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EARTH SCIENCES (466 journals)                  1 2 3 | Last

Showing 1 - 200 of 371 Journals sorted alphabetically
Acta Geochimica     Hybrid Journal   (Followers: 5)
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: 8)
Acta Geotechnica     Hybrid Journal   (Followers: 7)
Acta Meteorologica Sinica     Hybrid Journal   (Followers: 3)
Advances in High Energy Physics     Open Access   (Followers: 19)
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: 5)
AMBIO     Hybrid Journal   (Followers: 15)
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: 22)
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: 5)
Applied Geochemistry     Hybrid Journal   (Followers: 14)
Applied Geomatics     Hybrid Journal   (Followers: 3)
Applied Geophysics     Hybrid Journal   (Followers: 8)
Applied Ocean Research     Hybrid Journal   (Followers: 5)
Applied Petrochemical Research     Open Access   (Followers: 2)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 36)
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: 19)
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: 2)
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: 10)
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: 18)
Cadernos de Geociências     Open Access  
Canadian Mineralogist     Full-text available via subscription   (Followers: 4)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 20)
Carbonates and Evaporites     Hybrid Journal   (Followers: 3)
CATENA     Hybrid Journal   (Followers: 5)
Chemical Geology     Hybrid Journal   (Followers: 20)
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: 14)
Computational Mathematics and Mathematical Physics     Hybrid Journal   (Followers: 3)
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: 12)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 3)
Coral Reefs     Hybrid Journal   (Followers: 18)
Cretaceous Research     Hybrid Journal   (Followers: 8)
Cybergeo : European Journal of Geography     Open Access   (Followers: 6)
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: 129)
Earth and Space Science     Open Access   (Followers: 14)
Earth Interactions     Full-text available via subscription   (Followers: 13)
Earth Science Research     Open Access   (Followers: 7)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 5)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 19)
Earth System Dynamics     Open Access   (Followers: 9)
Earth System Dynamics Discussions     Open Access   (Followers: 6)
Earth's Future     Open Access   (Followers: 1)
Earth, Planets and Space     Open Access   (Followers: 71)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 9)
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: 27)
Environmental Geology     Hybrid Journal   (Followers: 20)
Environmental Geosciences     Full-text available via subscription   (Followers: 5)
Environmental Geotechnics     Hybrid Journal   (Followers: 5)
Erwerbs-Obstbau     Hybrid Journal  
Estuaries and Coasts     Hybrid Journal   (Followers: 17)
Estuarine, Coastal and Shelf Science     Hybrid Journal   (Followers: 33)
Estudios Geográficos     Open Access  
European Journal of Mineralogy     Full-text available via subscription   (Followers: 14)
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: 11)
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   (Followers: 2)
Geochemistry : Exploration, Environment, Analysis     Hybrid Journal   (Followers: 8)
Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 30)
Geochimica et Cosmochimica Acta     Hybrid Journal   (Followers: 48)
Geochronometria     Hybrid Journal   (Followers: 3)
Geoderma Regional : The International Journal for Regional Soil Research     Full-text available via subscription   (Followers: 4)
Geodinamica Acta     Open Access   (Followers: 3)
Geodynamics & Tectonophysics     Open Access   (Followers: 2)
Geoenvironmental Disasters     Open Access   (Followers: 3)
Geofluids     Open Access   (Followers: 4)
Geoforum     Hybrid Journal   (Followers: 23)
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: 27)
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: 34)
Geophysical Prospecting     Hybrid Journal   (Followers: 7)
Geophysics     Full-text available via subscription   (Followers: 19)
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: 4)
Geoscience Data Journal     Open Access   (Followers: 2)
Geoscience Frontiers     Open Access   (Followers: 10)
Geoscience Letters     Open Access   (Followers: 1)
Geoscience Records     Open Access  
Geosciences     Open Access   (Followers: 2)
Geosciences Journal     Hybrid Journal   (Followers: 11)
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: 19)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 15)
Gondwana Research     Hybrid Journal   (Followers: 9)
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: 3)
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: 21)
Hydrological Processes     Hybrid Journal   (Followers: 28)
Hydrology and Earth System Sciences     Open Access   (Followers: 28)
ICES Journal of Marine Science: Journal du Conseil     Hybrid Journal   (Followers: 53)
IEEE Journal of Oceanic Engineering     Hybrid Journal   (Followers: 13)
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: 16)
International Journal of Advanced Geosciences     Open Access   (Followers: 1)
International Journal of Advanced Remote Sensing and GIS     Open Access   (Followers: 36)
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: 29)
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: 18)
International Journal of Earth Sciences     Hybrid Journal   (Followers: 39)
International Journal of Earthquake and Impact Engineering     Hybrid Journal   (Followers: 2)
International Journal of Geo-Engineering     Open Access   (Followers: 3)

        1 2 3 | Last

Journal Cover Chemical Geology
  [SJR: 2.346]   [H-I: 145]   [20 followers]  Follow
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 0009-2541
   Published by Elsevier Homepage  [3043 journals]
  • Inside front cover (Editorial Board)
    • Abstract: Publication date: 15 November 2017
      Source:Chemical Geology, Volume 472

      PubDate: 2017-10-18T11:48:10Z
  • Short-lived juvenile effects observed in stable carbon and oxygen isotopes
           of UK oak trees and historic building timbers
    • Abstract: Publication date: 15 November 2017
      Source:Chemical Geology, Volume 472
      Author(s): Josie E. Duffy, Danny McCarroll, Alexander Barnes, Christopher Bronk Ramsey, Darren Davies, Neil J. Loader, Daniel Miles, Giles H.F. Young
      Stable carbon (δ13C) and oxygen (δ18O) isotope ratios were measured on the latewood α-cellulose of individual oak (Quercus robur L, Q. petraea Liebl.) samples from living trees and historic building timbers. This represents the type of material available to produce long tree-ring chronologies for north-western Europe including the UK and Ireland. Results from the juvenile rings, those located closest to the pith, were compared with results from equivalent sections (representing the same calendar years) from independent master isotope chronologies that do not contain any juvenile wood, allowing any juvenile offsets and trends to be separated from those caused by environmental change. Oak timbers from archaeological sources are often relatively short (<100years). Therefore, removing the first 50 rings, as is typical for Pinus sp., would severely constrain the material available for chronology construction. The aim of this study was to determine the magnitude and duration of juvenile effects, including the detection of trends, offsets and their influence upon signal strength. The results show clearly that juvenile effects for oak from central England are very small and short-lived and that removing merely the first five rings closest to the pith is sufficient to avoid them. This result greatly increases the potential for building long and well-replicated stable isotope chronologies using archived oak samples from historic building timbers, allowing high-resolution climate reconstructions to be produced for the highly-populated regions, where oak is abundant and which are currently under-represented in regional palaeoclimate reconstructions.

      PubDate: 2017-10-18T11:48:10Z
  • Improved accuracy of LA-ICP-MS U-Pb ages of Cenozoic zircons by alpha dose
    • Abstract: Publication date: 15 November 2017
      Source:Chemical Geology, Volume 472
      Author(s): J.T. Sliwinski, M. Guillong, C. Liebske, I. Dunkl, A. von Quadt, O. Bachmann
      While Laser Ablation Inductively-Coupled-Plasma Mass Spectrometry (LA-ICP-MS) is the method of choice for U-Pb dating of zircons in provenance analysis, its application to young (<100Ma) zircons is hindered by systematic analytical bias. In magmatic petrology, where the majority of studied units are young, this often places zircon dates at odds with established 40Ar/39Ar eruption ages or high-precision ID-TIMS crystallization ages. Zircon lattice properties, particularly the degree of lattice damage caused by the radioactive decay of U and Th, impart analytical bias by causing differential ablation rates and therefore differential fractionation of U and Pb throughout each analysis. Although it is possible to normalize the zircon lattice strengths to calibration reference zircons by thermal annealing to some extent, this may not entirely alleviate the problem. In this study, the effects of alpha decay dose (i.e., degree of radiation damage) on analytical biases in age determination are examined by analyzing a number of zircon reference materials under well-constrained analytical parameters. A regression-based, multi-standard correction method is demonstrated, which improves the accuracy of age data, particularly in young (Cenozoic) zircons. A novel data reduction scheme (Dose_Corrector.ipf) is introduced, which runs in conjunction with the widely-used Igor Pro/Iolite platform and performs a correction for alpha dose and Th disequilibrium. This scheme improves the accuracy of age data for unannealed zircons, and its utility is demonstrated by applying it to zircons from several well-studied units.
      Graphical abstract image

      PubDate: 2017-10-18T11:48:10Z
  • Temporal changes in Cr fluxes and δ53Cr values in runoff from a small
           serpentinite catchment (Slavkov Forest, Czech Republic)
    • Abstract: Publication date: 15 November 2017
      Source:Chemical Geology, Volume 472
      Author(s): Martin Novak, Pavel Kram, Ondrej Sebek, Alexandre Andronikov, Vladislav Chrastny, Eva Martinkova, Marketa Stepanova, Eva Prechova, Jan Curik, Frantisek Veselovsky, Oldrich Myska, Veronika Stedra, Juraj Farkas
      Headwater catchments underlain by ultramafic bedrock may release toxic amounts of hexavalent chromium [Cr(VI)] into runoff. Redox cycling of Cr during weathering, pedogenesis and runoff generation can be constrained by studying 53Cr/52Cr isotope systematics. We report a 12-month time-series of δ53Cr values of runoff from a small serpentinite-dominated catchment in Central Europe, complemented with a 5-year monitoring of Cr and DOC concentrations. δ53Cr values of bulk bedrock, rock-forming minerals, and bulk soil were also determined. Cr(VI) concentrations in runoff exceeded the 10-ppb maximum contaminant level by 70%. Concentrations of Cr(VI) in runoff were positively correlated with water discharge and DOC concentrations. Chromium export flux in winter was significantly higher than in summer. Runoff δ53Cr exhibited a temporal trend, with negative values (−0.6‰) in winter and positive values (0.5‰) in summer. On an annual basis, the mass-weighted δ53Cr value of runoff was −0.4‰. Dissolved Cr exported in winter was isotopically lighter than the within-site Cr sources: δ53Cr of bulk bedrock ranged from −0.2 to 0.0‰, and δ53Cr of soil ranged from −0.2 to 0.3‰. Chromium isotope analysis of seven rock-forming minerals separated from serpentinites, tremolite/actinolite schists and amphibolites revealed a much wider range of δ53Cr values, from −0.4‰ (hornblende) to 1.7‰ (albite). Most of the variability in runoff δ53Cr can be explained by incongruent oxidative weathering, with a contribution of isotope-selective processes related to pedogenesis. Isotopically light Cr was preferentially mobilized from weathered bedrock and soil in cold, water-rich periods and exported via stream water. In these periods, precipitation water more rapidly penetrates into the soil than in summer months, characterized by lower rainfall and larger evapotranspiration, and more efficiently leaches the accumulated Cr(VI).
      Graphical abstract image

      PubDate: 2017-10-18T11:48:10Z
  • Radiogenic isotopic compositions of low concentration dust and aerosol
           from the GISP2 ice core
    • Abstract: Publication date: 15 November 2017
      Source:Chemical Geology, Volume 472
      Author(s): Kyle W. Meyer, Sarah Aciego, Janne M. Koornneef
      We have produced a record of radiogenic isotope compositions of particulate and soluble dust fractions from the GISP2 ice core, from basal ice to the surface. For each respective dust fraction we measured Sr and Nd isotope values to constrain the provenance of mineral dust and sea salt aerosol transported to the Greenland Ice Sheet. We have produced high-precision measurements of ε Nd values on sub-ng quantities of Nd in ice core samples via TIMS with 1013 ohm resistors. The range of measured ε Nd values of soluble material (−12.9±2.6 to −7.3±0.9) is interpreted to represent sea salt aerosol based on Sr isotope compositions that resemble seawater values (0.711007±0.000030 to 0.709416±0.000072). Therefore, the ε Nd potentially provides important insights to the atmospheric circulation patterns at high latitudes at least through MIS 5. We compare ε Nd values of surface seawater from ocean basins and deltas at high latitudes to alternating relatively radiogenic and unradiogenic ε Nd at GISP2. We suggest these changes represent transitions between a climate system dominated by enhanced and persistent Arctic Oscillation positive anomalies (as determined from sea level and ocean bottom pressure measurements), and a state representative of modern atmospheric circulation, which typically exhibits either negative anomalies or none altogether.

