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Reid, William David Kenneth. "Trophodynamics on mid-ocean ridges." Thesis, University of Newcastle Upon Tyne, 2012. http://hdl.handle.net/10443/1744.
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The global mid-ocean ridge (MOR) system is ~60 000 km long and accounts for 9% of the seafloor. Deep-sea organisms living on MOR have two potential energy sources; chemosynthesis and the downward flux of photosynthetic organic matter. This study examines the trophodynamics of benthic fauna collected from non-vent sites north and south of the Charlie-Gibb Fracture Zone (CGFZ) on the Mid-Atlantic Ridge (MAR) and hydrothermal vents fields (E2 and E9) on the East Scotia Ridge (ESR) using stable isotopes of carbon (δ13C), nitrogen (δ15N) and sulphur (δ34S). δ13C and δ34S values revealed the MAR benthos was sustained by photosynthetic primary production and no chemosynthetic food source was detected. δ15N values of benthic invertebrates were lower than the surficial sediments at the southern site but this did not occur at the northern site. Benthic invertebrates appeared to comprise a separate food chain to bentho-pelagic fishes and crustaceans but size-based trends in δ13C and δ15N revealed at certain life history stages bentho-pelagic fishes may consume benthic fauna. Size-based trends in δ13C and δ15N trends varied spatially and temporally in some bentho-pelagic fishes, which suggested differences in feeding plasticity among the species. Spatial differences among sites were observed in δ13C, δ15N and δ34S of the ESR vent fauna. These were thought to reflect differences in the vent fluid chemistry, vent derived carbon fixation pathways and incorporation of photosynthetic organic matter into the vent system depending on the species and the magnitude of the difference among sites. Size and sex were important determinants of intra-population variability in stable isotope values of three species of vent fauna but this was not consistent among sites. Abstract ii The present study revealed the importance of undertaking a tri-isotope approach to deep-sea trophic studies in order to elucidate production sources and at different sizes deep-sea organisms can link different trophic pathways.
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Morgan, Jennifer Patricia. "Constructive volcanic processes at mid-ocean ridges." Thesis, University of Leeds, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.426820.
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Dusunur, Doga. "Thermal structure of Mid-Ocean Ridges (Lucky Strike, Mid-Atlantic Ridge) and magma chambers." Paris, Institut de physique du globe, 2008. http://www.theses.fr/2008GLOB0002.
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Modéliser la structure thermique de la croûte océanique est une étape importante pour comprendre la création de la croûte océanique qui couvre les deux tiers de la surface de notre planète. La production magmatique à l'axe de la dorsale et le refroidissement de la lithosphère océanique en fonction du temps et de l'espace sont des facteurs clés pour comprendre la création de la croûte océanique, qui est déterminée en premier lieu par la structure thermique des dorsales. Les modèles thermiques classiques ne prédisent pas l'existence de chambres magmatiques stationnaires le long des dorsales qui ont un taux d'expansion inférieur à 30 mm/an. L'identification par imagerie sismique d'une chambre magmatique sous le volcan et le champ hydrothermal de Lucky Strike sur la dorsale médio-Atlantique représente une opportunité unique d'étudier la structure thermique des dorsales lentes. Nous présentons ici conjointement des données microsismiques et une modélisation thermique qui nous permettent d'éclaircir la nature des chambres magmatiques éphémères aux dorsales lentes, de contraindre les échelles de temps associées aux changements dans l'alimentation magmatique et les paramètres nécessaires à l'existence d'une chambre magmatique, et nous renseignent sur les différents mécanismes qui peuvent conduire au refroidissement et à la disparition d'une chambre magmatique. Les résultats de l'analyse microsismique, de la modélisation thermique et les contraintes temporelles dérivées des contraintes géologiques suggèrent qu'il existe un apport focalisé au centre du segment, et ce de façon régulière, et que cet apport est maintenu sur des périodes de temps étendu, pouvant conduire à une chambre magmatique durable. Cette thèse, tout en traitant des processus actifs au segment Lucky Strike, fournit un modèle plus général pour comprendre et étudier d'autres segments de dorsales lentes, et comprendre comment se forme la croûte océanique le long de ces segmentsModeling the thermal state of the oceanic crust is an important task to understand the construction of oceanic crust which covers two thirds of the surface of our planet. The interplays among magma delivery to the axis and cooling of the oceanic lithosphere as a function of both space and time are key factors to understand the creation of the oceanic crust, which is primarily determined by the overall thermal structure of mid-ocean ridges. Classical thermal models do not predict steady state axial magma chambers (AMCs) along mid-ocean ridges at spreading rates less than 30 mm/year. The identification and seismic imaging of an axial magma chamber underlying the Lucky Strike central volcano and hydrothermal field at the Mid-Atlantic Ridge provides a unique opportunity to study the thermal structure of slow spreading ridges. Here we present coupled microseismic data and thermal modeling to provide insight on the nature of ephemeral magma chambers at slow-spreading ridges, to constrain the timescales associated with changes in melt supply and the parameters that can create them, and to shed light on the different mechanisms that can result on the cooling and disappearance of these structures. Both the coupled microseismic and thermal modeling results, and the time-constraints derived from the geological constraints put forward, suggest that focused melt supply to the segment center is required regularly, and that this supply is maintained over extended periods of time, that can lead to a durable magma chamber. This thesis, while focusing on the processes occurring at the Lucky Strike, provide a more general template to both understand and study other slow-spreading ridge segments, and to gain insight on how the oceanic crust is formed along them
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Turner, Ian Mark. "Crustal accretionary processes at mid-ocean ridges - Valu Fa Ridge, Lau Basin." Thesis, Durham University, 1998. http://etheses.dur.ac.uk/5002/.
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The structure of oceanic crust is surprisingly uniform, which suggests that crustal accretionary processes at mid-ocean ridges must be broadly similar, despite their different spreading rates and seafloor morphologies. Seismic studies have revealed the presence of sub-axial magma chambers at fast, slow and intermediate spreading ridges, but constraints on their shape and size are generally restricted to the fast spreading East Pacific Rise. The aim of this study is to compare the processes of crustal accretion at fast, slow and intermediate ridges by investigating the detailed crustal structure and magma chamber geometry of a magmatically active intermediate spreading ridge, the Valu Fa Ridge. A multidisciplinary geophysical experiment was conducted over the Central Valu Fa Ridge and its overlap with the Northern Valu Fa Ridge during R/V Maurice Swing Cruise EW9512, and wide-angle seismic data, recorded on a set of digital ocean bottom seismometers, were used to generate velocity-depth models on two across-axis, two along-axis and two axis-parallel profiles. These models were further constrained by modelling of the normal incidence seismic and gravity data and the resulting combined models of crustal structure were interpreted to reveal that a composite magma chamber exists beneath the Valu Fa Ridge crest. The magma chamber consists of a thin, narrow (1-1.5 km) melt lens, with an interconnected melt fraction, overlying a wider (-4 km) region of hot rock or low melt fraction. A reflection from the top of the melt lens is identified on both the normal incidence seismic and wide-angle seismic data and delay- time modelling indicates that velocities as high as 5.5 km s(^-1) are achieved -250 m below the top of the melt lens. The main body of the magma chamber corresponds to the region of hot rock below the melt lens and is delineated by anomalously low velocities, extending down through seismic layer 3 to within 1.5-2 km of the Moho. Moho reflections from beneath the overlapping spreading centre and a low on the mantle Bouguer anomaly map implies that this region is currently, or has recently been, the site of enhanced magmatism. This observation is contrary to popular models of ridge segmentation and melt delivery. The transition from pre-rift crust (both island arc and back-arc crust) to post-rift material, marked by considerable thinning of seismic layer 2, has also been uniquely identified in this study and describes the limit of VFR-generated crust. The size and temporal stability of magma chambers are largely dependent on their magma budget and the Valu Fa Ridge magma chamber model, developed as part of this study, may bridge the gap between the large, long-lived magma chambers identified at the East Pacific Rise and the more transitory magma chambers proposed at slow spreading ridges. Melt ascends as small isolated pockets through the main body of the magma chamber at the Valu Fa Ridge and resides in the melt lens until eruption. Seismic layer 2 is constructed solely from material 'erupted' from the melt lens, with the main body of the magma chamber cooling to form seismic layer 3. Convection currents, induced by large thermal gradients at the sides of the magma chamber, both accelerate the cooling process, thus limiting its size, and helps to generate the thick layered sequences as observed in ophiolite studies. The entire crust is emplaced within the axial region and a distinct Moho is formed at -0 Ma.
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Scott, Jameson Lee. "Petrological Constraints on Magma Plumbing Systems along Mid-Ocean Ridges." The Ohio State University, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=osu1322599745.
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Pasini, Valerio. "Biopetrology of the hydrating mantle along mid ocean ridges." Paris, Institut de physique du globe, 2013. http://www.theses.fr/2013GLOB0901.
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Cooper, Matthew John. "Geochemical investigations of hydrothermal fluid flow at mid-ocean ridges." Thesis, University of Cambridge, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.441508.
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Neves, Maria C. "Models of stress at mid-ocean ridges and their offsets." Thesis, Durham University, 2000. http://etheses.dur.ac.uk/4408/.
