for Journals by Title or ISSN
for Articles by Keywords
help
  Subjects -> EARTH SCIENCES (Total: 652 journals)
    - EARTH SCIENCES (470 journals)
    - GEOLOGY (71 journals)
    - GEOPHYSICS (27 journals)
    - HYDROLOGY (21 journals)
    - OCEANOGRAPHY (63 journals)

EARTH SCIENCES (470 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: 12)
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     Full-text available via subscription   (Followers: 12)
Annals of GIS     Hybrid Journal   (Followers: 19)
Annals of Glaciology     Full-text available via subscription   (Followers: 3)
Annual Review of Marine Science     Full-text available via subscription   (Followers: 10)
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: 16)
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: 25)
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  
Boletín de Ciencias de la Tierra     Open Access   (Followers: 1)
Boreas: An International Journal of Quaternary Research     Hybrid Journal   (Followers: 10)
Bragantia     Open Access   (Followers: 2)
Bulletin of Earthquake Engineering     Hybrid Journal   (Followers: 10)
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: 6)
Cybergeo : European Journal of Geography     Open Access   (Followers: 5)
Depositional Record     Open Access  
Developments in Geotectonics     Full-text available via subscription   (Followers: 3)
Developments in Quaternary Science     Full-text available via subscription   (Followers: 3)
Développement durable et territoires     Open Access   (Followers: 3)
Diatom Research     Hybrid Journal   (Followers: 1)
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: 98)
Earth and Space Science     Open Access   (Followers: 10)
Earth Interactions     Full-text available via subscription   (Followers: 11)
Earth Science Research     Open Access   (Followers: 6)
Earth Sciences Research Journal     Open Access  
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: 68)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 6)
Earthquake Science     Hybrid Journal   (Followers: 9)
Earthquake Spectra     Full-text available via subscription   (Followers: 17)
Ecohydrology     Hybrid Journal   (Followers: 10)
Ecological Questions     Open Access   (Followers: 7)
Electromagnetics     Hybrid Journal   (Followers: 4)
Energy Efficiency     Hybrid Journal   (Followers: 11)
Energy Exploration & Exploitation     Full-text available via subscription   (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: 3)
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)
Exploration Geophysics     Hybrid Journal   (Followers: 4)
Facies     Hybrid Journal   (Followers: 9)
Fieldiana Life and Earth Sciences     Full-text available via subscription   (Followers: 1)
Física de la Tierra     Open Access  
Folia Musei rerum naturalium Bohemiae occidentalis. Geologica et Paleobiologica     Open Access  
Folia Quaternaria     Open Access  
Forestry Chronicle     Full-text available via subscription   (Followers: 10)
Frontiers in Earth Science     Open Access   (Followers: 5)
Frontiers in Geotechnical Engineering     Open Access   (Followers: 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: 24)
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: 3)
Geoscience Data Journal     Open Access   (Followers: 2)
Geoscience Frontiers     Open Access   (Followers: 9)
Geoscience Letters     Open Access   (Followers: 1)
Geoscience Records     Open Access  
Geosciences     Open Access   (Followers: 2)
Geosciences Journal     Hybrid Journal   (Followers: 9)
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: 3)
Global and Planetary Change     Hybrid Journal   (Followers: 16)
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: 23)
Hydrology and Earth System Sciences     Open Access   (Followers: 26)
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: 2)
Indonesian Journal on Geoscience     Open Access   (Followers: 2)
Interdisciplinary Environmental Review     Hybrid Journal   (Followers: 3)
International Geology Review     Hybrid Journal   (Followers: 12)
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]
  • In situ multiple sulfur isotope analysis by SIMS of pyrite, chalcopyrite,
           pyrrhotite, and pentlandite to refine magmatic ore genetic models
    • Authors: Crystal LaFlamme; Laure Martin; Heejin Jeon; Steven M. Reddy; Vikraman Selvaraja; Stefano Caruso; Thi Hao Bui; Malcolm P. Roberts; Francois Voute; Steffen Hagemann; David Wacey; Sten Littman; Boswell Wing; Marco Fiorentini; Matthew R. Kilburn
      Pages: 1 - 15
      Abstract: Publication date: 9 December 2016
      Source:Chemical Geology, Volume 444
      Author(s): Crystal LaFlamme, Laure Martin, Heejin Jeon, Steven M. Reddy, Vikraman Selvaraja, Stefano Caruso, Thi Hao Bui, Malcolm P. Roberts, Francois Voute, Steffen Hagemann, David Wacey, Sten Littman, Boswell Wing, Marco Fiorentini, Matthew R. Kilburn
      With growing interest in the application of in situ multiple sulfur isotope analysis to a variety of mineral systems, we report here the development of a suite of sulfur isotope standards for distribution relevant to magmatic, magmatic-hydrothermal, and hydrothermal ore systems. These materials include Sierra pyrite (FeS2), Nifty-b chalcopyrite (CuFeS2), Alexo pyrrhotite (Fe(1−x)S), and VMSO pentlandite ((Fe,Ni)9S8) that have been chemically characterized by electron microprobe analysis, isotopically characterized for δ33S, δ34S, and δ36S by fluorination gas-source mass spectrometry, and tested for homogeneity at the micro-scale by secondary ion mass spectrometry. Beam-sample interaction as a function of crystallographic orientation is determined to have no effect on δ34S and Δ33S isotopic measurements of pentlandite. These new findings provided the basis for a case study on the genesis of the Long-Victor nickel-sulfide deposit located in the world class Kambalda nickel camp in the southern Kalgoorlie Terrane of Western Australia. Results demonstrate that precise multiple sulfur isotope analyses from magmatic pentlandite, pyrrhotite and chalcopyrite can better constrain genetic models related to ore-forming processes. Data indicate that pentlandite, pyrrhotite and chalcopyrite are in isotopic equilibrium and display similar Δ33S values +0.2‰. This isotopic equilibrium unequivocally fingerprints the isotopic signature of the magmatic assemblage. The three sulfide phases show slightly variable δ34S values (δ34Schalcopyrite =2.9±0.3‰, δ34Spentlandite =3.1±0.2‰, and δ34Spyrrhotite =3.9±0.5‰), which are indicative of natural fractionation. Careful in situ multiple sulfur isotope analysis of multiple sulfide phases is able to capture the subtle isotopic variability of the magmatic sulfide assemblage, which may help resolve the nature of the ore-forming process. Hence, this SIMS-based approach discriminates the magmatic sulfur isotope signature from that recorded in metamorphic- and alteration-related sulfides, which may not be resolved during bulk rock fluorination analysis. The results indicate that, unlike the giant dunite-hosted komatiite systems that thermo-mechanically assimilated volcanogenic massive sulfides proximal to vents and display negative Δ33S values, the Kambalda ores formed in relatively distal environments assimilating abyssal sulfidic shales.

      PubDate: 2016-10-04T04:59:28Z
      DOI: 10.1016/j.chemgeo.2016.09.032
      Issue No: Vol. 444 (2016)
       
  • Characterization of natural and synthetic floating iron surface films and
           their associated waters
    • Authors: R.B. Perkins; Z.N. Gray; G. Grathoff; R. Hugo
      Pages: 16 - 26
      Abstract: Publication date: 9 December 2016
      Source:Chemical Geology, Volume 444
      Author(s): R.B. Perkins, Z.N. Gray, G. Grathoff, R. Hugo
      Naturally occurring floating iron surface films can be found in wetland and estuarine environments throughout the world, though they are often mistaken for oil sheens. While formation of biofilms at the sediment-water interface has been well documented, there has been surprisingly little mention of floating surface iron films in the geochemical literature, despite their ubiquitous nature. Two earlier studies determined that these films are amorphous or nano-crystalline, but disagree as to whether or not they contain mixed-valent iron and the importance of organic matter in their formation. Here, we further characterize naturally occurring floating iron surface films in terms of their solid compositions and structure, as well as the aqueous environment in which they form. We then develop a methodology for synthesizing these films in order to examine how variations in key environmental conditions affect their formation and stability under controlled conditions. Our findings indicate that such films do contain mixed-valent iron and can form abiotically. We propose possible mechanisms for the formation and persistent occurrence of floating iron surface films. The first is essentially that humic acid coprecipitated with a ferric oxyhydroxide phase entraps and concentrates Fe(II) within diffusion limited microenvironments. The second is that of a cyclic transformation of initial ferrihydrite film to green rust via reduction by organic material followed by photo-/atmospheric-oxidation. Both processes may be important.

      PubDate: 2016-10-04T04:59:28Z
      DOI: 10.1016/j.chemgeo.2016.09.027
      Issue No: Vol. 444 (2016)
       
  • A kinetic analysis of the transformation from akaganeite to hematite: An
           in situ time-resolved X-ray diffraction study
    • Authors: Kristina M. Peterson; Peter J. Heaney; Jeffrey E. Post
      Pages: 27 - 36
      Abstract: Publication date: 9 December 2016
      Source:Chemical Geology, Volume 444
      Author(s): Kristina M. Peterson, Peter J. Heaney, Jeffrey E. Post
      The nucleation and growth of akaganeite and its transformation to hematite under hydrothermal conditions were monitored over a temperature range of 80 to 200°C using time-resolved synchrotron X-ray diffraction. In each experiment, akaganeite was the first phase to form and hematite was the final phase. No intermediate phases were identified. The induction time to akaganeite nucleation was ~5525s and 537s at 80°C and 100°C, respectively, yielding an activation energy of 129±15kJ/mol. However, akaganeite nucleated at a constant temperature of 123±5°C when the heater set point was 150°C or higher, suggesting an activation energy for akaganeite nucleation of 0kJ/mol between 150 and 200°C. Hematite nucleation induction times decreased with increasing temperature from 1723s to 110s between 150 and 200°C. Based on a JMAK analysis, the activation energies for the crystal growth and dissolution of akaganeite were 74±8kJ/mol and 125±7kJ/mol, respectively. Our calculated activation energies for hematite nucleation and crystal growth were 80±13kJ/mol and 110±21kJ/mol, respectively.
      Graphical abstract image

      PubDate: 2016-10-04T04:59:28Z
      DOI: 10.1016/j.chemgeo.2016.09.017
      Issue No: Vol. 444 (2016)
       
  • Adapting the diffusive exchange method for stable isotope analysis of pore
           water to brine-saturated rocks
    • Authors: Antoine de Haller; Monique Hobbs; Jorge E. Spangenberg
      Pages: 37 - 48
      Abstract: Publication date: 9 December 2016
      Source:Chemical Geology, Volume 444
      Author(s): Antoine de Haller, Monique Hobbs, Jorge E. Spangenberg
      The isotope diffusive exchange technique was originally developed to indirectly measure the hydrogen and oxygen stable isotope composition (δ 2H and δ 18O values) and content of pore water (WC) in low permeability rocks. This study aims to benchmark developments of the technique adapted for high salinity (low water activity) pore waters by: 1) investigating the factors that might influence the results of the isotope diffusive exchange technique for saline solutions through solution-solution experiments; and 2) apply the adapted diffusive exchange method to rock samples previously equilibrated with synthetic solutions of known chemical and isotopic composition. Two different rocks were used in the benchmarking tests: Queenston Formation red shale from Ontario, Canada; and Opalinus Clay from the Mont Terri Underground Research Laboratory (URL), Switzerland. Solution-solution experiments showed that the main sources of error included a mismatch between the water activities (a w ), a major chemical mismatch, and/or a significant weight difference between the test and sample solution. Samples with a w spanning from 1 to 0.5 can be matched with adequate precision using a set of test waters with an a w of 0.99, 0.805, and 0.5. The a w of the test water must be equal or lower than the sample because equilibration is faster at low a w (i.e. higher salinities). Experiments with contrasting chemistry between test and sample solutions (NaCl vs CaCl2), induced a deviation in the δ 2H and δ 18O values (<5‰ and <2‰, respectively) calculated for the sample water. This deviation can be minimized by matching the a w of the test solution with salts that correspond to the dominant dissolved species in the sample pore water. The mass of test water must be no <0.5 times the mass of the sample water to maintain acceptable errors. This means the technique is adapted for rocks having a WC >0.5wt%. The WCs are generally estimated with errors of about 10% for all solution-solution experiments. To prepare rock samples containing pore waters of known composition, rocks were equilibrated with 0.3 and 5m NaCl, and 2.5 and 5m CaCl2 synthetic solutions. Based on time series chemical analyses of the solutions, equilibrium was attained in about 1 and 4days for the Opalinus Clay and Queenston shale samples, respectively. Benchmarking experiments were then conducted by applying the diffusive exchange method to determine the pore water isotopic compositions of these equilibrated rock samples and comparing to the known values of the equilibrated solution. The water stable isotope compositions are identical within 2σ error and do not depend on salinity or type of salt (NaCl, CaCl2). The adapted diffusive exchange method allows reliable stable isotope analysis of pore waters at all salinities when a w and chemistries of the pore and test waters are similar.

      PubDate: 2016-10-04T04:59:28Z
      DOI: 10.1016/j.chemgeo.2016.09.033
      Issue No: Vol. 444 (2016)
       
  • Cu isotopic compositions in Elsholtzia splendens: Influence of soil
           condition and growth period on Cu isotopic fractionation in plant tissue
    • Authors: Shi-Zhen Li; Xiang-Kun Zhu; Long-Hua Wu; Yong-Ming Luo
      Pages: 49 - 58
      Abstract: Publication date: 9 December 2016
      Source:Chemical Geology, Volume 444
      Author(s): Shi-Zhen Li, Xiang-Kun Zhu, Long-Hua Wu, Yong-Ming Luo
      This study investigates the magnitude and direction of Cu isotopic fractionation in the Cu-tolerant, strategy I plant, Elsholtzia splendens, considering the effect of soil condition and plant growth cycle. Uptake of Cu by E. splendens from soil was found to favor light Cu isotopic enrichment (δ65Cu<0‰) due to reduction at the soil–root interface. The magnitude of fractionation between soil and plant was found to be dependent on free Cu ion species in soil solution correlated with pH value of soil other than the phytoavailable component of soil or total soil for the same parent soil. Cu isotopic fractionation occurs in plant, the fractionation direction and magnitude would vary between different plant organs (i.e., root and stem) as well as different plant tissues (i.e., xylem and phloem). Remobilization of Cu associated with plant senescence was found to have a considerable effect on Cu isotopic fractionation within the plant, and the fractionation factor was found to change with the degree of remobilization. Overall, the results show that the Cu isotopic composition is useful as a tracer for probing the mechanisms of Cu translocation and retranslocation between soil and plants, and within plants.

