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Sample records for oceanic crust

  1. The Oceanic Crust.

    ERIC Educational Resources Information Center

    Francheteau, Jean

    1983-01-01

    The earth's oceanic crust is created and destroyed in a flow outward from midocean ridges to subduction zones, where it plunges back into the mantle. The nature and dynamics of the crust, instrumentation used in investigations of this earth feature, and research efforts/findings are discussed. (JN)

  2. The Oceanic Crust.

    ERIC Educational Resources Information Center

    Francheteau, Jean

    1983-01-01

    The earth's oceanic crust is created and destroyed in a flow outward from midocean ridges to subduction zones, where it plunges back into the mantle. The nature and dynamics of the crust, instrumentation used in investigations of this earth feature, and research efforts/findings are discussed. (JN)

  3. Ophiolites and oceanic crust

    USGS Publications Warehouse

    Moores, E.M.; Jackson, E.D.

    1974-01-01

    OPHIOLITES consist of a pseudostratiform sequence, of harzburgite, tectonite, ultramafic and mafic cumulates sometimes including gabbro and quartz diorite (plagiogranite) intrusions, dolerite dyke swarms, pillow lava 1, and deep-sea sediments2-4. This assemblage occurs in all Phanerozoic mountain systems and is interpreted as fossil oceanic crust and uppermost mantle5-10. Outstanding problems include differences between the chemical properties of Ophiolites and rocks thought to represent present-day oceanic crust11,12, the lack in some complexes of recognised dyke swarms or cumulates, and the relative thinness of ophiolite mafic rocks compared with standard oceanic crustal sections5,8,13. ?? 1974 Nature Publishing Group.

  4. Aleutian basin oceanic crust

    USGS Publications Warehouse

    Christeson, Gail L.; Barth, Ginger A.

    2015-01-01

    We present two-dimensional P-wave velocity structure along two wide-angle ocean bottom seismometer profiles from the Aleutian basin in the Bering Sea. The basement here is commonly considered to be trapped oceanic crust, yet there is a change in orientation of magnetic lineations and gravity features within the basin that might reflect later processes. Line 1 extends ∼225 km from southwest to northeast, while Line 2 extends ∼225 km from northwest to southeast and crosses the observed change in magnetic lineation orientation. Velocities of the sediment layer increase from 2.0 km/s at the seafloor to 3.0–3.4 km/s just above basement, crustal velocities increase from 5.1–5.6 km/s at the top of basement to 7.0–7.1 km/s at the base of the crust, and upper mantle velocities are 8.1–8.2 km/s. Average sediment thickness is 3.8–3.9 km for both profiles. Crustal thickness varies from 6.2 to 9.6 km, with average thickness of 7.2 km on Line 1 and 8.8 km on Line 2. There is no clear change in crustal structure associated with a change in orientation of magnetic lineations and gravity features. The velocity structure is consistent with that of normal or thickened oceanic crust. The observed increase in crustal thickness from west to east is interpreted as reflecting an increase in melt supply during crustal formation.

  5. Continental crust generated in oceanic arcs

    NASA Astrophysics Data System (ADS)

    Gazel, Esteban; Hayes, Jorden L.; Hoernle, Kaj; Kelemen, Peter; Everson, Erik; Holbrook, W. Steven; Hauff, Folkmar; van den Bogaard, Paul; Vance, Eric A.; Chu, Shuyu; Calvert, Andrew J.; Carr, Michael J.; Yogodzinski, Gene M.

    2015-04-01

    Thin oceanic crust is formed by decompression melting of the upper mantle at mid-ocean ridges, but the origin of the thick and buoyant continental crust is enigmatic. Juvenile continental crust may form from magmas erupted above intra-oceanic subduction zones, where oceanic lithosphere subducts beneath other oceanic lithosphere. However, it is unclear why the subduction of dominantly basaltic oceanic crust would result in the formation of andesitic continental crust at the surface. Here we use geochemical and geophysical data to reconstruct the evolution of the Central American land bridge, which formed above an intra-oceanic subduction system over the past 70 Myr. We find that the geochemical signature of erupted lavas evolved from basaltic to andesitic about 10 Myr ago--coincident with the onset of subduction of more oceanic crust that originally formed above the Galápagos mantle plume. We also find that seismic P-waves travel through the crust at velocities intermediate between those typically observed for oceanic and continental crust. We develop a continentality index to quantitatively correlate geochemical composition with the average P-wave velocity of arc crust globally. We conclude that although the formation and evolution of continents may involve many processes, melting enriched oceanic crust within a subduction zone--a process probably more common in the Archaean--can produce juvenile continental crust.

  6. Drilling to gabbro in intact ocean crust.

    PubMed

    Wilson, Douglas S; Teagle, Damon A H; Alt, Jeffrey C; Banerjee, Neil R; Umino, Susumu; Miyashita, Sumio; Acton, Gary D; Anma, Ryo; Barr, Samantha R; Belghoul, Akram; Carlut, Julie; Christie, David M; Coggon, Rosalind M; Cooper, Kari M; Cordier, Carole; Crispini, Laura; Durand, Sedelia Rodriguez; Einaudi, Florence; Galli, Laura; Gao, Yongjun; Geldmacher, Jörg; Gilbert, Lisa A; Hayman, Nicholas W; Herrero-Bervera, Emilio; Hirano, Nobuo; Holter, Sara; Ingle, Stephanie; Jiang, Shijun; Kalberkamp, Ulrich; Kerneklian, Marcie; Koepke, Jürgen; Laverne, Christine; Vasquez, Haroldo L Lledo; Maclennan, John; Morgan, Sally; Neo, Natsuki; Nichols, Holly J; Park, Sung-Hyun; Reichow, Marc K; Sakuyama, Tetsuya; Sano, Takashi; Sandwell, Rachel; Scheibner, Birgit; Smith-Duque, Chris E; Swift, Stephen A; Tartarotti, Paola; Tikku, Anahita A; Tominaga, Masako; Veloso, Eugenio A; Yamasaki, Toru; Yamazaki, Shusaku; Ziegler, Christa

    2006-05-19

    Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Recent drilling in the eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate. The gabbros are the crystallized melt lenses that formed beneath a mid-ocean ridge. The depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid-ocean ridges: Melt lenses occur at shallower depths at faster spreading rates. The gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas, precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by Hole 1256D.

  7. The oceanic crust as a bioreactor

    NASA Astrophysics Data System (ADS)

    Staudigel, Hubert; Tebo, Bradley; Yayanos, Art; Furnes, Harald; Kelley, Katie; Plank, Terry; Muehlenbachs, Karlis

    Various lines of evidence suggest that large portions of hydrothermal systems in the oceanic crust acts like a giant bioreactor that mediates water-rock exchange and buffers the chemical composition of seawater. We review the current literature and present new chemical, biological and petrographic data on microbially mediated alteration of glass in the oceanic crust. Microbial alteration of glass displays characteristic bioalteration features, in particular in the alteration fronts around residual glass in the oceanic crust, suggesting that microbes take on an active role in its dissolution. Such features are found throughout the oceanic crust ranging in age from a few million to 170 Ma and they are found down to crustal depths of 500 m, possibly defining a Deep Oceanic Biosphere (DOB) that covers up to 2/3 of the earth's surface area. Microbial glass alteration substantially increases the active surface area of dissolving glass particles thereby enhancing the reaction rates during microbially aided dissolution. Microbially mediated glass alteration involves the establishment of two types of diffusion barriers including hydration rinds and biofilms that play an important role in mediating glass alteration. In particular biofilms may be very active by creating a localized chemical environment conducive to glass alteration, and by sequestering dissolved chemical inventory from solution. When compared with abiotic alteration of the oceanic crust, bio-alteration causes more rapid deposition of biotic and abiotic reaction products in the oceanic crust that result in a more effective removal of elements from seawater and a more rapid sealing of the oceanic crust. Thus, it is likely that microbial activity increases the fluxes of seawater components into the crust, while reducing the low—temperature flux of basalt components into seawater. However, much about the microbial activity and its relationships to the chemistry of hydrothermal systems still remains to be

  8. 1900-Ma ocean crust in Canada

    NASA Astrophysics Data System (ADS)

    Maggs, William Ward

    The oldest known occurrence in North America of an ophiolite, considered to be a piece of ancient ocean crust, has been reported in the Cape Smith Belt in northern Quebec, Canada.The recognition last summer of a key structural component of the characteristic ophiolite suite has buttressed confidence in the theory that the 1900-Ma fragments of an ocean basin were accreted to an early Proterozoic Canadian continent. The tectonic mixing of oceanic and continental crust is strong evidence for the operation of plate tectonics early in Earth's history.

  9. Frozen magma lenses below the oceanic crust.

    PubMed

    Nedimović, Mladen R; Carbotte, Suzanne M; Harding, Alistair J; Detrick, Robert S; Canales, J Pablo; Diebold, John B; Kent, Graham M; Tischer, Michael; Babcock, Jeffrey M

    2005-08-25

    The Earth's oceanic crust crystallizes from magmatic systems generated at mid-ocean ridges. Whereas a single magma body residing within the mid-crust is thought to be responsible for the generation of the upper oceanic crust, it remains unclear if the lower crust is formed from the same magma body, or if it mainly crystallizes from magma lenses located at the base of the crust. Thermal modelling, tomography, compliance and wide-angle seismic studies, supported by geological evidence, suggest the presence of gabbroic-melt accumulations within the Moho transition zone in the vicinity of fast- to intermediate-spreading centres. Until now, however, no reflection images have been obtained of such a structure within the Moho transition zone. Here we show images of groups of Moho transition zone reflection events that resulted from the analysis of approximately 1,500 km of multichannel seismic data collected across the intermediate-spreading-rate Juan de Fuca ridge. From our observations we suggest that gabbro lenses and melt accumulations embedded within dunite or residual mantle peridotite are the most probable cause for the observed reflectivity, thus providing support for the hypothesis that the crust is generated from multiple magma bodies.

  10. Q structure of the oceanic crust

    NASA Astrophysics Data System (ADS)

    Wepfer, W. W.; Christensen, N. I.

    1991-08-01

    Compressional wave attenuations and velocities have been measured as a function of confining pressure in ophiolite samples representing a cross-section of the oceanic crust and uppermost mantle. Data are presented for basalts, diabase dikes, gabbros and a suite of serpentinites and peridotites showing a range of serpentization. An ultrasonic pulse-echo spectral ratio technique was used to determine the attenuations to confining pressures of 500 MPa. From this data a Q profile for the oceanic crust and upper mantle is presented. Q is found to moderately increase with depth through the pillow basalts of the upper oceanic crust. The sheeted dike rocks of Layer 2C show an increase in Q with depth due to progressive metamorphism (from greenschist to amphibolite facies). Q drops abruptly from Layer 2C to Layer 3, though it is not clear why the gabbros have such low Q's. The crust-mantle boundary is a Q discontinuity; however, the Q contrast between Layer 3 and the upper mantle could be altered by upper mantle serpentinization, interlayered gabbros and peridotites at the boundary, or serpentinized peridotite diapirs intruding the gabbroic section. Q varies significantly with the percentage of serpentinization in the ultramafic samples, with the largest changes in Q being at the extremes of zero and full serpentinization. Q is sensitive to the overburden pressure for all of the samples.

  11. Chemical remanent magnetization of oceanic crust

    SciTech Connect

    Verhoef, J. ); Arkani-Hamed, J. )

    1990-10-01

    The effects of chemical remanent magnetization (CRM) of oceanic crust on the anomalous skewness of sea-floor spreading magnetic anomalies are investigated. Considering a realistic constraint that the actual magnetization at anomaly M0 is reversed, the CRM of layer 2A basalts fails to explain the anomalous skewness of the magnetic anomalies. The CRM of the deeper layers does contribute to the anomalous skewness of anomalies 33/34, but the major contribution comes from thermal remanent magnetization.

  12. Oceanic Crust in the Canada Basin of the Arctic Ocean

    NASA Astrophysics Data System (ADS)

    Hutchinson, Deborah; Chian, Deping; Jackson, Ruth; Lebedeva-Ivanova, Nina; Shimeld, John; Li, Qingmou; Mosher, David; Saltus, Richard; Oakey, Gordon

    2015-04-01

    Crustal velocities from 85 expendable sonobuoys in the Canada Basin of the Arctic Ocean acquired between 2007 and 2011 distinguish oceanic, transitional, and extended continental crust. Crustal type was based on objective assignments of diagnostic velocities - oceanic from the presence of layer 3 velocities (6.7-7.2 km/s); transitional from the presence of a lower-most, high velocity layer (7.2-7.7 km/s), and continental for velocities typical of continental crust (≤6.6 km/s). Combined interpretations of sonobuoys, coincident multichannel seismic reflection profiles and existing maps of potential field (gravity and magnetic) are used to refine the distribution of oceanic crust. Oceanic crust forms a polygon approximately 320-350 km wide (east-west) by ~500 km (north-south). The northern segment of the Canada Basin Gravity Low (CBGL) bisects this zone of oceanic crust, as would be expected from the axis of the spreading center. The multichannel profiles also image a prominent bathymetric valley along this segment of the CBGL, similar to axial valleys found on slow and ultra-slow spreading ridges. Paired magnetic anomalies are associated only with crust that has typical oceanic velocities and are interpreted to represent possibly Mesozoic marine magnetic anomalies M0r - M4 (?), for a duration of opening of 8 million years, and a half spreading rate of ~10 mm/a. The southern segment of the CBGL, where it trends toward the Mackenzie Delta/fan, is associated with transitional velocities that are interpreted to represent serpentinized peridotite (mantle). As a result of being close to the inferred pole of rotation, this southern area may have had a spreading rate too low to support magmatism, producing amagmatic transitional crust. Further north, near Alpha Ridge and along Northwind Ridge, transitional crust is interpreted to be underplated or intruded material related to the emplacement of the High Arctic Large Igneous Province. Seismic reflection profiles across the

  13. Quantifying glassy and crystalline basalt partitioning in the oceanic crust

    NASA Astrophysics Data System (ADS)

    Moore, Rachael; Ménez, Bénédicte

    2016-04-01

    The upper layers of the oceanic crust are predominately basaltic rock, some of which hosts microbial life. Current studies of microbial life within the ocean crust mainly focus on the sedimentary rock fraction, or those organisms found within glassy basalts while the potential habitability of crystalline basalts are poorly explored. Recently, there has been recognition that microbial life develops within fractures and grain boundaries of crystalline basalts, therefore estimations of total biomass within the oceanic crust may be largely under evaluated. A deeper understanding of the bulk composition and fractionation of rocks within the oceanic crust is required before more accurate estimations of biomass can be made. To augment our understanding of glassy and crystalline basalts within the oceanic crust we created two end-member models describing basalt fractionation: a pillow basalt with massive, or sheet, flows crust and a pillow basalt with sheeted dike crust. Using known measurements of massive flow thickness, dike thickness, chilled margin thickness, pillow lava size, and pillow lava glass thickness, we have calculated the percentage of glassy versus crystalline basalts within the oceanic crust for each model. These models aid our understanding of textural fractionation within the oceanic crust, and can be applied with bioenergetics models to better constrain deep biomass estimates.

  14. The global oxidation state of the upper oceanic crust

    NASA Astrophysics Data System (ADS)

    Rutter, J.; Harris, M.; Coggon, R. M.; Alt, J.; Smith-Duque, C. E.; Teagle, D. A.

    2012-12-01

    The oxidation state of the oceanic crust is an important component of the Earth system. The widespread oxidation of the crust is a major contributor to the redox state of the mantle due to the subduction of hydrothermally altered oceanic crust, which supplies 10 - 25 % of the net ferric iron flux to the global mantle Fe3+/FeTOT budget (Lécuyer and Ricard, 1999). Secondly, the degree of oxidation of the upper oceanic crust provides a measure of the biomass of microbial life sub-basement (Bach and Edwards, 2003). Thirdly, oxidation state analyses of oceanic basalt give information on the environment and relative timings of local hydrothermal alteration events. To date comprehensive measurements of Fe3+/FeTOT for the oceanic crust are lacking. Post crystallisation oxidation processes, occurring predominantly in the upper basaltic layers of the crust, elevate ratios of ferric to total iron (Fe3+/FeTOT) from mantle levels of 0.16 ± 0.01 (Cottrell and Kelley, 2011). Ferrous (Fe2+/) iron is oxidised to ferric (Fe3+/) iron during reaction with oxidised seawater, which circulates through oceanic crust for tens of millions of years following crustal formation. This study integrates published data with new analyses from six ocean crustal boreholes to categorise the global oxidation state of the upper crust. Samples range from <1 to 129 Ma, and represent basalt from medium to superfast spreading centres, depths between <100 - 2000 mbsf, and at a variety of sedimentary cover rates and thicknesses. Results show that by 1 Ma, the Fe3+/FeTOT ratio of the bulk crust is already raised to an average of 0.28 ± 0.07, implying that the oxidation state is established very early in the lifetime of the ocean crust. Post 1 Ma, Fe3+/FeTOT ratios are more variable, reflecting the effects of prolonged exposure to circulating seawater, but are on average ~0.35.

  15. Stable Chlorine Isotopes in Ocean Crust Processes

    NASA Astrophysics Data System (ADS)

    Bach, W.; Layne, G.; Kent, A.

    2003-12-01

    The study of natural variations of Cl isotopic composition in ocean crustal rocks has large potential to further our understanding of geochemical cycling of volatiles and elements soluble in saline aqueous solutions. Studies of oceanic basalt suites to date confirm that Cl abundances are highly sensitive to the addition of saline components - either from addition of subduction-related volatile fluxes in back-arc basins and volcanic arcs or via interaction between magmas and Cl-rich seawater-derived components during melting, magma storage and transport. Recent data suggest that δ 37Cl is much more variable in the marine environment than originally thought, with strongly negative δ 37Cl values (down to -7.5 ‰ ) in marine pore waters and positive values (up to +7 ‰ ) in hydrothermal fluids from oceanic spreading centers. Moreover, mantle-derived magmatic rocks reveal large variations in δ 37Cl (-3 to +11 ‰ ), reflecting mantle heterogeneity as well as assimilation of exogenic Cl by crystallizing magmas. The large isotopic variation in low-Cl basalts has been explained by isotopic heterogeneities of the mantle, with very light δ 37Cl values in rocks from the southwest Chile Ridge that have island arc geochemical affinities and heavy δ 37Cl values in Reykjanes Ridge samples (Stewart, 2000, PhD Thesis, Duke University). The inference is that a slab-flux carries a negative δ 37Cl signature while recycled ocean crust in mantle plumes carries a strongly positive δ 37Cl signature, although this is not well constrained at present. Preferential release of isotopically light Cl from the dewatering sediments is suggested by pore water data from the Barbados and Nankai accretionary prisms with δ 37Cl values down to -7.5 ‰ (Ransom et al. 1995, Geology, 23, 715). Volcanic fumaroles also appear to have negative δ 37Cl values. If this is the case then residual Cl in the subducting slab should become isotopically heavier as 35Cl is preferentially released in the

  16. Imaging proto-oceanic crust off the Brazilian Continental Margin

    NASA Astrophysics Data System (ADS)

    Klingelhoefer, F.; Evain, M.; Afilhado, A.; Rigoti, C.; Loureiro, A.; Alves, D.; Leprêtre, A.; Moulin, M.; Schnurle, P.; Benabdellouahed, M.; Baltzer, A.; Rabineau, M.; Feld, A.; Viana, A.; Aslanian, D.

    2015-01-01

    During the Sanba (Santos basin seismic transect) experiment in 2010-2011, a 380-km-long combined wide-angle and reflection seismic profile has been acquired using 30 ocean-bottom seismometers, a 4.5 km seismic streamer and a 8900 in.3 airgun array. The Sanba 3 profile crosses the southern flank of the Sao Paulo Plateau, the Sao Paulo Ridge and the easternmost Santos Basin in an east-west direction. Its eastern end is located on undisturbed oceanic crust. Tomographic and forward modelling of the wide-angle seismic data reveals that the sedimentary thickness is variable with only 1-2 km on top of the ridge and thickening to 4-5 km in the basin. Crustal thickness at the ridge is about 18 km and the relative layer thickness and velocity gradients indicate a continental origin of this ridge. The eastern Santos Basin is underlain by crust of only 5 km thickness, characterized by high seismic velocities between 6.20 km s-1 in the upper crust and 7.40 km s-1 in the lower crust. Three hypotheses for the nature of the crust in this region are tested here: (i) thinned continental crust, (ii) serpentinized upper mantle material, (iii) thin oceanic crust. As seismic velocity gradients seem to rule out a continental origin of this region, and clear Moho reflections argue against serpentinized upper mantle, we propose that the crust underlying the easternmost Santos Basin is of oceanic origin. Deviations from normal oceanic crustal velocities in the lower crust (6.70-7.00 km s-1) can be explained by accretion at slow spreading rates leading to the inclusion of serpentinite into the lower crust at the onset of organized seafloor spreading.

  17. Palaeozoic oceanic crust preserved beneath the eastern Mediterranean

    NASA Astrophysics Data System (ADS)

    Granot, Roi

    2016-09-01

    Subduction of oceanic crust into the mantle results in the relatively young Mesozoic-Cenozoic age of the current oceanic basins, thus, hindering our knowledge of ancient oceanic lithospheres. Believed to be an exception, the eastern Mediterranean Sea (containing the Herodotus and Levant basins) preserves the southern margin of the Neotethyan, or older, ocean. An exceptionally thick sedimentary cover and a lack of accurate magnetic anomaly data have led to contradicting views about its crustal nature and age. Here I analyse total and vector magnetic anomaly data from the Herodotus Basin. I identify a long sequence of lineated magnetic anomalies, which imply that the crust is oceanic. I use the shape, or skewness, of these magnetic anomalies to constrain the timing of crustal formation and find that it formed about 340 million years ago. I suggest that this oceanic crust formed either along the Tethys spreading system, implying the Neotethys Ocean came into being earlier than previously thought, or during the amalgamation of the Pangaea Supercontinent. Finally, the transition from the rather weak and stretched continental crust found in the Levant Basin to the relatively strong oceanic Herodotus crust seems to guide the present-day seismicity pattern as well as the plate kinematic evolution of the region.

  18. Microbial Life of North Pacific Oceanic Crust

    NASA Astrophysics Data System (ADS)

    Schumann, G.; Koos, R.; Manz, W.; Reitner, J.

    2003-12-01

    Information on the microbiology of the deep subsurface is necessary in order to understand the factors controlling the rate and extent of the microbially catalyzed reactions that influence the geophysical properties of these environments. Drilling into 45-Ma oceanic basaltic crust in a deepwater environment during ODP Leg 200 provided a promising opportunity to explore the abundance, diversity and activity of micro-organisms. The combined use of culture-independent molecular phylogenetic analyses and enrichment culture techniques is an advantageous approach in investigating subsurface microbial ecosystems. Enrichment culture methods allow the evaluation of potential activities and functions. Microbiological investigations revealed few aerobic cultivable, in part hitherto unknown, micro-organisms in deep submarine sediments and basaltic lava flows. 16S rDNA sequencing of isolates from sediment revealed the next relatives to be members of the genera Halomonas, Pseudomonas, and Lactobacillus. Within the Pseudomonadaceae the closest relative is Acinetobacter sp., which was isolated from a deep subsurface environment. The next phylogenetical relatives within the Halomonadaceae are bacteria typically isolated from Soda lakes, which are considered as model of early life conditions. Interestingly, not only sediment bacteria could be obtained in pure culture. Aerobic strains could also be successfully isolated from the massive tholeiitic basalt layer at a depth of 76.16 mbsf (46 m below the sediment/basement contact). These particular isolates are gram-positive with low G+C content of DNA, phylogenetically affiliated to the phylum Firmicutes. The closest neighbors are e.g. a marine Bacillus isolated from the Gulf of Mexico and a low G+C gram-positive bacterium, which belongs to the microbial flora in the deepest sea mud of the Mariana Trench, isolated from a depth of 10,897 m. Based on the similarity values, the isolates represent hitherto undescribed species of the deep

  19. Under the sea: microbial life in volcanic oceanic crust.

    PubMed

    Edwards, Katrina J; Wheat, C Geoffrey; Sylvan, Jason B

    2011-09-06

    Exploration of the microbiology in igneous, 'hard rock' oceanic crust represents a major scientific frontier. The igneous crust harbours the largest aquifer system on Earth, most of which is hydrologically active, resulting in a substantial exchange of fluids, chemicals and microorganisms between oceanic basins and crustal reservoirs. Study of the deep-subsurface biosphere in the igneous crust is technically challenging. However, technologies have improved over the past decade, providing exciting new opportunities for the study of deep-seated marine life, including in situ and cross-disciplinary experimentation in microbiology, geochemistry and hydrogeology. In this Progress article, we describe the recent advances, available technology and remaining challenges in the study of the marine intraterrestrial microbial life that is harboured in igneous oceanic crust.

  20. Deep-ocean ferromanganese crusts and nodules

    USGS Publications Warehouse

    Hein, James R.; Koschinsky, Andrea

    2014-01-01

    Ferromanganese crusts and nodules may provide a future resource for a large variety of metals, including many that are essential for emerging high- and green-technology applications. A brief review of nodules and crusts provides a setting for a discussion on the latest (past 10 years) research related to the geochemistry of sequestration of metals from seawater. Special attention is given to cobalt, nickel, titanium, rare earth elements and yttrium, bismuth, platinum, tungsten, tantalum, hafnium, tellurium, molybdenum, niobium, zirconium, and lithium. Sequestration from seawater by sorption, surface oxidation, substitution, and precipitation of discrete phases is discussed. Mechanisms of metal enrichment reflect modes of formation of the crusts and nodules, such as hydrogenetic (from seawater), diagenetic (from porewaters), and mixed diagenetic–hydrogenetic processes.

  1. Generation of continental crust in intra-oceanic arcs

    NASA Astrophysics Data System (ADS)

    Gazel, E.; Hayes, J. L.; Kelemen, P. B.; Everson, E. D.; Holbrook, W. S.; Vance, E.

    2014-12-01

    The origin of continental crust is still an unsolved mystery in the evolution of our planet. Although the best candidates to produce juvenile continental crust are intra-oceanic arcs these systems are dominated by basaltic lavas, and when silicic magmas are produced, the incompatible-element compositions are generally too depleted to be a good match for continental crust estimates. Others, such as the W. Aleutians, are dominated by andesitic melts with trace element compositions similar to average continental crust. In order to evaluate which intra-oceanic arcs produced modern continental crust, we developed a geochemical continental index (CI) through a statistical analysis that compared all available data from modern intra-oceanic arcs with global estimates of continental crust. Our results suggest that magmas from Costa Rica (<10 Ma) have a CI <50, closer to the CI (~20) computed from available average continental crust estimates. Transitional CI values of 50-100 were found in the Aleutians, the Iwo-Jima segment of Izu-Bonin, the L. Antilles, Panama, Nicaragua, and Vanuatu. The geochemical signature of the Costa Rican lavas is controlled by melts from the subducting Galapagos tracks. Iwo-Jima and Vanuatu are in a similar tectonic scenario with subducting intraplate seamounts. Melts from the subducting oceanic crust are thought to significantly control the geochemical signature in the W. Aleutians and Panama. In the L. Antilles and E. Aleutians the continental signature may reflect recycling of a component derived from subducting continental sediments. Most of Izu-Bonin, Marianas, S. Scotia and Tonga arcs with a CI >100 have the least continent-like geochemical signatures. In these arcs the subducting plate is old (>100 Ma), not overprinted by enriched intraplate volcanism and the geochemistry may be dominated by slab-derived, aqueous fluids. We also found a strong correlation between the CI and average crustal P-wave velocity, validating the geochemical index

  2. Another one bites the rust: Microbes weathering subsurface oceanic crust

    NASA Astrophysics Data System (ADS)

    Orcutt, B.

    2014-12-01

    The marine deep biosphere consists of deeply buried sediment and igneous oceanic crust; yet, the existence, extent, and activity of life hosted in igneous oceanic crust is considerably less understood than in the sedimentaty realm. The role of microbial metabolic reactions in altering basaltic crust is not yet clear. This presentation will summarize what is currently known or speculated about rates of microbial activity in crust, focusing on the cycling of iron and oxygen, based on data from a few basalt-dominated sites that have been examined recently, including the eastern flank of the Juan de Fuca Ridge, the western flank of the Mid-Atlantic Ridge, and the Dorado Outcrop. Microbial community diversity, activity and genomic data will seek to address the question: how do you think microbes get along without iron when it's gone?

  3. FERROMANGANESE CRUST RESOURCES IN THE PACIFIC AND ATLANTIC OCEANS.

    USGS Publications Warehouse

    Commeau, R.F.; Clark, A.; Johnson, Chad; Manheim, F. T.; Aruscavage, P. J.; Lane, C.M.

    1984-01-01

    Ferromanganese crusts on raised areas of the ocean floor have joined abyssal manganese nodules and hydrothermal sulfides as potential marine resources. Significant volumes of cobalt-rich (about 1% Co) crusts have been identified to date within the US Exclusive Economic Zone (EEZ) in the Central Pacific: in the NW Hawaiian Ridge and Seamount region and in the seamounts in the Johnston Island and Palmyra Island regions. Large volumes of lower grade crusts, slabs, and nodules are also present in shallow ( greater than 1000 m) waters on the Blake plateau, off Florida-South Carolina in the Atlantic Ocean. Data on ferromanganese crusts have been increased by recent German and USGS cruises, but are still sparse, and other regions having crust potential are under current investigation. The authors discuss economic potentials for cobalt-rich crusts in the Central Pacific and Western North Atlantic oceans, with special reference to US EEZ areas. Additional research is needed before more quantitative resource estimates can be made.

  4. The effect of thicker oceanic crust in the Archaean on the growth of continental crust through time

    NASA Technical Reports Server (NTRS)

    Wilks, M. E.

    1988-01-01

    Present crustal evolution models fail to account for the generation of the large volume of continental crust in the required time intervals. All Archaean plate tectonic models, whether invoking faster spreading rates, similar to today's spreading rates, or longer ridge lengths, essentially propose that continental crust has grown by island arc accretion due to the subduction of oceanic crust. The petrological differences that characterize the Archaean from later terrains result from the subduction of hotter oceanic crust into a hotter mantle. If the oceanic crust was appreciably thicker in the Archaean, as geothermal models would indicate, this thicker crust is surely going to have an effect on tectonic processes. A more valid approach is to compare the possible styles of convergence of thick oceanic crust with modern convergence zones. The best modern analog occurs where thick continental crust is colliding with thick continental crust. Oceanic crustal collision on the scale of the present-day Himalayan continental collision zone may have been a frequent occurrence in the Archaean, resulting in extensive partial melting of the hydrous underthrust oceanic crust to produce voluminous tonalite melts, leaving a depleted stabilized basic residuum. Present-day island arc accretion may not have been the dominant mechanism for the growth of the early Archaean crust.

  5. Primitive layered gabbros from fast-spreading lower oceanic crust

    NASA Astrophysics Data System (ADS)

    Gillis, Kathryn M.; Snow, Jonathan E.; Klaus, Adam; Abe, Natsue; Adrião, Álden B.; Akizawa, Norikatsu; Ceuleneer, Georges; Cheadle, Michael J.; Faak, Kathrin; Falloon, Trevor J.; Friedman, Sarah A.; Godard, Marguerite; Guerin, Gilles; Harigane, Yumiko; Horst, Andrew J.; Hoshide, Takashi; Ildefonse, Benoit; Jean, Marlon M.; John, Barbara E.; Koepke, Juergen; Machi, Sumiaki; Maeda, Jinichiro; Marks, Naomi E.; McCaig, Andrew M.; Meyer, Romain; Morris, Antony; Nozaka, Toshio; Python, Marie; Saha, Abhishek; Wintsch, Robert P.

    2014-01-01

    Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks--in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas--provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt.

  6. Primitive layered gabbros from fast-spreading lower oceanic crust.

    PubMed

    Gillis, Kathryn M; Snow, Jonathan E; Klaus, Adam; Abe, Natsue; Adrião, Alden B; Akizawa, Norikatsu; Ceuleneer, Georges; Cheadle, Michael J; Faak, Kathrin; Falloon, Trevor J; Friedman, Sarah A; Godard, Marguerite; Guerin, Gilles; Harigane, Yumiko; Horst, Andrew J; Hoshide, Takashi; Ildefonse, Benoit; Jean, Marlon M; John, Barbara E; Koepke, Juergen; Machi, Sumiaki; Maeda, Jinichiro; Marks, Naomi E; McCaig, Andrew M; Meyer, Romain; Morris, Antony; Nozaka, Toshio; Python, Marie; Saha, Abhishek; Wintsch, Robert P

    2014-01-09

    Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks--in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas--provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt.

  7. Anorthositic oceanic crust in the Archean Earth

    NASA Technical Reports Server (NTRS)

    Jagoutz, E.; Dawson, J. B.; Hoernes, S.; Spettel, B.; Waenke, H.

    1985-01-01

    Ultrapure minerals separated from eclogite inclusions in kimberlites were analyzed for Sm, Nd, Sr, and oxygen isotopes and for major and trace elements. Clinopyroxene (cpx) and garnet (gnt) are the only primary mineral phases in these rocks, and mineral phases and their alteration products. The WR sub calc. is the reconstructed bulk composition excluding all the contamination influences. Two groups of eclogites: are distinguished: (1) type A Noritic-anorthositic eclogites; and (2) type B Ti-ferrogabbroic eclogites. The oxygen isotopes are primary mantle-derived features of these rocks and are not caused by posteruption processes, as they were measured on unaltered, clean mineral separates and show a correlation with REE pattern and Sr and Nd isotopes. It is suggested that the variation of the oxygen isotopes are caused by crustal-level fluid-rock interaction at relatively low temperature. It is shown that oxygen isotopes variation in MORB basalts caused by the hydrothermal system are in the same range as the observed oxygen isotope variation in eclogites. A model to explain the new set of data is proposed. It is thought that some of these eclogites might be emplaced into the upper lithosphere or lower crust at the time corresponding to their internal isochron age. The calculated WR composition was used to estimate model ages for these rocks.

  8. Fluids from aging ocean crust that support microbial life.

    PubMed

    Cowen, James P; Giovannoni, Stephen J; Kenig, Fabien; Johnson, H Paul; Butterfield, David; Rappé, Michael S; Hutnak, Michael; Lam, Phyllis

    2003-01-03

    Little is known about the potential for life in the vast, low-temperature (<100 degrees C) reservoir of fluids within mid-ocean ridge flank and ocean basin crust. Recently, an overpressured 300-meter-deep borehole was fitted with an experimental seal (CORK) delivering crustal fluids to the sea floor for discrete and large-volume sampling and characterization. Results demonstrate that the 65 degrees C fluids from 3.5-million-year-old ocean crust support microbial growth. Ribosomal RNA gene sequence data indicate the presence of diverse Bacteria and Archaea, including gene clones of varying degrees of relatedness to known nitrate reducers (with ammonia production), thermophilic sulfate reducers, and thermophilic fermentative heterotrophs, all consistent with fluid chemistry.

  9. Magnetization of the oceanic crust: TRM or CRM?

    NASA Technical Reports Server (NTRS)

    Raymond, C. A.; Labrecque, J. L.

    1987-01-01

    A model was proposed in which chemical remanent magnetization (CRM) acquired within the first 20 Ma of crustal evolution may account for 80% of the bulk natural remanent magnetization (NRM) of older basalts. The CRM of the crust is acquired as the original thermoremanent magnetization (TRM) is lost through low temperature alteration. The CRM intensity and direction are controlled by the post-emplacement polarity history. This model explains several independent observations concerning the magnetization of the oceanic crust. The model accounts for amplitude and skewness discrepancies observed in both the intermediate wavelength satellite field and the short wavelength sea surface magnetic anomaly pattern. It also explains the decay of magnetization away from the spreading axis, and the enhanced magnetization of the Cretaceous Quiet Zones while predicting other systematic variations with age in the bulk magnetization of the oceanic crust. The model also explains discrepancies in the anomaly skewness parameter observed for anomalies of Cretaceous age. Further studies indicate varying rates of TRM decay in very young crust which depicts the advance of low temperature alteration through the magnetized layer.

  10. Helium isotopes in ferromanganese crusts from the central Pacific Ocean

    USGS Publications Warehouse

    Basu, S.; Stuart, F.M.; Klemm, V.; Korschinek, G.; Knie, K.; Hein, J.R.

    2006-01-01

    Helium isotopes have been measured in samples of two ferromanganese crusts (VA13/2 and CD29-2) from the central Pacific Ocean. With the exception of the deepest part of crust CD29-2 the data can be explained by a mixture of implanted solar- and galactic cosmic ray-produced (GCR) He, in extraterrestrial grains, and radiogenic He in wind-borne continental dust grains. 4He concentrations are invariant and require retention of less than 12% of the in situ He produced since crust formation. Loss has occurred by recoil and diffusion. High 4He in CD29-2 samples older than 42 Ma are correlated with phosphatization and can be explained by retention of up to 12% of the in situ-produced 4He. 3He/4He of VA13/2 samples varies from 18.5 to 1852 Ra due almost entirely to variation in the extraterrestrial He contribution. The highest 3He/4He is comparable to the highest values measured in interplanetary dust particles (IDPs) and micrometeorites (MMs). Helium concentrations are orders of magnitude lower than in oceanic sediments reflecting the low trapping efficiency for in-falling terrestrial and extraterrestrial grains of Fe-Mn crusts. The extraterrestrial 3He concentration of the crusts rules out whole, undegassed 4–40 μm diameter IDPs as the host. Instead it requires that the extraterrestrial He inventory is carried by numerous particles with significantly lower He concentrations, and occasional high concentration GCR-He-bearing particles.

  11. Magnetization of lower oceanic crust and upper mantle

    NASA Astrophysics Data System (ADS)

    Kikawa, E.

    2004-05-01

    The location of the magnetized rocks of the oceanic crust that are responsible for sea-floor spreading magnetic anomalies has been a long-standing problem in geophysics. The recognition of these anomalies was a key stone in the development of the theory of plate tectonics. Our present concept of oceanic crustal magnetization is much more complex than the original, uniformly magnetized model of Vine-Matthews-Morley Hypothesis. Magnetic inversion studies indicated that the upper oceanic extrusive layer (Layer 2A of 0.5km thick) was the only magnetic layer and that it was not necessary to postulate any contribution from deeper parts of oceanic crust. Direct measurements of the magnetic properties of the rocks recovered from the sea floor, however, have shown that the magnetization of Layer 2A, together with the observations that this layer could record geomagnetic field reversals within a vertical section, is insufficient to give the required size of observed magnetic anomalies and that some contribution from lower intrusive rocks is necessary. Magnetization of oceanic intrusive rocks were observed to be reasonably high enough to contribute to sea-floor spreading magnetic anomalies, but were considered somewhat equivocal until late 1980Os, in part because studies had been conducted on unoriented dredged and ophiolite samples and on intermittent DSDP/ODP cores. Since ODP Leg 118 that cored and recovered continuous 500m of oceanic intrusive layer at Site 735B, Southwest Indian Ridge with an extremely high recovery of 87 percent, there have been several ODP Legs (legs 147, 153, 176, 179 and 209) that were devoted to drilling gabbroic rocks and peridotites. In terms of the magnetization intensities, all of the results obtained from these ODP Legs were supportive of the model that a significant contribution must come from gabbros and peridotites and the source of the lineated magnetic anomalies must reside in most of the oceanic crust as well as crust-mantle boundary

  12. Layered basic complex in oceanic crust, romanche fracture, equatorial atlantic ocean.

    PubMed

    Melson, W G; Thompson, G

    1970-05-15

    A layered, basic igneous intrusion, analogous in mineralogy and texture to certain large, continental layered complexes, is exposed in the Romanche Fracture, equatorial Atlantic Ocean. Crustal intrusion of large masses of basic magmas with their subsequent gravity differentiation is probably one of a number of major processes involved in the formation of new oceanic crust during sea-floor spreading.

  13. Colonization of subsurface microbial observatories deployed in young ocean crust.

    PubMed

    Orcutt, Beth N; Bach, Wolfgang; Becker, Keir; Fisher, Andrew T; Hentscher, Michael; Toner, Brandy M; Wheat, C Geoffrey; Edwards, Katrina J

    2011-04-01

    Oceanic crust comprises the largest hydrogeologic reservoir on Earth, containing fluids in thermodynamic disequilibrium with the basaltic crust. Little is known about microbial ecosystems that inhabit this vast realm and exploit chemically favorable conditions for metabolic activities. Crustal samples recovered from ocean drilling operations are often compromised for microbiological assays, hampering efforts to resolve the extent and functioning of a subsurface biosphere. We report results from the first in situ experimental observatory systems that have been used to study subseafloor life. Experiments deployed for 4 years in young (3.5 Ma) basaltic crust on the eastern flank of the Juan de Fuca Ridge record a dynamic, post-drilling response of crustal microbial ecosystems to changing physical and chemical conditions. Twisted stalks exhibiting a biogenic iron oxyhydroxide signature coated the surface of mineral substrates in the observatories; these are biosignatures indicating colonization by iron oxidizing bacteria during an initial phase of cool, oxic, iron-rich conditions following observatory installation. Following thermal and chemical recovery to warmer, reducing conditions, the in situ microbial structure in the observatory shifted, becoming representative of natural conditions in regional crustal fluids. Firmicutes, metabolic potential of which is unknown but may involve N or S cycling, dominated the post-rebound bacterial community. The archaeal community exhibited an extremely low diversity. Our experiment documented in situ conditions within a natural hydrological system that can pervade over millennia, exemplifying the power of observatory experiments for exploring the subsurface basaltic biosphere, the largest but most poorly understood biotope on Earth.

  14. Primary carbonatite melt from deeply subducted oceanic crust.

    PubMed

    Walter, M J; Bulanova, G P; Armstrong, L S; Keshav, S; Blundy, J D; Gudfinnsson, G; Lord, O T; Lennie, A R; Clark, S M; Smith, C B; Gobbo, L

    2008-07-31

    Partial melting in the Earth's mantle plays an important part in generating the geochemical and isotopic diversity observed in volcanic rocks at the surface. Identifying the composition of these primary melts in the mantle is crucial for establishing links between mantle geochemical 'reservoirs' and fundamental geodynamic processes. Mineral inclusions in natural diamonds have provided a unique window into such deep mantle processes. Here we provide experimental and geochemical evidence that silicate mineral inclusions in diamonds from Juina, Brazil, crystallized from primary and evolved carbonatite melts in the mantle transition zone and deep upper mantle. The incompatible trace element abundances calculated for a melt coexisting with a calcium-titanium-silicate perovskite inclusion indicate deep melting of carbonated oceanic crust, probably at transition-zone depths. Further to perovskite, calcic-majorite garnet inclusions record crystallization in the deep upper mantle from an evolved melt that closely resembles estimates of primitive carbonatite on the basis of volcanic rocks. Small-degree melts of subducted crust can be viewed as agents of chemical mass-transfer in the upper mantle and transition zone, leaving a chemical imprint of ocean crust that can possibly endure for billions of years.

  15. Primary carbonatite melt from deeply subducted oceanic crust

    SciTech Connect

    Walter, M.J.; Bulanova, G.P.; Armstrong, L.S.; Keshav, S.; Blundy, J.D.; Gudfinnesson, G.; Lord, O.T.; Lennie, A.R.; Clark, S.M.; Smith, C.B.; Gobbo, L.

    2008-07-01

    Partial melting in the Earth's mantle plays an important part in generating the geochemical and isotopic diversity observed in volcanic rocks at the surface. Identifying the composition of these primary melts in the mantle is crucial for establishing links between mantle geochemical 'reservoirs' and fundamental geodynamic processes. Mineral inclusions in natural diamonds have provided a unique window into such deep mantle processes. Here they provide exper8imental and geochemical evidence that silicate mineral inclusions in diamonds from Juina, Brazil, crystallized from primary and evolved carbonatite melts in the mantle transition zone and deep upper mantle. The incompatible trace element abundances calculated for a melt coexisting with a calcium-titanium-silicate perovskite inclusion indicate deep melting of carbonated oceanic crust, probably at transition-zone depths. Further to perovskite, calcic-majorite garnet inclusions record crystallization in the deep upper mantle from an evolved melt that closely resembles estimates of primitive carbonatite on the basis of volcanic rocks. Small-degree melts of subducted crust can be viewed as agents of chemical mass-transfer in the upper mantle and transition zone, leaving a chemical imprint of ocean crust that can possibly endure for billions of years.

  16. Permanent components of the crust, geoid and ocean depth tides

    NASA Astrophysics Data System (ADS)

    Sun, Wenke; Sjöberg, Lars E.

    2001-04-01

    The tidal deformation caused by the luni-solar potential includes not only a periodic part, but also a time-independent part, called the permanent tide. How to deal with the tidal correction in gravimetric observations, especially the treatment of the permanent tide, has been discussed for a long time, since some practical and physical problems exist anyhow. A resolution adopted by IAG (1983) was that the permanent tidal attraction of the Moon and the Sun should be eliminated, but the permanent tidal deformation of the Earth be maintained. This is called zero gravity, and the geoid associated with it is the zero geoid. As to the crust deformation, Poutanen et al. (Poutanen, M., Vermeer, M., Mäkinen, J., 1996. The permanent tide in GPS positioning. Journal of Geodesy 70, 499-504.) suggested that co-ordinates should be reduced to the zero crust, i.e. the crust that includes the effect of the permanent tide. This research shows that horizontal components of the permanent earth tides, which are not considered in recent studies, are also important in GPS positioning and geoid determination. Since the tide-generating potential can be expanded into harmonics and divided into two parts (geodetic coefficients and the group of harmonic waves), the permanent earth tides can be easily obtained by multiplying the amplitude of the zero-frequency wavelength by the corresponding geoid geodetic coefficient. Formulas for both elastic and fluid cases are presented. Numerical results for the elastic case show that he vertical permanent crust (zero crust), geoid and ocean depth tides reach -12.0, -5.8 and 6.1 cm at the poles, and 5.9, 2.9 and -3.0 cm at the equator, respectively. The horizontal permanent crust, geoid and ocean depth tide components reach as much as 2.5, 8.7 and 6.3 cm, respectively. According to the solution of IAG (1983), the permanent vertical components are kept in GPS positioning and geoid computation. Thus, it is natural to include the horizontal components

  17. Marine Magnetic Anomalies, Oceanic Crust Magnetization, and Geomagnetic Time Variations

    NASA Astrophysics Data System (ADS)

    Dyment, J.; Arkani-Hamed, J.

    2005-12-01

    Since the classic paper of Vine and Matthews (Nature, 1963), marine magnetic anomalies are commonly used to date the ocean floor through comparison with the geomagnetic polarity time scale and proper identification of reversal sequences. As a consequence, the classical model of rectangular prisms bearing a normal / reversed magnetization has been dominant in the literature for more than 40 years. Although the model explains major characteristics of the sea-surface magnetic anomalies, it is contradicted by (1) recent advances on the geophysical and petrologic structure of the slow-spreading oceanic crust, and (2) the observation of short-term geomagnetic time variations, both of which are more complex than assumed in the classical model. Marine magnetic anomalies may also provide information on the magnetization of the oceanic crust as well as short-term temporal fluctuations of the geomagnetic field. The "anomalous skewness", a residual phase once the anomalies have been reduced to the pole, has been interpreted either in terms of geomagnetic field variations or crustal structure. The spreading-rate dependence of anomalous skewness rules out the geomagnetic hypothesis and supports a spreading-rate dependent magnetic structure of the oceanic crust, with a basaltic layer accounting for most of the anomalies at fast spreading rates and an increasing contribution of the deeper layers with decreasing spreading rate. The slow cooling of the lower crust and uppermost mantle and serpentinization, a low temperature alteration process which produces magnetite, are the likely cause of this contribution, also required to account for satellite magnetic anomalies over oceanic areas. Moreover, the "hook shape" of some sea-surface anomalies favors a time lag in the magnetization acquisition processes between upper and lower magnetic layers: extrusive basalt acquires a thermoremanent magnetization as soon as emplaced, whereas the underlying peridotite and olivine gabbro cool slowly

  18. Stability of hydrous phases in subducting oceanic crust

    USGS Publications Warehouse

    Liu, J.; Bohlen, S.R.; Ernst, W.G.

    1996-01-01

    Experiments in the basalt-H2O system at 600-950??C and 0.8-3.0 GPa, demonstrate that breakdown of amphibole represents the final dehydration of subducting oceanic tholeiite at T ??? 650??C; the dehydration H2O occurs as a free fluid or in silicate melt co-existing with an anhydrous eclogite assemblage. In contrast, about 0.5 wt% of H2O is stored in lawsonite at 600??C, 3.0 GPa. Our results suggest that slab melting occurs at depths shallower than 60 km for subducting young oceanic crust; along a subduction zone with an average thermal gradient higher than 7??C/km, H2O stored in hydrated low-potassium, metabasaltic layers cannot be subducted to depths greater than 100 km, then released to generate arc magma.

  19. Magnetism of the oceanic crust: Evidence from ophiolite complexes

    SciTech Connect

    Banerjee, S.K.

    1980-07-10

    The magnetic properties of six ophiolite complexes from around the world, ranging in age from Jurassic to Miocene, are presented. An emphasis is placed in our study on the petrologic and isotopic data from these ophiolite complexes in order to determine first whether the rock samples presently available represent the pristine ocean crust or whether they have been altered subaerially since their formation. Five of the ophiolites are found to be acceptable, and the conclusion is overwhelmingly in favor of a marine magnetic source layer that includes not only the pillow lavas but also the underlying dikes and gabbro. At the moment, however, our observations do not suggest that the magnetic contributions of the basaltic dikes should be overlooked in favor of gabbro. A second important conclusion is that nearly pure magnetite could indeed be a magnetic carrier which contributes to marine magnetic anomanies. It only awaits discovery by deeper ocean crustal penetration by future Deep Sea Drilling Project legs.

  20. Colonization of subsurface microbial observatories deployed in young ocean crust

    PubMed Central

    Orcutt, Beth N; Bach, Wolfgang; Becker, Keir; Fisher, Andrew T; Hentscher, Michael; Toner, Brandy M; Wheat, C Geoffrey; Edwards, Katrina J

    2011-01-01

    Oceanic crust comprises the largest hydrogeologic reservoir on Earth, containing fluids in thermodynamic disequilibrium with the basaltic crust. Little is known about microbial ecosystems that inhabit this vast realm and exploit chemically favorable conditions for metabolic activities. Crustal samples recovered from ocean drilling operations are often compromised for microbiological assays, hampering efforts to resolve the extent and functioning of a subsurface biosphere. We report results from the first in situ experimental observatory systems that have been used to study subseafloor life. Experiments deployed for 4 years in young (3.5 Ma) basaltic crust on the eastern flank of the Juan de Fuca Ridge record a dynamic, post-drilling response of crustal microbial ecosystems to changing physical and chemical conditions. Twisted stalks exhibiting a biogenic iron oxyhydroxide signature coated the surface of mineral substrates in the observatories; these are biosignatures indicating colonization by iron oxidizing bacteria during an initial phase of cool, oxic, iron-rich conditions following observatory installation. Following thermal and chemical recovery to warmer, reducing conditions, the in situ microbial structure in the observatory shifted, becoming representative of natural conditions in regional crustal fluids. Firmicutes, metabolic potential of which is unknown but may involve N or S cycling, dominated the post-rebound bacterial community. The archaeal community exhibited an extremely low diversity. Our experiment documented in situ conditions within a natural hydrological system that can pervade over millennia, exemplifying the power of observatory experiments for exploring the subsurface basaltic biosphere, the largest but most poorly understood biotope on Earth. PMID:21107442

  1. Dating low-temperature alteration of the upper oceanic crust

    NASA Astrophysics Data System (ADS)

    Coogan, L. A.; Hinton, R. W.; Gillis, K. M.; Dosso, S. E.

    2011-12-01

    Off-axis hydrothermal systems lead to extensive chemical exchange between the oceans and upper oceanic crust but it is unclear when this exchange occurs. We address this using a new dating approach and via the re-evaluation of existing data that contain age information. We have developed a method to directly date adularia, a common alkali-rich phase in old oceanic crust, using the 40K to 40Ca radiogenic decay system. In situ analysis, using the Cameca 1270 ion microprobe at the University of Edinburgh, allows small, replacive, secondary mineral grains to be analyzed. In comparison to previous radiogenic dating of low-temperature secondary minerals, using Rb-Sr and K-Ar approaches on mineral separates, this approach has the advantages that: (i) analysis is not limited to large, void filling, grains; (ii) the initial isotopic ratio is well constrained; (iii) contamination and phase heterogeneity are minimized; and (iv) the daughter isotope is relatively immobile. However, the requirement to analyse doubly charged ions, to reduce molecular interferences and suppress the presence of 40K on 40Ca, leads to low count rates [1]; e.g. single spot ages have uncertainties of 10's of millions of years. Combining all analyses for a given sample gives best fitting instantaneous precipitation "ages" of 102 and 70 Myr for DSDP Holes 417A and 543A (versus crustal ages of 120 and 80 Myr). The scatter in the data are consistent with adularia precipitation over >30 Myr. The timing of carbonate precipitation in the upper oceanic crust can be constrained from comparison of their 87Sr/86Sr to the seawater Sr-isotope curve if the proportion of basaltic Sr in the fluid can be constrained. Modeling such data from 12 drill cores shows that they are best fit by a model in which >90% of carbonate precipitation occurs over ≤20 Myr after crustal formation [2]. Evaluation of published Rb-Sr "isochron" data [3,4] shows that these data can be explained in different ways. The "isochron

  2. Biomineralisation of the ferromanganese crusts in the Western Pacific Ocean

    NASA Astrophysics Data System (ADS)

    Jiang, Xiao-Dong; Sun, Xiao-Ming; Guan, Yao; Gong, Jun-Li; Lu, Yang; Lu, Rong-Fei; Wang, Chi

    2017-04-01

    Ferromanganese (Fe-Mn) crusts are deep-sea sedimentary polymetallic minerals that are explored for their economic potential, particularly for Mn, Cu, Co, Ni and rare earth elements (REEs). The precipitation mechanism of the metallic elements in crusts has remained controversial between chemical oxidation (abiotic origin) and microbial enzymatic processes (biomineralization). In this study, the microbial mineralization in ferromanganese crusts from the Western Pacific Ocean was explored. Scanning electron microscope (SEM) and transmission electron microscope (TEM) analyses showed abundant micron-scale spherical aggregates of Mn-oxide filaments (20-80 nm), which are closely associated with filamentous cells within the biofilm (biofilm mineralization) exist within the stromatolitic structure. The high-throughput sequencing of 16S rRNA and phylogenetic analysis suggests that biofilms are dominated by three Mn-oxidizing bacterial species from the families Bacillus, Arthrobacter and Pseudomonas. In addition, Mn concentration in the biofilms is approximately 108 times that of the associated seawater (2.3 ppb Mn). Iron (16.2 wt%), Cu (0.11 wt%), Co (0.719 wt%) and Ni (0.459 wt%) were found in the biofilms via X-ray Fluorescence Spectrometer (XRF) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). We suggest that biomineralization provides a new perspective for understanding Fe-Mn crustal-related mineral deposits, and the ultra-high microbial trace element enrichment ability is noteworthy. Utilization of microbial activities in accumulating precious metals from seawater may offer a viable alternative for the world's metal production in the future.

  3. Chemical complexity of hotspots caused by cycling oceanic crust through mantle reservoirs

    NASA Astrophysics Data System (ADS)

    Li, Mingming; McNamara, Allen K.; Garnero, Edward J.

    2014-05-01

    Lavas erupted at ocean island hotspots such as Hawaii have diverse geochemical signatures. These ocean island basalts are thought to be derived from many sources with different chemical compositions within Earth's mantle and contain components of more primitive, less degassed material, as well as several recycled oceanic crustal components. Furthermore, the recycled oceanic crustal components display vastly different ages. The various components may be derived from different mantle reservoirs that are entrained and carried to the surface by mantle plumes, but it is unclear how individual plumes could successively sample each of these reservoirs or why the recycled oceanic crust would have variable ages. Here we use high-resolution numerical simulations to investigate the interaction between mantle plumes, subducted oceanic crust and a more primitive lower mantle reservoir. In our simulations, some subducted oceanic crust is entrained directly into mantle plumes, but a significant fraction of the crust--up to 10%--enters the more primitive reservoirs. As a result, mantle plumes entrain a variable combination of relatively young oceanic crust directly from the subducting slab, older oceanic crust that has been stirred with ancient more primitive material and background, depleted mantle. Cycling of oceanic crust through mantle reservoirs can therefore reconcile observations of different recycled oceanic crustal ages and explain the chemical complexity of hotspot lavas.

  4. Evidence for oceanic crust in the Herodotus Basin

    NASA Astrophysics Data System (ADS)

    Granot, Roi

    2016-04-01

    Some of the fundamental tectonic problems of the Eastern Mediterranean remain unresolved due to the extremely thick sedimentary cover (10 to 15 km) and the lack of accurate magnetic anomaly data. I have collected 7,000 km of marine magnetic profiles (2012-2014) across the Herodotus and Levant Basins, Eastern Mediterranean, to study the nature and age of the underlying igneous crust. The towed magnetometer array consisted of two Overhauser sensors recording the total magnetic anomaly field in a longitudinal gradiometer mode, and a fully oriented vector magnetometer. The total field data from the Herodotus Basin reveal a newly detected short sequence of long-wavelength NE-SW lineated anomalies that straddle the entire basin suggesting a deep two-dimensional magnetic source layer. The three components of the magnetic vector data indicate that an abrupt transition from a 2D to 3D magnetic structure occurs east of the Herodotus Basin, along where a prominent NE-SW gravity feature is found. Altogether, these new findings confirm that the Herodotus Basin preserves remnants of oceanic crust that formed along the Neotethyan mid-ocean ridge system. The continuous northward and counterclockwise motion of the African Plate during the Paleozoic and Mesozoic allow predicting the evolution of remanent magnetization directions, which in-turn dictate that shape of the anomalies. The shape of the Herodotus anomalies best fit Late Carboniferous to Early Permian (300±20 Myr old) magnetization directions. Finally, I will discuss the implications of these results on the tectonic architecture of the region as well as on various geodynamic processes.

  5. Partial melting of the mafic lower oceanic crust

    NASA Astrophysics Data System (ADS)

    Leuthold, J.; Lissenberg, J. C. J.; Falloon, T.; Ulmer, P.; O'Driscoll, B.; Klimentyeva, D.

    2016-12-01

    Along oceanic spreading ridges, magma batches underplate, ascend, stall and erupt (see Fig.1). Although fractional crystallization is frequently considered the most important process, MASH processes characterize open-system igneous complexes and affects the crustal evolution of magmas. We examine the record of partial melting in lower oceanic crust gabbro cumulate from slow- (Atlantic ocean at Kane Megamullion) and fast-spreading ridges (Pacific ocean at Hess Deep). At the crystal scale, eutectic minerals are most affected by partial melting (i.e. Cpx + Plg ± Ol). Cpx oikocryst microstructure show resorption embayments related to sharp geochemical variations: compatible Cr and Al and incompatible Ti, Zr and REE decrease from core to rim, with a late-stage TiO2and REE increase. Mg#, Eu* and Sr* increase from core to rim and strongly decrease towards the outer rim. Plg shows reverse zoning. Textures and geochemical evolution are incoherent with processes such as fractional crystallization or crystallization from successive mantle-derived melts. Zoned Cpx oikocrysts result from 1) partial melting of REE-poor lower crust gabbroic cumulate, 2) hybridization with intrusive hot Cpx-undersaturated primitive mantle-derived melt and 3) refertilization. Reaction products are a Cpx-poor gabbro/troctolite residue and a hybrid melt saturated in Cpx and Plg (see Eq.1). Mineralogy, texture and chemistry of cumulate primocrysts and hot intrusive melt are modified. Outer rim crystallized from a distinct percolating late-stage reacted melt, close to solidus conditions. Hybrid melt has high SiO2 and Mg# contents and is Cr-, Zr- and REE-depleted. MELTS calculations and geochemical modelling of the hybrid melt liquid line of descent match N-MORB chemistry. Hybridization with co-genetic gabbro partial melt is a valid alternative/complementary process to explain variations in the primitive MORB geochemistry. However, the lacks of strong mineralogical and geochemical contrast between co

  6. U Mineral Hosts and Enrichment Processes in Altered Oceanic Crust

    NASA Astrophysics Data System (ADS)

    Farr, L. C.; Plank, T.; Kelley, K.; Alt, J. C.

    2001-12-01

    The U-Pb-Th isotopic system is a primary tool for understanding mantle and continental evolution and for quantifying the flow of mass and heat through the Earth's reservoirs. One of the major sites of U-Pb-Th fractionation is the oceanic crust, which is a sink for seawater U. For example, the upper, oxidized oceanic crust (U ~0.4ppm) may be as much as 4x enriched over pristine igneous values (U ~0.09ppm) with a minor net change in Pb and Th. Little, however, is understood about the mechanisms controlling uranium enrichment, its mineral hosts, or the timing of the process. We have used laser ablation inductively coupled mass spectrometry (LA-ICP-MS) and electron microprobe data to study the distribution of U in the oldest sampled crust in the Pacific, Jurassic mid-ocean ridge basalts (MORB) from ODP Site 801C (1000 km seaward of the Mariana trench), formed at fast spreading rates. Seventeen thin sections, 8 with the highest whole rock U content (HUC) (0.61-1.7ppm) and 9 with low U content (LUC) (U <0.4ppm) were analyzed in detail. Twelve of the thin sections contain various types of carbonates (veins, vugs, vesicles, and inter-flow material) in a variety of crystal morphologies (radial, inclusion-rich, and sparry). A 50-micron laser beam was rastered across various alteration zones, such as halos, veins, and the surrounding host to provide in-situ multi-element analysis (U, Th, Pb, REE, alkalis, etc). HUC are exclusively associated with low-Mg calcites (< 1wt% Mg), whereas LUC are typically associated with high-Mg calcites (10-20 wt% Mg). U also correlates positively with Sr and negatively with total rare earth element (REE) concentration. High U carbonates inherit the seawater REE pattern with both HREE and LREE enrichment, whereas LUC inherit the LREE depletion of the MORB host. Thus, there appear to be distinct generations of fluids that precipitate carbonate; those with the high Sr and low REE of seawater tend to precipitate U-rich calcites (up to 4.5 ppm U

  7. The thickness of Enceladus' crust and ocean as seen by Cassini

    NASA Astrophysics Data System (ADS)

    Trinh, A.; Rivoldini, A.; Beuthe, M.; Baland, R. M.; Van Hoolst, T.

    2016-12-01

    The librations of Enceladus are so large that the icy crust must be decoupled from the rocky bulk by a global-scale ocean (Thomas et al. 2016). New models of Airy-isostatic compensation (assuming minimum stress within the crust) and librations (including additional non-hydrostatic torques) can reconcile Cassini's gravity, topography, and libration observations over a consistent picture of the crust (see Rivoldini et al., this session). Here we sum up our current view of the thickness of Enceladus's crust and ocean from a joint Bayesian inversion of all three kinds of observations.

  8. Evolving morphology of thermochemical piles caused by accumulation of subducted oceanic crust

    NASA Astrophysics Data System (ADS)

    Li, M.; McNamara, A. K.

    2015-12-01

    Seismic tomography results have shown two large low shear velocity provinces (LLSVPs) in the lowermost mantle beneath Africa and Pacific. The LLSVPs have been hypothesized to be caused by large-scale compositional heterogeneity. Two hypotheses have been proposed for the origin of this compositional heterogeneity: (1) primordial material formed during Earth's early differentiation, and (2) accumulations of subducted oceanic crust on the core-mantle boundary (CMB). Previous geodynamical calculations often show that stable thermochemical piles caused by primordial material have sharp boundaries. So, if the accumulation of subducted oceanic crust has different morphology than that of piles caused by primordial material, we may be able to constrain the origin of compositional heterogeneity from high resolution seismic observations of the boundaries of LLSVPs.Here, we performed geodynamic calculations to investigate the morphology of accumulation of subducted oceanic crust on the CMB. We found that the ability of subducted oceanic crust to accumulate on the CMB and the sharpness of the boundaries of the accumulations both strongly depends on the crustal thickness. A thick (e.g., ~30 km) oceanic crust produced from the early hot mantle can form into large-scale accumulations on the CMB, but with fuzzy and diffuse top boundaries. However, as the oceanic crust becomes thinner, it becomes more difficult to accumulate on the CMB, and the top boundaries of the accumulations of subducted oceanic crust also gradually become sharp, more like that of piles caused by primordial material. Thus, a sharp top boundaries of LLSVPs in the present-day Earth does not guarantee that they are caused by piles of primordial material. In addition, as the oceanic crust becomes thinner, more subducted oceanic crust is entrained and recycled to shallow depth, which may have important implications for geochemical observations on Earth's surface.

  9. Post-glacial ocean acidification and the decline of reefal microbial crusts

    NASA Astrophysics Data System (ADS)

    Riding, R.; Liang, L.; Braga, J.

    2011-12-01

    Data from Pacific, Indian Ocean and Caribbean coral reefs indicate marked Late Pleistocene to Holocene decline in the maximum thickness of microbial carbonate crusts in reef cavities. Using estimated values of pH, temperature, CO2, and ionic composition, we calculated calcite saturation ratio (Ωcalcite) of tropical surface seawater for the past 16 Ka. This shows a declining trend of Ωcalcite, paralleling that of reefal microbial crust thickness. We suggest that thinning of reefal microbial crusts could reflect decrease in seawater carbonate saturation due to ocean acidification in response to deglacial CO2 increase. Previously, decline in reefal microbial crusts, for example at Tahiti in the Pacific Ocean, has mainly been attributed to changes in nutrient supply associated with ocean upwelling and/or terrestrial run-off. Ocean acidification does not preclude such effects on microbial crust development produced by localized changes, but two features in particular are consistent with a global link with carbonate saturation state. Firstly, post-glacial decline in reefal microbial crust thickness affected tropical coral reefs in several oceans. Secondly, seawater carbonate saturation is a major long-term control on microbial carbonate abundance; microbially-induced biocalcification requires elevated seawater saturation for CaCO3 minerals and can be expected to fluctuate with carbonate saturation. In addition to compiling published crust thickness data, we measured thicknesses of microbial carbonate crusts in cavities in Tahiti reefs sampled by Integrated Ocean Drilling Program coring in 2005. This indicates halving of maximum crust thickness, during the same period as steep decline in mean-ocean calcite saturation, near the Pleistocene-Holocene transition. Reefal microbial crusts have been common since skeletal reefs became widespread during the Ordovician Period, 475 Ma ago. The habitat for cryptic crusts expanded as scleractinian corals developed cavernous

  10. Partial separation of halogens during the subduction of oceanic crust

    NASA Astrophysics Data System (ADS)

    Joachim, Bastian; Pawley, Alison; Lyon, Ian; Henkel, Torsten; Clay, Patricia L.; Ruzié, Lorraine; Burgess, Ray; Ballentine, Christopher J.

    2014-05-01

    Incompatible elements, such as halogens, have the potential to act as key tracers for volatile transport processes in Earth and planetary systems. The determination of halogen abundances and ratios in different mantle reservoirs gives us the ability to better understand volatile input mechanisms into the Earth's mantle through subduction of oceanic crust. Halogen partition coefficients were experimentally determined between forsterite, orthopyroxene and silicate melt at pressures ranging from 1.0 to 2.3 GPa and temperatures ranging from 1500-1600°C, thus representing partial melting conditions of the Earth's mantle. Combining our data with results of recent studies (Beyer et al. 2012; Dalou et al. 2012) shows that halogen partitioning between forsterite and melt increases by factors of about 1000 (fluorine) and 100 (chlorine) between 1300°C and 1600°C and does not show any pressure dependence. Chlorine partitioning between orthopyroxene and melt increases by a factor of about 1500 for a temperature increase of 100°C (anywhere between 1300°C and 1600°C), but decreases by a factor of about 1500 for a pressure increase of 1.0 GPa (anywhere between 1.0 GPa and 2.5 GPa). At similar P-T conditions, a comparable effect is observed for the fluorine partitioning behaviour, which increases by 500-fold for a temperature increase of 100°C and decreases with increasing pressure. Halogen abundances in mid-ocean ridge basalts (MORB; F=3-15, Cl=0.5-14ppm) and ocean island basalts (OIB; F=35-65, Cl=21-55 ppm) source regions were estimated by combining our experimentally determined partition coefficients with natural halogen concentrations in oceanic basalts (e.g. Ruzié et al. 2012). The estimated chlorine OIB source mantle concentration is in almost perfect agreement with primitive mantle estimates (Palme and O'Neill 2003). If we expect an OIB source mantle slightly depleted in incompatible elements, this suggests that at least small amounts of chlorine are recycled deep

  11. Ancient Fungal Life in North Pacific Eocene Oceanic Crust

    NASA Astrophysics Data System (ADS)

    Schumann, G.; Manz, W.; Reitner, J.; Lustrino, M.

    2003-12-01

    Little is known about the manifold life forms of the deep biosphere although there is increasing scientific evidence that an extensive biosphere does exist in extreme environments such as the rocks below the seafloor. The ODP Leg 200 was devoted to the study of Eocene oceanic crust of the North Pacific Ocean. Within a massive tholeiitic lava flow unit, at depth of 51 mbsf underneath a water column of about 5000 m, we found unique filamentous structures. Based on morphological traits like branching, septa and central pores the filaments are interpreted as fungi. These filaments were found within carbonate-filled vesicles ranging in size from 0.5 to 3 mm in diameter. The net of fungal hyphae completely fills the whole pore space from the basalt-carbonate boundary towards the center of the pores. The cross section dimension of these filaments is about 5-10 micrometer and the length differ from 50 to several hundreds micrometer. Thereby the cell septa of the hyphae are clearly visible. The number of hyphae ranges from some tenth to some hundreds per particular pore. The presence of pyrite within the carbonate cements points out anaerobic conditions in this habitat. After removing the carbonate by etching the vesicles with diluted formic acid, the 3-dimensional structure of the fungus could be clearly visualized. Fine structure analysis of the hyphae obtained by field emission scanning electron microscopy (FE-SEM) revealed a network of tiny small fibers coating the surface of the hyphae. Semi-quantitative chemical analyses of the etched hyphae were conducted with an energy dispersive spectrometer system (EDS) coupled with the FE-SEM. The results evidence a chemical composition of the hyphae different from the surrounding carbonate matrix. Undisturbed filamentous growth through different calcite crystals within the vesicles and small open space between the fungi and matrix indicate endolithic fungal growth after the calcium carbonate filling of the vesicles. To the best

  12. Pervasive reactive melt migration through fast-spreading lower oceanic crust (Hess Deep, equatorial Pacific Ocean)

    NASA Astrophysics Data System (ADS)

    Lissenberg, C. Johan; MacLeod, Christopher J.; Howard, Kerry A.; Godard, Marguerite

    2013-01-01

    Mid-ocean ridge basalt (MORB) is the most abundant magma on Earth, and provides a geochemical window into the mantle. Deriving mantle composition and melting processes from the erupted lavas requires correction to be made for their evolution as they pass through and generate the oceanic crust. This is typically done by assuming that modification of melts in crustal magma chambers occurs exclusively by fractional crystallisation. However, extensive mineral major- and trace element data from a full section of fast-spread lower crustal rocks exposed in Hess Deep (equatorial Pacific Ocean) demonstrate that their evolution is instead controlled by reactive porous flow. These reactions lead to a strong enrichment in, and fractionation of, incompatible trace elements in the melt (as recorded by clinopyroxene compositions), leading to melt compositions far outside of the compositional realm of MORB both in terms of trace element abundances and ratios. The reactive signature increases in strength up section, peaking in varitextured gabbros interpreted to represent the fossilised axial melt lens, indicating that reactive porous flow occurred on the scale of the entire lower crust. The enrichment of the melt is coupled with a strong trace element depletion of plagioclase, olivine, and, to a lesser extent, clinopyroxene cores, suggesting that these phases represent the residues of the reactions from which trace elements have been removed. The dominant role of reactive porous flow, and the resulting deviations from fractional crystallisation predictions, suggest that the lower oceanic crust plays a much more complex and significant role in modifying the compositions of MORB than previously expected, with consequent implications for models of mantle processes.

  13. Sediment stratigraphy of the Nansen Basin, Arctic Ocean and characterization of the ultraslow-spreading oceanic crust

    NASA Astrophysics Data System (ADS)

    Lutz, R.; Franke, D.; Berglar, K.; Schnabel, M.

    2015-12-01

    The Nansen Basin is the southern part of the Eurasia Basin in the Arctic Ocean. Opening of the Eurasia Basin started here with the tear-off of the continental Lomonossov ridge. Here we present a couple of multichannel reflection seismic lines, covering an area from the Barents Shelf to 83.2 deg N. The profiles extend for about 275 km and 170 km, respectively from the Barents Sea margin (Hinlopen margin) into northern direction and cover together ~300 km of oceanic crust on two parallel lines. One connecting profile was acquired on oceanic crust crossing anomaly C23 (~50-52 Ma). The data were acquired during ice-free conditions and reveal for the first time the architecture of the oldest sediments deposited on the oceanic crust. We discuss the seismic facies of the oldest sediments on the oceanic crust and determine their age by correlation of onlap contacts onto oceanic crust with well defined magnetic anomalies. The lowermost sedimentary unit can be subdivided by at least one more prominent seismic reflector in the distal part of the Nansen Basin and two more seismic reflectors in the proximal part. Furthermore we present images and interpretations of oceanic crust formed at the ultraslow-spreading Gakkel ridge (< 20 mm yr-1 full rate). We discuss the basement morphology, volcanic cones and major faults, bounding horsts and grabens in the light of our present understanding of melt-poor ultraslow-spreading ridges.

  14. Oceanic crust formation in the Egeria Fracture Zone Complex (Central Indian Ocean)

    NASA Astrophysics Data System (ADS)

    Le Minor, Marine; Gaina, Carmen; Sigloch, Karin; Minakov, Alexander

    2016-04-01

    This study aims to analyse in detail the oceanic crust fabric and volcanic features (seamounts) formed for the last 10 million years at the Central Indian Ridge between 19 and 21 latitude south. Multibeam bathymetry and magnetic data has been collected in 2013 as part of the French-German expedition RHUM-RUM (Reunion hotspot and upper mantle - Reunion's unterer mantel). Three long profiles perpendicular on the Central Indian Ridge (CIR), south of the Egeria fracture zone, document the formation of oceanic crust since 10 million years, along with changes in plate kinematics and variations in the magmatic input. We have inspected the abyssal hill geometry and orientation along conjugate oceanic flanks and within one fracture zone segment where we could identify J-shaped features that are indicators of changes in plate kinematics. The magnetic anomaly data shows a slight asymmetry in seafloor spreading rates on conjugate flanks: while a steady increase in spreading rate from 10 Ma to the present is shown by the western flank, the eastern part displays a slowing down from 5 Ma onwards. The deflection of the anti J-shaped abyssal hill lineations suggest that the left-stepping Egeria fracture zone complex (including the Egeria, Flinders and an un-named fracture zone to the southeast) was under transpression from 9 to 6 Ma and under transtension since 3 Ma. The transpressional event was triggered by a clockwise mid-ocean ridge reorientation and a decrease of its offset, whereas the transtensional regime was probably due to a counter-clockwise change in the spreading direction and an increase of the ridge offset. The new multibeam data along the three profiles reveal that crust on the eastern side is smoother (as shown by the abyssal hill number and structure) and hosts several seamounts (with age estimations of 7.67, 6.10 and 0.79 Ma), in contrast to the rougher conjugate western flank. Considering that the western flank was closer to the Reunion plume, and therefore

  15. Extending the deep biosphere through ocean drilling: Bioalteration of volcanic glass in the oceanic crust

    NASA Astrophysics Data System (ADS)

    Banerjee, N. R.; Furnes, H.; Muehlenbachs, K.; Staudigel, H.; French, J.

    2003-12-01

    Scientific ocean drilling through the Deep Sea Drilling Program (DSDP) and Ocean Drilling Program (ODP) has provided a window into the deep biosphere. Microbial communities have been identified within hydrothermal vent systems at ocean floor spreading centers, deep within oceanic sediments, and within glassy portions of the basaltic oceanic crust of varying age. Questions still remain regarding the utilized metabolic pathways, how long such microbial activity persists within the oceanic crust, and how well it may be preserved on Earth or other planets. Microbial alteration of basaltic glass from ODP/DSDP cores and ophiolites can be documented by petrographic and biogeochemical techniques. Microbial alteration is seen as either tubular or granular textures. Tubular textures are characterized by micron-scale channel-like features extending from palagonite alteration rims into fresh glass. Granular textures appear as irregular patches of spherical bodies protruding into fresh glass. Detailed SEM imaging of these features commonly reveals delicate filament-like structures and material resembling desiccated biofilm. X-ray element maps invariably show elevated levels of C, N, P, and K associated with suspected microbial alteration features. Comparison of glasses from different ODP/DSDP holes indicates that carbon isotope ratios of carbonates in samples of microbially altered volcanic glass are commonly depleted by as much as -20 per mil, except in samples from slow-spreading ridges where both elevated and depleted C-isotope ratios are observed. In general, the microbes appear to be living off of dissolved organic matter with some evidence of lithoautotrophy at slow-spreading ridges. Maximum microbial activity seems to occur at ˜70° C but is moderated by pore water flow. We have applied nucleic acid staining techniques to many samples and imaged them with epifluorescence and laser scanning confocal microscopy. This has produced exceptional images that provide high

  16. Diamondiferous eclogites from Siberia: Remnants of Archean oceanic crust

    NASA Astrophysics Data System (ADS)

    Jacob, D.; Jagoutz, E.; Lowry, D.; Mattey, D.; Kudrjavtseva, G.

    1994-12-01

    We have investigated eight diamond-bearing bimineralic eclogite xenoliths from the Udachnaya Mine, Yakutia, Siberia, in terms of major elements, 87Sr /86Sr- , 143Nd /144Nd and oxygen isotopic ratios. The β18O-values, measured with the new laser-fluorination technique, are different from mantle values and range between 5.19 and 7.38%. with an average error of 0.08%.. Strontium and neodymium initial isotopic ratios for cpx are between 0.70226 and 0.70699 and 0.51170 and 0.51257, respectively. Chemically and petrographically, the Siberian eclogites are very similar to the South African eclogite suite from Roberts Victor or Bellsbank, the most important similarities being the late Archean age (2.76 Ga) and the δ18O values that deviate from mantle values. However, differences exist in detail, as no samples with δ18O values lower than mantle values have yet been reported from Siberia and the cesium concentrations of the Siberian eclogites are generally lower than those of the Roberts Victor eclogite suite. The data obtained from the studied sample suite are best explained by a model proposing an origin from Archean oceanic crust that was intensely altered prior to subduction to mantle depths. Using oxygen isotopic values, the effects of seawater alteration can be shown and the composition of the unaltered protolith qualitatively estimated. We propose that mantle eclogites from kimberlites were generated by a globally operating subduction process during the late Archean and that differences between samples from different cratons are small compared to their similarities.

  17. Deformation and rupture of the oceanic crust may control growth of Hawaiian volcanoes

    USGS Publications Warehouse

    Got, J.-L.; Monteiller, V.; Monteux, J.; Hassani, R.; Okubo, P.

