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  Subjects -> EARTH SCIENCES (Total: 647 journals)
    - EARTH SCIENCES (461 journals)
    - GEOLOGY (75 journals)
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EARTH SCIENCES (461 journals)                  1 2 3 | Last

Showing 1 - 200 of 371 Journals sorted alphabetically
Acta Geochimica     Hybrid Journal   (Followers: 6)
Acta Geodaetica et Geophysica     Hybrid Journal   (Followers: 2)
Acta Geodaetica et Geophysica Hungarica     Full-text available via subscription   (Followers: 2)
Acta Geophysica     Open Access   (Followers: 9)
Acta Geotechnica     Hybrid Journal   (Followers: 7)
Acta Meteorologica Sinica     Hybrid Journal   (Followers: 3)
Advances in High Energy Physics     Open Access   (Followers: 19)
Advances In Physics     Hybrid Journal   (Followers: 19)
Aeolian Research     Hybrid Journal   (Followers: 5)
African Journal of Aquatic Science     Hybrid Journal   (Followers: 13)
Algological Studies     Full-text available via subscription   (Followers: 2)
Alpine Botany     Hybrid Journal   (Followers: 5)
AMBIO     Hybrid Journal   (Followers: 15)
Anales del Instituto de la Patagonia     Open Access   (Followers: 1)
Andean geology     Open Access   (Followers: 13)
Annales Henri Poincaré     Hybrid Journal   (Followers: 3)
Annales UMCS, Geographia, Geologia, Mineralogia et Petrographia     Open Access  
Annals of Geophysics     Open Access   (Followers: 13)
Annals of GIS     Hybrid Journal   (Followers: 23)
Annual Review of Marine Science     Full-text available via subscription   (Followers: 11)
Anthropocene     Hybrid Journal   (Followers: 3)
Anthropocene Review     Hybrid Journal   (Followers: 6)
Applied Clay Science     Hybrid Journal   (Followers: 5)
Applied Geochemistry     Hybrid Journal   (Followers: 14)
Applied Geomatics     Hybrid Journal   (Followers: 3)
Applied Geophysics     Hybrid Journal   (Followers: 8)
Applied Ocean Research     Hybrid Journal   (Followers: 5)
Applied Petrochemical Research     Open Access   (Followers: 2)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 36)
Arctic Science     Open Access   (Followers: 6)
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 9)
Artificial Satellites : The Journal of Space Research Centre of Polish Academy of Sciences     Open Access   (Followers: 20)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 20)
Asian Journal of Earth Sciences     Open Access   (Followers: 20)
Asian Review of Environmental and Earth Sciences     Open Access   (Followers: 2)
Atlantic Geology : Journal of the Atlantic Geoscience Society / Atlantic Geology : revue de la Société Géoscientifique de l'Atlantique     Full-text available via subscription   (Followers: 11)
Atmosphere-Ocean     Full-text available via subscription   (Followers: 12)
Atmospheric and Climate Sciences     Open Access   (Followers: 28)
Australian Journal of Earth Sciences: An International Geoscience Journal of the Geological Society of Australia     Hybrid Journal   (Followers: 11)
Boletim de Ciências Geodésicas     Open Access  
Boreas: An International Journal of Quaternary Research     Hybrid Journal   (Followers: 11)
Bragantia     Open Access   (Followers: 2)
Bulletin of Earthquake Engineering     Hybrid Journal   (Followers: 11)
Bulletin of Geosciences     Open Access   (Followers: 11)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (Followers: 1)
Bulletin of the Seismological Society of America     Full-text available via subscription   (Followers: 21)
Bulletin of Volcanology     Hybrid Journal   (Followers: 21)
Cadernos de Geociências     Open Access  
Canadian Mineralogist     Full-text available via subscription   (Followers: 5)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 21)
Carbonates and Evaporites     Hybrid Journal   (Followers: 3)
CATENA     Hybrid Journal   (Followers: 7)
Chemical Geology     Hybrid Journal   (Followers: 22)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 5)
Chinese Geographical Science     Hybrid Journal   (Followers: 5)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 4)
Ciencia del suelo     Open Access   (Followers: 2)
Ciencias Espaciales     Open Access  
Climate and Development     Hybrid Journal   (Followers: 12)
Coastal Management     Hybrid Journal   (Followers: 25)
Cogent Geoscience     Open Access  
Comptes Rendus Geoscience     Full-text available via subscription   (Followers: 9)
Computational Geosciences     Hybrid Journal   (Followers: 15)
Computational Mathematics and Mathematical Physics     Hybrid Journal   (Followers: 3)
Computers and Geotechnics     Hybrid Journal   (Followers: 10)
Contemporary Trends in Geoscience     Open Access   (Followers: 3)
Continental Shelf Research     Hybrid Journal   (Followers: 9)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 12)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 3)
Coral Reefs     Hybrid Journal   (Followers: 18)
Cretaceous Research     Hybrid Journal   (Followers: 8)
Cybergeo : European Journal of Geography     Open Access   (Followers: 6)
Depositional Record     Open Access  
Developments in Geotectonics     Full-text available via subscription   (Followers: 3)
Developments in Quaternary Science     Full-text available via subscription   (Followers: 4)
Développement durable et territoires     Open Access   (Followers: 3)
Diatom Research     Hybrid Journal   (Followers: 2)
Doklady Physics     Hybrid Journal   (Followers: 1)
Dynamics of Atmospheres and Oceans     Hybrid Journal   (Followers: 12)
E&S Engineering and Science     Open Access  
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (Followers: 140)
Earth and Space Science     Open Access   (Followers: 14)
Earth Interactions     Full-text available via subscription   (Followers: 13)
Earth Science Frontiers     Full-text available via subscription   (Followers: 7)
Earth Science Research     Open Access   (Followers: 8)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 6)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 20)
Earth System Dynamics     Open Access   (Followers: 9)
Earth System Dynamics Discussions     Open Access   (Followers: 6)
Earth's Future     Open Access   (Followers: 1)
Earth, Planets and Space     Open Access   (Followers: 71)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 9)
Earthquake Science     Hybrid Journal   (Followers: 12)
Earthquake Spectra     Full-text available via subscription   (Followers: 21)
Ecohydrology     Hybrid Journal   (Followers: 11)
Ecological Questions     Open Access   (Followers: 7)
Electromagnetics     Hybrid Journal   (Followers: 5)
Energy Efficiency     Hybrid Journal   (Followers: 11)
Energy Exploration & Exploitation     Hybrid Journal   (Followers: 4)
Environmental Earth Sciences     Hybrid Journal   (Followers: 27)
Environmental Geology     Hybrid Journal   (Followers: 20)
Environmental Geosciences     Full-text available via subscription   (Followers: 5)
Environmental Geotechnics     Hybrid Journal   (Followers: 5)
Erwerbs-Obstbau     Hybrid Journal  
Estuaries and Coasts     Hybrid Journal   (Followers: 17)
Estuarine, Coastal and Shelf Science     Hybrid Journal   (Followers: 33)
Estudios Geográficos     Open Access  
European Journal of Mineralogy     Full-text available via subscription   (Followers: 15)
European Journal of Remote Sensing     Open Access   (Followers: 3)
Exploration Geophysics     Hybrid Journal   (Followers: 4)
Facies     Hybrid Journal   (Followers: 9)
Fieldiana Life and Earth Sciences     Full-text available via subscription   (Followers: 1)
Física de la Tierra     Open Access  
Folia Musei rerum naturalium Bohemiae occidentalis. Geologica et Paleobiologica     Open Access  
Folia Quaternaria     Open Access  
Forestry Chronicle     Full-text available via subscription   (Followers: 11)
Frontiers in Earth Science     Open Access   (Followers: 5)
Frontiers in Geotechnical Engineering     Open Access   (Followers: 3)
Frontiers of Earth Science     Hybrid Journal   (Followers: 8)
Fundamental and Applied Limnology / Archiv für Hydrobiologie     Full-text available via subscription   (Followers: 4)
GEM - International Journal on Geomathematics     Hybrid Journal   (Followers: 1)
Geo-Marine Letters     Hybrid Journal   (Followers: 7)
Geoacta     Open Access   (Followers: 4)
Geobiology     Hybrid Journal   (Followers: 9)
Geocarto International     Hybrid Journal   (Followers: 22)
Geochemistry : Exploration, Environment, Analysis     Hybrid Journal   (Followers: 8)
Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 31)
Geochimica et Cosmochimica Acta     Hybrid Journal   (Followers: 51)
Geochronometria     Hybrid Journal   (Followers: 3)
Geoderma Regional : The International Journal for Regional Soil Research     Full-text available via subscription   (Followers: 4)
Geodinamica Acta     Open Access   (Followers: 3)
Geodynamics & Tectonophysics     Open Access   (Followers: 2)
Geoenvironmental Disasters     Open Access   (Followers: 3)
Geofluids     Open Access   (Followers: 4)
Geoforum     Hybrid Journal   (Followers: 23)
Géographie physique et Quaternaire     Full-text available via subscription   (Followers: 1)
Geography and Natural Resources     Hybrid Journal   (Followers: 8)
Geoheritage     Hybrid Journal   (Followers: 1)
Geoinformatica Polonica : The Journal of Polish Academy of Arts and Sciences     Open Access  
Geoinformatics & Geostatistics     Hybrid Journal   (Followers: 9)
Geological Journal     Hybrid Journal   (Followers: 16)
Geology Today     Hybrid Journal   (Followers: 29)
Geology, Geophysics and Environment     Open Access   (Followers: 1)
Geomagnetism and Aeronomy     Hybrid Journal   (Followers: 3)
Geomatics, Natural Hazards and Risk     Hybrid Journal   (Followers: 10)
Geomechanics for Energy and the Environment     Full-text available via subscription   (Followers: 1)
GEOmedia     Open Access   (Followers: 1)
Geomorphology     Hybrid Journal   (Followers: 30)
Geophysical & Astrophysical Fluid Dynamics     Hybrid Journal   (Followers: 2)
Geophysical Journal International     Hybrid Journal   (Followers: 34)
Geophysical Prospecting     Hybrid Journal   (Followers: 7)
GeoResJ     Hybrid Journal  
Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards     Hybrid Journal   (Followers: 9)
Geoscience Canada : Journal of the Geological Association of Canada / Geoscience Canada : journal de l'Association Géologique du Canada     Full-text available via subscription   (Followers: 5)
Geoscience Data Journal     Open Access   (Followers: 2)
Geoscience Frontiers     Open Access   (Followers: 10)
Geoscience Letters     Open Access   (Followers: 1)
Geoscience Records     Open Access  
Geosciences     Open Access   (Followers: 2)
Geosciences Journal     Hybrid Journal   (Followers: 11)
Geoscientific Instrumentation, Methods and Data Systems     Open Access   (Followers: 4)
Geoscientific Model Development     Open Access   (Followers: 2)
Geostandards and Geoanalytical Research     Hybrid Journal   (Followers: 2)
Geosystem Engineering     Hybrid Journal   (Followers: 1)
Geotectonic Research     Full-text available via subscription   (Followers: 5)
Geotectonics     Hybrid Journal   (Followers: 7)
GISAP : Earth and Space Sciences     Open Access   (Followers: 2)
Glass Physics and Chemistry     Hybrid Journal   (Followers: 4)
Global and Planetary Change     Hybrid Journal   (Followers: 19)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 15)
Gondwana Research     Hybrid Journal   (Followers: 9)
GPS Solutions     Hybrid Journal   (Followers: 17)
Grassland Science     Hybrid Journal  
Ground Water     Hybrid Journal   (Followers: 35)
Ground Water Monitoring & Remediation     Hybrid Journal   (Followers: 19)
Groundwater for Sustainable Development     Full-text available via subscription   (Followers: 3)
Helgoland Marine Research     Open Access   (Followers: 3)
History of Geo- and Space Sciences     Open Access   (Followers: 4)
Hydrobiologia     Hybrid Journal   (Followers: 18)
Hydrogeology Journal     Hybrid Journal   (Followers: 22)
Hydrological Processes     Hybrid Journal   (Followers: 30)
Hydrology and Earth System Sciences     Open Access   (Followers: 30)
ICES Journal of Marine Science: Journal du Conseil     Hybrid Journal   (Followers: 55)
IEEE Journal of Oceanic Engineering     Hybrid Journal   (Followers: 13)
Indian Geotechnical Journal     Hybrid Journal   (Followers: 4)
Indonesian Journal on Geoscience     Open Access   (Followers: 2)
Interdisciplinary Environmental Review     Hybrid Journal   (Followers: 3)
International Geology Review     Hybrid Journal   (Followers: 16)
International Journal of Advanced Geosciences     Open Access   (Followers: 1)
International Journal of Advanced Remote Sensing and GIS     Open Access   (Followers: 38)
International Journal of Advancement in Earth and Enviromental Sciences     Open Access   (Followers: 3)
International Journal of Advancement in Remote Sensing, GIS, and Geography     Open Access   (Followers: 31)
International Journal of Applied Earth Observation and Geoinformation     Hybrid Journal   (Followers: 36)
International Journal of Coal Geology     Hybrid Journal   (Followers: 2)
International Journal of Disaster Risk Reduction     Hybrid Journal   (Followers: 18)
International Journal of Earth Sciences     Hybrid Journal   (Followers: 39)
International Journal of Earthquake and Impact Engineering     Hybrid Journal   (Followers: 4)
International Journal of Geo-Engineering     Open Access   (Followers: 3)
International Journal of Geographical Information Science     Hybrid Journal   (Followers: 55)

