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

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 22, SJR: 2.156, h-index: 61)
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Journal Cover   Geochemistry, Geophysics, Geosystems
  [SJR: 2.156]   [H-I: 61]   [24 followers]  Follow
   Full-text available via subscription Subscription journal
   ISSN (Online) 1525-2027
   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • Structural heterogeneity of the midcrust adjacent to the central Alpine
           Fault, New Zealand: Inferences from seismic tomography and seismicity
           between Harihari and Ross
    • Authors: Sandra Bourguignon; Stephen Bannister, C. Mark Henderson, John Townend, Haijiang Zhang
      Pages: n/a - n/a
      Abstract: Determining the rates and distributions of microseismicity near major faults at different points in the seismic cycle is a crucial step towards understanding plate boundary seismogenesis. We analyse data from temporary seismic arrays spanning the central section of the Alpine Fault, New Zealand, using double‐difference seismic tomography. This portion of the fault last ruptured in a large earthquake in 1717 AD and is now late in its typical 330‐year cycle of Mw∼8 earthquakes. Seismicity varies systematically with distance from the Alpine Fault: (1) directly beneath the fault trace, earthquakes are sparse and largely confined to the footwall at depths of 4–11km; (2) at distances of 0–9km southeast of the trace, seismicity is similarly sparse and shallower than 8km; (3) at distances of 9–20km southeast of the fault trace, earthquakes are much more prevalent and shallower than 7km. Hypocentre lineations here are subparallel to faults mapped near the Main Divide of the Southern Alps, confirming that those faults are active. The region of enhanced seismicity is associated with the highest topography and a high‐velocity tongue doming at 3–5km depth. The low‐seismicity zone adjacent to the Alpine Fault trace is associated with Vp and Vs values at mid‐crustal depths about 8% and 6% lower than further southeast. We interpret lateral variations in seismicity rate to reflect patterns of horizontal strain rate superimposed on heterogeneous crustal structure, and the variations in seismicity cutoff depth to be controlled by temperature and permeability structure variations. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-28T00:48:31.190045-05:
      DOI: 10.1002/2014GC005702
  • Authigenesis of magnetic minerals in gas hydrate‐bearing sediments
           in the Nankai Trough, offshore Japan
    • Authors: Myriam Kars; Kazuto Kodama
      Pages: n/a - n/a
      Abstract: Gas hydrate occurrence is one of the possible mechanisms invoked for iron sulfide formation. A high resolution rock magnetic study was conducted in IODP Expedition 316 Hole C0008C located in the Megasplay Fault Zone of the Nankai Trough, offshore Japan. In this particular zone, no bottom simulating reflectors (BSR), indicating the base of the gas hydrate stability field, have been identified. 218 Pleistocene samples were collected from 70 to 110 m CSF in order to document the changes in the concentration, grain size and rock magnetic parameters of magnetic minerals, through the gas hydrate‐bearing horizons. Two different populations of magnetic grains are recognized in the pseudo‐single domain range. Three types of magnetic mineral assemblages are identified: iron oxides (magnetite), ferrimagnetic iron sulfides (greigite and pyrrhotite) and their mixture. Greigite and pyrrhotite are authigenic and constitute six layers, called IS1 to IS6. IS1, IS3, IS4 and IS6 are associated with pore water anomalies, suggesting the occurrence of gas hydrates and anoxic conditions. IS2 and IS5 are probable gas hydrates horizons, although there is no independent data to confirm it. The remaining intervals are mainly composed of detrital iron oxides and paramagnetic iron sulfides. Two scenarios based on different diagenetic stages are proposed to explain the variations in the magnetic properties and mineralogy over the studied interval. The results suggest that rock magnetism appears useful to better constrain the gas hydrate distribution in Hole C0008C, and counterbalances the low resolution of pore water analyses and the absence of a BSR. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-26T09:17:32.219882-05:
      DOI: 10.1002/2014GC005614
  • Hydrous upwelling across the mantle transition zone beneath the Afar
           Triple Junction
    • Authors: D.A. Thompson; J.O.S. Hammond, J‐M. Kendall, G.W. Stuart, G.R. Helffrich, D. Keir, A. Ayele, B. Goitom
      Pages: n/a - n/a
      Abstract: The mechanisms that drive the upwelling of chemical heterogeneity from the lower to upper mantle (e.g. thermal vs. compositional buoyancy) are key to our understanding of whole mantle convective processes. We address these issues through a receiver function study on new seismic data from recent deployments located on the Afar Triple Junction, a location associated with deep mantle upwelling. The detailed images of upper mantle and mantle transition zone structure illuminate features that give insights into the nature of upwelling from the deep Earth. A seismic low velocity layer directly above the mantle transition zone, interpreted as a stable melt layer, along with a prominent 520 km discontinuity suggest the presence of a hydrous upwelling. A relatively uniform transition zone thickness across the region suggests a weak thermal anomaly (
      PubDate: 2015-02-26T08:44:17.569043-05:
      DOI: 10.1002/2014GC005648
  • New mafic magma refilling a quiescent volcano: Evidence from
           He‐Ne‐Ar isotopes during the 2011–2012 unrest at
           Santorini, Greece
    • Authors: A.L. Rizzo; F. Barberi, M.L. Carapezza, A. Di Piazza, L. Francalanci, F. Sortino, W. D'Alessandro
      Pages: n/a - n/a
      Abstract: In 2011‐2012 Santorini was characterized by seismic‐geodetic‐geochemical unrest, which was unprecedented since the most‐recent eruption occurred in 1950 and led to fear an eruption was imminent. This unrest offered a chance for investigating the processes leading to volcanic reactivation and the compositional characteristics of involved magma. We have thus analyzed the He‐Ne‐Ar‐isotope composition of fluid inclusions in olivines and clinopyroxenes from cumulate mafic enclaves hosted in cogenetic dacitic lavas of the 1570–1573 and 1925–1928 eruptions of Nea Kameni. These unique data on Aegean volcanism were compared with those of gases collected in quiescent periods and during the unrest. The 3He/4He‐ratios (3.1–4.0Ra) are significantly lower than the typical arc‐volcano values (R/Ra∼7–8), suggesting the occurrence of magma contamination in Santorini plumbing system, which would further modify the 3He/4He‐ratio of parental magmas generated in the local metasomatized mantle. The 3He/4He‐values of enclaves (3.1–3.6Ra) are comparable to those measured in gases during quiescent periods, confirming that enclaves reflect the He‐isotope signature of magma residing at shallow depths and feeding passive degassing. A significant increase in soil CO2 flux from Nea Kameni and anomalous compositional variations in the fumaroles were identified during the unrest, accordingly with previous studies. Simultaneously, 3He/4He‐ratios up to 4.0Ra were also measured, demonstrating that the unrest was due to the intrusion into the shallow plumbing system of a more‐primitive 3He‐rich magma, which is even volatile richer and less contaminated than mafic magma erupted as enclaves. This new intrusion did not however trigger an eruption. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-26T07:57:47.210549-05:
      DOI: 10.1002/2014GC005653
  • Emplacement of submarine lava flow fields: A geomorphological model from
           the Niños eruption at the Galápagos Spreading Center
    • Authors: J. Timothy McClinton; Scott M. White
      Pages: n/a - n/a
      Abstract: In the absence of any direct observations of an active submarine eruption at a mid‐ocean ridge (MOR), our understanding of volcanic processes there is based on the interpretation of eruptive products. Submarine lava flow morphology serves as a primary indicator of eruption and emplacement processes; however, there is typically a lack of visual observations and bathymetric data at a scale and extent relevant to submarine lava flows, which display meter‐ to sub‐meter‐scale morphological variability. In this paper, we merge submersible‐based visual observations with high‐resolution multibeam bathymetry collected by an autonomous underwater vehicle (AUV) and examine the fine‐scale geomorphology of Niños, a submarine lava flow field at the Galápagos Spreading Center (GSC).We identify separate morphological facies (i.e. morphofacies) within the lava flow field, each having distinct patterns of lava flow morphology and volcanic structures. The spatial and stratigraphic arrangement of morphofacies suggests that they were emplaced sequentially as the eruption progressed, implying that the Niños eruption consisted of at least three eruptive phases. We estimate eruption parameters and develop a chronological model that describes the construction of the Niños lava flow field. An initial phase with high effusion rates emplaced sheet flows, then an intermediate phase emplaced a platform of lobate lavas, and then an extended final phase with low effusion rates emplaced a discontinuous row of pillow lava domes. We then compare this model to mapped lava flow fields at other MORs. Despite disparities in scale, the morphological similarities of volcanic features at MORs with different spreading rates suggest common emplacement processes that are primarily controlled by local magma supply. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-25T05:01:44.587778-05:
      DOI: 10.1002/2014GC005632
  • Observations of pore‐scale growth patterns of carbon dioxide hydrate
           using X‐ray computed microtomography
    • Authors: Xuan Hien Ta; Tae Sup Yun, Balasingam Muhunthan, Tae‐Hyuk Kwon
      Pages: n/a - n/a
      Abstract: Natural and artificial gas hydrates with internal pores of nano‐ to centimeters and weak grain‐cementation have been widely reported, while the detailed formation process of grain‐cementing hydrates remains poorly identified. Pore‐scale morphology of carbon dioxide (CO2) hydrate formed in a partially brine‐saturated porous medium was investigated via X‐ray computed microtomography (X‐ray CMT). Emphasis is placed on the pore‐scale growth patterns of gas hydrate, including the growth of dendritic hydrate crystals on pre‐formed hydrate and water‐wetted grains, porous nature of the hydrate phase, volume expansion of more than 200% during the water‐to‐hydrate phase transformation, preference of unfrozen water wetting hydrophilic minerals, and the relevance to a weak cementation effect on macro‐scale physical properties. The presented pore‐scale morphology and growth patterns of gas hydrate are expected in natural sediment settings where free gas is available for hydrate formation, such as active gas vents, gas seeps, mud volcanoes, permafrost gas hydrate provinces, and CO2 injected formation for the sake of geologic carbon storage; and in laboratory hydrate samples synthesized from partially brine‐saturated sediments or formed from water‐gas interfaces. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-24T02:21:30.431266-05:
      DOI: 10.1002/2014GC005675
  • Mid‐mantle anisotropy in subduction zones and deep water transport
    • Authors: Andy Nowacki; J.‐Michael Kendall, James Wookey, Asher Pemberton
      Pages: n/a - n/a
      Abstract: The Earth's transition zone has until recently been assumed to be seismically isotropic. Increasingly, however, evidence suggests that ordering of material over seismic wavelengths occurs there, but it is unclear what causes this. We use the method of source‐side shear wave splitting to examine the anisotropy surrounding earthquakes deeper than 200km in slabs around the globe. We find significant amounts of splitting (≤ 2.4s), confirming that the transition zone is anisotropic here. However, there is no decrease in the amount of splitting with depth, as would be the case for a metastable tongue of olivine which thins with depth, suggesting this is not the cause. The amount of splitting does not appear to be consistent with processes in the ambient mantle, such as lattice preferred orientation development in wadsleyite, ringwoodite or MgSiO3‐perovskite. We invert for the orientation of several mechanisms‐subject to uncertainties in mineralogy and deformation‐and the best fit is given by up‐dip flattening in a style of anisotropy common to hydrous phases and layered inclusions. We suggest that highly anisotropic hydrous phases or hydrated layering is a likely cause of anisotropy within the slab, implying significant water transport from the surface down to at least 660 km depth. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-24T01:49:32.368-05:00
      DOI: 10.1002/2014GC005667
  • Untangling boulder dislodgement in storms and tsunamis: Is it possible
           with simple theories?
