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  Subjects -> EARTH SCIENCES (Total: 644 journals)
    - EARTH SCIENCES (467 journals)
    - GEOLOGY (70 journals)
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EARTH SCIENCES (467 journals)                  1 2 3 | Last

Showing 1 - 200 of 371 Journals sorted alphabetically
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: 14)
Advances In Physics     Hybrid Journal   (Followers: 7)
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: 12)
Anadolu University Journal of Science and Technology     Open Access  
Anales del Instituto de la Patagonia     Open Access   (Followers: 1)
Andean geology     Open Access   (Followers: 3)
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: 9)
Annals of GIS     Hybrid Journal   (Followers: 16)
Annals of Glaciology     Full-text available via subscription   (Followers: 2)
Annual Review of Marine Science     Full-text available via subscription   (Followers: 9)
Anthropocene     Hybrid Journal   (Followers: 2)
Anthropocene Review     Hybrid Journal   (Followers: 4)
Applied Clay Science     Hybrid Journal   (Followers: 4)
Applied Geochemistry     Hybrid Journal   (Followers: 10)
Applied Geomatics     Hybrid Journal   (Followers: 3)
Applied Geophysics     Hybrid Journal   (Followers: 6)
Applied Ocean Research     Hybrid Journal   (Followers: 4)
Applied Petrochemical Research     Open Access   (Followers: 2)
Applied Remote Sensing Journal     Open Access   (Followers: 15)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 32)
Arctic Science     Open Access   (Followers: 5)
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: 14)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 7)
Asian Journal of Earth Sciences     Open Access   (Followers: 19)
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: 1)
Atmosphere-Ocean     Full-text available via subscription   (Followers: 11)
Atmospheric and Climate Sciences     Open Access   (Followers: 23)
Australian Journal of Earth Sciences: An International Geoscience Journal of the Geological Society of Australia     Hybrid Journal   (Followers: 11)
Boletim de Ciências Geodésicas     Open Access  
Boreas: An International Journal of Quaternary Research     Hybrid Journal   (Followers: 10)
Bragantia     Open Access   (Followers: 2)
Bulletin of Earthquake Engineering     Hybrid Journal   (Followers: 10)
Bulletin of Geosciences     Open Access   (Followers: 9)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (Followers: 1)
Bulletin of the Seismological Society of America     Full-text available via subscription   (Followers: 19)
Bulletin of Volcanology     Hybrid Journal   (Followers: 16)
Cadernos de Geociências     Open Access  
Canadian Journal of Plant Science     Full-text available via subscription   (Followers: 14)
Canadian Mineralogist     Full-text available via subscription   (Followers: 3)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 17)
Carbonates and Evaporites     Hybrid Journal   (Followers: 3)
CATENA     Hybrid Journal   (Followers: 4)
Chemical Geology     Hybrid Journal   (Followers: 13)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 4)
Chinese Geographical Science     Hybrid Journal   (Followers: 5)
Chinese Journal of Geochemistry     Hybrid Journal   (Followers: 3)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 2)
Ciencias Espaciales     Open Access  
Climate and Development     Hybrid Journal   (Followers: 12)
Coastal Management     Hybrid Journal   (Followers: 23)
Cogent Geoscience     Open Access  
Comptes Rendus Geoscience     Full-text available via subscription   (Followers: 7)
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: 2)
Continental Shelf Research     Hybrid Journal   (Followers: 8)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 9)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 2)
Coral Reefs     Hybrid Journal   (Followers: 16)
Cretaceous Research     Hybrid Journal   (Followers: 6)
Cybergeo : European Journal of Geography     Open Access   (Followers: 4)
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: 2)
Diatom Research     Hybrid Journal  
Doklady Physics     Hybrid Journal   (Followers: 1)
Dynamics of Atmospheres and Oceans     Hybrid Journal   (Followers: 9)
E&S Engineering and Science     Open Access  
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (Followers: 67)
Earth and Space Science     Open Access   (Followers: 6)
Earth Interactions     Full-text available via subscription   (Followers: 11)
Earth Science Research     Open Access   (Followers: 6)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 3)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 19)
Earth System Dynamics     Open Access   (Followers: 5)
Earth System Dynamics Discussions     Open Access   (Followers: 4)
Earth's Future     Open Access   (Followers: 1)
Earth, Planets and Space     Open Access   (Followers: 5)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 6)
Earthquake Science     Hybrid Journal   (Followers: 9)
Earthquake Spectra     Full-text available via subscription   (Followers: 16)
Ecohydrology     Hybrid Journal   (Followers: 9)
Ecological Questions     Open Access   (Followers: 5)
Electromagnetics     Hybrid Journal   (Followers: 2)
Energy Efficiency     Hybrid Journal   (Followers: 11)
Energy Exploration & Exploitation     Full-text available via subscription   (Followers: 4)
Environmental Earth Sciences     Hybrid Journal   (Followers: 17)
Environmental Geology     Hybrid Journal   (Followers: 9)
Environmental Geosciences     Full-text available via subscription   (Followers: 4)
Environmental Geotechnics     Open Access   (Followers: 3)
Erwerbs-Obstbau     Hybrid Journal  
Estuaries and Coasts     Hybrid Journal   (Followers: 16)
Estuarine, Coastal and Shelf Science     Hybrid Journal   (Followers: 31)
Estudios Geográficos     Open Access  
European Journal of Mineralogy     Full-text available via subscription   (Followers: 10)
Exploration Geophysics     Hybrid Journal   (Followers: 3)
Facies     Hybrid Journal   (Followers: 8)
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: 4)
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: 7)
Geocarto International     Hybrid Journal   (Followers: 4)
Geochemistry : Exploration, Environment, Analysis     Hybrid Journal   (Followers: 7)
Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 23)
Geochimica et Cosmochimica Acta     Hybrid Journal   (Followers: 22)
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  
Geoenvironmental Disasters     Open Access   (Followers: 3)
Geofluids     Hybrid Journal   (Followers: 4)
Geoforum     Hybrid Journal   (Followers: 19)
Géographie physique et Quaternaire     Full-text available via subscription  
Geography and Natural Resources     Hybrid Journal   (Followers: 4)
Geoheritage     Hybrid Journal   (Followers: 1)
Geoinformatica Polonica : The Journal of Polish Academy of Arts and Sciences     Open Access  
Geoinformatics & Geostatistics     Hybrid Journal   (Followers: 3)
Geological Journal     Hybrid Journal   (Followers: 13)
Geological Magazine     Hybrid Journal   (Followers: 15)
Geology Today     Hybrid Journal   (Followers: 17)
Geomagnetism and Aeronomy     Hybrid Journal   (Followers: 2)
Geomatics, Natural Hazards and Risk     Hybrid Journal   (Followers: 6)
GEOmedia     Open Access   (Followers: 1)
Geomorphology     Hybrid Journal   (Followers: 22)
Geophysical & Astrophysical Fluid Dynamics     Hybrid Journal   (Followers: 2)
Geophysical Journal International     Hybrid Journal   (Followers: 29)
Geophysical Prospecting     Hybrid Journal   (Followers: 7)
Geophysics     Full-text available via subscription   (Followers: 17)
GeoResJ     Hybrid Journal  
Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards     Hybrid Journal   (Followers: 7)
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: 8)
Geoscience Letters     Open Access  
Geoscience Records     Open Access  
Geosciences     Open Access   (Followers: 2)
Geosciences Journal     Hybrid Journal   (Followers: 9)
Geoscientific Instrumentation, Methods and Data Systems     Open Access   (Followers: 2)
Geoscientific Model Development     Open Access   (Followers: 2)
Geostandards and Geoanalytical Research     Hybrid Journal   (Followers: 2)
Geosystem Engineering     Hybrid Journal  
Geotectonic Research     Full-text available via subscription   (Followers: 4)
Geotectonics     Hybrid Journal   (Followers: 6)
GISAP : Earth and Space Sciences     Open Access   (Followers: 1)
Glass Physics and Chemistry     Hybrid Journal   (Followers: 3)
Global and Planetary Change     Hybrid Journal   (Followers: 11)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 8)
Gondwana Research     Hybrid Journal   (Followers: 6)
Grassland Science     Hybrid Journal   (Followers: 1)
Ground Water     Hybrid Journal   (Followers: 19)
Ground Water Monitoring & Remediation     Hybrid Journal   (Followers: 15)
GSA Today     Partially Free  
Helgoland Marine Research     Open Access   (Followers: 3)
History of Geo- and Space Sciences     Open Access   (Followers: 3)
Hydrobiologia     Hybrid Journal   (Followers: 17)
Hydrogeology Journal     Hybrid Journal   (Followers: 14)
Hydrological Processes     Hybrid Journal   (Followers: 20)
Hydrology and Earth System Sciences     Open Access   (Followers: 20)
ICES Journal of Marine Science: Journal du Conseil     Hybrid Journal   (Followers: 51)
IEEE Journal of Oceanic Engineering     Hybrid Journal   (Followers: 10)
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: 5)
International Journal of Advanced Geosciences     Open Access  
International Journal of Advanced Remote Sensing and GIS     Open Access   (Followers: 17)
International Journal of Advancement in Earth and Enviromental Sciences     Open Access   (Followers: 2)
International Journal of Advancement in Remote Sensing, GIS, and Geography     Open Access   (Followers: 15)
International Journal of Applied Earth Observation and Geoinformation     Hybrid Journal   (Followers: 14)
International Journal of Coal Geology     Hybrid Journal   (Followers: 2)
International Journal of Disaster Risk Reduction     Hybrid Journal   (Followers: 9)
International Journal of Earth Sciences     Hybrid Journal   (Followers: 27)
International Journal of Forest, Soil and Erosion     Open Access   (Followers: 4)
International Journal of Geo-Engineering     Open Access  
International Journal of Geographical Information Science     Hybrid Journal   (Followers: 45)

        1 2 3 | Last

Journal Cover Chemical Geology
  [SJR: 1.927]   [H-I: 123]   [13 followers]  Follow
    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 0009-2541
   Published by Elsevier Homepage  [2970 journals]
  • Enabling Gibbs energy minimization algorithms to use equilibrium constants
           of reactions in multiphase equilibrium calculations
    • Abstract: Publication date: 25 October 2016
      Source:Chemical Geology, Volume 437
      Author(s): Allan M.M. Leal, Dmitrii A. Kulik, Martin O. Saar
      The geochemical literature provides numerous thermodynamic databases compiled from equilibrium constants of reactions. These databases are typically used in speciation calculations based on the law of mass action (LMA) approach. Unfortunately, such LMA databases cannot be directly used in equilibrium speciation methods based on the Gibbs energy minimization (GEM) approach because of their lack of standard chemical potentials of species. Therefore, we present in this work a simple conversion approach that calculates apparent standard chemical potentials of species from equilibrium constants of reactions. We assess the consistency and accuracy of the use of apparent standard chemical potentials in GEM algorithms by benchmarking equilibrium speciation calculations using GEM and LMA methods with the same LMA database. In all cases, we use PHREEQC to perform the LMA calculations, and we use its LMA databases to calculate the equilibrium constants of reactions. GEM calculations are performed using a Gibbs energy minimization method of Reaktoro — a unified open-source framework for numerical modeling of chemically reactive systems. By comparing the GEM and LMA results, we show that the use of apparent standard chemical potentials in GEM methods produces consistent and accurate equilibrium speciation results, thus validating our new, practical conversion technique that enables GEM algorithms to take advantage of many existing LMA databases, consequently extending and diversifying their range of applicability.


      PubDate: 2016-06-26T15:23:47Z
       
  • Oxidative biotransformation of biotite and glauconite by alkaliphilic
           anaerobes: The effect of Fe oxidation on the weathering of phyllosilicates
           
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Daria G. Zavarzina, Natalya I. Chistyakova, Alexey V. Shapkin, Alla V. Savenko, Tatyana N. Zhilina, Vadim V. Kevbrin, Tatiana V. Alekseeva, Andrey V. Mardanov, Sergey N. Gavrilov, Andrey Yu. Bychkov
      Two alkaliphilic anaerobic bacteria, namely, the dissimilatory iron-reducer Geoalkalibacter ferrihydriticus and the fermentative hydrolytic Clostridium alkalicellulosi, along with their co-cultures, are studied to examine their ability to release Si and Fe from two main Fe-containing phyllosilicates in Earth's crust: biotite and glauсonite. The formation of magnetically ordered phase(s) within 200days of incubation was only observed in the presence of G. ferrihydriticus whether in a mono- or co-culture but not in the abiotic controls or a pure culture of C. alkalicellulosi. The co-culture of these organisms could represent a simple trophic chain in which C. alkalicellulosi decomposed microcrystalline cellulose to produce organic acids and ethanol, while G. ferrihydriticus, as we expected, utilized these products and reduces Fe(III) in phyllosilicate lattices. Unexpectedly, G. ferrihydriticus did not utilize but instead produced an additional 3mM of acetate during growth with phyllosilicates. An analysis of the Mössbauer spectra of biotite and glauconite that were weathered in the presence of G. ferrihydriticus revealed magnetically ordered phases that formed by Fe2+ oxidation rather than by Fe3+ reduction. The only possible explanation of this phenomenon could be in the ability of G. ferrihydriticus to produce acetate during anaerobic Fe2+ oxidation with carbonate as an electron acceptor. Thermodynamic calculations show the possibility of such a reaction. Thus, microorganisms with respiratory metabolism could play an active role in the bioweathering of phyllosilicates under alkaline anaerobic conditions. The bacterial anaerobic oxidation of ferrous iron with carbonate as an electron donor is supposed to have played a significant role in ancient environments, serving as one of the causes of banded iron formations.
      Graphical abstract image