      PubDate: 2017-10-18T11:48:10Z
  • Inside front cover (Editorial Board)
    • Abstract: Publication date: 5 November 2017
      Source:Chemical Geology, Volume 471

      PubDate: 2017-10-18T11:48:10Z
  • Geochemical behavior of Cu and sulfur isotopes in the tropical mining
           region of Taxco, Guerrero (southern Mexico)
    • Abstract: Publication date: 5 November 2017
      Source:Chemical Geology, Volume 471
      Author(s): Azucena Dótor-Almazán, María Aurora Armienta-Hernández, Oscar Talavera-Mendoza, Joaquin Ruiz
      This study was aimed to identify the main processes involved in the release of metals using sulfur isotopes and those acting on the fractionation of copper isotopes. We determined sulfur and copper isotopic ratios in rocks, tailings, precipitates and soils samples from the historical mining district of Taxco, Southern Mexico. Ratios of sulfur isotopes are reported in notation of δ per mil, and standardization was based on international standards OGS-1 and NBS123. Only copper ratio isotopes were studied in pyrite (Py), sphalerite (Sph) and galena. Results are reported as δ65Cu=((65Cu/63Cusample/65Cu/63CuNIST976standar)−1)∗1000. Error for all the analyzed samples was 0.01‰ and any mass bias was corrected trough bracketing method. The sulfur isotopic value of a sample from the Mexcala shale Formation was −7.7‰. The δ34S values in tailing ponds ranged from −2.6 to 1.2‰, in precipitates from −7.2 to −3.2‰, and in soils from −0.8 to 4.5‰. Isotopic δ34S values in rocks evidenced a magmatic influence of sulfides present in the Mexcala Formation. Oxidation of sulfides contained in tailings and precipitates was observed related with their low δ34S values close to zero, oxidation followed the order: sphalerite>pyrite>pyrite rich in copper>chalcopyrite>galena. In soils, oxidation is not the main process as supported by their positive δ34S values and low sulfate content. Results of δ65Cu values in rocks were −0.09 to 0.54‰; in sulfides values were δ65Cu=0.18‰ (Py) and δ65Cu=−0.20‰ (Sph), concentration of copper in galena was not enough to get a reliable isotopic value. In tailings δ65Cu ranged from −1.19 to 0.91‰ and in precipitates from −0.36 to 0.92‰; in soils values were between 0.26 and 1.72‰. Results in rocks indicate an igneous influence on copper provenance. The δ65Cu values in Py and Sph were within the range of terrestrial primary sulfides (from −1 to 1‰) confirming its igneous provenance. Processes related with fractionation of copper isotopes were oxidation of tailings and reprecipitation of 65Cu in tailings with a high content of Fe oxyhydroxides. Enrichment in 65Cu in precipitates and soils is favored by organic matter presence that retains 63Cu. Erosion of rocks and tailings is the main source of copper being rocks the initial and tailings the end members. The isotopic studies on environmental research provide a useful tool on: 1) The behavior of copper and sulfur isotopes in environments where both, natural and anthropogenic activities, are present. 2) Understanding surficial processes of dispersion of metals and identifying their source in mining areas surrounded by villages and agricultural soils.

      PubDate: 2017-10-18T11:48:10Z
  • The influence of seawater carbonate chemistry, mineralogy, and diagenesis
           on calcium isotope variations in Lower-Middle Triassic carbonate rocks
    • Abstract: Publication date: 5 November 2017
      Source:Chemical Geology, Volume 471
      Author(s): Kimberly V. Lau, Kate Maher, Shaun T. Brown, Adam B. Jost, Demir Altıner, Donald J. DePaolo, Anton Eisenhauer, Brian M. Kelley, Daniel J. Lehrmann, Adina Paytan, Meiyi Yu, Juan Carlos Silva-Tamayo, Jonathan L. Payne
      The geological calcium cycle is linked to the geological carbon cycle through the weathering and burial of carbonate rocks. As a result, calcium (Ca) isotope ratios (44Ca/40Ca, expressed as δ 44/40Ca) can help to constrain ancient carbon cycle dynamics if Ca cycle behavior can be reconstructed. However, the δ 44/40Ca of carbonate rocks is influenced not only by the δ 44/40Ca of seawater but also by diagenetic processes and fractionation associated with carbonate precipitation. In this study, we investigate the dominant controls on carbonate δ 44/40Ca in Upper Permian to Middle Triassic limestones (ca. 253 to 244Ma) from south China and Turkey. This time interval is ideal for assessing controls on Ca isotope ratios in carbonate rocks because fluctuations in seawater δ 44/40Ca may be expected based on several large carbon isotope (δ 13C) excursions ranging from −2 to +8‰. Parallel negative δ 13C and δ 44/40Ca excursions were previously identified across the end-Permian extinction horizon. Here, we find a second negative excursion in δ 44/40Ca of ~0.2‰ within Lower Triassic strata in both south China and Turkey; however, this excursion is not synchronous between regions and thus cannot be interpreted to reflect secular change in the δ 44/40Ca of global seawater. Additionally, δ 44/40Ca values from Turkey are consistently 0.3‰ lower than contemporaneous samples from south China, providing further support for local or regional influences. By measuring δ 44/40Ca and Sr concentrations ([Sr]) in two stratigraphic sections located at opposite margins of the Paleo-Tethys Ocean, we can determine whether the data represent global conditions (e.g., secular variations in the δ 44/40Ca of seawater) versus local controls (e.g., original mineralogy or diagenetic alteration). The [Sr] and δ 44/40Ca data from this study are best described statistically by a log-linear correlation that also exists in many previously published datasets of various geological ages. Using a model of early marine diagenetic water-rock interaction, we illustrate that this general correlation can be explained by the chemical evolution of bulk carbonate sediment samples with different initial mineralogical compositions that subsequently underwent recrystallization. Although early diagenetic resetting and carbonate mineralogy strongly influence the carbonate δ 44/40Ca values, the relationship between [Sr] and δ 44/40Ca holds potential for reconstructing first-order secular changes in seawater δ 44/40Ca composition.

      PubDate: 2017-10-18T11:48:10Z
  • Different partitioning behaviors of molybdenum and tungsten in a
           sediment–water system under various redox conditions
    • Abstract: Publication date: 5 November 2017
      Source:Chemical Geology, Volume 471
      Author(s): Yusuke Watanabe, Teruhiko Kashiwabara, Jun-ichiro Ishibashi, Oki Sekizawa, Kiyofumi Nitta, Tomoya Uruga, Yoshio Takahashi
      Molybdenum (Mo) and tungsten (W) are redox-sensitive elements that exhibit contrasting geochemical behaviors under different redox and/or sulfidic conditions despite belonging to the same group in the periodic table. In this study, the geochemistry of Mo and W in a sediment–porewater system was investigated using a core sample collected from sediment containing hydrothermal minerals (Izena Hole, Okinawa, Japan). The Mo contents in the sediment and the porewater were quantified to calculate the partition coefficient K d (concentration ratio of the solid phase to the aqueous phase) for Mo. However, the W concentration in the porewater was exceedingly low to be detected. Thus, we also conducted laboratory experiments simulating the sediment–porewater system to clarify the partitioning behaviors of Mo and W under redox conditions. X-ray absorption near-edge structure (XANES) spectra were obtained to determine the chemical species of Mo and W in sediment at different depths to understand the chemical processes of Mo and W. In particular, high-sensitivity XANES spectroscopy using wavelength-dispersive fluorescence mode was applied to W L 3 edge XANES spectra to reduce the interference signals from coexistent elements (Zn and Ni). This step thereby facilitating the identification of oxygen- and sulfur-coordinated species for W in the sediments. The collected core sample covered a broad range of redox conditions under various hydrogen sulfide (H2S) concentrations depending on the depth. The presence of iron oxides in the upper layer suggested an oxic condition above within 4cm depth below the seafloor (cmbsf). Iron and Zn sulfide minerals were found in deeper layers (16–24 cmbsf) and indicated the reductive conditions formed in such layer. Hydrogen sulfide was also observed in the porewater of the deeper layer. Analyses for Mo in the natural sediments, as well as XANES analyses, revealed that the K d was higher in the deeper layer, with high pyrite and H2S contents, than in the upper layer. These results implied that Mo was removed from the porewater under reductive and/or sulfidic conditions. The chemical species of Mo was also an oxygen-coordinated species in the upper layer (0–8 cmbsf). Meanwhile, the detection of sulfide in the deeper layer suggests that Mo sulfidation is an important reaction in Mo enrichment in sediments. On the other hand, at all the depths, W formed oxygen-coordinated species in the sediment. The subtle change of K d for W with depth suggested that H2S did not affect the W adsorption in our samples. Hence, the Mo/W ratio in the sediments increased with the development of reducing conditions and vice versa in the coexistent porewater. Speciation analysis revealed that the high stability of oxygen-coordinated species of W was responsible for the variations of the Mo/W ratio under various redox conditions.

      PubDate: 2017-10-18T11:48:10Z
  • Experimental determination of gold speciation in sulfide-rich hydrothermal
           fluids under a wide range of redox conditions
    • Abstract: Publication date: 5 November 2017
      Source:Chemical Geology, Volume 471
      Author(s): A.L. Trigub, B.R. Tagirov, K.O. Kvashnina, S. Lafuerza, O.N. Filimonova, M.S. Nickolsky
      The redox state of hydrothermal fluids, which form economic deposits of noble metals, varies in wide limits – from oxidized ones typical for porphyry mineralization to reduced, which form volcanogenic massive sulfide deposits. Sulfur-bearing species, along with chloride, are the most important ligands that form stable aqueous complexes with Au and determine the concentration of the latter in natural ore-generating fluids. Depending on the oxygen fugacity, in high-temperature fluids (t >300°C) the dominant forms of sulfur are sulfides (H2S, HS−), sulfites (SO2, HSO3 −, SO3 2−), sulfates (HSO4 −, SO4 2−), and the radical species (S2 −, S3 −). Here we report an investigation of Au complexation in high-temperature sulfide-bearing fluids of contrasting redox states. The solubility of Au was measured in a relatively oxidized sulfide fluid (H2S/SO4 2− buffer controls the redox state) at 450°C, 1000bar, and compared with the Au solubility in the “reduced” sulfide systems (H2S/HS− predominate) reported in the literature. The measured values of the Au solubility matches best the model of the formation of Au(HS)2 − at near-neutral to weakly acidic pH, and AuHS° in acidic solutions. The solubility constants have been determined for the reactions Au cr + H 2 S ° aq + HS − = Au HS 2 − + 0.5 H 2 g log K Au HS 2 − = − 0.9 ± 0.1 , Au cr + H 2 S ° aq = AuHS ° aq + 0.5 H 2 g log K AuHS = − 6.5 ± 0.1 . The average value of log K Au(HS)2 − =−1.3±0.5 was calculated for 450°C (P =500–1500bar) using all the available Au solubility constants obtained in both “reduced” and “oxidized” sulfide systems. The local atomic environment and electronic structure of Au in high-temperature hydrothermal fluids have been studied using X-ray absorption spectroscopy (XAS) in high energy resolution fluorescence detection (HERFD) mode in combination with ab initio molecular dynamics (AIMD) and Reverse Monte Carlo (RMC) simulations. Interpretation of Au L 3-edge extended X-ray absorption fine structure (EXAFS) spectra showed that, independently of the redox (sulfide or sulfide/sulfate systems) and PT – conditions (350–450°C, 500bar) two S atoms are located in the first coordination sphere of Au at 2.29±0.02Å. Comparison of the experimental spectra with those simulated by means of AIMD revealed that EXAFS spectroscopy is not sensitive to the presence of light atoms like S in the distant coordination spheres of Au. However, theoretical calculations indicated that the shape of Au L...
      PubDate: 2017-10-18T11:48:10Z
  • Precise selenium isotope measurement in seawater by carbon-containing
           hydride generation-Desolvation-MC-ICP-MS after thiol resin
    • Abstract: Publication date: 5 November 2017
      Source:Chemical Geology, Volume 471
      Author(s): Yan Chang, Jing Zhang, Jian-Quo Qu, Yun Xue
      The isotopic compositions of Se in natural materials can be measured by multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) in combination with hydride generation and desolvation device. Although oceans are important targets for research, precise isotopic analysis of Se in seawater is generally difficult owing to its low concentrations and the much higher concentrations of other elements. We developed a robust, accurate method for the precise determination of Se isotopic composition in seawater. A hydride generator combined with a desolvation device with methane addition was coupled to MC-ICP-MS to measure Se isotopes precisely. Two new thiol resins were used to preconcentrate and separate Se from the seawater matrix efficiently, and Se was quickly eluted by small amounts of concentrated HNO3. No detectable Se isotope fractionation was observed during the preconcentration and separation procedure. The carbon-containing plasma increased Se sensitivity, suppressed the spectral interference signal, and improved precision by >3 times. The precisions (2sd, n =24) of Merck Se standard solutions for δ82/78Se, δ82/77Se, and δ82/76Se were 0.07‰, 0.10‰, and 0.16‰, respectively. The δ82/76Se values of standard solutions agreed well with results obtained by other research groups. The method was used to obtain a depth profile for dissolved inorganic Se (DISe) isotopes in seawater from the Northwestern Pacific Ocean. The range of δ82/76DISe was 0.41‰ to 1.59‰, indicating that Se isotopes fractionate sufficiently to clarify the biogeochemical cycling of this element in the ocean.
      Graphical abstract image