    • Authors: R. Weiss; P. Diplas
      Pages: n/a - n/a
      Abstract: Boulders can move during storms and tsunamis. It is difficult to find a simple method to distinguish boulders moved by tsunami waves from those moved during storms in the field. In this contribution, we explore boulder dislodgement by storm and tsunami waves by solving an adapted version of Newton's Second Law of Motion in polar coordinates and defining a critical position for boulder dislodgement. We find that the boulder dislodgement is not only a function of the causative wave, but also of the roughness in the vicinity of the boulder and the slope angle. We employ the amplitude of storm and tsunami waves to dislodge boulders of given masses to evaluate if boulder dislodgement in storms can be untangled from boulder transport in tsunamis. As the main result of our numerical experiments, we find a significant difference between storm and tsunami waves to dislodge the same boulder for large masses and large roughness values. This allows us to conclude that simple theories are applicable to answer the questions asked in the title, but we argue only if they contain a critical dislodgement condition like the one presented here. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-24T00:43:12.953584-05:
      DOI: 10.1002/2014GC005682
  • Paleointensity determinations from the Etendeka province, Namibia, support
           a low magnetic field strength leading up to the Cretaceous normal
    • Authors: Sarah C. Dodd; Adrian R. Muxworthy, Conall Mac Niocaill
      Pages: n/a - n/a
      Abstract: Paleointensity estimates provide much needed information on field generation within Earth's core and upon the convective processes at work within the mantle. We present new paleointensity estimates from the early Cretaceous Etendeka large igneous province in Namibia (~135Ma) which add to the sparse southern hemisphere dataset. The Early Cretaceous marks an important change in the Earth's magnetic field from a state of rapid polarity reversals, to one of long‐term stability associated with the onset of the Cretaceous Normal Superchron at c. 121Ma. Paleointensity determinations, using the IZZI protocol, were carried out on a total of 172 specimens from 14 sites encompassing the exposed stratigraphy of the Etendeka province. Numerous checks of data reliability were considered before results were accepted, including partial thermoremanent magnetisation (pTRM) checks and pTRM tail checks, hysteresis properties, thermomagnetic analyses, observations under reflected light, and changes to room‐temperature susceptibility during the experiments. Following these checks a total of 64 individual samples from 5 sites were considered to provide reliable paleointensity determinations. These results were combined to provide site mean data with an overall average virtual dipole moment (VDM) for the study of 2.5 ± 1.0 x 1022 Am2. This value equates to approximately 30% of present Earth's field and, when considered alongside existing studies, suggests that Earth's field strength was low in the time leading up to the Cretaceous normal superchron. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-19T08:21:18.653493-05:
      DOI: 10.1002/2014GC005707
  • Viscosity measurements of crystallizing andesite from Tungurahua volcano
    • Authors: Magdalena Oryaëlle Chevrel; Corrado Cimarelli, Lea deBiasi, Jonathan Hanson, Yan Lavallée, Fabio Arzilli, Donald B. Dingwell
      Pages: n/a - n/a
      Abstract: Viscosity has been determined during isothermal crystallization of an andesite from Tungurahua volcano (Ecuador). Viscosity was continuously recorded using the concentric cylinder method and employing a Pt‐sheathed alumina spindle at 1 bar and from 1400 ºC to sub‐liquidus temperatures to track rheological changes during crystallization. The disposable spindle was not extracted from the sample but rather left in the sample during quenching thus preserving an undisturbed textural configuration of the crystals. The inspection of products quenched during the crystallization process reveals evidence for heterogeneous crystal nucleation at the spindle and near the crucible wall, as well as crystal alignment in the flow field. At the end of the crystallization, defined when viscosity is constant, plagioclase is homogeneously distributed throughout the crucible (with the single exception of experiment performed at the lowest temperature). In this experiments, the crystallization kinetics appear to be strongly affected by the stirring conditions of the viscosity determinations. A TTT (Time‐Temperature‐Transformation) diagram illustrating the crystallization “nose” for this andesite under stirring conditions and at ambient pressure has been constructed. We further note that at a given crystal content and distribution, the high aspect ratio of the acicular plagioclase yields a shear‐thinning rheology at crystal contents as low as 13 vol.%, and that the relative viscosity is higher than predicted from existing viscosity models. These viscosity experiments hold the potential for delivering insights into the relative influences of the cooling path, undercooling and deformation on crystallization kinetics and resultant crystal morphologies, as well as their impact on magmatic viscosity. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-19T08:13:41.83061-05:0
      DOI: 10.1002/2014GC005661
  • Historical bathymetric charts and the evolution of Santorini submarine
           volcano, Greece
    • Authors: A. B. Watts; P. Nomikou, J. D. P. Moore, M. M. Parks, M. Alexandri
      Pages: n/a - n/a
      Abstract: Historical bathymetric charts are a potential resource for better understanding the dynamics of the seafloor and the role of active processes, such as submarine volcanism. The British Admiralty, for example, have been involved in lead line measurements of seafloor depth since the early 1790s. Here, we report on an analysis of historical charts in the region of Santorini volcano, Greece. Repeat lead line surveys in 1848, late 1866 and 1925‐1928 as well as multibeam swath bathymetry surveys in 2001 and 2006 have been used to document changes in seafloor depth. These data reveal that the flanks of the Kameni Islands, a dacitic dome complex in the caldera center, have shallowed by up to ~175 m and deepened by up to ~80 m since 1848. The largest shallowing occurred between the late 1866 and 1925‐1928 surveys and the largest deepening occurred during the 1925‐1928 and 2001 and 2006 surveys. The shallowing is attributed to the emplacement of lavas during effusive eruptions in both 1866‐1870 and 1925‐1928 at rates of up to 0.18 and 0.05 km3 a‐1 respectively. The deepening is attributed to a load‐induced viscoelastic stress relaxation following the 1866‐1870 and 1925‐1928 lava eruptions. The elastic thickness and viscosity that best fits the observed deepening are 1.0 km and ~1016 Pa s respectively. This parameter pair, which is consistent with the predictions of a shallow magma chamber thermal model, explains both the amplitude and wavelength of the historical bathymetric data and the present day rate of subsidence inferred from InSAR analysis. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-19T08:13:29.44729-05:0
      DOI: 10.1002/2014GC005679
  • Episodic and multistaged gravitational instability of cratonic lithosphere
           and its implications for reactivation of the North China Craton
    • Authors: Yongming Wang; Jinshui Huang, Shijie Zhong
      Pages: n/a - n/a
      Abstract: Archean cratons are the most stable tectonic units and their lithospheric mantle is chemically depleted and buoyant relative to the underlying mantle. The chemical depletion leads to high viscosity that maintains the long‐term stability of cratons. However, the eastern part of the North China Craton (~1200 km in horizontal length scale) had been extensively reactivated and modified over a time scale of ~100 Myrs in the Mesozoic and Cenozoic. While the causes for the weakening of the North China Craton, a necessary condition for its reactivation, are still in debate, we investigate gravitational instability of compositionally buoyant lithosphere, by computing 2‐D thermochemical convection models with different buoyancy number, lithospheric viscosity and rheology. We find that the gravitational instability of cratonic lithosphere can happen over a larger range of buoyancy numbers with non‐Newtonian rheology, but lithospheric instability with Newtonian rheology only happens with relatively small buoyancy numbers. For cratonic lithosphere with non‐Newtonian rheology and relatively weak temperature‐dependent viscosity, the instability starts in the cold, shallow part of the lithosphere and has small horizontal length scale (
      PubDate: 2015-02-19T08:11:52.304994-05:
      DOI: 10.1002/2014GC005681
  • Issue Information
    • Pages: i - ii
      PubDate: 2015-02-16T06:36:44.806859-05:
      DOI: 10.1002/ggge.20557
  • Application of the cBΩ model to the calculation of diffusion
           parameters of Si in silicates
    • Authors: Baohua Zhang; Shuangming Shan
      Pages: n/a - n/a
      Abstract: Silicon diffusion in major mantle minerals plays an important role in understanding a number of physical and chemical processes in the Earth's interior. Inspection of existing experimental data reveals linear compensation law between the pre‐exponential factors and the activation energies for Si diffusion in various minerals by focusing on those of geophysical interest. On the basis of the observed compensation relationship, here, we propose a thermodynamic model, the so‐called cBΩ model that interconnects point defect parameters with the bulk properties to reproduce the Si self‐diffusion coefficients in different rock‐forming minerals. When the uncertainties are considered, the predicted results show that the temperature and pressure dependences of self‐diffusion coefficients concur with existing experimental data and theoretical calculations in most cases. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-12T19:27:21.282047-05:
      DOI: 10.1002/2014GC005551
  • Source and magma mixing processes in continental subduction factory:
           Geochemical evidence from postcollisional mafic igneous rocks in the Dabie
    • Authors: Li‐Qun Dai; Zi‐Fu Zhao, Yong‐Fei Zheng, Juan Zhang
      Pages: n/a - n/a
