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Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 25, SJR: 2.439, h-index: 91)
Geophysical Research Letters     Full-text available via subscription   (Followers: 105, SJR: 3.323, h-index: 185)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 12, SJR: 3.22, h-index: 136)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 4, SJR: 4.444, h-index: 18)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 116)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 27)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 50)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 48)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 102)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 43)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 121)
Paleoceanography     Full-text available via subscription   (Followers: 6, SJR: 3.067, h-index: 100)
Radio Science     Full-text available via subscription   (Followers: 36, SJR: 1.072, h-index: 59)
Reviews of Geophysics     Full-text available via subscription   (Followers: 34, SJR: 8.833, h-index: 107)
Space Weather     Full-text available via subscription   (Followers: 16, SJR: 1.341, h-index: 26)
Tectonics     Full-text available via subscription   (Followers: 14, SJR: 2.628, h-index: 96)
Water Resources Research     Full-text available via subscription   (Followers: 78, SJR: 2.661, h-index: 144)
Journal Cover Geochemistry, Geophysics, Geosystems
  [SJR: 2.439]   [H-I: 91]   [25 followers]  Follow
    
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   ISSN (Online) 1525-2027
   Published by AGU Homepage  [17 journals]
  • Thermal state of the Explorer segment of the Cascadia subduction zone:
           Implications for seismic and tsunami hazards
    • Authors: Dawei Gao; Kelin Wang, Earl E. Davis, Yan Jiang, Tania L. Insua, Jiangheng He
      Abstract: The Explorer segment of northernmost Cascadia is an end-member “warm” subduction zone with very young incoming plate and slow convergence rate. Understanding the megathrust earthquake potential of this type of subduction zone is of both geodynamic and societal importance. Available geodetic observations indicate that the subduction megathrust of the Explorer segment is currently locked to some degree, but the downdip extent of the fault area that is potentially seismogenic is not known. Here we construct finite element models to estimate the thermally allowed megathrust seismogenic zone, using available knowledge of regional plate kinematics, structural data, and heat flow observations as constraints. Despite ambiguities in plate interface geometry constrained by hypocenter locations of low-frequency earthquakes beneath Vancouver Island, the thermal models suggest a potential rupture zone of ∼60 km downdip width located fully offshore. Using dislocation modeling, we further illustrate that a rupture zone of this size, even with a conservative assumption of ∼100 km strike length, can cause significant tsunami-genic deformation. Future seismic and tsunami hazard assessment in northern Cascadia must take the Explorer segment into account. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-20T11:15:27.693403-05:
      DOI: 10.1002/2017GC006838
       
  • The potential of sedimentary foraminiferal rare earth element patterns to
           trace water masses in the past
    • Authors: Anne H. Osborne; Ed C. Hathorne, Johan Schijf, Yves Plancherel, Philipp Böning, Martin Frank
      Abstract: Dissolved rare earth element (REE) concentration data from intermediate and deep seawater form an array characterized by higher middle REE enrichments (MREE/MREE*) in the North Atlantic and a progressive increase in heavy-to-light REE ratios (HREE/LREE) as water masses age. The REEs in foraminifera are fractionated towards higher MREE/MREE* and lower HREE/LREE relative to seawater. Calculations based on a scavenging model show that the REE patterns in uncleaned core-top foraminifera resemble those adsorbed onto calcite, particulate organic material, and hydrous ferric oxides but the full extent of the REE fractionation measured in foraminifera was not reproduced by the model. However, differences in the HREE/LREE, MREE/MREE* ratios and the cerium anomaly between ocean basins are preserved and are in agreement with the seawater REE distribution. Under oxic conditions, the HREE/LREE and MREE/MREE* compositions of uncleaned foraminifera at the sediment/seawater boundary are preserved during burial but the cerium anomaly is sensitive to burial depth. In suboxic sedimentary environments, all uncleaned foraminiferal REE concentrations are elevated relative to core-top values indicating addition of REEs from pore waters. The HREE/LREE ratio is highest when sedimentation rates were greatest [Lippold et al., 2009] and when high Fe/Ca ratios in the uncleaned foraminifera indicate that Fe was mobile. In sediments that have not experienced suboxic conditions during burial, uncleaned foraminifera preserve the seawater signal taken up at the sediment/seawater interface and are therefore suggested to be a suitable archive of changes in the REE signal of past bottom waters. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T19:20:31.094052-05:
      DOI: 10.1002/2016GC006782
       
  • Differential response of corals to regional mass-warming events as evident
           from skeletal Sr/Ca and Mg/Ca ratios
    • Authors: Harry Clarke; Juan Pablo D'Olivo, James Falter, Jens Zinke, Ryan Lowe, Malcolm McCulloch
      Abstract: During the summer of 2010/11, a regional marine heat wave resulted in coral bleaching of variable severity along much of the western coastline of Australia. At Ningaloo Reef, a 300km long fringing reef system and World Heritage site, highly contrasting coral bleaching was observed between two morphologically distinct nearshore reef communities located on either side of the Ningaloo Peninsula: Tantabiddi (∼20% bleaching) and Bundegi (∼90% bleaching). For this study, we collected coral cores (Porites sp.) from Tantabiddi and Bundegi reef sites to assess the response of the Sr/Ca temperature proxy and Mg/Ca ratios to the variable levels of thermal stress imposed at these two sites during the 2010/11 warming event. We found that there was an anomalous increase in Sr/Ca and decrease in Mg/Ca ratios in the Bundegi record that was coincident with the timing of severe coral bleaching at the site, while no significant changes were observed in the Tantabiddi record. We show that the change in the relationship of Sr/Ca and Mg/Ca ratios with temperature at Bundegi during the 2010/11 event reflects changes in related coral ‘vital' processes during periods of environmental stress. These changes were found to be consistent with a reduction in active transport of Ca2+ to the site of calcification leading to a reduction in calcification rates and reduced Rayleigh fractionation of incorporated trace elements. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T19:20:26.74265-05:0
      DOI: 10.1002/2016GC006788
       
  • Yttrium and rare earth element partitioning in seawaters from the Bay of
           Bengal
    • Authors: Zhaojie Yu; Christophe Colin, Eric Douville, Laure Meynadier, Stéphanie Duchamp-Alphonse, Sophie Sepulcre, Shiming Wan, Lina Song, Qiong Wu, Zhaokai Xu, Frank Bassinot
      Abstract: The dissolved Yttrium (Y) and Rare Earth Element (REE) concentrations of seawater samples collected along a north-south hydrological transect within the Bay of Bengal (BoB) have been analyzed to estimate contributions of the Ganges and Brahmaputra (G-B) river inputs to the dissolved REE distribution of the Northern Indian Ocean. Surface water masses of the BoB are characterized by Y/Ho ratios (84) intermediate between the G-B river suspended sediment (41) and water mass from the South Indian Ocean (93). Co-variation of MREE (Middle REE, Sm) and LREE (Light REE, La) concentrations suggests that the dissolved REEs in surface waters (upper 100 m depth) of the BoB (Sm/La = 0.21) appear to derive mainly from the freshwater discharge of the G-B river system. In contrast, values obtained in the intermediate and deep-waters (Sm/La = 0.14) suggest a mixing of dissolved REEs deriving from the release of G-B river suspended particles (Sm/La = 0.16) and the contribution of Antarctic Bottom Water (AABW) (Sm/La = 0.12). Consequently, we propose that MREE/MREE* ratios in the BoB waters could be an accurate proxy to trace lithogenic inputs from the G-B river system. The dissolved and particle re-mineralization Nd fluxes from G-B river system are calculated to constitute about 9% and 4% of the global dissolved river discharge and ‘Boundary inputs' flux. Our estimation indicates that the massive G-B river system inputs could greatly alter the dissolved REEs distribution in the BoB and contribute to the dissolved REEs budget in the ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T19:15:33.493806-05:
      DOI: 10.1002/2016GC006749
       
  • Mantle heterogeneity in the source region of mid-ocean ridge basalts along
           the northern central Indian Ridge (8°–17°S)
    • Authors: Jonguk Kim; Sang-Joon Pak, Jai-Woon Moon, Sang-Mook Lee, Jihye Oh, Finlay M. Stuart
      Abstract: The northern Central Indian Ridge (CIR) between 8°S and 17°S is composed of seven segments whose spreading rates increase southward from ∼35 to ∼40 mm/yr. During expeditions of R/V Onnuri to study hydrothermal activity on the northern CIR in 2009 to 2011, high-resolution multibeam mapping was conducted and ridge axis basalts were dredged. The major and trace element, and Sr–Nd–Pb-He isotopic compositions of basaltic glasses dredged from the spreading axis require three mantle sources; depleted mantle and two distinct enriched mantle sources. The southern segments have Sr, Nd, and Pb that are a mix of depleted mantle and an enriched component as recorded in southern CIR MORB. This enrichment is indistinguishable from Rèunion plume mantle, except for He isotopes. This suggests that the southern segments have incorporated a contribution of the fossil Rèunion plume mantle, as the CIR migrated over hotspot-modified mantle. The low 3He/4He (7.5 to 9.2 RA) of this enriched component may result from radiogenic 4He ingrowth in the fossil Rèunion mantle component. Basalts from the northern segments have high 206Pb/204Pb (18.53-19.15) and low 87Sr/86Sr (0.70286-0.70296) that are distinct from the Rèunion plume but consistent with derivation from mantle with FOZO signature, albeit with 3He/4He (9.2 to 11.8 RA) that are higher than typical. The FOZO-like enriched mantle cannot be attributed to the track of a nearby mantle plume. Instead, this enrichment may have resulted from recycling oceanic crust, possibly accompanied by small plume activity. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T11:40:40.124914-05:
      DOI: 10.1002/2016GC006673
       
  • A pure dipole analysis of the Gondwana apparent polar wander path:
           Paleogeographic implications in the evolution of Pangea
    • Authors: L. C. Gallo; R. N. Tomezzoli, E. O. Cristallini
      Abstract: The paleogeography of pre break-up Pangea at the beginning of the Atlantic Spreading has been a subject of debate for the past 50 years. Reconciling this debate involves theoretical corrections that cast doubt on available data and paleomagnetism as an effective tool for performing paleoreconstructions. This 50-year-old debate focuses specifically on magnetic remanence and its ability to correctly record the inclination of the paleomagnetic field. In this paper, a selection of paleopoles was made to find the great circles containing the paleomagnetic pole and the respective sampling site. The true dipole pole (TDP) was then calculated by intersecting these great circles, effectively avoiding non-dipolar contributions and inclination shallowing, in an innovative method. The great circle distance between each of these TDPs and the paleomagnetic means show the accuracy of paleomagnetic determinations in the context of a dominantly geocentric, axial and dipolar geomagnetic field. The TDPs calculated allowed a bootstrap analysis to be performed to further consider the flattening factor that should be applied to the sedimentary derived paleopoles. It is argued that the application of a single theoretical correction factor for clastic sedimentary-derived records could lead to a bias in the paleolatitude calculation and therefore to incorrect paleogeographic reconstructions. The unbiased APWP makes it necessary to slide Laurentia to the West in relation to Gondwana in a B-type Pangea during the Upper Carboniferous, later evolving, during the Early Permian, to reach the final A-type Pangea configuration of the Upper Permian. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T11:40:36.100697-05:
      DOI: 10.1002/2016GC006692
       
