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  Subjects -> EARTH SCIENCES (Total: 656 journals)
    - EARTH SCIENCES (472 journals)
    - GEOLOGY (73 journals)
    - GEOPHYSICS (28 journals)
    - HYDROLOGY (21 journals)
    - OCEANOGRAPHY (62 journals)

EARTH SCIENCES (472 journals)                  1 2 3 | Last

Showing 1 - 200 of 371 Journals sorted alphabetically
Acta Geochimica     Hybrid Journal   (Followers: 3)
Acta Geodaetica et Geophysica     Hybrid Journal   (Followers: 1)
Acta Geodaetica et Geophysica Hungarica     Full-text available via subscription   (Followers: 2)
Acta Geophysica     Open Access   (Followers: 6)
Acta Geotechnica     Hybrid Journal   (Followers: 6)
Acta Meteorologica Sinica     Hybrid Journal   (Followers: 3)
Advances in High Energy Physics     Open Access   (Followers: 21)
Advances In Physics     Hybrid Journal   (Followers: 18)
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: 3)
AMBIO     Hybrid Journal   (Followers: 13)
Anadolu University Journal of Science and Technology     Open Access  
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: 12)
Annals of GIS     Hybrid Journal   (Followers: 20)
Annals of Glaciology     Full-text available via subscription   (Followers: 3)
Annual Review of Marine Science     Full-text available via subscription   (Followers: 9)
Anthropocene     Hybrid Journal   (Followers: 3)
Anthropocene Review     Hybrid Journal   (Followers: 4)
Applied Clay Science     Hybrid Journal   (Followers: 4)
Applied Geochemistry     Hybrid Journal   (Followers: 11)
Applied Geomatics     Hybrid Journal   (Followers: 3)
Applied Geophysics     Hybrid Journal   (Followers: 7)
Applied Ocean Research     Hybrid Journal   (Followers: 4)
Applied Petrochemical Research     Open Access   (Followers: 2)
Applied Remote Sensing Journal     Open Access   (Followers: 33)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 34)
Arctic Science     Open Access   (Followers: 6)
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 10)
Artificial Satellites : The Journal of Space Research Centre of Polish Academy of Sciences     Open Access   (Followers: 17)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 20)
Asian Journal of Earth Sciences     Open Access   (Followers: 20)
Asian Review of Environmental and Earth Sciences     Open Access   (Followers: 1)
Atlantic Geology : Journal of the Atlantic Geoscience Society / Atlantic Geology : revue de la Société Géoscientifique de l'Atlantique     Full-text available via subscription   (Followers: 10)
Atmosphere-Ocean     Full-text available via subscription   (Followers: 12)
Atmospheric and Climate Sciences     Open Access   (Followers: 27)
Australian Journal of Earth Sciences: An International Geoscience Journal of the Geological Society of Australia     Hybrid Journal   (Followers: 11)
Boletim de Ciências Geodésicas     Open Access  
Boreas: An International Journal of Quaternary Research     Hybrid Journal   (Followers: 10)
Bragantia     Open Access   (Followers: 2)
Bulletin of Earthquake Engineering     Hybrid Journal   (Followers: 11)
Bulletin of Geosciences     Open Access   (Followers: 8)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (Followers: 1)
Bulletin of the Seismological Society of America     Full-text available via subscription   (Followers: 21)
Bulletin of Volcanology     Hybrid Journal   (Followers: 17)
Cadernos de Geociências     Open Access  
Canadian Journal of Plant Science     Full-text available via subscription   (Followers: 15)
Canadian Mineralogist     Full-text available via subscription   (Followers: 3)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 20)
Carbonates and Evaporites     Hybrid Journal   (Followers: 3)
CATENA     Hybrid Journal   (Followers: 4)
Chemical Geology     Hybrid Journal   (Followers: 15)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 4)
Chinese Geographical Science     Hybrid Journal   (Followers: 5)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 3)
Ciencias Espaciales     Open Access  
Climate and Development     Hybrid Journal   (Followers: 12)
Coastal Management     Hybrid Journal   (Followers: 24)
Cogent Geoscience     Open Access  
Comptes Rendus Geoscience     Full-text available via subscription   (Followers: 8)
Computational Geosciences     Hybrid Journal   (Followers: 12)
Computational Mathematics and Mathematical Physics     Hybrid Journal   (Followers: 1)
Computers and Geotechnics     Hybrid Journal   (Followers: 8)
Contemporary Trends in Geoscience     Open Access   (Followers: 3)
Continental Shelf Research     Hybrid Journal   (Followers: 8)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 9)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 3)
Coral Reefs     Hybrid Journal   (Followers: 16)
Cretaceous Research     Hybrid Journal   (Followers: 7)
Cybergeo : European Journal of Geography     Open Access   (Followers: 5)
Depositional Record     Open Access  
Developments in Geotectonics     Full-text available via subscription   (Followers: 3)
Developments in Quaternary Science     Full-text available via subscription   (Followers: 3)
Développement durable et territoires     Open Access   (Followers: 3)
Diatom Research     Hybrid Journal   (Followers: 2)
Doklady Physics     Hybrid Journal   (Followers: 1)
Dynamics of Atmospheres and Oceans     Hybrid Journal   (Followers: 10)
E&S Engineering and Science     Open Access  
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (Followers: 102)
Earth and Space Science     Open Access   (Followers: 11)
Earth Interactions     Full-text available via subscription   (Followers: 11)
Earth Science Research     Open Access   (Followers: 6)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 4)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 19)
Earth System Dynamics     Open Access   (Followers: 6)
Earth System Dynamics Discussions     Open Access   (Followers: 4)
Earth's Future     Open Access   (Followers: 1)
Earth, Planets and Space     Open Access   (Followers: 65)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 7)
Earthquake Science     Hybrid Journal   (Followers: 10)
Earthquake Spectra     Full-text available via subscription   (Followers: 18)
Ecohydrology     Hybrid Journal   (Followers: 10)
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: 22)
Environmental Geology     Hybrid Journal   (Followers: 20)
Environmental Geosciences     Full-text available via subscription   (Followers: 3)
Environmental Geotechnics     Hybrid Journal   (Followers: 4)
Erwerbs-Obstbau     Hybrid Journal  
Estuaries and Coasts     Hybrid Journal   (Followers: 17)
Estuarine, Coastal and Shelf Science     Hybrid Journal   (Followers: 34)
Estudios Geográficos     Open Access  
European Journal of Mineralogy     Full-text available via subscription   (Followers: 12)
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: 2)
Frontiers of Earth Science     Hybrid Journal   (Followers: 8)
Fundamental and Applied Limnology / Archiv für Hydrobiologie     Full-text available via subscription   (Followers: 4)
GEM - International Journal on Geomathematics     Hybrid Journal   (Followers: 1)
Geo-Marine Letters     Hybrid Journal   (Followers: 7)
Geoacta     Open Access   (Followers: 4)
Geobiology     Hybrid Journal   (Followers: 8)
Geocarto International     Hybrid Journal   (Followers: 22)
Geochemical Perspectives     Hybrid Journal  
Geochemistry : Exploration, Environment, Analysis     Hybrid Journal   (Followers: 7)
Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 25)
Geochimica et Cosmochimica Acta     Hybrid Journal   (Followers: 43)
Geochronometria     Hybrid Journal   (Followers: 3)
Geoderma Regional : The International Journal for Regional Soil Research     Full-text available via subscription   (Followers: 3)
Geodinamica Acta     Hybrid Journal   (Followers: 3)
Geodynamics & Tectonophysics     Open Access   (Followers: 1)
Geoenvironmental Disasters     Open Access   (Followers: 3)
Geofluids     Hybrid Journal   (Followers: 4)
Geoforum     Hybrid Journal   (Followers: 21)
Géographie physique et Quaternaire     Full-text available via subscription   (Followers: 1)
Geography and Natural Resources     Hybrid Journal   (Followers: 5)
Geoheritage     Hybrid Journal   (Followers: 1)
Geoinformatica Polonica : The Journal of Polish Academy of Arts and Sciences     Open Access  
Geoinformatics & Geostatistics     Hybrid Journal   (Followers: 8)
Geological Journal     Hybrid Journal   (Followers: 14)
Geology Today     Hybrid Journal   (Followers: 25)
Geology, Geophysics and Environment     Open Access  
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  
GEOmedia     Open Access   (Followers: 1)
Geomorphology     Hybrid Journal   (Followers: 25)
Geophysical & Astrophysical Fluid Dynamics     Hybrid Journal   (Followers: 2)
Geophysical Journal International     Hybrid Journal   (Followers: 31)
Geophysical Prospecting     Hybrid Journal   (Followers: 7)
Geophysics     Full-text available via subscription   (Followers: 18)
GeoResJ     Hybrid Journal  
Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards     Hybrid Journal   (Followers: 8)
Geoscience Canada : Journal of the Geological Association of Canada / Geoscience Canada : journal de l'Association Géologique du Canada     Full-text available via subscription   (Followers: 4)
Geoscience Data Journal     Open Access   (Followers: 2)
Geoscience Frontiers     Open Access   (Followers: 9)
Geoscience Letters     Open Access   (Followers: 1)
Geoscience Records     Open Access  
Geosciences     Open Access   (Followers: 2)
Geosciences Journal     Hybrid Journal   (Followers: 10)
Geoscientific Instrumentation, Methods and Data Systems     Open Access   (Followers: 4)
Geoscientific Model Development     Open Access   (Followers: 2)
Geostandards and Geoanalytical Research     Hybrid Journal   (Followers: 2)
Geosystem Engineering     Hybrid Journal   (Followers: 1)
Geotectonic Research     Full-text available via subscription   (Followers: 4)
Geotectonics     Hybrid Journal   (Followers: 7)
GISAP : Earth and Space Sciences     Open Access   (Followers: 2)
Glass Physics and Chemistry     Hybrid Journal   (Followers: 4)
Global and Planetary Change     Hybrid Journal   (Followers: 17)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 12)
Gondwana Research     Hybrid Journal   (Followers: 6)
Grassland Science     Hybrid Journal   (Followers: 1)
Ground Water     Hybrid Journal   (Followers: 30)
Ground Water Monitoring & Remediation     Hybrid Journal   (Followers: 17)
Groundwater for Sustainable Development     Full-text available via subscription  
GSA Today     Partially Free  
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: 16)
Hydrological Processes     Hybrid Journal   (Followers: 24)
Hydrology and Earth System Sciences     Open Access   (Followers: 27)
ICES Journal of Marine Science: Journal du Conseil     Hybrid Journal   (Followers: 53)
IEEE Journal of Oceanic Engineering     Hybrid Journal   (Followers: 11)
Indian Geotechnical Journal     Hybrid Journal   (Followers: 3)
Indonesian Journal on Geoscience     Open Access   (Followers: 2)
Interdisciplinary Environmental Review     Hybrid Journal   (Followers: 3)
International Geology Review     Hybrid Journal   (Followers: 13)
International Journal of Advanced Geosciences     Open Access  
International Journal of Advanced Remote Sensing and GIS     Open Access   (Followers: 33)
International Journal of Advancement in Earth and Enviromental Sciences     Open Access   (Followers: 3)
International Journal of Advancement in Remote Sensing, GIS, and Geography     Open Access   (Followers: 27)
International Journal of Applied Earth Observation and Geoinformation     Hybrid Journal   (Followers: 35)
International Journal of Coal Geology     Hybrid Journal   (Followers: 2)
International Journal of Disaster Risk Reduction     Hybrid Journal   (Followers: 11)

        1 2 3 | Last

Journal Cover Chemical Geology
  [SJR: 2.346]   [H-I: 145]   [15 followers]  Follow
    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 0009-2541
   Published by Elsevier Homepage  [3039 journals]
  • Carbon isotope analysis of whole-coal and vitrinite from intruded coals
           from the Illinois Basin: No isotopic evidence for thermogenic methane
           generation
    • Abstract: Publication date: 20 March 2017
      Source:Chemical Geology, Volume 453
      Author(s): Mohammad W. Rahman, Susan M. Rimmer, Harold D. Rowe, William W. Huggett
      Igneous intrusions into coals and organic-rich rocks may have contributed to global warming in the geologic past through the release of greenhouse gases. Evidence for a large release of thermogenic CH4 from the organics would include significant δ13Corg enrichment in the residual organic matter (OM). However, δ13Corg values of thermally altered OM in coals and shales adjacent to intrusions often show negative trends or, in some cases, ambiguous or positive trends. Previous studies have evaluated the δ13Corg of whole-coal samples rather than that of individual organic components, or macerals. As different macerals have different isotopic compositions, maceral-specific trends may be masked by variations in maceral composition of the bulk samples. This study evaluates the hypothesis that, if a large-scale release of 13C-depleted thermogenic CH4 resulted from intrusion of the coal, then it should have produced 13C-enriched coal residuum and, specifically, vitrinite (the most abundant component of the coal) adjacent to the intrusion. This study reports geochemical and petrographic data for whole-coal samples and vitrinite macerals (separated via density-gradient centrifugation, DGC) from a transect of thermally altered Springfield (No. 5) Coal (Pennsylvanian) in the Illinois Basin. Approaching the dike contact, mean vitrinite reflectance (Rr) increases from background levels of 0.55% up to ~4.8% and liptinites become indistinguishable from vitrinite. Isotropic and fine-grained circular mosaic coke is observed in samples within ~1.3m of the coal/intrusion contact. Approaching the intrusion, volatile matter (VM) decreases and fixed carbon (FC) and %C increase, whereas %H decreases. Total organic carbon (TOC) for whole coal decreases from 77% to 34%, whereas TOC for separated vitrinite fractions increases from ~66% to 93% toward the coal/intrusion contact. Density of DGC-separated vitrinites ranges from 1.27g/mL in the unaltered coal to 1.52g/mL in the sample adjacent to the coal/intrusion contact. Vitrinite density increases with increased Rr and %C, and decreases with increased %H and H/C. Despite these marked geochemical and petrographic changes, no significant changes in δ13Corg values of the whole coal (−25.3‰ to −24.9‰) or the separated vitrinites (−25.3‰ to −25.0‰) occur proximal to the intrusion. Changes in the isotopic signatures are not of a magnitude that would be expected if significant quantities of isotopically depleted thermogenic CH4 had been generated by the intrusive event. Moreover, no petrographic evidence supports the occurrence of condensed or immobilized thermal products due to rapid pyrolysis (such as 12C-rich pyrolytic carbon) close to the intrusion that could have moderated any changes in δ13Corg. Geochemical and petrographic results suggest that only minimal loss of CH4 was associated with the rapid heating of the Springfield (No. 5) Coal by the intrusion. These findings have significant implications for models linking past global warming to the intrusion of coals and carbonaceous shales.