      PubDate: 2016-06-26T15:23:47Z
       
  • Os isotopic constraints on crustal contamination in Auckland Volcanic
           Field basalts, New Zealand
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Jenni L. Hopkins, Christian Timm, Marc-Alban Millet, André Poirier, Colin J.N. Wilson, Graham S. Leonard
      The Auckland Volcanic Field (AVF) represents the youngest and northernmost of three subjacent Quaternary intraplate basaltic volcanic fields in the North Island, New Zealand. Previous studies on AVF eruptive products suggested that their major- and trace-element, and Sr-, Nd- and Pb-isotopic signatures primarily reflect their derivation from the underlying asthenospheric and lithospheric mantle. All AVF lavas however ascend through a ca. 20–30km thick continental crust, and some do carry crustal xenoliths, posing the question whether or not crustal contamination plays a role in their formation. Here we present new Os and Pb isotopic data, and Os and Re concentrations for 15 rock samples from 7 AVF volcanic centres to investigate mantle and crustal petrogenetic processes. The samples include the most primitive lavas from the field (Mg# 59–69) and span a range of eruption sizes, ages, locations, and geochemical signatures. The data show a large range in Os concentrations (6–579ppt) and 187Os/188Os isotope ratios from mantle-like (0.123) to highly radiogenic (0.547). Highly radiogenic Os signatures together with relatively low Os contents in most samples suggest that ascending melts experienced contamination primarily from metasedimentary crustal rocks with high 187Os/188Os ratios (e.g., greywacke). We further demonstrate that <1% metasedimentary crustal input into the ascending melt can produce the radiogenic Os isotope signatures observed in the AVF data. This low level of crustal contamination has no measurable effect on the corresponding trace element ratios and Sr-Nd-Pb isotopic compositions. In addition, high Os contents (195–578ppt) at slightly elevated but mantle-like Os isotopic compositions (187Os/188Os=0.1374–0.1377) in some samples suggest accumulation of xenocrystic olivine-hosted mantle sulphides from the Permian-Triassic ultramafic Dun Mountain Ophiolite Belt, which traverses the crust beneath the Auckland Volcanic Field. We therefore infer that the AVF Os isotopic compositions and Os contents reflect contamination from varying proportions of heterogeneous crustal components, composed of Waipapa and Murihiku terrane metasediments, and ultramafic rocks of the Dun Mountain Ophiolite Belt. This demonstrates, contrary to previous models that primitive lavas from the Auckland Volcanic Field do show evidence for variable interaction with the crust.


      PubDate: 2016-06-26T15:23:47Z
       
  • Lithium isotope fractionation during incongruent melting: Constraints from
           post-collisional leucogranite and residual enclaves from Bengbu Uplift,
           China
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): He Sun, Yongjun Gao, Yilin Xiao, Hai-ou Gu, John F. Casey
      Lithium (Li) elemental and isotopic compositions of the Jurassic Jingshan leucogranites, including garnet-rich mafic enclaves and wall rock Wuhe gneisses from the southeast margin of North China Craton (NCC) were investigated to understand the behavior of Li isotopes during post-collisional magmatism. The Jingshan leucogranites have distinct U-shape REE patterns with Y and REE concentrations significantly lower yet Sr/Y ratios higher than their presumed source rocks, i.e., the Dabie-Sulu gneisses. Trace element modeling of REE and Sr/Y suggests these elemental signatures of the Jingshan leucogranites can be consistently explained by a fluid-present crustal incongruent partial melting: Bt+Qz+Pl+H2O=Grt+melt, leaving mainly Grt+Bt with minor allanite in the residuum. The mafic enclaves show identical Sr-Nd isotopic compositions with their host leucogranites, contrasting with the Wuhe gneiss and the exposed regional lower crust. The garnet-rich mafic enclaves are thus interpreted as entrained residual phases formed by this incongruent partial melting. The Jingshan leucogranites show relatively high δ7Li values (+4.0‰ to +9.0‰) and low Li concentrations (4.7–11.3ppm) in comparison to published data for worldwide granites. In contrast, the residual enclaves show low δ7Li values (as low as +0.6‰) and high Li concentrations (as high as 118ppm). Garnet separated from residual enclaves is characterized by a narrow range of low δ7Li values (−1.5‰ to −0.1‰) with high Li concentrations from 32.9 to 81.7ppm. By contrast, coexisting quartz shows relatively high δ7Li values (+15.0‰ to +16.6‰) with very low Li concentrations (~1ppm). Biotite from both leucogranite and residual enclaves shows high Li concentrations (195–382ppm) and relatively heavy Li isotope compositions (+3.2‰ to +7.5‰). The Li elemental and isotopic signatures of the residual enclaves can be modeled as a Grt-Bt rich residuum mixed with leucogranite melt in various proportions. This work indicates that the Li isotopic compositions for magmatic rocks that are derived from anatexis of mid to lower crustal gneisses may not be a faithful source indicator as commonly suggested.


      PubDate: 2016-06-22T21:05:18Z
       
  • A new method for estimating parent rock trace element concentrations from
           zircon
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): James B. Chapman, George E. Gehrels, Mihai N. Ducea, Nicky Giesler, Alex Pullen
      Zircon/bulk rock REE partition coefficients from natural samples correlate with REE concentration in zircon. The correlation is the strongest for the LREE and diminishes with decreasing ionic radius. The relationship between partition coefficient and REE concentration in zircon can be modeled as a power law and the coefficient (α) and exponent (β) terms for each of the REE are empirically determined using new and previously published data. A series of independent tests show that using variable partition coefficients based on the reported α and β terms commonly results in more accurate estimates of bulk rock REE concentrations than average partition coefficients, particularly for the LREE that have previously been difficult to constrain. These results provide a way to account for highly variable REE concentrations in zircon, which may be controlled by numerous processes such as surface enrichment, complex substitution mechanisms, or accidental sampling of sub-microscopic inclusions. The proposed method for estimating bulk rock REE concentrations is especially well-suited to detrital zircon investigations where there is no information available on the parent rock composition.
      Graphical abstract image

      PubDate: 2016-06-22T21:05:18Z
       
  • Beryllium desorption from minerals and organic ligands over time
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Vanessa Boschi, Jane K. Willenbring
      Beryllium isotopes sorbed to sediments have provided useful tools in the field of geochronology and geomorphology over the last few decades. The use of beryllium isotopes relies on the premise that beryllium sorbed to sediments is unaltered over large timescales. Changes in the environmental chemistry, either in-situ or en route from soil to fluvial system, to the ocean, can cause beryllium desorption and may preclude some beryllium isotopic applications. Four mechanisms were tested to determine the relative desorption potential of beryllium including a reduction in pH, an increase in ionic strength (NaCl) and complexation by soluble organic (malonic acid) and inorganic species (NaF). To assess the relative effect of each mechanism on beryllium desorption from both organic and mineral fractions, we prepared separate solutions of beryllium bound to minerals and organic compounds and measured beryllium concentrations in solution before and after each chemical perturbation. We conclude a reduction in pH resulted in the greatest amount of desorption among the four treatments, removing 97% and 75% of sorbed beryllium from illite and montmorillonite, respectively, and none from the organic ligands tested. The addition of malonic acid and increasing the ionic strength also resulted in desorption from montmorillonite. Although increasing the ionic strength did remove 32% and 8.4% of beryllium from montmorillonite and sulfonate, respectively, the presence of sodium significantly enhanced sorption to illite. The addition of NaF did not result in any beryllium desorption. Our results demonstrate that various chemical processes can promote the exchange of beryllium between solid and dissolved phases, the extent to which depends on the composition of the system. We also related differences in beryllium desorption behavior to complexation mechanisms driving retention among organic and mineral species. We estimate inner sphere complexation is the predominant sorption mechanism among the organic ligands tested due to the minimal amounts of desorption and the large stability constants previously reported in the literature. Additionally, we found that different complexation processes are involved in beryllium sorption to illite versus montmorillonite. Because beryllium desorbed from montmorillonite due to changes in pH, ionic strength and organic acid complexation, we hypothesize that a portion of beryllium-montmorillonite associations involve outer sphere processes, driven by weaker electrostatic attractions. However, beryllium exhibited a unique relationship with illite in that sorption not only involves inner sphere processes but also physical inclusion within collapsed interlayer spaces.


      PubDate: 2016-06-22T21:05:18Z
       
  • The geochemistry of nitrate deposits: I. Thermodynamics of
           Mg(NO3)2–H2O and solubilities in the
           Na+–Mg2+–NO3––SO42––H2O system
    • Abstract: Publication date: 15 October 2016
      Source:Chemical Geology, Volume 436
      Author(s): Michael Steiger
      The Na+–Mg2+–NO3 ––SO4 2––H2O quaternary reciprocal system is an important subsystem of several natural brine types associated with nitrate deposits on earth (e.g. in the Atacama Desert), in atmospheric aerosols, in natural stone or building stones subjected to atmospheric pollution and in cave nitrates. We report on an ion interaction (Pitzer) model that is used to calculate activities and solubilities in this reciprocal system. Model parameterization is based on binary and ternary experimental data. New parameters are reported for Mg(NO3)2–H2O and the ternary systems Na+–Mg2+–NO3 ––H2O and Mg2+–NO3 ––SO4 2––H2O. We also report on the details of the model parameterization of the NaNO3–H2O binary. Calculated solubility diagrams of the full reciprocal system are compared to experimental data. In most cases the calculations are within the experimental uncertainty. Several important invariant points of the reciprocal system are calculated and the stability ranges of the various solids are discussed in detail. It is shown that both darapskite, Na3NO3SO4·H2O, and bloedite, Na2Mg(SO4)2·4H2O, have large stability fields in the reciprocal system at near ambient temperatures. This is in agreement with their common occurrence in nitrate deposits.
      Graphical abstract image

      PubDate: 2016-06-17T18:00:47Z
       
  • Editorial Board
    • Abstract: Publication date: 15 October 2016
      Source:Chemical Geology, Volume 436




      PubDate: 2016-06-17T18:00:47Z
       
  • Acid-dissolution of antigorite, chrysotile and lizardite for ex situ
           carbon capture and storage by mineralisation
    • Abstract: Publication date: 25 October 2016
      Source:Chemical Geology, Volume 437
      Author(s): Alicja M. Lacinska, Michael T. Styles, Keith Bateman, Doris Wagner, Matthew R. Hall, Charles Gowing, Paul D. Brown
      Serpentine minerals serve as a Mg donor in carbon capture and storage by mineralisation (CCSM). The acid-treatment of nine comprehensively-examined serpentine polymorphs and polytypes, and the subsequent microanalysis of their post-test residues highlighted several aspects of great importance to the choice of the optimal feed material for CCSM. Compelling evidence for the non-uniformity of serpentine mineral performance was revealed, and the following order of increasing Mg extraction efficiency after three hours of acid-leaching was established: Al-bearing polygonal serpentine (<5%)≤Al-bearing lizardite 1T (≈5%)<antigorite (24–29%)<well-ordered lizardite 2H1 (≈65%)≤Al-poor lizardite 1T (≈68%)<chrysotile (≈70%)<poorly-ordered lizardite 2H1 (≈80%)<nanotubular chrysotile (≈85%). It was recognised that the Mg extraction efficiency of the minerals depended greatly on the intrinsic properties of crystal structure, chemistry and rock microtexture. On this basis, antigorite and Al-bearing well-ordered lizardite were rejected as potential feedstock material whereas any chrysotile, non-aluminous, widely spaced lizardite and/or disordered serpentine were recommended. The formation of peripheral siliceous layers, tens of microns thick, was not universal and depended greatly upon the intrinsic microtexture of the leached particles. This study provides the first comprehensive investigation of nine, carefully-selected serpentine minerals, covering most varieties and polytypes, under the same experimental conditions. We focused on material characterization and the identification of the intrinsic properties of the minerals that affect particle's reactivity. It can therefore serve as a generic basis for any acid-based CCSM pre-treatment.


      PubDate: 2016-06-17T18:00:47Z
       
  • Sr isotopic compositions of the interstitial water and carbonate from two
           basins in the Gulf of Mexico: Implications for fluid flow and origin
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Yan-Ping Li, Shao-Yong Jiang
      Strontium isotopic compositions of the interstitial water and carbonate from marine sediments sampled during IODP 308, in two basins (the normally-pressured Brazos-Trinity Basin IV and the over-pressured Ursa Basin) on the northern slope of the Gulf of Mexico, are present in this study. In the Brazos-Trinity Basin IV, the 87Sr/86Sr ratios of the interstitial water range from 0.70917 to 0.70954, with carbonates sharing similar or slightly lower values from 0.70851 to 0.70952. The interstitial water above 31m shows similar 87Sr/86Sr ratio close to that of seawater, whereas the interstitial water below 31m shows more radiogenic 87Sr/86Sr ratios, possibly indicating a water/rock interaction between the fluid and silicate component in the deep basin sediments. The Sr-isotope ratios of the carbonates are less radiogenic than the seawater, which may reflect a terrestrial carbonate input (such as limestone) transported through the Brazos and Trinity rivers. In the Ursa Basin, the 87Sr/86Sr ratios of the interstitial water range from 0.70887 to 0.70999, those of the carbonate vary from 0.70808 to 0.70930. Both the interstitial water and carbonate show a similar trend of 87Sr/86Sr ratios throughout the depth. The 87Sr/86Sr ratios of the interstitial water decrease from the sea floor surface to the minimum at the Seismic Reflector S10. A lateral fluid incursion with less radiogenic 87Sr/86Sr ratios along the Seismic Reflector S10 can be inferred in the Ursa Basin. It is suggested that the lateral fluid incursion shows a seawater origin, modified by diagenetic reactions including the dissolution of halite. Between the Seismic Reflector S10 and S40, 87Sr/86Sr ratios of the interstitial water increase linearly to the maximum. Along the Seismic Reflector S40, a lateral fluid incursion with more radiogenic 87Sr/86Sr ratios is again recognized. The fluid may have also originated from seawater, but modified by the diagenesis of terrigenous sediments, likewise characterized by highly radiogenic 87Sr/86Sr compositions. A two-dimension fluid-flow model in the Ursa Basin is established.


