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  Subjects -> EARTH SCIENCES (Total: 586 journals)
    - EARTH SCIENCES (433 journals)
    - GEOLOGY (65 journals)
    - GEOPHYSICS (27 journals)
    - HYDROLOGY (17 journals)
    - OCEANOGRAPHY (44 journals)

EARTH SCIENCES (433 journals)                  1 2 3 4 5 | Last

Acta Geodaetica et Geophysica     Hybrid Journal   (Followers: 1)
Acta Geodaetica et Geophysica Hungarica     Full-text available via subscription   (Followers: 2)
Acta Geophysica     Hybrid Journal   (Followers: 7)
Acta Geotechnica     Hybrid Journal   (Followers: 8)
Acta Meteorologica Sinica     Hybrid Journal   (Followers: 2)
Advances in High Energy Physics     Open Access   (Followers: 12)
Advances In Physics     Hybrid Journal   (Followers: 6)
Aeolian Research     Hybrid Journal   (Followers: 1)
African Journal of Aquatic Science     Hybrid Journal   (Followers: 13)
Algological Studies     Full-text available via subscription   (Followers: 2)
Alpine Botany     Hybrid Journal   (Followers: 2)
AMBIO     Hybrid Journal   (Followers: 13)
Anales del Instituto de la Patagonia     Open Access   (Followers: 2)
Andean geology     Open Access   (Followers: 5)
Annales Henri Poincaré     Hybrid Journal   (Followers: 2)
Annales UMCS, Geographia, Geologia, Mineralogia et Petrographia     Open Access   (Followers: 1)
Annals of Geophysics     Full-text available via subscription   (Followers: 9)
Annals of GIS     Hybrid Journal   (Followers: 14)
Annals of Glaciology     Full-text available via subscription  
Annual Review of Marine Science     Full-text available via subscription   (Followers: 9)
Anthropocene Review     Hybrid Journal  
Applied Clay Science     Hybrid Journal   (Followers: 2)
Applied Geochemistry     Hybrid Journal   (Followers: 6)
Applied Geomatics     Hybrid Journal   (Followers: 6)
Applied Geophysics     Hybrid Journal   (Followers: 6)
Applied Ocean Research     Hybrid Journal   (Followers: 6)
Applied Petrochemical Research     Open Access   (Followers: 3)
Applied Remote Sensing Journal     Open Access   (Followers: 8)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 20)
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 7)
Artificial Satellites     Open Access   (Followers: 13)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 2)
Asian Journal of Earth Sciences     Open Access   (Followers: 17)
Atlantic Geology : Journal of the Atlantic Geoscience Society / Atlantic Geology : revue de la Société Géoscientifique de l'Atlantique     Full-text available via subscription   (Followers: 3)
Atmosphere-Ocean     Full-text available via subscription   (Followers: 3)
Atmospheric and Climate Sciences     Open Access   (Followers: 14)
Australian Journal of Earth Sciences: An International Geoscience Journal of the Geological Society of Australia     Hybrid Journal   (Followers: 11)
Boletim de Ciências Geodésicas     Open Access  
Boreas: An International Journal of Quaternary Research     Hybrid Journal   (Followers: 8)
Bragantia     Open Access   (Followers: 2)
Bulletin of Earthquake Engineering     Hybrid Journal   (Followers: 9)
Bulletin of Geosciences     Open Access   (Followers: 8)
Bulletin of Marine Science     Full-text available via subscription   (Followers: 13)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (Followers: 1)
Bulletin of the Seismological Society of America     Full-text available via subscription   (Followers: 15)
Bulletin of Volcanology     Hybrid Journal   (Followers: 11)
Canadian Journal of Plant Science     Full-text available via subscription   (Followers: 10)
Canadian Mineralogist     Full-text available via subscription   (Followers: 1)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 17)
Carbonates and Evaporites     Hybrid Journal   (Followers: 3)
CATENA     Hybrid Journal   (Followers: 3)
Central European Journal of Geosciences     Hybrid Journal   (Followers: 5)
Central European Journal of Physics     Hybrid Journal   (Followers: 1)
Chemical Geology     Hybrid Journal   (Followers: 9)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 3)
Chinese Geographical Science     Hybrid Journal   (Followers: 3)
Chinese Journal of Geochemistry     Hybrid Journal   (Followers: 2)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 3)
Ciencia del suelo     Open Access  
Climate and Development     Hybrid Journal   (Followers: 9)
Coastal Management     Hybrid Journal   (Followers: 18)
Comptes Rendus Geoscience     Full-text available via subscription   (Followers: 5)
Computational Geosciences     Hybrid Journal   (Followers: 10)
Computational Mathematics and Mathematical Physics     Hybrid Journal   (Followers: 1)
Computers and Geotechnics     Hybrid Journal   (Followers: 5)
Contemporary Trends in Geoscience     Open Access  
Continental Journal of Earth Sciences     Open Access   (Followers: 2)
Continental Shelf Research     Hybrid Journal   (Followers: 7)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 7)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 2)
Coral Reefs     Hybrid Journal   (Followers: 18)
Cretaceous Research     Hybrid Journal   (Followers: 4)
Cybergeo : European Journal of Geography     Open Access   (Followers: 4)
Developments in Geotectonics     Full-text available via subscription   (Followers: 2)
Developments in Quaternary Science     Full-text available via subscription   (Followers: 3)
Développement durable et territoires     Open Access   (Followers: 2)
Diatom Research     Hybrid Journal  
Doklady Physics     Hybrid Journal   (Followers: 1)
Dynamics of Atmospheres and Oceans     Hybrid Journal   (Followers: 3)
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (Followers: 135)
Earth Interactions     Full-text available via subscription   (Followers: 8)
Earth Science Research     Open Access   (Followers: 6)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 1)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 11)
Earth System Dynamics     Open Access   (Followers: 4)
Earth System Dynamics Discussions     Open Access   (Followers: 3)
Earth's Future     Open Access   (Followers: 1)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 7)
Earthquake Spectra     Full-text available via subscription   (Followers: 11)
Ecohydrology     Hybrid Journal   (Followers: 11)
Ecological Questions     Open Access   (Followers: 6)
Electromagnetics     Hybrid Journal   (Followers: 1)
Energy Efficiency     Hybrid Journal   (Followers: 9)
Energy Exploration & Exploitation     Full-text available via subscription   (Followers: 4)
Environmental Earth Sciences     Hybrid Journal   (Followers: 10)
Environmental Geology     Hybrid Journal   (Followers: 11)
Environmental Geosciences     Full-text available via subscription   (Followers: 4)
Environmental Geotechnics     Open Access  
Erwerbs-Obstbau     Hybrid Journal  

        1 2 3 4 5 | Last

Journal Cover Chemical Geology
   [11 followers]  Follow    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
     ISSN (Print) 0009-2541
     Published by Elsevier Homepage  [2563 journals]   [SJR: 1.714]   [H-I: 111]
  • Determination of methane content in NaCl–H2O fluid inclusions by
           Raman spectroscopy. Calibration and application to the external part of
           the Central Alps (Switzerland)
    • Abstract: Publication date: 15 June 2014
      Source:Chemical Geology, Volumes 378–379
      Author(s): Marie-Camille Caumon , Pascal Robert , Emmanuel Laverret , Alexandre Tarantola , Aurélien Randi , Jacques Pironon , Jean Dubessy , Jean-Pierre Girard
      The reconstruction of the entrapment conditions of geological fluids requires determining the volumetric and composition properties of the inclusions containing these fluids. In some cases, the analytical data necessary for PVTX determination cannot be obtained from microthermometry. The quantification of the dissolved gases in aqueous fluid inclusions by Raman spectroscopy, following proper calibration of the instrument and methodology, can provide alternative data. In the present study, the intensity of the Raman signal of the symmetric stretching vibrational mode of methane was calibrated in order to (1) determine CH4 concentration in pure and saline water and (2) quantify the molar fractions of H2O and CH4 in the gas phase. High-pressure optical cell (HPOC), i.e. a pressurization system connected to a silica microcapillary heated on a customized heating–cooling stage, reveals to be much more convenient and accurate than synthetic fluid inclusions. Moreover, a wide range of pressure, temperature, and salinity can be covered by this methodology. Over 1000 measurements were produced to define the calibration curves, covering the ranges in temperature, pressure and salinity of 60–180°C, 30–1000bar, and 0–4mol.kg−1 NaCl, respectively, which corresponds to a CH4 molality up to 0.6mol.kg−1. The CH4 solubility vs. CH4/H2O Raman peak area ratio was fitted by a second-order polynomial curve (R2 =0.996). In the gas phase, the molar fraction of H2O vs. Raman peak area ratio is fitted by a straight line (R2 =0.990). The calibration was applied to a set of natural fluid inclusions trapped within late quartz Alpine fissure of the external part of the Central Alps (Switzerland). The determination of CH4 concentration in the studied fluids provided valuable insight on conditions of trapping and on the pressure regimes prevailing in this low-grade metamorphic setting.
      Graphical abstract image

      PubDate: 2014-06-14T15:32:48Z
       
  • Pseudomorphic replacement of diopside during interaction with (Ni,Mg)Cl2
           aqueous solutions: Implications for the Ni-enrichment mechanism in talc-
           and serpentine-type phases
    • Abstract: Publication date: 25 July 2014
      Source:Chemical Geology, Volume 380
      Author(s): Alik S. Majumdar , Helen E. King , Timm John , Christof Kusebauch , Andrew Putnis
      A hydrothermal experimental study of diopside interaction with (Ni,Mg)Cl2 aqueous solutions has been carried out to clarify the replacement mechanism and pattern of element mobilization and its relevance to peridotite alteration. Three different solution compositions were used with Ni/Mg ratios of 0, 0.5, and 1, respectively. Experiments were carried out in cold-seal pressure-vessels at 300–600°C and 100MPa pressure for 15days in gold capsules. For 600°C NiCl2 experiments, the solutions were also spiked with 50% H2 18O to study the behavior of isotopic exchange and hence the replacement mechanism. The diopside–pseudomorph interface is compositionally and structurally sharp. After the MgCl2 experiments pure talc and/or serpentine comprised the replacement phases, whereas reaction in the Ni-bearing fluids produced talc–willemseite and/or lizardite–nepouite series compositions. Additionally, a complex rim of Ni-poor and Ni-rich regions developed in the NiCl2 and (Ni,Mg)Cl2 experiments. Raman spectroscopy of the Ni-rich talc from the experiment with 18O-enriched solution showed a shift of ~10±1cm−1 for the SiO(bridging)Si band towards lower wavenumbers relative to unspiked solution experiments, indicating that 18O was incorporated into the silicate framework; thus the reaction progressed via an interface-coupled dissolution-precipitation mechanism. Overgrowths of an olivine-type phase at 500–600°C and serpentine-type phases in 300–400°C experiments were also observed. For a simple binary composition (e.g. Ni–Mg) of the interacting solution, the fluid composition (different Ni/Mg ratio) at the reacting interface and its silica activity has a strong control on the phase precipitated. The sequential formation of a Ni-poor inner phase and Ni-rich outer phase indicates a stepwise increase in Ni incorporation within the neo-formed phase that is representative of a cyclic dissolution-precipitation process during supergene enrichment of Ni in natural ore deposits. Additionally, Ca transport occurs in the opposite direction to Ni during alteration, which may be important as a geochemical tracer for economic Ni-deposits and as a source of Ca for the rodingitization processes that are often associated with peridotite occurrences.


      PubDate: 2014-06-14T15:32:48Z
       
  • Reduction of OH contamination in quantification of water contents using
           NanoSIMS imaging
    • Abstract: Publication date: 25 July 2014
      Source:Chemical Geology, Volume 380
      Author(s): Alice Stephant , Laurent Remusat , Aurélien Thomen , François Robert
      Quantification of water content is relevant in various topics in geology and planetary sciences. NanoSIMS has capabilities for high spatial resolution imaging and offers opportunities to accurately quantify water contents at fine scale on small surface areas. The main concern using ion microprobe techniques is to estimate and minimize contribution of water contamination, from residual gas in the sample chamber, sticking onto the surface of the sample. Here we tackle a set of sputtering/analytical parameters and we evaluate their relative influence on the OH−/Si− ratio. We demonstrate that a high erosion rate, reached using a primary beam intensity of ~25pA, is sufficient to lower this OH contamination for basaltic glass. This leads us to describe a procedure to correct for OH contamination and thus determine accurate values of OH/Si ratio in order to quantify water contents in silicate materials using NanoSIMS imaging.