      PubDate: 2017-02-16T07:08:54Z
       
  • Arsenic and thioarsenic species in the hot springs of the Rehai magmatic
           geothermal system, Tengchong volcanic region, China
    • Abstract: Publication date: 20 March 2017
      Source:Chemical Geology, Volume 453
      Author(s): Qinghai Guo, Britta Planer-Friedrich, Mingliang Liu, Jiexiang Li, Chao Zhou, Yanxin Wang
      Rehai is the sole geothermal area in mainland China that discharges both acidic and alkaline hot springs with pHs ranging from 1.4 to 10.0. The hot springs in Rehai are also characterized by a wide range of arsenic concentrations, with the highest being 1.35mg/L. Analysis of the arsenic geochemistry indicates that the alkaline and neutral hot springs in Rehai have high arsenic concentrations. The alkaline hot springs represent arsenic-rich boiled reservoir fluids, and the neutral hot springs represent mixtures of these reservoir fluids with non-thermal waters and/or condensed steam. The enrichment of arsenic in the deep reservoir fluids, also called “primary fluids”, is primarily attributable to the leaching of reservoir rocks at high temperatures, but a possible arsenic contribution from magmatic fluids cannot be ruled out. The acidic springs, in contrast, have much lower arsenic concentrations because they are a mixture of non-thermal oxygenated waters and condensed steam, both of which are depleted in arsenic. In addition to arsenite and arsenate, mono-, di-, and tri-thioarsenates have been detected in the Rehai geothermal waters. The arsenic speciation in the Rehai hot springs is primarily linked to the formation processes of the various secondary fluids at the surface and their sulfide concentrations. In addition, the speciation is also affected by the pH values and the redox potentials of the waters. Compared to the springs along secondary E-W-trending faults, the chloride-rich springs along the main N-S-trending faults are generally much less affected by mixing with non-thermal waters and/or condensed steam. They are also more enriched in sulfide, and thioarsenates consequently represent a greater proportion of the total arsenic. This is the first study to report the thioarsenic geochemistry of hot springs in China.

      PubDate: 2017-02-16T07:08:54Z
       
  • The role of Fe on the formation and diagenesis of interstratified
           glauconite-smectite and illite-smectite: A case study of Lower Cretaceous
           shallow-water carbonates
    • Abstract: Publication date: 20 March 2017
      Source:Chemical Geology, Volume 453
      Author(s): Andre Baldermann, Martin Dietzel, Vasileios Mavromatis, Florian Mittermayr, Laurence N. Warr, Klaus Wemmer
      The widespread formation of interstratified glauconite-smectite (Gl-S) and illite-smectite (I-S) in modern and ancient diagenetic settings records the physicochemical conditions prevailing during clay mineral authigenesis. To date, however, significant gaps in our knowledge persist in respect to the influence of interstitial solution chemistry, temperature and reaction kinetics on the evolution of composition, mineralogy and microstructure of Gl-S and I-S. Herein, we present a study on the reaction mechanisms and the physicochemical conditions that led to the precipitation of early diagenetic Gl-S and late diagenetic I-S on a stable carbonate platform during the Cenomanian at Langenstein in the Northern German Basin. The texture and the K-Ar age (95.0±1.8Ma) of the green glauconitized grains revealed that green-clay authigenesis progressed in initially organic-rich, semi-confined micromilieus, i.e., in fecal pellets and in foraminifera, close to the sediment-seawater interface. The composition of Gl-S varied in the range (K+ 0.20–0.74Na+ 0–0.10Ca2+ 0–0.05)0.28–0.75 (Fe3+ 0.63–1.20Fe2+ 0.08–0.24Al3+ 0.19–0.97 Mg2+ 0.29–0.52)2.01–2.12 [Al3+ 0.09–0.35Si4+ 3.65–3.91O10](OH2), and depended on the rate of aqueous Fe2+ and K+ ion diffusion, the micromilieu of glauconitization and on the bulk sedimentation rate. The mineralogical, microstructural and chemical changes of the ongoing Gl-S products revealed the following reaction for green-clay authigenesis at Langenstein: Fe(III)-smectite reacted with monosilicic acid, goethite and aqueous K+, Mg2+ and Fe2+ to form glauconite and aqueous Na+, Ca2+ and H+ ions. This process considers complex mineral transformations commonly associated with glauconitization, such as early diagenetic oxidation of organic matter and microbial-catalyzed dissolution of Fe-(oxy)hydroxides, carbonates and detrital silicates. In contrast, the K-Ar age of I-S (68.0±1.6Ma) and its compositional variability, (K+ 0.29–0.45Na+ 0–0.10Ca2+ 0–0.06)0.30–0.55 (Fe3+ 0.16–0.29Fe2+ 0–0.10Al3+ 1.37–1.68Mg2+ 0.18–0.43)2.00–2.12 [Al3+ 0.17–0.39Si4+ 3.61–3.83O10](OH2), indicate a burial diagenetic origin for this mineral phase, rather than transformation of illitic clays into I-S during weathering under warm and humid climatic conditions. The results from kinetic modelling support a diagenetic origin of I-S (50–60%I layers and 50–40%S layers) and imply its formation by the replacement of pre-existing K-feldspar at high pore-fluid activity K/Na ratios and at low Fe2+ concentrations. We propose that the substitution of Al3+ for Fe3+, Fe2+ and Mg2+ in the octahedral sheet shifts the stability field of the kaolinite–Fe-Al-Mg-smectite–Fe-Al-Mg-illite (or glauconite) triple point to much lower monosilicic acid activities, and stabilizes the I-S (or Gl-S) structure. This reaction supports the idea that the (bio)availability of Fe is the rate-limiting factor for glauconitization, which is not the case for the diagenetic growth of I-S, whereby the pore water Fe2+ concentration may be limited by the competing formation of Fe-(oxy)hydroxides and/or Fe-sulfides.

      PubDate: 2017-02-16T07:08:54Z
       
  • 238U-235U-234U fractionation between tetravalent and hexavalent uranium in
           seafloor phosphorites
    • Abstract: Publication date: 20 February 2017
      Source:Chemical Geology, Volume 451
      Author(s): Yehoshua Kolodny, Adi Torfstein, Keren Weiss-Sarusi, Yevgeni Zakon, Ludwik Halicz
      Variations in the 238U/235U ratio are mostly observed in association with changes in the uranium oxidation state and therefore controlled by changes in the redox conditions, although evidence for this process has so far been indirect. Here, the δ238U and δ234U isotope composition of different redox species is studied for the first time within the same geological samples: the bulk, reduced (U(IV)) and oxidized (U(VI)) uranium species in seafloor phosphorites. In all cases, δ238U(IV) is higher (−0.27 to −0.81) than corresponding δ238U(VI) (−0.64 to −1.07), with δ234U(VI) displaying extremely high values (~500–2000) relative to δ234U(IV) (−240 to −100). By comparison, the bulk δ238U, δ234U and U concentrations are −0.42 to −0.85, −10 to +20, and 63–328ppm, respectively. These values are mostly in the range of natural variations in previously reported samples, with the bulk δ238U and δ234U values corresponding with seawater values except for a tail in δ238U toward lower values. The main exception is displayed by the composition of the U(VI) fraction, which ranges toward relatively low δ238U values but has a very strong positive δ234U excursion relative to comparable samples elsewhere. Given that the studied phosphorites formed in high productivity environments, where oxygen consumption was high and hence anoxic conditions could have been favored, it is assumed that most of the initial uranium in the samples was in reduced form. Indeed, even if some of the U oxidized over time, the studied samples still consist of approximately 60–80% U(IV). The process of 238U radioactive decay resulted in the oxidation of the decay product, 234U, and consequently, the δ234U(VI) within the samples has very high values. The evolution of δ238U is related to the effect of the ‘nuclear field shift’, which predicts that nuclides with higher atomic masses will be reduced preferentially over those with lower atomic masses. Accordingly, it is easier to reduce 238U than 235U in the same environment, which would result in higher δ238U of U(IV). This is indeed the case observed here, although in addition, the evidence here shows that the δ238U fractionation occurred during U oxidation and that the final differences of δ238U between both oxidation states reflect the combined effect of the depositional nuclear field shift and the in-situ recoil-related oxidation. A set of quantitative models are used to evaluate the role and rate of the different processes and suggest that because of the independent evolution of U(IV) and U(VI) over time, their relative fraction and isotopic compositions can be used to evaluate the formation ages of seafloor phosphorites, which has so far not been possible using U-decay series.

      PubDate: 2017-02-16T07:08:54Z
       
  • Composition and orientation dependent annealing of ion tracks in apatite -
           Implications for fission track thermochronology
    • Abstract: Publication date: 20 February 2017
      Source:Chemical Geology, Volume 451
      Author(s): A. Nadzri, D. Schauries, P. Mota-Santiago, C. Trautmann, A.J.W. Gleadow, A. Hawley, P. Kluth
      The annealing behaviour of swift heavy-ion tracks in apatite from different origins is studied as a function of their crystallographic orientation and the mineral composition. The tracks were generated by irradiating the apatite samples with 2.3GeV Bi ions, which have a comparable rate of energy loss to fission tracks in this mineral. The track radius was investigated using synchrotron-based small-angle x-ray scattering (SAXS) combined with ex situ annealing. Results indicate that tracks parallel to the c-axis are initially larger and anneal slower than those perpendicular to the c-axis. Natural variation in the mineral composition shows stronger annealing resistance of ion tracks with higher chlorine content. The SAXS results are consistent with previous studies on etched tracks and provide evidence that the orientation and composition effects are directly linked to the property of the un-etched track and not to preferential etchability. The study helps to connect the empirical studies on etched fission tracks to more fundamental solid-state processes.

      PubDate: 2017-02-16T07:08:54Z
       
  • Redox state of seafloor hydrothermal fluids and its effect on sulfide
           mineralization
    • Abstract: Publication date: 20 February 2017
      Source:Chemical Geology, Volume 451
      Author(s): Shogo Kawasumi, Hitoshi Chiba
      The redox conditions (fO2-fS2) of seafloor hydrothermal fluids were calculated from dissolved H2 and H2S concentrations at conditions of 300 and 350°C, 71–409bars, and seawater salinity. The results were compared with fO2-fS2 conditions estimated from black smoker mineralogy as reported by previous studies and our own investigations for various geological settings, including the Okinawa Trough (sediment-rich Hakurei and sediment-poor Jade fields), the Manus back-arc basin (felsic-hosted PACMANUS and basalt-hosted Vienna Woods fields), and the Central Indian Ridge (mafic/ultramafic-hosted Kairei and basalt-hosted Edmond fields). In most cases, the calculated fluid fO2-fS2 conditions agree well with estimates from primary mineral assemblages and FeS contents in sphalerite/wurtzite. This correlation suggests that the redox states of sulfide mineralization in chimneys typically reflect dissolved H2 and H2S concentrations in vent fluids. It should be noted that variable H2 concentrations in vent fluids (0.01–19mmol/kg) dominantly control not only fluid fO2 but also fS2, whereas the more uniform H2S concentrations (1–20mmol/kg) have less of an influence on fluid fS2. Fluid-sediment interaction elevates H2 concentrations to over 1mmol/kg, which sufficiently lowers fO2-fS2 to the pyrrhotite stability field; however, low H2S concentrations do not necessarily account for low fS2. Calculations for the H2-poor (≤0.1mmol/kg) and H2S-rich (~10mmol/kg) fluids in some felsic-hosted arc-backarc systems indicate relatively high fO2-fS2 states consistent with the bornite-pyrite-chalcopyrite assemblages. These redox conditions result from a combination of fluid-mineral equilibria at relatively oxidizing conditions±elevated H2S concentrations from magmatic degassing. Redox calculations at 350°C indicate that neither pyrite nor pyrrhotite is stable in the initial hot fluids of the TAG and ultramafic-hosted Mid-Atlantic Ridge systems due to the low H2S concentrations. With cooling of these fluids, either conductively or by interacting with seawater, higher fS2 and fO2 result and Fe-sulfides are stable, in agreement with the higher-fO2-fS2 mineral assemblages observed.

      PubDate: 2017-02-16T07:08:54Z
       
  • Systematic variations in magmatic sulphide chemistry from mid-ocean
           ridges, back-arc basins and island arcs
    • Abstract: Publication date: 20 February 2017
      Source:Chemical Geology, Volume 451
      Author(s): Manuel Keith, Karsten M. Haase, Reiner Klemd, Ulrich Schwarz-Schampera, Henrike Franke
      Immiscible sulphide liquids preserved as magmatic sulphide globules are hosted in igneous rocks of highly variable composition formed during magmatic processes at different tectonic settings. Here we report on compositional in situ data of magmatic sulphides from mid-ocean ridge, back-arc and island arc magmatic systems. Iron-Ni-rich monosulphide solid solutions (mss) that mainly consist of pyrrhotite and pentlandite are the dominant phases in magmatic sulphide globules. In subduction-related magmas mss are most abundant in relatively evolved melts and characterised by low Ni and high Fe contents, as well as low Ni/Cu ratios (<1). In contrast, mss from mid-ocean ridges and back-arcs without subduction input occur in mafic rocks and have high Ni/Cu ratios (>1). Thus, mss chemistry varies systematically in lavas from the different tectonic settings in terms of their Fe-Ni distribution and Ni/Cu ratio. The observed mineralogical and chemical variations between mss from the different settings reflect early S saturation accompanied by olivine fractionation along mid-ocean ridges compared to later stage S saturation in arc systems associated with Fe-Ti oxide fractionation. These systematics are probably related to the relatively oxidised character of subduction zone magmas opposed to more reduced mid-ocean ridge melts. Large Ni/Cu variations in mss from the same locality and in individual samples suggest that S saturation is a multistage or continuous process leading to inclusions of magmatic sulphides that represent different fractionation stages in the ascending magma.