      PubDate: 2017-10-18T11:48:10Z
  • Stream and slope weathering effects on organic-rich mudstone geochemistry
           and implications for hydrocarbon source rock assessment: A Bowland Shale
           case study
    • Abstract: Publication date: 5 November 2017
      Source:Chemical Geology, Volume 471
      Author(s): Joseph F. Emmings, Sarah J. Davies, Christopher H. Vane, Melanie J. Leng, Vicky Moss-Hayes, Michael H. Stephenson, Gawen R.T. Jenkin
      This study contributes to the exploration and quantification of the weathering of organic-rich mudstones under temperate climatic conditions. Bowland Shales, exposed by a stream and slope, were sampled in order to develop a model for the effects of weathering on the mudstone geochemistry, including major and trace element geochemistry, Rock-Eval pyrolysis and δ13Corg. Four weathering grades (I – IV) are defined using a visual classification scheme; visually fresh and unaltered (I), chemically altered (II, III) and ‘paper shale’ that typifies weathered mudstone on slopes (IV). Bedload abrasion in the stream exposes of visually fresh and geochemically unaltered mudstone. Natural fractures are conduits for oxidising meteoric waters that promote leaching at the millimetre scale and/or precipitation of iron oxide coatings along fracture surfaces. On the slope, bedding-parallel fractures formed (and may continue to form) in response to chemical and/or physical weathering processes. These fractures develop along planes of weakness, typically along laminae comprising detrital grains, and exhibit millimetre- and centimetre-scale leached layers and iron oxide coatings. Fracture surfaces are progressively exposed to physical weathering processes towards the outcrop surface, and results in disintegration of the altered material along fracture surfaces. Grade IV, ‘paper shale’ mudstone is chemically unaltered but represents a biased record driven by initial heterogeneity in the sedimentary fabric. Chemically weathered outcrop samples exhibit lower concentrations of both ‘free’ (S1) (up to 0.6mgHC/g rock) and ‘bound’ (S2) (up to 3.2mgHC/g rock) hydrocarbon, reduced total organic carbon content (up to 0.34wt%), reduced hydrogen index (up to 58mgHC/gTOC), increased oxygen index (up to 19mgCO+CO2/gTOC) and increased Tmax (up to 11°C) compared with unaltered samples. If analysis of chemically weathered samples is unavoidable, back-extrapolation of Rock-Eval parameters can assist in the estimation of pre-weathering organic compositions. Combining Cs/Cu with oxygen index is a proxy for identifying the weathering progression from fresh material (I) to ‘paper shale’ (IV). This study demonstrates that outcrop samples in temperate climates can provide information for assessing hydrocarbon potential of organic-rich mudstones.
      Graphical abstract image

      PubDate: 2017-10-18T11:48:10Z
  • Silicate-sulfide liquid immiscibility in modern arc basalt (Tolbachik
           volcano, Kamchatka): Part II. Composition, liquidus assemblage and
           fractionation of the silicate melt
    • Abstract: Publication date: 5 November 2017
      Source:Chemical Geology, Volume 471
      Author(s): Vadim S. Kamenetsky, Michael Zelenski, Andrey Gurenko, Maxim Portnyagin, Kathy Ehrig, Maya Kamenetsky, Tatiana Churikova, Sandrin Feig
      Olivine-hosted inclusions of silicate and sulfide melts, Cr-spinel and pyroxene were studied to estimate magma composition, temperature, pressure, and fO2 at the onset and during the silicate-sulfide immiscibility in modern arc basalt from Tolbachik volcano, Kamchatka arc. We demonstrate that the olivine phenocrysts hosting sulfide and silicate melt inclusions belong to the same population. The compositions of the silicate melt inclusions in most primitive olivine (88–91mol% Fo) represent moderately oxidized (~QFM+1.1) high-MgO (up to 12–12.6wt%) and high CaO/Al2O3 (0.8–1.2) melt that has abundances and ratios of the lithophile trace elements typical of island arc magmas. The initial volatile contents in parental Tolbachik magma are estimated from the melt inclusions and mass-balance considerations to be at least 4.9wt% H2O, 2600ppm S, 1100ppm Cl, 550ppm F, and 1200ppm CO2. These data are used to calculate the temperature (~1220°C) and minimum pressure (3kbar) at which the beginning of crystallization and exsolution of sulfide melt took place. The presence of anhydrite, especially ubiquitous in the crystallized silicate melt associated with sulfide globules, suggest that much higher sulfur abundances prior to degassing and sulfate immiscibility and/or crystallization should be expected. We tentatively considered hydrothermal accumulations of sulfur (elemental, sulfate and sulfide) in the volcanic conduit responsible for local contamination and oversaturation of the Tolbachik magma in sulfur and related sulfide immiscibility. Coexisting sulfide and sulfate can be also interpreted in favor of the magmatic sulfide oxidation and related generation of S-rich fluids. Such fluids are expected to accumulate metals released from decomposed sulfide melts and supply significant epithermal mineralization, including native gold.

      PubDate: 2017-10-18T11:48:10Z
  • Adsorption of tetradecanoic acid on kaolinite minerals: Using flash
           pyrolysis to characterise the catalytic efficiency of clay mineral
           adsorbed fatty acids
    • Abstract: Publication date: 5 November 2017
      Source:Chemical Geology, Volume 471
      Author(s): Rabia Zafar, Jonathan S. Watson
      The clay mineral kaolinite is one of the major inorganic constituents of sedimentary rocks. Kaolinite-carboxylic acid interactions are of considerable importance from the geochemical perspective. The two-fold aim of this study was to quantify the adsorption of tetradecanoic acid on kaolinite and then the flash pyrolysis of adsorbed fatty acids-kaolinite samples to understand the transformation of adsorbed fatty acids on kaolinite. Adsorption of tetradecanoic acid on kaolinite results in an s-isotherm which reflects the multilayer adsorption. Adsorption of tetradecanoic acid on kaolinite involves its both functionalities i.e. siloxane (tetrahedral face) and hydroxyl surface (octahedral face) as indicated from the pyrolysis results. Flash pyrolysis of tetradecanoic acid adsorbed kaolinite mainly yielded saturated/unsaturated hydrocarbons, aromatic hydrocarbons, and ketones while pure tetradecanoic acid generated saturated/unsaturated hydrocarbons and a series of unsaturated and saturated low molecular weight fatty acids. We have successively tested an empirical approach to identify organic compounds formed from fatty acid adsorbed kaolinite to the organic compounds obtained from fatty acid adsorbed alumina and silica. Kaolinite mainly reflects the transformation of carboxylic acids into hydrocarbons and ketones via hydroxyl surface (octahedral face). Ketonisation is mainly observed at multilayer adsorption of tetradecanoic acid on kaolinite. The major implication of the work is the understanding of fatty acids adsorption on kaolinite via both surfaces of the mineral which is helpful to understand the fate of fatty acids as they pass into the geosphere during diagenesis.

      PubDate: 2017-10-18T11:48:10Z
  • Efficacy of in situ and meteoric 10Be mixing in fluvial sediment collected
           from small catchments in China
    • Abstract: Publication date: 5 November 2017
      Source:Chemical Geology, Volume 471
      Author(s): Thomas B. Neilson, Amanda H. Schmidt, Paul R. Bierman, Dylan H. Rood, Veronica Sosa Gonzalez
      Using measurements of in situ and meteoric 10Be in fluvial sand to measure erosion rates, quantify soil loss, and trace sediment sources and sinks relies on the assumption that such sediment is well-mixed and representative of the upstream area. We test this assumption at 13 river junctions in three tributary watersheds (200–2500km2) to the Mekong River, Yunnan, China, where human alteration of the landscape is significant and widespread. We find that two of the three watersheds mix well for in situ 10Be and none mix well for meteoric 10Be when considering the concentration of 10Be at the outlet compared to the area-weighted mean of headwater samples. We also assessed mixing at 13 river junctions by comparing the erosion rate-weighted isotopic concentration of sediment taken from tributaries upstream of a junction to the concentration in a sample taken downstream of the junction. With this metric, mixing is generally poor for both in situ and meteoric 10Be but is better for in situ 10Be than for meteoric 10Be (p<0.05). This is likely because in situ 10Be is measured in quartz, which is resilient to physical and chemical breakdown in river systems whereas meteoric 10Be is measured in grain coatings which can abrade and dissolve. Basins eroding faster (>100mm/kyr) tend to mix better than slowly eroding basins. We find no evidence that agricultural land use in sampled basins affects sediment mixing downstream. Mixing improves with increased basin area (particularly >200km2), increased sampling distance downstream from an upstream junction (>500m), and increased difference in size between tributaries (one tributary >3 times larger than the other). The most important factor affecting mixing efficacy for both in situ and meteoric 10Be is the fraction of the basin area contributing to the downstream sample that does not contribute to the upstream samples. Junctions with >2% of the basin area unsampled by upstream samples tend not to mix as well. Our data suggest specific sampling location strategies (such as amalgamation) likely to improve the outcome of fluvial network analysis using cosmogenic nuclides.

      PubDate: 2017-10-18T11:48:10Z
  • The noble gas isotope record of hydrocarbon field formation time scales
    • Abstract: Publication date: 5 November 2017
      Source:Chemical Geology, Volume 471
      Author(s): Igor N. Tolstikhin, Chris J. Ballentine, Boris G. Polyak, Edward M. Prasolov, Olga E. Kikvadze
      Noble gases may be considered as the most prominent tracers of natural fluids, including hydrocarbons. The atmosphere is the only source of 20Ne, 36Ar, 84Kr, 130Xe in subsurface environments, and their concentrations in pore waters after recharge are known from the solubility data. This allows modelling of noble gas partitioning between coexisting gas, oil and water phases in the course of hydrocarbon formation, migration, and storage. Radiogenic isotopes, 4He*, 21Ne*, 40Ar*, 136Xe*, after being released from source rocks, are mixed with air-derived noble gases already present in the pore space. Concentrations of radiogenic species in the pore space of “typical” hydrocarbon fields are generally so high, that they can hardly be accumulated in situ and thus indicate noble gas transfer from ground waters. The time bearing ratios 4He*/20Ne, 21Ne*/20Ne, 4He*/40ArAIR 40Ar*/40ArAIR in hydrocarbon fields are thus proportional to the time interval between the ground water recharge and noble gases partitioning into the hydrocarbon phase(s), the ‘recharge – partition interval’. The largest available data set allows the recharge-partition intervals to be constrained for a large number of hydrocarbon fields, situated in different tectonic settings (ancient plates, young plates, mobile belts). These intervals increase systematically with the ages of hydrocarbon source and trap lithologies and are comparable with these ages. This important feature, valid in general for different hydrocarbon fields, implies: (i) local sources of radiogenic noble gas isotopes in ground waters; (ii) relatively recent formation of hydrocarbon fields and (iii) their short formation time scales. In some cases the duration of formation of a hydrocarbon field can be constrained. For example, nearly constant 21Ne*/20Ne, 40Ar*/40ArAIR ratios, measured in samples from the Magnus oil field (North Sea), give an accumulation time scale≈10Ma. It should be emphasized that the above noble gas isotope ratios give the time estimates, which are independent of geological reconstructions. Sometimes the noble gas inventory in a hydrocarbon field and ground waters allows characterization of the source rock volume, involved in formation of the field; generally this volume exceeds that of the hydrocarbon field rocks by orders of magnitude.