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This thesis aims to investigate the stresses at mid-ocean ridge offsets, and particularly at the particular class of offsets represented by oceanic microplates. Amongthese, the Easter microplate is one of the best surveyed. This thesis first studies the stress field associated with mid-ocean ridges and simple types of ridge offsets, and then uses the stress field observed at Easter to constrain the driving mechanism of microplates. Two-dimensional finite element modelling is used to predict the lithospheric stress indicators, which are then compared with observations. Extensional structures at high angles (> 35 ) to ridge trends are often observed at ridge-transform intersections and non-tranform offsets, but remained unexplained until now. This study proposes that the topographic loading created by the elevation of mid-ocean ridges relative to old seafloor is a source of ridge parallel tensile stresses, and shows they can be explained by the rotation of ridge parallel tensile stresses at locked offsets. The elasto-plastic rheology is used to investigate the evolution of normal faults near mid-ocean ridges. It is shown that variations in the lithospheric strength, caused entirely by variations in the brittle layer thickness, can account for the observed variations in fault character with spreading rate and along-axis position. Plasticity is shown to prevent the achievement of large fault throws in thin brittle layers. Consequently, it may be important at fast spreading ridges. A new dynamic model is proposed for Easter microplate. It mainly consists of: 1) driving forces along the East and West Rifts, resulting from the combination of a regional tensile stress with an increasing ridge strength towards rift tips, 2) mantle basal drag resisting the microplate rotation, and contributing with less than 20% to the total resisting torque, and 3) resisting forces along the northern and southern boundaries. To explain both the earthquake focal mechanism evidence and theexistence of compressional ridges in the Nazca plate, the boundary conditions alongthe northern boundary are required to change with time, from completely locked tolocked in the normal direction only. This study does not invalidate the microplate kinematic model proposed by Schouten et al. (1993), but shows that normal resisting forces along the northern and southern boundaries of Easter microplate must exist in order to explain the stress observations. Also, it suggests that ridge strength variations play an important role in the dyamics of mid-ocean ridge overlap regions.
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Scott, Jameson Lee. "Towards a Petrologically Constrained Thermal Model of Mid-Ocean Ridges." The Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1496397674423802.
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Copley, Jonathan Timothy Peter. "Ecology of deep-sea hydrothermal vents." Thesis, University of Southampton, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.246235.
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Whittaker, Joanne. "Tectonic consequences of mid-ocean ridge evolution and subduction." Thesis, The University of Sydney, 2008. http://hdl.handle.net/2123/3971.
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Mid-ocean ridges are a fundamental but insufficiently understood component of the global plate tectonic system. Mid-ocean ridges control the landscape of the Earth's ocean basins through seafloor spreading and influence the evolution of overriding plate margins during midocean ridge subduction. The majority of new crust created at the surface of the Earth is formed at mid-ocean ridges and the accretion process strongly influences the morphology of the seafloor, which interacts with ocean currents and mixing to influence ocean circulation and regional and global climate. Seafloor spreading rates are well known to influence oceanic basement topography. However, I show that parameters such as mantle conditions and spreading obliquity also play significant roles in modulating seafloor topography. I find that high mantle temperatures are associated with smooth oceanic basement, while cold and/or depleted mantle is associated with rough basement topography. In addition spreading obliquities greater than > 45° lead to extreme seafloor roughness. These results provide a predictive framework for reconstructing the seafloor of ancient oceans, a fundamental input required for modelling ocean-mixing in palaeoclimate studies. The importance of being able to accurately predict the morphology of vanished ocean floor is demonstrated by a regional analysis of the Adare Trough, which shows through an analysis of seismic stratigraphy how a relatively rough bathymetric feature can strongly influence the flow of ocean bottom currents. As well as seafloor, mid-ocean ridges influence the composition and morphology of overriding plate margins as they are consumed by subduction, with implications for landscape and natural resources development. Mid-ocean ridge subduction also effects the morphology and composition of the overriding plate margin by influencing the tectonic regime experienced by the overriding plate margin and impacting on the volume, composition and timing of arc-volcanism. Investigation of the Wharton Ridge slab window that formed beneath Sundaland between 70 Ma and 43 Ma reveals that although the relative motion of an overriding plate margin is the dominant force effecting tectonic regime on the overriding plate margin, this can be overridden by extension caused by the underlying slab window. Mid-ocean ridge subduction can also affect the balance of global plate motions. A longstanding controversy in global tectonics concerns the ultimate driving forces that cause periodic plate reorganisations. I find strong evidence supporting the hypothesis that the plates themselves drive instabilities in the plate-mantle system rather than major mantle overturns being the driving mechanism. I find that rapid sub-parallel subduction of the Izanagi mid-ocean ridge and subsequent catastrophic slab break o_ likely precipitated a global plate reorganisation event that formed the Emperor-Hawaii bend, and the change in relative plate motion between Australia and Antarctica at approximately 50 Ma
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Whittaker, Joanne. "Tectonic consequences of mid-ocean ridge evolution and subduction." University of Sydney, 2008. http://hdl.handle.net/2123/3971.
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Doctor of Philosophy(PhD)Mid-ocean ridges are a fundamental but insufficiently understood component of the global plate tectonic system. Mid-ocean ridges control the landscape of the Earth's ocean basins through seafloor spreading and influence the evolution of overriding plate margins during midocean ridge subduction. The majority of new crust created at the surface of the Earth is formed at mid-ocean ridges and the accretion process strongly influences the morphology of the seafloor, which interacts with ocean currents and mixing to influence ocean circulation and regional and global climate. Seafloor spreading rates are well known to influence oceanic basement topography. However, I show that parameters such as mantle conditions and spreading obliquity also play significant roles in modulating seafloor topography. I find that high mantle temperatures are associated with smooth oceanic basement, while cold and/or depleted mantle is associated with rough basement topography. In addition spreading obliquities greater than > 45° lead to extreme seafloor roughness. These results provide a predictive framework for reconstructing the seafloor of ancient oceans, a fundamental input required for modelling ocean-mixing in palaeoclimate studies. The importance of being able to accurately predict the morphology of vanished ocean floor is demonstrated by a regional analysis of the Adare Trough, which shows through an analysis of seismic stratigraphy how a relatively rough bathymetric feature can strongly influence the flow of ocean bottom currents. As well as seafloor, mid-ocean ridges influence the composition and morphology of overriding plate margins as they are consumed by subduction, with implications for landscape and natural resources development. Mid-ocean ridge subduction also effects the morphology and composition of the overriding plate margin by influencing the tectonic regime experienced by the overriding plate margin and impacting on the volume, composition and timing of arc-volcanism. Investigation of the Wharton Ridge slab window that formed beneath Sundaland between 70 Ma and 43 Ma reveals that although the relative motion of an overriding plate margin is the dominant force effecting tectonic regime on the overriding plate margin, this can be overridden by extension caused by the underlying slab window. Mid-ocean ridge subduction can also affect the balance of global plate motions. A longstanding controversy in global tectonics concerns the ultimate driving forces that cause periodic plate reorganisations. I find strong evidence supporting the hypothesis that the plates themselves drive instabilities in the plate-mantle system rather than major mantle overturns being the driving mechanism. I find that rapid sub-parallel subduction of the Izanagi mid-ocean ridge and subsequent catastrophic slab break o_ likely precipitated a global plate reorganisation event that formed the Emperor-Hawaii bend, and the change in relative plate motion between Australia and Antarctica at approximately 50 Ma
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Letessier, Tom Bech. "The influence of mid-ocean ridges on euphausiid and pelagic ecology." Thesis, University of St Andrews, 2012. http://hdl.handle.net/10023/3229.
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1 & 2. Euphausiids comprise a major component of ecosystems in the pelagic realm, the world's largest habitat, but basin scale drivers of euphausiids diversity and abundance are poorly understood. Mid-Ocean Ridges are the largest topographical feature in the pelagic realm and their benthic and pelagic fauna have only just recently become the focus of research. This thesis present new analyses on the drivers of euphausiids species richness in the Atlantic and the Pacific, giving specific attention to the influence of Mid- Ocean Ridges. New information is given on the biogeography of euphausiids and pelagic food-web trophology of the Mid-Atlantic Ridge, and on the biogeography of pelagic decapods on the South-West Indian Ocean ridge. 3. A Generalized Additive Model framework was used to explore spatial patterns of variability in euphausiid species richness (from recognized areas of occurrence) and in numerical abundance (from the Continuous Plankton Recorder survey) in conjunction with variability in a suite of biological, physical and environmental parameters on, and at either side of, the Mid-Atlantic Ridge. Euphausiid species richness peaked in midlatitudes and was significantly higher on the ridge than in adjacent waters, but the ridge did not influence numerical abundance in the top 10 m significantly. Sea surface temperature (SST) was the most important single factor influencing both euphausiid numerical abundance (-76.7%) and species richness (34.44%). Dissolved silicate concentration, a proxy for diatom abundance, significantly increased species richness (29.46%). Increases in sea surface height variance, a proxy for mixing, increased the numerical abundance of euphausiids. GAM predictions of variability in species richness as a function of SST and depth of the mixed layer were consistent with present theories, which suggest that pelagic niche-availability is related to the thermal structure of the near surface water. 4. Using a Generalized Additive Model in the Pacific, the main drivers of species richness, in order of decreasing importance, were found to be sea surface temperature (explaining 29.53% in species variability), salinity (20.29%), longitude (-15.01%, species richness decreased from West to East), distance to coast (10.99%), and dissolved silicate concentration (9.03%). An additional linear model poorly predicted numerical abundance. The practical differences in drivers of species richness in the Atlantic and Pacific Ocean were compared. Predictions of future species richness changes in the Pacific and Atlantic were made using projected environmental change from the IPCC A1B climate scenario, suggesting an increase in species richness in temperature latitudes (30° to 60° N and S) and little to no change in low latitudes (20° N to 20° S). 5. New baseline information is presented on biogeography, abundance and vertical distribution of euphausiids along the Mid-Atlantic Ridge (40° to 62° N). 18 species were recorded, with Euphausia krohni and Thysanoessa longicaudata being most abundant. Eight species had not been recorded in the area previously. The Subpolar Front is a northern boundary to some southern species, but not a southern boundary to northern ubiquitous species that show submergence. Four major species assemblages were identified and characterised in terms of spatial distribution and species composition. Numerical abundance was highly variable but decreased by orders of magnitude with depth. The Mid-Atlantic Ridge showed only a marginal effect on euphausiid distribution and abundance patterns. 6. Zooplankton and micronektic invertebrate epi- and mesopelagic (0-200 and 200- 800m) vertical distribution (e.g. Euphausiacea, Decapoda, Amphipoda, Thecosomata, Lophogastrida) on either side of the Subpolar Front of the Mid-Atlantic Ridge is described. Dietary relationships are explored, using stable isotope ratios and fatty acid trophic marker (FATM) composition. An increase in trophic level with size was observed. Individuals from southern stations were higher in dinoflagellate Fatty Acid Trophic Markers (FATM) (22:6(n-3)) and individuals from northern stations were higher in Calanus spp and storage FATMs (20:1(n-9) and 22:1(n-9)) reflecting primary production patterns in the two survey sectors. Observations on the geographical and vertical variability in trophodynamics are discussed. 7. New baseline information is presented on the biogeography, abundance, and vertical distribution of mesopelagic (200-1000 m), crustacean micronekton on- and offseamounts of the South-West Indian Ocean Ridge (26° to 42° S). Species richness and numerical abundance were typically higher near seamounts and lower over the abyssal plains, with several species being caught uniquely on seamounts. Observations suggest that the ‘oasis effect' of seamounts conventionally associated with higher trophic levels is also applicable to pelagic micronektic crustaceans at lower trophic levels. Biophysical coupling of micronekton to seamounts may be an important factor controlling benthopelagic coupling in seamount food-webs. 8. Euphausiid and pelagic diversity is driven primarily by geographical variability in temperature, by longitudinal patterns in upwellings, and by variability in nutrient concentration. Mid-Ocean Ridges modify pelagic ecology, by raising the seafloor and by bringing in proximity true pelagic and bathypelagic predators associated with the seabed. The increase in specialized fauna and biomass associated with ridges and seamounts serves to deplete zooplankton in the near bottom layer (0-200 m) and affect systems in and above the benthic boundary layer (<200 m from the seafloor), and the benthopelagic faunal layer. Mid-Ocean Ridges may serve to structure pelagic faunal distribution, and increase the overall diversity of the world ocean. The influence of ridges in the ocean basin may be comparable to that of hedges in a farmland; whilst delimiting the extent of crops (or zooplankton assemblages), hedges serve as local hotspots of mammal and avian diversity.