      PubDate: 2017-02-16T07:08:54Z
       
  • Double-spike inversion for three-isotope systems
    • Abstract: Publication date: 20 February 2017
      Source:Chemical Geology, Volume 451
      Author(s): Christopher D. Coath, Tim Elliott, Remco C. Hin
      Double spiking is conventionally used to make accurate determinations of natural mass-dependent isotopic fractionations for elements with four or more stable isotopes. Here we document a methodology which extends the effective application of double spiking to three isotope systems. This approach requires making a mixture with isotope ratios that lie on a ‘critical curve’ where the sample – double-spike mixing line and the tangent to the instrumental mass-bias curve are coincident. Inversion of the mixing equations for such a mixture leads to a solution for the sample fractionation which is independent (to first order) of the uncertainty in the instrumental mass-bias and, hence, independent of any mass-dependent artefacts in the measurement such as those produced by residual matrix not completely removed by prior chemical purification. In practice, mixtures can be made which yield an accuracy conservatively estimated to be ∼ 0.005‰/amu. The precision of the method is explored as a function of double-spike composition for Mg, Si and K isotope systems. We show that for Mg and Si measurement precision is not compromised by the compositions of viable critical mixtures nor by uncertainty magnification during inversion of the equations. Thus, double spiking provides a valuable means to obtain robust, high precision isotopic measurements of Mg and Si. For K, however, the low abundance of 40K in the optimal critical mixture places a significant practical limitation on the application of double spiking to analyses of this element.

      PubDate: 2017-02-16T07:08:54Z
       
  • Conodont apatite δ88/86Sr and δ44/40Ca compositions and implications for
           the evolution of Palaeozoic to early Mesozoic seawater
    • Abstract: Publication date: Available online 14 February 2017
      Source:Chemical Geology
      Author(s): Sandrine Le Houedec, Malcolm McCulloch, Julie Trotter, Kai Rankenburg
      We present high precision (TIMS double spike) stable isotope measurements of both δ44/40Ca and δ88/86Sr together with radiogenic 87Sr/86Sr ratios determined from conodont apatite. These data represent five intervals ranging from the early Ordovician to late Triassic. The conodont δ44/40Ca values (relative to NIST 915a) range from −0.47‰ to +0.15‰, with an apparent shift to more positive values between the early Silurian and late Devonian/early Carboniferous, similar to the brachiopod-based marine δ44/40Ca record (Farkaš et al., 2007a). We calculated a δ44/40Ca seawater-bio-apatite fractionation factor of about −1.9‰, which allowed us to reconstruct a palaeo-seawater δ44/40Ca record from bio-apatites. Despite a slightly positive offset of about +0.2 to +0.5‰, the δ44/40Ca record obtained from bio-apatites is consistent with the previously reported δ44/40Ca seawater record inferred from carbonates. We find that unlike the carbonate δ44/40Ca records, the δ88/86Sr measurements from conodont apatite show unexpectedly large variations (up to ~1‰), with ratios ranging from −0.6‰ to 0.3‰. These reconnaissance data reveal a reasonable correlation between δ88/86Sr and radiogenic 87Sr/86Sr* (r2 =0.60, n =13, p =0.002), suggesting that the controls from differential weathering regimes and/or continental crustal compositions buffered ancient seawater compositions.

      PubDate: 2017-02-16T07:08:54Z
       
  • Geochemical characteristics of He and CO2 from the Ordos (cratonic) and
           Bohaibay (rift) basins in China
    • Abstract: Publication date: Available online 10 February 2017
      Source:Chemical Geology
      Author(s): Jinxing Dai, Yunyan Ni, Shengfei Qin, Shipeng Huang, Deyu Gong, Dan Liu, Ziqi Feng, Weilong Peng, Wenxue Han, Chenchen Fang
      According to the compositional and isotopic analyses of He, CO2 and CH4 of 95 gas samples from the Ordos Basin and 76 gas samples from the Bohaibay Basin in China, this study carried out a detailed investigation on the geochemical characteristics of helium and CO2 from the cratonic and rift basins, especially the cratonic Ordos Basin and rift Bohaibai Basin in China. Compared to the rift Bohaibay Basin, the cratonic Ordos Basin has lower level of helium and CO2, lower R/Ra ratio and lower δ13CCO2 maximum value. When the content of CO2 is <10%, both cratonic and rift basins have organic CO2 associated with the hydrocarbon generation process and inorganic CO2 from the thermal decomposition of carbonates or dissolution of organic acid. Rift basin can also have CO2 >20% and δ13CCO2 of −6±2‰ or −4‰–−9‰, which is of volcanic-magmatic or mantle-derived origin, while cratonic basin has no such type of CO2. The cratonic basins (Ordos, Sichuan) have CO2 <5% and relatively smaller variation range of δ13CCO2, i.e., 25.6‰ (−0.8‰–−26.4‰). The rift basins have higher level of CO2, R/Ra ratios and δ13CCO2 values. The content of CO2 can reach 100%, the R/Ra ratio can be up to 6.45 and the variation range of δ13CCO2 is 34‰ (7‰–−27‰). Bigger variation range of δ13CCO2 implies stronger tectonic activities, which is not favorable for the gas accumulation. While smaller variation range means more stable structure, which is favorable for the discovery of large gas fields. Sample in the cratonic basin in China generally has CH4/3He of 109–1012 and R/Ra<0.1 and in the rift basin it has CH4/3He of 105–1013 and R/Ra of 0.1–6.45. It is concluded that methane with CH4/3He≤108 is of inorganic origin and methane with CH4/3He≥1011 is of organic origin. The regression equation for the CH4/3He-R/Ra in the cratonic basin is: y=3E+11e−46.28x and it is: y=4E+10e−1.791x in the rift basin. Both Ordos and Bohaibay basins found gas wells with He>0.05%, which has significant commercial values and provides a clue for the exploration of helium-rich gas fields. Traps with R/Ra>2 is a favorable clue for the discovery of CO2 gas fields. Seven CO2 gas fields have been found in the Bohaibay Basin, but no systematic development has been carried out and only some liquid CO2 has been injected for tight oil from the Huagou CO2 gas field.

      PubDate: 2017-02-16T07:08:54Z
       
  • Setting a limit on anthropogenic sources of atmospheric 81Kr through Atom
           Trap Trace Analysis
    • Abstract: Publication date: Available online 9 February 2017
      Source:Chemical Geology
      Author(s): J.C. Zappala, K. Bailey, W. Jiang, B. Micklich, P. Mueller, T.P. O’Connor, R. Purtschert
      We place a 2.5% limit on the anthropogenic contribution to the modern abundance of 81Kr/Kr in the atmosphere at the 90% confidence level. Due to its simple production and transport in the terrestrial environment, 81Kr (half-life=230,000years) is an ideal tracer for old water and ice with mean residence times in the range of 105–106 years. In recent years, 81Kr-dating has been made available to the earth science community thanks to the development of Atom Trap Trace Analysis (ATTA), a laser-based atom counting technique. Further upgrades and improvements to the ATTA technique now allow us to demonstrate 81Kr/Kr measurements with relative uncertainties of 1% and place this new limit on anthropogenic 81Kr. As a result of this limit, we have removed a potential systematic constraint for 81Kr-dating.

      PubDate: 2017-02-16T07:08:54Z
       
  • The behaviour of zirconium, hafnium and rare earth elements during the
           crystallisation of halite and other salt minerals
    • Abstract: Publication date: Available online 9 February 2017
      Source:Chemical Geology
      Author(s): P. Censi, C. Inguaggiato, S. Chiavetta, C. Schembri, F. Sposito, V. Censi, P. Zuddas
      Halite crystals from Messinian and Tortonian evaporites from Sicily and Spain and current precipitated halite crystals and the relative parent brines (active evaporation systems) were investigated in order to evaluate the Zr, Hf and Rare Earth Element (REE) behaviour. Halite crystallisation from evaporating brines fractionates Zr, Hf and REE through a two-step process. During the first step, dissolved complexes of studied elements are scavenged onto the surfaces of crystallising halite. During the second step, elements are co-precipitated into the crystal lattice as it grows. The first step mechanism is determined by the dissolved REE speciation. In saltworks where carbonate-REE complexes occur, surface complexation of REE onto halite crystals does not occur. On the contrary, surface REE-complexes onto halite crystals are forming in the Dead Sea water where aqueous REE speciation is dominated by chloride-complexes. Under the latter conditions, halite crystallises with cubic and cubic-octahedral composite habitus. Octahedral planes involve the formation of strong coulombic interactions, mainly with [Hf(H2O)3(OH)5]− rather than with [Zr(H2O)4(OH)4]0 complexes. As a consequence, newly formed halite in the Dead Sea shows strong subchondritic Zr/Hf ratios. Based on these indications, analyses carried out on salt minerals from Messinian and Tortonian evaporites in Sicily and Spain show that their overall REE content can be considered a discriminating parameter between authigenic minerals and diagenetic modified materials. However, features of shale-normalised REE patterns are driven by the mineralogical composition of evaporites rather than their authigenic or secondary nature. On the contrary, the Zr/Hf signature of salt minerals is influenced by their origin. Indeed, subchondritic Zr/Hf values are found in primary salt minerals, whereas larger Zr/Hf values are recognised in those diagenetically modified. Calculated distribution coefficients of Zr, Hf and REE are employed for modelling the REE distribution in halite equilibrated with the deep-sea brines from Typo, Medee and Thetis basins (Eastern Mediterranean). The obtained indications allow us to discriminate brines formed by dissolution of evaporites relative to those representing relics of fossil evaporated seawater.

      PubDate: 2017-02-16T07:08:54Z
       
  • 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: Available online 8 February 2017
      Source:Chemical Geology
      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-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 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 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: 2017-02-16T07:08:54Z
       
  • Comparison of europium and neptunium adsorption to aluminum (hydr)oxide
           minerals
    • Abstract: Publication date: Available online 7 February 2017
      Source:Chemical Geology
      Author(s): Teresa Baumer, Patricia Kay, Amy E. Hixon
      Batch experiments and detailed solid-phase characterization (i.e., powder X-ray diffraction, scanning electron microscopy, and infrared spectroscopy) were used to determine the effect of secondary phase formation on the adsorption of europium and neptunium to a suite of aluminum (hydr)oxide minerals. Europium experiments were conducted as a function of gibbsite (γ-Al(OH)3), bayerite (α-Al(OH)3), corundum (α-Al2O3), and γ-alumina (γ-Al2O3) concentration (2.5–30m2 L−1), europium concentration (10−8–10−5 M), pH (3−12), and ionic strength (0.01–0.1M NaCl). Neptunium experiments were conducted at constant neptunium and mineral concentrations. The composition of the mineral phase had no apparent effect on europium adsorption whereas preferential adsorption of neptunium followed the trend bayerite>corundum>γ-alumina. The data presented here suggest that there are at least two different mechanisms controlling the adsorption of europium and neptunium and highlight the need to study both aqueous-phase chemistry and the properties of mineral surfaces in order to understand the behavior of lanthanides and actinides at the mineral-water interface.

      PubDate: 2017-02-16T07:08:54Z
       
  • Immobilization of nanoparticles by occlusion into microbial calcite
    • Abstract: Publication date: Available online 6 February 2017
      Source:Chemical Geology
      Author(s): Rebecca L. Skuce, Dominique J. Tobler, Ian MacLaren, Martin R. Lee, Vernon R. Phoenix
      Binding of nanoparticles (NPs) to mineral surfaces influences their transport through the environment. The potential, however, for growing minerals to immobilize NPs via occlusion (the process of trapping particles inside the growing mineral) has yet to be explored in environmentally relevant systems. In this study, the ureolytic bacteria Sporosarcina pasteurii was used to induce calcium carbonate precipitation in the presence of organo-metallic manufactured nanoparticles. As calcite crystals grew the nanoparticles in the solution became trapped inside these crystals. Capture of NPs within the calcite via occlusion was verified by transmission electron microscopy of thin foils. Nanoparticles with a negative surface charge were captured with greater efficiency than those with a positive surface charge, resulting from stronger attachment of negative nanoparticles to the positively charged calcite surfaces, which in turn facilitated occlusion. Thermodynamic and kinetic analysis, however, did not reveal a significant difference in k p (calcite precipitation rate constant) or the critical saturation at which precipitation initiates (S crit ), indicating the presence of different charged nanoparticles did not influence calcite precipitation at the concentrations used here. Overall, these findings demonstrate that microbially driven mineral precipitation has potential to immobilize nanoparticles in the environment via occlusion.

      PubDate: 2017-02-16T07:08:54Z
       
  • Theory of a methodology for initial lead determination, and a procedure to
           resolve it to some of its stages
    • Abstract: Publication date: 5 February 2017
      Source:Chemical Geology, Volume 450
      Author(s): Fouad Tera
      A method for determining the initial Pb of a terrain, on the basis of the measured isotopic compositions of its rocks, is put forward in this report. The method was inspired by the premise that the initial Pb inherited by the rocks from a reservoir from which they were extracted, is immutable and inerasable, irrespective of the multitude of disturbances that may have subsequently been superimposed on the terrain. This is because while these disturbances may have altered the isotopic composition of some or all the rocks, they lacked the vehemence to re-melt the entire terrain or at least a very large portion of it, which is a pre-requisite for altering the isotopic composition of initial Pb. If this rational is valid, then a large Pb isotope database (including data on mineral separates with low affinities for U and Th) that is representative of a terrain, when plotted on any Pb isotope correlation diagram (e.g., the conventional Pb/Pb plot), may define a dispersion field that tapers toward a single spot. That single spot (once unambiguously determined) is the initial Pb isotopic composition. Furthermore, from the equations of radioactive decay as applied in geochronology, the author provides evidence for the potential existence within a Pb isotope dispersion field, of three classes of lines that converge in different types of Pb isotope correlations, to always meet in a point that yields the composition of initial Pb. These lines are: (1) isochron: defined by samples that remained as closed systems since crystallization, (2) transposichrons: each made up of samples that experienced in a disturbance episode, fractionation by the same constant factor F=(238U/204Pb)pc/(238U/204Pb)cr, and/or the same constant factor K=(232Th/238U)pc/(232Th/238U)cr, where the subscripts stand for post-crystallization and crystallization, and (3) Heterochrons: each defined by samples produced by different evolution scenarios (including different multiple stages with different values for the geochemical parameters F and K), which happen to accidentally have the same average F and/or the same average K. As demonstrated by synthetic examples, heterochrons occur because the production of a Pb isotopic ratio by radioactive decay is controlled by multiple Independent variables. This circumstance allows for various combinations of the parameters, to accidentally produce same radiogenic isotopic ratio. An application to a terrestrial terrain (out of four, discussed in companion paper # 2), further illustrates in this report, the validity of the rational and the practicality of the method. The determination of the initial Pb composition is a necessary step toward elucidation of the early evolution history of a planet, but alone, such a step falls short of reaching the goal. Full chronometric elucidation of a planet's history, requires complete resolution of initial Pb into all the multiple stages that led to its final composition. As this is not readily tenable, a procedure (termed ‘Congruently Associated Profiles’, CAPs) in which initial Pb is partially resolved (into two or three stages) is introduced as an alternative. However, now that initial Pb is shown to be routinely determinable, could resolution of its multi-stages, someday become possible? To some, including the author, even the apparent ‘impossibility’ of an objective neither over rides hope nor stops prospecting for its realization.

