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

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 21, SJR: 2.156, h-index: 61)
Geophysical Research Letters     Full-text available via subscription   (Followers: 48, SJR: 2.668, h-index: 142)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 5, SJR: 2.4, h-index: 109)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 2, SJR: 0.126, h-index: 2)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 21)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 7)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 24)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 16)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 13)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 23)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 16)
Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 2.16, h-index: 82)
Radio Science     Full-text available via subscription   (Followers: 3, SJR: 0.527, h-index: 47)
Reviews of Geophysics     Full-text available via subscription   (Followers: 19, SJR: 8.837, h-index: 87)
Space Weather     Full-text available via subscription   (Followers: 3, SJR: 0.496, h-index: 16)
Tectonics     Full-text available via subscription   (Followers: 7, SJR: 2.16, h-index: 79)
Water Resources Research     Full-text available via subscription   (Followers: 176, SJR: 1.769, h-index: 110)
Journal Cover Geochemistry, Geophysics, Geosystems
   [23 followers]  Follow    
   Full-text available via subscription Subscription journal
     ISSN (Online) 1525-2027
     Published by American Geophysical Union (AGU) Homepage  [17 journals]   [SJR: 2.156]   [H-I: 61]
  • Thermodynamic calculations of the polybaric melting phase relations of
           spinel lherzolite
    • Authors: Kenta Ueki; Hikaru Iwamori
      Pages: n/a - n/a
      Abstract: This study presents a new thermodynamic model for the calculation of phase relations during the melting of anhydrous spinel lherzolite at pressures of 1–2.5 GPa. The model is based on the total energy minimization algorithm for calculating phase equilibria within multicomponent systems and the thermodynamic configuration of Ueki and Iwamori [2013]. The model is based on a SiO2 – Al2O3 – FeO–Fe3O4–MgO–CaO system that includes silicate melt, olivine, clinopyroxene, orthopyroxene, and spinel as possible phases. The molar Gibbs free energy of the melt phase is modeled quasi‐empirically, and the thermodynamic parameters for silicate melt end‐member components are calibrated with a polybaric calibration database. The temperatures and pressures used in this newly compiled calibration dataset are 1230–1600∘C and 0.9–3 GPa, corresponding to the stability range of spinel lherzolite. The modeling undertaken during this study reproduces the general features of experimentally determined melting phase relations of spinel lherzolite at 1–2.5 GPa, including the solidus temperature, the melt composition, the chemical reaction during melting and the degree of melting. This new thermodynamic modeling also reproduces phase relations of various bulk compositions from fertile to deplete spinel lherzolite and can be used in the modeling of polybaric mantle melting within various natural settings. Comparing the results derived from this new modeling with those produced using previous models indicates that the new approach outlined here, involving a combination of total energy minimization and the direct calibration of melt thermodynamic parameters at pressure and temperature conditions corresponding to mantle melting with a relatively simple melt thermodynamic equation, can accurately model polybaric melting phase relations. This article is protected by copyright. All rights reserved.
      PubDate: 2014-12-16T14:55:58.637236-05:
      DOI: 10.1002/2014GC005546
       
  • Issue Information
    • Pages: i - i
      PubDate: 2014-12-15T08:10:35.544656-05:
      DOI: 10.1002/ggge.20327
       
  • Mechanical properties and processes of deformation in shallow sedimentary
           rocks from subduction zones: An experimental study
    • Authors: Leslie Gadenne; Hugues Raimbourg, Rémi Champallier, Yuzuru Yamamoto
      Pages: n/a - n/a
      Abstract: To better constrain the mechanical behavior of sediments accreted to accretionary prism, we conducted triaxial mechanical tests on natural samples from the Miura‐Boso paleo‐accretionary prism (Japan) in drained conditions with confining pressures up to 200 MPa as well as post‐experiments P‐wave velocity (Vp) measurements. During experiments, deformation is principally non‐coaxial and accommodated by two successive modes of deformation, both associated with strain‐hardening and velocity‐strengthening behavior: (1) compaction‐assisted shearing, distributed in a several mm‐wide shear zone and (2) faulting, localized within a few tens of µm‐wide, dilatant fault zone. Deformation is also associated with (1) a decrease in Young's modulus all over the tests, (2) anomalously low Vp in the deformed samples compared to their porosity and (3) an increase in sensitivity of Vp to effective pressure. We interpret this evolution of the poroelastic properties of the material as reflecting the progressive breakage of intergrain cement and the formation of microcracks along with macroscopic deformation. When applied to natural conditions, these results suggest that the deformation style (localized vs distributed) of shallow (z 
      PubDate: 2014-12-13T02:40:55.456707-05:
      DOI: 10.1002/2014GC005580
       
  • Toward a unified hydrous olivine electrical conductivity law
    • Authors: Emmanuel Gardés; Fabrice Gaillard, Pascal Tarits
      Pages: n/a - n/a
      Abstract: It has long been proposed that water incorporation in olivine has dramatic effects on the upper mantle properties, affecting large scale geodynamics and triggering high electrical conductivity. But the laboratory‐based laws of olivine electrical conductivity predict contrasting effects of water, precluding the interpretation of geophysical data in term of mantle hydration. We review the experimental measurements of hydrous olivine conductivity and conclude that most of data are consistent when errors in samples water contents are considered. We report a new law calibrated on the largest database of measurements on hydrous olivine oriented single crystals and polycrystals. It fits most of measurements within uncertainties, and is compatible with most of geophysical data within petrological constraints on mantle olivine hydration. The isotropic conductivity (S/m) is where CH2O is the water concentration in olivine (wt.ppm) and T the temperature (K). The conductivity anisotropy of hydrous olivine might be higher than dry olivine, but preferential orientation should produce moderate anisotropy (~0–0.8 log unit). In the oceanic mantle, the enhancement of olivine conductivity is limited to ~1 log unit in the maximum range of mantle olivine water concentrations (0–500 wt.ppm). Strongest enhancements are expected in colder regions, like cratonic lithospheres and subduction settings. High conductivities in melt‐free mantle require great depths and high water concentrations in olivine (>0.1 S/m at >250 km and >200 wt.ppm). Thus, the hydration of olivine appears unlikely to produce the highest conductivities of the upper mantle. This article is protected by copyright. All rights reserved.
      PubDate: 2014-12-09T04:30:13.61405-05:0
      DOI: 10.1002/2014GC005496
       
  • Effects of crystal shape and size modality on magma rheology
    • Authors: P. Moitra; H. M. Gonnermann
      Pages: n/a - n/a
      Abstract: Erupting magma often contains crystals over a wide range of sizes and shapes, potentially affecting magma viscosity over many orders of magnitude. A robust relation between viscosity and the modality of crystal sizes and shapes remains lacking, principally because of the dimensional complexity and size of the governing parameter space. We have performed a suite of shear viscosity measurements on liquid‐particle suspensions of dynamical similarity to crystal‐bearing magma. Our experiments encompass five suspension types, each consisting of unique mixtures of two different particle sizes and shapes. The experiments span two orthogonal subspaces of particle concentration, as well as particle size and shape for each suspension type, thereby providing insight into the topology of parameter space. For each suspension type, we determined the dry maximum packing fraction and measured shear rates across a range of applied shear stresses. The results were fitted using a Herschel‐Bulkley model and augment existing predictive capabilities. We demonstrate that our results are consistent with previous work, including friction‐based constitutive laws for granular materials. We conclude that predictions for ascent rates of crystal‐rich magmas must take the shear‐rate dependence of viscosity into account. Shear‐rate dependence depends first and foremost on the volume fraction of crystals, relative to the maximum packing fraction, which in turn depends on crystal size and shape distribution. This article is protected by copyright. All rights reserved.
      PubDate: 2014-12-04T10:41:07.359378-05:
      DOI: 10.1002/2014GC005554
       
  • Ages and magnetic structures of the South China Sea constrained by deep
           tow magnetic surveys and IODP Expedition 349
    • Authors: Chun‐Feng Li; Xing Xu, Jian Lin, Xing Xu, Zhen Sun, Jian Zhu, Yongjian Yao, Xixi Zhao, Qingsong Liu, Denise K. Kulhanek, Jian Wang, Taoran Song, Junfeng Zhao, Ning Qiu, Yongxian Guan, Zhiyuan Zhou, Trevor Williams, Rui Bao, Anne Briais, Elizabeth A. Brown, Yifeng Chen, Peter D. Clift, Frederick S. Colwell, Kelsie A. Dadd, Weiwei Ding, Iván Hernández Almeida, Xiao‐Long Huang, Sangmin Hyun, Tao Jiang, Anthony A.P. Koppers, Qianyu Li, Chuanlian Liu, Zhifei Liu, Renata H. Nagai, Alyssa Peleo‐Alampay, Xin Su, Maria Luisa (Marissa) G. Tejada, Hai Son Trinh, Yi‐Ching Yeh, Chuanlun Zhang, Fan Zhang, Guo‐Liang Zhang
      Pages: n/a - n/a
      Abstract: Combined analyses of deep tow magnetic anomalies and International Ocean Discovery Program Expedition 349 cores show that initial seafloor spreading started around 33 Ma in the northeastern South China Sea (SCS), but varied slightly by 1‐2 myr along the northern continent‐ocean boundary (COB). A southward ridge jump of ∼ 20km occurred around 23.6 Ma in the East Subbasin; this timing also slightly varied along the ridge and was coeval to the onset of seafloor spreading in the Southwest Subbasin, which propagated for about 400km southwestward from ∼23.6 Ma to ∼ 21.5 Ma. The terminal age of seafloor spreading is ∼15 Ma in the East Subbasin and ∼16 Ma in the Southwest Subbasin. The full spreading rate in the East Subbasin varied largely from ∼20 to ∼80km/myr, but mostly decreased with time except for the period between ∼26.0 Ma and the ridge jump (∼23.6 Ma), within which the rate was the fastest at ∼70km/myr on average. The spreading rates are not correlated, in most cases, to magnetic anomaly amplitudes that reflect basement magnetization contrasts. Shipboard magnetic measurements reveal at least one magnetic reversal in the top 100m of basaltic layers, in addition to large vertical intensity variations. These complexities are caused by late‐stage lava flows that are magnetized in a different polarity from the primary basaltic layer emplaced during the main phase of crustal accretion. Deep tow magnetic modeling also reveals this smearing in basement magnetizations by incorporating a contamination coefficient of 0.5, which partly alleviates the problem of assuming a magnetic blocking model of constant thickness and uniform magnetization. The primary contribution to magnetic anomalies of the SCS is not in the top 100m of the igneous basement. This article is protected by copyright. All rights reserved.
      PubDate: 2014-12-04T10:25:15.58005-05:0
      DOI: 10.1002/2014GC005567
       
  • MELTS_Excel: A Microsoft Excel‐based MELTS interface for research
           and teaching of magma properties and evolution
    • Authors: Guilherme A. R. Gualda; Mark S. Ghiorso
      Pages: n/a - n/a
      Abstract: The thermodynamic modeling software MELTS is a powerful tool for investigating crystallization and melting in natural magmatic systems. Rhyolite‐MELTS is a recalibration of MELTS that better captures the evolution of silicic magmas in the upper crust. The current interface of rhyolite‐MELTS, while flexible, can be somewhat cumbersome for the novice. We present a new interface that uses web services consumed by a VBA backend in Microsoft Excel©. The interface is contained within a macro‐enabled workbook, where the user can insert the model input information and initiate computations that are executed on a central server at OFM Research. Results of simple calculations are shown immediately within the interface itself. It is also possible to combine a sequence of calculations into an evolutionary path; the user can input starting and ending temperatures and pressures, temperature and pressure steps, and the prevailing oxidation conditions. The program shows partial updates at every step of the computations; at the conclusion of the calculations, a series of data sheets and diagrams are created in a separate workbook, which can be saved independently of the interface. Additionally, the user can specify a grid of temperatures and pressures and calculate a phase diagram showing the conditions at which different phases are present. The interface can be used to apply the rhyolite‐MELTS geobarometer. We demonstrate applications of the interface using an example early‐erupted Bishop Tuff composition. The interface is simple to use and flexible, but it requires an internet connection. The interface is distributed for free from http://melts.ofm‐research.org. This article is protected by copyright. All rights reserved.
      PubDate: 2014-12-04T10:24:41.015814-05:
      DOI: 10.1002/2014GC005545
       
