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  Subjects -> EARTH SCIENCES (Total: 591 journals)
    - EARTH SCIENCES (437 journals)
    - GEOLOGY (66 journals)
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EARTH SCIENCES (437 journals)                  1 2 3 4 5 | Last

Acta Geodaetica et Geophysica     Hybrid Journal   (Followers: 1)
Acta Geodaetica et Geophysica Hungarica     Full-text available via subscription   (Followers: 2)
Acta Geophysica     Hybrid Journal   (Followers: 7)
Acta Geotechnica     Hybrid Journal   (Followers: 8)
Acta Meteorologica Sinica     Hybrid Journal   (Followers: 2)
Advances in High Energy Physics     Open Access   (Followers: 12)
Advances In Physics     Hybrid Journal   (Followers: 7)
Aeolian Research     Hybrid Journal   (Followers: 2)
African Journal of Aquatic Science     Hybrid Journal   (Followers: 13)
Algological Studies     Full-text available via subscription   (Followers: 2)
Alpine Botany     Hybrid Journal   (Followers: 3)
AMBIO     Hybrid Journal   (Followers: 12)
Anales del Instituto de la Patagonia     Open Access   (Followers: 2)
Andean geology     Open Access   (Followers: 5)
Annales Henri Poincaré     Hybrid Journal   (Followers: 2)
Annales UMCS, Geographia, Geologia, Mineralogia et Petrographia     Open Access   (Followers: 1)
Annals of Geophysics     Full-text available via subscription   (Followers: 9)
Annals of GIS     Hybrid Journal   (Followers: 15)
Annals of Glaciology     Full-text available via subscription  
Annual Review of Marine Science     Full-text available via subscription   (Followers: 9)
Anthropocene Review     Hybrid Journal   (Followers: 1)
Applied Clay Science     Hybrid Journal   (Followers: 2)
Applied Geochemistry     Hybrid Journal   (Followers: 6)
Applied Geomatics     Hybrid Journal   (Followers: 7)
Applied Geophysics     Hybrid Journal   (Followers: 6)
Applied Ocean Research     Hybrid Journal   (Followers: 6)
Applied Petrochemical Research     Open Access   (Followers: 3)
Applied Remote Sensing Journal     Open Access   (Followers: 9)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 21)
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 7)
Artificial Satellites     Open Access   (Followers: 15)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 2)
Asian Journal of Earth Sciences     Open Access   (Followers: 17)
Atlantic Geology : Journal of the Atlantic Geoscience Society / Atlantic Geology : revue de la Société Géoscientifique de l'Atlantique     Full-text available via subscription   (Followers: 3)
Atmosphere-Ocean     Full-text available via subscription   (Followers: 4)
Atmospheric and Climate Sciences     Open Access   (Followers: 14)
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: 8)
Bragantia     Open Access   (Followers: 2)
Bulletin of Earthquake Engineering     Hybrid Journal   (Followers: 10)
Bulletin of Geosciences     Open Access   (Followers: 8)
Bulletin of Marine Science     Full-text available via subscription   (Followers: 13)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (Followers: 1)
Bulletin of the Seismological Society of America     Full-text available via subscription   (Followers: 17)
Bulletin of Volcanology     Hybrid Journal   (Followers: 11)
Canadian Journal of Plant Science     Full-text available via subscription   (Followers: 12)
Canadian Mineralogist     Full-text available via subscription   (Followers: 1)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 20)
Carbonates and Evaporites     Hybrid Journal   (Followers: 4)
CATENA     Hybrid Journal   (Followers: 3)
Central European Journal of Geosciences     Hybrid Journal   (Followers: 5)
Central European Journal of Physics     Hybrid Journal   (Followers: 1)
Chemical Geology     Hybrid Journal   (Followers: 9)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 3)
Chinese Geographical Science     Hybrid Journal   (Followers: 3)
Chinese Journal of Geochemistry     Hybrid Journal   (Followers: 2)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 3)
Ciencia del suelo     Open Access  
Climate and Development     Hybrid Journal   (Followers: 9)
Coastal Management     Hybrid Journal   (Followers: 19)
Comptes Rendus Geoscience     Full-text available via subscription   (Followers: 5)
Computational Geosciences     Hybrid Journal   (Followers: 11)
Computational Mathematics and Mathematical Physics     Hybrid Journal   (Followers: 1)
Computers and Geotechnics     Hybrid Journal   (Followers: 5)
Contemporary Trends in Geoscience     Open Access  
Continental Journal of Earth Sciences     Open Access   (Followers: 2)
Continental Shelf Research     Hybrid Journal   (Followers: 8)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 7)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 2)
Coral Reefs     Hybrid Journal   (Followers: 18)
Cretaceous Research     Hybrid Journal   (Followers: 4)
Cybergeo : European Journal of Geography     Open Access   (Followers: 4)
Developments in Geotectonics     Full-text available via subscription   (Followers: 2)
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: 3)
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (Followers: 171)
Earth Interactions     Full-text available via subscription   (Followers: 8)
Earth Science Research     Open Access   (Followers: 6)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 1)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 11)
Earth System Dynamics     Open Access   (Followers: 4)
Earth System Dynamics Discussions     Open Access   (Followers: 3)
Earth's Future     Open Access   (Followers: 1)
Earth, Planets and Space     Open Access   (Followers: 1)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 7)
Earthquake Spectra     Full-text available via subscription   (Followers: 12)
Ecohydrology     Hybrid Journal   (Followers: 11)
Electromagnetics     Hybrid Journal   (Followers: 1)
Energy Efficiency     Hybrid Journal   (Followers: 11)
Energy Exploration & Exploitation     Full-text available via subscription   (Followers: 4)
Environmental Earth Sciences     Hybrid Journal   (Followers: 10)
Environmental Geology     Hybrid Journal   (Followers: 11)
Environmental Geosciences     Full-text available via subscription   (Followers: 4)
Environmental Geotechnics     Open Access  
Erwerbs-Obstbau     Hybrid Journal  

        1 2 3 4 5 | Last

Journal Cover Chemical Geology
   Journal TOC RSS feeds Export to Zotero [11 followers]  Follow    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
     ISSN (Print) 0009-2541
     Published by Elsevier Homepage  [2570 journals]   [SJR: 1.714]   [H-I: 111]
  • Testing provenance diagrams: Lessons from the well-constrained Cariaco
           Basin
    • Abstract: Publication date: 11 December 2014
      Source:Chemical Geology, Volume 389
      Author(s): A. Riboulleau , V. Bout-Roumazeilles , N. Tribovillard , F. Guillot , P. Recourt
      The fine-grained sediments of the Cariaco Basin, Venezuela, of the last 130ky, whose deposition history is well characterized, were analyzed geochemically in order to test the validity of sediment bulk geochemistry as an indicator of detrital provenance. Several binary and ternary diagrams as well as the chemical index of alteration (CIA) were tested for their capacity to discriminate the poorly contrasted detrital sources to the Cariaco Basin, and to describe the temporal evolution of the contributions of these different sources. Most of the diagrams tested did not allow a good discrimination of sources or, when sources were well discriminated, did not allow an interpretation of the temporal variations consistent with the known history. A relatively good discrimination of sources and a consistent interpretation of temporal variations were however obtained using Hf vs. Th and La/Yb vs. Gd/Yb binary diagrams, as well as Ti–Zr–Th, Ti–Zr–La, and Lu–Hf–Th ternary diagrams. Compared to the previous studies of the detrital content of the Cariaco Basin sediments, the geochemical approach permitted the recognition of a sediment contribution eroded from the Unare platform and Gulf of Cariaco during rapid sea level oscillations, and the contribution of Saharan eolian particles during the Younger Dryas–Preboreal and MIS6-5 transition. The choice of plotted elements was determined after considering carrier minerals, so that different elements may be informative in different sedimentary contexts. Overall, mineral sorting during transport appears as a major limit to quantitative estimation of the different contributions. In particular mineral sorting leads to the selective enrichment of elements associated with clays (Al, Rb, Th and LREE) in sediments deposited in the basin. Unless the geochemical effect of mineral sorting can be measured, it appears that quantitative provenance analysis should be performed on fractions of similar grain size instead of bulk sediment.


      PubDate: 2014-10-21T02:34:21Z
       
  • Zn sorption to biogenic bixbyite-like Mn2O3 produced by Bacillus CUA
           isolated from soil: XAFS study with constraints on sorption mechanism
    • Abstract: Publication date: 11 December 2014
      Source:Chemical Geology, Volume 389
      Author(s): Zhijun Zhang , Hui Yin , Wenfeng Tan , Luuk K. Koopal , Lirong Zheng , Xionghan Feng , Fan Liu
      Although most reported biogenic Mn oxides are hexagonal birnessites, other types of biogenic Mn oxides also commonly occur in the environment. Compared to hexagonal birnessites, the sorption characteristics and the underlying mechanism of adsorption of heavy-metal ions to those of the other biogenic Mn oxides are still rarely addressed. A strain of Mn-oxidizing bacteria isolated from Claypani-Udic Argosols was identified as Bacillus with 16S rRNA sequencing analysis. The bacterial Mn(II) oxidation product is a poorly crystallized bixbyite-like Mn2O3 (α-Mn2O3). The maximum adsorption capacities of Zn(II) onto the biogenic Mn oxide at pH6.00 and pH4.00 were 663mmol/kg and 629mmol/kg, respectively. The complex structure of adsorbed Zn2+ was constrained using Zn EXAFS analysis, combined with structural parameters of the biogenic Mn oxide with alternately arranged regular and distorted MnO6 octahedra obtained through multiple-FEFF fitting of Mn EXAFS data. At a relatively low Zn2+ loading (100mmol/kg, pH6.00), Zn2+ adsorbed onto the biogenic Mn oxide with two types of tetrahedrally coordinated complexes, i.e. (1) coordinated with one regular/distorted MnO6 octahedron as a monodentate–mononuclear complex and (2) with two MnO6 octahedra (two regular, two distorted or a regular and a distorted) as a bidentate–binuclear complex. While, at a relatively high Zn2+ loading (556mmol/kg, pH4.00; 635mmol/kg, pH6.00), two types of octahedrally coordinated complexes are constrained, i.e. (1) coordinated with one regular/distorted MnO6 octahedron as a monodentate–mononuclear complex and (2) with one regular MnO6 octahedron as a bidentate mononuclear complex. This research extends further understanding on the formation of biogenic Mn oxides in the environment and the adsorption mechanism of heavy metals onto low-valence Mn oxides with distorted structures. The application of low valence biogenic Mn oxides to efficiently remove heavy metals from water is also shown to be promising.


      PubDate: 2014-10-16T02:01:08Z
       
  • Use of two new Na/Li geothermometric relationships for geothermal fluids
           in volcanic environments
    • Abstract: Publication date: 11 December 2014
      Source:Chemical Geology, Volume 389
      Author(s): B. Sanjuan , R. Millot , R. Ásmundsson , M. Brach , N. Giroud
      This work has made it possible to obtain two new Na/Li geothermometric relationships in addition to the three already known (Fouillac and Michard, 1981; Kharaka et al., 1982) and confirms that the Na/Li geothermometer, unlike the Na/K, Na/K/Ca, K/Mg and silica geothermometers, or the isotope δ18O (H2O–SO4) geothermometer, also depends on the fluid salinity and the nature of the reservoir rocks reacting with the geothermal water. One of the relationships concerns the fluids derived from seawater–basalt interaction processes existing in emerged rifts such as those of Iceland (Reykjanes, Svartsengi, and Seltjarnarnes geothermal fields) and Djibouti (Asal-Ghoubbet and Obock geothermal areas), or in numerous oceanic ridges and rises (Mid-Atlantic and Middle-Valley ridges, East Pacific rise, etc.). The best adapted Na/Li relationship for geothermal fluids discharged from emerged rifts between 0 and 365°C is: T K = 920 / ⁢ log ⁡ N a / L i − 1.105 r 2 = 0.994 , n = 27 where Na and Li are the aqueous concentrations of these elements given in mol/L. The other Na/Li relationship was determined using dilute waters collected from wells located in different high-temperature (200–325°C) volcanic geothermal areas of Iceland (Krafla, Námafjall, Nesjavellir and Hveragerdi). This relationship can be expressed as follows:T(K)=2002/ [log(Na/Li)+1.322] (r2 =0.967, n=17). These two relationships give estimations of temperature with an uncertainty close to ±20°C. The second Na/Li relationship was also successfully applied to HT dilute geothermal waters from the East African Rift (Ethiopia, Kenya). Some case studies in the literature and thermodynamic considerations suggest that the Na/Li ratios for this type of fluids could be controlled by full equilibrium reactions involving a mineral assemblage constituting at least albite, K-feldspar, quartz and clay minerals such as kaolinite, illite (or muscovite) and Li-micas. Unlike the Na/Li ratios, no thermometric relationship using Li isotopes could be determined for this type of water. However, it was noticed that δ7Li values higher than 16‰ are always associated with low- to medium-temperature waters.