        1 2 3 | Last

Journal Cover Chemical Geology
  [SJR: 2.346]   [H-I: 145]   [22 followers]  Follow
    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 0009-2541
   Published by Elsevier Homepage  [3089 journals]
  • Inside front cover (Editorial Board)
    • Abstract: Publication date: 25 December 2017
      Source:Chemical Geology, Volume 475


      PubDate: 2017-12-13T01:58:36Z
       
  • Rare earth element partitioning and subsolidus exchange behaviour in
           olivine
    • Abstract: Publication date: 25 December 2017
      Source:Chemical Geology, Volume 475
      Author(s): Clare V. Stead, Emma L. Tomlinson, Cora A. McKenna, Balz S. Kamber
      The systematics of the rare earth element (REE) abundances in olivine are poorly understood and this undermines their usefulness in deciphering petrogenetic processes. Here we report a novel REE dataset for olivine spanning a wide range in Mg number (Mg# 1–59) from the Skaergaard Layered Intrusion. Olivine La and Lu concentrations vary from 0.68 to 56ppb and 66 to 571ppb, respectively. Apparent olivine/melt partition coefficients for the heavy to middle REE define near-symmetrical parabolas, consistent with lattice strain theory, indicating that these elements are structurally bound within the olivine lattice. The parabolas define peaks with variable optimal radii (r o ). This suggests that the Fe-content of olivine exerts a strong influence on REE partitioning, as fayalitic olivines display more open parabolas (higher apparent Young's modulus, E) than forsteritic olivines. The greater site elasticity is manifest as higher REE concentrations in the fayalitic samples. The observation that E and r o are not constant accross a range of Mg# has implications for predictive models for olivine REE. The CI chondrite-normalised olivine REE patterns show smooth trends from the heavy to the middle REE, with greater variation amongst the light REE (LREE). Variation and enrichment of the LREE are ubiquitous within published data for natural olivine, yet are not predicted by experimental results. This study also presents two-dimensional REE maps illustrating the behaviour of the REE in olivine in unprecedented detail. Mapping of magnesian olivine shows core to rim increase in Al, LREE, and Eu and the development of a positive Eu anomaly in olivine, resulting from solid-state, likely diffusive re-equilibration with adjacent plagioclase. We propose that secondary redistribution is the principal cause of elevated LREE patterns in natural olivine. The results from the present investigation show that from a petrological point of view, MREE to HREE systematics of olivine hold promise for study of olivine petrogenesis and that the LREE in olivine may be further developed to understand diffusion time scales and cooling rates in magmatic systems.

      PubDate: 2017-12-13T01:58:36Z
       
  • Hydrogen and oxygen stable isotope signatures of goethite hydration waters
           by thermogravimetry-enabled laser spectroscopy
    • Abstract: Publication date: 25 December 2017
      Source:Chemical Geology, Volume 475
      Author(s): Erik Oerter, Michael Singleton, Lee Davisson
      The hydrogen and oxygen stable isotope composition (δ2H and δ18O values) of mineral hydration waters can give information on the environment of mineral formation. Here we present and validate an approach for the stable isotope analysis of mineral hydration waters based on coupling a thermogravimetric analyzer with a laser-based isotope ratio infrared spectroscopy instrument (Picarro L-2130i), which we abbreviate as TGA-IRIS. TGA-IRIS generates δ2H and δ18O values of liquid water samples with precision for δ2H of ±1.2‰, and for δ18O of ±0.17‰. For hydration waters in goethite, precision for δ2H ranges from ±0.3‰ to 1.6‰, and for δ18O ranges from ±0.17‰ to 0.27‰. The ability of TGA-IRIS to generate detailed water yield data and δ2H and δ18O values of water at varying temperatures allows for the differentiation of water in varying states of binding on mineral surfaces and within the mineral matrix. TGA-IRIS analyses of hydrogen isotopes in goethite yields δ2H values that reflect the hydrogen of the OH− phase in the mineral and are comparable to that made by IRMS and found in the literature. In contrast, δ18O values on goethite reflect the oxygen in OH− groups bound to Fe (Fe-OH group), and not the oxygen bound only to Fe (Fe-O group) in the mineral crystal lattice, and may not be comparable to literature δ18O values made by IRMS that reflect the total O in the mineral. TGA-IRIS presents the possibility to isotopically differentiate the various oxygen reservoirs in goethite, which may allow the mineral to be used as a single mineral geothermometer. TGA-IRIS measurements of hydration waters are likely to open new avenues and possibilities for research on hydrated minerals.

      PubDate: 2017-12-13T01:58:36Z
       
  • Anoxic development of sapropel S1 in the Nile Fan inferred from redox
           sensitive proxies, Fe speciation, Fe and Mo isotopes
    • Abstract: Publication date: 25 December 2017
      Source:Chemical Geology, Volume 475
      Author(s): Alan Matthews, Irit Azrieli-Tal, Ayelet Benkovitz, Miryam Bar-Matthews, Derek Vance, Simon W. Poulton, Nadya Teutsch, Ahuva Almogi-Labin, Corey Archer
      Redox conditions and the mechanisms of redox development are a critical aspect of Eastern Mediterranean sapropels, whose formation in oxygen-depleted waters is closely related to water column stratification at times of global sea level rise and insolation maxima. Sapropels in the Nile Fan formed at relatively shallow water depths under the influence of the monsoon-driven freshwater output from the River Nile. This work evaluates the redox evolution of Holocene sapropel S1 in VALPAMED cruise core MD9509, recovered at 880 mbsl in the NE Nile Fan, using a combination of geochemical element proxies, Fe speciation, Fe and Mo isotopes studies. The productivity and redox proxies (Ba/Al, Mo/Al, U/Al, V/Al, Sb/Al) show well-defined enrichments in the sapropel, but with a marked minimum at ca 8.2ka indicative of reventilation corresponding to a well known global cooling event. Peak productivity and reducing signals occur close to the initiation of sapropel formation. The proxy signals in sapropel 9509 are stronger and of longer duration than those of a second sapropel S1, recovered at the same depth, but 380km to the north (MD9501), supporting the notion (suggested in previous studies) of more reduced conditions in the Nile Fan. The MoEF vs. UEF enrichment factor variations in core 9509 infer a transition from open marine suboxic conditions in the enclosing non-sapropel sediments to anoxic non-sulphidic water column conditions in the sapropel. Correspondingly, the highly reactive Fe pool (FeHR) measured in Fe speciation studies is dominated by Fe(oxyhydr) oxide minerals in the background sediments, whereas pyrite (Fepy) becomes the dominant component of the FeHR pool in the sapropel. Maximum Fepy values in the sapropel coincide with peak productivity and reducing conditions, implying a clear link between trace element uptake, diagenetic bacterial sulphate reduction in anoxic porewater and Fe mobilization in the sapropel. Iron isotope compositions (δ56Fe) in the sapropel do not show any departure from primary (marine and detrital) source sediment values, and the absence of an Fe/Al vs. δ56Fe trend strongly argues against an Fe shuttle. Molybdenum isotopes, however, show marked non-conservative fractionation patterns. Background sediment δ98/95Mo values (0.2 to 0.7‰) are compatible with fractionation upon absorptive uptake by Fe (oxyhydr)oxides and pyrite. In contrast, minimum δ98/95Mo values exhibited at peak sapropel (reducing and pyrite producing) conditions are most closely modeled by Mo isotope fractionation during kinetically controlled conversion of aqueous molybdate to thiomolybdate species. The conservative Fe isotope behavior/Mo isotope fractionation minima in the sapropel may be a characteristic of organic-rich sediment diagenesis below an anoxic non-sulphidic water body, without the operation of a benthic Fe shuttle.

      PubDate: 2017-12-13T01:58:36Z
       
  • Two-phase fluid induced by N2 in metamorphic rocks
    • Abstract: Publication date: 25 December 2017
      Source:Chemical Geology, Volume 475
      Author(s): Margaret V. Artimenko
      The first quantitative thermodynamic model of two-phase fluid induced by N2 and equilibrated with mineral assemblage in the system SiO2–TiO2–Al2O3–Fe2O3–MnO–MgO–CaO–Na2O–K2O–P2O5–H2O–CO2–N2 (STAFMMCNKPO–HCN) in the conditions of amphibolite facies rocks is proposed. Calculations are performed using the Gibbs free energy minimization technique. The fluid may consist of one phase, either aqueous solution (AS) or fluid with gas-like properties (FG), and also these two fluid phases (AS+FG) may co-exist, depending upon nitrogen content in the system. The values of total nitrogen at which AS co-exists with FG in the fluid were calculated at carbon saturation over a range in pressure and temperature of 3–7kb and 550–650°C. The model is capable of accurately describing various parameters of two fluid phases at each step of nitrogen increment in the system: density, partial pressure and mole fraction of constituents, pH, Eh, concentration of aqueous species. Increments of such an inert component as nitrogen cause the changes in the fluid resulting into subtle quantitative re-distribution of mineral phases in the assemblage plagioclase–biotite–quartz–ilmenite–garnet–apatite–graphite plus minor sillimanite and andalusite.

      PubDate: 2017-12-13T01:58:36Z
       
  • Fulvic acid induced the liberation of chromium from
           CrO42−-substituted schwertmannite
    • Abstract: Publication date: 25 December 2017
      Source:Chemical Geology, Volume 475
      Author(s): Yingying Xie, Guining Lu, Han Ye, Chengfang Yang, Di Xia, Xiaoyun Yi, John Reinfelder, Zhi Dang
      The cycling of Fe and Cr is intimately linked to the fate of schwertmannite in acid mine drainage (AMD) and acid sulfate soil (ASS) environments. Dissolved organic matter (DOM) can affect the stability of minerals and speciation, mobility and toxicity of heavy metals via redox reactions and complexation. However, knowledge about the fate of Fe and Cr upon reduction and complexation of CrO4 2−-substituted schwertmannite by fulvic acid (FA) is poorly understood. In this study, experiments were devised to investigate the interaction between FA and schwertmannite, with major degeneration of the schwertmannite structure as well as the formation of secondary minerals. For CrO4 2−-substituted schwertmannite, results indicated that the concentrations of total Fe and Cr in the solution were determined to be the maximum values at 360h and 96h, and then decreasing over the reaction time when FA was 10mg/L at pH of 6.5. The characterization on the solid phase by X-ray diffraction, scanning electron microscopy and X-ray photo electron spectroscopy technologies revealed that goethite was the dominant newly secondary mineral phases which played a vital role in controlling the fate and transport of Fe and Cr. The possible mechanism was proposed to be synergistic effect including ligand-promoted and reduction. This study provides new insights into the understanding of trace element behavior in environments affected by DOM and also has guiding significance for Cr-contaminant remediation.

      PubDate: 2017-12-13T01:58:36Z
       
  • Arsenite and arsenate immobilization by preformed and concurrently formed
           disordered mackinawite (FeS)
    • Abstract: Publication date: 25 December 2017
      Source:Chemical Geology, Volume 475
      Author(s): Alejandra S. Vega, Britta Planer-Friedrich, Pablo A. Pastén
      Arsenic (As) immobilization in aqueous environments is controlled by sorption and/or coprecipitation with iron (Fe) oxyhydroxides under oxic conditions and with ferrous iron (Fe(II)) and sulfide (S(-II)) minerals under anoxic conditions. Shifts to reducing conditions trigger the release of dissolved As by reductive dissolution of Fe oxyhydroxides. More stringent reducing conditions in sulfur (S) rich environments form solid S(-II) phases that immobilize As. This work studied the association of arsenite and arsenate with freshly-formed disordered mackinawite (FeS), a model Fe and reduced S mineral of environmental relevance. The association of As to preformed FeS (Set P experiments) was contrasted to the case when As is present during FeS precipitation (Set C experiments). Most abiotic studies of As reactivity under reducing conditions have been done under Set P setting, despite that redox shifts are likely to induce environments analogous to Set C experiments. Dissolved As removal was initially higher for Set C compared to Set P experiments at least by a factor of two, except for arsenate with low initial redox potential. Set C experiments had a rapid initial As sorption, followed by some As desorption. When stringent reducing conditions ensue, the extent and rate of As removal by FeS is misrepresented by sorption experiments to preformed FeS. A continuous release of As concurrent with the formation of thioarsenates - a dissolved form of As with weaker binding to Fe minerals - was observed in Set C experiments with arsenite. The formation of thioarsenates occurred preferably in Set C experiments at low redox potential, while it was negligible for arsenate experiments. Remarkably, the occurrence of thioarsenates disputes a widespread notion that their formation is inhibited by Fe(II)-S(-II) affinity and precipitation as FeS. This study shows the potential of FeS to scavenge arsenite and to decrease its mobility in reduced sediments and groundwater, and highlights the role of thioarsenates in As mobility in systems with changing redox conditions where the reactive solid forms in presence of the contaminant.

