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    - EARTH SCIENCES (468 journals)
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EARTH SCIENCES (468 journals)                  1 2 3 | Last

Showing 1 - 200 of 371 Journals sorted alphabetically
Acta Geochimica     Hybrid Journal   (Followers: 3)
Acta Geodaetica et Geophysica     Hybrid Journal   (Followers: 1)
Acta Geodaetica et Geophysica Hungarica     Full-text available via subscription   (Followers: 2)
Acta Geophysica     Open Access   (Followers: 6)
Acta Geotechnica     Hybrid Journal   (Followers: 7)
Acta Meteorologica Sinica     Hybrid Journal   (Followers: 3)
Advances in High Energy Physics     Open Access   (Followers: 20)
Advances In Physics     Hybrid Journal   (Followers: 19)
Aeolian Research     Hybrid Journal   (Followers: 5)
African Journal of Aquatic Science     Hybrid Journal   (Followers: 13)
Algological Studies     Full-text available via subscription   (Followers: 2)
Alpine Botany     Hybrid Journal   (Followers: 4)
AMBIO     Hybrid Journal   (Followers: 14)
Anales del Instituto de la Patagonia     Open Access   (Followers: 1)
Andean geology     Open Access   (Followers: 13)
Annales Henri Poincaré     Hybrid Journal   (Followers: 3)
Annales UMCS, Geographia, Geologia, Mineralogia et Petrographia     Open Access  
Annals of Geophysics     Open Access   (Followers: 13)
Annals of GIS     Hybrid Journal   (Followers: 21)
Annual Review of Marine Science     Full-text available via subscription   (Followers: 11)
Anthropocene     Hybrid Journal   (Followers: 3)
Anthropocene Review     Hybrid Journal   (Followers: 6)
Applied Clay Science     Hybrid Journal   (Followers: 4)
Applied Geochemistry     Hybrid Journal   (Followers: 12)
Applied Geomatics     Hybrid Journal   (Followers: 3)
Applied Geophysics     Hybrid Journal   (Followers: 7)
Applied Ocean Research     Hybrid Journal   (Followers: 5)
Applied Petrochemical Research     Open Access   (Followers: 2)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 34)
Arctic Science     Open Access   (Followers: 6)
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 9)
Artificial Satellites : The Journal of Space Research Centre of Polish Academy of Sciences     Open Access   (Followers: 18)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 20)
Asian Journal of Earth Sciences     Open Access   (Followers: 20)
Asian Review of Environmental and Earth Sciences     Open Access   (Followers: 1)
Atlantic Geology : Journal of the Atlantic Geoscience Society / Atlantic Geology : revue de la Société Géoscientifique de l'Atlantique     Full-text available via subscription   (Followers: 10)
Atmosphere-Ocean     Full-text available via subscription   (Followers: 12)
Atmospheric and Climate Sciences     Open Access   (Followers: 28)
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: 10)
Bragantia     Open Access   (Followers: 2)
Bulletin of Earthquake Engineering     Hybrid Journal   (Followers: 11)
Bulletin of Geosciences     Open Access   (Followers: 8)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (Followers: 1)
Bulletin of the Seismological Society of America     Full-text available via subscription   (Followers: 21)
Bulletin of Volcanology     Hybrid Journal   (Followers: 17)
Cadernos de Geociências     Open Access  
Canadian Mineralogist     Full-text available via subscription   (Followers: 3)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 21)
Carbonates and Evaporites     Hybrid Journal   (Followers: 3)
CATENA     Hybrid Journal   (Followers: 5)
Chemical Geology     Hybrid Journal   (Followers: 18)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 4)
Chinese Geographical Science     Hybrid Journal   (Followers: 5)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 4)
Ciencia del suelo     Open Access   (Followers: 2)
Ciencias Espaciales     Open Access  
Climate and Development     Hybrid Journal   (Followers: 12)
Coastal Management     Hybrid Journal   (Followers: 25)
Cogent Geoscience     Open Access  
Comptes Rendus Geoscience     Full-text available via subscription   (Followers: 8)
Computational Geosciences     Hybrid Journal   (Followers: 13)
Computational Mathematics and Mathematical Physics     Hybrid Journal   (Followers: 2)
Computers and Geotechnics     Hybrid Journal   (Followers: 10)
Contemporary Trends in Geoscience     Open Access   (Followers: 3)
Continental Shelf Research     Hybrid Journal   (Followers: 9)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 10)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 3)
Coral Reefs     Hybrid Journal   (Followers: 16)
Cretaceous Research     Hybrid Journal   (Followers: 6)
Cybergeo : European Journal of Geography     Open Access   (Followers: 5)
Depositional Record     Open Access  
Developments in Geotectonics     Full-text available via subscription   (Followers: 3)
Developments in Quaternary Science     Full-text available via subscription   (Followers: 3)
Développement durable et territoires     Open Access   (Followers: 3)
Diatom Research     Hybrid Journal   (Followers: 2)
Doklady Physics     Hybrid Journal   (Followers: 1)
Dynamics of Atmospheres and Oceans     Hybrid Journal   (Followers: 11)
E&S Engineering and Science     Open Access  
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (Followers: 112)
Earth and Space Science     Open Access   (Followers: 12)
Earth Interactions     Full-text available via subscription   (Followers: 11)
Earth Science Research     Open Access   (Followers: 5)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 4)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 19)
Earth System Dynamics     Open Access   (Followers: 6)
Earth System Dynamics Discussions     Open Access   (Followers: 4)
Earth's Future     Open Access   (Followers: 1)
Earth, Planets and Space     Open Access   (Followers: 67)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 8)
Earthquake Science     Hybrid Journal   (Followers: 11)
Earthquake Spectra     Full-text available via subscription   (Followers: 18)
Ecohydrology     Hybrid Journal   (Followers: 11)
Ecological Questions     Open Access   (Followers: 7)
Electromagnetics     Hybrid Journal   (Followers: 5)
Energy Efficiency     Hybrid Journal   (Followers: 11)
Energy Exploration & Exploitation     Hybrid Journal   (Followers: 4)
Environmental Earth Sciences     Hybrid Journal   (Followers: 24)
Environmental Geology     Hybrid Journal   (Followers: 20)
Environmental Geosciences     Full-text available via subscription   (Followers: 4)
Environmental Geotechnics     Hybrid Journal   (Followers: 5)
Erwerbs-Obstbau     Hybrid Journal  
Estuaries and Coasts     Hybrid Journal   (Followers: 18)
Estuarine, Coastal and Shelf Science     Hybrid Journal   (Followers: 33)
Estudios Geográficos     Open Access  
European Journal of Mineralogy     Full-text available via subscription   (Followers: 13)
European Journal of Remote Sensing     Open Access  
Exploration Geophysics     Hybrid Journal   (Followers: 4)
Facies     Hybrid Journal   (Followers: 9)
Fieldiana Life and Earth Sciences     Full-text available via subscription   (Followers: 1)
Física de la Tierra     Open Access  
Folia Musei rerum naturalium Bohemiae occidentalis. Geologica et Paleobiologica     Open Access  
Folia Quaternaria     Open Access  
Forestry Chronicle     Full-text available via subscription   (Followers: 10)
Frontiers in Earth Science     Open Access   (Followers: 5)
Frontiers in Geotechnical Engineering     Open Access   (Followers: 3)
Frontiers of Earth Science     Hybrid Journal   (Followers: 8)
Fundamental and Applied Limnology / Archiv für Hydrobiologie     Full-text available via subscription   (Followers: 4)
GEM - International Journal on Geomathematics     Hybrid Journal   (Followers: 1)
Geo-Marine Letters     Hybrid Journal   (Followers: 7)
Geoacta     Open Access   (Followers: 4)
Geobiology     Hybrid Journal   (Followers: 8)
Geocarto International     Hybrid Journal   (Followers: 22)
Geochemical Perspectives     Hybrid Journal  
Geochemistry : Exploration, Environment, Analysis     Hybrid Journal   (Followers: 7)
Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 26)
Geochimica et Cosmochimica Acta     Hybrid Journal   (Followers: 47)
Geochronometria     Hybrid Journal   (Followers: 3)
Geoderma Regional : The International Journal for Regional Soil Research     Full-text available via subscription   (Followers: 3)
Geodinamica Acta     Hybrid Journal   (Followers: 3)
Geodynamics & Tectonophysics     Open Access   (Followers: 1)
Geoenvironmental Disasters     Open Access   (Followers: 3)
Geofluids     Open Access   (Followers: 4)
Geoforum     Hybrid Journal   (Followers: 22)
Géographie physique et Quaternaire     Full-text available via subscription   (Followers: 1)
Geography and Natural Resources     Hybrid Journal   (Followers: 7)
Geoheritage     Hybrid Journal   (Followers: 1)
Geoinformatica Polonica : The Journal of Polish Academy of Arts and Sciences     Open Access  
Geoinformatics & Geostatistics     Hybrid Journal   (Followers: 9)
Geological Journal     Hybrid Journal   (Followers: 14)
Geology Today     Hybrid Journal   (Followers: 25)
Geology, Geophysics and Environment     Open Access   (Followers: 1)
Geomagnetism and Aeronomy     Hybrid Journal   (Followers: 3)
Geomatics, Natural Hazards and Risk     Hybrid Journal   (Followers: 9)
Geomechanics for Energy and the Environment     Full-text available via subscription   (Followers: 1)
GEOmedia     Open Access   (Followers: 1)
Geomorphology     Hybrid Journal   (Followers: 27)
Geophysical & Astrophysical Fluid Dynamics     Hybrid Journal   (Followers: 2)
Geophysical Journal International     Hybrid Journal   (Followers: 31)
Geophysical Prospecting     Hybrid Journal   (Followers: 7)
Geophysics     Full-text available via subscription   (Followers: 18)
GeoResJ     Hybrid Journal  
Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards     Hybrid Journal   (Followers: 8)
Geoscience Canada : Journal of the Geological Association of Canada / Geoscience Canada : journal de l'Association Géologique du Canada     Full-text available via subscription   (Followers: 3)
Geoscience Data Journal     Open Access   (Followers: 2)
Geoscience Frontiers     Open Access   (Followers: 9)
Geoscience Letters     Open Access   (Followers: 1)
Geoscience Records     Open Access  
Geosciences     Open Access   (Followers: 2)
Geosciences Journal     Hybrid Journal   (Followers: 10)
Geoscientific Instrumentation, Methods and Data Systems     Open Access   (Followers: 4)
Geoscientific Model Development     Open Access   (Followers: 2)
Geostandards and Geoanalytical Research     Hybrid Journal   (Followers: 2)
Geosystem Engineering     Hybrid Journal   (Followers: 1)
Geotectonic Research     Full-text available via subscription   (Followers: 5)
Geotectonics     Hybrid Journal   (Followers: 7)
GISAP : Earth and Space Sciences     Open Access   (Followers: 2)
Glass Physics and Chemistry     Hybrid Journal   (Followers: 4)
Global and Planetary Change     Hybrid Journal   (Followers: 17)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 15)
Gondwana Research     Hybrid Journal   (Followers: 7)
GPS Solutions     Hybrid Journal   (Followers: 17)
Grassland Science     Hybrid Journal  
Ground Water     Hybrid Journal   (Followers: 34)
Ground Water Monitoring & Remediation     Hybrid Journal   (Followers: 19)
Groundwater for Sustainable Development     Full-text available via subscription   (Followers: 1)
Helgoland Marine Research     Open Access   (Followers: 3)
History of Geo- and Space Sciences     Open Access   (Followers: 4)
Hydrobiologia     Hybrid Journal   (Followers: 18)
Hydrogeology Journal     Hybrid Journal   (Followers: 18)
Hydrological Processes     Hybrid Journal   (Followers: 27)
Hydrology and Earth System Sciences     Open Access   (Followers: 28)
ICES Journal of Marine Science: Journal du Conseil     Hybrid Journal   (Followers: 54)
IEEE Journal of Oceanic Engineering     Hybrid Journal   (Followers: 12)
Indian Geotechnical Journal     Hybrid Journal   (Followers: 4)
Indonesian Journal on Geoscience     Open Access   (Followers: 2)
Interdisciplinary Environmental Review     Hybrid Journal   (Followers: 3)
International Geology Review     Hybrid Journal   (Followers: 14)
International Journal of Advanced Geosciences     Open Access  
International Journal of Advanced Remote Sensing and GIS     Open Access   (Followers: 35)
International Journal of Advancement in Earth and Enviromental Sciences     Open Access   (Followers: 3)
International Journal of Advancement in Remote Sensing, GIS, and Geography     Open Access   (Followers: 27)
International Journal of Applied Earth Observation and Geoinformation     Hybrid Journal   (Followers: 36)
International Journal of Coal Geology     Hybrid Journal   (Followers: 2)
International Journal of Disaster Risk Reduction     Hybrid Journal   (Followers: 15)
International Journal of Earth Sciences     Hybrid Journal   (Followers: 35)
International Journal of Earthquake and Impact Engineering     Hybrid Journal   (Followers: 2)
International Journal of Forest, Soil and Erosion     Open Access   (Followers: 6)