      PubDate: 2017-02-16T07:08:54Z
       
  • Determination of initial leads of four terrestrial terrains, applying the
           Tulip methodology
    • Abstract: Publication date: 5 February 2017
      Source:Chemical Geology, Volume 450
      Author(s): Fouad Tera
      The recently developed TULIP methodology for determining Initial lead, based on the measured Pb isotopic compositions of rocks, was applied to four terrestrial terrains, and the results are shown and discussed in this report. Particular emphasis was given to the determination of initial Pb of the South Of Isua (SOI) terrain, because of the availability of a large high-quality database on its rocks and feldspar separates. The initial Pb results for SOI, are: 206Pb/204Pb=11.088±0.024, 207Pb/204Pb=12.983±0.002, and 208Pb/204Pb=31.196±0.014. Initial Pb was also precisely determined for the Beartooth Mountains, and the results are: 206/204=13.571±0.071, 207/204=14.891±0.003, and 208/204=32.41±0.08. These results demonstrate the feasibility of routine determination of initial Pb by the developed methodology, once large databases for the terrains are established. Extending the methodology to terrains of other planets should be possible. Projection of initial Pb by a database is caused by Th-U-Pb fractionation, in disturbing events superimposed on the rocks of a terrain. As demonstrated mathematically (in a companion paper), elemental fractionation can induce alignments of the data in linear trends of false ‘ages,’ which together with the rocks' isochron, converge (on various Pb plots) on a point that yields initial Pb. This may result in two counter effects: (1) precise determination of initial Pb, and (2) ambiguity in the exact meaning of an isochron (because of the possibility it was affected by fractionation, like the other lines). Consequently associating a Pb/Pb age with the determined initial may not always be meaningful. For SOI, the rocks' age is 3.84±0.05Ga. The ability to determine initial Pb opens the possibility for eventual unfolding of details of the evolutionary history of the Earth and other planets. However, that cannot be satisfactorily achieved without additionally developing a methodology for resolving initial Pb to its multi-stages. Because such methodology is still lacking, a procedure of Congruently Associated Profiles (CAPs), for resolving initial Pb to a maximum of three stages was developed and outlined in a companion paper (C, P, # 1, 2016). For SOI, only a two-stage CAP solution is possible. It indicates a U-Pb fractionation event at 67.5Ma after the Earth's accretion (that is a first stage lasting from 4.563 to 4.496Ga), with μ= 238 U/204Pb=0.45±0.25. For the second stage, extending from 4.496 to 3.8Ga, I obtain μ=9.25±0.02. The stated duration of 67.5Ma for the first stage is an upper limit, and in a multi- stage solution, if and when it becomes possible, the first stage duration would be shorter, and its meaning may be elucidated. For now it remains subject to qualitative speculations including the possibility of being ‘associated’ with core formation, or an early impact (resulting in formation of the moon?), or …or. This ambiguity underlines a crucial need for a methodology to resolve initial Pb to more of its multi stages.

      PubDate: 2017-02-16T07:08:54Z
       
  • Effect of dissolved H2SO4 on the interaction between CO2-rich brine
           solutions and limestone, sandstone and marl
    • Abstract: Publication date: 5 February 2017
      Source:Chemical Geology, Volume 450
      Author(s): E.M. Thaysen, Josep M. Soler, Marijn Boone, Veerle Cnudde, Jordi Cama
      The effect of pure and impure carbon dioxide (CO2) on geological storage is uncertain. Oxidation of impurities such as sulfur dioxide (SO2) and hydrogen sulfide (H2S), which may be catalyzed by co-injected oxygen or nitrogen oxides, leads to the formation of sulfuric acid (H2SO4) and a decrease in pH of the formation water. We investigated the effect of 0.005molL−1 H2SO4 (corresponding to a worst case of 0.4% SO2 in the flue gas, anticipating total conversion of SO2 into H2SO4) on the reactivity of the reservoir (limestone and sandstone) and cap (marl) rocks of Hontomin (Spain) at P=pCO2 =10bar and 60°C during 24days using flow-through column experiments. Aqueous element concentrations were measured from fluid extracts obtained periodically throughout the experiments to infer fluid-rock reactions over time. Results were modeled with the CrunchFlow reactive transport code. The added H2SO4 lowered the pH of the injected brine by ~1.5pH units with respect to the pH of ~3.6 of the H2SO4-free brine. In both H2SO4-free and H2SO4-rich brine, calcite dissolution fostered gypsum precipitation. A comparison between the reactivity of the rocks reacted in H2SO4-free and H2SO4-rich brine showed that calcite dissolution and gypsum precipitation rates were increased by 27–48% and 25–75%, respectively, in H2SO4-rich brine. Overall rock porosity increments in H2SO4-rich brine were 2.9–3.6%, 3.7–4.8%, and 2.1–2.7% for sandstone, limestone and marl, respectively. Porosities in H2SO4-rich brine were 6%, 23% and 250%, higher, respectively, than under pure CO2. Modeled porosity increments in the acid inlet zone in H2SO4-rich brine for sandstone, limestone and marl were 28–29%, 44–45% and 24–28%, respectively, corresponding to an increase of 1%, 250% and 25%, respectively, relative to H2SO4-free brine. Gypsum precipitation was consistently higher in marl than in limestone and sandstone, indicating kinetically favorable conditions for gypsum precipitation within the cap rock. Our results provide relevant data for long-term storage simulations of impure CO2 injection.

      PubDate: 2017-02-16T07:08:54Z
       
  • Impact of water-particle interactions on molybdenum budget in humid
           tropical rivers and estuaries: insights from Nethravati, Gurupur and
           Mandovi river systems
    • Abstract: Publication date: 5 February 2017
      Source:Chemical Geology, Volume 450
      Author(s): GP Gurumurthy, M Tripti, J Riotte, R Prakyath, K Balakrishna
      The study presents the seasonal and inter-annual monitoring of molybdenum (Mo) distribution and variability in humid tropical riverine and estuarine systems (Nethravati, Gurupur and Mandovi estuaries) of west coast of India. The study investigates the geochemical behaviour of Mo in the river and estuaries, and their ultimate fluxes into the ocean. The riverine flux of dissolved Mo (DMo) to the Nethravati, Gurupur and Mandovi estuaries are 1800molyr−1 (4.88molday−1), 195molyr−1 (0.53molday−1) and 10.5×103 molyr−1 (28molday−1) respectively, and the riverine particulate Mo (PMo) flux to Nethravati estuary is 10.8×103 molyr−1. The DMo in river (~30 to 40%) is scavenged onto particles under oxidized acidic river water conditions and subsequently released in the estuary, impacting the solute budget of Mo to the sea. In the estuaries, under low salinity conditions, DMo is sequestered onto particles during pre-monsoonal season. The DMo sequestration in the estuary is estimated to be ~2molday−1 in the Nethravati estuary and ~1.9molday−1 in the Mandovi estuary. During this season sequestration in the estuary is higher than the riverine supply, indicating the sequestration of both marine and river borne DMo. However, the mechanisms involved in the removal process are different in these estuaries viz. oxidative adsorption process in the Nethravati-Gurupur estuary and microbial utilization in the Mandovi estuary. The lower salinity region during monsoon and post-monsoon season shows slight excess of DMo, river borne particulate Mo could release up to 3 to 4nmolL−1 by desorption under alkaline higher ionic strength conditions. At higher salinity (>20psu) in both the estuaries and in all the seasons, DMo gain is systematic (~1 to 37nmolL−1). Mo release from river borne particles could contribute only up to 3 to 4nmolL−1, which is not sufficient to balance the observed Mo excess. On the other hand, the reductive Mo remobilization from bottom sediments (Mo=4mgkg−1) during sediment diagenesis and subsequent tidal activity, release up to 28nmolL−1 of DMo to the estuarine water. Mo release to water column is supported by the gradual enrichment of DMo with depth in the estuary. Therefore, diagenetic release of DMo forms the potential source of DMo excess in the estuary.

      PubDate: 2017-02-16T07:08:54Z
       
  • Was the Ediacaran Shuram Excursion a globally synchronized early
           diagenetic event? Insights from methane-derived authigenic carbonates in
           the uppermost Doushantuo Formation, South China
    • Abstract: Publication date: 5 February 2017
      Source:Chemical Geology, Volume 450
      Author(s): Huan Cui, Alan J. Kaufman, Shuhai Xiao, Chuanming Zhou, Xiao-Ming Liu
      The Ediacaran Period is characterized by the most profound negative carbon isotope (δ13C) excursion in Earth history, the Shuram Excursion. Various hypotheses – including the massive oxidation of dissolved organic carbon (DOC) in the oceans, the weathering of terrestrial organic carbon, or the release and oxidation of methane hydrates and/or expelled petroleum from the subsurface – have been proposed as sources of the 13C-depleted carbon. More recently, it has been suggested that global-scale precipitation of early authigenic carbonates, driven by anaerobic microbial metabolism in unconsolidated sediments, may have caused the Shuram Excursion, but empirical evidence is lacking. Here we present a comprehensive analysis of a Shuram-associated interval from the uppermost Doushantuo Formation in South China. Our study reveals petrographic evidence of methane-derived authigenic calcite (formed as early diagenetic cements and nodules) that are remarkably depleted in 13C – suggesting a buildup of alkalinity in pore fluids through the anaerobic oxidation of methane (AOM) – and systematically depleted in 18O relative to co-occurring dolomite. Early authigenesis of these minerals is likely to be driven by increased microbial sulfate reduction, triggered by enhanced continental weathering in the context of a marked rise in atmospheric oxygen levels. In light of the finding of methane-derived authigenic carbonates at Zhongling, and based on our basin-scale stratigraphic correlation, we hypothesize that the marked 13C and 18O depletion (including their co-variation noted worldwide) in the Shuram Excursion may reflect an episode of authigenesis occurring within a sulfate–methane transition zone (SMTZ). If true, the Shuram Excursion was then a global biogeochemical response to enhanced seawater sulfate concentration in the Ediacaran ocean driven by the Neoproterozoic oxidation of surface environments. This paleo-oceanographic transition may have therefore paved the way for subsequent evolution and diversification of animals. Our study highlights the significance of an integrated approach that combines petrography, mineralogy, and texture-specific micro-drilling geochemistry in chemostratigraphic studies.

      PubDate: 2017-02-16T07:08:54Z
       
  • Chemical alteration and mineral growth under high pCO2 conditions:
           Insights from the mineral chemistry of carbonate phases in the Caprese
           Reservoir (Northern Apennines, central Italy)
    • Abstract: Publication date: 5 February 2017
      Source:Chemical Geology, Volume 450
      Author(s): Gabriele Bicocchi, Orlando Vaselli, Giovanni Ruggieri, Marco Bonini, Franco Tassi, Antonella Buccianti, Francesco di Benedetto, Giordano Montegrossi
      Carbon cycle is one of the most complex geochemical cycles of the Earth as it involves atmosphere, hydrosphere, biosphere and lithosphere. Among these different geochemical spheres, mechanisms and reaction rates governing migration and fixation of carbon (i.e. fluxes), such as the stability of the different carbonates minerals under specific conditions in crustal reservoirs, are still matter of detailed studies. In this work, SEM and XRD investigations and EMP analyses were combined to study the mineralogical and chemical compositions of carbonate phases occurring in strongly altered Triassic volcanic rocks, hosting a CO2 reservoir (namely the Caprese Reservoir), located in eastern Tuscany (central Italy). The Caprese crustal fluid reservoir, lying at 3300m depth beneath the Quaternary San Cassiano basin, hosts a supercritical CO2-(N2)-rich phase and a Na-Cl-rich saline (up to 81.5g/L) brine. The mineralogical assemblage consists of authigenic microcrystalline quartz, Ca-Fe-Mg carbonates, Na-K-clay minerals and Fe-oxides, the primary minerals being almost completely chemically altered by secondary processes. Scanning electron microscope images and cation (Ca-Fe-Mg) contents in carbonates highlighted that zoned ankerite was re-grown on calcite crystals due to interactions processes with CO2-rich fluids. The geochemical and mineralogical data provided useful insights for understanding both the processes that formed the C-bearing minerals and their growth in deep, saline reservoirs.

      PubDate: 2017-02-16T07:08:54Z
       
  • Fluid evolution of the Neoarchean Pampalo orogenic gold deposit (E
           Finland): Constraints from LA-ICPMS fluid inclusion microanalysis
    • Abstract: Publication date: 5 February 2017
      Source:Chemical Geology, Volume 450
      Author(s): Tobias Fusswinkel, Thomas Wagner, Grigorios Sakellaris
      The sources of fluids and precious metals in orogenic gold deposits remain controversial, and a key question is whether auriferous magmatic-hydrothermal fluids are a decisive ingredient for economic orogenic gold mineralization. Contributions of magmatic-hydrothermal fluids have been proposed based on indirect arguments such as isotope data or emplacement of granites coevally with gold deposits. By contrast, complete elemental concentration data for orogenic gold fluids, which could be used to compare them directly with those of magmatic-hydrothermal fluids, are not yet available. This study reports the major and trace element chemistry of fluid inclusions from the Neoarchean Pampalo orogenic gold deposit, located in the Hattu schist belt in Eastern Finland. Three successive generations of gold-mineralized and barren veins provide the paragenetic and chronological framework for establishing the fluid evolution history of the Pampalo deposit. Based on petrography, microthermometry and LA-ICPMS fluid inclusion microanalysis, five major fluid types were identified. The data shows that all fluid types, despite characteristic differences, carry the signature of orogenic gold fluids, i.e., low- to intermediate-salinity, presence of CO2, CH4 or N2, high S contents, enrichment in B, As, Sb and W, and low Pb and Zn concentrations. Gold concentrations vary significantly between the oldest (ca. 0.6μg/g) fluid related to the zones with highest gold grades in the deposit and later fluid types (<0.03μg/g) unrelated to gold mineralization. This shows that gold mineralization was caused by episodic input of exceptionally gold-rich fluids and not by continuous and long-lasting hydrothermal activity. Comparing the fluid composition data from Pampalo with the well-established major and trace element characteristics of magmatic-hydrothermal fluids, the potential role of magmatic fluids in orogenic gold systems can be quantitatively evaluated. Mass balance calculations using metal concentrations as well as Cl/Br ratios demonstrate that the composition of the early Au-rich fluid at Pampalo is not compatible with any significant magmatic-hydrothermal fluid contribution, and that all fluids at Pampalo are metamorphogenic in origin. Molar Cl/Br ratios as low as 9–20 are found in some of the fluid types, which groups them among the most Br enriched crustal fluids reported so far. The exceptional Br enrichment is inconsistent with an evaporative origin but instead points to Br uptake from organic matter in metasedimentary rocks during metamorphism and fluid production. Taken together, the selective Au enrichment of only the earliest fluid type is well explained by metamorphic fluid production from Au-rich source rock lithologies, without any significant contributions from magmatic-hydrothermal fluids.