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Weatherley, Samuel. "Melting and melt migration in heterogeneous mantle beneath mid-ocean ridges." Thesis, University of Oxford, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.597100.
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Evidence for chemical heterogeneity in the mantle is widespread in oceanic basalts, yet its consequences for basalt petrogenesis are little understood. A significant unknown is the effect that heterogeneity has on the dynamics of magma flow in the mantle. Observations of oceanic crust and the upper mantle suggest that magma migrates to the surface through a network of high porosity channels. In this thesis, I use computational models of coupled magma/mantle dynamics beneath mid-ocean ridges to question whether a physical connection exists between channelized flow and mantle heterogeneity. The models are intialized with simple, hypothetical patterns of heterogeneity that cause the fusiblity of the model mantle to vary. The principal result is that channelized melt flow is a consequence of melting in a heterogeneous mantle. Magma from preferentially melted heterogeneities nucleates high porosity, high permeability channels that grow by a feedback between magma flux and dissolution. Using the models in various configurations, I explore the dynamics of channel formation and investigate how the topology of mantle heterogeneity affects melt segregation and focusing be- neath ridge axes. Additionally, I use the models to predict the speed and time scale of melt migration. A simple model of equilibrium partitioning is used to cast the results in terms of 230Th disequilibria. Comparisons of the modelled geochemistry against global measurements indicate that the models presented here provide a reasonable, first-order description of the dynamics of magma flow beneath ridges. I also explore a systematic connection between plate kinematics and global pat- terns of mid-ocean ridge bathymetry with three dimensional models of solid mantle flow beneath transform faults. The results provide new constraints on the scale of melt focusing and melt redistribution at ridge axes, and pose questions for future 3D studies of melt migration beneath ridges .
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Elkins, Lynne J. "Basalt petrogenesis beneath slow- and ultraslow-spreading Arctic mid-ocean ridges." Thesis, Massachusetts Institute of Technology, 2009. http://hdl.handle.net/1721.1/55327.
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Thesis (Ph. D.)--Joint Program in Marine Geology and Geophysics (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2009.Includes bibliographical references.
To explore the ability of melting mafic lithologies to produce alkaline ocean-island basalts (OIB), an experimental study was carried out measuring clinopyroxene (Cpx)melt and garnet (Gt)-melt partition coefficients during silica-poor garnet pyroxenite melting for a suite of trace elements, including U and Th, at 2.5GPa and 1420-1450°C. Partition coefficients range from 0.0083+0.0006 to 0.020+0.002 for Th and 0.0094+0.0006 to 0.024+0.002 for U in Cpx, and are 0.0032+0.0004 for Th and 0.013+0.002 for U in Gt. Forward-melting calculations using these experimental results to model time-dependent uranium-series isotopes do not support the presence of a fixed quantity of garnet pyroxenite in the source of OIB. To use U-series isotopes to further constrain mantle heterogeneity and the timing and nature of melting and melt transport processes, U-Th-Pa-Ra disequilibria, radiogenic isotopes, and trace-element compositions were measured for the slow-spreading Arctic mid-ocean ridges (MOR). A focused case study of 33 young (<10ka) MOR basalts (MORB) from the shallow endmember of the global ridge system, the Kolbeinsey Ridge (67°05'-70°26'N) found that unaltered Kolbeinsey MORB have universally high (230Th/238U) (1.165-1.296) and relatively uniform (230Th/232Th) (1.196-1.324), ENd (8.4310.49), 87Sr/86Sr (0.70274-0.70301), EHf(16.59-19.56), and Pb isotopes (e.g. 208Pb/206Pb 2.043-2.095). This suggests a homogeneous mantle source and a long peridotite melting column produces the thick Kolbeinsey crust. Trace element ratios suggest a young, depleted mantle source. Data from the slow- to ultraslow Mohns and Knipovich Ridges north of Kolbeinsey form a sloped array, and (230Th/232Th) correlates systematically with radiogenic isotopic variations.
(cont.) These data are readily reproduced by models for heterogeneous mantle melting. MORB from 85oE on the global ultraslow-spreading endmember Gakkel Ridge are homogeneously depleted with low (230Th/238U) and high (226Ra/230Th) that lie along a global negative correlation. Arctic data support a global mantle temperature control on mean (230Th/238U).
by Lynne J. Elkins.
Ph.D.
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Magde, Laura Suzan. "Mantle upwelling, melt generation, and magma transport beneath mid-ocean ridges." Thesis, Massachusetts Institute of Technology, 1997. http://hdl.handle.net/1721.1/55052.
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Hanson, Jeffrey Acton. "Seismic and hydroacoustic investigations near Ascension Island /." Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC campuses, 1998. http://wwwlib.umi.com/cr/ucsd/fullcit?p9823701.
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Chia, Chin-Swee 1968. "A comparison of bistatic scattering from two geologically distinct mid-ocean ridges." Thesis, Massachusetts Institute of Technology, 1999. http://hdl.handle.net/1721.1/80000.
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Gardiner, Alexander B. "Segmentation and cycles of crustal accretion at mid-ocean ridges : a study of the Reykjanes Ridge." Thesis, Durham University, 2003. http://etheses.dur.ac.uk/4121/.
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Early studies of mid-ocean ridges suggest a fundamental difference between crustal accretionary processes at slow- and fast-spreading ridges. Accretion, and the supply of melt to the crust itself, is thought to be highly episodic at slow-spreading ridges but steady-state at fast-spreading ridges. However, recent studies are beginning to question this model, with evidence for the temporal variation in crustal accretionary processes at all spreading rates emerging. This study provides evidence from bathymetry, TOBI sidescan, gravity and magnetic data, collected during different cruises to the Reykjanes Ridge, for the temporal nature of crustal accretion and its relationship to segmentation. Interpretation of TOBI images indicates that individual adjacent axial volcanic ridges (AVRs) vary in relative age, suggesting that they are at various stages of an evolutionary lifecycle, with episodic cycles of magmatic and tectonic activity. However, prior to investigating the possible effects of tectonomagmatic cycles on the crustal structure of AVRs, the effect of the Iceland hotspot on the ridge is examined. The along-axis free-air gravity anomaly is forward modelled in 2-D, revealing an along-axis increase in crustal thickness towards Iceland from 7.5 km to 10.5 km and a decrease in mantle densities from 3.30 to 3.23 g cm"^ between 57 30'N and 62 N. Calculation of the residual mantle Bouguer Anomaly (RMBA) and inversion of magnetic anomaly data, reveal intermediate-wavelength fluctuations in RMBA amplitude and magnetization intensity respectively that are attributed to hotspot pulses, with 59 N marking the southern most extent of the most recent pulse. Removal of the hotspot effect on the gravity data reveals short-wavelength RMBA lows, associated with individual AVRs, superimposed on a broad ridge-trending low. Along-AVR-axis gravity modelling shows that a number of these RMBA lows can be explained by a 200-800 m thickening of the crust and/or by the presence of 5-20% partial melt in the mid-crust. A correlation between relative AVR age and crustal structure is established, with longer, more mature AVRs having a thicker crust and shorter, younger AVRs having more partial melt in the mid-crust. Short-wavelength magnetization intensity highs, associated with younger AVRs, corroborate the TOBI age interpretations. Local spreading rate calculations reveal that total spreading rates for younger AVRs are up to 20% faster than for older AVRs over the last 1.42 Ma. On the basis of these results a model for the cyclicity of crustal accretion is presented, whereby far-field tectonic stresses result in spreading-orthogonal brittle deformation of the crust in the neovolcanic zone, and 3-D mantle upwelling, with a wavelength of -70 km, follows the ridge trend and results in second order segments that comprise ~5 AVRs. It is proposed that along-axis migration of melt within such a segment results in the observed variations in AVR age, length, RMBA amplitude, magnetization intensity and local spreading rate. The proposed model has implications for the temporal variability of crustal accretion at all spreading rates.
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MacLeod, Sarah J. "Characteristics of extinct spreading centers and the relationship between spreading ridges, hotspots and deep mantle structure." Thesis, The University of Sydney, 2017. http://hdl.handle.net/2123/17988.