      Abstract: Postcollisional mafic igneous rocks commonly exhibit petrological and geochemical heterogeneities, but their origin still remains enigmatic. While source mixing is substantial due to the crust‐mantle interaction during continental collision, magma mixing is also significant during postcollisional magmatism. The two processes are illustrated by Early Cretaceous mafic igneous rocks in the Dabie orogen. These mafic rocks show arc‐like trace element distribution patterns and enriched Sr‐Nd‐Pb isotope compositions, indicating their origination from enriched mantle sources. They have variable whole‐rock εNd(t) values of ‐17.6 to ‐5.2 and zircon εHf(t) values of ‐29.0 to ‐7.7, pointing to source heterogeneities. Such whole‐rock geochemical features are interpreted by the source mixing through melt‐peridotite reaction in the continental subduction channel. Clinopyroxene and plagioclase megacrystals show complex textural and compositional variations, recording three stages of mineral crystallization during magma evolution. Cpx‐1 core has low Cr and Ni but high Ba, Rb and K, indicating its crystallization from a mafic melt (Melt 1) derived from partial melting of hydrous peridotite rich in phlogopite. Cpx‐1 mantle and Cpx‐2 exhibit significantly high Cr, Ni and Al2O3 but low Rb and Ba, suggesting their crystallization from pyroxenite‐derived mafic melt (Melt 2). Whole‐rock initial 87Sr/86Sr ratios of gabbro lies between those of Pl‐1core (crystallized from Melt 1) and Pl‐1 mantle and Pl‐2 core (crystallized from Melt 2), providing isotopic evidence for magma mixing between Melt 1 and Melt 2. Taken together, a heterogeneously enriched mantle source would be generated by the source mixing due to reaction of the overlying subcontinental lithospheric mantle wedge peridotite with felsic melts derived from partial melting of different rocks of the deeply subducted continental crust during the continental collision. The magma mixing would occur between mafic melts that were derived from partial melting of the heterogeneously metasomatic mantle domains in the postcollisional stage. As a consequence, the source and magma mixing processes in the continental subduction factory are responsible for the significant variations in the whole‐rock and mineral geochemistries of postcollisional mafic igneous rocks. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-12T19:14:01.071431-05:
      DOI: 10.1002/2014GC005620
  • Germanium/silicon of the Ediacaran‐Cambrian Laobao cherts:
           Implications for the bedded chert formation and paleoenvironment
    • Authors: Lin Dong; Bing Shen, Cin‐Ty A. Lee, Xu‐jie Shu, Yang Peng, Yuanlin Sun, Zhuanhong Tang, Hong Rong, Xianguo Lang, Haoran Ma, Fan Yang, Wen Guo
      Pages: n/a - n/a
      Abstract: Sedimentary strata of the terminal Ediacaran (635–542 Ma) to early Cambrian (542–488 Ma) Laobao‐Liuchapo bedded cherts in the South China Block include the Ediacaran Oxidation Event and the Cambrian explosion. Understanding the origin and depositional environment of the bedded cherts may provide insight into how the Earth's surface environment changed between the Proterozoic and Phanerozoic. We measured major and trace element compositions and Ge/Si ratios of the Laobao cherts from northern Guangxi Province. The Laobao cherts were deposited in the deep basinal environment of the South China Block. We show that the composition of the Laobao cherts is determined by a mixture of four components: quartz, clay, carbonate, and pyrite/iron‐oxide. The quartz component is the dominant component of the Laobao cherts. The maximum estimated Ge/Si of the quartz component is between 0.4 and 0.5 μmol/mol, which is close to the Ge/Si of modern seawater and biogenic silica but one order of magnitude lower than that of hydrothermal fluids. These Ge/Si systematics suggest that normal seawater rather than mid‐ocean ridge hydrothermal fluids is the primary Si source for the Laobao cherts. The Ge/Si of the clay component varies between 1 and 10 μmol/mol, which is comparable to the Ge/Si of typical marine clays, but 10 to 100 times lower than that of chert nodules from early Ediacaran beds (the Doushantuo Formation) predating the terminal Ediacaran Labao cherts studied here. Our observations indicate that the clay component Ge/Si ratio decreased from the early Ediacaran to the late Ediacaran. We speculate that high Ge/Si ratios in clays reflect the preferential chelation of Ge by dissolved organic compounds adsorbed onto clays. If so, this suggests that the decrease in Ge/Si ratio of the clay component in the Ediacaran signifies a decrease in the total dissolved organic carbon content of seawater towards the Ediacaran‐Cambrian transition, consistent with oxidation of the oceans during the late Ediacaran. Finally, the seawater origin of the Laobao cherts also suggests that replacement of carbonate may not be the primary cause for bedded chert formation. Instead, direct precipitation from seawater or early diagenetic silicification of calcareous sediments, perhaps due to the emergence of Si‐accumulation bacteria, may have been responsible for the bedded Laobao‐Liuchapo chert formation in South China Block. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-12T07:01:06.744745-05:
      DOI: 10.1002/2014GC005595
  • Origin and evolution of the Kolbeinsey Ridge and Iceland Plateau,
    • Authors: Bryndís Brandsdóttir; Emilie E. E. Hooft, Rolf Mjelde, Yoshio Murai
      Pages: n/a - n/a
      Abstract: Variations in crustal structure along the 700 km long KRISE7 refraction/reflection and gravity profile, straddling 66.5°N across the Iceland Shelf, Iceland Plateau and western Norway Basin confirm that extinct spreading centers coexisted with the now extinct Ægir Ridge prior to the initiation of the Kolbeinsey Ridge at 26 Ma. The western 300 km of the profile, across the Iceland shelf, formed by rifting at the Kolbeinsey Ridge, whereas the eastern 400 km, across the Iceland Plateau and the western Norway Basin, formed by earlier rifting, possibly containing slivers of older oceanic or continental crust rifted off the central E‐Greenland margin along with the Jan Mayen Ridge. Crustal thickness increases gradually across the Iceland shelf, from 12–13 km near the Kolbeinsey Ridge to 24–28 km near the eastern shelf edge, decreasing abruptly across the shelf edge, to 12–13 km. The Iceland Plateau has crustal thickness ranging from 12–15 km decreasing to 5–8 km across the western Norway Basin and 4–5 km at the Ægir Ridge. We suggest that high‐velocity lower crustal domes and corresponding gravity highs across the Iceland plateau mark the location of extinct rift axes, that coexisted with the Ægir Ridge. Similar lower crustal domes are associated with the currently active rift segments within Iceland and the Kolbeinsey Ridge. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-10T08:41:44.373421-05:
      DOI: 10.1002/2014GC005540
  • The carbon‐isotope signature of ultramafic xenoliths from the
           Hyblean Plateau (southeast Sicily, Italy): Evidence of mantle
    • Authors: Alessandra Correale; Antonio Paonita, Andrea Rizzo, Fausto Grassa, Mauro Martelli
      Pages: n/a - n/a
      Abstract: We investigated the carbon isotope composition of mantle source beneath the Hyblean Plateau (southeast Sicily, Italy) by studying CO2 in fluid inclusions from ultramafic xenoliths recovered in some Miocene diatremes. In order to constrain the processes influencing the isotopic marker of carbon we combined δ13CCO2 results with information about noble gases (He and Ar) obtained in a previous investigation of the same products. Although Ar/CO2 and He/Ar ratios provide evidence of Rayleigh‐type fractional degassing, the isotopic geochemistry of carbon is poorly influenced by this process. Mixing related to metasomatic processes where MORB‐type pyroxenitic melts permeate a peridotite mantle probably contaminated by crustal fluids inherited from a fossil subduction can explain the measured δ13C and CO2/3He variations, ranging from −4‰ to −2‰ and from 109 to 1010, respectively. Simple mass‐balance calculations highlighted that the Hyblean peridotite source was mainly contaminated by the carbonate source, being carbonate and organic matter present at a ratio that varied within the range from 7:1 to 4:1. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-10T03:19:05.035556-05:
      DOI: 10.1002/2014GC005656
  • Stress drops for intermediate‐depth intraslab earthquakes beneath
           Hokkaido, northern Japan: Differences between the subducting oceanic crust
           and mantle events
    • Authors: Saeko Kita; Kei Katsumata
      Pages: n/a - n/a
      Abstract: Spatial variations in the stress drop for 1726 intermediate‐depth intraslab earthquakes were examined in the subducting Pacific plate beneath Hokkaido, using precisely relocated hypocenters, the corner frequencies of events, and detailed determined geometry of the upper interface of the Pacific plate. The results show that median stress drop for intraslab earthquakes generally increases with an increase in depth from ∼10 to 157 Mpa at depths of 70 to 300 km. More specifically, median stress drops for events in the oceanic crust decrease (9.9 to 6.8 MPa) at depths of 70 to 120km and increase (6.8 to 17 MPa) at depths of 120 to 170 km, whereas median stress drop for events in the oceanic mantle decrease (21.6 to 14.0 MPa) at depths of 70 to 170km, where the geometry of the Pacific plate is well determined. The increase in stress drop with depth in the oceanic crust at depths of 120–170 km, for which several studies have shown an increase in velocity, can be explained by an increase in the velocity and a decrease in the water content due to the phase boundary with dehydration in the oceanic crust. Stress drops for events in the oceanic mantle were larger than those for events in the oceanic crust at depths of 70–120 km. Differences in both the rigidity of the rock types and in the rupture mechanisms for events between the oceanic crust and mantle could be causes for the stress drop differences within a slab. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-05T02:02:07.678454-05:
      DOI: 10.1002/2014GC005603
  • PRIMELT3 MEGA.xlsm software for primary magma calculation: Peridotite
           primary magma MgO contents from the liquidus to the solidus
    • Authors: C. Herzberg; P.D. Asimow
      Pages: n/a - n/a
      Abstract: An upgrade of the PRIMELT algorithm [Herzberg and O'Hara, 2002] for calculating primary magma composition is given together with its implementation in PRIMELT3 MEGA.xlsm software. It supersedes PRIMELT2.xls [Herzberg and Asimow, 2008] in correcting minor mistakes in melt fraction and computed Ni content of olivine, it identifies residuum mineralogy, and it provides a thorough analysis of uncertainties in mantle potential temperature and olivine liquidus temperature. The uncertainty analysis was made tractable by the computation of olivine liquidus temperatures as functions of pressure and partial melt MgO content between the liquidus and solidus. We present a computed anhydrous peridotite solidus in T‐P space using relations amongst MgO, T and P along the solidus; it compares well with experiments on the solidus. Results of the application of PRIMELT3 to a wide range of basalts shows that the mantle sources of ocean islands and large igneous provinces were hotter than oceanic spreading centers, consistent with earlier studies and expectations of the mantle plume model. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-05T01:41:00.653967-05:
      DOI: 10.1002/2014GC005631
  • A comprehensive interpretative model of slow slip events on Mt. Etna's
           eastern flank
    • Authors: Mario Mattia; Valentina Bruno, Tommaso Caltabiano, Andrea Cannata, Flavio Cannavò, Walter D'Alessandro, Giuseppe Di Grazia, Cinzia Federico, Salvatore Giammanco, Alessandro La Spina, Marco Liuzzo, Manfredi Longo, Carmelo Monaco, Domenico Patanè, Giuseppe Salerno
      Pages: n/a - n/a
