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  Subjects -> EARTH SCIENCES (Total: 636 journals)
    - EARTH SCIENCES (462 journals)
    - GEOLOGY (68 journals)
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EARTH SCIENCES (462 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     Open Access   (Followers: 7)
Acta Geotechnica     Hybrid Journal   (Followers: 9)
Acta Meteorologica Sinica     Hybrid Journal   (Followers: 4)
Advances in High Energy Physics     Open Access   (Followers: 11)
Advances In Physics     Hybrid Journal   (Followers: 7)
Aeolian Research     Hybrid Journal   (Followers: 3)
African Journal of Aquatic Science     Hybrid Journal   (Followers: 12)
Algological Studies     Full-text available via subscription   (Followers: 2)
Alpine Botany     Hybrid Journal   (Followers: 6)
AMBIO     Hybrid Journal   (Followers: 13)
Anales del Instituto de la Patagonia     Open Access   (Followers: 2)
Andean geology     Open Access   (Followers: 5)
Annales Henri Poincaré     Hybrid Journal   (Followers: 1)
Annales UMCS, Geographia, Geologia, Mineralogia et Petrographia     Open Access   (Followers: 2)
Annals of Geophysics     Full-text available via subscription   (Followers: 10)
Annals of GIS     Hybrid Journal   (Followers: 18)
Annals of Glaciology     Full-text available via subscription   (Followers: 2)
Annual Review of Marine Science     Full-text available via subscription   (Followers: 11)
Anthropocene     Hybrid Journal  
Anthropocene Review     Hybrid Journal   (Followers: 3)
Applied Clay Science     Hybrid Journal   (Followers: 3)
Applied Geochemistry     Hybrid Journal   (Followers: 8)
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: 11)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 20)
Arctic Science     Open Access   (Followers: 4)
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 9)
Artificial Satellites     Open Access   (Followers: 14)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 2)
Asian Journal of Earth Sciences     Open Access   (Followers: 20)
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: 7)
Atmospheric and Climate Sciences     Open Access   (Followers: 15)
Australian Journal of Earth Sciences: An International Geoscience Journal of the Geological Society of Australia     Hybrid Journal   (Followers: 13)
Boletim de Ciências Geodésicas     Open Access  
Boreas: An International Journal of Quaternary Research     Hybrid Journal   (Followers: 10)
Bragantia     Open Access   (Followers: 2)
Bulletin of Earthquake Engineering     Hybrid Journal   (Followers: 10)
Bulletin of Geosciences     Open Access   (Followers: 9)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (Followers: 1)
Bulletin of the Seismological Society of America     Full-text available via subscription   (Followers: 18)
Bulletin of Volcanology     Hybrid Journal   (Followers: 16)
Canadian Journal of Plant Science     Full-text available via subscription   (Followers: 13)
Canadian Mineralogist     Full-text available via subscription   (Followers: 2)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 20)
Carbonates and Evaporites     Hybrid Journal   (Followers: 4)
CATENA     Hybrid Journal   (Followers: 4)
Chemical Geology     Hybrid Journal   (Followers: 10)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 4)
Chinese Geographical Science     Hybrid Journal   (Followers: 5)
Chinese Journal of Geochemistry     Hybrid Journal   (Followers: 3)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 2)
Ciencia del suelo     Open Access   (Followers: 1)
Ciencias Espaciales     Open Access  
Climate and Development     Hybrid Journal   (Followers: 12)
Coastal Management     Hybrid Journal   (Followers: 17)
Cogent Geoscience     Open Access  
Comptes Rendus Geoscience     Full-text available via subscription   (Followers: 6)
Computational Geosciences     Hybrid Journal   (Followers: 13)
Computational Mathematics and Mathematical Physics     Hybrid Journal   (Followers: 1)
Computers and Geotechnics     Hybrid Journal   (Followers: 7)
Contemporary Trends in Geoscience     Open Access   (Followers: 2)
Continental Shelf Research     Hybrid Journal   (Followers: 8)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 8)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 2)
Coral Reefs     Hybrid Journal   (Followers: 16)
Cretaceous Research     Hybrid Journal   (Followers: 6)
Cybergeo : European Journal of Geography     Open Access   (Followers: 5)
Depositional Record     Open Access  
Developments in Geotectonics     Full-text available via subscription   (Followers: 3)
Developments in Quaternary Science     Full-text available via subscription   (Followers: 3)
Développement durable et territoires     Open Access   (Followers: 2)
Diatom Research     Hybrid Journal  
Doklady Physics     Hybrid Journal   (Followers: 1)
Dynamics of Atmospheres and Oceans     Hybrid Journal   (Followers: 3)
E&S Engineering and Science     Open Access  
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (Followers: 77)
Earth and Space Science     Open Access  
Earth Interactions     Full-text available via subscription   (Followers: 11)
Earth Science Research     Open Access   (Followers: 7)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 3)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 13)
Earth System Dynamics     Open Access   (Followers: 7)
Earth System Dynamics Discussions     Open Access   (Followers: 4)
Earth's Future     Open Access   (Followers: 1)
Earth, Planets and Space     Open Access   (Followers: 3)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 7)
Earthquake Science     Hybrid Journal   (Followers: 8)
Earthquake Spectra     Full-text available via subscription   (Followers: 13)
Ecohydrology     Hybrid Journal   (Followers: 10)
Electromagnetics     Hybrid Journal   (Followers: 2)
Energy Efficiency     Hybrid Journal   (Followers: 12)
Energy Exploration & Exploitation     Full-text available via subscription   (Followers: 3)
Environmental Earth Sciences     Hybrid Journal   (Followers: 12)

        1 2 3 4 5 | Last

Journal Cover   Chemical Geology
  [SJR: 1.927]   [H-I: 123]   [10 followers]  Follow
    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 0009-2541
   Published by Elsevier Homepage  [2800 journals]
  • Isotopic and chemical composition of precipitation in Riyadh, Saudi Arabia
    • Abstract: Publication date: 15 October 2015
      Source:Chemical Geology, Volume 413
      Author(s): Nils Michelsen, Mustefa Reshid, Christian Siebert, Stephan Schulz, Kay Knöller, Stephan M. Weise, Randolf Rausch, Mohammed Al-Saud, Christoph Schüth
      Only limited data on the isotopic and chemical composition of Riyadh rain are currently available. In this study, we complement these data by analyzing integral samples covering 28 precipitation events between 2009 and 2013. Results of stable isotope analyses are used to establish a Local Meteoric Water Line: δ2H=5.22(±0.38)·δ18O+14.8(±0.9) ‰. Moisture source-related isotopic fingerprints are masked by the continental effect, the altitude effect, sub-cloud evaporation, and moisture recycling. The study of one event for intra-storm variability revealed strong isotopic depletion due to rainout and Rayleigh distillation processes, thus highlighting the general need for integral samples. Tritium analyses of grab samples from 12 events yielded concentrations between 2.8 and 6.4tritium units (TU), which are close to the natural background of a few TU. Major ion concentrations and ratios indicate that solutes are predominantly derived from atmospheric dust originating from limestone outcrops and sabkha deposits. The latter play a role with respect to the elevated Cl−and Na+ contents, but are probably also responsible for the SO4 2−and a part of the Ca2+ found in Riyadh rain. Observed intra- and inter-storm variabilities of major ion levels necessitate the collection of integral samples and the calculation of precipitation-weighted means, respectively. The obtained isotopic signatures and the precipitation-weighted mean Cl−concentration (9.5mgL−1) may be useful in groundwater assessments, e.g., for the identification of modern recharge and quantification thereof by means of the Chloride Mass Balance method.


      PubDate: 2015-09-03T18:51:34Z
       
  • Editorial Board
    • Abstract: Publication date: 27 September 2015
      Source:Chemical Geology, Volume 412




      PubDate: 2015-09-03T18:51:34Z
       
  • Nb and Ta diffusion in titanite
    • Abstract: Publication date: 15 October 2015
      Source:Chemical Geology, Volume 413
      Author(s): D.J. Cherniak
      Chemical diffusion of Nb and Ta under anhydrous, pO2-buffered conditions has been measured in natural titanite. The source of diffusants were mixtures of CaSiO3, TiO2, and Al2O3 powders, with either Nb or Ta oxides added. Experiments were run in evacuated sealed silica glass ampoules with solid buffers (buffered at NNO). Rutherford Backscattering Spectrometry (RBS) was used to measure Nb and Ta concentration profiles. The following Arrhenius parameters were obtained for Nb and Ta diffusion parallel to c over the temperature range 850–1252°C under NNO-buffered conditions: D N b = 3.23 × 10 − 9 exp − 299 ± 12 kJ mol − 1 / R T m 2 s − 1 D T a = 4.34 × 10 − 10 exp − 281 ± 10 kJ mol − 1 / R T m 2 s − 1 . In contrast to the findings of Marschall et al. (2013) for rutile, there do not appear to be large differences between Ta and Nb diffusivities in titanite, so the likelihood of diffusive fractionation of these elements will not be great. Values for diffusion coefficients measured parallel to the a-axis for both elements are similar to those parallel to c, indicating that Nb and Ta diffusivities in titanite do not exhibit significant anisotropy. Nb and Ta diffusion in rutile is faster than diffusion of these elements in titanite at higher temperatures (by about 2 and 1.5 orders of magnitude at 700°C for Nb and Ta, respectively), but because of the higher activation energies for diffusion of Nb and Ta in rutile, they will approach values for titanite with decreasing T. These diffusion data indicate that titanite should be moderately retentive of Nb and Ta chemical signatures, with diffusivities slower than those for Zr, O and Pb in titanite, but faster than those for the REE under most geologic conditions. These differences among diffusivities suggest that information derived from Zr thermometry, U–Pb geochronology, and geochemical tracers such as Ta, Nb and the REE may be decoupled in titanite under some conditions.


      PubDate: 2015-08-30T15:49:13Z
       
  • Archeological slag from Glinet: An example of silicate glass altered in an
           anoxic iron-rich environment
    • Abstract: Publication date: 15 October 2015
      Source:Chemical Geology, Volume 413
      Author(s): A. Michelin, E. Leroy, D. Neff, J.J. Dynes, P. Dillmann, S. Gin
      Vitreous slag wastes produced by a blast furnace working during the 16th century at the Glinet ironworks site (Normandy, France) are studied as analogs for vitrified nuclear waste. They display a specific interface between iron corrosion products and the glass matrix, which can contribute to understanding the influence of iron on glass alteration mechanisms and kinetics under anoxic conditions. Characterization of the archeological artifacts, especially of the interfacial area between the glass and corrosion products inside cracks, using micro and nanobeam techniques (micro-Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, scanning transmission X-ray microscopy) suggests that four main mechanisms have taken place: iron carbonate precipitation, glass hydration and interdiffusion, silicate network hydrolysis, and precipitation of iron silicates. Extrapolation using the results from short-term experiments and comparison with alteration thickness measurements carried out on archeological artifacts reveal that the presence of iron has a detrimental effect on glass durability. This can be attributed to the precipitation of iron silicates, consuming silicon and sustaining glass hydrolysis.
      Graphical abstract image

      PubDate: 2015-08-30T15:49:13Z
       
  • Evaluating the consequences of diagenesis and fossilization on bioapatite
           lattice structure and composition
    • Abstract: Publication date: 15 October 2015
      Source:Chemical Geology, Volume 413
      Author(s): Sarah W. Keenan, Annette S. Engel, Amitava Roy, G. Lisa Bovenkamp-Langlois
      Changes to hydroxylapatite mineral structure, composition, and lattice configuration must occur during the transformation of fresh bone to fossil bioapatite mineral phases. A paucity of analytical methods to characterize atomic-level lattice composition and bonding arrangements in modern and fossil bone has resulted in limited knowledge of how depositional environments, and thus diagenetic conditions, may influence fossilization processes. Synchrotron-based analyses can provide insight into bone fossilization, specifically X-ray absorption near edge structure (XANES) spectroscopy at the P and Ca K-edges. To aid in the interpretation of XANES spectral data from modern bones, and from Late Neogene and Late Cretaceous fossil bones, we applied other established techniques used to characterize bone mineralogy and composition, including X-ray diffraction, Fourier transform infrared spectroscopy, electron microprobe analysis, and thermal analysis. Specific bone depositional origins and ages likely influenced the varying elemental chemistry, preservation of organic matter, and mineral content. Fossilized bones, ranging from an almost pure fluorapatite to carbonated fluorapatite mineral phases, had less preserved organic material than modern bone. The atomic-level P and Ca lattice structures of fossilized bones differed from modern bones, specifically having shifted and shortened Ca–O bonds due to Ca2+ substitution by ionic phases, such as iron or rare earth elements, and carbonate substitution for phosphate. The modified lattice structure, regardless of bulk mineral chemistry, provides thermodynamic stability in geochemically dynamic depositional environments that may dictate bioapatite preservation over geologic time.