      PubDate: 2014-10-16T02:01:08Z
       
  • On the isotope composition of reactive iron in marine sediments: Redox
           shuttle versus 45 early diagenesis
    • Abstract: Publication date: 11 December 2014
      Source:Chemical Geology, Volume 389
      Author(s): Florian Scholz , Silke Severmann , James McManus , Anna Noffke , Ulrike Lomnitz , Christian Hensen
      The isotope composition of reactive iron (Fe) in marine sediments and sedimentary rocks is a promising tool for identifying Fe sources and sinks across ocean basins. In addition to cross-basinal Fe redistribution, which can modify Fe isotope signatures, Fe minerals also undergo diagenetic redistribution during burial. The isotope fractionation associated with this redistribution does not affect the bulk isotope composition, but complicates the identification of mineral-specific isotope signatures. Here, we present new Fe isotope data for Peru margin sediments and revisit previously published data for sediments from the California margin to unravel the impact of early diagenesis on Fe isotope compositions of individual Fe pools. Sediments from oxic California margin sites are dominated by terrigenous Fe supply with Fe release from sediments having a negligible influence on the solid phase Fe isotope composition. The highly reactive Fe pool (sum of Fe bound to (oxyhydr)oxide, carbonate, monosulfide and pyrite) of these sediments has a light isotope composition relative to the bulk crust, which is consistent with earlier studies showing that continental weathering shifts the isotope composition of Fe (oxyhydr)oxides to lighter values. Ferruginous sediments within the Peruvian oxygen minimum zone are depleted in Fe relative to the lithogenic background, which we attribute to extensive Fe release to the water column. The remaining highly reactive Fe pool has a heavier isotope composition compared to California margin sediments. This observation is in agreement with the general notion of an isotopically light benthic Fe efflux. Most of the reactive Fe delivered and retained in the sediment is transferred into authigenic mineral phases within the topmost 10 to 20cm of the sediments. We observe a first-order relationship between the extent of pyritization of Fe monosulfide and the isotope composition of authigenic pyrite. With increasing pyritization, the isotope composition of authigenic pyrite approaches the isotope composition of the highly reactive Fe pool. We argue that the isotope composition of authigenic pyrite or other Fe minerals that may undergo pyritization may only be used to trace water column sources or sinks if the extent of pyritization is separately evaluated and either close to 100% or 0%. Alternatively, one may calculate the isotope composition of the highly reactive Fe pool, thereby avoiding isotope effects due to internal diagenetic redistribution. In depositional settings with high Fe but low sulfide concentrations, source and sink signatures in the isotope composition of the highly reactive Fe pool may be compromised by sequestration of Fe within authigenic silicate minerals. Authigenic silicate minerals appear to be an important burial phase for reactive Fe below the Peruvian oxygen minimum zone.


      PubDate: 2014-10-16T02:01:08Z
       
  • Geochemistry of Alpine serpentinites from rifting to subduction: A view
           across paleogeographic domains and metamorphic grade
    • Abstract: Publication date: 11 December 2014
      Source:Chemical Geology, Volume 389
      Author(s): Jaime D. Barnes , Marco Beltrando , Cin-Ty A. Lee , Miguel Cisneros , Staci Loewy , Emily Chin
      Serpentinites from several tectono-metamorphic units of the Western Alps were studied to constrain their origin and tectonic setting of serpentinization. Study areas were selected to cover the whole width of the orogen and a wide range of metamorphic grades from anchizone (Canavese Zone) to greenschist facies (St. Barthelemy, Piemonte Zone) to blueschist facies (Rocca Canavese unit and Punta Rossa unit). Bulk rock serpentinite samples have high REE concentrations, compared to typical mid-ocean ridge serpentinites, with nearly flat REE patterns. Relict spinels from the Rocca Canavese unit have extremely low Cr#s (average=0.087) and high Mg#s (average=0.798) suggesting very low degrees of melt depletion. Both of these observations are consistent with an abyssal origin in a hyper-extended rifted margin with minimal melt depletion, or refertilization. Seafloor hydration between 150 and 200°C is indicated by oxygen isotope data (δ18O values=+5.2 to+9.4‰), supporting lithostratigraphic evidence of exhumation to the floor of the Alpine Tethys already available for the Canavese, St. Barthelemy and Punta Rossa serpentinites. Subsequent interaction with the metasediments during Alpine metamorphism resulted in variations in trace element concentrations and stable isotope compositions with decreasing distance to the interface between the sediment and serpentinite. The chemical gradient between the ultramafic rocks and the neighboring metasediments is best seen in the Punta Rossa unit, where Pb, Ba, Cs, U, and Rb concentrations increase, δ18O values increase, δ37Cl values decrease within the serpentinite with decreasing distance to the contact and a “blackwall” of pure chlorite is found at the contact. As these contacts between ultramafic rocks, continental basement and meta-sediments are analogous to the slab–mantle interface, our results support the mobility of Pb, Ba, Cs, U, Rb, Cl, and water at the scale of <10m across the interface during Alpine metamorphism. However, the preservation of geochemical gradients within the Punta Rossa serpentinite indicates a limited role for externally derived fluid flux.


      PubDate: 2014-10-12T01:51:23Z
       
  • Sm–Nd dating of hydrothermal carbonate formation: An example from
           the Breitenau magnesite deposit (Styria, Austria)
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Friedhelm Henjes-Kunst , Walter Prochaska , Andrea Niedermayr , Nora Sullivan , Ethan Baxter
      The geochemical evolution and chronology of the Breitenau sparry magnesite deposit has been investigated by means of major and trace element abundances, stable C–O isotope, and Sm–Nd isotope analysis. Chemically separated magnesite fractions (leachates) show consistent light-REE depleted REY patterns (marine carbonate normalized) and nearly constant normalized values for the middle and heavy REE without significant Ce, Eu or Y anomalies. Non-carbonate residues (composed of dark matrix material) are high in Al2O3 and show variably light to middle REE enriched REY patterns similar to shales. Leachates of limestone from nearby country rocks have normalized REY patterns similar to marine carbonates. Only Ce and Y are slightly fractionated. Based on their trace element chemistry and normalized REY patterns, leachates of dolomite from the margin of the deposit closely resemble either limestone (Type A) or magnesite (Type B). With one exception, δ18OVPDB for both dolomite types is within the range of the respective magnesite values (δ18OVPDB c. −14.2 to −15.7‰). Leachates of dolomite from druses within the deposit (Type C) show low REY abundances with normalized patterns similar to those of magnesite. Sm–Nd isotope data for magnesite leachates yield a well-defined isochron age of 222.5±9.8Ma (MSWD 3.1). Isochron calculations for leachate(s) and residue(s) from individual magnesite samples (“internal isochrons”) yield ages between 236±16Ma and 193.5±8.6Ma with a low degree of scatter (MSWD<1.3). The most precise estimate for the time of Sm–Nd isotopic equilibration between carbonates and their host assemblages, both on hand specimen and deposit scale, is obtained from the mean of internal isochron calculations for eight samples (229.3±2.4Ma; MSWD 0.65) and is interpreted to date hydrothermal formation of the magnesite deposit. The scatter in initial Nd isotopic composition of the eight internal isochrons (0.511891±0.000014; MSWD 7.7) indicates geologically based heterogeneities in isotopic composition on the deposit scale. Sm–Nd isotope data for Type C dolomite scatter off the upper right of the magnesite isochron. In combination with the respective O isotopic composition, the Sm–Nd isotope data of Type C dolomite can be modeled in terms of three distinct age groups: 1) 130±20Ma (δ18OVPDB c. −15‰), 2) 37.9±5.4Ma (δ18OVPDB c. −12.5‰), and 3) −1.7±6.8Ma (δ18OVPDB c. −11‰). Type C dolomite data indicate that druses were formed by open-system fluid flow throughout the deposit.


      PubDate: 2014-10-12T01:51:23Z
       
  • The role of Fe(III) bioreduction by methanogens in the preservation of
           organic matter in smectite
    • Abstract: Publication date: 11 December 2014
      Source:Chemical Geology, Volume 389
      Author(s): Jing Zhang , Hailiang Dong , Qiang Zeng , Abinash Agrawal
      Organic matter in sediments is largely associated with clay minerals and can be preserved over geological time. However, microbial activity can possibly influence this association and release organic matter from clay minerals via reductive or oxidative dissolution of clay minerals. In this study, the relationship between bioreduction of structural Fe(III) in smectite and organic matter release from smectite structure was investigated. A model organic compound, 12-aminolauric acid (ALA) was intercalated into the interlayer region of an iron-rich smectite (nontronite, NAu-2). Two methanogens: mesophilic Methanosarcina mazei and thermophilic Methanothermobacter thermautotrophicus were selected to reduce structural Fe(III) in ALA-intercalated nontronite. As a comparison, sodium dithionite was used to chemically reduce structural Fe(III) in the same mineral. The results showed that the intercalation of ALA into the nontronite interlayer decreased both the rate and the extent of Fe(III) bioreduction. Furthermore, methanogenesis was more inhibited by the presence of intercalated ALA in the nontronite structure relative to pure nontronite. After the bioreduction, the intercalated ALA was partially released, and the extent of release was positively correlated with the extent of Fe(III) reduction. A low reduction extent of bioreduction (<30%) resulted in little ALA release, whereas a nearly complete chemical reduction by sodium dithionite released all intercalated ALA. SEM observations and aqueous chemistry data suggested that reductive dissolution was a main mechanism for the observed ALA release. Because naturally prevalent biological reduction is a slow process with a low reduction extent relative to chemical reduction, the results of this study demonstrated that organic matter preserved within smectite structure should not be released by the mechanism of iron reduction.


      PubDate: 2014-10-08T01:40:24Z
       
  • A geostatistical framework for predicting variability in strontium
           concentrations and isotope ratios in Alaskan rivers
    • Abstract: Publication date: 11 December 2014
      Source:Chemical Geology, Volume 389
      Author(s): Clément P. Bataille , Sean R. Brennan , Jens Hartmann , Nils Moosdorf , M.J. Wooller , Gabriel J. Bowen
      Bataille and Bowen (2012) developed models to predict variations in the ratio of 87-strontium to 86-strontium (87Sr/86Sr) in rocks (bedrock model) and rivers (catchment water model) for regional provenance studies. Here, we revisit those models' formulation and calibration and apply them to predict Sr concentrations ([Sr]) and 87Sr/86Sr of Alaskan rivers. In a first step, we add several new components and/or improvements to resolve limitations of the model, including: 1) an independent siliciclastic sediment sub-model, 2) an explicit consideration of 87Sr/86Sr variability at the local scale, and 3) a fully-coupled assessment of prediction uncertainty. Tested against a compilation of 885 87Sr/86Sr rock analyses across Alaska, the new bedrock model significantly improves 87Sr/86Sr prediction accuracy in both igneous and sedimentary settings. In a second step, we develop a fully independent Sr chemical weathering model calibrated using a database of 339 [Sr] analyses from rivers of Northern Hemisphere high-latitude and predicting spatial variations in the rate of Sr release from rocks as a function of lithology, permafrost cover and slope. We combine the bedrock and Sr chemical weathering models to predict [Sr] and 87Sr/86Sr in Alaskan rivers. Tested on a dataset of 61 water samples, the resulting catchment water model explains 82% of 87Sr/86Sr variations in Alaskan rivers. We compare the average [Sr] and 87Sr/86Sr of Alaskan runoff estimated with the catchment water model to observed data of the Yukon River. The estimated average [Sr] and 87Sr/86Sr of Alaskan surface runoff – 104.3μg/L and 0.7098 respectively – differ significantly from those of the Yukon River — 139.3μg/L and 0.7137 respectively. This result calls into question the assumption that [Sr] and 87Sr/86Sr values estimated only from large rivers are representative of the Sr weathering flux from the entire Earth surface. The data products from this work provide an alternative basis for estimating 87Sr/86Sr values in rocks and rivers for regional provenance and chemical weathering studies across Alaska.


      PubDate: 2014-10-08T01:40:24Z
       
  • YBCs sanidine: A new standard for 40Ar/39Ar dating
    • Abstract: Publication date: 21 November 2014
      Source:Chemical Geology, Volume 388
      Author(s): Fei Wang , Fred Jourdan , Ching-Hua Lo , Sebastien Nomade , Herve Guillou , Rixiang Zhu , Liekun Yang , Wenbei Shi , Huile Feng , Lin Wu , Haiqing Sang
      The 40Ar/39Ar dating technique is based on neutron fluence monitors (standards). Recent investigation demonstrates that currently used standards are not as homogenous as believed and new standards are needed (Philips and Matchan, 2013). In this study, we report a new sanidine standard, YBCs, collected from a phonolite at Yabachi in Tibet, China, for single-grain 40Ar/39Ar dating. Aliquots were distributed to four international laboratories for analysis and intercalibration. The results show that YBCs crystals are homogenous in K content, 40Ar⁎/39ArK (F-value) and age at the single grain level. The standard deviations of the F-value and age have small ranges from 0.29% to 0.53% and from 0.42% to 0.52%, respectively. These show that YBCs is a suitable standard for 40Ar/39Ar geochronology. The calibrated age of YBCs is 29.286±0.206Ma, or neglecting the error in the decay constant, 29.286±0.045Ma. Finally, the intercalibration factors (which allow direct comparison between standards) between YBCs and FCs, GA1550, ACs and HB3gr are calculated as: R FCs YBCs =1.044296±0.003968, R GA1550 YBCs =0.291261±0.001148 R ACs YBCs =24.443066±0.068432 and R HB3gr YBCs = 0.020312±0.000885. These values can be used to compare YBCs with other standards directly.


      PubDate: 2014-10-02T01:21:03Z
       
  • Hydrogeochemistry of prairie pothole region wetlands: Role of long-term
           critical zone processes
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Martin B. Goldhaber , Christopher T. Mills , Jean M. Morrison , Craig A. Stricker , David M. Mushet , James W. LaBaugh
      This study addresses the geologic and hydrogeochemical processes operating at a range of scales within the prairie pothole region (PPR). The PPR is a 750,000km2 portion of north central North America that hosts millions of small wetlands known to be critical habitat for waterfowl and other wildlife. At a local scale, we characterized the geochemical evolution of the 92-ha Cottonwood Lake study area (CWLSA), located in North Dakota, USA. Critical zone processes are the long-term determinant of wetland water and groundwater geochemistry via the interaction of oxygenated groundwater with pyrite in the underlying glacial till. Pyrite oxidation produced a brown, iron oxide-bearing surface layer locally over 13m thick and an estimated minimum of 1.3×1010 g sulfate (SO4 2−) at CWLSA. We show that the majority of this SO4 2− now resides in solid-phase gypsum (CaSO4•2H2O) and gypsum-saturated groundwater. Results from the CWLSA were scaled up to a 9700km2 area surrounding CWLSA using ~1800 drill logs and literature data on wetland water chemistry for 178 wetlands within this larger area. The oxidized brown zone depth and wetland water compositional trends are very similar to the CWLSA. Additionally, surface water data from 176 southern Canadian pothole wetlands that conform to the same wetland water geochemical trends as those recorded in the CWLSA further corroborate that SO4 2− accumulation driven by pyrite oxidation is a nearly ubiquitous process in the prairie pothole region and distinguishes PPR wetlands from other wetlands worldwide that have a similar overall hydrology.


