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  Subjects -> EARTH SCIENCES (Total: 644 journals)
    - EARTH SCIENCES (467 journals)
    - GEOLOGY (69 journals)
    - GEOPHYSICS (26 journals)
    - HYDROLOGY (21 journals)
    - OCEANOGRAPHY (61 journals)

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: 6)
Aeolian Research     Hybrid Journal   (Followers: 5)
African Journal of Aquatic Science     Hybrid Journal   (Followers: 12)
Algological Studies     Full-text available via subscription   (Followers: 2)
Alpine Botany     Hybrid Journal   (Followers: 3)
AMBIO     Hybrid Journal   (Followers: 12)
Anadolu University Journal of Science and Technology     Open Access  
Anales del Instituto de la Patagonia     Open Access   (Followers: 1)
Andean geology     Open Access   (Followers: 3)
Annales Henri Poincaré     Hybrid Journal   (Followers: 3)
Annales UMCS, Geographia, Geologia, Mineralogia et Petrographia     Open Access  
Annals of Geophysics     Full-text available via subscription   (Followers: 8)
Annals of GIS     Hybrid Journal   (Followers: 14)
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: 13)
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: 9)
Artificial Satellites : The Journal of Space Research Centre of Polish Academy of Sciences     Open Access   (Followers: 14)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 6)
Asian Journal of Earth Sciences     Open Access   (Followers: 19)
Asian Review of Environmental and Earth Sciences     Open Access   (Followers: 1)
Atlantic Geology : Journal of the Atlantic Geoscience Society / Atlantic Geology : revue de la Société Géoscientifique de l'Atlantique     Full-text available via subscription   (Followers: 1)
Atmosphere-Ocean     Full-text available via subscription   (Followers: 11)
Atmospheric and Climate Sciences     Open Access   (Followers: 22)
Australian Journal of Earth Sciences: An International Geoscience Journal of the Geological Society of Australia     Hybrid Journal   (Followers: 11)
Boletim de Ciências Geodésicas     Open Access  
Boreas: An International Journal of Quaternary Research     Hybrid Journal   (Followers: 10)
Bragantia     Open Access   (Followers: 2)
Bulletin of Earthquake Engineering     Hybrid Journal   (Followers: 10)
Bulletin of Geosciences     Open Access   (Followers: 9)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (Followers: 1)
Bulletin of the Seismological Society of America     Full-text available via subscription   (Followers: 19)
Bulletin of Volcanology     Hybrid Journal   (Followers: 16)
Cadernos de Geociências     Open Access  
Canadian Journal of Plant Science     Full-text available via subscription   (Followers: 14)
Canadian Mineralogist     Full-text available via subscription   (Followers: 3)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 17)
Carbonates and Evaporites     Hybrid Journal   (Followers: 3)
CATENA     Hybrid Journal   (Followers: 4)
Chemical Geology     Hybrid Journal   (Followers: 13)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 4)
Chinese Geographical Science     Hybrid Journal   (Followers: 5)
Chinese Journal of Geochemistry     Hybrid Journal   (Followers: 3)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 2)
Ciencias Espaciales     Open Access  
Climate and Development     Hybrid Journal   (Followers: 12)
Coastal Management     Hybrid Journal   (Followers: 23)
Cogent Geoscience     Open Access  
Comptes Rendus Geoscience     Full-text available via subscription   (Followers: 7)
Computational Geosciences     Hybrid Journal   (Followers: 12)
Computational Mathematics and Mathematical Physics     Hybrid Journal   (Followers: 1)
Computers and Geotechnics     Hybrid Journal   (Followers: 8)
Contemporary Trends in Geoscience     Open Access   (Followers: 2)
Continental Shelf Research     Hybrid Journal   (Followers: 8)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 9)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 1)
Coral Reefs     Hybrid Journal   (Followers: 16)
Cretaceous Research     Hybrid Journal   (Followers: 6)
Cybergeo : European Journal of Geography     Open Access   (Followers: 3)
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: 8)
E&S Engineering and Science     Open Access  
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (Followers: 65)
Earth and Space Science     Open Access   (Followers: 5)
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: 18)
Earth System Dynamics     Open Access   (Followers: 5)
Earth System Dynamics Discussions     Open Access   (Followers: 4)
Earth's Future     Open Access  
Earth, Planets and Space     Open Access   (Followers: 5)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 6)
Earthquake Science     Hybrid Journal   (Followers: 9)
Earthquake Spectra     Full-text available via subscription   (Followers: 16)
Ecohydrology     Hybrid Journal   (Followers: 9)
Ecological Questions     Open Access   (Followers: 5)
Electromagnetics     Hybrid Journal   (Followers: 2)
Energy Efficiency     Hybrid Journal   (Followers: 11)
Energy Exploration & Exploitation     Full-text available via subscription   (Followers: 4)
Environmental Earth Sciences     Hybrid Journal   (Followers: 17)
Environmental Geology     Hybrid Journal   (Followers: 9)
Environmental Geosciences     Full-text available via subscription   (Followers: 4)
Environmental Geotechnics     Open Access   (Followers: 3)
Erwerbs-Obstbau     Hybrid Journal  
Estuaries and Coasts     Hybrid Journal   (Followers: 16)
Estuarine, Coastal and Shelf Science     Hybrid Journal   (Followers: 30)
Estudios Geográficos     Open Access  
European Journal of Mineralogy     Full-text available via subscription   (Followers: 10)
Exploration Geophysics     Hybrid Journal   (Followers: 3)
Facies     Hybrid Journal   (Followers: 8)
Fieldiana Life and Earth Sciences     Full-text available via subscription   (Followers: 1)
Física de la Tierra     Open Access  
Folia Musei rerum naturalium Bohemiae occidentalis. Geologica et Paleobiologica     Open Access  
Folia Quaternaria     Open Access  
Forestry Chronicle     Full-text available via subscription   (Followers: 10)
Frontiers in Earth Science     Open Access   (Followers: 4)
Frontiers in Geotechnical Engineering     Open Access   (Followers: 2)
Frontiers of Earth Science     Hybrid Journal   (Followers: 8)
Fundamental and Applied Limnology / Archiv für Hydrobiologie     Full-text available via subscription   (Followers: 4)
GEM - International Journal on Geomathematics     Hybrid Journal   (Followers: 1)
Geo-Marine Letters     Hybrid Journal   (Followers: 7)
Geoacta     Open Access   (Followers: 4)
Geobiology     Hybrid Journal   (Followers: 7)
Geocarto International     Hybrid Journal   (Followers: 3)
Geochemistry : Exploration, Environment, Analysis     Hybrid Journal   (Followers: 6)
Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 23)
Geochimica et Cosmochimica Acta     Hybrid Journal   (Followers: 22)
Geochronometria     Hybrid Journal   (Followers: 3)
Geoderma Regional : The International Journal for Regional Soil Research     Full-text available via subscription   (Followers: 3)
Geodinamica Acta     Hybrid Journal   (Followers: 3)
Geodynamics & Tectonophysics     Open Access  
Geoenvironmental Disasters     Open Access   (Followers: 3)
Geofluids     Hybrid Journal   (Followers: 4)
Geoforum     Hybrid Journal   (Followers: 19)
Géographie physique et Quaternaire     Full-text available via subscription  
Geography and Natural Resources     Hybrid Journal   (Followers: 4)
Geoheritage     Hybrid Journal   (Followers: 1)
Geoinformatica Polonica : The Journal of Polish Academy of Arts and Sciences     Open Access  
Geoinformatics & Geostatistics     Hybrid Journal   (Followers: 3)
Geological Journal     Hybrid Journal   (Followers: 13)
Geological Magazine     Hybrid Journal   (Followers: 15)
Geology Today     Hybrid Journal   (Followers: 17)
Geomagnetism and Aeronomy     Hybrid Journal   (Followers: 2)
Geomatics, Natural Hazards and Risk     Hybrid Journal   (Followers: 6)
GEOmedia     Open Access   (Followers: 1)
Geomorphology     Hybrid Journal   (Followers: 22)
Geophysical & Astrophysical Fluid Dynamics     Hybrid Journal   (Followers: 2)
Geophysical Journal International     Hybrid Journal   (Followers: 28)
Geophysical Prospecting     Hybrid Journal   (Followers: 7)
Geophysics     Full-text available via subscription   (Followers: 17)
GeoResJ     Hybrid Journal  
Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards     Hybrid Journal   (Followers: 6)
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  
Geosciences     Open Access   (Followers: 2)
Geosciences Journal     Hybrid Journal   (Followers: 9)
Geoscientific Instrumentation, Methods and Data Systems     Open Access   (Followers: 2)
Geoscientific Model Development     Open Access   (Followers: 2)
Geostandards and Geoanalytical Research     Hybrid Journal   (Followers: 2)
Geosystem Engineering     Hybrid Journal  
Geotectonic Research     Full-text available via subscription   (Followers: 4)
Geotectonics     Hybrid Journal   (Followers: 6)
GFF     Hybrid Journal  
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: 18)
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: 18)
Hydrology and Earth System Sciences     Open Access   (Followers: 19)
ICES Journal of Marine Science: Journal du Conseil     Hybrid Journal   (Followers: 51)
IEEE Journal of Oceanic Engineering     Hybrid Journal   (Followers: 10)
Indian Geotechnical Journal     Hybrid Journal   (Followers: 2)
Indonesian Journal on Geoscience     Open Access   (Followers: 2)
Interdisciplinary Environmental Review     Hybrid Journal   (Followers: 3)
International Geology Review     Hybrid Journal   (Followers: 5)
International Journal of Advanced Geosciences     Open Access  
International Journal of Advanced Remote Sensing and GIS     Open Access   (Followers: 13)
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: 11)
International Journal of Applied Earth Observation and Geoinformation     Hybrid Journal   (Followers: 9)
International Journal of Coal Geology     Hybrid Journal   (Followers: 2)
International Journal of Disaster Risk Reduction     Hybrid Journal   (Followers: 8)
International Journal of Earth Sciences     Hybrid Journal   (Followers: 26)
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: 42)

        1 2 3 | Last

Journal Cover Chemical Geology
  [SJR: 1.927]   [H-I: 123]   [13 followers]  Follow
    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 0009-2541
   Published by Elsevier Homepage  [2970 journals]
  • Precise titanium isotope compositions of refractory inclusions in the
           Allende CV3 chondrite by LA-MC-ICPMS
    • Abstract: Publication date: 15 October 2016
      Source:Chemical Geology, Volume 436
      Author(s): C.D. Williams, P.E. Janney, R.R. Hines, M. Wadhwa
      We present here analyses of mass-independent effects in the Ti isotope ratios of 17 coarse-grained (compact Type A and Type B) calcium–aluminum-rich inclusions (CAIs) from the Allende CV3 chondrite utilizing a method developed for rapid measurement of Ti isotopes by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS). Based on the analyses of the synthetic CMAS glasses doped with varying amounts of Ca, Cr and V, an empirical scheme for correction of potential isobaric interferences and matrix effects was developed. Using this scheme, mass-independent variations of the Ti isotope ratios 46Ti/47Ti, 48Ti/47Ti and 50Ti/47Ti were measured with external reproducibilities (2SD) of ±0.4, ±0.5, and ±1.8 for ε46Ti, ε48Ti, and ε50Ti, respectively, based on repeat measurements of standard glasses. The Ti isotope compositions of the 17 Allende refractory inclusions analyzed here show that most of these CAIs are “normal” with limited variation in their mass-independent Ti isotope composition. One CAI (designated as CMS-1) has a significantly larger mass-independent ε50Ti anomaly and is identified as having Fractionation and Unidentified Nuclear (FUN) effects. The limited range in the Ti isotope compositions of normal CAIs (ε50Ti excesses ranging from 2.9 to 11.4) suggests that they originated from a nebular reservoir that was relatively well-mixed, although not completely homogenized (at the level of precision of the analyses reported here), in its isotopic composition. The distinctive isotopic composition of the FUN CAI CMS-1 indicates that it formed from a reservoir in the protoplanetary disk that was spatially or temporally distinct from that from which the normal CAIs were formed.


