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

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 25, SJR: 2.56, h-index: 69)
Geophysical Research Letters     Full-text available via subscription   (Followers: 55, SJR: 3.493, h-index: 157)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 5, SJR: 3.239, h-index: 119)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 2, SJR: 1.944, h-index: 7)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 22)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 7)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 25)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 14)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 13)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 26)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 15)
Paleoceanography     Full-text available via subscription   (Followers: 3, SJR: 3.22, h-index: 88)
Radio Science     Full-text available via subscription   (Followers: 3, SJR: 0.959, h-index: 51)
Reviews of Geophysics     Full-text available via subscription   (Followers: 20, SJR: 9.68, h-index: 94)
Space Weather     Full-text available via subscription   (Followers: 3, SJR: 1.319, h-index: 19)
Tectonics     Full-text available via subscription   (Followers: 9, SJR: 2.748, h-index: 85)
Water Resources Research     Full-text available via subscription   (Followers: 79, SJR: 2.189, h-index: 121)
Journal Cover   Geochemistry, Geophysics, Geosystems
  [SJR: 2.56]   [H-I: 69]   [25 followers]  Follow
    
   Full-text available via subscription Subscription journal
   ISSN (Online) 1525-2027
   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • Continent scale strike‐slip on a low‐angle fault beneath New
           Zealand's Southern Alps: Implications for crustal thickening in oblique
           collision zones
    • Abstract: New Zealand's Southern Alps lie adjacent to the continent‐scale dextral strike‐slip Alpine Fault, on the boundary between the Pacific and Australian plates. We show with a simple 2‐D model of crustal balancing that the observed crustal root and erosion (expressed as equivalent crustal shortening) is up to twice that predicted by the orthogonal plate convergence since ∼11 Ma, and even since ∼23 Ma when the Alpine Fault formed. We consider two explanations for this, involving a strong component of motion along the length of the plate‐boundary zone. Geophysical data indicate that the Alpine Fault has a listric geometry, flattening at mid‐crustal levels, and has accommodated sideways underthrusting of Australian plate crust beneath Pacific plate crust. The geometry of the crustal root, together with plate reconstructions, require the underthrust crust to be the hyper‐extended part of an asymmetric rift system which formed over 500 km farther south during the Eocene – the narrow remnant part today forms the western margin of the Campbell Plateau. At ∼10 Ma, the hyper‐extended margin underwent shallow subduction in the Puysegur subduction zone, and then was dragged over 300 km along the length of the Southern Alps beneath a low angle (
      PubDate: 2015-08-22T01:08:10.434771-05:
      DOI: 10.1002/2015GC005990
       
  • Deeply dredged submarine HIMU glasses from the Tuvalu Islands, Polynesia:
           Implications for volatile budgets of recycled oceanic crust
    • Abstract: Ocean island basalts (OIB) with extremely radiogenic Pb‐isotopic signatures are melts of a mantle component called HIMU (high µ, high 238U/204Pb). Until now, deeply‐dredged submarine HIMU glasses have not been available, which has inhibited complete geochemical (in particular, volatile element) characterization of the HIMU mantle. We report major, trace and volatile element abundances in a suite of deeply‐dredged glasses from the Tuvalu Islands. Three Tuvalu glasses with the most extreme HIMU signatures have F/Nd ratios (35.6±3.6) that are higher than the ratio (∼21) for global OIB and MORB, consistent with elevated F/Nd ratios in endmember HIMU Mangaia melt inclusions. The Tuvalu glasses with the most extreme HIMU composition have Cl/K (0.11 to 0.12), Br/Cl (0.0024) and I/Cl (5‐6 × 10−5) ratios that preclude significant assimilation of seawater‐derived Cl. The new HIMU glasses that are least degassed for H2O have low H2O/Ce ratios (75‐84), similar to ratios identified in endmember OIB glasses with EM1 and EM2 signatures, but significantly lower than H2O/Ce ratios (119 to 245) previously measured in melt inclusions from Mangaia. CO2‐H2O equilibrium solubility models suggest that these HIMU glasses (recovered in two different dredges at 2,500 to 3,600 meters water depth) have eruption pressures of 295 to 400 bars. We argue that degassing is unlikely to significantly reduce the primary melt H2O. Thus, the lower H2O/Ce in the HIMU Tuvalu glasses is a mantle signature. We explore oceanic crust recycling as the origin of the low H2O/Ce (∼50 to 80) in the EM1, EM2 and HIMU mantle domains. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-21T03:16:24.19131-05:0
      DOI: 10.1002/2015GC005966
       
  • Evolution of permeability across the transition from brittle failure to
           cataclastic flow in porous siltstone
    • Authors: Marco M. Scuderi; Hiroko Kitajima, Brett M. Carpenter, Demian M. Saffer, Chris Marone
      Abstract: Porous sedimentary rocks fail in a variety of modes ranging from localized, brittle deformation to pervasive, cataclastic flow. To improve our understanding of this transition and its affect on fluid flow and permeability, we investigated the mechanical behavior of a siltstone unit within the Marcellus Formation, PA USA, characterized by an initial porosity ranging from 41 to 45%. We explored both hydrostatic loading paths (σ1=σ2=σ3) and triaxial loading paths (σ1>σ2=σ3) while maintaining constant effective pressure (Pe=Pc‐Pp). Samples were deformed with an axial displacement rate of 0.1 μm/s (strain rate of 2x10−6 s−1). Changes in pore water volume were monitored (drained conditions) to measure the evolution of porosity. Permeability was measured at several stages of each experiment. Under hydrostatic loading, we find the onset of macroscropic grain crushing (P*) at 39 MPa. Triaxial loading experiments show a transition from brittle behavior with shear localization and compaction to cataclastic‐flow as confining pressure increases. When samples fail by shear localization, permeability decreases abruptly without significant changes in porosity. Conversely, for cataclastic deformation, permeability reduction is associated with significant porosity reduction. Post‐experiment observation of brittle samples show localized shear zones characterized by grain comminution. Our data show how zones of shear localization can act as barriers to fluid flow and thus modify the hydrological and mechanical properties of the surrounding rocks. Our results have important implications for deformation behavior and permeability evolution in sedimentary systems, and in particular where the stress field is influenced by injection or pumping. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-20T11:12:37.356814-05:
      DOI: 10.1002/2015GC005932
       
  • Issue Information
    • PubDate: 2015-08-20T08:18:07.671342-05:
      DOI: 10.1002/ggge.20563
       
  • A 1400 year environmental magnetic record from varved sediments of Lake
           Xiaolongwan (northeast China) reflecting natural and anthropogenic soil
           erosion
    • Authors: Youliang Su; Guoqiang Chu, Qingsong Liu, Zhaoxia Jiang, Xing Gao, Torsten Haberzettl
      Abstract: Lake sediments can provide high‐quality information about human activities. In this study, we investigate a sediment core from Lake Xiaolongwan using magnetic and geochemical methods. The dominant magnetic minerals of this sediment core are stable single domain (SSD) and superparamagnetic (SP) magnetite particles. The increasing amount of SP particles reflected by the rise of magnetic susceptibility and frequency dependent magnetic susceptibility since AD 1500 can be attributed to an increasing influx in pedogenic soil, which is related to a regional‐scale increase in the intensity of human activity in Northeastern China. This extends the timing of human activities, which is independent from climate changes and its effects on local ecosystems in Northeastern China significantly. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-20T01:38:45.352313-05:
      DOI: 10.1002/2015GC005880
       
  • First hydrothermal discoveries on the Australian‐Antarctic Ridge:
           Discharge sites, plume chemistry, and vent organisms
    • Abstract: The Australian‐Antarctic Ridge (AAR) is one of the largest unexplored regions of the global mid‐ocean ridge system. Here, we report a multi‐year effort to locate and characterize hydrothermal activity on two 1st‐order segments of the AAR: KR1 and KR2. To locate vent sites on each segment, we used profiles collected by Miniature Autonomous Plume Recorders on rock corers during R/V Araon cruises in March and December of 2011. Optical and oxidation‐reduction‐potential anomalies indicate multiple active sites on both segments. Seven profiles on KR2 found 3 sites, each separated by ∼25 km. Forty profiles on KR1 identified 13 sites, some within a few km of each other. The spatial density of hydrothermal activity along KR1 and KR2 (plume incidence of 0.34) is consistent with the global trend for a spreading rate of ∼70 mm/yr. The densest area of hydrothermal activity, named “Mujin”, occurred along the 20‐km‐long inflated section near the segment center of KR1. Continuous plume surveys conducted in January‐February of 2013 on R/V Araon found CH4/3He (1‐15 × 106) and CH4/Mn (0.01‐0.5) ratios in the plume samples, consistent with a basaltic‐hosted system and typical of ridges with intermediate spreading rates. Additionally, some of the plume samples exhibited slightly higher ratios of H2/3He and Fe/Mn than others, suggesting that those plumes are supported by a younger hydrothermal system that may have experienced a recent eruption. The Mujin‐field was populated by Kiwa crabs and seven‐armed Paulasterias starfish previously recorded on the East Scotia Ridge, raising the possibility of circum‐Antarctic biogeographic connections of vent fauna. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-18T18:33:52.560033-05:
      DOI: 10.1002/2015GC005926
       
  • Magnetic unmixing of first‐order reversal curve diagrams using
           principal component analysis
    • Authors: Ioan Lascu; Richard J. Harrison, Yuting Li, Joy R. Muraszko, James E. T. Channell, Alexander M. Piotrowski, David A. Hodell
      Abstract: We describe a quantitative magnetic unmixing method based on principal component analysis (PCA) of first‐order reversal curve (FORC) diagrams. For PCA we resample FORC distributions on grids that capture diagnostic signatures of single‐domain (SD), pseudo‐single‐domain (PSD), and multi‐domain (MD) magnetite, as well as of minerals such as hematite. Individual FORC diagrams are recast as linear combinations of end‐member (EM) FORC diagrams, located at user‐defined positions in PCA space. The EM selection is guided by constraints derived from physical modeling and imposed by data scatter. We investigate temporal variations of two EMs in bulk North Atlantic sediment cores collected from the Rockall Trough and the Iberian Continental Margin. Sediments from each site contain a mixture of magnetosomes and granulometrically distinct detrital magnetite. We also quantify the spatial variation of three EM components (a coarse silt‐sized MD component, a fine silt‐sized PSD component, and a mixed clay‐sized component containing both SD magnetite and hematite) in surficial sediments along the flow path of the North Atlantic Deep Water (NADW). These samples were separated into granulometric fractions, which helped constrain EM definition. PCA‐based unmixing reveals systematic variations in EM relative abundance as a function of distance along NADW flow. Finally, we apply PCA to the combined dataset of Rockall Trough and NADW sediments, which can be recast as a four‐EM mixture, providing enhanced discrimination between components. Our method forms the foundation of a general solution to the problem of unmixing multi‐component magnetic mixtures, a fundamental task of rock magnetic studies. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-14T03:36:41.971052-05:
      DOI: 10.1002/2015GC005909
       
  • Intraplate volcanism of the western Pacific: New insights from geological
           and geophysical observations in the Pigafetta Basin
    • Authors: Timothy J. Stadler; Masako Tominaga
      Abstract: Understanding intraplate volcanism is a key to deciphering the Earth's magmatic history. One of the largest intraplate volcanic events occurred during the mid Cretaceous, roughly 75 to 125 Ma in the western Pacific. To investigate the origin and effects of this volcanism on various Earth systems, we present the first comprehensive study of volcanism in the Pigafetta Basin using seismic surveys, magnetic and gravity modeling, and Ocean Drilling Program drill core and well log data from Site 801. Our results show that intraplate volcanism in the Pigafetta Basin coincides with the rest of the western Pacific seamount provinces, supporting the previously suggested plumelets scenario for the origin of intraplate volcanism during the mid Cretaceous volcanic events. We also discover that the late stage volcanism does not overprint the remanant magnetization acquired by the Jurassic ocean crust in the Pigafetta Basin, and hence, marine magnetic anomalies recorded in the Jurassic basement are preserved. Also, the formerly identified Rough Smooth Boundary (RSB) is indistinguishable from any other rough‐smooth topographic boundaries throughout the survey area suggesting that the RSB is unlikely to be a Cretaceous sill‐Jurassic basement boundary. Lastly, the apparent ages and spatial distribution of volcanic features suggests a dynamic history of hydrothermal circulation in the Pigafetta Basin, indicating that hydrothermal circulation was ongoing well past 100 Ma. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-12T03:42:07.870133-05:
      DOI: 10.1002/2015GC005873
       