    2008-01-01

    Hawaiian volcanoes are formed by the eruption of large quantities of basaltic magma related to hot-spot activity below the Pacific Plate. Despite the apparent simplicity of the parent process - emission of magma onto the oceanic crust - the resulting edifices display some topographic complexity. Certain features, such as rift zones and large flank slides, are common to all Hawaiian volcanoes, indicating similarities in their genesis; however, the underlying mechanism controlling this process remains unknown. Here we use seismological investigations and finite-element mechanical modelling to show that the load exerted by large Hawaiian volcanoes can be sufficient to rupture the oceanic crust. This intense deformation, combined with the accelerated subsidence of the oceanic crust and the weakness of the volcanic edifice/oceanic crust interface, may control the surface morphology of Hawaiian volcanoes, especially the existence of their giant flank instabilities. Further studies are needed to determine whether such processes occur in other active intraplate volcanoes. ??2008 Nature Publishing Group.

  18. Deformation and rupture of the oceanic crust may control growth of Hawaiian volcanoes.

    PubMed

    Got, Jean-Luc; Monteiller, Vadim; Monteux, Julien; Hassani, Riad; Okubo, Paul

    2008-01-24

    Hawaiian volcanoes are formed by the eruption of large quantities of basaltic magma related to hot-spot activity below the Pacific Plate. Despite the apparent simplicity of the parent process--emission of magma onto the oceanic crust--the resulting edifices display some topographic complexity. Certain features, such as rift zones and large flank slides, are common to all Hawaiian volcanoes, indicating similarities in their genesis; however, the underlying mechanism controlling this process remains unknown. Here we use seismological investigations and finite-element mechanical modelling to show that the load exerted by large Hawaiian volcanoes can be sufficient to rupture the oceanic crust. This intense deformation, combined with the accelerated subsidence of the oceanic crust and the weakness of the volcanic edifice/oceanic crust interface, may control the surface morphology of Hawaiian volcanoes, especially the existence of their giant flank instabilities. Further studies are needed to determine whether such processes occur in other active intraplate volcanoes.

  19. Calcites from Ocean Crust Basalts: Reliable Proxy Archives?

    NASA Astrophysics Data System (ADS)

    Böhm, Florian; Eisenhauer, Anton

    2017-04-01

    Calcite cements in ocean crust basalts of the deep sea form from mixtures of cold seawater and warm hydrothermal fluids (about 0-70°C). These low temperature alteration (LTA) calcites have recently gained new interest as proxy recorders of seawater composition (Refs. 1-5). Recent LTA calcite reconstructions of the Sr/Ca and Mg/Ca evolution in ocean waters point to considerably lower Sr/Ca and Mg/Ca ratios during the Cretaceous and Paleogene than in the modern ocean. However, diagenetic alteration in contact with the basalt host rock may change the composition of the LTA calcites. For testing the reliability of LTA calcite records of seawater composition multi-proxy approaches are applied: oxygen isotopes indicate precipitation temperatures, strontium isotopes (87Sr/86Sr) and trace elements indicate influences from hydrothermal fluids. Additional information about the influence of basement rocks on LTA calcite composition can be derived from analyses of stable calcium and strontium isotopes (44/40Ca, 88/86Sr). We find low 44/40Ca values for DSDP and ODP sites where the 87Sr/86Sr ratios of LTA calcites indicate basement influence. On the other hand, for some sites the 87Sr/86Sr values inidicate precipitation from pristine seawater, while low 44/40Ca values indicate basement influence. All of these sites are either older than 50 Myr or show calcite precipitation temperatures >50°C. Sites that are younger than 25 Myr and had formation temperatures <10°C show high 44/40Ca values indicating no basement influence, in agreement with the 87Sr/86Sr ratios. Average 44/40Ca values of the latter sites are in good agreement with the Neogene seawater 44/40Ca evolution (Ref. 6). Stable strontium isotopes show little variability at most sites (88/86Sr= 0.30 to 0.35‰). Only at temperatures >50°C significantly higher 88/86Sr values were observed. The calcium isotope results indicate basement influence on LTA calcite composition at temperatures >10

  20. Inconsistent correlation of seismic layer 2a and lava layer thickness in oceanic crust.

    PubMed

    Christeson, Gail L; McIntosh, Kirk D; Karson, Jeffrey A

    2007-01-25

    At mid-ocean ridges with fast to intermediate spreading rates, the upper section of oceanic crust is composed of lavas overlying a sheeted dyke complex. These units are formed by dykes intruding into rocks overlying a magma chamber, with lavas erupting at the ocean floor. Seismic reflection data acquired over young oceanic crust commonly image a reflector known as 'layer 2A', which is typically interpreted as defining the geologic boundary between lavas and dykes. An alternative hypothesis is that the reflector is associated with an alteration boundary within the lava unit. Many studies have used mapped variability in layer 2A thickness to make inferences regarding the geology of the oceanic crust, including volcanic construction, dyke intrusion and faulting. However, there has been no link between the geologic and seismological structure of oceanic crust except at a few deep drill holes. Here we show that, although the layer 2A reflector is imaged near the top of the sheeted dyke complex at fast-spreading crust located adjacent to the Hess Deep rift, it is imaged significantly above the sheeted dykes section at intermediate-spreading crust located near the Blanco transform fault. Although the lavas and underlying transition zone thicknesses differ by about a factor of two, the shallow seismic structure is remarkably similar at the two locations. This implies that seismic layer 2A cannot be used reliably to map the boundary between lavas and dykes in young oceanic crust. Instead we argue that the seismic layer 2A reflector corresponds to an alteration boundary that can be located either within the lava section or near the top of the sheeted dyke complex of oceanic crust.

  1. Archean recycled oceanic crust sampled in Azores lavas

    NASA Astrophysics Data System (ADS)

    Beguelin, P.; Bizimis, M.; Beier, C.; Turner, S.

    2016-12-01

    Azores lava compositions extend below the mantle array in ɛNd-ɛHf space and define the steepest slope of all plume provinces [1], but this pattern is largely controlled by low ɛHf lavas from Eastern São Miguel island (SM). Here we present new Hf isotope data on well-characterized on-land and submarine Azores lavas from several islands, the Terceira Rift and João do Castro seamount (JdC), in order to further constrain this trend. While Azores lavas fall along the mantle array with relatively steep slopes (e.g. São Jorge slope = 2.1), both SM and JdC fall below the mantle array as two distinct steep arrays with slopes of 2.0 and 2.6 respectively, extending to ɛHf = 0 at ɛNd = 2 (SM) and 4 (JdC). This is a unique feature in OIBs. The new Hf-Nd data overlaps the HIMU-type Mangaia and St Helena compositions. However, SM and JdC have distinctly less radiogenic and more variable Pb isotopes (e.g. 206Pb/204Pb = 18.8 to 20.2) than HIMU. Hf-Nd isotope decoupling below the mantle array is therefore not an exclusive HIMU signature. The coupled Hf-Nd-Pb-Sr isotope compositions of the enriched SM and JdC end-members can be modeled by recycled 2.5-3.0 Ga N-MORB, with some E-MORB affinity for SM. Unlike HIMU however, no Pb-loss during subduction is required for recycled MORB to explain their Pb isotopes. The relatively high κ (232Th/238U 4.3) required by the Azores data is also consistent with a high Th/U Archean mantle [2]. Aged, metasomatised mantle lithosphere based on a global peridotite and pyroxenite compilation is too variable and only fortuitously could explain the Azores compositions. Both enriched JdC and SM endmembers can therefore be explained by a recycled Archean oceanic crust that is locally heterogeneous, as presently observed in some MOR segments where N-and E-MORB exist closely [3, 4]. The lack of mixing between SM and JdC end-members some 100 km apart further implies that this recycled crust has retained its distinct signature through mantle convection

  2. Noble gas transport during devolatilization of oceanic crust

    NASA Astrophysics Data System (ADS)

    Jackson, C.; Smye, A.; Shuster, D. L.; Parman, S. W.; Kelley, S. P.; Hesse, M. A.; Cooper, R. F.

    2014-12-01

    Here we examine the role of slab dehydration in determining the elemental pattern of recycled noble gases. As a first step, we apply newly reported measurements of He-Ne-Ar (light noble gases) solubility and diffusivity in amphibole to parameterize a 1D diffusive-reaction transport model that simulates noble gas behavior during fluid loss from down-going oceanic crust. Recent experiments demonstrate that noble gases are highly soluble in ring-structured minerals, such as amphibole and other common hydrothermal products in slabs [1]. These results suggest that ring-structured minerals have the potential to strongly influence the budget of noble gases input into subduction zones and the elemental fractionations associated with volatile loss from slabs New measurements of He-Ne-Ar solubility in a suite of amphiboles have been completed utilizing the methodology described in [1]. These new measurements confirm that all light noble gases are highly soluble in amphibole, and that noble gas solubility correlates with the availability of unoccupied ring sites. New experimental measurements of He and Ne diffusivity have also been completed using a step-degassing approach at the Berkeley Geochronology Center. These measurements suggest that vacant ring sites in amphibole act to slow noble gas diffusion. We combine the newly acquired He and Ne diffusivity measurements with literature values for Ar diffusivity [2] to parameterize the diffusive-reaction transport model. Application of these data to the diffusive-reaction transport model yields several new insights. The relative mobility of Ne compared to Ar allows for efficient extraction of Ne from "hot" slabs by shallow depths (<50 km), while Ar is effectively retained to deeper depths, potentially past sub-arc conditions. Noble gas partition coefficients sharply increase with depth, following their increasing non-ideality in supercritical fluids, causing noble gases to partition back into minerals from any fluids retained in

  3. Asymmetric Spreading, and the Construction of Oceanic Crust at the Kane Oceanic Core Complex

    NASA Astrophysics Data System (ADS)

    Cheadle, M.; John, B.; Lusk, M.; Wooden, J.

    2008-12-01

    Detachment faulting at the Kane Oceanic Core Complex (OCC) on the Mid-Atlantic Ridge (23° N) has exposed a tectonic window through oceanic crust. Here we present fourteen Pb/U zircon SHRIMP ages of evolved lower crustal gabbro collected by ROV and dredging during R/V Knorr Cruise 180-2 from the Babel, Cain and Abel, and Adam domes of the Kane Oceanic Core complex (up to 46 km off axis), and from ODP Hole 923A, near the present day ridge axis. These data allow us to constrain the construction history of oceanic crust at Kane. Weighted average 206Pb/238U ages range from 2.45 +/-0.06 to 3.70+/-0.16 Ma for samples from the Kane OCC, and 0.86+/-0.14 Ma for Hole 923A. Age-distance plots yield a North American plate-spreading rate of 15.3+/-2.0 cm/yr during formation of the Kane OCC, with a time-averaged rate of 14.3+/-0.95 cm/yr determined from ODP 923A near the ridge axis to the western margin of the OCC, consistent with rates determined from magnetic anomaly data (Williams, 2007). Both the zircon and magnetic data show that the Kane OCC formed during a period of asymmetric spreading with over 60% of the total plate-motion accommodated by detachment faulting, requiring associated ridge migration. The average 206Pb/238U age is consistently ~~250,000 yr older than the magnetic age, implying acquisition of magnetic remanence roughly 4 km off-axis. Ten samples have ages within error of the calculated spreading rate. However, two samples are significantly older, and two significantly younger than those that define the spreading rate trend, implying variability in both depth and location of crustal accretion. One of the older samples (3.7 +/-0.16Ma) comes from the northern Babel Dome, adjacent to the Kane Transform Fault. This single age is 0.6 Ma older than samples from the Cain and Abel domes (15 km) to the south, and may suggest that the gabbros forming Babel Dome crystallized ~~5km deeper in cooler lithosphere adjacent the transform fault. Additional

  4. IODP Expedition 345: Structural characteristics of fast spread lower ocean crust, implications for growth and cooling of ocean crust

    NASA Astrophysics Data System (ADS)

    John, B. E.; Ceuleneer, G.; Cheadle, M. J.; Harigane, Y.

    2013-12-01

    IODP Expedition 345 to the Hess Deep Rift sampled ~1 Ma, fast-spread East Pacific Rise gabbroic crust exposed as a dismembered, lower crustal section. Sixteen holes were drilled at Site U1415, centered on a sub-horizontal, 200-m wide E-W-trending bench between 4675 and 4850 mbsl. The bench was formed as a rotational slide within a 1km high slump along the southern wall of the intra-rift ridge. Primitive olivine gabbro and troctolite (Mg# 76-89) were sampled in four discrete, 30 to ≥ 65 m sized blocks formed by the mass wasting that dominates the southwestern slope of the ridge. Igneous fabric orientations (both layering and foliation) in the blocks vary from sub-vertical to gently dipping, suggesting some of the blocks have rotated at least 90°. Magmatic fabrics including spectacular modal and/or grain size layering are prevalent in >50% of the recovered core. Magmatic foliation in all blocks is defined by plagioclase crystal shape, but may also be defined by olivine and, to a lesser extent, orthopyroxene and clinopyroxene when the crystals have suitable habits. In all cases, this foliation is controlled by both the preferred orientation and shape anisotropy of the crystals. Fabric intensity varies from moderate to strong in the block with simple modal layering, weak to absent in the two blocks of troctolite, and largely absent in the block with heterogeneous textures and/or diffuse banding. Intrinsic to the layering and banding is the common development of dendritic and/or skeletal olivine textures (grain size up to 3 cm). The preservation of these delicate olivine grains showing only limited subgrain formation, and no kinking precludes significant low melt fraction (<20%) crystal plastic flow of the cumulates. This observation prohibits ocean crust formation models that require homogeneous deformation/flow at low melt fractions. Down-temperature sub-solidus crystal plastic deformation and/or shear zones are virtually absent from the recovered core. Significant

  5. Seismic structure of oceanic crust at ODP borehole 504B: Investigating anisotropy and layer 2 characteristics

    NASA Astrophysics Data System (ADS)

    Gregory, E. P. M.; Hobbs, R. W.; Peirce, C.; Wilson, D. J.

    2015-12-01

    Fracture and fault networks in the upper oceanic crust influence the circulation of hydrothermal fluids and heat transfer between crust and ocean. These fractures form by extensional stresses, with a predominant orientation parallel to the ridge axis, creating porosity- and permeability-derived anisotropy that can be measured in terms of seismic velocity. These properties change as the crust ages and evolves through cooling, alteration and sedimentation. The rate at which these changes occur and their effects on oceanic crustal structure and hydrothermal flow patterns are currently not well constrained. The NERC-funded OSCAR project aims to understand the development of upper oceanic crust, the extent and influence of hydrothermal circulation on the crust, and the behavior of fluids flowing in fractured rock. We show P-wave velocity models centered on DSDP/ODP Hole 504B, located ~200 km south of the Costa Rica Rift, derived from data acquired during a recent integrated geophysics and oceanography survey of the Panama Basin. The data were recorded by 25 four-component OBSs deployed in a grid, that recorded ~10,000 full azimuthal coverage shots fired by a combined high- and low-frequency seismic source. Both reflection and refraction data are integrated to reveal the seismic velocity structure of the crust within the 25 km by 25 km grid. The down-hole geological structure of 6 Ma crust at 504B comprises 571.5 m of extrusive basalts overlying a 209 m transition zone of mixed pillows and dikes containing a clear alteration boundary, which grades to >1050 m of sheeted dikes. Our model results are compared with this lithological structure and other previously published results to better understand the nature of velocity changes within seismic layer 2. The data provide a 3D framework, which together with analysis of the S-wave arrivals and particle motion studies, constrain estimates of the seismic anisotropy and permeability structure of the upper oceanic crust as it

  6. Estimation of seismic velocities of upper oceanic crust from ocean bottom reflection loss data.

    PubMed

    Dong, Hefeng; Chapman, N Ross; Hannay, David E; Dosso, Stan E

    2010-04-01

    This paper describes a Bayesian inversion of acoustic reflection loss versus angle measurements to estimate the compressional and shear wave velocities in young uppermost oceanic crust, Layer 2A. The data were obtained in an experiment on the thinly sedimented western flank of the Endeavor segment of the Juan de Fuca Ridge, using a towed horizontal hydrophone array and small explosive charges as sound sources. Measurements were made at three sites at increasing distance from the ridge spreading center to determine the effect of age of the crust on seismic velocities. The inversion used reflection loss data in a 1/3-octave band centered at 16 Hz. The compressional and shear wave velocities of the basalt were highly sensitive parameters in the inversion. The compressional wave velocity increased from 2547 + or - 30 to 2710 + or - 18 m/s over an age span of 1.4 million years (Ma) from the spreading center, an increase of 4.5 + or - 1.0%/Ma. The basalt shear wave velocity increased by nearly a factor of 2, from approximately 725 to 1320 m/s over the same age span. These results show a decreasing trend of Poisson's ratio with age, from a value of 0.46 at the youngest site closest to the ridge axis.

  7. Carbon fixation in oceanic crust: Does it happen, and is it important?

    NASA Astrophysics Data System (ADS)

    Orcutt, B.; Sylvan, J. B.; Rogers, D.; Lee, R.; Girguis, P. R.; Carr, S. A.; Jungbluth, S.; Rappe, M. S.

    2014-12-01

    The carbon sources supporting a deep biosphere in igneous oceanic crust, and furthermore the balance of heterotrophy and autotrophy, are poorly understood. When the large reservoir size of oceanic crust is considered, carbon transformations in this environment have the potential to significantly impact the global carbon cycle. Furthermore, igneous oceanic crust is the most massive potential habitat for life on Earth, so understanding the carbon sources for this potential biosphere are important for understanding life on Earth. Geochemical evidence suggests that warm and anoxic upper basement is net heterotrophic, but the balance of these processes in cooler and potentially oxic oceanic crust are poorly known. Here, we present data from stable carbon isotope tracer incubations to examine carbon fixation in basalts collected from the Loihi Seamount, the Juan de Fuca Ridge, and the western flank of the Mid-Atlantic Ridge, to provide a first order constraint on the rates of carbon fixation on basalts. These data will be compared to recently available assessments of carbon cycling rates in fluids from upper basement to synthesize our current state of understanding of the potential for carbon fixation and respiration in oceanic crust. Moreover, we will present new genomic data of carbon fixation genes observed in the basalt enrichments as well as from the subsurface of the Juan de Fuca Ridge flank, enabling identification of the microbes and metabolic pathways involved in carbon fixation in these systems.

  8. Trace element chemistry of zircons from oceanic crust: A method for distinguishing detrital zircon provenance

    NASA Astrophysics Data System (ADS)

    Grimes, C. B.; John, B. E.; Kelemen, P. B.; Mazdab, F. K.; Wooden, J. L.; Cheadle, M. J.; Hanghøj, K.; Schwartz, J. J.

    2007-07-01

    We present newly acquired trace element compositions for more than 300 zircon grains in 36 gabbros formed at the slow-spreading Mid-Atlantic and Southwest Indian Ridges. Rare earth element patterns for zircon from modern oceanic crust completely overlap with those for zircon crystallized in continental granitoids. However, plots of U versus Yb and U/Yb versus Hf or Y discriminate zircons crystallized in oceanic crust from continental zircon, and provide a relatively robust method for distinguishing zircons from these environments. Approximately 80% of the modern ocean crust zircons are distinct from the field defined by more than 1700 continental zircons from Archean and Phanerozoic samples. These discrimination diagrams provide a new tool for fingerprinting ocean crust zircons derived from reservoirs like that of modern mid-ocean ridge basalt (MORB) in both modern and ancient detrital zircon populations. Hadean detrital zircons previously reported from the Acasta Gneiss, Canada, and the Narryer Gneiss terrane, Western Australia, plot in the continental granitoid field, supporting hypotheses that at least some Hadean detrital zircons crystallized in continental crust forming magmas and not from a reservoir like modern MORB.

  9. Emergence of blueschists on Earth linked to secular changes in oceanic crust composition

    NASA Astrophysics Data System (ADS)

    Palin, Richard M.; White, Richard W.

    2016-01-01

    The oldest blueschists--metamorphic rocks formed during subduction--are of Neoproterozoic age, and 0.7-0.8 billion years old. Yet, subduction of oceanic crust to mantle depths is thought to have occurred since the Hadean, over 4 billion years ago. Blueschists typically form under cold geothermal gradients of less than 400 °C GPa-1, so their absence in the ancient rock record is typically attributed to hotter pre-Neoproterozoic mantle prohibiting such low-temperature metamorphism; however, modern analogues of Archaean subduction suggest that blueschist-facies metamorphic conditions are attainable at the slab surface. Here we show that the absence of blueschists in the ancient geological record can be attributed to the changing composition of oceanic crust throughout Earth history, which is a consequence of secular cooling of the mantle since the Archaean. Oceanic crust formed on the hot, early Earth would have been rich in magnesium oxide (MgO). We use phase equilibria calculations to show that blueschists do not form in high-MgO rocks under subduction-related geothermal gradients. Instead, the subduction of MgO-rich oceanic crust would have created greenschist-like rocks--metamorphic rocks formed today at low temperatures and pressures. These ancient metamorphic products can hold about 20% more water than younger metamorphosed oceanic crust, implying that the global hydrologic cycle was more efficient in the deep geological past than today.

  10. Trace element chemistry of zircons from oceanic crust: A method for distinguishing detrital zircon provenance

    USGS Publications Warehouse

    Grimes, Craig B.; John, Barbara E.; Kelemen, P.B.; Mazdab, F.K.; Wooden, J.L.; Cheadle, Michael J.; Hanghoj, K.; Schwartz, J.J.

    2007-01-01

    We present newly acquired trace element compositions for more than 300 zircon grains in 36 gabbros formed at the slow-spreading Mid-Atlantic and Southwest Indian Ridges. Rare earth element patterns for zircon from modern oceanic crust completely overlap with those for zircon crystallized in continental granitoids. However, plots of U versus Yb and U/Yb versus Hf or Y discriminate zircons crystallized in oceanic crust from continental zircon, and provide a relatively robust method for distinguishing zircons from these environments. Approximately 80% of the modern ocean crust zircons are distinct from the field defined by more than 1700 continental zircons from Archean and Phanerozoic samples. These discrimination diagrams provide a new tool for fingerprinting ocean crust zircons derived from reservoirs like that of modern mid-ocean ridge basalt (MORB) in both modern and ancient detrital zircon populations. Hadean detrital zircons previously reported from the Acasta Gneiss, Canada, and the Narryer Gneiss terrane, Western Australia, plot in the continental granitoid field, supporting hypotheses that at least some Hadean detrital zircons crystallized in continental crust forming magmas and not from a reservoir like modern MORB. ?? 2007 The Geological Society of America.

  11. Hydrothermal cooling of the ocean crust: Insights from ODP Hole 1256D

    NASA Astrophysics Data System (ADS)

    Harris, Michelle; Coggon, Rosalind M.; Wood, Martin; Smith-Duque, Christopher E.; Henstock, Timothy J.; Teagle, Damon A. H.

    2017-03-01

    The formation of new ocean crust at mid-ocean ridges is a fundamental component of the plate tectonic cycle and involves substantial transfer of heat and mass from the mantle. Hydrothermal circulation at mid-ocean ridges is critical for the advection of latent and sensible heat from the lower crust to enable the solidification of ocean crust near to the ridge axis. The sheeted dike complex (SDC) is the critical region between the eruptive lavas and the gabbros through which seawater-derived recharge fluids must transit to exchange heat with the magma chambers that form the lower ocean crust. ODP Hole 1256D in the eastern equatorial Pacific Ocean provides the only continuous sampling of in-situ intact upper ocean crust formed at a fast spreading rate, through the SDC into the dike-gabbro transition zone. Here we exploit a high sample density profile of the Sr-isotopic composition of Hole 1256D to quantify the time-integrated hydrothermal recharge fluid flux through the SDC. Assuming kinetically limited fluid-rock Sr exchange, a fluid flux of 1.5- 3.2 ×106 kgm-2 is required to produce the observed Sr-isotopic shifts. Despite significant differences in the distribution and intensity of hydrothermal alteration and fluid/rock Sr-isotopic exchange between Hole 1256D and SDC sampled in other oceanic environments (ODP Hole 504B, Hess Deep and Pito Deep), the estimated recharge fluid fluxes at all sites are similar, suggesting that the heat flux extracted by the upper crustal axial hydrothermal system is relatively uniform at intermediate to fast spreading rates. The hydrothermal heat flux removed by fluid flow through the SDCs, is sufficient to remove only ∼20 to 60% of the available latent and sensible heat from the lower crust. Consequently, there must be additional thermal and chemical fluid-rock exchange deeper in the crust, at least of comparable size to the upper crustal hydrothermal system. Two scenarios are proposed for the potential geometry of this deeper

  12. Controls on ferromanganese crust composition and reconnaissance resource potential, Ninetyeast Ridge, Indian Ocean

    NASA Astrophysics Data System (ADS)

    Hein, James R.; Conrad, Tracey; Mizell, Kira; Banakar, Virupaxa K.; Frey, Frederick A.; Sager, William W.

    2016-04-01

    A reconnaissance survey of Fe-Mn crusts from the 5000 km long (~31°S to 10°N) Ninetyeast Ridge (NER) in the Indian Ocean shows their widespread occurrence along the ridge as well as with water depth on the ridge flanks. The crusts are hydrogenetic based in growth rates and discrimination plots. Twenty samples from 12 crusts from 9 locations along the ridge were analyzed for chemical and mineralogical compositions, growth rates, and statistical relationships (Q-mode factor analysis, correlation coefficients) were calculated. The crusts collected are relatively thin (maximum 40 mm), and those analyzed varied from 4 mm to 32 mm. However, crusts as thick as 80 mm can be expected to occur based on the age of rocks that comprise the NER and the growth rates calculated here. Growth rates of the crusts increase to the north along the NER and with water depth. The increase to the north resulted from an increased supply of Mn from the oxygen minimum zone (OMZ) to depths below the OMZ combined with an increased supply of Fe at depth from the dissolution of biogenic carbonate and from deep-sourced hydrothermal Fe. These increased supplies of Fe increased growth rates of the deeper-water crusts along the entire NER. Because of the huge terrigenous (rivers, eolian, pyroclastic) and hydrothermal (three spreading centers) inputs to the Indian Ocean, and the history of primary productivity, Fe-Mn crust compositions vary from those analyzed from open-ocean locations in the Pacific. The sources of detrital material in the crusts change along the NER and reflect, from north to south, the decreasing influence of the Ganga River system and volcanic arcs located to the east, with increasing influence of sediment derived from Australia to the south. In addition, weathering of NER basalt likely contributed to the aluminosilicate fraction of the crusts. The southernmost sample has a relatively large detrital component compared to other southern NER crust samples, which was probably

  13. Millennial-scale ocean acidification and late Quaternary decline of cryptic bacterial crusts in tropical reefs.

    PubMed

    Riding, R; Liang, L; Braga, J C

    2014-09-01

    Ocean acidification by atmospheric carbon dioxide has increased almost continuously since the last glacial maximum (LGM), 21,000 years ago. It is expected to impair tropical reef development, but effects on reefs at the present day and in the recent past have proved difficult to evaluate. We present evidence that acidification has already significantly reduced the formation of calcified bacterial crusts in tropical reefs. Unlike major reef builders such as coralline algae and corals that more closely control their calcification, bacterial calcification is very sensitive to ambient changes in carbonate chemistry. Bacterial crusts in reef cavities have declined in thickness over the past 14,000 years with largest reduction occurring 12,000-10,000 years ago. We interpret this as an early effect of deglacial ocean acidification on reef calcification and infer that similar crusts were likely to have been thicker when seawater carbonate saturation was increased during earlier glacial intervals, and thinner during interglacials. These changes in crust thickness could have substantially affected reef development over glacial cycles, as rigid crusts significantly strengthen framework and their reduction would have increased the susceptibility of reefs to biological and physical erosion. Bacterial crust decline reveals previously unrecognized millennial-scale acidification effects on tropical reefs. This directs attention to the role of crusts in reef formation and the ability of bioinduced calcification to reflect changes in seawater chemistry. It also provides a long-term context for assessing anticipated anthropogenic effects. © 2014 John Wiley & Sons Ltd.

  14. Silicon isotopes reveal recycled altered oceanic crust in the mantle sources of Ocean Island Basalts

    NASA Astrophysics Data System (ADS)

    Pringle, Emily A.; Moynier, Frédéric; Savage, Paul S.; Jackson, Matthew G.; Moreira, Manuel; Day, James M. D.

    2016-09-01

    The study of silicon (Si) isotopes in Ocean Island Basalts (OIB) has the potential to discern between different models for the origins of geochemical heterogeneities in the mantle. Relatively large (∼several per mil per atomic mass unit) Si isotope fractionation occurs in low-temperature environments during biochemical and geochemical precipitation of dissolved Si, where the precipitate is preferentially enriched in the lighter isotopes relative to the dissolved Si. In contrast, only a limited range (∼tenths of a per mil) of Si isotope fractionation has been observed from high-temperature igneous processes. Therefore, Si isotopes may be useful as tracers for the presence of crustal material within OIB mantle source regions that experienced relatively low-temperature surface processes in a manner similar to other stable isotope systems, such as oxygen. Characterizing the isotopic composition of the mantle is also of central importance to the use of the Si isotope system as a basis for comparisons with other planetary bodies (e.g., Moon, Mars, asteroids). Here we present the first comprehensive suite of high-precision Si isotope data obtained by MC-ICP-MS for a diverse suite of OIB. Samples originate from ocean islands in the Pacific, Atlantic, and Indian Ocean basins and include representative end-members for the EM-1, EM-2, and HIMU mantle components. On average, δ30Si values for OIB (-0.32 ± 0.09‰, 2 sd) are in general agreement with previous estimates for the δ30Si value of Bulk Silicate Earth (-0.29 ± 0.07‰, 2 sd; Savage et al., 2014). Nonetheless, some small systematic variations are present; specifically, most HIMU-type (Mangaia; Cape Verde; La Palma, Canary Islands) and Iceland OIB are enriched in the lighter isotopes of Si (δ30Si values lower than MORB), consistent with recycled altered oceanic crust and lithospheric mantle in their mantle sources.

  15. Contraction or expansion of the Moon's crust during magma ocean freezing?

    PubMed Central

    Elkins-Tanton, Linda T.; Bercovici, David

    2014-01-01

    The lack of contraction features on the Moon has been used to argue that the Moon underwent limited secular cooling, and thus had a relatively cool initial state. A cool early state in turn limits the depth of the lunar magma ocean. Recent GRAIL gravity measurements, however, suggest that dikes were emplaced in the lower crust, requiring global lunar expansion. Starting from the magma ocean state, we show that solidification of the lunar magma ocean would most likely result in expansion of the young lunar crust, and that viscous relaxation of the crust would prevent early tectonic features of contraction or expansion from being recorded permanently. The most likely process for creating the expansion recorded by the dikes is melting during cumulate overturn of the newly solidified lunar mantle. PMID:25114310

  16. RESEARCH NOTE: On the roughness of Mesozoic oceanic crust in the western North Atlantic

    NASA Astrophysics Data System (ADS)

    Minshull, T. A.

    1999-01-01

    Seismic reflection profiles from Mesozoic oceanic crust around the Blake Spur Fracture Zone (BSFZ) in the western North Atlantic have been widely used in constraining tectonic models of slow-spreading mid-ocean ridges. These profiles have anomalously low basement relief compared to crust formed more recently at the Mid-Atlantic Ridge at the same spreading rate. Profiles from other regions of Mesozoic oceanic crust also have greater relief. The anomalous basement relief and slightly increased crustal thickness in the BSFZ survey area may be due to the presence of a mantle thermal anomaly close to the ridge axis at the time of crustal formation. If so, the intracrustal structures observed may be representative of an atypical tectonic regime.

  17. Contraction or expansion of the Moon's crust during magma ocean freezing?

    PubMed

    Elkins-Tanton, Linda T; Bercovici, David

    2014-09-13

    The lack of contraction features on the Moon has been used to argue that the Moon underwent limited secular cooling, and thus had a relatively cool initial state. A cool early state in turn limits the depth of the lunar magma ocean. Recent GRAIL gravity measurements, however, suggest that dikes were emplaced in the lower crust, requiring global lunar expansion. Starting from the magma ocean state, we show that solidification of the lunar magma ocean would most likely result in expansion of the young lunar crust, and that viscous relaxation of the crust would prevent early tectonic features of contraction or expansion from being recorded permanently. The most likely process for creating the expansion recorded by the dikes is melting during cumulate overturn of the newly solidified lunar mantle. © 2014 The Author(s) Published by the Royal Society. All rights reserved.

  18. A geological model for the structure of ridge segments in slow spreading ocean crust

    NASA Astrophysics Data System (ADS)

    Tucholke, Brian E.; Lin, Jian

    1994-06-01

    First-order (transform) and second-order ridge-axis discontinuities create a fundamental segmentation of the lithosphere along mid-ocean ridges, and in slow spreading crust they commonly are associated with exposure of subvolcanic crust and upper mantle. We analyzed available morphological, gravity, and rock sample data from the Atlantic Ocean to determine whether consistent structural patterns occur at these discontinuities and to constrain the processes that control the patterns. The results show that along their older, inside-corner sides, both first-and second-order discontinuities are characterized by thinned crust and/or mantle exposures as well as by irregular fault patterns and a paucity of volcanic features. Crust on young, outside-corner sides of discontinuities has more normal thickness, regular fault patterns, and common volcanic forms. These patterns are consistent with tectonic thinning of crust at inside corners by low-angle detachment faults as previously suggested for transform discontinuities by Dick et al. [1981] and Karson [1990]. Volcanic upper crust accretes in the hanging wall of the detachment, is stripped from the inside-corner footwall, and is carried to the outside comer. Gravity and morphological data suggest that detachment faulting is a relatively continuous, long-lived process in crust spreading at <25-30 mm/yr, that it rnay be intermittent at intermediate rates of 25-40 mm/yr, and that it is unlikely to occur at faster rates. Detachment surfaces are dissected by later, high-angle faults formed during crustal uplift into the rift mountains; these faults can cut through the entire crust and may be the kinds of faults imaged by seismic reflection profiling over Cretaceous North Atlantic crust. Off-axis variations in gravity anomalies indicate that slow spreading crust experiences cyclic magmatic/amagmatic extension and that a typical cycle is about 2 m.y. long. During magmatic phases the footwall of the detachment fault probably exposes

  19. Misho mafic complex - A part of paleotethyan oceanic crust or a magmatism in continental rift?

    NASA Astrophysics Data System (ADS)

    Azimzadeh, Zohreh; Jahangiri, Ahmad; Saccani, Emilio; Dilek, Yildirim

    2013-04-01

    Misho Mafic Complex (NW Iran) represents a significant component of the West Cimmerian domain in Paleo-Tethys. The Misho Mafic Complex (MMC) consists of gabbro (mainly) and norıte,olivine gabbro, anorthosite and diorite with the east- west sereight. MMC has ıntrussıved ın Kahar sedımrtery Infta- Cambrıan rocks, crosscut by abundant basaltic dykes and the overlying basaltic sheeted dyke complex. Kahar sedimentary rocks are representing the northern margin of Gondwana. Misho mafic complex are covered by Permian sedimentary rocks. The gabbros and basaltic dykes have MORB affinities. MMC formed as a product of interactions between a depleted MORB-type asthenosphere and plume-type material. Mafic rocks represent an early Carboniferous magmatic event developed during the continental break-up of the northern edge of Gondwanaland that led to the opening of Paleotethys. Alternatively, these magmas may have been emplaced into the continental crust at the continental margin soon after the oceanic crust was formed (that is the oceanic crust was still narrow). There is no data for discriminating between these two hypotheses. In first hypothesis MMC is a part of ophiolites related to paleotethyan oceanic crust and the rocks that were above this crustal level should have necessarily been eroded. In another hypothesis Misho complex represents an aborted rift in a triple junction. Above a mantle plume, the continental crust breaks along three directions at 120 degrees. But, soon after, the extension proceeds along two of these three direction. Between them is formed the oceanic crust. The continental extension along the third direction is aborted. Here no oceanic crust if formed and there is only rifted, thinned continental crust. But, also in the aborted branch MORB magmatism can occur for short time. In this hypothesis, the Misho complex was never associated with oceanic crust, but was anyway associated with the opening of the Paleotethys. This magmatism was originally

  20. Tectonic slicing of subducted oceanic crust along plate interfaces: numerical modeling

    NASA Astrophysics Data System (ADS)

    Ruh, Jonas B.; Burov, Evgenii; Gerya, Taras; Agard, Philippe; Le Pourhiet, Laetitia

    2015-04-01

    Remains of high-pressure low-temperature metamorphic oceanic crust are observed within orogenic belts and along recent subduction zones all around the globe. Such blueshist and eclogite "slivers" can strongly vary in extend and experienced P-T-t evolution. To reveal the surficial occurrence of oceanic rocks that occasionally have been at depths down to ~ 80 km within subduction zones, two main processes have to be investigated individually: (i) Detachment of oceanic slivers from the down going plate preventing rocks to be consumed by the mantle, and (ii) exhumation of detached oceanic material to the surface. It has often been stated that the process of slicing and exhumation of oceanic blueshists and eclogites is closely related to continental subduction. Nevertheless, several examples worldwide show exhumation (and therefore latest possible detachment from slab) occurring early or intermediate of the subduction process. We present new insight into the mechanical processes of detachment of oceanic slivers and their exhumation independent of continental collision by using numerical modelling tools. Large-scale thermo-mechanical models (600 * 200 km) based on finite difference, marker-in-cell technique, are applied to test how serpentinised upper slab mantle (mantle serpentinisation at oceanic ridges or/and along to outer-rise normal faults) influences shallow and deep crustal detachment. Preliminary results show that a through-going serpentinite layer below the oceanic crust, if serpentinisation exceeds 50%, triggers slicing at wedge depths. Even if mechanical coupling mainly occurs at intermediate depths along the subduction interface, stresses within the oceanic crust are dispersed upward due to the weak, decoupling serpentinite layer. Unconnected patches of serpentinised upper slab mantle eventually lead to deep slicing, depending on upper plate serpentinisation due to slab dewatering. Furthermore, we investigated the effect of pressure-dependent fluid migration

  1. Magnetic Properties of the Lower Oceanic Crust from Hess Deep (IODP Expedition 345)

    NASA Astrophysics Data System (ADS)

    Friedman, S. A.; Morris, A.; Horst, A. J.

    2014-12-01

    Hess Deep is located at a tectonic window created by rifting of the Cocos-Nazca into East Pacific Rise crust. IODP Expedition 345 drilled into this window to recover lower oceanic crust primitive plutonic rock lithologies such as gabbros, troctolites, and olivine gabbronorites. Differences in ferromagnetic carriers impact the ferromagnetic properties and the contribution of oceanic crust to magnetic anomalies. 57 samples have been magnetically examined using low-temperature magnetic remanence, hysteresis properties, and low field susceptibility. Results show varying properties based on lithology and alteration processes. Secondary processes such as metamorphism, hydrothermal alteration and cataclasis can alter the magnetic history and properties of these once primitive rocks. The least altered samples consistently show pure stiochemetric magnetic magnetite as the main ferromagnetic carrier. Whereas, altered samples may contain multiple ferromagnetic carriers, such as pyrrhotite and magnetite.

  2. Tectonic slicing of subducting oceanic crust along plate interfaces: Numerical modeling

    NASA Astrophysics Data System (ADS)

    Ruh, J. B.; Le Pourhiet, L.; Agard, Ph.; Burov, E.; Gerya, T.

    2015-10-01

    Multikilometer-sized slivers of high-pressure low-temperature metamorphic oceanic crust and mantle are observed in many mountain belts. These blueschist and eclogite units were detached from the descending plate during subduction. Large-scale thermo-mechanical numerical models based on finite difference marker-in-cell staggered grid technique are implemented to investigate slicing processes that lead to the detachment of oceanic slivers and their exhumation before the onset of the continental collision phase. In particular, we investigate the role of the serpentinized subcrustal slab mantle in the mechanisms of shallow and deep crustal slicing. Results show that spatially homogeneous serpentinization of the sub-Moho slab mantle leads to complete accretion of oceanic crust within the accretionary wedge. Spatially discontinuous serpentinization of the slab mantle in form of unconnected patches can lead to shallow slicing of the oceanic crust below the accretionary wedge and to its deep slicing at mantle depths depending on the patch length, slab angle, convergence velocity and continental geothermal gradient. P-T paths obtained in this study are compared to natural examples of shallow slicing of the Crescent Terrane below Vancouver Island and deeply sliced crust of the Lago Superiore and Saas-Zermatt units in the Western Alps.

  3. Europa's Crust and Ocean: Origin, Composition, and the Prospects for Life

    USGS Publications Warehouse

    Kargel, J.S.; Kaye, J.Z.; Head, J. W.; Marion, G.M.; Sassen, R.; Crowley, J.K.; Ballesteros, O.P.; Grant, S.A.; Hogenboom, D.L.

    2000-01-01

    We have considered a wide array of scenarios for Europa's chemical evolution in an attempt to explain the presence of ice and hydrated materials on its surface and to understand the physical and chemical nature of any ocean that may lie below. We postulate that, following formation of the jovian system, the europan evolutionary sequence has as its major links: (a) initial carbonaceous chondrite rock, (b) global primordial aqueous differentiation and formation of an impure primordial hydrous crust, (c) brine evolution and intracrustal differentiation, (d) degassing of Europa's mantle and gas venting, (e) hydrothermal processes, and (f) chemical surface alteration. Our models were developed in the context of constraints provided by Galileo imaging, near infrared reflectance spectroscopy, and gravity and magnetometer data. Low-temperature aqueous differentiation from a carbonaceous CI or CM chondrite precursor, without further chemical processing, would result in a crust/ocean enriched in magnesium sulfate and sodium sulfate, consistent with Galileo spectroscopy. Within the bounds of this simple model, a wide range of possible layered structures may result; the final state depends on the details of intracrustal differentiation. Devolatilization of the rocky mantle and hydrothermal brine reactions could have produced very different ocean/crust compositions, e.g., an ocean/crust of sodium carbonate or sulfuric acid, or a crust containing abundant clathrate hydrates. Realistic chemical-physical evolution scenarios differ greatly in detailed predictions, but they generally call for a highly impure and chemically layered crust. Some of these models could lead also to lateral chemical heterogeneities by diapiric upwellings and/or cryovolcanism. We describe some plausible geological consequences of the physical-chemical structures predicted from these scenarios. These predicted consequences and observed aspects of Europa's geology may serve as a basis for further analys is

  4. Europa's Crust and Ocean: Origin, Composition, and the Prospects for Life

    NASA Astrophysics Data System (ADS)

    Kargel, Jeffrey S.; Kaye, Jonathan Z.; Head, James W.; Marion, Giles M.; Sassen, Roger; Crowley, James K.; Ballesteros, Olga Prieto; Grant, Steven A.; Hogenboom, David L.

    2000-11-01

    We have considered a wide array of scenarios for Europa's chemical evolution in an attempt to explain the presence of ice and hydrated materials on its surface and to understand the physical and chemical nature of any ocean that may lie below. We postulate that, following formation of the jovian system, the europan evolutionary sequence has as its major links: (a) initial carbonaceous chondrite rock, (b) global primordial aqueous differentiation and formation of an impure primordial hydrous crust, (c) brine evolution and intracrustal differentiation, (d) degassing of Europa's mantle and gas venting, (e) hydrothermal processes, and (f) chemical surface alteration. Our models were developed in the context of constraints provided by Galileo imaging, near infrared reflectance spectroscopy, and gravity and magnetometer data. Low-temperature aqueous differentiation from a carbonaceous CI or CM chondrite precursor, without further chemical processing, would result in a crust/ocean enriched in magnesium sulfate and sodium sulfate, consistent with Galileo spectroscopy. Within the bounds of this simple model, a wide range of possible layered structures may result; the final state depends on the details of intracrustal differentiation. Devolatilization of the rocky mantle and hydrothermal brine reactions could have produced very different ocean/crust compositions, e.g., an ocean/crust of sodium carbonate or sulfuric acid, or a crust containing abundant clathrate hydrates. Realistic chemical-physical evolution scenarios differ greatly in detailed predictions, but they generally call for a highly impure and chemically layered crust. Some of these models could lead also to lateral chemical heterogeneities by diapiric upwellings and/or cryovolcanism. We describe some plausible geological consequences of the physical-chemical structures predicted from these scenarios. These predicted consequences and observed aspects of Europa's geology may serve as a basis for further analys is

  5. Mid-ocean ridges produced thicker crust in the Jurassic than in Recent times

    NASA Astrophysics Data System (ADS)

    Van Avendonk, H. J.; Harding, J.; Davis, J. K.; Lawver, L. A.

    2016-12-01

    We present a compilation of published marine seismic refraction data to show that oceanic crust was 1.7 km thicker on average in the mid-Jurassic (170 Ma) than along the present-day mid-ocean ridge system. Plate reconstructions in a fixed hotspot framework show that the thickness of oceanic crust does not correlate with proximity to mantle hotspots, so it is likely that mid-plate volcanism is not the cause of this global trend. We propose that more melt was extracted from the upper mantle beneath mid-ocean ridges in the Jurassic than in recent times. Numerical studies show that temperature increase of 1 degree C in the mantle can lead to approximately 50-70 m thicker crust, so the upper mantle may have cooled 15-20 degrees C/100 Myr since 170 Ma. This average temperature decrease is larger than the secular cooling rate of the Earth's mantle, which is roughly 10 degrees C/100 Myr since the Archean. Apparently, the present-day configuration and dynamics of continental and oceanic plates removes heat more efficiently from the Earth's mantle than in its earlier history. The increase of ocean crustal thickness with plate age is also stronger in the Indian and Atlantic oceans than in the Pacific Ocean basin. This confirms that thermal insulation by the supercontinent Pangaea raised the temperature of the underlying asthenospheric mantle, which in turn led to more magmatic output at the Jurassic mid-ocean ridges of the Indian and Atlantic oceans.

  6. Mission Moho: Rationale for drilling deep through the ocean crust into the upper mantle

    NASA Astrophysics Data System (ADS)

    Ildefonse, B.; Abe, N.; Kelemen, P. B.; Kumagai, H.; Teagle, D. A. H.; Wilson, D. S.; Moho Proponents, Mission

    2009-04-01

    Sampling a complete section of the ocean crust to the Moho was the original inspiration for scientific ocean drilling, and remains the main goal of the 21st Century Mohole Initiative in the IODP Science Plan. Fundamental questions about the composition, structure, and geophysical characteristics of the ocean lithosphere, and about the magnitude of chemical exchanges between the mantle, crust and oceans remain unresolved due to the absence of in-situ samples and measurements. The geological nature of the Mohorovičić discontinuity itself remains poorly constrained. "Mission Moho" is a proposal that was submitted to IODP in April 2007, with the ambition to drill completely through intact oceanic crust formed at a fast spreading rate, across the Moho and into the uppermost mantle. Although, eventually, no long-term mission was approved by IODP, the scientific objectives related to deep drilling in the ocean crust remain essential to our understanding of the Earth. These objectives are to : - Determine the geological meaning of the Moho in different oceanic settings, determine the in situ composition, structure and physical properties of the uppermost mantle, and understand mantle melt migration, - Determine the bulk composition of the oceanic crust to establish the chemical links between erupted lavas and primary mantle melts, understand the extent and intensity of seawater hydrothermal exchange with the lithosphere, and estimate the chemical fluxes returned to the mantle by subduction, - Test competing hypotheses of the ocean crust accretion at fast spreading mid-ocean ridges, and quantify the linkages and feedbacks between magma intrusion, hydrothermal circulation and tectonic activity, - Calibrate regional seismic measurements against recovered cores and borehole measurements, and understand the origin of marine magnetic anomalies, - Establish the limits of life in the ocean lithosphere. The "MoHole" was planned as the final stage of Mission Moho, which requires

  7. Lithospheric transition from the Variscan Iberian Massif to the Jurassic oceanic crust of the Central Atlantic

    NASA Astrophysics Data System (ADS)

    Fernàndez, M.; Marzán, I.; Torne, M.

    2004-08-01

    A 1000-km-long lithospheric transect running from the Variscan Iberian Massif (VIM) to the oceanic domain of the Northwest African margin is investigated. The main goal of the study is to image the lateral changes in crustal and lithospheric structure from a complete section of an old and stable orogenic belt—the Variscan Iberian Massif—to the adjacent Jurassic passive margin of SW Iberia, and across the transpressive and seismically active Africa-Eurasia plate boundary. The modelling approach incorporates available seismic data and integrates elevation, gravity, geoid and heat flow data under the assumptions of thermal steady state and local isostasy. The results show that the Variscan Iberian crust has a roughly constant thickness of ˜30 km, in opposition to previous works that propose a prominent thickening beneath the South Portuguese Zone (SPZ). The three layers forming the Variscan crust show noticeable thickness variations along the profile. The upper crust thins from central Iberia (about 20 km thick) to the Ossa Morena Zone (OMZ) and the NE region of the South Portuguese Zone where locally the thickness of the upper crust is <8 km. Conversely, there is a clear thickening of the middle crust (up to 17 km thick) under the Ossa Morena Zone, whereas the thickness of the lower crust remains quite constant (˜6 km). Under the margin, the thinning of the continental crust is quite gentle and occurs over distances of ˜200 km, resembling the crustal attitude observed further north along the West Iberian margins. In the oceanic domain, there is a 160-km-wide Ocean Transition Zone located between the thinned continental crust of the continental shelf and slope and the true oceanic crust of the Seine Abyssal Plain. The total lithospheric thickness varies from about 120 km at the ends of the model profile to less than 100 km below the Ossa Morena and the South Portuguese zones. An outstanding result is the mass deficit at deep lithospheric mantle levels required

  8. Glacial cycles drive variations in the production of oceanic crust.

    PubMed

    Crowley, John W; Katz, Richard F; Huybers, Peter; Langmuir, Charles H; Park, Sung-Hyun

    2015-03-13

    Glacial cycles redistribute water between oceans and continents, causing pressure changes in the upper mantle, with consequences for the melting of Earth's interior. Using Plio-Pleistocene sea-level variations as a forcing function, theoretical models of mid-ocean ridge dynamics that include melt transport predict temporal variations in crustal thickness of hundreds of meters. New bathymetry from the Australian-Antarctic ridge shows statistically significant spectral energy near the Milankovitch periods of 23, 41, and 100 thousand years, which is consistent with model predictions. These results suggest that abyssal hills, one of the most common bathymetric features on Earth, record the magmatic response to changes in sea level. The models and data support a link between glacial cycles at the surface and mantle melting at depth, recorded in the bathymetric fabric of the sea floor.

  9. Microbial community on oceanic ferro-manganese crusts from Takuyo-Daigo Seamount and Ryusei Seamount

    NASA Astrophysics Data System (ADS)

    Nitahara, S.; Kato, S.; Yamagishi, A.

    2012-12-01

    and Discussion We estimated the numbers of bacterial and archaeal cell on Mn crusts from Takuyo-Daigo seamount by QPCR. Bacterial cell number on Mn crust was estimated to be approximately 10^7 cells/g. Those of archaea were estimated to be between 10^6 and 10^7 cells/g. Archaea dominated in three of four Mn crust samples (50~83 % of total cell numbers). Microbial community of Mn crusts was different from those of sediment and seawater. This suggests that unique microbial community present on Mn crusts. Many phylotypes related to uncultured group were detected. Phylotypes closely related to Marine Group I (MGI) were detected from six Mn crust samples, collected from Takuyo-Daigo and Ryusei seamounts. MGI includes Ammonia-Oxidizing Archaea (AOA) and is ubiquitously distributed in ocean (Karner et al., 2001). Phylotypes closely related to Nitrosospira, ammonia-oxidizing bacteria (AOB), were detected from four Mn crusts collected from Takuyo-Daigo seamount. Presence of these ammonia oxidizers was supported by detection of bacterial and archaeal amoA genes. The copy numbers of bacterial and archaeal amoA genes were estimated to be approximately 10^5 -10^6 copy/g by QPCR. These facts suggest that ammonia oxidizers are present abundantly on Mn crusts. MGI and Nitrosospira include autotrophic ammonia oxidizers. These groups may play a role as primary producers in Mn crust ecosystems.

  10. Recycled oceanic crust observed in 'ghost plagioclase' within the source of Mauna Loa lavas

    PubMed

    Sobolev; Hofmann; Nikogosian

    2000-04-27

    The hypothesis that mantle plumes contain recycled oceanic crust is now widely accepted. Some specific source components of the Hawaiian plume have been inferred to represent recycled oceanic basalts, pelagic sediments or oceanic gabbros. Bulk lava compositions, however, retain the specific trace-element fingerprint of the original crustal component in only a highly attenuated form. Here we report the discovery of exotic, strontium-enriched melt inclusions in Mauna Loa olivines. Their complete trace-element patterns strongly resemble those of layered gabbros found in ophiolites, which are characterized by cumulus plagioclase with very high strontium abundances. The major-element compositions of these melts indicate that their composition cannot be the result of the assimilation of present-day oceanic crust through which the melts have travelled. Instead, the gabbro has been transformed into a (high-pressure) eclogite by subduction and recycling, and this eclogite has then been incorporated into the Hawaiian mantle plume. The trace-element signature of the original plagioclase is present only as a 'ghost' signature, which permits specific identification of the recycled rock type. The 'ghost plagioclase' trace-element signature demonstrates that the former gabbro can retain much of its original chemical identity through the convective cycle without completely mixing with other portions of the former oceanic crust.

  11. Deep mantle cycling of oceanic crust: evidence from diamonds and their mineral inclusions.

    PubMed

    Walter, M J; Kohn, S C; Araujo, D; Bulanova, G P; Smith, C B; Gaillou, E; Wang, J; Steele, A; Shirey, S B

    2011-10-07

    A primary consequence of plate tectonics is that basaltic oceanic crust subducts with lithospheric slabs into the mantle. Seismological studies extend this process to the lower mantle, and geochemical observations indicate return of oceanic crust to the upper mantle in plumes. There has been no direct petrologic evidence, however, of the return of subducted oceanic crustal components from the lower mantle. We analyzed superdeep diamonds from Juina-5 kimberlite, Brazil, which host inclusions with compositions comprising the entire phase assemblage expected to crystallize from basalt under lower-mantle conditions. The inclusion mineralogies require exhumation from the lower to upper mantle. Because the diamond hosts have carbon isotope signatures consistent with surface-derived carbon, we conclude that the deep carbon cycle extends into the lower mantle.

  12. Deep Mantle Cycling of Oceanic Crust: Evidence from Diamonds and Their Mineral Inclusions

    NASA Astrophysics Data System (ADS)

    Walter, M. J.; Kohn, S. C.; Araujo, D.; Bulanova, G. P.; Smith, C. B.; Gaillou, E.; Wang, J.; Steele, A.; Shirey, S. B.

    2011-10-01

    A primary consequence of plate tectonics is that basaltic oceanic crust subducts with lithospheric slabs into the mantle. Seismological studies extend this process to the lower mantle, and geochemical observations indicate return of oceanic crust to the upper mantle in plumes. There has been no direct petrologic evidence, however, of the return of subducted oceanic crustal components from the lower mantle. We analyzed superdeep diamonds from Juina-5 kimberlite, Brazil, which host inclusions with compositions comprising the entire phase assemblage expected to crystallize from basalt under lower-mantle conditions. The inclusion mineralogies require exhumation from the lower to upper mantle. Because the diamond hosts have carbon isotope signatures consistent with surface-derived carbon, we conclude that the deep carbon cycle extends into the lower mantle.

  13. Microbial Inventory of Deeply Buried Oceanic Crust from a Young Ridge Flank

    PubMed Central

    Jørgensen, Steffen L.; Zhao, Rui

    2016-01-01

    The deep marine biosphere has over the past decades been exposed as an immense habitat for microorganisms with wide-reaching implications for our understanding of life on Earth. Recent advances in knowledge concerning this biosphere have been achieved mainly through extensive microbial and geochemical studies of deep marine sediments. However, the oceanic crust buried beneath the sediments, is still largely unexplored with respect to even the most fundamental questions related to microbial life. Here, we present quantitative and qualitative data related to the microbial inventory from 33 deeply buried basaltic rocks collected at two different locations, penetrating 300 vertical meters into the upper oceanic crust on the west flank of the Mid-Atlantic spreading ridge. We use quantitative PCR and sequencing of 16S rRNA gene amplicons to estimate cell abundances and to profile the community structure. Our data suggest that the number of cells is relatively stable at ~104 per gram of rock irrespectively of sampling site and depth. Further, we show that Proteobacteria, especially Gammaproteobacteria dominate the microbial assemblage across all investigated samples, with Archaea, in general, represented by < 1% of the community. In addition, we show that the communities within the crust are distinct from the overlying sediment. However, many of their respective microbial inhabitants are shared between the two biomes, but with markedly different relative distributions. Our study provides fundamental information with respect to abundance, distribution, and identity of microorganisms in the upper oceanic crust. PMID:27303398

  14. Structure and thickness of the Earth's crust in the northeastern part of the Indian Ocean

    NASA Astrophysics Data System (ADS)

    Schreider, A. A.; Mazo, E. L.; Kulikova, M. P.; Gilod, D. A.

    2008-10-01

    A digital database on the seismostratigraphy of the oceanic crust of the northeastern part of the Indian Ocean is compiled. In the first layer of the crust, the interval seismic wave velocities are 3.02 ± 0.16 km/s; in the second layer, they equal to 5.31 ± 0.27 km/s; and, in the third layer, the values are 6.46 ± 0.30 km/s. The bottom of the third seismic layer is represented by mantle rocks with an average velocity of 8.10 ± 0.16 km/s. Schemes of the distribution of the thicknesses of the second and third layers of the oceanic crust, of the total thickness of the crust, of the surface of the basement, and of the Mohorovicic discontinuity for the area considered are presented. The schemes compiled allow one to update and complement the ideas about the configuration of the major tectonic structures of the area.

  15. Microbial Inventory of Deeply Buried Oceanic Crust from a Young Ridge Flank.

    PubMed

    Jørgensen, Steffen L; Zhao, Rui

    2016-01-01

    The deep marine biosphere has over the past decades been exposed as an immense habitat for microorganisms with wide-reaching implications for our understanding of life on Earth. Recent advances in knowledge concerning this biosphere have been achieved mainly through extensive microbial and geochemical studies of deep marine sediments. However, the oceanic crust buried beneath the sediments, is still largely unexplored with respect to even the most fundamental questions related to microbial life. Here, we present quantitative and qualitative data related to the microbial inventory from 33 deeply buried basaltic rocks collected at two different locations, penetrating 300 vertical meters into the upper oceanic crust on the west flank of the Mid-Atlantic spreading ridge. We use quantitative PCR and sequencing of 16S rRNA gene amplicons to estimate cell abundances and to profile the community structure. Our data suggest that the number of cells is relatively stable at ~10(4) per gram of rock irrespectively of sampling site and depth. Further, we show that Proteobacteria, especially Gammaproteobacteria dominate the microbial assemblage across all investigated samples, with Archaea, in general, represented by < 1% of the community. In addition, we show that the communities within the crust are distinct from the overlying sediment. However, many of their respective microbial inhabitants are shared between the two biomes, but with markedly different relative distributions. Our study provides fundamental information with respect to abundance, distribution, and identity of microorganisms in the upper oceanic crust.

  16. the Deep Biosphere Archaeal Microbial Community in Igneous Ocean Crust

    NASA Astrophysics Data System (ADS)

    Edwards, K. J.

    2014-12-01

    Ridge flank hydrothermal systems represent vast environments that may be habitable by 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. These potential ecosystems may play a significant role in biogeochemical processes and elemental fluxes that are known to be regulated by these systems. I will discuss the nature of ridge flank hydrothermal 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 conditions that help to determine the nature of subseafloor biomes: crustal age, extent of fluid flow, and thermal state. A brief overview of subseafloor conditions, within the context of these three characteristics for select sites will be described. Technical challenges remain and likely will limit progress in studies of microbial ridge flank hydrothermal ecosystems, which is why it is vital to select and design future studies so as to leverage as much general understanding as possible from work focused at a small number of sites. A characterization framework that perhaps includes alternative or additional physical or chemical characteristics is essential for achieving the greatest benefit from multidisciplinary microbial investigations of oceanic ridge flank hydrothermal systems.

  17. Dating the growth of oceanic crust at a slow-spreading ridge.

    PubMed

    Schwartz, Joshua J; John, Barbara E; Cheadle, Michael J; Miranda, Elena A; Grimes, Craig B; Wooden, Joseph L; Dick, Henry J B

    2005-10-28

    Nineteen uranium-lead zircon ages of lower crustal gabbros from Atlantis Bank, Southwest Indian Ridge, constrain the growth and construction of oceanic crust at this slow-spreading midocean ridge. Approximately 75% of the gabbros accreted within error of the predicted seafloor magnetic age, whereas approximately 25% are significantly older. These anomalously old samples suggest either spatially varying stochastic intrusion at the ridge axis or, more likely, crystallization of older gabbros at depths of approximately 5 to 18 kilometers below the base of crust in the cold, axial lithosphere, which were uplifted and intruded by shallow-level magmas during the creation of Atlantis Bank.

  18. Dating the growth of oceanic crust at a slow-spreading ridge

    USGS Publications Warehouse

    Schwartz, J.J.; John, Barbara E.; Cheadle, Michael J.; Miranda, E.A.; Grimes, Craig B.; Wooden, J.L.; Dick, H.J.B.

    2005-01-01

    Nineteen uranium-lead zircon ages of lower crustal gabbros from Atlantis Bank, Southwest Indian Ridge, constrain the growth and construction of oceanic crust at this slow-spreading midocean ridge. Approximately 75% of the gabbros accreted within error of the predicted seafloor magnetic age, whereas ???25% are significantly older. These anomalously old samples suggest either spatially varying stochastic intrusion at the ridge axis or, more likely, crystallization of older gabbros at depths of ???5 to 18 kilometers below the base of crust in the cold, axial lithosphere, which were uplifted and intruded by shallow-level magmas during the creation of Atlantis Bank.

  19. Glacial cycles drive variations in the production of oceanic crust

    NASA Astrophysics Data System (ADS)

    Katz, R. F.; Crowley, J. W.; Langmuir, C. H.

    2012-12-01

    Glacial cycles redistribute water between the oceans and continents, causing long-wavelength changes of static pressure in the upper mantle. Previous work has shown that subaerial, glaciated volcanoes respond to these changes with variation in eruption rates (Jull & McKenzie 1996, Huybers & Langmuir 2009), and has suggested that the magmatic flux at mid-ocean ridges may vary with changes in sea-level (Huybers & Langmuir 2009, Lund & Asimow 2011). The latter is speculative, however, because previous theory has assumed highly simplified melt transport and neglected the dependence of the ridge thermal structure on spreading rate. Moreover, it remains a challenge to connect model predictions of variations arising from sea-level change with sea-floor observations. Here we present results from a theoretical model of a mid-ocean ridge based on conservation of mass, momentum, energy, and composition for two phases (magma & mantle) and two thermodynamic components (enriched & depleted) (Katz 2008, 2010). The model is driven by imposed variations in the static pressure within the mantle. We consider both the geochemically inferred record of past sea-level variation, as well as simpler harmonic and instantaneous variations. The output of these models is compared with observations of bathymetry at ridges that are undisturbed by off-axis volcanism. The comparison is preliminary but suggests that some abyssal hills on the sea-floor are, at least in part, the result of glacial cycles. To understand the simulation results in more detail, we develop analytical solutions for a reduced-complexity model. This model is derived according to the idea that the melting induced by sea-level changes can be thought of as a small perturbation to a steady-state system. We obtain a Green's function solution for crustal thickness as a function of sea-level change with the associated dependencies on geophysical parameters of the magma/mantle system. We show that this solution captures much of the

  20. Forearc oceanic crust in the Izu-Bonin arc - new insights from active-source seismic survey -

    NASA Astrophysics Data System (ADS)

    Kodaira, S.; Noguchi, N.; Takahashi, N.; Ishizuka, O.; Kaneda, Y.

    2009-12-01

    Petrological studies have suggested that oceanic crust is formed in forearc areas during the initial stage of subduction. However, there is little geophysical evidence for the formation of oceanic crust in those regions. In order to examine crustal formation process associated with a subduction initiation process, we conducted an active-source seismic survey at a forearc region in the Izu-Bonin intra-oceanic arc. The resultant seismic image shows a remarkably thin crust (less than 10 km) at the northern half of the Bonin ridge (at the north of the Chichi-jima) and abrupt thickening the crust (~ 20 km thick) toward the south (at the Haha-jima). Comparison of velocity-depth profiles of the thin forearc crust of the Bonin ridge with those of typical oceanic crusts showed them to be seismologically identical. The observed structural variation also well corresponds to magmatic activities along the forearc. Boninitic magmatism is evident in the area of thin crust and tholeiitic-calcalkaline andesitic volcanism in the area of thick crust. Based on high precision dating studies of those volcanic rocks, we interpreted that the oceanic-type thin crust associated with boninitic volcanism has been created soon after the initiation of subduction (45-48 Ma) and and that the nonoceanic thick crust was created by tholeiitic-calcalkaline andesitic magmatism after the boninitic magmatism was ceased. The above seismological evidences strongly support the idea of forearc oceanic crust (or phiolite) created by forearc spreading in the initial stage of subduction along the intra-oceanic arc.

  1. Dismembered Archaean ophiolite in the southeastern Wind River Mountains, Wyoming: Remains of Archaean oceanic crust

    NASA Technical Reports Server (NTRS)

    Harper, G. D.

    1986-01-01

    Archean mafic and ultramafic rocks occur in the southeastern Wind River Mountains near Atlantic City, Wyoming and are interpreted to represent a dismembered ophiolite suite. The ophiolitic rocks occur in a thin belt intruded by the 2.6 Ga Louis Lake Batholith on the northwest. On the southeast they are in fault contact with the Miners Delight Formation comprised primarily of metagraywackes with minor calc-alkaline volcanics. The ophiolitic and associated metasedimentry rocks (Goldman Meadows Formation) have been multiply deformed and metamorphosed. The most prominant structures are a pronounced steeply plunging stretching lineation and steeply dipping foliation. These structural data indicate that the ophiolitic and associated metasedimentary rocks have been deformed by simple shear. The ophiolitic rocks are interpreted as the remains of Archean oceanic crust, probably formed at either a mid-ocean ridge or back-arc basin. All the units of a complete ophiolite are present except for upper mantle periodotities. The absence of upper mantle rocks may be the result of detactment within the crust, rather than within the upper mantle, during emplacement. This could have been the result of a steeper geothermal gradient in the Archean oceanic lithosphere, or may have resulted from a thicker oceanic crust in the Archean.

  2. Thin and layered subcontinental crust of the great Basin western north America inherited from Paleozoic marginal ocean basins?

    USGS Publications Warehouse

    Churkin, M.; McKee, E.H.

    1974-01-01

    The seismic profile of the crust of the northern part of the Basin and Range province by its thinness and layering is intermediate between typical continental and oceanic crust and resembles that of marginal ocean basins, especially those with thick sedimentary fill. The geologic history of the Great Basin indicates that it was the site of a succession of marginal ocean basins opening and closing behind volcanic arcs during much of Paleozoic time. A long process of sedimentation and deformation followed throughout the Mesozoic modifying, but possibly not completely transforming the originally oceanic crust to continental crust. In the Cenozoic, after at least 40 m.y. of quiescence and stable conditions, substantial crustal and upper-mantle changes are recorded by elevation of the entire region in isostatic equilibrium, crustal extension resulting in Basin and Range faulting, extensive volcanism, high heat flow and a low-velocity mantle. These phenomena, apparently the result of plate tectonics, are superimposed on the inherited subcontinental crust that developed from an oceanic origin in Paleozoic time and possibly retained some of its thin and layered characteristics. The present anomalous crust in the Great Basin represents an accretion of oceanic geosynclinal material to a Precambrian continental nucleus apparently as an intermediate step in the process of conversion of oceanic crust into a stable continental landmass or craton. ?? 1974.

  3. Archaean zircons in Miocene oceanic hotspot rocks establish ancient continental crust beneath Mauritius

    NASA Astrophysics Data System (ADS)

    Ashwal, Lewis D.; Wiedenbeck, Michael; Torsvik, Trond H.

    2017-01-01

    A fragment of continental crust has been postulated to underlie the young plume-related lavas of the Indian Ocean island of Mauritius based on the recovery of Proterozoic zircons from basaltic beach sands. Here we document the first U-Pb zircon ages recovered directly from 5.7 Ma Mauritian trachytic rocks. We identified concordant Archaean xenocrystic zircons ranging in age between 2.5 and 3.0 Ga within a trachyte plug that crosscuts Older Series plume-related basalts of Mauritius. Our results demonstrate the existence of ancient continental crust beneath Mauritius; based on the entire spectrum of U-Pb ages for old Mauritian zircons, we demonstrate that this ancient crust is of central-east Madagascar affinity, which is presently located ~700 km west of Mauritius. This makes possible a detailed reconstruction of Mauritius and other Mauritian continental fragments, which once formed part of the ancient nucleus of Madagascar and southern India.

  4. Mid-ocean ridge basalt trace element evolution controlled by melt-rock reaction in the lower oceanic crust

    NASA Astrophysics Data System (ADS)

    Lissenberg, C.; MacLeod, C. J.

    2013-12-01

    Mid-ocean ridge basalt (MORB) is the most abundant magma on Earth, and is the main geochemical window into the mantle. When evolution in crustal magma chambers is accounted for, its composition reflects a combination of mantle composition, melting processes and melt migration mechanisms. However, this approach assumes that modification of melts in crustal magma chambers can be corrected for. Recently, it has emerged that the trace element distributions in MORB do not follow fractional crystallization trends, being characterized by a relative over-enrichment of incompatible elements acquired during crustal processing (O'Neill and Jenner, 2012). This implies that it is no longer appropriate to use fractional crystallization models alone to correct for intra-crustal evolution. In order to continue using MORB as messenger from the mantle, it is critical to fully understand the origin of its trace element distributions. O'Neill and Jenner (2012) posit that the trace elements in MORB are the result of repeated replenishment-tapping-fractionation cycles in oceanic magma chambers. Here we explore the alternative hypothesis that the trace elements are instead controlled by melt-rock reaction in the lower oceanic crust. Our hypothesis is based on observations from a suite of lower crustal gabbroic rocks from the East Pacific Rise exposed in Hess Deep (equatorial Pacific Ocean). These gabbros preserve evidence for extensive reactions between ascending melts and a gabbroic framework in the crystal mush that forms the bulk of the lower crust (Lissenberg et al. 2013). In this contribution we compare the trace element distributions generated by melt-rock reaction with those documented in MORB. We treat MORB as mixtures between rapidly transported melts that escape melt-rock reaction and melts that ascend slowly by reactive porous flow, with the trace element enrichment for the latter constrained by the Hess Deep gabbro data. Our results display an excellent correlation with the

  5. Tracing the subducted oceanic crust beneath the central California continental margin: Results from ocean bottom seismometers deployed during the 1986 Pacific Gas and Electric EDGE experiment

    SciTech Connect

    Trehu, A. )

    1991-04-10

    Large aperture seismic data were collected on several ocean bottom seismometers (OBS) deployed along a deep crustal seismic profile that was shot across the central California continental margin. The line of shots extends from the oceanic crust seaward of the Santa Lucia Escarpment to the California coast near Morro Bay and crosses the Santa Lucia Basin, Santa Lucia Bank, Santa Maria Basin, and Hosgri fault zone. The OBS data permits one to trace the subducted oceanic crust from seaward of the Santa Lucia Escarpment to beneath the central part of the Santa Maria Basin. Just seaward of the Santa Lucia Escarpment, the oceanic crust is subhorizontal and covered by a thin layer of low-velocity sediment. The velocity (4.5 km/s) and gradient (1.20-1.25 km/s/km) of the upper oceanic crust in this region are well determined and agree with earlier determinations of the crustal structure of the eastern Pacific. Beneath the Santa Lucia Escarpment and Santa Lucia Basin, the oceanic crust dips approximately 16{degrees} to the east. It is overlain by material with a velocity that increases from 4.8 to 6.4 km/s at a depth of 1.7-5.5 km below the seafloor beneath the Santa Lucia Basin. A low-velocity zone may be sandwiched between the subducted crust and this shallow high-velocity material, which the authors interpret to represent obducted oceanic crustal material. Beneath the eastern edge of Santa Lucia Basin, the dip of the subducted oceanic crust decreases to less that 2{degrees}. The configuration of the subducted crust in this region is consistent with imbrication of the subducted crust. Beneath the central Santa Maria Basin, the top of the subducted oceanic crust is at a depth of about 14-16 km and the Moho is at 19-21 km.

  6. Abundance and diversity of microbial life in ocean crust.

    PubMed

    Santelli, Cara M; Orcutt, Beth N; Banning, Erin; Bach, Wolfgang; Moyer, Craig L; Sogin, Mitchell L; Staudigel, Hubert; Edwards, Katrina J

    2008-05-29

    Oceanic lithosphere exposed at the sea floor undergoes seawater-rock alteration reactions involving the oxidation and hydration of glassy basalt. Basalt alteration reactions are theoretically capable of supplying sufficient energy for chemolithoautotrophic growth. Such reactions have been shown to generate microbial biomass in the laboratory, but field-based support for the existence of microbes that are supported by basalt alteration is lacking. Here, using quantitative polymerase chain reaction, in situ hybridization and microscopy, we demonstrate that prokaryotic cell abundances on seafloor-exposed basalts are 3-4 orders of magnitude greater than in overlying deep sea water. Phylogenetic analyses of basaltic lavas from the East Pacific Rise (9 degrees N) and around Hawaii reveal that the basalt-hosted biosphere harbours high bacterial community richness and that community membership is shared between these sites. We hypothesize that alteration reactions fuel chemolithoautotrophic microorganisms, which constitute a trophic base of the basalt habitat, with important implications for deep-sea carbon cycling and chemical exchange between basalt and sea water.

  7. Subduction-related oceanic crust in the Khantaishir ophiolite (western Mongolia).

    NASA Astrophysics Data System (ADS)

    Gianola, O.; Schmidt, M. W.; Jagoutz, O. E.

    2014-12-01

    Most of the oceanic crust is generated at mid oceanic ridges and only a minor portion results from magmatism related to subduction zones (i.e. back-arc basins or in incipient arcs). However it has been observed that several ophiolites preserve an oceanic crust displaying a subduction zone signature. Such a signature is also found in the Khantaishir ophiolite located in western Mongolia. This ~570 m.y. old ophiolite is ~480 km2 in size and displays a complete sequence, tectonically slightly dismembered during the emplacement process. The ophiolite exposes ~130 km2 of highly refractory harzburgitic mantle with local dunite channels and lenses. Towards its top the mantle is replaced by sub-horizontal km-wide discrete zones of pyroxenites situated either in the mantle or forming a crust-mantle transition zone overlain by gabbros. The crust is then composed of various gabbros and minor gabbronorite (both in part replaced by pyroxenites and/or cut by intermediate dykes), by a dyke/sill-complex and by pillow lavas. The entire ophiolite is re-equilibrated at lower greenschist facies conditions. Major and trace elements of the crustal rocks of the Khantaishir ophiolite show trends similar to those observed for the Izu-Bonin-Mariana subduction system. Mafic dykes/sills and pillow lavas of the Khantaishir ophiolite have overall basaltic-andesite compositions, resembling high-Mg andesites with an average SiO2 of 57 wt%. Their low TiO2 (<1.2 wt%) and Ti/V ratios of ~10 contrast with MOR suites, which are basaltic (average SiO2 ~50 wt%) with TiO2 values usually higher than 1.2 wt% and a Ti/V ratio between 20 and 50. This difference is prominent in Th/Yb and Nb/Yb, where the lavas from the Khantaishir ophiolite plot at higher Th/Yb than the typical MORB-OIB array. Moreover, some volcanics with nearly primitive compositions (XMg = 0.69, SiO2 = 56.5 wt%) have trace element patterns comparable with those displayed in high-Mg andesites and boninites from modern island arcs. This

  8. Small, monogenetic volcanoes: building blocks of the upper oceanic crust

    NASA Astrophysics Data System (ADS)

    Yeo, Isobel A.; Achenbach, Kay L.; Searle, Roger C.; Le Bas, Tim P.

    2010-05-01

    The study of slow-spreading mid-ocean ridge volcanism provides important insights into the mechanisms of oceanic crustal accretion. This study uses a combination of sidescan sonar and recently developed methods of high resolution bathymetry and video data collection to describe the volcanic features on the Mid-Atlantic Ridge axis at 45°N in more detail than has previously been possible. Within most axial valleys lie axial volcanic ridges (AVRs), linear volcanic features thought to be the focus of volcanism at slow spreading ridges. AVR volcanic morphologies have been described independently in a number of studies, through combinations of remote sensing (predominantly through the use of sidescan sonar) and deep towed cameras or submersibles. These different methods have led to classification of volcanic features on two very different scales. While the resolution of the sidescan sonar studies allows only for the identification and classification of features tens to hundreds of metres in size, the photographic and submersible studies describe features from centimetre to metre scale. Until now it has been difficult to reliably link these observations together as no intermediate sensing method has been available. This study uses 1m resolution ROV multibeam bathymetry to address this problem and link features identified at different scales together. We identify a prominent 22km long axial volcanic ridge within a 1km deep axial valley that ranges from 6 to 14km across. We find that 'hummocks' described in previous sidescan sonar studies (of which the AVR is composed) are individual, monogenetic volcanic cones. These cones range from 2 to 200m in height and 40 to 400m in diameter and we identify over 8000 of them on the surface of the AVR. We calculate the average volume of a cone to be 220,000m3 and estimate the AVR is built of approximately 73,000 such cones. We estimate these edifices form on time scales ranging from less than one hour to several months so are likely

  9. Noble gases in the Oceanic Crust: Preliminary results from ODP Hole 1256D

    NASA Astrophysics Data System (ADS)

    Kurz, M. D.; Curtice, J. M.; IODP Expedition 335 Science Party

    2011-12-01

    Noble gas isotopes and abundance ratios have been extensively used as tracers of oceanic mantle sources and fluxes. Most of the existing data are from seafloor basalt glasses and hydrothermal fluids, and there are very few studies of noble gases in the oceanic crust, which is an important component in global subduction flux estimates. In an effort to determine the relative contributions of mantle, radiogenic, and atmospheric/hydrothermal noble gas components in the ocean crust, we have performed helium, neon and argon measurements on a suite of gabbros and granoblastic dikes collected during IODP Expeditions 312 and 335 to Hole 1256D, a deep crustal borehole drilled into 15 Ma ocean crust formed at the East Pacific Rise during an episode of superfast spreading (>200 mm/yr). All measurements were carried out by coupled vacuum crushing and melting of whole rock samples, in order to determine the distribution of noble gases within the ocean crust. Total helium abundances in the gabbros range from 0.46 to 1.22 micro cc STP/gram, which is 2 to 5 times higher than literature data, all of which are from the slow spreading Southwest Indian Ridge (Kumagai et al., 2003; Moreira et al., 2003). These strikingly higher helium concentrations place constraints on the thermal crustal history (due to rapid helium diffusivity) and are assumed to reflect fundamentally different emplacement/degassing processes within crust formed at a super fast spreading rate. Crushing releases 12-25 % of the total helium in the gabbros demonstrating that most of the helium resides in the solid mineral phases. Contact metamorphosed granoblastic dikes have total helium contents lower than the gabbros (typically ~ 0.15 micro cc STP/gram), but significantly higher than the assumed degassed basaltic protolith, thus suggesting that metamorphism actually adds helium to the crust, an important hypothesis that requires further testing. The helium isotopes obtained by crushing of both the gabbros and

  10. Anaerobic Fungi: A Potential Source of Biological H2 in the Oceanic Crust

    PubMed Central

    Ivarsson, Magnus; Schnürer, Anna; Bengtson, Stefan; Neubeck, Anna

    2016-01-01

    The recent recognition of fungi in the oceanic igneous crust challenges the understanding of this environment as being exclusively prokaryotic and forces reconsiderations of the ecology of the deep biosphere. Anoxic provinces in the igneous crust are abundant and increase with age and depth of the crust. The presence of anaerobic fungi in deep-sea sediments and on the seafloor introduces a type of organism with attributes of geobiological significance not previously accounted for. Anaerobic fungi are best known from the rumen of herbivores where they produce molecular hydrogen, which in turn stimulates the growth of methanogens. The symbiotic cooperation between anaerobic fungi and methanogens in the rumen enhance the metabolic rate and growth of both. Methanogens and other hydrogen-consuming anaerobic archaea are known from subseafloor basalt; however, the abiotic production of hydrogen is questioned to be sufficient to support such communities. Alternatively, biologically produced hydrogen could serve as a continuous source. Here, we propose anaerobic fungi as a source of bioavailable hydrogen in the oceanic crust, and a close interplay between anaerobic fungi and hydrogen-driven prokaryotes. PMID:27433154

  11. Magnetization of the oceanic crust - Thermoremanent magnetization of chemical remanent magnetization?

    NASA Technical Reports Server (NTRS)

    Raymond, C. A.; Labrecque, J. L.