      PubDate: 2016-05-14T18:03:00Z
       
  • Editorial Board
    • Abstract: Publication date: 1 September 2016
      Source:Chemical Geology, Volume 433




      PubDate: 2016-05-14T18:03:00Z
       
  • Temporal and compositional evolution of Jorullo volcano, Mexico:
           Implications for magmatic processes associated with a monogenetic eruption
           
    • Abstract: Publication date: 15 September 2016
      Source:Chemical Geology, Volume 434
      Author(s): Christine Rasoazanamparany, Elisabeth Widom, Claus Siebe, M.-N. Guilbaud, M.J. Spicuzza, J.W. Valley, G. Valdez, S. Salinas
      The 1759–1774 eruption of the Jorullo volcano in the Michoacán Guanajuato Volcanic Field (MGVF), Mexico, produced lavas that range in composition from basalt to basaltic andesite. We have conducted new major and trace element and isotopic studies (whole rock Sr–Nd–Pb–Hf–Os, and O isotopes in olivine separates) of the Jorullo lavas and tephras spanning the duration and compositional range of the eruption, to further constrain the potential roles of mantle source heterogeneity, subduction-related metasomatism, and crustal assimilation in the petrogenesis of the Jorullo magmas. This study presents the first Hf, high precision Pb and comprehensive oxygen isotope measurements for Jorullo volcanic rocks. All samples have arc-like trace element patterns with enrichments in large ion lithophile elements (e.g. Ba, Rb, and Pb) and depletions in fluid immobile elements (e.g. Nb, Ta). In addition, the samples show variations in 87Sr/86Sr (0.7038–0.7040), 143Nd/144Nd (0.51280–0.51285), 176Hf/177Hf (0.28297–0.28300), 206Pb/204Pb (18.62–18.66), 207Pb/204Pb (15.57–15.59) and 208Pb/204Pb (38.34–38.43). Osmium isotope signatures are, with one exception, more radiogenic than the depleted and primitive mantle (187Os/188Os=0.1231–0.1616). Oxygen isotope ratios of olivine phenocrysts (δ18OSMOW =5.70–6.02‰) show limited variation, but are isotopically heavier than normal mantle olivine. The samples define two geochemical groups: high-MgO samples with higher 87Sr/86Sr, lower 143Nd/144Nd and 176Hf/177Hf, and a positive correlation of Sr and Pb isotopes; and low-MgO samples displaying lower 87Sr/86Sr but higher 143Nd/144Nd and 176Hf/177Hf than the former group, and a negative correlation of Sr and Pb isotopes. The high-MgO group comprises most of the early tephra and lavas, whereas the low-MgO group includes most of the late tephra and lavas. These compositional variations are inconsistent with shallow level contamination, but rather are interpreted to reflect mantle source heterogeneity. Trace element and isotopic signatures are consistent with North Mexican Extensional Province (NMEP) mantle metasomatised by subduction components composed of sediment- and oceanic crust-derived hydrous fluid. The temporal–compositional variations observed in Jorullo magmas are inferred to result from a combination of variable degrees of fractional crystallization of magmas produced by tapping a progressively less metasomatised mantle source that is vertically and/or laterally heterogeneous.


      PubDate: 2016-05-14T18:03:00Z
       
  • Combining soil water balance and clumped isotopes to understand the nature
           and timing of pedogenic carbonate formation
    • Abstract: Publication date: 1 October 2016
      Source:Chemical Geology, Volume 435
      Author(s): Timothy M. Gallagher, Nathan D. Sheldon
      Pedogenic carbonate is an important archive for paleoclimate, paleoecology, and paleoelevation studies. However, it can form under seasonal environmental conditions that differ significantly from the mean growing season environment or mean annual conditions, potentially complicating its use for proxy reconstructions. The observed seasonal temperature is typically, but not always, biased high relative to mean annual air temperature (MAT). To evaluate the annual timing of pedogenic carbonate formation, ten different soils were sampled across the western United States. Sites were selected to span a variety of precipitation regimes and soil orders. Precipitation regimes ranged from arid sites (mean annual precipitation (MAP) <20cm) that receive the majority of precipitation during the winter to wetter sites (MAP >50cm) dominated by summer precipitation. Pedogenic carbonate formation temperatures derived from clumped isotope measurements ranged between 6 and 22°C, with most samples falling at or below MAT. Clumped isotope temperatures were compared to monthly precipitation normals and modeled monthly values of evapotranspiration and soil water content. Results show that carbonate formation temperatures agree with the annual timing of soil water depletion, suggesting soil moisture content is a primary control on the timing of pedogenic carbonate formation. Although the seasonal bias is a function of environmental factors that are difficult to reconstruct in paleo-studies, the use of other paleosol proxies can help to assess if changes in clumped isotope temperatures are a function of changes in air temperature or hydrology. These results have important implications for the production of accurate paleoclimate and paleoelevation estimates.


      PubDate: 2016-05-14T18:03:00Z
       
  • Assessment of coral δ44/40Ca as a paleoclimate proxy in the Great
           Barrier Reef of Australia
    • Abstract: Publication date: 1 October 2016
      Source:Chemical Geology, Volume 435
      Author(s): Xuefei Chen, Wenfeng Deng, Hongli Zhu, Zhaofeng Zhang, Gangjian Wei, Malcolm T. McCulloch
      The Ca isotopic composition (δ44/40Ca) in a Porites spp. coral from the Great Barrier Reef was analyzed at monthly intervals for two consecutive years. It was found that variations in skeletal δ44/40Ca values over the 2-year period are slightly greater than the analytical precision of the measurements, although other coralline geochemical records (i.e., δ13C, δ18O, δ11B and Sr/Ca ratios) show remarkable variations. To evaluate the potential of δ44/40Ca as a paleoclimate proxy in corals, we compared δ44/40Ca with other well-established indicators, and found that δ44/40Ca values show little relationship to δ18O, Sr/Ca, Δδ18O, or δ11B values, thus suggesting that the influence of seawater temperature, seawater pH and river inputs on skeletal δ44/40Ca is limited or overwhelmed by other factors. However, skeletal δ44/40Ca values are significantly related to δ13C values (r=0.46, p <0.05; n=27), thus indicating that Ca isotopic fractionation in corals is subjected to vital effect. The strong influence of biogenic factors on δ44/40Ca in corals, limit the applicability of coral δ44/40Ca to paleoclimate reconstruction.


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


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


      PubDate: 2016-05-08T09:18:17Z
       
  • Constraining porewater chemistry in a 250m thick argillaceous rock
           sequence
    • Abstract: Publication date: 15 September 2016
      Source:Chemical Geology, Volume 434
      Author(s): Paul Wersin, Martin Mazurek, Urs K. Mäder, Thomas Gimmi, Daniel Rufer, Catherine Lerouge, Daniel Traber
      The geochemistry of an argillaceous rock sequence from a deep borehole in NE-Switzerland was investigated. The focus was to constrain the porewater chemistry in low permeability Jurassic rocks comprising the Liassic, the Opalinus Clay formation, the ‘Brown Dogger’ unit and the Effingen Member (Malm). A multi-method approach including mineralogical analysis, aqueous and Ni-ethylenediamine extraction, squeezing tests and pCO2 measurements as well as geochemical modelling was applied for this purpose. A consistent dataset was obtained with regard to the main solutes in the porewaters. A fairly constant anion-accessible porosity of ~50% of the total porosity was deduced for all analysed samples which displayed variable clay-mineral contents. Sulphate concentrations were shown to be constrained by a sulphate-bearing phase, presumably by celestite or a Sr–Ba sulphate. Application of a simple equilibrium model, including cation exchange reactions, calcite and celestite equilibrium showed good agreement with squeezing data, indicating the suitability of the modelling approach to simulate porewater chemistry in the studied argillaceous rocks. The modelling highlighted the importance of correct determination of the exchangeable cation population. The analysis corroborates that squeezing of the studied rocks is a viable and efficient way to sample porewater.


      PubDate: 2016-05-08T09:18:17Z
       
  • Iron mineralogy across the oxycline of a lignite mine lake
    • Abstract: Publication date: 15 September 2016
      Source:Chemical Geology, Volume 434
      Author(s): Jennyfer Miot, Shipeng Lu, Guillaume Morin, Areej Adra, Karim Benzerara, Kirsten Küsel
      Iron-rich pelagic aggregates of microbial origin named “iron snow” are formed in the water column of some acidic lignite mine lakes. We investigated the evolution of Fe mineralogy across the oxycline of the Lusatian lake 77, Germany at two sampling sites differing by their pH and mixing profiles. The central basin (CB) of this lake shows a dimictic water regime with a non-permanent anoxic deep layer and a homogeneous acidic pH all over the water column (pH3). In contrast, the northern basin (NB) is meromictic with a permanently anoxic bottom layer and a pH increase from pH3 in the mixolimnion (superficial part of the lake) to pH5.5 in the monimolimnion (anoxic bottom layer). Fe minerals above and below the oxycline were identified using X-ray Absorption Spectroscopy (XAS) at the Fe K-edge and further characterized down to the atomic scale by High Resolution Transmission Electron Microscopy (HRTEM) and Scanning Transmission Electron Microscopy (STEM) coupled to Energy Dispersive X-ray Spectroscopy (EDXS). We explored local Fe redox state and C speciation using Scanning Transmission X-ray Microscopy (STXM) at the Fe L2,3-edges and C K-edge. Schwertmannite [Fe8O8(OH)8-2x(SO4)x] identified as the sole Fe mineral in CB, was polycrystalline, consisting in the aggregation of nanodomains of 2–3nm each one exhibiting the crystal structure of schwertmannite. In contrast, schwertmannite was partly (40%) converted to aluminous ferrihydrite when reaching the oxycline in NB. This mineralogical transformation was most probably due to a combination of abiotic and microbial anaerobic processes promoting pH increase and release of Fe(II) (e.g. via heterotrophic Fe(III) reduction) that induce the catalytic hydrolysis of schwertmannite to ferrihydrite. Mineral products were stabilized in the monimolimnion by the adsorption of aluminum, silicate and organic matter. Noteworthy, local Fe redox state heterogeneities were observed, with a few areas enriched in Fe(II) as evidenced by STXM analyses at the Fe L2,3-edges. These local redox heterogeneities could arise from microbial activity (e.g. Fe(III) and/or sulfate reduction). All these results provide an in-depth mineralogical overview of iron phases forming in lake 77 as a basis for future investigations of microbial vs. abiotic parameters controlling their stability and transformation.
      Graphical abstract image

