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  Subjects -> EARTH SCIENCES (Total: 636 journals)
    - EARTH SCIENCES (462 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: 10)
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: 19)
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: 12)
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: 3)
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  
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: 12)
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: 86)
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  [2807 journals]
  • 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 roles of decompression rate and volatiles (H2O+Cl±CO2±S) on
           crystallization in (trachy-) basaltic magma
    • Abstract: Publication date: 14 September 2015
      Source:Chemical Geology, Volume 411
      Author(s): Adrian Fiege, Francesco Vetere, Gianluca Iezzi, Adam Simon, François Holtz
      The decompression rates (r) and the amount and types of volatile species (H2O+Cl±CO2 ±S) are crucial parameters for the physical behavior of ascending magmas because they affect the formation of crystals and bubbles, as well as bulk viscosities of volcanic suspensions and their eruptive styles (e.g., effusive vs. hazardous explosive eruptions). However, the roles of CO2, S and Cl, in addition to H2O, on decompression induced crystallization (DIC) are still poorly understood. In this study, we investigated the DIC of a trachybasaltic magma, as a function of volatile content (H2O≈5wt.%; Cl≈0.7wt.%; ±CO2 ≈2000ppm; ±S≈3000ppm) and r. Isothermal decompression experiments were conducted at T =1030°C and log(fO2)≈QFM+2 (QFM: quartz–fayalite–magnetite buffer) by releasing pressure (P) continuously from 300 to 70MPa at r =0.01, 0.1 and 1MPa/s. The phase assemblages at 300MPa, before the onset of decompression were composed of 91 to 99area% melt/glass, 2 to 9area% clinopyroxene, ≪1 area% spinel and ≪1 area% bubbles; with higher cpx proportions in the CO2-bearing samples (i.e., at lower water activity). We compare our experimental results with numerical models of equilibrium degassing (using SolEx and DCompress), phase assemblages (using MELTS) and with literature data on phase stabilities in S-bearing and S-free systems. These comparisons reveal that near-equilibrium conditions are reached in all three investigated systems if decompressed at the lowest rate of 0.01MPa/s before quenching. The crystallization of clinopyroxene during decompression at 0.1 and 0.01MPa/s is strongly enhanced by the presence of S, whereas spinel shows less significant variations. Plagioclase, biotite and olivine only occur in the S-bearing samples decompressed at a r of 0.01MPa/s. At r =0.01MPa/s (near-equilibrium) a crystallinity of ~60 area% was reached in the S-bearing system, while the S-free systems are characterized by a ~20 area% crystallinity. The strong influence of S on the crystallinity is mainly explained by an increasing thermal stability of clinopyroxene during decompression to 70MPa, which is probably due to the lower water activity in the presence of S at P ≤70MPa and the increase of the liquidus T with decreasing water content in the melt (i.e., with decreasing P at volatile saturated conditions). Even though the viscosity of the S-bearing melt is relatively low during decompression, the increasing modal abundance of clinopyroxene (and other phases) can lead to a strong increase of the effective magma viscosity and, thus, can slow down magma ascent within the conduit and limit the escape of bubbles. Subsequently, the magma may either i) solidify at shallow depths or ii) new magma inputs and/or a P increase in the conduit owing to second boiling can induce an explosive eruption. Hence, the obtained experimental results indicate that S-bearing basaltic arc magmas have a higher chance to erupt explosively than S-poor basalts, especially if the ascent rate is relatively low.


      PubDate: 2015-07-28T08:55:05Z
       
  • 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
       
  • A nitrogen isotope fractionation factor between diamond and its parental
           fluid derived from detailed SIMS analysis of a gem diamond and theoretical
           calculations
    • Abstract: Publication date: Available online 25 June 2015
      Source:Chemical Geology
      Author(s): D.C. Petts , T. Chacko , T. Stachel , R.A. Stern , L.M. Heaman
      To determine the magnitude of N-isotope fractionation between diamond and its parental fluid, detailed C- and N-isotope analyses of a complexly-zoned, eclogitic diamond (JDE-25) were undertaken using secondary ion mass spectrometry. Combined C- and N-isotope and N-abundance measurements were made across four distinct growth zones and show the following range of values: δ 13C=−5.7 to −2.1‰; δ 15N=−7.0 to +5.5‰; [N]=104 to 5420at.ppm. The core zone displays a continuous, rimward increase in δ 13C and δ 15N values and decreases in N-abundance, and is interpreted to have formed by fractional crystallization of diamond from a single pulse of fluid (i.e., closed system). Modelling of the isotopic and abundance data from the core zone yields a diamond–fluid nitrogen partition coefficient (K N) of 4.4 and a N-isotope fractionation factor (∆15Ndiam–fluid) of −4.0±1.2‰ (2σ) at ~1100°C, for precipitation from a pure carbonate fluid. Calculated K N and ∆15Ndiam–fluid values would have larger magnitudes if JDE-25 formed from a more complex fluid, in which the carbonate species formed only a minor component. Theoretical calculations of N-isotope fractionation between the principal N-species associated with upper mantle fluids (N2, NH3 or NH4 +) and the CN− molecule, as an analogue for the carbon–nitrogen bond in diamond, yield the following ∆15Ndiam (CN)–fluid estimates at 1100°C: −3.6‰ for NH4 +, −2.1‰ for N2 and −1.4‰ for NH3. The theoretical calculations provide only minimum estimates of the true diamond–fluid N-isotope fractionation factor, given that the C–N single bond in diamond would have a lower affinity for 15 N than the stronger C–N triple bond in the CN− molecule. Accordingly, the theoretical N-isotope fractionation factors are consistent with the empirical fractionation factor derived from diamond JDE-25. As a consequence of the large magnitude of ∆15Ndiam–fluid, intracrystalline N-isotope variations in diamond should provide a sensitive test for fluid-related, fractional crystallization processes. Furthermore, the large magnitude of ∆15Ndiam–fluid could be reflected in the wide range of δ 15N values for natural diamonds and the absence of clearly defined modes for the N-isotope compositions of peridotitic and eclogitic diamonds.


