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  Subjects -> EARTH SCIENCES (Total: 646 journals)
    - EARTH SCIENCES (467 journals)
    - GEOLOGY (72 journals)
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EARTH SCIENCES (467 journals)                  1 2 3 | Last

Showing 1 - 200 of 371 Journals sorted alphabetically
Acta Geodaetica et Geophysica     Hybrid Journal   (Followers: 1)
Acta Geodaetica et Geophysica Hungarica     Full-text available via subscription   (Followers: 2)
Acta Geophysica     Open Access   (Followers: 6)
Acta Geotechnica     Hybrid Journal   (Followers: 6)
Acta Meteorologica Sinica     Hybrid Journal   (Followers: 3)
Advances in High Energy Physics     Open Access   (Followers: 14)
Advances In Physics     Hybrid Journal   (Followers: 7)
Aeolian Research     Hybrid Journal   (Followers: 5)
African Journal of Aquatic Science     Hybrid Journal   (Followers: 12)
Algological Studies     Full-text available via subscription   (Followers: 2)
Alpine Botany     Hybrid Journal   (Followers: 3)
AMBIO     Hybrid Journal   (Followers: 13)
Anadolu University Journal of Science and Technology     Open Access  
Anales del Instituto de la Patagonia     Open Access   (Followers: 1)
Andean geology     Open Access   (Followers: 3)
Annales Henri Poincaré     Hybrid Journal   (Followers: 3)
Annales UMCS, Geographia, Geologia, Mineralogia et Petrographia     Open Access  
Annals of Geophysics     Full-text available via subscription   (Followers: 10)
Annals of GIS     Hybrid Journal   (Followers: 16)
Annals of Glaciology     Full-text available via subscription   (Followers: 2)
Annual Review of Marine Science     Full-text available via subscription   (Followers: 9)
Anthropocene     Hybrid Journal   (Followers: 2)
Anthropocene Review     Hybrid Journal   (Followers: 4)
Applied Clay Science     Hybrid Journal   (Followers: 4)
Applied Geochemistry     Hybrid Journal   (Followers: 10)
Applied Geomatics     Hybrid Journal   (Followers: 3)
Applied Geophysics     Hybrid Journal   (Followers: 6)
Applied Ocean Research     Hybrid Journal   (Followers: 4)
Applied Petrochemical Research     Open Access   (Followers: 2)
Applied Remote Sensing Journal     Open Access   (Followers: 22)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 32)
Arctic Science     Open Access   (Followers: 5)
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 10)
Artificial Satellites : The Journal of Space Research Centre of Polish Academy of Sciences     Open Access   (Followers: 15)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 7)
Asian Journal of Earth Sciences     Open Access   (Followers: 19)
Asian Review of Environmental and Earth Sciences     Open Access   (Followers: 1)
Atlantic Geology : Journal of the Atlantic Geoscience Society / Atlantic Geology : revue de la Société Géoscientifique de l'Atlantique     Full-text available via subscription   (Followers: 2)
Atmosphere-Ocean     Full-text available via subscription   (Followers: 11)
Atmospheric and Climate Sciences     Open Access   (Followers: 23)
Australian Journal of Earth Sciences: An International Geoscience Journal of the Geological Society of Australia     Hybrid Journal   (Followers: 11)
Boletim de Ciências Geodésicas     Open Access  
Boreas: An International Journal of Quaternary Research     Hybrid Journal   (Followers: 10)
Bragantia     Open Access   (Followers: 2)
Bulletin of Earthquake Engineering     Hybrid Journal   (Followers: 10)
Bulletin of Geosciences     Open Access   (Followers: 9)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (Followers: 1)
Bulletin of the Seismological Society of America     Full-text available via subscription   (Followers: 20)
Bulletin of Volcanology     Hybrid Journal   (Followers: 17)
Cadernos de Geociências     Open Access  
Canadian Journal of Plant Science     Full-text available via subscription   (Followers: 14)
Canadian Mineralogist     Full-text available via subscription   (Followers: 3)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 17)
Carbonates and Evaporites     Hybrid Journal   (Followers: 3)
CATENA     Hybrid Journal   (Followers: 4)
Chemical Geology     Hybrid Journal   (Followers: 14)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 4)
Chinese Geographical Science     Hybrid Journal   (Followers: 5)
Chinese Journal of Geochemistry     Hybrid Journal   (Followers: 3)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 3)
Ciencias Espaciales     Open Access  
Climate and Development     Hybrid Journal   (Followers: 12)
Coastal Management     Hybrid Journal   (Followers: 23)
Cogent Geoscience     Open Access  
Comptes Rendus Geoscience     Full-text available via subscription   (Followers: 7)
Computational Geosciences     Hybrid Journal   (Followers: 12)
Computational Mathematics and Mathematical Physics     Hybrid Journal   (Followers: 1)
Computers and Geotechnics     Hybrid Journal   (Followers: 8)
Contemporary Trends in Geoscience     Open Access   (Followers: 2)
Continental Shelf Research     Hybrid Journal   (Followers: 8)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 10)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 2)
Coral Reefs     Hybrid Journal   (Followers: 16)
Cretaceous Research     Hybrid Journal   (Followers: 6)
Cybergeo : European Journal of Geography     Open Access   (Followers: 4)
Depositional Record     Open Access  
Developments in Geotectonics     Full-text available via subscription   (Followers: 3)
Developments in Quaternary Science     Full-text available via subscription   (Followers: 3)
Développement durable et territoires     Open Access   (Followers: 2)
Diatom Research     Hybrid Journal  
Doklady Physics     Hybrid Journal   (Followers: 1)
Dynamics of Atmospheres and Oceans     Hybrid Journal   (Followers: 9)
E&S Engineering and Science     Open Access  
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (Followers: 70)
Earth and Space Science     Open Access   (Followers: 6)
Earth Interactions     Full-text available via subscription   (Followers: 11)
Earth Science Research     Open Access   (Followers: 6)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 3)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 19)
Earth System Dynamics     Open Access   (Followers: 5)
Earth System Dynamics Discussions     Open Access   (Followers: 4)
Earth's Future     Open Access   (Followers: 1)
Earth, Planets and Space     Open Access   (Followers: 5)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 6)
Earthquake Science     Hybrid Journal   (Followers: 9)
Earthquake Spectra     Full-text available via subscription   (Followers: 17)
Ecohydrology     Hybrid Journal   (Followers: 9)
Ecological Questions     Open Access   (Followers: 5)
Electromagnetics     Hybrid Journal   (Followers: 2)
Energy Efficiency     Hybrid Journal   (Followers: 11)
Energy Exploration & Exploitation     Full-text available via subscription   (Followers: 4)
Environmental Earth Sciences     Hybrid Journal   (Followers: 19)
Environmental Geology     Hybrid Journal   (Followers: 9)
Environmental Geosciences     Full-text available via subscription   (Followers: 4)
Environmental Geotechnics     Open Access   (Followers: 3)
Erwerbs-Obstbau     Hybrid Journal  
Estuaries and Coasts     Hybrid Journal   (Followers: 18)
Estuarine, Coastal and Shelf Science     Hybrid Journal   (Followers: 33)
Estudios Geográficos     Open Access  
European Journal of Mineralogy     Full-text available via subscription   (Followers: 10)
Exploration Geophysics     Hybrid Journal   (Followers: 3)
Facies     Hybrid Journal   (Followers: 8)
Fieldiana Life and Earth Sciences     Full-text available via subscription   (Followers: 1)
Física de la Tierra     Open Access  
Folia Musei rerum naturalium Bohemiae occidentalis. Geologica et Paleobiologica     Open Access  
Folia Quaternaria     Open Access  
Forestry Chronicle     Full-text available via subscription   (Followers: 10)
Frontiers in Earth Science     Open Access   (Followers: 5)
Frontiers in Geotechnical Engineering     Open Access   (Followers: 2)
Frontiers of Earth Science     Hybrid Journal   (Followers: 8)
Fundamental and Applied Limnology / Archiv für Hydrobiologie     Full-text available via subscription   (Followers: 4)
GEM - International Journal on Geomathematics     Hybrid Journal   (Followers: 1)
Geo-Marine Letters     Hybrid Journal   (Followers: 7)
Geoacta     Open Access   (Followers: 4)
Geobiology     Hybrid Journal   (Followers: 7)
Geocarto International     Hybrid Journal   (Followers: 6)
Geochemistry : Exploration, Environment, Analysis     Hybrid Journal   (Followers: 7)
Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 24)
Geochimica et Cosmochimica Acta     Hybrid Journal   (Followers: 23)
Geochronometria     Hybrid Journal   (Followers: 3)
Geoderma Regional : The International Journal for Regional Soil Research     Full-text available via subscription   (Followers: 3)
Geodinamica Acta     Hybrid Journal   (Followers: 3)
Geodynamics & Tectonophysics     Open Access  
Geoenvironmental Disasters     Open Access   (Followers: 3)
Geofluids     Hybrid Journal   (Followers: 4)
Geoforum     Hybrid Journal   (Followers: 20)
Géographie physique et Quaternaire     Full-text available via subscription  
Geography and Natural Resources     Hybrid Journal   (Followers: 4)
Geoheritage     Hybrid Journal   (Followers: 1)
Geoinformatica Polonica : The Journal of Polish Academy of Arts and Sciences     Open Access  
Geoinformatics & Geostatistics     Hybrid Journal   (Followers: 5)
Geological Journal     Hybrid Journal   (Followers: 14)
Geological Magazine     Hybrid Journal   (Followers: 16)
Geology Today     Hybrid Journal   (Followers: 18)
Geomagnetism and Aeronomy     Hybrid Journal   (Followers: 2)
Geomatics, Natural Hazards and Risk     Hybrid Journal   (Followers: 7)
GEOmedia     Open Access   (Followers: 1)
Geomorphology     Hybrid Journal   (Followers: 23)
Geophysical & Astrophysical Fluid Dynamics     Hybrid Journal   (Followers: 2)
Geophysical Journal International     Hybrid Journal   (Followers: 30)
Geophysical Prospecting     Hybrid Journal   (Followers: 7)
Geophysics     Full-text available via subscription   (Followers: 18)
GeoResJ     Hybrid Journal  
Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards     Hybrid Journal   (Followers: 8)
Geoscience Canada : Journal of the Geological Association of Canada / Geoscience Canada : journal de l'Association Géologique du Canada     Full-text available via subscription   (Followers: 3)
Geoscience Data Journal     Open Access   (Followers: 2)
Geoscience Frontiers     Open Access   (Followers: 8)
Geoscience Letters     Open Access  
Geoscience Records     Open Access  
Geosciences     Open Access   (Followers: 2)
Geosciences Journal     Hybrid Journal   (Followers: 9)
Geoscientific Instrumentation, Methods and Data Systems     Open Access   (Followers: 2)
Geoscientific Model Development     Open Access   (Followers: 2)
Geostandards and Geoanalytical Research     Hybrid Journal   (Followers: 2)
Geosystem Engineering     Hybrid Journal   (Followers: 1)
Geotectonic Research     Full-text available via subscription   (Followers: 4)
Geotectonics     Hybrid Journal   (Followers: 6)
GISAP : Earth and Space Sciences     Open Access   (Followers: 1)
Glass Physics and Chemistry     Hybrid Journal   (Followers: 3)
Global and Planetary Change     Hybrid Journal   (Followers: 11)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 8)
Gondwana Research     Hybrid Journal   (Followers: 6)
Grassland Science     Hybrid Journal   (Followers: 1)
Ground Water     Hybrid Journal   (Followers: 22)
Ground Water Monitoring & Remediation     Hybrid Journal   (Followers: 15)
GSA Today     Partially Free  
Helgoland Marine Research     Open Access   (Followers: 3)
History of Geo- and Space Sciences     Open Access   (Followers: 3)
Hydrobiologia     Hybrid Journal   (Followers: 17)
Hydrogeology Journal     Hybrid Journal   (Followers: 14)
Hydrological Processes     Hybrid Journal   (Followers: 21)
Hydrology and Earth System Sciences     Open Access   (Followers: 21)
ICES Journal of Marine Science: Journal du Conseil     Hybrid Journal   (Followers: 51)
IEEE Journal of Oceanic Engineering     Hybrid Journal   (Followers: 11)
Indian Geotechnical Journal     Hybrid Journal   (Followers: 2)
Indonesian Journal on Geoscience     Open Access   (Followers: 2)
Interdisciplinary Environmental Review     Hybrid Journal   (Followers: 3)
International Geology Review     Hybrid Journal   (Followers: 5)
International Journal of Advanced Geosciences     Open Access  
International Journal of Advanced Remote Sensing and GIS     Open Access   (Followers: 23)
International Journal of Advancement in Earth and Enviromental Sciences     Open Access   (Followers: 2)
International Journal of Advancement in Remote Sensing, GIS, and Geography     Open Access   (Followers: 19)
International Journal of Applied Earth Observation and Geoinformation     Hybrid Journal   (Followers: 20)
International Journal of Coal Geology     Hybrid Journal   (Followers: 2)
International Journal of Disaster Risk Reduction     Hybrid Journal   (Followers: 10)
International Journal of Earth Sciences     Hybrid Journal   (Followers: 27)
International Journal of Forest, Soil and Erosion     Open Access   (Followers: 4)
International Journal of Geo-Engineering     Open Access  
International Journal of Geographical Information Science     Hybrid Journal   (Followers: 49)