      PubDate: 2015-08-26T15:48:53Z
       
  • An improved thermodynamic model for the complexation of trivalent
           actinides and lanthanide with oxalic acid valid to high ionic strength
    • Abstract: Publication date: 15 October 2015
      Source:Chemical Geology, Volume 413
      Author(s): Punam Thakur, Yongliang Xiong, Marian Borkowski
      The dissociation constants of oxalic acid (Ox), and the stability constants of Am3+, Cm3+ and Eu3+ with Ox2− have been determined at 25°C, over a range of concentration varying from 0.1 to 6.60m NaClO4 using potentiometric titration and extraction techniques, respectively. The experimental data support the formation of complexes, M(Ox)n 3−2n, where (M=Am3+, Cm3+ and Eu3+ and n=1 and 2). The dissociation constant and the stability constant values measured as a function of NaClO4 concentration were used to estimate the Pitzer parameters for the respective interactions of Am3+, Cm3+ and Eu3+ with Ox. Furthermore, the stability constants data of Am3+–Ox measured in NaClO4 and in NaCl solutions from the literature were simultaneously fitted in order to refine the existing actinide–oxalate complexation model that can be used universally in the safety assessment of radioactive waste disposal. The thermodynamic stability constant: log β0 101 =6.30±0.06 and log β0 102 =10.84±0.06 for Am3+ was obtained by simultaneously fitting data in NaCl and NaClO4 media. Additionally, log β0 101 =6.72±0.08 and log β0 102 =11.05±0.09 for the Cm3+ and log β0 101 =6.67±0.08 and log β0 102 =11.15±0.09 for the Eu3+ were calculated by extrapolation of data to zero ionic strength in NaClO4 medium only. For all stability constants, the Pitzer model gives an excellent representation of the data using interaction parameters β(0), β(1), and Cϕ determined in this work. The thermodynamic model developed in this work will be useful in accurately modeling the potential solubility of trivalent actinides and early lanthanides to ionic strength of 6.60m in low temperature environments in the presence of Ox. The work is also applicable to the accurate modeling transport of rare earth elements in various environments under the surface conditions.
      Graphical abstract image

      PubDate: 2015-08-26T15:48:53Z
       
  • Barium stable isotope composition of the Earth, meteorites, and
           calcium–aluminum-rich inclusions
    • Abstract: Publication date: 15 October 2015
      Source:Chemical Geology, Volume 413
      Author(s): Frédéric Moynier, Emily A. Pringle, Audrey Bouvier, Julien Moureau
      High-precision stable Ba isotope ratios are reported in a variety of terrestrial samples, undifferentiated primitive meteorites, and calcium–aluminum-rich inclusions (CAIs) from the Allende chondrite. All whole-rock terrestrial and meteorite samples are isotopically indistinguishable at a 50 parts per million (ppm) level per atomic mass unit (amu). Three CAIs are isotopically light, with δ138/137Ba (permil deviation of the 138Ba/137Ba ratio from a terrestrial standard) values down to −0.6‰ compared to whole-rock meteorites, whereas the matrix is enriched in heavy isotopes (δ138/137Ba: +0.2‰). Similar light isotope enrichments in CAIs have been previously observed for Eu, Sr, and Ca, while for most other elements CAIs are enriched in the heavier isotopes (e.g. Mg, Fe). Kinetic isotopic fractionation is a possible explanation for the enrichment in the lightest isotopes, either by condensation from a vapor phase enriched in light isotopes by kinetic effects or by kinetic fractionation during non-equilibrium condensation of an undercooled gas as suggested for Ca isotopes. However, the common property of Ba, Eu, and Sr is that they all have a low first ionization potential. We suggest that electromagnetic sorting of ionized species in the early Solar System is a possible alternative mechanism to explain the depletion in heavy isotopes observed in refractory inclusions for those elements.


      PubDate: 2015-08-26T15:48:53Z
       
  • Editorial Board
    • Abstract: Publication date: 14 September 2015
      Source:Chemical Geology, Volume 411




      PubDate: 2015-08-18T15:40:08Z
       
  • New experimental approach to study aqueous alteration of amorphous
           silicates at low reaction rates
    • Abstract: Publication date: 27 September 2015
      Source:Chemical Geology, Volume 412
      Author(s): Corentin Le Guillou, Ralf Dohmen, Detlef Rogalla, Thomas Müller, Christian Vollmer, Hans-Werner Becker
      Understanding the kinetics of silicate alteration in aqueous media is central to the study of oceanic hydrothermal systems, nuclear glass durability or carbonaceous asteroids from which chondrites are coming. We present a new experimental approach in combination with an integrated analytical protocol designed to study alteration processes at low temperatures (<200°C) and slow reaction rates. We used pulsed laser deposition (PLD) to produce micrometer thick films of amorphous silicate of controlled chemical composition. After reacting with water in sealed capsules, the films consist of a succession of compositionally different layers. The initial planar geometry of the film allows a complete characterization of the transformed materials at the nanometer scale. By combining Rutherford back-scattering (RBS), nuclear reaction analysis (NRA), transmission electron microscopy (TEM) and scanning transmission X-ray microscopy (STXM), it is possible to constrain the propagation rate of the reaction fronts, the thicknesses of individual layers, spatial variations in composition, the nature of the interface between the layers, the iron redox state, the water content along depth profiles, as well as the porosity and the density. We investigated the serpentinization of amorphous silicate films with stoichiometry close to olivine (~Fe1.1Mg0.9SiO4.15H0.3) at 90°C (2weeks) and 200°C (2h). In both cases, ~500nm of altered material is formed. At the reaction front, a hydrated, amorphous and oxidized Fe-rich layer forms. At the interface with the fluid, a more Mg-rich layer develops. The system evolves towards a biphasic assemblage of Fe-serpentine and Mg-saponite composition. Both layers remain amorphous. It is shown that water propagation is coupled to hydrolysis, iron oxidation (Fe3+/∑Fe ratio>50%) and H2 formation, whose quantifications are crucial to understand terrestrial serpentinization processes. Interfacial precipitation–dissolution seems to be the rate controlling mechanisms. In addition, we investigated a crystallized film reacted at 190°C (2h), which transformation rate is ten times slower than that of the amorphous silicate but is nevertheless readily observable. This approach can be used to understand alteration in terrestrial and extraterrestrial samples. In particular, we reproduced several features observed in carbonaceous chondrites (amorphous and oxidized hydrated silicates) and show that, at 90°C, alteration may be faster than usually considered. It should allow us in the future to constrain the temperatures and timescales of alteration in chondrites.


      PubDate: 2015-08-14T04:07:43Z
       
  • Trace metal concentration and partitioning in the first 1.5m of
           hydrothermal vent plumes along the Mid-Atlantic Ridge: TAG, Snakepit, and
           Rainbow
    • Abstract: Publication date: 27 September 2015
      Source:Chemical Geology, Volume 412
      Author(s): Alyssa J. Findlay, Amy Gartman, Timothy J. Shaw, George W. Luther
      To determine the significance of metal fluxes from hydrothermal vents, understanding the speciation, reactivity, and possible transformations of metals and metal sulfides within the hydrothermal plume is critical. In this study, we measure the concentration and partitioning of trace metals (Fe, Mn, Cu, Cd, Co, Pb, Ni) and sulfide phases within the first 1.5m of the rising plume at three vent fields (TAG, Snakepit, and Rainbow) along the Mid-Atlantic Ridge. A HCl/HNO3 leaching method was used to differentiate metals present in metal mono-sulfides from those in pyrite and chalcopyrite. At all three vent sites, Mn and Fe are primarily in the <0.2μm (filtered) portion, whereas Cu, Co, Cd, and Pb are mainly in the unfiltered fraction. Significant concentrations of HNO3-extractable metals were found in the <0.2μm fraction at all three vent sites, indicating that they likely exist in a recalcitrant nanoparticulate phase such as pyrite or chalcopyrite. At TAG and Snakepit, Cu is correlated with Co, as Co enters into chalcopyrite and other CuFeS phases and Zn is correlated with Cd and Pb as they form discrete metal sulfide phases. At Rainbow, Zn, Cd, and Pb are correlated, but Cu and Co are not correlated. The Rainbow data are consistent with the higher metal to sulfide ratio found at Rainbow. These speciation differences are significant as both mineral type and size will affect the amount of metal transported from the vent site and its availability for biogeochemical processes.


      PubDate: 2015-08-09T03:41:51Z
       
  • Subduction zone metamorphic pathway for deep carbon cycling: II. Evidence
           from HP/UHP metabasaltic rocks and ophicarbonates
    • Abstract: Publication date: 27 September 2015
      Source:Chemical Geology, Volume 412
      Author(s): Nathan C. Collins, Gray E. Bebout, Samuel Angiboust, Philippe Agard, Marco Scambelluri, Laura Crispini, Timm John
      Exposures of low-grade metabasalts and ophicarbonates in the Northern Apennines, and their high- and ultrahigh-pressure metamorphic equivalents in the Western and Ligurian Alps and Tianshan (representing an overall peak P–T range of ~0.2–3.0GPa, 200–610°C), allow investigation of the effects of prograde metamorphic devolatilization, and other fluid–rock interactions, on degrees of retention and isotopic evolution of C in subducting oceanic crust and associated mantle rocks. Such work can inform models of C cycling at convergent margins, helping to constrain the efficiency of return of initially subducted C via arc volcanism and the fraction of this subducted C entering the deeper mantle beyond arcs. In the metabasaltic rocks, the preservation of finely disseminated carbonate with δ13C overlapping that of seafloor-altered protoliths, and the minimal mineralogical evidence of decarbonation, indicates large degrees of carbonate retention in this suite extending to UHP conditions similar to those beneath modern volcanic fronts. For many of the metabasalts, the δ18O of this carbonate can be explained by closed-system equilibration with silicate phases (e.g., garnet, clinopyroxene) during HP/UHP metamorphism. Larger volumes of carbonate preserved in interpillow regions and as breccia-filling largely escaped decarbonation, showing little or no evidence for reaction with adjacent metabasalt. Calculated devolatilization histories demonstrate that, in a closed-system model, carbonate in metabasaltic rocks can largely be preserved to depths approaching those beneath volcanic fronts (80–90km). Modeling of open-system behavior indicates that episodic infiltration of such rocks by H2O-rich fluids would have greatly enhanced decarbonation. Trends in O–C isotope composition of carbonate in some metabasaltic suites likely reflect effects of infiltration by externally-derived fluid with or without resulting decarbonation. Most carbonated ultramafic rocks similarly show little mineralogical evidence for decarbonation, consistent with calculated reaction histories, and have δ13C largely overlapping that of seafloor equivalents. However, the high-grade ophicarbonates show more restricted ranges in δ18O consistent with some control by infiltrating fluids, likely during subduction. This combination of field, petrographic, and isotopic evidence, together with calculated decarbonation histories, is consistent with minimal loss of CO2 from these rocks via decarbonation during forearc metamorphism. Combining our results with those of Cook-Kollars et al. (2014; Chemical Geology) for associated W. Alps metasedimentary rocks, we suggest that the majority of the CO2 (perhaps 80–90%, considering the full range of rock types) could be retained through forearcs in more intact volumes of subducting sediment, basalt, and ophicarbonate experiencing closed- or limited open-system conditions. Deep in forearcs and beneath arcs, decarbonation (and also carbonate dissolution) could be enhanced in shear zones and highly fractured volumes experiencing larger fluid flux in part from dehydrating sub-crustal ultramafic rocks in slabs. Degrees of C loss by decarbonation, carbonate dissolution, and partial melting should be particularly significant as the subducting sections experience heating to >600°C at depths of 80–120km (i.e., approximately at depths beneath arcs).