        1 2 3 | Last

Journal Cover Chemical Geology
  [SJR: 2.346]   [H-I: 145]   [18 followers]  Follow
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 0009-2541
   Published by Elsevier Homepage  [3034 journals]
  • Revealing the conditions of Ni mineralization in the laterite profiles of
           New Caledonia: insights from reactive geochemical transport modelling
    • Abstract: Publication date: Available online 21 June 2017
      Source:Chemical Geology
      Author(s): Andrey Myagkiy, Laurent Truche, Michel Cathelineau, Fabrice Golfier
      New Caledonia is one of the world’s largest nickel laterite deposits that form from intense chemical and mechanical weathering of a peridotite bedrock. As a result of such a weathering process a subsequent downward migration of Si, Mg and Ni takes place, which eventually leads to redistribution of the elements in depth and over time depending on their mobility. Being released from ultramafic parent rock to groundwater, the mobility of nickel is to a great extent controlled by sorption, substitution and dissolution/precipitation processes. Therefore, the final profile of nickel enrichment is the result of the superposition of these possible fates of nickel. The way how Ni is redistributed in between them represents and defines its mineralization in laterite. Knowledge of these processes along with factors, controlling them appears to be a key to detailed understanding of laterite formation. In this study a numerical model, which solves the reaction-transport differential equations, is used to simulate the formation of laterite profile from ultramafic bedrock with particular emphasis on modelled Ni enrichment curve, its comparison with in situ observations, and detailed understanding of trace elements mobility. Since nickel deposits in New Caledonia is characterized by oxide and hydrous Mg silicate ores, three different concurrent fates of Ni deposition in a profile were taken into account in the modelling: i) Ni in a goethite crystal lattice, ii) Ni sorbed on weak and strong goethite sorption sites, and iii) Ni precipitated with silicates (garnierite). Simulations were performed using PHREEQC associated with llnl.dat thermodynamic database that has been edited in order to account garnierite minerals used in the calculations. The work outline is represented by: i) long term (10 Ma) simulation of nickel laterite formation and evolution, ii) analysis of mobility of the elements and understanding its controlling factors, iii) comparison of modelled and in situ Ni enrichment profile and analysis of nickel distribution in between different retention processes, iv) modelling and in depth understanding of these retention processes. The modelling reveals that the vertical progression of the pH front controls thickening of iron-rich zone, explains the vertical mobility of the elements and governs the Ni enrichment. Adsorption itself plays an important role in lateritization process retarding Ni mobility, but i) becomes significant in a narrow range of pH (slightly alkaline) due to competition of Mg and Ni for sorption sites and ii) does not explain such a high nickel content in limonite nowadays, suggesting that Ni is held in goethite mostly by stronger ties i.e. substituted for Fe in the crystal lattice of iron oxyde. 1-D modelling appears to be a powerful tool in understanding the general behavior of trace elements upon the formation of laterite and at the same time reveals that locally Ni mineralizations should be explained by more complex processes, such as lateral transfers, convective flows and preferential pathways.

      PubDate: 2017-06-21T03:43:46Z
  • Inside front cover (Editorial Board)
    • Abstract: Publication date: 20 June 2017
      Source:Chemical Geology, Volume 461

      PubDate: 2017-06-21T03:43:46Z
  • “Free” oxide ions in silicate melts: Thermodynamic considerations and
           probable effects of temperature
    • Abstract: Publication date: 20 June 2017
      Source:Chemical Geology, Volume 461
      Author(s): Jonathan F. Stebbins
      Reactions of simple metal oxides with silica often describe the melt solution in terms of equilibria among three energetically distinct oxygen ion species: bridging oxygens connecting two network cations, non-bridging oxygens shared by one network cation and multiple modifier cations of lower charge and larger radius, and “free” oxide ion bonded only to modifiers. Interest in the role and concentration of “free” oxide ions has recently been renewed by spectroscopic studies that have directly determined its abundance in a few glassy and amorphous silicates with low silica contents and high cation field strengths, as well as being inferred to be present by less direct measurements of bridging/non-bridging oxygen ratios in more silica-rich compositions. Here we review and evaluate recent evidence on “free” oxide concentrations in silicate glasses and melts, and discuss the important clues about effects of composition that come from comparisons of heats of formation of silicates and measurements of oxide component activities: relative proportions of oxygen species are expected to depend strongly on the charge and size of the modifier cation and its degree of covalent/ionic bonding. We also present simple thermodynamic approximations that allow temperature effects on “free” oxide concentrations to be estimated. In most compositions, this relative high energy species is expected to become more abundant at high temperatures, and may reach quite significant concentrations (>10% of total oxygens') in hot, ultramafic, Mg- and Fe-rich melts in nature. In contrast, in most common, glass-forming compositions the concentrations of “free” oxide ion are probably much lower, particularly on cooling to glass transition temperatures, but this species has been suggested to potentially have an important role in melt dynamics even at low abundances. In any case, further experimental determinations, and theoretical evaluations, of this somewhat elusive and controversial species are well warranted.

      PubDate: 2017-06-21T03:43:46Z
  • Bridging oxygen speciation and free oxygen (O2−) in K-silicate glasses:
           Implications for spectroscopic studies and glass structure
    • Abstract: Publication date: 20 June 2017
      Source:Chemical Geology, Volume 461
      Author(s): H. Wayne Nesbitt, Grant S. Henderson, G. Michael Bancroft, Ryan Sawyer, Richard A. Secco
      O 1s X-ray Photoelectron Spectroscopic (XPS) results for K-silicate glasses near the K-disilicate composition indicate the presence of at least two, and likely three, types of Bridging Oxygen (BO), which are distinguished by the number of K ions coordinated to BO (BO-K moieites). The relative abundances of these moieties cannot be accurately determined from XPS spectra, but based on crystalline K-disilicate, they are likely to be BO, BO-K1 and BO-K2 or BO-K1, BO-K2 and BO-K3 (subscript indicates the number of K+ coordinated to BO). The presence of three BO moieties is consistent with crystallographic results for crystalline K-disilicate (K2Si2SiO5) and their presence in K-silicate glass may be assessed using high quality molecular dynamics studies. The binding energy (BE) separating each BO-K peak is ~40 to ~70kJ and sufficient to affect stretching frequencies in Raman and IR spectra, and to effect the electronic properties of O and Si atoms in 17O and 29Si NMR spectra. The BO moieties may give rise to separate signals or they may coalesce to produce a broad, asymmetric band in Raman, 29Si NMR and 17O NMR spectra, just as they do in O 1s XPS spectra. The O 1s XPS and previously published 17O NMR spectra of glasses containing ~20 and ~35mol% K2O are well resolved. Both types of spectra yield the same NBO and BO abundances within experimental uncertainty. For 20mol% K2O glass, the O 1s XPS results yield ~21.5mol% NBO and ~78.5% BO whereas the 17O MAS NMR results are ~22.2% NBO and 77.8% BO. The 35mol% K2O glass yields similarly close agreement between O 1s XPS and 17O MAS NMR spectral fits. Both techniques can provide accurate oxygen species abundances provided the spectra are of high resolution, and provided complete fits are performed on the spectra. A 2D 29Si MAF NMR spectrum of K-disilicate glass indicates that two Q4 contributions are present in the spectrum, as previously observed in highly siliceous K-silicate glasses. One explanation for two peaks is that one Q4 signal represents Q4 tetrahedra attached to Q3 tetrahedra and the second peak represents Q4 attached to other Q4 species. An alternative explanation is offered based on these findings. The two Q4 signals may result from different types of BO-K moieties bonded to Si centers of Q4 species. K+, located in the second coordination sphere of BO, modifies the electronic (valence band) properties of the BO and Si atoms of the tetrahedron, thereby producing two Q4 signals. The 2D 29Si MAF NMR, 17O NMR and O 1s XPS spectra indicate ~2 (±1) mol% O2− in the K-disilicate glass. These three types of spectra yield remarkably similar results when high resolution spectra are collected and complete fitting of the spectra are performed.

      PubDate: 2017-06-21T03:43:46Z
  • The structure of Y- and La-bearing aluminosilicate glasses and melts: A
           combined molecular dynamics and diffraction study
    • Abstract: Publication date: 20 June 2017
      Source:Chemical Geology, Volume 461
      Author(s): Johannes Wagner, Volker Haigis, Marlène Leydier, Aleksei Bytchkov, Viviana Cristiglio, Henry E. Fischer, Najim Sadiki, Didier Zanghi, Louis Hennet, Sandro Jahn
      To understand the behavior of rare earth elements (REE) in magmatic systems it is important to characterize in a systematic way their incorporation into silicate melts and glasses. Here, we study the structural environment of the REE Y and La in four aluminosilicate glasses and melts with varying REE content, using a combined simulation and diffraction approach. Glasses are investigated by X-ray and neutron diffraction as well as classical molecular dynamics simulations using two different polarizable ion potentials. Structure models of the corresponding melts are derived from classical and first-principles molecular dynamics simulations. We discuss the effect of temperature on coordination numbers and rationalize the structural changes in response to variations in melt/glass composition in terms of cation field strengths. We find robust evidence that REE and Al coordination numbers decrease with increasing REE content in the investigated melts and glasses. Comparing the two classical potentials, one of them is able to reproduce features of the experimental structure factors with a mixture of corner- and edge-sharing Al-O/REE-O polyhedra, whereas the other potential predicts corner-sharing Al-O/REE-O polyhedra only.

