the eruption area are appropriate for a vapor phase in a seawater system undergoing subcritical liquid-vapor phase separation (boiling) and phase segregation. Hydrothermal fluids streamed directly from fissures and pits that may have been loci of lava drainback and/or hydrovolcanic explosions. These fissures and pits were lined with white mats of a unique fast-growing bacteria that was the only life associated with the brand-new vents. The prolific bacteria, which covered thousands of square meters on the ridge crest and were also abundant in subseafloor voids, may thrive on high levels of gases in the vapor-rich hydrothermal fluids initially escaping the hydrothermal system. White bacterial particulates swept from the seafloor by hydrothermal vents swirled in an unprecedented biogenic ‘blizzard’ up to 50 m above the bottom. The bacterial proliferation of April, 1991 is likely to be a transient bloom that will be checked quickly either by decline of dissolved gas concentrations in the fluids as rapid heat loss brings about cessation of boiling, and/or by grazing as other organisms are re-established in the biologically devastated area.

  16. The First-ever Detection and Tracking of a Mid-Ocean Ridge Volcanic Eruption Using the Recently Completed, NSF-Funded, Submarine Fiber-Optic Network in the Juan de Fuca Region.

    NASA Astrophysics Data System (ADS)

    Delaney, J. R.

    2015-12-01

    The most scientifically diverse and technologically advanced component of the Ocean Observatories Initiative involves 900 km of electro-optical fiber, extending from Pacific City, OR, across active portions of the JDF tectonic plate, and upward into the overlying ocean. Completed in 2014, on time and under budget, this network enables real-time, high-bandwidth, 2-way communication with seafloor/water-column sensor arrays across: 1. the Cascadia accretionary prism, 2. the JdF spreading center, and, 3. portions of the overlying NE Pacific. Oceanographic processes in coastal environments, the California Current, and 400 km offshore, are captured by six remote-controlled, profiling moorings covering full-ocean depths. In August, 2015, all sections of cable, all six operational primary nodes, all 17 junction boxes, and 97% of all 146 instruments are transmitting data ashore to the Internet via the Pacific Northwest Gigapop (http://www.pnwgp.net/). All data are archived at the U of Washington, pending completion of the OOI CyberInfrastructure System in October 2015. In 2014, community requests to access seismic and seafloor deformational information for assessment of progressive inflation at Axial Seamount (Chadwick et al, 2012), resulted in NSF releasing, through IRIS (http://www.iris.edu/hq/), real-time data from 7 seismometers and 3 pressure sensors. At a community-initiated meeting on April 20-22, 90 participants covering the spectrum of Ocean Sciences, met in Seattle to explore scientific responses in the event Axial actually erupted (http://novae.ocean.washington.edu). On April 24, Axial did erupt; seismic event counts rose dramatically to many hundreds/hour (Wilcock, AGU-2015), the Axial caldera floor dropped 2.2 m in ~20 hours (Nooner et al, AGU-2015), and water temperatures in the caldera rose slowly by ~0.7°C, then declined in 3 weeks to normal values. Unusual water-bourn acoustic signals indicated ongoing seafloor activity along the rift zone extending north

  17. Vesicularity and CO2 in mid-ocean ridge basalt

    USGS Publications Warehouse

    Moore, J.G.

    1979-01-01

    Vesicles and included CO2are enriched in deep-sea basalts that are also enriched in light rare earth and incompatible elements. This enrichment probably results from a unique deep mantle origin of such melts but may have been modified by CO2 bubbles rising in shallow magma chambers. ?? 1979 Nature Publishing Group.

  18. Microearthquake activity, lithospheric structure, and deformation modes at an amagmatic ultraslow spreading Southwest Indian Ridge segment

    NASA Astrophysics Data System (ADS)

    Schmid, Florian; Schlindwein, Vera

    2016-07-01

    While nascent oceanic lithosphere at slow to fast spreading mid-ocean ridges (MOR) is relatively well studied, much less is known about the lithospheric structure and properties at ultraslow MORs. Here we present microearthquake data from a 1 year ocean bottom seismometer deployment at the amagmatic, oblique supersegment of the ultraslow spreading Southwest Indian Ridge. A refraction seismic experiment was performed to constrain upper lithosphere P-velocities and results were used to construct a 1D velocity model for earthquake location. Earthquake foci were located individually and subsequently relocated relative to each other to sharpen the image of seismically active structures. Frequent earthquake activity extends to 31 km beneath the seafloor, indicating an exceptionally thick brittle lithosphere and an undulating brittle-ductile transition that implies significant variations in the along-axis thermal structure of the lithosphere. We observe a strong relation between petrology, microseismicity distribution, and topography along the ridge axis: Peridotite-dominated areas associate with deepest hypocenters, vast volumes of lithosphere that deforms aseismically as a consequence of alteration, and the deepest axial rift valley. Areas of basalt exposure correspond to shallower hypocenters, shallower and more rugged axial seafloor. Focal mechanisms deviate from pure extension and are spatially variable. Earthquakes form an undulating band of background seismicity and do not delineate discrete detachment faults as common on slow spreading ridges. Instead, the seismicity band sharply terminates to the south, immediately beneath the rift boundary. Considering the deep alteration, large steep boundary faults might be present but are entirely aseismic.

  19. Widespread active detachment faulting and core complex formation near 13 degrees N on the Mid-Atlantic Ridge.

    PubMed

    Smith, Deborah K; Cann, Johnson R; Escartín, Javier

    2006-07-27

    Oceanic core complexes are massifs in which lower-crustal and upper-mantle rocks are exposed at the sea floor. They form at mid-ocean ridges through slip on detachment faults rooted below the spreading axis. To date, most studies of core complexes have been based on isolated inactive massifs that have spread away from ridge axes. Here we present a survey of the Mid-Atlantic Ridge near 13 degrees N containing a segment in which a number of linked detachment faults extend for 75 km along one flank of the spreading axis. The detachment faults are apparently all currently active and at various stages of development. A field of extinct core complexes extends away from the axis for at least 100 km. Our observations reveal the topographic characteristics of actively forming core complexes and their evolution from initiation within the axial valley floor to maturity and eventual inactivity. Within the surrounding region there is a strong correlation between detachment fault morphology at the ridge axis and high rates of hydroacoustically recorded earthquake seismicity. Preliminary examination of seismicity and seafloor morphology farther north along the Mid-Atlantic Ridge suggests that active detachment faulting is occurring in many segments and that detachment faulting is more important in the generation of ocean crust at this slow-spreading ridge than previously suspected. PMID:16871215

  20. Widespread active detachment faulting and core complex formation near 13 degrees N on the Mid-Atlantic Ridge.

    PubMed

    Smith, Deborah K; Cann, Johnson R; Escartín, Javier

    2006-07-27

    Oceanic core complexes are massifs in which lower-crustal and upper-mantle rocks are exposed at the sea floor. They form at mid-ocean ridges through slip on detachment faults rooted below the spreading axis. To date, most studies of core complexes have been based on isolated inactive massifs that have spread away from ridge axes. Here we present a survey of the Mid-Atlantic Ridge near 13 degrees N containing a segment in which a number of linked detachment faults extend for 75 km along one flank of the spreading axis. The detachment faults are apparently all currently active and at various stages of development. A field of extinct core complexes extends away from the axis for at least 100 km. Our observations reveal the topographic characteristics of actively forming core complexes and their evolution from initiation within the axial valley floor to maturity and eventual inactivity. Within the surrounding region there is a strong correlation between detachment fault morphology at the ridge axis and high rates of hydroacoustically recorded earthquake seismicity. Preliminary examination of seismicity and seafloor morphology farther north along the Mid-Atlantic Ridge suggests that active detachment faulting is occurring in many segments and that detachment faulting is more important in the generation of ocean crust at this slow-spreading ridge than previously suspected.

  1. Explosive volcanism on the ultraslow-spreading Gakkel ridge, Arctic Ocean.

    PubMed

    Sohn, Robert A; Willis, Claire; Humphris, Susan; Shank, Timothy M; Singh, Hanumant; Edmonds, Henrietta N; Kunz, Clayton; Hedman, Ulf; Helmke, Elisabeth; Jakuba, Michael; Liljebladh, Bengt; Linder, Julia; Murphy, Christopher; Nakamura, Ko-Ichi; Sato, Taichi; Schlindwein, Vera; Stranne, Christian; Tausenfreund, Maria; Upchurch, Lucia; Winsor, Peter; Jakobsson, Martin; Soule, Adam

    2008-06-26

    Roughly 60% of the Earth's outer surface is composed of oceanic crust formed by volcanic processes at mid-ocean ridges. Although only a small fraction of this vast volcanic terrain has been visually surveyed or sampled, the available evidence suggests that explosive eruptions are rare on mid-ocean ridges, particularly at depths below the critical point for seawater (3,000 m). A pyroclastic deposit has never been observed on the sea floor below 3,000 m, presumably because the volatile content of mid-ocean-ridge basalts is generally too low to produce the gas fractions required for fragmenting a magma at such high hydrostatic pressure. We employed new deep submergence technologies during an International Polar Year expedition to the Gakkel ridge in the Arctic Basin at 85 degrees E, to acquire photographic and video images of 'zero-age' volcanic terrain on this remote, ice-covered ridge. Here we present images revealing that the axial valley at 4,000 m water depth is blanketed with unconsolidated pyroclastic deposits, including bubble wall fragments (limu o Pele), covering a large (>10 km(2)) area. At least 13.5 wt% CO(2) is necessary to fragment magma at these depths, which is about tenfold the highest values previously measured in a mid-ocean-ridge basalt. These observations raise important questions about the accumulation and discharge of magmatic volatiles at ultraslow spreading rates on the Gakkel ridge and demonstrate that large-scale pyroclastic activity is possible along even the deepest portions of the global mid-ocean ridge volcanic system.

  2. Active Gas Venting Through Hydrate-Bearing Sediments on the Vestnesa Ridge, Offshore W-Svalbard

    NASA Astrophysics Data System (ADS)

    Buenz, S.; Vadakkepuliyambatta, S.; Polyanov, S.; Mienert, J.

    2010-12-01

    Gas hydrate systems offshore western Svalbard are far more extensive (~4000km^2) than previously assumed and include the whole Vestnesa Ridge, an elongated sediment drift north of the Molloy Transform and just east of the Molloy Ridge, one of the shortest segments of the slow spreading North-Atlantic Ridge system. The crest of the Vestnesa Ridge at water depth between 1200-1300 m is pierced with fluid-flow features. Seafloor pockmarks vary in size up to 1 km in diameter. Pockmarks are generally larger at the onset of the Vestnesa Ridge than at its western termination. A recent cruise with R/V Jan Mayen discovered methane flares in the water column above the pockmark field at the onset of the Vestnesa Ridge. Over a period of one week at least 4 pockmarks were continuously active and methane flares in the water column reached a height of approximately 800 m. The extent of the active gas venting is much stronger than discovered earlier and it is still unclear what has triggered the increase in gas expulsion from seafloor sediments. Any connection to hundreds of active gas vents further to the east at the shelf edge in water depth of 250-400 m remains speculative at this point but cannot be ruled out. High -resolution 3D seismic data acquired in 2007 and 2010 also show significant dissimilarities of the sub-seafloor expression of these fluid leakage systems. At the western end of the Vestnesa Ridge, sub-seafloor fluid flow features resemble well-described pipe structures. However, the seismic expression of the active fluid flow features is much broader, much more chaotic, dome-shaped and is not very similar to a typical chimney structure. The Vestnesa Ridge gas-hydrate and free- gas system occurs within few km of a mid-oceanic ridge and transform fault, which makes this gas hydrate system unique on Earth. The close proximity to the spreading centre and its hydrothermal circulation system affects the dynamics of the gas-hydrate and free-gas system. The high heat flow

  3. Different cluster relocation techniques and their applications to mid-oceanic earthquakes

    NASA Astrophysics Data System (ADS)

    Pan, J.; Antolik, M.; Dziewonski, A. M.

    2002-12-01

    We examine and apply three multi-event relocation (cluster analysis) techniques: double difference (DD), hypocentral decomposition (HDC), and joint hypocenter determination based on master events constrained by seafloor bathymetry (JHDME) to mid-oceanic earthquakes. We compare them with single event relocation method coupled with three dimensional (3D) model-based station corrections (SER3D), in terms of event clustering against background bathymetry, location errors, root mean square (RMS) of the residuals and relocation vectors. We find that using virtually the same dataset, JHDME fits the data best and yields lower residual RMS than the other methods. Absolute locations cluster better for HDC and JHDME against the bathymetric features. In one of our experiments, HDC hypocentroid change little and this reduces HDC to JHDME in which only one master event is fixed. Because DD fits relative travel times, the absolute residual RMSs are relatively larger, relocation vectors are smaller and final locations scatter more. But the error ellipses for DD are smallest. SER3D uses more available phases for each event but no other inter-event constraint than model-based station corrections and usually generates smaller relocation vectors than the other methods, except DD. For the cluster on the Romanche Fracture Zone (RFZ), all methods leave several events off the main transform. But the cluster `collapses' more toward the linear RFZ bathymetric low for HDC and JHDME. For the cluster in the Gulf of Aden, distribution of the relocated events is more consistent with the geometry of closely spaced ridges and transforms. We introduce a measure for the mid-oceanic earthquake clustering based on the concept of Voronoi cells but adapted to the linear bathymetry features. Such measurement is set up to be a function of the volume of each Voronoi cell around an earthquake weighted by the minimum distance between the earthquake and any possible ridge/transform to which the earthquake is

  4. Evidence of recent volcanic activity on the ultraslow-spreading Gakkel ridge.

    PubMed

    Edwards, M H; Kurras, G J; Tolstoy, M; Bohnenstiehl, D R; Coakley, B J; Cochran, J R

    2001-02-15

    Seafloor spreading is accommodated by volcanic and tectonic processes along the global mid-ocean ridge system. As spreading rate decreases the influence of volcanism also decreases, and it is unknown whether significant volcanism occurs at all at ultraslow spreading rates (<1.5 cm yr(-1)). Here we present three-dimensional sonar maps of the Gakkel ridge, Earth's slowest-spreading mid-ocean ridge, located in the Arctic basin under the Arctic Ocean ice canopy. We acquired this data using hull-mounted sonars attached to a nuclear-powered submarine, the USS Hawkbill. Sidescan data for the ultraslow-spreading (approximately 1.0 cm yr(-1)) eastern Gakkel ridge depict two young volcanoes covering approximately 720 km2 of an otherwise heavily sedimented axial valley. The western volcano coincides with the average location of epicentres for more than 250 teleseismic events detected in 1999, suggesting that an axial eruption was imaged shortly after its occurrence. These findings demonstrate that eruptions along the ultraslow-spreading Gakkel ridge are focused at discrete locations and appear to be more voluminous and occur more frequently than was previously thought.

  5. Evidence of recent volcanic activity on the ultraslow-spreading Gakkel ridge.

    PubMed

    Edwards, M H; Kurras, G J; Tolstoy, M; Bohnenstiehl, D R; Coakley, B J; Cochran, J R

    2001-02-15

    Seafloor spreading is accommodated by volcanic and tectonic processes along the global mid-ocean ridge system. As spreading rate decreases the influence of volcanism also decreases, and it is unknown whether significant volcanism occurs at all at ultraslow spreading rates (<1.5 cm yr(-1)). Here we present three-dimensional sonar maps of the Gakkel ridge, Earth's slowest-spreading mid-ocean ridge, located in the Arctic basin under the Arctic Ocean ice canopy. We acquired this data using hull-mounted sonars attached to a nuclear-powered submarine, the USS Hawkbill. Sidescan data for the ultraslow-spreading (approximately 1.0 cm yr(-1)) eastern Gakkel ridge depict two young volcanoes covering approximately 720 km2 of an otherwise heavily sedimented axial valley. The western volcano coincides with the average location of epicentres for more than 250 teleseismic events detected in 1999, suggesting that an axial eruption was imaged shortly after its occurrence. These findings demonstrate that eruptions along the ultraslow-spreading Gakkel ridge are focused at discrete locations and appear to be more voluminous and occur more frequently than was previously thought. PMID:11236991

  6. Hydrothermal activity at slow-spreading ridges: variability and importance of magmatic controls

    NASA Astrophysics Data System (ADS)

    Escartin, Javier

    2016-04-01

    Hydrothermal activity along mid-ocean ridge axes is ubiquitous, associated with mass, chemical, and heat exchanges between the deep lithosphere and the overlying envelopes, and sustaining chemiosynthetic ecosystems at the seafloor. Compared with hydrothermal fields at fast-spreading ridges, those at slow spreading ones show a large variability as their location and nature is controlled or influenced by several parameters that are inter-related: a) tectonic setting, ranging from 'volcanic systems' (along the rift valley floor, volcanic ridges, seamounts), to 'tectonic' ones (rift-bounding faults, oceanic detachment faults); b) the nature of the host rock, owing to compositional heterogeneity of slow-spreading lithosphere (basalt, gabbro, peridotite); c) the type of heat source (magmatic bodies at depth, hot lithosphere, serpentinization reactions); d) and the associated temperature of outflow fluids (high- vs.- low temperature venting and their relative proportion). A systematic review of the distribution and characteristics of hydrothermal fields along the slow-spreading Mid-Atlantic Ridge suggests that long-lived hydrothermal activity is concentrated either at oceanic detachment faults, or along volcanic segments with evidence of robust magma supply to the axis. A detailed study of the magmatically robust Lucky Strike segment suggests that all present and past hydrothermal activity is found at the center of the segment. The association of these fields to central volcanos, and the absence of indicators of hydrothermal activity along the remaining of the ridge segment, suggests that long-lived hydrothermal activity in these volcanic systems is maintained by the enhanced melt supply and the associated magma chamber(s) required to build these volcanic edifices. In this setting, hydrothermal outflow zones at the seafloor are systematically controlled by faults, indicating that hydrothermal fluids in the shallow crust exploit permeable fault zones to circulate. While

  7. Constrained circulation at Endeavour ridge facilitates colonization by vent larvae.

    PubMed

    Thomson, Richard E; Mihály, Steven F; Rabinovich, Alexander B; McDuff, Russell E; Veirs, Scott R; Stahr, Frederick R

    2003-07-31

    Understanding how larvae from extant hydrothermal vent fields colonize neighbouring regions of the mid-ocean ridge system remains a major challenge in oceanic research. Among the factors considered important in the recruitment of deep-sea larvae are metabolic lifespan, the connectivity of the seafloor topography, and the characteristics of the currents. Here we use current velocity measurements from Endeavour ridge to examine the role of topographically constrained circulation on larval transport along-ridge. We show that the dominant tidal and wind-generated currents in the region are strongly attenuated within the rift valley that splits the ridge crest, and that hydrothermal plumes rising from vent fields in the valley drive a steady near-bottom inflow within the valley. Extrapolation of these findings suggests that the suppression of oscillatory currents within rift valleys of mid-ocean ridges shields larvae from cross-axis dispersal into the inhospitable deep ocean. This effect, augmented by plume-driven circulation within rift valleys having active hydrothermal venting, helps retain larvae near their source. Larvae are then exported preferentially down-ridge during regional flow events that intermittently over-ride the currents within the valley.

  8. CO2-filled vesicles in mid-ocean basalt

    USGS Publications Warehouse

    Moore, J.G.; Batchelder, J.N.; Cunningham, C.G.

    1977-01-01

    Volatile-filled vesicles are present in minor amounts in all samples of mid-ocean basalt yet collected (and presumably erupted) down to depths of 4.8 km. When such vesicles are pierced in liquid under standard conditions, the volume expansion of the gas is 0.2 ?? 0.05 times the eruption pressure in bars or 20 ?? 5 times the eruption depth in km. Such expansion could be used as a measure of eruption depth. A variety of techniques: (1) vacuum crushing and gas chromatographic, freezing separation, and mass spectrographic analyses; (2) measurements of phase changes on a freezing microscope stage; (3) microscopic chemical and solubility observations; and (4) volume change measurements, all indicate that CO2 comprises more than 95% by volume of the vesicle gas in several submarine basalt samples from the Atlantic and Pacific. The CO2 held in vesicles is present in quantities about equal to or greater than that presumed to be dissolved in the glass (melt) and amounts to 400-900 ppm of the rock. The rigid temperature of the glass is 800-1000??C and increases for shallower samples. A sulfur gas was originally present in subordinate amounts in the vesicles, but has largely reacted with iron in the vesicle walls to produce sulfide spherules. ?? 1977.

  9. A reduced crustal magnetization zone near the first observed active hydrothermal vent field on the Southwest Indian Ridge

    NASA Astrophysics Data System (ADS)

    Zhu, Jian; Lin, Jian; Chen, Yongshun J.; Tao, Chunhui; German, Christopher R.; Yoerger, Dana R.; Tivey, Maurice A.

    2010-09-01

    Inversion of near-bottom magnetic data reveals a well-defined low crustal magnetization zone (LMZ) near a local topographic high (37°47‧S, 49°39‧E) on the ultraslow-spreading Southwest Indian Ridge (SWIR). The magnetic data were collected by the autonomous underwater vehicle ABE on board R/V DaYangYiHao in February-March 2007. The first active hydrothermal vent field observed on the SWIR is located in Area A within and adjacent to the LMZ at the local topographic high, implying that this LMZ may be the result of hydrothermal alteration of magnetic minerals. The maximum reduction in crustal magnetization is 3 A/M. The spatial extent of the LMZ is estimated to be at least 6.7 × 104 m2, which is larger than that of the LMZs at the TAG vent field on the Mid-Atlantic Ridge (MAR), as well as the Relict Field, Bastille, Dante-Grotto, and New Field vent-sites on the Juan de Fuca Ridge (JdF). The calculated magnetic moment, i.e., the product of the spatial extent and amplitude of crustal magnetization reduction is at least -3 × 107 Am2 for the LMZ on the SWIR, while that for the TAG field on the MAR is -8 × 107 Am2 and that for the four individual vent fields on the JdF range from -5 × 107 to -3 × 107 Am2. Together these results indicate that crustal demagnetization is a common feature of basalt-hosted hydrothermal vent fields at mid-ocean ridges of all spreading rates. Furthermore, the crustal demagnetization of the Area A on the ultraslow-spreading SWIR is comparable in strength to that of the TAG area on the slow-spreading MAR.

  10. Discovery of abundant hydrothermal venting on the ultraslow-spreading Gakkel ridge in the Arctic Ocean.

    PubMed

    Edmonds, H N; Michael, P J; Baker, E T; Connelly, D P; Snow, J E; Langmuir, C H; Dick, H J B; Mühe, R; German, C R; Graham, D W

    2003-01-16

    Submarine hydrothermal venting along mid-ocean ridges is an important contributor to ridge thermal structure, and the global distribution of such vents has implications for heat and mass fluxes from the Earth's crust and mantle and for the biogeography of vent-endemic organisms. Previous studies have predicted that the incidence of hydrothermal venting would be extremely low on ultraslow-spreading ridges (ridges with full spreading rates <2 cm x yr(-1)-which make up 25 per cent of the global ridge length), and that such vent systems would be hosted in ultramafic in addition to volcanic rocks. Here we present evidence for active hydrothermal venting on the Gakkel ridge, which is the slowest spreading (0.6-1.3 cm x yr(-1)) and least explored mid-ocean ridge. On the basis of water column profiles of light scattering, temperature and manganese concentration along 1,100 km of the rift valley, we identify hydrothermal plumes dispersing from at least nine to twelve discrete vent sites. Our discovery of such abundant venting, and its apparent localization near volcanic centres, requires a reassessment of the geologic conditions that control hydrothermal circulation on ultraslow-spreading ridges.

  11. Geo-Morphological Analyses of the Gakkel Ridge and the Southwest Indian Ridge

    NASA Astrophysics Data System (ADS)

    Dorschel, B.; Schlindwein, V. S. N.; Eagles, G.

    2014-12-01

    The Gakkel Ridge in the Arctic Ocean and the Southwest Indian Ridge in the Southwest Indian Ocean between Africa and Antarctica are ultraslow-spreading (<20 mm yr-1) mid ocean ridges. This type of mid ocean ridge has distinct geo-morphologies that are influenced by the slow rate of plate divergence and by mantle potential temperature, which control the processes (peridotite diapirism and intersticial melt migration) by which material rises to fill the space vacated by plate divergence. These ridges are characterised by non-orthogonal spreading. Transform faults, typical of faster spreading mid ocean ridges, are far less common at ultraslow spreading mid ocean ridges. Thus in return, detailed geo-statistical analyses of the geo-morphology of ultraslow-spreading mid ocean ridges can provide valuable information towards a better understanding of these slowest of spreading ridges. We have generated high resolution bathymetric grids for the Gakkel and Southwest Indian ridges based on high resolution multibeam echosounder data from various expeditions with RV Polarstern. On the basis of these grids, geo-statistical analyses allow for an assessment of the geo-morphological elements of the ridges on various scales. The results of these analyses show that, approximately 200 km long medium-scale sections of the ridges can be characterised by the lengths and orientations of the short-scale (hundreds of meters to tens of kilometres) ridges and troughs. The geomorphologies of short-scale ridges and troughs situated at the junctions between medium scale sections often exhibit a mixture of the geomorphological elements seen in the neighbouring sections. These geo-morphological patterns provide insights into the overall spreading-geometry along the Gakkel Ridge and the Southwest Indian Ridge.

  12. The Sound Generated by Mid-Ocean Ridge Black Smoker Hydrothermal Vents

    PubMed Central

    Crone, Timothy J.; Wilcock, William S.D.; Barclay, Andrew H.; Parsons, Jeffrey D.

    2006-01-01

    Hydrothermal flow through seafloor black smoker vents is typically turbulent and vigorous, with speeds often exceeding 1 m/s. Although theory predicts that these flows will generate sound, the prevailing view has been that black smokers are essentially silent. Here we present the first unambiguous field recordings showing that these vents radiate significant acoustic energy. The sounds contain a broadband component and narrowband tones which are indicative of resonance. The amplitude of the broadband component shows tidal modulation which is indicative of discharge rate variations related to the mechanics of tidal loading. Vent sounds will provide researchers with new ways to study flow through sulfide structures, and may provide some local organisms with behavioral or navigational cues. PMID:17205137

  13. Compilation of Rare Earth Element Analyses from US Geothermal Fields and Mid Ocean Ridge Hydrothermal Vents

    DOE Data Explorer

    Andrew Fowler

    2015-10-01

    Compilation of rare earth element and associated major and minor dissolved constituent analytical data for USA geothermal fields and global seafloor hydrothermal vents. Data is in original units. Reference to and use of this data should be attributed to the original authors and publications according to the provisions outlined therein.

  14. Aqueous volatiles in hydrothermal fluids from the Main Endeavour Field, northern Juan de Fuca Ridge: temporal variability following earthquake activity

    NASA Astrophysics Data System (ADS)

    Seewald, Jeffrey; Cruse, Anna; Saccocia, Peter

    2003-12-01

    The Main Endeavour Field, northern Juan de Fuca Ridge, experienced intense seismic activity in June 1999. Hydrothermal vent fluids were collected from sulfide structures in September 1999 and July 2000 and analyzed for the abundance of H2, H2S, CH4, CO2, NH3, Mg and Cl to document temporal and spatial changes following the earthquakes. Dissolved concentrations of CO2, H2, and H2S increased dramatically in the September 1999 samples relative to pre-earthquake abundances, and subsequently decreased during the following year. In contrast, dissolved NH3 and CH4 concentrations in 1999 and 2000 were similar to or less than pre-earthquake values. Aqueous Cl abundances showed large decreases immediately following the earthquakes followed by increases to near pre-earthquake values. The abundances of volatile species at the Main Endeavour Field were characterized by strong inverse correlations with chlorinity. Phase separation can account for 20-50% enrichments of CO2, CH4, and NH3 in low-chlorinity fluids, while temperature- and pressure-dependent fluid-mineral equilibria at near-critical conditions are responsible for order of magnitude greater enrichments in dissolved H2S and H2. The systematic variation of dissolved gas concentrations with chlorinity likely reflects mixing of a low-chlorinity volatile-enriched vapor generated by supercritical phase separation with a cooler gas-poor hydrothermal fluid of seawater chlorinity. Decreased abundances of sediment-derived NH3 and CH4 in 1999 indicate an earthquake-induced change in subsurface hydrology. Elevated CO2 abundances in vent fluids collected in September 1999 provide evidence that supports a magmatic origin for the earthquakes. Temperature-salinity relationships are consistent with intrusion of a shallow dike and suggest that the earthquakes were associated with movement of magma beneath the ridge crest. These data demonstrate the large and rapid response of chemical fluxes at mid-ocean ridges to magmatic activity and

  15. Gondwana subduction-modified mantle domain prevents magmatic seafloor generation in the Central Indian Ridge

    NASA Astrophysics Data System (ADS)

    Morishita, T.; Nakamura, K.; Senda, R.; Suzuki, K.; Kumagai, H.; Sato, H.; Sato, T.; Shibuya, T.; Minoguchi, K.; Okino, K.

    2013-12-01

    The creation of oceanic crust at mid-ocean ridges is essential to understanding the genesis of oceanic plate and the evolution of the Earth. Detailed bathymetric measurements coupled with dense sample recovery at mid-ocean ridge revealed a wide range of variations in the ridge and seafloor morphologies, which cannot be simply explained by a spreading rate, but also by ridge geometry, mantle compositions and thermal structure (Dick et al., 2003 Nature; Cannat et al. 2006 Geology). It is now widely accepted that very limited magmatic activity with tectonic stretching generates oceanic core complex and/or smooth seafloor surface in the slow to ultraslow-spreading ridges, where serpentinized peridotite and gabbros are expected to be exposed associated with detachment faults (Cann et al., 1997 Nature; Cannat et al., 2006), although magmatism might be an essential role for the formation of oceanic core complexes (Buck et al., 2005 Nature; Tucholke et al 2008 JGR). A rising question is why magmatic activity is sometimes prevented during the oceanic plate formation. Ancient melting domain, that are too refractory to melt even in adiabatically upwelling to the shallow upper mantle, might cause the amagmatic spreading ridges (Harvey et al., 2006 EPSL, Liu et al.,2008 Nature). Its origin and effect on seafloor generations are, however, not well understood yet. We report an oceanic hill as an example of an ancient subduction-modified mantle domain, probably formed at continental margin of the Gondwanaland~Pangea supercontinent, existing beneath the Central Indian Ridge. This domain is the most likely to have prevented magmatic seafloor generation, resulting in creation of very deep oceanic valley and serpentine diaper (now the studied oceanic hill) at the present Central Indian ridge.