    1987-01-01

    A model was proposed in which chemical remanent magnetization (CRM) acquired within the first 20 Ma of crustal evolution may account for 80 percent of the bulk natural remanent magnetization (NRM) of older basalts. The CRM of the crust is acquired as the original thermoremanent magnetization (TRM) is lost through low temperature alteration. The CRM intensity and direction are controlled by the post-emplacement polarity history. This model explains several independent observations concerning the magnetization of the oceanic crust. The model accounts for amplitude and skewness dicrepancies observed in both the intermediate wavelength satellite field and the short wavelength sea surface magnetic anomaly pattern. It also explains the decay of magnetization away from the spreading axis, and the enhanced magnetization of the Cretaceous Quiet Zones while predicting other systematic variations with age in the bulk magnetization of the oceanic crust. The model also explains discrepancies in the anomaly skewness parameter observed for anomalies of Cretaceous age. Further studies indicate varying rates of TRM decay in very young crust which depicts the advance of low temperature alteration through the magnetized layer.

  12. Anaerobic Fungi: A Potential Source of Biological H2 in the Oceanic Crust.

    PubMed

    Ivarsson, Magnus; Schnürer, Anna; Bengtson, Stefan; Neubeck, Anna

    2016-01-01

    The recent recognition of fungi in the oceanic igneous crust challenges the understanding of this environment as being exclusively prokaryotic and forces reconsiderations of the ecology of the deep biosphere. Anoxic provinces in the igneous crust are abundant and increase with age and depth of the crust. The presence of anaerobic fungi in deep-sea sediments and on the seafloor introduces a type of organism with attributes of geobiological significance not previously accounted for. Anaerobic fungi are best known from the rumen of herbivores where they produce molecular hydrogen, which in turn stimulates the growth of methanogens. The symbiotic cooperation between anaerobic fungi and methanogens in the rumen enhance the metabolic rate and growth of both. Methanogens and other hydrogen-consuming anaerobic archaea are known from subseafloor basalt; however, the abiotic production of hydrogen is questioned to be sufficient to support such communities. Alternatively, biologically produced hydrogen could serve as a continuous source. Here, we propose anaerobic fungi as a source of bioavailable hydrogen in the oceanic crust, and a close interplay between anaerobic fungi and hydrogen-driven prokaryotes.

  13. In situ Detection of Microbial Life in the Deep Biosphere in Igneous Ocean Crust.

    PubMed

    Salas, Everett C; Bhartia, Rohit; Anderson, Louise; Hug, William F; Reid, Ray D; Iturrino, Gerardo; Edwards, Katrina J

    2015-01-01

    The deep biosphere is a major frontier to science. Recent studies have shown the presence and activity of cells in deep marine sediments and in the continental deep biosphere. Volcanic lavas in the deep ocean subsurface, through which substantial fluid flow occurs, present another potentially massive deep biosphere. We present results from the deployment of a novel in situ logging tool designed to detect microbial life harbored in a deep, native, borehole environment within igneous oceanic crust, using deep ultraviolet native fluorescence spectroscopy. Results demonstrate the predominance of microbial-like signatures within the borehole environment, with densities in the range of 10(5) cells/mL. Based on transport and flux models, we estimate that such a concentration of microbial cells could not be supported by transport through the crust, suggesting in situ growth of these communities.

  14. In situ Detection of Microbial Life in the Deep Biosphere in Igneous Ocean Crust

    PubMed Central

    Salas, Everett C.; Bhartia, Rohit; Anderson, Louise; Hug, William F.; Reid, Ray D.; Iturrino, Gerardo; Edwards, Katrina J.

    2015-01-01

    The deep biosphere is a major frontier to science. Recent studies have shown the presence and activity of cells in deep marine sediments and in the continental deep biosphere. Volcanic lavas in the deep ocean subsurface, through which substantial fluid flow occurs, present another potentially massive deep biosphere. We present results from the deployment of a novel in situ logging tool designed to detect microbial life harbored in a deep, native, borehole environment within igneous oceanic crust, using deep ultraviolet native fluorescence spectroscopy. Results demonstrate the predominance of microbial-like signatures within the borehole environment, with densities in the range of 105 cells/mL. Based on transport and flux models, we estimate that such a concentration of microbial cells could not be supported by transport through the crust, suggesting in situ growth of these communities. PMID:26617595

  15. The role of low temperature alteration of the oceanic crust in global carbon cycling

    NASA Astrophysics Data System (ADS)

    Coogan, L. A.; Gillis, K. M.; Parrish, R. R.; Austin-Giddings, W.; Dosso, S. E.; Moore, A.

    2013-12-01

    The role of low-temperature (10's of degrees Celsius) seafloor alteration in the long-term carbon cycle has received far less attention than the parallel process on the continents of 'chemical weathering'. This difference in the amount of study of these two sinks for CO2 degassed from the solid Earth does not appear to be based on their relative importance to global C-cycling. A large amount of CO2 is precipitated as carbonate minerals in the upper oceanic crust and this appears to be largely due to in situ alkalinity generation due to reactions within the oceanic crust [1,2]. Better understanding of the role of this CO2 sink in the global carbon cycle requires better constraints on: (i) the controls on alkalinity production during low-temperature alteration of the upper oceanic crust; and (ii) when in the life cycle of a piece of crust most low-temperature reaction occurs. Alkalinity production due to low-temperature alteration of the oceanic crust appears to have varied over the last 160 Myr [3], but whether this reflects differences in ocean composition or ocean bottom temperature is unclear. New data, largely from the Troodos ophiolite, are being collected to determine the reactions involved in alkalinity generation and how these vary as a function of the environmental conditions. To this end, a new suite of volcanic glass major and trace element data have been collected to determine the protolith composition; this is required in computing alkalinity generation from the altered rock compositions. Overall the data collection (bulk-rock compositions, carbonate O-isotopes) is designed to allow us to determine the roles of protolith composition and fluid temperature in alkalinity generation. Sample processing is underway and these new data will be presented. Understanding the timing of low-T alteration of the oceanic crust is important in understanding the impacts of this process on global geochemical cycling. We are applying novel dating approaches to this problem

  16. Hydrothermal fault zones in the lower oceanic crust: An example from the Samail ophiolite, Oman

    NASA Astrophysics Data System (ADS)

    Zihlmann, Barbara; Müller, Samuel; Koepke, Juergen; Teagle, Damon

    2017-04-01

    Hydrothermal circulation is a key process for the exchange of chemical elements between the oceans and the solid Earth and particularly for the extraction of heat from newly accreted crust at mid-ocean ridges. However, due to a dearth of samples from intact oceanic crust, or continuous samples from ophiolites, there remain major short comings in our understanding of hydrothermal circulation in the oceanic crust, especially in the deeper part. In particular, it is unknown whether fluid recharge and discharge occurs pervasively or if it is mainly channeled onto discrete zones such as faults. Here, we present a description of a hydrothermal fault zone that crops out in the layered gabbro section, of Wadi Gideah in the Samail ophiolite in Oman, which might be a channel of enhanced fluid flow. Field observations reveal an approximately one meter-thick chlorite - epidote normal fault with heavily altered gabbro clasts in the center. In places there is copper mineralization within the chlorite - epidote zone. In both, the hanging and the footwall the gabbro is heavily altered and veined, mainly with amphibole, epidote, prehnite and zeolite veins. Even though the fault zone is within the layered gabbro section, and perhaps only 1 km above the crust-mantle boundary, the gabbro around the fault zone shows highly variable textures. Preliminary strontium isotope whole rock data yield 87Sr/86Sr ratios of ˜ 0.7046, which are considerably more radiogenic than "fresh" gabbro from the Oman ophiolite (87Sr/86Sr = 0.7026 - 0.7030), and similar to black smoker hydrothermal signatures based on epidote, measured elsewhere in the ophiolite.

  17. The OSCAR experiment: using full-waveform inversion in the analysis of young oceanic crust

    NASA Astrophysics Data System (ADS)

    Silverton, Akela; Morgan, Joanna; Wilson, Dean; Hobbs, Richard

    2017-04-01

    The OSCAR experiment aims to derive an integrated model to better explain the effects of heat loss and alteration by hydrothermal fluids, associated with the cooling of young oceanic crust at an axial ridge. High-resolution seismic imaging of the sediments and basaltic basement can be used to map fluid flow pathways between the oceanic crust and the surrounding ocean. To obtain these high-resolution images, we undertake full-waveform inversion (FWI), an advanced seismic imaging technique capable of resolving velocity heterogeneities at a wide range of length scales, from background trends to fine-scale geological/crustal detail, in a fully data-driven automated manner. This technology is widely used within the petroleum sector due to its potential to obtain high-resolution P-wave velocity models that lead to improvements in migrated seismic images of the subsurface. Here, we use the P-wave velocity model obtained from travel-time tomography as the starting model in the application of acoustic, time-domain FWI to a multichannel streamer field dataset acquired in the east Pacific along a profile between the Costa Rica spreading centre and the Ocean Drilling Program (ODP) borehole 504B, where the crust is approximately six million years old. FWI iteratively improves the velocity model by minimizing the misfit between the predicted data and the field data. It seeks to find a high-fidelity velocity model that is capable of matching individual seismic waveforms of the original raw field dataset, with an initial focus on matching the low-frequency components of the early arriving energy. Quality assurance methods adopted during the inversion ensure convergence in the direction of the global minimum. We demonstrate that FWI is able to recover fine-scale, high-resolution velocity heterogeneities within the young oceanic crust along the profile. The highly resolved FWI velocity model is useful in the identification of the layer 2A/2B interface and low-velocity layers that

  18. Chemical composition of ferromanganese crusts in the world ocean: a review and comprehensive database

    USGS Publications Warehouse

    Manheim, Frank T.; Lane-Bostwick, Candice M.

    1989-01-01

    A comprehensive database of chemical and mineralogical properties for ferromanganese crusts collected throughout the Atlantic, Pacific, and Indian Oceans, and has been assembled from published and unpublished sources which provide collection and analytical information for these samples. These crusts, their chemical compositions and natural distribution, have been a topic of interest to scientific research, as well as to industrial and military applications. Unlike abyssal ferromanganese nodules, which form in areas of low disturbance and high sediment accumulation, crusts have been found to contain three to five times more cobalt than these nodules, and can be found on harder, steeper substrates which can be too steep for permanent sediment accumulation. They have also been documented on seamounts and plateaus within the U.S. exclusive economic zone in both Pacific and Atlantic Oceans, and these are therefore of strategic importance to the United States Government, as well as to civilian mining and metallurgical industries. The data tables provided in this report have been digitized and previously uploaded to the National Oceanic and Atmospheric Administration National Geophysical Data Center in 1991 for online distribution, and were provided in plain text format. The 2014 update to the original U.S. Geological Survey open-file report published in 1989 provides these data tables in a slightly reformatted version to make them easier to ingest into geographic information system software, converted to shapefiles, and have completed metadata written and associated with them.

  19. Evolution of porosity and seismic structure of upper oceanic crust: Importance of aspect ratios

    NASA Astrophysics Data System (ADS)

    Wilkens, Roy H.; Fryer, Gerard J.; Karsten, Jill

    1991-10-01

    Seismic properties of the uppermost igneous crust of the oceans are dominated by porosity effects, that is, the size, concentration, and shape of void spaces. Porosity is initially determined by the physics of extrusion (does an eruption form breccia, pillows, or massive flows?) but is very rapidly modified by alteration and hydrothermal deposition. Laboratory data provide insight into compressional wave velocity-porosity behavior of basalts at a hand sample scale, while well logs provide data at outcrop scale. Relating observations at all scales to porosity structure and extrapolating to seismic scale requires application of rock physics theory. Using information from ophiolites and deep ocean cores, we have defined rock physics parameters for two simple models of upper oceanic crust. The models approximate different levels of void filling by alteration products by differing in the amount of crack (low aspect ratio) porosity they contain. From the models we compute theoretical compressional wave velocity and porosity profiles. Calculated profiles agree well with both well logs and seismic data and illustrate that the increase in seismic velocities measured seismically in the upper crust need not be accompanied by large changes in total porosity.

  20. Melt flow and hypersolidus deformation in the lower ocean crust: Preliminary observations from IODP Leg 345

    NASA Astrophysics Data System (ADS)

    Snow, J. E.; Koepke, J.; Falloon, T.; Abe, N.; Hoshide, T.; Akizawa, N.; Maeda, J.; Jean, M. M.; Cheadle, M. J.

    2013-12-01

    Models for the construction of the fast-spreading lower ocean crust include the gabbro glacier model (GGM), in which most crystallization occurs within a shallow melt lens and the resulting crystal mush subsides downwards and outwards by crystal sliding. Second, the Sheeted Sill Model (SSM) predicts magmatic injection at many levels in the crust, and requires rapid cooling of the lithosphere. A second set of models seeks to reconcile the relatively unevolved nature of most MORB with the existence of an extensive lower crust with both layering (in the lower crust) and highly evolved gabbros (in the upper plutonic sequence). The mechanisms involved here are melt aggregation during vertical porous flow in the lower crust as opposed to lateral sill injection and in-situ crystallization. Here we report new observations from IODP Expedition 345 to the Hess Deep Rift, where propagation of the Cocos Nazca Ridge (CNR) into young, fast-spreading East Pacific Rise (EPR) crust exposes a dismembered lower crustal section. Drilling in ~4850 m water depth produced 3 holes of 35 to 100 mbsf with ~30% recovery of primitive (Mg# 79-87) plutonic lithologies including troctolite, olivine gabbro, and olivine gabbronorite, showing cumulate textures found in layered mafic intrusions and some ophiolite complexes including: 1. Spectacular modal layering 2. Orthopyroxene very early on the liquidus compared to canonical MORB. 3. Delicate large (2-5 cm) skeletal and hopper structures in olivine. 4. Oikocrystic clinopyroxene enclosing chadacrysts different from the host assemblage. These complex relationships are only hinted at in the existing observations from the ocean floor, and will require significant lab study, however some preliminary inferences can be drawn from the petrographic observations. First, the textures observed in olivine throughout the cores are consistent with rapid crystallization, possibly due to steep thermal gradients in the lower crust. They occur early in the

  1. Oceanic crust of the Grenada Basin in the Southern Lesser Antilles Arc Platform

    NASA Astrophysics Data System (ADS)

    Speed, R. C.; Walker, J. A.

    1991-03-01

    Seismic refraction data permit the southern Lesser Antilles arc and surrounding regions to be divided by the velocity of their basement. We propose that high-velocity basement of the arc platform beneath the Grenadine islands and below a part of the Tobago Trough forearc basin is oceanic and continuous and was originally connected with oceanic crust of the Grenada Basin. Low-velocity basements of the Tobago terrane and the arc platform from St. Vincent north lie south and north, respectively, of the high-velocity basement of the arc platform. An oceanic origin of this high-velocity crust in the Grenadines is argued to be more plausible than an origin as unroofed lower arc crust. The segment of probable oceanic crust in the arc platform was greatly uplifted during development of the present island arc, mainly in late Neogene time, relative to the Grenada Basin and Tobago Trough. Accepting the proposition of shallow oceanic crust in the Grenadines, early middle Eocene and possibly older pillow basalts of Mayreau, the oldest rock unit of the southern Lesser Antilles arc platform, may be an exposure of such basement. Major and minor element compositions of Mayreau Basalt are indicative of a spreading rather than arc origin. The stratigraphy of the pillow basalts indicates extrusion in an open marine environment, distant or shielded from sources of arc or continental sediment, followed by a period of pelagic sedimentation above the carbonate compensation depth. The Eocene basalt and pelagic cover formed a relatively deep floor of a marine basin in which arc-derived turbidites and pelagic sediments accumulated over the succeeding 25-30 ma. Such basalts thus indicate a probable spreading origin of the Grenada Basin and an age of cessation of spreading in the region of Mayreau in Eocene time. The configuration of the Eocene basin and the direction of spreading, however, are unknowns. Regional structural relationships imply the spreading was probably backarc, an origin also

  2. Comparison of Lunar Basalts and Gabbros with those of the Terrestrial Ocean Crust

    NASA Astrophysics Data System (ADS)

    Natland, J. H.

    2012-12-01

    Initial studies of lunar samples returned from the Apollo and Luna missions took place before rocks of the Earth's lower ocean crust, chiefly varieties of gabbro cumulates, were widely known or understood. Continuing exploration of the ocean crust invites some new comparisons. When volcanic rocks and glass from Apollo 11 and 17 were discovered to have very high TiO2 contents (8-14%), nothing comparable was known from Earth. The high-TiO2 lunar samples were soon described as primary melts derived from considerable depths in the lunar mantle. Other lunar samples have only very low TiO2 contents (~0.2%) and very low concentrations of highly incompatible elements such as Zr and Sr. Today, dredging and drilling results indicate that oxide gabbros rich in magmatic oxides and sulfides and with up to 12% TiO2 comprise a significant percentage of the gabbroic portion of the ocean crust especially at slowly spreading ridges. These are very late stage differentiates, and are commonly juxtaposed by high-temperature deformation processes with more primitive olivine gabbros and troctolites having only ~0.2% TiO2 and low concentrations of Zr and other incompatible elements. The rocks are mainly adcumulates, with very low concentrations of incompatible elements set by proportions of cumulus minerals, and with little contribution from the liquids that produced them. In addition, some lunar gabbros with highly calcic plagioclase (~An93-98) are similar to gabbros and troctolites found in island arcs. All of these similarities suggest that very few lunar basaltic rocks are pristine; instead they all could be nearly complete shock fusion products produced by meteorite impact into a diverse assemblage of lunar gabbros that included both low- and high-TiO2 gabbroic facies. On this hypothesis, no lunar basalt is a primary melt derived from the Moon's mantle. Although magmatic environments on the ancient Moon and in the modern ocean crust were different in important ways, the general

  3. Samples from the Jurassic ocean crust beneath Gran Canaria, La Palma and Lanzarote (Canary Islands)

    NASA Astrophysics Data System (ADS)

    Schmincke, Hans-Ulrich; Klügel, Andreas; Hansteen, Thor H.; Hoernle, Kaj; van den Bogaard, Paul

    1998-11-01

    Gabbro and minor metabasalt fragments of MORB composition were found on three of the seven Canary Islands. On Gran Canaria, they occur as metamorphosed (greenschist facies) metabasalt and metagabbro clasts in Miocene fanglomerates and sandstones overlying the shield basalts. On Lanzarote and La Palma, MORB gabbros occur as xenoliths in Pleistocene and historic basanite scoria cones and lava flows. The MORB xenoliths are interpreted as fragments of layers 2 and 3 of the underlying Mesozoic oceanic crust, based on mineral compositions (An-rich plagioclase, Ti- and Al-poor clinopyroxene, ± orthopyroxene ± olivine), depleted major and trace element signatures, and Jurassic ages (ca. 180 Ma) determined on single primary plagioclase and secondary amphibole crystals using the 40Ar/ 39Ar laser technique. The Lanzarote gabbros are very mafic (mg# 87 to 89 in clinopyroxene), moderately deformed, and highly depleted. Gran Canaria gabbros are more evolved (mg# 69 to 83 in clinopyroxene) and texturally mostly isotropic. La Palma MORB gabbros have a range of compositions (mg# 68 to 83 in clinopyroxene), some rocks being strongly metasomatized by interaction with basanite magma. The occurrence of MORB fragments on Lanzarote provides definite evidence that oceanic crust beneath the Canary Island archipelago continues at least as far east as the eastern Canary Islands. We postulate that MORB gabbros on Lanzarote which are commonly associated with peridotite xenoliths, represent the base of oceanic layer 3 where gabbros and peridotites were possibly tectonically interleaved. Such tectonic mixing would explain the enigmatic seismic velocities in this area. Gabbro xenoliths from La Palma were derived from within layer 3, probably from wall rock close to magma reservoirs emplaced during the Pleistocene/Holocene growth of La Palma. The Gran Canaria xenoliths are interpreted to represent the metamorphosed layer 2 and upper layer 3. The abundance of lower crustal xenoliths emphasizes

  4. Physical properties and seismic structure of Izu-Bonin-Mariana fore-arc crust: Results from IODP Expedition 352 and comparison with oceanic crust

    NASA Astrophysics Data System (ADS)

    Christeson, G. L.; Morgan, S.; Kodaira, S.; Yamashita, M.; Almeev, R. R.; Michibayashi, K.; Sakuyama, T.; Ferré, E. C.; Kurz, W.

    2016-12-01

    Most of the well-preserved ophiolite complexes are believed to form in suprasubduction zone (SSZ) settings. We compare physical properties and seismic structure of SSZ crust at the Izu-Bonin-Mariana (IBM) fore arc with oceanic crust drilled at Holes 504B and 1256D to evaluate the similarities of SSZ and oceanic crust. Expedition 352 basement consists of fore-arc basalt (FAB) and boninite lavas and dikes. P-wave sonic log velocities are substantially lower for the IBM fore arc (mean values 3.1-3.4 km/s) compared to Holes 504B and 1256D (mean values 5.0-5.2 km/s) at depths of 0-300 m below the sediment-basement interface. For similar porosities, lower P-wave sonic log velocities are observed at the IBM fore arc than at Holes 504B and 1256D. We use a theoretical asperity compression model to calculate the fractional area of asperity contact Af across cracks. Af values are 0.021-0.025 at the IBM fore arc and 0.074-0.080 at Holes 504B and 1256D for similar depth intervals (0-300 m within basement). The Af values indicate more open (but not necessarily wider) cracks in the IBM fore arc than for the oceanic crust at Holes 504B and 1256D, which is consistent with observations of fracturing and alteration at the Expedition 352 sites. Seismic refraction data constrain a crustal thickness of 10-15 km along the IBM fore arc. Implications and inferences are that crust-composing ophiolites formed at SSZ settings could be thick and modified after accretion, and these processes should be considered when using ophiolites as an analog for oceanic crust.

  5. IODP Expedition 345: Primitive Layered Gabbros From Fast-Spreading Lower Oceanic Crust

    NASA Astrophysics Data System (ADS)

    Ildefonse, Benoit; Gillis, Kathryn M.; Snow, Jonathan E.; Klaus, Adam

    2014-05-01

    Three-quarters of the ocean crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the seafloor. However, owing to the nearly continuous overlying extrusive upper crust, sampling in situ the lower crust is challenging. Hence, models for understanding the formation of the lower crust are based essentially on geophysical studies and ophiolites. Integrated Ocean Drilling Program (IODP) Expedition 345 recovered the first significant sections of primitive, modally layered gabbroic rocks from the lowermost plutonic crust formed at a fast-spreading ridge, and exposed at the Hess Deep Rift (Gillis et al., Nature, 2014, doi:10.1038/nature12778). Drilling Site U1415 is located along the southern slope of the intrarift ridge. The primary science results were obtained from coring of two ~110 m deep reentry holes and one 35-m-deep single-bit hole, all co-located within an ~100-m-wide area. Olivine gabbro and troctolite are the dominant plutonic rock types recovered, with minor gabbro, clinopyroxene oikocryst-bearing gabbroic rocks, and gabbronorite. All rock types are primitive to moderately evolved, with Mg# 89-76, and exhibit cumulate textures similar to ones found in layered mafic intrusions and some ophiolites. Spectacular modal and grain size layering, prevalent in >50% of the recovered core, confirm a long held paradigm that such rocks are a key constituent of the lowermost ocean crust formed at fast-spreading ridges. Magmatic foliation is largely defined by the shape-preferred orientation of plagioclase. It is moderate to strong in intervals with simple modal layering but weak to absent in troctolitic intervals and typically absent in intervals with heterogeneous textures and/or diffuse banding. Geochemical analysis of these primitive lower plutonics, in combination with previous geochemical data for shallow-level plutonics

  6. Investigating Compositional Links Between Arc Magmas And The Subducted Altered Oceanic Crust

    NASA Astrophysics Data System (ADS)

    Straub, S. M.

    2015-12-01

    Arc magmatism is causally related to the recycling of materials from the subducting plate. Numerous studies showed that the recycled material flux is dominated by recycled continental crust (oceanic sediment, eroded crust) and altered oceanic igneous crust (AOC). The crustal component is highly enriched, and thus its signal in arc magmas can readily be distinguished from mantle wedge contributions. In contrast, the impact of the AOC flux is much more difficult to detect, since the AOC isotopically resembles the mantle. Mass balance studies of arc input and output suggest that the recycled flux from the thick (6000 meter on average) AOC may buffer the flux of the recycled continental crust to the point of concealment in arc settings where the latter is volumetrically minor. In particular, highly fluid- mobile elements Sr and Pb in arc magmas are strongly influenced by the AOC, implying that the arc chemistry may allow for inferring the Sr and Pb isotopic composition of the subducted AOC. This hypothesis is being tested by a compilation of published data of high-quality trace element and isotope compositions from global arcs. In agreement with previous studies, our results confirm that the Sr-rich fluids released from the AOC control the arc Sr isotopes, whereby the slightly elevated 87Sr/86Sr (up to 0.705) of many arcs may principally reflect the similarly elevated Sr isotope ratios of the AOC rather than a recycled crustal component. In contrast, the arc Pb isotope ratios are influenced by both the AOC and the recycled crustal component which create the typical binary mixing arrays. These arrays should then point to the Pb isotope composition of the AOC and the recycled crust, respectively. However, as the proportions of these end members may strongly vary in arc magmas, the exact 206Pb/204Pb of the subducted AOC in a given setting is challenging. Remarkably, the Pb isotope systematics from well-constrained western Aleutian (minimal sediment subduction) and central

  7. Crack structure in oceanic crust: A seismic anisotropy study in the Costa Rica Rift

    NASA Astrophysics Data System (ADS)

    Zhang, Ling; Tong, Vincent; Hobbs, Richard; Lowell, Robert

    2017-04-01

    In 2015, an interdisciplinary cruise JC114 was carried out over the Costa Rica Rift (CRR) in the Panama Basin. A 5 x 5 grid of ocean-bottom seismographs (OBS) were deployed over the ridge axis, covering an area of 400 km2. Using tomography to invert 69,000 Pn and Pg arrivals, we obtained a three-dimensional velocity-depth model of the upper crust beneath the CRR, discovering a variable velocity structure beneath the axis. By fitting the traveltime residuals of our tomography result with a cosinusoid curve composed of cos(2θ) and cos(4θ) terms, we investigated the crack-induced azimuthal anisotropy in the upper oceanic crust, focusing on the bathymetric dome in the west of our survey region and the small-offset overlapping spreading centre (OSC) to the east. Our results reveal that in the upper crust on/near the axis, the fast direction of P-wave propagation is roughly aligned with the strike of the ridge axis, implying the presence of vertically aligned cracks that are nearly in parallel to the axis in the upper crust. At the dome, beneath which the axial magma chamber has been imaged on coincident seismic reflection data, a high velocity and aspect ratio of A4θ /A2θ region within the dyke section indicates the presence of thinner cracks. We interpret that this interval may be partly filled by the minerals precipitated from active hydrothermal circulation and/or magmatic intrusions. Conversely, in the dyke section beneath the OSC, we observe an anomalously low velocity yet a high A4θ /A2θ with the fast direction oriented oblique counter-clockwise to the axis. These observations suggest the presence of thin and water-saturated cracks under tectonic stress that may provide important pathways for the downflow of seawater into the crust. However, near the bottom of the dyke section, a lower ratio of A4θ /A2θ and higher anisotropy amplitude is evidence for the presence of thicker cracks, which may implicate lower rate of mineral precipitation and/or the absence

  8. Osmium isotope variations in the oceans recorded by Fe-Mn crusts

    USGS Publications Warehouse

    Burton, K.W.; Bourdon, B.; Birck, J.-L.; Allegre, C.J.; Hein, J.R.

    1999-01-01

    This study presents osmium (Os) isotope data for recent growth surfaces of hydrogenetic ferromanganese (Fe-Mn) crusts from the Pacific, Atlantic and Indian Oceans. In general, these data indicate a relatively uniform Os isotopic composition for modern seawater, but suggest that North Atlantic seawater is slightly more radiogenic than that of the Pacific and Indian Oceans. The systematic difference in the Os isotopic composition between the major oceans probably reflects a greater input of old continental material with a high Re/Os ratio in the North Atlantic Ocean, consistent with the distribution of Nd and Pb isotopes. This spatial variation in the Os isotope composition in seawater is consistent with a residence time for Os of between 2 and 60 kyr. Indian Ocean samples show no evidence of a local source of radiogenic Os, which suggests that the present-day riverine input from the Himalaya-Tibet region is not a major source for Os. Recently formed Fe-Mn crusts from the TAG hydrothermal field in the North Atlantic yield an Os isotopic composition close to that of modern seawater, which indicates that, in this area, the input of unradiogenic Os from the hydrothermal alteration of oceanic crust is small. However, some samples from the deep Pacific (???4 km) possess a remarkably unradiogenic Os isotope composition (187Os/186Os ratios as low as 4.3). The compositional control of Os incorporation into the crusts and mixing relationships suggest that this unradiogenic composition is most likely due to the direct incorporation of micrometeoritic or abyssal peridotite particles, rather than indicating the presence of an unradiogenic deep-water mass. Moreover, this unradiogenic signal appears to be temporary, and local, and has had little apparent effect on the overall evolution of seawater. These results confirm that input of continental material through erosion is the dominant source of Os in seawater, but it is not clear whether global Os variations are due to the input

  9. Metastable garnet in oceanic crust at the top of the lower mantle.

    PubMed

    Kubo, Tomoaki; Ohtani, Eiji; Kondo, Tadashi; Kato, Takumi; Toma, Motomasa; Hosoya, Tomofumi; Sano, Asami; Kikegawa, Takumi; Nagase, Toshiro

    As oceanic tectonic plates descend into the Earth's lower mantle, garnet (in the basaltic crust) and silicate spinel (in the underlying peridotite layer) each decompose to form silicate perovskite-the 'post-garnet' and 'post-spinel' transformations, respectively. Recent phase equilibrium studies have shown that the post-garnet transformation occurs in the shallow lower mantle in a cold slab, rather than at approximately 800 km depth as earlier studies indicated, with the implication that the subducted basaltic crust is unlikely to become buoyant enough to delaminate as it enters the lower mantle. But here we report results of a kinetic study of the post-garnet transformation, obtained from in situ X-ray observations using sintered diamond anvils, which show that the kinetics of the post-garnet transformation are significantly slower than for the post-spinel transformation. Although metastable spinel quickly breaks down at a temperature of 1,000 K, we estimate that metastable garnet should survive of the order of 10 Myr even at 1,600 K. Accordingly, the expectation of where the subducted oceanic crust would be buoyant spans a much wider depth range at the top of the lower mantle, when transformation kinetics are taken into account.

  10. Seismic constraints on the hydration of subducting oceanic crust and mantle

    NASA Astrophysics Data System (ADS)

    Garth, T.; Rietbrock, A.

    2014-12-01

    It is widely accepted that large amounts of water are delivered to the mantle through subduction. Some water is carried in the hydrated minerals in the oceanic crust, and is released to the mantle through dehydration reactions as the slab becomes warmer. It is also thought that the subducting lithospheric mantle is highly hydrated, and carries large amounts of water to the crust. There are however few observational constraints on the depth to which this water is delivered by the hydrated subducting slab, and the amount of water delivered to the mantle. Subduction zone guided waves spend longer interacting with the low velocity hydrous mineral assemblages than any other seismic phase, as so give us a unique opportunity to put new constraints on these features of the subduction zone. We use full waveform seismic modelling techniques to constrain these dispersed arrivals, and so image the velocity structure of the slab. This technique gives an observational constraint on both the hydration of the slab, and the onset of dehydration reactions. We have shown that both low velocity subducted oceanic crust, and low velocity outer rise normal faults that penetrate the lithospheric mantle can act as effective waveguides producing characteristic body wave dispersion. Analysis of the spatial coda decay associated with intermediate depth earthquakes recorded in Northern Japan suggest that these hydrated normal faults may in fact carry the majority of the water transported by the subducting slab to the mantle. Seismic constraints on the dehydration reactions occurring in the subducted oceanic crust also show that full dehydration may occur at much greater depth than predicted by current thermo petrological models, meaning that water may be carried to greater depths than previously thought. Together these methods suggest that much more water is delivered to the mantle by subduction than has previously been suggested, and that water may be carried to greater depths in the mantle

  11. Seismic structure of the crust and uppermost mantle of South America and surrounding oceanic basins

    USGS Publications Warehouse

    Chulick, Gary S.; Detweiler, Shane; Mooney, Walter D.

    2013-01-01

    We present a new set of contour maps of the seismic structure of South America and the surrounding ocean basins. These maps include new data, helping to constrain crustal thickness, whole-crustal average P-wave and S-wave velocity, and the seismic velocity of the uppermost mantle (Pn and Sn). We find that: (1) The weighted average thickness of the crust under South America is 38.17 km (standard deviation, s.d. ±8.7 km), which is ∼1 km thinner than the global average of 39.2 km (s.d. ±8.5 km) for continental crust. (2) Histograms of whole-crustal P-wave velocities for the South American crust are bi-modal, with the lower peak occurring for crust that appears to be missing a high-velocity (6.9–7.3 km/s) lower crustal layer. (3) The average P-wave velocity of the crystalline crust (Pcc) is 6.47 km/s (s.d. ±0.25 km/s). This is essentially identical to the global average of 6.45 km/s. (4) The average Pn velocity beneath South America is 8.00 km/s (s.d. ±0.23 km/s), slightly lower than the global average of 8.07 km/s. (5) A region across northern Chile and northeast Argentina has anomalously low P- and S-wave velocities in the crust. Geographically, this corresponds to the shallowly-subducted portion of the Nazca plate (the Pampean flat slab first described by Isacks et al., 1968), which is also a region of crustal extension. (6) The thick crust of the Brazilian craton appears to extend into Venezuela and Colombia. (7) The crust in the Amazon basin and along the western edge of the Brazilian craton may be thinned by extension. (8) The average crustal P-wave velocity under the eastern Pacific seafloor is higher than under the western Atlantic seafloor, most likely due to the thicker sediment layer on the older Atlantic seafloor.

  12. Seismic structure of the crust and uppermost mantle of South America and surrounding oceanic basins

    NASA Astrophysics Data System (ADS)

    Chulick, Gary S.; Detweiler, Shane; Mooney, Walter D.

    2013-03-01

    We present a new set of contour maps of the seismic structure of South America and the surrounding ocean basins. These maps include new data, helping to constrain crustal thickness, whole-crustal average P-wave and S-wave velocity, and the seismic velocity of the uppermost mantle (Pn and Sn). We find that: (1) The weighted average thickness of the crust under South America is 38.17 km (standard deviation, s.d. ±8.7 km), which is ˜1 km thinner than the global average of 39.2 km (s.d. ±8.5 km) for continental crust. (2) Histograms of whole-crustal P-wave velocities for the South American crust are bi-modal, with the lower peak occurring for crust that appears to be missing a high-velocity (6.9-7.3 km/s) lower crustal layer. (3) The average P-wave velocity of the crystalline crust (Pcc) is 6.47 km/s (s.d. ±0.25 km/s). This is essentially identical to the global average of 6.45 km/s. (4) The average Pn velocity beneath South America is 8.00 km/s (s.d. ±0.23 km/s), slightly lower than the global average of 8.07 km/s. (5) A region across northern Chile and northeast Argentina has anomalously low P- and S-wave velocities in the crust. Geographically, this corresponds to the shallowly-subducted portion of the Nazca plate (the Pampean flat slab first described by Isacks et al., 1968), which is also a region of crustal extension. (6) The thick crust of the Brazilian craton appears to extend into Venezuela and Colombia. (7) The crust in the Amazon basin and along the western edge of the Brazilian craton may be thinned by extension. (8) The average crustal P-wave velocity under the eastern Pacific seafloor is higher than under the western Atlantic seafloor, most likely due to the thicker sediment layer on the older Atlantic seafloor.

  13. Growth response of a deep-water ferromanganese crust to evolution of the Neogene Indian Ocean

    USGS Publications Warehouse

    Banakar, V.K.; Hein, J.R.

    2000-01-01

    A deep-water ferromanganese crust from a Central Indian Ocean seamount dated previously by 10Be and 230Th(excess) was studied for compositional and textural variations that occurred throughout its growth history. The 10Be/9Be dated interval (upper 32 mm) yields an uniform growth rate of 2.8 ?? 0.1 mm/Ma [Frank, M., O'Nions, R.K., 1998. Sources of Pb for Indian Ocean ferromanganese crusts: a record of Himalayan erosion. Earth Planet. Sci. Lett., 158, pp. 121-130.] which gives an extrapolated age of ~ 26 Ma for the base of the crust at 72 mm and is comparable to the maximum age derived from the Co-model based growth rate estimates. This study shows that Fe-Mn oxyhydroxide precipitation did not occur from the time of emplacement of the seamount during the Eocene (~ 53 Ma) until the late Oligocene (~ 26 Ma). This paucity probably was the result of a nearly overlapping palaeo-CCD and palaeo-depth of crust formation, increased early Eocene productivity, instability and reworking of the surface rocks on the flanks of the seamount, and lack of oxic deep-water in the nascent Indian Ocean. Crust accretion began (older zone) with the formation of isolated cusps of Fe-Mn oxide during a time of high detritus influx, probably due to the early-Miocene intense erosion associated with maximum exhumation of the Himalayas (op. cit.). This cuspate textured zone extends from 72 mm to 42 mm representing the early-Miocene period. Intense polar cooling and increased mixing of deep and intermediate waters at the close of the Oligocene might have led to the increased oxygenation of the bottom-water in the basin. A considerable expansion in the vertical distance between the seafloor depth and the CCD during the early Miocene in addition to the influx of oxygenated bottom-water likely initiated Fe-Mn crust formation. Pillar structure characterises the younger zone, which extends from 40 mm to the surface of the crust, i.e., ~ 15 Ma to Present. This zone is characterised by > 25% higher

  14. Pliocene granodioritic knoll with continental crust affinities discovered in the intra-oceanic Izu-Bonin-Mariana Arc: Syntectonic granitic crust formation during back-arc rifting

    NASA Astrophysics Data System (ADS)

    Tani, Kenichiro; Dunkley, Daniel J.; Chang, Qing; Nichols, Alexander R. L.; Shukuno, Hiroshi; Hirahara, Yuka; Ishizuka, Osamu; Arima, Makoto; Tatsumi, Yoshiyuki

    2015-08-01

    A widely held hypothesis is that modern continental crust of an intermediate (i.e. andesitic) bulk composition forms at intra-oceanic arcs through subduction zone magmatism. However, there is a critical paradox in this hypothesis: to date, the dominant granitic rocks discovered in these arcs are tonalite, rocks that are significantly depleted in incompatible (i.e. magma-preferred) elements and do not geochemically and petrographically represent those of the continents. Here we describe the discovery of a submarine knoll, the Daisan-West Sumisu Knoll, situated in the rear-arc region of the intra-oceanic Izu-Bonin-Mariana Arc. Remotely-operated vehicle surveys reveal that this knoll is made up entirely of a 2.6 million year old porphyritic to equigranular granodiorite intrusion with a geochemical signature typical of continental crust. We present a model of granodiorite magma formation that involves partial remelting of enriched mafic rear-arc crust during the initial phase of back-arc rifting, which is supported by the preservation of relic cores inherited from initial rear-arc source rocks within magmatic zircon crystals. The strong extensional tectonic regime at the time of intrusion may have allowed the granodioritic magma to be emplaced at an extremely shallow level, with later erosion of sediment and volcanic covers exposing the internal plutonic body. These findings suggest that rear-arc regions could be the potential sites of continental crust formation in intra-oceanic convergent margins.

  15. Hydrothermal heat flux through aged oceanic crust: where does the heat escape?

    NASA Astrophysics Data System (ADS)

    Villinger, Heinrich; Grevemeyer, Ingo; Kaul, Norbert; Hauschild, Jan; Pfender, Marion

    2002-08-01

    Recent publications suggest that most of the fluid flow in the upper oceanic crust is channelized through small volumes of rock and vented into the ocean. This implies that at flanks of generally thinly sedimented mid-ocean ridges, focused discharge at the seafloor should be concentrated most likely at outcrops, high-angle normal faults or seamounts. These vents should be associated with a significant heat flow signature. However, only few observations worldwide support this assumption up to now. On our quest for focused fluid exchange between young oceanic crust and the ocean we surveyed a 720 km long and 40-90 km wide off-axis portion of seafloor intersecting the East Pacific Rise near 14°14'S. A wealth of geophysical methods including high-resolution swath mapping bathymetry, single channel seismics, sediment echo sounding, magnetics and heat flow determinations were used. Heat flow data in the tectonic corridor cover crustal ages of 0.3-9.3 Ma. With respect to the conductive plate cooling model the data show the well-known pattern of low values close to the ridge, associated with vigorous hydrothermal circulation of cold seawater through the young upper crust, and a fast recovery to almost lithospheric conductive cooling values at a surprisingly young crustal age of 9.3 Ma. Although the sediment cover is fairly thin, measurements with a 3.6 m violin bow type heat probe were possible almost everywhere within the investigated area. A detailed survey between two large seamounts at 4.5 Ma revealed localized extremely high values of up to 618 mW/m 2 (275% of the expected heat flow) at the foot of the seamount. This is interpreted as a clear indication of focused discharge of hydrothermal fluid. If we, however, relate heat flow normalized by the expected conductive heat loss to the character of igneous basement, heat flow is highest in areas with an almost flat and sedimented basement, and lowest within ˜10-20 km of seamounts and other rough basement relief. We

  16. Nickel isotopic compositions of ferromanganese crusts and the constancy of deep ocean inputs and continental weathering effects over the Cenozoic

    NASA Astrophysics Data System (ADS)

    Gall, L.; Williams, H. M.; Siebert, C.; Halliday, A. N.; Herrington, R. J.; Hein, J. R.

    2013-08-01

    The global variability in nickel (Ni) isotope compositions in ferromanganese crusts is investigated by analysing surface samples of 24 crusts from various ocean basins by MC-ICPMS, using a double-spike for mass bias correction. Ferromanganese crusts have δ60Ni isotopic compositions that are significantly heavier than any other samples thus far reported (-0.1‰ to 0.3 ‰), with surface scrapings ranging between 0.9 ‰ and 2.5 ‰ (relative to NIST SRM986). There is no well resolved difference between ocean basins, although the data indicate somewhat lighter values in the Atlantic than in the Pacific, nor is there any evidence that the variations are related to biological fractionation, presence of different water masses, or bottom water redox conditions. Preliminary data for laterite samples demonstrate that weathering is accompanied by isotopic fractionation of Ni, which should lead to rivers and seawater being isotopically heavy. This is consistent with the slightly heavier than average isotopic compositions recorded in crusts that are sampled close to continental regions. Furthermore, the isotopic compositions of crusts growing close to a hydrothermal source are clustered around ∼ 1.5 ‰, suggesting that hydrothermal fluids entering the ocean may have a Ni isotopic composition similar to this value. Based on these data, the heavy Ni isotopic compositions of ferromanganese crusts are likely due to input of isotopically heavy Ni to the ocean from continental weathering and possibly also from hydrothermal fluids. A depth profile through one crust, CD29-2, from the north central Pacific Ocean displays large variations in Ni isotope composition (1.1 - 2.3 ‰) through the last 76 Myr. Although there may have been some redistribution of Ni associated with phosphatisation, there is no systematic difference in Ni isotopic composition between deeper, older parts and shallower, younger parts of the crust, which may suggest that oceanic sources and sinks of Ni have

  17. Validation of the BASALT model for simulating off-axis hydrothermal circulation in oceanic crust

    NASA Astrophysics Data System (ADS)

    Farahat, Navah X.; Archer, David; Abbot, Dorian S.

    2017-08-01

    Fluid recharge and discharge between the deep ocean and the porous upper layer of off-axis oceanic crust tends to concentrate in small volumes of rock, such as seamounts and fractures, that are unimpeded by low-permeability sediments. Basement structure, sediment burial, heat flow, and other regional characteristics of off-axis hydrothermal systems appear to produce considerable diversity of circulation behaviors. Circulation of seawater and seawater-derived fluids controls the extent of fluid-rock interaction, resulting in significant geochemical impacts. However, the primary regional characteristics that control how seawater is distributed within upper oceanic crust are still poorly understood. In this paper we present the details of the two-dimensional (2-D) BASALT (Basement Activity Simulated At Low Temperatures) numerical model of heat and fluid transport in an off-axis hydrothermal system. This model is designed to simulate a wide range of conditions in order to explore the dominant controls on circulation. We validate the BASALT model's ability to reproduce observations by configuring it to represent a thoroughly studied transect of the Juan de Fuca Ridge eastern flank. The results demonstrate that including series of narrow, ridge-parallel fractures as subgrid features produces a realistic circulation scenario at the validation site. In future projects, a full reactive transport version of the validated BASALT model will be used to explore geochemical fluxes in a variety of off-axis hydrothermal environments.

  18. Boron isotope geochemistry of the oceanic crust from DSDP/ODP Hole 504B

    SciTech Connect

    Ishikawa, Tsuyoshi; Nakamura, Eizo )

    1992-04-01

    Boron contents and boron isotopic compositions were determined for the uppermost 1.3 km section of typical 6.2 Ma oceanic crust from DSDP/ODP Hole 504B, Costa Rica Rift, Galapagos Spreading Center. Both the boron content and the {delta}{sup 11}B value in the oceanic crust are controlled by two types of alteration: (1) low-temperature alteration (0 to 60C; Zones 1 and 2) and (2) high-temperature hydrothermal alteration (200 to 400C; Zones 3 and 4). Basalts subjected to the low-temperature alteration are characterized by their relatively high boron contents (0.69 to 19.3 ppm) and high {delta}{sup 11}B values (+2.2 to +10.6{per thousand}), indicating uptake of boron into secondary phases in equilibrium with seawater or evolved seawater. Hydrothermally altered basalts contain less abundant boron (0.17 to 0.52 ppm) and relatively constant {delta}{sup 11}B values ({minus}0.1 to +1.0{per thousand}). Although basalts from the upper part of these hydrothermal zones (< 1,300 mbsf) show equilibrated boron content and {delta}{sup 11}B value with aqueous fluid, effective leaching of boron from basalt is predominant in the lower part (> 1,300 mbsf). Original boron content and {delta}{sup 11}B value of the Hole 504B MORB were 0.35 ppm and +0.2{per thousand}, respectively. The present data provide fundamental information in understanding of the distribution of boron and boron isotopes in the oceanic crust.

  19. Boron contents and isotopic compositions of the hydrothermally altered oceanic crust from the Troodos ophiolite, Cyprus

    NASA Astrophysics Data System (ADS)

    Matsukura, S.; Yamaoka, K.; Ishikawa, T.; Kawahata, H.

    2010-12-01

    The boron contents and isotopic compositions were determined for the hydrothermally altered oceanic crust through the Troodos ophiolite. The samples were represented by the International Crustal Research Drilling Group (ICRDG) drill-Holes CY1 (479m), CY2A (689m), CY4 (2263m), and selected outcrops along the Akaki river. Hole CY1 was composed upper and lower pillow lava, CY4 constituted sheeted dike complex and gabbro section, and the samples along Akaki river formed from pillow lava to sheeted dike complex. Hole CY2A was composed pillow lava and sheeted dike, drilled near Agrokipia ‘B’ deposit a stockwork type which completely enclosed within the lower pillow lava. The goal of this study is to understand the Boron geochemistry during hydrothermal alteration of the oceanic crust including hydrothermal ore deposit as Agrokipia ‘B’. The average boron contents of each sequence from Troodos ophiolite were pillow lava (63.2ppm), sheeted dike complex (4.5ppm), gabbro section (1.6ppm). But then, those of Oman ophiolite were 7.9ppm, 5.3ppm, 1.7ppm (Yamaoka et al., 2010 submitted). Thus, both of these ophiolites, the vertical profile of boron content decreased with depth, also the boron contents were much richer than fresh-MORB (0.5ppm) (Spivack and Edmond, 1987; Chaussidon and Jambon, 1994). This indicates boron rich of the altered oceanic crust were derived from seawater. And sheeted dike complex and gabbro section were similar value relatively, but pillow lava differed widely. These results may represent the difference of length being submarine, because these ophiolites were generated in deep water of the Tethys sea about 90Ma (Late Cretaceous) (Tilton et al., 1981; Mukasa and Ludden, 1987), and Oman ophiolite was obducted about 70Ma (Lanphere, 1981) but Troodos ophiolite uplifted about 10Ma (Middle Miocene) (Robertson and Woodcock, 1979).

  20. Thermal diffusion of the lunar magma ocean and the formation of the lunar crust

    NASA Astrophysics Data System (ADS)

    Zhu, D.; Wang, S.

    2010-12-01

    The magma ocean hypothesis is consistent with several lines of evidence including planet formation, core-mantle differentiation and geochemical observations, and it is proved as an inevitable stage in the early evolution of planets. The magma ocean is assumed to be homogeneous in previous models during solidification or crystallization[1]. Based on the recent advance and our new data in experimental igneous petrology[2], we question this assumption and propose that an gabbrotic melt, from which the anorthositic lunar crust crystallized, can be produced by thermal diffusion, rather than by magma fractionation. This novel model can provide explanations for the absence of the advection in lunar magma ocean[3] and the old age of the anorthositic lunar crust[4-5]. 1. Solomatov, V., Magma Oceans and Primordial Mantle Differentiation, in Treatise on Geophysics, S. Gerald, Editor. 2007, Elsevier: Amsterdam. p. 91-119. 2. Huang, F., et al., Chemical and isotopic fractionation of wet andesite in a temperature gradient: Experiments and models suggesting a new mechanism of magma differentiation. Geochimica Et Cosmochimica Acta, 2009. 73(3): p. 729-749. 3. Turcotte, D.L. and L.H. Kellogg, Implications of isotope data for the origin of the Moon, in Origin of the Moon, W.K. Hartmann, R.J. Phillips, and G.J. Taylor, Editors. 1986, Lunar and Planet. Inst.: Houston, TX. p. 311-329. 4. Alibert, C., M.D. Norman, and M.T. McCulloch, An ancient Sm-Nd age for a ferroan noritic anorthosite clast from lunar breccia 67016. Geochimica Et Cosmochimica Acta, 1994. 58(13): p. 2921-2926. 5. Touboul, M., et al., Tungsten isotopes in ferroan anorthosites: Implications for the age of the Moon and lifetime of its magma ocean. Icarus, 2009. 199(2): p. 245-249.

  1. Upper Paleozoic oceanic crust in the Polish Sudetes: NdSr isotope and trace element evidence

    NASA Astrophysics Data System (ADS)

    Pin, C.; Majerowicz, A.; Wojciechowska, I.

    1988-03-01

    The two main mafic-ultramafic complexes outcropping in the Polish Sudetes around the Sowie Góry high-grade massif (Mt. Sleza and Nowa Ruda) have been studied for trace elements and NdSr isotopes. Upper Paleozoic SmNd whole-rock isochron ages are obtained for both massifs: 353 ± 21 Ma (2σ) with ɛi = + 8.8 ± 0.1 (Mt. Sleza) and 351 ± 16 Ma with ɛi = 8.5 ± 0.1 (Nowa Ruda). The high initial ɛNd point to a source characterized by a strong time-integrated LREE depletion and, along with incompatible trace-element patterns, they substantiate the oceanic derivation of these massifs. Although a subduction-related marginal basin setting cannot be totally precluded, trace-element and isotopic data rather suggest a normal mid-ocean ridge origin for the Sudetic ophiolites. While these results are in marked contrast with previously inferred Precambrian or early Paleozoic ages, remnants of oceanic crust as young as the Early Carboniferous are consistent with the local sedimentary record of pelagic facies from the Frasnian to the Tournaisian, and they may provide a suitable explanation for the contrasted evolution displayed by different domains of the Sudetes. The Sudetic ophiolites might tentatively be correlated with the upper Paleozoic "prasinites" of the Saxon Lineament if a Late Hercynian ca. 150 km dextral offset is assumed along the Elbe Fracture. In that case, they might represent the latest-stage remnants of an oceanic suture zone between the Saxo-Thuringian and Moldanubian zones. Together with several other examples throughout Europe, these ophiolites provide compelling evidence for the involvement of oceanic crust and plate-tectonic processes in the Variscan belt.

  2. The Jamestown Ophiolite Complex, Barberton mountain belt - A section through 3.5 Ga oceanic crust

    NASA Astrophysics Data System (ADS)

    de Wit, Maarten J.; Hart, Roger A.; Hart, Rodger J.

    The Jamestown Ophiolite Complex of the Barberton greenstone belt, South Africa, is investigated, and the intrusive nature of mafic-ultramafic units from the Komati and Kromberg formations into overlying pillow lavas and sediments is documented. Evidence is presented for multiple intrusive events within the igneous sections, including crosscutting intrusives, multiple injection of magma in the Komati section, and sheeted intrusions in the Kromberg section. The thinness of the Jamestown complex suggests that, locally at least, the ca 3.5 Ga oceanic crust was also thin, consistent with the regionally extensive metasomatic alteration.

  3. Isotopic constraints on the formation of carbonates during low-temperature hydrothermal oceanic crust alteration

    NASA Astrophysics Data System (ADS)

    Stolper, D. A.; Antonelli, M. A.; Ramos, D. S.; Bender, M. L.; Schrag, D. P.; DePaolo, D. J.; Higgins, J. A.

    2016-12-01

    Low temperature (<100°C) water-rock reactions in oceanic crust have a potentially large influence on seawater chemical compositions and atmospheric pCO2. Quantification of the conditions (e.g., temperature) of oceanic crust alteration is needed to evaluate its importance for global silicate weathering fluxes. The isotopic and chemical compositions of secondary carbonates in oceanic crust reflect the temperature and chemistry of the circulating fluid and thus are used to reconstruct past conditions of crustal alteration. For example, temperatures are calculated via carbonate δ18O thermometry using measured δ18Ocarb vs. assumed δ18Ofluid. δ18Ofluid is usually assumed to be the seawater value at the time of carbonate formation. We present measured clumped-isotope temperatures (Tclump) and δ18O, δ13C, δ44Ca, and 87Sr/86Sr values of Jurassic carbonates from altered oceanic crust (ODP Site 801). Tclump measured at Caltech ranges from 24-51°C. Calculated δ18Ofluid (based on Tclump and δ18Ocarb) ranges from -0.4‰ (±0.4, 1σ) to -3.5‰ (±0.6). Higher temperatures correlate with lower δ18Ofluid (R2 = 0.75). This suggests that at elevated temperatures, δ18Ofluid was modified away from seawater values, likely via the preferential incorporation of 18O vs. 16O into secondary minerals relative to water. This indicates that δ18Ofluid values of circulating fluids are not necessarily identical to seawater δ18O. Tclump measurements are being replicated at Harvard for further verification. Carbonates with δ13C indicating a seawater C source (δ13C > 0‰) have average δ44Ca (relative to modern seawater) of -0.84‰ (±0.08). This is indistinguishable from igneous rock δ44Ca and suggests that carbonate Ca is derived from igneous Ca released during crustal alteration. Carbonates with δ13C indicating an organic C source (δ13C < -2.5‰) have lower δ44Cacarb (< -1‰). Carbonate 87Sr/86Sr ranges from 0.70742 to 0.70656. Based on the seawater 87Sr/86Sr curve, this

  4. Thorium isotope evidence for melting of the mafic oceanic crust beneath the Izu arc

    NASA Astrophysics Data System (ADS)

    Freymuth, Heye; Ivko, Ben; Gill, James B.; Tamura, Yoshihiko; Elliott, Tim

    2016-08-01

    We address the question of whether melting of the mafic oceanic crust occurs beneath ordinary volcanic arcs using constraints from U-Series (238U/232Th, 230Th/232Th and 226Ra/230Th) measurements. Alteration of the top few hundred meters of the mafic crust leads to strong U enrichment. Via decay of 238U to 230Th, this results in elevated (230Th/232Th) (where brackets indicate activity ratios) over time-scales of ∼350 ka. This process leads to the high (230Th/232Th), between 2.6 and 11.0 in the mafic altered oceanic crust (AOC) sampled at ODP Sites 801 and 1149 near the Izu-Bonin-Mariana arc. Th activity ratios in the Izu arc lavas range from (230Th/232Th) = 1.2-2.0. These values are substantially higher than those in bulk sediment subducting at the Izu trench and also extend to higher values than in mid-ocean ridge basalts and the Mariana arc. We show that the range in Th isotope ratios in the Izu arc lavas is consistent with the presence of a slab melt from a mixed source consisting of AOC and subducted sediments with an AOC mass fraction of up to approximately 80 wt.% in the component added to the arc lava source. The oceanic plate subducting at the Izu arc is comparatively cold which therefore indicates that temperatures high enough for fluid-saturated melting of the AOC are commonly achieved beneath volcanic arcs. The high ratio of AOC/sediments of the slab melt component suggested for the Izu arc lavas requires preferential melting of the AOC. This can be achieved when fluid-saturated melting of the slab is triggered by fluids derived from underlying subducted serpentinites. Dehydration of serpentinites and migration of the fluid into the overlying crust causes melting to start within the AOC. The absence of a significant sediment melt component suggests there was insufficient water to flux both AOC and overlying sediments.

  5. The Jamestown Ophiolite Complex, Barberton mountain belt - A section through 3.5 Ga oceanic crust

    NASA Technical Reports Server (NTRS)

    De Wit, Maarten J.; Hart, Roger A.; Hart, Rodger J.

    1987-01-01

    The Jamestown Ophiolite Complex of the Barberton greenstone belt, South Africa, is investigated, and the intrusive nature of mafic-ultramafic units from the Komati and Kromberg formations into overlying pillow lavas and sediments is documented. Evidence is presented for multiple intrusive events within the igneous sections, including crosscutting intrusives, multiple injection of magma in the Komati section, and sheeted intrusions in the Kromberg section. The thinness of the Jamestown complex suggests that, locally at least, the ca 3.5 Ga oceanic crust was also thin, consistent with the regionally extensive metasomatic alteration.

  6. Early evolution of the continental crust, the oxygenated atmosphere and oceans, and the heterogeneous mantle

    NASA Astrophysics Data System (ADS)

    Ohmoto, H.

    2011-12-01

    The current paradigm for the evolution of early Earth is that, only since ~2.5 Ga ago, the Earth began to: (a) form a large granitic continental crust; (b) form an oxygenated atmosphere; (c) operate oxidative weathering of rocks on land; (d) form Fe-poor, but S-, U- and Mo-rich, oceans; (e) operate large-scale transfers of elements between oceans and oceanic crust at MORs; (f) subduct the altered oceanic crust; (g) create the mantle heterogeneity, especially in the concentrations and isotopic compositions of Fe(III), Fe(II), U, Pb, alkali elements, C, S, REEs, and many other elements; (h) create chemical and isotopic variations among OIB-, OPB-, and MORB magmas, and between I- and S-type granitoid magmas; and (i) create variations in the chemical and isotopic compositions of volcanic gas. Submarine hydrothermal fluids have typically developed from seawater-rock interactions during deep (>2 km) circulation of seawater through underlying hot volcanic rocks. When the heated hydrothermal fluids ascend toward the seafloor, they mix with local bottom seawater to precipitate a variety of minerals on and beneath the seafloor. Thus, the mineralogy and geochemistry of submarine hydrothermal deposits and associated volcanic rocks can be used to decipher the chemistry of the contemporaneous seawater, which in turn indicate the chemistry of the atmosphere and the compositions and size of the continental crust. The results of mineralogical and geochemical investigations by our and other research groups on submarine hydrothermal deposits (VMS and BIF) and hydrothermally-altered submarine volcanic rocks in Australia, South Africa, and Canada, ~3.5-2.5 Ga in ages, suggest that the above processes (a)-(i) had began by ~3.5 Ga ago. Supportive evidence includes, but not restricted to, the similarities between Archean submarine rocks and modern ones in: (1) the abundance of ferric oxides; (2) the Fe(III)/Fe(I) ratios; (3) the abundance of barite; (4) the increased Li contents; (5) the

  7. Constraints on cooling of the lower ocean crust from epidote veins in the Wadi Gideah section, Oman Ophiolite

    NASA Astrophysics Data System (ADS)

    Diehl, Alexander; Bieseler, Bastian; Bach, Wolfgang

    2017-04-01

    Determining the depth, extent, and timing of high-temperature hydrothermal alteration in the ocean crust is key to understanding how the lower oceanic crust is cooled. We report data from 18 epidote veins from the Wadi Gideah section in the Wadi Tayin block, which is a reference section for alteration of the lower crust formed at a fast oceanic spreading center. 87Sr/86Sr ratios feature a narrow range from 0.70429 to 0.70512, while O isotope compositions vary between - 0.7 and +4.9‰ in δ18OSMOW. These compositions indicate uniform water-rock ratios between 1 and 2 and formation temperatures in the range of 300 to 450˚ C. There is no systematic trend in Sr and O isotope compositions down section. Fluid inclusion entrapment temperatures for a subset of four samples linearly increase from 338˚ C to 465˚ C in lowermost 3 km of crust of the Wadi Gideah section. Salinities are uniform throughout and scatter closely around seawater values. We developed a numerical cooling model to assign possible crustal ages to the thermal gradients observed. For pure conductive cooling, these ages range between 4 and 20 Ma. Our thermal model runs with a high Nusselt number (Nu) of 20 down to the base of the crust indicate that the epidote veins may record this near-axial deep circulation in crust of only 0.1 Ma (5-7 km off axis). When off-axis circulation is shut off in the more distal flanks, however, massive conductive reheating of the lower crust by as much as 200˚ C is predicted to take place. But there is no evidence for prograde metamorphic reactions in the samples we studied (or other hydrothermally altered oceanic gabbros). An intermediate model, in which Nu is 20 down to 2 km for the first 0.1 Ma and Nu is then 4 down to 6.5 km depth off axis to 1 Ma, is consistent with the permeability distribution within the ocean crust and predicts a thermal gradient for the lower crust that matches the observed one for ages between 1 and 3 Ma. The most plausible explanation for the

  8. 238U-234U-230Th disequilibrium in hydrogenous oceanic Fe-Mn crusts: Palaeoceanographic record or diagenetic alteration?

    USGS Publications Warehouse

    Chabaux, F.; O'Nions, R. K.; Cohen, A.S.; Hein, J.R.

    1997-01-01

    A detailed TIMS study of (234Uexc/238U), (230Th/232Th), and Th/U ratios have been performed on the outermost margin of ten hydrogenous Fe-Mn crusts from the equatorial Pacific Ocean and west-central Indian Ocean. Th/U concentration ratios generally decrease from the crust's surface down to 0.5-1 mm depth and growth rates estimated by uranium and thorium isotope ratios are significantly different in Fe-Mn crusts from the Peru Basin and the west-central Indian Ocean. Fe-Mn crusts from the same geographical area define a single trend in plots of Ln (234Uexc/238U) vs. Ln(230Th/232Th) and Th/U ratios vs. age of the analysed fractions. Results suggest that (1) hydrogenous Fe-Mn crusts remain closed-systems after formation, and consequently (2) the discrepancy observed between the 230Th and 234U chronometers in Fe-Mn crusts, and the variations of the Th/U ratios through the margin of Fe-Mn crusts, are not due to redistribution of uranium and thorium isotopes after oxyhydroxide precipitation, but rather to temporal variations of both Th/U and initial thorium activity ratios recorded by the Fe-Mn layers. Implications of these observations for determination of Fe-Mn crust growth-rates are discussed. Variations of both Th/U and initial Th activity ratios in Fe-Mn crusts might be related to changes in particle input to seawater and/or changes in ocean circulation during the last 150 ka. Copyright ?? 1997 Elsevier Science Ltd.

  9. Seismic wave velocity of rocks in the Oman ophiolite: constraints for petrological structure of oceanic crust

    NASA Astrophysics Data System (ADS)

    Saito, S.; Ishikawa, M.; Shibata, S.; Akizuki, R.; Arima, M.; Tatsumi, Y.; Arai, S.

    2010-12-01

    Evaluation of rock velocities and comparison with velocity profiles defined by seismic refraction experiments are a crucial approach for understanding the petrological structure of the crust. In this study, we calculated the seismic wave velocities of various types of rocks from the Oman ophiolite in order to constrain a petrological structure of the oceanic crust. Christensen & Smewing (1981, JGR) have reported experimental elastic velocities of rocks from the Oman ophiolite under oceanic crust-mantle conditions (6-430 MPa). However, in their relatively low-pressure experiments, internal pore-spaces might affect the velocity and resulted in lower values than the intrinsic velocity of sample. In this study we calculated the velocities of samples based on their modal proportions and chemical compositions of mineral constituents. Our calculated velocities represent the ‘pore-space-free’ intrinsic velocities of the sample. We calculated seismic velocities of rocks from the Oman ophiolite including pillow lavas, dolerites, plagiogranites, gabbros and peridotites at high-pressure-temperature conditions with an Excel macro (Hacker & Avers 2004, G-cubed). The minerals used for calculations for pillow lavas, dolerites and plagiogranites were Qtz, Pl, Prh, Pmp, Chl, Ep, Act, Hbl, Cpx and Mag. Pl, Hbl, Cpx, Opx and Ol were used for the calculations for gabbros and peridotites. Assuming thermal gradient of 20° C/km and pressure gradient of 25 MPa/km, the velocities were calculated in the ranges from the atmospheric pressure (0° C) to 200 MPa (160° C). The calculation yielded P-wave velocities (Vp) of 6.5-6.7 km/s for the pillow lavas, 6.6-6.8 km/s for the dolerites, 6.1-6.3 km/s for the plagiogranites, 6.9-7.5 km/s for the gabbros and 8.1-8.2 km/s for the peridotites. On the other hand, experimental results reported by Christensen & Smewing (1981, JGR) were 4.5-5.9 km/s for the pillow lavas, 5.5-6.3 km/s for the dolerites, 6.1-6.3 km/s for the plagiogranites, 6

  10. Changes in erosion and ocean circulation recorded in the Hf isotopic compositions of North Atlantic and Indian Ocean ferromanganese crusts

    USGS Publications Warehouse

    Piotrowski, Alexander M.; Lee, Der-Chuen; Christensen, John N.; Burton, Kevin W.; Halliday, Alex N.; Hein, James R.; Günther, Detlef

    2000-01-01

    High-resolution Hf isotopic records are presented for hydrogenetic Fe–Mn crusts from the North Atlantic and Indian Oceans. BM1969 from the western North Atlantic has previously been shown to record systematically decreasing Nd isotopic compositions from about 60 to ∼4 Ma, at which time both show a rapid decrease to unradiogenic Nd composition, thought to be related to the increasing influence of NADW or glaciation in the northern hemisphere. During the Oligocene, North Atlantic Hf became progressively less radiogenic until in the mid-Miocene (∼15 Ma) it reached +1. It then shifted gradually back to an ϵHf value of +3 at 4 Ma, since when it has decreased rapidly to about −1 at the present day. The observed shifts in the Hf isotopic composition were probably caused by variation in intensity of erosion as glaciation progressed in the northern hemisphere. Ferromanganese crusts SS663 and 109D are from about 5500 m depth in the Indian Ocean and are now separated by ∼2300 km across the Mid-Indian Ridge. They display similar trends in Hf isotopic composition from 20 to 5 Ma, with the more northern crust having a composition that is consistently more radiogenic (by ∼2 ϵHf units). Paradoxically, during the last 20 Ma the Hf isotopic compositions of the two crusts have converged despite increased separation and subsidence relative to the ridge. A correlatable negative excursion at ∼5 Ma in the two records may reflect a short-term increase in erosion caused by the activation of the Himalayan main central thrust. Changes to unradiogenic Hf in the central Indian Ocean after 5 Ma may alternatively have been caused by the expanding influence of NADW into the Mid-Indian Basin via circum-Antarctic deep water or a reduction of Pacific flow through the Indonesian gateway. In either case, these results illustrate the utility of the Hf isotope system as a tracer of paleoceanographic changes, capable of responding to subtle changes in erosional regime not readily resolved

  11. Archaean zircons in Miocene oceanic hotspot rocks establish ancient continental crust beneath Mauritius

    PubMed Central

    Ashwal, Lewis D.; Wiedenbeck, Michael; Torsvik, Trond H.

    2017-01-01

    A fragment of continental crust has been postulated to underlie the young plume-related lavas of the Indian Ocean island of Mauritius based on the recovery of Proterozoic zircons from basaltic beach sands. Here we document the first U–Pb zircon ages recovered directly from 5.7 Ma Mauritian trachytic rocks. We identified concordant Archaean xenocrystic zircons ranging in age between 2.5 and 3.0 Ga within a trachyte plug that crosscuts Older Series plume-related basalts of Mauritius. Our results demonstrate the existence of ancient continental crust beneath Mauritius; based on the entire spectrum of U–Pb ages for old Mauritian zircons, we demonstrate that this ancient crust is of central-east Madagascar affinity, which is presently located ∼700 km west of Mauritius. This makes possible a detailed reconstruction of Mauritius and other Mauritian continental fragments, which once formed part of the ancient nucleus of Madagascar and southern India. PMID:28140395

  12. Statistical averaging of marine magnetic anomalies and the aging of oceanic crust.

    USGS Publications Warehouse

    Blakely, R.J.

    1983-01-01

    Visual comparison of Mesozoic and Cenozoic magnetic anomalies in the North Pacific suggests that older anomalies contain less short-wavelength information than younger anomalies in this area. To test this observation, magnetic profiles from the North Pacific are examined from crust of three ages: 0-2.1, 29.3-33.1, and 64.9-70.3Ma. For each time period, at least nine profiles were analyzed by 1) calculating the power density spectrum of each profile, 2) averaging the spectra together, and 3) computing a 'recording filter' for each time period by assuming a hypothetical seafloor model. The model assumes that the top of the source is acoustic basement, the source thickness is 0.5km, and the time scale of geomagnetic reversals is according to Ness et al. (1980). The calculated power density spectra of the three recording filters are complex in shape but show an increase of attenuation of short-wavelength information as the crust ages. These results are interpreted using a multilayer model for marine magnetic anomalies in which the upper layer, corresponding to pillow basalt of seismic layer 2A, acts as a source of noise to the magnetic anomalies. As the ocean crust ages, this noisy contribution by the pillow basalts becomes less significant to the anomalies. Consequently, magnetic sources below layer 2A must be faithful recorders of geomagnetic reversals.-AuthorPacific power density spectrum

  13. Archaean zircons in Miocene oceanic hotspot rocks establish ancient continental crust beneath Mauritius.

    PubMed

    Ashwal, Lewis D; Wiedenbeck, Michael; Torsvik, Trond H

    2017-01-31

    A fragment of continental crust has been postulated to underlie the young plume-related lavas of the Indian Ocean island of Mauritius based on the recovery of Proterozoic zircons from basaltic beach sands. Here we document the first U-Pb zircon ages recovered directly from 5.7 Ma Mauritian trachytic rocks. We identified concordant Archaean xenocrystic zircons ranging in age between 2.5 and 3.0 Ga within a trachyte plug that crosscuts Older Series plume-related basalts of Mauritius. Our results demonstrate the existence of ancient continental crust beneath Mauritius; based on the entire spectrum of U-Pb ages for old Mauritian zircons, we demonstrate that this ancient crust is of central-east Madagascar affinity, which is presently located ∼700 km west of Mauritius. This makes possible a detailed reconstruction of Mauritius and other Mauritian continental fragments, which once formed part of the ancient nucleus of Madagascar and southern India.

  14. High-Albedo Salt Crusts on the Tropical Ocean of Snowball Earth: Measurements and Modeling

    NASA Astrophysics Data System (ADS)

    Carns, R.; Light, B.; Warren, S. G.

    2014-12-01

    During a Snowball Earth event, almost all of the ocean surface first freezes as sea ice. As in modern sea ice, trapped inclusions of liquid brine permeate the ice cover. As the ice grows and cools, salt crystals precipitate within the inclusions. At -23C, the most abundant salt in seawater, sodium chloride, begins to precipitate as the dihydrate mineral hydrohalite (NaCl·2H2O). Crystals of hydrohalite within the sea ice scatter light. Measurements of cold, natural sea ice show a broadband albedo increase of 10-20% when salt precipitates. Such snow-free natural sea ice with a surface temperature below -23C is rare on modern Earth, but would have been common in tropical regions of a Snowball Earth where evaporation exceeded precipitation. The persistent cold and lack of summer melt on the Snowball ocean surface, combined with net evaporation, is hypothesized to yield lag deposits of hydrohalite crystals on the ice surface. To investigate this process, we prepared laboratory-grown sea ice in a 1000 liter tank in a walk-in freezer laboratory. The ice was cooled below -23 C and the surface sprayed with a 23% NaCl solution to create a layer of hydrohalite-enriched ice, a proxy for lag deposits that would have formed over long periods of surface sublimation. We have developed a novel technique for measuring the spectral albedo of ice surfaces in the laboratory; this technique was used to monitor the evolution of the surface albedo of our salt crust as the ice matrix sublimated away leaving a layer of fine-grained hydrohalite crystals. Measurements of this hydrohalite surface crust show a very high albedo, comparable to fresh snow at visible wavelengths and significantly larger than fresh snow at near infrared wavelengths. Broadband albedos are 0.55 for bare artificial sea ice at -30C, 0.75 for ice containing 25% hydrohalite by volume, 0.84 after five days of desiccation and 0.93 after 47 days of desiccation. Using our laboratory measurements, along with estimates of

  15. Information on stress conditions in the oceanic crust from oval fractures in a deep borehole

    USGS Publications Warehouse

    Morin, R.H.

    1990-01-01

    Oval images etched into the wall of a deep borehole were detected in DSDP Hole 504B, eastern equatorial Pacific Ocean, from analysis of an acoustic televiewer log. A systematic inspection of these ovals has identified intriguing consistencies in appearance that cannot be explained satisfactorily by a random, coincidental distribution of pillow lavas. As an alternative hypothesis, Mohr-Coulomb failure criterion is used to account for the generation and orientation of similarly curved, stress-induced fractures. Consequently, these oval features can be interpreted as fractures and related directly to stress conditions in the oceanic crust at this site. The azimuth of the oval center corresponds to the orientation of maximum horizontal principal stress (SH), and the oval width, which spans approximately 180?? of the borehole, is aligned with the azimuth of minimum horizontal principal stress (Sh). The oval height is controlled by the fracture angle and thus is a function of the coefficient of internal friction of the rock. -from Author

  16. Quantifying alteration in the ocean crust through the use of wireling logs.

    NASA Astrophysics Data System (ADS)

    Harvey, P. K.; Brewer, T. S.; Barr, S. R.

    2003-04-01

    Alteration has long been known as a characteristic of ocean crust, the degree of alteration apparently increasing with age. Sampling of alteration is often biased by the lack of recovered core, and consequently quantitative estimates of the extent of alteration are difficult to obtain. The products of alteration change the petrophysical character of fresh basalt in such a way that they can be detected by some downhole measurements. In this contribution we describe a novel method for quantifying the degree of alteration in an oceanic section. The method is based on increases in the neutron absorption cross-section, as measured by a number of logging tools, as alteration develops. This increase is mainly due to the addition of trace elements, particularly boron, during the alteration process. The technique is applied to two sections in ODP Hole 801, (Leg 185, Mariana-Izu-Bonin Convergent Margin).

  17. Concept of biogenic ferromanganese crust formation: coccoliths as bio-seeds in crusts from Central Atlantic Ocean (Senghor Seamount/Cape Verde).

    PubMed

    Wang, Xiaohong; Peine, Florian; Schmidt, Alexander; Schröder, Heinz C; Wiens, Matthias; Schlossmacher, Ute; Müller, Werner E G

    2011-05-01

    At depths of 2,000 to 3,000 m, seamounts from the Cape Verde archipelago (Central Atlantic Ocean) are largely covered with ferromanganese crusts. Here we studied 60 to 150 mm thick crusts from the Senghor Seamount (depth: 2257.4 m). The crusts have a non lamellated texture and are covered with spherical nodules. The chemical composition shows a dominance of MnO2 (26.1%) and Fe2O3 (38.8%) with considerable amounts of Co (0.74%) and TiO2 (2.1%). Analysis by scanning electron probe microanalyzer (EPMA) revealed a well defined compositional zonation of micro-layers; the distribution pattern of Mn does not match that of Fe. Analysis by high resolution scanning electron microscopy (SEM) revealed that coccospheres/coccoliths exist in the crust material as microfossils; most of the coccospheres/coccoliths are not intact. The almost circular coccoliths belong to the type of heterococcoliths and are taxonomically related to species of the family Calcidiscaceae. By energy dispersive X-ray spectroscopic analysis an accumulation of the coccoliths in the Mn- and Fe rich micronodules was detected. Focused ion beam assisted SEM mapping highlighted that the coccoliths in the crust are Mn rich, suggesting that the calcareous material of the algal skeleton has been replaced by Mn-minerals. We conclude that a biologically induced mechanism has been involved in the formation of the crusts, collected from the Cape Verde archipelago from depths of 2,000 to 3,000 m in the mixing region between the oxygen-minimum surface zone and the oxygen-rich deep waters; the deposition process might have been triggered by chemical reactions during the dissolution of the Ca-carbonate skeletons of the coccoliths allowing Mn(II) to oxidize to Mn(IV) and in turn to deposit this element in the crust material.

  18. First Investigation of the Microbiology of the Deepest Layer of Ocean Crust

    PubMed Central

    Mason, Olivia U.; Nakagawa, Tatsunori; Rosner, Martin; Van Nostrand, Joy D.; Zhou, Jizhong; Maruyama, Akihiko; Fisk, Martin R.; Giovannoni, Stephen J.