      PubDate: 2017-06-21T03:43:46Z
  • Properties of magmatic liquids by molecular dynamics simulation: The
           example of a MORB melt
    • Abstract: Publication date: 20 June 2017
      Source:Chemical Geology, Volume 461
      Author(s): Thomas Dufils, Nicolas Folliet, Boris Mantisi, Nicolas Sator, Bertrand Guillot
      A new atom–atom interaction potential is introduced for describing by classical molecular dynamics (MD) simulation the physical properties of natural silicate melts. The equation of state, the microscopic structure, the viscosity, the electrical conductivity, and the self-diffusion coefficients of ions in a mid-oceanic ridge basalt (MORB) melt are evaluated by MD over a large range of temperature and pressure (1673–3273K and 0–60GPa). A detailed comparison with experimental data shows that the model reproduces the thermodynamic, structural and transport properties of a MORB with an unprecedented accuracy. In particular, it is shown that the MORB melt crystallizes at lower mantle conditions into a perovskite phase whose the equation of state (EOS) is compatible with those proposed in the experimental literature. Moreover, in accordance with experimental findings, the simulation predicts not only that the MORB viscosity exhibits a (slight) minimum with the pressure, but also that the viscosity at high temperature remains very low (<100mPa.s for T > 2273K) even at high pressure (up to 40GPa). However the evolution of the electrical conductivity with temperature and pressure is not always the symmetrical of that of the viscosity. In fact, the relationship between viscosity and electrical conductivity shows a crossover at around 2073 K.

      PubDate: 2017-06-21T03:43:46Z
  • Ni partitioning between metal and silicate melts: An exploratory
           ab initio molecular dynamics simulation study
    • Abstract: Publication date: 20 June 2017
      Source:Chemical Geology, Volume 461
      Author(s): Daniela Künzel, Johannes Wagner, Sandro Jahn
      Element partitioning is a key geochemical process. While partition coefficients between phases including melts have been measured in many experimental studies, new insight into the mechanisms of partitioning may be obtained by relating partitioning to melt structure. Here, we address this problem by exploring an ab initio molecular dynamics simulation approach. Combined with the thermodynamic integration method, these simulations provide a unique way to predict simultaneously thermodynamic properties related to element partitioning and information about the molecular structure of the melt. Thus, it should be possible not only to predict the partitioning of elements, but also to provide an explanation for this behavior based on atomic structures of the coexisting phases. Using this approach, we derive from first-principles the Ni partition coefficient between a metal and a silicate melt at 2500K and ambient pressure, which is at least in qualitative agreement with experiment. Structural analysis of various (Mg,Fe,Ni)2SiO4 silicate and (Fe,Ni) metal melts reveals that the Ni partitioning is mainly determined by its structural environment in the silicate melt, whereas the coordination environments of Ni and Fe are almost indistinguishable in the metal melt. Possible strategies to improve the predictive power of the proposed approach are discussed.

      PubDate: 2017-06-21T03:43:46Z
  • Inside front cover (Editorial Board)
    • Abstract: Publication date: 20 August 2017
      Source:Chemical Geology, Volume 465

      PubDate: 2017-06-15T14:25:12Z
  • Skeletal growth controls on Mg/Ca and P/Ca ratios in tropical Eastern
           Pacific rhodoliths (coralline red algae)
    • Abstract: Publication date: 20 August 2017
      Source:Chemical Geology, Volume 465
      Author(s): Hillary R. Sletten, David P. Gillikin, Jochen Halfar, C. Fred T. Andrus, Héctor M. Guzmán
      The Mg/Ca ratios of non-geniculate coralline red algae (CA) are often assumed to be solely dependent on the sea surface temperature (SST) in which they grow, suggesting that they are ideal proxies for SST reconstruction. Mg/Ca ratio cyclicity is also commonly used as a marker for annual growth band identification in many CA species. However, other controls on elemental uptake such as light, calcification mechanisms, and vital effects may control elemental ratios (e.g. Mg/Ca and P/Ca) in certain genera or species of CA, thus complicating the use of Mg/Ca and P/Ca ratios as a proxy source for SST and paleo-environmental reconstructions. Here we present evidence showing that the Mg/Ca and P/Ca ratio cyclicity in unattached living tropical Lithothamnion sp. (n=8) rhodoliths correlate >90% to the number of sub-annual growth bands (n=1 to 20) formed during a 172-day controlled growth experiment and not to temperature or other environmental conditions (e.g. salinity, pH, etc.). This pattern is also observed in the wild, pre-experimental growth further supporting that the bands are not annual. The Mg/Caalga (R2 =0.67; p-value=0.013; n=8) and P/Caalga (R2 =0.64; p-value=0.018; n=8) significantly correlate to skeletal extension during the experiment. This work has implications for trace elemental proxy development using CA as well as the influence of skeletal extension on elemental incorporation in biogenic carbonates, particularly rhodoliths of Lithothamnion sp. from the Gulf of Panama, Panama.

      PubDate: 2017-06-15T14:25:12Z
  • The chemical and isotopic compositions of volatiles in magmatic
           hydrothermal fluids beneath the Songliao Basin, northeastern China
    • Abstract: Publication date: 20 August 2017
      Source:Chemical Geology, Volume 465
      Author(s): Jinliang Gao, Jiaqi Liu, David R. Hilton, Fanchao Meng, Zhongping Li, Lina Zhai, Chunqing Sun, Lei Zhang
      It is controversial whether deep fluids can make contributions to natural gas accumulations in the Songliao Basin, NE China. The occurrence of magmatic hydrothermal quartz veins in crystalline basement of the basin provides a way to understand the characteristics of deep fluids and their significance for natural gas accumulation. Three types of fluid inclusions were identified in hydrothermal quartz: aqueous inclusions, H2O-CO2-rich inclusions and H2O-CO2-CH4-rich inclusions. The primary inclusions show high homogenization temperatures (280–433°C) which are typical of magmatic hydrothermal fluids. The oxygen and hydrogen isotopic compositions (δ18O and δD) of water are 2.0‰ to 4.6‰ and −91.6‰ to −75.7‰, respectively. The OH isotopic compositions suggest that hydrothermal fluids associated with the quartz veins were derived from the primary magmatic system and mixed by small amounts of meteoric water. Two types of magmatic hydrothermal volatiles were identified in the study. Both types of volatiles are dominated by CO2. CO2 in type I volatiles shows more negative δ13C values (−13.8‰ to −9.7‰) than typical magmatic CO2, which likely resulted from carbon isotopic fractionation during magma degassing. Type II volatiles have higher δ13C values (−5.5‰ to −3.3‰) and are similar to typical magmatic CO2. Small amounts of hydrocarbons were observed in both types of magmatic hydrothermal fluids. Hydrocarbons in type I volatiles have relatively high δ13CCH4 values (−30.6‰ to −24.1‰) and reversed carbon isotopic trends while hydrocarbons in type II volatiles have more negative δ13CCH4 values (−49.7‰ to −37.6‰) and orderly carbon isotopic trends. CO2 in hydrothermal fluids is a potential source for CO2 gas accumulations. Hydrocarbons in hydrothermal fluids, however, can hardly make significant contributions to the commercial gas reservoirs due to their low concentration.

      PubDate: 2017-06-15T14:25:12Z
  • Building biogenic beachrock: Visualizing microbially-mediated carbonate
           cement precipitation using XFM and a strontium tracer
    • Abstract: Publication date: 20 August 2017
      Source:Chemical Geology, Volume 465
      Author(s): Jenine McCutcheon, Luke D. Nothdurft, Gregory E. Webb, Jeremiah Shuster, Linda Nothdurft, David Paterson, Gordon Southam
      The fate of reef islands is a topic of ongoing debate in the face of climate change-induced sea-level rise and increased cyclone intensity. Increased erosion and changes to the supply of reef-derived sediment may put sand reef cays at risk of dramatic morphological changes. These changes may have negative implications for the existence of reef cay environments, which host vital sea turtle and bird rookery habitats. Beachrock, consolidated carbonate beach sediment in the intertidal zone, forms naturally on many tropical beaches and reduces the erosion rates of these beaches when compared to unconsolidated sand. In spite of the critical role beachrock plays in stabilizing some reef cay shores, the method of beachrock formation is still incompletely understood. In this investigation, beachrock was synthesized using beach sand and beachrock samples from Heron Island (Great Barrier Reef, Australia) in aquarium experiments in which natural beachrock formation conditions were simulated. Beachrock was produced in two aquaria wherein the water chemistry was influenced by microorganisms derived from the natural beachrock ‘inoculum’, whereas no cementation occurred in an aquarium that lacked a microbial inoculum and was controlled only by physicochemical evapoconcentration. The new cements in the synthesized beachrock were analyzed using synchrotron-based X-ray fluorescence microscopy and were identified using a Sr-tracer added to the experimental seawater. The resulting precipitates cement sand grains together and contain abundant microfossils of the microorganisms on whose exopolymer they nucleated, demonstrating the fundamental role microbes play in beachrock formation. These results are of interest because beachrock could be utilized as a natural coastline stabilization strategy on sand reef cays, and in turn, protect the unique habitats reef islands support.
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      PubDate: 2017-06-15T14:25:12Z
  • The impact of partition coefficient data on the interpretation of chemical
           tracer behaviour in carbon geosequestration projects
    • Abstract: Publication date: 20 August 2017
      Source:Chemical Geology, Volume 465
      Author(s): Matthew Myers, Cameron White, Tara La Force, Charles Heath, Se Gong, Linda Stalker, Bobby Pejcic
      Partition (or distribution) coefficients determine the relative equilibrium concentrations of chemical constituents (or chemical tracers) in each of the phases of a multi-phase system under dilute conditions. The various fluid phases in a reservoir have differing transport properties (e.g. varying relative permeability) and to correctly interpret the behaviour of injected chemical tracers it is essential that accurate partition coefficients are known. In the context of carbon geosequestration or long-term storage of CO2, chemical tracers will be predominantly exposed to an environment consisting of supercritical CO2 and formation water as the main fluid phases. To estimate/simulate the reservoir properties relevant to injected CO2 tagged with chemical tracers, it is therefore necessary to incorporate high pressure/temperature CO2/water partition coefficients into any model/simulation. In this paper, we present a method to determine these partition coefficients for gaseous chemical tracers using a variation of the widely used EPICS (or equilibrium partitioning in a closed system) method. With this method, only the concentration in one phase (in this case, CO2) needs to be measured. We then report these values for a series of representative chemical tracers (i.e. krypton, xenon, sulfur hexafluoride, perdeuterated methane and R134a) at pressure/temperature conditions that have been previously used at the CO2CRC's Otway CCS demonstration project in Victoria, Australia. These values were generally lower than the corresponding Henry's Law coefficients at comparable temperatures. Experiments also examined the impact of adding CH4 to the system to mimic feedstock gas at the CO2CRC Otway project and provide data pertinent to scenarios where CO2 is injected into depleted CH4 gas fields. These values are compared with Henry's Law coefficients and another recently published set of high pressure/temperature partition coefficients. With computational simulations, we have shown that these differences are potentially significant and demonstrate their impact in three typical CO2 geo-sequestration scenarios (i.e. injection into two types of aquifers and injection into a depleted reservoir with a gas cap).