      PubDate: 2017-10-18T11:48:10Z
  • Corrigendum to “Impact of electrogenic sulfur oxidation on trace metal
    • Abstract: Publication date: 5 November 2017
      Source:Chemical Geology, Volume 471
      Author(s): Sebastiaan van de Velde, Ine Callebaut, Yue Gao, Filip J.R. Meysman

      PubDate: 2017-10-18T11:48:10Z
  • The effect of sample treatments on the oxygen isotopic composition of
           phosphate pools in soils
    • Abstract: Publication date: Available online 18 October 2017
      Source:Chemical Geology
      Author(s): Zhao-Hua Jiang, Han Zhang, Deb P. Jaisi, Ruth E. Blake, Ai-Rong Zheng, Min Chen, Xi-Guang Zhang, An-Guo Peng, Xue-Tie Lei, Kai-Qiang Kang, Zhi-Gang Chen
      The oxygen isotopic composition of phosphate (δ18OP) has been increasingly used as an effective tracer for the biogeochemical cycling of phosphorus (P) in soils and other environments. However, diverse pretreatment methods (e.g. storage, preparation and extraction) are being used for soil samples. For the uniformity of methods as well as for the comparison of results, it is important to understand if specific treatment methods can compromise original δ18OP values. Here, Ag3PO4 and KH2PO4 were used to test whether a modified Hedley sequential extraction and purification procedure can alter the δ18OP values of phosphate standards. Additionally, to test the effect of sample storage and drying conditions, two types of soils were first processed by using eight different pretreatment methods including sterilizing, storing, drying, and sieving and then the δ18OP values of each soil P pool were measured. Results indicate that the extraction and purification procedure, drying temperature (<0°C to 80°C) and sieving mesh (20 to 100) had no significant effect on the δ18OP values of Pi (inorganic P) pools, but storage at room temperature (without microbial growth inhibitor-HgCl2 added) can lead to significant changes in δ18OP values of almost all P pools. For the two soils studied, the δ18OP values of Pi pools decrease from H2O (H2O-Pi) to NaHCO3 (NaHCO3-Pi), NaOH (NaOH-Pi) and HCl (HCl-Pi), and organic P was also found in the extraction solution of HCl. Furthermore, the δ18OP values calculated from isotope mass balance were different from the measured values suggesting variable extraction of different P pools during single and sequential extraction methods. Collectively these results highlight the need for a unified and standard processing and extraction methods for soil samples to allow meaningful intercomparison of results.

      PubDate: 2017-10-18T11:48:10Z
  • Insight into factors controlling formation rates of pedogenic carbonates:
           A combined geochemical and isotopic approach in dryland soils of the US
    • Abstract: Publication date: Available online 16 October 2017
      Source:Chemical Geology
      Author(s): Syprose Nyachoti, Lixin Jin, Craig E. Tweedie, Lin Ma
      Natural accumulation of pedogenic carbonates has been well documented but few studies have focused on carbonate formation in agricultural drylands. This study aims to determine accumulation rates of pedogenic carbonates in intensively irrigated soils, and to define key linkages between flood irrigation, salt loading and soil-atmospheric CO2 exchange in cultivated drylands of the southwestern United States. We used a combination of elemental chemistry (CaO, soil organic and inorganic carbon contents), mineralogy, and U-series (238U-234U-230Th) disequilibrium dating technique to investigate calcium sources, ages and formation rates of pedogenic carbonates. Study sites include an irrigated alfalfa field near El Paso in western Texas and a non-irrigated natural dryland site on the USDA Jornada Experimental Range of southern New Mexico. Our results showed that large amounts of dissolved calcium and inorganic carbon along with other soluble elements were loaded onto agricultural fields through irrigation waters in El Paso, TX while dust and rainfall were important for salt loading in natural soils of the Jornada. U-series activity ratios, (234U/238U) and (230Th/238U), in bulk soils suggested eolian deposits added U and modified U isotopes in shallow soils at both the irrigated and natural sites. Mobility of 234U within the soil profile is related to leaching of U (and by inference other soluble ions) and carbonate accumulation at depth. The U-series dating technique in pedogenic carbonates revealed the presence of much younger carbonates at the irrigated site compared to the natural site. Pedogenic carbonate formation rates in the irrigated soils were also much higher than those in the non-irrigated soils, likely a result of influxes from dissolved Ca and inorganic carbon in water used for irrigation. This study demonstrates the potential for agricultural expansion and land use change in drylands to increase rate of pedogenic carbonate accumulation. Such changes may have important implications to global carbon cycling since drylands are forecast to become the most expansive terrestrial biome by mid-century and dryland agriculture is expanding quickly.

      PubDate: 2017-10-18T11:48:10Z
  • Fraction-specific controls on the trace element distribution in iron
           formations: Implications for trace metal stable isotope proxies
    • Abstract: Publication date: Available online 16 October 2017
      Source:Chemical Geology
      Author(s): P.B.H. Oonk, H. Tsikos, P.R.D. Mason, S. Henkel, M. Staubwasser, L. Fryer, S.W. Poulton, H.M. Williams
      Iron formations (IFs) are important geochemical repositories that provide constraints on atmospheric and ocean chemistry, prior to and during the onset of the Great Oxidation Event. Trace metal abundances and their Mo-Cr-U isotopic ratios have been widely used for investigating ocean redox processes through the Archean and Paleoproterozoic. Mineralogically, IFs consist of three main Fe-bearing fractions: (1) Fe-Ca-Mg-Mn carbonates, (2) magnetite and/or hematite and (3) Fe-silicates. These fractions are typically fine-grained on a sub-μm scale and their co-occurrence in varying amounts means that bulk-rock or microanalytical geochemical and stable isotope data can be influenced by cryptic changes in mineralogy. Fraction specific geochemical analysis has the potential to resolve mineralogical controls and reveal diagenetic versus primary precipitative controls on IF mineralogy. Here we adapt an existing sequential extraction scheme for Fe-phases (Poulton and Canfield, 2005) to the high Fe-content in IF and the specific three-fraction mineralogy. We optimized the scheme for magnetite-dominated Archean IFs using samples from the hematite-poor Asbestos Hills Subgroup IF, Transvaal Supergroup, South Africa. Previously commonly-used hydroxylamine-HCl and dithionite leaches were omitted since ferric oxides are quantitatively insignificant in these IF samples. The acetate leach was tested at variable temperatures, reaction times and under different atmospheres in order to ensure that all micro-crystalline Fe-carbonates were effectively dissolved, resulting in an optimum extraction for 48h at 50°C under anoxic conditions. The dissolution of magnetite by NH4-oxalate was also tested, resulting in an optimum extraction for 24h under an ambient atmosphere. Finally, a HF-HClO4-HNO3 leach was used to dissolve the residual silicate fraction which has to date not been considered in detail in IF. Accuracy of the extraction technique was generally excellent, as verified using 1) elemental recoveries, 2) comparison of major and trace element distributions against mineralogy and 3) comparison to results from microanalytical techniques. This study focuses on the distribution of three frequently used geochemical proxies in IF; U, Mo and Cr. Molybdenum abundances in the Kuruman and Griquatown IF are low and show an apparent correlation with mineralogical variability, as determined by the sequential extraction. This suggests that changes in bulk-rock mineralogy, rather than redox chemistry might significantly affect Mo stable isotopes. For Cr, a minor bulk-rock stratigraphic increase can be related to the oxide and silicate fraction. However, a positive relationship with Zr indicates that this was also controlled by detrital or volcanic ash input. Uranium is predominantly bound to the silicate fraction and shows clear correlations with Zr and Sc implying detrital reworking under anoxic conditions. The discrepant behaviour of these three proxies indicate that mineralogy should be taken into account when interpreting heterogeneous bulk-rock samples and that fraction specific techniques will provide new insights into the evolution of atmosphere and ocean chemistry.

      PubDate: 2017-10-18T11:48:10Z
  • Testing the utility of geochemical proxies for paleoproductivity in oxic
           sedimentary marine settings of the Gulf of Aqaba, Red Sea
    • Abstract: Publication date: Available online 13 October 2017
      Source:Chemical Geology
      Author(s): Zvi Steiner, Boaz Lazar, Adi Torfstein, Jonathan Erez
      During the early stages of sediment diagenesis most of the organic matter reoxidizes, leaving behind a residual fraction of organic carbon which does not typically reflect its original quantities. Paleo-productivity reconstructions are therefore based on changes in the chemical composition of carbonate shells or, alternatively, use the abundances of inorganic elements in the bulk sediments, that have been shown to be proxies for organic matter contents. To examine the applicability of bulk inorganic elements composition for this task, we compare recorded changes in known anthropogenic nutrient fluxes to the oligotrophic and oxygenated Gulf of Aqaba in the north Red Sea, with the sedimentary records of barium, cadmium, copper and nickel over the last decades. Among these elements, nickel and copper strongly correlate with recorded nutrient fluxes and primary productivity in the region. In the present case, nickel is a more reliable proxy since part of the copper is possibly contributed from air-borne pollution sources. The applicability of cadmium to serve as a tracer for nutrient additions could not be reliably tested because contribution of cadmium associated with phosphate ore loading in the adjacent ports may be significant. We do not observe any bulk sediment barium enrichments associated with increased nutrient fluxes. Overall, it appears from these correlations that nickel and probably also copper reliably record past changes in nutrient availability and organic matter fluxes while sedimentary barium and barite, which are commonly attributed to productivity, do not.

      PubDate: 2017-10-18T11:48:10Z
  • Minimisation of pressure dependent mass discrimination in the ion source
           of the Helix MC Plus noble gas mass spectrometer
    • Abstract: Publication date: Available online 13 October 2017
      Source:Chemical Geology
      Author(s): Xiaodong Zhang, Masahiko Honda
      When tuned for maximum sensitivity, the electron bombardment ion source on the ANU (the Australian National University) Helix MC Plus noble gas mass spectrometer produces an Ar ion beam that is mass discriminated as a function of Ar partial pressure. This effect can be reduced substantially or eliminated by adjusting the repeller and trap voltages, although with a sensitivity loss of up to 50%. A recommended procedure for tuning an ion source to reduce pressure dependent mass discrimination is described. Similar procedures can be applied to other noble gases and other ion sources to ensure that pressure dependent mass discrimination is avoided.

      PubDate: 2017-10-18T11:48:10Z
  • The effect of fluorine and chlorine on trace element partitioning between
           apatite and sediment melt at subduction zone conditions
    • Abstract: Publication date: Available online 13 October 2017
      Source:Chemical Geology
      Author(s): Huijuan Li, Jörg Hermann
      The effect of F and Cl on trace element recycling during subduction-related sediment melting has been investigated by performing piston-cylinder experiments with a hydrous experimental pelite starting material (EPSM) with variable Cl (~0, 500, 1000, 2000, or 3000ppm) and F (~0, 700, or 1500ppm) concentrations, at 2.5GPa, 800°C. The variations of trace element concentrations in melt are systematically correlated with the variation of F (0.07–0.39wt%) and Cl (0.07–0.39wt%) contents. Trace elements Zn, V and Pb, and major elements Fe, Mg and Ca, show positive correlations with each other, and also with the Cl content in melt. The concentrations of light and medium rare earth elements (LMREE) increase with the Cl content in melt, whereas both F and Cl cause a decrease in the concentrations of high field strength elements (HFSE, such as Nb, Ta, Zr and Hf). Trace element (REE, Y, Sr, Th, U) concentrations in apatite are found to increase with the mole fraction of chlorapatite (ClAp). The preference for ClAp is stronger for cations with higher charge (e.g., Th4+, U4+ >REE3+) and larger ionic radii (e.g., LREE>HREE). Trace element partition coefficients between apatite and melt show up to 4 times variation between experiments, e.g., D La Ap-melt =77–281; D Sm Ap-melt =176–519; D Sr Ap-melt =4–12 and D Th,U Ap-melt =4–19. The REE partition coefficients between apatite and melt (D REE Ap-melt) display a concave pattern with the peak at Sm/Nd and a negative Eu anomaly, and are significantly higher than previously reported values for partitioning experiments conducted at lower pressures and higher temperatures. The high values of D LREE Ap-melt demonstrate the importance of apatite in terms of LREE partitioning during sediment melting, while allanite/monazite still dominates the partitioning of Th. In the absence of allanite/monazite, apatite-buffered melt is characterized by a significant enrichment of Th relative to La. Because of the contrasting behavior of LREE and HFSE in melt with the addition of Cl and F, the fractionation of these elements in slab-derived sediment melts will be enhanced by the presence of halogens.