      PubDate: 2015-07-03T00:23:31Z
       
  • 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
       
  • Chemical weathering controls on variations in the molybdenum isotopic
           composition of river water: evidence from large rivers in China
    • Abstract: Publication date: Available online 26 June 2015
      Source:Chemical Geology
      Author(s): Zhibing Wang , Jinlong Ma , Jie Li , Gangjian Wei , Xuefei Chen , Wenfeng Deng , Luhua Xie , Weijian Lu , Liang Zou
      Mo isotopic composition in large rivers is very important for understanding global Mo cycle. At present, temporal variation signatures in the δ98/95Mo in large rivers have not been investigated, which hinder a comprehensive understanding on the mechanism for the variations of Mo isotopic compositions in river water. In this study, we report a one-year-long time series (March 2010 to March 2011) of the δ98/95Mo of both the water and suspended particles collected at Guiping, from the middle reaches of the Xijiang River (XJR), and of the water δ98/95Mo from the lower reaches of the Huanghe River (HHR, or Yellow River) collected at Lijin, China. The results indicate that the temporal variations in the concentration and δ98/95Mo of dissolved Mo in the XJR range from 4.32 to 10.5nmol/L (mean 7.31nmol/L) and 1.04‰ to 1.31‰ (relative to NIST 3134) (mean 1.20‰), respectively, but that the particulates have a lower δ98/95Mo (-0.18‰ to 0.58‰). Analysis of the suspended particulates and other chemical parameters of the river water suggest that the weathering of silicates and sulfides is the main contributor to the dissolved Mo content in the XJR. Subsequently, the highly efficient preferential trapping of lighter δ98/95Mo by weathering products such as clay minerals, Fe–Mn oxides, and organic materials in soils and saprolites, which are abundant in the tropical catchment, is the key to the heavy Mo isotope signatures in the XJR. Furthermore, it seems that neither Mo-scavenging by suspended particulates during riverine transportation, nor the weathering of sulfates, significantly influence the δ98/95Mo of the XJR water. Cross comparison among the δ98/95Mo in the waters of the three largest rivers in China (the Changjiang (CJR), HHR, and XJR) indicates that the HHR has the highest Mo concentrations, but the lightest δ98/95Mo, the XJR has the lowest Mo concentrations but the heaviest δ98/95Mo, and the values for the CJR are intermediate between those for the other two rivers. This supports the conclusion that the efficiency of the selective trapping of Mo by soils and saprolites is the main factor controlling δ98/95Mo in these large rivers. The tropical/subtropical XJR catchment experiences intense chemical weathering, the semi-arid temperate HHR catchment experiences very little, and the CJR catchment falls somewhere between the two. Such a relationship between water Mo concentrations and δ98/95Mo agrees with previous observations from many large rivers worldwide. It is therefore suggested that chemical weathering on continents is the key to variations in the isotopic composition of Mo in the waters of large rivers.