      PubDate: 2016-10-11T09:35:38Z
      DOI: 10.1016/j.chemgeo.2016.09.036
      Issue No: Vol. 444 (2016)
       
  • Spatial domain analysis of carbon dioxide from soils on Vulcano Island:
           Implications for CO2 output evaluation
    • Authors: Roberto M.R. Di Martino; Giorgio Capasso; Marco Camarda
      Pages: 59 - 70
      Abstract: Publication date: 9 December 2016
      Source:Chemical Geology, Volume 444
      Author(s): Roberto M.R. Di Martino, Giorgio Capasso, Marco Camarda
      The carbon dioxide emissions of volcanoes have been targeted as effective contributors of CO2 to the atmosphere. However, different sources can be effective and active at the same time in the generation and release of CO2 in volcanic zones. Since isotopic fingerprinting of CO2 allows the precise identification of different sources, coupling carbon isotope and CO2 flux measurements enables the evaluation of the mass contribution of each source to the carbon dioxide emissions. This paper accounts for the first extensive spatial analysis of coupled measurements of carbon isotopologues of CO2 in the soil gases and CO2 fluxes discharged by soils on Vulcano Island. An innovative method has been designed, tested and fine-tuned in the laboratory to measure δ13C(CO2) values directly in field using a new type of laser-based isotopologues analyzer, namely a DeltaRay™ (Thermo Fisher Scientific). The method can be used to determine the carbon isotope composition across the full range of CO2 concentrations in the soil gases (0 – 100vol%). These data have been combined with measurements of the CO2 contents in the soil gases to distinguish CO2 from deep origins from CO2 of biogenic origin in the inhabited area of Vulcano Porto. The method of evaluating the amount of deep-origin CO2 in the soil gases is widely applicable in volcanic and geothermal zones for evaluation and monitoring purposes for both gas and volcanic hazards.

      PubDate: 2016-10-11T09:35:38Z
      DOI: 10.1016/j.chemgeo.2016.09.037
      Issue No: Vol. 444 (2016)
       
  • Diagenetic alteration affecting δ18O, δ13C and 87Sr/86Sr signatures of
           carbonates: A case study on Cretaceous seep deposits from Yarlung-Zangbo
           Suture Zone, Tibet, China
    • Authors: Hongpeng Tong; Qinxian Wang; Jörn Peckmann; Yuncheng Cao; Linying Chen; Weide Zhou; Duofu Chen
      Pages: 71 - 82
      Abstract: Publication date: 9 December 2016
      Source:Chemical Geology, Volume 444
      Author(s): Hongpeng Tong, Qinxian Wang, Jörn Peckmann, Yuncheng Cao, Linying Chen, Weide Zhou, Duofu Chen
      Seep carbonates provide excellent records of past seepage activities, and have been commonly considered to preserve primary, unaltered stable carbon isotope signatures. However, late diagenetic reactions may overprint original isotopic compositions, but the mode and effect of such alterations are poorly understood. In particular, there are significant uncertainties regarding how carbon and strontium isotopic compositions of seep carbonates respond to diagenesis. This study reports recently discovered Cretaceous hydrocarbon-seep deposits from the Yarlung-Zangbo Suture Zone, Tibet, China that have experienced substantial diagenetic alteration that is shown by recrystallization and secondary veins. Unitary linear recursive analysis was applied to δ13C values and 87Sr/86Sr ratios of the seep carbonates to evaluate the degree of secondary modification and to quantitatively constrain the compositions of primary carbonates and late diagenetic fluids. The δ18O values range from −11.8‰ to −2.2‰, δ13C values from −34.1‰ to −12.9‰ and 87Sr/86Sr ratios from 0.706221 to 0.706808. The heterogeneity in isotopic compositions and the observation that the most negative δ18O values occur in samples with the most extensive recrystallization indicate significant and spatially heterogeneous modification of isotope compositions during late diagenesis. The linear correlations between δ13C values and δ18O values for matrix micrites (R2 =0.54), and between bulk carbonate 87Sr/86Sr ratios and δ18O values (R2 =0.85) are best explained by burial diagenetic overprinting of oxygen, strontium, and even carbon isotopic compositions rather than by meteoric water hypergenesis. Extrapolated values of δ13C and ratios of 87Sr/86Sr against a δ18O value of −2‰ (average value of calcite precipitated in isotopic equilibrium with coeval Cretaceous seawater) that would characterize the primary carbonate, give an end member δ13C value of −34‰ and an end member 87Sr/86Sr ratio of 0.7072. The end member isotopic values obtained by this extrapolation suggest that the primary seep carbonates with low δ13C values and high 87Sr/86Sr ratios were formed by anaerobic oxidation of methane near the seafloor. In contrast, the measured δ18O values and 87Sr/86Sr ratios reflect late diagenetic fluids represented by burial pore water characterized by a low 87Sr/86Sr ratio and high temperature. Our findings reveal that δ13C values can only be moderately and 87Sr/86Sr ratios can be significantly altered during late diagenesis, and show that it is possible to quantitatively assess the primary composition of diagenetically altered seep carbonates.

      PubDate: 2016-10-16T14:12:52Z
      DOI: 10.1016/j.chemgeo.2016.10.003
      Issue No: Vol. 444 (2016)
       
  • Fluid evolution in the Strange Lake granitic pluton, Canada: Implications
           for HFSE mobilisation
    • Authors: O.V. Vasyukova; A.E. Williams-Jones; N.J.F. Blamey
      Pages: 83 - 100
      Abstract: Publication date: 9 December 2016
      Source:Chemical Geology, Volume 444
      Author(s): O.V. Vasyukova, A.E. Williams-Jones, N.J.F. Blamey
      Strange Lake is a mid-Proterozoic peralkaline granite pluton (Québec-Labrador, Canada) that underwent extreme enrichment in high field strength elements (HFSE), including the rare earth elements (REE). The HFSE mineralisation is confined to highly altered pegmatites and the most altered parts of the granites, implying a genetic association between hydrothermal fluids and HFSE enrichment. This study uses analyses of fluid inclusions to investigate the hydrothermal evolution of the Strange Lake pluton and the role of hydrothermal processes in concentrating the HFSE to potentially exploitable levels. Five groups of inclusions were distinguished. From earliest to latest, these groups are: primary aqueous inclusions (~25wt% NaCl eq.) associated with melt inclusions (Group 1); primary aqueous inclusions (13–23wt% NaCl eq.) associated with CH4 inclusions (Groups 2a and b); primary aqueous inclusions (~9wt% NaCl eq.) associated with CO2 inclusions (Group 3); primary aqueous inclusions (~9wt% NaCl eq.), which contain no carbonic component (Group 4); and finally, secondary aqueous inclusions (19wt% NaCl eq.), including inclusions that outline mineral pseudomorphs, also with no detectable carbonic component (Group 5). Most of the inclusions (except those in Group 5) have conspicuous ‘implosion’ haloes, evident as numerous tiny voids. This indicates that the inclusions re-equilibrated, most likely during isobaric cooling. Fluid evolution commenced with the exsolution of a saline aqueous liquid (~25wt% NaCl eq.) and an immiscible CH4 +H2 gas from the pegmatitic melt at temperatures of ~450–500°C and a pressure of ~1100bars. During isobaric cooling, the gas component of the fluid was gradually oxidised, evolving from being CH4-dominant to a CH4 fluid with a significant proportion of higher order hydrocarbons (due to oxidative coupling of methane induced by the consumption of O2 through the alteration of arfvedsonite to aegirine; ~325–360°C), and finally to a CO2-dominated fluid at ~300°C. The apparent salinity of the aqueous fluid decreased from ~25 to ~4.5wt% NaCl eq. due to fluid-rock interaction. The latter also caused precipitation of nahcolite (as a result of the reaction of newly formed CO2 with sodium from decomposing minerals), formation of pseudomorphs after primary Na-zirconosilicates and Na-titanosilicates and replacement of primary REE-silicates by bastnäsite-(Ce). Owing to this interaction, the carbonic component of the fluid was consumed which, together with the consumption of H2O to form Al-, K- and Fe-phyllosilicates, contributed to an increase in the fluid salinity (up to ~19wt% NaCl eq.). Light rare earth elements (LREE) were remobilised over 10s to 100s of metres by the high temperature high salinity fluid (preserved as Group 1 and 2a inclusions), whereas heavy rare earth elements (HREE) were remobilised on a much smaller scale by a late, low temperature high salinity fluid (trapped as inclusion Group 5). Fluid preserved as inclusion Group 3 may have been responsible for remobilisation of Zr and Ti.

      PubDate: 2016-10-16T14:12:52Z
      DOI: 10.1016/j.chemgeo.2016.10.009
      Issue No: Vol. 444 (2016)
       
  • The Cr-isotope signature of surface seawater — A global perspective
    • Authors: Cora Paulukat; Geoffrey J. Gilleaudeau; Pavel Chernyavskiy; Robert Frei
      Pages: 101 - 109
      Abstract: Publication date: 9 December 2016
      Source:Chemical Geology, Volume 444
      Author(s): Cora Paulukat, Geoffrey J. Gilleaudeau, Pavel Chernyavskiy, Robert Frei
      This study presents chromium-isotope data (expressed as δ53Cr) and chromium concentrations (Cr) of surface seawater (0–2m depth) from a variety of locations worldwide. In addition to samples from the open ocean (Arctic, Southern, Pacific, and Atlantic Oceans), samples were analysed from areas with more restricted water exchange rates (the Mediterranean Sea, Øresund, and Baltic Sea). The data indicate a heterogeneous distribution of δ53Cr in seawater with a total range from +0.13‰ to +1.24‰. The data are in agreement with a previous study, which focused on depth profiles, suggesting that seawater heterogeneity in δ53Cr and Cr concentration can be explained using a single fractionation factor for Cr-reduction (ε=−0.79±0.06‰ 2SD). In basins with limited water exchange with the open ocean, however, local factors seem to control the Cr-isotope composition.

      PubDate: 2016-10-16T14:12:52Z
      DOI: 10.1016/j.chemgeo.2016.10.004
      Issue No: Vol. 444 (2016)
       
  • Fe(II)-induced phase transformation of ferrihydrite: The inhibition
           effects and stabilization of divalent metal cations
    • Authors: Chengshuai Liu; Zhenke Zhu; Fangbai Li; Tongxu Liu; Changzhong Liao; Jey-Jau Lee; Kaimin Shih; Liang Tao; Yundang Wu
      Pages: 110 - 119
      Abstract: Publication date: 9 December 2016
      Source:Chemical Geology, Volume 444
      Author(s): Chengshuai Liu, Zhenke Zhu, Fangbai Li, Tongxu Liu, Changzhong Liao, Jey-Jau Lee, Kaimin Shih, Liang Tao, Yundang Wu
      The Fe(II)-induced phase transformation of iron (hydr)oxides is an important process in the geochemical iron cycle and is one in which the coexisting metal cations inhibit the phase transformation rates. However, the underlying affecting mechanisms by metal cations and the critical property of metal cations that is responsible for the inhibition remain unclear. In this study, we focus on the cation effect of seven divalent cations (denoted as Me(II), including Mg(II), Ca(II), Ba(II), Mn(II), Co(II), Ni(II), and Zn(II)) and their influencing mechanisms on Fe(II)-catalyzed transformation processes of ferrihydrite. At initial reaction conditions (i.e. pH6.5 and 2.0mM Fe(II)), the binding ability (the affinity of cations for ferrihydrite surface) of Me(II) was found to affect the ferrihydrite transformation. Me(II) with higher binding abilities reduced the bound-Fe(II) amount on ferrihydrite and decreased the redox potentials of the Fe(II)-catalyzed system to inhibit the transformation rates of ferrihydrite. In addition to the inhibition effect, the Me(II) was partly stabilized in the formed secondary iron minerals. The binding abilities of Me(II) also affected the Fe(II)-induced transformation pathways of ferrihydrite by affecting the amount of bound-Fe(II) on ferrihydrite. Ferrihydrite was first transformed to lepidocrocite and later to goethite and magnetite with Me(II) that had lower binding ability than Fe(II), whereas it was directly transformed to goethite and magnetite when with Me(II) that had higher binding ability than Fe(II).
      Graphical abstract image

      PubDate: 2016-10-16T14:12:52Z
      DOI: 10.1016/j.chemgeo.2016.10.002
      Issue No: Vol. 444 (2016)
       
  • Structural boron as factor controlling illite crystallinity in a mud
           volcano environment (Northern Apennine, Italy)
    • Authors: Stefano Battaglia; Maddalena Pennisi
      Pages: 120 - 127
      Abstract: Publication date: 9 December 2016
      Source:Chemical Geology, Volume 444
      Author(s): Stefano Battaglia, Maddalena Pennisi
      Mud extrusion is a well-known phenomenon whereby fluid-rich, fine grained sediments ascend within a lithological succession due to their buoyancy. In this study of the terrestrial mud volcanism associated to an active thrusted area in the northern Apennines, we investigated the effect of boron incorporation in illite crystalline lattice. A significant variation was recorded on the investigated mud samples with respect to both the amount of fixed boron (118–529μg/g) and the full width at half-maximum height value (FWHM) of illite (0.29–0.62°Δ 2Θ). We attribute these variations to the interaction between boron-rich formation waters and the silty-clayey lithologies of the local sedimentary succession. The lack of any geographical correlation between the selected geochemical and mineralogical parameters is likely due to intensive thrust and fold deformation of the study area. Comparison of FWHM values with those from Dellisanti et al. (2008); Santerno succession, northern Apennine) lead us to suggest that application of FWHM as a depth indicator could be misleading within B-rich hydro-geological environments.
      Graphical abstract image