      PubDate: 2017-02-16T07:08:54Z
       
  • Coupling DGT passive samplers and multi-collector ICP-MS: A new tool to
           measure Pb and Zn isotopes composition in dilute aqueous solutions
    • Abstract: Publication date: 5 February 2017
      Source:Chemical Geology, Volume 450
      Author(s): Anne-Marie Desaulty, Merlin Méheut, Catherine Guerrot, Catherine Berho, Romain Millot
      Using zinc (Zn) and lead (Pb) isotopes is a powerful tool to track metal pollution in environment. In this study we have developed the coupling between DGT passive samplers and multi-collector ICP-MS to measure Pb and Zn isotopic ratios in dilute aqueous solutions. The benefits of this coupling are multiple: the use of DGT device allows achieving an isotopic composition of natural water integrated over time and to pre-concentrate metals in situ. This development will greatly facilitate the field collection of samples and their preparations in cleanroom prior to their isotopic analyses. To test the capability of DGT samplers a series of experiments was achieved in cleanroom and in experimental pilot simulating a water flow. These tests have shown that there is no fractionation of Pb isotopes due to the use of DGT within the reported precision of MC-ICPMS measurements. For Zn, the diffusion process through a membrane, inherent to the use of DGT device, induces a fractionation between the isotopic composition obtained by the DGT and the natural composition. However, this bias can be easily corrected by using a simple relation independent of the time of exposure and the thickness of diffusion layer. The coupling DGT passive samplers and multi-collector ICP-MS is suitable to determine the Pb and Zn isotopic compositions in natural waters and offers new perspectives to track the anthropic pollutions in the hydrosphere.
      Graphical abstract image

      PubDate: 2017-02-16T07:08:54Z
       
  • Tungsten stable isotope compositions of terrestrial samples and meteorites
           determined by double spike MC-ICPMS
    • Abstract: Publication date: 5 February 2017
      Source:Chemical Geology, Volume 450
      Author(s): Nadine Krabbe, Thomas S. Kruijer, Thorsten Kleine
      Tungsten stable isotopes hold great potential to examine a variety of physical and chemical processes operating during the accretion and differentiation of asteroids and terrestrial planets, such as core formation and mantle–crust differentiation. To assess the magnitude and origin of W isotope fractionations, we determined the W stable isotopic compositions of six USGS geological reference materials, a NIST steel, and 24 iron meteorites and chondrites. The W isotope data were obtained using multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) and using a 180W–183W double spike. Chondrites and iron meteorites exhibit a very narrow range in W stable isotope compositions, resulting in a mean δ 184/183W=0.027±0.007‰ (95% conf.) relative to the NIST 3163 W standard. This value represents a good estimate for the W stable isotope composition of bulk planetary bodies from the inner solar system. The δ 184/183W of some iron meteorites slightly deviates from this value, most likely due to W isotope fractionations induced during crystallization of the metal cores of iron meteorite parent bodies. The investigated terrestrial silicate rocks exhibit a narrow range in δ 184/183W, which for most samples is indistinguishable from the mean value of chondrites and iron meteorites. However, felsic samples tend to be isotopically lighter than mafic samples, indicating that magmatic processes on Earth induced W isotope fractionations. These fractionations are possibly related to the fluid-mobility of W in subduction zones, but more data are needed to test this hypothesis. Given that most terrestrial igneous rocks are isotopically indistinguishable from chondrites and iron meteorites, core formation on Earth does not seem to have induced a measurable isotopic fractionation for W. However, more data are needed to firmly arrive at this conclusion.

      PubDate: 2017-02-16T07:08:54Z
       
  • The effect of growth rate on uranium partitioning between individual
           calcite crystals and fluid
    • Abstract: Publication date: 5 February 2017
      Source:Chemical Geology, Volume 450
      Author(s): Jeremy M. Weremeichik, Rinat I. Gabitov, Bruno M.J. Thien, Aleksey Sadekov
      Elemental to calcium ratios in calcium carbonate minerals are used to study environmental conditions. In particular, uranium to calcium ratio (U/Ca) has been proposed as a proxy for seawater carbonate ion concentration (CO3 2−) and seawater pH. This work is focused on the evaluation of growth rate and its effect on uranium partitioning between calcite and fluid. We grew inorganic calcite (by diffusion of CO2) isothermally from NH4Cl-CaCl2 doped with uranium. The growth rate of calcite (crystal extension rate, V) was monitored by sequentially spiking calcite-precipitating fluids with rare earth element (REE) dopants. The REE was analyzed with secondary ion mass spectrometry (SIMS) at spots matching those where U/Ca was determined. Partition coefficient KU =(U/Ca)calcite /(U/Ca)fluid increases with increasing growth rate (V). Specifically, KU increases from 0.02 to 0.06 when V increases from 0.01 to 0.14nm/s and remains nearly constant at faster rates. Numerical simulations using the growth entrapment model (GEM) and unified uptake kinetics model (UUKM) were undertaken to explain KU-V relationship in the recent data on calcite.

      PubDate: 2017-02-16T07:08:54Z
       
  • Secondary gold structures: Relics of past biogeochemical transformations
           and implications for colloidal gold dispersion in subtropical environments
           
    • Abstract: Publication date: 5 February 2017
      Source:Chemical Geology, Volume 450
      Author(s): Jeremiah Shuster, Frank Reith, Geert Cornelis, John E. Parsons, John M. Parsons, Gordon Southam
      Biogeochemical processes are known to drive the cycling of gold via dissolution/re-precipitation reactions that result in the transformation of gold grain in near-surface environments. In this study, placer gold grains were collected from West Coast Creek, Queensland, Australia and characterized using high-resolution electron microscopy. The outer surface of grains contains 98.1mol% Au and 1.9mol% Ag. Crevices occur on the surface of grains and are filled with organics and clay minerals in which nanometer-size gold colloids and micrometer-sized octahedral gold platelets are embedded. The formation of these secondary gold structures is attributed to gold precipitation by the availability of reducing agents such as microbiota, residual organics and clays. Bacterioform gold contains 74.7mol% Au and 25.3mol% Ag and is also embedded in clay minerals within crevices. From sonicated gold grains, grain surfaces beneath the clay minerals are striated and contain 68.5mol% Au and 31.5mol% Ag. This data suggests that dissolution processes occur at the gold grain interface. Varying sizes of gold colloids on the grains suggest that five ‘episodes’ of gold dissolution/re-precipitation processes occurred; each episode was estimated to be 7.64±4.1years. Therefore, these grains represent 17.9–58.5years of gold cycling and mobilization within this subtropical environment. Furthermore, laboratory experiments involving colloidal gold dispersion demonstrated that iron-oxides and organic material from West Coast Creek sediment adsorbed 94.5% of suspended gold colloids. In conclusion, this study highlights the value of nanophase gold characterization for the interpretation of biogeochemical processes affecting gold grain transformation and mobility in near-surface environments. Importantly, this study is the first to estimate the kinetics of biogeochemical gold cycling with regards to colloidal gold dispersion and re-concentration.

      PubDate: 2017-02-16T07:08:54Z
       
  • Petrogenesis of the Huili Paleoproterozoic leucogranite in the Jiaobei
           Terrane of the North China Craton: A highly fractionated albite granite
           forced by K-feldspar fractionation
    • Abstract: Publication date: 5 February 2017
      Source:Chemical Geology, Volume 450
      Author(s): Youlian Li, Huafeng Zhang, Jinghui Guo, Chaofeng Li
      A Paleoproterozoic leucogranite has been identified petrologically as highly fractionated granite near Huili in the Jiaobei Terrane of the North China Craton. This granite occurs as a 3×10km pluton in the metamorphic Paleoproterozoic Jingshan Group and was dated at 1.86Ga using zircon LA-ICP-MS U-Pb ages. The Huili leucogranite can be subdivided into three groups: groups I and II have normal leucogranite abundances with 2.22–2.57wt.% Na2O, 6.10–6.92wt.% K2O, and 71.68–73.07wt.% SiO2 and group III has a remarkably high Na2O concentration of 6.45–7.23wt.%, a low K2O content of 0.82–1.05wt.%, and 74.21–74.79%wt.% SiO2. Moreover, group III leucogranite comprises slightly lower Th, U, and Sr contents and considerably lower K-feldspar compatible elements, such as Rb, Ba, Pb, and Cs than those of groups I & II. All of these features strongly indicate the K-feldspar fractionation during the magma evolution from groups I and II to group III, which is strongly supported by the Pb isotope analysis. Group III leucogranite has a notably high U/Pb ratio of 0.30 to 0.45 with a normal U content of 1.42 to 2.21ppm and very low Pb content of 4 to 5ppm, indicating a significant K-feldspar fractionation of the magma. Group III leucogranite has high radiogenic Pb isotopes (i.e., 206Pb/204Pb=37.407–116.31), demonstrating that the rocks must have existed in a high U-Pb system for a notably long time. Fortunately, the whole rock Pb isotope values roughly define the Pb-Pb isochrons, not only when all three groups of samples are considered together but also when any individual group is considered. The ages are very close to the formation time of ~1.86Ga indicating the high U/Pb system required by these Pb isotopes must have been initiated during the formation of the Huili leucogranite, which has perfectly confirmed the K-feldspar fractionation during the magma evolution. All samples have consistent εNd values (t =1.86Ga) of −4.0 to −6.2 with two-stage Nd model ages (TDM2) of 2.67 to 2.85Ga. It is concluded that the 1.86Ga Huili leucogranite is highly fractionated, derived from ancient crustal material extracted from the depleted mantle at 2.7–2.8Ga, and experienced significant fractionation of K-feldspar in association with biotite and monazite fractionation during the magmatic evolution. Group III leucogranite is albite granite with albite phenocrysts, whereas groups I & II are K-feldspar granites with K-feldspar phenocrysts. It is here argued that the Huili albite granite (group III leucogranite) is derived from a highly differentiated residual melt separated from the K-rich magma represented by groups I and II leucogranite due to Kf fractionation during magma evolution.

      PubDate: 2017-02-16T07:08:54Z
       
  • The effect of crystal-plastic deformation on isotope and trace element
           distribution in zircon: Combined BSE, CL, EBSD, FEG-EMPA and NanoSIMS
           study
    • Abstract: Publication date: 5 February 2017
      Source:Chemical Geology, Volume 450
      Author(s): Elizaveta Kovaleva, Urs Klötzli, Gerlinde Habler, Benjamin Huet, Yunbin Guan, Dieter Rhede
      Plastically-deformed zircon grains from granulite-facies (Ivrea-Verbano Zone, Southern Alps, Italy) and amphibolite-facies (Tauern Window, Eastern Alps, Austria) shear zones have been investigated. The main focus was on the effects of crystal-plastic deformation on the distribution of trace elements and their isotopes. Zircon grains reveal crystal-plastic deformation patterns in form of (I) gradual bending of the lattice (high density of free dislocations), (II) highly-deformed margins revealing a combination of low- and high-angle boundaries and gradual bending of the lattice, and (III) a low-angle boundary (LAB) network. Chemical and isotopic maps and profiles show that trace elements in zircon are re-distributed in all plastically-deformed domains. Changes in trace element composition along LABs are sometimes revealed by brighter CL and darker BSE signal. LABs and domains of high free-dislocation density have depletion in U, Y, Yb and enrichment in Ce, La and Nd, while Ti and P are either enriched, depleted or remain unaffected, and Hf demonstrates stability. Y and Yb are decreased in concentration across LABs, and have oscillating concentration in domains of high free-dislocation density. Our observations confirm that crystal-plastically deformed domains in zircon act as effective pathways for trace cations. The Pb isotopic system is disturbed by crystal-plastic deformation microstructures, as indicated by relative 207Pb/206Pb ages showing significant discordance in plastically-deformed zircon domains. In deformed domains, a positive correlation between dislocation density and the degree of isotopic age distortion is observed. Fractured or porous domains and domains with high density of (sub)grain boundaries are enriched in common Pb from the matrix, and thus show significantly older relative 207Pb/206Pb ages than pristine domains. In contrast, Pb loss occurs in domains with high free-dislocation density at a distance of 5–10μm from the grain boundary, caused by out-diffusion of radiogenic Pb. This study adds to a growing database on the consequences of crystal-plastic deformation for trace elements and isotopic systems in zircon. We provide additional evidence that such deformation cannot be neglected and may have important implications for zircon geochronology and geochemistry.