      PubDate: 2017-10-18T11:48:10Z
  • Boron, lithium and nitrogen isotope geochemistry of NH4-illite clays in
           the fossil hydrothermal system of Harghita Bãi, East Carpathians, Romania
    • Abstract: Publication date: Available online 12 October 2017
      Source:Chemical Geology
      Author(s): Iuliu Bobos, Lynda B. Williams
      Coarse (2.0–0.2μm) and fine (<0.2μm) clay fractions of NH4-illite-smectite (I-S) mixed-layered and K-illite/(NH4,K)-illite (I) mixed phases that vary in age and trace element composition were collected from the fossil hydrothermal system of Harghita Bãi, East Carpathians. Boron and Li isotope ratios were measured by secondary ion mass spectrometry (SIMS), and N by isotope ratio mass spectrometry (IRMS) with the aim to characterize the isotope geochemistry and source of light elements fixed in authigenic NH4-illitic clays. Boron in NH4-I-S clays ranges from 513 to 1457ppm and reached ~1000ppm in the K-I/(NH4,K)-I. The δ11B (‰) measured in NH4-I-S ranges from −12.6 to −22.4 (±0.3‰) and in K-I/(NH4,K)-I is consistently −5.5 to −5.1 (±0.3‰). Boron isotopes systematically become lighter in the NH4-I-S series as temperature increased from 90 to 270°C. Low Li content (1 to 8ppm) was found in illitic clay fractions. The δ7Li (‰) shows negative values ranging from −8.6 to −12.3 (±0.8‰) for the coarser (2.0–0.2μm) NH4-I-S clays and from +4.3 to +14.1 (±1‰) for the finer (<0.2μm) fraction of NH4-I-S and K-I/(NH4,K)-I clays. The N (%) measured in the NH4-I-S clays ranges from 0.70 to 1.50 (±0.2%), whereas in the K-I/(NH4,K)-I is about 0.70 (±0.2%). The δ15N (‰) ranges from +4.8 to +7.4 (±0.6) for most NH4-I-S and NH4,K-I clays, with one outlier for NH4-I-S of +14.6 (±0.6). The δ11B of K-I/(NH4,K)-I clays reflect a magmatic source, whereas the NH4-I-S series is consistent with the influx of isotopically light-B waters derived from hydrothermal leaching of continental evaporites and/or organic-rich sediments. The δ7Li signature measured on K-I/(NH4,K)-I clays also support a magmatic fluid, enriched in heavy Li, followed by precipitation of coarser NH4-I-S from more recent sedimentary contributions of isotopically light Li. This interpretation is also supported by the δ15N, which reflect an influx of waters from an organic sediment origin. The δ15N of +14.6‰ (±0.6) measured on NH4-I-S could be attributed to the presence of meteoric waters mixed with hydrothermal fluids. The isotopic data obtained trace the mobility of magmatic and organic – sedimentary components in the upper continental crust.

      PubDate: 2017-10-18T11:48:10Z
  • Determination of Cr(III) solids formed by reduction of Cr(VI) in a
           contaminated fractured bedrock aquifer: Evidence for natural attenuation
           of Cr(VI)
    • Abstract: Publication date: Available online 11 October 2017
      Source:Chemical Geology
      Author(s): Jiujiang Zhao, Tom Al, Steven W. Chapman, Beth L. Parker, Katherine R. Mishkin, Diana Cutt, Richard T. Wilkin
      Hexavalent chromium Cr(VI) is toxic and can be highly mobile in many aquifer systems. Redox reactions with naturally occurring minerals and organic compounds can reduce Cr(VI) to Cr(III), forming labile Cr(III) oxyhydroxide precipitates, which is a natural attenuation process. In fractured bedrock aquifers, reduction of Cr(VI) in the rock matrix can enhance attenuation beyond that from matrix diffusion only, and potentially reduce back diffusion if concentrations in fractures decline following source reduction via natural processes or engineered remediation. In this study, we develop an extraction method for labile Cr(III) precipitates from Cr(VI) reduction using 5% hydrogen peroxide (H2O2). Combining Cr(III) extractions with an established sodium hydroxide (NaOH) method for determination of Cr(VI) concentrations in rock porewater, a measure of the labile Cr(III) and Cr(VI) fractions in geologic samples is achieved. The methods were applied to cores from a contaminated groundwater system in fractured porous bedrock in order to assess the effectiveness of natural attenuation and whether Cr(VI) mass that diffused into the bedrock matrix was undergoing reduction. Detailed vertical distributions display two depth intervals with corresponding elevated concentrations of Cr(VI) in the porewater and extractable total Cr. The correspondence of Cr(VI) and labile Cr(III) provides evidence for reduction of Cr(VI) contamination in the bedrock matrix. Mineralogical analysis suggests that Fe(II)-bearing minerals, chlorite and biotite are the most likely candidates for natural reductants. This study provides evidence for the natural attenuation of anthropogenic Cr(VI) contamination in the porewater of a fractured bedrock aquifer, and it outlines a quantitative method for evaluating the effectiveness of natural attenuation in groundwater systems.

      PubDate: 2017-10-11T18:32:35Z
  • Coupling Δ47 and fluid inclusion thermometry on carbonate cements to
           precisely reconstruct the temperature, salinity and δ18O of
           paleo-groundwater in sedimentary basins
    • Abstract: Publication date: Available online 10 October 2017
      Source:Chemical Geology
      Author(s): Xavier Mangenot, Magali Bonifacie, Marta Gasparrini, Alina Götz, Carine Chaduteau, Magali Ader, Virgile Rouchon
      Diagenetic minerals may provide information about the burial history of geological units and can have practical applications, for instance, for reconstructing the geochemical and thermal histories of sedimentary basins. Clumped isotope, or Δ47, thermometry on carbonates opens a new avenue for interpreting carbonate formation temperature and thermal history of rocks. Yet, most of current knowledge on Δ47 systematics has been acquired via theoretical or experimental studies with only limited validation by the rock-record at geological conditions/timescales. Here, we investigate calcitic and dolomitic cements representative of three genetically different cementation phases from a well-documented mineral paragenesis of a carbonate unit (Middle Jurassic, Paris basin, France). We compare Δ47 with fluid inclusions microthermometry (FIM) data that were independently obtained from the same calcite and dolomite crystal specimens. The range of homogenization temperatures (Th) found for Cal1, Cal2 and Dol1 fluid inclusions fit remarkably well (i.e. within <5°C) with the temperatures determined from the Δ47 measurements (TΔ47), for a temperature range between 60 and 100°C and salinities between 0 and 15wt% NaCl eq. This provides a consistent rock-based validation of the experimentally determined Δ47 calibration with formation temperature for both calcite and dolomite mineralogy. Such findings also confirm the applicability of Δ47 thermometry in low temperature diagenetic environments (i.e., below circa. 100°C), which provides higher precision than FIM measurements (i.e., typical uncertainties of ±6°C with three Δ47 measurements) though significantly less time-consuming. Importantly, this study underlines how the coupling of both techniques can help to evaluate the degree of preservation of the original temperature information captured by either fluid inclusions or Δ47 compositions, and interpret each proxy as confidently/accurately as possible. Moreover, because both FIM and Δ47 measurements can provide independent constraints on the geochemistry of diagenetic paleofluids (via their salinity and δ18O composition), this study also highlights the benefits of coupling both techniques to further unravel the nature of paleofuids. Finally, we propose a practical guideline as a basis for future applications of combined FIM and Δ47 thermometry.
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      PubDate: 2017-10-11T18:32:35Z
  • Inside front cover (Editorial Board)
    • Abstract: Publication date: 20 October 2017
      Source:Chemical Geology, Volume 470

      PubDate: 2017-10-04T01:17:51Z
  • Synthesis and characterization of redox-active ferric nontronite
    • Abstract: Publication date: 20 October 2017
      Source:Chemical Geology, Volume 470
      Author(s): A.G. Ilgen, R.K. Kukkadapu, D.R. Dunphy, K. Artyushkova, J.M. Cerrato, J.N. Kruichak, M.T. Janish, C.J. Sun, J.M. Argo, R.E. Washington
      Heterogeneous redox reactions on clay mineral surfaces control mobility and bioavailability of redox-sensitive nutrients and contaminants. Iron (Fe) residing in clay mineral structures can either catalyze or directly participate in redox reactions; however, chemical controls over its reactivity are not fully understood. In our previous work we demonstrated that converting a minor portion of Fe(III) to Fe(II) (partial reduction) in the octahedral sheet of natural Fe-rich clay mineral nontronite (NAu-1) activates its surface, making it redox-active. In this study we produced and characterized synthetic ferric nontronite (SIP), highlighting structural and chemical similarities and differences between this synthetic nontronite and its natural counterpart NAu-1, and probed whether mineral surface is redox-active by reacting it with arsenic As(III) under oxic and anoxic conditions. We demonstrate that synthetic nontronite SIP undergoes the same activation as natural nontronite NAu-1 following the partial reduction treatment. Similar to NAu-1, SIP oxidized As(III) to As(V) under both oxic (catalytic pathway) and anoxic (direct oxidation) conditions. The similar reactivity trends observed for synthetic nontronite and its natural counterpart make SIP an appropriate analog for laboratory studies. The development of chemically pure analogs for ubiquitous soil minerals will allow for systematic research of the fundamental properties of these minerals.
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      PubDate: 2017-10-04T01:17:51Z
  • Geochemistry and Genesis of Craton-derived Sediments from Active
           Continental Margins: Insights from the Mizoram Foreland Basin, NE India
    • Abstract: Publication date: 20 October 2017
      Source:Chemical Geology, Volume 470
      Author(s): Sariput S. Sawant, K. Vijaya Kumar, V. Balaram, D.V. Subba Rao, K.S. Rao, R.P. Tiwari
      This study investigates relative controls of source compositions and sedimentary processes (weathering+mineral sorting) on the bulk-rock geochemistry of clastic rocks deposited along an active continental margin setting by considering the Miocene Mizoram basin of NE India. It is a classic foreland accretionary sedimentary basin related to Himalaya orogeny and is ideally located to understand tectonic evolution of the Eastern Himalayas, Indo-Burmese arc, and Indian cratonic margin [the Meghalaya Plateau (also known as Shillong Plateau)]. These three tectonic domains represent potential suppliers of clastic sediments to the basin. The Mizoram basin comprises sandstone, siltstone and shale deposited under shallow marine conditions. Depletions in Ca and Sr indicate plagioclase-dominated weathering of the sources. The Mizoram strata are first-cycle sediments; however, quartz dilution has resulted in the decrease of both major (except SiO2) and trace elements, carbonate dilution has increased Ca and Sr, and heavy minerals have influenced Zr, HREE, Hf, U and Y abundances in a few of the studied Mizoram sediments. Newly formulated Rb/V-Zr/Zn-Sc/Nb and Rb/V-Zr/Zn-Eu/Eu* ternary plots distinguish sediments derived from mafic versus felsic sources, and allow the estimation of mafic:felsic proportions in the sedimentary provenances. We thus suggest an average tonalite-granodiorite source terrain for the Mizoram sediments. The geochemical evidences, including TiO2 (>0.7wt%), Cr/Cr* (>1), V/V* (>1), Th/U (10.23), Nb/U (9.01) and Th/Rb (0.124) values, indicate that the Mizoram sediments were derived chiefly from old cratonic crust of the Meghalaya Plateau. The source for the Miocene Mizoram sediments is slightly mafic than that supplying the modern Brahmaputra and Ganga sediments (dominated by the juvenile Himalayas). The Mizoram clastics provide indirect evidence for rise of the Meghalaya Plateau and a possible change in the Brahmaputra drainage as controlled by the uplift. We suggest that the Mizoram sediments are archives of the cratonic provenance in a foreland basin dominated by orogeny-derived detritus.
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      PubDate: 2017-10-04T01:17:51Z
  • Chemically-oscillating reactions during the diagenetic oxidation of
           organic matter and in the formation of granules in late Palaeoproterozoic
           chert from Lake Superior
    • Abstract: Publication date: 20 October 2017
      Source:Chemical Geology, Volume 470
      Author(s): Dominic Papineau, Zhenbing She, Matthew S. Dodd
      Filamentous and coccoidal microfossils have been reported since the 1950's from a range of granular cherts from the late Palaeoproterozoic southwestern Superior Craton, Canada-United States. However, the chemical and mineral compositions of granules, the presence of microfossils in granules, and the common presence of granules in intercolumnar space of stromatolitic chert are poorly documented and explained. Furthermore, the depositional model for the origin of granules in wave-agitated waters does not entirely explain their mineral diversity nor their characteristic morphologies and patterns. We report on the crystallinity of organic matter, mineral diversity, and compositions of microfossils in granules from three different kinds of late Palaeoproterozoic cherts, namely phosphatic, organic, and haematitic. Stromatolitic organic-rich chert from the Gunflint Fm contains granules with euhedral carbonate and equidistant concentric laminations of organic matter, akin to fractal patterns from the Belousov-Zhabotinsky (B-Z) chemically-oscillating reaction. These granules also contain authigenic anatase, ferric-ferrous silicates, and Fe-oxides. Filamentous and coccoidal microfossils similar to those of the Gunflint occur in chert from the Biwabik Formation and share morphology, and co-occur with Mn-siderite and apatite. Granules in phosphatic chert in the Michigamme Formation often contain filamentous and coccoidal microfossils composed of organic matter, sericite, and apatite. Bulk carbonate associated with these Michigamme granular phosphatic chert beds has systematically negative δ13Ccarb values around −3.1±0.9‰ (1σ) and δ18Ocarb-SMOW between +20.8 and +30.7‰, which suggest some contribution from the diagenetic oxidation of organic matter. Notably, residual carboxylic acid is detectable in C-XANES spectra of organic matter from granular phosphatic chert, which is a residual reactant of B-Z type reactions. Along with previously reported observations of pyritised microfossils from the Gunflint Formation, these distinct mineralogies indicate variable modes of preservation for the products of chemically-oscillating reactions that likely relate to the availability of different oxidants in the diagenetic environment. We conclude that the late Palaeoproterozoic shallow-marine environments of the Lake Superior area were populated by morphologically similar micro-organisms, and that the diagenetic oxidation of organic matter through chemically-oscillating reactions contributed to the formation of spheroidal rosettes, granules, and concretions during a late Palaeoproterozoic Great Putrefaction Event. Granules in chert that contain organic matter or microfossils thus provide a reliable petrographic context to search for a record of putrefaction of microbial life on the early Earth and on other ancient planetary surfaces.
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      PubDate: 2017-10-04T01:17:51Z
  • Foraminiferal εNd in the deep north-western subtropical Pacific Ocean:
           Tracing changes in weathering input over the last 30,000years
    • Abstract: Publication date: 20 October 2017
      Source:Chemical Geology, Volume 470
      Author(s): Qiong Wu, Christophe Colin, Zhifei Liu, Franck Bassinot, Quentin Dubois-Dauphin, Eric Douville, François Thil, Giuseppe Siani
      This study focuses on the neodymium isotopic composition (εNd) of mixed planktonic foraminifera species in sediment cores collected on the north-western margin of the South China Sea (SCS) and in the western equatorial Pacific. The aim was to identify temporal changes in the εNd of the Upper Circumpolar Deep Water (UCDW) during its northward penetration in the western subtropical Pacific and to re-evaluate the significance of deep-water εNd changes at the northern margin of the SCS over the past 25kyr. The past seawater εNd record derived from sediments of the subtropical western Pacific displays a narrow range from −2.9±0.3 to −1.6±0.4. This suggests that the εNd of the UCDW is greatly modified during its northward penetration through the process of boundary exchange along the western volcanic margin of the South Pacific and that the unradiogenic εNd signature of the UCDW observed in the South Pacific (about −6) has not been propagated beyond the equatorial Pacific, at least not over the last 25kyr. Seawater εNd values obtained from the northern SCS display large variations with values that are lower (−6.8±0.3 to −5.6±0.2) for the last glacial period than for the late Holocene (−5.7±0.3 to −4.1±0.2). Such glacial εNd cannot be attributed to a higher proportion of southern-sourced water (SSW) as εNd values of the UCDW in the equatorial western Pacific are more radiogenic than those observed in the SCS. The evolution of the seawater εNd values of the northern SCS during the last glacial period and the last climatic transition is coeval with variations in the contribution of sediments from the Pearl River. The low sea level stand of the last glacial period resulted in a more proximal position of the Pearl River mouth relative to core MD05-2904, which a higher input of freshwater and, in particular, detrital sedimentary particles originating from the Pearl River that partially dissolved during sinking led to a decrease in the seawater εNd along the north-western margin of the SCS. Enhanced rainfalls associated with intensified monsoon and more frequent typhoon activities during the early Holocene Climatic Optimum (from 9 to 7calkyrBP) are linked to a decrease in sea surface salinity (lower δ18O values of planktonic foraminifera) and a decrease in εNd values (−6.2±0.3) induced by lithogenic Nd input from pronounced freshwater and sediment discharge from distal rivers.