      PubDate: 2016-10-24T19:04:47Z
      DOI: 10.1016/j.chemgeo.2016.10.017
      Issue No: Vol. 444 (2016)
       
  • Exploring the role of mantle eclogite at mid-ocean ridges and hotspots:
           U-series constraints on Jan Mayen Island and the Kolbeinsey Ridge
    • Authors: L.J. Elkins; S.R. Scott; K.W.W. Sims; E.R. Rivers; C.W. Devey; M.K. Reagan; C. Hamelin; R.B. Pedersen
      Pages: 128 - 140
      Abstract: Publication date: 9 December 2016
      Source:Chemical Geology, Volume 444
      Author(s): L.J. Elkins, S.R. Scott, K.W.W. Sims, E.R. Rivers, C.W. Devey, M.K. Reagan, C. Hamelin, R.B. Pedersen
      Sensitivity to rates of melting and to the presence of garnet in the melting residue makes U-series isotope disequilibrium a helpful tool for assessing the presence and role of mantle eclogite rocks in generating magmatic crust at both mid-ocean ridges and hotspots. Here we present 238U-234U-230Th-226Ra-210Pb isotope disequilibria data for a suite of fresh lavas from the tectonically and geochemically complex Jan Mayen region, including Jan Mayen Island; the Northern Kolbeinsey Ridge, which hosts the anomalous Eggvin Bank bathymetric high; and the Southern Mohns Ridge. Age-constrained, unaltered samples from the Northern Kolbeinsey Ridge and Eggvin Bank, including fresh popping rocks from Eggvin Bank, are characterized by relatively high (230Th/238U)=1.23 to 1.36 and low (226Ra/230Th)=1.24 to 1.28. These data are best explained by contributions to magma mixtures from an eclogite-dominated, upwelling mantle source with only modest aging or crystal fractionation effects during transport or storage in the crust. Young lavas from Jan Mayen Island have lower (230Th/238U)=1.15 and (226Ra/230Th)=1.05 to 1.16, and forward modeling suggests a detectible, but smaller mafic source contribution to a peridotitic magma mixture in faster upwelling mantle. Globally, most ocean island basalts also show low (226Ra/230Th) disequilibria, suggesting that in addition to high upwelling rates, mafic lithologies make significant melt contributions at most hotspots and that upwelling mantle plumes likely entrain highly fusible, recycled mafic rocks. Eggvin Bank apparently overlies a highly unusual mantle source that likely contains a remnant of delaminated, formerly underplated, ancient subcratonic mafic rocks, entrained by upwelling beneath the northernmost Kolbeinsey Ridge.

      PubDate: 2016-10-24T19:04:47Z
      DOI: 10.1016/j.chemgeo.2016.09.035
      Issue No: Vol. 444 (2016)
       
  • Consumption of atmospheric methane in a limestone cave in Indiana, USA
    • Authors: Kevin D. Webster; Anmar Mirza; Jessica M. Deli; Peter E. Sauer; Arndt Schimmelmann
      Pages: 1 - 9
      Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): Kevin D. Webster, Anmar Mirza, Jessica M. Deli, Peter E. Sauer, Arndt Schimmelmann
      Recent observations suggest that karst landscapes may be an unaccounted sink for atmospheric CH4, but questions remain about the processes contributing to sub-atmospheric CH4 mole fractions in caves. The CH4 dynamics associated with karst environments were studied over 18months at 6 locations in Buckner Cave, Southern Indiana by measuring the mole fractions and stable isotopic composition of CH4 and carbon dioxide (CO2). CO2 mole fractions were used to infer seasonal changes in airflow. Samples were obtained on a monthly basis. CH4 mole fractions ranged from 1.9±0.1ppm near the cave entrance to 0.1±0.1ppm in the more remote parts of the cave. The carbon and hydrogen stable isotopic compositions of CH4 in the cave ranged from −58.7 to +7 ‰ (VPDB) and −170 to +10 ‰ (VSMOW), respectively. The isotopic data suggest that the CH4 dynamics of Buckner Cave can be described by a seasonally variable mixing system in which atmospheric CH4 enters primarily through the main entrance and is subsequently consumed by methanotrophs. Additionally, at least two smaller CH4 sources are evident when air has been stagnant. The results suggest that subterranean karst cavities are an important sink for atmospheric CH4.

      PubDate: 2016-09-22T23:29:42Z
      DOI: 10.1016/j.chemgeo.2016.09.020
      Issue No: Vol. 443 (2016)
       
  • Dissolved inert gases (He, Ne and N2) as markers of groundwater flow and
           degassing areas at Mt Etna volcano (Italy)
    • Authors: A. Paonita; M. Longo; S. Bellomo; W. D'Alessandro; L. Brusca
      Pages: 10 - 21
      Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): A. Paonita, M. Longo, S. Bellomo, W. D'Alessandro, L. Brusca
      Fractions of the volatiles ascending from magma chambers meet groundwaters flowing away from the volcano summit and are carried to great distance as dissolved gases. The complex interactions between ascending magmatic volatiles, tectonic structures, heterogeneities in rock permeability and flow lines of aquifers deeply affect the dispersion of the dissolved species. Studying the spatial distribution of such species can therefore provide valuable information on the circulation of fluids inside volcanic edifices. Our study focussed on the composition of dissolved inert gases (He, Ne and N2) and He isotope ratio (3He/4He) in groundwaters circulating at Mt Etna volcano (Italy), because the concentrations of these species differ markedly between magmatic and shallow (crustal and atmospheric) sources, and they do not interact chemically with rocks. We identified groundwaters that flow through anomalously degassing areas associated with clearly evident or known tectonic structures. These waters show a typically magmatic He isotope composition (high 3He/4He ratios) and high proportions of dissolved magmatic gases (He and CO2) compared to the atmospheric ones (Ne and N2). Downstream of the degassing structures, along the hydrological outflows, we found groundwaters that are progressively enriched in atmospheric-derived gases (Ne and N2) and exhibited lower 3He/4He ratios. On this basis, we set up a model of unidimensional dispersion-advection of inert volatile solutes, coupled with a two-layer model for the dynamic exchange of volatiles through the aquifer–atmosphere interface. The model is able to quantitatively explain the progressive dilution of the magmatic signal over distances of several kilometres from the source location of the anomaly towards the final part of the flow lines at the coast. Typical hydrogeological parameters such as the flow velocity, rock permeability and rate of air–groundwater interaction can be constrained, and underground pathways of waters can be identified. Waters that are anomalously rich in magmatic tracers with respect to their peripheral position along the flow lines reflect arrival of deep gases from below, and they therefore offer a powerful tool for revealing hidden tectonic structures.

      PubDate: 2016-09-22T23:29:42Z
      DOI: 10.1016/j.chemgeo.2016.09.018
      Issue No: Vol. 443 (2016)
       
  • Biosignatures and microbial fossils in endolithic microbial communities
           colonizing Ca-sulfate crusts in the Atacama Desert
    • Authors: Beatríz Cámara; Virginia Souza-Egipsy; Carmen Ascaso; Octavio Artieda; Asunción De Los Ríos; Jacek Wierzchos
      Pages: 22 - 31
      Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): Beatríz Cámara, Virginia Souza-Egipsy, Carmen Ascaso, Octavio Artieda, Asunción De Los Ríos, Jacek Wierzchos
      Since the description of microbial communities colonizing Ca-sulfate crusts in the Atacama Desert, there has been much interest in the mechanisms that could lead to the formation and preservation of biosignatures or microbial fossils of these communities. A key to understanding physico-chemical processes of taphonomy and early diagenesis is to examine microfossils in their natural environment. In this study, we characterize organomineral traces and microbial fossils found around microbial communities present in these Ca-sulfate crusts. Through scanning electron microscopy, microanalytical (EDS) and Raman spectroscopy techniques, calcium carbonate precipitates were detected around remnants of cryptoendolithic algae beneath the crust surface. As what seems to be the final step in the organomineralization of these cryptoendolithic communities, we also observed alga cell remains permineralized by Mg-Si-rich minerals inside gypsum crystals. Additionally, Mg-Si bearing minerals formed a web-like structure within the hypoendolithic cyanobacterial habitat via permineralization of extracellular polymeric substances. Our observations indicate that despite the extremely hyperarid environment, microenvironmental conditions may be appropriate for the formation of biosignatures and microbial fossils of extinct endolithic microbial communities. A model of the possible organomineralization processes involved is presented.
      Graphical abstract image

      PubDate: 2016-09-22T23:29:42Z
      DOI: 10.1016/j.chemgeo.2016.09.019
      Issue No: Vol. 443 (2016)
       
  • Calibration of dolomite clumped isotope thermometry
    • Authors: Ian Z. Winkelstern; Stephen E. Kaczmarek; Kyger C Lohmann; John D. Humphrey
      Pages: 32 - 38
      Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): Ian Z. Winkelstern, Stephen E. Kaczmarek, Kyger C Lohmann, John D. Humphrey
      Use of the clumped isotope thermometer to decipher environments of dolomite formation has been inhibited by a lack of empirical data from dolomites formed at known temperatures. Calibrations for aragonite, calcite, siderite, and apatite indicate that a universal Δ47 – temperature relationship may exist across all carbonate-bearing mineral phases. However, theoretical modeling and proposed acid fractionation differences have suggested that dolomites may have different Δ47 values relative to calcites precipitated at the same temperature. To resolve this question, we analyzed five synthetic and four natural dolomites formed at known temperatures. These included synthetic dolomites grown in Mg-Ca-Cl solutions at temperatures of 200–250°C, and natural samples constrained by fluid inclusion analyses (~70°C), climate (~28 and ~27°C), and deep ocean borehole temperature (~21°C). When using calcite Δ47 acid fractionation values, these data result in a calibration line that is statistically indistinguishable from other clumped isotope calibrations. At least with current precision, we find no evidence for a consistent dolomite Δ47 offset. These results further support a universal calibration for carbonate clumped isotope thermometry and enable new investigations into conditions of dolomite formation.

      PubDate: 2016-09-22T23:29:42Z
      DOI: 10.1016/j.chemgeo.2016.09.021
      Issue No: Vol. 443 (2016)
       
  • High-strain deformation and fluid infiltration diachronism of the middle
           crust: New Devonian–Permian Alice Springs ages (365–290Ma) of shear
           zones in the Strangways Metamorphic Complex, Central Australia
    • Authors: H.W. Fournier; A. Camacho; J.K.W. Lee
      Pages: 39 - 53
      Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): H.W. Fournier, A. Camacho, J.K.W. Lee
      The Proterozoic granulite-facies Strangways Metamorphic Complex (SMC) in the Arunta Region of Central Australia was partially hydrated during the Devonian–Carboniferous (380–320Ma) Alice Springs Orogeny by amphibolite-greenschist-facies high-strain deformation. Stable and radiogenic isotope analyses on coeval muscovite and biotite from shear-zone samples of the SMC were performed to analyse the composition and date the timing of fluid circulation and cooling. Stable isotope data (δ18O and δD) indicate that in the south of the SMC, the source of fluids appear to be the thick successions of Neoproterozoic Heavitree Quartzite, while another source in the north and central part of the SMC may be related to meteoric fluids. The heterogeneous δ18OFluid and homogenous δDFluid values for muscovite reflect changes in water/rock ratios with initial high values in the south and lower in the north, possibly suggesting a migration of fluids from south to north (as indicated by the evolving δ18OFluid and homogenous δDFluid values). Rb/Sr muscovite ages of 365–340Ma approximate the time of crystallization and fluid infiltration. They also indicate that the shear zones were active at different times during the Alice Springs Orogeny and that some were active prior to the peak Carboniferous amphibolite-facies metamorphism. 40Ar/39Ar muscovite ages of 350–325Ma are mostly interpreted as cooling ages. Rb/Sr biotite ages of 335–290Ma are interpreted as cooling ages at ~300°C (Sr closure temperature for biotite) and extend the activity of the Alice Springs Orogeny until the Permian. Cooling histories of the Wallaby Knob and Harry Creek shear zones, which serve as the tectonic boundaries of the SMC to the north and south respectively, indicate slow cooling (<3.5°CMa−1) for c. 30Ma (360–330Ma) at both boundaries continuing until 290Ma in the western portion of the Harry Creek shear zone. This slow cooling is associated with passive exhumation (i.e. erosion). Rapid cooling (20–10°CMa−1) then occurred over c. 10Ma (330–320Ma) in the Wallaby Knob shear zone and in the east portion of the Harry Creek shear zone. This change in cooling rate is associated with the combined initiation in the Carboniferous of dextral and thrusting deformation by the Pinnacles Bore and Erontonga shear zones.

