Showing papers on "Mid-ocean ridge published in 2012"
TL;DR: In this paper, the authors investigate tectonic events from the period ∼110 to 90 million years ago and show that the reorganization occurred between 105 and 100 million years, was global in scale, and affected all major plates.
150 citations
TL;DR: Crameri et al. as mentioned in this paper show that assuming a free surface, rather than the conventional free-slip surface, allows the dynamical behavior at convergent plate boundaries to change from double-sided to single-sided.
Abstract: [1] Earth’s lithosphere is characterized by the relative movement of almost rigid plates as part of global mantle convection. Subduction zones on present-day Earth are strongly asymmetric features composed of an overriding plate above a subducting plate that sinks into the mantle. While global self-consistent numerical models of mantle convection have reproduced some aspects of plate tectonics, the assumptions behind these models do not allow for realistic single-sided subduction. Here we demonstrate that the asymmetry of subduction results from two major features of terrestrial plates: (1) the presence of a free deformable upper surface and (2) the presence of weak hydrated crust atop subducting slabs. We show that assuming a free surface, rather than the conventional free-slip surface, allows the dynamical behavior at convergent plate boundaries to change from double-sided to single-sided. A weak crustal layer further improves the behavior towards steady single-sided subduction by acting as lubricating layer between the sinking and the overriding plate. This is a first order finding of the causes of single-sided subduction, which by its own produces important features like the arcuate curvature of subduction trenches. Citation: Crameri, F., P. J. Tackley, I. Meilick, T. V. Gerya, and B. J. P. Kaus (2012), A free plate surface and weak oceanic crust produce single-sided subduction on Earth, Geophys. Res. Lett., 39, L03306, doi:10.1029/2011GL050046.
142 citations
TL;DR: In this article, high-precision U/Pb zircon geochronology from the Oman-United Arab Emirates (U.A.E.) ophiolite provides insight into the timing and duration of magmatism and the tectonic setting during formation of the lower crust.
Abstract: [1] New high-precision U/Pb zircon geochronology from the Oman-United Arab Emirates (U.A.E.) ophiolite provides insight into the timing and duration of magmatism and the tectonic setting during formation of the lower crust. The new data come from a well-preserved and exposed crustal section in the center of the Wadi Tayin massif. Single grain and grain fragment 206Pb/238U dates from upper-level gabbros, tonalites/trondhjemites and gabbroic pegmatites, corrected for initial Th exclusion, range from 112.55 ± 0.21 to 95.50 ± 0.17 Ma, with most data clustered between 96.40 ± 0.17 to 95.50 ± 0.17 Ma. Zircon dates from upper-level gabbros are most consistent with the ophiolite forming at a fast spreading ridge with half-rates of 50–100 km/Ma. Dates from tonalites/trondhjemites and from a gabbroic pegmatite associated with a wehrlite intrusion overlap with dates from adjacent upper-level gabbros, suggesting that any age differences between these three magmatic series are smaller than the analytical uncertainties or intrasample variability in the dates. Three of the dated upper-level gabbros and a single gabbroic pegmatite from the base of the crust have >1 Ma intrasample variability in single grain dates, suggesting assimilation of older crust during the formation or crystallization of the magmas. Whole rock eNd(t) of seven samples, including the upper-level gabbros with variable zircon dates, have tightly clustered initial values ranging from eNd(96 Ma) = 7.59 ± 0.23 to 8.28 ± 0.31. The eNd values are similar to those from other gabbros within the ophiolite, suggesting that any assimilated material had a similar isotopic composition to primitive basaltic magmas. The new dates suggest that the studied section formed at a fast spreading mid-ocean ridge between ∼96.4–95.5 Ma. The large intrasample variability in zircon dates in some samples is unexpected in this setting, and may be related to propagation of a younger ridge into older oceanic lithosphere.
123 citations
TL;DR: The authors found that 25% of the melt inclusions from two Pacific Ocean mid-ocean ridges crystallize below the melt lens to form the lower oceanic crust, but there is little consensus on where crystallization of melt actually occurs within the crust or mantle.