  • Exploring the mineralogical heterogeneities of the Louisville Seamount
           Trail
    • Authors: Michael J. Dorais
      Abstract: Diopside phenocrysts of the Louisville Seamount Trail show an increase in Ti, Al, and Na with decreasing Mg/(Mg+Fe) as is typical for clinopyroxene in alkalic basalts. Chondrite‐normalized REE patterns of calculated liquids from LA‐ICPMS analyses are comparable to whole‐rock and glass values. Exceptions are clinopyroxene crystals from the Rigil Seamount, the second oldest seamount drilled at the northern end of the chain. Some crystals from this site are strongly zoned with distinct compositional boundaries between cores and mantles. The cores have high Mg/(Mg+Fe) and low Al and Ti concentrations compared to the mantles and phenocrysts. Major element, clinopyroxene discrimination diagrams indicate that the clinopyroxene mantles and phenocrysts crystallized from alkalic basalts. In contrast, the Mg‐rich cores have tholeiitic affinities. The REE abundances of the cores are similar to that of clinopyroxene from transitional tholeiites of the Kerguelen Archipelago. Calculated liquid La/Yb values for the cores have ratios that are similar to transitional tholeiites in Hawaii, whereas the mantles have higher La/Yb values similar to Hawaiian alkalic basalts. The major and trace element compositions of clinopyroxene cores from the Rigil seamount suggest that a transitional tholeiitic magma was present, but no evidence for Hawaiian shield‐type tholeiites was found. Plagioclase crystals from the Rigil seamount have 86Sr/86Sr from 0.70306 to 0.70363, within the range of FOZO. The transitional tholeiitic signature of the Mg‐rich clinopyroxene cores probably did not have a distinct source compared to other Louisville magmas, but more likely indicates a higher degree of partial melting (2‐5%) of that FOZO source. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-12T03:41:44.858747-05:
      DOI: 10.1002/2015GC005917
       
  • Tracing mantle‐reacted fluids in magma‐poor rifted margins:
           The example of Alpine Tethyan rifted margins
    • Authors: Victor Hugo; G. Pinto, Gianreto Manatschal, Anne Marie Karpoff, Adriano Viana
      Abstract: The thinning of the crust and the exhumation of subcontinental mantle in magma‐poor rifted margins is accompanied by a series of extensional detachment faults. We show that exhumation along these detachments is intimately related to migration of fluids leading to changes in mineralogy and chemistry of the mantle, crustal and sedimentary rocks. Using field observation and analytical methods, we investigate the role of fluids in the fossil distal margins of the Alpine Tethys. Using Cr‐Ni‐V, Fe and Mn as tracers, we show that fluids used detachment faults as pathways and interacted with the overlying crust and sediments. These observations allow us to discuss when, where and how this interaction happened during the formation of the rifted margin. The results show that: (i) serpentinization of mantle rocks during their exhumation results in the depletion of elements and migration of mantle‐reacted fluids that are channeled along active detachment system; (ii) in earlier‐stages, these fluids affected the overlying syn‐tectonic sediments by direct migration from the underlying detachments; (iii) in later‐stages, these fluids arrived at the seafloor, were introduced into, or “polluted” the seawater and were absorbed by post‐tectonic sediments. We conclude that a significant amount of serpentinization occurred underneath the hyperextended continental crust, and that the mantle‐reacted fluids might have modified the chemical composition of the sediments and seawater. We propose that the chemical signature of serpentinization related to mantle exhumation is recorded in the sediments and may serve as a proxy to date serpentinization and mantle exhumation at present‐day magma‐poor rifted margins. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-12T03:40:26.956968-05:
      DOI: 10.1002/2015GC005830
       
  • Geochemistry of river‐borne clays entering the East China Sea
           indicates two contrasting types of weathering and sediment transport
           processes
    • Authors: Lei Bi; Shouye Yang, Chao Li, Yulong Guo, Quan Wang, James T. Liu, Ping Yin
      Abstract: The East China Sea is characterized by wide continental shelf receiving a huge input of terrigenous matter from both large rivers and mountainous rivers, which makes it an ideal natural laboratory for studying sediment source‐to‐sink transport processes. This paper presents mineralogical and geochemical data of the clays and bulk sediments from the rivers entering the East China Sea, aiming to investigate the general driving mechanism of silicate weathering and sediment transport processes in East Asian continental margin. Two types of river systems, tectonically‐stable continental rivers and tectonically‐active mountainous rivers, co‐exist in East Asia. As the direct weathering products, clays can better reflect the silicate weathering regimes within the two river systems. Provenance rock types are not the dominant factor causing silicate weathering intensity difference existed in the East Asian rivers. The silicate weathering intensity of tectonically‐stable river basins is primarily driven by monsoon climate, and the sediment transfer is relatively slow because of natural trapping process and increasing damming effect. The geochemistry of these river‐borne sediments can thus indicate paleo‐weathering intensities in East Asian continent. In contrast, silicate weathering intensity in tectonically‐active mountainous rivers is greatly limited by strong physical erosion despite the high temperature and highest monsoon rainfall. The factors controlling silicate weathering in tectonically‐active catchments are complex and thus, it should be prudent to use river sediment records to decipher paleoclimate change. These two different silicate weathering regimes and sediment transport processes are manifestations of the landscape evolution and overall dominate the sedimentation in Asian continental margin. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-12T03:39:40.424822-05:
      DOI: 10.1002/2015GC005867
       
  • Heterogeneous and asymmetric crustal accretion: New constraints from
           multibeam bathymetry and potential field data from the Rainbow area of the
           Mid‐Atlantic Ridge (36°15′N)
    • Authors: M. Paulatto; J. P. Canales, R. A. Dunn, R. A. Sohn
      Abstract: At slow‐spreading mid‐ocean ridges, crustal accretion style can vary significantly along and across ridge segments. In magma‐poor regions, seafloor spreading can be accommodated largely by tectonic processes, however, the internal structure and formation mechanism of such highly tectonized crust are not fully understood. We analyze multi‐beam bathymetry and potential field data from the Rainbow area of the Mid‐Atlantic Ridge (35º40'N‐37º40'N), a section of the ridge that shows diverse accretion styles. We identify volcanic, tectonized and sedimented terrain and measure exposed fault area to estimate the tectonic strain, T, and the fraction of magmatic accretion, M. Estimated T values range from 0.2‐0.4 on ridge segments to 0.6‐0.8 at the Rainbow non‐transform discontinuity (NTD). At segment ends T is asymmetric, reflecting asymmetries in accretion rate, topography and faulting between inside and outside offset corners. Detachment faults have formed preferentially at inside corners, where tectonic strain is higher. We identify at least two oceanic core complexes on the fossil trace of the NTD, in addition to the Rainbow massif, which occupies the offset today. A gravity high and low magnetization suggest that the Rainbow massif, which hosts a high‐temperature hydrothermal system, was uplifted by a west dipping detachment fault. Asymmetric plate ages indicate localization of tectonic strain at the inside corners and migration of the detachment towards and across the ridge axis, which may have caused emplacement of magma into the footwall. Hydrothermal circulation and heat extraction is possibly favored by increased permeability generated by fracturing of the footwall and deep‐penetrating second‐generation faults. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-10T03:51:23.495754-05:
      DOI: 10.1002/2015GC005743
       
  • Influence of cratonic lithosphere on the formation and evolution of flat
           slabs: Insights from 3‐D time‐dependent modeling
    • Abstract: Several mechanisms have been suggested for the formation of flat slabs including buoyant features on the subducting plate, trenchward motion and thermal or cratonic structure of the overriding plate. Analysis of episodes of flat subduction indicate that not all flat slabs can be attributed to only one of these mechanisms and it is likely that multiple mechanisms work together to create the necessary conditions for flat slab subduction. In this study we examine the role of localized regions of cratonic lithosphere in the overriding plate in the formation and evolution of flat slabs. We explicitly build on previous models, by using time‐dependent simulations with three‐dimensional variation in overriding plate structure. We find that there are two modes of flat subduction: permanent underplating occurs when the slab is more buoyant (shorter or younger), while transient flattening occurs when there is more negative buoyancy (longer or older slabs). Our models show how regions of the slab adjacent to the sub‐cratonic flat portion continue to pull the slab into the mantle leading to highly contorted slab shapes with apparent slab gaps beneath the craton. These results show how the interpretation of seismic images of subduction zones can be complicated by the occurrence of either permanent or transient flattening of the slab, and how the signature of a recent flat slab episode may persist as the slab resumes normal subduction. Our models suggest that permanent underplating of slabs may preferentially occur below thick and cold lithosphere providing a built‐in mechanism for regeneration of cratons. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-07T04:33:07.19802-05:0
      DOI: 10.1002/2015GC005940
       
  • Coupled C‐S‐Fe geochemistry in a rapidly accumulating marine
           sedimentary system: Diagenetic and depositional implications
    • Authors: A. Peketi; A. Mazumdar, H.M. Joao, D. J. Patil, A. Usapkar, P. Dewangan
      Abstract: In the present study, we have investigated the C‐S‐Fe systematics in a sediment core (MD161‐13) from the Krishna‐Godavari (K‐G) basin, Bay of Bengal. The core covers the late Holocene period with high overall sedimentation rate of ∼573 cm ky−1. Pore fluid chemical analyses indicate that the depth of the present sulfate methane transition zone (SMTZ) is at ∼6 mbsf. The (ΔTA+ΔCa+ΔMg)/ΔSO42‐ ratios suggest that both organoclastic degradation and anaerobic oxidation of methane (AOM) drive sulfate reduction at the study site. The positive correlation between total organic carbon content (TOC) and chromium reducible sulfur (CRS) content indicates marked influence of organoclastic sulfate reduction on sulfidization. Coupled occurrence of 34S enriched iron sulfide (pyrite) with 12C enriched authigenic carbonate zones are the possible records of paleo‐sulfate methane transition zones where AOM driven focused sulfate reduction was likely fueled by sustained high methane flux from underlying gas rich zone. Aluminium normalized poorly reactive iron (FePR/Al) and La/Yb ratios suggest increasing contribution from Deccan basalts relative to that of Archean‐ Proterozoic granitic complex in sediment flux of Krishna‐Godavari basin during the last 4 ky. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-07T04:31:25.214918-05:
      DOI: 10.1002/2015GC005754
       
  • Effect of maghemization on the magnetic properties of nonstoichiometric
           pseudosingle‐domain magnetite particles
    • Abstract: The effect of maghemization on the magnetic properties of magnetite (Fe3O4) grains in the pseudo‐single‐domain (PSD) size range is investigated as a function of annealing temperature. X‐ray diffraction and transmission electron microscopy confirms the precursor grains as Fe3O4 ranging from ∼ 150 nm to ∼ 250 nm in diameter, whilst Mössbauer spectrometry suggests the grains are initially near‐stoichiometric. The Fe3O4 grains are heated to increasing reaction temperatures of 120 – 220 ºC to investigate their oxidation to maghemite (γ‐Fe2O3). High‐angle annular dark field imaging and localized electron energy‐loss spectroscopy reveals slightly oxidized Fe3O4 grains, heated to 140 ºC, exhibit higher oxygen content at the surface. Off‐axis electron holography allows for construction of magnetic induction maps of individual Fe3O4 and γ‐Fe2O3 grains, revealing their PSD (vortex) nature, which is supported by magnetic hysteresis measurements, including first‐order reversal curve analysis. The coercivity of the grains is shown to increase with reaction temperature up to 180 ºC, but subsequently decreases after heating above 200 ºC; this magnetic behavior is attributed to the growth of a γ‐Fe2O3 shell with magnetic properties distinct from the Fe3O4 core. It is suggested there is exchange coupling between these separate components that results in a vortex state with reduced vorticity. Once fully oxidized to γ‐Fe2O3, the domain states revert back to vortices with slightly reduced coercivity. It is argued that due to a core/shell coupling mechanism during maghemization, the directional magnetic information will still be correct, however, the intensity information will not be retained. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-06T11:12:26.282034-05:
      DOI: 10.1002/2015GC005858
       
  • S wave splitting in the offshore South Island, New Zealand: Insights into
           plate‐boundary deformation
    • Authors: S. C. Karalliyadda; M. K. Savage, A. Sheehan, J. Collins, D. Zietlow, A. Shelley
      Abstract: Local and regional S‐wave splitting in the offshore South Island of the New Zealand plate‐boundary zone provides constraints on the spatial and depth extent of the anisotropic structure with an enhanced resolution relative to land‐based and SKS studies. The combined analysis of offshore and land measurements using splitting tomography suggests plate‐boundary shear dominates in the central and northern South Island. The width of this shear zone in the central South Island is about 200 km, but is complicated by stress‐controlled anisotropy at shallow levels. In northern South Island, a broader (>200 km) zone of plate‐boundary parallel anisotropy is associated with the transitional faulting between the Alpine fault and Hikurangi subduction and the Hikurangi subduction zone itself. These results suggest S‐phases of deep events (∼ 90 km) in the central South Island are sensitive to plate‐boundary derived NE‐SW aligned anisotropic media in the upper‐lithosphere, supporting a “thin viscous sheet” deformation model. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-06T11:12:08.057655-05:
      DOI: 10.1002/2015GC005882
       
  • Wettability measurement under high P‐T conditions using X‐ray
           imaging with application to the brine‐supercritical CO2 system
    • Authors: Kuldeep Chaudhary; Eric J. Guiltinan, M. Bayani Cardenas, Jessica A. Maisano, Richard A. Ketcham, Philip C. Bennett
      Abstract: We present a new method for measuring wettability or contact angle of minerals at reservoir pressure‐temperature conditions using high‐resolution X‐ray computed tomography (HRXCT) and radiography. In this method, a capillary or a narrow slot is constructed from a mineral or a rock sample of interest wherein two fluids are allowed to form an interface that is imaged using X‐rays. After some validation measurements at room pressure‐temperature conditions, we illustrate this method by measuring the contact angle of CO2–brine on quartz, muscovite, shale, borosilicate glass, polytetrafluoroethylene (PTFE or Teflon), and polyether ether ketone (PEEK) surfaces at 60‐71°C and 13.8 – 22.8 MPa. At reservoir conditions, PTFE and PEEK surfaces were found to be CO2–wet with contact angles of 140° and 127°, respectively. Quartz and muscovite were found to be water–wet with contact angles of 26° and 58°, respectively under similar conditions. Borosilicate glass–air–brine at room conditions showed strong water‐wet characteristics with a contact angle of 9°, whereas borosilicate glass‐CO2–brine at 13.8 MPa and 60°C showed a decrease in its water‐wetness with contact angle of 54°. This method provides a new application for X‐ray imaging and an alternative to other methods. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-06T11:09:33.875019-05:
      DOI: 10.1002/2015GC005936
       