    2010-01-01

    The gabbroic layer comprises the majority of ocean crust. Opportunities to sample this expansive crustal environment are rare because of the technological demands of deep ocean drilling; thus, gabbroic microbial communities have not yet been studied. During the Integrated Ocean Drilling Program Expeditions 304 and 305, igneous rock samples were collected from 0.45-1391.01 meters below seafloor at Hole 1309D, located on the Atlantis Massif (30 °N, 42 °W). Microbial diversity in the rocks was analyzed by denaturing gradient gel electrophoresis and sequencing (Expedition 304), and terminal restriction fragment length polymorphism, cloning and sequencing, and functional gene microarray analysis (Expedition 305). The gabbroic microbial community was relatively depauperate, consisting of a low diversity of proteobacterial lineages closely related to Bacteria from hydrocarbon-dominated environments and to known hydrocarbon degraders, and there was little evidence of Archaea. Functional gene diversity in the gabbroic samples was analyzed with a microarray for metabolic genes (“GeoChip”), producing further evidence of genomic potential for hydrocarbon degradation - genes for aerobic methane and toluene oxidation. Genes coding for anaerobic respirations, such as nitrate reduction, sulfate reduction, and metal reduction, as well as genes for carbon fixation, nitrogen fixation, and ammonium-oxidation, were also present. Our results suggest that the gabbroic layer hosts a microbial community that can degrade hydrocarbons and fix carbon and nitrogen, and has the potential to employ a diversity of non-oxygen electron acceptors. This rare glimpse of the gabbroic ecosystem provides further support for the recent finding of hydrocarbons in deep ocean gabbro from Hole 1309D. It has been hypothesized that these hydrocarbons might originate abiotically from serpentinization reactions that are occurring deep in the Earth's crust, raising the possibility that the lithic microbial

  19. First investigation of the microbiology of the deepest layer of ocean crust.

    PubMed

    Mason, Olivia U; Nakagawa, Tatsunori; Rosner, Martin; Van Nostrand, Joy D; Zhou, Jizhong; Maruyama, Akihiko; Fisk, Martin R; Giovannoni, Stephen J

    2010-11-05

    The gabbroic layer comprises the majority of ocean crust. Opportunities to sample this expansive crustal environment are rare because of the technological demands of deep ocean drilling; thus, gabbroic microbial communities have not yet been studied. During the Integrated Ocean Drilling Program Expeditions 304 and 305, igneous rock samples were collected from 0.45-1391.01 meters below seafloor at Hole 1309D, located on the Atlantis Massif (30 °N, 42 °W). Microbial diversity in the rocks was analyzed by denaturing gradient gel electrophoresis and sequencing (Expedition 304), and terminal restriction fragment length polymorphism, cloning and sequencing, and functional gene microarray analysis (Expedition 305). The gabbroic microbial community was relatively depauperate, consisting of a low diversity of proteobacterial lineages closely related to Bacteria from hydrocarbon-dominated environments and to known hydrocarbon degraders, and there was little evidence of Archaea. Functional gene diversity in the gabbroic samples was analyzed with a microarray for metabolic genes ("GeoChip"), producing further evidence of genomic potential for hydrocarbon degradation--genes for aerobic methane and toluene oxidation. Genes coding for anaerobic respirations, such as nitrate reduction, sulfate reduction, and metal reduction, as well as genes for carbon fixation, nitrogen fixation, and ammonium-oxidation, were also present. Our results suggest that the gabbroic layer hosts a microbial community that can degrade hydrocarbons and fix carbon and nitrogen, and has the potential to employ a diversity of non-oxygen electron acceptors. This rare glimpse of the gabbroic ecosystem provides further support for the recent finding of hydrocarbons in deep ocean gabbro from Hole 1309D. It has been hypothesized that these hydrocarbons might originate abiotically from serpentinization reactions that are occurring deep in the Earth's crust, raising the possibility that the lithic microbial

  20. Thermal regime along the Antilles subduction zone: Influence of the oceanic lithosphere materials subducted in the oceanic crust

    NASA Astrophysics Data System (ADS)

    Biari, Youssef; Marcaillou, Boris; Klingelhoefer, Frauke; Francis, Lucazeau; Fréderique, Rolandone; Arnauld, Heuret; Thibaud, Pichot; Hélène, Bouquerel

    2017-04-01

    Heat-flow measurements acquired during the Antithesis Cruise in the Northern Lesser Antilles reveal an atypical heat-flow trend, from the trench to the margin forearc, where the subducting crust consists of exhumed and serpentinized mantle rocks (see Marcaillou et al. same session). We investigate the thermal structure of the Lesser Antilles subduction zone along two transects perpendicular to the margin located off Antigua and Martinique Islands. We perform 2-D steady-state finite elements thermal modelling constrained by newly-recorded and existing data: heat flow measurements, deep multichannel reflection and wide angle seismic data as well as earthquake hypocenters location at depth. Along the Martinique profile, the heat-flow decreases from the trench (45 mW.m-2) to minimum in the outer fore-arc (30 mW.m-2) and increases to a plateau (50 mW.m-2) toward the back-arc area. These trend and values are typical for the subduction of a steep 80-MYr old oceanic plate beneath an oceanic margin. As a result, the 150°-350°C temperature range along the interplate contact, commonly associated to the thermally-defined seismogenic zone, is estimated to be located between 200 - 350km from the trench. In contrast, along the Antigua profile, the heat-flow shows an atypical "flat" trend at 40 ± 15 mW.m-2 from the trench to the inner forearc. Purely conductive thermal models fail at fitting both the measured values and the flat trend. We propose that the subducting crust made of serpentinized exhumed mantle rock strongly affecting the heat-flow at the surface and the margin thermal structure. The geothermal gradient in the 5-km-thick serpentinized layer is expected to be low compared to "normal" oceanic crust because of cold water percolation and peridotite alteration. Moreover, from 50km depth, serpentine dehydration reactions provide significant amounts of hot water expelled toward the upper plate, generated heat beneath the forearc. As a result, in our preferred model: 1/ A

  1. Blocks in the Europan Crust Provide More Evidence of Subterranean Ocean

    NASA Technical Reports Server (NTRS)

    2000-01-01

    The image on the left shows a region of Europa's crust made up of blocks which are thought to have broken apart and 'rafted' into new positions. These features are the best geologic evidence to date that Europa may have had a subsurface ocean at some time in its past. Combined with the geologic data, the presence of a magnetic field leads scientists to believe an ocean is most likely present at Europa today. In this false color image, reddish-brown areas represent non-ice material resulting from geologic activity. White areas are rays of material ejected during the formation of the 25-km diameter impact crater Pwyll (see global view). Icy plains are shown in blue tones to distinguish possibly coarse-grained ice (dark blue) from fine-grained ice (light blue). Long, dark lines are ridges and fractures in the crust, some of which are more than 3,000 kilometers (1,850 miles) long. These images were obtained by NASA's Galileo spacecraft during September 7, 1996, December 1996, and February 1997 at a distance of 677,000 kilometers (417,489 miles).

  2. High resolution dating of young magmatic oceanic crust using near-seafloor magnetics

    NASA Astrophysics Data System (ADS)

    Dyment, J.; Kitazawa, M.; Hemond, C.; Guillou, H.; Chauvin, A.; Ravilly, M.; Honsho, C.

    2015-12-01

    We compare two independent dating methods on a section of oceanic crust created within the last million year on the Central Indian Ridge axis at 19°10'S, an area affected by the Reunion hotspot. First, near-seafloor magnetic anomalies display characteristic sequences of magnetic intensity variations that we confidently identified by comparison with published paleointensity curves for the Brunhes period and used as a dating tool. This approach is further confirmed by the linear trend relating the NRM (Natural Remanent Magnetization) and paleointensity measured on rock samples along the same section. Second, valid K-Ar and Ar-Ar ages are determined on enriched basalt samples collected by deep-sea submersible. They show an excellent coincidence with the magnetic ages and support the use of high-resolution, near-seafloor marine magnetic anomalies as an efficient tool to date the young magmatic oceanic crust, where radiometric methods are generally unpractical, with unprecedented resolution. The ages obtained on the CIR reveal a 150-200 kyr cyclicity in the magmatic and tectonic processes of seafloor formation, two ridge jumps of 2.5 km and 1.2 km, respectively, and a systematic spreading asymmetry in favor to the Indian flank which may result from the interaction of the CIR with the Reunion hotspot.

  3. Intraterrestrial life in igneous ocean crust: advances, technologies, and the future (Invited)

    NASA Astrophysics Data System (ADS)

    Edwards, K. J.; Wheat, C. G.

    2010-12-01

    The “next frontier” of scientific investigation in the deep sub-seafloor microbial biosphere lies in a realm that has been a completely unexplored until just the past decade: the igneous oceanic crust. Problems that have hampered exploration of the “hard rock” marine deep biosphere have revolved around sample access (hard rock drilling is technologically complex), contamination (a major hurdle), momentum (why take on this challenge when the relatively “easier” marine muds also have been a frontier) and suspicion that microbes in more readily accessed using (simpler) non-drilling technologies - like vents - are truly are endemic of subsurface clades/activities. Since the late 1990’s, however, technologies and resultant studies on microbes in the igneous ocean crust deep biosphere have risen sharply, and offer a new and distinct view on this biome. Moreover, microbiologists are now taking leading roles in technological developments that are critically required to address this biosphere - interfacing and collaborating closely with engineers, genomic biologists, geologists, seismologists, and geochemists to accomplish logistically complex and long-term studies that bring observatory research to this deep realm. The future of this field for the least decade is rich - opportunities abound for microbiologists to play new roles in how we study microbiology in the deep subsurface in an oceanographic and Earth system science perspective.

  4. Blocks in the Europan Crust Provide More Evidence of Subterranean Ocean

    NASA Technical Reports Server (NTRS)

    2000-01-01

    The image on the left shows a region of Europa's crust made up of blocks which are thought to have broken apart and 'rafted' into new positions. These features are the best geologic evidence to date that Europa may have had a subsurface ocean at some time in its past. Combined with the geologic data, the presence of a magnetic field leads scientists to believe an ocean is most likely present at Europa today. In this false color image, reddish-brown areas represent non-ice material resulting from geologic activity. White areas are rays of material ejected during the formation of the 25-km diameter impact crater Pwyll (see global view). Icy plains are shown in blue tones to distinguish possibly coarse-grained ice (dark blue) from fine-grained ice (light blue). Long, dark lines are ridges and fractures in the crust, some of which are more than 3,000 kilometers (1,850 miles) long. These images were obtained by NASA's Galileo spacecraft during September 7, 1996, December 1996, and February 1997 at a distance of 677,000 kilometers (417,489 miles).

  5. Positive geothermal anomalies in oceanic crust of Cretaceous age offshore Kamchatka

    NASA Astrophysics Data System (ADS)

    Delisle, G.

    2011-09-01

    Heat flow measurements were carried out in 2009 offshore Kamchatka during the German-Russian joint-expedition KALMAR. An area with elevated heat flow in oceanic crust of Cretaceous age - detected ~30 yr ago in the course of several Russian heat flow surveys - was revisited. One previous interpretation postulated anomalous lithospheric conditions or a connection between a postulated mantle plume at great depth (>200 km) as the source for the observed high heat flow. However, the positive heat flow anomaly - as our bathymetric data show - is closely associated with the fragmentation of the western flank of the Meiji Seamount into a horst and graben structure initiated during descent of the oceanic crust into the subduction zone offshore Kamchatka. This paper offers an alternative interpretation, which connects high heat flow primarily with natural convection of fluids in the fragmented rock mass and, as a potential additional factor, high rates of erosion, for which evidence is available from our collected bathymetric image. Given high erosion rates, warm rock material at depth rises to nearer the sea floor, where it cools and causes temporary elevated heat flow.

  6. Positive geothermal anomalies in oceanic crust of Cretaceous age offshore Kamchatka

    NASA Astrophysics Data System (ADS)

    Delisle, G.

    2011-05-01

    Heat flow measurements were carried out in 2009 offshore Kamchatka during the German-Russian joint-expedition KALMAR. An area with elevated heat flow in oceanic crust of Cretaceous age - detected ~30 years ago in the course of several Russian heat flow surveys - was revisited. One previous interpretation postulated anomalous lithospheric conditions or a connection between a postulated mantle plume at great depth (> 200 km) as the source for the observed high heat flow. However, the positive heat flow anomaly - as our bathymetric data show - is closely associated with the fragmentation of the western flank of the Meiji Seamount into a horst and graben structure, initiated during descend of the oceanic crust into the subduction zone offshore Kamchatka. This paper offers an alternative interpretation, which connects high heat flow primarily with natural convection of fluids in the fragmented rock mass and, as a potential additional factor, high rates of erosion, for which evidence is available from our collected bathymetric image. Given high erosion rates, warm rock material at depth rises to nearer the sea floor, where it cools and causes temporary elevated heat flow.

  7. Seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing.

    PubMed

    Audet, Pascal; Bostock, Michael G; Christensen, Nikolas I; Peacock, Simon M

    2009-01-01

    Water and hydrous minerals play a key part in geodynamic processes at subduction zones by weakening the plate boundary, aiding slip and permitting subduction-and indeed plate tectonics-to occur. The seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization. However, seismological observations bearing on the presence of water within the subducting plate itself are less well documented. Here we use converted teleseismic waves to obtain observations of anomalously high Poisson's ratios within the subducted oceanic crust from the Cascadia continental margin to its intersection with forearc mantle. On the basis of pressure, temperature and compositional considerations, the elevated Poisson's ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values. Combined with observations of a strong negative velocity contrast at the top of the oceanic crust, our results imply that the megathrust is a low-permeability boundary. The transition from a low- to high-permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization. These results may have important implications for our understanding of seismogenesis, subduction zone structure and the mechanism of episodic tremor and slip.

  8. The geochemical stratigraphy and bulk composition of the lower oceanic crust in the Wadi Khafifah section the Oman ophiolite

    NASA Astrophysics Data System (ADS)

    VanTongeren, J. A.; Kelemen, P. B.

    2016-12-01

    It has long been known that mid-ocean ridge basalts (MORB) are not the direct result of mantle melting, but rather have experienced some degree of crystal fractionation and mixing prior to eruption. Yet fundamental questions as to where this fractionation occurs within the lower oceanic crust and upper mantle, what degree of fractionation they undergo, and how much magma mixing occurs, remain to be answered. There are two ways of constraining the composition of the oceanic crust; and thus, the degree of fractional crystallization and other processes occurring in both the mantle and crust at spreading ridges: (1) from the bottom-up, investigating the nature and extent of mantle melting, or (2) from the top-down, quantifying the composition of the erupted and intruded oceanic crust. The first of these has been the most common approach to-date. This study is concerned with the top-down approach to constraining the composition and nature of the oceanic crust. We present a detailed, stratigraphically-constrained, bulk composition for the lower crust of the Wadi Khafifah section of the Oman ophiolite. Together with sheeted dikes and lavas having trace element contents similar to MORB, the bulk crustal composition meets two fundamental criteria for a mantle-derived melt: (1) It has Mg# in equilibrium with Fo90 mantle olivine; (2) it is multiply saturated in ol+aug+opx±plag/sp at shallow mantle pressures. In addition, clinopyroxene crystallizes early, eliminating the so-called `pyroxene paradox'. The parent magma represents an aggregate produced by polybaric decompression melting of depleted MORB mantle (DMM), which has crystallized approximately 5% olivine - probably by reactive fractionation (e.g., Collier & Kelemen, J Petrol 2010) in the crust-mantle transition zone - prior to emplacement within the crust. An additional 40-60% fractional crystallization (ol+aug+ plag) in the lower crust is required to produce the observed sheeted dike and lava compositions. Where data

  9. Constructional features of Troodos type oceanic crust: Relationships between dike density, alteration, magnetization, and ore body distribution and their implications for in situ oceanic crust

    NASA Astrophysics Data System (ADS)

    Hall, James M.; Yang, Jing-Sui

    1995-10-01

    Quantitative relationships are demonstrated between dike density and hydrothermal alteration, magnetization, location of volcanic massive sulfide (VMS) ore bodies and a number of other properties in a 40 by 15 km segment of the Extrusive Series of the Troodos Ophiolite. Low-temperature alteration extends from the sediment-extrusive interface to close to the 25% dike density surface, while the onset of greenstone type alteration is closely associated with the 50% dike density surface. These associations are explained in terms of the regional scale variations in the dike density contour surfaces following the form of the top of the gabbroic layer of the ophiolite. An upper magnetic zone, characterized by stable high remanence, terminates just below the 25% dike density surface. A deeper magnetic zone, characterized by high induced magnetization, straddles the top of the Sheeted Complex. Volcanic massive sulfide bodies are concentrated in a narrow depth range centered about 0.1 km above the 25% dike density surface. A model for the postridge crest evolution of ore bodies accounts for this optimum depth range for preservation. Physical property changes with dike density, and the depth distribution of VMS ore bodies and prospects for the ophiolite as a whole support this model. Similar relationships appear to be present in the Samail, Macquarie Island, Newfoundland and Chilean ophiolites and Icelandic crust. As a result of relationships found in the Troodos ophiolite, an alternative is proposed to the current lithologic profile for Ocean Drilling Program Hole 504B.

  10. Geophysical and geochemical nature of relaminated arc-derived lower crust underneath oceanic domain in southern Mongolia

    NASA Astrophysics Data System (ADS)

    Guy, Alexandra; Schulmann, Karel; Janoušek, Vojtech; Štípská, Pavla; Armstrong, Robin; Belousova, Elena; Dolgopolova, Alla; Seltmann, Reimar; Lexa, Ondrej; Jiang, Yingde; Hanžl, Pavel

    2016-04-01

    The Central Asian Orogenic Belt (CAOB) in southern Mongolia consists of E-W trending Neoproterozoic cratons and Silurian-Devonian oceanic tectonic zones. Previous study revealed that the Early Paleozoic accretionary wedge and the oceanic tectonic zone are underlain by a layer giving a homogeneous gravity signal. Forward gravity modelling suggests that this layer is not formed of high-density material typical of lower oceanic crust but is composed of low- to intermediate-density rocks resembling continental crust. The nature of this lower crust is constrained by the whole-rock geochemistry and zircon Hf isotopic signature of abundant Late Carboniferous high-K calc-alkaline and Early Permian A-type granitoids intruding the two Early Paleozoic domains. It is possible to explain the genesis of these granitoids by anatexis of juvenile, metaigneous (tonalitic-gabbroic) rocks of Late Cambrian age, the source of which is presumed to lie in the "Khantaishir" arc (520-495Ma) further north. In order to test this hypothesis, the likely modal composition and density of Khantaishir arc-like protoliths are thermodynamically modelled at granulite- and higher amphibolite-facies conditions. It is shown that the current average density of the lower crust inferred by gravity modelling (2730 ±20kg/m3) matches best metamorphosed leucotonalite to diorite. Based on these results, it is now proposed that Mongolian CAOB has an architecture in which the accretionary wedge and oceanic upper crust is underlain by allochthonous lower crust that originated in a Cambrian arc. A tectonic model explaining relamination of allochthonous felsic to intermediate lower crust beneath mafic upper crust is proposed.

  11. Secular variations of iron isotopes in ferromanganese crusts: evidences for deeply sourced iron in the Pacific Ocean?

    NASA Astrophysics Data System (ADS)

    Rouxel, O. J.; Gueguen, B.

    2016-12-01

    Ferromanganese (Fe-Mn) crusts are potential archive of the Fe isotope composition of deep seawater through time. Here, we report Fe isotope composition of two pairs of Fe-Mn crusts collected on two volcanic seamounts from the Northern Pacific Ocean (Apuupuu Seamount, Hawaii) and the Southern Pacific Ocean (near Rurutu Island, Austral archipelago of French Polynesia). This approach allows (a) a direct comparison of the Fe isotope record in Fe-Mn crusts from the same seamount in order to address local effects, and (b) a comparison of geochemical composition of crusts between North and South Pacific in order to address the effect of more global geochemical processes. The results show that, despite different growth rates, diagenetic history, textures and geochemical patterns, Fe-Mn crusts from both North and South Pacific Oceans have fairly homogenous Fe isotope compositions over the last 17 Ma, yielding average δ56Fe values of -0.22 ± 0.20‰ (1sd, n = 54). The results also show striking correlations between Fe and Pb isotope ratios, indicating that local mixing between water masses is the main factor controlling Fe isotope composition in FeMn crusts. Recently, Horner et al. (2015) reported a range of δ56Fe values from -1.12‰ to 1.54‰ along a 76 Ma-old FeMn crust from the central pacific. However, secular variations of Fe isotopes inferred from other FeMn crusts in the Central North Pacific and Western Pacific (Yang and Rouxel, unpublished) show different patterns over the last 40 Ma, with δ56Fe ranging from -0.07 to -0.61‰ (n=81). Hence, the application of Fe isotopes as paleoceanographic proxies to trace deeply sourced iron at the scale of oceanic basins should be used with caution, prompting for an integrative approach combining diverse yet complimentary geochemical proxies.

  12. The contribution of hydrothermally altered ocean crust to the mantle halogen and noble gas cycles

    NASA Astrophysics Data System (ADS)

    Chavrit, Déborah; Burgess, Ray; Sumino, Hirochika; Teagle, Damon A. H.; Droop, Giles; Shimizu, Aya; Ballentine, Chris J.

    2016-06-01

    Recent studies suggest that seawater-derived noble gases and halogens are recycled into the deep mantle by the subduction of oceanic crust. To understand the processes controlling the availability of halogens and noble gases for subduction, we determined the noble gas elemental and isotopic ratios and halogen (Cl, Br, I) concentrations in 28 igneous samples from the altered oceanic crust (AOC) from 5 ODP sites in the Eastern and Western Pacific Ocean. Crushing followed by heating experiments enabled determination of noble gases and halogens in fluid inclusions and mineral phases respectively. Except for He and Ar, Ne, Kr and Xe isotopic ratios were all air-like suggesting that primary MORB signatures have been completely overprinted by air and/or seawater interaction. In contrast, 3He/4He ratios obtained by crushing indicate that a mantle helium component is still preserved, and 40Ar/36Ar values are affected by radiogenic decay in the mineral phases. The 130Xe/36Ar and 84Kr/36Ar ratios are respectively up to 15 times and 5 times higher than those of seawater and the highest ratios are found in samples affected by low temperature alteration (shallower than 800-900 m sub-basement). We consider three possible processes: (i) adsorption onto the clays present in the samples; (ii) fluid inclusions with a marine pore fluid composition; and (iii) fractionation of seawater through phase separation caused by boiling. Ninety percent of the Cl, Br and I were released during the heating experiments, showing that halogens are dominantly held in mineral phases prior to subduction. I/Cl ratios vary by 4 orders of magnitude, from 3 × 10-6 to 2 × 10-2. The mean Br/Cl ratio is 30% lower than in MORB and seawater. I/Cl ratios lower than MORB values are attributed to Cl-rich amphibole formation caused by hydrothermal alteration at depths greater than 800-900 m sub-basement together with different extents of I loss during low and high temperature alteration. At shallower depths, I

  13. Microbiology of the lower ocean crust - Preliminary results from IODP Expedition 360, Atlantis Bank

    NASA Astrophysics Data System (ADS)

    Sylvan, J. B.; Edgcomb, V. P.; Burgaud, G.; Klein, F.; Schubotz, F.; Expedition 360 Scientists, I.

    2016-12-01

    International Ocean Discovery Program (IODP) Expedition 360 represents the first leg of a multi-phase drilling program, SloMo, aimed at investigating the nature of the lower crust and Moho at slow spreading ridges. The goal of Expedition 360 was to recover a representative transect of the lower oceanic crust formed at Atlantis Bank, an oceanic core complex on the SW Indian Ridge. We present here preliminary analysis of microbial communities sampled from Hole U1473A, drilled to 789.7 m below seafloor during Expedition 360. Sub-sampling of core sections was conducted in a newly designed plexiglass enclosure with positive air pressure and HEPA filtered air, providing a clean environment for microbiology sampling aboard the JOIDES Resolution. Adenosine triphosphoate, an indicator of microbial biomass, was quantified above detection in 23 of 66 samples analyzed. We measured exoenzyme activity for alkaline phosphatase (AP), leucine aminopeptidase and arginine aminopeptidase in 16 samples and found AP to be very low but above background for 14 of the samples, with highest activities measured between 10 and 70 m below seafloor (mbsf) and peaks again at 158 and 307 mbsf, while both peptidase enzymes were above detection for only one sample at 715 mbsf. Isolates of fungi obtained from core samples as well as analyses of lipid and DNA biomarkers, and Raman spectra for a few of our rock core samples provide initial insights into microbial communities in the lower oceanic crust. Finally, a new tracer of seawater and drilling mud contamination, perfluoromethyl decaline (PFMD), was tested for the first time and its performance compared with the commonly used tracer perfluoromethylcyclohexane (PMCH). PFMD was run during coring operations for ten samples and was routinely detected in the drilling fluids, usually detected on the outside of uncleaned cores, and rarely above detection on the cleaned outside of cores. It was below detection on the inside of cores, indicating

  14. Hybrid troctolites from mid-ocean ridges: inherited mantle in the lower crust

    NASA Astrophysics Data System (ADS)

    Sanfilippo, Alessio; Morishita, Tomoaki; Kumagai, Hidenori; Nakamura, Kentaro; Okino, Kyoko; Hara, Kaori; Tamura, Akihiro; Arai, Shoji

    2015-09-01

    Studies on olivine-rich troctolites from oceanic ridges propose that hybridized mantle rocks may locally constitute small portions of the lower oceanic crust. The exact reaction process by which they originate is still debated and their hybrid nature is controversial. We show that textural and chemical inheritances of the pre-existing mantle are preserved in olivine-rich troctolites recently sampled at the Central Indian Ridge. The occurrence of a large orthopyroxene of a probable mantle origin suggests that these rocks formed through the reactive overprint of a mantle peridotite. Combining our data with those of olivine-rich troctolites worldwide, we show that the clinopyroxenes from these rocks follow chemical trends slightly distinct to those of the oceanic gabbros. These chemical trends can be ascribed to crystallization from melts assimilating mantle peridotites, suggesting that a "mantle flavor" can be locally retained in these hybrid rocks. The present distribution of Ol-rich troctolites suggests that melt-mantle reaction processes by which these rocks originate is likely to be more diffuse at slower spreading environments, where extensive melt-rock reactions within a thick thermal boundary layer enhances the conversion of the shallow oceanic mantle into hybrid crustal rocks.

  15. The Evolution of Slow-Intermediate Oceanic Crust in the South Atlantic: A Continuous Seismic Reflection Transect

    NASA Astrophysics Data System (ADS)

    Reece, R.; Christeson, G. L.; Carlson, R. L.; Estep, J. D.

    2016-12-01

    High-quality seismic transects spanning a range of crustal ages are limited to intermediate-spreading and fast-spreading (28-85 mm/yr half rate), young (<10 Ma) ocean crust formed at the Juan de Fuca Ridge and East Pacific Rise. The CREST (Crustal Reflectivity Experiment Southern Transect) expedition in the South Atlantic acquired 2680 km of 2D multichannel seismic (MCS) data including a 1500 km east-west continuous MCS transect from east of the Mid Atlantic Ridge (MAR) to the Rio Grande Rise. The transect continuously images 70 myr of crust formed at the same ridge segment with spreading rates varying from 13- 28 mm/yr. This is the first geophysical study of oceanic crust produced at any spreading rate to cover such a long span of time. Additionally, with a 12.6 km, 1008 channel streamer, the CREST data provide a longer offset than that utilized for previous studies of oceanic crust. Preliminary seismic reflection results show complex crustal structure that appears to vary highly with spreading rate. Many of these crustal reflectors exhibit high amplitude and extensive lateral continuity deep into the crust. Interestingly, crustal structure is more evident in ridge parallel profiles than in ridge normal profiles, potentially providing insight into the process of deep crust formation at the ridge. In spite of this, the data have no indications of lower crustal dipping reflectors identified in other studies. In the western portion of the study area the seismic data exhibit extensive faults cutting the seafloor in sedimentary basins at the foot of Rio Grande Rise, possibly indicating the influence of Rise processes, ie, via isostasy, on the seafloor and crust of the surrounding region. This study of South Atlantic crust will extend studies of crustal structure and evolution to lower spreading rates, and address several key questions regarding the nature of oceanic crust, including: how structure of crust produced at slow-to-intermediate rates varies with age and

  16. Mid-Ocean Ridges: Dynamics of Processes Associated With Creation of New Ocean Crust

    NASA Astrophysics Data System (ADS)

    McClain, James S.

    In the study of mid-ocean ridges, the interdisciplinary approach has been embraced for many years. On research expeditions, it is not unusual to see geologists, geophysicists, physical oceanographers, chemists, and biologists working together on their respective experiments. The motivation for this approach is not only to combine efforts; it also allows specialists to appreciate the intellectual value of each other's disciplines.

  17. Widespread Occurrence of Zircon in Slow- and Ultraslow Spreading Ocean Crust: A Tool for Studying Ocean Lithospheric Processes

    NASA Astrophysics Data System (ADS)

    Grimes, C. B.; John, B. E.; Cheadle, M. J.; Schwartz, J. J.

    2005-12-01

    The presence of igneous zircon in oceanic gabbro and peridotite provides a new opportunity to constrain absolute ages, and the processes and rates of crustal accretion in oceanic environments. Our recent investigations show zircon to be common in slow and ultraslow spreading oceanic crust including several locations along the Mid-Atlantic Ridge (MAR) and Southwest Indian Ridge (SWIR), and in rock types ranging from trondjhemite dikes to peridotite. Zircon is typically found in felsic intrusions and oxide gabbro, and in many cases may be due to late stage saturation in small pockets of residual melt. We report the morphologic and chemical characteristics of zircon grains collected from >100 rock samples recovered both from the seafloor by manned submersible and ROV, and with depth by ODP/IODP drilling. Grains range from euhedral and faceted to anhedral and fractured, with internal zonation that may be homogeneous, concentric, or patchy, and rarely contain relict cores. Sizes range from <5 μm to >1 mm. Measurements of major, minor, and trace element concentrations and high-resolution Pb/U ages were collected with the SHRIMP-RG. Chondrite-normalized rare earth element (REE) patterns for more than 50 zircon grains are uniform in shape and closely resemble patterns for known terrestrial igneous zircon. This is in contrast to mantle affinity zircon (e.g. kimberlite), which typically show depleted and relatively unfractionated patterns. Observed total REE concentrations range from 330-3765 ppm. Patterns are convex upward and rise sharply towards the HREE, with normalized Sm/La ratios = 16-320 and Lu/Gd ratios = 20-51. Positive Ce and negative Eu anomalies are ubiquitous. Hf abundances range from 5988 to 14,266 ppm. Other elements occurring at minor abundance levels include Y (463-6949 ppm), P (253-2288 ppm), U (7-2827 ppm), and Th (3-7403 ppm). Preliminary Ti concentrations range from 13 to 270 ppm, indicating crystallization temperatures of 765 to 1147°C based on Ti in

  18. Seismic properties of the Nazca oceanic crust in southern Peruvian subduction system

    NASA Astrophysics Data System (ADS)

    Kim, YoungHee; Clayton, Robert W.

    2015-11-01

    The horizontal Nazca slab, extending over a distance of ∼800 km along the trench is one of enigmatic features in Peruvian subduction zone. Increased buoyancy of the oceanic lithosphere alone due to the subduction of Nazca Ridge is insufficient to fully explain such a lengthy segment. We use data from the recent seismic experiment in southern Peru to find that the subduction-related hydration plays a major role in controlling shear wave velocities within the upper part of the oceanic crust and overlying materials. We observe substantial velocity reductions of ∼20-40% near the top plate interface along- and perpendicular-to the trench from ∼40-120 km depths. In particular, significant shear wave velocity reductions and subsequently higher P-to-S velocity ratio (exceeding 2.0) at the flat slab region suggest that the seismically probed layer is fluid-rich and mechanically weak. The dominant source of fluid comes from metasediments and subducted crust (Nazca Ridge). Long-term supply of fluid from the southward migrating Nazca Ridge provides additional buoyancy of the subducting oceanic lithosphere and also lowers the viscosity of the overlying mantle wedge to drive and sustain the flat plate segment of ∼800 km along the trench. Also, by comparing calculated seismic velocities with experimentally derived mineral physics data, we additionally provide mechanical constraints on the possible changes in frictional behavior across the subduction zone plate interface. Observed low seismic velocities in the seismogenic zone suggest a presence of low strength materials that may be explained by overpressured pore fluids (i.e., accreted sediment included in the subduction channel).

  19. Genomic evidence for the Wood-Ljungdahl pathway for carbon fixation in warm basaltic ocean crust

    NASA Astrophysics Data System (ADS)

    Smith, A. R.; Fisk, M. R.; Mueller, R.; Colwell, F. S.; Mason, O. U.; Popa, R.

    2016-12-01

    Microbial life in the deep suboceanic aquifer can harness geochemical energy resulting from water-rock reactions and contribute to carbon cycling in the ocean via primary production, or chemosynthesis. Iron-bearing minerals such as olivine in oceanic crust can produce molecular hydrogen, small molecular weight hydrocarbons, and hydrogen sulfide as they react with seawater. Although this generally occurs in serpentinizing systems at very high temperatures deep in the subsurface, it has also been hypothesized to drive the subseafloor microbial ecosystems present in shallower basaltic aquifers. We present genome-based evidence for chemolithoautotrophic microbes present on the surface of olivine incubated in Juan de Fuca Ridge basaltic ocean crust for a 4-year period. These metagenome-derived genomes show dominant taxa capable of using both branches of the Wood-Ljungdahl pathway for carbon fixation and energy generation. This pathway uses molecular hydrogen potentially derived from the olivine surface as it reacts with seawater and CO2 which is inherent to seawater. These taxa were not reported from aquifer fluid samples, but have been found only in association with mineral surfaces in this study location. Most taxa in this simple community are distant relatives of cultured taxa; therefore this genome information is crucial to understanding how the subseafloor aquifer community is structured, how it obtains energy, how it cycles carbon, and gives us keys to help cultivate these organisms in the laboratory. Our findings also support the Subsurface Lithoautotrophic Microbial Ecosystem (SLiME) hypothesis and have implications for understanding life on early Earth and the potential for life in the Martian subsurface.

  20. Seismic properties of the Nazca oceanic crust in southern Peruvian subduction system

    NASA Astrophysics Data System (ADS)

    Kim, Y.; Clayton, R. W.

    2015-12-01

    The horizontal Nazca slab, extending over a distance of ~800 km along the trench is one of enigmatic features in Peruvian subduction zone. Increased buoyancy of the oceanic lithosphere alone due to the subduction of Nazca Ridge is insufficient to fully explain such a lengthy segment. We use data from the recent seismic experiment in southern Peru to find that the subduction-related hydration plays a major role in controlling shear wave velocities within the upper part of the oceanic crust and overlying materials. We observe substantial velocity reductions of ~20-40% near the top plate interface along- and perpendicular-to the trench from ~40-120 km depths. In particular, significant shear wave velocity reductions and subsequently higher P-to-S velocity ratio (exceeding 2.0) at the flat slab region suggest that the seismically probed layer is fluid-rich and mechanically weak. The dominant source of fluid comes from metasediments and subducted crust (Nazca Ridge). Long-term supply of fluid from the southward migrating Nazca Ridge provides additional buoyancy of the subducting oceanic lithosphere and also lowers the viscosity of the overlying mantle wedge to drive and sustain the flat plate segment of ~800 km along the trench. Also, by comparing calculated seismic velocities with experimentally derived mineral physics data, we additionally provide mechanical constraints on the possible changes in frictional behavior across the subduction zone plate interface. Observed low seismic velocities in the seismogenic zone suggest a presence of low strength materials that may be explained by overpressured pore fluids (i.e., accreted sediment included in the subduction channel).

  1. From mantle to crust: Tomographic image of a mid-ocean ridge volcano

    NASA Astrophysics Data System (ADS)

    Schmid, Florian; Koulakov, Ivan; Schlindwein, Vera

    2016-04-01

    Volcanoes are an integral part of mid-ocean ridges. At ultraslow spreading ridges, volcanic centres receive more melt than is produced locally and hence are centres of very efficient magmatism. The cause of melt focussing and the structure of the underlying magma plumbing systems at these volcanic centres are still enigmatic. We present microearthquake data and local earthquake tomography results, based on a one-year deployment of ocean bottom seismometers from 2012 to 2013 on a volcanic centre at the ultraslow Southwest Indian Ridge. In the period 1996-2001, several tectono-magmatic earthquake swarms including unusually strong teleseismically recorded events indicated recent magmatic activity at the experiment site. The distribution of recorded microearthquakes reveals a prominent gap in seismicity of approx. 20 km diameter immediately beneath the volcano indicating elevated temperatures. Tomography results show distinct velocity anomalies in the area of the seismicity gap. An eminent circular low Vs anomaly was found at 4-6 km depth beneath the volcano, imaging a potential crustal magma chamber. Another anomaly of high Vp/Vs-ratios is located at the eastern rim of the seismicity gap, capped by a cluster of microearthquakes and underlain by another low Vs anomaly in the upper mantle. We propose anomalies of reduced seismic velocity to result from recent magmatic activity that is further manifested in elevated temperatures beneath the volcano. Clustering microearthquake foci might be associated with steep temperature gradients and thermal fracturing, where hot upwelling material is confronted with a cold, rigid crust. Our results provide the first direct observation of a melt lens beneath the ultraslow type of mid-ocean ridge and give unprecedented insights to potential magma pathways from the upper mantle to the crust.

  2. Emplacement of Oceanic Crust Can Continue for Several Hundred Thousand Years

    NASA Astrophysics Data System (ADS)

    Durant, D. T.; Toomey, D. R.

    2005-12-01

    We report on the seismic imaging of magma-filled sills located ~20 km off axis near the East Pacific Rise (EPR) between the Siqueiros and Clipperton fracture zones. The seismic data were collected during the UNDERSHOOT experiment, which was designed to test competing magma supply models for the EPR by mapping the pattern of magma delivery from the mantle to the crust along the entire length of a transform-bounded ridge segment. Previously reported delay-time tomographic images reveal a mantle low-velocity zone (MLVZ) beneath the EPR that defines both on- and off-axis centers of magma accumulation spaced at along-axis intervals of 15 to 25 km. The tomographic results can be explained by a 1-3% change in melt fraction; however, melt concentrations could be greater if distributed anisotropically. To find evidence of off-axis magma bodies, we analyze seismic waveform data recorded on 12 three-component ocean-bottom seismometers (OBSs) located 10 and 20 km off axis. We search data for evidence of P-wave shadow zones and anomalous P-to-S conversions consistent with magma-filled sills. In a region where the MLVZ is located 10-15 km east of the rise, we find compelling evidence for anomalously low crustal and mantle velocities. Data from an OBS located 20 km east of the rise reveal an amplified secondary phase on the radial channel consistent with a solid-melt interface. Additionally, a crustal-level P-wave shadow zone is detected near the off-axis MLVZ. We find no evidence for off-axis low-velocity anomalies in regions where the MLVZ is located beneath the rise. To better map the location and physical properties of the anomalous features, we are using a combination of travel time and full waveform modeling. Our preliminary interpretation is that in regions where the MLVZ is located off axis, magmatic processes modify both the crust-mantle transition zone and the lower crust. An unusual aspect of this discovery is that there are no seamounts nor any other bathymetric

  3. A Cross-Hole, Multi-Year Tracer Injection Experiment in the Volcanic Ocean Crust

    NASA Astrophysics Data System (ADS)

    Fisher, A. T.; Neira, N. M.; Wheat, C. G.; Clark, J. F.; Becker, K.; Hsieh, C. C.; Rappe, M. S.

    2014-12-01

    We present preliminary results from the first cross-hole tracer injection experiment in the volcanic ocean crust. The test site is on 3.5 to 3.6 M.y. old seafloor on the eastern flank of the Juan de Fuca Ridge. Six borehole subseafloor observatories (CORKs) were installed during three scientific ocean drilling expeditions, five arrayed along a 1 km profile aligned with the strike of underlying abyssal hills (Holes 1026B, 1301A/B, and 1362A/B), and one offset 2.4 km to the east (1027C). Before installing the sixth CORK in Hole 1362B, in 2010, we injected a mixture of tracers (dissolved gas, metal salts, particles) during 24 hours into the upper ocean crust. Seafloor samplers connected CORKs, sampling from different locations in the crust, were recovered during servicing expeditions in 2011 and 2013; downhole samplers that contain records from the full four years following tracer injection will be recovered in Summer 2014. Analyses of dissolved gas tracers collected with wellhead samplers through 2013 suggest that the dominant flow direction in upper basement is south to north, as inferred from regional thermal data and the chemistry of geochemical (pore fluid and borehole) samples. The apparent tracer flow rate in upper basement is on the order of meters/day, but calculations are complicated by an incomplete CORK seal in Hole 1301A, which resulted in discharge from this system that also "pulled" water and tracer to the south. Samples were collected from the tracer injection borehole, Hole 1362B, and a sampling site 200 m to the north, Hole 1362A, beginning one year after tracer injection, after opening a large-diameter ball valve on the wellhead of Hole 1362B to initiate a long-term free flow experiment. Analyses of these samples suggest that much of the tracer injected in 2010 remained close to Hole 1362B rather than being advected and dispersed into the formation. It also appears that much of the tracer transport to Hole 1362A occurred within one or more

  4. Growth and Construction of Oceanic Crust at Atlantis Bank, Southwest Indian Ridge

    NASA Astrophysics Data System (ADS)

    Schwartz, J. J.; John, B. E.; Cheadle, M. J.; Miranda, E. A.; Grimes, C. B.; Wooden, J. L.; Dick, H. J.

    2005-12-01

    Magmatic zircon is a common accessory mineral in oceanic crustal rocks including gabbro, oxide gabbro, diabase and felsic veins. Its presence in these rocks provides an exceptional opportunity to document crustal growth processes at slow-spreading mid-ocean ridges. We present nineteen Pb/U zircon SHRIMP-RG ion probe ages of lower crustal rocks collected by manned submersible, ROV, dredging and ODP drilling from a 20 x 30 km2 area of Atlantis Bank, Southwest Indian Ridge, which allow us to constrain the growth and construction of oceanic crust. Weighted average 206Pb/238U ages of these samples range from 10.7 to 13.9 Ma, with errors of 0.1-0.6 m.y. (<1 - 4%). At least 75% of these gabbros accreted within error of the predicted sea-surface magnetic age, whereas up to 25% are between 700,000 and 2.5 m.y. older. In one sample, we identified zircon with inherited cores as much as 1.5 m.y. older than their corresponding rims. There is no observable correlation between age and lithology, and the anomalously old samples are not from any specific part of Atlantis Bank; they appear to be randomly distributed amongst the non-anomalous age samples and come from various structural depths. We consider two models to explain the presence of these anomalously old rocks: i) a stochastic intrusion model whereby magma was intruded at different spatial locations within the rift valley as the plates spread apart, resulting in the entrapment of older lower crust by subsequent intrusions; and/or ii) a model in which some gabbroic bodies originally crystallized at depths of ~5-18 km below the base of the crust in a thick, cold, axial lithosphere and were subsequently uplifted along flow-lines and intruded by shallow-level magmas during the creation of Atlantis Bank. In this model, the difference in time between the Pb/U zircon crystallization age and the magnetic age is a proxy for the depth at which zircon crystallized (assuming a constant mantle upwelling rate during the construction of

  5. Hydrothermal circulation in fast spread ocean crust - where and how much? Insight from ODP Hole 1256D

    NASA Astrophysics Data System (ADS)

    Harris, M.; Coggon, R. M.; Smith-Duque, C. E.; Teagle, D. A. H.

    2014-12-01

    Understanding and quantifying hydrothermal circulation is critical to testing models of the accretion of lower ocean crust and quantifying global geochemical cycles. However, our understanding is principally limited by a lack of direct observations from intact ocean crust. Key questions remain about the magnitude of hydrothermal fluid fluxes, the nature and distribution of fluid pathways and their global variability. ODP Hole 1256D in the eastern equatorial Pacific samples a complete section of 15 Myr old upper ocean crust down to the dike/gabbro transition zone. A high spatial resolution Sr isotope profile is integrated with wireline studies, volcanostratigraphy, petrography and mineral geochemistry to document fluid pathways and develop a model for the evolving hydrothermal system during volcanic construction of the crust. Major off-axis fluid conduits in the volcanic sequence are restricted to the flow margins of two anomalously thick (>25 m) massive flows, indicating that massive flows act as a permeability barrier for fluid flow. Dike margins are pathways for both recharge and discharge hydrothermal fluids. Sub-horizontal channeling of high temperature fluids at the dike/gabbro boundary is a common attribute of most cartoons of mid ocean ridge hydrothermal systems. Hole 1256D provides the first in situ observations of the dike/gabbro transition zone and records lateral fluid transport along intrusive boundaries. The time-integrated fluid flux in the sheeted dikes of Hole 1256D calculated using Sr isotope mass balance is ~1.8 x 106 kg/m2. This is similar to fluid fluxes from other studies (Hole 504B, Pito Deep, Hess Deep) despite large variations in the thickness and Sr isotope profiles of the sheeted dike complexes, suggesting that hydrothermal fluid fluxes are remarkably uniform and independent of the local structure of the crust. This fluid flux is not large enough to completely remove the heat flux from crystallizing and cooling the lower crust and requires

  6. The effects of post-accretion sedimentation on the magnetization of oceanic crust

    NASA Astrophysics Data System (ADS)

    Dyment, J.; Granot, R.

    2016-12-01

    The presence of marine magnetic anomalies related to seafloor spreading is often considered a key evidence to locate the continent-ocean boundary (COB) at passive margins. Conversely, thermal demagnetization is also advocated to explain the poor shape of such oceanic anomalies under thick sedimentary cover. To investigate the effects of post-accretion sedimentation on marine magnetic anomalies, we focus our study on two conjugate regions of the southern South Atlantic Ocean (Anomalies M4 to M0) that, although formed at the same time and along the same spreading segments, reveal contrasting characters. The anomalies exhibit strong amplitudes (>400 nT) and a well-marked shape off South Africa, where the sediments are less than 3 km-thick, but become weaker ( 200 nT) and much smoother off northern Argentina, where the sedimentary cover is thicker than 5 km. We interpret this observation as reflecting thermal demagnetization of the extrusive layer and its low Curie temperature titanomagnetite. We perform a series of thermo-magnetic models (Dyment and Arkani-Hamed, Geophys. J. Int., 1995, modified to include the sedimentary cover) to simulate the acquisition and loss of remanent magnetization in the oceanic lithosphere. We assume that most of the sediments accumulated shortly after crustal accretion. We investigate a range of possible thermal demagnetization temperatures for the extrusive layer and find that 200°C to 280ºC best explains the observations, in reasonable agreement with Curie temperatures of titanomagnetite, suggesting that most of the extrusive layer may be demagnetized under sediments thicker than 5 km. Thermal demagnetization should therefore be considered while interpreting marine magnetic anomalies for the age and nature of the crust (i.e., continental versus oceanic) in regions with thick sedimentary cover.

  7. The deep subsurface biosphere in igneous ocean crust: frontier habitats for microbiological exploration.

    PubMed

    Edwards, Katrina J; Fisher, Andrew T; Wheat, C Geoffrey

    2012-01-01

    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 selected 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 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 from work focused at a small number of sites. 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.

  8. The Deep Subsurface Biosphere in Igneous Ocean Crust: Frontier Habitats for Microbiological Exploration

    PubMed Central

    Edwards, Katrina J.; Fisher, Andrew T.; Wheat, C. Geoffrey

    2011-01-01

    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 selected 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 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 from work focused at a small number of sites. 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. PMID:22347212

  9. Formation of Oceanic Crust Geostructurs and Relation Between Submarine landslides and Tsunamis

    NASA Astrophysics Data System (ADS)

    Harutyunyan, Albert V.

    2010-05-01

    Numerous geological and geophysical data proved the presence of oceanic crust relicts of Tethys in the territory of Lesser Caucasus. To discover the deep structure, composition and evolution of the modern Earth crust, the elastic and density properties of basites, ultrabasites and serpentinites of lesser Caucasus at high pressures and temperatures were investigated. On the basis of this data, and numerous geological-geophysical factual data concerning Mid ridges, Transform faults, Subdaction zones, Island arcs and Marginal seas are presented as a possible mechanism of their formations and relation between submarine landslides and tsunamis. The numerous volcanic and seismic centers, serpentinized protrusions and also hydrothermal sources are dated in the rang of Mid ridges and Transform faults. The formation of serpentinized ultrabasites 3-rd layer affects an infiltration of oceanic waters on ultrabasites of the upper mantle. At the same time, on an axial part of the ridge, the horizontally cramping forces, on the 5-6км depth, are established, which step-by-step pass on expanding in the top of the ridge. Analyzing the data about composition and properties of oceanic crust, we suspect that during the formation of Mid ridges, the main role belongs to serpentinized rocks of the 3-rd oceanic layer. Owing to high plasticity and low density, the serpentinized masses, by tectonic faults, in the central zone of Mid ridges, from both parties, float up and by means of protrusions implanted in the oceanic crust, then the serpentinized masses are grasp by basalts lavas. Accumulation in the axial zone of Mid ridges large masses of basalts and serpentinits, under influence of gravitation forces make slides to downwards on the serpentinized layer to the foot of ridge and low-powered sedimentary layers between these masses are saved. In the proposed model we attempted to interpret the above mentioned phenomena in the following sense. 1. Because of serpentinization of ultrabasits

  10. Mantle flow and oceanic crust formation during the opening of the Tyrrhenian back-arc basin

    NASA Astrophysics Data System (ADS)

    Magni, Valentina

    2017-04-01

    The formation of the Tyrrhenian back-arc basin occurred through short-lived episodes of fast spreading alternated with periods of slow rifting. I present results from three-dimensional numerical models of laterally varying subduction to explain the mechanism of back-arc basin opening and its episodic spreading behaviour. Moreover, I explore the consequences of this alternation between fast and slow episodes of extension on the production of new oceanic crust in the back-arc basin. Results show that the presence of continental plates (i.e. Africa and Adria) nearby the oceanic subduction of the Ionian slab produces localised deformation within the overriding plate and it is, thus, crucial for the opening of the back-arc basin. Moreover, the occurrence of collision results in the formation of two slab windows at the ocean-continent boundaries, which is in very good agreement with what is observed in the Central Mediterranean, nearby the Calabrian slab. During the evolution of the system the trench velocity shows pulses of fast trench retreat that last a few millions of years. This is associated with episodes of more intense melting of the asthenosphere rising at the back-arc basin. Finally, these three-dimensional models are used to track the mantle flow throughout the model evolution and the source of the mantle melting at the spreading centre.

  11. Pillow basalts of the Angayucham terrane: Oceanic plateau and island crust accreted to the Brooks Range

    NASA Astrophysics Data System (ADS)

    Pallister, John S.; Budahn, James R.; Murchey, Benita L.

    1989-11-01

    from obduction faulting, but the lack of fault slabs of gabbro or peridotite suggests that obduction faults did not penetrate below oceanic layer 2, a likely occurrence if layer 2 were anomalously thick, as in the vicinity of an oceanic island. The presence of basaltic tuff interbeds indicates proximity to an explosive basaltic eruptive center. The juxtaposition of submarine basalts of differing chemical affinity and age, adjacent to higher-grade Paleozoic metamorphic rocks of the Brooks Range to the north, may be explained by obduction of internally complex (thickened) oceanic crust formed in an ocean plateau setting. Emplacement and rotation of thrust plates to steep attitudes occurred during accretion of the Brooks Range passive margin, probably beginning in the Late to Middle Jurassic.

  12. Fiskenaesset Anorthosite Complex: Stable isotope evidence for shallow emplacement into Archean ocean crust

    SciTech Connect

    Peck, W.H.; Valley, J.W.

    1996-06-01

    Oxygen and hydrogen isotope ratios indicate that unusual rocks at the upper contact of the Archean Fiskenaesset Anorthosite Complex at Fiskenaesset Harbor (southwest Greenland) are the products of hydrothermal alteration by seawater at the time of anorthosite intrusion. Subsequent granulite-facies metamorphism of these Ca-poor and Al- and Mg-rich rocks produced sapphirine- and kornerupine-bearing assemblages. Because large amounts of surface waters cannot penetrate to depths of 30 km during granulite-facies metamorphism, the isotopic signature of the contact rocks must have been obtained prior to regional metamorphism. The stable isotope and geochemical characteristics of the contact rocks support a model of shallow emplacement into Archean ocean crust for the Fiskenaesset Anorthosite Complex. 45 refs., 3 figs., 2 tabs.

  13. Mapping tectonic deformation in the crust and upper mantle beneath Europe and the North Atlantic Ocean.

    PubMed

    Zhu, Hejun; Tromp, Jeroen

    2013-08-23

    We constructed a three-dimensional azimuthally anisotropic model of Europe and the North Atlantic Ocean based on adjoint seismic tomography. Several features are well correlated with historical tectonic events in this region, such as extension along the North Atlantic Ridge, trench retreat in the Mediterranean, and counterclockwise rotation of the Anatolian Plate. Beneath northeastern Europe, the direction of the fast anisotropic axis follows trends of ancient rift systems older than 350 million years, suggesting "frozen-in" anisotropy related to the formation of the craton. Local anisotropic strength profiles identify the brittle-ductile transitions in lithospheric strength. In continental regions, these profiles also identify the lower crust, characterized by ductile flow. The observed anisotropic fabric is generally consistent with the current surface strain rate measured by geodetic surveys.

  14. Seismic reflection character of the Cameroon volcanic line: Evidence for uplifted oceanic crust

    SciTech Connect

    Meyers, J.B.; Rosendahl, B.R. )

    1991-11-01

    Deep-imaging multifold seismic lines across submarine parts of the Cameroon volcanic line (west Africa-Gulf of Guinea) show asymmetric uplift of oceanic crust associated with extensive magmatism. The main pulse of uplift occurred after creation of a regional sequence boundary believed to be Miocene in age. The apparent synchroneity of uplift argues against the Cameroon line being a simple hotspot trace, as previously inferred. One plausible theory of origin for the seaward part of the Cameroon volcanic line and its asymmetric uplift geometry combines regional asthenospheric upwelling with restriction of magmatic egress to regularly spaced weak spots, corresponding to fracture-zone crossings. Horizontal motion and buckling also may have occurred along the Cameroon volcanic line.

  15. Seismic array detection of subducted oceanic crust in the lower mantle

    NASA Astrophysics Data System (ADS)

    Rost, Sebastian; Garnero, Edward J.; Williams, Quentin

    2008-06-01

    We analyze short-period precursory energy to PP that can be observed in seismograms in the distance range from ˜95° to 105° to infer the behavior of subducted slabs beneath western Pacific subduction zones. PP is a P wave once reflected at the free surface between the source and receiver. Using high-resolution seismic array techniques, we analyze the incidence angle, timing, and azimuth of the PP precursors. The precursory energy is resolved to originate from off great circle path azimuths and is consistent with scattering by small-scale heterogeneities. Assuming single scattering, upper mantle- and midmantle-derived scatterer locations show a strong geographical and depth correlation to high seismic velocities in tomographic studies. Scattering locations beneath the Tonga and Mariana subduction zones outline continuous dipping structures to a depth of at least 1000 km, consistent with scattering associated with subducted former oceanic lithosphere. Scatterer locations uniquely explain the timing, slowness, and back azimuth of the PP precursors at the array. The observed reflections can be explained with the velocity impedance variations expected for high-pressure basalt juxtaposed with pyrolite or harzburgite and thus may be due to the paleo-Mohorovičić discontinuity within subducted slabs. These results are consistent with basaltic crust penetrating into the lower mantle. This method provides a means for tracking the location of geochemically enriched former oceanic crust in the lower mantle by using recordings of globally distributed seismic arrays and is complementary to longer-wavelength constraints on high seismic velocity slabs inferred from tomography.

  16. The extent of oceanization of the Egyptian northern Red Sea crust indicated by gravity and magnetic data

    NASA Astrophysics Data System (ADS)

    Khattab, M. M.

    1992-05-01

    A 50 mGal gravity minimum and a 200 nT total magnetic intensity anomaly in the northern Red Sea shelf near the port of Safaga and a 250 nT total intensity anomaly across the Brothers Island indicate a more advanced degree of oceanization of the Red Sea crust than proposed from the results of the latest seismic studies in the same area. The interpretation of this gravity and magnetic anomalies reveals and extremely thinned continental crust which transforms to a mixed continental-oceanic crust some 50 km from the coast line. This mixed crust is found to contain segregated basaltic bodies, normally and reversely magnetized by the ambient Earth's magnetic field. The magnetic anomaly over the Brothers Island is interpreted as related to a short 4 Ma old spreading axis. A review of the results of this investigation together with previous studies suggests three modes for the emplacement of the oceanic part of the Red Sea crust in the Egyptian Red Sea shelf. The calculated crustal model at the offshore Safaga is compared with refraction based models across Saudi Arabia and the western Atlantic.

  17. Fractal analysis on Enceladus: a global ocean underneath the icy crust

    NASA Astrophysics Data System (ADS)

    Lucchetti, Alice; Pozzobon, Riccardo; Mazzarini, Francesco; Cremonese, Gabriele; Simioni, Emanuele; Massironi, Matteo

    2016-04-01

    Plumes of water have been observed erupting from Enceladus' south polar terrain providing direct evidence of a reservoir of liquid below the surface, that could be considered global or just a small body of water concentrated at its south pole. Gravity data collected during the spacecraft's several close flyby over the south polar region determined that the icy shell above the liquid ocean must be 30-40 km thick extending from the south pole up to 50°S (Iess et al. 2014). The hypothesis of a global ocean beneath the icy crust has been raised even in a recent paper of Thomas et al. (2015) thanks to the measurements of the very slight wobble that Enceladus displays as it orbits Saturn. In this work we support the hypothesis of the presence of an ocean layer using the fractal percolation theory. This method allowed us to estimate the icy shell thickness values in different regions of Enceladus from the south polar terrain up to the north pole. The spatial distribution of fractures on Enceladus has been analyzed in terms of their self-similar clustering and a two-point correlation method was used to measure the fractal dimension of the fractures population (Mazzarini, 2004, 2010). A self-similar clustering of fractures is characterized by a correlation coefficient with a size range defined by a lower and upper cut-off, that represent a mechanical discontinuity and the thickness of the fractured icy crust, thus connected to the liquid reservoir. We mapped the fractures on Enceladus surface based on April 2010 global mosaic from Cassini mission and applied the fractal method firstly to the south polar terrain finding indeed a fractal correlation of fractures and providing an ice shell thickness of ~40 km. Then, we analyzed fractures of four different regions around the equator and around the north pole inferring an overall ice shell thickness ranging from 35 to 45 km. Our results are in agreement with the gravity observations (Iess et al., 2014) and the mechanical models

  18. How to identify oceanic crust-Evidence for a complex break-up in the Mozambique Channel, off East Africa

    NASA Astrophysics Data System (ADS)

    Klimke, Jennifer; Franke, Dieter; Gaedicke, Christoph; Schreckenberger, Bernd; Schnabel, Michael; Stollhofen, Harald; Rose, Jens; Chaheire, Mohamed

    2016-12-01

    The identification of oceanic crust at rifted margins plays a crucial role in academic research understanding rifting mechanisms and the architecture of continent-ocean boundaries, and is also important for hydrocarbon exploration extending into deeper water. In this paper, we provide a workflow for the determination of the crustal nature in the Mozambique Channel, east of Davie Ridge, by presenting a compilation of several geophysical attributes of oceanic crust at divergent margins. Previous reconstructions locate the Davie Ridge at the trace of a transform fault, along which Madagascar drifted to the south during the breakup of Gondwana. This implies a sharp transition from continental to oceanic crust seaward of Davie Ridge. Using new multichannel seismic profiles offshore northern Mozambique, we are able to identify distinct portions of stretched basement east of Davie Ridge. Two phases of deformation affecting the basement are observed, with the initial phase resulting in the formation of rotated fault blocks bounded by listric faults. Half-grabens are filled with wedge-shaped, syn-extensional sediments overlain by a prominent unconformity that northward merges with the top of highly reflective, mildly deformed basement, interpreted as oceanic crust. The second phase of deformation is associated with wrench faulting and probably correlates with the southward drift of Madagascar, which implies that the preceding phase affected basement generated or modified prior to the opening of the West Somali Basin. We conclude that the basement is unlikely to consist of normal oceanic crust and suggest that the first extensional phase corresponds to rifting between Madagascar and Africa. We find evidence for a wide area affected by strike-slip deformation, in contrast to the earlier proposed major single transform fault in the vicinity of Davie Ridge and suggest that the Mozambique Channel area to the north of Madagascar may be classified as an oblique rather than sheared

  19. Heatflow in Young Oceanic Crust. Is Earth's Heat Flux 44 TW or 31 TW?

    NASA Astrophysics Data System (ADS)

    Gosnold, W. D.

    2008-05-01

    We address the question of heat flow in young oceanic crust using a 2-D finite difference heat flow model of sea floor spreading. The model parameters include thermal conductivity variation with temperature, a fixed T-z profile at the ridge that follows the mantle liquidus, constant spreading rate, and constant heat flow into the base of the lithosphere. The output of the model is a 2-D temperature-depth grid that provides a comparison with various analytical models of oceanic heat flow. We tested the reliability of the computations using different half-spreading rates and different node spacings and verified that the models yield equivalent results at equivalent times and depths. To address the question of global heat flux with respect to heat flow at oceanic spreading centers, we summed heat flux on the oceanic ridge system assuming a length of 65000 km and a half-spreading rate of 2.5 cm/y. Our results show that the GDH1, HSC, and PSM models overestimate heat flow close to the ridge, but the differences are less than 5 percent beyond 2 ma. Our model converges with GDH1 at a young age of 4.32 my and actually shows slightly lower heat flow than HSC and PSM models for ages greater than 5 ma. Total heat flux for GDH1 between 80ky and 2.32 my is 6.24 TW and our model yields 5.56 TW. Assuming agreement with heat flux over the rest of the globe, our model is only 0.68 TW different from 44TW. However, our result for heat flux in a 2 km wide section over the global ridge crest is 0.161 TW while the analytical models predict infinite heat flow.

  20. Andesite Magmas are Produced along Oceanic Arcs where the Crust is Thin: Evidence from Nishinoshima Volcano, Ogasawara Arc, Japan

    NASA Astrophysics Data System (ADS)

    Tamura, Y.; Ishizuka, O.; Sato, T.; Nichols, A. R.

    2015-12-01

    The incentive for this study is the ongoing explosive eruption of Nishinoshima volcano, located about 1,000 km south of Tokyo along the Ogasawara (Bonin) Arc. The straightforward but unexpected relationship presented here relates crustal thickness and magma type in the Izu-Ogasawara Oceanic Arc. Volcanoes along the Ogasawara segment of the arc, which include Nishinoshima, are underlain by thin crust (16-21 km)—in contrast to those along the Izu segment, where the crust is ~35 km thick. Interestingly, andesite magmas are dominant products from the former volcanoes and mostly basaltic lavas erupt from the latter. Why and how do volcanoes on the thin crust erupt andesite magmas? An introductory petrology textbook might answer this question by suggesting that, under decreasing pressure and hydrous conditions, the liquidus field of forsterite expands relative to that of enstatite, with the result that, at some point, enstatite melts incongruently to produce primary andesite melt. According to the hypothesis presented here, however, rising mantle diapirs stall near the base of the oceanic arc crust at depths controlled by the thickness of the overlying crust. Where the crust is thin, as along the Ogasawara segment of the arc, pressures are relatively low, and magmas produced in the mantle wedge tend to be andesitic. Where the crust is thick, as along the Izu segment, pressures are greater, and only basaltic magmas tend to be produced. To examine this hypothesis, JAMSTEC cruise NT15-E02 on the R/V Natsushima took place from 11 June to 21 June 2015 to Nishinoshima. It's present island has an elevation of only ~150 m, but its submarine flanks extend to ocean depths of 2,000-3,000 m, so the great bulk of the volcano is submarine and yet-to-be explored. We present the new hypothesis and its evidence from Nishinoshima based on the primitive lavas collected from the submarine parts of the volcano.

  1. The Continental Crust.

    ERIC Educational Resources Information Center

    Burchfiel, B. Clark

    1983-01-01

    Continental crust underlies the continents, their margins, and also small shallow regions in oceans. The nature of the crust (much older than oceanic crust) and its dynamics are discussed. Research related to and effects of tectonics, volcanism, erosion, and sedimentation on the crust are considered. (JN)

  2. The Continental Crust.

    ERIC Educational Resources Information Center

    Burchfiel, B. Clark

    1983-01-01

    Continental crust underlies the continents, their margins, and also small shallow regions in oceans. The nature of the crust (much older than oceanic crust) and its dynamics are discussed. Research related to and effects of tectonics, volcanism, erosion, and sedimentation on the crust are considered. (JN)

  3. Controls on thallium uptake during hydrothermal alteration of the upper ocean crust

    NASA Astrophysics Data System (ADS)

    Coggon, Rosalind M.; Rehkämper, Mark; Atteck, Charlotte; Teagle, Damon A. H.; Alt, Jeffrey C.; Cooper, Matthew J.

    2014-11-01

    Hydrothermal circulation is a fundamental component of global biogeochemical cycles. However, the magnitude of the high temperature axial hydrothermal fluid flux remains disputed, and the lower temperature ridge flank fluid flux is difficult to quantify. Thallium (Tl) isotopes behave differently in axial compared to ridge flank systems, with Tl near-quantitatively stripped from the intrusive crust by high temperature hydrothermal reactions, but added to the lavas during low temperature reaction with seawater. This contrasting behavior provides a unique approach to determine the fluid fluxes associated with axial and ridge flank environments. Unfortunately, our understanding of the Tl isotopic mass balance is hindered by poor knowledge of the mineralogical, physical and chemical controls on Tl-uptake by the ocean crust. Here we use analyses of basaltic volcanic upper crust from Integrated Ocean Drilling Program Hole U1301B on the Juan de Fuca Ridge flank, combined with published analyses of dredged seafloor basalts and upper crustal basalts from Holes 504B and 896A, to investigate the controls on Tl-uptake by mid-ocean ridge basalts and evaluate when in the evolution of the ridge flank hydrothermal system Tl-uptake occurs. Seafloor basalts indicate an association between basaltic uptake of Tl from cold seawater and uptake of Cs and Rb, which are known to partition into K-rich phases. Although there is no clear relationship between Tl and K contents of seafloor basalts, the data do not rule out the incorporation of at least some Tl into the same minerals as the alkali elements. In contrast, we find no relationship between the Tl content and either the abundance of secondary phyllosilicate minerals, or the K, Cs or Rb contents in upper crustal basalts. We conclude that the uptake of Tl and alkali elements during hydrothermal alteration of the upper crust involves different processes and/or mineral phases compared to those that govern seafloor weathering. Furthermore

  4. Alteration and mineralization of an oceanic forearc and the ophiolite-ocean crust analogy

    USGS Publications Warehouse

    Alt, J.C.; Teagle, D.A.H.; Brewer, T.; Shanks, Wayne C.; Halliday, A.

    1998-01-01

    Mineralogical, chemical, and isotopic (O, C, S, and Sr) analyses were performed on minerals and bulk rocks from a forearc basement section to understand alteration processes and compare with mid-ocean ridges (MOR) and ophiolites. Ocean Drilling Program Hole 786B in the Izu-Bonin forearc penetrates 103 m of sediment and 725 m into volcanic flows, breccias, and basal dikes. The rocks comprise boninites and andesites to rhyolites. Most of the section was affected by low-temperature (<100??C) seawater alteration, with temperatures increasing downward. The rocks are partly (5-25%) altered to smectite, Fe-oxyhydroxide, calcite, and phillipsite, and exhibit gains of K, Rb, and P, loss of Ca, variable changes in Si, Na, Mg, Fe, Sr, and Y, and elevated ??18O and 87Sr/86Sr. Higher temperatures (???150??C) in the basal dikes below 750 m led to more intense alteration and formation of chlorite-smectite, corrensite, albite, K-feldspar, and quartz (??chlorite). A 5 m thick hydrothermally altered and pyritized zone at 815 m in the basal dikes reacted with mixtures of seawater and hydrothermal fluids to Mg-chlorite, albite, and pyrite, and gained Mg and S and lost Si and Ca. Focused flow of hydrothermal fluids produced sericitization halos (Na-K sericite, quartz, pyrophyllite, K-feldspar, and pyrite) along quartz veins at temperatures of 200??-250??C. High 87Sr/86Sr ratios of chloritized (???0.7055) and sericitized (???0.7065) rocks indicate involvement of seawater via mixing with hydrothermal fluids. Low ??34S of sulfide (???2 to -5.5???) and sulfate (12.5???) are consistent with input of magmatic SO2 into hydrothermal fluids and disproportionation to sulfide and sulfate. Alteration processes were generally similar to those at MORs, but the arc section is more intensively altered, in part because of the presence of abundant glassy rocks and mafic phases. The increase in alteration grade below 750 m and the mineralization in the basal dikes are analogous to changes that occur near

  5. Reactive overprint of the Central Indian Ridge mantle and formation of hybrid troctolites: reassessing the significance of bulk oceanic crust

    NASA Astrophysics Data System (ADS)

    Sanfilippo, A.; Morishita, T.; Kumagai, H.; Nakamura, K.; Okino, K.; Tamura, A.; Arai, S.

    2014-12-01

    The idea that hybridized mantle rocks can contribute to the oceanic crust composition has recently emerged thanks to studies on primitive (olivine-rich) troctolites [e.g. 1]. These rocks are considered to be formed by melt-rock interaction, but the exact reaction process by which they originate is still debated and their role on the bulk oceanic crust composition has been never defined. Olivine-rich troctolites have been mostly found at slow spreading ridges [2] or at their fossil analogues [3]. Similar rocks have been recently collected in the 25ºS area of the intermediate spreading Central Indian Ridge (CIR), and rarely characterize the crust mantle boundary at fast spreading ridges [4]. We show that textural and chemical inheritances of the pre-existing mantle are preserved in the CIR troctolites. In particular, the local occurrence of granular, mantle-derived orthopyroxenes and the composition of the associated clinopyroxene indicate that these crustal rocks formed through a direct (one-stage) conversion of a mantle peridotite. We use chemical evidence to infer the same origin of the olivine-rich troctolites worldwide, concluding that the reactive overprint of the oceanic mantle is a process diffused over the entire spreading rate spectrum. Bulk oceanic crust estimates of the Hess Deep (Pacific) and Atlantis Massif (Atlantic) crustal sections are used to quantify and compare the effect of these rocks on the bulk crust composition at fast and slow spreading ridges. Our inferences suggest that the significance of the bulk oceanic crust should be reassessed. When hybrid troctolites are included at crustal levels, the oceanic crust cannot be considered equal to the composition of the melt extracted from the mantle, but it results more primitive and importantly thicker. References: [1] Suhr G., Hellebrand E., Johnson K., Brunelli D., 2008, Geochem. Geophys. Geosyst. 9, doi:10.1029/2008GC002012; [2] Drouin M., Godard M., Ildefonse B., Bruguier O., Garrido C

  6. Formation of Fast-Spread Ocean Crust : Crystallographic Preferred Orientations From a Reference Lower Crust Section in the Oman Ophiolite

    NASA Astrophysics Data System (ADS)

    Ildefonse, B.; Mueller, T.; Mock, D.; Koepke, J.

    2016-12-01

    About 20 years ago, two competing models were proposed for the formation of the lower, gabbroic crust at fast-spreading ridges. The lower crust is either formed by downward flow of mushy material from the shallow axial melt lens (gabbro glacier), or by sill intrusions (sheeted sills). To further test these end-member models, we characterized the vertical distribution of Crystallographic Preferred Orientations (CPO) in Wadi Gideah gabbro section (Sumail ophiolite, Sultanate Oman), using the Electron Backscattered Diffraction (EBSD) technique. CPO were measured on 67 gabbro samples, documenting a 5 km thick section, with an average interval of 80 m between samples. EBSD data sets were processed using MTEX, a free Matlab toolbox. Average misorientation in grains (angle between each pixel orientation and mean orientation of the grain) is very low ( 0.25°). This is consistent with magmatic flow in these rocks, and the paucity of crystal-plastic overprint. The strength (J index) of plagioclase CPO increases down-section, with a more pronounced variability in the layered gabbros. For clinopyroxene, the difference between upper (foliated) and lower (layered) gabbros is stronger, with low J in upper gabbros, and higher and more variable J in lower gabbros. In upper gabbros the symmetry of plagioclase and clinopyroxene CPO progressively evolves downward to progressively more oblate. Continuing down-section, the trend reverses, with progressively more prolate CPO in lower gabbros. The crystallographic fabric variability in the lower crust section calls for distinct formation mechanisms in the upper and lower gabbros. It is consistent with a hybrid model for crustal formation (Boudier et al., 1996, doi:10.1016/0012-821X(96)00167-7). The genesis of the upper foliated gabbro can be at least partly explained by the gabbro glacier model, while the continuous emplacement of sheeted sills at various depths is a more plausible model for the lower layered gabbro section.

  7. Geological storage of CO2 within the oceanic crust by gravitational trapping

    NASA Astrophysics Data System (ADS)

    Marieni, Chiara; Henstock, Timothy J.; Teagle, Damon A. H.

    2013-12-01

    rise of atmospheric carbon dioxide (CO2) principally due to the burning of fossil fuels is a key driver of anthropogenic climate change. Mitigation strategies include improved efficiency, using renewable energy, and capture and long-term sequestration of CO2. Most sequestration research considers CO2 injection into deep saline aquifers or depleted hydrocarbon reservoirs. Unconventional suggestions include CO2 storage in the porous volcanic lavas of uppermost oceanic crust. Here we test the feasibility of injecting CO2 into deep-sea basalts and identify sites where CO2 should be both physically and gravitationally trapped. We use global databases to estimate pressure and temperature, hence density of CO2 and seawater at the sediment-basement interface. At previously suggested sites on the Juan de Fuca Plate and in the eastern equatorial Pacific Ocean, CO2 is gravitationally unstable. However, we identify five sediment-covered regions where CO2 is denser than seawater, each sufficient for several centuries of anthropogenic CO2 emissions.

  8. Predicting the Isotopic Composition of Subduction-Filtered Subducted Oceanic Crust and Sediment

    NASA Astrophysics Data System (ADS)

    White, W. M.

    2010-12-01

    The chemical and isotopic character of mantle plumes, which produce oceanic island volcanoes, are widely thought to reflect the presence of recycled oceanic crust and sediment. Isotopic systematics suggest the “cycle time” for this process is 1 Ga or longer, but it should be possible to use a simple mass balance approach to discern how the presently operating subduction zone filter affects the ratios of radioactive parent to radiogenic daughter isotopes. Simple uniformitarian assumptions can then be used to predict the present isotopic composition of anciently subducted lithosphere. Our underlying assumption in deciphering the subduction zone filter is that the flux of an element into the deep mantle is simply equal to the flux of element into the subduction zone less the flux of that element into subduction zone magmas. The former is readily calculated from published data. The latter can be calculated by estimating parental magma compositions, arc accretion rates, and the assumption that arc magma compositions differ from MORB only because of material derived from subducting crust and sediment. Using this approach for 8 intra-oceanic subduction zones, we find 73% of Th and Pb, 79% of U, 80% of Rb and Sr, 93% of Nd and 98% of Sm survive the subduction zone filter. The subduction zone filter systematically increases Sm/Nd ratios in all subduction zones, but the effect is small, with a weighted mean increase of 1.5%. The effect of subduction is to decrease the Sm/Nd of the mantle, but only slightly. The effect of subduction is to increase the Rb/Sr of the mantle, but the subduction zone filter does not have a systematic effect on Rb/Sr ratios: it significantly increases in Rb/Sr in 3 subduction zones and significantly decreases it in one; the weighted mean shows no significant change. The effect of the subduction zone filter on U/Pb is also not systematic. U/Pb ratios in the mantle fluxes are bimodal, with values equal to or lower than the bulk Earth value in 4

  9. A glimpse of Earth's primordial crust: The Nuvvuagittuq greenstone belt as a vestige of mafic Hadean oceanic crust

    NASA Astrophysics Data System (ADS)

    O'Neil, J.; Carlson, R. W.

    2010-12-01

    Investigation of Earth’s primitive crust is biased towards felsic rocks because they contain zircons that provide robust geochronological constraints. Felsic rocks, however, cannot be derived directly from the mantle thus the first crust had to be mafic in composition. Obtaining precise ages on old mafic rocks is however difficult due to their lack of zircon and the metamorphic overprinting they have suffered. 143Nd and 142Nd analyses on the Nuvvuagittuq greenstone belt suggests that its mafic components formed more than 4 billion years ago and that the dominant lithology of the belt, known as the “faux-amphibolite”, represents the only relict of Hadean crust formed at ~4.3 Ga. Its protolith is interpreted to be mafic volcanic rocks and volcanic pyroclastic deposits. The faux-amphibolite can be divided into three distinct geochemical group stratigraphically superimposed. The faux-amphibolite at the base of the sequence is tholeiitic with a composition indicating derivation of primary melts from an undepleted mantle and fractionation under dry conditions. Towards the top of the volcanic sequence, the faux-amphibolites are characterized by higher Al/Ti ratios. They appear to have fractionated under elevated water pressure and are geochemically similar to modern boninite and calc-alkaline volcanic rocks. A new series of faux-amphibolite was analysed for 142Nd isotopic composition. 21 samples have deficits in 142Nd ranging from -7 to -18 ppm compared to the terrestrial standard. These deficits have now been confirmed by measurements of the same samples at ETH (Roth et al., GCA, A886, 2010). A 146Sm-142Nd isochron constructed for all faux-amphibolite yields an age of 4368 +72-142 Ma (n=30). A line fit only to the faux-amphibolite compositional group that shows the widest range in LREE enrichment, including corresponding co-genetic ultramafic sills gives a 146Sm-142Nd age of 4381 +67-123 Ma (n=21). The Hadean age for the faux-amphibolite is supported by a 4079

  10. Mantle-crust differentiation of chalcophile elements in the oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Ciążela, J.; Dick, H. J.; Koepke, J.; Kuhn, T.; Muszynski, A.; Kubiak, M.

    2014-12-01

    The chalcophile elements, as associated with sulfides, are believed mainly from the study of ophiolites to be generally enriched in the upper mantle, but depleted by magmatic processes in the lower and upper ocean crust. However, studies of some orogenic lherzolites suggest a copper depletion of peridotites in relation to the primitive mantle, suggesting that a portion of the sulfides is melted during decompression and incorporated into the ascending magmas. The rarity of abyssal peridotites and the high degree of their alteration have not allowed these results to be verified in situ in the oceans.Here, we present the first complete study of chalcophile elements based on a suite of rocks from an oceanic core complex (OCC), the Kane Megamullion at 22°30'N at the Mid-Atlantic Ridge. OCCs provide large exposures of mantle and lower crustal rocks on the seafloor on detachment fault footwalls at slow and ultraslow spreading ridges. The Kane Megamullion is one of the best sampled OCCs in the world, with 1342 rocks from 28 dredge sites and 14 dives. We have made XRF, TD-MS and INAA analyses of 129 representative peridotites, gabbroic rocks, diabases and basalts. Our results suggest a depletion of some peridotites in relation to the primitive mantle (28 ppm Cu). Dunites, troctolites and olivine gabbros are relatively enriched in chalcophile elements. The amount of sulfides decreases gradually with progressive differentiation, reaching a minimum in gabbronorites and diabases. The highest bulk abundance of chalcophile elements in our sample suite was observed in dunites (up to ~ 300 ppm Cu in several samples) and a contact zone between residual peridotite and a mafic vein (294 ppm Cu). Plagioclase-bearing harzburgites, generally formed by late-stage melt impregnation in the mantle, are typically more enriched in Cu than unimpregnated residual peridotites. For these reasons, our initial results indicate sulfide melting during mantle melting, and their local precipitation in

  11. Transient Hydrothermal Alteration in Fault Zones Cutting the Lower Oceanic Crust, Hess Deep Rift

    NASA Astrophysics Data System (ADS)

    McCaig, Andrew; Titarenko, Sofya; Cliff, Robert; Ivan, Savov; Adrian, Boyce

    2015-04-01

    -axis alteration, common in any location where faults intersect the Moho, including transform faults, near axis normal faults at slow spreading ridges, and perhaps bending faults at subduction zones. [1] Gillis, K.M., Snow J. E. and Shipboard Science Party (2014) Primitive layered gabbros from fast-spreading lower oceanic crust. Nature, 505, 204-207.

  12. Transient Hydrothermal Alteration In Fault Zones Cutting The Lower Oceanic Crust, Hess Deep Rift

    NASA Astrophysics Data System (ADS)

    McCaig, A. M.; Titarenko, S.; Cliff, R. A.; Savov, I. P.; Boyce, A.; Dutt, R.

    2014-12-01

    location where faults intersect the Moho, including transform faults, near axis normal faults at slow spreading ridges, and perhaps bending faults at subduction zones. [1] Gillis, K.M., Snow J. E. and Shipboard Science Party (2014) Primitive layered gabbros from fast-spreading lower oceanic crust. Nature, 505, 204-207.