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Oceanic spreading ridges and mantle plumes represent the vehicles through which heat is lost from the deep Earth and govern the distribution of most basic volcanism at Earth’s surface. Despite numerous hotspots located in close proximity to spreading ridges, generally, hotspots are seen to be independent of them. This study catalogues extinct spreading ridges and their physical and spreading characteristics to understand ridge evolution. Variability of ridges related to tectonic subtype is described and compared with active ridges. Uncertain extinct spreading ridges are compared with ‘characteristic’ extinct ridges and a number of possible new ridges are identified. The spatial correlation of hotspots with active plate boundaries through time is assessed, including reorganizations of spreading ridges at times of major ridge jumps. This evaluation determines that over the last 100 m.y. spreading ridges have been closer to hotspots than expected by random distribution, as they are at present-day. After ridge jumps, spreading ridges are more often located closer to a hotspot, particularly when microcontinents are formed. In contrast, subduction zones are generally further from hotspots than expected by a random distribution over the last 100 m.y., particularly those likely to have a deep mantle origin. To explore relationships of plumes and deep mantle structure with surface tectonics spherical geodynamic models are evaluated, using different tectonic reconstructions for boundary conditions. First order behavior and motion of modelled hotspots are compared with observations of present-day hotspots and their trails. Evolution of large-scale modelled anomalous dense structures in the deep mantle are quantified. The direction and rate of boundary retreat or advance for the African, Pacific and ‘Perm’ anomalies are described and overlap with large low-shear wave seismic provinces evaluated. Insight is gained on behavior of plumes and deformation of deep mantle structures.
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Ramondenc, Pierre. "Effect of seismicity and diking on hydrothermal circulation at mid-ocean ridges." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2008. http://hdl.handle.net/1853/22619.
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Thesis (Ph. D.)--Civil and Environmental Engineering, Georgia Institute of Technology, 2008.Committee Chair: Germanovich, Leonid; Committee Co-Chair: Lowell, Robert; Committee Member: Di Iorio, Daniela; Committee Member: Huang, Haiying; Committee Member: Rix, Glenn; Committee Member: Xu, Wenyue.
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Genc, Gence. "Serpentinization-assisted deformation processes and characterization of hydrothermal fluxes at mid-ocean ridges." Diss., Georgia Institute of Technology, 2012. http://hdl.handle.net/1853/43725.
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Seafloor hydrothermal systems play a key role in Earth fs energy and geochemical budgets. They also support the existence and development of complex chemosynthetic biological ecosystems that use the mineral-laden fluids as a source of energy and nutrients. This dissertation focuses on two inter-related topics: (1) heat output and geochemical fluxes at mid-ocean ridges, and (2) structural deformation of oceanic lithosphere related to subsurface serpentinization in submarine settings.
The determination of heat output is important for several reasons. It provides important constraints on the physics of seafloor hydrothermal processes, on the nature of the heat sources at mid-ocean ridges, and on nutrient transport to biological ecosystems. Despite its importance, measurements of hydrothermal heat outputs are still scarce and cover less than 5% of active hydrothermal vent sites. In this work, we report development of two new devices designed to measure fluid flow velocities from the submersible at temperatures of up to 450 C and depths 5,000 m. By using these instruments on 24 Alvin dives, new measurements of hydrothermal heat output have been conducted at the Juan de Fuca Ridge, including first measurements from the High Rise and Mothra hydrothermal fields. The collected data suggest that the high-temperature heat output at the Main Endeavour Field (MEF) may be declining since the 1999 eruption. The flow measurement results, coupled with in-situ geochemical measurements, yielded the first estimates of geochemical fluxes of volatile compounds at MEF and Mothra. Our findings indicate that geochemical flux from diffuse flows may constitute approximately half of the net geochemical flux from Juan de Fuca Ridge.
It has recently been recognized that serpentinization of mantle peridotites, due to its exothermic nature, may be a mechanism contributing to the heat output at mid-ocean ridges. The tectonic response of the crust to serpentinization of extensively distributed peridotites at mid-ocean ridges and subduction zones could provide a means of characterizing serpentinized regions in the oceanic lithosphere. These regions are often associated with surface topographic anomalies that may result from the volume expansion caused by the serpentinization reactions. Although there is a clear correlation between tectonics and serpentinization, the link is complex and still not understood. In this dissertation, we calculated the transformation strain and surface uplift associated with subsurface serpentinization of variously shaped ultramafic inclusions. Application of the results to explain the anomalous topographic salient at the TAG hydrothermal field (Mid-Atlantic Ridge) suggests that this feature may result from a serpentinized body beneath the footwall of a detachment fault. Because the depth of the potential serpentinized region appears to be more than 1.5 times the size of the inclusion, the uplift profile is relatively insensitive to the exact location or shape of the serpentinized domain. The rate of exothermic heat release needed to produce the serpentinized volume may contribute to the ongoing diffuse flow. Application of the results to an uplift feature associated with the Kyushu ]Palau subduction zone in the western Pacific, shows that approximately 3% transformational strain in an inclined serpentinized region of the mantle wedge near the subducted Kyushu ]Palau Ridge may result in the observed uplift on the Miyazaki Plain. Using the uplift data may help to constrain the level of the subsurface serpentinization.
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Cordery, Matthew Jean. "Mantle convection, melt migration and the generation of basalts at mid-ocean ridges." Thesis, Massachusetts Institute of Technology, 1991. http://hdl.handle.net/1721.1/52936.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, and Woods Hole Oceanographic Institute, 1991.Includes bibliographical references (leaves 183-190).
by Matthew Jean Cordery.
Ph.D.
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Blackman, Donna K. "Axial structure of fast spreading mid-ocean ridges : implications for overlapping spreading centers." Thesis, Massachusetts Institute of Technology, 1986. http://hdl.handle.net/1721.1/55308.
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Thesis (M.S.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1986.Includes bibliographical references (leaves 88-91).
by Donna Kay Blackman.
M.S.
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Hosford, Allegra. "Crustal accretion and evolution at slow and ultra-slow spreading mid-ocean ridges." Thesis, Massachusetts Institute of Technology, 2001. http://hdl.handle.net/1721.1/58441.
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Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric and Planetary Sciences, and the Woods Hole Oceanographic Institution), 2001.Page 250 blank.
Includes bibliographical references.
Half of the ocean crust is formed at spreading centers with total opening rates less than 40 km/Myr. The objective of this Thesis is to investigate temporal variations in active ridge processes and crustal aging at slow-spreading centers by comparing axial crustal structure with that on conjugate flanks of the slow-spreading Mid-Atlantic Ridge (MAR) (full rate, 20 km/Myr) and the ultra-slow spreading Southwest Indian Ridge (SWIR) (full rate, 14 km/Myr). Seismic refraction data collected along the rift valley and flanking rift mountains of the OH-1 segment (35ʻN) at the MAR show that the entire crustal section is constructed within a zone that is less than 5 km wide. Shallow-level hydrothermal circulation within the axial valley is suggested by the rift mountain seismic profiles, which show that the upper crust is 20% thinner and 16% faster along strike than zero-age crust. These effects probably result from fissure sealing within the extrusive crust. Deeper crustal velocities remain relatively constant at the segment midpoint within the first 2 Myr, but are reduced near the segment offsets presumably by faulting and fracturing associated with uplift out of the rift valley.
(cont.) A temporal variation in axial melt supply is suggested by a 15% difference in along-strike crustal thickness between the rift valley and rift mountains, with relatively less melt supplied today than 2 Ma. Crustal accretion at the SWIR appears to occur in a similar manner as at the MAR, although gravity and seismic data indicate that the average crustal thickness is 2-4 km less at theultra-slow spreading SWIR. A 25 Myr record on both flanks of the ridge shows that seafloor spreading has been highly asymmetric through time, with 35% faster crustal accretion on the Antarctic (south) plate. A small-offset non-transform discontinuity between two ridge segments is just as stable as two neighboring transform discontinuities, although a single mantle Bouguer gravity anomaly centered over the non-transform offset indicates that this boundary does not significantly perturb underlying mantle flow. Off-axis magnetic anomalies are recorded with high fidelity despite the very low spreading rates and the absence of a basaltic upper crust in one area. The lower crust may be the dominant off-axis carrier of the magnetic signal, contrary to traditionalmodels of crustal magnetic structure. Morphological and gravity data show evidence of asymmetric crustal accretion across the SWIR ridge axis, with slightly warmer mantletemperatures beneath the slower-spreading African (north) plate.
by Allegra Hosford.
Ph.D.
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Day, Anthony James. "Seismic imaging of crustal structure at mid-ocean ridges : a three-dimensional approach." Thesis, Durham University, 2001. http://etheses.dur.ac.uk/4274/.