      PubDate: 2016-06-17T18:00:47Z
       
  • A comparison of Fe(III) reduction rates between fresh and aged biogenic
           iron oxides (BIOS) by Shewanella putrefaciens CN32
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Tarek Najem, Sean Langley, Danielle Fortin
      This study compared the chemical and physical properties of freshwater biogenic iron oxides (BIOS) aged at 4°C for ~5years in the dark to simulate early diagenesis to previously published findings from fresh samples. In addition, the reactivity of aged BIOS in the presence of a well-characterized iron reducing bacterium, Shewanella putrefaciens CN32, was investigated. The composition and proportion of the iron minerals were examined using XRD, Fe k-edge EXAFS, and chemical extractions, and results were similar in both fresh and aged BIOS. Aged BIOS were dominated by 2-line ferrihydrite indicating that the intermixed bacterial cells and their exudates inhibited the transformation of 2-line ferrihydrite into crystalline phases. However, despite the similarity of mineralogy between aged and fresh BIOS, the rates of bacterial iron reduction in aged BIOS were found to be significantly lower than that of their fresh counterparts. Further characterization of aged BIOS by TEM revealed unique morphologies which were not previously observed in fresh BIOS. TEM also showed thickly coated bacterial exudates and presumably those formed as a result of continued accretion or aggregation of iron oxide nanoparticles. Aggregation of the iron oxide particles and subsequent decrease in solubility are likely responsible for the low rates of bacterial iron reduction in aged BIOS. Phase stability and aggregation in BIOS have important implications for the long-term reactivity of BIOS and the preservation of organic matter in the environment.


      PubDate: 2016-06-17T18:00:47Z
       
  • Hydrothermal transport, deposition, and fractionation of the REE:
           Experimental data and thermodynamic calculations
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): A. Migdisov, A.E. Williams-Jones, J. Brugger, F.A. Caporuscio
      For many years, our understanding of the behavior of the REE in hydrothermal systems was based on semi-empirical estimates involving extrapolation of thermodynamic data obtained at 25°C (Haas et al., 1995; Wood, 1990a). Since then, a substantial body of experimental data has accumulated on the stability of aqueous complexes of the REE. These data have shown that some of the predictions of Haas et al. (1995) are accurate, but others may be in error by several orders of magnitude. However, application of the data in modeling hydrothermal transport and deposition of the REE has been severely hampered by the lack of data on the thermodynamic properties of even the most common REE minerals. The discrepancies between the predictions of Haas et al. (1995) and experimental determinations of the thermodynamic properties of aqueous REE species, together with the paucity of data on the stability of REE minerals, raise serious questions about the reliability of some models that have been proposed for the hydrothermal mobility of these critical metals. In this contribution, we review a body of high-temperature experimental data collected over the past 15 years on the stability of REE aqueous species and minerals. Using this new thermodynamic dataset, we re-evaluate the mechanisms responsible for hydrothermal transport and deposition of the REE. We also discuss the mechanisms that can result in REE fractionation during their hydrothermal transport and deposition. Our calculations suggest that in hydrothermal solutions, the main REE transporting ligands are chloride and sulfate, whereas fluoride, carbonate, and phosphate likely play an important role as depositional ligands. In addition to crystallographic fractionation, which is based on the differing affinity of mineral structures for the REE, our models suggest that the REE can be fractionated hydrothermally due to the differences in the stability of the LREE and HREE as aqueous chloride complexes.


      PubDate: 2016-06-17T18:00:47Z
       
  • Short-lived reheating events in the Sommartuva shear zone hosted in cold
           crust: Ar-diffusion modelling constraints for the exhumation of eclogites
           in Flakstadøy, Lofoten Islands, Norway
    • Abstract: Publication date: 25 October 2016
      Source:Chemical Geology, Volume 437
      Author(s): Herbert W. Fournier, James K.W. Lee, Alfredo Camacho
      The exhumation of the Norwegian Lofoten eclogites, formed during the continental collision of Laurentia and Baltica, is mainly due to the reactivation of shear zones during the collapse of the Caledonian orogen. The current cooling history [temperature–time (T–t) paths] for the exhumation of these rocks as determined by 40Ar/39Ar dating and geothermometry involves a protracted period of slow cooling (c. −1.7°CMa−1) from high T. The Sommartuva shear zone, hosted in a Palaeoproterozoic anorthosite, contains retrogressed eclogite and amphibolite-facies lenses and served as an open system for Ar with 40Ar-rich and 40Ar-poor hot fluid infiltration. Ca-rich amphiboles from amphibolites contain excess Ar and are mantled by phlogopites that yield Ar-diffusion loss profiles with plateau ages of 413–400Ma associated with an extensional event in Lofoten. Locally, the zone was intruded in its late-stage history (c. 333Ma) by a granitoid rock crystallizing annite and muscovite, previously inferred to be injected synchronously with amphibolite-facies metamorphism. An alternative cooling path related to the exhumation of the eclogites was determined by a combination of i) 40Ar/39Ar ages in minerals that grew during different thermal events, ii) thermal modelling of the Ar systematics in these minerals and iii) exploration of the thermal effects caused by the superposition of each event on pre-existing mineral phases. The best-fit T–t path as constrained by the Ar data reveals that, in order to preserve Ar (radiogenic and excess) within the minerals, the ambient T must have been relatively low (≤300°C) over 80Ma (413–333Ma). Likewise, the associated intrusion of the granitoid and the hot fluid that percolated the shear zone and crystallized the phlogopites were likely to have been short-lived (c. 100–25ka) and at relatively moderate T (500°C). In contrast to previous thermal models where the thermal events are represented as simple inflexions in a T–t curve, our cold-crust model incorporates short-lived thermal disturbances (including estimates of the magnitude of their maximum temperature and duration) superimposed on protracted periods of slow cooling from high T. A new proposed T–t path obtained for the exhumation of this terrane indicates that the eclogites resided for 80Ma (413–333Ma) at relatively cool ambient temperatures (c. 300°C), highlighting the enormous potential of coupling geochronological data from multiple isotopic systems with thermal modelling to identify and constrain the duration of short-lived events.


      PubDate: 2016-06-13T02:29:05Z
       
  • Laser ablation Rb/Sr dating by online chemical separation of Rb and Sr in
           an oxygen-filled reaction cell
    • Abstract: Publication date: 25 October 2016
      Source:Chemical Geology, Volume 437
      Author(s): Thomas Zack, K. Johan Hogmalm
      The Rb-Sr beta-decay dating system is one of the most attractive tools in geochronology, as Rb is sufficiently abundant in common K-bearing minerals like biotite, muscovite and K-feldspar. This allows dating of a wide variety of rocks (e.g., volcanic, magmatic, metamorphic, sedimentary and hydrothermal environments) without the need of preconcentration, as is often required for U-Pb dating of zircon. However, this advantage was to date negatively counteracted by the lack of a suitable in-situ technique, as beta decay systems by nature have isobaric interferences of the daughter isotope by their respective parent isotope. A reaction cell sandwiched between two quadrupoles within an inductively coupled plasma mass spectrometer (ICP-MS) allows exactly this, the online chemical separation of two different elements. Coupled to a laser ablation (LA) system, in-situ Rb-Sr dating is therefore possible if a suitable reaction gas within the reaction cell can be found that separates Sr from Rb. We present here a simple procedure in which Rb-Sr ages can be obtained from a suite of individual phases in regular thin sections. Using the most established reaction gas, oxygen (O2), it is possible to react part of the ablated Sr to SrO+ while no RbO+ is formed. Measurements of feldspars and micas with a spot size of 80μm were calibrated against glass standards NIST SRM 610, BCR-2G and biotite from La Posta (California; 91.6Ma). Results are presented for a variety of magmatic rocks with well-established thermal records: a sample each from the Klokken syenodiorite (Greenland; 1161Ma), the Ulvö alkaligabbro (Sweden; 1256Ma) and a pegmatite from the Bohus granite (Sweden; 920Ma). Obtained in situ Rb-Sr isochron ages are accurate <1.5%, while initial 87Sr/86Sr ratios are accurate <0.2% compared to published data. The methodology outlined in this study has significant implications for Rb-Sr studies due to the high spatial resolution: (1) quality of measurements can be better controlled by avoiding alterations and inclusions, (2) large sample numbers can be investigated simply by using regular thin sections, (3) several mineral generations can be targeted that would not be distinguishable in mineral separates, and (4) isotope zonation within single crystals can be revealed. This will open the Rb-Sr dating system to new fields of study. For example, thermochronologic studies can use 87Sr/86Sr and 87Rb/86Sr zonation within crystals to better work out cooling and/or reheating paths. Further applications are in the field of provenance studies and temporal evolution of shear zones.


      PubDate: 2016-06-13T02:29:05Z
       
  • Mg, Sr, and O isotope geochemistry of syenites from northwest Xinjiang,
           China: Tracing carbonate recycling during Tethyan oceanic subduction
    • Abstract: Publication date: 25 October 2016
      Source:Chemical Geology, Volume 437
      Author(s): Shan Ke, Fang-Zhen Teng, Shu-Guang Li, Ting Gao, Sheng-Ao Liu, Yongsheng He, Xuanxue Mo
      Magnesium isotopic compositions of igneous rocks could be potentially used to trace recycling of supracrustal materials. High-δ26Mg granitoids have been previously reported and explained to reflect the involvement of surface weathered materials in their sources. Low-δ26Mg granitoids, however, have not been reported. In this study, we report high-precision Mg isotopic analyses of Cenozoic alkaline syenites and syenogranites from the Kuzigan and Zankan plutons, northwest Xinjiang, China. The Kuzigan syenites were originated from the mantle metasomatized by recycled supracrustal materials, and the syenogranites are differentiated products of the syenites. Both syenites and syenogranites have δ26Mg values (−0.46 to −0.26‰ and −0.41 to −0.17‰, respectively) significantly lighter than the mantle (−0.25±0.07‰, 2SD). No correlation of δ26Mg with either SiO2 or MgO is observed, indicating limited Mg isotope fractionation during alkaline magmatic differentiation. The low δ26Mg of the syenites and syenogranites thus reflects a light Mg isotopic source. This, combined with high 87Sr/86Sr ratios (0.70814 to 0.71105) and negative correlation between δ26Mg and δ18O, suggests that the magma source contains recycled marine carbonates. Modeling of the Mg-O-Sr isotopic data indicates that the recycled carbonate is mainly limestone with minor dolostone, suggesting that the metasomatism occurred at depths shallower than 60 to 120km. Given that the plutons are located at the India–Eurasia collision zone, the carbonate recycling was most likely derived from the subducted Tethyan oceanic crust during the Mesozoic–Cenozoic. Our study suggests that the combined Mg, O, and Sr isotopic studies are powerful for tracing recycled carbonates and identifying their species in mantle sources.


      PubDate: 2016-06-13T02:29:05Z
       
  • Early entombment within silica minimizes the molecular degradation of
           microorganisms during advanced diagenesis
    • Abstract: Publication date: 25 October 2016
      Source:Chemical Geology, Volume 437
      Author(s): Julien Alleon, Sylvain Bernard, Corentin Le Guillou, Damien Daval, Feriel Skouri-Panet, Sylvain Pont, Ludovic Delbes, François Robert
      Most ancient organic microfossils delicately preserved in 3D have been found in cherts. Although entombment within silica has been shown to promote morphological preservation, the impact of early silicification on the molecular evolution of fossilized microorganisms during burial remains poorly constrained. Here, we report results of advanced fossilization experiments performed under pressure (250bars) and temperature (250°C) conditions typical of sub-greenschist facies metamorphism for different durations up to 100days on microorganisms experimentally entombed (or not) within a silica gel. The experimental residues have been characterized using XRD and XANES spectroscopy. The present study demonstrates that entombment within silica limits the degradation of microorganism molecular signatures, likely through specific chemical interactions, despite the progressive conversion of silica into quartz during the experiments. Extrapolation of the present results suggests that such protection may persist during geological timescales. The present experimental study provides molecular evidence that, in addition to morphologies, cherts may support the chemical preservation of remains of ancient life. The present results thus constitute a step forward towards the reconstruction of the original chemistry of putative fossilized microorganisms.
      Graphical abstract image

      PubDate: 2016-06-13T02:29:05Z
       
  • Editorial Board
    • Abstract: Publication date: 1 October 2016
      Source:Chemical Geology, Volume 435




      PubDate: 2016-06-13T02:29:05Z
       
  • Fluorine partitioning between eclogitic garnet, clinopyroxene, and melt at
           upper mantle conditions
    • Abstract: Publication date: 25 October 2016
      Source:Chemical Geology, Volume 437
      Author(s): C. Beyer, S. Klemme, T. Grützner, T.R. Ireland, C.W. Magee, D.J. Frost
      In this experimental study we obtained new mineral/melt (DF =cmineral/cmelt) partitioning data for fluorine in a bimineralic hydrous eclogite under Earth's upper mantle conditions (4–6GPa, 1460–1550°C). Omphacitic clinopyroxene displays mineral/melt partition coefficients between DF =0.056±0.005 and DF =0.074±0.001. Garnet partition coefficients are consistently lower with an average partition coefficient of DF =0.016±0.003. We found that omphacitic clinopyroxene is the dominant nominally fluorine-free phase in subducted oceanic crust and hence omphacite is expected to be the major fluorine carrier during subduction of crust into the deeper mantle. Together with previously obtained partitioning data we propose that the oceanic crust can host more fluorine per mass unit than the underlying depleted oceanic mantle. If the majority of entrained fluorine is recycled into Earth's transition zone it is possible that the fluorine is either incorporated into high-pressure transition zone phases or released during high-pressure phase transformations and forming fluorine-rich small degree partial melts. Both scenarios are supported by elevated fluorine concentration in ocean island basalts, kimberlites, and lamproites. Combining the fluorine partitioning data with water partitioning data yields a plausible process to generate lamproitic magmas with a high F/H2O ratio. The enrichment of fluorine relative to H2O is triggered by multiple episodes of small degree melting that deplete the residual more in H2O than in fluorine, caused by the approximately three times smaller mineral-melt partition coefficients of H2O.