  • Long‐term evolution of an Oligocene/Miocene maar lake from Otago,
           New Zealand
    • Authors: B. R. S. Fox; J. Wartho, G. S. Wilson, D. E. Lee, F. E. Nelson, U. Kaulfuss
      Pages: n/a - n/a
      Abstract: Foulden Maar is a highly resolved maar lake deposit from the South Island of New Zealand comprising laminated diatomite punctuated by numerous diatomaceous turbidites. Basaltic clasts found in debris flow deposits near the base of the cored sedimentary sequence yielded two new 40Ar/39Ar dates of 24.51 ± 0.24 Ma and 23.38 ±. 24 Ma (2σ). The younger date agrees within error with a previously published 40Ar/39Ar date of 23.17 ± 0.19 Ma from a basaltic dyke adjacent to the maar crater. The diatomite is inferred to have been deposited over several tens of thousands of years in the latest Oligocene/earliest Miocene, and may have been coeval with the period of rapid glaciation and subsequent deglaciation of Antarctica known as the Mi‐1 event. Sediment magnetic properties and SEM measurements indicate that the magnetic signal is dominated by pseudo‐single domain pyrrhotite. The most likely source of detrital pyrrhotite is schist country rock fragments from the inferred tephra ring created by the phreatomagmatic eruption that formed the maar. Variations in magnetic mineral concentration indicate a decrease in erosional input throughout the depositional period, suggesting long‐term (tens of thousands of years) environmental change in New Zealand in the latest Oligocene/earliest Miocene. This article is protected by copyright. All rights reserved.
      PubDate: 2014-12-04T10:24:05.397549-05:
      DOI: 10.1002/2014GC005534
       
  • Magmatic arc structure around Mount Rainier, WA, from the joint inversion
           of receiver functions and surface wave dispersion
    • Authors: Mathias Obrebski; Geoffrey A. Abers, Anna Foster
      Pages: n/a - n/a
      Abstract: The deep magmatic processes in volcanic arcs are often poorly understood. We analyze the shear wave velocity (VS) distribution in the crust and uppermost mantle below Mount Rainier, in the Cascades arc, resolving the main velocity contrasts based on converted phases within P coda via source normalization or receiver function (RF) analysis. To alleviate the trade‐off between depth and velocity we use long period phase velocities (25‐100s) obtained from earthquake surface waves, and at shorter period (7‐21s) use seismic noise cross‐correlograms. We use a transdimensional Bayesian scheme to explore the model space (VS in each layer, number of interfaces and their respective depths, level of noise on data). We apply this tool to 15 broadband stations from permanent and EarthScope temporary stations. Most results fall into 2 groups with distinctive properties. Stations east of the arc (Group I) have comparatively slower middle‐to‐lower crust (VS=3.4‐3.8km/s at 25km depth), a sharp Moho and faster uppermost mantle (VS=4.2‐4.4km/s). Stations in the arc (Group II) have a faster lower crust (VS=3.7‐4km/s) overlying a slower uppermost mantle (VS=4.0‐4.3km/s), yielding a weak Moho. Lower crustal velocities east of the arc (Group I) most likely represent ancient subduction mélanges mapped nearby. The lower crust for Group II ranges from intermediate to felsic. We propose that intermediate‐felsic to felsic rocks represent the pre‐arc basement, while intermediate composition indicates the mushy andesitic crustal magmatic system plus solidified intrusion along the volcanic conduits. We interpret the slow upper mantle as partial melt. This article is protected by copyright. All rights reserved.
      PubDate: 2014-12-04T10:22:01.469927-05:
      DOI: 10.1002/2014GC005581
       
  • Chalcophile behavior of thallium during MORB melting and implications for
           the sulfur content of the mantle
    • Authors: Sune G. Nielsen; Nobumichi Shimizu, Cin‐Ty A. Lee, Mark D. Behn
      Pages: n/a - n/a
      Abstract: We present new laser ablation ICPMS trace element concentration data for 28 elements in 97 mid ocean ridge basalt (MORB) glasses that cover all major spreading centers as well as Tl concentration data for all mineral phases in five lherzolites from the Lherz massif, France. The ratio between the elements thallium (Tl) and cerium (Ce) is nearly constant in MORB, providing evidence that the depleted MORB mantle (DMM) has uniform Ce/Tl. Lherzolite mineral data reveal that sulfides are heterogeneous and contain between 23 and 430ng/g of Tl while all other minerals contain Tl below the analytical detection limit of ∼1ng/g. We argue that Tl in MORB is controlled by residual sulfide during mantle melting. To investigate the observed relationship between Tl and Ce, we conduct models of fractional mantle melting, which show that the constant Ce/Tl in MORB is only reproduced if the ratio between clinopyroxene and sulfide in the upper mantle varies by less than 10%. In addition, the rate of melting for these two phases must be nearly identical as otherwise melt depletion and refertilization processes would lead to Ce/Tl fractionation. These model results allow us to establish a relationship for the sulfur content of DMM: [S]DMM = SCSS × Mcpx ⁄Rcpx, where SCSS is the sulfur concentration of a silicate melt at sulfide saturation, Rcpx is the melt reaction coefficient and Mcpx is the modal abundance of clinopyroxene in the DMM. Using this equation, we calculate that the average upper mantle sulfur concentration is 195±45μg/g. This article is protected by copyright. All rights reserved.
      PubDate: 2014-12-04T05:01:26.332794-05:
      DOI: 10.1002/2014GC005536
       
  • Three‐dimensional numerical simulations of crustal systems
           undergoing orogeny and subjected to surface processes
    • Authors: C. Thieulot; P. Steer, R.S. Huismans
      Pages: n/a - n/a
      Abstract: As several modeling studies indicate, the structural expression and dynamic behavior of orogenic mountain belts is dictated not only by their rheological properties or by far‐field tectonic motion, but also by the efficiency of erosion and sedimentation acting on its surface. Until recently, numerical investigations have been mainly limited to 2D studies because of the high computational cost required by 3D models. Here, we have efficiently coupled the landscape evolution model Cascade with the 3D thermo‐mechanically coupled tectonics code FANTOM. Details of the coupling algorithms between both codes are given. We present results of numerical experiments designed to study the response of viscous‐plastic crustal materials subjected to convergence and to surface processes including both erosion and sedimentation. In particular, we focus on the equilibration of both the tectonic structures and on the surface morphology of the orogen. We show that increasing the efficiency of fluvial erosion increases the frontal thrust angle, which in turn decreases the width of the orogen. In addition, the maximum summit elevation of the orogen during transient evolution is significantly higher in those models showcasing surface processes than those that do not. This illustrates the strong coupling between tectonic and surface processes. We also demonstrate that an along‐strike gradient of erosion efficiency can have a major impact upon the landscape morphology and the tectonic structure and deformation of the orogen, in both the across‐ and along‐strike directions. Overall, our results suggest that surface processes, by enhancing localization of deformation, can act as a positive forcing to topographic building. This article is protected by copyright. All rights reserved.
      PubDate: 2014-12-04T04:38:07.439385-05:
      DOI: 10.1002/2014GC005490
       
  • Surface slip and off‐fault deformation patterns in the 2013 MW 7.7
           Balochistan, Pakistan earthquake: Implications for controls on the
           distribution of near‐surface coseismic slip
    • Authors: Robert Zinke; James Hollingsworth, James F. Dolan
      Pages: n/a - n/a
      Abstract: Comparison of 398 fault offsets measured by visual analysis of WorldView high‐resolution satellite imagery with deformation maps produced by COSI‐Corr sub‐pixel image correlation of Landsat‐8 and SPOT5 imagery reveals significant complexity and distributed deformation along the 2013 Mw 7.7 Balochistan, Pakistan earthquake. Average slip along the main trace of the fault was 4.2 m, with local maximum offsets up to 11.4 m. Comparison of slip measured from offset geomorphic features, which record localized slip along the main strand of the fault, to the total displacement across the entire width of the surface deformation zone from COSI‐Corr reveals ~45 % off‐fault deformation. Whereas previous studies have shown that the structural maturity of the fault exerts a primary control on the total percentage of off‐fault surface deformation, large along‐strike variations in the percentage of strain localization observed in the 2013 rupture imply the influence of important secondary controls. One such possible secondary control is the type of near‐surface material through which the rupture propagated. We therefore compared the percentage off‐fault deformation to the type of material (bedrock, old alluvium, young alluvium) at the surface and the distance of the fault to the nearest bedrock outcrop (a proxy for sediment thickness along this hybrid strike‐slip/reverse slip fault). We find significantly more off‐fault deformation in younger and/or thicker sediments. Accounting for and predicting such off‐fault deformation patterns has important implications for the interpretation of geologic slip rates, especially for their use in probabilistic seismic hazard assessments, the behavior of near‐surface materials during coseismic deformation, and the future development of microzonation protocols for the built environment. This article is protected by copyright. All rights reserved.
      PubDate: 2014-12-04T04:37:32.943393-05:
      DOI: 10.1002/2014GC005538
       
  • The mantle transition zone beneath West Antarctica: Seismic evidence for
           hydration and thermal upwellings
    • Authors: E.L. Emry; A. A. Nyblade, J. Julià, S. Anandakrishnan, R. C. Aster, D. A. Wiens, A. D. Huerta, T. J. Wilson
      Pages: n/a - n/a
      Abstract: Although prior work suggests that a mantle plume is associated with Cenozoic rifting and volcanism in West Antarctica, the existence of a plume remains conjectural. Here, we use P‐wave receiver functions (PRFs) from the Antarctic POLENET array to estimate mantle transition zone thickness, which is sensitive to temperature perturbations, throughout previously unstudied parts of West Antarctica. We obtain over 8000 high‐quality PRFs using an iterative, time‐domain deconvolution method filtered with a Gaussian‐width of 0.5 and 1.0, corresponding to frequencies less than ∼0.24 Hz and ∼0.48 Hz, respectively. Single‐station and common conversion point stacks, migrated to depth using the AK135 velocity model, indicate that mantle transition zone thickness throughout most of West Antarctica does not differ significantly from the global average, except in two locations; one small region exhibits a vertically thinned (210±15km) transition zone beneath the Ruppert Coast of Marie Byrd Land and another laterally broader region shows slight, vertical thinning (225±25km) beneath the Bentley Subglacial Trench. We also observe the 520 discontinuity and a prominent negative peak above the mantle transition zone throughout much of West Antarctica. These results suggest that the mantle transition zone may be hotter than average in two places, possibly due to upwelling from the lower mantle, but not broadly across West Antarctica. Furthermore, we propose that the transition zone may be hydrated due to >100 million years of subduction beneath the region during the early Mesozoic. This article is protected by copyright. All rights reserved.
      PubDate: 2014-11-26T11:32:54.03111-05:0
      DOI: 10.1002/2014GC005588
       
  • Rock magnetism of tiny exsolved magnetite in plagioclase from a
           Paleoarchean granitoid in the Pilbara Craton
    • Authors: Yoichi Usui; Takazo Shibuya, Yusuke Sawaki, Tsuyoshi Komiya
      Pages: n/a - n/a
      Abstract: Granitoids are widespread in Precambrian terranes as well as the Phanerozoic orogenic belts, but they have garnered little attention in paleomagnetic studies, because granitoids often contain abundant coarse‐grained, magnetically unstable oxides. In this study, the first example of tiny, needle‐shaped, exsolved oxides in plagioclase in a Paleoarchean granitoid is reported. The magnetic properties of single plagioclase crystals with the exsolved oxide inclusions have been studied to determine their paleomagnetic recording fidelity. Demagnetization experiments and hysteresis parameters indicate that the oxide inclusions are near stoichiometric magnetite and magnetically very stable. First‐order reversal curve (FORC) diagrams reveal negligible magnetostatic interactions. Minimal interactions are also reflected by very efficient acquisition of anhysteretic remanent magnetization. Single plagioclase crystals exhibit strong magnetic remanence anisotropies, which require corrections to their paleodirectional and paleointensity data. Nonetheless, quantitative consideration of anisotropy tensors of the single plagioclase crystals indicates that the bias can be mitigated by properly averaging data from a few tens of single crystals. From the nonlinear thermoremanence acquisition of the plagioclase crystals, we estimate that the plagioclase crystals can reconstruct paleointensity up to 50 μT. Local metamorphic condition suggests that those magnetite may carry remanence of ca. 3.2 to 3.3 Ga. We suggest that exsolved magnetite in granitoids is potentially a suitable target for the study of the early history of the geomagnetic field, and prompt detailed microscopic investigations as well as paleomagnetic tests to constrain the age of remanence. This article is protected by copyright. All rights reserved.
      PubDate: 2014-11-24T04:20:44.136581-05:
      DOI: 10.1002/2014GC005508
       