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

    NASA Astrophysics Data System (ADS)

    Murton, Bramley J.; Rona, Peter A.

    2015-11-01

    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 26 mm/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.

  17. Enhanced East Pacific Rise hydrothermal activity during the last two glacial terminations

    NASA Astrophysics Data System (ADS)

    Lund, D. C.; Asimow, P. D.; Farley, K. A.; Rooney, T. O.; Seeley, E.; Jackson, E. W.; Durham, Z. M.

    2016-01-01

    Mid-ocean ridge magmatism is driven by seafloor spreading and decompression melting of the upper mantle. Melt production is apparently modulated by glacial-interglacial changes in sea level, raising the possibility that magmatic flux acts as a negative feedback on ice-sheet size. The timing of melt variability is poorly constrained, however, precluding a clear link between ridge magmatism and Pleistocene climate transitions. Here we present well-dated sedimentary records from the East Pacific Rise that show evidence of enhanced hydrothermal activity during the last two glacial terminations. We suggest that glacial maxima and lowering of sea level caused anomalous melting in the upper mantle and that the subsequent magmatic anomalies promoted deglaciation through the release of mantle heat and carbon at mid-ocean ridges.

  18. Enhanced East Pacific Rise hydrothermal activity during the last two glacial terminations.

    PubMed

    Lund, D C; Asimow, P D; Farley, K A; Rooney, T O; Seeley, E; Jackson, E W; Durham, Z M

    2016-01-29

    Mid-ocean ridge magmatism is driven by seafloor spreading and decompression melting of the upper mantle. Melt production is apparently modulated by glacial-interglacial changes in sea level, raising the possibility that magmatic flux acts as a negative feedback on ice-sheet size. The timing of melt variability is poorly constrained, however, precluding a clear link between ridge magmatism and Pleistocene climate transitions. Here we present well-dated sedimentary records from the East Pacific Rise that show evidence of enhanced hydrothermal activity during the last two glacial terminations. We suggest that glacial maxima and lowering of sea level caused anomalous melting in the upper mantle and that the subsequent magmatic anomalies promoted deglaciation through the release of mantle heat and carbon at mid-ocean ridges. PMID:26823422

  19. Enhanced East Pacific Rise hydrothermal activity during the last two glacial terminations.

    PubMed

    Lund, D C; Asimow, P D; Farley, K A; Rooney, T O; Seeley, E; Jackson, E W; Durham, Z M

    2016-01-29

    Mid-ocean ridge magmatism is driven by seafloor spreading and decompression melting of the upper mantle. Melt production is apparently modulated by glacial-interglacial changes in sea level, raising the possibility that magmatic flux acts as a negative feedback on ice-sheet size. The timing of melt variability is poorly constrained, however, precluding a clear link between ridge magmatism and Pleistocene climate transitions. Here we present well-dated sedimentary records from the East Pacific Rise that show evidence of enhanced hydrothermal activity during the last two glacial terminations. We suggest that glacial maxima and lowering of sea level caused anomalous melting in the upper mantle and that the subsequent magmatic anomalies promoted deglaciation through the release of mantle heat and carbon at mid-ocean ridges.

  20. Neogene magmatism northeast of the Aegir and Kolbeinsey ridges, NE Atlantic: Spreading ridge-mantle plume interaction?

    NASA Astrophysics Data System (ADS)

    Breivik, AsbjøRn Johan; Faleide, Jan Inge; Mjelde, Rolf

    2008-02-01

    According to mantle plume theory the Earth's interior cools partly by localized large vertical mass transport, causing extensive decompression melting. The Iceland melt anomaly is regarded as a typical example of a mantle plume. However, there are centers of Miocene to recent magmatism in the Norwegian-Greenland Sea not easily explained by the plume theory. Here we present new data to document diffuse late Miocene magmatic underplating of older oceanic crust located mostly north of the Aegir Ridge, an extinct seafloor spreading axis in the Norway Basin. There is also a region with similar magmatism northeast of the presently spreading Kolbeinsey Ridge north of Iceland. Intraplate magmatism in these locations is not easily explained by local plume models, edge-driven convection, or by asthenosphere flow-lithosphere thickness interaction. On the basis of correlation between the magmatism and the active or extinct spreading ridges, we propose the mid-ocean ridge basalt-capture model, in which this magmatism can be understood through plume-spreading ridge interaction: The asthenosphere flow out from Iceland captures deeper, low-degree partially molten asthenospheric regions from underneath the spreading ridges and carry these across the terminating fracture zones, to subsequently underplate oceanic crust or to intrude and build seamounts. This model is similar to lithospheric cracking models for intraplate magmatism in requiring that low-degree partial melt can be retained in the asthenosphere over time but differ in that the magma is extracted by internal magma movement processes and not by external tectonic forces.

  1. Enabling Integration and Synthesis through the Ridge 2000 Data Portal

    NASA Astrophysics Data System (ADS)

    Ryan, W. B.; Ferrini, V.; Carbotte, S. M.; O'Hara, S.; Arko, R. A.; Bonczkowski, J.; Chan, S.

    2009-12-01

    Achieving an integrated, holistic understanding of global mid-ocean ridge (MOR) processes through the Ridge 2000 Program relies heavily on the sharing of data across disciplinary boundaries. Although most Ridge 2000 research has been focused on three Integrated Study Sites (ISSs), data from throughout the global mid-ocean ridge (MOR) system will also be important in the integration and synthesis phase of the program. The Ridge 2000 Data Portal (www.marine-geo.org/portals/ridge2000) is a resource established to facilitate the sharing, discovery, and integration of Ridge 2000-relevant data by providing data documentation and access. The Data Portal includes an inventory of field programs and activities, raw and derived data files, and access to publications and remote data sets in specialized data systems (e.g. PetDB, www.petdb.org; GenBank, www.ncbi.nlm.nih.gov; NGDC, www.ngdc.noaa.gov). Access to the R2K database is provided through several pathways, including a web-based search interface (www.marine-geo.org/tools/search) that now includes basic and advanced search functionality. New versions of our data exploration and visualization tool GeoMapApp (www.geomapapp.org) includes a newly designed menu system with cascading and tear-off menus, a new Layer Manager that lists selected layers and enables re-ordering and rendering with variable transparency, a Go To option that allows direct zooming to areas of interest, and an Add Image Overlay option for importing georeferenced images. New data sets are continually added to both GeoMapApp and Virtual Ocean (www.virtualocean.org), a 3D virtual globe with GeoMapApp functionality. Several Ridge 2000 data sets are also bundled for viewing in Google Earth (www.marine-geo.org/tools/kmls.php). This poster will highlight new functionality and data sets relevant to Ridge 2000 Integration and Synthesis, and will provide comparisons of several data sets that exist for each ISS.

  2. Where are the undiscovered hydrothermal vents on oceanic spreading ridges?

    NASA Astrophysics Data System (ADS)

    Beaulieu, Stace E.; Baker, Edward T.; German, Christopher R.

    2015-11-01

    In nearly four decades since the discovery of deep-sea vents, one-third of the length of global oceanic spreading ridges has been surveyed for hydrothermal activity. Active submarine vent fields are now known along the boundaries of 46 out of 52 recognized tectonic plates. Hydrothermal survey efforts over the most recent decade were sparked by national and commercial interests in the mineral resource potential of seafloor hydrothermal deposits, as well as by academic research. Here we incorporate recent data for back-arc spreading centers and ultraslow- and slow-spreading mid-ocean ridges (MORs) to revise a linear equation relating the frequency of vent fields along oceanic spreading ridges to spreading rate. We apply this equation globally to predict a total number of vent fields on spreading ridges, which suggests that ~900 vent fields remain to be discovered. Almost half of these undiscovered vent fields (comparable to the total of all vent fields discovered during 35 years of research) are likely to occur at MORs with full spreading rates less than 60 mm/yr. We then apply the equation regionally to predict where these hydrothermal vents may be discovered with respect to plate boundaries and national jurisdiction, with the majority expected to occur outside of states' exclusive economic zones. We hope that these predictions will prove useful to the community in the future, in helping to shape continuing ridge-crest exploration.

  3. A History of Classified Activities at Oak Ridge National Laboratory

    SciTech Connect

    Quist, A.S.

    2001-01-30

    The facilities that became Oak Ridge National Laboratory (ORNL) were created in 1943 during the United States' super-secret World War II project to construct an atomic bomb (the Manhattan Project). During World War II and for several years thereafter, essentially all ORNL activities were classified. Now, in 2000, essentially all ORNL activities are unclassified. The major purpose of this report is to provide a brief history of ORNL's major classified activities from 1943 until the present (September 2000). This report is expected to be useful to the ORNL Classification Officer and to ORNL's Authorized Derivative Classifiers and Authorized Derivative Declassifiers in their classification review of ORNL documents, especially those documents that date from the 1940s and 1950s.

  4. Epithermal Neutron Activation Analysis of Some Geological Samples of Different Origin

    SciTech Connect

    Duliu, O. G.; Cristache, C. I.; Oaie, G.; Ricman, C.; Culicov, O. A.; Frontasyeva, M. V.

    2010-01-21

    Instrumental Epithermal Neutron Activation Analysis was used to investigate the distribution of six major elements and 34 trace elements in a set of eight igneous and metamorphic rocks collected from Carpathian and Macin Mountainsas well as unconsolidated sediments collected from anoxic zone of the Black Sea. All experimental data were interpreted within the Upper Continental Core and Mid Ocean Ridge Basalt model system that allowed getting more information concerning samples origin as well as the environmental peculiarities.

  5. Beryllium 10 in hydrothermal vent deposits from the East Pacific Ridges: Role of sediments in the hydrothermal processes

    SciTech Connect

    Valette-Silver, J.N.; Tera, F.; Klein, J.; Middleton, R.

    1987-10-10

    Beryllium 10 concentrations were determined for 31 handpicked hydrothermal sulfides, six oxyhydroxides, seven basalts, and five sediments collected from the hydrothermally active areas of the East Pacific ridges. The samples includes specimens from the East Pacific Rise (EPR) at 21 /sup 0/N and 13 /sup 0/N, the Galapagos Rift, the Guaymas Basin, and the Gorda and the Juan de Fuca ridges. Additional samples from massive sulfides associated with the Oman ophiolites were studied. In all samples, we obtained values ranging from 0.04 x 10/sup 6/ atoms/g to 125 x 10/sup 6/ atoms/g, with the lowest values being very close to our blank (0.015 x 10/sup 6/ atoms/g). The data show systematic variations with sample location and type. The /sup 10/Be concentrations measured for the mid-ocean basalts are of the order of 0.3 x 10/sup 6/ atoms/g and reach 3800 x 10/sup 6/ atoms/g for the pelagic deep-sea sediments collected near the EPR 21 /sup 0/N. Based on their /sup 10/Be concentrations, we can clearly distinguish two categories of sulfides: sulfides containing low /sup 10/Be concentration (<10 /sup 6/ atoms/g) sitting directly on the mid-ocean basalt (EPR of Juan de Fuca), and sulfides with high /sup 10/Be concentration (>10/sup 6/ atoms/g) located atop of a thick pile of young sediments (Guaymas Basin or Gorda Ridge).

  6. Magmatic and amagmatic seafloor generation at the ultraslow-spreading Gakkel ridge, Arctic Ocean.

    PubMed

    Michael, P J; Langmuir, C H; Dick, H J B; Snow, J E; Goldstein, S L; Graham, D W; Lehnert, K; Kurras, G; Jokat, W; Mühe, R; Edmonds, H N

    2003-06-26

    A high-resolution mapping and sampling study of the Gakkel ridge was accomplished during an international ice-breaker expedition to the high Arctic and North Pole in summer 2001. For this slowest-spreading endmember of the global mid-ocean-ridge system, predictions were that magmatism should progressively diminish as the spreading rate decreases along the ridge, and that hydrothermal activity should be rare. Instead, it was found that magmatic variations are irregular, and that hydrothermal activity is abundant. A 300-kilometre-long central amagmatic zone, where mantle peridotites are emplaced directly in the ridge axis, lies between abundant, continuous volcanism in the west, and large, widely spaced volcanic centres in the east. These observations demonstrate that the extent of mantle melting is not a simple function of spreading rate: mantle temperatures at depth or mantle chemistry (or both) must vary significantly along-axis. Highly punctuated volcanism in the absence of ridge offsets suggests that first-order ridge segmentation is controlled by mantle processes of melting and melt segregation. The strong focusing of magmatic activity coupled with faulting may account for the unexpectedly high levels of hydrothermal activity observed.

  7. Magmatic and amagmatic seafloor generation at the ultraslow-spreading Gakkel ridge, Arctic Ocean.

    PubMed

    Michael, P J; Langmuir, C H; Dick, H J B; Snow, J E; Goldstein, S L; Graham, D W; Lehnert, K; Kurras, G; Jokat, W; Mühe, R; Edmonds, H N

    2003-06-26

    A high-resolution mapping and sampling study of the Gakkel ridge was accomplished during an international ice-breaker expedition to the high Arctic and North Pole in summer 2001. For this slowest-spreading endmember of the global mid-ocean-ridge system, predictions were that magmatism should progressively diminish as the spreading rate decreases along the ridge, and that hydrothermal activity should be rare. Instead, it was found that magmatic variations are irregular, and that hydrothermal activity is abundant. A 300-kilometre-long central amagmatic zone, where mantle peridotites are emplaced directly in the ridge axis, lies between abundant, continuous volcanism in the west, and large, widely spaced volcanic centres in the east. These observations demonstrate that the extent of mantle melting is not a simple function of spreading rate: mantle temperatures at depth or mantle chemistry (or both) must vary significantly along-axis. Highly punctuated volcanism in the absence of ridge offsets suggests that first-order ridge segmentation is controlled by mantle processes of melting and melt segregation. The strong focusing of magmatic activity coupled with faulting may account for the unexpectedly high levels of hydrothermal activity observed. PMID:12827193

  8. Evidence for microbial carbon and sulfur cycling in deeply buried ridge flank basalt.

    PubMed

    Lever, Mark A; Rouxel, Olivier; Alt, Jeffrey C; Shimizu, Nobumichi; Ono, Shuhei; Coggon, Rosalind M; Shanks, Wayne C; Lapham, Laura; Elvert, Marcus; Prieto-Mollar, Xavier; Hinrichs, Kai-Uwe; Inagaki, Fumio; Teske, Andreas

    2013-03-15

    Sediment-covered basalt on the flanks of mid-ocean ridges constitutes most of Earth's oceanic crust, but the composition and metabolic function of its microbial ecosystem are largely unknown. By drilling into 3.5-million-year-old subseafloor basalt, we demonstrated the presence of methane- and sulfur-cycling microbes on the eastern flank of the Juan de Fuca Ridge. Depth horizons with functional genes indicative of methane-cycling and sulfate-reducing microorganisms are enriched in solid-phase sulfur and total organic carbon, host δ(13)C- and δ(34)S-isotopic values with a biological imprint, and show clear signs of microbial activity when incubated in the laboratory. Downcore changes in carbon and sulfur cycling show discrete geochemical intervals with chemoautotrophic δ(13)C signatures locally attenuated by heterotrophic metabolism.

  9. Revisiting the Ridge-Push Force Using the Lithospheric Geoid

    NASA Astrophysics Data System (ADS)

    Richardson, R. M.; Coblentz, D. D.

    2014-12-01

    The geoid anomaly and driving force associated with the cooling oceanic lithosphere ("ridge push") are both proportional to dipole moment of the density-depth distribution, and allow a reevaluation of the ridge push force using the geoid. The challenge with this approach is to isolate the "lithospheric geoid" from the full geoid signal. Our approach is to use a band-pass spherical harmonic filter on the full geoid (e.g., EGM2008-WGS84, complete to spherical harmonic degree and order 2159) between orders 6 and 80. However, even this "lithospheric geoid" is noisy, and thus we average over 100 profiles evenly spaced along the global ridge system to obtain an average geoid step associated with the mid-ocean ridges. Because the positive ridge geoid signal is largest near the ridge (and to capture fast-spreading ridges), we evaluate symmetrical profiles extending ±45 m.y. about the ridge. We find an average ridge geoid anomaly of 4.5m, which is equivalent to a 10m anomaly for 100 m.y. old oceanic lithosphere. This geoid step corresponds to a ridge push force of ~2.4 x1012N/m for old oceanic lithosphere of 100 m.y., very similar to earlier estimates of ~2.5 x1012N/m based on simple half-space models. This simple half-space model also predicts constant geoid slopes of about 0.15 m/m.y. for cooling oceanic lithosphere. Our observed geoid slopes are consistent with this value for ages up to 40-50 m.y., but drop off to lower values at greater ages. We model this using a plate cooling model (with a thickness of the order of 125km) to fit the observation that the geoid anomaly and ridge driving force only increase slowly for ages greater than 40 m.y. (in contrast to the half-space model where the linear dependence on age holds for all ages). This reduction of the geoid slope results in a 20% decrease in the predicted ridge push force. This decrease is due to the combined effects of treating the oceanic lithosphere as a cooling plate (vs. a half-space), and the loss of geoidal

  10. Structural processes at slow-spreading ridges.

    PubMed

    Mutter, J C; Karson, J A

    1992-07-31

    Slow-spreading (<35 millimeters per year) mid-ocean ridges are dominated by segmented, asymmetric, rifted depressions like continental rifts. Fast-spreading ridges display symmetric, elevated volcanic edifices that vary in shape and size along axis. Deep earthquakes, major normal faults, and exposures of lower crustal rocks are common only along slow-spreading ridges. These contrasting features suggest that mechanical deformation is far more important in crustal formation at slow-spreading ridges than at fast-spreading ridges. New seismic images suggest that the nature and scale of segmentation of slow-spreading ridges is integral to the deformational process and not to magmatic processes that may control segmentation on fast-spreading ridges.

  11. S-wave velocity, basalt chemistry and bathymetry along the Mid-Atlantic Ridge

    NASA Astrophysics Data System (ADS)

    Zhang, Yu-Shen; Tanimoto, Toshiro; Stolper, Edward M.

    1994-07-01

    Major element chemistry of mid-ocean ridge basalt, S-wave velocity and bathymetry along the Mid-Atlantic Ridge are correlated on an intermediate length scale, and the large anomalies in these observations are associated with hotspot locations. The best correlations are for at depths of 100-200 km, and there is no correlation for depths of 300 km or deeper. S-Wave velocities are low directly under the Mid-Atlantic Ridge above 100 km, but the low-velocity regions shift away from the ridge at greater depth, and a sinuous strip of asthenospheric low velocity extends along the Atlantic hotspots from the Azores to Tristan da Cunha. These features suggest that common physical processes in the upper mantle produce these anomalies. An inspection of the history of the Atlantic Ocean suggests that the asthenospheric low-velocity strip may record earlier ridge position or possible upwelling in the mantle. Possible interaction between hotspots and the ridge blurs the distinction between passive and active upwellings.

  12. Long-term seismicity of the Reykjanes Ridge (North Atlantic) recorded by a regional hydrophone array

    NASA Astrophysics Data System (ADS)

    Goslin, Jean; Lourenço, Nuno; Dziak, Robert P.; Bohnenstiehl, DelWayne R.; Haxel, Joe; Luis, Joaquim

    2005-08-01

    The seismicity of the northern Mid-Atlantic Ridge was recorded by two hydrophone networks moored in the sound fixing and ranging (SOFAR) channel, on the flanks of the Mid-Atlantic Ridge, north and south of the Azores. During its period of operation (05/2002-09/2003), the northern `SIRENA' network, deployed between latitudes 40° 20'N and 50° 30'N, recorded acoustic signals generated by 809 earthquakes on the hotspot-influenced Reykjanes Ridge. This activity was distributed between five spatio-temporal event clusters, each initiated by a moderate-to-large magnitude (4.0-5.6 M) earthquake. The rate of earthquake occurrence within the initial portion of the largest sequence (which began on 2002 October 6) is described adequately by a modified Omori law aftershock model. Although this is consistent with triggering by tectonic processes, none of the Reykjanes Ridge sequences are dominated by a single large-magnitude earthquake, and they appear to be of relatively short duration (0.35-4.5 d) when compared to previously described mid-ocean ridge aftershock sequences. The occurrence of several near-equal magnitude events distributed throughout each sequence is inconsistent with the simple relaxation of mainshock-induced stresses and may reflect the involvement of magmatic or fluid processes along this deep (>2000 m) section of the Reykjanes Ridge.

  13. Comment on "Sensitivity of seafloor bathymetry to climate-driven fluctuations in mid-ocean ridge magma supply".

    PubMed

    Tolstoy, Maya

    2016-07-15

    Olive et al (Reports, 16 October 2015, p. 310) and Goff (Technical Comment, 4 September 2015, p. 1065) raise important concerns with respect to recent findings of Milankovitch cycles in seafloor bathymetry. However, their results inherently support that the Southern East Pacific Rise is the optimum place to look for such signals and, in fact, models match those observations quite closely. PMID:27418497

  14. Comment on "Sensitivity of seafloor bathymetry to climate-driven fluctuations in mid-ocean ridge magma supply".

    PubMed

    Tolstoy, Maya

    2016-07-15

    Olive et al (Reports, 16 October 2015, p. 310) and Goff (Technical Comment, 4 September 2015, p. 1065) raise important concerns with respect to recent findings of Milankovitch cycles in seafloor bathymetry. However, their results inherently support that the Southern East Pacific Rise is the optimum place to look for such signals and, in fact, models match those observations quite closely.

  15. The Response of Two-Phase Hydrothermal Systems to Changing Magmatic Heat Input at Mid Ocean Ridges

    NASA Astrophysics Data System (ADS)

    Choi, J.; Lowell, R. P.

    2012-12-01

    Hydrothermal processes at oceanic spreading centers are largely influenced by changing magmatic heat input. We use the two-phase NaCl-H2O FISHES code to investigate the evolution of surface temperature and salinity as a function of time-varying heat flux at the base of the hydrothermal system. We consider a two-dimensional rectangular box that is 1.5 km deep and 2 km long with homogeneous permeability of 10-13 m2. Temperature and pressure at top boundary correspond to seafloor conditions of 10°C, 25MPa respectively. Upstream conditions are applied at the top so temperature and salinity of fluids exiting the surface is set equal to that at one cell below the top boundary. Impermeable, insulated conditions are imposed on the left and right hand boundaries. To simulate time-varying heat flux from a sub-axial magma chamber of 500 m long half-width, we considered a variety of basal boundary conditions: (1) a sinusoidal heat flux with a period of 6 years and an amplitude ranging between 100 and 50 W/m2; (2) step function, random function, and exponential function between 200 and 15 W/m2; and (3) an analytical function of temporally decaying heat flux resulting from a simulated cooling, crystallizing magmatic sill. As a result of the investigation we find: (1) changes in bottom temperature and salinity closely follow the temporal variations in magmatic heat inputs; (2) the surface temperature response is severely damped and high frequency variations in heat flow are not detected; (3) in regions where phase separation occurs, surface salinity variations may be recorded in response to changing conditions at depth, but these are smaller in amplitude than the changes at depth. These simulations represent an important forward toward understanding time-dependent behavior of coupled magma-hydrothermal processes at oceanic spreading centers.

  16. Comment on “Sensitivity of seafloor bathymetry to climate-driven fluctuations in mid-ocean ridge magma supply”

    NASA Astrophysics Data System (ADS)

    Tolstoy, Maya

    2016-07-01

    Olive et al. (Reports, 16 October 2015, p. 310) and Goff (Technical Comment, 4 September 2015, p. 1065) raise important concerns with respect to recent findings of Milankovitch cycles in seafloor bathymetry. However, their results inherently support that the Southern East Pacific Rise is the optimum place to look for such signals and, in fact, models match those observations quite closely.

  17. Drainage systems associated with mid-ocean channels and submarine Yazoos: Alternative to submarine fan depositional systems

    SciTech Connect

    Hesse, R. )

    1989-12-01

    Submarine drainage systems associated with mid-ocean channels and Yazoo River-type tributaries in small ocean basins represent a contrast to deep-sea fan depositional systems. Deep-sea fans are diverging sediment-dispersal systems of distributary fan valleys. Deep-sea channel-submarine-yazoo systems, on the other hand, form centripetally converging patterns of tributaries and yazoo-type satellite channels that join a major basin-draining (mid-ocean) channel. The facies model for such systems is characterized by randomly stacked fining-upward, gravelly, and sandy channel-fill and submarine point-bar sequences of the main channel encased in fine-grained overbank deposits. Second-order channels contain sandy proximal overbank deposits, whereas the levees of the main channel are predominantly composed of silt and clay. Second-order channels may be braided and may broaden into braid plains. Morphology and surficial sediment distribution have been studied within the Northwest Atlantic Mid-Ocean Channel of the Labrador Sea and its associated levees and yazoo-type (and other) tributaries.

  18. Melt generation beneath Arctic Ridges: Implications from U decay series disequilibria in the Mohns, Knipovich, and Gakkel Ridges

    NASA Astrophysics Data System (ADS)

    Elkins, L. J.; Sims, K. W. W.; Prytulak, J.; Blichert-Toft, J.; Elliott, T.; Blusztajn, J.; Fretzdorff, S.; Reagan, M.; Haase, K.; Humphris, S.; Schilling, J.-G.

    2014-02-01

    We present new 238U-230Th-226Ra-210Pb, 235U-231Pa, and Nd, Sr, Hf, and Pb isotope data for the slow- to ultraslow-spreading Mohns, Knipovich, and Gakkel Ridges. Combined with previous work, our data from the Arctic Ridges cover the full range of axial depths from the deep northernmost Gakkel Ridge shallowing upwards to the Knipovich, Mohns, and Kolbeinsey Ridges north of Iceland. Age-constrained samples from the Mohns and Knipovich Ridges have (230Th/238U) activity ratios ranging from 1.165 to 1.30 and 1.101 to 1.225, respectively. The high 230Th excesses of Kolbeinsey, Mohns, and Knipovich mid-ocean ridge basalts (MORB) are erupted from ridges producing relatively thin (Mohns, Knipovich) to thick (Kolbeinsey) oceanic crust with evidence for sources ranging from mostly peridotite (Kolbeinsey) to eclogite-rich mantle (Mohns, Knipovich). Age-constrained lavas from 85°E on the Gakkel Ridge, on the other hand, overlie little to no crust and range from small (˜5%) 230Th excesses to small 238U excesses (˜5%). The strong negative correlation between (230Th/238U) values vs. axial ridge depth among Arctic ridge basalts is controlled not only by solidus depth influence on 238U-230Th disequilibria, but also by variations in mantle source lithology and depth to the base of the lithosphere, which is expected to vary at ultra-slow spreading ridges. Small 231Pa excesses (65% excess) in age-constrained basalts support the presence of eclogite in the mantle source for this region. Conversely, the ultraslow-spreading Gakkel Ridge basalts are homogeneous, with Sr, Nd, and Hf radiogenic isotopic signatures indicative of a long time-averaged depleted mantle source. The Gakkel samples have minimum (226Ra/230Th) ratios ranging from 3.07 to 3.65 ± 3%, which lie along and extend the global negative correlation between 226Ra and 230Th excesses observed in MORB. The new 230Th-226Ra data support a model for global MORB production in which deep melts record interaction with shallower

  19. Minimum speed limit for ocean ridge magmatism from 210Pb-226Ra-230Th disequilibria

    NASA Astrophysics Data System (ADS)

    Rubin, K. H.; van der Zander, I.; Smith, M. C.; Bergmanis, E. C.

    2005-09-01

    Although 70 per cent of global crustal magmatism occurs at mid-ocean ridges-where the heat budget controls crustal structure, hydrothermal activity and a vibrant biosphere-the tempo of magmatic inputs in these regions remains poorly understood. Such timescales can be assessed, however, with natural radioactive-decay-chain nuclides, because chemical disruption to secular equilibrium systems initiates parent-daughter disequilibria, which re-equilibrate by the shorter half-life in a pair. Here we use 210Pb-226Ra-230Th radioactive disequilibria and other geochemical attributes in oceanic basalts less than 20 years old to infer that melts of the Earth's mantle can be transported, accumulated and erupted in a few decades. This implies that magmatic conditions can fluctuate rapidly at ridge volcanoes. 210Pb deficits of up to 15 per cent relative to 226Ra occur in normal mid-ocean ridge basalts, with the largest deficits in the most magnesium-rich lavas. The 22-year half-life of 210Pb requires very recent fractionation of these two uranium-series nuclides. Relationships between 210Pb-deficits, (226Ra/230Th) activity ratios and compatible trace-element ratios preclude crustal-magma differentiation or daughter-isotope degassing as the main causes for the signal. A mantle-melting model can simulate observed disequilibria but preservation requires a subsequent mechanism to transport melt rapidly. The likelihood of magmatic disequilibria occurring before melt enters shallow crustal magma bodies also limits differentiation and heat replenishment timescales to decades at the localities studied.

  20. Minimum speed limit for ocean ridge magmatism from 210Pb-226Ra-230Th disequilibria.

    PubMed

    Rubin, K H; van der Zander, I; Smith, M C; Bergmanis, E C

    2005-09-22

    Although 70 per cent of global crustal magmatism occurs at mid-ocean ridges-where the heat budget controls crustal structure, hydrothermal activity and a vibrant biosphere-the tempo of magmatic inputs in these regions remains poorly understood. Such timescales can be assessed, however, with natural radioactive-decay-chain nuclides, because chemical disruption to secular equilibrium systems initiates parent-daughter disequilibria, which re-equilibrate by the shorter half-life in a pair. Here we use 210Pb-226Ra-230Th radioactive disequilibria and other geochemical attributes in oceanic basalts less than 20 years old to infer that melts of the Earth's mantle can be transported, accumulated and erupted in a few decades. This implies that magmatic conditions can fluctuate rapidly at ridge volcanoes. 210Pb deficits of up to 15 per cent relative to 226Ra occur in normal mid-ocean ridge basalts, with the largest deficits in the most magnesium-rich lavas. The 22-year half-life of 210Pb requires very recent fractionation of these two uranium-series nuclides. Relationships between 210Pb-deficits, (226Ra/230Th) activity ratios and compatible trace-element ratios preclude crustal-magma differentiation or daughter-isotope degassing as the main causes for the signal. A mantle-melting model can simulate observed disequilibria but preservation requires a subsequent mechanism to transport melt rapidly. The likelihood of magmatic disequilibria occurring before melt enters shallow crustal magma bodies also limits differentiation and heat replenishment timescales to decades at the localities studied.