      PubDate: 2016-06-13T02:29:05Z
       
  • Determination of halogen abundances in terrestrial and extraterrestrial
           samples by the analysis of noble gases produced by neutron irradiation
    • Abstract: Publication date: 25 October 2016
      Source:Chemical Geology, Volume 437
      Author(s): Lorraine Ruzié-Hamilton, Patricia L. Clay, Ray Burgess, Bastian Joachim, Christopher J. Ballentine, Grenville Turner
      The lack of a reliable database for heavy halogens (bromine and iodine) in terrestrial and extraterrestrial samples is mainly due to the analytical challenges of determining their very low abundances (<1ppm) in the materials of interest. The neutron irradiation noble gas mass spectrometric (NI-NGMS) technique initially developed in the 1960s is the only viable technique currently capable of determining concentrations below 1ppb of iodine for small (<10mg) sample sizes. We describe in detail the analytical protocols and provide a comprehensive and transparent overview of the data reduction procedures in order to fully explore the uncertainties of the technique. We demonstrate how the capabilities of modern mass spectrometers used for Ar-Ar dating, can be readily extended to incorporate halogen measurements. A new and critical assessment of the use of standards is presented based on results from multiple irradiations, including a meteorite (Shallowater aubrite), scapolite minerals introduced by Kendrick (2012) and a novel internal calibration method based on using barium.


      PubDate: 2016-06-13T02:29:05Z
       
  • Isotopic analysis of sulfur cycling and gypsum vein formation in a natural
           CO2 reservoir
    • Abstract: Publication date: 15 October 2016
      Source:Chemical Geology, Volume 436
      Author(s): Florence Chen, Alexandra V. Turchyn, Niko Kampman, David Hodell, Fernando Gázquez, Alexandra Maskell, Mike Bickle
      In order to assess the long-term security of geologic carbon storage, it is crucial to study the geochemical behavior of sulfur in reservoirs that store CO2. Fossil fuel combustion may produce mixtures of carbon dioxide and sulfur gases, and the geochemical effects of sulfur–CO2 cosequestration are poorly understood. This study examines sulfur mineralization from a core drilled in a stacked sequence of natural CO2 reservoirs near the town of Green River, Utah. These reservoirs include the Entrada and Navajo Sandstone, which are separated by the Carmel Formation caprock and transected by a system of CO2-degassing normal faults, through which saline CO2-charged brines discharge. Our objective in this study is to evaluate the mechanisms and timing of secondary mineral formation, particularly gypsum formation, in the CO2 reservoirs and intervening caprock. The Carmel Formation contains beds of gypsum within a fault zone. Secondary veins of gypsum exist throughout the Entrada Sandstone and Carmel Formation. We report sulfur and oxygen isotope data (δ34SSO4 and δ18OSO4, respectively) measured in gypsum and δ34S measured in pyrite, and the oxygen and hydrogen isotope composition (δ18O and δD, respectively) of gypsum hydration water. The multiple isotope approach allows us to trace the sources of sulfur in the reservoirs and, when combined with structural and petrological evidence, the progress of fluid-rock reactions and relative timing of vein mineralization. The secondary gypsum veins in the Carmel Formation derive from mixing of fluids with two isotopically distinct sulfate sources: sulfate from the gypsum beds within the Carmel Formation and sulfate-rich brines that originate from evaporites in the underlying Carboniferous Paradox Formation. The gypsum veins in the Entrada Sandstone have a relatively wide range of δ18OSO4, both isotopically more enriched in 18O and more depleted in 18O than the primary gypsum sources. We suggest that some aqueous sulfate in the Entrada Sandstone may cycle through multiple valence states as it undergoes reduction and reoxidation, resulting in the replacement of its oxygen atoms and allowing the occasional formation of gypsum with anomalous low δ18OSO4. Our data from gypsum hydration water indicates that groups of gypsum veins formed at two different times. Gypsum veins in the Entrada Sandstone and some veins in the Carmel Formation likely formed during Quaternary CO2-charged brine discharge events, while other veins located close to the gypsum beds in the Carmel Formation formed earlier, likely during cycles of dehydration and rehydration associated with the Laramide-age (40Mya) faulting. We conclude that calcium-sulfate mineral formation in brine-filled fractures may play an important role in inhibiting fluid migration in geologic reservoirs that contain CO2.


      PubDate: 2016-06-13T02:29:05Z
       
  • Factors affecting B/Ca ratios in synthetic aragonite
    • Abstract: Publication date: 25 October 2016
      Source:Chemical Geology, Volume 437
      Author(s): M. Holcomb, T.M. DeCarlo, G.A. Gaetani, M. McCulloch
      Measurements of B/Ca ratios in marine carbonates have been suggested to record seawater carbonate chemistry, however experimental calibration of such proxies based on inorganic partitioning remains limited. Here we conducted a series of synthetic aragonite precipitation experiments to evaluate the factors influencing the partitioning of B/Ca between aragonite and seawater. Our results indicate that the B/Ca ratio of synthetic aragonites depends primarily on the relative concentrations of borate and carbonate ions in the solution from which the aragonite precipitates; not on bicarbonate concentration as has been previously suggested. The influence of temperature was not significant over the range investigated (20–40°C), however, partitioning may be influenced by saturation state (and/or growth rate). Based on our experimental results, we suggest that aragonite B/Ca ratios can be utilized as a proxy of [CO3 2–]. Boron isotopic composition (δ11B) is an established pH proxy, thus B/Ca and δ11B together allow the full carbonate chemistry of the solution from which the aragonite precipitated to be calculated. To the extent that aragonite precipitation by marine organisms is affected by seawater chemistry, B/Ca may also prove useful in reconstructing seawater chemistry. A simplified boron purification protocol based on amberlite resin and the organic buffer TRIS is also described.


      PubDate: 2016-06-13T02:29:05Z
       
  • Zircon from the Anorthosite zone II of the Stillwater Complex as a
           U–Pb geochronological reference material for Archean rocks
    • Abstract: Publication date: 15 October 2016
      Source:Chemical Geology, Volume 436
      Author(s): Corey J. Wall, James S. Scoates, Dominique Weis
      Well-characterized reference materials are essential for precise and accurate geochronology. The growing use of in situ techniques, especially U–Pb zircon dating by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), requires the availability of natural reference materials for matrix-matched calibration, age normalization, and quality control. Although there are a number of zircon reference materials available for U–Pb LA-ICP-MS (e.g., Plešovice, 91500, Temora, FC-1), there is currently no widely available Archean zircon reference material for U–Pb geochronology. Zircon from a leucogabbro in the Anorthosite zone II (AN2) of the Neoarchean Stillwater Complex (Montana, USA) is a candidate for an Archean reference material. The AN2 leucogabbro contains abundant zircon with simple sector zoning typical of mafic igneous rocks and with no inheritance of older grains; accessory baddeleyite and secondary rutile and titanite are also present. Trace element concentrations and ratios determined by LA-ICP-MS are variable in Stillwater AN2 zircon (e.g., U=54–585ppm, Hf=5500–9800ppm, Ti=9–22ppm, Th/U=0.38–0.70, Ce/Nd=0.76–18) and consistent with crystallization from highly fractionated interstitial melt at near-solidus temperatures (Ti-in-zircon thermometry=768–864°C) in plagioclase-rich cumulates. U–Pb dates for Stillwater AN2 zircon were established by several different methods, including (1) conventional isotope dilution–thermal ionization mass spectrometry (ID-TIMS) of grains that were air-abraded, (2) chemical abrasion-ID-TIMS or CA-TIMS where grains were annealed and acid-leached prior to dissolution, and (3) LA-ICP-MS. The U–Pb results by all methods for Stillwater AN2 zircon are concordant and yield indistinguishable concordia and weighted mean 207Pb/206Pb dates, with the latter ranging in relative precision from 2710.44±0.32Ma (n=9) by CA-TIMS to 2710.56±0.65Ma by ID-TIMS (n=4) to 2715±9Ma by LA-ICP-MS (n=58). The in situ dates are statistically identical both at a scale of tens to hundreds of microns within individual zircon grains and between grains, and there is only rare evidence for localized domains that have lost small amounts of Pb since crystallization. In contrast, baddeleyite from the AN2 leucogabbro has micron-scale overgrowths and internal replacement patches of secondary zircon that has lost Pb; the weighted mean 207Pb/206Pb date of 2709.73±0.48Ma for baddeleyite that was air-abraded to remove the secondary zircon rims is indistinguishable from the igneous zircon dates. Consequently, it is concluded that the Stillwater AN2 zircon grains represent a suitable geochronological reference material that can be used for a variety of U–Pb zircon analytical methods, especially for LA-ICP-MS dating of Archean zircon, and for comparison of U–Pb zircon dating results between different analytical laboratories worldwide.


      PubDate: 2016-06-13T02:29:05Z
       
  • Geochemical constraints on petrogenesis of marble-hosted eclogites from
           the Sulu orogen in China
    • Abstract: Publication date: 15 October 2016
      Source:Chemical Geology, Volume 436
      Author(s): Yi-Xiang Chen, Jun Tang, Yong-Fei Zheng, Yuan-Bao Wu
      Marble-hosted eclogite is volumetrically minor in collisional orogens, but its geochemistry has great bearing on the origin of deeply subducted crustal rocks and the fluid mobility of subduction zones. This paper presents a combined study of whole-rock major-trace elements and SrNd isotopes, mineral O isotopes, carbonate C and O isotopes, and zircon UPb ages and LuHf isotopes for marble-hosted ultrahigh-pressure metamorphic eclogites from Rongcheng and Sanqingge in the Sulu orogen. The results provide insights into the protolith nature of eclogites and the fluid mobility of subduction zones. Zircon UPb dating yields consistent middle Triassic ages for the two occurrences of eclogites, indicating new growth of metamorphic zircon during continental collision. The Sanqingge eclogite shows LREE-enriched patterns and negative εNd(t) of −16.6 to −14.3 for whole-rock and negative εHf(t) of −27.1 to −15.2 for metamorphic zircon. A few relict zircon domains show middle Neoproterozoic UPb ages and negative εHf(t) of −35.2 to −15.5. Thus, the Sangqingge eclogite was metamorphosed from a mafic rock that was derived from partial melting of an anciently enriched mantle source. In contrast, the Rongcheng eclogite exhibits flat or even LREE-depleted patterns with negative εNd(t) values of −12.2 to −1.0 for whole-rock but positive εHf(t) values of 5.4 to 10.4 for zircon. The occurrence of interstitial and highly cuspate plagioclase along grain boundaries indicates the presence of partial melting in the eclogite. Thus, its positive zircon εHf(t) values are ascribed to the eclogite protolith of juvenile origin, whereas the LREE depletion is due to extraction of LREE-rich anatectic melt from the eclogite during the Triassic continental collision. As such, the Rongcheng eclogite was metamorphosed from a mafic rock that was derived from partial melting of a less enriched mantle source. All the eclogites from both areas show variably high δ18O values of 9.4‰ to 19.5‰. Oxygen isotope fractionations between mineral pairs mostly yield eclogite-facies temperature of 600 to 800°C, suggesting that the high δ18O signature was inherited from their protoliths before the Triassic subduction. In combination with the field relation between the eclogite and marble, it is inferred that the eclogite protolith is probably basaltic tuff and its high δ18O value would be acquired together with the marble protolith during their deposition from the surface water. Therefore, there would be the limited isotopic exchange between marble and eclogite during continental collision.


      PubDate: 2016-06-13T02:29:05Z
       
  • Chromium isotope stratigraphy of Ediacaran cap dolostones, Doushantuo
           Formation, South China
    • Abstract: Publication date: 15 October 2016
      Source:Chemical Geology, Volume 436
      Author(s): A.S. Rodler, S.V. Hohl, Q. Guo, R. Frei
      The Yangtze Platform in South China is considered a key site for studying Neoproterozoic ocean oxygenation. Comprehensive Cr concentration and isotopic signatures are presented from cap dolostones of the Doushantuo Formation from a deep water and a shallow marine carbonate section, that have previously been used to constrain the redox evolution of Ediacaran seawater in space and time employing major and trace element abundances as well as Sr-, O- and C-isotope compositions. The majority of the studied samples yield elevated Cr concentrations and negatively fractionated δ53Cr signals due to (i) detrital contamination consistent with Cr signatures of other post-Marinoan cap dolostones, (ii) diagenetic carbonate alteration and post-depositional remobilization of Cr. Al concentrations were used as indicator of detrital contamination and to calculate authigenic Cr concentration and isotope signatures when the detrital contribution of Cr was <40%, which generally resulted in slightly lower Cr concentrations and slightly higher Cr isotopic signals in these samples. Diagenetic alteration of a majority of the studied cap dolostones is evident from low Sr concentrations and light δ18Ocarb signals that correlate with low Cr concentrations and negatively fractionated δ53Cr values. Post-depositional remobilization of Cr results in the loss of heavy Cr isotopes and consequently light Cr isotopes are retained during precipitation of secondary carbonates. However, some samples might still show pristine Ediacaran seawater signals. Two samples pertaining to the deep water cap dolostone section at Yanwutan and one sample pertaining to the shallow water section at Jijiawan yield slightly positively fractionated δ53Cr signals, particularly when corrected to presumably authigenic signals (δ53Crauth). These values are consistent with those from other Neoproterozoic carbonate sections deposited in an oxygenated environment. This short sequence of positively fractionated δ53Crauth is concurrent with a cap dolostone 87Sr/86Sr signal close to Ediacaran seawater values with relatively high Sr and Cr concentrations, with decreased detrital contamination and with a pronounced negative Ce anomaly. They indicate either light pulses of enhanced oxidative weathering or, inferred from a few least-altered samples, prevailing oxic conditions during cap dolostone deposition. The Cr isotopic composition of Ediacaran cap dolostones pertaining to the Doushantuo Formation reveals fine-scale fluctuations in post-glacial detrital contamination and diagenesis balanced against a clear signal of oxidative weathering. Cr isotopes have the potential to further enhance our understanding of Neoproterozoic weathering regimes and past ocean oxygenation, however, before the Cr isotope system can be faithfully applied to delineate changes in redox conditions recorded in ancient marine carbonates, possible diagenetic alteration of the Cr signal as well as Cr contribution from detrital contamination need to be taken into consideration.