      PubDate: 2017-10-04T01:17:51Z
  • Pyrite oxidation in air-equilibrated solutions: An electrochemical study
    • Abstract: Publication date: 20 October 2017
      Source:Chemical Geology, Volume 470
      Author(s): Paul Chiriță, Michel L. Schlegel
      Oxidative dissolution of pyrite (FeS2) in air-equilibrated solutions was investigated with electrochemical techniques (polarization measurements and electrochemical impedance spectroscopy) at different solution pH (1 to 5) and temperatures (25 to 40°C). We have found that, at a given temperature, the oxidation current density (jox) varies only slightly from initial pH1 to 4, but was significantly lower at initial pH5, while oxidation potential (Eox) decreases when pH increases. At the investigated temperatures, the reaction order with respect to hydrogen ions has been found to be zero, suggesting that H+ is not involved into the rate-determining step of the oxidation rate. The jox values increase with temperature, whereas Eox potentials are hardly affected by temperature from 25 to 40°C. Activation energies (Ea) were estimated from polarization data and vary between 57±28kJ/mol (initial pH5) and 19±3kJ/mol (initial pH4). The obtained values indicate that the rate-determining step of the overall oxidative dissolution of FeS2 involves a surface reaction. The importance of surface reaction is also supported by the electrochemical impedance spectroscopy measurements which show that interfacial transfer of electrons is dominated by surface charge transfer and the elementary surface reaction which controls the mineral oxidative dissolution is the electron transfer from the conduction band of pyrite to adsorbed O2.
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      PubDate: 2017-10-04T01:17:51Z
  • IG-Mapper: A new ArcGIS® toolbox for the geostatistics-based automated
           geochemical mapping of igneous rocks
    • Abstract: Publication date: 20 October 2017
      Source:Chemical Geology, Volume 470
      Author(s): Patrizia Fiannacca, Gaetano Ortolano, Mario Pagano, Roberto Visalli, Rosolino Cirrincione, Luigi Zappalà
      This work introduces a user-friendly, fast and effective tool for the automated geochemical mapping of igneous rocks, to provide a geospatial representation of petrological parameters through the application of either deterministic or stochastic interpolation techniques. The geostatistical approach allows mapping to be extended to areas where igneous rocks are hidden under sedimentary cover, also introducing an estimate of the interpolation reliability. This new GIS-based automated procedure, which is mostly constructed with the ArcGIS® Model-Builder as a graphical code designer, can generate three main types of outputs: 1) interpolation maps of major and trace elements, isotopic compositions, and derived parameters, to provide valuable geospatial information regarding the genetic and structural relationships between magmatic bodies; 2) lithological maps, that are based on the geostatistical interpolation of major element data and mirror the main geochemical classification diagrams of plutonic rocks; and 3) interpolation checking maps, which provide an estimate of the geostatistical interpolability for one specific parameter at a time. This last output is expressed in the form of an empirical index (Stochastic Interpolation Index), which highlights the randomness level of a specific value distribution. This automated lithological mapping function is an entirely new tool based on five original python scripts, which makes it possible to interpolate on a georeferenced map the compositions deriving from the most used geochemical classification diagrams. This new tool package is presented here by using a portion of the Serre Batholith in central Calabria (southern Italy), to illustrate the potential of this method to open new research perspectives in igneous petrology.
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      PubDate: 2017-10-04T01:17:51Z
  • Major transition of continental basalts in the Early Cretaceous:
           Implications for the destruction of the North China Craton
    • Abstract: Publication date: 20 October 2017
      Source:Chemical Geology, Volume 470
      Author(s): Kai Wu, Ming-Xing Ling, Weidong Sun, Jia Guo, Chan-Chan Zhang
      Geochemical compositions of continental basalts are generally considered as the best proxy record of the chemical and isotopic evolution of subcontinental lithosphere and convective mantle. There are voluminous Cretaceous continental basalts in the North China Craton (NCC), which are mainly composed of alkali basalts with minor sub-alkali basalts. Abrupt changes in chemical and isotopic compositions of these basalts were often ascribed to subcontinental lithosphere thinning of the NCC. However, processes responsible for such changes and its implications for subcontinental lithosphere evolution remain obscure. Here we report major geochemical changes at ~108Ma in the north part of the NCC. The >108Ma alkali basalts are characterized by negative εNd(t) and declining “island arc-like” geochemical characteristics from the east to the west, implying decreasing slab-derived components from westward subducting slabs in their metasomatized lithospheric mantle sources. In contrast, the <108Ma Cretaceous alkali basalts have depleted Sr-Nd isotopic compositions and “OIB-like” geochemical features. These observations suggest that westward subduction of the Paleo-Pacific plate was responsible for the Cretaceous basaltic activities in the NCC. Combined with plate reconstructions and geophysical observations, we propose that flat subduction of the “extinct” ridge between the Izanagi and the Pacific plates controlled this major transition as well as the destruction of the NCC. The transition from a supra-subduction zone environment to a within-plate extensional environment around 108Ma is probably due to the eastward slab rollback and the northward shift of the spreading ridge.

      PubDate: 2017-10-04T01:17:51Z
  • Gas hydrate estimates in muddy sediments from the oxygen isotope of water
    • Abstract: Publication date: 20 October 2017
      Source:Chemical Geology, Volume 470
      Author(s): Akihiro Kano, Reina Miyahara, Katsunori Yanagawa, Taiki Mori, Satoko Owari, Hitoshi Tomaru, Yoshihiro Kakizaki, Glen Snyder, Takaya Shimono, Yoshihiro Kakuwa, Ryo Matsumoto
      Gas hydrate in deep-sea sediments is a potential energy resource, and has been the focus of extensive drilling research. However, direct evaluation of the amount of the gas hydrate in marine sediments has been difficult because the gas hydrate in recovered sediment cores is at least partly dissociated due to the drop in pressure and increase in temperature during onboard recovery. In this study, we apply a new method based on oxygen isotopic composition of the H2O fraction of both hydrate and mud sub-samples (δ18OH and δ18OM, respectively) in order to evaluate the volume percentage of gas hydrate in core sections collected from the Japan Sea off Joetsu and Oki, which contain different fabrics of the hydrate within hemipelagic mud. We measured isotopic composition of CO2 equilibrated with H2O of the sub-samples of a small size (typically 0.3cm3) carefully separated from the core sediments and sealed in glass vials. The volume percentage of gas hydrate (H in %) was determined using porosity of the mud sub-samples and oxygen isotopic composition of the bulk pore water (δ18OPW) squeezed from a certain length of core sediment (typically 20cm) including dissociated hydrate. 28 out of the 29 examined core sections indicate the relation in the isotopic values of the three components, δ18OH >δ18OPW >δ18OM, as expected from isotopic fractionation that enriches 18O in the hydrate component. Evaluated H-values of the 28 sections ranged from 1.0% to 95.4% and, for most of the section, the H-value was clearly larger than the value estimated by the hydrate distribution on core images. Our new method can, in a simple manner, correct for the underestimation of hydrate amount caused as a result of dissociation during core handling. Our oxygen isotopic data of the hydrate and mud sub-samples fits poorly with the isotopic evolutional curve that assumes Rayleigh fractionation in a closed system. This implies that the pore water isotopic composition may have been homogenized by diffusion and advection of less 18O-depleted pore water from the surrounding sediments. Presence of micro-scale hydrate in the mud matrix was suspected for some sections from the Joetsu site, which present a small difference between δ18OH and δ18OM as well as high CH2/CO2 ratios in headspace gas. We suggest that this method, if carried out with careful and quick onboard sampling, is appropriate for the estimation of gas hydrate as an energy resource based on the amount of hydrate present in marine mud.