  • Density, temperature, and composition of the North American lithosphere:
           New insights from a joint analysis of seismic, gravity, and mineral
           physics data. 1: Density structure of the crust and upper mantle
    • Authors: Mikhail K. Kaban; Magdala Tesauro, Walter D. Mooney, Sierd A.P.L. Cloetingh
      Pages: n/a - n/a
      Abstract: We introduce a new method to construct integrated 3D models of density, temperature and compositional variations of the crust and upper mantle based on a combined analysis of gravity, seismic and tomography data with mineral physics constraints. The new technique is applied to North America. In the first stage we remove the effect of the crust from the observed gravity field and topography, using a new crustal model (NACr2014). In the second step, the residual mantle gravity field and residual topography are inverted to obtain a 3D density model of the upper mantle. The inversion technique accounts for the notion that these fields are controlled by the same factors but in a different way, e.g. depending on depth and horizontal dimension. This enables us to locate the position of principal density anomalies in the upper mantle. Afterwards, we estimate the thermal contribution to the density structure by inverting two tomography models for temperature (NA07 and SL2013sv), assuming a laterally and vertically uniform ‘fertile' mantle composition. Both models show the cold internal part and the hot western margin of the continent, while in some Proterozoic regions (e.g., Grenville province) NA07 at a depth of 100 km is >200˚C colder than SL2013sv. After removing this effect from the total mantle anomalies, the residual “compositional” fields are obtained. Some features of the composition density distribution, which are invisible in the seismic tomography data, are detected for the first time in the upper mantle. These results serve as a basis for the second part of the study, in which we improve the thermal and compositional models by applying an iterative approach to account for the effect of composition on the thermal model. This article is protected by copyright. All rights reserved.
      PubDate: 2014-11-20T22:28:36.119605-05:
      DOI: 10.1002/2014GC005483
       
  • Cogenetic late Pleistocene rhyolite and cumulate diorites from Augustine
           Volcano revealed by SIMS 238U‐230Th dating of zircon, and
           implications for silicic magma generation by extraction from mush
    • Authors: Michelle L. Coombs; Jorge A. Vazquez
      Pages: n/a - n/a
      Abstract: Augustine Volcano, a frequently active andesitic island stratocone, erupted a late Pleistocene rhyolite pumice fall that is temporally linked through zircon geochronology to cumulate dioritic blocks brought to the surface in Augustine's 2006 eruption. Zircon from the rhyolite yield a 238U‐230Th age of ca. 25 ka for their unpolished rims, and their interiors yield a bimodal age populations at ca. 26 ka and a minority at ca. 41 ka. Zircon from dioritic blocks, ripped from Augustine's shallow magmatic plumbing system and ejected during the 2006 eruption, have interiors defining a ca. 26 ka age population that is indistinguishable from that for the rhyolite; unpolished rims on the dioritic zircon are dominantly younger (≤12 ka) indicating subsequent crystallization. Zircon from rhyolite and diorite overlap in U, Hf, Ti, and REE concentrations, though diorites also contain a second population of high‐U, high temperature grains. Andesites that brought dioritic blocks to the surface in 2006 contain zircon with young (≤9 ka) rims and a scattering of older ages, but few zircon that crystallized during the 26 ka interval. Both the Pleistocene‐age rhyolite and the 2006 dioritic inclusions plot along a whole‐rock compositional trend distinct from mid‐Holocene–present andesites and dacites, and the diorites, rhyolite, and two early Holocene dacites define linear unmixing trends often oblique to the main andesite array and consistent with melt (rhyolite) extraction from a mush (dacites), leaving behind a cumulate amphibole‐bearing residue (diorites). Rare zircon antecrysts up to ca. 300 ka from all rock types indicate that a Quaternary center has been present longer than preserved surficial deposits. This article is protected by copyright. All rights reserved.
      PubDate: 2014-11-20T04:19:07.986339-05:
      DOI: 10.1002/2014GC005589
       
  • Density, temperature, and composition of the North American lithosphere:
           New insights from a joint analysis of seismic, gravity, and mineral
           physics data. 2: Thermal and compositional model of the upper mantle
    • Authors: Magdala Tesauro; Mikhail K. Kaban, Walter D. Mooney, Sierd A.P.L. Cloetingh
      Pages: n/a - n/a
      Abstract: Temperature and compositional variations of the North American (NA) lithospheric mantle are estimated using a new inversion technique introduced in Part I [Kaban et al., 2014], which allows us to jointly interpret seismic tomography and gravity data, taking into account depletion of the lithospheric mantle beneath the cratonic regions. The technique is tested using two tomography models (NA07 and SL2013sv) and different lithospheric density models. The first density model (Model I) reproduces the typical compositionally stratified lithospheric mantle, which is consistent with xenolith samples from the central Slave craton, while the second one (Model II) is based on the direct inversion of the residual gravity and residual topography [Part I, Kaban et al., 2014]. The results obtained, both in terms of temperature and composition, are more strongly influenced by the input models derived from seismic tomography, rather than by the choice of lithospheric density Model I versus Model II. The final temperatures estimated in the Archean lithospheric root, are up to 150°C higher than in the initial thermal models obtained using a laterally and vertically uniform “fertile” compositional model and are in agreement with temperatures derived from xenolith data. Therefore, the effect of the compositional variations cannot be neglected when temperatures of the cratonic lithospheric mantle are estimated. Strong negative compositional density anomalies (92, characterize the lithospheric mantle of the northwestern part of the Superior craton and the central part of the Slave and Churchill craton, according to both tomographic models. The largest discrepancies between the results based on different tomography models are observed in the Proterozoic regions, such as the Trans Hudson Orogen (THO), Rocky Mountains and Colorado Plateau, which appear weakly depleted (>‐0.025 g/cm3corresponding to Mg # ~91) when NA07 is used, or locally characterized by high density bodies when model SL2013sv is used. The former results are in agreement with those based on the interpretation of xenolith data. The high density bodies might be interpreted as fragments of subducted slabs or of the advection of the lithospheric mantle induced from the eastward‐directed flat slab subduction. The selection of a seismic tomography model plays a significant role when estimating lithospheric density, temperature and compositional heterogeneity. The consideration of the results of more than one model gives a more complete picture of the possible compositional variations within the NA lithospheric mantle. This article is protected by copyright. All rights reserved.
      PubDate: 2014-11-20T04:00:40.171624-05:
      DOI: 10.1002/2014GC005484
       
  • Imaging methane hydrates growth dynamics in porous media using synchrotron
           X‐ray computed microtomography
    • Authors: Prasad B. Kerkar; Kristine Horvat, Keith W. Jones, Devinder Mahajan
      Pages: n/a - n/a
      Abstract: Commercial‐scale methane (CH4) extraction from natural hydrate deposits remains a challenge due to, among other factors, a poor understanding of hydrate‐host sediment interactions under low‐temperature and high‐pressure conditions that are conducive to their existence. We report the use of synchrotron X‐ray computed microtomography (CMT) to image, for the first time, time‐resolved pore‐scale methane CH4 hydrate growth from an aqueous solution containing 5wt% barium chloride (BaCl2) and pressurized CH4 hosted in glass beads, all contained in an aluminum cell with an effective volume of 3.5mL. Multiple two‐dimensional (2‐D) cross sectional images show CH4 hydrates, with 7.5 µm resolution, distributed in patches throughout the system without dependence on distance from the cell walls. The time‐resolved three‐dimensional (3‐D) images, constructed from the 2‐D slices, exhibited pore‐filling hydrate formation from dissolved CH4 gas, similar to natural CH4 hydrates (sI) in the marine environment. Furthermore, the 3‐D images show that the aqueous phase was the wetting phase of the glass beads, i.e., the host and the formed hydrate were separated by an aqueous layer. These results provide some fundamental understanding of the nucleation phenomenon of gas hydrate formation at the pore scale. Pore‐filling CH4 hydrate growth is likely to result in a reduced bulk modulus, and thus could affect seafloor stability during the reverse phenomenon, i.e, dissociation of natural hydrate deposits. This article is protected by copyright. All rights reserved.
      PubDate: 2014-11-18T09:46:56.829667-05:
      DOI: 10.1002/2014GC005373
       
  • The ability of rock physics models to infer marine in situ pore pressure
    • Authors: Matthew J. Hornbach; Michael Manga
      Pages: n/a - n/a
      Abstract: Pore fluid pressure is an important parameter defining the mechanical strength of marine sediments. Obtaining high spatial resolution in situ pore pressure measurements in marine sediments, however, is a challenge, and as a result, only a handful of in situ pore pressure measurements exist at scientific drill sites. Integrating rock physics models with standard IODP measurements provides a potentially widely applicable approach for calculating in situ pore pressure. Here, we use a rock physics approach to estimate in situ pore pressure at two Scientific Ocean Drill Sites where in situ pressure is well‐constrained: ODP Site 1173, used as reference for normal (hydrostatic) fluid pressures, and ODP Site 948, where previous studies infer high fluid pressures (λ*∼ 0.45‐0.95, where the pore pressure ratio λ* is defined as the pore pressure above hydrostatic divided by the difference between the largest principal stress and hydrostatic stress). Our analysis indicates that the rock physics method provides an accurate, low‐precision method for estimating in situ pore pressure at these drill sites, and sensitivity analysis indicates this method can detect modestly high (λ*>0.6) pore pressure at the 95% confidence level. This approach has broad applicability because it provides an inexpensive, high‐resolution (meter‐scale) method for retrospectively detecting and quantifying high pore pressure at any drill site where quality wire‐line logs and ocean drilling data exists. This article is protected by copyright. All rights reserved.
      PubDate: 2014-11-18T09:46:49.975548-05:
      DOI: 10.1002/2014GC005442
       
  • Development and evolution of detachment faulting along 50km of the
           Mid‐Atlantic Ridge near 16.5°N
    • Authors: Deborah K. Smith; Hans Schouten, Henry Dick, Joe Cann, Vincent Salters, Horst R. Marschall, Fuwu Ji, Dana Yoerger, Alessio Sanfilippo, Ross Parnell‐Turner, Camilla Palmiotto, Alexei Zheleznov, Hailong Bai, Will Junkin, Ben Urann, Spencer Dick, Margaret Sulanowska, Peter Lemmond, Scott Curry
      Pages: n/a - n/a
      Abstract: A multifaceted study of the slow‐spreading Mid‐Atlantic Ridge (MAR) at 16.5ºN provides new insights into detachment faulting and its evolution through time. The survey included regional multibeam bathymetry mapping, high‐resolution mapping using AUV Sentry, seafloor imaging using the TowCam system, and an extensive rock‐dredging program. At different times, detachment faulting was active along ∼50km of the western flank of the study area, and may have dominated spreading on that flank for the last 5 Ma. Detachment morphologies vary and include a classic corrugated massif, non‐corrugated massifs, and back‐tilted ridges marking detachment breakaways. High‐resolution Sentry data reveal one other detachment morphology; a low‐angle, irregular surface in the regional bathymetry is shown to be a finely corrugated detachment surface (corrugation wavelength of only tens of meters and relief of just a few meters). Multi‐scale corrugations are observed 2‐3km from the detachment breakaway suggesting that they formed in the brittle layer, perhaps by anastomosing faults. The thin wedge of hanging wall lavas that covers a low‐angle (6º) detachment footwall near its termination are intensely faulted and fissured; this deformation may be enhanced by the low‐angle of the emerging footwall. Active detachment faulting currently is limited to the western side of the rift valley. Nonetheless, detachment fault morphologies also are present over a large portion of the eastern flank on crust > 2 Ma indicating that within the last 5 Ma parts of the ridge axis have experienced periods of two‐sided detachment faulting. This article is protected by copyright. All rights reserved.
      PubDate: 2014-11-18T09:46:12.024754-05:
      DOI: 10.1002/2014GC005563
       
