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Publisher: American Geophysical Union (AGU)   (Total: 17 journals)   [Sort by number of followers]

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 25, SJR: 2.56, h-index: 69)
Geophysical Research Letters     Full-text available via subscription   (Followers: 56, SJR: 3.493, h-index: 157)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 7, SJR: 3.239, h-index: 119)
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Paleoceanography     Full-text available via subscription   (Followers: 3, SJR: 3.22, h-index: 88)
Radio Science     Full-text available via subscription   (Followers: 4, SJR: 0.959, h-index: 51)
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Water Resources Research     Full-text available via subscription   (Followers: 66, SJR: 2.189, h-index: 121)
Journal Cover Geochemistry, Geophysics, Geosystems
  [SJR: 2.56]   [H-I: 69]   [25 followers]  Follow
   Full-text available via subscription Subscription journal
   ISSN (Online) 1525-2027
   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • Phosphorites, Co‐rich Mn nodules, and Fe‐Mn crusts from
           Galicia Bank, NE Atlantic: Reflections of Cenozoic tectonics and
    • Abstract: A wide variety of marine mineral deposits were recovered from 750 to 1400 m water depths on Galicia Bank, Iberian margin. Mineral deposits include: (1) carbonate fluorapatite phosphorite slabs and nodules that replaced limestone and preserved original protolith fabric. (2) Ferromanganese vernadite crusts with high Mn and Fe (Mn/Fe = 1) contents, and thick stratabound layers consisting mainly of Mn (up to 27% MnO) and Fe (15% Fe2O3), which impregnated and replaced the phosphorite. (3) Co‐rich Mn nodules are composed of romanechite and todorokite laminae. Mn‐rich layers (up to 58% MnO) contain up to 1.8% Co. (4) Goethite nodules with Fe up to 67% Fe2O3 have low Mn and trace metals. We interpret this mineralization paragenesis to be related to major changes in oceanographic and tectonic regimes. Three phosphatization generations formed hardgrounds dated by 87Sr/86Sr isotopes as late Oligocene, early Miocene and latest early Miocene. During the latest early Miocene, the hardground was fractured and breached due to regional intraplate tectonism, which was coeval with a widespread regional erosional unconformity. The stratabound layers and Co‐rich manganese nodules were derived from low‐temperature geothermally driven hydrothermal fluids, with fluid conduits along reactivated faults. During middle and late Miocene, the introduction of vigorous deep‐water flow from the Arctic generated growth of hydrogenetic ferromanganese crusts. Finally, growth of diagenetic Fe‐rich nodules (late Pliocene) was promoted by the introduction of hypersaline Mediterranean Outflow Water into the Atlantic Ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-29T08:33:37.525134-05:
      DOI: 10.1002/2015GC005861
  • High‐resolution images of tremor migrations beneath the Olympic
           Peninsula from stacked array of arrays seismic data
    • Authors: Yajun Peng; Allan M. Rubin
      Abstract: Episodic tremor and slip (ETS) in subduction zones is generally interpreted as the manifestation of shear slip near the base of earthquake‐generating portion of the plate interface. Here we devise a new method of cross‐correlating stacked Array of Arrays seismic data that provides greatly improved tremor locations, a proxy for the underlying slow slip, beneath the Olympic Peninsula. This increased resolution allows us to image many features of tremor that were not visible previously. We resolve the spatial transition between the rupture zones of the inter‐ETS and major ETS episodes in 2010, suggesting stress redistribution by the former. Most tremor migrations propagated along the slowly‐advancing main tremor front during both the inter‐ETS and the major ETS episodes, even though the main front of the former deviated strongly from its usual (along‐dip) orientation. We find a distinct contrast between along‐dip rupture extent of large‐scale rapid tremor reversals (RTRs) to the south and that to the north in our study region that anti‐correlates with the locations of inter‐ETS events. These RTRs originate from the main front, similar to smaller‐scale RTRs previously observed at high resolution, and many start by propagating along the main front. This could be consistent with RTRs being triggered by a cascading failure of brittle asperities. After initiation, the RTRs repeatedly occupy the same source region, and the early repetitions appear not to be tidally driven. Their stress drop may come from continuing fault weakening processes within the tremor zone, or loading by aseismic slip in surrounding regions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-28T18:20:15.181917-05:
      DOI: 10.1002/2015GC006141
  • A MATLAB toolbox and Excel workbook for calculating the densities, seismic
           wave speeds, and major element composition of minerals and rocks at
           pressure and temperature
    • Authors: Geoffrey A. Abers; Bradley R. Hacker
      Abstract: To interpret seismic images, rock seismic velocities need to be calculated at elevated pressure and temperature for arbitrary compositions. This Technical Report describes an algorithm, software and data to make such calculations from the physical properties of minerals. It updates a previous compilation and Excel® spreadsheet (Hacker and Abers, 2004) and includes new MATLAB® tools for the calculations. The database of 60 mineral endmembers includes all parameters needed to estimate density and elastic moduli for many crustal and mantle rocks at conditions relevant to the upper few hundred km of Earth. The behavior of α‐ and β‐quartz is treated as a special case, owing to its unusual Poisson's ratio and thermal expansion that vary rapidly near the α‐β transition. The MATLAB tools allow integration of these calculations into a variety of modeling and data analysis projects. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-28T18:18:51.469615-05:
      DOI: 10.1002/2015GC006171
  • Dehydration‐induced porosity waves and episodic tremor and slip
    • Authors: Rob M. Skarbek; Alan W. Rempel
      Abstract: Episodic tremor and slip (ETS) along the subduction interface take place where there is abundant evidence for elevated, near‐lithostatic pore pressures, at sufficiently great depths (30‐45 km) that chemical dehydration reactions must act as their dominant source. We simulate fluid and heat flow while tracking the location of a vertically oriented, one‐dimensional column of material as it subducts through the slow slip and tremor zone. The material in the column is transformed through a pressure and temperature dependent dehydration reaction that we describe with a generalized nonlinear kinetic rate law. Column deformation is largely dominated by viscous creep, with a closure rate that depends linearly on porosity. This behavior causes the dehydration reaction to generate traveling porosity waves that transport increased fluid pressures within the slow slip region. To explore the possibility that the observed periodicity of slow slip and tremor in subduction zones can be explained by the migration of such porosity waves, we derive a dispersion relation that accurately describes our numerical results. We also obtain an expression for how the thickness of the dehydrating layer is expected to vary as a function of the parameters in the reaction rate law. Although the amplitudes of pore pressure perturbations rival those that are produced by known external forcings (e.g. tides or passing surface waves), our analysis suggests that given reasonable estimates of rock viscosity, permeabilities in the range 6.5˟10−15 to 5˟10−10 m2 are required for porosity wavetrains to form at periods comparable to those of slow slip and tremor. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-27T09:42:12.787951-05:
      DOI: 10.1002/2015GC006155
  • Comment on “The insular shelves of the Faial‐Pico Ridge
           (Azores archipelago): A morphological record of its evolution” by
           Quartau et al
    • Authors: F.O. Marques; A. Hildenbrand, V. Zanon, T. Boulesteix
      Abstract: Several processes concur to shape an oceanic volcanic island, in particular the insular shelves, but the dominant process will be the one with the highest rate for a given period of time. Therefore, one has to estimate rates in order to conclude for the dominant process. We take advantage of the contrasting tectonic settings of two islands in the Azores, Pico (on the active diffuse Nubia‐Eurasia plate boundary) and Santa Maria (on the oldest and inactive eastern end of the Azores Plateau), in order to estimate two main rates and their effects on shaping insular shelves: subsidence and wave erosion rates. We conclude that, for subsidence rates ≥ 1 mm/yr and lava flow dips ≤ 5º, as in Pico Island, the subsidence rate can dominate. Despite the likely dominant effect of island subsidence, Quartau et al. (2015) recognized its existence in Pico but did not estimate its magnitude or use the subsidence rate to discuss the wave erosion rates reported in the paper. Therefore, the wave erosion rates reported in Quartau et al. (2015) may be greatly overestimated by incorporating indiscriminately two rates, island subsidence and wave erosion. The effects of subsidence on shaping insular shelves as discussed here can potentially be applied to other subsiding islands mostly made of shield volcanoes, like the Big Island in Hawaii. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-27T09:40:15.28659-05:0
      DOI: 10.1002/2015GC006052
  • Reply to comment by Marques et al. on “The insular shelves of the
           Faial‐Pico Ridge (Azores archipelago): A morphological record of its
    • Abstract: Shelves of the Azores volcanic islands were essentially formed by surf erosion of their slopes as sea level oscillated during glacial‐interglacial stages. A variety of secondary processes may have changed their primary erosional morphology, and in volcanically active islands such as Pico, volcanic progradation can dominate locally. To understand the actual roles of the different processes, there is a need to integrate all information available above and below sea level. In this reply we summarize onshore and offshore evidence, which allows us to reject the hypothesis that the Azorean islands' shelves formed mainly by subsidence, rather than by wave erosion. Assuming that these shelves started forming as soon as the main phase of volcanism decreased; their widths and shelf break depths can be used to infer, respectively, rates of shelf erosion and subsidence of the islands. The oldest edifices of the central group of islands in the Azores have subsided, but do not have shelf edges deeper than 400 m. A rough estimate of their subsidence rate can be calculated assuming that the shelf edges were formed at the lowstand that immediately followed their main volcanic phase. The calculations reveal average subsidence rates
      PubDate: 2016-01-27T03:46:10.068038-05:
      DOI: 10.1002/2015GC006180
  • Potential groundwater and heterogeneous heat source contributions to ice
           sheet dynamics in critical submarine basins of East Antarctica
    • Authors: Brad T. Gooch; Duncan A. Young, Donald D. Blankenship
      Abstract: We present the results of two numerical models describing contributions of groundwater and heterogeneous heat sources to ice dynamics directly relevant to basal processes in East Antarctica. A two‐phase, one‐dimensional hydrothermal model demonstrates the importance of groundwater flow in vertical heat flux advection near the ice‐bed interface. Typical, conservative vertical components of groundwater volume fluxes (from either topographical gradients or vertically channeled flow) on the order of ±1‐10 mm/yr can alter vertical heat flux by ±50‐500 mW/m2 given parameters typical for the interior of East Antarctica. This heat flux has the potential to produce considerable volumes of meltwater depending on basin geometry and geothermal heat production. A one‐dimensional hydromechanical model demonstrates that groundwater is mainly recharged into saturated, partially poroelastic (i.e. vertical stress only; not coupled to a deformation equation) sedimentary aquifers during ice advance. During ice retreat, groundwater discharges into the ice‐bed interface, which may contribute to water budgets on the order of 0.1‐1 mm/yr. We also present an estimated map of potentially heterogeneous heat flow provinces using radiogenic heat production data from East Antarctica and southern Australia, calculated sedimentary basin depths, and radar‐derived bed roughness. These are overlaid together to delineate the areas of greatest potential effect from these modeled processes on the ice sheet dynamics of the East Antarctic Ice Sheet. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-25T11:15:44.148768-05:
      DOI: 10.1002/2015GC006117
  • Comment on “Determination of low B/Ca ratios in carbonates using
           ICP‐QQQ)” by S. D. Fernandez et al.