  • Remanence acquisition efficiency in biogenic and detrital magnetite and
           recording of geomagnetic paleointensity
    • Authors: Liang Chen; David Heslop, Andrew P. Roberts, Liao Chang, Xiang Zhao, Helen V. McGregor, Gianluca Marino, Laura Rodriguez-Sanz, Eelco J. Rohling, Heiko Pälike
      Abstract: Relative paleointensity (RPI) variations of Earth's magnetic field are widely used to understand geomagnetic field behavior and to develop age models for sedimentary sequences. RPI estimation is based on a series of assumptions. One key assumption that is rarely considered is that all magnetic particles in the sediment acquired a magnetization in an identical manner. In this paper, we test this assumption for sediments from the eastern equatorial Pacific Ocean that record well-documented global RPI variations over the last ∼780 kyr. The magnetization is carried by two stable single domain magnetic components, which we identify as magnetite magnetofossils and titanomagnetite nanoparticle inclusions within larger silicate particles. By analyzing signals carried by the two components separately, we determine for the first time that magnetic nanoparticle inclusions can cause their host particles to record reliable but inefficient sedimentary paleomagnetic signals. The magnetization carried by biogenic magnetite is acquired more efficiently than that carried by the nanoparticle inclusions. Variations in the concentration of both components are modulated climatically so that they record nearly identical RPI signals. In many sediment types, there is no correlation between the concentrations of different magnetic components so that variable remanence acquisition efficiency will complicate RPI recording. Our work demonstrates that detailed assessment of paleomagnetic recording by each constituent magnetic component needs to become a routine part of sedimentary RPI analysis. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T11:35:33.94083-05:0
      DOI: 10.1002/2016GC006753
       
  • Impact of silica diagenesis on the porosity of fine-grained strata: An
           analysis of Cenozoic mudstones from the North Sea
    • Authors: Thilo Wrona; Kevin G. Taylor, Christopher A-L. Jackson, Mads Huuse, Jens Najorka, Indranil Pan
      Abstract: Silica diagenesis has the potential to drastically change the physical and fluid flow properties of its host strata and therefore plays a key role in the development of sedimentary basins. The specific processes involved in silica diagenesis are, however, still poorly explained by existing models. This knowledge gap is addressed by investigating the effect of silica diagenesis on the porosity of Cenozoic mudstones of the North Viking Graben, northern North Sea through a multiple linear regression analysis. First, we identify and quantify the mineralogy of these rocks by scanning electron microscopy and X-ray diffraction, respectively. Mineral contents and host rock porosity data inferred from wireline data of two exploration wells are then analyzed by multiple linear regressions. This robust statistical analysis reveals that biogenic opal-A is a significant control and authigenic opal-CT is a minor influence on the porosity of these rocks. These results suggest that the initial porosity of siliceous mudstones increases with biogenic opal-A production during deposition and that the porosity reduction during opal-A/CT transformation results from opal-A dissolution. These findings advance our understanding of compaction, dewatering and lithification of siliceous sediments and rocks. Moreover, this study provides a recipe for the derivation of the key controls (e.g. composition) on a rock property (e.g. porosity) that can be applied to a variety of problems in rock physics. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-14T11:31:05.615078-05:
      DOI: 10.1002/2016GC006482
       
  • Along-strike variations in the Himalayan orogenic wedge structure in
           Bhutan from ambient seismic noise tomography
    • Authors: Julia Singer; Anne Obermann, Eduard Kissling, Hongjian Fang, György Hetényi, Djordje Grujic
      Abstract: The geological units and tectonic structure exposed in the Bhutan Himalaya document significant regional variations, expressed primarily as tectonic windows and klippen. The along-strike variations of these structures and their metamorphic grade is usually associated with the formation of local duplexes in the underlying tectonic units. To investigate these variations and their extent in depth, we image the isotropic shear-wave velocity structure of the orogenic wedge by ambient noise tomography. Group velocities are extracted from cross-correlations of ambient seismic noise, recorded by the temporary GANSSER network in Bhutan. The upper crustal structure beneath Bhutan is mapped down to 18 km depth by directly inverting Rayleigh-wave group velocity measurements in the period range between 2-20 s with a ray-tracing based inversion approach. Our results reveal several distinct high shear-wave velocity anomalies (≥3.6 km/s) and reflect the along-strike variations in the upper crustal structure in relation to the alternating tectonic windows and klippen at the surface. In correlation with the surface geology in the northern part of Bhutan, we interpret shallow high shear-wave velocity anomalies as quarzite-dominated rocks or felsic migmatites with large intrusions of leucogranites. High velocity anomalies in the orogenic wedge in eastern and western Bhutan correlate with the local geometry of the Main Himalayan Thrust and provide evidence for the formation and depth extent of localized duplexes of quartzite dominated lithology in association with the formation of tectonic windows in the Bhutan Himalaya. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-12T19:45:39.033311-05:
      DOI: 10.1002/2016GC006742
       
  • Sub-ice shelf sediment geochronology utilizing novel radiocarbon
           methodology for detrital-rich sediments
    • Authors: C. Subt; H. I. Yoon, K. C. Yoo, J. I. Lee, A. Leventer, E.W. Domack, B. E. Rosenheim
      Abstract: Sub-ice shelf sediments near Larsen C ice shelf (LIS-C) show fine-scale rhythmic laminations that could provide a near-continuous seasonal-resolution record of regional ice mass changes. Despite the great potential of these sediments, a dependable Late Quaternary chronology is difficult to generate. As with many marginal Antarctic sediments, in the absence of preserved carbonate microfossils, the reliability of radiocarbon chronologies depends on presence of high proportions of autochthonous organic carbon with minimized detrital organic carbon. Consequently, acid insoluble organic (AIO) 14C dating works best where high productivity drives high sediment accumulation rates, but can be problematic in condensed sequences with high proportions of detrital organic carbon. Ramped PyrOx 14C dating has progressively been shown to improve upon AIO 14C dates, to the point of matching foraminiferal carbonate 14C dates, through differential thermochemical degradation of organic components within samples. But in highly detrital sediments, proportions of contemporaneously-deposited material are too low to fully separate autochthonous organic carbon from detrital carbon in samples large enough to 14C date. We introduce two modifications of the Ramped PyrOx 14C approach applied to highly-detrital sediments near LIS-C to maximize accuracy by utilizing ultra-small fractions of the highly-detrital AIO material. With minimization of the uncertainty cost, these techniques allow us to generate chronologies for cores that would otherwise go undated, pushing the limits of radiocarbon dating to regions and facies with high proportions of pre-aged detritus. Wider use of these techniques will enable more coordinated a priori coring efforts to constrain regional glacial responses to rapid warming where sediments had previously been thought too difficult to date. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-07T19:30:35.726316-05:
      DOI: 10.1002/2016GC006578
       
  • Lithospheric thickness estimation beneath Northwestern South America from
           an S-wave Receiver Function analysis
    • Authors: J. Faustino Blanco; Carlos A. Vargas, Gaspar Monsalve
      Abstract: We make use of the S-to-P receiver function technique beneath Colombia and neighboring regions to make a first-order approximation of the depth of the Lithosphere-Asthenosphere Boundary (LAB) and therefore, of lithospheric thickness. A deconvolution technique was used to calculate the receiver functions, and after a move-out correction and a time-depth conversion, LAB depths for different tectonic regions of northwestern South America were obtained. Results are typically between 65 and 110 km, consistent with previous estimates around the world and other regions in South America. Lithospheric thickness beneath an oceanic island in the Caribbean is ∼80 km, whereas for the Ecuador-Colombia Trench it is ∼65 km, and around 100 km for the Panama Arc. The transition to the continent is associated with an increase in LAB depth, where it can reach ∼110 km, with no significant differences among terranes and/or tectonic blocks. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-07T19:25:33.440473-05:
      DOI: 10.1002/2016GC006785
       
  • Rock magnetic investigation and its geological significance for vein-type
           uranium deposits in Southern China
    • Authors: Kunpeng Ge; Qingsong Liu, Juzhi Deng, David Nobes, Yang Wang, Yanguo Wang, Xiao Chen
      Abstract: To characterize the metallogenic environment of a typical vein-type uranium deposit, samples from diabase dykes, alteration zones including metamorphic diabase and uranium ore, and granites were systematically investigated for 6 boreholes from southeastern China. Rock magnetic results indicate that coarse-grained magnetites (pseudo-single domain, PSD, and multi-domain, MD) are dominant magnetic carriers in diabase. In contrast, the uranium ore is dominated by fine-grained magnetites (superparamagnetic, SP, and single-domain, SD). The concentration of magnetic particles in fresh granites is low. Magnetic properties of metamorphic diabases exhibit much greater variability of magnetic properties and higher degrees of sulfuration than unaltered diabase and granite, due to contact metasomatism and reduction effects close to the vein. Compared with diabase, magnetic remanence of the uranium ore is much lower, but displays much higher stability. The Koenigsberger ratio Q peaks in the uranium ore with a value of ∼1.00. Using the systematic rock magnetic results to constrain the interpretation, the contribution of the intersection zone of diabase dyke and silicified fault to magnetic anomalies was further modeled, and the effects of the ore body are significant for magnetic exploration. Overall, rock magnetic investigations of vein-type uranium deposit provide a better understanding of the interactions between different rock types, and further facilitate regional magnetic surveys on the ground. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-07T19:25:30.287372-05:
      DOI: 10.1002/2016GC006756
       
  • Two-layered oceanic lithospheric mantle in a Tibetan ophiolite produced by
           episodic subduction of Tethyan slabs
    • Authors: Qing Xiong; William L. Griffin, Jian-Ping Zheng, Norman J. Pearson, Suzanne Y. O'Reilly
      Abstract: The origin and evolution of the Yarlung Zangbo ophiolites (South Tibet, China) is the key to the tectonics of the Neo-Tethyan Ocean between Greater India and Asia and the underlying upper-mantle dynamics. This study presents a detailed investigation of the Zedang ultramafic body (comprising a harzburgitic and a lherzolitic domain) in the eastern Yarlung Zangbo Suture. Major-element compositions and Ti, Y and HREE concentrations in peridotites and their minerals indicate that the harzburgites experienced higher degrees of melting than the lherzolites (∼13-19% vs ∼7-12%). The overall enrichment of LREE, Zr and Sr in harzburgites and their clinopyroxenes suggest that the harzburgites were pervasively metasomatized (cryptically) by silicate melts. The harzburgites also record local strong metasomatism close to melt channels. Nd isotopes indicate that both metasomatic agents were derived from forearc basaltic magmas that intruded the harzburgites at ∼130-120 Ma. The lherzolites did not experience such metasomatism. Thermometry shows that the harzburgites experienced a thorough, lower-temperature re-equilibration process in lithosphere, while the lherzolites rapidly accreted from the asthenosphere and preserved high equilibration temperatures (up to ∼1320°C). Comparable enrichment in fluid-mobile elements and radiogenic Sr-isotope compositions in both harzburgitic and lherzolitic pyroxenes reflect slab-fluid infiltration into both mantle domains. All the evidence and the presence of subduction-related chromitites in the harzburgites suggest that the Zedang harzburgites formed in a possibly Jurassic mature subduction system, while the lherzolites accreted later in an early Cretaceous forearc during subduction initiation. The two-layered lithospheric mantle reflects the episodic subduction of the Tethyan slabs. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-06T11:10:36.283844-05:
      DOI: 10.1002/2016GC006681
       
  • Pressure evolution in shallow magma chambers upon buoyancy-driven
           replenishment
    • Authors: P. Papale; C.P. Montagna, A. Longo
      Abstract: The invasion of active magma chambers by primitive magma of deeper provenance is a frequent occurrence in volcanic systems, and it is commonly associated with pressurization. Chamber replenishment is driven by pressure and buoyancy forces that cause magma ascent towards shallow depths. We examine the end-member case of pure buoyancy-driven (natural) convection in crustal reservoirs deriving from the presence of degassed, dense magma at shallow level, that can originate a gravitational instability. Space-time-dependent numerical simulations of magma dynamics in composite underground systems reveal highly non-linear pressure evolution dominated by decompression at shallow depths. This counter-intuitive result originates from the compressible nature of multiphase magmas and their complex convection and mixing dynamics. Shallow magma chamber decompression upon replenishment is favored by large volatile contents of the uprising magma, resulting in large density contrasts among the resident and the incoming components. These results show that the intuitive concept of magma chamber pressurization upon replenishment may not always hold in real situations dominated by buoyancy, and provide new perspectives for the interpretation of geophysical records at active volcanoes. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-06T11:10:30.949284-05:
      DOI: 10.1002/2016GC006731
       