      PubDate: 2016-06-13T02:29:05Z
       
  • Chemical and isotopic signatures of waters associated with the carbonation
           of ultramafic mine tailings, Woodsreef Asbestos Mine, Australia
    • Abstract: Publication date: 15 October 2016
      Source:Chemical Geology, Volume 436
      Author(s): H.C. Oskierski, B.Z. Dlugogorski, T.K. Oliver, G. Jacobsen
      Extensive carbonate crusts have formed on the tailings of the Woodsreef Asbestos Mine, sequestering significant amounts of CO2 directly from the atmosphere. The physico-chemical (pH, T, conductivity), chemical (cations, dissolved inorganic carbon (DIC)) and isotopic (δ 2H, δ 18O, δ 13CDIC, F 14C) signatures of waters interacting with the tailings and associated carbonate precipitates provide insight into the processes controlling carbonation. We observe two distinct evolutionary pathways for a set of stream and meteoric-derived water samples, respectively, with both groups generally being characterised as moderately alkaline, bicarbonate-dominated and Mg-rich waters. Stream water samples are supersaturated with CO2 and therefore prone to degassing, which, in combination with evaporation, drives carbonate supersaturation and precipitation. Isotopic signatures indicate soil CO2 as the main carbon source in the stream waters entering the tailings pile, whereas water emerging downstream of the tailings pile may also contain carbon from the dissolution of isotopically light bedrock magnesite in an open system with respect to soil CO2. The evolution of meteoric-derived waters on the other hand, partly occurs under CO2-limited conditions, which results from reduced CO2 ingress at depth and/or a temporal lag between fluid alkalisation and kinetically hindered uptake of CO2 into alkaline solution. A high pH, Mg-rich meteoric water absorbs atmospheric CO2 after discharging into a tunnel within the tailings pile, resulting in high DIC concentrations with atmospheric carbon isotope signature. Evaporation of the water at the discharge point in the tunnel drives precipitation of hydromagnesite (Mg5(CO3)4(OH)2·4H2O), displaying a clear atmospheric isotope signature, broadly consistent with previous estimates of carbon and oxygen isotope fractionation during precipitation of hydrated Mg-carbonate.


      PubDate: 2016-06-13T02:29:05Z
       
  • Biological reduction of structural Fe(III) in smectites by a marine
           bacterium at 0.1 and 20MPa
    • Abstract: Publication date: 2 November 2016
      Source:Chemical Geology, Volume 438
      Author(s): Deng Liu, Fengping Wang, Hailiang Dong, Hongmei Wang, Linduo Zhao, Liuqin Huang, Lingling Wu
      Microbial iron reduction has been implicated as an important biochemical reaction on Earth. The influence of environmental parameters (e.g., pressure) on bioreduction of ferruginous clay minerals, however, is not well constrained. The objective of this study was to investigate microbial reduction of structural Fe(III) in smectite minerals and associated mineral transformations under elevated hydrostatic pressure. Bioreduction experiments were performed in a hydrostatic pressure system at 0.1 and 20MPa, in which lactate as the sole electron donor, two smectites having different iron contents (montmorillonite SWy-2 and nontronite NAu-2) as the sole electron acceptor, and a marine bacterium Shewanella piezotolerans strain WP3 as the reaction mediator with and without an electron shuttle anthraquinone-2,6-disulfonate (AQDS). Our results indicated that S. piezotolerans strain WP3 was capable of reducing structural Fe(III) in smectites, and AQDS enhanced the initial reduction rate and final extent. In the absence of AQDS, inhibitory effect of hydrostatic pressure on smectite reduction was observed. However, with AQDS, the reduction extents at 0.1 and 20MPa were approximately the same value, but the initial reduction rate at 20MPa was lower than that at 0.1MPa. Greater degree of smectite dissolution was found in NAu-2 reactors compared to SWy-2 experiments. Mineralogical analysis by X-ray diffraction (XRD), Sybilla simulation, and scanning and transmission electron microscopy (SEM and TEM) showed that neoformation of illite was present in bioreduced NAu-2, but not in SWy-2, and this smectite illitization was facilitated by higher hydrostatic pressure. These results have important implications for understanding iron cycling and low-temperature illite formation in marine settings.


      PubDate: 2016-06-13T02:29:05Z
       
  • Manganese accumulation and solid-phase speciation in a 3.5m thick mud
           sequence from the estuary of an acidic and Mn-rich creek, northern Baltic
           Sea
    • Abstract: Publication date: 25 October 2016
      Source:Chemical Geology, Volume 437
      Author(s): Changxun Yu, Joonas J. Virtasalo, Peter Österholm, Edward D. Burton, Pasi Peltola, Antti E.K. Ojala, Johan K. Hogmalm, Mats E. Åström
      In sediments, manganese (Mn) is typically enriched in the form of authigenic Mn hydroxides at the water-sediment interface where intensive redox cycling of Mn occurs. Here we show, based on existing hydrochemical and geochemical (sediment core) data and new detailed chemical and mineralogical characterization of a 3.5m long sediment core from a Boreal estuary, that the behavior of Mn can be profoundly different and more complex in estuarine settings receiving an abundance of terrestrial Mn. The most notable feature in the 3.5m long sediment core is two depth intervals (60–155cm and 181–230cm) where there are strong fine-scale variations in Mn concentrations with peaks episodically reaching up to 10–25gkg−1 and 6.7–12gkg−1, respectively. X-ray absorption spectroscopy and sequential chemical extraction show that Mn occurs mainly as authigenic rhodochrosite at these two depth intervals and is mainly surface-sorbed in other sections with relatively low and stable Mn concentrations. The data suggests that the strong fine-scale variations in Mn concentrations are a reflection of the extent of formation and settling of Mn hydroxides, the precursors of the authigenic rhodochrosite (and also of the surface-sorbed Mn), rather than Mn input to the estuary or redox-related Mn translocation within the sediment. There was agreement between the results of linear combination fitting of extended X-ray absorption fine structure data and a 7-step sequential chemical extraction (SCE) in terms of quantification of surface-sorbed Mn species, whereas the SCE experiment failed to fractionate a majority of rhodochrosite into SCE step-2 (1M NH4-acetate at pH6), which is frequently employed to dissolve carbonate. We ascribe this discrepancy to only partial dissolution of rhodochrosite in the weakly acidic (pH=6) NH4-acetate leach.
      Graphical abstract image

      PubDate: 2016-06-13T02:29:05Z
       
  • Uranium isotope composition of waters from South Texas uranium ore
           deposits
    • Abstract: Publication date: 25 October 2016
      Source:Chemical Geology, Volume 437
      Author(s): Christa J. Placzek, Jeffrey M. Heikoop, Brian House, Benjamin S. Linhoff, Mark Pelizza
      Redox conditions and associated changes in mobility of uranium (U) are tightly linked to a multitude of challenges connected with U mining in sandstone-hosted deposits and new methods that directly measure reduction or oxidation of U can inform on these questions. A novel proxy for understanding U redox chemistry has recently emerged, the volume dependent isotopic fractionation of uranium-238 (238U) from uranium-235 (235U). Novel measurements of 238U/235U isotopic ratio are combined with measurements of the more commonly utilized uranium-234/uranium-238 activity [(234U/238U)] ratio, as both isotopic ratios can be measured simultaneously. However, application of both U isotopic ratios in the contexts of exploration and environmental remediation of U ores requires characterization of these isotopic ratios across a variety of redox settings. Here, 238U/235U and (234U/238U) ratios are examined from eight transects in two U ore bodies (the Rosita and Kingsville Dome deposits) in South Texas; these sites are classic roll front deposits and exhibit a wide variety of both natural and altered redox conditions. Across all transects it is observed that (234U/238U) ratios decrease systematically towards the ore body from both the oxidizing and reducing sides, irrespective of whether the site has been mined or not. This pattern reflects geologically recent and significant U leaching and mobility and is characteristic of active roll fronts. Overall δ238U values in these transects decrease systematically towards the reducing zone. A simple Rayleigh fractionation model, where U ore is deposited from an increasingly isotopically depleted reservoir of dissolved U best explains the overall trend; very negative δ238U values likely reflect multiple cycles of U deposition and dissolution. The South Texas data set indicates that both (234U/238U) ratios and δ238U values can be variable at an individual mine site. However, overall low (234U/238U) ratios and negative δ238U values are characteristic of active roll front deposits. The comprehensive U isotopic composition of both ores and well waters represents a powerful new tool in prospecting of sandstone-hosted U ore and in environmental remediation following extraction of U ore.


      PubDate: 2016-06-13T02:29:05Z
       
  • Early Cretaceous potassic volcanic rocks in the Jiangnan Orogenic Belt,
           East China: Crustal melting in response to subduction of the
           Pacific–Izanagi ridge?
    • Abstract: Publication date: 25 October 2016
      Source:Chemical Geology, Volume 437
      Author(s): Liang Zhao, Feng Guo, Weiming Fan, Qingwen Zhang, Yangming Wu, Jingyan Li, Wen Yan
      Eastern China is characterized by widespread Mesozoic magmatism that was closely related to subduction of the Paleo-Pacific Plate, but the genetic relationship between magmatism and subduction is not always clear. The Jiangnan Orogenic Belt (JOB) is a Neoproterozoic collisional belt between the Yangtze and Cathaysian blocks. Early Cretaceous felsic magmas from the Tianmushan basin in the eastern JOB form a >3000m thick volcanic sequence erupted within an extremely short period of time (132–130Ma). These rocks are potassic (K2O=3.86–6.05%) and peraluminous (A/CNK=1.0–1.23), and are compositionally similar to experimental melts derived from predominantly clay-poor and K-rich metasediments or granodiorites at crustal conditions. They are LILE- and LREE-enriched ((La/Yb)CN =9.0–14.6) and display variable Eu anomalies and Nb–Ta depletion. The weakly variable and negative whole-rock εNd(t) (−5.4 to −2.5) and heterogeneous zircon εHf(t) values (−6.7 to +3.5) of these rocks are similar to those of basement rocks beneath the JOB, suggesting a genetic relationship with basement. No magmas with clear subduction-related composition occurred at this time in the area and the compositions of the potassic rocks more closely resemble high-temperature melts of extensively thinned crust in continental rift environments. Our data favors a model that the Early Cretaceous potassic magmas formed as subduction of the Pacific–Izanagi ridge resulted in upwelling asthenosphere, providing the high temperatures required to extensively melt the lower-middle crust in the eastern JOB.


      PubDate: 2016-06-13T02:29:05Z
       
  • Competitive adsorption of magnesium and calcium with phosphate at the
           goethite water interface: Kinetics, equilibrium and CD-MUSIC modeling
    • Abstract: Publication date: 25 October 2016
      Source:Chemical Geology, Volume 437
      Author(s): Malihe Talebi Atouei, Rasoul Rahnemaie, Esmaiel Goli Kalanpa, Mohammad Hossein Davoodi
      In natural environments, competitive interactions of ions with charged particles often control bioavailability and mobility of ions. In these systems, chemical reactions are often in a semi-equilibrium state; thus, along with equilibrium data, kinetic data are of great importance in predicting ion bioavailability and mobility. Therefore, in this research, kinetics and equilibrium adsorption interactions of magnesium, calcium and phosphate on goethite are investigated as a function of time, pH, ion loading and ionic strength in single and binary ion systems. The thermodynamically valid model parameters, obtained from single ion systems by applying the CD-MUSIC model, are used to predict ion interactions in more complex systems, i.e. binary ion systems. Results indicate that the kinetics of phosphate and magnesium adsorption is pH-dependent and electrostatic attractive and repulsive forces dominantly control the reaction. Thus, adsorption of magnesium and calcium influences significantly both the amount of adsorbed phosphate and the reaction time, and vice versa. Therefore, the equilibration time for phosphate and magnesium adsorption on goethite is approximately ≤1h when electrostatic forces are attractive and approximately 24h when electrostatic forces are repulsive. Variation in ionic strength (0.01–0.3M NaCl) has no significant effect on the magnesium and calcium adsorption, suggesting a large difference in the adsorption energy of Ca and Mg compared to Na, which is very likely due to the difference in their adsorption mechanism. Based on the model calculations, formation of two inner sphere surface complexes, as (FeOH)2Mg and (FeOH)2MgOH, is the dominant mechanism for magnesium adsorption. For calcium, however, a monodentate outer sphere surface complex dominates its adsorption at acid pH range and a monodentate inner sphere complex at basic pH range (FeOHCa). This difference results in a different amount of charge on electrostatic planes that influences strongly phosphate adsorption such that phosphate adsorption is significantly higher in Ca–PO4 systems than in Mg–PO4 systems.