      Abstract: Starting off from a review of previous literature on kinematic models of the unstable eastern flank of Mt. Etna, we propose a new model. The model is based on our analysis of a large quantity of multidisciplinary data deriving from an extensive and diverse network of INGV monitoring devices deployed along the slopes of the volcano. Our analysis had a twofold objective: first, investigating the origin of the recently observed slow‐slip events on the eastern flank of Mt. Etna; and secondly, defining a general kinematic model for the instability of this area of the volcano. To this end, we investigated the 2008 to 2013 period using data collected from different geochemical, geodetic and seismic networks, integrated with the tectonic and geologic features of the volcano and including the volcanic activity during the observation period. The complex correlations between the large quantities of multidisciplinary data have given us the opportunity to infer, as outlined in this work, that the fluids of volcanic origin and their interrelationship with aquifers, tectonic and morphological features play a dominant role in the large scale instability of the eastern flank of Mt. Etna. Furthermore, we suggest that changes in the strain distribution due to volcanic inflation/deflation cycles are closely connected to changes in shallow depth fluid circulation. Finally, we propose a general framework for both the short and long term modelling of the large flank displacements observed. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-04T22:29:31.764933-05:
      DOI: 10.1002/2014GC005585
  • The emergence of volcanic oceanic islands on a slow‐moving plate:
           The example of Madeira Island, NE Atlantic
    • Authors: Ricardo S. Ramalho; António Brum da Silveira, Paulo E. Fonseca, José Madeira, Michael Cosca, Mário Cachão, Maria M. Fonseca, Susana N. Prada
      Pages: n/a - n/a
      Abstract: The transition from seamount to oceanic island typically involves surtseyan volcanism. However, the geological record at many islands in the NE Atlantic ‐ all located within the slow‐moving Nubian plate ‐ does not exhibit evidence for an emergent surtseyan phase but rather an erosive unconformity between the submarine basement and the overlying subaerial shield sequences. This suggests that the transition between seamount and island may frequently occur by a relative fall of sea level through uplift, eustatic changes, or a combination of both, and may not involve summit volcanism. In this study we explore the consequences for island evolutionary models using Madeira Island (Portugal) as a case‐study. We have examined the geologic record at Madeira using a combination of detailed fieldwork, biostratigraphy, and 40Ar/39Ar geochronology in order to document the mode, timing, and duration of edifice emergence above sea level. Our study confirms that Madeira's subaerial shield volcano was built upon the eroded remains of an uplifted seamount, with shallow marine sediments found between the two eruptive sequences and presently located at 320–430 m above sea level. This study reveals that Madeira emerged around 7.0–5.6 Ma essentially through an uplift process and before volcanic activity resumed to form the subaerial shield volcano. Basal intrusions are a likely uplift mechanism, and their emplacement is possibly enhanced by the slow‐motion of the Nubian plate relative to the source of partial melting. Alternating uplift and subsidence episodes suggest that island edifice growth may be governed by competing dominantly volcanic and dominantly intrusive processes. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-04T02:50:03.936193-05:
      DOI: 10.1002/2014GC005657
  • Generic along‐strike segmentation of Afar normal faults, East
           Africa: Implications on fault growth and stress heterogeneity on
           seismogenic fault planes
    • Authors: I. Manighetti; C. Caulet, L. De Barros, C. Perrin, F. Cappa, Y. Gaudemer
      Pages: n/a - n/a
      Abstract: Understanding how natural faults are segmented along their length can provide useful insights into fault growth processes, stress distribution on fault planes, and earthquake dynamics. We use cumulative displacement profiles to analyze the two largest‐scales of segmentation of ∼900 normal faults in Afar, East Africa. We build upon a prior study by Manighetti et al. (2009) and develop a new signal processing method aimed at recovering the number, position, displacement and length of both the major (i.e., longest) and the subordinate, secondary segments within the faults. Regardless of their length, age, geographic location, total displacement and slip rate, 90% of the faults contain two to five major segments, whereas more than 70% of these major segments are divided into two to four secondary segments. In each hierarchical rank of fault segmentation, most segments have a similar proportional length, whereas the number of segments slightly decreases with fault structural maturity. The along‐strike segmentation of the Afar faults is thus generic at its two largest scales. We summarize published fault segment data on 42 normal, reverse and strike‐slip faults worldwide, and find a similar number (two to five) of major and secondary segments across the population. We suggest a fault growth scenario that might account for the generic large‐scale segmentation of faults. The observation of a generic segmentation suggests that seismogenic fault planes are punctuated with a deterministic number of large stress concentrations, which are likely to control the initiation, arrest and hence extent and magnitude of earthquake ruptures. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-04T02:11:22.275014-05:
      DOI: 10.1002/2014GC005691
  • Re‐Os isotope and platinum group elements of a Focal Zone mantle
           source, Louisville Seamounts Chain, Pacific Ocean
    • Authors: Maria Luisa G. Tejada; Takeshi Hanyu, Akira Ishikawa, Ryoko Senda, Katsuhiko Suzuki, Godfrey Fitton, Rebecca Williams
      Pages: n/a - n/a
      Abstract: The Louisville Seamount Chain (LSC) is, besides the Hawaiian‐Emperor Chain, one of the longest‐lived hotspot traces. We report here the first Re‐Os isotope and platinum group element (PGE) data for Canopus, Rigil, and Burton Guyots along the chain, which were drilled during IODP Expedition 330. The LSC basalts possess (187Os/188Os)i = 0.1245‐0.1314 that are remarkably homogeneous and do not vary with age. A Re‐Os isochron age of 64.9 ± 3.2 Ma was obtained for Burton seamount (the youngest of the three seamounts drilled), consistent with 40Ar‐39Ar data. Isochron‐derived initial 187Os/188Os ratio of 0.1272 ± 0.0008, together with data for olivines (0.1271‐0.1275), are within the estimated primitive mantle values. This (187Os/188Os)i range is similar to those of Rarotonga (0.124‐0.139) and Samoan shield (0.1276‐0.1313) basalts and lower than those of Cook‐Austral (0.136‐0.155) and Hawaiian shield (0.1283‐0.1578) basalts, suggesting little or no recycled component in the LSC mantle source. The PGE data of LSC basalts are distinct from those of oceanic lower crust. Variation in PGE patterns can be largely explained by different low degrees of melting under sulfide‐saturated conditions of the same relatively fertile mantle source, consistent with their primitive mantle‐like Os and primordial Ne isotope signatures. The PGE patterns and the low 187Os/188Os composition of LSC basalts contrast with those of Ontong Java Plateau (OJP) tholeiites. We conclude that the Re‐Os isotope and PGE composition of LSC basalts reflect a relatively pure deep‐sourced common mantle sampled by some ocean island basalts but is not discernible in the composition of OJP tholeiites. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-03T02:05:27.300155-05:
      DOI: 10.1002/2014GC005629
  • Intraplate volcanism due to convective instability of stagnant slabs in
           the Mantle Transition Zone
    • Authors: Matthew H. Motoki; Maxim D. Ballmer
      Pages: n/a - n/a
      Abstract: The study of volcanism can further our understanding of Earth's mantle processes and composition. Continental intraplate volcanism commonly occurs above subducted slabs that stagnate in the Mantle Transition Zone (MTZ), such as in Europe, eastern China, and western North America. Here, we use two‐dimensional numerical models to explore the evolution of stagnant slabs in the MTZ and their potential to sustain mantle upwellings that can support volcanism. We find that weak slabs may go convectively unstable within tens of Myr. Upwellings rise out of the relatively warm underbelly of the slab, are entrained by ambient‐mantle flow and reach the base of the lithosphere. The first and most vigorous upwellings rise adjacent to lateral heterogeneity within the slab. Ultimately, convective instability also acts to separate the compositional components of the slab, harzburgite and eclogite, from each other with harzburgite rising into the upper mantle and eclogite sinking into the lower mantle. Such a physical filtering process may sustain a long‐term compositional gradient across the MTZ. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-30T00:12:54.355354-05:
      DOI: 10.1002/2014GC005608
  • Temperature and velocity measurements of a rising thermal plume
    • Authors: Neil Cagney; William H. Newsome, Carolina Lithgow‐Bertelloni, Aline Cotel, Stanley R. Hart, John A. Whitehead
      Pages: n/a - n/a
      Abstract: The three‐dimensional velocity and temperature fields surrounding an isolated thermal plume in a fluid with temperature‐dependent viscosity are measured using Particle‐Image Velocimetry and thermochromatic liquid‐crystals, respectively. The experimental conditions are relevant to a plume rising through the mantle. It is shown that while the velocity and the isotherm surrounding the plume can be used to visualise the plume, they do not reveal the finer details of its structure. However, by computing the Finite‐Time Lyapunov Exponent fields from the velocity measurements, the material lines of the flow can be found, which clearly identify the shape of the plume head and characterise the behaviour of the flow along the plume stem. It is shown that the vast majority of the material in the plume head has undergone significant stretching and originates from a wide region very low in the fluid domain, which is proposed as a contributing factor to the small‐scale isotopic variability observed in ocean‐island basalt regions. Finally, the Finite‐Time Lyapunov Exponent fields are used to calculate the steady‐state rise velocity of the thermal plume, which is found to scale linearly with the Rayleigh number, in contrast to some previous work. The possible cause and the significance of these conflicting results are discussed, and it is suggested that the scaling relationship may be affected by the temperature‐dependence of the fluid viscosity in the current work. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-30T00:11:47.106618-05:
      DOI: 10.1002/2014GC005576
  • Along‐axis variation in crustal thickness at the ultraslow spreading
           Southwest Indian Ridge (50°E) from a wide‐angle seismic
    • Authors: Xiongwei Niu; Aiguo Ruan, Jiabiao Li, T. A. Minshull, Daniel Sauter, Zhenli Wu, Xuelin Qiu, Minghui Zhao, Y. John Chen, Satish Singh
      Pages: n/a - n/a