  • The role of mantle-derived magmas in the isotopic evolution of
           Yellowstone's magmatic system
    • Authors: Mark E. Stelten; Kari M. Cooper, Josh B. Wimpenny, Jorge A. Vazquez, Qing-Zhu Yin
      Abstract: Injection of mantle-derived magmas into the Earth's crust provides the heat necessary to develop and maintain large silicic magmatic systems. However, the role of mantle-derived magmas in controlling the compositional evolution of large silicic systems remains poorly understood. Here we examine the role of mantle-derived magmas in the post-caldera magmatic system at Yellowstone Plateau, the youngest magmatism associated with the Yellowstone hotspot. Using microbeam techniques we characterize the age and Hf isotope composition of single zircon crystals hosted in rhyolites from the most recent eruptive episode at Yellowstone Plateau, which produced the Central Plateau Member rhyolites. We place these zircon data into context by comparing them to new solution Hf isotope data for the Central Plateau Member glasses, Yellowstone basalts, and potential local crustal sources. Zircons in the Central Plateau Member rhyolites record a wide range of Hf isotope compositions relative to their host melts and extend from values similar to previously erupted Yellowstone rhyolites to values similar to Yellowstone basalts. Most zircons (∼90%) are in isotopic equilibrium with their host melt, but a significant proportion show εHf values higher than their host melt, thus providing the direct evidence that silicic derivatives of mantle-derived basalts have recharged Yellowstone's magmatic system. Mixing models confirm that the isotopic characteristics of the youngest Yellowstone rhyolites can be explained by recharge of Yellowstone's magma reservoir with silicic derivatives of underplating, mantle-derived basalts (∼5% to 10% material added by mass). This process helps drive the long-term isotopic evolution of Yellowstone's magmatic system. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-02T10:35:33.198874-05:
      DOI: 10.1002/2016GC006664
       
  • Origins of water content variations in the sub-oceanic upper mantle:
           Insight from Southwest Indian Ridge abyssal peridotites
    • Authors: Wei Li; Vincent Soustelle, Zhenmin Jin, Huaiming Li, Tao Chen, Chunhui Tao
      Abstract: To investigate the origin of heterogeneous water distribution in the sub-oceanic lithospheric mantle, we performed a detailed petrological and geochemical analysis of abyssal peridotites collected from two localities (53°E and 63.5°E) on the Southwest Indian Ridge. These serpentinized peridotites display primary olivine-orthopyroxene-clinopyroxene-spinel assemblage and record equilibrium temperature of 1150-1200°C and around 1000°C for the 53°E and 63.5°E locations, respectively. The rocks were thus equilibrated in the spinel stability field prior to their exhumation to the seafloor. Our FTIR analyses show variable water contents in orthopyroxene ranging from 24 to 262 wt. ppm H2O. Orthopyroxene in the 63.5°E peridotites is characterized by homogeneous and high water content (>200 ppm), whereas orthopyroxene in the 53°E peridotites displays a wider range of water contents (24-246 ppm). We first demonstrate that differences in equilibrium conditions (i.e. pressure and temperature) and mineral chemistry and serpentinization cannot explain the water content variations. Melting modeling show that a fractional melting in both garnet and spinel stability fields is needed to explain the MREE and HREE concentrations in clinopyroxene. Enrichment in LREE, high water contents and high H2O/Ce, however, require a post-melting rehydration event such as metasomatism. Based on petrographic evidence and investigation of chemical heterogeneities at segment/dredge scale, we suggest that this event involves a metasomatic agent enriched in water and incompatible elements. We infer that the small-scale heterogeneities in water and trace elements may result either from successive infiltration of various amounts of melt/fluids as described in the formation of oceanic core complex or from spatial heterogeneities of melt infiltration as observed in peridotite massifs. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-02T10:35:31.552898-05:
      DOI: 10.1002/2016GC006767
       
  • Removal of deep lithosphere in ancient continental collisional orogens: A
           case study from central Tibet, China
    • Authors: Sheng-Sheng Chen; Wei-Ming Fan, Ren-Deng Shi, Xiao-Han Gong, Kang Wu
      Abstract: Widespread but small-volume Late Cretaceous volcanic rocks in central Tibet contain important information on the Lhasa–Qiangtang collision process. In this contribution, we focus on Late Cretaceous volcanics in the southern Qiangtang subterrane, and present zircon LA–ICP–MS U–Pb ages, whole-rock major and trace element compositions, and Sr–Nd isotopic data. Zircon LA–ICP–MS U–Pb dating yielded a concordant age of 80 Ma, which postdates the Early Cretaceous collision of the Qiangtang and Lhasa terranes. The volcanic rocks are potassium-rich alkaline andesites with high contents of K2O (3.45–5.11 wt.%) and Th (13.39–25.02 ppm), as well as high K2O/Na2O ratios (0.6–0.9). They have higher REE and HFSE contents than coeval Mg-rich and adakite-like magmatic rocks that can be related to partial melting of a thickened lower crust. Moreover, they have higher values of Mg# and lower contents of SiO2 than lower continental crust-derived rocks in central Tibet and experimental data of mafic rocks. We argue that the andesites were generated after the removal of thickened lithospheric mantle and subsequent to the final Lhasa–Qiangtang amalgamation in a post-collisional setting. The high-K characteristics can be explained by producing the primitive andesite magmas from partial melting of the residual and shallow metasomatized lithospheric mantle (the K-rich layer) during heating by upwelling asthenosphere; subsequently, these primitive andesite magmas were subjected to fractional crystallization to generate the Amdo andesites. The way in which these andesites were formed provides evidence for the lithospheric thickening and uplifting of central Tibet during the Late Cretaceous prior to India–Asia collision. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-02T10:35:26.269102-05:
      DOI: 10.1002/2016GC006678
       
  • Comparison of thermal modeling, microstructural analysis, and Ti-in-quartz
           thermobarometry to constrain the thermal history of a cooling pluton
           during deformation in the Mount Abbot Quadrangle, CA
    • Authors: Johanna M. Nevitt; Jessica M. Warren, Steven Kidder, David D. Pollard
      Abstract: Granitic plutons commonly preserve evidence for jointing, faulting, and ductile fabric development during cooling. Constraining the spatial variation and temporal evolution of temperature during this deformation could facilitate an integrated analysis of heterogeneous deformation over multiple length-scales through time. Here, we constrain the evolving temperature of the Lake Edison granodiorite within the Mount Abbot Quadrangle (central Sierra Nevada, CA) during late Cretaceous deformation by combining microstructural analysis, titanium-in-quartz thermobarometry (TitaniQ), and thermal modeling. Microstructural and TitaniQ analyses were applied to 12 samples collected throughout the pluton, representative of either the penetrative “regional” fabric or the locally strong “fault-related” fabric. Overprinting textures and mineral assemblages indicate the temperature decreased from 400-500°C to
      PubDate: 2017-03-01T06:41:17.543138-05:
      DOI: 10.1002/2016GC006655
       
  • An activity index for geomagnetic paleosecular variation, excursions, and
           reversals
    • Authors: S. Panovska; C.G. Constable
      Abstract: Magnetic indices provide quantitative measures of space weather phenomena that are widely used by researchers in geomagnetism. We introduce an index focused on the internally generated field that can be used to evaluate long term variations or climatology of modern and paleomagnetic secular variation, including geomagnetic excursions, polarity reversals, and changes in reversal rate. The paleosecular variation index, Pi, represents instantaneous or average deviation from a geocentric axial dipole field using normalized ratios of virtual geomagnetic pole colatitude and virtual dipole moment. The activity level of the index, σPi, provides a measure of field stability through the temporal standard deviation of Pi. Pi can be calculated on a global grid from geomagnetic field models to reveal large scale geographic variations in field structure. It can be determined for individual time series, or averaged at local, regional, and global scales to detect long term changes in geomagnetic activity, identify excursions, and transitional field behavior. For recent field models Pi ranges from less than 0.05 to 0.30. Conventional definitions for geomagnetic excursions are characterized by Pi exceeding 0.5. Strong field intensities are associated with low Pi unless they are accompanied by large deviations from axial dipole field directions. σPi provides a measure of geomagnetic stability that is modulated by the level of PSV or frequency of excursional activity and reversal rate. We demonstrate uses of Pi for paleomagnetic observations and field models and show how it could be used to assess whether numerical simulations of the geodynamo exhibit Earth-like properties. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-28T03:40:47.643151-05:
      DOI: 10.1002/2016GC006668
       
  • Across-arc versus along-arc Sr-Nd-Pb isotope variations in the Ecuadorian
           volcanic arc
    • Authors: Marie-Anne Ancellin; Pablo Samaniego, Ivan Vlastélic, François Nauret, Mouhcine Gannoun, Silvana Hidalgo
      Abstract: Previous studies of the Ecuadorian arc (1°N - 2°S) have revealed across-arc geochemical trends that are consistent with a decrease in mantle melting and slab dehydration away from the trench. The aim of this work is to evaluate how these processes vary along the arc in response to small-scale changes in the age of the subducted plate, subduction angle, and continental crustal basement. We use an extensive database of 1524 samples containing 71 new analyses, of major and trace elements as well as Sr-Nd-Pb isotopes from Ecuadorian and South Colombian volcanic centers. Large geochemical variations are found to occur along the Ecuadorian arc, in particular along the front arc, which encompasses 99% and 71% of the total variations in 206Pb/204Pb and 87Sr/86Sr ratios of Quaternary Ecuadorian volcanics, respectively. The front arc volcanoes also show two major latitudinal trends: (1) the southward increase of 207Pb/204Pb and decrease of 143Nd/144Nd reflect more extensive crustal contamination of magma in the southern part (up to 14%); and (2) the increase of 206Pb/204Pb and decrease of Ba/Th away from ∼0.5°S result from the changing nature of metasomatism in the sub-arc mantle wedge with the aqueous fluid/siliceous slab melt ratio decreasing away from 0.5°S. Subduction of a younger and warmer oceanic crust in the Northern part of the arc might promote slab melting. Conversely, the subduction of a colder oceanic crust south of the Grijalva Fracture Zone and higher crustal assimilation lead to the reduction of slab contribution in southern part of the arc. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-27T18:45:42.056952-05:
      DOI: 10.1002/2016GC006679
       
  • Modelling the impact of melt on seismic properties during mountain
           building
    • Authors: Amicia L. Lee; Andrew M. Walker, Geoffrey E. Lloyd, Taija Torvela
      Abstract: Initiation of partial melting in the mid/lower crust causes a decrease in P-wave and S-wave velocities; recent studies imply that the relationship between these velocities and melt is not simple. We have developed a modelling approach to assess the combined impact of various melt and solid phase properties on seismic velocities and anisotropy. The modelling is based on crystallographic preferred orientation (CPO) data measured from migmatite samples, allowing quantification of the variation of seismic velocities with varying melt volumes, shapes, orientations, and matrix anisotropy. The results show non-linear behaviour of seismic properties as a result of the interaction of all of these physical properties, which in turn depend on lithology, stress regime, strain rate, pre-existing rock fabrics, and pressure-temperature conditions. This non-linear behaviour is evident when applied to a suite of samples from a traverse across a migmatitic shear zone in the Seiland Igneous Province, Northern Norway. Critically, changes in solid phase composition and CPO, and melt shape and orientation with respect to the wave propagation direction can result in huge variations in the same seismic property even if the melt fraction remains the same. A comparison with surface wave interpretations from tectonically active regions highlights the issues in current models used to predict melt percentages or partially molten regions. Interpretation of seismic data to infer melt percentages or extent of melting should, therefore, always be underpinned by robust modelling of the underlying geological parameters combined with examination of multiple seismic properties in order to reduce uncertainty of the interpretation. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-27T18:30:32.839069-05:
      DOI: 10.1002/2016GC006705
       