      PubDate: 2016-06-13T02:29:05Z
       
  • Fluid mixing in orogenic gold deposits: Evidence from the H-O-Sr isotope
           composition of the Val-d'Or vein field (Abitibi, Canada)
    • Abstract: Publication date: 25 October 2016
      Source:Chemical Geology, Volume 437
      Author(s): Georges Beaudoin, Massimo Chiaradia
      Quartz and tourmaline from the Val-d'Or, Québec (Canada) orogenic gold vein field have oxygen isotope fractionation indicating equilibrium over a range of temperature (280–492°C). The range of δ18O values is from 9.2 to13.8‰, and from 6.5 to 9.5‰, for quartz and tourmaline, respectively. The hydrogen isotope composition of tourmaline has a range from −63 to −13‰. The initial Sr isotope composition (87Sr/86Sr(i)) of vein tourmaline at the time of mineralization ranged from 0.700710 to 0.702246. Vein carbonates have a similar range in Sr isotope composition, form 0.701243 to 0.703641. A series of samples from the Sigma deposit displays no systematic variation with depth for δ18O values of quartz or tourmaline, δD values of tourmaline, temperature of equilibrium, or 87Sr/86Sr(i). The 87Sr/86Sr(i) of local komatiite, basalt, andesite, grauwacke and granodiorite, at 2.7Ga, ranged from 0.681971 to 0.7128706. Country rocks with low 87Sr/86Sr(i) are likely a consequence of hydrothermal resetting of the Rb/Sr system in these samples. Covariation of the calculated equilibrium δDH2O and δ18OH2O with quartz-tourmaline equilibrium temperatures indicates mixing between a low-δ18O (<1.5‰), high δD (>−10‰), low temperature (<280°C) upper crustal fluid, and a high-δ18O (>9.3‰), low δD (<−40‰), high temperature (>490°C) deep-seated metamorphic fluid. At temperatures below the critical point for low-salinity hydrothermal fluids, δDH2O are affected by liquid-vapour phase separation, yielding the high δD values characteristic of the upper crustal fluid. A broad covariation between δ18OH2O and the 87Sr/86Sr(i) of tourmaline is consistent with mixing of two fluids from two reservoirs with different Sr concentrations and δ18OH2O values. The low 87Sr/86Sr(i) (<0.7007) inferred for the deep-seated metamorphic fluid end-member is consistent with Archean prograde metamorphic dewatering of typical volcanic and sedimentary country rocks of the Val-d'Or area. The higher 87Sr/86Sr(i) (>0.7022) of the upper crustal fluid end-member most likely resulted from a long history of water-rock exchange between Archean seawater and carbonate and radiogenic plutonic rocks of the Abitibi sub-province.
      Graphical abstract image

      PubDate: 2016-06-13T02:29:05Z
       
  • Precise and fast determination of inorganic magnesium in coccolithophore
           calcite
    • Abstract: Publication date: 25 October 2016
      Source:Chemical Geology, Volume 437
      Author(s): Yan Li, Marius N. Müller, Brett Paull, Pavel N. Nesterenko
      Coccolithophores are calcifying marine phytoplankton playing a pivotal role in the production of calcium carbonate deposits. During the formation of calcium carbonate phase, calcium ions may be replaced by other divalent cations (e.g. Sr2+ and Mg2+) which results in a unique elemental signature showing environmental and, in case of biogenic precipitates, physiological conditions of its formation. It was shown that the ratio between magnesium and calcium is strongly correlated with surface sea temperature variation and can be used in paleoceanographic studies to reconstruct past environmental conditions and to understand biogeochemical cycles. The determination of the coccolithophore calcite Mg/Ca ratio, however, requires the efficient removal or quantification of organically bound magnesium, which is up to 400 times more abundant than the magnesium incorporated within the inorganic calcite. Classical methods, using oxidation to remove organic matter, are time consuming and require a considerable amount of sample material. So, the aim of our investigation is the development of a new reliable method for determination of inorganic magnesium and the Mg/Ca ratio in coccolithophore calcite. For this purpose, labile organic bound magnesium is replaced by the incubation with added Cu2+ for 10min, and the released Mg2+ is determined via high performance chelation ion chromatography (HPCIC). This method enabled the determination of the coccolithophore calcite Mg/Ca ratio within 1h using <5mg dry coccolithophore sample material. The method has been successfully tested with laboratory cultured calcifying and non-calcifying coccolithophores samples. The portable HPCIC instrumentation can be easily mounted on-board scientific research vessels and thus potentially represent a new tool for in-situ calcite analysis of coccolithophore surface bloom situations. However, a further work is required to make this method suitable for analysis of pelagic sediments and sediment traps, which may contain residues of Mg-rich clays.
      Graphical abstract image

      PubDate: 2016-06-13T02:29:05Z
       
  • Behavior of rare earth elements and yttrium during simulation of arctic
           estuarine mixing between glacial-fed river waters and seawater and the
           impact of inorganic (nano-)particles
    • Abstract: Publication date: 2 November 2016
      Source:Chemical Geology, Volume 438
      Author(s): Nathalie Tepe, Michael Bau
      Rivers are the major source of many trace elements to the oceans. As a consequence of climate change, increasing volumes of glacial meltwater enter the oceans after being affected by estuarine processes. Although the behavior of high field strength elements such as the rare earth elements and yttrium (REY) has been intensively studied in tropical, temperate and boreal estuaries, little is known about arctic estuarine mixing of glacial-fed river waters that are poor in organic nanoparticles and colloids (NPCs), but rich in inorganic NPCs. Here we provide the first data set from estuarine mixing experiments with glacial-fed river waters and seawater. The glacial-fed river water endmembers originate from southern Iceland (sampled in 2010 and 2013) and from West Greenland (sampled in 2013); these endmembers are rich in NPCs and larger particles of (glassy) volcanic ash and of ultra-fine rock flour, respectively. For the mixing experiments, 0.2 μm-filtered glacial-fed river water was mixed with seawater in different ratios to cover the full range of estuarine low- to high-salinity conditions. All 2010 freshwater endmembers show higher concentrations of individual REY (due to their high NPC load) than the respective seawater endmember, whereas the 2013 freshwater endmembers have higher light REY, but lower heavy REY concentrations (due to significantly less NPCs). Admixture of minute amounts of seawater (5% and 10% seawater (SW) admixture, i.e. salinities of ~1.7 psu and of ~3.4 psu, respectively) already has a strong impact on REY concentrations and REY signatures. In all experiments, a large amount of REY (e.g., up to 98.2% of Nd and 98.2% of Yb) is removed at very low salinities. At intermediate to high salinities, however, remobilization of REY from aggregated NPCs occurs if NPC-rich river waters are used in the experiments, whereas if the endmember is poor in NPCs, the REY mix almost conservatively. The REY removal is dependent on the amount of NPCs present in the glacial-fed river waters; NPC-rich river water shows larger REY removal than NPC-poorer river water. After the initial drop in REY concentrations under low-salinity conditions, the experiment with glacial-fed river water from Greenland reveals that heavy REY mix almost conservatively, but light REY are again remobilized from aggregated NPCs. Our results suggest that aggregated NPCs composed of rock flour (Greenland) and volcanic ash (Iceland) may cause different trace element behavior during estuarine processes. However, all experiments show similar Y-Ho fractionation during estuarine mixing, indicating that the actual type of particle is not a major constraint on Y-Ho behavior, but rather the availability of particle surfaces. Estuarine processes may cause the development of negative Ce anomalies during remobilization of REY at higher salinity if Ce(IV) compounds of low solubility are present in the (nano-)particle load. However, if only Ce(III) is present, such as in fresh mafic volcanic ash, no fractionation of Ce from its trivalent REY neighbors occurs.


      PubDate: 2016-06-13T02:29:05Z
       
  • Evaluation of sampling methods for sulfur speciation in volcanic gases
    • Abstract: Publication date: 2 November 2016
      Source:Chemical Geology, Volume 438
      Author(s): Sangchul Lee, Takeshi Ohba, Sung Hyo Yun, Kyounghee Yang, Hoon Young Jeong
      Determination of H2S and SO2 in fumarolic gases is necessary to predict volcanic activities. This study evaluated three gas sampling methods (CdS precipitation, Ag2S precipitation, and I2 oxidation) using artificial fumarolic gases with known compositions. H2S was initially fixed as CdS(s) or Ag2S(s) in the sulfide precipitations and S(0) in the I2 oxidation. On the other hand, SO2 was collected as dissolved species (SO3 2− and SO4 2−). The Ag2S precipitation, which did not allow HCl determination in fumarolic gases by forming AgCl(s), significantly underestimated H2S and SO2 due to the entrained loss of analytes as a result of water vaporization during H2O2 treatment. The I2 oxidation, which required duplicate sampling for complete analysis of H2S and SO2, tended to overestimate SO2 probably due to difficulties in separating S(0) particles from solutions. The CdS precipitation produced more satisfactory H2S and SO2 recoveries than the other methods. Additionally, the effect of delayed sample treatment was assessed by comparing H2S-to-SO2 molar ratios resulting from immediate and one-week-delayed treatments after gas collection. The delayed treatment significantly lowered the ratios in the CdS precipitation likely due to the susceptibility of CdS(s) to oxidation. In the meanwhile, such an effect was minimal in I2 oxidation. Similar results were also observed in the field evaluation of fumarolic gases at Mt. Hakone. Along with detailed sampling and treatment procedures, the causes for unsatisfactory H2S and SO2 recoveries were discussed in the context of the underlying reaction schemes as well as potential experimental artifacts. Thus, this study helps to select and further modify appropriate gas sampling methods to meet specific needs.
      Graphical abstract image

      PubDate: 2016-06-13T02:29:05Z
       
  • A scanning ion imaging investigation into the micron-scale U-Pb
           systematics in a complex lunar zircon
    • Abstract: Publication date: 2 November 2016
      Source:Chemical Geology, Volume 438
      Author(s): J.J. Bellucci, M.J. Whitehouse, A.A. Nemchin, J.F. Snape, R.T. Pidgeon, M. Grange, S.M. Reddy, N. Timms
      The full U-Pb isotopic systematics in a complex lunar zircon ‘Pomegranate’ from lunar impact breccia 73235 have been investigated by the development of a novel Secondary Ion Mass Spectrometry (SIMS) scanning ion imaging (SII) technique. This technique offers at least a four-fold increase in analytical spatial resolution over traditional SIMS analyses in zircon. Results from this study confirm the hypothesis that the Pomegranate zircon crystallized at 4.302±0.013Ga and experienced an impact that formed, U-enriched zircon around primary zircon cores at 4.184±0.007Ga (2σ, all uncertainties). The increase in spatial resolution offered by this technique has facilitated targeting of primary zircon that was previously inaccessible to conventional spot analyses. This approach has yielded results indicating that individual grains with a diffusive distance of less than ~4μm have been reset to the young impact age, while individual grains with a diffusive distance larger than ~6μm have retained the old crystallization age. Assuming a broad range in cooling rate of 0.5–50°C/year, which has been observed in a suite of similar lunar breccias, a maximum localized temperature generated by the impact that reset small primary zircon and created new, high-U zircon is estimated to be between 1100 and 1280°C.


      PubDate: 2016-06-13T02:29:05Z
       
  • Applying the Midas touch: Differing toxicity of mobile gold and platinum
           complexes drives biomineralization in the bacterium Cupriavidus
           metallidurans
    • Abstract: Publication date: 2 November 2016
      Source:Chemical Geology, Volume 438
      Author(s): B. Etschmann, J. Brugger, L. Fairbrother, C. Grosse, D.H. Nies, G. Martinez-Criado, F. Reith
      The β-Proteobacterium Cupriavidus metallidurans CH34, which dominates biofilm communities on natural gold (Au) grains, is a key species involved in their (trans)formation. Gold(III)-chloride complexes, with toxicity levels similar to those of Hg- and Ag-ions, are rapidly sorbed by C. metallidurans cells and detoxified by active reductive precipitation to metallic Au nanoparticles. In this study, we exposed C. metallidurans CH34 to a range of environmentally-relevant Au(I)- and Pt(II/IV)-complexes with differing toxicity levels, i.e., Au(I)-thiosulfate>Au(I)-cyanide, and cisplatin>Pt(IV)-chloride>Pt(II)-cyanide. The aim was to investigate how Au/Pt-complex toxicity, in combination with the metabolic state of cells, affects Au/Pt accumulation, speciation and biomineralization. Overall, more Au(I)- than Pt-complexes were accumulated. Significantly more Au(I)-thiosulfate was taken up by metabolically active vs. inactive or dead cells. Toxicity of Au(I)-complexes was ‘managed’ via the formation of intermediate species, e.g., Au(I)-C mixed ligand complexes. Over time Au(I) associated with active cells was reduced to metallic particles, with higher rates of transformation being observed in experiments amended with Au(I)-thiosulfate- compared to Au(I)-cyanide complexes. In contrast, Pt uptake did not differ with respect to metabolic state. Pt(IV)-complexes were reduced to Pt(II) within 1min of amendment; further reduction of the Pt(II) was not observed. In conclusion, toxicity of Au/Pt-complexes is linked to the ability of cells to take up and actively detoxify the complexes. Gold uptake was linked to the detoxification of the Au(I)-complexes via active reductive precipitation to Au(0). In contrast, metabolic activity/toxicity did not influence Pt accumulation and/or transformation. This indicates that the ability of bacteria to cycle Au via mobilization, accumulation and biomineralization provides a selective advantage for organisms able to detoxify highly mobile Au-complexes. Because Pt-complexes are not taken up as readily and are hence less toxic, they do not provide a similar selective advantage, and hence Pt is less readily cycled. This may explain the substantially higher environmental mobility of Au compared to Pt.