        1 2 3 | Last

Journal Cover Chemical Geology
  [SJR: 1.927]   [H-I: 123]   [14 followers]  Follow
    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 0009-2541
   Published by Elsevier Homepage  [2969 journals]
  • Simulation of geochemical banding I: Acidization-precipitation experiments
           in a ferruginous limestone rock
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Maysam Msharrafieh, Mazen Al-Ghoul, Farah Zaknoun, Houssam El-Rassy, Samia El-Joubeily, Rabih Sultan
      This paper is the first among two articles which aim at exploring the possible similarities between the well-known Liesegang banding phenomenon in precipitate systems, and the stripe formation observed in a large number of rocks. In the present (first) article, we review a comprehensive and long study wherein patterning experiments were performed in-situ (real rock systems), to simulate the band formation through the acidization of a ferruginous limestone rock, causing dissolution and precipitation reactions. The results are analyzed by microscopy, AA and XRD techniques. The correlation between the Liesegang gel experiment and the processes taking place inside the rock medium is established. In the second part (following paper), a theoretical model is set forth to support our experiments. The fractal nature of the contours of the various regions will be explored.
      Graphical abstract image

      PubDate: 2016-07-24T06:13:25Z
       
  • Evidence against an ancient non-chondritic mantle source for North
           Atlantic Igneous Province lavas
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): James M.D. Day
      North Atlantic Igneous Province (NAIP) lavas host olivine with the highest 3He/4He ever measured for terrestrial igneous rocks (up to 50 RA, or 4He/3He=~15,300). The relationship of high-3He/4He with Pb isotope compositions close to the terrestrial geochron and 143Nd/144Nd plausibly consistent with supra-chondritic mantle Sm/Nd in Baffin Island and West Greenland lavas has been interpreted to reflect an ancient ‘non-chondritic’ mantle source signature. Alternatively, assimilation of continental crustal rocks with unradiogenic Pb isotope compositions and low 143Nd/144Nd, into magmas with high-3He/4He, and derived from variably depleted mantle sources, could impart similar geochemical signatures. Radiogenic and stable isotope data for NAIP lavas are consistent with origins as melts from upper mantle sources that contain low-18O/16O recycled lithosphere and/or hydrothermally altered crust, or that have experienced pervasive contamination by crustal gneisses. Olivines from NAIP lavas with 3He/4He spanning from 8 to 48 RA have δ18O ranging from 3.5 to 5.5‰. These compositions are consistent with sources of ambient mantle and low-δ18O recycled lithosphere, or with concomitant crustal assimilation and He-loss during fractional crystallization. Limited assimilation (≤1%) of incompatible element rich crustal gneisses with low 206Pb/204Pb and 143Nd/144Nd by melts from variably depleted mantle sources can explain Nd-Pb isotope compositions of Baffin Island and West Greenland picrites. Icelandic lavas provide supporting evidence that the ancestral mantle plume responsible for generating NAIP magmatism sampled variably enriched and depleted mantle, with no evidence for ancient non-chondritic mantle sources. Pervasive crustal contamination and partial melting of heterogeneous mantle sources, generated by plate tectonic processes, can account for the compositions of continental flood basalts (CFB), without the requirement of a non-chondritic terrestrial reservoir. Combined with evidence that the 142Nd/144Nd composition of the bulk silicate Earth is due to nucleosynthetic S-process deficits in chondrite meteorites, these observations cast doubt that NAIP lavas sampled a non-chondritic mantle source with Sm/Nd higher than in chondrites. If short-lived radiogenic (e.g., 146Sm-142Nd, 182Hf-182 W) isotope anomalies are found in CFB, they must either reflect assimilation of isotopically anomalous crustal materials, or partial melting of early-formed mantle heterogeneities produced by differentiation and late accretion.