      PubDate: 2017-12-13T01:58:36Z
       
  • Effect of iron and nanolites on Raman spectra of volcanic glasses: A
           reassessment of existing strategies to estimate the water content
    • Abstract: Publication date: 25 December 2017
      Source:Chemical Geology, Volume 475
      Author(s): Danilo Di Genova, Stefania Sicola, Claudia Romano, Alessandro Vona, Sara Fanara, Laura Spina
      The effect of iron content and iron nanolites on Raman spectra of hydrous geologically-relevant glasses is presented. Current procedures to estimate the water content using Raman spectra were tested to explore potential effects of iron content, its oxidation state, and nanolites on models' reliability. A chemical interval spanning from basalt to rhyolite, including alkali- and iron-rich compositions, with water content up to 5.6wt% was investigated using two spectrometers. When considering nanolite-free samples, the area of the band at 3550cm−1 linearly correlates with the sample water content regardless of chemical composition. Using this approach, data were reproduced with a root-mean-square error (RMSE) of ~0.15wt%. Depending on the sample chemistry, water content, and acquisition conditions the laser-induced sample oxidation led to underestimating the water content up to ~90% with a long acquisition time (26min). Normalising the water band region to the silicate band region minimises such a limitation. The area ratio between these bands linearly correlates with the water content and the use of different baseline procedures does not remove the dependence of such a correlation by the iron content and its oxidation state. With this procedure, data were reproduced with a RMSE of ~0.16wt%. For both approaches, the presence of iron nanolites may result in underestimating the water content.

      PubDate: 2017-12-13T01:58:36Z
       
  • Links between deformation, chemical enrichments and Li-isotope
           compositions in the lithospheric mantle of the central Siberian craton
    • Abstract: Publication date: 25 December 2017
      Source:Chemical Geology, Volume 475
      Author(s): Dmitri A. Ionov, Luc S. Doucet, Philip A.E. Pogge von Strandmann, Alexander V. Golovin, Andrey V. Korsakov
      We report the concentrations ([Li]) and isotopic compositions of Li in mineral separates and bulk rocks obtained by MC-ICPMS for 14 previously studied garnet and spinel peridotite xenoliths from the Udachnaya kimberlite in the central Siberian craton as well as major and trace element compositions for a new suite of 13 deformed garnet peridotites. The deformed Udachnaya peridotites occur at >5GPa; they are metasomatized residues of melt extraction, which as a group experienced greater modal and chemical enrichments than coarse peridotites. We identify two sub-groups of the deformed peridotites: (a) mainly cryptically metasomatized (similar to coarse peridotites) with relatively low modal cpx (<6%) and garnet (<7%), low Ca and high Mg#, sinusoidal REE patterns in garnet, and chemically unequilibrated garnet and cpx; (b) modally metasomatized with more cpx and garnet, higher Ca, Fe and Ti, and equilibrated garnet and cpx. The chemical enrichments are not proportional to deformation degrees. The deformation in the lower lithosphere is caused by a combination of localized stress, heating and fluid ingress from the pathways of ascending proto-kimberlite melts, with metasomatic media evolving due to reactions with wall rocks. Mg-rich olivine in spinel and coarse garnet Udachnaya peridotites has 1.2–1.9ppm Li and δ7Li of 1.2–5.0‰, i.e. close to olivine in equilibrated fertile to depleted off-craton mantle peridotites from literature data, whereas olivine from the deformed peridotites has higher [Li] (2.4–7.5ppm) and a broader range of δ7Li (1.8–11.6‰), which we attribute to pre-eruption metasomatism. [Li] in opx is higher than in coexisting olivine while Δ7LiOl-Opx (δ7LiOl −δ7LiOpx) ranges from −6.6 to 7.8‰, indicating disequilibrium inter-mineral [Li] and Li-isotope partitioning. We relate these Li systematics to interaction of lithospheric peridotites with fluids or melts that are either precursors of kimberlite magmatism or products of their fractionation and/or reaction with host mantle. The melts rich in Na and carbonates infiltrated, heated and weakened wall-rock peridotites to facilitate their deformation as well as produce high [Li] and variable, but mainly high, δ7Li in olivine. The carbonate-rich melts preferentially reacted with the opx without achieving inter-mineral equilibrium because opx is consumed by such melts, and because of small volumes and uneven distribution of the metasomatic media, as well as short time spans between the melt infiltration and the capture of the wall-rock fragments by incoming portions of ascending kimberlite magma as xenoliths. Trapped interstitial liquid solidified as cryptic components responsible for high [Li] and the lack of δ7Li balance between olivine and opx, and bulk rocks. Unaltered δ26Mg values (0.20–0.26‰) measured in several olivine separates show no effects of the metasomatism on Mg-isotopes, apparently due to high Mg in the peridotites.
      Graphical abstract image

      PubDate: 2017-12-13T01:58:36Z
       
  • High precision U-series dating of scleractinian cold-water corals using an
           automated chromatographic U and Th extraction
    • Abstract: Publication date: 25 December 2017
      Source:Chemical Geology, Volume 475
      Author(s): Anne-Marie Wefing, Jennifer Arps, Patrick Blaser, Claudia Wienberg, Dierk Hebbeln, Norbert Frank
      High-precision U-series dating of scleractinian cold-water corals is a key chronological tool for studies of past environmental and climate conditions. Here, we tested and optimized an automated chemical extraction system (ESI prepFAST-MC equipped with an Eichrom TRU-resin chromatographic column) for its ability to purify U and Th isotopes for mass spectrometric U-series dating at the sub-‰ precision level. Chemical yields are constantly high, on average around 90% for both U and Th. Analytical blanks are comparable to manual purification (<0.15pg U, 0.15pg Th for a typical sample of 50mg) and memory effects due to the recycling of the column are mostly insignificant as the carry-over fraction is about 10−5 for Th and 10−3 for U isotopes. However, it was found that the built-in UTh (TRU-resin) column must be pre-conditioned for analysis using >1μgg−1 Th and U in order to achieve the above mentioned chemical yields. This conditioning has no impact on the Th/U data. The automated chemical preparation protocol described here is compared to conventional high precision U-series dating with manual sample purification. For the 34 cold-water corals extracted from a sediment core collected from a coral mound off Angola, the differences between 230Th/238U- and 234U/238U-ratios and U-series ages measured with the two analytical methods are smaller than the respective analytical uncertainty of <3.0‰, 0.8‰ and 3.0‰, respectively. Overall, ages of the studied corals span 34,000years and perfectly meet quality control constrains, such as initial seawater δ234U0. Finally, our record of coral ages indicates vigorous coral growth under warm and cold climate conditions in the temperate south-eastern Atlantic, contrasting climate influenced coral occurrences in the north-eastern Atlantic.

      PubDate: 2017-12-13T01:58:36Z
       
  • Synergistic adsorption of Cd(II) with sulfate/phosphate on ferrihydrite:
           An in situ ATR-FTIR/2D-COS study
    • Abstract: Publication date: Available online 8 December 2017
      Source:Chemical Geology
      Author(s): Jing Liu, Runliang Zhu, Xiaoliang Liang, Lingya Ma, Xiaoju Lin, Jianxi Zhu, Hongping He, Stephen C. Parker, Marco Molinari
      Elucidation of the co-adsorption characteristics of heavy metal cations and oxyanions on (oxyhydr)oxides can help to better understand their distribution and transformation in many geological settings. In this work, batch adsorption experiments in combination with in situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) were applied to explore the interaction mechanisms of Cd(II) with sulfate or phosphate at the ferrihydrite (Fh)–water interface, and the two-dimensional correlation spectroscopic analysis (2D–COS) was used to enhance the resolution of ATR-FTIR bands and the accuracy of analysis. The batch adsorption experiments showed enhanced adsorption of both sulfate (S) and phosphate (P) on Fh when co-adsorbed with Cd(II); additionally, the desorbed percentages of Cd(II) were much lower in the P+Cd adsorption systems than those in the S+Cd adsorption systems. The spectroscopic results suggested that in the single adsorption systems, sulfate primarily adsorbed as outer-sphere complexes with a small amount of bidentate inner-sphere complexes, while the dominant adsorbed species of phosphate were largely the bidentate nonprotonated inner-sphere complexes, although there was significant pH-dependence. In the co-adsorption systems, the synergistic adsorption of Cd(II) and sulfate was dominantly attributed to the electrostatic interaction, as well as the formation of Fe–Cd–S (i.e., Cd-bridged) ternary complexes. In contrast, Fe–P–Cd (i.e., phosphate-bridged) ternary complexes were found in all of the co-adsorption systems of phosphate and Cd(II); furthermore, electrostatic interaction should also contribute to the co-adsorption process. Our results show that in situ ATR-FTIR in combination with 2D–COS can be an efficient tool in analyzing the co-adsorption mechanisms of anions and heavy metal cations on iron (oxyhydr)oxides in ternary adsorption systems. The co-existence of Cd(II) with sulfate or phosphate can be beneficial for their accumulations on Fh, and phosphate is more efficient than sulfate for the long-term immobilization of Cd(II).
      Graphical abstract image

      PubDate: 2017-12-13T01:58:36Z
       
  • Factors influencing the precision and accuracy of Nd isotope measurements
           by thermal ionization mass spectrometry
    • Abstract: Publication date: Available online 7 December 2017
      Source:Chemical Geology
      Author(s): Marion Garçon, Maud Boyet, Richard W. Carlson, Mary F. Horan, Delphine Auclair, Timothy D. Mock
      Taking the example of Nd, we present a method based on a 4-mass-step acquisition scheme to measure all isotope ratios dynamically by thermal ionization mass spectrometry (TIMS); the aim being to minimize the dependency of all mass fractionation-corrected ratios on collector efficiencies and amplifier gains. The performance of the method was evaluated from unprocessed JNdi-1 Nd standards analyzed in multiple sessions on three different instruments over a period of ~1.5years (n=61), as well as from standards (18 JNdi-1 and 19 BHVO-2) processed through different chemical purification procedures. The Nd isotopic compositions of standards processed through fine-grained (25–50μm) Ln-spec resin show a subtle but clear fractionation caused by the nuclear field shift effect. This effect contributes to the inaccuracy of Nd isotope measurements at the ppm level of precision. Following a comprehensive evaluation of the mass spectrometer runs, we suggest several criteria to assess the quality of data acquired by TIMS, in particular to see whether the measurements were affected by domain mixing effects on the filaments. We define maximum tolerable Ce and Sm interference corrections and the minimum number of ratios to acquire to ensure the best possible accuracy and precision for all Nd isotope ratios. Changes in fractionation of Nd isotope ratios in between acquisition steps can result in significant inaccuracy and bias dynamic μ142 values by >15ppm. To correct for these effects, we developed a systematic drift-correction based on the monitoring of Nd isotope ratios through time. The residual components of scatter in the JNdi-1 and BHVO-2 datasets were further investigated in binary isotopic plots in which we modeled the theoretical effects of domain mixing on filaments, nuclear field shift and correlated errors from counting statistics using Monte-Carlo simulations. These plots indicate that the 4-step method returns precisions limited by counting errors only for drift-corrected dynamic Nd isotope ratios. Data acquired on three different TIMS instruments suggest the following composition for the JNdi-1 reference standard: 142Nd/144Nd=1.141832±0.000006 (2s), 143Nd/144Nd=0.512099±0.000005 (2s), 145Nd/144Nd=0.348403±0.000003 (2s), 148Nd/144Nd=0.241581±0.000003 (2s), and 150Nd/144Nd=0.236452±0.000006 (2s) when normalized to 146Nd/144Nd=0.7219. Measurements performed on different instruments (Triton™ vs. Triton Plus™) show resolvable differences of about 10ppm for absolute 143Nd/144Nd, 145Nd/144Nd and 148Nd/144Nd ratios. The different criteria and corrections developed in this study could be applied to other isotopic systematics to improve and better evaluate the quality of high-precision data acquired by TIMS.

      PubDate: 2017-12-13T01:58:36Z
       
  • Phosphorus cycling in marine sediments: Advances and challenges
    • Abstract: Publication date: Available online 7 December 2017
      Source:Chemical Geology
      Author(s): Delphine Defforey, Adina Paytan
      Phosphorus (P) is a critical macronutrient for all living cells, as it provides the phosphate-ester backbone of nucleic acids, plays a crucial role in the transmission of chemical energy by the ATP molecule and is also a structural constituent in many cell components. While P cycling in the marine water column has been the subject of many studies and reviews, many aspects of the sedimentary P cycling remain poorly understood. This is mainly due to low P concentrations in sediments and analytical difficulties associated with isolating distinct P forms from sediment samples. The aim of this review is to highlight the advances made in our understanding of P cycling in marine sediments as a result of improved instrumentation and novel approaches. Some of these techniques range from sediment sequential extractions (SEDEX) to spectroscopic techniques (31P NMR) and innovative isotope tracer experiments (stable oxygen ratios in phosphate, radioactive P isotopes). While our understanding of P cycling has considerably improved, numerous aspects of sedimentary P cycling including the fate of organic P in sediments as a function of redox conditions, organic matter content and burial depth, as well as turnover rates of different sedimentary P compounds remain poorly constrained. The techniques presented in this review, along with yet to be utilized innovative approaches used in soil and lacustrine sediment P research, will play an important role in addressing these important questions and further our understanding of this critical component of the P global biogeochemical cycle.