Abstract: Oceanic crust is formed at mid-ocean ridges, but there is little consensus on where crystallization of melt actually occurs within the crust or mantle. Geochemical analyses of melt inclusions from two Pacific Ocean mid-ocean ridges indicate that 25% of the melt crystallizes below the melt lens to form the lower oceanic crust.
118 citations
TL;DR: In this paper, the authors investigate the consequences of chemical heterogeneity of the mantle for the predicted pathways of melt transport and show that preferentially melted heterogeneities can nucleate magmatic channels that transport their melts.
103 citations
TL;DR: In the roots of the ocean crust, mantle-derived rocks are progressively hydrated by hydrothermal circulation as mentioned in this paper, which point to the hydration process as a possible energy source.
Abstract: In the roots of the ocean crust, mantle-derived rocks are progressively hydrated by hydrothermal circulation. Raman spectroscopic analyses of hydrated rocks sampled from the ocean floor reveal accumulations of organic matter, which point to the hydration process as a possible energy source.
101 citations
TL;DR: The Oceanography Society as mentioned in this paper published a survey of the state-of-the-art methods for ocean exploration and its application in the field of ocean exploration. The published article is copyrighted by the Oceanography society and can be found at http://www.tos.org
Abstract: This is the publisher’s final pdf. The published article is copyrighted by The Oceanography Society and can be found at: http://www.tos.org/.
101 citations
TL;DR: Measurements of the depths and thicknesses of the Pv → pPV transition in multiphase systems (San Carlos olivine, pyrolitic, and midocean ridge basaltic compositions) at the P - T conditions of the lowermost mantle, searching for candidate rocks with a sharp Pv - pPv discontinuity.
Abstract: The discovery of a phase transition in Mg-silicate perovskite (Pv) to postperovskite (pPv) at lowermost mantle pressure-temperature (P - T) conditions may provide an explanation for the discontinuous increase in shear wave velocity found in some regions at a depth range of 200 to 400 km above the core-mantle boundary, hereafter the D′′ discontinuity. However, recent studies on binary and ternary systems showed that reasonable contents of Fe2+ and Al for pyrolite increase the thickness (width of the mixed phase region) of the Pv - pPv boundary (400–600 km) to much larger than the D′′ discontinuity (≤ 70 km). These results challenge the assignment of the D′′ discontinuity to the Pv - pPv boundary in pyrolite (homogenized mantle composition). Furthermore, the mineralogy and composition of rocks that can host a detectable Pv → pPv boundary are still unknown. Here we report in situ measurements of the depths and thicknesses of the Pv → pPv transition in multiphase systems (San Carlos olivine, pyrolitic, and midocean ridge basaltic compositions) at the P - T conditions of the lowermost mantle, searching for candidate rocks with a sharp Pv - pPv discontinuity. Whereas the pyrolitic mantle may not have a seismologically detectable Pv → pPv transition due to the effect of Al, harzburgitic compositions have detectable transitions due to low Al content. In contrast, Al-rich basaltic compositions may have a detectable Pv - pPv boundary due to their distinct mineralogy. Therefore, the observation of the D′′ discontinuity may be related to the Pv → pPv transition in the differentiated oceanic lithosphere materials transported to the lowermost mantle by subducting slabs.
92 citations
TL;DR: The East Pacific Rise between 9°N and 10°N: Twenty-five years of integrated, multidisciplinary oceanic spreading center studies was studied in this article, where the authors proposed a model for the spreading center.
Abstract: Fornari, D.J., K.L. Von Damm, J.G. Bryce, J.P. Cowen, V. Ferrini, A. Fundis, M.D. Lilley, G.W. Luther III, L.S. Mullineaux, M.R. Perfit, M.F. Meana-Prado, K.H. Rubin, W.E. Seyfried Jr., T.M. Shank, S.A. Soule, M. Tolstoy, and S.M. White. 2012. The East Pacific Rise between 9°N and 10°N: Twenty-five years of integrated, multidisciplinary oceanic spreading center studies. Oceanography 25(1):18–43, http://dx.doi.org/10.5670/oceanog.2012.02.