      PubDate: 2016-07-24T06:13:25Z
       
  • Tracing provenance and sediment fluxes in the Irrawaddy River basin
           (Myanmar)
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Eduardo Garzanti, Jiang-Gang Wang, Giovanni Vezzoli, Mara Limonta
      This study illustrates the petrographic, heavy-mineral, geochemical and geochronological signatures of sand transported by various branches of the Irrawaddy (Ayeyarwadi) River, one of the first in the world for sediment flux. Intrasample and intersample compositional variability, weathering and hydraulic-sorting controls are also discussed. Feldspatho-quartzose sand in Irrawaddy headwaters is largely derived first-cycle from mid-crustal metamorphic and plutonic rocks of the Mogok Belt and Lohit complex, whereas feldspatho-litho-quartzose Chindwin sand is largely recycled from supracrustal, sedimentary and very low-grade metasedimentary units. Additional mafic to ultramafic detritus is derived from ophiolites and blueschists exposed from the Indo-Burman Ranges to the Jade Mines and Myitkyina belts, linked northward to the Yarlung-Tsangpo suture of the Himalaya. Volcanic detritus derived from the Popa-Wuntho arc or recycled from forearc-basin strata also occurs. Decreasing concentration of most chemical elements along the Irrawaddy reflects progressive addition of detritus recycled from sedimentary rocks, most evident downstream of the Chindwin confluence. REE patterns with LREE enrichment and negative Eu anomaly reflect the occurrence of allanite, largely derived from granitoid rocks in the Mali catchment. Chemical indices indicate moderate weathering in the monsoon-dominated climate of Myanmar. Young U-Pb ages (15–170Ma) represent 85% of detrital zircons in Irrawaddy headwater branches, reflecting long-lasting subduction-related magmatism along a ring of fire connecting with the southern and central Lhasa batholiths in Tibet and polyphase metamorphism in the Mogok belt. Chindwin sand contains larger amounts of finer-grained, recycled pre-Mesozoic zircons, also yielding early Mesoproterozoic to Archean ages. Such different petrographic, heavy-mineral, geochemical and geochronological fingerprints characterizing sand in different river branches allowed us to calculate bulk-sediment and zircon-provenance budgets that converge to indicate equivalent sand supply from the Nmai and Mali Rivers to the upper Irrawaddy, and from the Chindwin and upper Irrawaddy to the lower Irrawaddy. This implies that despite of higher erosion potential indicated by stream-profile analysis in high-relief Irrawaddy headwaters, sediment yields and erosion rates are detectably higher in the Chindwin catchment, which is mainly ascribed to higher erodibility of widely exposed siliciclastic rocks. Quantifying sediment provenance and defining erosion patterns based on an integrated compositional database in a big-river system such as the mighty Irrawaddy allows us to expand our understanding of sediment-generation processes with the ultimate goal to increase our capacity to read into the stratigraphic record.


      PubDate: 2016-07-24T06:13:25Z
       
  • Hf–Nd isotope decoupling in bulk abyssal peridotites due to
           serpentinization
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Carl Frisby, Michael Bizimis, Soumen Mallick
      Serpentinized abyssal peridotites are the product of the chemical interaction between the Earth's oceans and the peridotitic upper mantle. This reaction is dominated by the addition of water and fluid mobile elements to the peridotite, with implications on the cycling of water and fluid mobile elements in subduction zones. Rare earth elements (REE) were generally thought as immobile during serpentinization. However the identification of Nd (and Sr) isotopic disequilibrium between fresh clinopyroxene and their host bulk serpentinized peridotite suggest addition of Nd and other light REE to the peridotite during serpentinization (Frisby et al. (2016)). As with the REE, the high-field strength elements (HFSE) have also been considered immobile and non-reactive during serpentinization. However, this has yet to be explicitly tested. Here we present a modified dissolution and extraction method that improves on the yields of REE and HFSE from highly depleted ultramafic rocks (peridotites) through the use of B(OH)3 during digestion dry-down steps, Fe-precipitation for REE-HFSE sequestration, and liquid–liquid exchange for Fe removal. Using this method we report on the Nd and Hf isotope compositions of bulk rock serpentinized abyssal peridotites from the South-West Indian Ridge (SWIR), Mid-Atlantic Ridge (MIR) and Mid-Caymen Rise (MCR) and on samples where their clinopyroxene Hf, Nd isotopes were previously reported. The bulk rock Hf isotopes are equal to clinopyroxene to within 1 εHf, while Nd isotopes in the bulk peridotite are up to 8 εNd units less radiogenic than their clinopyroxene. Melt-rock reaction between the depleted peridotite endmember and local enriched lavas cannot generate the observed decoupling between Hf and Nd isotopes. Our data is instead consistent with seawater — peridotite interaction as this results in significant shift in Nd isotopes with little to no change in Hf isotopes until extremely high water/rock mass ratios of >100,000. Our data shows that Hf isotopes are not affected by serpentinization and that Hf isotopes in bulk peridotites better record the magmatic composition of peridotites than Nd or Sr. The largest Nd isotopic difference between clinopyroxene and bulk peridotite is observed in the most LREE-depleted samples, while refertilized samples show little change. Based on the bulk rock (serpentinized) and clinopyroxene (magmatic value) Nd isotope compositions we estimate that 1–34% of the bulk rock Nd budget is seawater derived, but only 1–6% of Sm and <2% of Lu and Hf in these samples. Therefore serpentinization will lower the Sm/Nd relative to Lu/Hf ratio in a peridotite and this may lead to decoupling between Hf and Nd isotopes upon recycling and aging, assuming no additional fractionation of these elements during the subduction process. Alternatively, if Nd becomes fluid-mobile upon dehydration during subduction then the excess Nd in the serpentinized peridotites may become an additional reservoir for Nd in arc volcanism.


      PubDate: 2016-07-24T06:13:25Z
       
  • The origin of N2-H2-CH4-rich natural gas seepages in ophiolitic context: A
           major and noble gases study of fluid seepages in New Caledonia
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Eric Deville, Alain Prinzhofer
      The study of natural gas seepages in New Caledonia has shown the occurrence of two gas families, one is N2-H2-CH4-rich, the other is N2-rich. The N2-H2-CH4-rich gases are bubbling in hyperalkaline springs (OH–-rich) known in the peridotite massif of the southern part of the island. This family of gas shows contents of N2 between 50 and 62%, H2 between 26 and 36%, and CH4 between 11 and 16%. δ13C values of methane are ranging from −39 to −32‰. We interpret the origin of H2 as a product of fluid-rock interaction between basic-ultrabasic rocks and water with oxidation of Fe2+ and reduction of underground water in the fracture system of the peridotites nappe. Methane is interpreted as the result of a reduction of dissolved inorganic and/or organic carbon in subsurface aquifers. The second family of gas emissions was found in thermal springs in the sedimentary units located structurally below the peridotites nappe. The gas is composed mostly of N2 (between 97 and 98%) associated with relatively high concentration of He. Both gas families show notably high N2 vs fossil noble gas contents (36Ar, 20Ne, 84Kr) suggesting that N2 is not directly issued from gas dissolved in aquifers equilibrated with atmosphere but most likely finds its origin in a deep source, probably within metamorphic sediments which are tectonically buried below the ophiolitic nappe. We interpret the N2-H2-CH4-rich family of gas as a result of a mixing between two end-members, (1) a H2-CH4-rich pole issued from the weathering of peridotite rocks and (2) a N2-rich pole which would be issued from the metamorphosed sediments buried below the ophiolitic units. An unusual inverse correlation between the atmospheric noble gas isotopes 20Ne and 36Ar is interpreted as the result of a degassing of a relatively shallow confined aquifer related to a bubble flow from depth.


      PubDate: 2016-07-24T06:13:25Z
       
  • High-precision measurement and standard calibration of triple oxygen
           isotopic compositions (δ18O, Δ′17O) of sulfate by F2 laser
           fluorination
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Benjamin R. Cowie, David T. Johnston
      A new approach for measuring the triple oxygen isotope composition of sulfate minerals was developed using fluorine gas and an infrared laser to generate O2. A correction for the mass-dependent isotope effect observed during barite fluorination was rigorously calibrated. Analyte gas purification was performed via numerous cryofocus steps and the introduction of an in-line gas chromatograph. As with previous methods, our fluorination technique still requires pairing with independent δ 18O measurements made by TC/EA conversion to carbon monoxide, but leads to higher yields than BrF5-based methods. We first calibrated our method against known silicate standards (UWG-2, SCO, NBS-28). Following from this, we report high-precision triple oxygen isotope ratios for international sulfate standards (IAEA-SO-5, IAEA-S O-6, NBS-127), and an internal laboratory standard, JMG. Replicate analyses of JMG yielded a Δ ′17O value of −0.057 ± 0.004‰ (standard error), demonstrating per-meg level precision that approaches instrumental limits. We provide δ 17O and δ 18O values that can be translated into any preferred reference frame for comparison with gases, water, or rocks/minerals. Quantification of differences in triple oxygen isotope composition of the standard reference materials further enables application of this measure and approach on environmental and geological materials.