  • Late Pleistocene ages for the most recent volcanism and
           glacial‐pluvial deposits at Big Pine volcanic field, California,
           USA, from cosmogenic 36Cl dating
    • Authors: J.A. Vazquez; J. M. Woolford
      Abstract: The Big Pine volcanic field is one of several Quaternary volcanic fields that poses a potential volcanic hazard along the tectonically active Owens Valley of east‐central California, and whose lavas are interbedded with deposits from Pleistocene glaciations in the Sierra Nevada range. Previous geochronology indicates an ∼1.2 Ma history of volcanism, but the eruption ages and distribution of volcanic products associated with the most‐recent eruptions are poorly resolved. To delimit the timing and products of the youngest volcanism, we combine field mapping and cosmogenic 36Cl dating of basaltic lava flows in the area where lavas with youthful morphology and well‐preserved flow structures are concentrated. Field mapping and petrology reveal approximately fifteen vents and six principal flow units with variable geochemical composition and mineralogy. Cosmogenic 36Cl exposure ages for lava flow units from the top, middle, and bottom of the volcanic stratigraphy indicate eruptions at ca. 17 ka, 27 ka, and 40 ka, revealing several different and previously unrecognized episodes of late Pleistocene volcanism. Olivine to plagioclase‐pyroxene phyric basalt erupted from several vents during the most recent episode of volcanism at ca. 17 ka, and produced a lava flow field covering ∼ 35 km2. The late Pleistocene 36Cl exposure ages indicate that moraine and pluvial shoreline deposits that overly or modify the youngest Big Pine lavas reflect Tioga stage glaciation in the Sierra Nevada and the shore of Owens Lake during the last glacial cycle. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-01T10:44:20.049941-05:
      DOI: 10.1002/2015GC005889
       
  • Propagation of back‐arc extension into the arc lithosphere in the
           southern New Hebrides volcanic arc
    • Authors: M. Patriat; J. Collot, M. Fabre, L. Danyushevsky, S. Meffre, T. Falloon, P. Rouillard, B. Pelletier, M. Roach, M. Fournier
      Abstract: New geophysical data acquired during three expeditions of the R/V Southern Surveyor in the southern part of the North Fiji Basin allow us to characterize the deformation of the upper plate at the southern termination of the New Hebrides subduction zone, where it bends eastward along the Hunter Ridge. Unlike the northern end of the Tonga subduction zone, on the other side of the North Fiji Basin, the 90° bend does not correspond to the transition from a subduction zone to a transform fault, but it is due to the progressive retreat of the New Hebrides trench. The subduction trench retreat is accommodated in the upper plate by the migration toward the southwest of the New Hebrides arc and toward the south of the Hunter Ridge, so that the direction of convergence remains everywhere orthogonal to the trench. In the back‐arc domain, the active deformation is characterised by propagation of the back‐arc spreading ridge into the Hunter volcanic arc. The N‐S spreading axis propagates southward and penetrates in the arc, where it connects to a sinistral strike‐slip zone via an oblique rift. The collision of the Loyalty Ridge with the New Hebrides arc, less than two million years ago, likely initiated this deformation pattern and the fragmentation of the upper plate. In this particular geodynamic setting, with an oceanic lithosphere subducting beneath a highly sheared volcanic arc, a wide range of primitive subduction‐related magmas has been produced including adakites, island arc tholeiites, back‐arc basin basalts, and medium‐K subduction‐related lavas. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-01T10:43:49.688434-05:
      DOI: 10.1002/2015GC005717
       
  • Multiple mantle upwellings in the transition zone beneath the northern
           East African Rift System from relative P wave travel time tomography
    • Authors: Chiara Civiero; James O. S. Hammond, Saskia Goes, Stewart Fishwick, Abdulhakim Ahmed, Atalay Ayele, Cecile Doubre, Berhe Goitom, Derek Keir, Michael Kendall, Sylvie Leroy, Ghebrebrhan Ogubazghi, Georg Rumpker, Graham W. Stuart
      Abstract: Mantle plumes and consequent plate extension have been invoked as the likely cause of East African Rift volcanism. However, the nature of mantle upwelling is debated, with proposed configurations ranging from a single broad plume connected to the large low‐shear‐velocity province beneath Southern Africa, the so‐called African Superplume, to multiple lower‐mantle sources along the rift. We present a new P‐wave travel‐time tomography model below the northern East‐African, Red Sea and Gulf of Aden rifts and surrounding areas. Data are from stations that span an area from Madagascar to Saudi Arabia. The aperture of the integrated dataset allows us to image structures of ∼100 km length scale down to depths of 700‐800 km beneath the study region. Our images provide evidence of two clusters of low‐velocity structures consisting of features with diameter of 100‐200 km that extend through the transition zone, the first beneath Afar and a second just west of the Main Ethiopian Rift, a region with off‐rift volcanism. Considering seismic sensitivity to temperature, we interpret these features as upwellings with excess temperatures of 100±50 K. The scale of the upwellings is smaller than expected for lower mantle plume sources. This, together with the change in pattern of the low‐velocity anomalies across the base of the transition zone, suggests that ponding or flow of deep‐plume material below the transition zone may be spawning these upper‐mantle upwellings. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-01T10:42:37.541849-05:
      DOI: 10.1002/2015GC005948
       
  • Factors affecting the rare earth element compositions in massive sulfides
           from deep‐sea hydrothermal systems
    • Authors: Zhigang Zeng; Yao Ma, Xuebo Yin, David Selby, Fancui Kong, Shuai Chen
      Abstract: To reconstruct the evolution of ore‐forming fluids and determine the physicochemical conditions of deposition associated with seafloor massive sulfides, we must better understand the sources of rare earth elements (REEs), the factors that affect the REE abundance in the sulfides, and the REE flux from hydrothermal fluids to the sulfides. Here, we examine the REE profiles of 46 massive sulfide samples collected from seven seafloor hydrothermal systems. These profiles feature variable total REE concentrations (37.2–4,092 ppb) and REE distribution patterns (LaCN/LuCN ratios = 2.00–73.8; (Eu/Eu*)CN ratios = 0.34–7.60). The majority of the REE distribution patterns in the sulfides are similar to those of vent fluids, with the sulfides also exhibiting light REE enrichment. We demonstrate that the variable REE concentrations, Eu anomalies, and fractionation between light REEs and heavy REEs in the sulfides exhibit a relationship with the REE properties of the sulfide‐forming fluids and the massive sulfide chemistry. Based on the sulfide REE data, we estimate that modern seafloor sulfide deposits contain approximately 280 tons of REEs. According to the flux of hydrothermal fluids at mid‐ocean ridges (MORs) and an average REE concentration of 3 ng/g in these fluids, hydrothermal vents at MORs alone transport more REEs (>360 tons) to the oceans over the course of just two years than the total quantity of REEs in seafloor sulfides. The excess REEs (i.e., the quantity not captured by massive sulfides) may be transported away from the systems and become bound in sulfate deposits and metalliferous sediments. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-01T10:42:11.191306-05:
      DOI: 10.1002/2015GC005812
       
  • Hydrogeological responses to incoming materials at the erosional
           subduction margin, offshore Osa Peninsula, Costa Rica
    • Authors: Jun Kameda; Robert N. Harris, Mayuko Shimizu, Kohtaro Ujiie, Akito Tsutsumi, Minoru Ikehara, Masaoki Uno, Asuka Yamaguchi, Yohei Hamada, Yuka Namiki, Gaku Kimura
      Abstract: Bulk mineral assemblages of sediments and igneous basement rocks on the incoming Cocos Plate at the Costa Rica subduction zone are examined by X‐ray diffraction analyses on core samples. These samples are from Integrated Ocean Drilling Program Expedition 334 reference Site U1381, ∼ 5 km seaward of the trench. Drilling recovered approximately 100 m of sediment and 70 m of igneous oceanic basement. The sediment includes two lithologic units: hemipelagic clayey mud and siliceous to calcareous pelagic ooze. The hemipelagic unit is composed of clay minerals (∼50 wt.%), quartz (∼5 wt.%), plagioclase (∼5 wt.%), calcite (∼15 wt.%) and ∼30 wt.% of amorphous materials, while the pelagic unit is mostly made up of biogenic amorphous silica (∼50 wt.%) and calcite (∼50 wt.%). The igneous basement rock consists of plagioclase (∼50‐60 wt.%), clinopyroxene (∼>25 wt.%), and saponite (∼15‐40 wt.%). Saponite is more abundant in pillow basalt than in the massive section, reflecting the variable intensity of alteration. We estimate the total water influx of the sedimentary package is 6.9 m3/yr per m of trench length. Fluid expulsion models indicate that sediment compaction during shallow subduction causes the release of pore water while peak mineral dehydration occurs at temperatures of approximately ∼100 °C, 40‐30 km landward of the trench. This region is landward of the observed updip extent of seismicity. We posit that in this region the presence of subducting bathymetric relief capped by velocity weakening nannofossil chalk is more important in influencing the updip extent of seismicity than the thermal regime. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-01T10:41:59.942064-05:
      DOI: 10.1002/2015GC005837
       
  • Chemical and Pb‐isotope composition of phenocrysts from bentonites
           constrain the chronostratigraphy around the Cretaceous‐Paleogene
           boundary in the Hell Creek region, Montana
    • Authors: Ryan B. Ickert; Sean R. Mulcahy, Courtney J. Sprain, Jessica F. Banaszak, Paul R. Renne
      Abstract: An excellent record of environmental and paleobiological change around the Cretaceous‐Paleogene boundary is preserved in the Hell Creek and Fort Union Formations in the western Williston Basin of northeastern Montana. These records are present in fluvial deposits whose lateral discontinuity hampers longdistance correlation. Geochronology has been focused on bentonite beds that are often present in lignites. To better identify unique bentonites for correlation across the region, the chemical and Pb isotopic composition of feldspar and titanite have been measured on 46 samples. Many of these samples have been dated by 40Ar/39Ar. The combination of chemical and isotopic compositions of phenocrysts has enabled the identification of several unique bentonite beds. In particular, three horizons located at and above the Cretaceous‐Paleogene boundary can now be traced – based on their unique compositions – across the region, clarifying previously ambiguous stratigraphic relationships. Other bentonites show unusual features, such as Pb isotope variations consistent with magma‐mixing or assimilation, that will make them easy to recognize in future studies. This technique is limited in some cases by more than one bentonite having compositions that cannot be distinguished, or bentonites with abundant xenocrysts. The Pb isotopes are consistent with a derivation from the Bitterroot Batholith, whose age range overlaps that of the tephra. These data provide an improved stratigraphic framework for the Hell Creek region and provide a basis for more focused tephrostratigraphic work, and more generally demonstrate that the combination of mineral chemistry and Pb isotope compositions is an effective technique for tephra correlation. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-01T10:41:45.112356-05:
      DOI: 10.1002/2015GC005898
       
  • Multiple‐scale hydrothermal circulation in 135 Ma oceanic crust of
           the Japan Trench outer rise: Numerical models constrained with heat flow
           observations
    • Authors: Labani Ray; Yoshifumi Kawada, Hideki Hamamoto, Makoto Yamano
      Abstract: Anomalous high heat flow is observed within 150 km seaward of the trench axis at the Japan Trench offshore of Sanriku, where the old Pacific plate (∼135 Ma) is subducting. Individual heat flow values range between 42 and 114 mW m−2, with an average of ∼70 mW m−2. These values are higher than those expected from the seafloor age based on thermal models of the oceanic plate, i.e., ∼50 mW m−2. The heat flow exhibits spatial variations at multiple scales: regional high average heat flow (∼100 km) and smaller‐scale heat flow peaks (∼1 km). We found that hydrothermal mining of heat from depth due to gradual thickening of an aquifer in the oceanic crust toward the trench axis can yield elevated heat flow of the spatial scale of ∼100 km. Topographic effects combined with hydrothermal circulation may account for the observed smaller‐scale heat flow variations. Hydrothermal circulation in high‐permeability faults may result in heat flow peaks of a sub‐kilometer spatial scale. Volcanic intrusions are unlikely to be a major source of heat flow variations at any scale because of limited occurrence of young volcanoes in the study area. Hydrothermal heat transport may work at various scales on outer rises of other subduction zones as well, since fractures and faults have been well developed due to bending of the incoming plate. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-30T03:59:32.900082-05:
      DOI: 10.1002/2015GC005771
       