      PubDate: 2015-08-09T03:41:51Z
       
  • Disruption of the geochemical metal cycle during mining: Multiple isotope
           studies of lake sediments from Schefferville, subarctic Québec
    • Abstract: Publication date: 27 September 2015
      Source:Chemical Geology, Volume 412
      Author(s): Stéphane Aebischer, Christophe Cloquet, Jean Carignan, Charles Maurice, Reinhard Pienitz
      Iron mining in Schefferville (1939–1977) in subarctic Quebec has left behind large numbers and quantities of tailing deposits. The impact of past mining activity on aquatic ecosystems in the Schefferville area has been studied using geochemical and isotopic (Pb, Zn and Fe) analyses of lake sediments, ore deposits, tailings and epiphytic lichens. Analysis of two sediment cores from Lakes Dauriat and Oksana reveals that the surface geochemical cycle of the Schefferville area has been profoundly disturbed by anthropogenic activity such as mining. Disturbances were particularly abrupt at the transitions from pre-mining to mining and mining to post-mining periods. Elemental and isotopic analyses of the lake sediments reveal four different end-member contributions to the lake sedimentation, with changes in terms of sources and source contribution observed throughout the sedimentation history. End-members were identified using Pb, Zn and Fe isotopes and are consistent for each element. Lead isotope ratios vary from highly radiogenic (206Pb/204Pb=27) to less radiogenic (206Pb/204Pb=17.7) from the bottom to the top of the sediment cores. Iron isotope compositions vary from −0.2‰ to 1‰, the latter value remaining constant throughout the sedimentary history of Lake Oksana. A systematic difference in the Zn isotope ratios of the two lakes is also observed, and can be explained by local differences in basin lithology. In order to identify pollution sources, samples from ores and tailings and epiphytic lichens were measured as proxies of mining activity, lithology, and atmospheric deposition, respectively. The impact of anthropogenic activity is clearly evident in the sediment records and results from mining activity, as well as local urban and industrial activities (waste water inputs). Long-range atmospheric deposition also accounts for some of the variations in isotopic composition measured in the sediments. The systematic coupling of Pb isotopes with Zn or Fe isotopes allows us to identify and constrain the metal sources that contributed to sediment contamination. This study demonstrates that disturbance due to mining activities is very effective in rapidly modifying lake sediment composition. Though the disturbances remained local in terms of geographic extent, the damage to the aquatic ecosystem has been significant and may persist for decades.
      Graphical abstract image

      PubDate: 2015-08-09T03:41:51Z
       
  • Applications and limitations of U–Th disequilibria systematics for
           determining ages of carbonate alteration minerals in peridotite
    • Abstract: Publication date: 27 September 2015
      Source:Chemical Geology, Volume 412
      Author(s): Evelyn M. Mervine, Kenneth W.W. Sims, Susan E. Humphris, Peter B. Kelemen
      238U–234U–230Th dating was conducted on carbonate alteration minerals in the peridotite layer of the Samail Ophiolite, Sultanate of Oman, in order to assess the applicability of U-series dating techniques to these types of Quaternary terrestrial carbonates and also to further constrain natural rates of carbonation of the peridotite. Due to their low U concentrations and relatively high Th/U ratios, Samail carbonates are challenging to date with the 230Th technique because of the sensitivity of ages to corrections for initial 230Th. Uncorrected 230Th ages for Ca-rich travertines are consistently older than previously obtained 14C ages. However, geologically reasonable initial 230Th corrections bring the two sets of ages into concordance. This age concordance suggests that the travertines are generally closed systems, adding a level of credence to the reliability of previously obtained 14C ages. In contrast, uncorrected 230Th ages for Mg-rich carbonate veins are generally younger than previously obtained 14C ages. These young ages are interpreted in terms of remobilization of hexavalent U, which is subsequently deposited as tetravalent U by reduced serpentinization fluids. Two Mg-rich carbonate veins sampled at a roadcut have near-equilibrium (230Th/238U) and (234U/238U) values, which indicate that these veins are >375,000years in age, consistent with their “14C dead” (>50,000yearsBP) ages. The variable young and old ages for these Mg-rich carbonate veins indicate that carbonation of the peridotite layer of the Samail Ophiolite is an ongoing process and that there have been multiple generations of subsurface carbonate vein formation. Overall, this study provides insights into some of the challenges associated with applying U-series dating methods to Quaternary terrestrial carbonates, in particular carbonate alteration minerals in peridotites, and highlights some areas where there is room for improvement, such as obtaining better constraints on the isotopic composition of admixed detritus, and also some advantages, such as the ability to identify open system behavior not apparent from 14C dating and stable C and O isotopic analysis alone.


      PubDate: 2015-08-09T03:41:51Z
       
  • In situ lithium and boron isotope determinations in mica, pyroxene, and
           serpentine by LA-MC-ICP-MS
    • Abstract: Publication date: 27 September 2015
      Source:Chemical Geology, Volume 412
      Author(s): Céline Martin, Emmanuel Ponzevera, George Harlow
      Boron and Li are light, incompatible elements that preferentially partition into the liquid phase, whether melt or aqueous fluid, and thus are useful for tracking fluid-related processes in rocks. Most of the Li isotopic data presently available on subduction-related rocks are from whole-rock analyses; and the B isotopic analyses of subduction material have been carried out either on whole-rocks or in-situ on an accessory phase, such as tourmaline. The new method presented here couples an ESI New Wave UP-193-FX ArF* (193nm) excimer laser-ablation microscope with a Neptune Plus (Thermo Scientific) MC-ICP-MS aiming to measure both Li and B isotopes in situ with good spatial resolution (metamorphic minerals are commonly chemically zoned, and whole-rock analyses lose this detail). The data thus obtained are compared with SIMS analyses on the same mineral samples for B, and with MC-ICP-MS analyses on whole-rock or mineral separates from the same sample for Li. Additionally, data acquired on tourmaline standards were compared to SIMS values. The results show that for B concentrations above 5μg/g, the data obtained by LA-MC-ICP-MS and by SIMS are identical within error, for mica (phengitic muscovite), pyroxene (jadeite), serpentine (antigorite), and tourmaline. For Li concentrations above 10μg/g, the data obtained by LA-MC-ICP-MS and by MC-ICP-MS are also identical, within error, for mica (phengitic muscovite), and pyroxene (jadeite). However, analyses of tourmaline standards have shown significant differences with reference values, so LA-MC-ICP-MS does not yet appear to be an appropriate method to analyze Li isotopes in tourmalines. Thus, LA-MC-ICP-MS is a suitable method to measure Li and B isotopes with good spatial resolution in major rock-forming silicates from subduction-related rocks where concentrations exceed 10μg/g and 5μg/g, respectively, with an error on individual measurements equal to or less than previously used methods, but obtainable in a significantly shorter amount of time. The external reproducibility is ±2.88 to 3.31‰ for B and ±1.50 to 1.75‰ for Li, which is lower than or equal to the variations encountered within a given chemically zoned sample (up to 10‰ of variation within a given natural sample).


      PubDate: 2015-08-05T03:28:57Z
       
  • The impact of Mg contents on Sr partitioning in benthic foraminifers
    • Abstract: Publication date: 27 September 2015
      Source:Chemical Geology, Volume 412
      Author(s): Antje Mewes, Gerald Langer, Gert-Jan Reichart, Lennart Jan de Nooijer, Gernot Nehrke, Jelle Bijma
      Foraminiferal calcite Mg/Ca (Mg/CaCC) is used in paleoceanographic studies to reconstruct temperature. Furthermore, the Mg/CaCC is influenced by different seawater Mg/Ca (Mg/CaSW). Foraminiferal calcite Sr/Ca (Sr/CaCC) can potentially be used to reconstruct Sr/Ca ratios of seawater (Sr/CaSW). As these elements are the most abundant of all elements incorporated into the calcium carbonate of the foraminiferal tests, they potentially might affect each other's incorporation. To investigate the effects of the Mg concentration in the test on Sr incorporation, we conducted a culture study with two species of benthic foraminifera producing carbonate tests with different Mg content. Foraminifers grew under controlled conditions in different Mg/CaSW, whereas Sr/CaSW was kept constant. By analyzing Sr/CaCC of cultured specimens with laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS), we show that Sr/CaCC increases with increasing Mg/CaCC. We explain this observation by a stress in the crystal lattice, caused by elevated Mg/CaCC, which may be compensated for by the additional incorporation of Sr. We discuss this finding in the context of biomineralization and evaluate the reliability of Sr/CaCC as a possible Sr/CaSW proxy.


      PubDate: 2015-08-05T03:28:57Z
       
  • Sulfate and sulfide sulfur isotopes (δ34S and δ33S) measured by
           solution and laser ablation MC-ICP-MS: An enhanced approach using external
           correction
    • Abstract: Publication date: 27 September 2015
      Source:Chemical Geology, Volume 412
      Author(s): Michael J. Pribil, W. Ian Ridley, Poul Emsbo
      Isotope ratio measurements using a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) commonly use standard-sample bracketing with a single isotope standard for mass bias correction for elements with narrow-range isotope systems measured by MC-ICP-MS, e.g. Cu, Fe, Zn, and Hg. However, sulfur (S) isotopic composition (δ34S) in nature can range from at least −40 to +40‰, potentially exceeding the ability of standard-sample bracketing using a single sulfur isotope standard to accurately correct for mass bias. Isotopic fractionation via solution and laser ablation introduction was determined during sulfate sulfur (Ssulfate) isotope measurements. An external isotope calibration curve was constructed using in-house and National Institute of Standards and Technology (NIST) Ssulfate isotope reference materials (RM) in an attempt to correct for the difference. The ability of external isotope correction for Ssulfate isotope measurements was evaluated by analyzing NIST and United States Geological Survey (USGS) Ssulfate isotope reference materials as unknowns. Differences in δ34Ssulfate between standard-sample bracketing and standard-sample bracketing with external isotope correction for sulfate samples ranged from 0.72‰ to 2.35‰ over a δ34S range of 1.40‰ to 21.17‰. No isotopic differences were observed when analyzing Ssulfide reference materials over a δ34Ssulfide range of −32.1‰ to 17.3‰ and a δ33S range of −16.5‰ to 8.9‰ via laser ablation (LA)-MC-ICP-MS. Here, we identify a possible plasma induced fractionation for Ssulfate and describe a new method using external isotope calibration corrections using solution and LA-MC-ICP-MS.
      Graphical abstract image

      PubDate: 2015-08-05T03:28:57Z
       
  • Spatial variability of African dust in soils in a montane tropical
           landscape in Puerto Rico
    • Abstract: Publication date: 27 September 2015
      Source:Chemical Geology, Volume 412
      Author(s): M.A. McClintock, G. Brocard, J. Willenbring, C. Tamayo, S. Porder, J.C. Pett-Ridge
      Dust deposition provides rock-derived nutrients such as phosphorus (P) to terrestrial ecosystems. Over pedogenic timescales, as bedrock sources of P are depleted, dust sources of P may support productivity in certain ecosystems, but controls on the spatial variability of dust in montane forested systems are largely unknown. Here, we use neodymium (Nd) isotope ratios in 31 ridgetop surface soils to evaluate the spatial variability of dust contributions to soil across ~100km2 in the Luquillo Mountains, Puerto Rico. Dust from the Sahara–Sahel region of Africa carries a distinct isotopic signature of −12 εNd. Local bedrock, in contrast, has a εNd value of ~+7. End-member mixing calculations based on εNd reveal a wide range in dust influence on surface soils, with between 0% and 92% of the top 20cm of ridgetop soil Nd derived from African dust. Using εNd paired with both dust and soil Nd content, the current soil dust content was calculated, ranging from 0 to 8%. There were no correlations between current dust content of soil and 10Be-based denudation rate, elevation, rainfall, longitude, or forest type. Current soil dust content in the Luquillo Mountains is significantly higher in soils developed on volcaniclastic sandstone, breccia and mudstone than in soils developed on quartz diorite bedrock, which we attribute to greater retention capacity in the volcaniclastic soils. Current soil dust content also increases with increasing ridge-width, implying that small-scale topographic effects and other factors such as wind speed and turbulence influence local dust deposition rates. Higher current dust content of soil is also positively correlated with biologically cycled fractions of soil P on quartz diorite bedrock (r2 =0.24 and p=0.002 for sum of extractable NaHCO3-P+NaOH-P), suggesting that atmospheric dust inputs contribute to the fertility of Luquillo Mountain ecosystems on the relatively P-poor quartz diorite bedrock.


      PubDate: 2015-08-05T03:28:57Z
       
  • A refined dissolution method for rare earth element studies of bulk
           carbonate rocks
    • Abstract: Publication date: 27 September 2015
      Source:Chemical Geology, Volume 412
      Author(s): Kan Zhang, Xiang-Kun Zhu, Bin Yan
      Extraction of primary geochemical signals from bulk carbonate rocks is a key task in palaeoenvironmental studies. REE behaviour is investigated during stepwise dissolution of carbonate rocks. The experiment is designed to achieve total carbonate dissolution in ten steps, aiming for approximately 10% of the total carbonate to be dissolved in each step using acetic acid. An additional eleventh step using excess acid completed each experiment. Results show that calcite is preferentially dissolved before dolomite though no significant effect on the REE pattern of the rock is observed. Secondary carbonate phases and adsorbates are likely to be dissolved at the beginning of the dissolution process but this does not increase REE concentration though the REE pattern may be altered e.g. lessening of negative Ce anomaly, altering of Eu anomaly, Y/Ho ratio and relative fractionation between LREE and HREE. Non-carbonate minerals e.g. terrestrial particulate matter, Fe–Mn oxyhydroxides and phosphates, are likely to be dissolved towards the end of the total dissolution, especially in the final excess acid step. This should increase REE concentrations and alter REE distribution patterns greatly. In contrast, solutions from intermediate steps are less contaminated. Further, compared with using 10% v/v acetic acid, the solutions obtained using 5%v/v acid return more pristine REE information from the carbonate. A proposed method for REE studies of bulk carbonate rocks requires an initial dissolution of 30%–40% followed by the sampling dissolution of 30%–40% of total carbonate using 5%v/v acetic acid to produce REE information considered to best represent that of the carbonate source water. Following these steps 20%–30% of the carbonate should remain undissolved, thus minimising contamination from non-carbonate minerals.