91 citations
TL;DR: In this paper, the authors use 3D spherical numerical simulations with self-consistently generated plates and compositionally and rheologically distinct continents floating at the top of the mantle in order to investigate the feedbacks between continental drift, oceanic plate tectonics and the thermal state of the Earth's mantle, by using different continent configurations ranging from one supercontinent to six small continents.
88 citations
TL;DR: In this paper, the formation and evolution of the Ninetyeast Ridge (NER) was analyzed using geophysical data for the Central Indian and Wharton Basins of the Indian Ocean.
Abstract: [1] Analysis of new and existing geophysical data for the Central Indian and Wharton Basins of the Indian Ocean were used to understand the formation and evolution of the Ninetyeast Ridge (NER), especially its relationship to the Kerguelen hot spot and the Wharton spreading ridge. Satellite gravity data and magnetic anomalies 34 through 19 define crustal isochrons and show fracture zones striking ∼N5°E. One of these, at 89°E, crosses the ∼N10°E trending NER, impacting the NER morphology. From 77 to 43 Ma the NER lengthened at a rate of ∼118 km/Myr, twice that of the ∼48–58 km/Myr accretion rate of adjacent oceanic crust. This difference can be explained by southward jumps of the Wharton spreading ridge toward the hot spot, which transferred portions of crust from the Antarctic plate to the Indian plate, lengthening the NER. Magnetic anomalies document a small number of large spreading ridge jumps in the ocean crust immediately to the west of the NER, especially two leaving observable 65 and 42 Ma fossil spreading ridges. In contrast, complex magnetic anomaly progressions and morphology imply that smaller spreading ridge jumps occurred at more frequent intervals beneath the NER. Comparison of the NER dates and magnetic anomaly ages implies that the hot spot first emplaced NER volcanoes on the Indian plate at a distance from the Wharton Ridge, but as the northward drifting spreading ridge approached the hot spot, the two interacted, keeping later NER volcanism near the spreading ridge crest by spreading center jumps.
TL;DR: In this article, it was shown that a limited number of spreading modes can form: transform faults, microplates, overlapping spreading centers, zigzag ridges and oblique connecting spreading centers.
TL;DR: In this article, single sulfides from Gakkel and Southwest Indian ridge peridotites were analyzed for Pb and Re-Os to constrain the evolution of oceanic mantle composition.
TL;DR: A characterization framework such that as presented in this paper, perhaps including alternative or additional physical or chemical characteristics, is essential for achieving the greatest benefit from multidisciplinary microbial investigations of oceanic ridge flanks.
Abstract: We discuss ridge flank environments in the ocean crust as habitats for subseafloor microbial life. Oceanic ridge flanks, areas far from the magmatic and tectonic influence of seafloor spreading, comprise one of the largest and least explored microbial habitats on the planet. We describe the nature of the ridge flank crustal environments, and present a framework for delineating a continuum of conditions and processes that are likely to be important for defining subseafloor microbial "provinces." The basis for this framework is three governing sets of conditions that help to determine the nature of subseafloor biomes: crustal age, extent of fluid flow, and thermal state. We present a brief overview of subseafloor conditions, within the context of these three characteristics, for five field sites where microbial studies have been done, are underway, or have been proposed. Technical challenges remain and likely will limit progress in studies of microbial ridge-flank ecosystems, which is why it is vital to select and design future studies so as to leverage as much general understanding as possible. A characterization framework such as presented in this paper, perhaps including alternative or additional physical or chemical characteristics, is essential for achieving the greatest benefit from multidisciplinary microbial investigations of the oceanic ridge flanks.
TL;DR: In this paper, the authors evaluate whether seismic attenuation can reconcile subridge seismic structure with low melt fractions, which is at odds with low productivity and high melt mobility inferred from petrology and geochemistry.