      PubDate: 2014-10-02T01:21:03Z
       
  • Zinc isotope systematics in snow and ice accretions in Central European
           mountains
    • Abstract: Publication date: 21 November 2014
      Source:Chemical Geology, Volume 388
      Author(s): Petra Voldrichova , Vladislav Chrastny , Adela Sipkova , Juraj Farkas , Martin Novak , Marketa Stepanova , Michael Krachler , Frantisek Veselovsky , Vladimir Blaha , Eva Prechova , Arnost Komarek , Leona Bohdalkova , Jan Curik , Jitka Mikova , Lucie Erbanova , Petra Pacherova
      Zinc (Zn) pollution negatively affects human and ecosystem health. We quantified atmospheric Zn inputs at six remote mountain-top locations in the Czech Republic (Central Europe), and used δ66Zn isotope ratios to identify Zn from different pollution sources. The study sites were located at an elevation of approximately 1000m near the state borders with Germany and Poland. During two winter seasons (2009–2010), over 400 samples of vertical deposition (snow) and horizontal deposition (ice accretions) were collected. Zinc pollution levels were generally low. Zinc concentrations in snow and ice accretions were less than twice as high in the east, compared to the west. Across the sites, over 90% of Zn was present in a weak-acid soluble form. Zinc concentrations were 5 times higher in ice accretions, which formed from small droplets originating in the basal cloud layer, rich in pollutants, than in snow. In contrast, droplets resulting in snow formation were larger and scavenged less pollution due to their smaller surface area. δ66Zn of Pribram sphalerite (west) and smelter-derived fly ash (west) were low, −0.23 and −0.47‰, respectively. Olkusz sphalerite (east) had a higher δ66Zn of 0.02‰. δ66Zn of snow ranged from −0.60 to 0.68‰. Ice accretions had δ66Zn between −0.67 and 0.14‰. At the three eastern sites, δ66Zn of ice accretions was lower than δ66Zn of snow, suggesting the presence of volatilized smelter-derived or coal-burning derived Zn. δ66Zn of ice accretions at two of the three western sites was higher than δ66Zn of snow. Different δ66Zn values of snow and ice accretions from the same site reflected different pollution sources, which may have been situated at different distances from the receptor site. δ66Zn of the soluble Zn fraction was higher than δ66Zn of the insoluble Zn fraction, possibly also indicating a different origin of these two Zn fractions. Zinc isotope heterogeneity in the atmosphere of remote areas indicates that δ66Zn can be a useful tool in pollution provenance studies.


      PubDate: 2014-10-02T01:21:03Z
       
  • Compositional and pressure effects on the solubility of H2O and CO2 in
           mafic melts
    • Abstract: Publication date: 21 November 2014
      Source:Chemical Geology, Volume 388
      Author(s): Tatiana A. Shishkina , Roman E. Botcharnikov , Francois Holtz , Renat R. Almeev , Aleksandra M. Jazwa , Artur A. Jakubiak
      The effect of the anhydrous composition on the solubilities of H2O and CO2 in mafic melts varying from MORB to nephelinite was investigated experimentally between 50 and 500MPa at 1200 to 1250°C. In all compositions, CO2 is only present as carbonate species in the quenched glasses. The concentrations of dissolved H2O and CO2 have been analyzed by KFT (Karl–Fischer titration) and FTIR (Fourier-transform infrared spectroscopy). The Mid-Infrared (MIR) absorption coefficients for the H2O band at 3500cm−1 are identical within error for all investigated melt compositions and equal to 59.2±4L/(mol∗cm). The absorption coefficients for the carbonate bands vary in the range 306±32 to 360±24L/(mol∗cm) for the 1430cm−1 band and in the range 349±25 to 394±27L/(mol∗cm) for the 1520cm−1 band. However, a simple correlation with the melt composition could not be determined. Water solubility in mafic to intermediate melts increases slightly with the total alkali content and the effect of composition is more pronounced at higher pressures. At 500MPa, the solubility of H2O in melts coexisting with nearly pure H2O fluids varies from 8.8 to 9.5wt.% H2O. A strong effect of melt composition on the solubility of CO2 is observed at all investigated pressures. For instance, at 500MPa, mafic melts coexisting with nearly pure CO2 fluids can dissolve from around 0.32 to more than 1.30wt.% CO2 as melt composition changes from tholeiite to nephelinite. The compositional effect on the solubility of CO2 in melts coexisting with pure CO2 fluid is best described by non-linear (exponential) correlations with compositional parameters such as the parameter Π proposed by Dixon (1997; American Mineralogist, 82: 368–378) or structural parameters (e.g., nonbridging oxygen per tetrahedrally coordinated cation). The obtained relationships are used to propose empirically derived equations of the form ln(CO2)=1.150·lnP +6.71· Π*−1.345, where CO2 is the solubility of CO2 in silicate melts in wt.% (at 1200 to 1250°C), P is pressure in MPa and Π* is a compositional parameter (Π*=Ca2+ +0.8K+ +0.7Na+ +0.4Mg2+ +0.4Fe2+)/(Si4+ +Al3+) with cations expressed as cation fractions in melt.


      PubDate: 2014-10-02T01:21:03Z
       
  • Neoproterozoic boninite-series rocks in South China: A depleted mantle
           source modified by sediment-derived melt
    • Abstract: Publication date: 21 November 2014
      Source:Chemical Geology, Volume 388
      Author(s): Jun-Hong Zhao , Paul D. Asimow
      A series of Neoproterozoic boninitic pillow lavas are hosted in Early Neoproterozoic strata along the southeastern margin of the Yangtze Block. These lavas record a period of subduction initiation during the secular evolution of South China. In this work we present analytical results from three units of pillow lavas (the Zhangyuan, Shexian and Lushan units) that form a common liquid line of descent with the previously reported 830-Ma boninites in the region, suggesting that they were produced by differentiation from similar parental melts. MELTS calculations show that major element compositions require fractionation of olivine+clinopyroxene+plagioclase+spinel from primary boninitic melts to have been accompanied by variable amounts of crustal contamination. The pillow lavas from the three localities show similar rare earth element patterns and arc-like primitive mantle normalized trace element patterns. They have high 206Pb/204Pb (18.41–19.33), 207Pb/204Pb (15.63–15.79) and 208Pb/204Pb ratios (38.78–40.26). Their εNd values decrease from the Lushan lavas (+0.7 to +1.9) to the Shexian lavas (−0.5 to −1.1) to the Zhangyuan lavas (−4.0 to −1.0), reaching values lower than the upper-level crustal contaminants and therefore indicating progressive enrichment of their sources by sediment melts. Compared with nearby but older (850Ma) MORB-like tholeiitic basalts, which yield mantle potential temperature below 1400°C and H2O contents nearly 0.13wt.%, the primary magmas of the boninite series rocks in this study were formed under higher pressures (3.5–1.0GPa) and mantle potential temperatures (1440–1500°C) with high H2O contents (3420 to 4830ppm), clear slab-derived sedimentary melt input, and harzburgite residues. The introduction of slab-derived water and sedimentary melt components into the mantle wedge at such high pressures, together with high temperatures and high degrees of melting, indicate that within a time span of only 20millionyears after subduction initiation (850 to 830Ma), the slab subducting under the southeastern margin of the Yangtze Block achieved substantial penetration to depth and interacted with hot overlying mantle, before subduction was abruptly ended by the amalgamation of the Yangtze and Cathaysia blocks.


      PubDate: 2014-10-02T01:21:03Z
       
  • The Archean magmatic-hydrothermal system of Lac Shortt (Au-REE), Abitibi,
           Canada: Insights from carbonate fingerprinting
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Olivier Nadeau , Ross Stevenson , Michel Jébrak
      Trace element and isotopic data from the Lac Shortt (Abitibi, Canada) gold and rare earth element (REE) deposit sheds light on the cogenetic association between alkali magmatism (syenites and lamprophyres), carbonatation (magmatic and carbothermal) and gold – REE mineralization. Magmatic and carbothermal carbonates from comagmatic lithologies from the Lac Shortt gold-REE deposit were analyzed for major, minor and trace elements concentrations as well as for their 87Sr/86Sr0, δ13C and δ18O isotopic ratios. Carbonates from Lac Shortt have δ13C values ranging from −4.89 to 2.38 ‰ VPDB and δ18O values ranging from 8.28 to 15.02. The lightest values are found in carbonate from the carbonatites and the heaviest values are associated with the ore-carbonates. Carbonates from the various lithologies lie between these two extremes. Initial 87Sr/86Sr ratios from the carbonates cluster tightly around 0.7013–0.7016, i.e., depleted Archean mantle signatures, although some have slightly more radiogenic values, from 0.7026 to 0.7029. The small variation in the strontium isotope ratios compared to the C and O isotopic values suggests that carbonate anions (CO3 2−) were decoupled from the cations during hydrothermal alteration. Cations such as Sr2+ and Ca2+ in carbonates from the high grade ore and from magmatic and hydro-carbothermal carbonatite appear to originate from the Late-Archean, depleted mantle. Since gold is a cation and is strongly associated with carbonatite magmatism and associated carbo-hydrothermal carbonatation, we suggest gold too has a depleted mantle origin. The carbonates are enriched in mobile high field strength elements (HFS; Ba, Sr) elements, depleted in immobile HFS elements (Th, U, Ta, Nb, Zr, Ti) and have enriched- to slightly depleted REE patterns with flat to strong light-REE enrichments. These trace element concentrations support the fact that all the lithologies were comagmatic and that the ore itself was genetically related to these lithologies. Based on present day knowledge of Archean tectonics of Abitibi, post collision extension tectonics and mantle delamination, isotope geochronology, on the magmatic-hydrothermal system of Lac Shortt itself and on the present paper's carbonate fingerprinting, we propose that Lac Shortt calc-alkaline dioritic magmatism-hydrothermalism was followed by alkaline syenitic, and eventually carbonatitic magmatism-hydro-carbothermalism, and that gold and REE originated from the carbonatite magmatic-carbo-hydrothermal system and its mantle roots, which agrees with mass balance calculations. The depleted-mantle-rooted, carbonatite exsolved a hydro-carbothermal fluid enriched in REE and gold which fenitized and oxidized the syenite and out of which the gold and REE mineralization was deposited.
      Graphical abstract image

      PubDate: 2014-09-25T00:58:03Z
       
  • Spatial, vertical and temporal variation of arsenic in shallow aquifers of
           the Bengal Basin: Controlling geochemical processes
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Ashis Biswas , Harald Neidhardt , Amit K. Kundu , Dipti Halder , Debashis Chatterjee , Zsolt Berner , Gunnar Jacks , Prosun Bhattacharya
      A detailed understanding of the geochemical processes that regulate the spatial, temporal and vertical variation of dissolved arsenic (As) in shallow aquifers (<50m) is a prerequisite for sustainable drinking water management in the Bengal Basin. The present study conducted at Chakdaha Block of the Nadia District, West Bengal, India, combined a high resolution hydrogeochemical monitoring study over 20months from two sets of piezometers (2×5) to the sediment geochemistry at areas with high (average: 146μg/L, n=5) and relatively low (average: 53.3μg/L, n=10) dissolved As concentrations in groundwater. The determination of the isotopic composition of δ2H and δ18O in groundwater of the two sites indicated the recharge of evaporative surface water to the aquifer. The concentrations of major aqueous solutes (Ca2+, Mg2+, Na+, K+, HCO3 − and Cl−) and electrical conductivity were considerably higher in wells at the high As site compared to the low As site. Additionally, at the high As site, the major ions, Fe, SO4 2−, electrical conductivity, δ2H and δ18O showed markedly greater enrichment in the shallowest part (<24m) of the aquifer compared to the deeper part, reflecting vertical layering of groundwater composition within the aquifer. The oxidation of pyrites has been attributed to the high rate of mineral dissolution resulting in such greater enrichments in this part of the aquifer. In addition, the anthropogenic input with recharge water possibly increased the concentrations of Cl − in this part of the aquifer. The vertical layering of groundwater was absent in the aquifer at the low As site. The absence of such layering and relatively low major ion concentrations and electrical conductivity could be linked to the enhanced aquifer flushing and decreased water–sediment interactions influenced by local-scale groundwater abstraction. The seasonal variations of As concentrations in groundwater were observed only in the shallowest part of the aquifers (<30m). Furthermore, the As concentrations in groundwater at the uppermost part of the shallow aquifers (<21m) increased continuously over the monitoring period at both sites. This study supports the view that the reductive dissolution of Fe oxyhydroxides coupled with competitive PO4 3− sorption reactions in the aquifer sediment enriches As in groundwater of the Bengal Basin. However, the additional Fe released by the weathering of silicate minerals, especially biotite, or the precipitation of Fe as secondary mineral phases such as siderite, vivianite and acid volatile sulfides may result in the decoupling of As and Fe enrichment in groundwater. The redox zonation within the aquifer possibly regulates the vertical distribution of As in the groundwater.