      PubDate: 2015-08-05T03:28:57Z
       
  • Remineralization of ferrous carbonate from bioreduction of natural
           goethite in the Lorraine iron ore (Minette) by Shewanella putrefaciens
    • Abstract: Publication date: 27 September 2015
      Source:Chemical Geology, Volume 412
      Author(s): Baptiste Maitte, Frédéric P.A. Jorand, Dragan Grgic, Mustapha Abdelmoula, Cédric Carteret
      Bacterial iron oxide reduction has been extensively studied over recent decades with the aim of improving knowledge of Fe-bearing mineral transformations. Chemically synthesized Fe(III) oxides such as ferrihydrite or goethite have mainly been used as iron oxide models but very few studies have focused on natural oxides. The scope of our work was to evaluate the ability of iron-reducing bacteria to transform iron ore and to identify the nature and outcome of the reduced phases. For this purpose, Lothringen (minette), the oolitic iron ore type found in the Lorraine area (North-East of France), was incubated with Shewanella putrefaciens CIP 80.40 as a model iron-reducing bacteria. Chemical and mineralogical analyses (ferrozine assay, X-ray diffraction, diffuse reflectance infrared Fourier transform spectroscopy, Mössbauer spectroscopy, transmission electron microscopy) were performed on both aged (i.e. iron ore oxidized by air during ~80years of mining exploitation) and intact iron ores, before and after bioreduction in anoxic conditions. The oolites of intact iron ore were composed of goethite (α-FeOOH), with siderite (FeCO3) and phyllosilicates as cement. Oolites of the aged iron ore contained hematite (α-Fe2O3), as well as goethite and significantly less siderite. We observed that 26% and 20% of goethite was bio-reduced for aged and intact iron ore respectively. No other Fe(III) phase was significantly reduced. Natural iron oxides constituting the iron ore can now be considered more available for iron-reducing bacteria than chemically synthesized oxides. The nano-crystallinity of iron ore goethite and the presence of sorption sites for Fe(II) were suggested as explanations for this discrepancy. Finally, a poorly crystalline ferrous carbonate was the main Fe(II) phase formed during the bio-reduction process of the iron ore (aged and intact). In the context of iron mines, the neo-formed ferrous carbonate could be a precursor of siderite and could recreate the diagenetic links and the inter-oolitic cohesion lost by the iron ore during its oxidizing process. The long-term stability of iron ore pillars would therefore be ensured.


      PubDate: 2015-07-31T20:52:08Z
       
  • Distribution of rare earth elements and other high field strength elements
           in glacial meltwaters and sediments from the western Greenland Ice Sheet:
           Evidence for different sources of particles and nanoparticles
    • Abstract: Publication date: 27 September 2015
      Source:Chemical Geology, Volume 412
      Author(s): Nathalie Tepe, Michael Bau
      Although global warming increases meltwater input into the oceans, rather little is known about the distribution of high field strength elements such as the rare earths and Y (REY), Zr, Hf, Th, and U in arctic glacial meltwaters and glacial-fed rivers. We studied glacial meltwater and glacial-fed rivers from the Kangerlussuaq area and a glacial-fed lake and its meltwater inflow in the Isua area, both in the western part of the Greenland Ice Sheet (GRIS). Trace element concentrations were determined in 0.2μm-filtered water samples (“dissolved” fraction), in the respective filter residues (“particulate” fraction) and in ambient sediments (including cryoconite from a meltwater pond on the surface of the GRIS). We also measured “truly dissolved” REY concentrations in a 10kDa-ultrafiltered sample from glacial-fed Watson River. Shale-normalized (“SN”) REY patterns of the particulate fraction and ambient sediments (including cryoconite) are rather similar to those of local Archean gneisses and show positive EuSN anomalies. This rules out Asian dust (which does not show positive EuSN anomalies) as a possible source of these aluminosilicate particles, but suggests that local Archean basement is eroded and currently transported by ice, water and wind to the depositional sites in front of and onto the GRIS. All 0.2μm-filtered glacial-fed rivers show very unusual REYSN patterns in comparison to tropical, temperate and boreal rivers, and are significantly enriched in light relative to heavy REY. For glacial-fed Watson River, the <10kDa-ultrafiltrate shows much lower REY concentrations than the <0.2μm-filtrate, suggesting that >99% of La and >78% of Yb in the latter are associated with nanoparticles and colloids. Although the REYSN patterns of the <0.2μm-filtrates are rather similar to those of the particulate fraction and ambient sediments, they lack any EuSN anomalies, whereas the <0.2μm-filtrates from a meltwater pond on the surface of the GRIS show positive EuSN anomalies similar to (but somewhat smaller than) the cryoconite. While the water from the meltwater pond carries nanoparticles derived from local Archean sources, the lack of EuSN anomalies in the glacial-fed rivers suggests that their nanoparticle and colloid load represents atmospheric dust that is remobilized from greater depth in the GRIS. Most of this dust probably originated from eastern Asia and shows a REY distribution similar to Post-Archean upper continental crust, i.e., lacks any EuSN anomaly. Hence, REY geochemistry suggests that the particulates and the nanoparticles/colloids in these arctic meltwaters are derived from different sources.


      PubDate: 2015-07-31T20:52:08Z
       
  • The role of bacterial sulfate reduction during dolomite precipitation:
           Implications from Upper Jurassic platform carbonates
    • Abstract: Publication date: 27 September 2015
      Source:Chemical Geology, Volume 412
      Author(s): Andre Baldermann, Artur P. Deditius, Martin Dietzel, Vanessa Fichtner, Cornelius Fischer, Dorothee Hippler, Albrecht Leis, Claudia Baldermann, Vasileios Mavromatis, Christian P. Stickler, Harald Strauss
      The early diagenetic formation of dolomite in modern aquatic environments is limited mostly to evaporitic and marine-anoxic, organic-rich sediments dominated by bacterial sulfate reduction (BSR). In such environments, bacterial activity lowers the energy barriers for the nucleation and growth of dolomite and thus promotes the formation of non-stoichiometric, highly disordered and metastable (proto)dolomite. Although the boundary conditions for the formation of modern (proto)dolomites are considered to be generally understood, the role of BSR during limestone dolomitization in ancient marine environments remains questionable. Herein, we present a study about the physicochemical conditions and processes, which led to the formation of partly dolomitized limestone and dolostone in the presence of BSR on a stable carbonate platform during the Upper Jurassic at Oker (Northern German Basin). The dolomite textures, the spatial trace elemental patterns of the dolomite and of the surrounding limestone and the results of δ18O and δ13C isotope analyses reveal that the Oker dolomite has been formed by the early diagenetic replacement of magnesian calcite precursors at temperatures between 26°C and 37°C. We interpret the mineralizing fluids responsible for dolomitization as pristine-marine to slightly evaporitic and reducing seawater being modified during shallow seepage reflux and/or evaporitic tidal pumping. The elevated δ34SCAS values (+17.9 to +19.7‰, V-CDT) of the Oker dolomite, compared to ambient Upper Jurassic seawater, indicate that BSR facilitated dolomite formation. For the first time, we show that a linear anti-correlation exists between decreasing carbonate-associated sulfate (CAS) contents in dolomite and increasing ordering ratio of the dolomite lattice structure, with the degree of cation order in dolomite to be given by: degree of cation order(Dol): =−0.018·CAS(Dol) +68.3 (R2 =0.98). This correlation implies that the CAS content of sedimentary dolomite can be used as a measure for dolomite maturity. The relationships between the ambient (paleo)environmental controls, the resultant dolomitization pathways and subsequently the structure and the composition of the precipitating dolomite are presented and discussed in relation to the stability of modern and ancient (proto)dolomites throughout burial diagenesis.


      PubDate: 2015-07-28T08:55:05Z
       
  • Strontium and oxygen isotopic profiles through 3km of hydrothermally
           altered oceanic crust in the Reykjanes Geothermal System, Iceland
    • Abstract: Publication date: 27 September 2015
      Source:Chemical Geology, Volume 412
      Author(s): Naomi Marks, Robert A. Zierenberg, Peter Schiffman
      The Iceland Deep Drilling Program well RN-17 was drilled 3km into a section of hydrothermally altered basaltic crust in the Reykjanes geothermal system in Iceland. The system is located on the landward extension of the Mid-Atlantic Ridge, and the circulating hydrothermal fluid is modified seawater, making Reykjanes a useful analog for mid-oceanic ridge hydrothermal systems. We have determined whole-rock Sr and O isotope compositions, and Sr isotope compositions of epidote grains from the RN-17 cuttings and RN-17B core. Whole rock oxygen isotope ratios range from −0.13 to 3.61‰ V-SMOW, and are isotopically lighter than fresh MORB (5.8±0.2‰). The concentrations of Sr in the altered basalt range from well below to well above concentrations in fresh rock, and appear to be strongly correlated with the dominant alteration mineralogy. Whole rock Sr isotope ratios ranged from 0.70329 in the least altered crystalline basalt, to 0.70609 in the most altered hyaloclastite samples; there is no correlation with depth. Sr isotope ratios in epidote grains measured by laser ablation MC-ICP-MS ranged from 0.70360 to 0.70731. Three depth intervals, at 1000m, 1350m, and 1650m depth, have distinctive isotopic signatures, where 87Sr/86Sr ratios are elevated (mean value>0.7050) relative to background levels (mean altered basalt value ~0.7042). These areas are proximal to geothermal feed zones, and the 1350m interval directly overlies the transition from dominantly extrusive to dominantly intrusive lithologies. Oxygen isotope measurements yield integrated water/rock ratios of 0.4 to 4.3, and suggest that hydrothermal fluids must have formerly had a component of meteoric water. Strontium isotopic measurements provide a more sensitive indication of seawater interaction and require significant exchange with seawater strontium. Both isotopic systems indicate that the greenschist-altered basalts were in equilibrium with hydrothermal fluids at a relatively high mean water/rock (Wt.) ratio ranging from about 0.5 to 4. These ratios are higher than estimates from ODP Hole 504B and IODP Hole 1256D, but are consistent with values inferred from vent fluids from 21° and 13°N on the East Pacific Rise (Albarède et al., 1981; Michard et al., 1984; Alt et al., 1996; Harris et al., 2015).


      PubDate: 2015-07-28T08:55:05Z
       
  • Alkali and alkaline earth metal chloride solutions influence sulfide
           mineral dissolution
    • Abstract: Publication date: 27 September 2015
      Source:Chemical Geology, Volume 412
      Author(s): Hariprasad Parthasarathy, David A. Dzombak, Athanasios K. Karamalidis
      Alkali and alkaline metal chlorides have been considered as inert electrolyte species with respect to sulfide mineral dissolution in the presence of oxidizing agents such as O2 and Fe3+. Under anoxic conditions in the laboratory or the field, as exist in most saline subsurface environments, the potential reactivity of alkali and alkaline metal chlorides with sulfide minerals has typically been ignored. Arsenopyrite (FeAsS(s)), galena (PbS(s)), and pyrite (FeS2(s)) are commonly encountered sulfide mineral phases, the dissolution of which affects many ecosystems. In this study, dissolution experiments with these minerals were conducted under anoxic conditions with 10mM solutions of NaCl, CaCl2, and MgCl2 at constant pH of 2.56. Results show that these electrolytes affect sulfide mineral dissolution under anoxic conditions, either increasing or decreasing the rate. The extent to which sulfide mineral dissolution is affected is small but measurable and depends on the anionic species in the mineral and cationic species in solution. Specifically, the dissolution of arsenic from arsenopyrite increased with an increase in cation activity in solution, while the dissolution of sulfur decreased with an increase in chloride ion activity. These results suggest that sulfide mineral dissolution under anoxic conditions is caused by an interaction of cations in solution with anions on the mineral surface, and inhibited by the presence of competing anions in solution.


      PubDate: 2015-07-28T08:55:05Z
       
  • The controls of post-entrapment diffusion on the solubility of
           chalcopyrite daughter crystals in natural quartz-hosted fluid inclusions
    • Abstract: Publication date: 27 September 2015
      Source:Chemical Geology, Volume 412
      Author(s): Edward T. Spencer, Jamie J. Wilkinson, John Nolan, Andrew J. Berry
      The presence of chalcopyrite daughter crystals in natural quartz-hosted fluid inclusions that do not dissolve when heated to trapping conditions suggests that inclusions are subject to post-entrapment modifications that affect chalcopyrite solubility. Previous double capsule experiments conducted by Mavrogenes and Bodnar (1994) concluded that the post-entrapment outward diffusion of H2 is responsible for the presence of non-dissolvable chalcopyrite crystals in natural, quartz-hosted brine inclusions. However, recent studies have shown that quartz-hosted inclusions can also be modified by diffusional H+ loss and Cu+ gain. This means that multiple factors may influence chalcopyrite solubility in different fluid inclusion types. In this study, the experimental procedure of Mavrogenes and Bodnar (1994) was recreated in order to rehydrogenate quartz-hosted, chalcopyrite-bearing fluid inclusions from the El Teniente Cu–Mo porphyry deposit, Chile. These inclusions had a range of salinities and densities. Results show that the experimental technique is successful for fluid inclusions that contain relatively small chalcopyrite daughter crystals and have moderate salinities (>5wt.% NaCleq). In contrast, chalcopyrite crystals do not dissolve in low density vapor inclusions even after rehydrogenation. The failure of chalcopyrite crystals to dissolve in these inclusions is attributed to their lower initial pH and higher sulfide concentrations, which led to greater post-entrapment H+ loss and Cu+ gain. This considered, Cu concentrations in moderate to high salinity inclusions are likely to reflect those present at trapping, suggesting that H2 loss is the primary control on the failed dissolution of chalcopyrite. By contrast, Cu concentrations in S-rich vapor inclusions can increase considerably via inward Cu+ diffusion in the presence of an external Cu-bearing fluid and a pH gradient between the inclusion and this fluid (Lerchbaumer and Audétat, 2012; Seo and Heinrich, 2013). In accordance with these studies, the post-entrapment modification of Cu concentrations in vapor inclusions may undermine the apparent importance of phase separated vapors as a key agent of Cu transport and deposition in porphyry systems.