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

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


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


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


      PubDate: 2016-05-08T09:18:17Z
       
  • Editorial Board
    • Abstract: Publication date: 15 August 2016
      Source:Chemical Geology, Volume 432




      PubDate: 2016-05-03T07:47:02Z
       
  • Surface complexation modeling of U(VI) adsorption on granite at
           ambient/elevated temperature: Experimental and XPS study
    • Abstract: Publication date: 1 September 2016
      Source:Chemical Geology, Volume 433
      Author(s): Qiang Jin, Lin Su, Gilles Montavon, Yufeng Sun, Zongyuan Chen, Zhijun Guo, Wangsuo Wu
      The Beishan granitic formation is being investigated as a potential host rock for a high-level radioactive waste repository in China. It is important to understand the retention processes, including influential parameters such as the metal ion concentration, pH, ionic strength (I) and temperature. The present study deals with U(VI) adsorption on Beishan granite using batch-type experiments in a CO2-free atmosphere. U(VI) adsorption on granite is shown to be insensitive to ionic strength. Temperature has a positive effect on U(VI) adsorption indicating that surface reactions are endothermic. Combining X-ray photoelectron spectroscopy (XPS) analysis and adsorption data at 25°C, a Generalized Composite (GC) model with three surface complexes, SOUO2 +, SO(UO2)2(OH)2 + and SO(UO2)3(OH)5, was constructed. The experimental data at 40°C and 60°C were fitted by the proposed model to obtain the equilibrium constants (K) of the surface reactions at these two temperatures. The enthalpy changes (ΔH) of the surface reactions were evaluated from the equilibrium constants obtained at three temperatures via the van't Hoff equation. Finally, blind modeling predictions were performed to test the robustness of the proposed model and ΔH. Satisfactory agreement with the literature data confirmed that this GC model with ΔH proving a useful tool to predict U(VI) adsorption on granite samples, especially on Beishan granite at ambient/high temperature.


      PubDate: 2016-05-03T07:47:02Z
       
  • Volcanic plume fingerprint in the groundwater of a persistently degassing
           basaltic volcano: Mt. Etna
    • Abstract: Publication date: 1 September 2016
      Source:Chemical Geology, Volume 433
      Author(s): M. Liotta, W. D'Alessandro, S. Bellomo, L. Brusca
      The chemical composition of the groundwater at Mt. Etna was investigated in order to determine the extent to which the persistent plume affects the chemical composition of circulating waters. Samples from 31 springs and wells were collected during June and July 2014 and analyzed for their chemical compositions. The content of dissolved elements derives from the bulk deposition (wet and dry deposition) at the recharge areas as well as from the weathering of volcanic rocks during the infiltration and transport of groundwater. In its early phase, the chemical weathering of volcanic rocks and ashes is promoted by the acid rain that characterizes the area and subsequently by the huge amount of deep magmatic carbon dioxide (CO2) coming up through the volcanic edifice and dissolving in the water. The high content of chlorine is mainly derived from interactions between the plume and rainwater, while the total alkalinity can be completely ascribed to the dissociation of carbonic acid (H2CO3) after the hydration of CO2. The relative contributions of plume-derived elements/weathering and CO2-driven weathering has been computed for each element. In addition, the comparison between the chemical compositions of the bulk deposition and of groundwater provides a new understanding about the mobility of volatile elements. The proposed approach has revealed that the persistent plume strongly affects the chemical composition of groundwater at Mt. Etna and probably also at other volcanoes characterized by huge open-conduit degassing activity.
      Graphical abstract image

      PubDate: 2016-05-03T07:47:02Z
       
  • A record of uranium-series transport at Nopal I, Sierra Peña Blanca,
           Mexico: Implications for natural uranium deposits and radioactive waste
           repositories
    • Abstract: Publication date: 15 September 2016
      Source:Chemical Geology, Volume 434
      Author(s): J.S. Denton, S.J. Goldstein, P. Paviet, A.J. Nunn, R.S. Amato, K.A. Hinrichs
      Studies of uranium-series (U-series) disequilibria within and around ore deposits provide valuable information on the extent and timing of actinide mobility, via mineral-fluid interaction, over a range of spatial and temporal scales. Such information is useful in studies of analogs of high-level nuclear-waste repositories, as well as for mining and mineral extraction sites, locations of previous nuclear weapons testing, and legacy nuclear waste contamination. In this study we present isotope dilution mass spectrometry U-series measurements for fracture-fill materials (hematite, goethite, kaolinite, calcite, dolomite and quartz) from one such analog; the Nopal I uranium ore deposit situated at Peña Blanca in the Chihuahua region of northern Mexico. The ore deposit is located in fractured, unsaturated volcanic tuff and fracture-fill materials from surface fractures as well as fractures in a vertical drill core have been analyzed. High uranium concentrations in the fracture-fill materials (between 12 and 7700ppm) indicate uranium mobility and transport from the deposit. Uranium concentrations generally decrease with horizontal distance away from the deposit but in this deposit there is no trend with depth below the surface. Isotopic activity ratios, 238U–234U-230Th–232Th linear age trends, and U-Th-Pa model ages indicate an actinide mobility evolution that can be characterized as complex and varied. There is evidence of both open and closed system behavior for uranium, depending on both sample and time-scale. The 234U/238U activity ratios indicate U mobility from 1.2–0.2Ma. In the last 0.2Ma there is isotopic evidence for some U mobility but mostly U immobility. In almost all cases the Th‐Pa radionuclides studied here have been immobile for the past 200ka. Radium appears to have been recently (0.1–8ka) mobile due to ongoing fluid flow in the fractures. Overall, the mainly closed system behavior for U-Th-Pa over the past ~200ka provides an indicator of the geochemical immobility of these actinides over time-scales relevant for potential nuclear waste repositories situated in fractured, unsaturated tuff.


      PubDate: 2016-05-03T07:47:02Z
       
  • Ab initio thermodynamics of magnesium carbonates and hydrates in
           water-saturated supercritical CO2 and CO2-rich regions
    • Abstract: Publication date: 15 September 2016
      Source:Chemical Geology, Volume 434
      Author(s): Anne M. Chaka, Andrew R. Felmy, Odeta Qafoku
      ab initio Thermodynamics is used to determine how free energies of magnesium carbonates and hydrates in a CO2-rich environment change with water concentration across a range temperature and pressure relevant to geochemistry and carbon sequestration (275K to 375K and pCO2 of 1 to 210bar). The methodology is based on first principles density-functional theory (DFT) calculations of the total energies and vibrational entropy of periclase, magnesite, brucite, nesquehonite, and hydromagnesite coupled to the experimental chemical potentials of CO2 and H2O. The impact of water in supercritical CO2 (scCO2), even though less than 1% at saturation, is found to have a significant impact on the stability of hydrated carbonate minerals. Hydromagnesite and nesquehonite are found to be more thermodynamically stable than periclase and brucite in water-saturated scCO2 and hence may be expected to result kinetically from carbonation of these minerals during CO2 sequestration. Under dehydrating conditions nesquehonite destabilizes rapidly, whereas hydromagnesite is much more likely to persist in a CO2-rich environment, consistent with the observations that hydromagnesite is widespread in nature and nesquehonite is relatively rare.
      Graphical abstract image

      PubDate: 2016-05-03T07:47:02Z
       
  • Effect of diagenetic phase transformation on the silicon isotope
           composition of opaline sinter deposits of Geysir, Iceland
    • Abstract: Publication date: 1 September 2016
      Source:Chemical Geology, Volume 433
      Author(s): Sonja Geilert, Pieter Z. Vroon, Manfred J. van Bergen
      Detectible δ 30Si variations in present-day chemical silica deposits have stimulated the application of silicon isotopes to infer environmental conditions from ancient equivalents. Interpretations of δ 30Si signatures remain problematic in view of potential post-depositional changes, of which magnitudes and underlying mechanisms are largely unknown. A critical issue in the interpretation of isotope data from cherts concerns the extent to which early-diagenetic processes modify original δ 30Si signatures. Here, we report δ 30Si variations in opal-A, opal-A/CT and opal-CT from fossil sinter deposits in an active discharge apron in the Geysir geothermal field, Iceland. Opal-A samples show an average δ 30Si of −0.7±0.2‰, while opal-CT samples are isotopically lighter with an average δ 30Si of −2.0±0.4‰, implying a sizable shift of approximately 1.3‰ between the different phases. This shift can be explained by repetitive dissolution/re-precipitation processes, diffusive transport or temperature differences during phase transitions. On average, the fossil opal-A tends to be less negative in δ 30Si than amorphous silica that recently precipitated from the hydrothermal water. The difference can be attributed to primary variability in isotopic fractionation that accompanies precipitation out of spring water at the surface, or to a post-depositional release of surface 28Si at the onset of diagenetic formation. Our results corroborate the perception that original silicon isotope signatures of silica, acquired during chemical precipitation from a saturated solution, may not be preserved in the geological record, and that post-depositional changes must be taken into account when interpreting data from ancient chert deposits.