  13. Subduction of hydrated basalt of the oceanic crust: Implications for recycling of water into the upper mantle and continental growth

    NASA Technical Reports Server (NTRS)

    Rapp, R. P.

    1994-01-01

    Subduction zones are presently the dominant sites on Earth for recycling and mass transfer between the crust and mantle; they feed hydrated basaltic oceanic crust into the upper mantle, where dehydration reactions release aqueous fluids and/or hydrous melts. The loci for fluid and/or melt generation will be determined by the intersection of dehydration reaction boundaries of primary hydrous minerals within the subducted lithosphere with slab geotherms. For metabasalt of the oceanic crust, amphibole is the dominant hydrous mineral. The dehydration melting solidus, vapor-absent melting phase relationships; and amphibole-out phase boundary for a number of natural metabasalts have been determined experimentally, and the pressure-temperature conditions of each of these appear to be dependent on bulk composition. Whether or not the dehydration of amphibole is a fluid-generating or partial melting reaction depends on a number of factors specific to a given subduction zone, such as age and thickness of the subducting oceanic lithosphere, the rate of convergence, and the maturity of the subduction zone. In general, subduction of young, hot oceanic lithosphere will result in partial melting of metabasalt of the oceanic crust within the garnet stability field; these melts are characteristically high-Al2O3 trondhjemites, tonalites and dacites. The presence of residual garnet during partial melting imparts a distinctive trace element signature (e.g., high La/Yb, high Sr/Y and Cr/Y combined with low Cr and Y contents relative to demonstrably mantle-derived arc magmas). Water in eclogitized, subducted basalt of the oceanic crust is therefore strongly partitioned into melts generated below about 3.5 GPa in 'hot' subduction zones. Although phase equilibria experiments relevant to 'cold' subduction of hydrated natural basalts are underway in a number of high-pressure laboratories, little is known with respect to the stability of more exotic hydrous minerals (e.g., ellenbergite) and

  14. Slow-Spreading Oceanic Crust Formed By Steady-State Axial Volcanic Ridges

    NASA Astrophysics Data System (ADS)

    Murton, B. J.; Schroth, N.; LeBas, T.; Van Calsteren, P. W.; Yeo, I. A.; Achenbach, K. L.; Searle, R. C.

    2012-12-01

    Oceanic crust originates at mid-ocean spreading ridges (MORs), covers almost three quarters of the earth's surface and dominates the global magmatic flux. Axial volcanic ridges (AVRs) are almost ubiquitous features of orthogonal slow-spreading ridges, which account for three quarters of the global mid-ocean spreading ridge system today. Typically 3-6 km wide, 200-500 m high and 10-20 km long, AVRs are the loci of recent volcanic activity and form the most prominent topography rising above the otherwise flat-lying Median Valley floor. Previous studies indicate that AVRs, and their related crustal magma reservoirs are episodic, on a time scale of 150-300 ka. Yet their near ubiquitous occurrence at slow-spreading ridge segments provides us with a paradox: if AVRs have a life cycle of formation and degradation, does their near ubiquitous presence at slow spreading ridges imply their life-cycles are synchronised? In this contribution, we report the findings from a high-resolution study of a well-developed axial volcanic ridge (AVR) at 45°N on the Mid-Atlantic Ridge (MAR). Here, the MAR is typical of most slow-spreading ridges: it spreads generally symmetrically and orthogonally, at a full rate of 23.6 mm per year, has second and third-order segmentation, and contains a typical AVR. Using a combination of detailed micro-bathymetry, sidescan sonar, visual surveying and petrology, we suggest that the AVR is the product of quasi-steady state volcanotectonic processes. Small volume lava flows, originating at or near the crest and with short run-out lengths, form ~60 m high hummocky pillow-lava mounds that dominate the construction of the AVR. The lavas are the product of moderate degrees of mantle melting that are typical for normal mid-ocean ridge basalt. Synchronous with these eruptions the flanks of the AVR subside forming a structural horst. Subsidence is partially accommodated by a series of outward-facing volcanic growth faults that step-down and away from the AVR

  15. Geochemistry of Dikes and Lavas in Ocean Crust: Implications for Dike Intrusion and Eruption at Fast to Superfast Spreading Centers

    NASA Astrophysics Data System (ADS)

    Pollock, M. A.; Klein, E. M.; Karson, J. A.; Hayman, N. W.

    2006-12-01

    Recent sampling of in situ ocean crust exposed in tectonic windows and by ocean drilling allows us to examine the relationship between dikes and lavas, revealing important insights into upper crustal construction at fast (~110 mm/yr full rate) to superfast (~220 mm/yr) spreading centers. Dikes and lavas from the Hess Deep Rift (~110 mm/yr, ~1 Ma crust, eastern equatorial Pacific) show unexpected differences in whole rock composition, with most lavas offset to lower abundances of FeO* compared to dikes with the same MgO content (Stewart et al., 2003). This chemical distinction results from plagioclase accumulation and reflects a difference in calculated magma density of ~0.04 g/cm3, suggesting that lower-density magmas are preferentially erupted as lavas due to their increased buoyancy, while higher-density magmas preferentially remain sub-surface and solidify as dikes. Strong partitioning of magma between dikes and lavas is also observed in data from the Oman ophiolite (northern Oman, Einaudi et al., 2000; Godard et al., 2003; Miyashita, et al., 2003; Umino et al., 2003), oceanic crust believed to be produced at a fast-spreading rate. Chemical differences between dikes and lavas are also observed at super-fast spread crust sampled at the Pito Deep Rift (>140 mm/yr, ~3 Ma crust, southeast Pacific), and IODP Hole 1256D (~220 mm/yr, ~15 Ma crust, eastern Pacific, Wilson et al., 2006), although the effect is less dramatic than that observed at Hess Deep and the Oman ophiolite. These observations suggest a model for partitioning of magma between dikes and lavas that depends on a complex balance among spreading-rate dependent factors that include magma pressure, regional tectonic stress, and elastic crustal stress (Ida, 1999), but also on factors that determine magma buoyancy (i.e. thermal state, conditions that lead to entrainment of low-density plagioclase crystals, bulk density of the overlying crust). Such a model has significant implications for the nature of the

  16. Petrology and geochemistry of primitive lower oceanic crust from Pito Deep: Implications for the accretion of the lower crust at the Southern East Pacific Rise

    USGS Publications Warehouse

    Perk, N.W.; Coogan, L.A.; Karson, J.A.; Klein, E.M.; Hanna, H.D.

    2007-01-01

    A suite of samples collected from the uppermost part of the plutonic section of the oceanic crust formed at the southern East Pacific Rise and exposed at the Pito Deep has been examined. These rocks were sampled in situ by ROV and lie beneath a complete upper crustal section providing geological context. This is only the second area (after the Hess Deep) in which a substantial depth into the plutonic complex formed at the East Pacific Rise has been sampled in situ and reveals significant spatial heterogeneity in the plutonic complex. In contrast to the uppermost plutonic rocks at Hess Deep, the rocks studied here are generally primitive with olivine forsterite contents mainly between 85 and 88 and including many troctolites. The melt that the majority of the samples crystallized from was aggregated normal mid-ocean ridge basalt (MORB). Despite this high Mg# clinopyroxene is common despite model predictions that clinopyroxene should not reach the liquidus early during low-pressure crystallization of MORB. Stochastic modeling of melt crystallisation at various levels in the crust suggests that it is unlikely that a significant melt mass crystallized in the deeper crust (for example in sills) because this would lead to more evolved shallow level plutonic rocks. Similar to the upper plutonic section at Hess Deep, and in the Oman ophiolite, many samples show a steeply dipping, axis-parallel, magmatic fabric. This suggests that vertical magmatic flow is an important process in the upper part of the seismic low velocity zone beneath fast-spreading ridges. We suggest that both temporal and spatial (along-axis) variability in the magmatic and hydrothermal systems can explain the differences observed between the Hess Deep and Pito Deep plutonics. ?? Springer-Verlag 2007.

  17. Transition from rifted continental to oceanic crust at the southeastern Korean margin in the East Sea (Japan Sea)

    NASA Astrophysics Data System (ADS)

    Cho, H.; Kim, H.; Jou, H.; Hong, J.; Baag, C.

    2004-12-01

    The southeastern Korean margin documents the processes of continental rifting and seafloor spreading that eventually led to the opening of the southern part of the East Sea (Japan Sea). In this study, we present the transitional structure of the southeastern Korean margin and its formation process from rifted continental to 10 km thick normal oceanic crust. The two-dimensional P velocity model of the southeastern Korean margin was computed from ocean bottom seismometer data by tomographic inversion and distilled by iterative forward modeling. The crustal structure shows the emplacement of high-velocity (>7 km/s) lower crust under the continental shelf and slope area associated with a rapid transition from rifted continental to oceanic crust. The high-velocity lower crust is interpreted as magmatic underplating formed by voluminous igneous activity during rifting. Magnetic modeling confirms its primary correlation with a prominent magnetic anomaly along the edge of the southeastern Korean Peninsula that is assumed to represent volcanic extrusives and intrusives. The continental margin featuring a rapid transition from continental to oceanic crust exhibits a remarkable decrease in crustal thickness accompanied by shallowing of the Moho over a distance of about 50 km. It thus appears that the Korean margin experienced intense tectonism comprising crustal deformation and volcanism associated with the opening of the East Sea and consequently registered the early history of continental rifting and subsequent sea floor spreading. We suggest that the rifting and subsequent seafloor spreading at the Korean margin was significantly controlled by the supply of magma in a region of hotter than normal mantle temperature.

  18. Separate zones of sulfate and sulfide release from subducted mafic oceanic crust

    NASA Astrophysics Data System (ADS)

    Tomkins, Andrew G.; Evans, Katy A.

    2015-10-01

    Liberation of fluids during subduction of oceanic crust is thought to transfer sulfur into the overlying sub-arc mantle. However, despite the importance of sulfur cycling through magmatic arcs to climate change, magma oxidation and ore formation, there has been little investigation of the metamorphic reactions responsible for sulfur release from subducting slabs. Here, we investigate the relative stability of anhydrite (CaSO4) and pyrite (FeS2) in subducted basaltic oceanic crust, the largest contributor to the subducted sulfur budget, to place constraints on the processes controlling sulfur release. Our analysis of anhydrite stability at high pressures suggests that this mineral should dominantly dissolve into metamorphic fluids released across the transition from blueschist to eclogite facies (∼450-650 °C), disappearing at lower temperatures on colder geothermal trajectories. In contrast, we suggest that sulfur release via conversion of pyrite to pyrrhotite occurs at temperatures above 750 °C. This higher temperature stability is indicated by the preservation of pyrite-bornite inclusions in coesite-bearing eclogites from the Sulu Belt in China, which reached temperatures of at least 750 °C. Thus, sulfur may be released from subducting slabs in two separate pulses; (1) varying proportions of SO2, HSO4- and H2S are released via anhydrite breakdown at the blueschist-eclogite transition, promoting oxidation of remaining silicates in some domains, and (2) H2S is released via pyrite breakdown well into the eclogite facies, which may in some circumstances coincide with slab melting or supercritical liquid generation driven by influx of serpentinite-derived fluids. These results imply that the metallogenic potential in the sub-arc mantle above the subducting slab varies as a function of subduction depth, having the greatest potential above the blueschist-eclogite transition given the association between oxidised magmas and porphyry Cu(-Au-Mo) deposits. We speculate

  19. High Temperature Logging and Monitoring Instruments to Explore and Drill Deep into Hot Oceanic Crust.

    NASA Astrophysics Data System (ADS)

    Denchik, N.; Pezard, P. A.; Ragnar, A.; Jean-Luc, D.; Jan, H.

    2014-12-01

    Drilling an entire section of the oceanic crust and through the Moho has been a goal of the scientific community for more than half of a century. On the basis of ODP and IODP experience and data, this will require instruments and strategies working at temperature far above 200°C (reached, for example, at the bottom of DSDP/ODP Hole 504B), and possibly beyond 300°C. Concerning logging and monitoring instruments, progress were made over the past ten years in the context of the HiTI ("High Temperature Instruments") project funded by the european community for deep drilling in hot Icelandic geothermal holes where supercritical conditions and a highly corrosive environment are expected at depth (with temperatures above 374 °C and pressures exceeding 22 MPa). For example, a slickline tool (memory tool) tolerating up to 400°C and wireline tools up to 300°C were developed and tested in Icelandic high-temperature geothermal fields. The temperature limitation of logging tools was defined to comply with the present limitation in wireline cables (320°C). As part of this new set of downhole tools, temperature, pressure, fluid flow and casing collar location might be measured up to 400°C from a single multisensor tool. Natural gamma radiation spectrum, borehole wall ultrasonic images signal, and fiber optic cables (using distributed temperature sensing methods) were also developed for wireline deployment up to 300°C and tested in the field. A wireline, dual laterolog electrical resistivity tool was also developed but could not be field tested as part of HiTI. This new set of tools constitutes a basis for the deep exploration of the oceanic crust in the future. In addition, new strategies including the real-time integration of drilling parameters with modeling of the thermo-mechanical status of the borehole could be developed, using time-lapse logging of temperature (for heat flow determination) and borehole wall images (for hole stability and in-situ stress determination

  20. Extent and impact of Cretaceous magmatism on the formation and evolution of Jurassic oceanic crust in the western Pacific

    NASA Astrophysics Data System (ADS)

    Feng, H.; Lizarralde, D.; Tominaga, M.; Hart, L.; Tivey, M.; Swift, S. A.

    2015-12-01

    Multi-channel seismic (MCS) images and wide-angle sonobuoy data acquired during a 2011 cruise on the R/V Thomas G. Thompson (TN272) show widespread emplacement of igneous sills and broadly thickened oceanic Layer 2 through hundreds of kilometers of oceanic crust in one of the oldest ocean basins in the western Pacific, a region known as the Jurassic Quiet Zone (JQZ). Oceanic crust from the JQZ has grown through at least two main magmatic phases: It was formed by mid-ocean ridge processes in the Jurassic (at ~170 Ma), and then it was added to by a substantial Cretaceous magmatic event (at ~75-125 Ma). The scale of Cretaceous magmatism is exemplified by massive seafloor features such as the Ontong Java Plateau, Mid-Pacific Mountains, Marshall-Gilbert Islands, Marcus-Wake Seamount Chain, and numerous guyots, seamounts, and volcaniclastic flows observed throughout the region. We use seismic data to image heavily intruded and modified oceanic crust along an 800-km-long transect through the JQZ in order to examine how processes of secondary crustal growth - including magmatic emplacement, transport, and distribution - are expressed in the structure of modified oceanic crust. We also model gravity anomalies to constrain crustal thickness and depth to the Moho. Our observations suggest that western Pacific crust was modified via the following modes of emplacement: (a) extrusive seafloor flows that may or may not have grown into seamounts, (b) seamounts formed through intrusive diking that pushed older sediments aside during their formation, and (c) igneous sills that intruded sediments at varying depths. Emplacement modes (a) and (b) tend to imply a focused, pipe-like mechanism for melt transport through the lithosphere. Such a mechanism does not explain the observed broadly distributed intrusive emplacement of mode (c) however, which may entail successive sill emplacement between igneous basement and sediments thickening oceanic Layer 2 along ~400 km of our seismic line

  1. In situ enrichment of ocean crust microbes on igneous minerals and glasses using an osmotic flow-through device

    NASA Astrophysics Data System (ADS)

    Smith, Amy; Popa, Radu; Fisk, Martin; Nielsen, Mark; Wheat, C. Geoffrey; Jannasch, Hans W.; Fisher, Andrew T.; Becker, Keir; Sievert, Stefan M.; Flores, Gilberto

    2011-06-01

    The Integrated Ocean Drilling Program (IODP) Hole 1301A on the eastern flank of Juan de Fuca Ridge was used in the first long-term deployment of microbial enrichment flow cells using osmotically driven pumps in a subseafloor borehole. Three novel osmotically driven colonization systems with unidirectional flow were deployed in the borehole and incubated for 4 years to determine the microbial colonization preferences for 12 minerals and glasses present in igneous rocks. Following recovery of the colonization systems, we measured cell density on the minerals and glasses by fluorescent staining and direct counting and found some significant differences between mineral samples. We also determined the abundance of mesophilic and thermophilic culturable organotrophs grown on marine R2A medium and identified isolates by partial 16S or 18S rDNA sequencing. We found that nine distinct phylotypes of culturable mesophilic oligotrophs were present on the minerals and glasses and that eight of the nine can reduce nitrate and oxidize iron. Fe(II)-rich olivine minerals had the highest density of total countable cells and culturable organotrophic mesophiles, as well as the only culturable organotrophic thermophiles. These results suggest that olivine (a common igneous mineral) in seawater-recharged ocean crust is capable of supporting microbial communities, that iron oxidation and nitrate reduction may be important physiological characteristics of ocean crust microbes, and that heterogeneously distributed minerals in marine igneous rocks likely influence the distribution of microbial communities in the ocean crust.

  2. The effect of recycled oceanic crust in the thermal evolution of the Galapagos Plume

    NASA Astrophysics Data System (ADS)

    Gazel, E.; Herzberg, C. T.; Vidito, C. A.

    2011-12-01

    Current models suggest that the massive basaltic production responsible for the emplacement of Large Igneous Provinces (LIPS) during the Permian-Paleocene may represent the initial phases (plume heads) of some of the mantle plumes that feed the current ocean island basalts (OIB). In many cases, magmatism associated with the initiation of mantle plumes was so voluminous that produced global environmental impacts. The origin of these intra-plate magmatism is still debated but recent petrological, geochemical and geophysical studies of some of these localities like Samoa, Hawaii, Galapagos, provide evidence that melting is related to a true mantle plume, representing a geochemically heterogeneous, hot-buoyant domain that originates from a boundary layer beneath the upper mantle. Thus, plume-related magmas produced in OIB and LIPS and their connecting plume tracks are windows into the Earth's mantle, providing evidence on mantle temperature, size and composition of heterogeneities, and the deep earth geochemical cycles. Our preliminary petrological modeling suggests that mantle plumes for LIPS with Permian-Paleocene ages were generally hotter and melted more extensively than plumes of more modern oceanic islands. Although a lot of work has been done on LIPS and OIB, no complete record of the evolution of a mantle plume is available to this point, mostly due to the inaccessibility of the submerged sections of almost all plume tracks. Galapagos-related lavas provide a complete record of the evolution of a mantle plume since the plume's initial stages in the Cretaceous. In the case of the Galapagos, our work suggests a decrease from TP(max) of1650 C in the Cretaceous to 1500 C in the present day. Our recent work on the Galapagos Islands and the preliminary work on older Galapagos-related terranes suggest that this secular cooling is directly related with increasing amounts of recycled crust in the plume.

  3. Diversity of microbial communities in ocean crust below ancient hotspot seamounts along the Louisville Seamount Chain

    NASA Astrophysics Data System (ADS)

    Sylvan, J. B.; Edwards, K. J.

    2012-12-01

    The goal of Integrated Ocean Drilling Expedition 330, Louisville Seamount Trail, was to understand the motion of the Louisville hotspot during 50-80 Ma. As such, >1 km of volcanic basement was collected from five sites on four seamounts, providing an excellent chance to study how microbial populations are effected by different lithologies, different seamounts and age of basement rock along the Louisville Seamount Chain (LSC). Analysis of bacteria growing in enrichment incubations that targeted oligotrophs (with 1% or 10% Marine Broth 2216 diluted with 3% NaCl) and sulfur oxidizers reveals the presence of a diverse array of bacteria, including ɛ-proteobacteria closely related to Sulfurimonas autotrophica, β-proteobacterial methylotrophs, ζ-proteobacteria and Bacteroidetes most closely related to organisms cultured from sediments. Many of these sequences are <97% similar to the closest cultured relatives, indicating the subsurface LSC may be home to unique new species. One isolate recently obtained in pure culture from the original enrichments is 99% similar to Halomonas sulfidaeris str. Esulfude1, a bacterium originally isolated from a hydrothermal sulfide chimney. A second isolate may be a new species of Bacillus. Initial molecular analysis of bacterial communities by pyrosequencing of the 16S rRNA gene as part of the Census of Deep Life (CoDL) supports the data from the culturing work; in one sample collected 174 meters below seafloor, the most abundant bacteria detected include species from the genera Pseudomonas, Sulfurimonas, Methyloversatilis and Desulfocapsa. More CoDL samples will be analyzed in the near future. We will describe results to date on subsurface microbial diversity along the Louisville Seamount Chain from the culturing work and CoDL project and draw comparisons to data derived from younger crustal sites to try to understand how the LSC ecosystem fits into our global picture of life in ocean crust.

  4. Dehydration of lawsonite could directly trigger earthquakes in subducting oceanic crust

    NASA Astrophysics Data System (ADS)

    Okazaki, Keishi; Hirth, Greg

    2016-02-01

    Intermediate-depth earthquakes in cold subduction zones are observed within the subducting oceanic crust, as well as the mantle. In contrast, intermediate-depth earthquakes in hot subduction zones predominantly occur just below the Mohorovičić discontinuity. These observations have stimulated interest in relationships between blueschist-facies metamorphism and seismicity, particularly through dehydration reactions involving the mineral lawsonite. Here we conducted deformation experiments on lawsonite, while monitoring acoustic emissions, in a Griggs-type deformation apparatus. The temperature was increased above the thermal stability of lawsonite, while the sample was deforming, to test whether the lawsonite dehydration reaction induces unstable fault slip. In contrast to similar tests on antigorite, unstable fault slip (that is, stick-slip) occurred during dehydration reactions in the lawsonite and acoustic emission signals were continuously observed. Microstructural observations indicate that strain is highly localized along the fault (R1 and B shears), and that the fault surface develops slickensides (very smooth fault surfaces polished by frictional sliding). The unloading slope during the unstable slip follows the stiffness of the apparatus at all experimental conditions, regardless of the strain rate and temperature ramping rate. A thermomechanical scaling factor for the experiments is within the range estimated for natural subduction zones, indicating the potential for unstable frictional sliding within natural lawsonite layers.

  5. Dehydration of lawsonite could directly trigger earthquakes in subducting oceanic crust.

    PubMed

    Okazaki, Keishi; Hirth, Greg

    2016-02-04

    Intermediate-depth earthquakes in cold subduction zones are observed within the subducting oceanic crust, as well as the mantle. In contrast, intermediate-depth earthquakes in hot subduction zones predominantly occur just below the Mohorovičić discontinuity. These observations have stimulated interest in relationships between blueschist-facies metamorphism and seismicity, particularly through dehydration reactions involving the mineral lawsonite. Here we conducted deformation experiments on lawsonite, while monitoring acoustic emissions, in a Griggs-type deformation apparatus. The temperature was increased above the thermal stability of lawsonite, while the sample was deforming, to test whether the lawsonite dehydration reaction induces unstable fault slip. In contrast to similar tests on antigorite, unstable fault slip (that is, stick-slip) occurred during dehydration reactions in the lawsonite and acoustic emission signals were continuously observed. Microstructural observations indicate that strain is highly localized along the fault (R1 and B shears), and that the fault surface develops slickensides (very smooth fault surfaces polished by frictional sliding). The unloading slope during the unstable slip follows the stiffness of the apparatus at all experimental conditions, regardless of the strain rate and temperature ramping rate. A thermomechanical scaling factor for the experiments is within the range estimated for natural subduction zones, indicating the potential for unstable frictional sliding within natural lawsonite layers.

  6. Observations of oceanic crust and mantle structures at a deep ocean seismic array in the Eastern Mid Atlantic

    NASA Astrophysics Data System (ADS)

    Hannemann, Katrin; Krüger, Frank; Dahm, Torsten

    2016-04-01

    In 2011, twelve ocean bottom stations (OBS) were installed approximately 100 km North of the Gloria Fault during the DOCTAR project (Deep OCean Test ARray). This fault marks the plate boundary between the Eurasian and African plate in the North Eastern Mid Atlantic. The experiment took place in water depth of 4-6 km, 800 km West of the Portuguese coast. The stations were equipped with broad band seismometers which recorded for ten months. We employ P and S receiver functions (RF) to have a closer look at the structure of crust and mantle. The ocean is a quite noisy environment, therefore the number of usable events is low (around 20) compared to RF studies on land. We use several quality criteria (e.g. signal to noise ratio, relative spike position) to select proper processing parameters for the calculation of the RF and carefully reviewed all later on used RF. Despite the low number of events, the usage of an array of OBS with an aperture of 75 km allows us to investigate deeper discontinuities (e.g. in 410 and 660 km depth) compared to single station approaches which are usually employed for OBS. Furthermore, we increase the number of usable events by applying array methods. We use move out corrected and stacked RF to have a closer look at the mantle transition zone, and estimate average depth values for the Moho, the lithosphere asthenosphere boundary (LAB) and the base of the asthenosphere. The Moho lies at depth of 7 km, the LAB at approximately 50 km and the asthenosphere has an approximated thickness of 110 km. We observe a slight increase in the time difference of the mantle discontinuity conversion times compared to PREM. RF give just information regarding the impedance contrast at a discontinuity instead of velocities. We additionally use P wave polarization of teleseismic events to estimate absolute S velocities beneath the single stations. All in all, we use the information gained by the RF analysis, and the analysis of the P wave polarization to

  7. Silicon Isotope Geochemistry of Ocean Island Basalts: Search for Deep Mantle Heterogeneities and Evidence for Recycled Altered Oceanic Crust

    NASA Astrophysics Data System (ADS)

    Pringle, E. A.; Savage, P. S.; Jackson, M. G.; Moreira, M. A.; Day, J. M.; Moynier, F.

    2014-12-01

    recycled altered oceanic crust in the plume source. However, the sampling of a primitive reservoir enriched in the light isotopes of Si, as suggested by [4], cannot be ruled out as a potential source of Si isotope variations in OIBs. References: [1] Ziegler et al., GCA 2005 [2] Savage et al., GCA 2011 [3] Savage et al., EPSL 2010 [4] Huang et al., GCA 2014

  8. Silicon Isotope Geochemistry of Ocean Island Basalts: Mantle Heterogeneities and Contribution of Recycled Oceanic Crust and Lithosphere

    NASA Astrophysics Data System (ADS)

    Pringle, E. A.; Moynier, F.; Savage, P. S.; Jackson, M. G.; Moreira, M. A.; Day, J. M.

    2015-12-01

    altered oceanic crust and lithosphere in the plume source. References: [1] Ziegler et al., GCA 2005 [2] Savage et al., GCA 2011 [3] Savage et al., EPSL 2010 [4] Day et al., Geology 2009 [5] Huang et al., GCA 2014

  9. Constraints on the accretion of the gabbroic lower oceanic crust from plagioclase lattice preferred orientation in the Samail ophiolite

    NASA Astrophysics Data System (ADS)

    VanTongeren, J. A.; Hirth, G.; Kelemen, P. B.

    2015-10-01

    Oceanic crust represents more than 60% of the earth's surface and despite a large body of knowledge regarding the formation and chemistry of the extrusive upper oceanic crust, there still remains significant debate over how the intrusive gabbroic lower oceanic crust is accreted at the ridge axis. The two proposed end-member models, the Gabbro Glacier and the Sheeted Sills, predict radically different strain accumulation in the lower crust during accretion. In order to determine which of these two hypotheses is most applicable to a well-studied lower crustal section, we present data on plagioclase lattice preferred orientations (LPO) in the Wadi Khafifah section of the Samail ophiolite. We observe no systematic change in the strength of the plagioclase LPO with height above the crust-mantle transition, no dominant orientation of the plagioclase a-axis lineation, and no systematic change in the obliquity of the plagioclase LPO with respect to the modal layering and macroscopic foliation evident in outcrop. These observations are most consistent with the Sheeted Sills hypothesis, in which gabbros are crystallized in situ and fabrics are dominated by compaction and localized extension rather than by systematically increasing shear strain with increasing depth in a Gabbro Glacier. Our data support the hypothesis of MacLeod and Yaouancq (2000) that the rotation of the outcrop-scale layering from sub-horizontal in the layered gabbros to sub-vertical near the sheeted dikes is due to rapid vertical melt migration through upper gabbros close to the axial magma chamber. Additionally, our results support the hypothesis that the majority of extensional strain in fast spreading ridges is accommodated in partially molten regions at the ridge axis, whereas in slow and ultra-slow ridges large shear strains are accommodated by plastic deformation.

  10. Geochemical investigation of Gabbroic Xenoliths from Hualalai Volcano: Implications for lower oceanic crust accretion and Hualalai Volcano magma storage system

    NASA Astrophysics Data System (ADS)

    Gao, Ruohan; Lassiter, John C.; Barnes, Jaime D.; Clague, David A.; Bohrson, Wendy A.

    2016-05-01

    The patterns of axial hydrothermal circulation at mid-ocean ridges both affect and are influenced by the styles of magma plumbing. Therefore, the intensity and distribution of hydrothermal alteration in the lower oceanic crust (LOC) can provide constraints on LOC accretion models (e.g., ;gabbro glacier; vs. ;multiple sills;). Gabbroic xenoliths from Hualalai Volcano, Hawaii include rare fragments of in situ Pacific lower oceanic crust. Oxygen and strontium isotope compositions of 16 LOC-derived Hualalai gabbros are primarily within the range of fresh MORB, indicating minimal hydrothermal alteration of the in situ Pacific LOC, in contrast to pervasive alteration recorded in LOC xenoliths from the Canary Islands. This difference may reflect less hydrothermal alteration of LOC formed at fast ridges than at slow ridges. Mid-ocean ridge magmas from slow ridges also pond on average at greater and more variable depths and undergo less homogenization than those from fast ridges. These features are consistent with LOC accretion resembling the ;multiple sills; model at slow ridges. In contrast, shallow magma ponding and limited hydrothermal alteration in LOC at fast ridges are consistent with the presence of a long-lived shallow magma lens, which limits the penetration of hydrothermal circulation into the LOC. Most Hualalai gabbros have geochemical and petrologic characteristics indicating derivation from Hualalai shield-stage and post-shield-stage cumulates. These xenoliths provide information on the evolution of Hawaiian magmas and magma storage systems. MELTS modeling and equilibration temperatures constrain the crystallization pressures of 7 Hualalai shield-stage-related gabbros to be ∼2.5-5 kbar, generally consistent with inferred local LOC depth. Therefore a deep magma reservoir existed within or at the base of the LOC during the shield stage of Hualalai Volcano. Melt-crust interaction between Hawaiian melts and in situ Pacific crust during magma storage partially

  11. Microbial diversity in deep-sea sediment from the cobalt-rich crust deposit region in the Pacific Ocean.

    PubMed

    Liao, Li; Xu, Xue-Wei; Jiang, Xia-Wei; Wang, Chun-Sheng; Zhang, Dong-Sheng; Ni, Jian-Yu; Wu, Min

    2011-12-01

    Cobalt-rich crusts are important metallic mineral resources with great economic potential, usually distributed on seamounts located in the Pacific Ocean. Microorganisms are believed to play a role in the formation of crusts as well as in metal cycling. To explore the microbial diversity related to cobalt-rich crusts, 16S ribosomal RNA gene clone libraries were constructed from three consecutive sediment layers. In total, 417 bacterial clones were obtained from three bacterial clone libraries, representing 17 distinct phylogenetic groups. Proteobacteria dominated in the bacterial communities, followed by Acidobacteria and Planctomycetes. Compared with high bacterial diversity, archaea showed a remarkably low diversity, with all 137 clones belonging to marine archaeal group I except one novel euryarchaeotal clone. The microbial communities were potentially involved in sulfur, nitrogen and metal cycling in the area of cobalt-rich crusts. Sulfur oxidation and metal oxidation were potentially major sources of energy for this ecosystem. This is the first reported investigation of microbial diversity in sediments associated with cobalt-rich crusts, and it casts fresh light on the microbial ecology of these important ecosystems.

  12. Magnetic mapping of (carbonated) oceanic crust-mantle boundary: New insights from Linnajavri, northern Norway

    NASA Astrophysics Data System (ADS)

    Tominaga, M.; Beinlich, A.; Tivey, M.; Andrade Lima, E.; Weiss, B. P.

    2012-12-01

    The contribution of lower oceanic crust and upper mantle to marine magnetic anomalies has long been recognized, but the detailed magnetic character of this non-volcanic source layer remains to be fully defined. Here, we report preliminary results of a magnetic survey and source characterization of a "carbonated" oceanic Moho (petrological "Mohorovicic discontinuity") sequence observed at the Linnajavri Serpentinite Complex (LSC), northern Norway. The LSC is located at 67° 36'N and 16° 24'E within the upper Allochthon of the Norwegian Caledonides and represents a dismembered ophiolite. Particularly in the southern ("Ridoalggicohkka") area of the LSC, gabbro, serpentinite and its talc-carbonate (soapstone) and quartz-carbonate (listvenite) altered equivalents are extraordinarily well-exposed [1]. An intact oceanic Moho is exposed here, despite its complex tectonic setting. The small degree of arctic rock weathering (≤ 2 mm weathering surface) allowed for detailed regional-scale surface magnetic mapping across alteration fronts (serpentinite-soapstone; soapstone-listvenite) and lithological contacts (soapstone-gabbro). Magnetic mapping was conducted using a handheld 3-axis magnetometer, surface-towed resistivity meter and Teka surface magnetic susceptometer with sample spacing of 1 m. Geophysical field mapping was combined with petrological observations and scanning SQUID microscopy (SM) mapping conducted on thin sections from rock samples that were drilled along the survey lines. Regional scale magnetic mapping indicates that the total magnetic field across both the "carbonated" Moho and the soapstone-serpentinite interfaces show higher frequency changes in their magnetic anomaly character and amplitudes than the surface-towed resistivity data. SQUID microscopy mapping of both natural remanence magnetization (NRM) and anhysteretic remanence magnetization (ARM) on gabbro, serpentinite, soapstone, and listvenite samples, with a sensor-sample separation of ˜190

  13. Lithium and Li-isotopes in young altered upper oceanic crust from the East Pacific Rise

    NASA Astrophysics Data System (ADS)

    Brant, C.; Coogan, L. A.; Gillis, K. M.; Seyfried, W. E.; Pester, N. J.; Spence, J.

    2012-11-01

    Lithium contents and Li-isotope ratios have been measured in a section of young oceanic upper crust formed at the East Pacific Rise and exposed in the Hess Deep rift. The Li contents of both lavas and sheeted dikes are lower than in the fresh protolith (by 43% and 74%, respectively) suggesting that axial hydrothermal systems leach Li from the entire upper crust. The Li-isotopic composition of the lavas (4.1 ± 1.7‰) is similar to the protolith (3.4 ± 1.4‰) whereas the dikes extend to both higher and lower isotopic compositions (δ7Li from -11 to 14‰). Loss of Li from all lava samples was unexpected due to the petrographically fresh appearance of these rocks and the relatively cool fluids generally thought to exist within the lava pile. This Li loss is interpreted as indicating that warm (50-100 °C) fluids pervade the lava pile, at least episodically, leaching Li from the lavas without recrystallization of the primary igneous minerals. This hypothesis has been tested experimentally. Hydrothermal experiments at 125 °C, using synthetic basalts cooled at different rates to produce different starting material texture (glass to crystalline rock), demonstrate that Li is leached rapidly at this temperature and substantially more rapidly from crystalline rock than from glass. The natural and experimental data are consistent with Li diffusion out of plagioclase into warm hydrothermal fluids acting to leach Li from the lavas without any concomitant mineralogical changes. The wide range of δ7Li in the sheeted dike complex, and the lack of a systematic correlation of δ7Li with the concentration of Li, suggest that more than one process leads to Li depletion. A combination of mineral-fluid reactions that break-down igneous minerals and produce secondary minerals (principally amphibole, chlorite and secondary plagioclase), along with diffusive loss of Li from plagioclase into the fluid, can explain the Li-systematics of the sheeted dike complex at Hess Deep.

  14. High Melt Porosity in the Lower Oceanic Crust Inferred from Phosphorus Zoning in Olivine

    NASA Astrophysics Data System (ADS)

    Hellebrand, E.; Welsch, B. T.; Hammer, J. E.

    2013-12-01

    The canonical view that the lower oceanic crust is composed of cumulates of fractional crystallization has been intensely debated in recent years. Migrating melts, reacting with previously crystallized minerals in the crystal mush, can modify the mineralogy and phase proportions inside the lower crust, as well as the composition of erupted MORB [1]. An extreme product of reactive melt migration was discovered during IODP Legs 304/305 at Atlantis Massif (MAR 30N). In this 1.5 km deep drillhole, there are several sequences of olivine-rich troctolite with ';textbook cumulate texture', which may be associated with the contact of a gabbroic pluton into peridotite [2,3]. While there is little ambiguity about the geological relationships, the exact mechanism for the in-situ reactive transformation of mantle peridotites into lower crustal gabbroic lithologies is still poorly understood. One widespread textural feature in support of the dominant role of reactive melt migration is the occurrence of interstitial and vermicular high-Mg# cpx, which form post-compaction at very low melt porosities. The rare screens of opx-bearing mantle peridotites in this drillhole also preserve evidence for low-porosity replacement of mantle opx by gabbroic cpx [4], with minimal volume change. However, we will show that a significant and possibly the main mass of the olivine crystals in the olivine-rich troctolites do not form at low melt porosities, but instead in a melt-rich local environment. Initially, olivines crystallize as rapidly grown dendrites, which is marked by distinct enrichments of the slowly diffusing element phosphorus. Subsequent slow growth produces the main mass of the otherwise P-free olivine crystal. Our observations on natural basalt-hosted and experimentally grown olivines indicate that strong undercooling in a crystal-poor environment is required for dendrite formation. By extrapolation, this would require a crystal-poor melt lens at the top of an evolving gabbroic

  15. Continental hyperextension, mantle exhumation and thin oceanic crust at the continent-ocean transition, West Iberia: new insights from wide-angle seismic

    NASA Astrophysics Data System (ADS)

    Davy, Richard; Minshull, Tim; Bayrakci, Gaye; Bull, Jon; Klaeschen, Dirk; Papenberg, Cord; Reston, Timothy; Sawyer, Dale; Zelt, Colin

    2016-04-01

    Anomalously thin oceanic crust and expanses of exhumed and serpentinised mantle material at magma-poor rift margins are now a globally observed phenomena that characterizes the seaward limit of the continent-ocean transition. Hyperextension of continental crust at the Deep Galicia rifted margin in the North Atlantic has been accommodated by the rotation of continental fault blocks, which are underlain by the S-reflector, an interpreted detachment fault, along which exhumed and serpentinized mantle peridotite is observed. West of these features, the enigmatic Peridotite Ridge has been suggested to delimit the seaward extent of the continent-ocean transition. An outstanding question at this margin is where oceanic crust begins, with little existing data to constrain this boundary and a lack of clear seafloor spreading magnetic anomalies. Here we present results from a 160-km-long wide-angle seismic profile (WE-1). Forward modelling and travel time tomography models of the crustal compressional velocity structure reveal highly thinned and rotated crustal blocks overlying the S-reflector, which correlates with the 6.0 - 7.0 kms-1 velocity contours, corresponding to peridotite serpentinization of 60 - 30 %, respectively. West of the Peridotite Ridge we observe a basement layer which is 2.8 - 3.5 km thick in which velocities increase smoothly and rapidly from ˜4.6 kms-1 to 7.3 - 7.6 kms-1,with an average velocity gradient of 1.00 s-1. Below this, velocities slowly increase toward typical mantle velocities. Such a downward increase into mantle velocities is interpreted as decreasing serpentinization of mantle material with depth. However, sparse Moho reflections indicate the onset of an anomalously thin oceanic crust, which increases in thickness from ˜0.5 km to ˜1.5 km over a distance of 35 km, seaward.

  16. Predictions of hydrothermal alteration within near-ridge oceanic crust from coordinated geochemical and fluid flow models

    USGS Publications Warehouse

    Wetzel, L.R.; Raffensperger, J.P.; Shock, E.L.

    2001-01-01

    Coordinated geochemical and hydrological calculations guide our understanding of the composition, fluid flow patterns, and thermal structure of near-ridge oceanic crust. The case study presented here illustrates geochemical and thermal changes taking place as oceanic crust ages from 0.2 to 1.0 Myr. Using a finite element code, we model fluid flow and heat transport through the upper few hundred meters of an abyssal hill created at an intermediate spreading rate. We use a reaction path model with a customized database to calculate equilibrium fluid compositions and mineral assemblages of basalt and seawater at 500 bars and temperatures ranging from 150 to 400??C. In one scenario, reaction path calculations suggest that volume increases on the order of 10% may occur within portions of the basaltic basement. If this change in volume occurred, it would be sufficient to fill all primary porosity in some locations, effectively sealing off portions of the oceanic crust. Thermal profiles resulting from fluid flow simulations indicate that volume changes along this possible reaction path occur primarily within the first 0.4 Myr of crustal aging. ?? 2001 Elsevier Science B.V. All rights reserved.

  17. Structure, porosity and stress regime of the upper oceanic crust: Sonic and ultrasonic logging of DSDP Hole 504B

    USGS Publications Warehouse

    Newmark, R.L.; Anderson, R.N.; Moos, D.; Zoback, M.D.

    1985-01-01

    The layered structure of the oceanic crust is characterized by changes in geophysical gradients rather than by abrupt layer boundaries. Correlation of geophysical logs and cores recovered from DSDP Hole 504B provides some insight into the physical properties which control these gradient changes. Borehole televiewer logging in Hole 504B provides a continuous image of wellbore reflectivity into the oceanic crust, revealing detailed structures not apparent otherwise, due to the low percentage of core recovery. Physical characteristics of the crustal layers 2A, 2B and 2C such as the detailed sonic velocity and lithostratigraphic structure are obtained through analysis of the sonic, borehole televiewer and electrical resistivity logs. A prediction of bulk hydrated mineral content, consistent with comparison to the recovered material, suggests a change in the nature of the alteration with depth. Data from the sonic, borehole televiewer, electrical resistivity and other porosity-sensitive logs are used to calculate the variation of porosity in the crustal layers 2A, 2B and 2C. Several of the well logs which are sensitive to the presence of fractures and open porosity in the formation indicate many zones of intense fracturing. Interpretation of these observations suggests that there may be a fundamental pattern of cooling-induced structure in the oceanic crust. ?? 1985.

  18. Imaging The Flat Slab Beneath The Sierras Pampeanas, Argentina, Using Receiver Function Analysis: Evidence For Overthickened Subducted Oceanic Crust

    NASA Astrophysics Data System (ADS)

    Gans, C.; Beck, S. L.; Zandt, G.; Gilbert, H. J.; Alvarado, P. M.; Linkimer, L.; Porter, R. C.

    2009-12-01

    The western margin of the South American continent between 30°and 32° S is characterized by the flat slab subduction of the ~43 Ma oceanic Nazca plate beneath the continental South American plate. Several arrays of broadband seismic instruments have been deployed in Chile and western Argentina to study this phenomenon (e.g., CHARGE, 2000-2002; SIEMBRA, 2007-2009; ESP, 2008-2010). The low angle subduction has prevented magmatism in the area since the late Miocene due to reduced mantle flow above the subducting slab, and spatially correlates with the formation of both thick-skinned (Sierras Pampeanas) and thin-skinned (Andean Precordillera) thrust belts within the region. In order to better constrain the crust and upper mantle structure in the transition region between flat slab and normal subduction to the south and east, we have calculated receiver functions (RFs) from teleseismic earthquakes. Using our dense SIEMBRA array, combined with the broader CHARGE and ESP arrays, we are able to image in detail the flat slab, which contains a distinct negative arrival (indicative of a low velocity zone) at the top of the flat slab, followed by a strong positive P-to-S conversion. While the exact causes of flat slab subduction continue to be debated, one overriding theme is the necessity of having an overthickened crust in order to increase the buoyancy of the subducting slab. In this region, the hotspot seamount chain of the Juan Fernandez Ridge (JFR) is thought to provide such a mechanism. Kopp et al. (2004), however, did not find overthickened crust in the offshore portion of the JFR, but rather moderately thick oceanic crust. Preliminary results from our receiver functions, compared with synthetic RFs containing either a normal (7 km) or an overthickened (17km) crust, indicate that the oceanic crust at the top of the slab (the low velocity zone) must be at least ~15 km thick. Our results support the idea of an overthickened crust in the subducted flat slab beneath

  19. The Generation of Oceanic Lithosphere in an Embryonic Oceanic Crust : the Example of the Chenaillet Ophiolite in the Western Alps

    NASA Astrophysics Data System (ADS)

    Masini, E.; Manatschal, G.; Muntener, O.

    2007-12-01

    fault. Across the whole Chenaillet Ophiolite, volcanic rocks directly overlie either the detachment fault or the sediments. In several places, N-S trending high-angle normal faults have been mapped. These faults truncate and displace the detachment fault leading to small domino-like structures. The basins, limited by these high-angle faults, are some hundreds to a few kilometres wide and few tens to some hundreds of meters deep. Because these high- angle faults are sealed locally by basalts and obliterated by volcanic structures, we interpret them as oceanic structures being active during the emplacement of the basalts. The alignment of porphyritic basaltic dykes parallel to, and their increasing abundance towards the high-angle faults suggest that they may have served as feeder channels for the overlying volcanic rocks. The complex poly-phase tectonic and magmatic processes observed in the Chenaillet Ophiolite are reminiscent of those reported from slow to ultraslow spreading ridges. The key result from our study is that mantle exhumation along detachment faults is followed by syn-magmatic normal faulting resulting in the emplacement of laterally variable, up to 300 meters thick massive lavas and pillow basalts covering the exhumed detachment fault. This implies that off-axis processes are more important as previously assumed and that large-scale detachment faults may be buried under massive volcanic sequences suggesting that detachment faulting is presumably more common than suggested by dredging or morpho-structural investigations of ultra- to slow- spreading oceanic crust.

  20. Zircon record of fractionation, hydrous partial melting and thermal gradients at different depths in oceanic crust (ODP Site 735B, South-West Indian Ocean)

    NASA Astrophysics Data System (ADS)

    Pietranik, A.; Storey, C.; Koepke, J.; Lasalle, S.

    2017-03-01

    Felsic veins (plagiogranites) are distributed throughout the whole oceanic crust section and offer insight into late-magmatic/high temperature hydrothermal processes within the oceanic crust. Despite constituting only 0.5% of the oceanic crust section drilled in IODP Site 735B, they carry a significant budget of incompatible elements, which they redistribute within the crust. Such melts are saturated in accessory minerals, such as zircon, titanite and apatite, and often zircon is the only remaining phase that preserves magmatic composition and records processes of felsic melt formation and evolution. In this study, we analysed zircon from four depths in IODP Site 735B; they come from the oxide gabbro (depth approximately 250 m below sea floor) and plagiogranite (depths c. 500, 860, 940 m below sea floor). All zircons have similar ɛHf composition of c. 15 units indicating an isotopically homogenous source for the mafic magmas forming IODP Site 735B gabbro. Zircons from oxide gabbro are scarce and variable in composition consistent with their crystallization from melts formed by both fractionation of mafic magmas and hydrous remelting of gabbro cumulate. On the other hand, zircon from plagiogranite is abundant and each sample is characterized by compositional trends consistent with crystallization of zircon in an evolving melt. However, the trends are different between the plagiogranite at 500 m bsf and the deeper sections, which are interpreted as the record of plagiogranite formation by two processes: remelting of gabbro cumulate at 500 m bsf and fractionation at deeper sections. Zircon from both oxide gabbro and plagiogranite has δ18O from 3.5 to 6.0‰. Values of δ18O are best explained by redistribution of δ18O in a thermal gradient and not by remelting of hydrothermally altered crust. Tentatively, it is suggested that fractionation could be an older episode contemporaneous with gabbro crystallization and remelting could be a younger one, triggered by

  1. Shallow oceanic crust: Full waveform tomographic images of the seismic layer 2A/2B boundary

    NASA Astrophysics Data System (ADS)

    Christeson, Gail L.; Morgan, Joanna V.; Warner, Michael R.

    2012-05-01

    We present results of full-waveform tomographic inversions of four profiles acquired over young intermediate- and fast spreading rate oceanic crust. The mean velocity-depth functions from our study include a 0.25-0.30 km-thick low-velocity, low-gradient region beneath the seafloor overlying a 0.24-0.28-km-thick high-gradient region; together these regions compose seismic layer 2A. Mean layer 2A interval velocities are 3.0-3.2 km/s. The mean depth to the layer 2A/2B boundary is 0.49-0.54 km, and mean velocities within the upper 0.25 km of layer 2B are 4.7-4.9 km/s. Previous velocity analyses of the study areas using 1-D ray tracing underestimate the thickness of the high-gradient region at the base of layer 2A. We observe differences in the waveform inversion velocity models that correspond to imaging of the layer 2A event; regions with a layer 2A event have higher velocity gradients at the base of layer 2A. Intermittent high velocities, which we interpret as massive flows, are observed in the waveform inversion velocity models at 0.05-0.10 km below the seafloor (bsf) over 10-25% of the intermediate-spreading profiles and 20-45% of the fast spreading profiles. The high-gradient region located 0.25-0.54 km bsf at the base of layer 2A may be associated with an increased prevalence of massive flows, the first appearance of dikes (lava-dike transition zone), or with increased crack sealing by hydrothermal products. The upper portion of layer 2B, which begins at 0.49-0.54 km bsf, may correspond to sheeted dikes or the top of the transition zone of lavas and dikes.

  2. Evidence for ocean-continent crust boundary beneath the abyssal plain of the East Central Atlantic

    NASA Astrophysics Data System (ADS)

    Storetvedt, K. M.

    1987-09-01

    A survey of geophysical results and basalt characteristics of the East Central Atlantic suggests that such data are at present unable to define the seaward limit of the thinned continental crust. The combined evidence from margin sedimentation, deep-sea diapirism, salinity concentration in DSDP-IPOD cores, and the distribution of deep-sea barite and palygorskite-sepiolite assemblages indicate that the Central Atlantic developed from a wide rift basin within a normal continental setting. The notion of an extensive pre-drift basin gains additional support from the occurrences of Lower Cretaceous black shales which are interpreted as resulting from a tectonomagmatic forerunner phase to the actual continental separation process. Seafloor spreading which appears to have commenced at around 90 Ma B.P. (Cenomanian-Turonian), following major phases of subsidence and crustal attenuation in Jurassic and Lower Cretaceous, is identified by an apparently sharp change-over from reducing to oxygenated deep-sea environment as well as by the 'onset' of a major sedimentary hiatus. The new development model of the East Central Atlantic is regarded as a representative example of a global pattern; commencement of seafloor spreading in the Upper Cretaceous probably explains the world wide 'Cenomanian' transgression as well as the formation of extensive Upper Cretaceous sedimentary basins in the interior of the major continental blocks. A consequence of this model is that vertical crustal dynamics seem to be as important as seafloor spreading in the development of the oceanic lithosphere. Thus, in the Central Atlantic spreading is probably confined only to the region of the elevated Mid-Atlantic Ridge.

  3. Geochronologic and isotopic study of the La Désirade island basement complex: Jurassic oceanic crust in the Lesser Antilles?

    NASA Astrophysics Data System (ADS)

    Mattinson, James M.; Fink, L. Kenneth; Hopson, Clifford A.

    1980-01-01

    La Désirade, a small island east of Guadeloupe, is underlain by the only exposed pre-Tertiary basement rocks in the Lesser Antilles. The basement complex comprises spilitic and keratophyric flows and pillow lavas (with interbedded and overlying radiolarian cherts), swarms of mafic to silicic dikes, and subjacent plagiogranite. These features, and the absence of carbonates, terrigenous clastic sediments, or tuffaceous sediments from the complex indicate that it developed in a deep marine environment beyond the reach of terrigenous sedimentation or emergent island arc pyroclastic deposition. Previous workers have suggested that the Désirade basement complex originated either as oceanic crust or during an early (tholeiitic) stage of island arc growth. The isotopic compositions of Sr and Pb from the complex, and previously reported rare earth data (Johnston and Schilling, 1974) do not provide a clear distinction between these two possibilities. Nor does the presence of siliceous keratophyre in the complex rule out an oceanic crustal origin-such rocks are common in well studied ophiolites that originated as oceanic crust. Hence we turn to the age relationships of the complex, the surrounding ocean floor, and adjacent island arcs in an attempt to resolve this problem. The age of the complex strongly supports an oceanic crustal (ophiolitic) origin. The ages of zircons and a previously reported K-Ar age indicate that the complex is 145±5 m.y. old. The complex predates the next oldest volcanic rocks of the Lesser Antilles arc by ca. 110 m.y., and the oldest known rocks of the Aves Ridge, a possible Mesozoic precursor of the Lesser Antilles arc, by 50 60 m.y. This makes it unlikely that the Désirade complex is related to an early phase of either of these arcs. Instead, the age of the complex falls in the range of ages expected for oceanic crust in the vicinity of the Lesser Antilles prior to the development of any subduction zone and resulting arc. Thus we interpret the

  4. Copper and zinc isotope systematics of altered oceanic crust at IODP Site 1256 in the eastern equatorial Pacific

    NASA Astrophysics Data System (ADS)

    Huang, Jian; Liu, Sheng-Ao; Gao, Yongjun; Xiao, Yilin; Chen, Sha

    2016-10-01

    This paper presents the first combined Cu and Zn isotopic study of altered oceanic crust at Integrated Ocean Drilling Program (IODP) Hole 1256D that penetrates a volcanic section, a lava-dyke transition zone, a sheeted dyke complex, and a plutonic complex. In the volcanic section, all but one rocks have Cu and Zn isotopic compositions similar to fresh mid-ocean ridge basalt (MORB), reflecting restricted seawater circulation and low oxygen fugacity. Rocks in the transition zone have MORB-like δ65Cu and δ66Zn, indicating the dominant influence of basalt-derived Cu and Zn during alteration. Rocks in the dyke complex have more variable δ65Cu (-0.50-0.90‰) and δ66Zn (0.19-0.55‰) and those in the plutonic complex have δ65Cu of -0.43 to 0.20‰ and δ66Zn of 0.21 to 0.41‰. The rocks with heavier δ66Zn and heavier or lighter δ65Cu relative to MORB are characterized by Cu-Zn depletions, low Li/Yb (<1.0) and low δ18O (<5‰), suggesting that hydrothermal extraction during high temperature alteration of oceanic crust can result in significant Cu and Zn isotope fractionation. Such large Cu and Zn isotopic variations are the results of redox transformation of Cu as well as Cu and Zn isotope fractionation between altered basaltic rocks and dissolved Cu and Zn species in hydrothermal fluids (e.g., [CuCl3]1-, Zn(HS)42-). This work is the first to define the distribution of Cu and Zn isotopes in an intact oceanic crust with concentration-weighted averages of δ65Cu (0.05 ± 0.03‰) and δ66Zn (0.27 ± 0.01‰). The potential implications of these new observations are discussed.

  5. Magmatic Diversity of the Wehrlitic Intrusions in the Oceanic Lower Crust of the Northern Oman Ophiolite

    NASA Astrophysics Data System (ADS)

    Kaneko, R.; Adachi, Y.; Miyashita, S.

    2014-12-01

    The Oman ophiolite extends along the east coast of Oman, and is the world's largest and best-preserved slice of obducted oceanic lithosphere. The magmatic history of this ophiolite is complex and is generally regarded as having occurred in three stages (MOR magmatism, subduction magmatism and intraplate magmatism). Wehrlitic intrusions constitute an important element of oceanic lower crust of the ophiolite, and numerous intrusions cut gabbro units in the northern Salahi block of this ophiolite. In this study area, we identified two different types of wehrlitic intrusions. One type of the intrusions mainly consists of dunite, plagioclase (Pl) wehrlite and mela-olivine (Ol) gabbro, in which the crystallization sequence is Ol followed by the contemporaneous crystallization of Pl and clinopyroxene (Cpx). This type is called "ordinary" wehrlitic intrusions and has similar mineral compositions to host gabbros (Adachi and Miyashita 2003; Kaneko et al. 2014). Another type of the intrusions is a single intrusion that crops out in an area 250 m × 150 m along Wadi Salahi. This intrusion consists of Pl-free "true" wehrlite, in which the crystallization sequence is Ol and then Cpx. The forsterite contents (Fo%) of Ol from the "ordinary" wehrlitic intrusions and "true" wehrlitic intrusions have ranges of 90.8-87.0 (NiO = 0.36-0.13 wt%) and 84.7 (NiO = 0.31 wt%), respectively. Cr numbers (Cr#) of Cr-spinel from the "true" wehrlitic intrusions show higher Cr# value of 0.85 than those of the "ordinary" wehrlitic intrusions (0.48-0.64). But the former is characterized by very high Fe3+ values (YFe3+ = 0.49-0.68). Kaneko et al. (2014) showed that the "ordinary" ubiquitous type has similar features to MOR magmatism and the depleted type in the Fizh block (Adachi and Miyashita 2003) links to subduction magmatism. These types are distinguished by their mineral chemistries (TiO2 and Na2O contents of Cpx). The TiO2 and Na2O contents of Cpx from the "true" wehrlitic intrusions have 0

  6. Velocity structure in upper ocean crust at Hole 504B from vertical seismic profiles

    NASA Astrophysics Data System (ADS)

    Swift, Stephen A.; Lizarralde, D.; Stephen, Ralph A.; Hoskins, Hartley

    1998-07-01

    Hole 504B provides the only opportunity to directly correlate seismic velocity structure to the lithology and physical properties of upper ocean crust, providing a baseline for comparison with seismic measurements elsewhere. We determine P and S velocities from vertical seismic profiles (VSPs) obtained on Ocean Drilling Program (ODP) Legs 111 and 148. Four issues are considered: the location of the seismic layer 2/3 boundary, P to S wave conversion by scattering, transverse isotropy, and Poisson's ratio as an indicator of lithology, porosity, and structure. (1) In the P velocity profile, the change in slope marking the layer 2/3 boundary coincides with the top of the sheeted dike unit. Seismic layer 2 is composed of the extrusives and the lithologic transition zone, the layer in which flows and dikes interfinger. (2) Even in these normal incident VSPs, several second arrivals with velocities indicative of vertically polarized shear energy are observed. P to S wave conversion within the upper 110 m of basement occurs by scattering from surface roughness and volume heterogeneities and does not depend on angle of incidence as predicted by plane boundary transmission coefficient analysis. (3) Vertical velocities determined from the VSP differ by <10% from horizontal velocities obtained from the oblique seismic experiment (OSE) on Deep Sea Drilling Project (DSDP) Leg 92. The P wave velocity structure is determined by small and intermediate (<1 cm) pore structure with no measurable anisotropy. The large-scale, well-oriented vertical fractures, which are formed tectonically, do not have a detectable effect on compressional wave velocities. (4) High Poisson's ratio in the upper 300 m of basement coincides with an extrusive layer composed of pillows and thin flows. Low Poisson's ratio at 850-1150 m below seafloor (mbsf) coincides with the downhole decrease in bulk porosity caused by the transition from extrusives to dikes. Relatively large-aspect ratio cracks are required

  7. Hydrofracturing of Sediment and Hydrated MORB's during Subduction of Ocean Crust

    NASA Astrophysics Data System (ADS)

    Nabelek, P. I.

    2015-12-01

    Dehydration accompanying metamorphism of sediment and hydrated mafic ocean crust during ocean plate subduction can potentially lead to fracturing and intraplate tremors. As rocks are buried, they lose porosity and permeability and eventually pass into the lithostatic pressure regime where the porosity and permeability are thought to be very small. Pore fluid pressure generation stemming from metamorphic reactions was modeled using the program SUTRAMET (Nabelek et al., 2014), which is a modified version of the program SUTRA (Voss & Provost, 2002). The program allows modeling of metamorphic reactions at high pressures and temperatures and incorporates transient porosity and permeability changes due to overpressure and changing volumes of metamorphic assemblages. Permeabilities (k) of 10-18 and 10-20 m2 within the brittle regime were modeled. The model assumes continuous subduction with fluids generated at horizons where specific P-T conditions for reactions exist. Temperature gradient was assumed to be 10°C/km and the vertical component of subduction velocity to be at 0.85 cm/y. The model reactions in sediment (Campito Formation of western California as a proxy) and average MORB were generated by the program Theriak-Domino (de Capitani and Petrakakis, 2010). The initial sediment mineralogy includes albite, K-feldspar, phengite, chlorite, and lawsonite. The final mineralogy for the 10°C/km gradient is quartz, phengite, garnet, and pyroxene with jadeite and acmite components. The initial mineralogy of MORB includes chlorite, amphibole, pumpelyite, albite and quartz and the final eclogite mineralogy includes garnet, cpx, kyanite and quartz. Calculations show that at k = 10-18 m2, pore fluid pressure can reach 450 MPa. Pore pressure is reduced by negative ∆V of mineral assemblages stemming from reactions; nevertheless, for the garnet-forming reaction, pore pressure is about 110 MPa. Assuming a tensile strength of 15 MPa and fracture aperture given by rock

  8. Global distribution of beryllium isotopes in deep ocean water as derived from Fe-Mn crusts

    USGS Publications Warehouse

    Von Blanckenburg, F.; O'Nions, R. K.; Belshaw, N.S.; Gibb, A.; Hein, J.R.

    1996-01-01

    The direct measurement of the ratio of cosmogenic 10Be (T1/2 = 1.5 Ma) to stable terrigenously sourced 9Be in deep seawater or marine deposits can be used to trace water mass movements and to quantify the incorporation of trace metals into the deep sea. In this study a SIMS-based technique has been used to determine the 10Be/9Be ratios of the outermost millimetre of hydrogenetic ferromanganese crusts from the worlds oceans. 10Be/9Be ratios, time-corrected for radioactive decay of cosmogenic 10Be using 234U/ 238U, are in good agreement with AMS measurements of modern deep seawater. Ratios are relatively low in the North and equatorial Atlantic samples (0.4-0.5 ?? 10-7). In the Southwest Atlantic ratios increase up to 1 ?? 10-7, they vary between 0.7 and 1.0 ?? 10-7 in Indian Ocean samples, and have a near constant value of 1.1 ?? 0.2 ?? 10-7 for all Pacific samples. If the residence time of 10Be (??10Be) in deep water is constant globally, then the observed variations in 10Be/9Be ratios could be caused by accumulation of 10Be in deep water as it flows and ages along the conveyor, following a transient depletion upon its formation in the Northern Atlantic. In this view both 10Be and 9Be reach local steady-state concentration in Pacific deep water and the global ??10Be ??? 600 a. An alternative possibility is that the Be isotope abundances are controlled by local scavenging. For this scenario ??10Be would vary according to local particle concentration and would ??? 600 a in the central Pacific, but ??10Be ??? 230 a in the Atlantic. Mass balance considerations indicate that hydrothermal additions of 9Be to the oceans are negligible and that the dissolved riverine source is also small. Furthermore, aeolian dust input of 9Be appears insufficient to provide the dissolved Be inventory. The dissolution of only a small proportion (2%) of river-derived particulates could in principle supply the observed seawater Be content. If true, ocean margins would be the sites for 9Be

  9. Detachment Fault Initiation and Control by Partially Molten Zones in the Lower Ocean Crust

    NASA Astrophysics Data System (ADS)

    Dick, H. J.; Natland, J. H.; MacLeod, C. J.; Robinson, P. T.

    2012-12-01

    The close association of oxide gabbro and deformation in interleaved ferrogabbro and olivine gabbro at Atlantis Bank on the SW Indian Ridge explains the formation of this enormous single-domed gabbroic oceanic core complex. ODP Holes 735B and 1105A show that the stratigraphy is defined by 100's of zones of intense deformation and strain localization in the upper 500-m where various melts percolated including late-stage iron-titanium rich melts. The latter created highly deformed oxide-rich gabbro zones at scales from millimeters to over 100 meters. Mapping by ROV, over-the-side rock drilling, dredging, and submersible shows that this stratigraphy exists uniformly over the bank. Deep drilling and sampling up the headwalls of major landslips cutting into the core complex show that the fault zone was imbricate, likely reflecting relocation of the active slip plane due to cyclic intrusion in the lower crust. The detachment originated as a high-angle fault on the rift valley wall that propagated into a zone of partially molten gabbro beneath the sheeted dikes. This zone then pinned the footwall block, creating a plutonic growth fault along which gabbro intruded beneath the ridge axis was continuously uplifted and exposed on the Antarctic plate for ~3.9 myr. The overlying basaltic carapace spread more slowly to the north on the African Plate. Textural evidence, particularly that provided by iron-titanium oxides, shows that melts migrated along complex shear zones in which several creep mechanisms operated, ranging from crystal plastic dislocation creep, diffusion creep, grain boundary sliding, and brittle deformation. More than one of these mechanisms may have occurred concurrently. Subsequently, these zones localized later solid-state creep, often producing texturally complex rocks where separation of the timing and duration of different creep mechanisms is difficult to unravel. As uplift of the plutonic section progressed, the footwall passed through the zone of diking

  10. Brittle ductile transition in experimentally deformed basalt under oceanic crust conditions

    NASA Astrophysics Data System (ADS)

    Violay, M.; Gibert, B.; Mainprice, D.; Evans, B.; Pezard, P. A.; Flovenz, O.

    2009-04-01

    experiments show (1) at a pressure of 100 MPa between 600°C to 900°C and at pressure of 300 MPa between 600°C and 700°C, the sample fails by localized rupture with a peak strength that depends on temperature and (2) at a pressure of 300 MPa between 800°C and 900°C homogeneous, distributed flow with strengths of 600 MPa and 300 MPa, respectively. Mechanical observations at a constant strain rate of 1 × 10- 5 s- 1 and a confining pressure of 100 MPa and 300 MPa indicate that the rocks are brittle and dilatant up to 700 to 800°C. This indicates that, in the context of the Icelandic geotherm, hydrothermal fluids may circulate, at least briefly, through the oceanic basaltic crust down to 6 to 8 km depth. These results are coherent with the lower limit of the Icelandic seismogenic zone which seems to be associated with a 750 ± 100 °C isothermal surface.

  11. Triple seismic source, double research ship, single ambitious goal: integrated imaging of young oceanic crust in the Panama Basin

    NASA Astrophysics Data System (ADS)

    Wilson, Dean; Peirce, Christine; Hobbs, Richard; Gregory, Emma

    2016-04-01

    Understanding geothermal heat and mass fluxes through the seafloor is fundamental to the study of the Earth's energy budget. Using geophysical, geological and physical oceanography data we are exploring the interaction between the young oceanic crust and the ocean in the Panama Basin. We acquired a unique geophysical dataset that will allow us to build a comprehensive model of young oceanic crust from the Costa Rica Ridge axis to ODP borehole 504B. Data were collected over two 35 x 35 km2 3D grid areas, one each at the ridge axis and the borehole, and along three 330 km long 2D profiles orientated in the spreading direction, connecting the two grids. In addition to the 4.5 km long multichannel streamer and 75 ocean-bottom seismographs (OBS), we also deployed 12 magnetotelluric (MT) stations and collected underway swath bathymetry, gravity and magnetic data. For the long 2D profiles we used two research vessels operating synchronously. The RRS James Cook towed a high frequency GI-gun array (120 Hz) to image the sediments, and a medium frequency Bolt-gun array (50 Hz) for shallow-to-mid-crustal imaging. The R/V Sonne followed the Cook, 9 km astern and towed a third seismic source; a low frequency, large volume G-gun array (30 Hz) for whole crustal and upper mantle imaging at large offsets. Two bespoke vertical hydrophone arrays recorded real far field signatures that have enabled us to develop inverse source filters and match filters. Here we present the seismic reflection image, forward and inverse velocity-depth models and a density model along the primary 330 km north-south profile, from ridge axis to 6 Ma crust. By incorporating wide-angle streamer data from our two-ship, synthetic aperture acquisition together with traditional wide-angle OBS data we are able to constrain the structure of the upper oceanic crust. The results show a long-wavelength trend of increasing seismic velocity and density with age, and a correlation between velocity structure and basement

  12. Cross-hole tracer experiment reveals rapid fluid flow and low effective porosity in the upper oceanic crust

    NASA Astrophysics Data System (ADS)

    Neira, N. M.; Clark, J. F.; Fisher, A. T.; Wheat, C. G.; Haymon, R. M.; Becker, K.

    2016-09-01

    Numerous field, laboratory, and modeling studies have explored the flows of fluid, heat, and solutes during seafloor hydrothermal circulation, but it has been challenging to determine transport rates and flow directions within natural systems. Here we present results from the first cross-hole tracer experiment in the upper oceanic crust, using four subseafloor borehole observatories equipped with autonomous samplers to track the transport of a dissolved tracer (sulfur hexafluoride, SF6) injected into a ridge-flank hydrothermal system. During the first three years after tracer injection, SF6 was transported both north and south through the basaltic aquifer. The observed tracer transport rate of ∼2-3 m/day is orders of magnitude greater than bulk rates of flow inferred from thermal and chemical observations and calculated with coupled fluid-heat flow simulations. Taken together, these results suggest that the effective porosity of the upper volcanic crust through which much tracer was transported is <1%, with fluid flowing rapidly along a few well-connected channels. This is consistent with the heterogeneous (layered, faulted, and/or fractured) nature of the volcanic upper oceanic crust.

  13. Effects of Sediment Layer and Shallow Portion of the Oceanic Crust on Waveforms of Broadband Ocean Bottom Seismometers in Northwest Pacific Ocean

    NASA Astrophysics Data System (ADS)

    Abe, Y.; Kawakatsu, H.

    2015-12-01

    Earthquake Research Institute, The University of Tokyo and Japan Agency for Marine-Earth Science and Technology have conducted seismic observation in the northwest Pacific Ocean with broadband ocean bottom seismometers (BBOBSs), for understanding the structure of the Earth's interior and the mechanism of plate motion (Normal Mantle Project). We have performed receiver function (RF) analyses using the waveform data, for detecting velocity discontinuities in the upper mantle, and have understood that it is essential to reveal shallower structure (especially structure of sediment) for elucidating the upper mantle structure using RFs (Abe et al., 2014, SSJ meeting; 2015, JpGU meeting). Therefore, we attempted to estimate the shallower structure by using power spectrum and auto correlation function (ACF) of ambient noise in addition to RFs. Power spectrum of horizontal seismogram of a BBOBS has several peaks due to resonances of S wave in the sediment. Godin & Chapman (1999, J. Acoust. Soc. Am.) introduced a method to estimate a 1-D velocity distribution in the sediment from the resonance frequencies. From the location of spectral peaks of a station (NM14), we estimated the velocity distribution to be Vs(z) = 0.519z0.473 (Vs: S wave velocity (km/s), z: depth (km)), assuming a sediment layer thickness of 0.3 km. Two way S wave travel time in this sediment corresponds to the arrival time of a prominent negative ACF peak of horizontal seismogram of the station. On the other hand, for P-wave RFs (0.4-2.0 Hz) of the station, the arrival time of the first positive peak is not explained only by the estimated sediment structure, and another discontinuity located a few hundred meters deeper than the bottom of the sediment is necessary to explain it. We attempt to constrain the structure of the sediment and shallow portion of the oceanic crust by analyzing RF waveforms in more detail that also explains power spectrum and ACF of ambient noise.

  14. Constant Molybdenum Isotope Composition of Ocean Water and Fe-Mn crusts for the Last 70 Myr

    NASA Astrophysics Data System (ADS)

    Siebert, C.; Nagler, T. F.; von Blankenburg, F.; Kramers, J. D.

    2001-12-01

    In the relatively new field of heavy stable isotope geochemistry, molybdenum (Mo) is one of the very promising elements. Molybdenum is a redox-sensitive trace metal. Isotope fractionation during terrestrial processes such as low-temperature redox transitions, chemical weathering, changes in the composition of the atmosphere, hydrothermal activity and sedimentary cycling is likely. Molybdenum is also an essential element for biological nitrogen fixation. Therefore, biogeochemical Mo isotope fractionation is also probable. The oceans represent an important terrestrial Mo reservoir. Dissolved concentrations in seawater are relatively high (0.01 ppm). The global ocean residence time is corresponding high with 800 kyr. The aim of this study is to characterise the principle present day oceanic Mo reservoirs and their changes with time. Molybdenum isotopic compositions were determined precisely using a Nu instruments MC-ICP-MS. Instrumental and laboratory mass fractionation is separated from natural mass dependent fractionation by addition of a molybdenum double spike prior to chemical separation (Siebert et al., 2001). The external standard reproducibility is at or below 0.1 per mil for the 98Mo/95Mo ratio (2s.d.). We analysed ocean water samples from the Atlantic (n=3, 0m-2400m depth), the Pacific and the Indian Ocean (deep water). These yield a homogeneous Mo isotopic composition as would be expexted from the long residence time of Mo in the oceans. Ocean water has the heaviest Mo isotopic composition measured to date (+2.3 per mil on the 98Mo/95Mo ratio relative to a Johnson Mattey ICP standard solution, lot 602332B). In view of the homogeneous ocean water ratios, we propose the use of present day ocean water as an reference standard (Mean Ocean Molybdenum: MOMO). Significantly lighter compositions from -2.7 to -3.1 per mil on the 98Mo/95Mo ratio relative to MOMO were determined for six Fe-Mn crust surface layers. Pelagic clay (-2.7 per mil) and clastic sediments (-2

  15. Magnesium isotope composition of the altered upper oceanic crust at ODP Holes 504B and 896A, Costa Rica Rift

    NASA Astrophysics Data System (ADS)

    Beaumais, Aurélien; Teagle, Damon A. H.; James, Rachael H.; Harris, Michelle; Pearce, Christopher R.; Milton, James A.; Cooper, Matthew J.; Alt, Jeffrey C.

    2017-04-01

    Chemical reactions between the oceanic crust and seawater play a major role in regulating the composition of the oceans that, in turn, influence important geochemical cycles (e.g., C, S, Mg). It is well established that alteration of the oceanic crust is the principal sink of Mg in seawater, but the effect of this process on the Mg isotope composition of the oceans remains unclear. Here we present the first measurements of Mg isotopes in altered oceanic crust from ODP Holes 504B and 896A. These holes are located in 5.9 Ma crust located 200 km south of the intermediate spreading rate Costa Rica Rift. Hole 504B penetrates: (i) A volcanic section, consisting of primitive to moderately altered mid-ocean ridge basalt (MORB) that was open to seawater alteration under oxic-suboxic conditions at temperatures <150 °C; (ii) A transition zone that marks the contact between volcanic flows and dikes and is characterised by mixing between upwelling hydrothermal fluids and seawater at temperatures between 100 and 350 °C; (iii) A sheeted dike complex consisting of diabase partially altered to greenschist facies minerals. Hole 896A is 1 km away from 504B, and penetrates volcanic rocks altered under low temperature (<100 °C) and oxic-suboxic conditions. Samples were selected from each of the different alteration zones and have been well characterized petrographically and geochemically. The overall range in δ26Mg values is -0.52 to -0.12‰, which is significantly greater than the restricted range of the unaltered MORB ( 0.25 ± 0.06‰ [1]). These values also extend towards significantly lighter compositions than published values for altered oceanic crust ( 0.25 ± 0.11‰ [2]) from basalts and gabbros recovered from the ODP Site 1256 on 15 Ma EPR crust on the Cocos Plate. Overall, δ26Mg values decrease with depth in the volcanic section in both Holes 504B and 896A, covering the entire range of Mg isotopic compositions. The highest δ26Mg values are found in saponite

  16. Modification of an oceanic plateau, Aruba, Dutch Caribbean: Implications for the generation of continental crust

    NASA Astrophysics Data System (ADS)

    White, R. V.; Tarney, J.; Kerr, A. C.; Saunders, A. D.; Kempton, P. D.; Pringle, M. S.; Klaver, G. T.

    1999-01-01

    The generation of the continental crust may be connected to mantle plume activity. However, the nature of this link, and the processes involved, are not well constrained. An obstacle to understanding relationships between plume-related mafic material and associated silicic rocks is that later tectonic movements are liable to obscure the original relationships, particularly in ancient greenstone belts. Studies of younger analogous regions may help to clarify these relationships. On the island of Aruba in the southern Caribbean, a sequence of partly deformed mafic volcanic rocks intruded by a predominantly tonalitic batholith is exposed. The mafic lavas show geochemical and isotopic affinities with other basaltic, picritic and komatiitic rocks that crop out elsewhere in the Caribbean—these are well documented as belonging to an 88-91 Ma plume-related oceanic plateau, which is allochthonous with respect to the Americas, and is thought to have been formed in the Pacific region. The ˜85 to ˜82 Ma tonalitic rocks share some geochemical characteristics (high Sr and Ba, low Nb and Y) with Archaean tonalite-trondhjemite-granodiorite (TTG) suites. Field relationships suggest that deformation of the plateau sequence, possibly related to collision with a subduction zone, was synchronous with intrusion of the Aruba batholith. New incremental heating 40Ar/ 39Ar dates, combined with existing palaeontological evidence, show that cooling of the batholith occurred shortly after eruption of the plateau basalt sequence. Sr-Nd isotopic data for both rock suites are uniform ( 87Sr/ 86Sr i≈0.7035 , ɛNd i≈+7), whereas Pb isotopes are more variable (Plateau sequence: 206Pb/ 204Pb =18.6-19.1 , 207Pb/ 204Pb =15.54-15.60 , 208Pb/ 204Pb =38.3-38.75 ; Aruba batholith: 206Pb/ 204Pb =18.4-18.9 , 207Pb/ 204Pb =15.51-15.56 , 208Pb/ 204Pb =38.0-38.5 ). This suggests that there has been a minor sedimentary input into the source region of the batholith. However, the limited time interval

  17. Early Permian mafic dikes in the Nagqu area, central Tibet, China, associated with embryonic oceanic crust of the Meso-Tethys Ocean

    NASA Astrophysics Data System (ADS)

    Chen, Sheng-Sheng; Shi, Ren-Deng; Fan, Wei-Ming; Gong, Xiao-Han; Wu, Kang

    2017-06-01

    During the latest Carboniferous to Early Permian, a possible mantle plume initiated continental rifting along the northern Gondwana margin, which subsequently developed into the Meso-Tethys Ocean. However, the nature and timing of the embryonic oceanic crust of the Meso-Tethys Ocean remain poorly understood. Here we present for the first time a combined analysis of petrological, geochronological, geochemical, and Sr-Nd isotopic data for mafic rocks from the Nagqu area, central Tibet. Zircons from the mafic rocks yield a concordant age of 277.8 ± 1.8 Ma, which is slightly younger than the age of mantle plume activity ( 300-279 Ma), as represented by the large igneous province (LIP) on the northern Gondwana margin. Geochemical features suggest that the Nagqu mafic rocks, which display normal mid-ocean ridge basalt affinities, are different from those of the LIP, which display oceanic island basalt-type affinities. The Nagqu mafic rocks result from a relatively high degree of melting of depleted asthenospheric mantle. Combined with observations from previous studies, we suggest that the late Early Permian Nagqu magmatism fully records processes of early stage rifting and incipient formation of oceanic crust. Moreover, the patterns of magmatism are consistent with patterns of rift-related sedimentation that records the transition from predominantly continental to marine deposition in the region during the Carboniferous-Permian. We therefore suggest that rifting of the eastern Cimmerian and northern Gondwana continents started at 277.8 Ma, and the rifting culminated in the opening of the Meso-Tethys Ocean.

  18. Platinum group elements and gold in ferromanganese crusts from Afanasiy-Nikitin seamount, equatorial Indian Ocean: Sources and fractionation

    USGS Publications Warehouse

    Banakar, V.K.; Hein, J.R.; Rajani, R.P.; Chodankar, A.R.