      PubDate: 2016-09-22T23:29:42Z
      DOI: 10.1016/j.chemgeo.2016.09.014
      Issue No: Vol. 443 (2016)
       
  • Chemical weathering rates and atmospheric/soil CO2 consumption of igneous
           and metamorphic rocks under tropical climate in southeastern Brazil
    • Authors: Alexandre Martins Fernandes; Fabiano Tomazini da Conceição; Eder Paulo Spatti Junior; Diego de Souza Sardinha; Jeferson Mortatti
      Pages: 54 - 66
      Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): Alexandre Martins Fernandes, Fabiano Tomazini da Conceição, Eder Paulo Spatti Junior, Diego de Souza Sardinha, Jeferson Mortatti
      Chemical weathering rates and atmosphere/soil CO2 consumption of igneous and metamorphic rocks under tropical climate in southeastern Brazil were evaluated using the chemical composition of surface waters and fresh rocks and soil (horizon C) in the Upper Sorocaba River basin. Surface water samples were collected between June/2009 and June/2010, and analyses were performed to assess pH, electrical conductivity (EC), temperature and total dissolved solids (TDS), including Na+, K+, Ca2+, Mg2+, Cl−, SO4 2−, PO4 3−, NO3 − and SiO2. Fresh rocks and C horizon samples were also collected, taking into account their geological context, abundance and spatial density, to analyze major elements and mineralogy. The concentration of TDS and dissolved cations, anions and silica increased during the dry period in relation to the wet period, and the same behavior was observed for pH, EC and temperature. After corrections of anthropogenic contributions (ca. 21t/km2/yr) and atmospheric inputs (ca. 19t/km2/yr), the annual flux due to chemical weathering involving the igneous and metamorphic rocks was ca. 29t/km2/yr. The CO2 atmospheric/soil consumption in the Upper Sorocaba River basin was ca. 0.2×106 mol/km2/yr, and when extrapolated to the entire Mantiqueira Orogenic Belt, accounted an estimated consumption of 0.07×1012 mol/yr, representing 0.6% of the total CO2 consumption flux derived from global average silicate weathering. The chemical weathering rates of igneous and metamorphic rocks in the Upper Sorocaba River basin were estimated at 15m/My, respectively. The main weathering process in this watershed was the monossialitization, with partial hydrolyses of bedrock minerals, except quartz, which was not weathered and remained in the soil profile. The annual specific flux derived from igneous and metamorphic rocks at Upper Sorocaba River basin could be compared with watersheds in tropical climates. However, this value is higher than in other North American, European, Asian and African granitoid watersheds, and lower than in montane watersheds.

      PubDate: 2016-09-27T23:34:16Z
      DOI: 10.1016/j.chemgeo.2016.09.008
      Issue No: Vol. 443 (2016)
       
  • Zircon U-Pb geochronological constraints on rapid exhumation of the mantle
           peridotite of the Xigaze ophiolite, southern Tibet
    • Authors: Tong Liu; Fu-Yuan Wu; Liang-Liang Zhang; Qing-Guo Zhai; Chuan-Zhou Liu; Wen-Bin Ji; Chang Zhang; Yang Xu
      Pages: 67 - 86
      Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): Tong Liu, Fu-Yuan Wu, Liang-Liang Zhang, Qing-Guo Zhai, Chuan-Zhou Liu, Wen-Bin Ji, Chang Zhang, Yang Xu
      The Xigaze ophiolite outcrops in the central segment of the Yarlung Zangbo suture zone, southern Tibet. It is characterized by large amounts of ultramafic units with minor mafic rocks. The mafic rocks consist of gabbros, diabases and basalts. The gabbroic rocks of the Xigaze ophiolite occur as layered bodies or isotropic dikes intruding into mantle sections. Hectometric-sized gabbroic bodies are well-preserved in Dazhuqu, Baigang and Jiding. However, their formation time and generation mechanism are not systematically proposed or well understood. In this study, nine samples of mafic rocks from the Xigaze ophiolite, including seven gabbros and two rodingites (altered from diabases), were selected for in situ zircon U-Pb and Hf isotopic analyses. The geochemical feature suggested that these rocks formed by the intrusion of melts of normal mid-ocean ridge (N-MORB) type. The U-Pb data yielded identical ages of 124–129Ma within uncertainties. Positive zircon ε Hf(t) values indicated that these samples had an origin of depleted mantle source. Combined with previous studies on mafic dikes, amphibolite blocks and radiolarian cherts, it can be concluded that the Yarlung Zangbo ophiolites formed over a short period of time from 119 to 132Ma. Hence, a rapid exhumation of the mantle peridotites and gabbroic rocks of the Xigaze ophiolite may have occurred to get intrusion of the diabase dikes and sills. It excludes the existence of a long-term ancient magma chamber or lens. It is more likely that the gabbroic rocks are a series of plutonic intrusions beneath a fossil slow-spreading ridge, rather than products of magma chambers. Therefore, the “Chapman detachment model” may be applied to the generation of the Yarlung Zangbo ophiolites.

      PubDate: 2016-09-27T23:34:16Z
      DOI: 10.1016/j.chemgeo.2016.09.015
      Issue No: Vol. 443 (2016)
       
  • Equilibrium zinc isotope fractionation in Zn-bearing minerals from
           first-principles calculations
    • Authors: Manoj Ducher; Marc Blanchard; Etienne Balan
      Pages: 87 - 96
      Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): Manoj Ducher, Marc Blanchard, Etienne Balan
      Isotopic composition analysis contributes significantly to the investigation of the biogeochemical cycle of zinc with its economical, environmental and health implications. Interpretation of isotopic measurements is however hindered by the lack of a set of equilibrium isotopic fractionation factors between Zn-bearing minerals. In this study, equilibrium mass-dependent Zn isotope fractionation factors in Zn-bearing minerals are determined from first-principles calculations within the density functional theory (DFT) scheme. A wide range of minerals belonging to sulfide, carbonate, oxide, silicate, sulfate and arsenate mineral groups are modelled to account for the natural diversity of Zn crystal-chemical environment. Calculated reduced partition function ratios (β-factors) span a range smaller than 2‰ at 22°C. All studied secondary minerals (adamite, gahnite, gunningite, hemimorphite, hydrozincite, zincite) but zinc carbonate (smithsonite) are isotopically heavier than zinc sulfide minerals (sphalerite and wurtzite) from which they could form through supergene processes. Zinc–aluminium spinel and zinc silicate are the isotopically heaviest minerals. The investigation of the crystal-chemical parameters at the origin of differences in isotopic properties shows an excellent linear correlation between lnβ and Zn interatomic force constants. β-factors are also observed to increase when the Zn-first neighbour bond lengths decrease and charges on atoms involved in the bonding increase and vice versa. These findings are in line with the observation of heavy isotope enrichment in systems having the largest bond strength.

      PubDate: 2016-09-27T23:34:16Z
      DOI: 10.1016/j.chemgeo.2016.09.016
      Issue No: Vol. 443 (2016)
       
  • Clumped isotope thermometry of carbonate-bearing apatite: Revised sample
           pre-treatment, acid digestion, and temperature calibration
    • Authors: Ulrike Wacker; Tanja Rutz; Niklas Löffler; Anika C. Conrad; Thomas Tütken; Michael E. Böttcher; Jens Fiebig
      Pages: 97 - 110
      Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): Ulrike Wacker, Tanja Rutz, Niklas Löffler, Anika C. Conrad, Thomas Tütken, Michael E. Böttcher, Jens Fiebig
      It has recently been shown that the clumped isotopic composition (Δ47) of carbonate-bearing hydroxyapatite (CHAP) from teeth and bones reveals important information about the body temperature of vertebrates (Eagle et al., 2010, 2011). In this study we reinvestigate the temperature dependence of Δ47 in CHAP, extending the temperature range from 2 to 59°C. In addition, the effects of chemical pre-treatment of CHAP on its bulk and clumped isotopic composition are studied. CHAP is best reacted with phosphoric acid at 90 to 110°C minimizing the potential of evolved CO2 or reaction intermediates to re-equilibrate with traces of water in the acid environment. Reaction at 110°C ensures that digestions of CHAP samples are complete within 60min. We determined a Δ47 ⁎ 90–110 value of 0.032±0.008‰ that is - within errors - indistinguishable from a Δ47 ⁎ 90–110 value of 0.019±0.007‰ received for aragonite. For tooth enamel pre-treated with H2O2 lower Δ47 values, and higher δ18O and δ13C values were measured than for pre-treated tooth dentine. In addition, similar trends were observed for pre-treated dentine and bone material: higher Δ47 values, and lower δ18O and δ13C values were determined compared to untreated samples. A new tentative clumped isotope temperature calibration based on a synthetic apatite, untreated tooth enamel of an African elephant and enameloid from teeth of a Greenland shark is presented using a reaction temperature of 110°C. It follows the equation: Δ 47 =0.0320(±0.0022)×106/T2 +0.1977(±0.0259)(Δ47 in‰andTinK). The slope of this regression line is identical to those previously obtained from 90°C digestions of calcite and/or aragonite in several laboratories (e.g., Henkes et al., 2013; Wacker et al., 2014; Defliese and Lohmann, 2015). The Δ47 data of untreated enamel(oid) samples reacted at 90°C closely match a Δ47-1/T2 relationship for calcite that was made at the same digestion temperature (Wacker et al., 2014). These preliminary results suggest that calcite calibrations made at a reaction temperature of 90°C might be directly applicable to CHAP samples to determine their formation temperatures.

      PubDate: 2016-09-27T23:34:16Z
      DOI: 10.1016/j.chemgeo.2016.09.009
      Issue No: Vol. 443 (2016)
       
  • Constraints on the sedimentary input into the Loki's Castle hydrothermal
           system (AMOR) from B isotope data
    • Authors: Tamara Baumberger; Gretchen L. Früh-Green; Andrea Dini; Chiara Boschi; Kirsten van Zuilen; Ingunn H. Thorseth; Rolf B. Pedersen
      Pages: 111 - 120
      Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): Tamara Baumberger, Gretchen L. Früh-Green, Andrea Dini, Chiara Boschi, Kirsten van Zuilen, Ingunn H. Thorseth, Rolf B. Pedersen
      Loki's Castle at 73°30′ N along the Arctic Mid-Ocean Ridge (AMOR) provides a natural laboratory to study the evolution of hydrothermal circulation in an ultraslow-spreading environment. In addition, a sedimentary input into the Loki's Castle hydrothermal circulation cell is indicated by vent fluid and gas chemistry. Here we present B and Sr isotope data to investigate interaction between fluid, MORB and sediment and to constrain mass transfer during hydrothermal circulation of a sediment-influenced, black smoker hydrothermal system. The high boron concentrations (1770–2170μmol/l) and the relatively low boron isotope composition of the vent fluids (δ11B values ranging from +11.6 to +15.2‰) reflect not only fluid/rock interaction with mafic crust but indicates a significant contribution from sediments, in agreement with other chemical and isotopic (87Sr/86Sr) analyses. We present a model of B extraction from sediments at a temperature of 300°C, which suggests a water/sediment ratio of approximately 2 to 3 at Loki's Castle. At these ratios, relative B extraction from sediment ranges from 35 to 55%.

      PubDate: 2016-09-27T23:34:16Z
      DOI: 10.1016/j.chemgeo.2016.09.026
      Issue No: Vol. 443 (2016)
       
  • Combined Sr-Nd isotopic and geochemical fingerprinting as a tool for
           identifying tephra layers: Application to deep-sea cores from Eastern
           Mediterranean Sea
    • Authors: M. D'Antonio; R. Mariconte; I. Arienzo; F.C. Mazzeo; A. Carandente; D. Perugini; M. Petrelli; C. Corselli; G. Orsi; M.S. Principato; L. Civetta
      Pages: 121 - 136
      Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): M. D'Antonio, R. Mariconte, I. Arienzo, F.C. Mazzeo, A. Carandente, D. Perugini, M. Petrelli, C. Corselli, G. Orsi, M.S. Principato, L. Civetta
      Tephra layers from six deep-sea cores, recovered between the Ionian and the Aegean Seas, and covering a time span between ~102 and ~8.2ka BP, were investigated with the aim of identifying their volcanic source. The stratigraphic position of each tephra layer defined by means of nannofossil biostratigraphy and occurrence of sapropel layers was integrated with a thorough geochemical characterization of glass shards based on major oxides and trace elements content, and Sr-Nd isotope ratios. Major oxides composition permitted their subdivision into three groups (K-trachyte, peralkaline rhyolite, andesite), confirmed by Principal Component Analysis on trace elements data. Primordial mantle-normalized trace elements distribution patterns allowed for precise identification of the geochemical affinity and geodynamical setting of each group. Although the Sr isotopic composition of some investigated tephras was deeply affected by seawater alteration as expected, the 143Nd/144Nd values discriminate well the three groups of tephras in combination with major oxides and trace elements data. Therefore, La/Yb and Th/Y vs. 143Nd/144Nd discrimination diagrams are proposed for identification of potential volcanic sources active in the 102–8.2ka BP time span in the Eastern Mediterranean area for unknown tephras. The obtained results have permitted the attribution of six K-trachytic tephras to the Y-5 stratigraphic marker (Campanian Ignimbrite eruption of Campi Flegrei, Southern Italy, ~39ka BP), and two pantelleritic (=peralkaline rhyolite) tephras to the Y-6 stratigraphic marker (Green Tuff eruption of Pantelleria Island, Sicily Channel, ~46ka BP). The andesitic tephra, stratigraphically constrained between ~39 and ~83–102ka BP, might correspond to the X-1 stratigraphic marker, found in several deep-sea cores of the Eastern Mediterranean Sea, although its attribution is still debated in the literature. On the basis of trace elements content, supported by Sr-Nd isotopic features, it is here hypothesized that this tephra could be related to a volcanic source located in the Aegean Sea area, probably the island of Santorini. More generally, this study demonstrates that the combination of Sr-Nd isotopic with major oxide and trace element geochemical fingerprinting on selected and purified glass shards is a very effective tool for identifying nature and source of doubtful tephra layers.
      Graphical abstract image

      PubDate: 2016-10-04T04:59:28Z
      DOI: 10.1016/j.chemgeo.2016.09.022
      Issue No: Vol. 443 (2016)
       
  • First-principles phase diagram calculations for the carbonate quasibinary
           systems CaCO3-ZnCO3, CdCO3-ZnCO3, CaCO3-CdCO3 and MgCO3-ZnCO3
    • Authors: Z.T.Y. Liu; B.P. Burton; S.V. Khare; P. Sarin
      Pages: 137 - 145
      Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): Z.T.Y. Liu, B.P. Burton, S.V. Khare, P. Sarin
      Quasibinary solid solutions of calcite-structure carbonate minerals play an important role in rock formation. We have investigated solid solutions with cations Ca2+, Cd2+, Mg2+ and Zn2+ by performing first-principles phase diagram calculations for Ca1−xZnxCO3, Cd1−xZnxCO3, Ca1−xCdxCO3 and Mg1−xZnxCO3 (0≤x≤1) with density functional theory, cluster expansion and Monte Carlo simulations. The end members and the dolomite structures were individually studied to analyze their structural parameters and bonding characteristics. Consolute temperatures and continuous order-disorder transition temperatures are 1450K for Ca1−xZnxCO3 and 1000K for Cd1−xZnxCO3, but below 100K for Ca1−xCdxCO3 and Mg1−xZnxCO3. In agreement with existing literature, consolute temperatures increase with increasing differences in cation radii. If the dolomite structures are assumed to be stable, the phase diagram calculations predict that they persist to 1150K for Ca1−xZnxCO3, and 900K for Cd1−xZnxCO3 before decomposition at peritectoid points. This confirms the conjectured phase diagram for Ca1−xZnxCO3 in (Goldsmith, 1983. Rev. Mineral. Geochemistry 11). In addition, formation energies of the dolomite structures were decomposed into two parts: first a volume change, then chemical exchange and relaxation. They were compared with the corresponding random solid solutions at the same bulk compositions. (Meta)stability of the dolomite structures was demonstrated by this analysis, and was also studied by examining the bond lengths and cation octahedral distortions.

