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Publisher: American Geophysical Union (AGU)   (Total: 17 journals)

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: 53, SJR: 3.493, h-index: 157)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 5, SJR: 3.239, h-index: 119)
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J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 6)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 24)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 15)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 13)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 26)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 15)
Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 3.22, h-index: 88)
Radio Science     Full-text available via subscription   (Followers: 3, SJR: 0.959, h-index: 51)
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Tectonics     Full-text available via subscription   (Followers: 9, SJR: 2.748, h-index: 85)
Water Resources Research     Full-text available via subscription   (Followers: 207, SJR: 2.189, h-index: 121)
Journal Cover   Geochemistry, Geophysics, Geosystems
  [SJR: 2.56]   [H-I: 69]   [27 followers]  Follow
   Full-text available via subscription Subscription journal
   ISSN (Online) 1525-2027
   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • The insular shelves of the Faial–Pico Ridge (Azores archipelago): A
           morphological record of its evolution
    • Authors: R. Quartau; J. Madeira, N. C. Mitchell, F. Tempera, P. F. Silva, F. Brandão
      Abstract: Shelves surrounding reefless volcanic ocean islands are formed by surf erosion of their slopes during changing sea levels. Post‐erosional lava flows, if abundant, can cross the coastal cliffs and fill partially or completely the accommodation space left by erosion. In this study, multibeam bathymetry, high‐resolution seismic reflection profiles and sediment samples are used to characterize the morphology of the insular shelves adjacent to Faial and Pico islands. The data shows offshore fresh lava flow morphologies, as well as an irregular basement beneath shelf sedimentary bodies and reduced shelf width adjacent to older volcanic edifices. These observations suggest that the shelves have been significantly filled by volcanic progradation and can thus be classified as “rejuvenated”. Despite the general volcanic infilling of the shelves around the islands, most of their edges are below the depth of the Last Glacial Maximum, revealing that that at least parts of the island have subsided after the shelves formed by surf erosion. Prograding lava deltas reached the shelf edge in some areas triggering small slope failures, locally decreasing the shelf width and depth of their edges. These areas can represent a significant risk for the local population; hence their identification can be useful for hazard assessment and contribute to wiser land‐use planning. Shelf morphology, subaerial geomorphology, magnetic anomalies and crustal structure data were also interpreted to reconstruct the longer term combined onshore and offshore evolution of the Faial‐Pico ridge. The subaerial emergence of this ridge is apparently older than previously thought, i.e., before ∼850 ka. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-15T10:57:56.793069-05:
      DOI: 10.1002/2015GC005733
  • Magnetic and geochemical characterization of Andosols developed on basalts
           in the Massif Central, France
    • Authors: Hana Grison; Eduard Petrovsky, Sarka Stejskalova, Ales Kapicka
      Abstract: Identification of Andosols is primarily based upon the content of their colloidal constituents – clay and metal‐humus complexes – and on the determining of andic properties. This needs time and cost‐consuming geochemical analyses. Our primary aim of this study is to describe the magnetic and geochemical properties of soils rich in iron oxides derived from strongly magnetic volcanic basement (in this case Andosols). Secondary aim is to explore links between magnetic and chemical parameters of Andic soils with respect to genesis factors: parent material age, precipitation and thickness of the soil profile. Six pedons of andic properties, developed on basaltic lavas, were analysed down to parent rock by a set of magnetic and geochemical methods. Magnetic data of soil and rock samples reflect the type, concentration and particle‐size distribution of ferrimagnetic minerals. Geochemical data include soil reaction (pH in H2O), cation exchange capacity, organic carbon and different forms of extractable iron and aluminum content. Our results suggest the following: (1) Magnetic measurements of low‐field mass‐specific magnetic susceptibility can be a reliable indicator for estimating andic properties, and in combination with thermomagnetic curves may be suitable for discriminating between alu‐andic and sil‐andic subtypes. (2) In the studied Andosols, strong relationships were found between (a) magnetic grain‐size parameters, precipitation and exchangeable bases; (b) concentration of ferrimagnetic particles and degree of crystallization of free iron; and (c) parameters reflecting changes in magneto‐mineralogy and soil genesis (parent material age + soil depth). This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-15T10:44:12.802248-05:
      DOI: 10.1002/2015GC005716
  • Abbot Ice Shelf, structure of the Amundsen Sea continental margin and the
           southern boundary of the Bellingshausen Plate seaward of West Antarctica
    • Authors: James R. Cochran; Kirsty J. Tinto, Robin E. Bell
      Abstract: Inversion of NASA Operation IceBridge airborne gravity over the Abbot Ice Shelf in West Antarctica for sub‐ice bathymetry defines an extensional terrain made up of east‐west trending rift basins formed during the early stages of Antarctica/Zealandia rifting. Extension is minor, as rifting jumped north of Thurston Island early in the rifting process. The Amundsen Sea Embayment continental shelf west of the rifted terrain is underlain by a deeper, more extensive sedimentary basin also formed during rifting between Antarctica and Zealandia. A well‐defined boundary zone separates the mildly extended Abbot extensional terrain from the deeper Amundsen Embayment shelf basin. The shelf basin has an extension factor, β, of 1.5‐1.7 with 80‐100 km of extension occurring across an area now 250 km wide. Following this extension, rifting centered north of the present shelf edge and proceeded to continental rupture. Since then, the Amundsen Embayment continental shelf appears to have been tectonically quiescent and shaped by subsidence, sedimentation and the advance and retreat of the West Antarctic Ice Sheet. The Bellingshausen Plate was located seaward of the Amundsen Sea margin prior to incorporation into the Antarctic Plate at about 62 Ma. During the latter part of its independent existence, Bellingshausen plate motion had a clockwise rotational component relative to Antarctica producing convergence across the north‐south trending Bellingshausen Gravity Anomaly structure at 94°W and compressive deformation on the continental slope between 94°W and 102°W. Farther west, the relative motion was extensional along an east‐west trending zone occupied by the Marie Byrd Seamounts. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-15T10:43:16.07419-05:0
      DOI: 10.1002/2014GC005570
  • Geochemical variations in Japan Sea back‐arc basin basalts formed by
           high‐temperature adiabatic melting of mantle metasomatized by
           sediment subduction components
    • Authors: Yuka Hirahara; Jun‐Ichi Kimura, Ryoko Senda, Takashi Miyazaki, Hiroshi Kawabata, Toshiro Takahashi, Qing Chang, Vaglarov S. Bogdan, Takeshi Sato, Shuichi Kodaira
      Abstract: The Yamato Basin in the Japan Sea is a back‐arc basin characterized by basaltic oceanic crust that is twice as thick as typical oceanic crust. Two types of ocean floor basalts, formed during the opening of the Japan Sea in the Middle Miocene, were recovered from the Yamato Basin during Ocean Drilling Program Legs 127/128. These can be considered as depleted (D‐type) and enriched (E‐type) basalts based on their incompatible trace element and Sr–Nd–Pb–Hf isotopic compositions. Both types of basalts plot along a common mixing array drawn between depleted mantle and slab sediment represented by a sand‐rich turbidite on the Pacific Plate in the NE Japan forearc. The depleted nature of the D‐type basalts suggests that the slab sediment component is nil to minor relative to the dominant mantle component, whereas the enrichment of all incompatible elements in the E‐type basalts was likely caused by a large contribution of bulk slab sediment in the source. The results of forward model calculations using adiabatic melting of a hydrous mantle with sediment flux indicate that the melting conditions of the source mantle for the D‐type basalts are deeper and hotter than those for the E‐type basalts, which appear to have formed under conditions hotter than those of normal mid‐oceanic ridge basalts (MORB). These results suggest that the thicker oceanic crust was formed by greater degrees of melting of a hydrous metasomatized mantle source at unusually high mantle potential temperature during the opening of the Japan Sea. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-15T10:42:48.440545-05:
      DOI: 10.1002/2015GC005720
  • Issue Information
    • PubDate: 2015-04-15T07:07:08.789262-05:
      DOI: 10.1002/ggge.20559
  • Basalt Volatile Fluctuations During Continental Rifting: An Example From
           the Rio Grande Rift, USA
    • Authors: Michael C. Rowe; John C. Lassiter, Kathleen Goff
      Abstract: Hydration and metasomatism of the lithospheric mantle potentially influences both the magmatic and tectonic evolution of southwestern North America. Prior studies have suggested that volatile enrichments to the mantle underlying western North America resulted from shallow subduction of the Farallon Plate during the Laramide (∼74‐40 Ma). This study examines temporal and spatial variations in volatile elements (H2O, Cl, F, S) determined from olivine‐ and orthopyroxene‐hosted melt inclusions along and across the Rio Grande Rift, the easternmost extent of Laramide shallow subduction. Maximum chlorine enrichments are observed in the southern rift with a Cl/Nb of ∼210 and reduce with time to MORB‐OIB levels (∼5‐17). Measured water abundances are
      PubDate: 2015-04-14T14:10:47.507959-05:
      DOI: 10.1002/2014GC005649
  • Estimates of future warming‐induced methane emissions from hydrate
           offshore West Svalbard for a range of climate models
    • Authors: Héctor Marín‐Moreno; Timothy A. Minshull, Graham K. Westbrook, Bablu Sinha
      Abstract: Methane hydrate close to the hydrate stability limit in seafloor sediment could represent an important source of methane to the oceans and atmosphere as the oceans warm. We investigate the extent to which patterns of past and future ocean‐temperature fluctuations influence hydrate stability in a region offshore West Svalbard where active gas venting has been observed. We model the transient behavior of the gas hydrate stability zone at 400‐500 m water depth (mwd) in response to past temperature changes inferred from historical measurements and proxy data and we model future changes predicted by seven climate models and two climate‐forcing scenarios (Representative Concentration Pathways RCPs 2.6 and 8.5). We show that over the past 2000 yr, a combination of annual and decadal temperature fluctuations could have triggered multiple hydrate‐sourced methane emissions from seabed shallower than 400 mwd during episodes when the multi‐decadal average temperature was similar to that over the last century (∼2.6°C). These temperature fluctuations can explain current methane emissions at 400 mwd, but decades to centuries of ocean warming are required to generate emissions in water deeper than 420 m. In the venting area, future methane emissions are relatively insensitive to the choice of climate model and RCP scenario until 2050 yr, but are more sensitive to the RCP scenario after 2050 yr. By 2100 CE, we estimate an ocean uptake of 97‐1050 TgC from marine Arctic hydrate‐sourced methane emissions, which is 0.06‐0.67% of the ocean uptake from anthropogenic CO2 emissions for the period 1750‐2011. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-10T01:19:54.574729-05:
      DOI: 10.1002/2015GC005737
  • Investigation of the records of earthquake slip in carbonaceous materials
           from the Taiwan Chelungpu fault by means of infrared and Raman
    • Authors: Tetsuro Hirono; Yuka Maekawa, Hikaru Yabuta
      Abstract: To understand the mechanism of fault lubrication during the 1999 Taiwan Chi‐Chi earthquake, we developed a new temperature proxy for carbonaceous materials by using infrared and Raman spectroscopies together with heating and friction experiments. We found marked anomalies in the infrared and Raman spectra of carbonaceous materials retrieved from the primary slip zone of the earthquake: the infrared spectra exhibited very weak aliphatic CH2 and CH3 peaks and aromatic C=C absorbance peaks, and the Raman spectra exhibited very weak disordered and graphitic bands and a high ratio of disordered band area to graphitic band area. Those weak peaks and bands and the band area ratio were reproduced by heating carbonaceous materials from the nearby host rock to 700 °C. These results suggest that the frictional heat in the slip zone reached approximately 700 °C. We characterized the host rock's carbonaceous materials by means of elemental analysis, pyrolysis–gas chromatography–mass spectrometry, and simultaneous thermogravimetry–differential scanning calorimetry and found that the H/C and O/C ratios were 0.108 and 0.400, respectively (which are close to the ratios for lignin) and that the volatile fraction was as high as 48 wt %. The pyrolysates obtained by heating from 100 to 400 °C were dominated by phenols, fatty alcohols, and n‐alkanes. When the residue from pyrolysis at 100–400 °C was rapidly heated to 700 °C, the resulting pyrolysate was dominated by phenols, aromatic compounds, heterocyclic compounds, and n‐alkenes. This information suggests that changes in the infrared and Raman spectra with increasing temperature may have been due to decomposition and aromatization reactions during pyrolysis. Rapid heating during earthquake slip may promote reactions of carbonaceous materials that are different from the reactions that occur during long‐term metamorphism. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-09T02:34:31.598074-05:
      DOI: 10.1002/2014GC005622
  • Occurrence of greigite in the Pliocene sediments of Lake Qinghai, China,
           and its paleoenvironmental and paleomagnetic implications
    • Authors: FU Chaofeng; Jan Bloemendal, Qiang Xiaoke, Mimi J. Hill, AN ZhiSheng
      Abstract: Lake Qinghai in North China is the largest interior plateau lake in Central Asia and is climatically sensitive. An almost continuous 626‐m long sediment core was drilled in an in‐filled part of the southern lake basin of Lake Qinghai. The magnetic susceptibility record reveals the presence of two distinct peaks within an interval of fine‐grained lacustrine sediments of Lower Pliocene age. We selected a depth interval of approximately 40 m spanning the magnetic susceptibility peaks for detailed rock magnetic and geochemical analyses in order to identify the magnetic mineralogy responsible and to assess its possible paleoenvironmental and paleomagnetic implications. Rock magnetic, X‐ray diffraction analysis, scanning electron microscopy (SEM) and energy dispersive analysis of X‐ray (EDAX) analyses reveal that the main magnetic carrier is greigite (Fe3S4). The greigite is of early diagenetic origin and formed in an interval of high lake level and inferred relatively warm, humid climate. The greigite‐enriched zones are separated by an interval of relatively high total sulfur and organic carbon content and we infer that in the adjacent greigite‐bearing zones the lower concentrations of sulfur and organic carbon, and high levels of reactive iron, arrested the process of pyritization resulting in the preservation of the greigite on a timescale of several million years. The greigite zones contain narrow intervals of normally‐magnetized sediments which may be previously unrecognized cryptochrons within the Gilbert Chron, or alternatively they may reflect the continued formation of greigite long after the age of deposition of the surrounding sediment matrix. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-09T02:33:28.330578-05:
      DOI: 10.1002/2014GC005677
  • Quantifying near‐field and off‐fault deformation patterns of
           the 1992 Mw 7.3 Landers earthquake
    • Authors: Christopher W.D. Milliner; James F. Dolan, James Hollingsworth, Sebastien Leprince, Francois Ayoub, Charles Sammis
      Abstract: Co‐seismic surface deformation in large earthquakes is typically measured using field mapping and with a range of geodetic methods (e.g., InSAR, lidar differencing, and GPS). Current methods, however, either fail to capture patterns of near‐field co‐seismic surface deformation or lack pre‐event data. Consequently, the characteristics of off‐fault deformation and the parameters that control it remain poorly understood. We develop a standardized method to fully measure the surface, near‐field, co‐seismic deformation patterns at high‐resolution using the COSI‐Corr program by correlating pairs of aerial photographs taken before and after the 1992 Mw 7.3 Landers earthquake. COSI‐Corr offers the advantage of measuring displacement across the entire zone of surface deformation and over a wider aperture than that available to field geologists. For the Landers earthquake, our measured displacements are systematically larger than the field measurements, indicating the presence of off‐fault deformation. We show that 46 % of the total surface displacement occurred as off‐fault deformation, over a mean deformation width of 154 m. The magnitude and width of off‐fault deformation along the rupture is primarily controlled by the macroscopic structural complexity of the fault system, with a weak correlation with the type of near‐surface materials through which the rupture propagated. Both the magnitude and width of distributed deformation are largest in stepovers, bends, and at the southern termination of the surface rupture. We find that slip along the surface rupture exhibits a consistent degree of variability at all observable length scales and that the slip distribution is self‐affine fractal with dimension of 1.56. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-07T06:22:20.782611-05:
      DOI: 10.1002/2014GC005693
  • What processes control the chemical compositions of arc front
    • Authors: Stephen J Turner; Charles H Langmuir
      Abstract: Arc front stratovolcanoes have global chemical systematics that constrain processes at convergent margins. Positive correlations exist for arc averages among “fluid mobile,” “high field strength,” and “large ion lithophile” elements. 143Nd/144Nd and 87Sr/86Sr from rear‐arc lavas lacking subduction signature align with the oceanic “mantle array,” and correlate with arc front 143Nd/144Nd. Most chemical parameters (but not isotopes) also correlate well with crustal thickness and slightly less well with the slab thermal parameter, but not with the depth of the slab or model slab surface temperatures. Successful models of arc volcanism should account for these global regularities. Two distinct models can quantitatively account for the observations—different extents of melting of the mantle wedge caused by variations in wedge thermal structure, or varying contributions from the subducting slab owing to variations in the slab thermal structure. The wedge melting model has constant contributions from ocean crust, sediment and mantle wedge to lavas globally, while the slab model varies slab contributions with slab temperature. The wedge melting model fit improves by incorporating convergence rate and slab dip, which should affect the wedge thermal structure; the slab model is not supported by a similar analysis. The wedge model also more easily accommodates the isotope data. The two models predict different primary H2O contents, with large variations in H2O for the wedge model, and relatively constant H2O for the slab model. An evaluation of the effects of varying sediment compositions on arc lavas will benefit from considering the very different consequences of the two models. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-07T05:31:15.394429-05:
      DOI: 10.1002/2014GC005633
  • A protocol for variable‐resolution first‐order reversal curve
           (FORC) measurements
    • Authors: Xiang Zhao; David Heslop, Andrew P. Roberts
      Abstract: High‐resolution first‐order reversal curve (FORC) diagrams are being increasingly used in rock and environmental magnetism, including for detection of biomagnetic signals in sediments. Resolution can be a major barrier to obtaining high‐quality FORC diagrams and time‐consuming measurements that employ small field steps are necessary to resolve the finest features of a FORC distribution. We present a new experimental protocol with irregularly spaced field steps that allow different parts of a FORC diagram to be measured at different resolutions. Larger numbers of measurements can, therefore, be made in key regions of a FORC distribution to resolve diagnostic features at higher resolution. Specification of the field steps in the irregular measurement grid is based on major hysteresis properties; no a priori knowledge concerning the underlying FORC distribution is required. FORC diagrams obtained with conventional measurements and with our new measurement protocol give consistent results. Because of its variable resolution, the irregular protocol provides a clear representation of fine‐scale features produced by quasi‐reversible superparamagnetic and non‐interacting single‐domain particles. Although the proposed irregular measurement protocol is not as efficient at suppressing noise as recently developed post‐processing techniques (e.g., VARIFORC, Egli [2013]), it enables efficient high‐resolution analysis for relatively strongly magnetized samples where measurement noise is not detrimental to FORC distribution estimation. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-07T04:58:19.327094-05:
      DOI: 10.1002/2014GC005680
  • Development of topography in 3‐D continental collision models
    • Authors: A. E. Pusok; Boris J. P. Kaus