      PubDate: 2016-04-28T07:29:26Z
       
  • Experimental determination of stable chlorine and bromine isotope
           fractionation during precipitation of salt from a saturated solution
    • Abstract: Publication date: 1 September 2016
      Source:Chemical Geology, Volume 433
      Author(s): H.G.M. Eggenkamp, M. Bonifacie, M. Ader, P. Agrinier
      In order to better understand the chlorine and bromine stable isotope fractionation that occurs when chloride and bromide salts precipitate from their saturated solutions, we determined experimentally the equilibrium fractionation factors between precipitating pure salt minerals and their coexisting saturated brine at 22°C. Fractionation factors (expressed as 103lnα(37Cl/35Cl) salt-brine and 103lnα81Br/79Br salt-brine) obtained for 11 chloride and 7 bromide salts of geological and industrial interest show a relatively largerange of variation (from −0.31 to +0.41), with the salt that precipitates having either a lower or a higher isotope ratio than the brine from which they precipitate. A negative fractionation factor indicates that the brine has a larger isotope ratio than the precipitate, a positive factor that the precipitate has a larger isotope ratio. The results of the chlorine and bromine isotope fractionation measurements (103lnα) for the various salts at a temperature of 22±2°C are: LiCl: +0.03±0.05, NaCl: +0.35±0.08, KCl: −0.12±0.05, NH4Cl: +0.09±0.03, RbCl: −0.31±0.05, CsCl: −0.23±0.02, MgCl2: −0.02±0.02, CaCl2: +0.04±0.02, SrCl2: +0.15±0.02, BaCl2: +0.41±0.03, FeCl3: −0.23±0.05, LiBr: −0.03±0.09, NaBr: +0.07±0.05, KBr: 0.00±0.01, NH4Br: +0.11±0.05, MgBr2: +0.06±0.06, CaBr2: −0.06±0.04 and SrBr2: +0.05±0.06. In these measurements the uncertainty is defined as the 1σ standard deviation of replicate determinations. We compare the results to previous theoretical calculations based on reduced partition coefficients as well as with thermodynamic properties of precipitated salts. Though most of our experimental fractionation data qualitatively agree with theoretical predictions (e.g., larger isotope fractionation for Cl than for Br species), it remains difficult to directly and finely compare those estimations which is likely due to the complexness of the processes that take place in saturated solutions. We use the data to predict the δ37Cl evolution of the Earth's oceans' through geological time under the condition that solely isotope fractionation during evaporation and salt precipitation took place, together with erosion of evaporite deposits. Under these conditions, we calculate a δ37Cl decrease of at most 0.25‰ for the Earth's oceans during the last 1billion years. Such a trend is not observed in old evaporite deposits, in part because δ37Cl variations in most evaporite deposits are larger than the expected trend, and we conclude that δ37Cl may also be influenced by other processes such as chloride exchange with the deep Earth. We only observed very small bromine isotope fractionation during precipitation of bromide salts from brine. This small fractionation cannot explain the large deviation of δ81Br of dissolved evaporite samples from precipitation of seawater with a modern bromine isotope composition, and suggests large bromine isotope variations in the oceans through time.


      PubDate: 2016-04-28T07:29:26Z
       
  • High-precision determination of the oxidation state of komatiite lavas
           using vanadium liquid-mineral partitioning
    • Abstract: Publication date: 1 September 2016
      Source:Chemical Geology, Volume 433
      Author(s): Robert W. Nicklas, Igor S. Puchtel, Richard D. Ash
      Oxygen fugacity of the mantle (fO2) is an important intensive variable in Earth sciences that is largely unconstrained throughout Earth history. Oxygen fugacity of modern basalts is determined by examining the Fe+3/Fe+2 ratios of fresh volcanic glass, but this method cannot be applied to older lavas that have experienced post-magmatic alteration and/or metamorphism. Here, we report the newly developed analytical techniques that, using the published experimental petrology data, have enabled us to determine the fO2 of mantle-derived lavas with a precision of better than 0.10 log fO2 units. This new method uses the partitioning behavior of the redox-sensitive transition metal vanadium between olivine and/or chromite and komatiitic melt as oxybarometers. In order to obtain accurate and precise results, a series of whole-rock samples was collected across differentiated komatiitic basalt Victoria's Lava Lake in Fennoscandia. Special attention was paid to ensure that the lava contained magmatic olivine and chromite that were in equilibrium with the emplaced melt composition, and that no re-equilibration has occurred during the lava differentiation. Vanadium and other transition metal abundances in the whole-rock samples were determined using standard addition solution ICP-MS technique with a precision of better than 5% (2SD), and in liquidus olivine and chromite by laser ablation ICP-MS, with a precision of better than 5% (2SE). The MgO and transition metal abundances in the emplaced komatiitic basalt lava were precisely calculated using several independent approaches; these approaches provided consistent results that agreed within the uncertainty of the method. The partition coefficients D V Ol, Chr-Liq were then calculated and used to determine the fO2 of the lava lake to be −0.24±0.04 and −0.21±0.03 ΔNNO log units, respectively (2SE). The two independent estimates are identical within the respective uncertainties and attest to the accuracy of the method; the average value of −0.22±0.04 ΔNNO log units (2SD) represents our best estimate for the redox state of the lava lake. Corrected for 4% crustal contamination, the redox state of the original komatiite is calculated to be −0.29±0.04 ΔNNO log units. The new method provides a high-resolution tool for constraining the evolution of the redox state of the mantle over Earth's history.


      PubDate: 2016-04-28T07:29:26Z
       
  • The effect of temperature and cataclastic deformation on the composition
           of upper crustal fluids — An experimental approach
    • Abstract: Publication date: 1 September 2016
      Source:Chemical Geology, Volume 433
      Author(s): Mathias Burisch, Michael A.W. Marks, Marcus Nowak, Gregor Markl
      We investigated the potential of common crystalline rocks to facilitate the geochemical evolution of continental basement brines and to serve as a metal source for hydrothermal ore deposits. We performed leaching experiments on typical crystalline basement rocks (granite and gneiss), a redbed sandstone and their mineral separates (feldspar, quartz and biotite) at variable T (25, 180, 275 and 350°C), P (ambient pressure, 0.9, 1.4 and 1.9kbar), grain-size fractions (<0.01mm, 0.063–0.125 and 2–4mm) and variable fluid/rock ratios (10 to 1.1) with ultrapure water and 25wt.% NaCl solution as solvents. The modification of the fluid chemistry during water–rock interaction strongly depends on grain-size: leachates (using pure H2O) of fine-grained rock powders have lower Na/Cl and Cl/Br ratios but much higher chlorinities (by a factor of up to 40) compared to leachates from coarse-grained rock powders. The Cl/Br ratios of all leachates are lower than that of their respective whole-rocks. Smaller grain-sizes of the starting materials yield element ratios (Cl/Br and Na/Cl) similar to those found in natural fluids, emphasizing the influence of cataclastic deformation on the fluid chemistry of crustal fluids. During our leaching experiments, Pb, Zn, Cu and W are released by felsic minerals, while biotite alteration releases Ni, As and additional Zn and Cu. Our experiments confirm that crystalline rocks may serve as metal source for hydrothermal ore deposits. Short-term water–rock interactions along cataclastic fault zones in the brittle crust may influence the geochemical evolution of upper crustal fluids. This is further suggested by low F/Cl and Cl/Br ratios in some of the leachates being very similar to halogen systematics in natural fluid samples.


      PubDate: 2016-04-28T07:29:26Z
       
  • Editorial Board
    • Abstract: Publication date: 1 August 2016
      Source:Chemical Geology, Volume 431




      PubDate: 2016-04-28T07:29:26Z
       
  • Zirconium–hafnium and rare earth element signatures discriminating
           the effect of atmospheric fallout from hydrothermal input in volcanic lake
           water
    • Abstract: Publication date: 1 September 2016
      Source:Chemical Geology, Volume 433
      Author(s): C. Inguaggiato, P. Censi, P. Zuddas, W. D'Alessandro, L. Brusca, G. Pecoraino, S. Bellomo
      Geochemical behaviour of rare earth elements (REE), Zr, and Hf was investigated in CO2-rich waters circulating in Pantelleria Island also including ‘Specchio di Venere’ Lake within a calderic depression. A wide range of total dissolved REE concentrations was found (2.77–12.07nmolL−1), with the highest contents in the lake. The main REE complexes in the CO2-rich waters are [REE(CO3)2]− and [REECO3]+, showing changeable proportions as a function of pH. The REE normalized to post-Archean Australian Shale (PAAS) showed similar features with heavy REE (HREE) enrichments in CO2-rich waters collected from springs and wells, whereas a different REE pattern was found in the ‘Specchio di Venere’ Lake water with middle REE (MREE) enrichments. The PAAS normalized concentration ratios (LREE/HREE)N and (MREE/HREE)N in waters are <1, except for the lake water in which (MREE/HREE)N >1. Positive Eu anomalies were found in the investigated waters owing to water–rock interactions with less evolved host rocks. Ce anomalies as a function of Eh values were recognized, with the highest Ce anomaly occurring in the lake water with respect to the CO2-rich waters. The Y/Ho and Zr/Hf molar ratios are higher in the investigated waters (except for lake water) than that in the local rocks, with values ranging from 35.4 to 77.9 and from 76.3 to 299, respectively. The precipitation of authigenic phases was considered to be responsible for the increase in the Y/Ho and Zr/Hf ratios owing to enhanced Hf and Ho removal with respect to Zr and Y. The REE patterns in the lake water show a similar shape (MREE-enriched and a positive Ce anomaly) as those found in the settling dust and in the desert varnish coating of the rocks in arid environments, which mainly contain Fe- and Mn-oxyhydroxides and clay minerals. Similarly, Y/Ho and Zr/Hf ratios in the ‘Specchio di Venere’ Lake (35.4 and 76.3, respectively) show a desert varnish signature. These data, coupled with the presence of iron oxyhydroxides and phyllosilicates in the shallowest water layer of the ‘Specchio di Venere’ Lake, verify the aeolian input from the Sahara Desert.


      PubDate: 2016-04-24T06:39:21Z
       
  • Geochemistry of fluid discharges from Peteroa volcano (Argentina-Chile) in
           2010–2015: Insights into compositional changes related to the fluid
           source region(s)
    • Abstract: Publication date: 15 August 2016
      Source:Chemical Geology, Volume 432
      Author(s): F. Tassi, F. Aguilera, O. Benavente, A. Paonita, G. Chiodini, S. Caliro, M. Agusto, F. Gutierrez, B. Capaccioni, O. Vaselli, A. Caselli, O. Saltori
      This study presents the first geochemical data of fluid discharges collected from February 2010 to March 2015 from the Planchón–Peteroa–Azufre Volcanic Complex (PPAVC), located in the Transitional Southern Volcanic Zone (TSVZ) at the border between Argentina and Chile. During the study period, from January 2010 to July 2011, Peteroa volcano experienced phreatic to phreatomagmatic eruption possibly related to the devastating Maule earthquake occurred on February 27, 2010. The compositional dataset includes low temperature (from 43.2 to 102°C) gas discharges from (i) the summit of Peteroa volcano and (ii) the SE flank of Azufre volcano, both marked by a significant magmatic fluid contribution, as well as bubbling gases located at the foothill of the Peteroa volcanic edifice, which showed a chemical signature typical of hydrothermal fluids. In 2012, strong compositional changes affected the Peteroa gases from the summit area: the acidic gas species, especially SO2, increased, suggesting an input of fluids from magma degassing. Nevertheless, the R/Ra and δ13C–CO2 values decreased, which would imply an enhanced contribution from a meteoric-hydrothermal source. In 2014–2015, the chemical and isotopic compositions of the 2010–2011 gases were partially restored. The anomalous decoupling between the chemical and the isotopic parameters was tentatively interpreted as produced by degassing activity from a small batch of dacitic magma that in 2012 masked the compositional signature of the magmatic fluids released from a basaltic magma that dominated the gas chemistry in 2010–2011. This explanation reliably justifies the observed geochemical data, although the mechanisms leading to the change in time of the dominating magmatic fluid source are not clear. At this regard, a geophysical survey able to provide information on the location of the two magma batches could be useful to clarify the possible relationships between the compositional changes that affected the Peteroa fluid discharges and the 2010–2011 eruptive activity.