    • Abstract: The use of 46Ca to determine the amount of Ca will have large uncertainties in B/Ca ratios, due to large uncertainty in isotope abundance of 46Ca. Also the large discrepancy of results between ICP‐QQQ MS data and those obtained by TIMS and SIMS cannot be explained by the tailing effect and isobaric interference from 11C at 11B in TIMS. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-25T03:15:54.939174-05:
      DOI: 10.1002/2015GC006139
  • Reply to comments by S. K. Aggarwal and C.‐F. You on
           “Determination of low B/Ca ratios in carbonates using
    • PubDate: 2016-01-25T02:57:22.324903-05:
      DOI: 10.1002/2015GC006187
  • Paleo‐methane emissions recorded in foraminifera near the landward
           limit of the gas hydrate stability zone offshore western Svalbard
    • Authors: Giuliana Panieri; Carolyn A. Graves, Rachael H. James
      Abstract: We present stable isotope and geochemical data from four sediment cores from west of Prins Karls Forland (ca. 340 m water depth), offshore western Svalbard, recovered from close to sites of active methane seepage, as well as from shallower water depths where methane seepage is not presently observed. Our analyses provide insight into the record of methane seepage in an area where ongoing ocean warming may be fueling the destabilization of shallow methane hydrate. The δ13C values of benthic and planktonic foraminifera at the methane seep sites show distinct intervals with negative values (as low as ‐27.8 ‰) that do not coincide with the present‐day depth of the Sulfate Methane Transition Zone (SMTZ). These intervals are interpreted to record long‐term fluctuations in methane release at the present‐day landward limit of the Gas Hydrate Stability Zone (GHSZ). Shifts in the radiocarbon ages obtained from planktonic foraminifera towards older values are related to methane‐derived authigenic carbonate overgrowths of the foraminifer tests, and prevent us from establishing the chronology of seepage events. At shallower water depths where seepage is not presently observed no record of past methane seepage is recorded in foraminifera from sediments spanning the last 14 ka cal. BP (14C‐AMS dating). δ13C values of foraminifera carbonate tests appear to be much more sensitive to methane seepage than other sediment parameters. By providing nucleation sites for authigenic carbonate precipitation, foraminifera thus record the position of even a transiently stable SMTZ, which is likely to be characteristic of the temporally variable methane fluxes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-23T11:11:04.858938-05:
      DOI: 10.1002/2015GC006153
  • Erosion rate study at the Allchar deposit (Macedonia) based on radioactive
           and stable cosmogenic nuclides (26Al,36Cl, 3He, and 21Ne)
    • Abstract: This paper focuses on constraining the erosion rate in the area of the Allchar Sb‐As‐Tl‐Au deposit (Macedonia). It contains the largest known reserves of lorandite (TlAsS2), which is essential for the LORanditeEXperiment (LOREX), aimed at determining the long‐term solar neutrino flux. Because the erosion history of the Allchar area is crucial for the success of LOREX, we applied terrestrial in situ cosmogenic nuclides including both radioactive (26Al and 36Cl) and stable (3He and 21Ne) nuclides in quartz, dolomite/calcite, sanidine and diopside. The obtained results suggest that there is accordance in the values obtained by applying 26Al, 36Cl and 21Ne for around 85% of the entire sample collection, with resulting erosion rates varying from several tens of m/Ma to ∼165 m/Ma. The samples from four locations (L‐8 CD, L1b/R, L1c/R and L‐4/ADR) give erosion rates between 300 m/Ma and 400 m/Ma. Although these localities reveal remarkably higher values, which may be explained by burial events that occurred in part of Allchar, the erosion rate estimates mostly range between 50 m/Ma and 100 m/Ma. This range further enables us to estimate the vertical erosion rate values for the two main ore bodies Crven Dol and Centralni Deo. We also estimate that the lower and upper limits of average paleo‐depths for the ore body Centralni Deo from 4.3 Ma to the present are 250‐290 m and 750‐790 m, respectively, whereas the upper limit of paleo‐depth for the ore body Crven Dol over the same geological age is 860 m. The estimated paleo‐depth values allow estimating the relative contributions of 205Pb derived from pp‐neutrino and fast cosmic‐ray muons, respectively, which is an important prerequisite for the LOREX experiment. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-23T11:06:22.399239-05:
      DOI: 10.1002/2015GC006054
  • Crustal anisotropy beneath Pacific Ocean islands from harmonic
           decomposition of receiver functions
    • Authors: Tolulope Morayo Olugboji; Jeffrey Park
      Abstract: Crustal anisotropy beneath ocean islands can be attributed to preferentially aligned minerals, cracks, or dike structures. Stacked with harmonic weighting, receiver functions from permanent ocean‐island stations display evidence of strong and distinct anisotropy parameters in the underlying crust and underplated layer. We analyze data for eleven IRIS‐GSN stations in the Pacific Ocean. We observe the prevalence of two‐lobed receiver function (RF) amplitude variations with back‐azimuth, consistent with “slow” tilted‐axis anisotropy. In most cases the anisotropy is accommodated in the underplated crust. Synthetic modeling of a representative station indicates that the strength of anisotropy of Vp=10% and Vs=5% is possible. The strike direction of the inferred symmetry axis tends to align with plate motion, with some scatter. At stations in the northwest Pacific i.e. KWAJ, TARA, and WAKE, the strike direction of the symmetry axis aligns with plate motion at the time of volcano emplacement. Beneath station POHA and the closest stations to the present‐day Hawaiian hotspot, alignment of the symmetry axis is almost orthogonal to the plate motion. We attribute the crustal anisotropy to the preferred alignment of dike structures that transported asthenospheric magma toward the seafloor volcanic edifice. Our results suggest that the thermal‐plume origin for ocean islands must be supplemented by tectonic‐stress heterogeneities that allow magma to penetrate the lithosphere via fractures. Magma‐transport fractures should align normal to the least‐compressive direction, which are predicted by theoretical models to align approximately with plate motion at the time of emplacement. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-20T18:06:29.002305-05:
      DOI: 10.1002/2015GC006166
  • Eocene to Pleistocene magmatic evolution of the Delarof Islands, Aleutian
    • Authors: Allen J. Schaen; Brian R. Jicha, Suzanne M. Kay, Brad S. Singer, Ashley Tibbetts
      Abstract: The Delarof Islands in the Aleutian Arc near 179º W record ∼37 million years of discontinuous arc magmatism along a SW‐NE cross‐arc transect from near the trench to the active volcanic front. Geochemical and geochronologic data from the pre‐Pleistocene volcanic record in this region are limited and the 40Ar/39Ar, isotopic, and trace element data presented here are the first from units older than the Pleistocene‐Holocene volcanoes (Tanaga, Gareloi). Twenty‐two new 40Ar/39Ar ages establish a temporal framework for geochemical data and reveal that magmatism in the Delarof region was coincident with two arc‐wide magmatic flare ups in the late Eocene/early Oligocene and latest Miocene/Pliocene. Mafic lavas and plutons in the southern Delarofs give 40Ar/39Ar plateau ages ranging from 36.8 ± 0.2 to 26.9 ± 0.6 Ma on Amatignak Island and 37.0 ± 0.2 to 29.3 ± 1.0 Ma on Ulak Island. To the north 25 km, 40Ar/39Ar ages from the central Delarof Islands, Kavalga, Ogliuga, and Skagul are late Miocene (6.28 ± 0.04 Ma) to Pliocene (4.77 ± 0.18 Ma) with younger ages to the northeast. A significant transition in arc chemistry occurs in the Pleistocene where lavas from active volcanoes Gareloi and Tanaga exhibit higher sediment and hydrous fluid signatures (Th/La, Cs/Ta, La/Sm, LILE abundances) and lower 143Nd/144Nd than older Delarof Island units closer to the trench. Similar findings from Eocene‐Miocene lavas from Amchitka to Adak suggest that a previously minor sediment melt component become more pronounced in the Quaternary. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-20T17:50:35.731893-05:
      DOI: 10.1002/2015GC006067
  • Effects of chaotic advection on the timescales of cooling and
           crystallization of magma bodies at mid crustal levels
    • Authors: Maurizio Petrelli; Kamal El Omari, Yves Le Guer, Diego Perugini
      Abstract: We numerically define the thermo‐chemical evolution of a subduction‐related crystal‐bearing magmatic mass at mid‐crustal levels (0.7 GPa, 20–25 km). Two main dynamic mechanisms are considered: 1) a pure buoyancy driven system where the convective flow is induced by density changes during magma cooling; 2) a buoyancy driven convective system governed by chaotic advection. The non‐Newtonian rheology of natural magmas is taken into account linking the Herschel–Bulkley formulation with the results of fractional crystallization experiments of magmas with the same composition and at the same conditions of temperature and pressure of the studied system. The latent heat of crystallization is also considered in order to address the thermal release in the system induced by the crystallization. Results indicate that the development of chaotic advection generates a complex thermo‐chemical evolution of the system speeding up the crystallization process and the timing required to reach the jamming condition relative to the pure buoyancy driven convective system (nearly 2 times faster). Our results have important implications for both the rheological history of the magmatic body and the refilling of shallower magmatic systems. In particular: 1) a time‐dependent composition ranging from basalt to andesite can be extracted from an initial basaltic magmatic batch; 2) at the attainment of the maximum packing fraction (i.e. just before the jamming condition), homogeneous andesitic melts can be potentially extracted from the system; 3) the development of chaotic advection within the system allows for the extraction of andesitic melt in shorter times compared to a buoyancy dominated system. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-20T17:49:35.516115-05:
      DOI: 10.1002/2015GC006109
  • Issue Information
    • PubDate: 2016-01-20T02:49:11.551909-05:
      DOI: 10.1002/ggge.20568
  • New insights into the tectonic inversion of North Canterbury and the
           regional structural context of the 2010–2011 Canterbury earthquake
           sequence, New Zealand
    • Authors: Philip M. Barnes; Francesca C. Ghisetti, Andrew R. Gorman
      Abstract: The 2010‐2011 Canterbury earthquake sequence highlighted the existence of previously unknown active faults beneath the North Canterbury plains and Pegasus Bay, South Island, New Zealand. We provide new insights into the geometry and kinematics of ongoing deformation by analysing marine seismic data to produce new maps of regional faults and cross‐sectional reconstructions of deformation history. Active faulting and folding extends up to 30 km offshore, and involves reactivation of sets of Late Cretaceous‐Paleogene normal faults under NW‐SE tectonic compression. The active faults consist predominantly of NE‐SW striking, SE‐dipping reverse faults, and less commonly E‐W to NW‐SE faults suitably oriented for strike‐slip reactivation. Additionally, newly developing reverse faults obliquely segment and overprint the inherited basement fabric and impose geometric and kinematic complexities revealed by mapping and reverse displacement profiles of markers. The Quaternary reverse slip rates decrease from 0.1‐0.3 mm/yr beneath northern Pegasus Bay to
      PubDate: 2016-01-19T10:48:44.321406-05:
      DOI: 10.1002/2015GC006069
  • Subduction of a buoyant plateau at the Manila Trench: Tomographic evidence
           and geodynamic implications
    • Authors: Jianke Fan; Dapeng Zhao, Dongdong Dong
      Abstract: We determined P‐wave tomographic images by inverting a large number of arrival‐time data from 2,749 local earthquakes and 1,462 teleseismic events, which are used to depict the three‐dimensional morphology of the subducted Eurasian Plate along the northern segment of the Manila Trench. Dramatic changes in the dip angle of the subducted Eurasian Plate are revealed from the north to the south, being consistent with the partial subduction of a buoyant plateau beneath the Luzon Arc. Slab tears may exist along the edges of the buoyant plateau within the subducted plate induced by the plateau subduction, and the subducted lithosphere may be absent at depths greater than 250 km at ∼19°N and ∼21°N. The subducted buoyant plateau is possibly oriented toward NW‐SE, and the subducted plate at ∼21°N is slightly steeper than that at ∼19°N. These results may explain why the western and eastern volcanic chains in the Luzon Arc are separated by ∼50 km at ∼18°N, whereas they converge into a single volcanic chain northward, which may be related to the oblique subduction along the Manila Trench caused by the northwestern movement of the Philippine Sea Plate. A low‐velocity zone is revealed at depths of 20‐200 km beneath the Manila Accretionary Prism at ∼22°N, suggesting that the subduction along the Manila Trench may stop there and the collision develops northward. The Taiwan Orogeny may originate directly from the subduction of the buoyant plateau, because the initial time of the Taiwan Orogeny is coincident with that of the buoyant plateau subduction. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-19T10:46:01.626482-05:
      DOI: 10.1002/2015GC006201
  • Linkages between mineralogy, fluid chemistry, and microbial communities
           within hydrothermal chimneys from the Endeavor Segment, Juan de Fuca Ridge
    • Authors: T. J. Lin; H. C. Ver Eecke, E. A. Breves, M. D. Dyar, J. W. Jamieson, M. D. Hannington, H. Dahle, J. L. Bishop, M. D. Lane, D. A. Butterfield, D. S. Kelley, M. D. Lilley, J. A. Baross, J. F. Holden