      PubDate: 2014-06-14T15:32:48Z
       
  • Geochemistry of the Abulangdang intrusion: Cumulates of high-Ti picritic
           magmas in the Emeishan large igneous province, SW China
    • Abstract: Publication date: 15 June 2014
      Source:Chemical Geology, Volumes 378–379
      Author(s): Christina Yan Wang , Mei-Fu Zhou , Shenghong Yang , Liang Qi , Yali Sun
      The ~260Ma Abulangdang ultramafic intrusion in the Panxi region, SW China, is part of the Emeishan large igneous province (ELIP). The intrusion is composed of dunite and lherzolite with minor olivine gabbro. Olivine crystals in the dunite have Fo values from 89.0 to 86.5mol%. Chromite grains in both the dunite and lherzolite contain more than 0.5wt.% TiO2 and show a trend of increasing Fe3+ with increasing Fe2+/(Mg+Fe2+). Rocks of the intrusion have γOs(t) values from +0.1 to +1.2 and εNd(t) values from −1.9 to +2.9, similar to the high-Ti picrites, high-Ti flood basalts and Fe–Ti oxide-bearing, gabbroic intrusions of the ELIP. Modeling indicates that the Abulangdang intrusion formed by accumulation of olivine and chromite from a high-Ti picritic magma, whereas the high-Ti flood basalts and Fe–Ti oxide-bearing, gabbroic intrusions formed from derivative, evolved magmas after the early fractionation of minerals from the high-Ti picritic magma. Both the dunite and lherzolite have relatively high PGE concentrations and primitive mantle-normalized chalcophile element patterns with positive Os and Ru anomalies, in contrast to PGE-poor high-Ti flood basalts with negative Os and Ru anomalies. This is consistent with the retention of Ru–Os–Ir trace phases with olivine and chromite in the formation of the Abulangdang intrusion. Using a clinopyroxene geobarometer, the crystallization pressure of the Abulangdang intrusion is calculated to be 7.1 to 8.3kbar, equivalent to a depth of ~21 to 24km, which is deeper than that estimated for the Fe–Ti oxide-bearing, gabbroic layered intrusions of the ELIP (~5kbar). We propose that large volumes of mantle plume-derived high-Ti picritic magma underplating along the Moho boundary underwent early fractionation before eruption, and some high-Ti picritic magma ascended through a fossil conduit where the Abulangdang intrusion formed. Residual, evolved magmas either from the conduits or underplating magma chamber formed Fe–Ti oxide-bearing, gabbroic layered intrusions or high-Ti flood basalts. In this fashion, the Abulangdang intrusion resembles ultramafic portions that are missing in the Fe–Ti oxide-bearing, gabbroic intrusions of the ELIP.


      PubDate: 2014-06-14T15:32:48Z
       
  • Editorial Board
    • Abstract: Publication date: 15 June 2014
      Source:Chemical Geology, Volumes 378–379




      PubDate: 2014-06-14T15:32:48Z
       
  • Nanogeochemistry: Nanostructures, emergent properties and their control on
           geochemical reactions and mass transfers
    • Abstract: Publication date: 15 June 2014
      Source:Chemical Geology, Volumes 378–379
      Author(s): Yifeng Wang
      Nanogeochemistry—a newly emerging research field—attempts to understand geochemical reactions and mass transfers at nanometer scales, especially with regards to the formation of nanostructures in geochemical systems, emergent properties of these structures, and their controls on geochemical processes. The research also includes use of nanotechnology to design new materials and engineering approaches for effective natural resource extraction and environmental management. At the core of this new research field is the concept that, as the size of a material is reduced to nanometers, novel physical or chemical properties of the material may emerge that can be drastically different from those of the corresponding bulk phase and the material properties then become size-dependent. Nanostructures, which frequently occur in geologic materials, may directly control mineral phase stability, mineral–water interface chemistry, geochemical reaction kinetics, geo-fluid migration and transport, and even global biogeochemical cycles as a whole. This paper aims to provide a comprehensive review of recent progress in nanogeochemical research. The review is focused on two general types of nanostructures—nano solid phases and nanopores (nanofluids)—with an emphasis on the occurrence of each nanostructure in natural environments, the associated emergent properties, and the potential geochemical implications. Stemming from an increasing interest in shale gas research, a special discussion is provided on gas/oil disposition and migration in unconventional low-permeability reservoirs, wherein shale is treated as a nanocomposite material. Nanogeochemistry is a relatively young research field, and much remains to be explored. There is an urgent need for systematically characterizing specific nanostructures over the whole nanometer-size range and developing a general theoretical framework for data analysis and synthesis. There is also a need for developing experimental and modeling techniques to extrapolate the knowledge obtained from simple model systems to complex natural systems.


      PubDate: 2014-06-14T15:32:48Z
       
  • Si stable isotope fractionation during adsorption and the competition
           between kinetic and equilibrium isotope fractionation: Implications for
           weathering systems
    • Abstract: Publication date: 25 July 2014
      Source:Chemical Geology, Volume 380
      Author(s): Marcus Oelze , Friedhelm von Blanckenburg , Daniel Hoellen , Martin Dietzel , Julien Bouchez
      The adsorption of Si onto amorphous Al-hydroxides is the cause for the light Si isotope signature that secondary crystalline clay minerals in weathering systems carry. We propose this hypothesis from a series of adsorption experiments in which the light isotopes are being favored during Si adsorption onto crystalline gibbsite and in which the associated fractionation factor depends on the solution's initial Si concentration. Three adsorption experiments were carried out at pH7 with different initial Si concentrations of 0.36, 0.71 and 1.42mmol/l Si start concentrations. As Al-hydroxide adsorbent, 30g/l crystalline gibbsite were used to provide equal surface area in all experiments. Adsorption rates are higher with higher initial Si concentration. At the same time, calculated apparent isotope fractionation factors 103 ln α adsorbed/solution decrease from −1.8 to −3‰ with increasing initial Si concentration. As care was taken to avoid isotope fractionation during transport of dissolved Si to the gibbsite surface, the mass dependence of the activation energy barrier at the interface is causing the kinetic isotope fractionation. Within the mass balance framework of DePaolo (2011) the shift in Si isotope fractionation with initial Si concentration is interpreted to be induced by different kinetic isotope fractionation factors associated with the forward reaction. Only after ca. two months do the isotope ratios begin to adjust to an equilibrium isotope fractionation factor that is close to 0‰. With such slow re-equilibration Si adsorption differs fundamentally from transition metals that re-equilibrate isotopically within hours after adsorption onto Fe and Mn oxide surfaces. These observations may provide an explanation for the light Si isotope signature clay minerals formed during weathering carry: the light Si isotope composition is being inherited early on during Si adsorption onto amorphous Al-hydroxides and is potentially carried over during all further stages of transformation.


      PubDate: 2014-06-14T15:32:48Z
       
  • Partial melting control of water contents in the Cenozoic lithospheric
           mantle of the Cathaysia block of South China
    • Abstract: Publication date: 25 July 2014
      Source:Chemical Geology, Volume 380
      Author(s): Yantao Hao , Qunke Xia , Qiwen Li , Huan Chen , Min Feng
      Major and trace element and the H2O contents of minerals in peridotite xenoliths hosted by the Cenozoic basalts of Jiande in the Cathaysia block were evaluated using electron microprobe, laser-ablation ICP-MS and Fourier transform infrared spectroscopy, respectively. The correlations among the major elements of the minerals define a melting trend, and modeling of the Y and Yb contents in the clinopyroxenes indicates that the degree of partial melting ranges from 1% to 15%. Most samples (22 out of 29) show depleted chondrite-normalized rare earth element patterns and a degree of partial melting <4%. The H2O contents (weight in ppm) of the clinopyroxenes, orthopyroxenes and olivines are 390–590ppm, 160–330ppm and ∼0ppm, respectively. Although potential H-loss during xenolith ascent cannot be excluded for olivines, pyroxenes largely preserve the H2O content they have in the mantle prior to sampling by the host basalts, as inferred from (1) the homogenous H2O content within single pyroxene grains, and (2) the equilibrium H2O partitioning between the clinopyroxene and orthopyroxene. Based on the mineral modes and assuming a partition coefficient of 10 for H2O between the clinopyroxene and olivine, the calculated whole-rock H2O contents range from 90 to 220ppm, similar to that of the MORB source. When combined with previously reported data for peridotites hosted by Cenozoic basalts at other localities of the Cathaysia block, the correlation of H2O content with melting index (such as Yb content in cpx, Cr# in spinel) suggests that extent of partial melting is the main factor controlling H2O abundance in these rocks. Nevertheless, the variations in whole-rock H2O contents cannot be perfectly modeled as a simple modal melting process using available experimental partition coefficients of H2O between peridotite and melt. The lack of coherent variations between the H2O contents of the whole rocks and the metasomatic index (La/Yb ratio in clinopyroxene) indicates that mantle metasomatism did not modify the initial H2O contents after the melting event(s). Based on the similarities in major and trace elements, and of H2O contents in the Cathaysia block peridotites and those inferred for the MORB source, we propose that the Cenozoic lithospheric mantle of the Cathaysia block is accreted from the upwelled and cooled asthenospheric mantle. In addition, the H2O contents of the Cenozoic lithospheric mantle of the Cathaysia block are much higher than those of the North China Craton of similar fertility (100ppm vs. 20ppm for whole rock H2O contents), where lithospheric thinning occurred during the Mesozoic. This implies that either the Cathaysia block did not undergo similar lithospheric thinning or that the mechanism of the lithospheric thinning was different.


      PubDate: 2014-06-14T15:32:48Z
       
  • Experimental dissolution of dolomite by CO2-charged brine at 100°C
           and 150bar: Evolution of porosity, permeability, and reactive surface area
           
    • Abstract: Publication date: 25 July 2014
      Source:Chemical Geology, Volume 380
      Author(s): Andrew J. Luhmann , Xiang-Zhao Kong , Benjamin M. Tutolo , Nagasree Garapati , Brian C. Bagley , Martin O. Saar , William E. Seyfried Jr.
      Hydrothermal flow experiments of single-pass injection of CO2-charged brine were conducted on nine dolomite cores to examine fluid–rock reactions in dolomite reservoirs under geologic carbon sequestration conditions. Post-experimental X-ray computed tomography (XRCT) analysis illustrates a range of dissolution patterns, and significant increases in core bulk permeability were measured as the dolomite dissolved. Outflow fluids were below dolomite saturation, and cation concentrations decreased with time due to reductions in reactive surface area with reaction progress. To determine changes in reactive surface area, we employ a power-law relationship between reactive surface area and porosity (Luquot and Gouze, 2009). The exponent in this relationship is interpreted to be a geometrical parameter that controls the degree of surface area change per change in core porosity. Combined with XRCT reconstructions of dissolution patterns, we demonstrate that this exponent is inversely related to both the flow path diameter and tortuosity of the dissolution channel. Even though XRCT reconstructions illustrate dissolution at selected regions within each core, relatively high Ba and Mn recoveries in fluid samples suggest that dissolution occurred along the core's entire length and width. Analysis of porosity–permeability data indicates an increase in the rate of permeability enhancement per increase in porosity with reaction progress as dissolution channels lengthen along the core. Finally, we incorporate the surface area–porosity model of Luquot and Gouze (2009) with our experimentally fit parameters into TOUGHREACT to simulate experimental observations.


      PubDate: 2014-06-14T15:32:48Z
       
  • In situ speciation of sulfur vapors by X-ray absorption near edge
           structure spectroscopy
    • Abstract: Publication date: 25 July 2014
      Source:Chemical Geology, Volume 380
      Author(s): Annette Summers Engel , Gudrun Lisa Bovenkamp , Alexander Prange , Josef Hormes
      Despite the tremendous importance for geologic systems, there is limited knowledge of sulfur-containing gas-phase geochemical reactions that take place at elevated temperatures, including up to 1000K. This deficit is at least partly caused by a lack of suitable experimental techniques to monitor and quantify potential reactions. We developed a new furnace design that can heat solid samples to specific temperatures while in situ X-ray absorption spectra of the gas-phase species are collected at the sulfur K-edge at about 2400eV for X-ray absorption near edge structure (XANES) spectroscopy. Our experimental design improves previous furnace systems developed to generate sulfur vapors for spectrometry measurements and earlier sulfur XANES spectroscopic analyses of sulfur-containing gases and vapors. The new design minimizes contamination by air, water vapor, and stainless steel. Our results demonstrate that vapors emitted from elemental sulfur heated to different temperatures are similar to previous findings, but we can now resolve long-held interpretative discrepancies for sulfur vapor speciation, some of which resulted from the misidentification of vapors due to reactions between the sulfur vapors and furnace materials. We also used the new design to quantify the sulfur vapor species produced from elemental sulfur reacted with water over a range of temperatures. At low temperatures, like 363K (~90°C), the gas consisted of different sulfur species, whereas at temperatures greater than 550K (~280°C), the gas present in the furnace was only SO2. These quantitative findings show promise that future reaction kinetics can be done by using our experimental spectrometry system.


      PubDate: 2014-06-14T15:32:48Z
       
  • Halogen variations in alkaline rocks from the Upper Rhine Graben (SW
           Germany): Insights into F, Cl and Br behavior during magmatic processes
    • Abstract: Publication date: 25 July 2014
      Source:Chemical Geology, Volume 380
      Author(s): Lian-Xun Wang , Michael A.W. Marks , Jörg Keller , Gregor Markl
      We present halogen compositions in a series of alkaline rocks from the Kaiserstuhl, Hegau and Urach areas of the Upper Rhine Graben region (South Germany). Most primitive rocks (olivine melilitites and olivine nephelinites) have lower Cl and Br concentrations (generally below 100μg/g and below 0.3μg/g, respectively) compared to more evolved tephrites, phonolites and related rocks (up to 7600μg/g Cl and 34μg/g Br). However, the Cl/Br ratios of the majority of the investigated samples are relatively uniform (371±120), regardless of rock type and sample locality, suggesting that partial melting, fractional crystallization, and degassing have limited effects on the fractionation of Cl from Br. The mean value of the Cl/Br ratio is similar to previous estimates for basaltic rocks representing MORB and OIB mantle signatures. Fluorine concentrations of the primitive rocks show limited variations (900–1100μg/g) and are within the range defined by the evolved rocks (400–2100μg/g), but are much higher than previous estimates for the MORB and OIB mantle (50–135μg/g). This may indicate a relatively F-rich mantle source beneath the Rhine Graben region. In contrast to Cl/Br ratios, the F/Cl ratios vary significantly over three orders of magnitudes (from <0.1 to around 100) and decrease from primitive rocks to more evolved ones, implying that magmatic processes such as fractional crystallization and degassing strongly effect this ratio. Positive correlations between F, Cl and Br contents in a series of tephritic rocks from Kaiserstuhl probably record degassing processes, since these samples have similar geochemical and isotopic signatures with the only difference being textural: they vary from glassy to phanerocrystalline. Simplified calculations imply that about 50% F and 90% Cl and Br were released during the crystallization process, resulting in increasing F/Cl, but relatively constant Cl/Br ratios.