      PubDate: 2016-04-24T06:39:21Z
       
  • Hf isotope analysis of small zircon and baddeleyite grains by conventional
           Multi Collector-Inductively Coupled Plasma-Mass Spectrometry
    • Abstract: Publication date: 1 September 2016
      Source:Chemical Geology, Volume 433
      Author(s): François-Xavier D'Abzac, Joshua H.F.L. Davies, Jörn-Frederik Wotzlaw, Urs Schaltegger
      Protocols for solution nebulization (SN) multi collector (MC) ICP-MS analysis of the Hf isotopic composition of accessory minerals (zircon and baddeleyite) selected for high-precision ID-TIMS UPb geochronology have improved significantly during the last two decades. Precise crystallization age and Hf isotopic composition from the exact same volume of mineral can be retrieved, provided that time-consuming sample preparation and purification is possible. The present work focuses on the analytical capabilities of SN-MC-ICP-MS for Hf isotope analysis on non-purified reference materials. Analytical issues arise for (i) igneous and metamorphic zircons, often enriched in trace elements and REEs, which increase the amount of isobaric interferences. (ii) Small minerals that challenge the limits of quantification of the apparatus. JMC475 Hf standard, Plešovice and Temora-2 zircons and Phalaborwa baddeleyite were used to characterize the sources of analytical bias and to set up an optimized protocol for Hf isotope analysis of small accessory minerals with accurate correction of the isobaric interferences and accounting for the matrix effects. Doping of JMC475 with major (Zr, Si), trace (U, Th, Y, Ti) and rare earth elements (Ce, Nd, Gd, Dy, Ho, Er) shows that oxide species produced in the ICP induce an offset of the measured 176Hf/177Hf ratios that is corrected by normalization to the reference value. The 176Yb isobaric interference is efficiently removed for samples with Yb/Hf ratios <0.6 using in-run determined 176Yb/173Yb reference ratios from Yb doped reference material. This isotopic ratio varies (i) with time and (ii) sample matrix. Plasma loading by Zr is not the source of these variations. We propose that Yb oxidation in the plasma torch is responsible for variations in the 176Yb/173Yb ratio. Our dataset shows that matrix matched standards should be preferred for data reduction, and that Plešovice is a good zircon standard because of its availability, quantifiable REEs composition and potential for wide range of Yb doping. Consequently, low Hf concentration solutions from Temora-2 zircons were accurately measured in a small sample volume (<120μl), using a short acquisition protocol (50×1s cycles), and sample-standard bracketing for mass bias coefficient determination in a [Hf]~10ng/g Plešovice solution measured through 80×4s cycles. The repeatability of the measurement (2SD) is improved by factors of up to ~4.4 and the limit of quantification is brought down to <2ng/g Hf, corresponding to zircon minerals of <24ng.


      PubDate: 2016-04-24T06:39:21Z
       
  • FeMnOx-1: A new microanalytical reference material for the investigation
           of Mn–Fe rich geological samples
    • Abstract: Publication date: 15 August 2016
      Source:Chemical Geology, Volume 432
      Author(s): K.P. Jochum, S.A. Wilson, H. Becker, D. Garbe-Schönberg, N. Groschopf, Y. Kadlag, D.S. Macholdt, R. Mertz-Kraus, L.M. Otter, B. Stoll, A. Stracke, U. Weis, G.H. Haug, M.O. Andreae
      Suitable Mn–Fe rich microanalytical reference materials (MRMs) as calibration material for laser ablation–inductively coupled plasma mass spectrometry (LA-ICPMS) have not been available. The United States Geological Survey (USGS) in collaboration with the Max Planck Institute for Chemistry has prepared a synthetic MRM, FeMnOx-1, with elevated mass fractions of MnO (25g/100g), Fe2O3 (8.5g/100g) and high mass fractions of 25 trace elements varying between 200 and 5000mg/kg. This new MRM has been designed as calibration material for a wide range of different Mn–Fe deposits, such as desert/rock varnish, ocean crusts and nodules as well as Mn accumulations in soils and lakes. Small-scale and large-scale homogeneity of FeMnOx-1 were tested with three LA systems (200nm femtosecond, and 193nm and 213nm nanosecond lasers) using different spot sizes and fluences. Our results demonstrate that FeMnOx-1 is homogeneous in the pg to μg and nm to μm range and therefore well suited for microanalytical applications. The relative standard deviation (RSD) values obtained from repeated measurements are about 2–3% for test portion masses of 5–100ng, and are comparable to those of the homogeneous NIST SRM 610 and USGS GSE-1G reference glasses. Homogeneity of FeMnOx-1 was also verified for a test portion of 0.1ng. Seven laboratories using five different bulk and microanalytical techniques were involved in the characterization of FeMnOx-1. Small amounts of this MRM can be obtained on request from the authors.
      Graphical abstract image

      PubDate: 2016-04-20T06:31:38Z
       
  • In-situ U–Pb dating and Nd isotopic analysis of perovskite from a
           rodingite blackwall associated with UHP serpentinite from southwestern
           Tianshan, China
    • Abstract: Publication date: 1 August 2016
      Source:Chemical Geology, Volume 431
      Author(s): Ting-ting Shen, Fu-yuan Wu, Li-fei Zhang, Joerg Hermann, Xu-ping Li, Jin-xue Du
      Metamorphic perovskite associated with diopside+chlorite occurs in blackwalls surrounding rodingitized mafic dikes that are included in ultrahigh-pressure (UHP) serpentinized ultramafic rocks from Changawuzi, Chinese southwestern Tianshan. Perovskite forms discrete crystals in the chlorite-rich matrix or appears at the rim of ilmenite. The metamorphic perovskites from the blackwalls are characterized by high concentrations of REE, Nb, Ta, Th and U. The primitive mantle normalized patterns of perovskite are two to three orders of magnitude higher but parallel to the bulk rock patterns, demonstrating that perovskite is a major host of these incompatible trace elements in the ultramafic rocks. The bulk-rock major and trace element compositions of rodingite dikes and associated blackwalls display variable patterns, consistent with different mafic protoliths of the rodingites. Perovskites from three samples were analysed for U–Pb by in-situ secondary ion mass spectrometry. Two samples contained essentially common Pb whereas one sample had enough radiogenic Pb to define a discordia in the Tera–Wasserburg diagram with an age intercept of 317±11Ma. If all analyses are pooled together, a 238U–206Pb age of 308±5 Ma is obtained. These ages are in agreement with the age of peak UHP metamorphism in Chinese southwestern Tianshan. In-situ Nd isotopes of perovskite from the three samples were obtained by laser ablation multi-collector inductively coupled plasma mass spectrometry. The ɛNd(t)310 of two samples are 8.37±0.14 and 7.88±0.25, respectively, which suggests that they have similar protoliths. These samples plot close to a 310Ma Sm–Nd isochron. In contrast, the third sample has a different ɛNd(t)310 of 0.90±0.51, indicating a completely different protolith. Thus, in-situ analyses of perovskite can provide important information on the protolith as well as the metamorphic evolution of reaction zones between mafic and ultramafic rocks.


      PubDate: 2016-04-20T06:31:38Z
       
  • Editorial Board
    • Abstract: Publication date: 15 July 2016
      Source:Chemical Geology, Volume 430




      PubDate: 2016-04-16T06:27:55Z
       
  • Re-evaluation of the ages of 40Ar/39Ar sanidine standards and
           supereruptions in the western U.S. using a Noblesse multi-collector mass
           spectrometer
    • Abstract: Publication date: 1 August 2016
      Source:Chemical Geology, Volume 431
      Author(s): Brian R. Jicha, Brad S. Singer, Peter Sobol
      The rewards from resolving the tempo of magmatic, paleobiologic, paleoclimatic, and tectonic processes at better than the per mil level of precision have spurred a quest within the geochronology community to improve the precision, accuracy, and calibration of several chronometers, including the 40Ar/39Ar variant of the K-Ar clock. A new generation of multi-collector mass spectrometers for 40Ar/39Ar geochronology provides the means to address these issues, as well as to evaluate the ages of 40Ar/39Ar standards and key marker horizons deposited by large volume, caldera-forming supereruptions in the western U.S. at unprecedented temporal resolution. We highlight the utility of a 5-collector Noblesse mass spectrometer that has a sufficiently high mass resolving power to distinguish 36Ar+H35Cl from a 36Ar+H35Cl+ 12C3 sum peak at m/e 36, maintains a remarkably low and stable background, and thus yields 40Ar/39Ar dates of exceptionally high precision. We have calibrated a reference gas and developed a simple analytical routine involving one peak hop, both of which are used to evaluate detector-bias correction factors. Because the 40Ar/39Ar chronometer is a relative dating technique that relies on calibration against mineral standards of known age, precise and accurate determination of the ages of these standards, and verification of isotopic composition and homogeneity, is essential. We present results from both total fusion and incremental heating experiments on single crystals of the widely used Alder Creek (ACs) and Fish Canyon (FCs) sanidine standards. Several recent, independent, measurements of ACs using multi-collector mass spectrometers, including those presented herein, favor an age of 1.1864±0.0003/0.0012Ma (95% confidence analytical/full external uncertainty), that is >1% younger than the widely used value obtained by Nomade et al. (2005) using a single collector mass spectrometer. Incremental-heating experiments on single crystals of FCs most commonly do not yield plateau ages, instead many crystals appear to be affected by Ar loss or excess Ar. Our findings, obtained using ion counters, independently confirm those recently obtained using a multiple-Faraday detectors or a combination of Faraday and ion counting detectors. Thus, careful consideration must be given to using FCs in future high-precision 40Ar/39Ar calibrations of the geologic timescale. Notwithstanding, 40Ar/39Ar dating of sanidine using the 5- collector Noblesse can now provide leverage critical to interpreting the large range of U–Pb zircon dates commonly found in rhyolitic lavas and tuffs.


      PubDate: 2016-04-16T06:27:55Z
       
  • Carbon speciation in saline solutions in equilibrium with aragonite at
           high pressure
    • Abstract: Publication date: 1 August 2016
      Source:Chemical Geology, Volume 431
      Author(s): Sébastien Facq, Isabelle Daniel, Gilles Montagnac, Hervé Cardon, Dimitri A. Sverjensky
      The solubility of carbonate minerals and aqueous carbonate speciation in deep fluids is of critical importance to the long-term carbon cycle. However, no experimental data exist in the calcium carbonate–water–salt system at pressures greater than 10kbar. Here we present an integrated experimental and theoretical study of carbon speciation and solubility during equilibration of aragonite with NaCl solutions at 300°C from 20 to 70kbar. Using in situ Raman spectroscopy in a diamond anvil cell with NaCl concentrations up to 1.0 m (5.5wt.%), bicarbonate and carbonate ions were the only C-bearing aqueous species spectroscopically detected in the fluids. The proportion of total dissolved bicarbonate to carbonate increased with NaCl concentration and was interpreted using a thermodynamic model to retrieve the dissociation constant of the NaHCO3 0 complex. The results were extended to higher temperatures using the Deep Earth Water (DEW) model to predict calcite solubility and speciation at 700°C and 10kbar in chloride solutions. With a one parameter extension of the Debye–Hückel model, the calculated solubilities agree with data from Newton and Manning (2002) up to 10 m NaCl where the solubility is about 250millimolal. Such solubilities might contribute substantial transport of carbon in fluids at depth in subduction zones.