      PubDate: 2017-12-13T01:58:36Z
       
  • Dual-phase mass balance modeling of small mineral particle losses from
           sedimentary rock-derived soils
    • Abstract: Publication date: Available online 6 December 2017
      Source:Chemical Geology
      Author(s): Carleton R. Bern, Tiffany Yesavage
      Losses of small mineral particles can be a significant physical process that affects the elemental composition of soils derived from sedimentary rocks. Shales, in particular, contain abundant clay-sized minerals that can be mobilized by simple disaggregation, and solutional weathering is limited because the parent rock is composed primarily of recalcitrant minerals previously subjected to continental weathering. Here, the dual-phase mass balance model is employed to quantify losses of small mineral particles as water dispersible colloids (WDCs) from three previously studied soil profiles along a hill slope at the Susquehanna Shale Hills Critical Zone Observatory (SSHO). WDCs were isolated from soil in the laboratory to determine their mineralogical and elemental compositions. Clay minerals dominated WDCs, including illite, vermiculite, and chlorite inherited from the parent shale, along with neoformed kaolinite. Quartz present in bulk soil was generally excluded from WDCs. Elements of low solubility and/or bound in recalcitrant forms, like Rb in illite, were employed in tracer ratios in the dual-phase model. Aluminum, Ga, and Rb were enriched in WDCs, and Zr and Hf were partially excluded. Six different combinations of elements into tracer ratios (Al/Zr, Ga/Zr, Rb/Zr, Al/Hf, Ga/Hf, Rb/Hf) each yielded similar model results. Mass losses of WDCs were large, ranging from −68±7% to −15±5% relative to soil parent material in different parts of the profiles. Mass losses via solution were smaller, ranging from −7±2% to a gain of 6±1% in part of one profile. Losses of WDCs account for >90% of total mass loss, surpassing chemical dissolution, and therefore dominate the weathering portion of denudation at SSHO. Zirconium concentrations were 97–158ppm in the generally ≤1μm WDCs, suggesting colloidal, Zr-bearing phases. Model-quantified losses of Zr via WDCs were large, with a median loss of 41% relative to parent material. Such losses indicate systematic underestimates of weathering by traditional mass balance that uses Zr as an index element. Losses of Ca, Mg, and K via WDCs exceeded losses via solution, countering assumptions of base cation losses primarily via mineral dissolution. The results illustrate a geochemical fingerprint of physical weathering and the ability of the dual-phase model to quantify that weathering process.
      Graphical abstract image

      PubDate: 2017-12-13T01:58:36Z
       
  • Multi-element isotopic evolution of magmatic rocks from Caviahue-Copahue
           Volcanic Complex (Chile-Argentina): Involvement of mature slab recycled
           materials
    • Abstract: Publication date: Available online 5 December 2017
      Source:Chemical Geology
      Author(s): E. Roulleau, D. Tardani, Ivan Vlastelic, N. Vinet, J. Sanchez, Y. Sano, N. Takahata
      In the Southern Volcanic Zone (Chile-Argentina), the active Caviahue-Copahue Volcanic Complex (CCVC) is approximately 30km east of the main N-S trending volcanic front, where the Nazca Plate subducts under the South American Plate. CCVC activity includes three major stages: the 5–4Ma old Ante-Caviahue series, the 2.6Ma old Caviahue series, and Copahue volcano that has been active since 1.2Ma ago. CCVC volcanism results from extension and slab steepening since 2.6Ma ago that produces the asthenospheric influx under Copahue. Here we investigate the link between the geochemical and multi-element (He, Li, N, Sr, Nd, Hf, and Pb) isotopic composition of CCVC magmas and evolution of the subduction regime since 5Ma ago. The CCVC magmatic source is characterized by a mid-ocean ridge basalt-like mantle signature (high 3He/4He=8 Ra) and a high δ15N (+5.8‰) related to subducted sediments. These data suggest a significant degree of N recycling, but low 4He recycling over time. Trace element and isotope modeling indicates that the influence of sediments is strongest in Copahue magmatism, whereas the mantle wedge contribution was strongest in Caviahue and Ante-Caviahue magmatism. The light rare earth element-enriched nature of Copahue rocks (compared to Caviahue and Ante-Caviahue rocks) is more likely due to incorporation of sediments from the slab rather than a very low degree of partial melting. The low δ7Li (0.44±0.31‰ to 1.42±0.17‰) of Copahue and Caviahue magmas indicates the contribution of a mature slab depleted in 7Li during previous dehydration events. Conversely, Ante-Caviahue rocks have a moderate δ7Li value (2.63‰), suggesting input from a less-dehydrated slab. This is consistent with trace element data, showing that the Copahue (and Caviahue) source is less enriched in fluid-mobile elements (low Ba/Th and U/Th ratios). We propose that the geochemical and isotopic evolution of CCVC magmas records the evolution of the subduction regime under the CCVC. Ante-Caviahue magmas were produced in an arc front extensional regime in which the shallow, extensively dehydrating slab delivered large amounts of fluids to the melting zone. Later slab steepening caused the volcanic front to migrate west of the CCVC, probably due to the beginning of subduction in the Mocha fault zone (south of CCVC) 2.5–5Ma ago. Crustal attenuation and asthenospheric influx under the CCVC produced Caviahue magmatism. Further progressive slab steepening and successive dehydration events produced a mature slab and dry mantle wedge that explain the particular signature of Copahue magmas.

      PubDate: 2017-12-13T01:58:36Z
       
  • Determining the optimum locations for pumping low-fluoride groundwater to
           distribute to communities in a fluoridic area in the Upper East Region,
           Ghana
    • Abstract: Publication date: Available online 5 December 2017
      Source:Chemical Geology
      Author(s): Laura Craig, James M. Thomas, Alexandra Lutz, David L. Decker
      Groundwater is the primary source of water in the Upper East Region of Ghana, and is generally considered a safe source of drinking water; but there are pockets where the groundwater contains high concentrations of fluoride due to the dissolution of minerals in the local granite. The goal of this study is to evaluate the hydrogeology and hydrogeochemistry of an area where dental fluorosis endemic, in order to identify the optimum locations to pump and distribute low-fluoride groundwater. As expected, the data indicate that the higher elevation recharge areas with outcrops of Bongo granite have elevated concentrations of fluoride in the groundwater (up to 4.6mgL−1), posing the highest risk of fluorosis in the nearby communities. The lower elevation areas, which are the farthest from the Bongo granitic, have the lowest groundwater fluoride (<0.5mgL−1) and the lowest risk of fluorosis. Groundwater flow models suggest that the steady decrease in fluoride is driven by dispersion, with the fluoride concentrations dropping to the World Health Organization's recommended drinking water limit of 1.5mgL−1 at about 400–500m from the source. The optimum locations to install boreholes (or use existing boreholes) for piping low fluoride groundwater to the higher fluoride areas, would be at or beyond this distance. Although the initial costs of developing such a water system would be substantial, this is a potentially viable option for providing low fluoride water to communities suffering from fluorosis.

      PubDate: 2017-12-13T01:58:36Z
       
  • Lead isotopes as particulate contaminant tracers and chronostratigraphic
           markers in lake sediments in northeastern North America
    • Abstract: Publication date: Available online 5 December 2017
      Source:Chemical Geology
      Author(s): Carol A.L. Cheyne, Alyson M. Thibodeau, Gregory F. Slater, Bridget A. Bergquist
      The utility of lead (Pb) isotopes as tracers of particulate contamination and as chronostratigraphic markers was assessed in five lake sediment cores from the Great Lakes and southern Ontario region. The marker for smelting of highly radiogenic ores in the Upper Mississippi Valley in the mid-19th century was seen in the Pb isotopes in four lakes expanding the geographic range of this established marker into southern and central Ontario. This marker is useful for age-dating sediments deposited in the 1800s in archives where it is present. The estimated dates for this ~1850 marker using other dating methods span a range of 57years, emphasizing the importance of the marker as a tie-point for extrapolated dates in the 19th century. Lead isotopes identified leaded gasoline as an important source only in one lake in the second half of the 20th century; its signal was not identified in all other lakes because the lakes were more affected by local and regional inputs. Lead isotopes combined with trace metal fluxes identified a potential marker for coal combustion or urban sources in the second half of the 20th century in southern Ontario, where these sources were significant enough to outweigh leaded gasoline input. The effects of Canadian mining and smelting activities were seen in the trace metal flux profiles of central Ontario lakes; however, these activities were not visible in the Pb isotopes, likely due to mixing of several sources with similar Pb isotopic signatures. This study demonstrates the usefulness of combining lead isotopes and trace metal flux profiles for identifying markers of historical and modern particulate contamination sources, especially in regions where several pollution sources exist.

      PubDate: 2017-12-13T01:58:36Z
       
  • Trace inheritance—Clarifying the zircon O-Hf isotopic fingerprint of
           I-type granite sources: Implications for the restite model
    • Abstract: Publication date: Available online 5 December 2017
      Source:Chemical Geology
      Author(s): Heejin Jeon, Ian S. Williams
      Early to mid Carboniferous I-type granites distributed in a broad meridional belt west of Sydney, southeastern Australia, represent the last phase of granite magmatism in the southern Lachlan Fold Belt. Rare inherited zircon in the granites, in combination with zircon precipitated from the melt phase of the magmas, provides direct evidence of the nature and age of the source rocks from which the granites were derived, and the isotopic compositions of those rocks. Most granites from the north and central parts of the belt, represented by the Wuuluman, Home Rule, Oberon and Lett plutons (334±3, 328±4, 339±2 and 328±2Ma, respectively), are characterized by scarce inherited zircon dominantly of Siluro-Devonian age, consistent with all being derived from a common zircon-poor source rock of mid to late Devonian age. Based on the isotopic compositions of the igneous zircon rims, that source was relatively homogeneous and immature (δ18Ozrn 6–7‰, εHf(t) 0–+4). Two samples of the Tarana pluton (331±2Ma), near the centre of the belt, show evidence for heterogeneity in that source and the presence of a more evolved component that is also more zircon rich. Together with the Lett pluton from the same geochemical suite, their zircon rim compositions define a mixing array between more and less evolved end members (δ18Ozrn 5.5–9.5‰, εHf(t) -4–+4). Only in the southern Chapmans Creek pluton (327±3Ma) is there clear evidence that the source of the granite also contained a minor component with a similar older protolith to the LFB early Palaeozoic sediments that host most of the LFB pre-Carboniferous granites. Presence of that component, which overwhelmingly dominates the inheritance in the pre-Carboniferous granites, is also reflected in a slightly elevated whole rock initial 87Sr/86Sr (>0.7050) and igneous δ18Ozrn (ca. 7.5‰), but not in the igneous zircon εHf(t) (ca. +2). The amount of restitic zircon in granite is determined by not only the proportion of restite present but also the zircon contents of the various source rock components. The process of restite unmixing does not necessarily mean that more mafic, restite-rich granites will contain more inherited zircon.

      PubDate: 2017-12-13T01:58:36Z
       
  • REE-Th-U and Nd isotope systematics of monazites in magnetite- and
           ilmenite-series granitic rocks of the Japan arc: Implications for its use
           as a tracer of magma evolution and detrital provenance
    • Abstract: Publication date: Available online 2 December 2017
      Source:Chemical Geology
      Author(s): Keita Itano, Tsuyoshi Iizuka, Mihoko Hoshino
      Monazite [(Ce, La, Th) PO4], a common accessory mineral in low-Ca felsic rocks and a wide range of metamorphic rocks, is suitable for in situ U–Pb dating, rare earth element (REE) geochemistry, and Nd isotope tracer studies. To link monazite composition with igneous petrogenesis and differentiation, we carried out comprehensive LA-ICP-MS measurements of REE-Th-U abundances and Sm-Nd isotope ratios in monazites from magnetite-series (oxidized type) and ilmenite-series (reduced type) granitic rocks across the Japan arc. The data revealed systematic differences in monazite composition between magnetite-series and ilmenite-series samples, and between pegmatites and granites. In ilmenite-series granitic rocks, monazites from pegmatites showed larger negative Eu anomalies, lower light-REE/middle-REE ratios, and higher middle-REE/heavy-REE ratios than those from granites. These geochemical variations were attributed to significant fractional crystallization of feldspars, monazite, xenotime, and garnet during differentiation in relatively reduced peraluminous granitic magmas. In contrast, there was no remarkable difference in the REE fractionation pattern between magnetite-series pegmatites and granites. Furthermore, the magnitudes of the negative Eu anomalies in the magnetite-series monazites were smaller than those observed in the ilmenite-series samples. These features were interpreted to reflect the suppression of monazite, xenotime, and garnet fractionation and limited Eu incorporation into fractionating feldspars in relatively oxidized and non-peraluminous magmas. In addition, our data demonstrated intra-grain variations in REE-Th-U compositions in a monazite from the Masaki ilmenite-series pegmatite, despite its homogenous Nd isotope composition. These variations suggest that the core and rim of the grain crystallized from distinct magmas that were generated from similar crustal rocks, but were differentiated under different conditions. A comparison of the present data with previously reported data indicates that igneous monazites from granitic rocks are distinct from a wide range of metamorphic monazites in REE-Th-U composition, especially due to their larger negative Eu anomalies. This finding highlights the potential utility of monazite REE-Th-U systematics as a provenance indicator for detrital monazites.