      PubDate: 2014-06-14T15:32:48Z
       
  • Origin of calcareous dust in Argentinean Pleistocene periglacial deposits
           traced by Sr, C and O isotopic compositions, and REE distribution
    • Abstract: Publication date: 25 July 2014
      Source:Chemical Geology, Volume 380
      Author(s): Isabelle Techer , Norbert Clauer , Thea Vogt
      Calcareous dust is often closely associated with periglacial features in the Pleistocene deposits of extra-Andean Argentina. Conventionally interpreted as resulting from continental pedogenic weathering and precipitation during interglacial episodes, little has yet been demonstrated about its origin. Strontium, oxygen and carbon isotopic compositions and REE distributions were determined for such calcareous dust from a wide geographic area including sites in Patagonia, the southern Pampa and the Mendoza Pre-Cordillera. The δ13C values between −1.9 and −7.2‰ (V-PDB) and δ18O values between −2.6 and −6.0‰ (V-SMOW) confirm a glaciogenic origin and not the generally advocated pedogenic genesis. The narrow 87Sr/86Sr ratio from 0.706463±0.000008 to 0.707477±0.000006 (2σ) and the constant REE distribution with a specific negative Ce anomaly both require crystallization in a large and homogeneous reservoir with some seawater supply during successive glacial episodes. The homogeneous Sr isotopic and REE data suggest that the Ca, Sr and REEs released from Andean basement rocks during glacial episodes: (1) were transported by rivers to the eastern continental shelf of which up to ~1million of km2 was probably emerged during the successive glacial episodes, (2) were mixed there in a semi-marine lacustrian meander-rich environment with seawater supply where they dissolved and re-precipitated, before (3) being eroded and dispersed by southeasterly winds over most of Argentina.


      PubDate: 2014-06-14T15:32:48Z
       
  • To bleach or not to bleach' Comparing treatment methods for isolating
           biogenic carbonate
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Brooke E. Crowley , Patrick V. Wheatley
      Stable carbon and oxygen isotope ratios in tooth enamel and bone hydroxyapatite are used to answer a variety of ecological questions. Researchers employ chemical treatments to reduce contaminants that may interfere with the isotope ratios in hydroxyapatite. Chemical treatment should remove exogenous as well as non lattice-bound molecules without affecting the isotopic composition of the biogenic carbonate or introducing secondary carbonate material. However, there is growing concern that some of the most frequently used chemicals may alter biogenic isotope ratios, thus compromising isotope data. Despite a considerable body of literature, there is still no consensus as to which chemical treatment methods are “the best”. Here we test the isotopic effects of a suite of chemical experimental treatments on modern and fossil enamel carbonate, including six published protocols. We also test the degree to which carbon and oxygen isotope ratios in bone and dentine are affected by commonly used treatment chemicals. We conclude that treatment with hydrogen peroxide followed by Ca-buffered acetic acid yields the “best” results. This combination results in less extreme carbon, oxygen, and weight percent carbonate values compared to other treatments. Although sequential treatment with sodium hypochlorite (bleach) and Ca-buffered acetic acid produces similar results for enamel, bleach is not recommended for tissues with a higher organic content, such as bone or dentine.


      PubDate: 2014-06-14T15:32:48Z
       
  • Oxygen and carbon isotope fractionation in calcitic deep-sea corals:
           Implications for paleotemperature reconstruction
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Justine B. Kimball , Robert B. Dunbar , Thomas P. Guilderson
      Inhabiting areas of the ocean where paleoenvironmental records are sparse, deep-sea corals represent valuable yet largely untapped Holocene records of intermediate and deep ocean variability. δ18O and δ13C were analyzed in nine live-collected deep-sea gorgonian corals (Isididae and Coralliidae) in order to further develop the “lines” paleotemperature method. Least squares linear regression analysis for full lifespan δ18O vs. δ13C (corrected for δ18Owater and δ13CDIC) was utilized to yield equations of the form y=mx+b. δ18O and intercept values were found to be a function of temperature, and to approximate calcite δ18O equilibrium. The corals in this study extend the previously reported calibration (Hill et al., 2011) over a broader range of temperatures from 5°C to 11.2°C. When combined with the data from Hill et al. (2011), a new expression for the relationship between the δ18Ointercept value and temperature is proposed: T ° C = − 4.12 ± 0.38 δ 18 O intercept + 12.33 ± 0.75 R 2 = 0.90 , p value < 0.0001 . Error estimates are ±0.7°C for corals living at cold temperatures (2°C), ±1.4°C in warmer waters (11°C), and ±0.5°C at the mean water temperature of the data set (4.6°C). The first multi-specimen verification of the “lines” method was performed on three co-located bamboo (Isididae) corals and found to give nearly coincident δ18O intercepts. Detailed intraspecimen sampling reveals δ18O and δ13C isotopic variability within coeval portions of the skeleton. In one specimen, “lines” method analysis was utilized on multiple samples taken from the same temporal increment of the skeleton, yielding multiple δ18O intercepts. Calculated temperatures using the calibration proposed here describe a temperature range of 7.9 to 10.3°C, which approaches the temperature range of 11.1±0.7°C at the coral collection location.


      PubDate: 2014-06-14T15:32:48Z
       
  • Syn-deformation fluid-assisted growth of monazite during renewed
           high-grade metamorphism in metapelites of the Central Rhodope (Bulgaria,
           Greece)
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): A. Didier , V. Bosse , Z. Cherneva , P. Gautier , M. Georgieva , J.L. Paquette , I. Gerdjikov
      We present textural, chemical and U–Th–Pb-age data on monazites from garnet–kyanite (Grt–Ky) metapelites, from the Chepelare Shear Zone (Bulgaria) and the Nestos Shear Zone (Greece), in the Central Rhodope. Samples from both locations have experienced two stages of high temperature metamorphism during Alpine times. The first event involved mid-Mesozoic granulite facies dehydration melting. The second event involved mid-Cenozoic lower-grade fluid-assisted partial melting. The latter is well expressed in adjacent felsic rocks but had limited impact on the Grt–Ky metapelites. Most samples display evidence for strong ductile shearing in the presence of fluids. Monazite is present in the highly foliated matrix and as inclusions in garnet and kyanite. Unlike the inclusions, matrix monazites display features of fluid-assisted dissolution–recrystallization. Y-poor domains with U–Th–Pb ages of between ca. 115 and 165Ma represent the largest part of the grains. Y-rich domains with mid-Cenozoic ages occur as rims, or as small satellite grains surrounding the Mesozoic grains. The Cenozoic monazite domains crystallized at the expense of the Mesozoic ones and simultaneously incorporated Y provided by the fluid-assisted resorption of garnet. An age of ca. 36Ma is obtained for the samples of both shear zones, interpreted as dating the main episode of monazite growth during the Cenozoic. Similar ages exist for the crystallization of leucosomes and pegmatites in the adjacent migmatitic gneisses, indicating that the fluids responsible for the precipitation of the Cenozoic monazites were probably released during the crystallization of nearby anatectic melts. Together with associated rutile and biotite, many newly grown monazites show a preferred orientation paralleling the matrix foliation. This supports the hypothesis of dynamic dissolution–precipitation as an efficient mean to promote renewed monazite crystallization during ductile deformation of the host rock at ca. 36Ma.


      PubDate: 2014-06-14T15:32:48Z
       
  • Iron–clay interactions under hydrothermal conditions: Impact of
           specific surface area of metallic iron on reaction pathway
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Franck Bourdelle , Laurent Truche , Isabella Pignatelli , Régine Mosser-Ruck , Catherine Lorgeoux , Christophe Roszypal , Nicolas Michau
      The long-term evolution of minerals in contact with metallic iron is important in various domains such as the earth sciences, materials science, cosmochemistry or industry. As an illustration, iron–clayey rock interactions are notable issues in the framework of secondary alteration processes in chondrites, or in the evolution of steel canister corrosion in projects for high-level nuclear waste repositories. In these contexts, interactions between the geological environment and metallic iron or engineered structures must be assessed with a high level of precision. Therefore, over the last decade, several experimental studies have focused on metallic iron–clay interactions showing the important relationship between the reaction progress and the iron/clay mass ratio. The present investigation demonstrates that, apart from this mass ratio, the specific surface area of metallic iron has a crucial influence, since it impacts the reaction pathway and the ambient physico-chemical parameters of the medium. For this purpose, two original continuous monitoring experiments were performed to measure pH and pressure in real-time, as well as analyze the gas and solution compositions, by bringing the same mass of (a) iron powder (S iron =0.07m2/g) or (b) iron grains (S iron ≈0.001m2/g) into contact with claystone (Callovo–Oxfordian claystone, Bure, France) at 90°C for 3months. Using iron powder, i.e. the more reactive cast iron (with a corrosion rate of 0.54mmol/day for iron powder as against 0.01mmol/day for iron grains), causes an Fe-enrichment of the clay particles, leading initially to the formation of new phases intermediate between interlayered illite–smectite and iron-rich serpentine, followed by conversion into odinite–greenalite. On the other hand, using iron grains make the clay compositions “kaolinitic” with a noticeable I.C. depletion. Meanwhile, the illite–smectite and quartz fractions of the claystone are destabilized, while the mineral transformations control the pH around 7 (+/- 0.3) and prevent the formation of magnetite, thus contradicting the thermodynamic predictions. The present study, which involves in situ monitoring of pH and H2 production, provides some important keys to obtaining better constraints on reaction mechanisms, kinetics and thermodynamic models, aimed at predicting accurate reaction paths and their long-term consequences.


      PubDate: 2014-06-14T15:32:48Z
       
  • Late season mobilization of trace metals in two small Alaskan arctic
           watersheds as a proxy for landscape scale permafrost active layer dynamics
           
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Amanda J. Barker , T.A. Douglas , A.D. Jacobson , J.W. McClelland , A.G. Ilgen , M.S. Khosh , G.O. Lehn , T.P. Trainor
      Increasing air temperatures in the Arctic have the potential to degrade permafrost and promote the downward migration of the seasonally thawed active layer into previously frozen material. This may expose frozen soils to mineral weathering that could affect the geochemical composition of surface waters. Determining watershed system responses to drivers such as a changing climate relies heavily on understanding seasonal controls on freshwater processes. The majority of studies on elemental concentrations in Arctic river systems have focused on sampling only from spring snowmelt to the summer season. Consequently, there remains a limited understanding of surface water geochemistry, particularly with respect to trace metals, during late fall and early winter. To examine the variability of metal concentrations as a function of seasonality, we measured trace metal concentrations from spring melt to fall freeze-up in 2010 in two high Arctic watersheds: Imnavait Creek, North Slope, Alaska and Roche Mountanee Creek, Brooks Range, Alaska. We focused on aluminum (Al), barium (Ba), iron (Fe), manganese (Mn), nickel (Ni) and zinc (Zn). Concentrations of ‘dissolved’ (<0.45μm) Al, Ba, Fe, and Mn in Imnavait Creek waters and Ba in Roche Mountanee waters were highest in late fall/early winter. To link observed surface water concentrations at Imnavait Creek to parent soil material we analyzed the elemental composition of a soil core from the watershed and tracked the soil temperatures as a function of time and depth. The results from this study show a distinct seasonal signature of trace metal concentrations in late fall that correlates with the depth of the thawed active layer.


      PubDate: 2014-06-14T15:32:48Z
       
  • Infrared spectroscopy and multivariate analysis to appraise
           α-cellulose extracted from wood for stable carbon isotope
           measurements
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Béatrice Richard , Fabienne Quilès , Cédric Carteret , Oliver Brendel
      Wood is a heterogeneous material mainly constituted of cellulose, hemicelluloses, lignin, and extractives which all have different isotopic signatures. For applications where δ13C measurement of α-cellulose is required, it is important that residues of the other constituents remain below an acceptable level. The laboratory method most widely used for the extraction of α-cellulose consists of several successive treatments: organic solvents and boiled water, acidified sodium chlorite (NaClO2) application, and finally a NaOH treatment. Different variants of this method were tested systematically to optimize the extraction of α-cellulose from wood samples of four tree species: oak, beech, poplar, and pine. Mid-infrared spectroscopy in Attenuated Total Reflection mode (IR-ATR) combined with a curve resolution method (Bayesian Positive Sources Separation statistical analysis) was used to monitor the residues of other wood constituents in extracted α-cellulose. IR-ATR spectroscopy was shown to be sensitive enough to detect residual compounds in α-cellulose extracts below a concentration which does not present a measurable bias for δ13C measurements. For all tree species, a residual concentration of lignin below the bias threshold for δ13C measurements was reached with fewer additions of acidic NaClO2 than usually reported. Further the NaOH treatment step was not necessary to remove the hemicelluloses from oak and beech. Infrared spectroscopy combined with a curve resolution method is appropriate to improve α-cellulose extraction species-specifically for reliable stable carbon isotope δ13C measurements. It allows to check the extracted α-cellulose and to reduce the consumption of chemicals, the extraction time and the loss of α-cellulose.
      Graphical abstract image

      PubDate: 2014-06-14T15:32:48Z
       
  • Seawater δ7Li: A direct proxy for global CO2 consumption by
           continental silicate weathering'
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Christoph Wanner , Eric L. Sonnenthal , Xiao-Ming Liu
      The fractionation of stable Li isotopes (6Li, 7Li) has become a promising proxy for assessing changes related to continental silicate weathering patterns. Recently, the first complete record of Cenozoic seawater Li isotopic composition (δ7Li) was reported (Misra and Froelich, 2012, Science 335, 818–821) showing a stepwise increase of +9‰ over the last 56Ma. This increase was attributed to a general change in continental silicate weathering behavior caused by tectonic uplift. In particular, the low global average riverine δ7Li inferred for the Paleocene–Eocene boundary was explained by congruent silicate weathering of primary silicate minerals, which is inconsistent with the stoichiometry of secondary minerals and the resultant water chemistry. In this study, we present a novel reactive transport modeling approach that explicitly includes Li isotopic fractionation to assess alternative geochemically-constrained interpretations that do not rely on congruent weathering. Simulations show that riverine δ7Li is mainly controlled by the subsurface residence time, the corresponding weathering intensity, and the concentration of a river's suspended load. Based on these factors, we suspect that the low δ7Li observed at the Paleocene–Eocene boundary was inherited from a high weathering intensity with predominant weathering of previously formed secondary mineral phases (e.g., clays, oxides) having low δ7Li values. Moreover, we conclude that the Cenozoic δ7Li increase was caused by an increasing amount of primary silicate mineral dissolution inherited from an increasing suspended river load concentration and a decreasing weathering intensity both likely induced by tectonic uplift. In contrast, Cenozoic cooling and corresponding pCO2 and precipitation variations do not seem to have a distinct control on the Cenozoic δ7Li record. Finally, our simulations revealed a close relation between δ7Li and CO2 consumption by silicate weathering implying that the Cenozoic seawater δ7Li record could be potentially used to quantify such CO2 consumption through time. However, more experimental and modeling work is required to quantify the correlation between seawater δ7Li and global CO2 consumption by silicate weathering. Key parameters are the temperature-dependent thermodynamic properties of specific Li-bearing primary and secondary minerals (e.g., crystallographic Li substitution reaction, maximum Li substitution, Li solubility, Li isotopic fractionation factor) as well as the determination of global average subsurface and river discharges through time.