      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
       
  • Impact of heterotrophic bacterium Pseudomonas aureofaciens on the release
           of major and trace elements from podzol soil into aqueous solution
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Olga Yu. Drozdova , Liudmila S. Shirokova , Audrey Сarrein , Sergey A. Lapitskiy , Oleg S. Pokrovsky
      The release of major and trace elements (TEs) from mineral and organic-rich podzol soil horizons has been studied in batch reactors with live and dead soil heterotrophic Pseudomonas aureofaciens bacteria and in bacteria-free systems. Dissolved organic carbon (DOC) concentrations decreased and dissolved inorganic carbon concentrations increased over the course of experiments with live bacteria due to on-going DOC mineralization processes. Several families of major and trace elements could be distinguished, depending on their release patterns in bacteria-free systems and live bacteria-bearing systems. Live bacteria enhanced the release of Mg, Rb, Cd, Pb, Al, Fe and V from the mineral soil horizon and the release of Rb, Ni, Pb, As, Fe, V and La from the organic soil horizon relative to bacteria-free soil or dead bacteria experiments. Unexpectedly, K, Ca, Sr, Cu, Ti, Mn, Zn and As release from the mineral horizon and Mg, K, Ca, Sr, Ba, Cr, Ti, Mn and Zn release from the organic horizon decreased in the presence of live P. aureofaciens compared to bacteria-free and dead bacteria systems. Finally, live bacteria exhibited no effect on the release of Si, Al, Cu and Mo from the humic horizon and Ni, Mo, Cr, Ba, Si and La from the mineral horizon relative to the bacteria-free system. These results can be interpreted via a combination of several simultaneous processes, which occur in the soil–bacteria suspension and lead to changes in TE speciation and affinities to mineral surfaces and bacteria and include the following: 1) a slight decrease in the pH due to exometabolite production; 2) degradation of DOC and TE organic complexes by heterotrophic bacteria; 3) element adsorption at cell surfaces and biological uptake; and 4) element release from the soil mineral and organic particles. Compared to abiotic systems, the observed decreases in the concentrations of major elements and many heavy metals that leach from the soil in the presence of bacteria have important consequences regarding our understanding of the role of bacteria in element mobilizations from soil to rivers. It follows that chemical weathering in both organic and mineral horizons of podzol soil may not be strongly affected by heterotrophic bacterial activity; rather, the solution pH and DOC levels may control the intensity of element mobilization. The aqueous concentrations of many TEs (Fe, Al, La, Cr, Ni, Cd, Pb, Cu, and Rb) at the end of live-bacteria experiments were comparable with reported compositions of interstitial podzol soil solutions.
      Graphical abstract image

      PubDate: 2015-07-03T00:23:31Z
       
  • Approaching the geochemical complexity of As(V)-contaminated systems
           through thermodynamic modeling
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Katherine Vaca-Escobar , Mario Villalobos , Teresa Pi-Puig , Rodolfo Zanella
      Arsenate mobility in oxic environments is largely controlled by its adsorption to iron (hydr)oxides, but precipitation as heavy metal arsenates represents a potentially significant competing mechanism. Predicting As geochemical behavior in heterogeneous contaminated systems where various simultaneous equilibria are taking place may be achieved in a thermodynamically sound manner by coupling adsorption and solid-aqueous equilibria, provided that accurate equilibrium constants are employed; especially challenging is the surface complexation model segment. The influence of adsorption and precipitation processes on As(V) mobility was modeled in the presence of Pb(II) and goethite by varying the As/Fe and As/Pb ratios, the goethite particle size, pH, and the inclusion of chloride and sulfate ions. A bottom-up approach is adopted here to gradually approximate the geochemical complexity of real contaminated scenarios. A unified surface complexation model for goethites of different particle sizes was used, which accounts for differences in their reactivity, and was coupled with a thermodynamic speciation model for aqueous and solid-phase equilibria. The geochemical conditions found that favor As(V) precipitation as Pb(II) arsenates were high As/Fe, low As/Pb, goethites of small particle size, and especially the presence of chloride, and low sulfate concentrations (at low pH). The opposite conditions favor As(V) adsorption. Surprisingly, precipitation processes are more prevalent than expected and are favored from relatively low As/Fe in the presence of chloride, or with small particle-sized goethites.
      Graphical abstract image

      PubDate: 2015-07-03T00:23:31Z
       
  • Sulfide metasomatism and the mobility of gold in the lithospheric mantle
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): J. Edward Saunders , N.J. Pearson , Suzanne Y. O'Reilly , W.L. Griffin
      Gold is a strongly chalcophile element and its concentration and behaviour in the mantle are intimately connected to the nature, abundance and mobility of sulfide phases. In this study Au has been analysed together with a suite of chalcophile and siderophile elements in sulfides hosted in 31 mantle-derived xenoliths from Spitsbergen in the Svalbard Archipelago. These data have been combined with analysis of silicate phases to assess the mobility of Au, and sulfides in general, during mantle processes. The 31 samples comprise lherzolites, harzburgites, dunites and a wehrlite. Several lherzolites contain amphibole and apatite, and two are cross-cut by amphibole veins. The REE patterns in clinopyroxene reveal three geochemically distinct groups of peridotites. Group I samples have flat or LREE depleted patterns ((La/Yb)n <1); Group II samples show sinuous REE patterns; and Group III samples show LREE enrichment with moderately steep slopes ((La/Yb)n >1). The sulfides hosted in the Group I samples have very heterogeneous Au compositions (1.9±1.4ppm), whereas those hosted in Groups II and III samples have lower and more homogenous Au concentrations (Group II Au: 0.66±0.60ppm; Group III Au: 0.53±0.57ppm). A chromatographic metasomatic model is proposed to explain the variations between the three groups of samples. The LREE enrichment, and common inclusion of amphibole in the Group III samples is consistent with modification by a carbonate-rich melt. It is interpreted that these samples were located close to the metasomatic conduit. The sulfide homogeneity in these samples results from extensive equilibration with the metasomatic agent; Au has partitioned into the fluid phase, reducing the Au content of these sulfides. The Group I samples show little to no metasomatic alteration in the silicate phases, indicating they were located far from the conduit. The agents that have affected these samples have been modified by extensive fractionation and wall–rock interaction during percolation through the mantle, becoming enriched in Au. Group II samples are inferred to have been located at an intermediate distance from the conduit with silicate and sulfide characteristics that fall in between those from the other two groups.