      PubDate: 2017-10-04T01:17:51Z
  • Helium trapping in apatite damage: Insights from (U-Th-Sm)/He dating of
           different granitoid lithologies
    • Abstract: Publication date: 20 October 2017
      Source:Chemical Geology, Volume 470
      Author(s): Alice Recanati, Cécile Gautheron, Jocelyn Barbarand, Yves Missenard, Rosella Pinna-Jamme, Laurent Tassan-Got, Andy Carter, Eric Douville, Louise Bordier, Maurice Pagel, Kerry Gallagher
      Apatite (U-Th-Sm)/He (AHe) thermochronometry is widely used to constrain thermal histories and rates of tectonic, exhumation, and erosion processes. However, data interpretation is often challenging, especially when the thermal history includes extended residence time in the He partial retention zone (HePRZ), with highly dispersed dates revealing the complexity of diffusion processes in natural systems. This study investigates chemical and physical factors that may have impacted He diffusion in apatite over long timescales in a context of protracted residence in the HePRZ. Nine samples from the Ploumanac'h pluton and North Tregor (Armorican Massif, France) were collected in granitoids, differing in petrography and chemisty. This area was chosen because these samples underwent a similar thermal history since ~300Ma. We report new (U-Th-Sm)/He dates, along with apatite fission-track (AFT) data, as well as lithological and chemical characterization. The results show dispersed (U-Th-Sm)/He dates, ranging from 87±7 to 291±23Ma, whereas central AFT dates vary from 142±6 to 199±9Ma. Current predictive models for He diffusion and fission-track annealing in apatite could not reproduce the two datasets together. However, this apparent discrepancy gives insight into the parameters influencing He diffusion at geological timescales. The data confirm that radiation damage enhances He trapping, as the AHe dates are positively correlated to effective uranium (eU) concentration. The He age dispersion for constant eU content cannot be explained just by variations in grain size or chemical composition. To explore the potential influence of recoil damage trapping behavior and annealing kinetics on AHe dates, we tested a new diffusion model from Gerin et al. (2017). Given the expected model of the thermal history provided by AFT inversion, we investigated the influence of the trapping energy on AHe dates. The AHe date variations can be explained only if the trapping energy evolves from one crystal to another, increasing with the amount of damage. For a given trapping energy, minor variations in the recoil-damage annealing rate can consistently explain most of the remaining dispersion of the AHe dates.

      PubDate: 2017-10-04T01:17:51Z
  • Adsorption isotherm, mechanism, and geometry of Pb(II) on magnetites
           substituted with transition metals
    • Abstract: Publication date: 20 October 2017
      Source:Chemical Geology, Volume 470
      Author(s): Xiaoliang Liang, Gaoling Wei, Juan Xiong, Fuding Tan, Hongping He, Chenchen Qu, Hui Yin, Jianxi Zhu, Runliang Zhu, Zonghua Qin, Jing Zhang
      Iron oxides are abundant in natural waters and soils and have high capacities for scavenging Pb(II) by adsorption, which affects the transport and fate of Pb on the earth's surface. We investigated the adsorption of Pb(II) on magnetites substituted with commonly incorporated transition metals such as Cr, Mn, Co, and Ni. The adsorption capacity, mechanism, and local coordination of Pb(II) were investigated by traditional macroscopic studies, i.e., acid-base titration and batch adsorption experiment, complemented with X-ray absorption fine structure (XAFS) spectrum analysis and surface complexation model (SCM). The substitution increased the surface site density, while pHpzc did not vary. Pb(II) adsorption was not suppressed by the presence of background electrolyte and improved as pH increased. The isotherms were well fit to the Langmuir adsorption model. The XAFS analysis demonstrated that Pb(II) ions were adsorbed on magnetite surface predominantly via inner-sphere complexation, where the adsorbed Pb(II) species was in bidentate binuclear corner-sharing geometry, independent of the adsorption capacity. This adsorption geometry can be applied to fit the experimental adsorption data well with the diffuse layer model (DLM). The substitutions improved the adsorption capacity in the following order: Cr>Ni>Mn>Co, and were discussed regarding the measured values of active site density and local coordination of adsorbed Pb(II). This study is the first documentation of Pb(II) adsorption on magnetite with different substitutions. The obtained results are of great significance for the understanding of Pb(II) surface complexation reactions on magnetite surface.
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      PubDate: 2017-10-04T01:17:51Z
  • Quantitative analysis of radiocesium retention onto birnessite and
    • Abstract: Publication date: 20 October 2017
      Source:Chemical Geology, Volume 470
      Author(s): Qianqian Yu, Toshihiko Ohnuki, Naofumi Kozai, Fuminori Sakamoto, Kazuya Tanaka, Keiko Sasaki
      Although the Cs adsorption onto soil minerals has been widely studied, the structural factor of Mn oxides affecting the adsorption behavior of trace amounts of Cs is unclear. In order to elucidate the adsorption mechanisms of radioactive Cs at trace levels and the role of Mn oxides on Cs migration in the terrestrial environment, the Cs adsorption onto birnessite and todorokite was investigated for a wide range of concentrations (1×10−10 mol/L to 0.1mol/L) and an ion-exchange model was used to interpret the adsorption data. Although birnessite showed a higher Cs adsorption capacity than todorokite, most of the Cs adsorbed onto birnessite was desorbed by ion exchange. Two types of adsorption sites were observed for todorokite. Despite low density, the selectivity coefficient was much higher for the T1 site (Log Na Cs K sel = 4.2) than for the T2 site (Log Na Cs K sel =−0.6). Sequential extraction was carried out at Cs concentrations of 1×10−9 mol/L and 1×10−3 mol/L. At lower concentrations, approximately 34% of the adsorbed Cs was residual in the todorokite after the sequential extraction; this value was much higher than the results for the Cs-adsorbed birnessite as well as the Cs-adsorbed todorokite at higher concentrations. The present results indicate that the structural factors of Mn oxides significantly affect the retention capacity of radioactive Cs. Aside from phyllosilicate minerals, todorokite also contributes to the fixation of radioactive Cs in soils.
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      PubDate: 2017-10-04T01:17:51Z
  • Manganese-calcium intermixing facilitates heteroepitaxial growth at the
           101¯4 calcite-water interface
    • Abstract: Publication date: 20 October 2017
      Source:Chemical Geology, Volume 470
      Author(s): Man Xu, Shawn L. Riechers, Eugene S. Ilton, Yingge Du, Libor Kovarik, Tamas Varga, Bruce W. Arey, Odeta Qafoku, Sebastien Kerisit
      In situ atomic force microscopy (AFM) measurements were performed to probe surface precipitates that formed on the 10 1 ¯ 4 surface of calcite (CaCO3) single crystals following reaction with Mn2+-bearing aqueous solutions. Three-dimensional epitaxial islands were observed to precipitate and grow on the surfaces. In situ time-sequenced measurements demonstrated that the growth rates were commensurate with those obtained for epitaxial islands formed on calcite crystals reacted with Cd2+-bearing aqueous solutions of the same range in supersaturation with respect to the pure metal carbonate phase. This finding was unexpected as rhodochrosite (MnCO3) and calcite display a 10% lattice mismatch, based on the area of their 10 1 ¯ 4 surface unit cells, whereas the lattice mismatch is only 4% for otavite (CdCO3) and calcite. Coatings of varying thicknesses were therefore synthesized by reacting calcite single crystals in calcite-equilibrated aqueous solutions with up to 250μM MnCl2. Ex situ X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), X-ray reflectivity (XRR), and AFM measurements of the reacted crystals demonstrated the formation of an epitaxial (Mn,Ca)CO3 solid solution. The epitaxial solid solution had a spatially complex composition, whereby the first few nanometers were rich in Ca and the Mn content increased with distance from the original calcite surface, culminating in a topmost region of almost pure MnCO3 for the thickest coatings. The effective lattice mismatch was therefore much smaller than the nominal mismatch thus explaining the measured growth rates. These findings highlight the strong influence played by the substrate on the composition of surface precipitates in aqueous conditions.

      PubDate: 2017-10-04T01:17:51Z
  • U-Th chronology and formation controls of methane-derived authigenic
           carbonates from the Hola trough seep area, northern Norway
    • Abstract: Publication date: 20 October 2017
      Source:Chemical Geology, Volume 470
      Author(s): Simone Sauer, Antoine Crémière, Jochen Knies, Aivo Lepland, Diana Sahy, Tõnu Martma, Stephen R. Noble, Jasmin Schönenberger, Martin Klug, Carsten J. Schubert
      We investigated methane-derived authigenic carbonate (MDAC) crusts and nodules from a cold seep site on the northern Norwegian continental shelf in ca. 220m water depth to determine the timing and mode of their formation. Gas bubbling observed during remotely operated vehicle (ROV)-assisted sampling of MDAC crusts revealed ongoing seep activity. Authigenic carbonates were present as crusts on the seafloor and as centimetre-size carbonate-cemented nodules at several intervals within an adjacent sediment core. Aragonite-dominated mineralogy of the MDAC crusts suggests formation close to the seafloor at higher rates of sulphate-dependent anaerobic oxidation of methane (AOM). In contrast, dolomite-cemented nodules are consistent with the formation at the sulphate-methane-transition zone deeper within the sediment at lower rates of AOM. The δ13C-carbonate values of bulk rock and of micro-drilled aragonite samples vary between −22.2‰ and −34.6‰ (VPDB). We interpret the carbon in aragonite to be mainly derived from the anaerobic oxidation of thermogenic methane, with a minor contribution from seawater dissolved inorganic carbon (DIC). AOM activity is supported by high concentrations of AOM-related biomarkers of archaea (archaeol and 2-sn-hydroxyarchaeol) and sulphate-reducing bacteria (iso and anteiso-C15:0 fatty acids) in the crusts. The dolomite nodules exhibit higher δ13C-carbonate values (−12‰ VPDB) suggesting a smaller amount of methane-derived carbon, presumably due to the contribution of DIC migrating from depth, and lower AOM rates. The latter is supported by orders of magnitude lower concentrations of archaeol and sn-2-hydroxyarchaeol in the sediment interval containing the largest dolomite nodules. δ18O values of pure aragonite samples and dolomite nodules indicate the precipitation of carbonate close to isotopic equilibrium with seawater and no influence of gas hydrate-derived water. U-Th dating of two MDAC crusts shows that they formed between 1.61±0.02 and 4.39±1.63ka BP and between 2.65±0.02 and 4.32±0.08ka BP. We infer both a spatial and temporal change in methane flux and related MDAC formation at this seep site. These changes might be caused by regional seismic events that can affect pore pressure or re-activation of migration pathways thus facilitating fluid flow from deep sources towards the seabed.

      PubDate: 2017-10-04T01:17:51Z
  • The solubility of apatite in H2O, KCl-H2O, NaCl-H2O at 800°C and 1.0GPa:
           Implications for REE mobility in high-grade saline brines
    • Abstract: Publication date: 20 October 2017
      Source:Chemical Geology, Volume 470
      Author(s): Philipp Mair, Peter Tropper, Daniel E. Harlov, Craig E. Manning
      Apatite is an important host for LREE, F, and Cl and thus can be used to monitor elemental mass transfer in high P-T settings. In this investigation, the solubilities of synthetic fluor- and chlorapatite and natural Durango fluorapatite were measured in H2O, KCl-H2O, and NaCl-H2O fluids at 800°C and 1.0GPa, using the piston-cylinder apparatus. The solubility of apatite is reported in parts/million by weight (ppm), and is calculated via the formula mAp =[(∆mAp /H2O+KCl or NaCl+∆mAp)∗106], where ∆mAp =mAp-in −mAp-out. The experimental results indicate a strong increase in apatite solubility in aqueous fluids with moderate KCl and NaCl mole fractions (XKCl and XNaCl). Under all conditions, synthetic fluorapatite and synthetic chlorapatite dissolve congruently. The solubility of synthetic fluorapatite increases from 19ppm in pure H2O to 2123ppm at XKCl =0.52. The solubility of synthetic chlorapatite rises from 37ppm in pure H2O to 2590ppm at XKCl =0.50. At XKCl >0.5 the solubility of synthetic fluorapatite decreases to 1691 (XKCl =0.72) and the solubility of synthetic chlorapatite decreases to 2284ppm (XKCl =0.65). In contrast, natural Durango fluorapatite dissolves incongruently at XKCl <0.2 to monazite+fluid and congruently at XKCl >0.2. The solubility of natural Durango fluorapatite increases from 70ppm in pure H2O to 2073ppm at XKCl =0.50 respectively. In contrast to the KCl-H2O system, apatite solubilities are considerably higher in the NaCl-H2O system. The solubility of synthetic fluorapatite increases to 4909ppm at XNaCl =0.51 and the solubility of synthetic chlorapatite rises to 5144ppm at XNaCl =0.50. Compared to fluorite, calcite and anhydrite, apatite solubility is significantly lower in H2O, NaCl-H2O, and KCl-H2O. After completion of each experiment, the pH was measured and a comprehensive step-by step procedure (limiting reactant concept) was used to model the observed quench pH in the system apatite-KCl-H2O.