      PubDate: 2016-06-13T02:29:05Z
       
  • Allanite UPb geochronology by 193nm LA ICP-MS using NIST610 glass for
           external calibration.
    • Abstract: Publication date: 2 November 2016
      Source:Chemical Geology, Volume 438
      Author(s): Christopher R.M. McFarlane
      The viability of using NIST610 glass as an external standard for allanite UThPb geochronology by 193nm laser ablation inductively-coupled plasma mass spectrometry was investigated. Laser ablation conditions were optimized to minimize time-dependent fractionation of Pb/U and Pb/Th. A data reduction scheme (VizualAge_UMod) was additionally used to apply Pb/U fractionation corrections when the standard contains an enriched 238U/235U ratio (NIST610=420) compared to natural materials. Optimization of ablation conditions was carried out on natural allanite material separated from a ~415Ma tonalite intrusion (Hartfield intrusion, New Brunswick Canada). Conditions of 25s of ablations at 1J/cm2, 3Hz, and a 45μm crater diameter were established. This produced 3μm deep craters, and a near-steady-state, low mass-load, material transfer to the ICP-MS producing negligible time-dependent Pb/U and Pb/Th fractionation. Increasing fluence only modestly up to 3J/cm2 induced time-dependent Pb/U fractionation. To test for any residual matrix dependencies, the long-term reproducibility, accuracy, and absolute precision of the method was verified using ~31.5Ma Siss3 allanite material (Bergell Intrusion, Switzerland). The method yielded accurate and precise Pb/U ad Pb/Th ages that compare favourably to alternative allanite dating methods. As a further test, allanite separated from the Hartfield tonalite was analyzed and yielded an age that overlaps within error of the TIMS titanite UPb age for the intrusion. This is despite a partially amorphous microstructure as documented by Raman spectroscopy and optical microscopy. The results of this study emphasize the need to empirically calibrate laser fluence conditions that yield minimal element fractionation and to establish robust plasma settings. These conditions are unlikely to yield highest sensitivity, but should minimize the effects of the numerous laser-induced and plasma ionization processes that lead to Pb/U fractionation.


      PubDate: 2016-06-13T02:29:05Z
       
  • Syndepositional diagenetic control of molybdenum isotope variations in
           carbonate sediments from the Bahamas
    • Abstract: Publication date: 2 November 2016
      Source:Chemical Geology, Volume 438
      Author(s): Stephen J. Romaniello, Achim D. Herrmann, Ariel D. Anbar
      Molybdenum (Mo) isotope variations recorded in black shales provide important constraints on marine paleoredox conditions. However, suitable shales are not ubiquitous in the geologic record. Moreover, reliable reconstruction of Mo isotope records from shales requires deposition from a water column containing very high concentrations of sulfide—a condition which is both rare and difficult to verify with certainty when examining preserved sediments. The utility of Mo isotopic records could be improved if reconstructions were possible using alternative lithologies, such as marine carbonates, which are more abundant in the geologic record. Here, we focus on the role of early diagenesis in determining the Mo isotopic composition preserved in shallow-water carbonate sediments from four push cores collected in different shallow-water depositional environments in the Bahamas. In contrast with carbonate primary precipitates, which generally contain <0.2ppm Mo, we find that carbonate sediments deposited under fully oxic shallow bottom water can experience extensive syndepositional authigenic Mo enrichment (1 to >10ppm Mo). The extent of this authigenic enrichment appears to be driven by high concentrations of hydrogen sulfide in the porewaters. In cores with the least authigenic Mo enrichment and lowest pore water sulfide, Mo isotopes are ~1–1.2‰ lighter than seawater, while cores with greater Mo enrichments and higher pore water sulfide approach seawater Mo isotope values (2.2–2.5‰), even under oxic bottom water conditions. However, the sensitivity of bulk carbonate δ98Mo to syndepositional diagenetic conditions potentially complicates interpretation of a carbonate Mo isotope paleoredox proxy. Robust reconstruction of seawater Mo isotopic composition from carbonates will thus require the ability to place constraints on early diagenetic conditions of pore waters at the time of deposition. We show that in order to record seawater Mo isotope values, carbonate pore waters must contain 50–100μM H2Saq, which is achieved only in organic- and sulfide-rich carbonate sediments.


      PubDate: 2016-06-13T02:29:05Z
       
  • Magnesium and oxygen isotopes in Roberts Victor eclogites
    • Abstract: Publication date: 2 November 2016
      Source:Chemical Geology, Volume 438
      Author(s): Jin-Xiang Huang, YuanXin Xiang, Yajun An, William L. Griffin, Yoann Gréau, Liewen Xie, Norman J. Pearson, Huimin Yu, Suzanne Y. O'Reilly
      Magnesium and oxygen are critical elements in the solid Earth and hydrosphere. A better understanding of the combined behavior of Mg and O isotopes will refine their use as a tracer of geochemical processes and Earth evolution. In this study, the Mg-isotope compositions of garnet and omphacite separated from well-characterized xenolithic eclogites from the Roberts Victor kimberlite pipe (South Africa) have been measured by solution multi-collector ICP-MS. The reconstructed whole-rock δ26Mg values of Type I (metasomatized) eclogites range from −0.61‰ to −0.20‰ (Type IA) and from −0.60‰ to −0.30‰ (Type IB) (mean−0.43‰±0.12‰), while δ26Mg of Type IIA (fresh, least metasomatized) eclogites ranges from −1.09‰ to −0.17‰ (mean−0.69‰±0.41‰); a Type IIB (fresh, least metasomatized) has δ26Mg of −0.37‰. Oxygen-isotope compositions of garnet were analyzed in situ by SIMS (CAMECA 1280) and cross-checked by laser fluorination. Garnets have δ18O of 6.53‰ to 9.08‰ in Type IA, 6.14‰ to 6.65‰ in Type IB, and 2.34‰ to 2.91‰ in Type IIB. The variation of δ26Mg and δ18O in Type IA and IB eclogites is consistent with the previously proposed model for the evolution of these samples, based on major and trace elements and radiogenic isotopes. In this model, the protoliths (Type II eclogites) were metasomatized by carbonatitic to kimberlitic melts/fluids to produce first Type IA eclogites and then Type IB. Metasomatism has changed the O-isotope compositions, but the Mg-isotope compositions of Type IA are mainly controlled by the protoliths; those of Type IB eclogites reflect mixing between the protoliths and the kimberlitic melt/fluid. The combination of a large range of δ26Mg and low δ18O in Type II eclogites cannot be explained easily by seawater alteration of oceanic crust, interaction of carbonate/silicate sediments with oceanic crust, or partial melting of mafic rocks.


      PubDate: 2016-06-13T02:29:05Z
       
  • Vivianite is a key sink for phosphorus in sediments of the Landsort Deep,
           an intermittently anoxic deep basin in the Baltic Sea
    • Abstract: Publication date: 2 November 2016
      Source:Chemical Geology, Volume 438
      Author(s): Nikki Dijkstra, Caroline P. Slomp, Thilo Behrends
      Phosphorus (P) is an essential nutrient for marine organisms. Its burial in hypoxic and anoxic marine basins is still incompletely understood. Recent studies suggest that P can be sequestered in sediments of such basins as reduced iron (Fe)-P but the exact phase and the underlying mechanisms that lead to its formation are unknown. In this study, we investigated sediments from the deepest basin in the Baltic Sea, the Landsort Deep (site M0063), that were retrieved during the Integrated Ocean Drilling Project (IODP) Baltic Sea Paleoenvironment Expedition 347. The record comprises the whole brackish/marine Littorina Sea stage including past intervals of extensive hypoxia in the Baltic Sea that occurred during the Holocene Thermal Maximum (HTMHI) and the Medieval Climate Anomaly (MCA1HI and MCA2HI). Various redox proxies (e.g. the presence of laminations and high Mo contents) suggest almost permanent bottom water hypoxia during the Littorina Sea stage in the Landsort Deep. The bottom waters were likely even seasonally anoxic or sulfidic during the MCA1HI and MCA2HI, and permanently sulfidic during the HTMHI. With the use of micro-analysis of sieved minerals (SEM-EDS, XRD and synchrotron-based XAS), we show that Mn- and Mg-rich vivianite crystals are present at various depths in the Littorina Sea sediments. We also have indications for vivianite in the MCA1HI, MCA2HI and HTMHI deposits. The formation of vivianite thus likely explains the high Fe-bound P fraction throughout the whole Littorina Sea stage. Shuttling of Fe and Mn from the shelves into the basin and high inputs of P in settling organic matter are likely key drivers for vivianite formation. Our study shows that vivianite can likely form in near-surface sediments under a broad range of bottom water redox conditions, varying from hypoxic and anoxic to sulfidic.
      Graphical abstract image

      PubDate: 2016-06-13T02:29:05Z
       
  • Enargite-luzonite hydrothermal vents in Manus Back-Arc Basin: submarine
           analogues of high-sulfidation epithermal mineralization
    • Abstract: Publication date: 2 November 2016
      Source:Chemical Geology, Volume 438
      Author(s): Vesselin M. Dekov, Olivier Rouxel, Kalin Kouzmanov, Luca Bindi, Dan Asael, Yves Fouquet, Joël Etoubleau, Gaëtan Burgaud, Markus Wälle
      Active and inactive hydrothermal chimneys composed almost entirely of enargite and luzonite, rare minerals in seafloor hydrothermal deposits, were found at the summits of two submarine volcanoes, North Su and Kaia Natai, in the Manus Back-Arc Basin. Detailed mineralogical and geochemical studies revealed that most probably these deposits precipitated at T=200°–330°C and high fS2. The negative δ34S values (−8.58 to −3.70‰) of the enargite-luzonite are best explained by disproportionation reactions of magmatic SO2 and suggest that the high fS2 is likely provided by direct magmatic input of SO2 into the hydrothermal system. Fractionation of Cu stable isotopes during the precipitation of enargite-luzonite (δ65Cu ranges from −0.20 to +0.35‰) is inferred to be associated with either Rayleigh-type fractionation, or redox processes (Cu+ oxidation to Cu2+) and the mass balance of dissolved Cu+ and Cu2+ species in the hydrothermal fluid. The trace element composition of enargite and luzonite indicates a temporal fluctuation of the chemistry of the ore-forming fluid with an increase of Fe, Ga, Tl, Au, Hg, Pb and Ag, and decrease of Sb, Sn, Te, Ge and V concentrations with time and points out that this type of deposits is the richest in Au (average 11.9ppm) and Te (average 169ppm) among all other types of seafloor metal deposits. In addition to the widespread inorganic precipitation of enargite and luzonite in this setting, there is evidence that this mineralization may be biogenically mediated on the external surfaces of the active vents. Fungi-like filaments mineralized by luzonite imply that the fungi (Dikarya subkingdom) may be implicated in a mechanism of bio-sequestration of As, S and Cu, and provide the initial substrate for luzonite precipitation. The studied enargite-luzonite deposits have characteristics similar to those of subaerial high-sulfidation epithermal mineralization: back-arc basin setting; acid-sulfate and boiling ore-forming fluids; altered (advanced argillic stage) dacitic host rocks; major enargite-luzonite and minor pyrite, barite and S0; δ34S<0‰. Therefore, they may be considered as submarine analogues of subaerial high-sulfidation epithermal deposits with the potential for concealed porphyry Cu(Au) mineralization at depth.


      PubDate: 2016-06-13T02:29:05Z
       
  • Evaluation of temperature-time conditions for the chemical abrasion
           treatment of single zircons for U–Pb geochronology
    • Abstract: Publication date: 2 November 2016
      Source:Chemical Geology, Volume 438
      Author(s): Magdalena H. Huyskens, Sonja Zink, Yuri Amelin
      Partial loss of radiogenic Pb and presence of older grains or cores (inheritance) are the main obstacles for accurate and precise U–Pb geochronology of magmatic zircons by ID-TIMS (isotope dilution thermal ionisation mass spectrometry). Sequential annealing and leaching (chemical abrasion, (Mattinson 2005)), restores crystallinity to zircons with low to medium radiation damage, and removes parts of the zircon with strongly radiation damaged crystal lattice, effectively eliminating Pb loss. The original version of chemical abrasion (Mattinson 2005) was designed for determination of precise 206Pb–238U plateau ages of multi-grain zircon populations, free from inheritance, using multiple dissolution steps. However, the main application of chemical abrasion is to date zircon populations potentially affected by both Pb loss and inheritance, which is commonly achieved by analysis of single zircon grains. Chemical abrasion of single zircon grains involves only one high temperature leaching step, since stepwise leaching is rarely possible or practical due to the small sample size. Despite the widespread use of single zircon chemical abrasion, the optimal temperature and duration of annealing and single step leaching have not been reported thus far. Furthermore, the temperature and time durations of chemical abrasion (apparently an ad-hoc adaptation of the findings of multi-grain, multi-step leaching experiments) vary substantially between published geochronological studies, leaving the possibility of incomplete removal of Pb loss and hence a potential bias in comparison of the ages. Here we report the results of a systematic study aimed at determination of optimal conditions for chemical abrasion of single zircon grains to ensure complete removal of zones that were affected by Pb loss in a single high temperature leaching step. The tests were performed on three different zircon standards with various ages (~418 to 3465Ma) using a three step partial dissolution technique. The optimal conditions for low to moderately radiation damaged zircons are annealing at 900°C for 48h and subsequent leaching at 190°C or slightly higher in concentrated HF in a pressurized vessel for 15h. Leaching at a lower temperature does not completely remove Pb loss. Annealing at higher temperature might be restoring crystallinity in the parts of zircon crystals that were affected by Pb loss.


      PubDate: 2016-06-13T02:29:05Z
       
  • An investigation of the laser-induced zircon ‘matrix effect’
    • Abstract: Publication date: 2 November 2016
      Source:Chemical Geology, Volume 438
      Author(s): E. Marillo-Sialer, J. Woodhead, J.M. Hanchar, S.M. Reddy, A. Greig, J. Hergt, B. Kohn
      This study aims to improve our understanding of the current limitations to high-precision U–Pb analysis of zircon by LA-ICP-MS by investigating the underlying causes of variation in ablation behaviour between different zircon matrices. Multiple factors such as: the degree of accumulated radiation damage; trace element composition; crystal colour; and crystallographic orientation are all systematically investigated. Due to the marked decrease in elastic moduli of natural zircon crystals with increasing radiation damage, the accumulation of this damage is the dominant factor controlling the rate of ablation for partially damaged to highly metamict zircon samples. There are slight differences, however, in ablation behaviour between highly crystalline matrices that cannot be attributed solely to differences in the degree of accumulated radiation damage. These differences are associated with structural weakening (i.e., decrease in elastic moduli and overall lower mechanical resistance) caused by an increasing degree of cation substitution in some of the zircon samples. Effects of crystallographic orientation and of crystal opacity (i.e., colour) on ablation behaviour are negligible compared to the combined influences of accumulated radiation damage and trace element substitution into the zircon structure. Experiments performed on natural and annealed zircon grains reveal that the reduction in ablation rates observed for the treated samples compared to the untreated grains is proportional to the degree of structural reconstitution achieved after annealing. Thermal annealing of natural zircon at temperatures >1000°C results in much more uniform ablation characteristics. This ‘homogenisation’ of ablation behaviour between zircon matrices produces a decrease in the laser-induced matrix effects and subsequent improvement in the accuracy of 206Pb/238U ratio determinations by LA-ICP-MS.