  • Tectonic structure, evolution, and the nature of oceanic core complexes
           and their detachment fault zones (13°20'N and 13°30'N, Mid-Atlantic
           Ridge)
    • Authors: J. Escartín; C. Mével, S. Petersen, D. Bonnemains, M. Cannat, M. Andreani, N. Augustin, A. Bezos, V. Chavagnac, Y. Choi, M. Godard, K. Haaga, C. Hamelin, B. Ildefonse, J. Jamieson, B. John, T. Leleu, C. J. MacLeod, M. Massot-Campos, P. Nomikou, J. A. Olive, M. Paquet, C. Rommevaux, M. Rothenbeck, A. Steinfuhrer, M. Tominaga, L. Triebe, R. Campos, N. Gracias, R. Garcia
      Abstract: Microbathymetry data, in-situ observations, and sampling along the 13°20'N and 13°20'N 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 along-extension 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 13°20‘N OCC, and gabbro and peridotite at 13°30'N, demonstrating that brittle strain localization in shallow lithosphere form corrugations, regardless of lithologies in the detachment zone. Finally, faulting and volcanism dismember the 13°30'N OCC, with widespread present and past hydrothermal activity (Semenov fields), while the Irinovskoe hydrothermal field at the 13°20'N core complex suggests a magmatic source within the footwall. These results confirm the ubiquitous relationship between hydrothermal activity and oceanic detachment formation and evolution. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-23T07:31:48.636698-05:
      DOI: 10.1002/2016GC006775
       
  • Revisit of rare earth element fractionation during chemical weathering and
           river sediment transport
    • Authors: Ni Su; Shouye Yang, Yulong Guo, Wei Yue, Xiaodan Wang, Ping Yin, Xiangtong Huang
      Abstract: Although rare earth element (REE) has been widely applied for provenance study and paleoenvironmental reconstruction, its mobility and fractionation during earth surface processes from weathering to sediment deposition remain more clarification. We investigated the REE fractionations during chemical weathering and river sediment transport based on the systematic observations from a granodiorite-weathering profile and Mulanxi River sediments in southeast China. Two chemical phases (leachates and residues) were separated by 1 N HCl leaching and the leachates account for 20∼70% of the bulk REE concentration. REEs in the weathering profile have been mobilized and fractionated to different extents during chemical weathering and pedogenesis. Remarkable cerium anomalies (Ce/Ce*=0.1∼10.6) occur during weathering as a result of co-precipitation with Mn (hydro)oxides in the profile, while poor or no Ce anomalies in the river sediments were observed. This contrasting feature sheds new light on the indication of Ce anomaly for redox change. The hydraulic sorting-induced mineral redistribution can further homogenize the weathering and pedogenic alterations and thus weaken the REE fractionations in river sediments. The mineral assemblage is the ultimate control on REE composition, and the Mn–Fe (hydro)oxides and secondary phosphate minerals are the main hosts of acid-leachable REEs, while the clay minerals could be important reservoirs for residual REEs. We thus suggest that the widely-used REE proxies such as (LREE/HREE)UCC ratio in the residues is reliable for the indication of sediment provenance, while the ratio in the leachates can indicate the total weathering process to some extent. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-23T07:10:54.201866-05:
      DOI: 10.1002/2016GC006659
       
  • Incipient mantle plume evolution: Constraints from ancient landscapes
           buried beneath the North Sea
    • Authors: G. Stucky de Quay; G. G. Roberts, J. S. Watson, C. A.-L. Jackson
      Abstract: Geological observations that constrain the history of mantle convection are sparse despite its importance in determining vertical and horizontal plate motions, plate rheology, and magmatism. We use a suite of geological and geophysical observations from the northern North Sea to constrain evolution of the incipient Paleocene-Eocene Icelandic plume. Well data and a three-dimensional seismic survey are used to reconstruct a 58–55 Ma landscape now buried ∼1.5 km beneath the seabed in the Bressay region. Geochemical analyses of cuttings from wells that intersect the landscape indicate the presence of angiosperm debris. These observations, combined with presence of coarse clastic material, interpreted beach ridges, and a large dendritic drainage network, indicate that this landscape formed subaerially. Longitudinal profiles of palaeo-rivers were extracted and inverted for an uplift rate history, indicating three distinct phases of uplift and total cumulative uplift of ∼350 m. Dinoflagellate cysts in the surrounding marine stratigraphy indicate that this terrestrial landscape formed in ∼150 km/Ma. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-23T07:10:44.459644-05:
      DOI: 10.1002/2016GC006769
       
  • 40Ar/39Ar geochronology reveals rapid change from plume-assisted to
           stress-dependent volcanism in the Newer Volcanic Province, SE Australia
    • Authors: K. F. Oostingh; F. Jourdan, E. L. Matchan, D. Phillips
      Abstract: Here, we present 40Ar/39Ar ages of volcanic features in the Cenozoic intraplate Newer Volcanic Province in southeast Australia. The 
      PubDate: 2017-02-21T03:30:38.933184-05:
      DOI: 10.1002/2016GC006601
       
  • Quantifying K, U, and Th contents of marine sediments using shipboard
           natural gamma radiation spectra measured on DV JOIDES Resolution
    • Authors: David De Vleeschouwer; Ann G. Dunlea, Gerald Auer, Chloe H. Anderson, Hans Brumsack, Aaron de Loach, Michael C. Gurnis, Youngsook Huh, Takeshige Ishiwa, Kwangchul Jang, Michelle A. Kominz, Christian März, Bernhard Schnetger, Richard W. Murray, Heiko Pälike,
      Abstract: During International Ocean Discovery Program (IODP) expeditions, shipboard-generated data provide the first insights into the cored sequences. The natural gamma radiation (NGR) of the recovered material, for example, is routinely measured on the ocean drilling research vessel DV JOIDES Resolution. At present, only total NGR counts are readily available as shipboard data, although full NGR spectra (counts as a function of gamma-ray energy level) are produced and archived. These spectra contain unexploited information, as one can estimate the sedimentary contents of potassium (K), thorium (Th), and uranium (U) from the characteristic gamma-ray energies of isotopes in the 40K, 232Th, and 238U radioactive decay series.Dunlea et al. [2013] quantified K, Th and U contents in sediment from the South Pacific Gyre by integrating counts over specific energy levels of the NGR spectrum. However, the algorithm used in their study is unavailable to the wider scientific community due to commercial proprietary reasons. Here, we present a new MATLAB algorithm for the quantification of NGR spectra that is transparent and accessible to future NGR users. We demonstrate the algorithm's performance by comparing its results to shore-based inductively coupled plasma-mass spectrometry (ICP-MS), inductively coupled plasma-emission spectrometry (ICP-ES), and quantitative wavelength-dispersive X-ray fluorescence (XRF) analyses. Samples for these comparisons come from eleven sites (U1341, U1343, U1366-U1369, U1414, U1428-U1430, U1463) cored in two oceans during five expeditions. In short, our algorithm rapidly produces detailed high-quality information on sediment properties during IODP expeditions at no extra cost. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-20T07:30:25.598961-05:
      DOI: 10.1002/2016GC006715
       
  • Olivine fabrics and tectonic evolution of forearc mantles: A natural
           perspective from the Songshugou dunite and harzburgite in the Qinling
           orogenic belt, central China
    • Authors: Yi Cao; Haemyeong Jung, Shuguang Song
      Abstract: To advance our understanding of deformation characteristics, rheological behaviors and tectonic evolution of the forearc lithospheric mantle, we analyzed mineral fabrics for a large spinel-bearing ultramafic massif in the Songshugou area in the Qinling orogenic belt, central China. In the spinel-poor coarse-grained dunite, stronger A-/D-type and weaker C-type-like fabrics were found, whereas the spinel-rich coarse-grained dunite displayed a comparatively stronger B-type-like fabric. These olivine fabrics are high-T fabrics influenced by the presence of melt, in which B- and C-type-like fabrics are inferred to be produced by melt-assisted grain boundary sliding during synkinematic high-T melt‒rock reactions. In contrast, the spinel-poor porphyroclastic and fine-grained dunites present weak AG- and B-type-like fabrics, respectively. Their olivine fabrics (low-T fabrics) are inferred to transform from A-/D-type fabric in their coarse-grained counterparts possibly through mylonitization process assisted by low-T fluid‒rock reactions, during which strain was accommodated by the fluid-enhanced dislocation slip and/or fluid-assisted grain boundary sliding processes. Combined with the tectonic results of our previous work [Cao et al., 2016], the high-T olivine fabrics are probably related to a young and warm forearc mantle where intense partial melting and high-T boninitic melt‒rock reactions prevalently occurred, whereas the low-T olivine fabrics likely reflect the evolving tectonic settings through the cooling forearc mantle to a continental lower crust in a collisional orogeny where low-T fluid‒rock reactions were pervasively activated. These low-T olivine fabrics imply that, though cold, the forearc lithospheric mantle may be locally weak (∼20‒30 MPa), allowing ductile deformation to occur at a geologically significant strain rate. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-17T03:32:17.450918-05:
      DOI: 10.1002/2016GC006614
       
  • Iron isotopic evolution during fractional crystallization of the uppermost
           Bushveld Complex layered mafic intrusion
    • Authors: Laura D. Bilenker; Jill A. VanTongeren, Craig C. Lundstrom, Adam C. Simon
      Abstract: We present δ56Fe (56Fe/54Fe relative to standard IRMM-014) data from whole rock and magnetite of the Upper and Upper Main Zones (UUMZ) of the Bushveld Complex. With it, we assess the role of fractional crystallization in controlling the Fe isotopic evolution of a mafic magma. The UUMZ evolved by fractional crystallization of a dry tholeiitic magma to produce gabbros and diorites with cumulus magnetite and fayalitic olivine. Despite previous experimental work indicating a potential for magnetite crystallization to drastically change magma δ56Fe, we observe no change in whole rock δ56Fe above and below magnetite saturation. We also observe no systematic change in whole rock δ56Fe with increasing stratigraphic height, and only a small variation in δ56Fe in magnetite separates above magnetite saturation. Whole rock δ56Fe (errors twice standard deviation, ±2σ) throughout the UUMZ ranges from -0.01 ±0.03‰ to 0.21 ±0.09‰ (δ56FeaverageWR = 0.10 ±0.09‰; n=21, isotopically light outlier: δ56FeWR = -0.15‰), and magnetites range from 0.28 ±0.04‰ to 0.86 ±0.07‰ (δ56FeaverageMgt = 0.50 ±0.15‰; n=20), similar to values previously reported for other layered intrusions. We compare our measured δ56FeWR to a model that incorporates the changing normative mineralogy, calculated temperatures, and published fractionation factors of Fe-bearing phases throughout the UUMZ and produces δ56FeWR values that evolve only in response to fractional crystallization. Our results show that the Fe isotopic composition of a multiply-saturated (multiple phases on the liquidus) magma is unlikely to change significantly during fractional crystallization of magnetite due to the competing fractionation of other Fe-bearing cumulus phases. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-16T01:05:56.84891-05:0
      DOI: 10.1002/2016GC006660
       