      PubDate: 2016-04-16T06:27:55Z
       
  • Fluid source and methane-related diagenetic processes recorded in cold
           seep carbonates from the Alvheim channel, central North Sea
    • Abstract: Publication date: 15 August 2016
      Source:Chemical Geology, Volume 432
      Author(s): Antoine Crémière, Aivo Lepland, Shyam Chand, Diana Sahy, Kalle Kirsimäe, Michael Bau, Martin J. Whitehouse, Stephen R. Noble, Tõnu Martma, Terje Thorsnes, Harald Brunstad
      Integrated petrography, mineralogy, geochronology and geochemistry of cold seep carbonate crusts and free gas from the Alvheim channel elucidate diagenetic carbonate precipitation and related seepage histories in the central North Sea. Free gas isotope characteristics coupled with carbonate δ 13C values as low as −66‰ VPDB, indicate a predominantly microbial methane source with minor thermogenic contribution. We estimate that ~70% of the carbon sequestered into carbonate precipitates was derived from local oxidation of methane. The early stage of crust growth is represented by microcrystalline aragonite and Mg-calcite (10 to 40% mol MgCO3) cementing seafloor sediments consisting of clays, quartz, feldspar, and minor detrital low Mg-calcite and dolomite. Typical association of aragonite cement with coarse-grained detritus may reflect elevated fluid flow and flushing of fine particles prior to cementation close to the seafloor. Middle rare earth element enrichment in early generation microcrystalline cements containing framboidal pyrite indicates diagenetic precipitation within the zone of anaerobic methane oxidation contiguous to iron reduction. The later generation diagenetic phase corresponds to less abundant radial fibrous and botryoidal aragonite which lines cavities developed within the crusts. In contrast to early generation cements, late generation cavity infills have rare earth elements and Y patterns with small negative Ce anomalies similar to seawater, consistent with carbonate precipitation in a more open, seawater dominated system. Aragonite U–Th ages indicate carbonate precipitation between 6.09 and 3.46kyr BP in the northern part of the channel, whereas in the southern part precipitation occurred between 1.94 and 0.81kyr BP reflecting regional changes in fluid conduit position.
      Graphical abstract image

      PubDate: 2016-04-16T06:27:55Z
       
  • Rapid lithospheric thinning of the North China Craton: New evidence from
           cretaceous mafic dikes in the Jiaodong Peninsula
    • Abstract: Publication date: 15 August 2016
      Source:Chemical Geology, Volume 432
      Author(s): Liang Ma, Shao-Yong Jiang, Albrecht W. Hofmann, Yi-Gang Xu, Bao-Zhang Dai, Ming-Lan Hou
      The North China Craton is a classic case for the destruction of an ancient craton, in that it records the loss of more than 100km of ancient refractory lithospheric mantle during the late Mesozoic and early Cenozoic. However, the mechanisms for this lithospheric thinning remain controversial in large part due to the lack of any systematic investigations of the Mesozoic asthenospheric mantle via its derived mafic rocks, which are key to understand the thinning processes. In this paper, we present detailed zircon U–Pb geochronology, elemental geochemistry, and Sr–Nd–Hf isotopic data for lamprophyres and diabase-porphyries of the Jiaodong Peninsula, in the eastern North China Craton in order to place constraints on models for lithospheric thinning. Our results show that the lamprophyres and diabase-porphyries are derived from the convective asthenospheric mantle via different degrees of partial melting, and that this mantle source was previously modified by carbonatitic liquids. Zircon LA-ICP-MS U–Pb dating suggests an emplacement age for these rocks of 123–121Ma, the earliest evidence for asthenospherically-derived melts in the Jiaodong Peninsula so far. This emplacement age indicates that the thickness of the lithosphere in the Jiaodong Peninsula was relatively thin at that time. Co-occurrence of the asthenospheric and lithospheric mantle-derived mafic rocks as well as high-Mg adakites record a rapid transition from lithospheric to asthenospheric mantle sources, indicating that the lithosphere beneath the Jiaodong Peninsula was rapidly detached just prior to ca. 120Ma. Lithospheric thinning of the North China Craton may have been initiated from the Jiaodong Peninsula and Bohai Sea and then propagated towards the interior of the craton.


      PubDate: 2016-04-16T06:27:55Z
       
  • Kinetics of precipitation of non-ideal solid-solutions in a liquid
           environment
    • Abstract: Publication date: 1 August 2016
      Source:Chemical Geology, Volume 431
      Author(s): C. Noguera, B. Fritz, A. Clément
      We present a theoretical formalism which, for the first time, accounts for the nucleation, growth and/or redissolution of binary non-ideal solid-solutions, whether mineral or bimetallic, in solution. It yields the time evolution of all ion activities, together with the particle population characteristics: number, size and composition profile of particles as a function of time and of their time of nucleation. It is shown that depending on the Guggenheim parameter values which drive the non-ideality of the solid-solution, on the ratio of the solubility products of the end-members and on initial conditions, different scenarios of precipitation may take place, in which particles display composition profiles which may be smooth or discontinuous. An illustration of the characteristics of precipitation in the various scenarios is given, by simulations performed under some simplifying assumptions and qualitative predictions are made for the precipitation of some mineral solid solutions of geochemical interest. To our knowledge, this is the first time, in the fields of both geochemistry and metallic alloys, that these out-of-equilibrium precipitation processes of non-ideal solid-solutions are fully described.


      PubDate: 2016-04-12T03:00:23Z
       
  • Effects of extraction protocols on the oxygen isotope composition of
           conodont elements
    • Abstract: Publication date: 1 August 2016
      Source:Chemical Geology, Volume 431
      Author(s): Page C. Quinton, Stephen A. Leslie, Achim D. Herrmann, Kenneth G. MacLeod
      Oxygen isotopic measurements of conodont elements can provide important paleoclimatic, paleoecological, and paleoenvironmental information. However, preparation techniques are often cited as a potential, albeit poorly constrained, source of error. Prior to isotopic measurements, conodont elements are typically liberated from a calcium carbonate matrix using dilute solutions of acetic or formic acid. These extraction techniques are well established for paleontological studies, but the protocols were established before conodonts became a common target for isotopic analyses. Whereas concerns are often raised about the possibility of bias introduced through phosphate leaching, especially with stronger acid, empirical observations regarding error in isotopic measurements introduced due to the preparation techniques used are limited and generally anecdotal. This study was designed to constrain the effects different types of acids used for carbonate digestion have on the measured oxygen isotopic compositions of bioapatite (specifically, conodont elements and brachiopod shells). We performed parallel analyses on splits from seven Late Ordovician samples that were processed with both buffered and unbuffered solutions of acetic and formic acid. In addition, to further assess the potential for δ18O alteration related to partial apatite dissolution, we forced partial dissolution of isolated conodont elements by placing them in an unbuffered formic solution for varying intervals of time prior to analysis. Measured δ18O values from conodont elements and brachiopod shell fragments show no statistically significant differences related to processing technique, but isolated conodont elements with visible evidence of partial dissolution due to prolonged exposure to unbuffered formic acid record consistently offset δ18O values. Such partially dissolved elements should be avoided in geochemical studies. If specimens exhibiting apparent etching are avoided, though, we conclude that standard digestion protocols for conodont element extraction from carbonates do not introduce a bias (either a systematic offset or an increase in variance) on measured δ18O values from phosphate-bound oxygen in bioapatite above the limits of the resolution of this study.


      PubDate: 2016-04-12T03:00:23Z
       
  • Editorial Board
    • Abstract: Publication date: 1 July 2016
      Source:Chemical Geology, Volume 429




      PubDate: 2016-04-08T10:10:14Z
       
  • Rhenium–osmium isotopes in pervasively metasomatized mantle
           xenoliths from the Bohemian Massif and implications for the reliability of
           Os model ages
    • Abstract: Publication date: 15 July 2016
      Source:Chemical Geology, Volume 430
      Author(s): Yulia V. Kochergina, Lukáš Ackerman, Vojtěch Erban, Magdalena Matusiak-Małek, Jacek Puziewicz, Patricie Halodová, Petr Špaček, Jakub Trubač, Tomáš Magna
      Peridotite xenoliths brought to the surface by basaltic lavas attest to a variety of mantle processes, including partial melting, melt percolation or refertilization. The whole rock Re–Os concentrations and Os isotopic compositions were determined for 30 xenoliths collected from 11 localities across the northern Bohemian Massif in order to evaluate the Os model ages and attempt to relate the results to major crustal tectonic events during the history. Most samples were affected by variable extent of metasomatic overprint, which is commonly paralleled by very low Os concentrations (<1ppb). Rhenium concentrations in the whole suite are below the primitive mantle value. A subset of samples shows evidence for Re addition from host basaltic rocks, consistent with the presence of abundant melt pockets with secondary sulphides. The 187Os/188Os ratios range from 0.1162 to 0.1330 and cannot be directly related to individual mantle domains, implying the inability of more recent tectonic events to reset the original Os isotopic systematics. The calculated mantle extraction ages (TMA) range from <0.1 to 2.1Ga, whereas future ages obtained for nine samples indicate metasomatic overprints. The Re depletion ages (TRD) vary between <0.1 and ~1.6Ga. However, the TRD is not well suited for direct comparison with crustal ages because it represents a minimum age limit rather than specific age estimate. Therefore, a modified model age (TRDII) was calculated assuming a non-zero Re content during the pre-metasomatic stage and using a composition of the most depleted sample in our suite. A prominent peak in the calculated TRDII ages ranges between 0.5 and 0.6Ga which corresponds to the Cadomian orogenic cycle.


      PubDate: 2016-04-04T06:23:24Z
       
  • Influence of cell's growth phase on the sulfur isotopic fractionation
           during in vitro microbial sulfate reduction
    • Abstract: Publication date: 1 August 2016
      Source:Chemical Geology, Volume 431
      Author(s): Fumihiro Matsu'ura, Michinari Sunamura, Yuichiro Ueno, Tetsuro Urabe
      Culture experiments of sulfate reducing bacteria were conducted to produce large sulfur isotope fractionation in marine sediments. We determined the sulfur isotope fractionation factor for both the exponential growth phase and maintenance growth phase. The results show that the sulfur isotope fractionation during the maintenance phase is larger than it is in the exponential phase, irrespective of a temperature anywhere from 25°C to 37°C. In the natural environment, sulfate reducing bacteria may not dominantly grow exponentially. We suggest that sulfate reducers in the natural environment only metabolize at the minimum level to maintain their body. This may solve the apparent discrepancy between the large sulfur isotope fractionation observed in marine sediments and the smaller fractionation obtained from culture experiments.