      PubDate: 2015-07-03T00:23:31Z
       
  • Carbonate composition and its impact on fluvial geochemistry in the NE
           Tibetan Plateau region
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Yibo Yang , Xiaomin Fang , Albert Galy , Gengxin Zhang , Shaochen Liu , Jinbo Zan , Fuli Wu , Qingquan Meng , Chengcheng Ye , Rongsheng Yang , Xiaoming Liu
      Using co-variations of Sr/Ca and Mg/Ca, we examined the carbonate compositions of various bedrocks (silicate and carbonate rocks) and sediments (eolian and fluvial sediments, sand, and topsoil) found in the NE Tibetan Plateau (TP) region. A combined carbonate composition dataset based on our results and other reported data shows that bedrock carbonate composition on the NE TP displays a much broader range of Sr/Ca and Mg/Ca ratios than restricted source carbonate endmembers reported upon in previous studies. This has clear implications for modern weathering studies in addition to paleo-reconstructions in this tectonically active and climatically variable area during the Late Cenozoic. Bedrock carbonate compositions are characterized by disseminated carbonates with higher Sr/Ca and Mg/Ca ratios, and sedimentary carbonates (mostly marine) with lower Sr/Ca, but variable Mg/Ca, ratios. The mostly authigenic carbonates found in sediments show similar trends, with a gradient ~0.97–1.00 in a plot of log (Sr/Ca) versus log (Mg/Ca), suggesting that ‘calcite precipitation’ processes – i.e. the sources of the dissolved cations in the water – control their chemistry. Based on observations and modeling, we conclude that the mixing of authigenic and bedrock carbonate endmembers, plus the incongruent dissolution of bedrock carbonates, accounts for the bulk carbonate composition of sediments (e.g. loess, sand and topsoil). A comparison of bedrock and sedimentary carbonate composition with reported fluvial water data in the NE TP suggests that weathering of carbonates in terrigenous sediments, rather than in bedrock, is mostly responsible for the changes in fluvial Sr, Mg and Ca compositions. Our study suggests that interactions between carbonates and water occur widely during the exposure, transport and deposition of sediments, significantly modifying regional carbonate compositions and fluvial geochemistry.


      PubDate: 2015-07-03T00:23:31Z
       
  • Abiotic and candidate biotic micro-alteration textures in subseafloor
           basaltic glass: A high-resolution in-situ textural and geochemical
           investigation
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Leif-Erik Rydland Pedersen , Nicola McLoughlin , Per Erik Vullum , Ingunn H. Thorseth
      The oceanic crust provides one of the largest habitats for subsurface microbial life on earth, where lithoautotrophs utilize redox gradients between reduced elements in volcanic rocks and oxygenated seawater to form the basis of a deep microbial biosphere. Progressive alteration of the oceanic crust is argued to be “in part” microbially mediated, but identifying robust textural and geochemical biosignatures with good fossilization potential is challenging. This study investigates pillow basalts from the Antarctic Australian Discordance (AAD) at the South East Indian Ridge (SEIR) containing candidate textural biosignatures in alteration products of the glassy margins (Thorseth et al., 2003). Samples include 2.5Ma dredged seafloor basalts, and 18–28Ma drill core samples from the Ocean Drilling Program (ODP) Leg 187. The focused ion beam (FIB) technique was used to prepare electron transparent foils across spherical microtextures in zeolite filled fractures and altered glass (palagonite), and across microtunnels at the interface of fresh and altered glass. Transmission electron microscopy (TEM) was used to map chemical and ultrastructural variations and to evaluate both biotic and abiotic origins of the candidate textural biosignatures in the FIB prepared foils. Three foils were cut from zeolite hosted, hollow microspheres, which comprise purely Fe-oxyhydroxides, or mixed Mn–Mg, and Fe–Mn oxyhydroxides. The microspheres are 1 to 4μm across, with a radiating ultrastructure, and have a denser inner surface and a more porous outer surface, suggesting outwards growth from a spherical initial surface. Amorphous organic carbon is associated with some of the microtextures both on the inner and outer walls. These microtextures are interpreted as mineral encrusted microbial cells. A FIB-foil was also cut from palagonite-hosted microspheres, which are more irregular in shape and partially infilled by palagonite. Amorphous organic carbon is abundant in the vicinity of the microtextures but is spatially unrelated, and may be derived from several sources. The results indicate that maturation of the palagonite, involving dehydration and recrystallization, overprints and destroys potential biosignatures in this alteration phase. In contrast, the zeolite-hosted microtextures appear to have a higher preservation potential. Tubular microtextures in the glass at the alteration front comparable to argued “bioalteration” textures are also abundant in the AAD basalts. However, their angular cross-sectional shape and lack of “bio-elements” in the palagonite infill, mean that an abiotic origin cannot be excluded. In summary, FIB-TEM provides multiple high-resolution lines of information to characterize alteration textures in ocean floor basalts. Comparing the evidence obtained from glass hosted microtunnels, zeolite and palagonite hosted microspheres we conclude that the zeolite hosted microtextures are the strongest candidate biosignature. The combination of the size, rim ultrastructure, and elemental composition is consistent with an origin as cell encrustations, resulting from the biologically induced mineralization of microbial cells that inhabited fractures in pillow lavas both at the seafloor and the subseafloor stage.