      PubDate: 2015-07-28T08:55:05Z
       
  • Mineralogical transformations set slow weathering rates in low-porosity
           metamorphic bedrock on mountain slopes in a tropical climate
    • Abstract: Publication date: 14 September 2015
      Source:Chemical Geology, Volume 411
      Author(s): Ricarda Behrens, Julien Bouchez, Jan A. Schuessler, Stefan Dultz, Tilak Hewawasam, Friedhelm von Blanckenburg
      In the Sri Lankan Highlands erosion and chemical weathering rates are among the lowest for global mountain denudation. In this tropical humid setting, highly weathered deep saprolite profiles have developed from high-grade metamorphic charnockite during spheroidal weathering of the bedrock. The spheroidal weathering produces rounded corestones and spalled rindlets at the rock–saprolite interface. We used detailed textural, mineralogical and chemical analyses to reconstruct the sequence of weathering reactions and their causes. The first mineral attacked by weathering was found to be pyroxene initiated by in situ Fe oxidation. Volumetric calculations suggest that this oxidation leads to the generation of porosity due to the formation of micro-fractures allowing for fluid transport and subsequent dissolution of biotite and plagioclase. The rapid ensuing plagioclase weathering leads to formation of high secondary porosity in the corestone over a distance of only a few cm and eventually to the final disaggregation of bedrock to saprolite. The first secondary phases are oxides or amorphous precipitates from which secondary minerals (mainly gibbsite, kaolinite and goethite) form. As oxidation is the first weathering reaction, the supply of O2 is a rate-limiting factor for chemical weathering. Hence, the supply of O2 and its consumption at depth connects processes at the weathering front with those at the Earth's surface in a feedback mechanism. The strength of the feedback depends on the relative weight of advective versus diffusive transport of O2 through the weathering profile. The feedback will be stronger with dominating diffusive transport. The low weathering rate is explained by the nature of this feedback that is ultimately dependent on the transport of O2 through the whole regolith, and on lithological factors such as low bedrock porosity and the amount of Fe-bearing primary minerals. Tectonic quiescence in this region and low pre-development erosion rate (attributed to a dense vegetation cover) minimize the rejuvenation of the thick and cohesive regolith column, finally leading to low denudation rates.


      PubDate: 2015-07-24T08:46:36Z
       
  • Evaluation of detrital thermochronology for quantification of glacial
           catchment denudation and sediment mixing
    • Abstract: Publication date: 14 September 2015
      Source:Chemical Geology, Volume 411
      Author(s): Eva Enkelmann, Todd A. Ehlers
      Thermochronometric methods have been applied successfully on bedrock samples as well as detrital material to study exhumation processes in mountain belts. The access to exposed bedrock can be a limiting factor in remote and rugged mountainous regions, or areas covered by ice. The analysis of detrital material provides an integrated signal of rock cooling from sediment source areas in a catchment. One advantage of detrital thermochronology is that the source areas can include regions that are inaccessible for bedrock dating, such as beneath glaciers. In this study we investigate the suitability of various detrital thermochronometer sampling approaches at the glacier terminus including sediments from the pro-glacial fluvial outwash, the ice-cored terminal moraine, and older moraines. Specifically we analyzed the detrital apatite fission track ages of sand-size material collected from the Tiedemann and Scimitar Glaciers that drain the eastern and northern flanks of Mt. Waddington British Columbia, Canada, respectively. We present 935 new apatite fission-track ages and compare the grain-age distributions of the various detrital sites among each other and with published bedrock ages from the Tiedemann Glacier catchment. We show that detrital apatite fission-track thermochronometry is a viable and powerful tool to obtain a robust cooling age distribution of a catchment or region that can elucidate age populations originating from those parts of the catchment that are covered by ice and therefore remain undetected by bedrock studies. We also show that sampling the ice-cored terminal moraine is an alternative sampling approach to the pro-glacial river sediments that provides cooling age distributions representative of the sediments sourced by the entire catchment including sub-glacially eroded material. Finally, samples collected from the modern glacial systems and terminal moraines of different ages are compared to assess temporal variations in the distribution of glacial erosion over the Late Holocene.


      PubDate: 2015-07-24T08:46:36Z
       
  • The oxidation state of europium in silicate melts as a function of oxygen
           fugacity, composition and temperature
    • Abstract: Publication date: 14 September 2015
      Source:Chemical Geology, Volume 411
      Author(s): A.D. Burnham, A.J. Berry, H.R. Halse, P.F. Schofield, G. Cibin, J.F.W. Mosselmans
      Europium L III-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of synthetic glasses and melts equilibrated over a range of oxygen fugacities ( f O2s, from −14 to +6 logarithmic units relative to the quartz–fayalite–magnetite, QFM, buffer) and temperatures (1250–1500°C). Eu3+/ΣEu (where ΣEu=Eu2+ +Eu3+) values were determined from the spectra with a precision of ±0.015. Eu3+/ΣEu varies systematically with f O2 from 0 to 1 over the range studied, increases with decreasing melt polymerisation and temperature, and can be described by the empirical equation: E u 3 + / Σ E u = 1 1 + 10 − 0.25 log f O 2 − 6410 T − 14.2 Λ − 10.1 , where Λ is the optical basicity of the melt and T is the temperature in K. Eu3+/ΣEu in glasses and melts equilibrated at the same conditions are in excellent agreement for Fe-free systems. For Fe-bearing compositions the reaction Eu2+ +Fe3+ =Eu3+ +Fe2+ occurs during quenching to a glass and the high temperature value of Eu3+/ΣEu is not preserved on cooling; in situ measurements are essential for determining Eu3+/ΣEu in natural melts.


      PubDate: 2015-07-24T08:46:36Z
       
  • Strontium incorporation during calcite growth: Implications for chemical
           mapping using friction force microscopy
    • Abstract: Publication date: 14 September 2015
      Source:Chemical Geology, Volume 411
      Author(s): Pablo Cubillas, Xiaoming Hu, Steven R. Higgins
      Sr partitioning on calcite crystals growing from Ca2+–Sr2+–CO3 2− solutions was studied by means of friction force microscopy (FFM). Experiments were performed with various Sr2+ (aq)/Ca2+ (aq) concentration ratios and total Sr2+ (aq) concentration in order to examine conditions under which Sr-calcite growth is self-limiting (e.g., the so-called “template effect”) and also to investigate continuous Sr-calcite growth, where spiral growth predominates and Sr incorporation is sector-dependent. In these latter experiments, the goal was to evaluate the utility of friction force microscopy to discriminate sector zoning. Results from the experiments show that friction increases with the incorporation of Sr into the growing calcite layers. The maximum increase in friction was measured at high Sr2+ (aq)/Ca2+ (aq), although a quantitative link between a specific amount of increase in friction to a specific amount of Sr incorporation was not possible to determine due to experimental uncertainties. Nevertheless, it was possible to establish that no change in friction is detectable when Sr incorporation yields a solid composition of Sr0.05Ca0.95CO3. Friction was found to increase during growth of several layers in an incremental fashion. The increase can be linked either to an incremental increase in Sr content in the newly formed calcite, controlled by the thermodynamics of the strained layers necessitated by the substitution of larger Sr cations into the calcite, or to the incremental increase in layer thickness which in turn leads to increases in the probe-surface contact area. No difference in friction could be observed between acute and obtuse sectors under any of the experimental conditions, which was primarily due to the limits of the friction measurement sensitivity.
      Graphical abstract image

      PubDate: 2015-07-24T08:46:36Z
       
  • Pore-scale heterogeneity in the mineral distribution and reactive surface
           area of porous rocks
    • Abstract: Publication date: 14 September 2015
      Source:Chemical Geology, Volume 411
      Author(s): Peter Lai, Kevin Moulton, Samuel Krevor
      The reactive surface area is an important control on interfacial processes between minerals and aqueous fluids in porous rocks. Spatial heterogeneity in the surface area can lead to complications in modelling reactive transport processes, but quantitative characterisation of this property has been limited. In this paper 3D images obtained using X-ray micro-tomography were used to characterise heterogeneity in surface area in one sandstone and five carbonate rocks. Measurements of average surface area from X-ray imagery were 1–2 orders of magnitude lower than measurements from nitrogen BET. A roughness factor, defined as the ratio of BET surface area to X-ray based surface area, was correlated to the presence of clay or microporosity. Co-registered images of Berea sandstone from X-ray and energy dispersive spectroscopy imagery were used to guide the identification of quartz, K-feldspar, dolomite, calcite and clays in X-ray images. In Berea sandstone, clay and K-feldspar had higher average surface area fractions than their volumetric fractions in the rock. In the Edwards carbonate, however, modal mineral composition correlated with surface area. By sub-sampling digital images, statistical distributions of the surface area were generated at various length scales of subsampling. Comparing these to distributions used in published modelling studies showed that the common practice of leaving surface area and pore volume uncorrelated in a pore has lead to unrealistic combinations of surface area and pore volume in the models. We suggest these models adopt a moderate correlation based on observations. In Berea sandstone, constraining ratios of surface area to pore volume to a range of values between that of quartz-lined and five times that of clay-lined spheres appeared sufficient.


      PubDate: 2015-07-24T08:46:36Z
       
  • Application of radon and radium isotopes to groundwater flow dynamics: An
           example from the Dead Sea
    • Abstract: Publication date: 14 September 2015
      Source:Chemical Geology, Volume 411
      Author(s): Yael Kiro, Yishai Weinstein, Abraham Starinsky, Yoseph Yechieli
      This study presents the behavior of radon and radium isotopes and their application to groundwater age and flow dynamics. The research was conducted in the complex Dead Sea groundwater system, which includes a large variety of sediments, groundwater salinities, flow mechanisms and groundwater ages. Groundwater around the Dead Sea contains high activities of radon (up to tens of thousands dpm/L) and radium (up to hundreds dpm/L). Adsorption of radium, which is partially salinity controlled, is an important source of unsupported 222Rn, which is used for estimating the adsorption partition coefficient of radium. In addition to salinity, the concentration of Mn and Fe oxides and aquifer heterogeneity are important factors controlling the adsorption partition coefficient. The different nature of the rocks on both sides of the Dead Sea transform, with lower Th/U ratios in the carbonate rocks on the western catchment of the Dead Sea compared to higher ratios in the sandstone aquifer east of the Dead Sea, is reflected in a higher 228Ra/226Ra activity ratio in the eastern compared with the western groundwaters (averages of 0.76 and 0.15, respectively). The different groundwater groups around the Dead Sea contain secular or non-secular equilibrium ratios, which depend on the age of the groundwater (the time since the groundwater entered the aquifer) or whether the groundwater system is in a steady state (the age of the groundwater system). Young groundwater, such as the Dead Sea water that circulates in the aquifer or freshwater springs, is depleted in the long-lived radium isotopes compared to the short-lived isotopes, whereas old groundwater contains relatively high activity of 226Ra (∼500dpm/L) and the radium activity ratios are close to secular equilibrium. The common secular equilibrium ratios between all four radium isotopes in the Dead Sea groundwaters suggest that many of the groundwater flow paths did not change significantly during the past 8000years.


      PubDate: 2015-07-19T16:09:55Z
       
  • Geochemical mapping of organic carbon in stalagmites using liquid-phase
           and solid-phase fluorescence
    • Abstract: Publication date: 14 September 2015
      Source:Chemical Geology, Volume 411
      Author(s): Marine Quiers, Yves Perrette, Emilie Chalmin, Bernard Fanget, Jérôme Poulenard
      The soil-derived organic matter incorporated in speleothems provides a proxy for the impacts of climate and environmental changes on the surrounding catchment. These organic proxies, combined with inorganic proxies, can be used to investigate variations in carbon fluxes. The present article describes a method for obtaining high spatial-resolution estimations of speleothem organic carbon concentrations by using the relationship between liquid phase fluorescence and the organic carbon contents of soil samples. Applying this method to soil and stalagmite samples from three locations in the French Prealps gave estimated stalagmite carbon concentrations of between 0.27 and 3.03mgC/g of calcite, which are consistent with measured concentrations reported in the literature. We assessed the high-resolution reliability of our procedure by comparing TOC estimations with solid phase fluorescence values for samples taken every 2-mm along one of the stalagmites (TAM). Due to variations in the optical properties (e.g., optical density) of the calcite, revealed by Near Infra Red Reflectance, it was necessary to draw up a nonlinear model in order to obtain good estimates (R2 =0.81) of organic carbon concentrations from solid-phase fluorescence results. The resulting high-resolution map of organic carbon concentrations along the TAM sample was consistent with the recent history of the area's environment. Our results show that variations in carbon flux in mountain karst environments are strongly linked to changes affecting the area's soils.