      PubDate: 2014-09-25T00:58:03Z
       
  • Climate-forced change in Hudson Bay seawater composition and temperature,
           Arctic Canada
    • Abstract: Publication date: 21 November 2014
      Source:Chemical Geology, Volume 388
      Author(s): Uwe Brand , Rosemarie E. Came , Hagit Affek , Karem Azmy , Randy Mooi , Kara Layton
      Global climate change impacts marine ecosystems, directly and indirectly, especially in the Arctic and Antarctic. We show the first long-term (1920–2011) time-series of oceanographic change in Hudson Bay, an arctic marine ecosystem, based on coupled brachiopod-calcite stable and clumped isotope results. Long-term decrease in brachiopod δ13C parallels that of seawater-DIC in Hudson Bay, and after considering its seasonal sea ice coverage, it is similar to that of the 13C-Suess effect observed in the North Atlantic and other regions. Acidification of Hudson Bay seawater leads warming by about 10–20years, and with intensified warming from the 1970s to 2010s closely coupled to earlier sea-ice breakup. Post-industrial warming of Hudson Bay is initially slow, but in later years, faster and of greater magnitude than of the coeval global oceans. Our observations for the past 90years suggest that climate-forced change contributed to an average increase of about 0.1°C and 3.6°C in sea-surface water temperature of Hudson Bay over the first 50 and subsequent 40years, respectively. This 3.7°C post-industrial warming of Hudson Bay seawater is about six times the 0.67°C increase observed during the past 100years in global ocean sea-surface temperature, which is about double the postulated increase of about 2°C for polar regions. Our results are consistent with the general notion that polar marine environments, such as Hudson Bay, can serve as sensitive indicators of change in climate, and of change still to come for lower latitude ecosystems.


      PubDate: 2014-09-25T00:58:03Z
       
  • A 9000-year carbon isotopic record of acid-soluble organic matter in a
           stalagmite from Heshang Cave, central China: Paleoclimate implications
    • Abstract: Publication date: 21 November 2014
      Source:Chemical Geology, Volume 388
      Author(s): Xiuli Li , Chaoyong Hu , Junhua Huang , Shucheng Xie , Andy Baker
      Organic matter preserved in speleothems has the potential to reflect the changes of the overlying vegetation and soil ecosystem. Here we report the first near 50-yr-resolution acid-soluble organic matter (ASOM) carbon isotope (δ13CASOM) sequence derived from a laminated stalagmite (HS-4) spanning the last 9ky in Heshang Cave, central China. The δ13CASOM values vary between −25.8‰ and −22.0‰, with lower values from 9 to 4ka BP and less negative values from 4 to 0ka BP. We postulate that the δ13CASOM sequence is mainly controlled by temperature and water balance. Temperature could affect both vegetation physiology and microbial degradation in soil horizons. The influence of temperature on the δ13CASOM is supported by the negative correlation (r =−0.48, p <0.001) between the δ13CASOM record and the paleotemperature record in the nearby Dajiuhu peatland. The water balance can affect the retention time of organic matter in soils. Under drier conditions, the soil organic matter will be retained longer and is more likely to be biologically degraded, resulting in more negative δ13CASOM values. Our results reveal that δ13CASOM in speleothems has the potential to reflect the response of vegetation and/or soil processes to paleoclimate changes.


      PubDate: 2014-09-25T00:58:03Z
       
  • Tracing Sr isotopic composition in space and time across the Yangtze River
           basin
    • Abstract: Publication date: 21 November 2014
      Source:Chemical Geology, Volume 388
      Author(s): Chao Luo , Hongbo Zheng , Ryuji Tada , Weihua Wu , Tomohisa Irino , Shouye Yang , Keita Saito
      Variation of dissolved 87Sr/86Sr in the Yangtze River is poorly documented compared to other Tibetan-sourced rivers. Here, we trace the Sr isotopic composition in space and time across the Yangtze River basin using a systematic sampling strategy. The 87Sr/86Sr values of samples collected at different depths within the water column at three gauge stations located in the upper, middle and lower reaches of the river are very similar, indicating a well-mixed water body at a given location. Data from basin wide-samples shows low 87Sr/86Sr values in the upper reaches while higher 87Sr/86Sr values in the middle-lower reaches which mainly reflect the controls of source rocks. A time series record at the lowermost reaches of the river indicates a temporal variation of 87Sr/86Sr values from 0.7101 to 0.7109, with a discharge weighted value of 0.7106. We suggest that temporal variations of the isotopic composition result from changes in the relative contributions from different terrains associated with spatial variability of rainfall within the basin. A mixing model based on Sr values deduced from basin-wide sampling from different seasons yields similar temporal variations of Sr isotopic composition compared to experimental data. Calculation of Sr fluxes indicates that the upper reaches contribute 3 times as much as the middle and lower reaches to total Sr flux. The annual Sr flux of the Yangtze River to the East China Sea is estimated to be 1.9×109 mol·a−1, which is one magnitude higher than that of the Brahmaputra and Ganges.


      PubDate: 2014-09-25T00:58:03Z
       
  • Pressure determination in HDAC experiments with haplogranite glass plus
           water by laser interferometry
    • Abstract: Publication date: 21 November 2014
      Source:Chemical Geology, Volume 388
      Author(s): Giulio Solferino , Alan J. Anderson
      The pressure in a series of Hydrothermal Diamond Anvil Cell (HDAC) experiments with water plus haplogranite was determined via in-situ observation of the alpha to beta quartz transformation in the range 130–900MPa and 600–800°C, by means of laser interferometry. The pressures measured at the alpha-beta quartz transformation temperature are found to be similar or less than the values estimated using the EOS of pure water (including the error) for runs where the transition temperature is ≤665°C. On the contrary, at ~780°C the pressure in the HDAC is ~100MPa greater than that estimated using pure water isochore. A simplified model of haplogranite dissolution in water for an isochoric sample chamber is proposed to explain the negative sign and the variation of the observed discrepancy between measured and calculated pressure. We suggest that, beyond the change in the intrinsic properties of the fluid phase with increasing solute concentration, two factors control the P–T path in the HDAC: (1) hydration of the glass prior to the glass transition; and (2) changing volume of the aqueous pressure medium. The measurements made in this study demonstrate that laser interferometry on quartz inversion is an effective tool for precise pressure determination in HDAC experiments aimed at the investigation of physical and chemical properties of quartz-saturated hydrous melts in the mid and upper continental crust (i.e., P<1GPa).


      PubDate: 2014-09-25T00:58:03Z
       
  • Nitrogen stable isotope composition and the origins of cupric nitrate
           mineralization
    • Abstract: Publication date: 21 November 2014
      Source:Chemical Geology, Volume 388
      Author(s): Erik B. Melchiorre , Becky C. Talyn
      Cupric nitrate minerals are rare, but when present may record evidence of past biological activity. The cupric nitrate minerals we examined from localities around the world are broadly divided into four main groups based upon their inferred conditions of formation and δ15N values. These groups, named for their probable source of nitrogen, include: 1) termite, 2) bat, 3) atmospheric and 4) coal mine fires and natural gas. It is clear that in three of these categories, the role of past or present biological activity is critical for formation of cupric nitrate minerals. For cupric nitrate minerals associated with an atmospheric source of nitrogen the link is not clear, though a microbial source is possible. Nitrogen isotope analyses of gerhardtite and buttgenbachite mineral pairs from two localities suggest fractionation between these two minerals at ambient temperatures is 1.1‰ (Australia) to 0.8‰ (Democratic Republic of Congo). Similarly, differences between δ15N values of sal ammoniac and gerhardtite pairs suggest fractionation between these minerals at higher temperatures in coal bed fires is 1.9‰ (Pennsylvania, USA) to 1.7‰ (Germany).


      PubDate: 2014-09-25T00:58:03Z
       
  • Experimental study on the pseudobinary H2O+NaAlSi3O8 at
           600–800°C and 0.3–2.4GPa
    • Abstract: Publication date: 21 November 2014
      Source:Chemical Geology, Volume 388
      Author(s): Christian Schmidt , Anke Wohlers , Katharina Marquardt , Anke Watenphul
      The pseudobinary H2O+NaAlSi3O8 was studied at compositions between 35 and 66 mass% NaAlSi3O8, at temperatures between 600 and 804°C, and at pressures ranging from 0.3 to 2.41GPa using a hydrothermal diamond-anvil cell (HDAC), cold-seal hydrothermal pressure vessels (CSPVs), and a piston-cylinder apparatus (PC). We propose a revised pseudosection for bulk compositions near 40 mass% NaAlSi3O8, the critical composition at 750°C obtained from the HDAC experiments. The corresponding critical pressure was 1.56GPa, as determined directly using zircon as Raman spectroscopic pressure sensor. Pressures at the melt-fluid homogenization temperature and along six isochoric P–T paths obtained from the Raman spectrum of zircon were close to or slightly lower than the pressure calculated from the EoS of H2O based on the vapor dissolution or ice melting temperature, which demonstrates that the effect of albite or NaAlSi3O8 melt dissolution is small. In an experiment up to 700°C and 2.28GPa, no clear evidence was found from optical observation for the hypothetical reappearance of melt-fluid immiscibility at higher pressure. The results consistently point to incongruent behavior at elevated pressures by the formation of paragonite. The HDAC experiments showed that paragonite nucleation was generally sluggish and occurred usually within several hours to days. For the critical composition of 40 mass% NaAlSi3O8, the temperature limit of paragonite stability was determined to a pressure of 1.2GPa using CSPVs and a PC and found to increase significantly with pressure. There was no discernible difference in its P–T location from that of the paragonite breakdown reaction to corundum, albite, and water reported in the literature for pressures to 0.7GPa. The paragonite stability limit intersected the critical curve at 764°C, 1.53GPa. Hence, our experiments indicate the existence of three previously not recognized stability fields in pseudosections for bulk compositions near 40 mass% NaAlSi3O8, namely (1) paragonite+albite+fluid, (2) paragonite+melt+fluid, and (3) paragonite+fluid. Formation of a substantial fraction of paragonite at these NaAlSi3O8 concentrations implies that the aqueous fluid must have a peralkaline composition, which has important implications for mass transfer in the crust an upper mantle, e.g. enhancement of the solubility of high field strength elements.


      PubDate: 2014-09-25T00:58:03Z
       
  • Origin and interactions of fluids circulating over the Amik Basin (Hatay,
           Turkey) and relationships with the hydrologic, geologic and tectonic
           settings
    • Abstract: Publication date: 21 November 2014
      Source:Chemical Geology, Volume 388
      Author(s): G. Yuce , F. Italiano , W. D'Alessandro , T.H. Yalcin , D.U. Yasin , A.H. Gulbay , N.N. Ozyurt , B. Rojay , V. Karabacak , S. Bellomo , L. Brusca , T. Yang , C.C. Fu , C.W. Lai , A. Ozacar , V. Walia
      We investigated the geochemical features of the fluids circulating over the Amik Basin (SE Turkey–Syria border), which is crossed by the Northern extension of the DSF (Dead Sea Fault) and represents the boundary area of three tectonic plates (Anatolian, Arabian and African plates). We collected 34 water samples (thermal and cold from natural springs and boreholes) as well as 8 gas samples (bubbling and gas seepage) besides the gases dissolved in the sampled waters. The results show that the dissolved gas phase is a mixture of shallow (atmospheric) and deep components either of mantle and crustal origin. Coherently the sampled waters are variable mixtures of shallow and deep ground waters, the latter being characterised by higher salinity and longer residence times. The deep groundwaters (from boreholes deeper than 1000m) have a CH4-dominated dissolved gas phase related to the presence of hydrocarbon reservoirs. The very unique tectonic setting of the area includes the presence of an ophiolitic block outcropping in the westernmost area on the African Plate, as well as basalts located to the North and East on the Arabic Plate. The diffuse presence of CO2-enriched gases, although diluted by the huge groundwater circulation, testifies a regional degassing activity. Fluids circulating over the ophiolitic block are marked by H2-dominated gases with abiogenic methane and high-pH waters. The measured 3He/4He isotopic ratios display contributions from both crustal and mantle-derived sources over both sides of the DSF. Although the serpentinization process is generally independent from mantle-type contribution, the recorded helium isotopic ratios highlight variable contents of mantle-derived fluids. Due to the absence of recent volcanism over the western side of the basin (African Plate), we argue that CO2-rich volatiles carrying mantle-type helium and enriched in heavy carbon, are degassed by deep-rooted regional faults rather than from volcanic sources.


      PubDate: 2014-09-25T00:58:03Z
       
  • In situ determination of K–Ar ages from minerals and rocks using
           simultaneous laser-induced plasma spectroscopy and noble gas mass
           spectrometry
    • Abstract: Publication date: 21 November 2014
      Source:Chemical Geology, Volume 388
      Author(s): Jesús Solé
      Geochronology is one of the foundations of Earth Sciences and is evolving constantly towards higher reliability and usefulness. Currently, Ar–Ar and U–Pb dating are the geochronological techniques most used. However, the classical K–Ar technique can be modified to measure the age of single minerals or rock sections in situ. This new technique combines the use of laser-induced plasma spectroscopy for the determination of potassium, with noble gas mass spectrometry for the determination of argon, both extracted simultaneously by laser ablation. This work constitutes both a proof-of-concept and a test of this method on 13 samples (53 analyses), with ages in the range 950–70Ma. Deviations from the conventional K–Ar age are lower than 5% for most samples. The method is characterized by easy analytical procedures and relatively low uncertainty with the equipment used. This method is excellent for exploratory chronology of the earth and extraterrestrial bodies owing to the simple sample preparation and low turnaround time for the analysis.
      Graphical abstract image

      PubDate: 2014-09-25T00:58:03Z
       
  • New aspects of sulfur biogeochemistry during ore deposition from δ34S
           of elemental sulfur and organic sulfur from the Here's Your Chance
           Pb/Zn/Ag deposit
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Alex I. Holman , Kliti Grice , Paul F. Greenwood , Michael E. Böttcher , John L. Walshe , Katy A. Evans
      Sulfur isotope studies of base metal sulfide deposits have mostly focussed on sulfide minerals, but elemental sulfur and organic sulfur are also potentially significant components of the sulfur cycle during ore deposition. The δ 34S of elemental sulfur and organic sulfur isolated from the Paleoproterozoic Here's Your Chance (HYC) Pb/Zn/Ag deposit (McArthur Basin, northern Australia) were measured to be between +5 and +8‰, approximately 6 to 7‰ heavier than the median values of first-generation HYC sulfides. Elemental sulfur and organic sulfur are thought to have been formed contemporaneously with the first generation of metal sulfides. The δ 34S of organic sulfur showed an increasing trend along the path of the mineralising fluid, as sulfate was progressively 34S-enriched due to Rayleigh distillation. The δ 34S data support a model in which bacterial sulfate reduction produced dissolved sulfide with δ 34S of 0 to +5‰. The subsequent oxidation of sulfide produced reactive sulfur species such as polysulfide ions, which were then incorporated into organic matter.