      PubDate: 2016-07-24T06:13:25Z
       
  • Geochemical anatomy of a spheroidally weathered diabase
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Anupam Banerjee, Ramananda Chakrabarti, Sourav Mandal
      Major, trace element concentrations and Nd, Sr isotope ratios were measured in micro-drilled samples of a 2.37Ga-old, hand-specimen sized spheroidally weathered diabase from southern India. A sample of the un-weathered diabase dike was also analyzed. X-ray micro-CT imaging of the weathered sample shows three dominant mineral phases which are plagioclase, pyroxene, and a Fe-bearing phase (possibly hematite and/or ilmenite). This imaging documents the pervasive nature of two generations of ribbon-like, cross-cutting fractures. The older fracture is sealed while the more recent fracture is open without any in-filling. The values of the Chemical Index of Alteration (CIA) of the samples show a wide range but are less than 50. Despite being a relatively less weathered rock, we observe that concentrations of major, minor and trace elements vary significantly with the percentage relative standard deviation (%RSD) for the elements ranging from 10.2–41.8. The CIA of the samples do not show any trend with the position of the sample in the hand-specimen. Barring Ca and Li, whose concentrations decrease from the core to the rim of the sample, there is no significant spatial trend in the concentrations of the elements. Concentrations of Na, Al, and Sr increase with increasing CIA values while concentrations of Mg, Fe, and Sc decrease with increasing CIA. The strong positive correlations of Na and Al, as well as Na and Sr indicates preferential weathering of plagioclase in the diabase. Na/Ca increases while Mg/Al, Mg/Na, Mg/Ca, Fe/Al and Sc/Sr decrease with increasing CIA values and the un-weathered rock plots in the middle of these trends. Such variations are explained in terms of differential weathering of plagioclase (in samples with lower CIA than the un-weathered rock, W1-type) and pyroxene (in samples with higher CIA than the un-weathered rock, W2-type) which have varying resistance to weathering. At the hand-specimen scale, the variability in the weathering indices like CIA are controlled by differential weathering of minerals and might not accurately reflect the intensity of weathering. Chondrite-normalized La/Sm and Gd/Lu co-vary with CIA values indicating mobility of the REEs during spheroidal weathering even at the hand-specimen scale. The Eu anomaly also increases with increasing CIA values which is explained by differential weathering of pyroxene and plagioclase. We observe large percentage deviations of the Nb-normalized concentrations of elements from the un-weathered rock in specific samples but no spatial trend is observed. Overall, the variations in element concentrations can be explained by varying fluid mobility of the elements, selective weathering of the minerals in the diabase, and ambient environmental conditions. Considerable Nd and Sr isotopic variability is observed at the hand-specimen scale and is explained in terms of weathering-related fractionation of parent/daughter ratios. This elemental fractionation must have happened long time ago to allow for radiogenic decay of the long-lived isotopes of 87Rb and 147Sm. The spread (%RSD) in the initial Sr and Nd isotope compositions of the weathered samples reach a minimum value around 1.2–1.3Ga which we interpret as the timing of the peak weathering event which led to fractionation of the parent/daughter ratios. For Nd isotopes, the average εNd (1.2 Ga) of the weathered samples coincides with the εNd (1.2 Ga) of the un-weathered rock. The timing of the weathering event coincides with the timing of the breakup of the Columbia supercontinent and follows wide-spread alkaline volcanism in the Indian subcontinent. This is the first such attempt to determine the timing of a weathering event in rocks using long-lived radioactive isotopes.


      PubDate: 2016-07-24T06:13:25Z
       
  • The controversial role of inter-diffusion in glass alteration
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): S. Gin, L. Neill, M. Fournier, P. Frugier, T. Ducasse, M. Tribet, A. Abdelouas, B. Parruzot, J. Neeway, N. Wall
      Current kinetic models for nuclear waste glasses (e.g. GM2001, GRAAL) are based on a set of mechanisms that have been generally agreed upon within the international waste glass community. These mechanisms are: hydration and ion exchange reactions (the two processes are referred as inter-diffusion), hydrolysis of the silicate network, and condensation/precipitation of partly or completely hydrolyzed species that produces a porous and amorphous layer and crystalline phases on surface of the altered glass. Recently, a new idea with origins in the mineral dissolution community has been proposed that excludes inter-diffusion processes as a potential rate-limiting mechanism. To understand how the newly proposed interfacial dissolution/precipitation model can change the current understanding of glass corrosion, a key experiment used to account for this model was replicated to further revisit the interpretation. This experiment was performed far from saturation, at 50°C, with SON68 glass, in static mode, deionized water, and a S/V ratio of 10m−1 for 6months. Results were repeatable and showed that glass dissolution rate progressively dropped by ~1 order of magnitude compared to the forward rate, suggesting that a dense surface layer was under construction. According to previous and new solids characterizations, it is concluded that neither a simple inter-diffusion model nor the interfacial dissolution/precipitation model can account for the observed elemental profiles within the alteration layer. More generally, far-from- and close-to-saturation conditions must be distinguished. This argument is bolstered by literature where evidence shows that inter-diffusion takes place in acidic conditions and far from saturation. However, closer to saturation, when a sufficiently dense layer is formed, a new approach is proposed requiring a full description of chemical reactions taking place within the alteration layer and an accurate budget of hydrous species along the profile as it is thought that the access of a sufficient amount of water to the pristine glass is the rate-limiting process in these conditions.


      PubDate: 2016-07-24T06:13:25Z
       
  • Basin redox and primary productivity within the Mesoproterozoic Roper
           Seaway
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Grant M. Cox, Amber Jarrett, Dianne Edwards, Peter W. Crockford, Galen P. Halverson, Alan S. Collins, André Poirier, Zheng-Xiang Li
      The ca. 1.4Ga Roper Group of the greater McArthur Basin in northern Australia comprises the sedimentary fill of one of the most extensive Precambrian hydrocarbon-bearing basins preserved in the geological record. It is interpreted to have been deposited in a large epeiric sea known as the Roper Seaway. Trace element data suggest that the redox structure of the basin was a shallow oxic layer overlying deeper suboxic to anoxic waters along with a prominent episode of euxinia. These anoxic and sulfidic conditions, as inferred by Mo, V, and U concentrations (molybdenum, vanadium and uranium), developed due to high organic carbon loading consistent with models that suggest that euxinic conditions cannot develop until the flux of organic matter is significantly greater than the flux of bioavailable iron, which permits sulphate reduction to proceed. Considering the high reactive iron and molybdenum contents of these shales and the requirement for S/Fe ratios>2 for euxinia to develop, suggests that sufficient atmospheric O2 was available for oxidative scavenging of S and Mo from the continents. This is further supported by prominent negative cerium anomalies within these shales, indicative of active oxidative redox cycling of cerium. We propose that the high organic matter flux was the result of increased nutrient loading to the Roper Seaway from weathering of the continental hinterland. Data from both major and high-field strength elements (niobium, tantalum, zirconium and, hafnium) together with neodymium isotopes (143Nd/144Nd) indicate that a likely mechanism for this enhanced nutrient delivery was a shift in sedimentary provenance to a more primitive (i.e. mafic) precursor lithology. This switch in provenance would have increased phosphorus delivery to the Roper Seaway, contributing to high primary productivity and the onset of euxinia. This dataset and model serve as a basis for understanding the temporal evolution of the deepest sections of the Roper Seaway and finer scale changes in the environment at this time.