      PubDate: 2016-10-04T04:59:28Z
      DOI: 10.1016/j.chemgeo.2016.09.024
      Issue No: Vol. 443 (2016)
       
  • Optimized sequential extraction for carbonates: Quantification and δ13C
           analysis of calcite, dolomite and siderite
    • Authors: Alejandra Morera-Chavarría; Jasper Griffioen; Thilo Behrends
      Pages: 146 - 157
      Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): Alejandra Morera-Chavarría, Jasper Griffioen, Thilo Behrends
      Siderite is present in diverse types of rocks and sediments, but its quantification is cumbersome when present in relatively low contents. A new analytical method for the sequential separation of different carbonate phases is presented. The separation, quantification and characterization of the carbonates is based on different kinetic reactions during acid digestion in combination with monitoring the solution composition and the concomitant release of CO2. Trapping the released gasses permits the isotopic characterization (δ13C) of the different phases. Based on published rate laws, an idealized model was built to identify pH and temperature conditions for optimal separation of the various carbonates by taking the size distribution of carbonate particles into consideration. Results from the theoretical survey were implemented in an extraction scheme, which was optimized and evaluated by tests on pure mineral phases and their mixtures. Further tests on a diverse array of sediment and rock samples validated the specificity and reproducibility of the method.

      PubDate: 2016-10-04T04:59:28Z
      DOI: 10.1016/j.chemgeo.2016.09.025
      Issue No: Vol. 443 (2016)
       
  • Beryllium isotopic systematics in island arc volcanic rocks from northeast
           Japan: Implications for the incorporation of oceanic sediments into island
           arc magmas
    • Authors: Akiko Shimaoka; Mineo Imamura; Ichiro Kaneoka
      Pages: 158 - 172
      Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): Akiko Shimaoka, Mineo Imamura, Ichiro Kaneoka
      We present beryllium (Be) isotopic ratios (10Be/9Be) for 55 volcanic rocks from 21 Quaternary volcanoes in the northeast Japan arc, including the Hokkaido and central Japan areas (the NEJH). Although Be isotopic ratios in these rocks are much lower than in other arcs reported in the literature, they are much higher than in control samples (ocean island basalt (OIB) and Tertiary basalts) that have no recent addition of 10Be. Hence, our results indicate the incorporation of a component derived from subducted sediments in the Japan arc magmas. Two kinds of Be isotopic variations were observed in the NEJH: 1) variations among lava flows of different ages in the same volcano; and 2) variations among volcanoes in an along-arc or cross-arc direction. In the first case, there are no apparent differences in Be isotopic ratios for lavas of the same age, but relatively large variations among lavas of different ages, and the degree of variation depends on the regional characteristics, indicating that regional differences in near-surface conditions may affect these Be isotopic variations. In the second case, the progressive decrease in Be isotopic ratios in a cross-arc direction suggests the continuous incorporation of subducted sediment, even in the back-arc region. The regional along-arc Be isotopic variations reflect not only the incorporation of sediments into the arc magma, but also regional tectonic and/or geological and geochemical processes. This is the first systematic study of Be isotopic systems in the NE Japan arc, and we provide direct evidence for the incorporation of sedimentary components from the subducting slab into the arc magma, including in the back-arc region at 300km from the volcanic front, where the Wadati–Benioff Zone is at a depth of 290km.

      PubDate: 2016-10-04T04:59:28Z
      DOI: 10.1016/j.chemgeo.2016.08.032
      Issue No: Vol. 443 (2016)
       
  • Evidence of intense methane seepages from molybdenum enrichments in gas
           hydrate-bearing sediments of the northern South China Sea
    • Authors: Fang Chen; Yu Hu; Dong Feng; Xin Zhang; Sihai Cheng; Jun Cao; Hongfeng Lu; Duofu Chen
      Pages: 173 - 181
      Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): Fang Chen, Yu Hu, Dong Feng, Xin Zhang, Sihai Cheng, Jun Cao, Hongfeng Lu, Duofu Chen
      The application of molybdenum (Mo) enrichment to sediments can potentially provide unique constraints on the methane seepage dynamics at continental margins. We report herein elemental and isotopic measurements for authigenic carbonates and sediment samples in drill cores (~95m long; Site GMGS2-08 at ~800m water depth) from gas hydrate-bearing sediments in the northern South China Sea. Five intervals that were impacted by methane seepages in the sediment profile were identified by the presence of strongly 13C-depleted carbonates (δ13C values as low as −56.8‰). The carbonates often cement chemosynthetic bivalve shell fragments and have anomalous δ18O positive values. These observations suggest that methane seepage was intense during those time intervals and that carbonate precipitation occurred in the uppermost sediments, probably due to gas hydrate dissociation. Interestingly, the five intervals were all characterized by strong Mo enrichments. We thus hypothesize that the presence of strong Mo enrichments is a good indicator of past episodes of methane seepage. Such an environment results in sulfidic conditions that occur in a narrow zone in proximity to seawater close to the seafloor, leading to resultant Mo enrichments in associated sediments. In contrast, environments with low methane flux would lead to the distribution of Mo throughout a wide sulfidic zone and, consequently, an absence of Mo enrichment in the sediments. The insights into the environment for Mo enrichments at seeps have significant implications for tracing past methane seepages, their intensities, and even possible occurrences of gas hydrate dissociation.
      Graphical abstract image

      PubDate: 2016-10-04T04:59:28Z
      DOI: 10.1016/j.chemgeo.2016.09.029
      Issue No: Vol. 443 (2016)
       
  • Argon redistribution during a metamorphic cycle: Consequences for
           determining cooling rates
    • Authors: Christopher S. McDonald; Clare J. Warren; Darren F. Mark; Alison M. Halton; Simon P. Kelley; Sarah C. Sherlock
      Pages: 182 - 197
      Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): Christopher S. McDonald, Clare J. Warren, Darren F. Mark, Alison M. Halton, Simon P. Kelley, Sarah C. Sherlock
      40Ar/39Ar thermochronology is commonly used to constrain the rates and times of cooling in exhumed metamorphic terranes, with ages usually linked to temperature via Dodson's closure temperature (TC) formulation. Whilst many metamorphic 40Ar/39Ar data are consistent with the timing of crystallisation or cooling within a chronological framework defined by other, higher temperature, chronometers, other 40Ar/39Ar data are more difficult to interpret. We report white mica and biotite single grain fusion and laser ablation 40Ar/39Ar ages from felsic gneisses from the Western Gneiss Region, Norway. The rocks record isothermal decompression from peak eclogite-facies conditions (white mica stable) to amphibolite-facies conditions (biotite stable) at c. 700°C. White mica and biotite yield dispersed single grain fusion dates from 416 to 373Ma and 437 to 360Ma respectively. In-situ laser ablation analyses provide a similar range, with white mica spot ages ranging from 424 to 370Ma and biotite spot ages ranging from 437 to 370Ma. The dates span the duration of the metamorphic cycle suggested by previous studies, and cannot be reconciled with the results of simple models of Ar loss by diffusion during cooling. Samples that show evidence for different physical processes, such as the chemical breakdown of white mica, partial melting, and fluid ingress, generated different age populations to samples that did not experience or record obvious petrological evidence for these processes. Samples that record significant recrystallization and deformation yielded younger white mica (but older biotite) single grain fusion ages than more pristine samples. Amphibolite-facies gneisses that preserve evidence for significant partial melting generated younger biotite ages than samples that recorded evidence for significant hydration. Our data support other reported observations that high-temperature metamorphic mica 40Ar/39Ar dates cannot be assumed to record the timing of cooling through a specific temperature window. Careful assessment of the petrographic context of the dated minerals and consideration of their post-crystallisation history may provide a more robust insight into whether ‘age’ links to ‘stage’ in a temporally meaningful way.

      PubDate: 2016-10-11T09:35:38Z
      DOI: 10.1016/j.chemgeo.2016.09.028
      Issue No: Vol. 443 (2016)
       
  • Distributions and compound-specific isotopic signatures of sedimentary
           chlorins reflect the composition of photoautotrophic communities and their
           carbon and nitrogen sources in Swiss lakes and the Black Sea
    • Authors: Sebastian Naeher; Hisami Suga; Nanako O. Ogawa; Yoshinori Takano; Carsten J. Schubert; Kliti Grice; Naohiko Ohkouchi
      Pages: 198 - 209
      Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): Sebastian Naeher, Hisami Suga, Nanako O. Ogawa, Yoshinori Takano, Carsten J. Schubert, Kliti Grice, Naohiko Ohkouchi
      We examined the distributions of tetrapyrrole pigments (i.e. intact chlorophylls and bacteriochlorophylls, pheopigments) as well as their compound-specific carbon and nitrogen isotopic compositions in the sediments of three Swiss lakes (Lakes Rotsee, Cadagno and Zurich) and the Black Sea to investigate the biogeochemical cycling of carbon and nitrogen mediated by phototrophic eukaryotes (algae) and bacteria. The factors controlling chlorin isotope variations are discussed and the feasibility to use chlorins as indicators for reconstructions of surface water environments is evaluated. Chlorophyll a and its derivatives including pheophytin a, a pheophytin a epimer, pyropheophytin a, 132,173-cyclopheophorbide-a-enol, chlorophyllone a as well as steryl and carotenol chlorin esters were detected in all sediments. The presence of bacteriochlorophylls e and their derivatives confirmed the presence of brown strains of green phototrophic sulfur bacteria (Chlorobiaceae; GSB) in all three lakes. In the shallower Lakes Rotsee and Cadagno, purple sulfur bacteria (Chromatiaceae; PSB) were also present as confirmed by bacteriochlorophyll a derivatives. Despite the different degrees of water column hypoxia at the studied sites, the chlorins in all sediments were attributed to rapid transformation of intact tetrapyrroles and the formation of related pheopigments. The scatter of compound-specific carbon isotopic compositions of Chl a and its derivatives resulted from different timing of pheopigment formation, likely due to the interaction of blooms of various phytoplankton communities at different times of the year and the variable degrees of carbon limitation and/or different contributions of recycled organic matter (OM). The nitrogen isotopic composition of the chloropigments mainly derived from nitrate assimilation in Lake Zurich and the Black Sea, whereas ammonium and nitrate assimilation were predominant in Lake Rotsee. In the epilimnion of the meromictic Lake Cadagno, dissolved organic nitrogen (DON) supplied to the surface water from ammonium assimilation in the chemocline may be the main nitrogen source. Phototrophic sulfur bacteria in Lakes Rotsee and Cadagno thrived mainly under dissolved organic carbon depleted conditions within the chemocline and in the hypolimnion. GSB may use predominantly ammonium and at least in Lake Cadagno also perform N2 fixation. In contrast, the nitrogen source of PSB could not be reconstructed with δ15N values of bacteriochlorins, because nitrogen isotopic fractionation during BChl a synthesis seems to be almost independent of the assimilated substrate.

      PubDate: 2016-10-16T14:12:52Z
      DOI: 10.1016/j.chemgeo.2016.04.029
      Issue No: Vol. 443 (2016)
       
  • WITHDRAWN: Celestine in a sulfidic spring barite deposit-A potential
           biomarker'
    • Authors: David M. Singer; Elizabeth M. Griffith; John M. Senko; Kaci Fitzgibbon; Inoka H. Widanagamage
      Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): David M. Singer, Elizabeth M. Griffith, John M. Senko, Kaci Fitzgibbon, Inoka H. Widanagamage


      PubDate: 2016-10-24T19:04:47Z
      DOI: 10.1016/j.chemgeo.2016.09.039
      Issue No: Vol. 442 (2016)
       
  • Ice core record of dust sources in the western United States over the last
           300years
    • Authors: S.M. Aarons; S.M. Aciego; P. Gabrielli; B. Delmonte; J.M. Koornneef; C. Uglietti; A. Wegner; M.A. Blakowski; C. Bouman
      Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): S.M. Aarons, S.M. Aciego, P. Gabrielli, B. Delmonte, J.M. Koornneef, C. Uglietti, A. Wegner, M.A. Blakowski, C. Bouman
      Over the past ~5000years, amplified dust generation and deposition in the American West has been linked to human activity. In recent decades, intensified rates of agriculture and livestock grazing have been correlated with greater dust production detected on seasonal to annual timescales. The combination of land use intensification and climate change (i.e. increased drought frequency) in North America highlights the importance of characterizing the sources of dust both before and after the influence of anthropogenic activity. We apply high-precision geochemical and isotopic (Sr and Nd isotopes) techniques to an ice core from the Upper Fremont Glacier (Wyoming, USA) to produce the first glacial dataset from the American West. Our Sr-Nd isotopic composition data indicates the evolving dust provenance to the Upper Fremont Glacier (UFG) from a long-range transport of mineral dust to a local source. This increasing input of dust from a local source is supported by a rise in average dust particle diameter combined with greater average dust concentration throughout the record. The greater presence of dust particles smaller than 2.5μm in the most recent samples from UFG ice core record support existing satellite and sediment core data regarding the effects of anthropogenic activity upon dust sources and pathways in the American West. Although the Sr-Nd isotope database in North America needs be expanded, our results provide a survey of windborne dust through the past 270years.