      Abstract: Understanding the formation and evolution of high mountain belts, such as the Himalayas and the adjacent Tibetan Plateau, has been the focus of many tectonic and numerical models. Here, we employ 3D numerical simulations to investigate the role that subduction, collision and indentation play on lithosphere dynamics at convergent margins, and to analyze the conditions under which large topographic plateaus can form in an integrated lithospheric and upper‐mantle scale model. Distinct dynamics are obtained for the oceanic subduction side (trench retreat, slab roll‐back) and the continental‐collision side (trench advance, slab detachment, topographic uplift, lateral extrusion). We show that slab‐pull alone is insufficient to generate high topography in the upper‐plate, and that external forcing and the presence of strong blocks such as the Tarim Basin are necessary to create and shape anomalously high topographic fronts and plateaus. Moreover, scaling is used to predict four different modes of surface expression in continental‐collision models: (I)‐low‐amplitude homogenous shortening, (II)‐high‐amplitude homogenous shortening, (III)‐Alpine‐type topography with topographic front and low plateau, and (IV)‐Tibet‐Himalaya‐type topography with topographic front and high plateau. Results of semi‐analytical models suggest that the Argand number governs the formation of high topographic fronts, while the amplitude of plateaus is controlled by the initial buoyancy ratio of the upper plate. Applying these results to natural examples, we show that the Alps belong to regime (III), the Himalaya‐Tibet to regime (IV), whereas the Andes‐Altiplano fall at the boundary between regimes (III)‐(IV). This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-06T10:53:20.148008-05:
      DOI: 10.1002/2015GC005732
  • Water in Hawaiian peridotite minerals: A case for a dry metasomatized
           oceanic mantle lithosphere
    • Authors: Anne H. Peslier; Michael Bizimis
      Abstract: The distribution of water concentrations in the upper mantle has drastic influence on its melting, rheology, and electrical and thermal conductivities and yet is primarily known indirectly from analyses of OIB and MORB. Here, actual mantle samples, eight peridotite xenoliths from Salt Lake Crater (SLC) and one from Pali in Oahu in Hawaii were analyzed by FTIR. Water contents of orthopyroxene, clinopyroxene and the highest measured in olivine are 116‐222, 246‐442, and 10‐26 ppm weight H2O respectively. Although pyroxene water contents correlate with indices of partial melting, they are too high to be explained by simple melting modeling. Mantle‐melt interaction modeling reproduces best the SLC data. These peridotites represent depleted oceanic mantle older than the Pacific lithosphere that has been refertilized by nephelinite melts containing
      PubDate: 2015-04-06T10:35:52.190899-05:
      DOI: 10.1002/2015GC005780
  • The electrical structure of the central Pacific upper mantle constrained
           by the NoMelt experiment
    • Authors: Emily Sarafian; Rob. L. Evans, John Collins, Jimmy Elsenbeck, Glenn Gaetani, James B. Gaherty, Greg Hirth, Daniel Lizarralde
      Abstract: The NoMelt Experiment imaged the mantle beneath 70 Ma Pacific seafloor with the aim of understanding the transition from the lithosphere to the underlying convecting asthenosphere. Seafloor magnetotelluric data from four stations were analyzed using 2‐D regularized inverse modeling. The preferred electrical model for the region contains an 80 km thick resistive (>103 Ωm) lithosphere with a less resistive (∼50 Ωm) underlying asthenosphere. The preferred model is isotropic and lacks a highly conductive (≤10 Ωm) layer under the resistive lithosphere that would be indicative of partial melt. We first examine temperature profiles that are consistent with the observed conductivity profile. Our profile is consistent with a mantle adiabat ranging from 0.3 °C/km‐0.5 °C/km. A choice of the higher adiabatic gradient means that the observed conductivity can be explained solely by temperature. In contrast, a 0.3 °C/km adiabat requires an additional mechanism to explain the observed conductivity profile. Of the plausible mechanisms, H2O, in the form of hydrogen dissolved in olivine, is the most likely explanation for this additional conductivity. Our profile is consistent with a mostly dry lithosphere to 80 km depth, with bulk H2O contents increasing to between 25 and 400 ppm by weight in the asthenosphere with specific values dependent on the choice of laboratory dataset of hydrous olivine conductivity and the value of mantle oxygen fugacity. The estimated H2O contents support the theory that the rheological lithosphere is a result of dehydration during melting at a mid‐ocean ridge with the asthenosphere remaining partially hydrated and weakened as a result. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-28T08:38:09.515254-05:
      DOI: 10.1002/2014GC005709
  • Tracking the weathering of basalts on Mars using lithium isotope
           fractionation models
    • Authors: Alberto G. Fairén; Elisabeth L. Adams, Carolina Gil, Luis Gago‐Duport, Esther R. Uceda, Steven W. Squyres, J. Alexis P. Rodríguez, Alfonso F. Davila, Christopher P. McKay
      Abstract: Lithium (Li), the lightest of the alkali elements, has geochemical properties that include high aqueous solubility (Li is the most fluid mobile element) and high relative abundance in basalt‐forming minerals (values ranking between 0.2 and 12 ppm). Li isotopes are particularly subject to fractionation because the two stable isotopes of lithium – 7Li and 6Li – have a large relative mass difference (∼15%) that results in significant fractionation between water and solid phases. The extent of Li isotope fractionation during aqueous alteration of basalt depends on the dissolution rate of primary minerals – the source of Li – and on the precipitation kinetics leading to formation of secondary phases. Consequently, a detailed analysis of Li isotopic ratios in both solution and secondary mineral lattices could provide clues about past martian weathering conditions, including weathering extent, temperature, pH, supersaturation and evaporation rate of the initial solutions in contact with basalt rocks. In this paper we discuss ways in which martian aqueous processes could have lead to Li isotope fractionation. We show that Li isotopic data obtained by future exploration of Mars could be relevant to highlighting different processes of Li isotopic fractionation in the past, and therefore to understanding basalt weathering and environmental conditions early in the planet's history. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-28T08:18:22.479653-05:
      DOI: 10.1002/2015GC005748
  • An open source Bayesian Monte Carlo isotope mixing model with applications
           in Earth surface processes
    • Authors: Carli A. Arendt; Sarah M. Aciego, Eric A. Hetland
      Abstract: The implementation of isotopic tracers as constraints on source contributions has become increasingly relevant to understanding Earth surface processes. Interpretation of these isotopic tracers has become more accessible with the development of Bayesian Monte Carlo (BMC) mixing models, which allow uncertainty in mixing end‐members and provide methodology for systems with multi‐component mixing. This study presents an open source multiple isotope BMC mixing model that is applicable to Earth surface environments with sources exhibiting distinct end‐member isotopic signatures. Our model is first applied to new δ18O and δD measurements from the Athabasca Glacier, which showed expected seasonal melt evolution trends and vigorously assessed the statistical relevance of the resulting fraction estimations. To highlight the broad applicability of our model to a variety of Earth surface environments and relevant isotopic systems we expand our model to two additional case studies: deriving melt sources from δ18O, δD and 222Rn measurements of Greenland Ice Sheet bulk water samples and assessing nutrient sources from ϵNd and 87Sr/86Sr measurements of Hawaiian soil cores. The model produces results for the Greenland Ice Sheet and Hawaiian soil datasets that are consistent with the originally published fractional contribution estimates. The advantage of this method is that it quantifies the error induced by variability in the end‐member compositions, unrealized by the models previously applied to the above case studies. Results from all three case studies demonstrate the broad applicability of this statistical BMC isotopic mixing model for estimating source contribution fractions in a variety of Earth surface systems. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-27T03:36:25.743217-05:
      DOI: 10.1002/2014GC005683
  • The thinning of subcontinental lithosphere: The roles of plume impact and
           metasomatic weakening
    • Authors: Hongliang Wang; Jeroen van Hunen, D. Graham Pearson
      Abstract: Geologically rapid (10s of Myr) partial removal of thick continental lithosphere is evident beneath Precambrian terranes such as North China craton, southern Africa and the North Atlantic Craton, and has been linked with thermo‐mechanical erosion by mantle plumes. We performed numerical experiments with realistic viscosities to test this hypothesis and constrain the most important parameters that influence cratonic lithosphere erosion. Our models indicate that the thermo‐mechanical erosion by a plume impact on typical Archean lithospheric mantle is unlikely to be more effective than long‐term erosion from normal plate‐mantle interaction. Therefore, unmodified cratonic roots that have been stable for billions of years will not be significantly disrupted by the erosion of a plume event. However, the buoyancy and strength of highly depleted continental roots can be modified by fluid‐melt metasomatism, and our models show this is essential for the thinning of originally stable continental roots. The long‐term but punctuated history of metasomatic enrichment beneath ancient continents makes this mode of weakening very likely. The effect of the plume impact is to speed up the erosion significantly and help the removal of the lithospheric root to occur within 10s of Myrs if affected by metasomatic weakening. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-27T03:36:14.766436-05:
      DOI: 10.1002/2015GC005784
  • Azimuthal anisotropy beneath north central Africa from shear wave
           splitting analyses
    • Authors: Awad A. Lemnifi; Kelly H. Liu, Stephen S. Gao, Cory A. Reed, Ahmed A. Elsheikh, Youqiang Yu, Abdala A. Elmelade
      Abstract: This study represents the first multi‐station investigation of azimuthal anisotropy beneath the interior of north central Africa, including Libya and adjacent regions, using shear wave splitting (SWS) analysis. Data used in the study include recently available broadband seismic data obtained from 15 stations managed by the Libyan Center for Remote Sensing and Space Science, and those from 5 other stations at which data are publicly accessible. A total of 583 pairs of high‐quality SWS measurements utilizing the PKS, SKKS, and SKS phases demonstrate primarily N‐S fast orientations with an average splitting delay time of approximately 1.2 s. An absence of periodic azimuthal variation of the observed splitting parameters indicates the presence of simple anisotropy, and lack of correlation between surficial features and the splitting parameters suggests that the origin of the observed anisotropy is primarily asthenospheric. This conclusion is enhanced by non‐periodic azimuthal variation of the splitting parameters observed at one of the stations located near the boundary of areas with different anisotropic properties. We interpret the observed anisotropy to be the consequence of northward movement of the African plate relative to the asthenosphere toward the Hellenic and Calabrian subduction zones. Local variance in fast orientations may be attributable to flow deflection by the northern edge of the African continental root. The observations provide critical and previously lacking constraints on mantle dynamic models in the vicinity of the convergent boundary between the African and Eurasian plates. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-27T03:36:03.520542-05:
      DOI: 10.1002/2014GC005706
  • Helium isotope, C/3He, and Ba‐Nb‐Ti signatures in the northern
           Lau Basin: Distinguishing arc, back‐arc, and hot spot affinities