      PubDate: 2014-09-21T00:37:27Z
       
  • Redistribution of Pb during transformation of monohydrocalcite to
           aragonite
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Takashi Munemoto , Keisuke Fukushi , Yoshiki Kanzaki , Takashi Murakami
      Monohydrocalcite was transformed to aragonite in the presence of Pb in solution to understand the Pb redistribution mechanisms and kinetics during the transformation. Monohydrocalcite (Ca0.98Mg0.02CO3·1.00H2O) was synthesized, and along with Pb stock solution, added to a buffer solution; batch sorption experiments were carried out (i) at different initial-Pb concentrations ranging from 1 to 100μM (1–30 and 50–100μM, referred to as low and high initial-Pb concentrations, respectively) for 24h and (ii) at different reaction time for 1–24h at 1 and 100μM of the initial Pb concentrations. The CO3 2− concentration (24.4meq/L), pH (9.50) and temperature (25°C) were kept constant within experimental errors throughout the experiments. The transformation of monohydrocalcite was completed in 24h, probably in 15h, at the low initial-Pb concentrations but not even in 24h at the high initial-Pb concentrations, suggesting that the aqueous Pb concentration affects the rates of dissolution of monohydrocalcite and/or nucleation and crystal growth of aragonite, and thus, the transformation rate. Aragonite was shortly formed as monohydrocalcite was dissolved and almost all amount of Ca dissolved from monohydrocalcite formed aragonite. ~90–96% of the initial Pb was adsorbed onto monohydrocalcite before the transformation started, and >95% of the initial Pb was distributed in solid phases at any elapsed time of the reaction and at any initial-Pb concentrations, indicating that the Pb redistribution rate was as fast as the transformation rate of monohydrocalcite. Pb carbonate (possibly hydrocerussite) was formed at the high initial-Pb concentrations but not at the low initial-Pb concentrations. The precipitation rate of the Pb carbonate is slower than the transformation rate of monohydrocalcite. The Pb sorption on aragonite occurred dominantly by incorporation. After the transformation, accordingly, almost all Pb was incorporated in aragonite at the low initial-Pb concentrations, and in aragonite and Pb carbonate at the high initial-Pb concentrations. The incorporation of Pb in aragonite is well explained by the formation of (Ca,Pb)CO3 solid-solution. The Pb redistribution during transformation of monohydrocalcite to aragonite revealed by the present study will give a deeper understanding of trace element transport at the Earth's surface when the transport is accompanied by mineral transformation.
      Graphical abstract image

      PubDate: 2014-09-21T00:37:27Z
       
  • Microbial dissolution and reduction of uranyl crystals by Shewanella
           oneidensis MR-1
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Yu Yang , Shuao Wang , Thomas E. Albrecht-Schmitt
      Dissimilatory metal-reducing bacteria (DMRB) can harvest energy for growth and activities by respiring metals, but it is so far unknown whether DMRB can acquire crystalline-phase actinides. In the present study, we used Shewanella oneidensis MR-1 to investigate microbially-mediated dissolution and reduction of U(VI) in three uranyl(VI) borate and boronate crystals (i.e., UO2(CH3BO2)(H2O) (UCBO); UO2B2O4 (UBO); and Na[(UO2)B6O10(OH)]·2H2O (NaUBO)). Comparison of the dissolved U(VI) concentrations between samples with and without bacteria indicates that MR-1 facilitated dissolution of UCBO and UBO. Based on the assumption that only dissolved U(VI) was reduced, U(VI) reduction was substantially underestimated for UCBO and NaUBO, indicating that MR-1 directly reduced crystalline U(VI) in these two compounds. Laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis implied that interactions occurred between microbial ligands and the residual particles of uranyl compounds. We found that S. oneidensis MR-1 can mediate the dissolution and reduction of crystalline U(VI) through facilitated dissolution and consequent reduction of crystals or direct reduction of U(VI) in crystals. These results help evaluate the environmental fate of solid-phase U(VI), critical for predicting U transport and remediating U-contaminated sites.


      PubDate: 2014-09-21T00:37:27Z
       
  • The curved 14C vs. δ13C relationship in dissolved inorganic carbon: A
           useful tool for groundwater age- and geochemical interpretations
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Liang-Feng Han , L. Niel Plummer , Pradeep Aggarwal
      Determination of the 14C content of dissolved inorganic carbon (DIC) is useful for dating of groundwater. However, in addition to radioactive decay, the 14C content in DIC (14CDIC) can be affected by many geochemical and physical processes and numerous models have been proposed to refine radiocarbon ages of DIC in groundwater systems. Changes in the δ13C content of DIC (δ13CDIC) often can be used to deduce the processes that affect the carbon isotopic composition of DIC and the 14C value during the chemical evolution of groundwater. This paper shows that a curved relationship of 14CDIC vs. δ13CDIC will be observed for groundwater systems if (1) the change in δ13C value in DIC is caused by a first-order or pseudo-first-order process, e.g. isotopic exchange between DIC and solid carbonate, (2) the reaction/process progresses with the ageing of the groundwater, i.e. with decay of 14C in DIC, and (3) the magnitude of the rate of change in δ13C of DIC is comparable with that of 14C decay. In this paper, we use a lumped parameter method to derive a model based on the curved relationship between 14CDIC and δ13CDIC. The derived model, if used for isotopic exchange between DIC and solid carbonate, is identical to that derived by Gonfiantini and Zuppi (2003). The curved relationship of 14CDIC vs. δ13CDIC can be applied to interpret the age of the DIC in groundwater. Results of age calculations using the method discussed in this paper are compared with those obtained by using other methods that calculate the age of DIC based on adjusted initial radiocarbon values for individual samples. This paper shows that in addition to groundwater age interpretation, the lumped parameter method presented here also provides a useful tool for geochemical interpretations, e.g. estimation of apparent rates of geochemical reactions and revealing the complexity of the geochemical environment.


      PubDate: 2014-09-21T00:37:27Z
       
  • Evidence for the control of the geochemistry of Amazonian floodplain
           sediments by stratification of suspended sediments in the Amazon
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Martin Roddaz , Jérôme Viers , Patricia Moreira-Turcq , Camille Blondel , Francis Sondag , Jean-Loup Guyot , Luciane Moreira
      Holocene and historical Amazonian floodplain deposits collected from two cores of the “Varzea do Curuai” flooded area (Brazil) were analysed for major and trace element geochemistry as well as Nd–Sr isotopic compositions (21 samples). The TA11 and TA14 cores (110cm and 270cm in depth, respectively) were collected at different locations in the Varzea, near a channel inlet connecting the Amazon River to the Varzea and at the centre of the Varzea, respectively. The two cores represent records of sedimentation on different time-scales, with TA11 covering the last 100years and TA14 extending back to 5600yearscal BP. Although the sediments are generally coarser in TA11 than in TA14, the major and trace element concentrations, Cr/Th and Th/Sc and Eu anomalies and Nd–Sr isotopic compositions in both cores fail to show any clear variations with depth. However, there are chemical differences between the two analysed cores. The TA14 sediments have higher Al/Si and Chemical Index of Alteration (CIA) values than those of TA11. The TA14 sediments are enriched in Th, U, Y, Nb, REE, Cs, Rb, V and Ni but show slightly depleted MgO, CaO and Sr and more strongly depleted Na2O, Zr and Hf compared to TA11. In addition, the Nd–Sr isotopic compositions of the TA11 sediment core are on the whole similar to the Solimões suspended particulate matter (SPM), whereas TA14 has a similar Nd–Sr isotopic composition compared with the SPM of the Amazon River at Obidos. These differences are best explained by chemical stratification of the SPM of the Amazon River. During flooding of the Amazon River, coarser grained particulates supplied by the Solimões River are deposited in the deepest environments near the channel inlet, as recorded in the TA11 sediment core. By contrast, finer grained suspended sediments derived from the Madeira River are transported into the shallower environments of the Varzea system and deposited as a result of flow expansion and loss of carrying power, as recorded in the TA14 sediment core. Using the Nd isotopic compositions and concentrations of each sediment core, we calculate between ~20 and ~85% of the sediment input to the TA14 core site is derived from the Madeira River, while between 52% and 100% of the sediment input to the TA11 core sample is supplied by the Solimões River. Similar calculation using the Sr isotopic compositions and concentrations of TA11 and TA14 sediments gave similar results with between ~32% and ~49% of TA14 sediments deriving from the Solimões River while between ~70% and ~100% of TA11 sediments originate from the Solimões River. Finally, our findings lead us to question whether particular Amazon deposits that do not integrate the entire range of SPM present in the river are representative of the overall sediment load provided today and in the past by global rivers.


      PubDate: 2014-09-21T00:37:27Z
       
  • Hydrothermal carbonate chimneys from a continental rift (Afar Rift):
           Mineralogy, geochemistry, and mode of formation
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): V.M. Dekov , N.M. Egueh , G.D. Kamenov , G. Bayon , S.V. Lalonde , M. Schmidt , V. Liebetrau , F. Munnik , Y. Fouquet , M. Tanimizu , M.O. Awaleh , I. Guirreh , B. Le Gall
      Carbonate chimney-like deposits up to 60m high are scattered or arranged in rows at the shores of a desiccating hypersaline and alkaline lake from a continental rift setting (Lake Abhé, Afar Rift, Djibouti). The chimneys formed sub-aqueously in the lake water body at a higher water level than observed today. Alternating calcite and low-Mg calcite+silica concentric layers compose the chimney structures. Mineralogical and geochemical investigations of the chimneys, lake water, and hot spring (hydrothermal) fluids suggest that the chimneys are a result of rapid carbonate precipitation during the mixing of hydrothermal fluids with lake water. In contrast to the hot spring fluid, lake water is enriched in HREE and possesses a pronounced positive Ce anomaly, features that are preserved in the carbonate chimney layers. Mixing calculations based on Sr-isotope and concentration data indicate a hydrothermal fluid contribution of ~45% in the chimney interior, which decreases to ~4% in the external chimney layer. Sr in the hydrothermal fluids is predominantly leached from the underlying volcanic rocks, whereas the lake's Sr budget is dominated by riverine input. Considering the fluid mixing ratios calculated by Sr-data, the measured C and O isotope compositions indicate that chimney carbonates precipitated at temperatures between 14°C (internal part) and 22°C (external part) with δ13C-carbonate mainly controlled by isotope equilibrium exchange of lake water with atmospheric CO2. The low-Mg calcite layers, including the outermost layer, have enhanced signals of lake water inheritance based on elevated concentrations of immobile elements, ΣREE, and Sr and Ca isotope compositions. Ca-isotope data reveal that internal chimney layers formed by non-equilibrium calcite precipitation with a predominantly hydrothermal Ca source. The external low-Mg calcite layer received Ca contributions from both hydrothermal fluid and lake water, with the latter being the dominant Ca source. Highly positive δ44/40Ca of lake water likely reflects non-equilibrium Ca-carbonate precipitation during lake water evaporation with resulting 44Ca enrichment of residual lake water. The strong degree of 44Ca enrichment may point towards multiple lake drying and Ca-reservoir depletion events. Coupled C–O–Ca-isotope data of the sampled carbonate chimney suggest late-stage (low-temperature) hydrothermal carbonate chimney formation during strongly evaporative lake conditions at the time of low-Mg calcite precipitation. U–Th age dating suggests that the chimneys formed no earlier than 0.82kyrBP (0.28±0.54).


      PubDate: 2014-09-21T00:37:27Z
       
  • Diagenesis of speleothems and its effect on the accuracy of 230Th/U-ages
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Denis Scholz , Jan Tolzmann , Dirk L. Hoffmann , Klaus P. Jochum , Christoph Spötl , Dana F.C. Riechelmann
      Speleothems can be precisely dated by U-series disequilibrium methods. One basic assumption of the 230Th/U-dating method is that the system remains closed and U and Th isotopes are neither lost nor added after deposition. For stalagmites, this requirement is usually fulfilled due to their protected environment in caves. However, undersaturated drip water may lead to diagenesis of speleothem CaCO3 and altered 230Th/U-ages. Here we present a detailed petrographic investigation using thin sections and 230Th/U-ages of a stalagmite from the Riesenberghöhle, northern Germany, which grew during Marine Isotope Stages 5e, c and a. The occurrence of a mosaic fabric in large parts of the stalagmite in combination with several age inversions is a clear indication for diagenetic alteration. We discuss the potential diagenetic processes and quantitatively model their influence on the 230Th/U ages. Post-depositional U loss leads to increasing (230Th/238U) activity ratios and, thus, apparently older ages. In contrast, post-depositional addition of U results in decreased (230Th/238U) activity ratios and apparently younger ages. Comparison with our modelling results suggests that diagenesis occurred early after deposition. Initial and post-depositional detrital contamination probably played a minor role for the observed age inversions.


      PubDate: 2014-09-21T00:37:27Z
       
  • Determination of in situ dissolved inorganic carbon concentration and
           alkalinity for marine sedimentary porewater
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Justine Sauvage , Arthur J. Spivack , Richard W. Murray , Steven D'Hondt
      Dissolved inorganic carbon (DIC) concentration and total alkalinity in marine sediment vary with biological activity, mineral diagenesis and past bottom ocean water composition. Reliable interpretation of this data is often compromised due to precipitation of calcium carbonate (CaCO3) during sediment recovery, processing and sample storage. Here we present and test a method that corrects for this precipitation and consequently allows quantification of in situ carbonate system chemistry. Our method relies on the over-determination of the dissolved carbonate system by (i) measuring DIC, alkalinity and calcium, and (ii) explicitly assuming CaCO3 saturation in the sediment. We experimentally tested this method using data from Integrated Ocean Drilling Program (IODP) Site U1368 in the South Pacific Gyre. Our results show that we can accurately reproduce in situ aqueous carbonate system chemistry if DIC, alkalinity and calcium concentration are measured simultaneously. At Site U1368, the correction for sampling associated precipitation is equivalent to 4.5 and 8.9% of the measured DIC and alkalinity, respectively. The method is well suited for any sediment porewater that is saturated with respect to calcium carbonate; consequently, it is applicable for approximately 50% of the global oceanic seafloor.