  1. Ancient, highly heterogeneous mantle beneath Gakkel ridge, Arctic Ocean.

    PubMed

    Liu, Chuan-Zhou; Snow, Jonathan E; Hellebrand, Eric; Brügmann, Gerhard; von der Handt, Anette; Büchl, Anette; Hofmann, Albrecht W

    2008-03-20

    The Earth's mantle beneath ocean ridges is widely thought to be depleted by previous melt extraction, but well homogenized by convective stirring. This inference of homogeneity has been complicated by the occurrence of portions enriched in incompatible elements. Here we show that some refractory abyssal peridotites from the ultraslow-spreading Gakkel ridge (Arctic Ocean) have very depleted 187Os/188Os ratios with model ages up to 2 billion years, implying the long-term preservation of refractory domains in the asthenospheric mantle rather than their erasure by mantle convection. The refractory domains would not be sampled by mid-ocean-ridge basalts because they contribute little to the genesis of magmas. We thus suggest that the upwelling mantle beneath mid-ocean ridges is highly heterogeneous, which makes it difficult to constrain its composition by mid-ocean-ridge basalts alone. Furthermore, the existence of ancient domains in oceanic mantle suggests that using osmium model ages to constrain the evolution of continental lithosphere should be approached with caution.

  2. The Davie Ridge: a Marginal Transform Ridge not Formed During Continental Breakup

    NASA Astrophysics Data System (ADS)

    Phethean, J. J. J.; Van Hunen, J.; McCaffrey, K. J. W.; Davies, R. J.

    2014-12-01

    The breakup of Gondwana translated Madagascar southwards relative to Africa along the Davie Fracture Zone (DFZ). This fracture zone now forms the Transform Passive Continental Margin (TPCM) from Kenya to Mozambique. The Davie Ridge (DR), a transform marginal ridge, has formed along the DFZ between 5 and 2°S and 22 and 11°S, but with little expression in-between. It has been proposed that this marginal ridge was formed by the thermal effects of a passing Mid Ocean Ridge (MOR) during the separation of Gondwana. Plate kinematic reconstructions, however, constrained by ocean magnetic anomalies, show that the MOR only passed between the north and south expressions of the DR. Therefore the positive linear gravity anomalies of the DR cannot be attributed to the effects of a passing MOR, and some other mechanism must be found to explain their formation. Interpretation of seismic reflection profiles along the DR shows that the gravity highs occur adjacent to large basin structures. In the north this correlates with a basin-bounding basement high of ~Albian age, and in the south with the rift flank uplifts of the currently active Quirimbas graben. This suggests that the northern and southern DR segments are instead shoulder uplifts resulting from two separate extensional episodes during different stress regimes. These are the Cretaceous NE-SW extension during the breakup of the south Atlantic, and the E-W extension of the Neogene-recent Afar-East Africa rift system, respectfully. The lack of deformation and DR formation along the region of the TPCM passed by the MOR suggests it has been coupled by thermal effects and/or the injection of magma.

  3. Age distribution of Ocean Drill sites across the Central Walvis Ridge indicates plate boundary control of plume volcanism in the South Atlantic

    NASA Astrophysics Data System (ADS)

    O'Connor, John M.; Jokat, Wilfried

    2015-08-01

    The Tristan-Gough hotspot trail on the African plate consists of the Walvis Ridge and a younger province of seamounts and islands. In order to determine the relative motion between the African plate and the Tristan-Gough hotspot it is essential to resolve changes in the age and morphology of the Walvis Ridge. A significant problem is, however, to establish how the vigor and flow of hotspot material to the mid-ocean ridge constructed the Walvis Ridge. We have addressed this issue by measuring an 40Ar/39Ar stratigraphy at three sites across the central Walvis Ridge sampled by Ocean Drilling (DSDP Leg 74). The age-distance relation of volcanism, together with geophysical, geochemical and paleodepth information, suggests collectively that hotspot volcanism was occurring locally c. 72 Ma on an elevated segment of the mid-ocean ridge located close to the Tristan-Gough hotspot. As the mid-ocean ridge migrated away from the hotspot (c. 36 km/Ma) between c. 72 Ma and 68 Ma, hotspot material continued flowing to the mid-ocean ridge and the Walvis Ridge shoaled rapidly (c. 500 m/Ma) to the west, on seafloor that might have been subsiding at a rate consistent with normal crustal cooling. This apparent correlation points to the possibility of an inverse relation between the volume flux of hotspot volcanism and the distance between the mid-ocean ridge and the Tristan-Gough hotspot. We infer that since c. 93 Ma the geometry and motion of the mid-ocean ridge determined where the hotspot material that built the Walvis Ridge was channeled to the plate surface. Furthermore, interplay between hotspot flow, and the changing geometry of the mid-ocean ridge as it migrated relative to the Tristan-Gough hotspot, might explain the age and morphology of the Walvis Ridge. Our finding provides further evidence that the distribution of hotspot volcanism in the southeast Atlantic expresses interaction between deep mantle (plume) and shallow plate tectonic and asthenosphere processes.

  4. Annual report on surveillance and maintenance activities at Oak Ridge National Laboratory, Oak Ridge, Tennessee, fiscal year 1996

    SciTech Connect

    1996-11-01

    In fiscal year (FY) 1995, the sites and facilities from both the Remedial Action (RA) and Decontamination and Decommissioning (D and D) programs were combined to form the Oak Ridge National Laboratory (ORNL) Environmental Restoration (ER) Surveillance and Maintenance (S and M) Program. Surveillance and Maintenance activities were conducted throughout FY 1996 at the RA facilities. Overall, the RA S and M Program consists of approximately 650 acres that include 14 waste area groupings with approximately 200 sites. These sites include 46 major facilities, several leak and contaminated soil sites, 38 inactive tanks, approximately 50 environmental study areas and approximately 2,973 wells and boreholes. Site inspections were conducted at established frequencies on appropriate sites in the RA S and M Program in accordance with the established S and M FY 1996 Incentive Task Order (ITO).

  5. Hydrothermal venting on the Juan de Fuca Ridge over the last 600,000 years

    NASA Astrophysics Data System (ADS)

    Costa, K.; McManus, J. F.; Winckler, G.; Huybers, P. J.; Langmuir, C. H.; Giosan, L.; Middleton, J. L.; Mukhopadhyay, S.

    2015-12-01

    Mid-ocean ridges provide a unique chemical, physical, and biological environment on the seafloor. They are a significant source of dissolved Fe, a critical micronutrient in the ocean, and they are a primary source of CO2 from the mantle, Earth's largest carbon reservoir. Although more than a hundred modern hydrothermal systems have been discovered, few records of their variation through geological time have been obtained. Sediments near ocean ridges hold the potential to provide such records, and here we investigate sediments near the Juan de Fuca Ridge through continuous XRF scans coupled with oxygen isotope temporal constraints to explore hydrothermal activity over the past 600,000 years. These are the first records over multiple glacial-interglacial cycles and permit investigation of potential feedbacks between glacial-interglacial climate change and hydrothermal activity. Today, hydrothermal activity on the Juan de Fuca Ridge is characterized by hydrothermal particles with high concentrations of Fe, Cu, and Zn (Feely et. al., 1987). Over longer time scales, Fe concentrations are positively correlated with Ti (r2≥0.75), so that the dominant variability in Fe is due to the input of lithogenic material. Additional Fe inputs from hydrothermal activity increase the Fe/Ti ratio above the lithogenic value of 11.7 wt%/wt%. Intense hydrothermal activity (Fe/Ti > 25 wt%/wt%) is observed on the Juan de Fuca Ridge from ~375-430ka, and less intense but still elevated hydrothermal activity (Fe/Ti > 17 wt%/wt%) recurs at near 100kyr cyclicity, from 80-120ka, 180-220ka, 290-330ka, and 470-520ka. These time periods correspond to times of sea level fall owing to expanding ice volume, supporting a link between sea level changes and ridge crest activity

  6. ­­­­Submarine Mass Wasting on Hovgaard Ridge, Fram Strait, European Arctic

    NASA Astrophysics Data System (ADS)

    Forwick, M.; Laberg, J. S.; Husum, K.; Gales, J. A.

    2015-12-01

    Hovgaard Ridge is an 1800 m high bathymetric high in the Fram Strait, the only deep-water gateway between the Arctic Ocean and the other World's oceans. The slopes of the ridge provide evidence of various types of sediment reworking, including 1) up to 12 km wide single and merged slide scars with maximum ~30 m high headwalls and some secondary escarpments; 2) maximum 3 km wide and 130 m deep slide scars with irregular internal morphology, partly narrowing towards the foot of the slope; 3) up to 130 m deep, 1.5 km wide and maximum 8 km long channels/gullies originating from areas of increasing slope angle at the margins of a plateau on top of the ridge. Most slide scars result presumably from retrogressive failure related to weak layers in contourites or ash. The most likely trigger mechanism is seismicity related to tectonic activity within the nearby mid-ocean fracture zone. Gully/channel formation is suggested to result from cascading water masses and/or from sediment gravity flows originating from failure at the slope break after winnowing on the plateau of the ridge.

  7. New data about small-magnitude earthquakes of the ultraslow-spreading Gakkel Ridge, Arctic Ocean

    NASA Astrophysics Data System (ADS)

    Morozov, Alexey N.; Vaganova, Natalya V.; Ivanova, Ekaterina V.; Konechnaya, Yana V.; Fedorenko, Irina V.; Mikhaylova, Yana A.

    2016-01-01

    At the present time there is available detailed bathymetry, gravimetric, magnetometer, petrological, and seismic (mb > 4) data for the Gakkel Ridge. However, so far not enough information has been obtained on the distribution of small-magnitude earthquakes (or microearthquakes) within the ridge area due to the absence of a suitable observation system. With the ZFI seismic station (80.8° N, 47.7° E), operating since 2011 at the Frantz Josef Land Archipelago, we can now register small-magnitude earthquakes down to 1.5 ML within the Gakkel Ridge area. This article elaborates on the results and analysis of the ZFI station seismic monitoring obtained for the period from December 2011 to January 2015. In order to improve the accuracy of the earthquakes epicenter locations, velocity models and regional seismic phase travel-times for spreading ridges in areas within the Euro-Arctic Region have been calculated. The Gakkel Ridge is seismically active, regardless of having the lowest spreading velocity among global mid-ocean ridges. Quiet periods alternate with periods of higher seismic activity. Earthquakes epicenters are unevenly spread across the area. Most of the epicenters are assigned to the Sparsely Magmatic Zone, more specifically, to the area between 1.5° E and 19.0° E. We hypothesize that assignment of most earthquakes to the SMZ segment can be explained by the amagmatic character of the spreading of this segment. The structuring of this part of the ridge is characterized by the prevalence of tectonic processes, not magmatic or metamorphic ones.

  8. Segment-scale volcanic episodicity: Evidence from the North Kolbeinsey Ridge, Atlantic

    NASA Astrophysics Data System (ADS)

    Yeo, I. A.; Devey, C. W.,; LeBas, T. P.; Augustin, N.; Steinführer, A.

    2016-04-01

    The upper oceanic crust is produced by magmatism at mid-ocean ridges, a process thought to be characterized by cyclic bouts of intense magmatic activity, separated by periods when faulting accommodates most or even all of the plate motion. It is not known whether there is a distinct periodicity to such magmatic-tectonic cycles. Here we present high-resolution sidescan sonar data from the neovolcanic zone of the North Kolbeinsey Ridge, a shallow slow-spreading ridge where high glacial and steady post-glacial sedimentation rates allow relative flow ages to be determined with a resolution of around 2 kyr using backscatter amplitude as a proxy for sediment thickness and hence age. We identify 18 lava flow fields covering 40% of the area surveyed. A group of 7 flow fields showing the highest (and similar) backscatter intensity are scattered along 75 km of axial valley surveyed, suggesting that at least this length of the segment was magmatically active within a 1.2 kyr time window. Based on conservative age estimates for all datable flows and estimated eruption volumes, the post-glacial volcanic activity imaged is insufficient to maintain crustal thickness, implying that episode(s) of enhanced activity must have preceded the volcanism we image.

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

    NASA Astrophysics Data System (ADS)

    Escartin, J.; Barreyre, T.; Cannat, M.; Garcia, R.; Gracias, N.; Deschamps, A.; Salocchi, A.; Sarradin, P. M.; Ballu, V.

    2015-12-01

    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. The Lucky Strike segment hosts three active hydrothermal fields: Capelinhos, Ewan, and the known Main Lucky Strike Hydrothermal Field (MLSHF). Capelinhos is located 1.3 km E of the axis and the MLSHF, and consists of a ~20 m sulfide mound with black smoker vents. Ewan is located ~1.8 km south from the MLSHF along the axial graben, and displays only diffuse flow along and around scarps of collapse structures associated with fault scarps. At the MLSHF we have identified an inactive site, thus broadening the extent of this field. Heat flux estimates from these new sites are relatively low and correspond to ~10% of the heat flux estimated for the Main field, with an integrated heatflux of 200-1200 MW. Overall, most of the flux (up to 80-90%) is associated with diffuse outflow, with the Ewan site showing solely diffuse flow and Capelinhos mostly focused flow. Microbathymetry also reveals a large, off-axis (~2.4 km) hydrothermal field, similar to the TAG mound in size, on the flanks of a rifted volcano. The association of these fields to a central volcano, and the absence of indicators of hydrothermal activity along the ridge segment, suggest that sustained hydrothermal activity is maintained by the enhanced melt supply and the associated magma chamber(s) required to build central volcanoes. Hydrothermal outflow zones at the seafloor are systematically controlled by faults, indicating that hydrothermal circulation in the shallow crust exploits permeable fault zones. Central volcanoes are thus associated with long-lived hydrothermal activity, and these sites may play a major role in the distribution and biogeography of vent communities.

  10. Alkalic Basalt in Ridge Axis of 53˚E Amagmatic Segment Center, Southwest Indian Ridge

    NASA Astrophysics Data System (ADS)

    Zhou, H.; Wang, J.; Liu, Y.; Ji, F.; Dick, H. J.

    2014-12-01

    Mid-ocean ridge basalt (MORB) is key tracer of composition and process in the mantle. It is interesting to notice that some alkalic basalts occur in amagmatic spreading center of ultraslow spreading ridges, for examples, 9-16˚E of the Southwest Indian ridge (Standish et al., 2008) and Lena Trough of Arctic Ocean (Snow et al., 2011). The latter is interpreted as the result of the pre-existence of continental transform fault or the especially cold thermal structure of ancient continental lithosphere. 53˚E segment, east of the Gallieni transform fault, was discovered as an amagmatic segment (Zhou and Dick, 2013). On both sides of the ridge axis, peridotites with a little gabbro are exposed in an area more than 3200 km2. Basalts exist in the southern portion of 53˚E segment, indicating the transformation from magmatic to amagmatic spreading about 9.4 million years ago. In April of 2014, Leg 4 of the RV Dayang Yihao cruise 30, basaltic glasses was dredged at one location (3500 m water depth) in the ridge axis of 53˚E segment center. It is shown by electric probe analysis that the samples have extremely high sodium content (4.0-4.49 wt% Na­2O ), relative higher potassium content (0.27-0.32 wt% K2O) and silica (50.67-51.87 wt% SiO2), and lower MgO content (5.9-6.4 wt% MgO). Mg-number is 0.55-0.59. It is distinctly different from the N-MORB (2.42-2.68 wt% Na2O, 0.03-0.06 wt% K2O, 48.6-49.6 wt% Si2O, 8.8-9.0 wt% MgO, Mg-numbers 0.63) distributed in the 560-km-long supersegment, west of the Gallieni transform fault, where the active Dragon Flag hydrothermal field was discovered at 49.6˚E in 2007. The reasons for the alkalic basalt in the ridge axis of 53˚E amagmatic segment center, either by low melting degree of garnet stability field, by melting from an ancient subcontinental lithospheric mantle, or by sodium-metasomatism or even other mantle processes or their combination in the deep mantle, are under further studies.

  11. Three-dimensional seismic structure of the Dragon Flag oceanic core complex at the ultraslow spreading Southwest Indian Ridge (49°39'E)

    NASA Astrophysics Data System (ADS)

    Zhao, Minghui; Qiu, Xuelin; Li, Jiabiao; Sauter, Daniel; Ruan, Aiguo; Chen, John; Cannat, Mathilde; Singh, Satish; Zhang, Jiazheng; Wu, Zhenli; Niu, Xiongwei

    2013-10-01

    The Southwest Indian Ridge (SWIR) is an ultraslow spreading end-member of mid-ocean ridge system. We use air gun shooting data recorded by ocean bottom seismometers (OBS) and multibeam bathymetry to obtain a detailed three-dimensional (3-D) P wave tomographic model centered at 49°39'E near the active hydrothermal "Dragon Flag" vent. Results are presented in the form of a 3-D seismic traveltime inversion over the center and both ends of a ridge segment. We show that the crustal thickness, defined as the depth to the 7 km/s isovelocity contour, decreases systematically from the center (˜7.0-8.0 km) toward the segment ends (˜3.0-4.0 km). This variation is dominantly controlled by thickness changes in the lower crustal layer. We interpret this variation as due to focusing of the magmatic activity at the segment center. The across-axis velocity model documents a strong asymmetrical structure involving oceanic detachment faulting. A locally corrugated oceanic core complex (Dragon Flag OCC) on the southern ridge flank is characterized by high shallow crustal velocities and a strong vertical velocity gradient. We infer that this OCC may be predominantly made of gabbros. We suggest that detachment faulting is a prominent process of slow spreading oceanic crust accretion even in magmatically robust ridge sections. Hydrothermal activity at the Dragon Flag vents is located next to the detachment fault termination. We infer that the detachment fault system provides a pathway for hydrothermal convection.

  12. Characterization of Active Hydrothermal Fluid Discharge and Recharge Zones in the Endeavour Axial Valley, Juan de Fuca Ridge

    NASA Astrophysics Data System (ADS)

    Salmi, M.; Hutnak, M.; Hearn, C.; Tivey, M.; Bjorklund, T.; Johnson, H. P.

    2012-12-01

    Sites where warm hydrothermal fluid vents at mid-ocean spreading centers are important for understanding a wide range of critical oceanic processes, but discharge zones represent a very limited portion of crustal fluid circulation pathways. Mapping the distribution of both fluid recharge and discharge sites within the axial valley provides wider insight into the larger scale features of hydrothermal circulation. Our 2011 survey consisted of 180 conductive heat flow stations within the Endeavour axial valley in roughly a 400 m by 1000 m grid, extending across the entire axial valley from the outer flank of the western boundary ridge to the eastern wall. Data acquisition used thermal blankets which measured conductive heat flow without requiring substantial sediment cover. A surprising result from this survey was zones of high heat flow extending across-strike, from the summit of the west valley wall across the entire axial valley floor. This trend was correlated with anomalously low seafloor magnetization from a near-bottom survey with the ROV JASON. Unexpectedly, over half of the axial valley floor was anomalously low at <50 mW m-2, while a small portion of the sites within the 'warm zone' had heat flow values >1 W m-2. The areas of extremely low heat flow values are interpreted as being directly influenced by recharge zones. Based on MCS estimates of partial melt depth below the axial valley and the assumption of no fluid advection, the purely conductive heat flow for this region should be on the order of 1 W m-2.The observation that conductive heat flux is suppressed over large portions of the axial valley floor suggests that heat transfer within the crustal sub-surface fluid reservoir is widespread, and impacts a large portion of our survey area. The largely bi-modal distribution of high and low conductive heat flow, coupled with geophysical and video observations, suggest current Endeavour axial valley crustal fluid circulation models need to be re-evaluated.

  13. Petrological and geochemical variations along the Mid-Atlantic Ridge between 46°S and 32°S: Influence of the Tristan da Cunha mantle plume

    NASA Astrophysics Data System (ADS)

    Humphris, Susan E.; Thompson, Geoffrey; Schilling, Jean-Guy; Kingsley, Richard H.

    1985-06-01

    Basalts from a section of the Mid-Atlantic Ridge close to the active volcanic island of Tristan da Cunha in the South Atlantic have been analysed to investigate the influence of the mantle plume on the geochemistry of basalts being erupted at the spreading center. Although petrographically the rocks show only limited variation, two basaltic types were determined to be erupting in this region based on their major, trace and REE compositions. One group shows depletion in the incompatible and LRE elements, and can be characterised as N-type mid-ocean ridge basalts. The second group shows "enriched" geochemical characteristics and is similar to T-type MORBs. Mixing hyperbolae for the incompatible element and REE ratios suggest that extensive mixing of an end-member, characteristic of a plume region with an end-member of normal depleted MORB, canaccount for the occurrence of the T-type MORBs in this region.Based on the nature and development of the Tristan da Cunha mantle plume over the past 100 Ma, a composite model of evolution is suggested,in which a ridge-centered hotspot progressed to a near ridge hotspot, and finally to a totally intraplate situation. The fact that Tristan da Cunha is highly alkalic now, but that an irregular geochemical anomalyis also present on the Mid-Atlantic Ridge at this latitude would suggest an intermediate stage between the near-ridge and totally intraplate situation. This model leads to the conclusion that, as the Mid-Atlantic Ridge migrated away from the Tristan hotspot, a preferential sublithospheric flow towards the Ridge was established. This discontinuous feature can explain the geochemical variations seen along the Mid-Atlantic Ridge by providing a mechanism for mixing of a depleted N-type MORB component with an enriched component originating through processes active at the Tristan da Cunha mantle plume.

  14. Jurassic zircons from the Southwest Indian Ridge.

    PubMed

    Cheng, Hao; Zhou, Huaiyang; Yang, Qunhui; Zhang, Lingmin; Ji, Fuwu; Dick, Henry

    2016-01-01

    The existence of ancient rocks in present mid-ocean ridges have long been observed but received less attention. Here we report the discovery of zircons with both reasonably young ages of about 5 Ma and abnormally old ages of approximate 180 Ma from two evolved gabbroic rocks that were dredged from the Southwest Indian Ridge (SWIR) in the Gallieni fracture zone. U-Pb and Lu-Hf isotope analyses of zircons were made using ion probe and conventional laser abrasion directly in petrographic thin sections. Young zircons and their host oxide gabbro have positive Hf isotope compositions (εHf = +15.7-+12.4), suggesting a highly depleted mantle beneath the SWIR. The spread εHf values (from-2.3 to-4.5) of abnormally old zircons, together with the unradiogenic Nd-Hf isotope of the host quartz diorite, appears to suggest an ancient juvenile magmatism along the rifting margin of the southern Gondwana prior to the opening of the Indian Ocean. A convincing explanation for the origin of the unusually old zircons is yet to surface, however, an update of the theory of plate tectonics would be expected with continuing discovery of ancient rocks in the mid-oceanic ridges and abyssal ocean basins. PMID:27185575

  15. Jurassic zircons from the Southwest Indian Ridge.

    PubMed

    Cheng, Hao; Zhou, Huaiyang; Yang, Qunhui; Zhang, Lingmin; Ji, Fuwu; Dick, Henry

    2016-05-17

    The existence of ancient rocks in present mid-ocean ridges have long been observed but received less attention. Here we report the discovery of zircons with both reasonably young ages of about 5 Ma and abnormally old ages of approximate 180 Ma from two evolved gabbroic rocks that were dredged from the Southwest Indian Ridge (SWIR) in the Gallieni fracture zone. U-Pb and Lu-Hf isotope analyses of zircons were made using ion probe and conventional laser abrasion directly in petrographic thin sections. Young zircons and their host oxide gabbro have positive Hf isotope compositions (εHf = +15.7-+12.4), suggesting a highly depleted mantle beneath the SWIR. The spread εHf values (from-2.3 to-4.5) of abnormally old zircons, together with the unradiogenic Nd-Hf isotope of the host quartz diorite, appears to suggest an ancient juvenile magmatism along the rifting margin of the southern Gondwana prior to the opening of the Indian Ocean. A convincing explanation for the origin of the unusually old zircons is yet to surface, however, an update of the theory of plate tectonics would be expected with continuing discovery of ancient rocks in the mid-oceanic ridges and abyssal ocean basins.

  16. Jurassic zircons from the Southwest Indian Ridge

    NASA Astrophysics Data System (ADS)

    Cheng, Hao; Zhou, Huaiyang; Yang, Qunhui; Zhang, Lingmin; Ji, Fuwu; Dick, Henry

    2016-05-01

    The existence of ancient rocks in present mid-ocean ridges have long been observed but received less attention. Here we report the discovery of zircons with both reasonably young ages of about 5 Ma and abnormally old ages of approximate 180 Ma from two evolved gabbroic rocks that were dredged from the Southwest Indian Ridge (SWIR) in the Gallieni fracture zone. U–Pb and Lu–Hf isotope analyses of zircons were made using ion probe and conventional laser abrasion directly in petrographic thin sections. Young zircons and their host oxide gabbro have positive Hf isotope compositions (εHf = +15.7–+12.4), suggesting a highly depleted mantle beneath the SWIR. The spread εHf values (from‑2.3 to‑4.5) of abnormally old zircons, together with the unradiogenic Nd-Hf isotope of the host quartz diorite, appears to suggest an ancient juvenile magmatism along the rifting margin of the southern Gondwana prior to the opening of the Indian Ocean. A convincing explanation for the origin of the unusually old zircons is yet to surface, however, an update of the theory of plate tectonics would be expected with continuing discovery of ancient rocks in the mid-oceanic ridges and abyssal ocean basins.

  17. Jurassic zircons from the Southwest Indian Ridge

    PubMed Central

    Cheng, Hao; Zhou, Huaiyang; Yang, Qunhui; Zhang, Lingmin; Ji, Fuwu; Dick, Henry

    2016-01-01

    The existence of ancient rocks in present mid-ocean ridges have long been observed but received less attention. Here we report the discovery of zircons with both reasonably young ages of about 5 Ma and abnormally old ages of approximate 180 Ma from two evolved gabbroic rocks that were dredged from the Southwest Indian Ridge (SWIR) in the Gallieni fracture zone. U–Pb and Lu–Hf isotope analyses of zircons were made using ion probe and conventional laser abrasion directly in petrographic thin sections. Young zircons and their host oxide gabbro have positive Hf isotope compositions (εHf = +15.7–+12.4), suggesting a highly depleted mantle beneath the SWIR. The spread εHf values (from−2.3 to−4.5) of abnormally old zircons, together with the unradiogenic Nd-Hf isotope of the host quartz diorite, appears to suggest an ancient juvenile magmatism along the rifting margin of the southern Gondwana prior to the opening of the Indian Ocean. A convincing explanation for the origin of the unusually old zircons is yet to surface, however, an update of the theory of plate tectonics would be expected with continuing discovery of ancient rocks in the mid-oceanic ridges and abyssal ocean basins. PMID:27185575

  18. Special collection on the June 1993 volcanic eruption on the CoAxial segment, Juan de Fuca Ridge

    NASA Astrophysics Data System (ADS)

    Fox, Christopher G.

    1995-01-01

    The mid-ocean ridge system is the primary site of oceanic crustal accretion and a major source of chemical/thermal flux from the Earth's interior to the hydrosphere. Although geological mapping, geophysical exploration, and hydrographie/chemical surveys have provided important insights into accretionary processes, until recently there has been no method of monitoring the dynamics of these systems over long periods and large areas. Insights into spreading center dynamics have depended on serendipitous discoveries of evidence of activity based on water column anomalies [Baker et al., 1987], bathymetric differences [Fox et al., 1992], or direct field observations [Haymon et al., 1993]. In situ instrumentation has been used to monitor dynamics at a few selected sites for long periods, but these instruments are generally unable to telemeter data in real time.

  19. DATA SHARING REPORT CHARACTERIZATION OF POPULATION 7: PERSONAL PROTECTIVE EQUIPMENT, DRY ACTIVE WASTE, AND MISCELLANEOUS DEBRIS, SURVEILLANCE AND MAINTENANCE PROJECT OAK RIDGE NATIONAL LABORATORY OAK RIDGE, TENNESSEE

    SciTech Connect

    Harpenau, Evan M

    2013-10-10

    The U.S. Department of Energy (DOE) Oak Ridge Office of Environmental Management (EM-OR) requested that Oak Ridge Associated Universities (ORAU), working under the Oak Ridge Institute for Science and Education (ORISE) contract, provide technical and independent waste management planning support under the American Recovery and Reinvestment Act (ARRA). Specifically, DOE EM-OR requested that ORAU plan and implement a sampling and analysis campaign targeting certain URS|CH2M Oak Ridge, LLC (UCOR) surveillance and maintenance (S&M) process inventory waste. Eight populations of historical and reoccurring S&M waste at the Oak Ridge National Laboratory (ORNL) have been identified in the Waste Handling Plan for Surveillance and Maintenance Activities at the Oak Ridge National Laboratory, DOE/OR/01-2565&D2 (WHP) (DOE 2012) for evaluation and processing to determine a final pathway for disposal. Population 7 (POP 7) consists of 56 containers of aged, low-level and potentially mixed S&M waste that has been staged in various locations around ORNL. Several of these POP 7 containers primarily contain personal protective equipment (PPE) and dry active waste (DAW), but may contain other miscellaneous debris. This data sharing report addresses the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA) specified waste in a 13-container subpopulation (including eight steel boxes, three 55-gal drums, one sealand, and one intermodal) that lacked sufficient characterization data for possible disposal at the Environmental Management Waste Management Facility (EMWMF) using the approved Waste Lot (WL) 108.1 profile.

  20. Evolution of magmatic and hydrothermal activity in the western Arctic and North Atlantic regions

    NASA Astrophysics Data System (ADS)

    Sorokhtin, N. O.; Lobkovsky, L. I.; Novikov, G. V.; Kozlov, E. E.; Bogdanova, O. Yu.; Nikiforov, S. L.

    2016-07-01

    This paper discusses the geodynamic evolution of the lithosphere in the Arctic region during the Phaneorozic and its polyphase lithotectonic reorganization. Spatiotemporal patterns of the mosaic junction of lithospheric plates of different age are presented for the Caledonian-Hercynian stage and for the Cenozoic evolution of the North Atlantic and Arctic oceanic basins. Special attention is given to the intersections of fault systems with different kinematics, which control the manifestation of peculiar magmatism and the formation of numerous mineral deposits. It is shown that the hydrothermal activity of the region is related to the ocean opening in the Eocene and is confined to the mid-ocean ridge.

  1. Sulphur Cycling at the Mid-Atlantic Ridge: Isotopic Evidence From the Logatchev and Turtle Pits Hydrothermal Fields

    NASA Astrophysics Data System (ADS)

    Eickmann, B.; Strauss, H.; Koschinsky, A.; Kuhn, T.; Petersen, S.; Schmidt, K.