      PubDate: 2017-12-13T01:58:36Z
       
  • U-series disequilibria in minerals from Gandak River sediments (Himalaya)
    • Abstract: Publication date: Available online 2 December 2017
      Source:Chemical Geology
      Author(s): Clio Bosia, François Chabaux, Eric Pelt, Antoine Cogez, Peter Stille, Etienne Deloule, Christian France-Lanord
      The impact of mineralogical sorting on U-Th-Ra variations observed for the Ganga sediments in the Himalayan alluvial plain due to transport and deposition processes highlights the importance to better understand the U-series isotopic system in Himalayan minerals. For this purpose, U, Th, and Ra concentrations, along with 238U–234U–230Th–226Ra radioactive disequilibria, were measured in several pure mineral fractions, i.e. monazite, zircon, titanite, ilmenite, rutile, garnet, magnetite, quartz, biotite and muscovite, separated from one bank sediment of the Gandak River (Ganga Basin). The data, obtained through a double digestion protocol in high-pressure and high-temperature vessels, confirm that U and Th concentrations and isotopic ratios of the sediments are mainly influenced by a few minor mineral species that are very enriched in U and Th, namely, monazite, titanite and epidote, as well as major minerals with lower U and Th concentrations, such as quartz. More importantly, the data clearly indicate that, similarly to whole rock samples, the U-Th-Ra systematics of individual minerals have likely recorded complex U-Th-Ra fractionations. In particular, the 226Ra enrichment observed in the biotite and clay fractions likely results from the affinity of Ra for phyllosilicate minerals, while the 230Th and, to a lesser extent the 234U enrichment observed for the other minerals might result from a nuclide adsorption process on the Fe-oxide coatings present on the grains or directly on the mineral surface. The linear trend outlined by the minerals phases in the (230Th/234U) vs (226Ra/230Th) diagram suggests that 230Th is probably mobilized from zircons, the only fraction presenting 230Th/U<1, and from monazites, and redistributed among the all minerals during weathering. Such a scenario suggest that zircon weathering, evidenced in this study, must have been very recent, estimated <2000years by a modeling approach, including simulation of nuclides mobility by alpha-recoil ejection and loss and gain processes.

      PubDate: 2017-12-13T01:58:36Z
       
  • Geochronology of Hadean zircon grains from the Jack Hills, Western
           Australia constrained by quantitative scanning ion imaging
    • Abstract: Publication date: Available online 2 December 2017
      Source:Chemical Geology
      Author(s): J.J. Bellucci, A.A. Nemchin, M.J. Whitehouse, R.B. Kielman, J.F. Snape, R.T. Pidgeon
      Five Hadean (>4Ga) aged zircon grains from the Jack Hills metasedimentary belt have been investigated by a secondary ion mass spectrometry scanning ion image technique. This technique has the ability to obtain accurate and precise full U-Pb systematics on a scale <5μm, as well as document the spatial distribution of U, Th and Pb. All five of the grains investigated here have complex cathodoluminescence patterns that correlate to different U, Th, and Pb concentration domains. The age determinations for these different chemical zones indicate multiple reworking events that are preserved in each grain and have affected the primary crystalized zircon on the scale of <10μm, smaller than conventional ion microprobe spot analyses. In comparison to the spot analyses performed on these grains, these new scanning ion images and age determinations indicate that almost half of the spot analyses have intersected several age and chemical domains in both fractured and unfractured parts of the individual crystals. Some of these unfractured, mixed domain spot analyses have concordant ages that are inaccurate. Thus, if the frequency of spot analyses intersecting mixed domains here is even close to representative of all other studies of the Jack Hills zircon population, it makes the interpretation of any trace element, Hf, or O isotopic data present in the literature tenuous. Lastly, all of the grains analysed here preserve at least two distinguishable 207Pb/206Pb ages. These ages are preserved in core-rim and/or complex internal textural relationships in unfractured domains. These secondary events took place at ca. 4.3, 4.2, 4.1, 4.0, 3.7, and 2.9Ga, which are coincident with previously determined statistically robust age peaks present in this zircon population.

      PubDate: 2017-12-13T01:58:36Z
       
  • Comparison of climate signals obtained from encrusting and free-living
           rhodolith coralline algae
    • Abstract: Publication date: Available online 2 December 2017
      Source:Chemical Geology
      Author(s): Siobhan Williams, Jochen Halfar, Thomas Zack, Steffen Hetzinger, Martin Blicher, Thomas Juul-Pedersen
      Coralline algae have been used for sclerochronological studies throughout the last decade. These studies have focused on two different growth morphologies of the photosynthetic coralline algae: massive crusts forming small buildups on hard substrate, and free-living branching algal nodules, known as rhodoliths. The latter are generally found on soft-substrate, where they are frequently overturned by water movement and bottom feeding organisms, leaving one side of the rhodolith partially buried in the sediment at any given time. Here we test whether either of these growth morphologies is more suitable for proxy reconstructions by comparing Mg/Ca ratios – a temperature proxy – in multiple replicates of rhodoliths of Lithothamnion glaciale and in rhodoliths as well as encrusting specimens of Clathromorphum compactum. With both species being widespread throughout the Temperate and Arctic regions, we have chosen two North Atlantic localities at Nuuk Fjord, Greenland (Subarctic), and off the southeastern coast of Newfoundland, Canada (Temperate), for this study. Two to three Mg/Ca ratio transects spanning 18years of growth were analysed on multiple specimens with encrusting morphologies and along different sides of rhodoliths using laser ablation inductively coupled mass spectrometry and compared to remotely sensed sea surface temperature (SST) data. The length of the common time span used for comparison was limited by growth interruptions in rhodoliths. Furthermore, our comparison is based on the assumption that rhodolith growth increments are annual – an assumption that has recently been challenged by mesocosm studies. Monthly Mg/Ca values from multiple transects within each individual were compared and in samples from Nuuk fjord significant correlations were found in 4 of 4 encrusting C. compactum, 4 of 4 C. compactum rhodoliths, and 2 of 3 L. glaciale rhodoliths. In Newfoundland significant correlations were found in 6 of 6 encrusting C. compactum comparisons (average: r=0.61, p<0.001), and in 6 of 6 L. glaciale rhodolith comparisons (average: r=0.43, p<0.001) for monthly resolved time series. The monthly Mg/Ca ratios (n=216) from each morphology were compared with instrumental Reynolds SST yielding the following correlations: encrusting C. compactum (r=0.64, p<0.001), C. compactum rhodolith (r=0.62, p<0.001) and L. glaciale (r=0.58, p<0.001). In Newfoundland both morphologies indicate a similar strength in recording SST: encrusting C. compactum (r=0.85, p<0.001) and rhodolith-forming L. glaciale (r=0.84, p<0.001). In summary, Mg/Ca ratios derived from both coralline algal growth forms can yield SST information, however, massive encrusting forms generally yield higher correlations to SST than transects measured on individual rhodoliths, which only allowed for the generation of short uninterrupted time series due to frequent growth irregularities.

      PubDate: 2017-12-13T01:58:36Z
       
  • Accurate stable tungsten isotope measurements of natural samples using a
           180W-183W double-spike
    • Abstract: Publication date: Available online 2 December 2017
      Source:Chemical Geology
      Author(s): Florian Kurzweil, Carsten Münker, Jonas Tusch, Ronny Schoenberg
      Tungsten is a moderately siderophile element and, thus, enriched in the Earth's core. Moreover, W behaves incompatibly during partial melting, causing relative enrichment in the Earth's crust compared to the mantle. However, little is known about the geochemical cycle of the redox-sensitive element W in the crust-mantle system and in modern to ancient low-temperature environments. High resolution stable W isotope measurements of rock samples from different geochemical reservoirs might be a powerful tool to better constrain this cycle. So far, low relative mass differences between the different W isotopes and analytical challenges hampered such high-resolution measurements. Notably, some pioneering studies on the stable W isotope composition of geological reference material show inconsistent results, calling for further verification of the true compositions of these materials. This study presents an analytical protocol for stable W isotope measurements including the calibration of a 180W-183W double-spike as well as W isotope and W concentration data of several geological reference materials (BHVO-2, AGV-2, SDC-1, W-2a, ScO-2, NOD-A-1, NOD-P-1). The reproducibility of stable W isotope measurements (±0.018‰ in δ186/184W; 2s.d.) is significantly improved compared to previous studies, which allows resolving between the stable W isotope compositions of various rock reservoirs on Earth. Relative to the NIST SRM 3163 standard, the highest δ186/184W value was observed for the Pacific Mn crust NOD-P-1 (+0.154±0.013‰; 2s.d.; n =6), which is significantly different from the δ186/184W value of the Atlantic Mn crust NOD-A-1 (+0.029±0.014‰; 2s.d.; n =6). Considering equilibrium fractionation between seawater WO4 2− and slowly growing Mn oxides, this indicates an isotopically heterogeneous distribution of W in the modern oceans. Igneous rocks also show a resolvable range in δ186/184W values. Magmatic reference materials range in δ186/184W between +0.016±0.032‰ (andesite AGV-2; 2s.d.; n =5) and +0.082±0.010‰ (basalt BHVO-2; 2s.d.; n=5) showing relative enrichment of light isotopes in more evolved magmatic rocks. These isotopic differences might result from isotope fractionation during magmatic differentiation. Alternatively, the mobilization of W by hydrothermal and/or magmatic fluids might be accompanied by isotope fractionation.

      PubDate: 2017-12-13T01:58:36Z
       
  • Sr and Nd isotope compositions of alluvial sediments from the Ganga Basin
           and their use as potential proxies for source identification and
           apportionment
    • Abstract: Publication date: Available online 28 November 2017
      Source:Chemical Geology
      Author(s): Neeraj Awasthi, Esha Ray, Debajyoti Paul
      The Sr (87Sr/86Sr) and Nd (εNd values) isotopic composition of sediments transported by the Ganga fluvial system and deposited within the Ganga Basin have been used by other studies to identify and characterize sediment provenance. Isotopic data have also been used for the purpose of quantitative source apportionment. Furthermore, isotopic signatures of sediments imply the Higher Himalaya (HHS) source to be the major contributor of sediments to the Ganga Basin and Bay of Bengal, contrary to the few studies from the adjacent Indus Basin to the west that inferred higher erosion and sediment contribution from the Lesser Himalaya (LHS) source in recent times. In the present study, we test the reliability of using 87Sr/86Sr and/or εNd of river sediments as proxies for source(s) characterization and their apportionment. First, we compiled all available Sr and Nd isotope ratios of the Ganga basin sediments as well as their probable sources reported in earlier studies. After subjecting the data set to statistical scrutiny and removing outliers, we carefully evaluated spatial variability in the isotopic composition vis-à-vis the composition of lithologies drained by a complex network of both the Himalayan- and the Peninsular-sourced tributaries in the Ganga fluvial system. Our analysis shows a large (isotope) compositional overlap among probable sources, which suggests that application of isotopic composition of the Ganga Basin sediments as a proxy for provenance is not straight forward and is fraught with complications. Following the general notion that the Ganga Basin sediments are primarily derived from the HHS and LHS sources, we performed Monte Carlo simulations aimed to model the isotopic composition of sediment mixture. These results show greater uncertainty associated with quantitative source apportionment estimates given the available set of isotopic constraints. Compared to the 87Sr/86Sr ratios that can be modified during fluvial transport, the εNd values serve as more reliable provenance indicator although the use of this parameter for source apportionment may still have a large amount of uncertainty. Nevertheless, a Kernel density estimate (KDE) plot of εNd of river sediments suggests the influence of various source lithologies on the εNd pattern of the Ganga main channel sediments and indicates presence of dominantly HHS- and LHS-sourced sediments. The average εNd of sediments along the Ganga main channel shows distinct change at places in the plains due to local influence of contributing tributaries, most likely Peninsular-sourced tributaries. At present, virtually no isotopic data is available on the sediments of Peninsular-sourced tributaries, which makes it difficult to assess the contribution of cratonic sources to the isotopic budget of the Ganga Basin sediments, particularly in the Ganga-Yamuna interfluve regions.

      PubDate: 2017-12-13T01:58:36Z
       
  • Melting phlogopite-rich MARID: Lamproites and the role of alkalis in
           olivine-liquid Ni-partitioning
    • Abstract: Publication date: Available online 28 November 2017
      Source:Chemical Geology
      Author(s): Michael W. Förster, Dejan Prelević, Harald R. Schmück, Stephan Buhre, Horst R. Marschall, Regina Mertz-Kraus, Dorrit E. Jacob
      In this study, we show how veined lithospheric mantle is involved in the genesis of ultrapotassic magmatism in cratonic settings. We conducted high pressure experiments to simulate vein+wall rock melting within the Earth's lithospheric mantle by reacting assemblages of harzburgite and phlogopite-rich hydrous mantle xenoliths. These comprised a mica-, amphibole-, rutile-, ilmenite-, diopside (MARID) assemblage at 3–5GPa and 1325–1450°C. Melting of the MARID assemblages results in infiltration of melt through the harzburgite, leading to its chemical alteration. At 3 and 4GPa, melts are high in K2O (>9wt%) with K2O/Na2O>>2 comparable to anorogenic lamproites. Higher pressures and temperatures (5GPa/1450°C) lead to increasing MgO contents of the melt and to some extent lower K2O contents (5–7wt%) at equally high K2O/Na2O ratios. Our experiments provide insights into the role of alkalis in nickel-partitioning (DNi) between olivine and ultrapotassic melt. We observe that the high contents of Na, K, and Al are indicative of high DNi values, implying that the melt polymerization is the dominant factor influencing the olivine/melt nickel partitioning. The change of DNi as a function of melt composition results in a pressure independent, empirical geothermometer: T ° C = − 88.14 · ln liq ( Na 2 O + K 2 O · liq 1 + SiO 2 TiO 2 · liq Al 2 O 3 liq MgO + FeO · D Ni + 1906.2 Element oxides represent the composition of the glass (in wt%), and DNi is the liquid/olivine Ni-partitioning coefficient. We propose that this geothermometer is applicable to all natural silicate melts that crystallized olivine in a temperature interval between 1000 and 1600°C. Application to glass-olivine pairs from calc-alkaline settings (Mexico), MORB (East Pacific Rise), and OIB (Hawaii) yielded reasonable values of 996–1199°C, 1265°C, and 1330°C, respectively.