      PubDate: 2017-06-15T14:25:12Z
  • Dolomite dissolution in aqueous solutions in the presence of nucleotides
           and their structural components at 25°C and pCO2 ~1atm
    • Abstract: Publication date: 20 August 2017
      Source:Chemical Geology, Volume 465
      Author(s): Sanoopkumar Puthiya Veetil, Alfonso Mucci, Takeshi Arakaki
      The kinetics and stoichiometry of dolomite dissolution were investigated in deionized water at 25°C and pCO2 ~1atm in the absence (control) and presence of various nucleotides and their structural components at low concentrations (1mM). Dissolutions were conducted in “free-drift” mode and rates estimated based on the accumulation rate of calcium and magnesium ions in solution. The additives considered in this study are likely present at low concentrations (<mM) in deep, carbonate aquifers targeted for CO2 geological sequestration. Results of our control experiment are consistent with rates and stoichiometries reported in earlier studies. In the presence of nucleotides, dolomite dissolution was inhibited and proceeded incongruently, yielding a Mg:Ca solution ratio ≥2. An investigation of the nucleotide structural components (nucleosides, nitrogenous bases and phosphates) revealed that, whereas they nearly all inhibited dolomite dissolution, only phosphate salts (mono-, di-, tri-, and hexametaphosphate) lead to incongruent dissolution. Examination of the surface morphology and phosphate analysis of the solids reacted in the presence of nucleotides and phosphate salts revealed the formation of a solid phosphate phase on the dolomite surface, likely a Ca rich-phosphate phase that accounts for the observed dissolution incongruency.

      PubDate: 2017-06-15T14:25:12Z
  • Inside front cover (Editorial Board)
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464

      PubDate: 2017-06-01T15:20:29Z
  • Dirac's dream: Understanding metal sorption by geomedia using density
           functional theory
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Kideok D. Kwon, Garrison Sposito
      Density functional theory (DFT) can provide accurate, computationally-manageable, first-principles characterizations of mineral surface speciation at molecular scales. This is accomplished by solving the Kohn-Sham equation, a tractable method of applying quantum mechanics to multi-electron systems that was inspired by some visionary remarks of P. A. M. Dirac, one of the founders of quantum theory. Fifty years of advances in DFT and the development of massively-parallel computing platforms have paved the way for molecular-scale modeling of the mineral surface that is directly relevant to the study of natural nanoparticles. In fact, DFT has matured sufficiently to be applied to metal sorption by geomedia as a “computer experiment,” yielding quantitative molecular-scale information that fully complements insights gained from surface spectroscopy. This powerful approach is illustrated first through a series of structural simulations of chalcophanite group minerals and hetaerolite–hausmannite solid solutions, then through several recent studies by the authors and their collaborators focusing on transition and heavy metal sorption by layer-type Mn(IV) oxide and Fe(II) sulfide nanoparticles. Our results show how DFT simulations reveal the mechanistic basis of metal partitioning trends observed experimentally while creating useful models for resolving experimental surface speciation conundrums.

      PubDate: 2017-06-01T15:20:29Z
  • A density functional theory investigation of oxalate and Fe(II) adsorption
           onto the (010) goethite surface with implications for ligand- and
           reduction-promoted dissolution
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): James D. Kubicki, Daniel Tunega, Stephan Kraemer
      Oxalic acid is an important, biologically-produced species in the natural environment. The deprotonated form, oxalate, is dominant in aqueous solutions under circumneutral pH conditions and is a strong ligand for Fe(III). The high affinity of oxalate for Fe(III) means that Fe(III)-oxalate surface and aqueous complexes are common and can lead to ligand-enhanced dissolution. Fe(II) adsorption onto goethite (α-FeOOH) has been shown to enhance dissolution-recrystallization reactions. The goethite (010) face is one of the more common and reactive surfaces on this environmentally critical Fe-hydroxide phase. Hence, this study models both separate and coordinated adsorption of oxalate and Fe(II) onto the (010) face of goethite in order to test for synergistic effects of ligand-promoted and reductive dissolution. Periodic and cluster density functional theory (DFT) energy minimizations were performed to determine the structure, vibrational frequencies and energies of various configurations. The adsorption mechanism of oxalate is verified via comparison to observed IR spectra. The potential roles of oxalate and Fe(II) in ligand-enhanced reductive dissolution of goethite are discussed.
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      PubDate: 2017-06-01T15:20:29Z
  • An integrated flow microcalorimetry, infrared spectroscopy and density
           functional theory approach to the study of chromate complexation on
           hematite and ferrihdyrite
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Nadine J. Kabengi, Maria Chrysochoou, Nefeli Bompoti, James D. Kubicki
      An approach to constructing comprehensive predictive models for contaminant interactions with mineral surfaces is to obtain multiples lines of evidence for surface properties and the types of complexes formed under different geochemical conditions. In this study, we used flow adsorption microcalorimetry (FAMC), attenuated total reflection infrared (ATR FTIR) spectroscopy, and density functional theory (DFT) calculations to study chromate complexation on ferrihydrite (FH) and hematite (HT). Under the experimental conditions used, chromate binds via an exothermic inner-sphere complexation on both surfaces, with similar enthalpy values that do not reveal dramatic differences in the sorption mechanism. Due to their underlying surface structure, FH favors more monodentate and HT more bidentate complexation. Chromate complexes were found by ATR and FAMC to be completely reversible, with substantially slower desorption compared to sorption. Both the FAMC and DFT indicated the presence of surface sites with different energetics, whereby lower surface coverages corresponded to higher enthalpies on both FH and HT. Both flow-based ATR and FAMC yielded different surface coverages than batch isotherms under the same conditions, highlighting the need to assess contaminant sorption under realistic conditions. Overall, this integrated approach proved to be an improved paradigm to study ion sorption on mineral surfaces.

      PubDate: 2017-06-01T15:20:29Z
  • Formation of a mixed Fe(II)-Zn-Al layered hydroxide: Effects of Zn
           co-sorption on Fe(II) layered hydroxide formation and kinetics
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Autumn N. Starcher, Evert J. Elzinga, Donald L. Sparks
      Previous research demonstrated the formation of single divalent metal (Co, Ni, and ZnAl) and mixed divalent metal (NiZnAl) layered double hydroxide (LDH) phases from reactions of the divalent metal with Al-bearing substrates and soils in both laboratory experiments and in the natural environment. Recently Fe(II)-Al-LDH phases have been found in laboratory batch reaction studies, and although they have yet to be found in the natural environment. Potential locations of Fe(II)-Al-LDH phases in nature include areas with suboxic and anoxic conditions. Because these areas can be environments of significant contaminant accumulation, it is important to understand the possible interactions and impacts of contaminant elements on LDH phase formation. One such contaminant, Zn, can also form as an LDH and has been found to form as a mixed divalent layered hydroxide phase. To understand how Zn impacts the formation of Fe(II)-Al-LDH phase formation and kinetics, 3mM or 0.8mM Fe(II) and 0.8mM Zn were batch reacted with either 10g/L pyrophyllite or 7.5g/L γ-Al2O3 for up to three months under anoxic conditions. Aqueous samples were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES) and solid samples were analyzed with X-ray absorption spectroscopy (XAS). Shell-by-shell fits of Fe(II) and co-sorption samples with pyrophyllite show the formation of a mixed divalent metal (Fe(II)-Zn-Al) layered hydroxide phase, while Fe(II) and Zn co-sorption samples with γ-Al2O3 produce Fe(II)-Al-LDH phases and Zn in inner-sphere complexation with the γ-Al2O3. This study demonstrates the formation of a mixed divalent metal layered hydroxide and further iterates the importance of sorbent reactivity on LDH phase formation.

      PubDate: 2017-06-01T15:20:29Z
  • Comparison of tungstate and tetrathiotungstate adsorption onto pyrite
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Minming Cui, Karen H. Johannesson
      Adsorption of tungstate (WO4 2−) and tetrathiotungstate (WS4 2−) onto pyrite was investigated as a function of W species concentrations, pH, and ionic strength. Additional experiments examined the adsorption of these W species onto goethite. The adsorption experiments indicate that pyrite is a strong scavenger of W in aquatic environments. Specifically, results showed that WO4 2− and WS4 2− adsorption onto pyrite increased with decreasing pH. The greatest amounts of WO4 2− and WS4 2− adsorbed onto pyrite occurred at pH4.95 and 5.2, respectively. Kinetic experiments indicate that WO4 2− adsorption onto pyrite occurs more rapidly than WS4 2−. The kinetic behavior of the adsorption of both W species onto pyrite is well described with a pseudo-second-order model. More specifically, rapid external adsorption followed by intraparticle diffusion were the rate-controlling steps during both WO4 2− and WS4 2− adsorption onto pyrite, and intraparticle diffusion of both W species onto pyrite took up >99% of time, suggesting that it was the major rate-limiting step. The mono-surface Langmuir model provided the best fit to the adsorption data indicating that WO4 2− and WS4 2− were mainly adsorbed onto one specific site on the pyrite surface. Our model calculations suggest that the specific adsorption of WO4 2− onto pyrite was greater than the corresponding adsorption of WS4 2− onto pyrite in all cases. The difference of specific adsorption between WO4 2− and WS4 2− may be attributed to their different inner-sphere complexation on the pyrite surface. The adsorption experiments also showed that W species were less adsorbed onto pyrite than goethite. The results of the adsorption experiments demonstrate that pyrite plays an important role in determining the fate and transport of WO4 2− and WS4 2− in natural waters. Furthermore, comparison of our results with previous studies of Mo indicates that WS4 2− is less particle reactive with respect to pyrite than MoS4 2−, which we propose can explain, in part, the apparent stability of W in sulfidic waters compared to Mo as well as the elevated Mo/W ratios in Black Sea sapropels.