      Abstract: The Southwest Indian Ridge (SWIR) is characterized by an ultraslow spreading rate, thin crust and extensive outcrops of serpentinized peridotite. Previous studies have used geochemical and geophysical data to suggest the presence of a thicker crust at the central and shallowest portions of the SWIR, from the Prince Edward (35°30′E) to the Gallieni (52°20′E) fracture zones. Here we present a new analysis of wide‐angle seismic data along the ridge 49°17′E ‐ 50°49′E. Our main conclusions are as follows: 1) we find an oceanic layer 2 of roughly constant thickness and steep velocity gradient, underlain by a layer 3 with variable thickness and low velocity gradient; 2) the crustal thickness varies from c. 5 km beneath non‐transform discontinuities (NTDs) up to c. 10km beneath a segment center; 3) the melt supply is focused in segment centers despite a small NTD between adjacent segments; 4) the presence of a normal upper mantle velocity indicates that no serpentinization occurs beneath this thick crust. Our observation of thick crust at an ultra‐slow spreading ridge adds further complexity to relationships between crustal thickness and spreading rate, and supports previous suggestions that the extent of mantle melting is not a simple function of spreading rate, and that mantle temperature or chemistry (or both) must vary significantly along‐axis. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-28T10:18:35.457422-05:
      DOI: 10.1002/2014GC005645
  • South Asian monsoon history over the past 60 kyr recorded by radiogenic
           isotopes and clay mineral assemblages in the Andaman Sea
    • Authors: Sajid Ali; Ed C. Hathorne, Martin Frank, Daniel Gebregiorgis, Karl Stattegger, Roland Stumpf, Steffen Kutterolf, Joel E. Johnson, Liviu Giosan
      Pages: n/a - n/a
      Abstract: The Late Quaternary variability of the South Asian (or Indian) monsoon has been linked with glacial‐interglacial and millennial scale climatic changes but past rainfall intensity in the river catchments draining into the Andaman Sea remains poorly constrained. Here, we use radiogenic Sr, Nd, and Pb isotope compositions of the detrital clay‐size fraction and clay mineral assemblages obtained from sediment core NGHP Site 17 in the Andaman Sea to reconstruct the variability of the South Asian monsoon during the past 60 kyr. Over this time interval εNd values changed little, generally oscillating between −7.3 and −5.3 and the Pb isotope signatures are essentially invariable, which is in contrast to a record located further northeast in the Andaman Sea. This indicates that the source of the detrital clays did not change significantly during the last glacial and deglaciation suggesting the monsoon was spatially stable. The most likely source region is the Irrawaddy river catchment including the Indo‐Burman Ranges with a possible minor contribution from the Andaman Islands. High smectite/(illite+chlorite) ratios (up to 14), as well as low 87Sr/86Sr ratios (0.711) for the Holocene period indicate enhanced chemical weathering and a stronger South Asian monsoon compared to marine oxygen isotope stages 2 and 3. Short, smectite‐poor intervals exhibit markedly radiogenic Sr isotope compositions and document weakening of the South Asian monsoon, which may have been linked to short‐term northern Atlantic climate variability on millennial time scales. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-28T08:47:14.833205-05:
      DOI: 10.1002/2014GC005586
  • Split‐beam echo sounder observations of natural methane seep
           variability in the northern Gulf of Mexico
    • Authors: Kevin Jerram; Thomas C. Weber, Jonathan Beaudoin
      Pages: n/a - n/a
      Abstract: A method for positioning and characterizing plumes of bubbles from marine gas seeps using an 18‐kHz scientific split‐beam echosounder (SBES) was developed and applied to acoustic observations of plumes of presumed methane gas bubbles originating at approximately 1400m depth in the northern Gulf of Mexico. A total of 161 plume observations from 27 repeat surveys were grouped by proximity into 35 clusters of gas vent positions on the seafloor. Profiles of acoustic target strength per vertical meter of plume height were calculated with compensation for both the SBES beam pattern and the geometry of plume ensonification. These profiles were used as indicators of the relative fluxes and fates of gas bubbles acoustically observable at 18kHz and showed significant variability between repeat observations at time intervals of 1 hour to 7.5 months. Active gas venting was observed during approximately one third of the survey passes at each cluster. While gas flux is not estimated directly in this study owing to lack of bubble size distribution data, repeat surveys at active seep sites showed variations in acoustic response that suggest relative changes in gas flux of up to one order of magnitude over time scales of hours. The minimum depths of acoustic plume observations at 18kHz averaged 875m and frequently coincided with increased amplitudes of acoustic returns in layers of biological scatterers, suggesting acoustic masking of the gas bubble plumes in these layers. Minimum plume depth estimates were limited by the SBES field of view in only five instances. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-28T07:08:40.20173-05:0
      DOI: 10.1002/2014GC005429
  • P and S velocity tomography of the Mariana subduction system from a
           combined land‐sea seismic deployment
    • Authors: Mitchell Barklage; Douglas A. Wiens, James A. Conder, Sara Pozgay, Hajime Shiobara, Hiroko Sugioka
      Pages: n/a - n/a
      Abstract: Seismic imaging provides an opportunity to constrain mantle wedge processes associated with subduction, volatile transport, arc volcanism, and backarc spreading. We investigate the seismic velocity structure of the upper mantle across the Central Mariana subduction system using data from the 2003‐2004 Mariana Subduction Factory Imaging Experiment, an 11‐month deployment consisting of 20 broadband seismic stations installed on islands and 58 semi‐broadband ocean bottom seismographs. We determine the three dimensional VP and VP/VS structure using over 25,000 local and over 2000 teleseismic arrival times. The mantle wedge is characterized by slow velocity and high VP/VS beneath the forearc, an inclined zone of slow velocity underlying the volcanic front, and a strong region of slow velocity beneath the backarc spreading center. The slow velocities are strongest at depths of 20‐30km in the forearc, 60‐70km beneath the volcanic arc, and 20‐30km beneath the spreading center. The forearc slow velocity anomalies occur beneath Big Blue seamount and are interpreted as resulting from mantle serpentinization. The depths of the maximum velocity anomalies beneath the arc and backarc are nearly identical to previous estimates of the final equilibrium depths of mantle melts from thermobarometry, strongly indicating that the low velocity zones delineate regions of melt production in the mantle. The arc and backarc melt production regions are well separated at shallow depths, but may be connected at depths greater than 80km. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-28T07:02:20.25662-05:0
      DOI: 10.1002/2014GC005627
  • The evolution of MORB and plume mantle volatile budgets: Constraints from
           fission Xe isotopes in Southwest Indian Ridge basalts
    • Authors: Rita Parai; Sujoy Mukhopadhyay
      Pages: n/a - n/a
      Abstract: We present high‐precision measurements of the fission isotopes of xenon (Xe) in basalts from the Southwest Indian Ridge (SWIR) between 16 and 25°E. Corrections for syn‐ to post‐eruptive atmospheric contamination yield the Xe isotopic compositions of SWIR mantle sources. We solve for the proportions of mantle Xe derived from the primordial mantle Xe budget, recycling of atmospheric Xe, decay of short‐lived 129I, fission of extinct 244Pu and fission of extant 238U. Xe isotope systematics evident in SWIR basalts and other mantle‐derived samples provide new insights into the integrated history of mantle source degassing and regassing. We find that recycled atmospheric Xe dominates the Xe inventories of the SWIR Western and Eastern Orthogonal Supersegment mantle sources (∼80‐90% of 132Xe is recycled in origin), consistent with results from studies of plume‐influenced basalts from Iceland (Mukhopadhyay, 2012) and the Rochambeau Rift (Pető et al., 2013). While significant regassing of the mantle is evident, we also find differences in the extent of degassing of the MORB and plume sources. MORB sources are consistently characterized by a lower fraction of fission Xe derived from Pu‐fission, indicating a greater extent of degassing relative to the plume source. The prevalence of recycled atmospheric Xe in mantle sources indicates incorporation of depleted recycled material even into mantle sources with primitive He and Ne isotopic compositions. Consequently, depleted lithophile isotopic compositions in mantle sources with primitive He and Ne cannot be interpreted as evidence for a non‐chondritic bulk silicate Earth. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-28T05:54:42.092539-05:
      DOI: 10.1002/2014GC005566
  • New Geophysical Constraints on a Failed Subduction Initiation: The
           Structure and Potential Evolution of the Gagua Ridge and Huatung Basin
    • Authors: Daniel H. Eakin; Kirk D. McIntosh, H. J. A. Van Avendonk, Luc Lavier
      Pages: n/a - n/a
      Abstract: We modeled the velocity structure of the Huatung Basin and Gagua Ridge using offshore wide‐angle seismic data along four ∼E‐W transects. These transects are accompanied by several multichannel seismic reflection (MCS) profiles that highlight the shallow deformation in this area east of Taiwan. Although it is agreed that the Gagua Ridge was the product of a transient compressional episode in the past, relatively few data have been collected that reveal the deeper structure resulting from this enigmatic process. The velocity models show evidence for normal, to thin, oceanic crustal thicknesses in the Huatung Basin and West Philippine Basin. Moho reflections from the associated MCS profiles confirm the thickness observed in the velocity models. The velocity models indicate significant crustal thickening associated with the Gagua Ridge, to 12‐18km along its entire length. Most importantly, the two central velocity models also show a significant asymmetry in the crustal thickening suggesting a westward underthrusting of >20km of WPB oceanic crust beneath that of the Huatung Basin. This geometry is extremely unexpected given interpretations that indicate the Huatung Basin could be significantly older than the West Philippine Basin (Early Cretaceous vs. Eocene). Our observations, along with recent geophysical data concerning the age of the Huatung Basin, indicate that the Gagua Ridge was the result of a failed subduction event during the Miocene that may have existed simultaneously and for a short time, competed with the Manila subduction zone to the west in accommodating convergence between the Eurasia and Philippine Sea plates. In this scenario, the present day Gagua Ridge represents a snapshot of a failed subduction initiation preserved in the geologic record. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-22T16:07:13.152925-05:
      DOI: 10.1002/2014GC005548
  • Time scales of intraoceanic arc magmatism from combined U‐Th and
           (U‐Th)/He zircon geochronology of Dominica, Lesser Antilles
    • Authors: Howe T.M; Schmitt A.K, Lindsay J.M, Shane P, Stockli D.F.