    2005-12-01

    Mid-ocean ridges and associated hydrothermal vent systems represent a unique scenario in which the interaction of hydrosphere, lithosphere and biosphere and the related element cycling can be studied. Sulphur participates in inorganic and microbially driven processes and plays, thus, an important role at these vent sites. The sulphur isotopic compositions of different sulphur-bearing minerals as well as dissolved sulphur compounds provide a tool for identifying the sulphur source and pertinent processes of sulphur cycling. Here, we present sulphur isotope data from an ongoing study of the Logatchev hydrothermal field at 14°45' N and the Turtle Pits hydrothermal field at 4°48' S. The former is located in 2900 to 3060 m water depth, hosted by ultramafic rocks, while the latter is situated in 2990 m water depth, hosted by basaltic rocks. Different metal sulphides (chalcopyrite, pyrite, pyrrhotite, various copper sulphides), either particles from the emanating hot fluid itself or pieces of active and inactive black smokers, display δ34S values between +2 and +9 ‰. So far, no significant difference is discernible between mineral precipitates from both hydrothermal fields. However, differences exist between different generations of sulphide precipitates. Based on respective data from other sites of hydrothermal activity at mid-ocean ridges, this sulphur isotope range suggests that sulphur in the hydrothermal fluid and mineral precipitates represents a mixture between mantle sulphur and reduced seawater sulphate. Anhydrite precipitates from hydrothermal chimneys, located inside sulphide conduits, and obvious late stage gypsum needles from voids, yielded sulphur isotope values between +17.5 and +20.0 ‰. This clearly identifies seawater sulphate as the principal sulphur source. Variable, but generally low abundances of sulphide and sulphate in differently altered mafic and ultramafic rocks point to a complex fluid-rock interaction. Sulphur isotope values for total

  2. The Morphology, Structure and Origin of Seamounts on the South-West Indian Ocean Ridge

    NASA Astrophysics Data System (ADS)

    Muller, L. A.; Watts, A. B.; JC066 shipboard scientific party

    2012-04-01

    The South-West Indian Ridge (SWIR) between longitude 46 and 57° East is an ultra-slow spreading (~16 mm/a) mid-ocean ridge system with a highly oblique (>50°) spreading direction and a large number of closely spaced transform faults. Previous swath bathymetry surveys onboard R/V Dr. Fridtjof Nansen in 2009 show that the ridge crest is characterised by a number of irregularly shaped seamounts which rise about 2500 m above the mean depth of the surrounding seafloor. However, the origin of these seamounts and whether they reflect passive cracking of the lithosphere or deep mantle processes is not clear. In November/December, 2011 we re-surveyed 5 of these seamounts onboard RRS James Cook using an EM120 swath bathymetry system, a Lacoste-Romberg air-sea gravimeter and a Remotely Operated Vehicle (ROV). Preliminary results show that the seamounts are highly fractured, with fault trends parallel and orthogonal to the spreading direction. There is evidence of both growth and collapse structures, including head scars, chutes and debris flows. We present here a preliminary analysis of the morphology, gravity field and rock sample data and its implications for tectonics, mass wasting and eruptive processes at young seamounts that have formed in an active extensional setting.

  3. Hydrological response to a seafloor spreading episode on the Juan de Fuca ridge.

    PubMed

    Davis, Earl; Becker, Keir; Dziak, Robert; Cassidy, John; Wang, Kelin; Lilley, Marvin

    2004-07-15

    Seafloor hydrothermal systems are known to respond to seismic and magmatic activity along mid-ocean ridges, often resulting in locally positive changes in hydrothermal discharge rate, temperature and microbial activity, and shifts in composition occurring at the time of earthquake swarms and axial crustal dike injections. Corresponding regional effects have also been observed. Here we present observations of a hydrological response to seafloor spreading activity, which resulted in a negative formation-fluid pressure transient during and after an earthquake swarm in the sediment-sealed igneous crust of the Middle Valley rift of the northernmost Juan de Fuca ridge. The observations were made with a borehole seal and hydrologic observatory originally established in 1991 to study the steady-state pressure and temperature conditions in this hydrothermally active area. The magnitude of the co-seismic response is consistent with the elastic strain that would be expected from the associated earthquakes, but the prolonged negative pressure transient after the swarm is surprising and suggests net co-seismic dilatation of the upper, permeable igneous crust. The rift valley was visited four weeks after the onset of the seismic activity, but no signature of increased hydrothermal activity was detected in the water column. It appears that water, not magma, filled the void left by this spreading episode.

  4. Hydrological response to a seafloor spreading episode on the Juan de Fuca ridge.

    PubMed

    Davis, Earl; Becker, Keir; Dziak, Robert; Cassidy, John; Wang, Kelin; Lilley, Marvin

    2004-07-15

    Seafloor hydrothermal systems are known to respond to seismic and magmatic activity along mid-ocean ridges, often resulting in locally positive changes in hydrothermal discharge rate, temperature and microbial activity, and shifts in composition occurring at the time of earthquake swarms and axial crustal dike injections. Corresponding regional effects have also been observed. Here we present observations of a hydrological response to seafloor spreading activity, which resulted in a negative formation-fluid pressure transient during and after an earthquake swarm in the sediment-sealed igneous crust of the Middle Valley rift of the northernmost Juan de Fuca ridge. The observations were made with a borehole seal and hydrologic observatory originally established in 1991 to study the steady-state pressure and temperature conditions in this hydrothermally active area. The magnitude of the co-seismic response is consistent with the elastic strain that would be expected from the associated earthquakes, but the prolonged negative pressure transient after the swarm is surprising and suggests net co-seismic dilatation of the upper, permeable igneous crust. The rift valley was visited four weeks after the onset of the seismic activity, but no signature of increased hydrothermal activity was detected in the water column. It appears that water, not magma, filled the void left by this spreading episode. PMID:15254534

  5. Numerical and laboratory experiments on the dynamics of plume-ridge interaction. Progress report

    SciTech Connect

    Kincaid, C.; Gable, C.W.

    1995-09-01

    Mantle plumes and passive upwelling beneath ridges are the two dominant modes of mantle transport and thermal/chemical fluxing between the Earth`s deep interior and surface. While plumes and ridges independently contribute to crustal accretion, they also interact and the dispersion of plumes within the upper mantle is strongly modulated by mid-ocean ridges. The simplest mode of interaction, with the plume centered on the ridge, has been well documented and modeled. The remaining question is how plumes and ridges interact when the plume is located off-axis; it has been suggested that a pipeline-like flow from the off-axis plume to the ridge axis at the base of the rigid lithosphere may develop. Mid-ocean ridges migrating away from hot mantle plumes can be affected by plume discharges over long times and ridge migration distances. Salient feature of this model is that off-axis plumes communicate with the ridge through a channel resulting from the refraction and dispersion of an axi-symmetric plume conduit along the base of the sloping lithosphere. To test the dynamics of this model, a series of numerical and laboratory dynamic experiments on the problem of a fixed ridge and an off-axis buoyant upwelling were conducted. Results are discussed.

  6. Are Axial Volcanic Ridges where all the (volcanic) action is?

    NASA Astrophysics Data System (ADS)

    Searle, R. C.

    2012-12-01

    Although axial volcanic ridges (AVRs) are generally recognised as the main loci for lithospheric generation at slow-spreading mid-ocean ridges, various recent studies have suggested that axial volcanism is not confined to them. Here I present evidence from three studies for significant amounts of off-AVR volcanism at three slow-spreading ridges. 1) Near-bottom side-scan sonar (TOBI) images of the Mid-Atlantic Ridge near 13°N show a complex pattern of closely-spaced, active oceanic core complexes (OCCs) where plate separation is largely a-volcanic, separated by short segments of vigorous volcanic spreading. In one such volcanic segment, the brightest sea floor and therefore inferred youngest volcanism occurs not on the topographic axis (an apparently 'old' AVR) but at the edge of a broad axial valley. 2) A similar TOBI survey of the Mid-Cayman Spreading Centre reveals AVRs in the north and south flanking an OCC (Mt. Dent) and a non-volcanic ridge interpreted as tectonically extruded peridotite ('smooth' sea floor). In both AVR segments there are clear, young lava flows that have erupted from perched sources part way up the median valley walls and have partly flowed down into the valley. 3) The third case is from the Mid-Atlantic Ridge at 45°N, where we conducted a detailed geophysical and geological study of an AVR and surrounding median valley floor. The AVR is largely surrounded by flat sea floor composed mainly of lobate and sheet flows, whereas the AVR comprises predominantly pillow lavas. Although we have no firm dates, various indicators suggest most lavas on the AVR are around 10ka old or somewhat less. The apparently youngest (brightest acoustic returns, thinnest sediment cover) of the flat-lying lava flows appears to have a similar age from its degree of sediment cover. Contact relations between these lavas and the AVR flanks show no evidence of a clear age difference between the two, and we think both types of eruption may have occurred roughly

  7. Human factors activities in teleoperator development at the Oak Ridge National Laboratory

    SciTech Connect

    Draper, J.V.; Herndon, J.N.

    1986-01-01

    The Consolidated Fuel Reprocessing Program (CFRP) at the Oak Ridge National Laboratory is developing advanced teleoperator systems for maintenance of future nuclear reprocessing facilities. Remote maintenance systems developed by the CFRP emphasize man-in-the-loop teleoperation. Consequently, human factors issues which affect teleoperator performance must be addressed. This papers surveys research and development activities carried out by the human factors group within the Remote Control Engineering Task of the CFRP.

  8. Detailed leak detection test plan and schedule for the Oak Ridge National Laboratory LLLW active tanks

    SciTech Connect

    Douglas, D.G.; Maresca, J.W. Jr. )

    1993-03-01

    This document provides a detailed leak detection test plan and schedule for leak testing many of the tanks that comprise the active portion of the liquid low-level waste (LLLW) system at the Oak Ridge National Laboratory (ORNL). This plan was prepared in response to the requirements of the Federal Facility Agreement (FFA) between the US Department of Energy (DOE) and two other agencies, the US Environmental Protection Agency (EPA) and the Tennessee Department of Environment and Conservation (TDEC).

  9. 78 FR 69447 - Agency Information Collection Activities; Existing Collection, Comments Requested: Friction Ridge...

    Federal Register 2010, 2011, 2012, 2013, 2014

    2013-11-19

    ... Requested: Friction Ridge Cards: Arrest and Institution; Applicant; Personal Identification; FBI Standard... form/collection: Friction Ridge Cards: Arrest and Institution; Applicant; Personal Identification;...

  10. Modelling of hydrothermal fluid circulation in a heterogeneous medium: Application to the Rainbow Vent site (Mid-Atlantic-Ridge, 36°14N)

    NASA Astrophysics Data System (ADS)

    Perez, F.; Mügler, C.; Jean-Baptiste, P.; Charlou, J. L.

    2012-04-01

    Hydrothermal activity at the axis of mid-ocean ridges is a key driver for energy and matter transfer from the interior of the Earth to the ocean floor. At mid-ocean ridges, seawater penetrates through the permeable young crust, warms at depth and exchanges chemicals with the surrounding rocks. This hot fluid focuses and flows upwards, then is expelled from the crust at hydrothermal vent sites in the form of black or white smokers completed by diffusive emissions. We developed a new numerical tool in the Cast3M software framework to model such hydrothermal circulations. Thermodynamic properties of one-phase pure water were calculated from the IAPWS formulation. This new numerical tool was validated on several test cases of convection in closed-top and open-top boxes. Simulations of hydrothermal circulation in a homogeneous-permeability porous medium also gave results in good agreement with already published simulations. We used this new numerical tool to construct a geometric and physical model configuration of the Rainbow Vent site at 36°14'N on the Mid-Atlantic Ridge. In this presentation, several configurations will be discussed, showing that high temperatures and high mass fluxes measured at the Rainbow site cannot be modelled with hydrothermal circulation in a homogeneous-permeability porous medium. We will show that these high values require the presence of a fault or a preferential pathway right below the venting site. We will propose and discuss a 2-D one-path model that allows us to simulate both high temperatures and high mass fluxes. This modelling of the hydrothermal circulation at the Rainbow site constitutes a first but necessary step to understand the origin of high concentrations of hydrogen issued from this ultramafic-hosted vent field.

  11. Abundant Hydrothermal Venting in the Southern Ocean Near 62°S/159°E on the Australian-Antarctic Ridge

    NASA Astrophysics Data System (ADS)

    Baker, E. T.; Hahm, D.; Rhee, T. S.; Park, S. H.; Lupton, J. E.; Walker, S. L.; Choi, H.

    2014-12-01

    Circum-Antarctic Ridges (CARs) comprise almost one-third of the global Mid-Ocean Ridge, yet remain terra incognita for hydrothermal activity and chemosynthetic ecosystems. The InterRidge Vents Database lists only 3 confirmed (visualized) and 35 inferred (plume evidence) active sites along the ~21,000 km of CARs. Here, we report on a multi-year effort to locate and characterize hydrothermal activity on two 1st-order segments of the Australian-Antarctic Ridge that are perhaps more isolated from other known vent fields than any other vent site on the Mid-Ocean Ridge. KR1 is a 300-km-long segment near 62°S/159°E, and KR2 a 90-km-long segment near 60°S/152.5°E. We used profiles collected by Miniature Autonomous Plume Recorders (MAPRs) on rock corers in March and December of 2011 to survey each segment, and an intensive CTD survey in Jan/Feb 2013 to pinpoint sites and sample plumes on KR1. Optical and oxidation-reduction potential (ORP, aka Eh) anomalies indicate multiple active sites on both segments. Seven profiles on KR2 found 3 sites, each separated by ~25 km. Forty profiles on KR1 identified 13 sites, some within a few km of each other. The densest site concentration on KR1 occurred along a relatively inflated, 90-km-long section near the segment center. CTD tows covered 20 km of the eastern, most inflated portion of this area, finding two 6-km-long zones centered near 158.6°E and 158.8°E with multiple plume anomalies. Three ORP anomalies within 50 m of the seafloor indicate precise venting locations. We call this area the Mujin "Misty Harbor" vent field. Vent frequency sharply decreases away from Mujin. 3He/heat ratios determined from 20 plume samples in the Mujin field were mostly <0.015 fM/J, indicative of chronic venting, but 3 samples, 0.021-0.034 fM/J, are ratios typical of a recent eruption. The spatial density of hydrothermal activity along KR1 and KR2 is similar to other intermediate-rate spreading ridges. We calculate the plume incidence (ph) along

  12. Ridges and Hot Spots: Reconciling Isotopes and Major Elements

    NASA Astrophysics Data System (ADS)

    Albarede, F.; Lee, C.; Agranier, A.; Blichert-Toft, J.

    2008-12-01

    Meyzen et al. (2007) combined the radiogenic isotope data of several hundred MORB samples along a single mid-ocean ridge profile extending from the northernmost Atlantic to the Indian over to the Pacific Ocean covering >400 degrees. A remarkable finding was that the total reduced variance on Sr-Nd-Hf-Pb data, hereafter referred to as "isotopic variance", showed conspicuous maxima and that a harmonic analysis of this variance showed a periodic spacing of the maxima with a mean value of ~35° (actually a doublet, which is a consequence of modulation by a hemispheric contrast). The strong but unexpected hint was that hot spots are nearly periodically spaced along the ridge systems. To explore whether the isotopic variations in the mantle are controlled by physical properties of the mantle source, such as its thermal state and major element composition, we estimated the apparent temperatures and pressures of equilibration using newly calibrated thermometers and barometers of Lee et al. (submitted) based on the most recent compilation of experimental data. We calculated T-P couples for over 3000 MORB samples after correcting for olivine fractionation. Peaks appear for pressure and even stronger for temperature at the precise same localities along the ridges where the presence of hot spots has been inferred. Periodograms of temperature estimates were calculated which produced a spectrum similar to that of the isotopic variance with the same conspicuous doublet and the same mean spacing of ~35° with a total power >40%. Pressure estimates show similar features with a lesser signal/noise ratio but we suspect that these features may largely reflect the rather strong correlation between errors on Tand P. These results, based on two independent data sets, leave little doubt about deep mantle upwellings with high potential temperatures underpinning mid-ocean ridges. However, the regular spacing of hot spots along mid-ocean ridges remains an unsolved conundrum. Meyzen etal

  13. Evidence of explosive seafloor volcanic activity from the Walvis Ridge, South Atlantic Ocean

    NASA Astrophysics Data System (ADS)

    Haxel, J. H.; Dziak, R. P.

    2005-07-01

    Hydrophones moored in the North Atlantic Ocean recorded a sequence of explosive, volcano-acoustic signals originated at the Walvis Ridge in the South Atlantic Ocean. 365 explosive signals were detected from the Walvis Ridge beginning 24 November 2001 continuing through March 2002. The largest swarm began on 19 December at 2329 GMT, and lasted 1.25 hrs producing 32 locatable events. Swarm locations are centered on the northern flank of an unnamed seamount (-32.96°S -5.22°W), northwest of Wüst Seamount. These signals are interpreted as volcanogenic explosions due to similarities with acoustic signals recorded from a confirmed submarine eruption in the Caribbean in 2001 (Kick'em Jenny volcano). The observations presented suggest recent magmatic activity along the Walvis Ridge may be unrelated to the Tristan da Cunha mantle plume. Furthermore, these events lend support for an extensional fracture-zone model resulting in the recurrence of volcanic activity along older segments of large-scale sea floor lineaments.

  14. First hydrothermal discoveries on the Australian-Antarctic Ridge: Discharge sites, plume chemistry, and vent organisms

    NASA Astrophysics Data System (ADS)

    Hahm, Doshik; Baker, Edward T.; Siek Rhee, Tae; Won, Yong-Jin; Resing, Joseph A.; Lupton, John E.; Lee, Won-Kyung; Kim, Minjeong; Park, Sung-Hyun

    2015-09-01

    The Australian-Antarctic Ridge (AAR) is one of the largest unexplored regions of the global mid-ocean ridge system. Here, we report a multiyear effort to locate and characterize hydrothermal activity on two first-order segments of the AAR: KR1 and KR2. To locate vent sites on each segment, we used profiles collected by Miniature Autonomous Plume Recorders on rock corers during R/V Araon cruises in March and December of 2011. Optical and oxidation-reduction-potential anomalies indicate multiple active sites on both segments. Seven profiles on KR2 found 3 sites, each separated by ˜25 km. Forty profiles on KR1 identified 17 sites, some within a few kilometer of each other. The spatial density of hydrothermal activity along KR1 and KR2 (plume incidence of 0.34) is consistent with the global trend for a spreading rate of ˜70 mm/yr. The densest area of hydrothermal activity, named "Mujin," occurred along the 20 km-long inflated section near the segment center of KR1. Continuous plume surveys conducted in January-February of 2013 on R/V Araon found CH4/3He (1 - 15 × 106) and CH4/Mn (0.01-0.5) ratios in the plume samples, consistent with a basaltic-hosted system and typical of ridges with intermediate spreading rates. Additionally, some of the plume samples exhibited slightly higher ratios of H2/3He and Fe/Mn than others, suggesting that those plumes are supported by a younger hydrothermal system that may have experienced a recent eruption. The Mujin-field was populated by Kiwa crabs and seven-armed Paulasterias starfish previously recorded on the East Scotia Ridge, raising the possibility of circum-Antarctic biogeographic connections of vent fauna.

  15. Geochemical Characterization of Hydrothermal Plume Fluids From Peridotite- and Basalt- Dominated Regions of the Ultra-Slow Spreading Gakkel Ridge

    NASA Astrophysics Data System (ADS)

    Upchurch, L.; Edmonds, H. N.; Resing, J.; Nakamura, K.; Buck, N.; Liljebladh, B.; Stranne, C.; Tupper, G.; Winsor, P.

    2007-12-01

    Geochemical characterization of hydrothermal plumes initially located during the 2001 AMORE cruise to the Gakkel Ridge was undertaken as part of the 2007 Arctic Gakkel Vents Expedition (AGAVE). One peridotite- and one basalt-dominated area were targeted for this exploration to constrain the range of venting environments found on the Gakkel Ridge, the ultra-slow spreading endmember of the global mid-ocean ridge. CTD hydrocasts at the 7 E peridotite-hosted site relocated the plumes found initially on the AMORE cruise. The target plume was located between 2800 and 2950 meters and exhibited a localized signal in temperature and light scattering. While shipboard analysis of dissolved gases was unavailable at the 7 E site, samples were preserved for manganese and helium measurements. No Eh signal was found at the 7 E site. The 85 E basalt-hosted site has experienced recent volcanic activity and was more extensively studied relative to the 7 E site during the AGAVE cruise. CTD casts detected numerous temperature, light scattering, and Eh plumes at 85 E indicative of multiple hydrothermal sources. Three of the plumes sampled exhibited methane concentrations ranging from 20 nM to greater than 250 nM and hydrogen concentrations ranging from 10nM to 100nM. In situ Eh measurements recorded negative excursions of at least 25 mV in each plume. Associated manganese and particle chemistry samples collected at both sites will be analyzed in time for this meeting.

  16. An off-axis hydrothermal vent field near the Mid-Atlantic Ridge at 30 degrees N.

    PubMed

    Kelley, D S; Karson, J A; Blackman, D K; Früh-Green, G L; Butterfield, D A; Lilley, M D; Olson, E J; Schrenk, M O; Roe, K K; Lebon, G T; Rivizzigno, P

    2001-07-12

    Evidence is growing that hydrothermal venting occurs not only along mid-ocean ridges but also on old regions of the oceanic crust away from spreading centres. Here we report the discovery of an extensive hydrothermal field at 30 degrees N near the eastern intersection of the Mid-Atlantic Ridge and the Atlantis fracture zone. The vent field--named 'Lost City'--is distinctly different from all other known sea-floor hydrothermal fields in that it is located on 1.5-Myr-old crust, nearly 15 km from the spreading axis, and may be driven by the heat of exothermic serpentinization reactions between sea water and mantle rocks. It is located on a dome-like massif and is dominated by steep-sided carbonate chimneys, rather than the sulphide structures typical of 'black smoker' hydrothermal fields. We found that vent fluids are relatively cool (40-75 degrees C) and alkaline (pH 9.0-9.8), supporting dense microbial communities that include anaerobic thermophiles. Because the geological characteristics of the Atlantis massif are similar to numerous areas of old crust along the Mid-Atlantic, Indian and Arctic ridges, these results indicate that a much larger portion of the oceanic crust may support hydrothermal activity and microbial life than previously thought.

  17. Tracking the Tristan-Gough Mantle Plume Using Discrete Chains of Intraplate Volcanic Centers Buried in the Walvis Ridge

    NASA Astrophysics Data System (ADS)

    O'Connor, John; Jokat, Wilfried; Wijbrans, Jan

    2016-04-01

    Explanations for hotspot trails range from deep mantle plumes rising from the core-mantle boundary (CMB) to shallow plate cracking. Such mechanisms cannot explain uniquely the scattered hotspot trails distributed across a 2,000-km-wide swell in the sea floor of the southeast Atlantic Ocean. While these hotspot trails formed synchronously, in a pattern consistent with movement of the African Plate over plumes rising from the edge of the African LLSVP, their distribution is controlled by the interplay between plumes and the motion and structure of the African Plate (O'Connor et al. 2012). A significant challenge is to establish how the vigor and flow of hotspot material to the mid-ocean ridge constructed the Walvis Ridge. 40Ar/39Ar stratigraphy for three sites across the central Walvis Ridge sampled by Ocean Drilling (DSDP Leg 74) (Rohde et al., 2013; O'Connor & Jokat 2015a) indicates an apparent inverse relation between the volume flux of hotspot volcanism and the distance between the mid-ocean ridge and the Tristan-Gough hotspot. Moreover, since ˜93 Ma the geometry and motion of the mid-ocean ridge determined where hotspot material was channeled to the plate surface to build the Walvis Ridge. Interplay between hotspot flow, and the changing geometry of the mid-ocean ridge as it migrated relative to the Tristan-Gough hotspot, might explain much of the age and morphology of the Walvis Ridge. Thus, tracking the location of the Tristan-Gough plume might not be practicable if most of the complex morphology of the massive Walvis Ridge is related to the proximity of the South Atlantic mid-ocean ridge. But 40Ar/39Ar basement ages for the Tristan-Gough hotspot track (Rohde et al., 2013; O'Connor & Jokat 2015b), together with information about morphology and crustal structure from new swath maps and seismic profiles, suggest that separated age-progressive intraplate segments track the location of the Tristan-Gough mantle plume. The apparent continuity of the inferred age

  18. Tracking the Tristan-Gough Mantle Plume Using Discrete Chains of Intraplate Volcanic Centers Buried in the Walvis Ridge

    NASA Astrophysics Data System (ADS)

    O'Connor, J. M.; Jokat, W.; Wijbrans, J. R.

    2015-12-01

    Explanations for hotspot trails range from deep mantle plumes rising from the core-mantle boundary (CMB) to shallow plate cracking. Such mechanisms cannot explain uniquely the scattered hotspot trails distributed across a 2,000-km-wide swell in the sea floor of the southeast Atlantic Ocean. While these hotspot trails formed synchronously, in a pattern consistent with movement of the African Plate over plumes rising from the edge of the African LLSVP, their distribution is controlled by the interplay between plumes and the motion and structure of the African Plate (O'Connor et al., 2012). A significant challenge is to establish how the vigor and flow of hotspot material to the mid-ocean ridge constructed the Walvis Ridge. 40Ar/39Ar ages for three sites across the central Walvis Ridge sampled by Ocean Drilling (DSDP Leg 74) (Rohde et al., 2013; O'Connor and Jokat, 2015a) indicate an apparent inverse relation between the volume flux of hotspot volcanism and the distance between the mid-ocean ridge and the Tristan-Gough hotspot. Moreover, since ca. 93 Ma the geometry and motion of the mid-ocean ridge determined where hotspot material was channeled to the plate surface to build the Walvis Ridge. Interplay between hotspot flow, and the changing geometry of the mid-ocean ridge as it migrated relative to the Tristan-Gough hotspot, might explain much of the age and morphology of the Walvis Ridge. Thus, tracking the location of the Tristan-Gough plume might not be practicable if most of the complex morphology of the massive Walvis Ridge is related to the proximity of the South Atlantic mid-ocean ridge. But 40Ar/39Ar basement ages for the Tristan-Gough hotspot track (Rohde et al., 2013; O'Connor and Jokat, 2015b), together with information about morphology and crustal structure from new swath maps and seismic profiles, suggest that separated age-progressive intraplate segments track the location of the Tristan-Gough mantle plume. The apparent continuity of the inferred age

  19. Crevasse-squeeze ridge corridors: Diagnostic features of late-stage palaeo-ice stream activity

    NASA Astrophysics Data System (ADS)

    Evans, David J. A.; Storrar, Robert D.; Rea, Brice R.

    2016-04-01

    A 200-km-long and 10-km-wide linear assemblage of till-filled geometrical ridges on the bed of the Maskwa palaeo-ice stream of the late Wisconsinan southwest Laurentide Ice Sheet are interpreted as crevasse-squeeze ridges (CSR) developed during internal flow unit reorganization, immediately prior to ice stream shutdown. Ridge orientations are predominantly orientated WNW-ESE, with a subordinate WSW-ENE alignment, both indicative of ice fracture development transverse to former ice stream flow, as indicated by NNE-SSW aligned MSGL. Subglacial till injection into basal and/or full depth, mode I and II crevasses occurred at the approximate centreline of the ice stream, in response to extension and fracturing. Landform preservation indicates that this took place during the final stages of ice streaming, immediately prior to ice stream shutdown. This linear zone of ice fracturing therefore likely represents the narrowing of the fast-flowing trunk, similar to the plug flow identified in some surging valley glaciers. Lateral drag between the final active flow unit and the slower moving ice on either side is likely recorded by the up-ice bending of the CSR limbs. The resulting CSR corridor, here related to an individual ice stream flow unit, constitutes a previously unreported style of crevasse infilling and contrasts with two existing CSR patterns: (1) wide arcuate zones of CSRs related to widespread fracturing within glacier surge lobes; and (2) narrow concentric arcs of CSRs and recessional push moraines related to submarginal till deformation at active temperate glacier lobes.

  20. Tidal Triggering and Statistical Patterns of Microseismicity at Axial Volcano on the Juan de Fuca Ridge

    NASA Astrophysics Data System (ADS)

    Bohnenstiehl, D. R.; Dziak, R. P.; Caplan-Auerbach, J.; Haxel, J. H.; Mann, M. E.; Pennington, C.; Weis, J.; Womack, N.; Levy, S.

    2015-12-01

    Tidal stress changes are known to modulate the timing of microearthquakes within many mid-ocean ridge volcanic systems. At Axial Volcano, located on the Juan de Fuca Ridge, earthquakes occur preferentially when volumetric extension peaks near times of low ocean tide. Autonomous ocean-bottom hydrophone (OBH, 2007-2011) and cabled ocean bottom seismometer (OBS, Nov. 2014-) data are used to quantify the strength of tidal triggering in time periods before the April 2011 and April 2015 eruptions at Axial Volcano. The mean percent excess at times of low ocean-tide is ~14% (16% std) in the four years prior to the 2011 eruption and ~18% (17% std) in the five months prior to the 2015 eruption. The sensitivity of earthquakes to tidal stress does not evolve systematically prior to either eruption; however, this pattern is disturbed by much larger stress changes associated with the onset of dike intrusion. Following dike injection and eruption, seismicity rates drop sharply. As seismicity rates continue to rise in the months following the 2015 eruption, real-time data available from the cabled OBS network will be used quantify temporal patterns in microearthquake activity as dike induced stresses are relaxed and the magma chamber inflates.

  1. Rare earth abundances and Rb-Sr systematics of basalts, gabbro, anorthosite and minor granitic rocks from the Indian Ocean Ridge System, Western Indian Ocean

    USGS Publications Warehouse

    Hedge, C.E.; Futa, K.; Engel, C.G.; Fisher, R.L.

    1979-01-01

    Basalts dredged from the Mid-Indian Ocean Ridge System have rare earth, Rb, and Sr concentrations like those from other mid-ocean ridges, but have slightly higher Sr87/Sr86 ratios. Underlying gabbroic complexes are similar to the basalts in Sr87/Sr86, but are poorer K, Rb, and in rare earths. The chemical and isotopic data, as well as the geologic relations suggest a cumulate origin for the bulk of the gabbroic complexes. ?? 1979 Springer-Verlag.