      PubDate: 2016-07-24T06:13:25Z
       
  • How sulfate-driven anaerobic oxidation of methane affects the sulfur
           isotopic composition of pyrite: A SIMS study from the South China Sea
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Zhiyong Lin, Xiaoming Sun, Jörn Peckmann, Yang Lu, Li Xu, Harald Strauss, Haoyang Zhou, Junli Gong, Hongfeng Lu, Barbara M.A. Teichert
      Sulfate-driven anaerobic oxidation of methane (SO4-AOM) in marine sediments commonly leads to the precipitation of pyrite. It is, however, frequently challenging to unequivocally unravel the entire history of pyritization, because of the common coexistence of SO4-AOM derived pyrite with pyrite resulting from organiclastic sulfate reduction (OSR). To better understand how SO4-AOM affects pyritization in methane-bearing sediments and how this can be identified, we applied secondary ion mass spectroscopy (SIMS) to analyze the sulfur isotope composition (δ34S) of authigenic pyrite in addition to sulfur isotope measurements of bulk sulfide and hand-picked pyrite aggregates from the two seafloor sites, HS148 and HS217, in the Shenhu seepage area, South China Sea. Authigenic, mostly tubular pyrite aggregates from these sites consist of three types of pyrite: framboids, zoned aggregates with radial overgrowths surrounding a framboidal core, and euhedral pyrite crystals. Framboids with low SIMS δ34S values (as low as −41.6‰ at HS148, and −38.8‰ at HS217) are dispersed throughout the cores, but are especially abundant in the shallow part of the sedimentary column (i.e. above 483cmbsf in HS148; above 670cmbsf in HS217). These patterns are interpreted to reflect the dominance of OSR during early diagenetic processes in the shallow sediments. With increasing depth, both δ34S values of bulk sulfide minerals and hand-picked pyrite aggregates increase sharply at 483cmbsf in core HS148, and at 700cmbsf in core HS217, respectively. Radial pyrite overgrowths and euhedral crystals become abundant at depth typified by high δ34S values for hand-picked pyrite. Moreover, SIMS analysis reveals an extreme variability of δ34S values for the three pyrite types on a small scale in these zones. Besides some moderately 34S enriched framboids, most of the overgrowths and euhedral crystals display extremely high SIMS δ34S values (as high as +114.8‰ at HS148, and +74.3‰ at HS217), representing the heaviest stable sulfur isotope composition of pyrite ever reported to the best of our knowledge. Such an abrupt and extreme increase in δ34Spyrite values with depth is best explained by an enrichment of 34S in the pool of dissolved sulfide caused by SO4-AOM in the sulfate methane transition zone (SMTZ). The increase in δ34S values from framboidal cores to overgrowth layers and euhedral crystals indicates continuous, and finally near to complete exhaustion of dissolved sulfate at the SMTZ following a Rayleigh distillation process. SO4-AOM allowed for subsequent growth of later stage pyrite over the initial framboids, part of which formed earlier and at shallower depth by OSR. The combination of a detailed petrographic study of authigenic pyrite with SIMS analysis of stable sulfur isotopes in organic-rich strata proves to be a powerful tool for reconstructing the dynamics of sulfur cycling in modern and, potentially, ancient sedimentary sequences.
      Graphical abstract image

      PubDate: 2016-07-24T06:13:25Z
       
  • Celestine in a sulfidic spring barite deposit - A potential biomarker?
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): David M. Singer, Elizabeth M. Griffith, John M. Senko, Kaci Fitzgibbon, Inoka H. Widanagamage
      We have documented the presence of celestine (SrSO4) within sediment accumulating at an artesian sulfidic spring (Zodletone Spring, Oklahoma) dominated by barite (BaSO4) precipitation associated with microbiological activity. The distribution and speciation of Sr in solid phases was determined by synchrotron-based micro-X-ray fluorescence spectroscopy and micro-X-ray diffraction, and particle morphology and texture was determined using electron microscopy. In all the natural sulfidic spring samples and lithified tufa sample, celestine was detected in fine-grained micron-scale Sr-rich phases but not in euhedral, Sr-poor grains. In parallel laboratory-based precipitation experiments, celestine was not observed even when solutions contained high Sr/Ba. Thermodynamic predictions alone do not account for the presence of micron-scale celestine in the sulfidic spring, and they do not account for the differences in Sr presence and distribution in naturally-occurring versus synthetic grains. While the mechanism is unclear, based on this evidence we hypothesize that the combination of bacterial surfaces and microenvironments within the crusts and microbial mats creates a synergistic effect where Sr is preferentially exchanged over Ba between the overlying stream water and the pore water within the mats allowing celestine to precipitate. Ultimately, our results point to an important role of biological activity for preferential Sr uptake. The presence of micron-scale celestine in ancient barite deposits can therefore potentially be used as a biomarker for conditions similar to modern sulfidic springs.
      Graphical abstract image

      PubDate: 2016-07-24T06:13:25Z
       
  • Redox conditions across the G–L boundary in South China: Evidence
           from pyrite morphology and sulfur isotopic compositions
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Hengye Wei, Xuemei Wei, Zhen Qiu, Huyue Song, Guo Shi
      The end-Guadalupian (middle Permian) mass extinction recorded the disappearance of shelf faunas such as fusulinids, small foraminifers, brachiopods, rugose corals which flourished in the warm shallow marine habitats after the late Pennsylvanian–early Permian glaciation. The causes for this mass extinction are still unclear. Marine anoxia, often as a cause for other mass extinctions, has been paid less attention in this extinction. Here we present a comprehensive study including pyrite morphology and sulfur isotopic compositions of pyrite across the Guadalupian–Lopingian (middle–upper Permian; G–L) boundary at Tianfengping in northern Yangtze Platform and at Penglaitan in southern Yangtze Platform in South China. Our results show that the sulfur isotope ratios of pyrite mainly track the framboid size distributions in these two sections. Anoxia and intermittent euxinia occur at the G–L boundary evidenced by the coincidence between reduction of framboid size and the extremely negative sulfur isotope values of pyrite involving disproportionation, and corresponds to the main mass extinction of the end-Guadalupian, suggesting a causal link between them. Long-term dysoxic conditions during the middle Capitanian are evidenced by the relative small framboid size and lower sulfur isotope ratios of pyrite, and coincide with the first pulse of the end-Guadalupian mass extinction, also suggesting a causal link between them. The oxygen-depletion conditions in the Capitanian during the global sea level falling may be the main cause for the end-Guadalupian mass extinction.


      PubDate: 2016-07-24T06:13:25Z
       
  • Stable sulfur isotope dynamics in an acid sulfate soil landscape following
           seawater inundation
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): C.A. Maher, L.A. Sullivan
      In 2002 a tidally driven seawater exchange remediation strategy was successfully implemented on a severely acidified tropical coastal landscape dominated by acid sulfate soils (ASS) in northern Australia. This study examined changes in the stable sulfur isotope signatures in a range of sulfide and sulfate (SO4) fractions at three sites with different levels of exposure to the tidally driven seawater exchange remediation. δ34S in the acid soluble SO4 fraction (e.g. jarosite) was less depleted in 34S than the corresponding sulfide, indicating a degree of fractionation during sulfide oxidation and jarosite precipitation. The δ34S of jarositic-SO4 was similar at all three sites indicating the appreciable stability of jarositic-SO4 even after extended exposure to seawater. δ34S of the water soluble, exchangeable and schwertmannitic-SO4 reflect conditions post remediation and indicate the relative contributions from two potential SO4 sources – a lighter SO4 derived from the oxidation of pyrite, and a heavier SO4 derived from the seawater. The δ34S of the contemporary surficial sulfide accumulations also reflect a SO4 contribution from seawater used for remediation and were isotopically different from the relict sulfides found at depth at all sites. δ34S of water soluble sulfate allowed the progress of the remediation to be traced down the soil profile. This study demonstrates the utility of stable sulfur isotope signatures in various sulfide and SO4 fractions to trace the sulfur geochemical pathways occurring in soils, in this case as a result of the introduction of tidally driven sea water.


      PubDate: 2016-07-13T14:00:17Z
       
  • Effective use of cerium anomalies as a redox proxy in carbonate-dominated
           marine settings
    • Abstract: Publication date: 2 November 2016
      Source:Chemical Geology, Volume 438
      Author(s): Rosalie Tostevin, Graham A. Shields, Gary M. Tarbuck, Tianchen He, Matthew O. Clarkson, Rachel A. Wood
      Rare earth elements and yttrium (REY) have a distinct distribution pattern in seawater, and this pattern may be faithfully preserved in carbonate sediments and rocks. Anomalous concentrations of redox-sensitive cerium (Ce) compared with neighbouring REY originate in oxic water column conditions, and as such, Ce anomalies can provide a potentially useful redox proxy in carbonate-dominated marine settings. The methods used to extract REY from carbonates vary widely, and may suffer from widespread leaching of REY from accessory non-carbonate minerals and organic matter, limiting the application of Ce anomalies for palaeo-redox reconstruction. We have systematically compared different methods on 195 carbonate samples with varying purity (% carbonate) from both modern and ancient environments. We used sequential leaching experiments in nitric acid to identify the most ‘seawater-like’ portion of the carbonate sample where contributions from non-carbonate minerals and organic matter are minimised. We also compared the results of sample dissolution in different types and strengths of acid. Our results confirm that REY concentrations can be inadvertently contaminated by partial leaching of clays and Fe (oxyhydr)oxides during a single-step digestion, and we suggest a pre-leach of 20% of the sample, followed by a partial leach of 40% of the sample to selectively dissolve carbonate. We suggest that REY studies are optimised in carbonates with >85% CaCO3, and show that dolomites behave differently during the leaching process and must be treated separately. We present REY patterns for modern carbonate-rich sediments from a range of environments, and show that seawater REY are faithfully preserved in some non-skeletal carbonate, but modified leaching procedures are necessary for impure, unlithified or organic rich carbonate sediments. We combine REY with Fe-speciation data to identify how Fe oxides and clays contribute to the REY signal and explore how the two proxies can be used together to provide a complex and high-resolution redox reconstruction in carbonate-dominated marine environments.