      PubDate: 2014-06-14T15:32:48Z
       
  • Calcium carbonate and calcium sulfate precipitation, crystallization and
           dissolution: Evidence for the activated steps and the mechanisms from the
           enthalpy and entropy of activation values
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Athinoula L. Petrou , Athina Terzidaki
      Calculation of the thermodynamic parameters Eact, ΔH≠, ΔS≠ and ΔG≠ for the precipitation, crystallization and dissolution processes of the salts CaCO3 and CaSO4, leads to very important conclusions about the activated steps and the mechanisms. The ΔG≠ values are almost the same for all the processes at the same temperature, suggesting that the electrostatic forces between the ions Ca2+ and CO3 2− (CaCO3) and Ca2+ and SO4 2− (CaSO4) are the most important factors governing the above processes. The values of ΔH≠ and ΔS≠ differ but the values of ΔG≠ which refer to the overall transformations are the same and are independent of the various steps that take place as well as the mechanisms (associative, dissociative). Dehydration and aquation of the ions are revealed by the values of ΔH≠ and ΔS≠. The precipitation of the two salts may take place both by a dissociative mechanism (ΔS≠ >0) or by an associative mechanism (ΔS≠ <0). For processes taking place without the need for diffusion of the ions, (Ca2+, CO3 2−, SO4 2−), the free energy of activation is about 85kJmol−1, whereas when diffusion of the ions is necessary, an additional amount of ~20kJmol−1 is required. This amount is the activation energy for the diffusion. In the case of CaCO3, a wide range of values is found for ΔH≠ from −66.00 to 162.00kJmol−1, and for ΔS≠ from −501.00 to +248.00JK−1 mol−1 while the ΔG≠ values cover only a small range from 75 to 90kJmol−1. Values of 120, 131 and 132kJmol−1 are reported for cases where retardation is caused due to the presence of foreign compounds. In the case of CaSO4, a wide range of values is found for ΔH≠ from 6.00 to 122.00kJmol−1 and for ΔS≠ from −342.00 to +117.00JK−1 mol−1 while the ΔG≠ values fall in the narrow range from 80.00 to 89.00kJmol−1. A value of 126kJmol−1 is reported for cases where retardation is caused due to the presence of foreign compounds. The Eact values vary between −63 and 164kJmol−1 for CaCO3 and between 8 and 184kJmol−1 for CaSO4, demonstrating once again that the ΔG≠ value is more realistic, being almost the same for similar processes. The various small differences for the values of ΔG≠ arise from the different ionic strengths due to the concentration and charge of the foreign ions affecting the rate constants and thus the activation parameters. The pH also has an effect, as does the nature of the solvent. The very large and very small absolute values of ΔH≠ and ΔS≠ suggest composite reactions. Composite reactions in the precipitation, crystallization and dissolution processes are the ones where dehydration takes place (positive values of ΔH≠ and ΔS≠) followed by association of the ions (negative values of ΔH≠ and ΔS≠). The algebraic sum of the relevant ΔH≠ and ΔS≠ values gives the total value of ΔH≠ and ΔS
      PubDate: 2014-06-14T15:32:48Z
       
  • Chemical and stable isotopic characteristics of syn-tectonic tourmaline
           from the Western fold belt, Mount Isa inlier, Queensland, Australia
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): R.J. Duncan , I.S. Buick , K. Kobayashi , A.R. Wilde
      Understanding hydrothermal fluid flow events related to orogenic events is important because they provide large-scale redistribution of metals which may become concentrated in ore deposits. In this study we determine the origin of tourmaline-bearing quartz veins and pegmatites in the Western fold belt of the Proterozoic Mount Isa inlier, a region which host numerous syn-orogenic Cu deposits. This paper uses tourmaline mineral chemistry and isotopic analyses (in-situ B and bulk O and H isotopes) to fingerprint the fluid reservoirs that contributed to the fluid budget during the Isan orogeny. All the tourmaline samples have schorl–dravite solid solution compositions. Quartz–tourmaline δ18O (11.0–12.6 and 7.7–9.4‰ V-SMOW, respectively) pairs yield equilibrium formation temperatures of 305 to 575°C with the higher temperatures related to pegmatitic tourmaline. Tourmaline δD values have a relatively narrow range from −68 to −56‰ V-SMOW. In most cases calculated δ18OFluid and δDFluid values are compatible with primary magmatic water with a small component of metamorphic/18O-enriched formation waters. However, there are large variations in tourmaline δ11B values which range from +10.7 to −17.0‰, with the majority of values between −10 and −15‰. Variations in δ11BTur in the pegmatites suggest that boron degassing generated relative 11B depletion during crystallization. Similar δ11BTur values in quartz–tourmaline veins spatially associated with the pegmatites, in cordierite- and sillimanite-bearing metamorphic rocks west of the Mount Isa fault, demonstrate a similar origin for B in this tourmaline. In contrast, quartz–tourmaline veins of similar timing hosted by greenschist grade metamorphic rocks east of the Mount Isa fault have much higher δ11BTur values (+6.2 to +10.7‰) which reflect the involvement of a distinct B isotopic reservoir; mostly likely a near-surface derived evaporitic brine. Thus, the B isotopic data reveal important differences in fluid origin that are not reflected in the δ18O values. Therefore, at least two isotopically distinct fluids, one sourced from fractionating anatectic pegmatites and the other from external basinal brines, circulated contemporaneously in separate areas during the Isan orogeny. There is no evidence, based on the tourmaline isotopic data, to suggest that these fluids mixed, which demonstrates that the hydrothermal plumbing system in the Western fold belt during the Isan orogeny was disconnected, probably due to differences in thermal structure/metamorphic grade across the Western fold belt during orogenesis.


      PubDate: 2014-06-14T15:32:48Z
       
  • Mercury speciation in the Mt. Amiata mining district (Italy): Interplay
           between urban activities and mercury contamination
    • Abstract: Publication date: 25 July 2014
      Source:Chemical Geology, Volume 380
      Author(s): Valentina Rimondi , Fabrizio Bardelli , Marco Benvenuti , Pilario Costagliola , John E. Gray , Pierfranco Lattanzi
      A fundamental step to evaluate the biogeochemical and eco-toxicological significance of Hg dispersion in the environment is to determine speciation of Hg in solid matrices. In this study, several analytical techniques such as scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), sequential chemical extractions (SCEs), and X-ray absorption spectroscopy (XANES) were used to identify Hg compounds and Hg speciation in samples collected from the Mt. Amiata Hg mining district, southern Tuscany, Italy. Different geological materials, such as mine waste calcine (retorted ore), soil, stream sediment, and stream water suspended particulate matter were analyzed. Results show that the samples were generally composed of highly insoluble Hg compounds such as sulphides (HgS, cinnabar and metacinnabar), and more soluble Hg halides such as those associated with the mosesite group. Other moderately soluble Hg compounds, HgCl2, HgO and Hg0, were also identified in stream sediments draining the mining area. The presence of these minerals suggests active and continuous runoff of soluble Hg compounds from calcines, where such Hg compounds form during retorting, or later in secondary processes. Specifically, we suggest that, due to the proximity of Hg mines to the urban center of Abbadia San Salvatore, the influence of other anthropogenic activities was a key factor for Hg speciation, resulting in the formation of unusual Hg-minerals such as mosesite.


      PubDate: 2014-06-14T15:32:48Z
       
  • A sequential extraction technique for mass-balanced stable selenium
           isotope analysis of soil samples
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Kathrin Schilling , Thomas M. Johnson , Paul R.D. Mason
      The sequential extraction (SEP) of selenium to determine the isotopic composition of different Se pools in soils is an important tool for tracking biogeochemical cycling of Se in soils. An appropriate Se extraction method for the stable Se isotope analysis was established and tested for different Se pools in soils in terms of reproducibility and recovery. Soil samples (n=4) from a seleniferous field site in Punjab, India and two standard reference materials were chosen because of their relatively high total Se concentration of 2 to 4.5mgkg−1. Additionally, a soil sample with low Se content was utilized to verify the procedure and reproducibility. The isotopic mass-balance calculation gives consistent δ82Se values for the sum of the extracted pools, relative to δ82Se of the bulk soils. For instance, in one studied soil (soil-1), the sum of δ82Se of all individual Se-pools (δ82Semass-balance =3.35‰) is in good agreement with the analysis of δ82Se of the bulk soil (δ82Sebulk of 3.56‰). We observed up to a 2‰ range in δ82Se among the different Se soil-pools (soil-3). Our Se isotopic results of Se soil pools provide more detailed observations of the Se linked processes in soils compared to bulk δ82Se. Thus, quantitative Se isotopic analysis of different Se pools can be an important tool for predicting the environmental cycling of selenium in soils or sediments.


      PubDate: 2014-06-14T15:32:48Z
       
  • Iron isotope fractionation during crystallization and sub-solidus
           re-equilibration: Constraints from the Baima mafic layered intrusion, SW
           China
    • Abstract: Publication date: 25 July 2014
      Source:Chemical Geology, Volume 380
      Author(s): Lie-Meng Chen , Xie-Yan Song , Xiang-Kun Zhu , Xiao-Qi Zhang , Song-Yue Yu , Jun-Nian Yi
      To better understand Fe isotope fractionation between mafic minerals and oxides during crystallization of mafic magma and sub-solidus re-equilibration, the Fe isotopes of whole-rocks and separated minerals (olivine, clinopyroxene, magnetite and ilmenite) of the Baima mafic layered intrusion, SW China, have been investigated. The separated minerals show a systematical decrease in δ57Fe values, from magnetite (0.15 to 0.51‰) to olivine (−0.11 to 0.15‰) and clinopyroxene (−0.35 to 0.05‰) and then to ilmenite (−0.82 to −0.10‰), demonstrating regular fractionation between these minerals. Except for a few of samples, most of the olivine and clinopyroxene are similar to those of mantle xenoliths in Fe isotopes, indicating that Fe isotope equilibrium reached during magma crystallization was well preserved. By contrast, the Fe isotopes of the magnetite and ilmenite may be evidently modified by sub-solidus re-equilibration via the Fe3+ versus Ti4+ and Fe2+ exchange between the oxides. Furthermore, the sub-solidus re-equilibration in Fe isotope is strongly controlled by the proportions of magnetite and ilmenite in rocks. Therefore, although the δ57FeIlm of the Lower Zone rocks with magnetite/ilmenite ratios as high as 6–10 was reduced evidently by the sub-solidus re-equilibration, the magnetite preserved their original Fe isotope compositions. By contrast, the Fe isotopes of both most magnetite and ilmenite in the Middle Zone had been markedly modified by sub-solidus re-equilibration owing to the moderate magnetite/ilmenite ratios (4–7). The decreases of both δ57FeMt and δ57FeOl upwards in the cyclic units of the Lower Zone reveal that extensive early fractional crystallization of the magnetite resulted in depletion of heavier Fe isotopes in the magma. On the other hand, early crystallization of olivine and clinopyroxene gave rise to the slight elevation of δ57FeOl values upwards in the cyclic units of the Middle and Upper zones. The stratigraphic reversals in the δ57FeMt and δ57FeOl values suggest multiple magma recharges. Additionally, the δ57FeOl values in the base of the Lower Zone (0.10 to 0.15‰) indicate that the parental magma were heavy in Fe isotope due to extensive silicate mineral fractionation at depth. This study indicates fractionation in Fe isotope between silicates and oxides during magma crystallization and sub-solidus re-equilibration.