      PubDate: 2016-04-04T06:23:24Z
       
  • Isotope amount effects in hydrologic and climate reconstructions of
           monsoon climates: Implications of some long-term data sets for
           precipitation
    • Abstract: Publication date: 15 July 2016
      Source:Chemical Geology, Volume 430
      Author(s): C.J. Eastoe, D.L. Dettman
      Many studies of Quaternary climate make use of terrestrial stable isotope records which are interpreted based on seasonal patterns of stable isotopes in modern precipitation. Multi-decade records of isotopes in rainfall allow testing of the assumed behavior of isotope signals used for this interpretation on multi-year to decadal scales. A 32-year record of stable O and H isotopes in precipitation in Tucson, Arizona permits a detailed examination of stable isotope amount effects, at time scales ranging from individual events to decades, in a location with summer monsoonal and winter frontal rainy seasons. Amount effects are weak to non-existent in Tucson at seasonal and longer time scales, and are not useful for discriminating either wetter or drier rainy seasons or wetter or drier decades. Amount effects are also weak to non-existent in published data for annual and multi-year amount-weighted averages for monsoonal precipitation in New Delhi and Hong Kong, but an annual amount effect appears to be present on Guam (U.S. Territory). In addition, site-specific amount effects do not correlate with measures of regional monsoon intensity. This data analysis challenges the general validity of paleoclimate reconstructions based on short-term (sub-annual) relationships observed in precipitation isotope data when applied to long-term records such as speleothem studies.
      Graphical abstract image

      PubDate: 2016-04-04T06:23:24Z
       
  • An experimental study of hydroxylbastnasite solubility in aqueous
           solutions at 25°C
    • Abstract: Publication date: 15 July 2016
      Source:Chemical Geology, Volume 430
      Author(s): Martin Voigt, Juan Diego Rodriguez-Blanco, Beatriz Vallina, Liane G. Benning, Eric H. Oelkers
      The mobility of rare earth elements (REEs) in natural aqueous solutions is of increasing interest due to their application in numerous high-tech and renewable energy technologies (e.g., wind turbines and electric vehicles), as well as their role as tracers in the Earth Sciences. Basic thermodynamic data for water–rock interactions are, however, sparse and partially inconsistent, even for carbonate minerals of the bastnasite group (REE(CO3)(OH,F)), which currently represent the most exploited REE ore. Towards the improved quantification of the REE in natural and industrial systems, pure hexagonal hydroxylbastnasite and orthorhombic kozoite (REE(CO3)(OH)) were synthesized at hydrothermal conditions and subsequently used as starting material for aqueous dissolution and precipitation experiments. All closed system experiments were performed in air equilibrated aqueous fluids. Reaction progress was followed by measuring pH and REE concentrations at regular intervals. Experiments at 25°C in the presence of selected concentrations of HCl and NaOH were run for up to 33days until steady-state pH and REE concentrations were attained. Results indicate that the solubility products (K SP ) of the reaction REE(CO3)(OH)=REE3+ +CO3 2 − +OH− are log(K SP )=−23.8±0.1, −24.1±0.3, and −22.3±0.2 for Nd-, La-hydroxylbastnasite, and Nd-kozoite, respectively at 25°C.


      PubDate: 2016-04-04T06:23:24Z
       
  • The formation conditions of the early Ediacaran cherts, South China
    • Abstract: Publication date: 15 July 2016
      Source:Chemical Geology, Volume 430
      Author(s): Hanjie Wen, Haifeng Fan, Shihong Tian, Qilian Wang, Ruizhong Hu
      During the early Ediacaran, abundant cherts (nodules and bands) were discontinuously hosted by dolostones and mudstones of the Doushantuo Formation (635–551Ma) in the Yangtze Three Gorges area, South China. The appearance of these cherts correlates with the distribution of acanthomorphic acritarch fossils and is potentially important for the reconstruction of the biogeochemical conditions and oceanic chemistry during the early Ediacaran. However, the formation mechanisms of these cherts are still poorly understood. In this study, silicon isotopes, Ge/Si molar ratios and rare earth element (REE)+Y features are used to constrain the origin mechanism of these cherts, silica geochemical cycling and seawater redox conditions during the Early Ediacaran. Our results suggest that the studied cherts were formed by the replacement of precursor carbonate minerals, which included the dissolution of a siliceous precursor and reprecipitation of microquartz. The positive silicon isotopic composition (typically between 0.4‰ and 1.4‰) and Ge/Si molar ratios (0.04–0.95×10−6) indicate a major seawater source of Ge and Si in these cherts with minor detrital and negligible hydrothermal contamination. In the early Ediacaran Ocean, the dissolved silica may be derived from the river input with high δ30Si values. On the other hand, the output of dissolved silica was controlled by inorganic precipitation, which was subsequently fixed as microquartz in cherts during early diagenesis. During the formation of cherts, the primary REE+Y signals of seawater were preserved with little detrital influence. Meanwhile, the REE+Y patterns, obvious negative Ce anomalies and near modern seawater Y/Ho ratios indicate an oxic marine chemistry during chert formation. This oxic seawater environment may have promoted the observed diversity of Ediacaran biological systems. Finally, temporal variations related to silica sources and silicon isotopic fractionation controls in the cherts and/or ocean were also evaluated.


      PubDate: 2016-04-04T06:23:24Z
       
  • A hundred year record of industrial and urban development in French Alps
           combining Hg accumulation rates and isotope composition in sediment
           archives from Lake Luitel
    • Abstract: Publication date: 1 August 2016
      Source:Chemical Geology, Volume 431
      Author(s): Stéphane Guédron, David Amouroux, Pierre Sabatier, Carole Desplanque, Anne-Lise Develle, Julien Barre, Caiyan Feng, Frederic Guiter, Fabien Arnaud, Jean Louis Reyss, Laurent Charlet
      This study reconstructs the history of multiple industrial and urban mercury (Hg) emissions recorded in the sediment archive of Lake Luitel (France) from AD ~1860 to AD 2011. For this purpose, we provide a well constrained short-lived radionuclides continuous age–depth relationship of the sediment sequence (mean accumulation rate of 5.18±0.28mm·yr−1) with Hg accumulation rates (Hg AR), Hg isotopic composition and extensive historical data. Hg AR were stable around 45μg·m−2·y−1 from 1860 to WWI and rose to reach their maximum at the end of WWII (250μgm−2 y−1) followed by a gradual decreased to reach about 90μgm−2 y−1 in the current period. Normalization to a terrigenous Hg proxy highlighted the dominance of atmospheric Hg inputs to the lake. The combination of Hg AR with isotopic signatures through the use of binary mixing (Δ199Hg vs 1/Hg AR) models and isotopic plots (and comparison to literature data) allowed us to identify the main industrial and urban historical inputs. The major outcome of this study is that the Hg mass independent fractionation (MIF) signature did not enable the identification of particular anthropogenic sources but reflected an integrated pool of industrial and urban emissions which tend to shift to less negative MIF values (mean: −0.15±0.04‰) during their period of maximum emissions. Temporal MIF and Hg AR variations depict the rising Hg emissions from the industrial revolution (1860–1910) to the modern industrial and urban development period (1950–1980). Mass dependent fractionation (MDF) signatures enabled the identification of major contributors in relation to their relative intensities lying between two endmember pools: (i) the combustion activities (smelters, cement factories and urban heating) with more negative δ202Hg values, and (ii) the chemical and electrometallurgical activities (electrochemistry, chlor-alkali) with higher δ202Hg values. Unconformities of MIF and MDF signatures were observed during WWI, WWII and interwar period, and were attributed to drastic and rapid changes in regional industrial activities. Finally, recent laws regarding Hg emissions (1998–2011) prove their efficiency as Hg AR decreased with a return to more negative MIF and MDF signatures such as during the industrial revolution period. Our study highlights that the combination of Hg isotopic data with Hg AR in sediment archives is a useful tool for reconstructing the history of anthropogenic Hg emissions, and has the potential to identifiy their relative contributions.


      PubDate: 2016-04-04T06:23:24Z
       
  • Consumption of atmospheric CO2 via chemical weathering in the Yellow River
           basin: The Qinghai–Tibet Plateau is the main contributor to the high
           dissolved inorganic carbon in the Yellow River
    • Abstract: Publication date: 15 July 2016
      Source:Chemical Geology, Volume 430
      Author(s): Liang Wang, Longjun Zhang, Wei-Jun Cai, Baosen Wang, Zhigang Yu
      The high HCO3 − concentrations in the Yellow River (China) are generally believed to originate primarily from the carbonate-rich Loess Plateau as a result of carbonate weathering reactions that consume atmospheric CO2. We studied chemical weathering across the entire Yellow River basin in 2007 and 2009 and found that the amounts of CO2 consumption by carbonate and silicate weathering were 87.61×109 mol/a and 10.70×109 mol/a, respectively. As the source of water for the Yellow River, although the Qinghai–Tibet Plateau accounts for only 30% of the area of the basin, it has a strong leaching effect and contributes a majority (61%) of the atmospheric CO2 consumption of the entire basin and a majority of the riverine DIC flux (66%) to the ocean. As a result, the dissolved inorganic carbon (DIC) concentrations in this river are among the highest of the world's major rivers. In contrast, despite the abundant carbonate mineral content in the Loess Plateau, the high ratio of evaporation to precipitation (4.2) limits the weathering processes, and the low amount of runoff restricts the transport of weathering products, which results in a relatively low contribution (38%) to the basin-wide atmospheric CO2 consumption, which is much lower than was previously thought. The Qinghai–Tibet Plateau is the origin of the high levels of HCO3 − in and acts as the main DIC contributor to the Yellow River.


      PubDate: 2016-03-30T09:20:09Z
       
  • The impact of ice-sheet dynamics in western Mediterranean environmental
           conditions during Terminations. An approach based on terrestrial long
           chain n-alkanes deposited in the upper slope of the Gulf of Lions
    • Abstract: Publication date: 15 July 2016
      Source:Chemical Geology, Volume 430
      Author(s): Aleix Cortina, Joan O. Grimalt, Andrés Rigual-Hernández, Anne-Marie Ballegeer, Belen Martrat, Francisco Javier Sierro, José Abel Flores
      During the PROMESS campaign (summer 2004) a borehole (PRGL1) was drilled in the upper slope of the Gulf of Lions. Previous studies showed that the deposition of the sedimentary units at borehole PRGL1 was determined by changes in the discharge of the Rhone river, induced by sea level and climate variability with a strong 100ka imprint. However, there are no detailed studies that focus on the glacial/interglacial transitions (i.e. Terminations) that define 100ka Quaternary ice-age cycles. Here we present a high-resolution record of terrestrial long chain n-alkanes (C21 to C33) from MIS 3 to MIS 11 in order to monitor changes in the sediments deposited on the upper slope of the Gulf of Lions with the aim to provide new insight into the climatic events occurring during Terminations in the western Mediterranean Sea. Principal component analysis grouped n-alkane on the basis of the degree of maturation of organic matter (fresh/degraded) that allowed to monitor past variability in the source of the sediment deposited in the upper slope as a consequence of sea level and climate change. This information was used to estimate the impact that northern ice-sheet variability had on the western Mediterranean basin. As sea level started to fall, fresh terrestrial organic matter increased owing to the seaward migration of Rhone's mouth. Simultaneously, the decrease in sea level exposed old and reworked organic material from the Rhone's catchment, including the delta plain and the continental shelf, that was eroded and transported into our study site. This resulted in an increase in the proportion of degraded terrestrial organic matter together with sea level decrease. Aside from sea level changes, the proportion of fresh vs degraded organic matter seems to be affected by the amount of continental runoff, with an increasing proportion of fresh terrestrial organic matter occurring at times of reduced continental runoff. During the last glacial, the increase of fresh terrestrial organic matter was contemporaneous with Heinrich events (HEs), suggesting dry and cold conditions on the western Mediterranean basin. These dry and cold millennial-scale episodes were reproduced during Terminations II, III and IV preceding flood events, most probably due to weakening/reinforcement of Atlantic Meridional Overturning Circulation (AMOC) that caused dry/humid conditions in western Mediterranean.