  • Composition law of oblique anhysteretic remanent magnetization and its
           relation to the magnetostatic interaction
    • Authors: Masahiko Sato; Nobutatsu Mochizuki, Minako Watanabe, Hideo Tsunakawa
      Abstract: The basic properties of oblique anhysteretic remanent magnetization (OARM) acquired in a weak and steady magnetic field with an arbitrary angle to the alternating field direction were studied. OARM and rock-magnetic experiments were conducted on samples of basalt, granite, and sediment containing non-interacting single-domain (SD), interacting SD, pseudo-single-domain, and multidomain low-Ti titanomagnetites. The intensity of OARM (MOARM) systematically increased or decreased with increasing angle between alternating and steady field directions (θSF), while the angle between alternating field and OARM directions (θOARM) increased with increasing θSF for all samples. During stepwise alternating field demagnetization, the OARM vector shows a single component parallel to the steady field direction for θSF = 0° (ARMǁ) and 90° (ARM⊥). The median destructive field of ARM⊥ is larger than that of ARMǁ. For intermediate angles (θSF = 30°, 45°, and 60°), the OARM vector was not parallel to the applied steady field; instead, it gradually increased with coercivity. These experiments indicate that the OARM vector is approximately given by the sum of two orthogonal magnetizations coinciding with ARMǁ and ARM⊥, respectively. Thus, the OARM vector can be determined by acquisition efficiencies of ARMǁ and ARM⊥ in an individual sample. Based on these experiments and associated rock-magnetic measurements, non-interacting SD samples show lower ARM⊥/ARMǁ ratios, compared to other samples. This result suggests that OARM can be used as a useful tool to detect non-interacting SD particles in the paleomagnetic samples. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-14T18:05:31.630523-05:
      DOI: 10.1002/2016GC006671
       
  • Geodynamic implications for zonal and meridional isotopic patterns across
           the northern Lau and North Fiji Basins
    • Authors: Allison A. Price; Matthew G. Jackson, Janne Blichert-Toft, Mark D. Kurz, Jim Gill, Jerzy Blusztajn, Frances Jenner, Raul Brens, Richard Arculus
      Abstract: We present new Sr-Nd-Pb-Hf-He isotopic data for sixty-five volcanic samples from the northern Lau and North Fiji Basin. This includes forty-seven lavas obtained from forty dredge sites spanning an east-west transect across the Lau and North Fiji basins, ten ocean island basalt (OIB)-type lavas collected from seven Fijian islands, and eight OIB lavas sampled on Rotuma. For the first time we are able to map clear north-south and east-west geochemical gradients in 87Sr/86Sr across the northern Lau and North Fiji Basins: lavas with the most geochemically enriched radiogenic isotopic signatures are located in the northeast Lau Basin, while signatures of geochemical enrichment are diminished to the south and west away from the Samoan hotspot. Based on these geochemical patterns and plate reconstructions of the region, these observations are best explained by the addition of Samoa, Rurutu, and Rarotonga hotspot material over the past 4 Ma. We suggest that underplated Samoan material has been advected into the Lau Basin over the past ∼4 Ma. As the slab migrated west (and toward the Samoan plume) via rollback over time, younger and hotter (and therefore less viscous) underplated Samoan plume material was entrained. Thus, entrainment efficiency of underplated plume material was enhanced, and Samoan plume signatures in the Lau Basin became stronger as the trench approached the Samoan hotspot. The addition of subducted volcanoes to the Cook-Austral Volcanic Lineament material, first from the Rarotonga hotspot, then followed by the Rurutu hotspot, contributes to the extreme geochemical signatures observed in the northeast Lau Basin. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:45:32.295224-05:
      DOI: 10.1002/2016GC006651
       
  • Neogene paleoceanography of the eastern equatorial Pacific based on the
           radiolarian record of IODP drill sites off Costa Rica
    • Authors: María I. Sandoval; Demetrio Boltovskoy, Alan T. Baxter, Peter O. Baumgartner
      Abstract: The Integrated Ocean Drilling Program (IODP) Expedition 344 drilled cores following a transect across the convergent margin off Costa Rica. Two of the five sites (U1381 and U1414) are the subject of the present study. Major radiolarian faunal breaks and characteristic species groups were defined with the aid of cluster analysis, nodal analysis and discriminant analysis of principal components. A middle-late Miocene to Pleistocene age (radiolarian zones RN5 to RN16) was determined for the sites, which agrees with the nannofossil zonations and 40Ar/39Ar and tephra layers. Considering the northward movement of the Cocos plate (∼7.3 cm/year), and a paleolatitude calculator, it is assumed that during the Miocene the two sites were located ∼1000 km to the southwest of their current position, slightly south of the equator. The radiolarian faunas retrieved were thus seemingly formed under the influence of different oceanic currents and sources of nutrients. Changes in the radiolarian assemblages at Site U1414 point at dissimilar environmental settings associated with the colder South Equatorial Current and the warmer Equatorial Countercurrent, as well as to coastal upwelling. These differences are best reflected by changes in the abundance of the morphotype Spongurus spp., with noticeably higher values during the Miocene, than in the Pliocene and the Pleistocene. Because Spongurus spp. is generally associated with cooler waters, these abundance variations (as well as those of several other species) suggest that during the Miocene the area had a stronger influence of colder waters than during younger periods. During the Pliocene and the lowermost Pleistocene, biogenic remains are scarce, presumably due to the terrigenous input, which could have diluted and affected the preservation of pelagic fossils, as well as to the displacement of the site to warmer waters. A typically tropical fauna characterized the Pleistocene, yet with widespread presence of colder water species, most probably indicative of the influence of coastal upwelling processes. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:40:58.811024-05:
      DOI: 10.1002/2016GC006623
       
  • Boron isotope variations in Tonga-Kermadec-New Zealand arc lavas:
           Implications for origin of subduction components and mantle influences
    • Authors: William P. Leeman; Sonia Tonarini, Simon Turner
      Abstract: The Tonga-Kermadec-New Zealand volcanic arc is an end-member of arc systems with fast subduction suggesting that the Tonga sector should have the coolest modern slab thermal structure on Earth. New data for boron concentration and isotopic composition are used to evaluate the contrasting roles of postulated subduction components (sediments and oceanic slab lithologies) in magma genesis. Major observations include: (a) Tonga-Kermadec volcanic front lavas are enriched in B (as recorded by B/Nb and similar ratios) and most have relatively high δ11B (> +4‰), whereas basaltic lavas from New Zealand have relatively low B/Nb and δ11B (< -3.5‰); (b) both δ11B and B/Nb generally increase northward from New Zealand along with convergence rate and overall slab flux; (c) δ11B and B/Nb decrease toward the back-arc, as observed elsewhere; and (d) low δ11B is observed in volcanic front samples from Ata, an anomalous sector where the back-arc Valu Fa Spreading Center impinges on the arc and the Louisville Seamount Chain is presently subducting. Otherwise, volcanic front lavas exhibit positive correlations for both B/Nb and δ11B with other plausible indicators of slab-derived fluid contributions (e.g., Ba/Nb, U/Th, (230Th/232Th) and 10Be/9Be), and with estimated degree of melting to produce the mafic lavas. Inferred B-enrichments in the arc magma sources are likely dominated by serpentinite domains deeper within the subducting slab (± altered oceanic crust), and B systematics are consistent with dominant transport by slab-derived aqueous fluids. Effects of this process are amplified by mantle wedge source depletion due to prior melt extraction. . This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:40:50.41082-05:0
      DOI: 10.1002/2016GC006523
       
  • Variations in pockmark composition at the Vestnesa Ridge: Insights from
           marine controlled source electromagnetic and seismic data
    • Authors: Bedanta K. Goswami; Karen A. Weitemeyer, Stefan Bünz, Timothy A. Minshull, Graham K. Westbrook, Stephan Ker, Martin C. Sinha
      Abstract: The Vestnesa Ridge marks the northern boundary of a known submarine gas hydrate province in the west Svalbard margin. Several seafloor pockmarks at the eastern segment of the ridge are sites of active methane venting. Until recently, seismic reflection data was the main tool for imaging beneath the ridge. Coincident controlled source electromagnetic (CSEM), high-resolution two-dimensional (2D) airgun, sweep frequency SYSIF and three-dimensional (3D) p-cable seismic reflection data were acquired at the south-eastern part of the ridge between 2011 and 2013. The CSEM and seismic data contains profiles across and along the ridge, passing several active and inactive pockmarks. Joint interpretation of resistivity models obtained from CSEM and seismic reflection data provides new information regarding the fluid composition beneath the pockmarks. There is considerable variation in transverse resistance and seismic reflection characteristics of the gas hydrate stability zone (GHSZ) between the ridge flanks and chimneys beneath pockmarks. Layered seismic reflectors on the flanks are associated with around 300Ωm2 transverse resistance, whereas the seismic reflectors within the chimneys exhibit amplitude blanking and chaotic patterns. The transverse resistance of the GHSZ within the chimneys vary between 400 and 1200 Ωm2. Variance attributes obtained from the 3D p-cable data also highlight faults and chimneys, which coincide with the resistivity anomalies. Based on the joint data interpretation, widespread gas hydrate presence is likely at the ridge, with both hydrates and free gas contained within the faults and chimneys. However, at the active chimneys the effect of gas likely dominate the resistive anomalies. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-10T17:35:40.546073-05:
      DOI: 10.1002/2016GC006700
       
  • Three-dimensional resistivity structure in Ishikari Lowland, Hokkaido,
           northeastern Japan - Implications to strain concentration mechanism
    • Authors: Yusuke Yamaya; Toru Mogi, Ryo Honda, Hideaki Hase, Takeshi Hashimoto, Makoto Uyeshima
      Abstract: The Ishikari Lowland on the island of Hokkaido in northeastern Japan is situated at the end of a westward-moving foreland fold-and-thrust belt from the Hidaka collision zone, where the northeastern Japan and Kurile arcs meet. This activity forms a tectonic zone under an east–west compression field in this region. A magnetotelluric resistivity survey was performed to investigate the mechanism for the strain concentration in this region. A three-dimensional (3D) resistivity inversion showed a conductive thick sedimentary layer and an underlying resistive basement. Remarkable conductors were found in the resistive basement beneath the Ishikari-teichi-toen fault zone (ITFZ) and the Shikotsu caldera. The conductors beneath the ITFZ were interpreted as aqueous fluids that accumulated in the damaged zone in connection with the formation of pull-apart faults and horst. In contrast, the conductor beneath the Shikotsu caldera corresponds to a magmatic fluid path from the upper mantle. These features suggest that the ductile deformation in the upper crust contribute to the strain concentration in this region. The soft thick sediments allow ductile deformations to occur. Furthermore, local fluid-rich zones in the basement cause the crustal strength to be heterogeneous. These thick sediments and local fluids in the basement both contribute to the strain concentration in this region. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-10T17:35:29.727286-05:
      DOI: 10.1002/2016GC006771
       
  • Origin of geochemical mantle components: Role of spreading ridges and
           thermal evolution of mantle
    • Authors: Jun-Ichi Kimura; James B. Gill, Peter E. van Keken, Hiroshi Kawabata, Susanne Skora
      Abstract: We explore the element redistribution at mid-ocean ridges (MOR) using a numerical model to evaluate the role of decompression melting of the mantle in Earth's geochemical cycle, with focus on the formation of the depleted mantle component. Our model uses a trace element mass balance based on an internally consistent thermodynamic-petrologic computation to explain the composition of MOR basalt (MORB) and residual peridotite. Model results for MORB-like basalts from 3.5 to 0 Ga indicate a high mantle potential temperature (Tp) of 1650–1500°C during 3.5–1.5 Ga before decreasing gradually to ∼1300°C today. The source mantle composition changed from primitive (PM) to depleted as Tp decreased, but this source mantle is variable with an early depleted reservoir (EDR) mantle periodically present. We examine a two-stage Sr-Nd-Hf-Pb isotopic evolution of mantle residues from melting of PM or EDR at MORs. At high-Tp (3.5–1.5 Ga), the MOR process formed extremely depleted DMM. This coincided with formation of the majority of the continental crust, the sub-continental lithospheric mantle, and the enriched mantle components formed at subduction zones and now found in OIB. During cooler mantle conditions (1.5–0 Ga), the MOR process formed most of the modern ocean basin DMM. Changes in the mode of mantle convection from vigorous deep mantle recharge before ∼1.5 Ga to less vigorous afterwards is suggested to explain the thermochemical mantle evolution. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-09T10:45:39.896012-05:
      DOI: 10.1002/2016GC006696
       