    • Authors: John Lupton; Ken H. Rubin, Richard Arculus, Marvin Lilley, David Butterfield, Joseph Resing, Edward Baker, Robert Embley
      Abstract: The northern Lau Basin hosts a complicated pattern of volcanism, including Tofua Arc volcanoes, several back‐arc spreading centers and individual “rear‐arc” volcanoes not associated with these structures. Elevated 3He/4He ratios in lavas of the NW Lau Spreading Center suggest the influence of a mantle plume, possibly from Samoa. We show that lavas from mid‐ocean ridges, volcanic arcs, and hotspots occupy distinct, non‐overlapping fields in a 3He/4He vs. C/3He plot. Applied to the northern Lau Basin, this approach shows that most of Lau back‐arc spreading systems have mid‐ocean ridge 3He/4He ‐ C/3He characteristics, except the NW Lau spreading center, which has 3He/4He ‐ C/3He similar to “high 3He” hotspots such as Loihi, Kilauea, and Yellowstone, but with slightly lower C/3He. Niua seamount, on the northern extension of the Tofua Arc, falls squarely in the arc field. All of the NE Lau rear‐arc volcanoes, including the recently erupting West Mata, also have arc‐like 3He/4He ‐ C/3He characteristics. Ba‐Nb‐Ti contents of the lavas, which are more traditional trace element indicators of mantle source enrichment, depletion and subduction input, likewise indicate arc and hot spot influences in the lavas of the Northern Lau Basin, but in a more ambiguous fashion because of a complex prior history. This verifies that 3He/4He ‐ C/3He systematics are useful for differentiating between mid‐ocean ridge, arc, and hotspot affinities in submarine volcanic systems, that all three of these affinities are expressed in the northern Lau Basin, and provides additional support for the Samoan plume influence in the region. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-21T09:13:28.854924-05:
      DOI: 10.1002/2014GC005625
  • Gravity anomalies, crustal structure, and seismicity at subduction zones:
           2. Interrelationships between fore‐arc structure and seismogenic
    • Authors: Dan Bassett; Anthony B. Watts
      Abstract: An ensemble averaging technique is used to remove the long‐wavelength topography and gravity field associated with subduction zones. Short‐wavelength residual anomalies are attributed to the tectonic structure of subducting and overthrusting plates. A paired (positive‐negative) forearc anomaly is observed consisting of a long (>1000 km), linear, trench‐parallel ridge landward of the deep‐sea‐terrace basin. Ridges have amplitudes of 1500‐3000 m and 160‐240 mGal, wavelengths of 150‐200 km, and high gravity anomaly to topography ratios (50‐75 mGal km−1). The ridge crests correlate with the down‐dip limit of coseismic slip and strong interplate coupling and in Cascadia, the up‐dip limit of tremor epicenters. The ridge crest may be interpreted as defining the boundary between the velocity‐weakening and seismogenic region of the subduction interface and the down‐dip frictional transition zone. In Tonga‐Kermadec, the Kuril Islands and Chile landward ridges are associated with extinct volcanic arcs. Paired anomalies are attributed to the preferential subduction erosion of the outer‐forearc and a spatially varying combination of a) lower‐crustal underplating beneath the inner forearc, b) the transformation of interseismic strain into permanent geologic strain via faulting, folding or buckling of the inner‐forearc, and c) the relative trenchward migration of extinct volcanic arcs in regions operating with a net crustal deficit. Along‐strike transitions in forearc morphology and seismogenic behavior are related to pre‐existing crustal structure of subducting and overthrusting plates. Forearcs have the added potential of recording the time‐integrated response of the upper‐plate to subduction processes, and forearc structure should be considered in tandem with seismological observations. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-21T08:32:39.658955-05:
      DOI: 10.1002/2014GC005685
  • Episodic Holocene eruption of the Salton Buttes rhyolites, California,
           from paleomagnetic, U‐Th, and Ar/Ar dating
    • Authors: Heather M. Wright; Jorge A. Vazquez, Duane E. Champion, Andrew T. Calvert, Margaret T. Mangan, Mark Stelten, Kari M. Cooper, Charles Herzig, Alexander Schriener
      Abstract: In the Salton Trough, CA, five rhyolite domes form the Salton Buttes: Mullet Island, Obsidian Butte, Rock Hill, and North and South Red Hills, from oldest to youngest. Results presented here include 40Ar/39Ar anorthoclase ages, 238U‐230Th zircon crystallization ages, and comparison of remanent paleomagnetic directions with the secular variation curve, which indicate that all domes are Holocene. 238U‐230Th zircon crystallization ages are more precise but within uncertainty of 40Ar/39Ar anorthoclase ages, suggesting that zircon crystallization proceeded until shortly before eruption in all cases except one. Remanent paleomagnetic directions require three eruption periods: (1) Mullet Island, (2) Obsidian Butte, (3) Rock Hill, North Red Hill, and South Red Hill. Borehole cuttings logs document up to two shallow tephra layers. North and South Red Hill likely erupted within 100 years of each other, with a combined 238U‐230Th zircon isochron age of: 2.83 ± 0.60 ka (2 sigma); paleomagnetic evidence suggests this age predates eruption by hundreds of years (1800 cal BP). Rock Hill erupted closely in time to these eruptions. The Obsidian Butte 238U‐230Th isochron age (2.86 ± 0.96 ka) is nearly identical to the combined Red Hill age, but its Virtual Geomagnetic Pole position suggests a slightly older age. The age of aphyric Mullet Island dome is the least well constrained: zircon crystals are resorbed and the paleomagnetic direction is most distinct; possible Mullet Island ages include ca. 2300, 5900, 6900, and 7700 cal BP. Our results constrain the duration of Salton Buttes volcanism to between ca. 5900 and 500 years. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-20T06:13:13.923188-05:
      DOI: 10.1002/2015GC005714
  • The impact of splay faults on fluid flow, solute transport, and pore
           pressure distribution in subduction zones: A case study offshore the
           Nicoya Peninsula, Costa Rica
    • Authors: Rachel M. Lauer; Demian M. Saffer
      Abstract: Observations of seafloor seeps on the continental slope of many subduction zones illustrate that splay faults represent a primary hydraulic connection to the plate boundary at depth, carry deeply sourced fluids to the seafloor, and are in some cases associated with mud volcanoes. However, the role of these structures in forearc hydrogeology remains poorly quantified. We use a 2‐D numerical model that simulates coupled fluid flow and solute transport driven by fluid sources from tectonically driven compaction and smectite transformation to investigate the effects of permeable splay faults on solute transport and pore pressure distribution. We focus on the Nicoya margin of Costa Rica as a case study, where previous modeling and field studies constrain flow rates, thermal structure, and margin geology. In our simulations, splay faults accommodate up to 33% of the total dewatering flux, primarily along faults that outcrop within 25‐km of the trench. The distribution and fate of dehydration‐derived fluids is strongly dependent on thermal structure, which determines the locus of smectite transformation. In simulations of a cold end‐member margin, smectite transformation initiates 30‐km from the trench, and 64% of the dehydration‐derived fluids are intercepted by splay faults and carried to the middle and upper slope, rather than exiting at the trench. For a warm end‐member, smectite transformation initiates 7‐km from the trench, and the associated fluids are primarily transmitted to the trench via the décollement (50%), and faults intercept only 21% of these fluids. For a wide range of splay fault permeabilities, simulated fluid pressures are near lithostatic where the faults intersect overlying slope sediments, providing a viable mechanism for the formation of mud volcanoes. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-20T05:21:37.912904-05:
      DOI: 10.1002/2014GC005638
  • A global model for cave ventilation and seasonal bias in speleothem
           paleoclimate records
    • Authors: Eric W. James; Jay L. Banner, Benjamin Hardt
      Abstract: Cave calcite deposits (speleothems) provide long and continuous records of paleoenvironmental conditions in terrestrial settings. Typical environmental proxy measurements include speleothem growth rate and variations in elemental and isotope geochemistry. Commonly the assumption is made that speleothems grow continuously and at a constant rate throughout the year. However, seasonal variation of growth rate may be the rule in many caves. Here we apply observations of modern calcite growth and cave‐air CO2 concentrations and a model of factors controlling cave ventilation to construct a global model predicting where cave calcite growth may be seasonal. Previous models and measurements of calcite precipitation in caves demonstrate the retardation of speleothem growth by high cave‐air CO2. Elevated CO2 is commonly dissipated by ventilation driven by density differences between cave and surface air. Seasonal cycles in atmospheric temperature, pressure and humidity commonly drive these density contrasts. Modeling these changes latitudinally and globally indicates a geographic control on seasonal cave ventilation and thus on a principal controlling factor of speleothem growth. The model predicts that given constant water, calcium and CO2 inputs, speleothems from temperate to boreal continental regions commonly accumulate more calcite in the cool season and less or none in the warm season. These models predict that proxies from temperate to boreal speleothems may be seasonally biased due to seasonal ventilation, whereas tropical and maritime records should not. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-16T21:41:20.318093-05:
      DOI: 10.1002/2014GC005658
  • Natural remanent magnetization acquisition in bioturbated sediment:
           General theory and implications for relative paleointensity
    • Authors: R. Egli; X. Zhao
      Abstract: We present a general theory for the acquisition of natural remanent magnetizations (NRM) in sediment under the influence of (a) magnetic torques, (b) randomizing torques, and (c) torques resulting from interaction forces. Dynamic equilibrium between (a) and (b) in the water column and at the sediment‐water interface generates a detrital remanent magnetization (DRM), while much stronger randomizing torques may be provided by bioturbation inside the mixed layer. These generate a so‐called mixed remanent magnetization (MRM), which is stabilized by mechanical interaction forces. During the time required to cross the surface mixed layer, DRM is lost and MRM is acquired at a rate that depends on bioturbation intensity. Both processes are governed by a MRM lock‐in function. The final NRM intensity is controlled mainly by a single parameter γ that is defined as the product of rotational diffusion and mixed layer thickness, divided by sedimentation rate. This parameter defines three regimes: (1) slow mixing (γ  10) with MRM acquisition and full DRM randomization, and (3) intermediate mixing. Because the acquisition efficiency of DRM is larger than that of MRM, NRM intensity is particularly sensitive to γ in case of mixed regimes, generating variable NRM acquisition efficiencies. This model explains (1) lock‐in delays that can be matched with empirical reconstructions from paleomagnetic records, (2) the existence of small lock‐in depths that lead to DRM preservation, (3) specific NRM acquisition efficiencies of magnetofossil‐rich sediments, and (4) some relative paleointensity artifacts. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-16T21:40:50.193541-05:
      DOI: 10.1002/2014GC005672
  • Gravity anomalies, crustal structure, and seismicity at subduction zones:
           1. Seafloor roughness and subducting relief
    • Authors: Dan Bassett; Anthony B. Watts
      Abstract: An ensemble averaging technique is used to remove the long‐wavelength topography and gravity field from subduction zones. >200 residual bathymetric and gravimetric anomalies are interpreted within forearcs, many of which are attributed to the tectonic structure of the subducting plate. The residual‐gravimetric expression of subducting fracture zones extends >200 km landward of the trench‐axis. The bathymetric expression of subducting seamounts with height ≥1 km and area ≥500 km2 (N=36), and aseismic ridges (N>10), is largest near the trench (within 70 km) and above shallow subducting slab depths (SLAB1.0
      PubDate: 2015-03-14T11:10:03.004997-05:
      DOI: 10.1002/2014GC005684
  • Evaluating the utility of B/Ca ratios in planktic foraminifera as a proxy
           for the carbonate system: A case study of Globigerinoides ruber
    • Authors: Michael J. Henehan; Gavin L. Foster, James W. B. Rae, Katherine C. Prentice, Jonathan Erez, Helen C. Bostock, Brittney J. Marshall, Paul A. Wilson
      Abstract: B/Ca ratios in foraminifera have attracted considerable scientific attention as a proxy for past ocean carbonate system. However, the carbonate system controls on B/Ca ratios are not straightforward, with Δ[CO32−] ([CO32−]in situ – [CO32−]at saturation) correlating best with B/Ca ratios in benthic foraminifera, rather than pH,B(OH)4−HCO3−, or B(OH)4−DIC (as a simple model of boron speciation in seawater and incorporation into CaCO3 would predict). Furthermore, culture experiments have shown that in planktic foraminifera properties such as salinity and [B]sw can have profound effects on B/Ca ratios beyond those predicted by simple partition coefficients. Here, we investigate the controls on B/Ca ratios in G. ruber via a combination of culture experiments and core‐top measurements, and add to a growing body of evidence that suggests B/Ca ratios in symbiont‐bearing foraminiferal carbonate are not a straightforward proxy for past seawater carbonate system conditions. We find that, while B/Ca ratios in culture experiments co‐vary with pH, in open ocean sediments this relationship is not seen. In fact, our B/Ca data correlate best with [PO43‐] (a previously undocumented association) and in most regions, salinity. These findings might suggest a precipitation rate or crystallographic control on boron incorporation into foraminiferal calcite. Regardless, our results underscore the need for caution when attempting to interpret B/Ca records in terms of the ocean carbonate system, at the very least in the case of mixed‐layer planktic foraminifera. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-13T21:01:24.640984-05:
      DOI: 10.1002/2014GC005514
  • Tracking the Late Jurassic apparent (or true) polar shift in U‐Pb
           dated kimberlites from Cratonic North America (superior province of
    • Authors: Dennis V. Kent; Bruce A. Kjarsgaard, Jeffrey S. Gee, Giovanni Muttoni, Larry M. Heaman
      Abstract: Different versions of a composite apparent polar wander (APW) path of variably selected global poles assembled and averaged in North American coordinates using plate reconstructions show either a smooth progression or a large (∼30°) gap in mean paleopoles in the Late Jurassic, between about 160 Ma and 145 Ma. In an effort to further examine this issue, we sampled accessible outcrops/subcrops of kimberlites associated with high precision U‐Pb perovskite ages in the Timiskaming area of Ontario, Canada. The 154.9±2.4 Ma Peddie kimberlite yields a stable normal polarity magnetization that is coaxial within less than 5° of the reverse polarity magnetization of the 157.5±1.2 Ma Triple B kimberlite. The combined ∼156 Ma Triple B and Peddie pole (75.5°N, 189.5°E, A95=2.8°) lies about midway between igneous poles from North America nearest in age (169 Ma Moat volcanics and the 146 Ma Ithaca kimberlites) showing that the polar motion was at a relatively steady yet rapid (∼1.5°/Myr) pace. A similar large rapid polar swing has been recognized in the Middle to Late Jurassic APW path for Adria‐Africa and Iran‐Eurasia, suggesting a major mass redistribution. One possibility is that slab breakoff and subduction reversal along the western margin of the Americas triggered an episode of true polar wander. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-13T21:01:10.899923-05:
      DOI: 10.1002/2015GC005734
  • Reconciling subduction dynamics during Tethys closure with
           large‐scale Asian tectonics: Insights from numerical modeling
    • Authors: F.A. Capitanio; A. Replumaz, N. Riel
      Abstract: We use three‐dimensional numerical models to investigate the relation between subduction dynamics and large‐scale tectonics of continent interiors. The models show how the balance between forces at the plate margins such as subduction, ridge push and far‐field forces, controls the coupled plate margins and interiors evolution. Removal of part of the slab by lithospheric break‐off during subduction destabilizes the convergent margin, forcing migration of the subduction zone, whereas in the upper plate large‐scale lateral extrusion, rotations and back‐arc stretching ensue. When external forces are modeled, such as ridge push and far‐field forces, indentation increases, with large collisional margin advance and thickening in the upper plate. The balance between margin and external forces leads to similar convergent margin evolutions, whereas major differences occur in the upper plate interiors. Here, three strain regimes are found: large‐scale extrusion, extrusion and thickening along the collisional margin, and thickening only, when negligible far‐field forces, ridge push, and larger far‐field forces, respectively, add to the subduction dynamics. The extrusion tectonics develop a strong asymmetry towards the oceanic margin driven by large‐scale subduction, with no need of pre‐existing heterogeneities in the upper plate. Because the slab break‐off perturbation is transient, the ensuing plate tectonics is time‐dependent. The modeled deformation and its evolution are remarkably similar to the Cenozoic Asian tectonics, explaining large‐scale lithospheric faulting and thickening, and coupling of indentation, extrusion and extension along the Asian convergent margin as a result of large‐scale subduction process. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-12T07:03:19.001821-05:
      DOI: 10.1002/2014GC005660
  • Grain‐size dynamics beneath mid‐ocean ridges: Implications for
           permeability and melt extraction
    • Authors: Andrew J. Turner; Richard F. Katz, Mark D. Behn
      Abstract: Grain size is an important control on mantle viscosity and permeability, but is difficult or impossible to measure in situ. We construct a two‐dimensional, single phase model for the steady‐state mean grain size beneath a mid‐ocean ridge. The mantle rheology is modelled as a composite of diffusion creep, dislocation creep, dislocation accommodated grain boundary sliding, and a plastic stress limiter. The mean grain size is calculated by the paleowattmeter relationship of Austin and Evans [2007]. We investigate the sensitivity of our model to global variations in grain growth exponent, potential temperature, spreading‐rate, and mantle hydration. We interpret the mean grain‐size field in terms of its permeability to melt transport. The permeability structure due to mean grain size may be approximated as a high permeability region beneath a low permeability region. The transition between high and low permeability regions occurs across a boundary that is steeply inclined toward the ridge axis. We hypothesise that such a permeability structure generated from the variability of the mean grain size may focus melt towards the ridge axis, analogous to Sparks and Parmentier [1991]‐type focusing. This focusing may, in turn, constrain the region where significant melt fractions are observed by seismic or magnetotelluric surveys. This interpretation of melt focusing via the grain‐size permeability structure is consistent with MT observation of the asthenosphere beneath the East Pacific Rise [Baba et al., 2006; Key et al., 2013]. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-11T01:40:46.036718-05:
      DOI: 10.1002/2014GC005692
  • An upper‐mantle seismic discontinuity beneath the Galápagos
           Archipelago and its implications for studies of the
           lithosphere‐asthenosphere boundary
    • Authors: Joseph S. Byrnes; Emilie E. E. Hooft, Douglas R. Toomey, Darwin R. Villagómez, Dennis J. Geist, Sean C. Solomon
      Abstract: An upper‐mantle seismic discontinuity (the Gutenberg or G discontinuity), at which shear wave velocity decreases with depth, has been mapped from S‐to‐p conversions in radial receiver functions recorded across the Galápagos Archipelago. The mean depth of the discontinuity is 91 ± 8km beneath the southeastern archipelago and 72 ± 5km beneath surrounding regions. The discontinuity appears deeper beneath the portion of the Nazca plate that we infer passed over the Galápagos mantle plume than elsewhere in the region. We equate the depth of the G discontinuity to the maximum depth extent of anhydrous melting, which forms an overlying layer of dehydrated and depleted mantle. We attribute areas of shallow discontinuity depth to the formation of the dehydrated layer near the Galápagos Spreading Center and areas of greater discontinuity depth to its modification over a mantle plume with an excess temperature of 115 ± 30 °C. The G discontinuity lies within a high‐seismic‐velocity anomaly that we conclude forms by partial dehydration and a gradual but steady increase in seismic velocity with decreasing depth after upwelling mantle first encounters the solidus for volatile‐bearing mantle material. At the depth of the solidus for anhydrous mantle material, removal of remaining water creates a sharp increase in velocity with decreasing depth; this discontinuity may also mark a site of melt accumulation. Results from seismic imaging, the compositions of Galápagos lavas, and rare‐earth‐element concentrations across the archipelago require that mantle upwelling and partial melting occur over a broad region within the dehydrated and depleted layer. We conclude that the G discontinuity beneath the archipelago does not mark the boundary between rigid lithosphere and convecting asthenosphere. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-02T15:17:43.168769-05:
      DOI: 10.1002/2014GC005694
  • Structural heterogeneity of the midcrust adjacent to the central Alpine
           Fault, New Zealand: Inferences from seismic tomography and seismicity
           between Harihari and Ross
    • Authors: Sandra Bourguignon; Stephen Bannister, C. Mark Henderson, John Townend, Haijiang Zhang