      PubDate: 2016-07-09T13:55:27Z
       
  • Extracting foraminiferal seawater Nd isotope signatures from bulk deep sea
           sediment by chemical leaching
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Patrick Blaser, Jörg Lippold, Marcus Gutjahr, Norbert Frank, Jasmin M. Link, Martin Frank
      The seawater radiogenic neodymium (143Nd/144Nd) isotope signature is an invaluable tool for the reconstruction of past deep water provenance. Sedimentary foraminifera or fish teeth are among the most reliable archives known for Nd isotope based reconstructions of past seawater. As the distribution and preservation of these archives are limited, the extraction of hydrogenetic ferromanganese oxyhydroxides from bulk sediments provide an easily applicable alternative. This method, however, implies the risk of generating artefacts due to the possible release of non-seawater derived Nd during the extraction procedure. Here we revisit and further investigate the reliability of the extraction of seawater derived Nd isotope signatures via leaching of bulk deep sea sediments with two commonly used buffered acetic acid and acid-reductive mix solutions. Repeated application of such stepwise leaching procedures to different non-decarbonated sediments from distinct settings across the deep Atlantic Ocean shows pronounced elemental and Nd isotope trends during the leaching process in the laboratory. Our results show that seawater Nd isotope compositions are extracted together with carbonates and manganese oxides only at the beginning of the leaching series. During chemical extraction, the carbonates effectively work as a buffer preventing acid-induced mobilisation of Fe oxides and volcanogenic material. Once this buffer is consumed, potentially present volcanogenic phases are considerably attacked, leading to shifts in the extracted Nd isotope signal of up to +12 epsilon units. Such volcanogenic phases are a significant source of contaminant Nd reflected by markedly elevated Al/Nd signatures. We consequently propose a revised weak leaching protocol for carbonate bearing deep sea sediments, which is simple to use, provides excellent agreement with data obtained from uncleaned foraminifera, and can be easily screened for contamination.


      PubDate: 2016-07-09T13:55:27Z
       
  • Chemical and stable isotopic (B, H, and O) compositions of tourmaline in
           the Maocaoping vein-type Cu deposit, western Yunnan, China: Constraints on
           fluid source and evolution
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Shiqiang Huang, Yucai Song, Zengqian Hou, Chuandong Xue
      This study determined the chemical and stable isotopic (B, H, and O) compositions of three stages of ore-related hydrothermal tourmaline in the newly discovered Maocaoping vein-type Cu deposit, western Yunnan, China, as well as the chemical and B isotopic compositions of magmatic tourmaline (tourmaline-G) from a neighboring peraluminous granitic intrusion that is contemporaneous with the Cu mineralization. The objective was to constrain the source and evolution of the CO2-rich ore-forming fluid. The Maocaoping cupriferous veins (types V1 and V2) are hosted in mylonitized and metamorphosed Jurassic sedimentary rocks. Early- to late-stage hydrothermal tourmaline occurs as vein-related alteration halos (tourmaline-A) in altered marble or schist, as tourmaline-V1 in type V1 veins, and as tourmaline-V2 in type V2 veins. Tourmaline-A, -V1, and -V2 belong to the alkalic group and consist mostly of dravite, with δ11B values of −3.9‰ to −1.1‰, −2.8‰ to +0.5‰, and −6.3‰ to −3.9‰, respectively. Tourmaline-G belongs to the alkalic group and consists of schorl, with δ11B values of −14.7‰ to −12.2‰. The similarities in chemical composition between marble- and schist-hosted tourmalines and between tourmalines in veins and alteration halos (indicating their formation at different water/rock ratios) suggest that the compositions of the hydrothermal tourmaline at Maocaoping were controlled mainly by the chemistry of ore fluid rather than that of the host rocks. The similarities together with the lack of systematic variations in δ11B values in individual tourmaline grains imply that mixing of multiple fluids cannot be important during the mineralization. The positive shift in δ11B values from tourmaline-A to tourmaline-V1 is interpreted to result from continuous precipitation of tourmaline in a closed fluid system. Relative to tourmaline-A and -V1, tourmaline-V2 is Fe-rich, Al-poor, and 11B-depleted. These differences may be explained by fluid phase separation and subsequent escape of the CO2-rich vapor phase during the formation of type V2 veins, consistent with the observations that these veins are extensional, contain large amounts of carbonate minerals, and include fluid inclusions trapped in the vapor–liquid two-phase field. The earliest tourmaline-A records the initial ore fluid chemistry. The calculated δ11B, δ18O, and δD values of the initial ore fluid in equilibrium with tourmaline-A at mineralization temperatures of 280 to 320°C are −0.3‰ to +3.0‰, +10.7‰ to +11.8‰ and −110‰ to −84‰, respectively. The δ11B values as well as the chemical compositions of tourmaline-A are much different from those of tourmaline-G, suggesting that the ore fluid at Maocaoping was not derived directly from the peraluminous granitic magma. The B–H–O isotopic features as well as enrichment of CO2 in fluid inclusions also suggest the ore fluid cannot solely be derived from basinal fluid or meteoric water. The simplest explanation is that the fluid was derived from a metamorphic fluid that had obtained isotopically lighter hydrogen via reaction with “organic” compounds (e.g., CH4, H2S). Alternatively, the ore fluid sourced from the mixed metamorphic fluid and basinal brine, meteoric water, or degassed magmatic fluid. It implies that the vein Cu ore formation in the Lanping Basin is unlikely as a result of a sole magmatic or basinal fluid activity and it needs the involvement of metamorphic fluid/process.


      PubDate: 2016-07-09T13:55:27Z
       
  • Chemical abrasion-SIMS (CA-SIMS) U-Pb dating of zircon from the late
           Eocene Caetano caldera, Nevada
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Kathryn E. Watts, Matthew A. Coble, Jorge A. Vazquez, Christopher D. Henry, Joseph P. Colgan, David A. John
      Zircon geochronology is a critical tool for establishing geologic ages and time scales of processes in the Earth's crust. However, for zircons compromised by open system behavior, achieving robust dates can be difficult. Chemical abrasion (CA) is a routine step prior to thermal ionization mass spectrometry (TIMS) dating of zircon to remove radiation-damaged parts of grains that may have experienced open system behavior and loss of radiogenic Pb. While this technique has been shown to improve the accuracy and precision of TIMS dating, its application to high-spatial resolution dating methods, such as secondary ion mass spectrometry (SIMS), is relatively uncommon. In our efforts to U-Pb date zircons from the late Eocene Caetano caldera by SIMS (SHRIMP-RG: sensitive high resolution ion microprobe, reverse geometry), some grains yielded anomalously young U-Pb ages that implicated Pb-loss and motivated us to investigate with a comparative CA and non-CA dating study. We present CA and non-CA 206Pb/238U ages and trace elements determined by SHRIMP-RG for zircons from three Caetano samples (Caetano Tuff, Redrock Canyon porphyry, and a silicic ring-fracture intrusion) and for R33 and TEMORA-2 reference zircons. We find that non-CA Caetano zircons have weighted mean or bimodal U-Pb ages that are 2–4% younger than CA zircons for the same samples. CA Caetano zircons have mean U-Pb ages that are 0.4–0.6Myr older than the 40Ar/39Ar sanidine eruption age (34.00±0.03Ma; error-weighted mean, 2σ), whereas non-CA zircons have ages that are 0.7–1.3Myr younger. U-Pb ages do not correlate with U (~100–800ppm), Th (~50–300ppm) or any other measured zircon trace elements (Y, Hf, REE), and CA and non-CA Caetano zircons define identical trace element ranges. No statistically significant difference in U-Pb age is observed for CA versus non-CA R33 or TEMORA-2 zircons. Optical profiler measurements of ion microprobe pits demonstrate consistent depths of ~1.6μm for CA and non-CA Caetano, R33 and TEMORA-2 zircons, and do not indicate variations in secondary ion sputtering rates due to chemical or structural changes from the CA treatment. Our new data underscore the potential for cryptic Pb-loss to go unrecognized in other geologically young magmatic centers that do not have zircons with high U, statistically discordant isotope ratios, high common Pb, or metamict textures.