  2. Data Sharing Report for the Quantification of Removable Activity in Various Surveillance and Maintenance Facilities at the Oak Ridge National Laboratory Oak Ridge TN

    SciTech Connect

    King, David A

    2013-12-12

    The U.S. Department of Energy (DOE) Oak Ridge Office of Environmental Management (OR-EM) requested that Oak Ridge Associated Universities (ORAU), working under the Oak Ridge Institute for Science and Education (ORISE) contract, provide technical and independent waste management planning support using American Recovery and Reinvestment Act (ARRA) funds. Specifically, DOE OR-EM requested that ORAU plan and implement a sampling and analysis campaign targeting potential removable radiological contamination that may be transferrable to future personal protective equipment (PPE) and contamination control materials—collectively referred to as PPE throughout the remainder of this report—used in certain URS|CH2M Oak Ridge, LLC (UCOR) Surveillance and Maintenance (S&M) Project facilities at the Oak Ridge National Laboratory (ORNL). Routine surveys in Bldgs. 3001, 3005, 3010, 3028, 3029, 3038, 3042, 3517, 4507, and 7500 continuously generate PPE. The waste is comprised of Tyvek coveralls, gloves, booties, Herculite, and other materials used to prevent worker exposure or the spread of contamination during routine maintenance and monitoring activities. This report describes the effort to collect and quantify removable activity that may be used by the ORNL S&M Project team to develop radiation instrumentation “screening criteria.” Material potentially containing removable activity was collected on smears, including both masselin large-area wipes (LAWs) and standard paper smears, and analyzed for site-related constituents (SRCs) in an analytical laboratory. The screening criteria, if approved, may be used to expedite waste disposition of relatively clean PPE. The ultimate objectives of this effort were to: 1) determine whether screening criteria can be developed for these facilities, and 2) provide process knowledge information for future site planners. The screening criteria, if calculated, must be formally approved by Federal Facility Agreement parties prior to use for

  3. An Early-Middle Guadalupian (Permian) isotopic record from a mid-oceanic carbonate buildup: Akiyoshi Limestone, Japan

    NASA Astrophysics Data System (ADS)

    Musashi, Masaaki; Isozaki, Yukio; Kawahata, Hodaka

    2010-08-01

    In order to understand the oceanographic changes before the Guadalupian-Lopingian (Permian) boundary mass extinction event, we investigated the isotopic compositions of the inorganic carbon and the oxygen ( δ13C carb and δ18O carb) of the Guadalupian (Middle Permian) shallow marine carbonates deposited on a seamount-top in the superocean Panthalassa. The drilled samples were obtained at Kaerimizu in the Akiyoshi area, SW Japan. We focused on the Roadian-Wordian (Middle Guadalupian) interval that spans over 7 fusuline zones; i.e. the Parafusulina kaerimizuensis Zone ( Pk Z.), Afghanella ozawai Zone ( Ao Z.), Neoschwagerina craticulifera robusta Zone ( Ncr Z.), Verbeekina verbeeki-Afghanella schenki Zone ( Vv-As Z.), Neoschwageina fusiformis Zone ( Nf Z.), Verbeekina verbeeki Zone ( Vv Z.), and Colania douvillei Zone ( Cd Z.), in ascending order. Analytical results showed that the δ13C carb values stayed almost constant around + 3.0‰ PDB in the Pk Z., Ao Z. and the lower half of the Ncr Z., and those in the upper-section gradually decreased down to -2.0‰, of which the lowest was found in the Cd Z. We statistically extracted the samples with presumably better preserved δ13C carb values in the Kaerimizu section ranged between + 0.5 and + 4.0‰ with average values of δ13C carb of + 2.7 ± 1.0‰, on the basis of δ13C carb- δ18O carb characterization. This interval shows a monotonous decrease in δ13C carb values from ca + 4.0‰ to + 2.0‰. This indicates that the primary productivity might be generally high in the Wordian mid-oceanic domain but slightly declined in the Late Wordian. The studied Early-Middle Guadalupian interval is chemostratigraphically correlated with the other mid-Pansalassan paleo-atoll limestone e.g. Iwato Formation in Japan, suggesting that the relatively high δ13C carb (over + 3.0‰) of seawater predominated in shallow mid-superocean during the middle Middle Permian.

  4. Volatile abundances and oxygen isotopes in basaltic to dacitic lavas on mid-ocean ridges: The role of assimilation at spreading centers

    USGS Publications Warehouse

    Wanless, V.D.; Perfit, M.R.; Ridley, W.I.; Wallace, P.J.; Grimes, Craig B.; Klein, E.M.

    2011-01-01

    Most geochemical variability in MOR basalts is consistent with low- to moderate-pressure fractional crystallization of various mantle-derived parental melts. However, our geochemical data from MOR high-silica glasses, including new volatile and oxygen isotope data, suggest that assimilation of altered crustal material plays a significant role in the petrogenesis of dacites and may be important in the formation of basaltic lavas at MOR in general. MOR high-silica andesites and dacites from diverse areas show remarkably similar major element trends, incompatible trace element enrichments, and isotopic signatures suggesting similar processes control their chemistry. In particular, very high Cl and elevated H2O concentrations and relatively light oxygen isotope ratios (~ 5.8‰ vs. expected values of ~ 6.8‰) in fresh dacite glasses can be explained by contamination of magmas from a component of ocean crust altered by hydrothermal fluids. Crystallization of silicate phases and Fe-oxides causes an increase in δ18O in residual magma, but assimilation of material initially altered at high temperatures results in lower δ18O values. The observed geochemical signatures can be explained by extreme fractional crystallization of a MOR basalt parent combined with partial melting and assimilation (AFC) of amphibole-bearing altered oceanic crust. The MOR dacitic lavas do not appear to be simply the extrusive equivalent of oceanic plagiogranites. The combination of partial melting and assimilation produces a distinct geochemical signature that includes higher incompatible trace element abundances and distinct trace element ratios relative to those observed in plagiogranites.

  5. Lateral variation in upper mantle temperature and composition beneath mid-ocean ridges inferred from shear-wave propagation, geoid, and bathymetry. Ph.D. Thesis

    NASA Technical Reports Server (NTRS)

    Sheehan, Anne Francis

    1991-01-01

    Resolution of both the extent and mechanism of lateral heterogeneity in the upper mantle constraints the nature and scales of mantle convection. Oceanic regions are of particular interest as they are likely to provide the closest glimpse at the patterns of temperature anomalies and convective flow in the upper mantle because of their young age and simple crustal structure relative to continental regions. Lateral variations were determined in the seismic velocity and attenuation structure of the lithosphere and astenosphere beneath the oceans, and these seismological observations were combined with the data and theory of geoid and bathymetry anomalies in order to test and improve current models for seafloor spreading and mantle convection. Variations were determined in mantle properties on a scale of about 1000 km, comparable to the thickness of the upper mantle. Seismic velocity, geoid, and bathymetry anomalies are all sensitive to variations in upper mantle density, and inversions were formulated to combine quantitatively these different data and to search for a common origin. Variations in mantle density can be either of thermal or compositional origin and are related to mantle convection or differentiation.

  6. Improving Geologic Mapping of Mid-ocean Ridges by Integrating sonar and Visual Observations through Seafloor Classification by Machine-learning Systems

    NASA Astrophysics Data System (ADS)

    White, S. M.; McClinton, J. T.

    2011-12-01

    Beyond the ability of modern near-bottom sonar systems to deliver air-photo-like images of the seafloor to help guide fieldwork, there is a tremendous amount of information hidden within sonar data that is rarely exploited for geologic mapping. Seafloor texture, backscatter amplitude, seafloor slope and roughness data can provide clues about seafloor geology but not straightforward to interpret. We present techniques for seafloor classification in volcanic terrains that integrate the capability of high-resolution, near-bottom sonar instruments to cover extensive areas of seafloor with the ability of visual mapping to discriminate differences in volcanic terrain. These techniques are adapted from the standard practices of terrestrial remote-sensing for use in the deep seafloor volcanic environment. A combination of sonar backscatter and bathymetry is used to supplement the direct seafloor visual observations by geologists to make quasi-geologic thematic maps that are consistent, objective, and most importantly spatially complete. We have taken two approaches to producing thematic maps of the seafloor for the accurate mapping of fine-scale lava morphology (e.g. pillow, lobate and sheet lava) and for the differentiation of distinct seafloor terrain types on a larger scale (e.g. hummocky or smooth). Mapping lava morphology is most accurate using fuzzy logic capable of making inferences between similar morphotypes (e.g. pillow and lobate) and where high-resolution side-scan and bathymetry data coexist. We present examples of lava morphology maps from the Galápagos Spreading Center that show the results from several analyses using different types of input data. Lava morphology is an important source of information on volcanic emplacement and eruptive dynamics. Terrain modeling can be accomplished at any resolution level, depending on the desired use of the model. For volcanic processes, input data needs to be at the appropriate scale to resolve individual volcanic features on the seafloor (e.g. small haystacks and lava channels). We present examples from the East Pacific Rise, which shows that the number of volcanic terrains differs from the tectonic provinces defined by following the spreading axis. Our terrain modeling suggests that differences in ocean crust construction and evolution can be meaningfully identified and explored without a priori assumptions about the geologic processes in a given region.

  7. Coupled Porosity and Chemical Evolution of Hydrothermal Circulation: Implications for the Morphology of Vents and Recharge Zones at Mid-Ocean Ridges

    NASA Astrophysics Data System (ADS)

    Montesi, L.; Liao, Y.; Bai, H.; Ma, Z.; Tao, R.; Syverson, D. D.; Lowell, R. P.; Fischer, T. P.

    2015-12-01

    While the clearest evidence for hydrothermal circulation resides in focused upwellings at high-temperature vents, which form chimneys, circulation also features less-understood low-temperature diffuse flow and recharge zones. Flow focusing depends on the subsurface porosity and permeability structure, which, in the reactive environment of hydrothermal circulation, is likely influenced by mineral dissolution and precipitation from hydrothermal fluids. We developed two-dimensional Finite Element models of coupled reactive flow and porosity evolution and discuss how reactions may influence flow focusing and the morphology of upwellings and downwellings. This work can also address the chemical and thermal flux provided to the ocean, and the grade and volume of metal sulfide deposition. Our coupled system (See image) considers 1) Darcy flow driven by fluid buoyancy; 2) Heat transport in a porous medium; 3) Evolution of dissolved mineral concentration; 4) Evolution of porosity and permeability in response to mineral precipitation or dissolution. We also include an "ocean" layer, which allows hot fluid to escape the system without being forced to cool dramatically as they approach the seafloor. Absent porosity evolution, hydrothermal circulation forms flame-like upwellings that bend to avoid downdrafts. The circulation varies at the time scale of decades. Assuming thermodynamic equilibrium is maintained, precipitation of amorphous silica takes place in the upwellings as they rise and cool down. When coupled with porosity and permeability evolution, silicate precipitation forces the upwellings to flatten and become diffuse. Localized recharge zones stabilize and develop an armor of low porosity rocks where high temperature fluids cooled rapidly and deposited silica as they approach the recharge zone. This morphology of localized, armored recharge zone and diffuse upwellings does not match observations at natural vent fields, which implies that a critical element of the hydrothermal system is missing from our models. Future models will also consider anhydrite which precipitates as seawater is heated and hence may counteract the effects of silica and may form localized upwellings and diffuse downwellings.

  8. An analysis of ridge axis gravity roughness and spreading rate

    NASA Technical Reports Server (NTRS)

    Small, Christopher; Sandwell, David T.

    1992-01-01

    Fast and slow spreading ridges have radically different morphologic and gravimetric characteristics. In this study, altimeter measurements from the Geosat Exact Repeat Mission are used to investigate spreading rate dependence of the ridge axis gravity field. Gravity roughness provides an estimate of the amplitude of the gravity anomaly and is robust to small errors in the location of the ridge axis. Gravity roughness as a weighted root mean square of the vertical deflection at 438 ridge crossings on the mid-ocean ridge system is computed. Ridge axis gravity anomalies show a decrease in amplitude with increasing spreading rate up to an intermediate rate of about 60-80 mm/yr and almost no change at higher rates; overall the roughness decreases by a factor of 10 between the lowest and highest rates. In addition to the amplitude decrease, the range of roughness values observed at a given spreading rate shows a similar order of magnitude decrease with transition between 60 and 80 mm/yr. The transition of ridge axis gravity is most apparent at three relatively unexplored locations on the Southeast Indian Ridge and the Pacific-Antarctic Rise; on these intermediate rate ridges the transition occurs abruptly across transform faults.

  9. Mantle control of a dynamically evolving spreading center: Mid-Atlantic Ridge 31-34 deg S

    NASA Astrophysics Data System (ADS)

    Michael, Peter J.; Forsyth, Donald W.; Blackman, Donna K.; Fox, Paul J.; Hanan, Barry B.; Harding, Alistair J.; MacDonald, Ken C.; Neumann, Gregory A.; Orcutt, John A.; Tolstoy, Maya

    1994-02-01

    A segment of the slow-spreading Mid-Atlantic Ridge (MAR) at 33 deg S changes dramatically as its center is approached. Towards the center of the segment, the axis shoals from 3900 to 2400 m and a deep median valley nearly disappears. There is a prominent bullseye gravity low centered over the shallow summit, indicating thicker crust or lower density mantle or both. Incompatible element and radiogenic isotope ratios in MORB increase, creating a 'spike high' centered on the summit of the segment. The basalts' enrichment is confined to this robust ridge sement alone and is geochemically unlike the nearby hotspots at Tristan da Cunha, Gough and Discovery Islands. The average extent of mantle melting for the entire segment, as determined from mid-ocean ridge basalt (MORB) major element chemistry, is slightly greater than for adjacent segments. The segment has lengthened to 100 km by ridge propagation at both ends during the past 3.5 m.y., and is presently the longest and shallowest segment in the region. Although the ridge crest anomalies of this ridge segment strongly resemble those caused by the interaction of mid-ocean ridges with mantle hotspots, the geochemical and geophysical evidence suggests that they may instead be related to interaction of the ridge with a passively embedded chemical heterogeneity in the mantle.

  10. The Pine Ridge-Mayo National Aeronautics and Space Administration Telemedicine Project: Program Activities and Participant Reactions

    NASA Technical Reports Server (NTRS)

    Kottke, T. E.; Little Finger, L.; Trapp, M. A.; Panser, L. A.; Novotny, P. J.

    1996-01-01

    OBJECTIVE: To determine the response of participants to the Pine Ridge-Mayo National Aeronautics and Space Administration telemedicine project. DESIGN: We describe a 3-month demonstration project of medical education and clinical consultations conducted by means of satellite transmission. Postparticipation questionnaires and a postproject survey were used to assess the success of the activity. MATERIAL AND METHODS: Patients and employees at the Pine Ridge Indian Health Service Hospital in southwestern South Dakota and employees at Mayo Clinic Rochester participated in a telemedicine project, after which they completed exit surveys and a postproject questionnaire to ascertain the acceptability of this mode of health care. RESULTS: Almost all Pine Ridge and Mayo Clinic participants viewed the project as beneficial. The educational sessions received favorable evaluations, and almost two-thirds of the patients who completed evaluations thought the consultation had contributed to their medical care. More than 90% of the respondents from Pine Ridge and more than 85% of the respondents from Mayo Clinic Rochester said that they would recommend participation in this project to others. More than 90% of respondents from Pine Ridge and 80% of Mayo respondents agreed with the statement that the project should continue. CONCLUSION: These data suggest that a program of clinical consultation services, professional education, and patient education available by telemedicine might be viewed as beneficial.

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

    NASA Astrophysics Data System (ADS)

    Escartin, J.; Barreyre, T.; Cannat, M.; Garcia, R.; Gracias, N.; Deschamps, A.; Salocchi, A.; Sarradin, P.-M.; Ballu, V.

    2015-12-01

    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 an active 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.

  12. Seismic evidence for hotspot-induced buoyant flow beneath the Reykjanes Ridge.

    PubMed

    Gaherty, J B

    2001-08-31

    Volcanic hotspots and mid-ocean ridge spreading centers are the surface expressions of upwelling in Earth's mantle convection system, and their interaction provides unique information on upwelling dynamics. I investigated the influence of the Iceland hotspot on the adjacent mid-Atlantic spreading center using phase-delay times of seismic surface waves, which show anomalous polarization anisotropy-a delay-time discrepancy between waves with different polarizations. This anisotropy implies that the hotspot induces buoyancy-driven upwelling in the mantle beneath the ridge.

  13. Middle Tertiary volcanism during ridge-trench interactions in western California

    SciTech Connect

    Cole, R.B.; Basu, A.R. )

    1992-10-01

    Bimodal volcanism in the Santa Maria Province of west-central California occurred when segments of the East Pacific Rise interacted with a subduction zone along the California margin during the Early Miocene (about 17 million years ago). Isotopic compositions of neodymium and strontium as well as trace-element data indicate that these volcanic rocks were derived from a depleted-mantle (mid-ocean ridge basalt) source. After ridge-trench interactions, the depleted-mantle reservoir was juxtaposed beneath the continental margin and was erupted to form basalts. It also assimilated and partially melted local Jurassic-Cretaceous sedimentary and metasedimentary basement rocks to form rhyolites and dacites. 28 refs.

  14. Middle tertiary volcanism during ridge-trench interactions in Western california.

    PubMed

    Cole, R B; Basu, A R

    1992-10-30

    Bimodal volcanism in the Santa Maria Province of west-central California occurred when segments of the East Pacific Rise interacted with a subduction zone along the California margin during the Early Miocene (about 17 million years ago). Isotopic compositions of neodymium and strontium as well as trace-element data indicate that these volcanic rocks were derived from a depleted-mantle (mid-ocean ridge basalt) source. After ridge-trench interactions, the depleted-mantle reservoir was juxtaposed beneath the continental margin and was erupted to form basalts. It also assimilated and partially melted local Jurassic-Cretaceous sedimentary and metasedimentary basement rocks to form rhyolites and dacites.

  15. Evidence of recent, off-axis volcanism on Gakkel Ridge, Arctic Ocean

    NASA Astrophysics Data System (ADS)

    Edwards, M.; Cochran, J. R.; Dick, H. J.

    2010-12-01

    In 1998 and 1999 the Science and Ice Exercise (SCICEX) programs used interferometric sonars installed on a U.S. Navy nuclear-powered submarine to map the morphology, texture and crustal structure of Gakkel Ridge from 6° E to 96° E with coverage out to ~50 km from the ridge axis (Edwards et al., 2001; Cochran et al., 2003). This effort represented the most comprehensive, systematic survey of this important end-member ridge on the spreading rate spectrum (Cochran et al., 2003). The SCICEX programs were followed by the Arctic Mid-Ocean Ridge Expedition (AMORE) in 2001 which used both the USCGC Healy and PFS Polarstern to map the axial valley floor and walls along Gakkel Ridge at high resolution (Michael et al., 2003; Jokat et al., 2003) from the Lena Trough to an inferred active volcanic construrct at 85°E (Müller and Jokat, 2000; Edwards et al., 2001). We have used the GPS-navigated AMORE data to refine the navigation of the SCICEX data, extending the coverage of both the SCICEX and AMORE datasets and improving the resolution and positional accuracy of the SCICEX data. The integrated dataset allows identification of several reflective, and thus relatively recent, off-axis lava flows. These flows are analogous to off-axis eruptions that have been reported on the Southwest Indian Ridge [Standish and Sims, 2010]. Several of the flows on Gakkel Ridge originate along fissures located at or near the top of the axial valley walls and spill down onto the axial valley floors. Other flows are associated with small (a few hundred meter or less in diameter) constructs contained entirely within the axial valley. We present a comparison of the integrated topographic and textural data with the results of dredge samples recovered during the AMORE expedition to document the petrology and relative age of these flow features. We further use the morphology of the reflective flow features, in combination with tectonic interpretations of the local terrain, to demonstrate the eruptive

  16. Intraplate compressional deformation in West-Congo and the Congo basin: related to ridge-puch from the South Atlantic spreading ridge?

    NASA Astrophysics Data System (ADS)

    Delvaux, Damien; Everaerts, Michel; Kongota Isasi, Elvis; Ganza Bamulezi, Gloire

    2016-04-01

    After the break-up and separation of South America from Africa and the initiation of the South-Atlantic mid-oceanic ridge in the Albian, at about 120 Ma, ridge-push forces started to build-up in the oceanic lithosphere and were transmitted to the adjacent continental plates. This is particularly well expressed in the passive margin and continental interior of Central Africa. According to the relations of Wiens and Stein (1985) between ridge-push forces and basal drag in function of the lithospheric age of oceanic plates, the deviatoric stress reaches a compressional maximum between 50 and 100, Ma after the initiation of the spreading ridge, so broadly corresponding to the Paleocene in this case (~70-20 Ma). Earthquake focal mechanism data show that the West-Congo margin and a large part of the Congo basin are still currently under compressional stresses with an horizontal compression parallel to the direction of the active transform fracture zones. We studied the fracture network along the Congo River in Kinshasa and Brazzaville which affect Cambrian sandstones and probably also the late Cretaceous-Paleocene sediments. Their brittle tectonic evolution is compatible with the buildup of ridge-push forces related to the South-Atlantic opening. Further inland, low-angle reverse faults are found affecting Jurassic to Middle Cretaceous cores from the Samba borehole in the Congo basin and strike-slip movements are recorded as a second brittle phase in the Permian cores of the Dekese well, at the southern margin of the Congo basin. An analysis of the topography and river network of the Congo basin show the development of low-amplitude (50-100 m) long wavelengths (100-300 km) undulations that can be interpreted as lithospheric buckling in response to the compressional intraplate stress field generated by the Mid-Atlantic ridge-push. Wiens, D.A., Stein, S., 1985. Implications of oceanic intraplate seismicity for plate stresses, driving forces and theology. Tectonophysics

  17. Topographic Analysis of Europa's Ridges

    NASA Astrophysics Data System (ADS)

    Bader, C. E.; Kattenhorn, S. A.; Schenk, P. M.

    2008-12-01

    lower limit of 0.05 that corresponds to the widest ridge examined (4.05 km). Conversely, the upper limit of this ratio is 0.13, corresponding to the highest ridge examined (362 m). This raises the possibility that tall ridges modify their morphology through time through some form of gravitational collapse, thus decreasing the height, increasing the width, and decreasing the ridge slopes. The low slopes and overall low average height to total width ratios suggest predominantly time-dependent viscoplastic gravitational collapse. Variability between ridges may be related to the mechanisms driving ridge development (whether dilational, contractional, or shear heating), which are likely to influence the rate of ridge construction as well as the temperature (and hence rheology) of the icy material involved. We also observe a distinct upper limit of 0.58 for the ratio of average ridge height to PTP width, suggesting that once an active ridge exceeds a certain height, the ridge peaks begin to geomorphically migrate apart in order to maintain a limiting slope of the inner and outer flanks through gravitational collapse (whether it be granular flow or viscoplastic). Lower ratios of average ridge height to PTP width may indicate underdeveloped ridge heights but may also be a sign of dilation across a ridge, causing a tectonic increase in PTP width. Finally, variability in ratios of average height to total width along individual ridges indicates that some ridge pairs may have evolved differently along their lengths, an effect partially related to changes in ridge orientation along the observable length and associated variability in ridge kinematics.

  18. Chemistry of Hydrothermal Plumes at 159°E on the Australian-Antarctic Ridge

    NASA Astrophysics Data System (ADS)

    Hahm, D.; Baker, E. T.; Rhee, T. S.; Lupton, J. E.; Resing, J. A.; Park, S. H.

    2014-12-01

    The Australian-Antarctic Ridge (AAR) is one of the largest unexplored regions of the global mid-ocean ridge system. In the present contribution, we present the geochemistry of the hydrothermal plumes over the KR1, an AAR segment at 159ºE and 62ºS. In 2011, we collected 48 Miniature Autonomous Plume Recorder profiles, measuring optical back scatter and oxidation-reduction potential, and identified the area between 158.5 and 159ºE as the densest concentration of active hydrothermal sites. In order to further characterize the chemistry of the hydrothermal plumes over the area, named 'Mujin', we conducted intensive vertical and tow-yo CTD casts in 2013. The maximum concentrations of the chemical tracers 3He, CH4, H2, and dissolved Mn, were 7.47 fmol/kg, 19.6 nmol/kg, 8.8 nmol/kg, 94.3 nmol/L, respectively. The CH4/3He (1 -10) and CH4/Mn (0.01 - 0.2) ratios were significantly lower than many ultra-mafic hosted systems, which are often found in slow spreading ridges. The lower ratios are consistent with a basaltic-hosted system, typical of the intermediate spreading rate of 6.8 cm/yr of KR1. Additionally, some of the plume samples collected around 158.6 and 158.8ºE exhibited slightly higher ratios of H2/3He than the others. Assuming that H2 is produced from the reduction of water by reduced iron compounds in the rock, the higher ratios suggest that those plumes are supported by a younger hydrothermal system, which may have experienced a recent eruption.

  19. Hydrothermal Activity and Volcanism on the Southern Mid-Atlantic Ridge

    NASA Astrophysics Data System (ADS)

    Haase, K. M.; Scientific Party, M.

    2005-12-01

    In April 2005 four recently discovered different hydrothermal fields on the slow-spreading Mid-Atlantic Ridge (MAR) south of the Equator were studied and sampled using a remotely operated vehicle (ROV) during cruise METEOR 64/1. Three of these hydrothermally active fields (called Turtle Pits, Red Lion, and Wideawake) occur at about 3000 m water depth in the centre of a MAR segment at 4° 48'S which appears to be volcanically very active. The youngest lava flow partly covers the low-temperature, diffuse flow Wideawake mussel field and is thus probably only a few years old. The high-temperature Turtle Pits hydrothermal field with four active vent structures lies some 300 m west of the diffuse vent field and is characterized by boiling fluids with temperatures close to 400° C. The mineral assemblage recovered from inactive hydrothermal mounds includes massive magnetite+hematite+sulfate and differs from that of the presently active vents and indicates more oxidizing conditions during the earlier activity. The vent fluids at Turtle Pits contain relatively high contents of hydrogen which may have formed during iron oxidation processes when basaltic magmas crystallized. The high fluid temperatures, the change to more reducing conditions, and the relatively high hydrogen contents in the fluids are most likely due to the ascent of magmas from the mantle that fed the very recent eruption. The high-temperature Red Lion hydrothermal field lies some 2 km north of the Turtle Pits field and consists of at least four active black smokers surrounded by several inactive sulfide mounds. The composition of the Red Lion fluids differs significantly from the Turtle Pits fluids, possibly owing largely to a difference in the temperature of the two systems. The fourth hydrothermally active field on the southern MAR, the Liliput field, was discovered near 9° 33'S in a water depth of 1500 m and consists of several low-temperature vents. A shallow hydrothermal plume in the water column

  20. Active and relict sea-floor hydrothermal mineralization at the TAG hydrothermal field, Mid-Atlantic Ridge

    SciTech Connect

    Rona, P.A. . Atlantic Oceanographic and Meteorological Labs.); Hannington, M.D. ); Raman, C.V. ); Thompson, G.; Tivey, M.K.; Humphris, S.E. ); Lalou, C. . Lab. CNRS-CEA); Petersen, S. Aachen Univ. of Technology )

    1993-12-01

    The TAG hydrothermal field is a site of major active and inactive volcanic-hosted hydrothermal mineralization in the rift valley of the slow-spreading Mid-Atlantic Ridge at 26[degree]N. The axial high is the principal locus of present magmatic intrusions. The TAG field contains three main areas of present and past hydrothermal activity: (1) an actively venting high-temperature sulfide mound; (2) two former high-temperature vent areas; (3) a zone of low-temperature venting and precipitation of Fe and Mn oxide deposits. The volcanic centers occur at the intersections between ridge axis-parallel normal faults and projected axis-transverse transfer faults. The intersections of these active fault systems may act as conduits both for magmatic intrusions from sources beneath the axial high that build the volcanic centers and for hydrothermal upwelling that taps the heat sources. Radiometric dating of sulfide samples and manganese crusts in the hydrothermal zones and dating of sediments intercalated with pillow lava flows in the volcanic center adjacent to the active sulfide mound indicate multiple episodes of hydrothermal activity throughout the field driven by heat supplied by episodic intrusions over a period of at least 140 [times] 10[sup 3] yr. The sulfide deposits are built by juxtaposition and superposition during relatively long residence times near episodic axial heat sources counterbalanced by mass wasting in the tectonically active rift valley of the slow-spreading oceanic ridge. Hydrothermal reworking of a relict hydrothermal zone by high-temperature hydrothermal episodes has recrystallized sulfides and concentrated the first visible primary gold reported in a deposit at an oceanic ridge.

  1. Composition and dissolution of black smoker particulates from active vents on the Juan de Fuca Ridge

    NASA Astrophysics Data System (ADS)

    Feely, Richard A.; Lewison, Maureen; Massoth, Gary J.; Robert-Baldo, Gillian; Lavelle, J. William; Byrne, Robert H.; Von Damm, Karen L.; Curl, Herbert C., Jr.

    1987-10-01

    During two Atlantis II/Alvin cruises to the Juan de Fuca Ridge in 1984 active high temperature (140°-284°C) vents were sampled for black smoker particulates using the Grassle Pump. Individual mineral phases were identified using standard X ray diffraction and petrographic procedures. In addition, elemental compositions and particle morphologies were determined by X ray energy spectrometry and scanning electron microscope/X ray energy spectrometry techniques. The vent particulates from the southern Juan de Fuca Ridge vent sites were highly enriched in S, Si, Fe, Zn, and Cu and were primarily composed of sphalerite, wurtzite, pyrite, pyrrhotite, barite, chalcopyrite, cubanite, hydrous iron oxides, and elemental sulfur. Two additional unidentified phases which were prevalent in the samples included an Fe-Si phase and a Ca-Si phase. The grain sizes of the individual particle phases ranged from < 2 μm for the sphalerite and Fe oxide particles to > 100 μm for the Fe-Si particles. Grain size and current meter data were used in a deposition model of individual phase dispersal. For many of the larger sulfide and sulfate particles, the model predicts dispersal to occur over length scales of only several hundreds of meters. The high-temperature black smokers from the more northerly Endeavour Segment vents were highly enriched in Fe, S, Ca, Cu, and Zn and were primarily composed of anhydrite, chalcopyrite, sphalerite, barite, sulfur, pyrite, and other less abundant metal sulfide minerals. The grain sizes of the individual particles ranged from < 10 μm to slightly larger than 500 μm. The composition and size distributions of the mineral phases are highly suggestive of high-temperature mixing between vent fluids and seawater. A series of field and laboratory studies were conducted to determine the rates of dissolution of several sulfate and sulfide minerals. The dissolution rates ranged over more than 3 orders of magnitude, from 3.2 × 10-8 cm s-1 for anhydrite to 1.2

  2. Chemosynthetic microbial activity at Mid-Atlantic Ridge hydrothermal vent sites

    NASA Astrophysics Data System (ADS)

    Wirsen, Carl O.; Jannasch, Holger W.; Molyneaux, Stephen J.