  • Cobalt‐based age models of pelagic clay in the South Pacific Gyre
    • Authors: Ann G. Dunlea; Richard W. Murray, Justine Sauvage, Robert A. Pockalny, Arthur J. Spivack, Steven D'Hondt, Robert N. Harris
      Abstract: Dating pelagic clay can be a challenge due to its slow sedimentation rate, post‐depositional alteration, and lack of biogenic deposition. Co‐based dating techniques have the potential to create age models in pelagic clay under the assumption that the flux of non‐detrital Co to the seafloor is spatially and temporally constant, resulting in the non‐detrital Co concentrations being inversely proportional to sedimentation rate. We apply a Co‐based method to the pelagic clay sequences from Sites U1365, U1366, U1369, and U1370 drilled during Integrated Ocean Drilling Program (IODP) Expedition 329 in the South Pacific Gyre. We distinguished non‐detrital Co from detrital Co using multivariate statistical partitioning techniques. We found that the non‐detrital flux of Co at Site U1370 is approximately twice as high than at the other sites, implying that the non‐detrital Co flux is not regionally constant. This regional variation reflects the heterogeneous distribution of Co in the water column, as is observed in the present day. We present an improved approach to Co‐based age modeling throughout the South Pacific Gyre and determine that the Co‐based method can effectively date oxygenated pelagic clay deposited in the distal open‐ocean, but is less reliable for deposition closer to continents. When extending the method to geologically old sediment, it is important to consider the paleolocation of a given site to ensure these conditions are met. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-29T19:48:36.917079-05:
      DOI: 10.1002/2015GC005892
       
  • A Bayesian, multivariate calibration for Globigerinoides ruber Mg/Ca
    • Abstract: The use of Mg/Ca in marine carbonates as a paleothermometer has been challenged by observations that implicate salinity as a contributing influence on Mg incorporation into biotic calcite and that dissolution at the sea‐floor alters the original Mg/Ca. Yet, these factors have not yet been incorporated into a single calibration model. We introduce a new Bayesian calibration for Globigerinoides ruber Mg/Ca based on 186 globally‐distributed core top samples, which explicitly takes into account the effect of temperature, salinity, and dissolution on this proxy. Our reported temperature, salinity, and dissolution (here expressed as deep‐water ΔCO32‐) sensitivities are (±2σ) 8.7±0.9%/°C, 3.9±1.2%/psu, and 3.3±1.3%/μmol.kg−1 below a critical threshold of 21 μmol/kg, in good agreement with previous culturing and core‐top studies. We then perform a sensitivity experiment on a published record from the western tropical Pacific to investigate the bias introduced by these secondary influences on the interpretation of past temperature variability. This experiment highlights the potential for misinterpretations of past oceanographic changes when the secondary influences of salinity and dissolution are not accounted for. Multi‐proxy approaches could potentially help deconvolve the contributing influences but this awaits better characterization of the spatio‐temporal relationship between salinity and δ18Osw over millennial and orbital timescales. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-22T03:58:44.749955-05:
      DOI: 10.1002/2015GC005844
       
  • Geology, sulphide geochemistry, and supercritical venting at the Beebe
           Hydrothermal Vent Field, Cayman Trough
    • Authors: Alexander P. Webber; Stephen Roberts, Bramley J. Murton, Matthew R.S. Hodgkinson
      Abstract: The Beebe Vent Field (BVF) is the world's deepest known hydrothermal system, at 4960m below sea level. Located on the Mid‐Cayman Spreading Centre, Caribbean, the BVF hosts high temperature (∼401°C) ‘black smoker' vents that build Cu, Zn and Au‐rich sulphide mounds and chimneys. The BVF is highly gold‐rich, with Au values up to 93 ppm and an average Au:Ag ratio of 0.15. Gold precipitation is directly associated with diffuse flow through ‘beehive' chimneys. Significant mass‐wasting of sulphide material at the BVF, accompanied by changes in metal content, results in metaliferous talus and sediment deposits. Situated on very thin (2‐3km thick) oceanic crust, at an ultraslow spreading centre, the hydrothermal system circulates fluids to a depth of ∼1.8km in a basement that is likely to include a mixture of both mafic and ultramafic lithologies. We suggest hydrothermal interaction with chalcophile‐bearing sulphides in the mantle rocks, together with precipitation of Au in beehive chimney structures, has resulted in the formation of a Au‐rich volcanogenic massive sulphide (VMS) deposit. With its spatial distribution of deposit materials and metal contents, the BVF represents a modern day analogue for basalt hosted, Au‐rich VMS systems. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-22T03:40:30.387821-05:
      DOI: 10.1002/2015GC005879
       
  • Water‐rich bending faults at the Middle America Trench
    • Authors: Samer Naif; Kerry Key, Steven Constable, Rob L. Evans
      Abstract: The portion of the Central American margin that encompasses Nicaragua is considered to represent an end‐member system where multiple lines of evidence point to a substantial flux of subducted fluids. The seafloor spreading fabric of the incoming Cocos plate is oriented parallel to the trench such that flexural bending at the outer rise optimally reactivates a dense network of normal faults that extend several kilometers into the upper mantle. Bending faults are thought to provide fluid pathways that lead to serpentinization of the upper mantle. While geophysical anomalies detected beneath the outer rise have been interpreted as broad crustal and upper mantle hydration, no observational evidence exists to confirm that bending faults behave as fluid pathways. Here, we use seafloor electromagnetic data collected across the Middle America Trench (MAT) offshore of Nicaragua to create a comprehensive electrical resistivity image that illuminates the infiltration of seawater along bending faults. We quantify porosity from the resistivity with Archie's law and find that our estimates for the abyssal plain oceanic crust are in good agreement with independent observations. As the Cocos crust traverses the outer rise, the porosity of the dikes and gabbros progressively increase from 2.7% and 0.7% to 4.8% and 1.7%, peaking within 20 km of the trench axis. We conclude that the intrusive crust subducts twice as much pore water as previously thought, significantly raising the flux of fluid to the seismogenic zone and the mantle wedge. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-21T08:23:13.310045-05:
      DOI: 10.1002/2015GC005927
       
  • Eduction, extension, and exhumation of ultrahigh‐pressure rocks in
           metamorphic core complexes due to subduction initiation
    • Authors: Kenni Dinesen Petersen; W. Roger Buck
      Abstract: The controversy over the exhumation of ultra‐high pressure (UHP) rocks centers on whether it involves rising of pieces of crust detached from subducted continental lithosphere or an entire subducted plate that undergoes “eduction”, i.e. reverse subduction. We present a new thermomechanical model of continental subduction showing that these apparently contrasting mechanisms can occur together: Crust subducted deep enough is heated and weakened, causing limited diapiric rise, while crust subducted to shallower depths retains strength and is exhumed only by eduction. The model also shows for the first time how eduction followed by seafloor spreading can occur in a zone of regional convergence. This occurs spontaneously when subduction of buoyant crust causes a subduction zone to “lock up” in one place causing a new subduction zone to form in another. The model is consistent with many features of the youngest region of UHP rock exhumation on earth: the D'Entrecasteaux Islands. UHP exhumation and the amount of regional extension, as well as the seismic structure around the islands, can be explained by eduction. Ductile flow fabrics, seen on the islands, would result from exhumation of the most deeply subducted crust heated enough to undergo partial melting. Reversal of motion on the north‐dipping continental subduction zone, required by this model, was likely triggered by initiation of the New Britain Trench, as suggested previously. Our model implies that the crust of Goodenough Basin, south of the islands, was exhumed by eduction in the last 5 Ma and this hypothesis can be tested by drilling. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-17T03:28:24.269522-05:
      DOI: 10.1002/2015GC005847
       
  • Reply to comment by Ivan Pineda‐Velasco, Tai T. Nguyen, Hiroshi
           Kitagawa, and Eizo Nakamura on “Diverse magmatic effects of
           subducting a hot slab in SW Japan: Results from forward modeling”
    • Abstract: The Comment by Pineda‐Velasco et al. [2015] examined Pb isotope data presented by Kimura et al. [2014]. The authors' points are that (1) there is uncertainty in the analytical results of Kimura et al. [2014] due to the effect of mass fractionation, and therefore, (2) the interpretations of Kimura et al. [2014] based on the extent of crustal assimilation and the estimated Pb isotopic composition of the crustal component are erroneous. In response to the Comment, we report a flaw in the original paper that the samples from the Aono, Daisen, and Kannabe regions were analyzed using conventional TIMS methods and all other samples were analyzed using thallium‐spiked multi‐collector inductively‐coupled‐plasma mass spectrometry (TS‐MC‐ICP‐MS). We have re‐analyzed the sample powders from Karasugasen, Daisen, and Aono using TS‐MC‐ICP‐MS. Our new results showed considerable overlap with the data in Pineda‐Velasco et al. [2015]. Therefore, the isotopic trends shown by the conventional TIMS in Kimura et al. [2014] were analytical artifacts from mass bias. We conclude that the crustal assimilation proposed by Kimura et al. [2014] was erroneous in terms of Pb isotopes, nevertheless some crustal assimilation in the Karasugasen lava is evident from the chemical zoning of hornblende phenocrysts. Although the original Pb isotope argument for crustal contamination was wrong, the ABS4 modeling is unaffected because of no to subtle changes in estimated mantle source compositions in their ABS4 model. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-17T02:22:07.781949-05:
      DOI: 10.1002/2015GC005984
       
  • Comment on “Diverse magmatic effects of subducting a hot slab in SW
           Japan: Results from forward modeling'' by J.‐I. Kimura et al.
    • PubDate: 2015-07-16T04:33:10.722884-05:
      DOI: 10.1002/2015GC005914
       
  • Varying styles of magmatic strain accommodation across the East African
           Rift
    • Authors: James D. Muirhead; Simon A. Kattenhorn, Nicolas Le Corvec
      Abstract: Observations of active dike intrusions provide present day snapshots of the magmatic contribution to continental rifting. However, unravelling the contributions of upper crustal dikes over the timescale of continental rift evolution is a significant challenge. To address this issue, we analyzed the morphologies and alignments of >1,500 volcanic cones to infer the distribution and trends of upper crustal dikes in various rift basins across the East African Rift (EAR). Cone lineament data reveal along‐axis variations in the distribution and geometries of dike intrusions as a result of changing tectono‐magmatic conditions. In younger (
      PubDate: 2015-07-16T04:28:13.092083-05:
      DOI: 10.1002/2015GC005918
       
  • River geochemistry, chemical weathering, and atmospheric CO2 consumption
           rates in the Virunga Volcanic Province (East Africa)
    • Abstract: We report a water chemistry data set from 13 rivers of the Virunga Volcanic Province (VVP) (Democratic Republic of Congo), sampled between December 2010 and February 2013. Most parameters showed no pronounced seasonal variation, whereas their spatial variation suggests a strong control by lithology, soil type, slope and vegetation. High total suspended matter (289‐1467 mg L−1) was recorded in rivers in the Lake Kivu catchment, indicating high soil erodibility, partly as a consequence of deforestation and farming activities. Dissolved and particulate organic carbon (DOC and POC) were lower in rivers from lava fields, and higher in non‐volcanic sub‐catchments. Stable carbon isotope signatures (δ13C) of POC and DOC mean δ13C of ‐22.5 and ‐23.5 ‰, respectively), are the first data to be reported for the highland of the Congo River basin and showed a much higher C4 contribution than in lowland areas. Rivers of the VVP were net sources of CH4 to the atmosphere (4‐ 5052 nmol L−1). Most rivers show N2O concentrations close to equilibrium, but some rivers showed high N2O concentrations related to denitrification in groundwaters. δ13C signatures of dissolved inorganic carbon suggested magmatic CO2 inputs to aquifers/soil, which could have contributed to increase basalt weathering rates. This magmatic CO2‐mediated basalt weathering strongly contributed to the high major cation concentrations and total alkalinity. Thus, chemical weathering (39.0 ‐2779.9 t km−2 yr−1) and atmospheric CO2 consumption (0.4‐ 37.0 x 106 mol km−2 yr−1) rates were higher than previously reported in literature for basaltic terrains. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-16T04:27:14.648344-05:
      DOI: 10.1002/2015GC005999
       
  • LA‐ICPMS Ba/Ca analyses of planktic foraminifera from the Bay of
           Bengal: Implications for late Pleistocene orbital control on monsoon
           freshwater flux
    • Abstract: Indian Summer Monsoon (ISM) indices are characterised by large secular variation during both glacials and interglacials. Although much information about palaeo‐monsoon intensity is derived from such indicies, current datasets do not relate simply to precipitation. In order to directly constrain the variability of ISM freshwater flux to the Bay of Bengal, we report Ba/Ca LA‐ICPMS data of the surface‐dwelling foraminifera G. ruber from core RC12‐343 (central Bay of Bengal) between 68‐47 ka. Planktic foraminifera Ba/Ca directly relates to seawater Ba/Ca, in turn principally controlled by freshwater flux. Our foraminifera‐derived Ba/Casw record for the central Bay of Bengal is highly coherent with that derived from δ18O measurements of the same material, implying that these reconstructions are not significantly biased by potential shifts in δ18Ofreshwater. Validating this method allows us to produce a freshwater stack for the last 80 ka for the Bay of Bengal, enabling the orbital controls on ISM precipitation to be examined for the first time. The highest freshwater flux in the last 80 ka was ∼3× larger than present and occurred during the early‐mid Holocene. We show that the orbital timing of this record is best explained by a combination of factors with a weighting of ∼45% given to the 30° N‐equator mean summer insolation gradient and ∼55% given to the variability of May‐July insolation at the equator and June‐July insolation at 30° N. These processes are consistent with current mechanistic understanding of ISM forcings and demonstrate a dominant orbital control on monsoon precipitation amount on millennial‐timescales. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-16T04:26:23.921548-05:
      DOI: 10.1002/2015GC005822
       