      PubDate: 2014-09-21T00:37:27Z
       
  • Key factors to understand in-situ behavior of Cs in
           Callovo–Oxfordian clay-rock (France)
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Z. Chen , G. Montavon , S. Ribet , Z. Guo , J.C. Robinet , K. David , C. Tournassat , B. Grambow , C. Landesman
      Understanding the behavior of 137Cs and 135Cs in soils and geological formations is of considerable interest in the context of nuclear accidents and nuclear waste repositories. Although the clay fraction is known to be responsible for sorption, there are still unanswered questions raised by the literature data concerning (i) the reversibility of the sorption process(es), (ii) the validity of the additivity rule (the overall distribution coefficient (Kd) for a radionuclide on a mixture of minerals is predicted from the distribution coefficients measured on individual minerals) and (iii) the validity of model transposition from dispersed systems to consolidated/intact systems. Because of these uncertainties, the validity of sorption models at equilibrium under in-situ conditions and for very long-term interaction is still pending. These different issues are studied in the present work for the Callovo–Oxfordian (COx) clay-rich rock Formation, which is under investigation in France as a geological barrier for a long-term nuclear waste repository. The work is based on sorption data measured on thirteen samples of different mineralogy taken from five different boreholes at several depths within the COx sedimentary layer. To our knowledge, it is the most extended Cs sorption dataset that has been published for a single clay formation in term of (i) sample locations (and thus natural variability), (ii) sorption conditions (powder dispersed in suspension, compacted powders and intact samples) and (iii) equilibration time (from one week to five years). Moreover, for the first time ever, radioactive Cs sorption results were compared to the natural distribution of non-radioactive Cs isotopes between pore water and the solid phase. The experimental system appeared to be in chemical equilibrium as much as can be expected for an ion-exchange reaction. More particularly, no kinetically-controlled process leading to partial Cs irreversibility was observed, in contrary to what was found in the literature for soils. This difference in behavior may be related to the difference in the illite studied, i.e. a soil-type illite which would be more altered than a sedimentary formation-type illite. No decrease in site capacity was observed between dispersed and intact/compacted states. A model based on exchange reactions with cations interacting with illite (frayed edge, type-II and planar sites) and mixed layer illite–smectite (I/S) (planar sites) using parameters published in the literature enabled the Kd variation to be described as a function of Cs concentration, the mineralogy of the samples, the change in water composition and the temperature (22–80°C). Our study clearly demonstrates that no frayed edge sites should be considered on the illite fraction of I/S, thus emphasizing the difference of sorption properties between an I/S mixed layer mineral and a corresponding mechanical mix of illite and smectite minerals. The robustness of the model was confirmed by data analysis describing the behavior of naturally-occurring Cs in the formation thereby demonstrating the effectiveness of the Cs sorption processes in a very long-time period prospective. Lastly, the model was used to predict the sorption of trace concentrations of Cs in the COx Formation on the time-scale relevant for nuclear waste disposal performance assessment. As expected, the retention was significant with Kd values ranging from 100 to 2000L/kg whatever the conditions that were probed and a simulation covering a period of over 105 years could show that the COx Formation is an efficient barrier to prevent Cs transport from the storage facility to the surrounding environment.
      Graphical abstract image

      PubDate: 2014-09-08T23:16:02Z
       
  • Non-equilibrium fractionation of stable carbon isotopes in chemosynthetic
           mussels
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): K. Nedoncelle , N. Le Bris , M. de Rafélis , N. Labourdette , F. Lartaud
      Chemosynthetic bivalves from deep-sea hydrothermal vents exploit the energy derived from chemical compounds, such as methane, sulfide or hydrogen, using symbiotic bacteria that are able to fix inorganic carbon. Available chemical resources in their habitat vary widely at various scales, from the vent field scale to the micro-habitat scale. Parallel to this environmental heterogeneity, Bathymodiolus species are considered to be flexible in their energy acquisition pathways. The goal of this study was to determine whether the isotopic compositions archived in the shells of hydrothermal vent mussels could trace chemical energy sources and their variability over spatial and temporal scales. Two different species (Bathymodiolus azoricus and Bathymodiolus thermophilus) inhabiting three vent fields with contrasted geochemical features on the Mid Atlantic Ridge (MAR; Rainbow and Menez Gwen) and the East Pacific Rise (EPR; 9°47′N), were considered for carbon isotopes and growth rate variation along the shell length. The study revealed that 13C fractionation between shells and seawater is higher than expected from calcite–bicarbonate equilibrium fractionation, suggesting a significant influence of the chemosynthetic pathway on the shell composition. Furthermore, significant differences in δ13Cshell fractionation with respect to seawater are observed between sites and habitats of the two MAR vent fields, suggesting that different chemosynthetic pathway (e.g. methanotrophic and thiotrophic) could lead to variable enrichments of the shell in 13C. Mussels supposed to rely more largely on methanotrophy (at Rainbow where free sulfide is unavailable) display a lower δ13Cshell values than mussels relying also on sulfide-oxidizing symbiosis (at Menez Gwen). Variability in δ13Cshell between habitats, or between individuals within the same assemblage, could thus reflect differences in the symbiosis activity at a micro-habitat scale. These isotopic signatures could provide useful information on the relationships between micro-habitat properties, symbiont activity and shell mineralization.


      PubDate: 2014-09-08T23:16:02Z
       
  • Formation of H2, CH4 and N-species during low-temperature experimental
           alteration of ultramafic rocks
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): I. Okland , S. Huang , I.H. Thorseth , R.B. Pedersen
      Hydrogen, methane and ammonium are important chemical species for chemolithotrophic microorganisms that sustain subsurface endolithic communities. It is well known that production of H2 through serpentinization of the primary Fe(II)-containing minerals, olivine and pyroxene, in ultramafic rocks and reduction of CO2 to CH4 are processes that occur at higher temperatures. Knowledge is, however, limited about these reactions under low-temperature conditions, and how they are affected by Fe(II)-containing secondary minerals, such as brucite and serpentine, which are commonly found in these environments. In this experimental study, we explored the formation of H2, CH4 and NH4 during low-temperature (25°C) reactions between deionised water and 1) unaltered, 2) medium altered and 3) highly altered dunites, over a period of 99days. Hydrogen was detected in all three experiments and the concentration increased over time. The O2 concentration decreased over time, but O2 was still present at significant levels when H2 started to form. This suggests that the H2 was formed locally at the surface of mineral grains rather than in the hypoxic solution. The highly altered dunite gave the highest and the unaltered dunite the lowest H2 concentrations, and while the O2 concentration decreased fast with the medium altered dunite, significant amounts of O2 still remained with the unaltered dunite at the end of the experiments. Both the medium and highly altered dunite gave O2 concentrations close to or below the detection limit during the last part of the experiment. Factors that likely affected the H2 production are Fe-content, mineralogy, dissolution rates and surface areas of the minerals as well as the presence of catalytic minerals such as chromite and magnetite. Methane was also detected and increased with time in all experiments. The results indicate that CH4 formation also occurred locally on the surface of minerals. Detection of both NH4 and NO3 in the aqueous solutions indicates that previously absorbed N-species were leached out during the experiment. Increasing NH4 and decreasing NO3 concentrations over time suggest NO3 reduction, likely catalysed by mineral surfaces. The results imply that low-temperature water–rock reactions in moderately to highly altered ultramafic rocks provide reduced chemical species that can be utilized as electron donors in energy-yielding metabolic processes by subsurface microorganisms. Such reactions could play an important role in sustaining subsurface microbial communities, particularly in ophiolites and near seafloor parts of the ultramafic oceanic lithosphere at slow-spreading mid-ocean ridges and possibly also in other rocks and sediments. The results suggest that brucite and possibly also serpentine may be more important sources of Fe(II) and thus H2 formation than olivine in low-temperature ultramafic systems. Formation of H2 in reducing microenvironments on mineral surfaces surrounded by oxic conditions increases the extent of H2 formation and the subsurface habitat for aerobic H2-oxidizing microorganisms.


      PubDate: 2014-09-08T23:16:02Z
       
  • Iron–clay interactions: Detailed study of the mineralogical
           transformation of claystone with emphasis on the formation of iron-rich
           T–O phyllosilicates in a step-by-step cooling experiment from
           90°C to 40°C
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Isabella Pignatelli , Franck Bourdelle , Danièle Bartier , Régine Mosser-Ruck , Laurent Truche , Enrico Mugnaioli , Nicolas Michau
      Iron–minerals–water interactions are of primary importance in the contexts of underground structure engineering (e.g. reactive barriers or deep geological storage) and for the understanding of secondary alteration processes in primitive meteorites. To improve our understanding of these systems, we determine the mineralogical transformations induced by the association of iron and silicates during a cooling through an experimental simulation of iron–clay interactions with a step-by-step procedure in the range of 90°C to 40°C. The run products and solutions are well characterised, by means of different techniques (X-ray diffraction, scanning and transmission electron microscopy, manocalcimetry, inductively coupled plasma optical emission spectrometry and ion chromatography), and the thermodynamic data concerning Fe-bearing phyllosilicates are well-tested comparing the modelling and experimental results. Therefore, the main mineralogical modifications observed include the remarkable formation of cronstedtite and greenalite, as well as the formation of magnetite at all temperatures, along with a significant dissolution of quartz, mixed-layer illite–smectite clays, illite (affecting more than 70% of each mineralogical phase) and a partial alteration of chlorite, kaolinite and dolomite. The experimental results confirm the reaction path predicted by thermodynamic modelling, i.e. the formation of iron-rich T–O phyllosilicates (cronstedtite and greenalite) and magnetite at the expense of metal iron and silicates. Both the experimental and thermodynamic results presented in this study provide important constraints to well predict the impact of nuclear waste canister corrosion in a claystone media and to better understand secondary alteration processes, which could also affect the mineralogical and chemical composition of primitive meteorites.


      PubDate: 2014-09-08T23:16:02Z
       
  • Subduction-related enrichment of the Neapolitan volcanoes (Southern Italy)
           mantle source: New constraints on the characteristics of the slab-derived
           components
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): F.C. Mazzeo , M. D'Antonio , I. Arienzo , M. Aulinas , V. Di Renzo , D. Gimeno
      The Neapolitan volcanic area (Southern Italy), which includes the Phlegrean Volcanic District and the Somma–Vesuvius complex, has been the site of intense Plio-Quaternary magmatic activity and has produced volcanic rocks with a subduction-related geochemical and isotopic signature. High-Mg, K-basaltic lithic lava fragments dispersed within hydromagmatic tuff of the Solchiaro eruption (Procida Island) provide constraints on the nature and role of both the mantle source prior to enrichment and the subduction-related components. The geochemical data (Nb/Yb, Nb/Y, Zr/Hf) indicate a pre-enrichment source similar to that of enriched MORB mantle. In order to constrain the characteristics of subducted slab-derived components added to this mantle sector, new geochemical and Sr–Nd-isotopic data have been acquired on meta-sediments and pillow lavas from Timpa delle Murge ophiolites. These represent fragments of Tethyan oceanic crust (basalts and sediments) obducted during the Apennine orogeny, and may be similar to sediments subducted during the closure of the Tethys Ocean. Based on trace element compositions (e.g., Th/Nd, Nb/Th, Yb/Th and Ba/Th) and Nd-isotopic ratio, we hypothesize the addition of several distinct subducted slab-derived components to the mantle wedge: partial melts from shales and limestones, and aqueous fluids from shales, but the most important contribution is provided by melts from pelitic sediments. Also, trace elements and Sr–Nd-isotopic ratios seem to rule out a significant role for altered oceanic crust. Modeling based on variations of trace elements and isotopic ratios indicates that the pre-subduction mantle source of the Phlegrean Volcanic District and Somma–Vesuvius was enriched by 2–4% of subducted slab-derived components. This enrichment event might have stabilized amphibole and/or phlogopite in the mantle source. 6% degree of partial melting of a phlogopite-bearing enriched source, occurring initially in the garnet stability field and then in the spinel stability field can generate a melt with trace elements and Sr–Nd-isotopic features matching those of high-Mg, K-basalts of Procida Island. Furthermore, 2% partial melting of the same enriched source can reproduce the trace elements and isotopic features of the most primitive magmas of Somma–Vesuvius, subsequently modified by assimilation of continental crust during fractional crystallization processes at mid-lower depth. Combined trace element and Sr–Nd isotope modeling constrains the age of the enrichment event to 45Ma ago, suggesting that the Plio-Quaternary magmatism of the Neapolitan area is post-orogenic, and related to the subduction of oceanic crust belonging to the Tethys Ocean.