  • Primary magmas and mantle temperatures through time
    • Authors: Jérôme Ganne; Xiaojun Feng
      Abstract: Chemical composition of mafic magmas is a critical indicator of physico-chemical conditions, such as pressure, temperature and fluid availability, accompanying melt production in the mantle and its evolution in the continental or oceanic lithosphere. Recovering this information has fundamental implications in constraining the thermal state of the mantle and the physics of mantle convection throughout the Earth's history. Here, a statistical approach is applied to a geochemical database of about 22,000 samples from the mafic magma record. Potential temperatures (Tps) of the mantle derived from this database, assuming melting by adiabatic decompression and a Ti-dependent (Fe2O3/TiO2=0.5) or constant redox condition (Fe2+/∑Fe = 0.9 or 0.8) in the magmatic source, are thought to be representative of different thermal “horizons” (or thermal heterogeneities) in the ambient mantle, ranging in depth from a shallow sublithospheric mantle (Tp minima) to a lower thermal boundary layer (Tp maxima). The difference of temperature (Δ Tp) observed between Tp maxima and minima did not change significantly with time (∼170°C). Conversely, a progressive but limited cooling of ∼150°C is proposed since ∼2.5 Gyr for the Earth's ambient mantle, which falls in the lower limit proposed by Herzberg et al [2010] (∼ 150 to 250°C hotter than today). Cooling of the ambient mantle after 2.5 Ga is preceded by a high-temperature plateau evolution and a transition from dominant plumes to a plate tectonics geodynamic regime, suggesting that subductions stabilized temperatures in the Archaean mantle that was in warming mode at that time. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-09T10:45:30.478804-05:
      DOI: 10.1002/2016GC006787
       
  • Linking basin-scale and pore-scale gas hydrate distribution patterns in
           diffusion-dominated marine hydrate systems
    • Authors: Michael Nole; Hugh Daigle, Ann E. Cook, Jess I.T. Hillman, Alberto Malinverno
      Abstract: The goal of this study is to computationally determine the potential distribution patterns of diffusion-driven methane hydrate accumulations in coarse-grained marine sediments. Diffusion of dissolved methane in marine gas hydrate systems has been proposed as a potential transport mechanism through which large concentrations of hydrate can preferentially accumulate in coarse-grained sediments over geologic time. Using one-dimensional compositional reservoir simulations, we examine hydrate distribution patterns at the scale of individual sand layers (1 to 20 m thick) that are deposited between microbially active fine-grained material buried through the gas hydrate stability zone (GHSZ). We then extrapolate to two- dimensional and basin-scale three-dimensional simulations, where we model dipping sands and multilayered systems. We find that properties of a sand layer including pore size distribution, layer thickness, dip, and proximity to other layers in multilayered systems all exert control on diffusive methane fluxes toward and within a sand, which in turn impact the distribution of hydrate throughout a sand unit. In all of these simulations, we incorporate data on physical properties and sand layer geometries from the Terrebonne Basin gas hydrate system in the Gulf of Mexico. We demonstrate that diffusion can generate high hydrate saturations (upward of 90%) at the edges of thin sands at shallow depths within the GHSZ, but that it is ineffective at producing high hydrate saturations throughout thick (greater than 10 m) sands buried deep within the GHSZ. Furthermore, we find that hydrate in fine-grained material can preserve high hydrate saturations in nearby thin sands with burial. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-07T11:31:21.528571-05:
      DOI: 10.1002/2016GC006662
       
  • The undatables: Quantifying uncertainty in a highly expanded Late
           Glacial-Holocene sediment sequence recovered from the deepest Baltic Sea
           basin: IODP Site M0063
    • Authors: S.P. Obrochta; T. Andrén, S.Z. Fazekas, B.C. Lougheed, I. Snowball, Y. Yokoyama, Y. Miyairi, R. Kondo, A.T. Kotilainen, O. Hyttinen, A Fehr
      Abstract: Laminated, organic-rich silts and clays with high dissolved gas content characterize sediments at IODP Site M0063 in the Landsort Deep, which at 459 m is the deepest basin in the Baltic Sea. Cores recovered from Hole M0063A experienced significant expansion as gas was released during the recovery process, resulting in high sediment loss. Therefore during operations at subsequent holes, penetration was reduced to 2 m per 3.3 m core, permitting expansion into 1.3 m of initially empty liner. Fully filled liners were recovered from Holes B through E, indicating that the length of recovered intervals exceeded the penetrated distance by a factor of >1.5. A typical down-core logarithmic trend in gamma density profiles, with anomalously low density values within the upper ∼1 m of each core, suggests that expansion primarily occurred in this upper interval. Thus, we suggest that a simple linear correction is inappropriate. This interpretation is supported by anisotropy of magnetic susceptibility data that indicate vertical stretching in the upper ∼1.5 m of expanded cores. Based on the mean gamma density profiles of cores from Holes M0063C and D, we obtain an expansion function that is used to adjust the depth of each core to conform to its known penetration. The variance in these profiles allows for quantification of uncertainty in the adjusted depth scale. Using a number of bulk 14C dates, we explore how the presence of multiple carbon source pathways leads to poorly constrained radiocarbon reservoir age variability that significantly affects age and sedimentation rate calculations. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-07T03:20:46.276395-05:
      DOI: 10.1002/2016GC006697
       
  • Geochemistry of Zr, Hf, and REE in a wide spectrum of Eh and water
           composition: The case of Dead Sea fault system (Israel)
    • Authors: P. Censi; M. Raso, Y. Yechieli, H. Ginat, F. Saiano, P. Zuddas, L. Brusca, W. D'Alessandro, C. Inguaggiato
      Abstract: Along the Jordan Valley-Dead Sea fault area several natural waters in springs, wells and catchments occur. The chemical-physical characters of the studied waters allowed for the first time the investigation of the Zr and Hf geochemical behaviour, apart from REE, extended to a wide range of Eh, temperature, salinity and pH conditions. The results of this study indicate that the dissolved Zr and Hf distribution in natural waters is strongly influenced by redox conditions since these in turn drive the deposition of Fe-oxyhydroxides or pyrite. In oxidizing waters saturated or oversaturated in Fe-oxyhydroxides [Group-1], superchondritic Zr/Hf values are measured. On the contrary, in waters where Eh
      PubDate: 2017-02-07T03:20:40.473874-05:
      DOI: 10.1002/2016GC006704
       
  • A geochemical and geophysical reappraisal to the significance of the
           recent unrest at Campi Flegrei caldera (Southern Italy)
    • Authors: Roberto Moretti; Giuseppe De Natale, Claudia Troise
      Abstract: Volcanic unrest at calderas involve complex interaction between magma, hydrothermal fluids and crustal stress and strain. Campi Flegrei caldera (CFc), located in the Naples (Italy) area and characterised by the highest volcanic risk on Earth for the extreme urbanisation, undergoes unrest phenomena involving several meters of uplift and intense shallow micro-seismicity since several decades. Despite unrest episodes display in the last decade only moderate ground deformation and seismicity, current interpretations of geochemical data point to a highly pressurized hydrothermal system. We show that at CFc, the usual assumption of vapour-liquid coexistence in the fumarole plumes leads to largely overestimated hydrothermal pressures and, accordingly, interpretations of elevated unrest. By relaxing unconstrained geochemical assumptions, we infer an alternative model yielding better agreement between geophysical and geochemical observations. The model reconciles discrepancies between what observed 1) for two decades since the 1982-84 large unrest, when shallow magma was supplying heat and fluids to the hydrothermal system, and 2) in the last decade. Compared to the 1980's unrest, the post-2005 phenomena are characterized by much lower aquifers overpressure and magmatic involvement, as indicated by geophysical data and despite large changes in geochemical indicators. Our interpretation points out a model in which shallow sills, intruded during 1969-1984, have completely cooled, so that fumarole emissions are affected now by deeper, CO2-richer, magmatic gases producing the modest heating and overpressure of the hydrothermal system. Our results have important implications on the short-term eruption hazard assessment and on the best strategies for monitoring and interpreting geochemical data. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-03T20:00:41.742925-05:
      DOI: 10.1002/2016GC006569
       
  • Mantle exhumation and metamorphism in the Basque-Cantabrian Basin (N
           Spain): Stable and clumped isotopic analysis in carbonates and comparison
           with ophicalcites in the North-Pyrenean Zone (Urdach and Lherz)
    • Authors: I. DeFelipe; D. Pedreira, J. A. Pulgar, E. Iriarte, M. Mendia
      Abstract: The presence of ophicalcites in serpentinized peridotites together with fragments of these rocks in Cretaceous breccias along several North-Pyrenean basins, led to the interpretation of complete mantle exhumation to the seafloor. The westernmost outcrop of peridotites in the Pyrenean-Cantabrian belt is located in Ziga (eastern Basque-Cantabrian Basin), associated to a band of HT metamorphism related to the Leiza fault (Marble Unit). We present a petrological and geochemical study of the marbles and Ziga peridotite-hosted calcite, including standard stable isotope composition and clumped isotope geothermometry. These isotopic techniques allow the determination of different types of formational fluids and crystallization temperatures, and are a useful tool for studying carbonation processes in hyperextended basins. Fieldwork and analytical studies lead us to conclude that during the opening of the Bay of Biscay, mantle rocks were unroofed at the base of the sedimentary pile of the eastern Basque-Cantabrian Basin. However, the ophicalcite veins were recrystallized from meteoric fluids at low temperatures (≤ 49 ºC). The primary carbonate phase may have been formed either during the mid-Cretaceous unroofing of the mantle or in a post-exhumation stage. The process of mantle exhumation was accompanied with HT-LP metamorphism and fluid circulation along major faults that reset the marine isotopic signature in the nearest marbles. For comparison, ophicalcites from Urdach and Lherz (North-Pyrenean Zone) were included in the clumped isotope study. Results show that they were recrystallized from hot (∼200-230 ºC), saline fluids, and from meteoric fluids at near ambient temperatures (∼32-42 ºC), respectively. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-31T06:52:42.549131-05:
      DOI: 10.1002/2016GC006690
       
  • Conductivity structure of the lithosphere-asthenosphere boundary beneath
           the eastern North American margin
    • Authors: Eric Attias; Rob. L. Evans, Samer Naif, Jimmy Elsenbeck, Kerry Key
      Abstract: Tectonic plate motion and mantle dynamics processes are heavily influenced by the characteristics of the lithosphere-asthenosphere boundary (LAB), yet this boundary remains enigmatic regarding its properties and geometry. The processes involved in rifting at passive margins result in substantial alteration of the lithosphere through the transition from continental to oceanic lithologies. Here we employ marine magnetotelluric (MT) data acquired along a ∼135 km long profile, offshore Martha's Vineyard, New England, USA, to image the electrical conductivity structure beneath the New England continental margin for the first time. We invert the data using two different MT 2-D inversion algorithms and present a series of models that are obtained using three different parameterizations: fully unconstrained, unconstrained with an imposed LAB discontinuity and a-priori constrained lithosphere resistivity. This suite of models infers variability in the depth of the LAB, with an average depth of 115 km at the eastern North America passive margin. Models robustly detect a ∼350 Ωm lithospheric anomalous conductivity zone (LACZ) that extends vertically through the entire lithosphere. Our preferred conductivity model is consistent with regional P-to-S receiver function data, shear-wave velocity, gravity anomalies and prominent geological features. We propose that the LACZ is indicative of paleo lithospheric thinning, either resulting from Kimberlite intrusions associated with rifting and the New England Great Meteor hotspot track, or from shear-driven localized deformation related to rifting. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-31T06:52:26.125212-05:
      DOI: 10.1002/2016GC006667
       