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Over recent years geological, geochemical and geophysical surveys of mid-ocean ridges have revealed a significant degree of along-axis variability not only in seabed morphology, but also in crustal structure, particularly Numerous geophysical surveys of the Valu Fa Ridge, southwest Pacific, have mapped the extent of an axial mid-crustal reflector. This reflector has been interpreted as representing the top of a sill-like melt lens, comprising a high percentage of partial melt, lying at the top of a crustal magma chamber. In 1995, a controlled-source, wide-angle seismic dataset was acquired at the Valu Fa Ridge during RN Maurice Ewing cruise EW9512, to investigate the mid-deep crustal structure at this ridge, and particularly the crustal magma chamber associated with the melt lens beneath the ridge axis. The EW9512 acquisition geometry was primarily two-dimensional in design, and modelling of these 2-D profiles revealed the presence of an axial low velocity zone beneath the melt lens. This low velocity zone is thought to represent a region of crystal mush comprising a much lower percentage of partial melt than is present in the overlying melt lens. Similar structures have been modelled beneath a number of other mid-ocean ridges. The primary aim of this study was to build on this 2-D interpretation by taking advantage of three-dimensional ray coverage in the axial region in order to assess the along-axis continuity of the magmatic system, correlate this to any ridge segmentation apparent in the seabed morphology, and determine if ridge segmentation is related to the magma supply. The 3-D data were analysed using a tomographic inversion technique. The inversion results suggest that the axial low velocity zone may be segmented on a scale of 5-10 km, which correlates with the morphological segmentation of the ridge crest and is believed to reflect episodic magma supply with different ridge segments at different stages of a cycle of magmatic and amagmatic extension. However, three- dimensional ray coverage is not ideal owing to the dominantly 2-D acquisition geometry. Therefore a detailed assessment of data uncertainty and resolution was undertaken to enable a meaningful interpretation of the inversion results in terms of which features have a geological origin and which are artefacts of the inversion process. P-S mode converted arrivals arising from mid-crustal interfaces were also modelled in order to obtain improved geological constraints on the crustal structure than is possible from P-wave studies alone. This modelling indicates that the uppermost crust is pervaded by thin cracks. In addition, techniques were developed for modeling the polarisation of 5-wave arrivals with low signal strength. Application of these methods suggests that the thin cracks have a preferred orientation parallel to the ridge crest on-axis, and oblique to the ridge crest off-axis which is thought to reflect the pattern of southward propagation of the ridge system inferred from regional tectonic and bathymetric studies. Modelling of P-S mode converted arrivals arising from conversion at the top of the melt lens provided additional constraints on the properties of the melt lens. In conjunction with the 3-D tomographic results, this work suggests that the southernmost ridge segment in the study area has recently become magmatically active following a period of amagmatic extension suggested by its morphology, thus providing evidence for episodic melt supply at this ridge. As part of the suggestions for further work, a theoretical investigation of survey resolution was undertaken to test commonly adopted acquisition geometries with a view to optimising the design and cost-effectiveness of future 3-D controlled-source tomographic experiments.
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Achenbach, Kay L. "Melt and deformation in the mantle beneath mid-ocean ridges a textural and lattice-preferred orientation study of abyssal peridotites /." Laramie, Wyo. : University of Wyoming, 2008. http://proquest.umi.com/pqdweb?did=1798966681&sid=1&Fmt=2&clientId=18949&RQT=309&VName=PQD.
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Thesis (Ph.D.)--University of Wyoming, 2008.Title from PDF title page (viewed on Mar. 22, 2010). Appendices are available as supplemental PDF files. Includes bibliographical references (p. 249-263).
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Singh, Shreya. "Exploring the relationship between crustal permeability and hydrothermal venting at mid-ocean ridges using numerical models." Diss., Virginia Tech, 2015. http://hdl.handle.net/10919/73586.
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Hydrothermal systems associated with oceanic spreading centers account for a quarter of Earth's total heat flux and one third of the heat flux through the ocean floor. Circulation of seawater through these systems alters both the crust and the circulating fluid, impacting global geochemical cycles. The warm vent fluids rich in nutrients support a wide variety of unique biological communities. Thus, understanding hydrothermal processes at oceanic spreading centers is important to provide insight into thermal and biogeochemical processes. In this dissertation I present the results of numerical modeling efforts for mid-ocean ridge hydrothermal systems. In the three manuscripts presented, permeability emerges as a key controlling factor for hydrothermal venting. In the first manuscript, I use 2-D numerical models to find that the distribution of permeability in the crust controls fluid velocity as well as the amount of mixing between hot hydrothermal fluids and cold seawater. This, in turn, effects the temperature and composition of fluids emerging on the surface. For the second manuscript, I construct single-pass 1-D models to show that a sudden increase in permeability caused due to magmatic or seismic events in the seafloor causes a sharp rise in the fluid output of the system. This, in conjunction with steep thermal gradients close to the surface, results in a rapid increase of venting temperatures. In the third manuscript, I develop a particle tracking model to study fluid trajectories in the subsurface. The results show that permeability distribution in the subsurface governs fluid paths and consequently, the residence time of fluids in the crust. Based on the work presented in this document, I conclude that permeability distribution, both local and field scale, exerts a major control on hydrothermal circulation in the subsurface and on the temperature and composition of venting fluids on the surface.Ph. D.
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Georgen, Jennifer E. "Interactions between mantle plumes and mid-ocean ridges : constraints from geophysics, geochemistry, and geodynamical modeling." Thesis, Massachusetts Institute of Technology, 2001. http://hdl.handle.net/1721.1/29052.
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Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2001."September 2001." Vita. Page 223 blank.
Includes bibliographical references.
This thesis studies interactions between mid-ocean ridges and mantle plumes using geophysics, geochemistry, and geodynamical modeling. Chapter 1 investigates the effects of the Marion and Bouvet hotspots on the ultra-slow spreading, highly-segmented Southwest Indian Ridge (SWIR). Gravity data indicate that both Marion and Bouvet impart high-amplitude mantle Bouguer anomaly lows to the ridge axis, and suggest that long-offset transforms may diminish along-axis plume flow. Building upon this observation, Chapter 2 presents a series of 3D numerical models designed to quantify the sensitivity of along-axis plume-driven mantle flow to transform offset length, spreading rate, and mantle viscosity structure. The calculations illustrate that long-offset transforms in ultra-slow spreading environments may significantly curtail plume dispersion. Chapter 3 investigates helium isotope systematics along the western SWIR as well as near a global array of hotspots. The first part of this study reports uniformly low 3He/4He ratios of 6.3-7.3 R/Ra along the SWIR from 9⁰-24⁰E, compared to values of 8 +/- 1 Ra for normal mid-ocean ridge basalt. The favored explanation for these low values is addition of (U+Th) into the mantle source by crustal and/or lithospheric recycling. Although high He/4He values have been observed along the SWIR near Bouvet Island to the west, there is no evidence for elevated 3He/4He ratios along this section of the SWIR. The second part of Chapter 3 investigates the relationship between 3He/4He ratios and geophysical indicators of plume robustness for nine hotspots.
(cont.) A close correlation between a plume's flux and maximum 3He/4He ratio suggests a link between plume upwelling strength and origination in the deep, relatively undegassed mantle. Chapter 4 studies 3D mantle flow and temperature patterns beneath oceanic ridge-ridge-ridge triple junctions (TJs). In non-hotspot-affected TJs with geometry similar to the Rodrigues TJ, temperature and upwelling velocity along the slowest-spreading of the three ridges are predicted to increase within a few hundred kilometers of the TJ, to approach those of the fastest-spreading ridge. Along the slowest-spreading branch in hotspot-affected TJs such as the Azores, a strong component of along-axis flow directed away from the TJ is predicted to advect a hotspot thermal anomaly away from its deep-seated source.
by Jennifer E. Georgen.
Ph.D.
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Johnston, Thomas Michael Shaun. "Internal tide scattering at midocean topography." Thesis, University of Hawaii at Manoa, 2003. http://proquest.umi.com/pqdweb?index=0&did=765959661&SrchMode=2&sid=6&Fmt=2&VInst=PROD&VType=PQD&RQT=309&VName=PQD&TS=1209407173&clientId=23440.
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VanderBeek, Brandon. "New Perspectives on Mid-Ocean Ridge Magmatic Systems and Deformation in the Uppermost Oceanic Mantle from Active- and Passive-Source Seismic Imaging in Cascadia." Thesis, University of Oregon, 2019. http://hdl.handle.net/1794/24205.
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In this dissertation, I use seismic imaging methods to constrain the evolution of the oceanic upper mantle across the Juan de Fuca (JdF) and Gorda plates. This work begins by studying the geometry of the mantle magmatic system and patterns of mantle flow beneath the northern JdF ridge in relation to ridge-parallel changes in accretionary processes. I find that the dynamics of lithospheric rifting exert the primary control on the distribution of shallow mantle melts and variations in crustal thickness and composition. The orientation of mantle divergence beneath the JdF ridge, as inferred from seismic anisotropy, is oblique to the overlying plate divergence direction. Similar observations made at the East Pacific Rise and Mid-Atlantic ridge suggest plate motions alone do not control mantle flow patterns. On the contrary, stresses exerted at the base of the plate by the asthenospheric flow field may contribute to changes in plate motion prompting a reorientation of oceanic spreading segments. The mantle anisotropic fabric of the JdF plate interior is then investigated to identify whether the rotated mantle flow field observed beneath the JdF ridge persisted throughout the recent geologic past. However, observations suggest that the anisotropic structure created at the ridge partially reorganizes off-axis obscuring the paleo-flow geometry. Next, I focus on how the physical state of the oceanic lithosphere evolves with time. Using local earthquake arrival times I test whether the seismic velocity structure of the upper mantle lithosphere is thermally controlled or dominated by heterogeneities introduced upon accretion at the ridge or by subsequent deformation off axis. Despite extensive surficial evidence of faulting across the Gorda plate, deformation appears to be restricted to crustal depths and mantle velocities are explained by conductive cooling. In contrast, the velocity structure of the JdF plate is inconsistent with conductively-cooled mantle. Hydration of the mantle lithosphere associated with tectonic discontinuities is invoked to explain anomalously slow P-wave speeds. Lastly, a joint inversion of teleseismic body and surface wave data is proposed to image the geometry of mantle upwelling and melt production beneath the JdF and Gorda Ridges.
This dissertation includes previously published and unpublished coauthored material.
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Crowell, Brendan William. "Biological sulfur reactions and the influence on fluid flow at mid-ocean ridge hydrothermal systems." Thesis, Available online, Georgia Institute of Technology, 2007, 2007. http://etd.gatech.edu/theses/available/etd-07092007-085654/.
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Arnoux, Gillean. "Novel Insights into Mass and Energy Transfer and Mid-Ocean Ridges from Seismic Imaging of the East Pacific Rise and Juan de Fuca Ridge." Thesis, University of Oregon, 2019. http://hdl.handle.net/1794/24532.