    2007-01-01

    The major element relationships in ferromanganese (Fe-Mn) crusts from Afanasiy-Nikitin seamount (ANS), eastern equatorial Indian Ocean, appear to be atypical. High positive correlations (r = 0.99) between Mn/Co and Fe/Co ratios, and lack of correlation of those ratios with Co, Ce, and Ce/Co, indicate that the ANS Fe-Mn crusts are distinct from Pacific seamount Fe-Mn crusts, and reflect region-specific chemical characteristics. The platinum group elements (PGE: Ir, Ru, Rh, Pt, and Pd) and Au in ANS Fe-Mn crusts are derived from seawater and are mainly of terrestrial origin, with a minor cosmogenic component. The Ru/Rh (0.5-2) and Pt/Ru ratios (7-28) are closely comparable to ratios in continental basalts, whereas Pd/Ir ratios exhibit values ( 0.75) correlations between water depth and Mn/Co, Fe/Co, Ce/Co, Co, and the PGEs. Fractionation of the PGE-Au from seawater during colloidal precipitation of the major-oxide phases is indicated by well-defined linear positive correlations (r > 0.8) of Co and Ce with Ir, Ru, Rh, and Pt; Au/Co with Mn/Co; and by weak or no correlations of Pd with water depth, Co-normalized major-element ratios, and with the other PGE (r < 0.5). The strong enrichment of Pt (up to 1 ppm) relative to the other PGE and its positive correlations with Ce and Co demonstrate a common link for the high concentrations of all three elements, which likely involves an oxidation reaction on the Mn-oxide and Fe-oxyhydroxide surfaces. The documented fractionation of PGE-Au and their positive association with redox sensitive Co and Ce may have applications in reconstructing past-ocean redox conditions and water masses.

  19. Fe-XANES analyses of Reykjanes Ridge basalts: Implications for oceanic crust's role in the solid Earth oxygen cycle

    NASA Astrophysics Data System (ADS)

    Shorttle, Oliver; Moussallam, Yves; Hartley, Margaret E.; Maclennan, John; Edmonds, Marie; Murton, Bramley J.

    2015-10-01

    The cycling of material from Earth's surface environment into its interior can couple mantle oxidation state to the evolution of the oceans and atmosphere. A major uncertainty in this exchange is whether altered oceanic crust entering subduction zones can carry the oxidised signal it inherits during alteration at the ridge into the deep mantle for long-term storage. Recycled oceanic crust may be entrained into mantle upwellings and melt under ocean islands, creating the potential for basalt chemistry to constrain solid Earth-hydrosphere redox coupling. Numerous independent observations suggest that Iceland contains a significant recycled oceanic crustal component, making it an ideal locality to investigate links between redox proxies and geochemical indices of enrichment. We have interrogated the elemental, isotope and redox geochemistry of basalts from the Reykjanes Ridge, which forms a 700 km transect of the Iceland plume. Over this distance, geophysical and geochemical tracers of plume influence vary dramatically, with the basalts recording both long- and short-wavelength heterogeneity in the Iceland plume. We present new high-precision Fe-XANES measurements of Fe3+ / ∑ Fe on a suite of 64 basalt glasses from the Reykjanes Ridge. These basalts exhibit positive correlations between Fe3+ / ∑ Fe and trace element and isotopic signals of enrichment, and become progressively oxidised towards Iceland: fractionation-corrected Fe3+ / ∑ Fe increases by ∼0.015 and ΔQFM by ∼0.2 log units. We rule out a role for sulfur degassing in creating this trend, and by considering various redox melting processes and metasomatic source enrichment mechanisms, conclude that an intrinsically oxidised component within the Icelandic mantle is required. Given the previous evidence for entrained oceanic crustal material within the Iceland plume, we consider this the most plausible carrier of the oxidised signal. To determine the ferric iron content of the recycled component ([Fe2O

  20. Transformation of juvenile Izu-Bonin-Mariana oceanic arc into mature continental crust: An example from the Neogene Izu collision zone granitoid plutons, Central Japan

    NASA Astrophysics Data System (ADS)

    Saito, Satoshi; Tani, Kenichiro

    2017-04-01

    Granitic rocks (sensulato) are major constituents of upper continental crust. Recent reviews reveal that the average composition of Phanerozoic upper continental crust is granodioritic. Although oceanic arcs are regarded as a site producing continental crust material in an oceanic setting, intermediate to felsic igneous rocks occurring in modern oceanic arcs are dominantly tonalitic to trondhjemitic in composition and have lower incompatible element contents than the average upper continental crust. Therefore, juvenile oceanic arcs require additional processes in order to get transformed into mature continental crust enriched in incompatible elements. Neogene granitoid plutons are widely exposed in the Izu Collision Zone in central Japan, where the northern end of the Izu-Bonin-Mariana (IBM) arc (juvenile oceanic arc) has been colliding with the Honshu arc (mature island arc) since Middle Miocene. The plutons in this area are composed of various types of granitoids ranging from tonalite to trondhjemite, granodiorite, monzogranite and granite. Three main granitoid plutons are distributed in this area: Tanzawa plutonic complex, Kofu granitic complex, and Kaikomagatake granitoid pluton. Tanzawa plutonic complex is dominantly composed of tonalite and trondhjemite and characterized by low concentration of incompatible elements and shows geochemical similarity with modern juvenile oceanic arcs. In contrast, Kofu granitic complex and Kaikomagatake granitoid pluton consists mainly of granodiorite, monzogranite and granite and their incompatible element abundances are comparable to the average upper continental crust. Previous petrogenetic studies on these plutons suggested that (1) the Tanzawa plutonic complex formed by lower crustal anatexis of juvenile basaltic rocks occurring in the IBM arc, (2) the Kofu granitic complex formed by anatexis of 'hybrid lower crust' comprising of both basaltic rocks of the IBM arc and metasedimentary rocks of the Honshu arc, and (3) the

  1. Oceanic Character of Sub-Salt Crust in the NW Gulf of Mexico (GOM) Using Seismic Refraction and Reflection Data

    NASA Astrophysics Data System (ADS)

    Karner, G. D.; Johnson, C. A.

    2015-12-01

    Significant renewed interest in the geological development of the NW GOM is exemplified by the acquisition of academic seismic refraction and oil industry seismic reflection data. There is agreement that the GOM formed by Jurassic separation of North America and Yucatan, but disagreements remain on the distribution and timing of extended continental versus oceanic crust. Van Avendonk et al. (Geology, v43, 2015) interpreted seismic refraction data from the 2010 "GUMBO" expedition as rifted continental crust thinned by large-scale extensional faulting and syn-rift magmatism beneath the NW GOM. However, seismic reflection evidence for this extension is non-existent, and diagnostic fault-controlled syn-rift packages are not resolved. A very different interpretation of basement type and basin evolution is possible by applying geological process linked to hyper-extended margin formation to the same data. We note: 1) Base salt and Moho interfaces are well imaged; top basement is not resolved. We interpret a pre-salt sedimentary sequence 5-10 km thick, with velocities up to 6 km/s; high velocities in this sequence likely relate to greenschist-facies metamorphism associated with early high heat flow and deep burial. 2) Velocities of 6-8 km/s characterize crystalline basement but do not uniquely determine crustal type (i.e., velocity does not equate to rock type). Lateral variations (0-8 km) in crustal thickness are consistent with slow/ultra-slow seafloor spreading. 3) The undeformed base salt reflector and pre-salt sediment sequence imply a post-kinematic setting and a substantial delay between breakup and Callovian salt deposition. 4) Liassic Central Atlantic breakup is kinematically linked to the GOM and related SDR magmatism. Inboard SDRs, observed on both conjugate margins of the GOM, imply outboard oceanic crust. Together, these observations are consistent with regional sub-salt basement of early-mid Jurassic slow/ultra-slow spreading oceanic crust, associated with

  2. Activity and phylogenetic diversity of sulfate-reducing microorganisms in low-temperature subsurface fluids within the upper oceanic crust

    PubMed Central

    Robador, Alberto; Jungbluth, Sean P.; LaRowe, Douglas E.; Bowers, Robert M.; Rappé, Michael S.; Amend, Jan P.; Cowen, James P.

    2015-01-01

    The basaltic ocean crust is the largest aquifer system on Earth, yet the rates of biological activity in this environment are unknown. Low-temperature (<100°C) fluid samples were investigated from two borehole observatories in the Juan de Fuca Ridge (JFR) flank, representing a range of upper oceanic basement thermal and geochemical properties. Microbial sulfate reduction rates (SRR) were measured in laboratory incubations with 35S-sulfate over a range of temperatures and the identity of the corresponding sulfate-reducing microorganisms (SRM) was studied by analyzing the sequence diversity of the functional marker dissimilatory (bi)sulfite reductase (dsrAB) gene. We found that microbial sulfate reduction was limited by the decreasing availability of organic electron donors in higher temperature, more altered fluids. Thermodynamic calculations indicate energetic constraints for metabolism, which together with relatively higher cell-specific SRR reveal increased maintenance requirements, consistent with novel species-level dsrAB phylotypes of thermophilic SRM. Our estimates suggest that microbially-mediated sulfate reduction may account for the removal of organic matter in fluids within the upper oceanic crust and underscore the potential quantitative impact of microbial processes in deep subsurface marine crustal fluids on marine and global biogeochemical carbon cycling. PMID:25642212

  3. Activity and phylogenetic diversity of sulfate-reducing microorganisms in low-temperature subsurface fluids within the upper oceanic crust.

    PubMed

    Robador, Alberto; Jungbluth, Sean P; LaRowe, Douglas E; Bowers, Robert M; Rappé, Michael S; Amend, Jan P; Cowen, James P

    2014-01-01

    The basaltic ocean crust is the largest aquifer system on Earth, yet the rates of biological activity in this environment are unknown. Low-temperature (<100°C) fluid samples were investigated from two borehole observatories in the Juan de Fuca Ridge (JFR) flank, representing a range of upper oceanic basement thermal and geochemical properties. Microbial sulfate reduction rates (SRR) were measured in laboratory incubations with (35)S-sulfate over a range of temperatures and the identity of the corresponding sulfate-reducing microorganisms (SRM) was studied by analyzing the sequence diversity of the functional marker dissimilatory (bi)sulfite reductase (dsrAB) gene. We found that microbial sulfate reduction was limited by the decreasing availability of organic electron donors in higher temperature, more altered fluids. Thermodynamic calculations indicate energetic constraints for metabolism, which together with relatively higher cell-specific SRR reveal increased maintenance requirements, consistent with novel species-level dsrAB phylotypes of thermophilic SRM. Our estimates suggest that microbially-mediated sulfate reduction may account for the removal of organic matter in fluids within the upper oceanic crust and underscore the potential quantitative impact of microbial processes in deep subsurface marine crustal fluids on marine and global biogeochemical carbon cycling.

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

    NASA Astrophysics Data System (ADS)

    Stroncik, N. A.; Niedermann, S.

    2016-01-01

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

  5. How concentration of porosity, crack shape, and crack wall asperity control the seismic structure of the upper oceanic crust

    NASA Astrophysics Data System (ADS)

    Fontana, E.; Gilbert, L. A.; Marinoni, N.; Tartarotti, P.; Mancini, L.

    2015-12-01

    Surprisingly little is known about several important aspects of the architecture of oceanic crust, including controls on lithology, heterogeneity of hydrothermal alteration, and thickness of the lava section and sheeted dike complex and their influences on the seismic structure. Ophiolites provide useful analogs of ancient oceanic crust, with more exposure and access than deep-sea drill holes or submarine fracture zones. We examine the brittle structure of the lava, sheeted dikes, and lava-dike transition zone of the Troodos ophiolite (Cyprus) in the Lythrodontas area. The lava zone consists of pillow basalts; the transition zone consists of sheeted basaltic dikes cutting pillow lava through a large interval, and of hyaloclastitic breccia; the sheeted dikes are composed of 1-2 m thick basaltic or dacitic dikes. Detailed sample measurements of P- and S-wave velocity, permeability, porosity, and structures, show that porosity decreases drastically, as does permeability from lava to sheeted dikes. Variation of P-wave velocity mainly depends on the porosity, as well as the shapes of the cracks or pores. To better understand the relationships among P-wave velocity, porosity, and crack shape and aspect ratio, we made laboratory measurements using synchroton X-ray computed microtomography (micro-CT). Micro-CT images enhance the phase-contrast between primary (igneous) minerals and alteration minerals now filling the pores. Overall, quantitative data of the volume and shape of the pores (at the sample scale) allow us to evaluate the "empty" (effective) porosity as well as the paleo-porosity (pores now partially or completely filled with secondary minerals). We also quantify the asperity of crack walls. These measurements allow us to construct 3D structural patterns in the investigated ophiolite, which we use to examine the influence of cracks in the upper oceanic crust on seismic structure.

  6. Sensitivity of elastic surface deformations caused by atmospheric, hydrologic, and oceanic loads to the Earth's crust and mantle properties

    NASA Astrophysics Data System (ADS)

    Dill, Robert; Klemann, Volker; Kaban, Mikhail; Dobslaw, Henryk; Thomas, Maik

    2016-04-01

    The elastic deformation of the Earth's surface due to atmospheric surface pressure, terrestrial water storage, and ocean bottom pressure on seasonal or shorter time scales is usually represented by a set of elastic load Love numbers or the corresponding Green's function, determined from a radial Earth structure like PREM. Thereby, the influence of local deviations of the Earth's crustal and mantle properties is assumed to be negligible. However, local Green's functions derived individually for 1° grid cells from the 3D crustal structure model CRUST1 show large variations for in particular smaller distance angles. The loading response due to small-scale surface loads extending over less than 2500km2 significantly depends on the heterogeneous shallow structure of the Earth. In this contribution, we discuss the influence of lateral variations in the crust and mantle structure on atmospheric, hydrologic, and oceanic surface loads with regard to their spatial scales and distribution. Non-tidal atmospheric loading is calculated from an atmospheric surface pressure time series covering four decades (1976 - 2015) based on 3-hourly atmospheric data of ECMWF that has been homogenized by mapping surface pressure to a common reference orography. Hydrological loading is calculated for daily terrestrial water storage from LSDM over the same time period, where the surface water compartment is mapped from the 0.5° model resolution to a 0.125° GIS-based river network. Ocean tidal loading is exemplarily calculated based on the FES2014 ocean tidal model (0.0625°). Especially along the coasts of the oceans; in regions with steep orographic gradients; and in areas with thick crustal layers or sediments we will show the significant influence of the Earth's structure on small-scale deformation features caused by surface loads.

  7. Sulphide mineral evolution and metal mobility during alteration of the oceanic crust: Insights from ODP Hole 1256D

    NASA Astrophysics Data System (ADS)

    Patten, C. G. C.; Pitcairn, I. K.; Teagle, D. A. H.; Harris, M.

    2016-11-01

    Fluxes of metals during the hydrothermal alteration of the oceanic crust have far reaching effects including buffering of the compositions of the ocean and lithosphere, supporting microbial life and the formation of sulphide ore deposits. The mechanisms responsible for metal mobilisation during the evolution of the oceanic crust are complex and are neither fully constrained nor quantified. Investigations into the mineral reactions that release metals, such as sulphide leaching, would generate better understanding of the controls on metal mobility in the oceanic crust. We investigate the sulphide and oxide mineral paragenesis and the extent to which these minerals control the metal budget in samples from Ocean Drilling Program (ODP) Hole 1256D. The ODP Hole 1256D drill core provides a unique sample suite representative of a complete section of a fast-spreading oceanic crust from the volcanic section down to the plutonic complex. The sulphide population at Hole 1256D is divided into five groups based on mineralogical assemblage, lithological location and texture: the magmatic, metasomatised, high temperature hydrothermal, low temperature and patchy sulphides. The initiation of hydrothermal alteration by downward flow of moderate temperature (250-350 °C) hydrothermal fluids under oxidising conditions leads to metasomatism of the magmatic sulphides in the sheeted dyke and plutonic complexes. Subsequent increase in the degree of hydrothermal alteration at temperatures >350 °C under reducing conditions then leads to the leaching of the metasomatised sulphides by rising hydrothermal fluids. Mass balance calculations show that the mobility of Cu, Se and Au occurs through sulphide leaching during high temperature hydrothermal alteration and that the mobility of Zn, As, Sb and Pb is controlled by silicate rather than sulphide alteration. Sulphide leaching is not complete at Hole 1256D and more advanced alteration would mobilise greater masses of metals. Alteration of oxide

  8. Distribution and sources of pre-anthropogenic lead isotopes in deep ocean water from Fe-Mn crusts

    USGS Publications Warehouse

    Von Blanckenburg, F.; O'Nions, R. K.; Hein, J.R.

    1996-01-01

    The lead isotope composition of ocean water is not well constrained due to contamination by anthropogenic lead. Here the global distribution of lead isotopes in deep ocean water is presented as derived from dated (ca. 100 ka) surface layers of hydrogenetic Fe-Mn crusts. The results indicate that the radiogenic lead in North Atlantic deep water is probably supplied from the continents by river particulates, and that lead in Pacific deep water is similar to that characteristic of island and continental volcanic arcs. Despite a short residence time in deep water (80-100 a), the isotopes of lead appear to be exceedingly well mixed in the Pacific basin. There is no evidence for the import of North Atlantic deep water-derived lead into the Pacific ocean, nor into the North Indian Ocean. This implies that the short residence time of lead in deep water prohibits advection over such long distances. Consequently, any climate-induced changes in deep-water flow are not expected to result in major changes in the seawater Pb-isotope record of the Pacific Ocean.

  9. The spectral albedo of sea ice and salt crusts on the tropical ocean of Snowball Earth: II. Optical modeling

    NASA Astrophysics Data System (ADS)

    Carns, Regina C.; Light, Bonnie; Warren, Stephen G.

    2016-07-01

    During the Snowball Earth events of the Neoproterozoic, tropical regions of the ocean could have developed a precipitated salt lag deposit left behind by sublimating sea ice. The major salt would have been hydrohalite, NaCl•2H2O. The crystals in such a deposit can be small and highly scattering, resulting in an allwave albedo similar to that of snow. The snow-free sea ice from which such a crust could develop has a lower albedo, around 0.5, so the development of a crust would substantially increase the albedo of tropical regions on Snowball Earth. Hydrohalite crystals are much less absorptive than ice in the near-infrared part of the solar spectrum, so their presence at the surface would increase the overall albedo as well as altering its spectral distribution. In this paper, we use laboratory measurements of the spectral albedo of a hydrohalite lag deposit, in combination with a radiative transfer model, to infer the inherent optical properties of hydrohalite as functions of wavelength. Using this result, we model mixtures of hydrohalite and ice representing both artificially created surfaces in the laboratory and surfaces relevant to Snowball Earth. The model is tested against sequences of laboratory measurements taken during the formation and the dissolution of a lag deposit of hydrohalite. We present a parameterization for the broadband albedo of cold, sublimating sea ice as it forms and evolves a hydrohalite crust, for use in climate models of Snowball Earth.

  10. Igneous Cooling Rate constraints on the Accretion of the lower Oceanic Crust in Mid-ocean Ridges: Insights from a new Thermo-mechanical Model

    NASA Astrophysics Data System (ADS)

    Garrido, C. J.; Machetel, P.

    2005-12-01

    We report the results of a new thermo-mechanical model of crustal flow beneath fast spreading mid-ocean ridges to investigate both the effect of deep, near off-axis hydrothermal convection on the thermal structure of the magma chamber and the role of variable number of melt intrusions on the accretion of the oceanic crust. In our model the melt is injected at the center of the axial magma chamber with a 'needle' with adjustable porosity at different depths allowing the simulation of different arrangements of melt injection and supply within the magma chamber. Conversely to previous models, the shape of the magma chamber -defined as the isotherm where 95% solidification of the melt occurs- is not imposed but computed from the steady state reached by the thermal field considering the heat diffusion and advection and the latent heat of crystallization. The motion equation is solved for a temperature and phase dependent viscosity. The thermal diffusivity is also dependent on temperature and depth, with a higher diffusivity in the upper plutonic crust to account for more efficient hydrothermal cooling at these crustal levels. In agreement with previous non-dynamic thermal models, our results show that near, deep off-axis hydrothermal circulation strongly affects the shape of the axial magma by tightening isotherms in the upper half of the plutonic oceanic crust where hydrothermal cooling is more efficient. Different accretion modes have however little effect on the shape of the magma chamber, but result in variable arrangements of flow lines ranging from tent-shape in a single-lens accretion scenario to sub-horizontal in "sheeted-sill" intrusion models. For different intrusion models, we computed the average Igneous Cooling Rates (ICR) of gabbros by dividing the crystallization temperature interval of gabbros by the integrated time, from the initial intrusion to the point where it crossed the 950 °C isotherm where total solidification of gabbro occurs, along individual

  11. Proterozoic crustal evolution of the Eucla basement, Australia: Implications for destruction of oceanic crust during emergence of Nuna

    NASA Astrophysics Data System (ADS)

    Kirkland, C. L.; Smithies, R. H.; Spaggiari, C. V.; Wingate, M. T. D.; Quentin de Gromard, R.; Clark, C.; Gardiner, N. J.; Belousova, E. A.

    2017-05-01

    younger magmatism; 2) recycled c. 1950 Ma crust reworked in primitive arcs and in intra-plate settings and; 3) minor evolved material representing fragments of hyperextended continent. The observed isotopic evolution pattern is comparable to that of other central Australian Proterozoic provinces, including the Musgrave Province, the northern margin of the Gawler Craton, and components within the Rudall Province. Linking these isotopic signatures defines the Mirning Ocean, and its subducted and underplated equivalents. In a global context we suggest c. 1950 Ma crust production reflects the onset of ordered oceanic spreading centres, which swept juvenile crustal fragments into Nuna.

  12. Continental growth through time by underplating of subducted oceanic crust: Evidence from kimberlites in South Africa and SW Pacific

    NASA Technical Reports Server (NTRS)

    Taylor, Lawrence A.; Neal, Clive R.

    1988-01-01

    In the dynamic model of plate tectonics, it is evident that crustal components are returned to the mantle by subduction. Chemical signatures of these subducted components were identified in ocean island volcanics and in island arc volcanics. Indeed, an origin involving a subducted protolith was postulated for certain types of xenoliths in kimberlite, including diamonds. Recent studies of eclogite xenoliths in kimberlite from southern Africa and megacrysts form the Malaitan alnoite, Solomon islands, indicate that lithospheric underplating by subducted oceanic crust has occurred in these two contrasting areas. The results of new eclogite studies from the Bellsbank kimberlite, South Africa, and isotopic data from the Malaitan alnoite megacryst suite. This forms the basis for discerning the role of lithospheric underplating in the growth of cratons and in the evolution of mantle-derived magma.

  13. How thick is the magmatic crust at slow spreading oceanic ridges?

    SciTech Connect

    Cannat, M.

    1996-02-10

    This report explores the composition and structure of ultramafic and gabbroic rocks used to constrain the geology of thick lithosphere, thin crust regions of slow spreading ridges, and proposes a geological model that would affect the understanding of the relation between axial segmentation and magma supply variations along slow spreading ridges.

  14. The Igneous Architecture of IODP Hole U1309D: Constructing Oceanic Crust from Multiple Sills

    NASA Astrophysics Data System (ADS)

    Christofferson, C. A.; John, B. E.; Cheadle, M. J.; Swapp, S. M.; Grimes, C. B.

    2010-12-01

    Gabbroic rocks comprise a significant portion of the footwall of many oceanic core complexes. The decreasing age of these gabbros from the breakaway to the termination suggests that they are continuously accreted as the bounding detachment fault slips. But, questions remain; how are these large (>1 km diameter) gabbroic bodies constructed and at what scale and frequency is melt added to the system? Here we report a detailed lithologic analysis of IODP Hole U1309D drilled into the Atlantis Massif core complex (30° N, MAR). We present new, high resolution (1m scale) downhole lithological diagrams, compiled from observations of the archive core, refined using other available data including magnetic susceptibility. These data show that the thickness of the individual magmatic units is on the order of 1 to a few 10’s of m. Contacts between many of these units are sharp; many of the units are interpreted to result from small injections of melt as opposed to in-situ fractionation. However, these units are often disaggregated by later intrusive bodies and hence, it is likely that individual gabbro units were initially somewhat thicker. Downhole plots of magmatic fabric dip from both shipboard data and new electron backscatter diffraction (EBSD) data are consistent with the units having initially been intruded as sills, and subsequently rotated by ~40-50°, as constrained by paleomagnetic data (Morris et al., 2009). Using all of the available data, we propose the IODP Hole 1309D gabbro section is a composite body that grew episodically by relatively small (10’s m) repeated sill-like injections of melt. We conclude that the melt lens that formed these gabbros was relatively small at any one time. The EBSD data also provide textural constraints on the model for crustal accretion. Gabbroic samples analyzed so far show a moderate to weak plagioclase and clinopyroxene foliation, similar to those in continental mafic intrusions and are thus interpreted to be magmatic fabrics

  15. Preliminary Results on the Structure of Ocean Crust from new Holes Drilled in Fast-Spread Crust During ODP Leg 206

    NASA Astrophysics Data System (ADS)

    Tartarotti, P.; Crispini, L.

    2003-12-01

    ODP Leg 206 successfully accomplished the initial phase of a multi-leg drilling program that aims to sample a complete section of upper oceanic crust through the extrusive lavas, the sheeted dike complex, and into the gabbros. Drilling was conducted at Site 1256 (6.7N, 91.9W), which resides on 15-Ma oceanic lithosphere of the Cocos plate that was created by superfast seafloor spreading (~220 mm/yr). Two holes, 1256C and 1256D were drilled into the basement to a depth of 340.3 mbsf (89.6 m sub-basement) and 752 mbsf (502 m sub-basement) respectively. The main stratigraphy of upper oceanic crust at Site 1256 consists of a sequence of massive flows and thin sheet flows with minor amounts of pillow basalts and breccias. The sequence is slightly altered and has N-MORB composition. Structural analysis carried out on board on the recovered cores from both Holes 1256C and 1256D revealed the occurrence of primary igneous as well as post-magmatic structures. Primary igneous features include magmatic fabrics, laminations and flattened vesicles, folds and shear-related structures, late magmatic veins, and fracturing. Postmagmatic structures include veins, shear veins, microfaults, joints, and breccia. Veins are the most prominent structural features and show a variety of morphologies ranging from planar and curviliear to anastomosing. In many cases veins are oriented in en echelon, and Riedel-shear arrays, giving in such case useful shear indications. Shear veins are mostly present in massive coarser-grained lithologic units and are filled with fibrous clay minerals. Shear veins and microfaults indicate both strike-slip and oblique apparent senses of shear. In Hole 1256D shear veins show a change in the sense of shear, from reverse to normal, from ~645 mbsf to the bottom of the hole. More than 600 veins and joints from the basement units of Hole C and more than 1700 features from Hole D were measured in the archive half relative to the core barrel reference frame. True dip

  16. Consequences of Rift Propagation for Spreading in Thick Oceanic Crust in Iceland

    NASA Astrophysics Data System (ADS)

    Karson, J. A.

    2015-12-01

    Iceland has long been considered a natural laboratory for processes related to seafloor spreading, including propagating rifts, migrating transforms and rotating microplates. The thick, hot, weak crust and subaerial processes of Iceland result in variations on the themes developed along more typical parts of the global MOR system. Compared to most other parts of the MOR, Icelandic rift zones and transform faults are wider and more complex. Rift zones are defined by overlapping arrays of volcanic/tectonic spreading segments as much as 50 km wide. The most active rift zones propagate N and S away from the Iceland hot spot causing migration of transform faults. A trail of crust deformed by bookshelf faulting forms in their wakes. Dead or dying transform strands are truncated along pseudofaults that define propagation rates close to the full spreading rate of ~20 mm/yr. Pseudofaults are blurred by spreading across wide rift zones and laterally extensive subaerial lava flows. Propagation, with decreasing spreading toward the propagator tips causes rotation of crustal blocks on both sides of the active rift zones. The blocks deform internally by the widespread reactivation of spreading-related faults and zones of weakness along dike margins. The sense of slip on these rift-parallel strike-slip faults is inconsistent with transform-fault deformation. These various deformation features as well as subaxial subsidence that accommodate the thickening of the volcanic upper crustal units are probably confined to the brittle, seismogenic, upper 10 km of the crust. At least beneath the active rift zones, the upper crust is probably decoupled from hot, mechanically weak middle and lower gabbroic crust resulting in a broad plate boundary zone between the diverging lithosphere plates. Similar processes may occur at other types of propagating spreading centers and magmatic rifts.

  17. The spectral albedo of sea ice and salt crusts on the tropical ocean of Snowball Earth: 1. Laboratory measurements

    NASA Astrophysics Data System (ADS)

    Light, Bonnie; Carns, Regina C.; Warren, Stephen G.

    2016-07-01

    The ice-albedo feedback mechanism likely contributed to global glaciation during the Snowball Earth events of the Neoproterozoic era (1 Ga to 544 Ma). This feedback results from the albedo contrast between sea ice and open ocean. Little is known about the optical properties of some of the possible surface types that may have been present, including sea ice that is both snow-free and cold enough for salts to precipitate within brine inclusions. A proxy surface for such ice was grown in a freezer laboratory using the single salt NaCl and kept below the eutectic temperature (-21.2°C) of the NaCl-H2O binary system. The resulting ice cover was composed of ice and precipitated hydrohalite crystals (NaCl · 2H2O). As the cold ice sublimated, a thin lag-deposit of salt formed on the surface. To hasten its growth in the laboratory, the deposit was augmented by addition of a salt-enriched surface crust. Measurements of the spectral albedo of this surface were carried out over 90 days as the hydrohalite crust thickened due to sublimation of ice, and subsequently over several hours as the crust warmed and dissolved, finally resulting in a surface with puddled liquid brine. The all-wave solar albedo of the subeutectic crust is 0.93 (in contrast to 0.83 for fresh snow and 0.67 for melting bare sea ice). Incorporation of these processes into a climate model of Snowball Earth will result in a positive salt-albedo feedback operating between -21°C and -36°C.

  18. Yin of birthing and the Yang of destroying continental crust at ocean-margin and crust-suturing subduction zones—exploring evidence about processes, amounts, and the Phanerozoic balance

    NASA Astrophysics Data System (ADS)

    Scholl, D. W.; Stern, R. J.

    2009-12-01

    INTRODUCTION: Field evidence implies that the creation of juvenile continental crust by mantle melting is paired with the destruction of older crust and its recycling to the mantle. The tectonic notion of birthing and annihilation is philosophically expressed by the Chinese concept of the twained and inseparable processes of yin-yang. Since at least the Phanerozoic these opposing processes appear to have been most active at the subduction zones (SZs) of ocean margins (i.e., Peru-Chile margin) and suture-building collision zones (i.e. closing of Tethys). Crust is also created and destroyed by non-plate tectonic processes, in particular additions by hotspot melting and losses by foundering of lower crust. BASIC OBSERVATIONS: Evidence exists that the creation of juvenile crust is impressively voluminous at newly formed SZs, in particular those that build the massifs of offshore arcs. Modern intra-oceanic arcs (e.g., the Aleutian and IBM arcs) have since the early Tertiary grown at a rate of 100-150 km3/Myr/km of SZ, thus adding to the inventory of continental crust at a global rate of about 1.5 km3/yr (= 1.5 Armstrong units or AU). The arc massifs forming along continental margin (e.g., Andean arc) compile at a much slower rate (30 km3/Myr/km of SZ) but globally account for about +1.0 AU of juvenile growth. An additional +0.7 AU is estimated supplied by continental rift and hotspot volcanism. The opposite or destructive Yang processes of sediment subduction and subduction erosion remove and transport crustal material toward and into the mantle at modern ocean-margin SZs. The global rate is evidentially estimated at about -2.5 AU. Similar observations of missing material estimate that a large volume, at least -0.7 AU or greater, of continental crust is loss at the SZs of colliding or suturing crustal blocks. The greater volume of the loss is effected by the detachment and en-mass sinking of deeply underthrust continental edges. THE BALANCE: During the Phanerozoic

  19. The age and emplacement of obducted oceanic crust in the Urals from Sm-Nd and Rb-Sr systematics

    NASA Technical Reports Server (NTRS)

    Edwards, R. L.; Wassburg, G. J.

    1985-01-01

    The Sm-Nd and Rb-Sr isotopic characteristics of two mafic-ultramafic bodies, the Kemperai Massif in the South Ural Mountains and the Voykar-syninsky Massif in the Polar Ural mountains are examined. The data are found to be consistent with the hypothesis that these bodies represent fragments of old oceanic crust. Whole rock samples of pillow basalt, troctolite, gabbros, diabase, and a metasediment give Sm-Nd values which lie on this isochron indicating that the rock units are genetically related and formed 397 My ago. Basic and ultrabasic rocks from Kempersai and Voykar-Syninsky have an initial isotopic composition at time T, epsilon Nd(397 My), of +8.4, indicating derivation for an ancient depleted mantle source. The Sr isotopic data and the correlation with epsilon Nd indicate extensive alteration by seawater which is particularly strong on ultrabasic rocks. The results show that the segments of oceanic crust formed at least 80 My before the collision that produced the Urals.

  20. Age dependent porosity of young upper oceanic crust: Insights from seafloor gravity studies of recent volcanic eruptions

    NASA Astrophysics Data System (ADS)

    Pruis, Matthew J.; Johnson, H. Paul

    2002-03-01

    An October 1997 ALVIN program collected a series of seafloor gravity stations over the February, 1996, volcanic eruption site on the northern Gorda Ridge (46°41'N, 126°47'W). These measurements yielded estimates of the density and porosity of mid-ocean ridge crust in the first few decades following formation. The estimated crustal density for this pillow eruption of 2270 +/- 260 kg/m3 is very low, but in good agreement with the estimated densities of other recent seafloor volcanic flows. Rock matrix densities, measured from hand samples retrieved from the study area, give a bulk porosity for this flow of 34% +/- 16%. This data, when included with similar estimates of upper crustal porosity, argue for a rapid evolution of upper crustal porosities from very high values (>=30%) for recent eruptions to much lower values (~10-15%) where the seafloor is older than 0.5 million years. These variations of density and porosity are of a magnitude consistent with previously observed changes in seismic compressional wave velocity and permeability of ocean crust.

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  2. On the Use of Boreholes in Controlled Electromagnetic Source Soundings of the Ocean Crust,

    DTIC Science & Technology

    1982-09-01

    the passive magnetotelluric method (Cox et al. 197C Filloux 1980, Chave et al. 1981). All such studies indicate rising electrical con-duct-ivity at...floor of the deep ocean have been used to infer the electrical conductivity of the upper mantle using the pas- sive magnetotelluric method (Cox 1_ 81...fields caused by large scale oceanic flows and magnetotelluric experiments in the ocean-continent transition region (Cox 1980, 1981). If the conductivity

  3. Understanding Fracturing and Alteration at ODP Borehole 504B: 3D Seismic Structure and Anisotropy of 5.9 Ma Oceanic Crust

    NASA Astrophysics Data System (ADS)

    Gregory, E. P. M.; Hobbs, R. W.; Peirce, C.; Wilson, D. J.; Zhang, L.

    2016-12-01

    Faults and fracture networks within the oceanic crust influence the pattern of hydrothermal circulation. This circulation changes the primary composition and structure of the crust as it evolves, particularly the upper crust (layer 2), through the secondary alteration of minerals and the infilling and 'sealing' of cracks. Processes influencing the extent and the depth within the crust of these changes are currently not well known. Alteration can be quantified by observing changes in the seismic velocity structure of the crust, and analysis of seismic anisotropy within the upper crust reveals the nature of ridge-parallel aligned faults and fractures. Here we show a 3D P-wave velocity model and anisotropy maps for 5.9 Ma crust at ODP borehole 504B, situated 200 km south of the Costa Rica Rift, derived from an active-source wide-angle seismic survey in the Panama Basin conducted in 2015. The seismic structure reveals relatively homogeneous, 5 km thick oceanic crust with upper crustal velocity boundaries occurring coincident with alteration fronts observed in 504B. Correlations between basement topography, velocity anomaly and anisotropy indicate that a distinct relationship between hydrothermal alteration, basement ridges, fractures, and the velocity structure of layer 2 exists in this location. A significant difference is seen in the velocity and anisotropic structure between regions to the east and west of the borehole, that correlates with patterns in heat flow observations and indicates that: 1) these two regions of crust have inherited differences in crustal fabric during accretion; and/or 2) different regimes of hydrothermal circulation have been active in each part of the crust as they have aged. This research is part of a major, interdisciplinary NERC-funded research collaboration entitled: Oceanographic and Seismic Characterisation of heat dissipation and alteration by hydrothermal fluids at an Axial Ridge (OSCAR).

  4. Redox processes in subducting oceanic crust recorded by sulfide-bearing high-pressure rocks and veins (SW Tianshan, China)

    NASA Astrophysics Data System (ADS)

    Li, Ji-Lei; Gao, Jun; Klemd, Reiner; John, Timm; Wang, Xin-Shui

    2016-09-01

    The oxidized nature of the sub-arc mantle and hence arc magmas is generally interpreted as a result of the migration of subduction-related oxidizing fluids or melts from the descending slab into the mantle wedge. This is of particular importance seeing that the oxidization state of sub-arc magmas seems to play a key role in the formations of arc-related ore deposits. However, direct constraints on the redox state of subducted oceanic crust are sparse. Here, we provide a detailed petrological investigation on sulfide- and oxide-bearing eclogites, blueschists, micaschists, eclogite-facies and retrograde veins from the Akeyazi high-pressure (HP) terrane (NW China) in order to gain insight into the redox processes recorded in a subducting oceanic slab. Sulfides in these rocks are mainly pyrite and minor pyrrhotite, chalcopyrite, bornite, molybdenite, sphalerite and chalcocite, including exsolution textures of bornite-chalcopyrite intergrowth. Magnetite, ilmenite and pyrite occur as inclusions in garnet, whereas sulfides are dominant in the matrix. Large pyrite grains in the matrix contain inclusions of garnet, omphacite and other HP index minerals. However, magnetite replacing pyrite textures are commonly observed in the retrograded samples. The eclogite-facies and retrograde veins display two fluid events, which are characterized by an early sulfide-bearing and a later magnetite-bearing mineral assemblage, respectively. Textural and petrological evidences show that the sulfides were mainly formed during HP metamorphism. Mineral assemblage transitions reveal that the relative oxygen fugacity of subducted oceanic crust decreases slightly with increasing depths. However, according to oxygen mass balance calculations, based on the oxygen molar quantities ( nO2), the redox conditions remain constant during HP metamorphism. At shallow levels (<60 km) in the subduction channel, interaction with oxidized fluid seems to have caused an increase of the oxygen fugacity and the

  5. Architecture of the Igneous Lower Crust at Oceanic Core Complexes: constraints from IODP Hole U1309D

    NASA Astrophysics Data System (ADS)

    Christofferson, C. A.; Cheadle, M. J.; John, B. E.; Schoolmeesters, N.; Swapp, S. M.; Grimes, C. B.

    2011-12-01

    Slow spreading mid-ocean ridges are a ubiquitous part of the global ridge system, yet remain poorly understood. The crust produced at these ridges is fundamentally different than that produced at fast spreading ridges where the crustal architecture largely conforms to the standard Penrose type crust. At slow spread ridges, a reduced magma supply to the crust allows extensional faulting to play a much more important role in accommodating plate separation than at fast spread ridges. Oceanic core complexes (OCCs), a product of such faulting, denude lower crust to the surface via detachment faulting, and thus provide a means to study the architecture of slow spread lower crust. We report a detailed lithologic analysis of IODP Hole U1309D drilled into the Atlantis Massif OCC (30°N MAR). The abundance of sharp contacts between thin inter-layered gabbroic and ultramafic rocks throughout the core supports crustal construction via small (10-40m thick) injections of magma. Paleomagnetic remanance data allows re-orientation of the observed contacts and igneous fabrics to their original orientation at intrusion, and suggest that most contacts and fabrics were sub-vertical. These data therefore imply that construction of slow spread gabbroic lower crust at OCCs is dominated by dike-like intrusions rather than by sills. Combined U-Pb and (U-Th)/He zircon thermochronometry are used to predict a 3-4km thick zone of accretion that lies 6-7 km below seafloor, at the root of the detachment fault. Existing seismic data estimate the depth of the Moho to be 5km at Atlantis Massif constraining the width of the accretion zone in the footwall of the detachment fault to be 4km. Igneous fabrics from unfaulted gabbroic rocks provide an additional major constraint on the processes occurring within this zone of magmatic accretion. Electron backscatter diffraction (EBSD) was used to characterize the fabrics of slow-spread gabbros from the Atlantis Massif OCC, and other OCCs on the MAR (the 15

  6. Continental basalts record the crust-mantle interaction in oceanic subduction channel: A geochemical case study from eastern China

    NASA Astrophysics Data System (ADS)

    Xu, Zheng; Zheng, Yong-Fei

    2017-09-01

    Continental basalts, erupted in either flood or rift mode, usually show oceanic island basalts (OIB)-like geochemical compositions. Although their depletion in Sr-Nd isotope compositions is normally ascribed to contributions from the asthenospheric mantle, their enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) is generally associated with variable enrichments in the Sr-Nd isotope compositions. This indicates significant contributions from crustal components such as igneous oceanic crust, lower continental crust and seafloor sediment. Nevertheless, these crustal components were not incorporated into the mantle sources of continental basalts in the form of solidus rocks. Instead they were processed into metasomatic agents through low-degree partial melting in order to have the geochemical fractionation of the largest extent to achieve the enrichment of LILE and LREE in the metasomatic agents. Therefore, the mantle sources of continental basalts were generated by metasomatic reaction of the depleted mid-ocean ridge basalts (MORB) mantle with hydrous felsic melts. Nevertheless, mass balance considerations indicate differential contributions from the mantle and crustal components to the basalts. While the depleted MORB mantle predominates the budget of major elements, the crustal components predominate the budget of melt-mobile incompatible trace elements and their pertinent radiogenic isotopes. These considerations are verified by model calculations that are composed of four steps in an ancient oceanic subduction channel: (1) dehydration of the subducting crustal rocks at subarc depths, (2) anataxis of the dehydrated rocks at postarc depths, (3) metasomatic reaction of the depleted MORB mantle peridotite with the felsic melts to generate ultramafic metasomatites in the lower part of the mantle wedge, and (4) partial melting of the metasomatites for basaltic magmatism. The composition of metasomatites is quantitatively dictated by

  7. Ocean-continent transition and tectonic framework of the oceanic crust at the continental margin off NE Brazil: Results of LEPLAC project

    NASA Astrophysics Data System (ADS)

    Gomes, Paulo Otávio; Gomes, Benedito S.; Palma, Jorge J. C.; Jinno, Koji; de Souza, Jairo M.

    In 1992, Brazilian Navy and PETROBRAS carried out a geophysical survey along the continental margin off northeastern Brazil, as part of a governmental plan to delineate the "Legal Continental Shelf" according to the international Law of the Sea. This data set is leading to a better understanding of the crustal transition processes and on the evolution of the oceanic crust over that part of the Brazilian continental margin. On our seismic transects, we show a rifted marginal plateau (Pernambuco Plateau) where crustal extension was controlled by detachment faulting, possibly in a non-volcanic margin setting. Farther north, dealing with the ocean-continent transition nearby a major transform margin, we found a normal passive margin-style transition zone instead of transform-related structures. With the support of multichannel seismic profiles and gravity data derived from GEOSAT altimetry, several well-known oceanic fracture zones and structural lineaments were properly located and correlated. The relationship of these structures with volcanic ridges and extensional, compressive and strike-slip tectonic reactivations suggests that fracture zones at this area behaved either as zones of weakness or as locked transform fault scars. Striking lithospheric flexural deformation is also related to FZs in this region. In the surroundings of the Fernando de Noronha Ridge, lithospheric flexure represents an isostatic response to volcanic loading, while bending across Ascension FZ is likely to have been caused by differential subsidence in crustal segments of contrasting ages. We also correlate some other deformation of the oceanic crust with changes in spreading directions that possibly took place at the Upper Cretaceous.

  8. Uranium isotope systematics of ferromanganese crusts in the Pacific Ocean: Implications for the marine 238U/235U isotope system

    NASA Astrophysics Data System (ADS)

    Goto, Kosuke T.; Anbar, Ariel D.; Gordon, Gwyneth W.; Romaniello, Stephen J.; Shimoda, Gen; Takaya, Yutaro; Tokumaru, Ayaka; Nozaki, Tatsuo; Suzuki, Katsuhiko; Machida, Shiki; Hanyu, Takeshi; Usui, Akira

    2014-12-01

    Variations of 238U/235U ratio (δ238U) in sedimentary rocks have been proposed as a possible proxy for the paleo-oceanic redox conditions, although the marine δ238U system is not fully understood. Here we investigate the spatial variation of δ238U in modern ferromanganese (Fe-Mn) crusts by analyzing U isotopes in the surface (0-3 mm depth) layer of 19 Fe-Mn crusts collected from 6 seamounts in the Pacific Ocean. δ238U values in the surface layers show little variation and range from -0.59‰ to -0.69‰. The uniformity of δ238U values is consistent with the long residence time of U in modern seawater, although the δ238U values are lighter than that of present-day seawater by ∼0.24‰. The light δ238U values are consistent with the isotope offset observed in previously reported adsorption experiment of U to Mn oxide. These results indicate that removal of U from seawater to Mn oxide is responsible for the second largest U isotope fractionation in the modern marine system, and could contribute to isotopically heavy U to seawater. Depth profiles of U isotopes (δ234U and δ238U) in two Fe-Mn crusts (MR12-03_D06-R01 and MC10_CB07_B), dated by Os isotope stratigraphy, were investigated to reconstruct the evolution of the oceanic redox state during the Cenozoic. The δ238U depth profiles show very limited ranges (-0.57‰ to -0.67‰ for MR12-03_D06-R01 and -0.56‰ to -0.69‰ for MC10_CB07_B), and have values that are similar to those of the surface layers of Fe-Mn crusts. The absence of any resolvable variation in the δ238U depth profiles may suggest that the relative amounts of oxic and reducing U sinks have not varied significantly over the past 45 Myr. However, the δ234U depth profiles of the same samples show evidence for the possible redistribution of 234U after deposition. Therefore, the depth profile of δ238U in Fe-Mn crusts may have been also overprinted by later chemical exchange with pore-water or seawater, and may not reflect the paleo-oceanic

  9. Investigation of Collisional Styles of the Caribbean Large Igneous Province (CLIP) vs. Normal Oceanic Crust from Seismic Reflection Profiles

    NASA Astrophysics Data System (ADS)

    Mataracioglu, M.; Magnani, M.; DeShon, H. R.; Cox, R. T.

    2011-12-01

    The Caribbean plate subducts beneath the North American and the South American plates at the Muertos Trough and the South Caribbean Deformed Belt (SCDB), respectively. During subduction, large amount of crustal material may enter the subduction zone with the subducting plate or may be incorporated into the accretionary prism. To investigate the changes in collisional style and structures associated with subduction of the Caribbean Large Igneous Province (CLIP) versus normal oceanic crust, we interpret seismic reflection profiles collected around the northern and southern margins of the Venezuelan Basin. We refine the extent of the CLIP in the central and eastern Caribbean by identifying the structural differences at the top of the acoustic basement (Horizon B") on a dataset of 150 multi-channel seismic time stack and migrated marine reflection profiles acquired in eight cruises from 1975 to 2004, as well as some selected Integrated Ocean Drilling Program (IODP) drilling data. We will also attempt to determine whether sedimentary material enters the trench and is recycled back into the mantle, and therefore characterize the northern and southern subduction zones as accretionary or erosive. Our preliminary results show that the CLIP extends spatially to most of the Venezuelan Basin, the western part of the Columbian Basin, and a part of the Beata Ridge, but that it does not extend as far south as suggested by previous interpretations. Furthermore, some portions of the CLIP at the northern and southern boundaries subduct beneath the North and the South American plates at the Muertos Trough and the SCDB, respectively. The change in nature of the subducting plate (CLIP or normal oceanic crust) causes variations in the collisional style (i.e., accretionary versus erosive) and in structures at the accretionary wedge and on the downgoing plate.

  10. What happens to the Juan de Fuca plate boundary beneath northern Cascadia? Insight into the methamorphism of the oceanic crust

    NASA Astrophysics Data System (ADS)

    Piana Agostinetti, N.; Miller, M. S.

    2012-12-01

    In subduction zones, the plate boundary is the locus where the largest earthquakes occur worldwide. The exact knowledge of the geometry of the plate boundary and the physical properties of the materials brought together in contact are fundamental to understand the earthquake nucleation process and to better asses the seismic hazard in densely populated areas. Due to the metamorphism of the subducted materials and the release of fluids from the subducted plate (e.g. breakout of the serpentinized oceanic upper mantle), the seismic properties of the materials distributed along the plate boundary change with depth (i.e. at different age of subduction). Thus, the clear recognition of the plate boundary at depth using indirect method (e.g. seismic tomography) might be a very complex task. In Northern Cascadia, the position of the plate boundary between the North America (NAM) plate and the Juan de Fuca (JdF) plate is still debated. While along the coast, plate boundary models almost agree, differences in the estimated depth of such interface arise in-land between different models, exactly where large earthquakes enucleate. In this study, we investigate the seismic properties of the subducted JdF crust as it plunges in the upper mantle beneath Northern Cascadia. Harmonic decomposition of a huge Receiver function data-set is used to image both isotropic and anisotropic structures along a trench-normal profile, to better constrain the metamorphism of the crustal materials during the subduction process. The analysis of the seismic anisotropy of the materials is fundamental to better recognize the different components of the subducted plate, where large changes in the seismic velocity of the materials are expected. Our results confirm the 2-layer structure of the JdF crust: basalts over gabbros, and allow us to depict their metamorphism as they plunge into the upper mantle. In the western part of out profile an East-dipping low S-velocity layer is interpreted as fluid

  11. Iron speciation and redox state of mantle eclogites: Implications for ancient volatile cycles during mantle melting and oceanic crust subduction

    NASA Astrophysics Data System (ADS)

    Aulbach, Sonja; Woodand, Alan; Vasilyev, Prokopiy; Viljoen, Fanus

    2017-04-01

    Kimberlite-borne mantle eclogite xenoliths of Archaean and Palaeoproterozoic age are commonly interpreted as representing former oceanic crust. As such, they may retain a memory of the redox state of the ancient convecting mantle sources that gave rise to their magmatic protoliths and which controls the speciation of volatiles in planetary interiors. Mantle eclogite suites commonly include both cumulate and variably evolved extrusive varieties [1], which may be characterised by initial differences in Fe3+/Fetotal. Recent Fe-based oxybarometry shows mantle eclogites to have fO2 relative to the fayalite-magnetite-quartz buffer (ΔFMQ) of -3 to 0, whereby low fO2 relative to modern MORB may relate to subduction of more reducing Archaean oceanic crust or loss of ferric Fe during partial melt loss [2]. Indeed, using V/Sc as a redox proxy, it was recently shown that Archaean mantle eclogites are more reduced than modern MORB (ΔFMQ-1.3 vs. ΔFMQ -0.4) [3]. However, in the warmer ancient mantle, they were also subject to modification due to partial melt loss upon recycling and, after capture in the cratonic mantle lithosphere, may be overprinted by interaction with metasomatic melts and fluids. In order to help further constrain the redox state of mantle eclogites and unravel the effect of primary and secondary processes, we measured Fe3+/Fetotal by Mössbauer in garnet from mantle eclogites from the Lace kimberlite (Kaapvaal craton), comprising samples with melt- and cumulate-like oceanic crustal protoliths as well as metasomatised samples. Fe3+/ΣFe in garnet shows a strong negative correlation with jadeite content and bulk-rock Li and Cu abundances, suggesting increased partitioning of Fe3+ into jadeite in the presence of monovalent cations with which it can form coupled substitutions. Broad negative correlation with whole-rock Al2O3/TiO2 and positive correlation with ΣREE are interpreted as incompatible behaviour of Fe3+ during olivine-plagioclase accumulation

  12. Seismic properties of subducting oceanic crust: Constraints from natural lawsonite-bearing blueschist and eclogite in Sivrihisar Massif, Turkey

    NASA Astrophysics Data System (ADS)

    Cao, Yi; Jung, Haemyeong

    2016-01-01

    Investigating the seismic properties of natural lawsonite (Lws)-bearing blueschist and eclogite is particularly important for constraining the seismic interpretation of subducting oceanic crust based on seismological observations. To achieve this end, we analyzed in detail the mineral fabrics and seismic properties of foliated Lws-blueschist and Lws-eclogites from Sivrihisar Massif in Turkey. In both blueschists and eclogites, the lawsonite fabric is characterized by three different patterns: [0 0 1] axes aligning sub-normal to foliation, and [0 1 0] axes aligning sub-parallel to lineation (normal type); [0 0 1] axes aligning sub-parallel to lineation, and [1 0 0] axes aligning sub-normal to foliation with a girdle sub-normal to lineation (abnormal type); and [0 0 1] axes aligning both sub-normal to foliation and sub-parallel to lineation, [0 1 0] axes aligning sub-parallel to lineation, and [1 0 0] axes aligning sub-normal to foliation (transitional pattern). In contrast, glaucophane and omphacite mostly present consistent axial fabrics with the [0 0 1] axes aligning to lineation. These mineral fabrics produce whole-rock seismic anisotropies with similar patterns. However, the variations in seismic anisotropies are mainly controlled by the rock type, to a lesser extent are determined by the lawsonite fabric type, and to only a small extent are affected by mineral fabric strength. Despite the constructive abnormal-type lawsonite fabric on whole-rock seismic anisotropies, because of their weaker mineral fabric strength (or deformation degree), the abnormal-type Lws-blueschist still exhibit comparatively lower seismic anisotropies than those normal-type Lws-blueschist from other localities. Based on the calculated seismic anisotropies and velocities, we estimated that when oceanic crust transforms from Lws-blueschist to Lws-eclogite with increasing subduction depth, (1) P-wave and max. S-wave polarization anisotropies reduce about 70% and 40%, respectively; and (2

  13. 3D P- and S-wave Velocity Structure and Anisotropy of 5.9 Ma Oceanic Crust at ODP Borehole 504B

    NASA Astrophysics Data System (ADS)

    Gregory, Emma; Hobbs, Richard; Peirce, Christine; Wilson, Dean

    2017-04-01

    Faults and fracture networks within the oceanic crust influence the pattern of hydrothermal circulation. This circulation changes the primary composition and structure of the crust as it evolves, particularly the upper crust (layer 2), through the secondary alteration of minerals and the infilling and 'sealing' of cracks. Processes influencing the extent and depth within the crust of these changes are not currently well known. Alteration can be investigated by observing changes in the seismic velocity structure of the crust, while analysis of seismic anisotropy within the upper crust reveals the nature of aligned faults and fractures acting as fluid pathways. Here we show 3D Vp and Vs models for 5.9 Ma crust at ODP borehole 504B, situated 200 km south of the Costa Rica Rift, derived from an active-source wide-angle seismic survey in the Panama Basin conducted in 2015. The P-wave seismic structure reveals relatively homogeneous, 5 km thick oceanic crust with upper crustal velocity boundaries occurring coincident with alteration fronts observed in 504B. A correlation between basement highs and faster upper crustal velocity suggests a shallowing of the layer 2b/2c transition in these locations, potentially linked to more intense hydrothermal alteration of the upper crust. Upper crustal P-wave azimuthal anisotropy is also present with a strong cos(2ϴ) pattern, indicating aligned open fractures with a ridge-parallel fast direction. The calculation of Vp/Vs and Poisson's ratios details variations in fracturing and alteration and, combined with anisotropy analysis, distinct relationships can be drawn between hydrothermal alteration, basement topography, fracturing, and the velocity structure of layer 2 as a whole. This research is part of a major, interdisciplinary NERC-funded research collaboration entitled: Oceanographic and Seismic Characterisation of heat dissipation and alteration by hydrothermal fluids at an Axial Ridge (OSCAR).

  14. Controls on ferromanganese crust composition and reconnaissance resource potential, Ninetyeast Ridge, Indian Ocean

    USGS Publications Warehouse

    Hein, James; Conrad, Tracey A.; Mizell, Kira; Banakar, Virupaxa K.; Frey, Frederick A.; Sager, William W.

    2016-01-01

    The southern third of NER has Fe-Mn crusts with the highest Co (0.91%), Ni (0.43%), ΣREY (0.33%), Cu (0.22%), Te (146 ppm), Pt (1.5 ppm), Ru (52 ppb), and Rh (99 ppb) contents. These are among the highest Pt, Ru, and Rh concentrations measured in marine Fe-Mn deposits. Because of these high metal concentrations, exploration is warranted for the southern sector of the NER, especially at shallower-water sites where the platinum group elements (PGE) and Co are likely to be even more enriched.

  15. Freshly brewed continental crust

    NASA Astrophysics Data System (ADS)

    Gazel, E.; Hayes, J. L.; Caddick, M. J.; Madrigal, P.

    2015-12-01

    Earth's crust is the life-sustaining interface between our planet's deep interior and surface. Basaltic crusts similar to Earth's oceanic crust characterize terrestrial planets in the solar system while the continental masses, areas of buoyant, thick silicic crust, are a unique characteristic of Earth. Therefore, understanding the processes responsible for the formation of continents is fundamental to reconstructing the evolution of our planet. We use geochemical and geophysical data to reconstruct the evolution of the Central American Land Bridge (Costa Rica and Panama) over the last 70 Ma. We also include new preliminary data from a key turning point (~12-6 Ma) from the evolution from an oceanic arc depleted in incompatible elements to a juvenile continental mass in order to evaluate current models of continental crust formation. We also discovered that seismic P-waves (body waves) travel through the crust at velocities closer to the ones observed in continental crust worldwide. Based on global statistical analyses of all magmas produced today in oceanic arcs compared to the global average composition of continental crust we developed a continental index. Our goal was to quantitatively correlate geochemical composition with the average P-wave velocity of arc crust. We suggest that although the formation and evolution of continents may involve many processes, melting enriched oceanic crust within a subduction zone, a process probably more common in the Achaean where most continental landmasses formed, can produce the starting material necessary for juvenile continental crust formation.

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  17. Pillow basalts of the Angayucham terrane: oceanic plateau and island crust accreted to the Brooks Range

    USGS Publications Warehouse

    Pallister, J.S.; Budahn, J.R.; Murchey, B.L.

    1989-01-01

    The Angayucham Mountains (north margin of the Yukon-Koyukuk province) are made up of an imbricate stack of four to eight east-west trending, steeply dipping, fault slabs composed of Paleozoic, Middle to Late Triassic, and Early Jurassic oceanic upper crustal rocks. Field relations and geochemical characteristics of the basaltic rocks suggest that the fault slabs were derived from an oceanic plateau or island setting and were emplaced onto the Brooks Range continental margin. The basalts are variably metamorphosed to prehnite-pumpellyite and low-greenschist facies. Major element analyses suggest that many are hypersthene-normative olivine tholeiites. The Triassic and Jurassic basalts are geochemically most akin to modern oceanic plateau and island basalts. Field evidence also favors an oceanic plateau or island setting. The great composite thickness of pillow basalt probably resulted from obduction faulting, but the lack of fault slabs of gabbro or peridotite suggests that obduction faults did not penetrate below oceanic layer 2, a likely occurrence if layer 2 were anomalously thick, as in the vicinity of an oceanic island. -from Authors

  18. Continental hyperextension, mantle exhumation, and thin oceanic crust at the continent-ocean transition, West Iberia: New insights from wide-angle seismic

    NASA Astrophysics Data System (ADS)

    Davy, R. G.; Minshull, T. A.; Bayrakci, G.; Bull, J. M.; Klaeschen, D.; Papenberg, C.; Reston, T. J.; Sawyer, D. S.; Zelt, C. A.

    2016-05-01

    Hyperextension of continental crust at the Deep Galicia rifted margin in the North Atlantic has been accommodated by the rotation of continental fault blocks, which are underlain by the S reflector, an interpreted detachment fault, along which exhumed and serpentinized mantle peridotite is observed. West of these features, the enigmatic Peridotite Ridge has been inferred to delimit the western extent of the continent-ocean transition. An outstanding question at this margin is where oceanic crust begins, with little existing data to constrain this boundary and a lack of clear seafloor spreading magnetic anomalies. Here we present results from a 160 km long wide-angle seismic profile (Western Extension 1). Travel time tomography models of the crustal compressional velocity structure reveal highly thinned and rotated crustal blocks separated from the underlying mantle by the S reflector. The S reflector correlates with the 6.0-7.0 km s-1 velocity contours, corresponding to peridotite serpentinization of 60-30%, respectively. West of the Peridotite Ridge, shallow and sparse Moho reflections indicate the earliest formation of an anomalously thin oceanic crustal layer, which increases in thickness from ~0.5 km at ~20 km west of the Peridotite Ridge to ~1.5 km, 35 km further west. P wave velocities increase smoothly and rapidly below top basement, to a depth of 2.8-3.5 km, with an average velocity gradient of 1.0 s-1. Below this, velocities slowly increase toward typical mantle velocities. Such a downward increase into mantle velocities is interpreted as decreasing serpentinization of mantle rock with depth.

  19. Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago.

    PubMed

    Wilde, S A; Valley, J W; Peck, W H; Graham, C M

    2001-01-11

    No crustal rocks are known to have survived since the time of the intense meteor bombardment that affected Earth between its formation about 4,550 Myr ago and 4,030 Myr, the age of the oldest known components in the Acasta Gneiss of northwestern Canada. But evidence of an even older crust is provided by detrital zircons in metamorphosed sediments at Mt Narryer and Jack Hills in the Narryer Gneiss Terrane, Yilgarn Craton, Western Australia, where grains as old as approximately 4,276 Myr have been found. Here we report, based on a detailed micro-analytical study of Jack Hills zircons, the discovery of a detrital zircon with an age as old as 4,404+/-8 Myr--about 130 million years older than any previously identified on Earth. We found that the zircon is zoned with respect to rare earth elements and oxygen isotope ratios (delta18O values from 7.4 to 5.0%), indicating that it formed from an evolving magmatic source. The evolved chemistry, high delta18O value and micro-inclusions of SiO2 are consistent with growth from a granitic melt with a delta18O value from 8.5 to 9.5%. Magmatic oxygen isotope ratios in this range point toward the involvement of supracrustal material that has undergone low-temperature interaction with a liquid hydrosphere. This zircon thus represents the earliest evidence for continental crust and oceans on the Earth.

  20. RESEARCH NOTE : Shear-wave velocity in marine sediments on young oceanic crust: constraints from dispersion analysis of Scholte waves

    NASA Astrophysics Data System (ADS)

    Heinz-Essen, H.; Grevemeyer, Ingo; Herber, Rolf; Weigel, Wilfried

    1998-01-01

    An experiment with a newly developed implosive source, located about 1 m above the seafloor at 3665 m depth, revealed a slow interface wave. This wave is attributed to S waves in a soft sediment layer covering the hard rock sub-bottom. Dispersion analysis by means of the multiple-filter technique yields the group velocity as a function of frequency. Model calculations showed that the observed dispersion curve can be reproduced by considering a steep gradient of S velocity within the sediment layer. Nearly perfect agreement of experimental and model data could be achieved with a sediment layer thickness of 21.5 m, an S speed of 225 m s- 1 at the water-sediment interface and an increase by 23 s- 1 within the layer. These values are relatively high compared to data from the literature. However, previous estimates of in situ S-wave velocity have been obtained on old oceanic crust in the vicinity of continents or islands, while our experiment was carried out on young Pacific crust. Therefore, we suggest two mechanisms which could support a relatively high S speed in sediments: (1) the input of hydrothermally generated metalliferous sediments from the adjacent spreading axis; and (2) post-depositional diagenesis which has accelerated the induration of sediments.

  1. Uranium isotopic compositions of the crust and ocean: Age corrections, U budget and global extent of modern anoxia

    NASA Astrophysics Data System (ADS)

    Tissot, François L. H.; Dauphas, Nicolas

    2015-10-01

    The 238U/235U isotopic composition of uranium in seawater can provide important insights into the modern U budget of the oceans. Using the double spike technique and a new data reduction method, we analyzed an array of seawater samples and 41 geostandards covering a broad range of geological settings relevant to low and high temperature geochemistry. Analyses of 18 seawater samples from geographically diverse sites from the Atlantic and Pacific oceans, Mediterranean Sea, Gulf of Mexico, Persian Gulf, and English Channel, together with literature data (n = 17), yield a δ238U value for modern seawater of -0.392 ± 0.005‰ relative to CRM-112a. Measurements of the uranium isotopic compositions of river water, lake water, evaporites, modern coral, shales, and various igneous rocks (n = 64), together with compilations of literature data (n = 380), allow us to estimate the uranium isotopic compositions of the various reservoirs involved in the modern oceanic uranium budget, as well as the fractionation factors associated with U incorporation into those reservoirs. Because the incorporation of U into anoxic/euxinic sediments is accompanied by large isotopic fractionation (ΔAnoxic/Euxinic-SW = +0.6‰), the size of the anoxic/euxinic sink strongly influences the δ238U value of seawater. Keeping all other fluxes constant, the flux of uranium in the anoxic/euxinic sink is constrained to be 7.0 ± 3.1 Mmol/yr (or 14 ± 3% of the total flux out of the ocean). This translates into an areal extent of anoxia into the modern ocean of 0.21 ± 0.09% of the total seafloor. This agrees with independent estimates and rules out a recent uranium budget estimate by Henderson and Anderson (2003). Using the mass fractions and isotopic compositions of various rock types in Earth's crust, we further calculate an average δ238U isotopic composition for the continental crust of -0.29 ± 0.03‰ corresponding to a 238U/235U isotopic ratio of 137.797 ± 0.005. We discuss the implications of

  2. Warm springs discovered on 3.5 Ma oceanic crust, eastern flank of the Juan de Fuca Ridge

    NASA Astrophysics Data System (ADS)

    Mottl, M. J.; Wheat, G.; Baker, E.; Becker, N.; Davis, E.; Feely, R.; Grehan, A.; Kadko, D.; Lilley, M.; Massoth, G.; Moyer, C.; Sansone, F.

    1998-01-01

    We have located warm springs on an isolated basement outcrop on 3.5 Ma crust on the eastern flank of the Juan de Fuca Ridge in the northeast Pacific Ocean. These are the first ridge-flank hydrothermal springs discovered on crust older than 1 Ma. The springs are venting altered seawater at 25.0 °C along a fault near the summit of Baby Bare outcrop, a high point along a ridge-axis-parallel basement ridge that is otherwise buried by turbidite sediment. Baby Bare is a small volcano that probably erupted off-axis ca. 1.7 Ma; it is thermally extinct, but acts as a high-permeability conduit for venting of basement fluids. The springs have been sampled from the manned submersible Alvin. Compared with the ambient ocean bottom water, they are heavily depleted in Mg, alkalinity, CO2, sulfate, K, Li, U, O2, nitrate, and phosphate, and enriched in Ca, chlorinity, ammonia, Fe, Mn, H2S, H2, CH4, 222Rn, and 226Ra. The springs appear to support a community of thysirid clams. Although we saw no obvious bacterial mats, the surficial sediments contain the highest biomass concentrations ever measured in the deep sea, based on their phospholipid phosphate content. Areal integration of Alvin heat-flow and pore-water velocity data yields flux estimates of 4 13 L/s and 2 3 MW for the total (diffuse and focused) hydrothermal output from Baby Bare, comparable to that from a black smoker vent on the ridge axis. Warm springs such as those on Baby Bare may be important for global geochemical fluxes.

  3. New constraints on the sources and behavior of neodymium and hafnium in seawater from Pacific Ocean ferromanganese crusts

    USGS Publications Warehouse

    van de Flierdt, T.; Frank, M.; Lee, D.-C.; Halliday, A.N.; Reynolds, B.C.; Hein, J.R.

    2004-01-01

    The behavior of dissolved Hf in the marine environment is not well understood due to the lack of direct seawater measurements of Hf isotopes and the limited number of Hf isotope time-series obtained from ferromanganese crusts. In order to place better constraints on input sources and develop further applications, a combined Nd-Hf isotope time-series study of five Pacific ferromanganese crusts was carried out. The samples cover the past 38 Myr and their locations range from sites at the margin of the ocean to remote areas, sites from previously unstudied North and South Pacific areas, and water depths corresponding to deep and bottom waters. For most of the samples a broad coupling of Nd and Hf isotopes is observed. In the Equatorial Pacific ENd and EHf both decrease with water depth. Similarly, ENd and EHf both increase from the South to the North Pacific. These data indicate that the Hf isotopic composition is, in general terms, a suitable tracer for ocean circulation, since inflow and progressive admixture of bottom water is clearly identifiable. The time-series data indicate that inputs and outputs have been balanced throughout much of the late Cenozoic. A simple box model can constrain the relative importance of potential input sources to the North Pacific. Assuming steady state, the model implies significant contributions of radiogenic Nd and Hf from young circum-Pacific arcs and a subordinate role of dust inputs from the Asian continent for the dissolved Nd and Hf budget of the North Pacific. Some changes in ocean circulation that are clearly recognizable in Nd isotopes do not appear to be reflected by Hf isotopic compositions. At two locations within the Pacific Ocean a decoupling of Nd and Hf isotopes is found, indicating limited potential for Hf isotopes as a stand-alone oceanographic tracer and providing evidence of additional local processes that govern the Hf isotopic composition of deep water masses. In the case of the Southwest Pacific there is

  4. The relationship between the age and depth of the oceanic crust in the central South China Sea

    NASA Astrophysics Data System (ADS)

    Peng, Yi-Jui; Hsu, Shu-Kun; Chiao, Ling-Yun

    2016-04-01

    South China Sea (SCS) is the largest marginal basin in the western Pacific. The onset of seafloor spreading in the central part of the SCS was suggested at 32 Ma. After a ridge jump around 25 Ma, the southwestern sub-basin started to open. The spreading of the entire basin ended at ~16 Ma, then a phase of post-magmatic seamount formation occurred (eg., Taylor and Hayes, 1983; Briais et al.,1993; Barckhausen et al., 2014). In this study, we want to find the relationship between the age and depth of the oceanic crust in the central SCS. We will also study a fracture zone trending NW-SE near to Manila trench and to understand how did the fracture zone affect the development of the SCS. We have analyzed five reflection seismic profiles collected by R/V Ocean Researcher 1 during the cruise ORI-1115. We have correlated the age of seismic strata in the central SCS by comparing to the seismic phase of profile MCS1115-7 that has crossed the IODP drilling site U1431. To understand the characteristics of the fracture zone, we have also applied the analytic signal and Euler deconvolution methods to the gravity and magnetic anomalies related to the fracture zone. We suggest that the fraction zone was formed in order to accommodate the spreading in the east sub-basin. However, this fracture zone is somewhat curved concave southwestward. According to the collision-extrusion model of Tapponnier et al. (1982), the formation of Indochina is followed with the constitution of Ailao Shan-Red River Shear Zone. We suppose that the formation of the fracture zone in this study is similar to the Ailao Shan-Red River Shear Zone. The fan-shaped crustal fabric is distinct in the younger portions of the oceanic basin. Both Ailao Shan-Red River Shear Zone and the fracture zone in northeastern SCS may share the same rotation pole. Furthermore, we have tried to find a relationship between oceanic crust depth and age in this area. The preliminary result shows that the relationship between depth and

  5. The origin of layered gabbros from the mid lower ocean crust, Hess Deep, East Pacific Rise

    NASA Astrophysics Data System (ADS)

    Cheadle, M. J.; Brown, T. C.; Ceuleneer, G.; Meyer, R.

    2014-12-01

    IODP Exp. 345 Holes U1415 I & J cored a ~30m thick unit of conspicuously layered gabbroic rocks from the lower plutonic crust at Hess Deep. These rocks likely come from >1500m below the dike gabbro transition and thus provide an unique opportunity to study the origin of layering and the formation of relatively deep, fast spread plutonic crust formed at the EPR. Here we report the initial results of a comprehensive high-resolution petrologic, geochemical and petrographic study of this unit, which focuses on a fairly continuous 1.5m long section recovered at Hole I. The rocks consist of opx-bearing olivine gabbro, olivine gabbro and gabbro and exhibit 1-10cm scale modal layering. Some layers host spectacular 2-3 cm diameter cpx oikocrysts encapsulating partially resorbed plagioclase laths. Downhole variations in mineral chemistry are complicated. Olivine, cpx and opx Mg#'s partly reflect equilibration and show a subtle metre-scale variation (1-2 Mg#), whereas, for example, plagioclase anorthite, and cpx TiO2 contents reveal a more complicated 10-20 cm-scale variation (2-4 An, and 0.2 TiO2). Mineral zonation, for all but Mg# in equilibrated olivine, is of higher magnitude than downhole variations in average mineral compositions. Trace element geochemistry reveals rather homogeneous plagioclase and opx compositions; however cpx exhibits variation at the mineral scale. Cpx shows an increased range of, and highest REE concentrations, in the more olivine rich, near cotectic, composition gabbros, whereas the more plagioclase rich, cumulates show no variation of, and low REE, concentrations.Plagioclase fabrics are moderate to weak and partially modally controlled, but the strength of the plagioclase crystallographic preferred orientation (CPO) varies dramatically, within the 1.5m core showing a significant part of the variation recorded by Oman ophiolite plutonic crust. Plagioclase shape preferred orientation and CPO match well suggesting that diffusion enabled compaction

  6. Constraints on the accretion of the gabbroic lower oceanic crust from plagioclase lattice preferred orientation in the Samail ophiolite

    NASA Astrophysics Data System (ADS)

    VanTongeren, J. A.; Hirth, G.; Kelemen, P. B.

    2015-12-01

    The debate over the processes of igneous accretion of gabbroic lower crust at submarine spreading centers is centered on two end-member hypotheses: Gabbro Glaciers and Sheeted Sills. In order to determine which of these two hypotheses is most applicable to a well-studied lower crustal section, we present newly published data (VanTongeren et al., 2015 EPSL v. 427, p. 249-261) on plagioclase lattice preferred orientations (LPO) in the Wadi Khafifah section of the Samail ophiolite, Oman. Based on our results we provide five critical observations that any model for the accretion of the lower oceanic crust must satisfy: (1) There is a distinctive change in the orientation of the outcrop-scale layering from near-vertical to sub-horizontal that is also reflected in the plagioclase fabrics in the uppermost ~1000-1500 m of the gabbroic crust; (2) The distinction between the upper gabbros and lower gabbros is not a geochemical boundary. Rather, the change in outcrop-scale orientation from near-vertical to sub-horizontal occurs stratigraphically lower in the crust than a change in whole-rock geochemistry; (3) There is no systematic difference in plagioclase fabric strength in any crystallographic axis between the upper gabbros and the lower gabbros; (4) Beneath the abrupt transition from sub-vertical to sub-horizontal fabric, there is no systematic change in the geographic orientation of the plagioclase fabric, or in the development of a dominant lineation direction within the upper gabbros or the lower gabbros; (5) In the lower gabbros, the obliquity between the (010) and the modal layering remains approximately constant and indicates a consistent top to the right sense of shear throughout the stratigraphy. Our observations are most consistent with the Sheeted Sills hypothesis, in which the majority of lower crustal gabbros are crystallized in situ and fabrics are dominated by compaction and localized extension rather than by systematically increasing shear strain with

  7. Characterization of the in situ magnetic architecture of oceanic crust (Hess Deep) using near-source vector magnetic data

    NASA Astrophysics Data System (ADS)

    Tominaga, Masako; Tivey, Maurice A.; MacLeod, Christopher J.; Morris, Antony; Lissenberg, C. Johan; Shillington, Donna J.; Ferrini, Vicki

    2016-06-01

    Marine magnetic anomalies are a powerful tool for detecting geomagnetic polarity reversals, lithological boundaries, topographic contrasts, and alteration fronts in the oceanic lithosphere. Our aim here is to detect lithological contacts in fast-spreading lower crust and shallow mantle by characterizing magnetic anomalies and investigating their origins. We conducted a high-resolution, near-bottom, vector magnetic survey of crust exposed in the Hess Deep "tectonic window" using the remotely operated vehicle (ROV) Isis during RRS James Cook cruise JC21 in 2008. Hess Deep is located at the western tip of the propagating rift of the Cocos-Nazca plate boundary near the East Pacific Rise (EPR) (2°15'N, 101°30'W). ROV Isis collected high-resolution bathymetry and near-bottom magnetic data as well as seafloor samples to determine the in situ lithostratigraphy and internal structure of a section of EPR lower crust and mantle exposed on the steep (~20°dipping) south facing slope just north of the Hess Deep nadir. Ten magnetic profiles were collected up the slope using a three-axis fluxgate magnetometer mounted on ROV Isis. We develop and extend the vertical magnetic profile (VMP) approach of Tivey (1996) by incorporating, for the first time, a three-dimensional vector analysis, leading to what we here termed as "vector vertical magnetic profiling" approach. We calculate the source magnetization distribution, the deviation from two dimensionality, and the strike of magnetic boundaries using both the total field Fourier-transform inversion approach and a modified differential vector magnetic analysis. Overall, coherent, long-wavelength total field anomalies are present with a strong magnetization contrast between the upper and lower parts of the slope. The total field anomalies indicate a coherently magnetized source at depth. The upper part of the slope is weakly magnetized and magnetic structure follows the underlying slope morphology, including a "bench" and lobe

  8. Constraints on Lu-Hf and Nb-Ta systematics in globally subducted oceanic crust from a survey of orogenic eclogites and amphibolites

    NASA Astrophysics Data System (ADS)

    Zirakparvar, N. Alex

    2016-04-01

    To further understand Lu-Hf and Nb-Ta systematics in globally subducted oceanic crust, this paper evaluates all available Lu-Hf garnet isochron ages and initial ɛHf values in conjunction with present-day bulk-rock Lu-Hf isotope and trace element (K, Nb, Ta, Zr, and Ti in addition to Lu-Hf) data from the world's orogenic eclogites and amphibolites (OEAs). Approximately half of OEAs exhibit Lu-Hf and Nb-Ta systematics mimicking those of unsubducted oceanic crust whereas the rest exhibit variability in one or both systems. For the Lu-Hf system, mixing calculations demonstrate that subduction-related phase transformations, in conjunction with open system behavior, can shift subducted oceanic crust toward higher Lu/Hf, or toward lower Lu/Hf that can also be associated with unradiogenic ɛHf values. However, evaluation of potential mechanisms for fractionating Nb from Ta is more complicated because many of the OEAs have Nb-Ta systematics that are decoupled from Lu-Hf and the behavior of K, Zr, and Ti. Nonetheless, the global data set demonstrates that the association between unradiogenic ɛHf and elevated Nb/Ta observed in some kimberlitic eclogite xenoliths can be inherited from processes that occurred during subduction of their oceanic crustal protoliths. This allows for a geologically based estimate of the Nb concentration in a reservoir composed of deeply subducted oceanic crust. However, mass balance calculations confirm that such a reservoir, when considered as a whole, likely has a Nb concentration similar to unsubducted oceanic crust and is therefore not the solution to the problem of the Earth's "missing" Nb.

  9. Venus trough-and-ridge tessera - Analog to earth oceanic crust formed at spreading centers?

    NASA Technical Reports Server (NTRS)

    Head, James W.

    1990-01-01

    The similarity between the morphologies of Venus trough-and-ridge tessera and the earth's ocean floor is discussed. The hypothesis that tessera texture might be related to a crustal fabric produced at spreading centers is examined. It is suggested that the proccesses that produce the ocean floor fabric on earth are good candidates for the origin and production of the trough-and-ridge tessera. To support this hypothesis, the characteristics of the trough-and-ridge terrain in Laima Tessera are described and compared to the seafloor at spreading centers.

  10. Coupling of oceanic and continental crust during Eocene eclogite-facies metamorphism: evidence from the Monte Rosa nappe, western Alps

    NASA Astrophysics Data System (ADS)

    Lapen, Thomas J.; Johnson, Clark M.; Baumgartner, Lukas P.; Piaz, Giorgio V. Dal; Skora, Susanne; Beard, Brian L.