      PubDate: 2017-06-01T15:20:29Z
  • Molecular-level understanding of malic acid retention mechanisms in
           ternary kaolinite-Fe(III)-malic acid systems: The importance of Fe
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Jianjun Yang, Jin Liu, Yongfeng Hu, Cornelia Rumpel, Nanthi Bolan, Donald Sparks
      Multivalent cation interaction and mineral association play a vital role in organic carbon (OC) stability in tropical soils. However, the retention mechanisms of OC in mineral-cation-OC systems remain largely unclear at the molecular level. Of all OC forms, low molecular weight organic acids (LMWOAs) are readily biodegradable. Given the general acidic conditions and dominance of kaolinite in tropical soils, we investigated the retention mechanisms of malic acid (MA) in kaolinite-Fe(III)-MA systems with various Fe/MA molar ratios at pH~3.5 using Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. Our results indicated the formed ferrihydrite mainly contributed to the enhanced MA retention through adsorption and/or coprecipitation at high Fe/MA molar ratios in kaolinite-Fe(III)-MA systems relative to kaolinite-MA systems. With the decrease of Fe/MA molar ratios, ternary complexation of MA with kaolinite via a tetrahedral Fe bridge occurred in kaolinite-Fe(III)-MA systems. This study highlighted the significant impact of Fe speciation on MA retention mechanisms in kaolinite-Fe(III)-MA systems under acidic conditions, and clearly showed the impact of ferrihydrite-induced adsorption/coprecipitation and Fe-bridged ternary complexation on MA retention in ternary kaolinite-Fe(III)-MA systems, which will enhance our understanding of the dynamics of MA and other LMWOAs in tropical soils.
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      PubDate: 2017-06-01T15:20:29Z
  • Sorption of Se(IV) and Se(VI) to coal fly ash/cement composite: Effect of
           Ca2+ and high ionic strength
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Weiling Sun, Jay E. Renew, Wenlong Zhang, Yuanzhi Tang, Ching-Hua Huang
      The sorption behavior and mechanisms of Se from wastewater with high salinity are unclear, a scenario that can be encountered in the flue-gas-desulfurization (FGD) blowdown at coal-fired power plants. In this study, we investigated the uptake behavior and mechanisms of Se(IV) and Se(VI) by a coal fly ash/cement composite at high ionic strengths (IS) of up to 3.0M NaCl or CaCl2 using batch sorption experiments, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and X-ray absorption spectroscopy. NaCl, regardless of IS, only caused slight changes in Se(IV) and Se(VI) uptake. Presence of CaCl2 significantly enhanced Se(IV) uptake, but had minor effect at low IS (≤0.3M) and no effect at high IS (>0.3M) on Se(VI) uptake. Both Se(IV) and Se(VI) mainly formed outer-sphere surface complexes in the presence of NaCl. The enhanced Se(IV) uptake by CaCl2 is likely due to the formation of Ca-SeO3 precipitate(s). Outer-sphere complexation is the main mechanism for Se(VI) sorption in the presence of CaCl2, with additional formation of minor amounts of inner-sphere complex at IS >0.3M. The results added new mechanistic insights for Se removal from industrial wastewaters with high salinity and will be useful for immobilization of Se in the co-management of FGD blowdown and coal fly ash wastes.
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      PubDate: 2017-06-01T15:20:29Z
  • Comparison of europium and neptunium adsorption to aluminum (hydr)oxide
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Teresa Baumer, Patricia Kay, Amy E. Hixon
      Batch experiments and detailed solid-phase characterization (i.e., powder X-ray diffraction, scanning electron microscopy, and infrared spectroscopy) were used to determine the effect of secondary phase formation on the adsorption of europium and neptunium to a suite of aluminum (hydr)oxide minerals. Europium experiments were conducted as a function of gibbsite (γ-Al(OH)3), bayerite (α-Al(OH)3), corundum (α-Al2O3), and γ-alumina (γ-Al2O3) concentration (2.5–30m2 L−1), europium concentration (10−8–10−5 M), pH (3−12), and ionic strength (0.01–0.1M NaCl). Neptunium experiments were conducted at constant neptunium and mineral concentrations. The composition of the mineral phase had no apparent effect on europium adsorption whereas preferential adsorption of neptunium followed the trend bayerite>corundum>γ-alumina. The data presented here suggest that there are at least two different mechanisms controlling the adsorption of europium and neptunium and highlight the need to study both aqueous-phase chemistry and the properties of mineral surfaces in order to understand the behavior of lanthanides and actinides at the mineral-water interface.

      PubDate: 2017-06-01T15:20:29Z
  • Bonding properties of humic acid with attapulgite and its influence on
           U(VI) sorption
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Liqiang Tan, Xiangxue Wang, Xiaoli Tan, Huiyang Mei, Changlun Chen, Tasawar Hayat, Ahmed Alsaedi, Tao Wen, Songsheng Lu, Xiangke Wang
      Humic acid (HA) can markedly change the surface charge of clay minerals and influences the sorption of uranium(U) on clay minerals in aquatic environments. Furthermore, HA can potentially affect the mobility of U(VI) in the natural environment. The effect of HA on the surface charge, aggregation, sorption and bonding properties of attapulgite [(Mg,Al)4(Si)8(O,OH,H2O)26·nH2O] was investigated using fluorescence spectroscopy technique in combination with other analytical tools. The UV–vis absorbance spectra and fluorescence peaks obtained in the experiments are attributed to a large number of electron donating substitutes, such as carboxylic acid-like and phenolic acid-like fluorophores, in HA. Electrostatic attraction and ligand exchanges are the primary interaction mechanisms between attapulgite and HA. A significant modification of excitation-emission matrix (EEM) spectra and emission intensity of HA are observed after the interaction, indicating a different binding complex of HA on attapulgite at the molecular level. The sorption kinetics indicate that strong chemical interactions and/or surface complexation primarily contributed to U(VI) sorption. An analysis of U(VI) sorption isotherms and model fitting reveals that the sorption of U(VI) onto attapulgite is a spontaneous and endothermic process. The presence of HA is an important factor that greatly influences the immobilization and removal of U(VI) ions from aqueous solutions.
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      PubDate: 2017-06-01T15:20:29Z
  • The reactivity of Fe(II) associated with goethite formed during short
           redox cycles toward Cr(VI) reduction under oxic conditions
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Elizabeth J. Tomaszewski, Seungyeol Lee, Jared Rudolph, Huifang Xu, Matthew Ginder-Vogel
      Chromium (Cr) is a toxic metal that causes a myriad of health problems and enters the environment as a result of anthropogenic activities and/or natural processes. The toxicity and solubility of chromium is linked to its oxidation state; Cr(III) is poorly soluble and relatively nontoxic, while Cr(VI) is soluble and a known carcinogen. Solid Fe(II) in iron-bearing minerals, such as pyrite, magnetite, and green rusts, reduce the oxidation state of chromium, reducing its toxicity and mobility. However, these minerals are not the only potential sources of solid-associated Fe(II) available for Cr(VI) reduction. For example, ferric (Fe(III)) (hydr)oxides, such as goethite or hematite, can have Fe(II) in the solid without phase transformation; however, the reactivity of Fe(II) within Fe(III) (hydr)oxides with contaminants, has not been previously investigated. Here, we cyclically react goethite with dissolved Fe(II) followed by dissolved O2, leading to the formation of reactive Fe(II) associated with goethite. In separate reactors, the reactivity of this Fe(II) is probed under oxic conditions, by exposure to chromate (CrO4 2−) after either one, two, three or four redox cycles. Cr is not present during redox cycling; rather, it is introduced to a subset of the solid after each oxidation half-cycle. Analysis of X-ray absorption near edge structure (XANES) spectra reveals that the extent of Cr(VI) reduction to Cr(III) depends not only on solid Fe(II) content but also surface area and mean size of ordered crystalline domains, determined by BET surface area analysis and X-ray diffraction (XRD), respectively. Shell-by-shell fitting of the extended X-ray absorption fine structure (EXAFS) spectra demonstrates chromium forms both single and double corner sharing complexes on the surface of goethite, in addition to sorbed Cr(III) species. Finally, transmission electron microscope (TEM) imaging and X-ray energy-dispersive spectroscopy (EDS) illustrate that Cr preferentially localizes on the (100) face of goethite, independent of the number of redox cycles goethite undergoes. This work demonstrates that under oxic conditions, solid Fe(II) associated with goethite resulting from rapid redox cycling is reactive and available for electron transfer to Cr(VI), suggesting Fe(III) (hydr)oxides may act as reservoirs of reactive electron density, even in oxygen saturated environments.
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      PubDate: 2017-06-01T15:20:29Z
  • Surface area effects on the reduction of UVI in the presence of synthetic
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Maxim I. Boyanov, Drew E. Latta, Michelle M. Scherer, Edward J. O'Loughlin, Kenneth M. Kemner
      The redox transformations that affect the environmental mobility of metal or radionuclide contaminants typically take place in the presence of mineral or biological surfaces. Adsorption can alter the speciation and free energy of a dissolved ion and can thus change the occurrence, rate, or products of redox reactions relative to those expected in homogenous solution. Here, we investigated the effect of SYn-1, a redox-inactive synthetic montomorillonite clay mineral, on the reduction of UVI by dihydroanthraquinone-2,6 disulfonate (AH2QDS, the reduced form of the soluble electron shuttle AQDS). We varied the surface:U ratio in a circumneutral bicarbonate solution and measured the valence and atomic coordination of U in the solids using X-ray absorption spectroscopy (XANES and EXAFS). The spectra show that UIV was the predominant product both in the presence or absence of the clay mineral, indicating that adsorption of UVI to clay mineral surface sites does not affect its ability to be reduced by AH2QDS. In the absence of the clay mineral and at low-to-intermediate surface:U ratios the predominant UIV product was nanoparticulate uraninite, UO2. At the highest surface:U ratio tested (100g/L clay mineral and 50μMU) we observe a decrease in the proportion of uraninite down to 50%, with the remaining UIV present as adsorbed, non-uraninite species. These results are similar to previous findings with magnetite and rutile, where adsorbed UIV were the predominant species below a specific surface coverage and U in excess of this coverage was precipitated as uraninite. The threshold coverage determined here for the SYn-1 clay mineral is 10-100× lower than that determined for magnetite and rutile, suggesting that clay mineral surfaces may be less important than metal oxides for stabilizing non-uraninite UIV species in natural sediments.
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      PubDate: 2017-06-01T15:20:29Z
  • Coupled dynamics of iron and iron-bound organic carbon in forest soils
           during anaerobic reduction
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Qian Zhao, Dinesh Adhikari, Rixiang Huang, Aman Patel, Xilong Wang, Yuanzhi Tang, Daniel Obrist, Eric E. Roden, Yu Yang
      The behavior of iron (Fe)-bound organic carbon (OC) under anoxic conditions in natural soils and sediments represents a critical knowledge gap for understanding the biogeochemical cycles of OC and Fe. In this study, we investigated the dynamics of Fe and OC in four forest soils in the presence of the dissimilatory Fe-reducing bacterium, Shewanella oneidensis MR-1. Over an 8-day reduction period, 3.8–9.9% of total OC was released to solution in conjunction with the reduction of 12.5–37.7% of reactive Fe. The fraction of OC released was correlated with the fraction of Fe reduced, indicating that the reductive release was the controlling factor for the mobilization of OC upon the anaerobic microbial reaction. During the reduction, the fractions of poorly crystalline Fe oxides decreased, coupling with an increase in the relative abundance of crystalline Fe oxides. Lability of OC (as reflected by water-extractable OC content) increased after microbial reduction, indicating the decreased stability of OC because of changes in mineral-OC interactions and the conformation of mineral-OC complexes. The reduction of Fe was closely related to bulk soil electron accepting capacity (0.15–0.34mmole−/mol C). Our findings demonstrate that the redox reactions of Fe, modified by the redox reactivity of OC, play an important role in regulating the stability and transformation of OC.