      PubDate: 2014-09-04T23:06:07Z
       
  • Boron isotopic geochemistry of the McMurdo Dry Valley lakes, Antarctica
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Deborah Leslie , W. Berry Lyons , Nathaniel Warner , Avner Vengosh , John Olesik , Kathleen Welch , Kelly Deuerling
      The geochemistry of boron was investigated in the ice-covered lakes and glacier meltwater streams within Taylor and Wright Valley of the McMurdo Dry Valleys (MCM), Antarctica, in order to achieve a greater understanding of the source of boron to these aquatic systems and how in-lake processes control boron concentration. Selected lake depths (surface and bottom water) and streams were analyzed for boron geochemistry. Boron stable isotope values in these waters span the range of +12.3‰ to +51.4‰, which corresponds to the variations from glacier meltwater streams to the hypolimnion of a highly evaporated hypersaline lake. The data demonstrate that the major sources of B to the aquatic system are via terrestrial chemical weathering of aluminosilicates within the stream channels, and a marine source, either currently being introduced by marine-derived aerosols or in the form of ancient seawater. Lakes Fryxell, Hoare, and upper waters of Lake Joyce, which experience more terrestrial influence of aluminosilicate chemical weathering via glacial meltwater streams, display a mixture of these two major sources, while the source of B in the bottom waters of Lake Joyce appears to be primarily of marine origin. Lakes Bonney and Vanda and the Blood Falls brine have a marine-like source whose δ11B values have become more positive by mineral precipitation and/or adsorption. Don Juan Pond displays a terrestrial aluminosilicate influence of a marine-like source. These hypersaline lake waters from Antarctica are similar in δ11B to other hypersaline lake waters globally, suggesting that similar processes control their B geochemistry.


      PubDate: 2014-09-04T23:06:07Z
       
  • Biosorption mechanisms of Cu(II) by extracellular polymeric substances
           from Bacillus subtilis
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Linchuan Fang , Shanshan Yang , Qiaoyun Huang , Aifang Xue , Peng Cai
      Biosorption mechanisms of Cu(II) by extracellular polymeric substances (EPS) from Bacillus subtilis were investigated using a combination of batch experiments, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, isothermal titration calorimetry (ITC), and X-ray absorption fine structure (XAFS) spectroscopy. A three discrete site non-electrostatic model fit the potentiometric titration data best, with the pK a values of 4.12±0.12, 6.60±0.06, and 9.09±0.03, and site concentrations of 4.81±0.62×10−3, 2.16±0.14×10−3, and 2.87±0.21×10−3 mol per gram dry mass of EPS, respectively. The ATR–FTIR results confirmed the presence of the functional groups with above pK a values within the EPS molecules, and indicated that Cu(II) binding onto the EPS involves either phosphoryl or carboxyl sites, or both. The calculated enthalpies and entropies of Cu(II) adsorption onto EPS suggest that Cu(II) binds with anionic oxygen-bearing ligands and forms inner-sphere complexes with the EPS functional groups. The XAFS results were consistent with inner-sphere binding of Cu(II) by carboxyl sites with 2.03 C atoms at distance of 2.96Å in the second shell, which further suggests that the carboxyl groups are the dominant sites for Cu(II) adsorption by EPS at pH5.0, and that a five-membered chelate ring structure is the most likely binding environment for Cu(II) bound to EPS. The molecular binding mechanisms obtained in this study will add fundamental knowledge of understanding the fate of heavy metals in natural environments.


      PubDate: 2014-09-04T23:06:07Z
       
  • Silicon isotopes and trace elements in chert record early Archean basin
           evolution
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Sonja Geilert , Pieter Z. Vroon , Manfred J. van Bergen
      Silicon isotopes of chemical sediments have received growing attention, given their applicability in the search for properties of ancient seawater. An important target is the reconstruction of secular changes in surface temperature of the Precambrian Earth, but interpretations are problematic since controls of the isotopic signals are potentially manifold. Here we report the existence of significant silicon isotope variability in chemically precipitated chert layers covering a continuous stratigraphic section across the ~3.42Ga Buck Reef Chert (BRC) in the Barberton Greenstone Belt (South Africa). Black chert bands are variably but consistently higher in δ30Si (up to ca. 0.9‰) than co-existing translucent counterparts at the same stratigraphic level, which is interpreted as a primary feature acquired during deposition upon interaction between submarine discharging hydrothermal water and a stratified water body. Averaged δ30Si values range from +0.3‰ (maximum +0.5‰) for shallow-marine sediments in the lower part of the section to −0.5‰ (minimum −1.3‰) for deeper water deposits in the upper part when results from translucent and black cherts are taken together. An average shift of ca. 0.8‰ (maximum 1.8‰) thus accompanied the formation of chemical cherts in a single Archean basin. Cherts at the base of the section represent silicified felsic volcanics and volcaniclastics and have exclusively positive δ30Si values between +0.1‰ and +0.7‰. Rare-earth-elements-and-yttrium patterns confirm the marine origin of the cherts and support the presumed basin evolution. From the combined evidence we infer that the δ30Si variations in the BRC reflect changes in the predominant origin of the silica, with terrigenous input supplying positive δ30Si to shallow waters and seafloor hydrothermal sources negative δ30Si to deeper levels. Our findings demonstrate the viability of silicon isotopes in cherts for reconstructing the evolution of ancient marine basins.


      PubDate: 2014-09-04T23:06:07Z
       
  • Comparative geochemistry of rhenium in oxidized arc magmas and MORB and
           rhenium partitioning during magmatic differentiation
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Yuan Li
      The geochemical behavior of Re in oxidized arc magmas (Eastern Manus basin) and MORB has been reexamined. In the early differentiation stage of oxidized arc magmas, Yb, Cu, Au, Ag, and Re have a bulk partition coefficient (D) between the crystallized mineral assemblage (olivine, clinopyroxene, and plagioclase) and the residual melt on the order of 0.33=DYb ~DAu >DRe >DAg >DCu, while in the late stage they have a bulk D between the crystallized mineral assemblage (clinopyroxene, plagioclase, magnetite, and monosulfide solid solution — MSS) and the residual melt on the order DRe >DCu >DAu ≥1>DYb =0.29>DAg. In oxidized arc magmas, Cu, Au, and Ag are primarily controlled by crystalline MSS, whereas Re is primarily controlled by magnetite and it has a MSS/silicate melt partition coefficient significantly lower than 20. Unlike oxidized arc magmas, most MORB are saturated with sulfide liquid. Based on the positive correlation of log (DCu /DRe) and ∆FMQ regardless of fS2, log (DCu /DRe)=0.997ΔFMQ+0.181 (R2 =0.99) (D=partition coefficient between sulfide liquid and silicate melt; FMQ=fayalite–magnetite–quartz oxygen buffer), the partition coefficient of Re between sulfide liquid and silicate melt for MORB is estimated to be in the range of 600–10,000 at fO2 of FMQ to FMQ−1, increasing with decreasing fO2. These partitioning data constrain that during MORB differentiation Re may be as compatible as Cu or Ag at relatively oxidized conditions (FMQ−0.5< fO2 <FMQ) or as Au at relatively reduced conditions (FMQ−1< fO2 <FMQ−0.5), and its concentration in MORB maybe expected to decrease with decreasing MgO in a similar manner as Cu, Ag, or Au. Yb is an incompatible element during MORB differentiation. Consequently, during MORB differentiation Yb/Re ratio may be expected to increase with decreasing MgO. However, the previously observed nearly constant Yb/Re ratio in a suite of MORB indicates similar incompatibility of Yb and Re and thus a minor role of sulfide liquid in controlling the geochemical behavior of Re during MORB differentiation. More work is required to resolve this paradox.


      PubDate: 2014-09-04T23:06:07Z
       
  • Trace metal distribution in the Atlantis II Deep (Red Sea) sediments
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Tea E. Laurila , Mark D. Hannington , Sven Petersen , Dieter Garbe-Schönberg
      The Atlantis II Deep is one of the only locations on the modern seafloor where active formation of a brine pool-type stratiform ore deposit can be studied. The presence of the brine pool causes retention of the hydrothermally released metals within the brine covered area, resulting in the accumulation of 90Mt of low-grade metalliferous sediment (2.06% Zn, 0.46% Cu, 41g/t Ag, and 0.5g/t Au; Guney et al., 1988). Almost all metals are derived from hydrothermal input, but some are also derived from seawater (e.g., Mo), pelagic phytoplankton (Ni) and detrital input (Cr). The hydrothermal fluid that is vented into the pool is rich in metals but relatively low in reduced sulfur compared to open ocean black smokers. Metals are deposited as sulfides from the cooling hydrothermal fluid but also by adsorption onto non-sulfidic “surface-active” particles (Si–Fe-OOH) in the brine pool. An unexpected increase in the Cu/Zn ratio of the sediments with distance from the vent source(s) may reflect pulses of higher-temperature venting and increased Cu fluxes to the brine pool, which are recorded as higher Cu/Zn ratios in the distal sediments or, alternatively, more efficient adsorption of Cu to Fe-OOH particles in the distal brine. During early diagenesis (a few thousand years) metals that are loosely bound to surface-active particles in the sediment apparently react with H2S to form sulfides. Proximal to the inferred vents, the ambient pore water is highly concentrated in trace metals such as Cd, Ag and Hg that are incorporated in diagenetic sulfides, including chalcopyrite and sphalerite. At greater distance from the vents, trace metals such as Mo, As, and Ga are taken up by framboidal pyrite. High concentrations of Au (up to 3ppm) are found in both proximal and distal metalliferous sediments, indicating that both primary deposition with sulfides and adsorption by diagenetic pyrite are important depositional processes. Some of the inferred pathways for metal precipitation in the Atlantis II Deep sediments, especially adsorption onto surface-active particles and subsequent incorporation in sulfides during diagenesis, may have been important unrecognized processes for metal accumulation in ancient stratiform ore deposits thought to have formed in brine pools.
      Graphical abstract image

      PubDate: 2014-09-04T23:06:07Z
       
  • Novel heating/cooling stage designed for fluid inclusion microthermometry
           of large stalagmite sections
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Yves Krüger , Beat Hiltbrunner , Andres Luder , Dominik Fleitmann , Martin Frenz
      Liquid–vapour homogenisation temperatures of fluid inclusions in stalagmites are used for quantitative temperature reconstructions in paleoclimate research. Specifically for this application, we have developed a novel heating/cooling stage that can be operated with large stalagmite sections of up to 17×35mm2 to simplify and improve the chronological reconstruction of paleotemperature time-series. The stage is designed for use of an oil immersion objective and a high-NA condenser front lens to obtain high-resolution images for bubble radius measurements. The temperature accuracy of the stage is better than ±0.1°C with a precision (reproducibility) of ±0.02°C.


      PubDate: 2014-09-04T23:06:07Z
       
  • Formation of the Troodos Ophiolite at a triple junction: Evidence from
           trace elements in volcanic glass
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): M. Regelous , K.M. Haase , S. Freund , M. Keith , C.G. Weinzierl , C. Beier , P.A. Brandl , T. Endres , H. Schmidt
      Fresh volcanic glasses from the extrusive section of the Troodos Ophiolite in Akaki Canyon are tholeiitic and basaltic to dacitic in composition. Compared to normal MORB they have extremely low fractionation corrected Na8, Fe8 and Ti8 and are enriched in fluid-mobile trace elements, including U, Ba, Rb, Sr and Pb, relative to non-fluid mobile elements of similar incompatibility. Trace element compositions of Akaki lavas define an array extending between ‘back-arc lava’-like compositions, and the field defined by Troodos boninites from the upper part of the lava sequence. Troodos lavas were derived from a mantle source that underwent early melt depletion, and later enrichment by both fluids and small degree melts. These processes can explain the unusual negative correlation of Pb/Ce with Zr/Nb and Ba/Nb in Troodos extrusives. Although some Troodos lavas are similar in composition to lavas from back-arc spreading centres, the boninites from the upper parts of the lava pile do not appear to have exact compositional equivalents among lavas from fore-arcs, back-arcs or other tectonic settings where similar rocktypes have been recovered. We suggest that the geochemical evolution inferred for the mantle source of Troodos lavas, together with geological evidence is most consistent with an origin for the Troodos Ophiolite at a spreading centre close to a ridge–trench–trench, or ridge–trench–transform triple junction, where highly depleted, subduction-modified, fluid-enriched mantle wedge material was able to upwell and decompress to shallow depths in a ‘fore-arc’ location. In such a tectonic setting, arc volcanism is captured by the spreading centre, explaining the lack of evidence for subaerial arc magmatism in Troodos. Rapid lateral migration of the triple junction could account for the similar ages of other Tethyan supra-subduction zone ophiolites.


      PubDate: 2014-09-04T23:06:07Z
       
  • Mantle dynamics and secular variations beneath the East African Rift:
           Insights from peridotite xenoliths (Mega, Ethiopia)
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Gianluca Bianchini , Julia G. Bryce , Janne Blichert-Toft , Luigi Beccaluva , Claudio Natali
      Segments of the Main Ethiopian Rift (MER), a key sector of the East African Rift (EAR) linking the Afar and Turkana depressions, record different stages of lithospheric evolution from initiation to continental break-up and incipient oceanic spreading. Although EAR tectonic and magmatic activity is generally ascribed to a manifestation of sublithospheric mantle processes, the number, depth of provenance and triggering mechanisms of the related mantle upwellings remain topics of debate. We present new Hf and Pb isotope data for EAR mantle xenolith clinopyroxenes from Southern Ethiopia (Mega, the Sidamo region) that compellingly testify to multiple episodes of mantle depletion and metasomatic enrichment. Radiogenic values (εHf up to +1076) suggest that present-day MER lithospheric mantle domains underwent melting, possibly in the presence of residual garnet, billions of years ago, thereby fractionating (increasing) the Lu/Hf ratio and ultimately leading to extremely high 176Hf/177Hf. Although the precise dating of these depletion episodes is hampered by possible metasomatic overprinting, positively correlated Lu/Hf and 176Hf/177Hf indicates apparent ingrowth at ca. 1.9Ga, providing a minimum age for the delineated Proterozoic melting events. Our findings of Early Proterozoic melting episodes are complementary to previously determined Os model ages on xenoliths from the same site indicating melt extraction between 2.4 and 2.8Ga (Reisberg et al., 2004; Chem. Geol. 208, 119–140). Taken together, the geochemical and isotopic characteristics of the Mega xenoliths therefore record melt extraction events and preserve memory of ancient mantle dynamics beneath this site, where important lithospheric discontinuities exist between the Archean/Early Proterozoic Tanzanian craton and the Late Proterozoic Panafrican mobile belt. Subsequent metasomatic reactions variably affected the Nd and Hf isotopic compositions of some samples and completely overprinted the Pb isotopic composition of the whole xenolith suite. The persistence of these geochemical heterogeneities within the investigated suites of xenoliths precludes pervasive melt refertilization, which would have homogenized existing compositions and obliterated the record of previous petrologic processes. This suggests that the MER segment considered here developed on a lithospheric section isolated by pre-existing tectonic structures, far from the influence of plumes originating deep in the convecting mantle.