      Abstract: Rock and fluid samples were collected from three hydrothermal chimneys at the Endeavour Segment, Juan de Fuca Ridge to evaluate linkages among mineralogy, fluid chemistry, and microbial community composition within the chimneys. Mössbauer, mid‐infrared thermal emission, and visible‐near infrared spectroscopies were utilized for the first time to characterize vent mineralogy, in addition to thin‐section petrography, x‐ray diffraction, and elemental analyses. A 282°C venting chimney from the Bastille edifice was composed primarily of sulfide minerals such as chalcopyrite, marcasite, and sphalerite. In contrast, samples from a 300°C venting chimney from the Dante edifice and a 321°C venting chimney from the Hot Harold edifice contained a high abundance of the sulfate mineral anhydrite. Geochemical modeling of mixed vent fluids suggested the oxic‐anoxic transition zone was above 100°C at all three vents, and that the thermodynamic energy available for autotrophic microbial redox reactions favored aerobic sulfide and methane oxidation. As predicted, microbes within the Dante and Hot Harold chimneys were most closely related to mesophilic and thermophilic aerobes of the Beta‐ and Gammaproteobacteria and sulfide‐oxidizing autotrophic Epsilonproteobacteria. However, most of the microbes within the Bastille chimney were most closely related to mesophilic and thermophilic anaerobes of the Deltaproteobacteria, especially sulfate reducers, and anaerobic hyperthermophilic archaea. The predominance of anaerobes in the Bastille chimney indicated that other environmental factors promote anoxic conditions. Possibilities include the maturity or fluid flow characteristics of the chimney, abiotic Fe2+ and S2‐ oxidation in the vent fluids, or O2 depletion by aerobic respiration on the chimney outer wall. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-11T17:51:46.606565-05:
      DOI: 10.1002/2015GC006091
  • Small‐scale spatial variation in near‐surface turbidites
           around the JFAST site near the Japan Trench
    • Authors: Shuro Yoshikawa; Toshiya Kanamatsu, Takafumi Kasaya
      Abstract: This paper aims to improve our understanding of the depositional processes associated with turbidites related to recent earthquake events. A series of short sediment cores (ca. 20–30 cm long) were recovered from the landward slope of the Japan Trench around JFAST (Japan Trench Fast Drilling Project) site C0019 by a remotely operated vehicle, KAIKO 7000 II, and the sample sites were accurately located using an LBL (long base line) acoustic navigation system. The properties of the cores were analyzed using visual observations, soft X‐ray radiographs, smear slides, measurement of anisotropy of magnetic susceptibility, and analysis of radioactive elements (134Cs, 137Cs, and excess 210Pb). For the first time, small‐scale (ca. 200–1000 m) spatial variations in recent earthquake‐triggered deep‐sea turbidites, the formation of which was probably linked to the 2011 Tohoku‐oki earthquake, are described. We also examine the submarine landslide that probably generated the sediment unit below the turbidites, which is thought to be an important process in the study area. The spatial distribution and characteristics of the near‐surface seismoturbidite obtained immediately after the earthquake, presented here, will enable precise calibration of offshore evidence of recent earthquakes, and thus facilitate the use of the sedimentary archive for paleoseismic interpretations. Furthermore, although sampling for turbidite seismology on steep slopes has not been widely performed previously, our results suggest that the recent event deposits may be continuously tracked from the slope to the basin using a combination of the present sampling method and conventional large‐scale investigation techniques. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-11T17:51:04.299981-05:
      DOI: 10.1002/2015GC006114
  • Temperature fluctuation of the Iceland mantle plume through time
    • Authors: Holly E. Spice; J. Godfrey Fitton, Linda A. Kirstein
      Abstract: The newly developed Al‐in‐olivine geothermometer was used to find the olivine‐Cr‐spinel crystallisation temperatures of a suite of picrites spanning the spatial and temporal extent of the North Atlantic Igneous province (NAIP), which is widely considered to be the result of a deep‐seated mantle plume. Our data confirm that start‐up plumes are associated with a pulse of anomalously hot mantle over a large spatial area before becoming focused into a narrow upwelling. We find that the thermal anomaly on both sides of the province at Baffin Island/West Greenland and the British Isles at ∼61 Ma across an area ∼2000 km in diameter was uniform, with Al‐in‐olivine temperatures up to ∼300°C above that of average mid‐ocean ridge basalt (MORB) primitive magma. Furthermore, by combining our results with geochemical data and existing geophysical and bathymetric observations, we present compelling evidence for long‐term (>107 year) fluctuations in the temperature of the Iceland mantle plume. We show that the plume temperature fell from its initial high value during the start‐up phase to a minimum at about 35 Ma, and that the mantle temperature beneath Iceland is currently increasing. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-08T19:14:35.14319-05:0
      DOI: 10.1002/2015GC006059
  • Surface Slip during Large Owens Valley Earthquakes
    • Abstract: The 1872 Owens Valley earthquake is the third largest known historical earthquake in California. Relatively sparse field data and a complex rupture trace, however, inhibited attempts to fully resolve the slip distribution and reconcile the total moment release. We present a new, comprehensive record of surface slip based on lidar and field investigation, documenting 162 new measurements of laterally and vertically displaced landforms for 1872 and prehistoric Owens Valley earthquakes. Our lidar analysis uses a newly developed analytical tool to measure fault slip based on cross‐correlation of sub‐linear topographic features and produce a uniquely shaped probability density function (PDF) for each measurement. Stacking PDFs along strike to form cumulative offset probability distribution plots (COPDs) highlights common values corresponding to single‐ and multiple‐event displacements. Lateral offsets for 1872 vary systematically from ∼1.0–6.0 m and average 3.3 ± 1.1 m. Vertical offsets are predominantly east‐side down between ∼0.1–2.4 m, with a mean of 0.8 ± 0.5 m. The average lateral‐to‐vertical ratio compiled at specific sites is ∼6:1. Summing displacements across sub‐parallel, overlapping ruptures implies a maximum of 7–11 m and net average of 4.4 ± 1.5 m, corresponding to a geologic Mw ∼7.5 for the 1872 event. We attribute progressively higher‐offset lateral COPD peaks at 7.1 ± 2.0 m, 12.8 ± 1.5 m, and 16.6 ± 1.4 m to three earlier large surface ruptures. Evaluating cumulative displacements in context with previously dated landforms in Owens Valley suggests relatively modest rates of fault slip, averaging between ∼0.6–1.6 mm/yr (1σ) over the late Quaternary. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-08T19:13:33.397692-05:
      DOI: 10.1002/2015GC006033
  • Geochemical Behavior of Dissolved Manganese in the East China Sea:
           Seasonal Variation, Estuarine Removal, and Regeneration under suboxic
    • Abstract: To better understand the geochemical cycle of dissolved manganese (Mn) in the East China Sea (ECS), the distribution of dissolved Mn across the ECS was investigated during three field studies in 2011 (May, August, and November). The concentration of dissolved Mn decreased across the ECS with distance from the coast. Mn‐rich ECS shelf waters could export to the Kuroshio waters, and had the potential to influence the northwest Pacific Ocean as well as the Japan Sea. The Kuroshio waters were devoid of dissolved Mn, so its incursion could be tracked as it entered the ECS continental shelf region (approximately 50 m isobath). Seasonal variations of dissolved Mn in the ECS were significant, with the highest concentrations occurring in summer. Dissolved Mn in the Changjiang Estuary was non‐conservative, and significant quantities were removed by net sorption onto suspended particulate matter. A model describing the sorption processes was applied to data for the Changjiang Estuary. Regeneration of dissolved Mn took place in near bottom waters of the suboxic zone in August 2011, following extensive consumption of oxygen. The benthic flux of dissolved Mn was estimated based on Mn concentrations in the overlying waters and the near bottom waters. A preliminary box model was established to develop a dissolved Mn budget for the ECS. Based on the dissolved Mn content in the ECS and the total input flux, a residence time of 76–350 days for dissolved Mn in the ECS was inferred. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-08T19:10:52.353848-05:
      DOI: 10.1002/2015GC006128
  • Stable and clumped isotopes in shell carbonates of land snails Cathaica
           sp. and Bradybaena sp. in north China and implications for
           ecophysiological characteristics and paleoclimate studies
    • Authors: Xu Wang; Linlin Cui, Jixuan Zhai, Zhongli Ding
      Abstract: Knowledge of eco‐physiological characteristics of different land snail species is crucial for defining climatic significance of snail faunal assemblages. However, little work has been done in this aspect, hindering our obtaining unambiguous paleoclimatic information using these proxy indicators. Here we document for the first time the different eco‐physiological characteristics of Cathaica sp. and Bradybaena sp. land snails using the stable isotopes and clumped isotope (Δ47) of the shell carbonates. The Δ47‐derived temperatures for both species revealed a robust correlation with environmental temperatures. Moreover, the temperatures for Cathaica sp. are 3∼5˚C higher than those for Bradybaena sp. land snails, indicating different eco‐physiological adaptations or growing seasons of the two species. Specifically, Cathaica sp. snails prefer living in a warm‐humid summer, whereas Bradybaena sp. snails are active in the relatively cool‐arid spring and/or autumn. The result testifies to the Δ47 in snail shell carbonates as a promising paleothermometer in monsoonal region and presents new insight into paleoclimatic explanation of these land snail species. This finding highlights the importance of climatic seasonality in the changes of the faunal assemblages of land snails. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-08T04:35:33.451777-05:
      DOI: 10.1002/2015GC006182
  • Magnetic properties of sediments of the Red River, Vietnam:Effect
           of sorting on the source‐to‐sink pathway and its implications
           for environmental reconstruction
    • Authors: Nguyen Thi Thu Hien; Weiguo Zhang, Zhen Li, Jie Li, Can Ge, Jinyan Liu, Xuexin Bai, Huan Feng, Lizhong Yu
      Abstract: We conducted a mineral magnetic study of river bank and subaqueous delta sediments from the Red River, Vietnam, in order to examine the role of sedimentary sorting on the variation of sedimentary magnetic properties from source to sink. The magnetic mineralogy mainly consists of magnetite and hematite. Bulk sediment particle‐size variations have a strong influence on magnetic properties, with the frequently‐used magnetic parameters χfd%, χARM, χARM/χ and χARM/SIRM exhibiting positive correlations with the
      PubDate: 2016-01-02T10:44:17.096383-05:
      DOI: 10.1002/2015GC006089
  • Detrital zircon provenance of the Paleogene synrift sediments in the
           northern South China Sea
    • Authors: Lei Shao; Licheng Cao, Xiong Pang, Tao Jiang, Peijun Qiao, Meng Zhao
      Abstract: The early rift sedimentation history of the South China Sea is still not well understood due to restricted borehole coverage of the Paleogene strata and lack of reliable stratigraphic dating. We use detrital zircon U–Pb geochronology to explore the source‐to‐sink characteristics of syn‐rift sequences in the northern South China Sea. The results reveal significant intrabasinal provenances in addition to the well‐perceived terrigenous supply from the north. The Dongsha Uplift is considered to account for the dominance of the Early Cretaceous zircons in the Eocene samples. The Lower Oligocene sediments in the Qiongdongnan Basin could have been sourced from Hainan Island and local uplifts, but their distinction cannot be confirmed by the U–Pb age spectra. Contemporary sediments in the northern Pearl River Mouth Basin were most likely transported from southeastern South China with well‐rounded zircon grains showing U–Pb age similarity to those from the northeastern tributaries of the Pearl River. By contrast, intrabasinal sources from the west and east are suggested to have contributed the infill of the southern part of the Pearl River Mouth Basin based on generally euhedral zircon shapes. These sedimentary source patterns appear to change very little in the Oligocene northern South China Sea. However, the newly detected Neoproterozoic zircons in the Upper Oligocene sediments from borehole L21 tend to indicate a southern source. The episodic and diachronic nature of rifting and erosion processes in the early South China Sea is the cause of complex patterns in the Paleogene provenance history. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-24T11:23:16.305732-05:
      DOI: 10.1002/2015GC006113
  • Compositional variation and 226Ra‐230Th model ages of axial lavas
           from the southern Mid‐Atlantic Ridge, 8°48′S
    • Abstract: We present geological observations and geochemical data for the youngest volcanic features on the slow‐spreading Mid‐Atlantic Ridge at 8°48'S that shows seismic evidence for a thickened crust and excess magma formation. Young lava flows with high sonar reflectivity cover about 14 km2 in the axial rift and were probably erupted from two axial volcanic ridges each of about 3 km in length. Three different lava units occur along an about 11 km long portion of the ridge, and lavas from the northern axial volcanic ridge differ from those of the southern axial volcanic ridge and surrounding lava flows. Basalts from the axial rift flanks and from a pillow mound within the young flows are more incompatible element depleted than those from the young volcanic field. Lavas from this volcanic area have 226Ra‐230Th disequilibria model ages of 1,000 and 4,000 years whereas the older lavas from the rift flank and the pillow mound, but also some of the lava field, are older than 8,000 years. Glasses from the northern and southern ends of the southern lava unit indicate up to 100°C cooler magma temperatures than in the center and increased assimilation of hydrothermally altered material. The compositional heterogeneity on a scale of 3 km suggests small magma batches rising vertically from the mantle to the surface without significant lateral flow and mixing. The observations on the 8°48'S lava field support the model of low frequency eruptions from single ascending magma batches that has been developed for slow‐spreading ridges. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-18T18:12:59.590606-05:
      DOI: 10.1002/2015GC006016
  • The mantle wedge's transient 3‐D flow regime and thermal structure
    • Authors: D. R. Davies; G. Le Voci, S. Goes, S. C. Kramer, C. R. Wilson
      Abstract: Arc volcanism, volatile cycling, mineralisation and continental crust formation are likely regulated by the mantle wedge's flow regime and thermal structure. Wedge flow is often assumed to follow a regular corner‐flow pattern. However, studies that incorporate a hydrated rheology and thermal buoyancy predict internal small‐scale‐convection (SSC). Here, we systematically explore mantle‐wedge dynamics in 3‐D simulations. We find that longitudinal ‘Richter‐rolls’ of SSC (with trench‐perpendicular axes) commonly occur if wedge hydration reduces viscosities to $\lesssim1\cdot10^{19}$Pa s, although transient transverse rolls (with trench‐parallel axes) can dominate at viscosities of $\sim 5\cdot10^{18} ‐ 1\cdot10^{19}$Pa s. Rolls below the arc and back‐arc differ. Sub‐arc rolls have similar trench‐parallel and trench‐perpendicular dimensions of 100–150 km and evolve on a 1–5 Myr time‐scale. Sub‐back‐arc instabilities, on the other hand, coalesce into elongated sheets, usually with a preferential trench‐perpendicular alignment, display a wavelength of 150–400 km and vary on a 5–10 Myr time‐scale. The modulating influence of sub‐back‐arc ridges on the sub‐arc system increases with stronger wedge hydration, higher subduction velocity and thicker upper plates. We find that trench‐parallel averages of wedge velocities and temperature are consistent with those predicted in 2‐D models. However, lithospheric thinning through SSC is somewhat enhanced in 3‐D, thus expanding hydrous melting regions and shifting dehydration boundaries. Sub‐arc Richter‐rolls generate time‐dependent trench‐parallel temperature variations of up to $\sim150$K, which exceed the transient 50–100 K variations predicted in 2–D and may contribute to arc‐volcano spacing and the variable seismic velocity structures imaged beneath some arcs. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-18T18:12:37.758415-05:
      DOI: 10.1002/2015GC006125
  • Horizontal principal stress orientation in the Costa Rica Seismogenesis
           Project (CRISP) transect from borehole breakouts
    • Authors: A. Malinverno; S. Saito, P. Vannucchi
      Abstract: The Costa Rica Seismogenesis Project (CRISP) drilled the Pacific margin of the Middle America Trench just north of where the Cocos Ridge enters the subduction zone, resulting in basal erosion of the upper plate. Here we report the orientations of the maximum horizontal principal stress (SHmax) from borehole breakouts detected by logging‐while‐drilling and wireline downhole measurements. All SHmax directions were estimated in the sediment cover of the margin, above the deeper rocks of the deformed margin wedge. We observe three overall SHmax orientations: NNE‐SSW (25° azimuth) in the deepest interval drilled at the upper slope Site U1379; ENE‐WSW (82°) in the rest of Site U1379 and in Site U1413, also drilled in the upper slope; and NNW‐SSE (157°) in the mid‐slope Site U1378. Our preferred interpretation is that the deepest interval of Site U1379 records the stress conditions in the underlying margin wedge, as SHmax is parallel to the direction of the Cocos‐Caribbean plate convergence and of the compressional axes of plate boundary fault earthquakes. The variable SHmax directions observed elsewhere are likely due to the effect of a network of normal faults that subdivide the sediment cover into a number of independently deforming blocks. In addition, the observed SHmax directions may be influenced by the subducting Cocos Ridge, which acts as an indenter causing oblique deformation, and by the transition to seismogenic subduction along the plate boundary fault. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-15T10:19:22.010186-05:
      DOI: 10.1002/2015GC006092
  • Crystallization depth beneath an oceanic detachment fault (ODP Hole 923A,
           Mid‐Atlantic Ridge)
    • Authors: C. Johan Lissenberg; Matthew Rioux, Christopher J. MacLeod, Samuel A. Bowring, Nobumichi Shimizu
      Abstract: Oceanic detachment faults are increasingly recognised as playing an integral role in the seafloor spreading process at slow and ultraslow spreading mid‐ocean ridges, with significant consequences for the architecture of the oceanic lithosphere. Although melt supply is considered to play a critical control on the formation and evolution of oceanic detachments, much less well understood is how melts and faults interact and influence each other. Few direct constraints on the locus and depth of melt emplacement in the vicinity of detachments are available. Gabbros drilled in ODP Hole 923A near the intersection of the Mid‐Atlantic Ridge and the Kane transform fault (23°N; the MARK area) represent magmas emplaced into the footwall of such a detachment fault and unroofed by it. We here present U‐Pb zircon dates for these gabbros and associated diorite veins which, when combined with a tectonic reconstruction of the area, allow us to calculate the depths at which the melts crystallized. Th‐corrected single zircon U‐Pb dates from three samples range from 1.138 ± 0.062 Ma to 1.213 ± 0.021 Ma. We find a crystallization depth of 6.4 +1.7/‐1.3 km, and estimate that the melts parental to the gabbros were initially emplaced up to 1.5 km deeper, at
      PubDate: 2015-12-15T10:18:59.751049-05:
      DOI: 10.1002/2015GC006027
  • A possible transoceanic tsunami directed toward the U.S. west coast from
           the Semidi segment, Alaska convergent margin
    • Authors: Roland von Huene; John J. Miller, Peter Dartnell
      Abstract: The Semidi segment of the Alaska convergent margin appears capable of generating a giant tsunami like the one produced along the nearby Unimak segment in 1946. Reprocessed legacy seismic reflection data and a compilation of multibeam bathymetric surveys reveal structures that could generate such a tsunami. A 200 km long ridge or escarpment with crests >1 km high is the surface expression of an active out‐of‐sequence fault zone, recently referred to as a splay fault. Such faults are potentially tsunamigenic. This type of fault zone separates the relatively rigid rock of the margin framework from the anelastic accreted sediment prism. Seafloor relief of the ridge exceeds that of similar age accretionary prism ridges indicating preferential slip along the splay fault zone. The greater slip may derive from Quaternary subduction of the Patton Murray hot spot ridge that extends 200 km toward the east across the north Pacific. Estimates of tsunami repeat times from paleotsunami studies indicate that the Semidi segment could be near the end of its current inter‐seismic cycle. GPS records from Chirikof Island at the shelf edge indicate 90% locking of plate interface faults. An earthquake in the shallow Semidi subduction zone could generate a tsunami that will inundate the US west coast more than the 1946 and 1964 earthquakes because the Semidi continental slope azimuth directs a tsunami southeastward. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-15T10:17:00.430602-05:
      DOI: 10.1002/2015GC006147
  • Torsion of a cylinder of partially molten rock with a spherical inclusion:
           Theory and simulation
    • Authors: Laura Alisic; Sander Rhebergen, John F. Rudge, Richard F. Katz, Garth N. Wells
      Abstract: The processes that are involved in migration and extraction of melt from the mantle are not yet fully understood. Gaining a better understanding of material properties of partially molten rock could help shed light on the behavior of melt on larger scales in the mantle. In this study, we simulate three‐dimensional torsional deformation of a partially molten rock that contains a rigid, spherical inclusion. We compare the computed porosity patterns to those found in recent laboratory experiments. The laboratory experiments show emergence of melt‐rich bands throughout the rock sample, and pressure shadows around the inclusion. The numerical model displays similar melt‐rich bands only for a small bulk‐to‐shear‐viscosity ratio (five or less). The results are consistent with earlier two‐dimensional numerical simulations; however, we show that it is easier to form melt‐rich bands in three dimensions compared to two. The addition of strain‐rate dependence of the viscosity causes a distinct change in the shape of pressure shadows around the inclusion. This change in shape presents an opportunity for experimentalists to identify the strain‐rate dependence and therefore the dominant deformation mechanism in torsion experiments with inclusions. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-15T10:13:11.499645-05:
      DOI: 10.1002/2015GC006061
  • New Insights into the mineralogy of the Atlantis II deep metalliferous
           sediments, Red Sea
    • Abstract: The Atlantis II Deep of the Red Sea hosts the largest known hydrothermal ore deposit on the ocean floor and the only modern analog of brine pool‐type metal deposition. The deposit consists mainly of chemical‐clastic sediments with input from basin‐scale hydrothermal and detrital sources. A characteristic feature is the mm‐scale layering of the sediments, which bears a strong resemblance to banded iron formation (BIF). Quantitative assessment of the mineralogy based on re‐logging of archived cores, detailed petrography, and sequential leaching experiments shows that Fe‐(oxy)hydroxides, hydrothermal carbonates, sulfides, and authigenic clays are the main “ore” minerals. Mn‐oxides were mainly deposited when the brine pool was more oxidized than it is today, but detailed logging shows that Fe‐ and Mn‐deposition also alternated at the scale of individual laminae, reflecting short‐term fluctuations in the Lower Brine. Previous studies underestimated the importance of non‐sulfide metal‐bearing components, which formed by metal adsorption onto poorly crystalline Si‐Fe‐OOH particles. During diagenesis, the crystallinity of all phases increased, and the fine layering of the sediment was enhanced. Within a few meters of burial (corresponding to a few thousand years of deposition), biogenic (Ca)‐carbonate was dissolved, manganosiderite formed, and metals originally in poorly crystalline phases or in pore water were incorporated into diagenetic sulfides, clays and Fe‐oxides. Permeable layers with abundant radiolarian tests were the focus for late‐stage hydrothermal alteration and replacement, including deposition of amorphous silica and enrichment in elements such as Ba and Au. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-14T03:31:45.783995-05:
      DOI: 10.1002/2015GC006010
  • Geologic evolution of the Lost City Hydrothermal Field
    • Abstract: The Lost City Hydrothermal Field (LCHF) is a novel serpentinite‐hosted vent field located on the Atlantis Massif southern wall. Results of 2 m resolution bathymetry, sidescan, and video and still imagery, integrated with direct submersible observations provide the first high‐resolution geologic map of the LCHF. These data form the foundation for an evolutionary model for the vent system over the past >120,000 years. The field is located on a down‐dropped bench 70 m below the summit of the massif. The bench is capped by breccia and pelagic carbonate deposits underlain by variably deformed and altered serpentinite and gabbroic rocks. Hydrothermal activity is focused at the 60 m‐tall, 100 m‐across, massive carbonate edifice ‘Poseidon', which is venting 91°C fluid. Hydrothermal activity declines south and west of the Poseidon complex and dies off completely at distances greater than 200 m. East of Poseidon, the most recent stage of hydrothermal flow is characterized by egress of diffuse fluids from narrow fissures within a low‐angle, anastomosing mylonite zone. South of the area of current hydrothermal activity, there is evidence of two discrete previously unrecognized relict fields. Active venting sites defined by carbonate‐filled fissures that cut the carbonate cap rock at the summit of the massif mark the present‐day northernmost extent of venting. These spatial relationships reflect multiple stages of field development, the northward migration of venting over time, and the likely development of a nascent field at the massif summit. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-14T03:30:30.853502-05:
      DOI: 10.1002/2015GC005869
  • Evolution of fracture permeability of ultramafic rocks undergoing
           serpentinization at hydrothermal conditions: An experimental study
    • Authors: A. Farough; D. E. Moore, D. A. Lockner, R. P. Lowell
      Abstract: We performed flow‐through laboratory experiments on 5 cylindrically cored samples of ultramafic rocks, in which we generated a well‐mated through‐going tensile fracture, to investigate evolution of fracture permeability during serpentinization. The samples were tested in a triaxial loading machine at a confining pressure of 50 MPa, pore pressure of 20 MPa, and temperature of 260 °C, simulating a depth of 2 km under hydrostatic conditions. A pore‐pressure difference of up to 2 MPa was imposed across the ends of the sample. Fracture permeability decreased by one to two orders of magnitude during the 200 to 340 hour experiments. Electron microprobe and SEM data indicated the formation of needle‐shaped crystals of serpentine composition along the walls of the fracture, and chemical analyses of sampled pore fluids were consistent with dissolution of ferro‐magnesian minerals. By comparing the difference between fracture permeability and matrix permeability measured on intact samples of the same rock types, we concluded that the contribution of the low matrix permeability to flow is negligible and essentially all of the flow is focused in the tensile fracture. The experimental results suggest that the fracture network in long‐lived hydrothermal circulation systems can be sealed rapidly as a result of mineral precipitation, and generation of new permeability resulting from a combination of tectonic and crystallization‐induced stresses is required to maintain fluid circulation. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-14T03:23:56.189572-05:
      DOI: 10.1002/2015GC005973