      Abstract: Determining the rates and distributions of microseismicity near major faults at different points in the seismic cycle is a crucial step towards understanding plate boundary seismogenesis. We analyse data from temporary seismic arrays spanning the central section of the Alpine Fault, New Zealand, using double‐difference seismic tomography. This portion of the fault last ruptured in a large earthquake in 1717 AD and is now late in its typical 330‐year cycle of Mw∼8 earthquakes. Seismicity varies systematically with distance from the Alpine Fault: (1) directly beneath the fault trace, earthquakes are sparse and largely confined to the footwall at depths of 4–11km; (2) at distances of 0–9km southeast of the trace, seismicity is similarly sparse and shallower than 8km; (3) at distances of 9–20km southeast of the fault trace, earthquakes are much more prevalent and shallower than 7km. Hypocentre lineations here are subparallel to faults mapped near the Main Divide of the Southern Alps, confirming that those faults are active. The region of enhanced seismicity is associated with the highest topography and a high‐velocity tongue doming at 3–5km depth. The low‐seismicity zone adjacent to the Alpine Fault trace is associated with Vp and Vs values at mid‐crustal depths about 8% and 6% lower than further southeast. We interpret lateral variations in seismicity rate to reflect patterns of horizontal strain rate superimposed on heterogeneous crustal structure, and the variations in seismicity cutoff depth to be controlled by temperature and permeability structure variations. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-28T00:48:31.190045-05:
      DOI: 10.1002/2014GC005702
  • Authigenesis of magnetic minerals in gas hydrate‐bearing sediments
           in the Nankai Trough, offshore Japan
    • Authors: Myriam Kars; Kazuto Kodama
      Abstract: Gas hydrate occurrence is one of the possible mechanisms invoked for iron sulfide formation. A high resolution rock magnetic study was conducted in IODP Expedition 316 Hole C0008C located in the Megasplay Fault Zone of the Nankai Trough, offshore Japan. In this particular zone, no bottom simulating reflectors (BSR), indicating the base of the gas hydrate stability field, have been identified. 218 Pleistocene samples were collected from 70 to 110 m CSF in order to document the changes in the concentration, grain size and rock magnetic parameters of magnetic minerals, through the gas hydrate‐bearing horizons. Two different populations of magnetic grains are recognized in the pseudo‐single domain range. Three types of magnetic mineral assemblages are identified: iron oxides (magnetite), ferrimagnetic iron sulfides (greigite and pyrrhotite) and their mixture. Greigite and pyrrhotite are authigenic and constitute six layers, called IS1 to IS6. IS1, IS3, IS4 and IS6 are associated with pore water anomalies, suggesting the occurrence of gas hydrates and anoxic conditions. IS2 and IS5 are probable gas hydrates horizons, although there is no independent data to confirm it. The remaining intervals are mainly composed of detrital iron oxides and paramagnetic iron sulfides. Two scenarios based on different diagenetic stages are proposed to explain the variations in the magnetic properties and mineralogy over the studied interval. The results suggest that rock magnetism appears useful to better constrain the gas hydrate distribution in Hole C0008C, and counterbalances the low resolution of pore water analyses and the absence of a BSR. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-26T09:17:32.219882-05:
      DOI: 10.1002/2014GC005614
  • Hydrous upwelling across the mantle transition zone beneath the Afar
           Triple Junction
    • Authors: D.A. Thompson; J.O.S. Hammond, J‐M. Kendall, G.W. Stuart, G.R. Helffrich, D. Keir, A. Ayele, B. Goitom
      Abstract: The mechanisms that drive the upwelling of chemical heterogeneity from the lower to upper mantle (e.g. thermal vs. compositional buoyancy) are key to our understanding of whole mantle convective processes. We address these issues through a receiver function study on new seismic data from recent deployments located on the Afar Triple Junction, a location associated with deep mantle upwelling. The detailed images of upper mantle and mantle transition zone structure illuminate features that give insights into the nature of upwelling from the deep Earth. A seismic low velocity layer directly above the mantle transition zone, interpreted as a stable melt layer, along with a prominent 520 km discontinuity suggest the presence of a hydrous upwelling. A relatively uniform transition zone thickness across the region suggests a weak thermal anomaly (
      PubDate: 2015-02-26T08:44:17.569043-05:
      DOI: 10.1002/2014GC005648
  • New mafic magma refilling a quiescent volcano: Evidence from
           He‐Ne‐Ar isotopes during the 2011–2012 unrest at
           Santorini, Greece
    • Authors: A.L. Rizzo; F. Barberi, M.L. Carapezza, A. Di Piazza, L. Francalanci, F. Sortino, W. D'Alessandro
      Abstract: In 2011‐2012 Santorini was characterized by seismic‐geodetic‐geochemical unrest, which was unprecedented since the most‐recent eruption occurred in 1950 and led to fear an eruption was imminent. This unrest offered a chance for investigating the processes leading to volcanic reactivation and the compositional characteristics of involved magma. We have thus analyzed the He‐Ne‐Ar‐isotope composition of fluid inclusions in olivines and clinopyroxenes from cumulate mafic enclaves hosted in cogenetic dacitic lavas of the 1570–1573 and 1925–1928 eruptions of Nea Kameni. These unique data on Aegean volcanism were compared with those of gases collected in quiescent periods and during the unrest. The 3He/4He‐ratios (3.1–4.0Ra) are significantly lower than the typical arc‐volcano values (R/Ra∼7–8), suggesting the occurrence of magma contamination in Santorini plumbing system, which would further modify the 3He/4He‐ratio of parental magmas generated in the local metasomatized mantle. The 3He/4He‐values of enclaves (3.1–3.6Ra) are comparable to those measured in gases during quiescent periods, confirming that enclaves reflect the He‐isotope signature of magma residing at shallow depths and feeding passive degassing. A significant increase in soil CO2 flux from Nea Kameni and anomalous compositional variations in the fumaroles were identified during the unrest, accordingly with previous studies. Simultaneously, 3He/4He‐ratios up to 4.0Ra were also measured, demonstrating that the unrest was due to the intrusion into the shallow plumbing system of a more‐primitive 3He‐rich magma, which is even volatile richer and less contaminated than mafic magma erupted as enclaves. This new intrusion did not however trigger an eruption. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-26T07:57:47.210549-05:
      DOI: 10.1002/2014GC005653
  • Emplacement of submarine lava flow fields: A geomorphological model from
           the Niños eruption at the Galápagos Spreading Center
    • Authors: J. Timothy McClinton; Scott M. White
      Abstract: In the absence of any direct observations of an active submarine eruption at a mid‐ocean ridge (MOR), our understanding of volcanic processes there is based on the interpretation of eruptive products. Submarine lava flow morphology serves as a primary indicator of eruption and emplacement processes; however, there is typically a lack of visual observations and bathymetric data at a scale and extent relevant to submarine lava flows, which display meter‐ to sub‐meter‐scale morphological variability. In this paper, we merge submersible‐based visual observations with high‐resolution multibeam bathymetry collected by an autonomous underwater vehicle (AUV) and examine the fine‐scale geomorphology of Niños, a submarine lava flow field at the Galápagos Spreading Center (GSC).We identify separate morphological facies (i.e. morphofacies) within the lava flow field, each having distinct patterns of lava flow morphology and volcanic structures. The spatial and stratigraphic arrangement of morphofacies suggests that they were emplaced sequentially as the eruption progressed, implying that the Niños eruption consisted of at least three eruptive phases. We estimate eruption parameters and develop a chronological model that describes the construction of the Niños lava flow field. An initial phase with high effusion rates emplaced sheet flows, then an intermediate phase emplaced a platform of lobate lavas, and then an extended final phase with low effusion rates emplaced a discontinuous row of pillow lava domes. We then compare this model to mapped lava flow fields at other MORs. Despite disparities in scale, the morphological similarities of volcanic features at MORs with different spreading rates suggest common emplacement processes that are primarily controlled by local magma supply. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-25T05:01:44.587778-05:
      DOI: 10.1002/2014GC005632
  • Observations of pore‐scale growth patterns of carbon dioxide hydrate
           using X‐ray computed microtomography
    • Authors: Xuan Hien Ta; Tae Sup Yun, Balasingam Muhunthan, Tae‐Hyuk Kwon
      Abstract: Natural and artificial gas hydrates with internal pores of nano‐ to centimeters and weak grain‐cementation have been widely reported, while the detailed formation process of grain‐cementing hydrates remains poorly identified. Pore‐scale morphology of carbon dioxide (CO2) hydrate formed in a partially brine‐saturated porous medium was investigated via X‐ray computed microtomography (X‐ray CMT). Emphasis is placed on the pore‐scale growth patterns of gas hydrate, including the growth of dendritic hydrate crystals on pre‐formed hydrate and water‐wetted grains, porous nature of the hydrate phase, volume expansion of more than 200% during the water‐to‐hydrate phase transformation, preference of unfrozen water wetting hydrophilic minerals, and the relevance to a weak cementation effect on macro‐scale physical properties. The presented pore‐scale morphology and growth patterns of gas hydrate are expected in natural sediment settings where free gas is available for hydrate formation, such as active gas vents, gas seeps, mud volcanoes, permafrost gas hydrate provinces, and CO2 injected formation for the sake of geologic carbon storage; and in laboratory hydrate samples synthesized from partially brine‐saturated sediments or formed from water‐gas interfaces. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-24T02:21:30.431266-05:
      DOI: 10.1002/2014GC005675
  • Mid‐mantle anisotropy in subduction zones and deep water transport
    • Authors: Andy Nowacki; J.‐Michael Kendall, James Wookey, Asher Pemberton
      Abstract: The Earth's transition zone has until recently been assumed to be seismically isotropic. Increasingly, however, evidence suggests that ordering of material over seismic wavelengths occurs there, but it is unclear what causes this. We use the method of source‐side shear wave splitting to examine the anisotropy surrounding earthquakes deeper than 200km in slabs around the globe. We find significant amounts of splitting (≤ 2.4s), confirming that the transition zone is anisotropic here. However, there is no decrease in the amount of splitting with depth, as would be the case for a metastable tongue of olivine which thins with depth, suggesting this is not the cause. The amount of splitting does not appear to be consistent with processes in the ambient mantle, such as lattice preferred orientation development in wadsleyite, ringwoodite or MgSiO3‐perovskite. We invert for the orientation of several mechanisms‐subject to uncertainties in mineralogy and deformation‐and the best fit is given by up‐dip flattening in a style of anisotropy common to hydrous phases and layered inclusions. We suggest that highly anisotropic hydrous phases or hydrated layering is a likely cause of anisotropy within the slab, implying significant water transport from the surface down to at least 660 km depth. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-24T01:49:32.368-05:00
      DOI: 10.1002/2014GC005667
  • Untangling boulder dislodgement in storms and tsunamis: Is it possible
           with simple theories?