      PubDate: 2016-09-22T23:29:42Z
      DOI: 10.1016/j.chemgeo.2016.09.006
      Issue No: Vol. 442 (2016)
       
  • The effect of melt composition on mineral-melt partition coefficients: The
           case of beryllium
    • Authors: A.D. Burnham; H. St C. O'Neill
      Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): A.D. Burnham, H. St C. O'Neill
      The divalent cation Be2+ is considerably smaller than other divalent cations (Mg2+, Fe2+, Ca2+, et cetera), leading to a strong preference for tetrahedral coordination in minerals. Its thermodynamic properties in silicate melts may accordingly differ from these other divalent cations, potentially distinguishing its mineral/melt partition coefficients. In order to investigate this possibility, the partitioning of Be between silicate melt and forsterite was examined for 16 melt compositions in the systems CaO–MgO–Al2O3–SiO2 at 1400°C with additional experiments to investigate the effect of added Na2O and TiO2, and temperature at 1300°C. The relative activity coefficient of BeO in the melts decreases with increasing CaO and NaO1.5. The results are compared to Mg and Ca partitioning in the same experiments, and to the partitioning of other divalent cations (Ni, Co, Mn) from the literature. While the partition coefficient of the latter correlate positively with the Mg partition coefficient, Be shows only a weak negative correlation. Compared to Ca, Be partitions less strongly into forsterite when the melt has high Na and/or Ca. Partition coefficients for Na, Al and Ti are also reported.

      PubDate: 2016-09-22T23:29:42Z
      DOI: 10.1016/j.chemgeo.2016.09.012
      Issue No: Vol. 442 (2016)
       
  • pH-dependent control of feldspar dissolution rate by altered surface
           layers
    • Authors: Bastien Wild; Damien Daval; François Guyot; Kevin G. Knauss; Marion Pollet-Villard; Gwenaël Imfeld
      Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): Bastien Wild, Damien Daval, François Guyot, Kevin G. Knauss, Marion Pollet-Villard, Gwenaël Imfeld
      Relevant modeling of mass and energy fluxes involved in pedogenesis, sequestration of atmospheric CO2 or geochemical cycling of elements partly relies on kinetic rate laws of mineral dissolution obtained in the laboratory. Deriving an accurate and unified description of mineral dissolution has therefore become a prerequisite of crucial importance. However, the impact of amorphous silica-rich surface layers on the dissolution kinetics of silicate minerals remains poorly understood, and ignored in most reactive transport codes. In the present study, the dissolution of oriented cleavage surfaces and powders of labradorite feldspar was investigated as a function of pH and time at 80°C in batch reactors. Electron microscopy observations confirmed the formation of silica-rich surface layers on all samples. At pH=1.5, the dissolution rate of labradorite remained constant over time. In contrast, at pH=3, both the dissolution rates at the external layer/solution interface and the internal layer/mineral interface dramatically decreased over time. The dissolution rate at the external interface was hardly measurable after 4weeks of reaction, and decreased by an order of magnitude at the internal interface. In another set of experiments conducted in aqueous silica-rich solutions, the stabilization of silica-rich surface layers controlled the dissolution rate of labradorite at pH=3. The reduction of labradorite dissolution rate may result from a gradual modification of the textural properties of the amorphous surface layer at the fluid/mineral interface. The passivation of the main cleavage of labradorite feldspar was consistent with that observed on powders. Overall, our results demonstrate that the nature of the fluid/mineral interface to be considered in the rate limiting step of the process, as well as the properties of the interfacial layer (i.e. its chemical composition, structure and texture) to be taken into account for an accurate determination of the dissolution kinetics may depend on several parameters, such as pH or time. The dramatic impact of the stabilization of surface layers with increasing pH implies that the formation and the role of surface layers on dissolving feldspar minerals should be accounted for in the future.

      PubDate: 2016-09-22T23:29:42Z
      DOI: 10.1016/j.chemgeo.2016.08.035
      Issue No: Vol. 442 (2016)
       
  • 186Os/188Os variations in upper mantle peridotites: Constraints on the
           Pt/Os ratio of primitive upper mantle, and implications for late veneer
           accretion and mantle mixing timescales
    • Authors: Rudra Chatterjee; John C. Lassiter
      Pages: 11 - 22
      Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): Rudra Chatterjee, John C. Lassiter
      186Os/188Os variations in mantle peridotites provide constraints on the long-term Pt/Os evolution of the depleted mantle and the Pt/Os ratio of the primitive upper mantle (PUM). We report new 186Os/188Os data for mantle peridotites from continental (Rio Grande Rift and Colorado Plateau) and oceanic (Lena Trough and Hawaiian Islands) settings that span a wide range in fertility (Al2O3 ≈0.67-4.42 wt.%) and 187Os/188Os ratios (0.1138-0.1305). Although peridotite 186Os/188Os values span only a narrow range (from 0.1198345 to 0.1198384), 186Os/188Os broadly correlates with indices of melt depletion including bulk rock Al2O3, spinel Cr#, and clinopyroxene Cr#, consistent with Pt depletion in residual peridotites. PUM 186Os/188Os is estimated to be 0.1198378±23 (2 SD) based on extrapolation of 186Os/188Os-fertility trends, which is very slightly lower than H-chondrites [≈0.1198398±16 (2 SD); Brandon et al., 2006]. This value is consistent with a PUM Pt/Os of 1.7±0.2, similar to average Pt/Os ratios of fertile continental peridotites. The inferred PUM Pt/Os is slightly lower than but within error of Pt/Os values measured in several classes of chondrites [Carbonaceous ≈1.8±0.2, Ordinary ≈1.9±0.1, and Enstatite ≈1.9±0.1 (Brandon et al., 2006)] indicating that PUM Pt/Os is broadly chondritic. In contrast, estimates for PUM Ru/Ir and Pd/Ir (cf. Becker et al., 2006) are suprachondritic. The addition of a chondritic late veneer alone cannot create a combination of chondritic and suprachondritic HSE ratios for the PUM. Instead, minor core segregation occurring concurrently with the addition of a late veneer may explain the observed mantle HSE abundances and ratios. Combined 186Os/188Os-187Os/188Os isotopic and Pt/Os and Re/Os variability in peridotites suggest an average mantle homogenization timescale of ~1.2 Ga. In contrast, combined Hf-Nd isotopic and Lu/Hf and Sm/Nd variability in peridotites indicate much shorter homogenization timescales (<0.4 Ga), potentially reflecting enhanced homogenization by melt-rock interaction to which the Pt-Os and Re-Os systems are relatively immune. The mechanical mixing timescale inferred from Os isotopes is consistent with timescales predicted for whole mantle convection.

      PubDate: 2016-09-11T05:59:54Z
      DOI: 10.1016/j.chemgeo.2016.08.033
      Issue No: Vol. 442 (2016)
       
  • Rare earth elements concentrations and speciation in rainwater from
           Guiyang, an acid rain impacted zone of Southwest China
    • Authors: Zhaozhou Zhu; Cong-Qiang Liu; Zhong-Liang Wang; Xiaolong Liu; Jun Li
      Pages: 23 - 34
      Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): Zhaozhou Zhu, Cong-Qiang Liu, Zhong-Liang Wang, Xiaolong Liu, Jun Li
      Rare earth elements (REEs) and major ions were determined in rainwater samples collected from Guiyang in the acid rain impacted zone of Southwest China. The concentrations of total dissolved and acid-soluble REEs in rainwater are higher than those of most of the world's cities as well as the local surface water. The dissolved REE concentrations are negatively correlated to pH in rainwater. This is consistent with acidification of rainwater resulting in increasing REE concentrations. Speciation calculations using the PHREEQC-Model predict that the free REE metal ion (i.e., Ln3+), sulfate (LnSO4 +), oxalate (LnOx+ and LnOx2 −) and fluoride (LnF2+) complexes were relatively important forms of dissolved REE. Although the contribution to rainwater acidity of oxalic acid is much lower than that of sulfuric acid, the proportion of REE-oxalate complexes is not lower than that of REE-sulfate complexes in rain waters. The rainwater is enriched in the middle REEs (MREE) compared to both the light REEs (LREE) and heavy REEs (HREE). REE-phosphate complexes, phosphatic minerals and a Fe-Mn-oxyhydroxide coating are more likely controls on the development of MREE enrichment in the rainwater. Complexation of sulfate and REE played little role in the development of MREE enrichment, even though sulfate is the most important anion in acidic rainwater. In the shale-normalized REE pattern plot, La showed a clear positive anomaly. La might have two main anthropogenic sources in rainwater. One of the important sources could be automobile emission; another main source of excess La in rainwater could be the REE fertilizer that has been widely used in the agriculture of China for approximately 30years.

      PubDate: 2016-09-11T05:59:54Z
      DOI: 10.1016/j.chemgeo.2016.08.038
      Issue No: Vol. 442 (2016)
       
  • Geochemistry of Campanian–Maastrichtian brachiopods from the Rørdal-1
           core (Denmark): Differential responses to environmental change and
           diagenesis
    • Authors: R. Harlou; C.V. Ullmann; C. Korte; B.W. Lauridsen; N.H. Schovsbo; F. Surlyk; N. Thibault; L. Stemmerik
      Pages: 35 - 46
      Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): R. Harlou, C.V. Ullmann, C. Korte, B.W. Lauridsen, N.H. Schovsbo, F. Surlyk, N. Thibault, L. Stemmerik
      Past marine environmental conditions can be reconstructed from geochemical signatures of both benthic and planktic organisms and particularly brachiopods have been extensively used for this purpose. However, it is seldom possible to investigate the links between environmental conditions and shell geochemistry in monospecific records over extended periods of time. Here, we present geochemical data from the ~100m thick, brachiopod-rich upper Campanian – upper Maastrichtian chalk (~73.8–68.6Ma) of the Rørdal-1 drill core (northern Denmark). Two species that lived attached to small substrate (Argyrotheca bronnii Roemer and Gisilina jasmundi Steinich) and one secondarily free-living species (Magas chitoniformis Schlotheim) have been studied for element concentrations as well as C and O isotope signatures. Each species has a unique chemical signature, with M. chitoniformis showing lowest Mg and Mn concentrations, A. bronnii the highest Mn concentrations and G. jasmundi the highest Sr concentrations. All three species display decreasing Mg/Ca ratios throughout the studied interval and distinct Mn enrichments in the Campanian–Maastrichtian boundary (CMB) interval. In the Campanian part of the succession, the three species are slightly enriched in 13C (median Δ13C values of +0.0 to +0.5‰) and 18O (median Δ18O values of +0.3 to +0.6‰) with respect to coeval chalk. In the course of the Maastrichtian, the two attached species acquire progressively lighter isotopic compositions with A. bronnii reaching δ13C and δ18O values >1.0‰ lower than coeval chalk. The secondarily free-living species M. chitoniformis, on the other hand, becomes isotopically heavier and reaches δ13C and δ18O enrichments of ~1.0‰ at the top of the core. The observed differences between the species illustrate species-specific signatures of biomineralisation, diagenesis and response to environmental change. This geochemical complexity illustrates that a comprehensive geochemistry-based picture of the Late Cretaceous Chalk Sea can only be painted using detailed multi-species and multi-proxy datasets.