      PubDate: 2017-06-01T15:20:29Z
  • Advanced biotic ligand models: Using surface complexation modeling to
           quantify metal bioavailability to bacteria in geologic systems
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Jeremy B. Fein
      There is considerable evidence that metal adsorption onto microbial cell envelopes represents the first step in bioavailability of the metal to the cell. Traditional biotic ligand models were the first attempts to relate metal bioavailability, as manifested by metal toxicity, to the concentration of adsorbed metal on organisms, using a single generic ‘biotic’ ligand as the lone metal binding site on the organism. Because biotic ligand models represent the bound metal using a non-mechanistic reaction, these models cannot be extrapolated accurately to predict metal adsorption, and hence metal bioavailability, under conditions that were not directly measured in the laboratory. Over the past 20years, surface complexation modeling has been applied to model metal adsorption onto bacteria and other microorganisms, yielding a mechanistic understanding of the adsorption reactions that have been studied. The development of mechanistic surface complexation models for bacteria provides an opportunity to develop advanced biotic ligand models which are similar to traditional biotic ligand models but which incorporate the mechanistic approach of surface complexation modeling to quantify metal adsorption. This review summarizes the studies that have been conducted to date which explore and calibrate this approach, and each of these studies demonstrates a strong correlation between the concentration and/or speciation of adsorbed metal and the bioavailability of that metal to bacteria. Using a surface complexation approach for modeling metal bioavailability to bacteria creates powerful quantitative models of metal bioavailability which can be extrapolated to complex realistic geologic or engineered systems.

      PubDate: 2017-06-01T15:20:29Z
  • Stoichiometry of mercury-thiol complexes on bacterial cell envelopes
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Bhoopesh Mishra, Elizabeth Shoenfelt, Qiang Yu, Nathan Yee, Jeremy B. Fein, Satish C.B. Myneni
      We have examined the speciation of Hg(II) complexed with intact cell suspensions (1013 cellsL−1) of Bacillus subtilis, a common gram-positive soil bacterium, Shewanella oneidensis MR-1, a facultative gram-negative aquatic organism, and Geobacter sulfurreducens, a gram-negative anaerobic bacterium capable of Hg-methylation at Hg(II) loadings spanning four orders of magnitude (120nM to 350μM) at pH5.5 (±0.2). The coordination environments of Hg on bacterial cells were analyzed using synchrotron based X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy at the Hg LIII edge. The abundance of thiols on intact cells was determined by a fluorescence-spectroscopy based method using a soluble bromobimane, monobromo(trimethylammonio)bimane (qBBr) to block thiol sites, and potentiometric titrations of biomass with and without qBBr treatment. The chemical forms of S on intact bacterial cells were determined using S k-edge XANES spectroscopy. Hg(II) was found to complex entirely with cell bound thiols at low Hg:biomass ratios. For Bacillus subtilis and Shewanella oneidensis MR-1 cells, the HgS stoichiometry changed from HgS3 to HgS2 and HgS (where ‘S’ represents a thiol site such as is present on cysteine) progressively as the Hg(II) loading increased on the cells. However, Geobacter sulfurreducens did not form HgS3 complexes. Because the abundance of thiol was highest for Geobacter sulfurreducens (75μM/g wet weight) followed by Shewanella oneidensis MR-1 (50μM/g wet weight) and Bacillus subtilis (25μM/g wet weight), the inability of Hg(II) to form HgS3 complexes on Geobacter sulfurreducens suggests that the density and reactivity of S-amino acid containing cell membrane proteins on Geobacter sulfurreducens are different from those of Bacillus subtilis and Shewanella oneidensis MR-1. Upon saturation of the high affinity thiol sites at higher Hg:biomass ratios, Hg(II) was found to form a chelate with α-hydroxy carboxylate anion. The stoichiometry of cell envelope bound Hg-thiol complexes and the associated abundance of thiols on the cell envelopes provide important insights for understanding the differences in the rate and extent of uptake and redox transformations of Hg in the environment.
      Graphical abstract image

      PubDate: 2017-06-01T15:20:29Z
  • Thermodynamic modeling of Mn(II) adsorption onto manganese oxidizing
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Angélica Vázquez-Ortega, Jeremy B. Fein
      Bacterial cell membranes display a strong affinity for a wide variety of aqueous metal cations and have the potential to affect the mass transport of these cations through adsorption reactions in water–rock systems. Prior studies have focused on determining the thermodynamic stability constants of heavy metals and radionuclides; however, the constants for Mn-bacterial surface complexes formed on manganese oxidizing bacteria remain unmeasured. We measured the rate, extent, and reversibility of Mn(II) adsorption onto a bacterial species capable of Mn-oxidization (Pseudomonas putida), and onto a bacterial species that does not promote Mn-oxidization (Bacillus subtilis). The extent of adsorption was measured as a function of both pH and metal loading in order to determine the thermodynamic stability constants of the Mn-bacterial surface complexes that form on the bacteria and to test whether Mn oxidizers exhibit unusual Mn adsorption behavior relative to species that do not oxidize Mn. Furthermore, we determined the effect of bacterial extracellular polymeric substances (EPS) on Mn(II) adsorption by conducting experiments with and without EPS removal from the biomass. The experimental results indicated that Mn(II) adsorption onto B. subtilis and P. putida was rapid and reversible. The extent of Mn(II) adsorption onto both bacterial species increased with increasing pH, but P. putida adsorbed significantly more Mn(II) than did B. subtilis across most of the pH range studied. Both the adsorption measurements and the calculated stability constants indicate that P. putida, a Mn oxidizing bacterial species, exhibited significantly enhanced Mn adsorption relative to that observed for B. subtilis. The enhanced Mn(II) adsorption exhibited by P. putida suggests that bacteria may adapt the metal binding environments within their cell envelopes in order to optimize bioavailability of metals that are beneficial to their metabolism. Experiments conducted at low metal-loading conditions yielded stability constants for the Mn(II)-bacterial surface complexes that were less than or similar to those calculated for the high metal-loading conditions, suggesting that Mn(II)-sulfhydryl binding does not dominate under low metal loading conditions as it does for other metals. Removal of EPS molecules from P. putida led to significantly reduced extents of Mn(II) adsorption, suggesting that Mn(II)-EPS binding plays at least some role in the overall adsorption of Mn(II) onto P. putida biomass.

      PubDate: 2017-06-01T15:20:29Z
  • Inside front cover (Editorial Board)
    • Abstract: Publication date: 5 July 2017
      Source:Chemical Geology, Volume 463

      PubDate: 2017-06-01T15:20:29Z
  • Changes in mineral reactivity driven by pore fluid mobility in partially
           wetted porous media
    • Abstract: Publication date: 5 July 2017
      Source:Chemical Geology, Volume 463
      Author(s): A.L. Harrison, G.M. Dipple, W. Song, I.M. Power, K.U. Mayer, A. Beinlich, D. Sinton
      Microfluidics experiments were used to examine mineral dissolution-precipitation reactions under evaporative conditions and identify pore-scale processes that control reaction rate. The entrainment of reacting mineral particles by a mobile water-gas interface driven by evaporation dramatically altered the relative abundance of reactive mineral surface area to fluid reservoir volume. This ratio, which directly influences reaction rate and reaction progress, was observed to vary by nearly two orders of magnitude as evaporation progressed in the experiments. Its dynamic evolution may have a correspondingly large impact on mineral-fluid reaction in Earth's shallow subsurface. We predict that the spatial and temporal variability of pore-scale reaction rates will be significant during evaporation, imbibition, or drainage in the vadose zone, with implications for chemical weathering, soil quality, and carbon cycling. Variable reaction rates during particle mobility are likely to be of increased significance as global rainfall patterns and soil moisture contents evolve in response to climate change.
      Graphical abstract image

      PubDate: 2017-06-01T15:20:29Z
  • Enrichment of germanium and associated arsenic and tungsten in coal and
           roll-front uranium deposits
    • Abstract: Publication date: 5 July 2017
      Source:Chemical Geology, Volume 463
      Author(s): Barbara Etschmann, Weihua Liu, Kan Li, Shifeng Dai, Frank Reith, Donna Falconer, Gemma Kerr, David Paterson, Daryl Howard, Peter Kappen, Jeremy Wykes, Joël Brugger
      Most of the World's germanium (Ge) is mined from Ge-rich lignite, where it is commonly associated with elevated arsenic (As), tungsten (W) and beryllium (Be) contents. Over the past decade, new evidence showing that World-class Ge deposits result from the interaction of hydrothermal fluids with organic matter in coal seams has emerged. Yet, the chemical state of Ge and associated metals in lignite remains poorly understood. We used Mega-pixel Synchrotron X-ray Fluorescence (MSXRF), X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) to characterize the oxidation states and chemical bonding environment of Ge, As, and W in two world-class Mesozoic Ge-in-lignite deposits (Lincang, Yunnan, southwestern China; Wulantuga, Inner Mongolia, northeastern China); in lignite-bearing uranium (U) ores from the Beverley deposit (South Australia) hosted in Eocene sandstones; and in lignite and preserved wood in late Oligocene-Miocene fluviatile sediments (Gore, Southland, New Zealand). The aim was to improve our understanding of the enrichment mechanism of Ge in lignite and better evaluate the environmental mobility of Ge and some of the associated metals (specifically As and W) in lignite ores. In all samples, chemical maps show that Ge is distributed homogeneously (down to 2μm) within the organic matter. XANES and EXAFS data show that Ge exists in the tetravalent oxidation state and in a distorted octahedral coordination with O, consistent with complexing of Ge by organic ligands. In some pyrite-bearing samples, a minor fraction of Ge is also present as Ge(IV) in association with pyrite. In contrast, As displays a more complex speciation pattern, sometimes even in a single sample, including As(III), As(V), and As(−I/+II) in solid solution in sulfides. Arsenic in sulfides occurs in anionic and cationic forms, i.e., it shows both the common substitution for S2 2− and the substitution for Fe recently discovered in some hydrothermal pyrites. Tungsten was present as W(VI) in distorted octahedral (3+3) coordination. The EXAFS data confirm the absence or minor contribution of individual W-rich minerals such as scheelite or ferberite to W mass balance in the studied samples. These data show that Ge, W, and probably some As are scavenged via formation of insoluble, oxygen-bridged metal organic complexes in lignite. Destruction of the organic ligands responsible for fixing Ge and W (As) in these lignites is required for liberating the metals, e.g. from waste materials. Geochemical modelling suggests that Ge, W, Be and As all can be extracted from granitic rocks by dilute, low temperature hydrothermal fluids. Germanium is transported mainly as the tetrahedral Ge(OH)4(aq) complex, but fixed as an octahedral oxy-bridged organic complex. The same situation is valid for W, which is transported at the tetrahedral tungstate ion, but most likely scavenged via formation of a 6-coordinated metal-organic species. The Ge-Be-W±As association in Ge-rich coals reflects the source of the metals as well as related scavenging mechanisms.