      PubDate: 2016-06-13T02:29:05Z
       
  • Precise titanium isotope compositions of refractory inclusions in the
           Allende CV3 chondrite by LA-MC-ICPMS
    • Abstract: Publication date: 15 October 2016
      Source:Chemical Geology, Volume 436
      Author(s): C.D. Williams, P.E. Janney, R.R. Hines, M. Wadhwa
      We present here analyses of mass-independent effects in the Ti isotope ratios of 17 coarse-grained (compact Type A and Type B) calcium–aluminum-rich inclusions (CAIs) from the Allende CV3 chondrite utilizing a method developed for rapid measurement of Ti isotopes by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS). Based on the analyses of the synthetic CMAS glasses doped with varying amounts of Ca, Cr and V, an empirical scheme for correction of potential isobaric interferences and matrix effects was developed. Using this scheme, mass-independent variations of the Ti isotope ratios 46Ti/47Ti, 48Ti/47Ti and 50Ti/47Ti were measured with external reproducibilities (2SD) of ±0.4, ±0.5, and ±1.8 for ε46Ti, ε48Ti, and ε50Ti, respectively, based on repeat measurements of standard glasses. The Ti isotope compositions of the 17 Allende refractory inclusions analyzed here show that most of these CAIs are “normal” with limited variation in their mass-independent Ti isotope composition. One CAI (designated as CMS-1) has a significantly larger mass-independent ε50Ti anomaly and is identified as having Fractionation and Unidentified Nuclear (FUN) effects. The limited range in the Ti isotope compositions of normal CAIs (ε50Ti excesses ranging from 2.9 to 11.4) suggests that they originated from a nebular reservoir that was relatively well-mixed, although not completely homogenized (at the level of precision of the analyses reported here), in its isotopic composition. The distinctive isotopic composition of the FUN CAI CMS-1 indicates that it formed from a reservoir in the protoplanetary disk that was spatially or temporally distinct from that from which the normal CAIs were formed.


      PubDate: 2016-05-14T18:03:00Z
       
  • Modern aragonite formation at near-freezing conditions in an alpine cave,
           Carnic Alps, Austria
    • Abstract: Publication date: 1 October 2016
      Source:Chemical Geology, Volume 435
      Author(s): Christoph Spötl, Jens Fohlmeister, Hai Cheng, Ronny Boch
      Aragonite is a common mineral in caves hosted in dolomitic bedrock, and in particular in those from warm climate regions. It is much less common in cold caves, such as in the Alps, where the vast majority of speleothems consist of low-Mg calcite. Here we report the occurrence of modern aragonite at very low temperatures, the first of its kind from an alpine cave. In the interior part of Obstanser Eishöhle, a high-alpine cave containing perennial ice in its near-entrance zone, aragonite forms stalactites and flowstones, which coexist with calcite and locally also hydromagnesite. Precipitation of aragonite occurs at saturation indices between 0.4 and 0.6 at 1.0±0.4°C. Modern 230Th dates of the youngest aragonite deposits demonstrate that aragonite formation is an ongoing process. δ13C values are high and overlap with those of the host rock. A comparison between δ18O values of aragonite and drip water showed that modern aragonite formation does not occur at isotopic equilibrium. Surprisingly, no radiocarbon “bomb peak” was detected in these speleothems, despite their young age. In addition, these samples are characterized by extremely high reservoir ages between 8800 and 10,100yr, the highest ever reported for a speleothem worldwide. These data are best reconciled with a model of karst dissolution largely decoupled from the soil zone. Oxidation of disseminated pyrite in the dark gray host rock gives rise to acidic solutions driving carbonate dissolution. The dolomitic nature of the bedrock, prior calcite precipitation and ventilation-enhanced degassing leads to high Mg/Ca ratios in the drip water favoring aragonite formation. These in-situ observations confirm previous findings that temperature is not the dominant control on calcite versus aragonite formation in caves. They also suggest that ancient aragonite speleothems should be examined carefully and ideally be compared to coeval calcite samples prior to interpreting their stable isotope data in terms of environmental/climatic changes.


      PubDate: 2016-05-14T18:03:00Z
       
  • Rare earth elements in deep-water articulated brachiopods: An evaluation
           of seawater mass
    • Abstract: Publication date: 1 October 2016
      Source:Chemical Geology, Volume 435
      Author(s): Amir H. Zaky, Karem Azmy, Uwe Brand, Jörundur Svavarsson
      Modern articulated brachiopods of the orders Rhynchonellids and Terebratulids were obtained from below the neritic zone of the Caribbean Sea, North Atlantic, South Pacific and Southern Oceans to evaluate the proxy potential of their rare earth element (REE) contents. Shells of the two orders display similar REE distribution coefficients that vary consistently within the series in response to changes in ionic radii relative to that of the Ca2+ cation, which, in turn, excludes the possibility of a biological control on their REE incorporation. The calculated log K D values for the brachiopods are similar to those documented in other investigations, and as a consequence, shell calcite is enriched in REE concentrations relative to ambient water masses by an average of 0.81×105. Brachiopods from the four areas yielded LREESN and MREESN characteristics of their ambient water mass, exhibiting gradual enrichments with increasing atomic number as well as negative CeSN excursions. Their HREESN values, on the other hand, define a trend that varies with water depth. Overall, the Ce/Ce* profile of open-water deep-sea brachiopods is coupled with that of the ambient seawater. Our study confirms the robustness of REEs in deep-water (>500m) articulated brachiopods and supports their use as a proxy of paleo-oceanography, specifically water mass and redox.


      PubDate: 2016-05-08T09:18:17Z
       
  • Mineral formation during bacterial sulfate reduction in the presence of
           different electron donors and carbon sources
    • Abstract: Publication date: 1 October 2016
      Source:Chemical Geology, Volume 435
      Author(s): Xiqiu Han, Logan Schultz, Weiyan Zhang, Jihao Zhu, Fanxu Meng, Gill G. Geesey
      Sulfate-reducing bacteria have long been known to promote mineral precipitation. However, the influence of electron donors (energy sources) and carbon sources on the minerals formed during sulfate reduction is less well understood. An investigation was therefore undertaken to determine how these nutrients affect sulfate reduction by the bacterium Desulfovibrio alaskensis G20 in a marine sediment pore water medium. Monohydrocalcite and a small amount of calcite formed during sulfate reduction with formate as the electron donor; Mg-phosphates and calcite precipitated when hydrogen served as the electron donor and when acetate and dissolved inorganic carbon served as carbon sources; and greigite and elemental sulfur were deposited when lactate was used as the electron donor and carbon source. The experimental results were generally consistent with geochemical modeling, suggesting that it may be possible to predict the processes and conditions during formation of these minerals in natural environments.
      Graphical abstract image

      PubDate: 2016-05-08T09:18:17Z
       
  • Determination of position-specific carbon isotope ratios in propane from
           hydrocarbon gas mixtures
    • Abstract: Publication date: 1 October 2016
      Source:Chemical Geology, Volume 435
      Author(s): Li Gao, Panqing He, Yongbin Jin, Yanqi Zhang, Xiaoqun Wang, Shuichang Zhang, Yongchun Tang
      Position-specific isotope ratios (PSIRs), also termed intramolecular isotope ratios, provide novel information to probe molecular structure, reaction mechanics, and molecular signatures for gas to source correlation. Successful application of this technique to natural gas may provide key insights into gas origin and formation mechanisms, which often cannot be satisfactorily addressed using bulk or compound-specific isotope analyses (CSIA) alone. In this study, we present a method to determine the PSIR of propane from hydrocarbon gas mixtures at natural abundance using a step-wise quantification and compound-specific isotope monitoring approach. First, we purify/enrich propane using a proprietary cryogenic gas processing unit, and then convert it to acetic acid (AA) by both enzyme-catalyzed and chemical reactions. Our method of PSIR analysis makes progress in several regards, including efficient propane separation from low-propane concentration gas mixtures (0.5% v/v) using relatively small sample quantities (below 8mL propane). This advance in methodology enables more routine analysis and an optimized workflow for isotope analysis with strict quality control. Results obtained from oil-derived natural gas show that the center carbon in propane is more 13C-enriched than the terminal carbons by about 19.2‰. Our results are discussed in the context of previous efforts in propane intramolecular analysis and potential future uses of this novel technique to improve understanding of the origin of gases, their formation, and the isotope reversal phenomenon.


      PubDate: 2016-05-08T09:18:17Z
       
  • (LA,Q)-ICPMS trace-element analyses of Durango and McClure Mountain
           apatite and implications for making natural LA-ICPMS mineral standards
    • Abstract: Publication date: 1 October 2016
      Source:Chemical Geology, Volume 435
      Author(s): David M. Chew, Michael G. Babechuk, Nathan Cogné, Chris Mark, Gary J. O'Sullivan, Isadora A. Henrichs, Daniel Doepke, Cora A. McKenna
      Apatite, the most abundant phosphate mineral in the Earth's crust and uppermost mantle, is able to accept a wide variety of trace elements into its crystal structure. Many of these trace element substitutions are below the detection limit of Electron Microprobe Analysis, but can be determined by laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS). LA-ICPMS elemental abundance determinations typically employ sample-standard bracketing using either standard glasses or an appropriate matrix-matched reference material. In this study we have undertaken laser ablation (>3000 analyses) and low-blank solution Q(quadrupole)-ICPMS trace-element analyses on crushed 150–300μm aliquots of Durango and McClure Mountain apatite to assess the accuracy of apatite elemental abundance determinations when using NIST 612 standard glass as the primary LA-ICPMS trace element standard. An accuracy (relative to the solution data) and precision of <5% can be obtained for most trace elements (Y, the REE, Sr, Mn, V, Th and U) in LA-ICPMS analyses of crushed Durango separates; the McClure Mountain data are similarly accurate for most trace elements but yield larger intra-crystal variability. Durango raster and image mapping experiments demonstrate some Durango crystals are more homogenous than others; the raster experiments also show that Durango typically exhibits less zoning parallel to the C-axis compared to perpendicular to the C-axis. A protocol for developing a homogenous Durango apatite trace-element reference material is suggested, and involves slicing the interior portions of several Durango crystals parallel to the C-axis, undertaking rapid LA-ICPMS raster experiments to characterize trace-element zoning, crushing the most homogenous crystal to 150–300μm and determining its trace-element contents by low-blank solution ICPMS. This generic approach can easily be modified and applied to characterize other natural LA-ICPMS mineral standards.


      PubDate: 2016-05-08T09:18:17Z
       
  • Kinetic inhibition of dolomite precipitation: Insights from Raman
           spectroscopy of Mg2+–SO42− ion pairing in MgSO4/MgCl2/NaCl
           solutions at temperatures of 25 to 200°C
    • Abstract: Publication date: 1 October 2016
      Source:Chemical Geology, Volume 435
      Author(s): Xiaolin Wang, I-Ming Chou, Wenxuan Hu, Shunda Yuan, Hao Liu, Ye Wan, Xiaoyu Wang
      The origin of dolomite has been an issue for hundreds of years, and its kinetic inhibition is a critical aspect of this issue. Dissolved sulfate is regarded as an inhibitor for dolomite formation because it can bind Mg2+ to form tight ion pairs and thus prevent the incorporation of Mg2+ into dolomite. Using Raman spectroscopy, we investigated the Mg2+–SO4 2− association in vapor-saturated aqueous MgSO4/MgCl2/NaCl solutions at temperatures of 25 to 200°C. The Mg2+–SO4 2− association is highly temperature and concentration dependent: the fractions of contact ion pairs (CIPs) and triple ion pairs (TIs) increase with increasing temperature and MgSO4 concentration. The presence of MgCl2 increases the Mg2+/SO4 2− ratio and favors Mg2+–SO4 2− interactions to produce CIPs and TIs, whereas the presence of NaCl exerts a negative effect on Mg2+–SO4 2− interactions, particularly at high temperatures (i.e., ≥150°C). The primary sulfate species in concentrated MgSO4 solutions at high temperatures (i.e., ≥2mol/kg, 200°C) are various contact ion pairs, whereas those in diluted solutions at Earth surface temperature appear to be unassociated SO4 2− and weakly associated solvent-separated and solvent-shared ion pairs. We propose that dissolved sulfate can inhibit the incorporation of Mg2+ into dolomite crystals by attracting Mg2+ to form tight contact ion pairs under hydrothermal conditions. However, thermochemical sulfate reduction (TSR) can effectively remove sulfate and free Mg2+ to enhance the precipitation of hydrothermal dolomite from sulfate-bearing fluids. The inhibiting effect of dissolved sulfate on the formation of massive low-temperature dolomite appears to have been overestimated. Removal of sulfate by anaerobic bacterial sulfate reduction (BSR) may not be responsible for the formation of microbial dolomite at surface temperatures. These new understandings also have implications for the study of thermochemical sulfate reduction because the formation of CIPs can increase the activity of sulfate in reactions with hydrocarbons.


      PubDate: 2016-05-08T09:18:17Z
       
 
 
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