      PubDate: 2017-02-16T07:08:54Z
       
  • Evaluation of potential monazite reference materials for oxygen isotope
           analyses by SIMS and laser assisted fluorination
    • Abstract: Publication date: 5 February 2017
      Source:Chemical Geology, Volume 450
      Author(s): Amélie Didier, Benita Putlitz, Lukas P. Baumgartner, Anne-Sophie Bouvier, Torsten W. Vennemann
      Monazite can record several episodes of fluid/rock interaction within a single grain. Coupled in situ determination of age and oxygen isotope composition by SIMS (secondary ion mass spectrometer) may characterize such events. Yet, monazite reference materials for oxygen isotope analyses are scarce. Ion probe (CAMECA IMS 1280) and laser fluorination measurements of δ18O values from four monazites are presented. Monazites (YREEPO4) with variable cheralite [CaTh(PO4)2] and huttonite (ThSiO4) substitutions, which span a wide compositional range covering most magmatic and metamorphic, as well as hydrothermal monazites were selected. Three monazites, Moacyr, Manangoutry and UNIL-Mnz1, have a homogeneous chemical (determined by electron microprobe analysis and back-scattered imaging) and oxygen isotopic composition with uncertainties (2 SD) in the range of 0.3 to 0.4‰ for ion probe measurements. These three samples are thus potentially suitable as reference materials, and sample UNIL-Mnz1 is available for inter-laboratory comparison. Sample UNIL-Mnz2 is less homogeneous, but it is still useful for internal monitoring. Uncertainties (2 SD) on the ion probe data are in the range of 0.5 to 0.6‰. Using the investigated monazites as well as literature data, a new calibration curve has been established to account for instrumental mass fractionation (IMF) due to the solid solution in natural monazites. The data illustrate that the IMF is primarily a function of the YREEPO4 content, while the influence of the Th-content seems to be less important. The δ18O values for the four studied monazites have been determined by laser fluorination (LF). They are as follows (1 SD): 1.5±0.1‰ for Moacyr, 10.2±0.1‰ for Manangoutry, 8.5±0.2‰ for UNIL-Mnz1, and 9.5±0.3‰ for the slightly less homogenous UNIL-Mnz2. The analytical protocol adopted here results in consistently high yields (>90%) and reproducible values, and hence it is argued that LF is a viable method for the calibration of phosphate reference materials.

      PubDate: 2017-02-16T07:08:54Z
       
  • Reassessment of the Raman CO2 densimeter
    • Abstract: Publication date: 5 February 2017
      Source:Chemical Geology, Volume 450
      Author(s): H.M. Lamadrid, L.R. Moore, D. Moncada, J.D. Rimstidt, R.C. Burruss, R.J. Bodnar
      Raman spectroscopy has proven to be an effective tool to confirm the presence and abundance of CO2 in fluid and melt inclusions. The Raman method for quantifying CO2 abundance is based on the observation that the distance between the two Raman bands comprising the Fermi diad varies systematically with CO2 density. In recent years, several Raman densimeters have been developed by different research groups to determine the density of CO2 in fluid and melt inclusions. The different densimeters that have been proposed predict different densities for the same Fermi diad splitting, leading to large differences in estimated CO2 contents for inclusions, depending on which densimeter is used to interpret the Raman data. In this study, we examine potential causes for variations in the various densimeters and show that these differences are mainly the result of using different Raman instruments and settings, different collection parameters, and different analytical methods. Twelve experiments were conducted to test the variability associated with changing instrumental and analytical conditions, as well as to understand the differences between the various densimeters, using three different Raman instruments, with different laser sources and dispersion gratings. In all of the experiments, the splitting of the Fermi diad of CO2 and CO2 density at pressures from the liquid-vapor curve (6.0MPa to 0.06MPa) at ambient temperature (~22°C) was calibrated using a high-pressure optical cell. The results show a consistent behavior whereby all analytical configurations show parallel trends in terms of the variation in Fermi diad splitting as a function of CO2 density. The slopes of the lines representing the variation in Fermi diad splitting as a function of CO2 density, as well as low density (pressure) data from other densimeters (Kawakami et al., 2003; Yamamoto and Kagi, 2006; Song et al., 2009; Fall et al., 2011; Wang et al., 2011) are remarkably similar, with a variation of about ~10% and a standard deviation of 3%. The differences observed in all densimeters, including previously published densimeters and the 12 experiments from this study, are most likely a function of variations in instrumentation, laser excitation wavelength, gratings, and analytical protocols used during the experimental calibration of the splitting of the Fermi diad. Based on results of this study, we recommend against using any published densimeter to interpret Raman data collected using an instrument other than that on which the calibration is based, and suggest that researchers develop a calibration that is applicable and specific to their instrument and data collection protocol.

      PubDate: 2017-02-16T07:08:54Z
       
  • Sulfur isotope fractionation in pyrite during laser ablation: Implications
           for laser ablation multiple collector inductively coupled plasma mass
           spectrometry mapping
    • Abstract: Publication date: 5 February 2017
      Source:Chemical Geology, Volume 450
      Author(s): Zhi-Yong Zhu, Shao-Yong Jiang, Cristiana L. Ciobanu, Tao Yang, Nigel J. Cook
      This study reports a detailed evaluation of how key parameters of operation influence the measurement of sulfur isotopes using laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). Sulfur isotopes are observed to display a fractionation up to 2‰ δ34S during analysis of pyrite with different laser parameters using a 193nm ArF excimer laser. In order to understand why the laser parameters affect S isotope fractionation when measuring S isotopes in pyrite, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) techniques were used to characterize debris formed during the ablation of pyrite, i.e., morphology and speciation of phases. The results show that pyrite decomposes to two phases: ball-like troilite (FeS) and a sulfur-rich floc-like agglomeration surrounding the troilite. The measured δ34S values vary due to the different proportions of troilite balls and the floc-like material generated under different laser parameters. The proportion of troilite and S was evaluated with a LA-(Quadrupole)-ICP-MS through direct comparison of the counts per second (CPS) ratio of 56Fe to 32S. In contrast to pyrite, natural pyrrhotite shows no decomposition process and the particle size of the debris from pyrrhotite is nearly 10 times larger than that of pyrite (~5μm for pyrrhotite compared to <1μm for pyrite). Therefore, a biased analysis of pyrite may happen using laser ablation although this problem can be minimized using high raster velocity. Last but not least, we provide a case study of S isotope mapping using high raster velocity, which extends the application of the in-situ S isotope analysis technique. The results here carry implications for the choice of settings needed to obtain accurate LA-MC-ICP-MS S-isotope maps of pyrite.
      Graphical abstract image

      PubDate: 2017-02-16T07:08:54Z
       
  • Contrasting particle size distributions and Fe isotope fractionations
           during nanosecond and femtosecond laser ablation of Fe minerals:
           Implications for LA-MC-ICP-MS analysis of stable isotopes
    • Abstract: Publication date: 5 February 2017
      Source:Chemical Geology, Volume 450
      Author(s): Xin-Yuan Zheng, Brian L. Beard, Seungyeol Lee, Thiruchelvi R. Reddy, Huifang Xu, Clark M. Johnson
      Laser ablation coupled to a multi-collector inductively coupled mass spectrometer (LA-MC-ICP-MS) is a promising tool for in situ analysis of metal and metalloid stable isotope ratios. Potential isotopic fractionation associated with laser ablation may, however, cause biased sampling of the substrate, posing a major challenge for precise and accurate isotope ratio measurements. To better characterize the nature of laser ablation induced isotopic fractionation, this study compared particle morphologies, sizes, and size-dependent Fe isotope fractionations produced by ablation of a suite of semi-conductive samples, including natural Fe oxide, sulfide, and carbonate minerals, under various conditions using a 193nm ArF nanosecond (ns) laser and a 266nm Ti:sapphire femtosecond (fs) laser. Ablation-produced particles were sorted based on aerodynamic size using a cascade impactor, and Fe isotope compositions of size-sorted particles were then measured offline using conventional solution nebulization ICP-MS to quantify isotopic fractionation produced by the laser ablation. Particle morphology and size distributions produced by ns-laser ablation are more substrate and fluence dependent as compared to fs-laser ablation, resulting from the thermal nature of ns-laser ablation. Often, a higher proportion of the ablated Fe mass resides in particles with large aerodynamic sizes during ns-laser ablation as compared to fs-laser ablation, posing a potential difficulty for LA-ICP-MS analysis due to the increased possibility of incomplete ionization of large particles. Significant size-dependent Fe isotope fractionations of up to several per mil can occur during both ns- and fs-laser ablation, highlighting the importance of quantitative transport of particles to the ICP-MS for accurate Fe isotope analysis. Size-dependent Fe isotope fractionation observed for fs-laser ablation of all Fe minerals can be explained by particle formation through a condensation model, but multiple processes need to be considered to explain the observed Fe isotope fractionation during ns-laser ablation. Mass-balance calculations suggest that ns-laser ablation does not sample magnetite stoichiometrically for Fe isotope compositions at low fluence (1J/cm2), but does at higher fluences for all minerals. In contrast, fs-laser ablation always provides stoichiometric sampling for Fe isotopes regardless of fluence. Results of this study demonstrate that ns-laser ablation is substrate- and fluence-dependent, resulting in variable particle size distributions and Fe isotope fractionations, and possible non-stoichiometric sampling of semi-conductive samples for Fe isotope analysis. Instead, fs-laser ablation largely minimizes the substrate and fluence dependence, providing more consistent ablation.

      PubDate: 2017-02-16T07:08:54Z
       
  • CO2 flow during orogenic gravitational collapse: Syntectonic decarbonation
           and fluid mixing at the ductile-brittle transition (Lavrion, Greece)
    • Abstract: Publication date: 5 February 2017
      Source:Chemical Geology, Volume 450
      Author(s): Christophe Scheffer, Alexandre Tarantola, Olivier Vanderhaeghe, Thomas Rigaudier, Adonis Photiades
      The knowledge of CO2 transfer between carbonate reservoirs is a key issue to understanding climate evolution through geologic times. Convergent plate boundaries represent major zones for carbon recycling via burial of carbonates. In this paper, we document syntectonic decarbonation of marble at the ductile-brittle transition during the emplacement of the low-angle mylonitic to cataclastic detachment exposed in the Lavrion peninsula along the western border of the Attico-Cycladic Metamorphic Complex. This process is evidenced by the presence, in a quartz vein boudinaged and transposed in the mylonitic marble, of CO2-rich fluid inclusions (i) dismembered and deformed along quartz subgrain boundaries, (ii) within deformation lamellae and (iii) in planes crosscutting subgrains. Microstructural analysis, stable isotope equilibrium and VX properties show that CO2 results from syntectonic decarbonation under a 70–115°C·km−1 thermal gradient during exhumation accommodated by regional NNE-SSW extension. We propose that decarbonation and CO2 release under HT/LP conditions were triggered by the increase in temperature caused by thermal relaxation of the thickened crust further enhanced by the intrusion of a granodiorite pluton during Miocene gravitational collapse of the Hellenic orogenic belt. Subsequent circulation of surface-derived fluid equilibrated with carbonaceous material of organic origin above the ductile-brittle transition led to precipitation of low 13C carbonates. This study points that the production of CO2 by syntectonic decarbonation of marble layers, representing tectonically accreted carbonates along convergent plate boundaries, should be considered in the carbon cycle and might thus impact the climate.

      PubDate: 2017-02-16T07:08:54Z
       
  • The reactivity of Fe(II) associated with goethite formed during short
           redox cycles toward Cr(VI) reduction under oxic conditions
    • Abstract: Publication date: Available online 5 February 2017
      Source:Chemical Geology
      Author(s): Elizabeth J. Tomaszewski, Seungyeol Lee, Jared Rudolph, Huifang Xu, Matthew Ginder-Vogel
      Chromium (Cr) is a toxic metal that causes a myriad of health problems and enters the environment as a result of anthropogenic activities and/or natural processes. The toxicity and solubility of chromium is linked to its oxidation state; Cr(III) is poorly soluble and relatively nontoxic, while Cr(VI) is soluble and a known carcinogen. Solid Fe(II) in iron-bearing minerals, such as pyrite, magnetite, and green rusts, reduce the oxidation state of chromium, reducing its toxicity and mobility. However, these minerals are not the only potential sources of solid-associated Fe(II) available for Cr(VI) reduction. For example, ferric (Fe(III)) (hydr)oxides, such as goethite or hematite, can have Fe(II) in the solid without phase transformation; however, the reactivity of Fe(II) within Fe(III) (hydr)oxides with contaminants, has not been previously investigated. Here, we cyclically react goethite with dissolved Fe(II) followed by dissolved O2, leading to the formation of reactive Fe(II) associated with goethite. In separate reactors, the reactivity of this Fe(II) is probed under oxic conditions, by exposure to chromate (CrO4 2−) after either one, two, three or four redox cycles. Cr is not present during redox cycling; rather, it is introduced to a subset of the solid after each oxidation half-cycle. Analysis of X-ray absorption near edge structure (XANES) spectra reveals that the extent of Cr(VI) reduction to Cr(III) depends not only on solid Fe(II) content but also surface area and mean size of ordered crystalline domains, determined by BET surface area analysis and X-ray diffraction (XRD), respectively. Shell-by-shell fitting of the extended X-ray absorption fine structure (EXAFS) spectra demonstrates chromium forms both single and double corner sharing complexes on the surface of goethite, in addition to sorbed Cr(III) species. Finally, transmission electron microscope (TEM) imaging and X-ray energy-dispersive spectroscopy (EDS) illustrate that Cr preferentially localizes on the (100) face of goethite, independent of the number of redox cycles goethite undergoes. This work demonstrates that under oxic conditions, solid Fe(II) associated with goethite resulting from rapid redox cycling is reactive and available for electron transfer to Cr(VI), suggesting Fe(III) (hydr)oxides may act as reservoirs of reactive electron density, even in oxygen saturated environments.
      Graphical abstract image

      PubDate: 2017-02-16T07:08:54Z
       
  • Probing the aluminum complexation by Siberian riverine organic matter
           using solid-state DNP-NMR
    • Abstract: Publication date: Available online 4 February 2017
      Source:Chemical Geology
      Author(s): Frédérique Pourpoint, Joëlle Templier, Christelle Anquetil, Hervé Vezin, Julien Trébosc, Xavier Trivelli, François Chabaux, Oleg S. Pokrovsky, Anatoly S. Prokushkin, Jean-Paul Amoureux, Olivier Lafon, Sylvie Derenne
      In a Siberian river, the concentrations of chemical species vary with the hydrological regime. Dissolved Organic Matter (DOM) and aluminum ions show a parallel trend in the course of the hydrological year. However, the speciation of aluminum in this natural environment remains an open question. We propose here a combination of spectroscopic techniques to investigate the proximity between the aluminum atoms and DOM. First, one-dimensional (1D) solid-state Nuclear Magnetic Resonance (NMR), Electron Paramagnetic Resonance (EPR) and 1D and 2D solution NMR spectra were acquired, providing a clear overview of the DOM composition. Second, the sensitivity enhancement yielded by Dynamic Nuclear Polarization enabled the NMR detection of proximities between the 27Al and 13C nuclei. Hence, we show that 8.3±1.3% of the carboxylate groups observed by NMR are connected to the Al3+ ions in the DOM sample. We here demonstrate for the first time how advanced solid-state NMR methods can provide key information about the localization of aluminum in such complex natural materials.
      Graphical abstract image