  • Crystallographic preferred orientations may develop in nanocrystalline
           materials on fault planes due to surface energy interactions
    • Authors: Virginia G. Toy; Thomas M. Mitchell, Anthony Druiventak, Richard Wirth
      Abstract: A layer of substantially non‐crystalline material, composed of partially annealed nanopowder with local melt, was experimentally generated by comminution during ∼1.5 mm total slip at ∼2.5x10−6 ms−1, Pconf ∼ 0.5 GPa, and 450°C or 600°C, on sawcut surfaces in novaculite. The partially annealed nanopowder comprises angular grains mostly 5‐200 nm diameter in a variably dense packing arrangement. A sharp transition from wall rock to partially annealed nanopowder illustrates the nanopowder effectively localizes shear, consistent with generation of nanoparticles during initial fragmentation, not by progressive grain size reduction. Dislocation densities in nanopowder grains or immediate wall rock are not significantly high, but there are planar plastic defects spaced at 5‐200 nm parallel to the host quartz grain's basal plane. We propose these plastic defects developed into through‐going fractures to generate nanocrystals. The partially annealed nanopowder has a crystallographic preferred orientation (CPO) that we hypothesise developed due to surface energy interactions to maximize coincident site lattices (CSL) during annealing. This mechanism may also have generated CPO recently described in micro/nanocrystalline calcite fault gouges. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-14T10:01:35.047042-05:
      DOI: 10.1002/2015GC005857
       
  • Effects of chemical composition, water, and temperature on physical
           properties of continental crust
    • Authors: Mattia Guerri; Fabio Cammarano, James A.D. Connolly
      Abstract: We explore the influence of major elements chemistry and H2O‐content on the density and seismic velocity of crustal rocks by computing stable and metastable crustal mineralogy and elastic properties as a function of pressure and temperature (P‐T). Proposed average compositions of continental crust result in significantly different properties, for example a difference in computed density of ∼ 4% is obtained at a given P‐T. Phase transformations affect crustal properties at the point that crustal seismic discontinuities can be explained with mineral reactions rather than chemical stratification. H2O, even if introduced in small amount in the chemical system, has an effect on physical properties comparable to that attributed to variations in major elements composition. Thermodynamical relationships between physical properties differ significantly from commonly used empirical relationships. Density models obtained by inverting CRUST 1.0 compressional wave velocity [Laske et al., 2013] are different from CRUST 1.0 density and translate into variations in isostatic topography and gravitational field that ranges ±600 m and ±150 mGal respectively. Inferred temperatures are higher than reference geotherms in the upper crust and in the deeper portions of thick orogenic crust, consistently with presence of metastable rocks. Our results highlight interconnections/dependencies among chemistry, pressure, temperature, seismic velocities and density that need to be addressed to better understand the crustal thermo‐chemical state. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-14T08:04:31.82752-05:0
      DOI: 10.1002/2015GC005819
       
  • Spatiotemporal variations of the slow slip event between 2008 and 2013 in
           the southcentral Alaska subduction zone
    • Authors: Yuning Fu; Zhen Liu, Jeffrey T. Freymueller
      Abstract: We apply a Kalman filter based time‐dependent slip inversion method to model a long‐term Slow Slip Event (SSE) in the southcentral Alaska subduction zone from 2008 to 2013. This event occurred downdip of the asperity that ruptured in the 1964 earthquake, the same part of plate interface that slipped during a previous SSE between 1998 and 2001. Most of the slip deficit that accumulated during the steady period between 2001 and 2008 (8 years total) in the SSE source region was released by this SSE. Our results indicate both lateral and down dip propagation during this event. The SSE started at the end of 2008 at the upper section of the slip patch, and gradually propagated to the east and to the deeper part of the interface. Our results indicate no connection between this SSE in Upper Cook Inlet and another SSE in Lower Cook Inlet that started in 2010. Analysis of the earthquake catalog in the southcentral Alaska subduction zone shows a clear increase in seismicity associated with the 2008‐2013 SSE. With the data from a newly available continuous GPS site, we now can better constrain the start time of the 1998‐2001 SSE as ∼1998.58. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-14T08:04:23.37089-05:0
      DOI: 10.1002/2015GC005904
       
  • Profiling planktonic foraminiferal crust formation
    • Abstract: Planktonic foraminifera migrate vertically through the water column during their life, thereby growing and calcifying over a range of depth‐associated conditions. Some species form a calcite veneer, crust or cortex at the end of their lifecycle. This additional calcite layer may vary in structure, composition and thickness, potentially accounting for most of their total shell mass and thereby dominating the element and isotope signature of the whole shell. Here we apply laser ablation ICP‐MS depth profiling to assess variability in thickness and Mg/Ca composition of shell walls of three encrusting species derived from sediment traps. Compositionally, Mg/Ca is significantly lower in the crusts of Neogloboquadrina dutertrei and Globorotalia scitula, as well as in the cortex of Pulleniatina obliquiloculata, independent of the species‐specific Mg/Ca of their lamellar calcite shell. Wall thickness accounts for nearly half of the total thickness in both crustal species and nearly a third in cortical P. obliquiloculata, regardless of their initial shell wall thickness. Crust thickness and crustal Mg/Ca decreases towards the younger chambers in N. dutertrei and to a lesser extent, also in G. scitula. In contrast, the cortex of P. obliquiloculata shows a nearly constant thickness and uniform Mg/Ca through the complete chamber wall. Patterns in thickness and Mg/Ca of the crust indicate that temperature is not the dominant factor controlling crust formation. Instead, we present a depth‐resolved model explaining compositional differences within individuals and between successive chambers as well as compositional heterogeneity of the crust and lamellar calcite in all three species studied here. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-14T07:26:03.966367-05:
      DOI: 10.1002/2015GC005752
       
  • The Manihiki Plateau—A multistage volcanic emplacement history
    • Abstract: The formation history of the Manihiki Plateau, a Large Igneous Province, is poorly understood. New high resolution seismic reflection data across the High Plateau, the largest edifice of the Manihiki Plateau, provides evidence for multistage magmatic emplacement. Improved data quality allows for an identification of an earlier volcanic phase, the initial formation phase (>125 Ma), in addition to the previously known volcanic formation phases: the expansion phase (125‐116) formerly called main‐phase and the secondary volcanic phase (100‐65 Ma). This enhances the understanding of the emplacement scenario. An intrabasement reflection band IB1 reveals the end of initial volcanic formation and forms the nucleus of the High Plateau. This feature provides indications that it continued beyond the Manihiki Scarp and thus supports the hypothesis of an extension of the Manihiki Plateau to the East during the initial formation and expansion phases. The expansion phase is characterized by massive volcanic outpourings leveling and extending the basement throughout the High Plateau and the neighboring Western Plateaus, which in contrast shows massive tectonic alteration. Extrusion centers formed within the secondary volcanic phase (ending ∼65 Ma) are mainly concentrated along the margins of the High Plateau, suggesting the magmatic sources shifted from those being related to the initial emplacement and expansion phases of the High Plateau to induced volcanism at the tectonically altered margins. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-14T07:25:43.009957-05:
      DOI: 10.1002/2015GC005852
       
  • Particle dynamics in the rising plume at Piccard Hydrothermal Field,
           Mid‐Cayman Rise
    • Authors: M. L. Estapa; J. A. Breier, C. R. German
      Abstract: Processes active in rising hydrothermal plumes, such as precipitation, particle aggregation, and biological growth, affect particle size distributions and can exert important influences on the biogeochemical impact of submarine venting of iron to the oceans and their sediments. However, observations of particle size distribution within these systems, to date, are both limited and conflicting. In a novel buoyant hydrothermal plume study at the recently discovered high‐temperature (398°C) Piccard Hydrothermal Field, Mid‐Cayman Rise, we report optical measurements of particle size distributions (PSDs). We describe the plume PSD in terms of a simple, power‐law model commonly used in studies of upper‐ and coastal ocean particle dynamics. Observed PSD slopes, derived from spectral beam attenuation and laser diffraction measurements, are among the highest found to date anywhere in the ocean and ranged from 2.9 to 8.5. Beam attenuation at 650 nm ranged from near zero to a rarely‐observed maximum of 192 m−1 at 3.5 m above the vent. We did not find large (>100 μm) particles that would settle rapidly to the sediments. Instead, beam attenuation was well‐correlated to total iron, suggesting the first‐order importance of particle dilution, rather than precipitation or dissolution, in the rising plume at Piccard. Our observations at Piccard caution against the assumption of rapid deposition of hydrothermal, particulate metal fluxes, and illustrate the need for more particle size and composition measurements across a broader range of sites, globally. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-14T07:25:40.4561-05:00
      DOI: 10.1002/2015GC005831
       
  • Imaging continental breakup using teleseismic body waves: The Woodlark
           Rift, Papua New Guinea
    • Authors: Zachary Eilon; Geoffrey A Abers, James B Gaherty, Ge Jin
      Abstract: This study images the upper mantle beneath the D'Entrecasteax Islands, Papua New Guinea, providing insight into mantle deformation beneath a highly rifted continent adjacent to propagating spreading centers. Differential travel times from P‐ and S‐wave teleseisms recorded during the 2010‐2011 CDPapua passive seismic experiment are used to invert for separate VP and VS velocity models of the continental rift. A low‐velocity structure marks the E‐W axis of the rift, correlating with the thinnest crust, high heat flow, and a linear trend of volcanoes. This slow region extends 250 km along strike from the oceanic spreading centers, demonstrating significant mantle extension ahead of seafloor breakup. The rift remains narrow to depth indicating localization of extension, perhaps as a result of mantle hydration. A high‐VP structure at depths of 90‐120 km beneath the north of the array is more than 6.5% faster than the rift axis and contains well‐located intermediate depth earthquakes. These independent observations place firm constraints on the lateral thermal contrast at depth between the rift axis and cold lithosphere to the north that may be related to recent subduction, although the polarity of subduction cannot be resolved. This geometry is gravitationally unstable; downwelling or small‐scale convection could have facilitated rifting and rapid lithospheric removal, although this may require a wet mantle to be realistic on the required timescales. The high‐V structure agrees with the maximum P,T conditions recorded by young ultra‐high pressure rocks exposed on the rift axis and may be implicated in their genesis. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-03T09:50:57.704778-05:
      DOI: 10.1002/2015GC005835
       
  • Effects of inherited cores and magmatic overgrowths on zircon
           (U‐Th)/He ages and age‐eU trends from Greater Himalayan
           sequence rocks, Mt. Everest region, Tibet
    • Authors: D.A. Orme; P.W. Reiners, J.K. Hourigan, B. Carrapa
      Abstract: Previous constraints on the timing and rate of exhumation of the footwall of the South Tibetan detachment system (STDS) north of Mt. Everest suggest rapid Miocene cooling from ∼ 700°C to 120°C between ca. 14‐17 Ma. However, twenty‐five new single grain zircon He ages from leucogranites intruding Greater Himalayan Sequence rocks in the footwall of the STDS are between 9.9 to 15 Ma, with weighted mean ages between 10 and 12 Ma. Zircon grains exhibit a positive correlation between age and effective uranium (eU). Laser ablation zircon U‐Pb geochronology, detailed SEM observations and laser ablation depth‐profiling of these zircons reveal low‐eU 0.5‐2.5 Ga inherited cores overgrown by high‐eU 17‐22 Ma rims. This intragranular zonation produces ages as much as 32% too young when a standard alpha‐ejection correction assuming uniform eU distribution is applied. Modeling of the effects of varying rim thickness and rim eU concentration on the bulk grain eU and alpha‐ejection correction suggests that zonation also exerts the primary control on the form of the age‐eU correlation observed. Application of grain‐specific zonation‐dependent age corrections to our data yields zircon He ages between 14‐17 Ma, in agreement with AFT and 40Ar/39Ar ages. Growth of magmatic rims followed by cooling to 
      PubDate: 2015-07-02T07:54:12.871204-05:
      DOI: 10.1002/2015GC005818
       
  • Thermal dependency of shell growth, microstructure, and stable isotopes in
           laboratory‐reared Scapharca broughtonii (Mollusca: Bivalvia)
    • Authors: Kozue Nishida; Atsushi Suzuki, Ryosuke Isono, Masahiro Hayashi, Yusuke Watanabe, Yuzo Yamamoto, Takahiro Irie, Yukihiro Nojiri, Chiharu Mori, Mizuho Sato, Kei Sato, Takenori Sasaki
      Abstract: We experimentally examined the growth, microstructure, and chemistry of shells of the bloody clam, Scapharca broughtonii (Mollusca: Bivalvia), reared at five temperatures (13, 17, 21, 25, and 29°C) with a constant pCO2 condition (∼450 ppm). In this species, the exterior side of the shell is characterized by a composite prismatic structure, on the interior side it has a crossed lamellar structure on the interior surface. We previously found a negative correlation between temperature and the relative thickness of the composite prismatic structure in field‐collected specimens. In the reared specimens, the relationship curve between temperature and the growth increment of the composite prismatic structure was humped‐shaped, with a maximum at 17°C, which was compatible with the results obtained in the field‐collected specimens. In contrast, the thickness of the crossed lamellar structure was constant over the temperature range tested. These results suggest that the composite prismatic structure principally accounts for the thermal dependency of shell growth, and this inference was supported by the finding that shell growth rates were significantly correlated with the thickness of the composite prismatic structure. We also found a negative relationship between the rearing temperature and δ18O of the shell margin, in close quantitative agreement with previous reports. The findings presented here will contribute to the improved age determination of fossil and recent clams based on seasonal microstructural records. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-02T07:19:22.934613-05:
      DOI: 10.1002/2014GC005634
       