    2007-02-01

    ophiolitic rocks through buoyancy-driven uplift. Coupling of oceanic and continental crust may therefore be critical in preventing permanent loss of oceanic crust to the mantle.

  11. The Cool Early Earth: Oxygen Isotope Evidence for Continental Crust and Oceans on Earth at 4.4 Ga

    NASA Astrophysics Data System (ADS)

    Valley, J. W.; King, E. M.; Peck, W. H.; Graham, C. M.; Wilde, S. A.

    2001-05-01

    oceans at that time. The evidence for liquid water and possibly oceans at 4.4 Ga suggests a Cool Early Earth. This contrasts with the Hot Early Earth and global magma oceans envisioned at 4.5-4.3 Ga based on: an impact origin of the Moon (4.45-4.50 Ga), core formation, higher Hadean radioactive heat production, and intense early meteorite bombardment. Magma on the surface of the Earth cools quickly by radiation to form a crust, but a magma ocean caused by these processes might persist beneath the initially thin crust for up to 400 m.y. and might erupt as massive flood basalts in response to major meteorite impacts, boiling surface waters. The thermal contrasts presented by these lines of evidence are minimized if the Moon and core formed earlier (\\sim4.5 Ga), if the Moon formed by a process not involving a Mars-size impactor, or if the early meteorite bombardment was less intense or irregular in timing. It is possible that periods of Cool vs. Hot Early Earth alternated, with boiling of early oceans after major impact events followed by periods of cooler surface conditions. If life evolved in these seas, multiple extinctions before 3.9 Ga are suggested.

  12. Geomicrobiology of the Ocean Crust: The Phylogenetic Diversity, Abundance, and Distribution of Microbial Communities Inhabiting Basalt and Implications for Rock Alteration Processes

    DTIC Science & Technology

    2007-06-01

    1256D has been subsequently deepened to 1507 meters below seafloor through sheeted dikes and into gabbro (Wilson et al., 2006). Independent of...et al., 2006. Drilling to gabbro in intact ocean crust. Science, 312: 1016- 1020. 139 140 Chapter 5. Characterization of Low-Temperature Alteration

  13. Oceanic crust within the paleozoic Granjeno Schist, northeastern Mexico. Remnants of the Rheic and paleo-Pacific Ocean.

    NASA Astrophysics Data System (ADS)

    Torres Sanchez, Sonia Alejandra; Augustsson, Carita; Rafael Barboza Gudiño, Jose; Jenchen, Uwe; Torres Sanchez, Dario; Aleman Gallardo, Eduardo; Abratis, Michael

    2015-04-01

    Late Paleozoic metamorphic rocks in Mexico are related to the Laurentia-Gondwana collision in Carboniferous time, during Pangaea amalgamation. Vestiges of the Mexican Paleozoic continental configuration are present in the Granjeno Schist, the metamorphic basement of the Sierra Madre Oriental. Field work and petrographic analysis reveal that the Granjeno Schist comprises metamorphic rocks with both sedimentary (psammite, pelite, turbidite, conglomerate, black shale) and igneous (tuff, lava flows, pillow lava and ultramafic bodies) protoliths. The chlorite geothermometer and the presence of phengite in the metasedimentary units as well as 40Ar/39Ar ages on metavolcanic and metaultramafic rocks indicate that the Granjeno Schist was metamorphosed under sub-greenschist to greenschist facies with temperatures ranging from 250-345°C with 2.5 kbar during Carboniferous time (330±30 Ma). The presence of metabasalt, metacumulate, serpentinite and talc bodies suggests an oceanic tectonic setting for the evolution of the Granjeno Schist. Serpetinite rocks have mesh, granular and ribbon textures which indicate recrystallization and metasomatic events. The serpentinite rocks are enriched in the very large incompatible elements Cs, U, and Zr and depleted in Ba, Sr, Pb, Zr and Ce. Normalized REE patterns (LaN/YbN = 0.51 - 19.95 and LaN/SmN = 0.72 - 9.08) of the serpentinite and talc/soapstone are characteristic of peridotite from both suprasubduction and mid-ocean ridge zones. Serpentinite from the Granjeno Schist have spinel content which can reveal different stages of evolution in host serpentinite. The composition of chromite indicates that they belong to podiform chromite that may have crystallized from mid-ocean ridge magma. Al-chromite in the serpentinite is characterized by #Cr 0.48 to 0.55, which indicates a depleted mantle source affected by 17 to 18% of partial melting. The ferritchromite has #Cr values of 0.93 to 1.00 which indicates a metamorphic origin. Our study

  14. Comparative geochemistry of four ferromanganese crusts from the Pacific Ocean and significance for the use of Ni isotopes as paleoceanographic tracers

    NASA Astrophysics Data System (ADS)

    Gueguen, Bleuenn; Rouxel, Olivier; Rouget, Marie-Laure; Bollinger, Claire; Ponzevera, Emmanuel; Germain, Yoan; Fouquet, Yves

    2016-09-01

    Ferromanganese (Fe-Mn) crusts are potential archive of the Ni isotope composition of seawater through time. In this study we aim at (1) understanding Ni isotope fractionation mechanisms and metal enrichment processes in Fe-Mn deposits, (2) addressing global vs. local control of Ni isotope composition of these deposits. Two Fe-Mn crusts from the North Pacific Ocean (Apuupuu Seamount, Hawaii) and two Fe-Mn crusts from the South Pacific Ocean (near Rurutu Island, Austral archipelago of French Polynesia) were characterized for their elemental geochemistry and Ni isotope composition. Geochemical analyses were performed at millimeter intervals in order to provide time-resolved record of Ni isotopes. Chronology and growth rates were determined using cosmogenic 10Be isotope abundances. The results show that, despite different growth rates, textures and geochemical patterns, Fe-Mn crusts from both North and South Pacific Oceans have fairly homogenous Ni isotope compositions over the last ∼17 Ma, yielding average δ60/58Ni values of 1.79 ± 0.21‰ (2sd, n = 31) and 1.73 ± 0.21‰ (2sd, n = 21) respectively. In one crust sample, however, layers directly in contact with the altered substrate show anomalously light δ60/58Ni values down to 0.25 ± 0.05‰ (2se) together with rejuvenated 10Be/9Be ratios correlating with elevated Ni/Mn ratios. Such patterns are best explained by protracted fluid-rock interactions leading to alteration of Mn-phases after crust formation. Isotopically light Ni would be the result of Ni isotope fractionation during adsorption rather than the contribution of external Ni sources (e.g. hydrothermal sources) having light Ni isotope compositions. The combination of our results with previously published data on Fe-Mn crusts indicates that the average Ni isotope composition in deep waters has not changed through the Cenozoic (∼70 Ma). We propose that Ni isotope variations in Fe-Mn crusts may not only record variations of Ni sources to the oceans, but

  15. Lithium isotope as a proxy for water/rock interaction between hydrothermal fluids and oceanic crust at Milos, Greece

    NASA Astrophysics Data System (ADS)

    Lou, U.-Lat; You, Chen-Feng; Wu, Shein-Fu; Chung, Chuan-Hsiung

    2014-05-01

    Hydrothermal activity at Milos in the Aegean island (Greece) is mainly located at rather shallow depth (about 5 m). It is interesting to compare these chemical compositions and the evolution processes of the hydrothermal fluids at deep sea hydrothermal vents in Mid-ocean Ridge (MOR). Lithium (Li) is a highly mobile element and its isotopic composition varies at different geological settings. Therefore, Li and its isotope could be used as an indicator for many geochemical processes. Since 6Li preferential retained in the mineral phase where 7Li is leached into fluid phase during basalt alteration, the Li isotopic fractionation between the rocks and the fluids reflect sensitively the degree of water-rock interaction. In this study, Bio-Rad AG-50W X8 cation exchange resin was used for purifying the hydrothermal fluids to separate Li from other matrix elements. The Li isotopic composition (δ7Li) was determined by Multi-collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) with precision better than 0.2‰ (2σ, n=20). The Li concentration in the hydrothermal fluids falls between 0.02 to 10.31 mM. The δ7Li values vary from +1.9 to +29.7‰, indicating significant seawater contamination have occurred. These hydrothermal fluids fit well with seawater and brine two end-member binary mixing model. During phase separation, lithium, boron, chlorine, iodine, bromine, sodium and potassium were enriched in the brine phase. On the other hand, aluminum, sulphur and iron were enriched in the vapor phase. There is no significant isotope fractionation between the two phases. The water/rock ratio (W/R) calculated is low (about 1.5 to 1.8) for the Milos fluids, restricted seawater recharge into the oceanic crust. Moreover, the oceanic crust in the region becomes less altered since the W/R is low. The δ7Li value of the hydrothermal fluids can be used as a sensitive tool for studying water-rock interaction.

  16. Comment on "Glacial cycles drive variations in the production of oceanic crust".

    PubMed

    Goff, John A

    2015-09-04

    Crowley et al. (Reports, 13 March 2015, p. 1237) propose that abyssal hill topography can be generated by variations in volcanism at mid-ocean ridges modulated by Milankovitch cycle-driven changes in sea level. Published values for abyssal hill characteristic widths versus spreading rate do not generally support this hypothesis. I argue that abyssal hills are primarily fault-generated rather than volcanically generated features.

  17. Ophiolites and the oceanic crust: New evidence from the Tyrrhenian sea and the Western Alps

    NASA Astrophysics Data System (ADS)

    Mascle, G.; Lemoine, M.; Mascle, J.; Rehault, J. P.; Tricart, P.

    The succession recovered in ODP hole 107-651 in the young oceanic Vavilov basin (Tyrrhenian Sea) comprises, beneath a thick Pleistocene to Upper Pliocene sedimentary cover (chiefly volcanoclastics), four basement units: (1) MORB-type basaltic pillows and breccias; (2) a complex succession made of dolerites, albitites, basaltic breccias, metadolerite pebbles (including an intercalated sandy layer with periodotite clasts); (3) MORB-type basaltic pillows and breccias; (4) highly serpentinized peridotite. Between units 3 and 4, granitoid pebbles occur. This sequence is surprisingly similar to successions known in the Western Alps' Tethyan ophiolites. There, the sediments (Callovian-Oxfordian radiolarian cherts) lie stratigraphically upon breccias mostly derived from underlying serpentinite, and sometimes gabbroic basement. At some places, thin basaltic (tholeiitic) pillows and breccias occur between the radiolarian cherts and the breccias. From the comparison between a present day setting (the central Tyrrhenian Sea) and a formerly emplaced basement succession (the Western Alps), we stress the following (a) both the here-discussed ophiolites and oceanic basement are different from classical ophiolite sequences; (b) both occurrences imply unroofing of mantle rocks that therefore were directly outcropping on the seafloor; (c) such a comparison may indicate a very slow spreading rate for the Alpine Tethyan ocean.

  18. Thickness of the oceanic crust and the mantle transition zone in the vicinity of the Tristan da Cunha hot spot estimated from ocean-bottom and ocean-island seismometer receiver functions

    NASA Astrophysics Data System (ADS)

    Geissler, Wolfram; Jokat, Wilfried; Jegen, Marion; Baba, Kiyoshi

    2016-04-01

    According to classical plume theory, the Tristan da Cunha hotspot is thought to have played a major role in the rifting of the South Atlantic margins and the creation of the aseismic Walvis Ridge by impinging at the base of the continental lithosphere shortly before or during the breakup of the South Atlantic margins. However, Tristan da Cunha is enigmatic as it cannot be clearly identified as a hot spot but may also be classified as a more shallow type of anomaly that may actually have been caused by the opening of the South Atlantic. The equivocal character of Tristan da Cunha is largely due to a lack of geophysical and petrological data in this region. We therefore staged a multi-disciplinary geophysical study of the region by acquiring passive marine electromagnetic and seismic data, and bathymetric data within the framework of the SPP1375 South Atlantic Margin Processes and Links with onshore Evolution (SAMPLE) funded by the German Science foundation. The experiment included two ship expeditions onboard the German R/V MARIA S. MERIAN in 2012 and 2013. In our contribution we will present results on the thickness of the oceanic crust in the vicinity of the Tristan da Cunha archipelago derived from ocean-bottom seismometer data. Using the Ps receiver function method we estimate a thickness of 5 to 7 km for the oceanic crust at 17 ocean-bottom stations surrounding the islands in an area where the ocean floor has an age of approximately 10 to 30 Ma (from west to east). This indicates normal to slightly lowered magmatic activity at the mid-ocean ridge during the crust formation. There seems to be no major contribution of a mantle plume to the melting conditions at the ridge, which should cause the formation of thickened oceanic crust. The magmatic activity at the archipelago and surrounding seamounts seems to have only local effects on the crustal thickness. Furthermore, we imaged the mantle transition zone discontinuities analysing receiver functions at the

  19. Thermal modelling of a transform-divergent interaction zone, the Demerara Plateau, French Guiana margin: architecture of oceanic and continental crusts

    NASA Astrophysics Data System (ADS)

    Grall, Céline; Marcaillou, Boris; Loncke, Lies; Mercier de Lepinay, Marion; Basile, Christophe; Roest, Walter R.; A. M Van Wees, Jan Diederik; A. P. L Cloetingh, Sierd

    2014-05-01

    The crustal architecture of passive margins is a key to constrain their origin and subsequent evolution, as well as their thermal subsidence. The square shaped continental Demerara Plateau, French Guiana margin, surmounts Central and Equatorial Atlantic oceanic crusts surrounding it. Bounded to the northeast by a WNW-ESE-trending transform fault segment and to both the west and the east by N-S divergent fault segments, the Demerara Plateau is a complex transform-divergent interaction zone. The aim of this study is to refine the crustal architecture of this region as derived from gravity and seismic data, by thermal modelling, and by using surface heat flow data as an additional constraint. Previous studies show that the transform transition domain from continental to oceanic crust occurs across a region of approximately 70-km wide, where the Moho deepens abruptly from 25-27 km beneath the plateau (thinned continental crust), to 11-12 km in the abyssal oceanic domain (3-4 km thick oceanic crust). During the IGUANES cruise (onboard R/V L'Atalante in 2013) 10 surface heat flow measurements crossing the plateau have been carried out. These data are combined with borehole heat flows values around. Measures indicate that surface heat flow values range between 47 and 80 mW/m2 (with an uncertainty on the measurements of ~4mW/m2 on average), and slightly decreases in the continental domain toward the ocean. Preliminary 1D thermal modelling results indicate that these heat flow values are consistent with crustal and sediment thicknesses observed on the Plateau. Along the transform domain, at the transition towards the oceanic crust, heat flow values are lower than model results, if we consider an oceanic crust of more than hundred million years and with a thickness of around 3-4 km. We examine, using a 2D approach, whether this low heat flow could be reasonably accounted for by thermal exchange between oceanic and continental lithospheres.

  20. Potential serpentinization, degassing, and gas hydrate formation at a young (<20 Ma) sedimented ocean crust of the Arctic Ocean ridge system

    NASA Astrophysics Data System (ADS)

    Rajan, Anupama; Mienert, Jürgen; Bünz, Stefan; Chand, Shyam

    2012-03-01

    Global assessment of methane must consider the role of mid-ocean ridges. Fluids from serpentinized mantle and gabbro material are noteworthy on ocean ridges, although they are not very well understood. Only a few sedimented ocean ridges exist worldwide, and they may document past and ongoing serpentinization-driven migration of gas-rich fluids. This study is based on two multichannel reflection seismic profiles acquired across a sedimented segment of the ultraslow spreading Knipovich Ridge offshore NW Svalbard. Seismic data allow suggesting a potential link between inferred areas of serpentinization, transfer of carbon from the deep-seated host rocks through the sediments above by diapirism, and methane capture within the gas hydrate stability zone at the eastern flank of the Knipovich Ridge. The origin of sediment remobilization features can be related to intrusions and the degassing process from mantle serpentinization. These disturbances in sediments overlying the oceanic crust can be observed in seismic data and are interpreted as diapirs. In shallower sediments, at the predicted base of the gas hydrate stability zone, the seismic data show a bright spot with all the characteristics of a gas hydrate related bottom-simulating reflector (BSR), such as enhanced reflection amplitude, phase reversal relative to the seabed reflection, and crosscutting of sedimentary strata. The BSR occurs at about 200 ms two-way time within a sequence of marine sediments. Two-dimensional concentration models of methane hydrate using the differential effective medium theory predict saturations of up to 26% of methane hydrate in the pore space of sediments in the gas hydrate reservoir.

  1. Silica-rich lavas in the oceanic crust: experimental evidence for fractional crystallization under low water activity

    NASA Astrophysics Data System (ADS)

    Erdmann, Martin; Koepke, Jürgen

    2016-10-01

    We experimentally investigated phase relations and phase compositions as well as the influence of water activity ( aH2O) and redox conditions on the equilibrium crystallization path within an oceanic dacitic potassium-depleted system at shallow pressure (200 MPa). Moreover, we measured the partitioning of trace elements between melt and plagioclase via secondary ion mass spectrometry for a highly evolved experiment (SiO2 = 74.6 wt%). As starting material, we used a dacitic glass dredged at the Pacific-Antarctic Rise. Phase assemblages in natural high-silica systems reported from different locations of fast-spreading oceanic crust could be experimentally reproduced only in a relatively small range of temperature and melt-water content ( T ~950 °C; melt H2O < 1.5 wt%) at redox conditions slightly below the quartz-fayalite-magnetite buffer. The relatively low water content is remarkable, because distinct hydrothermal influence is generally regarded as key for producing silica-rich rocks in an oceanic environment. However, our conclusion is also supported by mineral and melt chemistry of natural evolved rocks; these rocks are only congruent to the composition of those experimental phases that are produced under low aH2O. Low FeO contents under water-saturated conditions and the characteristic enrichment of Al2O3 in high aH2O experiments, in particular, contradict natural observations, while experiments with low aH2O match the natural trend. Moreover, the observation that highly evolved experimental melts remain H2O-poor while they are relatively enriched in chlorine implies a decoupling between these two volatiles during crustal contamination.

  2. Can Fractional Crystallization of a Lunar Magma Ocean Produce the Lunar Crust?

    NASA Technical Reports Server (NTRS)

    Rapp, Jennifer F.; Draper, David S.

    2013-01-01

    New techniques enable the study of Apollo samples and lunar meteorites in unprecedented detail, and recent orbital spectral data reveal more about the lunar farside than ever before, raising new questions about the supposed simplicity of lunar geology. Nevertheless, crystallization of a global-scale magma ocean remains the best model to account for known lunar lithologies. Crystallization of a lunar magma ocean (LMO) is modeled to proceed by two end-member processes - fractional crystallization from (mostly) the bottom up, or initial equilibrium crystallization as the magma is vigorously convecting and crystals remain entrained, followed by crystal settling and a final period of fractional crystallization [1]. Physical models of magma viscosity and convection at this scale suggest that both processes are possible. We have been carrying out high-fidelity experimental simulations of LMO crystallization using two bulk compositions that can be regarded as end-members in the likely relevant range: Taylor Whole Moon (TWM) [2] and Lunar Primitive Upper Mantle (LPUM) [3]. TWM is enriched in refractory elements by 1.5 times relative to Earth, whereas LPUM is similar to the terrestrial primitive upper mantle, with adjustments made for the depletion of volatile alkalis observed on the Moon. Here we extend our earlier equilibrium-crystallization experiments [4] with runs simulating full fractional crystallization

  3. Distributions of REE, Nd, Hf and Pb isotopes in the surfaces of Fe-Mn crusts from across the Pacific Ocean

    NASA Astrophysics Data System (ADS)

    Chu, N.; Nesbitt, R. W.; German, C. R.; Halbach, P.

    2001-12-01

    Over the past decade, numerous studies have investigated radiogenic isotopes (i.e. Nd, Pb, Hf, Os) in marine Fe-Mn deposits in an attempt to infer changes in ocean circulation throughout the Cenozoic. The use of radiogenic isotopes as paleoceanographic proxies has been challenged recently by evidence that the isotopic composition of crusts from oceanic domains near major river systems (e.g. Amazon, Congo) or old cratonic areas (i.e. the North Atlantic area) could be influenced significantly by continental inputs. Therefore, for any given crust, it is difficult to deduce the extent to which changes in weathering processes rather than ocean circulation may be responsible for observed isotopic variations. This is partly due to the fact that the oceanic budgets for some of these elements remain poorly constrained. In particular, the influence of both the eastward aeolian transport of Chinese loess and the erosion of the young West-Pacific volcanic belt on the isotopic composition of Pacific water masses has been poorly documented. The deep Pacific Ocean is composed of 4 principal water masses: North Pacific Intermediate Water (NPIW), Antarctic Intermediate Water (AAIW), Pacific Deep Water (PDW) and Antarctic Bottom Water (AABW). AABW, the main source of PDW, flows northward and enters the Central Basin through the Samoan Passage, where it is diverted into two branches: eastward to Line Island Passage and westward through Wake Passage. We will present Nd, Hf and Pb isotopic ratios combined with REE data from the surfaces of 16 Fe-Mn crusts taken at different depths from key areas of the Pacific Ocean. Two crusts were collected from the Izu-Bonin back-arc basin in the western Pacific and, hence, are particularly suitable for monitoring the influence of both continental aeolian and weathering inputs. Two other groups of crusts are from north and south of the equatorial Pacific region. The southern group is situated at the exit of the Samoan Passage, whereas the northern

  4. Lateral Variability of the Lower Ocean Crust at Atlantis Bank, SW Indian Ridge, Results of IODP Expedition 360

    NASA Astrophysics Data System (ADS)

    Dick, H. J.; MacLeod, C. J.; Blum, P.; Scientific Party, E.

    2016-12-01

    IODP Hole U1473A drilled 809.4 m into a 700-m depth wave-cut platform at Atlantis Bank on the SW Indian Ridge. It is an oceanic core complex where massive gabbro was emplaced into the footwall of a single detachment fault for ≥2.7 Myr, with total slip ≥39 km. It was then uplifted to its present position flanking the 6,100 m deep 199-km Atlantis II Transform. The gabbros are back-tilted 20°S, while a sub-horizontal 15 km long mantle peridotite-gabbro contact lies along the transform wall at 4200 m depth 11.5 km west of Hole U1473A. Hole U1473A is 1.4 km north of 158-m deep Hole 1105A and 2.2 km NNE of 1508-m deep Hole 735B. Thus we examine the lateral continuity of the lower ocean crust at ultraslow rates ( 15-16 mm/yr.), and compare it to 1400-m Hole U1309D in the Atlantis Massif MAR core complex (24 mm/yr.) flanking the 63-km Atlantis Transform. The three Atlantis Bank holes are very similar, consisting of a complex series of oxide-rich gabbros and olivine gabbros. Several dikes crosscutting the gabbro sections show that they passed through the dike-gabbro transition after crystallizing and cooling deeper in the crust. They all show extensive high-temperature crystal-plastic deformation predating dike intrusion. A small amount of troctolite was recovered only in Hole 735B. By contrast, gabbro, rather than olivine gabbro was the dominant lithology in Hole U1309D, with intercalations of troctolite and mantle peridotite, and subordinate oxide gabbro. Oxide gabbro is often associated with crystal-plastic deformation. While these are concentrated in the upper 1/3 of Hole 735B, they are more uniformly distributed in Hole U1309D. While one section cannot be traced directly to the other at Atlantis Bank, it appears that they can be correlated based on chemical and structural similarities, with the 1105A and 1473A sections lying some hundreds of meters deeper structurally than Hole 735B, consistent with erosion on the platform. All these sections represent sequential

  5. Experimental Simulations of Lunar Magma Ocean Crystallization: The Plot (But Not the Crust) Thickens

    NASA Technical Reports Server (NTRS)

    Draper, D. S.; Rapp, J. F.; Elardo, S. M.; Shearer, C. K., Jr.; Neal, C. R.

    2016-01-01

    Numerical models of differentiation of a global-scale lunar magma ocean (LMO) have raised as many questions as they have answered. Recent orbital missions and sample studies have provided new context for a large range of lithologies, from the comparatively magnesian "purest anorthosite" reported by to Si-rich domes and spinel-rich clasts with widespread areal distributions. In addition, the GRAIL mission provided strong constraints on lunar crustal density and average thickness. Can this increasingly complex geology be accounted for via the formation and evolution of the LMO? We have in recent years been conducting extensive sets of petrologic experiments designed to fully simulate LMO crystallization, which had not been attempted previously. Here we review the key results from these experiments, which show that LMO differentiation is more complex than initial models suggested. Several important features expected from LMO crystallization models have yet to be reproduced experimentally; combined modelling and experimental work by our group is ongoing.

  6. Sulu-Celebes-Banda basins: a trapped piece of Cretaceous to Eocene oceanic crust

    SciTech Connect

    McCabe, R.J.; Hilde, T.W.; Cole, J.T.; Sager, W.; Lee, C.S.

    1986-07-01

    The Sulu-Celebes-Banda basin is composed of three poorly understood marginal basins located between northwest Australia and southeast Asia. Recent studies have proposed that these three basins are remnants of once-continuous ocean basin. The on-land geology of this region is complicated. However, numerous stratigraphic and paleomagnetic studies on pre-Oligocene rocks are consistent with the interpretation that older landmasses presently dissecting the basin were translated into their present position during the middle to late Tertiary. Paleomagnetic data from the Philippines suggest that the Philippine arc is a composite of Early Cretaceous to Holocene arcs that were translated clockwise and from the southeast. Paleomagnetic and stratigraphic data from Kalimantan and Sulawesi suggest that these landmasses share a common origin and that Sulawesi was rifted eastward off of Borneo during the late Tertiary. Stratigraphic studies from the Sula microcontinent, Buru, Ceram, and Timor show close correlation to the stratigraphy of northwest Australia or New Guinea. In addition, paleomagnetic studies from Timor suggest that a portion of the island was part of Australia since the early Mesozoic.

  7. Magnetic properties of the Bay of Islands ophiolite suite and implications for the magnetization of oceanic crust

    USGS Publications Warehouse

    Swift, B. Ann; Johnson, H. Paul

    1984-01-01

    Rock magnetic properties, opaque mineralogy, and degree of metamorphism were determined for 101 unoriented samples from the North Arm and Blow-Me-Down massifs of the Bay of Islands ophiolite complex, Newfoundland. The weathered and metamorphosed extrusive basalt samples have a weak, secondary magnetization arising from oxidation and exsolution of ilmenite of unknown origin. The initial magnetization of the underlying sheeted dike complex appears to have been destroyed by hydrothermal alteration soon after formation. The magnetic intensity of the gabbroic samples increases as the degree of alteration increases, with the highly altered upper metagabbros having an average intensity of 3×10−3 emu/c3. Because magnetization of the metagabbro samples is related to nonpervasive, variable alteration, these crustal units are unlikely to make a significant contribution to lineated magnetic anomalies. A compilation of our results and other studies suggests a model in which oceanic crust magnetization results from an upper extrusive basalt source layer, roughly 600 m thick, with no contribution from a deeper source layer recognizable from these Bay of Islands data.

  8. Tomographic image of crust and upper mantle off the Boso Peninsula using data from an ocean-bottom seismograph array

    NASA Astrophysics Data System (ADS)

    Ito, Aki; Yamamoto, Yojiro; Hino, Ryota; Suetsugu, Daisuke; Sugioka, Hiroko; Nakano, Masaru; Obana, Koichiro; Nakahigashi, Kazuo; Shinohara, Masanao

    2017-08-01

    We determined the three-dimensional structure of the crust and upper mantle off the Boso Peninsula, Japan, by analyzing seismograms recorded by ocean-bottom seismometers and land stations between 2011 and 2013. We employed seismic tomography to determine the P- and S-wave velocity structures and earthquake locations simultaneously. The tomographic image shows that the mantle parts of the Pacific and the Philippine Sea plates have high-velocity anomalies. The upper boundary of the Philippine Sea plate is delineated as approximately 2-6 km shallower than that previously estimated from land-based data for the area 140.5°E-141.5°E and 35°N-35.5°N. A pronounced low-velocity anomaly in P- and S-waves with low- V p/ V s ratio (1.5-1.6) was observed at depths shallower than 20 km in the overriding North American plate. This anomaly may be caused by the presence of rocks with a low- V p/ V s ratio, such as quartzite, and the water expelled from the subducted Pacific and Philippine Sea plates.[Figure not available: see fulltext.

  9. Self-organization of hydrothermal outflow and recharge in young oceanic crust: Constraints from open-top porous convection analog experiments

    NASA Astrophysics Data System (ADS)

    Mittelstaedt, E. L.; Olive, J. A. L.; Barreyre, T.

    2016-12-01

    Hydrothermal circulation at the axis of mid-ocean ridges has a profound effect on chemical and biological processes in the deep ocean, and influences the thermo-mechanical state of young oceanic lithosphere. Yet, the geometry of fluid pathways beneath the seafloor and its relation to spatial gradients in crustal permeability remain enigmatic. Here we present new laboratory models of hydrothermal circulation aimed at constraining the self-organization of porous convection cells in homogeneous as well as highly heterogeneous crust analogs. Oceanic crust analogs of known permeability are constructed using uniform glass spheres and 3-D printed plastics with a network of mutually perpendicular tubes. These materials are saturated with corn syrup-water mixtures and heated at their base by a resistive silicone strip heater to initiate thermal convection. A layer of pure fluid (i.e., an analog ocean) overlies the porous medium and allows an "open-top" boundary condition. Areas of fluid discharge from the crust into the ocean are identified by illuminating microscopic glass particles carried by the fluid, using laser sheets. Using particle image velocimetry, we estimate fluid discharge rates as well as the location and extent of fluid recharge. Thermo-couples distributed throughout the crust provide insights into the geometry of convection cells at depth, and enable estimates of convective heat flux, which can be compared to the heat supplied at the base of the system. Preliminary results indicate that in homogeneous crust, convection is largely confined to the narrow slot overlying the heat source. Regularly spaced discharge zones appear focused while recharge areas appear diffuse, and qualitatively resemble the along-axis distribution of hydrothermal fields at oceanic spreading centers. By varying the permeability of the crustal analogs, the viscosity of the convecting fluid, and the imposed basal temperature, our experiments span Rayleigh numbers between 10 and 10

  10. Morphology and genesis of slow-spreading ridges-seabed scattering and seismic imaging within the oceanic crust

    NASA Astrophysics Data System (ADS)

    Peirce, Christine; Sinha, Martin; Topping, Simon; Gill, Christopher

    2007-01-01

    A grid of 32 across-axis and five axis-parallel multichannel seismic (MCS) reflection profiles were acquired at an axial volcanic ridge (AVR) segment at 57° 45'N, 32° 35'W on the slow-spreading Reykjanes Ridge, Mid-Atlantic Ridge, to determine the along-axis variation and geometry of the axial magmatic system and to investigate the relationship between magma chamber structure, the along-axis continuity and segmentation of melt supply to the crust, the development of faulting and the thickness of oceanic layer 2A. Seismic reflection profiles acquired at mid-ocean ridges are prone to being swamped by high amplitude seabed scattered noise which can either mask or be mistaken for intracrustal reflection events. In this paper, we present the results of two approaches to this problem which simulate seabed scatter and which can either be used to remove or simply predict events within processed MCS profiles. The 37 MCS profiles show clear intracrustal seismic events which are related to the structure of oceanic layer 2, to the axial magmatic system and to the faults which dismember each AVR as it ages through its tectono-magmatic life cycle and which form the median valley walls. The layer 2A event can be mapped around the entirety of the survey area between 0.1 and 0.5 s two-way traveltime below the seabed, being thickest at AVR centres, and thinning both off-axis and along-axis towards AVR tips. Both AVR-parallel and ridge-parallel trends are observed, with the pattern of on-axis layer 2A thickness variation preserved beneath relict AVRs which are rafted off-axis largely intact. Each active AVR is underlain by a mid-crustal melt lens reflection extending almost along its entire length. Similar reflection events are observed beneath the offset basins between adjacent AVRs. These are interpreted as new AVRs at the start of their life cycle, developing centrally within the median valley. The east-west spacings of relict AVRs and offset basins is ~5-7 km, corresponding to

  11. The magmatic-hydrothermal transition in the lower oceanic crust: Clues from the Ligurian ophiolites, Italy

    NASA Astrophysics Data System (ADS)

    Tribuzio, Riccardo; Renna, Maria Rosaria; Dallai, Luigi; Zanetti, Alberto

    2014-04-01

    The gabbroic bodies from the Jurassic Ligurian ophiolites are structurally and compositionally similar to the gabbroic sequences from the oceanic core complexes of the Mid Atlantic Ridge. Initial cooling of the Ligurian gabbros is associated with local development of hornblende-bearing felsic dykes and hornblende vein networks. The hornblende veining is correlated with the widespread development of hornblende as coronas/pseudomorphs after the clinopyroxene in the host gabbros. In addition, the studied gabbroic body includes a mantle sliver locally containing hornblende gabbros and hornblendite veins. The hornblendes from the felsic dykes and the hornblende-rich rocks within the mantle sliver show a similar geochemical signature, characterized by low Mg#, CaO and Al2O3, negligible Cl, and high TiO2, K2O, REE, Y, Zr and Nb concentrations. The whole-rock Sm-Nd isotopic compositions of the felsic dykes and the hornblende-rich rocks define a Sm-Nd isochron corresponding to an age of 154 ± 20 Ma and an initial ɛNd of 9.2 ± 0.5. The δ18O of the hornblendes and coexisting zircons from these rocks (about +4.5‰ and +5.8‰, respectively) do not indicate the presence of a seawater component in these melts. The formation of the felsic dykes and of the hornblende-rich rocks within the mantle sliver involved SiO2-rich silicate melts with negligible seawater component, which presumably were derived from high degree fractional crystallization of MOR-type basalts. The vein and the coronitic/pseudomorphic hornblendes show high Mg# and CaO, significant Cl (0.02-0.17 wt%) and low TiO2 and K2O concentrations. The coronitic/pseudomorphic hornblendes have trace element compositions similar to those of the clinopyroxenes from the gabbros and δ18O values (+1.0‰ to 0.7‰) close to seawater, suggesting an origin by reaction between migrating seawater-derived fluids and the host gabbros. The vein hornblendes commonly show slight LREE enrichment, relatively high concentrations of Nb

  12. Fluid flow pathways through the oceanic crust: reaction permeability and isotopic tracing

    NASA Astrophysics Data System (ADS)

    McCaig, Andrew; Castelain, Teddy; Klein, Frieder

    2013-04-01

    It is generally assumed that the dominant means of creating permeability in ocean floor hydrothermal systems is fracturing, induced either by cooling or by tectonic stress. Here we show textural evidence that metamorphic reactions can create a hierarchy of permeable pathways through gabbroic rocks similar to a fracture hierarchy. Isotopic microsampling shows that just as with fractures, most flow occurs through the larger channelways, and that even at the microscale, flow can be extremely heterogeneous with alteration affecting only certain minerals in the framework, leaving others untouched. Reaction permeability is created in three ways; dissolution creating open porosity, microcracking due to volume increase reactions involving olivine, and expansion of water due to rapid heating in dyke margins, particularly when intruded into brecciated rocks. Our data comes from IODP Hole U1309D, which was drilled to 1400 mbsf in the footwall of the Atlantis Massif detachment fault at the Mid-Atlantic Ridge 30°N. The core is composed of gabbroic rocks interlayered with olivine rich troctolites, with several basalt/diabase sills in the top 130 m. The dominant alteration occurred in the greenschist facies, at depths at least 1 km below seafloor, and decreases in intensity downhole. Whole rock oxygen isotope values range from +5.5 permil to +1.5 permil, indicating variable degrees of interaction with seawater at temperatures generally > 250 °C. Gabbroic rocks and diabases exhibit a range of Sr isotope ratios from MORB values (0.70261) to intermediate ratios (0.70429). Microsampling shows that amphiboles are often more radiogenic than coexisting plagioclase and can sometimes be isotopically altered in the same rock as completely unaltered primary minerals. Large (10 cm) amphibole-filled vugs show values ranging up to 0.708, close to seawater. In some cases however the secondary minerals are virtually unaltered indicating low fluid fluxes in pervasive alteration. SEM textures in

  13. Petrology of the crossite schist from Fru\\vska Gora Mts (Yugoslavia), relic of a subducted slab of the Tethyan oceanic crust

    NASA Astrophysics Data System (ADS)

    Milovanovic, Dragan; Marchig, Vesna; Stevan, Karamata

    1995-11-01

    The mineral assemblage formed during the main stage of metamorphic recrystallization in the crossite schist from the Fru\\vska Gora Mts corresponds to the epidote-blueschist subfacies or high- T epidote-bearing segment of blueschist facies ( P = 7-9 kbar and T = ± 400 °C). Geochemical investigations indicate a primary alkaline character of the crossite schist, which suggests a volcanic arc or mid ocean ridge basalt tectonic setting. These rocks most probably represent a small relic of the former Mezozoic Tethyan oceanic crust, a discrete slice of mafic rocks which was metamorphosed under subducting conditions in the Early Cretaceous.

  14. First findings of Paleo- and Mesoarchean zircons in the rocks from the Central Arctic province of oceanic rises as an evidence of the ancient continental crust

    NASA Astrophysics Data System (ADS)

    Sergeev, S. A.; Presnyakov, S. L.; Antonov, A. V.; Belyatsky, B. V.; Rodionov, N. V.; Shevchenko, S. S.

    2015-07-01

    This report presents the results of local U-Pb zircon dating (SIMS SHRIMP II) for a sample of migmatite gneiss dredged on the western slope of Alpha Ridge in the Arctic Ocean in the course of the "Arktika-2012" Russian polar expedition. The distribution of U-Pb ages of the examined zircon points to the Early Precambrian origin of this gneiss, for the bulk of the zircon was crystallized at least 3450 Ma ago from a magmatic melt under acidic volcanism at the primary crust formation. Zircon of the second generation was crystallized 3300 Ma ago under the remelting of acid volcanics and appearance of migmatite gneisses under the amphibolite facies of metamorphism. Most likely, a partial recrystallization of zircon and formation of microfolded structures and foliation took place 3000 Ma ago at the stage of rocks deformation. The latest zircon was formed 1900 Ma ago from the crust fluid or melt under the low-gradient metamorphism. In view of the possibility of the appearance of the treated clastogenic gneiss fragment under current oceanic erosion, the obtained results allow one to affirm that the occurrence of a fragment of the most ancient sialic continental crust formed at least 3450 Ma ago is possible at the submarine rises of the Arctic Ocean (Alpha Ridge and the Mendeleev Rise).

  15. The role of low-temperature (off-axis) alteration of the oceanic crust in the global Li-cycle: Insights from the Troodos ophiolite

    NASA Astrophysics Data System (ADS)

    Coogan, L. A.; Gillis, K. M.; Pope, M.; Spence, J.

    2017-04-01

    Changes in the global Li-cycle, as recorded in the Li concentration and/or isotopic composition of seawater, have the potential to provide important insight into the controls on the long-term C-cycle. Understanding the magnitude and isotopic composition of the fluxes of Li into and out-of the ocean, and the controls on any variability in these, is necessary if we are to correctly interpret the paleo-record of the Li-cycle. Here the low-temperature (off-axis) hydrothermal sink is investigated using the volcanic section of the exceptionally preserved Troodos ophiolite. Using glass to define the protolith Li content, the uptake flux of Li is determined using bulk-rock analyses from four hydrologically distinct sections through the lava pile of the ophiolite. Differences in paleo-hydrological conditions in the crust appear to have played a significant role in controlling the uptake flux of Li with an 'average' uptake flux of equivalent to 14-21 × 109 mol yr-1 - this is considerably larger than generally assumed. Bulk-rock samples that contain a large seawater Li component have δ7Li of ∼10 ± 2‰. Celadonite separates have a δ7Li of ∼6 ± 1‰, considerably lighter than bulk-rock samples with the same Li content. Because celadonite is a significant repository for Li within the Troodos upper crust this means that another phase(s) must have markedly heavier δ7Li than the average bulk-rock; i.e. changes in the average mineralogy of altered crust will lead to changes in the bulk isotopic fractionation between the Li added to the upper oceanic crust and seawater (ΔSW-lava). The shallowest samples in three of the four studied sections are isotopically lighter than deeper samples (but do not contain significant celadonite), again indicating that variations in alteration conditions and/or mineralogy can lead to variations in ΔSW-lava. Comparison with other studies of altered upper oceanic crust suggests that changes in alteration conditions lead to significant

  16. Unstable fault slip induced by lawsonite dehydration in blueschist: Implication for the seismicity in the subducting oceanic crusts

    NASA Astrophysics Data System (ADS)

    Okazaki, K.; Hirth, G.

    2015-12-01

    Intermediate-depth earthquakes in cold subduction zones are observed within the subducting oceanic crust, as well as the subducting mantle In contrast, intermediate-depth earthquakes in hot subduction zones predominantly occur just below the Moho. These observations have stimulated interest in potential relationships between blueschist-facies metamorphism and seismicity, particularly through the dehydration reactions involving lawsonite. The rheology of these high-pressure and low-temperature metamorphic minerals is largely unknown. We conducted experiments on lawsonite accompanied by monitoring of acoustic emission (AE) in a Griggs-type deformation apparatus. Deformation was started at the confining pressure of 1.0 GPa, the temperature of 300 ˚C, and constant displacement rates of 0.16 to 0.016 μm/s, that correspond to equivalent strain rates (ɛ) of 9 × 10-5 to 9 × 10-6 1/s. In these experiments, temperature was increased at the temperature ramp rate of 0.5 to 0.05˚C/s above the thermal stability of lawsonite (600˚C) while the sample was deforming to test whether the dehydration reaction induces unstable fault slip. In contrast to similar tests on antigorite, unstable fault slip (i.e., stick-slip) occurred during dehydration reactions in the lawsonite gouge layer, and AE signals were continuously observed. Microstructural observations indicate that strain is highly localized along the fault (R1 and B shear), and the fault surface shows mirror-like slickensides. The unloading slope (i.e., rate of stress drop as a function of slip) during the unstable slip follows the stiffness of the apparatus at all experimental conditions regardless of the strain rate and temperature ramping rate. A thermal-mechanical scaling factor in the experiments covers the range estimated for natural subduction zones, indicating the potential for unstable frictional sliding within natural lawsonite layers to induce seismicity in cold subduction zones.

  17. Mobility of Au and related elements during the hydrothermal alteration of the oceanic crust: implications for the sources of metals in VMS deposits

    NASA Astrophysics Data System (ADS)

    Patten, Clifford G. C.; Pitcairn, Iain K.; Teagle, Damon A. H.; Harris, Michelle

    2016-02-01

    Volcanogenic massive sulphide (VMS) deposits are commonly enriched in Cu, Zn and Pb and can also be variably enriched in Au, As, Sb, Se and Te. The behaviour of these elements during hydrothermal alteration of the oceanic crust is not well known. Ocean Drilling Program (ODP) Hole 1256D penetrates a complete in situ section of the upper oceanic crust, providing a unique sample suite to investigate the behaviour of metals during hydrothermal alteration. A representative suite of samples was analysed for Au, As, Sb, Se and Te using low detection limit methods, and a mass balance of metal mobility has been carried out through comparison with a fresh Mid-Oceanic Ridge Basalt (MORB) glass database. The mass balance shows that Au, As, Se, Sb, S, Cu, Zn and Pb are depleted in the sheeted dyke and plutonic complexes by -46 ± 12, -27 ± 5, -2.5 ± 0.5, -27 ± 6, -8.4 ± 0.7, -9.6 ± 1.6, -7.9 ± 0.5 and -44 ± 6 %, respectively. Arsenic and Sb are enriched in the volcanic section due to seawater-derived fluid circulation. Calculations suggest that large quantities of metal are mobilised from the oceanic crust but only a small proportion is eventually trapped as VMS mineralisation. The quantity of Au mobilised and the ratio of Au to base metals are similar to those of mafic VMS, and ten times enrichment of Au would be needed to form a Au-rich VMS. The Cu-rich affinity of mafic VMS deposits could be explained by base metal fractionation both in the upper sheeted dykes and during VMS deposit formation.

  18. Seismic and thermal evidences for subduction of exhumed mantle oceanic crust beneath the seismically quiet Antigua-St Martin Margin segment in the Northern Lesser Antilles

    NASA Astrophysics Data System (ADS)

    Marcaillou, Boris; Klingelhoefer, Frauke; Laurencin, Muriel; Biari, Youssef; Graindorge, David; Lebrun, Jean-Frederic; Laigle, Mireille; Lallemand, Serge

    2017-04-01

    Wide-angle, multichannel reflection seismic data and heat-flow measurements from the Lesser Antilles subduction zone depict a large patch of atypical oceanic basement in the trench and beneath the outer fore-arc offshore of the Antigua-Saint Martin active margin segment. This segment triggers a very low number of earthquakes compared to the seismicity beneath the Virgin Island Platform to the north or in the Central Antilles (Martinique-Guadeloupe) to the south. Seven along-dip and two along-strike multichannel seismic lines acquired in this region show high amplitude steep reflectors that extend downward to 15-km depth in the downgoing slab. These lines also substantiate the absence of any reflections at Moho depth. Based on the wide-angle velocity model, the oceanic basement consists of a 5-km-thick unique layer with p-wave velocities ranging from 5.2 to 7.4 km/s, which is atypical for an oceanic crust. Heat-flow measurements along a transect perpendicular to the margin indicate a "flat" heat-flow trend from the trench to the fore-arc at 40 ± 15 mW.m-2 (Biari et al., same session). This heat flow profile contrasts with the expected trench-to-forearc decreasing heat-flow and the 50% higher heat-flow values measured in the trench offshore off the central Antilles. Calculated heat-flow for an incoming oceanic plate with a depressed geothermal gradient in the trench and heat source at depth in the subduction zone corresponding with temperatures of 200-250°C fit the measurements. We propose that a large patch of exhumed and serpentinized mantle rocks solidified at the slow-spreading mid-Atlantic Ridge is currently subducting beneath the studied margin segment. The fact that the crust here consists of one single layer and comprises velocities higher than found in igneous rocks (> 7.2 km/s) are consistent with this hypothesis. The plate bending possibly triggers long and deep delamination planes that extend into the mantle beneath the serpentinization front, which has

  19. Updated maps of Moho topography and the earth crust thickness in the Deep Arctic Ocean based on results of potential field zoning and 3-D gravity modeling

    NASA Astrophysics Data System (ADS)

    Glebovsky, Yury; Astafurova, Ekaterina; Chernykh, Andrey; Egorova, Alena; Kaminsky, Valeriy; Korneva, Mariya; Redko, Anton

    2014-05-01

    Both initial (Glebovsky et al., 2013) and updated maps and digital models (DM) of Moho topography and earth crust thickness in the deep Arctic Ocean were compiled using the same procedure. It included several steps: analysis of potential fields information compiled under CAMPGM and ArcGP projects and updating by new Russian data; separation of the study area into individual geostructures; calculation of gravitational effects from two main boundaries lying above Moho, presented by IBCAO grid, and by grid of basement relief (Kaminsky et al., 2012); subtraction of these effects from observed gravity anomalies, and converting of residual anomalies to depths to Moho using Parker's (1974) algorithm. Averaged depth to Moho required by Parker's algorithm to estimate its relative variations was determined from available deep refraction seismic data. It varies for different regional geological structures (basins, ridges and rises) which boundaries were contoured based on results of potential fields zoning. Modeling process for each structure was iterative and calibrated by seismic data. Results that best fit with seismic sections were merged to compile the grid of depths to Moho. This grid was specified by estimation of gravitational effects related both with increasing of density of sediments with depth and with uplift of asthenosphere beneath the Gakkel Ridge (GR). Grids of total and consolidated crust thickness were computed by sequential subtracting the IBCAO and sediment thickness grids from the final grid of depths to Moho. Updated versions of maps and DM of Moho topography and earth crust thickness are specified by recent Russian multi-channel and DSS seismic data collected in 2011-2012. It is confirmed the significant differences in crustal structure between the Eurasian (EB) and Amerasian Basins (AB). The thickness of the consolidated crust in the EB shows a fairly clear bilateral symmetry with respect to the GR. In the Nansen and Amundsen basins it varies from 3 to

  20. Investigation of the Oceanic Crust and Mantle in the Eastern Mid Atlantic Next to a Major Transform Fault (Gloria Fault) By Receiver Function Analysis

    NASA Astrophysics Data System (ADS)

    Hannemann, K.; Krueger, F.; Dahm, T.

    2014-12-01

    Within the project Deep OCean Test ARray (DOCTAR), we want to test how much we can enhance the signal to noise ratio (SNR) of teleseismic and regional events recorded at the ocean bottom by using broad band array methods. Furthermore, we want to learn more about the structure of the oceanic crust and mantle 100 km North of the Gloria Fault (major transform fault at the plate boundary between Eurasian and African plate in the Atlantic ocean). For the latter, we employ receiver functions and apparent P-wave incidence angles. We deployed 12 ocean bottom stations (OBS) as a mid aperture array (75 km) in the deep Eastern Mid Atlantic (4-6 km) in 2011. Each free fall station consists of a broad band seismometer and a hydrophone. After 10 month of recording, the stations were recovered. We use P phase and Rayleigh phase polarization to estimate the orientation of the stations. Different data quality and site effects at the stations need a careful review of the processing parameters (filter, deconvolution length) used for the calculation of the receiver functions. We defined different criteria as relative spike position within the deconvolution time window, and energy ratios of several time windows of the deconvolved traces to assess an evaluation of the receiver function quality in dependence on the used processing parameters. Additionally, we had a look at the relationship between the apparent incidence angle and the S-velocity and find that it differs for the ocean bottom in comparison to the free surface. Surprisingly, the densities of the oceanic crust and the water column, as well as the P-velocity of the water column have also an influence on the apparent incidence angle. We measured incidence angles for several events and find that the angles show a dependence on the dominant frequency of the event. By comparison with synthetic receiver functions, we find that water multiples have a small or no influence at all on the real data receiver functions. We identify the

  1. Insights into magmatic processes and hydrothermal alteration of in situ superfast spreading ocean crust at ODP/IODP site 1256 from a cluster analysis of rock magnetic properties

    NASA Astrophysics Data System (ADS)

    Dekkers, Mark J.; Heslop, David; Herrero-Bervera, Emilio; Acton, Gary; Krasa, David

    2014-08-01

    analyze magnetic properties from Ocean Drilling Program (ODP)/Integrated ODP (IODP) Hole 1256D (6°44.1' N, 91°56.1' W) on the Cocos Plate in ˜15.2 Ma oceanic crust generated by superfast seafloor spreading, the only drill hole that has sampled all three oceanic crust layers in a tectonically undisturbed setting. Fuzzy c-means cluster analysis and nonlinear mapping are utilized to study down-hole trends in the ratio of the saturation remanent magnetization and the saturation magnetization, the coercive force, the ratio of the remanent coercive force and coercive force, the low-field magnetic susceptibility, and the Curie temperature, to evaluate the effects of magmatic and hydrothermal processes on magnetic properties. A statistically robust five cluster solution separates the data predominantly into three clusters that express increasing hydrothermal alteration of the lavas, which differ from two distinct clusters mainly representing the dikes and gabbros. Extensive alteration can obliterate magnetic property differences between lavas, dikes, and gabbros. The imprint of thermochemical alteration on the iron-titanium oxides is only partially related to the porosity of the rocks. Thus, the analysis complements interpretation based on electrofacies analysis. All clusters display rock magnetic characteristics compatible with an ability to retain a stable natural remanent magnetization suggesting that the entire sampled sequence of ocean crust can contribute to marine magnetic anomalies. Paleointensity determination is difficult because of the propensity of oxyexsolution during laboratory heating and/or the presence of intergrowths. The upper part of the extrusive sequence, the granoblastic dikes, and moderately altered gabbros may contain a comparatively uncontaminated thermoremanent magnetization.

  2. Compositional variations in spinel-hosted pargasite inclusions in the olivine-rich rock from the oceanic crust-mantle boundary zone

    NASA Astrophysics Data System (ADS)

    Tamura, Akihiro; Morishita, Tomoaki; Ishimaru, Satoko; Hara, Kaori; Sanfilippo, Alessio; Arai, Shoji

    2016-05-01

    The crust-mantle boundary zone of the oceanic lithosphere is composed mainly of olivine-rich rocks represented by dunite and troctolite. However, we still do not fully understand the global variations in the boundary zone, and an effective classification of the boundary rocks, in terms of their petrographical features and origin, is an essential step in achieving such an understanding. In this paper, to highlight variations in olivine-rich rocks from the crust-mantle boundary, we describe the compositional variations in spinel-hosted hydrous silicate mineral inclusions in rock samples from the ocean floor near a mid-ocean ridge and trench. Pargasite is the dominant mineral among the inclusions, and all of them are exceptionally rich in incompatible elements. The host spinel grains are considered to be products of melt-peridotite reactions, because their origin cannot be ascribed to simple fractional crystallization of a melt. Trace-element compositions of pargasite inclusions are characteristically different between olivine-rich rock samples, in terms of the degree of Eu and Zr anomalies in the trace-element pattern. When considering the nature of the reaction that produced the inclusion-hosting spinel, the compositional differences between samples were found to reflect a diversity in the origin of the olivine-rich rocks, as for example in whether or not a reaction was accompanied by the fractional crystallization of plagioclase. The differences also reflect the fact that the melt flow system (porous or focused flow) controlled the melt/rock ratios during reaction. The pargasite inclusions provide useful data for constraining the history and origin of the olivine-rich rocks and therefore assist in our understanding of the crust-mantle boundary of the oceanic lithosphere.

  3. Increase of seismic velocities in upper oceanic crust: The “superfast” Spreading East Pacific Rise at 14°14'S

    NASA Astrophysics Data System (ADS)

    Grevemeyer, Ingo; Weigel, Wilfried

    One of the most important observations of oceanic crustal evolution is that seismic compressional wave velocities in the upper igneous crust, layer 2A, increase as the crust ages. The most viable explanation for this phenomenon is decreasing porosity due to filling of open void spaces with hydrothermally generated minerals. But the idea that seismic velocity could be a function of plate age, however, depends primarily on airgun/sonobuoy profiles collected more than 20 years ago [Houtz and Ewing, 1976]. Therefore, in late 1995 new seismic refraction measurements were obtained on the eastern flank of the “superfast” spreading East Pacific Rise south of the Garrett transform. We found that velocities rapidly increase by about 45-50% within only 0.5-1 m.y., double within 5 m.y. or less and remain constant thereafter.

  4. Reconstruction of seawater chemistry from deeply subducted oceanic crust; hydrogen and oxygen isotope of lawsonite eclogites preserving pillow structure

    NASA Astrophysics Data System (ADS)

    Hamabata, D., VI; Masuyama, Y.; Tomiyasu, F.; Ueno, Y.; Yui, T. F.; Okamoto, K.

    2014-12-01

    In order to understand evolution of life, change of seawater chemistry from Hadean, Archean to present is significant. Pillow structure is well-preserved in the Archean greenstone belt (e.g. Komiya et al., 1999). Oxygen and hydrogen isotope of rims in the pillow is useful conventional tool to decipher chemistry of Paleao-seawater from Archean to Present. However, Archean greenstone belt suffered regional metamorphism from greenschist to Amphibolite facies conditions. Therefore, it is necessary to testify the validity of pillow chemistry from recent (Phanerozoic) metamorphosed greenstone. We have systematically collected pillowed greenstone from blueschist and eclogites. Two eclogite exhibiting pillow structures were chosen for oxygen and hydrogen isotope analysis. One is from Corsica (lawsonite eclogite collected with Dr. Alberto Vidale Barbarone) and another is from Cazadero, Franciscan belt (collected by Dr. Tatsuki Tsujimori). The both are ascribed as MORB from major and trace bulk chemistry and Ca is rich in the core and Na is poor in the rims. The former exhibits garnet, omphacite, lawsonite, and glacophane. Phengite is in core of the pillow and chlorite is in the rims. In the latter, besides garnet, omphacite, epdiote and glaucophane, chlorite is recognized with phengite in the core. Glaucophane is richer in the rims from the both samples, therefore istope analysis of glaucophane was done. Mineral separation was carefully done using micro-mill, heavy liquid and isodynamic separator. 20 mg specimens were used for oxygen isotope analysis and 2mg were for hydrogen analysis. δ18O of the all analysis (7.7 to 8.3) is within the range of unaltered igneous oceanic crust and high temperature hydrothermal alteration although rims (8.3 for Franciscan and 8.0 for Corsica) are higher than cores (7.7 for Franciscan and Corsica). δD data is also consistent with hydrothermal alteration. It is relative higher in core from the Corsica and Franciscan (-45 and -56) than of the

  5. Strontium and oxygen isotopic profiles through 3 km of hydrothermally altered oceanic crust in the Reykjanes Geothermal System, Iceland

    NASA Astrophysics Data System (ADS)

    Marks, N. E.; Zierenberg, R. A.; Schiffman, P.

    2010-12-01

    The Iceland Deep Drilling Program well of opportunity RN-17 was drilled 3 km into a section of hydrothermally altered basaltic crust in the Reykjanes geothermal system in Iceland. The system is located on the landward extension of the Mid-Atlantic Ridge, and the circulating hydrothermal fluid is modified seawater, making Reykjanes a useful analogue for mid-oceanic ridge hydrothermal systems. Whole rock oxygen isotope ratios range from -0.13 to 3.61‰, which are significantly depleted relative to fresh MORB (5.8±0.2‰). If oxygen isotope exchange between fluid and rock proceeded under equilibrium in a closed system, the bulk of the exchange must have occurred in the presence of a meteoric- as opposed to seawater-derived fluid. The concentrations of Sr in the altered basalt range from well below to well above concentrations in fresh rock, and appear to be strongly correlated with the dominant alteration mineralogy, although there is no correlation with 87Sr/86Sr isotopic ratios. Whole rock Sr isotopic ratios ranged from 0.70329 in the least altered crystalline basalt, to 0.70609 in the most altered hyaloclastite samples; there is no correlation with depth. Sr isotopic variation in epidote grains measured by laser ablation MC-ICP-MS ranged from 0.70353 to 0.70731. Three depth intervals have distinctive isotopic signatures, at 1000 m, 1350 m, and 2000 m depth, where 87Sr/86Sr ratios are elevated (mean value >0.7050) relative to background levels (mean altered basalt value ~0.7042). These areas are proximal to feed zones, and the 1350 m interval directly overlies the transition from dominantly extrusive to intrusive lithologies. Strontium and oxygen isotope data indicate that the greenschist-altered basalts were in equilibrium with modified hydrothermal fluids at a relatively high mean water/rock mass ratios (generally in the range 1-3), and require the presence of both meteoric- and seawater-derived recharge fluids at various stages in the hydrothermal history.

  6. North Atlantic Deep Water export to the Southern Ocean over the past 14 Myr: Evidence from Nd and Pb isotopes in ferromanganese crusts

    USGS Publications Warehouse

    Frank, M.; Whiteley, N.; Kasten, S.; Hein, J.R.; O'Nions, K.

    2002-01-01

    The intensity of North Atlantic Deep Water (NADW) production has been one of the most important parameters controlling the global thermohaline ocean circulation system and climate. Here we present a new approach to reconstruct the overall strength of NADW export from the North Atlantic to the Southern Ocean over the past 14 Myr applying the deep water Nd and Pb isotope composition as recorded by ferromanganese crusts and nodules. We present the first long-term Nd and Pb isotope time series for deep Southern Ocean water masses, which are compared with previously published time series for NADW from the NW Atlantic Ocean. These data suggest a continuous and strong export of NADW, or a precursor of it, into the Southern Ocean between 14 and 3 Ma. An increasing difference in Nd and Pb isotope compositions between the NW Atlantic and the Southern Ocean over the past 3 Myr gives evidence for a progressive overall reduction of NADW export since the onset of Northern Hemisphere glaciation (NHG). The Nd isotope data allow us to assess at least semiquantitatively that the amount of this reduction has been in the range between 14 and 37% depending on location.

  7. Oceanic crust in the mid-mantle beneath west-central Pacific subduction zones: evidence from S to P converted waveforms

    NASA Astrophysics Data System (ADS)

    Yang, Zhongtao; He, Xiaobo

    2015-10-01

    The fate of subducted slabs is enigmatic, yet intriguing. We analyse seismic arrivals at ˜20-50 s after the direct P wave in an array in northeast China (NECESSArray) recordings of four deep earthquakes occurring beneath the west-central Pacific subduction zones (from the eastern Indonesia to Tonga region). We employ the array analysing techniques of fourth root vespagram and beam-forming analysis to constrain the slowness and backazimuth of later arrivals. Our analyses reveal that these arrivals have a slightly lower slowness value than the direct P wave and the backazimuth deviates slightly from the great circle direction. Along with calculation of 1-D synthetic seismograms, we conclude that the later arrival is corresponding to an energy of S-to-P converted at a scatterer below the sources. Total five scatterers are detected at depths varying from ˜700 to 1110 km in the study region. The past subducted oceanic crust most likely accounts for the seismic scatterers trapped in the mid-mantle beneath the west-central subduction zones. Our observation in turn reflects that oceanic crust at least partly separated from subducted oceanic lithosphere and may be trapped substantially in the mid-mantle surrounding subduction zones, in particular in the western Pacific subduction zones.

  8. Oceanic crust in the mid-mantle beneath Central-West Pacific subduction zones: Evidence from S-to-P converted waveforms

    NASA Astrophysics Data System (ADS)

    He, X.

    2015-12-01

    The fate of subducted slabs is enigmatic, yet intriguing. We analyze seismic arrivals at ~20-50 s after the direct P wave in an array in northeast China (NECESSArray) recordings of four deep earthquakes occurring beneath the west-central Pacific subduction zones (from the eastern Indonesia to Tonga region). We employ the array analyzing techniques of 4th root vespagram and beam-form analysis to constrain the slowness and back azimuth of later arrivals. Our analyses reveal that these arrivals have a slightly lower slowness value than the direct P wave and the back azimuth deviates slightly from the great-circle direction. Along with calculation of one-dimensional synthetic seismograms, we conclude that the later arrival is corresponding to an energy of S-to-P converted at a scatterer below the sources. Total five scatterers are detected at depths varying from ~700 to 1110 km in the study region. The past subducted oceanic crust most likely accounts for the seismic scatterers trapped in the mid-mantle beneath the west-central subduction zones. Our observation in turn reflects that oceanic crust at least partly separated from subducted oceanic lithosphere and may be trapped substantially in the mid-mantle surrounding subduction zones, in particular in the western Pacific subduction zones.

  9. Trace element systematics in the plutonic section of fast-spread oceanic crust - evidence from the Wadi Gideah reference profile (Wadi Tayin Massif, Oman ophiolite)

    NASA Astrophysics Data System (ADS)

    Garbe-Schoenberg, C. D.; Koepke, J.; Mueller, T.; Wolff, P. E.; Strauss, H.

    2014-12-01

    The crystallization processes during the accretion of fast spreading oceanic crust are still not well constrained and a continous geochemical profile through recent oceanic crust that could provide deeper insights into such processes is still missing. We sampled a complete section of gabbros, dikes, and basalts along the Wadi Gideah in the Cretaceous ophiolite of Oman that is regarded as the best example of fast-spread oceanic lithosphere on land. Here we present trace element data displaying systematic compositional trends that are correlated with stratigraphic depth (distance to MOHO) in the plutonic suite. Concentrations of incompatible trace elements Th, Nb, Ta, REE, Zr, Hf show little variation in the layered gabbro section but display a continuous increase from foliated to varitextured gabbros. In contrast, Nb/Ta, Nb/La, Zr/Hf, La/Yb, La/Nd show significant fractionation in the layered and foliated gabbros suggesting in situ crystallization in a lower crustal filter layer and formation of small portions of highly evolved interstitial melt also favoring crystallization of zircon. Further up, fractional crystallization processes in a well-mixed magma chamber control the composition of isotropic gabbros and sheeted dikes where fractionation of HFSE element ratios is minor.

  10. Sulfur geochemistry and microbial sulfate reduction during low-temperature alteration of uplifted lower oceanic crust: Insights from ODP Hole 735B

    USGS Publications Warehouse

    Alford, Susan E.; Alt, Jeffrey C.; Shanks, Wayne C.

    2011-01-01

    Sulfide petrography plus whole rock contents and isotope ratios of sulfur were measured in a 1.5 km section of oceanic gabbros in order to understand the geochemistry of sulfur cycling during low-temperature seawater alteration of the lower oceanic crust, and to test whether microbial effects may be present. Most samples have low SO4/ΣS values (≤ 0.15), have retained igneous globules of pyrrhotite ± chalcopyrite ± pentlandite, and host secondary aggregates of pyrrhotite and pyrite laths in smectite ± iron-oxyhydroxide ± magnetite ± calcite pseudomorphs of olivine and clinopyroxene. Compared to fresh gabbro containing 100–1800 ppm sulfur our data indicate an overall addition of sulfide to the lower crust. Selection of samples altered only at temperatures ≤ 110 °C constrains microbial sulfate reduction as the only viable mechanism for the observed sulfide addition, which may have been enabled by the production of H2 from oxidation of associated olivine and pyroxene. The wide range in δ34Ssulfide values (− 1.5 to + 16.3‰) and variable additions of sulfide are explained by variable εsulfate-sulfide under open system pathways, with a possible progression into closed system pathways. Some samples underwent oxidation related to seawater penetration along permeable fault horizons and have lost sulfur, have high SO4/ΣS (≥ 0.46) and variable δ34Ssulfide (0.7 to 16.9‰). Negative δ34Ssulfate–δ34Ssulfide values for the majority of samples indicate kinetic isotope fractionation during oxidation of sulfide minerals. Depth trends in sulfide–sulfur contents and sulfide mineral assemblages indicate a late-stage downward penetration of seawater into the lower 1 km of Hole 735B. Our results show that under appropriate temperature conditions, a subsurface biosphere can persist in the lower oceanic crust and alter its geochemistry.

  11. Relics of Triassic-Jurassic Meliata Ocean crust in the Western Carpathians: some constraints on the back-arc basin closure in the western Tethysn

    NASA Astrophysics Data System (ADS)

    Ivan, P.

    2003-04-01

    Hypothetical Meliata (or Meliata-Hallstatt) Ocean was deduced from isolated occurrences of small tectonic slices, mélanges and recycled sedimentary material (clasts) located along the northwest and southeast borders of the lithospheric block referred to as ALCAPA, which included the area of the Eastern Alps, Western Carpathians and also Panonnian area northwest of the Middle Hungarian Zone. Fragmentary character of preserved relics of the Meliata Ocean crust in the form of an incomplete dismembered ophiolite sequence is typical feature. Deep-sea sediments, basaltic pillow lavas and lava flows are most widespread rock types. Most outcrops of these rocks are concentrated in the inner Western Carpathians (Meliatic Unit). Trace element distribution in basalts reveal suprasubduction zone environment of the Meliata Ocean formation. This ocean was probably founded and opened inboard of the Permian volcanic arc, the vestiges of which have been identified in the inner and southern central Western Carpathians. Analogically to recent back arc basins arc-like (IAT) or back-arc basin basalts (BABB) were generated during initial stage of the basin opening and were erupted in the environment of carbonaceous and pelitic clastic sediments. In the evolved stage of opening dated as Ladinian basalts close to typical N-MORB occur in association with red radiolarites. The Meliata Ocean closure in the middle Jurassic was related with accretionary prism formation and HP/LT metamorphism of the subducted slab. Basinal margin rocks produced originally in the initial stage of opening represent the majority of preserved blueschists, whereas LP/LT metamorphosed basalts of the accretionary wedge came originally from the central part of the basin. In the final stage of the Meliata Ocean closure also continental crust complexes represented by arc-related volcano-sedimentary complexes of the Permian and Early Paleozoic(?) age were involved in the subduction zone and metamorphosed in HP

  12. Permeabilities of Young Oceanic Crust on the Flanks of the Juan de Fuca Ridge and Costa Rica Rift Determined from Borehole Temperature and Pressure Measurements

    NASA Astrophysics Data System (ADS)

    Becker, K.; Davis, E. E.

    2002-12-01

    In 1996, long-term CORK hydrological observatories were installed in four cased holes drilled by the Ocean Drilling Program through sediment cover into young oceanic crust east of the Juan de Fuca Ridge. Initial temperature measurements indicated vertical fluid flow in all of the holes prior to sealing with CORKs. Warm water was being produced from igneous crust of two sediment-covered basement ridges, and cool seawater was being drawn into basement at the other two holes. Rates were estimated from the perturbed temperature profiles at ~60 to 200 m/hr. Pressure differentials driving the flow were also measured with the CORK installations, allowing estimates of permeabilities of the upper crustal sections penetrated by the holes. Values vary systematically with age, ranging from about 10-10 m2 in the youngest site (0.9 Ma) to 10-12 m2 in the oldest site (3.6 Ma), confirming an apparent reduction of permeability with age determined with packer experiments at three of the same sites. Combined with other estimates of permeabilities in the same holes using methods with different scales of investigation, the new permeability estimates provide further evidence for a significant scale dependence of permeability in the upper oceanic crust. In 2001, wireline-deployable analogs of the CORK experiments were installed in two cased holes in well-sedimented ~6-m.y.-old crust on the south flank of the Costa Rica Ridge. The structural setting was very similar to the older pair of the four Juan de Fuca CORKs, and the results from initial temperature measurements were also comparable: warm water was produced from a hole in a sediment-covered basement ridge and cool seawater was being drawn into a neighboring hole in a basement trough. Rates of flow were slower than at the Juan de Fuca sites, probably because the holes had been left open for years. Pressure data are scheduled to be collected in November 2002 using DSV Alvin and should allow permeability calculations similar to those

  13. Insights into Oceanic Crust Accretion from a Comparison of Rock Magnetic and Silicate Fabrics from Lower Crustal Gabbros from Hess Deep Rift

    NASA Astrophysics Data System (ADS)

    Horst, A. J.; Morris, A.; Friedman, S. A.; Cheadle, M. J.

    2014-12-01

    The mechanisms of lower crustal accretion remain a long-standing question for those who study fast-spreading mid-ocean ridges. One of the goals of Integrated Ocean Drilling Program (IODP) Expedition 345 is to test accretionary models by investigating the structure of the lower oceanic crust exposed within the Hess Deep Rift. Located near the tip of the westward-propagating Cocos-Nazca spreading center, Hess Deep Rift exposes crust formed at the East Pacific Rise. During IODP Expedition 345, primitive gabbroic rocks were recovered from a dismembered lower crustal section at ~4850 meters below sealevel. Constraints on physical processes during magmatic accretion are provided by the relative orientation and strength of rock fabrics. We present anisotropy of magnetic susceptibility (AMS) fabric data from gabbros recovered from the two deepest holes (U1415J and U1415P). AMS measurements provide petrofabric data that may be used to constrain magma emplacement and subsequent magmatic flow. Bulk susceptibility ranges from 1.15 x 10-4 to 5.73 x 10-2 SI, with a majority of the samples having susceptibility greater than 10-3 SI, suggesting magnetite is the dominant contributor to the AMS signal. Low-temperature demagnetization data show Verwey transitions near 125K indicating the presence of nearly stoichiometric magnetite in most samples. AMS reveals dominantly oblate fabrics with a moderate degree of anisotropy (P') ranging from 1.01 to 1.38 (average P' = 1.13). Fabric strength varies within each of the petrologically-defined units recovered from different crustal blocks. Additional remanence anisotropy fabric analyses of a few specimens reveal nearly identical directions of principal axes compared to AMS, but with larger degrees of anisotropy. Electron backscatter diffraction (EBSD) data from one sample shows a moderate plagioclase crystallographic preferred orientation best defined by a b-axis maxima that is coincident with the AMS minimum principal axis. This comparison

  14. Diffusive Transfer of Oxygen From Seamount Basaltic Crust Into Overlying Sediments: an Example From the Clarion-Clipperton Fracture Zone, Equatorial Pacific Ocean

    NASA Astrophysics Data System (ADS)

    Kasten, S.; Mewes, K.; Mogollón, J.; Picard, A.; Rühlemann, C.; Eisenhauer, A.; Kuhn, T.; Ziebis, W.

    2015-12-01

    Within the Clarion-Clipperton Fracture Zone (CCFZ) located in the equatorial Pacific Ocean numerous seamounts, with diameters ranging from 3 to 30 km and varying heights above the surrounding seafloor of up to 2500 m, occur throughout the deep-sea plain. There is evidence that these may serve as conduits for low-temperature hydrothermal circulation of seawater through the oceanic crust. During RV SONNE cruise SO205 in April/May 2010 and BIONOD cruise with RV ĹATALANTE in spring 2012 we took piston and gravity cores for geochemical analyses, as well as for high-resolution pore-water oxygen and nutrient measurements. Specifically, we took cores along a transect at three sites, located 400, 700 and 1000 m away from the foot of a 240 m high seamount, called 'Teddy Bare'. At all 3 sites oxygen penetrates the entire sediment column of the organic carbon-poor sediment. More importantly, oxygen concentrations initially decrease with sediment depth but increase again at depths of 3 m and 7 m above the basaltic basement, suggesting an upward diffusion of oxygen from seawater circulating within the seamount crust into the overlying basal sediments. This is the first time this has been shown for the deep subsurface in the Pacific Ocean. Mirroring the oxygen concentrations nitrate concentrations accumulate with sediment depth but decrease towards the basement. Transport-reaction modeling revealed that (1) the diffusive flux of oxygen from the basaltic basement exceeds the oxygen consumption through organic matter oxidation and nitrification in the basal sediments and (2) the nutrient exchange between the sediment and the underlying basaltic crust occurs at orders-of-magnitude lower rates than between the sediment surface and the overlying bottom water. We furthermore show that the upward diffusion of oxygen from the basaltic basement affects the preservation of organic compounds within the oxic sediment column at all 3 sites. Our investigations indicate that an upward

  15. Intermediate crust (IC); its construction at continent edges, distinctive epeirogenic behaviour and identification as sedimentary basins within continents: new light on pre-oceanic plate motions

    NASA Astrophysics Data System (ADS)

    Osmaston, Miles F.

    2014-05-01

    Introduction. The plate tectonics paradigm currently posits that the Earth has only two kinds of crust - continental and oceanic - and that the former may be stretched to form sedimentary basins or the latter may be modified by arc or collision until it looks continental. But global analysis of the dynamics of actual plate motions for the past 150 Ma indicates [1 - 3] that continental tectospheres must be immensely thicker and rheologically stiffer than previously thought; almost certainly too thick to be stretched with the forces available. In the extreme case of cratons, these tectospheric keels evidently extend to 600 km or more [2, 3]. This thick-plate behaviour is attributable, not to cooling but to a petrological 'stiffening' effect, associated with a loss of water-weakening of the mineral crystals, which also applies to the hitherto supposedly mobile LVZ below MORs [4, 5]. The corresponding thick-plate version of the mid-ocean ridge (MOR) process [6 - 8], replacing the divergent mantle flow model, has a deep, narrow wall-accreting axial crack which not only provides the seismic anisotropy beneath the flanks but also brings two outstanding additional benefits:- (i) why, at medium to fast spreading rates, MOR axes become straight and orthogonally segmented [6], (ii) not being driven by body forces, it can achieve the sudden jumps of axis, spreading-rate and direction widely present in the ocean-floor record. Furthermore, as we will illustrate, the crack walls push themselves apart at depth by a thermodynamic mechanism, so the plates are not being pulled apart. So the presence of this process at a continental edge would not imply the application of extensional force to the margin. Intermediate Crust (IC). In seeking to resolve the paradox that superficially extensional structures are often seen at margins we will first consider how this MOR process would be affected by the heavy concurrent sedimentation to be expected when splitting a mature continent. I reason

  16. Crust and upper-mantle structure in the Gulf of California from ambient noise Rayleigh waves recorded on land and ocean-bottom seismographs

    NASA Astrophysics Data System (ADS)

    Carriero, N.; Gaherty, J. B.; Calkins, J. A.

    2009-12-01

    The Pacific-North America plate boundary, a young oblique rift system running lengthwise along the Gulf of California (GoC), is an excellent modern example of continental breakup and drifting. The central gulf (Guaymas transect) and the southern gulf (Alarcon transect) display robustly magmatic ocean crust production, but between these two transects are deep grabens with apparently limited magmatic production. To explain these along-axis variations in deformation and spreading, we utilize surface-wave techniques to image the structure of the mantle beneath the Gulf. Using the data from eight ocean-bottom seismometers, part of the 12-month deployment of the Sea of Cortez Ocean-Bottom Array (SCOOBA) seismic experiment, and the data from onshore seismometers of the NARS-Baja experiment, we use ambient seismic noise to estimate the phase and group velocities of surface waves propagating through the crust and upper mantle beneath the GoC. We cross-correlate one year of continuous data from over 90 station pairs in 6-hour time windows and stack it. As inter-station distance decreases, the noise recorded becomes more coherent, and the resulting cross-correlation signal-to-noise ratio (SNR) increases. The cross-correlations are most successful, as measured by high SNR, for stations less than 400 km apart. We use two methods to determine the surface-wave velocities. We find the phase velocity using a spectral method based on Aki’s original expression for the cross-correlation of stochastic surface waves. We also use frequency-time analysis to estimate group velocity, from which we can extract phase velocity. Preliminary maps of group-velocity variation are dominated by the abrupt thinning of the crust within the Gulf. Along-axis variations suggest a localization of high velocities in the central southern Gulf, most prominent at 20-sec. period, that may be consistent with lower temperatures and/or less melt along the portion of the GoC that displays less robust magmatism.

  17. Insights on the Formation and Evolution of the Upper Oceanic Crust from Deep Drilling at ODP/IODP Hole 1256D

    NASA Astrophysics Data System (ADS)

    Teagle, D. A. H.

    2009-04-01

    Deep drilling of Hole 1256D on ODP Leg 206 and IODP Expeditions 309/312 provides the first complete section of intact upper oceanic crust down to gabbros. Site 1256 is located on ocean crust of the Cocos Plate that formed at the East Pacific Rise (EPR) 15 million years ago during an episode of superfast rate ocean spreading in excess of 200 mm/yr. Past deep drilling of intact ocean crust has been fraught with difficulties due to the highly fractured nature of oceanic lavas. Site 1256 was specifically chosen because the observed relationship between spreading rate and the depth to axial seismic low velocity zones at modern mid-ocean ridges (thought to be magma chambers), suggests that gabbroic rocks should occur at the shallowest levels in ocean crust formed at the highest spreading rates. In line with pre-drilling predictions, gabbroic rocks were first encountered 1157 m into the basement. Hole 1256D penetrates 754 m of lavas, a 57-m thick transition zone and a thin (346 m) sheeted dike complex. The lower ~60 m of the sheeted dikes are contact metamorphosed to granoblastic textures. After encountering gabbros the hole was deepened a further 100 m before the cessation of drilling operations and the plutonic section comprises two gabbroic sills, 52 and 24 m-thick, intruded into a 24 m screen of granoblastic dikes. The gabbro sills have chilled margins and compositions similar to the overlying lavas and dikes, precluding formation of the cumulate lower oceanic crust from the melt lenses so far penetrated by Hole 1256D. A vertical seismic experiment conducted in Hole 1256D indicates that the bottom of the Hole is still within seismic layer 2 despite gabbroic rocks having been recovered. These data together with 1-D and imaging wire-line logs, have been used to construct a continuous volcano-stratigraphy for Site 1256. Comparison of this data with the recovered cores and the styles of eruption occurring at the modern EPR indicate that ~50% of lava sequences were formed

  18. MORB mantle hosts the missing Eu (Sr, Nb, Ta and Ti) in the continental crust: New perspectives on crustal growth, crust-mantle differentiation and chemical structure of oceanic upper mantle

    NASA Astrophysics Data System (ADS)

    Niu, Yaoling; O'Hara, Michael J.