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In this dissertation, I use seismic imaging and waveform modeling methods to investigate melt migration processes and the structure of the magma plumbing system beneath the East Pacific Rise (EPR) and Endeavour segment of the Juan de Fuca Ridge, respectively. This work begins by studying shallow mantle reflections beneath the EPR. I find the amplitude versus offset and waveform characteristics of the reflections to be consistent with a sub-horizontal dunite channels located up to 20 km off-axis. The depth of the dunite channels correlate with patterns of mantle melt delivery and the predicted base of the thermal lithosphere, suggesting the channels are thermally controlled and may have formed in situ via dissolution by focused flow at the base of the lithosphere. This interpretation is consistent with field observations in ophiolites and numerical modeling of melt-focusing channels. The three-dimensional velocity structure of the Endeavour segment is then investigated to identify how patterns of mantle melt delivery influence the segment-scale distribution of crustal melt and crustal accretion. The results from this study indicate that the mantle magmatic system is skewed relative to the ridge-tracking crustal magmatic system and that this skew exerts primary control on magmatic, tectonic, and hydrothermal activity at the Endeavour segment. In regions where mantle melt delivery is axis-centered, mantle-derived melts are efficiently transported from the mantle to the crust, resulting in frequent crustal melt replenishment, associated seismogenic cracking, and enhanced crustal melt content that drives vigorous hydrothermal activity. Conversely, sites of off-axis melt delivery are characterized by less efficient vertical melt transport, resulting in infrequent crustal melt injection and hence, reduced crustal melt content and hydrothermal activity. Next, I focus on how along-axis variations in magma replenishment modulate crustal permeability and the intensity of hydrothermal circulation. Using full-waveform inversion, I show that sites of localized magma replenishment to the axial magma lens, along with induced seismogenic cracking, coincide with enhanced permeability. I conclude that the frequency of magma injection governs hydrothermal circulation patterns and heat flux at mid-ocean ridges.
This dissertation includes previously published and unpublished coauthored material.
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Loocke, Matthew Phillip. "The role of the axial melt lens in crustal accretion at fast-spreading mid-ocean ridges." Thesis, Cardiff University, 2016. http://orca.cf.ac.uk/97663/.
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Fast-spreading mid-ocean ridges (MOR) are underlain by a thin, quasi-steady-state melt or crystal mush body at the base of the sheeted dykes, referred to as the axial melt lens (AML). Although the AML is thought to play a key role in the development of MOR basalts (MORB), debate persists regarding the composition of the AML and the role it plays in the accretion of the lower crust. I address this question by studying a suite of varitextured gabbronorites from the Hess Deep rift valley in the equatorial Pacific Ocean which are interpreted to have formed in the AML of the East Pacific Rise. This unique sample set provides an unparalleled opportunity to conduct the first comprehensive investigation of the AML at a fast-spreading MOR. To facilitate this study, I here develop a method for the quantitative assessment of compositional distribution (QACD) in whole-thin-section element maps. QACD facilitates rapid data collection and processing to generate mineral modes, element and molar-ratio maps, and quantifying full-sample compositional distributions. My application of QACD to the Hess Deep AML suite reveals that mineral phases within the AML here are too evolved to be in equilibrium with MORB. I test the broader applicability of this conclusion by conducting detailed mapping and sampling of an analogous AML horizon in the Oman Ophiolite (Wadi Saq, Ibra Valley). This section is characterised by an evolved sheeted dyke complex rooting into a quartz diorite-hosted AML, supporting the supposition that the AML accommodates the fractionation of highly-evolved melts. I propose a model wherein the AML is predominantly fed by small volumes of evolved interstitial melts expelled from the underlying crystal mush. In the months preceding decadal eruption events, short-lived, focused injections of primitive melts into the AML mix with the extant highly-fractionated melt and trigger eruptions. This model reconciles the apparent mismatch between the volcanic and plutonic records and inferences made on geophysical and petrological grounds. I suggest that the AML is an active player in the development of MORB, permitting the fractionation and storage of evolved melts expelled from the underlying crystal mush and recording the mixing of that material with primitive melt, hence fulfilling more of a passive role with respect to lower crustal accretion than previously proposed.
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Choi, Jaewoon. "The response of two-phase hydrothermal systems to changing magmatic heat input at mid-ocean ridges." Thesis, Virginia Tech, 2013. http://hdl.handle.net/10919/50575.
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Hydrothermal processes at oceanic spreading centers are largely influenced by changing magmatic heat input. I use the FISHES code to investigate the evolution of surface temperature and salinity as a function of time-varying heat flux at the base of a two-phase, vapor-brine hydrothermal system. I consider a two-dimensional rectangular box that is 1.5 km deep and 4 km long with homogeneous permeability. 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, I consider a variety of basal boundary conditions: (1) a constant heat flux with an value of 130 W/m2; (2) a sinusoidal heat flux with a period of 6 years and an amplitude ranging between 100 and 50 W/m2; (3) step, random, and exponential heat fluxes ranging between 200 and 15 W/m2; and (4) an analytical function of temporally decaying heat flux resulting from a simulated cooling, crystallizing magmatic sill. As a result of the investigation I 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 of vapor and brine occurs, surface salinity variations may be recorded in response to changing conditions at depth, but these are smaller in amplitude.Master of Science
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Grimes, Craig B. "Duration, rates, and patterns of crustal growth at slow-spreading mid-ocean ridges using zircon to investigate the evolution of in situ ocean crust /." Laramie, Wyo. : University of Wyoming, 2008. http://proquest.umi.com/pqdweb?did=1799840381&sid=1&Fmt=2&clientId=18949&RQT=309&VName=PQD.
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Byrnes, Joseph. "Mantle flow and melting beneath young oceanic lithosphere: Seismic studies of the Galápagos Archipelago and the Juan de Fuca Plate." Thesis, University of Oregon, 2017. http://hdl.handle.net/1794/22638.
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In this dissertation, I use seismic imaging techniques to constrain the physical state of the upper mantle beneath regions of young oceanic lithosphere. Mantle convection is investigated beneath the Galápagos Archipelago and then beneath the Juan de Fuca (JdF) plate, with a focus on the JdF and Gorda Ridges before turning to the off-axis asthenosphere. In the Galápagos Archipelago, S-to-p receiver functions reveal a discontinuity in seismic velocity that is attributed to the dehydration of the upper mantle. The depth at which dehydration occurs is shown to be consistent with prior constraints on mantle temperature. A comparison between results from receiver functions, seismic tomography and petrology shows that mantle upwelling and melt generation occur shallower than the depth of the discontinuity, despite the expectation of high viscosities in the dehydrated layer. Beneath the JdF and Gorda Ridge, low Vs anomalies are too large to be explained by the cooling of the lithosphere and are attributed to partial melt. The asymmetry, large Vs gradients, and sinuosity of the anomalies beneath the JdF Ridge are consistent with models of buoyancy-driven upwelling. However, deformation zone processes appear to dominate mantle flow over seafloor spreading beneath the Explorer and Gorda diffuse plate boundaries. Finally, S-to-p receiver functions reveal a seismic discontinuity beneath the JdF plate that can only be attributed to seismic anisotropy. Synthesis of the receiver function results with prior SKS splitting results requires heterogeneous anisotropy between the crust and the discontinuity. Models of anisotropy feature increasing anisotropy before the decrease at the discontinuity, but well below the base of the lithosphere, and a clockwise rotation of the fast direction with increasing depth. In these results and even in the SKS splitting results, additional driving mechanisms for mantle flow such as density or pressure anomalies are required.
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Keeton, Jane A. "The use of image analysis techniques to characterise mid-ocean ridges from multibeam and sidescan sonar data." Thesis, Durham University, 1994. http://etheses.dur.ac.uk/1620/.
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Dannowski, Anke [Verfasser]. "Processes of magmatic and tectonic accretion of oceanic lithosphere at mid-ocean ridges : constraints from a seismic refraction study at the Mid-Atlantic Ridge near 21.5° N / Anke Dannowski." Kiel : Universitätsbibliothek Kiel, 2009. http://d-nb.info/1019867175/34.
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Combier, Violaine. "Mid-ocean ridges processes : insights from 3D reflection seismic at the 9°N OSC on the East Pacific Rise, and the Lucky Strike volcano on the Mid-Atlantic Ridge." Paris, Institut de physique du globe, 2007. http://www.theses.fr/2007GLOB0012.