  • A Cenozoic uplift history of Mexico and its surroundings from longitudinal
           river profiles
    • Authors: Simon N. Stephenson; Gareth G. Roberts, Mark J. Hoggard, Alexander C. Whittaker
      Pages: n/a - n/a
      Abstract: Geodynamic models of mantle convection predict that Mexico and western North America share a history of dynamic support. We calculate admittance between gravity and topography, which indicates that the elastic thickness of the plate in Mexico is 11 km and in western North America it is 12 km. Admittance at wavelengths > 500 km in these regions suggests that topography is partly supported by sub‐crustal processes. These results corroborate estimates of residual topography from isostatic calculations and suggest that the amount of North American topography supported by the mantle may exceed 1 km. The Cenozoic history of magmatism, sedimentary flux, thermochronometric denudation estimates and uplifted marine terraces imply that North American lithosphere was uplifted and eroded during the last 30 Ma. We jointly invert 533 Mexican and North American longitudinal river profiles to reconstruct a continent‐scale rock uplift rate history. Uplift rate is permitted to vary in space and time. Erosional parameters are calibrated using incision rate data in southwest Mexico and the Colorado Plateau. Calculated rock uplift rates were 0.15–0.2 mm/yr between 25–10 Ma. Central Mexico experienced the highest uplift rates. Central and southern Mexico continued to uplift at 0.1 mm/yr until recent times. This uplift history is corroborated by independent constraints. We predict clastic flux to the Gulf of Mexico and compare it to independent estimates. We tentatively suggest that the loop between uplift, erosion and deposition can be closed here. Mexico's staged uplift history suggests that its dynamic support has changed during the last 30 Ma. This article is protected by copyright. All rights reserved.
      PubDate: 2014-11-17T09:30:55.873118-05:
      DOI: 10.1002/2014GC005425
       
  • Sensitivity experiments on true polar wander
    • Authors: Marianne Greff‐Lefftz; Jean Besse
      Pages: n/a - n/a
      Abstract: Using sensitivity experiments based on the position of subductions and of superplumes, we derive models for the temporal evolution of 3D mass anomalies in the mantle and compute the associated inertia perturbations and polar wander. We show that although the large length‐scale mantle dynamics during the Earth's history may have been dominated by coupled supercontinent‐superplume cycles, subductions alone are sufficient to trigger major True Polar Wander (TPW) episodes, or rotation of the whole lithosphere and mantle with respect to the earth's rotation axis. We present two examples. We speculate that the distribution of continents with respect to the equator on the Earth's surface is driven by episodic subductions during the Wilson cycle: alternating fast subduction girdles around continents and upwellings during the divergence phases, with both reduced or stopped subductions activity around continents and moderate inter‐continental subductions during the convergence phases, lead to successive equatorial or polar distributions of continents, both configurations being separated by strong episodes of TPW. Finally, using plate reconstructions and geologic maps, over the period 1100–720 Ma, the period of amalgamation and destruction of the Rodinia supercontinent, we explain with our model the observed large eastward/westward and poleward/equatorward motions of the rotation axis. This article is protected by copyright. All rights reserved.
      PubDate: 2014-11-17T07:37:03.048088-05:
      DOI: 10.1002/2014GC005504
       
  • Lifting the cover of the cauldron: Convection in hot planets
    • Authors: Yanick Ricard; Stéphane Labrosse, Fabien Dubuffet
      Pages: n/a - n/a
      Abstract: Convection models of planetary mantles do not usually include a specific treatment of near‐surface dynamics. In all situations where surface dynamics is faster than internal dynamics, the lateral transport of material at the surface forbids the construction of a topography that could balance the internal convective stresses. This is the case if intense erosion erases the topography highs and fills in the depressions or if magma is transported through the lithosphere and spreads at the surface at large distances. In these cases, the usual boundary condition of numerical simulations, that the vertical velocity cancels at the surface should be replaced by a condition where the vertical flux on top of the convective mantle equilibrates that allowed by the surface dynamics. We show that this new boundary condition leads to the direct transport of heat to the surface and changes the internal convection that evolves toward a heat‐pipe pattern. We discuss the transition between this extreme situation where heat is transported to the surface to the usual situation where heat diffuses through the lithosphere. This mechanism is much more efficient to cool a planet and might be the major cooling mechanism of young planets. Even the modest effect of material transport by erosion on Earth is not without effect on mantle convection and should affect the heat flow budget of our planet. This article is protected by copyright. All rights reserved.
      PubDate: 2014-11-17T07:34:55.511295-05:
      DOI: 10.1002/2014GC005556
       
  • Large‐scale mechanical buckle fold development and the initiation of
           tensile fractures
    • Authors: Andreas Eckert; Peter Connolly, Xiaolong Liu
      Pages: n/a - n/a
      Abstract: Tensile failure associated with buckle folding is commonly associated to the distribution of outer arc extension but has also been observed on fold limbs. This study investigates whether tensile stresses and associated failure can be explained by the process of buckling under realistic in‐situ stress conditions. A 2D plane strain finite element modeling approach is used to study single‐layer buckle folds with a Maxwell visco‐elastic rheology. A variety of material parameters are considered and their influence on the initiation of tensile stresses during the various stages of deformation is analyzed. It is concluded that the buckling process determines the strain distribution within the fold layer but is not solely responsible for the initiation of tensile stresses. The modeling results show that tensile stresses are most dependent on the permeability, viscosity and overburden thickness. Low permeability (
      PubDate: 2014-11-14T04:21:45.079039-05:
      DOI: 10.1002/2014GC005502
       
  • Rock magnetic properties and paleomagnetic behavior of Neogene marine
           sediments from northern Chile
    • Authors: Claudio A. Tapia; Gary S. Wilson
      Pages: n/a - n/a
      Abstract: Neogene sediments from La Portada and Bahia Inglesa formations located at Mejillones Peninsula and Caldera Port, Northern Chile carry multicomponent magnetizations which can lead to mis‐ identification of the characteristic paleomagnetic vector and consequently, errors in chronostratigraphic and structural interpretation of the strata. Here we present a complete rock magnetic and paleomagnetic behavior study of Neogene sediments from La Portada and Bahia Inglesa formations. Isothermal remanent magnetization, thermal‐magnetic susceptibility, magnetic hysteresis and X ray diffractometer experiments are combined with paleomagnetic, alternating field and thermal demagnetization data to determine magnetization history. Remanent magnetizations generally comprises three components: a low blocking temperature (Tb) component, below 150∘C, close to the present day field in direction, interpreted as a thermo‐ viscous component; an intermediate Tb component, between 150∘‐290∘C, overprinted by the low and the high Tb components, considered as the survival of the characteristic detrital magnetization; and a high Tb component, above 290∘C, thought to be of diagenetic origin. Detrital magnetite, titanomagnetite and low titanium maghemite are identified as the main carrier of the magnetic remanence. Rock magnetic results of a minor group of samples at both locations, detected the presence of a high‐coercivity mineral, possibly hematite, interpreted to carry the high Tb component and to represent oxidation of minerals in post‐depositional processes.
      PubDate: 2014-11-11T01:21:56.43228-05:0
      DOI: 10.1002/2014GC005336
       
  • Crustal structure beneath the Rif Cordillera, North Morocco, from the
           RIFSIS wide‐angle reflection seismic experiment
    • Authors: Alba Gil; Josep Gallart, Jordi Diaz, Ramon Carbonell, Montserrat Torne, Alan Levander, Mimoun Harnafi
      Pages: n/a - n/a
      Abstract: The different geodynamic models proposed since the late 90's to account for the complex evolution of the Gibraltar Arc System lack definite constraints on the crustal structure of the Rif orogen. Here we present the first well‐resolved P‐wave velocity crustal models of the Rif cordillera and its southern continuation towards the Atlas made using controlled‐source seismic data. Two 300+ km‐long wide‐angle reflection profiles crossed the Rif along NS and EW trends. The profiles recorded simultaneously 5 land explosions of 1Tn each using ~850 high frequency seismometers. The crustal structure revealed from 2D forward modeling delineates a complex, laterally‐varying crustal structure below the Rif domains. The most surprising feature, seen on both profiles, is a ~50 km deep crustal root localized beneath the External Rif. To the east, the crust thins rapidly by 20 km across the Nekkor fault, indicating that the fault is a crustal scale feature. On the NS profile the crust thins more gradually to 40 km thickness beneath Middle Atlas and 42 km beneath the Betics. These new seismic results are in overall agreement with regional trends of Bouguer gravity and are consistent with recent receiver function estimates of crustal thickness. The complex crustal structure of the Rif orogen in the Gibraltar Arc is a consequence of the Miocene collision between the Iberian and African plates. Both the abrupt change in crustal thickness at the Nekkor fault and the unexpectedly deep Rif crustal root can be attributed to interaction of the subducting Alboran slab with the North African passive margin at late Oligocene‐early Miocene times.
      PubDate: 2014-11-11T01:21:30.519946-05:
      DOI: 10.1002/2014GC005485
       
  • Changing tectonic controls on the long‐term carbon cycle from
           Mesozoic to present
    • Authors: Benjamin Mills; Stuart J. Daines, Timothy M. Lenton
      Pages: n/a - n/a
      Abstract: Tectonic drivers of degassing and weathering processes are key long‐term controls on atmospheric CO2. However, there is considerable debate over the changing relative importance of different carbon sources and sinks. Existing geochemical models have tended to rely on indirect methods to derive tectonic drivers, such as inversion of the seawater87Sr/86Sr curve to estimate uplift or continental basalt area. Here we use improving geologic data to update the representation of tectonic drivers in the COPSE biogeochemical model. The resulting model distinguishes CO2 sinks from terrestrial granite weathering, total basalt weathering and seafloor alteration. It also distinguishes CO2 sources from subduction zone metamorphism and from igneous intrusions. We reconstruct terrestrial basaltic area from data on the extent of large igneous provinces, and use their volume to estimate their contribution to degassing. We adopt a recently‐published reconstruction of subduction‐related degassing, and relate seafloor weathering to ocean crust creation rate. Revised degassing alone tends to produce unrealistically high CO2, but this is counteracted by the inclusion of seafloor alteration and global basalt weathering, producing a good overall fit to Mesozoic‐Cenozoic proxy CO2 estimates and a good fit to 87Sr/86Sr data. The model predicts that seafloor alteration and terrestrial weathering made similar contributions to CO2 removal through the Triassic and Jurassic, after which terrestrial weathering increased and seafloor weathering declined. We predict that basalts made a greater contribution to silicate weathering than granites through the Mesozoic, before the contribution of basalt weathering declined over the Cenozoic due to decreasing global basaltic area.
      PubDate: 2014-11-11T00:53:59.284964-05:
      DOI: 10.1002/2014GC005530
       
  • The Iceland Basin excursion: Age, duration and excursion field geometry
    • Authors: J.E.T. Channell
      Pages: n/a - n/a
      Abstract: The Iceland Basin geomagnetic excursion coincided with the marine isotope stage (MIS) 6/7 boundary. The age and duration of the excursion, at seven North Atlantic sites with sufficient isotope data, are estimated by matching marine isotope stage (MIS) 7a‐c to a calibrated template. Two criteria for defining the excursion, virtual geomagnetic pole (VGP) latitudes
      PubDate: 2014-11-11T00:41:01.939089-05:
      DOI: 10.1002/2014GC005564
       