  • Classification of geochemical data based on multivariate statistical
           analyses: Complementary roles of cluster, principal component, and
           independent component analyses
    • Authors: Hikaru Iwamori; Kenta Yoshida, Hitomi Nakamura, Tatsu Kuwatani, Morihisa Hamada, Satoru Haraguchi, Kenta Ueki
      Abstract: Identifying the data structure including trends and groups/clusters in geochemical problems is essential to discuss the origin of sources and processes from the observed variability of data. An increasing number and high dimensionality of recent geochemical data require efficient and accurate multivariate statistical analysis methods. In this paper, we show the relationship and complementary roles of k-means cluster analysis (KCA), principal component analysis (PCA), and independent component analysis (ICA) to capture the true data structure. When the data are preprocessed by primary standardization (i.e., with the zero mean and normalized by the standard deviation), KCA and PCA provide essentially the same results, although the former returns the solution in a discretized space. When the data are preprocessed by whitening (i.e., normalized by eigenvalues along the principal components), KCA and ICA may identify a set of independent trends and groups, irrespective of the amplitude (power) of variance. As an example, basalt isotopic compositions have been analyzed with KCA on the whitened data, demonstrating clear rock‒tectonic occurrence‒mantle end-member discrimination. Therefore, the combination of these methods, particularly KCA on whitened data, is useful to capture and discuss the data structure of various geochemical systems, for which an Excel program is provided. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-28T03:21:49.561039-05:
      DOI: 10.1002/2016GC006663
       
  • An evaluation of benthic foraminiferal U/Ca and U/Mn proxies for deep
           ocean carbonate chemistry and redox conditions
    • Authors: Pujiao Chen; Jimin Yu, Zhangdong Jin
      Abstract: The deep ocean is thought to have played a crucial role in modulating atmospheric CO2 changes, and thus reconstructions of deep ocean conditions can place important constraints on the past global carbon cycle. Some previous studies suggested that foraminiferal U/Ca could be used to infer seawater carbonate chemistry changes, but others showed complications from diagenesis and temperature. A recent down-core study suggested that foraminiferal U/Mn may be used for sedimentary redox-conditions, but no core-top work has been done to investigate factors affecting U/Mn. We investigate controlling factors on U/Ca and U/Mn in two benthic foraminiferal species from 120 global core-tops and three Atlantic sediment cores. Our core-top data reveal no significant correlation between core-top benthic U/Ca and carbonate system parameters. The lack of an influence of deep-water [CO32-] on U/Ca is further supported by our down-core results. Together, our data highlight complications to use benthic U/Ca for deep-water carbonate chemistry reconstructions. Although no correlation is found between core-top U/Mn and hydrographic data, high-resolution U/Mn and U/Ca in core TNO57-21 show similar patterns to authigenic U (aU) and vary in tandem with atmospheric CO2 on millennial timescales. Changes in U/Mn, U/Ca and aU in TNO57-21 may reflect post-depositional diagenesis linked to sedimentary oxygen, which is controlled by subantarctic surface productivity and ventilation of deep South Atlantic in the past. We suggest that benthic U/Mn and U/Ca may be used as auxiliary indicators for past sedimentary redox-conditions and along with other proxies could reflect deep-water oxygenation. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-28T03:21:44.224751-05:
      DOI: 10.1002/2016GC006730
       
  • How can satellite imagery be used for mineral exploration in thick
           vegetation areas?
    • Authors: A. N. H. Hede; K. Koike, K. Kashiwaya, S. Sakurai, R. Yamada, D. A. Singer
      Abstract: The Hokuroku district, northern Japan, is globally recognized for rich ore deposits (kuroko and vein types), which have been thoroughly explored under thick vegetation cover. This situation is ideal to evaluate the effects of ore deposits on vegetation anomalies through geobotanical remote sensing. Here we present novel methods to detect vegetation anomalies caused by ore deposits and verify their usefulness by comparing the anomalies with a deposit potential map produced from multiple geological data. We use the reflectance spectra of Landsat ETM+ images acquired in summer and autumn to calculate a vegetation index for plant physiological activity. A key variable to detect the anomalies is a variation of vegetation index with time at each pixel. Difference in variation is enlarged by a sequence of image enhancement methods for the detection. We find that the vegetation anomalies, defined by the large ratios, correspond well to the high potential zones of ore deposits and known major deposits. Consequently, our methods can extend the applicability of remote sensing-based mineral exploration to the areas covered by thick vegetation, in addition to traditional arid and semi-arid areas. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-28T03:21:39.857568-05:
      DOI: 10.1002/2016GC006501
       
  • The GMT/MATLAB toolbox
    • Authors: Paul Wessel; Joaquim F. Luis
      Abstract: The GMT/MATLAB toolbox is a basic interface between MATLAB® (or Octave) and GMT, the Generic Mapping Tools, which allows MATLAB users full access to all GMT modules. Data may be passed between the two programs using intermediate MATLAB structures that organize the metadata needed; these are produced when GMT modules are run. In addition, standard MATLAB matrix data can be used directly as input to GMT modules. The toolbox improves interoperability between two widely used tools in the geosciences and extends the capability of both tools: GMT gains access to the powerful computational capabilities of MATLAB while the latter gains the ability to access specialized gridding algorithms and can produce publication-quality PostScript-based illustrations. The toolbox is available on all platforms and may be downloaded from the GMT website. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-28T03:21:30.437174-05:
      DOI: 10.1002/2016GC006723
       
  • Upper mantle seismic anisotropy beneath the northern transantarctic
           mountains, Antarctica from PKS, SKS, and SKKS splitting analysis
    • Authors: Jordan H. Graw; Samantha E. Hansen
      Abstract: Using data from the new Transantarctic Mountains Northern Network, this study aims to constrain azimuthal anisotropy beneath a previously unexplored portion of the Transantarctic Mountains (TAMs) to assess both past and present deformational processes occurring in this region. Shear-wave splitting parameters have been measured for PKS, SKS, and SKKS phases using the eigenvalue method within the SplitLab software package. Results show two distinct geographic regions of anisotropy within our study area: one behind the TAMs front, with an average fast axis direction of 42 ± 3° and an average delay time of 0.9 ± 0.04 s, and the other within the TAMs near the Ross Sea coastline, with an average fast axis oriented at 51 ± 5° and an average delay time of 1.5 ± 0.08 s. Behind the TAMs front, our results are best explained by a single anisotropic layer that is estimated to be 81-135 km thick, thereby constraining the anisotropic signature within the East Antarctic lithosphere. We interpret the anisotropy behind the TAMs front as relict fabric associated with tectonic episodes occurring early in Antarctica's geologic history. For the coastal stations, our results are best explained by a single anisotropic layer estimated to be 135-225 km thick. This places the anisotropic source within the viscous asthenosphere, which correlates with low seismic velocities along the edge of the West Antarctic Rift System. We interpret the coastal anisotropic signature as resulting from active mantle flow associated with rift-related decompression melting and Cenozoic extension. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-28T03:21:11.861349-05:
      DOI: 10.1002/2016GC006729
       
  • Bubble formation and decrepitation control the CO2 content of
           olivine-hosted melt inclusions
    • Authors: J. Maclennan
      Abstract: The CO2 contents of olivine-hosted melt inclusions have previously been used to constrain the depth of magma chambers in basaltic systems. However, the vast majority of inclusions have CO2 contents which imply entrapment pressures that are significantly lower than those obtained from independent petrological barometers. Furthermore, a global database of melt inclusions compositions from low H2O settings, indicates that the distribution of saturation pressures varies surprisingly little between mid-ocean ridges, ocean islands and continental rift zones. 95% of the inclusions in the database have saturation pressures of 200 MPa or less, indicating that melt inclusion CO2, does not generally provide an accurate estimate of magma chamber depths. A model of the P-V-T-X evolution of olivine-hosted melt inclusions was developed so that the properties of the inclusion system could be tracked as the hosts follow a model P-T path. The models indicate that the principal control on the saturation of CO2 in the inclusion and the formation of vapour bubbles is the effect of post-entrapment crystallisation on the major element composition of the inclusions and how this translates into variation in CO2 solubility. The pressure difference between external melt and the inclusion is likely to be sufficiently high to cause decrepitation of inclusions in most settings. Decrepitation can account for the apparent mismatch between CO2-based barometry and other petrological barometers, and can also account for the observed global distribution of saturation pressures. Only when substantial post-entrapment crystallisation occurs can reconstructed inclusion compositions provide an accurate estimate of magma chamber depth. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-25T20:20:40.772611-05:
      DOI: 10.1002/2016GC006633
       
  • M2Di: Concise and efficient MATLAB 2D Stokes solvers using the Finite
           Difference method
    • Authors: Ludovic Räss; Thibault Duretz, Yury Y. Podladchikov, Stefan M. Schmalholz
      Abstract: Recent development of many multiphysics modeling tools reflects the currently growing interest for studying coupled processes in Earth Sciences. The core of such tools should rely on fast and robust mechanical solvers. Here we provide M2Di, a set of routines for 2D linear and power law incompressible viscous flow based on Finite Difference discretizations. The 2D codes are written in a concise vectorized MATLAB fashion and can achieve a time to solution of 22 seconds for linear viscous flow on 10002 grid points using a standard personal computer. We provide application examples spanning from finely resolved crystal-melt dynamics, deformation of heterogeneous power law viscous fluids to instantaneous models of mantle flow in cylindrical coordinates. The routines are validated against analytical solution for linear viscous flow with highly variable viscosity and compared against analytical and numerical solutions of power law viscous folding and necking. In the power law case, both Picard and Newton iterations schemes are implemented. For linear Stokes flow and Picard linearization, the discretization results in symmetric positive-definite matrix operators on Cartesian grids with either regular or variable grid spacing allowing for an optimized solving procedure. For Newton linearization, the matrix operator is no longer symmetric and an adequate solving procedure is provided. The reported performance of linear and power law Stokes flow are finally analyzed in terms of wall time. All MATLAB codes are provided and can readily be used for educational as well as research purposes. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-20T05:00:47.983454-05:
      DOI: 10.1002/2016GC006727
       
  • Biomarkers in Lake Van sediments reveal dry conditions in Eastern Anatolia
           during 110.000-10.000 years B.P.
    • Authors: Marie-Eve Randlett; Achim Bechtel, Marcel T.J. van der Meer, Francien Peterse, Thomas Litt, Nadine Pickarski, Ola Kwiecien, Mona Stockhecke, Bernhard Wehrli, Carsten J. Schubert
      Abstract: Lipid biomarkers were analyzed in Lake Van sediments covering the last 600 ka, with a focus on the period between 110 and 10 ka, when a broad maximum in pore water salinity as a relict from the past suggests dry conditions. The occurrence and distribution of biomarkers indicative for terrestrial plants (long-chain n-alkane C29), haptophyte algae (methyl alkenones C37) and halophilic archaea (archaeol) all point towards a dry climate in Lake Van region during this time interval. The hydrogen isotopic composition of C29 n-alkanes (δDC29) and C37 alkenones (δDC37) is enriched between MIS 4 and MIS 2, which is interpreted as a decrease in the regional ratio of precipitation to evaporation. Similarly, the low abundance of the acyclic glycerol dialkyl glycerol tetraether GDGT-0 relative to archaeol, quantified by the Archaeol and Caldarchaeol Ecometric (ACE) is assumed to reflect the presence of halophilic euryarchaeota adapted to high salinity water. The climate around Lake Van appears in phase with the Yammouneh basin 800 km southwest and Lake Urmia 250 km southeast of Lake Van over the last two glacial periods. The results highlight the potential of combining ACE, δDC29, and δDC37 for reconstructing salinity changes and regional precipitation to evaporation ratio from lake sediments. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-16T04:15:54.883525-05:
      DOI: 10.1002/2016GC006621
       