Abstract: The first comprehensive seismic experiment sampling subridge mantle revealed a pronounced low-velocity zone between 40 and 100 km depth below the East Pacific Rise (EPR) that has been attributed to substantial retained melt fractions of 0.3–2%. Such high melt fractions are at odds with low melt productivity and high melt mobility inferred from petrology and geochemistry. Here, we evaluate whether seismic attenuation can reconcile subridge seismic structure with low melt fractions. We start from a dynamic spreading model which includes melt generation and migration and is converted into seismic structure, accounting for temperature-, pressure-, composition-, phase-, and melt-dependent anharmonicity, and temperature-, pressure-, frequency- and hydration-dependent anelasticity. Our models predict a double low-velocity zone: a shallow—approximately triangular—region due to dry melting, and a low-velocity channel between 60 and 150 km depth dominantly controlled by solid state high-temperature seismic attenuation in a damp mantle, with only a minor contribution of (<0.1%) melt. We test how tomographic inversion influences the imaging of our modeled shear velocity features. The EPR experiment revealed a double low-velocity zone, but most tomographic studies would only resolve the deeper velocity minimum. Experimentally constrained anelasticity formulations produce VSas low as observed and can explain lateral variations in near-ridge asthenospheric VS with ±100 K temperature variations and/or zero to high water content. Furthermore, such QS formulations also reproduce low asthenospheric VS below older oceans and continents from basic lithospheric cooling models. Although these structures are compatible with global QS images, they are more attenuating than permitted by EPR data.
TL;DR: In this paper, the authors investigated the relationship between mantle heterogeneity, melting, and magmatic channelization based on conservation mass, momentum, energy, and composition in a system with two phases and two thermodynamic components in local thermodynamic equilibrium.
Abstract: [1] Beneath mid-ocean ridges, magma is thought to rise through a network of high porosity channels that form by reactive flow Partial mantle melts travel rapidly through these channels to the surface, and retain the geochemical signature of their source rock Global analyses of mid-ocean ridge lavas indicates that the mantle is chemically heterogeneous, but the consequences of this heterogeneity for reactive porous flow remain unclear Using numerical models of coupled magma/mantle dynamics, we investigate the relationships between mantle heterogeneity, melting, and magmatic channelization The models are based on conservation mass, momentum, energy, and composition in a system with two phases and two thermodynamic components in local thermodynamic equilibrium One of these components is more fusible than the other In this context, we find that heterogeneities enriched in the more fusible component can nucleate magmatic channels To understand this result we consider an expression for the melting rate derived from the conservation principles This expression quantifies the relationship of decompression, reactive flow, and thermal diffusion to the melting rate With it, we assess their relative importance in the ambient mantle, channels, and enriched heterogeneities In our models, heat diffuses into fertile channels and powers melting, in combination with reactive flow These results suggest that thermal diffusion influences the dynamics of magmatic channelization
TL;DR: In this paper, the authors used less mobile elements and a filter to remove the effects of recent slab additions, and provided new Hf isotope data for 30 Mariana Trough (MT) backarc basin lavas, showing that Hf and Nd isotope ratios are highly correlated, of Indian mid-oceanic ridge basalt character, and display variations similar to ocean ridges of comparable lengths.
Abstract: In the study of geochemical mass balances at subduction zones, the composition of the mantle wedge prior to additions from the slab is a critically important yet poorly constrained parameter. Deconvolving the influence of ancient versus modern enrichments is particularly
difficult, especially when considering elements that are highly mobile. Here we provide an alternative approach, using less mobile elements, and a filter to remove the effects of recent slab additions. We provide new Hf isotope data for 30 Mariana Trough (MT) backarc basin lavas. Once filtered, Hf and Nd isotope ratios are highly correlated, of Indian mid-oceanic ridge basalt character, and display variations similar to ocean ridges of comparable lengths.
The isotopic variability observed in this “ambient mantle” provides a new paradigm for the interpretation of the varied volcanic products of the arc. Thus, shoshonites associated with the northern termination of the backarc basin rift axis reflect the interaction of a subducted
sediment melt with an isotopically enriched mantle source. In contrast, the large volcanoes of the Central Island province have a consistent offset in Nd isotope compositions from the MT array resulting from fluid addition. Existing data for smaller edifices in the submarine portion of the arc have larger variations resulting from fluid addition on a more local scale. We suggest that the similar characterization of ambient mantle elsewhere may help to resolve many conflicting geochemical observations in arc lavas worldwide.
TL;DR: In this article, the authors studied the influence of slab properties, oceanic ridge position, continental keels, and a weak asthenospheric layer on NR and trench migration in the present-day plate setting.