      PubDate: 2015-07-03T00:23:31Z
       
  • Preservation of successive diagenetic stages in Middle Triassic bonebeds:
           Evidence from in situ trace element and strontium isotope analysis of
           vertebrate fossils
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Jeremy Michael McCormack , André Bahr , Axel Gerdes , Thomas Tütken , Peter Prinz-Grimm
      Bonebeds comprise reworked and time averaged constituents of different phosphatic vertebrate remains and, hence, provide an ideal substrate for the study of long and short term diagenetic processes. To test whether trace elements (U, Sr, REE) and 87Sr/86Sr ratios can be used for distinguishing between successive diagenetic signals, we performed geochemical analyses on vertebrate remains (bones, teeth, scales and coprolites) from two Triassic bonebeds located near Palzem (Germany). Trace element analysis was done on 170 vertebrate remains by LA-ICP-MS while 87Sr/86Sr ratios were determined for 39 bioclasts using LA-MC-ICP-MS. Although a low inter- and intra-bioclast REE variability indicates long-term open system behaviour, the observed REE patterns suggest the preservation of different diagenetic stages. A distinct difference in the geochemical composition exists between dark luminescent SEM-CL domains and greyish ones. Dark domains represent dense, diagenetically more resistant material of low porosities such as enamel and ganoine. These domains display distinctly lower ƩREE contents and a higher inter-clast variability than those of various grey tones. Together with lower, hence more seawater-like 87Sr/86Sr ratios this argues for a preservation of an earlier diagenetic signal. Notably, most dark domains are characterised by negative Eu anomalies while all grey ones display positive Eu anomalies. With the exception of Eu both domains display almost identical REE patterns with different concentrations indicating that late diagenetic REE uptake is not necessarily accompanied by fractionation processes in vertebrate bioclasts. Negative Eu anomalies are most likely inherited from Eu-depleted surface waters, possibly as a result of Eu-depleted aeolian input. Conversely, positive Eu anomalies were presumably affected by Eu-enriched late diagenetic fluids with a possible high temperature origin causing a fractionation of europium. Our study therefore demonstrates the potential of high-resolution geochemical analysis in bonebed strata to disentangle different stages of diagenesis and to detect least altered areas in vertebrate fossils for recovering pristine palaeoenvironmental signals.


      PubDate: 2015-07-03T00:23:31Z
       
  • Interpretations and implications of LA ICP-MS analysis of chert for the
           origin of geochemical signatures in banded iron formations (BIFs) from the
           Meadowbank gold deposit, Western Churchill Province, Nunavut
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): B. Gourcerol , P.C. Thurston , D.J. Kontak , O. Côté-Mantha
      Among the many types of mineral deposits within Archean cratons, gold mineralization is an important economic commodity with over 20,000metric tons of gold produced from greenstone belts in 2001. Of the Archean–early Paleoproterozoic gold deposits, several different types of mineralization are known, which includes the important Algoma-type banded iron formation (BIF) where gold is locally associated with sulfide-facies zones within regionally extensive oxide-facies. It is commonly accepted that the shale-normalized chemical signature of REE+Y of chert bands in Algoma-type BIFs may reflect one of the three processes, each of which may be relevant to the nature and origin of the gold mineralization: (1) direct seawater precipitation; (2) involvement of and contribution from hydrothermal fluids; and (3) replacement of precursor volcanic units due to silicification. An essential question in regard to the mineralization is, therefore, whether the gold mineralizing fluids have a preference for one geochemical type of iron formation versus another. In order to assess the relevance of these competing models, we report herein the results of a LA ICP-MS study of chert samples within different Algoma-type BIFs from the Meadowbank deposit (24.5Mt proven/probable ore reserves grading 2.8g/t (2011)) hosted in the Neoarchean Woodburn Lake Group of the Rae Domain of the western Churchill Province, Canada. This study used 39 carefully selected and characterized (i.e., petrography and SEM-EDS imaging) chert samples from the main deposit, the Central BIF, and four additional BIFs, the Far West, West, East and Grizzly zones, with data collected using line traverses along the chert bands. The geochemical data indicate that an ambient seawater signature (i.e., enrichment in HREE relative to LREE associated with positive La and Y anomalies) dominates the samples with a lesser hydrothermal component (characterized by a positive Eu anomaly) and the influence of detrital contamination can also be detected. These initial results indicate that the methodology and protocol employed provides a reliable means to assess and interpret the chemical signature of BIFs hosting gold mineralization. In the present case, the results for the Meadowbank deposit suggest that chert from mineralized BIF units does not record a typical chemical signature that may be used as a vector for potential gold mineralization.