  • FORCulator: A micromagnetic tool for simulating first‐order reversal
           curve diagrams
    • Authors: Richard J. Harrison; Ioan Lascu
      Pages: n/a - n/a
      Abstract: We describe a method for simulating first‐order reversal curve (FORC) diagrams of interacting single‐domain particles. Magnetostatic interactions are calculated in real space, allowing simulations to be performed for particle ensembles with arbitrary geometry. For weakly interacting uniaxial particles, the equilibrium magnetization at each field step is obtained by direct solution of the Stoner‐Wohlfarth model, assuming a quasi‐static distribution of interaction fields. For all other cases, the equilibrium magnetization is calculated using an approximate iterated solution to the Landau‐Lifshitz‐Gilbert equation. Multithreading is employed to allow multiple curves to be computed simultaneously, enabling FORC diagrams to be simulated in reasonable time using a standard desktop computer. Statistical averaging and post processing lead to simulated FORC diagrams that are comparable to their experimental counterparts. The method is applied to several geometries of relevance to rock and environmental magnetism, including densely packed random clusters and partially collapsed chains. The method forms the basis of FORCulator, a freely available software tool with graphical user interface that will enable FORC simulations to become a routine part of rock magnetic studies.
      PubDate: 2014-11-10T22:39:09.987862-05:
      DOI: 10.1002/2014GC005582
       
  • The affects of alteration and porosity on seismic velocities in oceanic
           basalts and diabases
    • Authors: R. L. Carlson
      Pages: n/a - n/a
      Abstract: Seismic velocities in the lavas that cap normal oceanic crust are affected by both crack porosity and alteration of the primary mineral phases, chiefly to clays. Porosity accounts for 75‐80% of the velocity variation in sonic log velocities in the lava sections of Holes 504B and 1256D, but the effect of alteration on the properties of the basalts has not been assessed. In this analysis, the grain velocities in basalt and diabase samples are estimated from an empirical linear relationship between grain density and the P‐wave modulus. The theoretical velocity in fresh, zero‐porosity basalt or diabase is 6.96±0.07 km/sec. Grain velocities in the diabase samples are statistically indistinguishable from the theoretical velocity, and show no variation with depth; alteration does not significantly affect the velocities in the diabase samples from Hole 504B. This result is consistent with previous analyses, which demonstrated that velocities in the dikes are controlled by crack porosity. In basalt lab samples, alteration reduces the average sample grain velocity to 6.74±0.02 km/sec; cracks at the sample scale further reduce the velocity to 5.86±0.03 km/sec, and large‐scale cracks in the lavas reduce the average in situ velocity to 5.2±0.3 km/sec. Cracks account for nearly 90% of the difference between seismic (in situ) velocities and the theoretical velocity in the unaltered solid material. Basalt grain velocities show a small, but significant systematic increase with depth; the influence of alteration decreases with depth in the lavas, reaching near zero at the base of the lavas in Holes 504B and 1256D.
      PubDate: 2014-11-08T11:26:30.580818-05:
      DOI: 10.1002/2014GC005537
       
  • A joint Monte Carlo analysis of seafloor compliance, Rayleigh wave
           dispersion, and receiver functions at ocean bottom seismic stations
           offshore New Zealand
    • Authors: Justin S. Ball; Anne F. Sheehan, Joshua C. Stachnik, Fan‐Chi Lin, John A. Collins
      Pages: n/a - n/a
      Abstract: Teleseismic body‐wave imaging techniques such as receiver function analysis can be notoriously difficult to employ on ocean‐bottom seismic data due largely to multiple reverberations within the water and low‐velocity sediments. In lieu of suppressing this coherently scattered noise in ocean‐bottom receiver functions, these site effects can be modeled in conjunction with shear velocity information from seafloor compliance and surface wave dispersion measurements to discern crustal structure. A novel technique to estimate 1D crustal shear‐velocity profiles from these data using Monte Carlo sampling is presented here. We find that seafloor compliance inversions and P‐S conversions observed in the receiver functions provide complimentary constraints on sediment velocity and thickness. Incoherent noise in receiver functions from the MOANA ocean bottom seismic experiment limit the accuracy of the practical analysis at crustal scales, but synthetic recovery tests and comparison with independent unconstrained nonlinear optimization results affirm the utility of this technique in principle.
      PubDate: 2014-11-08T11:15:56.574721-05:
      DOI: 10.1002/2014GC005412
       
  • Regional‐scale input of dispersed and discrete volcanic ash to the
           Izu‐Bonin and Mariana subduction zones
    • Authors: Rachel P. Scudder; Richard W. Murray, Julie C. Schindlbeck, Steffen Kutterolf, Folkmar Hauff, Claire C. McKinley
      Pages: n/a - n/a
      Abstract: We have geochemically and statistically characterized bulk marine sediment and ash layers at Ocean Drilling Program Site 1149 (Izu‐Bonin Arc) and Deep Sea Drilling Project Site 52 (Mariana Arc), and have quantified that multiple dispersed ash sources collectively comprise ~30‐35% of the hemipelagic sediment mass entering the Izu‐Bonin‐Mariana subduction system. Multivariate statistical analyses indicate that the bulk sediment at Site 1149 is a mixture of Chinese Loess, a second compositionally distinct eolian source, a dispersed mafic ash, and a dispersed felsic ash. We interpret the source of these ashes as respectively being basalt from the Izu‐Bonin Front Arc (IBFA) and rhyolite from the Honshu Arc. Sr‐, Nd‐, and Pb isotopic analyses of the bulk sediment are consistent with the chemical/statistical‐based interpretations. Comparison of the mass accumulation rate of the dispersed ash component to discrete ash layer parameters (thickness, sedimentation rate, and number of layers) suggests that eruption frequency, rather than eruption size, drives the dispersed ash record. At Site 52, the geochemistry and statistical modeling indicates that Chinese Loess, IBFA, dispersed BNN (boninite from Izu‐Bonin), and a dispersed felsic ash of unknown origin are the sources. At Site 1149 the ash layers and the dispersed ash are compositionally coupled, whereas at Site 52 they are decoupled in that there are no boninite layers, yet boninite is dispersed within the sediment. Changes in the volcanic and eolian inputs through time indicate strong arc‐ and climate‐related controls.
      PubDate: 2014-11-08T00:37:27.408732-05:
      DOI: 10.1002/2014GC005561
       
  • Curie temperatures of titanomagnetite in ignimbrites: Effects of
           emplacement temperatures, cooling rates, exsolution, and cation ordering
    • Authors: Mike Jackson; Julie A. Bowles
      Pages: n/a - n/a
      Abstract: Pumices, ashes and tuffs from Mt. St. Helens and from Novarupta contain two principal forms of titanomagnetite: homogeneous grains with Curie temperatures in the range 350‐500°C; and oxyexsolved grains with similar bulk composition, containing ilmenite lamellae, and having Curie temperatures above 500°C. Thermomagnetic analyses and isothermal annealing experiments in combination with stratigraphic settings and thermal models show that emplacement temperatures and cooling history may have affected the relative proportions of homogeneous and exsolved grains, and have clearly had a strong influence on the Curie temperature of the homogeneous phase. The exsolved grains are most common where emplacement temperatures exceeded 600°C, and in laboratory experiments, heating to over 600°C in air causes the homogeneous titanomagnetites to oxyexsolve rapidly. Where emplacement temperatures were lower, Curie temperatures of the homogeneous grains are systematically related to overburden thickness and cooling timescales, and thermomagnetic curves are generally irreversible, with lower Curie temperatures measured during cooling, but little or no change is observed in room‐temperature susceptibility. We interpret this irreversible behavior as reflecting variations in the degree of cation ordering in the titanomagnetites, although we cannot conclusively rule out an alternative interpretation involving fine‐scale subsolvus unmixing. Short‐range ordering within the octahedral sites may play a key role in the observed phenomena. Changes in the Curie temperature have important implications for the acquisition, stabilization and retention of natural remanence, and may in some cases enable quantification of the emplacement temperatures or cooling rates of volcanic units containing homogeneous titanomagnetites.
      PubDate: 2014-11-06T18:45:01.929005-05:
      DOI: 10.1002/2014GC005527
       
  • Microstructures, composition, and seismic properties of the Ontong Java
           Plateau mantle root
    • Authors: Andréa Tommasi; Akira Ishikawa
      Pages: n/a - n/a
      Abstract: To study how an impacting plume modifies the mantle lithosphere, we analyzed the microstructures and crystal preferred orientations (CPO) of 29 peridotites and 37 pyroxenites that sample the mantle root of the Ontong Java Plateau (OJP) from 60 to 120 km depth. The peridotites show a strong compositional variability, but homogeneous coarse‐granular to tabular microstructures, except for those equilibrated at the shallowest and deepest depths, which are porphyroclastic. All peridotites have clear olivine CPO, with dominant fiber‐[010] patterns. Low intragranular misorientations and straight grain boundaries in olivine suggest that, above 100 km depth, annealing often followed deformation. Calculated density and P‐wave velocities of the peridotites decrease weakly with depth. S‐wave velocities decrease faster, resulting in increasing Vp/Vs ratio with depth. Calculated densities and seismic velocity profiles are consistent with those estimated for normal mantle compositions under a cold oceanic geotherm. Enrichment in pyroxenites may further increase seismic velocities. The calculated seismic properties cannot therefore explain the low S‐waves velocities predicted by Rayleigh wave tomography and ScS data in the mantle beneath the OJP. Calculated P‐ and S‐wave anisotropy is variable (2‐12%). It is higher on average in the deeper section of the lithosphere. Because olivine has dominantly [010]‐fiber CPO patterns, if foliations are horizontal, vertically propagating S‐waves and Rayleigh waves will sample very weak anisotropy in the OJP mantle lithosphere. Moreover, if the orientation of the lineation changes with depth, the anisotropy‐induced contrast in seismic properties might produce an intralithospheric reflector marking the stratification of the OJP mantle root.
      PubDate: 2014-11-05T08:08:11.184537-05:
      DOI: 10.1002/2014GC005452
       
  • Soil CO2 emissions as a proxy for heat and mass flow assessment,
           Taupō Volcanic Zone, New Zealand
    • Authors: Bloomberg S; Werner C, Rissmann C, Mazot A, Horton T, Gravley D, Kennedy B, Oze C.
      Pages: n/a - n/a
      Abstract: The quantification of heat and mass flow between deep reservoirs and the surface is important for understanding magmatic and hydrothermal systems. Here, we use high‐resolution measurement of carbon dioxide flux (ϕCO2) and heat flow at the surface to characterize the mass (CO2 and steam) and heat released to the atmosphere from two magma‐hydrothermal systems. Our soil gas and heat flow surveys at Rotokawa and White Island in the Taupō Volcanic Zone, New Zealand, include over 3,000 direct measurements of ϕCO2 and soil temperature and 60 carbon isotopic values on soil gases. Carbon dioxide flux was separated into background and magmatic/hydrothermal populations based on the measured values and isotopic characterization. Total CO2 emission rates (ΣCO2) of 441 ± 84 t d‐1 and 124 ± 18 t d‐1 were calculated for Rotokawa (2.9 km2) and for the crater floor at White Island (0.3 km2), respectively. The total CO2 emissions differ from previously published values by +386 t d‐1 at Rotokawa and +25 t d‐1 at White Island, demonstrating that earlier research underestimated emissions by 700% (Rotokawa) and 25% (White Island). These differences suggest that soil CO2 emissions facilitate more robust estimates of the thermal energy and mass flux in geothermal systems than traditional approaches. Combining the magmatic/hydrothermal‐sourced CO2 emission (constrained using stable isotopes) with reservoir H2O:CO2 mass ratios and the enthalpy of evaporation, the surface expression of thermal energy release for the Rotokawa hydrothermal system (226 MWt) is 10 times greater than the White Island crater floor (22.5 MWt).
      PubDate: 2014-11-05T01:46:56.453817-05:
      DOI: 10.1002/2014GC005327
       