Abstract: [1] Absolute plate motion models with respect to a deep mantle reference frame (e.g., hot spots) typically contain some net rotation (NR) of the lithosphere. Global mantle flow models for the present-day plate setting reproduce similarly oriented NRs but with amplitudes significantly smaller than those found in some high NR Pacific hot spot reference frames. It is therefore important to understand the mechanisms of NR excitation, which we attempt here with two-dimensional cylindrical models of an idealized Pacific domain. We study the influence of slab properties, oceanic ridge position, continental keels, and a weak asthenospheric layer on NR and trench migration. Fast slab return flow develops in models with stiff slabs and moderate slab dips. Rapid NRs, comparable to the high NR Pacific hot spot reference frames, are primarily induced by asymmetric slab dips, in particular a shallow slab beneath South America and a steep slab in the western Pacific. A scaling relationship links the amplitude of NR to plate size, slab dip angle, and slab viscosity. Asymmetric ridge positions also promote NR through asymmetric plate sizes. Continental keels have less impact, in contrast to what has been found in earlier global studies. Several models yield unidirectional Pacific trench motions, such as slab advance in the western Pacific and, simultaneously, slab retreat in the eastern Pacific. Our model provides a physical explanation for NR generation in the present-day Pacific setting and hints at mechanisms for the temporal evolution of the basin.
TL;DR: In this article, a suite of samples from the 2005-06 eruption of the East Pacific Rise (EPR), collected at ~200m intervals along two well-defined flow pathways within a single seafloor eruption to examine syn-emplacement degassing, using measurements of vesicularity, crystallinity, volatile contents, and helium.
TL;DR: In this article, the authors used high-resolution magnetic data that were collected across the Tendaho Graben in the Afar Depression, Ethiopia, to document one of the fi rst examples of subaerial magnetic lineations similar in pattern and amplitude to those that characterize seafl oor spreading centers.
Abstract: Magnetic stripes parallel to mid-ocean ridges are one of the most signifi cant consequences of seafl oor spreading, and have played an essential role in the establishment of the plate tectonics theory and the determination of seafl oor spreading rates. Similar magnetic anomaly patterns have not been well documented subaerially in continental rifts transitioning into seafl oor spreading centers. Here, using high-resolution magnetic data that were collected across the Tendaho Graben in the Afar Depression, Ethiopia, we document one of the fi rst examples of subaerial magnetic lineations similar in pattern and amplitude to those that characterize seafl oor spreading centers. The ~50-km-wide graben is the southernmost structural and geomorphological expression of the on-land continuation of the Red Sea propagator, which is taken to represent the Arabian-Nubian plate boundary within Afar. The graben is bounded by northwest-trending border faults, with the footwalls dominated by ca. 1.7 Ma basalts and the downthrown blocks constituting progressively younger basalts toward the center of the graben, reaching ca. 35 ka. The Tendaho magnetic fi eld is characterized by an ~10-km-wide linear negative magnetic anomaly that corresponds to a normal-polarity zone that is fl anked by two parallel, ~20-km-wide linear positive magnetic anomalies of reversed polarity. This work shows that magnetic stripes can be developed in transitional continental rifts before the development of oceanic spreading centers. The common assumption that magnetic stripes can be used to date the onset of seafl oor spreading may need to be re-evaluated in light of the evidence provided here.
TL;DR: In order to constrain the early history of the mantle domains sampled by ocean island basalts and mid-ocean ridge basalts (MORB), the authors presented high-precision142Nd/144Nd measurements on 11 different lavas from five hot spots, and one lava each from the Indian and Atlantic ridges.