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Aux dorsales rapides, telle la dorsale Est Pacifique, la nature et la géométrie de la chambre magmatiqe sont relativement bien connues, grâce notamment au fait que des concentrations de magma ont été identiiées dans la croûte à laxe de ces dorsales au moyen de méthodes sismiques depuis la fin des années 1970. En revanche, la nature de la chambre magmatique aux dorsales lentes, telle la dorsale Médio-Atlantique, et moins bien comprise, les méthodes sismiques nayant révélé jusquà une période récente que de rares indicesattestant de la présence de magma dans la croûte. Lors de la campagne océanographique SISMOMAR sur la dorsale Médio-Atlantique en 2005, il a été découvert un réflecteur interprété comme le toit dune chambre magmaique, sous le Volcan Lucky Strike , apportant de nouvelles contraintes sur les processus daccrétion aux dorsles lentes. Une première partie de la thèse concerne loverlapping spreading center (OSC) à 9°N sur la dorsale Est Pacifique. Les processus daccrétion aux dorsales rapides sont souvent considérés comme bidimensionnels, sans variation selon la direction de laxe de la dorsale. Un OSC est une discontinuité non transformante de laxe des dorsales rapides; du fait de la discontinuité, les processus volcaniques et tectoniques deviennent tridimensionnels, fournissant de nouvelles contraintes sur le couplage entre l'alimentation magmatique et la tectonique de la lithosphère. Nous présenterons une étude sur les relations entre les sructures volcaniques et tectoniques du plancher océanique et la géométrie des concentrations en magma dans la croûte. Les données utilisées sont principalement issues de notre nouvelle interprétation des données de sismique réflexion en trois dimensions (3D) acquises lors de la campagne ARAD à lOSC 9°N en 1997. Nous avons produit une carte bathymétrique haute résolution dérivée des données sismiques et une carte en profondeur des lentilles de magma situées au toit de la chambre magmatique. Les principaux résultats de cette étude sont d'une part, la mise en évidence d'un découplage des contraintes entre la croûte supérieure fragile et la croûte inférieure, la lentille de magma agissant comme une zone de découplage efficace; et dautre part, l'identification des facteurs contrôlant la géométrie de la lentille de magma: la position de la source dalimentation sous la lentille, le champ de contraintes ambiant à la profoneur de la lentille, qui est le champ de contraintes régional lié à l'expansion des plaques, et le champde contraintes local dans la croûte supérieure fragile. Une deuxième partie de la thèse concerne le traitement en 3D, puis l'interprétation des données de sismque réflexion acquises sur le Volcan Lucky Strike pendant la campagne SISMOMAR. Notre séquence de traiement en 3D a permis dobtenir un cube de la réflectivité sismique en profondeur sous le volcan. Nous montrerons les principaux résultats qui incluent d'une part, la détermination de la profondeur et de la géométrie du réflecteur présent au toit de la chambre magmatique; d'autre part l'imagerie en profondeur de failles, dont les failles bordières de la vallée axiale et des failles présentes sur le volcan. Les relations spatiales entre la chambre magmatique et les failles suggèrent l'existence d'un refroidissement vigoureux de la chambre magmatique par la circulation hydrothermaleAt fast spreading ridges such as the East Pacific Rise, the nature and geometry of the magma chamber are relatively well constrained, due in part to the fact that melt has been quasi ubiquitously identified in the crust beneath the ridge axis with seismic methods experiments since the end of the 1970s. In contrast, at slow-spreading ridges such as the Mid-Atlantic Ridge, the nature of the magma chamber is less well understood, because until recently, seismic methods revealed little evidence of melt in the crut. During the SISMOMAR scientific cruise in 2005, a seismic reflector interpreted as the roof of a maga chamber was discovered beneath the Lucky Strike Volcano at the Mid-Atlantic Ridge, providing new contraints on accretion processes at slow-spreading ridges. The first part of this thesis concerns the overlapping spreading center (OSC) at 9°N on the East-Pacifc Rise. Accretion at fast-spreading ridges is often treated as bi-dimensional, with no variation along the ridge axis. At OSCs, which are non-transform discontinuities of the ridge axis found at fast-spreaing ridges, volcanic and tectonic processes are tri-dimensional (3D). The study of OSCs provides new insights into the coupling between the melt supply and the tectonics of the brittle lithosphere. I will present my study of the relationships between seafloor volcanic and tectonic structures and the geometry of melt concentrations in the crust at the 9°N OSC. My study is based mainly on a new interpretation of 3D reflection seismic data acquired during the 1997 ARAD cruise. I produced a high-resolution bathymtric map derived from the seismics and a map of the melt lens reflectors in depth. The main results inlude firstly, the identification of decoupled stresses between the brittle upper crust and the lower cust, with the melt lens acting as an effective decoupling zone; and secondly, the determination of factors controlling the geometry of the melt lens: the location of the melt source beneath the melt lens, the ambient stress-field at melt lens depth which is the regional stress-field controlled by plate separation, and the brittle upper crust local stress-field. A second part of the thesis concerns the 3D processing and interpretation of the seismic reflection data acquired at the Lucky Strike Volcano during the SISMOMAR cruise. The newly processed data constrain the depth and geometry of the reflector corresponding to the roof of the magma chamber. Faults are also imaged at depth, including the axial valley bounding faults and faults on the volcano itself. The spatial relationships between the magma chamber roof and the faults suggest a vigorous cooling of the magma chamber through hydrothermal circulation
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Zwan, Froukje Marieke van der [Verfasser]. "Hydrothermal activity at slow-spreading mid-ocean ridges : evidence from chlorine in basalt / Froukje Marieke van der Zwan." Kiel : Universitätsbibliothek Kiel, 2014. http://d-nb.info/1052529224/34.
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Nowacki, Andy. "Plate deformation from cradle to grave : seismic anisotropy and deformation at mid-ocean ridges in the lowermost mantle." Thesis, University of Bristol, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.558079.
Full textAbstract:
The lowermost mantle-also known as D"-comprises the few hundred kilometres above the core-mantle bound- ary, and is known to show significant seismic anisotropy. In this thesis I attempt to use observations of shear wave splitting to constrain flow in this region. In order to accurately measure splitting in D", it is necessary to constrain that present at the top of the mantle. Using the method of source-side shear wave splitting, over 100 novel measurements of anisotropy in the mantle beneath mid-ocean ridges are made across the globe. Splitting mostly increases away from the axis on ridges themselves, and fast directions become increasingly parallel to the spreading direction. This is consistent with the alignment of olivine a-axes being parallel to flow. However, models based purely on lattice preferred orientation of olivine cannot predict the observed difference between splitting in SKS waves by previous authors and the new S wave measurements; one possible explanation is the presence or horizontal pockets or melt at ~80-150 km depth. These new measurements allow us to vastly improve our understanding of both ridges processes and the lowermost mantle, as they provide the means to use mid-ocean ridge earthquakes to probe regions of D" inaccessible with only deep events. Over 700 measurements of differential S-ScS splitting are then made, using corrections obtained for the upper mantle, beneath North and Central America. Fast orientations along paths from South American earthquakes are consistent with previous observations (showing horizontally-polarised shear waves travel faster than vertically- polarised ones), but measurements made along paths from mid-ocean ridge earthquakes constrain the possible symmetries of anisotropy in the lowermost mantle. They show that radial anisotropy is not an adequate approximation to the style of anisotropy present in D" under the Caribbean. Using elastic constants obtained from ab initio calculations and deformation experiments for a variety of candidate lowermost mantle phases, the shear planes and directions which are compatible with the observations are shown. Assuming horizontal shear, slip on (001) planes in post-perovskite seems the likeliest mechanism to produce the observed splitting. These and earlier measurements of shear wave splitting are then tested against recent models of mantle flow derived from seismic and other geophysical constraints. The flow model is used to derive elastic constants using three different candidate sets of slip systems in post-perovskite. A method to forward-model the shear wave splitting in an arbitrarily anisotropic Earth is developed and used to show that the plasticity model which favours slip on (010) in post-perovskite produces fast orientations best compatible with observations. However, not all observations can be accurately reproduced, suggesting that deformation-induced texturing in post-perovskite may not be the only mechanism producing anisotropy in DU I suggest possible routes towards addressing our current lack of understanding of lowermost mantle processes, some of which are developed in the current work.
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Xu, Min Ph D. Massachusetts Institute of Technology. "Advanced geophysical studies of accretion of oceanic lithosphere in Mid-Ocean Ridges characterized by contrasting tectono-magmatic settings." Thesis, Massachusetts Institute of Technology, 2012. http://hdl.handle.net/1721.1/70780.
Full textAbstract:
Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2012.Cataloged from PDF version of thesis.
Includes bibliographical references.
The structure of the oceanic lithosphere results from magmatic and extensional processes taking place at mid-ocean ridges (MORs). The temporal and spatial scales of the variability of these two processes control the degree of heterogeneity of the oceanic lithosphere, represented by two end-member models: the classical Penrose Model exemplified by layered magmatic crust formed along fast-spreading MORs, e.g., East Pacific Rise (EPR); and the recently defined Chapman Model describing heterogeneous mafic and ultramafic lithosphere formed in settings of oceanic detachment faulting common along slow-spreading MORs, e.g., Mid-Atlantic Ridge (MAR). This thesis is using advanced marine geophysical methods (including finite-difference wave propagation modeling, 3D multi-channel seismic reflection imaging, waveform inversion, streamer tomography, and near-bottom magnetics) to study lithospheric accretion processes in MORs characterized by contrasting tectono-magmatic settings: the magmatically dominated EPR axis between 9°30'-10°00°N, and the Kane Oceanic Core Complex (KOCC), a section of MAR lithosphere (23°20°-23°38°N) formed by detachment faulting. At the EPR study area, I found that the axial magma chamber (AMC) melt sill is segmented into four prominent 2-4-km-long sections spaced every -5- 10 km along the ridge axis characterized by high melt content (>95%). In contrast, within the intervening sections, the AMC sill has a lower melt content (41-46%). The total magma volume extracted from the AMC sill was estimated of ~46 x 106 M3, with ~24 x 106 M3 left unerupted in the upper crust as dikes after 2005-06 eruption. At the KOCC, I used streamer tomography to constrain the shallow seismic velocity structure. Lithological interpretation of the seismic tomographic models provides insights into the temporal and spatial evolution of the melt supply at the spreading axis as the KOCC formed and evolved. Investigation of a magnetic polarity reversal boundary in crosssection at the northern boundary of KOCC suggests that the boundary (representing both a frozen isotherm and an isochron) dips away from the ridge axis along the Kane transform fault scarp, with a west-dipping angle of ~45° in the shallow (<1 km) crust and <20° in the deeper crust.
by Min Xu.
Ph.D.
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Liu, Lei. "Heat transfer from a convecting crystallizing, replenished magmatic sill and its link to seafloor hydrothermal heat output." Diss., Georgia Institute of Technology, 2010. http://hdl.handle.net/1853/37215.
Full textAbstract:
Hydrothermal systems at oceanic spreading centers play an important role in the composition of seawater, the formation of ore deposits, the support of microbial and macrofaunal ecosystems, and even for the development of life on early earth. These circulation systems are driven by heat transport from the underlying magma chamber, where latent heat of crystallization and sensible heat from cooling are transferred by vigorous, high Rayleigh number convection through a thin conductive boundary layer.
The traditional study of magmatic-hydrothermal systems is primarily based on the time-series observation, which takes the form of repeat visits, continuous offline monitoring by autonomous instruments, or continuous online monitoring by instruments with satellite or cable links to shore. Although a number of studies have deployed autonomous monitoring instruments at vents and around mid-ocean ridges to investigate geophysical and hydrothermal processes, the data are still rather limited and a comprehensive understanding of magma-hydrothermal processes at oceanic spreading centers is lacking. Numerical modeling needs to be employed to elucidate the dynamic behavior of magmatic hydrothermal systems and for testing completing hypotheses in these complex, data-poor environments.