      PubDate: 2017-12-13T01:58:36Z
       
  • Provenance and depositional environments of Quaternary sediments in the
           southern Kalahari Basin
    • Abstract: Publication date: Available online 28 November 2017
      Source:Chemical Geology
      Author(s): Shlomy Vainer, Yigal Erel, Ari Matmon
      The vast Kalahari Basin is characterized by uplifted margins, terrestrial sedimentation within semi endorheic sub-basins, subdued morphology and tectonic quiescence. This intracratonic basin has been subjected to a prolonged period of subsidence affecting its sedimentary fill by plate motion and climatic cycles. Provenance studies of Kalahari Group sediments mainly focused on the surficial deposits that represent only the last phase of sedimentation, leaving unresolved questions related to the rest of the strata. The southern Kalahari Group succession exposed along the walls of the Mamatwan Mine, Northern Cape, South Africa, reveals three main depositional environments: a bottom low-energy water-body, a middle fluvial, high-energy environment and an upper aeolian sandy unit. The entire section, which was deposited during the early to middle Quaternary, records significant environmental and depositional changes suggesting a highly dynamic landscape. The fully exposed section (55m) of the Kalahari Group at Mamatwan Mine was analysed for its mineralogy, elemental composition, Sr, Nd and Pb isotopic ratios and iron species. Isotopic fingerprints were used to determine the source rocks of the sediments. The basal part of the section was derived from the distant Angolan highlands supporting previous suggestions for the existence of a paleo, trans-Kalahari drainage system. Weathering sensitive indices show that these sediments underwent considerable chemical weathering typical to humid areas. Mineralogical assemblage and iron phases imply that a brackish and alkaline shallow water-body existed during the early-middle Pleistocene contemporaneous with relative dense hominin occupation of the area. During the Early-Middle Pleistocene transition, the lacustrine environment was rapidly filled with volcano-sedimentary Archean clasts that were derived mainly from the surrounding hills and experienced limited degree of source rock chemical weathering, but underwent subsequent groundwater alteration by iron, silica, and calcium-rich solutions and precipitation of calcrete and silcrete duricrusts. These changes in provenance and depositional environments may be related to tectonic adjustments along the southern margins of the Kalahari basin. Ultimately, a shift into an aeolian-transport dominated landscape occurred in the region. This was accompanied by reduced surface water availability and a supply of sediments from Paleoproterozoic western and north-western source areas following the establishment of the modern wind regime.

      PubDate: 2017-12-13T01:58:36Z
       
  • Bathyal records of enhanced silicate erosion and weathering on the exposed
           Luzon shelf during glacial lowstands and their significance for
           atmospheric CO2 sink
    • Abstract: Publication date: Available online 27 November 2017
      Source:Chemical Geology
      Author(s): Zhaokai Xu, Tiegang Li, Peter D. Clift, Shiming Wan, Xiaohua Qiu, Dhongil Lim
      We present a new high-resolution multiproxy data set of mass accumulation rates, Sr-Nd isotopes, as well as major and trace elements for the siliciclastic sediment fraction from International Marine Global Change Study Core MD06-3052, located on the continental slope of the western Philippine Sea. We integrate our new data with published grain sizes and sea surface temperatures from the same core, as well as with Equatorial Pacific sea level, and East Asian summer monsoon precipitation, in order to constrain at high-resolution changes in physical erosion and chemical weathering intensities on Luzon, and sediment source-to-sink processes. We assess the potential significance of chemical weathering of arc silicates in regulating global atmospheric CO2 since 156kyr BP. Sr-Nd isotopes show that the siliciclastic sediments were dominantly sourced from volcanic rocks exposed on Luzon (~68–100%), with a lesser contribution from Asian dust (~0–32%). Different indices indicate that stronger physical erosion and chemical weathering occurred during Marine Isotope Stage (MIS) 6 (130–156kyr BP), as well as in the latter stage of MIS 3 and MIS 2 (14–40kyr BP). The large sea-level lowstands and associated significant exposure of continental shelf in the western Philippine Sea during these two cold periods should favor physical erosion and chemical weathering of unconsolidated sediments on the exposed shelf. Furthermore, we notice the relatively good coherence between chemical weathering intensities on Luzon and global atmospheric CO2 concentrations over these cold intervals. We suggest that strengthening of chemical weathering of silicates on Luzon and other tropical arcs (within 20° of the Equator) during the Quaternary glacial sea-level lowstands may significantly contribute to the lowering of atmospheric CO2 concentrations during ice ages. We estimate that a significant fraction, up to ~16% (i.e., ~8ppmv), of all atmospheric CO2 ultimately sequestered by silicate weathering may be processed through an area corresponding to only ~1% of the exorheic drainage area worldwide.

      PubDate: 2017-12-13T01:58:36Z
       
  • The trace element and Sr-Nd-Pb isotope geochemistry of Juan Fernandez
           lavas reveal variable contributions from a high-3He/4He mantle plume
    • Abstract: Publication date: Available online 26 November 2017
      Source:Chemical Geology
      Author(s): Thi B. Truong, Paterno R. Castillo, David R. Hilton, James M.D. Day
      The Juan Fernandez Islands in the southeastern Pacific are an atypical linear volcanic chain that exhibits a considerable range in 3He/4He ratios (8 to 18 RA, where RA is the 3He/4He ratio of air), but limited ranges of 87Sr/86Sr and 143Nd/144Nd. Here we report new trace element abundance data and Sr-Nd-Pb isotope data for mafic lavas previously analyzed for their 3He/4He and He contents from the two main islands of Robinson Crusoe and Alexander Selkirk. Lavas from these islands have been previously grouped based on geochemical and petrological classification into Group I and III basalts, and Group II basanites. In general, samples have overlapping Sr-Nd-Pb isotope compositions that suggest a common, albeit slightly heterogeneous mantle source. In detail, the Group I and III tholeiitic and alkalic basalts have nearly identical incompatible trace element patterns, whereas the Group II basanites show elevated incompatible trace element abundances. Major and incompatible trace element modeling indicates that Group III basalts (3He/4He=7.8–9.5 RA) from younger Alexander Selkirk Island were produced by the highest degree of partial melting (>10%) of a common mantle source, followed by Group I basalts (13.6–18.0 RA) and Group II basanites (11.2–12.5 RA) from older Robinson Crusoe Island. The 206Pb/204Pb of Group I basalts and Group II basanites are slightly more radiogenic and limited in range (19.163 to 19.292) compared with those of Group III (18.939 to 19.221). The Group I and II lavas from Robinson Crusoe are consistent with an origin from the so-called focus zone (FOZO) mantle component, whereas the Alexander Selkirk basalts additionally contain contributions from a less-enriched or relatively depleted mantle component. Juan Fernandez lavas reveal limited ranges of Sr-Nd-Pb isotopes but variable 3He/4He as their parental magmas originated mainly from the FOZO component with high 3He/4He (>9 RA) and variably polluted with a depleted component with lower 3He/4He (ca. 8 RA). Contributions from high-3He/4He mantle sources to ocean island basalts can therefore vary both spatially and temporally, over meter to kilometer lengths and hundred to million-year time scales, and may not be strongly correlated to radiogenic lithophile isotope systematics.

      PubDate: 2017-12-13T01:58:36Z
       
  • Lu-Hf ratios of crustal rocks and their bearing on zircon Hf isotope model
           ages: The effects of accessories
    • Abstract: Publication date: Available online 26 November 2017
      Source:Chemical Geology
      Author(s): F. Bea, P. Montero, J.F. Molina, J.H. Scarrow, A. Cambeses, J.A. Moreno
      All other factors being equal, the calculation of zircon Hf two stage model ages (T DM Hf) depends on the particular Lu/Hf value assumed for the magmatic source, the effect being more pronounced as the age difference between zircon and magmatic source increases. It is generally considered that the Lu/Hf measured in the zircon-hosting rock does not represent the composition of the source because of potential garnet or zircon fractionation. Accordingly, most authors either assume a single fixed value for Lu/Hf source , often Lu/Hf≈0.079 to 0.108, or use two alternative models, one for felsic sources, often Lu/Hf≈0.09, and the other for mafic sources, often Lu/Hf≈0.165. In contrast with these opinions, however, here we show that partial melting of peraluminous sources causes little decoupling of Lu from Hf because of similar solubilities of zircon and monazite. Furthermore, the effects of residual garnet are largely compensated by the numerous zircon inclusions that garnet and other residual minerals almost always contain. Partial melting of metaluminous sources may significantly decouple Lu from Hf if allanite and/or titanite are not present in the source, but the effect decreases as the melt fraction increases. Similarly, fractional crystallization of metaluminous magmas may decouple Lu from Hf if amphibole or clinopyroxene begin to crystallize before zircon saturation. The Lu/Hf distribution in 4784 rocks from different regions and ages is lognomal rather than normal, and the calculated medians, i.e. the maximum of the probability density function for the logarithmically transformed Lu/Hf, are Lu/Hf mafic rocks ≈0.08, Lu/Hf felsic rocks ≈0.05, i.e. notably lower than the above-mentioned felsic and mafic magmatic source averages. Magmatic sources may be remarkably heterogeneous with respect to Lu/Hf. Our calculations show that fixed Lu/Hf source values translate the Lu/Hf heterogeneity of the source to the T DM Hf thus producing an artificial distribution of model ages that may be erroneously interpreted as different episodes of crustal growth. Therefore, we propose that the best strategy to calculate two stage Hf model ages of zircon is to use the analytically determined whole-rock Lu/Hf ratio as a proxy of the source. In the case of detrital or inherited zircons, for which no whole-rock information is available, it is advisable first to determine whether they come from a mafic or felsic rock by interpreting cathodoluminescence images, Th/U ratios and other chemical parameters, and then venture an estimate of the Lu/Hf source from the SiO2 average.

      PubDate: 2017-12-13T01:58:36Z
       
  • The fate of Cu pesticides in vineyard soils: A case study using δ65Cu
           isotope ratios and EPR analysis
    • Abstract: Publication date: Available online 26 November 2017
      Source:Chemical Geology
      Author(s): Simon Blotevogel, Priscia Oliva, Sophie Sobanska, Jerome Viers, Hervé Vezin, Stéphane Audry, Jonathan Prunier, José Darrozes, Laurent Orgogozo, Pierre Courjault-Radé, Eva Schreck
      Copper (Cu) based pesticides are widely used in viticulture and are the only pesticides permitted in organic viticulture. Due to its extensive long term use, Cu accumulates in vineyard soils and ecotoxicological implications are growing. In this study, the cycling of Cu based pesticides was investigated in vineyard environments using copper mass balance, Electron Paramagnetic Resonance (EPR) spectroscopy and Cu isotope analyses. Different soils (i.e. Vertic Cambisol and Calcaric Cambisol) from the Soave vineyard (Italy) were studied. Kinetic extractions were performed on soil samples using Na3-citrate to assess Cu bioavailable fraction. Results show that isotope ratios of pesticides depend on Cu speciation and their manufacturing date, covering a large range of isotope ratios (δ65Cu from −0.49±0.05‰ to 0.89±0.01‰) making it difficult to trace sources of Cu in soils. Mass balance calculations based on Ti as invariant element permitted to put in evidence large excess Cu stocks in both studied soils. Excess Cu is transported to depth with approximately the same apparent rate (0.0092myr−1) in both soils, faster than formerly reported in literature. A substantial amount of Cu was missing from calcaric cambisols (6 to 48%) when compared to vertic cambisols, implying a relative loss of Cu from such soils via the soil solution. In bulk soils, there are slight but significant differences in mean Cu isotope ratios depending on soil type (δ65Cu from 0.28 vs 0.18‰ in vertic and calcaric soils respectively), illustrating the loss of heavy Cu from carbonated soils. EPR analysis confirms a difference in Cu speciation between vertic and carbonate-rich soils, indicating an influence of carbonates on Cu retention besides the role of Cu-organic matter interactions. Kinetic extractions showed that the bioavailable fraction displays isotopically heavier Cu isotopes signature than bulk soil, whatever the soil type.
      Graphical abstract image

      PubDate: 2017-12-13T01:58:36Z
       
  • Density of hydrous magma
    • Abstract: Publication date: Available online 11 November 2017
      Source:Chemical Geology
      Author(s): Tatsuya Sakamaki
      Magmas in the interior of the Earth have been geophysically observed. The generation of magmas is closely related to H2O because the water-induced decrease in the melting temperature of mantle rocks is an essential process, which forms hydrous magmas due to H2O incorporation into the molten rocks. Knowledge on physical properties of hydrous magmas, such as density, can give us better understanding of the magma behavior inside the Earth. The density of hydrous magma can be controlled by the partial molar volume of H2O ( V ¯ H 2 O ) and its pressure- and temperature-dependences. The new compression behavior of V ¯ H 2 O using Vinet equation of state is proposed based on compiled data. Since H2O is one of the most compressible oxides in the magma at lower pressure, a rapid densification of hydrous magma can be observed at shallower mantle. The dense hydrous magma can be gravitational stable above and below mantle transition zone.