      PubDate: 2014-06-14T15:32:48Z
       
  • Weathering on land and transport of chromium to the ocean in a subtropical
           region (Misiones, NW Argentina): A chromium stable isotope perspective
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Robert Frei , Daniel Poiré , Karin Margarita Frei
      We have investigated the pathway of chromium from its mobilization on land and along its riverine transport in a subtropical region of South America (Misiones Province, Argentina), in an attempt to link Cr stable isotope compositions recently measured in seawater with signals prevailing in rivers and, ultimately, with Cr isotope effects observed during oxidative surface weathering in subtropical red soils. Cr concentrations and stable Cr isotopic compositions (expressed as δ53Cr ‰ values) in two typical and representative surface profiles of weathered basalt show significant depletion of Cr in the soils of up to 50%, together with pronounced negatively fractionated δ53Cr values which are indicative of oxidative mobilization of heavy Cr(VI) into the run-off. The behavior of Cr in the studied weathering profiles is not correlated with that of other redox sensitive elements, such as Ce and U; this is essentially due to the affinity of REE and U, but not Cr with secondary phosphates which form during weathering processes. Smaller tributaries in NW Argentina to the Paraná River (second largest river in South America) carry dissolved Cr in the order of 0.7–1.4ppb (13–27nM) with δ53Cr values of +0.2 to +0.4‰, balancing the negatively fractionated weathering products. The isotope composition and concentration of dissolved Cr in the ca. 1200km long Paraná River from Misiones to its estuary and discharge area into the South Atlantic Ocean remains relatively constant with an average Cr concentration around 2.4ppb (46nM) and an average δ53Cr value of +0.32‰. The Cr concentration in the estuary itself drops by ca. 50% but with only minor change in its Cr isotope composition. Results from the Paraná estuary are identical with recently analyzed surface seawater from the Argentine Basin with Cr contents of ~0.3ppb (~6nM) and δ53Cr values ~+0.4‰ (Bonnand et al., 2013), and indicate that there is only a minimal Cr isotopic variability during riverine transport, even during long transport distances as shown in our example of the Paraná River. Simple Cr input flux calculations reveal that the Paraná River accounts for ~5% of the total yearly Cr flux to the world's oceans today and that its isotopic signature seems to be, at least locally, imparted to the surface seawater of the Argentine Basin. Whether or not this Cr isotope signature is generally exhibited by the world's oceans needs further investigations, particularly the characterization of seawater around the globe.


      PubDate: 2014-06-14T15:32:48Z
       
  • Measurement of in-situ oxygen isotope ratios in monazite by SHRIMP ion
           microprobe: Standards, protocols and implications
    • Abstract: Publication date: 25 July 2014
      Source:Chemical Geology, Volume 380
      Author(s): Daniela Rubatto , Benita Putlitz , Laure Gauthiez-Putallaz , Céline Crépisson , Ian S. Buick , Yong-Fei Zheng
      Monazite forms at sub-solidus conditions in a variety of metamorphic rocks and has been proven to be reactive to fluids, and thus is a potential monitor of fluid–rock interaction. As monazite can preserve multiple growth zones, in order to explore the potential of monazite as a fluid tracer in metamorphic conditions, microbeam analysis is required. We performed oxygen isotope analysis of monazite using the SHRIMP ion microprobe and newly characterized standards, for which we obtained laser fluorination δ18O values (USGS-44069 monazite 7.67±0.26‰ and Itambé monazite 0.46±0.20‰). Reproducibility of δ18O ion microprobe analyses for USGS-44069 and Itambé monazites is in the order of 0.4–0.6‰, standard deviation at 95%c.l., similar to what is routinely obtained for silicates. This reproducibility is comparable to that of the analyses of experimental P-rich glasses, which are assumed to be homogeneous and free of geological imperfections. The variable composition of natural monazite has the potential to produce matrix effects during ion microprobe measurements. Monazite grains from the Malagasy syenite (Madagascar) and the Dora Maira whiteschists (Italy) display a scatter in δ18O values that show a negative correlation with Th content and is related to the huttonite [ThSiO4] and cheralite [CaTh(PO4)2] substitutions in monazite. The matrix effect on oxygen isotope measurements can be significant and is estimated to produce a shift in δ18O of circa −0.85 or −1.9‰ for every 10wt.% Th introduced by the huttonite and cheralite components, respectively. Corrections for this matrix effect are proposed on the basis of the natural samples investigated. Oxygen isotope fractionation factors for monazites of different compositions, cheralite and huttonite were calculated with the increment method. The results suggest that the substitution of trivalent LREE by tetravalent Ce and Th results in consistent enrichment of 18O in the monazites, whereas the substitution of tetravalent Th by divalent Ca results in the depletion of 18O in cheralite-rich monazite. Monazites from high-grade metasediments (Mount-Stafford, Central Australia) preserve inherited cores, but are homogeneous in oxygen composition. This suggests that diffusion may efficiently erase the oxygen isotope signature in monazite that experienced ~800°C metamorphism.


      PubDate: 2014-06-14T15:32:48Z
       
  • Boron isotope geochemistry of salt sediments from the Dongtai salt lake in
           Qaidam Basin: Boron budget and sources
    • Abstract: Publication date: 25 July 2014
      Source:Chemical Geology, Volume 380
      Author(s): Hai-Zhen Wei , Shao-Yong Jiang , Hong-Bing Tan , Wen-Jie Zhang , Bin-Kai Li , Tang-Li Yang
      Dongtai salt lake in Qaidam Basin contains abundant boron and lithium resources, and has the highest reserve of lithium found in the world, but the origin of these resources is highly controversial. In this study, we carried out a detailed study on chemical compositions and boron isotope characteristics of salt sediments collected from a drill core in the Dongtai salt lake. The evaporite, carbonate and silicate phases of the sediments show δ11B values of −6.4‰ to +2.2‰, −17.3‰ to +0.5‰ and −14.6‰ to −2.8‰, respectively, which suggest a non-marine origin for the salt lake. The variation of δ11B vs. K% identified the evolution process of the salt lake. The geochemical/hydrochemical signatures and the reserve estimation of boron resources approved the dominant sources of saline resources containing abundant B, K and Li from the inflowing Nalenggele River. The boron isotopic fractionation factors between evaporite and brine (i.e. αevaporite–brine) vary from 0.9869 to 0.9955, and the linear variation of αevaporite–brine vs. Li/Mg molar ratios reflects the boron isotopic fractionation during crystallization of the salts. The positive correlation between δ11B and [B] in the carbonate phase reflects the pH control on the incorporation of boron into the carbonates, and the equilibrium isotope fractionation follows the Rayleigh isotope fractionation rule in a closed system. The variation of δ11B values in the silicate phase behaved as an oscillating pattern, and the more negative shift of δ11B values corresponds to the mudstone strata deposited under warm–humid climate conditions, indicating the influence of intensive weathering.
      Graphical abstract image

      PubDate: 2014-06-14T15:32:48Z
       
  • Melting versus contamination effects on 238U–230Th–226Ra and
           235U–231Pa disequilibria in lavas from São Miguel, Azores
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Julie Prytulak , Riccardo Avanzinelli , Govert Koetsier , Katharina Kreissig , Christoph Beier , Tim Elliott
      The island of São Miguel, Azores piques geochemical interest due to a strikingly large range in the long-lived radiogenic isotope ratios of its lavas. The ‘enriched’ signatures (e.g., radiogenic 87Sr/86Sr and unradiogenic 143Nd/144Nd) observed in lavas found in the east of the island have been proposed to originate from recycled crustal components in their mantle sources. Such fertile lithologies should have higher melt productivities (amount of melt generated per decrement in pressure) than peridotitic mantle and this heritage ought to be evident in the U-series signatures of melts derived from such materials. Specifically, combined 238U–230Th and 235U–231Pa disequilibria in erupted lavas should identify significant differences in the melting rate and/or the mineralogical makeup of their sources (e.g., the amount of garnet). To this end, new 238U–230Th–226Ra and 235U–231Pa disequilibria are presented for sixteen mafic lavas (<50wt.% SiO2) that encompass the full range of São Miguel's long-lived radiogenic isotope variability. However, primary U-series signatures of mafic lavas can be influenced by assimilation of more differentiated products. Notably, syenite nodules and cumulate alkali feldspar xenocrysts have been identified in São Miguel basalts. We argue that secondary contamination of basalts is most evident in the hugely variable 230Th–226Ra disequilibria (~18% deficits to >300% excesses of 226Ra) that strongly correlate with Ba/Th ratios. The longer-lived U-series nuclides are only slightly perturbed in a number of the more enriched lavas, resulting in increased noise in the dataset. In apparently uncontaminated samples, isotopically depleted lavas from the west of São Miguel display slightly higher 230Th and 231Pa excesses than the enriched samples from the east. The U-series signature of the enriched lavas can be modeled with a combination of higher absolute values of D ¯ U U and D ¯ Th , coupled with higher melting rates compared to the depleted western source. These parameters are consistent with a peridotite source enriched with contributions from more fusible lithologies such as eclogite and/or pyroxenite in the eastern source, but not with direct derivation from such fusible lithologies. The only subtle variations in U-series isotopes coupled with large, systematic variations in radiogenic isotope compositions suggest that enriched components and/or their melts are well-mixed with mantle peridotite, thus masking their distinctive melting behavior. Finally, basalts from Sete Cidades volcano in the west of São Miguel have very similar long-lived isotope compositions and U-series disequilibria to basalts from the island of Pico, Azores. The geochemical similarity of lavas from islands at the center and periphery of the archipelago argue against systematic differences in melting behavior related to sampling different portions of an underlying plume structure.


      PubDate: 2014-06-14T15:32:48Z
       
  • Origin and accumulation of trace elements in sediments of the northwestern
           Mediterranean margin
    • Abstract: Publication date: 25 July 2014
      Source:Chemical Geology, Volume 380
      Author(s): D. Cossa , R. Buscail , P. Puig , J.-F. Chiffoleau , O. Radakovitch , G. Jeanty , S. Heussner
      Continental margins receive natural and anthropogenic trace elements (TEs) from direct atmospheric deposition of aerosols onto the sea surface and from advection of riverine suspended particles and/or resuspended sediments from the continental shelf/slope. When the margin is incised by submarine canyons, as for example in the Northwestern Mediterranean Sea, most of these particles are preferentially transferred via these topographic features towards their final repositories in the abyssal plain. The Gulf of Lions (GoL) shelf receives the largest particulate riverine input to the Western Mediterranean, with its associated chemical contaminants originating from the industrialized and urbanized Rhone Valley. Sediment samples (grabs, cores and moored traps) collected in the Cap de Creus (CdC) Canyon and its adjacent areas at the Southwestern exit of the GoL were analyzed to explore the origin, dispersion, transfer and accumulation of a suite of TEs (Ag, Cd, Co, Cr, Cu, Ni, Pb, Zn and V) from the GoL shelf to the adjacent continental rise. Distributions of Cu, Cr, Ni, Pb, Zn and V in the surface sediments of the shelf confirm their terrigenous origin in association with clay minerals, whereas Ag and Cd are more associated with organic matter (OM). All these TEs are anthropogenically enriched in the Rhone prodelta sediments. Anthropogenic influence remains clearly discernible in the GoL shelf surface sediments for Ag, Pb and Zn. Hydrodynamical resuspension and sorting of shelf sediments occur at the head of the CdC Canyon during dense shelf-water cascading events. During these events, the material collected in moored sediment traps contains a higher coarse carbonate fraction slightly impoverished in TEs compared to the clays of the nepheloid layer and the organically-rich particles deposited before and at the end of the cascading period. Upper and middle canyon sediments are characterized by high sedimentation rates (~0.2cmyr−1) of fine clay material. Conversely, sediments from the lower continental slope and rise exhibit low sedimentation rates (~0.06cmyr−1) and receive carbonaceous planktonic detritus from the water column. At the lower continental slope, coarse material includes foraminifers and pteropods, whereas at the continental rise finer planktonic-derived material is more abundant. Both in the CdC Canyon and in its adjacent lower continental slope/rise sediments, Co, Cu, Cr, Ni and V are associated with clay, whereas Ag, Cu and Pb are preferentially associated with OM. Cadmium, Cr, and Zn are also associated with OM in canyon sediments. Carbonaceous plankton appears to be especially efficient for scavenging Ag, whereas, Cr, V, Zn and Pb are diluted by biogenic carbonates. An authigenic Mn fraction is enriched with Co and Ni. Lead and Zn concentration levels and vertical profile patterns, along with Pb stable isotopic ratios, indicate that significant parts of Pb and Zn are of anthropogenic origin. A sediment chronology based on 210Pb dating reveals that Pb anthropization, mainly from gasoline additives, culminated between 1960 and 1980, being the current concentrations >40% lower than 30years ago. A similar distribution is observed for Zn, which originates mainly from combustion processes; but the reduction of Zn contamination amounts to only 20% during the same period. The largest anthropogenic Pb accumulation occurs in the middle part of CdC Canyon, with an inventory of 200μgcm−2. At the most distal part of the continental rise anthropogenic Pb accumulation within the first ~10cm below the surface sediment is estimated around 10μgcm−2, which is similar to the direct atmospheric deposition estimate.