  • Age of the Lava Creek supereruption and magma chamber assembly from
           combined 40Ar/39Ar and U‐Pb dating of sanidine and zircon crystals
    • Authors: Naomi E. Matthews; Jorge A. Vazquez, Andrew T. Calvert
      Abstract: The last supereruption from the Yellowstone Plateau formed Yellowstone caldera and ejected the >1000 km3 of rhyolite that composes the Lava Creek Tuff. Tephra from the Lava Creek eruption is a key Quaternary chronostratigraphic marker, in particular for dating the deposition of mid‐Pleistocene glacial and pluvial deposits in western North America. To resolve the timing of eruption and crystallization history for the Lava Creek magma, we performed (1) 40Ar‐39Ar dating of single sanidine crystals to delimit eruption age and (2) ion microprobe U‐Pb and trace‐element analyses of the crystal faces and interiors of single zircons to date the interval of zircon crystallization and characterize magmatic evolution. Sanidines from the two informal members composing Lava Creek Tuff yield a preferred 40Ar/39Ar isochron date of 631.3 ± 4.3 ka. Crystal faces on zircons from both members yield a weighted mean 206Pb/238U date of 626.5 ± 5.8 ka, and have trace element concentrations that vary with the eruptive stratigraphy. Zircon interiors yield a mean 206Pb/238U date of 659.8 ± 5.5 ka, and reveal reverse and/or oscillatory zoning of trace element concentrations, with many crystals containing high U concentration cores that likely grew from highly evolved melt. The occurrence of distal Lava Creek tephra in stratigraphic sequences marking the marine isotope stage 16—15 transition supports the apparent eruption age of ca. 631 ka. The combined results reveal that Lava Creek zircons record episodic heating, renewed crystallization, and an overall up‐temperature evolution for Yellowstone's subvolcanic reservoir in the 103—104 interval before eruption. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-02T06:36:59.453925-05:
      DOI: 10.1002/2015GC005881
       
  • Shock wave synthesis of amino acids from solutions of ammonium formate and
           ammonium bicarbonate
    • Authors: Chizuka Suzuki; Yoshihiro Furukawa, Toshimori Sekine, Hiromoto Nakazawa, Takeshi Kakegawa
      Abstract: The emergence of life's building blocks, such as amino acids and nucleobases, on the prebiotic Earth was a critical step for the beginning of life. Reduced species with low mass, such as ammonia, amines, or carboxylic acids, are potential precursors for these building blocks of life. These precursors may have been provided to the prebiotic ocean by carbonaceous chondrites and chemical reactions related to meteorite impacts on the early Earth. The impact of extraterrestrial objects on Earth occurred more frequently during this period than at present. Such impacts generated shock waves in the ocean, which have the potential to progress chemical reactions to form the building blocks of life from reduced species. To simulate shock‐induced reactions in the prebiotic ocean, we conducted shock‐recovery experiments on ammonium bicarbonate solution and ammonium formate solution at impact velocities ranging from 0.51 to 0.92 km/s. In the products from the ammonium formate solution, several amino acids (glycine, alanine, ß‐alanine, and sarcosine) and aliphatic amines (methylamine, ethylamine, propylamine, and butylamine) were detected, although yields were less than 0.1 mol% of the formic acid reactant. From the ammonium bicarbonate solution, smaller amounts of glycine, methylamine, ethylamine, and propylamine were formed. The impact velocities used in this study represent minimum cases because natural meteorite impacts typically have higher velocities and longer durations. Our results therefore suggest that shock waves could have been involved in forming life's building blocks in the ocean of prebiotic Earth, and potentially in aquifers of other planets, satellites, and asteroids. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-02T04:31:41.68426-05:0
      DOI: 10.1002/2015GC005783
       
  • Appreciation of peer reviewers for 2014
    • Abstract: The editorial and scientific publishing process relies on the sustained work of volunteer reviewers, and evaluating the inter‐disciplinary and broad interest papers published in G‐Cubed can be a particular challenge. As editors and associated editors, we are therefore hugely appreciative of the efforts of our reviewers, and would like to thank and acknowledge them in this editorial. G‐Cubed published 257 manuscripts out of 431 submissions in 2014, and for this we were able to rely on the efforts of 710 dedicated reviewers. Their names are listed below, in italics those 27 who provided three or more reviews. A big thank you from the G‐Cubed team!. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-01T10:36:24.123782-05:
      DOI: 10.1002/2015GC005982
       
  • Lithospheric shear wave velocity and radial anisotropy beneath the
           northern part of North China from surface wave dispersion analysis
    • Authors: Yuanyuan V. Fu; Yuan Gao, Aibing Li, Yutao Shi
      Abstract: Rayleigh and Love wave phase velocities in the northern part of the North China are obtained from ambient noise tomography in the period range of 8 to 35 s and two plane wave earthquake tomography at periods of 20 to 91 s using data recorded at 222 broadband seismic stations from the temporary North China Seismic Array and permanent China Digital Seismic Array. The dispersion curves of Rayleigh and Love wave from 8 to 91 s are jointly inverted for the 3‐D shear wave structure and radial anisotropy in the lithosphere to 140 km depth. Distinct seismic structure are observed from the Fenhe Graben and Taihang Mountain to North China Basin. The North China Basin from the lower crust to the depth of 140 km is characterized by high velocity anomaly, reflecting mafic intrusion and residual materials after the extraction of melt, and by strong radial anisotropy with Vsh > Vsv implying horizontal layering of intrusion and alignment of minerals due to vigorous extensional deformation and subsequent thermal annealing. However, low velocity anomaly and positive radial anisotropy are observed in the Fenhe Graben and Taihang Mountain, suggesting the presence of partial melt in the lithosphere due to the mantle upwelling and horizontal flow pull. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-19T17:51:39.82515-05:0
      DOI: 10.1002/2015GC005825
       
  • Nonlinear attenuation from the interaction between different types of
           seismic waves and interaction of seismic waves with shallow ambient
           tectonic stress
    • Authors: Norman H. Sleep; Nori Nakata
      Abstract: Strong seismic waves bring rock into frictional failure at the uppermost few hundred meters. Numerous small fractures slip with the cumulative effect of anelastic strain and nonlinear attenuation; these fractures should not distinguish between remote sources of stress. Still, frictional failure criteria are not evident especially when seismic waves change the normal traction on fractures. We identify three earthquakes as examples where consideration of interaction among dynamic stresses from different wave types and ambient tectonic stress provides theoretical predictions of nonlinear attenuation that are potentially testable with single station seismograms. For example, because Rayleigh waves produce shallow horizontal dynamic tension and compression, frictional failure should preferentially occur on the tensile half‐cycle if no shallow tectonic stress is present and on the compressional half‐cycle if the tectonic stress is already near thrust‐faulting failure. We observed neither effect on records from the 2011 Mw 9.0 Great Tohoku earthquake. However, Rayleigh waves from this event appear to have brought rock beneath MYGH05 station into frictional failure at ∼10 m depth and thus suppressed high‐frequency S‐waves. The tensile half‐cycle of high frequency P‐waves reduced normal traction on horizontal planes beneath station IWTH25 during the 2008 Mw 6.9 Iwate‐Miyagi earthquake, weakening the rock in shear and suppressing high‐frequency S‐waves. The near‐field velocity pulse from the 1992 Mw 7.3 Landers earthquake brought the uppermost few hundred meters of granite beneath Lucerne station into frictional failure, suppressing high frequency S‐waves. These mildly positive examples support the reality of nonlinear wave interaction, warranting study future strong ground motions. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-19T17:51:00.336745-05:
      DOI: 10.1002/2015GC005832
       
  • Mantle helium along the Newport‐Inglewood fault zone, Los Angeles
           basin, California—A leaking paleo‐subduction zone
    • Authors: J. R. Boles; G. Garven, H. Camacho, J. E. Lupton
      Abstract: Mantle helium is a significant component of the helium gas from deep oil wells along the Newport‐Inglewood fault zone (NIFZ) in the Los Angeles (LA) basin. Helium isotope ratios are as high as 5.3 Ra (Ra=3He/4He ratio of air) indicating 66% mantle contribution, (assuming R/Ra = 8 for mantle), and most values are higher than 1.0 Ra. Other samples from basin margin faults and from within the basin have much lower values (R/Ra 
      PubDate: 2015-06-19T17:50:43.830667-05:
      DOI: 10.1002/2015GC005951
       
  • A community benchmark for viscoplastic thermal convection in a 2‐D
           square box
    • Abstract: Numerical simulations of thermal convection in the Earth's mantle often employ a pseudo‐plastic rheology in order to mimic the plate‐like behavior of the lithosphere. Yet the benchmark tests available in the literature are largely based on simple linear rheologies in which the viscosity is either assumed to be constant or weakly dependent on temperature. Here we present a suite of simple tests based on non‐linear rheologies featuring temperature‐, pressure‐, and strain rate dependent viscosity. Eleven different codes based on the finite volume, finite element, or spectral methods have been used to run five benchmark cases leading to stagnant lid, mobile lid, and periodic convection in a 2‐D square box. For two of these cases, we also show resolution tests from all contributing codes. In addition, we present a bifurcation analysis, describing the transition from a mobile lid regime to a periodic regime, and from a periodic regime to a stagnant lid regime, as a function of the yield stress. At a resolution of around 100 cells or elements in both vertical and horizontal directions, all codes reproduce the required diagnostic quantities with a discrepancy of at most ∼ 3% in the presence of both linear and non‐linear rheologies. Furthermore they consistently predict the critical value of the yield stress at which the transition between different regimes occurs. As the most recent mantle convection codes can handle a number of different geometries within a single solution framework, this benchmark will also prove useful when validating viscoplastic thermal convection simulations in such geometries. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-15T11:26:14.973399-05:
      DOI: 10.1002/2015GC005807
       
  • Variations of the lithospheric strength and elastic thickness in North
           America
    • Authors: Magdala Tesauro; Mikhail K. Kaban, Walter D. Mooney
      Abstract: We evaluate the effect of temperature variations on strength and effective elastic thickness (Te) of the lithosphere of the North American (NA) continent. To this purpose, we use two thermal models that are corrected for compositional variations and anelasticity effects in the upper mantle. These thermal models are obtained from a joint inversion of gravity data and two recent seismic tomography models (NA07 and SL2013sv). The crustal rheology was defined using NACr14, the most recent NA crustal model. This model specifies seismic velocities and thickness for a three‐layer model of the crystalline crust. Strength in the lithosphere and in the crust has similar distributions, indicating that local geotherms play a dominant role in determining strength rather than crustal composition. A pronounced contrast is present in strength between cratonic and off‐cratonic regions. Lithospheric strength in the off‐cratonic regions is prevalently localized within the crust and Te shows low values (150 km). In contrast to previous results, our models indicate that Phanerozoic regions located close to the edge of the cratons, as the Appalachians, are characterized by low strength. We also find that locally weak zones exist within the cratons (e.g., beneath the intracratonic Illinois Basin and Midcontinent rift). Seismic tomography models NA07 and SL2013sv differ mainly in some peripheral parts of the cratons, as the Proterozoic Canadian Platform, the Grenville and the western part of the Yavapai‐Mazatzal province, where the integrated strength for the model NA07 is ten times larger than in model SL2013sv due to a temperature difference (>200˚C) in the uppermost mantle. The differences in Te between the two models are less pronounced. In both models, Proterozoic regions reactivated by Meso‐Cenozoic tectonics (e.g., Rocky Mountains and the Mississippi Embayment) are characterized by a weak lithosphere due to the absence of the mechanically strong part of the mantle lithospheric layer. Intraplate earthquakes are distributed along the edges of the cratons, demonstrating that tectonic stress accumulates there, while the cores of the cratons remain undeformed. In both models intraplate earthquakes occur in weak lithosphere (∼0.5 x10^13 Pa s, Te ∼15 km) or near the edges of strong cratonic blocks, characterized by pronounced contrasts of strength and Te. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:18:36.596803-05:
      DOI: 10.1002/2015GC005937
       
  • What happens to soil organic carbon as coastal marsh ecosystems change in
           response to increasing salinity? An exploration using ramped pyrolysis
           