      PubDate: 2015-07-03T00:23:31Z
       
  • Contrasting effects of extracellular polymeric substances on the surface
           characteristics of bacterial pathogens and cell attachment to soil
           particles
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Wenqiang Zhao , Sharon L. Walker , Qiaoyun Huang , Peng Cai
      Extracellular polymeric substances (EPSs) have been confirmed to affect bacterial surface properties and cell attachment to minerals. However, no systematic work has been done to clarify the contrasting roles of EPS in cell attachment to natural soil between different pathogenic strains. This study compared the different surface properties and attachment behaviors of two bacterial pathogens (with full or partial EPS) using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, potentiometric titration, zeta potential, hydrophobicity analysis, DLVO theory, and attachment tests. Cation exchange resin (CER) was employed to remove the EPS on Streptococcus suis and Escherichia coli such that the contribution of EPS to cell attachment to soil could be determined. ATR-FTIR confirmed the binding sites differed between S. suis and E. coli EPS. Notably, after partial EPS removal the absorption bands of S. suis between 1800cm−1 and 800cm−1 shifted or disappeared, whereas the lack of EPS did not affect the infrared absorption peaks for E. coli. This result suggests the overall surface site types within the E. coli EPS were similar to the residual EPS fractions or cell wall. The partial removal of EPS also changed the proton-active site concentrations of both cell types, and reduced the bacterial surface charge densities by 7%–17%. The negative charges on bacterial surfaces followed the order of full EPS-S. suis <partial EPS-S. suis <partial EPS-E. coli <full EPS-E. coli (ionic strength 1–100mM; pH5.6–5.8). With the removal of EPS, the average hydrophobicities of S. suis increased by 5% while those of E. coli decreased by 11%. EPS removal inhibited the attachment of S. suis to soil particles (<2mm) but enhanced E. coli attachment across the IS range of 1–100mM, which was attributed to the alteration in electrostatic repulsion. At IS 60–100mM, a sudden reduction in the attachment was observed only for full EPS-S. suis, which could be ascribed to the steric hindrance derived from EPS. However, full EPS-E. coli and partial EPS-E. coli showed similar increasing attachment trends at IS 1–100mM. This study clearly showed the distinct contribution of EPS to pathogen attachment to soil as a function of cell type and EPS present.
      Graphical abstract image

      PubDate: 2015-07-03T00:23:31Z
       
  • Vanadium geochemistry along groundwater flow paths in contrasting aquifers
           of the United States: Carrizo Sand (Texas) and Oasis Valley (Nevada)
           aquifers
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Katherine Telfeyan , Karen H. Johannesson , T. Jade Mohajerin , C. Dianne Palmore
      Vanadium (V) concentrations were measured along the flow paths in two contrasting aquifer systems to study how changing solution composition and redox conditions affect V cycling and transport in groundwater flow systems. The Oasis Valley groundwater system in Nevada is an unconfined system where groundwaters generally remain oxic along the flow path, whereas the Carrizo Sand aquifer is a deep, confined coastal plain aquifer in Texas that contains well-defined Fe(III) and SO4 2− reduction zones along the flow path. Speciation modeling predicts that in groundwaters from the Oasis Valley, V chiefly occurs as the vanadate oxyanion (e.g., H2VO4 −, HVO4 2−), which is mobile along the entire flow path because of vanadate's solubility in oxic and alkaline waters. Furthermore, the data indicate that V concentrations increase in groundwaters along the flow path suggesting that mineral dissolution and/or desorption reactions progressively contribute V to groundwaters of the Oasis Valley system. Geochemical modeling is consistent with dissolution of mafic phenocrysts that are common in the local tuffaceous aquifer rocks as a probable source of V to Oasis Valley groundwaters. Within the Carrizo Sand aquifer, groundwater V concentrations decrease with flow beyond the recharge zone and into the confined portion of the aquifer. Thereafter, total V concentrations remain low and relatively constant along the remainder of the studied flow path, exhibiting no systematic variations across the Fe(III) and SO4 2− reduction zones. Groundwaters from the Carrizo Sand aquifer are generally reducing, and as such, geochemical modeling suggests that both V(V) and V(IV) species occur in Carrizo Sand groundwaters, and furthermore, that the proportion of V(IV) species increases concurrent with the decrease in total V concentrations along the flow path. Results from sequential extraction experiments on Carrizo Sand aquifer sediment samples suggest that V adsorption onto Fe(III)/Mn(IV) oxides/oxyhydroxides acts to decrease the aqueous V concentrations in Carrizo Sand groundwater. Furthermore, geochemical modeling is consistent with complexation of V(IV) with dissolved organic matter (DOM) within the Carrizo Sand aquifer groundwaters, which, owing to the low DOM concentrations, appears to buffer V at low, but relatively constant concentrations within the confined and reducing Carrizo Sand groundwaters. These results support previous reports of V(IV)–DOM binding and imply that aquifer material maintains an essential role in not only determining the availability of V to enter aqueous solution but also in influencing partitioning between solid and aqueous phases.