      Pages: n/a - n/a
      Abstract: The island of Dominica, located in the intra‐oceanic Lesser Antilles arc, has produced a series of intermediate (mostly andesitic) lava domes and ignimbrites since the early Pleistocene. (U‐Th)/He eruption ages from centres across the island range from ca. 3 ka to ca. 770 ka, with at least 10 eruptions occurring in the last 80 ka. Three eruptions occurred near the southern tip of Dominica (Plat Pays Volcanic Complex) in the past 15 ka alone. Zircon U‐Th ages from individual centres range from near‐eruption to secular equilibrium implicating protracted storage and recycling of zircons within the crust. Overlapping zircon crystallization peaks within deposits from geographically separated vents (up to 40km apart) indicate that magma associated with separate volcanic edifices crystallized zircon contemporaneously. Two lava domes from the southern sector of the island display exclusively young zircon rim ages (
      PubDate: 2015-01-20T17:39:17.582388-05:
      DOI: 10.1002/2014GC005636
  • Intraplate volcanism at the edges of the Colorado Plateau sustained by a
           combination of triggered edge‐driven convection and
           shear‐driven upwelling
    • Authors: Maxim D. Ballmer; Clinton P. Conrad, Eugene I. Smith, Racheal Johnsen
      Pages: n/a - n/a
      Abstract: Although volcanism in the southwestern United States has been studied extensively, its origin remains controversial. Various mechanisms such as mantle plumes, upwelling in response to slab sinking, and small‐scale convective processes have been proposed, but have not been evaluated within the context of rapidly shearing asthenosphere that is thought to underlie this region. Using geodynamic models that include this shear, we here explore spatio‐temporal patterns of mantle melting and volcanism near the Colorado Plateau. We show that the presence of viscosity heterogeneity within an environment of asthenospheric shearing can give rise to decompression melting along the margins of the Colorado Plateau. Our models indicate that eastward shear flow can advect pockets of anomalously low viscosity toward the edges of thickened lithosphere beneath the plateau, where they can induce decompression melting in two ways. First, the arrival of the pockets critically changes the effective viscosity near the plateau to trigger small‐scale edge‐driven convection. Second, they can excite shear‐driven upwelling (SDU), in which horizontal shear flow becomes redirected upward as it is focused within the low‐viscosity pocket. We find that a combination of “triggered” edge‐driven convection and SDU can explain volcanism along the margins of the Colorado Plateau, its encroachment toward the plateau's southwestern edge, and the association of volcanism with slow seismic anomalies in the asthenosphere. Geographic patterns of intraplate volcanism in regions of vigorous asthenospheric shearing may thus directly mirror viscosity heterogeneity of the sublithospheric mantle. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-20T17:38:31.131061-05:
      DOI: 10.1002/2014GC005641
  • Slip‐rate‐dependent melt extraction at oceanic transform
    • Authors: Hailong Bai; Laurent G. J. Montési
      Pages: n/a - n/a
      Abstract: Crustal thickness differences between oceanic transform faults and associated mid‐ocean ridges may be explained by melt migration and extraction processes. Slow‐slipping transform faults exhibit more positive gravity anomalies than the adjacent spreading centers, indicating relative thin crust in the transform domain, whereas at intermediate‐ and fast‐spreading ridges, transform faults are characterized by more negative gravity anomalies than the adjacent spreading centers, indicating thick crust in the transform domain. We present numerical models reproducing these observations and infer that melt can be extracted at fast‐slipping transforms, but not at slow‐slipping ones. Melt extraction is modeled as a three‐step process [Montési et al., 2011]. 1) Melt moves vertically through buoyancy‐driven porous flow enhanced by sub‐vertical dissolution channels. 2) Melt accumulates in and travels along a decompaction channel lining a low‐permeability barrier at the base of the thermal boundary layer. 3) Melt is extracted to the surface when it enters a melt extraction zone. A melt extraction width of 2 – 4km and a melt extraction depth of 15 – 20km are needed to fit the tectonic damages associated with oceanic plate boundaries that reach into the upper mantle. Our conclusions are supported by the different degrees of magmatic activities exhibited at fast‐ and slow‐slipping transforms as reflected in geological features, geochemical signals and seismic behaviors. We also constrain that the maximum lateral distance of crust‐level dike propagation is about 50 to 70km. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-20T17:37:51.280279-05:
      DOI: 10.1002/2014GC005579
  • Full vector low‐temperature magnetic measurements of geologic
    • Authors: Joshua M. Feinberg; Peter A. Solheid, Nicholas L. Swanson‐Hysell, Mike J. Jackson, Julie A. Bowles
      Pages: n/a - n/a
      Abstract: The magnetic properties of geologic materials offer insights into an enormous range of important geophysical phenomena ranging from inner core dynamics to paleoclimate. Often it is the low‐temperature behavior (
      PubDate: 2015-01-14T03:50:11.699045-05:
      DOI: 10.1002/2014GC005591
  • Ocean Basalt Simulator version 1 (OBS1): Trace element mass balance in
           adiabatic melting of a pyroxenite‐bearing peridotite
    • Authors: Jun‐Ichi Kimura; Hiroshi Kawabata
      Pages: n/a - n/a
      Abstract: We present a new numerical trace element mass balance model for adiabatic melting of a pyroxenite‐bearing peridotite for estimating mantle potential temperature, depth of melting column, and pyroxenite fraction in the source mantle for a primary ocean basalt/picrite. The Ocean Basalt Simulator version 1 (OBS1) uses a thermodynamic model of adiabatic melting of a pyroxenite‐bearing peridotite with experimentally/thermodynamically parameterized liquidus–solidus intervals and source mineralogy. OBS1 can be used to calculate a sequence of adiabatic melting with two melting models, including (1) melting of peridotite and pyroxenite sources with simple mixing of their fractional melts (melt–melt mixing model), and (2) pyroxenite melting, melt metasomatism in the host peridotite, and melting of the metasomatized peridotite (source–metasomatism model). OBS1 can be used to explore (1) the fractions of peridotite and pyroxenite, (2) mantle potential temperature, (3) pressure of termination of melting, (4) degree of melting, and (5) residual mode of the sources. In order to constrain these parameters, the model calculates a mass balance for 26 incompatible trace elements in the sources and in the generated basalt/picrite. OBS1 is coded in an Excel spreadsheet and runs with VBA macros. Using OBS1, we examine the source compositions and conditions of the mid‐oceanic ridge basalts, Loihi–Koolau basalts in the Hawaiian hotspot, and Jurassic Shatsky Rise and Mikabu oceanic plateau basalts and picrites. The OBS1 model shows the physical conditions, chemical mass balance, and amount of pyroxenite in the source peridotite, which are keys to global mantle recycling. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-14T03:25:30.729522-05:
      DOI: 10.1002/2014GC005606
  • Confocal Raman microscopy in sclerochronology: A powerful tool to
           visualize environmental information in recent and fossil biogenic archives
    • Authors: Lars Beierlein; Gernot Nehrke, Thomas Brey
      Pages: n/a - n/a
      Abstract: Biological hard‐parts and skeletons of aquatic organisms often archive information of past environmental conditions. Deciphering such information forms an essential contribution to our understanding of past climate conditions and thus our ability to mitigate the climatic, ecological and social impacts of a rapidly changing environment. Several established techniques enable the visualization and reliable use of the information stored in anatomical features of such biogenic archives, i.e., its growth patterns. Here, we test whether confocal Raman microscopy (CRM) is a suitable method to reliably identify growth patterns in the commonly used archive Arctica islandica and the extinct species Pygocardia rustica (both Bivalvia). A modern A. islandica specimen from Norway has been investigated to verify the general feasibility of CRM, resulting in highly correlated standardized growth indices (r > 0.96; P < 0.0001) between CRM derived measurements and measurements derived from the established methods of fluorescence microscopy and Mutvei's solution staining. This demonstrates the general suitability of CRM as a method for growth pattern evaluation and cross‐dating applications. Moreover, CRM may be of particular interest for paleo‐environmental reconstructions, as it yielded superior results in the analysis of fossil shell specimens (A. islandica and P. rustica) compared to both Mutvei staining and fluorescence microscopy. CRM is a reliable and valuable tool to visualize internal growth patterns in both modern and fossil calcium carbonate shells that notably also facilitates the assessment of possible diagenetic alteration prior to geochemical analysis without geochemically compromising the sample. We strongly recommend the CRM approach for the visualization of growth patterns in fossil biogenic archives, where conventional methods fail to produce useful results. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-09T06:52:20.898977-05:
      DOI: 10.1002/2014GC005547
  • Lithospheric structure across the California Continental Borderland from
           receiver functions
    • Authors: Zachary Reeves; Vedran Lekić, Nicholas Schmerr, Monica Kohler, Dayanthie Weeraratne
      Pages: n/a - n/a
      Abstract: Due to its complex history of deformation, the California Continental Borderland provides an interesting geological setting for studying how the oceanic and continental lithosphere responds to deformation. We map variations in present‐day lithospheric structure across the region using Ps and Sp receiver functions at permanent stations of the Southern California Seismic Network as well as ocean bottom seismometer (OBS) data gathered by the Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment (ALBACORE), which enhances coverage of the borderland and provides first direct constraints on the structure of the Pacific plate west of the Patton Escarpment. Noisiness of OBS data makes strict handpicking and bandpass filtering necessary in order to obtain interpretable receiver functions. Using H‐κ and common‐conversion point stacking, we find pronounced lithospheric differences across structural blocks, which we interpret as indicating that the Outer Borderland has been translated with little to no internal deformation, while the Inner Borderland underwent significant lithospheric thinning, most likely related to accommodating the 90 clockwise rotation of the Western Transverse Range block. West of the Patton Escarpment, we find that the transition to typical oceanic crustal thickness takes place over a lateral distance of ∼ 50km. We detect an oceanic seismic lithosphere‐asthenosphere transition at 58km depth west of the Patton Escarpment, consistent with only weak age‐dependence of the depth to the seismic lithosphere‐asthenosphere transition. Sp common conversion point stacks confirm wholesale lithospheric thinning of the Inner Borderland and suggest the presence of a slab fragment beneath the Outer Borderland. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-08T15:13:34.327624-05:
      DOI: 10.1002/2014GC005617
  • Radial fast‐neutron fluence gradients during rotating 40Ar/39Ar
           sample irradiation recorded with metallic fluence monitors and geological
