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  • 1
    Publication Date: 2018-10-22
    Description: An International Ocean Discovery Program (IODP) workshop was held at Sydney University, Australia, from 13 to 16 June 2017 and was attended by 97 scientists from 12 countries. The aim of the workshop was to investigate future drilling opportunities in the eastern Indian Ocean, southwestern Pacific Ocean, and the Indian and Pacific sectors of the Southern Ocean. The overlying regional sedimentary strata are underexplored relative to their Northern Hemisphere counterparts, and thus the role of the Southern Hemisphere in past global environmental change is poorly constrained. A total of 23 proposal ideas were discussed, with 12 of these deemed mature enough for active proposal development or awaiting scheduled site survey cruises. Of the remaining 11 proposals, key regions were identified where fundamental hypotheses are testable by drilling, but either site surveys are required or hypotheses need further development. Refinements are anticipated based upon regional IODP drilling in 2017/2018, analysis of recently collected site survey data, and the development of site survey proposals. We hope and expect that this workshop will lead to a new phase of scientific ocean drilling in the Australasian region in the early 2020s.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 2
    Publication Date: 2016-12-06
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 3
    Publication Date: 2016-12-06
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 4
    Publication Date: 2019-02-01
    Description: Microbathymetry data, in situ observations, and sampling along the 138200N and 138200N oceanic core complexes (OCCs) reveal mechanisms of detachment fault denudation at the seafloor, links between tectonic extension and mass wasting, and expose the nature of corrugations, ubiquitous at OCCs. In the initial stages of detachment faulting and high-angle fault, scarps show extensive mass wasting that reduces their slope. Flexural rotation further lowers scarp slope, hinders mass wasting, resulting in morphologically complex chaotic terrain between the breakaway and the denuded corrugated surface. Extension and drag along the fault plane uplifts a wedge of hangingwall material (apron). The detachment surface emerges along a continuous moat that sheds rocks and covers it with unconsolidated rubble, while local slumping emplaces rubble ridges overlying corrugations. The detachment fault zone is a set of anostomosed slip planes, elongated in the alongextension direction. Slip planes bind fault rock bodies defining the corrugations observed in microbathymetry and sonar. Fault planes with extension-parallel stria are exposed along corrugation flanks, where the rubble cover is shed. Detachment fault rocks are primarily basalt fault breccia at 138200N OCC, and gabbro and peridotite at 138300N, demonstrating that brittle strain localization in shallow lithosphere form corrugations, regardless of lithologies in the detachment zone. Finally, faulting and volcanism dismember the 138300N OCC, with widespread present and past hydrothermal activity (Semenov fields), while the Irinovskoe hydrothermal field at the 138200N core complex suggests a magmatic source within the footwall. These results confirm the ubiquitous relationship between hydrothermal activity and oceanic detachment formation and evolution.
    Type: Article , PeerReviewed
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  • 5
    Publication Date: 2019-02-01
    Description: Handheld energy dispersive portable X-ray spectrometers (pXRF) are generally designed and used for qualitative survey applications. We developed shipboard quantitative analysis protocols for pXRF and employed the instrument to make over 2000 individual abundance measurements for a selection of major and trace elements on over 1200 m of recovered core during the eight weeks of the International Ocean Discovery Program (IODP) Expedition 352 to the Izu-Bonin forearc. pXRF analytical performance, accuracy and precision were found to be the same on powdered rock samples and on freshly cut rock surfaces, and sample results were similar within error to measurements made via shipboard ICP-OES analysis save at low abundance levels for a few elements. Instrument performance was optimal for elements between Z = 19 and Z = 40, and the system yielded reproducible data for K, Ca, Ti, V, Cr, Mn, Fe, Cu, Zn, Rb, Sr, and Zr on both powdered samples and rock surfaces. Working curves developed via pXRF measurement of a suite of geologic standard reference materials and well-characterized lavas permitted accurate quantitative measurements for many of the examined elements on both sample powders and rock surfaces. Although pXRF has been sporadically employed on previous cruises, Expedition 352 is the first time a detailed, high-density chemostratigraphy of recovered core samples was collected using pXRF measurements of rock core surfaces. These high-resolution data allowed the recognition of chemically distinct eruptive units in near real-time. The rapid identification of geochemical trends vastly improved our selection of samples for shipboard and shore-based analysis, permitted a more comprehensive interpretation of our Expedition results, and provided key decision-making information for drilling operations.
    Type: Article , PeerReviewed
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  • 6
    Publication Date: 2017-01-04
    Description: Author Posting. © Elsevier B.V., 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Chemical Geology 234 (2006): 179-210, doi:10.1016/j.chemgeo.2006.04.011.