      PubDate: 2014-06-14T15:32:48Z
       
  • Understanding parafluvial exchange and degassing to better quantify
           groundwater inflows using 222Rn: The King River, southeast Australia
    • Abstract: Publication date: 25 July 2014
      Source:Chemical Geology, Volume 380
      Author(s): Ian Cartwright , Harald Hofmann , Benjamin Gilfedder , Brittany Smyth
      222Rn is an important tracer for quantifying groundwater inflows to rivers, especially where groundwater and surface water have similar major ion and stable isotope geochemistry. Uncertainties in the 222Rn mass balance arise, however, from not accurately estimating the degree of degassing of 222Rn to the atmosphere and the extent to which interaction within the parafluvial zone provides an additional source of 222Rn. This study estimates both 222Rn production in the parafluvial zone and degassing along a 75km stretch of the King River, Australia, in order to more precisely quantify groundwater inflows. The contribution of 222Rn from the parafluvial zone (Fp ) was estimated using 222Rn emanation rates from near-river sediments and assessment of the residence time of water in the parafluvial zone from 222Rn activities and Cl concentrations in water the alluvial sediments. Values of Fp range from 40,400Bq/m/day in the upper King to 8500Bq/m/day in the lower King, corresponding to differences in the mineralogy and the volume of the parafluvial zone. The gas transfer coefficient (k) was estimated by matching groundwater inflows to observed increases in river discharge during a period of low discharge; k decreases from 25day−1 in the upper King to 3day−1 in the lower King. These k values are higher than those from most empirical formulations, probably due to the extensive pool and riffle sections that promote degassing. The King River is gaining in its upper and middle sections but several of the lower reaches are losing. Groundwater inflows on a reach scale are as high as 10m3/m/day and cumulative inflows along the 75km stretch are up to 19,000m3/day. Groundwater inflows increase proportional to total flow reflecting the response of both groundwater and surface water systems to rainfall. Uncertainties in the calculated groundwater inflows are reduced by independently estimating k from the discharge data; however, calculated inflows are up to 40% higher if parafluvial flow were not taken into account.


      PubDate: 2014-06-14T15:32:48Z
       
  • A new aqueous activity model for geothermal brines in the system
           Na-K-Ca-Mg-H-Cl-SO4-H2O from 25 to 300°C
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Ferdinand F. Hingerl , Thomas Wagner , Dmitrii A. Kulik , Kaj Thomsen , Thomas Driesner
      A revised formulation (named REUNIQUAC) of the Extended Universal QUAsiChemical (EUNIQUAC) activity model has been developed, which fits excess thermodynamic properties of binary and selected aqueous ternary electrolyte solutions in the system Na-K-Ca-Mg-H-Cl-SO4-H2O over temperatures from 298 to 573K and concentrations to 5 molal (or up to saturation if solubility is below 5 molal) for saturated water vapor conditions. Compared to the original EUNIQUAC model, REUNIQUAC employs an extended version of the Debye–Hückel model using effective ionic radii of solute species, a concentration dependence of a UNIQUAC parameter, an additional empirical term for strongly complexing salts, as well as a simple quadratic temperature dependence of the fitting parameters. REUNIQUAC considers only pairwise interactions between solute species, as opposed to the Pitzer activity model, which additionally needs to account for ternary interactions. Since REUNIQUAC uses also species-specific parameters, extension of the existing parameter set to solutions that are composed of different combinations of the fitted species, is straightforward, and involves only the parameterization of the pairwise interaction terms. All systems could be fitted with accuracy comparable to the Pitzer model or better, although with much fewer parameters.


      PubDate: 2014-06-14T15:32:48Z
       
  • Distinguishing silicate and carbonatite mantle metasomatism by using
           lithium and its isotopes
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Ben-Xun Su , Hong-Fu Zhang , Etienne Deloule , Nathalie Vigier , Yan Hu , Yan-Jie Tang , Yan Xiao , Patrick Asamoah Sakyi
      To investigate the effects of silicate and carbonatite metasomatism on mantle heterogeneity, we report lithium (Li) concentrations and isotopic compositions for olivine (Ol), orthopyroxene (Opx) and clinopyroxene (Cpx) from two suites of mantle xenoliths (Hannuoba, the North China Craton, and Haoti, the Western Qinling Orogen). The Hannuoba xenoliths range from lherzolite to pyroxenite and were affected by silicate metasomatism, whereas the Haoti xenoliths vary from harzburgite to wehrlite and were affected by carbonatite metasomatism. Lithium concentrations and isotopic compositions display a dichotomy between Hannuoba and Haoti xenoliths, and the overall variation exceeds what was previously reported. The minerals from Haoti xenoliths are more enriched in Li (Ol: 1.23–13.2ppm; Opx: 3.00–82.8ppm; Cpx: 1.39–112ppm) than those from Hannuoba samples (Ol: 1.34–5.52ppm; Opx: 0.23–16.1ppm; Cpx: 1.18–79.8ppm). Lithium isotopic compositions of these samples are highly variable in both suites of samples. δ7Li ranges from +3.0‰ to +41.9‰ in Ol, from −21.0‰ to +20.2‰ in Opx and from −17.4‰ to +18.9‰ in Cpx for Hannuoba samples. Haoti minerals display a similar degree of variation with δ7Li ranging from −29.1‰ to +19.9‰ in Ol, −16.9‰ to +18.0‰ in Opx and −45.1‰ to +19.6‰ in Cpx. On average, Li isotopic compositions of minerals from Hannuoba xenoliths follow the sequence of δ7LiOl >δ7LiOpx >δ7LiCpx, whereas those from Haoti xenoliths are characterized by the opposite sequence of δ7LiCpx >δ7LiOpx >δ7LiOl; in particular there is considerable difference in δ7Li values of Ol. The Li elemental and isotopic data suggest that mantle metasomatism by distinct agents is an important process for generating the large heterogeneity of Li abundances and isotopic distribution in the lithospheric mantle. The distinct geochemical characteristics of Li isotopes in silicate and carbonatite metasomatism are closely related to the preferential incorporation of Li into minerals from distinct melts. These findings further demonstrate that the Li isotopic systematics may in turn help to discriminate between silicate and carbonatite metasomatism.


      PubDate: 2014-06-14T15:32:48Z
       
  • New insights into cerium anomalies and mechanisms of trace metal
           enrichment in authigenic carbonate from hydrocarbon seeps
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Yu Hu , Dong Feng , Jörn Peckmann , Harry H. Roberts , Duofu Chen
      Authigenic carbonates that form at marine hydrocarbon seeps provide a unique geological archive of past local environmental conditions and pore fluid geochemistry. Recent work on such carbonates revealed variable cerium (Ce) anomalies and anomalous enrichments of certain trace metals. However, the mechanisms accounting for such anomalies remain poorly constrained. Here, we characterize the rare earth element (REE) patterns of carbonate phases and the trace metal patterns of bulk carbonate rocks sampled at three hydrocarbon seeps located at Congo Fan pockmarks (CF) and the Gulf of Mexico sites AC645 and GB425. The analyzed CF, GB425, and AC645 carbonates yielded different REE patterns, displaying positive, no, as well as negative Ce anomalies. The covariation of molybdenum (Mo) with uranium (U), including authigenic Mo (Moauth) and U (Uauth) enrichments as well as (Mo/U)auth ratios proved useful to obtain new insight into the applicability of Ce anomalies to constrain past redox conditions. Trace element patterns suggest that (1) CF carbonates formed in a restricted sulfidic environment, while (2) the AC645 site experienced intermittent oxygenation causing negative Ce anomalies, and (3) environmental conditions were variable at the GB425 mud volcano site. Interestingly, GB425 carbonates show significant Mo, arsenic (As), and antimony (Sb) enrichments with the enrichment factor of As (AsEF) correlating well with the authigenic Fe fraction. These results suggest that iron oxyhydroxides played an important role in the adsorption of Mo, As, and Sb in the water column and their transfer to the sediment. The combination of trace metal and REE geochemistry of authigenic carbonates used here is a promising tool to better assess past variability of redox conditions and biogeochemical processes at marine hydrocarbon seeps.


      PubDate: 2014-06-14T15:32:48Z
       
  • An online solid phase extraction method for the determination of
           ultratrace level phosphate in water with a high performance liquid
           chromatograph
    • Abstract: Publication date: 25 July 2014
      Source:Chemical Geology, Volume 380
      Author(s): Satoshi Asaoka , Yoshiaki Kiso , Tsubasa Oomori , Hideo Okamura , Toshiro Yamada , Masahiro Nagai
      Phosphorous monitoring is important for eutrophication control in aquatic ecosystems, but ultratrace level concentrations may not be detected by conventional analytical methods. A method for measuring ultratrace level phosphate by online solid-phase extraction combined with HPLC was developed. A short column (50mm) packed with octadecylsilane (ODS) was used for extraction of phosphoantimonylmolybdenum blue and dodecyltrimethylammonium hydrophobic ion-pair complexes. The ion-pair complexes entrapped on the ODS column were eluted with CH3CN/H2O (35/65; flow rate, 1.0mlmin−1) and monitored by an ultraviolet/visible spectrophotometer (λ=872nm). Phosphate concentration was determined from the peak area of the ion pair. The limit of detection for orthophosphate was 0.15μg PO4 l−1 and the dynamic range was 0.15–100μg PO4 l−1. Although our method was susceptible to silicate interference, it could be corrected by a proposed interference correction equation.
      Graphical abstract image

      PubDate: 2014-06-14T15:32:48Z
       
  • An eclogitic diamond from Mir pipe (Yakutia), recording two growth events
           from different isotopic sources
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): G.P. Bulanova , D.F. Wiggers de Vries , D.G. Pearson , A. Beard , S. Mikhail , A.P. Smelov , G.R. Davies
      An eclogitic octahedral macrodiamond from the Mir kimberlite (Yakutia) has a complex growth structure with distinctive core, intermediate and rim zones. Carbon isotope ratios change abruptly from depleted δ13C values of −16.6‰ in the core to a mean mantle-like signature of −6.8‰ in the intermediate and −7.5‰ in the rim areas, establishing that two main stages of diamond formation took place from fluids/melts derived from different carbon isotope reservoirs. The core and intermediate growth areas are separated by a zone with oscillatory growth that records an episode of diamond dissolution and regrowth. The Mir kimberlite pipe is known to contain two populations of eclogitic diamonds, with light and heavy carbon isotopes, but this is the first case where both populations have been found in a single diamond monocrystal. Multiple sulphides and two silicate inclusions occur at the boundary of the oscillatory zone with the intermediate area of the diamond. Silicate inclusions are composite and there is textural and compositional disequilibrium between the mineral phases. A primary omphacite phase has Mg# 66, a high jadeite content, 0.3wt.% K2O and contains no Cr. Texturally later omphacites in the composite inclusions have Mg# 70, lower jadeite, no potassium, 0.3wt.% Cr2O3 and occur together with interstitial albite and single grains of phlogopite. Late omphacites are 1.6–3 times higher in Ba, Ti, and Yb. This later mineral assemblage was formed as the result of decompression of primary omphacite in a partly open system in the presence of a fluid/melt enriched in Ba, K, Ti, Yb, Cr, and possibly water. Estimated formation pressure for the primary omphacite based on experimental data is 6GPa and significantly less than 6GPa for the later omphacites. A mantle residence time of 1.7Ga is implied for the core formation of the studied diamond, using published results that couple a 2.1Ga Re–Os sulphide inclusion age for Mir eclogitic diamonds with δ13C values of <−10‰, relative to the 0.36Ga emplacement age of the kimberlite. The re–growth and formation of the intermediate and rim areas took place at 0.9Ga based on a Re–Os isochron age determined from the sulphide inclusions of these zones, from a carbon source with mean mantle δ13C values. It is proposed that an early eclogitic diamond population characterised by low N content, negative δ15N and low δ13C values formed at a mantle depth of ~180km during the final assembly of the Siberian Craton associated with active subduction at ~2.1Ga. Formation of the second, 0.9Ga aged population of eclogitic diamonds characterised by moderate N content, less negative δ15N and typical mean-mantle δ13C values (−5‰) took place at a shallower depth of ~120km and is associated with rift-related magmatism at the end of the Proterozoic. Evidence for both episodes of Mir eclogitic diamond growth is found recording its exhumation from lower to shallower upper mantle depths and formation in two distinct geochemical environments.


      PubDate: 2014-06-14T15:32:48Z
       
  • Linking soil element-mass-transfer to microscale mineral weathering across
           a semiarid environmental gradient
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Rebecca A. Lybrand , Craig Rasmussen
      Understanding controls on silicate weathering is critical to characterizing critical zone evolution. The objective of this study was to investigate how climate, vegetation, and landscape position control feldspar transformations across a semiarid environmental gradient. Granitic surface soil and saprock samples were collected from desert scrub and mixed conifer sites within the Santa Catalina Mountain Critical Zone Observatory where mean annual temperature ranges from 24°C to 10°C and mean annual precipitation from 25 to 85cm. Quantitative X-ray diffraction, X-ray fluorescence, and electron microprobe analyses were employed to quantify elemental changes in bulk soils and across plagioclase grains. The chemical depletion of Na in bulk soils ranged from 5.4 – 15% in the desert scrub sites relative to 16–33% in the mixed conifer sites. Plagioclase grain alteration was classified into unaltered, edge, and altered sections to compare microscale weathering and elemental variation. The Na/Al and Si/Al ratios decreased from unaltered, to edge, to altered grain sections in the mixed conifer sites, whereas the element ratios of the desert scrub system were similar between unaltered and edge grain sections, and only exhibited significant decreases in Na/Al and Si/Al ratios between edge and altered materials. The microscale depletion of Na and Si suggested increased silicate weathering in the cooler, wetter, and more biologically productive mixed conifer system compared to the hot, dry desert scrub system. The results also demonstrated a topographic control on mineral transformation where increased plagioclase weathering occurred in convergent footslope landscapes with little change in elemental depletion of soils in divergent summit sites.