  • The role of elasticity in slab bending
    • Authors: Loic Fourel; Saskia Goes, Gabriele Morra
      Pages: n/a - n/a
      Abstract: Previous studies showed that plate rheology exerts a dominant control on the shape and velocity of subducting plates. Here, we perform a systematic investigation of the role of elasticity in slab bending, using fully dynamic 2D models where an elastic, viscoelastic or viscoelastoplastic plate subducts freely into a purely viscous mantle. We derive a scaling relationship between the bending radius of viscoelastic slabs and the Deborah number, De, which is the ratio of Maxwell time over deformation time. We show that De controls the ratio of elastically stored energy over viscously dissipated energy and find that at De >‐2, substantially less energy is required to bend a viscoelastic slab to the same shape as a purely viscous slab with the same intrinsic viscosity. Elastically stored energy at higher De favours retreating modes of subduction via unbending, while trench advance only occurs for some cases with De < 10‐2. We estimate the apparent Deborah numbers of natural subduction zones and find values ranging from 10‐3 to > 1, where most zones have low De < 10‐2, but a few young plates have De > 0.1. Slabs with De < 10‐2 either have very low viscosities or they may be yielding, in which case our De estimates may be underestimated by up to an order of magnitude potentially pointing towards a significant role of elasticity in ~60% of the subduction zones. In support of such a role of elasticity in subduction, we find that increasing De correlates with increasing proportion of larger seismic events in both instrumental and historic catalogues.
      PubDate: 2014-11-01T08:19:57.555072-05:
      DOI: 10.1002/2014GC005535
       
  • Influence of late Pleistocene glaciations on the hydrogeology of the
           continental shelf offshore Massachusetts, USA
    • Authors: Jacob Siegel; Mark Person, Brandon Dugan, Denis Cohen, Daniel Lizarralde, Carl Gable
      Pages: n/a - n/a
      Abstract: Multiple late Pleistocene glaciations that extended onto the continental shelf offshore Massachusetts, USA may have emplaced as much as 100 km3 of freshwater (salinity less than 5 ppt) in continental shelf sediments. To estimate the volume and extent of offshore freshwater, we developed a three‐dimensional, variable‐density model that couples fluid flow and heat and solute transport for the continental shelf offshore Massachusetts. The stratigraphy for our model is based on high‐resolution, multi‐channel seismic data. The model incorporates the last 3 Ma of climate history by prescribing boundary conditions of sea‐level change and ice sheet extent and thickness. We incorporate new estimates of the maximum extent of a late Pleistocene ice sheet to near the shelf‐slope break. Model results indicate that this late Pleistocene ice sheet was responsible for much of the emplaced freshwater. We predict that the current freshwater distribution may reach depths of up to 500 m below sea level and up to 30 km beyond Martha's Vineyard. The freshwater distribution is strongly dependent on the three‐dimensional stratigraphy and ice‐sheet history. Our predictions improve our understanding of the distribution of offshore freshwater, a potential non‐renewable resource for coastal communities along recently glaciated margins.
      PubDate: 2014-10-30T04:12:52.645546-05:
      DOI: 10.1002/2014GC005569
       
  • Imaging of CO2 bubble plumes above an erupting submarine volcano, NW
           Rota‐1, Mariana Arc
    • Authors: William W. Chadwick; Susan G. Merle, Nathaniel J. Buck, J. William Lavelle, Joseph A. Resing, Vicki Ferrini
      Pages: n/a - n/a
      Abstract: NW Rota‐1 is a submarine volcano in the Mariana volcanic arc located ~100 km north of Guam. Underwater explosive eruptions driven by magmatic gases were first witnessed there in 2004 and continued until at least 2010. During a March 2010 expedition, visual observations documented continuous but variable eruptive activity at multiple vents at ~560 m depth. Some vents released CO2 bubbles passively and continuously, while others released CO2 during stronger but intermittent explosive bursts. Plumes of CO2 bubbles in the water column over the volcano were imaged by an EM122 (12 kHz) multibeam sonar system. Throughout the 2010 expedition numerous passes were made over the eruptive vents with the ship to document the temporal variability of the bubble plumes and relate them to the eruptive activity on the seafloor, as recorded by an in situ hydrophone and visual observations. Analysis of the EM122 mid‐water dataset shows: (1) bubble plumes were present on every pass over the summit and they rose 200‐400 m above the vents but dissolved before they reached the ocean surface, (2) bubble plume deflection direction and distance correlate well with ocean current direction and velocity determined from the ship's acoustic doppler current profiler, (3) bubble plume heights and volumes were variable over time and correlate with eruptive intensity as measured by the in situ hydrophone. This study shows that mid‐water multibeam‐sonar data can be used to characterize the level of eruptive activity and its temporal variability at a shallow submarine volcano with robust CO2 output.
      PubDate: 2014-10-30T03:57:55.269164-05:
      DOI: 10.1002/2014GC005543
       
  • Understanding which parameters control shallow ascent of silicic effusive
           magma
    • Authors: Mark E. Thomas; Jurgen W. Neuberg
      Pages: n/a - n/a
      Abstract: The estimation of the magma ascent rate is key to predicting volcanic activity and relies on the understanding of how strongly the ascent rate is controlled by different magmatic parameters. Linking potential changes of such parameters to monitoring data is an essential step to be able to use these data as a predictive tool. We present the results of a suite of conduit flow models Soufrière that assess the influence of individual model parameters such as the magmatic water content, temperature or bulk magma composition on the magma flow in the conduit during an extrusive dome eruption. By systematically varying these parameters we assess their relative importance to changes in ascent rate. We show that variability in the rate of low frequency seismicity, assumed to correlate directly with the rate of magma movement, can be used as an indicator for changes in ascent rate and, therefore, eruptive activity. The results indicate that conduit diameter and excess pressure in the magma chamber are amongst the dominant controlling variables, but the single most important parameter is the volatile content (assumed as only water). Modelling this parameter in the range of reported values causes changes in the calculated ascent velocities of up to 800%.
      PubDate: 2014-10-30T02:38:09.834649-05:
      DOI: 10.1002/2014GC005529
       
  • The flow structure of jets from transient sources and implications for
           modeling short‐duration explosive volcanic eruptions
    • Authors: K. N. Chojnicki; A. B. Clarke, R. J. Adrian, J.C. Phillips
      Pages: n/a - n/a
      Abstract: We used laboratory experiments to examine the rise process in neutrally‐buoyant jets that resulted from an unsteady supply of momentum, a condition that defines plumes from discrete Vulcanian‐ and Strombolian‐style eruptions. We simultaneously measured the analog‐jet discharge rate (the supply rate of momentum) and the analog‐jet internal velocity distribution (a consequence of momentum transport and dilution). Then, we examined the changes in the analog‐jet velocity distribution over time to assess the impact of the supply‐rate variations on the momentum‐driven rise dynamics. We found that the analogue jet velocity distribution changes significantly and quickly as the supply rate varied, such that the whole‐field distribution at any instant differed considerably from the time‐average. We also found that entrainment varied in space and over time with instantaneous entrainment coefficient values ranging from 0 to 0.93 in an individual unsteady jet. Consequently, we conclude that supply rate variations exert first‐order control over jet dynamics, and therefore cannot be neglected in models without compromising their capability to predict large‐scale eruption behavior. These findings emphasize the fundamental differences between unsteady and steady jet dynamics, and show clearly that: (i) variations in source momentum flux directly control the dynamics of the resulting flow; and (ii) impulsive flows driven by sources of varying flux cannot reasonably be approximated by quasi‐steady flow models. New modeling approaches capable of describing the time‐dependent properties of transient volcanic eruption plumes are needed before their trajectory, dilution and stability can be reliably computed for hazards management.
      PubDate: 2014-10-30T02:35:21.635712-05:
      DOI: 10.1002/2014GC005471
       
  • Understanding a submarine eruption through time series hydrothermal plume
           sampling of dissolved and particulate constituents: West Mata,
           2008–2012
    • Authors: Tamara Baumberger; Marvin D. Lilley, Joseph A. Resing, John E. Lupton, Edward T. Baker, David A. Butterfield, Eric J. Olson, Gretchen L. Früh‐Green
      Pages: n/a - n/a
      Abstract: Four cruises between 2008 and 2012 monitored the continuing eruption of West Mata volcano in the NE Lau Basin as it produced plumes of chemically altered water above its summit. Although large enrichments in 3He, CO2, Fe, and Mn were observed in the plumes, the most notable enrichment was that of H2, which reached concentrations as high as 14843 nM. Strongly enriched H2 concentrations in the water column result from reactions between seawater or magmatic water and extremely hot rocks. In 2008, the observation of elevated H2 concentrations in the water column above West Mata pointed to vigorous ongoing eruptions at the volcano's summit. The eruption was confirmed by visual observations made by the ROV Jason 2 in 2009 and demonstrated that H2 measurements are a vital instrument to detect ongoing volcanic eruptions at the seafloor. Elevated H2 in 2010 showed that the eruption was ongoing, although at a reduced level given a maximum H2 concentration of 4410 nM. In 2012, H2 levels in the water column declined significantly, to a maximum of only 7 nM, consistent with visual observations from the Quest‐4000 ROV that found no evidence of an ongoing volcanic eruption. Methane behaved independently of other measured gases and its concentrations in the hydrothermal plume were very low. We attribute its minimal enrichments to a mixture of mantle carbon reduced to CH4 and biological CH4 from diffuse flow sites. This study demonstrates that ongoing submarine volcanic eruptions are characterized by high dissolved H2 concentrations present in the overlying water column.
      PubDate: 2014-10-30T00:42:49.816508-05:
      DOI: 10.1002/2014GC005460
       
  • Anthropophile elements in river sediments: Overview from the Seine River,
           France
    • Authors: Jiu‐Bin Chen; Jérôme Gaillardet, Julien Bouchez, Pascale Louvat, Yi‐Na Wang
      Pages: n/a - n/a
      Abstract: In contrast to larger river systems that drain relatively pristine basins, little is known about the sediment geochemistry of rivers impacted by intense human activities. In this paper, we present a systematic investigation of the anthropogenic overprints on element geochemistry in sediments of the human‐impacted Seine River, France. Most elements are fractionated by grain size, as shown by the comparison between suspended particulate matter (SPM) and riverbank deposits (RBD). The RBD are particularly coarse and enriched in carbonates and heavy minerals and thus in elements such as Ba, Ca, Cr, Hf, Mg, Na, REEs, Sr, Ti, Th, and Zr. Although the enrichment/depletion pattern of some elements (e.g. K, REEs, Zr, etc.) can largely be explained by a binary mixture between two sources, other elements such as Ag, Bi, Cr, Cd, Co, Cu, Fe, Mo, Ni, Pb, Sb, Sn, W and Zn in SPM in Paris show that a third end‐member having anthropogenic characteristics is needed to account for their enrichment at low water stage. These “anthropophile” elements, with high enrichment factors (EF) relative to the upper continental crust (UCC), display a progressive enrichment downstream and different geochemical behaviors with respect to the hydrodynamic conditions (e.g. grain size) compared to elements having mainly a natural origin. Our findings emphasize the need for systematic studies of these anthropophile elements in other human‐impacted rivers using geochemical normalization techniques, and stress the importance of studying the chemical variability associated with hydrodynamic conditions when characterizing riverine element geochemistry and assessing their flux to the ocean.
      PubDate: 2014-10-30T00:42:43.72233-05:0
      DOI: 10.1002/2014GC005516
       
  • Rayleigh wave phase velocities in the Atlantic upper mantle
    • Authors: Esther K. James; Colleen A. Dalton, James B. Gaherty
      Pages: n/a - n/a
      Abstract: Phase velocity in the period range 30‐130 seconds is measured for approximately 10,000 fundamental‐mode Rayleigh waves traversing the Atlantic basin. In order to isolate the signal of the oceanic upper mantle, paths with >30% of their length through continental upper mantle are excluded. The lateral distribution of Rayleigh wave phase velocity in the Atlantic upper mantle is explored with two approaches. One, phase velocity is allowed to vary only as a function of seafloor age. Two, a general two‐dimensional parameterization is utilized in order to capture perturbations to age‐dependent structure. In both scenarios, phase velocity shows a strong dependence on seafloor age at all periods, with higher velocity associated with older seafloor. Removing age‐dependent velocity from the 2‐D phase‐velocity maps highlights areas of anomalously low velocity, almost all of which are proximal to locations of hotspot volcanism. The age‐dependent phase velocities for the Atlantic are not consistent with a half‐space cooling model and are best explained by a plate cooling model with thickness of 75 km and mantle temperature of 1400oC. In contrast, age‐dependent phase velocities for the Pacific basin determined by Nishimura and Forsyth [1989] can be fit reasonably well by a half‐space cooling model with mantle temperature approximately 50oC warmer than the Atlantic. Comparison of Rayleigh wave phase velocity and fractionation‐corrected Na concentrations in mid‐ocean‐ridge basalts erupted at 87 axial ridge segments reveals a positive correlation coefficient that increases with period, as expected if along‐ridge variations in mantle potential temperature are controlling both quantities.
      PubDate: 2014-10-27T11:51:07.780232-05:
      DOI: 10.1002/2014GC005518
       