      PubDate: 2017-06-01T15:20:29Z
  • Using monazite geochronology to test the plume model for carbonatites: The
           example of Gifford Creek Carbonatite Complex, Australia
    • Abstract: Publication date: 5 July 2017
      Source:Chemical Geology, Volume 463
      Author(s): Jian-Wei Zi, Courtney J. Gregory, Birger Rasmussen, Stephen Sheppard, Janet R. Muhling
      Carbonatites are carbonate-dominated igneous rocks derived by low-degree partial melting of metasomatized mantle, although the geodynamic processes responsible for their emplacement into the crust are disputed. Current models favor either reactivation of lithospheric structures in response to plate movements, or the impingement of mantle plumes. Geochronology provides a means of testing these models, but constraining the age of carbonatites and related metasomatic events is rarely straightforward. We use in situ U-Th-Pb analysis of monazite by SHRIMP to constrain the emplacement age and hydrothermal history of the rare earth element-bearing Gifford Creek Carbonatite Complex in Western Australia, which has been linked to plume magmatism at ca. 1075Ma. Monazite in carbonatites and related metasomatic rocks (fenites) from the carbonatite complex dates the initial emplacement of the carbonatite at 1361±10Ma (n =22, MSWD=0.91). The complex was subjected to multiple stages of magmatic/hydrothermal overprinting from ca. 1300Ma to 900Ma during later regional tectonothermal events. Carbonatite emplacement at ca. 1360Ma appears to be an isolated igneous event in the region, and occurred about 300 million years before intrusion of the ca. 1075Ma Warakurna large igneous province, thus precluding a genetic connection. The Gifford Creek Carbonatite Complex occurs within a major crustal suture, and probably formed in response to reactivation of this suture during plate reorganization. Our study demonstrates the veracity of monazite geochronology in determining the magmatic and hydrothermal histories of a carbonatite complex, critical for evaluating competing geodynamic models for carbonatites. The approach involving in situ SHRIMP U-Th-Pb dating of monazite from a wide spectrum of rocks in a carbonatite complex is best suited to establishing the intrusive age and hydrothermal history of carbonatites.
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      PubDate: 2017-06-01T15:20:29Z
  • Diagenesis of carbonate associated sulfate
    • Abstract: Publication date: 5 July 2017
      Source:Chemical Geology, Volume 463
      Author(s): Vanessa Fichtner, Harald Strauss, Adrian Immenhauser, Dieter Buhl, Rolf D. Neuser, Andrea Niedermayr
      Carbonate associated sulfate (CAS) is a proxy for the seawater redox conditions of ancient oceans. Despite its frequent utilization from altered carbonate archives, the impact of diagenetic and anchimetamorphic overprint on CAS itself has not yet been calibrated in a systematic manner. In the present study, CAS abundances and sulfur and oxygen isotopic compositions from early diagenetic sabkha type dolomicrites (Triassic Dolomia Principale Formation) were compared with geochemical (87Sr/86Sr, δ13Ccarb, δ18Ocarb), optical (cathodoluminescence), and crystallographical data to assess the impact of burial alteration. Data shown here document that δ34SCAS withstands burial diagenesis or anchimetamorphosis (350°C) and reliably preserves a record of ambient seawater sulfate and early diagenetic redox processes. In contrast, δ18OCAS ratios of early diagenetic dolomites exposed to burial temperatures of 200°C and more are enriched in 18O likely due to exchange between δ18OCAS and δ18Ocarb. Carbonate associated sulfate concentrations are directly affected by burial conditions: Under increasing burial depth, temperature, and cation order in the increasingly stoichiometric dolomite crystal lattice, CAS concentration decreases linearly from a mean value of 470ppm in samples that experienced a burial temperature of 100°C to values below analytical detection in samples exposed to burial fluids with temperatures in excess of 350°C. Significant variations of δ34SCAS in samples from the northern Alps are further attributed to local sulfide oxidation triggered by the influence of meteoric water as based on the δ34S and δ18O values of the water soluble sulfate (WSS). The data shown here are of significance for those concerned with the chemical parameters of ancient oceans and shed light on processes during dolomite formation and diagenetic pathways.

      PubDate: 2017-06-01T15:20:29Z
  • A new approach to laser-ablation inductively-coupled-plasma
           mass-spectrometry (LA-ICP-MS) using the flexible map interrogation tool
    • Abstract: Publication date: 5 July 2017
      Source:Chemical Geology, Volume 463
      Author(s): J.A. Petrus, D.M. Chew, M.I. Leybourne, B.S. Kamber
      A new flexible interrogation tool for 2-dimensional LA-ICP-MS element and isotope maps is presented. Called Monocle, the tool works as an add-on in the software Iolite. Initial inspection of map data is undertaken with a size and shape adjustable loupe that is moved across the map. As the loupe is panned over the map, histograms and density/probability plots of the element concentration (or ratio, or age) contained within the area under the loupe are displayed. The data of any loupe position can be saved as a region of interest (ROI) in a table. For maps with rare earth element (REE) data a live normalised REE graph can be shown. Similarly, live graphs (e.g. Wetherill or Tera-Wasserburg concordiae) can be displayed for U/Pb data. The second step of map interpretation refines the initial data selection, for example by drawing polygons over irregular zones of similar chemistry. Where areas are separated (e.g. by cracks) several polygons can be nested into one ROI to improve counting statistics. Additional means of data pooling exist. Firstly, by growing a ROI from a seed, whereby the data distribution in the seed is used to find adjacent pixels that are within a user-defined statistical boundary (e.g. 1 standard deviation). Secondly, multiple criteria can be set (e.g. element A>0.1ppm and element B<40wt%) to determine which pixels (connected or unconnected) are included in a ROI. This new approach is illustrated with several examples. A Mariana Arc oceanic Mn crust is used to show how elemental maps resolve alternating periods of precipitation from variably intense hydrothermal input and how their REE systematics reflect mixing between two distinct sources. A map of a garnet from a lherzolite illustrates that with modern LA-ICP-MS instruments it is possible to obtain reproducible REE data from strongly depleted mantle minerals (e.g. La≈14ppb) with a 20×20μm2 beam and how mineral inclusions, metasomatism and serpentinisation can be avoided to obtain data from the most pristine parts of a grain. A map of a zoned augite crystal from Mt. Etna was used to examine the distribution of Ni and Cr, whose interrelationship in the strong oscillatory zoning can be modelled with extreme fractional crystallisation. A U/Pb map of a complex zircon was used to separate data from the complex inherited core and the much younger magmatic rim. Extraction of data from 2-dimensional maps (<1μm deep) avoids limitations of conventional LA-ICP-MS analysis that typically ablates much deeper. In combination with fast aerosol transfer systems that permit high repetition rate mapping (50–80Hz), extraction of quantitative data with map analysis tools heralds a new era of in situ chemical and isotopic analysis.
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      PubDate: 2017-06-01T15:20:29Z
  • Inside front cover (Editorial Board)
    • Abstract: Publication date: 25 June 2017
      Source:Chemical Geology, Volume 462

      PubDate: 2017-05-27T04:54:44Z
  • First evidence of the trisulfur radical ion S3− and other sulfur
           polymers in natural fluid inclusions
    • Abstract: Publication date: 25 June 2017
      Source:Chemical Geology, Volume 462
      Author(s): Guillaume Barré, Laurent Truche, Elena F. Bazarkina, Raymond Michels, Jean Dubessy
      Sulfur plays a key role in numerous processes occurring in the Earth's crust. However, its speciation in deep and hot geological fluids remains poorly constrained. Here, we used quantitative in-situ Raman spectroscopy on natural fluid inclusions from deep sedimentary environments where thermochemical sulfate reduction (TSR) occurred to determine the sulfur speciation at temperatures representative of their entrapment conditions (100–300°C). Results unambiguously demonstrate the presence of the trisulfur ion S3 − and other polymeric S species (Sn 2 − ±Sn 0) at temperature (T)>100°C, whereas only sulfide and sulfate were detected at 25°C. From 200 to 300°C, sulfate and sulfide, the two dominant S species, contribute to 41±9% and 59±9% of the mean total dissolved S concentration ([Stot]=0.25mol/kgH2O =0.8wt%), respectively. The S3 − concentration accounts for 0.2 to 3% of Stot in this T range, with a maximum recorded concentration of 2.9×10−2 mol/kgH2O (2780ppm) at 300°C. This observation implies that the TSR process occurs under physico-chemical conditions that enhanced the stability of S3 − and other polymeric S species. This conclusion has important consequences for the genesis of base metal sulfide deposits and sour gas fields where reduced sulfur originates from TSR.

      PubDate: 2017-05-27T04:54:44Z
  • Uptake of nickel by synthetic mackinawite
    • Abstract: Publication date: 25 June 2017
      Source:Chemical Geology, Volume 462
      Author(s): Richard T. Wilkin, Douglas G. Beak
      The uptake of aqueous Ni(II) by synthetic mackinawite (FeS) was examined in anaerobic batch experiments at near-neutral pH (5.2 to 8.4). Initial molar ratios of Ni(II) to FeS ranged from 0.008 to 0.83 and maximum Ni concentrations in mackinawite, expressed as the cation mol fraction, were as high as X Ni =0.56 (Fe1− x Ni x S; 0≤ x ≤1). Greater than 99% Ni removal from solution occurred when Ni loading remained below 0.13±0.03 (1σ) mol Ni per mol FeS due to sorption of Ni at the mackinawite surface. Characterization of experimental solids using X-ray diffraction and Raman spectroscopy showed patterns characteristic of nanocrystalline mackinawite; no evidence of nickel monosulfide (α-NiS or millerite), polydymite (Ni3S4), or godlevskite [(Ni,Fe)9S8] formation was indicated regardless of the amount of Ni loading. Slight expansion of the c-axis correlated with increasing Ni content in synthetic mackinawite, from c =5.07±0.01Å at X Ni =0.02 to c =5.10±0.01Å at X Ni =0.38. Ni K-edge extended X-ray absorption fine structure (EXAFS) spectra of synthetic Ni-bearing mackinawite are similar in phase and amplitude to the Fe K-edge EXAFS spectrum of Ni-free mackinawite, indicating that the molecular environment of Ni2+ in Ni-bearing mackinawite is similar to that of Fe2+ in Ni-free mackinawite. EXAFS data fitting of Ni-bearing mackinawite with X Ni =0.42 indicated a coordination number of 4.04±0.30 and an average NiS bond distance of 2.28Å, in good agreement with the FeS bond distance of 2.26Å in mackinawite, tetrahedral Fe coordination, and slight lattice expansion along the c-axis. At lower Ni loadings (X Ni =0.05–0.11), EXAFS analysis showed a decrease in NiS coordination towards CN=3, which reflects the influence of sorbed Ni. Continued Ni uptake, past the maximum amount of sorption, was accompanied by proportional molar release of Fe to solution. Interstitial occupancy of Ni within the mackinawite interlayer may be transitional to structural substitution of Fe. The Ni-mackinawite solid-solution is described by a one-site binary mixing model: ln K d = ln K e − W RT 1 − 2 X Ni where K d is the distribution coefficient, K e is the ratio of equilibrium constants for Ni-mackinawite and mackinawite (14.4±1.3), W is an ion interaction parameter, and X Ni is the mole fraction of end-member NiS in the solid solution. The experimentally determined value of W is 17.74±1.15kJ/mol and indicates significant non-ideality of the solid solution. Transformation processes were evaluated by aging Ni-mackinawite with polysulfides and solutions saturated with air. Reaction of Ni-mackinawite with polysulfides led to the formation of pyrite (FeS2) and Ni retention in the solid phase. When Ni-mackinawite was aged in the presence of dissolved oxygen, transformation to goethite (FeOOH) and violarite (FeNi2S4) was observed.