      PubDate: 2017-11-16T15:09:07Z
       
  • Phosphorylation on the early earth
    • Abstract: Publication date: Available online 10 November 2017
      Source:Chemical Geology
      Author(s): Matthew A. Pasek, Maheen Gull, Barry Herschy
      Phosphorus is an element critical to the formation of several biomolecules, including nucleic acids, the energy transfer molecule ATP, and phospholipids. It hence lies at the heart of several biomolecular functions. However, the formation of these key biomolecules is hindered by the geochemical properties of phosphorus, including its low solubility and poor reactivity. Here we review the approaches that have been taken to overcome some of these difficulties, and place them in the context of the geochemistry of the early earth. We find many experiments rely on phosphate minerals unlikely present on the Hadean earth, or employ conditions that depend on unusual geochemical environments. We propose the most likely routes to forming organophosphate molecules in the context of a plausible prebiotic geochemistry may rely instead on ephemeral phosphate minerals, or redox reactions of phosphorus to form the necessary soluble and reactive phosphorus reagents.

      PubDate: 2017-11-16T15:09:07Z
       
  • Inside front cover (Editorial Board)
    • Abstract: Publication date: 10 December 2017
      Source:Chemical Geology, Volume 474


      PubDate: 2017-11-09T04:31:01Z
       
  • Determination of Cr(III) solids formed by reduction of Cr(VI) in a
           contaminated fractured bedrock aquifer: Evidence for natural attenuation
           of Cr(VI)
    • Abstract: Publication date: 10 December 2017
      Source:Chemical Geology, Volume 474
      Author(s): Jiujiang Zhao, Tom Al, Steven W. Chapman, Beth L. Parker, Katherine R. Mishkin, Diana Cutt, Richard T. Wilkin
      Hexavalent chromium Cr(VI) is toxic and can be highly mobile in many aquifer systems. Redox reactions with naturally occurring minerals and organic compounds can reduce Cr(VI) to Cr(III), forming labile Cr(III) oxyhydroxide precipitates, which is a natural attenuation process. In fractured bedrock aquifers, reduction of Cr(VI) in the rock matrix can enhance attenuation beyond that from matrix diffusion only, and potentially reduce back diffusion if concentrations in fractures decline following source reduction via natural processes or engineered remediation. In this study, we develop an extraction method for labile Cr(III) precipitates from Cr(VI) reduction using 5% hydrogen peroxide (H2O2). Combining Cr(III) extractions with an established sodium hydroxide (NaOH) method for determination of Cr(VI) concentrations in rock porewater, a measure of the labile Cr(III) and Cr(VI) fractions in geologic samples is achieved. The methods were applied to cores from a contaminated groundwater system in fractured porous bedrock in order to assess the effectiveness of natural attenuation and whether Cr(VI) mass that diffused into the bedrock matrix was undergoing reduction. Detailed vertical distributions display two depth intervals with corresponding elevated concentrations of Cr(VI) in the porewater and extractable total Cr. The correspondence of Cr(VI) and labile Cr(III) provides evidence for reduction of Cr(VI) contamination in the bedrock matrix. Mineralogical analysis suggests that Fe(II)-bearing minerals, chlorite and biotite are the most likely candidates for natural reductants. This study provides evidence for the natural attenuation of anthropogenic Cr(VI) contamination in the porewater of a fractured bedrock aquifer, and it outlines a quantitative method for evaluating the effectiveness of natural attenuation in groundwater systems.

      PubDate: 2017-11-09T04:31:01Z
       
  • The effect of sample treatments on the oxygen isotopic composition of
           phosphate pools in soils
    • Abstract: Publication date: 10 December 2017
      Source:Chemical Geology, Volume 474
      Author(s): Zhao-Hua Jiang, Han Zhang, Deb P. Jaisi, Ruth E. Blake, Ai-Rong Zheng, Min Chen, Xi-Guang Zhang, An-Guo Peng, Xue-Tie Lei, Kai-Qiang Kang, Zhi-Gang Chen
      The oxygen isotopic composition of phosphate (δ18OP) has been increasingly used as an effective tracer for the biogeochemical cycling of phosphorus (P) in soils and other environments. However, diverse pretreatment methods (e.g. storage, preparation and extraction) are being used for soil samples. For the uniformity of methods as well as for the comparison of results, it is important to understand if specific treatment methods can compromise original δ18OP values. Here, Ag3PO4 and KH2PO4 were used to test whether a modified Hedley sequential extraction and purification procedure can alter the δ18OP values of phosphate standards. Additionally, to test the effect of sample storage and drying conditions, two types of soils were first processed by using eight different pretreatment methods including sterilizing, storing, drying, and sieving and then the δ18OP values of each soil P pool were measured. Results indicate that the extraction and purification procedure, drying temperature (<0°C to 80°C) and sieving mesh (20 to 100) had no significant effect on the δ18OP values of Pi (inorganic P) pools, but storage at room temperature (without microbial growth inhibitor-HgCl2 added) can lead to significant changes in δ18OP values of almost all P pools. For the two soils studied, the δ18OP values of Pi pools decrease from H2O (H2O-Pi) to NaHCO3 (NaHCO3-Pi), NaOH (NaOH-Pi) and HCl (HCl-Pi), and organic P was also found in the extraction solution of HCl. Furthermore, the δ18OP values calculated from isotope mass balance were different from the measured values suggesting variable extraction of different P pools during single and sequential extraction methods. Collectively these results highlight the need for a unified and standard processing and extraction methods for soil samples to allow meaningful intercomparison of results.

      PubDate: 2017-11-09T04:31:01Z
       
  • Fraction-specific controls on the trace element distribution in iron
           formations: Implications for trace metal stable isotope proxies
    • Abstract: Publication date: 10 December 2017
      Source:Chemical Geology, Volume 474
      Author(s): Paul B.H. Oonk, Harilaos Tsikos, Paul R.D. Mason, Susann Henkel, Michael Staubwasser, Lindi Fryer, Simon W. Poulton, Helen M. Williams
      Iron formations (IFs) are important geochemical repositories that provide constraints on atmospheric and ocean chemistry, prior to and during the onset of the Great Oxidation Event. Trace metal abundances and their Mo-Cr-U isotopic ratios have been widely used for investigating ocean redox processes through the Archean and Paleoproterozoic. Mineralogically, IFs consist of three main Fe-bearing fractions: (1) Fe-Ca-Mg-Mn carbonates, (2) magnetite and/or hematite and (3) Fe-silicates. These fractions are typically fine-grained on a sub-μm scale and their co-occurrence in varying amounts means that bulk-rock or microanalytical geochemical and stable isotope data can be influenced by cryptic changes in mineralogy. Fraction specific geochemical analysis has the potential to resolve mineralogical controls and reveal diagenetic versus primary precipitative controls on IF mineralogy. Here we adapt an existing sequential extraction scheme for Fe-phases (Poulton and Canfield, 2005) to the high Fe-content in IF and the specific three-fraction mineralogy. We optimized the scheme for magnetite-dominated Archean IFs using samples from the hematite-poor Asbestos Hills Subgroup IF, Transvaal Supergroup, South Africa. Previously commonly-used hydroxylamine-HCl and dithionite leaches were omitted since ferric oxides are quantitatively insignificant in these IF samples. The acetate leach was tested at variable temperatures, reaction times and under different atmospheres in order to ensure that all micro-crystalline Fe-carbonates were effectively dissolved, resulting in an optimum extraction for 48h at 50°C under anoxic conditions. The dissolution of magnetite by NH4-oxalate was also tested, resulting in an optimum extraction for 24h under an ambient atmosphere. Finally, a HF-HClO4-HNO3 leach was used to dissolve the residual silicate fraction which has to date not been considered in detail in IF. Accuracy of the extraction technique was generally excellent, as verified using 1) elemental recoveries, 2) comparison of major and trace element distributions against mineralogy and 3) comparison to results from microanalytical techniques. This study focuses on the distribution of three frequently used geochemical proxies in IF; U, Mo and Cr. Molybdenum abundances in the Kuruman and Griquatown IF are low and show an apparent correlation with mineralogical variability, as determined by the sequential extraction. This suggests that changes in bulk-rock mineralogy, rather than redox chemistry might significantly affect Mo stable isotopes. For Cr, a minor bulk-rock stratigraphic increase can be related to the oxide and silicate fraction. However, a positive relationship with Zr indicates that this was also controlled by detrital or volcanic ash input. Uranium is predominantly bound to the silicate fraction and shows clear correlations with Zr and Sc implying detrital reworking under anoxic conditions. The discrepant behaviour of these three proxies indicate that mineralogy should be taken into account when interpreting heterogeneous bulk-rock samples and that fraction specific techniques will provide new insights into the evolution of atmosphere and ocean chemistry.

      PubDate: 2017-11-09T04:31:01Z
       
  • The role of sulfide minerals in the genesis of groundwater with elevated
           geogenic arsenic in bedrock aquifers from western Quebec, Canada
    • Abstract: Publication date: 10 December 2017
      Source:Chemical Geology, Volume 474
      Author(s): Raphaël Bondu, Vincent Cloutier, Mostafa Benzaazoua, Eric Rosa, Hassan Bouzahzah
      The primary arsenic-bearing minerals and their relation to the mobilization of arsenic in groundwater are investigated in a mineralized area of the Canadian Shield (Province of Quebec, Canada). Bedrock core and groundwater samples were collected from three mining exploration boreholes completed in the fractured bedrock aquifer. Chemical analyses of groundwater reveal that the boreholes contain high arsenic concentrations ranging from 0.11 to 0.46mg/l. Mineralogical analyses of core samples indicate that arsenic is mainly present as arsenopyrite (FeAsS) and gersdorffite (NiAsS), with arsenic and sulfur contents ranging from a few mg/kg to more than 3.1 and 1.3wt%, respectively. Core leaching experiments show that the release of arsenic in oxidizing and weak alkaline water largely depends on the mineralogy of arsenic. Gersdorffite-rich samples release larger amount of arsenic than arsenopyrite-rich samples, which is mainly attributed to the high reactivity of gersdorffite compared to arsenopyrite. However, the high arsenic concentrations in groundwater are not associated with the occurrence and extent of gersdorffite-rich zones in the bedrock. These results are interpreted as evidence that the local weathering of sulfide minerals is not the main mechanism of arsenic mobilization in borehole waters. The groundwater composition strongly suggests that the mobilization of arsenic is related to the reductive dissolution of Fe and Mn oxyhydroxides in the downgradient part of the aquifer. Thus, it is proposed that the high arsenic concentrations in groundwater are mainly driven by (1) the initial release of arsenic through sulfide oxidation under oxidizing conditions (recharge areas), (2) the sequestration by sorption onto Fe-Mn oxyhydroxides and clay minerals, (3) the transport of arsenic in dissolved and particulate forms in groundwater through fractures, and (4) the (re)mobilization by dissolution of Fe-Mn oxyhydroxides under reducing conditions (downgradient areas). This study highlights the potential role of sulfide stability and the importance of redox conditions in the mobilization of arsenic in groundwater from areas of sulfide mineralization. More detailed information is needed regarding the geochemical behavior of arsenic-bearing sulfides in bedrock aquifers under natural conditions and its implication for geogenic arsenic in groundwater.

      PubDate: 2017-11-09T04:31:01Z
       
  • Corrigendum to “Isotope and trace element variations in lavas from
           Raivavae and Rapa, Cook-Austral islands: constraints on the nature of
           HIMU- and EM-mantle and the origin of mid-plate volcanism in French
           Polynesia” [CHEMGE: 202, Issues 1-2 (15 Dec 2003); pages 115-138]
    • Abstract: Publication date: 10 December 2017
      Source:Chemical Geology, Volume 474
      Author(s): JC Lassiter, J Blichert-Toft, EH Hauri


      PubDate: 2017-11-09T04:31:01Z
       
  • Serpentinization of olivine at 300°C and 500bars: An experimental study
           examining the role of silica on the reaction path and oxidation state of
           iron
    • Abstract: Publication date: Available online 7 November 2017
      Source:Chemical Geology
      Author(s): Drew D. Syverson, Benjamin M. Tutolo, David M. Borrok, William E. Seyfried
      The generation of H2 during serpentinization of ultramafic oceanic crust along Earth's mid-ocean ridges provides a key energy source for the deep biosphere and has important implications for metal transport and mineral stability in the subseafloor hydrothermal system. Nonetheless, the driving forces and rates of the coupled processes of Fe partitioning and H2 generation during serpentinization are complicated by many factors, but especially, the activity of SiO2(aq) in the serpentinizing fluid. To examine the role of this key variable in H2 generation and Fe partitioning between magnetite and Fe-serpentine during serpentinization, San Carlos olivine was reacted with a NaCl-SiO2(aq)-bearing fluid at 300°C and 500bars for approximately 90days using flexible gold cell hydrothermal equipment. Time-series changes in solution chemistry and dissolved H2 are coupled with magnetic susceptibility, Mössbauer spectroscopy, and Fe isotope measurements of reactant olivine and coexisting alteration products to provide a complete description of Fe mass transfer and oxidation as a function of reaction progress. Utilization of talc as reactant not only provided a source of SiO2(aq) for serpentinization, but effectively prevented brucite formation, in keeping with constraints imposed by many natural hydrothermal systems, which are inherently open systems. Moreover, this experimental strategy allowed Fe derived from olivine dissolution to transfer completely to serpentine and magnetite, facilitating reaction path modeling. Initial changes in solution chemistry indicate “buffering” of dissolved SiO2(aq), owing to the apparently balanced rates of olivine dissolution and serpentine precipitation. During this stage of reaction, a negligible amount of H2 is generated, suggesting that Fe2+-serpentine is stable and the dominant Fe-bearing product. With continued reaction, the rate of SiO2(aq) production lessens likely due to reduction in surface area of talc, such that the continued hydration and hydrolysis of olivine titrates the previously buffered activity of SiO2(aq), eventually approaching serpentine-brucite coexistence. During this secondary stage of reaction, the rate of H2 generation abruptly increases, which reflects the formation of magnetite and Fe3+-serpentine. Temporal changes in the isotope composition of dissolved Fe, throughout these stages, likely reflect different mineral alteration and formation processes, where initially, 1) preferential mass transfer of isotopically light Fe into solution occurred during olivine alteration, without considerable oxidation, as demonstrated by the low H2 production rate, and, 2) the preferential removal of isotopically light Fe from solution in connection with enhanced magnetite and H2 formation during the later stages of reaction. Post experimental analysis of the Fe isotopic composition of Fe-bearing minerals demonstrates statistically negligible enrichment of δ56Fe of magnetite relative to olivine and serpentine, consistent with observations from natural altered oceanic crust. Observations from this study demonstrate the utility of using coupled chemical, isotopic, and analytical approaches to ascertain time series changes in rates of mass transfer and mechanisms of oxidation of Fe during serpentinization of olivine. Ultimately, these insights provide important constraints for the chemical and Fe isotopic evolution of fluids and minerals in ultramafic hosted hydrothermal systems at mid-ocean ridges.