  • Neodymium isotope analyses after combined extraction of actinide and
           lanthanide elements from seawater and deep‐sea coral aragonite
    • Authors: Torben Struve; Tina van de Flierdt, Laura F. Robinson, Louisa I. Bradtmiller, Sophia K. Hines, Jess F. Adkins, Myriam Lambelet, Kirsty C. Crocket, Katharina Kreissig, Barry Coles, Maureen E. Auro
      Abstract: Isotopes of the actinide elements protactinium (Pa), thorium (Th) and uranium (U), and the lanthanide element neodymium (Nd) are often used as complementary tracers of modern and past oceanic processes. The extraction of such elements from low abundance matrices, such as seawater and carbonate, is however labor‐intensive and requires significant amounts of sample material. We here present a combined method for the extraction of Pa, Th and Nd from 5 to 10 L seawater samples, and of U, Th and Nd from
      PubDate: 2015-12-14T03:19:20.545008-05:
      DOI: 10.1002/2015GC006130
  • Imaging the magmatic system of Newberry Volcano using joint active source
           and teleseismic tomography
    • Authors: Benjamin A. Heath; Emilie E. E. Hooft, Douglas R. Toomey, Maximiliano J. Bezada
      Abstract: In this paper, we combine active and passive source P wave seismic data to tomographically image the magmatic system beneath Newberry Volcano, located east of the Cascade arc. By using both travel times from local active sources and delay times from teleseismic earthquakes recorded on closely spaced seismometers (300 to 800 m) we significantly improve recovery of upper crustal velocity structure (
      PubDate: 2015-12-14T03:19:09.996511-05:
      DOI: 10.1002/2015GC006129
  • Paleomagnetic constraints on the tectonic evolution of the Costa Rican
           subduction zone: New results from sedimentary successions of IODP drill
           sites from the Cocos Ridge
    • Abstract: The near flat subduction of the Cocos Ridge (CR) along the Middle American Trench (MAT) plays a pivotal role in governing the geodynamic evolution of the Central American convergent margin. Elucidating the onset of its subduction is essential to understand the tectonic evolution and seismogenesis of the Costa Rican convergent margin, a typical erosive convergent margin and modern example of a flat‐slab subduction. Initial subduction of the CR has been previously investigated by examining upper plate deformation that was inferred to have resulted from the initial CR subduction. However, little attention has been paid to the extensive sedimentary archives on the CR that could hold important clues to the initial CR subduction. Drilling on the CR during IODP Expedition 344 discovered a pronounced sedimentary hiatus at Site U1381. Here we present paleomagnetic and rock magnetic results of the Cenozoic sedimentary sequences at this site that bracket the hiatus between ca. 9.61 and 1.52 Ma. We also examine the areal extent, timing, and geologic significance of the hiatus by analyzing sedimentary records from five other ODP/IODP sites on CR and Cocos plate. The analyses show that the hiatus appears to be regional and the presence/absence of the sedimentary hiatus at different locations on CR implies a link to the onset of CR shallow subduction, as a result of either bottom current erosion or CR buckling upon its initial collision at the MAT. Records directly from CR thus provide a new window to unravelling the geodynamic evolution of the Central American margin. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-14T03:17:38.421405-05:
      DOI: 10.1002/2015GC006058
  • SKS splitting beneath Transportable Array stations in eastern North
           America and the signature of past lithospheric deformation
    • Authors: Maureen D. Long; Kenneth G. Jackson, John F. McNamara
      Abstract: Seismic anisotropy in the upper mantle beneath continental interiors is generally complicated, with contributions from both the lithosphere and the asthenosphere. Previous studies of SKS splitting beneath the eastern United States have yielded evidence for complex and laterally variable anisotropy, but until the recent arrival of the USArray Transportable Array (TA) the station coverage has been sparse. Here we present SKS splitting measurements at TA stations in eastern North America and compare the measured fast directions with indicators such as absolute plate motion, surface geology, and magnetic lineations. We find few correlations between fast directions and absolute plate motion, except in the northeastern US and southern Canada, where some stations exhibit variations in apparent splitting with backazimuth that would suggest multiple layers of anisotropy. A region of the southeastern US is dominated by null SKS arrivals over a range of backazimuths, consistent with previous work. We document a pattern of fast directions parallel to the Appalachian mountain chain, suggesting a contribution from lithospheric deformation associated with Appalachian orogenesis. Overall, our measurements suggest that upper mantle anisotropy beneath the eastern United States is complex, with likely contributions from both asthenospheric and lithospheric anisotropy in many regions. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-14T03:17:26.95453-05:0
      DOI: 10.1002/2015GC006088
  • Questions on the existence, persistence, and mechanical effects of a very
           small melt fraction in the asthenosphere
    • Authors: Benjamin K. Holtzman
      Abstract: This paper integrates current questions in rock physics on the effects and behavior of very small melt fractions (≪1%) in the asthenosphere. In experiment and theory, it has been shown that a very small melt fraction forming a connected network has a large effect on the diffusion creep shear viscosity, as well as in the anelastic behavior. Because small concentrations of volatiles, particularly H2O and CO2, significantly lower the peridotite solidus, a small melt fraction is expected in the asthenosphere. Even with connected networks, permeability will be low and surface tension will generate a strong force resisting complete draining of small melt fractions. The anelastic reduction of shear velocity due to melt could cause a ≥ 5% shear velocity contrast across the solidus, consistent with the contrast measured on features in the shallow sub‐oceanic upper mantle that are often interpreted as the lithosphere‐asthenosphere boundary. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-14T03:15:17.583615-05:
      DOI: 10.1002/2015GC006102
  • Synorogenic morphotectonic evolution of the Gangdese batholith, South
           Tibet: Insights from low‐temperature thermochronology
    • Authors: Guangwei Li; Barry Kohn, Mike Sandiford, Zhiqin Xu, Yuntao Tian, Christian Seiler
      Abstract: The uplift history of the Tibetan Plateau remains one of most intriguing and controversial issues in the Cenozoic history of our planet, and has a significant impact on regional and global climate. Here, we report new low‐temperature thermochronology apatite and zircon data from the Gangdese batholith in southern Tibet. Thermal history modeling of the data show that the batholith experienced a phase of rapid Early Cenozoic cooling probably associated with exhumation resulting from the initial India‐Asia's collision, but possibly also due to post‐arc volcanic activity in the region. The batholith, then transitioned to low erosion rates (
      PubDate: 2015-12-14T03:15:05.017455-05:
      DOI: 10.1002/2015GC006047
  • The impact of climatic and atmospheric teleconnections on the brine
           inventory over the Laptev Sea shelf between 2007 and 2011
    • Authors: Benoit Thibodeau; Dorothea Bauch
      Abstract: Export of brine‐enriched water from Siberian shelves is thought to be a key parameter in maintaining the Arctic Halocline, which isolates the fresh and cold surface water from the warm Atlantic water and thus prevent dramatic change in the Arctic sea‐ice thermodynamic. In this study, we used five years of oxygen isotope and hydrological summer surveys to better understand the factors controlling the brine inventory and distribution over the Laptev Sea shelf. The inventory was maximal in 2011 and 2007 and minimal in 2010. The brine inventory interannual variations are coherent with the winter Arctic Oscillation index that was maximal in 2011 and 2007 and minimal in 2010, which is known to modulate Arctic winds and sea‐ice export pattern. While we should remain cautious since our record is limited to 5‐years, our results suggest that the combined effect of the Arctic Oscillation and of the Arctic Dipole is the main factor controlling the annual variations in the inventory of brine‐enriched waters from the Laptev Sea shelf between 2007 and 2011, especially during extreme negative Arctic Oscillation and Arctic Dipole conditions as in 2010. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-09T06:40:30.931514-05:
      DOI: 10.1002/2015GC006063
  • Seismicity distribution and locking depth along the Main Marmara Fault,
    • Abstract: The seismicity along the Main Marmara Fault (MMF) below the Marmara Sea is analyzed during the 2007‐2012 period to provide insights on the recent evolution of this important regional seismic gap. High precision locations show that seismicity is strongly varying along strike and depth providing fine details of the fault behavior that are unaccessible from geodetic inversions. The activity strongly clusters at the regions of transition between basins. The Central basin shows significant seismicity located below the shallow locking depth inferred from GPS measurements. Its b‐value is low and the average seismic slip is high. All observations are consistent with a deep creep of this segment. On the contrary, the Kumburgaz basin at the center of the fault shows sparse seismicity with the hallmarks of a locked segment. In the eastern Marmara Sea, the seismicity distribution along the Princes Island segment in the Cinarcik basin, is consistent with the geodetic locking depth of 10km and a low contribution to the regional seismic energy release. The assessment of the locked segment areas provide an estimate of the magnitude of the main forthcoming event to be about 7.3 assuming that the rupture will not enter significantly within creeping domains. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-09T06:39:04.370361-05:
      DOI: 10.1002/2015GC006120
  • Stress state and its anomaly observations in the vicinity of a fault in
           NanTroSEIZE Expedition 322
    • Abstract: To better understand the stress state and geological properties within the shallow Shikoku Basin, southwest of Japan, two sites, C0011A and C0011B, were drilled in open‐ocean sediments using Logging While Drilling (LWD) and coring, respectively. Resistivity image logging was performed at C0011A from sea floor to 950 meters below sea floor (mbsf). At C0011B, the serial coring was obtained in order to determine physical properties from 340 to 880 mbsf. For the LWD images, a notable breakout anomaly was observed at a depth of 615 meters. Using resistivity images and a stress polygon, the potential horizontal principal stress azimuth and its magnitude within the 500 – 750 mbsf section of the C0011A borehole was constrained. Borehole breakout azimuths were observed for the variation by the existence of a fault zone at a depth of 615 mbsf. Out of this fracture zone, the breakout azimuth was located at approximately 109° ± 12°, sub‐parallel to the Nankai Trough convergence vector (300° to 315°). Our calculations describe a stress drop was determined based on the fracture geometry. A close 90° (73° ± 12°) rotation implied a 100% stress drop, defined as a maximum shear stress drop equal to 1MPa. The magnitude of the horizontal principal stresses near the fracture stress anomaly ranged between 49 and 52 MPa, and the bearing to the vertical stress (Sv = 52MPa) was found to be within the normal faulting stress regime. Low rock strength and a low stress level are necessary to satisfy the observations. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-09T02:22:25.911041-05:
      DOI: 10.1002/2015GC006072
  • Neodymium isotope constraints on provenance, dispersal, and
           climate‐driven supply of Zambezi sediments along the Mozambique
           Margin during the past ∼45,000 years
    • Authors: (H.) Jeroen. J. L. van der Lubbe; Martin Frank, Rik Tjallingii, Ralph R. Schneider
      Abstract: Marine sediments deposited off the Zambezi River that drains a considerable part of the southeast African continent provide continuous records of the continental climatic and environmental conditions. Here, we present time series of neodymium (Nd) isotope signatures of the detrital sediment fraction during the past ∼45,000 years, to reconstruct climate driven changes in the provenance of clays deposited along the Mozambique Margin. Coherent with the surface current regime, the Nd isotope distribution in surface sediments reveals mixing of the alongshore flowing Zambezi suspension load with sediments supplied by smaller rivers located further north. To reconstruct past changes in sediment provenances, Nd isotope signatures of clays that are not significantly fractionated during weathering processes have been obtained from core 64PE304‐80, which was recovered just north of the Zambezi Mouth at 1329 m water depth. Distinctly unradiogenic clay signatures (εNd values
      PubDate: 2015-12-09T02:20:15.344699-05:
      DOI: 10.1002/2015GC006080
  • Italian and Alpine three‐dimensional crustal structure imaged by
           ambient‐noise surface‐wave dispersion
    • Authors: I. Molinari; J. Verbeke, L. Boschi, E. Kissling, A. Morelli
      Abstract: We derive the 3D crustal structure (S wave velocity) underneath Italy and the Alpine region, expanding and exploiting the database of ambient noise Rayleigh‐wave phase‐ and group‐velocity of verbeke et al. [2012]. We first complement the database of verbeke et al. [2012] with a dense set of new ambient‐noise‐based phase‐velocity observations. We next conduct a suite of linear least squares inversion of both phase‐ and group‐velocity data, resulting in 2D maps of Rayleigh‐wave phase and group velocity at periods between 5 and 37 s. At relatively short periods, these maps clearly reflect the surface geology of the region, e.g. low velocity zones at the Po Plain; at longer periods, deeper structures such as Moho topography under Alps and Apennines, and lower‐crust anomalies are revealed. Our phase‐ and group‐velocity models are next inverted via the Neighbourhood Algorithm to determine a set of one‐dimensional shear‐velocity models (one per phase/group‐velocity pixel), resulting in a new three‐dimensional model of shear velocity (υS) parameterized in the same way as the European reference crustal model EPcrust [Molinari and Morelli, 2011]. We also show how well υS is constrained by phase and group dispersion curves. The model shows the low velocity area beneath the Po Plain and the Molasse basin; the contrast between the low‐velocity crust of the Adriatic domain and the high‐velocity crust of the Tyrrhenian domain is clearly seen, as well as an almost uniform crystalline crust beneath the Alpine belt. Our results are discussed from the geological/geodynamical standpoint, and compared to those of other, interdisciplinary studies. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-08T07:34:29.189271-05:
      DOI: 10.1002/2015GC006176
  • New evidence of CO2 soil degassing anomalies on Piton de la Fournaise
           volcano and the link with volcano tectonic structures
    • Authors: M. Liuzzo; A. Di Muro, G. Giudice, L. Michon, V. Ferrazzini, S. Gurrieri
      Abstract: Piton de la Fournaise (PdF) is recognised as one of the world's most active volcanoes in terms of eruptive frequency and the substantial quantity of lava produced. Yet, with the sole exception of rather modest intracrateric fumarole activity, this seems to be in contrast with an apparent absence of any type of natural fluid emission during periods of quiescence. Measurement campaigns were undertaken during a long‐lasting quiescent period (2012‐2014) and just after a short lived summit eruption (June 2014) in order to identify potential degassing areas in relation to the main structural features of the volcano (ex. rift zones) with the aim of developing a broader understanding of the geometry of the plumbing and degassing system. In order to assess the possible existence of anomalous soil CO2 flux, 513 measurements were taken along transects roughly orthogonal to the known tectonic lineaments crossing PdF edifice. In addition, 53 samples of gas for C isotope analysis were taken at measurement points that showed a relatively high CO2 concentration in the soil. CO2 flux values range from 10 to 1300 g m−2 d−1 while δ13C are between ‐26.6 to ‐8‰. The results of our investigation clearly indicate that there is a strong spatial correlation between the anomalous high values of diffusive soil emissions and the main rift zones cutting the PdF massif and, moreover, that generally high soil CO2 fluxes show a δ13C signature clearly related to a magmatic origin. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-08T07:31:36.666482-05:
      DOI: 10.1002/2015GC006032
  • 87Sr/86Sr in recent accumulations of calcium sulfate on landscapes of
           hyperarid settings: A bimodal altitudinal dependence for northern Chile
    • Authors: N. J. Cosentino; T. E. Jordan, L. A. Derry, J. P. Morgan
      Abstract: An elevation‐dependent relationship of the 87Sr/86Sr ratio of Holocene surface accumulations of sulfate salts is demonstrated for a continental margin hyperarid setting. In the Atacama Desert of northern Chile, gypsum and anhydrite of multiple origins exist widely on superficial materials that originated during the last 10,000 years. An important source of calcium sulfate is from offshore‐generated stratocumulus clouds that are advected onto the continent, where they generate fog that transfers water droplets to the ground surface which, upon evaporation, leaves calcium sulfate crystals. Meteorological measurements of the cloud base and top altitudes average ∼400 meters and ∼1100 meters above sea level (m.a.s.l.), respectively. The seawater ratio of 87Sr/86Sr (0.70917) is distinctively higher than that reported for weathered mean Andean rock (less than 0.70750). Samples of 28 modern surface salt accumulations for locations between 200–2950 m.a.s.l. and between ∼19°30' and ∼21°30' S verify that 87Sr/86Sr varies as a function of site altitude. Sites below 1075 m.a.s.l. and above 225 m.a.s.l. display calcium sulfate 87Sr/86Sr of mean value 0.70807 ± 0.00004, while the ratio outside this altitudinal domain is 0.70746 ± 0.00010. Thus, the 87Sr/86Sr ratio of Holocene salt accumulations differentiates two altitudinal domains. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-26T11:07:43.891345-05:
      DOI: 10.1002/2015GC005954
  • Subduction system and flat slab beneath the Eastern Cordillera of Colombia
    • Abstract: Seismicity at the northern terminus of the Nazca subduction is diffused over a wide area containing the puzzling seismic feature known as the Bucaramanga nest. We relocate about 5000 earthquakes recorded by the Colombian national seismic network and produce the first 3D velocity model of the area to define the geometry of the lithosphere subducting below the Colombian Andes. We found lateral velocity heterogeneities and an abrupt offset of the Wadati‐Benioff zone at 5°N indicating that the Nazca plate is segmented by an E‐W slab tear, that separates a steeper Nazca segment to the south from a flat subduction to the north. The flat Nazca slab extends eastward for about 400 km, before dip increases to ∼50° beneath the Eastern Cordillera, where it yields the Bucaramanga nest. We explain this puzzling locus of intermediate‐depth seismicity located beneath the Eastern Cordillera of Colombia as due to a massive dehydration and eclogitization of a thickened oceanic crust. We relate the flat subducting geometry to the entrance at the trench at ca. 10 Ma of a thick ‐ buoyant oceanic crust, likely a volcanic ridge, producing a high coupling with the overriding plate. Sub‐horizontal plate subduction is consistent with the abrupt disappearance of volcanism in the Andes of South America at latitudes > 5°N. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-26T10:52:49.178612-05:
      DOI: 10.1002/2015GC006048
  • Composition, geometry, and emplacement dynamics of a large volcanic island
           landslide offshore Martinique: From volcano flank‐collapse to
           seafloor sediment failure
    • Abstract: Landslides are common features in the vicinity of volcanic islands. In this contribution, we investigate landslides emplacement and dynamics around the volcanic island of Martinique based on the first scientific drilling of such deposits. The evolution of the active Montagne Pelée volcano on this island has been marked by three major flank‐collapses that removed much of the western flank of the volcano. Subaerial collapse volumes vary from 2 to 25 km3 and debris avalanches flowed into the Grenada Basin. High‐resolution seismic data (AGUADOMAR – 1999, CARAVAL – 2002 and GWADASEIS – 2009) is combined with new drill cores that penetrate up to 430 m through the three submarine landslide deposits previously associated to the aerial flank‐collapses (Site U1399, Site U1400, Site U1401, IODP Expedition 340, Joides Resolution, March‐April 2012). This combined geophysical and core data provide an improved understanding of landslide processes offshore a volcanic island. The integrated analysis shows a large submarine landslide deposit, without debris avalanche deposits coming from the volcano, comprising up to 300 km3 of remobilized seafloor sediment that extends for 70 km away from the coast and covers an area of 2100 km2. Our new data suggest that the aerial debris avalanche deposit enter the sea but stop at the base of submarine flank. We propose a new model dealing with seafloor sediment failures and landslide propagation mechanisms, triggered by volcanic flank‐collapse events affecting Montagne Pelée volcano. Newly recognized landslide deposits occur deeper in the stratigraphy, suggesting the recurrence of large‐scale mass‐wasting processes offshore the island and thus, the necessity to better assess the associated tsunami hazards in the region. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-25T17:44:35.763497-05:
      DOI: 10.1002/2015GC006034
  • The nonconservative property of dissolved molybdenum in the western Taiwan
           Strait: Relevance of submarine groundwater discharges and biological
    • Abstract: This study examined dissolved Mo and sedimentary Mo along with hydro‐chemical parameters in the western Taiwan Strait (WTS) in May and August 2012. The results demonstrate that dissolved Mo could be depleted of as high as 10‐20 nM during our May sampling period when the nutrient‐enriched Min‐Zhe coastal current ceased and spring blooms developed. The negative correlation between Chl‐a and dissolved Mo suggests of the possible involvement of high algal productivity into removing dissolved Mo out of the water column. Specific oceanographic settings (little currents) permitted a high sedimentary enrichment of Mo (>6 µg/g Mo) within the highly productive waters outside the Jiulong River mouth. Possibly, the high algal productivities and consequent organic matter sinks provide a pathway of Mo burial from water columns into sediments. Dissolved Mo was relatively high in groundwater samples, but we observed that submarine groundwater discharges (SGDs) only contributed to a relatively small percentage of the total dissolved Mo pool in WTS. It is probably attributable to the immediate removal of SGD‐released Mo ions via adsorption onto newly formed Mn oxides once exposed to oxygenated seawater, followed by an elevated sedimentary Mo accumulation near the SGDs (∼5 µg/g). In addition to metal oxide particle scavenging and sulfide precipitation, we estimated that biological uptake along with Mo adsorption onto organic matter carriers could finally provide more than 10% of the annual sedimentary Mo accumulation in WTS. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-23T18:34:04.602886-05:
      DOI: 10.1002/2014GC005708
  • Li partitioning in the benthic foraminifera Amphistegina lessonii
    • Authors: Gerald Langer; Aleksey Sadekov, Silke Thoms, Antje Mewes, Gernot Nehrke, Mervyn Greaves, Sambuddha Misra, Jelle Bijma, Henry Elderfield
      Abstract: The shallow water benthic foraminifer Amphistegina lessonii was grown in seawater of variable Li and Ca concentration and shell Li/Ca was determined by means of LA‐ICPMS. Shell Li/Ca is positively correlated to seawater Li/Ca only when the Li concentration of seawater is changed. If the seawater Ca concentration is changed, shell Li/Ca remains constant. This indicates that Li does not compete with Ca for incorporation in the shell of A. lessonii. A recently proposed calcification model can be applied to divalent cations (e.g. Mg and Sr), which compete for binding sites of ion‐transporters and positions in the calcite lattice. By contrast, the transport pathway of monovalent cations such as Li is probably diffusion‐based (e.g. ion‐channels), and monovalent cations do not compete with Ca for a position in the calcite lattice. Here, we present a new model for Li partitioning into foraminiferal calcite which predicts our experimental results and should also be applicable to other alkali metals. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-23T18:33:56.768154-05:
      DOI: 10.1002/2015GC006134
  • New methods for unmixing sediment grain size data
    • Authors: Greig A. Paterson; David Heslop
      Abstract: Grain size distribution (GSD) data are widely used in Earth sciences and although large data sets are regularly generated, detailed numerical analyses are not routine. Unmixing GSDs into components can help understand sediment provenance and depositional regimes/processes. End member analysis (EMA), which fits one set of end members to a given data set, is a powerful way to unmix GSDs into geologically meaningful parts. EMA estimates end members based on co‐variability within a data set and can be considered as a non‐parametric approach. Available EMA algorithms, however, either produce sub‐optimal solutions, or are time consuming. We introduce unmixing algorithms inspired by hyperspectral image analysis that can be applied to GSD data and which provide an improvement over current techniques. Non‐parametric EMA is often unable to identify unimodal grain size sub‐populations that correspond to single sediment sources. An alternative approach is single specimen unmixing (SSU), which unmixes individual GSDs into unimodal parametric distributions (e.g., lognormal). We demonstrate that the inherent non‐uniqueness of SSU solutions renders this approach unviable for estimating underlying mixing processes. To overcome this, we develop a new algorithm to perform parametric EMA, whereby an entire data set can be unmixed into unimodal parametric end members (e.g., Weibull distributions). This makes it easier to identify individual grain size sub‐populations in highly mixed data sets. To aid investigators in applying these methods, all of the new algorithms are available in AnalySize, which is GUI software for processing and unmixing grain size data. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-23T18:33:39.364885-05:
      DOI: 10.1002/2015GC006070
  • Visualizing tephra deposits and sedimentary processes in the marine
           environment: The potential of X‐ray microtomography
    • Authors: Adam J. Griggs; Siwan M. Davies, Peter M. Abbott, Mark Coleman, Adrian P. Palmer, Tine L. Rasmussen, Richard Johnston
      Abstract: Localised tephra deposition in marine sequences is the product of many complex primary and secondary depositional processes. These can significantly influence the potential applicability of tephra deposits as isochronous marker horizons and current techniques, used in isolation, may be insufficient to fully unravel these processes. Here, we demonstrate the innovative application of X‐ray microtomography (µCT) to successfully identify tephra deposits preserved within marine sediments and use these parameters to reconstruct their internal three‐dimensional structure. Three‐dimensional visualizations and animations of tephra dispersal in the sediment permits a more thorough assessment of post‐depositional processes revealing a number of complex micro‐sedimentological features that are not revealed by conventional methods. These features include bioturbation burrows and horizontally discontinuous tephra packages, which have important ramifications for the stratigraphic placement of the isochron in a sedimentary sequence. Our results demonstrate the potential for utilising rigorous two‐ and three‐dimensional micro‐sedimentological analysis of the ichnofabric to enhance and support the use of tephra deposits as isochronous marker horizons and to identify the stratigraphic position that best reflects the primary fallout of ash. The application also provides an exceptional insight into the style and rate of sedimentation processes and permits an assessment of the stratigraphic integrity of a tephra deposit. We discuss the possibility of applying these µCT methods to the identification of cryptotephras within various palaeoclimatic sequences and to enhance our understanding of marine sedimentation processes. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-20T10:46:19.51505-05:0