      PubDate: 2017-05-27T04:54:44Z
  • The experimental determination of REE partition coefficients in the
           water-calcite system
    • Abstract: Publication date: 25 June 2017
      Source:Chemical Geology, Volume 462
      Author(s): Martin Voigt, Vasileios Mavromatis, Eric H. Oelkers
      To better quantify the behaviour of rare earth elements (REE) in low temperature natural systems, calcite was precipitated from NaCl-bearing aqueous solutions in the presence of La and Yb using a constant addition technique at pH ~6 and 25°C under steady-state conditions. The presence of REE in the fluid significantly slows calcite growth rates. Partition coefficients (D REE), determined from analysis of the aqueous solution compositions, range from 102.8 at the lowest aqueous La concentration to 101.4 at the highest aqueous concentration of Yb. D REE was found to correlate with aqueous REE concentrations, as well as with the calcite saturation state. Experimental results, together with previous spectroscopic evidence, suggest the incorporation of REE3+ into the Ca2+ site of calcite while maintaining charge balance by the concurrent incorporation of Na+. The value of the equilibrium constants for the corresponding exchange reaction was determined for La, Eu and Yb by combining our results with previously published experimental data. Subsequently, these constants were used together with a lattice strain model to estimate corresponding values for the other REE. Application of this exchange model suggests that D REE vary significantly with the composition of natural waters including seawater. Such variations are likely the origin for the large differences of D REE values reported in the literature based on experimental and field evidence.

      PubDate: 2017-05-27T04:54:44Z
  • Oxidative dissolution of amorphous FeS and speciation of secondary Fe
           minerals: Effects of pH and As(III) concentration
    • Abstract: Publication date: 25 June 2017
      Source:Chemical Geology, Volume 462
      Author(s): Xue-Liang An, Fu-Gen Huang, Hai-Tao Ren, Yan-Fang Wang, Yao Chen, Zhong-Min Liu, Hong-Wei Zhang, Xu Han
      Oxidative dissolution of Fe sulfides is an important geochemical process mediating As mobility in aqueous environments. However, little is known regarding the presence of As(III) on the oxidative dissolution rate of FeS and on speciation of secondary Fe minerals under pH-neutral and alkaline conditions. In this study, oxidative dissolution of FeS under different concentrations of As(III) and pHs were carried out. The enhanced dissolution rates of Fe(II) from FeS with increasing As(III) concentrations (≤4.7mM) indicate that strong interactions between As(III) and S(−II) in FeS enhance the dissolution rates of structural Fe(II), whereas such an enhancement cannot be observed in the As(III)-FeS system under anoxic conditions at pHs7.0 and 9.0, suggesting the importance of dissolved oxygen in the enhanced dissolution of FeS by As(III). On the other hand, >98.5% of 6.7 and 4.7mM As(III) can be immobilized by 2gL−1 FeS within 1.5h at pHs7.0 and 9.0, respectively, demonstrating the high efficiency of amorphous FeS in the immobilization of concentrated As(III) under oxic conditions. XPS analysis further reveals that, irrespective of pH and initial As(III) concentrations, >60% As(III) was oxidized to As(V) before it was immobilization in secondary Fe minerals, and no As-S compound could be observed after 48h of oxidation. As(III) also influences speciation of secondary Fe minerals during oxidative dissolution of FeS. Analyses of XRD, FTIR, Raman, SEM and TEM demonstrate that the plate lepidocrocite is the primary oxidation product in the absence of As(III), but shows degenerations with increasing As(III) concentrations, and completely changes to ferric arsenate in the presence of 6.7mM As(III), revealing the inhibition of As(III) and/or As(V) on the development of lepidocrocite at pHs7.0 and 9.0. This also suggests that, apart from adsorption, coprecipitation of ferric arsenate is another important pathway to immobilize concentrated As(III) during oxidative dissolution of FeS.
      Graphical abstract image

      PubDate: 2017-05-27T04:54:44Z
  • Continental weathering and terrestrial (oxyhydr)oxide export: Comparing
           glacial and non-glacial catchments in Iceland
    • Abstract: Publication date: 25 June 2017
      Source:Chemical Geology, Volume 462
      Author(s): Scott M. Hawley, Philip A.E. Pogge von Strandmann, Kevin W. Burton, Helen M. Williams, Sigurdur R. Gíslason
      Glaciers enhance terrestrial erosion and sediment export to the ocean. Glaciers can also impact mineral specific weathering rates relative to analogous non-glacial terrains. In tandem these processes affect continent sediment export to the oceans over glacial-interglacial cycles. This study summarizes field data from glacial and non-glacial Icelandic river catchments to quantify the impact of weathering regime on iron and aluminium (oxyhydr)oxide mineral formation and flux rates. Aluminium and iron (oxyhydr)oxides are strong indicators of organic carbon preservation in soils and marine sediments. Tracing changes in (oxyhydr)oxide formation and deposition therefore provides a means of evaluating potential changes in organic carbon sequestration rates over glacial-interglacial cycles. Overall, there are several measurable chemical differences between the studied glacial and non-glacial catchments which reflect the key role of soil formation on terrestrial weathering. One of the noted chemical differences is that weathering in non-glacial catchments is characterized by higher apparent rates of iron and aluminium (oxyhydr)oxide formation relative to glacial catchments. However, the offset in (oxyhydr)oxide formation does not appear to be transferred into river sediment compositions, and physical weathering appears to be the dominant control of river sediment composition and export. Glacial rivers export far more total sediment to nearshore marine environments than analogous non-glacial rivers suggesting glacial weathering enhances carbon burial by increasing nearshore marine (oxyhydr)oxide accumulation.

      PubDate: 2017-05-27T04:54:44Z
  • On the use of different spectral windows in DOAS evaluations: Effects on
           the estimation of SO2 emission rate and mixing ratios during strong
           emission of Popocatépetl volcano
    • Abstract: Publication date: 25 June 2017
      Source:Chemical Geology, Volume 462
      Author(s): Matthias Fickel, Hugo Delgado Granados
      Reported SO2 emission values at strongly degassing volcanoes might be underestimated. Scanning spectroscopic instruments are widely used at volcanoes to quantify SO2 emission rates and, occasionally, molar ratios between SO2 and other gases such as BrO. Frequently, Differential Optical Absorption Spectroscopy is applied to evaluate spectra in the ultraviolet range. If radiative transfer effects are not considered in the evaluations, the retrieved column densities may vary depending on which spectral window is used. We compare the use of different windows for data collected at Popocatépetl volcano during strong degassing periods, when high column densities lead to especially high dependence on the applied evaluation window. We propose the use of three different windows (310–322nm, 314.7–326.7nm and 322–334nm) and a simple algorithm to choose the most accurate column density from them. The SO2 evaluation using proper spectral windows would allow a more realistic estimation of the SO2 emission rates.

      PubDate: 2017-05-27T04:54:44Z
  • An experimental study of trace element distribution during partial melting
           of mantle heterogeneities
    • Abstract: Publication date: 25 June 2017
      Source:Chemical Geology, Volume 462
      Author(s): Carl Spandler, Johannes Hammerli, Greg M. Yaxley
      Trace elements are widely used to interpret the origin of mantle-derived magmas, yet we lack detailed understanding of how trace elements behave during melting of mantle source components. Here, we present new data on trace element distribution and partitioning between phases from high pressure (3.0 to 5.0GPa), high-temperature (1230 to 1550°C) melting experiments on starting compositions that represent altered oceanic crust and metasedimentary protoliths. These compositions are expected to be recycled into the mantle via subduction or delamination to form heterogeneous mantle domains that are implicated in the genesis of intraplate and/or ocean floor magmas. In most of the experiments, the investigated trace elements behave incompatibly, expect for HREE and Y, which are compatible in garnet, and V, Cr and Zn, which partition into both garnet and clinopyroxene. Relative to Nd, P is more compatible in garnet than clinopyroxene, leading to fractionation of P/Nd with melting in some cases. Melt compositions in some experiments with low melt fractions feature distinctive negative anomalies for Nb, and for Sr, Ba and Eu, due to retention of these elements in minor/accessory rutile and feldspar, respectively. We also show that highly incompatible trace element (e.g., Cs, Th, U, LREE) concentrations in melts are strongly controlled by melt fraction, whereas moderately incompatible (M-HREE, Zr) to compatible (Cr, V) element concentrations are controlled by temperature and/or phase composition. Pressure has relatively little influence on trace element behaviour at the investigated conditions. Based on our results, we suggest that partial melting of eclogitic components of mantle domains may ultimately produce magmas with trace element compositions that are unlike peridotite-sourced magmas. Therefore, the trace element systematics of mantle-derived magmas should not only be interpreted in terms of mantle source compositions, but also with consideration to source petrology (e.g., mineral compositions and accessory phase stability) and melting conditions (e.g., melt fraction, pressure, temperature).
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      PubDate: 2017-05-27T04:54:44Z
  • Corrigendum to “The influence of O2 availability and Fe (III) mineralogy
           on Fe metabolism by an acidophilic bacterial culture” [CHEMGE: 457 (10
           May 2017); pages 107–117]
    • Abstract: Publication date: 25 June 2017
      Source:Chemical Geology, Volume 462
      Author(s): John E. Burwick, Shagun Sharma, Christopher M. Menge, Alper Buldum, John M. Senko

      PubDate: 2017-05-27T04:54:44Z
  • Branched GDGT-based paleotemperature reconstruction of the last
           30,000years in humid monsoon region of Southeast China
    • Abstract: Publication date: Available online 17 May 2017
      Source:Chemical Geology
      Author(s): Mengyuan Wang, Zhuo Zheng, Meiling Man, Jianfang Hu, Quanzhou Gao
      The use of bacterial branched glycerol dialkyl glycerol tetraethers (brGDGTs) to reconstruct mean annual air temperatures (MAATs) and sediment pH from soils has sparked significant interest in the terrestrial paleoclimate community. However, the reconstruction of these climate proxies from peat sediments is rare in monsoonal regions of the East Asia. This research was carried out on a core from the Shuizhuyang (SZY) peat bog located in Fujian Province. Branched GDGT (brGDGT) indexes were used for reconstructing the paleoclimate of the last 30calka. The aim was to evaluate quantitatively the MAAT (mean annual air temperature) and pH values since the Last Glacial Maximum (LGM) in the subtropical zone of China. Results show that the CBT-MBT′-derived MAAT at MIS 3 is about 15.6°C on average, which is followed by a significant fall at the LGM (11.7–12.1°C). The temperature difference between the LGM and the present-day value is as high as 5.8°C. The synchronous variation of biomarker and pollen proxies indicates that replacement of subtropical evergreen broadleaved forests by cold-tolerant, deciduous broadleaved forests was driven by the significant drop in air temperature. Our results also indicate that the Younger Dryas event lasted from about 12.9 to about 11.3calka, and cooling events at 3.2calka in the late Holocene were detected, showing the sensitivity of peat bogs to rapid cooling. Our pH reconstructions indicate that the pH of the bog rose during Heinrich 1 and Bølling-Allerød periods, probably due to low precipitation, and were lowest in the Holocene thermal maximum between 8ka and 2.5ka, probably due to higher precipitation. The decoupling of reconstructed MAAT and pH during particularly deglaciation and YD periods supports the hypothesis that the variations in temperature and precipitation are not always synchronous.

      PubDate: 2017-05-22T03:39:31Z
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