      PubDate: 2016-09-11T05:59:54Z
      DOI: 10.1016/j.chemgeo.2016.08.039
      Issue No: Vol. 442 (2016)
       
  • Three whole-wood isotopic reference materials, USGS54, USGS55, and USGS56,
           for δ2H, δ18O, δ13C, and δ15N measurements
    • Authors: Haiping Qi; Tyler B. Coplen; James A. Jordan
      Pages: 47 - 53
      Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): Haiping Qi, Tyler B. Coplen, James A. Jordan
      Comparative measurements of stable hydrogen and oxygen isotopes in wood are hampered by the lack of proper reference materials (RMs). The U.S. Geological Survey (USGS) has prepared three powdered, whole-wood RMs, USGS54 (Pinus contorta, Canadian lodgepole pine), USGS55 (Cordia cf. dodecandra, Mexican ziricote), and USGS56 (Berchemia cf. zeyheri, South African red ivorywood). The stable isotopes of hydrogen, oxygen, carbon, and nitrogen in these RMs span ranges as δ 2HVSMOW from –150.4 to –28.2mUr or ‰, as δ 18OVSMOW from +17.79 to +27.23mUr, as δ 13CVPDB from –27.13 to –24.34mUr, and as δ 15N AIR-N2 from –2.42 to +1.8mUr. These RMs will enable users to normalize measurements of wood samples to isotope–delta scales, and they are intended primarily for the normalization of δ 2H and δ 18O measurements of unknown wood samples. However, they also are suitable for normalization of stable isotope measurements of carbon and nitrogen in wood samples. In addition, these RMs are suitable for inter-laboratory calibration for the dual-water suilibration procedure for the measurements of δ 2HVSMOW values of non-exchangeable hydrogen. The isotopic compositions with 1-σ uncertainties, mass fractions of each element, and fractions of exchangeable hydrogen of these materials are: USGS54 (Pinus contorta, Canadian Lodgepole pine) δ 2HVSMOW =–150.4±1.1mUr (n=29), hydrogen mass fraction=6.00±0.04 % (n=10) Fraction of exchangeable hydrogen=5.4±0.6 % (n=29) δ 18OVSMOW =+17.79±0.15mUr (n=18), oxygen mass fraction=40.4±0.2 % (n=6) δ 13CVPDB =–24.43±0.02mUr (n=18), carbon mass fraction=48.3±0.4 % (n=12) δ 15NAIR- N2 =–2.42±0.32mUr (n=17), nitrogen mass fraction=0.05 % (n=4) USGS55 (Cordia cf. dodecandra, Mexican ziricote) δ 2HVSMOW =–28.2±1.7mUr (n=30), hydrogen mass fraction=5.65±0.06 % (n=10) Fraction of exchangeable hydrogen=4.1±0.5 % (n=30) δ 18OVSMOW =+19.12±0.07mUr (n=18), oxygen mass fraction=35.3±0.2 % (n=6) δ 13CVPDB =–27.13± 0.02mUr (n=18), carbon mass fraction=53.3±0.6 % (n=12) δ 15NAIR-N2
      PubDate: 2016-09-11T05:59:54Z
      DOI: 10.1016/j.chemgeo.2016.07.017
      Issue No: Vol. 442 (2016)
       
  • Elemental and isotopic composition of surface soils from key Saharan dust
           sources
    • Authors: A. Gross; D. Palchan; M.D. Krom; A. Angert
      Pages: 54 - 61
      Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): A. Gross, D. Palchan, M.D. Krom, A. Angert
      Saharan dust contains significant amount of P, an important macronutrient to all living organisms, which has been shown to exert large effects on nearby and remote ecosystems located across the dust transport pathways. The biological effect of Saharan dust depends on the amount and nature of the P speciation of the dust. However, thus far relatively small numbers of samples from potential source areas (PSA) has been analyzed. Here we report the P speciation (resin-P, HCl-P, Fe-bound-P and organic-P), the δ18OP values, the elemental composition, and the 87Sr/86Sr and 143Nd/144Nd of the fine fraction and bulk soil from 5 important PSAs across Northern Africa. We found the HCl-P concentrations between different source areas were relatively constrained but that these concentrations were higher in the fine fraction, which here is used a surrogate for dust. The δ18OP values for soils from sand dunes varied from 15.0 to 21.4‰, which is in the range of phosphate minerals from sedimentary origin. The δ18OP values of soils from dry lakes were significantly higher (24.0–28.5‰), probably since their P is derived from fossilized plankton that lived in the lake as it dried up. The 87Sr/86Sr and εNd values ranged from 0.7219 to 0.7276 and −12.7 to −14.0 in eastern samples and from 0.7146 to 0.7185 and −11.9 to −13.4 in western samples, suggesting a different source for the siliciclastic material of eastern and western samples. Our analysis indicates that the δ18OP values are decoupled from the Sr and Nd isotopic systems. Together, the new chemical and isotope data are specific for different PSAs and thus are used for source apportionment purposes. Such data can be used to provide more accurate estimates of the flux of potentially bioavailable P to marine and terrestrial ecosystems. These estimates can be used in global climate models to determine the magnitude and distribution of P control on carbon uptake.

      PubDate: 2016-09-11T05:59:54Z
      DOI: 10.1016/j.chemgeo.2016.09.001
      Issue No: Vol. 442 (2016)
       
  • Early Jurassic carbon-isotope excursion in the Qiangtang Basin (Tibet),
           the eastern Tethys: Implications for the Toarcian Oceanic anoxic event
    • Authors: Xiugen Fu; Jian Wang; Xinglei Feng; Dong Wang; Wenbin Chen; Chunyan Song; Shengqiang Zeng
      Pages: 62 - 72
      Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): Xiugen Fu, Jian Wang, Xinglei Feng, Dong Wang, Wenbin Chen, Chunyan Song, Shengqiang Zeng
      The Early Jurassic was characterized by a global disturbance of the carbon cycle known as the Toarcian oceanic anoxic event (T-OAE). This event is recorded by a large negative carbon-isotope excursion (CIE) in marine carbonates and marine organic matter, as well as fossil wood. Here, we present new high-resolution carbonate (δ13Ccarb) and organic (δ13Corg) carbon-isotope records and total organic content from the Bilong Co section in the Qiangtang Basin, the eastern Tethys. This section is high in organic carbon, and black shale facies are also identified. The Bilong Co section shows heavy δ13Corg values compared to other reported marine organic carbon-isotope values for the T-OAE. This is explained by variable mixing of terrestrial and marine organic matter. Compared to European sections, the magnitude of the negative CIE in the Bilong Co section is slightly different, which could be explained by different local paleoenvironmental conditions and strata missing. Although these, the carbon-isotope profile from the Bilong Co area displays a similar trend to those in contemporaneous strata in the western Tethyan and Boreal realms of NW Europe, which is characterized by a distinct negative excursion in the Early Toarcian. Our new data from the Bilong Co section in N Tibet provide the first high-resolution record of the Early Toarcian Oceanic Anoxic Event in the eastern Tethys.

      PubDate: 2016-09-18T19:55:54Z
      DOI: 10.1016/j.chemgeo.2016.09.007
      Issue No: Vol. 442 (2016)
       
  • Viscosity of pantelleritic and alkali-silicate melts: Effect of Fe redox
           state and Na/(Na+K) ratio
    • Authors: Paola Stabile; Sharon Webb; Jaayke L. Knipping; Harald Behrens; Eleonora Paris; Gabriele Giuli
      Pages: 73 - 82
      Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): Paola Stabile, Sharon Webb, Jaayke L. Knipping, Harald Behrens, Eleonora Paris, Gabriele Giuli
      The viscosity of two series of synthetic alkali silicates, corresponding to Al-bearing pantelleritic and Al-free tri-silicate compositions, has been investigated as a function of temperature, iron redox and Na/(Na+K) ratio. Low temperature (708–1000K) viscosities were determined by the micropenetration technique in the 109.6 to 1013.6 Pas range. The effect of Fe2+/Fetot, from 0.15 to 0.86, was explored for [Na/(Na+K)] ratios from 0 to 1. The results demonstrate a strong decrease of viscosity with the replacement of Na for K in both the Al-bearing and Al-free compositions. In the Al-bearing pantelleritic glasses (Ebu) the viscosity as well as the activation energy for viscous flow decrease with an increase of Fe2+/Fetot from 0.15 to 0.76. In contrast, no measurable changes in viscosity occur in the Al-free tri-silicate glasses (NFS and KFS) as Fe2+/Fetot ratio varies from 0.15 to 0.86. The comparison of the pantelleritic glasses with peralkaline compositions from the literature stresses the strong influence of iron redox on the overall viscosity of such melts. This suggests that the contribution of iron species should be accounted for in the calculation of agpaitic index (AI) if magma Fe3+ contents are known. One peculiarity of peralkaline magmas is the excess of alkali to alumina, which generally results in lower viscosities of pantelleritic liquids compared with more common metaluminous rhyolites and exerts a primary control of the rheological behaviour of these melts. Our new viscosity data, combined with existing literature, allow more accurate constraints on the nature and eruption of the pantelleritic magmas. Moreover, the present experimental study can be used to improve the viscosity models that mostly do not take in consideration parameters as iron redox in the viscosity prediction.

      PubDate: 2016-09-18T19:55:54Z
      DOI: 10.1016/j.chemgeo.2016.09.003
      Issue No: Vol. 442 (2016)
       
  • Absolute isotopic abundance ratios and the accuracy of Δ47
           measurements
    • Authors: M. Daëron; D. Blamart; M. Peral; H.P. Affek
      Pages: 83 - 96
      Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): M. Daëron, D. Blamart, M. Peral, H.P. Affek
      Clumped isotope measurements aim to quantify some statistical properties of the isotopologue population in a given sample, which requires prior knowledge of the absolute isotopic abundance ratios in reference materials such as VSMOW or VPDB. In the case of CO2, matters are further complicated by the need to define a mass-dependent fractionation law linking the three stable isotopes of oxygen. Conversion from raw mass spectrometric data to mass-47 anomalies (Δ47) thus relies on four external parameters: the (13C/12C) ratio of VPDB, the (17O/16O) and (18O/16O) ratios of VSMOW (or VPDB-CO2), and the slope of the triple oxygen isotope line (λ). Here we investigate the influence that these isotopic parameters exert on measured Δ47 values, using (a) real-world data corresponding to seven months of measurements; (b) simple simulations based on randomly generated data; (c) precise comparisons between water-equilibrated CO2 samples and between carbonate standards believed to share quasi-identical Δ47 values; and (d) reprocessing of two carbonate calibration data sets with different slopes of Δ47 versus T. We demonstrate that the use of different sets of isotopic parameters generally produces systematic offsets as large as 0.04‰ in measured Δ47 values, even after following the established standardization procedures. What's more, even using a single set of isotopic parameters can produce intra- and inter-laboratory discrepancies in measured Δ47 values, if some of these parameters are inaccurate, and depending on the isotopic compositions of the standards used for conversion to the absolute scale of Dennis et al. (2011), these errors should correlate strongly with either δ13C or δ18O, or more weakly with both. Based on measurements of samples expected to display quasi-identical Δ47 values, such as 25°C water-equilibrated CO2 with different carbon and oxygen isotope compositions, or high-temperature standards ETH-1 and ETH-2, we conclude that the traditional set of isotopic parameters used in all early clumped isotope studies produces large, systematic errors controlled by the relative bulk isotopic compositions of samples and standards. These errors are likely to be one of the key factors responsible for current inter-laboratory discrepancies, but cannot easily explain the conflicting carbonate calibration slopes obtained by different groups. By contrast, the isotopic parameters of Brand et al. (2010) appear to yield accurate Δ47 values regardless of bulk isotopic composition. Based on these findings, we offer recommendations aiming to minimize errors related to the choice of isotopic parameters.

      PubDate: 2016-09-18T19:55:54Z
      DOI: 10.1016/j.chemgeo.2016.08.014
      Issue No: Vol. 442 (2016)
       
  • Experimental study on the kinetics of silica polymerization during cooling
           of the Bouillante geothermal fluid (Guadeloupe, French West Indies)
    • Authors: Christelle Dixit; Marie-Lise Bernard; Bernard Sanjuan; Laurent André; Sarra Gaspard
      Pages: 97 - 112
      Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): Christelle Dixit, Marie-Lise Bernard, Bernard Sanjuan, Laurent André, Sarra Gaspard
      Despite many studies, our understanding of silica precipitation from natural waters remains limited, in particular for geothermal waters. Here we present a detailed study on the kinetics of silica polymerization as a function of fluid temperature and pH using high-temperature (250-260°C) seawater-derived geothermal fluids as those discharged from the Bouillante geothermal site. We monitored the on-site decrease in monomeric silica concentration (initial SiO2 concentration of about 600mgl−1) with time using the molybdenum blue spectrophotometric method on samples of separated water collected from the high-pressure separator at 167°C and cooled to 25, 50, 75, and 90°C and for pH values ranging from 5 to 8. During all these experiments, only silica polymerization was observed, with the formation of colloidal particles in suspension in the solutions. No scaling of amorphous silica was formed. The collected data were after modeled in order to determine the useful kinetic parameters for predicting and preventing amorphous silica precipitation in the production wells during fluid exploitation in the specific context of Bouillante. Results under the investigated experimental conditions show that the kinetics of silica precipitation is affected more strongly by pH than by temperature change. The reaction in the acidic Bouillante solution begins with a transition period that significantly decreases the kinetics of silica polymerization. Modeling the experimental data indicates that the silica polymerization up to the state of equilibrium is characterized by a 2nd-order kinetic law relative to dissolved silica; this could indicate that the polymerization is controlled mainly by the formation of dimers. However, the first hour of the experiments is better characterized by a 4th-order kinetic law, suggesting more complex polymerization reactions in the initial stage with the formation of nanocolloidal particles containing 3 to 4 monomers. In both cases, the corresponding kinetics rate constant k is linearly dependent on pH for pH between 5 and 7. The activation energy E a for the overall reaction, calculated using the Arrhenius equation under the considered pH and temperature conditions, ranges between 41±9.8 and 54±9.6kJmol−1. To complete this work, the colloid particles formed at the end of the kinetics experiments were extracted and analyzed by SEM and TEM microscopy, X-ray Diffraction and the BET method. Results show that the size of the colloids increase and their specific surface decrease with increasing pH and are thus dependent on pH. Globally, our work provides a reliable database for understanding silica polymerization kinetics in natural geothermal brines or geologic waters characterized by a near neutral pH and moderate dissolved silica concentrations.