           age standards
    • Authors: Daniel Rutte; Jörg A. Pfänder, Michal Koleška, Raymond Jonckheere, Sepp Unterricker
      Pages: n/a - n/a
      Abstract: Characterizing the neutron‐irradiation parameter J is one of the major uncertainties in 40Ar/39Ar dating. The associated uncertainty of the individual J‐value for a sample of unknown age depends on the accuracy of the age of the geological standards, the fast‐neutron fluence distribution in the reactor and the distances between standards and samples during irradiation. While it is generally assumed that rotating irradiation evens out radial neutron fluence gradients, we observed axial and radial variations of the J‐values in sample irradiations in the rotating channels of two reactors. To quantify them, we included three‐dimensionally distributed metallic fast‐ (Ni) and thermal‐ (Co) neutron fluence monitors in three irradiations and geological age standards in three more. Two irradiations were carried out under Cd‐shielding in the FRG1 reactor in Geesthacht, Germany, and four without Cd‐shielding in the LVR‐15 reactor in Řež, Czech Republic. The 58Ni(nf,p)58Co activation reaction and γ‐spectrometry of the 811 keV peak associated with the subsequent decay of 58Co to 58Fe allow one to calculate the fast‐neutron fluence. The fast‐neutron fluences at known positions in the irradiation container correlate with the J‐values determined by mass‐spectrometric 40Ar/39Ar measurements of the geological age standards. Radial neutron fluence gradients are up to 1.8%/cm in FRG1 and up to 2.2%/cm in LVR‐15; the corresponding axial gradients are up to 5.9 and 2.1%/cm. We conclude that sample rotation might not always suffice to meet the needs of high‐precision dating and gradient monitoring can be crucial. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-07T12:05:56.419572-05:
      DOI: 10.1002/2014GC005611
  • Gulf of Alaska continental slope morphology: Evidence for recent trough
           mouth fan formation
    • Authors: John M. Swartz; Sean P. S. Gulick, John A. Goff
      Pages: n/a - n/a
      Abstract: Glaciated continental shelves are host to numerous morphologic features that help understand past glacier dynamics. Southeastern Alaska is home to the St. Elias mountains, an active orogen that also hosts temperate marine glaciers. During glacial periods ice streams advance across the continental shelf, carving shelf‐crossing troughs that reach the shelf edge. We use high‐resolution multi‐beam data to develop the relationship between the Yakutat and Alsek Sea Valleys and the resulting continental slope morphology. The shelf and slope geomorphology can be divided into statistical groupings that relate to the relative balance of erosion and deposition. Our analysis indicates that only the Yakutat system has been able to build an incipient trough‐mouth fan. The extreme sediment supply from this region was able to overwhelm the steep initial topography of the transform margin, while further to the east sediment slope‐bypass dominates. This analysis provides an extreme end member to existing studies of temperate glaciation along continental margins. The unique interplay between rapid uplift due to ongoing collision and the massive erosion caused by temperate glaciers provides for sedimentary flux far above most other systems. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-07T09:58:33.957443-05:
      DOI: 10.1002/2014GC005594
  • Submarine record of volcanic island construction and collapse in the
           Lesser Antilles arc: First scientific drilling of submarine volcanic
           island landslides by IODP Expedition 340
    • Authors: Le Friant A; Ishizuka O, Boudon G, Palmer M.R, Talling P.J, Villemant B, Adachi T, Aljahdali M, Breitkreuz C, Brunet M, Caron B, Coussens M, Deplus C, Endo D, Feuillet N, Fraas A.J, Fujinawa A, Hart M.B, Hatfield R.G, Hornbach M, Jutzeler M, Kataoka K. S, Komorowski J‐C, Lebas E, Lafuerza S, Maeno F, Manga M, Martínez‐Colón M, McCanta M, Morgan S, Saito T, Slagle A, Sparks S, Stinton A, Stroncik N, Subramanyam K. S.V, Tamura Y, Trofimovs J, Voight B, Wall‐Palmer D, Wang F, Watt S.F.L.
      Pages: n/a - n/a
      Abstract: IODP Expedition 340 successfully drilled a series of sites offshore Montserrat, Martinique and Dominica in the Lesser Antilles from March to April 2012. These are among the few drill sites gathered around volcanic islands, and the first scientific drilling of large and likely tsunamigenic volcanic island‐arc landslide deposits. These cores provide evidence and tests of previous hypotheses for the composition and origin of those deposits. Sites U1394, U1399, and U1400 that penetrated landslide deposits recovered exclusively seafloor‐sediment, comprising mainly turbidites and hemipelagic deposits, and lacked debris avalanche deposits. This supports the concepts that i/ volcanic debris avalanches tend to stop at the slope break, and ii/ widespread and voluminous failures of pre‐existing low‐gradient seafloor sediment can be triggered by initial emplacement of material from the volcano. Offshore Martinique (U1399 and 1400), the landslide deposits comprised blocks of parallel strata that were tilted or micro‐faulted, sometimes separated by intervals of homogenized sediment (intense shearing), while Site U1394 offshore Montserrat penetrated a flat‐lying block of intact strata. The most likely mechanism for generating these large‐scale seafloor‐sediment failures appears to be propagation of a decollement from proximal areas loaded and incised by a volcanic debris avalanche. These results have implications for the magnitude of tsunami generation. Under some conditions, volcanic island landslide deposits comprised of mainly seafloor sediment will tend to form smaller magnitude tsunamis than equivalent volumes of subaerial block‐rich mass flows rapidly entering water. Expedition 340 also successfully drilled sites to access the undisturbed record of eruption fallout layers intercalated with marine sediment which provide an outstanding high‐resolution dataset to analyze eruption and landslides cycles, improve understanding of magmatic evolution as well as offshore sedimentation processes. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-06T00:41:56.426968-05:
      DOI: 10.1002/2014GC005652
  • Decadal‐scale variations in geomagnetic field intensity from ancient
           Cypriot slag mounds
    • Authors: Ron Shaar; Lisa Tauxe, Erez Ben‐Yosef, Vasiliki Kassianidou, Brita Lorentzen, Joshua M. Feinberg, Thomas E. Levy
      Pages: n/a - n/a
      Abstract: Geomagnetic models based on direct observations since the 1830s show that the averaged relative change in field intensity on Earth's surface over the past 170 years is less than 4.8% per decade. It is unknown if these rates represent the typical behavior of secular variations due to insufficient temporal resolution of archaeomagnetic records from earlier periods. To address this question we investigate two ancient slag mounds in Cyprus ‐ Skouriotissa Vouppes (SU1, 4th ‐ 5th centuries CE, 21 meter in height), and Mitsero Kokkinoyia (MK1, 7th ‐ 5th BCE, 8 meter in height). The mounds are multi‐layered sequences of slag and charcoals that accumulated near ancient copper production sites. We modeled the age‐height relation of the mounds using radiocarbon dates, and estimated paleointensities using Thellier‐type IZZI experiments with additional anisotropy, cooling rate, and non‐linear TRM assessments. To screen out ambiguous paleointensity interpretations we applied strict selection criteria at the specimen/sample levels. To ensure objectivity, consistency, and robust error estimation we employed an automatic interpretation technique and put the data available in the MagIC database. The analyses yielded two independent sub‐century scale paleointensity time series. The MK1 data indicate relatively stable field at the time the mound accumulated. In contrast, the SU1 data demonstrate changes that are comparable in magnitude to the fastest changes inferred from geomagnetic models. We suggest that fast changes observed in the published archaeomagnetic data from the Levant are driven by two longitudinally‐paired regions, the Middle East and South Africa, that show unusual activity in geomagnetic models. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-05T05:15:35.657671-05:
      DOI: 10.1002/2014GC005455
  • Bulk particle size distribution and magnetic properties of
           particle‐sized fractions from loess and paleosol samples in Central
    • Authors: Jinbo Zan; Xiaomin Fang, Shengli Yang, Maodu Yan
      Pages: n/a - n/a
      Abstract: Recent studies demonstrate that particle size separation based on gravitational settling and detailed rock magnetic measurements of the resulting fractionated samples constitutes an effective approach to evaluating the relative contributions of pedogenic and detrital components in the loess and paleosol sequences on the Chinese Loess Plateau. So far, however, similar work has not been undertaken on the loess deposits in Central Asia. In this paper, seventeen loess and paleosol samples from three representative loess sections in Central Asia were separated into four grain size fractions and then systematic rock magnetic measurements were made on the fractions. Our results demonstrate that the content of the 75 μm) exhibits the minimum values of χ, χARM and SIRM, demonstrating that the concentrations of ferrimagnetic grains are not positively correlated with the bulk particle size in the Central Asian loess deposits. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-02T01:17:40.689603-05:
      DOI: 10.1002/2014GC005616
  • Paleolatitudes of the Tibetan Himalaya from primary and secondary
           magnetizations of Jurassic to Lower Cretaceous sedimentary rocks
    • Authors: Wentao Huang; Douwe J.J. van Hinsbergen, Mark J. Dekkers, Eduardo Garzanti, Guillaume Dupont‐Nivet, Peter C. Lippert, Xiaochun Li, Marco Maffione, Cor G. Langereis, Xiumian Hu, Zhaojie Guo, Paul Kapp
      Pages: n/a - n/a
      Abstract: The Tibetan Himalaya represents the northernmost continental unit of the Indian plate that collided with Asia in the Cenozoic. Paleomagnetic studies on the Tibetan Himalaya can help constrain the dimension and paleogeography of ‘Greater India', the Indian plate lithosphere that subducted and underthrusted below Asia after initial collision. Here, we present a paleomagnetic investigation of a Jurassic (limestones) and Lower Cretaceous (volcaniclastic sandstones) section of the Tibetan Himalaya. The limestones yielded positive fold test, showing a pre‐folding origin of the isolated remanent magnetizations. Detailed paleomagnetic analyses, rock magnetic tests, end‐member modeling of acquisition curves of isothermal remanent magnetization, and petrographic investigation reveal that the magnetic carrier of the Jurassic limestones is authigenic magnetite, whereas the dominant magnetic carrier of the Lower Cretaceous volcaniclastic sandstones is detrital magnetite. Our observations lead us to conclude that the Jurassic limestones record a prefolding remagnetization, whereas the Lower Cretaceous volcaniclastic sandstones retain a primary remanence. The volcaniclastic sandstones yield an Early Cretaceous paleolatitude of 55.5°S [52.5°S, 58.6°S] for the Tibetan Himalaya, suggesting it was part of the Indian plate at that time. The size of ‘Greater India' during Jurassic time cannot be estimated from these limestones. Instead, a paleolatitude of the Tibetan Himalaya of 23.8°S [21.8°S, 26.1°S] during the remagnetization process is suggested. It is likely that the remagnetization, caused by the oxidation of early diagenetic pyrite to magnetite, was induced during 103‐83 Ma or 77‐67 Ma. The inferred paleolatitudes at these two time intervals imply very different tectonic consequences for the Tibetan Himalaya. This article is protected by copyright. All rights reserved.