      DOI: 10.1002/2015GC006073
  • Submarine and subaerial lavas in the West Antarctic Rift System: Temporal
           record of shifting magma source components from the lithosphere and
    • Abstract: The petrogenesis of Cenozoic alkaline magmas in the West Antarctic Rift System (WARS) remains controversial, with competing models highlighting the roles of decompression melting due to passive rifting, active plume upwelling in the asthenosphere, and flux melting of a lithospheric mantle metasomatized by subduction. In this study, seamounts sampled in the Terror Rift region of the Ross Sea provide the first geochemical information from submarine lavas in the Ross Embayment in order to evaluate melting models. Together with subaerial samples from Franklin Island, Beaufort Island, and Mt. Melbourne in Northern Victoria Land (NVL), these Ross Sea lavas exhibit ocean island basalt (OIB)‐like trace element signatures and isotopic affinities for the C or FOZO mantle endmember. Major‐oxide compositions are consistent with the presence of multiple recycled lithologies in the mantle source region(s), including pyroxenite and volatile‐rich lithologies such as amphibole‐bearing, metasomatized peridotite. We interpret these observations as evidence that ongoing tectonomagmatic activity in the WARS is facilitated by melting of subduction‐modified mantle generated during 550 – 100 Ma subduction along the paleo‐Pacific margin of Gondwana. Following ingrowth of radiogenic daughter isotopes in high‐µ (U/Pb) domains, Cenozoic extension triggered decompression melting of easily fusible, hydrated metasomes. This multistage magma generation model attempts to reconcile geochemical observations with increasing geophysical evidence that the broad seismic low‐velocity anomaly imaged beneath West Antarctica and most of the Southern Ocean may be in part a compositional structure inherited from previous active margin tectonics. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-20T10:37:15.137957-05:
      DOI: 10.1002/2015GC006076
  • Diking‐induced moderate‐magnitude earthquakes on a youthful
           rift border fault: The 2002 Nyiragongo‐Kalehe sequence, D.R. Congo
    • Authors: C. Wauthier; B. Smets, D. Keir
      Abstract: On 24 October 2002, a Mw 6.2 earthquake occurred in the central part of the Lake Kivu basin, Western Branch of the East African Rift. This is the largest event recorded in the Lake Kivu area since 1900. An integrated analysis of radar interferometry (InSAR), seismic and geological data, demonstrates that the earthquake occurred due to normal‐slip motion on a major pre‐existing east‐dipping rift border fault. A Coulomb stress analysis suggests that diking events, such as the January 2002 dike intrusion, could promote faulting on the western border faults of the rift in the central part of Lake Kivu. We thus interpret that dike‐induced stress changes can cause moderate to large‐magnitude earthquakes on major border faults during continental rifting. Continental extension processes appear complex in the Lake Kivu basin, requiring the use of a hybrid model of strain accommodation and partitioning in the East African Rift. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-20T10:37:02.542431-05:
      DOI: 10.1002/2015GC006110
  • Coupling changes in densities and porosity to fluid pressure variations in
           reactive porous fluid flow: Local thermodynamic equilibrium
    • Authors: Benjamin Malvoisin; Yury Yu. Podladchikov, Johannes C. Vrijmoed
      Abstract: Mineralogical reactions which generate or consume fluids play a key role during fluid flow in porous media. Such reactions are linked to changes in density, porosity, permeability and fluid pressure which influence fluid flow and rock deformation. To understand such a coupled system equations were derived from mass conservation and local thermodynamic equilibrium. The presented mass conservative modelling approach describes the relationships between evolving fluid pressure, porosity, fluid and solid density, and devolatilization reactions in multi‐component systems with solid solutions. This first step serves as a framework for future models including aqueous speciation and transport. The complexity of univariant and multi‐variant reactions is treated by calculating look‐up tables from thermodynamic equilibrium calculations. Simplified cases were also investigated to understand previously studied formulations. For non‐deforming systems or systems divided into phases of constant density the equations can be reduced to porosity wave equations with addition of a reactive term taking the volume change of reaction into account. For closed systems an expression for the volume change of reaction and the associated pressure increase can be obtained. The key equations were solved numerically for the case of devolatilization of three different rock types that may enter a subduction zone. Reactions with positive Clapeyron slope lead to increase in porosity and permeability with decreasing fluid pressure resulting in sharp fluid pressure gradients around a negative pressure anomaly. The opposite trend is obtained for reactions having a negative Clapeyron slope during which sharp fluid pressure gradients were only generated around a positive pressure anomaly. Coupling of reaction with elastic deformation induces a more efficient fluid flow for reactions with negative Clapeyron slope than for reactions with positive Clapeyron slope. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-20T10:36:38.123727-05:
      DOI: 10.1002/2015GC006019
  • Vesiculation in rhyolite at low H2O contents: A thermodynamic model
    • Abstract: We present experimental data on the thermodynamics and kinetics of bubble nucleation and growth in weakly H2O‐oversaturated rhyolitic melts. The high‐temperature (900‐1100°C) experiments involve heating of rhyolitic obsidian from Hrafntinnuhryggur, Krafla, Iceland to above their glass transition temperature (Tg ∼ 690°C) at 0.1 MPa for times of 0.25‐24 hours. During experiments, the rhyolite cores increase in volume as H2O vapour‐filled bubbles nucleate and expand. The extent of vesiculation, as tracked by porosity, is mapped in temperature‐time (T‐t) space. At constant temperature and for a characteristic dwell time, the rhyolite cores achieve a maximum volume where the T‐t conditions reach thermochemical equilibrium. For each T‐t snapshot of vesiculation, we use 3D analysis of X‐ray computed tomographic (XCT) images of the quenched cores to obtain the bubble number density (BND) and bubble size distribution (BSD). BNDs for the experimental cores are insensitive to T and t, indicating a single nucleation event. All BSDs converge to a common distribution, independent of T, melt viscosity (η), or initial degree of saturation, suggesting a common growth process. We use these data to calibrate an empirical model for predicting the rates and amounts of vesiculation in rhyolitic melts as a function of η and thermochemical affinity (A): two computable parameters that are dependent on T, pressure and H2O content. The model reproduces the experimental dataset and data from the literature to within experimental error, and has application to natural volcanic systems where bubble formation and growth are not diffusion limited (e.g., lavas, domes, ignimbrites, conduit infill). This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-20T10:36:21.889149-05:
      DOI: 10.1002/2015GC006024
  • Structure of the mantle beneath the Alboran basin from magnetotelluric
    • Abstract: We present results of marine MT acquisition in the Alboran sea that also incorporates previously acquired land MT from southern Spain into our analysis. The marine data show complex MT response functions with strong distortion due to seafloor topography and the coastline, but inclusion of high resolution topography and bathymetry and a seismically defined sediment unit into a 3D inversion model has allowed us to image the structure in the underlying mantle. The resulting resistivity model is broadly consistent with a geodynamic scenario that includes subduction of an eastward trending plate beneath Gibraltar, which plunges nearly vertically beneath the Alboran. Our model contains three primary features of interest: a resistive body beneath the central Alboran, which extends to a depth of ∼150 km. At this depth, the mantle resistivity decreases to values of ∼100 Ohm‐m, slightly higher than those seen in typical asthenosphere at the same depth. This transition suggests a change in slab properties with depth, perhaps reflecting a change in the nature of the seafloor subducted in the past. Two conductive features in our model suggest the presence of fluids released by the subducting slab or a small amount of partial melt in the upper mantle (or both). Of these, the one in the center of the Alboran basin, in the uppermost‐mantle (20‐30km depth) beneath Neogene volcanics and west of the termination of the Nekkor Fault, is consistent with geochemical models, which infer highly thinned lithosphere and shallow melting in order to explain the petrology of seafloor volcanics. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-14T03:25:06.058303-05:
      DOI: 10.1002/2015GC006100
  • Formation of andesite melts and Ca‐rich plagioclase in the submarine
           Monowai Volcanic System, Kermadec Arc
    • Authors: Fabian Kemner; Karsten M. Haase, Christoph Beier, Stefan Krumm, Philipp A. Brandl
      Abstract: Andesites are typical rocks of island arcs and may either form by fractional crystallization processes or by mixing between a mafic and a felsic magma. Here, we present new petrographic and geochemical data from lavas of the submarine Monowai volcanic system in the northern Kermadec island arc that display a continuous range in composition from basalt to andesite. Using petrology, major, trace and volatile element data we show that basaltic magmas mostly evolve to andesitic magmas by fractional crystallization. Our thermobarometric calculations indicate that the formation of the large caldera is related to eruption of basaltic‐andesitic to andesitic magmas from a magma reservoir in the deeper crust. Small variations in trace element ratios between the caldera and the large active cone imply a homogeneous mantle source. Contrastingly, resurgent dome melts of the caldera stagnated at shallower depths, are more depleted and show a stronger subduction input than the other edifices. The Monowai basaltic glasses contain less than 1 wt.% H2O and follow typical tholeiitic fractionation trends. High‐An plagioclase crystals observed in the Monowai lavas likely reflect mixing of H2O‐saturated melt batches with hot and dry tholeiitic, decompression melt batches. The result is a relatively H2O‐poor mafic magma at Monowai implying that partial melting of the mantle wedge is only partly due to the volatile flux and that adiabatic melting may play a significant role in the formation of the parental melts of the Monowai volcanic system and possibly other arc volcanoes. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-05T03:48:10.846402-05:
      DOI: 10.1002/2015GC005884
  • Anisotropic low‐velocity lower crust beneath the northeastern margin
           of Tibetan Plateau: Evidence for crustal channel flow
    • Authors: Xuzhang Shen; Xiaohui Yuan, Junsheng Ren
      Abstract: Detailed seismic structure in the crust beneath the northeastern margin of Tibetan Plateau was revealed by receiver functions of a regional permanent seismic network. At most stations, negative P‐to‐S converted phases can be detected in the radial receiver functions, prior to the Moho phases, indicating low velocities in the mid‐lower crust. Prominent azimuthal variations in the transverse receiver functions with polarity reversal suggest azimuthal anisotropy in the crust. We used time variations of the P‐to‐S converted phases at the Moho in the radial receiver functions and the azimuth‐weighted stacking of transverse receiver functions to determine the fast direction and magnitude of anisotropy. The low‐velocity mid‐lower crust with the coherent azimuthal anisotropy in the northeastern margin of Tibetan Plateau is consistent with the lower‐crustal channel flow model. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-05T03:47:44.514968-05:
      DOI: 10.1002/2015GC005952
  • Constraints on the evolution of crustal flow beneath northern Tibet
    • Authors: Florian Le Pape; Alan G. Jones, Martyn J. Unsworth, Jan Vozar, Wenbo Wei, Jin Sheng, Gaofeng Ye, Jianen Jing, Hao Dong, Letian Zhang, Chengliang Xie
      Abstract: Crustal flow is an important tectonic process active in continent‐continent collisions and which may be significant in the development of convergent plate boundaries. In this study, the results from multi‐dimensional electrical conductivity modeling have been combined with laboratory studies of the rheology of partially molten rocks to characterize the rheological behavior of the middle‐to‐lower crust of both the Songpan‐Ganzi and Kunlun terranes in the northern Tibetan Plateau. Two different methods are adopted to develop constraints on melt fraction, temperature and crustal flow velocity in the study area. The estimates of these parameters are then used to evaluate whether crustal flow can occur on the northern margin of the Tibetan plateau. In the Songpan‐Ganzi crust, all conditions are satisfied for topography‐driven channel flow to be dominant, with partial melt not being required for flow at temperature above 1000°C. Further north, the Kunlun fault defines the southern boundary of a transition zone between the Tibetan plateau and the Qaidam basin. Constrained by the estimated melt fractions, it is shown that channel injection across the fault requires temperatures close to 900°C. The composition of igneous rocks found at the surface confirm those conditions are met for the southern Kunlun ranges. To the north, the Qaidam basin is characterized by colder crust that may reflect an earlier stage in the channel injection process. In the study area at least 10% of the eastward directed Tibetan crustal flow could be deflected northwards across the Kunlun Fault and injected into the transition zone defining the northern margin of the Tibetan plateau. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-05T03:47:14.054077-05:
      DOI: 10.1002/2015GC005828
  • The meaning of mid‐lithospheric discontinuities: A case study in the
           northern U.S. craton
    • Authors: Emily Hopper; Karen M. Fischer
      Abstract: Converted wave imaging has revealed significant negative velocity gradients, often termed mid‐lithospheric discontinuities, within the thick, high velocity mantle beneath cratons. In this study we investigate the origins and implications of these structures with high resolution imaging of mantle discontinuities beneath the Archean Wyoming, Superior and Medicine Hat and Proterozoic Yavapai and Trans‐Hudson terranes. Sp phases from 872 temporary and permanent broadband stations, including the EarthScope Transportable Array, were migrated into three‐dimensional common conversion point stacks. Four classes of discontinuities were observed. 1) A widespread, near‐flat negative velocity gradient occurs largely at 70‐90 km depth beneath both Archean and Proterozoic cratons. This structure is consistent with the top of a frozen‐in layer of volatile‐rich melt. 2) Dipping negative velocity gradients are observed between 85‐200 km depth. The clearest examples occur at the suture zones between accreted Paleoproterozoic Yavapai arc terranes and the Wyoming and Superior cratons. These interfaces could represent remnant subducting slabs, and together with eclogite in xenoliths, indicate that subduction‐related processes likely contributed to cratonic mantle growth. 3) Sporadic positive velocity gradients exist near the base of the lithospheric mantle, perhaps due to laterally‐variable compositional layering. In contrast to off‐craton regions, clear Sp phases are typically not seen at lithosphere‐asthenosphere boundary depths beneath Archean and Proterozoic terranes, consistent with a purely thermal contrast between cratonic mantle lithosphere and asthenosphere. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-05T03:43:18.433534-05:
      DOI: 10.1002/2015GC006030
  • Bivalve shell horizons in seafloor pockmarks of the last
           glacial‐interglacial transition suggest a 1000 years of methane
           emissions in the Arctic Ocean
    • Abstract: We studied discrete bivalve shell horizons in two gravity cores from seafloor pockmarks on the Vestnesa Ridge (ca. 1200 m water depth), western Svalbard (79° 00' N, 06° 55' W) to provide insight into the temporal and spatial dynamics of seabed methane seeps. The shell beds, dominated by two genera of the family Vesicomyidae: Phreagena s.l. and Isorropodon sp., were 20‐30cm thick and centered at 250‐400cm deep in the cores. The carbon isotope composition of inorganic (δ13C from ‐13.02‰ to +2.36‰) and organic (δ13C from ‐29.28‰ to ‐21.33‰) shell material and a two‐end member mixing model indicate that these taxa derived between 8% and 43% of their nutrition from chemosynthetic bacteria. In addition, negative δ13C values for planktonic foraminifera (‐6.7‰ to ‐3.1‰), micritic concretions identified as methane‐derived authigenic carbonates, and pyrite encrusted fossil worm tubes at the shell horizons indicate a sustained paleo‐methane seep environment. Combining sedimentation rates with 14C ages for bivalve material from the shell horizons, we estimate the horizons persisted for about 1000 years between approximately 17,707 to 16,680 yrs. BP (corrected). The seepage event over a 1000‐year time interval was most likely associated with regional stress‐related faulting and the subsequent release of over‐pressurized fluids. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-05T03:43:08.147595-05:
      DOI: 10.1002/2015GC005980
  • The importance of a Ni correction with ion counter in the double spike
           analysis of Fe isotope compositions using a 57Fe/58Fe double spike
    • Authors: V.A. Finlayson; J.G. Konter, L. Ma
      Abstract: We present a new method capable of measuring iron isotope ratios of igneous materials to high precision by multicollector inductively coupled plasma mass spectrometry (MC‐ICP‐MS) using a 57Fe‐58Fe double spike. After sample purification, near‐baseline signal levels of nickel are still present in the sample solution, acting as an isobaric interference on 58 amu. To correct for the interference, the minor 60Ni isotope is monitored and used to subtract a proportional 58Ni signal from the total 58 amu beam. The 60Ni signal is difficult to precisely measure on the Faraday detector due to Johnson noise occurring at similar magnitude. This noise‐dominated signal is subtracted from the total 58 amu beam, and its error amplified during the double spike correction. Placing the 60Ni beam on an ion counter produces a more precise measurement, resulting in a near‐threefold improvement in δ56Fe reproducibility, from ±0.145‰ when measured on Faraday to 0.052‰. Faraday detectors quantify the 60Ni signal poorly, and fail to discern the transient 20Ne40Ar interference visible on the ion counter, which is likely responsible for poor reproducibility. Another consideration is instrumental stability (defined herein as drift in peak center mass), which affects high resolution analyses. Analyses experiencing large drift relative to bracketing standards often yield non‐replicating data. Based on this, we present a quantitative outlier detection method capable of detecting drift‐affected data. After outlier rejection, long‐term precision on individual runs of our secondary standard improves to ±0.046‰. Averaging 3‐4 analyses further improves precision to 0.019‰, allowing distinction between ultramafic minerals. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-29T18:35:57.47474-05:0
      DOI: 10.1002/2015GC006012
  • Magmatic consequences of the transition from orthogonal to oblique
           subduction in Panama
    • Authors: Tyrone O. Rooney; Kristin D. Morell, Paulo Hidalgo, Pastora Fraceschi
      Abstract: The closure of the Central American Seaway is linked with tectonic and magmatic processes that have controlled the evolution of the Isthmus of Panama. We focus on the terminal stages of arc activity in the Central Panama region, and present new geochemical data from ∼9 Ma explosive silicic volcanism preserved in three syngenetic tuff beds from the Gatun Formation. The magmatic evolution of the Gatun Formation is controlled by a series of magma mushes where pyroxene is the dominant early forming mafic mineral, with amphibole appearing only relatively late in the fractionation sequence. Our data shows Gatun lavas exhibit a strong subduction signature, consistent with plate reconstruction models showing arc‐normal subduction from Costa Rica to Panama pre‐8.5 Ma. However, large ion lithophile elements are depleted in the Gatun Formation in comparison to other regional suites, indicative of a lower flux of subduction fluid to the Gatun Formation mantle source, which is explained by a shift towards magma generation by decompression following the collision of the arc with South America. Oblique subduction commencing ∼8.5 Ma resulted in the shutdown of normal arc activity throughout Panama. We interpret subsequent regional Quaternary adakitic volcanism as a response to this oblique subduction. The now more refractory mantle wedge required greater fluid flux in order to melt. The resultant volatile‐rich melts were more prone to deep fractionation of amphibole and garnet cumulates forming adakites. Deep fractionation was potentially enhanced by changing stress regimes on the upper‐plate caused by oblique subduction. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-26T18:48:58.45387-05:0
      DOI: 10.1002/2015GC006150
  • A random‐walk algorithm for modeling lithospheric density and the
           role of body forces in the evolution of the Midcontinent Rift
    • Authors: Will Levandowski; Oliver Boyd, Rich Briggs, Ryan Gold
      Abstract: This paper develops a Monte Carlo algorithm for extracting three‐dimensional lithospheric density models from geophysical data. Empirical scaling relationships between velocity and density create a 3D starting density model, which is then iteratively refined until it reproduces observed gravity and topography. This approach permits deviations from uniform crustal velocity‐density scaling, which provide insight into crustal lithology and prevent spurious mapping of crustal anomalies into the mantle. We test this algorithm on the Proterozoic Midcontinent Rift (MCR), north‐central U.S. The MCR provides a challenge because it hosts a gravity high overlying low shear‐wave velocity crust in a generally flat region. Our initial density estimates are derived from a seismic velocity/crustal thickness model based on joint inversion of surface‐wave dispersion and receiver functions. By adjusting these estimates to reproduce gravity and topography, we generate a lithospheric‐scale model that reveals dense middle crust and eclogitized lowermost crust within the rift. Mantle lithospheric density beneath the MCR is not anomalous, consistent with geochemical evidence that lithospheric mantle was not the primary source of rift‐related magmas and suggesting that extension occurred in response to far‐field stress rather than a hot mantle plume. Similarly, the subsequent inversion of normal faults resulted from changing far‐field stress that exploited not only warm, recently faulted crust but also a gravitational potential energy low in the MCR. The success of this density modeling algorithm in the face of such apparently contradictory geophysical properties suggests that it may be applicable to a variety of tectonic and geodynamic problems. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-23T03:54:27.800404-05:
      DOI: 10.1002/2015GC005961
  • Reactive transport in a partially molten system with binary solid solution
    • Authors: Jacob S. Jordan; Marc A. Hesse
      Abstract: Melt extraction from the Earth's mantle through high‐porosity channels is required to explain the composition of the oceanic crust. Feedbacks from reactive melt transport are thought to localize melt into a network of high‐porosity channels. Recent studies invoke lithological heterogeneities in the Earth's mantle to seed the localization of partial melts. Therefore, it is necessary to understand the reaction fronts that form as melt flow across the lithological interface between the heterogeneity and the ambient mantle. Here, we present a chromatographic analysis of reactive melt transport across lithological boundaries, using the theory of hyperbolic conservation laws. This is an extension of linear trace element chromatography to the coupling of major elements and energy transport. Our analysis allows the prediction of the non‐linear feedbacks that arise in reactive melt transport due to changes in porosity. This study considers the special case of a partially molten porous medium with binary solid solution. As melt traverses a lithological contact, binary solid solution leads to the formation of a reacted zone between an advancing reaction front and the initial contact. The analysis also shows that the behavior of a fertile heterogeneity depends on its absolute concentration, in addition to compositional differences between itself and the refractory background. We present a regime diagram that predicts if melt emanating from a fertile heterogeneity localizes into high porosity channels or develops a zero porosity shell. The theoretical framework presented here provides a useful tool for understanding non‐linear feedbacks in reactive melt transport, because it can be extended to more complex and realistic phase behaviors. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-23T03:54:04.897884-05:
      DOI: 10.1002/2015GC005956
  • Evidence and models for lower crustal flow beneath the Galápagos
    • Abstract: The volcanic Galápagos Islands are constructed upon a broad platform, with their active westernmost islands marking the current position of the hotspot. Built upon young oceanic lithosphere (
      PubDate: 2015-10-23T03:50:51.133605-05:
      DOI: 10.1002/2015GC006136
  • Identifying cryptotephra units using correlated rapid, nondestructive
           methods: VSWIR spectroscopy, X‐ray fluorescence, and magnetic
    • Authors: Molly C. McCanta; Robert G. Hatfield, Bradley J. Thomson, Simon J. Hook, Elizabeth Fisher
      Abstract: Understanding the frequency, magnitude, and nature of explosive volcanic eruptions is essential for hazard planning and risk mitigation. Terrestrial stratigraphic tephra records can be patchy and incomplete due to subsequent erosion and burial processes. In contrast the marine sedimentary record commonly preserves a more complete historical record of volcanic activity as individual events are archived within continually accumulating background sediments. While larger tephra layers are often identifiable by changes in sediment color and/or texture, smaller fallout layers may also be present that are not visible to the naked eye. These cryptotephra are commonly more difficult to identify and often require time‐consuming and destructive point counting, petrography, and microscopy work. Here we present several rapid, non‐destructive, and quantitative core‐scanning methodologies (magnetic susceptibility, visible to shortwave infrared spectroscopy, and XRF core scanning) which, when combined, can be used to identify the presence of increased volcaniclastic components (interpreted to be cryptotephra) in the sedimentary record. We develop a new spectral parameter (BDI1000VIS) that exploits the absorption of the 1 µm near‐infrared band in tephra. Using pre‐determined mixtures BDI1000VIS can accurately identify tephra layers in concentrations >15–20%. When applied to the upper ∼270 kyr record of IODP core U1396C from the Caribbean Sea, and verified by traditional point counting, twenty‐nine potential cryptotephra layers were identified as originating from eruptions of the Lesser Antilles Volcanic Arc. Application of these methods in future coring endeavors can be used to minimize the need for physical disaggregation of valuable drill core material and allow for near‐real time recognition of tephra units, both visible and cryptotephra. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-14T09:56:49.771912-05:
      DOI: 10.1002/2015GC005913
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