      PubDate: 2015-07-03T00:23:31Z
       
  • Arsenate and phosphate adsorption on ferrihydrite nanoparticles.
           Synergetic interaction with calcium ions
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Juan Antelo , Florencio Arce , Sarah Fiol
      The geochemical behaviour of phosphate and arsenate ions in soil and aquatic systems is determined by the presence of mineral surfaces and major ions. Information about the distribution of oxyanions over the solid and solution phases is essential for understanding the transport, bioavailability and toxicity of these compounds in the environment. Here, we studied the adsorption of both arsenate and phosphate on ferrihydrite nanoparticles in the presence of calcium ions. The presence of calcium ions enhanced the retention of these oxyanions on ferrihydrite and vice versa. The arsenate–calcium and phosphate–calcium multi-component systems were described using a mechanistic surface complexation model. Use of this type of model enables prediction of the solution and surface speciation, along with analysis of oxyanion mobility in relation to environmental conditions. We were able to calibrate the charge distribution model with the macroscopic data obtained for the single-component systems, thus obtaining surface complexation constants for later use to simulate multi-component systems. The mutual interactions between arsenate and calcium were successfully described with these parameters, indicating that changes in the electrostatic forces at the solid/solution interface caused the observed enhanced adsorption. However, adsorption in the phosphate–calcium system was underestimated with the parameters obtained for the single-component systems, indicating that additional mechanisms or processes should be considered. Formation of insoluble mineral phases was ruled out, but the inclusion of a phosphate–calcium ternary surface complex improved the modelling predictions.


      PubDate: 2015-07-03T00:23:31Z
       
  • Ge/Si ratios indicating hydrothermal and sulfide weathering input to
           rivers of the Eastern Tibetan Plateau and Mt. Baekdu
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Yeongcheol Han , Youngsook Huh , Louis Derry
      Concentrations of dissolved silicon in river waters reflect a complex interplay among chemical weathering of primary silicate minerals, formation and weathering of secondary clay minerals, hydrothermal input and biological cycling (formation and dissolution of opal phytoliths and growth of diatoms). We applied the Ge/Si ratio to assess the different sources of dissolved Si in rivers hailing from the eastern Tibetan Plateau — the Salween, Mekong, Chang Jiang (Yangtze), Hong (Red) and Huang He (Yellow) and from Mt. Baekdu — the Duman. Elevated riverine Ge/Si ratios were observed in arid regions with high geothermal activity in the Salween, Chang Jiang and Mt. Baekdu streams. In the Huang He and Hong River basins geothermal influence was not as pronounced, but weathering of sulfide- and coal-bearing minerals may be responsible for the high Ge/Si ratios. In rivers where inputs from hydrothermal and sulfide weathering are minimal, our data mostly fall in the weathering-limited regime of high riverine Si concentrations and low Ge/Si ratios.


      PubDate: 2015-07-03T00:23:31Z
       
  • Selenium isotope ratios, redox changes and biological productivity across
           the end-Permian mass extinction
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Eva E. Stüeken , Julien Foriel , Roger Buick , Shane D. Schoepfer
      The causes of the greatest mass extinction in Earth's history, in the latest Permian, remain actively debated. Here we use Se isotopes and abundances in marine sediments from an outer-shelf environment to test one of the most common hypotheses for the collapse of the biosphere, i.e. widespread euxinia in the open ocean. Our data show a small positive excursion in δ82/78Se prior to the extinction, consistent with local euxinia. However, this is followed by a significant negative excursion with a minimum of −1.8‰ (relative to NIST SRM 3149), immediately preceding the principal extinction horizon. A net fractionation of this magnitude likely resulted from partial reduction of Se oxyanions dissolved in the water column. Due to their low abundance, Se oxyanions are rapidly scavenged in anoxic basins or regions of high biological productivity with little net isotopic fractionation. We therefore interpret the uniquely negative fractionations in this section as an indicator for relatively oxygenated conditions in this marine basin at the time when biological productivity declined. The offset between the peak excursion and the major extinction horizon possibly reflects a slow-down in ocean circulation leading to nutrient limitation, which may thus have prohibited a rapid recovery of the local biosphere in the early Triassic. Although we are unable to extrapolate to the global ocean due to the short residence time of Se in seawater, our data are consistent with the newly emerging view that euxinia developed along ocean margins and in oxygen minimum zones before the extinction, but was probably replaced by (sub-)oxic conditions during the ~1kyr peak productivity decline and was thus not solely responsible for the extinction event.