    • Authors: R. Weiss; P. Diplas
      Abstract: Boulders can move during storms and tsunamis. It is difficult to find a simple method to distinguish boulders moved by tsunami waves from those moved during storms in the field. In this contribution, we explore boulder dislodgement by storm and tsunami waves by solving an adapted version of Newton's Second Law of Motion in polar coordinates and defining a critical position for boulder dislodgement. We find that the boulder dislodgement is not only a function of the causative wave, but also of the roughness in the vicinity of the boulder and the slope angle. We employ the amplitude of storm and tsunami waves to dislodge boulders of given masses to evaluate if boulder dislodgement in storms can be untangled from boulder transport in tsunamis. As the main result of our numerical experiments, we find a significant difference between storm and tsunami waves to dislodge the same boulder for large masses and large roughness values. This allows us to conclude that simple theories are applicable to answer the questions asked in the title, but we argue only if they contain a critical dislodgement condition like the one presented here. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-24T00:43:12.953584-05:
      DOI: 10.1002/2014GC005682
  • Paleointensity determinations from the Etendeka province, Namibia, support
           a low magnetic field strength leading up to the Cretaceous normal
    • Authors: Sarah C. Dodd; Adrian R. Muxworthy, Conall Mac Niocaill
      Abstract: Paleointensity estimates provide much needed information on field generation within Earth's core and upon the convective processes at work within the mantle. We present new paleointensity estimates from the early Cretaceous Etendeka large igneous province in Namibia (~135Ma) which add to the sparse southern hemisphere dataset. The Early Cretaceous marks an important change in the Earth's magnetic field from a state of rapid polarity reversals, to one of long‐term stability associated with the onset of the Cretaceous Normal Superchron at c. 121Ma. Paleointensity determinations, using the IZZI protocol, were carried out on a total of 172 specimens from 14 sites encompassing the exposed stratigraphy of the Etendeka province. Numerous checks of data reliability were considered before results were accepted, including partial thermoremanent magnetisation (pTRM) checks and pTRM tail checks, hysteresis properties, thermomagnetic analyses, observations under reflected light, and changes to room‐temperature susceptibility during the experiments. Following these checks a total of 64 individual samples from 5 sites were considered to provide reliable paleointensity determinations. These results were combined to provide site mean data with an overall average virtual dipole moment (VDM) for the study of 2.5 ± 1.0 x 1022 Am2. This value equates to approximately 30% of present Earth's field and, when considered alongside existing studies, suggests that Earth's field strength was low in the time leading up to the Cretaceous normal superchron. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-19T08:21:18.653493-05:
      DOI: 10.1002/2014GC005707
  • Viscosity measurements of crystallizing andesite from Tungurahua volcano
    • Authors: Magdalena Oryaëlle Chevrel; Corrado Cimarelli, Lea deBiasi, Jonathan Hanson, Yan Lavallée, Fabio Arzilli, Donald B. Dingwell
      Abstract: Viscosity has been determined during isothermal crystallization of an andesite from Tungurahua volcano (Ecuador). Viscosity was continuously recorded using the concentric cylinder method and employing a Pt‐sheathed alumina spindle at 1 bar and from 1400 ºC to sub‐liquidus temperatures to track rheological changes during crystallization. The disposable spindle was not extracted from the sample but rather left in the sample during quenching thus preserving an undisturbed textural configuration of the crystals. The inspection of products quenched during the crystallization process reveals evidence for heterogeneous crystal nucleation at the spindle and near the crucible wall, as well as crystal alignment in the flow field. At the end of the crystallization, defined when viscosity is constant, plagioclase is homogeneously distributed throughout the crucible (with the single exception of experiment performed at the lowest temperature). In this experiments, the crystallization kinetics appear to be strongly affected by the stirring conditions of the viscosity determinations. A TTT (Time‐Temperature‐Transformation) diagram illustrating the crystallization “nose” for this andesite under stirring conditions and at ambient pressure has been constructed. We further note that at a given crystal content and distribution, the high aspect ratio of the acicular plagioclase yields a shear‐thinning rheology at crystal contents as low as 13 vol.%, and that the relative viscosity is higher than predicted from existing viscosity models. These viscosity experiments hold the potential for delivering insights into the relative influences of the cooling path, undercooling and deformation on crystallization kinetics and resultant crystal morphologies, as well as their impact on magmatic viscosity. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-19T08:13:41.83061-05:0
      DOI: 10.1002/2014GC005661
  • Historical bathymetric charts and the evolution of Santorini submarine
           volcano, Greece
    • Authors: A. B. Watts; P. Nomikou, J. D. P. Moore, M. M. Parks, M. Alexandri
      Abstract: Historical bathymetric charts are a potential resource for better understanding the dynamics of the seafloor and the role of active processes, such as submarine volcanism. The British Admiralty, for example, have been involved in lead line measurements of seafloor depth since the early 1790s. Here, we report on an analysis of historical charts in the region of Santorini volcano, Greece. Repeat lead line surveys in 1848, late 1866 and 1925‐1928 as well as multibeam swath bathymetry surveys in 2001 and 2006 have been used to document changes in seafloor depth. These data reveal that the flanks of the Kameni Islands, a dacitic dome complex in the caldera center, have shallowed by up to ~175 m and deepened by up to ~80 m since 1848. The largest shallowing occurred between the late 1866 and 1925‐1928 surveys and the largest deepening occurred during the 1925‐1928 and 2001 and 2006 surveys. The shallowing is attributed to the emplacement of lavas during effusive eruptions in both 1866‐1870 and 1925‐1928 at rates of up to 0.18 and 0.05 km3 a‐1 respectively. The deepening is attributed to a load‐induced viscoelastic stress relaxation following the 1866‐1870 and 1925‐1928 lava eruptions. The elastic thickness and viscosity that best fits the observed deepening are 1.0 km and ~1016 Pa s respectively. This parameter pair, which is consistent with the predictions of a shallow magma chamber thermal model, explains both the amplitude and wavelength of the historical bathymetric data and the present day rate of subsidence inferred from InSAR analysis. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-19T08:13:29.44729-05:0
      DOI: 10.1002/2014GC005679
  • Episodic and multistaged gravitational instability of cratonic lithosphere
           and its implications for reactivation of the North China Craton
    • Authors: Yongming Wang; Jinshui Huang, Shijie Zhong
      Abstract: Archean cratons are the most stable tectonic units and their lithospheric mantle is chemically depleted and buoyant relative to the underlying mantle. The chemical depletion leads to high viscosity that maintains the long‐term stability of cratons. However, the eastern part of the North China Craton (~1200 km in horizontal length scale) had been extensively reactivated and modified over a time scale of ~100 Myrs in the Mesozoic and Cenozoic. While the causes for the weakening of the North China Craton, a necessary condition for its reactivation, are still in debate, we investigate gravitational instability of compositionally buoyant lithosphere, by computing 2‐D thermochemical convection models with different buoyancy number, lithospheric viscosity and rheology. We find that the gravitational instability of cratonic lithosphere can happen over a larger range of buoyancy numbers with non‐Newtonian rheology, but lithospheric instability with Newtonian rheology only happens with relatively small buoyancy numbers. For cratonic lithosphere with non‐Newtonian rheology and relatively weak temperature‐dependent viscosity, the instability starts in the cold, shallow part of the lithosphere and has small horizontal length scale (
      PubDate: 2015-02-19T08:11:52.304994-05:
      DOI: 10.1002/2014GC005681
  • Application of the cBΩ model to the calculation of diffusion
           parameters of Si in silicates
    • Authors: Baohua Zhang; Shuangming Shan
      Abstract: Silicon diffusion in major mantle minerals plays an important role in understanding a number of physical and chemical processes in the Earth's interior. Inspection of existing experimental data reveals linear compensation law between the pre‐exponential factors and the activation energies for Si diffusion in various minerals by focusing on those of geophysical interest. On the basis of the observed compensation relationship, here, we propose a thermodynamic model, the so‐called cBΩ model that interconnects point defect parameters with the bulk properties to reproduce the Si self‐diffusion coefficients in different rock‐forming minerals. When the uncertainties are considered, the predicted results show that the temperature and pressure dependences of self‐diffusion coefficients concur with existing experimental data and theoretical calculations in most cases. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-12T19:27:21.282047-05:
      DOI: 10.1002/2014GC005551
  • Source and magma mixing processes in continental subduction factory:
           Geochemical evidence from postcollisional mafic igneous rocks in the Dabie
    • Authors: Li‐Qun Dai; Zi‐Fu Zhao, Yong‐Fei Zheng, Juan Zhang
      Abstract: Postcollisional mafic igneous rocks commonly exhibit petrological and geochemical heterogeneities, but their origin still remains enigmatic. While source mixing is substantial due to the crust‐mantle interaction during continental collision, magma mixing is also significant during postcollisional magmatism. The two processes are illustrated by Early Cretaceous mafic igneous rocks in the Dabie orogen. These mafic rocks show arc‐like trace element distribution patterns and enriched Sr‐Nd‐Pb isotope compositions, indicating their origination from enriched mantle sources. They have variable whole‐rock εNd(t) values of ‐17.6 to ‐5.2 and zircon εHf(t) values of ‐29.0 to ‐7.7, pointing to source heterogeneities. Such whole‐rock geochemical features are interpreted by the source mixing through melt‐peridotite reaction in the continental subduction channel. Clinopyroxene and plagioclase megacrystals show complex textural and compositional variations, recording three stages of mineral crystallization during magma evolution. Cpx‐1 core has low Cr and Ni but high Ba, Rb and K, indicating its crystallization from a mafic melt (Melt 1) derived from partial melting of hydrous peridotite rich in phlogopite. Cpx‐1 mantle and Cpx‐2 exhibit significantly high Cr, Ni and Al2O3 but low Rb and Ba, suggesting their crystallization from pyroxenite‐derived mafic melt (Melt 2). Whole‐rock initial 87Sr/86Sr ratios of gabbro lies between those of Pl‐1core (crystallized from Melt 1) and Pl‐1 mantle and Pl‐2 core (crystallized from Melt 2), providing isotopic evidence for magma mixing between Melt 1 and Melt 2. Taken together, a heterogeneously enriched mantle source would be generated by the source mixing due to reaction of the overlying subcontinental lithospheric mantle wedge peridotite with felsic melts derived from partial melting of different rocks of the deeply subducted continental crust during the continental collision. The magma mixing would occur between mafic melts that were derived from partial melting of the heterogeneously metasomatic mantle domains in the postcollisional stage. As a consequence, the source and magma mixing processes in the continental subduction factory are responsible for the significant variations in the whole‐rock and mineral geochemistries of postcollisional mafic igneous rocks. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-12T19:14:01.071431-05:
      DOI: 10.1002/2014GC005620
  • Germanium/silicon of the Ediacaran‐Cambrian Laobao cherts:
           Implications for the bedded chert formation and paleoenvironment
    • Authors: Lin Dong; Bing Shen, Cin‐Ty A. Lee, Xu‐jie Shu, Yang Peng, Yuanlin Sun, Zhuanhong Tang, Hong Rong, Xianguo Lang, Haoran Ma, Fan Yang, Wen Guo