    • Authors: Elizabeth K Williams; Brad E Rosenheim
      Abstract: Coastal wetlands store vast amounts of organic carbon, globally, and are becoming increasingly vulnerable to the effects of anthropogenic sea‐level rise. To understand the effect of sea‐level rise on organic carbon fate and preservation in this global sink, it is necessary to characterize differences in the biogeochemical stability of coastal wetland soil organic carbon (SOC). Here, we use ramped pyrolysis/oxidation decomposition characteristics as proxies for SOC stability to understand the fate of carbon storage in coastal wetlands comprising the Mississippi River deltaic plain, undergoing rapid rates of local sea level rise. Soils from three wetland types (fresh, brackish, and salt marshes) along a salinity gradient were subjected to ramped pyrolysis analysis to evaluate decomposition characteristics related to thermochemical stability of SOC. At equivalent soil depths, we observed that fresh marsh SOC was more stable than brackish and salt marsh SOC. Depth, isotopic, elemental, and chemical compositions, bulk density, and water content of SOC all exhibited different relationships with SOC stability across the marsh salinity gradient, indicative of different controls on SOC stability within each marsh type. The differences in stability imply stronger preservation potential of fresh marsh soil carbon, compared to that of salt and brackish marshes. Considering projected marsh ecosystem responses to sea‐level rise, these observed stability differences are important in planning and implementing coastal wetland carbon‐focused remediation and improving climate model feedbacks with the carbon cycle. Specifically, our results imply that ecosystem changes associated with sea‐level rise will initiate the accumulation of less stable carbon in coastal wetlands. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:18:21.635345-05:
      DOI: 10.1002/2015GC005839
       
  • Miocene climate change on the Chinese Loess Plateau: Possible links to the
           growth of the northern Tibetan Plateau and global cooling
    • Authors: Youbin Sun; Long Ma, Jan Bloemendal, Steven Clemens, Xiaoke Qiang, Zhisheng An
      Abstract: The evolution of the Asian monsoon‐arid environmental system during the Cenozoic was closely related to the growth of the Himalayan‐Tibetan Plateau and global climate change. However, due to inconsistencies in paleoclimatic reconstructions and to various constraints on the timing of the growth of the Tibetan Plateau, the relative impacts of regional uplift and global cooling on Asian climate change remain controversial. Here we investigate the mineralogical composition of a Miocene Red Clay deposit on the western Chinese Loess Plateau in order to infer changes in chemical weathering and monsoon intensity. Variations of four mineralogical ratios (chlorite/quartz, illite/quartz, calcite/quartz, protodolomite/quartz) reveal that the summer monsoon intensity was relatively strong during the early Miocene (23.5‐18.5 Ma), weakened gradually until ∼9.5 Ma, and strengthened again in the late Miocene. We synthesized previously published thermochronological data from the northeastern Tibetan Plateau and surrounding mountains, and the results suggest that two phases of the rapid growth of northern Tibet occurred around 24‐17 Ma and 13‐7 Ma. Comparison of paleoclimatic proxies and thermochronological data suggests that the gradual weakening of the summer monsoon intensity from 18.5 to 9.5 Ma paralleled global cooling, whereas two intervals of strengthened monsoon in the early and late Miocene were possibly related to the rapid growth of northern Tibet. Our combination of paleoenvironmental proxies and thermochronological data reveals possible links between Miocene Asian monsoon evolution, phased growth of the Tibetan Plateau, and global climate change, and confirms the interconnection of geodynamic and atmospheric processes in the geological past. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:18:05.962522-05:
      DOI: 10.1002/2015GC005750
       
  • Lithospheric structure of the Texas‐Gulf of Mexico passive margin
           from surface wave dispersion and migrated Ps receiver functions
    • Authors: Mohit Agrawal; Jay Pulliam, Mrinal K. Sen, Harold Gurrola
      Abstract: The seismic velocity structure beneath Texas Gulf Coastal Plain (GCP) is imaged by migrating Ps receiver functions with a seismic velocity model found by fitting surface wave dispersion. We use seismic data from a linear array of 22 broadband stations, spaced 16‐20 km apart. A Common Conversion Point (CCP) stacking technique is applied to earthquake data to improve the S/N ratios of receiver functions. Using an incorrect velocity model for depth migration of a stacked CCP image may produce an inaccurate image of the subsurface. To find sufficiently accurate P‐ and S‐velocity models, we first apply a nonlinear modeling technique to fit Rayleigh wave group velocity dispersion via Very Fast Simulated Annealing. Vs ranges from 1.5 km/s in shallow layers of the GCP in to 4.5 km/s beneath the Llano uplift and just outboard of the Balcones Fault Zone (BFZ). The BFZ is characterized by slow velocities that persist to nearly 100 km depth. In the stacked image, the largest‐amplitude positive‐polarity event ranges from the surface, at the Llano uplift, to a maximum depth of ∼ 16 km beneath Matagorda Island. We attribute this event to the sediment‐basement contact, which is expected to produce a large impedance contrast. Another large‐amplitude and positive‐polarity event at ∼35 km depth, which likely marks the Moho, disappears outboard of the Luling Fault Zone. The disappearance of the Moho beneath the GCP may be due to serpentinization of the upper mantle, which would reduce the impedance contrast between the lower crust and upper mantle dramatically. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:17:52.13368-05:0
      DOI: 10.1002/2015GC005803
       
  • New insights into the nature of debris‐avalanche deposits offshore
           Montserrat using Remotely Operated Vehicles (ROVs)
    • Abstract: Submarine landslide deposits have been mapped around many volcanic islands, but interpretations of their structure, composition and emplacement are hindered by the challenges of investigating deposits directly. Here, we report on detailed observations of four landslide deposits around Montserrat collected by Remotely Operated Vehicles, integrating direct imagery and sampling with sediment‐core and geophysical data. These complementary approaches enable a more comprehensive view of large‐scale mass wasting processes around island‐arc volcanoes than has been achievable previously. The most recent landslide occurred at 11.5–14 ka (Deposit 1; 1.7 km3) and formed a radially‐spreading hummocky deposit that is morphologically similar to many subaerial debris‐avalanche deposits. Hummocks comprise angular lava and hydrothermally‐altered fragments, implying a deep‐seated, central subaerial collapse, inferred to have removed a major proportion of lavas from an eruptive period that now has little representation in the subaerial volcanic record. A larger landslide (Deposit 2; 10 km3) occurred at ∼130 ka and transported intact fragments of the volcanic edifice, up to 900 m across and over 100 m high. These fragments were rafted within the landslide, and are best exposed near the margins of the deposit. The largest block preserves a primary stratigraphy of subaerial volcanic breccias, of which the lower parts are encased in hemipelagic mud eroded from the seafloor. Landslide deposits south of Montserrat (Deposits 3 and 5) indicate the wide variety of debris‐avalanche source lithologies around volcanic islands. Deposit 5 originated on the shallow submerged shelf, rather than the terrestrial volcanic edifice, and is dominated by carbonate debris. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:17:43.063172-05:
      DOI: 10.1002/2015GC005781
       
  • Crustal‐scale degassing due to magma system destabilization and
           magma‐gas decoupling at Soufrière Hills Volcano, Montserrat
    • Authors: T.E. Christopher; J. Blundy, K. Cashman, P. Cole, M. Edmonds, P.J. Smith, R.S.J. Sparks, A. Stinton
      Abstract: Activity since 1995 at Soufrière Hills Volcano (SHV), Montserrat has alternated between andesite lava extrusion and quiescence, which are well‐correlated with seismicity and ground deformation cycles. Large variations in SO2 flux do not correlate with these alternations, but high and low HCl/SO2 characterising lava dome extrusion and quiescent periods respectively. Since lava extrusion ceased (February 2010) steady SO2 emissions have continued at an average rate of 374 tonnes/day (± 140 t/d), and incandescent fumaroles (temperatures up to 610oC) on the dome have not changed position or cooled. Occasional short bursts (over several hours) of higher (∼ 10x) SO2 flux have been accompanied by swarms of volcano‐tectonic earthquakes. Strain data from these bursts indicate activation of the magma system to depths up to 10 km. SO2 emissions since 1995 greatly exceed the amounts that could be derived from 1.1 km3 of erupted andesite, and indicating extensive partitioning of sulfur into a vapour phase, as well as efficient decoupling and outgassing of sulfur‐rich gases from the magma. These observations are consistent with a vertically‐extensive, crustal magmatic mush beneath SHV. Three states of the magmatic system are postulated to control degassing. During dormant periods (103 to 104 years) magmatic vapour and melts separate as layers from the mush and decouple from each other. In periods of unrest (years) without eruption, melt and fluid layers become unstable, ascend and can amalgamate. Major destabilisation of the mush system leads to eruption, characterised by magma mixing and release of volatiles with different ages, compositions and sources. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:17:24.359999-05:
      DOI: 10.1002/2015GC005791
       
  • Lithospheric architecture beneath Hudson Bay
    • Authors: Robert W. Porritt; Meghan S. Miller, Fiona A. Darbyshire
      Abstract: Hudson Bay overlies some of the thickest Precambrian lithosphere on Earth, whose internal structures contain important clues to the earliest workings of plate formation. The terminal collision, the Trans‐Hudson Orogen, brought together the Western Churchill craton to the northwest and the Superior craton to the southeast. These two Archean cratons along with the Paleo‐Proterozoic Trans‐Hudson internides, form the core of the North American craton. We use S to P converted wave imaging and absolute shear velocity information from a joint inversion of P to S receiver functions, new ambient noise derived phase velocities, and teleseismic phase velocities to investigate this region and determine both the thickness of the lithosphere and the presence of internal discontinuities. The lithosphere under central Hudson Bay approaches ∼350 km thick but is thinner (∼200‐250 km) around the periphery of the Bay. Furthermore, the amplitude of the LAB conversion from the S receiver functions is unusually large for a craton, suggesting a large thermal contrast across the LAB, which we interpret as direct evidence of the thermal insulation effect of continents on the asthenosphere. Within the lithosphere, mid‐lithospheric discontinuities, significantly shallower than the base of the lithosphere, are often imaged, suggesting the mechanisms that form these layers are common. Lacking time‐history information, we infer that these discontinuities reflect re‐activation of formation structures during deformation of the craton. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:16:40.884185-05:
      DOI: 10.1002/2015GC005845
       
  • Magmatic recharge in continental flood basalts: Insights from the Chifeng
           igneous province in Inner Mongolia
    • Abstract: Eruptive sequences can be used as windows into the thermal and chemical evolution of magma chambers. We examined a continuous vertical section of the Baichahe basalt flow associated with the late Cenozoic Chifeng flood basalt in Inner Mongolia, North China. From oldest to youngest, MgO increases, K2O, light rare earths and other incompatible elements decrease, and Nb/La and radiogenic Pb isotopic ratios increase, all of which indicate increasing primitiveness and decreasing contribution of crustal contamination with time. The variable Pb isotope and incompatible element ratios require a component of crustal contamination, most likely of a lower crustal component (unradiogenic Pb, and low Ce/Pb) in the earliest lavas. Fractional crystallization can explain some of the elemental systematics, but alone cannot explain variable incompatible element ratios and Pb isotopes, nor the temporal trend to more primitive compositions. Crustal assimilation with or without fractional crystallization also cannot explain all the elemental systematics. We find instead that recharge by a primitive magma, in combination with fractional crystallization and decreasing rates of crustal assimilation, is needed to explain the observed geochemical systematics. Our observations suggest that the delivery of fresh basalt to the magma chamber must increase at rates faster than the crust can be assimilated or that the rates of crustal assimilation must decrease. However, progressive addition of primitive magma should heat up the crust and lead to more crustal assimilation. We suggest that during the initial stages of forming a magma chamber, the magma cools and develops an outer crystalline rind of mafic to ultramafic cumulates. This results in a thickening non‐convecting chemical boundary layer, which serves to insulate the magma chamber from further assimilation of crust and cooling, the latter resulting in the reduction of crystallization rates and the buffering of magma compositions at more primitive compositions. We show that certain segments of other large igneous provinces also display an evolution towards more primitive magmas with time, indicating that magmatic recharge may be a common feature of basaltic magma chambers. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-09T00:55:33.881344-05:
      DOI: 10.1002/2015GC005805
       
  • Origins of felsic magmas in Japanese subduction zone: Geochemical
           
    • Abstract: Dacitic to rhyolitic glass shards from 80 widespread tephras erupted during the past 5 Mys from calderas in Kyushu, and SW, central, and NE Japan were analyzed. Laser ablation inductively coupled plasma mass spectrometry was used to determine 10 major and 33 trace elements and 207Pb/206Pb–208Pb/206Pb isotope ratios. The tephras were classified into three major geochemical types and their source rocks were identified as plutonic, sedimentary, and intermediate amphibolite rocks in the upper crust. A few tephras from SW Japan were identified as adakite and alkali rhyolite and were regarded to have originated from slab melt and mantle melt, respectively. The Pb isotope ratios of the tephras are comparable to those of the intermediate lavas in the source areas but are different from the basalts in these areas. The crustal assimilants for the intermediate lavas were largely from crustal melts and are represented by the rhyolitic tephras. A large heat source is required for forming large volumes of felsic crustal melts and is usually supplied by the mantle via basalt. Hydrous arc basalt formed by cold slab subduction is voluminous, and its heat transfer with high water content may have melted crustal rocks leading to effective felsic magma production. Coincidence of basalt and felsic magma activities shown by this study suggests caldera‐forming eruptions are ultimately the effect of a mantle‐driven cause. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-05T03:05:22.453762-05:
      DOI: 10.1002/2015GC005854
       