    1993-06-01

    Chemosynthetic production of microbial biomass, determined by 14CO2 fixation and enzymatic (RuBisCo) activity, at the Mid-Atlantic Ridge (MAR) 23° and 26°N vent sites was found in various niches: warm water emissions, loosely rock-attached flocculent material, dense morphologically diverse bacterial mats covering the surfaces of polymetal sulfide deposits, and filamentous microbes on the carapaces of shrimp (Rimicaris exoculata). The bacterial mats on polymetal sulfide surfaces contained unicellular and filamentous bacteria which appeared to use as their chemolithotrophic electron or energy source either dissolved reduced minerals from vent emissions, mainly sulfur compounds, or solid metal sulfide deposits, mainly pyrite. Moderately thermophilic Chemosynthetic activity was observed in carbon dioxide fixation experiments and in enrichments, but no thermophilic aerobic sulfur oxidizers could be isolated. Both obligate and facultative chemoautotrophs growing at mesophilic temperatures were isolated from all chemosynthetically active surface scrapings. The obligate autotrophs could oxidize sterilized MAR natural sulfide deposits as well as technical pyrite at near neutral pH, in addition to dissolved reduced sulfur compounds. While the grazing by shrimp on the surface mats of MAR metal sulfide deposits was observed and deemed important, the animals' primary occurrence in dense swarms near vent emissions suggests that they were feeding at these sites, where conditions for Chemosynthetic growth of their filamentous microbial epiflora were optimal. The data show that the transformation of geothermal energy at the massive polymetal sulfide deposits of the MAR is based on the lithoautotrophic oxidation of soluble sulfides and pyrites into microbial biomass.

  3. Annual report of decommissioning and remedial action S&M activities for the Environmental Management Program at Oak Ridge National Laboratory, Oak Ridge, Tennessee

    SciTech Connect

    1997-11-01

    The Oak Ridge National Laboratory (ORNL) Surveillance and Maintenance (S&M) Program performs a variety of activities to ensure that sites and facilities within its responsibility remain in a safe condition and in compliance with applicable regulations. All S&M Program activities during fiscal year (FY) 1997 were accomplished safely, with no health and safety incidents, no lost work days, and no environmental noncompliances. In addition, all activities were performed within schedule thresholds and under budget. Many remedial action (RA) sites and decontamination and decommissioning (D&D) facilities are inspected and maintained by the S&M Program. RA sites encompass approximately 650 acres and 33 D&D facilities, including 4 inactive reactors. During FY 1997, routine, preventative, and emergency maintenance activities were performed as needed at these sites and facilities. Stabilization activities were also performed to reduce risks and reduce future S&M costs. Major activities at the RA sites during FY 1997 included maintaining proper liquid levels in surface impoundments and inactive -liquid low-level waste storage tanks as well as installing a new cover at the tumulus pads in Waste Area Grouping (WAG) 6, planting trees in the First Creek Riparian Corridor, and performing over 900 well inspections. Postremediation monitoring was conducted at the 3001 Canal, Core Hole 8, the WAG 6 Resource Conservation and Recovery caps, and WAG 5 Seeps C and D; groundwater monitoring was performed in WAGs 4, 5, and 6 and at the 3001 Canal Well. At ORNL D&D facilities, significant accomplishments included contaminated lead brick removal, asbestos abatement, contaminated equipment and debris removal, and radiologically contaminated area painting.

  4. Surveillance and maintenance report on decontamination and decommissioning and remedial action activities at the Oak Ridge Y-12 plant, Oak Ridge, Tennessee. Fiscal year 1996

    SciTech Connect

    King, H.L.; Sollenberger, M.L.; Sparkman, D.E.; Reynolds, R.M.; Wayland, G.S.

    1996-12-01

    The Oak Ridge Y-12 Plant Decontamination and Decommissioning (D&D) and Remedial Action (RA) programs are part of the Environmental Restoration (ER) Division and are funded by the Office of Environmental Management (EM-40). Building 9201-4 (known as Alpha-4), three sites located within Building 9201-3 (the Oil Storage Tank, the Molten Salt Reactor Experiment Fuel Handling Facility, and the Coolant Salt Technology Facility), and Building 9419-1 (the Decontamination Facility) are currently the facilities at the Y-12 Plant included in the D&D program. The RA program provides surveillance and maintenance (S&M) and program management of ER sites at the Y-12 Plant, including selected sites listed in Appendix C of the Federal Facilities Agreement (FFA), sites listed in the Hazardous and Solid Waste Amendment (HSWA) permit Solid Waste Management Unit (SWM-U) list, and sites currently closed or undergoing post-closure activities under the Resource Conservation and Recovery Act of 1976 (RCRA) or the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA). This report communicates the status of the program plans and specific S&M activities for the D&D and RA programs.

  5. SWEAP: Southwest Indian Ridge Earthquakes and Plumes: First Results from a Comparative Seismicity Study of Magmatic and Amagmatic Spreading

    NASA Astrophysics Data System (ADS)

    Schlindwein, V. S. N.; Scholz, J. R.; Schmid, F.

    2014-12-01

    Cruise ANT-29/8 (SWEAP) of RV Polarstern in November 2013 headed for the Oblique Supersegment of the Southwest Indian Ridge (SWIR) in the "Furious Fifties" to recover 10 ocean bottom seismometers (OBS) after recording earthquake activity for a period of one year. The OBS recovery was flanked by a multidisciplinary science program that searched in difficult sea conditions for signs of hydrothermal venting, examined deep-sea fauna and determined the thermal regime of this rift section. The seismic activity that accompanies crustal generation at ultraslow spreading mid-ocean ridges is hardly known. We present here preliminary results from the first-ever long-term deployment of OBS networks at two locations of the SWIR. We instrumented the segment 8 volcano near 65°E with 8 OBS and the amagmatically spreading Oblique Supersegment with 10 OBS, two of which returned no data. The networks had dimensions of 60 km x 40 km and a comparable station spacing of about 15 km. A first data analysis suggests that the seismic activity of the magmatic segment is about 4 times as high as that of the magmatically starved Oblique Supersegment. Interestingly, the segment 8 volcano itself displays a prominent seismic gap with a complete absence even of small earthquakes while the adjacent rift valley hosts earthquakes down to 15 km depth indicating a cold lithosphere. This spatial earthquake distribution may reflect an up-doming lithosphere-asthenosphere boundary that has been postulated to guide melts towards the widely spaced volcanoes of ultraslow spreading ridges.

  6. Along ridge variation of the seafloor cooling and subsidence

    NASA Astrophysics Data System (ADS)

    Huang, Po-Ju; Chiao, Ling-Yun

    2013-04-01

    Bathymetry is linearly proportional to the square root of the seafloor age according to decades of observations. It is well explained by the essentially one-dimensional thermal contraction such as that demonstrated in the classical half-space cooling model and the subsequent 2-D modifications such as the plate model, GDH1 model, PSM model etc. However, much less efforts have been undertaken on study of variation of seafloor cooling along the ridge axis. We carefully examine corridors in the spreading direction that avoid seamounts and other some secondary structures, in addition to the sediment correction. We find that subsidence rates vary along major mid-ocean ridges. It would require a range of 400 to -600°C difference if the subsidence rate variations are attributed entirely to sub-ridge mantle temperature anomalies. Pronounced anomalies include the noticeable lows at the equator in the mid-Atlantic ridge and the northern section of East Pacific Rise that might be attributed to the close by continental lithosphere. The eastern section in mid-Indian ridge is also significantly cooler within regions of the Australian-Antarctic Discordance (AAD), which has been attributed to an ancient slab stalled beneath the present-day Southeast Indian Ridge (SEIR). Further researches that take into account of trends of geoid data is underway to make consistent interpretations.

  7. Microbiological characterization of post-eruption "snowblower" vents at Axial Seamount, Juan de Fuca Ridge.

    PubMed

    Meyer, Julie L; Akerman, Nancy H; Proskurowski, Giora; Huber, Julie A

    2013-01-01

    Microbial processes within the subseafloor can be examined during the ephemeral and uncommonly observed phenomena known as snowblower venting. Snowblowers are characterized by the large quantity of white floc that is expelled from the seafloor following mid-ocean ridge eruptions. During these eruptions, rapidly cooling lava entrains seawater and hydrothermal fluids enriched in geochemical reactants, creating a natural bioreactor that supports a subseafloor microbial "bloom." Previous studies hypothesized that the eruption-associated floc was made by sulfide-oxidizing bacteria; however, the microbes involved were never identified. Here we present the first molecular analysis combined with microscopy of microbial communities in snowblower vents from samples collected shortly after the 2011 eruption at Axial Seamount, an active volcano on the Juan de Fuca Ridge. We obtained fluid samples and white flocculent material from active snowblower vents as well as orange flocculent material found on top of newly formed lava flows. Both flocculent types revealed diverse cell types and particulates when examined by phase contrast and scanning electron microscopy (SEM). Distinct archaeal and bacterial communities were detected in each sample type through Illumina tag sequencing of 16S rRNA genes and through sequencing of the sulfide oxidation gene, soxB. In fluids and white floc, the dominant bacteria were sulfur-oxidizing Epsilonproteobacteria and the dominant archaea were thermophilic Methanococcales. In contrast, the dominant organisms in the orange floc were Gammaproteobacteria and Thaumarchaeota Marine Group I. In all samples, bacteria greatly outnumbered archaea. The presence of anaerobic methanogens and microaerobic Epsilonproteobacteria in snowblower communities provides evidence that these blooms are seeded by subseafloor microbes, rather than from microbes in bottom seawater. These eruptive events thus provide a unique opportunity to observe subseafloor microbial

  8. Surficial permeability of the axial valley seafloor: Endeavour Segment, Juan de Fuca Ridge

    NASA Astrophysics Data System (ADS)

    Hearn, Casey K.; Homola, Kira L.; Johnson, H. Paul

    2013-09-01

    Hydrothermal systems at mid-ocean spreading centers play a fundamental role in Earth's geothermal budget. One underexamined facet of marine hydrothermal systems is the role that permeability of the uppermost seafloor veneer plays in the distribution of hydrothermal fluid. As both the initial and final vertical gateway for subsurface fluid circulation, uppermost seafloor permeability may influence the local spatial distribution of hydrothermal flow. A method of deriving a photomosaic from seafloor video was developed and utilized to estimate relative surface permeability in an active hydrothermal area on the Endeavour Segment of the Juan de Fuca Ridge. The mosaic resolves seafloor geology of the axial valley seafloor at submeter resolution over an area greater than 1 km2. Results indicate that the valley walls and basal talus slope are topographically rugged and unsedimented, providing minimal resistance to fluid transmission. Elsewhere, the axial valley floor is capped by an unbroken blanket of low-permeability sediment, resisting fluid exchange with the subsurface reservoir. Active fluid emission sites were restricted to the high-permeability zone at the base of the western wall. A series of inactive fossil hydrothermal structures form a linear trend along the western bounding wall, oriented orthogonal to the spreading axis. High-temperature vent locations appear to have migrated over 100 m along-ridge-strike over the decade between surveys. While initially an expression of subsurface faulting, this spatial pattern suggests that increases in seafloor permeability from sedimentation may be at least a secondary contributing factor in regulating fluid flow across the seafloor interface.

  9. Microbiological characterization of post-eruption "snowblower" vents at Axial Seamount, Juan de Fuca Ridge.

    PubMed

    Meyer, Julie L; Akerman, Nancy H; Proskurowski, Giora; Huber, Julie A

    2013-01-01

    Microbial processes within the subseafloor can be examined during the ephemeral and uncommonly observed phenomena known as snowblower venting. Snowblowers are characterized by the large quantity of white floc that is expelled from the seafloor following mid-ocean ridge eruptions. During these eruptions, rapidly cooling lava entrains seawater and hydrothermal fluids enriched in geochemical reactants, creating a natural bioreactor that supports a subseafloor microbial "bloom." Previous studies hypothesized that the eruption-associated floc was made by sulfide-oxidizing bacteria; however, the microbes involved were never identified. Here we present the first molecular analysis combined with microscopy of microbial communities in snowblower vents from samples collected shortly after the 2011 eruption at Axial Seamount, an active volcano on the Juan de Fuca Ridge. We obtained fluid samples and white flocculent material from active snowblower vents as well as orange flocculent material found on top of newly formed lava flows. Both flocculent types revealed diverse cell types and particulates when examined by phase contrast and scanning electron microscopy (SEM). Distinct archaeal and bacterial communities were detected in each sample type through Illumina tag sequencing of 16S rRNA genes and through sequencing of the sulfide oxidation gene, soxB. In fluids and white floc, the dominant bacteria were sulfur-oxidizing Epsilonproteobacteria and the dominant archaea were thermophilic Methanococcales. In contrast, the dominant organisms in the orange floc were Gammaproteobacteria and Thaumarchaeota Marine Group I. In all samples, bacteria greatly outnumbered archaea. The presence of anaerobic methanogens and microaerobic Epsilonproteobacteria in snowblower communities provides evidence that these blooms are seeded by subseafloor microbes, rather than from microbes in bottom seawater. These eruptive events thus provide a unique opportunity to observe subseafloor microbial

  10. Chemistry of hydrothermal vent fluids from the Main Endeavour Field, northern Juan de Fuca Ridge: Geochemical controls in the aftermath of June 1999 seismic events

    NASA Astrophysics Data System (ADS)

    Seyfried, W. E.; Seewald, J. S.; Berndt, M. E.; Ding, Kang; Foustoukos, D. I.

    2003-09-01

    In June 1999, an intense swarm of earthquakes occurred on the Endeavour segment of the Juan de Fuca Ridge influencing hydrothermal activity in and around the Main Endeavour Field (MEF). Here we report the dissolved concentrations of 31 species from five high-temperature vents sampled 3 months after the seismic event. The spatial variability of vent fluid chemistry is extreme. Vapor-dominated vent fluids at Cantilever and Sully sites have high measured temperatures (375°-379°C), high dissolved gas and boron concentrations, but low SiO2. Modeling results indicate that these fluids can be accounted for by supercritical phase separation and brine condensation. Other vent fluids have moderate temperatures (340°-366°C) and chloride concentrations (208-426 mmol/kg), and may result from mixing of supercritical, vapor-rich fluids with evolved seawater. Phase equilibria calculations indicate that in addition to chloride, redox, temperature, and especially pressure play key roles in accounting for compositional variability of vent fluids at MEF. In comparison with earlier (1988) data, the 1999 data set reveals significantly lower chloride concentrations and higher boron, whereas alkali and alkaline earth cations are lower by 10-20% in keeping with chloride decrease. That dissolved chloride, boron, and other elements returned to preevent levels when again sampled in 2000 provide additional data documenting the inherently dynamic nature of hydrothermal systems at mid-ocean ridges.

  11. Endeavour Segment, Juan de Fuca Ridge, Integrated Studies Site (ISS) Update and Opportunities

    NASA Astrophysics Data System (ADS)

    Butterfield, D.; Ridge Community

    2003-12-01

    The Ridge 2000 (R2K) Integrated Studies bull's eye on the Juan de Fuca Ridge is focused on the Main Endeavour hydrothermal field, located on the central portion of the Endeavour Segment. This vent field is one of the most vigorously venting systems along the global mid-ocean ridge spreading network, hosting at least 18 large sulfide structures that contains more than100 smokers. Prior to a magmatic event in 2000 some of the edifices had been venting 380C, volatile-rich fluids with extremely low chlorinities for a decade. In addition to the Main Endeavour Field there are four other known high temperature vent fields spaced approximately 2 kilometers apart along the segment (with hints of more) and abundant areas of diffuse flow, both nearby and distal to the high temperature venting. Diffuse flow from the structures and from a variety of basaltic-hosted sites provides rich habitats abundant with microbial and macrofaunal communities. There are well-developed gradients in volatile concentrations along axis that may reflect influence from a sedimentary source to the north, and high chlorinity fluids vent from the most southern (Mothra) and northern fields (Sasquatch). Twenty years of research have laid a firm base for the 5-year plans of R2K at this site, which include examining the response of this segment to perturbations induced by tectonic and magmatic events, identification of the reservoirs, fluxes, and feedbacks of mass and energy at this site, and predictive modeling coupled with field observations. Since designation as an IS site, high-resolution bathymetric mapping (EM300) and an extensive multi-channel seismic survey have been conducted along the entire segment. Smaller focused areas have also been mapped at meter resolution by SM2000 sonar. Intense field programs in 2003 established the first in-situ seismic array along a mid-ocean ridge, which includes installation of a buried broadband seismometer and 7 short-period seismometers emplaced within basaltic

  12. Oak Ridge National Laboratory's (ORNL) Weigh-In-Motion (WIM) Configuration and Data Management Activities

    SciTech Connect

    Abercrombie, Robert K; Sheldon, Frederick T; Schlicher, Bob G

    2006-01-01

    The Oak Ridge National Laboratory (ORNL) involvement in the Weigh-in-Motion (WIM) research with both government agencies and private companies dates back to 1989. The discussion here will focus on the US Army's current need for an automated WIM system to weigh and determine the center-of-balance for military wheeled vehicles and cargo and the expanded uses of WIM data. ORNL is addressing configuration and data management issues as they relate to deployments for both military and humanitarian activities. The transition from the previous WIM Gen I to the current Gen II system illustrates a configuration and data management solution that ensures data integration, integrity, coherence and cost effectiveness. Currently, Army units use portable and fixed scales, tape measures, and calculators to determine vehicle axle, total weights and center of balance for vehicles prior to being transshipped via railcar, ship, or airlifted. Manually weighing and measuring all vehicles subject to these transshipment operations is time-consuming, labor-intensive, hazardous and is prone to human errors (e.g., misreading scales and tape measures, calculating centers of balance and wheel, axle, and vehicle weights, recording data, and transferring data from manually prepared work sheets into an electronic data base and aggravated by adverse weather conditions). Additionally, in the context of the military, the timeliness, safety, success, and effectiveness of airborne heavy-drop operations can be significantly improved by the use of an automated system to weigh and determine center of balance of vehicles while they are in motion. The lack of a standardized airlift-weighing system for joint service use also creates redundant weighing requirements at the cost of scarce resources and time. This case study can be judiciously expanded into commercial operations related to safety and enforcement. The WIM program will provide a means for the Army to automatically identify/weigh and monitor

  13. Heat Flow on the South West Indian Ridge at 14°E and the Consequences for Microbiological Activity

    NASA Astrophysics Data System (ADS)

    Kaul, N. E.; Molari, M.; Boetius, A.

    2014-12-01

    During RV POLARSTERN cruise PS81 to the South West Indian Ridge (SWIR) at 52°S, 14°E an integrated study was carried out in more than 4000 m water depth employing seismology, geology, microbiology, deep-sea ecology, heat flow and others. Heat flow is supposed to be an indicator for the varying depth of the magma chamber beneath the ridge axis. Bottom observations from previous work on the SWIR are scarce and visual information about geostructures, habitat landscapes, benthic faunal communities and their distribution in this area have so far been missing. Vigorous fluid flow in the form of black smokers or shimmering water could not be detected but enhanced heat flow due to upward pore water migration occurred. This leads to values of very high heat flow (up to 850 mW/m2) and advection rates up to 25 cm/a Darcy velocity. Enhanced biomass and a greater variation of megafauna along those sites of high heat flow could be inferred from reconnaissance observations with a camera sledge. A closer investigation of microbial activity in the material of gravity corers revealed favorable living conditions for microorganisms. We find the inorganic carbon fixation rates, here applied like a proxy of microbial metabolic activity, were significantly higher (up to 7 times higher) in surficial sediments in proximity of the station PS 81/640 compared to other stations along the ridge. Conversely the extracellular enzymatic activities did not show any significant difference in the potential organic matter degradation between the stations investigated. These results suggest an increase of chemosynthetic activities at St PS 81/649, possibly related to increase of availability of reduced compounds (i.e. sulphide, reduced metals) in presence of pore water flow.

  14. Microbial Communities at Non-Volcanic and Volcanic Sites of the Gakkel Ridge

    NASA Astrophysics Data System (ADS)

    Helmke, E.; Juergens, J.; Tausendfreund, M.; Wollenburg, J.; Shank, T.; Edmonds, H.; Humphris, S.; Nakamura, K.; Liljebladh, B.; Winsor, P.; Singh, H.; Reves-Sohn, R. A.

    2007-12-01

    The Gakkel Ridge in the eastern Arctic Ocean is the slowest spreading, deepest, and most isolated portion of the global mid-ocean ridge system and therefore predestined for comparative investigations on deep-sea vent communities. However, the perennial cover of thick sea ice has made this area largely inaccessible to science. The Arctic Gakkel Vents Expedition (AGAVE) utilized the icebreaker ODEN and newly developed vehicles for exploration and sampling in connection with a CTD/rosette equipped with different sensors and a high-resolution multi-beam bathymetry system. We focused our studies on the peridotite-hosted region at 85°N, 7°E and on the basaltic volcanism area at 85°N, 85°E. Water, sediment, and rock samples were taken to describe the microbial communities in different zones of these two sites. Sampling was guided by anomalies of backscattering, temperature, Eh, as well as by high-resolution seafloor imagery. Samples were preserved or processed on board immediately after sampling. Molecular analyses, cultural methods, total bacterial counts, and activity measurements were employed to describe the structure of the microbial communities, their phylogeny, potential adaptations, and possible role in biogeochemical cycles. The first molecular biological results of the bacterial communities of the 85°E site indicated atypical of deep- sea venting communities. These preliminary results were supported by the images of the under-ice vehicle "Camper" which showed thick yellow "fluffy" mats (often > 5cm thick) and orange "pebbly" material without any smell of H2S markedly different than the white, consolidated Beggiatoa mats often observable at deep venting sites. Foraminifera occurred regularly on top of basalt rocks as well as within the bacterial mats.

  15. Annual Report on Environmental Monitoring Activities for FY 1995 (Baseline Year) at Waste Area Grouping 6 at Oak Ridge National Laboratory, Oak Ridge, Tennessee

    SciTech Connect

    1996-06-01

    This report describes baseline contaminant release conditions for Waste Area Grouping (WAG) 6 at Oak Ridge National Laboratory (ORNL). The sampling approach and data analysis methods used to establish baseline conditions were presented in ``Environmental Monitoring Plan for Waste Area Grouping 6 at Oak Ridge National Laboratory, Oak Ridge, Tennessee (EMP).`` As outlined in the EMP, the purpose of the baseline monitoring year at WAG 6 was to determine the annual contaminant releases from the site during fiscal year 1995 (FY95) against which any potential changes in releases over time could be compared. The baseline year data set provides a comprehensive understanding of release conditions from all major waste units in the WAG through each major contaminant transport pathway. Due to a mandate to reduce all monitoring work, WAG 6 monitoring was scaled back and reporting efforts on the baseline year results are being minimized. This report presents the quantified baseline year contaminant flux conditions for the site and briefly summarizes other findings. All baseline data cited in this report will reside in the Oak Ridge Environmental Information system (OREIS) database, and will be available for use in future years as the need arises to identify potential release changes.

  16. 4D Time-Lapse Seismic Analysis of Active Gas Seepage Systems on the Vestnesa Ridge, Offshore W-Svalbard

    NASA Astrophysics Data System (ADS)

    Bunz, S.; Hurter, S.; Plaza-Faverola, A. A.; Mienert, J.

    2014-12-01

    Active gas venting occurs on the Vestnesa Ridge, an elongated sediment drift north of the Molloy Transform and just east of the Molloy Ridge, one of the shortest segments of the slow spreading North-Atlantic Ridge system. The crest of the Vestnesa Ridge at water depth between 1200-1300 m is pierced with fluid-flow features. Seafloor pockmarks vary in size up to 1 km in diameter with significant morphological features consisting of small ridges, diapiric structures and small pits. Detailed hydro-acoustic surveying shows that gas mostly emanates from the small-scale pits, where also hydrates have been recovered by sediment sampling. High-resolution P-Cable 3D seismic data acquired in 2012 show vertical focused fluid flow features beneath the seafloor pockmarks. These co-called chimneys extend down to the free-gas zone underneath a bottom-simulating reflection (BSR). Here, they link up with small fault systems that might provide pathways to the deeper subsurface. The chimney features show a high variability in their acoustic characteristics with alternating blanked or masked zones and high-amplitude anomalies scattered through the whole vertical extent of the chimneys. The amplitude anomalies indicate high-impedance contrasts due to the likely presence of gas or a high-velocity material like gas hydrates or carbonates. In most cases, the high-amplitude anomalies line up along specific vertical pathways that connect nicely with the small-scale pits at the surface where gas bubbles seep from the seafloor. We re-acquired the 3D seismic survey in 2013 for time-lapse seismic studies in order to better understand the origin of the amplitude anomalies and in order to track potentially migrating gas fronts up along the chimney structure. The time-lapse seismic analysis indicates several areas, where gas migration may have led to changes in acoustic properties of the subsurface. These areas are located along chimney structures and the BSR. This work provides a basis for better

  17. Changes in microbial communities, including both uncultured and culturable bacteria, with mid-ocean ballast-water exchange during a voyage from Japan to Australia.

    PubMed

    Tomaru, Akiko; Kawachi, Masanobu; Demura, Mikihide; Fukuyo, Yasuwo

    2014-01-01

    We assessed changes in the microbial communities in ballast water during a trans-Pacific voyage from Japan to Australia that included a mid-ocean ballast-water exchange. Uncultured (i.e., total) and culturable bacteria were counted and were characterized by using denaturing gradient gel electrophoresis (DGGE). There was a clear decrease over time in numbers of uncultured microorganisms, except for heterotrophic nanoflagellates, whereas the abundance of culturable bacteria initially decreased after the ballast-water exchange but then increased. The increase, however, was only up to 5.34% of the total number of uncultured bacteria. Cluster analysis showed that the DGGE profiles of uncultured bacteria clearly changed after the exchange. In contrast, there was no clear change in the DGGE profiles of culturable bacteria after the exchange. Multidimensional scaling analysis showed changes in microbial communities over the course of the voyage. Although indicator microbes as defined by the International Convention for the Control and Management of Ships' Ballast Water and Sediments were occasionally detected, no coliform bacteria were detected after the exchange.

  18. Changes in Microbial Communities, Including both Uncultured and Culturable Bacteria, with Mid-Ocean Ballast-Water Exchange during a Voyage from Japan to Australia

    PubMed Central

    Tomaru, Akiko; Kawachi, Masanobu; Demura, Mikihide; Fukuyo, Yasuwo

    2014-01-01

    We assessed changes in the microbial communities in ballast water during a trans-Pacific voyage from Japan to Australia that included a mid-ocean ballast-water exchange. Uncultured (i.e., total) and culturable bacteria were counted and were characterized by using denaturing gradient gel electrophoresis (DGGE). There was a clear decrease over time in numbers of uncultured microorganisms, except for heterotrophic nanoflagellates, whereas the abundance of culturable bacteria initially decreased after the ballast-water exchange but then increased. The increase, however, was only up to 5.34% of the total number of uncultured bacteria. Cluster analysis showed that the DGGE profiles of uncultured bacteria clearly changed after the exchange. In contrast, there was no clear change in the DGGE profiles of culturable bacteria after the exchange. Multidimensional scaling analysis showed changes in microbial communities over the course of the voyage. Although indicator microbes as defined by the International Convention for the Control and Management of Ships' Ballast Water and Sediments were occasionally detected, no coliform bacteria were detected after the exchange. PMID:24817212

  19. Active neutron interrogation for verification of storage of weapons components at the Oak Ridge Y-12 Plant

    SciTech Connect

    Mihalczo, J.T.; Valentine, T.E.; Mattingly, J.K.; Mullens, J.A.; Hughes, S.S.

    1998-02-23

    A nuclear weapons identification system (NWIS), under development since 1984 at the Oak Ridge Y-12 Plant and presently in use there, uses active neutron interrogation with low-intensity {sup 252}Cf sources in ionization chambers to provide a timed source of fission neutrons from the spontaneous fission of {sup 252}Cf. To date, measurements have been performed on {approximately}15 different weapons systems in a variety of configurations both in and out of containers. Those systems included pits and fully assembled systems ready for deployment at the Pantex Plant in Amarillo, Texas, and weapons components at the Oak Ridge Y-12 Plant. These measurements have shown that NWIS can identify nuclear weapons and/or components; nuclear weapons/components can be distinguished from mockups where fissile material has been replaced by nonfissile material; omissions of small amounts (4%) of fissile material can be detected; changes in internal configurations can be determined; trainer parts can be identified as was demonstrated by verification of 512 containers with B33 components at the Y-12 Plant (as many as 32 in one 8-hour shift); and nonfissile components can be identified. The current NWIS activities at the Oak Ridge Y-12 Plant include: (1) further development of the system for more portability and lower power consumption, (2) collection of reference signatures for all weapons components in containers, and (3) confirmation of a particular weapons component in storage and confirmation of receipts. This paper describes the recent measurements with NWIS for a particular weapons component in storage that have resolved an Inspector General (IG`s) audit finding with regard to performance of confirmation of inventory.

  20. Upper-mantle dynamics revealed by helium isotope variations along the southeast Indian ridge.

    PubMed

    Graham, D W; Lupton, J E; Spera, F J; Christie, D M

    2001-02-01

    Helium isotope variations in igneous rocks are important for relating isotopic heterogeneity to convective mixing in the Earth's mantle. High 3He/4He ratios at many ocean islands, along with lower and relatively uniform values in mid-ocean-ridge basalts (MORBs), are thought to result from a well mixed upper-mantle source for MORB and a distinct deeper-mantle source for ocean island basalts. At finer scales, 3He/4He variations along mid-ocean ridges have been related to underlying mantle heterogeneity, but relationships between the scales of geochemical segmentation and mantle convection remain enigmatic. Here we present helium isotope data for MORB glasses recovered along approximately 5,800 km of the southeast Indian ridge, and develop an approach to quantitatively relate spatial variations in geochemical and geophysical parameters at the Earth's surface. A point-to-point correlation analysis reveals structure in the helium isotope data at length scales of approximately 150 and approximately 400 km that appears to be related to secondary convection in the underlying mantle.