      PubDate: 2015-07-19T16:09:55Z
       
  • Impact of atmospheric pCO2, seawater temperature, and calcification rate
           on the δ18O and δ13C composition of echinoid calcite
           (Echinometra viridis)
    • Abstract: Publication date: 14 September 2015
      Source:Chemical Geology, Volume 411
      Author(s): T. Courtney, J.B. Ries
      The tropical echinoid Echinometra viridis was reared in controlled laboratory experiments at temperatures of approximately 20°C and 30°C to mimic winter and summer temperatures and at carbon dioxide (CO2) partial pressures of approximately 487ppm-v and 805ppm-v to simulate current and predicted-end-of-century levels. Spine material produced during the experimental period and dissolved inorganic carbon (DIC) of the corresponding culture solutions were then analyzed for stable oxygen (δ18Oe, δ18ODIC) and carbon (δ13Ce, δ13CDIC) isotopic composition. Fractionation of oxygen stable isotopes between the echinoid spines and DIC of their corresponding culture solutions (Δ18O=δ18Oe −δ18ODIC) was significantly inversely correlated with seawater temperature but not significantly correlated with atmospheric pCO2. Fractionation of carbon stable isotopes between the echinoid spines and DIC of their corresponding culture solutions (Δ13C=δ13Ce −δ13CDIC) was significantly positively correlated with pCO2 and significantly inversely correlated with temperature, with pCO2 functioning as the primary factor and temperature moderating the pCO2–Δ13C relationship. Echinoid calcification rate was significantly inversely correlated with both Δ18O and Δ13C across treatments, with effects of pCO2 and temperature controlled for through ANOVA. Therefore, calcification rate and potentially the rate of co-occurring dissolution appear to be important drivers of the kinetic isotope effects observed in the echinoid spines. Study results suggest that echinoid Δ18O monitors seawater temperature, but not atmospheric pCO2, and that echinoid Δ13C monitors atmospheric pCO2, with temperature moderating this relationship. These findings, coupled with echinoids' long and generally high-quality fossil record, support prior assertions that fossil echinoid Δ18O is a viable archive of paleo-seawater temperature throughout Phanerozoic time, and that Δ13C merits further investigation as a potential proxy of paleo-atmospheric pCO2. However, the apparent impact of calcification rate on echinoid Δ18O and Δ13C suggests that paleoceanographic reconstructions derived from these proxies in fossil echinoids could be improved by incorporating the effects of growth rate.


      PubDate: 2015-07-19T16:09:55Z
       
  • Controls on stable Sr-isotope fractionation in continental barite
    • Abstract: Publication date: 14 September 2015
      Source:Chemical Geology, Volume 411
      Author(s): Inoka H. Widanagamage, Elizabeth M. Griffith, David M. Singer, Howie D. Scher, Wayne P. Buckley, John M. Senko
      Barite precipitation typically occurs when barium rich fluids mix with sulfate rich fluids, however barite found in the modern continental environment suggests that biological activity can play an important role in barite formation by oxidizing sulfur and/or concentrating barium within microenvironments. These activities induce barite precipitation, and carry with them implications for studies of barite genesis. Strontium (Sr) is incorporated into the barite crystal structure during barite formation preserving a radiogenic and stable Sr-isotope signature in barite, providing information about its formation. Here we present Sr-isotope results from three artesian sulfidic springs with ongoing barite precipitation (Zodletone Spring, Oklahoma; Stinking Spring, Utah; and Doughty Springs, Colorado) to explore the controls on stable Sr-isotope fractionation during barite precipitation in a continental setting. Apparent stable Sr-isotope fractionation for all three sites ranged from −0.6‰ to ~0.0‰ similar to previously published calculated values for equilibrium conditions and measured values of synthetic barite. However, clear relationships do not exist between water and barite chemistry in the natural systems, indicating that barite does not precipitate directly from solution, but heterogeneously within diverse microenvironments created by microbial biomass or on sediment surfaces. The dynamic microenvironments in a continental setting influence the apparent stable Sr-isotope fractionation during barite precipitation because of changing saturation conditions, Sr concentration and/or precipitation of different mineral phases (e.g., celestine). In order to better understand the geochemistry of barite deposits, future work is necessary to study the controls on radiogenic and stable Sr-isotope signatures of barite in the context of the temporally and spatially dynamic nature of the continental setting.


      PubDate: 2015-07-19T16:09:55Z
       
  • Geochemical processes following freshwater reflooding of acidified inland
           acid sulfate soils: An in situ mesocosm experiment
    • Abstract: Publication date: 14 September 2015
      Source:Chemical Geology, Volume 411
      Author(s): Nathan L. Creeper, Warren S. Hicks, Paul Shand, Rob W. Fitzpatrick
      In their oxidised form, inland acid sulfate soils (IASS) with sulfuric horizons (pH≤3.5) contain substantial acidity and pose a number of threats to surrounding ecosystems. In their reduced form, IASS with sulfidic material are relatively benign. Freshwater reflooding has the potential to return oxidised IASS with sulfuric horizons to a reduced and benign state. This study uses mesocosms installed in situ to simulate reflooding in two sulfuric IASS profiles, one sandy textured and the other a cracking clay, and to document key geochemical consequences resulting from their reflooding. During the assessed period of 200days of subaqueous conditions, reducing conditions were established in parts of the former sulfuric horizons in both the sandy textured and clayey textured IASS. In the permeable sandy IASS, acidity was removed from the sulfuric horizon and displaced downward in the profile by advective piston flow, and thus not completely neutralised. The removal of acidity away from the soil surface was critical in preventing surface water acidification. In contrast, solute transport in the less permeable clayey IASS was diffusion dominated and acidity was not removed from the sulfuric horizon following reflooding and no increase in pH was observed. In the absence of piston flow, a diffusive flux of acidity, from the soil to surface water, resulted in surface water acidification. In the acidic porewaters of the reflooded sulfuric horizons, results indicated dissolved aluminium was controlled by an aluminium species with stoichiometry Al:OH:SO4 (e.g. jurbanite). In the same acidic porewaters, iron and sulfate activity appeared to be regulated by the dissolution of natrojarosite. Following the establishment of reducing conditions, the reductive dissolution of natrojarosite and schwertmannite was responsible for large increases in total dissolved iron. We did not observe any indirect evidence indicating the existence of sulfate reduction during the assessed period. It is likely that insufficiently reducing conditions, competitive exclusion by iron-reducing bacteria, and persisting low pH inhibited sulfate reduction during the assessed period. With insufficient in situ alkalinity generation, IASS are likely to continue to pose an environmental hazard following reflooding and remediation is likely to be slow. A number of geochemical processes involved in the remediation of sulfuric horizons were observed in this study. The key geochemical and physical processes affecting porewater chemistry, in particular Fe and Al, are summarised in a conceptual hydrogeochemical model, so that observations made in this study may be applied to other regions containing IASS with sulfuric horizons that are expected to be reflooded with freshwater.


      PubDate: 2015-07-19T16:09:55Z
       
  • In search of late-stage planetary building blocks
    • Abstract: Publication date: 14 September 2015
      Source:Chemical Geology, Volume 411
      Author(s): Richard J. Walker , Katherine Bermingham , Jingao Liu , Igor S. Puchtel , Mathieu Touboul , Emily A. Worsham
      Genetic contributions to the final stages of planetary growth, including materials associated with the giant Moon-forming impact, late accretion, and late heavy bombardment are examined using siderophile elements. Isotopic similarities between the Earth and Moon for both lithophile and siderophile elements collectively lead to the suggestion that the genetics of the building blocks for Earth, and the impactor involved in the Moon-forming event were broadly similar, and shared some strong genetic affinities with enstatite chondrites. The bulk genetic fingerprint of materials subsequently added to Earth by late accretion, defined as the addition of ~0.5wt.% of Earth's mass to the mantle, following cessation of core formation, was characterized by 187Os/188Os and Pd/Ir ratios that were also similar to those in some enstatite chondrites. However, the integrated fingerprint of late accreted matter differs from enstatite chondrites in terms of the relative abundances of certain other HSE, most notably Ru/Ir. The final ≤0.05wt.% addition of material to the Earth and Moon, believed by some to be part of a late heavy bombardment, included a component with much more fractionated relative HSE abundances than evidenced in the average late accretionary component. Heterogeneous 182W/184W isotopic compositions of some ancient terrestrial rocks suggest that some very early-formed mantle domains remained chemically distinct for long periods of time following primary planetary accretion. This evidence for sluggish mixing of the early mantle suggests that if late accretionary contributions to the mantle were genetically diverse, it may be possible to isotopically identify the disparate primordial components in the terrestrial rock record using the siderophile element tracers Ru and Mo.


      PubDate: 2015-07-16T10:46:07Z
       
  • Diffusive fractionation of H2O and CO2 during magma degassing
    • Abstract: Publication date: 14 September 2015
      Source:Chemical Geology, Volume 411
      Author(s): Shumpei Yoshimura
      H2O–CO2 systematics of melt inclusions and obsidian pyroclasts has been used widely to explore the behaviour of fluids in magmatic systems under the assumption that a fluid–melt equilibrium is attained. However, fluid exsolution is a process involving the diffusive transfer of volatiles, and kinetic effects may control the composition of volatiles. In this study, single-step decompression experiments were carried out on H2O–CO2-bearing basaltic and rhyolitic melts to investigate the evolution of H2O–CO2 compositions during vesiculation. Microanalysis of the volatiles using Raman and infrared spectroscopy showed that CO2 content decreased towards bubbles, while H2O content was almost constant throughout the quenched glass samples. This resulted in higher CO2/H2O ratios than estimated from the equilibrium degassing, which is interpreted as a kinetic effect: the diffusivity of H2O is higher than that of CO2. A simple model for the diffusive degassing of ascending magma was developed to investigate the disequilibrium evolution of its H2O–CO2 content. CO2/H2O ratios were strongly dependent on the magma ascent velocity. The model was applied to melt inclusions with high CO2/H2O ratios from Etna and Stromboli volcanoes, and it was shown that the high ratios could not be explained by diffusive fractionation under a typical magma ascent velocity. Rather, these ratios are affected by other processes, such as CO2 fluxing.


      PubDate: 2015-07-16T10:46:07Z
       
  • Real-time measurements of δ13C, CO2 concentration, and CO2/SO2 in
           volcanic plume gases at Mount Etna, Italy, over 5 consecutive days
    • Abstract: Publication date: 14 September 2015
      Source:Chemical Geology, Volume 411
      Author(s): A.L. Rizzo , M. Liuzzo , M.A. Ancellin , H.J. Jost
      We present new real-time measurements of the CO2 concentration and δ13C made from July 16 to 20, 2014 in diluted gases of the active plume emitted by the Central Craters at Mount Etna volcano, Italy. This innovative study involved measuring δ13C in plume gases at a very high frequency over 5days of measurements. The carbon-isotope composition calculated for volcanic CO2 ranged from −1.3‰ to +1.5‰, with uncertainties in the repeated single measurements (i.e., made over periods from 4 to 20min) that were generally <0.7‰, and yet surprisingly varied by larger amounts over the 5-day study period. The range of calculated δ13C values mostly overlaps with that indicated for the plume of the Central Craters obtained by discrete sampling and using the isotope-ratio mass spectrometry technique (−2.5‰<δ13C<−0.5‰). However, we propose that during particular conditions of volcanic activity, the carbon-isotope composition of CO2 degassed from magma can reach values (up to +1.5‰) that are higher than those reported previously. During this campaign we also made simultaneous measurements of the CO2 and SO2 concentrations using the MultiGAS technique. The volcanic δ13C and CO2/SO2 ratios exhibited similar trends over the 5days of measurements, with the ratios of both tracers peaking on July 16, possibly as a result of the early degassing of CO2 while an eruption was ongoing at Mount Etna. The observed variations and the highest δ13C values measured at Mount Etna during this campaign lead to new questions about the variability of this geochemical tracer. The comparisons with the CO2/SO2 ratio also confirm that monitoring δ13C in plume gases in real time, coupled to other geochemical tracers, is important for elucidating the magma dynamics at depth.
      Graphical abstract image