      PubDate: 2014-09-04T23:06:07Z
       
  • Subduction zone metamorphic pathway for deep carbon cycling: I. Evidence
           from HP/UHP metasedimentary rocks, Italian Alps
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Jennie Cook-Kollars , Gray E. Bebout , Nathan C. Collins , Samuel Angiboust , Philippe Agard
      Metamorphosed Jurassic oceanic sediment in the Italian Alps experienced peak P–T conditions (1.5–3.0GPa; 330–550°C) similar to those experienced by sediments subducting through forearcs in most modern subduction zones. Integrated field, petrologic, and geochemical study of the devolatilization history of these rocks provides evidence regarding the extents of loss and mobility of oxidized and reduced C during subduction of sediments to depths beneath volcanic fronts, thus constraining models of the release of initially subducted C into the atmosphere via arc volcanism or contributing to the C budget of the deeper mantle. In this suite, occurrences of lawsonite and grossular-rich garnet in higher grade calcschists indicate some decarbonation reaction along a P–T gradient of ~8°C/km. Across grade, carbonates largely retain δ13CVPDB values typical of marine carbonates (mostly ranging from −1.5 to +1.5‰), with some shifts to lower values in low-carbonate samples possibly in part reflecting decarbonation but likely largely due to isotopic exchange with abundant reduced C. Carbonaceous matter in some carbonate-poor samples shows increase in δ13C at higher grades, from values of −25 to −22‰ typical of marine organic matter, to values as high as −10‰, consistent with the expected effects of devolatilization involving release of varying fractions of the original reduced C into fluids as CH4. In more carbonate-rich samples, shifts in the carbonaceous matter to higher values, by up to 15‰, reflect varying degrees of exchange with the carbonate in these samples. Carbonate across grade has δ18OVSMOW of +17 to +22‰ significantly lower than values typical of marine carbonates (the latter +28 to +30‰), likely in part reflecting exchange with silicate phases. Such shifts in the more carbonate-rich samples cannot be explained by closed-system exchange with silicates and require exchange with an external reservoir. The observed extents of decarbonation in these carbonate-bearing sediments subducted to depths of up to 90–100km are consistent with limited open-system behavior, hence intermediate between models of Rayleigh-like CO2 loss and recent models supporting open-system flushing of carbonate-rich rocks by H2O-rich fluid from underlying subducting oceanic lithosphere. Based on a separate study of devolatilization history in the metapelites intercalated with the metacarbonates, and consideration of the O isotope compositions of the two lithologies, the metapelites appear to be sources for the H2O-rich fluid that drove decarbonation reactions and shifted the δ18O values of the carbonates to their present values. Calculated decarbonation histories of the calcschists, using the Perple_X software, predict significant release of CO2 related to the breakdown of CO2-bearing phases (dolomite, aragonite) and the growth of Ca-rich silicate phases (lawsonite, grossular-rich garnet). This decarbonation occurred at very low XCO2 (<0.01), for the P–T range considered here, perhaps attained during pulses of infiltration by H2O-rich fluids from the nearby metapelites. These results demonstrate that a large fraction of the initially subducted oxidized and reduced C in pelitic–carbonate sediment can be retained during forearc metamorphism and thus be available for loss beneath arcs or addition to the deeper mantle. In forearcs, the C inventory in these sediments experience minimal isotopic shift due to devolatilization, and isotopic reequilibration of the oxidized and reduced reservoirs during heating likely results in little net change in the bulk δ13C of the subducting section. Further attention should be paid to understanding the extent of C release, and possible isotope fractionation, during decarbonation, partial melting, and carbonate dissolution at the 80–100km depths over which subducting slabs experience greater heating by exposure to the convecting mantle wedge.


      PubDate: 2014-09-04T23:06:07Z
       
  • Determination of boron isotope compositions of geological materials by
           laser ablation MC-ICP-MS using newly designed high sensitivity skimmer and
           sample cones
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Lin Lin , Zhaochu Hu , Lu Yang , Wen Zhang , Yongsheng Liu , Shan Gao , Shenghong Hu
      The effects of addition of nitrogen gas with use of three different combinations of sample and skimmer cones on the performance of LA-MC-ICP-MS for in situ B isotope ratio measurements were investigated in detail. Compared to the standard arrangement (H skimmer cone+standard sample cone), sensitivities of B isotopes were improved by a factor of 2.4 and 3.8 with use of X skimmer cone+standard sampler cone and the X skimmer cone+Jet sample cone, respectively. The best within-run precision (2s) of 11B/10B was approximately 50ppm for B4 reference material (B content=31,400μg/g) with use of X skimmer cone+Jet sample cone. It was found that different cone combinations had significant effects on the instrument mass bias, but had little effect on the precisions of B isotope ratios. Addition of nitrogen (2–4ml/min) in the central channel gas did not improve the sensitivity of B. However, significant wider ion axial distribution profile and more stable mass-bias for B isotope ratios were evident. Under selected optimum conditions, good agreements between the measured δ11B values and the reference values were obtained for the international reference minerals (B4, Dravite, Elbaite, IMMRB1 and Schorl), the NIST SRM 610–612 synthetic soda-lime glasses and the MPI-DING glasses (GOR-128-G, GOR-132-G, and StHs6/80). It was found that for low-B containing (11ppm and 31ppm) materials, the precisions (2s) of 11B/10B can be improved by a factor of 2–10 by using the newly designed X-skimmer cone and Jet sample cone, compared to that of using the standard cones. The international reference mineral Danburite (δ11B of −14.92±6.77 2SD, n=32) showed a significant non-uniformity of B isotope ratio distribution. Our first boron isotopic results from standards GSD-1G (δ11B: 11.92±1.03, n=30), GSE-1G (δ11B: 1.63±0.32, n=30), GP-4 (δ11B: −5.85±1.01, n=40) and DD-1 (δ11B: −13.21±0.56, n=35) showed these materials were fairly homogeneous in boron isotope ratios, making them as good candidates as reference materials. The proposed LA-MC-ICPMS method is suitable for the direct determination of B isotope ratio in a variety of geological materials.


      PubDate: 2014-09-04T23:06:07Z
       
  • The gypsum–anhydrite paradox revisited
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): M. Ossorio , A.E.S. Van Driessche , P. Pérez , J.M. García-Ruiz
      Despite much experimentation precipitation of anhydrite from solution in conditions similar to those occurring in sedimentary environments has not yet been reproduced in the laboratory. To resolve this long-standing contradiction we have monitored the precipitation and stability behaviors of calcium sulfate during experiments lasting up to two years. Calcium sulfate was precipitated from solution between 40 and 120°C at three different salinities and the formed solid phase was sampled at different time intervals (from 2min up to 2yr). We found that below 80°C gypsum is the sole primary phase and in the range of 80 to 120°C gypsum and bassanite are the primary phases. The stability of the latter increased with increasing salinity. As expected, we did not observe primary anhydrite precipitation, but over time phase transition occurred and anhydrite eventually appeared at temperatures >80°C. We show that intrinsic thermodynamic and kinetic properties severely constrain the precipitation of anhydrite (compared to gypsum and bassanite), and consequently, a considerable amount of time (e.g. >2yr at 60°C) is needed for anhydrite to form. Even so, at a geological time-scale, anhydrite can be considered as a pseudo-primary phase thus resolving the long-standing paradox of our inability to directly precipitate anhydrite in the laboratory at temperatures below 120°C and the abundant presence of anhydrite in evaporitic environments. Our results also show that at low water activity, bassanite becomes an important phase, which could be relevant to explain its presence on the surface of Mars.


      PubDate: 2014-09-04T23:06:07Z
       
  • Ferrihydrite precipitation in groundwater-fed river systems (Nete and
           Demer river basins, Belgium): Insights from a combined
           Fe-Zn-Sr-Nd-Pb-isotope study
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): V.M. Dekov , E. Vanlierde , K. Billström , C.-D. Garbe-Schönberg , D.J. Weiss , G. Gatto Rotondo , K. Van Meel , E. Kuzmann , D. Fortin , L. Darchuk , R. Van Grieken
      Two groundwater-fed river systems (Nete and Demer, Belgium) carry red suspended material that settles on the river bed forming red sediments. The local aquifer that feeds these river systems is a glauconite-rich sand, which provides most of the dissolved Fe to the rivers. The solid component of these systems, i.e., the red suspended material and sediments, has a simple mineralogy (predominantly ferrihydrite), but shows a complex geochemistry pointing out the different processes contributing to the river chemistry: (1) the red sediments have higher transition metal (excluding Cu) and detrital element (e.g., Si, Al, K, Rb, etc.) concentrations than the red suspended matter because of their longer residence time in the river and higher contribution of the background (aquifer) component, respectively; (2) the red suspended material and sediments have inherited their rare earth element (REE) patterns from the aquifer; (3) the origin of Sr present in the red suspended matter and red sediments is predominantly marine (i.e., Quaternary calcareous rocks), but a small amount is geogenic (i.e., from detrital rocks); (4) Pb in both solids originates mostly from anthropogenic and geogenic sources; (5) all of the anthropogenic Pb in the red suspended material and sediments is hosted by the ferrihydrite; (6) Nd budget of the red riverine samples is controlled by the geogenic source and shows little anthropogenic component; (7) the significant Fe- and Zn-isotope fractionations are in line with the previous studies. Their fractionation patterns do not correlate, suggesting that the processes controlling the isotope geochemistry of Fe and Zn are different: oxidation/reduction most likely governs the Fe-isotope fractionation, whereas adsorption/desorption or admixing of anthropogenic sources controls the isotope fractionation of Zn.


      PubDate: 2014-09-04T23:06:07Z
       
  • The role of bacterial extracellular polymeric substances in
           geomicrobiology
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Janette Tourney , Bryne T. Ngwenya
      The last two decades have seen geomicrobiology evolve into a broad field encompassing a wide range of environmentally significant processes such as proton and metal adsorption onto cell surfaces, and the effect of microbes on mineralisation, redox cycling, and contaminant transport. Within this sphere, research groups have conducted studies using bacteria, fungi, diatoms and sludges, all of which can play a part in geochemical processes. Here, we review research on the role played by microbial extracellular polymeric substances (EPS), focussing on bacterial and cyanobacterial EPS. We conclude by outlining future research directions in order to investigate unresolved questions in the field. The effect of EPS on metal adsorption is complex; whereas some studies report an increase in metal adsorption when cells contain EPS, some report no differences; yet others report a decrease. These discrepancies may reflect differences in molecular and functional group composition of the EPS. EPS provides a template for adsorption of metal cations to which carbonate ions are attracted to induce local mineral supersaturation. This may be behind observed changes in both crystal polymorphism favouring formation of less stable polymorphs, and crystal morphology where the presence of EPS promotes the formation of rounded, smoothed crystals or spheroids. The role of EPS in mineral dissolution, bioleaching and corrosion is equivocal. EPS alone appears to have little effect on mineral dissolution, bioleaching or biocorrosion. Instead, it appears that EPS increases rates of mineral weathering and leaching by forming complexes with metallic ions released by the mineral surface, which may themselves catalyse bioleaching in the case of sulphides. EPS in biofilms forms effective barriers to the transport of particulate phases, and exerts important controls on the transport and deposition of natural and engineered nanoparticles. Much less is known about the role of EPS in the cycling of redox-sensitive trace metals. Intuitively, and as observed in microbial mats with sulphate reducing bacteria, EPS is a labile electron donor for microbial metabolism that could affect metal biogeochemistry but this remains to be conclusively demonstrated in laboratory experiments. Hence, in this and other areas, further studies are required to develop a mechanistic basis for including the effects of EPS in biogeochemical models.


      PubDate: 2014-09-04T23:06:07Z
       
  • Characterising the nickel isotopic composition of organic-rich marine
           sediments
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Sarah J. Porter , David Selby , Vyllinniskii Cameron
      New Ni stable isotope data (δ60Ni) determined by double-spike MC-ICP-MS for two geologically distinct suites of organic-rich marine sediments from the Sinemurian–Pliensbachian (S–P) Global Stratotype Section and Point (GSSP; Robin Hood's Bay, UK) and the Devonian–Mississippian Exshaw Formation (West Canada Sedimentary Basin) is presented herein. These sediments yield δ60Ni values of between 0.2‰ and 2.5‰, and predominantly have Ni isotopic compositions that are heavier than those of abiotic terrestrial and extraterrestrial samples (0.15‰ and 0.27‰), and in some cases present-day seawater (1.44‰) and dissolved Ni from riverine input (0.80‰). In addition, the observed degree of isotopic fractionation in the marine sediments is far greater than that of these other sample matrices. However, a strong similarity is exhibited between the δ60Ni values of the organic-rich sediments studied here and those of ferromanganese crusts (0.9–2.5‰), suggesting that factors ubiquitous to the marine environment are likely to play a key role in the heightened level of isotopic fractionation in these sample matrices. A lack of correlation between the Ni stable isotope compositions of the organic-rich sediments and Ni abundance suggests that isotopic fractionation in these sediments is not controlled by incorporation or enrichment of Ni during sediment accumulation. Further, no relationship is observed between δ60Ni and TOC concentrations or bottom–water redox conditions, indicating that the organic carbon reservoir and levels of oxygenation at the sediment–water interface do not exert a primary control on Ni isotope fractionation in marine sediments. Following examination of these relationships, it is therefore more likely that the heavy Ni isotope compositions of marine sediments are controlled by the weathering environment and the dominant sources of dissolved Ni into the global ocean reservoir.


      PubDate: 2014-09-04T23:06:07Z
       
 
 
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