  • Tectonic evolution of 200 km of Mid‐Atlantic Ridge over 10 million
           years—Interplay of volcanism and faulting
    • Authors: Johnson R Cann; Deborah K. Smith, Javier Escartin, Hans Schouten
      Abstract: We reconstruct the history of the mode of accretion of an area of the Mid‐Atlantic Ridge south of the Kane fracture zone using bathymetric morphology. The area includes 200 km of the spreading axis and reaches to 10 Ma on either side. We distinguish three tectonic styles: 1) volcanic construction with eruption and intrusion of magma coupled with minor faulting, 2) extended terrain with abundant large‐offset faults, 3) detachment faulting marked by extension on single long‐lived faults. Over 40% of the seafloor is made of extended terrain and detachment faults. The area includes products of seven spreading segments. The spreading axis has had detachment faulting or extended terrain on one or both sides for 70% of the last 10 Ma. In some parts of the area, regions of detachment faulting and extended terrain lie close to segment boundaries. Regions of detachment faulting initiated at 10 Ma close to the adjacent fracture zones to the north and south, and then expanded away from them. We discuss the complex evidence from gravity, seismic surveys and bathymetry for the role of magma supply in generating tectonic style. Overall we conclude that input of magma at the spreading axis has a general control on the development of detachment faulting, but the relationship is not strong. Other factors may include a positive feedback that stabilizes detachment faulting at the expense of volcanic extension, perhaps through the lubrication of active detachment faults by the formation of low friction materials (talc, serpentine) on detachment fault surfaces. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-05T02:59:43.509141-05:
      DOI: 10.1002/2015GC005797
       
  • Semiautomatic fracture zone tracking
    • Abstract: Oceanic fracture zone traces are widely used in studies of seafloor morphology and plate kinematics. Satellite altimetry missions have resulted in high‐resolution gravity maps in which all major fracture zones and other tectonic fabric can be identified, and numerous scientists have digitized such lineaments. We have initiated a community effort to maintain low‐cost infrastructure that allows seafloor fabric lineaments to be stored, accessed and updated. A key improvement over past efforts is our processing software (released as a GMT5 supplement) that allows for semi‐automatic corrections to previously digitized fracture zone traces given improved gridded data sets. Here we report on our seafloor fabric processing tools, which complement our database of seafloor fabric lineations, magnetic anomaly identifications and plate kinematic models. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-05T02:59:06.663321-05:
      DOI: 10.1002/2015GC005853
       
  • Pore water geochemistry at two seismogenic areas in the Sea of Marmara
    • Abstract: Within the Sea of Marmara, the highly active North Anatolian Fault (NAF) is responsible for major earthquakes (Mw >=7), and acts as a pathway for fluid migration from deep sources to the seafloor. This work reports on pore water geochemistry from three sediment cores collected in the gulfs of Izmit and Gemlik, along the Northern and the Middle strands of the NAF, respectively. The resulting dataset shows that anaerobic oxidation of methane (AOM) is the major process responsible for sulfate depletion in the shallow sediment. In the Gulf of Gemlik, depth concentration profiles of both sulfate and alkalinity exhibit a kink‐type profile. The Sulfate Methane Transition Zone (SMTZ) is located at moderate depth in the area. In the Gulf of Izmit, the low concentrations observed near the seawater‐sediment interface for sulfate, calcium, strontium and magnesium results from rapid geochemical processes, AOM and carbonate precipitation, occurring in the uppermost part of the sedimentary column and sustained by free methane accumulation. Barite dissolution and carbonate recrystallization have also been identified at deeper depth at the easternmost basin of the Gulf of Izmit. This is supported by the profile of the strontium isotope ratios (87Sr/86Sr) as a function of depth which exhibits negative anomalies compared to the modern seawater value. The strontium isotopic signature also shows that these carbonates had precipitated during the reconnection of the Sea of Marmara with the Mediterranean Sea. Finally, a first attempt to interpret the sulfate profiles observed in the light of the seismic activity at both sites is presented. We propose the hypothesis that seismic activity in the areas is responsible for the transient sulfate profile, and that the very shallow SMTZ depths observed in the Gulf of Izmit is likely due to episodic release of significant amount of methane. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-03T03:30:03.089073-05:
      DOI: 10.1002/2015GC005798
       
  • Magmatic water contents determined through clinopyroxene: Examples from
           the Western Canary Islands, Spain
    • Abstract: Water is a key parameter in magma genesis, magma evolution, and resulting eruption styles, because it controls the density, the viscosity, as well as the melting and crystallization behavior of a melt. The parental water content of a magma is usually measured through melt inclusions in minerals such as olivine, a method which may be hampered, however, by the lack of melt inclusions suitable for analysis, or post‐entrapment changes in their water content. An alternative way to reconstruct the water content of a magma is to use nominally anhydrous minerals (NAMs), such as pyroxene, which take up low concentrations of hydrogen as a function of the magma's water content. During magma degassing and eruption, however, NAMs may dehydrate. We therefore tested a method to reconstruct the water contents of dehydrated clinopyroxene phenocrysts from the Western Canary islands (n=28) through re‐hydration experiments followed by infrared and Mössbauer spectroscopy. Employing currently available crystal/melt partitioning data, the results of the experiments were used to calculate parental water contents of 0.71 ±0.07 to 1.49 ±0.15 wt. % H2O for Western Canary magmas during clinopyroxene crystallization at upper mantle conditions. This H2O range is in agreement with calculated water contents using plagioclase‐liquid‐hygrometry, and with previously published data for mafic lavas from the Canary Islands and comparable ocean island systems elsewhere. Utilizing NAMs in combination with hydrogen treatment can therefore serve as a proxy for pre‐eruptive H2O contents, which we anticipate becoming a useful method applicable to mafic rocks where pyroxene is the main phenocryst phase. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-03T02:54:40.735061-05:
      DOI: 10.1002/2015GC005800
       
  • Effects of simple acid leaching of crushed and powdered geological
           materials on high‐precision Pb isotope analyses
    • Authors: Erin Todd; Andreas Stracke, Erik E. Scherer
      Abstract: We present new results of simple acid leaching experiments on the Pb isotope composition of USGS standard reference material powders and on ocean island basalt whole rock splits and powders. Rock samples were leached with cold 6N HCl in an ultrasonic bath, then on a hot plate, and washed with ultrapure H2O before sample digestion in HF‐HNO3 and chromatographic purification of Pb. Lead isotope analyses were measured by Tl‐doped MC‐ICPMS. Intra‐ and inter‐session analytical reproducibilities of repeated analyses of both synthetic Pb solutions and Pb from single digests of chemically processed natural samples were generally better than 100 ppm (2 S.D.). The comparison of leached and unleached samples shows that leaching consistently removes variable amounts of contaminants that differ in Pb isotopic composition for different starting materials. For repeated digests of a single sample, analyses of leached samples reproduce better than those of unleached ones, confirming that leaching effectively removes most of the heterogeneously distributed extraneous Pb. Nevertheless, the external reproducibility of leached samples is still up to an order of magnitude worse than that of Pb solution standards (∼100 ppm). More complex leaching methods employed by earlier studies yield Pb isotope ratios within error of those produced by our method and at similar levels of reproducibility, demonstrating that our simple leaching method is as effective as more complex leaching techniques. Therefore, any Pb isotope heterogeneity among multiple leached digests of samples in excess of the external reproducibility is attributed to inherent isotopic heterogeneity of the sample. The external precision of ∼100 ppm (2 S.D.) achieved for Pb isotope ratio determination by Tl‐doped MC‐ICPMS is thus sufficient for most rocks. The full advantage of the most precise Pb isotope analytical methods is only realized in cases where the natural isotopic heterogeneity among samples in a studied suite is substantially below 100 ppm. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-28T10:32:02.269031-05:
      DOI: 10.1002/2015GC005804
       
  • Paleomagnetic evidence for vertical‐axis rotations of crustal blocks
           in the Woodlark Rift, SE Papua New Guinea: Miocene to Present‐Day
           kinematics in one of the world's most rapidly extending plate boundary
           zones
    • Authors: Elizabeth A. Cairns; Timothy A. Little, Gillian M. Turner, Laura M. Wallace, Susan Ellis
      Abstract: The continental Woodlark Rift, in SE Papua New Guinea lies west of a propagating oceanic spreading centre in the Woodlark Basin, and is currently one of few places on Earth where active continental breakup is thought to be occurring. Here, north‐south extension is localized on a few major normal faults. We determined characteristic remanent magnetization (ChRM) components from demagnetization profiles of >300 individual specimens. From these, 157 components contribute to paleomagnetic directions for six formations. We compare Early Miocene (∼20 Ma) to Late Pliocene (3.0 ± 0.5) ChRM mean directions, at four localities, with contemporaneous expected field directions corresponding to the Australian Plate. Time‐varying finite rotations from Cape Vogel Peninsula (28‐12°) suggest anticlockwise rotation had begun by ∼15 Ma. This rotation may have been accompanied by rifting, ∼7 Ma earlier than previously inferred. Furthermore that early extension may have occurred south of the present rift, and that deformation later migrated north of the Peninsula. Pliocene vertical‐axis rotations are consistent with GPS‐determined plate motions, suggesting that contemporary rift kinematics were established by ∼3 Ma. Finite anticlockwise rotation (10.1 ± 7.6°) in the Amphlett Islands is accordant with sea floor spreading in the Woodlark Basin, suggesting this locality has seen the full Woodlark plate motion since 3 Ma. Clockwise rotation of the Goodenough Bay Block (‐6.5 ± 11.2°) since the Late Miocene has accomplished transfer of deformation between major extensional corridors, and an especially rapid local rotation (‐16.3 ± 9.5°) in NW Normanby Island may suggest an incipient dextral transfer fault. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-28T10:31:40.868853-05:
      DOI: 10.1002/2015GC005788
       
  • Geochemical heterogeneities in magma beneath Mount Etna recorded by
           2001‐2006 melt inclusions
    • Authors: Federica Schiavi; Alberto Rosciglione, Hiroshi Kitagawa, Katsura Kobayashi, Eizo Nakamura, Pasquale Mario Nuccio, Luisa Ottolini, Antonio Paonita, Riccardo Vannucci
      Abstract: We present a geochemical study on olivine‐ and clinopyroxene‐hosted melt inclusions (MIs) from 2001‐2006 Etna basaltic lavas and pyroclastites. Three MI suites are distinguished on the basis of trace element fingerprinting. Type‐1 MIs (from 2001 Upper South and 2002 Northeast vents) share their trace element signature with low‐K lavas erupted before 1971. Critical trace element ratios (e.g., K/La, Ba/Nb), along with Pb isotope data of Type‐1 MIs provide evidence for a heterogeneous mantle source resulting from mixing of three end‐members with geochemical and isotopic characteristics of EM2, DMM and HIMU components. Type‐1 MIs composition does not support involvement of subduction‐related components. Type‐2 (from 2001 Lower and 2002 South vents) and Type‐3 (2004 eruption) MIs reveal “ghost plagioclase signatures”, namely lower concentrations in strongly incompatible elements, and positive Sr, Ba and Eu anomalies. Both Type‐1 and Type‐2 MIs occur in 2006 olivines, which highlight the occurrence of mixing between Type‐1 and Type‐2 end‐members. Type‐2/Type‐3 MIs testify to en‐route processes (plagioclase assimilation and volatile fluxing) peculiar for “deep dike fed” eruptions. The latter are strongly controlled by tectonics or flank instability that occasionally promote upraise of undegassed, more radiogenic primitive magma, which may interact with plagioclase‐rich crystal mush/cumulates before erupting. Type‐2/Type‐3 MIs approach the less radiogenic Pb isotopic composition of plagioclase from prehistoric lavas, thus suggesting geochemical overprinting of present‐day melts by older products released from distinct mantle sources. Our study emphasizes that MIs microanalysis offers new insights on both source characteristics and en‐route processes, allowing to a link between melt composition and magma dynamics. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-21T18:18:13.685358-05:
      DOI: 10.1002/2015GC005786
       
  • Strain decoupling reveals variable seismogenic risk in SE Japan (Nankai
           Trough)
    • Authors: James Van Tuyl; Tiago M. Alves, Gregory F. Moore
      Abstract: The determination of in situ stress states is vital in understanding the behavior of faults and subsequent seismogenesis of accretionary prisms. In this paper, a high quality 3D seismic volume is used to map the depth of the extensional‐compressional decoupling (ECD) boundary in the accretionary prism of Nankai, with the prior knowledge that strike‐slip and compressional stresses occur deeper than 1250 meters below seafloor (mbsf) in the Kumano Basin, changing to extension towards the seafloor. A total of 1108 faults from the accretionary prism are analyzed to estimate paleostresses via fault inversion and slip tendency techniques. A key result is this paper is that the ECD boundary can be used as a proxy to identify active structures on accretionary prisms as its depth depends on: a) local tectonic uplift in areas adjacent to active faults, and b) on the thickness of sediment accumulated above active thrust anticlines. The depth of the ECD boundary ranges from 0 to ∼650 mbsf, being notably shallower than in the Kumano Basin. In Nankai, frontal regions of the imbricate thrust zone, and the megasplay fault zone, reveal the shallower ECD depths and correlate with the regions where faulting is most active. As a corollary, this work confirms that estimates of stress state variability based on the analysis of 3D seismic data are vital to understand the behavior of faults and potential seismogenic regions on convergent margins. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-18T04:01:39.566209-05:
      DOI: 10.1002/2015GC005778
       
 
 
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