      PubDate: 2016-09-18T19:55:54Z
      DOI: 10.1016/j.chemgeo.2016.08.031
      Issue No: Vol. 442 (2016)
       
  • Surface reactivity of the anaerobic phototrophic Fe(II)-oxidizing
           bacterium Rhodovulum iodosum: Implications for trace metal budgets in
           ancient oceans and banded iron formations
    • Authors: Raul E. Martinez; Kurt O. Konhauser; Nataliya Paunova; Wenfang Wu; Daniel S. Alessi; Andreas Kappler
      Pages: 113 - 120
      Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): Raul E. Martinez, Kurt O. Konhauser, Nataliya Paunova, Wenfang Wu, Daniel S. Alessi, Andreas Kappler
      It is widely accepted that anoxygenic photosynthetic bacteria (photoferrotrophs) oxidized dissolved Fe(II) in anoxic Archean seawater, leading to the sedimentation of cellular biomass and the Fe(III) minerals that comprised banded iron formations (BIF). However, it remains unknown to which extent the sedimented bacteria may have transported trace metals to BIF through their ability to sorb ions from seawater. We therefore chose the marine anoxygenic photoferrotroph Rhodovulum iodosum to quantify Cd2+, Co2+, Cu2+, Mn2+, Ni2+ and Zn2+ adsorption to bacteria/Fe(III)-oxyhydroxide composites at neutral pH and an ionic strength of 0.1M. Acid base titration data were modeled using a linear programming optimization method to yield pKa values of 4.83±0.13, 6.21±0.18, 7.74±0.24 and 9.28±0.27 and corresponding site densities of 5.7, 12.0, 3.3 and 6.5×10−4 mol/g, respectively. FTIR spectroscopy confirmed the presence of carboxyl groups as the most acidic sites corresponding to the lowest pKa values. The analysis of Cd2+ sorption data showed two metal complexing sites with pKS values of 3.44±0.14 and 4.80±0.21 and corresponding binding site concentrations of 1.1±0.3×10−4 and 0.9±0.2×10−4 mol/g on the ferrihydrite/R. iodosum composite. The pKS values were used to calculate metal-ligand binding constants, Km. This confirmed Cd2+ binding to the most acidic carboxyl groups on the ferrihydrite/R. iodosum composite surface, as the calculated Km was consistent with reported Cd2+ binding constants for simple organic acids (e.g., lactic, acetic, pyruvic and citric acid). This was used to further calculate the concentration of Co2+, Cu2+, Mn2+, Ni2+ and Zn2+ sorbed to ferrihydrite/R. iodosum composites. The resulting concentrations were ultimately compared to the concentrations of trace elements in BIF to demonstrate that is plausible that the trace metal content in BIF was derived from plankton growing in the ancient ocean's photic zone.

      PubDate: 2016-09-18T19:55:54Z
      DOI: 10.1016/j.chemgeo.2016.09.004
      Issue No: Vol. 442 (2016)
       
  • A synthetic fluid inclusion study of the solubility of monazite-(La) and
           xenotime-(Y) in H2O-Na-K-Cl-F-CO2 fluids at 800°C and 0.5GPa
    • Authors: Li Zhou; John Mavrogenes; Carl Spandler; Heping Li
      Pages: 121 - 129
      Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): Li Zhou, John Mavrogenes, Carl Spandler, Heping Li
      Alkali-rich F-Cl-CO2-bearing aqueous fluids are thought to be common ore-forming fluids of carbonatite-associated rare earth element (REE) deposits. Solubilities of synthetic monazite-(La) and xenotime-(Y) end-members in such fluids were determined at 800°C and 0.5GPa in a series of piston-cylinder experiments. At run conditions, solutions of NaCl/KCl (1.9mol/kg)±CO2 (5% mole), NaCl/KCl (1.7mol/kg)±NaF/KF (0.2mol/kg) were trapped as synthetic fluid inclusions in quartz, and subsequently analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). The resultant concentrations in these fluids are extremely low, ranging from 2.92×10−6 to 3.05×10−5 molLa/kg H2O and 3.97×10−6 to 2.74×10−5 molY/kg H2O. Such low solubilities indicate that fluids expelled from carbonatite melts contain insufficient phosphate and REE simultaneously to form REE deposits. Mixing REE-bearing fluids with phosphate-bearing aqueous fluids or interacting with apatite in the wall rock may be a key control on the deposition of REE phosphate minerals. In addition, our results show that, at nearly equal total ligand molality, solubilities of La and Y are both higher in F or CO2-bearing solutions than in Cl-only solutions, and the highest solubilities obtained overall were in NaCl+NaF solutions, which significantly increase by a factor of 9.4 for La and 6.1 for Y relative to NaCl-only solutions. Moreover, the solubility differences between xenotime and monazite in F/CO2-bearing solutions, shown as Y/La (molality ratio), are higher in K-bearing solutions than in Na-bearing solutions. Y/La in the NaCl+F/CO2 solutions are close to 1, whereas in the KCl+F/CO2 solutions Y/La increase to 2.9 and 2.0, respectively. These results indicate that during the evolution of carbonatitic fluids, enrichment of Na and F can significantly increase solubilities of all REE, whilst enrichment of K may promote the fractionation of HREE from LREE in the presence of F or CO2.

      PubDate: 2016-09-18T19:55:54Z
      DOI: 10.1016/j.chemgeo.2016.09.010
      Issue No: Vol. 442 (2016)
       
  • Isotopic behavior during the aragonite-calcite transition: Implications
           for sample preparation and proxy interpretation
    • Authors: Philip T. Staudigel; Peter K. Swart
      Pages: 130 - 138
      Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): Philip T. Staudigel, Peter K. Swart
      At the Earth's surface, aragonite, a polymorph of calcium carbonate, is thermodynamically unstable, and readily converts to calcite, a process accelerated by heating and mechanical stress. This study has investigated the behavior of the 13C/12C, 18O/16O ratios, and the clumped isotope temperature proxy (∆47) during this mineral transition. Aragonite samples were partially or completely converted to calcite in an oven (normal atmospheric composition and pressure) or in a sealed tube in the presence of a vacuum or pure CO2 of varying isotopic compositions. Other samples were converted from aragonite to calcite using a drill. These experiments demonstrate a relationship between the δ13C and δ18O values and percent transition from aragonite to calcite. The ∆47 value of the aragonite changed independently from the mineral transition at temperatures as low as 125°C and was accompanied by a 0.5‰ decrease in the δ18O value. This change occurred irrespective of the presence of CO2 suggesting that water trapped in the mineral may facilitate some of this alteration. Results show that sampling techniques, such as drilling, produce sufficient heat to convert aragonite to calcite and can considerably alter ∆47. Even small degrees of conversion (~10%) can increase clumped isotope derived paleotemperature estimates by nearly 10°C.

      PubDate: 2016-09-18T19:55:54Z
      DOI: 10.1016/j.chemgeo.2016.09.013
      Issue No: Vol. 442 (2016)
       
  • Nitrate, perchlorate, and iodate co-occur in coastal and inland deserts on
           Earth
    • Authors: Rebecca A. Lybrand; James G. Bockheim; Wensheng Ge; Robert C. Graham; Stephan R. Hlohowskyj; Greg Michalski; Joel S. Prellwitz; Jason A. Rech; Fan Wang; David R. Parker
      Pages: 174 - 186
      Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): Rebecca A. Lybrand, James G. Bockheim, Wensheng Ge, Robert C. Graham, Stephan R. Hlohowskyj, Greg Michalski, Joel S. Prellwitz, Jason A. Rech, Fan Wang, David R. Parker
      Deserts accumulate soluble salts from atmospheric deposition that impact human health, are a source of nutrients for organisms, and provide insight into how landscapes evolved on Earth and Mars. We quantified perchlorate, nitrate, and iodate abundances and co-occurrence in terrestrial deserts to identify fundamental controls on soluble salt deposition and post-depositional cycling. Soils and nitrate deposits were examined in Death Valley, USA; Atacama Desert, Chile; Kumtag Desert, China; and along an environmental gradient in the Transantarctic Mountains, Antarctica. Concentrations of soluble salts were highest in the Transantarctic Mountains and Atacama Desert, where stable, hyper-arid landscapes accumulate atmospheric salts over million-year time scales. Average nitrate concentrations of 61.3gkg−1 in the Transantarctic Mountains and 53.0gkg−1 in the Atacama Desert were significantly greater than respective averages of 8.60gkg−1 and 5.14gkg−1 in Kumtag Desert and Death Valley. Perchlorate and iodate concentrations in the Atacama Desert averaged 206mgkg−1 and 344mgkg−1, respectively, which were two to three orders of magnitude greater than in Antarctica and other sites. Our findings suggest that local processes in the Atacama Desert result either in higher rates of perchlorate and iodate deposition, or a greater preservation of these salts relative to nitrate when compared to Antarctic landscapes. Lower salt concentrations in the Death Valley and Kumtag Desert deposits likely result from relatively wet present-day and paleoclimatic conditions, a more active geologic history, and a greater likelihood that biocycling disrupted long-term salt accumulation. Associations of perchlorate and nitrate were significantly higher than iodate-nitrate and iodate-perchlorate correlations in the four deserts. Perchlorate-nitrate relationships ranged from insignificant to highly significant with stronger correlations in the Atacama Desert and Kumtag Desert compared to the Transantarctic Mountains and Death Valley. Weaker geochemical associations with iodate were attributed to differences in local deposition rates or post-depositional cycling. Interestingly, relationships among perchlorate, nitrate, and iodate were generally stronger when examined by site within each desert compared to analyzing the soils for each desert as a whole, suggesting more localized controls on soluble salt preservation. We conclude that soluble salts vary in concentration and type across Earth's deserts as a result of present-day environment, paleoclimate conditions, biocycling, and geologic age.

      PubDate: 2016-10-24T19:04:47Z
      DOI: 10.1016/j.chemgeo.2016.05.023
      Issue No: Vol. 442 (2016)
       
  • Compound-specific carbon and nitrogen isotopic compositions of chlorophyll
           a and its derivatives reveal the eutrophication history of Lake Zurich
           (Switzerland)
    • Authors: Sebastian Naeher; Hisami Suga; Nanako O. Ogawa; Carsten J. Schubert; Kliti Grice; Naohiko Ohkouchi
      First page: 138
      Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): Sebastian Naeher, Hisami Suga, Nanako O. Ogawa, Carsten J. Schubert, Kliti Grice, Naohiko Ohkouchi
      To reconstruct the impact of eutrophication on phototrophic communities and the biogeochemical cycling of carbon and nitrogen in the surface water, we investigated the distributions and carbon and nitrogen isotopic compositions (δ13C and δ15N values) of chlorins in the sediments of Lake Zurich. The chlorin distributions were dominated by chlorophyll a (Chl a) and its derivatives, which reflect rapid degradation to the pheopigments in the water column and sediments of the lake. The δ13C values of these sedimentary chlorins followed the historical trends of eutrophication and reoligotrophication, except in the surface sediments, which were characterised by higher relative contributions of aged, redeposited organic matter (OM). The δ13C values of the sedimentary chlorins together with bulk sediment δ13C values and C/N ratios indicate that the phototrophic communities in the lake used a 13C-depleted carbon source, which is mainly of aquatic origin. The δ15N values of chlorins reflect the predominance of nitrate assimilating phototrophs, especially the non-N2-fixing cyanobacterium Planktothrix rubescens prevalent during sediment deposition. Shifts in δ15N values of Chl a followed mostly the trends in eutrophication and reoligotrophication, but were also affected by community assemblage shifts to diatoms and/or other cyanobacteria at the end of the 19th century and during the eutrophication maximum in the 1970s. The lower δ15NChl-a values in the surface sediments coincide with increasing nitrogen to phosphorus ratios and reduced water column mixing that characterise the recent reoligotrophication period and may explain the predominance of P. rubescens in Lake Zurich. In contrast, the higher contributions of laterally transported OM explains the large offset of δ15N values of the pheopigments relative to Chl a, which is supported by the high radiocarbon age of the surface sediments.

      PubDate: 2016-10-16T14:12:52Z
      DOI: 10.1016/j.chemgeo.2016.08.018
      Issue No: Vol. 441 (2016)
       
  • Celestine in a sulfidic spring barite deposit — A potential
           biomarker?
    • Authors: David M. Singer; Elizabeth M. Griffith; John M. Senko; Kaci Fitzgibbon; Inoka H. Widanagamage
      Pages: 15 - 25
      Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): David M. Singer, Elizabeth M. Griffith, John M. Senko, Kaci Fitzgibbon, Inoka H. Widanagamage
      We have documented the presence of celestine (SrSO4) within sediment accumulating at an artesian sulfidic spring (Zodletone Spring, Oklahoma) dominated by barite (BaSO4) precipitation associated with microbiological activity. The distribution and speciation of Sr in solid phases was determined by synchrotron-based micro-X-ray fluorescence spectroscopy and micro-X-ray diffraction, and particle morphology and texture was determined using electron microscopy. In all the natural sulfidic spring samples and lithified tufa sample, celestine was detected in fine-grained micron-scale Sr-rich phases but not in euhedral, Sr-poor grains. In parallel laboratory-based precipitation experiments, celestine was not observed even when solutions contained high Sr/Ba. Thermodynamic predictions alone do not account for the presence of micron-scale celestine in the sulfidic spring, and they do not account for the differences in Sr presence and distribution in naturally-occurring versus synthetic grains. While the mechanism is unclear, based on this evidence we hypothesize that the combination of bacterial surfaces and microenvironments within the crusts and microbial mats creates a synergistic effect where Sr is preferentially exchanged over Ba between the overlying stream water and the pore water within the mats allowing celestine to precipitate. Ultimately, our results point to an important role of biological activity for preferential Sr uptake. The presence of micron-scale celestine in ancient barite deposits can therefore potentially be used as a biomarker for conditions similar to modern sulfidic springs.
      Graphical abstract image

      PubDate: 2016-09-11T05:59:54Z
      DOI: 10.1016/j.chemgeo.2016.06.022
      Issue No: Vol. 440 (2016)
       
 
 
JournalTOCs
School of Mathematical and Computer Sciences
Heriot-Watt University
Edinburgh, EH14 4AS, UK
Email: journaltocs@hw.ac.uk
Tel: +00 44 (0)131 4513762
Fax: +00 44 (0)131 4513327
 
Home (Search)
Subjects A-Z
Publishers A-Z
Customise
APIs
Your IP address: 54.163.166.22
 
About JournalTOCs
API
Help
News (blog, publications)
JournalTOCs on Twitter   JournalTOCs on Facebook

JournalTOCs © 2009-2016