  • Mid-Brunhes magnetic excursions in marine isotope stages 9, 13, 14, and 15
           (286, 495, 540, and 590 ka) at North Atlantic IODP Sites U1302/3, U1305
           and U1306
    • Authors: J.E.T. Channell
      Abstract: Integrated Ocean Drilling Program (IODP) Site U1302/3 (Orphan Knoll, off Newfoundland) recorded magnetic excursions in marine isotope stages (MIS) 9a (at 286 ka) and 13a (at 495 ka). Sites U1306 and U1305 (Eirik Drift, off SE Greenland) record excursions in MIS 14a/b (at 540 ka) and 15b/c (at 590 ka). In the excursion intervals, magnetic measurements of continuous “u-channel” samples from multiple holes within site are augmented by measurements of cubic (8-cm3) discrete samples. The excursions lie in relative paleointensity (RPI) minima at each site and in RPI reference stacks, and correspond to dated intervals of 10Be overproduction in other deep-sea sediment records. Although observed at multiple holes at each site, and from u-channel and discrete samples, the excursions are not observed at all three sites, and often at only one of the three sites. Sporadic recording of these magnetic excursions, and excursions in general, is attributed to a combination of filtering by the process of acquisition of detrital remanent magnetization (DRM), post-depositional overprint of weak excursion magnetizations, the millennial or even centennial duration of directional excursions, and non-uniform sedimentation rates at these timescales in North Atlantic sediment drifts. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-13T05:06:23.809131-05:
      DOI: 10.1002/2016GC006626
       
  • Anisotropy in the lowermost mantle beneath the Indian Ocean Geoid Low from
           ScS splitting measurements
    • Authors: B. Padma Rao; M. Ravi Kumar, Arun Singh
      Abstract: The Indian Ocean Geoid Low (IOGL) to the south of Indian sub-continent is the world's largest geoid anomaly. In this study, we investigate the seismic anisotropy of the lowermost mantle beneath the IOGL by analyzing splitting of high quality ScS phases corrected for source and receiver side upper mantle anisotropy. Results reveal significant anisotropy (1.01 in the $D^{\prime\prime}$layer. The observed fast axis polarization azimuths in the ray coordinate system indicate a TTI (transverse isotropy with a tilted axis of symmetry) style of anisotropy. Lattice Preferred Orientation (LPO) deformation of the palaeo-subducted slabs experiencing high shear strain is a plausible explanation for the observed anisotropy beneath the IOGL. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-13T03:40:25.071435-05:
      DOI: 10.1002/2016GC006604
       
  • Shear localization in a mature mylonitic rock analogue during fast slip
    • Authors: M. Takahashi; M. van den Ende, A. R. Niemeijer, C. J. Spiers
      Abstract: Highly localized slip zones developed within ductile shear zones, such as pseudotachylite bands occurring within mylonitic fabric rocks, are frequently interpreted as evidence for earthquake nucleation and/or propagation within the ductile regime. To understand brittle/frictional shear localization processes in ductile shear zones and to relate these to earthquake nucleation and propagation, we performed tests with large change in velocity on a brine-saturated, 80:20 (wt.%) mixture of halite and muscovite gouge after forming a mature mylonitic structure through frictional-viscous flow. The direct effect a on shear strength that occurs in response to an instantaneous upward velocity-step is an important parameter in determining the nature of seismic rupture nucleation and propagation. We obtained reproducible results regarding low velocity mechanical behavior compared with previous work, but also obtained new insights into effects of sudden increases in slip velocity on localization and strength evolution, at velocities above a critical velocity Vc (∼20 μm/s). We found that once a ductile, mylonitic structure has developed in a shear zone, subsequent cataclastic deformation is consistently localized in a narrow zone. This switch to localized deformation is controlled by the imposed velocity, and becomes most apparent at velocities above Vc. In addition, the direct effect drops rapidly when the velocity exceeds Vc. This implies that slip can accelerate towards seismic velocities almost instantly and without much loss of fracture energy, once Vc is exceeded. Obtaining a measure for Vc in natural faults is therefore of key importance for understanding earthquake nucleation and propagation in the brittle-ductile transitional regime.
      PubDate: 2017-01-10T18:00:33.237004-05:
      DOI: 10.1002/2016GC006687
       
  • Mineralogical, geochemical and magnetic signatures of surface sediments
           from the Canadian Beaufort Shelf and Amundsen Gulf (Canadian Arctic)
    • Authors: Adriana Gamboa; Jean-Carlos Montero-Serrano, Guillaume St-Onge, André Rochon, Pierre-Arnaud Desiage
      Abstract: Mineralogical, geochemical, magnetic, and siliciclastic grain-size signatures of 34 surface sediment samples from the Mackenzie-Beaufort Sea Slope and Amundsen Gulf were studied in order to better constrain the redox status, detrital particle provenance, and sediment dynamics in the western Canadian Arctic. Redox-sensitive elements (Mn, Fe, V, Cr, Zn) indicate that modern sedimentary deposition within the Mackenzie-Beaufort Sea Slope and Amundsen Gulf took place under oxic bottom-water conditions, with more turbulent mixing conditions and thus a well-oxygenated water column prevailing within the Amundsen Gulf. The analytical data obtained, combined with multivariate statistical (notably, principal component and fuzzy c-means clustering analyses) and spatial analyses, allowed the division of the study area into four provinces with distinct sedimentary compositions: (1) the Mackenzie Trough-Canadian Beaufort Shelf with high phyllosilicate-Fe oxide-magnetite and Al-K-Ti-Fe-Cr-V-Zn-P contents; (2) Southwestern Banks Island, characterized by high dolomite-K-feldspar and Ca-Mg-LOI contents; (3) the Central Amundsen Gulf, a transitional zone typified by intermediate phyllosilicate-magnetite-K-feldspar-dolomite and Al-K-Ti-Fe-Mn-V-Zn-Sr-Ca-Mg-LOI contents; and (4) mud volcanoes on the Canadian Beaufort Shelf distinguished by poorly sorted coarse-silt with high quartz-plagioclase-authigenic carbonate and Si-Zr contents, as well as high magnetic susceptibility. Our results also confirm that the present-day sedimentary dynamics on the Canadian Beaufort Shelf is mainly controlled by sediment supply from the Mackenzie River. Overall, these insights provide a basis for future studies using mineralogical, geochemical, and magnetic signatures of Canadian Arctic sediments in order to reconstruct past variations in sediment inputs and transport pathways related to late Quaternary climate and oceanographic changes. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-03T03:01:02.726976-05:
      DOI: 10.1002/2016GC006477
       
  • TerraFERMA: The Transparent Finite Element Rapid Model Assembler for
           multiphysics problems in Earth sciences
    • Authors: Cian R. Wilson; Marc Spiegelman, Peter E. van Keken
      Abstract: We introduce and describe a new software infrastructure TerraFERMA, the\emph{Transparent Finite Element Rapid Model Assembler}, for the rapid and reproducible description and solution of coupled multiphysics problems. The design of TerraFERMA is driven by two computational needs in Earth sciences. The first is the need for increased flexibility in both problem description and solution strategies for coupled problems where small changes in model assumptions can lead to dramatic changes in physical behavior. The second is the need for software and models that are more transparent so that results can be verified, reproduced, and modified in a manner such that the best ideas in computation and Earth science can be more easily shared and reused. TerraFERMA leverages three advanced open-source libraries for scientific computation that provide high-level problem description (FEniCS), composable solvers for coupled multiphysics problems (PETSc), and an options handling system (SPuD) that allows the hierarchical management of all model options. TerraFERMA integrates these libraries into an interface that organizes the scientific and computational choices required in a model into a single options file from which a custom compiled application is generated and run. Because all models share the same infrastructure, models become more reusable and reproducible, while still permitting the individual researcher considerable latitude in model construction. TerraFERMA solves partial differential equations using the finite element method. It is particularly well suited for nonlinear problems with complex coupling between components. TerraFERMA is open-source and available at http://terraferma.github.io, which includes links to documentation and example input files. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-03T03:00:41.43675-05:0
      DOI: 10.1002/2016GC006702
       
  • Issue Information
    • Pages: 471 - 472
      PubDate: 2017-03-15T06:12:11.422377-05:
      DOI: 10.1002/ggge.21118
       
  • Rapid variations in fluid chemistry constrain hydrothermal phase
           separation at the Main Endeavour Field
    • Authors: Brooke Love; Marvin Lilley, David Butterfield, Eric Olson, Benjamin Larson
      Abstract: Previous work at the Main Endeavour Field (MEF) has shown that chloride concentration in high-temperature vent fluids has not exceeded 510 mmol/kg (94% of seawater), which is consistent with brine condensation and loss at depth, followed by upward flow of a vapor phase toward the seafloor. Magmatic and seismic events have been shown to affect fluid temperature and composition and these effects help narrow the possibilities for sub-surface processes. However, chloride-temperature data alone are insufficient to determine details of phase separation in the upflow zone. Here we use variation in chloride and gas content in a set of fluid samples collected over several days from one sulfide chimney structure in the MEF to constrain processes of mixing and phase separation. The combination of gas (primarily magmatic CO2 and seawater-derived Ar) and chloride data, indicate that neither variation in the amount of brine lost, nor mixing of the vapor phase produced at depth with variable quantities of (i) brine or (ii) altered gas rich seawater that has not undergone phase separation, can explain the co-variation of gas and chloride content. The gas-chloride data require additional phase separation of the ascending vapor-like fluid. Mixing and gas partitioning calculations show that near-critical temperature and pressure conditions can produce the fluid compositions observed at Sully vent as a vapor-liquid conjugate pair or as vapor-liquid pair with some remixing, and that the gas partition coefficients implied agree with theoretically predicted values. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:28:27.915162-05:
      DOI: 10.1002/2016GC006550
       
  • The biokarst system and its carbon sinks in response to pH changes: A
           simulation experiment with microalgae
    • Authors: Tengxiang Xie; Yanyou Wu
      Abstract: This study aims to explore the changes in a microalgal biokarst system as a potential carbon sink system in response to pH changes. The bidirectional isotope labeling method and mass balance calculation were adopted in a simulated biokarst environment with a series of set pH conditions and three microalgal species. Three key processes of the microalgal biokarst system, including calcite dissolution, CaCO3 reprecipitation, and inorganic carbon assimilation by microalgae, were completely quantitatively described. The combined effects of chemical dissolution and species-specific bio-dissolution caused a decrease in overall dissolution rate when the pH increased from 7 to 9. CaCO3 reprecipitation and the utilization of dissolved inorganic carbon originating from calcite dissolution decreased when the pH increased from 7 to 9. The three processes exhibited different effects in changing the CO2 atmosphere. The amount of photosynthetic carbon sink was larger at high pH values than at low pH values. However, the CO2 sequestration related to the biokarst process (biokarst carbon sink) increased with decreasing pH. Overall, the total amount of sequestered CO2 produced by the biokarst system (CaCO3-CO2-microalgae) shows a minimum at a specific pH then increases with decreasing pH. Therefore, various processes and carbon sinks in the biokarst system are sensitive to pH changes, and biokarst processes play an important negative feedback role in the release of CO2 by acidification. The results also suggest that the carbon sink associated with carbonate weathering cannot be neglected when considering the global carbon cycle on the scale of thousands of years (
      PubDate: 2016-12-20T13:21:08.2333-05:00
      DOI: 10.1002/2016GC006628
       
 
 
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