      PubDate: 2014-06-14T15:32:48Z
       
  • Separation and analysis of chlorine isotopes in higher plants
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Aide Sun , Qingcai Xu , Shujian Xu , Hongyuan Shen , Jing Sun , Yanling Zhang
      A novel chemical mass spectrographic method was used in the determination of chlorine (Cl) isotopes in plant tissues. The procedure includes dry ashing, three-step ion chromatographic separation of Cl isotopes, and isotope ratio determination based on Cs2Cl+ ion in positive thermal ionization mass spectrometry. The recovery of the method and the fractionation of Cl isotopes were validated using certified reference standard materials. The pretreatment strongly eliminated the effects of organic impurities and other anionic interferences, especially soluble nitrates and sulfates. The results show that there was severe fractionation of Cl isotopic composition in the tissues of plant samples, which might be caused by different molecular mechanisms of uptake and translocation of Cl within plants. The observed Cl isotopic variation is considered to be a useful isotope signature of living systems, which may be used to understand better the Cl cycling process in the environment.
      Graphical abstract image

      PubDate: 2014-06-14T15:32:48Z
       
  • Effects of Co and Ni co-doping on the structure and reactivity of
           hexagonal birnessite
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Hui Yin , Hui Li , Yan Wang , Matthew Ginder-Vogel , Guohong Qiu , Xionghan Feng , Lirong Zheng , Fan Liu
      Natural hexagonal birnessites are enriched in various transition metals (TMs). Many studies have examined the effects of single metal doping on the structures and properties of birnessites, but none focused on the simultaneous interaction mechanism of coprecipitation of two different TMs with birnessite. In this work Co and Ni co-doped hexagonal birnessites were synthesized and characterized by powder X-ray diffraction (XRD), elemental analysis, field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) spectroscopy to investigate the effects of co-doping on the structure and reactivity of birnessite and the crystal chemistry of Co and Ni. These co-doped birnessites have lower crystallinity, i.e., fewer manganese layers stacking in the c* direction, larger specific surface areas (SSAs) and increased Mn average oxidation states (AOSs) than the undoped birnessite, and Co exists in a valence of +3. Co, Ni and Mn K-edge extended X-ray absorption fine structure spectroscopy (EXAFS) spectra demonstrate an increase in edge-sharing Ni–Me (Me=Ni, Co and Mn) distances in birnessite layers with the increase of the contents of dopants while Mn–Me distances first decrease and then increase while those of Co–Me pairs are nearly constant, coupled with first a decrease and then increase of the in-plane unit-cell parameter b. The effect of co-doping on the amounts of structural Mn and K+, numbers of [MnO6] layers stacked in c* axis, and SSAs, is larger than the effects of doping with Co alone, but less than singly Ni doping. In birnessites doped with both Co and Ni, ~74–79% of the total Co and ~23–39% of the total Ni are present within the manganese layers. Compared with the spatial distribution of TM in singly doped birnessites, the coexistence of Ni hinders the incorporation of Co into the layers during birnessite crystallization; however, coprecipitation with Co has little effects, neither hindrance nor promotion, on the insertion of Ni into the layers. These results provide insight into the interaction mechanism between coexisting Co, Ni within layered Mn oxides. It further helps us to interpret the geochemical characteristics of multi-metal incorporation into natural Mn oxides and their effects on the structures and physicochemical properties of these minerals.


      PubDate: 2014-06-14T15:32:48Z
       
  • Discriminating between different genetic types of marine ferro-manganese
           crusts and nodules based on rare earth elements and yttrium
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): M. Bau , K. Schmidt , A. Koschinsky , J. Hein , T. Kuhn , A. Usui
      Marine ferro-manganese (Fe–Mn) crusts and nodules are metal-rich chemical sediments that are archives of paleoceanographic proxies and potential metal resources and targets of deep-sea mining. Traditionally, crusts and nodules are subdivided into hydrogenetic, diagenetic, and hydrothermal types. Because these are characterized by different compositions and hence economic resource potential, a useable genetic classification is not only a tool for geochemical studies but may also help to characterize potential exploration targets. We propose two easy-to-use, yet robust discrimination diagrams based on geochemical relationships controlling the rare earths and yttrium (REY) inventory of marine Fe–Mn (oxyhydr)oxide deposits. The REY are a set of trace elements that show coherent behavior, are routinely determined in geochemical studies, and for which the analytical quality can easily be evaluated. Graphs of Ce anomaly vs Nd concentration and Ce anomaly vs Y anomaly effectively discriminate between the different types of precipitates, regardless of their diverse mineralogical composition. Both hydrogenetic crusts and nodules show positive Ce anomalies, negative Y anomalies and high Nd concentrations of >100mgkg−1, although nodules tend to have slightly lower Nd concentrations than crusts. In marked contrast, hydrothermal deposits generally yield negative Ce anomalies, positive Y anomalies, and low Nd concentrations of <10mgkg−1, in spite of their large geochemical, mineralogical and sedimentological diversity. Diagenetic nodules show negative Ce anomalies, negative Y anomalies, and intermediate Nd concentrations, between 10 and 100mgkg−1. We discuss the geochemical background of these diagrams, the respective sources of REY in the different precipitates, and address the processes that control their REY inventory. Besides the three end-members, we also use REY systematics to define mixed-type diagenetic–hydrogenetic nodules and classify well-studied international reference standards.


      PubDate: 2014-06-14T15:32:48Z
       
  • Dating iron skarn mineralization using hydrothermal allanite-(La)
           U–Th–Pb isotopes by laser ablation ICP-MS
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): Xiao-Dong Deng , Jian-Wei Li , Guang Wen
      Trace elements and U–Th–Pb isotopes of hydrothermal allanite from the Beiminghe iron skarn deposit (eastern North China Craton) were simultaneously analyzed using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) to demonstrate the suitability and robustness of the U–Th–Pb system in this phase as a geochronometer of ore-forming processes. Allanite grains from the Beiminghe deposit, typically exhibit optical and chemical zoning and are intergrown or texturally equilibrated with prograde and/or retrograde skarn assemblages mainly consisting of tremolite, diopside, magnetite, and garnet. In addition, they contain abundant aqueous two-phase or daughter mineral-bearing three-phase aqueous fluid inclusions, confirming their hydrothermal origin. Most grains have high La/Ce atomic ratios (1.19–1.74) and thus are classified as allanite-(La). A notable feature of the hydrothermal allanite-(La) is the lack of common Pb, with values comparable with those of the zircon standards 91500 and GJ-1 used in this study. LA-ICPMS spot analyses reveal remarkable variation in Ca, REEs (rare earth elements), Fe, Al, and Mg within a single grain, consistent with the substitution of REE3+ +Mg+Fe2+ for Ca+Al+Fe3+ in the mineral structure. Traverses of zoned grains show that the rims have Th/U ratios ranging from 0.73 to 11.55, significantly higher than the core areas (0.07–1.81). Fifty-nine spot analyses (beam size of 32μm or 60μm) on 5 allanite-(La) grains yield reproducible 206Pb/238U ages of 134±7 to 139±6Ma, with a weighted mean 206Pb*/238U age (206Pbexcess-corrected 206Pb/238U age) of 136±1Ma (2σ). Of these, 34 analyses on allanite rims have a weighted mean 208Pb/232Th age of 139±2Ma (2σ) that is within error of the 206Pb*/238U age. The dating results showed that an increase of beam size from 32 to 60μm significantly reduces the analytical uncertainties (2σ). Petrographic and textural data have demonstrated that the allanite-(La) grains formed during skarn alteration and notably have close paragenetic relationships with retrograde skarn assemblages and iron oxide minerals; their U–Th–Pb ages therefore are interpreted as the timing of iron skarn mineralization at Beiminghe. Thirteen zircon grains from the ore-related diorite intrusion have a weighted mean 206Pb/238U age of 136±2Ma (2σ), consistent with and thus confirming the reliability of the allanite-(La) U–Th–Pb ages when interpreted as the formation age of the Beiminghe iron deposit. Our results suggest that hydrothermal allanite with low Th and U is a promising U–Th–Pb geochronometer and can provide reliable constraints on the timing of hydrothermal mineralization without requiring a matrix-matched external standard. The present dating results also indicate that the U–Pb, rather than Th–Pb, isotope system is preferable for U–Th–Pb dating of low Th/U allanite.


      PubDate: 2014-06-14T15:32:48Z
       
  • Geochemical response to pulsed sedimentation: Implications for the use of
           Mo as a paleo-proxy
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): J.L. McKay , T.F. Pedersen
      Deglacial sediments in piston core JT96-02 from the continental slope off western Canada contain numerous centimeter-thick, iron sulfide-rich layers. Major and minor element chemistry, as well as C and N isotopic data suggest that these layers represent background hemipelagic sedimentation. Intervening gray silty clay layers, each of which exhibits a fining-upward trend, are distal turbidites. The iron sulfides precipitated in response to the emplacement of each turbidite, which slowed oxygen diffusion into the sediment allowing anoxic conditions to develop in the underlying relatively organic-rich deposits. Resulting sulfate reduction and precipitation of Fe-sulfide fostered Mo enrichment (>2μg/g). It may also explain Ni, Cr, V, and Co enrichments, although differences in detrital mineralogy cannot be ruled out, particularly for Ni and Cr. Rhenium is also enriched, albeit slightly, within the sulfide layers, but low Re/Mo ratios (~0.78ng/g/μg/g) suggest that anoxic conditions developed very rapidly. Metals with high lithogenic concentrations (Cd and U) are not obviously enriched in the sulfide layers because there was insufficient time for noticeable authigenic accumulation before deposition of the next turbidite which severed diffusive communication with overlying seawater, the primary source of metals. Silver is also slightly higher in the sulfide layers, possibly linked to higher marine organic matter content. In summary, the pulsed character of sedimentation during the deglaciation led to episodic enrichment of Mo and occurred independently of changes in bottom water oxygen or organic carbon content. This observation potentially complicates the use of Mo as a paleo-proxy in some continental margin environments.


      PubDate: 2014-06-14T15:32:48Z
       
  • Fractionation of Sr and Hf isotopes by mineral sorting in Cascadia Basin
           terrigenous sediments
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): Marion Carpentier , Dominique Weis , Catherine Chauvel
      Oceanic sediments deposited on continental margins consist mainly of erosion products of the nearby exposed continental areas. Detrital input usually dominates their geochemical budget, and the composition of these sediments should record potential changes in their continental sources. However, along margins, mineral sorting associated with transport and sedimentary processes induces significant chemical and isotopic fractionation over a few tens of kilometers. The study of margin sediments should help to quantify the extent of modification of the continental terrigenous supply when it reaches deep oceans. Reported Sr, Nd, Hf and Pb isotopic compositions of fifty-seven sediments from the northernmost part of the Cascade forearc (Ocean Drilling Program, ODP, Sites 888 and 1027) suggest the involvement of two dominant end-members coming from the nearby Canadian Cordillera. Erosion products of the depleted, western part of the Cordillera dominate the detrital input, while the eastern enriched terranes of the Cordillera contribute only 10 to 28% of the input. There is no marked change of provenance of sediments during the last 3.5Myr and they all appear unaffected by glacial–interglacial climate cycles. The average isotopic compositions of the two sites are slightly different, but are both dominated by continental signature; these values can be used in future studies to identify any subducted sediment contribution to the Cascades Arc. On a finer scale, there are differences in the isotopic signature between samples dominated by clay minerals and those with coarser lithologies. For a given Nd isotopic composition, fine sediments have more radiogenic Sr and Hf isotope ratios than sands, and we interpret the difference as resulting from mineral sorting during transport of the particles. Fine sediments concentrate minerals with radiogenic Sr and Hf such as clays and micas, while coarse-grained detritus carry the unradiogenic mineral component of a given source rock through plagioclase–epidote and zircon. ODP Site 1027 is located 100km further away from continent than ODP Site 888 and contains more clay. As a consequence, it has significantly more radiogenic Sr and Hf bulk composition than ODP Site 888. Similar differences in isotopic signatures related to the distance to continent certainly occur in other areas in the word, and will account for a large part of differences known between continental sources and deep-sea sediments.


      PubDate: 2014-06-14T15:32:48Z
       
  • Carbon dioxide triggered metal(loid) mobilisation in a mofette
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): Judith Mehlhorn , Felix Beulig , Kirsten Küsel , Britta Planer-Friedrich
      Carbon capture and geologic storage is a frequently discussed option to reduce atmospheric CO2 concentrations with the long-term risk of leakage from storage sites to overlying aquifers and soils. We chose natural CO2 exhalations, so-called mofettes, in a wetland area in the Czech Republic as analogues to follow the fate of metal(loid)s under CO2-saturated conditions. Compared to the reference fluvisol at the study site, mofette soils exhibited lower pH (4.9±0.05) and redox potential (300±40mV), as well as higher organic carbon contents. Poorly crystalline and crystalline Fe (hydr)oxides, the most important metal(loid) sorbents in the CO2-unaffected soils (7.9±5.9gkg−1), showed significantly lower concentrations under the acidic and reducing conditions in the mofettes (1.2±0.4gkg−1). In turn, this increased the mobility of As and resulting concentrations were up to 2.5 times higher than in the CO2-unaffected pore water (58±18μgL−1). Methylation (up to 11% of total As) and thiolation (up to 9%) contributed to net As mobilisation. Dissolved Mn (131±53μgL−1), Ni (9.1±3.1μgL−1) and especially Cu (2.2±1.0μgL−1) concentrations remained low, likely due to complexation and/or adsorption to organic carbon and the small amount of Fe (hydr)oxides. A one-month-in-situ mobilisation experiment showed mobilisation of all investigated elements to the aqueous phase suggesting that desorption is the faster and initially dominating process while resorption is a secondary, slower process. We conclude that the CO2-induced mobilisation of toxic As and net-immobilisation of essential micro-nutrients (Mn, Ni, Cu) constitute serious risks and must be tested for transferability and relevance at geologic carbon storage sites.