      PubDate: 2017-11-09T04:31:01Z
       
  • Metasomatic PGE mobilization by carbonatitic melt in the mantle: Evidence
           from sub-μm-scale sulfide–carbonaceous glass inclusion in Tahitian
           harzburgite xenolith
    • Abstract: Publication date: Available online 4 November 2017
      Source:Chemical Geology
      Author(s): Norikatsu Akizawa, Akira Miyake, Akira Ishikawa, Akihiro Tamura, Yasuko Terada, Kentaro Uesugi, Akihisa Takeuchi, Shoji Arai, Chima Tanaka, Yohei Igami, Katsuhiko Suzuki, Tetsu Kogiso
      Platinum-group elements (PGEs) are one of the key tracers to reveal early differentiation processes of the Earth due to their preferential distribution into the metallic core. Meanwhile, informative evidence for early differentiation has been greatly disturbed through metasomatic PGE disturbance, which has been demonstrated through a number of PGE data for natural mantle peridotites as well as base-metal sulfide and platinum group mineral grains therein. The mechanism and process of metasomatic PGE mobilization should be investigated in detail for an appropriate estimation of PGE abundance in the primitive upper mantle. However, this has not yet been achieved, because sub-micrometer-scale (i.e. scale of less than a micrometer) descriptions for metasomatic effects imprinted in the mantle peridotites have not been sufficiently recorded. Here, we report a sub-micrometer-sized sulfide–glass inclusion array in a Tahitian harzburgite xenolith. The textural and chemical characteristics were disclosed with employing synchrotron X-ray and transmission electron microscope analyses. The results demonstrate that the sulfide and glass contain appreciable amounts of PGE (9.7 at.% Ir, 4.3 at.% Rh and 5.8 at.% Pt) and carbon (21.2 at.% C), respectively. The sulfide–glass inclusion array is hosted in sodium-enriched clinopyroxene (up to 1.8wt% Na2O) that shows vein-like distribution and partly replaces orthopyroxene. Primitive mantle-normalized trace-element patterns of the clinopyroxene show a general increase from heavy rare-earth elements (REEs) to light REEs with negative anomalies in Pb and high-field-strength elements such as Zr, Hf and Ti, which indicate equilibration with Mg-rich carbonatitic melt. These results suggest that Na-bearing Mg-rich carbonatitic melts were involved in the harzburgite formation and that Ir, Rh and Pt were mobilized through carbonatitic metasomatism and eventually distributed in the sulfides.

      PubDate: 2017-11-09T04:31:01Z
       
  • Inside front cover (Editorial Board)
    • Abstract: Publication date: 30 November 2017
      Source:Chemical Geology, Volume 473


      PubDate: 2017-11-02T04:35:59Z
       
  • Response of microbial communities to supercritical CO2 and biogeochemical
           influences on microbially mediated CO2-saline-sandstone interactions
    • Abstract: Publication date: 30 November 2017
      Source:Chemical Geology, Volume 473
      Author(s): Chenyang Li, Shuang Zhong, Fengjun Zhang, Zhi Wang, Fuhuan Jiang, Yuyu Wan
      In this study, laboratory experiments were conducted to investigate the influences of supercritical CO2 (scCO2) on the indigenous microbial communities. Results revealed that the abundance of gene copies (mL−1) in samples and the shifts in the microbial community were strongly affected by scCO2 injection. Furthermore, Proteobacteria showed better ability of tolerance to the scCO2 injection than Firmicutes in initial period and both of them were the predominant phyla after 180days' experiments. In addition, the acidogenic mineral-attached biofilms formed by Pseudomonas stutzeri, Acinetobacter soli etc. surrounding the feldspars and clays could lower the pH in solution and accelerate the dissolution of silicate minerals. As the secondary carbon sink minerals, amount of CaCO3 and FeCO2 were observed in the microbially mediated CO2-saline-sandstone interactions, demonstrated the Fe(III)-reducing microbes (Shewanella putrefaciens, Citrobacter sp. LAR-1 and Pseudomonas fluorescens) could reduce the Fe(III) released from clays to Fe(II) and induce siderite biomineralization through metabolic processes.

      PubDate: 2017-11-02T04:35:59Z
       
  • Dissolved oxygen in water and its stable isotope effects: A review
    • Abstract: Publication date: 30 November 2017
      Source:Chemical Geology, Volume 473
      Author(s): Michael Mader, Christian Schmidt, Robert van Geldern, Johannes A.C. Barth
      Dissolved oxygen (DO) is one of the most commonly measured parameters in aqueous studies, however its sources and sinks such as atmospheric exchange, photosynthesis or respiration often remain unknown. A large number of studies have combined concentration measurements with investigations of stable isotope ratios (18O/16O and 17O/16O) of DO that are expressed as per mille [‰] deviation versus international standards. These isotope shifts can provide important additional information about oxygen sources and sinks, aqueous productivity and also of turnover rates of related material such as carbon. This review provides an overview of oxygen isotope systematics and the Dole effect that is still unresolved and explores the discrepancy between the isotope composition of ocean water and atmospheric O2. Here we summarize the most relevant processes of influence on the isotope composition of DO, including gas-water exchange, photosynthesis and respiration. The latter is known to exert the strongest isotope effects with isotope discriminations of up to −29‰. New developments suggest that they can be cancelled out with recent applications of a 17O excess method for the evaluation of aqueous productivity. Furthermore, variations of DO stable isotope ratios can serve as a valuable additional tracer in a wide range of biogeochemical and aqueous studies. Among aquatic systems lakes, estuarine and shallow groundwater environments are the most challenging reservoirs as they usually expose steep oxygen gradients. So far causes and effects of these gradients remain often unexplored and could be revealed with DO isotope methods. Particularly in groundwater, additional oxygen sources from the soil zone may be quantified with new oxygen isotope signals that were influenced by diffusion. Future investigations of groundwater-surface water interactions and of ecological functions can benefit from this method. Other new applications include investigations of mineral oxidations and of engineered approaches such as photo-bioreactors.

      PubDate: 2017-11-02T04:35:59Z
       
  • The forgotten variable: Impact of cleaning on the skeletal composition of
           a marine invertebrate
    • Abstract: Publication date: Available online 22 October 2017
      Source:Chemical Geology
      Author(s): Jennifer Loxton, Jens Najorka, Emma Humphreys-Williams, Piotr Kuklinski, Abigail M. Smith, Joanne S. Porter, Mary Spencer Jones
      For centuries, invertebrate collections have been subjected to various post-collection and curatorial cleaning techniques. Cleaning, however, may damage or even dissolve skeletal calcium carbonate and consequently influence any subsequent geochemical analysis. We investigated the combined effects of three cleaning variables: water (deionized and tap water), bleach (10% and 78%) and ultrasound (all for a range of durations), on the skeleton of Flustra foliacea (Linnaeus, 1758), a marine bryozoan. Treated and control carbonates were analysed both before and after cleaning, measuring: MgCO3 in calcite (X-ray diffractometry and staining); organic:inorganic carbon ratio, using elemental analysis for total carbon by combustion and for organic carbon by acid dissolution and combustion. Treatment solutions were analysed using inductively coupled plasma atomic emission spectroscopy (ICP-AES) to detect any Ca2+ and Mg2+ that may have leached out. Significantly more weight loss and removal of MgCO3 from calcite occurred in bleach concentrations of 10% or higher, especially in longer duration treatments and with use of ultrasound. Specimens with higher initial MgCO3 in calcite were especially susceptible to Mg leaching. We suggest that the interaction between bryozoan skeletal MgCO3 and cleaning solutions is controlled by a combination of solution chemistry and reaction kinetics, and that when cleaning specimens prior to geochemical analysis, less is better.

      PubDate: 2017-10-25T20:36:58Z
       
  • Tracing sulfate recycling in the hypersaline Pétrola Lake (SE Spain): A
           combined isotopic and microbiological approach
    • Abstract: Publication date: Available online 22 October 2017
      Source:Chemical Geology
      Author(s): N. Valiente, R. Carrey, N. Otero, M.A. Gutiérrez-Villanueva, A. Soler, D. Sanz, S. Castaño, J.J. Gómez-Alday
      Sulfur (S) plays a significant role in saline environments, and sulfate (SO4 2−) is an important component of the biogeochemical S-cycle since it acts as the main electron acceptor in anoxic sediments. The purpose of this paper is to evaluate the fate of S, its origin, and processes affecting sulfate outcome in the hypersaline Pétrola Lake in the Castilla-La Mancha region (High Segura Basin, SE Spain). The lake is the terminal discharge zone of an endorheic basin with considerable anthropogenic pressures. Anthropogenic activities (mainly agricultural inputs and wastewater discharge), together with bedrock leaching of sulfate and sulfide-rich sediments, increase dissolved SO4 2− in surface and groundwater up to 123,000mg/L. The source and fate of sulfate in this environment was investigated coupling hydrochemistry, including hydrogen sulfide (H2S) microprofiles, isotopic analyses (δ34S, δ18OSO4, δ2HH2O, δ18OH2O, and tritium), mineralogical determinations, and molecular biology tools (16S rDNA amplification and sequencing). The origin of dissolved SO4 2− in water is related to pyrite oxidation from Lower Cretaceous sediments, and secondary gypsum dissolution. Under the lake, dissolved SO4 2− decreases with depth, controlled by three main processes: (1) seasonal evaporation cycles, (2) hydrodynamic instability caused by the different density-driven groundwater flow, and (3) sulfate-reduction processes, i.e. dissimilatory bacterial sulfate reduction (BSR). These processes control the continuous recycling of sulfur in the system. Lake water and groundwater are in hydraulic connection, and a density-driven flow (DDF) is able to transport reactive organic matter and dissolved SO4 2− towards the underlying aquifer. Hydrochemical evolution in depth, H2S production (up to 0.024nmol/cm3·s) and the presence of sulfate-reducing bacteria suggest the existence of BSR processes. However, isotope techniques are insufficient to elucidate BSR processes since their isotopic effect is masked by low isotope fractionation and high SO4 2− concentrations. The pattern here described may be found in other saline basins worldwide.

      PubDate: 2017-10-25T20:36:58Z
       
  • Testing the utility of geochemical proxies for paleoproductivity in oxic
           sedimentary marine settings of the Gulf of Aqaba, Red Sea
    • Abstract: Publication date: Available online 13 October 2017
      Source:Chemical Geology
      Author(s): Zvi Steiner, Boaz Lazar, Adi Torfstein, Jonathan Erez
      During the early stages of sediment diagenesis most of the organic matter reoxidizes, leaving behind a residual fraction of organic carbon which does not typically reflect its original quantities. Paleo-productivity reconstructions are therefore based on changes in the chemical composition of carbonate shells or, alternatively, use the abundances of inorganic elements in the bulk sediments, that have been shown to be proxies for organic matter contents. To examine the applicability of bulk inorganic elements composition for this task, we compare recorded changes in known anthropogenic nutrient fluxes to the oligotrophic and oxygenated Gulf of Aqaba in the north Red Sea, with the sedimentary records of barium, cadmium, copper and nickel over the last decades. Among these elements, nickel and copper strongly correlate with recorded nutrient fluxes and primary productivity in the region. In the present case, nickel is a more reliable proxy since part of the copper is possibly contributed from air-borne pollution sources. The applicability of cadmium to serve as a tracer for nutrient additions could not be reliably tested because contribution of cadmium associated with phosphate ore loading in the adjacent ports may be significant. We do not observe any bulk sediment barium enrichments associated with increased nutrient fluxes. Overall, it appears from these correlations that nickel and probably also copper reliably record past changes in nutrient availability and organic matter fluxes while sedimentary barium and barite, which are commonly attributed to productivity, do not.

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

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

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

      PubDate: 2017-10-18T11:48:10Z
       
 
 
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