  • Apparent timing and duration of the Matuyama‐Brunhes geomagnetic
           reversal in Chinese loess
    • Authors: Hui Zhao; Xiaoke Qiang, Youbin Sun
      Pages: n/a - n/a
      Abstract: The Matuyama‐Brunhes (MB) geomagnetic reversal in Chinese loess has been studied extensively as an important boundary for land‐ocean stratigraphic and paleoclimatic correlations. However, the apparent timing and duration of the MB boundary remains controversial in Chinese loess deposits due to its inconsistent stratigraphic position and the uncertain chronologies. Here we synthesized high‐resolution paleomagnetic records from four loess sequences in the central Chinese Loess Plateau and synchronized the loess‐paleosol chronology by matching the grain size variations to orbitally‐tuned grain size time series. The synthesized paleomagnetic results reveal consistent features of the MB transition in Chinese loess, including the stratigraphic position (L8/S8 transition), timing (˜808‐826 ka), duration (˜14‐16 ka), and rapid directional oscillations. Compared with the MB transition in marine records (770‐775 ka), the timing of the MB transition is relatively older and longer in Chinese loess, due to a complex interplay between different remanence acquisition mechanisms which occurred during the course of post‐depositional physical and chemical processes.
      PubDate: 2014-10-27T11:41:23.690395-05:
      DOI: 10.1002/2014GC005497
       
  • Volatile cycling of H2O, CO2, F, and Cl in the HIMU mantle: A new window
           provided by melt inclusions from oceanic hot spot lavas at Mangaia, Cook
           Islands
    • Authors: Rita A. Cabral; Matthew G. Jackson, Kenneth T. Koga, Estelle F. Rose‐Koga, Erik H. Hauri, Martin J. Whitehouse, Allison A. Price, James M.D. Day, Nobumichi Shimizu, Katherine A. Kelley
      Pages: n/a - n/a
      Abstract: Mangaia hosts the most radiogenic Pb‐isotopic compositions observed in ocean island basalts and represents the HIMU (high μ = 238U/204Pb) mantle end‐member, thought to result from recycled oceanic crust. Complete geochemical characterization of the HIMU mantle end‐member has been inhibited due to a lack of deep submarine glass samples from HIMU localities. We homogenized olivine‐hosted melt inclusions separated from Mangaia lavas and the resulting glassy inclusions made possible the first volatile abundances to be obtained from the HIMU mantle end‐member. We also report major and trace element abundances and Pb‐isotopic ratios on the inclusions, which have HIMU isotopic fingerprints. We evaluate the samples for processes that could modify the volatile and trace element abundances post‐mantle melting, including diffusive Fe and H2O‐loss, degassing, and assimilation. H2O/Ce ratios vary from 119 to 245 in the most pristine Mangaia inclusions; excluding an inclusion that shows evidence for assimilation, the primary magmatic H2O/Ce ratios vary up to ~200, and are consistent with significant dehydration of oceanic crust during subduction and long‐term storage in the mantle. CO2 concentrations range up to 2346 ppm CO2 in the inclusions. Relatively high CO2 in the inclusions, combined with previous observations of carbonate blebs in other Mangaia melt inclusions, highlight the importance of CO2 for the generation of the HIMU mantle. F/Nd ratios in the inclusions (30 ± 9; 2σ standard deviation) are higher than the canonical ratio observed in oceanic lavas, and Cl/K ratios (0.079 ± 0.028) fall in the range of pristine mantle (0.02‐0.08).
      PubDate: 2014-10-25T03:01:20.280804-05:
      DOI: 10.1002/2014GC005473
       
  • The role of viscoelasticity in subducting plates
    • Authors: R. J. Farrington; L.‐N. Moresi, F.A. Capitanio
      Pages: n/a - n/a
      Abstract: Subduction of tectonic plates into Earth's mantle occurs when one plate bends beneath another at convergent plate boundaries. The characteristic time of deformation at these convergent boundaries approximates the Maxwell relaxation time for olivine at lithospheric temperatures and pressures, it is therefore by definition a viscoelastic process. While this is widely acknowledged, the large‐scale features of subduction can, and have been, successfully reproduced assuming the plate deforms by a viscous mechanism alone. However the energy rates and stress profile within convergent margins are influenced by viscoelastic deformation. In this study viscoelastic stresses have been systematically introduced into numerical models of free subduction, using both the viscosity and shear modulus to control the Maxwell relaxation time. The introduction of an elastic deformation mechanism into subduction models produces deviations in both the stress profile and energy rates within the subduction hinge when compared to viscous only models. These variations result in an apparent viscosity that is variable throughout the length of the plate, decreasing upon approach and increasing upon leaving the hinge. At realistic Earth parameters, we show that viscoelastic stresses have a minor effect on morphology yet are less dissipative at depth and result in an energy transfer between the energy stored during bending and the energy released during unbending. We conclude that elasticity is important during both bending and unbending within the slab hinge with the resulting stress loading and energy profile indicating that slabs maintain larger deformation rates at smaller stresses during bending, retaining their strength during unbending at depth.
      PubDate: 2014-10-25T02:55:32.019261-05:
      DOI: 10.1002/2014GC005507
       
  • Spatial extent and degree of oxygen depletion in the deep
           proto‐North Atlantic basin during Oceanic Anoxic Event 2
    • Authors: Niels A.G.M. van Helmond; Itzel Ruvalcaba Baroni, Appy Sluijs, Jaap S. Sinninghe Damsté, Caroline P. Slomp
      Pages: n/a - n/a
      Abstract: Massive organic matter burial due to widespread ocean anoxia across the Cenomanian/Turonian boundary event (~94 Ma), resulted in a major perturbation of the global carbon cycle: the so‐called Oceanic Anoxic Event 2 (OAE2). The characteristics and spatial distribution of the OAE2 deposits that formed in the deep basin of the proto‐North Atlantic remain poorly described, however. Here, we present proxy data of redox sensitive (trace) elements (e.g., Mo, Fe/Al, Corg/Ptot and Mn) for OAE2 sediments from five Deep Sea Drilling Project and Ocean Drilling Program sites located in the deep proto‐North Atlantic basin. Our results highlight that bottom waters in the entire deep proto‐North Atlantic were anoxic during most of OAE2. Furthermore, regressions of Mo with total organic carbon content (TOC), previously shown to document the degree of water mass restriction, confirm that the water circulation in the proto‐North Atlantic basin was severely restricted during OAE2. Comparison of these values to Mo/TOC ratios in the present‐day Black Sea suggest a renewal frequency of the deep proto‐North Atlantic water mass of between 0.5 and 4 ka, compared to a maximum of ~200 years for the present‐day northern Atlantic. The Plenus Cold Event, a cooler episode during the early stages of OAE2 hypothesized to be caused by declining pCO2 due to extensive burial of organic matter, appears to have led to temporary re‐oxygenation of the bottom water in the deep proto‐North Atlantic basin during OAE2.
      PubDate: 2014-10-23T09:30:29.293047-05:
      DOI: 10.1002/2014GC005528
       
  • Continental breakup and UHP rock exhumation in action: GPS results from
           the Woodlark Rift, Papua New Guinea
    • Authors: Laura M. Wallace; Susan Ellis, Tim Little, Paul Tregoning, Neville Palmer, Robert Rosa, Richard Stanaway, John Oa, Edwin Nidkombu, John Kwazi
      Pages: n/a - n/a
      Abstract: We show results from a network of campaign Global Positioning System (GPS) sites in the Woodlark Rift, southeastern Papua New Guinea, in a transition from seafloor spreading to continental rifting. GPS velocities indicate anticlockwise rotation (at 2‐2.7 º/Myr, relative to Australia) of crustal blocks north of the rift, producing 10‐15 mm/yr of extension in the continental rift, increasing to 20‐40 mm/yr of seafloor spreading at the Woodlark Spreading Center. Extension in the continental rift is distributed among multiple structures. These data demonstrate that low‐angle normal faults in the continents, such as the Mai'iu Fault, can slip at high rates nearing 10 mm/yr. Extensional deformation observed in the D'Entrecasteaux Islands, the site of the world's only actively exhuming Ultra‐High Pressure (UHP) rock terrane, supports the idea that extensional processes play a critical role in UHP rock exhumation. GPS data do not require significant interseismic coupling on faults in the region, suggesting that much of the deformation may be aseismic. Westward transfer of deformation from the Woodlark Spreading Center to the main plate boundary fault in the continental rift (the Mai'iu fault) is accommodated by clockwise rotation of a tectonic block beneath Goodenough Bay, and by dextral strike‐slip on transfer faults within (and surrounding) Normanby Island. Contemporary extension rates in the Woodlark Spreading Center are 30‐50% slower than those from seafloor spreading‐derived magnetic anomalies. The 0.5 Ma‐present seafloor spreading estimates for the Woodlark Basin may be overestimated, and a re‐evaluation of these data in the context of the GPS rates is warranted.
      PubDate: 2014-10-23T06:35:48.361968-05:
      DOI: 10.1002/2014GC005458
       
  • Segmentation and eruptive activity along the East Pacific Rise at
           16°N, in relation with the nearby Mathematician hot spot
    • Authors: M. Le Saout; A. Deschamps, S. A. Soule, P. Gente
      Pages: n/a - n/a
      Abstract: The 16°N segment of the East Pacific Rise is the most over‐inflated and shallowest of this fast‐spreading ridge, in relation with an important magma flux due to the proximity of the Mathematician hotspot. Here, we analyze the detailed morphology of the axial dome and of the Axial Summit Trough (AST), the lava morphology and the geometry of fissures and faults, in regard to the attributes of the magma chamber beneath and of the nearby hotspot. The data used are 1‐meter‐resolution bathymetry combined with seafloor photos and videos. At the dome summit, the AST is highly segmented by ten 3rd and 4th ‐order discontinuities over a distance of 30 km. Often, two contiguous and synchronous ASTs coexist. Such a configuration implies a wide (1100 m minimum) zone of diking. The existence of contiguous ASTs, their mobility, their general en echelon arrangement accommodating the bow shape of the axial dome toward the hotspot, plus the existence of a second magma lens under the western half of the summit plateau, clearly reflect the influence of the hotspot on the organization of the spreading system. The different ASTs exhibit contrasted widths and depths. We suggest that narrow ASTs reflect an intense volcanic activity that produces eruptions covering the tectonic features and partially filling the ASTs. AST widening and deepening would indicate a decrease in volcanic activity but with continued dike intrusions at the origin of abundant sets of fissures and faults that are not masked by volcanic deposits.
      PubDate: 2014-10-21T11:01:18.682752-05:
      DOI: 10.1002/2014GC005560
       
  • Incorporating 3‐D parent nuclide zonation for apatite 4He/3He
           thermochronometry: An example from the Appalachian Mountains
    • Authors: Matthew Fox; Ryan E. McKeon, David L. Shuster
      Pages: n/a - n/a
      Abstract: The ability to constrain km‐scale exhumation with apatite 4He/3He thermochronometry is well established and the technique has been applied to a range of tectonic and geomorphic problems. However, multiple sources of uncertainty in specific crystal characteristics limit the applicability of the method, especially when geologic problems require identifying small perturbations in a cooling path. Here we present new 4He/3He thermochronometric data from the Appalachian Mountains, which indicate significant parent nuclide zonation in an apatite crystal. Using LA‐ICPMS measurements of U and Th in the same crystal, we design a 3‐D model of the crystal to explore the effects of intra‐crystal variability in radiation damage accumulation. We describe a numerical approach to solve the 3‐D production‐diffusion equation. Using our numerical model and a previously determined time temperature path for this part of the Appalachians, we find excellent agreement between predicted and observed 4He/3He spectra. Our results confirm this time‐temperature path and highlight that for complex U and Th zonation patterns, 3‐D numerical models are required to infer an accurate time‐temperature history. In addition, our results provide independent and novel evidence for a radiation damage control on diffusivity. The ability to exploit intra‐crystal differences in 4He diffusivity (i.e., temperature sensitivity) greatly increases the potential to infer complex thermal histories.
      PubDate: 2014-10-20T03:44:39.218353-05:
      DOI: 10.1002/2014GC005464
       