    2009-09-01

    We have examined the high quality data of 306 mid-ocean ridge basalt (MORB) glass samples from the East Pacific Rise (EPR), near-EPR seamounts, Pacific Antarctic Ridge (PAR), near-PAR seamounts, Mid-Atlantic Ridge (MAR), and near-MAR seamounts. The data show a correlated variation between Eu/Eu* and Sr/Sr*, and both decrease with decreasing MgO, pointing to the effect of plagioclase crystallization. The observation that samples with MgO > 9.5 wt.% (before plagioclase on the liquidus) show Eu/Eu* > 1 and Sr/Sr* > 1 and that none of the major phases (i.e., olivine, orthopyroxene, clinopyroxene, spinel and garnet) in the sub-ridge mantle melting region can effectively fractionate Eu and Sr from otherwise similarly incompatible elements indicates that the depleted MORB mantle (DMM) possesses excess Sr and Eu, i.e., [Sr/Sr*]DMM > 1 and [Eu/Eu*]DMM > 1. Furthermore, the well-established observation that DNb ≈ DTh, DTa ≈ DU and DTi ≈ DSm during MORB mantle melting, yet primitive MORB melts all have [Nb/Th]PMMORB > 1, [Ta/U]PMMORB > 1 and [Ti/Sm]PMMORB > 1 (where PM indicates primitive mantle normalized), also points to the presence of excess Nb, Ta and Ti in the DMM, i.e., [Nb/Th]PMDMM > 1, [Ta/U]PMDMM > 1 and [Ti/Sm]PMDMM > 1. The excesses of Eu, Sr, Nb, Ta and Ti in the DMM complement the well-known deficiencies of these elements in the bulk continental crust (BCC). These new observations, which support the notion that the DMM and BCC are complementary in terms of the overall abundances of incompatible elements, offer new insights into the crust-mantle differentiation. These observations are best explained by partial melting of amphibolite of MORB protolith during continental collision, which produces andesitic melts with a remarkable compositional (major and trace element abundances as well as key elemental ratios) similarity to the BCC, as revealed by andesites in southern Tibet produced during the India-Asia continental collision. An average amphibolite of MORB

  19. Preliminary Results from Downhole Osmotic Samplers in a Gas Tracer Injection Experiment in the Upper Oceanic Crust on the Eastern Flank of the Juan de Fuca Ridge.

    NASA Astrophysics Data System (ADS)

    de Jong, M. T.; Clark, J. F.; Neira, N. M.; Fisher, A. T.; Wheat, C. G.

    2015-12-01

    We present results from a gas tracer injection experiment in the ocean crust on the eastern flank of the Juan de Fuca Ridge, in an area of hydrothermal circulation. Sulfur hexafluoride (SF6) tracer was injected in Hole 1362B in 2010, during IODP Expedition 327. Fluid samples were subsequently collected from a borehole observatory (CORK) installed in this hole and similar CORKs in three additional holes (1026B, 1362A, and 1301A), located 300 to 500 m away. This array of holes is located on 3.5 My old seafloor, as an array oriented subparallel to the Endeavor Segment of Juan de Fuca Ridge. Borehole fluid samples were collected in copper coils using osmotic pumps. In addition to pumps at seafloor wellheads, downhole sampling pumps were installed in the perforated casing in the upper ocean crust. These downhole samplers were intended to produce a high-resolution continuous record of tracer concentrations, including records from the first year after tracer injection in Holes 1362A and 1362B. In contrast, wellhead samplers were not installed on these CORKs holes until 2011, and wellhead records from all CORKs have a record gap of up to one year, because of a delayed expedition in 2012. The downhole samples were recovered with the submersible Alvin in August 2014. SF6 concentrations in downhole samples recovered in 2014 are generally consistent with data obtained from wellhead samples. Of particular interest are the results from Hole 1362B, where a seafloor valve was opened and closed during various recovery expeditions. High resolution tracer curves produced from the 1362B downhole samples confirm that these operations produced an SF6 breakthrough curve corresponding to a classic push-pull test used to evaluate contaminant field locations in terrestrial setting. Complete analyses of downhole samples from these CORKs are expected to produce high-resolution breakthrough curves that will allow more precise analysis and modeling of hydrothermal flow in the study area.

  20. Icelandic-type crust

    USGS Publications Warehouse

    Foulger, G.R.; Du, Z.; Julian, B.R.

    2003-01-01

    Numerous seismic studies, in particular using receiver functions and explosion seismology, have provided a detailed picture of the structure and thickness of the crust beneath the Iceland transverse ridge. We review the results and propose a structural model that is consistent with all the observations. The upper crust is typically 7 ?? 1 km thick, heterogeneous and has high velocity gradients. The lower crust is typically 15-30 ?? 5 km thick and begins where the velocity gradient decreases radically. This generally occurs at the V p ??? 6.5 km s-1 level. A low-velocity zone ??? 10 000 km2 in area and up to ??? 15 km thick occupies the lower crust beneath central Iceland, and may represent a submerged, trapped oceanic microplate. The crust-mantle boundary is a transition zone ???5 ?? 3 km thick throughout which V p increases progressively from ???7.2 to ???8.0 km s-1. It may be gradational or a zone of alternating high- and low-velocity layers. There is no seismic evidence for melt or exceptionally high temperatures in or near this zone. Isostasy indicates that the density contrast between the lower crust and the mantle is only ???90 kg m-3 compared with ???300 kg m-3 for normal oceanic crust, indicating compositional anomalies that are as yet not understood. The seismological crust is ???30 km thick beneath the Greenland-Iceland and Iceland-Faeroe ridges, and eastern Iceland, ???20 km beneath western Iceland, and ???40 km thick beneath central Iceland. This pattern is not what is predicted for an eastward-migrating plume. Low attenuation and normal V p/V s ratios in the lower crust beneath central and southwestern Iceland, and normal uppermost mantle velocities in general, suggest that the crust and uppermost mantle are subsolidus and cooler than at equivalent depths beneath the East Pacific Rise. Seismic data from Iceland have historically been interpreted both in terms of thin-hot and thick-cold crust models, both of which have been cited as supporting the plume

  1. Two stage melt-rock interaction in the lower oceanic crust of the Parece Vela basin (Philippine sea), evidence from the primitive troctolites from the Godzilla Megamullion

    NASA Astrophysics Data System (ADS)

    Sanfilippo, A.; Dick, H. J.; Ohara, Y.

    2011-12-01

    Godzilla Megamullion is a giant oceanic core complex exposed in an extinct slow- to intermediate-spreading segment of the Parece Vela Basin (Philippine sea) [1; 2]. It exposes lower crust and mantle rocks on the sea-floor, offering a unique opportunity to unravel the architecture and the composition of the lower oceanic lithosphere of an extinct back arc basin. Here we present data on primitive troctolites and associated olivine-gabbros from the breakaway area of the Godzilla Megamullion. On the basis of the olivine/plagioclase volume ratio, the troctolites are subdivided into Ol-troctolites (Ol/Pl >1) and Pl-troctolites (Ol/Pl<1), which show evident textural differences. Ol-troctolites have rounded to polygonal olivine, subhedral plagioclase, and poikilitic clinopyroxene. This texture suggests chemical disequilibrium between the olivine and a melt crystallizing plagioclase and clinopyroxene. We interpret these rocks as reaction products of a dunite matrix with transient basaltic melts [e.g. 3; 4]. Pl-troctolites have euhedral plagioclase and poikilitic olivine and clinopyroxene. Irregular shapes and inverse zoning of the plagioclase chadacrysts within the olivine indicate disequilibrium between existing plagioclase and an olivine-clinopyroxene saturated melt. The occurrence of plagioclase chadacrysts within clinopyroxene ranging from irregular to euhedral in shape suggests crystallization of new lower-Na plagioclase with the clinopyroxene. Olivine oikocrysts in the Pl-troctolites have low-NiO olivine in equilibrium with a high-MgO melt. The Pl-troctolites, then, may be the product of reaction between a plagioclase cumulate and a basaltic melt produced by mixing the high-MgO melt residual to the formation of the Ol-troctolites with new magma. The effect of melt-rock reaction in the Pl- and Ol- troctolites explains the sharp decrease in plagioclase An with respect to Mg# in clinopyroxene and olivine. Furthermore, the melt is shifted towards lower Na, which is

  2. Channelling of hydrothermal fluids during the accretion and evolution of the upper oceanic crust: Sr isotope evidence from ODP Hole 1256D

    NASA Astrophysics Data System (ADS)

    Harris, Michelle; Coggon, Rosalind M.; Smith-Duque, Christopher E.; Cooper, Matthew J.; Milton, James A.; Teagle, Damon A. H.

    2015-04-01

    ODP Hole 1256D in the eastern equatorial Pacific is the first penetration of a complete section of fast spread ocean crust down to the dike-gabbro transition, and only the second borehole to sample in situ sheeted dikes after DSDP Hole 504B. Here a high spatial resolution record of whole rock and mineral strontium isotopic compositions from Site 1256 is combined with core observations and downhole wireline geophysical measurements to determine the extent of basalt-hydrothermal fluid reaction and to identify fluid pathways at different levels in the upper ocean crust. The volcanic sequence at Site 1256 is dominated by sheet and massive lava flows but the Sr isotope profile shows only limited exchange with seawater. However, the upper margins of two anomalously thick (>25 m) massive flow sequences are strongly hydrothermally altered with elevated Sr isotope ratios and appear to be conduits of lateral low-temperature off-axis fluid flow. Elsewhere in the lavas, high 87Sr/86Sr are restricted to breccia horizons. Mineralised hyaloclastic breccias in the Lava-Dike Transition are strongly altered to Mg-saponite, silica and pyrite, indicating alteration by mixed seawater and cooled hydrothermal fluids. In the Sheeted Dike Complex 87Sr/86Sr ratios are pervasively shifted towards hydrothermal fluid values (∼0.705). Dike chilled margins display secondary mineral assemblages formed during both axial recharge and discharge and have higher 87Sr/86Sr than dike cores, indicating preferential fluid flow along dike margins. Localised increases in 87Sr/86Sr in the Dike-Gabbro Transition indicates the channelling of fluids along the sub-horizontal intrusive boundaries of the 25 to 50 m-thick gabbroic intrusions, with only minor increases in 87Sr/86Sr within the cores of the gabbro bodies. When compared to the pillow lava-dominated section from Hole 504B, the Sr isotope measurements from Site 1256 suggest that the extent of hydrothermal circulation in the upper ocean crust may be

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

    NASA Astrophysics Data System (ADS)

    Schroeder, Timothy John

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

  4. Evolution of the continental crust.

    PubMed

    Hawkesworth, C J; Kemp, A I S

    2006-10-19

    The continental crust covers nearly a third of the Earth's surface. It is buoyant--being less dense than the crust under the surrounding oceans--and is compositionally evolved, dominating the Earth's budget for those elements that preferentially partition into silicate liquid during mantle melting. Models for the differentiation of the continental crust can provide insights into how and when it was formed, and can be used to show that the composition of the basaltic protolith to the continental crust is similar to that of the average lower crust. From the late Archaean to late Proterozoic eras (some 3-1 billion years ago), much of the continental crust appears to have been generated in pulses of relatively rapid growth. Reconciling the sedimentary and igneous records for crustal evolution indicates that it may take up to one billion years for new crust to dominate the sedimentary record. Combining models for the differentiation of the crust and the residence time of elements in the upper crust indicates that the average rate of crust formation is some 2-3 times higher than most previous estimates.

  5. The role of mantle temperature and lithospheric thickness during initial oceanic crust production: numerical modelling constraints from the southern South Atlantic

    NASA Astrophysics Data System (ADS)

    Taposeea, C.; Armitage, J. J.; Collier, J.

    2015-12-01

    Evidence from seaward dipping reflector distributions has recently suggested that segmentation plays a major role in the pattern of volcanism during breakup, particularly in the South Atlantic. At a larger scale, variations in mantle temperature and lithosphere thickness can enhance or reduce volcanism. To understand what generates along strike variation of volcanism at conjugate margins, we measure the thickness of earliest oceanic crust in the South Atlantic, south of the Walvis and Rio Grande ridges. We use data from 29 published wide-angle and multichannel seismic profiles and at least 14 unpublished multichannel seismic profiles. A strong linear trend between initial oceanic crustal thickness and distance from hotspot centres, defined as the commencement of Walvis and Rio Grande ridges, with a regression coefficient of 0.7, is observed. At 450km south of the Walvis Ridge, earliest oceanic crustal thickness is found to be 11.7km. This reduces to 7.0km in the south at a distance of 1,420km. Such a linear trend suggests rift segmentation plays a secondary role on volcanism during breakup. To explore the cause of this trend, we use a 2D numerical model of extension capable of predicting the volume and composition of melt generated by decompressional melting during extension to steady state seafloor spreading. We explore the effect of both mantle temperature and lithosphere thickness on melt production with a thermal anomaly (hot layer) 100km thick located below the lithosphere with an excess temperature of 50-200°C, and lithospheric thickness ranging from 125-140km, covering the thickness range estimated from tomographic studies. By focusing on a set of key seismic profiles, we show a reduction in hot layer temperature is needed in order to match observed oceanic crustal thickness, even when the effect of north to south variations in lithosphere thickness are included. This model implies that the observed oceanic thickness requires the influence of a hot layer up

  6. Coupling of Oceanic and Continental Crust During Eocene Eclogite-Facies Metamorphism: Evidence From the Monte Rosa Nappe, Western Alps, Italy

    NASA Astrophysics Data System (ADS)

    Lapen, T. J.; Johnson, C. M.; Baumgartner, L. P.; Skora, S.; Mahlen, N. J.; Beard, B. L.

    2006-12-01

    Subduction of continental crust to HP-UHP metamorphic conditions requires overcoming density contrasts that are unfavorable to deep burial, whereas exhumation of these rocks can be reasonably explained through buoyancy-assisted transport in the subduction channel to more shallow depths. In the western Alps, both continental and oceanic lithosphere has been subducted to eclogite-facies metamorphic conditions. The burial and exhumation histories of these sections of lithosphere bear directly on the dynamics of subduction and the stacking of units within the subduction channel. We address the burial history of the continental crust with high precision U-Pb rutile and Lu-Hf garnet geochronology of the eclogite-facies Monte Rosa nappe (MR), western Alps, Italy. U-Pb rutile ages from quartz-carbonate-white mica-rutile veins that are hosted within eclogite and schist of the MR, Gressoney Valley, Italy, indicate that it was at eclogite-facies metamorphic conditions at 42.6 +/- 0.6 Ma. The sample area (Indren glacier, Furgg zone; Dal Piaz, 2001) consists of eclogite boudins that are surrounded by micaceous schist. Associated with the eclogite and schist are quartz-carbonate-white mica-rutile veins that formed in tension cracks in the eclogite and along the contact between eclogite and surrounding schist. Intrusion of the veins occurred at eclogite-facies metamorphic conditions (480-570°C, >1.3-1.4 GPa) based on textural relations, oxygen isotope thermometry, and geothermobarometry. Lu-Hf geochronology of garnet from a chloritoid-talc-garnet-phengite-quartz-calcite-pyrite - chalcopyrite bearing boudin within talc-chloritoid whiteschists of the MR, Val d'Ayas, Italy (Chopin and Monie, 1984; Pawlig, 2001) yields an age of 40.54 +/- 0.36 Ma. The talc-chloritoid whiteschists from the area record pressures and temperatures of 1.6-2.4 GPa and 500-530°C (Chopin and Monie, 1984; Le Bayon et al., 2006) indicating near UHP metamorphic conditions. Based on the age, P-T, and textural

  7. Paleomagnetic constraints on deformation models for uppermost oceanic crust exposed at the Hess Deep Rift: Implications for axial processes at the East Pacific Rise

    NASA Astrophysics Data System (ADS)

    Varga, Robert J.; Karson, Jeffrey A.; Gee, Jeffrey S.

    2004-02-01

    Studies of oceanic crust exposed in tectonic windows and in ophiolites have revealed the importance of normal faulting and attendant tilting of upper crustal rock units in the accretion process at oceanic spreading centers. We present paleomagnetic remanence data from 45 fully oriented samples from dikes, gabbros and a small number of basaltic lavas from fast spread crust exposed along the Hess Deep Rift. Over ˜25 km along this escarpment, dikes and dike-subparallel fault zones dip consistently away from the East Pacific Rise (EPR) while lava flows dip toward the ridge. Underlying gabbro is less deformed but contains widely spaced, low-angle fractures, tentatively interpreted as shear zones. As expected from the crustal age (˜1.07-1.48 Ma), most remanence data indicate reversed polarity magnetization and are compatible with the expected range of secular variation at the site. Overly steep and directionally scattered gabbro remanence and observed low-angle shear structures within this unit are tentatively interpreted as the manifestation of three-dimensional strain along anastomosing shear zones. Although some remanence directions are incompatible with any plausible deformation history, and thus likely reflect orientation errors, the overall data set is consistent with a model involving sequential rotations on (1) outward dipping, EPR-parallel (˜N-S) normal faults and (2) Hess Deep Rift-parallel (˜E-W) normal faults Average rotations for these sequential events are 22° to the east (defined by the mean dike attitude) and 10° to the south (estimated by bathymetry), respectively. This model best explains the remanence data, observed dikes and lava orientations, presence of dike-parallel fault zones, and the observation of steep, little deformed dikes cutting both east dipping dikes and faults. The data support a structural model for spreading at the EPR in which outcrop-scale faulting and rotation is linked to subaxial subsidence and to consequent development of

  8. Are the Element Budget and the Occurrence of Polymetallic Nodules influenced by Fluids Circulating through the Oceanic Crust or/and Sediments?

    NASA Astrophysics Data System (ADS)

    Heller, C.; Kuhn, T.

    2016-12-01

    Hydrothermal fluids can extract significant amounts of heat from oceanic lithosphere by lateral fluid flow through permeable basaltic crust of an age of up to 65 Ma. Fluid recharge and discharge occur at basement outcrops in between impermeable pelagic sediments. Recharge of oxic seawater causes upward oxygen diffusion into sediments overlying the permeable basalt in areas proximal to recharge sites. It is suggested that this oxygen have a strong impact on sediments and Mn nodules during fluid exposure time. The aim of this study is to investigate if and how fluid flow through oceanic crust influence the distribution and element budget of the Mn nodules. For that purpose, Mn nodules were examined which were collected during the research cruise SO240 in the equatorial NE Pacific at sites with and without faults in the upper basement and overlying sediments. Faults are thought to be preferred fluid pathways. Nodules were found on the sediment surface as well as in the sediment and consist of different nm- to µm-thick, dense and porous layers. The geochemical composition of bulk nodules and single nodule layers were determined by XRF, ICP-MS/OES and by high resolution analyses with EMPA and LA-ICP-MS. Dense layers have low Mn/Fe ratios (<4) and high concentrations of Co, Zr and REY, while porous layers are characterized by high Mn/Fe ratios (> 10) and high Ni+Cu and Li concentrations (Koschinsky et al., 2010; Kuhn et al., 2010). The different compositions depends on different formation processes of the layers. Dense layers are formed by element precipitation from oxygen rich seawater and/or pore water and are called hydrogenetic, while porous layers were formed by precipitation from almost oxygen-free (suboxic) pore water (Burns & Burns, 1978; Glasby, 2006) and are called diagenetic (Halbach et al., 1988). Preliminary results show that there are significant differences between the geochemical composition of nodules grown at sediment surface and those found within

  9. Subduction-zone crust-mantle interaction is a common mechanism for the origin of oceanic arc and island basalts

    NASA Astrophysics Data System (ADS)

    Zheng, Y. F.; Zhao, Z. F.

    2014-12-01

    We present a generalized model for the origin of oceanic arc basalts (OAB) and oceanic island basalts (OIB). This is realized by an integrated study of their major-trace element and stable-radiogenic isotope compositions. Many continental basalts are geochemically indistinguishable from common OIB, a fact that requires part of the upper mantle to have been a common reservoir beneath both oceans and continents. In addition, this reservoir must have been isolated from the convective asthenosphere for preservation of geochemical anomalies. Common OAB and OIB show consistent enrichment of LILE and LREE relative to normal MORB. On the primitive mantle-normalized spidergram, however, OAB are characterized by negative Nb and Ta anomalies but a positive Pb anomaly, whereas OIB show positive or no Nb and Ta anomalies but a negative Pb anomaly. Such differences are attributed to the difference in the property of metasomatic agents (aqueous solutions, hydrous melts and supercritical fluids) derived from subducting crustal rocks. The metasomatic agents are highly enriched in fluid/melt-mobile incompatible trace elements such as LILE and LREE, transferring enriched components from the crustal rocks to the mantle sources of OAB and OIB. The stability of rutile in the subducting crustal rocks dictates the abundance of Nb and Ta in the metasomatic agents. Lead is preferentially partitioned into the metasomatic agents when released at subarc depths, whereas dehydrated Pb-poor restites were subducted to greater depths. This explains the positive Pb anomaly in OAB but the negative Pb anomaly in OIB. We accept the assumption that normal MORB are derived from partial melting of the normal asthenospheric mantle, a common reservoir of isotopic depletion. We extend the chemical reaction at the slab-mantle interface in subduction channel from subarc depths to those above the mantle transition zone, generating metasomatic ultramafic rocks (metasomes) in the upper mantle. The reaction at

  10. Permeability changes due to mineral diagenesis in fractured crust: implications for hydrothermal circulation at mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Fontaine, Fabrice Jh.; Rabinowicz, Michel; Boulègue, Jacques

    2001-01-01

    The hydrothermal processes at ridge crests have been extensively studied during the last two decades. Nevertheless, the reasons why hydrothermal fields are only occasionally found along some ridge segments remain a matter of debate. In the present study we relate this observation to the mineral precipitation induced by hydrothermal circulation. Our study is based on numerical models of convection inside a porous slot 1.5 km high, 2.25 km long and 120 m wide, where seawater is free to enter and exit at its top while the bottom is held at a constant temperature of 420°C. Since the fluid circulation is slow and the fissures in which seawater circulates are narrow, the reactions between seawater and the crust achieve local equilibrium. The rate of mineral precipitation or dissolution is proportional to the total derivative of the temperature with respect to time. Precipitation of minerals reduces the width of the fissures and thus percolation. Using conventional permeability versus porosity laws, we evaluate the evolution of the permeability field during the hydrothermal circulation. Our computations begin with a uniform permeability and a conductive thermal profile. After imposing a small random perturbation on the initial thermal field, the circulation adopts a finger-like structure, typical of convection in vertical porous slots thermally influenced by surrounding walls. Due to the strong temperature dependence of the fluid viscosity and thermal expansion, the hot rising fingers are strongly buoyant and collide with the top cold stagnant water layer. At the interface of the cold and hot layers, a horizontal boundary layer develops causing massive precipitation. This precipitation front produces a barrier to the hydrothermal flow. Consequently, the flow becomes layered on both sides of the front. The fluid temperature at the top of the layer remains quite low: it never exceeds a temperature of 80°C, well below the exit temperature of hot vent sites observed at

  11. The Archean Dongwanzi ophiolite complex, North China craton: 2.505-billion-year-old oceanic crust and mantle.

    PubMed

    Kusky, T M; Li, J H; Tucker, R D

    2001-05-11

    We report a thick, laterally extensive 2505 +/- 2.2-million-year-old (uranium-lead ratio in zircon) Archean ophiolite complex in the North China craton. Basal harzburgite tectonite is overlain by cumulate ultramafic rocks, a mafic-ultramafic transition zone of interlayered gabbro and ultramafic cumulates, compositionally layered olivine-gabbro and pyroxenite, and isotropic gabbro. A sheeted dike complex is rooted in the gabbro and overlain by a mixed dike-pillow lava section, chert, and banded iron formation. The documentation of a complete Archean ophiolite implies that mechanisms of oceanic crustal accretion similar to those of today were in operation by 2.5 billion years ago at divergent plate margins and that the temperature of the early mantle was not extremely elevated, as compared to the present-day temperature. Plate tectonic processes similar to those of the present must also have emplaced the ophiolite in a convergent margin setting.

  12. Active slivering of oceanic crust along the Molucca ridge (Indonesia-Philippine): Implication for ophiolite incorporation in a subduction wedge?

    NASA Astrophysics Data System (ADS)

    Bader, Anne GaëLle; Pubellier, Manuel; Rangin, Claude; Deplus, Christine; Louat, RéMy

    1999-08-01

    A recent marine geophysical survey in the northern Molucca Sea revealed the structure to be that of a classical active convergent margin. We observe from west to east a volcanic arc (Sangihe), a forearc basin resting on an outer ridge (the Molucca ridge), which serves as a buttress for an accretionary wedge, and a composite downgoing plate (Snellius Ridge and Philippine Sea Basin). Gravity modeling indicates a strong negative anomaly above the wedge, which cannot be explained with reasonable density values. Modeling imposes a basement deepening and a rupture of the 700-km-long subducting lithosphere. This process individualized the lithospheric slab from the Snellius Ridge, which in turn was separated recently from the south Philippine Basin by the incipient Philippine Trench. This induces a deformation of the forearc region with backthrusting of the outer ridge and forearc basin, visible on bathymetry and seismic data. We extrapolate the tectonic emplacement of such oceanic blocks to the Oligocene times in order to explain the origin of the Pujada Miangas outer ridge as a sliver previously incorporated to the margin, and we discuss the possibility of this deformation process being fabric for terrane accretion.

  13. Recycling of oceanic crust from a stagnant slab in the mantle transition zone: Evidence from Cenozoic continental basalts in Zhejiang Province, SE China

    NASA Astrophysics Data System (ADS)

    Li, Yan-Qing; Ma, Chang-Qian; Robinson, Paul T.; Zhou, Qin; Liu, Ming-Liang

    2015-08-01

    Cenozoic continental basalts from Zhejiang Province, southeast China are tholeiitic to weakly alkalic in composition, with moderate MgO contents (6-11 wt.%) and an average Mg# of 62. They display typical OIB-like trace element features, including enrichment in most incompatible elements, both LILE and LREE, and negative K, Pb, Zr, Hf anomalies. In particular, they are characterized by high Fe/Mn (73 ± 5), La/Yb (19 ± 6) and Nb/Ta (18.8 ± 0.4) ratios, which can be attributed to the presence of residual clinopyroxene, garnet and rutile in the mantle source. Based on these minerals, the following hybrid source rocks are hypothesized: garnet pyroxenite/eclogite and peridotite. Clinopyroxene-liquid thermobarometry indicates clinopyroxene crystallization temperatures of > 1257 °C. This is higher than the assumed temperature at the base of the sub-continental lithospheric mantle (SCLM) ( 1220 °C) beneath Zhejiang, thus the magmas were presumably derived from the asthenosphere. Some typical geochemical features such as negative K, Pb anomalies, positive Ba, Sr, Nb, Ta anomalies and the extremely high Os isotopic signatures, suggest participation of EM-like mantle sources, indicative of ancient subducted oceanic crust. (87Sr/86Sr)i (0.7037-0.7046) and 143Nd/144Nd (0.512832-0.512990) isotope ratios point to the presence of mixed components in the source region, i.e., DMM, EM1 and EM2. Recent seismic tomographic images of the mantle beneath Zhejiang suggest the presence of a subducted slab of oceanic lithosphere in the transition zone. Based on the combined geophysical and geochemical evidence, we propose that the major source of the Zhejiang basaltic magmas was the ancient subducted oceanic slab in the transition zone with an EM-like signature. The other magma sources include depleted asthenospheric peridotite possessing a DMM-like signature. The dynamics of this upwelling hybrid magma was apparently related to westward subduction of the Pacific plate underneath the

  14. Eclogite xenoliths from Orapa: Ocean crust recycling, mantle metasomatism and carbon cycling at the western Zimbabwe craton margin

    NASA Astrophysics Data System (ADS)

    Aulbach, S.; Jacob, D. E.; Cartigny, P.; Stern, R. A.; Simonetti, S. S.; Wörner, G.; Viljoen, K. S.

    2017-09-01

    Major- and trace-element compositions of garnet and clinopyroxene, as well as 87Sr/86Sr in clinopyroxene and δ18O in garnet in eclogite and pyroxenite xenoliths from Orapa, at the western margin of the Zimbabwe craton (central Botswana), were investigated in order to trace their origin and evolution in the mantle lithosphere. Two groups of eclogites are distinguished with respect to 87Sr/86Sr: One with moderate ratios (0.7026-0.7046) and another with 87Sr/86Sr >0.7048 to 0.7091. In the former group, heavy δ18O attests to low-temperature alteration on the ocean floor, while 87Sr/86Sr correlates with indices of low-pressure igneous processes (Eu/Eu∗, Mg#, Sr/Y). This suggests relatively undisturbed long-term ingrowth of 87Sr at near-igneous Rb/Sr after metamorphism, despite the exposed craton margin setting. The high-87Sr/86Sr group has mainly mantle-like δ18O and is suggested to have interacted with a small-volume melt derived from an aged phlogopite-rich metasome. The overlap of diamondiferous and graphite-bearing eclogites and pyroxenites over a pressure interval of ∼3.2 to 4.9 GPa is interpreted as reflecting a mantle parcel beneath Orapa that has moved out of the diamond stability field, due to a change in geotherm and/or decompression. Diamondiferous eclogites record lower median 87Sr/86Sr (0.7039) than graphite-bearing samples (0.7064) and carbon-free samples (0.7051), suggesting that interaction with the - possibly oxidising - metasome-derived melt caused carbon removal in some eclogites, while catalysing the conversion of diamond to graphite in others. This highlights the role of small-volume melts in modulating the lithospheric carbon cycle. Compared to diamondiferous eclogites, eclogitic inclusions in diamonds are restricted to high FeO and low SiO2, CaO and Na2O contents, they record higher equilibrium temperatures and garnets have mostly mantle-like O isotopic composition. We suggest that this signature was imparted by a sublithospheric melt with

  15. Geochemistry of the Jurassic Mirdita Ophiolite (Albania) and the MORB to SSZ evolution of a marginal basin oceanic crust

    NASA Astrophysics Data System (ADS)

    Dilek, Yildirim; Furnes, Harald; Shallo, Minella

    2008-01-01

    Ti and Zr contents decreasing upsection (TiO 2 = 3.5-0.5%, Zr = 300-50 ppm), while ɛNd( T) (+ 8 to + 6.5) varies little. These magmas were derived from partial melting of fertile MORB-type mantle. Fractional crystallization was important in the evolution of WMO magmas. The low Ti and HREE abundances and Cs and Ba enrichments in the uppermost basaltic andesites may indicate an increased subduction influence in the evolution of the late-stage WMO magmas. Basaltic andesites in the lower 700 m of the EMO volcanic sequence have lower TiO 2 (˜ 0.5%) and Zr (˜ 50 ppm) contents but ɛNd( T) values (+ 7 to + 6.5) are similar to those of the WMO lavas. These rocks show variable enrichment in subduction-enriched incompatible elements (Cs, Ba, Th, U, LREE). The basaltic andesites through dacites and boninites within the upper 400 meters of EMO lavas show low TiO 2 (˜ 0.8-0.3%) and ɛNd( T) (+ 6.5 to + 3.0). The mantle source of these rocks was variably enriched in Th by melts derived from subducted sediments as indicated by the large variations in Ba, K, and Pb contents. EMO boninitic dikes and lavas and some gabbroic intrusions with negative ɛNd ( T) values (- 1.4 and - 4.0, respectively) suggest that these magmas were produced from partial melting of previously depleted, ultra-refractory mantle. The MORB to SSZ transition (from west to east and stratigraphically upwards in the Mirdita Ophiolite and the progression of the ɛNd( T) values from + 8.0 to - 4.0 towards the east resulted from an eastward shift in protoarc-forearc magmatism, keeping pace with slab rollback in this direction. The mantle flow above the retreating slab and in the arc-wedge corner played a major role in the evolution of the melting column, in which melt generation, aggregation/mixing and differentiation occurred at all levels of the sub-arc/forearc mantle. The SSZ Mirdita Ophiolite evolved during the intra-oceanic collapse and closure of the Pindos marginal basin, which had a protracted tectonic

  16. Chlorine-rich amphibole in deep layered gabbros as evidence for brine/rock interaction in the lower oceanic crust: a microstructural and experimental study

    NASA Astrophysics Data System (ADS)

    Currin, Adriana; Koepke, Juergen; Almeev, Renat; Ildefonse, Benoit; Wolff, Paul Eric

    2017-04-01

    The occurrence of amphiboles in the lower oceanic crust indicates the presence of water during rock formation or alteration, either in the parental melt or in reacting hydrothermal fluids. Amphibole-rich high temperature metamorphic veins and magmatic dykes are found in MOR-associated layered olivine gabbros from Wadi Wariyah of the deep palaeocrust of the Sumail ophiolite, Sultanate of Oman. These veins contain different types of amphiboles: pargasite, hornblende, actinolite and Cl-rich pargasite. In some cases, amphibole grains are zoned and contain a range of compositions, from pargasite formed at magmatic conditions to Cl-rich pargasite formed at subsolidus conditions. Pargasite and ferropargasite are found containing up to 5 wt% Cl (1.4 a.p.f.u.). This variety of amphibole compositions occurring in layered gabbros evidences a complex history of brine/rock interaction and hydrothermal cooling at the base of the oceanic crust. However, the precise origin of these amphibole- and Cl-rich veins associated to hydrothermal activity is still poorly understood. We investigated the formation of Cl-rich amphibole by means of a microstructural study using EBSD (electron backscattered diffraction) and, in addition, an experimental simulation of hydrothermal processes affecting amphibole formation in gabbro at subsolidus conditions. The microstructural EBSD study on natural gabbro from Oman confirms that the hydrothermally-formed amphiboles (hornblende, actinolite, Cl-pargasite) are epitactic in relation to high-temperature pargasite areas adjacent to them. This implies a coherent process, where magmatic pargasite formed initially was subsequently transformed by a Cl-rich hydrothermal brine and fluid at decreasing temperatures, ranging from the magmatic regime down to greenschist facies. Experiments were performed with a starting material of millimetre-sized pargasite pieces (with Cl content <0.02 Cl wt%) and added olivine gabbro powder (ol+plg+cpx+mt) with an excess of

  17. High Temperature Hydrothermal Circulation in the Deep Oceanic Crust - Sr Isotopes and Trace Elements Modelisation Constraints on the Origin of the Fluids

    NASA Astrophysics Data System (ADS)

    Bosch, D.; Lamour, M.; Jamais, M.; Bodinier, J.

    2003-12-01

    Previous field, petrological and geochemical works have identified high temperature hydrous alteration traces throughout the gabbros of the Samail ophiolite. Temperatures have been calibrated for the successive stages of alteration, starting with orthopyroxene-pargasite coronas (above 975 \\deg C) and ending with the low temperature (LT) lizardite serpentinisation (below 500 \\deg C). Sr isotopic analyses performed on massive gabbros, dikes and veins and associated minerals depart from typical mantle signatures and are characterized by radiogenic Sr isotopic ratios suggesting seawater as the most likely hydrothermal contaminant. The main water channels may be submillimetric microcracks with a dominantly vertical attitude and constituting the recharge hydrothermal system, whereas dikes and veins represent the discharge part. This model requires that these dikes have been generated by hydration of the crystallizing gabbros via seawater penetration, near the internal wall of the LVZ-magma chamber, i.e. at temperatures well above the near 1000 \\deg C temperature recorded so far. We used the numerical plate model of VerniŠres et al. (1997) to simulate the chemical evolution of Sr isotopes and some trace elements in fluids through the gabbro column. This approach takes into account mineralogical and porosity variations due to dissolution-precipitation processes, as well as variations of partition coefficients as a function of distance from the fluid source. The aim of modelling was twofold: (1) to provide estimates of the chemical evolution of fluids as a result of high-temperature interaction with gabbros, and (2) to constrain the fluid-rock ratios throughout the gabbros sequence. Such an approach sheds new lights on the importance of high temperature hydrothermal processes and on the geochemical modifications they induced during oceanic crust formation at fast spreading ridge. VerniŠres J., Godard M., Bodinier J.-L., 1997. A plate model for the simulation of trace

  18. Vertical tectonics at a continental crust-oceanic plateau plate boundary zone: Fission track thermochronology of the Sierra Nevada de Santa Marta, Colombia

    NASA Astrophysics Data System (ADS)

    Villagómez, Diego; Spikings, Richard; Mora, AndréS.; GuzmáN, Georgina; Ojeda, GermáN.; CortéS, Elizabeth; van der Lelij, Roelant

    2011-08-01

    The topographically prominent Sierra Nevada de Santa Marta forms part of a faulted block of continental crust located along the northern boundary of the South American Plate, hosts the highest elevation in the world (˜5.75 km) whose local base is at sea level, and juxtaposes oceanic plateau rocks of the Caribbean Plate. Quantification of the amount and timing of exhumation constrains interpretations of the history of the plate boundary, and the driving forces of rock uplift along the active margin. The Sierra Nevada Province of the southernmost Sierra Nevada de Santa Marta exhumed at elevated rates (≥0.2 Km/My) during 65-58 Ma in response to the collision of the Caribbean Plateau with northwestern South America. A second pulse of exhumation (≥0.32 Km/My) during 50-40 Ma was driven by underthrusting of the Caribbean Plate beneath northern South America. Subsequent exhumation at 40-25 Ma (≥0.15 Km/My) is recorded proximal to the Santa Marta-Bucaramanga Fault. More northerly regions of the Sierra Nevada Province exhumed rapidly during 26-29 Ma (˜0.7 Km/My). Further northward, the Santa Marta Province exhumed at elevated rates during 30-25 Ma and 25-16 Ma. The highest exhumation rates within the Sierra Nevada de Santa Marta progressed toward the northwest via the propagation of NW verging thrusts. Exhumation is not recorded after ˜16 Ma, which is unexpected given the high elevation and high erosive power of the climate, implying that rock and surface uplift that gave rise to the current topography was very recent (i.e., ≤1 Ma?), and there has been insufficient time to expose the fossil apatite partial annealing zone.

  19. Recycled oceanic crust and marine sediment in the source of alkali basalts in Shandong, eastern China: Evidence from magma water content and oxygen isotopes

    NASA Astrophysics Data System (ADS)

    Liu, Jia; Xia, Qun-Ke; Deloule, Etienne; Chen, Huan; Feng, Min

    2015-12-01

    The magma water contents and cpx δ18O values in alkali basalts from the Fuyanyshan (FYS) volcano in Shandong, eastern China, were investigated by an inverse calculation based on the water content of clinopyroxene (cpx) phenocrysts, the ivAlcpx-dependent water partitioning coefficient Dwatercpx>/melt, and secondary ion mass spectrometer, respectively. The calculated water content (H2O wt.) of magma ranges from 0.58% to 3.89%. It positively correlates with heavy rare earth element concentrations and bulk rock 87Sr/86Sr ratios, and it negatively correlates with Nb/U ratios. However, it is not correlated with bulk Mg# (Mg# = 100 × Mg / (Mg + Fe)) and (La/Yb)n (n represents primitive mantle normalization). Combined with the rather homogenous distribution of water content within cpx grains, these correlations indicate that the water variations among different samples represent the original magma signature, rather than results of a shallow process, such as degassing and diffusion. The δ18O of cpx phenocrysts varies from 3.6‰ to 6.3‰ (±0.5‰, 2SD), which may be best explained by the involvement of components from the lower and upper oceanic crust with marine sediments within the mantle source. The H2O/Ce ratios of the calculated melts range from 113 to 696 and form a positive trend with bulk rock 87Sr/86Sr, which cannot be explained by the recycled Sulu eclogite or by the metasomatized lithospheric mantle. Our modeling calculation shows that the decoupling of ɛHf and ɛNd could be caused by the involvement of marine sediments. Combing the high Ba/Th ratios, positive Sr spikes, and low Ce/Pb ratios for the Fuyanshan basalts, we suggest that the hydrous nature of the FYS basalts was derived from the hydrous mantle transition zone with ancient sediments.

  20. Porosity estimates on basaltic basement samples using the neutron absorption cross section (Σ): Implications for fluid flow and alteration of the oceanic crust

    NASA Astrophysics Data System (ADS)

    Reichow, M. K.; Brewer, T. S.; Marvin, L. G.; Lee, S. V.

    2008-12-01

    Little information presently exists on the heterogeneity of hydrothermal alteration in the oceanic crust or the variability of the associated thermal, fluid, and chemical fluxes. Formation porosities are important controls on these fluxes and porosity measurements are routinely collected during wireline logging operations. These estimates on the formation porosity are measures of the moderating power of the formation in response to bombardment by neutrons. The neutron absorption macroscopic cross-section (Σ = σρ) is a representation of the ability of the rock to slow down neutrons, and as such can be used to invert the porosity of a sample. Boron, lithium and other trace elements are important controls on σ-values, and the distribution of these is influenced by secondary low-temperature alteration processes. Consequently, computed σ-values may be used to discriminate between various basalt types and to identify areas of secondary alteration. Critical in this analysis is the degree of alteration, since elements such as B and Li can dramatically affect the sigma value and leading to erroneous porosity values. We analysed over 150 'pool-samples' for S, Li, Be and B element concentrations to estimate their contribution to the measured neutron porosity. These chemical analyses allow the calculation of the model sigma values for individual samples. Using a range of variably altered samples recovered during IODP Expeditions 309 and 312 we provide bulk estimates of alteration within the drilled section using the measured neutron porosity. B concentration in Hole 1256D increases with depth, with sharp rises at 959 and 1139 mbsf. Elevated wireline neutron porosities cannot always be directly linked with high B content. However, our preliminary results imply that increased neutron porosity (~15) at depths below 1100 mbsf may reflect hydrothermal alteration rather than formation porosity. This interpretation is supported when compared with generally lower computed

  1. Widely distributed thrust and strike-slip faults within subducting oceanic crust in the Nankai Trough off the Kii Peninsula, Japan

    NASA Astrophysics Data System (ADS)

    Tsuji, Takeshi; Kodaira, Shuichi; Ashi, Juichiro; Park, Jin-Oh

    2013-07-01

    We identified widely distributed thrust and strike-slip faults within subducting oceanic crust in the Nankai Trough, southeast of the Kii Peninsula, Japan, on the basis of 2D and 3D seismic reflection data. The seafloor seaward of the trough axis is deformed by displacement on these intraoceanic reverse faults, producing topographic highs (part of Kashinosaki Knoll). Because the thrust faults extend to the Moho and offset the Moho reflection, they may be related to serpentinization of the mantle due to seawater invasion. These faults are seismically active, given that their geometries are consistent with the focal mechanisms of intraplate earthquakes and microearthquakes. The thrust faults appear to extend landward to a high-density dome within the accretionary prism off the Kii Peninsula. Because the dome and the associated thick accretionary prism are expected to generate high friction at the plate interface due to their large vertical load, the intraoceanic thrusts are likely to have grown with ongoing subduction. Furthermore, because the geometry of the fault system we identified off the Kii Peninsula has characteristics similar to faults at Zenisu Ridge east of our study area, the thrusts observed in the study area may be considered to be the westward continuation of those at Zenisu Ridge. Since the Euler rotation pole of relative motion between the Philippine Sea plate and Zenisu Ridge is consistent with the high-density dome off the Kii Peninsula, we interpret the high-density dome as well as Kashinosaki Knoll as a westward termination of the Zenisu compression zone.

  2. Isotopic evidence ( 87Sr/ 86Sr, δ 7Li) for alteration of the oceanic crust at deep-rooted mud volcanoes in the Gulf of Cadiz, NE Atlantic Ocean

    NASA Astrophysics Data System (ADS)

    Scholz, Florian; Hensen, Christian; Reitz, Anja; Romer, Rolf L.; Liebetrau, Volker; Meixner, Anette; Weise, Stephan M.; Haeckel, Matthias

    2009-09-01

    The chemical and isotopic composition of pore fluids is presented for five deep-rooted mud volcanoes aligned on a transect across the Gulf of Cadiz continental margin at water depths between 350 and 3860 m. Generally decreasing interstitial Li concentrations and 87Sr/ 86Sr ratios with increasing distance from shore are attributed to systematically changing fluid sources across the continental margin. Although highest Li concentrations at the near-shore mud volcanoes coincide with high salinities derived from dissolution of halite and late-stage evaporites, clayey, terrigenous sediments are identified as the ultimate Li source to all pore fluids investigated. Light δ 7Li values, partly close to those of hydrothermal vent fluids (δ 7Li: +11.9‰), indicate that Li has been mobilized during high-temperature fluid/sediment or fluid/rock interactions in the deep sub-surface. Intense leaching of terrigenous clay has led to radiogenic 87Sr/ 86Sr ratios (˜0.7106) in pore fluids of the near-shore mud volcanoes. In contrast, non-radiogenic 87Sr/ 86Sr ratios (˜0.7075) at the distal locations are attributed to admixing of a basement-derived fluid component, carrying an isotopic signature from interaction with the basaltic crust. This inference is substantiated by temperature constraints from Li isotope equilibrium calculations suggesting exchange processes at particularly high temperatures (>200 °C) for the least radiogenic pore fluids of the most distal location. Advective pore fluids in the off-shore reaches of the Gulf of Cadiz are influenced by successive exchange processes with both oceanic crust and terrigenous, fine-grained sediments, resulting in a chemical and isotopic signature similar to that of fluids in near-shore ridge flank hydrothermal systems. This suggests that deep-rooted mud volcanoes in the Gulf of Cadiz represent a fluid pathway intermediate between mid-ocean ridge hydrothermal vent and shallow, marginal cold seep. Due to the thicker sediment

  3. Crusts: biological

    USGS Publications Warehouse

    Belnap, Jayne; Elias, Scott A.

    2013-01-01

    Biological soil crusts, a community of cyanobacteria, lichens, mosses, and fungi, are an essential part of dryland ecosystems. They are critical in the stabilization of soils, protecting them from wind and water erosion. Similarly, these soil surface communities also stabilized soils on early Earth, allowing vascular plants to establish. They contribute nitrogen and carbon to otherwise relatively infertile dryland soils, and have a strong influence on hydrologic cycles. Their presence can also influence vascular plant establishment and nutrition.

  4. Devonian Nb-enriched basalts and andesites of north-central Tibet: Evidence for the early subduction of the Paleo-Tethyan oceanic crust beneath the North Qiangtang Block

    NASA Astrophysics Data System (ADS)

    Zhang, Hongrui; Yang, Tiannan; Hou, Zengqian; Bian, Yeke

    2016-07-01

    The early evolution of the Tethyan Ocean in north-central Tibet is currently poorly constrained. A sequence of volcanic rocks ranging from basic to intermediate in composition has been identified in the Zaduo area of the North Qiangtang Block. SHRIMP U-Pb dating of zircons from a sample of Zaduo andesite suggests an eruption age of Late Devonian ( 380 Ma). The Zaduo volcanic rocks exhibit geochemical characteristics similar to those of typical Nb-enriched basalts, with relatively high Nb, Ta, and Zr contents, resulting in high Nb/La ratios (0.70-1.08) and Nb/U ratios (10.57-34.37). The relative enrichment in high field strength elements, together with positive εNd(t) values of + 4.6 to + 5.8 and low (87Sr/86Sr)i ratios of 0.70367-0.70532, indicates the Zaduo volcanic rocks were derived from a depleted mantle source metasomatized by silicate melts of a subducted oceanic slab. The occurrence of Nb-enriched volcanic rocks in the North Qiangtang Block suggests that the subduction of Paleo-Tethyan oceanic crust was initiated in the Late Devonian. Available geochronological data from ophiolites surrounding the North Qiangtang Block suggest that the subducted slab is most likely the Longmucuo-Shuanghu Paleo-Tethyan oceanic crust.

  5. Shearing within lower crust during progressive retrogression: Structural analysis of gabbroic rocks from the Godzilla Mullion, an oceanic core complex in the Parece Vela backarc basin

    NASA Astrophysics Data System (ADS)

    Harigane, Yumiko; Michibayashi, Katsuyoshi; Ohara, Yasuhiko

    2008-10-01

    Microstructural and petrological analyses of gabbroic rocks sampled from the Godzilla Mullion, located along the Parece Vela Basin spreading ridge (Parece Vela Rift), Philippine Sea, reveal the development of a ductile shear zone in the lower crust. The shear zone is interpreted to represent a detachment fault within an oceanic core complex. Microstructures indicative of intense deformation, characterized by porphyroclastic textures consisting dominantly of coarse plagioclase porphyroclasts and lesser clinopyroxene porphyroclasts in a fine-grained matrix, are observed within samples of gabbroic rocks dredged near the breakaway area of the Godzilla Mullion (dredge site D6). Samples are classified into three types based upon the grain-size of fine-grained plagioclase in the matrix: coarse (80-130 µm), medium (25 µm), and fine (˜ 10 µm). Although the chemical composition of plagioclase porphyroclasts is consistently An 40-50 among all sample types, the compositions of fine grains in the matrix vary with decreasing grain-size, being An 40-50 for the coarse-type, An 30-40 for the medium-type, and An 5-30 for the fine-type. This finding implies that the composition of fine-grained plagioclase in the matrix is related to the following retrograde reaction that occurred during deformation: clinopyroxene + plagioclase + Fe-Ti oxide + fluid → hornblende + plagioclase. Plagioclase crystal-preferred orientations also show a gradual change with grain-size, varying from a (010)[100] pattern for the coarse-type, (010)[100] and (001)[100] patterns for the medium-type, and a weak (001)[100] pattern or random orientations for the fine-type. These patterns are interpreted to result from a change in the deformation mechanism of plagioclase from dislocation creep to grain-size-sensitive creep with decreasing temperature, thereby leading to strain softening and localization during cooling. Although secondary amphibole occurs ubiquitously within all samples, the chemical composition

  6. Shearing within lower crust during progressive retrogression: structural analysis of gabbroic rocks from the Godzilla Mullion, an oceanic core complex in the Parece Vela backarc basin

    NASA Astrophysics Data System (ADS)

    Harigane, Y.; Michibayashi, K.; Ohara, Y.

    2008-12-01

    Microstructural and petrological analyses of gabbroic rocks sampled from the Godzilla Mullion, located along the Parece Vela Basin spreading ridge (Parece Vela Rift), Philippine Sea. We reveal the development of a ductile shear zone in the lower crust. The shear zone is interpreted to represent a detachment fault within an oceanic core complex. Microstructures indicative of intense deformation, characterized by porphyroclastic textures consisting dominantly of coarse plagioclase porphyroclasts and lesser clinopyroxene porphyroclasts in a fine-grained matrix, are observed within samples of gabbroic rocks dredged near the breakaway area of the Godzilla Mullion (dredge site D6). Samples are classified into three types based upon the grain size of fine-grained plagioclase in the matrix: coarse (80--130micron), medium (25micron), and fine (~10micron). Although the chemical composition of plagioclase porphyroclasts is consistently An 40--50 among all sample types, the compositions of fine grains in the matrix vary with decreasing grain size, being An 40--50 for the coarse-type, An 30--40 for the medium-type, and An 5--30 for the fine-type. This finding implies that the composition of fine-grained plagioclase in the matrix is related to the following retrograde reaction that occurred during deformation: clinopyroxene + plagioclase + Fe-Ti oxide + fluid - hornblende + plagioclase. Plagioclase crystal-preferred orientations also show a gradual change with grain size and plagioclase composition, varying from a (010)[100] pattern for the coarse-type, (010)[100] and (001)[100] patterns for the medium-type, and a weak (001)[100] pattern or random orientations for the fine-type. These patterns are interpreted to result from a change in the deformation mechanism of plagioclase from dislocation creep to grain-size-sensitive creep with decreasing temperature, thereby leading to strain softening and localization during cooling. Although secondary amphibole occurs ubiquitously within

  7. Can high-temperature, high-heat flux hydrothermal vent fields be explained by thermal convection in the lower crust along fast-spreading Mid-Ocean Ridges?

    NASA Astrophysics Data System (ADS)

    Fontaine, Fabrice J.; Rabinowicz, M.; Cannat, M.

    2017-05-01

    We present numerical models to explore possible couplings along the axis of fast-spreading ridges, between hydrothermal convection in the upper crust and magmatic flow in the lower crust. In an end-member category of models corresponding to effective viscosities μM lower than 1013 Pa.s in a melt-rich lower crustal along-axis corridor and permeability k not exceeding ˜10-16 m2 in the upper crust, the hot, melt-rich, gabbroic lower crust convects as a viscous fluid, with convection rolls parallel to the ridge axis. In these models, we show that the magmatic-hydrothermal interface settles at realistic depths for fast ridges, i.e., 1-2 km below seafloor. Convection cells in both horizons are strongly coupled and kilometer-wide hydrothermal upflows/plumes, spaced by 8-10 km, arise on top of the magmatic upflows. Such magmatic-hydrothermal convective couplings may explain the distribution of vent fields along the East (EPR) and South-East Pacific Rise (SEPR). The lower crustal plumes deliver melt locally at the top of the magmatic horizon possibly explaining the observed distribution of melt-rich regions/pockets in the axial melt lenses of EPR and SEPR. Crystallization of this melt provides the necessary latent heat to sustain permanent ˜100 MW vents fields. Our models also contribute to current discussions on how the lower crust forms at fast ridges: they provide a possible mechanism for focused transport of melt-rich crystal mushes from moho level to the axial melt lens where they further crystallize, feed eruptions, and are transported both along and off-axis to produce the lower crust.

  8. Anisotropy in the subducted oceanic crust and the overlying continental crust explain the existence of a double tectonic tremor zone in the flat portion of the Mexican subduction zone.

    NASA Astrophysics Data System (ADS)

    Husker, A. L.; Castillo, J. A.; Perez-Campos, X.; Frank, W.; Kostoglodov, V.

    2015-12-01

    Tectonic tremor (TT) in Mexico has a complicated behavior due to the shape of the subducted plate. In the flat section the slab dives from the trench to a depth of 40 km at 150 km from the trench where it turns to be flat. It remains at 40 km depth till about 290 - 300 km from the trench where it continues to steeply dive into the mantle. All TT activity is within the flat slab section. An LFE catalog and the vertically averaged shear wave anisotropy observed from receiver functions at the slab interface are used to divide the region into 4 zones. (1) The Transient Zone located at the corner of the slab when it first arrives at 40 km depth (~130 km - 165 km from the trench) where the majority of LFE's are seen in small bursts that produce TT. (2) The Buffer Zone has almost no LFE and is located ~165 km - 190 km from the trench. (3) The sub-Sweet Spot is located ~190 - 204 km from the trench and seems to share many characteristics of the Sweet Spot, but has less than half the LFE activity observed in the Sweet Spot in addition to different anisotropy. (4) The Sweet Spot has the overwhelming majority of LFE and is located ~204 km - 245 km from the trench. No LFE is found from 245 km to 300 km from the trench despite the plate still being at 40 km depth. The anisotropy percentage in the continental crust drops significantly above the Transient Zone and Sweet Spot suggesting the crust acts as a seal in those two zones permitting trapped fluids to generate TT/LFE activity there as has been observed in other zones. The Buffer Zone coincides with a region of high fluid flow in the crust (Jodicke et al., 2005) suggesting that there is no seal in this zone allowing fluids to escape thereby limiting TT/LFE generation. The convergence of the zone would imply that the anisotropy preferred orientation at the plate interface should be perpendicular to the trench as much of it is. However, the fast azimuth direction rotates to be trench parallel in the region of the large SSE

  9. Joint Inversion for VP/VS Structure of the Upper Oceanic Crust beneath the Endeavour Segment of the Juan de Fuca Ridge

    NASA Astrophysics Data System (ADS)

    Kim, E.; Toomey, D. R.; Hooft, E. E. E.; Wilcock, W. S. D.; Weekly, R. T.; Lee, S. M.; Kim, Y.

    2016-12-01

    We have obtained models of VP and VS in the upper oceanic crust beneath the Endeavour segment of the Juan de Fuca Ridge by simultaneously inverting first arriving Pg and P-to-S converted Sg travel times obtained during a seismic tomography experiment. In the inversion, two types of VP to VS coupling are tested. First, VP/VS variations are assumed to be spatially smooth. Second, dlnVs/dlnVp is assumed to be constant. Of the two approaches, the constant dlnVs/dlnVp recovers VP/VS anomalies and also reduces Sg misfits the most. Preliminary results show that the horizontally averaged VP/VS in a 10 km by 20 km area at the center segment decreases from 3.0 in the seismic layer 2A (≤ 0.4 km depth) to 1.72 at the bottom of the seismic layer 2B (2 km). To understand the VP/VS variations with depth, we model the relationship between porosity and VP/VS using differential effective medium (DEM) theory. To explain our seismic observation, we use basalt (VP = 6.4 km/s, VS = 3.5 km/s, density = 2971 kg/m3 [Johnston and Christensen, 1997]) and seawater (VP = 1.5 km/s, VS = 0 km/s, density = 1030 kg/m3 [Telford and Sheriff, 1990]) as a host medium and fluid-filled crack (aligned vertically). At zero porosity (φ = 0%), VP/VS of basalt is set to 1.83. The observed VP/VS of 3.0 in the layer 2A can be explained by increased porosity of 0.8%, 2.3%, 7%, or 22% due to water-filled cracks with an aspect ratio (d) of 0.001, 0.002, 0.005, or 0.01, respectively. These crack aspect ratios are consistent with a porosity that is reduced to 0.1%, 0.3%, 1.1%, or 3.3% at the top of the layer 2B (0.6 km) where the observed VP/VS is 2.0. We also estimate variations in crack density with depth. The crack density of thin cracks (d ≤ 0.01) abruptly decreases by 85% between the layers 2A and 2B, whereas that of thick cracks (d ≥ 0.1) decreases by only 27%. Based on the modeling result, the observed variations in VP/VS of the layer 2 can be explained by the presence and sealing characters of cracks

  10. Habitability Of Europa's Crust

    NASA Astrophysics Data System (ADS)

    Greenberg, R.; Tufts, B. R.; Geissler, P.; Hoppa, G.

    Physical characterization of Europa's crust shows it to be rich in potentially habitable niches, with several timescales for change that would allow stability for organisms to prosper and still require and drive evolution and adaptation. Studies of tectonics on Europa indicate that tidal stress causes much of the surface cracking, that cracks pen- etrate through to liquid water (so the ice must be thin), and that cracks continue to be worked by tidal stress. Thus a global ocean is (or was until recently) well linked to the surface. Daily tidal flow (period~days) transports substances up and down through the active cracks, mixing surface oxidants and fuels (cometary material) with the oceanic reservoir of endogenic and exogenic substances. Organisms moving with the flow or anchored to the walls could exploit the disequilibrium chemistry, and those within a few meters of the surface could photosynthesize. Cracks remain active for at least ~10,000 yr, but deactivate as nonsynchronous rotation moves them to different stress regimes in less than a million yr. Thus, to survive, organisms squeezed into the ocean must migrate to new cracks, and those frozen in place must hibernate. Most sites remelt and would release captive organisms within about a million yr based on the prevalence of chaotic terrain, which covers nearly half of Europa. Linkage of the ocean to the surface also could help sustain life in the ocean by delivering oxidants and fuels. Suboceanic volcanism (if any) could provide additional sites and support for life, but is not necessary. Recent results support this model. We further constrain the non-synchronous rotation rate, demonstrate the plausibility of episodic melt-through, show that characteristics of pits and uplift features do not imply thick ice, and demonstrate polar wander, i.e. that the ice crust is detached from the solid interior and has slipped as a unit relative to the spin axis. Thus Europa's biosphere (habitable if not inhabited) likely

  11. Untangling Magmatic Processes and Hydrothermal Alteration of in situ Superfast Spreading Ocean Crust at ODP/IODP Site 1256 with Fuzzy c-means Cluster Analysis of Rock Magnetic Properties

    NASA Astrophysics Data System (ADS)

    Dekkers, M. J.; Heslop, D.; Herrero-Bervera, E.; Acton, G.; Krasa, D.

    2014-12-01

    Ocean Drilling Program (ODP)/Integrated ODP (IODP) Hole 1256D (6.44.1' N, 91.56.1' W) on the Cocos Plate occurs in 15.2 Ma oceanic crust generated by superfast seafloor spreading. Presently, it is the only drill hole that has sampled all three oceanic crust layers in a tectonically undisturbed setting. Here we interpret down-hole trends in several rock-magnetic parameters with fuzzy c-means cluster analysis, a multivariate statistical technique. The parameters include the magnetization ratio, the coercivity ratio, the coercive force, the low-field susceptibility, and the Curie temperature. By their combined, multivariate, analysis the effects of magmatic and hydrothermal processes can be evaluated. The optimal number of clusters - a key point in the analysis because there is no a priori information on this - was determined through a combination of approaches: by calculation of several cluster validity indices, by testing for coherent cluster distributions on non-linear-map plots, and importantly by testing for stability of the cluster solution from all possible starting points. Here, we consider a solution robust if the cluster allocation is independent of the starting configuration. The five-cluster solution appeared to be robust. Three clusters are distinguished in the extrusive segment of the Hole that express increasing hydrothermal alteration of the lavas. The sheeted dike and gabbro portions are characterized by two clusters, both with higher coercivities than in lava samples. Extensive alteration, however, can obliterate magnetic property differences between lavas, dikes, and gabbros. The imprint of thermochemical alteration on the iron-titanium oxides is only partially related to the porosity of the rocks. All clusters display rock magnetic characteristics in line with a stable NRM. This implies that the entire sampled sequence of ocean crust can contribute to marine magnetic anomalies. Determination of the absolute paleointensity with thermal techniques is

  12. The Oligocene gap in the formation of Co-rich ferromanganese crusts and sedimentation in the Pacific Ocean and the effects of bottom currents

    NASA Astrophysics Data System (ADS)

    Kononov, M. V.; Lobkovskii, L. I.; Novikov, G. V.

    2017-02-01

    The Marcus Wake and Magellan guyots formed about 129-74 Ma ago at 10°-30° S and drifted 1700-4400 km to their present-day latitudinal position across the equatorial zone of maximum deposition. Cooling of the Pacific plate brought these guyots to the northern arid zone during the Turonian-Maastrichtian, to depths at which sediment accumulation rates were low and the conditions promoted precipitation of Co-rich Fe-Mn crusts from the Campanian to the present. Nonprecipitation of Co-rich Fe-Mn crusts during the Oligocene was caused by the action of bottom currents. The presence of a hiatus identified in cores from drill holes was used as the basis for reconstruction of the directions of bottom currents in the Oligocene.

  13. Composition of the Continental Crust

    NASA Astrophysics Data System (ADS)

    Rudnick, R. L.; Gao, S.

    2003-12-01

    The Earth is an unusual planet in our solar system in having a bimodal topography that reflects the two distinct types of crust found on our planet. The low-lying oceanic crust is thin (˜7 km on average), composed of relatively dense rock types such as basalt and is young (≤200 Ma old) (see Chapter 3.13). In contrast, the high-standing continental crust is thick (˜40 km on average), is composed of highly diverse lithologies (virtually every rock type known on Earth) that yield an average intermediate or "andesitic" bulk composition (Taylor and McLennan (1985) and references therein), and contains the oldest rocks and minerals yet observed on Earth (currently the 4.0 Ga Acasta gneisses (Bowring and Williams, 1999) and 4.4 Ga detrital zircons from the Yilgarn Block, Western Australia (Wilde et al., 2001)), respectively. Thus, the continents preserve a rich geological history of our planet's evolution and understanding their origin is critical for understanding the origin and differentiation of the Earth.The origin of the continents has received wide attention within the geological community, with hundreds of papers and several books devoted to the topic (the reader is referred to the following general references for further reading: Taylor and McLennan (1985), Windley (1995), and Condie (1997). Knowledge of the age and composition of the continental crust is essential for understanding its origin. Patchett and Samson (Chapter 3.10) review the present-day age distribution of the continental crust and Kemp and Hawkesworth (Chapter 3.11) review secular evolution of crust composition. Moreover, to understand fully the origin and evolution of continents requires an understanding of not only the crust, but also the mantle lithosphere that formed more-or-less contemporaneously with the crust and translates with it as the continents move across the Earth's surface. The latter topic is reviewed in Chapter 2.05.This chapter reviews the present-day composition of the

  14. Internal time marker (Q1) of the Cretaceous super chron in the Bay of Bengal - a new age constraint for the oceanic crust evolved between India and Elan Bank

    NASA Astrophysics Data System (ADS)

    Krishna, K. S.; Ismaiel, M.; Karlapati, S.; Saha, D.; Mishra, J.

    2014-12-01

    Analysis of marine magnetic data of the Bay of Bengal (BOB) led to suggest two different tectonic models for the evolution of lithosphere between India and East Antarctica. The first model explains the presence of M-series (M11 to M0) magnetic anomalies in BOB with a small room leaving for accommodating the crust evolved during the long Cretaceous Magnetic Quiet Period. Second model explains in other way that most part of the crust in BOB was evolved during the quite period together with the possible presence of oldest magnetic chron M1/ M0 in close vicinity of ECMI. It is with this perspective we have reinvestigated the existing and recently acquired magnetic data together with regional magnetic model of BOB for identification of new tectonic constraints, thereby to better understand the evolution of lithosphere. Analysis of magnetic data revealed the presence of spreading anomalies C33 and C34 in the vicinity of 8°N, and internal time marker (Q1) corresponding to the age 92 Ma at 12°N in a corridor between 85°E and Ninetyeast ridges. The new time marker and its location, indeed, become a point of reference and benchmark in BOB for estimating the age of oceanic crust towards ECMI. The magnetic model further reveals the presence of network of fracture zones (FZs) with different orientations. Between 85°E and Ninetyeast ridges, two near N-S FZs, approximately followed 87°E and 89.5°E are found to extend into BOB up to 12°N, from there the FZs reorient in N60°W direction and reach to the continental margin region. Along ECMI two sets of FZs are identified with a northern set oriented in N60°W and southern one in N40°W direction. This suggests that both north and south segments of the ECMI were evolved in two different tectonic settings. The bend in FZs marks the timing (92 Ma) of occurrence of first major plate reorganisation of the Indian Ocean and becomes a very critical constraint for understanding the plate tectonic process in early opening of the

  15. Pacific ferromanganese crust geology and geochemistry

    SciTech Connect

    Andreev, S.I.; Vanstein, B.G.; Anikeeva, L.I. )

    1990-06-01

    Cobaltiferous ferromanganese crusts form part of a large series of oceanic ferromanganese oxide deposits. The crusts show high cobalt (commonly over 0.4%), low nickel and copper sum (0.4-0.8%), considerably high manganese (18-20%), and iron (14-18%). Less abundant elements in crusts are represented by molybdenum and vanadium; the rare-earth elements cerium, lanthenum, and yttrium; and the noble metals platinum and rhodium. Co-rich crusts form at water depths of 600 to 2,500 m. Crust thicknesses range from millimeters to 15-17 cm, averaging 2-6 cm. The most favorable conditions for 4-10 cm thick crusts to occur is at water depths of 1,200-2,200 m. The crusts formed on basaltic, calcareous, siliceous, and breccia bedrock surfaces provided there were conditions preventing bottom sedimentation at them. If the sedimentation takes place, it may be accompanied by nodules similar in composition to the crusts. The most favorable topography for extensive crust formation is considered to be subdued (up to 20{degree}) slopes and summit platforms of conical seamounts, frequently near faults and their intersection zones. Subhorizontal guyot summits do not usually favor crust growth. Crust geochemistry is primarily defined by mineralogy and manganese hydroxides (vernadite)/iron ratio. The first associated group of compounds includes cobalt, nickel, molybdenum, vanadium, cerium, and titanium; the other is strontium, yttrium, cerium, and cadmium. The aluminosilicate phase is associated with titanium, iron, chromium, and vanadium; phosphate biogenic phase includes copper, nickel, zinc, lead, and barium. The crucial point in cobaltiferous crust formation is their growth rate on which is dependent the degree of ferromanganese matrix sorption saturation with cobalt. The optimum for cobalt-rich ferromanganese ores is the conditions facilitating long-term and continuous hydrogenic processes.

  16. Fine-scale velocity structure of upper oceanic crust from full waveform inversion of downward continued seismic reflection data at the Lucky Strike Volcano, Mid-Atlantic Ridge

    NASA Astrophysics Data System (ADS)

    Arnulf, A. F.; Harding, A. J.; Singh, S. C.; Kent, G. M.; Crawford, W.

    2012-04-01

    We present a fine-scale 2D velocity structure beneath the Lucky Strike Volcano on the Mid-Atlantic Ridge (MAR) using an elastic full waveform inversion (FWI) method. The FWI is a data driven procedure that allows simultaneous exploitation of both reflections and refractions energy in multi-channel seismic data to create a single self-consistent, high-resolution velocity image of the upper crust that can be used for geologic interpretation. The long-wavelength background P-wave velocity model required by the local optimization approach was created using a combination of downward continuation and 3D first-arrival travel-time tomography. The elastic waveform inversion was applied to carefully windowed downward continued data, where wide-angle reflections and refractions arrive in front of the water-wave and are thus isolated from the high-amplitude seafloor scattering energy that is particularly acute in areas of rough igneous seafloor. Waveform inversion reduces the misfit of the initial model by 76% after 19 iterations and strongly reduced the size of the residuals relative to the signal size. The final model shows fine scale structure beneath the northern part of the Lucky Strike volcano on a resolution of tens of meters. Evidence for successive lava sequences testifies to the constructional origin of the upper section of layer 2A. Normal faults are revealed within the shallow crust and are strongly correlated with seafloor observations.

  17. Inferences about the geometry and distribution of magma supply to oceanic crust at slow and intermediate spreading rates from AMS studies in the Troodos Ophiolite, Cyprus.

    NASA Astrophysics Data System (ADS)

    Morton, V.; Fialko, Y.; Staudigel, H.

    2004-12-01

    We investigate magma flow directions in the sheeted dike complex of the Troodos Ophiolite, Cyprus, using anisotropy of magnetic susceptibility (AMS) data. Suites of oriented cores collected from the dike chilled margins were analyzed to determine the AMS orientations, and infer the magma flow directions. With newly collected data from the Kionia Peak area, our dataset includes 115 dikes sampled over an area of about 8 km along the ridge axis, and 2 km across the ridge axis. The inferred magma flow vectors are corrected for tectonic rotation and tilting, and present day elevation variations serve as estimators of individual site depths in the pseudostratigraphy along the paleo ridge axis. Analysis of flow directions as a function of depth indicates more vertical flow directions in the shallow section of the sheeted dike complex. Also, the horizontal component of magma velocity is found to correlate with the along-axis distance from the presumed segment center. The flow vectors rotate from more vertical directions by about 30 degrees toward horizontal over a distance of 8 km from the presumed magma source toward the bounding transform fault. Chemical analysis shows horizontal flow directions in the shallow section also correlate with low Ti magma chemistry. We use the observed magma flow orientations in the sheeted dike complex to constrain the along-axis distribution of magma supply from the mantle source to the crust. The two end-member models are: a predominantly vertical magma supply to the crust from the underlying mantle, and a highly focused supply to the center of the segment with subsequent lateral redistribution via blade-like dike injections in the shallow magma plumbing system. We show that the flow directions inferred from AMS are not a direct proxy for the mode of magma supply to the crust, as some vertical flow directions may be imprinted at the leading edge of a horizontally propagating dike, while horizontal flow directions may result from magma

  18. The structure of 0- to 0. 2-m. y. -old oceanic crust at 9 degree N on the East Pacific Rise from expanded spread profiles

    SciTech Connect

    Vera, E.E.; Mutter, J.C.; Buhl, P. ); Orcutt, J.A.; Harding, A.J.; Kappus, M.E. ); Detrick, R.S. ); Brocher, T.M. )

    1990-09-10

    At the seafloor the authors find very low V{sub P} and V{sub S}/V{sub P} values around 2.2 km/s and {le} 0.43. In the topmost 100-200 m of the crust, V{sub P} remains low then rapidly increases to 5 km/s at {approximately}500 m below the seafloor. High attenuation values (Q{sub P} < 100) are suggested in the topmost {approximately}500 m of the crust. The layer 2-3 transition probably occurs within the dike unit, a few hundred meters above the dike-gabbro transition. This transition may mark the maximum depth of penetration by a cracking front and associated hydrothermal circulation in the axial region above the axial magma chamber (AMC). The top of the AMC lies 1.6 km below the seafloor and consists of molten material where V{sub P} {approx} 3 km/s and V{sub S} = 0. Immediately above the AMC, there is a zone of large negative velocity gradients where, on the average, V{sub P} decreases from {approximately}6.3 to 3 km/s over a depth of approximately 250 m. Associated with the AMC there is a low velocity zone (LVZ) that extends to a distance no greater than 10 km away from the rise axis. At the top of the LVZ, sharp velocity contrasts are confined to within 2 km of the rise axis and are associated with molten material or material with a high percentage of melt which would be concentrated only in a thin zone at the apex of the LVZ, in the axial region where the AMC event is seen in reflection lines. The bottom of the LVZ is probably located near the bottom of the crust and above the Moho. Moho arrivals are observed in the profiles at zero and at 10 km from the rise axis. Rather than a single discontinuity, these arrivals indicate an approximately 1-km-thick Moho transition zone.

  19. Serpentinization and carbonation of pristine continental ultramafic rocks and applications to the oceanic crust; H2O-CO2 alteration of dunites and re-distribution of Ni-Cu-PGE in sulphide deposits

    NASA Astrophysics Data System (ADS)

    Grant, Thomas; McEnroe, Suzanne; Eske Sørensen, Bjørn; Larsen, Rune; Pastore, Zeudia; Rune Grannes, Kim; Nikolaisen, Even

    2017-04-01

    Here, we document carbonation and serpentinization within a suite of ultramafic rocks from a continental setting. These ultramafic rocks vary from pristine dunites to varying degrees of serpentinization which locally penetrates the ultramafic complex. Hence, it allows us to observe a number of delicate serpentinization and carbonation reactions, otherwise lost during more extensive alteration or tectonic events. We use a multi-disciplinary approach using petrographic, EPMA, thermodynamic modelling and geophysical data to reveal how the initial stages of serpentization and carbonation in dunites affects the distribution of economic to sub-economic deposits of Ni-Cu and PGE. The data can then be applied to oceanic crust. The samples are dunites and poikilitic wehrlites from the Reinfjord Ultramafic complex, Seiland Igneous Province Northern Norway. The complex formed through crystallization of picritic melts in the lower continental crust. The dunites contain small amounts of interstitial clinopyroxene, sulphides and spinel, with local enrichments in Ni, Cu and PGE. Late magmatic CO2-H2O-S fluids reacted with the dunite forming clots of amphibole + dolomite + sulphides + enstatite, reaction rims of enstatite + dolomite, and inclusions trails of dolomite + enstatite + magnetite + CO2 fluid. Thermodynamic modelling reveals that these textures formed at pressures of >12 kbar and temperatures 850-950 °C, which would be consistent with the late magmatic history of the Reinfjord complex. The clots and reactions have local association with enrichments in gold-rich PGMs. A second stage of alteration involved H2O-dominated fluids. These formed predominantly lizardite serpentinization, as is often concentrated within highly localized fracture zones. Thermodynamic modelling shows that these formed <400°C, after the complex had been exhumed towards the surface of the crust. Local and more pervasive serpentinization interacted with the earlier formed carbonate bearing

  20. Evolution of the Archaean crust by delamination and shallow subduction.

    PubMed

    Foley, Stephen F; Buhre, Stephan; Jacob, Dorrit E

    2003-01-16

    The Archaean oceanic crust was probably thicker than present-day oceanic crust owing to higher heat flow and thus higher degrees of melting at mid-ocean ridges. These conditions would also have led to a different bulk composition of oceanic crust in the early Archaean, that would probably have consisted of magnesium-rich picrite (with variably differentiated portions made up of basalt, gabbro, ultramafic cumulates and picrite). It is unclear whether these differences would have influenced crustal subduction and recycling processes, as experiments that have investigated the metamorphic reactions that take place during subduction have to date considered only modern mid-ocean-ridge basalts. Here we present data from high-pressure experiments that show that metamorphism of ultramafic cumulates and picrites produces pyroxenites, which we infer would have delaminated and melted to produce basaltic rocks, rather than continental crust as has previously been thought. Instead, the formation of continental crust requires subduction and melting of garnet-amphibolite--formed only in the upper regions of oceanic crust--which is thought to have first occurred on a large scale during subduction in the late Archaean. We deduce from this that shallow subduction and recycling of oceanic crust took place in the early Archaean, and that this would have resulted in strong depletion of only a thin layer of the uppermost mantle. The misfit between geochemical depletion models and geophysical models for mantle convection (which include deep subduction) might therefore be explained by continuous deepening of this depleted layer through geological time.

  1. Channelized lava flows at the East Pacific Rise crest 9°-10°N: the importance of off-axis lava transport in developing the architecture of young oceanic crust

    USGS Publications Warehouse

    Soule, S.A.; Fornari, D.J.; Perfit, M.R.; Tivey, M.A.; Ridley, W.I.; Schouten, Hans

    2005-01-01

     Submarine lava flows are the building blocks of young oceanic crust. Lava erupted at the ridge axis is transported across the ridge crest in a manner dictated by the rheology of the lava, the characteristics of the eruption, and the topography it encounters. The resulting lava flows can vary dramatically in form and consequently in their impact on the physical characteristics of the seafloor and the architecture of the upper 50–500 m of the oceanic crust. We have mapped and measured numerous submarine channelized lava flows at the East Pacific Rise (EPR) crest 9°–10°N that reflect the high-effusion-rate and high-flow-velocity end-member of lava eruption and transport at mid-ocean ridges. Channel systems composed of identifiable segments 50–1000 m in length extend up to 3 km from the axial summit trough (AST) and have widths of 10–50 m and depths of 2–3 m. Samples collected within the channels are N-MORB with Mg# indicating eruption from the AST. We produce detailed maps of lava surface morphology across the channel surface from mosaics of digital images that show lineated or flat sheets at the channel center bounded by brecciated lava at the channel margins. Modeled velocity profiles across the channel surface allow us to determine flux through the channels from 0.4 to 4.7 × 103m3/s, and modeled shear rates help explain the surface morphology variation. We suggest that channelized lava flows are a primary mechanism by which lava accumulates in the off-axis region (1–3 km) and produces the layer 2A thickening that is observed at fast and superfast spreading ridges. In addition, the rapid, high-volume-flux eruptions necessary to produce channelized flows may act as an indicator of the local magma budget along the EPR. We find that high concentrations of channelized lava flows correlate with local, across-axis ridge morphology indicative of an elevated magma budget. Additionally, in locations where channelized flows are located dominantly to the east

  2. Oxygen consumption in subseafloor basaltic crust

    NASA Astrophysics Data System (ADS)

    Orcutt, B. N.; Wheat, C. G.; Hulme, S.; Edwards, K. J.; Bach, W.

    2012-12-01

    Oceanic crust is the largest potential habitat for life on Earth and may contain a significant fraction of Earth's total microbial biomass, yet little is known about the form and function of life in this vast subseafloor realm that covers nearly two-thirds of the Earth's surface. A deep biosphere hosted in subseafloor basalts has been suggested from several lines of evidence; yet, empirical analysis of metabolic reaction rates in basaltic crust is lacking. Here we report the first measure of oxygen consumption in young (~ 8 Ma) and cool (<25 degrees C) basaltic crust, calculated from modeling oxygen and strontium profiles in basal sediments collected during Integrated Ocean Drilling Program (IODP) Expedition 336 to 'North Pond', a sediment 'pond' on the western flank of the Mid-Atlantic Ridge (MAR), where vigorous fluid circulation within basaltic crust occurs. Dissolved oxygen concentrations increased towards the sediment-basement interface, indicating an upward diffusional supply from oxic fluids circulating within the crust. A parametric reaction-transport model suggests oxygen consumption rates on the order of 0.5-500 nmol per cubic centimeter fluid per day in young and cool basaltic crust, providing sufficient energy to support a subsurface crustal biosphere.