      PubDate: 2017-02-16T07:08:54Z
       
  • In-situ trace element and Fe-isotope studies on magnetite of the
           volcanic-hosted Zhibo and Chagangnuoer iron ore deposits in the Western
           Tianshan, NW China
    • Abstract: Publication date: Available online 3 February 2017
      Source:Chemical Geology
      Author(s): T. Günther, R. Klemd, X. Zhang, I. Horn, S. Weyer
      The Carboniferous Zhibo and Chagangnuoer iron deposits are situated within a caldera centre and along the flank of the same volcanic edifice, respectively, in the Awulale Iron Metallogenic Belt of the Western Tianshan orogen. Several stratiform 10 to 100m large, tabular to lenticular shaped magnetite-dominated ore-bodies occur in (trachy-) andesitic to rhyolitic host rocks. The magnetite mineralization mainly occurs as massive iron ores, partly with columnar-network or flow textures, and as disseminated magnetite ores. Trace element and isotope investigations of the different ore types reveal two major groups of magnetite: Group I, represented by the massive, partly brecciated ores from both deposits, is enriched in Ti, V, Ni, and HFSE such as Y, with concentrations similar to Iron Oxide-Copper-Gold (IOCG) ores. The δ56Fe values (up to 0.4‰) support an ortho-magmatic origin corresponding with an isotopic source calculation at ~800°C. Positive correlations between Fetotal and δ56Fe (from +0.4‰ to −0.1‰) and incompatible trace element contents (e.g. Si, Al, Nb, Ti and Y) in Group I magnetite are interpreted to be the consequence of a Raleigh-type fractionation process. Decreasing V, Ni and Mn values indicate changing fO2 conditions at the time of ore genesis. Group II, which is represented by the disseminated ores from Chagangnuoer, is - compared to Group I - relatively depleted in elements like Ti, V, Ni and Y and further spans a dominant δ56Fe range from about 0‰ to −0.5‰. These textural and chemical characteristics and the garnet-actinolite-diopside-epidote-carbonate-K-feldspar paragenesis are in accordance with hydrothermal Fe-skarn ores. The similar multi-element patterns of magnetite from all investigated samples, the overlapping δ56Fe ratios of the same massive ore-type from Zhibo and Chagangnuoer and the close proximity of both deposits indicate a common source of Fe-enrichment for the different iron ore types. In contrast to the ortho-magmatic Group I magnetite, reverse trace element trends with decreasing δ56Fe ratios (from 0‰ to −0.5‰) among the disseminated ores cannot simply be explained by a straightforward Raleigh fractionation or alteration processes. Therefore, a bimodal formation model is suggested for the Group II magnetite formation, including a partial remobilization of iron from the proximal, ortho-magmatic ore bodies and a subsequent distal re-precipitation. These processes were driven by late-stage hydrothermal fluids, which originated from deeper- seated granitic/granodioritic intrusions in the immediate vicinity.

      PubDate: 2017-02-16T07:08:54Z
       
  • Stromboli volcanic activity variations inferred from observations of fluid
           geochemistry: 16years of continuous monitoring of soil CO2 fluxes
           (2000–2015)
    • Abstract: Publication date: Available online 3 February 2017
      Source:Chemical Geology
      Author(s): Salvatore Inguaggiato, Fabio Vita, Marianna Cangemi, Agnes Mazot, Aldo Sollami, Calderone Lorenzo, Sabina Morici, Mariana P. Jacome Paz
      Stromboli volcano is characterized by a persistent strombolian activity that was interrupted by effusive eruptions in 1985, 2002–2003, 2007, and 2014. The considerable amount of soil CO2 flux data acquired by the continuous geochemical network installed in the summit and peripheral areas of Stromboli Island have allowed us to thoroughly investigate and to model the plumbing system. This study analyzed 16years of soil CO2 fluxes from the summit area, which showed a wide range of values (from 2000 to 85,000gm−2 day−1). A set of discontinuous pCO2 data (ranging from 0.1 to 0.35atm) from a dedicated thermal well (COA) drilled in the Stromboli village has been taken into account for the peripheral degassing evaluation. One of the major accomplishments of this study is the confirmation of the previous formulated geochemical model based on of the soil CO2 fluxes database and the recent effusive eruptions. The analysis of soil summit degassing data recorded at the summit STR02 station throughout the monitoring period (2000–2015) revealed that the log(CO2 flux) values conformed to a polimodal distribution with different mean values of CO2 fluxes being recorded for each sub-period during the different effusive eruptions. The three selected periods (2000–2004, 2005–2010, and 2011–2015) were characterized by considerable increases in soil CO2 degassing coinciding with the periods of effusive eruptions, indicating volatiles overpressure and disequilibrium of the volcanic system. Moreover, the CO2 output evaluations based on soil CO2 fluxes and pCO2 values of thermal waters (COA), have showed a long increasing trend both in the summit and peripheral areas. From 2007 to 2014, culminating in the last effusive eruption (August–November 2014).

      PubDate: 2017-02-16T07:08:54Z
       
  • Whole rock basalt alteration from CO2-rich brine during flow-through
           experiments at 150°C and 150bar
    • Abstract: Publication date: Available online 3 February 2017
      Source:Chemical Geology
      Author(s): Andrew J. Luhmann, Benjamin M. Tutolo, Chunyang Tan, Bruce M. Moskowitz, Martin O. Saar, William E. Seyfried
      Four flow-through experiments at 150°C were conducted on intact cores of basalt to assess alteration and mass transfer during reaction with CO2-rich fluid. Two experiments used a flow rate of 0.1ml/min, and two used a flow rate of 0.01ml/min. Permeability increased for both experiments at the higher flow rate, but decreased for the lower flow rate experiments. The experimental fluid (initial pH of 3.3) became enriched in Si, Mg, and Fe upon passing through the cores, primarily from olivine and titanomagnetite dissolution and possibly pyroxene dissolution. Secondary minerals enriched in Al and Si were present on post-experimental cores, and an Fe2O3-rich phase was identified on the downstream ends of the cores from the experiments at the lower flow rate. While we could not specifically identify if siderite (FeCO3) was present in the post-experimental basalt cores, siderite was generally saturated or supersaturated in outlet fluid samples, suggesting a thermodynamic drive for Fe carbonation from basalt-H2O-CO2 reaction. Reaction path models that employ dissolution kinetics of olivine, labradorite, and enstatite also suggest siderite formation at low pH. Furthermore, fluid-rock interaction caused a relatively high mobility of the alkali metals; up to 29% and 99% of the K and Cs present in the core, respectively, were preferentially dissolved from the cores, likely due to fractional crystallization effects that made alkali metals highly accessible. Together, these datasets illustrate changes in chemical parameters that arise due to fluid-basalt interaction in relatively low pH environments with elevated CO2.

      PubDate: 2017-02-16T07:08:54Z
       
  • Chemostratigraphy and palaeo-environmental characterisation of the
           Cambrian stratigraphy in the Amadeus Basin, Australia
    • Abstract: Publication date: Available online 24 January 2017
      Source:Chemical Geology
      Author(s): Susanne Schmid
      The sedimentary succession of the central Australian Amadeus Basin consists of Neoproterozoic to Carboniferous sedimentary rocks and contains shallow marine, subtidal carbonates of Middle to Late (Series 2 to Furongian) Cambrian age. A combination of sequence stratigraphy, geochemistry and mineralogy shows a transgressive 2nd order cyclicity deposited between ~511–490Ma and a change from arid, low energy to humid, high energy depositional environments. This is reflected in an initially evaporitic sequence with upward decreasing ha lite and anhydrite abundance and transition from oxygenated to anoxic conditions, reflected by the Fe mineral species change from hematite to pyrite during transgression. Sequence boundaries of several 3rd order cycles consisting of HST carbonate rocks and LST siltstones, correlate with globally recognised sequence boundaries linked to the inferred eustatic sea level record for the upper two series of the Cambrian System. The carbon isotope record for this ~1400m thick succession in combination with biostratigraphic age correlation allowed the identification of the globally recognised Steptoean Positive Carbon Isotope Excursion (SPICE), Drumian Carbon Isotope Excursion (DICE) and Redlichiid-Olenellid Extinction Carbon Isotope Excursion (ROECE).

      PubDate: 2017-01-27T09:00:18Z
       
  • Differences in groundwater and chloride residence times in saline
           groundwater: The Barwon River Catchment of Southeast Australia
    • Abstract: Publication date: Available online 24 January 2017
      Source:Chemical Geology
      Author(s): William Howcroft, Ian Cartwright, L. Keith Fifield, D.I. Cendón
      The residence times of groundwater and chloride and the processes contributing to the development of saline (total dissolved solids (TDS) up to 45,379 mg/L) groundwater within the Barwon River Catchment of southeast Australia were investigated using major ion, stable isotope (δ18O, δ2H, and δ13C) and radioactive isotope (3H, 14C, 36Cl) geochemistry. The elevated groundwater salinity in the region is primarily due to evapotranspiration and recycling of solutes in saline lakes with minor contributions from weathering of halite, silicate and calcite minerals. Groundwater residence times estimated from 14C vary from modern to ~20 ka; for groundwater with lower 14C activities, the estimated residence times vary significantly depending on the assumed flow model and the 14C activity of recharge. Chloride residence times downgradient of Lake Murdeduke (a saline through-flow lake in the centre of the catchment) are greater than the corresponding groundwater residence times due to the recycling of Cl within the lake. Precise estimates of chloride residence time could not be determined using 36Cl due to R36Cl in precipitation being lower than that of groundwater. This is most likely due to R36Cl values in rainfall having been higher in the past than they are at present due to climate variability. δ18O, δ2H, and δ13C values also suggest that the region has experienced increasingly more evaporative conditions with time. The results of this study demonstrate that, while Cl is a useful tracer of hydrological processes, it must be applied carefully in arid and semi-arid regions of the world. In particular, recharge rates calculated using chloride mass balance may be underestimated where recycling of Cl has occurred.

      PubDate: 2017-01-27T09:00:18Z
       
  • Temperature-related changes of Ca and P release in synthesized
           hydroxylapatite, geological fluorapatite, and bone bioapatite
    • Abstract: Publication date: Available online 23 January 2017
      Source:Chemical Geology
      Author(s): Weikun Chen, Quanzhi Wang, Shiting Meng, Ping Yang, Liu Jiang, Xiang Zou, Zhen Li, Shuijin Hu
      Solubility of apatite is highly addressed in mineralogical and material studies. Heating is one of the major processes in apatite industry. In this study, synthesized hydroxylapatite (HAp), geological fluorapatite (FAp), and bone bioapatite (BAp) were heated at various temperature (100–900°C) for analyses. The mineralogy and solubility of the three apatites were analyzed by XRD, ATR–IR, and ICP. Release of Ca and P in water for BAp reach the maximum when heated at 200°C, i.e., 0.215mmol/L for Ca and 0.106mmol/L for P. The value is higher than the maximum values (heated at 900°C) of solubility for HAp and FAp. The heating temperature at 600°C is a re-crystallization point for all the three types of apatites. Especially, the crystallinity of BAp is significantly elevated at >600°C. Phase of geological FAp is relatively stable during heating up to 900°C. Phase of β-TCP is present when heating HAp at 800 to 900°C. In addition, BAp is transformed to the resemblance of HAp. However, no β-TCP was detected for BAp during heating between 800 and 900°C, which is probably due to its Ca-deficiency. This study elucidates the correlation of phase changes of BAp and its solubility during heating, which sheds the light on its application as materials and fertilizer.

      PubDate: 2017-01-27T09:00:18Z
       
  • Across-arc geochemical variation in the Jurassic magmatic zone, Southern
           Tibet: Implication for continental arc-related porphyry CuAu
           mineralization
    • Abstract: Publication date: Available online 19 January 2017
      Source:Chemical Geology
      Author(s): Rui Wang, Reza Tafti, Zeng-qian Hou, Zhi-chao Shen, Na Guo, Noreen J. Evans, Heejin Jeon, Qiu-yun Li, Wei-kai Li
      The Jurassic Gangdese arc hosts the giant Xietongmen (Xiongcun) porphyry CuAu magmatic-hydrothermal centre. In order to understand the tectonomagmatic framework of the arc and its controls on the origin and temporal-spatial distribution of porphyry mineralization we conducted extensive major and trace, and isotopic analysis on a suite of samples collected regionally. Geochemical variations were observed across the Jurassic arc from south to north, controlled by northward subduction of the Neo-Tethyan oceanic plate and overlying Lhasa lithosphere. In the frontal arc (latitude 29°–29.55°N), the magmas had high Ba/La and Ba/Th ratios, high εNd(t) (+3 to +7) values, and their zircons have high εHf(t) (+10 to +18) values, and heterogeneous O isotopic compositions (δ18O=+3.6 to +6.6‰). The low titanium-in-zircon temperature and zircon saturation temperature (both average=~700°C) suggest that a greater flux of dehydration fluids from the subducting slab triggered partial melting of the mantle wedge in the southern arc. High zircon Eu/Eu* and Ce/Nd ratios (>0.4 and >20, respectively) indicate highly oxidized, fertile magmas, from which the Xietongmen porphyry CuAu deposit formed. In contrast, the interior arc (north of latitude 29.55°), close to the ancient Tibetan basement, shows low Ba/La and Ba/Th ratios, low εNd(t) (−0.1 to +2.5) and zircon εHf(t) (+2 to +12) values, and less heterogeneous zircon O isotopic compositions with higher δ18O values. Higher titanium-in-zircon and zircon saturation temperatures (mostly >750°C) suggest that less water was involved during the partial melting of mantle wedge in this region. Low zircon Eu/Eu* and Ce/Nd ratios (mostly <0.4 and <20, respectively) point towards less oxidized magmatic conditions. We conclude that across-arc geochemical and fluid-flux variations controlled the formation and spatial distribution of Jurassic porphyry deposits in the Gangdese belt.