  1. SeaVOICE: Sea-going Experiments to Test Potential Linkages among Sea Level Change, Ocean Ridge Volcanism, and Hydrothermal Activity.

    NASA Astrophysics Data System (ADS)

    Langmuir, C. H.; Carbotte, S. M.; Huybers, P. J.; McManus, J. F.; Mukhopadhyay, S.; Winckler, G.; Boulahanis, B.; Costa, K.; Ferguson, D.; Katz, R. F.; Li, Y.; Middleton, J. L.

    2015-12-01

    Changes in sea level influence the pressure of the solid Earth over entire ocean basins. While the absolute changes in sea level caused by glacial cycles are small relative to ocean depths, the temporal variations in sea level can lead to pressure changes of similar order to mantle upwelling rates, with the potential to significantly perturb short term rates of melt production at ocean ridges (Huybers and Langmuir, EPSL, 2009). Such changes could then lead to fluctuations in crustal thickness, magma composition and hydrothermal activity. To investigate possible relationships between glacial cycles and ocean ridge processes, we carried out an 18 day cruise of mapping and sediment coring to the Cleft Segment of the Juan de Fuca ridge. High resolution bathymetry was obtained on the west side of the ridge axis to beyond 1Ma to test whether abyssal hill fabric shows periodicities consistent with glacial cycles. Nine successful piston cores up to 7.6m in length provide a sedimentary record back to more than 600kyr to test for spatial and temporal variations in hydrothermal activity. Oxygen isotope stratigraphy on these cores is systematic and provides good age constraints. Short cores near the ridge axis provide a record of the current trace of hydrothermal activity in youngest sediments. Several of the cores impacted basement and recovered a basement sample. Above basement, basaltic glass shards were recovered in the bottom meter of sediment, raising the possibility of temporal records of basalt chemical compositions using the age constraints the sediments provide. The glass samples provide a unique and new perspective on ridge volcanism, since previous off-axis samples were restricted to dredging old fault scarps. Cores can be taken anywhere, raising the potential for global time series studies of ridge volcanism. The coupled bathymetry, sediment geochemistry and magmatic glass compositions hold the promise of a definitive advance in our understanding of the

  2. Visual Observations and Geologic Settings of the Newly-Discovered Black Smoker Vent Sites Across the Galapagos Ridge-Hotspot Intersection

    NASA Astrophysics Data System (ADS)

    Anderson, P.; Haymon, R.; MacDonald, K.; White, S.

    2006-12-01

    Nearly one-fifth of the global mid-ocean ridge is hotspot-affected, yet very little is known about how hotspots affect quantity and distribution of high-temperature hydrothermal vents along the ridge. During the 2005-06 GalAPAGoS expedition, acoustic and plume sensor surveys were conducted across the Galapagos ridge- hotspot intersection, lon. 94.5ºW- lon. 89.5ºW, to map fine scale geologic features and locate hydrothermal plumes emanating from the ridge crest. Where significant plumes were detected, the Medea fiber-optic camera sled was used successfully to find and image high-temperature vents on the seafloor. With Medea we discovered and imaged the first active and recently extinct black smokers known along the entire Galapagos Spreading Center (GSC), and documented the geologic setting of these vents. The Medea survey imaged numerous inactive vents as well as 3 active high-temperature vent fields along the ridge at 94º 04.5'W (Navidad Site), 91º56.2'W (Iguanas Site) and 91º54.3'W (Pinguinos Site). Two recently extinct vent fields also were identified at 91º23.4'-23.7'W and 91º13.8'W. All of the high-temperature vent sites that we identified along the GSC are found above relatively shallow AMC reflectors and are located in the middle 20% of ridge segments. Without exception the vent sites are located along fissures atop constructional axial volcanic ridges (AVR's) composed of relatively young pillow basalts. In some cases, the vents were associated with collapses adjacent to the fissures. The fissures appear to be eruptive sources of the pillow lavas comprising the AVR's. Video images of the chimneys show mature, cylindrical structures, up to 14m high; little diffuse flow; few animals; and some worm casts and dead clam shells, suggesting prior habitation. We conclude that distribution of the vents is controlled by magmatic processes, (i.e., by locations of shallow AMC magma reservoirs and eruptive fissures above dike intrusions), and that there is

  3. Different TDM/CH4 hydrothermal plume signatures: TAG site at 26N and serpentinized ultrabasic diapir at 15 degrees 05'N on the Mid-Atlantic ridge

    SciTech Connect

    Charlou, J.L.; Bougault, H. ); Appriou, P. ); Nelsen, T.; Rona, P. )

    1991-11-01

    As a part of the 1988 NOAA VENTS Program, CH{sub 4} and Mn tracers were used to identify and compare hydrothermal plumes found above the TAG Field (26{degrees}N) and in the rift valley at 15{degrees}N close to the eastern intersection of the ridge axis with the 15{degrees}20'N Fracture Zone at the Mid-Atlantic Ridge (MAR). Active hydrothermal venting was confirmed at TAG, based on elevated concentrations of total dissolved Mn (TDM up to 30 nmol/kg), high CH{sub 4} concentrations (up to 200 nL/L), and elevated nephelometry signals. Plumes of a different composition were identified at 15{degree}N with high CH{sub 4} concentrations (up to 400 nL/L), low total dissolved Mn concentrations (TDM < 1 nmol/kg) and no significant nephelometry signal. The different properties of these tracers and the different tracer ratios can be used to deduce vent fluid characteristics and compare one hydrothermal area to another. TDM/CH{sub 4} and Nephel/CH{sub 4} ratios at TEG are of the same order of magnitude as those observed at other spreading axis hydrothermal fields. At 15{degrees}N, the low TDM/CH{sub 4} ratio provides evidence of fluid circulation into ultrabasic rocks and offers a potentially useful and single method of exploring for hydrothermal activity associated with serpentinization. Mantle degassing through hydrothermal activity associated with serpentinization is an important process with respect to chemical and thermal exchanges between the upper mantle and the ocean. Different ratios of hydrothermal tracers (i.e., TDM/CH{sub 4}) provide a useful framework for identifying subseafloor processes along mid-oceanic ridges.

  4. The mean composition of ocean ridge basalts

    NASA Astrophysics Data System (ADS)

    Gale, Allison; Dalton, Colleen A.; Langmuir, Charles H.; Su, Yongjun; Schilling, Jean-Guy

    2013-03-01

    mean composition of mid-ocean ridge basalts (MORB) is determined using a global data set of major elements, trace elements, and isotopes compiled from new and previously published data. A global catalog of 771 ridge segments, including their mean depth, length, and spreading rate enables calculation of average compositions for each segment. Segment averages allow weighting by segment length and spreading rate and reduce the bias introduced by uneven sampling. A bootstrapping statistical technique provides rigorous error estimates. Based on the characteristics of the data, we suggest a revised nomenclature for MORB. "ALL MORB" is the total composition of the crust apart from back-arc basins, N-MORB the most likely basalt composition encountered along the ridge >500 km from hot spots, and D-MORB the depleted end-member. ALL MORB and N-MORB are substantially more enriched than early estimates of normal ridge basalts. The mean composition of back-arc spreading centers requires higher extents of melting and greater concentrations of fluid-mobile elements, reflecting the influence of water on back-arc petrogenesis. The average data permit a re-evaluation of several problems of global geochemistry. The K/U ratio reported here (12,340 ± 840) is in accord with previous estimates, much lower than the estimate of Arevalo et al. (2009). The low Sm/Nd and 143Nd/144Nd ratio of ALL MORB and N-MORB provide constraints on the hypothesis that Earth has a non-chondritic primitive mantle. Either Earth is chondritic in Sm/Nd and the hypothesis is incorrect or MORB preferentially sample an enriched reservoir, requiring a large depleted reservoir in the deep mantle.

  5. Data base management activities for the Remedial Action Program at Oak Ridge National Laboratories (ORNL)

    SciTech Connect

    Hook, L.A.; Voorhees, L.D.; Gentry, M.J.; Faulkner, M.A.; Shaakir-Ali, J.A.; Newman, K.A.; McCord, R.A.; Goins, L.F.; Owen, P.T.

    1990-07-01

    The Oak Ridge National Laboratory (ORNL) Remedial Action Program (RAP) was established in 1985 in response to state and federal regulations requiring comprehensive control over facility discharges and cleanup of contaminated sites. A computerized Data and Information Management System (DIMS) was developed for RAP to (1) provide a centralized repository for data pertinent to RAP and (2) provide support for the investigations and assessments leading to the long-term remediation of contaminated facilities and sites. The current status of DIMS and its role in supporting RAP during 1989 are described. The DIMS consists of three components: (1) the Numeric Data Base, (2) the Bibliographic Data Base, and (3) the Records Control Data Base. This report addresses all three data bases, but focuses on the contents of the Numeric Data Base. Significant progress was made last year with the geographic information system (GIS) and ARC/INFO, which can be interfaced with SAS/GRAPH to provide combined mapping and statistical graphic products. Several thematic layers of GIS data for the Oak Ridge Reservation are now available. 18 refs., 8 figs., 19 tabs.

  6. Diffuse Crustal Accretion at the Southern Terminus of the Malaguana-Gadao Ridge, Mariana Trough

    NASA Astrophysics Data System (ADS)

    Sleeper, J. D.; Martinez, F.; Fryer, P. B.

    2014-12-01

    The mode of extension and crustal accretion in backarc basins is strongly affected by proximity to the arc volcanic front. The factor that likely has the strongest control on these processes is mantle water content. At Mid-Ocean Ridges, the small amount of water in the mantle is efficiently extracted into the melt, dehydrating the residual material and increasing the viscosity and strength of the lithosphere. This may aid in focusing melt generated over a broad (~200+ km wide) zone in the mantle toward a narrow zone of crustal accretion ~1-2 km wide. In the near-arc setting, the continuous flux of water into the mantle wedge should oppose lithospheric dehydration and inhibit strengthening of the lithosphere, which may allow deformation, volcanism, and crustal accretion to occur over a broad area instead of along a narrow axis. A possible example of this process can be observed at the southern terminus of the Malaguana-Gadao Ridge, a backarc spreading center in the Southern Mariana Trough, at the southern end of the Izu-Bonin-Mariana convergent margin. The spreading axis, which forms an axial high in this area, abruptly terminates at 143˚20'E, 12˚37'N and is replaced by a broad zone of active volcanism and tectonism characterized by short volcanic ridges, volcanic cones, and low-relief grabens. This study uses deep-towed and ship multibeam sonar, gravity, and magnetics data collected during an early 2012 cruise on R/V Thomas G. Thompson (TN273) along with available geophysical and geochemical data in the Southern Mariana Trough to gain insight into the nature of the diffuse crustal accretion process. Evidence of a similar transition from organized to "disorganized" spreading can also be observed at Valu Fa Ridge in the southern Lau basin and other backarc spreading centers. This suggests that this process is not unique to the Southern Mariana Trough, and may be an important mode of crustal accretion in a variety of backarc settings where there is extension in

  7. Magmatic effects of the Cobb hot spot on the Juan de Fuca Ridge

    USGS Publications Warehouse

    Chadwick, J.; Perfit, M.; Ridley, I.; Jonasson, I.; Kamenov, G.; Chadwick, W.; Embley, R.; le, Roux P.; Smith, M.

    2005-01-01

    The interaction of the Juan de Fuca Ridge with the Cobb hot spot has had a considerable influence on the magmatism of the Axial Segment of the ridge, the second-order segment that overlies the hot spot. In addition to the construction of the large volcanic edifice of Axial Seamount, the Axial Segment has shallow bathymetry and a prevalence of constructional volcanic features along its 100-km length, suggesting that hot spot-derived magmas supplement and oversupply the ridge. Lavas are generally more primitive at Axial Seamount and more evolved in the Axial Segment rift zones, suggesting that fractional crystallization is enhanced with increasing distance from the hot spot because of a reduced magma supply and more rapid cooling. Although the Cobb hot spot is not an isotopically enriched plume, it produces lavas with some distinct geochemical characteristics relative to normal mid-ocean ridge basalt, such as enrichments in alkalis and highly incompatible trace elements, that can be used as tracers to identify the presence and prevalence of the hot spot influence along the ridge. These characteristics are most prominent at Axial Seamount and decline in gradients along the Axial Segment. The physical model that can best explain the geochemical observations is a scenario in which hot spot and mid-ocean ridge basalt (MORB) magmas mix to varying degrees, with the proportions controlled by the depth to the MORB source. Modeling of two-component mixing suggests that MORB is the dominant component in most Axial Segment basalts. Copyright 2005 by the American Geophysical Union.

  8. Evidence for melt channelization in Galapagos plume-ridge interaction

    NASA Astrophysics Data System (ADS)

    Mittal, T.; Richards, M. A.

    2015-12-01

    Many present-day hot spots are located within ~ 1000 km of a mid-ocean ridge, either currently or in the geologic past, leading to frequent interaction between these two magmatic regimes. The consequent plume-ridge interactions provide a unique opportunity to test models for asthenosphere-lithosphere dynamics, with the plume acting as a tracer fluid in the problem, and excess magmatism reflecting otherwise unsampled sub-surface phenomena. Galapagos is an off-ridge hotspot with the mantle plume located ~150-250 km south of the plate boundary. Plume-ridge interaction in Galapagos is expressed by the formation of volcanic lineaments of islands and seamounts - e.g., the Wolf-Darwin lineament (WDL) - providing a direct probe of the plume-ridge interaction process, especially in regards to geochemical data. Although several models have been proposed to explain plume-ridge interaction in Galapagos, none adequately explain the observed characteristics, especially the WDL. In particular, predicted lithospheric fault orientations and melt density considerations appear at odds with observations, suggesting that lithospheric extension is not the primary process for formation of these islands. Other off-ridge hotspots interacting with nearby spreading ridges, such as Reunion and Louisville, also exhibit volcanic lineaments linking the plume and the ridge. Thus these lineament-type features are a common outcome of plume-ridge interaction that are indicative of the underlying physics. We propose that the lineaments are surface expressions of narrow sub-lithospheric melt channels focused towards the spreading ridge. These channels should form naturally due to the reactive infiltration instability in a two-phase flow of magma and solid mantle as demonstrated in two-phase flow simulations (e.g., Katz & Weatherley 2012). For Galapagos, we show that melt channels can persist thermodynamically over sufficient length-scales to link the plume and nearby ridge segments. We also show that

  9. Hydrodynamic properties and grain-size characteristics of volcaniclastic deposits on the mid-Atlantic Ridge north of Iceland (Kolbeinsey Ridge)

    SciTech Connect

    Oehmig, R.; Wallrabe-Adams, H. )

    1993-01-01

    Surface sediments from a transect across the mid-ocean ridge north of Iceland (Kolbeinsey Ridge) have been analyzed according to their compositional, textural and hydromechanical characteristics. The results were used to reconstruct sediment formation and depositional processes. The ridge sediments are dominated by volcaniclastic particles of hyaloclastic and pyroclastic origin. These particles show a wide variety in size, shape and density. Single-grain settling velocities of the different glass types reveal the suitability of this parameter as a reflector of the particle properties of size, shape and density, which are also known to be relevant to grain transport. Observations concerning different current expositions of central ridge sediments, combined with the parameters of settling velocity distribution, grain-size distribution and sediment particle composition, were applied to distinguish between transport association with rare, easily movable glass shards and poorly sorted sediments in sheltered ponds. A bimodal settling velocity distribution of steep ridge-flank sediments probably indicates the effect of sediment admixture from poorly sorted mass flows. Alternating coarse- and fine-grained layers characterize the transition between ridge-glass sands and the ridge-adjacent plain, which is dominated by slow-settling pelagic material.

  10. Enhanced hydrothermal activity along the East Pacific Rise during the last two glacial terminations

    NASA Astrophysics Data System (ADS)

    Lund, D. C.; Asimow, P. D.; Farley, K. A.

    2015-12-01

    Mid-ocean ridge magmatism is driven by seafloor spreading and decompression melting of the upper mantle. Scaling estimates [1-2] and model results [3-4] indicate that glacial-interglacial changes in sea level should modulate melt production at mid-ocean ridges, an idea that has been confirmed with detailed surveys of ridge bathymetry [4-5]. The nature and timing of associated changes in hydrothermal activity have remained unknown, however, precluding a clear understanding of whether ridge magmatism can act as a negative feedback on ice sheet size. Here we present multiple records of hydrothermal sedimentation spanning 1300 km of the East Pacific Rise (EPR). At each location, the flux of Fe, Mn, and As increased beginning at ~25 kyr BP, reached maximum values by 15 kyr BP, and then decreased into the Holocene. Lateral sediment focusing is an unlikely explanation given the similar signal in multiple cores and the lack of evidence for anomalous horizontal transport in 3He-based focusing factors. Coherent variations in Fe, Mn, and As suggest that diagenetic overprinting is not the primary driver of the down core signal. Elevated metal fluxes also occur during Termination II. The time series of hydrothermal sedimentation bear a strong resemblance to a record of seafloor bathymetry from 17ºS [5], suggesting that both have a common driver. The simplest explanation is glacial-interglacial variations in sea level, which apparently modulates sub-ridge melting, seafloor bathymetry, and hydrothermal activity at the EPR. Our results imply that geothermal heat flux from ridges increases during the last two glacial terminations, which should act to erode the deep ocean stratification, enhance the abyssal circulation, and transmit excess heat to the Southern Ocean, thereby setting the stage for deglaciation. [1] Lund and Asimow (2008) AGU Fall Meeting, Abstract #PP11D-08. [2] Huybers and Langmuir (2009) Earth and Planetary Science Letters 286, 479-491. [3] Lund and Asimow (2011

  11. COMSOL-Related Activities within the Research Reactors Division of Oak Ridge National Laboratory

    SciTech Connect

    Freels, James D

    2015-01-01

    Our group at Oak Ridge National Laboratory (ORNL) started using COMSOL shortly after version 3.0 was released in the Spring of 2004. Over 11 years later and several new releases of the code, the application usage has grown along with the number of licenses we are responsible for. This paper focuses not on details of results and modeling methods, but instead, takes a look at our past and present applications, and evaluates where we are headed with COMSOL in the future. In doing so, we reveal some lessons learned along our pathway, provide some insight on how best to use COMSOL in a group setting, and perhaps help both users and developers to improve how the code is utilized.

  12. Follow-on Research Activities for the Rensselaer Isothermal Dendritic Growth Experiment (RIDGE)

    NASA Technical Reports Server (NTRS)

    LaCombe, J. C.; Koss, M. B.; Lupulescu, A. O.; Frei, J. E.; Giummarra, C.; Glicksman, M. E.

    2001-01-01

    The RIDGE effort continues the aegis of the earlier, NASA-sponsored, Isothermal Dendritic Growth Experiment (IDGE) series of experiments through the continued analysis of microgravity data acquired during these earlier space flights. The preliminary observations presented here demonstrate that there are significant differences between SCN and the more anisotropic PVA dendrites. The side branch structure becomes amplified only further behind the tip, and the interface shape is generally wider (i.e. more hyperbolic than parabolic) in PVA than in SCN. These characteristics are seen to affect the process of heat transport. Additionally, the dendrites grown during the fourth United States Microgravity Payload (USMP-4) exhibit time-dependent growth characteristics and may not always have reached steady-state growth during the experiment.

  13. Seismicity and active accretion processes at the ultraslow-spreading Southwest and intermediate-spreading Southeast Indian ridges from hydroacoustic data

    NASA Astrophysics Data System (ADS)

    Tsang-Hin-Sun, Eve; Royer, Jean-Yves; Perrot, Julie

    2016-08-01

    Volcanic and tectonic events are the main processes involved in the generation of the oceanic crust and responsible for the seismicity associated with seafloor spreading. To monitor this activity, usually not or poorly detected by land-based seismological stations, we deployed from February 2012 to February 2013 a network of autonomous hydrophones to compare the behaviour of the ultraslow-spreading Southwest Indian ridge (SWIR) with that of the intermediate-spreading Southeast Indian ridge (SEIR). The rate of seismicity is similar for both ridges, suggesting that there is no systematic relationship between seismicity and spreading rates. The along-axis distribution of the seismic events, however, does differ, reflecting the rate dependence of accretion modes. Earthquakes are sparse and regularly spaced and scattered along the SWIR, reflecting prevailing tectonic processes. By contrast, along the SEIR, events are irregularly distributed and focus at ridge-segment ends and transforms faults, reflecting the ridge segmentation; only two swarms occurred at a segment centre and are probably caused by a magmatic event. This seismicity distribution thus looks controlled by segment-scale crustal heterogeneities along the SEIR and by regional-scale contrasting accretion processes along the SWIR, probably driven by different lithospheric and asthenospheric dynamics on either side of the Melville fracture zone. The comparison of hydroacoustic and teleseismic catalogues shows that, along these spreading ridges, the background seismicity observed in 1 yr by a hydroacoustic network is representative of the seismicity observed over two decades by land-based networks.

  14. Microbiological characterization of post-eruption “snowblower” vents at Axial Seamount, Juan de Fuca Ridge

    PubMed Central

    Meyer, Julie L.; Akerman, Nancy H.; Proskurowski, Giora; Huber, Julie A.

    2013-01-01

    Microbial processes within the subseafloor can be examined during the ephemeral and uncommonly observed phenomena known as snowblower venting. Snowblowers are characterized by the large quantity of white floc that is expelled from the seafloor following mid-ocean ridge eruptions. During these eruptions, rapidly cooling lava entrains seawater and hydrothermal fluids enriched in geochemical reactants, creating a natural bioreactor that supports a subseafloor microbial “bloom.” Previous studies hypothesized that the eruption-associated floc was made by sulfide-oxidizing bacteria; however, the microbes involved were never identified. Here we present the first molecular analysis combined with microscopy of microbial communities in snowblower vents from samples collected shortly after the 2011 eruption at Axial Seamount, an active volcano on the Juan de Fuca Ridge. We obtained fluid samples and white flocculent material from active snowblower vents as well as orange flocculent material found on top of newly formed lava flows. Both flocculent types revealed diverse cell types and particulates when examined by phase contrast and scanning electron microscopy (SEM). Distinct archaeal and bacterial communities were detected in each sample type through Illumina tag sequencing of 16S rRNA genes and through sequencing of the sulfide oxidation gene, soxB. In fluids and white floc, the dominant bacteria were sulfur-oxidizing Epsilonproteobacteria and the dominant archaea were thermophilic Methanococcales. In contrast, the dominant organisms in the orange floc were Gammaproteobacteria and Thaumarchaeota Marine Group I. In all samples, bacteria greatly outnumbered archaea. The presence of anaerobic methanogens and microaerobic Epsilonproteobacteria in snowblower communities provides evidence that these blooms are seeded by subseafloor microbes, rather than from microbes in bottom seawater. These eruptive events thus provide a unique opportunity to observe subseafloor microbial

  15. Implications of spinel compositions for the petrotectonic history of abyssal peridotite from Southwest Indian Ridge (SWIR)

    NASA Astrophysics Data System (ADS)

    Chen, T.; Jin, Z.; Wang, Y.; Tao, C.

    2012-12-01

    Abyssal peridotites generate at mid-ocean ridges. Lherzolite and harzburgite are the main rock types of peridotites in the uppermost mantle. The lherzolite subtype, less depleted and less common in ophiolites, characterizes mantle diapirs and slow-spreading ridges. Along the Earth's mid-ocean ridges, abyssal peridotites undergo hydration reactions to become serpentinite minerals, especially in slow to ultraslow spreading mid-ocean ridges. Spinel is common in small quantities in peridotites, and its compositions have often been used as petrogenetic indicators [1]. The Southwest Indian Ridge (SWIR) is one of the two ultraslow spreading ridges in the world. The studied serpentinized peridotite sample was collected by the 21st Voyage of the Chinese oceanic research ship Dayang Yihao (aka Ocean No. 1) from a hydrothermal field (63.5°E, 28.0°S, and 3660 m deep) in SWIR. The studied spinels in serpentinized lherzolite have four zones with different compositions: relic, unaltered core is magmatic Al-spinels; micro- to nano- sized ferrichromite zoned particles; narrow and discontinuous magnetite rim; and chlorite aureoles. The values Cr# of the primary Al-spinels indicate the range of melting for abyssal peridotites from SWIR extends from ~4% to ~7% [2]. The alteration rims of ferrichromite have a chemical composition characterized by Fe enrichment and Cr# increase indicating chromite altered under greenschist-amphibolite facies. Magnetites formed in syn- and post- serpentinization. Chlorite (clinochlore) formed at the boundary and crack of spinel indicating it had undergone with low-temperature MgO- and SiO2-rich hydrothermal fluids [3]. It suggests that serpentinized lherzolite from SWIR had undergone poly-stage hydration reactions with a wide range of temperature. Acknowledgments: EMPA experiment was carried out by Xihao Zhu and Shu Zheng in The Second Institute of Oceanography and China University of Geosciences, respectively. The work was supported by NSFC

  16. Influence of magma ascent rate on carbon dioxide degassing at oceanic ridges: Message in a bubble

    NASA Astrophysics Data System (ADS)

    Chavrit, D.; Humler, E.; Morizet, Y.; Laporte, D.

    2012-12-01

    In order to quantify the magma ascent rate beneath oceanic ridges, we propose a new method based on vesicle size distribution (VSD) and volatiles measurements (CO2 and H2O) on 65 fresh glasses from the global Mid-Ocean Ridge system. Comparisons of VSD between the main oceanic basins reveal that Mid-Ocean-Ridge-Basalts (MORB) from the Pacific Ocean have significantly higher bubble densities (148-124+739 bubbles/cm2) and initial population densities ln(n0)=19.6±2.8 cm-4 but lower vesicularities (˜0.1%). We determine the residence time of bubbles in the magma using the linear relationship between the logarithm of the density population and the vesicle diameters. Assuming a constant bubble growth rate (G=1.5×10-7 cm/s), we suggest that the transit times of the magmas through the oceanic crust are shorter beneath Pacific ridges (˜2 h) than elsewhere (˜15 h). In addition, the CO2 and H2O contents of the studied glasses allow the carbon saturation pressure to be calculated. Pacific MORB display a significantly lower carbon dioxide saturation pressure (Psat=843-221+300 bars) than Atlantic and Indian MORB (Psat=1898-827+1466 bars) but identical pressures of eruption (Pe˜310 bars). Consequently, the distance traveled by bubbles from the vesiculation depth to the seafloor is shorter for the Pacific (˜1.9 km) than for the Atlantic and Indian oceans (˜5.2 km): the longer the transit time, the longer the distance traveled. A closer inspection of the data revealed that the decompression rate (dP/dt) varied from 5.0×102 to 2.3×104 Pa/s and correlated positively with the measured bubble densities (nb/m3) as expected from experimental data and numerical modeling. At the global scale, most of the Pacific samples are characterized by high ascent rates (0.25-0.13+0.26 m/s) relative to those from the Atlantic (0.11-0.06+0.15 m/s) and the Indian samples (0.09-0.03+0.05 m/s). However, at the local scale some samples from the Mid-Atlantic Ridge at 37 °N (where the reflections of

  17. Moytirra: Discovery of the first known deep-sea hydrothermal vent field on the slow-spreading Mid-Atlantic Ridge north of the Azores

    NASA Astrophysics Data System (ADS)

    Wheeler, A. J.; Murton, B.; Copley, J.; Lim, A.; Carlsson, J.; Collins, P.; Dorschel, B.; Green, D.; Judge, M.; Nye, V.; Benzie, J.; Antoniacomi, A.; Coughlan, M.; Morris, K.

    2013-10-01

    Geological, biological, morphological, and hydrochemical data are presented for the newly discovered Moytirra vent field at 45oN. This is the only high temperature hydrothermal vent known between the Azores and Iceland, in the North Atlantic and is located on a slow to ultraslow-spreading mid-ocean ridge uniquely situated on the 300 m high fault scarp of the eastern axial wall, 3.5 km from the axial volcanic ridge crest. Furthermore, the Moytirra vent field is, unusually for tectonically controlled hydrothermal vents systems, basalt hosted and perched midway up on the median valley wall and presumably heated by an off-axis magma chamber. The Moytirra vent field consists of an alignment of four sites of venting, three actively emitting "black smoke," producing a complex of chimneys and beehive diffusers. The largest chimney is 18 m tall and vigorously venting. The vent fauna described here are the only ones documented for the North Atlantic (Azores to Reykjanes Ridge) and significantly expands our knowledge of North Atlantic biodiversity. The surfaces of the vent chimneys are occupied by aggregations of gastropods (Peltospira sp.) and populations of alvinocaridid shrimp (Mirocaris sp. with Rimicaris sp. also present). Other fauna present include bythograeid crabs (Segonzacia sp.) and zoarcid fish (Pachycara sp.), but bathymodiolin mussels and actinostolid anemones were not observed in the vent field. The discovery of the Moytirra vent field therefore expands the known latitudinal distributions of several vent-endemic genera in the north Atlantic, and reveals faunal affinities with vents south of the Azores rather than north of Iceland.

  18. Evidence for chemically heterogeneous Arctic mantle beneath the Gakkel Ridge

    NASA Astrophysics Data System (ADS)

    D'Errico, Megan E.; Warren, Jessica M.; Godard, Marguerite

    2016-02-01

    Ultraslow spreading at mid-ocean ridges limits melting due to on-axis conductive cooling, leading to the prediction that peridotites from these ridges are relatively fertile. To test this, we examined abyssal peridotites from the Gakkel Ridge, the slowest spreading ridge in the global ocean ridge system. Major and trace element concentrations in pyroxene and olivine minerals are reported for 14 dredged abyssal peridotite samples from the Sparsely Magmatic (SMZ) and Eastern Volcanic (EVZ) Zones. We observe large compositional variations among peridotites from the same dredge and among dredges in close proximity to each other. Modeling of lherzolite trace element compositions indicates varying degrees of non-modal fractional mantle melting, whereas most harzburgite samples require open-system melting involving interaction with a percolating melt. All peridotite chemistry suggests significant melting that would generate a thick crust, which is inconsistent with geophysical observations at Gakkel Ridge. The refractory harzburgites and thin overlying oceanic crust are best explained by low present-day melting of a previously melted heterogeneous mantle. Observed peridotite compositional variations and evidence for melt infiltration demonstrates that fertile mantle components are present and co-existing with infertile mantle components. Melt generated in the Gakkel mantle becomes trapped on short length-scales, which produces selective enrichments in very incompatible rare earth elements. Melt migration and extraction may be significantly controlled by the thick lithosphere induced by cooling at such slow spreading rates. We propose the heterogeneous mantle that exists beneath Gakkel Ridge is the consequence of ancient melting, combined with subsequent melt percolation and entrapment. Initial modes of depleted mantle composition from Hellebrand et al. (2002b). Melt compositions are from Brunelli et al. (2014) in

  19. Summary of environmental characterization activities at the Oak Ridge National Laboratory Solid Waste Storage Area Six, FY 1986 through 1987

    SciTech Connect

    Davis, E.C.; Solomon, D.K.; Dreier, R.B.; Lee, S.Y.; Kelmers, A.D.; Lietzke, D.A. ); Craig, P.M. )

    1987-09-30

    The Oak Ridge National Laboratory (ORNL) Remedial Action Program (RAP), has supported characterization activities in Solid Waste Storage Area (SWSA 6) to acquire information necessary for identification and planning of remedial actions that may be warranted, and to facilitate eventual closure of the site. In FY 1986 investigations began in the areas of site hydrology, geochemistry, soils, geology, and geohydrologic model application. This report summarizes work carried out in each of these areas during FY's 1986 and 1987 and serves as a status report pulling together the large volume of data that has resulted. Characterization efforts are by no means completed; however, a sufficient data base has been generated to begin data interpretation and analysis of site contaminants.