      PubDate: 2016-07-05T14:49:38Z
       
  • Evidence of organic matter control on As oxidation by iron oxides in
           riparian wetlands
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Hélène Guénet, Mélanie Davranche, Delphine Vantelon, Mathieu Pédrot, Maya Al-Sid-Cheikh, Aline Dia, Jacques Jestin
      Soils in riparian wetlands are periodically flooded, resulting in the establishment of reducing conditions and the solubilization of As, subsequently to the reductive dissolution of Fe(III)-oxyhydroxides. When the water level decreases, the wetlands are reoxidized. However, although the behavior of As under the reducing period is well documented, there is a lack of information regarding its behavior during the oxidizing period. In this study, we investigated As speciation in oxidation products from an initially reduced wetland soil solution recovered from the Naizin-Kervidy riparian wetland (France). The oxidation products were studied using NanoSIMS analysis and synchrotron X-ray techniques. These products were enriched in organic carbon, Fe and As compared with the soil and soil solution. The NanoSIMS analysis showed a colocalization of As and Fe but also revealed the presence of As hotspots where As was either associated with Fe or organic matter (OM). X-ray absorption spectroscopy (XAS) showed that As was sorbed to Fe(III)-oxyhydroxides. The linear combination fitting (LCF) of the As K-edge XANES revealed that As was not totally oxidized (i.e. between 65 and 100% of As(V)). Shell-by-shell fits of the As K-edge EXAFS showed that As formed binuclear edge-sharing 2E (RAs-Fe =2.74–2.95Å) and corner-sharing 2C (RAs-Fe =3.28–3.43Å) complexes with Fe. In addition to study of natural samples, oxidized reference samples were analyzed and demonstrated the role of OM on As speciation. The persistence of As(III) was explained by OM control on the As carrying phase during the oxidation, via the formation of nano-lepidocrocite and small Fe-clusters bound to OM. The small size of the Fe phase led to an increased capacity for As adsorption and an increase in 2E sites compared to 2C active sites for As(III) oxidation.


      PubDate: 2016-07-05T14:49:38Z
       
  • Fe(II) sorption on pyrophyllite: Effect of structural Fe(III) (impurity)
           in pyrophyllite on nature of layered double hydroxide (LDH) secondary
           mineral formation
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Autumn N. Starcher, Wei Li, Ravi K. Kukkadapu, Evert J. Elzinga, Donald L. Sparks
      Fe(II)-Al(III)-LDH (layered double hydroxide) phases have been shown to form from reactions of aqueous Fe(II) with Fe-free Al-bearing minerals (phyllosilicate/clays and Al-oxides). To our knowledge, however, the effect of small amounts of structural Fe(III) in natural clays on such reactions were not studied. In this study to understand the role of structural Fe(III) in clay, laboratory batch studies with pyrophyllite (10g/L), an Al-bearing phyllosilicate, containing small amounts of structural Fe(III) and 0.8mM and 3mM Fe(II) (both natural and enriched in 57Fe) were carried out at pH7.5 under anaerobic conditions (4% H2–96% N2 atmosphere). Samples were taken up to 4weeks for analysis by Fe-X-ray absorption spectroscopy and 57Fe Mössbauer spectroscopy. In addition to the precipitation of Fe(II)-Al(III)-LDH phases as observed in earlier studies with pure minerals (no Fe(III) impurities in the minerals), the analyses indicated the formation of small amounts of Fe(III) containing solids, most probably a hybrid Fe(II)-Al(III)/Fe(III)-LDH phase. The mechanism of Fe(II) oxidation was not apparent but most likely was due to either interfacial electron transfer from the spiked Fe(II) to the structural Fe(III) and/or surface-sorption-induced electron-transfer from the sorbed Fe(II) to the clay lattice. This research provides evidence for the formation of both Fe(II)-Al(III)-LDH and Fe(II)-Fe(III)/Al(III)-LDH-like phases during reactions of Fe(II) in systems that mimic the natural environments. Better understanding Fe phase formation in complex laboratory studies will improve models of natural redox systems.


      PubDate: 2016-07-05T14:49:38Z
       
  • Divalent metal cation adsorption onto Leptothrix cholodnii SP-6SL
           bacterial cells
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Ryan M. Nell, Jennifer E.S. Szymanowski, Jeremy B. Fein
      Bacteriogenic iron oxide (BIOS) particles have a high affinity to adsorb inorganic nutrients such as Ca, Mg, Zn, and Ni, raising the question of how Fe(II) oxidizing bacteria (FeOB) compete for these same cations while the BIOS are in such close proximity to the cells. Answering this question requires a detailed understanding of metal binding properties of FeOB. In this study, the adsorption behaviors of aqueous Ni, Cu, Zn, Sr, Cd, and Pb onto the sheathless FeOB Leptothrix cholodnii SP-6SL were measured separately, and the adsorption behaviors were compared to that of Bacillus subtilis in order to determine if the iron oxidizing bacterial species exhibits similar binding properties to those of most previously studied bacterial species. The experiments for both species were performed aerobically; ionic strength was held constant with 0.1M NaClO4; the experiments were conducted as a function of pH over the range of 2 to 9; and biomass and metal concentrations were 10g (wet weight)/L and 2ppm, respectively. Our results show that the two studied bacterial species exhibit similar adsorption of the tested metals, and that although some iron oxides formed during L. cholodnii SP-6SL liquid culture growth, they are present in low enough concentrations not to significantly affect the extent of metal adsorption. We apply a linear free-energy approach to define relationships between the stability constants for site-specific metal-bacterial surface complexes and corresponding metal-acetate binding constants. We use these relationships to estimate binding constants for L. cholodnii for some metals that have not been studied, and we apply these results to calculate the ability of L. cholodnii to compete for nutrients with BIOS particles that form during Fe(II) oxidation by the bacteria.


      PubDate: 2016-07-05T14:49:38Z
       
  • Extreme differences in 87Sr/86Sr between Samoan lavas and the magmatic
           olivines they host: Evidence for highly heterogeneous 87Sr/86Sr in the
           magmatic plumbing system sourcing a single lava
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): A.A. Reinhard, M.G. Jackson, J. Harvey, C. Brown, J.M. Koornneef
      Investigations of mantle heterogeneity in ocean island basalts (OIB) frequently compare heavy radiogenic isotopes (i.e. 87Sr/86Sr), often measured in whole rock powders, with 3He/4He and δ18O, commonly measured in olivines. However, the 87Sr/86Sr in the olivines, which is dominated by Sr in melt inclusions, may not be in equilibrium with the 87Sr/86Sr in the whole rock. Here we present new 87Sr/86Sr measurements made on Samoan magmatic olivines, where multiple olivine crystals are aggregated for a single isotopic measurement. The olivines host abundant melt inclusions, and yielded relatively large quantities of Sr (13.0 to 100.6ng) in 19 to 185mg aliquots of fresh olivine, yielding high Srsample/Srblank ratios (≥427). These new data on olivines show that samples can exhibit significant 87Sr/86Sr disequilibrium: in one extreme sample, where the basaltic whole rock 87Sr/86Sr (0.708901) is higher than several different aliquots of aggregate magmatic olivines (0.707385 to 0.707773), the whole rock-olivine 87Sr/86Sr disequilibrium is >1590ppm. The 87Sr/86Sr disequilibrium observed between whole rocks and bulk olivines relates to the isotopic disequilibrium between whole rocks and the average 87Sr/86Sr of the population of melt inclusions hosted in the olivines. Therefore, a population of olivines in a Samoan lava must have crystallized from (and trapped melts of) a different 87Sr/86Sr composition than the final erupted lava hosting the olivines. A primary question is how melts with different 87Sr/86Sr can exist in the same magmatic plumbing system and contribute heterogeneous 87Sr/86Sr to a lava and the magmatic olivines it hosts. We explore potential mechanisms for generating heterogeneous melts in magma chambers. The reliance, in part, of chemical geodynamic models of the relationships between isotopic systems measured in whole rocks (87Sr/86Sr) and systems measured in olivines (3He/4He and δ18O) means that whole rock-olivine Sr-isotopic disequilibrium will be important for evaluating relationship among these key isotopic tracer systems. Moving forward, it will be important to evaluate whether whole rock-olivine Sr-isotopic disequilibrium is a pervasive issue in OIB globally.