      PubDate: 2016-03-30T09:20:09Z
       
  • Determination of mineral dissolution regimes using flow-through
           time-resolved analysis (FT-TRA) and numerical simulation
    • Abstract: Publication date: 15 July 2016
      Source:Chemical Geology, Volume 430
      Author(s): Bart De Baere, Sergi Molins, K. Ulrich Mayer, Roger François
      Flow-through time resolved analysis (FT-TRA) involves subjecting small mineral samples (<10mg) inserted in a miniature flow-through cell (50μL) to controlled flows of eluent analyzed on-line by ICP-MS. In this study, FT-TRA is used to empirically determine the dissolution regimes for the two well-studied minerals forsterite and calcite, representing minerals with relatively slow and fast dissolution kinetics. A proportional increase in steady-state effluent [Mg, Si] concentrations with increasing flow-through cell eluent residence times confirms a dominantly surface-controlled dissolution regime for a powdered forsterite sample at pH2.3, implying that transport limitations are negligible. In contrast, the relationship between flow rates and dissolution rates for single grain calcite samples at pH2.3–4 reveals that transport limitations affect the rate of calcite dissolution. To provide a quantitative and process-based assessment of the effect of diffusive transport limitations, simulations of the calcite experiments were performed with a high resolution, pore-scale model that considers the geometry of the calcite grain and the FT-TRA flow-through reactor. The pore-scale model reproduces the observed effluent [Ca] concentrations for all experimental conditions using a single set of surface kinetic parameters, by accounting for the formation of a diffusive boundary layer (DBL) that varies in thickness as a function of flow rates. These results demonstrate that combining FT-TRA with pore-scale modeling makes it possible to obtain unprecedented insights not achievable by either method separately, including quantification of DBL thicknesses and the determination of transport controls as a function of pH, flow velocity and residence times.


      PubDate: 2016-03-26T01:48:42Z
       
  • Interpretation of the nitrogen isotopic composition of Precambrian
           sedimentary rocks: Assumptions and perspectives
    • Abstract: Publication date: 1 July 2016
      Source:Chemical Geology, Volume 429
      Author(s): M. Ader, C. Thomazo, P. Sansjofre, V. Busigny, D. Papineau, R. Laffont, P. Cartigny, G.P. Halverson
      Nitrogen isotope compositions in sedimentary rocks (d15Nsed) are routinely used for reconstructing Cenozoic N-biogeochemical cycling and are also being increasingly applied to understanding the evolution of ancient environments. Here we review the existing knowledge and rationale behind the use of d15Nsed as a proxy for the Precambrian N-biogeochemical cycle with the aims of (i) identifying the major uncertainties that affect analyses and interpretation of nitrogen isotopes in ancient sedimentary rocks, (ii) developing a framework for interpreting the Precambrian d15Nsed record, (iii) testing this framework against a database of Precambrian d15Nsed values compiled from the literature, and (iv) identifying avenues of focused research that should increase confidence in interpreting Precambrian d15Nsed data. This review highlights the intrinsic complexity of the d15Nsed proxy and the significant effort that remains to realize its potential. Specifically, it is crucial to gain a better understanding of how and when diagenesis and metamorphism affect the d15N of bulk and kerogen-bound nitrogen. Ultimately, more data are required to apply statistics to interpreting d15Nsed variability within given geological time intervals. Finally, numerical modeling of the d15Nsed variability expected in different environments under varying redox scenarios is necessary to establish a predictive template for interpreting the ancient nitrogen isotope record. In spite of the challenges facing the application of this proxy to the Precambrian, the existing d15Nsed record shows several features possibly related to the stepwise oxygenation of the surface environment, underlining the potential for nitrogen isotopes to reveal clues about the evolution of early Earth.


      PubDate: 2016-03-26T01:48:42Z
       
  • Asynchronous reductive release of iron and organic carbon from
           hematite–humic acid complexes
    • Abstract: Publication date: 15 July 2016
      Source:Chemical Geology, Volume 430
      Author(s): Dinesh Adhikari, Simon R. Poulson, Samira Sumaila, James J. Dynes, Joyce M. McBeth, Yu Yang
      Solid-phase iron (Fe) plays an important role in the accumulation and stabilization of soil organic matter (SOM). However, ferric minerals are subject to redox reactions, which can compromise the stability of Fe-bound SOM. To date, there is limited information available concerning the fate of Fe-bound SOM during redox reactions at Fe mineral surfaces. In this study, we investigated the release kinetics of hematite-bound organic carbon (OC) during the abiotic reduction of hematite–humic acid (HA) complexes by dithionite, to elucidate important processes governing the stability and fate of organic matter during the redox processes. Our results indicate that the reductive release of Fe obeyed first-order kinetics with release rate constants of 6.67–13.0×10−3 min−1. The Fe-bound OC was released rapidly during the initial stage with release rate constants of 0.011–1.49min−1, and then became stable with residual fractions of 4.6–58.2% between 120 and 240min. The release rate of aromatic OC was much faster than for the non-aromatic fraction of HA, and 90% of aromatic OC was released within the first hour for most samples. Our findings show that in the reductive reaction the mobilization of Fe-bound OC was asynchronous with the reduction of Fe, and aromatic OC was released more readily than other components of SOM. This study highlights the importance of evaluating the release of SOM bound with Fe during the redox reactions, especially the influence of the physicochemical properties of SOM.
      Graphical abstract image

      PubDate: 2016-03-26T01:48:42Z
       
  • Quantifying aluminium phosphate–sulphate minerals as markers of
           acidic conditions during the Permian–Triassic transition in the
           Iberian Ranges, E Spain
    • Abstract: Publication date: 1 July 2016
      Source:Chemical Geology, Volume 429
      Author(s): Violeta Borruel-Abadía, José F. Barrenechea, Ana Belén Galán-Abellán, Jacinto Alonso-Azcárate, Raúl De la Horra, Francisco Javier Luque, José López-Gómez
      In this paper, a method based on element mapping of randomly selected areas of thin sections on electron microprobe is proposed to quantify the relative contents of strontium-rich hydrated aluminium phosphate-sulphate (APS) minerals in siliciclastic continental sedimentary rocks. The main problems for these minerals to be quantified are related to their small size, low concentrations, and optical properties. By comparing the element maps obtained for the rocks in the study area of the Iberian Ranges (E Spain) with the results of whole rock analysis and with factors indicating the presence of life (bioturbation, palaeosols, and macro-plant remains), it has been possible to correlate relatively high levels of APS minerals in the first sedimentary record (Cañizar Formation) after the Permian–Triassic boundary, with the lack of living organisms. The APS are related to early diagenetic phases precipitated at low pH conditions and are therefore markers of formation in an acidic environment. Our findings suggest a long period of sustained acidic conditions followed by an environmental change linked with the recovery of life and with lower APS mineral contents. This change is detected at the top of the Cañizar Formation, at the end of the Spathian. The method proposed could be used as a tool to address the environmental changes that took place during the Permian–Triassic transition in continental environments.


      PubDate: 2016-03-22T08:32:24Z
       
  • Antimonate and arsenate speciation on reactive soil minerals studied by
           differential pair distribution function analysis
    • Abstract: Publication date: 1 July 2016
      Source:Chemical Geology, Volume 429
      Author(s): Case M. van Genuchten, Jasquelin Peña
      The pentavalent oxyanions antimonate (Sb(V)) and arsenate (As(V)) are toxic metalloids in environmental systems. In this study, we compare their sorption and surface speciation on 2-line ferrihydrite (Fh), δ-MnO2 and Na-montmorillonite (Mont) at pH5.5. The Sb(V) and As(V) sorption affinity and the highest measured surface excess (mmol:g) show that mineral reactivity towards Sb(V) or As(V) sorption increases in the order of Mont<δ-MnO2 <Fh. In addition, Sb(V) showed greater uptake relative to As(V) by both Fh and δ-MnO2. Using differential pair distribution function (d-PDF) analysis of high-energy X-ray scattering data, we found that the larger SbO6 octahedron (RSb–O =1.95Å) can bind in both single edge-(1E) and double corner-sharing (2C) geometries to Fh and δ-MnO2, whereas the smaller AsO4 tetrahedron (RAs–O =1.69Å) binds only in the 2C geometry. Thus, a greater number of surface sites support Sb(V) adsorption relative to As(V) adsorption on Fh and δ-MnO2. Conversely, Mont supports only 2C adsorption geometries, which is consistent with the similar uptake behaviors of Sb(V) and As(V) on this mineral. Finally, we indexed atomic pairs from adsorbed Sb(V) and As(V) at R>6Å in the d-PDFs. The analysis of the intermediate-ranged surface structure of our samples substantiated our interpretations of oxyanion adsorption geometries and provided information regarding changes in the underlying mineral surface upon ion adsorption. Our results show that the differences between Sb(V) and As(V) interactions with reactive soil minerals, including their sorption geometries, are linked to their distinct first-shell coordination environments.
      Graphical abstract image

      PubDate: 2016-03-22T08:32:24Z
       
  • Density of magmas at depth
    • Abstract: Publication date: 1 July 2016
      Source:Chemical Geology, Volume 429
      Author(s): Chrystèle Sanloup
      Knowing the density of silicate liquids at high pressure is essential to answer questions relevant to the presence of magmas at depth, whether that be in the present Earth or in its earliest times, during differentiation of the planet. Melts have unique physical and chemical properties, which vary as a function pressure, and chemical composition. The focus here will be on in situ measurements of the density of magmas, with a presentation of the available methods and of the main results obtained so far, including why some magmas may be trapped at depth. Understanding the macroscopical physical properties of magmas requires an accurate microscopic structural description. Structural descriptions of compressed magmas are becoming more widely available, from experiments and from theoretical calculations. These structural inputs are used to understand the compression mechanisms at stake in the densification of magmas, e.g. the collapse of voids, coordination increase for the major cations, and bond compressibility. These densification processes profoundly affect not only the physical properties of the melt, but also its chemical properties, i.e. the way element partition between the magma and a metallic melt or between the magma and crystals.


      PubDate: 2016-03-22T08:32:24Z
       
 
 
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