      PubDate: 2014-12-30T02:17:17.696434-05:
      DOI: 10.1002/2014GC005624
  • Precessional changes in the western equatorial Pacific hydroclimate: A 240
           kyr marine record from the Halmahera Sea, East Indonesia
    • Authors: Haowen Dang; Zhimin Jian, Catherine Kissel, Franck Bassinot
      Pages: n/a - n/a
      Abstract: Within the precession band, an inter‐hemispheric anti‐phase pattern in the tropical hydro‐climate is supported by many paleo‐records, and optimally explained by the forcing of precessional insolation change. However, scenarios within the western equatorial Pacific (WEP), which plays the role of the ascending center of atmospheric convection, remain poorly determined. In this study, a marine sediment core from the Halmahera Sea, East Indonesia, was analyzed with high‐resolution XRF scanning, quantitative discrete XRF and ICP‐AES/MS measurements. The terrigenous fractions in this core are constrained by their trace elemental characteristics to be locally sourced from Halmahera Island, and hence reflect variations in the local riverine runoff and precipitation. On this basis, a continuous record of precipitation changes of the western equatorial Pacific was reconstructed with multi‐decadal resolution over the last ~240 ka, using an age model established by the correlation between an adjusted ice volume model and benthic δ18O constrained by 14C dating. The records of terrigenous input show a dominant ~23 kyr periodicity with a 90°~100° phase lag to the boreal summer (i.e., in‐phase with the boreal autumn) insolation change. This pattern can be explained by the variability in the convective activity over the WEP, which might be primarily controlled by precessional changes in the El Niño and Southern Oscillation (ENSO) system. A dynamic linkage is implied between the precessional variations in the convective activity in the WEP and the East Asian and Australia‐Indonesian summer monsoons (EASM and AISM), in the sense of their distinct stable phase relationship to precession. This article is protected by copyright. All rights reserved.
      PubDate: 2014-12-29T02:38:47.764371-05:
      DOI: 10.1002/2014GC005550
  • A 3‐D Lagrangian finite element algorithm with remeshing for
           simulating large‐strain hydrodynamic instabilities in power law
           viscoelastic fluids
    • Authors: M. von Tscharner; S. M. Schmalholz
      Pages: n/a - n/a
      Abstract: We present a three‐dimensional (3‐D) numerical algorithm (PINK‐3D) that is based on the finite element method. The algorithm is designed to simulate hydrodynamic instabilities in power‐law viscoelastic fluids under gravity. These instabilities are caused by large and sharp contrasts in mechanical strength and/or density between different materials (e.g. folding, necking or Rayleigh‐Taylor diapirism). The instabilities are controlled by the geometry of the material interfaces and the related intra‐layer stress distribution when amplitudes of the material interfaces are still low. The presented algorithm combines a deformable Lagrangian mesh with re‐meshing in order to accurately simulate the low‐amplitude stages of the emerging instabilities, and also to simulate the large strain evolution of the structures emerging from these instabilities. The re‐meshing is based on material interfaces that accurately track the boundaries between materials with strongly varying material properties (e.g. effective viscosity or power‐law stress exponent). We describe here the main technical details of the 3‐D algorithm. The accuracy of the 3‐D algorithm is demonstrated with comparisons between the numerical results and 2‐D and 3‐D analytical solutions for folding, necking, Rayleigh‐Taylor diapirism and circular inclusions in viscous medium. We also benchmark the 3‐D algorithm with results of a different 2‐D finite element algorithm to test the accuracy of the large strain results with re‐meshing. Furthermore, two tests are presented that show the accuracy of the viscoelasticity implementation. PINK‐3D is also used to study 3‐D necking applied to lithospheric slab detachment, and 2‐D and 3‐D folding applied to fold nappe formation. In particular, we apply the 3‐D code to quantify and visualize the evolution of the 3‐D finite strain ellipsoid for the developing 3‐D structures. This article is protected by copyright. All rights reserved.
      PubDate: 2014-12-29T01:41:41.00192-05:0
      DOI: 10.1002/2014GC005628
  • Oxygen isotope thermometry, speedometry, and hygrometry: apparent
           equilibrium temperature versus closure temperature
    • Authors: Huaiwei Ni
      Pages: n/a - n/a
      Abstract: Rather than indicating formation/peak temperature, oxygen isotope fractionations preserved in mineral assemblages of slowly cooled plutonic and metamorphic rocks yield apparent equilibrium temperatures (Tae). The isotopic fractionations and Tae values deliver information about cooling history, as the extent of diffusive exchange of oxygen isotopes during cooling is controlled by the cooling time scale or cooling rate. Despite that several models, such as the Fast Grain Boundary (FGB) model, have been developed to simulate oxygen isotope exchange between coexisting minerals during cooling, extraction of cooling rate remains far from straightforward. On the other hand, there is a well‐defined quantitative relationship between the Dodson closure temperature (Tc) and the cooling rate, but Tc cannot be directly measured. Based on simulation results of existing models for a variety of rock systems, including open systems (with an infinite fluid reservoir), closed systems (with negligible fluid participation) and semi‐open systems (with moderate fluid participation), this study demonstrates that Tae of the mineral pair with the largest equilibrium isotope fractionation (PLEIF) is always bounded by their Tc values, regardless of how mineral proportions vary or how significant a role fluid has played in isotopic exchange. If the two Tc values happen to be similar, Tae will serve as a good approximation of both Tc, provided that the equilibrium fractionation factor has been precisely determined as a function of temperature. One such pair is quartz‐magnetite. By contrast, a mineral pair with similar Tc but relatively small fractionation is susceptible to the disturbance from other minerals, hence does not always have Tae confined within their Tc range. The relationship of Tae‐Tc correspondence for PLEIF with similar Tc can be used to constrain either cooling rate (i.e., as a speedometry method) or oxygen isotope diffusivity if one of them has been independently determined. In the latter case, the inferred oxygen diffusivity may be an index of water fugacity (i.e., as a hygrometry method) when compared with experimental diffusivity values measured under different fluid conditions. This article is protected by copyright. All rights reserved.
      PubDate: 2014-12-19T20:50:04.0963-05:00
      DOI: 10.1002/2014GC005574
  • Magmatic plumbing at Lucky Strike volcano based on olivine‐hosted
           melt inclusion compositions
    • Authors: V.D. Wanless; A.M. Shaw, M.D. Behn, S.A. Soule, J. Escartín, C. Hamelin
      Pages: n/a - n/a
      Abstract: Here we present volatile, major and trace element concentrations of 64 olivine‐hosted melt inclusions from the Lucky Strike segment on the mid‐Atlantic ridge. Lucky Strike is one of two locations where a crustal melt lens has been seismically imaged on a slow‐spreading ridge. Vapor‐saturation pressures, calculated from CO2 and H2O contents of Lucky Strike melt inclusions, range from approximately 300–3000 bars, corresponding to depths of 0.5–9.9 km below the seafloor. Approximately 50% of the melt inclusions record crystallization depths of 3–4 km, corresponding to the seismically‐imaged melt lens depth, while an additional ~35% crystallize at depths > 4 km. This indicates that while crystallization is focused within the melt lens, significant crystallization also occurs in the lower crust and/or upper mantle. The melt inclusions span a range of major and trace element concentrations from normal to enriched basalts. Trace element ratios at all depths are heterogeneous, suggesting that melts are not efficiently homogenized in the mantle or crust, despite the presence of a melt lens. This is consistent with the transient nature of magma chambers proposed for slower‐spreading ridges. To investigate the petrogenesis of the melt inclusion compositions, we compare the measured trace element compositions to theoretical melting calculations that consider variations in the melting geometry and heterogeneities in the mantle source. The full range of compositions can be produced by slight variations in the proportion of an Azores plume and depleted upper mantle components and changes in the total extent of melting. This article is protected by copyright. All rights reserved.
      PubDate: 2014-12-19T20:49:54.113663-05:
      DOI: 10.1002/2014GC005517
  • 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
  • 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‐ 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
  • 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
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