    Description: Abyssal peridotite from the 15°20’N area of the Mid-Atlantic Ridge show complex geochemical variations among the different sites drilled during ODP Leg 209. Major element compositions indicate variable degrees of melt depletion and refertilization as well as local hydrothermal metasomatism. Strongest evidence for melt-rock interactions are correlated Light Rare Earth Element (LREE) and High Field Strength Element (HFSE) additions at sites 1270 and 1271. In contrast, hydrothermal alteration at Sites 1274, 1272, and 1268 causes LREE mobility associated with minor HFSE variability, reflecting the low solubility of HFSE in aqueous solutions. Site 1274 contains the least-altered, highly refractory, peridotite with strong depletion in LREE and shows a gradual increase in the intensity of isochemical serpentinization; except for the addition of H2O which causes a mass gain of up to 20 g/100 g. The formation of magnetite is reflected in decreasing Fe2+/Fe3+ ratios. This style of alteration is referred to as rock-dominated serpentinization. In contrast, fluid-dominated serpentinization at Site 1268 is characterized by gains in sulfur and development of U-shaped REE pattern with strong positive Eu anomalies which are also characteristic for hot (350 to 400°C) vent-type fluids discharging from black smoker fields. Serpentinites at Site 1268 were overprinted by talc alteration under static conditions due to interaction with high aSiO2 fluids causing the development of smooth, LREE-enriched patterns with pronounced negative Eu anomalies. These results show that hydrothermal fluid-peridotite and fluid-serpentinite interaction processes are an important factor regarding the budget of exchange processes between the lithosphere and the hydrosphere in slow spreading environments.
    Description: ODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI), Inc.
    Keywords: Serpentinization ; Slow spreading ridges ; Abyssal peridotite ; Hydrothermal alteration ; Geochemistry ; Ocean Drilling Program Leg 209
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
    Format: 43241 bytes
    Format: 1741493 bytes
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  • 7
    Publication Date: 2019-08-08
    Description: The objectives for Expedition 352 were to drill through the entire volcanic sequence of the Bonin fore arc to 1. Obtain a high-fidelity record of magmatic evolution during subduction initiation and early arc development, 2. Test the hypothesis that fore-arc basalt lies beneath boninite and understand chemical gradients within these units and across the transition, 3. Use drilling results to understand how mantle melting processes evolve during and after subduction initiation, and 4. Test the hypothesis that the fore-arc lithosphere created during subduction initiation is the birthplace of suprasubduction zone (SSZ) ophiolites. Expedition 352 successfully cored 1.22 km of igneous basement and 0.46 km of over-lying sediment, providing diverse, stratigraphically controlled suites of fore-arc basalts (FAB) and boninite related to seafloor spreading and earliest arc development. FAB were recovered at the two deeper water sites (U1440 and U1441) and boninites at the two sites (U1439 and U1442) drilled upslope to the west. FAB lavas and dikes are depleted in high-field strength trace elements such as Ti and Zr relative to mid-ocean-ridge basalt but have relatively diverse concentrations of trace elements bezcause of variation in degrees of melting and amount of subducted fluids involved in their genesis. All FAB magmas underwent significant crystal fractionation in a persistent magma chamber system. Holes U1439C and U1442A yielded entirely boninitic lavas. We defined three boninite differentiation series based on variations in MgO, SiO2, and TiO2 concentrations of the parental magmas. Lavas in both pairs of holes have compositions that generally become more primitive and have lower TiO2 concentrations upward. The presence of dikes at the base of the sections at Sites U1439 and U1440 provides evidence that boninitic and FAB lavas are both underlain by their own conduit systems and that FAB and boninite group lavas are likely offset more horizontally than vertically. We thus propose that seafloor spreading related to subduction initiation migrated from east to west after subduction initiation and during early arc development. Initial spreading was likely rapid, and an axial magma chamber was present. Melting was largely decompressional during this period, but subducted fluids affected some melting. As subduction continued and spreading migrated to the west, the embryonic mantle wedge became more depleted, and the influence of subducted constituents dramatically increased, causing the oceanic crust to be built of boninitic rather than tholeiitic magma. The general decrease in fractionation upward reflects the eventual disappearance of persistent magma chambers, either because spreading rate was decreasing with distance from the trench or because spreading was succeeded by off-axis magmatism trenchward of the ridge. The extreme depletion of the sources for all boninitic lavas was likely related to the incorporation of mantle residues from FAB generation. This mantle depletion continued during generation of lower silica boninitic magmas, exhausting clinopyroxene from the mantle such that the capping high-Si, low-Ti boninites were generated from harzburgite. Additional results of the cruise include recovery of Eocene to recent deep-sea sediment that records variation in sedimentation rates with time resulting from variations in climate, the position of the carbonate compensation depth, and local structural control. Three phases of highly explosive volcanism (latest Pliocene to Pleistocene, late Miocene to earliest Pliocene, and Oligocene) were identified, represented by 132 graded air fall tephra layers. Structures found in the cores and reflected in seismic profiles show that this area had periods of normal, reverse, and strike-slip faulting. Finally, basement rock P-wave velocities were shown to be slower than those observed during logging of normal ocean crust sites.