      PubDate: 2015-07-16T10:46:07Z
       
  • Effect of oxygen fugacity on the coordination and oxidation state of iron
           in alkali bearing silicate melts
    • Abstract: Publication date: 14 September 2015
      Source:Chemical Geology, Volume 411
      Author(s): Jaayke L. Knipping , Harald Behrens , Max Wilke , Jörg Göttlicher , Paola Stabile
      In this study the effect of oxygen fugacity (fO2) on the oxidation state and coordination of Fe was investigated in different alkali trisilicate glasses (Rb2Si3O7 =RFS, K2Si3O7 =KFS; Na2Si3O7 =NFS; Li2Si3O7 =LFS) doped with ~5wt.% of Fe2O3 with main focus on K- and Na-bearing compositions. Most of the experiments were conducted at ambient pressure in a gas mixing furnace at 1250°C with controlled redox conditions (log fO2/bar: −0.68 to −16.18). The quenched glasses were analyzed using several methods. Analyses by a colorimetric wet chemistry method revealed a continuous increase in Fe2+/Fetotal towards more reducing conditions without reaching 100% Fe2+ even at extremely reducing conditions (range of Fe2+/Fetotal: from 0.08 in air to 0.93 in H2 atmosphere). X-ray absorption near edge structure (XANES) spectroscopy shows an increase of Fe coordination with decreasing ionic radius of the coexisting alkali, while the average coordination number seems to be independent on the oxidation state of iron aside from the largest studied alkali Rb, which seems to support lower coordinated Fe (tetrahedral) at more oxidizing conditions. The Fe2+/Fetotal ratios inferred by XANES, using an intensity ratio based calibration of Wilke et al. (2004), are systematically higher by 10% compared to the wet chemistry results of this study, which may be due to the different external Fe2+/Fetotal determination method (Mössbauer spectroscopy) used in their calibration. A new calibration curve based on wet chemistry and centroid positions is proposed for alkali silicate glasses. In optical spectroscopy, the position of the main Fe2+-related peak shifts to lower wavenumbers with increasing ionic radius of the incorporated alkali and with increasing abundance of ferrous iron. Absorption coefficients εFe(II) and εFe(III) were calculated for the absorbance band at ~9000 and ~26,000cm−1, respectively. A decrease in εFe(II) was detected with decreasing ionic radius of the incorporated alkalis (εFe(II) KFS =31.8±2.6L·mol−1·cm−1, εFe(II) NFS =30.7±2.3L·mol−1·cm−1 and εFe(II) LFS =23.6±1.7L·mol−1·cm−1). Finally, the results of this study are compared with recent models, which predict fO2 based on the knowledge of the Fe2+/Fetotal ratio. All models overestimate Fe2+/Fetotal in alkali silicate melts at very reducing conditions probably due to an unanticipated stabilization of Fe3+ by adjacent Fe2+.


      PubDate: 2015-07-16T10:46:07Z
       
  • SO2–CO2 and pure CO2 reactivity of ferroan carbonates at carbon
           storage conditions
    • Abstract: Publication date: 14 September 2015
      Source:Chemical Geology, Volume 411
      Author(s): Julie K. Pearce , Alison C.K. Law , Grant K.W. Dawson , Suzanne D. Golding
      Carbon dioxide captured and stored geologically from industrial sources such as coal fired and oxyfuel power stations or cement processing may contain impurity gases such as SO2. Carbonates including siderite and ankerite are some of the most reactive minerals present in siliciclastic reservoirs. The reactivity of crushed siderite–ankerite to pure supercritical CO2 and impure SO2–CO2 dissolved in water was compared in a combined experimental and kinetic modelling study. Ankerite dissolution and fine-grained Fe-bearing mineral precipitation was observed with pure CO2–water reaction at 80°C and 200bar. Dissolved Ni, Cr, and Zn concentrations initially increased but subsequently decreased by 192h. The solution pH was predicted to be buffered from 3.1 to 4.4, with siderite and trace Fe-oxide precipitation. With the presence of SO2 in the CO2 stream, greater ankerite dissolution and additionally siderite dissolution were observed and predicted initially. Solution pH initially decreased on gas injection and was subsequently buffered from 2.5 to 5 during reaction, with the predicted pH3.9. Gypsum and amorphous FeS were precipitated along with an Fe–Mn carbonate cement, potentially re-precipitated siderite. Dissolved Cr concentration initially increased but subsequently decreased by 192h, with Cr signatures observed in precipitated FeS. Conversion of ankerite or calcite to gypsum initially (rather than anhydrite) could decrease rock porosity. Theoretically complete conversion of 10% ankerite or 15% calcite could reduce horizontal permeability in potential CO2 storage cap-rock by 3–170mD favourably self-sealing and reducing gas migration. Changes to rock permeability through geochemical reactions however require validation data from experimental measurements. These results have implications for the co-injection of flue gas impurities during CO2 storage in reservoirs containing ferroan carbonates.
      Graphical abstract image

      PubDate: 2015-07-16T10:46:07Z
       
  • Carbon isotope composition in modern brachiopod calcite: A case of
           equilibrium with seawater?
    • Abstract: Publication date: 14 September 2015
      Source:Chemical Geology, Volume 411
      Author(s): Uwe Brand , K. Azmy , E. Griesshaber , M.A. Bitner , A. Logan , M. Zuschin , E. Ruggiero , P.L. Colin
      We examined a large number of modern, shallow-water articulated brachiopods representing the orders Terebratulida, Rhynchonellida, Thecideida and one inarticulated brachiopod of the order Craniida from polar to tropical regions for their carbon isotope compositions. Based on our detailed investigation, we recommend avoiding fast growth areas such as the youngest shell increments; in addition, the primary layer and transition zone calcites of brachiopods must be avoided because they are in carbon and oxygen isotope disequilibrium with ambient seawater. After adjusting isotope compositions for the Mg effect, we observed no significant difference (p >0.05) in δ13C values between dorsal and ventral valves of our articulated brachiopods. Using the calcite-bicarbonate enrichment factor (ε) in conjunction with δ13C values of dissolved inorganic carbon of habitat seawater, we conclude that modern shallow-water articulated and some inarticulated brachiopods incorporate oxygen (Brand et al., 2013) and carbon isotopes into shell calcite of secondary and/or tertiary layers in apparent equilibrium with ambient seawater. Within the general concept of equilibrium incorporation, with seawater, shell δ13C values are an excellent recorder of local/global seawater environments and water mass circulation. Thus, application of the Mg-effect permits brachiopods to be an extremely powerful archive, and δ13C values more precise proxy and tracer of past changes in marine productivity, evolution of seawater carbon chemistry and variation in the global carbon cycle.


      PubDate: 2015-07-12T11:57:21Z
       
  • Structures and acidity constants of arsenite and thioarsenite species in
           hydrothermal solutions
    • Abstract: Publication date: Available online 10 July 2015
      Source:Chemical Geology
      Author(s): Xiandong Liu , Mengjia He , Xiancai Lu , Rucheng Wang
      We report a first principles molecular dynamics (FPMD) study of structures and acidity constants of arsenite and thioarsenite species in liquid water from ambient temperature to 573K. The analyses show that at all temperatures, the OH ligands of arsenite species form H-bonds with solvating water molecules as both donors and acceptors, whereas there are only very weak H-bonds between the SH ligands of thioarsenite species and water. For both arsenites and thioarsenites, the dangling O/S sites form strong H-bonds with hydrogen atoms of water molecules, but the As atoms have almost no interaction with water molecules. The FPMD based vertical energy gap method was applied to calculate the acidity constants. With the evaluated acidity, the species distributions with respect to pH have been derived. The pKa1s of H3AsO3 and H3AsS3 demonstrate a decreasing trend with temperature. For arsenites, H3AsO3 and H2AsO3 - can coexist, whereas HAsO3 2- almost does not exist, due to the notably high pKa2s of H3AsO3. For thioarsenites, H2AsS3 - and HAsS3 2- are always the dominant species in the near neutral pH range from ambient temperature to 573 K.


      PubDate: 2015-07-12T11:57:21Z
       
  • The genesis of LCT-type granitic pegmatites, as illustrated by lithium
           isotopes in micas
    • Abstract: Publication date: Available online 3 July 2015
      Source:Chemical Geology
      Author(s): Sarah Deveaud , Romain Millot , Arnaud Villaros
      Isotopic compositions in the Monts d’Ambazac Pegmatite Field (French Massif Central) exhibit a narrow range of mica δ7Li values, ranging from -3.6 to +3.4‰. The value obtained in biotite from the host Saint Sylvestre granite falls within this range (δ7Li = -1.5‰). Lithium concentrations are consistent with the degree of magmatic evolution of each pegmatite type: from 630 ppm in Type II up to 13,500 ppm in the more evolved Type VI pegmatite. Although the rare-element contents e.g., Li, Cs, Ta of the micas are consistent with pegmatite differentiation, δ7Li (‰) are firstly, independent of the degree of magmatic differentiation (independent of pegmatite type) and secondly, independent of the content of Li and other flux-elements such as Be and Cs. Muscovite sampled in pegmatite V from the Chabannes locality is the only pegmatite to exhibit a δ7Li variation from intermediate unit (-1.7‰) to internal pegmatitic unit (+3.4‰). The nature of this δ7Li variation suggests that there was extensive fractional crystallisation during the pegmatite’s consolidation. The independence of δ7Li (‰) evolution from the degree of magmatic evolution and the presence of distinct major rare-element bearing phases throughout the pegmatite field tend to confirm that the δ7Li (‰) values recorded in mica are inherited from crustal source rocks common to the granite and pegmatite-forming melts. We propose that the distinct pegmatite subtypes (beryl columbite vs lepidolite-petalite subtypes) observed throughout the Monts d’Ambazac Pegmatite Field reflect the diverse contributions of crustal protoliths. The lack of evidence of surrounding alteration combined with the absence of increasing Li-content within the host granite tend to confirm that the δ7Li values obtained within this pegmatite field are primary, and that no Li-diffusional process and/or mixing-driven Li-isotope fractionation has overprinted these isotopic compositions. In light of these results, the process of partial melting of protoliths enriched in rare-element bearing phases, e.g., mica, garnet, seems to be more responsible for Li-isotope fractionation than Li-diffusion or fractional crystallisation at the temperature of pegmatite consolidation. Finally, we discuss the use of Li isotopic compositions to identify the most highly evolved pegmatitic systems.


      PubDate: 2015-07-06T11:46:21Z
       
  • Crystallisation temperatures of the most Mg-rich magmas of the Karoo LIP
           on the basis of Al-in-olivine thermometry
    • Abstract: Publication date: 14 September 2015
      Source:Chemical Geology, Volume 411
      Author(s): Jussi S. Heinonen , Eleanor S. Jennings , Teal R. Riley
      Calculating reliable temperatures of Mg-rich magmas is problematic because melt composition and KD(Fe–Mg)ol–liq, the key parameters of many traditional thermometers, are difficult to constrain precisely. The recently developed Al-in-olivine thermometer [Coogan, L.A., Saunders, A.D., Wilson, R.N., 2014. Aluminium-in-olivine thermometry of primitive basalts: Evidence of an anomalously hot mantle source for large igneous provinces. Chemical Geology 368, 1–10] circumvents these problems by relying on the temperature-dependent exchange of Al between olivine and spinel crystallising in equilibrium with each other. This thermometer is used to re-evaluate the crystallisation temperatures of most Mg-rich magma type identified from the Karoo large igneous province (LIP), known as the Vestfjella depleted ferropicrite suite. Previous temperature estimates for the suite were based on olivine–melt equilibria and indicated anomalously high crystallisation temperatures in excess of 1600°C. We also present crystallisation temperatures for another Antarctic Karoo magma type, Group 3 dykes from Ahlmannryggen, which are derived from a pyroxene-rich mantle source. Our high-precision analysis of Al in olivine–spinel pairs indicate crystallisation temperatures from 1391±42°C to 1481±35°C for the Vestfjella depleted ferropicrite suite (Fo88–92) and from 1253±64°C to 1303±40°C for the Group 3 dykes (Fo79–82). Although the maximum temperature estimates for the former are over 100°C lower than the previously presented estimates, they are still ~200°C higher than those calculated for mid-ocean ridge basalts using the same method. Although exact mantle potential temperatures are difficult to estimate, the presented results support elevated sub-Gondwanan upper mantle temperatures (generated by a mantle plume or internal mantle heating) during the generation of the Karoo LIP.


      PubDate: 2015-07-06T11:46:21Z
       
  • Fate of Adsorbed Arsenate during Phase Transformation of Ferrihydrite in
           the Presence of Gypsum and Alkaline Conditions
    • Abstract: Publication date: Available online 3 July 2015
      Source:Chemical Geology
      Author(s): Soumya Das , Joseph Essilfie-Dughan , M. Jim Hendry
      We investigated the fate of adsorbed arsenate during phase transformation of ferrihydrite in the presence of calcium (Ca2+) and sulfate (SO4 2-), a condition frequently encountered in oxic mine tailings, waste rocks, and metallurgical operations. Ferrihydrite transformation under conditions that mimic those present during milling at the Key Lake uranium mine, Canada (i.e., pH, porewater chemistry, Fe/As molar ratio) was examined using batch experiments. Solid samples were analyzed using X-ray diffraction (XRD), Raman spectroscopy (RS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller surface area analyses (BET), inductively coupled plasma mass spectrometry (ICP-MS), and X-ray absorption spectroscopic analyses (XAS). Aqueous samples were analyzed for arsenic (As) and iron (Fe) concentrations using ICP-MS. X-ray diffraction and Raman spectroscopy analyses show ferrihydrite completely transforms to hematite (with traces of goethite) after 168 h of aging (75 °C and pH ~10). Hematite generated in the presence of As is spindle shaped, in contrast to the hexagonal or rhomb shaped morphology in the absence of As. The rate of ferrihydrite transformation is enhanced in the presence of partially dissolved gypsum (possibly due to an ionic strength effect) and the kinetics of transformation follow a logistic decay model. BET analyses suggest that the reactive surface area is reduced by ~94% by the end of the batch experiment. Despite the loss of reactive surface area, the aqueous concentrations of As decreased from 1.4 to 0.1 mg/L during 168 h of aging. XAS analyses suggest the As is incorporated into the newly formed hematite via both bidentate-mononuclear and binuclear corner sharing complexes. Although this transformation is a slow process and could take years to complete, this structural incorporation mechanism could nevertheless be a relevant pathway of As sequestration in the environment.