Abstract: [1] 146Sm decays to 142Nd with a relatively short half-life (∼68 Ma). The142Nd/144Nd of modern terrestrial mantle-derived lavas is 18 ± 5 ppm higher than the chondrite reservoir. The difference in142Nd/144Nd between Earth and chondrites likely owes to Sm/Nd ratios 6% higher in the accessible Earth that arose within the first 30 million years following accretion. In order to constrain the early history of the mantle domains sampled by ocean island basalts (OIB) and mid-ocean ridge basalts (MORB), we present high-precision142Nd/144Nd measurements on 11 different lavas from five hot spots, and one lava each from the Indian and Atlantic ridges. The lavas examined in this study bracket much of the known Sr-Nd-Pb-He isotopic variability the in mantle. These data complement existing high-precision142Nd/144Nd data on MORB and OIB lavas. In agreement with previous studies, we find that MORB and OIB lavas examined for high-precision142Nd/144Nd exhibit ratios that are indistinguishable from the terrestrial standard and are 15–20 ppm higher than the average obtained for ordinary and enstatite chondrites. The uniform, superchondritic 142Nd/144Nd data in OIB and MORB are consistent with derivation from a common, early formed (<30 Ma after accretion) progenitor reservoir with Sm/Nd ∼6% higher than chondrites. If there exists any variability in 142Nd/144Nd in the OIBs and MORBs examined to date, it is too small to be resolved with the precision currently available.
TL;DR: In this paper, microstructural and petrological characteristics of a large number of samples from ultramafic exposures in the walls of the Mid-Atlantic Ridge (MAR) axial valley at three distinct locations at lat. 13°N and 14°45′N were reported.
Abstract: [1] Outcrops of deeply derived ultramafic rocks and gabbros are widespread along slow spreading ridges where they are exposed in the footwall of detachment faults. We report on the microstructural and petrological characteristics of a large number of samples from ultramafic exposures in the walls of the Mid-Atlantic Ridge (MAR) axial valley at three distinct locations at lat. 13°N and 14°45′N. One of these locations corresponds to the footwall beneath a corrugated paleo-fault surface. Bearing in mind that dredging and ROV sampling may not preserve the most fragile lithologies (fault gouges), this study allows us to document a sequence of deformation, and the magmatic and hydrothermal history recorded in the footwall within a few hundred meters of the axial detachment fault. At the three sampled locations, we find that tremolitic amphiboles have localized deformation in the ultramafic rocks prior to the onset of serpentinization. We interpret these tremolites as hydrothermal alteration products after evolved gabbroic rocks intruded into the peridotites. We also document two types of brittle deformation in the ultramafic rocks, which we infer could produce the sustained low magnitude seismicity recorded at ridge axis detachment faults. The first type of brittle deformation affects fresh peridotite and is associated with the injection of the evolved gabbroic melts, and the second type affects serpentinized peridotites and is associated with the injection of Si-rich hydrothermal fluids that promote talc crystallization, leading to strain localization in thin talc shear zones. We also observed chlorite + serpentine shear zones but did not identify samples with serpentine-only shear zones. Although the proportion of magmatic injections in the ultramafic rocks is variable, these characteristics are found at each investigated location and are therefore proposed as fundamental components of the deformation in the footwall of the detachment faults associated with denudation of mantle-derived rocks at the MAR.
TL;DR: In this article, the Galapagos Spreading Center (GSC) has been used to study how magma supply affects mid-ocean ridge (MOR) volcanic eruptions.
Abstract: [1] Mapping and sampling of 18 eruptive units in two study areas along the Galapagos Spreading Center (GSC) provide insight into how magma supply affects mid-ocean ridge (MOR) volcanic eruptions. The two study areas have similar spreading rates (53 versus 55 mm/yr), but differ by 30% in the time-averaged rate of magma supply (0.3 × 106 versus 0.4 × 106 m3/yr/km). Detailed geologic maps of each study area incorporate observations of flow contacts and sediment thickness, in addition to sample petrology, geomagnetic paleointensity, and inferences from high-resolution bathymetry data. At the lower-magma-supply study area, eruptions typically produce irregularly shaped clusters of pillow mounds with total eruptive volumes ranging from 0.09 to 1.3 km3. At the higher-magma-supply study area, lava morphologies characteristic of higher effusion rates are more common, eruptions typically occur along elongated fissures, and eruptive volumes are an order of magnitude smaller (0.002–0.13 km3). At this site, glass MgO contents (2.7–8.4 wt. %) and corresponding liquidus temperatures are lower on average, and more variable, than those at the lower-magma-supply study area (6.2–9.1 wt. % MgO). The differences in eruptive volume, lava temperature, morphology, and inferred eruption rates observed between the two areas along the GSC are similar to those that have previously been related to variable spreading rates on the global MOR system. Importantly, the documentation of multiple sequences of eruptions at each study area, representing hundreds to thousands of years, provides constraints on the variability in eruptive style at a given magma supply and spreading rate.