  20. Summary of environmental characterization activities at the Oak Ridge National Laboratory Solid Waste Storage Area Six, FY 1986 through 1987

    SciTech Connect

    Davis, E.C.; Solomon, D.K.; Dreier, R.B.; Lee, S.Y.; Kelmers, A.D.; Lietzke, D.A.; Craig, P.M.

    1987-09-30

    The Oak Ridge National Laboratory (ORNL) Remedial Action Program (RAP), has supported characterization activities in Solid Waste Storage Area (SWSA 6) to acquire information necessary for identification and planning of remedial actions that may be warranted, and to facilitate eventual closure of the site. In FY 1986 investigations began in the areas of site hydrology, geochemistry, soils, geology, and geohydrologic model application. This report summarizes work carried out in each of these areas during FY`s 1986 and 1987 and serves as a status report pulling together the large volume of data that has resulted. Characterization efforts are by no means completed; however, a sufficient data base has been generated to begin data interpretation and analysis of site contaminants.

  1. Summary of activities of the life cycle costing workshop conducted by the Environmental Restoration Program of Oak Ridge National Laboratory

    SciTech Connect

    Not Available

    1992-08-01

    A five-day life cycle workshop was conducted by the Environmental Restoration (FR) Program of Oak Ridge National Laboratory (ORNL) to develop appropriate remediation scenarios for each Waste Area Grouping (WAG) at ORNL and to identify associated data needs (e.g., remedial investigations, special studies, and technology demonstrations) and required interfaces. Workshop participants represented the Department of Energy, Martin Marietta Energy Systems, Inc., Bechtel National, Radian Corporation, EBASCO Corporation, and M-K Ferguson. The workshop was used to establish a technical basis for remediation activities at each WAG. The workshop results are documented in this report and provide the baseline for estimating the technical scope for each WAG. The scope and associated budgets and schedules will be summarized in baseline reports for each WAG, which, in turn, will be compiled into an overall strategy document for ORNL ER.

  2. Seismic reflection images of a near-axis melt sill within the lower crust at the Juan de Fuca ridge.

    PubMed

    Canales, J Pablo; Nedimović, Mladen R; Kent, Graham M; Carbotte, Suzanne M; Detrick, Robert S

    2009-07-01

    The oceanic crust extends over two-thirds of the Earth's solid surface, and is generated along mid-ocean ridges from melts derived from the upwelling mantle. The upper and middle crust are constructed by dyking and sea-floor eruptions originating from magma accumulated in mid-crustal lenses at the spreading axis, but the style of accretion of the lower oceanic crust is actively debated. Models based on geological and petrological data from ophiolites propose that the lower oceanic crust is accreted from melt sills intruded at multiple levels between the Moho transition zone (MTZ) and the mid-crustal lens, consistent with geophysical studies that suggest the presence of melt within the lower crust. However, seismic images of molten sills within the lower crust have been elusive. Until now, only seismic reflections from mid-crustal melt lenses and sills within the MTZ have been described, suggesting that melt is efficiently transported through the lower crust. Here we report deep crustal seismic reflections off the southern Juan de Fuca ridge that we interpret as originating from a molten sill at present accreting the lower oceanic crust. The sill sits 5-6 km beneath the sea floor and 850-900 m above the MTZ, and is located 1.4-3.2 km off the spreading axis. Our results provide evidence for the existence of low-permeability barriers to melt migration within the lower section of modern oceanic crust forming at intermediate-to-fast spreading rates, as inferred from ophiolite studies.

  3. Metopic ridge

    MedlinePlus

    ... infant is made up of bony plates. The gaps between the plates allow for growth of the skull. The places where these plates connect are called sutures or suture lines. They do not fully close until the 2nd or 3rd year of life. A metopic ridge occurs when the ...

  4. Tank waste treatment R and D activities at Oak Ridge National Laboratory

    SciTech Connect

    Jubin, R.T.; Lee, D.D.; Beahm, E.C.; Collins, J.L.; Davidson, D.J.; Egan, B.Z.; Mattus, A.J.; Walker, J.F. Jr.

    1997-08-01

    Oak Ridge National Laboratory (ORNL) served as the pilot plant for the Hanford production facility during the 1940s. As a result, the waste contained in the ORNL storage tanks has similarities to waste found at other sites, but is typically 10 to 100 times less radioactive. It is estimated that nearly 4.9 million liters of legacy of waste is stored on the site of ORNL. Of this volume about one-fifth is transuranic sludges. The remainder of the waste volume is classified as low-level waste. The waste contains approximately 130,000 Ci, composed primarily of {sup 137}Cs, {sup 90}Sr, and small amounts of other fission products. The wastes were originally acidic in nature but were neutralized using Na{sub 2}CO{sub 3}, NaOH, or CaO to allow their storage in tanks constructed of carbon steel or concrete (Gunite). In addition to the legacy waste, about 57,000 L of concentrated waste is generated annually, which contains about 13,000 Ci, consisting primarily of {sup 137}Cs, {sup 90}Sr, and small amounts of other fission products. As part of the US department of Energy`s (DOE`s) Environmental Management Tanks Focus Area and Efficient Separations and Processing programs, a number of tasks are under way at ORNL to address the wastes currently stored in tanks across the DOE complex. This paper summarizes the efforts in three of these tasks: (1) the treatment of the tank supernatant to remove Cs, Tc, and Sr; (2) the leaching or washing of the sludges to reduce the volume of waste to be vitrified; and (3) the immobilization of the sludges.

  5. Mantle flow geometry from ridge to trench beneath the Gorda-Juan de Fuca plate system

    NASA Astrophysics Data System (ADS)

    Martin-Short, Robert; Allen, Richard M.; Bastow, Ian D.; Totten, Eoghan; Richards, Mark A.

    2015-12-01

    Tectonic plates are underlain by a low-viscosity mantle layer, the asthenosphere. Asthenospheric flow may be induced by the overriding plate or by deeper mantle convection. Shear strain due to this flow can be inferred using the directional dependence of seismic wave speeds--seismic anisotropy. However, isolation of asthenospheric signals is challenging; most seismometers are located on continents, whose complex structure influences the seismic waves en route to the surface. The Cascadia Initiative, an offshore seismometer deployment in the US Pacific Northwest, offers the opportunity to analyse seismic data recorded on simpler oceanic lithosphere. Here we use measurements of seismic anisotropy across the Juan de Fuca and Gorda plates to reconstruct patterns of asthenospheric mantle shear flow from the Juan de Fuca mid-ocean ridge to the Cascadia subduction zone trench. We find that the direction of fastest seismic wave motion rotates with increasing distance from the mid-ocean ridge to become aligned with the direction of motion of the Juan de Fuca Plate, implying that this plate influences mantle flow. In contrast, asthenospheric mantle flow beneath the Gorda Plate does not align with Gorda Plate motion and instead aligns with the neighbouring Pacific Plate motion. These results show that asthenospheric flow beneath the small, slow-moving Gorda Plate is controlled largely by advection due to the much larger, faster-moving Pacific Plate.

  6. Age, Episodicity and Migration of Hydrothermal Activity within the Axial Valley, Endeavour Segment, Juan de Fuca Ridge

    NASA Astrophysics Data System (ADS)

    Jamieson, J. W.; Hannington, M. D.; Kelley, D. S.; Clague, D. A.; Holden, J. F.; Tivey, M. K.; Delaney, J. R.

    2011-12-01

    Hydrothermal sulfide deposits record the history of high-temperature venting along the Endeavour Segment. Active venting is currently located within five discreet vent fields, with minor diffuse venting occurring between the fields. However, inactive and/or extinct sulfide structures are found throughout the entire axial valley of the ridge segment, suggesting that hydrothermal activity has been more vigorous in the past or focused venting has migrated with time. Here, we present age constraints from U-series dating of 44 sulfide samples collected by manned submersible from between the Mothra Field in the south to Sasquatch in the north. Samples are dated using 226Ra/Ba ratios from hydrothermal barite that precipitates along with the sulfide minerals. Most samples have been collected from within or near the active vent fields. Fifteen samples from the Main Endeavour Field (MEF) show a spectrum of ages from present to 2,430 years old, indicating that this field has been continuously active for at least ~2,400 years. MEF appears to be oldest currently active field. This minimum value for the age of hydrothermal activity also provides a minimum age of the axial valley itself. Ages from thirteen samples from the High-Rise Field indicate continuous venting for at least the past ~1,250 years. These age data are used in conjunction with age constraints of the volcanic flows to develop an integrated volcanic, hydrothermal and tectonic history of the Endeavour Segment. The total volume of hydrothermal sulfide within the axial valley, determined from high-resolution bathymetry, is used in conjunction with the age constraints of the sulfide material to determine the mass accumulation rates of sulfide along the Endeavour Segment. These data can be used to calibrate the efficiency of sulfide deposition from the hydrothermal vents, and provide a time-integrated history of heat, fluid and chemical fluxes at the ridge-segment scale. The comparison of time-integrated rates with

  7. Short-lived Radium Isotopes in the Hawaiian Margin: Evidence for Large Fluid Fluxes Through the Puna Ridge

    NASA Astrophysics Data System (ADS)

    Moore, W. S.; Paull, C. K.; Ussler, W.

    2001-12-01

    were considerably greater than 224Ra in spite of the expected higher production rate of 224Ra from basalt. 223Ra was not supported by dissolved 227Ac. The highest enrichments of 223Ra were measured over the Puna Ridge (2100 m depth) east of Hawaii. Here 223Ra activities reached 2 dpm/100L, similar to activities measured near sites of active submarine groundwater discharge in the South Atlantic Bight. The high 223Ra values were not associated with significant thermal anomalies. To explain the high activities of 223Ra unaccompanied by 224Ra, we postulate that thermally-driven circulation of sea water through the Puna Ridge deposits 231Pa on basalt surfaces. With time the 231Pa produces 227Ac and 223Ra, which desorbs into circulating fluids. These fluids then transport 223Ra into the overlying ocean. Based on the inventory of 223Ra above the Puna Ridge, we estimate the flow of fluids through the ridge to be on the order of 40cm3cm-2day-1. In less than 100 years the incoming seawater could provide enough 231Pa to basalt surfaces to balance the inventory of 223Ra above the ridge if all of the 223Ra was transported to the overlying water. These observations have significant implications for quantifying fluid fluxes from the flanks of the mid ocean ridge. By mapping 223Ra inventories in the ocean above ridge flanks and the activity of 223Ra in the emerging fluids, the fluid flux can be obtained. These measurements could help resolve the debate of the relative importance of high and low temperature venting from the ridge.

  8. Using glacial morphology to constrain the impact of the Chile active spreading ridge subduction in Central Patagonia

    NASA Astrophysics Data System (ADS)

    Scalabrino, B.; Ritz, J. F.; Lagabrielle, Y.

    2009-04-01

    The Central Patagonian Cordillera is a unique laboratory to study interaction between oceanic and continental lithospheres during the subduction of an active spreading ridge beneath a continent. The subduction of the South Chile spreading Ridge, which separates the Nazca plate from the Antarctic plate, started ca. 15-14 Ma at the southern tip of Patagonia (55°S latitude). The northwards migration of the Chile Triple Junction induces the subduction of several segments especially around 46°S latitude. There, three segments subducted at ca. 6, 3 and 0.3 Ma, leading to the formation of a large asthenospheric slab-window beneath Central Patagonia. Contemporaneously, the Central Patagonia reliefs are undergoing major glacial events since at least 7 Ma. These events are evidenced to the east of the Central Patagonian morphotectonic front within perched relict surfaces. Inset in these perched glacial surfaces are found mid-Pleistocene glacial valleys, as the Lake General Carrera-Buenos Aires amphitheatre (LGCBA), which formed between 1.1 Ma and 16 ka. We used the relationships between the glacial valleys and the volcanism associated with the asthenospheric slab-window to better constraints the structural evolution of the Patagonian Cordillera related to the subduction of the Chili active spreading Ridge. The present work focused within two well-preserved perched flat surfaces named Meseta del Lago Buenos Aires and Meseta del Cerro Galera: (i) The meseta del Lago Buenos Aires defines a plateau made of interbedded units of tills and lavas dated between 12 Ma and 3 Ma. The top surface of the meseta, ˜2000 meters high is dated at 3 Ma, and is shaped by four NE-SW trending glacial lobes characterized with kettles, lineations and moraines. The glacial valleys are beheaded westwards and define perched valleys 200 to 400 meters higher than the western Cordillera. This suggests recent vertical movement along N160 extensive/transtensive corridor located between the morphotectonic

  9. Seismicity and active accretion processes at the ultraslow-spreading Southwest and intermediate-spreading Southeast Indian ridges from hydroacoustic data

    NASA Astrophysics Data System (ADS)

    Tsang-Hin-Sun, Eve; Royer, Jean-Yves; Perrot, Julie

    2016-05-01

    Volcanic and tectonic events are the main processes involved in the generation of the oceanic crust and responsible for the seismicity associated with seafloor spreading. To monitor this activity, usually not or poorly detected by land-based seismological stations, we deployed from February 2012 to February 2013 a network of autonomous hydrophones to compare the behaviour of the ultraslow-spreading Southwest (SWIR) with that of the intermediate-spreading Southeast Indian ridges (SEIR). The rate of seismicity is similar for both ridges, suggesting that there is no systematic relationship between seismicity and spreading rates. The along-axis distribution of the seismic events, however, does differ, reflecting the rate-dependence of accretion modes. Earthquakes are sparse and regularly spaced and scattered along the SWIR, reflecting prevailing tectonic processes. By contrast, along the SEIR, events are irregularly distributed and focus at ridge-segment ends and transforms faults, reflecting the ridge segmentation; only two swarms occurred at a segment centre and are probably caused by a magmatic event. This seismicity distribution thus looks controlled by segment-scale crustal heterogeneities along the SEIR and by regional-scale contrasting accretion processes along the SWIR, probably driven by different lithospheric and asthenospheric dynamics on either side of the Melville FZ. The comparison of hydroacoustic and teleseismic catalogues shows that, along these spreading ridges, the background seismicity observed in one year by a hydroacoustic network is representative of the seismicity observed over two decades by land-based networks.

  10. Moderation of Neogene Deep-Water Overflow at the Greenland-Scotland Ridge by the Icelandic Plume

    NASA Astrophysics Data System (ADS)

    Henstock, T.; White, N. J.; Jones, S. M.; Murton, B. J.; Maclennan, J.

    2010-12-01

    A global analysis of Neogene δ13C records shows that overflow of Northern Component Water (NCW), the ancient precursor of North Atlantic Deep Water, varies with time. It is generally accepted that the Icelandic plume, which has dominated the tectonic evolution of the North Atlantic Realm since its inception ˜60 million years ago, has played a key role in moderating deep-water overflow. The V-shaped ridges, which straddle the mid-oceanic ridge system on either side of Iceland, are an important window into transient convective circulation associated with this plume. Knowledge of the evolving pattern of convective circulation can be used to constrain vertical displacement of the Greenland-Scotland Ridge through time. Although some progress has been made in refining the variation of NCW with time, any link between overflow and convective circulation has relied upon vintage seismic reflection profiles acquired in the 1960s. In order to construct a more accurate chronology of plume activity through time, we have acquired a set of regional seismic reflection profiles along flowlines which traverse the Iceland and Irminger basins between 60° and 62° N. We used a single generator-injector airgun operating at 3,000 p.s.i. and a 2 km streamer with a group spacing of 12.5 m with an average fold of 20. The porcessed seismic images are excellent. The sediment-basement interface can be accurately mapped and fine details of the sedimentary cover are resolved. We have identified a series of V-shaped ridges and erected a chronology of plume activity on an astronomical timescale. This chronology correlates with the Neogene history of deep-water overflow and confirms that convective circulation of the mantle has played a significant role in moderating deep-water overflow. Our seismic profiles also cross the major contourites of the North Atlantic Ocean, notably the Gardar, Bjorn and Eirik Drifts. Excellent images of their detailed internal stratigraphy demonstrate that distinct

  11. Contrasting origins of the upper mantle revealed by hafnium and lead isotopes from the Southeast Indian Ridge.

    PubMed

    Hanan, Barry B; Blichert-Toft, Janne; Pyle, Douglas G; Christie, David M

    2004-11-01

    The origin of the isotopic signature of Indian mid-ocean ridge basalts has remained enigmatic, because the geochemical composition of these basalts is consistent either with pollution from recycled, ancient altered oceanic crust and sediments, or with ancient continental crust or lithosphere. The radiogenic isotopic signature may therefore be the result of contamination of the upper mantle by plumes containing recycled altered ancient oceanic crust and sediments, detachment and dispersal of continental material into the shallow mantle during rifting and breakup of Gondwana, or contamination of the upper mantle by ancient subduction processes. The identification of a process operating on a scale large enough to affect major portions of the Indian mid-ocean ridge basalt source region has been a long-standing problem. Here we present hafnium and lead isotope data from across the Indian-Pacific mantle boundary at the Australian-Antarctic discordance region of the Southeast Indian Ridge, which demonstrate that the Pacific and Indian upper mantle basalt source domains were each affected by different mechanisms. We infer that the Indian upper-mantle isotope signature in this region is affected mainly by lower continental crust entrained during Gondwana rifting, whereas the isotope signature of the Pacific upper mantle is influenced predominantly by ocean floor subduction-related processes.

  12. An authoritative global database for active submarine hydrothermal vent fields

    NASA Astrophysics Data System (ADS)

    Beaulieu, Stace E.; Baker, Edward T.; German, Christopher R.; Maffei, Andrew

    2013-11-01

    The InterRidge Vents Database is available online as the authoritative reference for locations of active submarine hydrothermal vent fields. Here we describe the revision of the database to an open source content management system and conduct a meta-analysis of the global distribution of known active vent fields. The number of known active vent fields has almost doubled in the past decade (521 as of year 2009), with about half visually confirmed and others inferred active from physical and chemical clues. Although previously known mainly from mid-ocean ridges (MORs), active vent fields at MORs now comprise only half of the total known, with about a quarter each now known at volcanic arcs and back-arc spreading centers. Discoveries in arc and back-arc settings resulted in an increase in known vent fields within exclusive economic zones, consequently reducing the proportion known in high seas to one third. The increase in known vent fields reflects a number of factors, including increased national and commercial interests in seafloor hydrothermal deposits as mineral resources. The purpose of the database now extends beyond academic research and education and into marine policy and management, with at least 18% of known vent fields in areas granted or pending applications for mineral prospecting and 8% in marine protected areas.

  13. Microearthquakes at the active Trans-Atlantic Geotraverse (TAG) hydrothermal mound, Mid-Atlantic Ridge, 26°08'N

    NASA Astrophysics Data System (ADS)

    Pontbriand, C.; Reves-Sohn, R. A.

    2010-12-01

    A small 200 m aperture network of five ocean bottom seismometers around the periphery the active TAG hydrothermal mound on the Mid-Atlantic Ridge (26°08’N) detected microearthquake events that may be associated with the subsurface hydraulics of the massive hydrothermal deposit. Seismic data were sampled at 100 Hz for a period of eight months spanning June, 2003 to February, 2004, during which time 24,191 locatable events were detected. Microearthquake hypocenters are concentrated within a 300 m radius of the sulfide mound in the top 250 m of crust, and exhibit a conical shape with the deepest events beneath the mound center. Event rates are steady at 180 events per day at the beginning of the study period and decline slightly to 116 events per day after whale calls elevate background noise levels about 2/3 of the way through the deployment. The mean local magnitude of events is -1.2 with a range of -2.9≦ML≦0.3. We suggest that events may be largely due to hydraulic fracturing of clogged flow conduits in the mineral deposit, which provides the possibility of using the microearthquake data to constrain subsurface flow parameters and the permeability structure of the active TAG deposit. Figure: A bathymetric map of the TAG area depicts a small aperture network of 5 ocean bottom seismometers (white triangles) around the periphery of the active TAG hydrothermal mound. High resolution bathymetry is from Roman and Singh, 2005.

  14. Zinc, copper, and lead geochemistry of oceanic igneous rocks - ridges, islands, and arcs

    USGS Publications Warehouse

    Doe, B.R.

    1995-01-01

    Variations in the abundances of Zn, Cu, and Pb are found to be useful in identifying tectonic regimes and separating ocean-island basalts into enriched- and depleted-source categories. The average Zn, Cu, and Pb contents of normal mid-ocean ridge basalts (N-MORB) are 84, 70, and 0.35 ppm, respectively. Differences in average Zn contents for various ridges reflect more the varying degrees of differentiation than variations of Zn content in the source rocks. At a Mg# of 70, or Mg#70, which is taken to represent primitive MORB, many MORB sequences converge at a Zn content of 58??6 ppm, which is close to the value for primitive mantle (50 ppm) and ordinary chondrites (~55 ppm). -from Author

  15. Gabbroic xenoliths from the northern Gorda Ridge: implications for magma chamber processes under slow spreading centers

    USGS Publications Warehouse

    Davis, A.S.; Clague, D.A.

    1990-01-01

    Abundant gabbroic xenoliths in porphyritic pillow basalt were dredged from the northern Gorda Ridge. The host lava is a moderately fractionated, normal mid-ocean ridge basalt with a heterogeneous glass rind (Mg numbers 56-60). Other lavas in the vicinity range from near primary (Mg number 69) to fractionated (Mg number 56). On the basis of textures and mineral compositions, the xenoliths are divided into five types. The xenoliths are not cognate to the host lava, but they are genetically related. Chemistry of mineral phases in conjunction with textural features suggests that the xenoliths formed in different parts of a convecting magma chamber that underwent a period of closed system fractionation. The chamber was filled with a large proportion of crystalline mush when new, more primitive, and less dense magma was injected and mixed incompletely with the contents in the chamber, forming the hybrid host lava. -from Authors

  16. Biological and Geological Characteristics of the Gakkel Ridge

    NASA Astrophysics Data System (ADS)

    Shank, T. M.; Bailey, J.; Edmonds, H.; Forte, P.; Helmke, E.; Humphris, S.; Kemp, J.; Nakamura, K.; Reves-Sohn, R.; Singh, H.; Willis, C.

    2007-12-01

    The Gakkel Ridge (Arctic Ocean) is one of the slowest (1.0 cm per yr), deepest (5000 m axial depth), and most hydrographically and tectonically isolated mid-ocean ridge systems on earth. This isolation from the global ridge system should have profound implications for the evolution and ecology of resident chemosynthetic fauna. The July 2007 Arctic GAkkel Vents Expedition (AGAVE) sought to define this Arctic biogeographic province and the relationship of Arctic vent fauna to Atlantic, Pacific, and hydrocarbon seep fauna through the use of an new under- ice vehicle `Camper', a fiber-optic video-guided sampling system drift towed 1 to 3 m above the seafloor. The imaging, sampling, and sensing capabilities were used to obtain high-resolution seafloor imagery to identify and collect benthic samples with a clamshell `grab' sampler and a suction 'slurp' sampler. Imagery from five video cameras, including obliquely-mounted video and downlooking digital high-definition color cameras were used to construct maps of seafloor features and faunal composition during 3 dives in the peridotite-hosted 7°E region and 13 dives in the volcanic 85°E region. The 7°E site was dominated by an almost continuous cover of pelagic sediment with abundant animal tracks, brittle stars, anemones, and shrimp. The explored 85°E area was dominated by relatively diverse and young lava morphologies- from large pillows hosting delicate surface ornamentation to lobates, long lava tubes, and fresh sheet flows, all with the upper surfaces covered (often cm thick) of fresh volcanic glass 'sediment' suggestive of explosive volcanic activity in the `recent' past. Fauna in these areas consisted mainly of sponges, anemones, amphipods and shrimp. Characterization of the newly-discovered Asgard volcanic chain, including `Oden', `Thor', and `Loke' volcanoes, in the 85°E axial valley revealed extensive microbial mats in the form of: 1) yellow `fluffy' material (often >5 cm thick) in places; and 2) yellow

  17. Inter-Rifting and Inter-Seismic Strain Accumulation in a Propagating Ridge System: A Geodetic Study from South Iceland

    NASA Astrophysics Data System (ADS)

    Travis, M. E.; La Femina, P. C.; Geirsson, H.

    2012-12-01

    The Mid-Atlantic Ridge, a slow spreading (~19 mm/yr) mid-ocean ridge boundary between the North American and Eurasian plates, is exposed subaerially in Iceland as the result of ridge-hotspot interaction. Plate spreading in Iceland is accommodated along neovolcanic zones comprised of central volcanoes and their fissure swarms. In south Iceland plate motion is partitioned between the Western Volcanic Zone (WVZ) and Eastern Volcanic Zone (EVZ). The EVZ is propagating to the southwest, while the WVZ is dying out from the northeast. Plate motion across both systems has been accommodated by repeated rifting events and fissure eruptions. In this study we investigate whether the WVZ is active and accumulating strain, and how strain is partitioned between the WVZ and EVZ. We also test how strain is accumulating along fissure swarms within the EVZ (i.e. is strain accumulation localized to one fissure swarm, or are multiple systems active?). We use GPS data and elastic block models run using the program DEFNODE to investigate these issues. GPS data are processed using the GIPSY-OASIS II software, and have been truncated to the 2000.5-2011 time period to avoid co-seismic displacement from the two June 2000 South Iceland Seismic Zone earthquakes. We also truncate the time series for sites within 20 km of Eyjafjallajökull to the beginning of 2010 to eliminate deformation associated with the March 2010 eruption of that volcano. We correct for co-seismic displacement from the two May 2008 SISZ earthquakes, inflation at Hekla volcano and the horizontal component of glacial isostatic rebound (GIA). Our best-fit model for inter-rifting and inter-seismic elastic strain accumulation suggests 80-90% of spreading is accommodated in the EVZ with the other 10-20% accommodated by the WVZ. The best-fit location of the EVZ is between Veidivotn and Lakigigar in an area of no Holocene volcanic activity. We suggest the WVZ is only active at Hengill and its associated fissure swarm. Geologic and

  18. FY 1994 annual summary report of the surveillance and maintenance activities for the Oak Ridge National Laboratory Environmental Restoration Program

    SciTech Connect

    Not Available

    1994-11-01

    The Oak Ridge National Laboratory (ORNL) Environmental Restoration (ER) Surveillance and Maintenance (S and M) Program was initiated to manage former waste management and environmental research sites contaminated with radioactive materials and/or hazardous chemicals. The S and M Program is responsible for managing designated sites/facilities from the end of their operating lives until final disposition or site stabilization. To effectively manage and perform the various S and M Program responsibilities, five summary-level work breakdown structure (WBS) elements have been established: S and M Preliminary Investigations, Special Projects, Routine S and M, Inactive Groundwater Wells, and Project Management. Routine S and M activities were conducted as scheduled throughout fiscal years (FY) 1994 at applicable inactive waste management (WM) and other contaminated areas. Overall, the ER S and M Program maintains 47 facilities, performs vegetation maintenance on approximately 230 acres, maintains 54 inactive tanks, and provides overall site management on over 700 acres. In addition to the routine S and M activities, detailed site inspections were conducted at established frequencies on appropriate sites in the ER S and M Program. This document provides a summary of the FY 1994 ORNL ER S and M Program accomplishments.

  19. 1993 annual report of hazardous waste activities for the Oak Ridge K-25 site

    SciTech Connect

    Not Available

    1994-02-01

    This report is a detailed listing of all of the Hazardous Waste activities occurring at Martin Marietta`s K-25 site. Contained herein are hazardous waste notification forms, waste stream reports, generator fee forms and various TSDR reports.

  20. Southeast Indian Ocean-Ridge Earthquake Sequences from Cross-correlation Analysis of Hydro-acoustic Data

    NASA Astrophysics Data System (ADS)

    Yun, S.; Ni, S.; Park, M.

    2006-12-01

    Earthquake sequences (location and timing of foreshocks and aftershocks) are critical for understanding dynamics of mid-ocean ridge and transform faults. Unfortunately whole sequences (including very small earthquakes) in the ocean can not be well recorded by land-based seismometers mostly because of large epicentral distances. Recent hydro-acoustic studies have demonstrated that T waves are very effective in detecting small submarine earthquakes because of little energy loss of T waves propagating in SOFAR channel. For example, a Mw6.2 (03/06/2006, Latitude -40.11, Longitude 78.49) transform earthquake occurred at the Southeastern Indian Ocean Ridge (an intermediate spreading rate ridge, 58-76 mm/year), but NEIC only reports 3 aftershocks in the first following week. We applied progressive multi-channel cross-correlation methods to hydro-acoustic data from the IMS arrays in Indian Ocean to detect the whole earthquake sequence. We also correlate waveform envelopes to accurately locate aftershocks and found consistent pattern of earthquake migration along the transform fault. In contrast to transform fault earthquake sequences at fast spreading ridge (East Pacific Rise, 142 mm/year) where foreshocks are observed, we failed to detect any foreshocks for the Mw6.2 earthquake though we found many aftershocks. The lack of foreshocks may be caused by lower spreading rate (hence lower temperature) or too small scale ridge segmentation. Still the number of aftershocks is much less than that of typical tectonic earthquake such as subduction or continental earthquakes, arguing different fault dynamics for mid ocean ridge systems, perhaps due to higher water content or presence of melt.