      PubDate: 2015-07-06T11:46:21Z
       
  • Refining the extraction methodology of carbonate associated sulfate:
           Evidence from synthetic and natural carbonate samples
    • Abstract: Publication date: Available online 25 June 2015
      Source:Chemical Geology
      Author(s): Bethany P. Theiling , Max Coleman
      Sulfur and oxygen isotope analyses of trace and whole mineral sulfate are valuable in investigating diagenetic processes and the microbial communities that produced them, seawater anoxia, and paleoclimate. Oxygen isotopes are particularly useful in that they may also record alterations to the original isotope ratio, be it from post-depositional processes or oxidation of sulfide minerals during the chemical extraction procedure. Here we rigorously test several common methodological procedures of extracting carbonate associated sulfate (CAS) for sulfur and oxygen isotope analysis in order to generate a method that will extract only the CAS, while preserving associated organic reduced sulfur and sulfides for analysis. The results of these experiments on synthetically generated carbonates demonstrate that our proposed protocol sufficiently removes all non-CAS sulfate and does not result in oxidation of included sulfides. Analytical reproducibility (standard deviation) of synthetic carbonates is 0.1‰ (1σ) for δ34S and 0.3‰ (1σ) for δ18O. Extractions of low pyrite, high organic matter geologic samples from the Monterey Formation across a range of facies types demonstrate a reproducibility (1σ) of 0.4‰-0.7‰ for δ34S and 0.8‰-1.3‰ for δ18O, resulting from sample heterogeneity. δ34S and δ18O from Monterey Formation samples do not demonstrate oxidation of organic matter, suggesting our proposed protocol will preserve organic sulfur. A high pyrite-concentration Jet Rock concretion demonstrates that additional sulfate can be produced during the non-CAS leaching processes from oxidation of pyrite. We show that pyrite from the Jet Rock concretion ceases to oxidize when the sample is leached under an anoxic environment.


      PubDate: 2015-07-03T00:23:31Z
       
  • Post-earthquake anomalies in He-CO2 isotope and relative abundance
           systematics of thermal waters: the case of the 2011 Van earthquake,
           eastern Anatolia, Turkey
    • Abstract: Publication date: Available online 25 June 2015
      Source:Chemical Geology
      Author(s): Harun Aydın , David R. Hilton , Nilgün Gülecxç , Halim Mutlu
      We report the helium and carbon isotope (3He/4He, δ13C) and relative abundance (CO2/3He) characteristics of hydrothermal gases from eastern Anatolia sampled ~1 month after the October 23, 2011 Van earthquake (Mw: 7.2, focal depth: 19 km). Seven sites were sampled which comprise three localities along the Çaldıran fault zone, at a distance of 58-66 km to the epicenter (Group I), two localities north of the Çaldıran fault, about 90-113 km from the epicenter (Group II), and two localities in vicinity of the historically-active Nemrut Caldera at a distance of 110-126 km from the epicenter (Group III). All sites were previously sampled for their He-CO2 systematics in 2009 (Mutlu et al., 2012) facilitating direct comparison with the post-earthquake dataset. The post-earthquake values cover a wide range of 3He/4He, δ13C and CO2/3He ratios, from 0.84 to 6.37 RA (where RA =air 3He/4He), -5.30 to +0.49‰ (vs. VPDB), and 4.9x1010 to 6.85x1013, respectively. Group I samples show a consistent post-earthquake increase in 3He/4He whereas both Group II localities decreased in 3He/4He. No change was recorded for the two Group III localities. He isotope variations are consistent with simple changes in the proportions of mantle and crustal volatiles, with all Group I sites showing an increase in the mantle He contribution. We hypothesize that the enhanced mantle He signal is derived from asthenospheric melts intruded into the crust, with seismic perturbations responsible for bubble formation and growth leading to overpressure and gas loss. The strike-slip Çaldıran fault zone provides the permeable pathway for the liberated volatiles to reach hydrothermal systems at shallow levels of the crust and the surface. Release of crustal He dominates the He mass balance of Group II samples as locations are further from the earthquake epicenter. Group III samples are even further away from the earthquake and show no perturbations in He isotopes. Whereas binary mixing dominates the He isotope systematics, CO2 shows additional effects involving the hydrothermal system. Consequently, changes in the balance between mantle and crustal CO2 are masked and more difficult to discern. The results emphasize the sensitivity of He isotopes to seismic perturbations in the crust and illustrate how location of sampling sites – on permeable segments of faults – and distance from seismic events influence resulting changes involving gas chemistry.


      PubDate: 2015-07-03T00:23:31Z
       
  • [Cobalt(III)-EDTA]− reduction by thermophilic methanogen
           Methanothermobacter thermautotrophicus
    • Abstract: Publication date: Available online 30 June 2015
      Source:Chemical Geology
      Author(s): Rajesh Singh , Hailiang Dong , Deng Liu , Amy R. Marts , David L. Tierney , Catherine Almquist
      Cobalt is a metal contaminant at high temperature radioactive waste disposal sites. Past studies have largely focused on mesophilic microorganisms to remediate cobalt, despite the presence of thermophilic microorganisms at such sites. In this study, Methanothermobacter thermautotrophicus, a thermophilic methanogen, was used to reduce Co(III) in the form of [Co(III)-EDTA]−. Bioreduction experiments were conducted in a growth medium with H2/CO2 as a growth substrate at initial Co(III) concentrations of 1, 2, 4, 7, and 10 mM. At low Co(III) concentrations (<4 mM), a complete reduction was observed within a week. Wet chemistry, X-ray absorption near-edge structure (XANES) and electron paramagnetic resonance (EPR) analyses were all consistent in revealing the reduction kinetics. However, at higher concentrations (7 and 10 mM) the reduction extents only reached 69.8% and 48.5%, respectively, likely due to the toxic effect of Co(III) to the methanogen cells as evidenced by a decrease in total cellular protein at these Co(III) concentrations. Methanogenesis was inhibited by Co(III) bioreduction, possibly due to impaired cell growth and electron diversion from CO2 to Co(III). Overall, our results demonstrated the ability of M. thermautotrophicus to reduce Co(III) to Co(II) and its potential application for remediating 60Co contaminant at high temperature subsurface radioactive waste disposal sites.


      PubDate: 2015-07-03T00:23:31Z
       
  • Microanalytical methods for in-situ high-resolution analysis of rock
           varnish at the micrometer to nanometer scale
    • Abstract: Publication date: Available online 30 June 2015
      Source:Chemical Geology
      Author(s): D.S. Macholdt , K.P. Jochum , C. Pöhlker , B. Stoll , U. Weis , B. Weber , M. Müller , M. Kappl , S. Buhre , A.L.D. Kilcoyne , M. Weigand , D. Scholz , A.M. Al-Amri , M.O. Andreae
      A wide range of analytical techniques were used to investigate rock varnish from different locations (Negev, Israel; Knersvlakte, South Africa; Death Valley and Mojave Desert, California): a 200nm-femtosecond laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS), an electron probe microanalyzer (EPMA), focused ion beam (FIB) slicing, and scanning transmission X-ray microscopy–near edge X-ray absorption fine structure spectroscopy (STXM–NEXAFS). This combination enables comprehensive high-spatial-resolution analysis of rock varnish. Femtosecond LA-ICP-MS and EPMA were used for quantitative determination of element concentrations. In-situ measurements were conducted on thick and thin sections with a resolution of 10–40μm and 2μm, respectively. The results demonstrate that some elements, such as Mn, Co, Pb, Ni, and Cu, are highly enriched in varnish relative to the upper continental crust (up to a factor of 100). The varnish composition is not influenced by the composition of the underlying rock, which is witnessed by plots of MnO2 vs. SiO2 contents. Furthermore, the Mn-free end members fall in the range of average dust compositions. The varnishes from the various locations show distinct differences in some elemental ratios, in particular Mn/Fe (0.3–25.1), Mn/Ba (4–170), Ni/Co (0.03–1.8) and Pb/Ni (0.4–23). The rare earth element (REE) patterns vary with LaN/YbN =3.5–12 and different degrees of Ce anomalies (Ce/Ce*=1.5–5.3). To study the internal structure of the varnish, 100–200nm thick FIB slices were prepared and mappings of Fe, Mn, N, CO3 2−, Ca, C, and Si at the nm scale performed. Banded internal structures of Mn, Fe and organic C were observed in the Israeli and Californian samples, however, no Fe-rich layers are present in the South African rock varnish samples. Furthermore, cavities were found that are partly filled by C, Fe, and Mn rich material. Internal structures are different for varnish from different locations, which might reflect different types of genesis. The results of the combined microanalytical techniques give important detailed insights towards unraveling the genesis of rock varnish.


      PubDate: 2015-07-03T00:23:31Z
       
  • Separation of monomethylmercury from estuarine sediments for mercury
           isotope analysis
    • Abstract: Publication date: Available online 20 June 2015
      Source:Chemical Geology
      Author(s): Sarah E. Janssen , Marcus W. Johnson , Joel D. Blum , Tamar Barkay , John R. Reinfelder
      Estuarine sediments support the production of monomethylmercury (MeHg) which accumulates in aquatic organisms. While natural variation in mercury stable isotope ratios can potentially be used to track sources and transformations of mercury in the environment, the isotopic signature of MeHg in sediments has not been measured directly. The isotopic composition of MeHg has been studied in laboratory experiments and fish using tandem gas chromatography-multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) systems; however, the precision and sensitivity of this method may be too low for the analysis of many environmental samples including sediments in which MeHg constitutes 1% or less of the total mercury. In this study, we developed an offline separation method for the precise measurement of the Hg isotopic composition of MeHg in estuarine sediments. Separation of MeHg from inorganic species was accomplished by distillation and chemical ethylation-GC, and was followed by gold amalgam trapping to collect and preconcentrate pyrolyzed MeHg, which was then released into an oxidizing solution. MeHg standards processed in this way were collected with an average yield of 97.5%. External precision for all replicate isotope analyses of MeHg process standards was ±0.14‰ (2SD, n=8) for δ202Hg and no detectable fractionation of Hg stable isotopes occurred during the separation. δ202Hg values for MeHg separated from estuarine sediments using our approach varied from -0.41 to +0.41‰ and were generally higher, and spatially and temporally more variable, than those for total Hg (-0.21 to -0.48‰).


      PubDate: 2015-07-03T00:23:31Z
       
  • Size distribution of alkali elements in riverbed sediment and its
           relevance to fractionation of alkali elements during chemical weathering
    • Abstract: Publication date: Available online 19 June 2015
      Source:Chemical Geology
      Author(s): Kazuya Tanaka , Naoko Watanabe
      In this study, we analyzed major elements, Rb and Cs in riverbed sediments after size separation to investigate the release and retention of alkali elements (K, Rb, and Cs) during chemical weathering. The degree of chemical weathering (measured using the chemical index of alteration) increased with decreasing particle size of the sediment, which is consistent with the mineralogy of the size-fractioned sediments. The analytical results from the size-fractioned sediments show that K and Rb are released from primary minerals during chemical weathering, whereas Cs is retained and accumulates in the weathering products (i.e., fine particle fractions), possibly because of a strong fixation by clay minerals. The analytical results show that the durability toward chemical weathering increases in the order of K, Rb, and Cs. As a result, the Rb/K and Cs/K ratios in the river water samples were low relative to the sediments, reflecting ongoing chemical weathering in the catchment basin. The Rb/K and Cs/K ratios of the sediments and soils would increase with progressive chemical weathering. Thus, the degree of fractionation between K, Rb, and Cs can be used to evaluate the degree of chemical weathering for granitic rocks. The behavior of stable Cs and radiocesium isotopes, both of which were distributed more in the fine particles of sediments, was coherent. However, isotope equilibrium was not reached, possibly because of the presence of stable Cs at the inner (nonreactive) sites of the minerals, depending on the particle size. Thus, the Cs isotope ratio of 137Cs/133Cs decreased with increasing particle size. In conclusion, the strong affinity of Cs for the weathering products of clay minerals causes a large fractionation of alkali elements during chemical weathering, and the transportation of particulates and dissolved fractions results in a further fractionation between the alkali elements.


      PubDate: 2015-07-03T00:23:31Z
       
 
 
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