    Type: Report , NonPeerReviewed
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  • 8
    Publication Date: 2015-05-20
    Description: Peridotites exhumed in the footwall of axial detachment faults at slow-spreading ridges are highly serpentinized. Most mid-ocean ridge detachment settings are magmatically active and hydrous fluid circulation in and near the fault has been shown to be influenced by the presence of melt or magmatic lithologies. Our working area along the Southwest Indian Ridge (62–65°E) is nearly amagmatic and represents an end-member to study the hydrous alteration of exhumed peridotites without these magmatic influences. We use an integrated petrological approach combining microstructural, mineralogical and chemical observations to unravel the sequence of serpentinization in 272 dredged samples of variably serpentinized peridotites and to document the circulation of serpentinizing fluids in and near the exhumation faults. We find that serpentine recrystallization and veins overprint the initial serpentinite mesh texture in ~25% of the samples. Oxygen isotope data suggest that this sequence developed at relatively high temperatures (271–336°C) and under increasing fluid–rock ratios, from near stoichiometry for mesh texture formation to 〉10 during recrystallization. Increasing fluid supersaturation relative to serpentine favors the replacement of mesh texture lizardite by chrysotile and polygonal or polyhedral serpentine. We attribute local recrystallization into antigorite to moderate Si-metasomatism, possibly following pyroxene serpentinization. We do not observe the more pronounced Si-metasomatism leading to talc replacing serpentine that is reported for the more magmatically active Mid-Atlantic Ridge detachment settings and is attributed to prior leaching of magmatic rocks. Scales of preferential fluid pathways in our samples evolved from pervasive and close-spaced (〈500 µm) microfractures during the formation of the initial serpentine mesh texture, to centimeter-thick planar domains of enhanced fluid flux, spaced at ~10 cm intervals and probably grouped in corridors that may be up to ~100 m across. Serpentine minerals are enriched in some fluid-mobile elements (Cl, B, U) relative to the peridotite protolith, and several elements (Al, Fe, Si, Cu, As, Sb, REE) are redistributed at the millimeter to decimeter scale. Serpentinizing fluids were seawater-derived, probably mildly alkaline (small to no europium anomalies), reducing and H 2 -enriched (formation of magnetite). These fluids may have been similar to, though warmer than, those venting at the ultramafic-hosted Lost City hydrothermal fluid (30°N, Mid-Atlantic Ridge).
    Print ISSN: 0022-3530
    Electronic ISSN: 1460-2415
    Topics: Geosciences
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  • 9
    Publication Date: 2014-02-09
    Description: At Cima di Gagnone, garnet peridotite and chlorite harzburgite lenses within pelitic schists and gneisses correspond to eclogite-facies breakdown products of hydrated peridotites and are suitable for studying dehydration of serpentinized mantle. Thermobarometry and pseudosection modelling yield peak temperatures of 750–850°C and pressures 〈3 GPa. The minimum temperature recorded by the garnet peridotite corresponds to the maximum conditions experienced by the chlorite harzburgite, suggesting that these rocks recrystallized cofacially at ~800°C. Alternatively, they might have decoupled during subduction, as achieved in tectonically active plate interface boundaries. The major and rare earth element (REE) variability of the peridotites was mostly acquired during pre-subduction mantle evolution as a result of partial melting and reactive melt flow. The ultramafic suite is also characterized by fluid-mobile element enrichments (B, Pb, As, Sb, Cs, Li, U, Be), which confirm derivation from variably serpentinized protoliths. Similarity in the U, Pb, B, Li and Sr contents of the Gagnone peridotites to present-day oceanic serpentinites suggests that these elements were partly taken up during initial serpentinization by seawater-derived fluids. Positive Be, As and Sb anomalies suggest involvement of fluids equilibrated with crustal (metasedimentary) reservoirs during subsequent subduction metamorphism and peridotite entrainment in (meta)sediments. Fluid-mobile element enrichment characterizes all peak eclogitic minerals, implying that multiple hydration events and element influx pre-dated the eclogite-facies dehydration. Peak anhydrous minerals retain B, Li, As and Sb concentrations exceeding primitive mantle values and may introduce geochemical anomalies into the Earth’s mantle. The relatively low contents of large ion lithophile elements and light REE in the Gagnone peridotites with respect to much higher enrichments shown by metasomatized garnet peridotite pods hosted in migmatites (Ulten Zone, Eastern Alps) suggest that the crustal rocks at Gagnone did not experience partial melting. The Gagnone garnet peridotite, despite showing evidence for chlorite dehydration, retains significant amounts of fluid-mobile elements documenting that no partial melting occurred upon chlorite breakdown. We propose that the Gagnone ultramafic rocks represent a prime example of multi-stage peridotite hydration and subsequent dehydration in a plate interface setting.
    Print ISSN: 0022-3530
    Electronic ISSN: 1460-2415
    Topics: Geosciences
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  • 10
    Publication Date: 2013-10-02
    Description: Author(s): F. Berenguer, P. Godard, M. Allain, J.-M. Belloir, A. Talneau, S. Ravy, and V. Chamard X-ray coherent diffraction imaging including ptychography provides the nanoscale resolved three-dimensional description of matter. The combination of these approaches to the Bragg geometry case arouses a strong interest for its capability to provide information about strain state in crystals. Among ... [Phys. Rev. B 88, 144101] Published Tue Oct 01, 2013
    Keywords: Structure, structural phase transitions, mechanical properties, defects
    Print ISSN: 1098-0121
    Electronic ISSN: 1095-3795
    Topics: Physics
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