      PubDate: 2017-10-04T01:17:51Z
  • Abrupt permafrost collapse enhances organic carbon, CO2, nutrient and
           metal release into surface waters
    • Abstract: Publication date: Available online 3 October 2017
      Source:Chemical Geology
      Author(s): Sergey V. Loiko, Oleg S. Pokrovsky, Tatiana V. Raudina, Artyom Lim, Larisa G. Kolesnichenko, Liudmila S. Shirokova, Sergey N. Vorobyev, Sergey N. Kirpotin
      Thawing of frozen peat in discontinuous permafrost zones may significantly modify the environment at local (slumps and engineering damages) and global (greenhouse gases regime) scales. We studied the aquatic geochemistry of CO2, CH4, dissolved organic carbon (DOC), P, Si, and colloidal trace metal from hollows, depressions, permafrost subsidences and soil waters in the actively thawing discontinuous permafrost zone of Western Siberia Lowland (WSL). This site of abrupt permafrost collapse is dominated by minerotrophic fens located within the flat mound peat bog. The CO2, DOC, major and trace metal concentrations decreased with the increase of the surface area of the water body, along the hydrological continuum (soil water→hollows→depressions and permafrost subsidences→thaw ponds→thermokarst lakes). Aqueous concentrations of CO2, CH4, Ca, Si, P, Al, Fe, Nd, and U were a factor of 4 to 10 higher in the site of catastrophic thaw compared to the steady thawing of a palsa peat bog that was previously studied in the same region. The colloids (1kDa–0.45μm) formed in hot spots were strongly enriched in Fe, Al, and trivalent and tetravalent hydrolysates relative to organic carbon. Because the increase in the thickness of the thawing depth intensifies the input of inorganic components from deep mineral horizons, abrupt permafrost thaw enriches the surface waters in Al-rich colloids and low molecular weight organic complexes. As a result, the WSL's surface water colloidal composition may shift from DOM-rich and DOM-Fe-rich to DOM-Al-rich, and the release of low-soluble trivalent and tetravalent hydrolysates from the soil to the river will increase. We hypothesize that in sites of abrupt permafrost thaw, there is direct mobilization of soil waters to a hydrological network (rivers and lakes) and there is minimal transformation by autochthonous processes, which is unlike the case of steady permafrost thawing. Therefore, the change in physical factors, such as water pathways and the water residence time, within a given elementary landscape will likely control the overall impact of on-going permafrost thaw on both the surface water chemistry and dissolved greenhouse gas pattern of the territory. For this, high-resolution (<2m) remote sensing analysis of water dynamics in the permafrost landscape is necessary.

      PubDate: 2017-10-04T01:17:51Z
  • Inside front cover (Editorial Board)
    • Abstract: Publication date: 10 October 2017
      Source:Chemical Geology, Volume 469

      PubDate: 2017-09-26T10:24:31Z
  • Regional distribution pattern of carbon and helium isotopes from different
           volcanic fields in the French Massif Central: Evidence for active mantle
           degassing and water transport
    • Abstract: Publication date: 10 October 2017
      Source:Chemical Geology, Volume 469
      Author(s): Karin Bräuer, Horst Kämpf, Samuel Niedermann, Hans-Ulrich Wetzel
      We report new data of the gas and isotope (δ13C and 3He/4He) composition of 26 free gas emanations escaping from bubbling springs and mofettes in six different Cenozoic volcanic areas of the French Massif Central, which belongs to the European Cenozoic Rift System (ECRIS). In all six units the CO2 concentrations in the free gas phase are >96vol% throughout, at most locations even >99vol%. The δ13C values range between −3.6 and −6.3‰ (relative to PDB) and the 3He/4He ratios between 1.05 and 6.36Ra (Ra= 3He/4He in air). These isotope signatures indicate an origin of helium and CO2 predominantly in the subcontinental mantle. In addition, the gas composition and the isotope ratios of C, N, He, Ne, and Ar were monitored approximately once a year at two locations in the Mont-Dore Massif during a time span of five and twelve years, respectively. As a result robust data have been obtained, suggesting mixing of variable fractions of atmosphere-derived and sedimentary nitrogen. 3He/4He ratios overlapping the characteristic range of the subcontinental lithospheric mantle (SCLM) indicate fluid transport in deep reaching faults from the magmatic reservoir to the surface and suggest that minor fractions of mantle-derived nitrogen are present also. In comparison with MORB (Mid-Ocean Ridge Basalt), the CO2 is slightly enriched in 13C, confirming the findings in the Eifel area (Germany) and the western Eger Rift (Czech Republic), two other regions of the ECRIS where SCLM–derived gases escape at the surface.

      PubDate: 2017-09-26T10:24:31Z
  • Geochemical constraints on origin of hydrothermal volatiles from southern
           Tibet and the Himalayas: Understanding the degassing systems in the
           India-Asia continental subduction zone
    • Abstract: Publication date: 10 October 2017
      Source:Chemical Geology, Volume 469
      Author(s): Maoliang Zhang, Zhengfu Guo, Lihong Zhang, Yutao Sun, Zhihui Cheng
      Hydrothermal activities (e.g., hot springs and geysers) are extensively distributed in southern Tibet and the Himalayas, forming an east-west trending, ~2000-km-long, hydrothermal belt that represents the ongoing degassing systems in the India-Asia continental subduction zone (IACSZ). In this study, we report new data of chemical compositions and He-C-N isotopes for gas samples from representative hot springs in the hydrothermal belt, aimed at understanding volatile origin of the hydrothermal degassing systems in the IACSZ and their tectonic implications. According to spatial location, the samples are divided into western, central and eastern subgroups, which display spatially discernible geochemical characteristics, such as 3He/4He and CO2/3He ratios. The essential components required for volatile evolution include silicate rocks, carbonate rocks, sedimentary organic matter and a mantle-derived end-member, which are all expected in the IACSZ following a tectonic model incorporating an accretionary wedge and a magmatic front. On the basis of He-C-N isotope mixing calculations, together with constraints from petrogeochemical and geophysical studies, an enriched mantle wedge (EMW) is proposed as a potential candidate for the source of the mantle-derived components, which highlights the importance of recycled Indian continental materials compared to previous models that do not take this possibility for mantle source enrichment in account. The regional crustal rock assemblages composed of silicate rocks, carbonate rocks and sedimentary organic matter are interpreted as contaminants of the EMW-derived volatiles (as exemplified by degassing systems in the magmatic front) or as source materials of hydrothermal volatiles from degassing systems in the accretionary wedge. Following tectonic framework of the IACSZ, we suggest that spatial variations in volatile geochemistry (e.g., 3He/4He) are predominantly controlled by tectonic affinities (i.e., the accretionary wedge and magmatic front) of the hydrothermal degassing systems in southern Tibet and the Himalayas. The crustal-like 3He/4He ratios and high N2 and 4He contents agree well with the high sedimentary contributions to degassing systems in the accretionary wedge (including the Himalayas and fore-arc basins), whereas the high mantle-derived helium emissions in southern Tibet exhibit close affinities with contributions from the EMW-derived melts beneath the magmatic front. Moreover, regional fault systems with variable scales and depths of penetration would act as an extra factor that may perturb the across-IACSZ profile of 3He/4He ratios controlled by tectonic settings. Our interpretation on degassing systems in southern Tibet and the Himalayas may have the potential to provide constraints from volatile geochemistry for understanding material recycling mechanism and tectonic settings of the IACSZ.
      Graphical abstract image

      PubDate: 2017-09-26T10:24:31Z
  • Noble gases and rock geochemistry of alkaline intraplate volcanics from
           the Amik and Ceyhan-Osmaniye areas, SE Turkey.
    • Abstract: Publication date: 10 October 2017
      Source:Chemical Geology, Volume 469
      Author(s): F. Italiano, G. Yuce, M. Di Bella, B. Rojay, G. Sabatino, A. Tripodo, M. Martelli, A.L. Rizzo, M. Misseri
      The SE part of Turkey is characterized by a transtensional regime within the complex collision zone between the Anatolian, Arabian and African plates, which is bounded by two main faults, Dead Sea Fault and its splays on east and the Karasu Fault on west. In this tectonic and geodynamic context developed the Amik and further North, Erzin and Toprakkale districts, which are located onshore the Iskenderun Gulf, with the occurrence of a widespread and young alkaline volcanism dated from 1.57 to 0.05Ma in Amik, and 2.25 to 0.61Ma in Toprakkale. Here we present the results of a petrochemical and noble gases (HeAr) study performed in basalts and basanites from the Basins in order to constrain the features of the mantle source. The major and trace elements composition indicate that the involved mafic melts could be the result of 0.8–2% partial melting of a predominantly spinel and garnet+spinel mantle, which has typical features of intra-plate OIB magmatism. The 4He/40Ar* ratios display two distinct ranges, roughly <1 and >1, for basalts and basanites respectively. The 3He/4He ratio of fluid inclusions in olivine crystals ranges from 7.29 to 8.03Ra (being Ra the atmospheric 3He/4He ratio of 1.39×10−6), which implies a rather homogenous helium isotope signature of the mantle source. Such values are commonly recorded in MORB-like reservoirs (namely 8±1Ra), confirming that lavas erupted in SE Turkey did not suffer any important process of crustal contamination. Combining the main evidences from the geochemical data of both rocks and noble gases of fluids inclusions, we suggest that the melt generation of the alkaline magmas was triggered by a stretching lithosphere resulting from asthenosphere upwelling and decompression melting, analogous to geodynamic models of the coeval Syrian alkaline volcanic rocks and the older NW Anatolian rocks, rather than by subduction or plume-related processes. Finally, the combined results of noble gas-rock geochemistry point to a widespread mantle metasomatism as well as absence of crustal assimilation and negligible crustal contamination in contrast to former conclusions based only on the solid rock geochemistry.

      PubDate: 2017-09-26T10:24:31Z
  • The origin of the Zhubu mafic-ultramafic intrusion of the Emeishan large
           igneous province, SW China: Insights from volatile compositions and
           C-Hf-Sr-Nd isotopes
    • Abstract: Publication date: 10 October 2017
      Source:Chemical Geology, Volume 469
      Author(s): Qingyan Tang, Mingjie Zhang, Yuekun Wang, Yunsheng Yao, Li Du, Liemeng Chen, Zhongping Li
      The Zhubu mafic-ultramafic intrusion of the Permian Emeishan large igneous province (LIP), SW China hosts a magmatic Ni, Cu and platinum-group element (PGE) deposit. It consists of a layered sequence with sub-horizontal modal layering wrapped by a sub-vertical marginal zone. Our new zircon U-Pb isotopic dating gives a mean age of 263.2±5.6Ma for the layered sequence. The volatiles extracted from the mineral separates of the Zhubu intrusion are composed of predominant H2O (11,769.84mm3·STP/g, STP-standard temperature and pressure) and minor H2 and CO2. The layered sequence has lower H2O but higher H2, CO2, H2S and SO2 contents than the marginal zone. The CO2 and CH4 extracted at 400–900°C and 900–1200°C have light δ13CCO2 varying from −17.45‰ to −7.10‰ and δ13CCH4 varying from −41.35‰ to −22.88‰. The pyroxene separates have significantly lower (87Sr/86Sr) i values (0.705882 to 0.708912) and slightly higher εNd (t=263Ma) values (−2.8 to 0.7) than the whole rocks. The εHf (t=263Ma) values of zircon crystals vary from −3.05 to +1.90. The chemical compositions of volatiles from the Zhubu intrusion indicate a slightly reduced condition and a H2O-rich parental magma. The C-Hf-Sr-Nd isotopes for the Zhubu intrusion are consistent with ~15wt% assimilation of the upper crust by a mantle-derived magma. The carbon isotopes indicate that a thermogenic component from sedimentary organic materials was present in the contaminated magma. A weak positive correlation between volatile and PGE contents, together with the C-Hf-Sr-Nd isotopes, indicates that sulfide saturation in the Zhubu magma was triggered by crustal contamination including the addition of volatiles.

      PubDate: 2017-09-26T10:24:31Z
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