      PubDate: 2014-06-14T15:32:48Z
       
  • Understanding biogeochemical gradients of sulfur, iron and carbon in an
           oil sands tailings pond
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): Sebastian Stasik , Nadine Loick , Kay Knöller , Christopher Weisener , Katrin Wendt-Potthoff
      Oil sands tailings ponds in Alberta (Canada) are strongly stratified ecosystems structured in an upper water layer and underlying mud layers that harbour a diversity of microorganisms, contributing to hydrocarbon degradation and elemental cycling. Until now not much is known about the biogeochemistry of the ponds and their spatial structure is not well explored yet. An understanding of microbial activity and community composition is important, in particular, in order to determine potential effects on pond properties and long term development. Therefore, the purpose of the present study was to identify reactive zones of iron, carbon and sulfur cycling in an active tailings pond, by comparing biogeochemical data along two depth profiles. For both profiles a zone of intense sulfur cycling was substantiated by maxima of: (a) dissolved and solid sulfides; (b) sulfate reduction rates and thiosulfate oxidation potentials; and (c) viable counts of sulfate reducers and relative abundances of functional genes. In addition, methanogenesis and microbial iron reduction were shown to be important electron accepting processes in the ponds. All processes coexisted in a zone of intense elemental cycling at a depth of 1–4m below the water–mud interface, where fresh tailings are likely to accumulate. Microbial activity and biomass decreased with depth, where tailings had higher age and density. While the upper mud layers were influenced by the presence of different archaea, the microbial communities showed an increased presence of bacterial species at depth. Insights from qPCR, 35S radiotracer technique and stable isotope analysis mirrored some differences between the profiles, regarding sulfur and carbon cycling. Despite this, both profiles showed remarkably similar patterns of microbial community composition and activity, revealing a good reproducibility of biogeochemical cycling within a few metres.


      PubDate: 2014-06-14T15:32:48Z
       
  • Records of trace metals in sediments from the Oregon shelf and slope:
           Investigating the occurrence of hypoxia over the past several thousand
           years
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): Andrea M. Erhardt , Clare E. Reimers , David Kadko , Adina Paytan
      Hypoxic (<62μmol/kg or 1.43mL/L O2) to anoxic conditions have been repeatedly observed over the last 10years on the Oregon shelf, while similar conditions are absent in historical records from 1950 to 1999. This study seeks to identify whether similar instances of decadal length hypoxia/anoxia have occurred in the Oregon coastal zone prior to recorded history and to shed light on potential causes for these events. We have measured redox-sensitive metals, uranium, vanadium, and molybdenum concentration profiles in 7 cores across the coastal affected region and in 3 cores from deeper water sites. Results indicate regional variability in redox conditions through time. The northern sites show no metal enrichment throughout the cores, while the southern sites show strong metal enrichment at the base of the cores, indicative of previous hypoxic/anoxic conditions. The southern sites indicate progression in time toward less hypoxic/anoxic burial, in conflict with recent hydrographic trends. Analysis of offshore sediments representing sites beneath the California Undercurrent shows an opposite trend to that observed in the coastal sites. Excess Mo concentrations generally increase toward the present in cores collected within the upwelling source waters at ~300m water depth, suggesting a trend toward oxygen depletion. The Mo enrichment corresponds to increases in δ13C, total organic carbon (TOC), and declines in carbon:nitrogen (C:N) ratios which may indicate that a localized rise in marine productivity has contributed to oxygen drawdown. However, these metal and associated geochemical enrichments are not clearly seen in other cores collected in deeper water in the same region, suggesting that widespread changes in productivity or hypoxia may not have occurred. When the Mo enrichment records for two dated mid-depth sites are compared to climatic indicators such as the Pacific Decadal Oscillation no clear relationships are found on decadal time scales. These results are consistent with the hypothesis that modern hypoxic conditions observed on the continental shelf throughout the region are driven by modern climate changes, not observed in this region for the past hundreds to thousands of years.


      PubDate: 2014-06-14T15:32:48Z
       
  • Methane sources and sinks in the subtropical South Pacific along 17°S
           as traced by stable isotope ratios
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): Chisato Yoshikawa , Elena Hayashi , Keita Yamada , Osamu Yoshida , Sakae Toyoda , Naohiro Yoshida
      We analyzed the concentration and stable carbon isotopic ratio (δ13C-CH4) of methane in the atmosphere and in dissolved methane in water column along 17°S in the subtropical South Pacific. Additionally, the hydrogen isotopic ratios (δD-CH4) of some water samples were analyzed. The sea–air CH4 flux is high in the eastern region and off the west coast of Australia, which is related to the high concentrations of dissolved CH4 and high wind speeds. Moreover, there is a positive correlation between the CH4 and chlorophyll a concentrations at the surface. This consistency suggested that active CH4 productions related to the primary production cause surface CH4 accumulation. CH4 shows a decrease in concentration and an increase in δ13C-CH4 and δD-CH4 values from the surface to the depth of about 1000m. The relationship between δ13C-CH4 values and CH4 concentration indicates that the isotopic enrichment of CH4 reflects microbial oxidation of CH4 with isotopic fractionation during vertical transport via vertical sinking and/or zooplankton migration. East of 120°W, δ13C-CH4 values at around 1000m exceed −30.0‰. The relationships among the δ13C-CH4 values, CH4 concentrations, and oxygen concentrations indicate that the 13C-enriched CH4 originates not only from in situ CH4 production and oxidation but also from CH4 transported from the eastern margin off Peru. Furthermore, at a site near the Central Lau Spreading Centers in the Lau Basin, high δ13C-CH4 values (up to −21.4‰) are observed in the benthic water, suggesting a hydrothermal field source.


      PubDate: 2014-06-14T15:32:48Z
       
  • Chalcophile elemental compositions of MORBs from the ultraslow-spreading
           Southwest Indian Ridge and controls of lithospheric structure on
           S-saturated differentiation
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): Alexandra Yang Yang , Mei-Fu Zhou , Tai-Ping Zhao , Xi-Guang Deng , Liang Qi , Ji-Feng Xu
      Sulfide globules in mid-ocean ridge basalts (MORBs) from the ultra-slow spreading Southwest Indian Ridge (SWIR) are present in olivine crystals and in the matrix glass, indicating S-saturated fractionation during magmatic evolution. Most of the sulfides were in equilibrium with the magmas based on the Ni contents of the sulfides. The major element trends of MORBs from Zone C (48–51°E) of the SWIR define two distinct liquid lines of descent (LLD), including one which is composed of MORBs depleted in HREE. The MORBs have variable PGE concentrations with Pd contents ranging from 0.056 to 1.572ppb. They are PGE-depleted relative to Ni and Cu with high Cu/Pd ratios (55,000–1,785,000). Quantitative modeling using Pd vs. Ni correlations yields relatively low amounts of segregated sulfides for both groups assuming complete equilibration between the sulfides and the melt. However, the correlation of calculated sulfur contents with MgO suggests much higher amounts of segregated sulfides, indicating that the segregated sulfides are incompletely equilibrated with the magma. There are no obvious correlations between PGE concentrations and spreading rates for MORBs globally. MORBs from the SWIR show slowly decreasing PGE contents during magmatic evolution but exhibit higher sulfide segregation rates compared to MORBs from faster-spreading ridges, largely due to the poor equilibration between the segregated sulfides and the magma. Such incomplete equilibration could result from thicker lithosphere and an absence of steady magma chambers and conduits beneath ultra-slow spreading ridges.


      PubDate: 2014-06-14T15:32:48Z
       
  • Evaluation of foraminiferal trace element cleaning protocols on the Mg/Ca
           of marine ostracod genus Krithe
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): William Gray , Jonathan Holmes , Amelia Shevenell
      The Mg/Ca of calcite from the marine ostracod genus Krithe may be an important tool for reconstructing past changes in oceanic bottom water temperature (150–4000m water depth). Rigorous cleaning procedures, routinely used to remove clays, organic matter and Fe–Mn oxyhydroxide coatings in trace element studies of foraminifera, are not regularly applied to marine ostracods despite the potential for Mg contamination. Here we apply standard oxidative and reductive foraminiferal cleaning procedures to core top Krithe pernoides valves from boxcore OCE205-50BC (26.23°N, 77.7°W, 817m water depth) to evaluate the effects of contamination on Mg/Ca ratios and assess the impact of cleaning techniques on contaminant removal and ostracod valve chemistry. Our results show that clays and Fe–Mn oxyhydroxides influence the Mg/Ca of Krithe. Following sonication in methanol/ultrapure water, there is a 1.6mmol/mol (11%) decrease in Mg/Ca (equivalent to a reduction in reconstructed temperature of 1.5°C), indicating that this is a critical step in the preparation of Krithe valves for Mg/Ca analyses. Oxidation with buffered hydrogen peroxide has little effect on the Mg/Ca of valves from our site. Reductive cleaning reduces inter-valve variability from 12% to 5%, resulting in an equivalent temperature precision of ±0.6°C. However, reductive cleaning also decreases Mg/Ca ratios due to the partial dissolution of the valve surface. Reductive cleaning offers the potential to improve Krithe Mg/Ca paleotemperature reconstructions and should be utilised in future Krithe Mg/Ca studies. Future work should also aim to constrain the effects of partial dissolution of the valve surface.


      PubDate: 2014-06-14T15:32:48Z
       
  • The genesis of gold mineralisation hosted by orogenic belts: A lead
           isotope investigation of Irish gold deposits
    • Abstract: Publication date: Available online 25 April 2014
      Source:Chemical Geology
      Author(s): C.D. Standish , B. Dhuime , R.J. Chapman , C.J. Hawkesworth , A.W.G. Pike
      Lead isotope analyses have been performed on 109 gold and 23 sulphide samples from 34 Irish gold occurrences, including 27 placers, and used to shed light on the sources of mineralising fluids and metals associated with gold mineralisation hosted by orogenic belts. The Pb isotope ratios of lode and placer gold range from 206Pb/204Pb=17.287-18.679, 207Pb/204Pb=15.382-15.661, and 208Pb/204Pb=37.517-38.635, consistent with the Pb isotopic data on previously reported Irish sulphide mineralisation. There is no evidence that gold mineralisation is associated with distinctive source regions, and it appears to have been derived from similar sources to those responsible for the widespread sulphide mineralisation in Ireland. It is inferred that the principal controls on the Au mineralisation are structural and not related to the distribution of Au in their source rocks. The range of Pb isotope ratios favours the interaction of multiple source reservoirs predominantly during the Caledonian Orogeny (c.475-380Ma). Underlying basement was the primary control on two key sources of Pb. Gold occurrences located to the south-east of the Iapetus Suture are characterised by Pb compositions that derive predominantly from the Late Proterozoic crustal basement or overlying Lower Palaeozoic sediments, whilst those located north-west of the Iapetus Suture are characterised by less radiogenic Pb signatures derived predominantly from Late Proterozoic or older crustal basement. A third source, relatively enriched in radiogenic Pb, also played a role in the formation of a number of Irish gold occurrences, and may have been associated with syn- to post-orogenic intrusives. Magmatic processes may therefore have played an important role in the formation of some gold occurrences located in orogenic settings.


      PubDate: 2014-04-29T07:12:42Z
       
  • Controls on Sr isotopic evolution in lacustrine systems: Eocene green
           river formation, wyoming
    • Abstract: Publication date: Available online 26 April 2014
      Source:Chemical Geology
      Author(s): Amalia C. Doebbert , Clark M. Johnson , Alan R. Carroll , Brian L. Beard , Jeffrey T. Pietras , Meredith Rhodes Carson , Brooke Norsted , L. Ashley Throckmorton
      Strontium isotopes from lacustrine carbonates record detailed weathering histories of exposed bedrocks, and thus are potentially useful for understanding interactions between tectonics and climate that may be driven by local or regional factors in basin-scale hydrologic systems. We combine extensive 87Sr/86Sr and δ18O datasets from the Green River Formation in order to identify environmental factors driving the evolution of Sr concentration and isotopic composition in lacustrine systems through the use of Sr mass balance modeling. Two models are developed. The first tests the effect of drainage capture that drove a constant-volume lake from balance-filled to overfilled conditions, and is applied to the Laney Member of the Green River Formation. The second model focuses on lacustrine evolution in response to changing the concentration, composition, and mass of water influx in a variable-size Sr reservoir (lake). This model is applied to the Wilkins Peak Member of the Green River Formation. 87Sr/86Sr ratios are correlated with δ18O values only during the Laney Member, when both are readily explained by a common forcing mechanism (drainage capture). Modeling of Sr isotope compositions and concentrations indicates a short residence time for Sr (~103-104 years or less) in both balance-overfilled and underfilled phases of the Green River Formation lacustrine system. This in turn suggests that paleo-lacustrine sediments in most lakes can preserve Sr isotope records with high-resolution (~102 years) timescales. Rates of Sr sequestration in carbonate are shown to have a strong influence on lacustrine Sr concentration, and high Sr concentrations of lacustrine carbonate consistent with high salinity are observed in the underfilled Wilkins Peak Member and the balance-filled Tipton and lower Laney Members. In the Laney Member, 87Sr/86Sr mass-balance modeling results provide additional support for previous interpretations that the introduction of a large drainage system produced an isotopic shift across the lower LaClede/upper LaClede boundary. Major drainage reorganization is not required to drive high variability in 87Sr/86Sr ratios, however. Modeling shows that variability in 87Sr/86Sr ratios of ~0.004 observed in the Wilkins Peak Member can be explained by change in the characteristics of intrabasinal water sources during highstand vs. lowstand conditions.


      PubDate: 2014-04-29T07:12:42Z
       
 
 
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