      PubDate: 2015-07-03T00:23:31Z
       
  • Fate of sulfide in the Frasassi cave system and implications for sulfuric
           acid speleogenesis
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Daniel S. Jones , Lubos Polerecky , Sandro Galdenzi , Brian A. Dempsey , Jennifer L. Macalady
      The oxidation of hydrogen sulfide (H2S) has led to the formation of some of the world's largest caves through a process known as sulfuric acid speleogenesis (SAS). Here we present a multi-year study of the large, sulfidic, and actively-forming Frasassi cave system, Italy. We show that despite the presence of abundant sulfide-oxidizing biofilms in Frasassi streams, H2S(g) degassing to the cave atmosphere was the major sink for dissolved sulfide. Degassing rates ranged from 0.9 to 80μmolm−2 s−1, whereas microbial oxidation rates were between 0.15 and 2.0μmolm−2 s−1. Furthermore, microsensor measurements showed that sulfuric acid is not a major end product of microbial sulfide oxidation in the streams. Our results suggest that subaerial SAS will be important for karstification, and more important than subaqueous SAS, wherever ground waters with high sulfide concentrations emerge as flowing streams in contact with cave air.


      PubDate: 2015-07-03T00:23:31Z
       
  • High Co-doping promotes the transition of birnessite layer symmetry from
           orthogonal to hexagonal
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Hui Yin , Yu Liu , Luuk K. Koopal , Xionghan Feng , Shengqi Chu , Mengqiang Zhu , Fan Liu
      Despite its presence in limited amounts, birnessite has a wide spread distribution and is often highly enriched in trace metals such as Co in diverse geological environments. This study investigated the effects of Co doping on the layer structure and properties of birnessites synthesized through the oxidation of Mn2+ by O2 under alkaline conditions, by using powder X-ray diffraction (XRD) and X-ray absorption spectroscopy (both XANES and EXAFS). The Co doped, high-pH birnessites are composed of platy crystals, and have lower crystallinity, larger specific surface areas (SSAs) and higher Mn average oxidation states (AOSs) than pure birnessite. Cobalt K-edge EXAFS analysis reveals that no CoOOH is formed, and ~76% of the total Co is located in the layers of the Co-doped birnessites. Careful examination of these Co-doped samples by XRD and EXAFS analyses demonstrates that the presence of Co during the synthesis of high-pH birnessite promotes the structural transition of the birnessite layer symmetry from orthogonal to hexagonal. This is due to the decrease in Mn(III) in the layers of the doped solids, leading to the attenuation of Jahn–Teller effect. The decrease in Mn(III) in the layers might be attributed to the substitution of Mn(III) by Co(III) in the layers. The competitive adsorption of Co2+/3+ with Mn2+ might also decrease the oxidation of Mn2+ to Mn(III) and the subsequent migration of Mn(III) into the birnessite layers. The results provide new insights into the interaction mechanisms between transition metals and birnessite-like minerals, and improve our understanding of the abiotic oxidation of Mn2+ as well as the prevalence of birnessites with hexagonal symmetry in natural environments.


      PubDate: 2015-07-03T00:23:31Z
       
  • The effect of lithium on the viscosity of pegmatite forming liquids
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Alexander Bartels , Harald Behrens , Francois Holtz , Burkhard C. Schmidt
      Viscosity experiments were conducted to investigate the effect of Li2O on the viscosity of pegmatite forming melts and the possible difference to K2O. The individual effects of Li2O and K2O on melt viscosity were investigated by adding the same molar proportion of both components to a starting composition representing natural pegmatite forming melts. The viscosity experiments on hydrous melts (~1 to ~6wt.% H2O) were carried out in the low temperature range at ambient pressure using the micropenetration technique and in the high temperature range at 200–300MPa pressure using the falling sphere method. The addition of Li2O and K2O results in a decrease of melt viscosity and, on a molar basis, the effect of Li2O is comparable to that of K2O. Viscosity-related properties such as activation energy of viscous flow, melt fragility and glass transition temperature are affected in a similar way by both alkali oxides. The observed effects of alkalis on the viscosity of melt are smaller when compared to an equal molar amount of OH− groups at low water content but become equal with increasing water content. Finally, it is shown that the low viscosity of pegmatite forming melts can only be explained by the combined effects of H2O, F and of non-charge balancing alkalis.


      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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