      PubDate: 2017-01-27T09:00:18Z
       
  • Metasomatic changes during periodic fluid flux recorded in grandite garnet
           from the Weondong W-skarn deposit, South Korea
    • Abstract: Publication date: Available online 19 January 2017
      Source:Chemical Geology
      Author(s): Changyun Park, Yungoo Song, Il-Mo Kang, Jaecheon Shim, Donghoon Chung, Chan-Soo Park
      The Weondong skarn deposit primarily consists of grandite (andradite-grossular) garnets, which exhibit poikilitic/intergrowth textures and compositional zoning in metasomatism. The garnets can be classified into three main types based on their textural characteristics and rare earth element (REE) concentrations: (1) exoskarn garnet near quartz porphyry with a poikilitic texture and oscillatory zoning (core: And31–50, rim: And54–63) (T1 garnet), (2) exoskarn garnet near quartz porphyry with intergrowths (core: And55–83, mantle: And55–59, rim: And77–97) and poikilitic textures (T2 garnet), and (3) vein-hosted garnet with reverse zoning (core: And70–96, rim: And31–36) and epitaxial growth on distinct cores (T3 garnet). The textural features of the garnets are considered to have been caused by periodic fluctuations. The chondrite-normalized REE patterns of the garnet types show that the Al-rich garnets are HREE-enriched and formed at equilibrium with slow growth rates according to the kinetics of the garnet's growth. In contrast, Fe-rich garnets are slightly LREE-enriched and HREE-depleted. In terms of the kinetics of the garnet's growth, the Fe-rich garnets grew rapidly from externally derived fluids during infiltration metasomatism. The enriched W contents in the grandite garnets also indicate the type of growth mechanism, the infiltration process, and a specific skarn environment (such as advective metasomatism). Furthermore, each textural type of garnet was influenced differently by fluid/rock interactions, the magnitude of the fluid flow and the fluid compositions in the skarn system. Consequently, periodic fluid fluctuation in the skarn system affected the morphology and composition of individual garnet grains. Thus, the geochemical features of the grandite garnet from the Weondong polymetallic deposit provide key information to understand the evolution of the skarn deposit and its metasomatic history.
      Graphical abstract image

      PubDate: 2017-01-27T09:00:18Z
       
  • Properties of lithium under hydrothermal conditions revealed by in situ
           Raman spectroscopic characterization of Li2O-SO3-H2O (D2O) systems at
           temperatures up to 420°C
    • Abstract: Publication date: Available online 18 January 2017
      Source:Chemical Geology
      Author(s): Xiaolin Wang, Xiaoyu Wang, I-Ming Chou, Wenxuan Hu, Ye Wan, Zhen Li
      Lithium (Li) is an important component of hydrothermal fluids, especially submarine hydrothermal fluids. Investigation of the species and ion complexation of Li+ at elevated temperature and pressure can improve our knowledge on the behavior of Li under hydrothermal conditions. In this study, in situ optical and Raman spectroscopic experiments were conducted on the Li2SO4-H2SO4-H2O system and its D2O analogue at temperatures≤420°C. An unexpected liquid-liquid phase separation (immiscibility) was observed at temperatures above 336.5°C; the aqueous phase was separated into a sulfate-rich heavy liquid phase and a sulfate-poor light liquid phase at vapor-saturation pressures. The liquid-liquid phase separation temperature decreased as the Li2SO4 concentration increased in dilute solutions (≤1.25 m), increased as the Li2SO4 concentration increased in concentrated solutions (>1.25 m), and exhibited a lower critical solution temperature (LCST) at ~336.5°C. The presence of excess H2SO4 (or D2SO4) increased the liquid-liquid phase separation temperature at a constant Li2SO4 concentration. Liquid-liquid phase separation is common in organic-bearing solutions, and LCST is considered to be a macro-scale property of polymer solutions, indicating complicated ion pairing between Li+ and SO4 2− at high temperature. In situ Raman spectra of the v 1(SO4 2−) band indicated the presence of “free” SO4 2−, LiSO4 −, possible Li2SO4 0 and other poly-ion associations. The Li+-SO4 2− association increased with increasing temperature at a constant Li2SO4 concentration. These results indicate that Li+ can form contact ion pairs with ligands despite its strong hydration tendency. In submarine hydrothermal systems, various contact ion pairs should be important Li species during the high-temperature leaching of Li from basalts (e.g., >200°C). The formation of strong and various Li+-SO4 2− pairs can enhance the leaching of Li because the mobility and diffusion of the Li+-SO4 2− complex has been reported to be stronger than that of “free” ions. During the low-temperature (e.g., <150°C) uptake of Li+ into secondary minerals (e.g., clays), free Li+ and outer-sphere ion pairs are the dominant Li+ species. Contact ion pairs are also important Li species during the hydrothermal alteration/precipitation of Li-rich minerals in pegmatite systems. Moreover, liquid-liquid phase separation was found to play important roles in the formation of low-temperature minerals (e.g., calcite). However, similar crystallization pathways have rarely been documented for inorganic systems under high temperatures. Anhydrous Li2SO4 was observed to precipitate within the dense liquid phase at ≥360°C in 1.5 m Li2SO4, confirming that the immiscible dense liquid phase can serve as a precursor for the crystallization of sulfates and other minerals under hydrothermal conditions. The possible processes for the dense-liquid-based crystallization by particle attachment can be summarized as (1) strong and reversible ion association induces liquid-liquid phase separation, (2) nucleation and crystallization within the dense liquid phase, and (3) continued growth on the surface of early formed solid phase. Submarine hydrothermal fluids often contain low-dielectric components such as methane and other organic components, which lower the dielectric constant of hydrothermal fluids and promote ion association. Therefore, liquid-liquid phase separation may occur in submarine hydrothermal fluids and can play important roles in the precipitation of sulfates and other minerals.

      PubDate: 2017-01-27T09:00:18Z
       
  • Geochemical evidence for initiation of the modern Mekong delta in the
           southwestern South China Sea after 8Ma
    • Abstract: Publication date: Available online 16 January 2017
      Source:Chemical Geology
      Author(s): Chang Liu, Peter D. Clift, Richard W. Murray, Jerzy Blusztajn, Thomas Ireland, Shiming Wan, Weiwei Ding
      Sedimentary records in the southwestern South China Sea reflect the evolving erosion and drainage systems that have operated in Southeast Asia during the Neogene. Analyses of the chemistry and clay mineral composition of sediments from International Ocean Discovery Program (IODP) Site U1433 allow us to examine these processes over the last 17Ma. Sediment older than 8Ma was deposited relatively slowly. Sr and Nd isotopes indicate a variable provenance with sequences of less and more altered material accompanied by strong changes in the proportion of smectite. Sediment flux was probably from Indochina, as well as from a more primitive volcanic source, most likely the Palawan ophiolite and/or Luzon. Sediments younger than 8Ma show a more stable Sr and Nd isotope character, indicating sources close to those seen in the modern Mekong River, although with some influx from smaller rivers draining the Indochina margin especially from 4 to 8 Ma. Our data are consistent with seismic estimates for an onset to the Mekong in its present location after 8Ma, following an avulsion from the Gulf of Thailand.

      PubDate: 2017-01-27T09:00:18Z
       
  • Application of a handheld X-ray fluorescence spectrometer for real-time,
           high-density quantitative analysis of drilled igneous rocks and sediments
           during IODP Expedition 352
    • Abstract: Publication date: Available online 15 January 2017
      Source:Chemical Geology
      Author(s): J.G. Ryan, J.W. Shervais, Y. Li, M.K. Reagan, H.Y. Li, D. Heaton, M. Godard, M. Kirchenbaur, S. Whattam, J.A. Pearce, T. Chapman, W. Nelson, J. Prytulak, K. Shimizu, K. Petronotis
      Handheld energy dispersive portable X-ray spectrometers (pXRF) are generally designed and used for qualitative survey applications. We developed shipboard quantitative analysis protocols for pXRF and employed the instrument to make over 2000 individual abundance measurements for a selection of major and trace elements on over 1200m of recovered core during the eight weeks of the International Ocean Discovery Program (IODP) Expedition 352 to the Izu-Bonin forearc. pXRF analytical performance, accuracy and precision were found to be the same on powdered rock samples and on freshly cut rock surfaces, and sample results were similar within error to measurements made via shipboard ICP-OES analysis save at low abundance levels for a few elements. Instrument performance was optimal for elements between Z=19 and Z=40, and the system yielded reproducible data for K, Ca, Ti, V, Cr, Mn, Fe, Cu, Zn, Rb, Sr, and Zr on both powdered samples and rock surfaces. Working curves developed via pXRF measurement of a suite of geologic standard reference materials and well-characterized lavas permitted accurate quantitative measurements for many of the examined elements on both sample powders and rock surfaces. Although pXRF has been sporadically employed on previous cruises, Expedition 352 is the first time a detailed, high-density chemostratigraphy of recovered core samples was collected using pXRF measurements of rock core surfaces. These high-resolution data allowed the recognition of chemically distinct eruptive units in near real-time. The rapid identification of geochemical trends vastly improved our selection of samples for shipboard and shore-based analysis, permitted a more comprehensive interpretation of our Expedition results, and provided key decision-making information for drilling operations.

      PubDate: 2017-01-27T09:00:18Z
       
  • Evidence of multiple halogen sources in scapolites from iron
           oxide-copper-gold (IOCG) deposits and regional NaCl metasomatic
           alteration, Norrbotten County, Sweden
    • Abstract: Publication date: Available online 13 January 2017
      Source:Chemical Geology
      Author(s): Nelson F. Bernal, Sarah A. Gleeson, Martin P. Smith, Jaime D. Barnes, Yuanming Pan
      Scapolites from barren regional NaCl metasomatic assemblages (RM), iron oxide-copper-gold deposits (IOCG), scapolite altered metabasic rocks (IOCG-M), and from IOCG-proximal alteration/Na-Skarns (IOCG-PS) from Norrbotten County in Northern Sweden have been analysed for halogen content and Cl stable isotope composition. The aim of the study was to constrain the source of halogens within alteration assemblages, and to investigate the possible fractionation of Cl isotopes between scapolite and the hydrothermal fluid. Scapolite separates were analysed for Cl, Br, and major oxide concentrations using electron probe micro-analysis (EPMA) and micro-X-ray fluorescence (XRFM) spectrometry. Chlorine was extracted from the scapolite separates via pyrohydrolysis and then analysed for their stable Cl isotope compositions by isotope ratio mass spectrometry (IRMS). All samples of scapolite investigated in this study are marialitic in composition. One of the scapolites from the Gruvberget deposit (IOCG-PS) had a Cl/Br molar ratio of 2363, which is the highest amongst all scapolites reported in the literature to date. Cl/Br molar ratios lower than seawater (650), were identified in two IOCG-PS scapolite samples (Cl/Br=554 and 271), as well as in two IOCG-M scapolites (Cl/Br=393 and 565). Three RM scapolites had Cl/Br molar ratios very close to, or slightly higher than, seawater values (639 to 770). Samples with Cl/Br molar ratios less than seawater are inferred to have halogens derived from evaporative residual brines; whereas samples with molar ratios higher than seawater may have halogens derived from fluids that have dissolved halite and/or are from magmatic systems. Considering the wide variation of the Cl/Br molar ratios in the IOCG-PS and IOCG-M scapolites compared to the restricted composition of the regional alteration (RM), it is proposed that the hydrothermal fluids interacted with several different protoliths to generate the IOCG alteration. RM alteration scapolites had δ37Cl values from −0.1‰ to +0.3‰, two IOCG-M scapolites had values of 0.2‰ and IOCG-PS scapolites had δ37Cl values from −0.1‰ to +1.0‰. Using a previously published δ37Cl value from fluid inclusion leachates (−1.7‰) from the IOCG-M mineralisation at Pahtohavare and the δ37Cl value of co-existing scapolite measured in this study (0.2‰), an empirical fluid-scapolite fractionation factor was calculated to be +1.9‰. This large fractionation factor is not supported by previous predictions for monovalent chlorides and, assuming equilibrium, indicates that 37Cl was preferentially accommodated in the A site of the scapolite structure. This indicates that either the stable Cl isotope partitioning between the CaCl2-rich brine and the scapolite may differ from currently available estimates for NaCl brines in equilibrium with silicate minerals, or that the scapolite and brine are not in isotopic equilibrium. Overall, the data in this study suggest that halogens in early scapolites were derived from residual brines and halite, during metamorphism of evaporites linked to the RM alteration. Later in the history of the Norrbotten district components of the RM alteration were recycled and mixed during magmatic and local metasomatic events to varying extents, resulting in the brines associated with IOCG alteration.

      PubDate: 2017-01-27T09:00:18Z
       
  • Measurements of bacterial mat metal binding capacity in alkaline and
           carbonate-rich systems
    • Abstract: Publication date: Available online 6 January 2017
      Source:Chemical Geology
      Author(s): Shannon L. Flynn, Qiyang Gao, Leslie J. Robbins, Tyler J. Warchola, Johanna N.J. Weston, Md. Samrat Alam, Yuxia Liu, Kurt O. Konhauser, Daniel S. Alessi
      Measuring the metal binding potential and reactivity of bacterial mats is challenging in alkaline and carbonate-rich systems. Traditional methods used to measure these parameters, such as potentiometric titrations and metal adsorption pH edges, are difficult to implement due to the presence of the carbonate minerals that buffer pH and prevent assessment of mat surface reactivity. Additionally, under alkaline conditions metals may form hydroxide and/or carbonate precipitates. In this study we examined the metal binding capacity of four distinct bacterial mats collected from Fairmont Hot Springs, BC, Canada. To prevent metal precipitation, the bacterial mat concentration was varied under a constant initial cadmium (Cd) concentration of 8.89μM and at pH8. In addition to the intact bacterial mats, a carbonate mineral sample and two bacterial mats in which the carbonate mineral was removed via acid-treatment, were used as end-members to assess the mechanisms of reactivity in the whole system. Freundlich adsorption isotherms were used to fit metal adsorption data and directly compare surface reactivity among intact mats and mat components. Two of the intact mats exhibited a higher affinity for Cd compared to the mineral at metal equilibrium concentrations above 2.5μM, while the other two intact mats had lower affinities under all experimental conditions. Generally, we found the acid-treated mats had higher Cd adsorption capacities than the carbonate mineral. When compared to their equivalent intact mats, only one acid-treated mat had a higher affinity for Cd. Further, we modeled whether metal adsorption in the intact mats, containing microbes and carbonate mineral, could be explained by a linear combination of the observed metal uptake by the organic and inorganic components through end-member experiments. Metal adsorption additivity results were mixed. Metal uptake by one intact mat was found to be additive, while for the other mat the additive model significantly underestimated the observed Cd accumulation. Our study demonstrates the potential, as well as the limitations, of using modified metal adsorption edges to determine the metal binding affinity and surface reactivity of bacterial mats in alkaline and carbonate-rich systems.

      PubDate: 2017-01-27T09:00:18Z
       
 
 
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