      PubDate: 2016-07-01T13:05:17Z
       
  • Editorial Board
    • Abstract: Publication date: 25 October 2016
      Source:Chemical Geology, Volume 437




      PubDate: 2016-07-01T13:05:17Z
       
  • Ancient depleted mantle as a source of boninites in the Izu-Bonin-Mariana
           arc: Evidence from Os isotopes in Cr-spinel and magnetite
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Ryoko Senda, Kenji Shimizu, Katsuhiko Suzuki
      Boninite is a volcanic rock derived from shallow melting of highly depleted hydrous mantle, fluxed with water from subducted slabs. The eruption of boninite early in the history of the Izu-Bonin-Mariana (IBM) arc (~48–45Ma), suggests generation by melting of upper mantle material that was relatively unmodified by subducted components. Thus, the boninite composition should largely reflect that of the sub-arc mantle. For better understanding of the mantle sources of nascent arc settings and the contributions of different components to arc melts, we analyzed Os isotope ratios (187Os/188Os) of bulk rocks and mineral separates (euhedral Cr-spinel from boninites and euhedral Cr-spinel/magnetite mixtures from tholeiites younger than 45Ma that erupted after boninites) from the Bonin Islands and Guam. The age-corrected (initial) Os isotope ratios of the whole-rock samples (0.1179–0.2050) were more radiogenic and variable than those of the mineral separates, possibly because of contamination with crustal materials during magma ascent or alteration after emplacement. The age-corrected Os isotope ratios of euhedral Cr-spinel in boninite from the Bonin Islands (0.1187–0.1254) and from Guam (0.1220–0.1269) are unradiogenic relative to primitive mantle, and those of the Cr-spinel/magnetite mixtures from the tholeiites from the Bonin Islands are similar to or slightly more radiogenic (0.1224–0.1382). The most depleted Os isotope ratio of the Cr-spinel from boninite yielded a model Re depletion (TRD) age of 1.4Ga, suggesting that the mantle source of the boninite experienced melt extraction prior to 1.4Ga. The source of the boninites is interpreted to be mostly highly depleted mantle with a small contribution of slab flux arising from altered oceanic crust that has radiogenic Os components, with or without contributions from components with relatively unradiogenic Os such as volcaniclastics of oceanic island basalt affiliation or very young mid-ocean ridge basalt.
      Graphical abstract image

      PubDate: 2016-07-01T13:05:17Z
       
  • Oxidative biotransformation of biotite and glauconite by alkaliphilic
           anaerobes: The effect of Fe oxidation on the weathering of phyllosilicates
           
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Daria G. Zavarzina, Natalya I. Chistyakova, Alexey V. Shapkin, Alla V. Savenko, Tatyana N. Zhilina, Vadim V. Kevbrin, Tatiana V. Alekseeva, Andrey V. Mardanov, Sergey N. Gavrilov, Andrey Yu. Bychkov
      Two alkaliphilic anaerobic bacteria, namely, the dissimilatory iron-reducer Geoalkalibacter ferrihydriticus and the fermentative hydrolytic Clostridium alkalicellulosi, along with their co-cultures, are studied to examine their ability to release Si and Fe from two main Fe-containing phyllosilicates in Earth's crust: biotite and glauсonite. The formation of magnetically ordered phase(s) within 200days of incubation was only observed in the presence of G. ferrihydriticus whether in a mono- or co-culture but not in the abiotic controls or a pure culture of C. alkalicellulosi. The co-culture of these organisms could represent a simple trophic chain in which C. alkalicellulosi decomposed microcrystalline cellulose to produce organic acids and ethanol, while G. ferrihydriticus, as we expected, utilized these products and reduces Fe(III) in phyllosilicate lattices. Unexpectedly, G. ferrihydriticus did not utilize but instead produced an additional 3mM of acetate during growth with phyllosilicates. An analysis of the Mössbauer spectra of biotite and glauconite that were weathered in the presence of G. ferrihydriticus revealed magnetically ordered phases that formed by Fe2+ oxidation rather than by Fe3+ reduction. The only possible explanation of this phenomenon could be in the ability of G. ferrihydriticus to produce acetate during anaerobic Fe2+ oxidation with carbonate as an electron acceptor. Thermodynamic calculations show the possibility of such a reaction. Thus, microorganisms with respiratory metabolism could play an active role in the bioweathering of phyllosilicates under alkaline anaerobic conditions. The bacterial anaerobic oxidation of ferrous iron with carbonate as an electron donor is supposed to have played a significant role in ancient environments, serving as one of the causes of banded iron formations.
      Graphical abstract image

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


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


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

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


      PubDate: 2016-06-22T21:05:18Z
       
  • Sr isotopic compositions of the interstitial water and carbonate from two
           basins in the Gulf of Mexico: Implications for fluid flow and origin
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Yan-Ping Li, Shao-Yong Jiang
      Strontium isotopic compositions of the interstitial water and carbonate from marine sediments sampled during IODP 308, in two basins (the normally-pressured Brazos-Trinity Basin IV and the over-pressured Ursa Basin) on the northern slope of the Gulf of Mexico, are present in this study. In the Brazos-Trinity Basin IV, the 87Sr/86Sr ratios of the interstitial water range from 0.70917 to 0.70954, with carbonates sharing similar or slightly lower values from 0.70851 to 0.70952. The interstitial water above 31m shows similar 87Sr/86Sr ratio close to that of seawater, whereas the interstitial water below 31m shows more radiogenic 87Sr/86Sr ratios, possibly indicating a water/rock interaction between the fluid and silicate component in the deep basin sediments. The Sr-isotope ratios of the carbonates are less radiogenic than the seawater, which may reflect a terrestrial carbonate input (such as limestone) transported through the Brazos and Trinity rivers. In the Ursa Basin, the 87Sr/86Sr ratios of the interstitial water range from 0.70887 to 0.70999, those of the carbonate vary from 0.70808 to 0.70930. Both the interstitial water and carbonate show a similar trend of 87Sr/86Sr ratios throughout the depth. The 87Sr/86Sr ratios of the interstitial water decrease from the sea floor surface to the minimum at the Seismic Reflector S10. A lateral fluid incursion with less radiogenic 87Sr/86Sr ratios along the Seismic Reflector S10 can be inferred in the Ursa Basin. It is suggested that the lateral fluid incursion shows a seawater origin, modified by diagenetic reactions including the dissolution of halite. Between the Seismic Reflector S10 and S40, 87Sr/86Sr ratios of the interstitial water increase linearly to the maximum. Along the Seismic Reflector S40, a lateral fluid incursion with more radiogenic 87Sr/86Sr ratios is again recognized. The fluid may have also originated from seawater, but modified by the diagenesis of terrigenous sediments, likewise characterized by highly radiogenic 87Sr/86Sr compositions. A two-dimension fluid-flow model in the Ursa Basin is established.


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


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


      PubDate: 2016-06-17T18:00:47Z
       
  • Factors affecting B/Ca ratios in synthetic aragonite
    • Abstract: Publication date: 25 October 2016
      Source:Chemical Geology, Volume 437
      Author(s): M. Holcomb, T.M. DeCarlo, G.A. Gaetani, M. McCulloch
      Measurements of B/Ca ratios in marine carbonates have been suggested to record seawater carbonate chemistry, however experimental calibration of such proxies based on inorganic partitioning remains limited. Here we conducted a series of synthetic aragonite precipitation experiments to evaluate the factors influencing the partitioning of B/Ca between aragonite and seawater. Our results indicate that the B/Ca ratio of synthetic aragonites depends primarily on the relative concentrations of borate and carbonate ions in the solution from which the aragonite precipitates; not on bicarbonate concentration as has been previously suggested. The influence of temperature was not significant over the range investigated (20–40°C), however, partitioning may be influenced by saturation state (and/or growth rate). Based on our experimental results, we suggest that aragonite B/Ca ratios can be utilized as a proxy of [CO3 2–]. Boron isotopic composition (δ11B) is an established pH proxy, thus B/Ca and δ11B together allow the full carbonate chemistry of the solution from which the aragonite precipitated to be calculated. To the extent that aragonite precipitation by marine organisms is affected by seawater chemistry, B/Ca may also prove useful in reconstructing seawater chemistry. A simplified boron purification protocol based on amberlite resin and the organic buffer TRIS is also described.


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


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

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


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


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


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

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

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


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

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


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


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


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


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


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

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


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


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


      PubDate: 2016-06-13T02:29:05Z
       
 
 
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