      Abstract: Sedimentary strata of the terminal Ediacaran (635–542 Ma) to early Cambrian (542–488 Ma) Laobao‐Liuchapo bedded cherts in the South China Block include the Ediacaran Oxidation Event and the Cambrian explosion. Understanding the origin and depositional environment of the bedded cherts may provide insight into how the Earth's surface environment changed between the Proterozoic and Phanerozoic. We measured major and trace element compositions and Ge/Si ratios of the Laobao cherts from northern Guangxi Province. The Laobao cherts were deposited in the deep basinal environment of the South China Block. We show that the composition of the Laobao cherts is determined by a mixture of four components: quartz, clay, carbonate, and pyrite/iron‐oxide. The quartz component is the dominant component of the Laobao cherts. The maximum estimated Ge/Si of the quartz component is between 0.4 and 0.5 μmol/mol, which is close to the Ge/Si of modern seawater and biogenic silica but one order of magnitude lower than that of hydrothermal fluids. These Ge/Si systematics suggest that normal seawater rather than mid‐ocean ridge hydrothermal fluids is the primary Si source for the Laobao cherts. The Ge/Si of the clay component varies between 1 and 10 μmol/mol, which is comparable to the Ge/Si of typical marine clays, but 10 to 100 times lower than that of chert nodules from early Ediacaran beds (the Doushantuo Formation) predating the terminal Ediacaran Labao cherts studied here. Our observations indicate that the clay component Ge/Si ratio decreased from the early Ediacaran to the late Ediacaran. We speculate that high Ge/Si ratios in clays reflect the preferential chelation of Ge by dissolved organic compounds adsorbed onto clays. If so, this suggests that the decrease in Ge/Si ratio of the clay component in the Ediacaran signifies a decrease in the total dissolved organic carbon content of seawater towards the Ediacaran‐Cambrian transition, consistent with oxidation of the oceans during the late Ediacaran. Finally, the seawater origin of the Laobao cherts also suggests that replacement of carbonate may not be the primary cause for bedded chert formation. Instead, direct precipitation from seawater or early diagenetic silicification of calcareous sediments, perhaps due to the emergence of Si‐accumulation bacteria, may have been responsible for the bedded Laobao‐Liuchapo chert formation in South China Block. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-12T07:01:06.744745-05:
      DOI: 10.1002/2014GC005595
  • Origin and evolution of the Kolbeinsey Ridge and Iceland Plateau,
    • Authors: Bryndís Brandsdóttir; Emilie E. E. Hooft, Rolf Mjelde, Yoshio Murai
      Abstract: Variations in crustal structure along the 700 km long KRISE7 refraction/reflection and gravity profile, straddling 66.5°N across the Iceland Shelf, Iceland Plateau and western Norway Basin confirm that extinct spreading centers coexisted with the now extinct Ægir Ridge prior to the initiation of the Kolbeinsey Ridge at 26 Ma. The western 300 km of the profile, across the Iceland shelf, formed by rifting at the Kolbeinsey Ridge, whereas the eastern 400 km, across the Iceland Plateau and the western Norway Basin, formed by earlier rifting, possibly containing slivers of older oceanic or continental crust rifted off the central E‐Greenland margin along with the Jan Mayen Ridge. Crustal thickness increases gradually across the Iceland shelf, from 12–13 km near the Kolbeinsey Ridge to 24–28 km near the eastern shelf edge, decreasing abruptly across the shelf edge, to 12–13 km. The Iceland Plateau has crustal thickness ranging from 12–15 km decreasing to 5–8 km across the western Norway Basin and 4–5 km at the Ægir Ridge. We suggest that high‐velocity lower crustal domes and corresponding gravity highs across the Iceland plateau mark the location of extinct rift axes, that coexisted with the Ægir Ridge. Similar lower crustal domes are associated with the currently active rift segments within Iceland and the Kolbeinsey Ridge. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-10T08:41:44.373421-05:
      DOI: 10.1002/2014GC005540
  • The carbon‐isotope signature of ultramafic xenoliths from the
           Hyblean Plateau (southeast Sicily, Italy): Evidence of mantle
    • Authors: Alessandra Correale; Antonio Paonita, Andrea Rizzo, Fausto Grassa, Mauro Martelli
      Abstract: We investigated the carbon isotope composition of mantle source beneath the Hyblean Plateau (southeast Sicily, Italy) by studying CO2 in fluid inclusions from ultramafic xenoliths recovered in some Miocene diatremes. In order to constrain the processes influencing the isotopic marker of carbon we combined δ13CCO2 results with information about noble gases (He and Ar) obtained in a previous investigation of the same products. Although Ar/CO2 and He/Ar ratios provide evidence of Rayleigh‐type fractional degassing, the isotopic geochemistry of carbon is poorly influenced by this process. Mixing related to metasomatic processes where MORB‐type pyroxenitic melts permeate a peridotite mantle probably contaminated by crustal fluids inherited from a fossil subduction can explain the measured δ13C and CO2/3He variations, ranging from −4‰ to −2‰ and from 109 to 1010, respectively. Simple mass‐balance calculations highlighted that the Hyblean peridotite source was mainly contaminated by the carbonate source, being carbonate and organic matter present at a ratio that varied within the range from 7:1 to 4:1. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-10T03:19:05.035556-05:
      DOI: 10.1002/2014GC005656
  • A comprehensive interpretative model of slow slip events on Mt. Etna's
           eastern flank
    • Authors: Mario Mattia; Valentina Bruno, Tommaso Caltabiano, Andrea Cannata, Flavio Cannavò, Walter D'Alessandro, Giuseppe Di Grazia, Cinzia Federico, Salvatore Giammanco, Alessandro La Spina, Marco Liuzzo, Manfredi Longo, Carmelo Monaco, Domenico Patanè, Giuseppe Salerno
      Abstract: Starting off from a review of previous literature on kinematic models of the unstable eastern flank of Mt. Etna, we propose a new model. The model is based on our analysis of a large quantity of multidisciplinary data deriving from an extensive and diverse network of INGV monitoring devices deployed along the slopes of the volcano. Our analysis had a twofold objective: first, investigating the origin of the recently observed slow‐slip events on the eastern flank of Mt. Etna; and secondly, defining a general kinematic model for the instability of this area of the volcano. To this end, we investigated the 2008 to 2013 period using data collected from different geochemical, geodetic and seismic networks, integrated with the tectonic and geologic features of the volcano and including the volcanic activity during the observation period. The complex correlations between the large quantities of multidisciplinary data have given us the opportunity to infer, as outlined in this work, that the fluids of volcanic origin and their interrelationship with aquifers, tectonic and morphological features play a dominant role in the large scale instability of the eastern flank of Mt. Etna. Furthermore, we suggest that changes in the strain distribution due to volcanic inflation/deflation cycles are closely connected to changes in shallow depth fluid circulation. Finally, we propose a general framework for both the short and long term modelling of the large flank displacements observed. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-04T22:29:31.764933-05:
      DOI: 10.1002/2014GC005585
  • Temperature and velocity measurements of a rising thermal plume
    • Authors: Neil Cagney; William H. Newsome, Carolina Lithgow‐Bertelloni, Aline Cotel, Stanley R. Hart, John A. Whitehead
      Abstract: The three‐dimensional velocity and temperature fields surrounding an isolated thermal plume in a fluid with temperature‐dependent viscosity are measured using Particle‐Image Velocimetry and thermochromatic liquid‐crystals, respectively. The experimental conditions are relevant to a plume rising through the mantle. It is shown that while the velocity and the isotherm surrounding the plume can be used to visualise the plume, they do not reveal the finer details of its structure. However, by computing the Finite‐Time Lyapunov Exponent fields from the velocity measurements, the material lines of the flow can be found, which clearly identify the shape of the plume head and characterise the behaviour of the flow along the plume stem. It is shown that the vast majority of the material in the plume head has undergone significant stretching and originates from a wide region very low in the fluid domain, which is proposed as a contributing factor to the small‐scale isotopic variability observed in ocean‐island basalt regions. Finally, the Finite‐Time Lyapunov Exponent fields are used to calculate the steady‐state rise velocity of the thermal plume, which is found to scale linearly with the Rayleigh number, in contrast to some previous work. The possible cause and the significance of these conflicting results are discussed, and it is suggested that the scaling relationship may be affected by the temperature‐dependence of the fluid viscosity in the current work. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-30T00:11:47.106618-05:
      DOI: 10.1002/2014GC005576
  • Split‐beam echo sounder observations of natural methane seep
           variability in the northern Gulf of Mexico
    • Authors: Kevin Jerram; Thomas C. Weber, Jonathan Beaudoin
      Abstract: A method for positioning and characterizing plumes of bubbles from marine gas seeps using an 18‐kHz scientific split‐beam echosounder (SBES) was developed and applied to acoustic observations of plumes of presumed methane gas bubbles originating at approximately 1400m depth in the northern Gulf of Mexico. A total of 161 plume observations from 27 repeat surveys were grouped by proximity into 35 clusters of gas vent positions on the seafloor. Profiles of acoustic target strength per vertical meter of plume height were calculated with compensation for both the SBES beam pattern and the geometry of plume ensonification. These profiles were used as indicators of the relative fluxes and fates of gas bubbles acoustically observable at 18kHz and showed significant variability between repeat observations at time intervals of 1 hour to 7.5 months. Active gas venting was observed during approximately one third of the survey passes at each cluster. While gas flux is not estimated directly in this study owing to lack of bubble size distribution data, repeat surveys at active seep sites showed variations in acoustic response that suggest relative changes in gas flux of up to one order of magnitude over time scales of hours. The minimum depths of acoustic plume observations at 18kHz averaged 875m and frequently coincided with increased amplitudes of acoustic returns in layers of biological scatterers, suggesting acoustic masking of the gas bubble plumes in these layers. Minimum plume depth estimates were limited by the SBES field of view in only five instances. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-28T07:08:40.20173-05:0
      DOI: 10.1002/2014GC005429
  • P and S velocity tomography of the Mariana subduction system from a
           combined land‐sea seismic deployment
    • Authors: Mitchell Barklage; Douglas A. Wiens, James A. Conder, Sara Pozgay, Hajime Shiobara, Hiroko Sugioka
      Abstract: Seismic imaging provides an opportunity to constrain mantle wedge processes associated with subduction, volatile transport, arc volcanism, and backarc spreading. We investigate the seismic velocity structure of the upper mantle across the Central Mariana subduction system using data from the 2003‐2004 Mariana Subduction Factory Imaging Experiment, an 11‐month deployment consisting of 20 broadband seismic stations installed on islands and 58 semi‐broadband ocean bottom seismographs. We determine the three dimensional VP and VP/VS structure using over 25,000 local and over 2000 teleseismic arrival times. The mantle wedge is characterized by slow velocity and high VP/VS beneath the forearc, an inclined zone of slow velocity underlying the volcanic front, and a strong region of slow velocity beneath the backarc spreading center. The slow velocities are strongest at depths of 20‐30km in the forearc, 60‐70km beneath the volcanic arc, and 20‐30km beneath the spreading center. The forearc slow velocity anomalies occur beneath Big Blue seamount and are interpreted as resulting from mantle serpentinization. The depths of the maximum velocity anomalies beneath the arc and backarc are nearly identical to previous estimates of the final equilibrium depths of mantle melts from thermobarometry, strongly indicating that the low velocity zones delineate regions of melt production in the mantle. The arc and backarc melt production regions are well separated at shallow depths, but may be connected at depths greater than 80km. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-28T07:02:20.25662-05:0
      DOI: 10.1002/2014GC005627
  • The evolution of MORB and plume mantle volatile budgets: Constraints from
           fission Xe isotopes in Southwest Indian Ridge basalts
    • Authors: Rita Parai; Sujoy Mukhopadhyay
      Abstract: We present high‐precision measurements of the fission isotopes of xenon (Xe) in basalts from the Southwest Indian Ridge (SWIR) between 16 and 25°E. Corrections for syn‐ to post‐eruptive atmospheric contamination yield the Xe isotopic compositions of SWIR mantle sources. We solve for the proportions of mantle Xe derived from the primordial mantle Xe budget, recycling of atmospheric Xe, decay of short‐lived 129I, fission of extinct 244Pu and fission of extant 238U. Xe isotope systematics evident in SWIR basalts and other mantle‐derived samples provide new insights into the integrated history of mantle source degassing and regassing. We find that recycled atmospheric Xe dominates the Xe inventories of the SWIR Western and Eastern Orthogonal Supersegment mantle sources (∼80‐90% of 132Xe is recycled in origin), consistent with results from studies of plume‐influenced basalts from Iceland (Mukhopadhyay, 2012) and the Rochambeau Rift (Pető et al., 2013). While significant regassing of the mantle is evident, we also find differences in the extent of degassing of the MORB and plume sources. MORB sources are consistently characterized by a lower fraction of fission Xe derived from Pu‐fission, indicating a greater extent of degassing relative to the plume source. The prevalence of recycled atmospheric Xe in mantle sources indicates incorporation of depleted recycled material even into mantle sources with primitive He and Ne isotopic compositions. Consequently, depleted lithophile isotopic compositions in mantle sources with primitive He and Ne cannot be interpreted as evidence for a non‐chondritic bulk silicate Earth. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-28T05:54:42.092539-05:
      DOI: 10.1002/2014GC005566
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