In this dissertation, I develop a mathematical framework for investigating heat transport from a vigorously convecting, crystallizing, cooling, and replenished magma chamber to an overlying hydrothermal system at an oceanic spreading center. The resulting equations are solved numerically using MATLAB. The simulations proceed step-by-step to investigate several different aspects of the system.
First, I consider a hydrothermal system driven by convection, cooling and crystallization in a ~ 100 m thick basaltic magma sill representing an axial magma chamber (AMC) at an oceanic spreading center. I investigate two different crystallization scenarios, crystal-suspended and crystal-settling, and consider both un-replenished and replenished AMCs. In cases without magma replenishment, the simulation results for crystals-suspended models show that heat output and the hydrothermal temperature decrease rapidly and crystallinity reaches 60% in less than ten years. In crystals-settling models, magma convection may last for decades, but decreasing heat output and hydrothermal temperatures still occur on decadal timescales. When magma replenishment is included, the magmatic heat flux approaches steady state on decadal timescales, while the magma body grows to double its original size. The rate of magma replenishment needed ranges between 5 x 10⁵ and 5 x 10⁶ m³/yr, which is somewhat faster than required for seafloor spreading, but less than fluxes to some terrestrial and subseafloor volcanoes on similar timescales. The heat output from a convecting, crystallizing, replenished magma body that is needed to drive observed high-temperature hydrothermal systems is consistent, with gabbro glacier models of crustal production at mid-ocean ridges.
Secondly, I study the heat transfer model from a parametric perspective and examine the effects of both initial magma chamber thickness and magma replenishment rate on the hydrothermal heat output. The initial rate of convective heat transfer is independent of the initial sill thickness; but without magma replenishment, the rate of decay of the heat output varies linearly with thickness, resulting in short convective lifetimes and decaying hydrothermal temperatures for sills up to ~ 100m thick. When magma replenishment is included in crystals settling scenarios at constant or exponentially decreasing rates of ~ 10⁻⁸ m/s to the base of the sill, growth of the sill results in stabilized heat output and hydrothermal temperature on decadal timescales and a relatively constant to increasing thickness of the liquid layer. Sills initially ~ 10 m thick can grow, in principal, to ~ 10 times their initial size with stable heat output and a final melt thickness less than 100m. Seismic data provides evidence of AMC thickness, but it can not discriminate whether it denotes initial magma thickness or is a result of replenishment. These results suggest that magma replenishment might not be seismically detectable on decadal time scales. Periodic replenishment may also result in quasi-stable heat output, but the magnitude of the heat output may vary considerably in crystals suspended models at low frequencies; compared to crystals settling models. In these models the direct coupling between magmatic and hydrothermal heat output suggests that heat output fluctuations might be recorded in hydrothermal vents; but if damping effects of the basal conductive boundary layer and the upflow zone are taken into account, it seems unlikely that heat output fluctuations on a time scale of years would be recorded in hydrothermal vent temperatures or heat output.
Thirdly, I extend the work to the binary system motivated by the fact that the real magmas are multi-component fluids. I focus on the extensively studied binary system, diopside-anorthite (Di-An), and investigate the effects of convection of a two-component magma system on the hydrothermal circulation system through the dynamic modeling of both temperature and heat output. I model the melt temperature and viscosity as a function of Di concentration, and incorporate these relations in the modeling of the heat flux. Simulations comparing the effects of different initial Di concentrations indicate that magmas with higher initial Di concentrations convect more vigorously, which results in faster heat transfer, more rapid removal of Di from the melt and growth of crystals on the floor. With magma replenishment, I assume that the magma chamber grows either horizontally or vertically. In either case magma replenishment at a constant rate of ~ 10⁻⁸ m³/a can maintain relatively stable heat output of 10⁷-10⁹ Watts and reasonable hydrothermal vent temperatures for decades. The final stabilized heat flux increases with increasing Di content of the added magma. Periodic replenishment with a 10 year period results in temperature perturbations within the magma that also increase as a function of increasing Di. With the simple magma model used here, one can not discern conclusively whether the decrease in magma temperature between the 1991/1992 and the 2005/2006 eruptions at EPR 9°50'N involved replenishment with more or less evolved magmas.
Fourthly, I investigate a high-silica magma chamber as the hydrothermal circulation driver. I construct viscosity models for andesite and dacite melts as a function of temperature and water content and incorporate these expressions into a numerical model of thermal convective heat transport from a high Rayleigh number, well-mixed, crystallizing and replenished magma sill beneath a hydrothermal circulation system. Simulations comparing the time dependent heat flux from basalt, 0.1wt.% andesite, 3wt.% andesite, and 4wt.% dacite, indicate that higher viscosity magmas convect less vigorously, which results not only in lower heat transport and hydrothermal vent temperatures, but also in a lower decay rate of the vent temperature. Though somewhat colder, hydrothermal systems driven by unreplenished high-silica melts tend to have a longer lifetime than those driven by basalts, assuming a heat output cutoff of 10⁷ Watts. As in the basaltic case, magma replenishment at a rate of ~ 3 x 10⁵ - 3 x 10⁶ m³/a can maintain relatively stable heat output of 10⁷-10⁹ Watts and hydrothermal vent temperatures for decades. Idealized models of porous flow through the lower crust suggest such replenishment rates are not likely to occur, especially for high-viscosity magmas such as andesite and dacite. Long term stability of hydrothermal systems driven by these magmas requires an alternate means of magma replenishment.
Finally, the dissertation concludes by discussing some avenues for future work. Most important of these are to: (1) couple magma convection with more realistic hydrothermal models and (2) link magma chamber processes to better physical models of replenishment and eruption.
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Babcock, Jeffrey Matthew. "Magma chamber structure and Moho reflections along the East Pacific Rise /." Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC campuses, 1997. http://wwwlib.umi.com/cr/ucsd/fullcit?p9737307.
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Lassner, Lisa A. "Examining the effects of mid ocean ridge topography on 3D marine magnetometric resistivity model responses." Thesis, Massachusetts Institute of Technology, 2004. http://hdl.handle.net/1721.1/58867.
Full textAbstract:
Thesis (S.M.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2004.Includes bibliographical references (leaves 68-69).
Methods which measure seafloor resistivity are uniquely suited to studying hydrothermal circulation in the crust. The magnetometric resistivity (MMR) technique is a galvanic method which uses a bipole current source with a magnetometer receiver. The resistivity of the subsurface can be estimated from the magnetic field read in MMR. In order to analyze and invert MMR data taken near Mid Ocean Ridges, it is important to understand the effects of ridge topography on MMR models. To analyze these effects a 3D MMR forward modeling program MMR3Df̲wd is used to model Mid Ocean Ridges with varying slopes, resistivities, and source/receiver geometries. The modeled magnetic fields are compared with models with a flat seafloor to determine the impact of the ridge topography. Results show that for some of the ridges modeled, the effects of the topography were significant, suggesting that in some instances it is important to include ridge topography in forward models to obtain accurate results from data inversion.
by Lisa A. Lassner.
S.M.
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Beynon, J. "Mineral reactions and isotopic tracer transport in the Troodos ophiolite as analogues of hydrothermal systems at mid-ocean ridges." Thesis, University of Cambridge, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.596622.
Full textAbstract:
The structure and evolution of mid-ocean ridge hydrothermal systems and the magnitude of crustal alteration associated with different temperature fluid fluxes are poorly understood. This project has used strontium and oxygen isotopes together with metamorphic assemblages to investigate fluid flow geometry, fluid evolution and fluid-rock interaction during hydrothermal circulation in the Troodos Alteration Zone, the mesostasis has been altered to saponite but coarse clinopyroxene and plagioclase phenocrysts are unaltered. Homogeneous whole rock 90 Ma 87Sr/86Sr profiles from the cores to rims of saponite-bearing pillows and dykes indicate the fluid flux passed through a matrix porosity rather than being channelled around the margins of the pillows or dykes. Fluid was channelled along cracks within glassy margins. A later celadonite-forming fluid was channelled along the margins of pillows and dykes. Within the Transition Zone, the mesostasis and clinopyroxene are altered to smectite/chlorite or chlorite, and plagioclase is variably altered to albite and alkali feldspar. Increasing whole rock 90 Ma 87Sr/86Sr correlates with increasing albitisation of plagioclase indicating that magmatic strontium is retained in anorthite. There is no 90 Ma 87Sr/86Sr gradient perpendicular to dyke margins suggesting that fluid flowed through the matrix porosity and was not channelled along dyke margins. Within the Basal Group and Sheeted Dyke Zone a greenschist facies assemblage is located structurally higher than an actinolite facies assemblage. Increasing 90 Ma 87Sr/86Sr correlates with increasing albitisation of plagioclase indicating that magmatic strontium is retained in anorthite. Samples with albitised plagioclase had similar 90 Ma 87Sr/86Sr values to the previously determined Sheeted Dyke Zone range of 0.7047-0.7059 (Bickle & Teagle, 1992).
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Williams, Clare Margaret. "Oceanic lithosphere magnetization marine magnetic investigations of crustal accretion and tectonic processes in mid-ocean ridge environments /." Thesis, Massachusetts Institute of Technology, 2007. http://hdl.handle.net/1912/2031.
Full textAbstract:
Thesis (Ph. D.)--Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2007."Joint Program in Oceanography/Applied Ocean Science and Engineering"--Cover. Title from Web page (viewed on Mar. 24, 2008). "September 2007". Includes bibliographical references.
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France, Lydéric [Verfasser]. "Magmatic/ hydrothermal interactions at fast spreading mid-ocean ridges : implications on the dynamics of the axial melt lens / Lydéric France." Hannover : Technische Informationsbibliothek und Universitätsbibliothek Hannover, 2010. http://d-nb.info/1010987348/34.
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Sheehan, Anne Frances. "Lateral variation in upper mantle temperature and composition beneath mid-ocean ridges inferred from shear-wave propagation, geoid, and bathymetry." Thesis, Massachusetts Institute of Technology, 1991. http://hdl.handle.net/1721.1/58232.
Full textAbstract:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1991.Includes bibliographical references (p. 208-224).
by Anne Frances Sheehan.
Ph.D.