TL;DR: In this article, the authors propose that the large dykes that initiate near the mid-segment magma source are attracted toward segment ends as a result of a thickening of the elastic-brittle lithosphere in the along-rift direction.
TL;DR: In this paper, the authors report evidence for two separate vent fields, one near 3°42′N, 63°40′E and another near 3.41.5′N and 63°50′E, on a segment of the Carlsberg Ridge that is apparently sparsely magmatic.
Abstract: [1] Indian Ocean ridges north of the Rodriguez Triple Junction remain poorly explored for seafloor hydrothermal activity, with only two active sites confirmed north of 25°S. We conducted water column surveys and sampling in 2007 and 2009 to search for hydrothermal plumes over a segment of the Carlsberg Ridge. Here we report evidence for two separate vent fields, one near 3°42′N, 63°40′E and another near 3°41.5′N, 63°50′E, on a segment that is apparently sparsely magmatic. Both sites appear to be located on off-axis highs at the top of the southern axial valley wall, at depths of ∼3600 m or shallower (∼1000 m above the valley floor). At the 63°40′E site, plume sampling found local maxima in light scattering, temperature anomaly, oxidation-reduction potential (ORP), dissolved Mn, and3He. No water samples are available from the 63°50′E site, but it showed robust light-scattering and ORP anomalies at multiple depths, implying multiple sources. ORP anomalies are very short-lived, so the strong signals at both sites suggest that fluid sources lie within a few kilometers or less from the plume sampling locations. Although ultramafic rocks have been recovered near these sites, the light-scattering and dissolved Mn anomalies imply that the plumes do not arise from a system driven solely by exothermic serpentinization (e.g., Lost City). Instead, the source fluids may be a product of both ultramafic and basaltic/gabbroic fluid-rock interaction, similar to the Rainbow and Logatchev fields on the Mid-Atlantic Ridge.
TL;DR: The segmentation pattern of the Central Indian Ridge (CIR) between 3°S and 11°S latitudes in the Indian Ocean has been studied using multibeam bathymetry and magnetic data as discussed by the authors.
TL;DR: In this paper, a detailed analysis of the Ophiolite at Wadi Fizh in the northern Oman ophiolites is presented, which reveals the across-axis accretionary process.
Abstract: [1] Detailed lithological study combined with geochemical variations of lavas reveals the across-axis accretionary process at Wadi Fizh in the northern Oman ophiolite. The >900 m thick V1 sequence is divided into the lower V1 (LV1), middle V1 (MV1) and upper V1 (UV1) sequence by 0.4 m and 0.8 m thick umbers at 410 mab (meters above the base of the extrusive rocks) and 670 mab, respectively. The lowest part of the LV1 (LV1a) consists of lobate sheet and pillow lava flows extruded on the relatively flat ridge crest. Elongate pillows at 230 mab are flows draping downslope from the ridge crest and characterize the lithofacies on the ridge flank. Just above a jasper layer at 270 mab, 130 m thick evolved lavas were transported from the crest and emplaced on the ridge flank (LV1b). Off-axial accretionary processes recorded in the MV1 resulted in alternating flows of less evolved, depleted lava and evolved lava, suggesting that the MV1 off-axial lava sequence comprises flows emanated from both on- and off-axis source vents. The less evolved and depleted UV1 flows suggest independent sources distinct from the axial lavas. The Lasail Unit is regarded as a subunit of the V1 because it is comparable to the UV1 in the geological, petrological, and geochemical characteristics. The broad compositional range of the V1 sequence endorses a view that the Wadi Fizh area corresponds to a segment end of the Oman paleospreading system accompanied by off-axis volcanism as in segment boundaries of the present East Pacific Rise.
TL;DR: In this article, remote observations of seismic anisotropy beneath mid-ocean ridges are made using measurements of source-side shear wave splitting, but are observed at teleseismic distances.