  • Assessment of relative Ti, Ta, and Nb (TITAN) enrichments in ocean island
           basalts
    • Authors: Bradley J. Peters; James M.D. Day
      Pages: n/a - n/a
      Abstract: The sensitivity of trace element concentrations to processes governing solid‐melt interactions has made them valuable tools for tracing the effects of partial melting, fractional crystallization, metasomatism and similar processes on the composition of a parental melt. Recent studies of ocean island basalts (OIB) have sought to correlate Ti, Ta and Nb (TITAN) anomalies to isotopic tracers, such as 3He/4He and 187Os/188Os ratios, which may trace primordial deep mantle sources. A new compilation of global OIB trace element abundance data indicates that positive TITAN anomalies, though statistically pervasive features of OIB, may not be compositional features of their mantle sources. OIB show a range of Ti (Ti/Ti*=0.28‐2.35), Ta (Ta/Ta*=0.11‐93.4) and Nb (Nb/Nb*=0.13‐17.8) anomalies that show negligible correlations with 3He/4He ratios, indicating that TITAN anomalies are not derived from the less‐degassed mantle source traced by high‐3He/4He. Positive TITAN anomalies can be modelled using variable degrees (0.1‐10%) of non‐modal batch partial melting of garnet‐spinel lherzolite at temperatures and pressures considered typical for OIB petrogenesis, and subjecting this partial melt to fractional crystallization and assimilation of mid‐ocean ridge basalt‐like crust (AFC). Correlations of TITAN anomalies with modal abundances of olivine and clinopyroxene in porphyritic Canary Islands lavas provide empirical support for this process and indicate that high abundances of these phases in OIB may create misleading trace element anomalies on primitive mantle‐normalized spider diagrams. Because partial melting and AFC are common to all mantle‐derived magmas, caution should be used when attributing TITAN anomalies to direct sampling of recycled or deep mantle sources by hotspots.
      PubDate: 2014-10-20T03:44:37.277039-05:
      DOI: 10.1002/2014GC005506
       
  • Segmentation of plate coupling, fate of subduction fluids, and modes of
           arc magmatism in Cascadia, inferred from magnetotelluric resistivity
    • Authors: Philip E. Wannamaker; Rob L. Evans, Paul A. Bedrosian, Martyn J. Unsworth, Virginie Maris, R Shane McGary
      Pages: n/a - n/a
      Abstract: Five magnetotelluric (MT) profiles have been acquired across the Cascadia subduction system and transformed using 2D and 3D non‐linear inversion to yield electrical resistivity cross sections to depths of ˜200 km. Distinct changes in plate coupling, subduction fluid evolution, and modes of arc magmatism along the length of Cascadia are clearly expressed in the resistivity structure. Relatively high resistivities under the coasts of northern and southern Cascadia correlate with elevated degrees of inferred plate locking, and suggest fluid‐ and sediment‐deficient conditions. In contrast, the north‐central Oregon coastal structure is quite conductive from the plate interface to shallow depths offshore, correlating with poor plate locking and the possible presence of subducted sediments. Low‐resistivity fluidized zones develop at slab depths of 35‐40 km starting ˜100 km west of the arc on all profiles, and are interpreted to represent prograde metamorphic fluid release from the subducting slab. The fluids rise to forearc Moho levels, and sometimes shallower, as the arc is approached. The zones begin close to clusters of low frequency earthquakes, suggesting fluid controls on the transition to steady sliding. Under the northern and southern Cascadia arc segments, low upper mantle resistivities are consistent with flux melting above the slab plus possible deep convective backarc upwelling toward the arc. In central Cascadia, extensional deformation is interpreted to segregate upper mantle melts leading to underplating and low resistivities at Moho to lower crustal levels below the arc and near backarc. The low‐ to high‐temperature mantle wedge transition lies slightly trenchward of the arc.
      PubDate: 2014-10-14T12:02:11.596984-05:
      DOI: 10.1002/2014GC005509
       
  • Deep water recycling through time
    • Authors: Valentina Magni; Pierre Bouilhol, Jeroen van Hunen
      Pages: n/a - n/a
      Abstract: We investigate the dehydration processes in subduction zones and their implications for the water cycle throughout Earth's history. We use a numerical tool that combines thermo‐mechanical models with a thermodynamic database to examine slab dehydration for present‐day and early Earth settings and its consequences for the deep water recycling. We investigate the reactions responsible for releasing water from the crust and the hydrated lithospheric mantle and how they change with subduction velocity (vs), slab age (a) and mantle temperature (Tm). Our results show that faster slabs dehydrate over a wide area: they start dehydrating shallower and they carry water deeper into the mantle. We parameterize the amount of water that can be carried deep into the mantle, W (x105 kg/m2), as a function of vs (cm/yr), a (Myrs), and Tm (°C): W=1.06υs+0.14α−0.023Tm+17 We generally observe that a 1) 100°C increase in the mantle temperature, or 2) ~15 Myr decrease of plate age, or 3) decrease in subduction velocity of ~2 cm/yr all have the same effect on the amount of water retained in the slab at depth, corresponding to a decrease of ~2.2x105 kg/m2 of H2O. We estimate that for present‐day conditions ~26% of the global influx water, or 7x108 Tg/Myr of H2O, is recycled into the mantle. Using a realistic distribution of subduction parameters, we illustrate that deep water recycling might still be possible in early Earth conditions, although its efficiency would generally decrease. Indeed, 0.5‐3.7x108 Tg/Myr of H2O could still be recycled in the mantle at 2.8 Ga.
      PubDate: 2014-10-14T12:02:04.52594-05:0
      DOI: 10.1002/2014GC005525
       
  • Shallow methane hydrate system controls ongoing, downslope sediment
           transport in a low‐velocity active submarine landslide complex,
           Hikurangi Margin, New Zealand
    • Authors: Joshu J. Mountjoy; Ingo Pecher, Stuart Henrys, Gareth Crutchley, Philip M. Barnes, Andreia Plaza‐Faverola
      Pages: n/a - n/a
      Abstract: Morphological and seismic data from a submarine landslide complex east of New Zealand indicate flow‐like deformation within gas hydrate‐bearing sediment. This “creeping” deformation occurs immediately downslope of where the base of gas hydrate stability reaches the seafloor, suggesting involvement of gas hydrates. We present evidence that, contrary to conventional views, gas hydrates can directly destabilize the seafloor. Three mechanisms could explain how the shallow gas hydrate system could control these landslides. 1) Gas hydrate dissociation could result in excess pore pressure within the upper reaches of the landslide. 2) Overpressure below low‐permeability gas hydrate‐bearing sediments could cause hydrofracturing in the gas hydrate zone valving excess pore pressure into the landslide body. 3) Gas hydrate‐bearing sediment could exhibit time‐dependent plastic deformation enabling glacial‐style deformation. We favor the final hypothesis, that the landslides are actually creeping seafloor glaciers. The viability of rheologically controlled deformation of a hydrate sediment mix is supported by recent laboratory observations of time‐dependent deformation behavior of gas‐hydrate‐bearing sands. The controlling hydrate is likely to be strongly dependent on formation controls and inter‐sediment hydrate morphology. Our results constitute a paradigm shift for evaluating the effect of gas hydrates on seafloor strength which, given the widespread occurrence of gas hydrates in the submarine environment, may require a re‐evaluation of slope stability following future climate‐forced variation in bottom water temperature.
      PubDate: 2014-10-14T02:58:27.905801-05:
      DOI: 10.1002/2014GC005379
       
  • Lucky Strike seamount: Implications for the emplacement and rifting of
           
    • Authors: J. Escartín; A. Soule, M. Cannat, D. J. Fornari, D. Düşünür, R. Garcia
      Pages: n/a - n/a
      Abstract: The history of emplacement, tectonic evolution, and dismemberment of a central volcano within the rift valley of the slow‐spreading Mid‐Atlantic Ridge at the Lucky Strike Segment is deduced using near‐bottom sidescan sonar imagery and visual observations. Volcano emplacement is rapid (
      PubDate: 2014-10-14T01:04:00.438503-05:
      DOI: 10.1002/2014GC005477
       
  • Eruptive history and magmatic stability of Erebus volcano, Antarctica:
           Insights from englacial tephra
    • Authors: Nels A. Iverson; Philip R. Kyle, Nelia W. Dunbar, William C. McIntosh, Nicholas J.G. Pearce
      Pages: n/a - n/a
      Abstract: A tephrostratigraphy of the active Antarctic volcano, Mt. Erebus, was determined from englacial tephra on the ice‐covered flanks of Erebus and an adjacent volcano. The tephra are used to reconstruct the eruptive history and magmatic evolution of Erebus. More fine grained and blocky particles define tephra formed in phreatomagmatic eruptions and larger fluidal shards are characteristic of magmatic eruptions and in some cases both eruptive types are identified in a single mixed tephra. The eruptions forming the mixed tephra likely started as phreatomagmatic eruptions which transitioned into Strombolian eruptions as the non‐magmatic water source was exhausted. We reconstructed the eruptive history of Erebus using the tephra layers stratigraphic position, 40Ar/39Ar ages, shard morphology and grain size. Major and trace element analyses of individual glass shards were measured by electron probe microanalysis and LA‐ICP‐MS. Trachybasalt, trachyte and phonolite tephra were identified. All phonolitic tephra are Erebus‐derived with compositions similar to volcanic bombs erupted from Erebus over the past 40 years. The tephra show that Erebus magma has not significantly changed for 40ka. The uniformity of the glass chemical composition implies that the phonolite magma has crystallized in the same manner without change throughout the late Quaternary, suggesting long‐term stability of the Erebus magmatic system. Trachyte and trachybasalt tephra were likely erupted from Marie Byrd Land and the McMurdo Sound area, respectively. The trachytic tephra can be regionally correlated, and could provide an important time‐stratigraphic marker in Antarctic ice cores.
      PubDate: 2014-10-08T04:25:27.372547-05:
      DOI: 10.1002/2014GC005435
       
  • Quantifying temporal variations in landslide‐driven sediment
           production by reconstructing paleolandscapes using tephrochronology and
           lidar: Waipaoa River, New Zealand
    • Authors: Corina Cerovski‐Darriau; Joshua J. Roering, Michael Marden, Alan S. Palmer, Eric L. Bilderback
      Pages: n/a - n/a
      Abstract: Hillslope response to climate‐driven fluvial incision controls sediment export and relief generation in most mountainous settings. Following the shift to a warmer, wetter climate after the Last Glacial Maximum (LGM) (˜18 ka), the Waipaoa River (New Zealand) rapidly incised up to 120 meters, leaving perched, low‐relief hillslopes unadjusted to that base level fall. In the Mangataikapua—a 16.5 km2 tributary principally comprised of weak mélange—pervasive post‐LGM landslides responded to >50 m of fluvial incision by sculpting and denuding >99% of the catchment. By reconstructing LGM and younger paleosurfaces from tephra identified by electron microprobe analysis (EMPA) and lidar‐derived surface roughness, we estimate the volume, timing, and distribution of hillslope destabilization in the Mangataikapua and the relative contribution of landslide‐prone terrain to post‐LGM landscape evolution. We calculate volume change between four paleosurfaces constrained by tephra age (Rerewhakaaitu, 17.5 ka; Rotoma, 9.4 ka; Whakatane, 5.5 ka; and Waimihia, 3.4 ka). From the paleosurface reconstructions, we calculate the total post‐LGM hillslope sediment contribution from the Mangataikapua catchment to be 0.5 ± 0.06 (s.d.) km3, which equates to a sub‐catchment averaged erosion rate of ˜1.6 mm yr‐1. This is double the previous hillslope volume when normalized by study area, demonstrating that landslide‐prone catchments disproportionately contribute to the terrestrial post‐LGM sediment budget. Finally, we observe particularly rapid post‐Waimihia erosion rates, likely impacted by human settlement.
      PubDate: 2014-10-08T03:52:44.066793-05:
      DOI: 10.1002/2014GC005467
       
 
 
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